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Marine Biological Laboratory Library

Woods Hole, Mass.

Presented by

the estate of

Dr. Herbert W. Rand
Jan. 9, 1964

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COMPARATIVE ANATOMY

OF

VERTEBRATES

"

MACMILLAN AND CO., LIMITED

LONDON . tOMBAY . CALCUTTA
MELBOURNE

THE 'MACMILLAN COMPANY

NEW YORK . BOSTON . CHICAGO
ATLANTA . SAN FRANCISCO

THE MACMILLAN CO. OF CANADA, LTD.

TORONTO

COMPARATIVE ANATOMY

VERTEBRATES

ADAPTED FROM THE GERMAN OF

DR. ROBERT WIEDERSHEIM

PROFESSOR OF ANATOMY, AND DIRECTOR OF THE INSTITUTE OF HUMAN AND COMPARATIVE. ANATOMV

IN THE UNIVERSITY OF FREIBURG-IN-BADEN

BY

W. N. PARKER, PH.D.

PROFESSOR OF ZOOLOGY AT THE UNIVERSITY COLLEGE OF SOUTH WALES AND MONMOUTHSHIRE

IN THE UNIVERSITY OF WALES

THIRD EDITION

(FOUNDED ON THE SIXTH GERMAN EDITION)

IBRARY =

*

WITH THREE HUNDRED AND SEVENTY-TWO FIGURES
AND A BIBLIOGRAPHY

MACMILLAN AND CO., LIMITED
ST. MARTIN'S STREET, LONDON

1907

All rights reserved

RICHARD CLAY ANL> SONS, LIMITED,

BREAD STREET HILL, E.C., AND

BUNGAY, SUFFOLK.

PREFACE

THE developmental history of this book has been somewhat
complicated. The first German edition appeared in 1882, under
the title of Lehrbuch der vergleichendc Anatomic der Wirbeltiere>
and a second edition followed in 1886. In 1884, a short
Grwidriss was published, which, after passing through four
editions and gradually increasing in size, replaced the Lehrbuch
under the title of Vergleichende Anatomic der Wirbeltiere, in 1902.
A further edition appeared in 1906, and this was followed by a
shorter Einfuhrung in 1907 : the latter was written to meet the
requirements of beginners, and it contains no bibliography.

The first and second English editions (1886 and 1897) were
based respectively on the first and third editions of the Grundriss,
and considerable modifications in detail were introduced. The
present (third) edition, which has been almost entirely re-written,
was prepared from the German editions of 1906 and 1907, and I
am much indebted to Professor Wiedersheim for allowing me to
make such alterations as seemed desirable in the interests of
English students, and for the pleasure of his collaboration last
summer, when I had the advantage of discussing various points
with him personally.

The general plan of the original has been retained throughout,
but I found it advisable to extend some portions and to abridge
others, besides making various minor modifications.

After re-editing by Professor Wiedersheim, the bibliography
given in the German original is inserted entire, except that the
titles and references have been abbreviated and slight rearrange-
ments made in order to save space : I have also ventured to
introduce a few additions. Though rather extensive for a work of

vi PREFACE

the kind, the list must not be regarded as anything approaching a
complete one of the more important papers relating to Vertebrate
Comparative Anatomy ; but I trust that such a list of references
in an easily accessible form will be found useful to advanced
students.

My thanks are due to Mr. T. H. Burlend, Demonstrator and
Assistant Lecturer in Zoology at this College, for preparing the
index.

W. N. PARKER.

UNIVERSITY COLLEGE, CARDIFF,
August, 1907.

CONTENTS

Preface v

INTRODUCTION 1

I. On the Meaning and Scope of Comparative Anatomy 1

II. Development and Structural Plan of the Vertebrate Body ... 2

III. Classified List of the Principal Vertebrate Groups 14

IV. Table showing the Gradual Development of the Vertebrata in

Time 16

SPECIAL PART.

A. INTEGUMENT 17

of Amphioxus 17

of Fishes 18

of Amphibians 20

of Reptiles 23

of Birds 25

of Mammals 28

Mammary Glands 34

B. SKELETON 39

1. EXOSKELETON 39

2. ENDOSKELETON 44

I. VERTEBRAL COLUMN 45

of Fishes 48

of Amphibians 54

of Reptiles 57

of Birds 59

of Mammals . . 60

II. 'RIBS 63

of Fishes 63

of Amphibians 66

of Reptiles 67

of Birds .... 69

of Mammals 69

8249

viii CONTENTS

PAGE
III. STERNUM 70

V. SKULL 74

General part 74

a. Brain -case 7(i

/*. Visceral Skeleton 80

c. Bones of the Skull 82

Special part 84

A. The Skull of Fishes . . . . 84

i:. ,, of Amphibians ... ... 97

c. ,, of Reptiles . . 108

D. ,, of Birds . . 120

E. ,, of Mammals 123

VI. APPENDICULAR SKELETON 135

a. Unpaired Fins 136

b. Paired Fins or Limbs 137

Pectoral Arch . . 140

of Fishes 140

of Amphibians 141

of Reptiles 142

of Birds 143

of Mammals 143

Pelvic Arch 145

of Fishes . . 145

of Amphibians 146

of Reptiles . .... 149

of Birds i;,2

of Mammals 154

Paired Fins of Fishes 155

Paired Limbs of the higher Vertebrata 159

of Amphibians 161

of Reptiles . 163

of Birds 165

of Mammals 168

C. MUSCULAR SYSTEM . 173

INTEliUMENTARY MUSCLES . .... 175

MUSCLES OF THE TRUNK 17'.)

of Amphioxus and Fishes 17 ( .t

of Amphibians . 181

of Reptiles 181

of Birds ... . .182

of Mammals . 183

MUSCLKS OF TIII: HI \I-IIKAI;M . 184

CONTENTS ix

i' \<.i<:

MUSCLES (IF THE APPENDAGES 185

BYE-MUSCLES . . 186

VISCERAL MUSCLES .... 187

<>f Fishes .... ... 187

of Amphibians 188

of Amniota 189

D. ELECTRIC ORGANS ... . . 190

E. NERVOUS SYSTEM AND SENSORY ORGANS . . 193

T. CENTRAL NERVOUS SYSTEM . . ... 195

Membranes of the brain and spinal cord 195

1. SPINAL CORD . . 198

2. BRAIN (general description and development) 199

of Cyclostomes 204

of Elasmobranehs . 208

of Ganoids ... 211

ofTeleosts 211

of Dipnoans 214

of Amphibians .... 215

of Reptiles ... 220

of Birds .221

of Mammals 225

II. PERIPHERAL NERVOUS SYSTEM . . 230

1. SPINAL NERVES .... 233

2. CEREBRAL NERVES . 234

Sympathetic 247

III. SENSORY ORGANS (general description and development) . 249

SENSE-ORGANS OF THE INTEGUMENT 250

<(. Nerve-Eminences 250

I. End-buds and gustatory organs . . 254

c. Tactile Cells and Corpuscles 255

'/. Club-shaped or lamellar Corpuscles 257

OLFACTORY ORGAN (general description and development) . . 258

of Cyclostomes 259

of Fishes .... . . 261

of Amphibians ... ... 263

of Reptiles . . 265

of Birds .... ... 266

of Mammals 267

Vomero-nasal (Jacobson's) Organ 271

EYE (general description and development) .... . 273

of Amphioxus ... 277

of Cyclostomes .... 278

of Fishes . 279

x CONTENTS

PAGE

EYE (continued)

of Amphibians 281

of Reptiles and Birds 282

of Mammals 283

Retina 284

Accessory Organs in Connection with the Eye 286

a. Eye-Muscles 286

6. Eyelids 287

c. Glands 288

AUDITORY ORGAN (general description and development) . . . 290

of Cyclostomes 295

of Fishes .... 295

of Amphibians 297

of Reptiles and Birds 299

of Mammals 301

F. ORGANS OF NUTRITION 308

ALIMENTARY CANAL AND ITS APPENDAGES (general

description) 308

I. ORAL CAVITY 312

Teeth (general description) 313

of Fishes and Amphibians 314

of Reptiles and Birds 316

of Mammals ... 318

Glands of the Mouth 324

of Amphibians 325

of Reptiles 325

of Birds . . 326

of Mammals 327

Tongue 327

THYROID 331

THYMUS . . . 333

II. CKSOPHAGUS, STOMACH, AND INTESTINE 335

of Ichthyopsida 335

of Reptiles 339

of Birds 339

of Mammals 341

Histology of the Mucous Membrane of the Alimentary Canal 344

LIVER 346

PANCREAS ... 348

G. ORGANS OF RESPIRATION . . 351

I. GILLS 351

of Amphioxus 352

CONTENTS xi

PAOE

GILLS (continued)

of Cyclostomes ". . 352

of Fishes 355

of Amphibians . '. . . 358

II. SWIM-BLADDER AND LUNGS 361

1. SWIM-BLADDER 361

2. LUNGS -' . 362

Air-Tubes and Larynx 364

of Amphibians 366

of Reptiles 369

of Birds ' . , 370

of Mammals ...'.. '371

The Lungs 377

of Dipnoans 377

of Amphibians :...... 377

of Reptiles 379

of Birds 382

of Mammals ; . 387

CCELOME 388

SEROUS MEMBRANES . . 388

ABDOMINAL PORES - 389

H. ORGANS OF CIRCULATION ... 393

General Description and Development 393 '

Heart, together with origin of Main Vessels 400

of Fishes . . 400

of Amphibians 403

of Reptiles 407

of Birds and Mammals 411

Arterial System 414

Venous System 419

of Amphioxus 419

of Fishes 419

of Amphibians 426

of Amniota 428

Retia Mirabilia 431

Lymphatic System 432

MODIFICATIONS FOR THE INTER-UTERINE NUTRITION

OF THE EMBRYO : FCETAL MEMBRANES 436

1. Anamnia 436

2. Amniota 437

xii CONTENTS

PAGE

I. URINOGENITAL ORGANS (general description and develop-
ment) 441

Pronephros 443

Mesonephros . 445

Metanephrous 446

Male and Female Generative Ducts 446

Gonads 450

URINARY ORGANS ... ... 452

of Amphioxus .... 452

of Fishes . 452

of Amphibians 455

of Repti]es and Birds 459

of Mammals . . . 461

GENITAL ORGANS ... 463

of Amphioxus . 463

of Fishes . 463

of Amphibians . ... 469

of Reptiles and Birds 474

of Mammals . 477

AITESSORY UENITAL GLANDS OF MAMMALS . ... 484

COPULATORY ORUANS .... . 486

ADRENAL BODIES 4!'L'

APPENDIX (Bibliography) 497

INDEX 565

ERRATA

PAGE

5 5th line from bottom, for "walls," read " wall."

13 Fig. 11, insert " gonad (ovary)" below " kidnc\'."

33 ,, 25, for " N and G nerves," read " JV, nerves ; G, blood-vessels."

40 8th line from top, for "disappear," i-l "disappears."

40 llth ,, ,, /or " ossifications," read "ossification."

.').'> Fig. 42, transpose A and B.

72 Line 6 from bottom, dfkte " the first " and " cervical. 1 '

73 ,, 10 .. for "are," read ~ is."

73 ,, 3 ,, for " proximal," read " anterior. "

73 ,, 2 ,, for " distal," rend " posterior.''

92 Fig. 69, insert "0" before "orbital ring," and " *</in)~>/. , symplcclic."

96 ,, 71, ,, "A", external gills."

112 3rd line from top, /or " system/' rratl " septum."

119 Fig. 86, for " condyles," read " condyle."

145 10th line from bottom, delete " B."

151 4th ,, top, for " show," read "shows."

155 18th ,, ,, for " stright," read " straight."

164 Fig. 129, for " 1st," read "5th."

17P 17th line from top, for "and thus effect.' 1 ,-< nil " thus effecting."

298 Fig. 218A, for " P r ," read " Ph."

304 ,, 223, for " vestibula," read " vestibnli."

315 3rd line from top, for " cartilaginious," read "cartilaginous."

340 Fig. 250n, for " tendrinous," read "tendinous."

43s 20th line from bottom, for " Phalcolarctos," ,-, ad " Phascolarctos."

445 6th and 7th lines from bottom, for " parosphoi'on," read " paroophoron."

450 13th line from top, for " epididennis." nail " epididymis."

^

but also to understand, tne meaning 01 numerous

B

xii CONTENTS

I 1 Ac.K

I. URINOGENITAL ORGANS (general description and develop-
ment) 441

Proiie[)hros 443

Mesonephros 445

Mctanephrous 44fi

Male and female Generative Ducts 44(5

(ronsuls

COMPARATIVE ANATOMY

INTRODUCTION.

I. ON THE MEANING AND SCOPE OF COMPARATIVE ANATOMY.

A KNOWLEDGE of the natural relationships and ancestral history
of animals can only be gained by a comparative study of their
parts (Comparative Anatomy) and of their mode of develop-
ment (Embryology or Ontogeny). In addition to existing
animals, fossil forms must also be taken into consideration (Pa-
laeontology), and by combining the results obtained under these
three heads, it is possible to make an attempt to trace out the
development of the various races or groups in time (Phylogeny).
As the different phases of development of the race may be repeated
to a greater or less extent in those of the individual, the depart-
ments of Phylogeny and Ontogeny help to complete one another.

It must, however, be borne in mind that in many cases the
phases of development are not repeated accurately in the individual
that is, are not palinyendic, but that ' ; falsifications " of the re-
cord, acquired by adaptation, very commonly occur along with
them, resulting in cccnogenetic modifications in which the original
relations are either no longer to be recognised at all, or are more
or less obscured. In this connection, two important factors must
be taken into consideration, viz., heredity and variability. The
former is conservative, and tends to the retention of ancestral
characters, while the latter, under the influence of change in
external conditions, results in modifications of structure which are
not fixed and unalterable, but are in a state of constant change.
The resulting adaptations so far as they are useful to the
organism concerned, are transmitted to future generations, and
thus in the course of time gradually lead to still further modifica-
tions. Thus heredity and adaptation are parallel factors, and a
conception of the full meaning of this fact helps us not only to
gain an insight into the blood-relationships of animals in general,
but also to understand the meaning of numerous degenerated and

P,

2 COMPARATIVE ANATOMY

rudimentary or vestigial organs and parts in the adult organism
which would otherwise remain totally inexplicable.

Histology is a subdivision of anatomy which concerns the
structural elements the building-stones of the organism, and the
combination of these to form tissues. Various combinations of
the tissues give rise to organs, and the organs, again, combine to
form systems of organs.

The structural elements consist primarily of cells and second-
arily of cells and fibres often enclosed in an intercellular substance
or matrix', and the different tissues may be divided into four
principal groups :

1. Epithelium, and its derivative, glandular tissue.

2. Supporting tissue (connective tissue, adipose tissue, cartilage,
bone).

3. Muscular tissue.

4. Nervous tissue.

In accordance with the functions they perform, epithelium and
supporting tissue may be described as passive, and muscular and
nervous tissue as active.

By an organ we understand an apparatus which performs a
definite function : as, for instance, the liver, which secretes bile ;
the gills and lungs, in which an exchange of gases is effected with
the surrounding medium ; and the heart, which pumps blood
through the body.

The organ-systems, which will be treated of in order in this
book, are as follows : 1. The outer covering of the body, or inte-
gument ; 2. The skeleton; 3. The muscles, together with electric
organs ; 4. The nervous system and sense-organs ; 5. The organs of
nutrition, respiration, circulation, excretion, and reproduction.

The closely-allied branches of science denned above are in-
cluded under the term Morphology, as opposed to Physiology
which concerns the functions of organs, apart from their morpho-
logical relations. The combined results obtained from these two
fields of study throw light on the organisation of animals in
general that is, on Zoology in its widest sense.

II. DEVELOPMENT AND STRUCTURAL PLAN OF THE
VERTEBRATE BODY.

The structural elements described in the preceding section as
the building-stones of the organism, i.e. the cells, all arise from a
single primitive cell, the egg-cell or ovum. This forms the
starting-point for the entire animal-body, and a general account of
its structure and subsequent development must therefore be given
here.

The ovum consists of a rounded, nucleated, protoplasmic body

INTRODUCTION 3

(Fig. I), consisting of the vitellus, in the interior of which is the
germinal vesicle, enclosing one or more germinal spots: the
membrane which covers the vitellus is spoken of as the vitelline
membrane. Since the ovum corresponds to a single cell, we may
speak of the vitellus as the protoplasm of the egg-cell, the
germinal vesicle as its nucleus, and the germinal spot as its
nucleolus : the vitellus, however, consists of two different sub-
stances protoplasm and deuteroplasm (yolk} in varying propor-
tion and relative distribution in different animals.

The nucleus is enclosed by a delicate nuclear membrane, and is
made up of two chief constituents the spongioplasm or chromatin,
and the hyaloplasm or achromatin. A
small particle, the ccntrosome, is also
present in the protoplasm of the cell,
and takes an important part in the
process of cell-division. An outer
limiting membrane, corresponding to
the vitelline membrane, is not an in-
tegral part of the cell, but may be
differentiated from the peripheral pro-

toplasm ^ IC{ - 1- DIAGRAM OF THE

In the process of sexual reproduc-

tion, which occurs in all Vertebrates, A vitellus; KB, germinal
the fusion with the ovum of the vesicle ; KF > S erminal s P ot '
sperm-cell or spermatozoon, con-

taining the generative substance of the male, is an absolute
necessity for the development of the former.

Before this fusion can occur, certain changes take place in
the ovum which constitute what is known as its maturation, the
essential result of which is a reduction in mass of the chromatin
in the germinal vesicle. The ovum undergoes a twice-repeated
process of cell division (karyokinesis or mitosis) similar to that
which occurs in tissue-cells, except that the resulting daughter-
cells, in addition to the reduction in their chromatin, are of
different sizes, two small evanescent polar-cells (Fig. 2) being suc-
cessively thrown off from the larger ovum. A similar process also
occurs in the development of the sperm-cell, except that there is
no difference in size between the products of division. The
portion of the original nucleus remaining in the ovum or sperm
is then known respectively as the female (or maternal) and male (or
paternal) pronucleus.

A sperm-cell then makes its way into the ovum, and its pro-
nucleus unites with the female pronucleus to form the segmentation
nucleus. This process, which is known as impregnation or
fertilisation, thus consists in a material intermingling of the
generative substances of both sexes, or more accurately of the
sperm-nucleus and egg- nucleus. The essential cause of inherit-
ance can thus be traced to the molecular structure of the nuclei of

B 2

COMPARATIVE ANATOMY

is

the

both male and female germinal cells. This structure
morphological expression of the characters of the species.

After fertilisation has taken place development begins. The
segmentation nucleus divides into two equal parts, each of which
forms a new centre for the division of the oosperm, as it must
now be called, into two halves or Uastomcres. This division, the
beginning of the process of segmentation, takes place by the
formation of a furrow round the egg which becomes deeper and
deeper until the division is complete. (Fig. 2, A).

The first stage in the process of segmentation is thus com-
pleted ; the second takes place in exactly the same way, and

FIG. 2. DIAGRAMS OF THE SEGMENTATION OF THE OOSPERM.

A, first stage (two segments) : JfK, polar cells. B, second stage (four segments).

C, further stage. D, morula stage.

results in a division of the oosperm into four parts ; and by a similar
process are formed eight, then sixteen, then thirty-two blastomeres,
and so on, the cells becoming smaller and smaller, and each being
provided with a nucleus. (Fig. 2, c D). In short, out of the
original oosperm a mass of cells is formed which represents the
building-material of the animal body, and which, from its likeness
in appearance to a mulberry, is spoken of as a morula.

In the interior of the morula a cavity (segmentation cavity or
llastoccele) filled with fluid is formed, and the morula is now spoken
of as the blastosphere or blastula (Fig. 3). The peripheral cells
enclosing this cavity form the germinal membrane or blasto-

INTRODUCTION

TIT)

FIG. 3. BLASTOSPHERE.

BD, blastoderm ; FH, segmentation
cavity.

derm. Consisting primarily of a single layer of cells, the blasto-
derm later on becomes two-layered, and then three-layered. From
the relative position of these, they are spoken of respectively as
the outer, middle, and inner
germinal layers, or as ecto-
derm (epiblast), mesoderm
(mesoUast*), and endoderm
(hypoblasf).

The mode of distribution
of the yolk-particles in the
ovum, and an increase in their
amount, result in certain
modifications of the primitive
form of segmentation as de-
scribed above. Yolk is an
inert substance, and its pre-
sence tends to hinder or even
entirely to prevent segmenta-
tion in those parts of the
ovum in which it is abundant.

When the whole ovum undergoes division, the segmentation is
known as entire or lioloblastic ; when division is restricted to part
of the ovum only, the segmentation is said to be partial or mcro-

blastic l (Fig. 4).

The question as to the origin of the
germinal layers, on account of its im-
portant signification, is one of the most
burning problems in morphology, and as
yet we cannot arrive at any full and
satisfactory conclusion on the subject.
It may, however, be stated that in all
Vertebrates the blastosphere passes or
did so in earlier times into a stage
called the gastrula, which is retained
in an unmodified form only in the lowest
Vertebrate (Amphioxus, cf. p. 14). The
gastrula is derived primitively from the
blastula by the walls of the latter
(Fig. 3) becoming pushed in or invaginated at one part, thus
giving rise to a double-walled sac (Fig. 5). The outer wall
then represents the ectoderm, which serves as an organ of
protection and sensation, while the inner, or endoderm, encloses

1 In holoblastic segmentation the resulting cells are approximately equal in
the Lancelet and in Mammals (with the exception of Monotremes and some
Marsupials) ; and unequal in the Cyclostomes, Sturgeon, Lepidosteus, Dipnoans,
and nearly all Amphibians, the segmentation sometimes approaching the mero-
blastic type. In Elasmobranchs, Teleosts, Reptiles, Birds, and Monotremes
the segmentation is from the first meroblastic and discoid, i.e.., restricted to the,
upper pole of the ovum (Fig. 4).

B*

No,

FIG. 4. DIAGRAM OF A MERO-
BLASTIC OOSPERM WITH
DISCOID SEGMENTATION.

Bla, blastoderm ; Do, yolk.

6 COMPARATIVE ANATOMY

a central space, the primitive digestive cavity (arckenterpn). The
opening of the latter to the exterior, where the two germinal
layers are continuous, represents the primitive mouth or Nastopore,
which is represented to a certain extent by the primitive streak of
higher forms.

From the ectoderm arise the epiderm and its derivatives, the
entire nervous system, the sensory cells, the lens and certain muscles
of the eye, the oral and anal involutions (stomodceum and procto-
dceuiii), and the oral portion of the pituitary body attached to the
brain. In an early stage the endoderm gives rise to an axial rod,
the notochord (Figs. 6 and 7), and eventually to the epithelium of the

Z'At-

I o

FIG. 5. GASTBULA.
Blp, blastopore ; Ekt, ectoderm ; Ent, endoderm ; U, archenteron.

greater part of the alimentary canal (Figs. 6 and 10) and its
glands, including the thyroid, thymus, liver and pancreas, as well
as to the epithelial parts of the gill-sacs and lungs.

Though we may look upon the ectoderm and endoderm that
is, both the primary germinal layers as arising primitively in the
manner above described, various modifications occur, depending
largely on the type of segmentation, and known as overgrowth
(epiboly\ dclamination, and partial delamination. The middle layer
or mesoderm is a secondary formation, and is phylogenetically
younger than the other two germinal layers ; both as regards the
origin of its cells and histologically, it is of a compound nature,
and thus forms a marked contrast to the two germinal layers proper.
One of its first and most important functions is the formation of
blood-cells ; later it gives rise to the heart, vessels, and to nearly
all the supporting and connecting substances (connective tissue,
adipose tissue, cartilage, bone), serous membranes (peritoneum,

INTRODUCTION

K

Ekt

Fi<;. 6, A AND B. DIAGRAMMATIC TRANSVERSE SKCTIONS THROUGH A DEVELOMM;

VERTEBRATE EMBRYO.

A, aorta; Ch l , (Fig. B), the notochord now constructed oft' from the endoderm ;
Go, Co'l, ca'lome ; J), alimentary canal ; Ekf, ectoderm ; Ent, endoderm,
showing in Fig. A the thickening (Gh) which will form the notochord ; H,
remains of the upper part of the cu'lomu in the interior of the mesodei inic
somites; M<><1, central nervous system (medullary <-<>rd) : in Fig. A it is
shown still connected with the ectoderm, from which il has become constricted
off in Fig. B; So P, somatic, and $pP, splanchnic mesoderm ; U<!, primary
urinary duct (pronephric duct); U\V, mesodermic somite.

P,**

8

COMPARATIVE ANATOMY

pleura, pericardium and muscles, as well as to almost the entire
excretory and reproductive apparatus.

A space in the mesodermic tissue divides it into a parietal or
somatic layer, lying along the inner side of the ectoderm, and
into a visceral or splanchnic layer, which becomes attached to the
outer side of the endoderm (Fig. 6, A and B). The former, together
with the ectoderm to which it is united, constitutes the somato-

mes

D

Fi<;. 7. DIAGRAMMATIC SECTIONS OF THK PKIMITIVK KNDODKKM AND ITS

DERIVATIVES OF AMPHIOXUS (A C, AFTER GOTTK).

From the dorsal part of the endoderm (unshaded) arise the notochcml (<//) and
the mesodermic somites (mrx) ; from the ventral part (shaded), the epithelial
lining of the gut. The entire endoderm in stages R and G thus shows four
hollow projections : a median (chordal) and paired lateral (mesodermal) out-
growths communicating at first with the larger ventral intestinal cavity (tlh),
but later becoming constricted off from the latter (D). The notochordal
outgrowth becomes solid, while the cavities in the mesodermal segments
represent the rudiments of the ctelome.

pi cure, and the latter, together with the endoderm, the splanclmo-
plcure. The cavity separating these is the body-cavity or ccclomc,
and is lined by an epithelium.

The ccelomc may arise as a segmen tally arranged series of
pouches (enteroccelcs) from the archenteron, in which case its lining
epithelium is at first continuous with the endoderm, as is most
plainly seen in Amphioxus (Fig. 7); or it may be formed
secondarily by a splitting (delamination) of the mesodermic tissue
(schizocasle). The former of these must be considered as the more
primitive.

INTRODUCTION 9

The dorsal part of the mesoderm which lies on either side of
the middle line in all cases early becomes metamerically
segmented to form a longitudinal series of mesodermic somites or
protovertebrce, which lose their cavities (cf. Fig. 6, A and B), and are
concerned in the formation of the vertebral column, body-muscles,
and part of the urinogeni tal apparatus. This primary segmenta-
tion must be distinguished from the segmentation which appears
later, on the formation of the vertebral column, ribs, spinal nerves,
&c. The ventral parts of the mesoderm are known as the lateral
plates ; in Ainphioxus alone do they exhibit segmentation.

As a general rule, a thickened disc-shaped region can be
recognised at a certain stage of development on the dorsal pole of
the oosperm : this is the so-called embryonic area, on which the
first indications of the body are seen (Fig. 4). This region
gradually sinks into the underlying yolk, from which it becomes
constricted off all round by the formation of furrows, and conse-
quently the connection of the body- rudiment with the ventral
yolk-sac (by the vitcllo -intestinal duct] is gradually reduced in size,
and when the yolk is eventually entirely absorbed disappears
altogether (Fig. 8, f). In the higher Vertebrates (Reptiles, Birds,
and Mammals) folds of the somatopleure arise externally to these
furrows, and are known respectively as the head-, tail-, and lateral
folds; these gradually grow upwards and eventually unite with
one another dorsally so as to form a membranous dome-like sac,
the amnion (Fig. 8) which encloses the embryo and contains a
fluid (liquor amnii).

Owing to the presence of this structure, the above-named
Vertebrates are usually distinguished as Amniota from the
Anamnia (Fishes and Amphibians), in which no amnion is
developed (p. 14).

A network of blood-vessels becomes developed over the yolk-
sac, which may therefore serve as an organ of respiration as well
as of nutrition. But in the higher Mammals (Eutlicria^) this func-
tion is only a very subsidiary one, as at a very early stage a
vascular sac-like outgrowth, the allantois (Fig. 8), arises from
the hinder part of the intestine (i.e. from the splanchnopleure).
This serves not only for respiration, but also for the reception of
excretory matters derived from the primitive kidney. It is also
present in Amphibians, but in them remains small, and does not
extend beyond the body-cavity of the embryo ; while in the
Amniota it gradually increases in size and grows round the embryo
as a stalked vesicle, which in Reptiles, Birds, and Monotremes
comes to lie close beneath the egg-shell and acts as an efficient
respiratory organ during the rest of the embryonic period.
Towards the close of this period the allantois gradually undergoes
more or less complete reduction.

In the higher Mammalia, an important vascular connection
takes place between the mother and foetus by means of the

10

COMPARATIVE ANATOMY

&F/.

B

C

Fig. S, A, I',, AND ('. PIAOKA.MS ILU T STI:ATIN<! THE FORMATION OK THE ASJNJON,
ALLANTOIS, AM> YOLK-SAC. A AND I'., IN F>ONCITI-IIN AI, SKI Ti<r; ; C, IN
TRANSX KIISK SIOCTION.

A, amnion ; .IF, amniotic fold: Alt, aimiiolii- cavity: At, allantois ; <J, noto-
chord ; l)h, alimentary cavity; /><>, y<> k-sa : A\ liody "t embryo; M,
medullary cord : />}>, ccclonu' ; ", somatopleuve ; ', splanchnopleure ; t, vitello-
jntestinal duct.

INTRODUCTION

11

allantois. The latter becomes attached to a definite region of
the uterine wall, and from it vascular processes or villi arise, so
that the foetal and maternal blood-vessels come into very close
relations with one another. Thus an allantoic placenta is

Pa

PC(CLf)

Dv

FIG. 9. DIAGRAMMATIC .SECTION THROUGH THE HUMAN GRAVID UTERUS.

A, aorta ; A, A, A, the cavity of the amnion filled with fluid : in the interior of the
amnion is seen the embryo suspended by the twisted umbilical cord ; Al,
allantoic (umbilical) arteries ; Chi, chorion Izeve ; D, the remains of the
yolk-sack (umbilical vesicle) ; Dr, decidua reflexa ; Dv, decidua vera, which
at Pu passes into the uterine portion of the placenta ; H, heart ; Pf, f<etal
portion of the placenta (chorion frondosum, Chf) : Tb, Fallopian tube ;
U, uterine wall ; UH, uterine cavity ; <:i, postcaval, cs, precaval, and p.
portal vein ; f, the liver, perforated by the umbilical vein.

formed, which serves both for the respiration and nutrition of the
foetus (Fig. 9).

The following important points must now be noted as regards
the structure of the Vertebrate body. After the above-mentioned
main organs have appeared, a smaller dorsal neural tube and
a larger ventral visceral tube are seen to extend longitudinally
through the body, and between the two is a rod-like support-
ing structure, the notochord (p. 6), which forms the primary

12 COMPARATIVE ANATOMY

skeletal axis ; it is usually replaced by a vertebral column
consisting of rcrtebrce, at a later stage of development (Fig. 10).
All these structures are median in position, and the body is
thus bilaterally symmetrical. The neural tube, or cerebro-sp'uml
cavity, enclosed by the skull and arches of the vertebra:, contains
the central nervous system (brain and spinal cord] ; the visceral tube
(co:lome) encloses the viscera (alimentary canal, heart, urinogenital
organs, &c.), and its muscular walls may be strengthened by a
series of ribs, articulating dorsally with the vertebral column.

Mtd

KW

FIG. 10. DIAGRAMMATIC TRANSVERSE SECTION THROUGH THE BODY OF AN ADULT

VERTEBRATE.

Ao, aorta ; Co, derm ; DH, lumen of intestine ; Ep, endodermic epithelium of
intestine; KW, body- wall ; Med, spinal cord; A/"*, mesentery; Msc, mus-
cular coat of intestine ; JV7?, neural tube ; Per, parietal layer of the peri-
toneum ; Per 1 , visceral layer of the peritoneum ; Stiln, connective-tissue
coat of intestine ; VR, visceral tube ; d', vertebra.

Certain of the ribs may reach the mid-ventral line and come into
connection with a breast-bone or sternum, and thus form complete
rings or hoops around the visceral tube.

The anterior end of the central nervous system (brain) and of
the alimentary tract enter into close relations with the outer
world, the former being connected with the higher sense-organs,
while from the latter are developed the mechanisms for taking in
nutriment and for respiration.

The anterior portion of the body, or head, passes behind into
the trunk, either with or without the intermediation of a neck. The
coelome is practically restricted to the trunk, in the hinder part of

INTRODUCTION

13

which the intestinal (anal) and urinogenital apertures are situated,
and posterior to which again is the tail. Head, trunk, and tail
constitute the body-axis, as distinguished from the fins or limbs
(appendages), which arise from the trunk and of which there are
typically two pairs. In addition to the paired appendages,
median fins are present in aquatic forms.

SPINAL CORD

NEURAL TUBE (CCREBRO SPINAL CANAL)

NOTOCHORD VISCERAL

BRAIN

ORAL CAVITY

INTERNAL GILL SLITS,

HEART LlVEK

ENITALCUCT

^ CLOACA
URINARY BLADDER

F. OUCT
PANCREAS

FIG. 11. DIAGRAMMATIC LONGITUDINAL SECTION OF A VERTEBRATE ( ? ).

SYSTEMATIC ZOOLOGY is concerned with the classification of
animals, on the basis of their relationship to one another, into
certain divisions and subdivisions, the chief of which are
designated as Phyla, Classes, Orders, Families, Genera, and Species.
In this connection, structural resemblance (homology) is taken into
account, and not mere functional resemblance (analogy}.

A general classification of the principal Vertebrate groups,
sufficient for the purposes of this book, and also a table showing
the order of their appearance in past time, are given on the
following pages.

u

COMPARATIVE ANATOMY

Division A.
B.

S
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65
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PM
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ACRANIA (CEPHALOCHORDATA).
Lancelet (Amphioxus).

GRANT ATA.

/ o. A N A M N I A.

Class I. CYCLOSTOMATA (Suctorial Fishes).

Order 1. Myxinoidei (Hag Fishes Myxine, Bdello-

stoina).

,, 2. Petromyzontes (Lampreys).
Class II. PISCES (True Fishes).
Sub-class 1. ELASMOBBANCHII.

Order 1. Pluyioxtomi (Sharks Selachii, and Rays

Batoidei).

,, 2. Holot-ephali (Chimzera, Callorhynchus).
Sub-class 2. TELEOSTOMF.

( Order 1. Cro8sopterygii(Polypterus,Cala.michihys).}
,, Chrondrostei (Acipenser, Polyodon).

3. Holoxtt-i (Lepidosteus, Amia).

4. Te.lv.oxli i. ^
Sub-order a. Physostomi (Carp, Pike, Salmon,

Herring, Eel, Silnroids).
., /*. Anocanthini (Cod, Flat-fishes).
,, c. A canthopteri (Perch, Stickleback,

Gurnard, Blenny).
., d. Pharynyoynatlii (Wrasse).
,, e. Plectognuthi (Q'runk- and File-

tishes).
,, /. Lophobranchii (Pipe-fish, Sea-

horse).
Sub-class 3. DIPNOI.

Order 1. Monopneumona (Ceratodus).

,, 2. Dijmeumona (Protopterus, Lepidosiren).
Class III. AMPHIBIA.

Order 1. Urodela.

a. Perennibraiichiatu (Proteus, Siren, Necturus).

{Derotremata (Amphiuina,
Menopoma)
Myctodera (Salamandra, Tri-
ton, Amblystoma),
2. Anuru (Frogs and Toads).
., 3. Gymnophiona (Limbless Csecilians).

/ /3. A M N i o T A.

Class IV. REPTILIA.

Order 1. Rhynchocephali (Hatteria).
,, 2. Lacertilia (Lizards).
,, 3. O/thidia (Snakes).
,, 4. Chelonin (Turtles and Tortoises).
,, 5. Grocodilia (Crocodiles and Alligators).
(.'lass V. AVES.

a. Jfiititu (Cursorial Birds Ostrich, Rhea, Emu, &c. ).
I. Carinatte (Birds of Flight).

Class VI. MAMMALIA.

Sub-class 1. PROTOTHERIA or ORNITHODELPHIA (the Ovi-
parous Monotremes -- Ornithorhynchus,

Echidna).

,, 2. METATHERIA or DIDELPHIA (Marsupials-
Kangaroo, Phalanger, Opossum. ).

,, 3. ElTTIIKKIA 01' MitNOUELPHIA.

INTRODUCTION

15

s

I

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w

Order 1. EdtuJatn (Sloth, Anteater).

2. Sirenia (Dugong, Manatee).

3. Cetacea (Porpoise, Whale).

4. Ungnlata (Rhinoceros, Horse, Ruminants.)

5. Hyracoidea (Hyrax).

6. Proboscidea (Elephant).

7. fiodentia (Rabbit, Mouse, Beaver, Cavies).
S. Cheiroptera (Bats).

9. Insectivora (Shrew, Mole, Hedgehog).

10. Camivora (Bear, Dog, Cat, Seal).

11. Prosimii (Lemurs).

12. Primate* (Monkeys and Man).

16

COMPARATIVE ANATOMY

Kainozoic Mesozoic

Palaeozoic

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SPECIAL PART.
A. INTEGUMENT.

THE skin consists of a superficial ectodermal layer, and a
deeper mesodermal layer. The former is called the epiderm
(scarf-skin) and the latter the derm (corium, cutis).

In the epiderm, which consists of cells only, two parts may in
gene'ral be distinguished : an external layer, composed of flattened
and hardened cells (stratum corneum, horny layer}, and a deeper, of
more columnar, formative cells (stratum Malpighii s. germativum,
mucous layer). The latter serves for the regeneration of the
horny layer, the superficial part of which is continually scaling
off, as well as for the formation of such horny structures as hairs,
bristles, nails, claws, and hoofs, and of the integumentary glands.
The peripheral sensory end-organs also arise by a differentiation
of this layer of cells.

The derm, which is usually thicker and tougher than the
epiderm, is made up principally of connective tissue and smooth
muscular fibres : it is usually not sharply marked off from the
subcutaneous connective tissue, which commonly encloses more or
less fat. Externally, the derm may give rise to numerous papillae
projecting into the epiderm, especially in higher forms. Apart
from the horny and glandular structures extending into it from
the epiderm, the derm encloses vessels, nerves, and often bony
structures also.

Pigment cells (chromatophores) and free pigment occur in both
layers of the skin : they correspond to modified connective tissue
cells, and in them a temporary shifting of the contained pigment
may occur, this process being under the control of the nervous
system.

In Amphioxus, the epiderm differs from that of all the Craniata
in the fact that it consists of a single layer of cells : its surface is
covered with cilia in the larval (gastrula) stage, and this must
undoubtedly be considered as an inheritance from Invertebrate
ancestors.

C

18

COMPARATIVE ANATOMY

Fishes.

The character of the epiderm varies greatly in the different
groups (Fig. 12). The striated cuticnlar border (present e.g. in
Cyclostomes, Teleosts, and Dipnoans) possibly indicates the
former possession of cilia. 1 Cornification of the superficial layer
occurs, especially in Teleosts, over those parts of the scales (Fig. 13)
which are not overlapped by their fellows. Numerous lymph-

-JTo

Co

FIG. 12. DIAGRAMMATIC TRANSVERSE SECTION ILLUSTRATING THE STRUCTURE OP

THE SKIN IN FISHES.

B, B, goblet-cells opening on the surface ; Co, derm ; CS, cuticular margin ;
Ep, epiderm ; F, subcutaneous fat ; G, vessels which pass upwards in the
vertical connective tissue bundles (8) of the derm ; Ko, goblet-cells ; Ko,
granular cells ; W, horizontal connective-tissue bundles.

cells (leucocytes) are found in the epiderm which have wandered
out of the derm, and some of them contain pigment. The derm
consists mainly of horizontal and vertical layers of connective
tissue, and encloses the other structures already referred to (Figs.
12 and 13).

Various kinds of mucus-secreting cells are formed in the
epiderm, and in addition to the relatively larger or smaller goblet

1 Cilia are occasionally present in very early stages in Teleosts.

INTEGUMENT

19

cells commonly present may be mentioned the granular cells in
the Lamprey (the nature of which is not understood), and in
Myxinoids the numerous slime-sacs, formed as invaginations of the
epiderm and containing peculiar thread-cells.

Special aggregations of gland-cells occur in relation with the
copulatory organs or claspers of male Elasmobranchs (glandulce
2rterygopodii), and on the operculum and dorsal fin-rays of certain
Acanthopteri (e.g. Trachinus, Thalassophryne, Synanceia), in
which latter they constitute a poison-apparatus serving for offence

defence, and consist of modified epidermic cells enclosed in

or

grooves of the spines of the operculum and dorsal fins. Most

Scales.

Mucus cells.

Pocket enclosing
scale.

Epiderm.

Muscles.

FIG. 13. LONGITUDINAL SECTION OF SKIN OF YOUNG TROUT (15 CM. LONG),

FROM THE TAIL.

g

of these poison-fishes are inhabitants of the temperate and
warmer seas : in fresh-water forms (e.g. Perca, Cottus), the
apparatus has apparently undergone partial or total degeneration.
Poison-organs are also said to occur in a number of other Teleosts
(e.g. in connection with the dorsal and pectoral spines of many
Siluridte) and in certain Elasmobranchs ; a closer examination will
probably prove their existence in many other Fishes.

Phosphorescent organs, formerly known from their appearance
as " accessory eyes/' occur on various parts of the head, body, and
tail of several families of deep-sea Teleosts (e.g. Stomiatida?,
Halosauridae, Anomalopidse), and in certain species of Elasmo-
branchs belonging to the family Spinacidse. Their arrangement,
distribution, and structure is very varied in different forms. The
luminous part consists of gland-cells, supplied by the trigeminal,

c 2

20 COMPARATIVE ANATOMY

facial, and spinal nerves ; and a number of accessory parts may be
present, e.g. a pigment- layer, a reflecting apparatus, a vitreous
body, and structures resembling a lens and an iris. These organs
probably serve to attract prey or to help their possessors in seek-
ing food in the darkness of the deep sea. 1

In addition to very numerous goblet-cells, the epiderm of the
South African Dipnoan, Protopterus, gives rise to cup-shaped
muUiccllidar glands, resembling those of the Amphibia. During
the dry season, this animal (like its South American ally,
Lepidosiren 2 ), buries itself in the river-bottom : its integumentary
glands then produce a varnish-like secretion and an enclosing
cocoon or capsule, by means of which it is protected during the
torpid period.

Pigment-cells, which, as already mentioned, are under the
control of the nervous system, and are able to cause a change of
colour, are present sometimes in both layers of the integument,
sometimes in one only. Colour may also be produced by reflecting
bodies consisting of excretory products (guanin) and known as
iridocytes.

The presence of scales (see under Exoskeleton) may affect the
epiderm when they project from the surface, and in some cases it
may disappear so that the scales become superficial (e.g. in Elas-
mobranchs, Ganoids, and some Teleosts).

Amphibians.

The epiderm of Amphibians differs markedly from that of
Fishes, inasmuch as nearly all the special forms of cells so
characteristic of the latter are wanting. Both epiderm and derm,
moreover, differ in the larva and in the adult. The epiderm at
first consists of a single layer of cells, and then of two layers, the
superficial one being provided with a ciliated or a striated cuticular
border, 3 and remaining throughout the larval period as a covering
layer (Fig. 14). The deeper layer, on the other hand, undergoes
various modifications : it becomes stratified, and replaces the super-
ficial cells as they are lost. Slime-secreting goblet-cells, such as are
characteristic of the epiderm of fishes, are typically wanting, 4 and
leucocytes are not abundant. Unicellular glands, known in
Urodeles as Leydiys cells, are, however, abundant in the larva ;

1 For the electric organs of Malopterurus, which are said to be epidermic in
origin, cf. under Electric Or/jans.

2 In the breeding season the posterior extremities of the male Lepidosiren are
provided with numerous long vascular papillae.

3 Cilia occur abundantly in Salamander larvae over parts of the head and
body, and their distribution is related to that of the integumentary sense-organs ;
they are also found in very young Anuran larva?.

4 In older Urodele larvte, after the epiderm has become thickened, numerous
goblet-like cells can be recognised and probably represent the Leydig's cells
described above.

INTEGUMENT

21

and cells with thread-like and vacuolated contents have also been
described in Anuran larvse. Later on, immediately before meta-
morphosis, numerous muliicdlular glands of alveolar structure

FIG. 14. SKIN OF LARVA OF SALAMANDER (Salamandra maculosa).

a, stratum corneum ; It, stratum Malpighii ; Co, derm ; OS, striated border ;
Ep, epiderm ; LZ, Leydig's cells (unicellular mucus glands).

(cf. p. 20) appear in adaptation for terrestrial life. These have
nothing to do with the unicellular larval glands : their great
abundance is very characteristic of the Amphibian skin (Figs. 15
and 16). As regards their distribution, they may be scattered singly
throughout the skin, or arranged in groups in Anurans chiefly

FIG. 15. SEMIDIAGRAMMATIC SECTION THROUGH THE SKIN OF ADULT
SALAMANDER (S. mactilom).

Co, derm, in the connective tissue stroma of which (B) the various sized integii-
mentary glands (A, C, D, D) lie embedded ; E, epithelium of glands ; Ep,
epiderm ; M l , the muscular, and Pr the connective-tissue layer of the
glands ; M, the same, seen from the surface ; Mm, subcutaneous layer of
muscles, through which vessels (G) extend into the derm; Pi, Piy, pigment
cells in the derm ; S, secretion of glands.

along the back, in Urodeles (and Toads) at the junction of head and
trunk (" parotoids") or laterally along the body and in the caudal
region (e.g. Spelerpes, Plethodon). These aggregated glands vary

COMPARATIVE ANATOMY

not only in relative size, but also in the structure of their cells
and in function. Mucus-glands, and much larger ^>ois0?i-(/Zfmrfs
(the secretion of which is granular), can usually be distinguished,
the latter serving as a passive means of defence ; but intermediate
forms may be recognised. Smooth muscle-cells are very numerous
in the derm, certain of them surrounding these glands, and form-
ing constrictors and dilators.

In the Anura, the blood-vessels are not always confined to the
derm, but in connection with the respiratory function of the skin
extend far into the epiderm before metamorphosis, during which
process the capillary loops in the epiderm increase markedly,
decreasing again subsequently. This may be explained by the
fact that during metamorphosis the gills are no longer functional

Mucus glands. Pigment. Sensory organs.

I

Superficial and deep layer
of epiderm.

.

'mi ,^^r~. .

- ^ _--r^' *-*;=*.' ~-^:^~~* f '~ : ~- ;.:.:"--

DI I'm.

FIG. 16. SECTION' THROUGH THE SKIN OF A Triton alpestris IN THE BREEDING

SEASON.

and pulmonary respiration alone is apparently insufficient ; an
additional vicarious arrangement is therefore temporarily neces-
sary.

The coloration of the skin, which may undergo change, is due
to chromatophore* of different tints in the derm. The derm is
similar to that of Fishes, and is, moreover, characterised by an
abundance of blood-vessels and nerves, as well as of smooth
muscle-fibres. Calcifications and even ossifications (e.g. in Cera-
tophrys dorsata) may occur in the derm, and in the Gymnophiona
definite dermal scales are present.

A stratified stratum corneum becomes developed even in
perennibranchiate types, and may become more pronounced in
adaptation to a terrestrial existence in caducibrarichiate forms at
metamorphosis. The cornification is especially marked along the
back, and may result in the formation of warts and papillae ;
occasionally claw-like structures are developed on the digits
(Xenopus, Onychodactylus). The horny layer of the epiderm is
shed periodically, either in pieces or entire.

INTEGUMENT

23

Reptiles.

In adaptation from the first to a terrestrial in place of an
aquatic existence, the skin of Reptiles is dry and more or less
pneumatic. Integumentary glands are practically wanting. The
" femoral pores " of Lizards, which were formerly looked upon as
glands, are now known to be merely subcutaneous, branched,
tube-like cavities lined by cornified cells which project from the
pores in the form of solid cones, and possibly serve as clasping
organs during copulation : it is doubtful whether these structures
originated from glands in the first instance. In the Crocodilia a
row of about twenty small, gland-like sacs are present under the
skin along the back from the neck to the base of the tail, at the
boundary between the first and the second rows of scutes.
Nothing is known as to the function of these, or of the evaginable

FIG. 17. DIAGRAMMATIC SECTIONS THROUGH VARIOUS KINDS OF EPIDERMIC
SCALES OF REPTILES. (From Boas's Zoology.)

A, rounded scales ; B, shields; C, imbricating scales; D, the same, with bony
scutes in the underlying derm ; h. horny layer, and s, Malpighian layer of
the epiderm ; /, derm ; o, bony scutes.

and odoriferous " musk-gland" on the lower jaw of Crocodiles and
the imaginations of the integument on the margins of the carapace
in Chelonians.

A further characteristic difference between the skin of
Reptiles and that of most Amphibians is seen in the presence of
scales (Fig. 17). Horny epidermic scales and dermal bony
structures, both of which may be present in the same animal,
must be distinguished from one another (D). In all cases, the first
traces of the scales are due to the formation of dermal papilla?,
which may or may not become calcified or ossified. In the former
case, the resulting bony scale or scute still remains covered by the
more or less horny epiderm (e.g. Anguis, Scincidse). As a general
rule, the epidermal cornification is much more marked than the
ossification.

COMPARATIVE ANATOMY

The dermal papilla is thus always the primary part of the
scale which causes the elevation of the epiderm (Fig. 18). At

Horny papilla.

Malpigltian layer Horny layer
Papilla 1 . of eviderm. of epiderm.

^'VW'

Derm.

_

Subcutaneous conntrtire
tissue with vessels.

,

FIG. IS. SECTION OF SKIN OF A YOUNG TORTOISE (Testudo yra'ca), FROM THE

NECK.

the same time, a marked proliferation of the epiderm occurs, at
first uniformly and later in different degrees on the upper and
lower surfaces of scales when they overlap one another (Fig. 19).

Loose connective Epitrk-hous
tissue layer. lai/er.

Horny layer (scale).

Piyment In;/' r.

Muscles.
FIG. 19. SECTION OF SKIN OF LIZARD (Laccrla ayi/i*).

As in Amphibians, a periodic casting of the superficial part of
the many-layered horny epiderm occurs, either in shreds, or

INTEGUMENT 25

entire (e.g. Snakes, Anguis). Most Lizards simply creep out of
their cast skin, as out of a sack ; while in Snakes it becomes
turned inside out while being shed.

The horny substance may undergo a variety of modifications,
and may give rise to such structures as ridges, prickles, warts,
claws, shields (e.g. the " tortoiseshell " of Chelonians l ), and
rattles (Rattlesnakes); or it may develop bunches of cuticular
hair-like bodies, such as those found on the toe-discs of Geckos.

In the derm, a superficial and a deeper layer may be dis-
tinguished. The latter is composed mainly of strong bundles of
connective tissue fibres which as a rule cross one another at right
angles, as in Fishes and Amphibians. The superficial or sub-
epidermic layer is looser in structure, and in addition to con-
nective tissue fibres, encloses smooth muscles and chromatophores
(Fig. 19), the degree of development of the latter differing greatly :
several rows of them may be present, e.g. in the Chameleon. The
power of changing colour, so characteristic of the last-named, is,
however, possessed to a greater or less extent by many other
Reptiles.

Birds.

The skin of Birds is characterised by giving rise to feathers, as
well as by the relatively thin epiderm and derm, the connective
tissue fibres of the latter being irregularly felted. A uropygial
gland, peculiar to Birds, and situated at the base of the rudi-
mentary tail (uropygium) is present in nearly all, being wanting
only in a few groups (e.g. Ratitse) : its secretion serves to oil the
feathers, and it is especially well developed in Water-Birds. A
gland is also present in the neighbourhood of the auditory passage
in certain Gallinaceo?, but otherwise integumentary glands are
wanting in Birds. Characteristic of the derm is its richness in
sensory organs (tactile corpuscles) and muscle-fibres, most of which
latter are inserted into the feather-sacs and serve to erect the
feathers (arrectores plumarum). Epidermic scales are present
on the feet.

The feather is foreshadowed in the reptilian epidermic scale,
of which it is merely a further modification. That scales and
feathers are homologous structures is, at any rate to a certain
extent, indicated by their mode of development, which is briefly as
follows.

In the region where a feather is to be formed, the dermal tissue
becomes slightly raised up towards the thickened epiderm (Fig.
20, A), and thus gives rise to a vascular papilla. As this papilla
grows out to form an elongated cone, the feather-germ (B), its

The individual epidermic shields of Chelonians are independent of the
underlying bony plates (Fig. 33), and do not correspond with them in arrange'
ment.

26

COMPARATIVE ANATOMY

vascular base gradually sinks deeper and deeper into the derm,
and thus becomes surrounded by a sort of pocket- -the feather-
follicle. The horny, as well as the Malpighian layer of the
epiderm extends into the base of the follicle, and thence on to the
feather-germ, the interior of which is throughout filled by cells of
the derm which give rise to the pulp.

As the feather-germ keeps on growing, the cells of the
Malpighian layer begin to proliferate rapidly, giving rise to a
series of radial folds arranged along a central axis and extending
inward towards the pulp (Fig. 21, A). These folds, between
which the nutritive pulp extends, then become cornified and
separated from above downwards from the surrounding cells (B) ;
and, on a gradual drying of the central pulp-substance, give rise
to a tuft of horny rays (C), which are, however, at first bound
together by the enclosing stratum corneum, which forms a sheath
around them. Most Birds are hatched when the feathers are in

IB P c

FIG. 20. Two EARLY STAGES IN THE DEVELOPMENT OF THE FEATHEK

(SEMIDIAGRAMMATIC).

B, blood-vessel ; C, derm ; E, proliferating epiderm ; F, rudiment of follicle ;
h, horny, and m, Malpighian layer of epiderm ; P, pulp of the papilla.

this stage of development, and they thus appear as if covered with
brush-like hairs.

By the shedding of the investing horny sheath, the rays or
barbs on which smaller secondary rays or barbulcs become
developed become free, and thus an embryonic down-feather
(pluma) is formed. The whole feather-germ, however, does not
become divided up in this manner : its lower portion, embedded in
the skin, forms the quill (calamus), the interior of which contains
a peculiar flaky and air-containing horny substance, the dried
remains of the pulp.

Thus the earlier stages of development of the feather and
reptilian scale are very similar, but during later stages the
feather becomes adaptively specialised. The warm-blooded but
still flightless ancestors of Birds probably possessed a covering of
down-feathers which served as a protection against the cold, and
which only later become adapted in connection with flight.

In many Birds the feathers retain throughout life the essential
characters of down, with more or less differentiation (e.g. Ratitas,
and more especially the Cassowaries) ; but in most cases the

INTEGUMENT

27

down 1 becomes covered or replaced by the more complicated
definitive contour-feathers, the proximal barbs of which usually
still retain their down-like character. A contour-feather (penna)
at first closely resembles a down-feather, but in the course of
further growth, two adjacent rays become enlarged to form, with
the relatively longer or shorter quill, a main axis or stem (scapus):
the part distal to the quill, to which the barbs are attached in
a double row opposite one another is called the shaft (rackis).
At the base of the quill is a small aperture, into which the
vascular papilla extends ; and a second very small aperture is
present at the junction of quill and shaft on the inner surface.

St

A

FIG. 21. THREE STAGES IN THE DEVELOPMENT OF THE EMBRYONIC DOWN
DIAGRAMMATIC (after Davies). A, B, IN TRANSVERSE, and C IN LONGI-
TUDINAL SECTION.

F, dried remains of pulp; F.S, follicle; F.Sp, quill; P, pulp, with its ex-
tensions towards the feather sheath at t in A, which separate the developing
barbs (St) : these have become free in C.

The barbs together constitute the vane (vesdllum), and the
barbules arise obliquely, in a double row on each barb, so as to have
relations to the latter similar to those of the barbs to the shaft.
On the barbules a double series of barbicels are developed, certain
of which may bear minute booklets which interlock with one
another, and so connect the barbs together into a continuous
sheet : this is particularly the case in the row of large wing-
feathers (remiges) on the fore-arm and manus and of tail-feathers
(rcctriccs) on the rump or uropygium.

In many Birds each quill of the ordinary feathers of the body
bears two vexilla, which may be equal in size (Cassowary) ; but
usually one, the aftershaft (hyporachis), is smaller than the main
shaft.

1 Various modifications of the down feathers occur (e.g. floplnmex, which
by some zoologists are supposed to represent the last remains of a primitive
feather-covering from which both down-feathers and contour-feathers have
become differentiated).

28 COMPARATIVE ANATOMY

The contour feathers are generally not distributed irregularly
over the body, but are arranged in definite feather- tracts (ptcrylce)
separated by down-covered spaces (apteria), having a more or less
different arrangement in the various groups. 1

A periodic casting of feathers, or moulting, takes place in all
Birds, and corresponds to the similar process of the casting of the
horny epidermic layer in Amphibians and Reptiles : the papilla per-
sists, and in connection with it the replacing feather is developed.

The feather-covering of Birds must have been acquired in very
early geological periods, for Archasopteryx, found in the Jurassic
strata of Bavaria, possessed well-formed feathers with a delicate
shaft and vane (Fig. 49).

The colours of feathers are due in part to the presence of
various pigments (viz., red, yellow, orange, black, and brown), and
in part to the phenomenon of interference, which may produce
white, grey, blue, and metallic or iridescent tints.

Mammals.

The integument of Mammals is characterised by the presence
of hairs, and the question as to how far scales, feathers, and hairs
are comparable to one another is an interesting one. No inter-
mediate forms are known, but there is no doubt that the
feather is much nearer to the scale than is the hair. The study
of their development, however, shows that the origin both of hair
and feather may be traced in the first instance to similar scale-
like structures, in spite of their very different final form. Thus
phylogenetically both are closely related to the horny scales of
Reptiles.

The development of hairs, as well as their grouping and distri-
bution, indicates certain topographical relations to scales, and
also that they first arose in relation with a primitive scaly coat.
Secondarily they appeared on or behind the scales, which were
gradually reduced as the hairs underwent increasing differentia-
tion. Hairs, which, like feathers, are arranged in groups, are not,
however, individually homologous with scales, but arise from parts
of a scale-area, while the feather possibly corresponds to an
entire scale. There can be little doubt that the earliest
Mammals, which arose from primitive Reptiles, possessed an
extensive scaly covering in addition to a sparse coat of hairs.

In development, the first essential indication of the hair is
seen in the epiderm, which may or may not become raised up at

1 In some Birds bristle-like feathers occur on the head, and the foot-scales
or shields may bear feathers of a peculiar form. In insectivorous and nocturnal
forms, tactile or sinus-feathers are present around the eye and ear and at the base
of the beak, analogous to the sinus-hairs of Mammals (q.v.).

INTEGUMENT

29

the point in question 1 (Fig. 22). This thickening of the epiderm
grows downwards in the form of a papilla (hair-germ) and
is surrounded by the cells of the derm, so that, as in the case
of the feather, it comes to lie within a kind of pocket, the hair
follicle. The originally uniform mass of cells of the hair-germ is
later differentiated into a peripheral and a central portion : the
latter (bulb-cone, C) gives rise later (D) to the hair-shaft with its
medulla and pith, and to the cortex, as well as to the cuticle of
the shaft and the so-called inner root-sheath ; from the former
arises the outer root-sheath. The origin of both sheaths, as well
as the sebaceous glands, can be traced to the Malpighian layer
(cf. Fig. 23.) The base of the hair-shaft which fills up the bottom
of the follicle is broadened out to form the hair-bulb, which extends

A

B

D

FIG. 22. DIAGRAM OF FOUR STAGES IN THE DEVELOPMENT OF THE HAIR

(founded on Stohr's figures).

A, hair germ ; B, hair-cone ; C, bulb-cone, showing formation of bulb, papilla, and
hair-cone, which latter is becoming cornified at the apex. D, later stage in
which the hair is further differentiated, but has not yet reached the surface.

round the highly vascular hair-papilla like a cap. The hair
usually breaks through the skin obliquely, the direction differing
in different parts of the body.

Thus the more or less cylindrical hair-shaft consists of three
parts medulla, cortex, and cuticle : the medulla is the most
important part of the hair, and on its structure mainly depend
the differences seen in the hair of individual species.

The colour of the hair is due to three causes : firstly, to a
greater or less accumulation of pigment in the cortical layer;
secondly, to air contained in the intercellular spaces of the
medulla ; and lastly to the nature of the surface of the hair, i.e.
whether it is rough or smooth.

The mode of formation of new hairs in post-embryonic stages is
not thoroughly understood : when the hair is shed, it is not known

1 This hair-rudiment at first more or less resembles the rudiment of an
integumentary sense-organ of a Fish or gilled Amphibian ; and this fact has led to
the expression of a view that the origin of hairs may be traced phylogenetically
to such sensor v organs of the lower Vertebrates.

30

COMPARATIVE ANATOMY

whether the old papilla remains, or whether a new one is formed.
The former method seems to occur in most instances, although not

ip ; ,/A

.
FIG. 23. LONGITUDINAL SECTION THROUGH A HAIR. (Diagrammatic.)

Ap, arrectores pili ; Co, derm ; F, outer longitudinal layer, and F', inner trans-
verse layer of connective tissue fibres of follicle ; Ft, adipose tissue ; GH,
hyaline layer, which lies between the inner and outer hair-sheaths, i.e.,
between the root-sheath and the follicle ; HBD, sebaceous glands ; HP,
hair-papilla, containing vessels ; M, medulla ; 0, cuticle of shaft ; It. cortex ;
Sc, stratum corneum ; Sch, hair shaft ; SM, stratum Malpighii ; WS, WS 1 ,
external and internal root-sheath the latter reaches above only as far as the
sebaceous ducts, and is not continuous with the epiderm.

infrequently new hairs are formed throughout life direct from the
epiderm, as in the embryo. From a primary hair-germ an entire
group of hairs may be formed by subsequent division.

The special tactile hairs (mbrissce, sinus-hairs) present on parts

INTEGUMENT 31

of the face are usually much longer and stronger than the
others, and are provided with striped muscle-fibres. They are the
first to appear in the embryo, and the last to be retained in those
forms which have lost their hairy covering in connection with an
aquatic life (e.g. Cetacea). Between the outer and inner layers of
their follicles are blood-spaces and cavernous tissue, and they are
well supplied with branches of the trigeminal _ nerve. The
ordinary hairs are also well innervated, especially in the case of
nocturnal animals, and aie sensory as well as protective in
function. Other modifications of the hairs are seen, e.g. in the
eye-lashes, the long tail-hairs of most Ungulates, and various other
forms of bristles : spines, such as are characteristic of the Hedge-
hog and Porcupine, are merely especially strongly developed
bristles.

Hairs, like feathers, are arranged in definite tracts (flumina
pilorum), and the fur often consists of finer and coarser elements.
A richer hairy covering (lanugo] is often met with in the
embryonic condition than in the adult (e.g. in the human foetus) ;
and this fact, together with the occasional appearance of abnorm-
ally hairy individuals, indicates that at one time Man was dis-
tinguished by a far more abundant clothing of hair than at the
present day.

Hairs are most scanty in the Cetacea and Sirenia, in the
former of which they are often limited to a ; few bristles (sinus-
hairs) in the region of the lips (Toothed Whales) or chin (Whale-
bone-Whales), or may be entirely wanting except in embryonic
stages. In the Sirenia, apart from the persistent hairs, a thick
coat of fine hairs is present in the embryo, and modified traces of
these can be recognised in the epiderm of the adult.

The hairy coat may be shed and renewed periodically (e.g. in
the case of Mammals exhibiting differences in their summer and
winter fur), or the shedding and renewal may take place con-
stantly, and so result in no marked change of coat.

Epidermic scales may also occur in Mammals, but are rarely
present on parts which are well covered with hair. They are large
and well marked in Manis, covering the dorsal surface of the head
and body, the sides of the latter and the whole tail, and are
present on the tail of various Rodents (e.g. Beaver, Anomalurus,
Muridse), Insectivores, Anteaters, 1 and Marsupials. Other epidermic
structures formed as thickenings of the horny layer also play an
important part in Mammals : such are, claws, nails, hoofs, the horn-
sheaths of Ruminants, the so-called whalebone (baleen) of the
Mystacoceti, the palatal plates of Sirenia, the thickened regions of
the epiderm in Cetaceans and Pachyderms, the ischial callosities of
certain Apes, and the nasal horns of the Rhinoceros, the last-
mentioned of which consist of numerous hair-like horny fibres.

According to the form taken by the horny covering of the

1 Vestiges of horny scales also occur in Armadilloes.

32

COMPARATIVE ANATOMY

distal ends of the digits, the Mammalia may be subdivided into
Unguiculata and Ungulata, the former group including those with
claws or nails, and the latter those with hoofs. But no hard and
fast line can be drawn between these structures, which in all cases
are derivable from a simple form of claw, like that of Reptiles and
Birds. The terminations of the digits are without a horny
covering in Cetacea, though rudiments are present in the embryo
of Toothed Whales ; while among the Sirenia the Manatee possesses
vestigial and variable nails.

The horny nail-plate is situated on the dorsal side of the
digit, while ventrally is the softer sole-horn, which is continuous
proximally with the pads or tori on which the foot partially or

D

FIG. 24. DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH THE DISTAL ENDS
OF THE DIGITS OF A, ECHIDNA ; B, AN UNGUICULATE MAMMAL ; C, MAN ;
AND Z>, HORSE (after Gegenbaur and Boas).

1 3, phalanges ; b, torus ; N, nail-plate ; S, sole-horn ; W, bed of claw or nail.

entirely rests when on the ground. The essential relations and
chief modifications and reductions of these parts in various
Mammals are illustrated in Fig. 24. Tori are present in most
Mammals, and have a definite arrangement on the palms and soles
(apical, interdigital, and proximal), and in them the dermal
papillae are either irregular or are definitely grouped, and may
give rise to a series of concentric lines and arches. 1

When pigment is present (e.g. on the snout, external genitals,
and teats), it is chiefly situated in cells of the Malpighian layer

1 Compare those of Man, which yield the characteristic "finger-prints."

INTEGUMENT

33

into which it wanders from the derm, which may also contain
pigment.

In the derm, as may be seen by a glance at Fig. 25, an outer
papillary and an inner reticular portion may be distinguished.
The papilla? of the former are accurately adapted to the over-lying
epiderm : some of them contain blood- and lymph-capillaries, and
others, nerves with tactile corpuscles. The latter, on the other hand,
becomes lost without any
sharp boundary line in the
subdermal connective tissue
and in the more or less
w e 1 1-d e v e 1 o p e d fatty lay er
(panniculus adiposus). The
panniculus may be very
largely developed in aquatic
Mammals e.g. in the Ce-
tacea, in which it serves to
preserve the heat of the
body, and at the same time
to reduce the specific gravity
of the animal.

As in Birds, the con-
nective tissue fibres of the
derm are irregularly felted.
Most of the smooth muscle-
fibres are inserted into the
hair-follicles (arrectores pilo-
rum, Fig. 23), but may #0, derm ; F, subcutaneous fat ; GP, vascular

FIG. 25. SECTION THROUGH THE HUMAN
SKIN.

papilla ; H, hair with sebaceous glands
( D) ; N and G, nerves ; NP, sensory -
papillae ; Sc, stratum corneum ; SD, sweat-
glands, with their ducts (SD l );SM, stratum
Malpighii.

occur independently of hairs,
e.g., in the scrotum and
teats.

In the great abundance
of integumentary glands,

Mammals differ greatly from Reptiles and Birds, and more nearly
resemble Amphibians. They may be present in all parts of the
skin, and differ greatly with regard to the consistency, composition,
colour, and odour of their secretions. Those which serve for the ex-
cretion of products of destructive metabolism in general, and for the
formation of odoriferous substances, are either tubular or alveolar
in structure. The former, which were probably derived from those
of ancestral Amphibians, possess a muscular investment, have
mostly the form of the characteristically coiled sweat-glands, and
are rarely entirely wanting (e.g. Cetacea) : the latter, which are a
new acquisition and are known as sebaceous glands, appear to be
not only functionally, but also ontogenetically and phylogenetic-
ally closely connected with the hairs (Figs. 23 and 25).
Various modifications of both kinds are met with, and they
are often arranged in groups. Thus the iiasolabial glands of

D

34

COMPARATIVE ANATOMY

cattle, the lateral glands of the Shrew, and the dorsal glands of
Hyrax resemble sweat-glands ; while the preputial and Meibo-
mian glands, the inguinal glands of certain Rodents, and the facial
glands of Bats, are largely, at any rate, modified sebaceous glands. 1
Another important modification of the integumentary glands
is seen in the characteristic mammary ylands, to the possession of

A B

g.m

d.

FIG. 26. A, VENTRAL VIEW OF A BROODING FEMALE OF Echidna hystrix. B,
DISSECTION OF THK VENTRAL INTEGUMENT FROM THE DORSAL (INNER)
SIDE. (After W. Haacke.)

cl, cloaca ; t, T, the two tufts of hair in the lateral folds of the marsupial pouch
(b.m.) from which the secretion flows. On either side of the pouch, which
is surrounded by strong muscles, a group of mammary glands (y.m.) opens.

which the Class owes its name, and which secrete milk for the
nourishment of the young. Nothing is known of their phylogeny
in the ancestors of Mammals, but in all cases they correspond to

1 Amongst many other modifications of these glands of both types may be
mentioned the anal glands (especially well developed, e.ij., in Manis and the
Skunk) ; the perineal or prescrotal glands of Viverra ; the caudal gland of
the Fox and Wolf ; the suborbital or ant-orbital glands situated in the cavity of
the lacrymal bone in Cervidse ; and the interdigital glands of many Ruminants.
The preputial glands of the Beaver and Musk-deer also deserve special mention.

A peculiar tubular, femoral or spur-gland is present at the knee in Echidna
and dorsal to the hip-joint in Ornithorhynchus, near the vertebral column. It
opens by means of a long duct on to the tarsal spur, and, though present in the
embryo of both sexes, undergoes reduction in the female.

g. m.

INTEGUMENT

35

modified integumentary glands which have a certain similarity to
sweat glands, and like these, are probably specialised forms of
primitive tubular glands. The sebaceous glands arise much later
ontogenetically, but it is interesting to note that in the develop-
ment of the mammary area, traces of early stages of hairs may be
observed : these disappear later, but their sebaceous glands become
connected with the mammary ducts.

Potentially, therefore, mammary organs may be developed in
any part of the skin, but as a matter of fact, they are limited to
the ventral side in adaptation to the
method of suckling the young.

Amongst the oviparous Mono-
tremes, a marsupial poucli appears in
the embryo of Echidna as an infold-
ing of the abdominal wall (Fig. 26).
This pouch, which serves to shelter
the egg and young, becomes tempor-
arily enlarged in the breeding season
as the offspring increases in size,
and has the form of a deep sac ex-
tending backwards and provided
with closing muscles. On its lateral
walls are a pair of depressions, the
so-called mammary pockets, which also
arise periodically. A bunch of hairs
is present in each pocket, the follicles
of which open along with the mam-
mary ducts Oil these two glandular
areas, which are sharply marked off
from the rest of the pouch. The
mammary glands themselves, which
are almost equally developed in the

two sexes, consist in Echidna of long, coiled, and much branched
tubes, the blind ends of which are swollen. Both they and the
mammary pockets are acted upon by a strong integumentary com-
pressor muscle, the presence of which is all the more necessary
as Monotremes possess no teats. The manner in which the young
take in the milk is uncertain : it has been supposed that the milk
drops from the two bunches of hairs and is then licked off; or
it may be that a temporary teat is formed by suction. 1

_ Amongst Marsupials, in which teats are present, the pouch is
evidently homologous with that of Echidna, but it reaches a higher

1 Nothing definite is known as to the manner in which Ornithorhynchus cares
for its young. The eggs are laid in burrows in the earth, and it appears that
110 marsupial pouch is formed at any time ; possibly it may have gradually
disappeared as the animal acquired an aquatic habit. The sieve-like apertures
of the mammary gland are distributed over two spindle-shaped areas on the
ventral body-wall which are covered by the fur and surrounded by integumentary
muscles.

D 2

FIG. 27. RUDIMENT OF THE
MARSUPIUM IN Diddphys
marsupialis, RECONSTRUCTED
FROM TRANSVERSE SECTIONS.
(After Bresslau. )

M, marsupial pockets ; Z, teats
or mammary pockets ; f,
lateral walls of the marsu-
pial pockets which fuse to

. form the walls of the pouch.

36

COMPARATIVE ANATOMY

stage of development. A solid ring-like ridge of the epiderm is
formed around each teat (or mammary pocket from which the teat
arises), and later on the areas thus enclosed sink inwards so as to
give rise to a row of hollows or marsupial pockets, which together
constitute what has been called the marsupial line, (Fig. 27). By
a fusion of the adjacent lateral walls of the pockets arise the
marsupial folds which form the pouch (Fig. 28). The lowest
Marsupials show no trace of a pouch, and their ancestors probably
never possessed one.

Marsupial pockets persist permanently in various degrees of
perfection in a number of the Eutheria ( e.g. Manida?, Murida?,

FIG. 28. POUCH OF Thylacinus, AFTER REMOVAL OF THE SKIN.
(After Cunningham.)

r, compressor mammas ( = cremaster of male), passing over which are seen blood-
vessels and the genito-crural nerve ; /, lymphatic glands ; s, sphincter
marsupii ; z, teats.

CervidaB, Carnivora). In Mice they can be recognised until the
beginning of lactation, and are then evaginated and thus lengthen
the teats.

The teats may become developed in one of two ways. Either
the skin surrounding the mammary pocket (Fig. 29, A) becomes
raised up to form a circular rampart, and thus gives rise to a teat
perforated by a canal, into the base of which the ducts of the
gland open (B) ; or the gland surface itself becomes elevated into

INTEGUMENT

37

a papilla, while the surrounding skin remains almost on a level

with the rest of the integument (c). In the latter case, therefore,

there is an evagination of the mammary pocket, and the teats

may be described as secondary or true (Marsupials, Rodents,

Lemurs, Monkeys, and

Man), and in the former

as primary or pseudo-

teats (Carnivores,

Pigs, Horses, and Ru-

minants). The latter

condition is already in-

dicated in certain Mar-

supials (e.g. Phalangista

vulpina).

The teats- are often
situated in two nearly
parallel rows along the
ventral side of the
thorax and abdomen
which slightly converge
towards the inguinal
region (e.g. Carnivores,
Pigs) : in other cases
they may be restricted
either to the inguinal
(Ungulates and Ceta-
ceans) or to the thoracic
region (Sloths, Manis,

rjlephants, oirenia, JT IG 09. DIAGRAMMATIC REPRESENTATIONS OF
many Lemurs, Cheirop- THE EARLY DEVELOPMENT OF THE LEADING
" TYPES OF MAMMARY GLANDS. (Modified from

ueeenbaur.)

B

G

tera and Primates):
, .,' . , .

while m others, again,

they mav be axillary or

i j i
abdominal, or they may

OCClir in various com-
binations of all these

> first or undifferentiated (mammary pocket)
stage ; B, stage of the pseudo- (primary) teat ;
^ | tage ' of th fe e true (sec ondary) teat ; d, mam-
mary canal ; f.y, glandular area; gl, mammary
glands ; v, rim (or rampart) of the glandular area.

regions.

The number of teats varies greatly : there may be as few as
one pair, or as many as eleven pairs (Centetes) : in general, their
number corresponds to that of the young produced at one
birth.

Not infrequently, supernumerary or accessory mammae and
teats can be recognised (e.g. in Sheep and Cattle), and there may
be indications in the foetus of a greater number of teats than that
which occurs in the adult: thus in the embryo of Whalebone-
Whales there are eight pairs, while the adult possesses only a
single pair on either side of the vulva. Cases of such a hyper-
maslism and hyperthclism are also well known in the human

38 COMPARATIVE ANATOMY

subject, as frequently in men as in women. The accessory
mammas and teats are usually anterior (above) or posterior to (below)
the normal ones, and thus form with them two converging rows
from the axillary to the inguinal region, just as in many other
Mammals and in the human embryo, at a certain stage of which
four pairs of additional rudiments of mammary organs can always
be recognised. There is thus an indication of normal hyper-
mastism and hyperthelism in human ontogeny, which is paralleled
in those numerous mammals in which a " mammary ridge " or
"line" is formed, a structure which is probably comparable to the
" marsupial line " forming the rudiment of the pouch in Mar-
supials.

In the male, the mammary apparatus becomes absorbed,
though frequently at birth and at puberty milk is produced in
the human subject. Male goats and castrated sheep have also
been known to give milk.

B. SKELETON.

1. EXOSKELETON.

THE hard exoskeleton, consisting of bone or of other calcified
tissues, must be distinguished from the horny exoskeletal parts
already described. Thus it will be remembered that the term
" scale " is sometimes used for a horny epidermal structure, and
sometimes for a bony dermal one (pp. 20 and 23).

In Cyclostomes, scales are entirely wanting.

Elasmobranchs. The integument of most Elasmobranch
Fishes encloses certain hard structures each consisting of a basal

FIG. 30. PLACOID SCALES FROM THE SKIN OF AN ELASMOBRANCH.

(Semi-diagrammatic. )

S, basal or socket-plates in the dermal connective tissue (By) ; X,

denticles.

plate or socket bearing a pointed spine or denticle (Fig. 30), differ-
ing considerably in form and relative size in the various members
of the Order, and known as a placoid scale. These placoid organs
are continually being formed anew throughout life, and are pro-
tective in function. The basal plate is rhomboid or rounded in
form and consists of bony tissue, while the denticle itself is
composed of dentine covered over with enamel. In many Rays,
there is a relatively small number of these placoids, while in most
Sharks and Dogfishes they are much more numerous and close-
set. In the Electric Rays they are wanting. The primary part

40

COMPARATIVE ANATOMY

is the enamel, which is formed as an excretion of the epidermic
cells (Fig. 31), while the later formed mesodermal dentinal and
bony portions become closely connected with the enamel secondarily.
Thus the enamel is the first and originally the only hard substance
of the placoid organ.

In the Holocephali (Chimsera, Callorhynchus) a double row
of placoids is developed along parts of the dorsal region in the
embryo, but disappear in later stages : in the adult these organs
apart from the spine on the anterior margin of the dorsal fin, occur
only on the claspers and frontal organ of the male. 1

Teleostomes. In these Fishes, ossifications in the derm to
form bony scales takes place independently of any stimulus from

FIG. 31. VERTICAL SECTION THROUGH THE SKIN OF AN EMBRYO SHARK.
(From Gegenbaur's Comp. Anatomy.}

C, derm ; c, c, c, d, layers of the derm ; E, epiderm ; e, its layer of columnar
cells ; o, enamel layer ; p, dermal papilla.

the epiderm. Thus the denticle, which in Elasmobranchs is the
primary cause of the development of the basal plate, gradually
disappears in ontogeny, and the latter is the only part of the
placoid organ which remains, its independence being retained
in the formation of bony skeletal substance in higher Vertebrates.
In Lepidosteus, denticles are still developed in the skin but
are quite transitory, and this primitive method of starting the
formation of bony tissue is again met with in the Vertebrate series
in connection with certain parts of the Amphibian skull ; here
certain bones (vomer, palatine, pterygoid, &c.) which originally
served as supports for oral teeth, persist even if the teeth dis"-
appear, as they have become an integral part of the facial
skeleton.

1 In addition to the ordinary placoid scales, larger or smaller spines of a
similar structure may become developed in connection with the dorsal tin, around
the first cartilaginous ray (e.g. Acanthias, Trygon, Chimera). In the Saw-
fish (Pristis), there is a double row of large denticles on the long rostrum.

EXOSKELETON 41

It is therefore evident that the first bony hard substances
of the Vertebrata arise in connection with the external skin and
oral mucous membrane, and that the bony integumentary skeleton,
or exoskeleton, is therefore phylogenetically older than the bony
internal skeleton or cndoskeleton. The latter owes its origin
to a gradual extension of the exoskeleton from the surface to
deeper parts, where it takes on relations to the cartilaginous endo-
skeleton. An independent ossification of the perichondrium, or
membrane which invests the cartilage, may also take place, so
that bone and cartilage now combine in the formation of the
skeletal framework and thus further complications arise. To
the original dermal ossification is added a formation of bone in
the perichondrium, and finally even a secondary endochondral
ossification may occur, replacing the cartilage : the former is most
marked in the Anamnia, the latter in the Amniota, and the result
of both is usually the subjection of the cartilaginous tissue in
the struggle of the tissues in the organism.

In most Ganoids, thick plates, usually rhombic in form, are
present in the skin ; in the bony Ganoids (Polypterus, Lepidosteus) l
these cover the entire body, their margins being in apposition.
These ganoid scales correspond to the deeper part of the placoid
basal plates. Their surface is dense and smooth, owing to the
presence of a layer of ganoin, of mesodermal origin, and formerly
erroneously described as enamel. The exoskeleton was largely
developed amongst fossil Ganoids.

The scales of Teleosts are usually thin, and of the form known
as cycloid or ctenoid ; in the former the whole margin is smooth,
while in the latter the posterior margin is toothed and comb-like,
but various intermediate stages occur. The scales are arranged in

FIG. 32. DIAGRAMMATIC LONGITUDINAL, SECTION THROUGH THE SKIN OF A
TELEOSTEAN, TO SHOW THE RELATION OF THE BONY SCALES. (From Boas's

Zoology.}

I, derm ; o, epiderm ; s, scale.

oblique rows and are situated directly beneath the epiderm, the
individual scales not touching one another. Secondarily, they
usually come to lie within definite pockets or sacs, and to overlap
one another like tiles on a roof (Figs. 13 and 32). The surface
of the scales may be sculptured.

In the developing scale, a superficial dense portion, formed

1 In Amia the scales have a "cycloid" form, and in the adult Polyoclon
they are absent.

42 COMPARATIVE ANATOMY

from cells and corresponding to the dentinal layer of the basal
plate, can be distinguished from a deeper part composed of several
layers of connective tissue : each of these becomes independently
ossified in a typical manner.

Numerous other forms of the dermal skeleton are met with in
Teleosts. In some of these Fishes (e.g. Plectognathi, Lopho-
branchii, certain Siluroids), 1 as in many of the earliest Paleozoic
Vertebrates (Ostracodermi), bony scutes are developed and form a
strong cuirass. In others, again (e.g. many Siluroids and Eels), the
scales may be reduced or absent. The bony dermal fin-rays or
" leptotrichia " of Teleostomes possibly correspond to modified
scales.

Dipnoans. In the Dipnoi, as in the Teleostomi, the scales
are not directly derivable from the Elasmobranch placoids. In
form, in their overlapping arrangement, and in their situation in
pockets of the derm, they resemble the cycloid scales of Teleosts ;
but this similarity must have come about independently in the
two cases, that is, must be due to convergence.

Amphibians. Recent Amphibians have retained only very
slight traces of such a dense integumentary bony armour as was
present in the fossil Stegocephali. Amongst these, specially strong
dermal plates were formed in the region of the shoulder-girdle, and
very commonly most of the body was covered with scales. A series
of oblique and bilaterally symmetrical rows of scales covered the
entire ventral surface between the shoulder- and hip-girdles, a
further differentiation of which results in the scales no longer
overlapping, but forming short parallel rods, which correspond
to the so-called " abdominal ribs " (" parasternal elements ") of
certain Reptiles. As examples of exoskeletal structures in exist-
ing forms may be mentioned the bony plates in the skin of the
back of certain Anura (Ceratophrys dorsata and Brachycephalus
ephippium), as well as the scales lying between the ring-like folds
of the limbless Amphibia (Gymnophiona). The latter resemble
in many respects the scales of Fishes, and may be derived from
such a scaly covering as that of the Permian Salamander Dis-
cosaurus.

Reptiles. The dermal skeleton was very highly developed
amongst fossil Reptiles, e.g. certain Dinosaurs, such as the Jurassic
Stegosauridae, in which enormous bony plates and spines covered
with horn, sometimes as much as sixty-three centimetres long,
were present in the dorsal region. Teleosaurus and Aetosaurus
(Crocodilia), as well as some of the gigantic Cretaceous Dinosaurs,
(Ceratopsidoe), possessed a strong exoskeleton.

Amongst existing Reptiles a series of well-developed "abdo-

' In these Siluroids, the bony plates may bear sockets in which denticles,
consisting of dentine and enamel, are implanted. But although denticles are
retained in these cases, they do not contribute to the formation of the basal plates,
as in Elasmobranchs.

EXOSKELETON

43

minal ribs" (cf. p. 42) are present in Hatteria in the rectus
abdominis muscle, each consisting of a median and of a paired
lateral bar, and being considerably more numerous than the body
metameres. In Crocodiles similar bars are present, their number
corresponding with that of the ribs: they no longer reach the
middle line, and with the exception of the first, each consists, on
either side, of two firmly united portions. Evidently these structures
have here begun to undergo reduction.

Crocodiles, many Lizards (Anguis, Cyclodus, Scincus), and more
especially Chelonians, exhibit a well-developed dermal skeleton,
the scutes composing which cover the body more or less completely.
In the last-mentioned group there is a dorsal and ventral shield
(carapace and plastron} consisting of numerous more or less closely
united pieces, and completely encircling the body (Fig. 33). The

FIG. 33. A, CARAPACE, and B, PLASTRON OF A YOUNG Testudo c/rceca;

C, PLASTRON OF Chelone midas.

G, costal plates ; E, entoplastron ; Ep, epiplastron ; Hp, hypoplastron ; Hy,
hyoplastron ; M, marginal plates ; N, neural plates ; Np, nuchal plate ;
Py, pygal plates ; R, ribs ; Xi, xiphiplastron. ( V indicates the anterior,
and If the posterior margin.)

plastron, the larger posterior portion of which probably corresp
to greatly modified abdominal ribs, arises entirely by ossification
of the derm ; while parts of the carapace have a close relation to the
endoskeleton (neural arches and ribs), which early in development
became broadened out into plates. At the same time, the inter-
costal muscles disappear completely, and the muscles of the back
undergo partial reduction, while the articulations of the vertebrae
and ribs disappear. The nuchal, pygal, and marginal plates (cf.
Fig. 33) are entirely independent of the endoskeleton that is, are
purely exoskeletal bones ; while the costal and neural plates
correspond to much thickened periosteal bones developed around
the cartilaginous ribs and neural spines respectively : though they
are subcutaneous in position, they have nothing to do with the
skin genetically.

44 COMPARATIVE ANATOMY

Reference has already been made to the dermal bones amongst
fossil Fishes, Amphibians, and Reptiles, and certain of these in
the antero-ventral region of the trunk are of special interest, as
they are represented in certain Reptiles by a bone known as the
episternum, which underlies the sternum. Though always arising
as a paired structure, the episternum, which is present, e.g., in
Palaeohatteria, most Lizards (Fig. 56) and Crocodiles, forms in the
adult an unpaired plate of varied form. It is wanting in
Chamaeleo, Anguis, Ophidia, and Chelonia. 1

Birds. Reduced abdominal ribs occur in the primitive
Archa^opteryx, otherwise no fossil or existing Birds possess a
dermal exoskeleton, and no independent elements corresponding
to an episternum can be recognised even in the embryo: they
have evidently long ago disappeared.

Mammals." Armadillos are the only Mammals possessing a
bony exoskeleton, 3 which consists of a series of five movable
transverse bony scutes covering the head, neck, and body, and of
smaller plates on the tail and limbs. Sparse hairs occur between
these plates. It is very doubtful whether this exoskeleton has
been derived from that of Reptiles: more probably it, like the
horny exoskeleton of Manis (p. 31), has arisen secondarily, and in
consequence of its development the hairs have become reduced.
In Glyptodon, a large fossil member of this group, the dermal
plates were firmly united together to form a shield which covered
the whole body.

2. ENDOSKELETON.

Under the term exoskeleton are included the bony parts
which, as a rule, remain throughout life in connection with the
integument : the endosMcton consists mainly of cartilaginous and
bony parts, all of which have a deeper position. The cartilaginous
portions, which in their entirety constitute the primordial endo-
sMcton, have undoubtedly from the first arisen in this position,
and for a long period formed, together with the notochord,
the entire internal skeleton, as they practically do at the present
day in Elasmobranchs as well as in Cyclostomes. As already

1 Unless the element of the plastron marked E in Fig. 33 is to be interpreted
as such.

2 A certain part of the anterior end of the sternum, as well as certain carti-
laginous and bony elements in the region of the sterno-clavicular articulation, are
sometimes said to correspond to the last remains of the dermal episternum of
lower forms ; but as further proofs are required before such a homology can be
definitely accepted, the term proxttrnum has been proposed to include the
elements in question (cf. under .Sternum).

3 It is possible that the peculiar horny tubercle in the region of the dorsal
fin in certain Cetaceans may represent the last vestiges of a dermal bony arma-
ture such as was present in the extinct Zeuglodon.

VERTEBRAL COLUMN 45

mentioned (p. 41), bone may be developed in connection with
this primordial skeleton and may arise in two different ways. It
may be formed in parts which are not superficial ; or bony elements
may become associated with the cartilaginous skeleton which are
derived phylogenetically from the exoskeleton but which in course
of time have taken up a deeper position : these become secondarily
connected with the bones which have arisen independently in this
position. In order to determine as to which of these two
categories any particular bone belongs, an appeal to comparative
Embryology is necessary.

The relations of a bone to the cartilaginous skeleton may be of
such a nature that it merely becomes applied to the outside of the
cartilage, when it may be described as an investing lone. A bone
may, however, originate in the perichondrium or membrane which
covers the cartilage, and then, in the course of phylogenetic
development, may invade and replace the cartilage : in other
words, perichondral bone may become endochondral. 1 The cartilage
beneath investing bones may gradually disappear in the course of
time, and occasionally a bone which was originally perichondral
may attain apparent independence by the loss of the cartilage
around which it was formed in the first instance.

I. VERTEBRAL COLUMN.

An elastic rod, the notochord, or chorda dorsalis, lying in the
longitudinal axis of the embryo between the neural and visceral
tubes (cf. p. 6), is the first part of the endoskeleton to be formed,
and is the primitive forerunner of the vertebral column. It is
developed as a ridge of the primary endoderm, from which it
becomes constricted off, and is therefore of epithelial origin. In
the large parenchyma-like cells of which it is composed vacuoles
soon appear, and eventually only the walls of the cells persist in
the greater part of the notochord ; these become flattened by
mutual pressure, so that they appear like a meshwork of pith-cells
(Fig. 34, A, B). At the periphery, however, the cells retain their
protoplasm, becoming flattened and arranged like an epithelium.
Around the notochord two homogeneous, cuticular sheaths are
successively developed from its cells. The primary sheath is first
secreted by the peripheral notochordal cells : the thicker secondary
sheath, which has a similar origin from the so-called " notochordal
epithelium," appears later.

From the surrounding mesoderm a skdetogenous layer is
developed : this not only surrounds the notochord, but extends
dorsally to it as well as ventrally. Thus a continuous tube of
embryonic connective tissue is formed enclosing the spinal cord,

1 The unsatisfactory terms "membrane-bones" and " cartilage " bones are
usually used in describing the investing and replacing bones.

46

COMPARATIVE ANATOMY

B

sk.l

sh.1

Ir.p

nc.

sk.l

FIG. 34. DIAGRAMS ILLUSTRATING THE DEVELOPMENT OF THE NOTOCHORDAL
SHEATHS AND VERTEBRAL COLUMN.

A. Early stage, showing notochordal cells (nc) and primary sheath (.s/t 1 ), as well
as the mesodermic skeletogenous layer (sk.l).

B. Later stage, in which the central notochordal cells (nc) have become
vacuolated, and the peripheral cells have given rise to the " notochordal
epithelium " (nc. ep. ) from which the secondary sheath (sh 2 ) is derived :
paired dorsal and ventral cartilages, or arcualia (d.a, v.a) have arisen in the
skeletogenous layer (Cyclostomes, Cartilaginous Ganoids).

C. Cartilage cells have passed through the primary sheath, and are invading
the secondary sheath (Elasmobranchii, Dipnoi).

D. The cartilages are growing round the notochord, outside its sheaths, which
gradually become reduced (Bony Ganoids, Teleostei, Amphibia, Amniota).

(A D represent the caudal region.)

E. A later stage in the development of a pre-caudal vertebra. The notochord
(nc) has become constricted, and the cartilages have united into a single
mass and have given rise to a centrum (<), neural arch (n.a), neural spine
(n.sp), transverse processes (tr.p), and articular processes (art).

and only broken through at the points of exit of the spinal
nerves. This stage is often known as the membranous stage, and
in it no indication is seen of the metameric segmentation which

VERTEBRAL COLUMN 47

occurs later in the vertebral axis. The cause of this segmentation
is to be traced primarily to the muscular system ; and it is evident,
on mechanical grounds, that the segmentation of the vertebral
column must alternate with that of the muscular segments or
myotomes. Small paired and segmentally arranged masses of
cartilage later appear in the skeletogenous tissue dorsally and
ventrally to the notochord, and these represent the rudiments of
the vertebra} (Fig. 34, B, E). This is the beginning of the second
or cartilaginous stage of the vertebral column ; and now ossification
may occur (bony stage). Those parts of the fibrous tissue which
do not become consolidated in this manner give rise to the ligaments
of the vertebral column.

Two different modes of development of the vertebral column
from the above-mentioned dorsal and ventral cartilages or arcualia
may be observed. In Elasmobranchs and Dipnoans the secondary
notochordal sheath undergoes a fibrillar degeneration, and becomes
invaded by cartilage-cells from the arcualia which break through
the primary sheath at these points (Fig. 34, c) and gradually extend
so as to surround the notochord, thus forming a cartilaginous
sheath which may undergo segmentation to form a series of
vertebral bodies or centra ; the arcualia at the same time extend
dorsally and ventrally respectively to form the vertebral (neural
and hcemal) arches. In other cases (e.g. Bony Ganoids, Teleosts,
Amphibians, and Amniota), the arcualia extend at their bases
round the notochord so as completely to surround it without
penetrating the primary and secondary sheaths (D). The centra
which are then formed by segmentation of this perichordal
cartilage may be described as perichordal centra, to distinguish
them from the chordal centra of Elasmobranchs and Dipnoans.

During these differentiations of the skeletogenous tissue, the
notochord suffers a very different fate in the various Vertebrate
groups ; it may increase in size and persist as a regular cylindrical
rod, or it may become metamerically constricted by the formation
of the vertebral bodies, and even entirely disappear. In all Verte-
brates above Elasmobranchs, the embryonic vertebral column is
relatively shorter than the notochord around which it is formed,
and thus there has been a phylogenetic reduction in length of the
axial skeleton. 1

Amphioxus. The notochord of Amphioxus exhibits many
primitive as well as special characters. It extends along the
whole length of the animal, whereas in the Craniata it always
ends anteriorly below the brain just behind the pituitary body.

1 An ephemeral structure, the subnotocliordal rod or hypocJiorda, occurring in
embryos of Fishes and Amphibians, may be briefly referred to in this place. It
arises as a longitudinal furrow or ridge of the endoderm in the head and trunk,
on the dorsal side of the gut, with which it may for a time remain in connection,
but eventually becomes constricted off as a rod lying beneath the notochord. It
soon undergoes degeneration, but traces of it may persist as an elastic band. It
seems probable that this structure is a vestige of the epipharyngeal groove of
Amphioxiis.

48

COMPARATIVE ANATOMY

A delicate primary sheath is present and is surrounded by con-
nective tissue which is continuous with that enclosing the neural
canal and separating the muscular segments or myotomes.

Cyclostomes. In these, as in all the true Fishes, only two
regions can be distinguished in the vertebral axis, a trunk- or pre-
caudal region, and a caudal region. An advance on the primitive
condition in Amphioxus is seen in the development of a thick
secondary sheath and, at any rate in the caudal region, of
cartilaginous elements : in the adult Petromyzon these are present
all along the notochord in the form of rudimentary neural arches,
which, however, do not meet above the spinal cord (cf. Fig. 34, B),
and of which there are two pairs to each muscular segment

Tc

FIG. 35. PORTION OF THE VERTEISRAL COLUMN OF Polyodon. Side view.

FIG. 36. TRANSVERSE SECTION OF THE VERTEBRAL COLUMN OF Acipenser
ruthenus (in the anterior part of the body).

Ao, aorta; C, notochord ; Ee, primary, and Cs, secondary sheath of the noto-
chord ; EL, longitudinal elastic band ; Fo, median ingrowths of the lo wet-
arches enclosing the aorta ventrally ; Tc, intercalary pieces (inter-dorsal and
inter-ventral); M, spinal cord; Ob, upper arch (basi-dorsal) ; P, pia
mater ; P,s, neural spine ; SS, skeletogenous layer ; Ub, lower arch (basi-
ventral ; Z, "basal stiimps" of the lower arches.

FIG. 37. PORTION OF THE VERTEBRAL COLUMN OF Protopterus. Side view.
C, notochord ; DF, neural spine ; FS, fin-ray ; FT, interspinous bone.

or myotome (cf. Elasmobranchs). In the tail haemal arches, enclos-
ing the caudal aorta and vein, are also present, and fusion of the
cartilaginous elements occurs.

Fishes. To the condition found in Cyclostomes, that seen in
the Cartilaginous Ganoids, Holocepliali, and Dipnoi, is directly
comparable : the notochord is persistent, no centra being formed,
as was also the case in the most primitive Palaeozoic Elasmo-
branchs, and thus the metameric character of the skeletal axis
is only seen in the arches (Figs. 35, 36, and 37). In the
Holocephali and Dipnoi, however, the thick secondary sheath
encloses cartilage cells amongst its fibres. In Chimasra narrow

VERTEBRAL COLUMN 49

calcified rings arc also developed in the sheath : these are con-
siderably more numerous than the arches. The latter remain
cartilaginous in the Cartilaginous Ganoids and Holocephali, but
become densely ossified in the Dipnoi (Fig. 37). The upper arches
may be completed above by neural spines. In the caudal region
the hcemal arches usually enclose the caudal aorta and vein com-
pletely ; further forwards the cartilages do not meet in the middle
line below, thus only surrounding the coelome to a slight extent, and
consequently the lower arches end on either side in a laterally-
directed cartilaginous projection, the transverse process or " basal
stump."

The relations of the arches in Plagiostomes, Bony Ganoids, and
Tcleosts are similar to those described above, but their structure
is more complicated in all Fishes than is there indicated.

The upper arches, which in many Fishes are not closed in
dorsal ly, 1 consist on either side of several distinct elements, which

06 Je

asg^spsppigsiEg^^^'ltSJSIS^'" 1

. ,

Jr

FK;. 38. PORTION OF THE VERTEBRAL COLUMN OF Scymnm.

Ic, intercalary pieces (interdorsals) ; Ob, neural arches (basidorsals) ; WK,
centra. The apertures for the roots of spinal nerves are shown.

are most plainly distinguishable in Cartilaginous Fishes. One of
these (directly above the centrum where such is developed) may
be described as the basidorsal or neural plate, and is usually
perforated by the foramen for the motor root of a spinal nerve.
Intercalated between successive basidorsals is another cartilage,
the interdorsal (intercalary piece, or interneural plate}, through
which the sensory root of a spinal nerve, situated anteriorly to the
corresponding dorsal root, usually passes : both these cartilages
may meet above, so as to complete the arch (Fig. 38). In some
cases, more than one intercalary piece is present on either side,
and frequently another series of cartilages becomes segmented off
from the basidorsals and interdorsals respectively to form the
keystone of the arch : these give rise in the median line to more
or less marked neural spines. The bases of this series of upper
intercalary pieces or supradorsals fit in alternately between the

1 A longitudinal elastic ligament is constantly present in this region (Figs.
36 and 42) and also in relation with the ventral arches.

E

50

COMPARATIVE ANATOMY

basidorsals and interdorsals, and thus may be twice as numerous
as the centra (Fig. 3D). 1 The lower arches consist of basivcntral
cartilages, between which are sometimes intercalated a series of
interventrals (Fig. 35), and which, in the tail, are produced into
JicKmal spines: these may be formed of distinct infra/central
elements.

In Dipnoans the interdorsals and interventrals are fused or
wanting, and in Bony Ganoids and Teleosts the various elements
usually become united in the fully-formed vertebra. Distinct fused

/crfcr 'i.n.p np^^

JL_J^

A/jfe

M '.''"

'":/

?" \

n.a

n.a,

nlc

n.a

n.a

h.a

Wiim-h-Bp

FIG. 39. PORTIONS OF THE VERTEBRAL COLUMN OF Scyllhun
(From Parker's Practical Zooloyy.)

A and B, from the trunk ; C and /), from the middle of the tail ; A and C, two
vertebrae in longitudinal section ; B and D, single vertebrae viewed from one
end ; b, calcined portion of centrum ; c, centrum ; for, foramen for dorsal,
and for', for ventral root of spinal nerve; h.a, haemal arch (basi-ventral) ;
Ji.c, ha-mal canal; h.*p, haemal spine; i.it.p, intercalary piece (interdorsal,
or interneural plate) ; n.a, neural arch ; n.c, neural canal; n.}), neural plate
(basi- dorsal) ; n.*p, neural spine ; nt<-, intervertebral substance (remains of
notochord) ; r, proximal portion of rib ; tr.pr, transverse process (basal
stump).

pairs of supradorsals, however, persist and remain unossified in
Lepidosteus (Fig. 42), and in the caudal region of Amia the basi-
dorsals and basiventrals remain separate from the interdorsals and
interventrals, thus giving rise to double vertebral bodies (pre-
centra, bearing the arches, and archless postcentra}. A somewhat
similar condition is seen in the Jurassic Eurycormus and other
fossil Ganoids. From what has been said above, it will be seen
that the number of arch elements does not necessarily correspond

1 It was mentioned on p. 48 that in the Lamprey there are two pairs of
arcualia to each myotome : it is possible that they correspond to alternating
basidorsals and interdorsals.

VERTEBRAL COLUMN

with that of the centra, or the number of the latter with that of
the myotomes.

Articular processes (zygapophyses) are usually present on the
neural arches of Bony Fishes.

In Plagiostomes, the cartilage which has invaded the sheath of
the notochord is segmented into definite vertebral centra, which
become partially calcified in various ways. The calcification (Fig.
40) may in each centrum take the form of a double cone, con-
stricted in the middle, as in Scymnus and Acanthias (cyclo-
spondylic form) ; concentric layers may be added to this, as in the
Rays (tectospondylic} ; or longitudinal plates may be formed radiating
outwards from the double cone, as in Scyllium (aster ospondylic).
The dorsal and ventral arches usually extend round the centrum

ex m

ha

na -

ha-

ha-

A B C

FIG. 40. DIAGRAMMATIC TRANSVERSE SECTION THROUGH THE MIDDLE OF A
CvCLOSPONDYLIC (A), A TKCTOSPONDYLIC (B), AND AN ASTEROSPONDYLIC
VERTEBRA (C). (From Zittel, .after Hasse. )

<1, middle portion of the calcined double cone ; d\ additional concentric calcified
layers ; d", double cone with radia-ting calcified layers ; ex.m, external elastic
membrane; h.a, haemal arch ; n.a, neural arch ; n.c, notochordal cavity.

so as to enclose it, and in the tail there may be two or more sets of
vertebral elements to each body segment. 1

In Bony Ganoids and Telcosts, there is a tendency towards a
reduction of the cartilage ; that which forms the centra is entirely
outside the notochordal sheaths, and the vertebras become more or
less densely ossified.

In the course of development of the centra in all cases, the
notochord becomes constricted by the growth of the cartilage at
regular intervals while the latter undergoes segmentation into
centra. Each point of constriction corresponds to the middle of a
centrum, i.e., it is intravertebral in position, and the notochord
may here disappear entirely ; intervertebrally it remains expanded

1 In Rays and Chima?roids the anterior vertebral elements become fused
into a single mass, on which a definite condyle is formed for articulation with the
skull ; amongst Sharks and in Dipnoans also, a concrescence of the anterior vertebral
elements with one another and with the skull may occur.

E 2

52

COMPARATIVE ANATOMY

XnKn 1 Li

and so persists as a kind of connecting or packing substance
between contiguous centra, which are consequently of a deeply
biconcave or amphicalous form (Fig. 41).

One of the Bony Ganoids, Lepidosteus, forms a marked excep-
tion to other Fishes as regards its
vertebral column, inasmuch as de-
finite articulations are formed be-
tween the centra (Fig. 42). A con-
cavity is formed at the hinder end of
each centrum which articulates with
a convexity on the vertebra next be-
hind (opisthoccelous form). The noto-
chord (except in the caudal region)
entirely disappears in the adult; in
the larva it is seen to be expanded
intravertebrally and constricted in-

FIG. 41. -PORTION OF THE VERTE- tervertebrallv, a condition of things

BRAL COLUMN OF A YOUNG ... .

DOGFISH (Scyllwm caiw-iila). whl ch appears again in the higher

(After Cartier.) types, as, for instance, in Reptiles.

C, notochord; FK, the fibro-car- In a sti11 earlier larval stage, how-

tilaginous mass lying between ever, the constrictions are intra-

the cartilaginous zones which ver tebral, as in other Fishes,
is undergoing calcification ; FTM i i ^ r .1.1 i

Kn, oute?, and Kn', inner, ^he skeleton of the posterior end

zone of cartilage ; Li, inverte- of the tail in Fishes requires special
bral ligament. notice, and the condition in Amphi-

oxus, Cyclostomes, and Dipnoans,

may be taken as a starting-point. In these, the notochord extends
straight backwards to the hinder end of the body and is sur-

f.S.

i.e.

A

-li.tt.

Fia. 42. PORTION OF THE VERTEBRAL COLUMN OF Lepidoxtvus. (After Balfour

and Parker.)

A, vertebra from anterior surface ; B, two vertebra from the side, en, anterior
convex face, and en', posterior concave face of centrum ; h.n, transverse
process; i.c, intercalary cartilages (fused supra-interdorsals) ; i.x, inter-
spinous bone ; /, /, longitudinal ligament ; n.a, upper arch (basi -dorsal).

rounded quite symmetrically by the tail-fin, and the tail is
therefore spoken of as diphyccreal : this condition is also met

VERTEBRAL COLUMN

53

with in certain Palaeozoic Fishes. In most other Fishes the
ventral part of the tail-fin with its supporting skeleton, as a
result of unequal growth, is more strongly developed than the dorsal
part, and the vertebral column becomes bent up dorsally, giving
rise to a Itcterocercal tail. This form of tail may be recognised
externally in most Elasmobranchs, Ganoids, and numerous fossil
Fishes, or may be masked by a more or less symmetrical tail-tin,
as in Lepidosteus, Arriia, and more particularly in most Teleosts,

FIG. 43A. TAIL OF Lvpidostvm.

n

Fir;. 43fi. CAUDAL END OF VERTEBRAL COLUMN OF SALMON. (From Boas's

Zoology. )

h, centrum ; h', urostyle ; n, haemal arch ; n', hypural bone ; o", neural arch ;

t, neural spine.

in which the heterocercal character is only visible internally,
and the tail is described as homocercal (cf. Figs. 43, A and !>}.
The posterior end of the vertebral column is then frequently
represented by a rod-like urostyJc, and in Teleosts one or more
wedge-shaped hypnral bones (enlarged hannal arches) generally
occur directly -beneath it. 1

1 The diphycercal character of the tail in Dipnoi and curtain Teleostuini is
probably not primitive (protocercal), but has been acquired secondarily.

54 COMPARATIVE ANATOMY

As a rule Elasmobranchs and Ganoids possess a greater number
of vertebras (up to nearly 400) than Teleosts, in which we seldom
meet with more than 70 : the Eel, however, possesses more than
200, while amongst the Plectognathi there may be as few as 15.

The tendency towards a fusion of the various components such
as occurs in the ossified vertebra? of Bony Ganoids and Teleosts is
also seen in the Amphibia and Amniota, the notochord being of
less importance and the vertebra? becoming more consolidated and
secondarily modified in various directions. Thus the homology of
the different elements of which they are composed can only be
traced by a study of their development ; but even in the adult,
parts are frequently present which recall the primarily composite
nature of the vertebrae, as will be seen in the following pages.

Amphibians. Amongst Amphibians, the vertebral column is
more or less distinctly differentiated into cervical, thoraco-lumbar,
sacral, and caudal regions, and these regions can be recognised,
except in certain modified types, in all the higher Vertebrates. On
account of the absence of extremities in Ca?cilians, there is no sacral
region, and in Anura, the caudal portion is modified to form a
urostyle (Fig. 45).

The notochord of Urodele larvae, like that of most Fishes,
undergoes intravertebral constrictions, while intervertebrally it
remains thicker, and accordingly appears expanded. Thus the
centra here also are ampliiccelous. In the course of their develop-
ment, a gradual reduction of the cartilage may be observed, and
the bone, originally perichondral in origin, becomes correspond-
ingly independent (Fig. 44). The cartilage is more and more
limited on the one hand to the arches (Fig. 52), and on the other
to the intervertebral regions round the notochord, extending to a
greater or less degree into the anterior and posterior ends of the
individual bony vertebral bodies, thus constricting or even entirely
obliterating the notochord in these regions. The bony centra are
formed from the bases of the arches, which, before ossification,
only rest on the notochord and do not enclose it. Finally a
differentiation, as well as a resorption, extending inwards from the
periphery, occurs in these cartilaginous parts : in the interior of
each an articular cavity is formed, so that in the vertebra? of many
higher Urodeles an anterior convexity and a posterior concavity
may be distinguished, both covered with cartilage ; they are,
therefore, opisthocoelous (Fig. 44).

In the development of the vertebral column of Urodeles we
can thus distinguish three stages: (1) A connection of the indi-
vidual vertebra? by means of the intervertebrally expanded
notochord ; (2) a connection by means of intervertebral masses
of cart i Inge; and finally (3) an articular connection. These
three different stages of development find a complete parallel
in the phylogeny of tailed Amphibians, inasmuch as many of

VERTEBRAL COLUMN

55

the Stegocephali of the Carboniferous period, as well as the
Perennibranchiata, Derotremata, and many Myctodera, possess
simple biconcave bony centra without differentiation of definite
articulations. 1

Thus the bony parts of the vertebrae of Urodeles are not
formed from the cartilage .surrounding the notochord, but in

-Liyf

11

FKI. 44. LONCUTUDINAL SECTION THROUGH THK VERTEBRAL CENTRA of VARIOUS
URODELES. A, Ra/nodon xiltericns ; B, Amblystoma tigrinum ; C, GyrinophUus
porphyriticus (1, II, III, the three anterior vertebrae) ; L), Salamandrina
perspicillata.

Ch, notochord ; CK, intravertebral cartilage and fat-cells ; Gp, concave posterior
face, and Gk, convex anterior face of centrum with articular socket and head ;
Jvk, invertebral cartilage ; K, superficial bone of centrum ; Liyt, intervertebral
ligament ; Mh, marrow cavity ; If, transverse process ; S, intravertebral
constriction of notochord in Amblystoma, without cartilage and fat-cells ;
**, intervertebral cartilage.

connective tissue, there being only an intervertebral cartilaginous
zone, extending into the ends of the centra. In the Anura, on the

1 In certain of the Stegocephali incomplete hoops of bone, the intercentra,
and pleurocentra, twice as numerous as the arches, surrounded the persistent
notochord (cf. the caudal region in Amia and Elasmobranchs, p. 50),

56

COMPARATIVE ANATOMY

other hand, as in Elasmobranchs, Teleosts, Bony Ganoids, and the
higher Vertebrates, the vertebrae are preformed in cartilage, and
true articulations are always formed between them : as a rule, but
by no means always, the convexity is posterior and the concavity
anterior (procodous form). A further difference is seen in the

relations of the notochord, which persists
intravertebrally longer than interverte-
brally, in this respect resembling Lepi-
dosteus and Reptiles.

The configuration of the caudal region
of the vertebral column must also be
remarked upon, as it differs in tailed and
tailless Amphibians. The long caudal
portion of the vertebral column in Anuran
larvae, which is very similar to that of
Urodeles, undergoes during metamorphosis
a gradual retrogressive modification, and
the vertebras of its proximal end become
fused and co-ossified to form a long, un-
segmented, dagger-like bone, the urostvle
(Fig. 45). '

Haemal arches are present in the
caudal region of Urodeles only. The
neural spines, as well as the transverse
processes, which in Urodeles are as a rule
bifurcated at the base and are present
from the second vertebra onwards, show
the greatest variety as regards shape and
size, differing in the several regions of the

FIG. 45. - VERTEBRAL body. The transverse processes of the
COLUMN OF Discoylossu* single sacral vertebra, which give attach-
pi ctus - ment to the pelvis, are particularly

Ob, upper arch of first ver- strongly developed, especially in the An-
tebra; Pa, articular ura (Fig. 45), in which the number of

s

neural spine ; Pt, Articular processes (zygapophyses) are
transverse processes of well developed in all Vertebrates from

trunk vertebra; Ptc, Uroc [eles onwards, and consist of two

t ran verse processes of ...

caudal vertebra (uro- pairs of projections arising from the an-

style, Oc) ; R, ribs ; Sy, terior and posterior edges respectively of

condylar facets of first th { h Thir surfaces are

vertebra ; S W , sacral

vertebra. covered with cartilage and overlap one

another from vertebra to vertebra, and

in some Urodeles the neural spines also articulate with one
another: thus a well-articulated and mobile, chain-like vertebral
column results.

The first, or ci'rcic.td vertebra becomes differentiated from the
others, and consists of a comparatively simple ring which articulat es

VERTEBRAL COLUMN 57

by means of lateral facets with the two condyles of the skull, and
also, in Urodeles, with the base of the latter by means of a projec-
tion, of varying size and form, the so-called " odontoid " process ;
thus a freer movement between the skull and vertebral column is
rendered possible. This vertebra, however, is not homologous with
the first vertebra (i.e. the atlas) of the higher Vertebrates, as is
demonstrated by a study of its development, which shows that the
real atlas, with the exception of the part which forms the " odon-
toid," loses its individuality as a separate mass, and becomes
united with the occipital region of the skull.

The number of vertebras present in Urodeles is inconstant, and
varies greatly : it may reach to nearly 100 (Siren), and in Cascilians
may be very much greater (up to 275).

Reptiles. In many fossil Reptiles (Theromorpha, Ichthyo-
sauria, &c.) the centra were biconcave, and this primitive form,
with an intervertebrally expanded notochord, is retained in the
Ascalabota amongst existing forms : the Rhynchocephali also
possess amphicoelous vertebrae, but intercentral fibro-cartilagin-
ous discs occur in their existing representative, Hatteria. A
primitive character of the Rhynchocephalian vertebral column is
seen in the retention throughout of the primary components of the
centra as distinct elements, wedge-shaped intercentra being inter-
calated between the centra proper or pleurocentra : in the majority
of Lacertilia intercentra also occur, but are usually only recognis-
able in the neck and tail ; in Chelonians a few intercentra are
present in the neck region. A pair of elements interposed between
the upper part of the first vertebra (atlas) and the skull in
Crocodiles, usually known as the " pro-atlas " (Fig. 46), which is re-
presented also in Hatteria, Chameleons, and many fossil forms, corre-
sponds to a disconnected pair of " supra-dorsal " elements (p. 49).

In the majority of Reptiles, the notochord remains expanded
longer in the intravertebral regions than intervertebrally, but in
the adult it becomes entirely aborted and replaced by bony tissue.
This stronger and more solid ossification of the whole skeleton
forms a characteristic difference between the Ichthyopsida on the
one hand and the Amniota on the other. As a rule the centra
of Reptiles are of the procoelous type and become definitely articu-
lated with one another : the forms with intervertebral remains of
the notochord and those with fibro-cartilaginous intervertebral discs,
(e.g. Crocodiles) form an exception to this rule. In Crocodiles the
vertebras are mostly procoelous, an exception being seen in the two
sacrals and first caudal. In Chelonians there is great variation in
the form of the individual centra of the cervical vertebras even in
the same individual procoelous, opisthocoelous, biconcave, and even
biconvex centra, with intervertebral discs, may occur ; while the
thoracic and lumbar vertebra- have flattened faces, and are firmly
united with one another by cartilage, and also with the
(p. 43).

58

COMPARATIVE ANATOMY

What has been said as to the classification of the vertebrae into
different regions in Urodeles, as well as to the presence of the
various processes, usually applies here also to a still greater extent.
Except in limbless form, there are always several cervical vertebra}
instead of a single one, and also typically at least two sacral
vertebrae. The two first cervical vertebra become differentiated
to form an atlas bearing a single occipital facet and usually
formed of three pieces, and an axis with an odontoid bone
belonging morphologically to the centrum of the atlas (cf. p. 57).

The neural spines vary in size, and transverse processes arise
from the centra themselves or close to them. Lower arches, or
chevron bones, corresponding to the intercentra, are present in the
tail in Lizards, Crocodiles, and some Chelonians : and besides

Po

FIG. 46. ANTERIOR PORTION OF THE VERTEBRAL COLUMN OF A YOUNG

CROCODILE.

atlas; Ep, axis, articulating with the atlas at h ; /, intervertebral disc; o,
" pro-atlas " ; Ob, neural arch ; Po, odontoid bone ; />*, neural spine ; Pt,
transverse process, arising from the base of the arch and articulating with
the rib at t ; R l , ^ 2 , ^, ribs ; u, ventral element, and s, arch of atlas ; WK,
centrum.

these, median inferior processes of the centra themselves are seen
in many of the vertebras of Lizards, Crocodiles, and Snakes :
in the last mentioned paired processes partly enclose the caudal
vessels. The arches in Snakes, Lizards, and usually in Chekmians,
become united with the centra by synostosis, while in Crocodiles
they remain, at any rate for a long time, separated from them by
sutures (Fig. 46).

In Snakes, Hatteria, and some Lizards (Iguana) extra articular
processes (zygosphenes and zygantrob) are developed on the neural
arches ; and in the caudal region of Hatteria and Lizards an
unossified septum remains in the middle of each centrum (which
really corresponds to two primary vertebral elements), so that the
tail easily breaks off' at these points. When this happens the tail
grows again, but true vertebras are not formed.

VERTEBRAL COLUMN

59

The greatest number of vertebrae is seen in Snakes, in which
there may be over 400.

Birds. The vertebral column of Birds has many points of
resemblance with that of Reptiles both phylogenetically and onto-
genetically. In both groups the notochord usually eventually
disappears entirely, and the whole skeleton becomes strongly
ossified. Archffiopteryx, as well as the Cretaceous Ichthyornis,
possessed biconcave vertebrae, but in existing adult Birds this
character never occurs except in the free caudal vertebrae.
Cervical, thoracic, lumbar, sacral, and caudal regions can be distin-
guished. The arches always become united into a single mass
with the corresponding centra, not remaining separated from them
by sutures, as is the case in certain Reptiles : even the ligament
which keeps the odontoid process of the axis in its place may

FIG. 47, A. ATLAS AND Axis (from the left side), and B, THIRD CERVICAL
VERTEBRA (ANTERIOR FACE) OF WOODPECKER (Picux viridis).

A, Ob, A, arch and centrum of atlas ; Po, odontoid process ; Pa, neural -spine of

axis ; Pt, transverse process ; WK, centrum of axis, and Su, its saddle-
shaped articular surface for the third vertebra ; t, condylar facet.

B, Ft, vertebrarterial foramen ; Ob, neural arch ; Pa, articular process ; Pt, Pi,

the two bars of the transverse process, shown on one side ankylosed with
the cervical rib (R) ; Pxi, median inferior process (hypapophysis).

become ossified. Fibro-cartilaginous discs or menisci, perforated
in the centre by a ligament, are present between the centra.

In the cervical region, which is extremely flexible and often
very long, the centra are in nearly all cases connected by means of
saddle-shaped (heteroccelous) synovial articulations ; the upper part
of each bifurcated transverse process arises from the arch, the
lower from the centrum, and these may unite with the correspond-
ing forked rib, the vertebral artery and vein extending through
the foramen thus formed (Fig. 47). The ring-like atlas, with its
single facet for the occipital condyle, is relatively small, and the
odontoid is fused with the axis. In the thoracic and lumbar
regions more or fewer of the vertebrae usually become immovably
united together.

The sacral region in Bird-embryos, like that in existing adult
Reptiles, consists of two vertebrae only, the transverse processes of

60

COMPARATIVE ANATOMY

which ossify separately and correspond to fused ribs, as in
Amphibia and Amniota. During further development, however,
a number of other (secondary sacral) vertebra? (thoracic, lumbar,
and caudal), with their rudimentary ribs, become fused with the
too primary ones (Fig. 48), so that the entire number of vertebra?
in the sacrum may be as many as twenty-three. In Archa?opteryx

the sacrum was much shorter than in
existing Birds, and fewer vertebra? were
united with it.

In existing Birds the actual caudal
region always exhibits a more or less
rudimentary character, and in its posterior
portion the vertebra? usually fuse together
to form a flattened bone, the pyc/osty/e,
which supports the tail quills (Fig. 132).
In the Batita? there is never more than
an insignificant pygostyle (Struthio), and
all the caudal vertebra? may remain dis-
tinct. That the latter is the more original
condition in Birds is shown by a study
of their development as well as by the
condition of the tail in Archa?opteryx, in
which it was supported by numerous
elongated free vertebra? (Fig. 49). It
must, however, be borne in mind that
the pygostyle may be made up of from
six to ten fused vertebra?, and in the
sacrum even a greater number may be
included, so that as many as twenty or
more caudal vertebra? may be represented.
Mammals. The notochord here per-
sists longer intervertebrally than intra-
vertebrally, but it disappears entirely by
the time the adult condition is reached.
A jelly-like pulpy mass, the nucleus pul-
posus, persists, however, throughout life
in the centre of the fibre-cartilaginous
menisci which are developed between the centra. The whole
vertebral column is preformed in cartilage, and the arches
develop in continuity with the centra but become ossified from
separate centres, as do also the various processes ; these separate
ossifications are no longer recognisable in the adult. The
presence of bony discs or epipliyscs on the flattened ends of the
centra which unite with the latter comparatively late, is very
characteristic of Mammals; they are, however, absent or only
imperfectly developed in Monotremes and in existing Sirenians.

True articulations between the centra ;iiv usually only formed
Hi the atlas and anterior face of the axis: well-developed 'articular

FIG. 48. PELVIS OF OWL
(Strix bubo). Ventral
view.

It, ilium ; /.y, ischium ; P,
pubis ; H, last two pairs
of ribs ; W, position of
the primary sacral verte-
br;f : between ft and //,
and behind W, are seen
the secondary sacral ver-
tebra^, fused with the pri-
mary (IF); f foramen
between ilium and ubis.

VERTEBRAL COLUMN

61

processes (zygapophyses) are present on the neural arches. 1 The
cervical region is usually the most movable, and the rcnlm
may hen; possess articulations and have a,n opisthoccelous form

FIG. 49. Archtcopteryx lithorjraj)hica. From the Solenhofen slates (Jurassic).
After Dames, from the specimen in the Berlin Museum.

c, carpus; d, clavicle; co, coracoid; h, humerus; r, radius; .sr, scapula; u, ulna'
/ ///, digits of manus; / IV, digits of pes.

(Ungulata). In some cases, on the other hand, the cervical
vertebra may become firmly fused with one another (e.g. Dasypus,
Talpa, Cetacea).

1 In certain Edentata (e.gr. Myrmecophaga, Dasypus) extra articular processes
are present besides the ordinary zygapophyses on the posterior thoracic and
lumbar vertebra-.

62 COMPARATIVE ANATOMY

The atlas 1 and axis essentially resemble those of Birds,
except that the condylar facet on the former is paired ; in
many Marsupials the ventral part of the axis may consist merely
of a fibrous band. The differentiation of the vertebral column
into regions characterised by difference of form is much more
sharply marked than in any other Vertebrates. There are as
a rule seven cervical vertebrae ; amongst the Edentata, however,
Bradypus possesses eight to nine, and Tamandua bivittata, eight,
while in Choloepus (and also in the Manatee) there are only six.

The transverse processes are simple except in the cervical
region and arise from the base of the arch. In the neck, they are
united with the vestiges of the cervical ribs, and in nearly all cases
enclose a vertebrarterial canal, as in Birds (p. 59): in Monotremes
these rib- vestiges remain distinct at any rate for a long time. In
the thoracic region the transverse processes are tipped with
cartilage on the ventral side of their distal ends for articula-
tion with the tubercle of the rib (q.v.). In the lumbar and
sacral regions they arise from the centra, and contain fused rib-
elements.

The number of thoraco-lumbar vertebrae varies greatly in
different Mammals ; there may be as few as fourteen (Armadillo)
or as many as thirty (Hyrax). In Ungulates the number is con-
stantly nineteen. In the lumbar vertebra? the transverse processes
are especially long, and other processes (anapophyses, metapophyses,
hypapophyses) may be characteristically present in this region.

Thus, as in Amphibians, Reptiles and Birds, the pelvis is con-
nected with the sacrum by means of vestigial ribs. As in the two
last-mentioned groups, there are not more than two primary sacral
vertebra?, but except in Ornithorhynchus and most Marsupials a
few caudal vertebrae become later included in the sacrum and are
usually more or less closely united with it by synostosis. The
various processes of the sacral vertebrae are more or less reduced.
In Anthropoids, and still more markedly in Man, the first sacral
vertebra is plainly marked off from the last lumbar by the forma-
tion of the so-called promontory. A sacrum is wanting in the
Cetacea and Sirenia, in correspondence with the absence of hind-
limbs : the horizontal tail-fin in these forms is not supported by
hard parts.

The caudal vertebra? vary extremely in their development, and
excepting in most long-tailed Mammals such as Kangaroos,
Sirenians, Cetaceans, and certain Apes no longer develop lower
arches. When present these " chevron bones " are intervertebral
in position. 2 The greatest number of caudal vertebra? is found in

1 A nodule of bone in the atlanto-occipital ligament of the Hedgehog may
represent the vestige of a "pro-atlas" (p. 57).

' The question as to homology of the chevron bones, as well as of certain
bony elements present in some Mammals beneath the intervertebral discs in the
tail (e.g. Dasypus, Erinaceus) and lumbar region (e.q. Talpa), requires further
investigation : it is doubtful to what extent they represent the lower arches or the
intercontra of other Vertebrates, or are structures peculiar to Mammals.

RIBS 63

Manis macrura (about fifty), and the caudal region is most reduced
in the higher Primates, in which it forms a stump-like coccyx
consisting of at most five to six vestigial vertebrae, all fused
together, and these may even (e.g. in Man) unite with the sacrum.
In the human embryo of 4-6 mm. in length, a distinct tail
is present, consisting of all the characteristic parts ; it gradually
undergoes reduction, and what is left no longer projects
externally.

II. RIBS.

Some doubt still exists as to whether the ribs are to be
considered as primitively independent skeletal structures, arising
in the intermuscular septa or myocommas, or as parts of certain
processes of the vertebrae which have become segmented off from
the latter, as is plainly seen to be the case, for example, in embryos
of Hatteria. Their relations to the axial skeleton, whether
primary or secondary, are of the very closest kind.

The ribs are situated in the septa between the great lateral
muscles of the body, and present much variation in the various
vertebrate Classes: they may be short and stump-like and almost
horizontal in position, or may grow ventralwards as delicate rod-
like structures, so as to encircle the body-cavity more or less
completely. Primitively, ribs may be present all along the
vertebral column, but, especially in the higher types, they become
reduced in certain regions.

A careful study of the ribs, in which their relations to the soft
parts (muscles) is taken into consideration, shows that they are
not completely homologous throughout the vertebrate series,
and that those of most Fishes are not exactly morphologically
comparable to those of Elasmobranchii, Amphibia, and Amniota.

Fishes. Two kinds of ribs, situated at different levels, may
be distinguished amongst Fishes dorsal ribs and ventral ribs (or
plcural arches} : the former extend into the transverse septa which
separate the epaxial or dorso-lateral from the hypaxial or ventro-
lateral muscles, while the latter are situated internally to the
muscles, just outside the peritoneum, but never more than
partially encircle the ccelome (Fig. 50). Both kinds of ribs are
usually considered as corresponding to prolongations of the transverse
processses (basal stumps) of the vertebral axis, from which they
have become segmented off but with which they remain closely
connected : another view as to their primary origin has been
stated above. The ventral ribs appear to be phylogenetically older
structures than the dorsal ribs, which can only have originated
after the differentiation of the intermuscular septa in which they
are situated.

Towards the caudal region, the ventral ribs, together with the
corresponding transverse processes, gradually take on the form of

RIBS

65

haemal arches, which in Teleosts, as in Elasmobranchs, arc
developed from the transverse processes alone (Fig. 50, B).
The dorsal ribs take no part in the formation of the hsemal arches :
towards the posterior part of the trunk they become rudimentary,
but may sometimes still be recognised in the tail as lateral
processes at the bases of the h.Temal arches.

In most Ganoids and in Dipnoans (Fig. 50, A, c) ventral ribs only
are present. In Crossopterygians (Polypterus, Figs. 50, E, anil
51) larger dorsal and smaller ventral ribs occur, so that there are

-JV

FIG. 51. ANTERIOR END OF THE VERTEBRAL COLUMN OF POLYPTEIIUS. From

the ventral side.

Ps, parasphenoid ; WK, centra ; / I 7 ", first five pairs of dorsal ribs ; ft, ventral

ribs.

two pairs of ribs to each body-segment. Dorsal ribs can also be
recognised in certain Tclcosts (Salmonidse, Clupeoidei) in addition
to ventral ribs, and like these, are always preformed in cartilage. 1
In many forms, the ventral ribs may undergo reduction, and in
Elasmdbrancks they are wanting, while dorsal ribs are usually
present. 2 In Chima3roids and many Rays, as is also the case in
Cyclostomes, a fibrous band extends outwards from the vertebral

1 This fact alone is sufficient to distinguish them from the intermuscular
bones often present in this region in Teleosts. In addition to these epicetitni/
intermuscular bones, others the epineurals and epipleurals are situated moi'o
dorsally and more ventrally respectively, and all of them are merely ossifications
in the septa.

! The hfemal arches of these Fishes, as well as of Ganoids, Dipnoans, and
Amphibians, apparently contain components corresponding to ventrals rib.

F

66

COMPARATIVE ANATOMY

axis in the position usually occupied by dorsal ribs : thus these
forms are ribless, and also in certain Teleosts and Ganoids the ribs
are wanting (e.g. Lophobranchii) or quite vestigial (Polyodon).

Amphibians. The ribs in the Amphibia correspond to the
dorsal ribs of Fishes, and are alwaj's connected with transverse
processes or at any rate with the vestiges of the basal stumps
(Fig. 50, F). The latter arc originally situated, as in Fishes,
towards the ventral side of the vertebral axis, nnd in the tail give

B

FIG. 52. A, VERTEBRA FROM ANTERIOR PART OF TAIL OF LARVA (43 MM.)
OF Nc<-tiiru$\ B, SACRAL VERTEBRA FROM LARVA (43 MM.) OF Nedum*;
C, FOURTH TRUNK-VERTEBRA FROM NEWLY-BORN LARVA OF Salanwiuli-n
maculosa. (After Goppert. )

Art. n/i., vertebral artery ; B, cartilage of basal stump ; B 1 . vestige of same in
larva of Salamander; B'*, bony bar which replaces the same functionally ;
Ch, notochord ; DRX, dorsal bar of ril> : .//. ilium ; N, neural arch ; /.'. rib ;
RT, A'7' 1 , ventral and dorsal rib-bearing portions of vertebra ; .s'/- 1 , lateral
process of hamial arch (//).

rise to haemal arches (Necturus, Salamander-larvae). In connection,
apparently, with the more dorsal position of the horizontal inter-
muscular septum in which they are situated, the transverse
processes, even in Salamanders, tend to arise more from the neural
arches than from the centra, and this upward displacement is carried
still further in the Gymnophiona and Anura. In TJrocleles (Fig.
52) the cartilaginous, rib-bearing basal stump is in close connection
with the centrum, but gives off secondarily an upwardly directed

RIBS 67

process which becomes connected with the neural arch and on
further development may serve as the chief point of attachment
for the rib. The proximal part of the primitive basal stump is
correspondingly reduced, and, with rare exceptions, is no longer
developed : in its' place is formed a bony bar, arising from the
centrum, and generally not preformed in cartilage.

The ribs of the Urodela and Gymnophiona are bifurcated
at their proximal ends, the ventral bar corresponding to the
primary rib-rudiment, while the dorsal bar is a secondary
structure formed in order to give the rib a firmer connection with
the vertebra : in Urodeles it becomes connected with the rib-
bearing portion of the vertebra, and in the Gymnophiona with the
neural arch itself. 1

The ribs of Amphibians are never very highly developed :
they are only slightly curved and do not encircle the body-cavity
to any extent. In Anurans they are not bifurcated and are
very short and stump-like (Fig. 45), usually becoming fused
with the transverse processes : they have doubtless undergone
reduction.

In many Urodeles the ribs are limited to the trunk, but
occasionally one or more pairs occur in the anterior part of the
tail, where the basal stumps have already extended ventralwards
to form the hasmal arches.

Finally, reference must be made to the cartilaginous " abdominal
ribs " (cf. p. 42) developed in the ventral intermuscular septa in
many Amphibians (Necturus, Menopoma, Bombinator).

Reptiles. As already mentioned, the ribs of the Amniota are
comparable to those of the Amphibia, but they grow further
ventralwards and so encircle the body-cavity to a greater or less
extent. Ribs may also be present in the tail : in Hatteria, for
instance, there are seven or more pairs of caudal ribs.

The dorsal (proximal) section of the rib may also become
segmented from the distal (ventral) portion, 2 and the former is
plainly homologous with the Urodele-rib. As a rule a certain
number of the ribs unite together ventrally to form a sternum
(cf. p. 72 and Fig. 56) : these are usually distinguished as " true "
ribs from the others, or " false " ribs.

The ribs of Snakes show the least amount of differentiation ;
for, without giving rise to a sternum, they extend along the
whole trunk from the third vertebra to the anal region, having
a similar form and size throughout. In Lizards, in which a dorsal,
unforked. bony portion and a ventral, cartilaginous portion can be
distinguished, three or four ribs reach the sternum, and are not
always completely segmented off from it. The proximal ends of

1 According to another view, the bifurcated amphibian rib is originally a
double structure, the dorsal bar of the fork originating independently and only
uniting secondarily with the ventral bar.

- An intermediate section also occurs in Crocodiles and man}' Lizards.

F 2

68

COMPARATIVE ANATOMY

the ribs of Hattcria are broadened out and articulate both with
the centra and arches, thus indicating a differentiation into a
capitulum and a tuberculum (cf. p 69.).

In Chelonians the cervical ribs unite with the vertebrae more
or less completely, and in the region of the trunk the ribs become
broadened out to form the costal plates of the carapace (p. 43).

FIG. 53. SKELETON OF THE TRUNK OF A FALCON.

Ca, coracoid, which articulates with the sternum (St) at t ; Cr, keel of sternum ;
Fu(Cl), furcula (clavicles) ; G, glenoitl cavity for humerus ; S, scapula :
I r , vertebral, and Sp, sternal, portion of rib ; Un, uncinate process.

Their proximal unbifurcated ends are attached between the centra,
at the junction of centrum and arch. There is no sternum.

The proximal ends of the cervical ribs in the Crocodilia are
bifurcated, in correspondence with the double transverse processes
in this region, and thus a vertebrarterial canal is formed (cf. p. 59).
Further back, the ribs increase in length, and become segmented
into two or three articulated portions. In passing from before
backwards, their point of origin becomes gradually shifted, so that
while the anterior thoracic ribs are attached to the centra, the

RIBS 69

posterior ones arise entirely from the transverse processes, which
increase in size correspondingly. Eight or nine ribs reach the
sternum, and from the eighteenth vertebra backwards the trans-
verse processes no longer bear ribs, but only short cartilaginous
apophyses.

Flat, curved cartilages, or uncinates, are present in connection
with the ribs in the Crocodilia as well as in Hatteria.

Birds. The ribs of Birds exhibit a much more marked
segmentation into vertebral and sternal portions, both of which
become ossified, and this evidently stands in relation to their
more active respiration. Bony uncinates, comparable to those
mentioned above, arise from, and usually become ankylosed to,
the vertebral portions in nearly all Birds, and overlap the ribs
next behind them (Fig. 53). The whole costal apparatus is
usually rendered still firmer by the
fusion of many of the trunk vertebrae
(p. 60), by the individual ribs often
being very broad, as well as by the
form and arrangement of the sternum
and pectoral arch. The last three or
four cervical vertebra? may bear com-
paratively large and movable ribs.
The number of ribs which articulate
with the sternum varies between two
(Dinornis elephantopus) and nine

(Cygnus). The delicate ribs of Archse-

'. . FIG. 54. COSTAL ARCH OF

opteryx (v ig.49; more nearly resembled MAN.

those of Lizards.

__ . m1 . , ., . (Ja, capitulum ; Co, neck, Cp,

Mammals. 1 he Cervical ribs in bony vertebral, and Kn,

nearly all cases unite completely with cartilaginous sternal por-

the vertebras, and a vertebrarterial tion of "J ; p *> neural

, . , f , nil i spine ; ft, transverse

canal is thus rormed. 1 he last cervical process; St, sternum ; Tb,

rib may be well developed and may tuberculum; f-F/if, centrum

articulate with the corresponding of vertebra -

vertebra (e.g. Choloepus hofraanni). 1 The seventh cervical rib
is also long in Bradypus, and the eighth and ninth ribs do
not reach the sternum ; they may therefore be counted as
cervical. There is considerable variation with regard to the
number of ribs which reach the sternum (e.g. in Manatus
2-3, in Cebus and Ateles 10): and in some cases the sternal,
as well as the vertebral ribs may become ossified. In the
vertebral portion a capitulum, a neck, and a tuberculum. may be
distinguished (Fig. 54). The capitulum usually articulates
with its own centrum as well as with that next in front,
in the region of the epiphysis ; the tuberculum articulates with

1 As amongst Reptiles, the ventral cartilaginous portions of some of the
anterior " false ribs " are connected with those in front, while the posterior ribs
end freely in the body-wall (" cost;e fluctuantes").

70 COMPARATIVE ANATOMY

the cartilaginous facet on the trans.verse process. In the " false "
ribs, these characters become to a greater or less extent lost in
passing from before backwards, so that the posterior ribs have a
more rudimentary character. As already mentioned (p. 62),
vestiges of ribs are present in the lumbar and sacral regions, and
unite with the corresponding transverse processes. There are
usually thirteen pairs of ribs, but their number may vary between
nine (Hyperoodon) and twenty-four (Choice pus). These facts
indicate that there has been a gradual phylogenetic reduction in
the number of ribs, and the occasional presence of supernumerary
ribs is to be explained as a reversion. 1

III. STERNUM.

Never present in Fishes, the sternum appears for the first time
in Amphibians in the form of a small variously-shaped plate of
cartilage situated in the middle line of the chest (Fig. 55). It
arises as a paired cartilaginous plate 2 derived in the first in-
stance from chondrifications in an intermuscular septum on the
median border (linea alba) of the rectus abdominis muscle, and
therefore may be looked upon as comparable to a pair of
" abdominal ribs." Such cartilaginous structures must have been
present in greater numbers in the ancestors of existing Urodeles
(cf. p. 67). In many tailless Batrachians (.#., Ranidas) the ventral
portion of the pectoral arch is continued forwards in the middle
line, from where the two clavicles meet, as a slender rod, the
omosternum (Fig. 55, D) : this has a similar origin, and the
proximal portion both of it and of the sternum becomes ossified.
Thus the sternum and omosternum of Amphibians are not to be
considered as corresponding to differentiations of the pectoral arch
(coraco-sternum), a view which is often held, but as consisting of
skeletal parts which primarily belong to the body-wall, and only
secondarily come into connection with the limb-skeleton.

In most Urodeles and certain Anurans (e.g. Pipa, Discoglossus,
Bombinator, Alytes), this cartilaginous sternal plate is inserted into
the grooved median margins of the two overlapping coracoids (Fig.
55, B, c). In Rana, on the other hand (D), in which the two

1 A primitive and a secondary type of thorax may be distinguished. The
former is the more usual, and occurs in most Mammals even up to the lower
Apes : it is characterised by an elongated form, and by the dorse-ventral
diameter being much greater than the transverse diameter. The latter occurs in
anthropoid Apes and Man, in which the dorso-ventral diameter has, both
ontogenetically and phylogenetically, become considerably reduced relatively :
the broad thorax is thus more cask-like in form, and may often even be flattened
dorso-ventrally. A somewhat similar modification is seen amongst insectivorous
Bats.

- It is unpaired from the first in Triton and Rana, but this is probably due
to an abbreviation of development.

Km. 55. PECTORAL ARCH OF VARIOUS AMPHIBIANS. From the ventral side.
A Urodele (diagrammatic) ; B Axolotl (Amblystoma) ; C Bomlnnnin,-
iy news ; U, J'nnn ewii/cnta.

C, coracoirl ; Cl, procoracoid ; C7 1 (Cl in U), clavicle ; EC, Co 1 , epicoracoid ;
Ep, omostermun ; Fe, fenestra between procoracoid and coracoid bars ; /\n,
cartilaginous xiphisternum ; t, Pf, O, glenoid cavity for the humerus ;
S, scapula ; SS, supraacapula ; St, Sf l , sternum. *, j (in B) indicate nerve-
apertures.

72

COMPARATIVE ANATOMY

halves of the pectoral arch are much more closely connected in the
middle line, by far the greater part of the sternum lies entirely
posterior to the coracoids, which do not overlap one another. In
the Perennibranchiata and Derotremata the sternum is much
simpler than in other Amphibians, and in Proteus and Amphiuma
it is entirely wanting.

In the Amniota, the form of the sternum, like that of the
pectoral arch, depends largely on the nature and function of the
forelimbs. It is usually considered as arising primarily by a
number of ribs running together ventrally so as to form a con-
tinuous cartilaginous longitudinal tract on either side. By the
more or less complete fusion of these two tracts, an unpaired sternal

FIG. 56. PECTORAL ARCH AND STERNUM OF A GECKO (Hemidactyhis

n rriicoaus). From the ventral side.

o, b, c, membranous fenestne in the coracoid ; Co. coracoid ; Co 1 , cartilaginous
epicoracoid ; Cl, clavicle ; Ep, episternum ; G, glenoid cavity for the
humerus ; R, ribs ; 8, scapula ; Si, cartilaginous cornua to which the last
pair of ribs is attached ; SS, suprascapula ; St, sternum.

band or plate is formed, from which the ribs are secondarily
segmented off by the formation of articulations, and beneath which
a dermal episternum is present in some cases (p. 44). The main
part of the anterior end of the sternum of Mammals is formed by
the median union of the first two or three pairs of cervical ribs,
and beneath and in front of this region in Monotremes is a large
T-shaped bone, the prosternum (" episternum "), the lateral parts of
which come into relation with the clavicles (Fig. 103). l

The sternum may become calcified (Reptiles), or converted
into true bone (Birds, Mammals). In Reptiles,' 2 Birds, and Mono-

1 Cf. note on p. 44.

- In (Snakes ami Chelonians there is no trace of a sternum.

STERNUM

73

tremes, the coracoids, as in Amphibians, come into direct connec-
tion with the lateral edges of the sternum (Figs. 53, 56, and 103),
and in other Mammals, the clavicles, when present, are connected
with it directly or indirectly.

The sternum is greatly developed in Birds, and consists of a
broad, more or less fenestrated plate, provided in the vast majority
of Carinata3 with a projecting keel, which forms an additional
surface for the origin of the wing-muscles (Fig. 53). In contrast
to these, the cursorial Ratitse are characterised by a broad, more
or less arched, shield-like sternum without a keel. In some
flightless Carinatse, however, the keel is rudimentary or even
absent, and the vestige of a keel may occur, though not constantly,

FIG. 57. A, STERNUM OF Fox; B, OF WALKUS ; AND C, OF MAN.

From the ventral side.

C, body ; J/Z>, manubrium ; Pe, xiphoid process ; B, ribs.

in certain Ratitse. The presence or absence of a keel is not, there-
fore, a constant character separating these two groups of Birds
from one another. 1

A far greater number of ribs are as a rule concerned in the
formation of the sternum of Mammals than is the case in Reptiles
and Birds. Consisting at first of a simple cartilaginous plate, it
later becomes segmented into definite bony portions (stcrnclircK)
the number of which may correspond to the affixed ribs (Fig. 57,
A, B) : in other cases as, for instance, amongst Primates (c), the
individual bony segments may run together to form a long plate
(corpus sterni}. Its proximal end forms a more or less distinct
manubrium, and the distal end a partly cartilaginous xiphoid or
ensiform

1 A keel was also present in the Pterosauria, and may be developed
wherever a larger surface fur the origin of the pectoral muscles is required (e.g.
Cheiroptera).

74 COMPARATIVE ANATOMY

IV. SKULL.
General Part.

The question as to the primary origin of the skull in the
Craniata has always taken a foremost place amongst the morpho-
logical problems relating to the structure of Vertebrates; and
the first point which requires elucidation relates to the nature of
the head whether it is a structure sui generis, or whether its
parts are due to modifications and further developments of parts
present in the trunk.

Until past the middle of the present century the theory which
held the field was the " vertebral theory " of Goethe and Oken,
according to which the skull consisted of a number of modified
vertebras (" cranial vertebrae "). On this theory, therefore, the
skull was regarded as a special modification of the anterior part
of the vertebral column, and a large number of facts were brought
forward in support of it : even when morphological science had
made further considerable advances, there still seemed to be a
certain amount of justification for this view.

The arguments in support of the vertebral theory of the skull
may be briefly stated as follows. As in the vertebral column, a
cartilaginous and a bony stage may be distinguished in the skull,
ontogenetically as well as phylogenetically. There is thus an
important correspondence between these two parts of the cranio-
spinal axis, and this is further emphasised by the fact that the
notochord always extends for a certain distance into the base of
the skull, so that the latter is developed on the same skeletogenous
basis as, and in direct continuation of, the vertebral axis. More-
over, the cranial cavity, enclosing the brain, may evidently be
considered as a continuation of the neural canal.

For a long time it was not recognised that as this theory
depended on giving an exact account merely of the skeletogenous
elements taking part in the formation of the skull, it could
not possibly lead to a true interpretation of the origin of the
vertebrate head. Any such attempt meant " putting the cart
before the horse," by looking upon the last acquisition of the head
its skeleton as the leading point for future researches.

Although it gradually became evident that, except occasion-
ally in the hinder (occipital) region, no trace of segmentation
of the cartilaginous elements can be recognised in the head
of any existing Craniata, it still seemed to be an open question
whether such a segmentation may not have occurred in early
phylogenetic stages and have gradually become suppressed owing
to deep-seated physiological and morphological modifications. 1

1 It is still by no means clear whether or not the sense-capsules were primi-
tively independent of the rest of the axial part of the skull, and it is quite
conceivable that the part of the latter anterior to the vagus foramen consisted
originally of independent skeletal portions which only secondarily became
connected with one another.

SKULL 75

The original segmentation of the head i.e., the segmentation of
the mesoderm into somites may have more or less closely resembled
that seen in Amphioxus ; but it must be borne in mind that there
is no direct connection between the Acrania and Craniata, and that
there must have been a whole series of intermediate forms. As a
matter of fact, only vestiges of the primary metamerism of the
head have persisted, and are more or less plainly indicated onto-
genetically by the ganglia, nerves, gill-arches, and myomeres. It
is nevertheless certain that the structural plan of the head, like
that of the trunk, is based on a condition of metamerism, although
it is doubtful how many primary segments are included, and
whether segmentation is not limited to the post-auditory region
(chordal or " spinal " portion) of the skull, and does not concern
the more anterior (prechordal or " prespinal ") portion.

In any case, however, the metameric character is much more
plainly seen in the post-auditory (occipital) region than in the
more anterior part of the head, in which the primary relations are
no longer recognisable owing to parts having become reduced,
displaced, fused, lost, or functionally changed in connection with
the modifications resulting from the development of the brain,
skull, the olfactory, optic, and auditory organs, and the oral muscles.
A reduction, fusion, or loss of cephalic myotomes has also occurred
in the post-auditory parts, the occipital region being of a very varied
and fluctuating nature, and it may even include spinal elements.
It is therefore evidently impossible in this place to give more than
the briefest sketch of the problem under consideration before
making a detailed study of the parts composing the head.

The portion of the skull which is situated along the main axis
in continuation of the vertebral column and which encloses the
brain, is known as the brain-case or cranium (neurocranium), and
is primarily composed of cartilage. A series of cartilaginous
arches arise in serial order on the ventral side of the brain- case ;
these encircle the anterior part of the alimentary tract like hoops,
incomplete dorsally, and are distinguished as the visceral portion
of the skull {splanchnocranium}. This bears an important relation
to branchial respiration, as between each consecutive pair of
arches a passage (gill-cleft) lined by endoderm, is present,
communicating between the pharynx and the exterior, and
through which the water passes in branchiate forms : the fore-
most visceral arch, which bounds the aperture of the mouth,
becomes modified to form the skeleton of the jaws. The arches,
therefore, serve primarily as gill-supports. Ossification may occur
in connection with the cranial and visceral portions later.

Before the cartilaginous skeleton begins to be formed in the
embryo, the greater part of the head consists of a mesodermic
formative tissue, which gives rise to a membranous capsule around
the brain and in which the rudiments of the individual cerebral
nerves can be plainly distinguished. The paired olfactory, optic,
and auditory organs also appear at a very early stage ; and these,

76

COMPARATIVE ANATOMY

in the course of further development, become situated in bays
or cavities within the head and enclosed by definite sense-capsules,
which take on close relations with the cranium, and thus are of
extreme importance in modifying the configuration of the skeletal
structures which are formed around them later.

The relations of the visceral to the cranial skeleton, and those
of both to the primary metamerism of the head, must be taken
into consideration. Both cranial and visceral regions must have
been originally segmented, and each myotome at one time included
a ventral portion (lateral plate of the mesoderm) which enclosed a
corresponding section of the cranial coelome, or t; head-cavity."
Later, however, the visceral region became relatively shifted to a
greater or less degree, especially in the anterior part of the head,

V-----A'

C

FIG. 58. FIRST CARTILAGINOUS RUDIMENTS OF THE SKULL.

C, notochord ; X, A, O, the three sense-capsules (olfactory, optic, and auditory) ;
PE, parachordal elements ; PR, primary pituitary space ; Tr, trabecuke
cranii.

so that its segments no longer corresponded to those of the cranial
region, which is in general more conservative as regards its
metamerism. Thus we find that the segmentation of the nervous,
muscular, and visceral parts of the head do not correspond with
one another.

a. Brain Case (Neurocranium).

The first cartilaginous rudiments of the primordial skull or
chondrocranium are seen in the form of an anterior and a posterior
pair of bars the trabeculce cranii and the imrachordal cartilages

SKULL

77

(Figs. 58 and 59), which may be continuous with one another.
They lie along the base of the brain, the parachordals embracing
the anterior end of the notochord. The parachordals soon unite,
more or less completely, to form a basal plate, which grows round
the notochord dorsally and ventrally, and thus early forms a solid
support for the hinder part of the brain. The trabeculaB project
forwards and enclose a space, which, as the pituitary body extends
from the brain through its posterior part, may be spoken of as the
primitive pituitary s/tace (anterior basicranial fontanelle). In the
parachordal region, an anterior auditory or otic, and a posterior
occipital portion, may be recognised on either side. The occipital
region, as already mentioned, may show indications of segmentation

Occipital arch

Cornu tra'jecv.lce

Notochord

Auditory capsule
Otic process
Palatoquadrate

Articular process

FKI. 59. NEUROCRANIUM AND PALATOQUADRATE OF LARVAL AMBLYSTOMA,
9 MM. IN LENGTH, SEEN OI'.LK.H'KLY FROM THE LEFT SIDE AND AP.OVE.
: ABOUT 35. (From copy by Fr. Ziegler of a model by Ph. St<ihr. )

II, foramen for optic nerve, and III, for oculomotor nerve.

in the nerve-apertures which perforate the cartilage as well as in
the surrounding myomeres : its anterior limit is marked by the
vagus nerve.

Around the auditory organ of either side is developed, usually
independently, a cartilaginous auditory capsule (Figs. 59-61),
which is relatively larger in the lower than in the higher Vertebrates.
It is situated laterally to the otic region, between the trigeminal
and vagus nerves, and becomes closely connected with the corres-
ponding parachordal which may help to complete it.

The trabeculoe may remain separated from one another or may
become united along the greater part of their length under the
influence of the developing optic capsules. In the former case the
skull may be described as platybasic, and the brain extends for-
ward interorbitally to the ethmoidal region (many Elasmobranchii,
Dipnoi, Amphibia, Fig. 60) ; in the latter, or tropibasic type (Fig.
61), the trabeculas give rise to a thin interorbital septum, and the

78

COMPARATIVE ANATOMY

brain only extends as far forward as this septum, in the dorsal part
of which a narrow canal encloses the olfactory lobes or nerves.

The further development of the wall of the orbital or orbito-
temporal fossa takes place either from the trabecute or indepen-
dently of them, and during its growth, the cartilage extends
round a number of the cerebral nerves. The anterior ends of the
trabecula?, which are continuous with the ethmoidal re y ion of the

Intei'iiasal cavity

Olfactory fenestra

Antorbilal process

Prootic foramen

Notochord

MecTcd's cart.
11

Ascen/lina process} 2

( S
Otic process J "3

I Auditory caps.

Tectum synoticum Occipital condyle

Fin. GO. NEUROCRANIUM (PLATYBASIC TYPE) AND MANDIBTTLAR ARCH OF
LARVAL NEWT (Triton titnintux), '2 CM. IN LENGTH, SEEN FROM ABOVE.
: 25. (From a model by E. Gaupp. )

skull, vary much in form in different Vertebrates according to
whether the skull is of the platybasic or tropibasic type. The
ethmoidal skeleton may be completed in various ways by the
cartilaginous nasal or olfactory capsules, the chondrification of which
takes place independently in the connective tissue surrounding
the nasal sacs. Anteriorly, the ethmoidal region may extend
forwards to form a rostrum, or prenasal cartilages may be formed.
The olfactory and auditory capsules, especially in higher types,
then become more and more drawn in to the skull proper, and
the lateral edges of the basal plate begin to grow upwards round
the brain on both sides, eventually extending even to the dorsal

SKULL

79

region. Thus a continuous cartilaginous neurocranium is formed
such as persists throughout life in Elasmobranchs, for example.
But in by far the greater number of Vertebrates, the cartilage
does not play so great a part, and is, as a rule, confined to the base
and lower parts of the sides of the skull and to the sense capsules,
except in the occipital region, where it always extends over the
brain. The rest of the skull, more particularly the roof, becomes

Hum'/

...ttnl. raps.

/s;

fj Columella

J#%

Notochora

FIG. 61. DIAGRAM OF A TROPIBASIC PRIMORDIAL SKULL, BASED ON THE
CONDITIONS SEEN IN THE AMNIOTA, AND SHOWING THE RELATIONS OF THE
MORE IMPORTANT FORAMINA. (After E. Gaupp. )

The cerebral nerves or their foramina are indicated by Roman numerals : Sp. o<\,

foramina for spino-occipital nerves.

directly converted from membrane into bone (investing bones) : at
the same time, bones may become differentiated in connection with
the chondrocranium itself, which is thus more or less completely
replaced by an osteocranium. In general, the higher the systematic
position of the animal, the less extensive are the cartilaginous
constituents and the more important the bony elements.

The investing bones (allostoses) have originally no direct con-
nection with the chondrocranium, and thus may be contrasted with
those bony elements which are formed in close relation with it,

80

COMPARATIVE ANATOMY

both phylogenetically and ontogenetically. These perichondral
bones extend into the cartilage and many even entirely replace
it (" endochondral " bones) ; they may therefore be called replac-
ing or substituting bones (autostoses~) (cf. p. 45). It must,
however, be borne in mind that the stage of development at
which the different bones appear gives no accurate basis for
phylogenetic speculations : bones which correspond in position as
well as in other respects and thus appear to be homologous, may
be developed in different ways in different Vertebrates.

The development of bones starts from so-called " centres of
ossification," of which there may be several in a single resulting
bony territory. An earlier or later fusion of these centres or
even of entire bones, leads to a reduction in number ; while on
the other hand, " supernumerary bones " may occur owing to
the absence of fusion between separate centres

'< The Visceral Skeleton (Splanchnocranium).

The primarily cartilaginous visceral arches, which are developed
successively from before backwards in the lateral plates of the

mesoderm, encircle the anterior section
of the alimentary canal, and are situ-
ated in the interbranchial septa (Figs.
62 and 63). The branchial apparatus
at first lies beneath the hind-brain, and
the arches are thus included under the
cranial skeleton, of which, however, they
are mostly genetically independent.
Later, owing to unequal growth, most of
the gill-sacs become relatively shifted
backwards so as to be situated in the
region of the trunk. The visceral arches
FIG. 62. - - DIAGRAMMATIC are always more numerous (in some
TRANSVERSE SECTION OF cases there are as many as nine) in
THE PRIMORDIAL SKULL. P , i -n ii -1-1

rorms which possess gills than in higher

G, notoehord ; 0, auditory , (Amniota), in which their number

capsule ; RH, cavity of the J 1 .

pharynx, enclosed 'by the gradually becomes reduced irom behind
visceral skeleton ; l to 4, forwards : they may, moreover, undergo

the individual elements a change of function, certain of them in

composing each visceral o . .

arch, which is united with some cases taking on definite relations

its fellow by a basal piece to the auditory organ, larynx, and

(Cp) ; Tr. trabeculre, which tnno .. ](

enclose the brain (0) ven- =

trally and laterally, I he most anterior arch arises

first and serves as a support for the

walls of the mouth which receive their nerve supply from the
trigeminal : it is distinguished from the other or post-oral arches
as the mandibular arch (Fig. 63). The post-oral arches serve

SKULL

81

as gill-bearers in the Anamnia, but even the first of them, the
hyoid, along which the facial nerve extends, becomes modified from
those lying behind it : these, the branchial arches proper, of which
there are usually five in Fishes, are in relation with the glosso-
pharyngeal and vagus. All the visceral arches must originally,
however, have borne gills.

Primarily unsegmented, the individual post- oral arches may
become divided up into portions (phoryngo-, epi-, cerato-, and hypo-
hyal or branchial), the uppermost of which lies under the base of
the skull, while the lowermost is connected with its fellow by a
median basal piece or copula (basihyal, basibranchial).

The mandibular arch also undergoes segmentation, and becomes
divided into a short proximal piece, the quadrate, and a long distal

V

FIG. 63. DIAGRAMMATIC FIGURE OF AN EMBRYONIC ELASMOBRANCH SKULL,
SHOWING THE RELATIONS OF THE VlSCERAL ARCHES.

A, eye ; a to e, branchial arches, between which the gill clefts (7 to V) are seen ;
br, brain ; C, inter-vertebral remains of notochord ; H, hyomandibular ; K,
hyoid arch ; L, labial cartilages ; M, Meckel's cartilage ; N, nasal capsule ;
0, auditory capsule ; PQ, Q, palatoquadrate, connected with the trabecula
by ligaments at f ; S, spiracle ; sp.c, spinal cord ; Tr, trabecula ; V, vertebral
arches.

mandibular or Meckel's cartilage (Fig. 63). The quadrate gives
rise anteriorly to a process, the palatoquadrate or palatopterygoid,
which usually becomes fixed in various way to the base of the
skull and gives rise to the primary upper jaw, Meckel's cartilage
forming the lower jaw.

The quadrate, which primarily serves as a support (suspcn-
sorium) for the jaws, either remains separated from the skull, being
only connected with it by an articulation or by connective tissue,
or becomes united with it.

The hyoid may also take part in the suspensorial apparatus,
and thus come into close relation to the mandibular arch and

G

82 COMPARATIVE ANATOMY

cranium : its upper element (pharyngohyal), articulating with
the skull to form the suspensorium, is then known as the hyo-
mandibular (Fig. 63), and from it (e.g., in Teleosts) a symplcdic
may be differentiated distally. 1 In the mid-ventral line there is a
basihyal connecting the arch of either side and embedded in the
tongue (entoglossal or glossohyaT).

Certain smaller or larger skeletal parts of doubtful homology
form a kind of outwork to the skull anteriorly, and have been
described as prccranial or preoral elements. Under this category
are included the labial cartilages of Elasmobranchs (Fig. 63), and
similar structures amongst Teleostomes and in certain Anuran
larvse.

Relations of the Chief Investing Bones to the Chondocranium.

The primary relations of the investing bones to definite parts
of the cartilaginous skull are not in all cases sufficiently known,
but the following scheme, formulated by Gaupp, probably holds
good as regards the more important elements (cf. under Special
Part and Figs. 67-95).

The parietals and frontals are primarily situated on the roof of
the skull in the auditory and orbitotemporal regions, beyond which
they may, however, extend. The squamosal, which in the Amniota
is developed on the outer wall of the auditory capsule, is also
present in Bony Ganoids and Teleosts, but in them loses the
character of an investing bone (cf. p. 83). 2

The investing bones of the ethmoidal region are : the nasal,
supracthmoid of Teleostei, prefrontal of Amphibia and Sauropsida,
septomaxillavy (situated in the posterior region of the nasal
fenestra in Amphibia and Reptilia, and sometimes extending into
and beyond the nasal capsule), and the lacrymal of Mammalia ; it
is doubtful whether the last mentioned corresponds to the
similarly-named bone of Lizards and Crocodiles. The relations of
the prcmaxilla, maxilla, and vomer to the ethmoidal skeleton are
possibly of a secondary nature (cf. p. 83). The parasphenoid arises
in the mucous membrane beneath the cranial floor.

On the lateral surface of the palatoquadrate of Amphibians is
an extensive bone, usually known as the squamosal, but called
the paraquadrate by Gaupp, who considers it to be homologous
with the quadratojugal of Reptiles. The vomer, palatine, and
pterygoid (including the ccto- and cnto-pterygoids of Teleosts)
probably arose as tooth-bearing investing bones on the palatine
region of the palatoquadrate, and are therefore traceable to the
teeth which in Elasmobranchs are present along the cartilaginous

1 It is possible that the hyoid arch is the derivative of two arches which were
primarily separated by a cleft.

' The primary relations of the postfrontal and jugal of the Amniota cannot
at present be stated.

SKULL 83

upper jaw. In Teleosts, however, the vomers, and in Amphibians the
palatines also, are no longer situated on parts of palatoquadrate,
but are related to the ethmoidal skeleton, and lie beneath the
olfactory capsules. The pterygoid alone retains its original
relations to the palatine bar in Amphibians and many Reptiles.

The palatoquadrate cartilage, which in Elasmobranchs is
situated along the upper margin of the mouth, represents the
primary upper jaw, and does not correspond to i.he premaxillo-
maxillary bar which lies externally to it in the higher Fishes and
in all Vertebrates above them. It is possible that the premaxilhe
and maxillae were originally laid down in relation with certain of
the labial cartilages referred to above (p. 82, Figs. 63, 65, and 66.)

In the lower jaw, investing and tooth-bearing bones are formed
around Meckel's cartilage. As in the upper jaw, two bars, an
outer and an inner, may be distinguished, the former represented
by the dentary and the latter by the splenial, in connection with
which there may be a varying number of presplenials. As the
teeth on the primordial lower jaw of Elasmobranchs maybe looked
upon as corresponding to splenial teeth, it is possible that the
dentary, like the premaxilla and maxilla, may have been originally
formed around a primary skeletal element situated anteriorly to
the primitive jaw.

The purely integumentary ossifications also of the lower jaw
are investing bones of Meckel's cartilage : great confusion exists
as regards their nomenclature (" dcrmamjular" " dcrmarticular,"
" supra-angular" " coronary ").

Certain tooth-bearing bones are formed in connection with the
hyobranchial skeleton of Teleosts (superior and inferior pliarynijm I
I/ones, " dermobranchials," " dermentoglossal "). In higher forms,
investing bones are only exceptionally present in connection with
this part of the primordial skeleton.

Relations of the Replacing Bones.

The following are usually looked upon as ossifications of the
occipital region (cf. Figs. 67-95) : the basioccipital, exoccipitals
(pleuroccipitcds\ and supraoccipital : the last mentioned, however,
usually arises from the ossification of that part of the roof of the
skull (tectnm synoticum, Fig. 60) which belongs to the auditory
region. The supraoccipitals and exoccipitals usually extend into
the auditory capsules, in each of which (otic region) arise the
periotic bones, including an opisthotic, epiotic, prootic, sphenotic, and
pterotic : of these, the most constant element is the prootic. The
sphenotic and pterotic occur only amongst Fishes, in which, how-
ever, the pterotic does not remain independent, but unites with a
dermal bone to form the squamosal, which is thus made up of an
autosquamosal and a dermosqiiamosal.

G 2

84 COMPARATIVE ANATOMY

In the orbito-temporal region, a basisphenoid, a prcsphenoid,
alisphenoids, and orbitosplienoid s may occur. In the ethmoidal
region lateral ^cthmoids and pre-ethmoids are present in bony Fishes
and a single ethmoid in Mammals. 1

The quadrate region of the palatoquadrate usually becomes
ossified as a quadrate bone. In Bony Ganoids and Teleosts there
is also a metapterygoid, and an autopalatine at the anterior end of
the palatine bar which mostly fuses with a dermopalatine.

An articular is usually formed at the proximal end of Meckel's
cartilage, and anteriorly and posteriorly to this zone of ossification
an autocoronary and other bones may be developed (Teleosts).
The anterior end of Meckel's cartilage usually becomes ossified as
a mentomandibular (mentomeckelian) which may become fused with
the dentary.

In the hyobranchial skeleton, the individual segments may
be uniformly ossified (e.g. styloliyal, glossohyal, and the segments of
the branchial arches) ; but frequently several ossifications may occur
in a single segment.

SPECIAL PART.

In Amphioxus the vestigial brain is merely surrounded by a
thin layer of connective tissue, and there is no proper cranial
skeleton. The margin of the oral funnel and the cirri arising from
it are supported by cartilage-like skeletal rods. The branchial
skeleton consists of a series of elastic rods of a cuticular nature,
which are connected together dorsally by arched portions and by
transverse bars at different levels, but which remain separate
ventrally. A comparison between these and the branchial skeleton
of higher forms is rendered all the more impossible by the fact
that no definite boundary between the head and trunk can be
recognised.

Fishes (including Cyclostomes).

The skull of Fishes exhibits very great differences in the various
groups, and in many cases reaches a high degree of complication.
It is therefore only possible to give here the merest sketch of its
characteristic structure in the different Orders.

In Cyclostomes, the skull is developed essentially in the
manner already described. Later, however, it shows many special
peculiarities (Fig. 64), probably in consequence of the suctorial
(Petromyzon) or parasitic (Myxine) mode of life of these animals :
the most important of these is the absence of jaws such as are

1 It is still uncertain as to how far the replacing bones of the cranium called
by the same names in different Vertebrate groups are really homologous.

SKULL

85

present in all other Craniata ; for this reason these forms are
spoken of as Cydostomata to distinguish them from the other
craniate Vertebrates or Gnathostomata.

In addition to the peculiar histological differences in the struc-
ture of the cartilage, many other special characters of the skull are
seen not only in the Class as a whole, but also in the two Orders,
which must have become specialised very early along different
lines, so that a comparison between them is rendered difficult.
The low character of the skull is marked by the imperfect develop-
ment of the cartilaginous brain-box, and thus most of the cerebral
nerves on making their exit from the skull are but slightly or not
at all surrounded by cartilage : moreover, the anterior region of
the spinal axis, which in Gnathostomes becomes assimilated to the
cranium, remains undifferentiated, so that an occipital region is
wanting and the vagus makes its exit behind the skull (" palceo-
cranium ") and not through its walls.

The jaw-apparatus has doubtless become degenerated, and
indications of its former presence may possibly lie recognised.

I. c.l

l.C.3

Fit;. 04. SKULL WITH BRANCHIAL BASKET OF Petromyzon marinus.

(After W. K. Parker.)

The cartilaginous parts are dotted. a.d.c, anterior dorsal cartilage; a.lat.c,
anterior lateral cartilage; an.c, annular cartilage; ait.c, auditory capsule;
br.li, 1 7, vertical bars of branchial basket ; br.cl, 1 7, external branchial
clefts; cn.c, cornual cartilage; cr.r, cranial roof; l.c, 1 4, longitudinal
bars of branchial basket ; fy. c, lingual cartilage ; mv.c, median ventral
cartilage; iia.ap, nasal aperture ; nch, notochord ; Nv. 2, foramen for optic
nerve; olf.c, olfactory capsule; pc.c, pericardial cartilage ; pdc, posterior
dorsal cartilage ; p.lat.c, posterior lateral, cartilage ; sb.oc.a, subocular arch ;
tst.p, styloid process; sly.c, styliform cartilage; t, teeth.

Thus certain similarities between the subocular arch (Fig. 64,
sb. oc. a.} and the palatoquadrate of the Frog-tadpole were pointed
out by Huxley ; the posterior lateral cartilage (p.lat.c) is perhaps
comparable with Meckel's cartilage and the styloid process (st.p)
and cornual cartilage (cn.c) with the hyoid. A number of other
cartilages supporting the anterior parts of the head cannot well be
directly compared with parts of the gnathostomatous skull.

In the adult Lamprey, for instance, the suctorial mouth is

86

COMPARATIVE ANATOMY

supported by various skeletal elements, amongst which may
be mentioned a ring-like cartilage (an.c) around the margin
of the dome-shaped oral funnel, between the dorsal side of
which and the brain-case are a couple of large overlapping carti-
lages (a.d.c, p.d.c} : the tongue is supported by a long, lingual
cartilage (Ig.c), and besides the other elements shown in Fig. 64,
cartilages are present in the velum at the base of the oral funnel.
On the mucous membrane covering the annular and lingual
cartilages inside the oral funnel are a number of horny teeth.
The fibro -cartilaginous olfactory sac is unpaired, and opens on the
dorsal surface of the head by a single nostril. The visceral
skeleton also shows many exceptional peculiarities : it consists of
a series of unsegmented rods produced into short processes and

Nv.S

ex.b.

brr 1

fiy.cn.

c&.br

FIG. 05. SKULL OF DOGFISH (Scyllium canicula). (From T. J. Parker's
Biology, after W. K. Parker. )

aud.cp, auditory capsule; br.a, 1 5 branchial arches ; br.r, br.r', branchial rays
arising from the hyoid and branchial arches; Cr, cranium; ex.br, extra-
branchial cartilages ; hy.cn, ventral part of hyoid arch ; hy.m, hyoman-
dibular ; ll>, labial cartilage; ff/.fy', ligaments supporting the jaws from
the cranium ; Lj, MeckePs cartilage; Nv. 2, optic foramen ; Nv. 5, foramen
for trigeminal and facial nerves; olf.cp, olfactory capsule; or, orbit; r,
rostral cartilage ; up.j, palatoquadrate. (The spiracular cartilage is not
indicated.)

connected with one another by longitudinal bars, the whole forming
a delicate cartilaginous basket-work : the last bar is connected
with a cartilage in the walls of the pericardium. This basket-
work has a very superficial position.

In Myxine the branchial skeleton is rudimentary, and amongst
other peculiarities, the long nasal passage is surrounded by
cartilaginous rings and communicates with the pharynx by a
naso-palatine duct.

No fossil Cyclostomes are known, but Palceospondylus gunni

SKULL

87

from the Old Red Sandstone of Caithness possibly shows affinities
with this group.

In Elasmobranchs the skull presents the simplest conditions
and most easily comprehensible relations, so that it may be taken
as the starting-point for the study of the skull of all other Verte-
brates. It consists of a simple cartilaginous and fibrous capsule
more or less movably articulated with the vertebral column, the
chondrocranium here reaching its highest development (Figs. 65
and 66), while from the Elasmobranchs onwards it undergoes, on
the whole, a gradual reduction owing to the increasing import-
ance of the osteocranium. The skull may become more
or less calcified, but true bones are never formed. The fibrous

V.JO

r

FIG. 66. SKULL OF Chinuxra monstrosa, LATERAL VIEW. (From
Parker and Haswell's Zoology, after Hubrecht. )

a.s.c, position of anterior semicircular canal ; rh.y, ceratohyal ; ep.hy, epi-
hyal ; fr.cl, frontal clasper ; h.n.c, position of horizontal semicircular canal ;
i.o.s, interorbital septum; tb. 1, Ib. 2, Ib. 3, labial' cartilages ; Mck.C,
mandible ; No. 2, optic foramen ; Nv. 10, vagus foramen ; olf.cp, olfactory
capsule; op.r, opercular rays; pal.qn, palatoquadrate ; ph.hy, pliaryngo-
hyal ; p.x.c, position of posterior semicircular canal; qu, quadrate region;
r, rostrum.

portions (fontanelles) are most marked in the prefrontal region,
except in the tropibasic skull of the Holocephali, in which there is
no prefrontal fontanelle and the interorbital region consists of a
thin membranous septum between the large eyes (Fig. 66).

As in all Vertebrates above Cyclostomes, an assimilation of
vertebral elements has taken in the occipital region, so that the
nerves belonging to the vagus-group perforate the skull ; the part
of the skull situated posteriorly to these foramina has therefore
been described as a " neocranium " (cf. p. 85).

The nasal region is often elongated to form a cut-water or

88 COMPARATIVE ANATOMY

rostrum, at the proximal end of which the olfactory sacs are
situated, their cavities being separated from the cranial cavity by a
membrane. Behind them are the deep orbital hollows, which are
bounded posteriorly by the strongly projecting auditory capsules.
Labial cartilages (cf. p. 82) are present in connection with the lips,
nostrils, and jaws.

The palatoquadrate meets with its fellow in the middle line and
is usually connected with the basis cranii by ligaments (Fig. 65). A
process may be present on it which articulates at some point with
the trabecular region. In the Chimseroids (Fig. 66) it becomes
immovably fused with the cranium, whence their name of
Holocephali. In the Sharks and Rays the palatoquadrate is not
directly united to the skull, but is suspended from it by the
hyomandibular (p. 82, and Fig. 65). In this case the skull may be
described as hyostylic, to distinguish it from autostylic skulls, in
which the hyoid takes no part in the suspensorium. In Notidanus,
both mandibular and hyoid arches are independently connected
with the skull, which is therefore spoken of as ampliistylic. A
vestigial cleft, the spiracle, is situated in front of the hyomandi-
bular, and leads into the pharynx ; on its anterior wall may be
found remnants of the embryonic spiracular gill, beneath which
are one or more spiracular cartilages which probably represent gill-
rays (cf. below). 1

In Plagiostomes the palatoquadrate and lower jaw are provided
with numerous teeth, arranged in rows ; in the Holocephali the
teeth have the form of strong and sharp-edged plates.

The branchial skeleton is relatively smaller in the Holocephali
than in other Elasmobranchs, in which it is always richly developed,
and owing to secondary segmentation and also to fusion of its
parts, exhibits characteristic modifications. On the outer circum-
ference of each branchial arch, as well as on the hyomandibular
and hyoid, radially-arranged cartilaginous rays are developed, which
serve as supports for the gill-sacs (Fig. 65). Externally to these
rays rod-like " extra-branchial " cartilages are present : these
correspond to the displaced uppermost and lowermost gill-rays.

In Plagiostomes the gill-slits open freely on to the surface of
the body, but in the Holocephali a fold of skin, the gill-cover or
opcrculum arising from the hinder border of the hyomandibular,
overlies them. In the frilled Shark (Chlamydoselachus) there is
an indication of an operculum.

Amongst Ganoids, the lowest condition is met with in
those forms in which the hyaline primordial skull is still retained,
immovably fixed to the vertebral axis, part of which becomes
secondarily assimilated to it. These forms are spoken of as
Cartilaginous Ganoids. The presence of definite bones, however,

1 A small basimandibular element has been described in Laanargus, and
mandibular rays can be recognised in the primitive Pleuracanthidtt from the
Permian formation.

SKULL

89

divides them sharply off from Elasmobranchs, and shows that
their skull has reached a much higher stage of development.
These bones have the form of richly sculptured plates and shields,
and are developed partly from the mucous membrane lining the
mouth and covering the visceral skeleton, and partly from the skin
covering the roof of the skull, where the arrangement of the bones
(e.g. frontals and parietals) typical of higher forms can to some
extent be recognised. A narrow parasphenoid forms a roof to the
oral cavity. The operculurn is more pronounced than in the
Holocephali, and is also supported by bones (cf. p. DO). The
whole palato-mandibular apparatus which is comparatively small,

M(g

FIG. 67. CRANIAL SKELETON OK STURGEON (Acipemer) AFTER REMOVAL OF THE

EXOSKELETAI. PARTS.

Ar, articular; C, notochorcl ; Cop, basal elements of the visceral skeleton ; De,
dentary ; GK, auditory capsule ; Hm, hyomandibular ; hy, hyoid ; 7 to V,
first to fifth branchial arches, with their segments the double pharyngo-
branchial (a), the epibranchial (b), the ceratobranchial (c), and the hypo-
branchial (d) ; Ih, interhyal; //, optic foramen ; Md, mandible ; Na, nasal
cavity ; Ob, neural arches ; Orb, orbit ; PF, AF, postorbital and antorbital
processes; PQ, palatoquadrate ; P8,'Ps', Ps", parasphenoid; Psp, neural
spines ; Qu, quadrate ; JR, rostrum ; Ri, ribs ; /S?jJV, apertures for spinal
nerves ; Sy, symplectic ; WS. vertebral column ; x, vagus foramen ; *, pro-
minent ridge on the basis cranii.

bears no teeth, and in relation with which bones are formed is
connected very loosely with the skull by means of a hyoman-
dibular and symplectic, as well as by ligaments (Fig. 67).

The dermal skeleton attains a much more considerable develop-
ment in the Bony Ganoids (Crossopterygii and Holostei), and
gives rise to a dense armour composed of numerous bones lying
on the roof and extending into all parts of the skull and jaws
(Fig. 68, A and B) : amongst these may be noted a median
(Amia) or paired (Polypterus) jugular plate between the rami of
the mandible. In addition to the investing bones, replacing bones
are present in the occipital, obic, orbitotemporal, and ethmoidal
regions. Investing and replacing bones very similar to those

90

COMPARATIVE ANATOMY

of Teleosts (p. 92) also occur in connection with the palatoquad-
rate and the entire visceral skeleton, including Meckel's cartilage

and the branchial arches.
Though still largely re-
tained, especially in Amia,
the cartilage thus becomes
relatively reduced as com-
pared with the cartilagin-
ous Ganoids (Fig. 68, B).

At the posterior end of
the trabeculge, which only
remain separated from one
another by a narrow slit,
a lateral basipterygoid
process arises in Lepidos-
teus for articulation with
the palatoquadrate arch,
which is thus connected
with the skull not only
indirectly, through the
hyomandibular, but also
directly.

The opercular bones are
more highly developed than
in cartilaginous Ganoids,
and may include an oper-
culum, a preoperculum, a
suboperculum, and an in-
teroperculurn as well as
branchiostegal rays : these
in part correspond to in-
vesting bones of the carti-
laginous byoid rays. A
symplectic, an interopercu-
lum, and branchiostegal
rays are wanting in Poly-
pterus.

The branchial skeleton
in Ganoids consists of four
or five more or less strong-
ly ossified and segmented
gill-arches, decreasing in
size antero - posteriorly
(Fig. 67) ; in Bony Ganoids
the surface which looks towards the throat is beset with teeth.

The Ganoidei are of special interest, as they, with the Elasmo-
branchii, constitute almost the entire Fish-fauna through the
Silurian, Devonian, and Carboniferous periods, and as the Teleostei,

Fin. 68A. SKULL OF Polypter-u* Iiirkir FROM
THE DORSAL SIDE.

a, b, c, (I, supraoccipital shields. The two
arrows pointing downwards under the
spiracular shields show the position of
the openings of the spiracles on to the
outer surface of the skull. F, frontal ;
M, maxilla; N, nasal; Xa, external
nostril ; Op, operculum ; Or!>, orbit ; 1\
parietal ; Pm.c, prenmxilla : PO, pre-
operculum; Sb, Sl>', anterior and posterior
suborbital ; SO, suboperculum ; Sj), pre-
spiracular bones.

SKULL

91

which appear later, are doubtless derived from them. They show,
moreover, a connection with the Dipnoi and with the oldest
Amphibia from the Carboniferous and Trias (Stegocephali).

In the Teleosts, the skull (Figs. 69 and 70) presents a large
amount of variation ; its ground-plan, however, may always be
derived from that of the Bony Ganoids, as is best seen by a com-

sp.eth-(-

op.o-

Ofl.O-

occ.-\-

Pa s P r fr

Ptf

Sb

Sf> Na

O.t

f/a

FIG. 68B. SKULL OF POLYPTERITS. A, LATERAL, AND B AND C DORSAL VIEWS,
THE LATTER AFTER REMOVAL OF THE DERMAL BONES, THE CARTILAGE
DOTTED. (From Traquair.)

An, angular ; Ar, articular ; D, dentary ; E, mesethmoid ; f.m, foramen magnum ;
Fr, frontal ; l.e, lateral ethmoid ; MX, maxilla ; Na, Na', nasal and accessory
nasals ; occ, occipital : ol, nasal aperture ; op, operculum ; op.o, opisthotic ;
O.t, "os terminals"; Pa, parietal; Pmx, premaxilla ; P.t, posttemporal ;
Ptf, postparietal ; Qu, quadrate ; S.b, S.b', suborbitals ; S.Op, sub-operculum ;
Sp, splenial; *}>.eth, "sphenethmoid," in the orbitosphenoid and alisphenoid
region, resembling the like-named bone of Anura (q.v.) ; sp.o, sphenotic ; Spr,
prespiracular ossicles ; S.t, supratemporals ; Y, preoperculum (cheek plate);
Y', Y", smaller cheek plates ; z, postspiracular ossicles ; z', prespiracular
ossicles.

parison of the Siluroids with Amia. On the other hand, no
relations with the Amphibia are observable, and we must consider
the whole group of the bony Fishes as a side branch of the piscine
phylum.

Much of the cartilaginous primordial skull persists in many

92

COMPARATIVE ANATOMY

Teleostei (Fig. 70), and in this respect such forms as Argyropelecus
and Cyclothone acclinidens deserve special mention. The cranial
cavity may reach between the eyes as far as the ethmoidal
region, or may become reduced to a narrow cartilaginous and
fibrous interorbital septum.

In addition to the general account of the various investing and
replacing bones of the skull on pp. 82-84, the following points may

Sphot pat' soec

dent

FIG. 69. CRANIAL SKELETON OF THE SALMON. From the left side.

art, articular ; branchiost, branchiostegal rays ; dent, dentary ; epiot, epiotic ; tth,
supraethmoid ; fr, frontal; hyom, hyomandibular ; into}), interoperculum ;
'".'/, jugal ; mpt, mesopterygoid ; mt/>t, metapterygoid ; mx, maxilla ; nas,
nasal ; orbital ring ; op, operculum ; jjal, palatine ; par, parietal ;
P.iiix, premaxilla ; jtra'p, preoperculum ; pt, Y J terygoid ; pier, pterotic
(squamosal) ; Quail, quadrate ; socc, snpraoccipital ; sphot, sphenotic ; sultoj),
suboperculum ; Zitnge, tongue.

be mentioned, and the reader is referred to Figs. 69 and 70 for
further details.

As in Ganoids, the chief roofing bones of the skull are the
parietals and frontals, the former of which may be separated from
one another by a process of the supraoccipital. Laterally to the
frontal is a sphenotic, which extends backwards to the pterotic
(squamosal, cf. p. 83). Supratemporals and jugular plates are
never present.

Forming the lateral walls of the skull in the orbital region is

SKULL

93

A

' /,''-" ^f*y,'~
, ' *^zz-- I

- / / f fe- ^r

p'roo'f flsjvh alsjtli

N.olf orl.sph '

socc . /

enioT

* i

B

mf. eocene

-Col.verf

I a son

Of!C

FIG. 70. A. CRANIAL SKELETON OF SALMON AFTER REMOVAL OF THE JAWS
AND ORBITAL AND OPERCULAR BONES. From the right side.

B. The same in longitudinal section. The cartilaginous parts are dotted in

both figures.

alsph, alisphenoid ; basocc, basioccipital ; basph, basisphenoid ; Col. vert, point
of connection of the skull with the vertebral column ; ekteth, ectoethmoid ;
epiof, epiotic ; exocc, exoccipital ; fr, frontal ; N.olf, canal for the olfactory
nerve ; opiafh, opisthotic ; orbsph, orbitosphenoid ; pfero, pterotic (squamosal) ;
prool, prootic ; jwph, parasphenoid ; socc, supraoccipital ; sy>hot, sphenotic ;
co, vomer.

an ossified zone, the anterior and posterior parts of which are
usually known respectively as the orbitosphenoid and alisphenoid.
On the base of the skull is a basisphenoid, ventrally to which
is a parasphenoid, developed in the mucous membrane of the
mouth. More anteriorly is a vomer, and laterally the palato-

94 COMPARATIVE ANATOMY

quadrate bar, which remains separate from its fellow and is con-
nected with the skull-base anteriorly. In connection with the
anterior part of this bar the palatine (investing and replacing
bone) is formed, and with the posterior part a quadrate.
Between these, bony elements are developed which are known as
pterygoids, of which may be distinguished a replacing meta-
pterygoid, an entopterygoid, and an investing mesopterygoid or
ectopterygoid (cf. pp. 82-84). These bones are already represented
in Bony Ganoids, and form, together with the base of the skull,
the roof of the oral cavity.

The olfactory sacs are sunk in the ethmoid cartilage, in which
region supraethmoid and lateral ethmoid (ectethmoid) bones are
developed.

In the auditory region, as in Bony Ganoids, are a prootic, an
epiotic, and an opisthotic, the most important of which is the
prootic. The opisthotic usually does not form an actual part of
the auditory capsule, with which, however, as already mentioned,
other bones (pterotic, sphenotic) may come into relation.

In the occipital region, with which vertebral elements are
assimilated, are exoccipitals, which largely or entirely surround the
occipital foramen, and a basioccipital, as in Bony Ganoids, as well
as a very variable supraoccipital, which is wanting in the last-
mentioned group (Fig. 68, B). Where the basioccipital is in contact
with the vertebral column, it presents a concavity containing
notochordal tissue. 1

Forming the margin of the upper jaw are a premaxilla and a
maxilla. These play an important part in all Vertebrates from
the Bony Ganoids onwards, but in Teleosts more particularly they
show considerable variation with regard to their relative develop-
ment, form, and arrangement, and in many cases the maxilla takes
no part in bounding the actual gape of the mouth, and does not
form a continuous bar with the premaxilla. Of the bones in relation
with the oral cavity, the vomer, the parasphenoid, the premaxilla,
arid the maxilla may bear teeth. The maxilla, however, is
edentulous except in the Physostomi. 2

Besides the above-mentioned bones in connection with the
jaws, the cranial capsule of Teleosts is surrounded by other out-
works consisting of bony plates and bars. These arise as true
dermal bones in the region of the eyes (orbital ring), and in the
gill-covers (opercular bones) : the latter are similar in number and
name to those of many Bony Ganoids (p. 90). A large number of

1 A curious asymmetry is seen in the head of adult Pleuronectidfe. When
hatched, these Fishes are quite symmetrical, but later on the eye of one side
becomes rotated, so that eventually both eyes are situated 011 the same side ; in
consequence of this, the skull also becomes asymmetrical. In many Teleosts a
canal, lying in the axis of the base of the skull, encloses the eye-muscles, and
opens on either side into the orbits.

2 The tactile barbules present on the head of many Fishes, (e.g., Siluroids)
are supported by skeletal parts (cf. p. 82.)

SKULL 95

branchiostegal rays are developed in the ventral parts of the
opercular fold or branchiostegal membrane (Fig. G9).

Anteriorly, the opercular apparatus lies against a bony chain
consisting of three pieces the hyomandibular, symplectic, and
quadrate which serves as a suspensorial apparatus for the lower
jaw (Fig. 69). The latter consists of Meckel's cartilage and of
several bony elements, the largest of which is the toothed dentary :
the others are the articular, angular, and coronary. The last two,
however, may be wanting. The articular is developed in the
articular portion of Meckel's cartilage, which latter is ensheathed
by the dentary and angular.

The hyoid arch is usually followed by four branchial arches and
a rudimentary fifth which forms the " inferior pharyngeal bone."
The dorsal segments of these arches become fused together to
form the "superior pharyngeal bone," which, like the inferior
pharyngeal, usually bears teeth.

The skull of Dipnoans is in a sense intermediate between
that of ChimaBroids and Teleostomes on the one hand, and that of
Amphibians (more especially Urodeles) on the other. In various
respects, however, it presents special characters, such as the marked
metameric segmentation of the occipital region and the relations
of certain of the investing bones.

The chondrocranium is retained almost entirely in the most
primitive existing representative of this group Ceratodus, and to
a large extent in the other two genera : the only perichondral
bones being a pair of exoccipitals (Fig. 71). The occipital region
is firmly connected with the vertebral axis, and the two or
three anterior vertebral elements which are united with the skull
may possess more or less distinct neural arches and spines (e.g.
Protopterus) : the vagus nerve passes through a space between
the auditory capsule and first neural arch. 1 A large " cranial
rib " articulates with the hinder part of the skull on either
side, in a position corresponding to the third occipital neural
arch.

The cranial cavity extends forwards between the orbits to the
ethmoidal region (platybasic type), and its front wall (lamina
cribrosa} is largely cartilaginous. The cartilaginous nasal capsules
are lattice-like, and as in all Vertebrates higher in the scale, each
nasal cavity communicates with the mouth by internal nostrils :
the external nostrils are covered by the upper lip. 2

The ethmo-nasal region is covered by a median dermal supra-
ethmoid, postero-laterally to which is a supraorbital bony lamella
(" dermal lateral ethmoid "), and articulating with it posteriorly
in the median line in Ceratodus is another unpaired bony lamella

1 In the embryo of Ceratodus it has been shown that there are five myotomes
anterior to this point.

! There are two so-called "labial cartilages," one of which arises from the
trabecular region, passing behind and to the outer side of the external nostril, and
the other probably belongs to the nasal skeleton.

90

COMPARATIVE ANATOMY

in

(" scleroparietal "). The last mentioned element is wanting
the other two genera, in which an unpaired frontoparietal covers
the roof and part of the side walls of the chondrocranium, on the
ventral side of which is a large parasphenoid.

The squamosal is closely applied to the solid palatoquadrate
cartilage, which becomes fused with the cranium (autostylic type),

FIG. 71. SKULL, WITH THE PECTORAL ARCH AND Fix, OF PROTOPTERUS.

A, splenial ; AF, antorbital process (the labial cartilage in this region is not in-
dicated) ; a, b, SL. teeth ; B, co, fibrous bands ; Z>, angular, FP, fronto-
parietal ; Ht, membranous fontanelle, perforated by the optic foramen (II) ;
Hy, ceratohyoid ; KR, cranial rib; Kn, coraco-scapular cartilage; LK,
clavicle ; MK, supraclavicle ; NK, fenestrated cartilaginous nasal capsule ;
Ob, auditory capsule ; Occ, exoccipital, with the hypoglossal foramina ; Op,
operculum ; Op', interoperculum, overlying cartilaginous vestiges of hyoid
rays ; PQ, palatopterygoid, which converges towards its fellow at PQ' ; SE,
dermal supraethmoid ; SK, supraorbital (dermal lateral ethmoid) ; Sq,
squamosal, overlying the quadrate cartilage ; TV, palatoquadrate cartilage ;
W, W, vertebral elements with their neural spines (P*p) united with the
skull ; x, facet on the pectoral arch for articulation with the basal^ segment
(b) of the fin ; **, vestigial lateral rays on the basal segment of the fin ; 13,
the three following segments : ft, projections of Meckel's cartilage ; I V,
branchial arches : I and II are segmented (concerning the bar arising from I
anteriorly, cf. note on p. 97).

and in connection with which a palatopterygoid bone is present.
A premaxillo-maxillary arch is wanting.

The strong lower jaw is ossified by an angular and a splenial,
and in Ceratodus a dentary is also present. Meckel's cartilage
extends freely for a short distance anteriorly.

The teeth, which are sharp and blade-like, are borne on the
palatopterygoid and mandible ; small " vomerine " teeth are also
present, though there is no actual vomer.

SKULL 97

The hyoicl arch consists on either side of a large ceratohyal,
and in Ceratodus a small hyomandibular and hypohyal, as well as
a median basihyal, are also present. The five branchial arches l
are comparatively small and weak, and some, or even all of them,
may be entirety unsegmented (Lepidosiren) .

The Dipnoi constitute a very ancient group, which must have
diverged from the main piscine stem at a very early period, for
they occur in the Trias and Carboniferous, and even extend into
the Devonian and possibly into the Silurian.

Amphibians.

Urodela. The skull of tailed Amphibians is distinguished
from that of bony Fishes in general principally by negative
characters, on the one hand by the presence of less cartilage in the
adult, and on the other by a reduction in the number of bones
(Fig. 72). In brief, its structure is in many respects simpler, and
becomes modified in adaptation to the different mode of life.
Moreover, no nerve-apertures are present in the occipital region
behind that for the vagus ; but as this region extends to a slight
extent posteriorly to the vagus foramen, it appears that a reduction
has here taken place. The occipital part of the skull has the form
of a neural arch, united with the auditory capsules above and
broadening out below where it abuts against the notochord, form-
ing a basal plate primarily including vertebral elements, on the
posterior surface of which are two occipital condyles, as in all other
Amphibians (Figs. 60 and 72). An exoccipital bone is developed
on either side.

The platybasic cranium is not laterally compressed in the
orbital region, and the brain, flanked by the cartilaginous and
bony cranial walls, extends between the orbits as far as the olfac-
tory capsules, at which point the cranial cavity is closed by a mem-
branous (Triton) or cartilaginous (Salamandra) ethmoid region
(lamina cribrosa), perforated by the olfactory nerves, or in certain
cases by special modifications of the frontal bones (Proteus, Sala-
mandra perspicillata). The anterior part of the lateral cranial
walls may be ossified as an orbitosphenoid. The well-developed
auditory capsules are connected with one another dorsally by a
narrow cartilaginous bar (tectitm synoticum) all that remains
of such an extensive cartilaginous roof as is seen in Elasmo-
branchs : this is retained in all the higher Vertebrates. In the
ossification of the capsules the prootics take the chief part, and

1 In Protopterus, a delicate cartilaginous rod arises from the first branchial
arch (Fig. 71), concerning the homology of which opinions differ. It may
represent the first branchial arch (and in this case the number of branchial
arches is six) ; or it may belong to the hyoid arch, thus indicating that the latter
is primarily double ; or, again, it may possibly correspond to a branchiostegal ray.

H

98 COMPARATIVE ANATOMY

later unite with the exoccipitals. A new and important modifica-
tion as compared with Fishes is the presence of an aperture, the
fenestra ovalis, s. vestibnli, 011 the outer and lower side of each
capsule, and corresponding to part of the original space between
the capsule and the parachordal cartilage. This fenestra is closed
by a cartilaginous plug, the stapcclial plate, which is connected with
the quadrate and paraquadrate (see p. 82) by ligament, or by a
cartilage or bone (coluinelln a/iris), the two structures probably
together corresponding phylogenetically to the upper section of the
hyoid arch (hyomandibular^ though this homology can no longer
be traced ontogenetically. The olfactory capsules are well
developed and arise in part independently and partly in connection
with the converging trabecula?. In Necturus and Proteus they are
delicate and fenestrated, and united with the cranium by connec-
tive tissues only.

The snout is limited anteriorly by the toothed premaxillas,
which usually more or less completely enclose a cavity (inter-
maxillary or internasal sinus) containing a gland. Each external
nostril is bounded by the nasal process of the premaxilla, the
nasal, and the toothed maxilla, and a small investing bone, the
septomaxillary, is also present between the maxilla and nasal in
relation with the nostril. The premaxilla? and maxilla form the
upper boundary of the gape. Between the nasal and maxilla is a
prefrontal, and medially to this a frontal, followed behind by a
parietal, which partly covers the auditory capsules.

Forming the greater part of the skeletal roof of the oral cavity
and strengthening the skull-base is a large and broad parasphenoid
(Fig. 72), which, as in Fishes, is sometimes provided with teeth.
It extends forwards from the occipital region to the olfactory cap-
sules, closing over the basicranial fbntanelle, and ventral to it is
the paired and toothed vomero-palatine bar, the two elements
comprising each of which become fused in adult Urodeles, but vary
much in form and arrangement. The vomerine part of this bar
is situated beneath the olfactory capsule and is in contact with the
premaxilla and maxilla, thus helping to strengthen this region, at
the posterior part of which is the internal nostril, situated much
more posteriorly than in Dipnoans. Internally to the suspen-
sorium is a pterygoid bone, a process of which extends forward
towards the maxilla.

The suspensorium is much more simple than that of Fishes
(Figs. 72 and 73). It consists of the palatuquadrate only, with
a quadrate ossification, and has usually four typical processes con-
necting it with surrounding parts (pedicle or basal process, otic,
ascending, and pterygoid processes). The quadrate l becomes fused
.secondarily with the skull, and on its outer surface is an investing

1 In Tylototriton i'tiTnco*ti* the quadrate sends forwards a process which
connects it with the maxilla, and thus forms a lower zygomatic arch or infra-
temporal arcade.

SKULL

99

M

7\. SKULL OF"A YOUM; AXOLOTL
(.\ini>ly*stomQ). Ventral view.

CoccOsp

Fin. 7-i;. SKULL OF Salamandrajatra
(ADULT). Dorsal view.

Cl

FIG. 7'2c. SKULL OF .W^///'//" ntf.i (ADULT). Ventral view.
posterior part of " alisphenoid " region ; Bp, cartilaginous basal plate between
the auditory capsules ; Can, nasal cavity ; Cocc, occipital condyles ; /',
frontal ; Fl, foramen for the olfactory nerve ; Fov, fenestra ovalis, closed
on one side by the stapedial plate (St) ; IN, internasal plate, which extends
laterally to form processes (TF and AF) bounding the internal nostrils
(C'/t) ; Lyt, ligament between the stapes and suspensorium ; M, maxilla ;
N, nasal ; Na, external nostrils ; NK, nasal capsule ; OB, auditory capsule
and exoccipital ; On, orbitosphenoid ; Osp, tectum synoticum ; P, parietal ;
Pf, prefrontal, perforated ac D for the lacrymal duct ; PI, palatine ; Pmx,
premaxilla ; Pot, otic process, PED, pedicle, and Pa, ascending process of
the quadrate ; Pp, palatine process of maxilla ; PS parasphenoid ; Pt, bony
pterygoid ; Ptc, cartilaginous pterygoid ; Qu, quadrate ; Jit, point of entrance
nt the ophthalmic branch of the fifth nerve into the nasal capsule; Squ,
paraquadrate (" squamosal'') ; TV, trabecula ; To, vomer ; Vop, vomeropala-
tine ; Z, tongue-like outgrowth of the internasal plate, which forms a roof
for the internasal cavity ; II, optic, V, trigeminal, and VII, facial foramina.

H 2

100

COMPARATIVE ANATOMY

bone, the paraquadrate (Gaupp), usually described as a squamosal.
The quadrate, exoccipital, prootic, orbitosphenoid, and columella
arise in the perichondrium and are replacing bones, while all the
others are investing bones.

The temporal region is either uncovered by skeletal parts,
or an upper zygomatic bar (supratemporal arcade) is formed by
processes of the paraquadrate and frontal respectively, and indi-
cates a reduction of a more marked development of bone in this
region such as occurred in the Stegocephali.

In connection with the lower jaw are usually developed a
replacing articular at the proximal end of Meckel's cartilage, and
investing splenial and dentary bones. The rest of the visceral
skeleton of Urodeles undergoes various modifications in the

gu.

mk,

FIG. 73. SKULL AND VISCERAL AKCHES OF Ah Koroma. From the side.

I, mandible ; II, hyoid ; III- VI, branchial arches ; qii, quadrate, covering which
is the paraquadrate (" squamosal") ; ar, articular; ml', Meckel's cartilage
enclosed by the dentary bone.

different types. We may consider the ground-form, as exhibited
in the larva, to consist of five pairs of bars in addition to the
mandibular arch (Fig. 73), in which latter the palatoquadrate and
Meckel's cartilage chondrify independently. The anterior bar, or
hyoid, consists of two pieces (Fig. 74. A), as do also the two first
branchial arches. The third and fourth branchial arches are much
smaller, and even vestigial in Salamanders. All these bars are
connected with a single or double basal piece. At the close of
larval life, that is, when the gills are lost, the two hinder pairs of
arches disappear entirely, while the two anterior pairs undergo
changes as regards form and position, 1 and may become more or
less densely ossified (Fig. 74, B D).

Gymnophiona. In contrast to the extensive and compact
chondrocranium of most Urodela and of Anura, that of
the limbless Amphibians consists of delicate cartilaginous rods

1 In the genus Spelerpes, which possesses a sling-like tongue, the dorsal
segment of the first branchial arch grows out into a long cartilaginous filament,
which extends far back under the dorsal integument (Fig. 74, D).

SKULL

101

separated by wide spaces ; and even where connected sheets
of cartilage are present, they are very delicate and thin. At the
same time the skull nearly resembles that of Urodeles (more

Cp

K, !-. l-l V

Cp Rad.I

Fit;. 7-4. HYOIJRANOHIAL APPARATUS OF UKODELKS. A, Axolotl (Sirndon. stage
of Amblystoma) ; B, Salamandra macvlosa, ; C, Triton cristatus ; D, Spelerpes

fuscus.

Cp, Cps, O.th, basihyobranchial or copula ; G.th, thyroid gland ; Hpbr. I and II,
first and second hypobranchial ; HpH, Bad I, hypohyal ; Kebr. I IV, first
to fourth ceratobranchial ; Keif, ceratohyal ("anterior cornu " of hyoid in
Caducibranchs the "posterior cornu " being made up of Hpbr I and II and
Kebr I). Rad. II arises in Salamandra secondarily during metamorphosis.

especially perennibranchiate forms) in spite of a considerable
reduction in its parts especially in the occipital, auditory, and
orbital regions, as well as of the peculiar and characteristic

II*

102

A

COMPARATIVE ANATOMY
B A*.

Co

n

- 1

*

& ^::

ii
in

IV

Fio. 7">. A, DORSAL, B, VENTRAL, C, LATERAL VIEW OK SKULL OF Sip

Jy<, external nostril ; tiny, angular ; Car, carotid foramen : Ch, internal nostril ;
Co, occipital condyle : deitf, dentary ; JJK, apertures for ducts of tentacular
gland ; E, ethmoid region ; F, frontal ; M, maxilla ; Xji>; naso-preniaxilla ;
Orl>, orbit ; Pal, palatine ; Po, petroso-occipital ; Pj>. Pp', palatine process
of the naso-premaxilla ami of the maxilla ; /'*, parasphenoid, united
posteriorly with the auditory and occipital elements ; J't, pterygoid ; Qn,
quadrate; <SV/, paraquadrate ( " squamosal ") ; Sfji, stapedial plate; I'o,
vomer ; X, vagus foramen.

D, Hyobranchial apparatus of the larva of IcJithyophis H/iifiiioxa. After
P. and F. Sarasin.); Co 1, 2, first and second basal elements (copula-); //,
hyoid ; I IV, branchial arches,

SKULL

103

solidarity of the osteocranium, doubtless due to adaptation in
connection with the burrowing habits of this group (Fig. 75).
The differences are due essentially to the more extensive ossifi-
cation, and to the fusion of certain of the bones, e.g. the para-
sphenoid, otic, and occipital elements, and the premaxilla and nasal.
The temporal region, moreover, is more or less covered by bone,
and the ethmoid skeleton is well developed, the solid nasal septum
enclosing no gland.

The post-mandibular visceral skeleton consists in the larva,
as in that of [Jrodeles, of a hyoid and of four branchial arches
which, however, have a more primitive and piscine form and
arrangement and are less reduced than in Urocleles (Fig. 75, D).
In the adult, the median basal connection between the first and

-Oc

FIG. 76. RESTORATION OF THE SKCLL OF A SHALL STEGOCEPHALAN (PROTKITON)
from the Carboniferous of Bohemia. (After Fritsch.)

Br, branchial apparatus ; F, frontal ; Fp, parietal foramen ; J/, maxilla ; JV,
nasal ; Nu, nostril ; Oc, sclerotic ring (orbital bones) ; P, parietal ; Pf t
prefrontal ; Pmx, premaxilla ; Socc, supraoccipital.

second branchial arches disappears, and the vestige of the fourth
arch unites with its predecessor.

The skull of the fossil Stegocephali, some of which were
comparatively gigantic, was covered by a much larger number of
dense and firm bony shields, and a foramen was present in
the parietal region, which, like that in modern Lizards, is in
relation to the pineal apparatus (Fig. 76). A row of bony
sclerotic plates was usually present around the orbit like those
of Birds and certain recent and fossil Reptiles (e.g. Lizards,
Ichthyosaurus). Many of these forms possessed the same number
of visceral arches as TJrodeles, and it has been shown that they
(e.g. Branchiosaurus) underwent a metamorphosis.

The class Amphibia, like the Reptilia, was of much greater
importance in earlier periods of the earth's history than at present,
but existing forms cannot be traced directly to those of the
Carboniferous and Permian strata.

ii

-*-*

10-t

COMPARATIVE ANATOMY

A

Mute.

Pr. front;

.
Fr.par.

II. Pier.
AVI
J^JIL

- til ff %;

FIG,

AXD

x

2),

B, DORSAL AND VENTRAL VIEWS OF THE SKULL OF
IN B THE INVESTING BONES AKE REMOVED ON THE RIGHT SITE.

Ajj.n3x.int, internal nostril: AI-<:.HH!>O<-, subocular or palatopterygoid arch;
C.2J)'-<>(f, inferior prenasal cartilage; Cr.x. a, subnasal crest ; Eth, spheneth-
moid ; F II l r , foramina for cerebral nerves ; Foss.cond, condyloid fossa, in
which are the foramina for the IXth and Xth cerebral nerves ; Fr.ji'n;
frontoparietal ; I. max, premaxilla ; J/o.c. maxilla ; Nn, nasal ; Occ.lat, ex-
occipital ; Pal, palatine ; Para, parasphenoid ; Pr. front, frontal process of
maxilla ; Proof, prootic ; Pr.-.t/<j, zygomatic process of paraquadrate ;
Pr.l><ix.lJ, basal process of quadrate; .Pr^ier. <J), pterygoid process of quad-
rate ; Ptf.r, pterygoid : <J nm.<-, quadratojugal (or quadratomaxilla) ; ijuailr,
quadrate; TnH.xyii, tectum synoticura ; Ty, paraquadrate (" squamosal ") ;
Vom, vomer.

Anura. The skull of adult tailless Batrachia is at first sight
very similar to that of Urodeles : it is platybasic, and the occipital
region has the same morphological value. It undergoes, however,

SKl T LL

105

especially in the parts surrounding the gapu, an essentially
different and much more complicated development, and cannot in
any way be directly derived from that of tailed Amphibians, so
that the common ancestral form must be sought ^n very^early
geological periods. The chondrocranium (Figs. 77 and 78) is
much more extensive than in Urodeles, and is largely retained in
the adult, the bones not gaining the upper hand to such an
extent.

The larva, or tadpole, in adaptation to the nature of its food
and mode of feeding, possesses a suctorial mouth, provided with

E.i't. i<o*1nt

\

\

Parietal fontanellt

A far cart.

\Sup. prt-

nasal cart,

Inf. pre-
nasal cart.

<-</rt. Frontal ontanett

, Tect.

--,. Fenestra
nvoUis

s tympanicus Artie, proces^'of

quadrate,
FIG. 77, C.

LATERAL VIE\V OF A WAX MODEL, RECONSTRUCTED FROM SECTIONS, OF TIIK
CHONDROCRANIUM OF A YOUNG liana ttmporan'a, ABOUT -2 CM. LONG. The
Stapedial Plate and Columella are removed, and the Ossifications are not
indicated. (After (-Jaupp. )

labial cartilages and horny jaws and denticles. The articulation
of the lower jaw with the palatoquadrate is situated very; far
forwards, and at metamorphosis becomes shifted backwards,
thus causing a considerable widening of the gape of the
mouth.

An important advance on other Amphibia is seen amongst
Anura (e.g. Ranidoe ) in connection with the auditory organ. The
first visceral (hyomandibular or spiracular) cleft disappears entirely
during development in the Urodela and Gymnophiona, but
becomes modified in many Anura to form a EustacMan t-ulc,
opening into the pharynx and leading into a tympanic cavity.
The latter is closed externally by a tympanic membrane supported
by a cartilaginous ring (annulus tympcmicus^ Figs. 77, C, and
78), and to it the distal end of the columella (p. 98) is attached.

1 Cf. under Auditory Organ. The tympanic ring is a derivative of the
palatoquadrate.

106

COMPARATIVE ANATOMY

On the dorsal side of the chondrocraniutn are a large median
anterior fontanelle or fenestra, and a smaller paired posterior
fontanelle, and the cartilage becomes replaced by bones in the
occipital, auditory, and ethmoid regions (Figs. 77 and 78). There
are paired exoccipital and prootics and a sphenethmoid, the
lateral parts of which correspond in position to the orbitosphenoids
of Urodeles, and which encircles the whole skull ; it also forms a
front wall to the cranial capsule, perforated by the olfactory nerves.

Alar cart.

Oblique <:<trt

A at.

/"/// process

II

/'>< i'//yoid
cart.

Ill

V, VII

Aiunthi-".

Prtmaxilla

Septonmx'Ma

Nasal

Max if /a

Artie pro-

cess of
quadrate

Pterygoid

Frontoparietal

f- Columella

Quadratojugal
Paraquadrate

Aud. 1-npxule

Exoccipital

FIG. 78. SKULL UK A YOUNG Raua dmporaria, 2 CM. IN LENGTH, JUST AFTER
METAMORPHOSIS, KROM THE DOKSAL SIDE. THE INVESTING BONES ARE
REMOVED ON THE LEFT SIDE (x ABT. 11.) After Gaupp, from a model by
Fr. Ziegler.)

Cartilage blue ; replacing bones ymy ; investing bones yellow.

The frontal and parietal of either side are as a rule fused,
thus giving rise to a frontoparietal. The maxillary bar grows
backwards much further than in Urodeles, and becomes connected
with the suspensorium by means of a small intermediate bone, the
quadratojugal, or quadratomaxilla (Figs. 77 and 78). There is
thus a lower sygomatic arch (cf. p. 98) ; an upper zygoma like
that of many Urodeles is never developed, and consequently
the temporal region is uncovered by skeletal parts. The palato-
quadrate is united anteriorly with the cartilaginous nasal capsule

SKULL

107

this is characteristic for the Anura (except Ranodon) as compared
with the Urodela. (For the relations of the bones bounding the
mouth-ca\dty, cf. Figs. 77, A and B). 1

,'l'r.anthy.

fom.ttrmS.

B

^~ Pr. ant.

Corn,

Fia. 79. A. HYOBRANOHIAL .SKELETON OF A LARVAL Kana ttmporaria, 29 MM.
IN LENGTH, FROM THE DORSAL SIDE. B. THE SAME OF A LARVA, 15 MM. IN
LENGTH, AT THE END OF METAMORPHOSIS, AFTER DISAPPEARANCE OF THE
TAIL. C. HVOID CARTILAGE OF A YOUNG FROG, 2 CM. IN LENGTH, FROM
THE VENTRAL SIDE.

(All these figures are from wax models after (Jaupp. )

A and B (in part), Brunch I IV, branchial arches ; Com. term. I ///, terminal
commissures of same; Cop, basal plate (copula); Hy, hyoid ; Pr.aut.hy,
Pr.lat.hy, Pr.jiont.hy, anterior, lateral, and posterior processes of the hyoid ;
S-pic. I IV, cartilaginous processes.

B (in part) and C. Corp.cart.hy, body of hyoid cartilage; Corn princ, anterior
cornu ; Alan, " manubrium " ; Pr.al, alary process; Pr.ant, anterior pro-
cess; Pr.poxt .lat , post ero- lateral process; Pr.thyr.post.med, thyroid or
postero-medial process (posterior cornu.)

The bones of the lower jaw are a dentary and an angular (or
angulosplenial). At the distal end of Meckel's cartilage a small

1 A septomaxillary, helj>ing to close the nasal fenestra on the outer side, is
present, as in Urodeles (p. 98),

108 COMPARATIVE ANATOMY

portion (" lower labial cartilage " of larva) is bent inwards towards
the median line and unites with its fellow in a symphysis, forming
in the adult the mentomandibular (p. 84).

There is a much greater reduction of the branchial skeleton at
the close of larval life than inUrodeles. In the larva, representatives
of the hyoid and of four branchial arches can be recognised, but
these are all united together and form a continuous structure (Fig.
79, A). The greater part of the broad basal parts of this
apparatus, as well as the four branchial arches, disappear during
metamorphosis. The hyoid cartilage of the adult -(B, c) is
formed partly from the remains of the hyobranchial cartilages of
the larva and partly by new outgrowths from it.

Reptiles.

The skull in Reptiles is extremely complex and varied as
regards its bones and their relations. Although differing markedly
in many important respects from the cranial skeleton of Am-
phibians, the ground-form of the latter is distinctly recognisable,
especially in the primitive Hatteria and in Lizards. On the other
hand, numerous points of similarity are seen in the skull, as
well as many other parts, of Reptiles and Birds, which are,
therefore, included together under the term Sauropsida.

In spite, however, of the similarity of plan of the amphibian
and reptilian skull, it must be borne in mind that no recent
Amphibian lies on the direct line of descent of the Reptiles,
though certain fossil Amphibians (Stegocephali) and Reptiles, as
Avell as the existing Hatteria, help to bridge over the space between
the two Classes.

In order not to cause confusion by reference to the multi-
farious details which present themselves in dealing with the
reptilian skull, it will be as well to consider first its more
important characteristics, many of which are common to the
Amniota in general, before treating specially of the various Orders.
In this general description, the lacertilian skull will be chiefly
referred to as a typical form (Figs. 80 and 82).

Apart from its naso-ethmoidal region, the chondrocranium
plays no important part in Reptiles subsequently to the embryonic
period, and it no longer forms such a complete structure as, c.y.,
in Anura, but is considerably reduced and frequently largely
fenestrated (Fig. 80). This want of completeness, however, is
later partly compensated for by the investing bones, and as the
ossification is very considerable, a firm and solid skull results.

The cranium includes three more vertebral elements than in
Amphibia (p. 97), so that the foramina for the three roots of the
compound hypoglossal nerve perforate the skull. In all Amniota

SKULL

109

the cranio-vertebral boundary is in a similar relative position, in
spite of differences of form in this region. The cerebro-nasal
axis, which is horizontal in Amphibians, becomes more or less bent

Sup. olf. cart.
Dorsal nasal fenestra

Lat. nasal fenestra

Orb. nas.Jissurt
Spheneth. carl

Max. proc.*~

Pituitary fenestra

Asc. proc. s
of pal. quad.

Columella.

Optic fenestra

FICJ. 80. SKULL OF AN EMBRYO Lacerta agilis, 47 MM. IN LENGTH. A,
DORSAL, AND B, VENTRAL VIE\V. C, LOWER JAW AND HYOBRANCHIAL
SKELETON, FROM THE VENTRAL SIDE.

(After Gaupp, from a wax model by Ziegler x ^.) Cartilage, blue ; 1'eplacing bones,

gray ; investing bones, ytUow.

downwards in front of the interorbital region, and thus causes
various modifications as regards the relations of the nasal and
cranial cavities (cf. Fig. 90).

110

COMPARATIVE AN ATOM Y

The cranial bones (Figs. 80-86) are much more numerous and
varied in form than in recent Amphibia. The solid base of the
skull is formed by bones developed on a cartilaginous foundation,
viz., of a basioccipital and a basisphenoid, on which latter there
may be a liasiptcrygold process on either side for articulation with
the pterygoid bone. An alisphenoid ossification may be present ;
presphenoids and orbitosphenoids are usually wanting. The

Pr, ma -

.In rial '
Transpal.

/</,;. tal
Pti, <o d

Postfroiti. -2 -

. proc.r"

cart.

I'll ,'<!.<' ptl't ffti't.

Optic " n ~tru
A-.prot. ofpal.quad.

Quad, (artic.

jiroc.)

1 PI i-il/mpli. jo,'.

Post. basicran.foiitnne!/( u<-. cond.
Fin. SO, B.

parasphenoid, which plays so important a part in the Anamnia
as an investing bone on the roof of the mouth, undergoes a
gradual reduction, but can still be recognised, although more or
less included within the basisphenoid : it may be represented by
lasitemporal ossifications which unite with the latter bone, and
its anterior part may form a lasisphenoidal rostrum. Above the

SKULL

111

basioccipital are a pair of exoccipitals, and above these again a
supraoccipital, which arises in the cartilage of this region (tectum
synotiemn, cf. p. 97). In contradistinction to the Amphibia,
only a single condyle connects the skull with the vertebral column
in the Sauropsida : this, on close examination, is usually seen to be

Denlary

Pi'esplenial

Meckel's cart

'Entoglossiit

Supra-angular^
Angular

Postsplemal
Articular

Hy. cornu

Branch
1-0,-nu 11 ?

FIG. 80, C.

formed of three parts, derived from the basioccipital and soccipitals
respectively.

The auditory capsules are ossified from three centres, the
most important of which, as in Amphibians, is the prootic, which
usually remains free, the epiotic uniting with the supraoccipital
and the opisthotic with the exoccipital. A fenestra rotunda is
present in the walls of the capsule in addition to a fenestra ovalis,
into which latter the stapedial plate of the bony columella is
inserted (cf. p. 98), and the tympanic cavity in most Reptiles

112 COMPARATIVE ANATOMY

communicates with the pharynx by means of a Eustachian
tube. 1

In most Reptiles a fibro-cartilaginous interorbital system is
present (tropibasic character, p. 77), but there is much variation
in its development, and the anterior part of the cranial cavity
tends to become more or less reduced and shifted dorsalwards.

On the roof of the skull the parietals usually, and the frontals
sometimes, are fused together : a median so-called " parietal
foramen " (under Brain) is present in certain recent and fossil
forms. The frontals and parietals are either confined to the skull
roof, or may also take part in forming its lateral walls. Pre frontals
and postfrontals are also present, and there may be a postorbital
in addition, as well as a series of supraorbital bones posteriorly to
the prefrontal and laterally to the frontal, forming the upper
margin of the orbit. Anteriorly to the frontals are, in most cases,
a pair of nasals, and laterally to the largely cartilaginous ethmoid
region, the lacrymals.

Anteriorly to the basisphenoid is a paired or unpaired vomer,
postero-laterally to which is the palatine, followed by a pterygoid,
which may articulate with the basipterygoid process of the basi-
sphenoid. Connecting each pterygoid and maxilla in most
Reptiles is a so-called transpalatine or ectopterygoid, and in certain
forms a rod-like epipterygoid (or antipterygoid) extends in a
vertical direction between the parietal and pterygoid, and is
comparable to the ascending process of the quadrate in Urodeles.

The premaxillse, which are in many cases fused together, come
into contact with the nasals dorsally and the maxilla laterally:
the latter usually bounds the greater part of the gape. A septo-
maxillary (p. 98) may be present. Teeth occur in nearly all
Reptiles, and may be borne on the palatine and pterygoid, as well
as on the premaxilla, maxilla, and dentary.

The quadrate may be fixed to the surrounding bones, or
niovably articulated : the pterygopalatine bar is frequently more
or less movable and free from the base of the skull. In some
Reptiles its elements become broadened out, and, together with a
shelf-like palatine process of the maxilla, may in certain cases
even take part in forming a secondary roof to the oral cavity, or
hard palate, distinct from the true (sphenoidal) base of the skull.
Thus the vomer becomes cut off from the cavity of the mouth and
forms a vertical plate situated above the hard palate, and the
internal nostrils are thrown further back.

In the temporal region, typical differences are seen in the
various Orders which indicate a very early divergence from the
common reptilian ancestor. The bones in this region may form a
series of bars, or arcades, separated by spaces or fossce. The main

1 The form of the columella varies considerably : it consists primarily of a
bony rod, to which a cartilaginous extrastapedial is attached distally and applied
to the tympanic membrane : both parts are derivatives of the hyoid.

SKULL 113

temporal fossa is usually separated from the orbit by a bar formed
from the postfrontal or postorbital and the jugal ; it may be
further subdivided into a dorsal and a ventral portion by a bridge
of bone, the upper zygomatic arch or superior temporal arcade, formed
by processes of the postfrontal and squamosal (or paraquadrate).
The jugal may be connected with the quadrate by the quadrato-
jugal (or paraquadrate), thus forming a lower zygomatic arch or
inferior temporal arcade}-

In the nomenclature of the cranial bones of Reptiles much
confusion exists. According to Gaupp, all Crocodiles, Chelonians,
Lizards, and most Snakes possess a squamosal ; all Crocodiles and
almost all Chelonians and Lizards a paraquadrate (cf. p. 82) ; but
none of the above possesses a quadrato-jugal (quadrato-maxillary).
In Hatteria the latter is present united with the well-developed
paraquadrate, while the squamosal appears to be entirely wanting.
In narrow-mouthed Snakes all three of these elements are absent

A number of bones arise in connection with the lower jaw
which includes remains of Meckel's cartilage ; viz., a dentary
angular, supra-angular, splenial, coronary, and articular.

Thus it will be seen that the number of investing bones in the
reptilian skull is very considerable, and it may be convenient to
enumerate them in this place : parietal, frontal, nasal, squamosal,
prefrontal, septomaxillary, postorbital, jugal and quadratojugal.
lacrymal, paraquadrate, parasphenoid, premaxilla, maxilla, vomer,
palatine, pterygoid, transpalatine, dentary, angular, supra-angular,
splenial, and coronary. In addition to these integral and typical
elements, a number of less constant accessory bones occur in
many forms (e.g. among Lizards) e.g. supraorbitals, supraoculars,
supratemporals : they arise as ossifications in the derm compara-
tively late.

In consequence of the absence of branchial respiration during
development, the hyobranchial apparatus plays a less important
part in Reptiles than in the Anamnia and thus undergoes reduc-
tion, in some cases only traces of it remaining.

The chief points characteristic of the skull in the different
Orders of Reptiles are enumerated below, and for further details
the reader is referred to Figs. 80-86.

Rhynchocephali. In Hatteria (Fig. 81) a fenestrated inter-

The squamosal and paraquadrate take part in various ways in the forma-
tion of upper and lower zygomatic arches or superior and inferior temporal
arcades (cf. p. 106), and according to the relations of the temporal bones three
types may be distinguished amongst the Amniota in general, as well as in
Amphibia, as follows : (1) xteyokrotaphic type (temporal region covered) Gym-
nophiona, Stegocephali, the most primitive Reptilia, and Marine Chelonia ; (2)
zyyokrotaphic type (with one or two zygomatic arches, derivable from (1) : a,
with lower zygoma only Anura and Aves ; b, with upper zygoma only many
Tritonidie, most Lacertilia, Chelonia, and Mammalia ; c, with upper and lower
zygomatic arches Tylototri ton, Rhynchocephali, many fossil Reptilia, Croco-
dilia ; (3) yymnokrotaphii- type (temporal region uncovered) most Urodela,
all Ophidia, some Lacertilia, Chelonia, and Mammalia.

114

COMPARATIVE ANATOMY

orbital septum is present. The frontals and parietals are paired,
and there is a parietal foramen. Three bony arcades are seen in
the temporal region a superior, an inferior, and a posterior. The
quadrate is large, and is firmly fixed to the skull by means of the

Pmx

Pmx

Prf

Q,

FIG. 81. SKULL OF HATTERIA. A, dorsal; B, ventral ; and (J, lateral view. xf.
(From Gadow, Cambridge Natural History. )

col, columella auris ; Cond, occipital condyle ; E.P, transpalatine or ectopterygoid ;
F , frontal ; Jug, jugal ; Max, maxilla ; Na, nasal ; No, external nostrils ;
Pal, palatine ; Par, parietal ; Pmx, premaxilla ; Prf, prefrontal ; Pt.f, post-
frontal and postorbital ; Pty, pterygoid or endopterygoid ; Q, quadrate
and quadrate- jugal ; *S'g, paraquadrate (squamosal) ; Vo, vomer.

quadratojugal, pterygoid, paraquadrate (" squamosal "), and ex-
occipital. The vomers, palatines, and pterygoids are broadened
out and form a bony roof to the mouth. An epipterygoid is
present. The premaxillae are separated by a suture. Teeth are
borne on the maxilla and palatine, as well as on the premaxilla

SKULL

115

A pmaz

e.jct.nar

trans

supr.oc, / oc \ cond
for.rncig

oc.cond bas.oi:

pt.orb

art

rienl

any

FIG. 82. SKULL OF Lncerta agilis. (From Parker and Haswell's Zooloyy,

after W. K. Parker. )

A, from above ; B, from below ; C, from the side, any, angular ; art, articular ;
bas.oc, basi-occipital ; bas.pt y, basipterygoid processes; bas.sph, basi-
sphenoid ; col, epipterygoid ; cor, coronary ; dent, dentary ; eth, ethmoid ;
ex.oc, exoccipital ; ext.nar, external nares ; for. may, foramen magnum;
fr, frontal ; int.itar, internal nares ; jti, jugal ; Icr, lacrymal ; max, maxilla ;
uas, nasal ; oc.cond, occipital condyle ; olf, olfactory capsule ; opi.ot, opis-
thotic ; opt.n, optic nerve ; pal, palatine ; par, parietal ; para, para-
sphenoid ; par.f, parietal foramen ; p.mx, premaxillaj ; pr.fr, prefrontal ;
ptg, pterygoid ; pt.orb, postorbital ; q-u, quadrate ; s.any, supra-angular ;
s.orb, supraorbitals ; - sq, paraquadrate (" squamosal ") ; ittpra.oc, supra-
occipital; supra. t. 1 , supratemporal ; 8upra.t. 2 ,sq\i&w.osal ("supratemporal"");
trans, transpalatine ; vom, vomer.

and vomer in young stages. The two dentaries are united by
ligament. The columella is continuous with the anterior cornu of

i 2

116 COMPARATIVE ANATOMY

the hyoid, the latter consisting of a body (basi-hyobranchial) and

of two pairs of cornua.

Lacertilia (Figs. 80 and 82). There is an interorbital septum
except in the AmphisbsenidaB, and in it there may be indications
of orbitosphenoids or alisphenoids : the orbit is usually separated
from the temporal fossa. Various modifications are seen as
regards the temporal arcades, but an inferior arcade, or lower
zygoma, is never present. 1 The quadrate is in most cases movably
articulated to the distal end of a parotic process from the temporal
region. There is a " parietal foramen " (situated in the frontal
region in Chameleons) ; the parietals are usually fused, and the
frontals may also become united in the adult. Lacrymals are
generally present, and epipterygoids occur except, e.g. in Amphis-
basnians and Chameleons. The pterygoid is movably articulated
to the quadrate and basipterygoid process of the basisphenoid,
and there is a basisphenoidal rostrum. The premaxillss are usually
fused. Teeth occur on the premaxilla, maxilla, dentary, and
frequently also on the palatine.

The hyobranchial skeleton (Fig. 80, c) consists of a narrow
basal portion extending forwards into the tongue, from which three
pairs of cornua may arise, belonging respectively to the hyoid and
branchial arches 1 and 2.

Ophidia (Fig. 83). The embryonic chondrocranium is much
less extensive than in Lizards. In the orbito-temporal region there
is hardly any cartilage, the parietals and frontals forming lateral
walls to the cranium, which extends to the ethmoid region, and
even here the amount of cartilage is small. The trabeculas remain
separate for the greater part of their length, forming narrow rods
lying side by side, which in many cases can be recognised even in
the adult. The skull, however, is here also of the tropibasic type
(p. 77): a thin interorbital septum is developed, but is compara-
tively little marked and does not become chondrified. There is a
large parasphenoidal rostrum on -the basisphenoid, the parietals
are united, and there are no temporal arcades or supratemporal
bones. The great strength and solidity of the investing bones is
doubtless correlated with the modifications of the jaws in con-
nection with the mode of feeding.

In most Snakes, the quadrate is only indirectly connected with
the skull by means of the squamosal, with the distal end of which
it articulates and which extends backwards, thus resulting in a
very wide gape ; moreover, the facial bones are capable of move-
ment upon one another and upon the cranium. The pterygoid
articulates posteriorly with the quadrate without the intervention
of ajugal. The premaxilla is unpaired and usually edentulous
(except, e.g. in Pythons). The rami of the mandible are connected

1 In Chameleons, characteristic processes arise from the cranial roof and
temporal regions, ami unite above the posterior part of the skull in the median
line.

SKULL

117

by an elastic ligament. Teeth are usually present on the relatively
short maxilla, the palatine, pterygoid, and dentary. The hyo-
branchial apparatus is much reduced, and consists merely of
a pair of cartilaginous rods situated beneath the trachea.

In Viperine Snakes the maxillopalatine apparatus is par-
ticularly mobile, and the very short maxilla is articulated with
the prefrontal and can swing forward in order to erect the
tang. In the burrowing Typhlopidus, on the other hand, amongst

Fov

Fiu. 83. SKDLL OF COLUBIUNE SNAKE (Tropidonotus natrix), dorsal and

ventral views.

Ay, angular; Art, articular; Bp, basioccipital ; /?*, basisphenoid ; Ch, internal
nostrils ; 6'ov, occipital condyle ; Dt, dentary ; E/h, ethmoid ; F, frontal ;
F l , postorbital ; For, fenestra ovalis ; //, optic foramen ; M, maxilla ; N,
nasal ; 01, exoccipital ; Ovp, supraoccipital ; P, parietal ; Pe, periotic ; Pf,
prefrontal ; PI, palatine ; Pni.r, premaxilla ; Pt, pterygoid ; Qu, quadrate ;
SA, supra-angular ; Sqn, s(juamosal ; Tx, transpalatine ; \ r o, vomer.

other characters in which they differ from the majority of
Snakes, the facial bones are immovably connected with the
cranium, and there is no transpalatine.

Chelonia (Fig. 84). The chondrocranium essentially re-
sembles that of Li/an Is and Crocodiles, and much of the
naso-ethmoidal cartilage persists. There is a large supraoccipital
crest and usually a lower temporal arcade or jugal arch, except
in certain cases where the paraquadrate ("quadrato-jugal") is
wanting. The nearly vertical quadrate, which is grooved pos-
teriorly for the columella auris, is very firmly united with the
neighbouring bones, and above it is a squamosal. There are no
separate nasals or lacrymals, their position being taken by a " pre-
frontal." The parietals do not unite. The orbit is completely

118 COMPARATIVE ANATOMY

surrounded by bones, and in some forms (e.g. Cheloiie, Sphargis) the
whole temporal region is covered by an additional roof, above the
cranial roof proper, formed by the parietal, squamosal, and post-
frontal. An ossification between the pterygoid and a descending
lamella of the parietal (which latter boric forms part of the wall of
the cranial cavity) apparently represents an epipterygoid. The
palatines give rise to palatine plates, which together with lateral
parts of the median voiner help in forming a short hard-palate.
The opisthotic remains distinct from the exoccipital, and the pre-

'Cccc

FIG. 84. SKULL OF YOUNG WATER-TORTOISE (Emys euro/urn). Side view.

BP, cartilaginous interval between basioccipital and basisphenoid ; f 1 , post-
frontal ; UK, horny sheaths of jaws ; /, point of entrance of the olfactory
nerve into the nasal capsule ; In;/, jugal ; Aid, mandible ; Mt, tympanic mem-
brane ; Na, external nostril ; 0*p, supraoccipital, which gives rise to a crest ;
Pf, pref rental, which forms a great part of the anterior boundary of the
orbit ; (Jju, paraquadrate (" quadrate- jugal") ; Si, interorbital septum. Other
letters as in Fig. 83.

maxilla is usually paired, while the dentaries are united at the
symphysis. No indications of a parasphenoid are known, and
a transpalatine is wanting.

There are no teeth, which are replaced functionally by horny
beaks, as was also the case in certain fossil Reptiles (e.g. Cerato-
psidre).

The hyobranchial skeleton is comparatively large, and consists
of a body, very closely related to the larynx and produced
anteriorly into a short lingual process, of a pair of small hyoid
cornua, and of two pairs of larger cornua belonging to the first
and second branchial arches (Fig. 85).

Crocodilia (Fig. 86). The chondrocranium is less fenestrated
than in Lizards, and a basipterygoid process is not strongly
marked : the interorbital septum, which is inserted vertically
between the anterior parts of the trabeculas, attains a consider-
able development. Alisphenoids are present.

The bones are very massive and firmly united together by
sutures : those on the dorsal and lateral regions are pitted.

SKULL

119

The parietals and frontals are unpaired in the adult, and lacrymaLs,
prefrontals, and postfrontals are present. There is an upper and
a lower temporal arcade or zygomatic arch, and between the orbit
and temporal fossa is a bony pillar formed by the postfrontal, jugal,

FIG. So. HYOBRANCHIAL APPARATUS,
WITH LARYNX AND TRACHEA, OF
Emys curoptt'a. From the dorsal
side.

.4 A', arytenoid cartilage
Cbr.I, 6V.//, first
branchial cornua; Chi/,
P.I, lingual process ;
cartilage of larynx ;
ZB, broader anterior,
rower posterior part
branchial.

of larynx ;

and second
hyoid cornu ;

RK, cricoid
Tr, trachea ;
and ZK, nar-

of basihyo-

Coce

FIG. 86. SKULL OF A YOUNG
CROCODILE. Ventral view.

Ch, internal nostrils ; Coce, oc-
cipital condyles ; J<j, jugal ;
J/, palatine process of max-
illa ; Ob, basioccipital ; Orb,
orbit ; PI, palatine ; Pmx,
premaxilla ;. Pt, pterygoid ;
>.>/, paraquadrate ("quadrato-
jugal"); (Jit, quadrate; Ts,
transpalatine.

and transpalatine : the vomer is paired. The palatines and ptery-
goids are very firmly attached to the base of the skull, and both
these bones, as well as the paired premaxilke and maxillae, take
part in the formation of the hard palate. 1 Thus the internal
nostrils open far back, beneath the basioccipital, from which alone
the occipital condyle is formed. The exoccipitals meet above the
foramen magnum, thus shutting out the supraoccipital. Teeth are
present in sockets on the premaxillae, maxillae, and dentaries.

A series of air-passages extends into the bones from the

1 In the pre-Cretaceous Crocodiles the pterygoids did not form palatal
plates.

,**

120

COMPARATIVE ANATOMY

tympanic cavity, and the Eustachian canals open into the pharynx
by a median aperture behind the internal nostrils.

The hyobranchial skeleton is much reduced, and consists of a
body with a single pair of cornua : it is not known whether these
belong to the hyoid or to the first branchial arch

[Birds.

As already mentioned, the skull of Birds is formed on a similar
plan to that of Reptiles more particularly of Lizards, but it
exhibits certain special characteristics (Figs. 87 and 88).

In correspondence with the higher type of brain, with its
well developed cerebral hemispheres, the brain-case is relatively

Interorl}. septum

Prefont (lac.)

Squamosal ^

aritt

Dent.

And. caps

Pteryr/.

Supra- Ang. " Paraiph.

Ang.

5 *"*

<&>

FIG. ST. -SKULI, OF AX EMBRYO CHICK 65 MM. IN LENGTH. From the right
side. (From a model b\- W. Toukoflf, x i). Cartilage, l/>n : investing bones,
yellmo.

Prentax.

large, and correlative modifications occur, especially in the occipital
and auditor}- regions. The relatively large size of the eyes,
moreover, has resulted in a limitation of the cranial cavity in an
anterior direction and in the expansion of the brain laterally and

SKULL

121

rtff'i -r.

^

;JPV

^p'V:" fc

i?ur^

ec:'(p \ y?

Atf

//

^?^

,- />

Fio. 88. SKULL OF A WILD DUCK (Anas boschas). A, from above ; B, from
below ; C, from the side. (From a preparation by W. K. Parker. )

ag, angular : als, alisphenoid ; a.p.f, anterior palatine foramen ; ar, articular ;
6.0, basioccipital ; b.py, basipterygoid ; b.t>, basisphenoid ; b.t, basitemporal ;
d, dentary ; e.n, external nostril; e.o exoccipital ; eth, ethmoid; e.n,
Eustachian aperture ; f.m, foramen magnum ; fr, frontal ; i.c, foramen for
internal carotid artery ; j, jugal ; fc, lacrymal ; mx, maxilla ; mx.p,
maxillopalatine process ; n, nasal ; n.px, nasal process of the premaxilla ; p.
parietal ; py, pterygoid ; pi, palatine ; p.n, internal nostrils ; px, presphenoid ;
px, premaxilla ; q, quadrate; q.j, quadrate- jugal ; s.o, supraoccipital ; $q,
squamosal ; ty, tympanic cavity ; v, vomer ; //, foramen for optic nerve ;
V, for trigeminal ; IX, X, for glossopharyngeal and vagus ; XII, for hypo-
glossal.

122 COMPARATIVE ANATOMY

posteriorly. There is thus a well-developed interorbital septum,
and the tropibasic type of skull reaches its extreme. The cranial
cavity has become further enlarged at the cost of parts formerly
situated extracranially than is the case amongst Reptiles.
The bones show a tendency to run together by the obliteration
of the sutures originally present between them : they are usually
delicate and spongy (" pneumatic "), thus contrasting greatly with
those of Reptiles. 1 Only in the ethmoidal region does the cartilage
persist throughout life to any extent.

The unpaired occipital condyle no longer lies at the posterior
boundary of the skull, but becomes relatively shifted forward along
the base, so that the axis of the latter lies at an angle with that of
the vertebral column.

The basis cranii is formed by a basioccipital and a basisphenoid,
from which latter a bony rostrum, the remains of the anterior part
of the parasphenoid. extends forwards : near the base of this,
basipterygoid processes, articulating with the pterygoids, may be
present. The posterior part of the parasphenoid persists as a
large and primarily paired plate, the lasitemporal, which underlies
the basisphenoid and part of the basioccipital.

The interorbital septum is thin, as in Lizards, but is more solid
and less membranous than in the latter : it becomes ossified
anteriorly by a mesethmoid and posteriorly by a presphenoid.
Orbitosphenoids and alisphenoids are also developed. The auditory
capsules, which are more drawn in to the cranial cavity than in
Reptiles, ossify by three centres (prootic, epiotic, and opisthotic)
which later become fused with one another and with neighbouring
bones, and the relations of the tympanic cavity, auditory fenestrge
and columella, including the stapes and extracolumella, are very
similar to those of Reptiles. The two Eustachian tubes open
together in the middle line.

The quadrate is movable upon the skull as is also the whole
maxillopalatine apparatus, the delicate palatopterygoid bar, which
is always more or less separated from its fellow in the middle line,
sliding on the rostrum of the basisphenoid, and so allowing the
beak to be raised or lowered to a greater or less extent : thus
a complete bony palate is never present. This mobility 'of the
upper jaw is most marked in Parrots, in which the frontonasal
joint forms a regular hinge.

The vomers, which may be absent, usually unite with one
another and with the palatines to a greater or less degree. 2 The

1 It should, however, be remembered that the development of air spaces
within the bones of the skull is hinted at in Crocodiles as well as in certain fossil
Reptiles.

2 The differences in details as regards the arrangement of the bones of the
palate are important for purposes of classification, as are also the mode of con-
nection of the lacrymal with surrounding parts, including the small bones (like
those in Lizards) which may be present in the neighbourhood of the lacrymal
(supraorbital, infraorbital, lacry mo-palatine).

SKULL

123

(basibrcmchial)

posterior nostrils are always situated between the vomers and
palatines. The two premaxillse, on the form of which depends
that of the beak, are fused, and a maxillopalatine process arises
from the maxilla anteriorly. The maxilla and quadrate are
connected by a delicate jugal and quadratojugal, and a squamosal
is present. Other investing bones are the nasals, frontals, parietals,
and lacrymals or prefrontals.

Teeth were present in Jurassic and Cretaceous Birds (Archae-
opteryx, Hesperornis, Ichthyornis), but were no longer developed
from the Tertiary period on-
wards, their place being taken
functionally by horny sheaths
covering the bones of the jaws,
and thus forming a beak, much
as in Chelonians.

In Meckel's cartilage, two
replacing bones are formed, viz.,
an articular and a mentoman-
dibular: the investing bones
are a dentary, splenial, coronary,
supra-angular, and an angular,
and their relations are essenti-
ally similar to those seen in
Reptiles : they, however, become
fused in the adult, and the two
rami of the mandible unite
distally by synostosis.

The hyobranchial skeleton

(Fig. 89) is greatly reduced. FIO. 89. -HYOBRANCHIAL SKELETON OF
The median body consists of an FOWL. (After Gegenbaur ; lettering

entoglossal (basihyoid) passing after Kallius. )
anteriorly into a primarily paired

paraglossal, which extends into the tongue, and posteriorly into a
urohyal (basibranchial). The single pair of cornua belongs to
the first branchial arch, and may, as in the Woodpecker, give rise
to long, jointed rods extending far over the skull. The columella
is the only part of the hyoid which persists, and even in the
embryo there is no trace of a second branchial arch.

branchial nrcli

Mammals.

In Mammals, the skull of which in many respects indicates an
origin of the Order from reptile-like ancestors, there is a much
closer connection between the cranial and visceral regions than is
the case in the Vertebrates already described. In the fully-
developed skull both maxillary and palatopterygoid regions are
closely united to the cranium, so that the facial and cranial portions

124

COMPARATIVE ANATOMY

are firmly united with one another. The higher we pass in
the Mammalian series, the more does the former come to lie below
instead of in front of the latter, the facial skeleton becoming
proportionately small as contrasted with the large cranial portion

B

FIG. 90.\. LONGITUDINAL VERTICAL SECTIONS THROUGH THE SKULLS OF A,
Salamandra maculosa. B, Tvxtndo i/wcu, AND C, Cornt-f corone, TO SHOW THE
RELATIONS BETWEEN THE CRANIAL AND NASAL PORTIONS.

of the skull, and the reduction of the angle between the basi-
cranial and vertebral axes being carried still further than in Birds
(cf. Fig. 90).

The base of the skull is mainly preformed in cartilage (Fig. 91),
and is but little interrupted except for the passage of vessels and
nerves. It consists of basioccipital, basisphenoidal and ethmoidal

SKULL

125

regions, continuous with one another and with the nasal septum.
Side walls are also partly formed by the chondrocranium, but are

FIG. 9i)B. -LONGITUDINAL VERTICAL SECTIONS THROUGH THE SKULLS OF A, DEER,
B, BABOON, AND C, MAN, TO SHOW THE RELATIONS BETWEEN THE CRANIAL
AND NASAL PORTIONS.

considerably fenestrated. The occipital region includes the
equivalents of four vertebra.

126

COMPARATIVE ANATOMY

Apart from a median cartilaginous bridge connecting the
anterior orbital region with the nasal capsules, and corresponding
to the interorbital septum of the Sauropsida (by the ossification of
which a presphenoid may arise), these capsules are connected

Kasal cups.

Sejituiii nasi.
Orbitonas. fissure
Ala orbitalis.
Ala temp, (alisph.)

Malleus
Incus.

Parotic crest

And. caps.

Jug. for.

Sup. orb. Jissurt
Carotid for.

Parietal

Int. aud. meatua
Jug. for.

X Endolymph.for.
flvpofflostal for

Tect. synot.

Fit;, ill, A. SKULL OK AN EMBRYO MOLE (4'2'3 MM. ix LENGTH FROM NOSE TO
BASE OK TAIL). FROM AN ENLARGED MODEL, x ^. A, dorsal, and B, ventral

view. The investing bones are removed on the left side. (After E. Fischer.)

with the cerebral part of the chondrocranium merely by thin bars
on either side (sphenethmoid cartilage) : the Mammalian skull is
therefore of the tropibasic type l (p. 77).

The anterior part of the basis cranii is formed by the ossifica-
tion of the cartilage, which either gives rise to a distinct pre-

1 Certain facts in the development of the skull in Apes indicate that the
Primates diverged very early from the common mammalian stem, many primi-
tive characters being present which are no longer recognisable in other "lower"
Mammals.

SKULL

127

sphenoid, as already mentioned, or may be due to a union of the
basal parts of the two orbitosphenoids. Alisphenoids, as well as
a basisphenoid, a basioccipital, a supraoccipital, and exoccipitals
(often with paroccipital or paramastoid processes) are always present,
the paired condyle x being furnished by the exoccipitals (Fig. 92).

A'arial fenestra

Preitiax

Basal fenestra

Paraseptal cart.
Nasal caps.

Vomer

Splteneth. cart.

Orbitonas. fissure
Meckel's cart.
__ Ala temp, (alisplt.)
Malleus
Carotid for.

Max. ...

Jugal

Pal 0-

1 P teri/g. " (parasph. ) ~

9

Tympanic

Squamosal

Fen. cochlea
Jug. for. -

Mypoglostal for.

for. may. Tect. synot.
FlG. 91, IJ.

The enlargement of the cranial cavity in correspondence with
the increased size of the brain affects the form of the skull in
various respects. Thus the supraoccipital becomes shifted

1 The presence of two condyles appears at first sight to form an important
difference to Reptiles, and this is the more remarkable as the occipital region has a
similar primary constitution in both groups and differs from that of Amphibians.
But in the case of the Sauropsida there axe four points of connection between the
occipital and the vertebral column. The single condyle is usually formed of
three parts (p. Ill), the median or axial of which articulates parti}' with the
centrum proper of the atlas and partly with the odontoid process, with
which it is connected by ligament. In Mammals, the lateral articulations
are alone developed, and in the Mole embryo there is a single continuous
articulation between the skull and vertebral column.

128

COMPAPxATIVE ANATOMY

J'ai

C.occ

Occ.las.

C.occ*

Jm

tSr/.oc

For.m
C.occ

SKULL 129

Fi<;. 92. SKULL OF GREYHOUND. A, from above; B, from the side; C, from
below ; and D, in longitudinal section.

B.occ, Occ.bas, basioccipital ; Cav.gl, glenoid cavity for the lower jaw; Cho,
posterior narial passage ; C.occ, occipital condyles (exoccipitals) ; Eth, lamina
perpendicular-is of the ethmoid; Eth', cribriform plate ; F, frontal; For.m,
foramen magnum ; Jg, jugal ; Jm, premaxilla ; L, lacrymal, surrounding
the lacrjnnal canal ; M, maxilla, with the infraorbital foramen (Finf) ; Maud,
external auditory meatus ; Md, mandible ; N, nasal ; P, parietal ; Pal (P in
C), palatine ; Pet, petrous portion of periotic ; Pjt, zygomatic process of the
squamosal ; Pt, pterygoid ; Sph, alisphenoid ; Sph 1 , basisphenoid ; Sph",
presphenoid ; Sq, squamosal; Sq.occ, supraoccipital ; T, tympanic; Vo,
vomer.

relatively backwards and the auditory region downwards to a
varied extent, so that the squamosal (as is also the case in Birds)
now usually helps to a greater or less extent to complete the walls
of the brain-case dorsally to the displaced auditory capsule.
Moreover, the course taken by the facial and auditory nerves
through the skull-walls has become altered. 1

In adaptation to the characteristic high development of the
olfactory organs amongst Mammals, the ethmoidal portion of the
skull is specially developed for enclosing the nasal cavities. The
ethmoid is formed from the anterior part of the chondrocranium,
which is continued forwards as the olfactory chamber, divided into
right and left halves by a cartilaginous septum (mesethmoid), and
separated from the cranial cavity by the cribriform plate (lamina
cribrosa), which, however, is not directly homologous with that of
lower types (p. 97) : this has a more or less oblique or vertical
position, according to the form and relations of the cerebral hemi-
spheres and olfactory lobes. The posterior part of the mesethmoid
becomes ossified as the lamina perpendicularis, and lateral ethmoids
are present at the sides of the nasal region ; the vomer, which
is unpaired in the adult, arises as a paired perichondral bone
ventrally to the nasal septum, 2 and the latter is thus in part bony.

The auditory capsules are ossified from prootic, epiotic, and
opisthotic centres, which early unite together to form the periotic
OTpetromastoid bone. The denser internal (petrous) portion of this
bone, which corresponds mainly to the prootic, encloses the essential
part of the organ of hearing, and a fenestra ovalis and fenestra
rotunda are present on its outer surface : the more spongy mastoid

1 Considerable differences exist amongst Mammals as regards the number
and arrangement of the nerve apertures. Thus amongst Carnivores, for example,
the following foramina are distinct from one another : opticum (II), foramen
lacerum anterius or uphenoidal fissure (III, IV, V 1 , VI), rotundum (V' 2 ), ovale
(V 3 ), meatus auditorius interims (VII, VIII), foramen lacerum posterius (IX, X,
XI), and the condylar foramina (XII). In the lower Mammals (e.g. Monotremes,
Marsupials, and certain Insectivores), the optic foramen and sphenoidal fissure are
not separate from one another, or, in some cases (Echidna, certain Insectivores,
Dasypus, Lemurs), from the foramen rotundum. The cribriform plate of the
ethmoid has numerous perforations for the olfactory nerve in all Mammals
but Ornithorhynchus.

- In Ornithorhynchus a small dumb-bell shaped bone (prevomer) is present
between the diverging premaxillaj.

K

130 COMPARATIVE ANATOMY

(opisthotic) portion reaches the surface of the skull between the
exoccipital and the lympanic bone, the homology of which is open
to doubt, but which possibly corresponds to the paraquadrate or
quadratojugal. The tympanic overlies the petrous portion of the
periotic, and gives attachment to the tympanic membrane : in
the Placentalia it forms the tubular external auditory passage or
meatus below which it may expand into a bulla tympani, which
encloses the tympanic cavity and communicates with the pharynx
by means of the Eustachian tube. The " temporal bone " of
human anatomy represents the fused periotic, tympanic, and
squamosal, the two last of which are investing bones.

The cranial cavity is roofed in by frontals, parietals, and a
supraoccipital : a primarily paired interparietal, not preformed in
cartilage, may remain distinct or may unite with the supraoccipital
or frontals. These roofing elements, like many of the other cranial
bones, are united by sutures which usually persist, at any rate for
a long time. Many of the bones are more or less spongy internally,
and may contain definite air-sinuses (e.g. in Ungulates).

The parietals and frontals vary much as regards form and
relative size in the different orders. In Primates, amongst many
others, the parietals are well developed, while in Insectivores they
are small : in toothed Whales they become separated from one
another by a large bone formed by the fusion of the supra-
occipital and interparietal, which reaches to the frontal (Fig. 94).
In many Mammals there is a large parietal and supraoccipital
crest in correlation with the strongly-developed muscles of the jaws
and neck. The frontals, which, like many of the neighbouring
bones, may become united together, extend downwards towards the
orbital region and cribriform plate, and thus take part in forming
the walls of the cranium and orbit.

Most of the true Ruminants are provided with horns or antlers
projecting from the frontal bones, the formation of which is to be
traced primarily to the integument (Fig. 93).

In the Camcornia (Bovina?, Antelopina 3 , Caprinse, Ovina?)
bony processes arise from the frontals, which become hollow and
are enveloped by horn formed from the epiderm. They are usually
present in both sexes, but in Tragelaphus, Neotragus, and others
are absent in the female. In the Ccrvidw a solid integumentary
bone is developed and becomes united with the frontal, growing
out to form the antler. After attaining its full development,
the investing skin dries up owing to the development of the
" burr " at its base ; this constricts the vessels, and the antler,
being deprived of nutriment, falls off periodically at the close of
the breeding season. In the young animal the antlers are simple,
but year by year they become more or less complicated and
branched. They are confined to the male except in the case of the
Reindeer. Amongst Giraffes, in addition to a short median " frontal
horn " present in many of the sub-species, both sexes possess small

SKULL

131

lateral horns covered with hair : these are usually described as
separate ossifications which become united with the frontals ; but
it has recently been shown that they originate in connection with
the fibrous osteogenetic tissue of the parietal bones.

Dorsally and laterally to the cartilaginous olfactory capsules
investing bones arise, viz., the variously-shaped nasals and the

FIG. 93. EARLY STAGES IN THE DEVELOPMENT OF ANTLERS (A, B) AND HORNS

(C, 1), E). (After M. Weber.)

Cor, derm ; Ep, epiclerm ; #$, horny sheath ; HZ, bony process of the frontal,
with the epiphysis-like " os oornu" (00} at its apex : the latter is comparable
to the beam of the antler, and the former to the pedicle : in E the two are
already fused (HZ +00) ; ^?, zone of resorption, at which point the antler
is shed ; SZ, process of the frontal still covered with the integument ; &2 1 ,
the same after loss of the integument.

lacrymals, each of the latter perforated by a lacryrnal foramen ; in
this region also are the lateral plates of the ethmoid (lamina;
papyracece). The scroll-like turbinals which are usually well-
developed within the olfactory chambers will be described later.
Cartilage persists in the adult only in the nasal septum, in the
form of the alinasal and aliscptal cartilages. 1 A septomaxillary

1 An external nose is peculiar to certain Mammals (e.g. Man). Representa-
tives of the cartilages mentioned above are present amongst other Amiiiota and
in Reptiles, in which, however, they do not extend anteriorly to the rest of the

K 2

132 COMPARATIVE ANATOMY

(cf. p. 82) can be recognised in embryos of Echidna close behind
the external nostrils : it unites later with the premaxilla, forming
its extra-nasal process, which in other Mammals possibly has a
similar independent origin.

The premaxillse, which may become fused, still take an
important part in enclosing the nasal cavities, and in the Dugong
(Halicore) they are very large and are bent downwards in corre-
lation with the large pair of incisor tusks. The maxillae form the
larger part of the facial skeleton, and are also important in
contributing to the walls of the nasal chambers and orbits. Each
maxilla is connected by means of a jugal (malar) with a process of
the squamosal, instead of with the quadrate, as in the Amphibia
and Sauropsida ; thus a zygomatic arch is formed from these
three bones. The orbit and temporal fossa are marked off from one
another in varying degrees : they are continuous, e.g. in Rodents,
Insectivores, and Carnivores, while in Perissodactyles, Ruminants,
and especially Primates, they are more or less completely separated
from one another by a process of the frontal meeting the jugal.

As regards the structure of the hard palate, Mammals agree
essentially with Crocodiles, and more or less complete palatine
plates are formed by the premaxillse, maxillse, and palatines ; but
the small " pterygoids " l (except, e.g. in Anteaters and some
Cetaceans) do not take part in its formation : in Echidna the
pterygoids form part of the basis cranii. The palate is very long
in Echidna and in certain Edentata and Cetacea, and often (e.g.
Marsupialia) presents unossified vacuities.

The general form of the skull differs very greatly amongst
Mammals. It is sometimes short and broad, sometimes elongated
especially in the region of the snout (e.g. Myrmecophaga, Cetacea).
Amongst the Cetacea (Fig. 94), the facial bones are of so great a
relative length that the skull may be one-third as long as the
whole animal (e.g. Balasna) ; the external nostrils are situated far
back, and there are numerous other secondary modifications apart
from those seen in the lower jaw, which is not used for purposes
of mastication and in certain respects shows traces of degeneration.

The genesis of the lower jaw is briefly as follows (Fig. 95). In
the embryo, the proximal end of Meckel's cartilage is differentiated
into two portions, corresponding to the articular and suspensorial

skull, and are entirely covered by bones, the most important of which in this
respect is the median nasal process of the premaxilla (Figs. 7"2, 80, 82, and 83).
This prenasal process is present only in the Monotremes amongst Mammals, and
with its fellow forms the transitory o.s caruncidcB. On the loss of this ascending
process of the premaxilla, a freer development of the cartilaginous skeleton is
rendered possible ; and under the influence of muscles, certain parts of it become
separated off to form the independent cartilages of the external nose (cf. under
Olfactory Organ).

1 True pterygoid bones, corresponding to those of the Sauropsida, are
apparently only known to occur in Monotremes. The so-called pterygoid (or
internal lamina of the pterygoid process of the basisphenoid) of other Mammals
has been shown to correspond to a posterior part of the parasphenoid.

SKULL

133

parts of the jaw in the lower Vertebrates : these become ossi-
fied and enclosed within the tympanic cavity situated within the
tympanic bone externally to the periotic. They thus represent

Fio. 94. A, SKULL OF DELPHINUS. (From M. Weber, after Boas.) B, SKULL
OF POSTAL Balcena japonica. (From M. Weber, after Eschricht. )

C, occipital condyle ; Fr, frontal ; Ju, jugal ; L, lacrymal : MX, maxilla ; n,
external nostril ; JVrt, nasal ; oe, exoccipital ; 0*, supraoccipital ; Pa, parietal ;
Pal, palatine ; Ft, pterygoid ; Px, premaxilla ; Sq, squamosal ; Ty, tym-
panic and bulla tympani.

the articular and quadrate, and are known as the malleus and incus
respectively. Having undergone a change of function, they form,
together with a third element the usually stirrup-shaped stapes,
a connected and articulated chain of auditory ossicles extending

134

COMPARATIVE ANATOMY

between the fenestra ovalis and the tympanic membrane, and
serving to conduct sound vibrations to the inner ear. 1 An investing
bone, the dentary, is developed around the main part of Meckel's
cartilage, distal to the malleus ; the cartilage itself may undergo
partial ossification, but gradually disappears, the dentary forming
the bony mandible, which develops a new articulation with the

e.n

or.c

(Modified from

FIG. 95. SKULL OF EMBRYO OF ARMADILLO ( Tatusia hybrida).

a drawing by W. K. Parker.)

a.ty, tympanic annulus ; au, auditory capsule ; b.hy, basihyal ; c.hy, ceratohyal ;
cr, cricoid ; d, dentary ; e.hy, epihyal ; e.n, external nostril ; eo, exoccipital :
/, frontal ; h.hy, liypohyal ; i, jugal ; in, incus; lc, lacrymal ; ink, Meckel's
cartilage; ml, malleus; mx, maxilla; n, nasal; oc.c, occipital condyle ;
2>, parietal ; pa, palatine ; px, premaxilla ; so, supraoccipital ; st, stapes ;
s.t, ethmoturbinal ; nt.m, stapedius muscle; sq, squamosal; (h, thyroid;
tr, trachea ; //, optic foramen ; V 1 , V~, foramina through which the first
and second divisions of the trigeminal pass out from the orbit.

squamosal, characteristic of, and confined to, the Mammalia, all
other Craniata possessing the more primitive quadrato-mandibular
articulation. The two rami of the lower jaw may remain distinct
at the symphysis, or many unite with one another (e.g. Bats,

1 There is some doubt as to how far it is justifiable to consider the tympano-
eustachian cavity as homologous with the spiracle of Fishes, and the tympanic
cavity and membrane of Amphibia, Sauropsida, and Mammalia as homologous
with one another.

The stirrup form of the stapes is due to its being perforated by an artery
(as in the case of the stapedial plate of the Gymnophiona), which in
certain cases persists in the adult. The stapes, however, is not perforated in
Monotremes and certain Marsupials and Edentates. The homology of this
element is by no means clear, but there are reasons for considering it to correspond
to the stapedial plate of the Sauropsida and to the whole columella of Amphibia ;
it is possible that all these structures are derivatives of the hyomandibular of
Fishes.

APPENDICULAR SKELETON 135

Perissodactyles, Primates) ; and on each a condylar, a coronoid,
and often an angular process (Marsupials, Rodents, Insectivores)
may be distinguished. 1 Teeth, which are only exceptionally
wanting (e.g. Echidna, certain Edentates), are confined to the pre-
maxilla, maxilla, and mandible. They present marked differences
in number, form, and size ; together with the muscles, they are
the cause of considerable modifications in the form of the jaws
and their articulation and may indirectly influence the entire
skull, in the study of which the law of correlation must always
be borne in mind.

The hyoid arch (Fig. 95) is connected proximally with the
base of the auditory capsule and sometimes becomes more
or less ossified, but the greater part is usually reduced to a
fibrous band, and may be quite rudimentary ; its dorsal end forms
the styloid process of the periotic, and its ventral end the lesser
(anterior) cornu of the so-called hyoid bone of the adult. The
body of this bone represents the basal parts of the hyoid and first
branchial arch, the greater (posterior) cornua belonging to the
latter. The hyoid apparatus is connected with the larynx by a
membrane, the thyro-hyal ligament, and the thyroid cartilage of
the larynx arises in the blastema of the second and third branchial
arches.

V. APPENDICULAR SKELETON

The problem of the evolution and morphology of the fins and
limbs of Vertebrates is one which, in point of interest and im-
portance, is comparable to that relating to the head. During the
last thirty years it has been attacked vigorously both from the
embryological and the paleeontological sides, and has given rise to
so many speculations often of a very contradictory nature that
only the barest outline of some of the more important results
obtained can be given in the course of the present chapter.

The fins or limbs, which are distinguished from the axial organs
(head, neck, and body) as appcndicular organs, serve mainly for
locomotion, and may be divided into two groups, the unpaired
and the paired (pectoral and pelvic). They arise independently of
the axial skeleton. 2

1 Two or more small bones ("ossa mentalia") occur in Man between the
distal ends of the mandibular rami, with which they unite, taking part in the
formation of the mental prominence.

' 2 Numerous and varied modifications of the tins occur amongst Wishes to
form, e.g. organs for protection, support, attachment, offence, defence, or for
alluring prey.

136

COMPARATIVE ANATOMY

A. Unpaired Fins.

The unpaired, or median fins, which are mainly characteristic
of Fishes, arise in the embryo as a ridge of the integument (ecto-
derm and mesoderm) extending along the median dorsal line from
the anterior part of the trunk backwards to the tail, around the
apex of which it is continued forwards for some distance along the
ventral side : thus a dorsal, caudal, and ventral portion can be
distinguished. In the course of further development, these
portions either remain continuous, or else certain parts undergo
reduction, so that the ridge only persists in certain regions, where
it forms independent dorsal, caudal, and ventral or anal fins (Fig.

BF An

FIG. 96. DIAGRAM SHOWING (A) THE UNDIFFERENTIATED CONDITION OF THE
PAIRED AND UNPAIRED FINS IN THE EMBRYO, AND (B) THE MANNER IN
WHICH THE PERMANENT FINS ARE FORMED FROM THE CONTINUOUS FOLDS.

AF, anal fin ; An, anus ; BF, pelvic fin ; Bi'F, pectoral fin ; Z>, dorsal fin-fold ;
RF, FF, dorsal fins ; SF, tail-fin ; S, S, lateral folds, which unite together
at S 1 to form the ventral fold.

96, A, B) : in these regions muscles and skeletal parts become
developed in Fishes.

These skeletal parts consist of supporting rays of two kinds.
In the base of the fin cartilaginous radii, or pteryyiophores, usually
segmented (typically into three portions), are formed ; these may
unite proximally to form one or more basipterygia, and in bony
Fishes they become extensively ossified : they frequently come
into secondary connection with the vertebral column. Except in
Cyclostomes, the peripheral part of the fin is supported by dermal
rays, which may consist of numerous delicate horny fibres (Elas-
mobranchs), or of bony rods, entire or jointed, often cleft at the
base, and articulating with the pterygiophores, and not preformed

APPENDICULAR SKELETON 137

in cartilage (Teleostomes) : recent researches indicate that the
latter are ectodermal in origin. 1

Median fins are also present in the Amphibia, in which they
may persist throughout life (e.g. Perennibranchiata), or only occur
in the larval stage ; occasionally also they become specially devel-
oped during the breeding season (e.g. Newt). They have the
form of a continuous integumentary fold extending round the tail
and along the back for a greater or less distance, but enclose no
skeletal elements.

Amongst Reptiles one or more median fins were present in
Ichthyosaurus, and these are comparable to the dorsal fins occur-
ring in the Cetacea amongst Mammals : in both cases they, like
the horizontal tail fin of these forms, must be looked upon as
structures acquired secondarily in connection with an aquatic
existence.

B. Paired Fins or Limbs.

As regards the origin of the paired fins, there is much difference
of opinion. According to one view, they correspond to modified
gill-arches and rays, the former giving rise to the pectoral and
pelvic arches or girdles, and the latter to the free portion of each
fin, one of the rays becoming enlarged so that the others are
attached in a row on either side of it, instead of to the arch. This
would result in a biserial form of fin, the " archipterygium " of
Gegenbaur, such as is most nearly retained in Ceratodus (Fig. 118),
and is also indicated in many Elasmobranchs. The fact that the
branchial arches are situated in the pharyngeal wall and the limb
arches in the body- wall, alone forms an important objection to this
theory.

Another view, which seems to be the more likely one, is
as follows. It is highly probable that primitive Vertebrates at
one time possessed, in addition to the median fins, a pair of con-
tinuous lateral fin-folds, traces of which, beginning with a prolifera-
tion of the mesoderm, can still be recognised in young embryos of
Elasmobranchs (Fig. 97) and to a less extent in those of other
Fishes and of Amphibians, and which, though never continuous, are
indicated by muscle-buds on the intermediate myotomes. They
extended backwards along the sides of the body from just behind
the head, gradually converging towards the anal region, where
they became continuous with the ventral part of the median fin-
fold (Fig. 96, A), and in this respect resembled the persistent lateral
or metaplcural folds present in the adult Amphioxus, though it is

1 The dermal fin rays or dermotrichia are classified by Goodrich as follows :
1. Horny ceratotrichia in Elasmobranchs ; 2. Bony leptotrichia in Teleostomes ;
3. Horny actinotrichia occurring in the embryo and in the margins of the fins of
adult Teleostomes, in addition to (2) ; 4. Fibrous, calcified,.or horny camjdot ricliia
in Dipnoans : it is doubtful whether the last-mentioned correspond to (1) or to (2).

138

COMPARATIVE ANATOMY

m. ~

doubtful how far this comparison is justifiable. As is usually the
case in the median fins, certain parts of these lateral folds have
undergone reduction, only the anterior and posterior portions
remaining to form respectively the pectoral arid pelvic fins, which

must therefore be looked upon as
the localised remains of a con-
tinuous lateral fin-fold on either
side of the body. 1

Into these paired folds extend
metameric processes of the myo-
meres, which undergo further
development in those regions
which will give rise to the pectoral
and pelvic fins, and disappear in
the intermediate region. More or
fewer spinal nerves pass into the
fins, and finally also cartilaginous
supports (pterygiophores), as in
the case of the median fins. These
radii appear first of all at the base
of the fin, gradually extending
centrifugally into the latter, and
also, becoming fused, centripetally
into the body- wall (Fig. 98). 2 An
articulation is then formed second-
arily between the fused basal part
of the skeleton situated in the free
portion of the fin (basipterygium)
and that which extends into the
lateral body-wall and serves as a
support for the limb proper : this
constitutes the limb-arch or girdle,
The arch may remain compara-
tively small and not extend far

dorsally ; but when the extremity is destined to perform more im-
portant movements in locomotion or to give a more definite

1 The essential part of this conception as to the origin of the paired
extremities is due to Thacher, Mivart, Balfour, Haswell, and Dohrn, and a
somewhat similar idea was put forward by Goodsir as early as 1856. The
Palffiozoic Cladoselache is very suggestive in this respect.

' Thus phylogenetically both anterior and posterior extremities can be traced
to a metameric ground-plan. At the same time it must be borne in mind that the
above account is not altogether borne out oiitogenetically. The muscle-buds are
not strictly metameric, as they fuse together before coming into connection with
the skeletal parts, with which they do not always correspond numerically and
which appear to consist at first of a single unsegmented basipterygium : in other
words, the radii arise secondarily. Moreover, it is held by some embryologists
that oiitogenetically the girdle is the primary part of the extremity from which
the free portion grows out secondarily, and a similar axifugal growth can be
recognised in the median tins. All this, however, may only mean that recapitula-
tion is incomplete, and the arguments against the lateral fin-theory are still not
conclusive.

FIG. 97. TRANSVERSE SECTION
THROUGH THE EMBRYO OF A
SHARK (Pristiurus melanosto-
mns), 9 MM. LONG, SHOWING
THE MODE OF ORIGIN OF THE
PECTORAL LIMB-BUDS.

ap, limb-buds ; ch, notochord ; co,
ccelome ; m, myomeres, which
are extending ventrally ; my,
spinal cord ; re', re", rudiment
of kidney tubule and duct.

APPENDICULAR SKELETON

139

support to the body, in addition to meeting with its fellow
ventrally, the arch may extend upwards so as to come into
connection with the axial skeleton, thus forming an almost
complete girdle around the body. The parts of the limb-skeleton

rd.

FIG. 98. A, B, C. DIAGRAM OF THREE SUCCESSIVE STAGES IN THE DEVELOP-
MENT OF THE PELVIC FIN OF A SHARK.

cl, cloacal aperture ; fo, obturator foramen ; rd, primitive radii, which in A are
beginning to fuse into a basal plate (7). In B this fusion has taken place
on both sides, and at * the proximal ends of the two basals are approximat-
ing to form the arch. In C the process is completed, and at f an articulation
has been formed between the arch and the free portion of the fin. On the
left side in C the radii are becoming secondarily segmented.

may become ossified later. The pelvic fin of Fishes as a rule
remains at a simpler and more embryonic stage than the pec-
toral fin.

The paired extremities are not connected with any particular
body-segments, but vary greatly as to their relative positions and
the nerves which supply them.

140

COMPARATIVE ANATOMY

Pectoral Arch.

Fishes. Paired fins and arches are wanting in Cyclostomes.
In Elasmobranchs the pectoral arch consists of a comparatively
simple cartilaginous bar (Fig. 99), situated just behind the
branchial apparatus, the two halves of which are united ventrally
by cartilage or fibrous tissue, 1 and in embryos of Teleostomes it
has at first a similar structure. Later, however, in the last-named

FIG. 99. PECTORAL ARCH AND Fix OF Heptanckus.

n, l>, the main fin-ray, lying in the axis of the metapterygium (3ft) ; t, single ray on
the other side of the axis (indication of a biserial type) ; F8, horny rays,
cut through ; Pr, Ms, Mf, the three basal elements of the fin (pro-, meso-,
and metapterygium) ; Ra, fin-rays ; SB, SB 1 , pectoral arch, with a nerve
aperture at NL.

Order, bony structures originating from the integument are
developed in this region ; so that a secondary or bony pectoral arch
may be distinguished from a primary or cartilaginous one, the
latter becoming less marked in proportion to the development of
the former (Fig. 100).

In all Fishes the free extremity, or fin, is connected with the
hinder and outer circumference of the (primary) arch, convex

1 In Heptanchus there is a small ventral element which has been compared
to a "sternum."

PECTORAL ARCH

141

articulations being formed on the arch which fit into concave
facets on the fin. The point of attachment of the extremity may
be taken as separating the arch into an upper dorsal and a lower
ventral section. The former, which may exceptionally be con-
nected with the vertebral
column (viz., Raiidre), cor-
responds to the scapula, and
the latter to the coracoid plus
procoracoid of the higher
Vertebrata.

In Teleosts and bony
Ganoids the secondary arch,
consisting of a row of bones,
forms the principal support
of the fin in the adult, the
main element being a large
clavicle. The arch becomes
secondarily connected with
the skull. (For further
details, cf. Fig. 100.) In
Dipnoans, clavicles and
supra-clavicles invest the

cartilaginous arch (Fig. 71). FR; m _ LEFT PECTOKAL ARCH AND FlN

OF THE TROUT. (From the outer side.)

Amphibians. In this
Class the pectoral arch shows
no direct connection with
that of Fishes, but is similar
in fundamental plan to that
of all the higher Vertebrates.

It always consists on
either side of a cartilagin-
ous or bony dorsal plate
(scapula and suprascapula),
which curves round the side
of the body and is con-
tinuous with two ventral
plates an anterior (procoracoid} and a posterior (coracoid} (Figs.
101 and 102). The ventral part of the arch becomes connected with
the sternal apparatus. The humerus articulates with a concave
glenoid facet at the junction of the scapula and coracoid. The
two coracoid plates either overlap one another in the mid-
ventral line (Urodeles, Fig. 55, A, B and certain Anura e.g.
Hyla, Bombinator, Fig. 55, c), or else their free edges come into
apposition and unite (other Anura, e.g. Rana, Fig. 55, D). In
Anurans the procoracoid has a more transverse position than
in Urodeles, and comes into connection with the coracoid in the
mid-ventral line, thus giving rise to a fenestra between the two.

Co(Cl)

D, D l , D 2 , chain of secondary bones of the
pectoral arch (clavicle and supraclavicle),
which is connected with the skull by
means of the post-temporal (Cm) ; F,
S, bony fin-rays, shown cut away from
their attachments ; HS, bony ray on the
border of the fin which is connected
with the fourth basal element ; L,
foramen in scapula ; M l , metapterygium ;
Ra, Rn, the second and third, and 4,
the fourth basal element of the fin ;
Ha 1 , the second cartilaginous row of
radii ; 8 and Co(Cl), bony scapula and
coracoid, which have become developed
in the cartilage Kn.

142

COMPARATIVE ANATOMY

E

The whole arch is, moreover, more strongly ossified, the procoracoid
being covered by an investing bone the clavicle, which may
more or less completely replace it. This integumentary bone

corresponds to the part of the secondary
arch which first appears in Ganoids : in the
Stegoeephali there was a well-developed
clavicle connected with the episternum (see
p. 44) and peripherally with another bony
rod (cleithrum), which also occurred in the

FIG.IOI.-DIAGRAMOFTHE fossil Re P tile Pareiasaurus
GROUND-TYPE OF PEC- Reptiles. As in Amphibians, the most
TOKAL ARCH MET WITH IN essential parts of the pectoral arch of

Re R t!les . are the sca p la u and coracoid -

arising in connection with a continuous
;1 cartiiagincras bar or plate, as is well seen in
scapula. Lizards (Fig. 56). A procoracoid may also

be formed, and in Chelonians a bone

usually described as the procoracoid is strongly developed, but
is firmly united with the pillar-like scapula, the two being
separated from the coracoid by a suture ; hence the bone in

ri

FIG. 102. PECTORAL ARCH OF THE RIGHT SIDE OF Salamandra maculosa,
considerably magnified, and flattened out.

a, b, bony processes extending into the procoracoid and coracoid respectively ;
Cl, procoracoid ; Co, coracoid ; G, glenoid cavity, surrounded by a rim of
cartilage (L); S, scapula (ossified); SS, suprascapula.

question is sometimes spoken of as a proscapida. In other recent
Reptiles the procoracoid is much reduced or even absent.

Traces of the relations of the procoracoid to the clavicle can
still be seen in some cases, but the latter, when present, arises
mainly from a connective-tissue blastema unconnected with a
procoracoid (Fig. 56). Nevertheless a primary and a secondary
part of the pectoral arch can also be recognised in Reptiles, the

PECTORAL ARCH 143

former represented by the more constant elements, while the latter
tends to become reduced and may even entirely disappear.
Clavicles are absent in Chelonians, and are either wanting or rudi-
mentary in Crocodiles and Chameleons.

On the loss of the extremities (certain Skinks, Amphisbsenians,
Snakes), the primary shoulder-girdle becomes reduced or even
entirely lost, the reduction beginning with the sternum.

The shifting backwards of the pectoral arch, which is already
to some extent seen in Amphibians as compared with Fishes, is
still more marked in Reptiles, in which it is situated some distance
from the head; this is especially seen in Chelonians and many
fossil forms, and reaches its maximum in Birds.

In Lizards, unossified spaces are left in the coracoid, giving
rise to fenestras closed over by fibrous membrane. A main fenestra
(cf. Fig. 56, a, dorsal to which a bony process, the vestigial pro-
coracoid, can be seen) maybe distinguished from accessory fenestrse
of varied form and number, and is typical of all Lizards : it
arises in the primary arch and corresponds to that occurring in
Amphibians (Fig. 55).

Birds. In Birds, the scapula consists of a thin and narrow
plate of bone often extending far backwards, the strong coracoid
being bent at an acute angle and united by ligament with it in
typical Carinate Birds (Fig. 53). In the Ratitre the relatively
small scapula and coracoid are ankylosed with one another.
The lower end of the latter bone is firmly articulated in a groove
on the anterior edge of the sternum, while its upper end takes
part with the scapula in forming the glenoid cavity, beyond
which it is produced in the Carinatas and in Archeopteryx to form
an acrocorcicoid process.

In Struthio the broad coracoid is fenestrated, and its anterior
part may be looked upon as a procoracoid: in other Ratitas the
latter is considerably reduced, and may be represented merely by
a ligament ; in Carinatas it can often no longer be recognised.

In almost all Flying Birds the clavicle, a purely dermal bone,
is well developed, and becomes united with its fellow to form a
furcula (Fig. 53). Amongst the Cursorial Birds, the Emu and
Cassowary possess vestigial clavicles : in the others they are
wanting, and they have also undergone more or less complete
reduction in some Carinate Birds (e.g. certain Parrakeets and
Owls).

Mammals. In Monotremes the pectoral arch retains primi-
tive characters, and in them only amongst Mammals does the
coracoid extend ventrally to reach the sternum (Fig. 103) ; in all
other members of this Class it characteristically becomes reduced, 1
and simply forms a prominent process on the scapula (coracoid

1 In early stages of certain Marsupials (e.g. Trichosaurus), and possibly in
all, the coracoid is well developed and articulates with the sternum, but it sub-
sequently undergoes reduction.

144

COMPARATIVE ANATOMY

process), which is ossified from a separate centre, apparently repre-
senting an epicoracoid, while the coracoid proper may be occa-
sionally indicated by a small centre of ossification on the glenoid
margin of the scapula.

Thus the scapula becomes freer from the rest of the skeleton,
and it alone serves to support the extremity ; it becomes at the
same time greatly broadened, and gives rise on its outer side,
in connection with the highly differentiated mu.scles of the limb,
to a strong ridge (spina scapulae), which extends downwards to
form the so-called acromion. The distal end of the clavicle usually

SC.

St.

FIG. 103. PECTORAL, ARCH AND STERNUM OF OrnithorJiynchus paradoxus.

c 1 , c 1 , c 3 , first, second, and third ribs; d, clavicle; e.c, epicoracoid; e* 1 and es^,
prosternum (episternum) ; m.c, coracoid (metacoracoid) ; m.s, manubrium
stern i ; sc, scapula ; st, sternebra.

becomes connected with the acromion, its proximal end articulat-
ing with the anterior edge of the sternum.

In those Mammals in which the fore-limbs are capable of very
varied and free movements (Lemurs, certain Marsupials, many
Rodents and Insectivores. Bats, and Primates) the clavicles are
strongly developed. 1 In others (e.g. Ungulates, Cetaceans, Carni-
vores, most Edentates, Rodents, Marsupials) they may be en-
tirely wanting or only vestigial, and in the latter case their rela-
tions to the scapula become altered.

1 The clavicle is primarily independent of the coraco-scapular portion of the
pectoral arch. Its original dermal character is retained in Monotremes, but in
all other Mammals it is developed on a cartilaginous basis.

PELVIC ARCH

145

Pelvic Arch.

Fishes. In Cartilaginous Ganoids, indications of a pelvis
are seen, but are very variable, even in individuals of the same
species. They consist of two calcified or ossified pelvic plates,
which correspond to portions segmented off from the basal
cartilage (basipterygium) of the fin. In some cases even this
segmentation does not take place, and thus the pelvis remains

FIG. 104. SIMPLE FORMS OF PELVIS AMONUST FISHES AND AMPHIBIANS.

A, Pleuracanthus the pelvis is here not differentiated from the proximal end
(tf) of the basipterygium ; B, Scapliirhynclms cataphrati H-S ; 0, Polypttrnx
bichir ; D, Neil tint* (Menobranchus). Ap, apophysis of the basipterygium ;
Sas 1 , basipterygium ; Fo, obturator foramen ; P, pelvis ; Had, radii.

undifferentiated. This simple condition is also met with in the
ancient forms Pleuracanthus and Xenacanthus (Fig. 104, A, B).

In Polypterus the pelvis shows some advance on that of
Sturgeons. Owing, doubtless, to the necessity of a firmer connec-
tion of the fin with the body-wall, the two pelvic plates become
united together in the mid-ventral line (Fig. 104, c). In spite,
however, of the rudimentary character of the pelvis of Polypterus,
the essential form of that of the Dipnoi and Amphibia (D) is
already sketched out (for Teleosts, cf. p. 159).

The pelvis of Elasmobranchs consists of a tranverse bar extend-
ing between the two basipterygia, from which it has become

146

COMPARATIVE ANATOMY

segmented off secondarily (Fig. 98) : it is perforated by nerves,
and gives rise on either side to an iliac process (most marked in the

Holocephali) extend-

Cep ing upwards into the

lateral walls of the
body (Fig. 105). A
prepubic process is also
present, and there is
apparently also an in-
dication of a median
epipubic process (cf.
in fret}. The whole
pelvic plate essentially
corresponds, more or
FIG. 105. DIAGRAM OF THE ELASMOBKANCH PELVIS. less completely, with

From the ventral side.

the

of

ischiopubis
Bus, Pro, Rad, basipterygium, propterygium, and higher forms.

radii of the fin ; BP, pelvic plate (ischiopubis) ; J n the Dipnoi the

Oep, epipubic process ; Fo l . obturator foramen ; , -i

cartilaginous

7, iliac process; PP, prepubic process;
region of the ischiopubic symphysis.

fly,

narrow

pelvic plate (Fig. 106)

is provided with a long

and delicate anterior median, a short posterior median process,

and two pairs of lateral processes. Of the latter the anterior

(prepubic processes) are much

longer in Protopterus than in

Ceratodus, and each is embedded
intermuscular septum ;

n an

with the posterior process the
skeleton of the fin is articu-
lated by means of an inter-
mediate piece. The anterior
unpaired process may be looked
upon as an epipubic process,
corresponding with that of Am-
phibia and Amniota (q. v.}. The
posterior or hypoischiatic process
bears a ridge for the attachment
of muscles.

Amphibians. It will be

seen by a glance at Fig. 104 D, Fu! 1()(i _p ELVIS()F Protopfeni ,<. From
that the ventral portion of the
pel vie arch of Necturus is formed
on the same plan as the pelvic

plate Of the Dipnoi and CrOSSO-

.. , , ii TT 11 j

pterygll, but 111 all Lrodela and

Amniota it is perforated by the
obturator nerve. Like the pelvis
of all Vertebrates, it has a paired origin, and in Proteus and

the ventral side.

prepubic p rocess , which may become
forked at its distal end ; b, process to
which the pelvic fin (HE) is attached ;
c, epipubic process; Gfr, ridge tor
attachment of muscles ; M, myotomes ;
M l , intermuscular septa.

PELVIC ARCH

147

ft -\-\-(Crp)

Sllfa)
PP

JP

i '' 1 i :tl i ( '.'| l /''/'"fl' li ni '(

C :>: ,' '<''[';>

I , ).!, ; ;!Y.

V:-W
r !'r ' I US'SSSiG

oV/tf ii' ijiij i .v^vT'v.'v

^ ' ' ... '- %..;"':"

~" ('=*<

fHSy)

--J

Ac, acetabulum ; 6V (*S'y), muscular ridge
on the ventral side of the ischiopubis ;
Fo, Fo l , obturator foramen ; J, J 1 ,
ilium ; JP, JP 1 , ventral pelvic plate
(ischiopubis) ; Lalb, linea alba ; My,
intermuscular septa ; PP, prepubis ; Sy,
S3'inphysis, in which region a strong
tendinous area (SH) exists in Amphiunm,
the pubic regions only coming together in
the middle line at * ;*' (in A), ossified
region of the ischium ; ** (in C and D),
secondary bifurcation of the epipubis ;
z, outgrowth from this bifurcation ; t,
(in C), hypoischiatic process, present in
the l)erotremata and Nee turns ; ft
(Cep), Ep, epipubis.

Fm. 107. PELVIS OK (A Proteus-. (B) Amphiuma; (C) Cryp'olirmwlius; AND
(D) Salamandra mwnloxa. From the ventral side.

L 2

148

COMPARATIVE ANATOMY

Amphiuma this is indicated by the fact that its anterior epipubic
process is paired throughout life (Fig. 107, A, B). In the
Derotrema and Myctodera, the anterior end of the median epi-
pubic process is bifurcated (c, D).

As already indicated, the ischiopubic plate is phylogenetically
the oldest part of the pelvis, and various modifications as regards
the degree of its fusion into a median unpaired plate and of its
ossification occur amongst Amphibians ; the typical triradiate
arrangement of the pelvic bones (ilium, ischinm, and pulis\ such

B

C

FIG. 108. PELVIS OF ANURA. A, Xenopus, from below; B, the same from
the front ; C, Eana i xculenta, from the right side.

Ac, acetabulum ; Cep, epipubic cartilage ; /, ilium ; P (in Xenopus), the
proximal end of the ilium, which is separated from its fellow and from
the pubis by a + -shaped zone of cartilage, f, * ; /*, ischium ; P, pubis (/" in
Kaiia, pubic end of ilium).

as is further differentiated in certain Stegocephali and in Reptiles,
is already sketched out.

One of the most characteristic differences between the pelvis
of Fishes and that of Amphibians is seen in the marked develop-
ment of the iliac region in the latter group. The ilium, like the
scapula, extends upwards in the lateral walls of the body ; and in
Proteus and Amphiuma, owing to the reduction of the limbs in
these forms, does not reach the vertebral column (Fig. 107, A, B).
In all other Amphibia, as in the Amniota, it comes into connection
with the sacrum, owing to the necessity for the hind-limb to act
as a support for the body in terrestrial animals.

The pelvis of the Anura differs from that of Urodela in the
following characteristics. In correspondence with their mode of
progression, the ilium of each side becomes extended so as to form

PELVIC ARCH

149

a long rod (Fig. 108, c) ; and the ischiopubic plate, which in
Urodeles lies in the plane of the abdominal walls, becomes closely
pressed together in the middle line and gives rise to a well-marked
ventral keel : it is not perforated by the obturator nerve. The
pubic region, moreover, though often calcified, is independently
ossified only in the case of Xenopus (Fig. 108, A, B).

Reptiles. The chief characteristics of the Reptilian pelvis as
compared with that of Amphibians consist in: (1) a much more
marked differentiation of the pubis, which is more distinctly
separated from the ischium by an ischiopubic foramen ; (2) the
greater development of the ilium, which is sometimes broadened
out at its vertebral end ; and (3) the more intense and solid
ossification of the arch as a whole.

Points of connection with the pelvis of Amphibians are seen
in certain fossil forms (e.g. Palseohatteria, Plesiosauria), and also

PP

Fo*.

FIG. 109. PELVIC AKCH OF Hatltrin. (After Credner.) From the

ventral side.

Cep, epipubic cartilage ; Fo l , obturator foramen ; /, ilium ; /*, ischium ; P,
pubis ; i>i>, prepubis ; *, hypoischiatic process, which becomes segmented
off from the pelvis in other Reptiles, t, t, ischiopubic foramina.

in Hatteria and the Chelonia. In the Plesiosauria and Hatteria
(Fig. 109) the pubes are not very widely separated from the
ischia, so that the ischiopubic foramina are not so extensive as in
many other Reptiles.

From this condition that seen in the Chelonia, more especially
in Macrochelys and Chelydra, maybe easily derived (Fig. 110, A).
In both cases the epipubis and prepubis are strongly marked. In
other respects there is great variation in the form of the pelvis in
Chelonians, but the obturator and ischiopubic foramina are never
distinct from one another (Fig. 110).

The pelvis of the typical Lacertilia (Fig. Ill) is characterised by

150

COMPARATIVE ANATOMY

a lightness of build. The rod-like pubis and ischium are separated
from one another by a large ischiopubic foramen, and between them
in the middle line is a longitudinal fibre-cartilaginous ligament,
continuous anteriorly with the plug-like epipubic cartilage and
posteriorly with the hypoischium or os doacce (absent in Chame-
leons). This tract represents the remnant of the median ends of

.Is

FIG. 110. PELVIC ARCH OF VARIOUS CHKLOMANS. From the ventral side.
A, J\fr.icrorliff/i/n (after G. Baur) ; B, S}ihar<jii* i-orinwa (after Hoffmann) ;
C, T(*tnrlo ; D, Chelone.

Ceft, epipubic cartilage ; Fopi. ischiopubic foramen ; Jfpltj, hypoischiatic process ;
Is, ischium ; P, pubis ; PP, prepubis.

the pubis and ischium which are present in the embryo ; and thus
in this, as in certain other respects, the pelvis of the Lacertilia
may be said to pass through a Hatteria-like stage in the course of
development. The ilium in some cases is almost vertical in posi-
tion : in others it is more oblique, sloping upwards and backwards
from the acetabulum.

PELVIC ARCH

151

In Snake-like Lizards the pelvis undergoes degeneration, and
in Amphisbsenians vestiges merely of the ilium and pubis are
present ; in certain Snakes vestiges of a pubis alone occur.

While the pelvis of Lizards and Chelonians show a certain
amount of similarity, that of Crocodiles exhibits special character-
istics and is of particular interest, as in some points it resembles
that of certain extinct forms. The pubes, which have at first a
transverse position, become later directed forwards much more
markedly than in Chelonians and Lizards, and thus the ischio-
pubic foramina (in which the obturator foramina are included) are
very wide, and are separated from one another by a fibrous cord

FIG. 111. PELVIS OF Lacerfa viripara. From the ventral side.

Ac, acetabulum, in which the three pelvic bones come together ; Cep, epipubis,
composed of calcified cartilage ; Fo l , obturator foramen ; Hpls, hypoischium,
which becomes segmented off from the hinder ends of the ischia in the
embryo as a paired structure ; /, ilium, with its small preacetabular process
tf, much more strongly developed in Crocodiles, Dinosaurians and Birds ;
Is, Ischium, forming a symphysis at 81s ; Lg, fibrous ligament ; P, pubis ;
PP, prepubis.

(Fig. 112). All three elements chondrify independently, and then
unite in the acetabulum, which is perforated. The pubic bone
becomes shut out from the acetabulum by a cartilaginous pars
acetabularis, not represented in lower Vertebrates, formed from the
acetabular process of the ilium. The epipubis is contained in a
cartilaginous apophysis at the anterior (distal) end of the pubis,
which does not, like the ischium, unite with its fellow in a
symphysis : there is no hypoischium.

The ilium becomes greatly broadened out in the antero-
posterior direction dorsally, where it is attached to the sacrum ; a
similar extension of the ilium occurs still more markedly in the
Theromorpha, Dinosauria, and Birds (Fig. 113), in which it

152

COMPARATIVE ANATOMY

becomes connected with an increasing number of vertebrae, and so
forms a firmer support to the body.

Birds. The pelvis of Birds is chiefly characterised by the
relatively large development of the iliac region and by the position
of the delicate pubis, which in the course of development becomes

FIG. 112. PELVIS OF A YOUNG Alligator lurittx. A, ventral, and B,

lateral view.

B, fibrous band between the pubis and symphysis ischii ; /<, foramen in the
acetabulum, bounded posteriorly by the two processes, and b, of the ilium
and ischium respectively ; F, ischiopubic foramen ; G, acetabulum ; 77, ilium ;
/S ischium ; M, fibrous membrane extending between the anterior margin of
the pubis and the last pair of "abdominal ribs" (.#/'); P, pubis ; Xy.
symphysis of ischium ; *, indication of a forward growth of the ilium, such
as is met with in Dinosaurians and Birds ; f, pars acetabularis, which is
interposed between the process a of the ilium and the pubis ; /, //, first and
second sacral vertebra?.

directed backwards, parallel to the ischium and postacetabular
process of the ilium. The preacetabular portion of the ilium
extends forward for a considerable distance, and a number of
vertebrae belonging to other than the true s;icral region become-
secondarily connected with the ilium (cf. p. 60). The acetabulum
is perforated, and the pars acetabularis forms a pectineal pro-
cess which is retained in the adult in Apteryx (Fig. 113). The

PELVIC ARCH

153

elements of the pelvis usually become ankylosed. The pubis actually
meets its fellow in the middle line only in Struthio, and the ischium

i7

Fir;. 113. PELVIS OF Aptery.c au>slralis. Lateral view. (After Marsh.)

a, acetabulum ; if, ilium ; in, ischium ; p, pectineal process from the pars

acetabularis ; p*, pubis.

(dorsally) only in Rhea. A process given off from the posterior end
of the pubis in the Emu, and extending forwards, may represent an
epipubis.

In Archseopteryx, all the elements of the pelvis were
independent and relatively small, the ilium coming into relation

FIG. 114. RIGHT HALF OF THE
HUMAN PELVIS. LATERAL VIEW.

Fo, obturator foramen ; the three
bones ilium (//), ischium (/.$), and
pubis (P) are shown distinct from
one another in the acetabulum.

J

Fi<;. 115. DIAGRAM SHOWING THE
RELATIONS OF THE PARS ACE-
TAPULAKIS in Viverra ciretta.

A, acetabular bone ; Ar, aoetabuluni ;
-/, ilium ; /<, ischium ; P, pubis.

with about six vertebras only, and the pubis and ischium being
less backwardly directed than in recent Birds.

154

COMPARATIVE ANATOMY

Mammals. The ilium and ischium of Mammals, like those
of the Anura and Sauropsida, are respectively preacetabular and
postacetabular in position, and the elements of the pelvis remain
separated for a long time by cartilage, but later become fused
(Fig. 114). The pubis always takes less part in the formation of
the acetabulum than do the other two bones, and may be more or
less entirely shut out from it by an ossification of the pars aceta-
bularis, which subsequently unites with either the ilium, ischium,
or pubis (Fig. 115). This acctabular bone is especially well

Tlll.il).

B

FIG. 116. PELVIS OF A, Echidna hystrix (ADULT), AND B, Didelphya azarce
5 '5 CM. IN LENGTH). From the ventral side.

Ep, epipubis (marsupial bone) ; Fobt, obturator foramen ; J, ilium ; J.y,

ischium ; LIJ and Lyf, ligament between the pubis and epipubis ; P, pubis ;

Sy, ischiopubic symphysis ; Tub.il.p, iliopectineal tubercle; **, cartila-

ginous apophysis at the anterior end of the epipubis.
In Fig. A, GH, articulation between the pubis and epipubis ; Tb, cartilaginous

tuber ischii ; Z, process on the anterior border of the pubis ; t*, t, tt, ilio-

and ischio-pubic sutures.
In Fig B, b, b 1 , cartilaginous base of the epipubis, continuous with the inter-

pubic cartilage at t ; *, *t, ischio-pubic and ischio-iliac sutures.

developed in the Mole, in which it shuts the ilium, as well as the
pubis, out of the acetabulum ; in Monotremes the acetabulum is
perforated. The angle between the axes of the ilium and sacrum
is largest in Ornithorhynchus, and most acute in Rodents ; the
ilium is connected with a varied number of vertebrae in the
different forms.

The original type with both pubic and ischiatic symphyses,
indicating an elongated form of pelvis, is seen in Monotremes,
Marsupials (Fig. 116), many Rodents, Insectivores, and Ungulates.
In many other Insectivores, in Carnivores, and more particularly

FINS 155

in the Primates, the ischia no longer meet below, and the broaden-
ing of the ilia seen in the higher forms of the last named Order
culminates in Man. The greatest amount of variety in the form
of the pelvis in any one order with typical appendages is seen in
Irisectivores, in some of which (e.g. Mole, Shrew), as well as in
most Bats, there is no symphysis pubis, so that the relatively small
pelvic cavity is not enclosed ventrally by bone. The obturator
foramen is always surrounded by bone.

In the Cetacea, in which hind limbs are wanting, paired
vestiges of the ischiopubic region of the pelvis are present : they
are unconnected with one another and with the vertebral column.
In the Sirenia.a paired bony rod (Manatus) or plate (Halicore) repre-
sents the last vestige of an ilium, in which an ischium is included
in the latter genus.

In Monotremes and Marsupials of both sexes, two strong so-
called "marsupial bones" (Fig. 116) arise from the anterior
border of the pubes, right and left of the middle line, and extend
forward in a stright or oblique direction embedded in the body-
walls, serving for the attachment of muscles. They form an
integral part of the pelvis, and in the embryo are seen to be in
direct connection with its cartilaginous symphysis (Fig. 116, B) ;
but later on definite articulations are formed between them and
the pubes (A). It is not improbable that these structures are
the homologues of the epipubis of lower Vertebrates, which has
been retained in non-placental Mammals in order to serve as a
support for the abdominal walls in connection with the marsupial
pouch.

PAIRED FINS OF FISHES.
Fishes.

The development of the extremities has already been alluded
to (p. 137). The pelvic fin usually retains a simpler and more
primitive form than the pectoral fin.

Elasniobmnchs. The cartilaginous skeleton of the fins is the
most richly segmented in these Fishes. There are usually two main
elements (basalia)in the pelvic fin which articulate with the arch and
with which a variable number of segmented rays are connected, the
latter passing towards the periphery of the fin (Fig. 117). Both
the larger, posterior lasipterygium or rnetaptcryyiuiii, and the
smaller, inconstant propterygium must be looked upon as originat-
ing phylogenetically by a fusion of the proximal ends of the primary
cartilaginous rays of the fin ; and the form and relations of these
main elements vary according to the degree in which such a fusion

156

COMPARATIVE ANATOMY

has taken place. 1 This is also true as regards the pectoral fin, in
which an additional basal piece, or mesopterygium, is usually present
(Fig. 99) : there may even be four basalia. These complications
arise in connection with the greater importance of the pectoral
fin as an organ of locomotion. The distal portions of both fins
are supported by horny fibres (cf. note on p. 137). With the

a,

Raff

FIG. 117. RIGHT PELVIC FIN OF
Heptanckus. From the ventral
side.

BP, pelvic plate ; Fo l , f, nerve-for-
amina ; Pr, propterygium ; 1'ml,
radii, which show secondary seg-
mentation ; S /'(!,, basi- or meta-
pterygium.

FIG. 118. PECTORAL Fix OF Cera-
todus fosteri.

a, 1>, the two first segments of the
main axial ray ; FS, dermal rays,
shown only on one side ; f, f,
lateral rays.

exception of one or at most of very few all the rays are
situated on the same side of the basalia (uniscrial type).

In Rays, the propterygium of the pectoral fin, and usually also the
metapterygium, are strongly developed, the former extending far
forwards so as to be connected with the skull by ligament, and in
some cases even uniting with its fellow in front of the skull.

Dipnoans. The cartilaginous pectoral and pelvic fins are here

1 In male Elasmobranchii a number of pieces of cartilage are connected witli
the distal snd of the basipterygium of the pelvic iin as a support for the
copulatory organs or claspers (q. n. ) : these may become more or less calcified.

FINS

157

also essentially similar to one another, the latter being rather
the simpler of the two. From a segmented main ray or axis a
number of segmented secondary rays arise on either side in
Ceratodus: these are not, however, strictly symmetrical (Fig. 118).
Beyond them dermal rays are present (p. 137). A proximal
(basal) segment of the axis, which bears no rays, articulates with
the arch. In Protopterus and Lepidosiren the fins, with their
skeleton, have undergone a marked reduction, so that little more
than the segmented axis remains.

Thus the fins of Dipnoans differ from those of most Elasmo-
branchs (as well as of Teleostomes) in being formed on a biserial
type, indications of which are, however, as already stated,

FKJ. 119. RIGHT PELVIC FIN OF A Y/orxo Pol yodon folium.
From the dorsal side.

F8, bony dermal rays ; M, metapterygium ; Pru, uncinate ("iliac") processes;
Ra, Ha}, radii of the first and second orders.

seen in the embryos and adults of certain Elasmobranchs.
Physiologically, the Dipnoan fin, like that of the young Polypterus,
serves not merely as a swimming organ, but also to support the
body when the animal is resting on the bottom, as do the limbs of
a Urodele.

Ganoids. The skeleton of the fin is much simpler and the
primary rays much fewer in number in Ganoids than in Elasmo-
branchs. This is, however, compensated for by the formation of
secondary dermal bony structures, as in the case of the pectoral
arch and skull : these arise on either side of the fin and may or
may not be segmented: they are always more strongly developed on
the anterior than on the posterior border of the fin. The most

158

COMPARATIVE ANATOMY

anterior or marginal ray comes into close connection with the
cartilage of the primary fin-skeleton (Sturgeons) or entirely
replaces it (Amia).

In the pelvic fin of cartilaginous Ganoids (Fig. 119) more or
fewer of the radii are connected proximally with a segmented
basale, which is perforated by nerves, and from which a very
primitive pelvic plate may in some cases become differentiated
(Fig. 104, B). It is important to bear in mind that the distinction
between an axis and secondary rays cannot, therefore, be strictly
recognised, as the basale corresponds to a number of fused radii,
and is perhaps not comparable to the metapterygium of Elasmo-

A

FIG. 120. LEFT PECTORAL FIN OF A, Polyodon, AND B, Amia.

a ff, radii which do not reach the arch and are connected with the most
posterior ray (IV. in A, ///. in B) ; 7 IV, cartilaginous radii connected
with the arch (S') ; KS, bony dermal rays.

branchs : but it is doubtful whether this character is primitive or
secondary.

The primitive relations have to a certain extent disappeared in
the pectoral fin of cartilaginous Ganoids, which, however, also
consists of a varied number of rays. Of these, four reach the arch
in Polyodon (Fig. 120, A), and five in Acipenser.

In the pectoral fin of Amia (Fig. 120, B) two large converging-
marginal rays articulate with the shoulder-girdle, and only one
intermediate ray reaches the arch : this condition may be compared
with that seen in the highly-developed pectoral fin of Polypterus
(Fig. 121), which is flanked on either side by a strong, ossified ray,
between which is an intermediate region. The fin, therefore,
resembles that of an Elasinobranch Avith its propterygium, meso-
pterygium, and metapterygium. 1

1 Even if it should be proved that the intermediate region (MS) no longer
arises in the embryo by a fusion of separate rays, it is possible that this was the
ease phylogenetically.

LIMBS

159

Oss

FIG. 121. PECTORAI, FIN OF
Polypterus.

The form of the pelvic fin in Polypterus and other bony
Ganoids may be easily derived from that seen in the cartilaginous
representatives of this order, and it
may be assumed that the basale is -F

due to the concrescence of a larger
number of separate radii, which are,
therefore, much less numerous than
in the Sturgeons (Fig. 104). Bony
rays support the distal part of both
pairs of fins (p. 137).

Tcleosts. A still further reduc-
tion has taken place in the primitive
skeleton of the paired fins in Tele-
osts, there being at most only a few
radials articulating with the arch
(Tig. 100), and even these (especi-
ally in the case of the pelvic fin, in
which the arch is usually considered
to be undifferentiated), may be want-
ing. The main part of each fin is
supported by bony rays, as in Ganoids.
The skeleton of the fins of Siluroids,
Cyprinoids, and Gymnotidse conies

nearest to that of Ganoids.

FS, bony dermal rays ; Nf, nerve

foramina ; 0**, centre of ossi-
fication in MS ; Pr, Mt, bony
marginal rays, which meet at f,
so that the intermediate region
(MS) does not reach the arch ;
Ra, JRa 1 , radii.

Though it is possible to derive

the skeleton of the fin of all the Orders of Fishes from a single
ground -type, it is a far more difficult task to trace the connection
of the latter with the extremities of Amphibia and Amniota.
Between these two types of extremity there seems to be a wide
gap, in consequence of the different conditions of life existing
between aquatic and terrestrial Vertebrates. We do not know
how the pentadactyle limb of an air-breathing Vertebrate (cluiro-
pterygium), adapted for progression upon land, has been derived
from the fin (icMhyopterygimn\ only fitted for use in the water,
and Paleontology has so far furnished no solution to this problem.

There is, however, a certain amount of probability in the view
that the cheiropterygium has arisen from such an ichthyopterygium
as that seen in cartilaginous Fishes, although it is quite un-
certain as to how far the individual parts are comparable to one
another (Fig. 122), and how the fin, which is practically a single-
jointed lever, amply sufficient for the movement of the body in a
fluid medium, became gradually transformed into a many-jointed
system of levers.

PAIRED LIMBS OF THE HIGHER
VERTEBRATA.

160

COMPARATIVE ANATOMY

As the function of the limb is now no longer simply to propel
the body, but also to lift it up from the ground, the firmly con-
nected elements of its skeleton are placed at an angle to one
another (elbow and knee, in which the angle is directed backwards
and forwards respectively), definite articulations being formed
between them in a proximo-distal direction. The fore-limb serves
in typical cases mainly for pulling and the hind-limb for pushing
the body along the ground, and on this fact depend the various
differences between the two as regards their relation as a whole to

the trunk and of their various parts
to one another. Instead of project-
ing horizontally outwards, the limb
extends downwards, and thus the
angle between it and the median
plane of the trunk is gradually re-
duced, until in Mammals eventually,
the longitudinal axis of the limb,
when at rest, is parallel with the
median plane of the body. In the
higher types this is more particularly
the case as regards the posterior
THE extremities, the anterior limbs under-
THE going the most varied adaptative
modifications, and giving rise to
prehensile or to flying organs or,
as in aquatic Mammals, becoming
once more converted into paddles.
The fore-limbs and hind-limbs of all
Vertebrates above Fishes may, how-
ever, be reduced to a single ground- type.

A division into four principal sections can always be recog-
nised : in the case of the fore- limb these are spoken of as upper
arm (Irachium), fore-arm (antibrachiuni), wrist (carpus), and hand
(mantis') ; and in the hind-limb as thigh (femur}, shank (cms),
ankle (tarsus), and foot (pcs) (Fig. 123). The bone of the upper
arm (humerus'), like that of the thigh (femur) is always unpaired,
but two bones are present in the fore-arm and shank. The former
are called radius and ulna, and the latter tibia and fibula. The
hand and foot are also respectively divisible into two sections, a
proximal metacarpus and metatarsus, and a distal series of
phalanges, which form the skeleton of the fingers and toes (digits).
Both manus and pes are made up of several series of cylin-
drical bones. There are never more than five complete series,
which except as regards number present essentially similar
primary relations throughout the higher Vertebrates. The
skeleton of the carpus and tarsus, each of which typically consists
of a series of small cartilages or bones, shows much variation ; but
the following arrangement may be taken as typical (Fig. 123).

FIG. 122. DIAGRAMMATIC FIGURES
TO SHOW THE RELATIONS OF
ANTERIOR EXTREMITY TO
TRUNK IN FISHES (A), AND THE
HIGHER VERTEBRATES (B).

Mt, metapterygium ; I'd, radialia
in A, radius in B ; S, pectoral
arch : Ul, ulna ; proximally to Ul
and Ed is the humerus.

arrangement

LIMBS

161

Fe-

FL

Round a centra /e, which may be double, is arranged a series of
other elements, of which three are proximal, and a varying number
(four to six) distal. The proximal, in correspondence with their
relations to the bones of the fore-arm and shank respectively, are
spoken of as radiale or tili'tl<', ulnare or fibula re, and intermedium ;
while the distal are called carpalia or tarsalia (in the narrower
sense). They are counted
beginning from the pre-
axial (radial or tibial) side
of the limb.

Amphibians. The
anterior and posterior
extremities of Urodela
are formed essentially on
the ground-plan described
above, but more or fewer
of the carpals or tarsals
may undergo fusion. In
them, as in Anura, there
are five digits in the
hind-limb, and usually
only four in the fore-
limb. In the Anura the
radius and ulna become
united, and a separate
intermedium is not re-
cognisable; the proximal
row of the tarsus, more-
over, consists of only two
cylindrical bones, one of
which (astragalus) corre- Fl(1
sponds to a tibiale, and
the other (cakaneum) to %, + digits ^/V-.^femur; Fi^, fibula;

a fibulare (Fig. 124).

In the distal row of
the carpus four separate
elements are formed in

Anura, but this number may become reduced owing to secondary
fusion ; in rare cases a fifth carpal may also be present. Tarsalia
// and /// are the most constant elements, but even these may
undergo fusion, and tarsalia /Fand Fare generally represented
by a ligament. 1

In Anura the metat:\rsals and phalanges, between which the
web of the foot is stretched, are, like the proximal tarsals, very
long and slender. The femur, as well as the fused bones of the

1 Very different views are held with regard to the homologies of the
individual carpals and tarsals in Amphibians, and the older numbering and
nomenclature are therefore provisionally retained here.

M

^

1-23. HIND LIMB OF A URODKLE (Spderpes
fuscus).

, meta-

tarsals (7 V) : Ph, phalanges ; T, tibia ;
Ta, tarsus, consisting of c, centrale ; f,
fibulare ; i, intermedium ; t, tibiale ; and
1 5, distal tarsalia.

162

COMPARATIVE ANATOMY

shank, are also exceedingly long, in correlation with the mode of
progression of these animals. The skeleton of the extremities
is more strongly ossified in Anurans than in Urodeles, in which
many of the elements remain cartilaginous.

Traces of an extra element (" prehallux"') occur on the tibial
side of the tarsus, and in both Urodeles and Anurans indications

B

. Twicde

; Tui'xiile

Tui-xule I

Centrals

Groove be-

t if I 1,1 ,'llllill.1
Hill/ I'/llll

Ulna

UlHfl,:

Car pal t
III

Fir;. 124. A, RIUHT FORE-ARM AND HAND, AND B, RICHT FOOT, OF
I'tina escitlenta. From the dorsal side, x 2. After E. Glaupp.

of an additional pre-axial ray in the manus are occasionally met
with. The number of phalanges in the individual digits varies in
different Amphibians.

Vestiges of the extremities can be recognised externally in
embryos of the limbless Gymnophiona.

LIMBS

163

Reptiles. In existing Reptiles as a general rule the
body is only slightly raised from the ground in locomotion., but
in some the limbs serve as more highly organised organs of
support, and in certain of the Dinosauria the hind limbs were the
main organs of progression. The fore limbs in such cases tend to
take on other functions, and in the Hying Pterosauria the fifth
finger was produced into a long, jointed rod which supported a
wing-like expansion of the integument.

Chelonians, and more particularly Hatteria, come nearest to
the Urodeles in the structure of the carpus. 1 Five digits are
usually present in Reptiles in both manus and pes, and traces also
of the former possession of an extra ray both on the radial and

7

Fie. 125. CARPUS OF A, Hatteria jjtmrfata, AND B, Emydnru krf/t'ti.

(After Baur.)

c 1 , radial cent rale ; c 2 , ulnar centrale ; i, intermedium ; p, ulnar sesamoid
(pisiform) ; /?, radius ; r, radiale ; U, ulna; , ulnare ; 15, carpalia ; /--T,
metacarpals.

ulnar side ("pisiform") can usually be recognised (Figs. 125-
130). The tibia and fibula always remain separate.

In Lizards and Crocodiles the carpus and tarsus diverge more
from the primitive form. In the latter, which, like Anurans,
possess no trace of an intermedium, the proximal row of the
carpus consists of two hour-glass-shaped bones a larger radiale,
and a smaller ulnare (Fig. 128). The centrale, as in An ura, comes
to be situated in the distal row, which, like the fourth and fifth
digits, is much reduced.

In Ichthyosaurus and Plesiosaurus the limbs were modified to

1 In Hatteria and certain Chelonians, as well as in the extinct Protcrosaurus,
a double centrale is present in the carpus, and more or less distinct traces of a
double condition of this element are seen in certain other Chelonians. Indica-
tions even of a third centrale occur in Hatteria.

M 2

164

COMPARATIVE ANATOMY

n >

v

FIG. 126. RIGHT CARPUS OF Emy*
From above.

i, intermedium ; 7?, radius ; r.c, fused
radiale and centrale ; U, ulna ; n,
ulnare ; t and *, elements on the
radial and ulnar side respectively,
indications of additional radial and
ulnar (pisiform 1 ! rays ; 1--5, the
carpalia, of which 4 and 5 are
fused ; / V, metacarpals.

FIG. 127. LEFT CARPUS OK Lm-i rln
agilis. From above.

c, centrale ; i, intermedium ; li,
radius ; r, radiale, formed by the
fusion of two elements, one of
which corresponds to a prepollex ;
U, ulna ; u, ulnare ; t, pisiform ;
1 5, carpalia; / V, the meta-
carpals.

form paddles: the radius and ulna were very short, and there were
numerous phalanges l (cf. Cetacea), additional rays being present
in the former genus.

Amongst the snake-like kinds of Lizards, various degrees of

JT

u

FIG. 128. RIGHT CARPUS OF A YOUNG
Afliynfor lui-iitx. From above.

C, centrale ; R, radius ; r, radiale ;
U, ulna ; 11, ulnare ; t, pisiform ;
1 to 5, the five carpalia, as yet un-
ossified, of which 1 and 2, as well
as 3, 4, and 5, have become fused ;
7 V, metacarpals.

FIG. 129. RIGHT TARSUS OF Emy
enropwa. From above.

F, fibula ; (i\f.f.f, the fused inter-
medium^), fibulare, tibiale, and
centrale ; Ph l , phalanx of 1st digit ;
T, tibia ; 1 4, distal tarsals ; / I',
metatarsals.

reduction of the extremities occur, and in such forms as Anguis
and Amphisba^na they have practically disappeared entirely, as in

1 An indication of this condition is seen in the embryo Crocodile.

LIMBS

165

most Snakes. In certain of the latter, howevei, traces of the
hind limbs exist (e.g. Python).

The tibia gradually becomes of relatively greater size than the
fibula in the reptilian series. The tarsus always undergoes a
marked reduction, especially in its proximal portion, and gradually
leads to the type seen in Birds. Thus in Chelonians and Lizards
(Figs. 129 and 130) the proximal tarsals may all run together into
a single mass, which in the former corresponds to the tibiale,
intermedium, fibulare, and centrale. In Lizards a centrale can
no longer be recognised, even in the embryo, and there is no distinct
trace of an intermedium. In the distal row three or four separate
tarsals are developed, but these may unite with one another
to a greater or less extent, and there is an increasing tendency

J7T

FIG. 130. RIGHT TARSUS OF LacerUt
II fit/ it. From above.

F, fibula; T, tibia; f.f.i.c, fused
tibiale, intermedium, fibulare, and
centrale ; t, trace of a 6th ray
present in Geckos ; 3 5, distal
tarsals ; / V, metatarsals.

FIG. 131. RIGHT TARSUS OF CROCO-
DILE. From above.

F, fibula ; f, fibulare (calcaneum) ;
T, tibia ; t,i,c, astragalus, corre-
sponding to fused tibiale, inter-
medium, and centrale ; 1, 2, 3.
fused 1st 3rd distal tarsals ; 4, 4th
distal tarsal ; / IV, metatarsals ;
F?, ath tarsal and metatarsal.

for the movement of the foot to take place by means of an inter -
tarsal articulation, as in the Dinosauria and also in Birds.

In Crocodiles (Fig. 131) there are two bones in the proximal
row of the tarsus, one of which corresponds to a tibiale, inter-
medium, and centrale, the other to a fibulare. The former is
spoken of as the astragalus, the latter as the calcaneum, and on it
a definite heel (calcaneal process) is seen for the first time in the
animal series. The distal row consists originally of four small
cartilages, but these later undergo a partial fusion.

Birds. In consequence of the fore limb of Birds having
become adapted for Might, the manus loses its primitive character
and undergoes reduction, while the humerus and the bones
of the fore arm more particularly the ulna, as well as the
entire pectoral arch and sternum, are extraordinarily developed,

166

COMPARATIVE ANATOMY

the wings in good fliers being considerably longer than the legs,
which alone bear the entire weight of the body when on the
ground (Fig. 132). In Cursorial Birds (Ratitse), however, the wing
has undergone regressive changes in connection with their habits,

ScJi.

-MF

- Z'

FlG. 132. -SKELKTiiN OF THK LtMB.S AND TAIL OV A C'ARINATE BlKU. (The

skeleton of the body is indicated by dotted lines.)

F, digits ; Fl, fibula ; H W, carpus ; MF, tarsometatarsus ; MH, carpometa-
carpus ; OA, humerus ; OS, femur ; Py, pygostyle ; #, coracoid ; Rd, ulna ;
Sch, scapula ; >SV, sternum, with its keel (Cr) ; T, tibiotarsus ; //, radius ;
~ ] , :., digits.

and in the extinct New Zealand Moa (J)inornis) no trace of it
has been found : in Penguins it serves as a paddle.

The relation of the superficial surface of the wings to the
weight of the body is far from constant, and depends largely on
the relative power of flight ; on the whole, the wings are relatively
largest in small, light Birds than in large, heavy ones.

LIMBS

167

In the carpus, at least seven elements are recognisable. In
the proximal row is an intermeclio-radiale and a centro-ulnare,
each of which consists of two parts in the embryo. In the distal
row there are also two elements, one of which (carpale 2 + 3) is
evidently primarily double : the other corresponds to carpale 4.
In early stages four distinct metacarpals can be seen, and these

-Klaue

Klaue

B

FIG. 133. CARPUS OF EMBRYO OF Sterna wilaoni. A, STAGK AT WHICH
OSSIFICATION BEGINS ; B, STAGE IMMEDIATELY T.EFORE HATCHING. (After
V. L. Leighton. )

r, distal carpals ; Ktaite, claw ; Had, radius ; rad, intermedio-radiale : Ufn,
ulna ; -tiln, centro-ulnare ; II V, metacarpals (in B, / V and V have become
fused).

seem to correspond to the 2nd-5th rather than to the lst-4th : the
5th metacarpal soon fuses with the 4th (Fig, 133).

The distal carpals become fused with the corresponding meta-
carpals, thus forming a carpomeiaearpus (Figs. 132, 133), and in
the adult only the two proximal elements remain separate as a
radiale and an ulnare. The three metacarpals themselves become
united proximally, and the second ('///) and third (IV) distally:
they only bear a limited number of phalanges at their free ends.

Claws were present on the terminal phalanges of all three
digits in Archaeopteryx (Fig. 49). In certain recent adult Birds
(e.g, Chauna) the first digit (//) bears a claw, and more rarely

168 COMPARATIVE ANATOMY

(Ratitse) the second (HI], and even the third (IV") also (e.g.
Struthio). Claws may be present in the young only (e.g. Opistho-
comus, Sterna, Fig. 133).

The tarsus is still more reduced in Birds than in Reptiles, and
consists in the embryo of three elements, two small proximal and
a broader distal, which in many cases (e.g. Penguin) consists
primarily of four distinct pieces. The former (tibiale and fibulare)
unite later with the distal end of the tibia, thus forming a tibiotarsus,
while the latter, which corresponds to tarsalia / to V, becomes
included in the base of the metatarsus. Thus the foot of adult
Birds no longer possesses any distinct tarsal elements, though, as
in Chelonians and Lizards, it really moves by an intertarsal
articulation. Of the original five metatarsals, the fifth soon dis-
appears, while the second, third, and fourth become united with
one another and with the distal element of the tarsus to form a
single bone, the tarsometatarsus (Fig. 132), grooves at the ends
of which indicate its compound nature, which is especially well
seen in Penguins. The first metatarsal remains to a greater
or less extent independent.

The number of toes varies between two (Struthio) and four :
that of the phalanges is normally 2, 3, 4, 5, reckoning from the
first to the fourth digit. The tibia, even from the first, greatly
exceeds the splint-like fibula in size, and the two bones become
united distally. 1

Mammals. In Mammals the anterior extremity either
remains in the condition of a simple organ of locomotion, serving
for progression on land, or it may give rise to a digging or a
prehensile organ ; or, again, may become modified in adaptation to
an aerial (Bats) or aquatic (Pinnipedia, Cetacea, Sirenia) mode
of life.

The humerus, which may possess a supracondyloid foramen near
its distal end, is variously modified as regards form and relative
length and the presence of ridges and elevations for the insertion
of muscles ; and the same is true as regards the femur and its
ridges or trochanters.

The tibia is the more important bone of the shank, and
the fibula often becomes united with it to a greater or less
extent distally and sometimes proximally also, usually taking
no part in the knee joint. The two shank-bones lie parallel,
and are at most very slightly movable on one another (e.g.
climbing Marsupials). The fibula never disappears entirely,
but in some cases (Bats, Ruminants) only its distal end is
recognisable as the lateral (external) malleolns.

The radius and ulna are connected with the humerus by a
hinge-joint at the elbow, only allowing movement in one plane,
and primarily their relations to one another are similar to those
of the tibia and fibula. This is the case in Monotremes and in

1 For the pneumatic character of Birds' bones, of, under Air-sacs,

LIMBS 169

all Mammals in which the radius is fixed in a position of pronation
(vide infra). In certain Mammals --more particularly the
Primates, in which the fore limb is prehensile, the bones of the
forearm, instead of being firmly connected together, articulate
with one another, the distal end of the radius being capable ot
rotation round the ulna. When the two bones lie parallel and the
wrist is not bent, the palmar surface of the manus looks inwards,
and when rotated on one another towards the body, backwards : the
former position is spoken of as that of supination, the latter that
of pronation. Indications of these movements are seen even in
climbing Marsupials. 1 The radius is the more important in
supporting the hand, while the ulna forms the chief connection
with the humerus. The ulna extends proximally beyond the
elbow joint as the olecranon, on which the extensor muscles are
inserted. The ulna may undergo more or less reduction and fusion
with the radius, so that in some cases only the olecranon is
distinguishable.

In addition to the power of rotation of the forearm, the
Prosimii and Primates proper are characterised by a higher
differentiation of the first finger (pollex), which becomes more
independent and is capable, not only of abduction and adduction,
but also of being brought into opposition with the palm of the
hand to a greater or less extent. As regards the pes, the hallux
even in Marsupials may be opposable, but never as markedly so
as in Lemurs and Monkeys, which are often spoken of as
Quadrumana. 2

A brief account of the mammalian carpus and tarsus must
suffice in this place, as considerable differences exist in the various
groups, and there is no consensus of opinion as regards the
homologies of the various components.

The carpus and tarsus most nearly correspond with those of
Urocleles, Hatteria and Chelonians. Primarily the centrale can be

1 The rotation of the radius on the ulna has doubtless come about largely
owing to the gradual increased differentiation of the muscles during phylogeny ;
but this does not sufficiently account for the different relative positions, of the
two bones of the fore-arm and shank respectively. The tibia lies on the inner
side of the shank, while the corresponding bone of the fore-arm, the radius,
owing to secondary shifting, is external when in the position of supination. The
reason of this cannot be due to a rotation of the distal end of the humerus, for
even in Amphibians the same conditions are plainly seen. The crossing of radius
and ulna has rather resulted in consequence of the manus becoming rotated in
a contrary direction to that of the limb as a whole as it extends inwards towards
the body in order to act as a support for the latter. Consequent!}', the originally
parallel position of the two bones of the forearm is not retained, as it is in the
case of those of the shank, in which the rotation follows the same direction as
that of the entire limb.

2 In the Marmosets (Arctopithecini) the thumb is not opposable, and the
opposable hallux is the only digit which bears a flat nail, all the others having
claws. In Ateles the pollex is vestigial and possesses only a single small phalanx,
while in Colobus it may even be wanting. In consequence of the erect position
of Man, and of the foot being used merely as an organ of support and loco-
motion, the prehensile character of the pes has become lost,

170 COMPARATIVE ANATOMY

recognised as a typical element in all pentadactyle Mammals ; but
as a rule it later becomes fused with one, or even with two, of the
neighbouring carpals generally with the radiale, less frequently
with carpal e 2 or 3. Occasionally indications of a second centrale
are seen, which usually fuses with the intermedium (Homo).
Similar fusion and shifting in relative position may also occur
in other carpals and tarsals (e.g. radiale and intermedium).
The " pisiform " corresponds to an additional ulnar ray, and not
to a sesamoid.

In the tarsus the centrale (navicular) is retained, and is usually
situated on the inner (tibial) border : it may be primarily double.
The astragalus possibly corresponds to the tibiale and intermedium,
and the calcaneum to the fibulare, while the ciiboid represents
tarsalia 4 and 5.

Traces of a " prepollex " and " prehallux " are present in all
pentadacyle Mammals, especially in lower forms, in which they may
each consist of two or more elements : in the higher Mammals
there is never more than one such bone, which usually becomes
fused with its neighbours. 1

There are typically five complete digits on each foot, but this
number may be reduced, the disappearance taking place in the
following order, 1, 5, 2, 4 : thus in the Horse the third is the only
complete digit remaining (Fig. 134). The number of phalanges
is similar in both hand and foot : in the first digit there are only
two, while in the others there are three. An exception to this
rule is seen in Cetacea, in which the phalanges are numerous.
The short humerus is enclosed in the body-wall in Toothed
Whales, which possess five digits, the fourth of which commonly
bifurcates in the embryo ; Whalebone-Whales possess only four
digits.

It is interesting to trace the reduction which has taken place
in the feet of the true Ungulates in the course of time. This
order has been undoubtedly derived from that of the Carnivora,
the fossil Condylarthra from the American Eocene and the tri-
tubercular Creodonta from the Cretaceous forming connecting
links between the two. In the Eocene, the Ungulata vera
diverged into two groups, the Perissodactyla (Tapir, Rhinoceros,
Horse) and Artiodactyla (Pigs, Hippopotami, Ruminants). In
Fig. 134 sketches of the stages in the phylogenetic development
of the fore-foot of the Horse are given, showing how it has been
gradually derived from a tetradactyle form : the embryo passes
through these stages in the course <>f its development. While in
this case the third digit becomes greatly enlarged relatively

1 Different views have been expressed as to the morphological nature of the
prepollex and prehallux, which m consequence of functional adaptation may
undergo further development in some Mammals (e.g. Talpa). It is not possible
in all cases to make a satisfactory comparison between individual elements of
the carpus and tarsus, or tohomologise the so-called "accessory elements."

LIMBS

171

(verissodactylc form}, 1 and eventually is the only complete one
remaining, in cloven-footed Ungulates the third and fourth digits

8

c

FIG. 134. FORE-FOOT OF ANCESTRAL FORMS OK THK HORSE. 1. OROHIPPUS
(Eocene). 2. MESOHIPPUS (Upper Eocene). 3. MIOHIPPUS (Miocene).
4. PROTOHIPPUS (Upper Pliocene). 5. PLIOHIPPUS (Uppermost Pliocene).
6. EQUUS.

are both functional and equally strongly developed (artipdactyU
form, Fig. 135) ; their metacarpals may be united with one
another and with the vestiges of
the proximal ends of the second
and fifth to form a " cannon-
bone," while the other digits are
gradually reduced. A similar re-
duction takes place in the hind-
foot, and is here as a rule more
rapid.

The Protungulata must origin- ?
ally have been pentadactyle and
plantigrade (i.e. the whole foot
rested on the ground) or semi-
plantigrade, with ungual phal-
anges but little broadened. On
the gradual elongation and
straightening out of the limbs
and unequal development of the
digits, they become digit-igrade
(as in most Carnivora), and
eventually unguli grade, only the J t /l\.

hoofs at the extremity of the
distal phalanges bearing the
weight of the body.'

The Tylopoda, as well as Ele- OFF
phants (Subungulata) have not F[( , 135. SKELETON OF THE LEFT
reached the unguligrade stage: FORE-LIMB OF A, PIG ; B, HYO-

they are practically digitigrade, a MOSCHUS ; C , TRAOULUS ; D, ROE-
J . i. j i VUCK ; E, SHEEP ; F, CAMEL.

large integumentary pad or sole (From Bell aftei . , ,. m . 0(L ,

(t-f. Fig. 24), from which the small

" hoofs" project, bearing the main weight of the body (Fig. 13(i).
Some of the many other adaptive modifications of the limbs in

1 The Tapir has four digits on the fore-foot and three on the hind-foot ; the
Rhinoceros has three on ea.ch foot,

172

COMPARATIVE ANATOMY

Mammals must also be briefly mentioned. In Bats, the phalanges
are greatly elongated to support the wing-membrane ; the fore
limbs are modified for digging in certain Mammals (e.g. Echidna,
Mole) ; and in the Cetacea (cf. p. 170) and Sirenia the digits are

not free, and serv r e as supports
for the fin-like paddles. Hind
limbs are absent in the two last-
mentioned orders (cf. p. 155),
but indications of them can be
seen even externally in very
young embryos of the Porpoise.
In the leaping Jerboa (Dipus),
the metatarsals are much elon-
gated, and may even become
ankylosed, as in Birds.

A bony knee-cap or patella,
such as occurs in certain Lizards
(e.g. Varanus) and in Birds, is
present in most Mammals, being
wanting only in Cetacea, Sirenia,
Cheiroptera, and some Marsu-
pialia. It has no genetic con-
nection with the bones of the
136. -LONGITUDINAL SECTION t hi g h and shank, and so is in no

OUGH THE M.ANUS OF THE LLAMA 1 i - , i , , ,

(Am-hema). (After M. Weber.) wa J comparable with the oleo-

cranon of the ulna, as was

1, metacarpal ; 2, 3, 4, phalanges; 5, so- f nrmpr ] v *, irmri op r ] T> i<s n trn,
called " hoof "; 6, horny part of sole ; ] r mei V Supposed.
7, cushion composed of elastic con- sesamoid bone, such as occurs in
nective tissue (more strongly de- connection with many of the
veloped in the Elephant, in which . _j- i i c ,\ >

also the phalanges are more vertical), individual joints of the digits,

which has arisen m the tendon

of the quadriceps femoris muscle in consequence of the friction
between this tendon and the condyle of the femur.

THROU

C. MUSCULAR SYSTEM.

THE muscles, commonly spoken of as " flesh," may be divided
into two groups, according to the histological character of their
elements, which consist of cells elongated to form contractile
fibres : namely, into those with smooth and those with transversely-
striated fibres. The former are phylogenetically the older, and are
to be looked upon as the precursors of the latter. The action of
both in causing movements is dependent on the nervous system, a
nerve entering each muscle at a definite point.

The smooth or involuntary muscle-fibres preponderate in the
viscera, derm, and vessels, and are not under the control of the
will ; the striated muscles occur chiefly in the body-walls and
organs of locomotion, and are almost without exception under the
control of the will (voluntary muscles). 1 The following general
statements refer exclusively to the latter kind of muscles, which
may, according to their mode of development, be arranged in the
following groups :

a. Muscles of the trunk, including the
coracohyoid of Fishes ( = sterno-
hyoid) and its derivatives in

I. Parietal muscles de-
rived from the meso-
dermic somites.

higher Vertebrates : these repre-
sent the oldest and most primitive
part of the muscular system.

b. Muscles of the diaphragm.

c. Muscles of the extremities.

d. Eye-muscles.

II. Visceral muscles, de- (Cranial muscles, with the exception
rived from the lateral -j of those included under a and d
plates of the mesoderm. [ above.

In its simplest form an origin, a belly, and an insertion, may be
distinguished in each muscle. The muscles of the trunk are as a

1 Exceptions are seen in the muscles characteristic of the heart, and in
those of the alimentary canal in the Tench. More or less of the anterior part
of the digestive canal may contain striated fibres in various Vertebrates.

174

COMPARATIVE ANATOMY

A

MUSCULAR SYSTEM 175

FIG. 137. A C. (After P. Buffa.)

A, Diagram showing the various phases in the movement of the scutes in Snakes.
a and b, two consecutive scutes ; c, the intervening integument ; d, fixed
point at free margin of scute ; e, distance along which the scute a is moved ;
A, resting stage; B, stage in which n is raised and in which there is the
greatest forward extension of the skin (c), while the free margin of the scute
catches against the ground ; 6', stage in which the scute a again takes on a
horizontal position, the skin (<) shows the greatest backward extension, and
the scute l> is moved forwards along the distance e.

15, Semi-diagrammatic figure of a longitudinal section through the ventral and
lateral parts of the skin of Tropidonotiis natrix, and of the costo-cutaneous
muscles in connection with the rib. c, rib ; c.c.i, r.r.,s, inferior and superior
costo-cutaneous muscle ; m.c.i, intrinsic musculature of the skin ; .s.</, longi-
tudinal sections of ventral scutes ; y.l, transverse sections of lateral scutes ;
v, vertebra.

C, Inner side of part of the ventral integument of Tropidonotus natrix. The
intrinsic muscles of the skin are not indicated, c, a pair of ribs with the
corresponding inferior (c.c.l) and superior (c.c.s) costo-cutaneous muscles ;
If, free raised border of the ventral (>?.</) and r of the lateral (*./) scutes.

rule flat, while those of the extremities have usually an elongated,
cylindrical, or prismatic form. In some cases, however, they assume
the most varied shapes ; for instance, there may be more than
one origin (bicipital, tricipital, or quadricipital forms), the belly
may be double (biveritral or digastric form), or the muscle may
be saw-shaped, or have its fibres arranged in a single or double
series like the barbs of a feather.

Most of the muscles are separated by fibrous sheaths, or fasciae,
and may be continuous with tendons for connecting the muscles to
the skeleton, or with flattened membranous expansions (aponeuroses).
Wherever marked friction occurs, ossifications (scsamoids) may be
developed in the course of a muscle or tendon.

The differentiation of independent muscles may take place

(1) by the separation of the originally single muscle into proximal
and distal parts by the formation of an intermediate tendon ;

(2) by the splitting of a muscular mass into layers; (8) by a
longitudinal splitting: or (4) by a fusion of primarily distinct
muscles. A muscle may undergo very considerable modification
both in form and position by a change of origin and insertion ;
and when the action of a muscle becomes unnecessary, it either
disappears, or what remains of it contributes to the strengthening
of a neighbouring muscle.

The most important point in determining the morphological
value of a muscle is its nerve-supply ; but other factors must also
be taken into consideration e.g. the homologies of the parts of
the skeleton, and the relative positions of the neighbouring parts.

Integumentary Muscles.

While most muscles have intimate relations to the skeleton,
which usually forms their points of origin and insertion and on
which they act directly, certain others are found in the derm or

176 COMPARATIVE ANATOMY

subdermal connective tissue, in which they end and usually also
arise : these are known as integumentary muscles, the first traces of
which are seen in the Anura. Their relations to the integument
have apparently been acquired secondarily, and they are to be
looked upon, at any rate in the Amniota, as originating from true
skeletal muscles : this is most plainly indicated in Monotremes
(Fig. 138), in which there is a close connection between the
epidermic exoskeleton and marsupial and mammary apparatus on
the one hand, and the integumentary musculature on the other. 1

Apart from the cutaneous striated muscles of the trunk and
limbs, an apparatus composed of smooth muscle-fibres is present
in Urodeles, and is more highly developed in Reptiles, in connection
with the nostrils, serving as dilators and constrictors. In Anurans
these muscles have become reduced, and play only a subsidiary
part, the movements of the alinasal cartilages here depending upon
those of the lower jaw, which presses upon the movable pre-
maxillaB and thus effect the closing of the nostrils : their opening-
is due essentially to the elasticity of the parts. The only other
integumentary muscles amongst the Anura, apart from a superior
labial muscle composed of smooth elements, are certain bands in
the regions of the trunk (rutaneus ^crtoris, c. atdominis) and
thigh (gracilis minor), and these are only present in the higher
forms.

In the Sauropsida the integumentary muscles play a great part
owing to their relations to the scutes, scales, and feathers ; and
those of Snakes will now be briefly dealt with as an example.

Considerable variations in arrangement, form, and insertion of
the integumentary muscles occur amongst the Ophidia. These
differences depend mainly on the number, form, and arrangement
of the ventral and lateral scales or scutes, and on the manner in
which they are utilised in locomotion according to the habits of
the snake in question. The muscles of the skin are most markedly
developed in those snakes which can creep rapidly over the ground
or burrow under it, and in which, by erection of the ventral scutes,
firm points of contact are formed between the hinder edges of
the latter and the ground, so that the body can be pulled or pushed
forwards (Fig. 137, A).

The muscles extending from the ribs to the scutes also aid in
progression ; they serve to throw the body into curves and to
straighten it, to draw it forwards over the integument, and con-
versely to move the integument forwards ventrally and laterally
over the body, thus aiding in giving the scutes a firm hold on
the ground (Fig. 137, B, c).

The integumentary musculature reaches its greatest develop-

1 It is, however, held by some Morphologists that the integumentary muscles
of Reptiles and Mammals are derivatives of a superficial part of the lateral
muscles of Fishes and Amphibians : in certain Anurans, Lizards, and Snakes,
relations between the integument and the rectus and superficial external oblique
certainly exist.

MUSCULAR SYSTEM

.77

B

Sphincter
colli

Pe.ctmalix

a I-, a

1 Spin i>i-/i i-

11 Mlllllllllll'll

gland

cloficce

C/oara

A, ventral view of male Or-
niihorhynchus ; B, ventral
view of male Echidna ; C,
lateral view of the head
and neck of Echidna, all
the muscles shown in which
are supplied by the facial
nerve.

Sphincter colli

Fid. 138. A C, THE INTEGUMENTARY MUSCLES OF MONOTREMES. (After Huge.

N

178

COMPARATIVE ANATOMY

ment in Mammals, and exhibits numerous modifications in passing
from Monotremes to Man. In lower forms (Monotremes, Fig. 138,
as well as, e.g. Dasypus, Centetes, Erinaceus, Pinnipedia, &c.),
it extends over the trunk and limbs (panniculus carnosus), while
in Primates it becomes reduced, and confined essentially to the
neck (jplatysma myoidcs) and head (mimetic muscles) : these
muscles are closely related genetically, and are all supplied by the
facial nerve. Two layers can be distinguished in the platysma
(Figs. 138 and 139), the more superficial of which has an oblique
or longitudinal direction, while the deeper layer (sphincter colli] is
circular : the two layers together correspond to the sphincter colli

MJevatwlqbii

M.orMo auric. M. helms

M.oiit.ocuH \ M.auric.sup /

\. M.maiicli-
bulo-miricnl

Via. 139. SUPERFICIAL FACIAL MUSCLES OF Lepilcmin- mnxtefiint.t. The deep
layer is recognisable on the neck. (After Huge.)

of the Sauropsida. They are continued on to the head, and there
give rise to a number of new muscles which are mainly grouped
around the eye, mouth, nose, and ear (Fig. 139). These mimetic
muscles are most highly differentiated in Man, but at the same
time reduction or tendinous transformation of certain of them
takes place, and some disappear entirely.

The action of the integumentary muscles is very varied in
different Vertebrates. It may serve to roll up the body into a ball
(e.g. Hedgehog, Armadillo), or aid in the movements of the limbs
and tail in swimming (e.g. Ornithorhynchus), or serve to erect the

MUSCULAR SYSTEM

179

integumentary spines (e.g. Echidna) ; or may cause local movements
(" twitching ") of the skin (many Mammals).

Parietal Muscles.

A. Muscles of the Trunk.

In Amphioxus the body muscles are made up of a series (60 or
more) of lateral muscular segments or myomercs separated by
> -shaped connective-tissue septa or myocommas, between which
the fibres run longitudinally. The myomeres have an alternating

Mo

RM

D

Fi<;. 140. THE MUSCULATURE OF LARVAL AMKLYSTOMA (AXOLOTL).

the side.

From

CV>, external ceratohyoid muscle ; Oph, cervical origin of the constrictor of the
pharynx ; Cu, cucullaris ; I), dorsal, and V, ventral portion of caudal
muscles ; Dg, digastric ; Z>.s, dorsalis scapulre ; LI, lateral line ; Lf, latissiinus
dorsi ; Lr, levator arcuum branchialium ; ttt> levator branchiarum ; Ma,
masseter ; Mr, myocommas between the myomeres of the dorsal portion of
the lateral muscles ; Mh\ mylohyoid (posterior portion) ; 0, superficial
layer of the external oblique muscle, arising from the lateral line, and ex-
tending to the fascia, F; at * a piece of this layer is removed, exposing the
deeper layer of this muscle (Oh) ; at Re the oblique fibres of the latter pass
into longitudinal fibres, indicating the beginning of the differentiation of a
rcctus abdominis ; at Re 1 the rectus-system is seen passing to the visceral
skeleton ; J^h, procoraco-humeralis ; RM, dorsal portion of lateral muscles of
the trunk ; SS, suprascapula ; T, temporal muscle ; Th, thymus.

arrangement on the two sides. On the ventral region of the
anterior two-thirds of the body is a thin transverse sheet of fibres.

In Fishes the myomeres and myocommas, arising exclusively
from the mesodermic somites (p. 9), have a zigzag arrangement on

N 2

180

COMPARATIVE ANATOMY

either side of the body, each of the former consisting, in its simplest
condition, of dorsal and ventral portions, separated from one another

FIG. 141. THE MUSCULATURE OF LARVAL AMBLYSTOMA (AXOLOTL). Ventral

Add, adductor arcuum branchialium ; C, constrictor arcuum branchialium ; Cbb,
coracobrachialis brevis ; Ce, Ci, Ci l , external and internal ceratohyoid : the
former is inserted on to the hyoid (Hy) ; do, cloaca; Cph, portion of the
constrictor of the pharynx, arising from the posterior branchial arch ; Dp,
depressores branchiarum ; Gh, geniohyoid ; La, linea alba ; Mh, J/A 1 ,
anterior and posterior portions of the mylohyoid, which is cut through in
the middle line, and removed on the left side, so as to show the proper
visceral musculature ; 0, superficial layer of the external oblique, passing
into the fascia, which is shown cut through at F ; Oh, second layer of
the same muscle ; Ph, claviculo-lmmeralis ; Spc, supracoracoideus ; Re, rectus
abdominis, passing into the visceral musculature (sternohyoid) at Re 1 , and
into the pectoralis major at P.

MUSCULAR SYSTEM 181

by a connective tissue septum extending from the axial skeleton
to the integument at the region of the " lateral line " l (cf. Fig. 140).
The myomeres meet together in the mid-dorsal and mid-ventral
lines, and constitute the great lateral muscles of the trunk.

This primitive metameric arrangement of the lateral muscles of
the trunk forms a characteristic feature in Vertebrates, and stands
in close relation with the segmentation of the axial skeleton and
spinal nerves, the number of vertebral segments and pairs of
nerves corresponding primitively to that of the myomeres.

The lateral muscles largely retain their primitive relations in
Fishes, but on the ventral side of the trunk, where they enclose
the body-cavity, certain differentiations occur which indicate the
formation of the recti and obliqui abdominis of higher types. The
dorsal portions of these parietal muscles, as well as the ventral
portions in the caudal region, retain a more primitive condition.

Amphibians. In Urodeles (Figs. 140 and 141) primary and
secondary ventral trunk-muscles can be distinguished, and both of
these groups, like the dorsal muscles, are segmented. The former
consists of internal obliques, arising directly from the muscle-plate
of the somite, and of external obliques developed from the ventral
border of the myomeres ; the obliqui towards the ventral middle
line are connected with the rectus abdominis.

The secondary muscles arise by delamination from the primary,
and give rise to a superficial external cb/iquc, a superficial rectus, a
transversalis, and a sulvcrtcbralis. These, however, only attain im-
portance in caducibranchiate forms, in which they become marked
during metamorphosis, and the primary musculature then under-
goes more or less reduction. Thus various conditions of the ventral
musculature are found amongst Urodeles.

In the broad-bodied Anura, on the other hand, both primary
and secondary muscles present a marked uniformity and relative
simplicity ; in the adult they give rise to a segmented rectus, in
part passing into a sternohyoid, a non-segmented obliquus externus,
and a transversalis, as well as to a cutaneus abdominis derived
from the external oblique. No trace of an internal oblique can be
seen in the adult.

Reptiles. In Reptiles, the lateral muscles of the trunk attain
a much higher grade of development. This is to be accounted for
by the more perfect condition of the skeleton, more especially of
the ribs and pectoral arch. The ribs and intercostal muscles now
play an important part in respiration, and changes, necessitated by
the higher development of the lungs, are thus brought about.

1 This septum is not present in Myxinoids, and is absent in Petromyzon and
Lepidosteus posteriorly to the gills.

182 COMPARATIVE ANATOMY

The ventral muscles of Reptiles represent the primary as well
as. the secondary muscles of Amphibians, though differing in their
further development, in consequence of which and of the course
taken by the nerves, relations of the parts are seen which lead up
to the condition occurring in Mammals. The primitive segmen-
tation may be retained or more or less completely lost, in which
latter case the muscles in question run together to form broad
plates.

The distinction between thoracic and abdominal regions becomes
gradually more plainly marked, and, in addition to the four
muscular layers present in Amphibians, well-marked external and
internal intercostal muscles are present : these are homologous with
the primary abdominal muscles of the last-named Order, as are also
the obliquus profundus (belonging to the system of the internal
intercostals) and the median, deep rectus abdominis. A transvcrsus
is present except in Snakes. A subvertebralis extends from rib to
rib, but is wanting in the lumbar region. A quadratus lumborum
(lumbar portion of the intercostalis) appears first in Reptiles, and
from it a psoas major and psoas minor may become differentiated.

The rectus muscle, which in Amphibia extends anteriorly to
the pectoral arch and is in part continuous with the neck muscles,
is in Reptiles interrupted at the sternum, so that pre- and post-
sternal portions can be distinguished. The rectus abdominis is
always well developed, and may consist of a segmented median and
of unsegmented lateral portions : it is not strictly comparable to
that of Urodeles, and the pyramidalis does not correspond to the
like-named muscle of Mammals.

While no important differentiation is noticeable in the dorsal
portion of the lateral body-muscles in Urodeles, a marked sub-
division of these muscles is seen in Reptiles. In them may be
distinguished a longissimus, an iliocostalis, interspinalcs, scmispinalcs,
iiiidtifidi splenii, and levatores costarum, together with the scaleni,
which belong to the last-mentioned group.

The muscles of the main part of the tail retain primitive
relations similar to those seen in Fishes : at the root of the tail
and in the cloacal region, however, new muscles become differ-
entiated, viz., the ilio-,ischio-, -Andpubo-catula/is and muscles of the
anus (already indicated in Anura) and generative organs.

Birds. In Birds the primitive character of the trunk-muscles
has disappeared far more than in Reptiles. This is mainly to be
accounted for by the excessive development of the muscles of the
anterior extremity the pectoralis major l more particularly and
the corresponding backward extension of the breast-bone.

External and internal oblique muscles are both present in the

1 The relative size of the pectoralis major does not always correspond to
the power of flight. It is very compact in Carinatas, and contains elements
corresponding to the pectorales major and minor of Man.

MUSCULAR SYSTEM 183

abdominal region, but only slightly developed : this is more par-
ticularly true of the internal oblique, which appears to be under-
going degeneration. No trace of a transversalis can be distinguished
in the abdominal region, but, on the other hand, a distinct, paired,
unsegmented rectus is present, reduced anteriorly and posteriorly.
External and internal intercostals are well developed, and a
triang-ularis stcrni (last trace of the transversus) appears for the
first time on the inner surface of the sternal ends of the ribs.
The dorsal portion of the trunk-musculature is only slightly
developed in the region of the body, though very strongly marked

in the neck.

All these modifications in Birds seem to be accounted for by
the specialisation of the mechanisms for flight and respiration, to
assist which the greatest possible number of muscles are brought
into play, and thereby influence the whole organism : an essential
difference is thus brought about between Birds and Reptiles.

Mammals. In general, there is a reduction of the ventral
musculature in Mammals. Three lateral abdominal muscles are
always present, an external and internal oblique and a transversalis.
In many cases, more particularly in Tupaia and in Lemurs, the
external oblique possesses tendinous intersections, thus indicating-
its primitive segmental character ; but in general all these muscles
consist of broad, uniform sheets. Towards the middle line, they
pass into strong aponeuroses which ensheath the rectus abdommis
The latter consists of a single band on either side and possesses a
varying number of myocommas ; it is no longer connected with the
axial muscles of the neck belonging to the same system (sterno-
hyoid, sternothyroid, &c.) as is the case in Urodeles, for the sternum
is always interposed between them, as in the Sauropsida. It, how-
ever, may occasionally (e.g. in lower Primates) reach as far forwards
as the region of the first rib : in higher forms it becomes more pi-
less shortened, the greatest loss of myomeres being seen in
Anthropoids and Man, in connection with the development and
relations of the great adductor (pectoralis major) of the fore limb.

In Monotremes and Marsupials, the strong pyramidaiis muscle
lies on the ventral side of the rectus abdominis. It arises from
the inner border of the marsupial bones (p. 155) and may
extend forwards as far as the sternum. In the higher Mammals,
in which marsupial bones are wanting, the pyramidaiis usually
becomes greatly reduced or entirely lost. Traces of it are, how-
ever, commonly to be met with even in the Primates, arising from
the anterior border of the pubis, right and left of the middle line. 1

The external and internal oblique muscles are represented in
the thoracic region in Mammals, as in the Sauropsida, in the form

1 A sphincter marsupii muscle is developed in connection with the marsupixim

(Figs. 28 and 138).

184 COMPARATIVE ANATOMY

of external and internal intercostals. 1 The subvertebral muscle is
represented by a longus colli and rccti capitis antici. What
has been said above with regard to the quadratus lumborum
and to the differentiation of the dorsal portion of the trunk-
muscles in Reptiles applies essentially also to Mammals, in which
also the metamerism of the dorsal body-wall is retained longer
than that of the ventral.

In the caudal musculature, flexors, extensors, and abductors
may be distinguished, and their degree of development is pro-
portional to that of the tail : in Man, for example, they become
reduced, and some of them (the pubo- and ilio-coccygeus) have
undergone a change of function, giving rise to the levator ani or
" pelvic diaphragm," consisting morphologically and phylogenetic-
ally of three portions (pubic, ischiatic, and iliac). 2

B. Muscles of the Diaphragm.

The formation of a diaphragm results from a gradual sub-
division of the coelome (pleuroperitoneal cavity) into pleuro-
pericardial and abdominal portions, and the differentiation of the
serous membranes which line these (pleura, pej'icardiiiin, peritoneum^)
can only be understood in connection with the complicated
development of the primitive urinogenital folds, liver, lungs, and
great veins, and so cannot be dealt with in this place.

From the Sauropsida onwards, a more or less distinct
separation of the pleural and peritoneal cavities is seen. In
Chelonians and Lizards a partition is present between these
chambers, but this is complete only in Crocodiles and Birds.
Subperitoneal muscular elements are present which connect it
with the vertebral column and ribs, but the innervation of these
precludes any homology with the diaphragmatic muscles of
Mammals. 3 It is here therefore only a case of analogy ; and it must
be remembered that in the Sauropsida the pericardium lies in the
general peritoneal cavity.

The very variable serrati postici superior and inferior are peculiar to
Mammals above Moiiotremes. They do not form a single layer, but are indepen-
dent of one another, and are derived respectively from the external and internal
intercostals.

2 It is doubtful how far the external sphincter of the anus, the muscles in
connection with the external generative organs, and the transi'trsu-f pcrittci pro-
f a mlus are derivable from the original sphincter cloacre of the Amphibia and
Sauropsida. In Mammals the pubo-coccygeux (or the pubic portion of the levator
ani), as well as the Mjilniu'tur ani c.i-fi-.niHx and bnlbo- and ischio-cavernosi, are
considered to represent separate portions of the integumentary muscle which
primarily extended over the greater part of the trunk.

3 Amongst the Amphibia (Rana) fibres from the transversus which extend on
to the gullet have been compared to a diaphragm, but the relations are here
quite different to those of the muscles of the mammalian diaphragm, in the forma-
tion of which the rectus abdominis plays an important part. In Birds, two
entirely different structures have been described as diaphragms (cf. under
Air-sacs).

MUSCULAR SYSTEM 185

A complete diaphragm dividing the coelome into thoracic and
abdominal cavities occurs only in the Mammalia. It is dome-
shaped and muscular, its muscles arising from the vertebral column,
ribs, and sternum. The diaphragm is of great importance in
respiration, as it allows of a lengthening of the thoracic cavity in
a longitudinal direction. It is supplied by paired phrenic nerves
arising from one or more of the cervical nerves (usually the 4th or
4th and 5th, but varying from the 3rd to the 8th) ; and is per-
forated by the oesophagus, aorta, postcaval and azygos veins,
thoracic duct, &c. In most cases it consists of a central tendon
from which muscular fibres radiate to the periphery and form
dorsally two strong " pillars " of the diaphragm. In some Mammals
(e.g. Echidna, PhocEena) the diaphragm is entirely muscular : in
the higher Primates the central tendon unites secondarily with
the pericardium.

The nerve-supply of the diaphragm indicates a polymeric
origin from the ventral portions of several myomeres. In the
course of development, it, like the pericardium, becomes shifted
backwards. The first rudiment of the diaphragm (" septum
transversum ") is composed of connective tissue into which the
musculature extends secondarily, and is situated ventrally on
either side of the median line: eventually it becomes closed in
laterally and posteriorly. 1 It is important to note that in the
innervation, as well as the grouping of the muscles, a costo-
sternal and a lumbar portion can be recognised in the mammalian
diaphragm.

Although in many respects the mode of evolution of the
mammalian diaphragm still requires elucidation, it is at any rate
certain that a close connection exists between its development and
that of the thorax and the changed respiratory conditions. The
diaphragm acts as an important respiratory muscle, and also aids
the abdominal muscles in the compression of the abdomen.

c. Muscles of the Appendages.

All the muscles of the appendages of Vertebrates are primarily
to be looked upon as derivatives of the ventral muscles of the
trunk, i.e., of the myomeres. This is indicated, apart from the
nerve-supply, by their mode of development in numerous Anamnia,
although in the Amniota the primitive mode of formation is not
clearly recognisable owing to an abbreviation of development.

Two principal groups of appendicular muscles may always be
distinguished : one lying in the region of the pectoral and pelvic
arches, dorsally and ventrally, the other in the free extremity. In

1 ri-

This mode of formation can be recognised not infrequently in those terato-
logical cases in which the costal and lumbar portions of the diaphragm do not
become united.

186 COMPARATIVE ANATOMY

Fishes the latter group consist essentially of elevators, adductors, and
depressors of the fins, and these again may become differentiated
into several layers. From the Amphibia onwards, in correspondence
with the more highly differentiated organs of locomotion, consider-
able complication is seen, and there is a much more marked separa-
tion into individual muscles corresponding with the different sections
of the extremity. Thus elevators, depressors, rotators, flexors, extensors,
protractors, retractors, abductors, and adductors are present in con-
nection with the pectoral and pelvic arches, the upper arm and
thigh, forearm and shank, and hand and foot : the digits are
also moved by a highly-differentiated musculature. The number
of muscles gradually increases in passing from the Urodela through
the Sauropsida to the Mammalia, and greatly influences the form
of the skeleton.

The most important muscles of the shoulder, the origin of
which from the trunk gradually becomes broader in the higher
forms, are the cucullaris, the stcrnocleidomastoideus (belonging
morphologically to the cucullaris, and, like it, supplied by the
spinal accessory nerve), the rhomboidci, and the Icvator anguli
scapulae : these act as rotators, protractors, and retractors of the
scapula.

The muscles connected with the pelvic arch cannot all be
looked upon as the serial homologues of those of the more movable
shoulder, for in many respects the different mechanical relations
of the hind limb have caused modifications in the muscles. Thus,
representatives of the Icvator anguli scapukv, rhomboideus, and
sc.rratus magnus are not present.

A much greater similarity especially marked in Urodeles
exists between the muscles of the free portions of the fore and
hind limbs. In correspondence with the fact that the angle formed
by the upper and middle sections points in opposite directions in
the pectoral and pelvic limbs, the extensor muscles of the former
are on its posterior border, and those of the latter on its anterior
border, while the flexors have the converse arrangement. From
the latter the pronators have arisen : these are more specialised
in the fore limb than in the hind limb. The supinaturs originated
from the extensors.

A very varied differentiation of the individual layers of muscle
takes place in different Vertebrates in connection with the shank
and foot, as well as the fore-arm and hand. The degree of differ-
entiation of the muscles in question in general corresponds to the
functional specialisations of the foot and hand, and is most marked
in the hand of Primates, more especially of Man.

D. The Eye-Muscles.

(These will be dealt with in connection with the organ of
vision.)

MUSCULAR SYSTEM 137

Visceral Muscles.

Fishes. The visceral muscles of Fishes l have been most
satisfactorily investigated in Elasmobranchs, and are classified by
Fiirbringer as follows :

A. Cranial or cerebral muscles (consisting originally of trans-
verse or circular fibres) supplied by the V th , VII th , IX th ,
and X th cerebral nerves.

1 . Constrictor arcuum visceralium, incl. constrictor superficialis

dorsalis et ventralis.

Innervation,
Levator labii superioris ~\

,, maxilke ,, V.

, , ] >alpebne nictitantis 2 J
,, rostri ^
,, hyomandibularis VT1

Depressor rostri j
,, mandibularis et hyomandibularis J
Interbranchiales IX, X.

Trapezius X.

2. Arcuales dorsales IX, X.

3. Adductores, iitd. adductor mandibulse ... V.

and abductores arcuum branchialium . . IX, X.

It. S/rinal muscles, originally longitiid- ^ c -,

i j j- -j j 1-1 .a. j.1. f opino-occipital 6 and
inal, and divided, like the other > - ,

11 I spinal nerves,

trunk-muscles, into myomeres. J

() Epibranchial spinal muscles, dorsal to visceral skeleton.

4. Subspinalis Spino-occipital nerves.

T Spino - occipital nerves,

5. Interbasales as well as the first

spinal nerve.
(b} Hypobranchial spinal muscles, ventral to visceral skeleton.

r ri , ( Spinal nerves, and partly

b. Uoraco - arcuales. me/, coraco- \ r , , , J

i i-i i -j the last one or more

branchiales, coraco - hyoideusX c ,, ., ,

and coraco-mandibularis . / f the spmo-occipital

^ nerves.

In the Ganoiclei, Dipnoi, Teleostei, Amphibia, and Amniota
there are no epibranchial spinal muscles, and the hypobranchial
muscles have a very different form from those of Elasmobranchs :
in Teleosts, for instance, they are much simplified. In Amphibians,
as already mentioned, the rectus system of the trunk is only

In Cyclostomes there is a remarkable transformation of the cranio-visceral
musculature in correspondence with their peculiar cranial skeleton (suctorial
apparatus) and branchial basket. It is covered over secondarily by the trunk
muscles.

This muscle lias nothing to do with the other eye-muscles.
1 These are spinal nerves emerging from the occipital region of the skull (cf,
under Nervous Sj'stem).

188 COMPARATIVE ANATOMY

partially interrupted by the sternum and pectoral arch, and is
continuous with the sternohyoid. These different conditions of
the muscles result from the varied adaptations of the visceral
skeleton and respiratory organs. 1

Amphibians. It is to be expected, a priori, that the muscu-
lature of the visceral skeleton should be more highly developed in
branchiate than in air-breathing Amphibians ; in the former,
more primitive relations are met with, while in the latter a greater
modification, or rather reduction, of these muscles takes place.

The muscles of the hyoid and branchial arches may be divided
into three groups a dorsal (levatorcs arcuum), a middle (muscles
of the external gills and the external ccratohyoid\ and a ventral
(internal ccratohyoid, subarcuales, and interbranchialis 3 or 4). The
nerve-supply of the dorsal group is strictly branchiomeric ; in the
middle, and still more in the ventral group, this condition is not
retained.

Between the two rami of the lower jaw is situated a muscle
with transverse fibres (the myloliyoid or intermandibular muscle),
supplied by the third division of the trigeminal and the facial
nerve ; this represents the last remnants of the ventral superficial
constrictor muscle of Fishes. As elevator of the floor of the
mouth, it stands in important relation to respiration and deglutition,
and is retained throughout the rest of the Vertebrata up to Man
(Figs. 140, 141).

A continuation of the trunk-musculature (the omo-, stcrno-, and
gcnio-hyoid), provided with tendinous intersections, lies above the
mylohyoid (Fig. 141). These muscles, which serve to pull the
visceral skeleton forwards and backwards, are supplied by the first
and second spinal nerves.

In contrast to Fishes, there is in Amphibians a definite differ-
entiation into muscles of the tongue, that is, into a Jii/oglossns and
a genioglossus ; these also must be considered as originating from
the anterior end of the ventral muscles of the trunk ; they are
present in all Vertebrates from the Amphibia onwards, and are
supplied by the hypoglossal that is, the first (or second, Anura)
spinal nerve.

In the Perennibranchiata and in Salamander larvae the muscles
of the hyoid and of the visceral arches may, by analogy with Fishes,
be divided into a ventral and a dorsal group ; the latter disappears
in adult Salamanders and Anurans, only the ventral persisting.
Their function is to raise and depress the branchial arches, as well
as to draw them forwards and backwards. To these may be added,
in branchiate forms, levators and adductors of the external gills
(Figs. 140 and 141). They are innervated by the vagus and
glossopharyngeal.

1 The visceral muscles of Polypterus are of especial interest, as they present
an intermediate condition between those of Elasmobranchs and Urodeles.

MUSCULAR SYSTEM 189

The jaw-muscles may be divided into a depressor (digastric, or
biventer mandibulcc, which here has only a single belly, Fig. 140),
supplied by the facial nerve, and into several elevators of the lower
jaw (masseter, temporal, and pterygoid muscles), supplied by the
third division of the trigeminal. The last-mentioned muscles may
be derived from the adductor of the mandible of Elasmobranchs,
and the biventer from the portion of the superficial constrictor of
Fishes which passes to the lower jaw : it arises from the same
matrix as the platysma, and serves to open the mouth.

Amniota. With the simplification of the visceral skeleton in
Amniota there is a considerable reduction of the musculature
belonging to it. All muscles connected with branchial respiration
are of course wanting, and the ventral trunk-muscles, as mentioned
above, are always interrupted in their forward extension by the
sternum and pectoral arch. At the same time, the muscles along
the neck and on the floor of the mouth met with in Amphibia are
present here also ; they are the mylo-, sterno-, omo-, and genio-
hyoid, as well as the hyoglossus and genioglossus. To these may
be also added a sternothyroid, and a thyrohyoid continued for-
wards from it.

The stylohyoid, styloglossus, and stylopharyngeus of Mam-
mals, arising from the styloid process and stylohyoid ligament and
undergoing numerous variations, are peculiar to Mammals. They
are supplied partly by the facial nerve, partly by the glossopharyn-
geal, and act as retractors of the tongue and levators of the pharynx
and hyoid. 1

The muscles of the jaws resemble those of Amphibia, although,
especially in the case of the pterygoids, they are much more sharply
differentiated into superficial and deep or external and internal
portions, and may become subdivided secondarily (e.g. in the region
of the temporal muscle) : they are throughout more strongly
developed. 2

1 For the tensor tympani and stapedius muscles, cf. under Auditory Organ.
The latter muscle, together with the stylohyoid, is possibly derived from the dorsal
portion of the deep constrictor layer of Fishes which passes to the hyoid, but
more probably corresponds to the ventral portion of this muscle.

- An anterior belly of the biventer appears in Mammals in consequence of a
shifting of the superficial layer of the mylohyoid, the fibres of which are originally
transverse. Its connection with the tendon of the posterior belly is therefore
secondary, as are also the relations of the mylohyoid to the hyoid bone.

D. ELECTRIC ORGANS.

ELECTRIC organs arc present in some Fishes, being most
strongly developed in certain Rays (Torpedinidte. e.g. Torpedo,
Hypnos) found in the Atlantic Ocean and various southern seas,
in a South American Eel (Gymnotus ckdricus) and in an African
Siluroid (Malopterurus electricus). Gymnotus possesses by far the
strongest electric power, next to it comes Malopterurus, and then
Torpedo. The electric batteries of these three Fishes are situated

in different parts of the body : in
Torpedo they have the form of a
broad mass, extending throughout
the substance of that part of the
body lying between the gill-sacs

and the propterygium on either
side of the head (Fig. 142); in
Gymnotus they lie in the ventral
region of the enormously long tail
(Fig. 143), that is, in the position
usually occupied by the ventral por-
tion of the great lateral muscles ;
and finally, in Malopterurus, the
electric organ extends between the
skin and muscles round almost the
entire circumference of the body,
thus enclosing the Fish like a
mantle : it is especially strongly
developed along the sides, but is
separated by the branchial apparatus

Fio. 142. -- Torpedo marmorata, into dorsal and ventral portions.
WITH THE ELECTRIC ORGANS (E) The electric power of t i loso

EXPOSED. . , , . , ,, , ,

Wishes which were formerly known
Au, eye ; KK, gill clefts ; s, skull ; as pseudo-electric " has now been

fully demonstrated, though it is

much feebler than in the forms described above. To this category
belong, e.g. all the Rays, excluding Torpedo, and the various
species of Mormyrus and Gymnarchus (both belonging to the
Teleostei). In all these the electric organs lie on either side of the
end of the tail and have a metameric arrangement like that of the
caudal muscles ; in the Mormyridse, for example, there is on either

ELECTRTC ORGANS

191

A

side an upper and lower row of electric organs. In addition to the
Fishes here referred to, electric organs have also been described
in other Teleosts (e.g. Astroscopus).

With the possible exception of Malopterurus, in which the
electric apparatus is said to be derived from the epiderm, the
electric organs of Fishes
consist of metamor-
phosed striated muscu-
lar fibres, and the nerve-
endings belonging to
them are the homo-
logues of the motor end-
plates which are ordin-
arily found on muscles.

As regards the struc-
ture of the electric
organs, the same essen-
tial arrangements are
met with in all. The
framework is formed of
fibrous tissue enclosing
numerous cells, which,
running partly longi-
tudinally, partly trans-
versely through the
organ, gives rfse to
numerous polygonal or
more or less rounded
chambers or compart-
ments. These latter are
arranged in rows, either FIG. 143, A and B. THE ELECTRIC ORGAN OF
along the longitudinal Gynmo/ux elu-trim*. (B, from a preparation by

axis of the body (Gym-

notus, Malopterurus) or A, anus ; DM, DM 1 , dorsal portions of the great

Ff

lateral muscles, seen partly in transverse, partly
in longitudinal, section ; E, the electric organ,
seen in transverse section at E (B), and from
the side at E 1 ; Ft, fin ; H, skin ; LH, posterior
end of body-cavity ; Sep, median longitudinal
fibrous septum between the left and right
electric organ and lateral trunk-muscles ; VJ\I,
VM 1 , ventral portions of the great lateral
muscles, seen partly in transverse, partly in
longitudinal, section ; WS, vertebral column
from the side, showing the spinal nerves, and
in transverse section.

iii a dorso-ventral direc-
tion (Torpedo), forming
definite prismatic col-
umns (Fig. 144). The
compartments are filled
with a homogeneous
fluid or semi-fluid sub-
stance, the nature of
which is not thoroughly
understood. It is known

to correspond to modified muscle-substance, and it contains
numerous large, round and oval nuclei, as well as certain highly
refracting bodies.

Numerous vessels and nerves ramify in the connective tissue

192

COMPARATIVE ANATOMY

lying between these compartments, the nerves being enclosed in
thick sheaths, and having a great variety of origin according to
the species of Fish under consideration. In Torpedo, in which
the electric organs probably arise in connection with the great
adductor muscle of the mandible and the constrictor of the gill-
arches, the nerves arise from the " electric lobe " of the medulla
oblongata, a single branch coming also from the trigeminal ; in all
" pseudo-electric " Fishes, as well as in Gymnotus, in which over
two hundred nerves pass to the electric organ, they arise from the
spinal cord, and most probably are in the closest relation with the
ventral cornua, which are particularly well developed in the last-
named Fish/ It is remarkable that the electric nerves of Malop-
terurus arise on either side from a single enormous lens-shaped
nerve-cell, which, situated in the neighbourhood of the second
spinal nerve, is continued into a very large
primitive-fibre, which passes towards the end of
the tail, dividing as it goes. The latter is invested,
by a thick sheath.

It is stated that in all electric Fishes the side
of the electric plate on which the nerve branches
out is negative at the moment of discharge, while
the opposite side is positive. Thus the different
arrangement of the parts in Gymnotus and
Malopterurus renders it clear that the electric
shock must pass in different directions in these
Fishes: in Malopterurus it passes from the head
to the tail, and in Gymnotus in bhe contrary direction, while in
Torpedo the discharge passes from below upwards.

Experiments have shown that all electric Fishes are proof
against the electric current, with the limitation that muscles and
nerves even the electric nerves themselves separated out from
the body are capable of being excited by the current. ' The last
and most important question with regard to the electric Fishes
naturally concerns the mechanism whereby the electric plates
become temporarily charged with electricity. The reply to this
question, although probably not so difficult a one as that relating
to the mechanism of muscular contraction, is still far from being
answered" (Du Bois-Reymond). The only thing that can be stated
with certainty is, that the electromotive force is under the control
of the will.

i

FIG. 144. ELEC-
TRIC PRISMS OF
Torpedo <"/-
morata. (Semi-
diagrammatic. )

E. NERVOUS SYSTEM.

The nervous system has the important function of placing the
organism in communication with its surroundings, stimuli received
by the sensory organs being transformed into nerve-impulses which
are conducted along the afferent or sensory nerve-tracks to the
central organ of the system. In the latter these stimuli are
transformed, or new ones are originated, and they travel along the
efferent or motor nerve-tracks to muscular elements, thus causing
their contraction, or to glands, causing them to secrete. The
intimate connection between muscle and nerve has already been
referred to.

It was pointed out in the Introduction that the nervous system
arises from the ectoderm. The parts of it which first become
differentiated histologically are the nerve-cells (ganglion-cells},
from which nerve-fibres arise later and serve as the conductors
of nervous impulses. The most important constituent of the
nerve-fibre is a central neuraxis or axis-fibre, and in those nerve -
fibres which are spoken of as medullated this is surrounded by a
highly refractile, fat-like substance (myelin), which forms the
medullary sheath. In certain (non-medullated) nerve-fibres this
sheath is wanting, but the two kinds of fibres are not sharply
marked off from one another, either locally or genetically : a fibre
may be medullated in one part of its course, and non-medullated
in another. Externally each nerve-fibre is enclosed by a delicate
sheath, the neiirilemma.

Part of the ectodermic tissue which forms the nervous system
of the embryo does not become transformed into nervous tissue,
but gives rise to an epithelial layer (ependyme) and also to a
supporting, connecting, or isolating framework the neuroglia,
which plays a very important part in the central nervous system ;
externally, investing membranes as well as blood- and lymph- vessels
are formed from the mesoderm. As compared with the central
organs, the peripheral tracks are comparatively poorly supplied with
blood.

The nervous system thus consists of central and peripheral
portions (Fig. 14.5). The central part (brain and spinal cord) is

o

194

COMPARATIVE ANATOMY

XT

Fie. 14.1. THE ENTIRE NERVOUS SYSTEM OF THE FROG.

From the ventral side.

(After A. Ecker.)

/', facial nerve ; G, ganglion of the vagus ; He, cerebral hemispheres ; 7 to A', first
to tenth eerebral nerves; Lop, optic lobes; M, spinal cord; Ml to J/ln,
spinal nerves, which are connected at SM by branches (ranii communicantcs)
with the ganglia (SI toSlO) of the sympathetic (>s r ) ; A r , nasal sac ; Ni, sciatic
nerve ; No, femoral nei^ve ; o, eye ; Va to Ve, the different branches of the tri-
geminal ; Vy, Gasserian ganglion ; Fx, connection of the sympathetic with the
< iasserian ganglion ; A'l to A'4, the different brandies of the vagus. Some of the
fibres of the sympathetic should be shown accompanying the vagus peripherally.

NERVOUS SYSTEM 195

the first to arise, and is torn KM I MS a direct product of the ectoderm ;
the peripheral portion (<:crcbra/, .y>in/, and sympathetic wr>rx, and
their ganglia) becomes established later.

1. THE CENTRAL NERVOUS SYSTEM.

The first indication of the central nervous system is a longi-
tudinal furrow (medullary groove, Fig. 6, A) which appears on the
dorsal side of the embryo, and which gradually becomes converted
into a tube by the meeting of its edges ; this tube, consisting
originally of epithelial cells like the ectoderm from which it arises,
then becomes separated from the latter, and gives rise to the
hollow medullary cord ] (Fig. 6, B), in which nerve-cells and fibres
become differentiated ; it comprises a more expanded anterior,
and a longer and more slender posterior section. From the former
arises the brain, from the latter the spinal cord.

In an early stage of development the lumen of the medullary
cord is primitively continuous posteriorly with that of the primary
intestine by the ncurenteric canal, but this connection soon dis-
appears. The cord consists of a cylindrical or more or less flattened
tube, the cavity of which expands in front to form the ventricles of
the brain, and is lined by ciliated epithelium. With the thickening
of the walls of the tube, this cavity becomes greatly reduced, and
in the spinal cord is spoken of as the central canal.

Some of the cells in the brain and spinal cord serve as sensory
centres, others as motor-centres, new centres being added
which complicate the originally simple reflex circuit, and various
other modifications gradually occur in the course of development of
the head.

Membranes of the brain and spinal cord (meninges).

In Amphioxus, the central nervous system is enclosed by an
undirFerentiated investment of connective tissue. In the lowest
Craniata, a differentiation takes place into a primitive meninx,
which closely invests the spinal cord, and a second membrane
(endorachis), which lines the vertebral canal : the latter, formerly
known as the "dura vertebralis," is comparable merely to the
perichondrium or periosteum, and has nothing to do with the
meninges proper. The blood-vessels supplying the spinal cord are
contained in the primitive meninx, the space and tissue directly
external to which may be spoken of as the perimeningeal space and
tissue. This condition is retained in Fishes (Fig. 146, A).

A further process of differentiation takes place in Urodeles
and is more marked in Anurans, reaching a higher stage in Reptiles
and a still higher one in Birds. This process consists in the

1 The cord is at first solid in certain Fishes (e.y. Petromyzon, Lepidosteus,
Amia, Teleostei, Lepidosiren), its cavity appearing later.

O 2

190

COMPARATIVE ANATOMY

appearance of a lymph-space in the primitive meningeal membrane,
dividing it into an outer dura mater spinalis and an inner primitive

pia mater. There is thus a pi //</// /'//I or <'j>/</n./-I xjxtir, external to
the dura., and a xitlitlii.nil .syvmr between it an<l the primitive pia

(B).

B

iiiiitire pin mater

space

Fi(!. 146. THE INVESTING MEMBRANES OF THE SPINAL CORD IN THE CHIEF
VERTEBRATE GROCPS. A, Fishes ; B, Amphibia and Sauropsida ; C, Mammalia.
Semidiagrammatic. (After Ster/i.) Riickenmark, spinal cord.

In Mammals, the pia is considerably thicker, and the lymph
spaces which it encloses are more marked : it becomes separated
into two layers, the inner of which gives rise to the vascular
secondary or definitive pia, and the outer to the arachnoid.

NERVOUS SYSTEM 197

In consequence of this process of differentiation, the originally
single perimedullary space is now represented by three 1 lymph
spaces : a peridural, a subdural, and a sukarachnoid space (c). The
last mentioned encloses a network of trabeculae. 1

The brain-membranes are formed in essentially the same way
as those of the spinal cord. The dura mater, however, possibly owing
to the rapid growth of the brain, becomes pressed against the
periosteum (so-called "endocranium" corresponding to the
endorachis of the spinal column), with which it may become fused
(e.g. Mammals) and thus was formerly known as the outer layer of
the dura. 2

In Fishes and tailed Amphibians, the whole subdural space is
rilled by a loose tissue consisting of meshes enclosing lymphoid
and fatty tissue, while in Anurans this is only the case in the
anterior part of the skull. Further back, to the end of the spinal
canal, there is a definite continuous space filled with lymph, and
this is especially well developed on the dorsal side of the brain
and spinal cord.

Functionally, the dura serves as a kind of internal periosteum,
while the vascular pia is important in connection with the
nutrition of the central nervous system. In places where the
walls of the brain are epithelial, and do not become converted into
nervous tissue, the pia also takes part secondarily in lining
the ventricles, into which it may extend, pushing the epithelial
wall (ependyme) before it, and thus giving rise to telcv or plexus
choroidci, which are very important throughout the Vertebrate
series. 3

The arachnoid, which, as mentioned above, becomes differenti-
ated between the dura and pia, is not really a membrane : it
consists of an extensive system of meshes of lymphoid (adenoid)
tissue, the cavities in which are lined by an epithelium and con-
tain a serous or lymphoid fluid. The meshes bridge over all

The cause of this gradual increase in complication is to be traced in the first
instance to the increasing vascularisation of the spinal cord in passing from the
lower to the higher Vertebrates : this results in an increase in the quantity of
lymph and necessitates an arrangement of larger channels for carrying it off.

' The morphological difference between these two membranes is seen most
plainly at the optic foramen, at which the periosteum of the cranium is directly
continuous with that of the orbit, while the dura proper forms the sheath of the
optic nerve. The mode of origin of the so-called simi* dune matris is not known
with certainty, but they appear to have nothing to do with the dura proper and to
correspond to sinuous enlargements of the periostea! " endocranium," as is indicated
by similar spaces in the endorachis of Anamnia, Reptiles, and mammalian embryos,
which are especially well seen around parts of the brain in Urodeles. In Anurans
they even extend beyond the cranial cavity along the whole length of the spinal
canal. (Of. under Auditory Organ perilymphatic and endolymphatic ducts.)

' The brain-membranes in Sauropsida require further investigation. In
Mammals the dura gives rise to folds extending between parts of the brain, known
as the fnl.r and the tentorium. The former, slight indications of which arc also
seen in Birds, extends between the two hemispheres, the latter between the hind-
brain and the occipital lobes of the hemispheres: both may become ossified
(<:</. in Carnivora).

198

COMPARATIVE ANATOMY

involutions and irregularities of the surface of the brain, and form
a delicate external lamella separating- them from the subdural
space all along the cerebro-spinal canal.

In all Vertebrates the subarachnoid space communicates in the
region of the hind-brain (medulla oblongata) with the ventricles and
central canal, so that there is a free passage into these for the

albuminous cerebro-spinal or arachnoid
fluid, which is mainly formed in connec-
tion with the ependyme cells covering
the choroid plexuses in the ventricles.

1. The Spinal Cord.

The spinal cord is at first of a uni-
form diameter throughout, but later,
when a richer nerve-supply becomes
needed for the extremities, it exhibits in
these regions definite swellings the
brack i a I and lumbo-sacral enlargements
(Fig. 147). The cord originally extends
along the whole length of the neural
canal, but its growth is usually less rapid
than that of the vertebral axis, so that
it is eventually considerably shorter than
the latter. In such cases (e.g. Primates,
Cheiroptera, Insectivora, Anura Figs.
145 and 147) it passes at its posterior
end into a brush-like mass of nerves, the
so-called cauda cquina ; these lie within
the neural canal, and the sacral nerves
arise from them. An axial prolongation
of the spinal cord nevertheless extends
far back, but is reduced to a thin thread-
like filum terminale.

The bilaterally-symmetrical form of
the spinal cord is pronounced by the
presence of a longitudinal fissure extend-
ing along it ventral ly ; this ventral fissure
is not always present, and the so-called
" dorsal fissure"

FIG. 147. DIAGRAMS OF THE
SPINAL CORD AND ITS
NERVES. In A the cord
passes to the end of the
tail, and at B it ends more
anteriorly and passes be-
hind into a longer filum
terminale.

Ce, caucla equina ; F.t, filum
terminale ; M.o, medulla
oblongata ; Pb, brachial
plexus ; PC, cervical
nerves ; PI, lumbo-sacral
plexus ; P.th, thoracic
nerves.

*/ -L

which is formed by

obliteration of the greater part of the primitive central canal, is
due to the presence of a septum formed from connecting sub-
stance, the ependyme. 1 If one imagines the points of exit of the

1 A secondary ventricle-like enlargement of the posterior end of the central
canal in many Mammals and other Vertebrates must not be confused with the
so-called N/J/HS rhomboidalis in the lumbar region uf theeurd in Birds, which arises
by a separation uf (lie lateral halves of the cord and is filled with modified
neuroglia, thus forming a much (hiekened dorsal septum.

BRAIN 199

dorsal (sensory) and ventral (motor) nerve-roots to be respectively
connected together by a longitudinal line, each half of the spinal
cord would thus be divided into three columns, a ventral, lateral,
and dorsal. Anteriorly the cord becomes continuous with the
posterior portion of the brain (medulla oblongata), with the
ventricle of which the central canal communicates.

As regards its minute structure, two kinds of nervous tissue can
be distinguished in the spinal cord, a white substance, consisting
of nerve-fibres only, and a gray substance, composed of nerve-cells
as well as fibres. 1 Their relative positions vary in the various
vertebrate groups, as well as in different regions of the cord ; the
white substance, however, has in general a more peripheral, the gray
a more central position, the latter usually presenting a pair of
dorsal and ventral cornua in transverse section, its groups of cells
showing in many places a metameric arrangement. 2

2. The Brain.

Before the medullary groove becomes closed, three swellings
may be seen at the anterior enlarged part of the medullary tube :
these are spoken of as the primary anterior, middle, and posterior
cerebral vesicles, or fore-, mid-, and hind-brain. (Fig. 148). The

FIG. 148. DIAGRAM OF THE EMBRYONIC CONDITION OF THE CENTRAL NERVOUS

SYSTEM.

G, brain, with its three primary vesicles. 7, //, /// ; R, spinal cord.

cavities of the vesicles are in direct connection with the central
canal of the spinal cord.

A differentiation of the primary fore-brain and hind-brain
respectively into two parts then takes place, and th us five divisions
of the brain may be distinguished. Counted from before back-
wards these are the telenccphalon (secondary fore-brain or cerebral
rudiment), diencephalon (primary fore-brain or twixt-brain), mes-
encephalon (mid-brain), metencephalon (secondary hind-brain), and

| Peculiar structures, known as Reissner's fibres, arising in the brain and ex-
tending along the spinal cord, occur in all vertebrate Classes. They represent an
ancient but highly differentiated apparatus, the function of which is to pass optic
stimuli by reflex action to the motor tracks of the cord. These fibres are largest
in Elasmobranchs and Teleosts, and are reduced or wanting in blind animals and
those in which the eyes have undergone reduction.

- In Struthioua Birds a row of elevations occurs in the liunbo-sacral region,
recalling the metameric segmentation of the cord in the Teleost Tr'ujln. The origin-
ally metameric character of the cord is also indicated phylogenetically and onto-
genetically by the primary, segmental blood-vessels. Tin- dorsal and ventral
longitudinal vascular trunks arise secondarily.

200

COMPARATIVE ANATOMY

myelcnccphalon 1 (primary hind-brain). The telencephalon usu-
ally gives rise to a pair of lobes, the cerebral hemispheres, and the
mid-brain to a pair of optic lobes or corpora bigemina dorsally, and to
two longitudinal bands, the crnra cerelri, ventrally. The meten-
cephalon is also spoken of as the cerebellum, and the myelencephalon
as the bulb or medulla oblongata. From the secondary fore-brain
paired olfactory lobes 2 are given off anteriorly, and its floor or basal

TIE Z ZimCJIKS 1 !) JIK

FIG. 149. LONGITUDINAL SECTION THROUGH THE SKULL AND BRAIN OK AN
(IDEAL) VERTEBRATE EMBRYO. (In part after Huxley.)

Be, basis cranii ; Cc, central canal of spinal cord ; Ch, notochord ; HC, posterior
commissure ; HH, cerebellum (metencephalon, secondary hind-brain) ; MH .
mid-brain (mesencephalon) ; NH, primary hind-brain ( myelencephalon J ;
NH 1 , nasal cavity ; SD, roof of skull ; VH, secondary fore-brain (telencepha-
lon), showing the corpus striatum (Cx) at the base, and the olfactory lobe
(Olj) anteriorly; ZH, diencephalon (primary fore-brain), which has given
rise dorsally to the pineal body (epiphysis, Z), and ventrally to the infundi-
bulum (7), to which the pituitary body (hypophysis, H) is attached : anteriorly
to this is seen the optic nerve (Opt), arising from the optic thalamus ( Tho).

portion becomes thickened to form a large " basal ganglion/' the
corpus striatum, while its peripheral part is distinguished as the
" mantle " or pallium (Fig 149). 3

The pallial region undergoes an important process of develop-
ment in passing upwards from lower to higher forms, gradually
becoming differentiated histologically into a layer of cortical gri/
matter of great physiological importance, the relative differentiation
of which stands in close relation to the mental development of
the animal. The telencephalon reaches its greatest perfection in
Mammals, more especially in Man, while in certain lower Vertebrates
the cortex is partially or entirely non-nervous and retains its

1 There is still a want of unanimity in the use of this nomenclature, and
many of the terms given above, as well as others with a similar ending, are used
in different senses by different morphologists.

2 The olfactory lobes are of great importance phylogenetically, as the origin
of the telencephalon, and more especially of its deeper basal portion, is closely con-
nected with the olfactory organ.

3 From the primarily epithelial pallium arises a median dorsal outgrowth,
the paraph ysi* (Fig. 150), just anteriorly to a transverse fold of the epithelial
roof (t'tlum trniixri rxmii) which separates the ventricles of the primary and
secondary fore-brain, and on the edge of which a posterior pallial commissure can
)>e recognised (<.</. in Lizards) : this fold encloses a vascular thickening of the
pia mater (rlioruit/ />/< .>-n.v), from which the paraphysisis not always distinguishable
(cf. Fig. 165). The latter apparently represents a glandular organ, recalling that
connected with the infiindibulum : whether it also includes the vestige of a sensory
apparatus, like the parietal and pineal organs, is doubtful (cf. pp. '20'2 and 2

BRAIN

201

embryonic epithelial character : this is usually regarded as being-
due to regressive metamorphosis, the cause of which, however, is
difficult to explain. The relative distribution of the gray and white
matter differs in various pares of the brain.

Connecting the two lateral halves of the brain are certain
transverse bands of nerve-fibres or commissures. In addition to a
small superior or habenular commissure in the pallium (Fig. 150),
an anterior commissure is present in the posterior region of the
secondary fore-brain, a middle in the primary fore-brain (in
Mammals only), and a posterior in the anterior part of the mid-
brain. In addition to these, others may be developed in the pallial
region (e.g. anterior and posterior pallial commissures, cf. Fig. 165) ;
and amongst Mammals those known as the corpus callosum and
fornix are of great importance.

Mesencephalon

Diencephalon

Pineal- undPftrietal-
/ organ,

,'Zirb

Veuan. tmnsversam
'piihelMe QuErlhlL

Fit;. 150. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH PART OF THE

EMBRYONIC BRAIN.

Zirbefpolster, pineal cushion.

The outer surface of the hemispheres is more or less smooth,
except amongst the Mammalia, in which fissures (sulci) and convolu-
tions (gyri) may be present. These consist of folds of the entire
pallium or cortex, and they cause a greater or less increase of the
superficial area.

From the primary fore-brain, the ventricle of which is walled
in anteriorly by the lamina terminalis, the following structures also
arise (Fig. 149) : the optic thai ami, formed as thickenings of its
basal walls, and the ganglia- haJicntt/cc on the posterior lateral
margin of the dorsal region, with the superior conuiiixxurc between
them; the primary optic vesicles, arising as paired ventro-lateral
outgrowths, from which the optic nerves and retina with its pigment
epithelium are derived later; the pineal apparatus, developed as

202

COMPARATIVE ANATOMY

outgrowths of the roof; and finally, the infundibulutn, formed as an
extension of the floor, together with part of the pituitary body
(hypophysis). Another portion of the pituitary body is derived from
the epithelium of the primary oral involution (stomodseum). 1

The pineal apparatus consists of the cpiphysis or pineal organ
proper, which persists in a more or less vestigial condition in
all Vertebrates, and of a more anterior outgrowth which may be
called the parietal organ, arising from the epiphysis or inde-
pendently from the roof of the diencephalon, and becoming
atrophied in the majority of Vertebrates. Each of these structures

H

n~

"13 J.

-H

FIG. 151. DIAGRAM ILLUSTRATING THE STRUCTURE OF THE HYPOPHYSIS OF
VERTEURATKS. A , PETROMYZON ; B, PISCES; G, AMPHIBIA; D, SAUROP-
SIDA ; E, MAMMALIA. (After Sterzi.)

H, " chromophilous " portion, and H 1 , " chromophobic " portion of hypophysis;
P, infundibular process ; S, saccus vasculosus.

represents a vestigial sensory organ, and may retain to a greater
or less extent the characters of a median eye, which in some cases
has probably a light-perceiving function. Certain facts indicate
that these organs may have been paired primitively or that the
two convspond to members of a pair ; but further researches are
desirable on this point, as well as on the relation of the two organs

1 Possibly the endodermie epithelium nt' the primary fore-gut may also take
part in its format ion.

BRAIN 203

to one another and the nature of certain accessory vesicles
in this region found in certain forms (e.g. Anguis). Both pineal
and parietal organs are in the embryo connected with the brain by
a special nerve or tract which grows out from the organ and
becomes connected with the brain secondarily (cf. Figs. 165 and
168).

As already stated, a nervous and an epithelial portion are to be
distinguished in the hypophysis, the former originating from the
infundibulum, the latter from the epithelium of the stomodteum
(Fig. 151). In Cyclostomes the nervous portion consists of a thin-
walled sac arising from the infundibulum (infundibular sac or
process), which in all the true Fishes is in part thrown into folds
by the invasion of numerous vessels. Thus arises the so-called
saccus vasculosus, the development of which shows great variation
amongst the different groups of Fishes. In the higher Vertebrates
the infundibular process undergoes various modifications, espe-
cially as regards the saccus vasculosus, 1 only traces of which may
still be recognisable (e.g. Mammals).

Both the primary and the secondary fore-brain are situated in
the prechordal region of the skull, all the other divisions of the
brain lying in its chordal portion (p. 75). The mid-brain and
medulla oblongata undergo fewer modifications than the fore-brain,
though each optic lobe becomes subdivided into an anterior and a
posterior lobe in Mammals ; only the anterior part of the thin roof
of the medulla (valve of Vieussens) is nervous, while its floor
becomes greatly thickened, and in Mammals gives rise anteriorly
to a transverse band of fibres (pons Varolii). It is important to
note that the greater number of the cerebral nerves arise from the
medulla oblongata. The cerebellum may be more or less distinctly
folded and subdivided into median and lateral lobes.

In the course of further development, the walls of the cerebral
vesicles become more and more thickened, so that their cavities,
transformed into the ventricles of the brain, undergo a gradual
reduction (Fig. 152).

A series of unpaired ventricles (tdocoele, diaccele, mesoccde,
metaccele, and myeloccde) lying in the longitudinal axis of the brain,
as well as paired ventricles, can be distinguished. When cerebral
hemispheres are more or less distinctly developed, the teloccele
gives rise to paired cavities extending into them and known as the
lateral ventricles (= ventricles 1 and 2); each of these communi-
cates with the diacoele or third ventricle (which extends into the

1 Various hypotheses have been put forward with regard to the primary
nature of the hypophysis : it may represent a sensory organ, or may correspond
to the primitive mouth ("palseostoma") of the Protovertebrata, which is to a
greater or less extent represented by the combined unpaired nasal and pituitary
passage of Cyclostomes (see under Olfactory Organ, and Fig. 190) : on the latter
hypothesis the mouth of existing Vertebrates is a "neostoma." It is very
probable, especially in the higher Vertebrates, that the epithelial part of the
hypophysis lias an important function as a ductless gland, which gives off its
secretion into neighbouring blood- and lymph-capillaries.

204

COMPARATIVE ANATOMY

infundibulum) by means of an opening, the foramen of Monro, and
may be continued into the corresponding olfactory lobe as an
olfactory ventricle or rhinoccelc. Each optic lobe also usually con-
tains an optic ventricle, or optoccele, communicating with the

mesocoele (iter or aqueduct of Sylvius). There
may be a distinct metacoele in the cere-
bellum opening into the myelocoele or fourth
ventricle. 1

All five cerebral vesicles lie at first in the
same horizontal plane, but in the course of
development a cerebral flexure takes place,
the axis of the vesicles becoming bent down-
wards, so that at a certain stage the mesen-
cephalon forms the apparent apex of the
brain. In Mammals, the parts of the brain
become still further folded on one another,
so that a parietal, a Varolian, and a cervical
bend may be distinguished (Fig. 153): this
process is connected with the further develop-
ment of the skull and the rapid longitudinal
growth of the brain.

In Fishes and Amphibians the cerebral
flexure later becomes practically obliterated,
but it persists more or less markedly in the
Cc, central canal of the higher types, more particularly in Mammals,
spinal cord (R); HH, T 4-U l , ri 4.1.

cerebellum ; MH, mid- In the latter Class > moreover, the original

brain, which encloses relation of the parts becomes still further

the iter (Aq), communi- complicated by the large development of the
eating between the 1,11 i_ i.- i i i i

third and fourth ven- cer ebral hemispheres, which grow backwards,

tricles; Nil, medulla and thus gradually overlie all the other
oblongata, with the p ar t s o f the brain: this condition of things

-fit
'. i?

n;. 1.V2. DIAGRAM
THE YENTKICLES OF TUE
VERTEBRATE BRAIN.

OF

each lateral
communicates with the

men of Monro (FM).

hem'i- attains its greatest perfection in Man.
spheres, with the lateral

ventricles (,SF) ; Zff, Amphioxus. The conical and enlarged

anterior end of "the spinal cord of the Lancelet
ventricle contains a widened portion of the central
canal which must be looked upon as a
ventricle. This opens freely to the exterior
dorsal ly by a neuropore, which represents
the last indication of the primitive connec-
tion of the central nervous system with the outer skin. It is
impossible to say with any degree of certainty to what extent
this " brain " of Amphioxus corresponds to parts of the Craniate
brain.

Cyclostomes. The brain of these forms remains in many

1 A so-called "fifth" ventricle, lying between the corpus callosum and fornix,
is found in Mammals, but morphologically it has nothing to do with the ventricles
proper, and simply represents a spaee between the thin internal walls (npjitiun
I ii<-iil a in) of the two hemispheres.

BRAIN

205

VH ZH

MH

respects in ;in embryonic condition : a dorsal connection of its two
halves by nervous elements is very incomplete, narrow bridges of
nervous substance only occurring in the primitive hind-brain, the
posterior portion of the mid-brain, the posterior commissure, and
the ganglion habenuhe of the right side. The main part of the
roof consists of membrane and vessels.

In the larval Petromyzon or Ammocoete, very primitive condi-
tions are met with (Fig. 154), and, as is also the
case in the adult, the individual vesicles lie in
an almost horizontal direction one behind the
other ; the telencephalon consists of a median
part and of small paired hemispheres con-
tinuous anteriorly with the larger, rounded
olfactory lobes. The median portion of the
teloccele is continued transversely outwards
into each hemisphere, in which it gives rise
to a lateral ventricle : this is continued for-
wards for a short distance into the base of the
olfactory lobe, as well as backwards into the
hemisphere. The roof (pallium) of the median
portion of the ventricle is non-nervous, and
consists of a single layer of epithelial cells,
which, together with the pia mater, has been
removed in the preparation represented in
Fig. 154, A. The mid-brain and elongated
medulla oblongata are relatively broad, and
the cerebellum is represented by a mere narrow
ledge overhanging the fourth ventricle anteri-
orly. The roof of the mesoccele is formed
mainly by a layer of epithelial cells, and, like
that of the third and fourth ventricles, is
covered by a thickened and vascular portion
of the pia mater, or choroid plexus.

The brain of Myxinoids (Fig. 155) shows
many peculiarities, and the morphology of its
parts requires further investigation. Its sub-
divisions are broader and more closely approxi-
mated than in the Lamprey, and its right
and left halves are more plainly marked off
from one another by a continuous longitudinal dorsal furrow. No
pallium is recognisable, at any rate in the adult. The ventricles
have undergone reduction, and present individual variations : in
the ventral region of the fore-brain there is a small isolated cavity
which probably represents the vestige of a third ventricle. The
broad olfactory lobes are separated from the telencephalon by a
transverse furrow. The diencephalon is not visible from the
dorsal side, but ventrally there is a distinct infundibular process.
The mid-brain is the most prominent division : the mesoccele ends

li

Fia. 153. DIAGRAM
TO ILLUSTRATE THE
CEREBRAL FLEX-
URE OF A MAMMAL.

HH, metencephalon ;
MH, mesenceph-
alon, which at SB
forms the most
projecting portion
of the brain, re-
presenting the so-
called " parietal
bend " ; NH, my-
eleneephalon, f orm -
ing the "cervical
bend" (NB): the
" Varolian bend "
(BB) arises on the
ventral circumfer-
ence, at the junc-
tion between HH
and NH; R, spinal
cord ; VH, telen-
cephalon ; ZH,
diencephalon, with
the pituitary body
(H) at its base.

206

COMPARATIVE ANATOMY

- L.ol.

NH IV Z.ff
lilt Mil

'

-Lot

B

IT/ r/

L.ol

FIG. 154. BRAIN OF LARVAL LAMPREY. A, from above ; B, from below;

C, from the side.

, ganglion habenulae ; Op, pineal organ ; HH, cerebellum ; Hyp, hypophysis ;
L.ol, olfactory lobe ; ^f^<l, spinal cord; MH, mid-brain; NH, medulla
oblongata ; S l , S', first and second spinal nerves ; &v, saccus vasculoslis ;
VH (Jias.Gf), cerebral hemispheres between which, in A, the median portion
of the telencephalon is seen, with the pallium removed ; ZH ', diencephalon ;
I-X, cerebral nerves.

blindly in it anteriorly. The cerebellum is relatively much larger
than in the Lamprey, and projects posteriorly so as to cover the
fourth ventricle completely : it resembles that in certain embryonic

BRAIN

20'

r.o

A"

FIG. 155. BRAIN OF MVXINE. (After G. Retains.) A, dorsal; B, ventral;
C, lateral view. The olfactory organ and its cartilaginous skeleton (r.o) is
left in situ.

a, eye; g.h, ganglion habenulse ; HH, cerebellum; //, vestigial optic nerve;
M H, mid-brain ; NH, medulla oblongata ; pi, infundibular process ; r,
spinal cord ; rb, olfactory lobe ; a, spinal nerves (dorsal roots and ganglia) ;
F 1 , 2 , 3 , trigeminal ; VII, telencephalon ; VII, facial ; VIII, auditor}- ; X,
vagus ; A' 1 , sensory branch of vagus or dorsal branch of a spino-occipital
nerve.

208

COMPARATIVE ANATOMY

stages of Teleosts. A large, paired, dorsal lobe is present <>n the
medulla. 1

In Petromyzon the pineal apparatus is represented by t\\"
vesicles, which arc probably the displaced members of a pair, each
connected with the dorsal surface of the diencephalon (ganglion
habenulre) and lying one above the other just beneath the roof of
the skull : the integument immediately above these vesicles is
pigmentless. The cells on the ventral side of the larger dorsal
vesicle (epipkysis OY pineal oryan probably representing the organ
of the right side) are arranged radially and contain pigment, form-

Tz&zzm&z

" PeUucn'n -

%##?,

"Rctiiin" -i&fW-
'4 ' , tff

" PilJi'fiiJn" ~

" Retiii '

Parapineal

Fin. 156. TRANSVERSE SECTION OF PINEAL APPARATUS OF Petromy~on

(After Studnicka. )

The connective tissue roof of the skull (.) is seen above, the roof of the third

ventricle (di) below.

ing a kind of retina, but they show signs of degeneration ; the
lower vesicle or parapineal (left) or^cm, which probably corresponds
to the parietal organ of Lizards (q.v.) is simpler and more variable,
and is without pigment. The pineal stalk, arising just in front of
the posterior commissure, passes directly into the dorsal vesicle,
and the ventral vesicle is also connected with the roof of the
diencephalon (habenular ganglion) by a stalk. In Myxinoids the
pineal apparatus is evidently much degenerated, and nothing is
known of an epiphysis proper. 2

Elasmobranchs. The brain of these Fishes, like that of
Cyclostomes, is in many respects of a specialised form, character-

The homology of the parts as given above has recently been questioned :
it is possible that the division described as the telencephalon corresponds to the
diencephalon, and that the " cerebellum " belongs to the mid-brain.
- For the hypophysis of Cyclostomes, see under Olfactory Organ.

BRAIN

209

istic or, and confined to, the group, though the particular regions
are much more highly developed than in Cyclostomes : the pallium

FIG. 157. BRAIN OF Scylltiun <-)ii<-nla. A, dorsal ; B, ventral ; C, lateral view.

6.0, olfactory bulb; ep, base of epiphysis ; /. 6, telencephalon ;//, fourth ventricle ;
h.li, cerebellum ; h.p, hypophysis; if, lobi inferiores ; m.b, mid-brain (optic
lobes) ; m.d, medulla oblongata ; *<-, saccus vasculosus ; th, diencephalon ; to,
olfactory tract (very short in Scy Ilium). The epithelial and vascular roof of
the third and of the fourth ventricle has been removed, ii-x, cerebral nerves
(the ventral vagus roots are omitted in B).

is almost exclusively nervous. According to its external form, two
main types can be distinguished. Thus in Spinax, Scymnus, Noti-

210

COMPARATIVE ANATOMY

melcL

danus, and the Holocephali, it is very narrow and elongated, while in
the rest of the Plagiostomi the individual parts are more closely
compressed and approximated together (Fig. 157). In almost all
Sharks the telencephalon is relatively much larger than any of the
other parts. The olfactory lobes arise from the anterior or antero-
lateral ends of the telencephalon, and in some Elasmobranchs
remain in close connection with it : in others, in which the

olfactory capsules are situated further
forwards, they become drawn out into
long olfactory tracts, each arising from
a basal olfactory tubercle and continuous
anteriorly with an olfactory bulb from
which the olfactory nerves arise.

A division of the telencephalon into
paired halves is hardly indicated at all
in Rays, and only slightly so in the
commoner Dogfishes (e.g. Scyllium,
Acanthias), in which, however, lateral
and olfactory ventricles are present
(Fig. 158). Only in Scymnus, and to
some extent in the Notidanidre, is there
a distinct separation of the pallium
into two hemispheres. In Rays there
is only a small unpaired telocoele, the
telencephalon consisting of a practic-

Tf ni - 7 ally solid mass, and the olfactory lobes

bio. Io8. BRAIN or (Jlieilo- > ' . .. J . . ,

scyllium, (From Parker and are also solid ; in the Myliobatidse

there is no trace at all of a telocoele.

The narrow diencephalon is roofed
over by a choroid plexus, and the tube-

veiiLricies removeu so as to i -i ' i / m i \

show the relations of the llke epiphysis (wanting in Torpedo)

cavities (semi-diagrammatic), may reach such a length as to extend

r, dilatation from which the be y ond the anterior end of the brain

metacoele is given off; dia, for a considerable distance, and pass

diaccele the reference line distally into or beyond the roof of the

points to the opening leading i 11 . inrliontinn pan hp CJPPTI nf a

, fl i * f* I'l 1 V oK-llll . 11O lllLUL/dulUil Cfill Ut; otJtJll Ul tl

iter(mesocoele), into which the parietal organ. A pair of small lobes

optocu?les (opt) open; met a, the lobi infcriorcs are present on

, i ^ -\ , utiG iiiiiiiicti on 1 uiii , nnci ti sa ecus / L''(tsc f it~

of teloccele ; /-A,' rhinoccele. losus or infundibular gland, surrounded

by a blood sinus, is present on the

sides and floor of the infundibulum, with the ventricle in which it
communicates and with which a pituitary body is connected
posteriorly.

The cerebellum is a.lwa\*s very large, overlapping the optic
lobes and medulla oblongata to a greater or less extent : it is
divided into several lobes lying one behind the other, and
usually contains a metacoele opening into the fourth ventricle.

Haswell's Zoology. )

Viewed from the dorsal side,
and the roofs of the various
ventricles removed so as to

Cf)

BRAIN

211

olf.l
c-.h

prs

In Sharks, especially in Scymnus and the Notidanidse, the
medulla oblongata is elongated and cylindrical, while in Rays it is
more compressed and triangular ; at its anterior end are a number
of elevations corresponding to origins of the nerves arising from
the gray matter of the floor of the fourth ventricle in this region.
In electric Rays a pair of electric lobes (p. 192) are present at this
point, and these enclose a mass of giant nerve-cells.

Ganoids. The pallium covering the median teloccele consists
mainly or entirely of epithelial and connective tissue elements, much
as in Cyclostomes ; and the telencephalon,
which may be produced dorso-laterally into
lobes (Fig. 159), gives rise anteriorly to
cerebral hemispheres containing lateral ven-
tricles and continuous with the olfactory lobes.

The well-developed diencephalon has a
marked ventral flexure, and from its roof
arises a strong pineal peduncle, the distal
end of which extends into a hollow in the
cranial roof, but undergoes atrophy in Amia,
becoming completely separated off from the
brain. 1 Well-marked lobi inferiores are pre-
sent, and the hypophysis and saccus vascu-
losus are voluminous : the latter consists
largely of glandular tubules which open into
the mfundibulum, as in Elasmobranchs.

The optic lobes are well-marked in most
Ganoids. The large cerebellum gives rise to
a mlvuhi cerebelli (cf. Fig. 161) extending
forwards into the ventricle of the mid-brain.

Except that only the median wall of the
pallium is epithelial, the brain of Amia on
the whole most nearly approaches that of the
Teleosts in structure.

opt. I

cbl

7TL.O

FIG. 159. BRAIN OF
Lepidosteus. Dorsal
view. (After Balfour
and Parker.)

rW, cerebellum ; c.h, cere-
bral hemispheres ; ill,
diencephalon ; m.o,
medulla oblongata ;
olf. /, olfactory lobes ;
opt. I, optic lobes ; prs,
lobes of telencephalon.

Teleosts. As is the case in many other
Fishes, the brain in most Teleosts by no
means fills the cranial cavity, and it is separ-
ated from the roof of the skull by a greater or less amount of a fat-
like tissue and lymph : it never attains to so large a relative size
as does that of Elasmobranchs. Its form varies greatly, more by
far than in any other Vertebrate group, and only the following-
essential points can be mentioned here.

1 In Polypterus and Calamichthys the pineal body gives rise to a peculiar
and extremely large epithelial vesicle, and the hypophysis communicates with
the mouth-cavity by a hollow duct, even in the adult. The brain of these
forms presents other special characters, and requires further investigation. In
Devonian (ianoids, as well as in the Placoderms, there was a parietal foramen
(p. 103).

p 2

212

COMPARATIVE ANATOMY

The pallium is entirely epithelial in structure (Figs. 160 162) :
it presents no median involution dividing the anterior part of the

A

7

L.ol.

W

M

</jr

. FIG. 160. BRAIN OF SALMON. A, dorsal ; B, ventral ; and C, lateral view.

Ch, chiasma; G.p, pineal body; ffff, cerebellum; Hyp, hypophysis; Jnf,
infundibulum ; L.ol, olfactory lobe ; Med, spinal cord ; MH, rnid-brain ; NH,
medulla oblongata ; Pall, pallium (in part removed), and SGund Ba*G, basal
ganglia (corpora striata) of the telencephalon ; ,SV, saccus vascnlosus ; Tr.opt,
optic tract; UL, lobi inferiores; VH, telencephalon ; I-X, cerebral nerves ;
1 and 2, first and second spinal nerves (the first represents the hypoglossal,

telencephalon into two lateral hemispheres, and is therefore further
reduced than in Ganoids: there is a median ventricle. The lower

BRAIN

213

b^^

part of the telencephalon consists of large paired basal ganglia
(corpora striata) connected together by an anterior commissure
The olfactory lobes are either closely applied to the telencephalon
and contain a small ventricle, or they become differentiated into
olfactory tract and bulb, as in Elasmobranchs.

FIG. 161. LONGITUDINAL VERTICAL SECTION THROUGH THE ANTERIOR PART OF
THE TELEOSTEAN BRAIN. (Founded on a figure of the Trout's brain by Rabl-
Riickhard.)

Aq, iter (mesoccele); B.ol, N.of, olfactory lobe and nerve ; Ca, anterior commissure ;
Ch, posterior optic; Ch.n.opt, optic chiasma ; Ci, "inferior com-
missure"; Op, posterior commissure; O.st, corpus striatum, whicli lies on
either side of the middle line ; E)>, the epithelium (ependyme), lining the walls
of the ventricles ; Gp, pineal body, with a cavity (Gj) 1 ) in its interior ; ff, //',
hypophysis ; J, infundibulum ; Li, lobi inferiores ; Sv, saccus vasculosus ;
TVo, roof of the optic lobes ; Tl, torus longitudinalis ; fr, pathetic nerve ; Val,
valvula cerebelli ; V.cm, common ventricle of the secondary fore-brain
(teloctele) ; V.t, third ventricle ; t, point at which the epithelial roof of the
secondary fore-brain (pallium, Pa) becomes continuous with the lining of the
anterior wall of the pineal tube ; above /is seen an outgrowth which represents
a rudimentary parietal organ.

The diencephalon is very small, and is depressed between the
telencephalon and mid-brain. The epiphysis (Figs. 160, 161) is
plainly distinguishable, but it usually does not pass into the roof
of the skull ; an outgrowth arising from the roof of the brain in
front of the epiphysis represents the parietal organ, but this

214

COMPARATIVE ANATOMY

becomes constricted off from the brain and disappears during
development. 1 Lobi inferiores, as well as a hypophysis and a
glandular saccus vasculosus are present, but these vary much in
the degree of their development. The saccus vasculosus here too,
opens by several apertures into the infundibulum and is surrounded
by a blood-sinus.

The mid-brain and cerebellum are extremely large relatively ;
the latter is bent upon itself, overlies the medulla oblongata behind,

and is prolonged in front into the ventricle
of the mid-brain as a valvula cerebelli
(Fig. 161), as in Ganoids.

The Teleostean brain is a further
specialisation of the type seen in Ganoids,
and has no direct connection with that of
Cyclostomes or Elasmobranchs.

Dipnoans. Both as regards external
and internal structure, certain points of
resemblance may be seen between the
brain of Dipnoans and that of Elasmo-
branchs and Ganoids, but in other respects
it is specialised. In Ceratodus, there is a
considerable space between the walls of
the cranium and the brain except in the
region of the large olfactory lobes. The
telencephalon is well developed, and the
thin pallium, which is mainly nervous, is
involuted along the median longitudinal
line so as to completely separate the two
hemispheres from one another dorsally in
Protopterus and Lepidosiren: in Ceratodus
they are partly united together dorsally and
posteriorly by a narrow bridge formed by
the choroid plexus. Olfactory lobes arise
from the telencephalon anteriorly, and contain ventricles : in
Ceratodus they overlie the hemispheres. Postero-laterally each
hemisphere gives rise to a distinct hippocampal lobe. The pineal
body has a long stalk, and its distal vesicle perforates the carti-
laginous roof of the skull : in the embryo Ceratodus it even
reaches as far as the integument. The complicated choroid plexus
in this region gives rise to a large vesicle over which the pineal
stalk extends. Lobi inferiores are present.

The well-marked mid-brain is indistinctly paired in Ceratodus,
but is unpaired in Protopterus and Lepidosiren. The cerebellum

1 A parietal foramen is present in the embryo in several Teleosts (e.y.
Cottus, Salmo), and in some others (e.y. Callichthys) persists in the adult without
a corresponding development of the pineal organ. In certain deep-sea forms (e.g.
Argyropelecus, Cyclothone), however, the latter is comparatively complicated,
and, as in Petromyzon, consists of two vesicles, showing regressive characters.

FIG. 162. - TRANSVERSE
SECTION THROUGH THE
FORE-PAKT OF THE TELE-
OSTEAN BRAIN.

G.t, corpora striata ; Ep,
ependyme ; fr, frontal
bone, underneath which
the pineal tube, Gp, is
visible in transverse sec-
tion, and below this the
perimeningeal tissue, Pm;
Pa, the pallium, formed
of a simple epithelial
layer ; T, T, olfactory
tracts ; V.cm, teloccele.

BRAIN

215

\

is relatively much smaller than in Elasmobranchs and Teleosts : it
gives rise to a valvula cerebelli, and a complicated choroid plexus
roofs over the fourth ventricle.

Amphibians. The cerebral hemispheres of the Amphibia are
distinguished from those of the Dipnoi by a higher development
of the pallium, which, however, is differentiated even in the latter
group into an external layer of nerve fibres and an internal cellular
layer (central gray matter). The basal
ganglia (corpora striata) are less marked,
except in the Gymnophiona, and merely
form a more or less prominent thickening
of the wall of each hemisphere projecting
into the lateral ventricle. A hippocampal
lobe is not distinctly developed, but a
hippocampus is represented by elevations
of the central gray matter, which are con-
nected right and left by a small anterior
pallial commissure just above the anterior
commissure (Fig. 164, ]>)

The Amphibian brain docs not, how-
ever, lead directly towards that of Reptiles.
Although the telencephalon is more highly
differentiated than in lower forms, the cli-
encephalon and mesencephalon are sim-
pler than in Fishes ; and, on the whole,
the brain of Amphibians is less com-
plicated than that of any other Verte-
brates, except Lampreys.

In Urodeles the individual parts are
more elongated and separated from one
another than in Anurans, and the dien-
cephalon is therefore more freely exposed.
The hemispheres are almost cylindrical,
and the olfactory lobes are distinct from
one another, while in the Anura they are
fused for a short distance anteriorly
(Fig. 164). The diencephalon and optic
lobes are much broader in Anurans than

Urodeles. The cerebellum consists

- G.UI

Fid. 163. BKAIN OF Cera-
tod'iis foxteri. Dorsal
view. (From Parker
and Haswell's Zoology. )

and, auditory nerve ; chl,
cerebellum ; fac, facial
nerve ; ijf, glossopharyn-
geal ; med, medulla ob-
longata ; mes, mesen-
cephalon ; or, oculomotor
nerve ; opt, optic nerve ;
}iroi, cerebral hemis-
pheres ; rh, olfactory
lobes ; <vy, vagus nerve.

in

simply of a small transverse fold, and is especially rudimentary
in Urodeles.

The infundibulum and hypophysis are well developed :_ a saccus
vasculosus (infundibular gland) is no longer distinct as in Fishes,
but is represented by the so-called infundibular process. The
epiphysis does not extend beyond the skull in Urodeles, but in
Anuran larvae it reaches the integument, undergoing reduction
later, when the bony skull-roof is formed; indications of its
extracranial portion can, however, be more or less distinctly

216

COMPARATIVE ANATOMY

\lcct.cbl

rifi

i- < X' c b chplx.2

Sp cd

FIG. 164. BKAIX OF FROG[(Rana temjiontriit). A, from above; B, from below ;
C, from the side ; D, in longitudinal vertical section. (A C, after (Jraupp ;
1), after Osborn.)

Cb, cerebellum; Cer.H, cerebral hemispheres; ch.plx 1 , in A, paraphysis, in 1),
anteroid choroid plexus (removed in A) ; rh.jdx*', posterior choroid plexus
(tela choroidea) ; Com (above), superior and posterior commissures; Com
(below), anterior commissure and anterior pallia! commissure just above it ;
Cr.C, crura cerebri ; JJi, diencephalon ; for. J/, foramen of Monro ; i, iter ;
inf, infundibulum ; Med.obl, medulla oblongata ; Off. I, olfactory lobe ; opt.ch,
optic chiasma ; Opt. I, optic lobe ; opt.v, optic ventricle ; pin, stalk pineal of
body ; pit, pituitary body ; Sp.cd, spinal cord : r\ third ventricle ; v 4 , fourth
ventricle; I-X, cerebral nerves ; 1 Sp, 2 Sp, first and second (= embryonic
second and third) spinal nerves.

BRAIN

217

recognised even in the adult as a " brow-spot " : thus its
intracranial portion does not represent the entire epiphysis. 1 A
parietal organ appears to be entirely wanting in all Amphibians
with the possible exception of some few Anura, in which traces of
it have been described. 2

FIG. 165. LONGITUDINAL SECTION OF BRAIN OF AN EMBRYO OF Lacerta
vivipara ^ 26. (After K. von Kupffer. )

ao, olfactory area ; hi, blood-sinus ; bo, olfactory lobe ; c, cerebellar commissures ;
ra, anterior commissure ; ch, superior (habenular) commissure ; ck, central
canal of spinal cord ; cp, posterior commissure ; cpa,, anterior pallial (hippo-
campal) commissure ; cpo, posterior optic commissure ; cpp, posterior pallial
commissure ; cs, spinal commissure ; <:n; swelling of optic chiasma ; c 1 , para-
physis ; hm, cerebral hemisphere ; hi/, hypophysis ; in, interorbital septum ;
J (and ) ventricle of infundibulum ; opt, optic chiasma ; pa.-, parietal organ ;
pi'h, choroid plexus on medulla oblongata ; /?, hind-brain ; ro, optic recess ;
si, sulcus intraencephalicus posterior ; tp, tuberculum posterius superius :
tr, torus transversus ; vb, ventral flexure of medulla oblongata ; vc, valvula
cerebelli ; vp, posterior medullary velum ; vt, velum transversum ; Z, epi-
physis.

In the Gymnophiona the olfactory lobes and hemispheres are
relatively larger than in other Amphibians, and the hemispheres
overlap the posterior parts of the brain to a larger extent. The

1 A rounded vascular body arising from the roof of the third ventricle and
representing a paraphysis (p. 200) has been mistaken for the epiphysis (Fig. 164,
A, ch. plx').

2 A parietal foramen was, however, present in the Palaeozoic Stegocephali
and other extinct Amphibia.

218

COMPARATIVE ANATOMY

-Sol B

rar-

&p

vi r

CkTrJnf ILyp VJf

Fid. 166. BRAIN OF Hatteria, punctata. A, dorsal ; B, ventral ; and C,

lateral view.

Bol, olfactory bulb ; Ch, optic chiasma ; GHS, cerebral peduncles ; G.p, pineal
body, shown in C continuous with the parietal eye (Pa}, and only indicated
diagrammatically in A ; h, small elevation in front of the cerebellum ; HH,
cerebellum ; Hyp, hypophysis ; //, infundibulum ; Lp, process of the hemi-
sphere representing a hippocampal lobe ; between Lp and GHS is a deep groove
(fovea limbica) between the olfactory lobe and pallium ; Med, spinal cord ;
MH, optic lobes ; NH, medulla oblongata ; Nopt, optic nerve ; R 1 , curved
ridge at the base of the optic lobe ; Tr, optic tract ; VH, cerebral hemi-
spheres ; I-XII, cerebral nerves.

BRAIN

219

A

FIG. 167. BRAIN OF ALLIGATOR. A, dorsal; B, ventral; and C, lateral view.

B.ol, olfactory bulb; G.p, pineal body; HH, cerebellum; Hyp, hypophysis;
Jnf, infundibulum ; Med, spinal cord ; MH, optic lobes ; NH, medulla
oblongata ; Tro, olfactory tract ; VH, cerebral hemispheres, each of which
gives rise postero-laterally to a hippocampal lobe partially overlying the corres-
ponding optic tract, Tr.opt ; ZH, diencephalon ; 1-XII, cerebral nerves ;
I, 2, first and second spinal nerves.

220 COMPARATIVE ANATOMY

infundibulum and hypophysis extend far backwards : details
regarding the pineal apparatus are not known.

Reptiles. The brain of Reptiles reaches a considerably higher
stage of development than that of the forms already described,
and the individual parts overlie one another to a greater extent,
especially in the Agama^ and Ascalabota?.

In the brain of Dipnoi and Amphibia there are comparatively
few cellular elements in the outer layer of the pallium, and the
larger masses of cells (central gray matter) line the ventricles :
in the Reptilia a peripheral shifting takes place, giving rise to the

Parict'll >,,, -^jfl&^^^jjfo E.n. ,,<! ,,,,A intenwl

tii.yer of optic i-i>p

. Nerve

Epithelial roof of jgf3fa< f j^ Wy .Jt3 pineal organ (epiphysis)
3rd ventricle V*ae* \\1KI

Superior commissure _ - _

FIG. lf>8. SKKTCH UK THE PINEAL APPAKATUS OF HATTERIA. (After Dendy. )

formation of a definite cortex, containing the characteristic pyra-
midal cells such as are present in all the higher Vertebrates. It
appears that the first differentiation of a cortex phylogenetically
was connected with the olfactory sense : while in Fishes, for
example, the olfactory tracts terminate in the corpora striata, most
of their fibres extend into a definite region of the pallium from
Reptiles onwards. Thus an " olfactory cortex " is formed, to which
other centres are gradually added in the ascending series of
Vertebrates.

The pallial commissures (Fig. 165), like those of Amphibians,
are not large relatively, but in addition to an anterior pallial or
hippocampal commissure, traces are present of a so-called "fornix"
(posterior pallial commissure, p. 201); the hippocampal lobes

BRAIN 221

with their choroid plexuses are much more distinct in many cases
(e.g. Hatteria, Chelonia, Crocodilia).

The olfactory lobes may be closely applied to the hemispheres
(e.g. Anguis, Amphisbaena, Typhlops), or may consist of a well-
marked olfactory tract, passing anteriorly into an olfactory bulb
from which the nerves of smell arise (e.g. Hatteria, Lacerta,
Crocodilus). Olfactory ventricles are usually present.

The diencephalon is always depressed, and is hardly, or not at
all, visible from the dorsal side. A distinct hypophysis and in-
fundibulum as well as an epiphysis are present, and in most
Lizards the parietal organ (cf. p. 202) retains more or less distinctly,
even in the adult, the structure of a median eye. 1

This parietal eye (Fig. 168) is situated in the parietal foramen
of the skull, and is in close connection with the more posteriorly
situated pineal organ, though in the embryo the nerve which
supplies it is seen to arise independently from the brain, in front
of the pineal outgrowth. It has the form of a vesicle, the dorsal
wall of which may become thickened to form a transparent lens-
like body, while the rest of the wall consists of several layers and
forms a pigmented "retina," with which the more or less rudi-
mentary nerve is continuous. The vesicle is surrounded by a
vascular connective tissue capsule, and in many cases the integu-
ment immediately overlying it is pigmentless and transparent,
forming a kind of cornea. Traces of a vitreous body have also
been observed. 2 Various degrees of reduction of the " retina "
and other parts as they occur, c.t/. in Hatteria, are seen amongst
Lizards (e.g. Lacerta, Anguis), and the organ may be recognised in
a simpler form in embi'yo Snakes.

As in all the Amniota, two chief divisions can usually be recog-
nised in the infundibulum of Reptiles : a dorsal vascular and
glandular body, corresponding to the saccus vasculosus of the
Anamnia, and a more ventral infundibular portion, the glandular
character of which is still retained to some extent in the Sauropsida,
but there is no opening into the ventricular cavity.

In the mid-brain the two well-marked optic lobes in some cases
show indications of a further subdivision into four ; from them the
optic tracts pass downwards and forwards to the chiasma. The
cerebellum is relatively small, except in the Crocodilia (Fig.
167), in which it consists of a thicker median lobe, and of two
lateral portions. The medulla oblongata has a marked ventral
flexure.

Birds. The avian brain (Fig. 169) is of a very peculiar type :
it has few points of resemblance to that of Mammals, and is very
different from that of Reptiles, though especially as regards its

1 A parietal organ is wanting in Gecko, Ameida and Tejus, and there is no
pineal organ in the Crocodile.

! The parapliysis gradually extends beneath the epiphysial outgrowth, and
forms a sort of cushion under the parietal eye.

222

COMPARATIVE ANATOMY

individual sections it is more or less comparable to that of certain
of the latter (e.g. Chelonia). The basal ganglia (corpora striata) of

H yp\

Tr.opt J/if
FIG. 169. BRAIN OF PIGEON. A, dorsal; B, ventral ; and C, lateral view.

HH, cerebellum ; Hyp, hypophysis ; luf, infundibuluin ; Z.o/, olfactory lobes;
Med, spinal cord ; MH, optic lobes ; Nil, medulla oblongata ; Tr.ojtt, optic
tract; VH, cerebral hemispheres; I-X1I, cerebral nerves; 1, 2, first and
second spinal nerves.

the hemispheres reach a relatively larger size in Birds than in
any other Vertebrates. An advance on Reptiles is seen in the

BRAIN

223

connections of the pallial cortex in various directions, and in- many
Birds indications of cortical centres can already be recognised.

In the well-developed hemispheres frontal, parietal, and temporal
regions can be recognised : their surface is perfectly smooth, and

f-P-

\

I lu

h.l.

FIG. 170. BRAIN OF RABBIT. A, dorsal ; B, ventral ; and C, lateral view.

6.0, olfactory bulb ; rb' superior verniis, and <!>", lateral lobe of cerebellum ; r/-,
crura cerebri ; fp, pineal body ; f.b, cerebral hemispheres ; f.p, pallial fissure ;
h. b, cerebellum ; h.l, hippocampal lobe ; lip, hypophysis; m.b, optic lobes;
iii.tl, medulla oblongata ; ji.r, pons Varolii ; r.f, rhinal fissure ; tr.o, olfactory
tract; i-xii, cerebral nerves.

the lateral ventricles are not extensive. The different parts of the
brain overlie one another much more markedly than in any Reptile,
and the hemispheres are much larger relatively, covering over the
diencephalon and part of the mid-brain. The olfactory lobes are

224

COMPARATIVE ANATOMY

poorly developed. 1 The distal enlarged end of the pineal body
extends as far as the dura mater, and the structure of the internal

B

H PO vim

FIG. 171. BRAIN OF DOG (POINTER). A, dorsal; B, ventral; and C,

lateral view.

Bo, Bo 1 , arcuate fissures ; B.ol, olfactory bulb ; Cr.ee, crura cerebri ; Fi.p, pallial
fissure ; FS, Sylvian fissure ; HH, lateral lobe, and HH 1 , flocculus of cere-
bellum ; Hyp, hypophysis ; LH, hippocampal lobe ; Med, spinal cord ; XH,
medulla oblongata ; Po, pons Varolii ; fiF, rhinal fissure; 8c, sulcus cruci-
atus ; TO, olfactory tract ; VH, cerebral hemisplieres ; Wu, superior vermis ;
I-XII, cerebral nerves.

part of the organ resembles that of a tubular gland, penetrated by
fibrous tissue and blood-vessels. There is no trace of a parietal
organ.

1 The toothed Birds of the Cretaceous period, with Hesperornis at their
head, possessed a very lowly organised, reptilian form of brain, with small
hemispheres and large olfactory lobes.

THE BRAIN

225

The cerebellum consists of a well-developed and folded median
lobe, and of two lateral portions (flocculi), which vary much both
in form and size. Posteriorly it completely covers the fourth
ventricle. The two optic lobes, in which, as in certain Reptiles, a
subdivision is indicated, are separated from one another and
pressed downwards, so as to lie at the sides of the brain in the
angle between the hemispheres, cerebellum, and medulla oblongata,
and they are connected by a broad commissure. The ventral side
of the short medulla shows a marked flexure, bending upwards to
the spinal cord.

Mammals. The brain in embryo Mammalia is very similar
to that of the Sauropsida, but the subsequent differentiation
of its parts, and more particularly that of the pallium, gives it a

epi 7tip.com

\mid.com
c.ytt

cbl

artl.com

rnecl

c.mam

Fio. 17'2. LONGITUDINAL SECTION OF BRAIN OF ROCK WALLABY (Petrogale

(From Parker and Hasvvell's Zoology.)

ant.com, anterior commissure ; cbl, cerebellum ; i-.inntn, corpus mammillare ; c.qit,
optic lobes ; cnir, orura cerebri ; epi, epiphysis, with the posterior com-
missure immediately behind it; f.mon, position of foramen of Monro ;
hip.i-otn, hippocampal commissure, consisting here of two layers, continuous
at a posterior bend, the splenium, somewhat divergent in front where the
septum lucidum extends between them ; hypo, hypophysis ; med, medulla
oblongata ; mtiL<-om, middle commissure ; olf, olfactory lobe ; opt, optic
chiasma ; <<?. 3, third ventricle.

very special character. The cerebral cortex becomes much more
highly developed and in many Mammals is more or less markedly
convoluted 1 (Figs. 171 and 173, B). In others, again, the
surface of the hemispheres remains smooth (Fig. 170), but a
subdivision into lobes (frontal, parietal, occipital, and temporal), as
well as certain fissures (e.g. rhinal, hippocampal, callosal) can
always be recognised to a greater or less extent, and the
hemispheres are relatively so large as to cover over the more
posterior parts of the brain ; in some of the lower forms, the mid-
brain can still be seen from above, while in the higher types

1 It is only possible to homologise the -main, sulc-i to a greater or less extent
amongst the various types of convolution seen in the Mammalian brain (cf. p. '228).

Q

2-_>6

COMPARATIVE ANATOMY

(Primates) even part of the cerebellum is hidden (Figs. 173, A
and B), although this is to a greater extent the case in some of
the lower Apes, with smooth hemispheres (e.g. Hapale, Chrysothrix),
than in Man. No satisfactory explanation has so far been given
for the different degrees of convolution seen amongst Mammals:
as a general rule, the brain in lower and smaller types (except, e.g.
in Echidna) is less convoluted than in higher and larger ones.

The number of fibres radiating from the cortex (corona radiata)
is very small in lower types (e.g. Rodents), and largest in Man. A
complex network of fibres in the cortex itself connects its various
parts together, and other strong bundles extend through the

MH

Jiff

FIG. 173A. HUMAN BRAIN. Median longitudinal vertical section.
(Mainly after Reichert. )

corpus callosum ; G, fornix, which extends antero-ventrally to the lamina
terminalis (Col), in the upper part of which is seen the anterior commissure
(Ca), and between the latter and the optic thalami (To) the foramen of
Monro (FM) ; H, pituitary body ; ////, cerebellum ; MH, corpora bigemina,
with the iter (Aq], anterior to which is seen the posterior commissure (Cp) ;
NH, medulla oblongata, with the pons Varolii (P) ; -ff, spinal cord; T,
infundibulum ; Teh, tela choroidea ; To, optic thalamus (diencephalon), with
the middle commissure (Cm); VH, cerebral hemisphere; Z, pineal bod}';
/, olfactory nerve ; //, optic nerve.

hemispheres connecting individual regions of the pallium with one
another. The commissures between the hemispheres known as
the corpus callosum and fornix (Fig. 173, A) are also much more
highly developed than in other Vertebrates. The former is an
important structure in the higher Mammalia, its development
corresponding to that of the pallium: ifc extends upwards and then
backwards from the region of the lamina terminalis in the form of
a thin plate, and reaches its highest development in Primates.
A corpus callosum is apparently wanting in Monotremes and
Marsupials, in which a hippocampal commissure is present in
the position of the body of the fornix, just above the anterior

THE BRAIN

227

commissure (Fig. 172, and cf. pp. 201 and 220), and the brain of
these forms remains at a comparatively low stage of development :
the anterior (basal) commissure is comparatively large, whereas
in the Eutheria its relative size is in inverse proportion to
that of the more important corpus callosum. In Edentates the
brain is also of a low type, and the same is true of that of Rodents,
Insectivores, and Bats, though a considerable advance is here
seen as compared with Marsupials. A large middle commissure
connects the two optic thalami.

In addition to the lobes mentioned above, a central lole of the
hemispheres is present in Primates, and increases in development
in passing from the Gibbon, Orang, Chimpanzee, and Gorilla, up
to Man. But there is no justification for the statement that " the
human brain is only an enlarged anthropoid brain," for in the

JUL

FIG. 173B. CONVOLUTIONS OF THE HUMAN BRAIN. (After A. Ecker.)

a,l,r, superior, middle, and inferior frontal gyri ; em, the calloso-marginal sulcus
on the dorsal surface ; FS, Sylvian fissure ; HH, cerebellum ; Lj\ frontal
lobe ; Lo, occipital lobe ; Lp, parietal lobe ; NH, medulla oblongata ; Po,
parieto-occipital fissure ; P, J n , superior and inferior parietal gyri separated
from one another by the interparietal fissure (/) ; P,, spinal cord ; T, tem-
poral lobe ; A', # I, anterior and posterior central convolutions, separated from
one another by the fissure of Rolando (A'); 1 to 3, superior, middle, and
inferior temporal convolutions.

former a number of entirely new regions have been acquired,
especially as regards the frontal, temporal, and central lobes, which
have consequently undergone extension.

In correspondence with the division of the hemispheres into
lobes, there is a differentiation of the lateral ventricles, 1 so that an
anterior, a posterior, and an inferior cornu can be distinguished
in each ; the inferior cornu extends into the temporal portion,
which corresponds to the hippocampal lobe of Reptiles, and an
eminence on its floor, the hippocampus major, 2 formed as an

1 The ventricles are lined by epithelium (epeudyme), which, strengthened by
connective tissue layers derived from the pia mater (tc/a 1 choroidecK), also forms
the roofs of the third and fourth ventricles and extends into the lateral ventricles
as plexus choroi(l< i.

"* The hippocampal system has important relations to the olfactory centre.
The (/tji-ux dt'-nfatiift (fascia dent at a) and i\\ejiml>ria arise in close connection with
the hippocampus, the fimbria having intimate relations to the fornix.

228 COMPARATIVE ANATOMY

involution of the median wall of the hemisphere, is much more
marked than in lower forms : the line of involution is known as
the hippocampal fissure.

The central olfactory apparatus (rkincnccphalon) in its entirety
is represented by the olfactory bulb, peduncle, and tubercle, the
piriform lobe, and the hippocampus, and is separated from the
pallium by the rhinal fissure (Figs. 170, 171). This fissure is in
close relation to the splenial (cattosal) fissure, which bounds the
supracallosal gyrus dorsally, extending more or less parallel to the
corpus callosum. The Sylvian fossa or fissure is also a typical
fissure : it is situated at about the middle of the rhinal fissure,
and in the higher Mammals is overlapped by the pallium so that
the fossa is converted into a fissure. In Carnivores, Cetaceans, and
Ungulates, three gyri arch over the Sylvian fissure, one above the
other, and are separated by the so-called arcuate fissures (Fig. 171).
The upper of these, bounded above by the longitudinal pall in I
fissure, is spoken of as the marginal gyrus. Along the lateral sur-
face of the hemisphere, the cruciate sulcus (the homologue of the
central sulcus or fissure of Rolando of Primates) extends upwards
to the pallial fissure. Characteristic of the brains of all Apes except-
ing those with smooth hemispheres is the parieto-occipital sulcus,
between the parietal and occipital lobes : in Man, the lateral parts
of this fissure are more or less indistinct (cf. 173, B, in which other
gyri and sulci of the human brain are shown).

The corpus striatum is surrounded and perforated by fibres
passing down from the pallium (anterior limb of the internal
capsule of Primates). Unlike the corresponding structure in
other Vertebrates, the corpus striatum of Mammals becomes
gradually more deeply situated, and is comparatively small as com-
pared with the rest of the brain.

The olfactory lobes usually extend forwards freely from the base
of the telencephalon, and each may retain throughout life a
prolongation of the lateral ventricle (e.g. Perissodactyles) ; in
some cases (e.g. numerous aquatic forms and Primates) they are
completely covered by the frontal lobes. The degree of their
development is in proportion to that of the olfactory sense, and
they may even be entirely reduced (cf. under Olfactory Organ).

The pineal body is displaced downwards by the hemispheres
and lies against the anterior lobes of the mid-brain, not reaching
to the roof of the skull and brain-membranes. Its bifurcated
peduncle connects it with the roof of the diencephalon and
contains nervous substance : its distal end has the form of a
rounded or oval sac, consisting of compact epithelial tissue and
containing concretions. A parietal organ is wanting. Traces of
the saccus vasculosus and lobi inferiores still occur, even in Man,
in connection with the infundibulum.

The mid-brain (corpora bigemina) is of smaller relative size
than in other Vertebrates. A transverse furrow across the solid

THE BRAIN

229

optic lobes subdivides them into an anterior larger and a posterior
smaller pair of lobes (cf. p. 221).

The division of the large cerebellum into a median and two
lateral portions, already indicated in Reptiles, is carried to a still
further extent in Mammals. The median portion gives rise to the
so-called superior veruiis, while the lateral parts form the lateral
lobes and ftocculi (Figs. 170, 171). In Carnivores, certain
Edentates, Pigs, and Lemurs, the verm is is relatively large as
compared with the lateral portions ; while in Cetaceans, Elephants,
Apes, and Man the latter are more highly developed and the
median lobe reduced. The two lateral lobes of the cerebellum are
connected by a large commissure, the pons Varolii : this extends
round the medulla oblongata ventrally, and is more largely
developed the higher we pass in the mammalian series. Other
bands of nerve-fibres connecting the cerebellum with various parts
of the brain are spoken of as the anterior, middle, and posterior

Tfv

LCar

Jftf

Cacb

Fio. 174. DIAGRAM OK THE CHIKK SYSTEMS OK FIBRES OF THE MAMMALIAN*
(HUMAN) BRAIN. (From a drawing by A. Ecker.)

Cac, crura cerebelli ad corpora bigemina ; Cacb, crura medulla ad cerebellum ;
Cap, crura cerebelli ad poiitem ; CC, crura (pedunculi) cerebri ; CS, corpus
striatum ; HH, cerebellum ; HM, hemisphere ; L, lemniscus : P, pons ;
Th, optic thalamus.

peduncles of the cerebellum, the relations of which, and of the
crura cerebri, are indicated in Fig. 174.

A study of the brain-casts in certain North American Eocene
forms is very instructive from an evolutional point of view, and
shows that the brain, and more especially the fore-brain, in these
animals was of extremely small size relatively (Fig. 175). The
brain of Dinoceras mirabile might easily be mistaken for that of
a Lizard, and was so small that it could easily be drawn through
the greater part of the neural canal : in the Cretaceous Dinosaurian
Triceratops, the brain was apparently still smaller relatively. The
olfactory nerves were extremely well developed in these forms.

In connection with the importance of the brain in modelling
the form of the skull, it may be mentioned that in many
Mammals, including Man, the outer surface of the skull in various

230

COMPARATIVE ANATOMY

parts shows a kind of relief of certain underlying portions of the
brain. In some cases, only the larger divisions of the brain
(cerebrum and cerebellum) are thus indicated externally: in others,
a relief of the convolutions is also seen, and in Mustela and Lutra. for
example, it is almost complete on the lateral portions of the skull

FIG. 175. CASTS OF THE BRAINS OF VARIOUS EOCENE MAMMALS.

(After Marsh.)

Skull, with brain indicated, of A, Tillotherium fodium ; B, Rronfotha-iiuii in</enx ;
C, (Joryphodoii . hamatnx ; and I), Dinoceras mirabile. E and F, ventral and
lateral views of the brain of Dinoi'tra-s mirabile.

covered by the temporal muscles, and may even extend dorsal ly
to the middle line.

II. PERIPHERAL NERVOUS SYSTEM.

Two principal groups of peripheral nerves may be distinguished,
viz., spinal and cerebral, that is, those which arise from the
spinal cord and brain respectively : between them is an inter-

PERIPHERAL NERVOUS SYSTEM

FlG. 176. DIAGRAM ILLUSTRATING THE ORIGIN, COURSE, AND TERMINATION OF
THE MOTOR AND SENSORY FIBRES OF THE SPINAL NERVES, AS WELL AS THE
RELATIONS OF THE SENSORY COLLATERAL FIBRES TO THE POINTS OF ORIGIN
<IF THE VENTRAL ROOTS. (After M. v. Lenhossek. )

The spinal cord is shown as if transparent. The fibres of the ventral roots
arise from the motor cells of the ventral cornua of the gray matter (a) and
end in fine branches 011 the striated muscle-fibres (c). The spinal ganglion
(//I is shown relatively much larger than in reality, and in it only a single
unipolar nerve-cell is represented : the centripetal fibre of the latter is seen
entering the dorsal root, and at e bifurcates in the spinal cord into an anterior
(/) and a posterior (g) branch, each of which ends freely in the gray substance,
first giving off numerous collateral fibres (h). The centrifugal fibre of the cell
in the spinal ganglion forms a peripheral sensory fibre extending to the skin,
where part of it is shown ending in fine branches in the epidenn (/), anothei
part forming a coil in connection with a tactile corpuscle ().

mediate group known as the spino-occipital nerves. By their means
a physiological connection is established between the periphery of
the body and -the central nervous system both in centripetal and
centrifugal directions.

o*

232

COMPARATIVE ANATOMY

Kein'ttl vidgc

The spinal nerves retain the more primitive and simple
relations, and all show a similar arrangement along both dorsal
and ventral regions of the spinal cord, so that each segment of the
trunk possesses a dorsal and a ventral pair. The former consist
mainly of afferent (centripetal) or sensory, the latter of efferent
(centrifugal) or motor fibres (Fig. 176).

Each dorsal or sensory nerve has a ganglion in connection
with it, while on the ventral nerves a ganglion is wanting, except
in the embryo of certain Fishes. The ventral nerves, which
mainly supply the great lateral muscles of the trunk and their
derivatives, arise as direct outgrowths from the spinal cord, while
the dorsal nerves first appear as outgrowths from their ganglia,
coming into connection with the cord secondarily. The ganglia them-
selves are developed from a neural ridge of ectoderm cells lying
close to the junction of the medullary cord and outer ectoderm
(Fig. 177). On the distal side of each ganglion, both nerve-roots

almost always become bound
up in a common sheath, though
many facts seem to indicate
that in the ancestors of exist-
ing Vertebrates the dorsal and
ventral roots remained dis-
tinct, as in fact is still the
case in Amphioxus and Petro-
myzon.

In addition to the somatic
afferent and efferent fibres,
which innervate the skin and
muscles of the dorsal and
lateral portions of the myo-
tomes, certain afferent and
efferent visceral components
also pass through the dorsal
and ventral nerves. The former
are in connection with ganglion cells of the sympathetic system,
situated more peripherally, and pass along the dorsal roots ;
while the latter arise from nerve-cells in the ventro-lateral regions
of the spinal cord, and pass out through both the dorsal and
ventral roots, without, however, having any physiological connection
with the ganglia of the former. These supply the muscles of the
viscera and blood-vessels which originate from the lateral plates of
the mesoderm (p. 9), but do not innervate the skeletal muscles.

The common nerve-trunk formed by the junction of the two
roots divides up again into a dorsal and a ventral branch, containing
both afferent and efferent somatic fibres, and a visceral branch,
containing afferent and efferent visceral fibres. The first of these
goes to the muscles and skin of the back, the second supplies the
lateral and ventral portions of the body- wall, while the visceral

l cord

FIG. 177. DIAGRAM OF THE EMBRYONIC
SPINAL CORD AND THE NEURAL
RIDGE FROM WHICH THE SPINAL
GANGLIA ARE DIFFERENTIATED.
(After J. S. Kingsley.)

SPINAL NERVES 233

branch supplies the viscera and is in connection with the
sympathetic.

1. SPINAL NERVES.

As a general rule, each corresponding pair of dorsal and ventral
nerves or nerve-roots lies in the same transverse plane : an excep-
tion to this is seen, however, in Cyclostomes, Elasmobranchs, and
Dipnoans, in which each ventral pair alternates with a dorsal
pair. 1 In Ganoids also, lateral displacements of the nerve-roots
are met with.

In Fishes the greatest variations are seen as regards the mode
of exit of the nerves, which pass through the intercalary pieces
of the vertebral column, or between the arches ; but from Amphi-
bians onwards the nerves always make their exit on either side
between the arches, through the iritervertebral foramina.

In their primitive undifferentiated condition the spinal nerves
have a strictly metameric arrangement, and are equally developed
all along the body. In the region of the developing extremities,
the nerves bifurcate and extend into the limb, each forming a
dorsal extensor and a ventral flexor. This primary condition
becomes complicated by the fact that the extensions of the
myotomes into the limbs do not all become differentiated simply
into a dorsal and a ventral portion, and thus it becomes impossible
to draw a hard and fast line between the dorsal and ventral
branches of the nerves of the extremities. As a rule, a number of
the ventral branches of the spinal nerves in the region of the
appendages become connected together to form plc.mses which,
according to their position, are spoken of as cervical, brachial,
lumbar, and sacred (Figs. 145 and 147). The number of nerves
composing these corresponds to the number of body-segments
taking part in the formation of the appendages, and their relative
size is usually directly proportional to the development of the
latter. A further important factor in the formation of the limb-
plexuses can be traced to the ontogenetic and phylogenetic shifting
of the extremities along the trunk, whereby they assimilate nerves
from other segments and lose some of those which primarily
belonged to them.

In contrast to Fishes, the great variation in the comparatively
slightly developed plexuses of which renders it impossible to reduce
them to a common plan, we find from the Amphibia onwards a
typical grouping of the branches of the cervico-brackial and lumbo-

1 In Amphioxus, the dorsal and ventral nerves alternate right and left, so
that a right dorsal and a left ventral nerve are in the same transverse plane and
vice versa, and the fibres composing the ventral nerves are not surrounded by a
common sheath. The dorsal nerves of Amphioxus and Petromyzon, like many
of the cerebral nerves of all Craniata, are throughout of a mixed nature, and this
was possibly the case in the Protovertebrata.

234 COMPARATIVE ANATOMY

moral plexuses, in which dorsal and ventral branches can usually
be recognised.

In the brachial plexus, which from the Sauropsida onwards is
more sharply differentiated from the cervical plexus, the following
may be distinguished: (1) anterior (superior) thoracic nerves
(dorsalis scupula: and thoraclcus latcralis of human anatomy); (2)
posterior (inferior) thoracic nerves (subclamus, thorr/<'<' anteriores) ;
(3) ventral (anterior) brachial nerves (mcdianus, with the mnsculo-
cutaneus, ulnaris, and cutaneus medius et interims) ; (4) dorsal
(posterior) brachial nerves (subscapulares, axillaris, and radialis).

The lumbo-sacral plexus shows in general, and more particularly
in Mammals, much greater variations than does the brachial plexus.
The nerves arising from it are also arranged in a dorsal and a
ventral series, the larger ones being spoken of as the obturator,
crural, sciatic, and pudendic. The sciatic divides up in the hind-
limb into a tibial and afibular nerve. The numerous individual
variations seen in the lumbo-sacral plexus (e.g. of Man) are due to
the fact that the pelvic girdle has not yet acquired such a fixed
position with regard to the trunk as has the shoulder-girdle, and
shows a tendency to become shifted forwards. 1

2 CEREBRAL NERVES.

It is customary to distinguish the following twelve pairs of
cerebral nerves, and of these the eleventh have a distinct origin
only in the Amniota, and the twelfth are represented b} 7 the
anterior spinal nerves in certain Fishes and in all Amphibians :

I. Olfactory.

II. Optic.

III. Oculomotor.

IV. Pathetic or trochlear.
V. Trigeminal.

VI. Abducent.

VII. Facial.

VIII. Auditory.

IX. Glossopharyngeal.

X. Vagus or pneumogastric.

XI. Spinal accessory.

XII. Hypoglossal, in close connection with which
are certain " spino-occipital " nerves.

Of these, the purely sensory olfactory and optic nerves have
an isolated position as regards their mode of development from the
secondary and primary fore-brain respectively, and they are dealt

1 In animals in which the extremities have disappeared, all traces of the
corresponding plexuses have also usually vanished : Snakes, however, still retain
remnants of them.

CEREBRAL NERVES 235

with in the sections on the brain and the olfactory and optic
organs.

The auditory is also a purely sensory nerve.

The oculomotor, pathetic and abducent supply the muscles of
the eye, and probably, like the spinal accessory and hypoglossal,
consist exclusively of somatic motor fibres. All the other cerebral
nerves, like the spinal nerves, are mixed, and contain, in different
proportions, somatic afferent (general cutaneous) and efferent, and
also visceral afferent and efferent components.

In their mode of origin the cerebral nerves (except I and II)
resemble the spinal nerves in many respects (p. 232), and a gradual
transition between the two groups is indicated in the lower Verte-
brata. Certain of them, like the motor portions of the spinal
nerves, arise as direct outgrowths from the extension of the ventral
horns of the spinal cord into the embryonic brain (III, VI, XII,
and probably IV). 1

Others again (V and VII in part, VIII, IX, and X) arise in
the embryo from the dorso-lateral region of the brain, and thus
resemble the dorsal roots of the spinal nerves, but differ from
these (except in the cases of Amphioxus and Petromyzon) in being
developed opposite to and not between the corresponding somites :
later on their origin becomes shifted to the ventral side of the
brain. In all these (except VIII) the visceral system predominates
over the somatic owing to visceral muscles occupying the place of
somatic muscles. They further resemble the dorsal roots of the
spinal nerves in arising from a neural ridge ; this, however, is not
continuous with that of the spinal cord, which ends in the auditory
region, but is more dorsal in position and extends into the trunk-
region for a short distance. But in the course of development
these two ridges become no longer distinguishable from one another,
and complications arise owing to a kind of struggle between the
two for mastery, in which the cerebral neural ridge gains the upper
hand ; thus the rudiments of the spinal ganglia in this region,
together with the corresponding somites, become reduced to mere
vestiges, the development of the nerves of the visceral arches
excluding that of the spinal nerves.

From the cerebral neural ridge certain ganglia arise, viz., the
Gasscrian (V), the ycniculate (VII), the petrosal (IX), and the
jugular (X) ; and, as in the case of the spinal ganglia, afferent
(sensory) fibres are formed which grow into the brain and are there
connected with their cerebral centres. In the formation of these
mixed cerebral nerves (arid also of the Vlllth), certain ganglionic
zones of proliferation of the external ectodermal epithelium

1 The fourth nerve is peculiar in appearing from the dorsal surface of the
brain, but this is probably a secondary condition. Originally, this and the third
nerve possibly belonged to the trigeminal, the sixth to the facial, and all three
nerves of the eye-muscles perhaps represent vestiges of primitive cerebral nerves
of a mixed nature, as is indicated by the fact that some few sensory fibres may
be present in the fourth, and probably the sixth, amongst the Anamnia.

236

COMPARATIVE ANATOMY

nit

nil'

(placodes) take part, in addition to the main ganglia already referred
to. Two rows of these accessory ganglia (which probably corre-
spond to the vestiges of primitive in-
tegumentary sense-organs) may often
be distinguished in the embryo in
either side of the head a dorso-lateral,
and a more ventral row just above the
gill-clefts: the former arc spoken of as
the lateral ganglia, and the latter as
the ventral or epibranchial ganglia,
which are associated with transitory
epibranchial sense-organs. In their
primitive superficial position the two
rows are connected with one another and
with the central organ by cellular cords.
It is probable that the motor
fibres of these mixed nerves (as well
as the cerebral portion of XI) grow
out secondarily from the brain into
the primary ganglionic nerve-rudi-
ments arising from the upper row of
nerve-centres formed loy the bifurca-
tion of the band-like spinal motor
tract. From the ventral row, which
lies in the same plane as the ventral
horns of gray matter, arise the Illrd,
IVth, Vlth, and Xllth nerves, as
already mentioned. 1

It must be remembered that the
head is primitively composed of a
series of metameres (p. 75), and it
is therefore important to ascertain, as
far as is possible in the present state
of our knowledge, to which individual
metameres the different cerebral nerves
belong. As already mentioned, the
olfactory and optic nerves present cer-
tain peculiarities which bring them
under another category, and they will
be treated of later in connection with
the corresponding sensory organs.
The following general summary gives a scheme of the prob-
able primitive relations of the head-segments and cerebral nerves,

1 An additional pair of vestigial cerebral nerves has been shown to exist in
Protopterus, Ceratodus, Anura, and many Elasmobranchs, arising in the region of
the lamina terminalis, crossing the olfactory lobes, and ending in the region of
the olfactory mucous membrane. Along the course of this nerve, the function
of which is unknown and which has been described as the " nervus terminalis,"
are one or two ganglia.

FIG. 178. DIAGRAM TO SHOW
THE MODE OF DEVELOPMENT
OF THE DORSAL CEREBRAL
NERVES AND THEIR GANGLIA
IN THE LAMPREY. (After
C. v. Kupffer.)

c/t, notochord ; d, gut ; f/c, epi-
branchial ganglion (ventral
placode) ; yl, lateral vagus
ganglion (lateral placode) ;
c/s, sympathetic ganglion ;
h, hind-brain ; /, neural ridge
(rudiment of the spinal gang-
lion proper) ; m, mesoderm ;
'), branchial nerve : ltd,
subepidermoid layer,derived
from the ectoderm, and giv-
ing rise to the peripheral
part of the branchial nerve ;
Ha, dorsal spinal nerve.

CEREBRAL NERVES

237

founded mainly on the conditions existing in Elasmobranch
embryos.

TABLE SHOWING THE SEGMENTAL ARRANGEMENT OF THE CEREBRAL NERVES,
WITH THEIR RELATION TO THE METAMERES OF THE HEAD.

!*< Metamere (superior, in-
ferior, and anterior rec-
tus, and inferior oblique
muscle).

2nd Metamere (superior
oblique).

3rd Metamere (posterior
rectus).

\th Metamere (muscles
which are early aborted).

5/h Metamere (muscles
which are early aborted).

Ventral branch.

Oculomotor (III).

Pathetic (IV).

Abducent (VI).
Wanting.
Wanting.

Dorsal branch.

Ramus ophthalmicus pro-
fundus of the trigeminal
( V), together with the
ciliary ganglion.

Trigeminal (with its gang-
lion, mini(-t the ramus
ophthalmicus profnn-
dus).

Facial (VII), and audi-
tory ( VIII), with their
ganglia.

Glossopharyngeal (IX ),
with its ganglion.

Nerves of the Eye-muscles. The oculomotor (III) pathetic
or trochlear (IV) and abducent (VI) nerves (Figs. 179181)
supply the muscles which move the bulb of the eye (cf. table
above).

The oculomotor arises from the base of the mid-brain, and is
in intimate relation with an oculomotor or ciliary ganglion which
primarily belongs to the sympathetic system, and from which oculo-
motor fibres pass out to the iris and ciliary muscles of the eye. 1

The pathetic nerve, 2 although actually arising in the interior
of the ventral part of the mid-brain, near the nucleus of the
oculomotor, appears externally on the dorsal side of the anterior
margin of the hind-brain (valve of Vieussens, p. 203).

The abducent nerve arises far back on the ventral side of the
medulla oblongaia : in addition to the posterior rectus, it supplies
the retractor bulbi and the muscles of the nictitating membrane in
the Sauropsida. In the Anura it becomes intimately connected
with the Gasserian ganglion of the trigeminal.

1 Further researches are desirable with regard* to the ciliary ganglion of
Anamnia and Sauropsida. It appears to be made up of two distinct ganglia
(eerebro-spinal and sympathetic), one of which, in Mammals, fuses with the
Gasserian ganglion during development, the other remaining as the ciliary gang-
lion of the adult.

- Before emerging on the roof of the brain, the fibres of each trochlear nerve
cross those of its fellow (c/tiitxnia trorh/nii-t), so that the right nerve supplies the
left superior oblique muscle, and ri<-i /< /:/. Originally this muscle may be a
derivative of muscles which once supplied the parietal eye.

238

COMPARATIVE ANATOMY

CEREBRAL NERVES

239

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240

COMPARATIVE ANATOMY

The trigeminal, facial, glossopharyngeal, and vagus nerves are
usually described as branchial or branch iomeric nerves, i.e. they are
primarily related to gill-clefts. A typical branchial nerve has a
ganglion near its origin from the brain and divides into (1) a
dorsal (somatic sensory) branch to the skin, (2) a palatine (visceral
sensory) branch to the oral mucous membrane, and (3) a branch
associated with the epibranchial ganglion (p. 236) which bifurcates

IV

G.L.IX.X.XI

FIG. 181. PERIPHERAL NERVES OF A HUMAN EMBRYO OF 4 WEEKS (6'99 MM.
IN LENGTH), RECONSTRUCTED FROM SECTIONS. (After G. S. Streeter. )

1, 2, 3, mamlilmlar, hyoid, and 1st branchial ridges; ///, oculomotor; IV,
trochlear ; l n , T 7 ' 2 , V s , the three main branches of the trigeminal ; Fniot,
motor portion of trigeminal ; (JV, ganglion of trigeminal ; VII, facial ; VIII,
auditory ganglion ; IX, glossopharyngeal with the petrosal ganglion
(Gy.petr) ; A', vagus with the ganglion nodosum ((Jg.nod) and the anterior
(superior) laryngeal nerve (Lar.siit))) ; G. L, ganglion ic ridge of the IXth, Xth,
and Xlth nerves ; XII, hypoglossal. The abducent (VI) is not visible.

over the corresponding branchial cleft into a prebranchial (visceral
sensory) and a, postbranchial (visceral motor) nerve.

Trigeminal Nerve. This is one of the largest of the cerebral
nerves. It arises from the ventro-lateral region of the anterior
part of the medulla oblongata (pons Varolii of Mammals) by a large
sensory and a smaller (ventral) motor root, has a large primarily

CEREBRAL NERVES 241

double intra-cranial or extra-cranial Gasserian ganglion at the origin
of the former, and in Fishes (Fig. 179), divides into two main
branches, an ophthalmic (including a superficial and a deep or
profundus portion), and a maxillo-mandibular : in most terrestrial
forms (Figs. 180 and 181) the maxillary and mandibular nerves
arise separately. From the presence of these three characteristic
branches, often known as the first, second, and third divisions of the
trigeminal, its name is derived. It passes out from the skull some-
times through a single aperture, and sometimes by two or three dis-
tinct ones. On the supposition that the mouth corresponds to a fused
pair of branchial clefts, the ophthalmic would correspond to the dorsal
branch of a branchial nerve, and the maxillary and mandibular to
the prebranchial and postbranchial branches ; the palatine branch
may be represented by a branch going to the roof of the mouth.

The superficial branch of the first division is usually distinct in
Fishes, in many of which, however, and in higher forms, it may be
united with the deep branch. In Amphibians its homology has not
been clearly made out. 1 It passes dorsally over the eye-ball, crosses
the superficial ophthalmic branch of the facial, with which it may
become secondarily connected, and is distributed to the skin
anterior to and above the orbits. The deep branch passes below
the superior and anterior recti and superior oblique muscles, and
supplies the integument of the snout, the eyelids and conjunctiva,
the mucous membrane of the nose, and the lacrymal glands (e.g.
in Mammals). A connection of the profundus with the ciliary
ganglion arises secondarily.

The second division of the trigeminal, like the superficial
and deep ophthalmic, is purely sensory. On it is a sphenopalatine
ganglion derived from the sympathetic, and it is connected with
the facial. It extends first along the floor of the orbit, supplying
the lacrymal and Harderian glands when present, the conjunctiva,
the mucous membrane of the nose, and the roof of the mouth ; it
then passes to the upper jaw, supplying the teeth ; and finally,
as the infraorbital branch, perforates the skull to reach the integu-
ment in the region of the upper jaw, snout, and upper lip.

The third division of the trigeminal is of a mixed nature ; its
motor portion, which has the character of a visceral nerve, supplies
certain masticatory muscles and some of the muscles of the palate
and floor of the mouth. The sensory portion extends along the
rami of the lower jaw and divides into two main parts, a lingual
and a mandibular proper, the former of which is not well differenti-
ated in the Anamnia and Sauropsida. The lingual or gustatory
nerve innervates the mucous membrane of the mouth and tongue,
containing gustatory fibres from the chorda tympana (cf. under
facial nerve).

The special mandibular branch, which may pass through the
inferior dental canal of the mandible, supplies the teeth and

1 It possibly corresponds to the ramus frontalis of Mammals.

R

242 COMPARATIVE ANATOMY

mucous membrane of the lower jaw, and then gives off one or
more branches to the integument of the latter and of the lower
lip : in Mammals, a smaller branch passes upwards in front of the
ear to the temporal region, supplying the adjacent skin and the
pinna of the ear. Two ganglia, the submaxillary and the otic (Fig.
180), derived from the sympathetic, are connected with its sensory
portion, the former being situated close beneath the exit of the
nerve from the skull, the latter on the lingual nerve at the point
where it passes into the tongue. The otic ganglion is connected
with the glossopharyngeal nerve, but it is doubtful whether the
gustatory fibres in connection with the lingual ganglion are
derived from this nerve or from the facial.

Facial Nerve. This, which is also a mixed nerve, presents
important differences in branchiate and pulmonate forms respec-
tively. In many Fishes (e.g. Cyclostomes, Elasmobranchs, many
Teleosts, Dipnoans) and in perennibranchiate Urodeles, it possesses
two distinct ganglia at its origin in connection with the sensory and
mixed portions respectively. In other Fishes (e.g. Chimrera, Polyp-
terus, Lepidosteus, certain Teleosts) and more especially in Anurans,
the facial nerve comes into such close connection with the trigeminal
that the ganglia in question are no longer distinguishable from the
Gasserian ganglion, and such complications arise that the original re-
lations of many of the components of the two nerves are no longer
traceable and cannot be analysed by dissection. Another (the gcnicu-
late) ganglion of the facial nerve is retained in all Vertebrates.

In aquatic branchiate Vertebrates the facial nerve consists of
the following main branches (Fig. 179):

I. A system of sensory branches for the supply of the in-
tegumentary sense-organs of the head (q.v.). These branches,
together with the auditory nerve and the lateral line branches of
the glossopharyngeal and vagus (p. 245), arise from the same centre
in the medulla oblongata (tuber acusticum), each originally possessing
its own ganglion, and together forming a primitive acustico-lateral
sensory nervous system, arising, like the sensory organs which they
supply, direct from the ectoderm. The following branches may be
distinguished : (a) a superficial ophthalmic, running parallel to the
like-named branch of the trigeminal and sometimes (e.g. in
Chimsera) becoming very closely connected with its deep portion :
(6) a buccal, close to the maxillary portion of the trigeminal, and,
giving off near its origin an otic branch ; and (c) an external
mandibular, in the region of the hyomandibular nerve, dividing
into an anterior and a posterior branch and frequently anastomos-
ing with the mandibular branch of the trigeminal.

II. A sensory (a) palatine * which may anastomose with the

1 There can be no doubt that the palatine branch of the facial in the Anamnia,
comparable to the visceral or pharyngeal branches of the glossopharyngeal and
vagus, corresponds to the greater superficial petrosal of Mammals, which is
a purely sensory nerve : the motor fibres which are said to arise from it probably
belong to the vagus.

CEREBRAL NERVES

243

maxillary branch of the trigeminal and which innervates the
mucous membrane of the pharynx, and (b) chorda tympani, 1 going
to the mucous membrane of the floor of the pharynx. These two
nerves correspond to the " portio intermedia " of the facial of
Mammals (Fig. 182), and are closely related at their origin with
the geniculate ganglion. The chorda tympani corresponds to the

/'. -1,11,1 VII

in nil.

FIG. 182. DIAGRAM SHOWING THE RELATIONS OF THE PORTIO INTERMEDIA OP
THE FACIAL NERVE IN MAX. (After A. F. Dixon ; slightly modified. )

I, II, III, the three branches of the trigeminal ; *, geniculate ganglion of the
facial ; t, sphenopalatine ganglion, in the neighbourhood of //; Ch.fi/, chorda
tympani; C.t, tympanic cavity, outlined; G'. ti-iij, (iasserian ganglion of
the trigeminal ; P.int.mVII, intermediate (sensory) portion of the facial;
P. mot. VII, motor portion of the facial (hyomandibular) ; H.lintj, lingual
branch of ///; Jl. ma ml, mandibular branch of /// ; li.yxil, palatine (greater
superficial petrosal) branch of facial. The motor portion of trigeminal ///
is not indicated.

prebranchial and the hyomandibular to the postbranchial branch,
but from Amphibians onwards the chorda tympani becomes post-
spiracular in position.

III. A main post-spiracular hyomandibular trunk, extending
along the hyoid arch, and essentially motor, except for the com-
ponents which give rise to the sensory external mandibular and
a few twigs supplying the mucous membrane of the spiracle,

1 The chorda tympani ("alveolar" branch of the facial) passes internally to
the lower jaw in Elasmobranchii, Ganoidei, Perennibranehiata, and Anura. In
other Amphibians, as in Reptiles, it passes into the bony lower jaw.

244 COMPARATIVE ANATOMY

anterior wall of the pharynx, floor of the mouth, and the skin. Its
motor fibres supply visceral muscles in connection with the
mandibular and hyoid arches.

In correspondence with the change from an aquatic to a terres-
trial mode of life, the integumentary sense-organs in caduci-
branchiate Urodela, Anura, and in Amniota, become more or
less completely lost, and the corresponding branches of the facial
nerve are reduced. The parts which persist, in addition to the
large motor hyomandibular, are the palatine and the chorda
tympani (cf. Figs. 179-182).

In the Amniota the chorda tympani has a very different
position from that seen in the Anamnia, and becomes character-
istically related to the tympanic cavity ; in Birds it is absent, and
is replaced functionally by the glossopharyngeal. From the Amphi-
bia and Reptilia onwards, a gradual development of the facial
muscles leads to the characteristic mimetic muscles of Mammals
and more especially of Primates, which are supplied by the hyoman-
dibular nerve. The complicated networks of this nerve, however,
appears late phylogenetically, and are wanting even in certain
embryonic stages in Man. In addition to the mimetic muscles, the
hyomandibular nerve in Mammals supplies the platysma, the stylo-
hyoid, the posterior belly of the digastic, and the stapedius.

Auditory Nerve. This large nerve, which has a ganglion at
its origin, arises in close connection with the facial, and comes'
under the same category as the sensory portion of the latter nerve,
inasmuch as it is probable that the auditory organ is a modified
portion of the lateral line organs. Soon after its origin it divides
into a vcstilular and a cochlea/ 1 branch. The latter passes to the
lagena or cochlea of the ear, while the former supplies the rest of
the auditory labyrinth.

Vagus group. This group includes the glossopharyngeal,
vagus, and spinal accessory, which stand in the closest relation to
one another, and, owing to the fact that the head in this region has
undergone fewer phylogenetic modifications, are less specialised
than the cerebral nerves already described. These nerves all
consist of both afferent and efferent fibres, the former being con-
nected with ganglia (the petrosal of IX, and the jugular and
cervical of X).

In Fishes and perennibranchiate Amphibians the glosso-
pharyngeal leaves the skull through a special foramen, and not
along with the vagus, as in other Vertebrates. In branchiate forms,
in addition to a palatine branch, it is distributed to the region of
the first (hyobranchial) gill-cleft, over which it bifurcates into a
smaller prebranchial and a larger postbranchial branch (Fig. 179).
In other Vertebrates it is distributed to the pharynx and tongue.
and as a rule anastomoses with the vagus and also with the geni-

CEREBRAL NERVES 245

culate ganglion or palatine branch of the facial and the otic gang-
lion of the third division of the trigeminal (Jacobsons anastomosis},
a continuation of this branch extending forwards, close to the
palatine branch of the facial. 1 In the higher Vertebrates, the
large lingual branch forms a gustatory nerve supplying the tongue,
tonsils, and epiglottis : this nerve is apparently already indicated
in Dipnoans.

The vagus has a very wide distribution, and is not limited to
the head but extends into the trunk. It includes a sensory lateral
line branch, pharyngeal ( = palatine), and branchial branches ; the
last-mentioned fork over the second and following gill-clefts and
supply the mucous membrane and muscles of the branchial appar-
atus in branchiate forms. Its visceral branch supplies the larynx,
heart, swim-bladder or lungs, and a considerable portion of the
digestive tract (gullet, stomach, and more or less of the intestine).
In pulmonate Vertebrates a reduction takes place of the motor
components of the branchial nerves along with the corresponding
muscles (Fig. 180).

The origin of both glossopharyngeal and vagus by numerous
roots, and the fact that they give off branches in the region of the
pharynx and visceral arches in which a metameric arrangement
can be recognised, indicates that they correspond originally to a
number of separate nerves.

The lateral branch of the vagus, as already mentioned (p. 242),
does not belong originally to this nerve, but to the lateral nervous
system of the head, having a similar central origin to that of the
acustico-facial group, with which it may even be directly con-
nected by a commissure outside the auditory capsule (Protopterus).
There is a special ganglion at its origin from the medulla
(Fig. 179), and its exit from the skull by the same foramen as the
vagus is evidently secondary. After giving off a supratemporal
branch, it extends along the trunk to the apex of the tail, and may
subdivide into several branches, some of which may be situated
directly under the skin and others (like the main lateral nerve of
Elasmobranchs and Dipnoans), beneath the lateral muscles close
to the vertebral column. All these branches supply the sensory
organs belonging to the lateral line system. 2

The so-called spinal accessory (accessorius Willisii) is a true
cerebral nerve, and can already be recognised in Elasmobranchs,
in which it is included in the vagus, from the posterior roots

1 It is possible that the lateral line fibres which may be associated with the
glossopharyngeal, and even with the trigeminal, are always derived from the
vagus and facial.

- Certain nerves present in Teleostomes and formerly described under the
term " ramns lateralis trigemini," may be included under the. term "ramus
lateralis accefssoriiix." They form a sensory system of nerves, provided with
ganglia, which are formed typically from somatic sensory fibres derived from the
Vth, Vllth, IXth, and Xth cerebral nerves and a varied number of spinal nerves.
Branches pass to some of, or even all, the fins, and supply sensory end-buds
(q.v.). The so-called lateral nerve of Petromyzon belongs to this system.

246 COMPARATIVE ANATOMY

of which it arises : it is therefore primitively a cerebral and not a
spinal nerve. It presents certain characteristic peculiarities in
the Amphibia, Sauropsida, and Mammalia respectively, so that
a direct comparison of the nerve in these groups is impossible. 1

Owing to secondary differentiations, the accessory of Mammals
takes on a very different character from that of the Sauropsida :
only that part of it in the former arising from the spinal cord can
properly be described as the accessory, while its cerebral portion
must be included under the vagus group. In the Sauropsida, the
nerve is better described as the spinal portion of the vagus. In
Mammals, the accessory contains viscero-motor elements from
the dorsal roots of the 5th to 7th spinal nerves, and extending
along the course of the vagus gives off branches to the larynx and
to the trapezius and sternocleidomastoid muscles.

Spino-occipital and Hypoglossal Nerves. Under the
term " spino- occipital nerves " is understood a group of nerve-
roots in the occipital region and anterior trunk-myotomes which
are in close relation to the hypoglossal. As most of their com-
ponents are bound up in the vagus-group, they were formerly
erroneously described as " ventral roots of the vagus."

In Cyclostomes they have either not been assimilated by the
cranium (cf. p. 85) or are not even differentiated from the cerebral
nerves, so that in this case they cannot be spoken of as spino-
occipital. In Plagiostomes, in which, as in Amphibians, vertebral
elements are fused with the occipital region of the skull, a series of
intracranial spinal nerves can be recognised which maybe described
as " occipital," a reduction in which, from before backwards, can
already be observed. In the Holocephali, owing to a still greater
assimilation of vertebral elements to the skull, three additional
spinal nerves later became intracranial, while the number of
occipital nerves is reduced to two. 2 The relative number of these
two series varies in Ganoidei, Dipnoi, and Amniota, the occipital
nerves having entirely disappeared in the Teleostei.

In Fishes the first spinal nerve, which corresponds to the hypo-
glossal of higher forms, supplies the muscles of the trunk, the floor

1 The evolution of the spinal accessory in the higher Vertebrates must have
taken place somewhat as follows. Beginning with the Amphibia, in which the
vagus group does not extend into the spinal cord, the accessory in the primitive
Amniota must have possessed the following characters : close connection with the
vagus, extension backwards at least as far as the first cervical segment, origin
from a lateral collection of cells of the ventral cornu, and course on the ventral
side of the dorsal cornu of the gray substance. From this primitive form the
nerve must have developed along two different lines in the Sauropsida and Mam-
malia respective!}', in both of which, however, in contrast to the Amphibia, it thus
forms a kind of connecting link between the cerebral and the spinal nerves, this
region including in the Sauropsida at most three, in Mammalia seven, segments.

' These additional nerves have been described as " occipito-spinal " to dis-
tinguish them from the "occipital" nerves : each series constitutes a sub-section
>f tln j spino-occipital group. In Amphibians (except Ichthyophis) the occipital
nerves are no longer recognisable, even in the embryo,

CEREBRAL NERVES 247

of the mouth, and the skin of the back, and also sends twigs to the
brachial plexus. In higher Vertebrates the hypoglossal becomes
gradually more differentiated from the other cervical nerves, and
innervates the intrinsic muscles of the tongue, takes up cervical
elements, and with them gives rise to the so-called n.inius descendcns
and the ansa hypoglossi, from which arise branches to the sterno-
hyoid and other muscles.

In the Gymnophiona, Urodela, and Aglossa amongst the Anura,
the first spinal nerve perforates the first vertebra : in other Anurans
this nerve has disappeared, though occasionally recognisable in the
embryo, and the nerve which arises behind the vagus and emerges
between the first and second vertebrae in reality corresponds to the
second spinal nerve (hypoglossal, cf. Figs. 145 and 164).

From the Sauropsida onwards, the hypoglossal, which arises
postero-ventrally to the vago-accessory group, leaves' the skull
through one or more apertures : it has three roots, 1 which corre-
spond to three spino-occipital nerves of the Anamnia.

Dorsal roots may be present temporarily or permanently in con-
nection with the hypoglossal of Sauropsida and Mammalia, and may
be provided with ganglia, as in the case of the accessory and of the
spino-occipital nerves of many Fishes. A reduction of dorsal roots
may also take place further backwards : in many Mammals, including
Man, that of the first cervical nerve (and even of the second in e.g.
the Orang) may be reduced or entirely wanting.

Sympathetic.

The sympathetic system is a derivative of the spinal system, with
which it remains throughout life in close connection by means of
raini communicantes (Fig. 145). It is distributed mainly to the
alimentary tract, the vascular system, and the glandular organs
of the body.

The sympathetic ganglia are derived from the developing spinal
ganglia, and, like these, show originally a segmental arrangement.
They contain typical ganglion-cells, 2 and usually become united
together secondarily by longitudinal commissures, thus giving rise
to a chain-like paired sympathetic cord lying on either side of the
vertebral column and aorta. From its ganglia nerves pass off
to the above-mentioned organs, and form plexuses. Numerous
peripheral ganglia, derived from the others, are also present in
the plexuses.

1 Other, more anterior elements occur in the embryo in Sauropsida.

- A special small form of cell occurs in the embryonic sympathetic ganglia,
and may extend beyond them to a greater or less degree into other parts. Thus
these chromaffin edit are found, e.ff., in the pancreas (islets of Langerhans),
coccygeal gland, hypophysis (Fig. 151), and suprarenals (medullary substance, q.v. ) :
in fact, in all " glands with internal secretion."

248 COMPARATIVE ANATOMY

The sympathetic, accompanying the arterial trunks, extends
along the vertebral column and passes anteriorly into the
skull, where it comes into relation with a series of the cerebral
nerves (cf. pp. 237, 241 and Fig. 180) similar to those which it
forms further back with the spinal nerves.

The original segmental character frequently disappears later on,
and this is especially the case in those regions where marked
modifications of the earlier metameric arrangement of the body
have taken place viz., in the neck and certain regions of the trunk,
especially towards the tail : thus in Mammals there are never more
than three cervical ganglia.

Nothing is known of a sympathetic in Amphioxus. In
Petromyzon typical ganglion-cells occur more or less sparsely along
the dorsal and ventral spinal nerves in the lateral walls of the body.
Nests of cells are present more frequently in the region where the
parietal veins open into the cardinal veins than alongside the aorta,
and they also occur along the caudal vein and its branches : these
ganglia are connected with the suprarenal organ (fj.v.}. The
sympathetic extends into the head.

In Elasmobranclis the sympathetic reaches a higher stage of
development, and it has been shown that the ganglia first appear
after the dorsal and ventral roots have united to form the spinal
nerve-trunks, just at their point of union, each ganglion containing
from the first both afferent and efferent elements. Except in
its most anterior embryonic segment, in which the ciliary ganglion
represents a part of this system, the head is without sympathetic
ganglia. A sympathetic cord, connecting the ganglia, is not de-
veloped in Elasmobranchs, although some of the individual ganglia
may become united together, while others disappear at an early
stage.

A cranial portion of the sympathetic exists in Tdcosts, arising
from the trigemino-facial system of nerves and possessing three
ganglia : in the trunk, too, there is a well-developed cord of ganglia,
frequently connected with its fellow by transverse commissures, the
two cords gradually converging antero-posteriorly. A similar con-
dition has been found to occur in the Dipnoi (Protopterus), in which
the delicate longitudinal sympathetic cords, with occasional ganglia,
extend along the aorta and notochord : nothing is known of
their connection with the cerebral nerves.

In Amphibians (Fig. 145), the sympathetic reaches a high stage
of development. It ends anteriorly in the ciliary ganglion, extends
along the aorta through the trunk and caudal regions as a
ganglionated cord, and has numerous anastomoses with the spinal
and cerebral nerves ; it is intimately related with the suprarenal
and abdominal veins (postcaval and revehent renal' veins).

In the Sauropsida the cervical portion of the sympathetic is
usually double, one part running within the vertebrarterial canal
alongside the vertebral artery, In all other Vertebrates the whole

SENSORY ORGANS 249

curd lies along the ventral and lateral region of the vertebral
column : it is generally situated close to the latter, overlying the
vertebral end of the ribs.

In Mammals, the cervical portion of the cord may have an
independent course from the vagus, or it may be more or less
closely applied to the latter nerve, the anterior cervical ganglion of
the sympathetic and the vagus-ganglion forming a single mass ; the
posterior cervical ganglion commonly fuses with the first thoracic.
From the anterior cervical ganglion the sympathetic passes into
the skull along with the internal carotid artery, and its cranial
portion takes on relations to the cerebral nerves more particularly
the Vth, IXth, and Xth, as in other Vertebrates. Numerous
branches also pass from the anterior cervical ganglion to the hypo-
glossal, the anterior cervical nerves, and to the pharynx, larynx, &c.

III. SENSORY ORGANS.

The specific elements of the sensory organs originate, like the
nervous system in general, from the ectoderm ; the peripheral
terminations of the sensory nerves are thus always to be found in
relation with cells of ectodermic origin, which become secondarily
connected by means of nerve-fibres with the central nervous
system. 1

The sensory apparatus was primarily situated on a level with
the. epiderm, and served to receive sensory impressions of but
slightly specialised kinds ; but in the course of phylogeny parts of
it passed inwards beneath the epiderm, certain of these becoming
differentiated into organs of a higher physiological order, viz.,
those connected with smell, sight, hearing, and taste. These are
situated in the head, and except the last mentioned, become
enclosed in definite mesodermic sense-capsules (p. 77) ; they must
be distinguished from the simpler integumentary sense-organs, which
are concerned with the senses of touch, pressure, and temperature.
In addition to free nerve-endings in the skin, various specific
forms of sensory cells occur, and these may be surrounded by
supporting or isolating cells, both kinds, however, being ectodermic.
The mesoderm may also take part in the formation of the sensory
organs, giving rise not only to the above-mentioned sense-capsules,
but also to various protective coverings and canals as well as to
contractile and nutritive elements (muscles, blood- and lymph-
channels).

In the sensory organs of the integument of branchiate Verte-
brates, as well as in all the higher sensory organs, the surrounding

1 The vertebrate eye forms an exception to the other sense-organs in that it
arises from a part of the ectoderm which has been involuted, to form the medullary
tube.

250

COMPARATIVE ANATOMY

medium is always moist, and in both cases, rod-, club-, or pear-
shaped sensory cells are met with.

In those animals which in the course of development give up an
aquatic life and come on land (most Amphibians), the external
layers of the epiderm dry up, and the integumentary sense-organs
pass further inwards from the surface, undergoing at the same
time changes of form. Thus from Reptiles onwards other kinds
of sense-organs are met with in the skin.

SENSE-ORGANS OF THE INTEGUMENT.
a. Nerve-eminences.

In Amphioxus certain rod -shaped or pear-shaped cells can be
recognised in the epiderm, especially in the anterior part of the
animal ; each of these is provided distally with a hair-like process
and proximally is in connection with a nerve. The cells are
distributed irregularly, but in the neighbourhood of the mouth
and cirri they form groups.

stz sz stz

FIG. 183. VERTICAL SECTION THROUGH THE SKIN AND A LATERAL LINE ORGAN
OK THE LARVA OF Triton tceniatus, 3 CM. IN LENGTH. (After F. Manrer. )

BO, blood-vessel ; Ep, epiderm ; SZ, sensory cells; St.Z, supporting cells.

It is doubtful whether these structures in Amphioxus are
directly comparable to the integumentary sense-organs of Fishes
and Amphibians, but it is important to note that each of the latter
always arises in the first instance from a single cell which forms a
group by division. Thes