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UNIVERSITY OF CALIFORNIA

SAN FRANCISCO MEDICAL CENTER

LIBRARY

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COMPARATIVE ANATOMY

OF

VERTEBRATES

MACMILLAN AND CO., Limited

LONDON . BOMBAY . CALCUTTA
MELBOURNE

THE MACMILLAN COMPANY

NEW YORK . BOSTON . CHICAGO
ATLANTA . SAN FRANCISCO

THE MACMILLAN CO. OF CANADA, Ltd.

TORONTO

COMPARATIVE ANATOMY

OF

VERTEBRATES

ADAPTED FROM THE GERMAN OF

DR. ROBERT [WIEDERSHEIM

PROFESSOR OF ANATOMY, AND DIRECTOR OF THE INSTITUTE OF HUMAN AND COMPARATIVE ANATOMY
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)

WITH THREE HUNDRED AND SEVENTY-TWO FIGURES
AND A BIBLIOGRAPHY

W44v

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

1907

All rights reserved

j/iiO-^

iC

Richard Clay and Sons, Limited,

bread street hill, e.g., 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 LehrhiLch der vergleichsnde Anatom/ie der Wirheltierey
and a second edition followed in 1886. In 1884, a short
Grundriss was published, which, after passing through four
editions and gradually increasing in size, replaced the Lehrbuch
under the title of Vergleichende Anatomic der Wirheltiere, in 1902.
A farther edition appeared in 1906, and this was followed by a
shorter Einfilhrung 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 G-ruiidriss,
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 pei'sonally.

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.

Univbrsity College, Cardiff,
August, 1907.

CONTENTS

PAO«

Preface v

INTRODUCTION 1

L On the Meaning and Scope of Comparative Anatomy 1

n. Development and Structural Plan of the Vei-tebrate 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

n.'RiBS 63

of Fishes 63

of Amphibians 66

of Reptiles 67

of Birds • . . • ..... 69

of Mammals 69

viii CONTENTS

PAGE

III. STERNUM 70

V. SKULL 74

General part 74

a. Brain -case 76

h. Visceral Skeleton 80

c. Bones of the Skull 82

Special part 84

A. The Skull of Fishes . . . . • 84

B. ,, of Amphibians 97

V. ,, 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 Fi&hes 145

of Amphibians 146

of Reptiles 149

of Birds 152

of Mammals 154

Paired Fins of Fishes 155

Paired Limbs of the higher Yertebi-ata 159

of Amphibians • • 161

of Reptiles 163

of Birds 165

of Mammals 168

C. MUSCULAR SYSTEM .... 173

INTEGUMENTARY MUSCLES • 175

MUSCLES OF THE TRUNK 179

of Amphioxus and Fishes 179

of Amphibians . • . . 181

of Reptiles 181

of Birds 182

of Mammals 183

MUSCLES OF THE DIAPHRAGM 184

CONTENTS ix

PAGE

MUSCLES OF THE APPENDAGES 185

EYE-MUSCLES . . 186

VISCERAL ]MUSCLBS • 187

of Fishes 187

of Amphibians 188

of Amniota 189

D. ELECTRIC ORGANS 190

E. NERVOUS SYSTEM AND SENSORY ORGANS 193

I. 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 Elasmobranchs 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

a. Nerve-Eminences • 250

h. End-buds and gustatory organs 254

c. Tactile Cells and Corpuscles 255

(I. 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 CyclostfMtnes 278

of Fishes 279

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

b. 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. (ESOPHAGUS, 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

L GILLS 351

of Amphioxus 352

CONTENTS xi

PAOE

GiLi^ {continued)

of Cyclostomes 362

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

ofReptUes 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 4(X)

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

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

h. 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. CESOPHAGUS, 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

PAOB

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. TJRINO GENITAL ORGANS (general description and develop-
ment) 441

Pronephros 443

Mesonephros 445

Metanephrous 446

Male and lemale Generative Ducts 446

Gonads 450

URINARY ORGANS 452

of Amphioxus 452

of Fishes 452

of Amphibians 455

of Reptiles 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

ACCESSORY GENITAL GLANDS OF MAMMALS 484

COPULATORY ORGANS 486

ADRENAL BODIES 492

APPENDIX (Bibliography) 497

INDEX 565

EKRATA

PArjR

5 oth line from bottom, for "walls," rtad " wall.""

13 Fig. 1], insert " gonad (ovary)" helovi " kidne\."

33 ,, 2iy,for "^and G nerves," read "-AT, nerves : G, blood-vessels.'"

40 8th line from top, /or "disappear," read "disappears."

40 11th ,, ,, /or " ossifications," rearf "ossification."

53 Fig. 42, transpose A and B.

72 Line 6 from bottom, delete " the first" and " cervical."

73 ,, 10 ,. /or "are," rearf "is."

73 ,, 3 ,. /or "proximal," reafZ " anterior."

73 ,, 2 ,, /or " distal," rear/ " posterior."

92 Fig. 69, insert " O "' before "orbital ring," and ".sy/»/>/. , .symplectic."'

96 ,, 71, ,, "AT, external gills."

112 3rd line from top, /or " systeni," read " septum."

119 Fig. 86, /or "condyles," read "condyle."

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

151 4th ,, top, /or " show," reofZ "shows."

155 18tli ,, ,, /)r " stright," reoc? " straight."

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

176 17th line from top, /or "and thus eflFect," read " thu.s effecting."

298 Fig. 218a, for '' Pp," read " Pft."

304 , , 223, for ' ' vestibula, " read ' ' vestibuli. "

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

340 Fig. 250b, /or "tendrinous," rearf "tendinous."

438 20th line from bottom, for " Phalcolarctos," read " Phasoolarctos. "

445 6th and 7th lines from bottom, /or " parosphoron," read "paroophoron.

450 13th line from top. /or " epididermis," ren/l " epididymis."

xii CONTENTS

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

Pronephros 443

Mesoncphros 445

Metanephrous 446

Male and I'emale Generative Ducts 446

Gonads 450

URINARY ORGANS . . . 452

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 paUiigenetic, — but that '' falsifications "of the re-
cord, acquired by adaptation, very commonly occur along with
them, resulting in coenogenetic 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 imalterable, 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 animal's in general,
but also to understand the meaning of numerous degenerated and

1?

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. Upithelmm, and its derivative, glandular tissue.

2. Swp'porting 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 bod}^

The organ-systems, which will be treated of in order in this
book, are as follows : — 1. The outer covering of the body, or i7ite-
gument ; 2. The skeleton; 3. The muscles, together with electric
organs ; 4. The ncrvoits system and sense-organs ; 5. The organs of
nutrition, 7^es'piration, circulation, excretion, and reproduction.

The closely-allied branches of science defined 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 stud}^ 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. 1), 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 memhrme, and is
made up of two chief constituents — the spongioplasm or chrmnatin,
and the hyaloplasm or achromatin. A

small particle, the centi'osome, is also ^^__^..^^ j^^

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. Fig. 1.— Diagram of the

In the process of sexual reproduc- Ovum.

tion, which occui-s in all Vertebrates, D, vitellus; KB, genninal

the fusion with the ovum of the vesicle ; iT/', germinal spot.

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 {kaiijokinesis 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 i\\Q female (or maternal) a.nd male (or
paternal) 2'>ronuchus.

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

4 COMPARATIVE ANATOMY

both male and female germinal cells. This structure is the
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 Uastomeres. 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

MIC

UK

Fig. 2. —Diagrams of the Segmentation of the Oosperm.

A, first stage (two segments) : BK, 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
blastoccele) 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

5

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 moter, middle, and inner

germinal layers, or as ecto- ,'J/2>

derm {epiblast), mesoderm
(mesohlast), and endoderm
(hypoblast).

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 holoUastic ; when division is restricted to part
of the ovum only, the segmentation is said to be partial or mero-

hlastic^ (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
did so in earlier times — into a
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

^ 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).

B7a.

mDo

Fig. 4. — Dial i:a.m of a Mero-
blastic Oosperm with
Discoid SEciMENTATiox.

Bla, blastoderm ; Do, yolk.

or

6 COMPARATIVE ANATOMY

a central space, the primitive digestive cavity {archenteron). The
opening of the latter to the exterior, where the two germinal
layers are continuous, represents the primitive mouth or hlastopore,
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 {stomodceitm and procto-
dceum), 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

JZ>//

IM-

Fig. 5.— Gastrula.
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), delaminatio^i, 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

— SoF

JEiit

SpP

Me<i

:E'n£ ' '

Ekt

--Coel

Fk;. 6, A AND B.

-Diagrammatic Transverse Sections through a Develoi'Ikg
Vertebrate Embryo.

A^ aorte ; O/i^, (Fig. B), the notochord now constructed off from the endoderni ;
Co, C<yU ccelonie ; />, alimentary canal ; Eki, ectoderm ; Ent, endoderm,
showing in Fig. A the thickening (CA) which will form the notochord ; H,
remains of the upper part of the cielome in the interior of the mesodermic
somites ; Med, central nervous system (medullary cord) : — in Fig. A it is
shown still connected with the ectoderm, from which it has become constricted
off in Fig. B ; iS'oP, somatic, and SpP^ splanchnic mesoderm ; UG^ primary
urinary duct (pronephric duct) ; UW. mesodermic somite.

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 imrietal or
somatic layer, lying along the inner side of the ectoderm, and
into a visceral or splanclmic 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-

B

iiies

D

Fig. 7.— Diagrammatic Skctions of the Primitive Endoderm and its
Derivatives of Amphioxus (A— C, afteii Gotti<;),
From the dorsal part of tlie endoderm (unshaded) arise the notochord {ch) and
the mesodermic somites {me..s) ; from the ventral part (shaded), the epithelial
lining of the gut. The entire endoderm in stages /? and G thus shows four
hollow projections : a median (chordal) and paired hxteral (mesodermal) out-
growths communicating at first with the larger ventral intestinal cavity (dh),
but later becoming constricted off from the latter (/>). The notochordal
outgrowth becomes solid, while the cavities in the mesodermal segments
represent the rudiments of the coelome.

pleure, and the latter, together with the endoderm, the splanclmo-
pleure. The cavity separating these is the body-cavity or ca^lome,
and is lined by an epithelium.

The coelome may arise as a segmentally arranged series of
pouches {enterocmlcs) 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
{schizocmlc). 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 77iesodermic somites or
jjrotovertehrcf^, 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 urinogenital 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
lilatcs ; in Amphioxus 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-intcstinal 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 Mamuials) 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 {Eutheria) 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

AF-

C

Fig. 8, A, B, AND C— Diagrams illustratino the Formation of the Amnion,
Allantois, and Yolk Sac. A and B, in Longititdinal Section; C, in
Transverse Section.

Af amnion; AF, aniniotic fold ; Ah^ amniotic cavity; AJ , allantois; C, noto-
chord ; Dh, alimentary cavity ; Do, yolk-sao ; E, body of embryo ; M,
medullary cord ; -pp, coelome ; a, somatopleuie ; h, splanchnopleure ; f, vitello-
intestinal 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 fcetal and maternal blood-vessels come into very close
relations with one another. Thus an allantoic placenta is

rrichf)

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 ; CM, chorion lieve ; D, the remains of the
yolk-sack (umbilical vesicle) ; Dr, decidua reflexa ; Dr, decidua vera, which
at Pxi passes into the uterine portion of the placenta ; H, heart ; P/, fcetal
portion of the placenta (chorion frondosum, Chf) ; 77>, Fallopian tube ;
U, uterine wall ; UH, uterine cavity ; ei, 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
fcetus (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 vertehrce, at a later stage of development (Fig. 10).
All these structures are median in position, and the body is
thus Ulatemlly symmetrical. The neural tube, or cerehro- spinal
cavity, enclosed by the skull and arches of the vertehra^, contains
the central nervous system {hrain and spinal cord) ; the visceral tube
(coelome) encloses the viscera (alimentary canal, heart, urinogenital
organs, &c.), and its muscular walls may be strengthened by a
series of rihs, articulating dorsally with the vertebral column.

Fig. 10. — Diagrammatic Transverse Section through the Body of an Adult

Vertebrate.

AOy aorta ; Co, derm ; DH, lumen of intestine ; Lp, endodermic epithelium of
intestine; KW, body-wall; Med, spinal cord; Ms, mesentery; 31sc, mus-
cular coat of intestine ; XB, neural tube ; Per, parietal layer of the peri-
toneum ; Per^, visceral layer of the peritoneum ; Suhni, connective-tissue
coat of intestine ; VB, visceral tube ; W, vertebra.

Certain of the ribs may reach the mid-ventral line and come into
connection with a hreast-hone 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 wdth or without the intermediation of a neck. The
ccelome 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 /ws or limhs
(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.

BRAIN

SPINAL CORD

\

NEURAL TUBE (CLREBRO SPINAL canal)

NOTOCHORD VISCERAL TUBtfCOaovi";

ORAL CAVITY

INTERNAL GILL SLITS,

HEART

CLOACA

URINARY BLADDER

IRILE nuCT
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 Vertebi-ate 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.

14

COMPARATIVE ANATOMY

Division A. ACRANIA (CEPHALOCHORDATA).

Lancelet (Amphioxus).

„ B. CRANIATA.

-Myxine, Bdello-

and Rays

;s

Sub-class 3.
Order 1

la. Anamnia.

Class I. Cyclostomata (Suctorial Fishes).
Order 1. Myxinoidei (Hag Fishes-
stonia).
,, 2. Petromyzontes (Lampreys).
Class II. Pisces (True Fishes).
Sub-class 1. Elasmobranchii.

Order 1. Plagiostomi (Sharks— Selachii,
— Batoidei).
,, 2. Holocephali (Chimaera, Callorliynchus).
Sub-class 2. Teleostomi.

r Order 1. Cros.so?j<ery^u(Polypterus,Calainichthys). 1 -^
-| ,, 2. Chrondro.stei (Acipenser, Polyodon). j-'o

I ., 3. Holostei (Lepidosteus, Amia).
, , 4. TeleoHtei.
Sub-order a. Physostomi (Carp, Pike, Salmon,
Herring, Eel, Siluroids).
,, h. Anacanthini (Cod, Flat-fishes).

,, c. ^ca?i</to/>^eW (Perch, Stickleback,

Gurnard, Blenny).
,, d. Pharyngognathi (Wrasse).
,, e. Plectognathi (Trunk- and File-

fishes).
,, /. Lophobranchii (Pipe-fish, Sea-
horse).
Dipnoi.
1. Monopneumona (Ceratodus).
,, 2. i>ip?ie«fwo?ia (Protopterus, Lepidosireu).
Class III. Amphibia.

Order 1. Urodela.

a. Perennibranchiata (Proteus, Siren, Necturus).

{Derotremata (Amphiuma,
Alyctodera (Salamandra, Tri-
ton, Amblystoma).
,, 2. Aiiura (Frogs and Toads).
,, 3. Gymnophiona (Limbless Cwcilians).

//8. Amniota.

Class IV. Reptilia.

Order 1. Bhynchocephali (Hatteria).
,, 2. Lacertilia (Lizards).
,, 3. Ophidia (Snakes).
,, 4. Chelonia (Turtles and Tortoises).
,, 5. Crocodilia (Crocodiles and Alligators).
Class V. Aves.

a. EatiUf (Cursorial Birds— Ostrich, Rhea, Emu, &c. ).
h. Carinatce (Birds of Flight).

Class VI. Mammalia.

Sub-class 1. Prototheria or Ornithodelphia (the Ovi-
parous Monotremes — Ornithorhj'nehus,
Echidna).
,, 2. Metatheria or Didelphia (Marsupials —
Kangaroo, Phalanger, Opossum.).

,, 3. EUTHEKIA or lVIONt)DELPHIA.

INTRODUCTION

15

Order 1., Edentata (Sloth, Aiiteater).

2. Sirenia (Dugong, Manatee).

3. Cetacea (Porpoise, Whale).

4. Ungulata (Rhinoceros, Horse, Ruminants.)

5. Hyracoidta (Hyrax).

6. Proboscidea (Elephant).

7. Hodentia (Rabbit, Mouse, Beaver, Cavies).

8. Cheiroptera (Bats).

9. Insectivora (Shrew, Mole, Hedgehog).

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

11. Prosimii (Lemurs).

12. Primates (Monkeys and Man).

16

COMPARATIVE ANATOMY

Kainozoic

Mesozoic

Palteozoic

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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 {corktm, cutis).

In the epiderm, which consists of cells only, two parts may in
general be distinguished : — an external layer, composed of flattened
and hardened cells (stratum corneum , hoimy 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 2)eripheral 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 bo7iy
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.

0

18

COMPARATIVE ANATOMY

Fishes.

The character of the epiderm varies greatly in the different
groups (Fig. 12). The striated cuticular border (present e.g. in
Cyclostomes, Teleosts, and Dipnoans) possibly indicates the
former possession of cilia.^ 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-

H

Ji

<f^

Fig. 12.— Diagrammatic Transverse Section illustrating the Structure of
THE Skin in Fishes.

5, B, goblet-cells opening on the surface ; Go, derm ; CS, cuticular margin ;
Ep, epiderm ; F, subcutaneous fat ; G, vessels which pass upwards in the
vertical connective tissue bundles (^S') 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 gobUt

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

INTEGUMENT

19

cells commonly present may be mentioned the gramclar 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
ptoygopodii), and on the operculum and dorsal fin-rays of certain
Acanthopteri (e.g. Trachinus, Thalassophryne, Synanceia), in
which latter they constitute a i^oison-apparatus serving for offence
or defence, and consist of modified epidermic cells enclosed in
grooves of the spines of the operculum and dorsal fins. Most

Scales.

Pocket enclosing
Mucus cells. scale.

Epiderm.

Muscles.

Fia. 13.— LoNoiTUDiN.\L Section of Skin of Young Trout (15 cm. long),

FROM THE Tail.

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
Siluridae) 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. Stomiatidae,
Halosaurida3, Anomalopidae), 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.^

In addition to very numerous goblet-cells, the epiderm of the
South African Dipnoan, Protopterus, gives rise to cup-shaped
w.ulticellular 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,^ 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,^ and
leucocytes are not abundant. Unicellular glands, known in
Urodeles as Leydigs cells, are, however, abundant in the larva ;

^ For the electric organs of Malopterurus, which are said to be epidermic in
origin, cf. under Electtic Organs.

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

^ 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 larvae.

^ In older Urodele larvie, 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 larvae. Later on, immediately before meta-
morphosis, numerous multicellular glands of alveolar structure

CJ'

Fig. 14. — Skin of Larva of Salamander {Salamandra maculosa).

a, stratum corneum ; b, stratum Malpighii ; Co, derm ; CS, 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

'Min

Fig. 15. — Semidiagrammatic Section through the Skin of Adult
Salamander {S. maculosa).

Co, derm, in the connective tissue stroma of which {B) the various sized integu-
mentary glands (A, C, D, D) lie embedded ; E, epithelium of glands'; Ep,
epiderm ; J/\ the muscular, and Pr the connective-tissue layer of the
glands ; M, the same, seen from the surface ; Mm, subcutaneous laj^er of
muscles, through which vessels {G) extend into the derm; Pi, Pig, 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

22

COMPARATIVE ANATOMY

not onh^ in relative size, but also in the structure of their cells
and in function. Mucus- glands, and much larger poison-glands
(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.

Superficial and deep layer
of epiderm.

Muscles.

Dti-ui.

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 chromatophores 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 caducibranchiate 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 Keptiles 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 throfgh Various Kinds of Epidermic
Scales of Reptiles. (From Boas's Zoology. )

A^ rounded scales ; B, shields; O, imbricating scales; Z), the same, with bony
scutes in the underlying derm ; A, horny layer, and s, Malpighian layer of
the epiderm ; I, derm ; o, bony scutes.

and odoriferous " musk-gland " on the lower jaw of Crocodiles and
the invaginations 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 papillae,
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, Scincidae). As a general
rule, the epidermal comification.is much more marked than the
ossification.

24

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.

Malpighian layer Horny layer
Papillw. of eoiderm. of epiderm.

•'\-V

V/ "'

/i.'

Derm. Subcutaneous connective

tissue with vessels.

Fig. 18.— Section of Skin of a Young Tortoise [Ttstudo grceca), 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 Epitrichous
tissue layer. layer.

Horny layer (scale).

Pitjment layer.

Muscles.

Fig. 19.— Section of Skin of Lizard {Lacerta agilis).

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 i), 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. Ratitae) : its secretion serves to oil the
feathers, and it is especially w^ell developed in Water- Birds. A
gland is also present in the neighbourhood of the auditory passage
in certain Gallinaceae, 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 {arredores plumarura). 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 extendmg
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

Fig. 20. — Two Early Stages in the Development of the Feather
( Semid I agramm atic ).

B, blood-vessel ; G, 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
harhs — on which smaller secondary rays or harbides become
developed — become free, and thus an embryonic doivnfeather
(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 qitill (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. Ratitae,
and more especially the Cassowaries) ; but in most cases the

INTEGUMENT

27

down^ 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 (scapits) :
the part distal to the quill, to which the barbs are attached in
a double row opposite one another is called the shaft (rachis).
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.

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 + in A, which separate the developing
barbs {St) : these have become free in C.

The barbs together constitute the vane (vcxilhim), 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 harhiccls 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
(rectriccs) 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.

^ Various modifications of the down feathers occur {e.g. Jiloplumes, 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 (pterylce)
separated by down-covered spaces (apteria), having a more or less
different arrangement in the various groups.^

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 Archseopteryx, 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

^ 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 ej'e and ear and at the base
of the beak, analogous to the sinus-hairs of Mammals [q.v.).

INTEGUMENT

29

the point in question ^ (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 {hnlb-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

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

^ 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}' 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

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 ; O, cuticle of shaft ; R, cortex ;
Sc, stratum corneum ; Sch, hair shaft ; SM, stratum Malpighii ; W8, WS^,
• 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 (vibrissce, 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 embiyo, 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 welt 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 chamcteristic of the Hedge-
hog and Porcupine, are merely especially strongly developed
bristles.

Hairs, like feathers, are arranged in definite tracts (Jlumina
piloimiJi), 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 w^ell marked in Man is, 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,
Muridae), Insectivores, Anteaters,^ 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 oiasal 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
^ Vestiges of horny scales also occur in Armadilloes.

32

COMPARATIVE ANATOMY

distal ends of the digits, the Mammalia may be subdivided into
Unguiculata and Ungidata, 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

m

Fig. 24. — Diagrammatic Longitudinal Sections through the Distal Ends
OF THE Digits of — A, Echidna ; B, an Unguiculate Mammal ; O, Man;
and D, Horse (after Gegenbaur and Boas).

1 — 3, phalanges ; h, torus ; N, nail-plate ; aS', 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.^

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 papillae 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
well-developed fatty layer
{panniculus advposus). The jvF,
panniculus may be very GR^
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 {arredores pilo-

ricm, Fig. 23), but may Oo, derm ; F, subcutaneous fat ; GP
occur independently of hairs

SM

Fig. 25. — Sectiox through the Human
Skin.

the scrotum and

vascular
papillae ; H, hair with sebaceous glands
(D) ; N and G, nerves; NP, sensor}^-
papillae ; Sc, stratum comeum ; SD, sweat-
glands, with their ducts {SD^) ; SM, stratum
Malpighii.

e.g., m
teats.

In the great abundance
of integumentary glands,

Mammals differ gi-eatly 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 glaiids, 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 nasolabial 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.^
Another important modification of the integumentary glands
is seen in the characteristic mammary glands, to the possession of

A

g.m

Fig. 26. — A, Ventral View of a Brooding Female of Echidna hystrix. B,
Dissection of thr Ventral Integument from the Dorsal (Inner)
Side. (After W. Haacke.)

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

g.m.

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

^ Amongst manj' other modifications of these glands of both types may be
mentioned the anal glands (especially well developed, e.g., in Manis and the
Skunk) ; the perineal or prescrotal glands of Viverra ; the caudal gland of
the Fox and Wolf ; the suborbital or antorbital glands situated in the cavity of
the lacryraal 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
embr^'o of both sexes, undergoes reduction in the female.

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 pouch appears in
the embryo of Echidna as an infold-
ing of the abdominal wall (Fig. 26).
This pouch, which serves to shelter
the Qgg 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 maiiwiary 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 on 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.^

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

^ Nothing definite is known as to the manner in which Omithorhynchus cares
for its young. The eggs are laid in burrows in the earth, and it appears that
no 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. Manidse, Murida3,

Fig. 28.— Pouch of Thylacinus, after Removal of the Skin.
(After Cunningham. )

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

Cervidse, 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 suiTOUDding 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 time (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,

Elephants, Sirenia, fig. 29. -Diagrammatic Representations of
many Lemurs, Cheirop- the Early Development of the Leading
tera," and Primates) : I^^^^ "^ Mammary Glands. (Modified from

1 -1 • ,1 • Gegenbaur.)

while m others, agam, *

they may be axillary or ^' ^^^^ o^ undifferentiated (mammary pocket)
abHnminnl nr fhpv mav stage; B, stage of the pseudo- (primary) teat;

aoaommai, or tney may ^^ ^^^g^ ^^ ^^^ ^^^^ (secondary) teat ; rf, mam-
occur m various com- mary canal ;/^, glandular area; gl, mammary
binations of all these glands; r, 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 paii-s (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
Tvhich 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-
mastism and hyperthelism are also well known in the human

38 COMPARATIVE ANATOMY

subject, as frequently in men as in women. The accessory
mammae 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 lasal

Fig. 30.— Placoid Scales from the Skin of an Elasmobranch.
(Semi-diagrammatic. )

<S', basal or socket-plates in the dermal connective tissue {Bg) ; Z^

denticles.

2olatc 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.^

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 Ejibryo Shark.
(From Gegenbaur's Comp. Anatomy.)

C, derm ; c, c, c, d, layers of the derm ; E, epiderm ; e, its layer of columnar
cells ; 0, 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.

^ In addition to the ordinary placoid scales, larger or smaller spines of a
similar structure may become developed in connection with the dorsal iin, around
the first cartilaginous ray [e.g. Acanthias, Trygon, Chimwra). 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 endoskeleton. 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) ^
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 Boiis's
Zoology. )

I, derm ; o, epiderm ; «, 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

^ In Amia the scales have a "cycloid" form, and in the adult Polyodon
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),^ as in many of the earliest Palaeozoic
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
Stegosauridge, 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,
(Ceratopsidse), 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. Bvit 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.—^, Carapace, and B, Plastron of a Young Testudo qrcuca;
C, Plastron of Chelone midas.

C, costal plates ; E, entoplastron ; E]}, epiplastron ; Hp, hypoplastron ; Hy,
hyoplastron ; M, marginal plates ; N, neural plates ; Xp, nuchal plate ;
Py, P3'gal plates ; B, ribs ; Xi, xiphiplastron. ( V indicates the anterior,
and H the posterior margin.)

plastron, the larger posterior portion of which probably corresponds
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
Palseohatteria, most Lizards (Fig. 56) and Crocodiles, forms in the
adult an unpaired plate of varied form. It is wanting in
Chamseleo, Anguis, Ophidia, and Chelonia.^

Birds. — Reduced abdominal ribs occur in the primitive
Archa3opteryx, 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,^ 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
homy 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 endoskcleton 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-
skeleton, 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

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

^ 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
^ometimes said to correspond to the last remains of the dermal episternum of
^ower forms ; but as further proofs are required before such a homology can be
definitely accepted, the term prostermim has been proposed to include the
elements in question (cf. under Sternum).

^ 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 hone, 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} 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 mesh work of pith-cells
(Fig. 34, A, b). At the periphery, however, the cells retain their
protoplasm, becoming flattened and an-anged like an epithelium.
Around the notochord two homogeneous, cuticular sheaths are
successively developed from its cells. The p^-imary 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 skcletogenous 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

sh.i

trip

sh.l

Fig. 34. — Diagrams illustrating the Development of the Notochordal
Sheaths and Vertebral Column.

A. — Early stage, showing notochordal cells {nc) and primary sheath (6^^), 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^) 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
(?ic) has become constricted, and the cartilages have united into a single
mass and have given rise to a centrum (c), neural arch («.«), 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 a,xis. 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 vertebrce (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 haemal) arches. In other cases (e.g. Bony Ganoids, Teleosts,
Amphibians, and Amniota), the arcualia extend at their bases
round the notochord ^ 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.^

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.

^ An ephemeral structure, the suhnotochordal rod or hypochorda, 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
Amphioxus.

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 tritnk- or irre-
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

Fig. 35. — Portion of the Veeteijkal Column of Polyodon. Side view.

Fig. 36. — Transverse Section of the Vertebral Column of Acipenser
riitheiuis (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 lower
arches enclosing the aorta ventrally ; 7c, intercalary pieces (inter-dorsal and
inter-ventral); M, spinal cord; Oh, upper arch (basi-dorsal) ; P, pia
mater ; Ps, neural spine ; »S^*S', skeletogenous layer ; tlb, lower arch (basi-
ventral ; Z, "basal stumps" 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 hsemal 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 Gartilaginotcs Ganoids, Holocephali, 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 Chimsera narrow

VERTEBRAL COLUMN 49

calcified rings are 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 neitral spines. In the caudal region
the haemal 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 " hasal
stwnip"

The relations of the arches in Plagiostomes, Bony Ganoids, and
Teleosts 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
doi*sally,^ consist on either side of several distinct elements, which

Jim:

Fig. 38. — Portion of the Vertebral Column of Scymniui.

Ic, intercalary pieces (interdorsals) ; Oh, 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 hasidorsal or neural plate, and is usually
jjerforated by the foramen for the motor root of a spinal nerve.
Intercalated between successive basidorsals is another cartilage,
the inter dor sal {intercalary piece, or internenral 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

^ 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. 39).^ The lower arches consist of hasiveniral
cartilages, between which are sometimes intercalated a series of
intervcntrals (Fig. 35), and which, in the tail, are produced into
hcemal spines : these may be formed of distinct infravcntral
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

/iPr f^^'inp np 77 sp

Tltc

Jcrfcr'^."Pn.p

h.a

h.sp

7i.a

h.a

Fig. 39. — Portions of the Vertebral Column of Scyllkim canicnla.
(From Parker's Practical Zoology.)

A and B, from the trunk ; G and Z>, from the middle of the tail ; A and C, two
vertebrjB in longitudinal section ; B and D, single vertebras viewed from one
end ; h, calcified portion of centrum ; r, centrum ; for, foramen for dorsal,
and /or', for ventral root of spinal nerve; h.a, haemal arch (basi-ventral) ;
h.c, haemal canal; h.sp, hsemal spine; Lilj), intercalary piece (interdorsal,
or interneural plate) ; n.a, neural arch ; n,c, neural canal; n.p, neural plate
(basi-dorsal) ; n.sp, neural spine ; ntc, 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 {prc-
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

^ 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

51

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

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

In Flagiodomes, the cartilage which has invaded the sheath of
the notochord is segmented into definite vertebral centra, which
become partially calcified in various w^ays. 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-
spo7ulylic form) ; concentric layers may be added to this, as in the
R^ys {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

Fio. 40.— Diagrammatic Transverse Section through the Middle of a

CvCLOSPONDYLIC (A), A TeOTOSPONDYLIC (B), AND AN ASTEROSPONDYLIC

Vertebra (C). (From Zittel, after Hasse. )

fZ, middle portion of the calcified double cone ; d', additional concentric calcified
laj-ers ; d", double cone with radiating calcified layers ; ex.m, external elastic
membrane; h.a, hjemal 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.^

Li Bony Ganoids and Teleosts, there is a tendency towards a
reduction of the cartilage ; that which forms the centra is entirely
outside the notochordal sheaths, and the vertebrae 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

^ In Rays and Chimseroids the anterior vertebral elements become fused
into a single mass, on which a definite cond3de is formed for articulation with the
skull ; amongst Sharks and in Dipnoans also, a concrescence of the anterior vertebral
elements with one another and witli the skull may occur.

E 2

52

COMPARATIVE ANATOMY

KnJiji^

and so persists as a kind of connecting or packing substance
between contiguous centra, which are consequently of a deeply
biconcave, or am'phicmlous 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 {opisthocceloicsform). 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-
tervertebrally, a condition of things
which appears again in the higher
types, as, for instance, in Reptiles.
In a still earlier larval stage, how-
ever, the constrictions are intra-
vertebral, as in other Fishes.

The skeleton of the posterior end

of the tail in Fishes requires special

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-

Fig. 41. — Portion of the Verte-
bral Column of a Young
Dogfish [Scyllmm canicula).
(After Cartier. )

C, notochord ; FK, the fibro-car-
tilaginous mass lying between
the cartilaginous zones which
is undergoing calcification ;
Kn, outer, and Kn', inner,
zone of cartilage ; Li, inverte-
bral ligament.

Fig. 42. — Portion of the Vertebral Column of Lepidostetis. (After Balfour

and Parker.)

A , vertebra from anterior surface ; B, two vertebra3 from the side, en, anterior
convex face, and ai', posterior concave face of centrum; h.a, transverse
process ; i.c, intercalary cartilages (fused supra-interdorsals) ; i.s, inter-
spinous bone ; I, i, longitudinal ligament ; n.a, upper arch (basi-dorsal).

rounded quite symmetrically by the tail-fin, and the tail is
therefore spoken of as diphycercal : 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 Jieterocercal 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-fin,
as in Lepidosteus, Amia, and more particularly in most Teleosts,

Fig. 43a.— Tail of Lepidosteus.

Fig. 43b. — Caudal End of Vertebral Column of Salmon. (From Boas's

Zoology.)

h, centrum ; h', urostyle ; ?i, ha?mal 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 B).
The posterior end of the vertebral column is then frequently
represented by a rod-like urostyle, and in Teleosts one or more
wedge-shaped hypural hones (enlarged hremal arches) generally
occur directly beneath it.^

1 The diphjcercal character of the tail in Dipnoi and certain Teleostomi is
probablj' not primitive {protocercal), but has been acquired secondarily.

54 COMPARATIVE ANATOMY

As a rule Elasmobranchs and Ganoids possess a greater number
of vertebrae (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 vertebrae of Bony Ganoids and Teleosts is
also seen in the Amphibia and Amniota, the notochord being of
less importance and the vertebrae 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-lumhar,
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 Caecilians, 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 intervertebral ly it
remains thicker, and accordingly appears expanded. Thus the
centra here also are amphiccelotcs. 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 vertebrae of many
higher Urodeles an anterior convexity and a posterior concavity
may be distinguished, both covered with cartilage ; they are,
therefore, opisthoccelous (Fig. 44).

In the development of the vertebral column of Urodeles we
can thus distinguish three stages : — (1) A connection of the indi-
vidual vertebrae by means of the inter vertebrally expanded
notochord; (2) a connection by means of intervertebral masses
of cartilage ; 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.^

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

Fig. 44. — Longitudinal Section through the Vertebral Centra of Various
Urodeles. A, Ranodon sihericus ; B, Amhly stoma tigrinum ; C, Gri/rinophilus
porphyriticiis {I, II, III, the three anterior vertebrae) ; D, ScUamandriivob
per-'ipicillata.

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 ; Ligt, intervertebral
ligament ; Mh, marrow ca\"ity ; B, transverse process ; S, intravertebral
constriction of notochord in AmMydoma, 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

^ In certain of the Stegocephali incomplete hoops of bone, the intercenfra,
ajid pleurocentra, twice as numerous as the arches, surrounded the persistent
not.ochord (cf. the caudal region in Amia and Elasmobranchs, p. 50).

56

COMPARATIVE ANATOMY

SfT

Pfe

Oe

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 {procceloits 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 vertebrae 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
body. The transverse processes of the
single sacral vertebra, which give attach-
ment to the pelvis, are particularly
strongly developed, especially in the An-
ura (Fig. 45), in which the number of
presacral vertebrae is usually only eight.

Articular processes (zygapophyses) are
well developed in all Vertebrates from
Urodeles onwards, and consist of two
pairs of projections arising from the an-
terior and posterior edges respectively of
the neural arch. Their surfaces are
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 cervical vertebra becomes differentiated from the
others, and consists of a comparatively simple ring which articulates

Fig. 45. — Vertebral
Column of Discoglossiis
jnctus.

Ob, upper arch of first ver-
tebra ; Pa, articular
processes ; Po, anterior
process of first vertebra;
Ps, neural spine ; Pf,
transverse processes of
trunk vertebrae ; Ptc,
tran verse processes of
caudal vertebrae (uro-
style, Oc) ; P, ribs ; Stf,
condylar facets of first
vertebra ; 8 W, sacral
vertebra.

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 vertebrae present in Urodeles is inconstant, and
varies greatly : it may reach to nearly 100 (Siren), and in Caecilians
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 vertebras, 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, w^edge-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 intra vertebral 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 proccelous 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
vertebrae are mostly proccelous, an exception being seen in the two
sacrals and first caudal. In Chelonians there is gi'eat variation in
the form of the individual centra of the cervical vertebrae — even in
the same individual proccelous, opisthoccelous, biconcave, and even
biconvex centra, with intervertebral discs, may occur ; while the
thoracic and lumbar vertebrae have flattened faces, and are firmly
united with one another by cartilage, and also with the carapace
(p. 43).

58

COMPARATIVE ANATOMY

What has been said as to the classification of 'the vertebra3 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 vertebrae
instead of a single one, and also typically at least two sacral
vertebrae. The two first cervical vertebrae 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 hones, corresponding to the intercentra, are present in the
tail in Lizards, Crocodiles, and some Chelonians ; and besides

Fig. 46. — Anterior Portion of the Vertebral Column of a Young

Crocodile.

A, atlas; Ep, axis, articulating with the atlas at A; Is, intervertebral disc; o,
" pro-atlas " ; Oh, neural arch ; Po, odontoid bone ; Ps, neural spine ; Pt,
transverse process, arising from the base of the arch and articulating with
the rib at t ; -R^ R^, H-, 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 vertebrae of Lizards, Crocodiles, and Snakes :
in the last mentioned paired processes partly enclose the caudal
vessels. The arches in Snakes, Lizards, and usually in Chelonians,
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 (zyfjosplienes and zygantrd) 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 vertebrae 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. Archaeopteryx, 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 {Picus viridis).

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

axis ; Pf, transverse process ; WK, centrum of axis, and Sa, its saddle-
shaped articular surface for the third vertebra ; +, condylar facet.

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

the two bars of the transverse process, shown on one side ankylosed with
the cervical rib (/?) ; Psi, 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 Qieterocmlous) 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) vertebrae (thoracic, lumbar,
and caudal), with their rudimentary ribs, become fused with the
two primary ones (Fig. 48), so that the entire number of vertebrae
in the sacrum may be as many as twenty-three. In Archaeopteryx

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 vertebrae usually fuse together
to form a flattened bone, the pygostyle,
which supports the tail quills (Fig. 132).
In the Ratitae there is never more than
an insignificant pygostyle (Struthio), and
all the caudal vertebrae 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 Archaeopteryx, in
which it was supported by numerous
elongated free vertebrae (Fig. 49). It
must, however, be borne in mind that
the pygostyle may be made up of from
six to ten fused vertebrae, and in the
sacrum even a greater number may be
included, so that as many as twenty or
more caudal vertebrae 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 fibro-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 epiphyses 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 are usually only formed
on the atlas and anterior face of the axis ; well-developed articular

Fig. 48. — Pelvis of Owl
{Strix huho). Ventral
view.

II, ilium ; Is, ischium ; P,
pubis ; B, last two pairs
of ribs ; W, position of
the primary sacral verte-
brae : between E and //,
and behind W, are seen
the secondary sacral ver-
tebrae, fused with the pri-
mary {W) ', t foramen
between ilium and pubis.

VERTEBRAL COLUMN

61

processes (zygapophyses) are present on the neural arches.^ The
cervical region is usually the most movable, and the centra
may here possess articulations and have an opisthoccelous form

Fig. 49. — Archcvopteryx litJiographka. From the Solenhofen slates (Jurassic).
After Dames, from the specimen in the Berlin Museum.

c, carpus; d, clavicle; co, coracoid; A, humerus; r, radius; sc, scapula; u, ulna;
/ — ///, digits of manus; / — IV, digits of pes.

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

^ In certain Edentata (e.j/. Mjrmecophaga, Dasypus) extra articular processes
are present besides the ordinary zygapophj'ses on the posterior thoracic and
lumbar vertebra:?.

62 COMPARATIVE ANATOMY

The atlas ^ 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 vertebras 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
vertebrae, 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 vertebras 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 vertebrae 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.^ The greatest number of caudal vertebrae is found in

^ 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
intercentra 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
2)leural 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 fonii of

s '^

^ '=<2^^

RIBS

65

haemal arches, which in Teleosts, as in Elasmobranchs, are
developed from the transverse processes alone (Fig. 50, b).
The dorsal ribs take no part in the formation of the haemal 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 haemal arches.

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

Fig. 51.— Anterior End of the Vertebral Column of Polypterus. From

the ventral side.

P-9, parasphenoid ; WK, centra ; 7— F, first five pairs of dorsal ribs ; tt, ventral

ribs.

two pairs of ribs to each body-segment. Dorsal ribs can also be
recognised in certain Teleosts (Salmonidae, Clupeoidei) in addition
to ventral ribs, and like these, are always preformed in cartilage.^
In many forms, the ventral ribs may undergo reduction, and in
Elasmohranchs they are wanting, while dorsal ribs are usually
present.'^ In Chimaeroids 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 intermiLSCular
hones often present in this region in Teleosts. In addition to these epicentrcd
intermuscular bones, others — the epineurals and epiplenrals — are situated more
dorsall}' and more ventrally respectively, and all of them are merely ossifications
in the septa.

^ The h?emal arches of these Fishes, as well as of Ganoids, Dipnoans, and
Amphibians, apparently contain components corresponding to ventrals rib.

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 always connected with transverse
processes or at any rate with the vestiges of the basal stumps
(Fig. 50, f). The latter are originally situated, as in Fishes,
towards the ventral side of the vertebral axis, and in the tail give

Fig. 52.— a, Vertebra from Anterior Part of Tail of Larva (43 mm.)
OF Necturus ; B, Sacral Vertebra from Larva (43 mm. ) of Necturus ;
C, Fourth Trunk- vertebra from Newly-born Larva of Salamandra
maculosa. (After Goppert. )

Art. vert., vertebral artery ; B, cartilage of basal stump ; B^, vestige of same in
larva of Salamander ; B'\ bony bar which replaces the same functionally ;
Ch, notochord ; DBS, dorsal bar of rib ; //, ilium ; ^V, neural arch ; /?, rib ;
RT, BT\ ventral and dorsal rib-bearing portions of vertebra; SF, lateral
process of hjemal arch {H).

rise to haemal arches (Necturus, Salamander-larvse). 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 upvA ard displacement is carried
still further in the Gymnophiona and Anura. In Urodeles (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 ^

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 haemal arches.

Finally, reference must be made to the cartilaginous " abdominal
ribs " (c£ 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,^ 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 originall}' 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 Hatteria 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 ; Or, keel of sternum ;
Fu{Cl), furcula (clavicles) ; G, glenoid cavity for humerus ; S, scapula ;
F, 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

TsPt

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
apojjhyses.

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 vertebrae 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 Archae-
opteryx (Fig. 49) more nearly resembled
those of Lizards.,

Mammals. — The cervical ribs in
nearly all cases unite completely with
the vertebra?, and a vertebrarterial
canal is thus formed. The last cervical
rib may be well developed and may
articulate with the corresponding
vertebra {e.g. Choloepus hofmanni).^ The seventh cervical rib
is also long in Bradj^us, 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 ttchercidu/n 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

Fig. 54. — Costal Arch of
Man.

Ca, capitulum ; Co, neck, Cp,
bony vertebral, and Kn,
cartilaginous sternal por-
tion of rib ; Ps, neural
spine ; Pt, transverse
process; St, sternum ; Tb,
tuberculum ; WK, centrum
of vertebra.

^ 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 (" costse fluctuantes ").

70 COMPARATIVE ANATOMY

the cartilaginous facet on the transverse 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.^

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 ^ 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 {e.g., Ranidge) 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

^ A primitive and a secondary type of thorax may be distinguislied. The
former is the more usual, and occurs in most Mammals even up to tlie lower
Apes : it is characterised by an elongated form, and by the dorso- 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.

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

Fig. 55.— Pectoral Arch of Variots Amphibians. From the ventral side.
A — Urodele (diagrammatic) ; B — Axolotl {Amhlystoma) ; C — Bomhinator
igneus ; D, Baiia esrulenta.

C, coracoid ; CI, proeoracoid ; Cl^ {CI in U), clavicle ; EC, Co^, epicoracoid ;
Ep, omosteruum ; Fe, fenestra between proeoracoid and coracoid bars ; Kn,
cartilaginous xiphisternum ; t, Pf, G, glenoid cavity for the humerus ;
.S, scapula ; >S^«S', suprascapula ; St, St^, sternum. *, f (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 {Hemidactylus
verrucosus). From the ventral side.

a, b, c, membranous fenestras in the coracoid ; Co, coracoid ; Co^, cartilaginous
epicoracoid ; CI, clavicle ; Ep, episternum ; G, glenoid cavity for the
humerus ; i?, ribs ; S, 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).^

The sternum may become calcified (Reptiles), or converted
into true bone (Birds, Mammals). In Rep til es,^ Birds, and Mono-

' Cf. note on p. 44.

2 T»> Snakes and 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 Carinatse with a projecting keel, which forms an additional
surface for the origin of the wing-muscles (Fig. 53). In contrast
to these, the cursorial Ratitae 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/?>, manubrium ; Pe, xiphoid process ; B, ribs.

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

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 (stcrnehw)
the number of which may correspond to the afiixed 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
manuhrium, and the distal end a partly cartilaginous xiphoid or
ensiform process.

^ A keel was also present in the Pterosauria, and may be developed
wherever a larger surface for the origin of the pectoral mu^jcles 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
vertebrae (*' cranial vertebra3 "). 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 hony 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.^

^ 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 pereisted, 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 occuiTed
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 w^hich is situated along the main axis
in continuation of the vertebral column and which encloses the
brain, is known as the hrain-casc or ci'anium {neiirocraniiciii), 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 (spknichnocranitom). 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 jaics. The arches,
therefore, serve primarily as gill-supports. Ossification may occur
in connection w4th 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 ''• head-cavity."
Later, however, the visceral region became relatively shifted to a
greater or less degree, especially in the anterior part of the head.

P£

Fig. 58.— First Cartilaginous Rudiments of the Skull.

C, notochord ; N, A, O, the three sense-capsules (olfactory, optic, and auditory) ;
PE, parachordal elements; PB, primary pituitary space; 7V, trabeculse

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
cho7idrocranmm are seen in the form of an anterior and a posterior
pair of bars — the trabemlce cranii and the parachordal 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 hasal plate, which grows round
the notochord dorsally and ventrally, and thus early forms a solid
support for the hinder part of the brain. The trabeculae 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 yihiitary space (anterior hasicranial fmitanelle). 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

Comu trabecula

Occipital arch
Notochord

Avdxloi-y capstUe
Otic process
Palatoquadrate

Articular process

Fig. 59. — Neurocrakium and Palatoquadrate of Larval Amblystoma,
9 MM. IN Length, seen obliquely from the Left Side and Above.
X about 35. (From copy by Fr. Ziegler of a model by Ph. Stohr. )

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),
w^hich 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 trabeculse 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 platyhasic, 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 trabeculse give rise to a thin interorbital septum, and the

78 COMPARATIVE ANATOMY

brain only extends as far forward as this septam, 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 trabeculfe or indepen-
dently of them, and during its growth, the cartilage extends
round a number of the cerebral nerves. The anterior ends of the
trabeculse, which are continuous with the ethmoidal region of the

Internaml cavity

MeckeV$ cart.
Antorbitcd procest —M' ^~^ji^M WS.-^ ^^S^^ -^-'^ ^^

Prootic foramen ^j|P^S||^ jf U § l^l^^^^^^B— Ascending process^ |

Otic process

}^

Notochord 111 .■■■■I . . ^^^^^^m^^^l^^^m- ^^^'iitory caps.

Tectum synoticum Occipital condyle

Fig. 60. — Neurocranium (Platybasic Type) and Mandibular Arch of
Larval Newt [Triton tceniatus), 2 cm. in Length, seen from Above.
X 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 7iasa/ or olfactory capsules, the chondrifi cation 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

Septum nasi ,.jSC

■\ Nasal caps.

Trabeeula • ^

Pituitary space ,
Trabecuia ."^ ,

Carotid foramen

D'

Basal plate .

'"^^

Notochora

I

'^

. in

- IV

, V

.

^jAud. caps.

"i

-^VIII

M-

_ Columella

Cl>#^

r-

^

■-"--'•-.'^ Sp.

oc.

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 ;
foramina for spino-occipital nerves.

>S^. oc,

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 hones (allostoses) have originally no direct con-
nection with the chondrocranium, and thus maybe 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 rq:)lac-
ing or substituting hones {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.

h. 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
are always more numerous (in some
cases there are as many as nine) in
forms which possess gills than in higher
types (Amniota), in which their number
gradually becomes reduced from behind
forwards : they may, moreover, undergo
a change of function, certain of them in
some cases taking on definite relations
to the auditory organ, larynx, and
tongue.

The 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 2^ost-oral arches
as the mandibular arch (Fig. 63). The post-oral arches serve

Fig. 62. — Diagrammatic
Transverse Section of
THE Primordial Skull.

C, notochord ; 0, auditory
capsule ; BH, cavity of the
pharynx, enclosed by the
visceral skeleton ; 1 to 4,
the individual elements
composing each visceral
arch, which is united with
its fellow by a basal piece
{Cp) ; Tr, trabeculce, which
enclose the brain (C) ven-
trally and laterally.

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 (pharyngo-, 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, basibranchicd).

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

Fio. 63. — Diagrammatic Figure of an Embryonic Elasmobranch Skull,

SHOWING THE RELATIONS OF THE ViSCERAL ArCHES.

A, eye ; a to e, branchial arches, between which the gill clefts [I to TO are seen ;
br, brain ; C, intervertebral remains of notochord ; H, hyomandibular ; K^
hyoifl 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 ; F, 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 (suspen-
soriuiii) for the jaws, either remains separated fi-om 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

82 COMPARATIVE ANATOMY

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

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 2)'>^eGral elements. Under this category
are included the labial cartilages of Elasmobranchs (Fig. 63), and
similar structures amongst Teleostomes and in certain Anuran
larvae.

Relations of the Chief Investing Bones to the Chondocraniiim.

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 frontal s 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).^

The investing bones of the ethmoidal region are : the nasal,
supraethmoid of Teleostei, prefrontal of Amphibia and Sauropsida,
septoinaxillco-y (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 premaxillcc, maxilla, and vomer to the ethmoidal skeleton are
possibly of a secondary nature (cf p. 83). The parasphe/ioid 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 paraqtoadrate by Gaupp, who considers it to be homologous
with the quadratojugal of Reptiles. The vomer, palatine, and
pterygoid (including the ecto- 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

^ It is possible that the hj'oid arch is the derivative of two arches which were
primarily separated by a cleft.

2 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 ihe premaxillo-
maxillary bar which lies externally to it in the higher Fishes and
in all Vertebrates above them. It is possible that the premaxillas
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. x\s 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 &plcnial, in connection with
Avhich there may be a varying number of j)'i^esplenials. 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 (" dermanf/ular," " dermurticular"
" supra-angular," " coronary ").

Certain tooth-bearing bones are formed in connection with the
hyobranchial skeleton of Teleosts (superior a.nd inferior pharyngeal
J^o'iu'S, " dermohranchiah" " dermentoglcssal "). In higher forms,
investing bones are only exceptionally present in connection with
this part of the primordial skeleton.

Relations of the Replacing Bones.

The follo\ving are usually looked upon as ossifications of the
occipital region (cf. Figs. 67-95) : the hasiaccijntal, cxoccipitcds
(pleuroccipitcds), and suprnoccipifal : the last mentioned, however,
usually arises from the ossification of that part of the roof of the
skull (tectum 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 hones, including an op)isthotic, 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
autosq^uamosal and a dermosquamosal.

G 2

84 COMPARATIVE ANATOMY

In the orbito-temporal region, a hasisjphenoid, a presphenoid,
alisphenoids, and 07'bitosphenoids may occur. In the ethmoidal
region lateral jthmoids and pre-ethrnoids are present in bony Fishes
and a single ethmoid in Mammals.^

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 aiitocoronary and other bones may be developed (Teleosts).
The anterior end of Meckel's cartilage usually becomes ossified as
a mentomandihular {mentomeckelian) which may become fused with
the dentary.

In the hyobranchial skeleton, the individual segments may
be uniformly ossified (e.g. stylohyal, 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 dorsal ly 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 Cydostmnata 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 be recognised.

sb.oc.a

Ic*

Lc.3

brcLr

Fjg. C4.— 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; au.c, auditory capsule;
hr.h, 1 — 7, vertical bars of branchial basket ; hr.d, 1 — 7, external branchial
clefts; cn.c, cornual cartilage; cr.r, cranial roof; l.c, 1—4, longitudinal
bars of branchial basket ; Ig.c, lingual cartilage ; mv.c, median ventral
cartilage ; na.ap, nasal aperture ; nch, notochord ; Nv. £, foramen for optic
nerve; olf.c, olfactory capsule; pc.c, pericardial cartilage ; jocZc, posterior
dorsal cartilage ; p.Iat.c, posterior lateral, cartilage ; sb.oc.a, subocular arch ;
st.p, styloid process ; sty.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.Iat.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 margm
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, Imgual
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 homy 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

JVv.S

etudejb

brr'

fitf.crv

-bra J

Fig. 65.— Skull of Dogfish {Scyllium canicula). (From T. J. Parker's
Biology, after W. K. Parker. )

aud.cp, auditory capsule ; hr.a, 1—5 branchial arches ; hr.r, hr.r', branchial rays
arising from the hyoid and branchial arches ; Cr, cranium ; ex. br, extra-
branchial cartilages; hy.ai, ventral part of hyoid arch; hy.7n, hyoman-
dibular; Ih, labial cartilage; tgJg', ligaments supporting the jaws from
the cranium ; l.j, Meckel's cartilage ; Nv. 2, optic foramen ; Nv. 5, foramen
for trigeminal and facial nerves; oJf.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 Palceospoiidylus 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, uii
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

^^cl

Op.T-

Fig. 66.— Skull' of Chimcera monstrosa. Lateral View.
Parker and Haswell's Zoology, after Hubrecht. )

(From

a.s.c, position of anterior semicircular canal ; ch.y, ceratohyal ; ep.ky, epi-
hyal ; fr.cl, frontal clasper ; h.a.c, position of horizontal semicircular canal ;
i.0.8, interorbital septum; Ih. 1, Ih. 2, Ih. 3, labial • cartilages ; Mck.C,
mandible ; Nc. 2, optic foramen ; Nv 10, vagus foramen ; olf. cp, olfactory
capsule; op.r, opercular rays; pal.qu, palatoquadrate ; ph.hy, pharyngo-
hyal ; p.f^.r, 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 " neoeranium " (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. QQ) it becomes
immovably fused with the cranium, whence their name of
Holocephali. Iq 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 amphistylic. 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 spirac2clar cartilages which probably represent gill-
rays (cf. below).^

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-ra3^s.

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
opercidimi 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,

^ A small basimandibular element has been described in La?margus, and
mandibular rays can be recognised in the primitive Pleuracanthidte 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 operculum is more pronounced than in the
Holocephali, and is also supported by bones (cf. p. 90). The
whole palato-mandibular apparatus — which is comparatively small,

^zr

Fig. 67.

-Cranial Skeleton ok Sturgeon (Acipenser) after Removal of the

EXOSKELETAI. PaRTS.

Ar, articular; C, notochord ; Co/?, basal elements of the visceral skeleton ; De,
dentary ; GK, auditor}^ capsule ; Hm, hj^omandibular ; hj/, hyoid ; / to F,
first to fifth branchial arches, with their segments— the double pharyngo-
branchial (a), the epibranchial (6), 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 ; Ps,Ps', Ps", parasphenoid; Psp, neural
spines ; Qii, quadrate ; H, rostrum ; Pi, ribs ; SpN, apertures for spinal
nerves ; Sy, symplectic ; [VS. 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, otic, orbitotemporal, and ethmoidal
regions. Investing and replacing bones very similar to those

90

COMPARATIVE ANATOMY

J^JC

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 trabeculae, 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-
teroperculum 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
posteriorly

Fig. 68a. — Skull of Polypferus hichir from
THE Dorsal Side.

a,

h, c, d, supraoccipital shields. The two
arroM^s 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 ; JVor, external
nostril ; Op, operculum ; Orb, orbit ; P,
parietal ; Fmx, premaxilla ; PO, pre-
operculum ; Sb, Sb', anterior and posterior
suborbital ; SO, suboperculum ; Sp, pre-
spiracular bones.

Size antero
(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,

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-

FiG. 68b.— Skull of Polypterus, 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 ; /.?», foramen magnum ;
Fr, frontal ; l.e, lateral ethmoid ; JIx, maxilla ; Xa, 2ia', nasal and accessory
nasals; occ, occipital: ol, nasal aperture; ojj, operculum ; op.o, opisthotic ;
O.t, "os terminale"; Pa, parietal; Pmx, premaxilla ; P.t, posttemporal ;
Pff, postparietal ; Qu, quadrate ; S.h, S.b', suborbitals ; S. Op, sub-operculum ;
Sj), splenial ; sp.eth, " sphenethmoid," in the orbitosphenoid and alisphenoid
region, resembling the like-named bone of Anura {q.v.) ; sj^.o, sphenotic ; Spr,
prespiracular ossicles ; >S'.^ supratemporals; F, preoperculum (cheek plate);
y, Y", smaller cheek plates ; z, post?piracular 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

jostei (Fig. 70), and in this respect such forms as Argyropelecus
Cyclothone acclinidens deserve special mention. The cranial

Teleostei

and

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

dent a?'t/ ^vraop

Fig. 69.— Cranial Skeleton of the Salmon. From the left side.

art, articular ; hranchiost, branchiostegal rays ; dent, dentary ; epiot, epiotic ; eth,
supraethmoid ; fr, frontal ; hyom, hyomandibular ; intoj), interopereulum ;
Jug, jugal ; mpt, mesopterygoid ; mtpt, metapterygoid ; mx, maxilla ; nas,
nasal ; orbital ring ; op, operculum ; pal, palatine ; par, parietal ;
P. mx, premaxilla ; proip, preoperculum ; 'pt, pterygoid ; pttr, pterotic
(squamosal) ; Quad, quadrate ; soqc, supraoccipital ; sphot, sphenotic ; suhop,
suboperculum ; Zunge, 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

Col.vert

psp.

Kasph yl^^f

Fig. 70. — A. Cranial Skeleton of Salmon after Removal of the Jaws
a?:d Orbital and Opercui,ab Bones. From the right side.

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

both figures.

aJsph, alisphenoid ; basoce, basioccipital ; basph, basisphenoid ; Col.vert, point
of connection of the skull with the vertebral column ; ekteth, ectoethmoid ;
epiof, epiotic ; exocc, exoccipital ; fr, frontal ; N.o^f, canal for the olfactory
nerve ; opisth, opisthotic ; orbsph, orbitosphenoid ; pfero, pterotic (squamosal) ;
proof, prootic ; psjth, parasphenoid ; socc, supraoccipital ; sphot, sphenotic ;
ro, 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.^

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,
and the maxilla may bear teeth. The maxilla, however is
edentulous except in the Physostomi.^

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 {orMtcd ring), and in the
gill-covers {opercular hones) : the latter are similar in number and
name to those of many Bony Ganoids (p. 90). A large number of

^ A curious asymmetry is seen in the head of adult Pleuronectidse. When
hatched, these Fishes are quite symmetrical, but later on the eye of one side
becomes rotated, so that eventually both eyes are situated on 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 ej^e-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

hranchiostegal rays are developed in the ventral parts of the
opercular fold or hranchiostegal membrane (Fig. 69).

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 Chimseroids 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.^ 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 (platy basic 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.^

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

^ 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.

96

COMPARATIVE ANATOMY

(" scleroparietal "). The last mentioned element is wanting in
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).

.W^^fs!]^ Jf

JStK

Fig. 71.— Skull, with the Pectoral Arch and Fin, of Protopterus.

A, splenial ; AF, antorbital process (the labial cartilage in this region is not in-
dicated) ; a, 6, SL, teeth ; B, co, fibrous bands ; D, 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 ; ^A", fenestrated cartilaginous nasal capsule ;
Oh, 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 ; Tr, palatoquadrate cartilage ;
W, W, vertebral elements with their neural spines {Psp) 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 ; 1 — 3,
the three following segments : ff, 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 h} oid 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 ^
are comparatively small and weak, and some, or even all of them,
may be entirely 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
{lamino. 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 {tectum 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

^ In Protopterus, a delicate cartilaginous rod arises from the first branchial
arch (Fig. 71), concerning the homology of which opinions diflFer. 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. vestibuli, on 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 stapedial plate, which is connected with
the quadrate and paraquadrate (see p. 82) by ligament, or by a
cartilage or bone {columella auris), 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 trabeculjB. 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 premaxillse,
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 premaxillse and maxillre 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 fontanelle, 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 palatoquadrate 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 ^ becomes fused
secondarily with the skull, and on its outer surface is an investing

^ In Tylototritoii verrucosus 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

Pmr ^

Pnu- Jo IX

Sit

Cwc ' CdecOsp

72a. — Skull OF A Young Ax OLOTL Fig. 72b. — Skull of Scdamandra atra
{Amhly stoma). Ventral view. (Adult). Dorsal view.

toci' 'OB

Fig. 7*2c. — Skull of Salamandra atra (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 ; For, fenestra ovalis, closed
on one side by the stapedial plate {St); IX, internasal plate, which extends
laterally to form processes (TF and AF) bounding the internal nostrils
(Ch) ; Lijt, ligament between the stapes and suspensorium ; 21, maxilla ;
X, nasal ; Xa, external nostrils ; XK, nasal capsule ; OB, auditory capsule
and exoccipital ; Os, 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 ; Rt, point of entrance
of the ophthalmic branch of the fifth nerve into the nasal capsule ; Squ,
para(|uadrate (" squamosal") ; 7^/', trabecula ; Fo, vomer ; Fo/?, vomeropala-
tine ; Z, tongue-like outgrowth of the internasal plate, which forms a roof
for tlie 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 ii^pper 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

Fig. 73. — Skull and Visceral Arches of Menopoma. From the side.

I, mandible ; II, hyoid ; III- VI, branchial arches ; qii, quadrate, covering which
is the paraquadrate (" squamo
enclosed by the dentary bone.

is the paraquadrate (" squamosal") ; ar, articular; 7nk, Meckel's cartilage
'be

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 w^iich 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,^ 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

^ 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 ^\dde spaces ; and even where connected sheets
of cartilage are present, they are very delicate and thin. At the
same time the sknli nearly resembles that of Urodeles (more

Cp

Op Rnd.l

JTpX

Fig. 74. — Hyobranchial Apparatus of Urodeles. A, Axolotl {Siredon stage
of Amhlystoma) ', B, SaJamnndra maculosa', C, Triton cristatus ; D, Spelerpes

fuscu-s.

Cp, Cps, O.th, basihyobranchial or copula ; G.fh, thyroid gland ; Hphr. I and //,
first and second hypobranchial ; HpH, Bad I, hypohyal ; Kehr. I— IV, first
to fourth ceratobranchial ; KeH, ceratohyal ("anterior cornu" of hj^oid in
Cadueibranchs — the " posterior cornu " being made up of Hphr I and // and
Kehr I). Rod. 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

102

COMPARATIVE ANATOMY
B Ap.

Fig. 75. — A, Dorsal

B, Ventral, C, Lateral View of Skull of Siphoiiojjs
annnlatus.

Ap, external nostril ; amj, angular ; Car, carotid foramen ; Ch, internal nostril ;
Co, occipital condyle : dent, dentary ; DK, apertures for ducts of tentacular
gland ; E, ethmoid region ; F, frontal ; M, maxilla ; Xj/r, naso-premaxilla ;
Orb, orbit ; Pal, palatine ; Po, petroso occipital ; Pp. Pp', palatine process
of the naso-preniaxilla and of the maxilla ; P,s, parasphenoid, united
posteriorly with the auditory and occipital elements ; Pt, pterygoid ; Qu,
quadrate; Sq, paraquadrate ("squamosal"); Sfp, stapedial plate ; Vo,
vomer ; X, vagus foramen.

D, Hyobranchial apparatus of the larva of IchthyophU y/ntinom. After
P. and F. 8arasin.); Co I, 2, first and second basal elements (copuhe) ; //,
' 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 CJrodeles, of a hyoid and of four branchial arches
which, however, have a more primitive and piscine fonn and
arrangement and are less reduced than in Urodeles (Fig. 75, d).
In the adult, the median basal connection between the first and

J^TiPji'e^

Fig. 76. — Restoration of the Skdll of a small Stegocephalan (Protriton)
from the Carboniferous of Bohemia. (After Fritsch.)

Br, branchial apparatus ; F, frontal ; Fp, parietal foramen ; M, maxilla ; N,
nasal ; Na, nostril ; Oc, gclerotic ring (orbital bones) ; P, parietal ; Pf^
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 w^hich were
comparatively gigantic, w^as 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 Urodeles, 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.

104

COMPARATIVE ANATOMY

Max. -
Fnf,'oul.~A

P^.xygT

^y- Tectm/,i.<^^<^'f<^t, Q^^^n

C.prn.inf. J^-'n^f^-^'-

Ct. s. n.

Ap.iias.inti
Maw,

Jb*c.s\tLoc

Q-^flifiV,

Occ.lat,

Fig. 77.— A and B, Dorsal and Ventral Views of the Skull of Bana
escidenta ( x 2), In B the Investing Bones are removed on the RightTSide.

Ap.nas.int , internal nostril ; Arc.suhoc, subocular or palatopterjgoid arch ;
C.prn.inf, inferior prenasal cartilage ; Cr.s.n, subnasal crest; Efh, spheneth-
moid ; F II — V, foramina for cerebral nerves ; Foss.cond, condyloid fossa, in
which are the foramina for the IXth and Xtli cerebral nerves ; Fr.par,
frontoparietal; I. max, premaxilla ; Ma.v, maxilla; V«, nasal; Occ.lat, ex-
occipital ; Pal, palatine ; Para, parasphenoid ; Pr.front, frontal process of
maxilla ; Proof, prootic ; Pr.zyg, zygomatic process of paraquadrate ;
Pr.has.Q, basal process of quadrate; P/'.^p^er. ^, pterygoid process of quad-
rate ; Pter, pterygoid ; Q max, quadratojugal (or quadratomaxilla) ; Qiiadr,
quadrate; Tect.syn, tectum synoticum ; 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,

8KULL

105

especially in the parts surrounding the gape, 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 c\ncestral form must be sought m 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

Ext. nostril Oblique cart. Frontal fontaneUe Parietal fontaneUe

Alar cart. ^

\Sujj. pr^:-
nasal cart.

Inf. prt-
nasal cart.

Tect. syn.

Fenestra
oralis

Maxillary 2yi'Oce>s8

Arc, suhoc.

Annulu^ tyinjMuicus Artie, process of
quadrate.
Fig. 77, C.

Lateral View of a Wax Model, reconstructed from Sections, of the
Chondrocranium of a Young Rana temporaria, about 2 cm. long. The
Stapedial Plate and Columella are removed, and the Ossifications are not
indicated. (After Gaupp.)

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. Ranidse) in connection with the auditory organ. The
first visceral (hyomandibular or spiracular) cleft disappeai^s entirely
during development in the Urodela and Gymnophiona, but
becomes modified in many iVnura to form a Eustachian tube,
opening into the pharynx and leading into a tympanic cavity.
The latter is closed externally by a tympanic membrane supported
by a cartilaginous ring (annidus tympanicus^ Figs. 77, C, and
78), and to it the distal end of the columella (p. 98) is attached.

- ^ Cf. under Auditory Organ. The tjmpanic ring is a derivative of the
palatoquadrat e.

106

COMPARATIVE ANATOMY

On the dorsal side of the chondrocraniuin 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.

Premaxilla

Alar cart

Oblique cart

Ant. maxil-
lary process

Septomaxilla

Nasal
Maxilla

Pterygoid
cart.
Ill

V, VII

Annidiis
tympanicu

Artie pro-
cess of
quadrate

Pterygoid
Frontoparietal

Columella

Qiiadratojugal
Paraquadrate

Aud. caps-ule

Exoccipital

Fig. 78. — Skull or a Young Pana temporaria, 2 cm. in Length, just after

Metamorphosis, from the Dorsal Side. The Investing Bones are

REMOVED ON THE Left Side (x abt. 11.) After Gaupp, from a model by
Fr. Ziegler.)

Cartilage — blue ; replacing boues — gray ; 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 coimected
with the suspensorium by means of a small intermediate bone, the
quadratojugal, or quadratomaxilla (Figs. 77 and 78). There is
thus a lower zygomatic 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-cavity, cf. Figs. 77, A and b).^

R-anthi

Pr. ant.

Com. princ.

Fig. 79.— a. Hyobraxchial Skeleton of a Larval Bana tcmjyoraria, 29 mm.
IX 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. Hyoid Cartilage of a Young Frog, 2 cm. in Length, from
THE Ventral Side.

(All these figures are from wax models after Gaiipp.)
A and B (in part), Branch I— IV, branchial arches ; Com. term. I— III, terminal
commissures of same; Co]), basal plate (copula); Hy, hyoid; Pr.ant.hy,
Pr.Jat.hy, Pr.po.st.hy, anterior, lateral, and posterior processes of the hyoid ';-
Spic. I — / V, cartilaginous processes.
B (in part) and C. Corp.rart.hy, body of hyoid cartilage; Com jiriiic, anterior
cornu; Man, "manubrium"; Pr.al, alary process; Pr.ant, anterior pro-
cess; Pr.po'it.lat, posterolateral process; Pr. thy r. pout. med^ thyroid or
posteromedial 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

^ A septomaxillary, helping 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 Saiiropsida.

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
well 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, e.g.,
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

Pituitary fenestra

Asc. proc. ^
of pal. quad.

Columdla.

Paraquad.

Squamosal

Aud. caps

Endolymph
for.

Fig. 80.— Skull of an Embryo Lacerta agilis, 47 mm. in Length. A,
Dorsal, and B, Ventral View. C, Lower Jaw and Hyobranchial
Skeleton, from the Ventral Side.

(After Gaupp, from a wax model by Ziegler x J.) Cartilage, blue ; replacing bones,
gray ; investing bones, yeUoic.

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 ANATOMY

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 hasipterygold jjrocess on either side for articulation with
the pterygoid bone. An alisphenoid ossification may be present ;
presphenoids and orbitosphenoids are usually wanting. The

Premax

Sfptomay

Vomer

Frontal

Basiptg. proi

Inf. alar. cart.

EctocJioanal cart.

Paraseptal cart.

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
hasitemporal ossifications which unite with the latter bone, and
its anterior part may form a lasisvlienoidal 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
synoticmn, 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

Dentary «i.— .^HH

r

y

^

..^.m

r

■r- iUckeVi tort

CoYonnrif —.^^^^^^^^^m

\

^^'Entoglostal

Sv.pra-angvMr ^BSMSw

\

--'"■■7^^

1

itsplen'."!^^^gW ^^^^^ i

0

%^

\^ w^ j^^ M

\S

^^

Hy. corau^M AF ^

V

Si n

Branch cornu im — -m 'J i

\

l\ 1

^^ 1 cornv.

i

K^

Branch ^^
cornu 11 ?

Fig.

80,

V.

formed of three parts, derived from the basioccipital and exoccipitals
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.i

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. Prefrontals
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 anUpterygoid) 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
movably 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
Jiard 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

^ The form of the columella varies considerably : it consists primarily of a
bony rod, to which a cartilaginous exirastapedial 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 postfroiital 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 lovjer 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 quadra to-jugal (quadrato-maxillary).
In Hatteria the latter is present united with the well-developed
paraquadrate, w^hile 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, ptervgoid, transpalatine, dentar}', 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 squamoscil aud 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) ><t(-jjokrotaplik type (temporal region covered) — Gym-
nophiona, Stegocephali, the most primitive Reptilia, and Marine Chelonia ; (2)
zygoh'otaphk type (with one or two zygomatic arches, derivable from (1) : — a,
with lower zygoma only — Anura and Aves ; h, with upijer zygoma onlj- — many
Tritonidag, most Lacertilia, Chelonia, and Mammalia ; e, with upper and lower
zygomatic arches — Tylototriton, Rhynchocepliali, many fossil Reptilia, Croco-
dilia ; (3) yyinnoh-otrqAk 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

Pttdc

Fig. 81. — Skull of Hatteria. A, dorsal ; B, ventral ; and C, lateral view. x|.
(From Gadow, Cambridge Natural History.)

coif 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 ; P/^/, prefrontal ; Pt.f, post-
frontal and postorbital ; Ptg, pterygoid or endopterygoid ; Q, quadrate
and quadrato- jugal ; Sq, 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 p.mnc:

^acinar j^^^^^ ejctnar

aupr.oc

J'or.mag

oc.cond

oc.cond bas.or

-^ -op Lot

7^ basoc \^
basptj^

ptff COL

■le^rtL

anff

Fig. 82. — Skull of Lactrta agUis. (From Parker and Haswell's Zoology,
after W. K. Parker. )

A, from above ; B, from below ; C, from the side, ang, angular ; art, articular
bas.oc, basi-occipital ; bas.ptg, basipterygoid processes; h*xs.S2>h, basi
sphenoid ; col, epipterygoid ; cor, coronary ; dent, dentary ; tth, ethmoid
ex.oc, exoccipital ; ext.nar, external nares ; for.mag, foramen magnum
fr, frontal ; int.nar, internal nares ; ju, jugal ; Icr, lacrymal; max, maxilla
lias, nasal ; oc.cond, occipital condyle ; olf, olfactory capsule ; opi.ot, opis
thotic ; opt.n, optic nerve ; pal, palatine ; par, parietal ; para, para
sphenoid ; p>a^-f'> parietal foramen ; p.mx, premaxill^e ; pr.fr, prefrontal
ptg. pterj'goid ; ptt.orh, postorbital,; qu, quadrate ; s.ang, supra-angular
a.orh, supraorbitals; s^, parat^uadrate ("squamosal"); supra. or, supra
occipital; supra. t.^, supratemporal ; bW^ra.^-, squamosal ("supratemporal-")
trans, transpalatine ; vom, vomer.

and vomer in young stages. The two dentaries are united by
ligament. The cohimella 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 Amphisbsenidse, 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.^ 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
generalh^ present, and epipterygoids occur except, e.g. in Amphis-
baenians and Chameleons. The pterygoid is movably articulated
to the quadrate and basipterygoid process of the basisphenoid,
and there is a basisphenoidal rostrum. The premaxillse 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 forw^ards 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). — Tlio embryonic chondrocranium is much
less extensive than in Lizards. In the orbito-temporal regiou there
is hardly any cartilage, tlie 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 trabeculse remain
separate for the greater part of their length, forming narrow^ rods
lying side by side, w^hicli in many cases can be recognised even in
the adult. The skull, liow^ever, 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 wdth 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 wdiich extends backw^ards, thus resulting in a
very w4de gape ; moreover, the facial bones are capable of move-
ment upon one another and upon the cranium. The pterygoid
articulates posteriorly wath the quadrate without the intervention
of a jugal. The premaxilla is unpaired and usually edentulous
(except, e.g. in Pythons). The rami of the mandible are connected

^ In Chameleons, characteristic processes arise from the cranial roof and
temporal regions, and nnite above the posterior part of the sknll 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 Typhlopidse, on the other hand, amongst

JVnjf

Fov

88. — Skull of Colubrise Snake {Tropklonotiis natrix), dorsal and
vential views.

J[/, angular; Art, articular; Bp, basioccipital ; Bs, basisphenoid ; CA, internal
nostrils ; Core, occipital condyle ; Dt, dentary ; Efh, ethmoid ; F, frontal ;
F^, postorbital ; For, fenestra ovalis ; //, optic foramen ; 31, maxilla ; JV,
nasal ; 01, exoccipital ; Osp, supraoccipital ; P, parietal ; Pe, periotic ; Pf,
prefrontal ; P/, palatine ; Pmx, premaxilla ; Pt, pterygoid ; Qu, quadrate ;
SA, supra-angular ; Squ, squamosal ; Ts, transpalatine ; Vo, 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 Lizards and Crocodiles, and much of the
nasoethmoidal cartilage pei-sists. 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. Chelone, 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 bone 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 vomer help in forming a short hard-palate.
The opisthotic remains distinct from the exoccipital, and the pre-

^_

Cocc

Fig. 84. — Skull of Young Water-Tortoise [Emys europ(ea). Side view.

BP, cartilaginous interval between basioccipital and basisphenoid ; F^, post-
frontal ; UK, horny sheaths of jaws ; /, point of entrance of the olfactory
nerve into the nasal capsule ; Ituj, jugal ; JUd, mandible ; Mt, tympanic mem-
brane ; Na, external nostril ; Osp, supraoccipital, which gives rise to a crest ;
Pf, prefrontal, which forms a great part of the anterior boundary of the
orbit ; Qjg, paraquadrate (" quadrato-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-
psidoe).

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 trabeculse, 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,

MK-~

Fig. 85. — Hyobranchial Apparatus,
WITH Larynx and Trachea, of
Emys euroxKea. From the dorsal
side.

>4A', arytenoid cartilage of larynx ;
Chr.I, Cbr.II, first and second
branchial cornua; Chy, hj^oid cornu;
P.I, lingual process ; BK, cricoid
cartilage of larynx ; Tr, trachea ;
ZB, broader anterior, and ZK, nar-
rower posterior part of basihyo-
brauchial.

C&ee

Fig. 86. — Skull of a Young
Crocodile. Ventral view.

Ch, internal nostrils ; Cocr, oc-
cipital condyles ; Jg, jugal ;
M, palatine process of max-
illa ; Ob, basioccipital ; Orb,
orbit ; PI, palatine ; Pmx,
preraaxilla ; Pt, pterygoid j
Qj, paraquadrate (*' quadrato-
jugal"); Qii, 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 maxilloe, take
part in the formation of the hard palate.^ 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 premaxillse, maxillje, and dentaries.

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

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

120

COMPARATIYE 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

FrontiX

Squamosal

Inter orb.

septum

Pre/ont {lac.)

M

fS^ .rfs^ ^^o-sal

^^'

IvA^H^^v

Vomer

Pariei

Aud. caps

Fig. 87. — Skull of ax Embryo Chick 65 mm. in Length. From the right

side. (From a model by W. Tonkoff,
yelloio.

[). Cartilage, hhie ; investing bones,

large, and correlative modifications occur, especially in the occipital
and auditory 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

9.0 P

eik T?;V

Fig. 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, basioccipitul ; 6.^^, basipterygoid ; b.s, basisphenoid ; b.t, basitemporal ;
d, dentary ; e.n, external nostril; e.o exoecipital ; eth, ethmoid; e.n,
Eustachian aperture ; f.m, foramen magnum ; fr, frontal ; i.c, foramen for
internal carotid artery ; j, jugal ; Ic, lacrymal ; mx, maxilla ; mx.p,
maxillopalatine process ; 7i, nasal ; n.px, nasal process of the premaxilla ; p.
parietal ; pg, pterygoid ; pi, palatine ; p.n, internal nostrils ; ps, presphenoid ;
pa-, premaxilla ; q, quadrate; q.j, quadrato-jugal ; s.o, supraoccipital ; sq,
squamosal ; ty, tympanic cavity ; v, vomer ; //, foramen for optic nerve ;
V, for trigeminal ; IX, X, for glossopharyngeal and vagus ; XII, for hji^-
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 extracranial ly 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.^ 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 hasitemporal, 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 fenestras
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 PaiTots, 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.^ 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

posterior nostrils are always situated between the vomers and
palatines. The two premaxillae, 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.
The median body consists of an
entoglossal (basihyoid) passing
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.

paraglossaZ

entoglossal
'(basihyoid)

urohyal
(basibranchial)

'abranchial arch

Fig. 89

Hyobranchial Skeleton of
Fowl. (After Gegenbaur ; lettering
after Kallius.)

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

Fig. 90a. — Longitudinal Vertical Sections through the Skulls of— A^
Salamaiidra maculosa. B, Testudo grceca, and C, Gorvu-s 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. 90b.— Longitudinal Vertical Sections through the Skulls of A, Deer,
B, Baboon, and C, Man, to show the Relations between the Cranial
AND Xasal Portions.

considerably fenestrated. The occipital region includes the
equivalents of four vertebrae.

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

Premax.

Nasal caps.

Nasal
... Max.

Spheneth. cart

Septum nasi,
Orbitonus. fissure
Ala orbitalis,
Ala temp, {alisph.)

Malleus
Incus.

Jug. for.

-- Frontal

Sup. orb. fissure
Carotid for.

- Parietal

•^ Int. aud. meatus
■ Jug. for.

\ Endolympk.for.
Hupoglossal for

Tect. synot.

Fig. 91, A. — Skull of AxV Embryo Mole (42-3 mm. in Length from Nose to
Base of 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^ (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-

^ Certahi facts in the development of the skull in Apes indicate that the
Primates diverged very earlyJiom the common mannnalian 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
(ofteQ with parocci-pital or paramastoid processes) are always present,
the paired condyle ^ being furnished by the exoccipitals (Fig. 92).

Xarial fenestra

Basal fenestra

Paraseptal cart.
Nasal cap».

Vomer

SpJieneth. cart.

Orbitona^. fissure
Meckel's cart.
^ Ala temp, (alisph.)
JJaileus
Carotid for.

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

^ 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 diffei*s from that of Amphibians.
But in the case of the Sauropsida there are 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 partly with the
centrum proper of the atlas and partly with the odoutoid 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

COMPARATIVE ANATOMY

S'pl^ €

For.m
C.occ

SKULL 129

Fig. 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
perpendicularis of the ethmoid; Eth', cribriform plate ; F, frontal; For.m,
foramen magnum ; Jg, jugal ; Jm, premaxilla ; L, lacrymal, surrounding
the lacrymal 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^, basisphenoid ; Sph'\
presphenoid ; Sq, squamosal; Sq.occ, supraoccipital ; T, tympanic; Vo,

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.^

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
crihrosa), 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,^ 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
or petromastoid 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 : opticiun (II), foramen
lacerum anterius or sphenoidal fissure (III, IV, V^, VI), rotundum (V^), ovale
(V^), meatus auditorius internus (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 premaxillse.

K

130 COMPARATIVE ANATOMY

(opisthotic) portion reaches the surface of the skull between the
exoccipital and the tympanic 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 hulla tymjpani, 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 dififerent 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 Cavicornia (BovinjB, Antelopina?, Caprinse, Ovinse)
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 Gervidce 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, D, E). (After M. Weber.)

(7o>', derm ; Ep, epiderm ; HS, horny sheath ; HZ, bony process of the frontal,
with the epiphysis-like " os cornu " [OC) 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+OC) ; R, zone of resorption, at which point the antler
is shed ; SZ, process of the frontal still covered with the integument ; SZ^,
the same after loss of the integument.

lacrymals, each of the latter perforated by a lacrymal foramen ; in
this region also are the lateral plates of the ethmoid (lamince
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} 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 Amniota 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, Avhich 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, maxillae, and palatines ; but
the small "pterygoids"^ (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. Balaena) ; 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. 72, 80, 82, and 83).
This prenasal process is present only in the Monotremes amongst Mammals, and
with its fellow forms the transitory os caruncuke. 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).

^ True pterygoid bones, corresponding to those of the Sauropsida, are
apparently only known to occur in Monotremes. The so-called pterygoid (or
internal l&,mina 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 thp
tympanic bone externally to the periotic. They thus represent

Fig. 94. — A, Skull of Delphinus. (From M. Weber, after Boas.) B, Skull
OF FoiTAL Balania japonica. (From M. Weber, after Eschricht.)

C, occipital condjde ; Fr, frontal ; Ju, jugal ; L, lacrymal : Mx, maxilla ; w,
external nostril ; Na, nasal ; oe, exoccipital ; Os, supraoccipital ; Pa, parietal ;
Pal, palatine ; Pt, pterygoid ; Px, premaxilla ; Sq, squamosal ; Ty, tym-
panic and bulla tympani.

the articular and quadrate, and are known as the malleus and inmis
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.^ 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

Fig. 95. — Skull of Embryo of Armadillo ( 7\x^?(8Ja %6r«c?a). (Modified from
a drawing by W. K. Parker. )

a.tyy tympanic annulus ; au, auditory capsule ; h.hy, basihyal ; c.liy, ceratohyal ;
cr, cricoid ; cZ, dentary ; e.hy^ epihyal ; e.??, external nostril ; eo, exoccipital :
y, frontal ; h.hy^ hypohyal ; i, jugal ; in, incus; Ic, lacrymal ; mk, Meckel's
cartilage; ml, malleus; mx, maxilla; ??, nasal; oc.c, occipital condyle;
-p, parietal ; joa, palatine ; 'px, premaxilla ; ho, supraoccipital ; st, stapes ;
s.t, ethmoturbinal ; ^t.m, stapedius muscle; sq, squamosal; ih, thyroid;
tr, trachea ; //, optic foramen ; F^ F^, 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.^ 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 palaeontological 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 appendicular (rrgans, serve mainly for
locomotion, and may be divided into two' groups, the unpaired
and the piaired (pectoral and pelvic). They arise independently of
the axial skeleton.^

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 fins occur amongst Fishes 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 dm^sal, 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 ; BrF, pectoral fin ; D, dorsal fin-fold;
RF, FF, dorsal fins ; SF, tail-fin ; >S', S, lateral folds, which unite together
at S^ 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 cartilagino^ijS radii, or pteryijio'phores, usually
segmented (typically into three portions), are formed ; these may
unite proximally to form one or more hasipterygia, 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 hy dermal
rays, which may consist of numerous delicate horny fibres (Elas-
mobranchs), or of hony 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.^

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 phar^mgeal 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, gi-adually 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 metapleural folds presenfin the adult Amphioxus^ though it is

^ The dermal fin rays or dermotrichia are classified bj' Goodrich as follows : —
1. Horny cerafotrichia in Elasmobranchs ; 2. Bony leptotrichia in Teleostomes ;
3. Horny actinotvkhia occurring in the embrj^o and in tlie margins of the fins of
adult Teleostomes, in addition to (2) ; 4. Fibrous, caXci^ed^^ov \\ovny camptotrichia
in Dipnoans ; it is doubtful whether the last-mentioned correspond to (1) or to (2).

138

COMPARATIVE ANATOMY

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 and j^elvic fins, which

must therefore be looked upon as
the localised remains of a con-
tinuous lateral fin-fold on either
side of the body.^

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
centrifu gaily 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 (hasipterygium)
and that which extends into the
lateral body-wall and serves as a
support for the limb proper : this
constitutes the limh-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 clue to Thacher, Mivart, Balfour, Haswell, and Dohrn, and a
somewhat similar idea was put forward by Goodsir as early as 1856. The
Palaeozoic Cladoselache is very suggestive in this respect.

2 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 ontogenetically. 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 hasipterygium : in other
words, the radii arise secondarily. Moreover, it is held by some embryologists
that ontogenetically 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 fins. 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-
mus), 9 MM. Long, showing
the Mode of Origin of the
Pectoral Limb-Buds.

ap, limb-buds ;
ca'lome : m

ch, notochord ; co,
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

Fig. 98.— a, B, C. Diagram of Three Successive Stages in the Develop-
ment OF the Pelvic Fin of a Shark.

cZ, cloacal aperture ; /o, obturator foramen ; rcZ, primitive radii, which in A are
beginning to fuse into a basal plate (66). 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 fias 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,^ and in embryos of Teleostomes it
has at first a similar structure. Later, however, in the last-named

Fig. 99.— Pectoral Arch and Fin of Heptanchiis.

a, b, the main tin-ray, lying in the axis of the metapterygium (Mt) ; +, single ray on
the other side of the axis (indication of a biserial type) ; FS, horny rays,
ciit through ; Pr, Ms, Mt, the three basal elements of the fin (pro-, meso-,
and metapterygium) ; Ra, fin-rays ; SB, SB^, 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

^ In Heptanchus there is a small ventral element which has been compared
to a "sternum."

PECTORAL ARCH

lil

Cm

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., Raiidae), cor-
responds to the scapula, and
the latter to the coracoid plus
procoracoid of the higher
Vertebrata.

In Tel costs 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).

cbrctj

J?//

Fig. 100.— Left Pectorai. Arch and Fin
OF THE Trout. (From the outer side.)

D, D^, B^, 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^, metapterygium ;
Ba, Ra, the second and third, and 4,
the fourth basal element of the fin ;
Ra}, the second cartilaginous row of
radii ; S and Co{Cl), bony scapula and
coracoid, which have become developed
in the cartilage Kn.

tinuous

plates — an anterior {procoi^acoid) and a posterior (coi^acoid) (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.

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-
with two ventral

142

COMPARATIVE ANATOMY

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
Stegocephali 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. lOl.-DiAGRAM of the ^^^^il Reptile Pareiasaurus
Ground-Type of Pec- Reptiles. — As m Amphibians, the most

TORAL Arch MET WITH IN essential parts of the pectoral arch of
Reptiles are the scapula and coracoid,
arising in connection with a continuous
cartilaginous bar or plate, as is well seen in
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

ALL VeRTEBRATA FROM

Amphibia to Mammalia.

CI, procoracoid ; Co, cor-
acoid ; H, humerus ; S,
scapula.

Fig. 102.-

-Pectoral Arch of the Right Side of Salamandra maculosa,
considerably magnified, and flattened out.

a, h, bony processes extending into the procoracoid and coracoid respectively ;
CI, 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 proscapidcc. 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, Amphisbaenians,
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
H}o 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 fenestrse closed over by fibrous membrane. A main fenestra
(c£ Fig. 56, a, dorsal to which a bony process, the vestigial pro-
coracoid, can be seen) may be 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 Ratitse 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 Carinatse and in Archeopteryx to form
an acrocoracoid process.

In Struthio the broad coracoid is fenestrated, and its anterior
part may be looked upon as a procoracoid : in other Ratitae the
latter is considerably reduced, and may be represented merely by
a ligament ; in Carinatae it can often no longer be recognised.

In almost all Flying Birds the clavicle, a purely dermal bone,
is Avell 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 ventrall}^ to reach the sternum (Fig. 103) ; in all
other members of this Class it characteristically becomes reduced,^
and simply forms a prominent process on the scapula (coracoid

^ 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 ^NATOMY

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 muscles 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

Fig. 103.— Pectoral Arch and Sternum of OniUhorhynchu-s paradoxus.

c^, c^, c^, first, second, and third ribs; d, clavicle; e.c, epicoracoid; es^ and es'^,
prosternum (episternum) ; m.c, coracoid (metacoracoid) ; in.s, manubrium
sterni ; 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.^ 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.

^ 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 p/«^es,
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

t +

Fig. 104. — Simple Forms of Pelvis amongst Fishes and Amphibians.

A, Pienracavthus — the pelvis is here not diiOFerentiated from the proximal end
(tr) of the basipter^'gium ; B, Scaphirhynchus cafaphrarfns ; C, Pofypfenis
hichir ;. D, Necfnrus {Menohranchus). Ap, apophysis of the basipterj-gium ;
Bas^, basipterygium ; Fo, obturator foramen ; P, pelvis ; Bad, 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 tran verse bar extend-
ing between the two basipterygia, from which it has become

L

146

COMPARATIVE ANATOMY

Rad.--

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-
ing upwards into the
lateral walls of the
body (Fig. 105). A
prepubic process is also
present, and there is
v^ apparently also an in-
dication of a median
epipubic process (cf.
infra). The whole
pelvic plate essentially
corresponds, more or
Fig. 105.— Diagram of the Elasmobranch Pelvis, less completely, with
From the ventral side. the ischiopuhis of

Bas, Pro, Bad, basipterygium, propterygiuni, and higher forms.

radii of the fin ; BP, pelvic plate (ischiopuhis) ; Jjj ^he Dionoi the

Cep, epipubic process ; Fo^, obturator foramen ; -i •'

/, iliac process; PP, prepubic process; Sy, narrow cartilaginous
region of the ischiopubic symphysis. 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
in an intermuscular septum ;
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 (g. v.). The
posterior orhypoischiatic process
bears a ridge for the attachment
of muscles.

Amphibians. — It will be
seen by a glance at Fig. 104, D,
that the ventral portion of the
pelvic arch of Necturus is formed
on the same plan as the pelvic
plate of the Dipnoi and Crosso-
pterygii, but in all Urodela and
Amniota it is perforated by the
obturator nerve. Like the pelvis
of all Vertebrates, it has a paired origin, and in Proteus and

Fig. 106.

N^e

Pelvis of Protopteruis. From
the ventral side.

a, prepubic process, which may become
forked at its distal end ; b, process to
vv^hich the pelvic fin {HE) is attached ;
c, epipubic process ; Gr, ridge for
attachment of muscles ; M, myotomes ;
M^, intermuscular septa.

PELVIC ARCH

147

n 1rf(Ce^)

SJf(Sy)
PP

Ac, acetabulum ; Or {Sy), muscular ridge
on the ventral side of the ischiopubis ;
Fo, Fo^, obturator foramen ; /, J^,
ilium ; JP, JP^, ventral pelvic plate
(ischiopubis) ; LaW, linea alba ; Mi/,
intermuscular septa ; PP, prepubis ; Sy,
symphysis, in which region a strong
tendinous area (SH) exists in Amphiuma,
the pubic regions only coming together in
the middle line at * ; ** (in A), ossitied
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 Derotremata and Necturus ; ft
(Cep), Ep, epipubis.

107.— Pelvis of (A Proteus; (B) Amphiuma; (C) Cryptohranchus; and
(D) ScUamandra maculosa. 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, ischium, and pubis), such

Fig. 108.— Pelvis of Anura. A, Xeiiopm, from below; B, the same from
the front ; C, Bana escufenta, from the right side.

Ac, acetabulum ; Cep, epipubic cartilage ; /, ilium ; P (in Xenopus), the
proximal end of the ilium, which is sepaiated from its fellow and from
the pubis by a + -shaped zone of cartilage, f, * ; /.s, ischium ; P, pubis {P^ in
liana, 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
gi'eater 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. Palaeohatteria, Plesiosauria), and also

Fig. 109. — Pelvic Arch of Hatteria. (Aft«r Credner.) From the
ventral side.

Cej}, epipubic cartilage ; Fo^, obturator foramen ; /, ilium ; Is, ischium ; P,
pubis ; pp, 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 fibro-cartilaginous ligament,
continuous anteriorly with the plug-like epipubic cartilage and
posteriorly with the hypoischium or os cloacce (absent in Chame-
leons). This tract represents the remnant of the median ends of

FiCx. 110.— Pelvic Arch of Various Chelonians. From the ventral side.
A, Macrochelys (after G. Baur) ; B, SpharuU coriacea (after Hoffmann) ;
C, Te,studo ; D, Chdone.

Cep, epipubic cartilage ; Fopi, ischiopubic foramen ; i/p/.s, 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 Lacerta vivipara. From the ventral side.

Ac, acetabulum, in which the three pelvic bones come together ; Cep, epipubis,
composed of calcified cartilage ; Fo^, obturator foramen ; Hpis, hypoischium,
which becomes segmented oflF from the hinder ends of the ischia in the
embryo as a paired structure ; I, ilium, with its small preacetabular process
+t, much more strongl}' developed in Crocodiles, Dinosaurians and Birds ;
Is, Ischium, forming a symphysis at Sis ; 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 ^Jrtrs
acetahularis, 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 vertebra^, 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 lucia>i.
lateral view.

A, ventral, and B,

B,

fibrous band between the pubis and s^miphysis ischii ; h, foramen in the
acetabulum, bounded posteriorly by the two processes, a and h, of the ilium
and ischium respectively ; F, ischiopubic foramen ; G, acetabulum ; //, ilium ;
/.s", ischium ; J/, fibrous membrane extending between the anterior margin of
the pubis and the last pair of "abdominal ribs" (^A') ; 7^, pubis ; Sy,
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 sacral 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 Strutbio, and the ischium

Fi(

IS.— Felvih OF A ptert/x au.^trali's. Lateral view, (After Marsh.)

a, acetabulum ; if, ilium ; if, ischium ; ]), 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 Archaeopteryx, all the elements of the pelvis were
independent and relatively small, the ilium coming into relation

Fio. 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.

Fi(i. 115.— Dl\gram showing the
Relations of the Pars Ace-
tabularis in Viverra civetfa.

A, acetabular bone ; Ac, acetabulum ;
./, ilium ; Js, ischium ; P, pubis.

with about six vertebrae only, and the pubis and ischium bein^
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 preace tabular 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 acetahnlar hone is especially well

TuUl/i

Fig. 116. — Pelvis of A, Echidna hystrix (Adult), and B, Didelphys azarcn
(FcETUS, 5*5 CM. IN Length). From the ventral side.

Ep, epipubis (marsupial bone) ; t'oht, obturator foramen ; /, ilium ; Ja,
ischium ; Lg and Lyt, ligament between the pubis and epipubis ; P, pubis ;
Sy, ischiopubic symphysis; Tiib.iLp, 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, ft, ilio-
and ischiopubic sutures.

In Fig B, b, b^, 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
Insectivores, in some of which {e.g. Male, 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.

Elasmohranchs. — 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 hasiiderygium or meta'pterygmm, and the
smaller, inconstant prc>29^e?\y/7iit??i 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. ^ This is also true as regards the pectoral fin, in
which an additional basal piece, or mesapterygium, 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

Rad.

t*.'

Bad4-

FiG. 117.— Right Pelvic Fin of
Heptanchus. From the ventral
side.

BP, pelvic plate ; Fo^, f, nerve-for-
aniiua ; Pr, propterygium ; Rad,
radii, which show secondary seg-
mentation ; S Bad, basi- or meta-
pterygium.

Fig. 118. — Pectoral Fin of Cera-
todus fonleri.

a, h, 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 {uniserial 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

' In male Elasmobranchii a number of pieces of cartilage are connected with
the distal and of the basipterygiiim of the pelvic fin as a support for the
copulatory organs or claspers {q.v.) : 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 dififer from those of most Elasmo-
branchs (as well as of Teleostomes) in being formed on a hiserial
type, indications of which are, however, as already stated,

Trii

Fig. 119.— Right Pelvic Fin of a Young Polyodon folium.
From the dorsal side.

FS, boii}'^ dermal rays ; M, metapter^'gium ; Pru, uncinate ("iliac
i?a, Ra^, radii of the first and second orders.

processes

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-

FiG. 120. — Left Pectoral Fin of A, Polyodon, and B, Amia.
a — g, radii which do not reach the arch and are connected with the most
posterior ray {IV. in A, ///. in B) ; / — 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 Elasmobranch with its propterygium, meso-
pterygium, and metapterygium.^

^ 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
case phylogenetically.

LI^BS

159

F^

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 -h

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).

Tdeosts. — 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
(Fig. 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, „ ^ ^

n.T^^^r.'.Ac. A n ^.^A Fig. 121. — Pectoral Fin of

Oypnnoids, and Gymnotidge comes Polypterus.

nearest to that of Ganoids. „^ ,

FS, bony dermal rays ; Nl, nerve
foramina ; Oss, 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 ;
Ha, Ra}, 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 (cheiro-
pterygium), adapted for progression upon land, has been derived
from the fin (^ichthyopterygium), only fitted for use in the w^ater,
and Palaeontology 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, bec/ime 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
extremities, the anterior limbs under-
going the most varied adaptative
modifications, and giving rise to
prehensile or to flying organs — or,
as m 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 {hrachium), fore-arm (antibrachmm), wrist [carpus), and hand
(mamcs) ; and in the hind-limb as thigh (femur), shank (crus),
ankle (tarsus), and foot (pes) (Fig. 123). The bone of the upper
arm (humerus), like that of the thigh (fermir) is always unpaired,
but two bones are present in the fore-arm and shank. The former
are called radius and idna, 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. 128).

Fig. 122. — Diagrammatic Figures
to show the relations of the
Anterior Extremity to the
Trunk in Fishes (A), and the
Higher Vertebrates (B).

Mt, metapterygium ; Bd, radialia
in A, radius in B ; *S', pectoral
arch : Ul, ulna ; proximally to Ul
and Bd is the humerus.

LIMBS

161

Round a centralc, 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 tibialr, ulnare or fihulare, and intermedmm ;
\vhile 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-
sponds to a tibiale, and
the other (calcaneum) to
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 IV nud V are generally represented
by a ligament.!

In Anura the metatarsals and phalanges, between which the
web of the foot is stretched, are, like the pi'oximal tarsals, very
long and slender. The femur, as well as the fused bones of the

Y/

Fig. 123.

Hind Limb of a Urodele {Spelerpes

Dg,

digits ; Fe, femur ; Fi, fibula ; Mt, meta-
tarsals (/ — V) : Ph, phalanges ; T, tibia ;
Ta, tarsus, consisting of — c, centrale ; /,
fibulare ; i, intermedium ; t, tibiale ; and
1 — 5, distal tarsalia.

^ Ver}' 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.

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 (" prehalhtx'') occur on the tibial
side of the tarsus, and in both Urodeles and Anurans indications

Groove be-
tween rwlivs
and ulna

Ulnare.

Tibiale

Tnrsale II and III

Tarsah I

CentraJe

Prehallux

Carpal
ILL -

Fig. 124. — A, Right Fore-arm and Hand, and B, Right Foot, of
Rana esciilenta. From the dorsal side, x 2. After E. Gaupp.

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 flying 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.^ 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

Fig. 125. — Carpus of A, Hatteria punctata^ and B, Emydura hrefftii.
•« (After Baur.)

f\ radial centrale ; c^, ulnar centrale ; z, intermedium ; j9, ulnar sesamoid
(pisiform) ; i?, radius ; r, radiale ; U, ulna; ti, ulnare ; 1 — 5, carpalia ; / — F,
metacarpals.

ulnar side ("pisiform") can usually be recognised (Figs. 125 —
180). 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 Anura, 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

^ In Hatteria and certain Chelonians, as well as in the extinct Proterosaurus,
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

/p

1 fl 111 ^

Fio. 126. — Right Carpus of Emy^
europiva. From above.

i, intermedium ; R, radius ; r.r, fused
radiale and centrale ; U, ulna ; u,
ulnare ; t and *, elements on the
radial and ulnar side respectively,
indications of additional radial and
ulnar (pisiform^ rays ; 1-5, the
carpalia, of which 4 and 5 are
fused ; / — V, metacarpals.

Fig. 127. — Left Carpus of Lacerta
agilis. From above.

c, centrale ; r, intermedium ; R,
radius ; r, radiale, formed by the
fusion of two elements, one of
which corresponds to a prepollex ;
U, ulna ; u, ulnare ; f, pisiform ;
1 — 5, carpalia; / — F, the meta-
carpals.

form paddles : the radius and ulna were very short, and there were
numerous phalanges ^ (cf. Cetacea), additional rays being present
in the former genus.

Amongst the snake-like kinds of Lizards, various degrees of

Ph'

<V

(i}ftj:.

Fig. 128.— Kight Carpus of a Young
Alligator Incius. From above.

C, centrale ; i?, radius ; r, radiale ;
U, ulna ; ii, ulnare ; f, 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 ;
/ — V, metacarpals.

Fig. 129.— Right Tarsus of Emys
europwa. From above.

F, fibula ; {i\f.f.c, the fused inter-
medium!?), fibulare, tibiale, and
centrale ; Ph^, phalanx of 1st digit ;
T, tibia ; 1 — 4, distal tarsals ; / — V,
metatarsals.

reduction of the extremities occur, and in such forms as Anguis
and Amphisbsena they have practically disappeared entirely, as in

^ An indication of this condition is seen in the embrj'o 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 tai-sus 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

jsr ir

Fio. 130. — Right Tar-sus of Lactrta
muixUU. From above.

F, fibula; T, tibia; t.f.i.c, fused
tibiale, intermedium, fibulare, and
centrale ; f, 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 ;
r?, oth 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 (c^dcaneal 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 flight, 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 (Ratita?), however, the wing
has undergone regressive changes in connection with their habits,

Fig. 132. - Skeleton of thk Lcmbs and Tail of a Carinate Bird. (The
skeleton of the body is indicated by dotted lines. )

F, digits; Fi, fibula; HW, carpus; MF, tarsometatarsus ; MH, carpometa-
carpus ; OA, humerus ; OS, femur ; Py, pygostyle ; B, coracoid ; Rd, ulna ;
Sch, scapula ; St, sternum, with its keel {Cr) ; T, tibiotarsus ; UI, radius ;
z^, z, digits.

and in the extinct New Zealand Moa (Dinornis) 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 intermedio-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

Fig. 133. — Carpus of Embryo of Sterna ivUsoni. A, Stage at which
Ossification begins; B, Stage immediately before Hatching. (After
V. L. Leighton.)

(', distal carpals ; Klaue, claw ; Rod, radius ; rad, intermedio-radiale : Uln,
ulna ; idn, centro-ulnare ; // — F, metacarpals (in B, /Tand Fhave 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 carpometa carpus (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 Archseopteryx (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 {III), and even the third {IV) also {<'.(j.
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
primaril}^ of four distinct pieces. The former (tibiale and fibulare)
unite later with the distal end of the tibia, thus forming a tihiotarsus,
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 tarsometatarsics (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.^

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) malleolus.

The radius and ulna are connected with the humerus by a

hinge-joint at the elbow, onl}^ 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

^ For the pneumatic chti-racter of Birds' bones, of. under Aii-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 siqnnation, the latter that
of ji^onation. Indications of these movements are seen even in
climbing Marsupials.^ 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.-

A brief account of the mammalian carpus and tarsus must
suffice in this place, as considerable difierences 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
Urodeles, Hatteria and Chelonians. Primarily the centrale can be

^ The rotation of the radius on the uhia 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. Consequently, 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.

■^ In the Marmosets (Arctopithecini) the thumb is not oppo.sable, 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 onl}- a single small phalanx,
while in Colobus it may even be wanting. In consequence of the erect position
of Man, and of tlie foot being used merely as an organ of support and loco-
motion, tlie prehensile chaiacter of the pes has beconiQ 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 cuboid 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.^

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 of its development. While in
this case the third digit becomes greatly enlarged relatively

^ Different views have been expressed as to the morphological nature of the
prepollex and prehallux, which in 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 toUomologise the so-called "accessory elements,"

LIMBS

171

(verissodaetyle fm^m)} and eventually is the only complete one
remaining, in cloven-footed Ungulates the third and fourth digits

I

Fig. 134. — Fore-foot of Ancestral Forms of the Horse. 1. Orohippus
(Eocene). 2. Mesohippus (Upper Eocene). 3. Miohippds (Miocene).
4. Protohippus (Upper Pliocene). 5. Pliohippus (Uppermost Pliocene).
6. Equus.

are both functional and equally strongly developed (artiodactyle
fm^m. 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 Pro tungu lata must origin-
ally have been pentadactyle and
lylantigrade {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 digitigrade
(as in most Carnivora), and
eventually unguligrade, only the
hoofs at the extremity of the
distal phalanges bearing the
weight of the body.

The Tylopoda, as well as Ele-
phants (Subungulata) have not
reached the unguligrade stage :
they are practically digitigrade, a
large integumentary pad or sole
(cf Fig. 24), from which the small
" hoofs" project, bearing the main weight of the body (Fig. 136).

Some of the many other adaptive modifications of the limbs in

^ The Tapir has four digits on the fore-foot and three on the hind-foot ; the
Rhinoceros ha^s three on eeich foot,

n £ F

Fift. 135. — Skeleton of the Left
Fore-limb of A, Pig; B, Hyg-
MoscHus ; C, Traoulus ; D, Roe-
buck ; E, Sheep ; F, Camel.
(From Bell, after Garrod. )

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 serve 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-cajj or ^^a^^/Zrit,
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
Fio. 136. -Longitudinal Section thigh and shank, and SO is in no

THROUGH THE MaNUS OF THE LlAMA " i 1 •, , , , i

{Aui-henia). .(After M. Weber. ) ^ay comparable with the oleo-

cranon of the ulna, as was

1, metacarpal ; 2, 3, 4, phalanges ; 5, so- f^^^p^l^ «nTmn^pH It is a triiP
called " hoof "; 6, horny part of sole ; lOI^meriy feUppoSCG. It IS a true

7, cushion composed of elastic con- Sesamoid bone, such as occurs m
nective tissue (more strongly de- connection with many of the

:^„Thept,tge!'atr;tMcin': -div-idual joints of the digits,

which has arisen in the tendon
of the quadriceps femoris muscle in consequence of the friction
between this tendon and the condyle of the femur.

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 precureore 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).^ 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
higher Vertebrates : these repre-
ived from the meso- 1 sent the oldest and most primitive
dermic somites. part of the muscular system.

h. Muscles of the diaphragm.

c. Muscles of the eo:tremit>ies,

d. Fye-mioscles.

II. Visceral muscles, de- (Cranial muscles, with the exception
rived from the lateral \ of those included under a and d
plates of the mesoderm. [ above.

In its simplest form an origin, a helly, and an insertion, may be
distinguished in each muscle. The muscles of the trunk are as a

^ 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.

Parietal muscles de-

174

COMPARATIVE ANATOMY

MUSCULAR SYSTEM 175

Fig. 187.— 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 a 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 ; C, stage in which the scute a again takes on a
horizontal position, the skin (c) shows the greatest backward extension, and
the scute h is moved forwards along the distance e.

B, Semi-diagrammatic figure of a longitudinal section through the ventral and

lateral parts of the skin of Tropidonotus Tiatrix, and of the costo-cutaneous
muscles in connection with the rib. c, rib ; c.c.i, c.c.s, inferior and superior
costo-cutaneous muscle ; m.c.i, intrinsic musculature of the skin ; s.g, longi-
tudinal sections of ventral scutes ; s.l, transverse sections of lateral scutes ;
»', 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 {cc.s) costo-cutaneous muscles ;
R, free raised border of the ventral (v.y) and r of the lateral {s.l) 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 (biventral 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, ovfascice,
and may be continuous with tendcnis for connecting the muscles to
the skeleton, or with flattened membranous expansions {aponeuroses).
Wherever marked friction occurs, ossifications (sesamoids) 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
disappear, 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 musculattire on the other.^

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-
maxillse 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 {cutaneus pectoris^ c. abdominis) 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-

^ 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

177

incter
colli

Mar«upium

A, ventral view of male Or
nithorhynchu.s ; 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

Fig. 138.— A— C, The Integumentary Muscles of Monotremes. (After Riige.)

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 myoides] and head (mimetic muscles) : these
muscles are closely related genetically, and are all Supplied b}^ 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

M.orbitoiiuric.

M.helicis

M.orb.omli ; M.aunc.sup /
M.lcmtorlabii \ ^~

aunciiM-f

M.spMnctercoUiy
Platf/s7Jta

M.trag.
anxtitray.

M.mandi:
bulo-aurimL

Fig. 139.— Superficial Facial Muscles of Lepilemur muMelinns. The deep
layer is recognisable on the neck. (After Ruge.)

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 Trunh.

In Amphioxus the body muscles are made up of a series (60 or
more) of lateral muscular segments or myomeres separated by
> -shaped connective-tissue septa or myocommas, between which
the fibres run longitudinally. The myomeres have an alternatiflg

Me

m£

Fio. UO.— The Musculature of Larval Amblystoma (AjfOLOTL).

the side.

From

Cf, external ceratohyoid muscle ; Cph, cervical origin of the constrictor of the
pharynx ; Cu, cucuUaris ; D, dorsal, and F, ventral portion, of caudal
muscles ; Dg, digastric ; Ds, dorsalis scapula? ; LI, lateral line ; Lt, latissimus
dorsi ; Lr, levator arcuum branchialium ; ttf, levator branchiarum ; Ma,
masseter ; Mc, mAocommas between the myomeres of the dorsal portion of
the lateral muscles ; J/A', mylohyoid (posterior portion) ; O, 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 lajer of this muscle {()h) ; at Re the oblique fibres of the latter pass
into longitudinal fibres, indicating the beginning of the dififerentiation of a
rectus abdominis ; at Re^ the rectus-sj'stem is seen passing to the visceral
skeleton ; Ph, procoraco-huraeralis ; RM, dorsal portion of lateral muscles of
tlie 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).
view.

Ventral

Add, adductor arcuum branchialium ; C, constrictor arcuum branchialiiira ; Cl)b,
coracobrachialis brevis ; Ce, Gi, Gi^, external and internal ceratohjoid : the
former is inserted on to the hyoid {Hy) ; Glo, cloaca ; Gph, portion of the
constrictor of the pharynx, arising from the posterior branchial arch ; Dp,
depressores branchiarum ; Gh, geniohyoid ; La, linea alba ; Mh, Mh^,
anterior and posterior portions of the mylohj^oid, 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-humeralis ; iSpc, supracoracoideus ; Be, rectus
abdominis, passing into the visceral musculature (sternohyoid) at Re^, 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 " ^ (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 nerv^es, 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 bodA^-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 14fl) pHmary and
sceondary 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 oblique, a superficial rectus, a
transversalis, and a suhvertehraHs. 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 ex tern us,
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.

^ 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 wliich
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 ohliquns profundus (belonging to the system of the internal
intercostals) and the median, deep rectus ahdominis. A transversus
is present except in Snakes. A suhvertehralis extends from rib to
rib, but is wanting in the lumbar region. A quadratus lumhoruvi
(lumbar portion of the intercostalis) appears first in Reptiles, and
from it a psoas major and 2^soas 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, interspinales, semisjmmlcs,
midtifidi splenii, and levatores costarum, together with the scalcni,
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-, amd pubo-caudalis 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 pectoiulis major '^ more particularly — and
the corresponding backward extension of the breast-bone.

External and internal oblique muscles are both present in the

^ The relative size of the pectoralis major does not always correspond to
the power of flight. It is very compact in Carina tae, 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 appeal's 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
triayuiulai'is sterni (last trace of the transversus) appears for the
fii-st time on the inner surface of the sternal ends of the ribs.
The doi'sal 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 abdominis
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 hrst rib : in higher forms it becomes more or
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 pyramida /is 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 pyramidalis 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.^

The external and internal oblique muscles are represented in
the thoracic region in Mammals, as in the Sauropsida, in the form

^ A sphincter marsupii muscle is developed in connection with the marsupium
(Figs. 28 and 138).

184 COMPARATIVE ANATOMY

of external and internal intercostals.i The subvertebral muscle is
represented by a longus colli and recti 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).^

B. Muscles of the Diaphragm.

The formation of a diaphragm results from a gradual sub-
division of the ccelome (pleuroperitoneal cavity) into pleuro-
pericardial and abdominal portions, and the differentiation of the
serous membranes which line these {pleura, pericardium, p)eTitoneum)
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.^ 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 Monotremes. They do not form a single layer, but are indepen-
dent of one another, and are derived respectively from the external and internal
intercostals.

'^ It is doubtful how far the external sphincter of the anus, the muscles in
connection with the external generative organs, and the transvtrsm perinei jiro-
fitnduH are derivable from the original sphincter cloacte of the Amphibia and
Sauropsida. In Mammals the pubo-coccygeiis (or the pubic porti(m of the levator
ani), as well as the sphincter ani externus and hulbo- and ischio-cavernosi, are
considered to represent separate portions of the integumentary muscle which
primarily extended over the greater part of the trunk.

^ Amongst the Amphibia (Rana) fibres from the trans versus 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. Ifc 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. lb 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.ff. Echidna, Phocsena) 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.^ 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, ic, 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

^ 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 ditferentiated 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, ahductors, 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 cucidlaris, the sternocleidofnadoidcus (belonging
morphologically to the cucullaris, and, like it, supplied by the
spinal accessory nerve), the rhoniboidei, and the levator anguli
scapidce : 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 levator anguli scapulae, rhomhoideus, and
serraius 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 supinators 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 187

Visceral Muscles.

Fishes. — The visceral muscles of Fishes ^ have been most
satisfactorily investigated in Elasmobranchs, and are classified by
Fiirbringer as follows : —

A. Cranial or cerebral tiiusclcs (consisting originally of trans-
verse or circular fibres) supplied by the V'^^ VII"', IX^*",
and X*'^ cerebral nerves.

1 . Constrictor arcuum visceralium,mc/. constrictor superficialis

dorsalis et ventralis.

Innervation.
Levator labii superioris \

,, maxilla3 ,, J- V.

,, ])alpebrie nictitaiitis- J
,, rostri ^

,, hyoiiiandibularis | VII

Depressor rostri |

,, mandibularis et hyomaudibularis j

Interbranchiales • IX, X.

Trapezius X.

2. Arcuales dorsales IX, X.

3. Adductores, ind. adductor mandibular ... V.

and abductores arcuum branchialium . . IX, X.
D. Sphml ,««scfes,originally jongitud- 1 g i„„.o^ci jt^, 3 and

inal, and divided, like the other > ^ ^^ ^

, \ 1 •' ( spinal nerves.

trunk-muscles, into myomeres. J ^

(a) Epibranchial spinal muscles, dorsal to visceral skeleton.

4. Subspinalis Spino-occipital nerves.

( Spino - occipital nerves,

5. Interbasales < as well as the first

( spinal nerve.

(b) Hypobranchial spinal muscles, ventral to visceral skeleton.

r. r^ 1 • 7 ^ Spinal nerves, and partly

o. Ooraco - arcuales. mcl. coraco- I "^.i i , ' r j

1 1 . 1 1 -J ; the last one or more

branchiales, coraco- hyoide us, < r. , i • • -i. i

1 ' JM 1 • ' J of the spmo- occipital

and coraco-mandibularis . . / ^ ^

\^ nerves.

In the Ganoidei, 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-viseeral
musculature in correspondence with their peculiar cranial skeleton (suctorial
apparatus) and branchial basket. It is covered over secondarily by the trunk
muscles.

- This muscle has nothing to do with the other eye-muscles.

^ These are spinal nerves emerging from the occipital region of the skull (cf.
under Nervous Svstem),

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.^

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 {levatores arcuum), a middle (muscles
of the external gills and the external ccratohyoid), and a ventral
{internal ceratokyoid, 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 mylohyoid 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-, sterna-, and
genio-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 hyoglossus 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 .

^ 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
bive7iter maTidibulce, 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.^

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."^

^ For the tensor fympani and stapedius muscles, cf. under Auditory Organ.
The latter muscle, together with the styloliyoid, 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 m^dohyoid, 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 are present in some Fishes, being most
strongly developed in certain Rays (Torpedinidae, e.g. Torpedo,
Hypnos) found in the Atlantic Ocean and various southern seas,
in a South American Eel (Gymnotus electricns) and in an African
Siluroid {Malopteritrns electricns). 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
into dorsal and ventral portions.

The electric power of those
Fishes which were formerly known
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

Fig. 142. — Torpedo marmorata,
WITH THE Electric Organs (E)

EXPOSED.

Au, eye ; KK, gill clefts ; ♦S', skull ;
Sp, spiracle.

ELECTRIC ORGANS

191

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. Adroscopus).

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 rise 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 Gyinuotus elertHcus. (B, from a preparation by
axis of the body (Gym-

A. Ecker.)

notus, Malopterurus) or A, anus ; DM, DAP, dorsal portions of the great
in 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

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^ ; 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 ; VM,
VAP, 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
WS^, in transverse section.

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

Fig. 144.— Elec- ^ x r xt.^ ^ • r^ ^ i

TRIG Prisms of arrangement or the parts in Gymnotus and

Torpedo mar- Malopterurus renders it clear that the electric

morata. (Semi- ghock must pass in different directions in these

Fishes : in Malopterurus it passes from the head

to the tail, and in Gymnotus in the contrary direction, while in

Torpedo the discharge passes from below upwards.

Experiments have shown fliat 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.

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 seTisory 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 ao:is-jibre, and in those nerve -
fibres which are spoken of as medulla ted 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 neurilemma.

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. 145). The central part (hrain and spin/il cord) is

O

194

COMPARATIVE ANATOMY

Fig. 145. — The Entire Nervous System of the Frog.
From the ventral side.

(After A. Ecker.)

facial nerve ; G, ganglion of the vagus ; He, cerebral liemispheres ; I to A', first
to tenth cerebral nerves ; Lop, optic lobes ; M, spinal cord ; M\ to Jf 10,
spinal nerves, which are connected at SM by branches (rami communicantes)
with the ganglia (>S^1 to>S^10) of the sympathetic {S) ; N, nasal sac ; Ni, sciatic
nerve ; No, femoral nerve ; o, eye ; Fa 'to Ve, the different branches of the tri-
geminal ; Vy, Gasserian ganglion ; Vs, connection of the sympathetic with the
Gasserian ganglion ; X\ to X4, the different branches of the vagus. Some of the
fibres of the sympathetic should be shown accompanying the vagus peripherall}'.

NERVOUS SYSTEM 195

the first to arise, and is formed as a direct product of the ectoderm ;
the peripheral portion (cerebraly spinal, and sympathetic nerves, and
their ganglia) becomes established later.

1. THE CENTRAL NERVOUS SYSTEM.

The first indication of the central nervous system is a longi-
tudinal furrow (medullaiy groavc, 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 neurenteric 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 becoQies 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.

Memhranes of the brain and spinal cord {meninges).

In Amphioxus, the central nervous system is enclosed by an
undifferentiated 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

^ The cord is at first solid in certain Fishes {e.g. Petromj'zon, Lepidosteus,
Amia, Teleostei, Lepidosiren), its cavity appearing later.

O 2

196

COMPARATIVE ANATOMY

appearance of a lymph-space in the primitive meningeal membrane,
dividing it into an outer dura mater sjnnalis and an inner 2yTimitive
pia mater. There is thus a peridural or epidural space external to
the dura, and a subdural space between it and the primitive pia
(B).

Primitive pin matei'
f

Endorachis

Subarachnoid
space

Seconda-jy
pia mate)-

'' Dv.ra mate)'

^ Subdural space

Arachiwid

Fig. 146. — The Investing Membranes of the Spinal Cord in the Chief
Vertebrate Groups. A, Fishes ; B, Amphibia and Sauropsida ; C, Mammalia.
Semidiagrammatic. (After Sterzi.) Riickeiimark, 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 lymph
spaces : — a ^:>c?'zrf«m7, a subdural, and a suharachnoid space (c). The
last mentioned encloses a network of trabeculye.^

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), w^ith which it may become fused
{e.g. Mammals) and thus was formerly known as the outer layer of
the dura.-

In Fishes and tailed AmphibianS; the whole subdural space is
filled 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 telce or plexus
choruidei, which are very important throughout the Vertebrate
series.^

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 oflF.

- 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 sinus diiroi 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 periosteal " 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. (Cf . iinder Auditory Organ— perilymphatic and endolymphatic ducts. )

•' The ]jrain-membiane.s in Sauropsida require further investigation. In
Mammals the dura gives rise to folds extending between parts of the brain, known
as the falx and the tentorium. The former, slight indications of which are also
seen in Birds, extends between the two hemispheres, the latter l>etween the hind-
brain and the occipital lobes of the hemispheres : both may become ossified
{<i.g. in Carnivora).

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
j\^ B 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
hrachial and lumho-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 equina ; 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 terminalc.

The bilaterally-symmetrical form of
the spinal cord is pronounced by the
presence of a longitudinal fissure extend-
ing along it ventrally ; this ventral fissure
is not always present, and the so-called
" dorsal fi,ssitre," 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.^ If one imagines the points of exit of the

^ A secondary ventricle-liko enlargement of the posterior end of the central
canal in many Mammals and other Vertebrates must not be confused with the
so-called sinun rhomboidaUs in the liiudmr region of the cord in Birds, which arises
by a separation of the lateral halves of the cord and is filled with modified
neuroglia, thus forming a much thickened dorsal septum.

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, Cauda equina; F.t, filum
terminale ; M.o, medulla
oblongata ; Pb, brachial
plexus ; Pc, cervical
nerves ; PI, lumbo-sacral
plexus ; P.th, thoracic
nerves.

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 ichite substance, consisting
of nerve-fibres only, and a gray substance, composed of nerve-cells
as well as fibres.^ 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. 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 /crc-, mid-, and hind-brain. (Fig. 148). The

Fk;. U8.— Diagram of the Embryonic Conditiox of the Central Nervous

System.

G, brain, with its three primary vesicles.. /, //, /// ; R, spinal cord.

cavities of the vesicles are in direct connection with the centi'al
canal of the spinal cord.

A differentiation of the primary fore-brain and hind-brain
respectively into two parts then takes place, and thus five divisions
of the brain may be distinguished. Counted from before back-
wards these are the telencephalon (secondary fore-brain or cerebml
rudiment), diencephalon (primary fore-brain or tmxt-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 Elasniobranchs and Teleosts, and are reduced or wanting in blind animals and
those in wliich the eyes have undergone reduction.

- In Struthious Birds a row of elevations occurs in the lumbosacral region,
recalling the metameric segmentation of the cord in the Teleost Trig/a. The origin-
ally metameric character of the cord is also indicated phylogenetically and onto-
genetically by the primary, segmental blood-vessels. The dorsal and ventral
longitudinal vascular trunks arise secondarily.

200 COMPARATIVE ANATOMY

myelencephcclon^ (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 Ugemina dorsally, and to
two longitudinal bands, the criora cerebri, ventrally. The meten-
cephalon is also spoken of as the cerebelhtm, and the myelencephalon
as the bulb or medtolla oblongata. From the secondary fore-brain
paired olfactory lobes '^ are given off anteriorly, and its floor or basal

0^ VE Z ZBKCJIKS'p :hk /T^

Fig. 149. — Longitudinal Section through the Skull and Brain of 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) ;
NH^, nasal cavity ; SD, roof of skull ; VH, secondary fore-brain (telencepha-
lon), showing the corpus striatum (Cs) 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 (/), 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 striaticm, while its peripheral part is distinguished as the
" mantle " or pallium (Fig 149).^

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 gray
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 diffei-ent morphologists.

- 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 paraphysi-H (Fig. 150), just anteriorly to a transverse fold of the epithelial
roof (velum transversum) which separates the ventricles of the primary and
secondary fore-brain, and on the edge of m hich a posterior pallial commissure can
be recognised {f.jj. in Lizards) : this fold encloses a vascular thickening of tlie
pia mater {choroid pfexnfi), from which the paraphysisis not always distinguisliable
(cf. Fig. 165). The latter apparently represents a glandular organ, recalling that
connected with the infundibulum : whether it aLso includes the vestige of a sensory
apparatus, like the parietal and pineal organs, is doubtful (cf. pp. 202 and 203).

BRAIN

!01

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 parts 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 IwMmdar 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 callositm and
fornix ai'e of great importance.

Mesencephalon

Diencephalaii

Fig. 150.— Diagrammatic Longitudinal Section through part of the

Embryonic Brain.

Zirbelpolster, pineal cushion.

The outer surface of the hemispheres is more or less smooth,
except amongst the Mammalia, in which fissures {sidci) 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 haltenulw on the posterior lateral
margin of the dorsal region, with the supeonm^ commissttre between
them ; the primary o^Jtic vesicles, arising as paired ventro-lateral
outgrowths, from which the 02)tic no'ves and retina with its pigment
epithelium are derived later ; the pineal apparatus, developed as

202

COMPARATIVE ANATOMY

outgrowths of the roof; and finally, the infundibulwn, formed as an
extension of the floor, together with part of the pituitary hody
{hypophysis). Another portion of the pituitary body is derived from
the epithelium of the primary oral involution (stomodseum).^

The pineal apparatus consists of the epiphysis 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 Verbebrates. Each of these structures

Fig. 151.— Diagram illustrating the Structure of the Hypophysis of
Vertebrates. A, Petromyzon ; B, Pisces ; C. Amphibia ; D, Saurop-
sida; E, Mammalia. (After Sterzi.)

H, " chromophilous " portion, and IP, " 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 correspond to members of a pair ; but further researches are
desirable on this point, as well as on the relation of the two organs

^ Possibly the endodermic epithelium of the primary fore-gut may also take
part in its formation.

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 (c£ 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 stomodaeum
(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
saccics vascvjlosus, 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,^ 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 {teloccele, diacmle, mesoccele,
metaccele, and myeloccele) Ipngin the longitudinal axis of the brain,
as well as paired ventricles, can be distinguished. When cerebral
hemispheres are more or less distinctly developed, the telocoele
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

^ Various hj-potheses 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 ( " palfeostoma " ) 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
hj-pothesis the mouth of existing Vertebrates is a "neostoma." It is very
probable, especially in the higher Vertebrates, that the epithelial part of the
liypophysis has 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 rhinocodle. Each optic lobe also usually con-
tains an optic ventricle, or optocoele, communicating with the
mesocoBle {iter or aqtoedttct of Sylvius). There
may be a distinct metacoele in the cere-
bellum opening into the myelocoele or fourth
ventricle}

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
higher types, more particularly in Mammals.
In the latter Class, moreover, the original
relation of the parts becomes still further
complicated by the large development of the
cerebral hemispheres, which grow backwards,
and thus gradually overlie all the other
parts of the brain : this condition of things
attains its greatest perfection in Man.

Fig. 152. — Diagram of
THE Ventricles of the
Vertebrate Brain.

Cc, central canal of the
spinal cord {R) ; HH,
cerebellum ; MH, mid-
brain, which encloses
the iter {Aq), communi-
cating between the
third and fourth ven-
tricles ; NH, medulla
oblongata, with the
fourth ventricle {IV) ;
VH, cerebral hemi-
spheres, with the lateral
ventricles {SV) ; ZH,
diencephalon, with the
third ventricle (///) ;
each lateral ventricle
communicates with the
third ventricle by a
smallaperture, the fora-
men of Monro {FM).

Amphioxus. — The conical and enlarged
anterior end of the spinal cord of the Lancelet
contains a widened portion of the central
canal which must be looked upon as a
ventricle. This opens freely to the exterior
dorsally 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

^ 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 space between the thin internal walls {septnvi
lucidum) of the two hemispheres.

BRAIN

205

respects in an 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 habenuhie of the right side. The main part of the
roof consists of membrane and vessels.

In the larval Petromyzon or Ammoccete, 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 (palHum) of the median
portion of the ventricle is non-nervous, and
consists of a single layer of epithelial cells,
w^hich, 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 nan-ow
ledge overhanging the fourth ventricle anteri-
orly. The roof of the mesocoele 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 mesocoele ends

Fig. 153.— Diagram
TO Illustrate the
Cerebral Flex-
ure OF a Mammal.

HH, metencephalon ;
MIT, mesenceph-
alon, which at SB
forms the most
projecting portion
of the brain, re-
presenting the so-
called *' parietal
bend " ; NH, my-
elencephalon, form-
ing the " cervical
bend" {NB): the
' ' Yarolian bend "
{BB) arises on the
ventral circumfer-
ence, at the junc-
tion between HH
and i>r^>i?, spinal
cord ; VH, telen-
cephalon ; ZH,
diencephalon, with
the pituitary body
{H) at its base.

206

COMPARATIVE ANATOMY

-lot

m N z.ff a.p

YEYl m\ 1 YII

c

Fig. 154.— Brain of Larval Lamprey. A, from above ; B, from below;
C, from the side.

Ohb, ganglion habeniilse ; Gp, pineal organ ; HH, cerebellum ; Hyp, hypophysis ;
L.ol, olfactory lobe ; Med, spinal cord; MH, mid-brain; NH, medulla
oblongata ; S^, S', first and second spinal nerves ; Sv, saccus vasculosus ;
VH {Bas.G), 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^

72 YII J-.

Fig. 155. — Brain of Myxine. (After G. Retzius.) A, dorsal; B, ventral;
C, lateral view. The olfactory organ and its cartilaginous skeleton (r.o) is
left 171 situ.

a, eye ; g.h, ganglion habenulge ; HH, cerebellum; //, vestigial optic nerve;
MH, mid-brain ; NH, medulla oblongata ; pi, infundibular process ; r,
spinal cord ; rh, olfactory lobe ; s, spinal nerves (dorsal roots and ganglia) ;
F\'-,^, trigeminal ; VH, telencephalon ; VII, facial ; VIII, auditory ; A',
vagus ; X^, sensory branch of vagus or dorsal branch of a spino-occipital

r

^■^V'

208

COMPARATIVE ANATO^?*^

stages of Teleosts. A large, paired, dorsal lobe is present on the
medulla.^

In Petromyzon the pineal apparatus is represented by two
vesicles, which are probably the displaced members of a pair, each
connected with the dorsal surface of the diencephalon (ganglion
habenulse) 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 {epiphysis or pineal organ — probably representing the organ
of the right side) are arranged radially and contain pigment, form-

~^ Pineal orgm

Para-pineal
organ

Fig. 156.— Transverse Section of Pineal Apparatus of Petromyzon marinuft.

(After Studnicka.)

The connective tissue roof of the skull (s) 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) organ, 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.^

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.

2 For the hypophysis of Cyclostomes, see under Olfactory Organ.

BRAIN

209

istic of, and confined to, the group, though the particular regions
are much more highly developed than in Cyclostomes : the pallium

h.0. f,h. c,, "' ^' h.h.

i-<^y th. m.b. \

m.d.

Fio

Braix of Scyllhim canicida. A, dorsal ; B, ventral ; C, lateral view

b.o, olfactory bulb; tp, base of epiphysis ; f.h, telencephalon ;//-, fourth ventricle ;
h.h, cerebellum ; h.p, hypophysis ; (/', lobi inferiores ; m.h; mid-brain (optic
lobes) ; m.d, medulla oblongata; sr, saccus vasculosus ; th, diencephalon ; to,
olfactory tract (very short in Scyllium) The epithelial and vascular roof of
th^ 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-

P

210

COMPARATIVE ANATOMY

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 tuhercle and continuous
anteriorly with an olfactory hulb 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 Notidanidae, is there
a distinct separation of the pallium
into two hemispheres. In Rays there
is only a small unpaired teloccele, the
telencephalon consisting of a practic-
ally solid mass, and the olfactory lobes
are also solid ; in the Myliobatidre
there is no trace at all of a teloccele.

The narrow diencephalon is roofed
over by a choroid plexus, and the tube-
like epiphysis (wanting in Torpedo)
may reach such a length as to extend
beyond the anterior end of the brain
for a considerable distance, and pass
distally into or beyond the roof of the
skull : no indication can be seen of a
parietal organ. A pair of small lobes
— the lohi inferior es — are present on
the infundibulum, and a saccus vasctc-
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 always 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.

Fig. 158.— Brain of Cheilo-
scyllium. (From Parker and
Haewell's Zoology. )

Viewed from the dorsal side,
and the roofs of the various
ventricles removed so as to
show the relations of the
cavities (semi-diagrammatic).

cer, ■ dilatation from which the
metacoele is given off; dia,
diacoele — the reference line
points to the opening leading
into the infundibulum ; iter,
iter (mesocoele), into which the
optocoeles {opt) open ; meta,
myeloccele ; para, lateral
ventricle ; j^f^os, median part
of teloccele ; rh, rhinocoele.

BRAIN

211

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 oiigins 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.i 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 infundibulum, as in Elasmobranchs.

The optic lobes are well-marked in most
Ganoids. The large cerebellum gives rise to
a vahmla 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.

-prs

^pt.l

chl

Fig. 159. — Brain of
Lfpidosttxis. Dorsal
view. (After Balfour
and Parker.)

cW, cerebellum ; c./i, cere-
bral hemispheres ; rf»,
diencephalon ; jm.o,
medulla oblongata ;
o//!/, olfactory lobes;
(ypt.l^ optic lobes ; prsi,
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 b}^
far than in any other Vertebrate group, and only the following
essential points can be mentioned here.

^ In Polypterus and Calamichthys the pineal body gives rise to a peculiar
and extremely large epithelial vesicle, and the hj^pophysis communicates with
the mouth-cavity b}- a holloAV duct, even in the aduft. The brain of these
forms presents otlier special characters, and requires further investigation. In
Devonian Ganoids, 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

B

Tr.opt
ME 3/y/

MR

im

VH(PaU)

Med

jpTvopt

^ Jnf\ \1V\

Fig. 160. — Brain of Salmon. A, dorsal; B, ventral; and C, lateral view.

Ch, chiasma ; G.p^ pineal body ; HH^ cerebellum ; Hyp, liypophj'sis ; Jnfy
infundibnlum ; L.ol, olfaotory lobe ; Med, spinal cord ; MH, mid-brain ; NH,
medulla oblongata ; Pall, pallium (in part removed), and ^Cr and BasG, basal
ganglia (corpora striata) of the telencephalon ; »SV', saccus vasculosus ; 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

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.

b:oI

Not

Fig. 161. — Longitudinal Vertical Section through the Anterior Part of
THE Teleostean Brain. (Founded on a 6gxire of the Trout's brain by Rabl-
Ruckhard.)

Aq, iter (mesoca4e); B.ol, N.ol, olfactory lobe and nerve ; Ca, anterior commissure ;
Ch, posterior optic; Ch.n.opt, optic chiasma ; C«, "inferior com-
missure"; Cp, posterior commissure; C.st, corpus striatum, which lies on
either side of the middle line ; Ep, the epithelium (ependyme), lining the walls
of the ventricles ; Gp, pineal bodj', with a cavity [Gp^) in its interior ; H, W,
hypophysis ; /, infundibulum ; Li, lobi inferiores ; Si\ saccus vasculosus ;
Tro, roof of the optic Icbes ; Tl, torus longitudinalis ; tr, 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. 3 60, 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.^ 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 valmda cercbelli

(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 lohc. 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

^ A parietal foramen is present in the embryo in several Teleosts (e.g.
Cottus, Salmo), and in some others (e.g. 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- PART of the Tele-
ostean Brain.

C.st, 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.cni, telocoele.

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
hip2?ocampus is represented by elevations
of the central gtay matter, which are con-
nected right and left by a small anterior
pallial commissure ]\\&t above the anterior
commissure (Fig. 164, D)

The Amphibian brain does not, how-
ever, lead directly towards that of Reptiles.
Although the telencephalon is more highly
differentiated than in lower forms, the di-
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

Fi(}. 163.— Brain of Cera-
totlus fofiteri. Dorsal
view. (From Parker
and Has well's Zoology.)

and, auditory nerve ; cbl,
cerebellum ; fac, facial
nerve ; gl, glossopharyn-
geal ; med, medulla ob-
longata ; mes, mesen-
cephalon ; oc, oculomotor
nerve ; opt, optic nerve ;
pros, cerebral hemis-
pheres ; rh, olfactory
lobes ; vg, vagus nerve.

m

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

4W/

2S^

—Sped

Cer.f/ fcr^^ cpiv f/P^J: cb ch.pix.2

Ch.plx'

Fig. 164. — Brain of Frog {Rana temporaria). A, from above; B, from below ;

C, from the side ; D, in longitudinal vertical section. (A^ — C, after Gaupp ;

D, after Osborn.)

Ch, cerebellum; Cer.H, cerebral hemispheres; ch.plx^, in A, paraphysis, in D,
anteroid choroid plexus (removed in A) ; ch.plx'\ posterior choroid plexus
(tela choroidea) ; Cojyi (above), superior and posterior commissures ; Com
(below), anterior commissure and anterior pallial commissure just above it ;
Cr. C, crura cerebri ; Di, diencephalon ; for. M, foramen of Monro ; i, iter ;
ivf, infundibulum ; Med.ohl, medulla oblongata ; Olf.l, olfactory lobe ; opt.ch,
optic chiasma ; Opt. I, optic lobe ; opt.v, optic ventricle ; ^wi, stalk pineal of
body ; pit, pituitary body ; Sp.cd, spinal cord ; v^, third ventricle ; i;'*, fourth
ventricle; I-X, cerebral nerves ; 1 Sp^ 2 Sj), first and second ( = embryonic
second and third) spinal nerv^es.

BRAIN

217

recognised even in the adult as a " brow-spot " : thus its
intracranial portion does not represent the entire epiphysis.^ 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.^

Fig. 165. — Longitudinal Section of Brain of an Embryo of Lacerta
vivipara x 26. (After K. von KupflFer. )

ao, olfactory area ; hi, blood-sinus ; ho, olfactory lobe ; c, cerebellar commissures ;
ca, anterior commissure ; ch, superior (habenular) commissure ; ch, central
canal of spinal cord ; cp, posterior commissure ; cpa, anterior pallial (hippo-
campal) commissure ; c/>o, posterior optic commissure ; cpp, posterior pallial
commissure ; cs, spinal commissure ; or, swelling of optic chiasma ; e^, para-
physis ; hm, cerebral hemisphere ; hy, hx'pophj'sis ; in, interorbital septum ;
J (and s) ventricle of infundibulum ; ojit, optic chiasma ; jm, parietal organ ;
}Kh, choroid plexus on medulla oblongata ; R, hind-brain ; ro, optic recess ;
.<i, sulcus intraencephalicus posterior ; tp, tuberculum posterius superius :
tr, torus transversus ; vh, 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

^ 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. jylx').

•^ A parietal foramen was, however, present in the Palaeozoic Stegocephcdi
and other extinct Amphibia.

218

COMPARATIVE ANATOMY

nrt-m

—I

Bol

€>

—J

K XU-XT)

Fig. 166.— Brain of Halteria punctata. A, dorsal ; B, ventral ; and C,

lateral view.
Bol, olfcictory bulb ; Ch, optic chiasma ; GHS, cerebral peduncles ; G.p, pineal
b(Wy, shown in C continuous with the parietal eye {Pa), and only indicated
diagrammatically in A ; ^, small elevation in front of the cerebellum ; ffH,
cerebellum ; Hyp, hypophysis ; /»/, infundibulum ; Lp, process of the hemi-
sphere representing a hippocampal lobe ; between Lp and GHS is a deep groove
{fovea limhica) between the olfactory lobe and pallium ; Med, spinal cord ;
MH, optic lobes ; NH, medulla oblongata ; Nopt, optic nerve ; B}, curved
ridge at the base of the optic lobe ; Tr, optic tract ; VH, cerebral hemi-
spheres ; I- XII, cerebral nerves.

BRAIN

219

Fig. 167. — Brain of Alligator. A, dorsal r B, ventral ; and C, lateral view.

B.ol, olfactory bulb; G.p, pineal body; HH, cerebellum; Hyp, hypopbj^sis ;
Jnf, infundibulum ; Med, spinal cord ; MH, optic lobes ; NH, medulla
oblongata ; 7Vo, 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 ; I-XII, cerebral nerves ;
1, 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 Agamse and Ascalabotse.

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

Parietal organ _ ^ BO^J'^^'^^ Extn-nal and internal

wit^^s^ ^<twyi B ~^^^ lojjer oj optic cv-p

Nerve

Spithelialroofof^^C:^f^^S,yt7>i, \Stif --^^ Pineal organ (epiphijsis)
3rd ventricle n3>^ wS m

Superior commissure _ ^ .

Fig. 168.— Sketch of the Pineal Apparatus 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. Angiiis, 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.^

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.- Various degrees of reduction of the "retina"
and other parts as they occur, e.g. in Hatteria, are seen amongst
Lizards {e.g. Lacerta, Anguis), and the organ ma}^ be recognised in
a simpler form in embryo 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 venti'al 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 t}^ :
it has few points of resemblance to that of Mammals, and is ver}'
different from that of Reptiles, though especially as regards its

* A parietal organ is wanting in Gecko, Ameida and Tejus, and there is no
pineal organ in the Crocodile.

- The paraphysis 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

Wj

Mff^ m

Tr.opt Jnf
Fig. 169. — Brain of Pigeon. A, dorsal; B, ventral ; and C, lateral view.

jy^, cerebellum ; H^yjL*, hypophysis ; Inf, infundibuliim ; L.ol, olfactory lobes;
Med, spinal cord ; MH, optic lobes ; NH, medulla oblongata ; Tr.opt, optic
tract ; VH, cerebi-al hemispheres ; I- XI I, 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

p.v. vi vii ix

Fig. 170.— Brain of Rabbit. A, dorsal ; B, ventral ; and C, lateral view.

h.o, olfactory bulb ; rh' superior vermi.s, and ch'\ lateral lobe- of cerebellum ; cr,
crura cerebri ; ep, pineal bodj' ; /. b, cerebral hemispheres ; f.p, pallial fissure ;
h.b, cerebellum ; hJ, hippocampal lobe ; hp, hypophysis; m.b, optic lobes;
m.d, medulla oblongata ; p.i', 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.^ The distal enlarged end of the pineal body
extends as far as the dura mater, and the structure of the internal

YH -^^_jipi I^ ^«' ETI

Fig. 171. — Brain of Dog (Pointer). A, dorsal ; B, ventral ; and C,
lateral view.

Bo, Bo^, arcuate fissures ; B.ol, olf actor}' bulb ; C7\ce, crura cerebri ; Fi.p, pallial
fissure ; FS, Sylvian fissure ; HH, lateral lobe, and HH^, flocculus of cere-
bellum ; Hyp, hypophysis ; LH, hippocampal lobe ; Med, spinal cord ; NH,
medulla oblongata ; Po, pons Varolii ; BF, rhinal fissure ; Sc, sulcus cruci-
atus ; TO, olfactory tract ; VH, cerebral hemispheres ; Wu, superior \ ermis ;
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.

^ The toothed Birds of the Cretaceous period, with Hesperornis at their
head, possessed a very lowly oi^ganised, 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

^pi hip.com

ctTtt^com

cmex-m '^'^'•"^-^

Fig. 172. — Longitudinal Section of Brain of Rock Wallaby {Petrogale
jieniciUata). (From l*arker and Haswell's Zoology. )

aut.com, anterior commissure ; rbl, cerebellum ; c.mam, corpus mammillare ; c.qtij
optic lobes ; crur, (;rura cerebri ; tpi, epiphysis, with the posterior com-
• missure immerliately behind it; f.mon, position of foramen of Monro;
hip.com, 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 ; 77ied, medulla
oblongata; mid.com, middle commissure; olf, olfactory lobe; opt, optic
chiasma ; vent.^, third ventricle.

very special character. The cerebral cortex becomes much more
highly developed and in many Mammals is more or less markedly
convoluted ^ (Figs. 171 and 173, b). In othei-s, again, the
surface of the hemispheres remains smooth (Fig. 170), but a
subdivision into lobes (frontal, parietal, occipital, and temporal), as
Avell 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

^ It is onlj' possible to homologise the main sulci to a greater or less extent
amongst the various types of convolution seen in the Mammalian brain (cf. p. 228).

226 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

2M'

Jiff

Fig. 173a. — Human Brain. Median longitudinal vertical section.
( Mai nl}^ 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 ; HH, 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) ; R, spinal cord ; T,
infundibulum ; Teh, tela choroidea ; 7'o, optic thalamus (diencephalon), with
the middle commissure {Cm) ; VH, cerebral hemisphere ; Z, pineal body ;
/, 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: it 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 compamtively 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 lobe 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

Fig. 173b.— Cox volutions of the Human Brain. (After A. Ecker.)

ci,h,c, superior, middle, and inferior frontal gj'ri ; em, the calloso-niarginal sulcus
on the dorsal surface ; FS, Sylvian fissure ; HH, cerebellum ; Lf, frontal
lobe ; Lo, occipital lobe ; Lp, parietal \ohe ; NH, medulla oblongata ; Po,
parietooccipital fissure ; P, P^, superior and inferior parietal gyri separated
from one another by the interparietal fissure (/) ; B, spinal cord ; T, tem-
poral lobe ; X, ^I, anterior and posterior central convolutions, separated from
one another by the fissure of Rolando {P) ; 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 latei*al ventricles,^ so that an
anterior, a 'posterior, and an inferior cmmu can be distinguished
in each ; the inferior comu extends into the temporal portion,
which corresponds to the hippocampal lobe of Reptiles, and an
eminence on its floor, the hixriwcamp^is major, - formed as an

^ The ventricles are lined by epithelium (ependyme), which, strengthened by
connective tissue laj^ers derived from the pia mater {tehe. choroid ece), also forms
the roofs of the third and fourth ventricles and extends into the lateral ventricles
as plexus choroidei.

'• The hippocampal system has important relations to the olfactory centre.
The gyrus dentatus {J'a~<tria dentata) and the fimbria arise in close connection Mith
the liippocampus, the fimbria having intimate relations to the fornix.

Q 2

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 {rhincncephcdon) 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 rhmal fissure (Figs. 170, 171). This fissure is in
close relation to the splenial (callosal) fissure, which bounds the
suj^racallosal 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 b}^ 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 pallial
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 imrieto-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 sojid

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 suiKvior vermis, while the lateral parts form the lateral
lobes and flocculi (Figs. 170, 171). In Carnivores, certain
Edentates, Pigs, and Lemurs, the vermis 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 2^ons 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

JD*f O TTv

LCac

Caeb

Fio. 174. — Diagram of the Chief Sy.stems of Fibres of the Mammalian
(Human) Brain. (From a drawing by A. Ecker.)

Cac, crura cerebelli ad corpora bigemina ; Cach, crura medulla ad cerebellum ;
Cap, crura cerebelli ad pontem ; 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

IS'

Fig. 175. — Casts of the Braims of Various Eocene Mammals.
(After Marsh.)

Skull, with brain indicated, of A, T illother mm f odium ; B, Brontotherium ingens ;
G, Goryphodon hamatu-s ; and D, Dinoceras mirabile. E and F, ventral and
lateral views of the brain of Dinoceras mirabile.

covered by the temporal muscles, and may even extend dorsally
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

231

Fig. 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
OF 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 tine branches on the striated muscle-fibres (c). The spinal ganglion
{(l) 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 epiderm (i), anothei
part forming a coil in connection with a tactile corpuscle {k).

mediate group known as the sjnno-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.

232

COMPARATIVE ANATOMY

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 develojDed from a oieicral 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

Ectoden

Neural riOge

Fig.

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-trunh 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

Spinal cord

177. — Diagram of the Embryonic
Spixal 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 transvei*se plane : an excep-
tion to this is seen, however, in Cyclostomes, Elasmobranchs, and
Dipnoans, in which each ventral pair alternates w4th a doi*sal
pair.^ 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 intervertebral foi'amina.

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 doi^sal 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 ple.imses which,
according to their position, are spoken of as cervical, hrachial,
lumhar, and scf.cml (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 rendei's it impossible to reduce
them to a common plan, we find from the Amphibia onwards a
typical grouping of the branches of the cervico-b^^achial and htmho-

^ In ^mphioxus, 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 verm, 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 ease in the Protovertebrata.

234 COMPARATIVE ANATOMY

saeral 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 scapidcv and thoracicits lateralis of human anatomy) ; (2)
posterior (inferior) thoracic nerves (suhclavius, thoracici anteriores) ;
(3) ventral (anterior) brachial nerves {mcdlanus, with the musculo-
cutaneus, idnaris, and cittaneiis mediiis et intermts) ; (4) dorsal
(posterior) brachial nerves (suhscapidares, 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 Sifihidar nerve. The numerous individual
variations seen in the lumbo-sacral plexus {e.g. of Man) are due to
tne 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.^

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 by 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

^ 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 outgi-owths from the extension of the ventral
horns of the spinal cord into the embryonic brain (III, VI, XII,
and probably IV).^

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
Gasserian (V), the geniculate (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 (and also of the Vlllth), certain ganglionic
zones of proliferation of the external ectodermal epithelium

^ 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

{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 are 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 by 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 Ilird,
IVth, Vlth, and Xllth nerves, as
already mentioned.^

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,

^ An additional pair of vestigial cerebral nerves has been shown to exist in
Protopterus, Ceratodus, Anura, and many PHasmobranchs, 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.)

ch, notochord ; d, gut ; ge, epi-
branchial ganglion (ventral
placode) ; gl, lateral vagus
ganglion (lateral placode) ;
gs, sympathetic ganglion ;
A, hind-brain ; /, neural ridge
(rudiment of the spinal gang-
lion proper) ; m, mesoderm ;
nb, branchial nerve : iid,
subepidermoid lay er, derived
from the ectoderm, and giv-
ing rise to the peripheral
part of the branchial nerve ;
118, 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.

Ventral branch.

Dorsal branch.

1st Mttamere (superior, in-
ferior, and anterior rec-
tus, and inferior oblique
muscle).

2nd Metamere (superior
oblique).

i 3rrf Metamere (posterior
\ rectus).

4//i Metamere (muscles
which are early aborted).

' bth Metamere (muscles
which are early aborted).

Oculomotor (///).

Pathetic {IV).

Abducent (F/).
Wanting.
Wanting.

Ramus ophthalmicus pro-
fundus of the trigeminal
(F), together with the
ciliary ganglion.

Trigeminal (with its gang-
lion, minus the ramus
ophthalmicus profun-
dus).

■

Facial {VII), and audi-
tory ( VIII), with their
ganglia.

Glossopharj^ngeal {IX ),
with its ganglion.

Nerves of the Eye-muscles. — The oculomotor (III) pathetic
or trochlear (IV) and abducent (VI) nerves (Figs. 179 — 181)
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
y)rimarily belongs to the sympathetic system, and from which oculo-
motor fibres pass out to the iris and ciliary muscles of the eye.^

The pathetic nerve,'^ 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 oblongata : 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.

^ Furtlier researches are desirable with regard to the ciliary ganglion of
Anamnia and Sauropsida. It appears to be made up of two distinct ganglia
(cerel)ro-spinal and sympathetic), one of which, in Mammals, fuses with the
(iasserian ganglion during development, the other remaining as the ciliar}' gang-
lion of the adult.

- Before emerging on the roof of tlie brain, the fibres of eacli trochlear nerve
cross those of its fellow {chiasma trochhare), so that the riglit nerve supplies the
left superior oblique muscle, and vice versa. Originally this muscle maj' be a
derivative of muscles which once supplied the parietal eye.

238

COMPARATIVE ANATOMY

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CEREBRAL NERVES

239

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240

COMPARATIVE ANATOMY

The trigeminal, facial, glossopharyngeal, and vagus nerves are
usually described as branchial or branch iomanc 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, mandibular, hyoid, and 1st branchial ridges; ///, oculomotor; IV,
trocldear ; V^, V'^, V^, the three main branches of the trigeminal ; Fmot,
motor portion of trigeminal ; G V, ganglion of trigeminal ; VII, facial ; VIII,
auditory ganglion ; IX, glossopharyngeal with the petrosal ganglion
[Gg.petr) ; X, vagus with the ganglion nodosum [Gg.nod] and the anterior
(superior) laryngeal nerve (Lar.sup) ; G.L, ganglionic ridge of the IXth, Xtli,
and Xlth nerves ; XII, hypoglossal. The abducent (VI) is not visible.

over the corresponding branchial cleft into a •prebrancliial (visceral
sensory) and a posth'anchial (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 Gasscrian ganglion at the origin
of the former, and in Fishes (Fig. 179), divides into two main
branches, an ophthalmic (including a superficial and a deepf or
profundus portion), and a maodllo-mandihular : 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 Jlrst, second, and third divisions of the
trigeminal, its name is derived. It passes oat 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.^ 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 sphcnopcdatinc
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 mandihdar 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

^ 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 sitbmaxillary 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. Chimsera, 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 genimt-
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 {titber acusticuw), 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

^ There can be no doubt that the palatine branch of the facial in the Anamnia,
comparable to the visceral or pharyngeal branches of the glossopharj^ngeal 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) cliorda tympani} 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

P.int.m VII

Fig. 182. — Diagram showing the Relations of the Portio Intermedia of
THE Facial Nerve in Man. (After A. F. Dixon; slightlj-modified. )

1, II, III, the three branches of the trigeminal ; *, geniculate ganglion of the
facial ; t, sphenopalatine ganglion, in the neighbourhood of // ; Ch.ly, chorda
tj'inpani ; C.t, tympanic cavity, outlined ; G. trig, Gasserian ganglion of
the trigeminal ; P.int.mVII, intermediate (sensory) portion of the facial;
P. mot. VII, motor portion of the facial (hyomandibular) ; R.liny, lingual
branch of///; ^,7?ia»</, mandibular branch of ///; R.pal, 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,

^ The chorda tympani ("alveolar" branch of the facial) passes intemall}' to
the lower jaw in Elasmobranchii, Ganoidei, Perennibranchiata, and Anura. In
other Amphibians, as in Reptiles, it passes into the bony lower jaw.

R 2

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 vestihdar and a cochlear 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
cewical 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 {Jacobsmis anastomosis),
a continuation of this branch extending forwards, close to the
palatine branch of the facial.^ 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 veiy wide distribution, and is not limited to
the head but extends into the trunk. It includes a sensory lateral
line branch, phar}Tigeal ( = 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 larjTix,
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-fiicial 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 latei-al muscles close
to the vertebral column. All these branches supply the sensory
organs belonging to the lateral line system.-

The so-called spinal accessory (accessorms WiUisii) is a true
cerebral nerve, and can already be recognised in Elasmobranchs,
in which it is included in the vagus, from the posterior roots

^ It is possible that the lateral line fibres which may be associated M'ith the
glossophanngeal, and even with the trigeminal, are always derived from the
vagus and facial.

- Certain nerves present in Teleostomes and formerly described under the
term "ramus latenilis trigemini,*' may be included under the term ^^ ramus
lateralis «rres.so>-/«.s." Thej' form a sensory system of nerves, provided with
ganglia, whicli 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 pas? to some of, or even all, the fins, and supply sensory end-buds
(^.i*.). 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, Saiiropsida, and Mammalia respectively, so that
a direct comparison of the nerve in these groups is impossible.^

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 7 th 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 may be 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.^ 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
vagvis, extension backwards at least as far as the first cervical segment, origin
from a lateral collection of cells of the v^entral cornu, and course on the ventral
side of the dorsal cornu of the gray substance. From this primitive form the
nerve nmst have developed along two different lines in the Sauropsida and Mam-
malia respectively, 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
of the 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 ramtts descendens
and the ansa hyi^oglossi, from which arise branches to the sterno-
hyoid and other muscles.

In the Gymnophiona, Urodela, and Aglossa amongst the Anura,
the fii*st 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,^ which coitc-
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
rami communicantes (Fig. 145). It is distributed mainly to the
alimentary tract, the vascular system, and the glandular organs
of the body.

The sympathetic gaTiglia are derived from the developing spinal
ganglia, and, like these, show originally a segmental arrangement.
They contain typical ganglion-cells,^ and usually become united
together secondarily by longitudinal commissicres, thus giving rise
to a chain-like paired sympath-etic 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.

^ 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 cella are found, e.g., in the pancreas (islets of Langerhans),
coccygeal gland, hj'pophysis (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 {q.v.). The
sympathetic extends into the head.

In Elasmohranchs 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 Elasmohranchs, 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
3,longside the vertebral artery. In all other Vertebrates the whole

SENSORY ORGANS 349

cord 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.i

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 smelly 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 te7nperature.
In addition to free net^e-endings in the skin, various specific
forms of sensory cells occur, and these may be surrounded by
suppm^ting 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

^ The vertebrate eye forms an exception to the other sense-organs in that it
arises from a part of the ectoderm which has been invohUed 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.

x5VZ SZ SiZ

Fig. 183.— Vertical Section through the Skin and a Lateral Line Organ
OF the Larva of Triton tceniatua, 3 cm. in Length. (After F. Manrer. )

BG, blood-vessel ; ISp, 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. These organs always consist of central cells,
arranged in the form of a rounded and depressed pyramid, and
of a peripheral mass grouped around the former like a mantle.
The central cells are surrounded by a network of nerve-fibres ;
each of them bears at its free end a stiff cuticular hair, and they
are the sensory cells proper : the others have merely an isolating,
supporting and slime-secreting function (Figs. 183 and 184).

In Dipnoi, perennibranchiate Amphibia, and amphibian larvae
these organs retain their peripheral free position, on a level with

A

SENSE-ORGANS OF THE INTEGUMENT
Ai: JE SZ StZ

251

Fig. 184, A.— Longitudinal Vertical Section of the Skin and a Lateral
Line Organ of Triton cristatus during the Breeding Season, when
THE Animal lives in the Water. (After Maurer.)

A E and JE, external and internal layers of epiderm abutting against the sensory
organs; BG, blood-vessel; SN, nerve; SZ, sensory cells; /S^^Z^, . supporting
cells.

B.— Isolated Supporting {StZ) and Sensory {SZ) Cells from a Later.yl
Line Organ of Triton.

C. — Vertical Section through the Lateral Canal of Amia calva.
(After AUis ; slightly modified.)

Nl, lateral nerve, and N, branches to sensory organs ; Oe, apertures of the lateral
canal to the exterior ; S, scales ; SO, sensory organs in the lateral canal.

252 COMPARATIVE ANATOMY

the epiderm/ but in other Fishes (as is also the case on the head
in Dipnoans) they may eventually become enclosed in depressions
or complete canals: these are often branched, and are formed
either by the epiderm only, or more usually, by the scales and
bones of the head, and they open externally from point to point
(Fig. 184, c).

Fig. 185. — Sensory Canals of Chima'ra monstrosa. (After F. J. Cole.) The
innervation is indicated by the different kinds of shading.

(1.) — Supra-orbital canal (innervated by superficial ophthalmic of facial —
cross-hatched : the black segment is the portion innervated by the pro-
fundus) = cranial (C) -f rostral (i?)-f sub- rostral [SR).

(2.) Infra-orbital canal (buccal + otic of facial — dotted) = orbital {Or) +
sub-orbital (/SO) -f- portion of angular (^)+ nasal (iV).

(3.) Hyomandibular or operculo-mandibular canal (external mandibular of
facial — black) = remainder of angular (^4)-t-oral (0)4- jugular (/).

(4.) Lateral canal (lateral line branch of vagus — oblique shading) = lateral
(iy)-f occipital (Oc)-f aural (^M)-t- post-aural [PAu).
The small dots on the snout represent the apertures of ampuUary tubes.

The distribution of these sensory organs extends over the whole
body, but (except, e.g. in Petromyzon), they are situated character-
istically along certain tracts, the position of which is very constant:
on the head, supra-orhital, infra-orhitcd, and hyomandibular tracts

^ At the time when an Amphibian undergoes metamorphosis and gives up its
aquatic habits, these sensory organs sink downwards into the deeper layer of the
skin, and, as the epiderm grows together over them, they become shut off from
the exterior and reduced, and may finally disappear (Anura and certain Caduci-
branchiata). In others of the latter group, in which they are retained and also new
ones are formed, they come to the surface when the animal returns to the water
during the breeding season (Fig. 184, a). Peculiar sense-organs are present in
the aquatic Xenopus and in Ichthyophis glutinosus. The horny wart-like
structures arising periodically during the breeding season in Cyprinoids, and
known as "pearl-organs," are due to a modification of the reduced nerve-
eminences. Similar structures occur in Anura.

SENSE-ORGANS OF THE INTEGUMENT 253

can be recognised, and along the sides of the body and tail
are one or more lateral lines ^ (Figs. 185 and 186). These struc-
tures are thus often spoken of as segmental scn^oi^y <rrgans, or oi^gans
of the lateral liyie-: primarily they have not a metameric arrange-
ment, and where such is seen, it is always secondary. The portions
lying in the region of the head are the first to be developed. They
are innervated by the lateral line branches of the facial, glosso-
pharyngeal and vagus (cf. pp. 242, 245).

It is thus clear that the entire lateral nervous system and its
modifications, including the auditory organ, is a specialised system
differing morphologically and histologically from all the other
integumentary sense-organs.

The so-called Savi's vesieles of Torpedo, the nerve-sacs or
liit-organs of Teleostomes, and the amimllary tuhes of Elasmo-
branchs correspond to modified nerve-eminences. These are

Fig. 186.— Distribution of the Lateral Sense-Organs in a Saiamander

Larva.

all limited in their distribution to the head and anterior portion
of the trunk, being most numerous on the snout : they arise from
thickenings of the epiderm which later become invaginated and
in which a sensory epithelium is differentiated. In Teleostomes
these organs retain a simple sac-like form, and are situated
abundantly on the head and along accessory lateral lines on the
trunk : in Toi-pedo they become completely separated from the
epiderm, while in other Elasmobranchs they are tubular, each
tube giving rise to one or more swellings or ampullar of varied
form enclosing radial folds of connective tissue and containing the
nerve-end organs, which are supplied by the lateral-line system
of nerves. The tubes are filled with a gelatinous substance.

These integumentary sense - organs are extremely ancient
structures, for traces of them have been observed in Jurassic
Elasmobranchs, and even in the Devonian Cephalaspidse and

^ There are several lateral lines in Polypterus and various other Fishes, in
Proteus, and in all amphibian larvje,

- The lateral canal system of Polyodon comes nearest to that of Elasmo-
branchs, and in Acipenser it shows certain resemblance to that of Bonj^ Ganoids,
in that the sensory organs become embedded within cranial elements. In Lepi-
dosteus, branched secondary canals arising from the main canals of the head
extend into the cranial bones : this is not the case in Polypterus. In Teleosteans
the system is very different from that of Elasmobranchs, but resembles that of
Ganoids in many respects — e.g. in often having the canals protected by bony
structures : in other respects the different families and species differ much from
one another, and reductions of the lateral line organs may occur {e.g. in
Siluroids).

254 COMPARATIVE ANATOMY

Pteraspidse : the so-called " spectacles " of Archegosaurus probably
belong to the same category. Their function is not thoroughly
understood, but there is no doubt that they are concerned with
the perception of mechanical stimuli from the surrounding water,
and are thus probably important in appreciating the direction of
these stimuli. As already mentioned (p. 242), they and the auditory
organ are genetically related to one another, the ear being merely
a specialised portion of the lateral line system.

h. End-buds and gustatory organs.

Although various intermediate forms between the nerve-
eminences and end-buds occur, it is an open question as to whether
there is any genetic connection between these two kinds of sense-
organs, and it is important to bear in mind that the nerve-supply
in the two cases is a very different one.

In contrast to the nerve-eminences, which tend to sink below
the surface, the end-buds usually form a dome-like elevation pro-
jecting above the general level of the epiderm. A central sensory
epithelium, provided with sensory hairs, and peripheral supporting
cells can be recognised, but the former are as long as the latter.
These organs are supplied by the facial nerve.

In Lampreys and most Elasmobranchs they remain at a primitive
stage of development, but become of great importance in Teleos-
tomes, in which they are scattered irregularly over the whole body
and are particularly numerous on the fins, lip-folds, barbules, and
mouth.

From Dipnoans onwards they are limited to the oral, pharyngeal,
and nasal cavities, and are supplied by the IXth and Xth as well
as by the Vllth nerve. Thus in Amphibians they occur on the
papillse of the oral and pharyngeal mucous membrane, on the
margins of the jaws, and on the apices of the fungiform papillae of
the tongue, which may possibly have a gustatory function.

In Reptiles the distribution of these organs is somewhat more
limited, and in Birds true taste-buds, though present in the mouth
and pharynx, are wanting on the horny tongue. The lingual
branch of the glossopharyngeal nerve is, however, strongly de-
veloped in many groups (e.g. Lamellirostres), and functionally
replaces the sensory branch of the trigeminal, which is wanting in
the avian tongue.^

^ In Lizards and Crocodiles typical gustatory organs are present, chiefly on
the soft, glandular mucous membrane of the pharynx : they are wanting on
the tongue and in the anterior part of the oral cavity. In Birds their dis-
tribution is apparently dependent on the size and form of the tongue : when the
latter is narrow, they are situated on the thin, glandular mucous membrane
of the lower beak ; and when it is broad, on that of the upper beak or of the
pharynx. Their arrangement is irregular, and their number varies greatly in
the different groups, being greatest in Parrots (300-400), in which their structure
resembles that of the gustatory organs of Mammals.

SENSE-ORGANS OF THE INTEGUMENT 255

In Mammals, organs of taste are still found on the soft palate,
on the walls of the pharynx, and even extend into the larynx ; but
they are most numerous on the tongue, where they occur on the
circumvallate and fungiform papillae as well as on the papilla
foliata.^

Thus the specific integumentary sense-organs of aquatic Verte-
brates have not entirely disappeared in terrestrial forms, certain of
these (end-buds) being retained even in Mammals, under the
necessary condition of a moist medium.^

c. Tactile cells and corpuscles.
(Terminal ganglion-cells.)

In these structures there is no longer any direct connection
with the surface of the epiderm, and supporting cells are wanting.

" Tactile spots," consisting of groups of tactile cells, are met with
for the first time in tailless Amphibians, in which they are
usually situated on small elevations, and are distributed over the
skin of the whole body (Fig. 187, a). Phylogenetically they are
probably derivable from the integumentary sense-organs of the
Ichthyopsida. In Reptiles, amongst which they retain the simplest
form in Hatteria and are arranged along the margins of the scales,
they are found chiefly on the lips and sides of the face and on the
snout, but in some cases (as in Blindworms, Snakes, and young
Crocodiles) they are present on the scales over the whole body,
and are usually arranged symmetrically. In Snakes and Birds
the tactile cells are confined to the mouth-cavity (tongue) and to
the beak (cere), and are much closer together, forming definite
masses, or tactile corpuscles. Each of these is surrounded by a
nucleated connective tissue investment from which septa extend
into the interior, partially separating the individual tactile cells
from one another : the " Grandry's coi-puscles " occurring on the
beak (Fig. 187, d) are modified tactile corpuscles.

In Mammals the tactile cells are either isolated — as, for instance,
on the hairless portions of the body, or they give rise to oval
corpuscles, each consisting of a many-layered and nucleated invest-
ment, into which a nerve passes, becomes twisted up, and comes
into relation with one or more terminal cells (Fig. 187, B, c). These
are most numerous and highly developed on the volar and plantar

^ Two circumvallate papillie are present in Monotremes, three in Marsupials,
and a variable number in the Eutheria. Foliate papillae are especially well
developed in Rodents, but in many Mammals are little marked or wanting. The
relative functional importance of the different kinds of papillae varies in the
course of the individual life in Man, In Cetaceans {e.g. Dolphin) only vestiges
of the gustatory organs are retained.

^ Nothing is known as to whether certain of these organs in lower Verte-
brates are concerned with the sense of taste, or whether a change of function has
taken place in passing to the higher forms : the nerv^e-supply is, however,
interesting.

256

COMPARATIVE ANATOMY

Fig. 187a. — A Tactile
Spot from the Skin of
THE Frog. Semi-dia-
grammatic. (Modified
from Merkel.)

a, a, neuro-epithelium ; b,
epiderm; jV, nerve, which
loses its medullary sheath
at NK

Fig. 187c.— a Tactile CoRruscLE (End-Bulb,
OR Krause's Corpuscle) from the Margin
OF THE Conjunctiva of Man. (After
Dogiel.)

b, nucleated fibrous investment ; n, meduUated
nerve fibre, the axis-fibre of which
into a closely coiled terminal fikein.

Fig. 187c.— Dermal Papilla
FROM THE Human Finger
enclosing a Tactile Cor-
puscle (Meissner's Cor-
puscle). (After Lawdowski.)

a, fibrous and cellular invest-
ment ; b, tactile corpuscle,
with its cells ; n, nerve-fibre ;
n', the further course of the
nerve-fibre, showing its curv-
ing branches ; n", terminal
twigs of the nerve-fibres with
club-shaped endings.

Fig. 187 d. — Transverse Section
through a tactile corpuscle
(Grandrys' Corpuscle) from
THE Beak of a Duck. (After
Carriere. )

n, nerve, entering the capsule K,
its sheath (S) becoming con-
tinuous with the latter. The
nerve passes between the two
covering-cells, DZ, DZ, widen-
ing out to form a tactile plate
at n}.

surfaces of the hand and foot respectively and on the conjunctiva
and snout, and are especially well-marked on the snout in the Mole :

SENSE-ORGANS OF THE INTEGUMENT 257

they are simplest on the glans penis and glans clitoridis. It is
doubtful whether they occur on hairy portions of the skin, though
it is certain that the hairs (and more especially the vibrissa?, cf. p. 30),
have a sensory function, which is very marked in the case of those
on the wings and ears of Bats.

Circumscribed and well-innervated areas, each composed of a
cap of thickened and curiously fhodified epithelium covering a
dermal papilla, have been observed in numerous Mammals close to
the hairs : these hair-discs represent a special kind of sensory
apparatus. In Man, each disc is a small, rounded structure situ-
ated in the acute angle between the oblique hair-shaft and the
skin-surfece : in a corresponding position in the obtuse angle
opposite to it is another well-marked, smooth area, probably corre-
sponding to a scale-rudiment. These two structures, together with
the other accessory organs of the hair (glands, muscles, nerves,,
vessels, &c.), constitute a well-defined " hair-area" which is probably
the morphological equivalent of the reptilian scale.

d. Club-shaped or lamellar corpuscles.

(Pacinian corpuscles.)

In Lizards and Snakes club-shaped corpuscles are present in
addition to the above-described tactile organs, occurring chiefly in
the region of the lips and teeth and also on the body {e.g. Lacerta) ;
they have an elongated, oval form, and their structure is simple.

Fig. 188. — Pacinian Corpuscle from Mesorectum of Kitten — A, two
days, and B, three days old. (After Guido Sala, )

In A the nerve-network is seen surrounding the main fibre. In B, knob-like
outgrowths are shown on the fibre at its distal end.

When more highly developed, the interior of each corpuscle
shows the continuation of the axis-fibre of the nerve surrounded

>258 COMPARATIVE ANATOMY

by numerous lamellae : on it either knob-like buds or networks can
be recognised, the latter surrounding the main fibre (Fig. 188).
Often also there is a double column of cells surrounding the axis.

Organs of this kind are universally present in the deeper layers
of the derm in Birds and Mammals : in the former they are
particularly abundant on the beak and at the bases of the
contour- feathers of the wings 'and tail, and are also found on
the tongue. They occur, moreover, in various other regions, both
in Birds and Mammals {e.g. on the various organs of the ab-
dominal cavity, the conjunctiva, the fasciae, tendons, ligaments,
the vas deferens, periosteum, submaxillary glands, mesentery, peri-
cardium, pleura, corpus cavernosum and spongiosum, the wing-
membrane of Bats, &c.). Their size varies greatly even in the same
individual.

The tactile cells and tactile and club-shaped corpuscles are all
concerned with the sense of touch and pressure. It is impossible to
say definitely what nerve-endings have to do with the perception
of temperature ; it is not improbable that the touch-cells, as well as
the nerve-fibres often provided with varicose swellings which end
freely in the epiderm, are here concerned. Suchyrec nerve-endings
occur in the skin of all Craniata and consist of a branched, inter-
cellular network, no direct connection between nerve and epithelial
cell having been observed.

OLFACTORY ORGAN.

The olfactory nerves are connected with the olfactory lobes
which arise as prolongations of the secondary fore-brain, the
ventricle of which is temporarily or permanently continued into
them: as already mentioned, each olfactory lobe is, in some cases,
differentiated into bulb, tract, and tubercle.

The filaments of the olfactory nerves are arranged in a single
bundle on either side, or in two more or less distinct bundles.
The individual fibres pass from the cranial cavity into the nasal
cavity either separately, through a cribriform plate of the ethmoid
(p. 129), or through a single aperture on either side. The latter
is the case, e.g. in Fishes, all Amphibians except Menopoma,
Reptiles, all Birds except Apteryx and the extinct Dinornis, and
Ornithorhynchus : in all Mammals except the last mentioned a
cribriform j)late is present.

The primary origin of the olfactory organ is by no means
understood, and it is doubtful whether it can be said to have a true
olfactory function in aquatic types. In its simplest form, the organ
consists of a ventral, paired, pit-like depression of the integument
of the snout opening on to the surface by an external nostril. It
is lined by ectodermal epithelium, which gives rise to a " primary
olfactory ganglion," the individual elements of which at first re-
semble unipolar nerve-cells : from these, the olfactory fibres grow

OLFACTORY ORGAN

259

centripetally towards the olfactory lobes and pass into the fore -brain,
when they become connected with the olfactory centre. The
individual olfectory cell and fibre thus form an organic unit — a
primitive condition such as occure in certain integumentary sense-
organs of Worms and Molluscs, but not in any of the other sensory
cells of Vertebrates. The olfactory cells thus constitute the only
true neuro-epithelmm in Vertebrates, as the nerve arises in connec-
tion with the cell itself, with which it remains continuous {ijrimary
sensory cell) : in other secondary nerve-cells, the relation of cell and
nerve is one of apposition merely.

In their final form, the olfactory cells are elongated, swollen in
the region of the nucleus, and bear hair-like processes on their free
ends, while proximally each is continuous
with a nerve-fibre (Fig. 189). Between
them are isolating or supporting cells,
which have a similar origin, and ciliated
cells may also be present.

The olfactory organs in all Fishes are
of a simple sac-like form, but from the
Dipnoi onwards they come to communicate
with the cavity of the mouth as well as
with the exterior. In consequence of this,
(interior or external nostrils, and posterior or
internal nostrils (choance) can be distin-
guished: as a free passage is thus formed
through which air can pass, the olfactory
organ takes on an important relation to
the respiratory apparatus, and in it olfac-
tory and respiratory regions can be dis-
tinguished.^ From the Amphibia onwards
glandular elements are present, the secre-
tion of which serves to keep the nasal
cavity moist.

In Amphioxus, the ciliated pit sup-
plied by a nerve and situated above the
anterior end of the central nervous system
probably represents an unpaired olfactory organ.

Cyclostomes. — In these forms the olfactory organ consists of a
sac enclosed by a fibrocartilaginous capsule containing numerous
radial folds of the mucous membrane enclosing tun-shaped sense-
buds, and is unique in being unpaired (Fig. 190). It lies just in
fi-ont of the cranial cavity, and opens on the doi'sal surface of the
head by a chimney-like tube, which in Myxine is long and is

^ The mode of formation of the primitive choance is already indicated in
many Elasmobranchs as well as in the embryo of Ceratodus, in which a groove,
bounded by folds of the skin, extends backwards from each external nostril to
the mouth (Fig. 191, a), through which water passes. In Mammals there is no
naso-oral funnel, and the development of the choanal in the higher Vertebrates is
accompanied by a secondary perforation of the primarily blind nasal sac.

s 2

Fig. 189.— Epithelium of
THE Olfactory Mucors
Membrane. A, of Petro-
myzoii planeri ; B, of
Salamandra atra.

E, interstitial epithelial
cells ; B, olfactory cells.

260

COMPARATIVE ANATOMY

supported by rings of cartilage. This tube is continued backwards
from the ventral side of the olfactory organ above the mucous

EnHmcr

Fig. 190. — A, B, C, Median Longitudinal Section through the Head of a
Larva of Petromyzon planeri in Three Successive Stages, to show the
Mode of Development and Relations of the Olfactory and Pituitary
Sacs and their Gradual Shifting from the Ventral to the Dorsal
Side, owing to the Growth of the Oral Funnel. (Mainly after Kiipffer
and Dohrn.)

Ch, notochord ; Chias, optic chiasma ; Gjj, pineal body ; HH, hind-brain ; ITi/p,
pituitary sac ; Jvf, infundibulum ; MB, stomodjeum ; M/I, mid-brain ; OL,,
UL, upper and lower margins of oral funnel ; HO, olfactory sac ; VET,
position of endodermic part of gut shown in C ( VOJJ) opening into the
stomod.ieum ; VH, fore-brain.

membrane of the mouth : in Petromyzon it forms a blind pouch,
but in Myxine opens into the oral cavity as a naso-palatinc duct.

Although the olfactory nerve is paired, the first trace of the
olfactory sac in the embryo is seen as an unpaired plate, which soon

OLFACTORY ORGAN

261

becomes grooved : its unpaired character is probably secondary.
In Petromyzon it arises on the ventral side of the head in front of
the oral involution (stomodaeum), and between it and the mouth is
another ectodermal inYs.gm£ition, the pituitari/ sac (Fig. 190). In
the course of development the olfactory and pituitary invaginations
become sunk in a common pit, which, owing to the growth of the
large oral funnel, gi'adually becomes shifted to the dorsal side. On
the further elongation of the naso-pituitary sac to form the above-
mentioned tube, the olfactory sac opens into it posteriorly, and is
incompletely divided into right and left halves by a septum which

Hyp BO

~~ Chios

^ VOD

Fio. 190, C.

grows down from the dorsal side. The pituitary body arises by
the formation of follicles from the pituitary sac where it passes
below the infundibulum.

Fishes. — The position of the olfactory organ in Elasmobranchs
(Fig. 191, a) differs from that seen in Cyclostomes in being on the
under instead of the upper surface of the snout, and thus retains
a more primitive position. In many forms each nostril is con-
nected with the mouth by a naso-oral groove (cf. Note on p. 259).
From these Fishes onward the organ is always paired, each sac
being more or less completely enclosed within a cartilaginous or
bony investment forming an outwork of the skull, and being situated
between the eye and the end of the snout, either laterally or more
or less doi-sally.

In the course of development each external nostril of Teleo-
stomes becomes completely divided into two portions, an anterior
and a posterior, by a fold of skin. The anterior aperture is often,
and the posterior sometimes, situated at the summit of a longer or
shorter tube, lined by ciliated cells, and the distance between the

262

COMPARATIVE ANATOMY

% <5+ «

Fig. 191.— J, Ventral View of the Head of a Dogfish {Sajllinm canicula).
HSO, integumentary sense-organs ; M, mouth ; N, nostril.

B, Lateral View of the Head of a Pike (Efio.v laciK.'i). Ay, eye ; a and h, the
anterior and posterior openings of the external nostrils, and t, fold of skin
separating them.

Fig. 191. — C, Lateral View of the Head of Marcuna hdena.

A, eye ; HSO^ integumentary sense-organs ; VR and HR, anterior and posterior

narial tubes.

OLFACTORY ORGAN 263

two apertures varies greatly, according to the width of the fold of
skin which separates them (Fig. 191, B and c). A passage for
the water is thus here also present, but, unlike that of Elasmo-
branchs, is not connected with the mouth.

The mucous membrane of the nasal organ of Fishes is always
raised up into a more or less complicated system of folds, which
may have a transverse, radial, rosette-like, or longitudinal arrange-
ment, and on which the sensory cells, as well as ciliated cells, are
situated.^

A nasal skeleton which is well differentiated from the skull
proper is met with for the first time in Dipnoans. In Protopterus
it consists of a cartilaginous trellis-work, enclosing the olfactory
sac and united with its fellow in the median line by a solid septum :
the floor is formed mainly by the pter}^'gopalatine and by con-
nective tissue. The mucous membrane is raised into numerous
transverse folds connected with a longitudinal fold, and the olfac-
tory organ in general most nearly resembles that of Elasmobranchs,
except that, as already mentioned (p. 259), internal as well as
external nostrils are present. The latter open beneath the upper
lip, and so cannot be seen when the mouth is closed ; the former
open into the oral cavity rather further back.-

Amphibians. — The olfactory organ of Perennibranchiates
resembles in many respects that of the Dipnoans : it is always
enclosed within a complete or perforated cartilaginous capsule
situated laterally to the snout close beneath the skin, and is not
protected by the bones of the skull (Fig. 192). Its floor is largely
fibrous, and the mucous membrane is raised into radial folds like
those of Cyclostomes and Polypterus.

In the higher Amphibia the olfactory organ is more com-
pletely included within the cranial skeleton, and its structure
becomes modified in correspondence with the change in the
mode of respiration, the nasal chamber giving rise to a special
respiratory portion, into which the external and internal nostrils
open. In Urodeles, the lumen of the organ is from the first
simple, while in Anurans, dorsal, middle, and ventral portions
may early be distinguished ; but in both cases the cavity becomes

^ The olfactory organ probably reaches its highest development and most
complicated form amongst Fishes in Polypterus. The nostril leads into an outer
cavity, which communicates with the olfactory sac proper, and the latter is
divided up into six radial compartments arranged around a central spindle and
separated by complicated septa, so that a transverse section of the organ some-
what resembles in appearance that of an orange. In certain representativ^es of
the Plectognathi and Gymnodontes amongst Teleosts, on the other hand, the organ
shows various stages of degeneration, and may even undergo almost entire
reduction.

- The peculiar position of the anterior nares has a physiological significance,
at any rate in Protopterus, in connection with the habits of the animal ; during its
summer sleep the animal breathes through a tube, passing between the lips, formed
from the capsule or cocoon which encloses it. The necessary moisture for the
olfactory mucous membrane during this time is provided by the numerous goblet
cells which line the walls of both nostrils (cf. p. 20),

264

COMPARATIVE ANATOMY

complicated later by the development of blind pouches or grooves,
which are, however, more marked in Anura, and more especially
in Gymnophiona, than in Urodela. The prominences or ridges

projecting into the nasal lumen
between the pouches are analo-
gous to the turbinals of higher
forms. A main chamber and a
more laterally situated accessory
cavity can be distinguished, the
latter extending into the maxil-
lary bone (Fig. 193). In certain
Gymnophiona the accessory cham-
ber becomes entirely shut off from
the main cavity and receives a
special branch of the olfactory
nerve, so that in these cases two
separate nasal cavities can be
distinguished. The external nos-
trils are always lateral in the
adult, but in Urodeles, this
position is attained secondarily.
They are opened and closed by
muscles.

Glands, situated under the
olfactory mucous membrane, are
now met with in terrestrial forms ;
these are either diffused, or united to form definite masses. They
either open directly into the nasal cavity, their secretion serving
for the necessary moistening of the mucous membrane (effected
in Fishes and in larval and perennibranchiate Amphibians by

Fig. 192.— Olfactory Organ of Nec-
turus macjilatus. From the dorsal
side.

A F, antorl)ital process ; F, frontal ;
iV, olfactory sac ; 01, olfactory
nerve ; P, process of the parietal ;
Pmz, preniaxilla ; PP, palatoptery-
goid.

Fig. 193. — Transverse Section through the Olfactory Cavities of
Plethedon ghUiiwmis.

C, cartilaginous, and »9\ fibrous portion of the turbinal, which causes the
olfactory epithelium {E) to project far into the nasal cavity ; /', frontal ;
ID, intermaxillary gland, shut off from the cavity of the mouth by the oral
mucous membrane {AIS) , K, maxillary cavity ; M, maxilla ; N, main nasal
cavity ; Pf, prefrontal ; Sp, nasal septum ; S, S^, olfactory mucous mem-
brane ; Vop, vomero-palatine.

the external medium), or they pour their secretion into the
pharynx or posterior nostrils. The latter are always situated

OLFACTORY ORGAN

265

tolerably far forwards on the roof of the mouth, and are for
the most part enclosed by the vomer, or vomero-palatine.

A naso-lacoymal duct passes out from the anterior angle of
the orbit, through the lateral wall of the nose, and opens into
the nasal cavity on the inner side of the upper jaw. It conducts
the lacrymal secretion from the conjunctival sac of the eye into
the nasal cavity, and arises in all Vertebrates, from the Myctodera
onwards, as an epithelial cord which is separated off from the
epiderm, and, growing down into the derm, becomes hollow
secondarily. A naso-lacrymal duct is wanting in Proteus and
Siren.

zv

Reptiles. — Owing to the growth of the brain and facial region
and to the formation of a secondary j3alate (p. 112), the olfactory
organs, from Reptiles onwards, gradually come to extend more
ventrally beneath the cranium. As in
Amphibians, a lateral or ventral respira-
tory, and a median olfactory portion can
be recognised.

The simplest olfactory organs amongst
Reptiles are seen in Lizards, Snakes, and
many Chelonians. The nasal cavity of
Lizards, for example, is divided into two
portions, a smaller outer (anterior), and a
larger inner (posterior) or olfactory cham-
ber proper (Fig. 194j. The latter alone
is provided with sensory cells, the former
being lined by ordinary stratified epi-
thelium continuous with the epiderm : it
may contain goblet-cells, but encloses no
aggregated glands, and externally to the
epithelial layer are muscular elements and
cavernous tissue. A large fold or tur-
linal, slightly rolled on itself, arises from
the outer wall of the inner nasal chamber,
and extends far into its lumen ; this is also

well developed in Ophidia, in which a distinct outer nasal chamber
is wanting, but is very simple in the Amphisbsenidse. The
skeletal supports of the turbinals in Reptiles as in all higher forms,
are developed secondarily.

A large gland which opens at the boundary between the inner
and outer nasal cavities lies within the turbinal (except in
Hatteria), and corresponds to the superior nasal gland of Urodeles.
Below the turbinal is the aperture of the lacrymal duct : in some
Reptiles this opens on the roof of the pharynx (Ascalabota),
and in othere into the internal nostrils (Ophidia), which, as in
Amphibians, are usually situated on the anterior part of the roof
of the mouth.

Ca MS

Fig. 194. — Diagram of the
Olfactory Organ of a
Lizard. (Longitmlinal
vertical section. )

^4^, IN, outer and inner
nasal chambers ; f , tube-
like connection between
them ; Ch, internal nos-
trils ; MS, oral mucous
membrane ; P, papilla of
Jacobson's organ {q.r.),
and Ca, its aperture of
communication with the
mouth.

266 COMPARATIVE ANATOMY

The structure of the nose in Chelonians is very complicated
and varied. In marine Chelonians the organ is divided into two
passages, one above the other, and connected by means of a per-
foration of the septum. The comparative paucity of glands in
the olfactory organ of Lizards and Snakes forms a marked
contrast to the condition seen in Chelonians, the nasal organ of
which is characterised by a great abundance of them.

The extension downwards and backwards of the olfactory organ
is most marked in Crocodiles, in correspondence with the forward
growth of the facial region and the formation of the palate ; its
posterior part thus lies below the brain and base of the skull, the
naso-pharyngeal passage being so much elongated that the
posterior nostrils open far backwards into the pharynx. Each
nasal chamber is divided posteriorly into two superimposed cavities,
the upper of which represents the proper olfactory chamber, and is
lined by sensory epithelium, while the lower serves as a respiratory
passage only. Certain accessory air-chambers are connected with
the nasal cavity. A large gland is present between the olfactory
chamber and its investing bones, and opens on either side of the
nasal septum, posteriorly to the external nostrils, by one or two
apertures. As in other Reptiles, there is only a single true
turbinal, but externally to it lies a second
OJir prominence, which may be spoken of as a

pseudo-hcrbioial, and which possibly corre-
sponds to the upper turbinal of Birds.

-^^^ M(f^\V II Birds. — In all Birds, as in Lizards, there

is an outer chamber lined by stratified e{)i-

thelium, and an olfactory chamber proper,

situated above the former. In addition to

Fig. 195.— Transverse a turbinal corresponding to that of Reptiles

Section THROUGH THE and usually known as the middle turbinal,

SJTIh'IikbMITS th-^re is a so-called upper iurUnal (Fig. 195) :

minor). the former is comparable to the maxiUo-

, , , turbinal and the latter to the naso-turbinal

a, upper, and />, lower „,^- \ / \ a -i • ^^

nasal passage; LR, of Mammals {q.y.). A special projection

air-chamber, which composed of undifferentiated epithelium and

oftheli '"er Uirbiiab situated in the outer chamber may be dis-

OM, MM, upper and tinguished as the vestibular turbinal. There

middle turbinals. is no longer any olfactory epithelium on

the middle turbinal in the adult, and the

upper turbinal during development gradually passes backwards

relatively to the middle turbinal, which is usually supported by

cartilage or more rarely by bone, and the form of which varies

greatly. It may be represented by a moderate-sized prominence,

or may become more or less rolled on itself: the lacrymal duct

opens below and anteriorly to it. The narrow, slit-like internal

nostrils open comparatively far back.

OLFACTORY ORGAN

267

Fig. 196a. — Lateral View of the Nasal
Chamber in the Human Embryo.

/, inferior (maxillary), //, middle, and
///, superior turbinal ; cr, base of
skull ; n, tip of nose ; os. Eustachian
aperture ; pi, hard palate ; f, super-
numerary ridge (ectoturbinal) which
occurs in the embryo.

The so-called external nasal gland of Birds is situated on the
frontal or nasal bones, along the upper margin of the orbit. It
is supplied by the first and
second branches of the tri-
geminal, and corresponds to
the lateral nasal gland of
Lizards.

Mammals. — Correspond-
ing to the more marked de-
velopment of the facial por-
tion of the skull, the nasal
cavity of Mammals is propor-
tionately much larger than
in the forms described above,
and consequently there is
much more room for the ex-
tension of the turbinals.
These give rise to a spongy
labyrinth, with cell-like com-
partments lined by mucous membrane; and thus variously-
shaped projections, supported partly by cartilage and partly by
bone, are seen extending into the nasal cavity, and the olfactory

organ reaches its highest
development amongst Ver-
tebrates. As in other ter-
restrial forms, however, a
posterior (upper) olfactory
region of considerable ver-
tical extent, and an anterior
(lower) respiratory region
can be recognised, in each
of which turbinals, with
their skeletal supports, are
developed (Figs. 196-198).
In the olfactory region
are found a series of true
olfactory ridges, or scrolls,
situated in the posterior
or median parts of each
nasal chamber. The muc-
ous membrane covering
these contains olfactory
cells and is supplied by
the olfactory nerve ; their
skeletal supports become
united later to the ethmoid.
In the respiratory portion,

Fig. 196b.— Sagittal Section through the
Nasal and Buccal Cavities of the
Human Head.

/, inferior (maxillary), //, middle, and ///,
superior turbinal ; Ik', entrance to mouth ;
Uj, tongue ; os, aperture of Eustachian
tube ; su', frontal sinus ; sn'', sphenoidal
sinus; v.i, atlas vertebra; v.ii, axis
vertebra.

268

COMPARATIVE ANATOMY

which communicates with the pharynx by the posterior nostrils,
the turbinals arise from the lateral walls of the chamber and are
developed later than the ethmoid turbinals : their skeletal frame-
work unites with the maxillary bone, while a less complicated
ridge, which may unite with the nasal bone, can usually be
recognised. The cthmotitrlinah project forwards between the
oiasoturlinnls and maxillohcrhincds; the two last-mentioned no longer
possess an olfactory epithelium, and have plainly undergone a
change of function in connection with the perception of the
warmth and moisture of the inspired air. When well-developed,
the maxilloturbinal forms a single or double coil, and may even
be more or less branched (Fig. 197) ; fibres of the maxillary division
of the trigeminal supply its mucous membrane.

The ethmoturbinals referred to above (endoturhinals) are peculiar
to Mammals, as are also certain accessory folds situated laterally
to them and also belonging to the ethmoid (Fig. 198) : these may

B

C

D

E

Fig. 197. — Various Forms of the Maxilloturbinal Bone in Mammals.

-^1, double coil ; B, transition from latter to single coil, E, F ', C, transition from
double coil to the dendritic form U. (After Zuckerkandl.)

be described as the postero-lateral or edohcrhiiials to distinguish
them from the endoturhinals and from the antero-lateral maxillo-
turbinals and nasoturbinals, which correspond to those of the
Sauropsida.

The ethmoid turbinals are arranged in a row more or less
parallel or obliquely to the palate : their number and relative
development varies considerably amongst Mammals and is propor-
tionate to the development of the olfactory lobes and sense of
smell. In Monotremes two extreme types are seen : Echidna
possesses a highly developed and complicated labyrinth of six or
more scrolls, while in Ornithorynchus the labyrinth is greatly
reduced in adaptation to an aquatic mode of life. A definite type
with five endoturhinals occurs in Marsupials, and this may be
taken as primarily typical for the Eutheria : it is approached most
nearly in Insectivores, in which there are from four to six, and a
very similar condition is seen in Hyrax, Bats, Carnivores, Rodents,

OLFACTORY ORGAN

269

and Leinui-s. In Ungulates, Elephants, and Edentates, further
complications have arisen, and the number of endoturbinals has
considerably increased secondarily {e.g. to nine in Orj^cteropus). In
Primates (Figs. 196 and 199), on the other hand, reduction has
occurred from the condition found in Lemurs.^

The ectoturbinals differ so greatly in the individual orders and
even species that they cannot be reduced to a common type. In
Marsupials, Insectivores, Hyrax and Bats, they are few in number,
while in Ungulates, Elephants, Carnivores, Seals, Edentates,
Rodents, and in Echidna, they are more numerous : in Ornitho-
rhynchus the ectoturbinals are entirely wanting, as is also the case
in most Primates and to a less extent in Lemurs.

According to the degree of development of the olfactory appa-
ratus, taking specially into account its cerebral portion (olfactory

Ectotui'biiials - -

Endoturbinnls

^"^ EiuiotiD-bhials

Septum na»i

Fig. 198.— Diagrammatic Transverse Section through the Nasal Cavity
OF A Mammal, to show the Relations of the Endo- and Ecto-
turbinals. (Modified from PauUi. )

lobes, &c., cf pp. 200, 228), we may distinguish between Mammals
which are macrosmatic (the majority of the mammalian orders),
microsmatic {e.g. Seals, Whalebone-Whales, Primates), and anos-
matic (most Toothed Whales).

Except in Monotremes, the nasal chamber communicates
with neighbouring cavities, such as the maxillary, frontal, and
sphenoidal sinuses (Figs. 196b and 199) : the two last-mentioned
cavities arise in connection with the nasal apparatus, and in forms
with a well-developed sense of smell may enclose olfactory folds ;

^ Primates possess one to three ethmoturbinals, but traces of as many as five
have been recognised in the embryo. The nasoturbinal in anthropoid Apes and
Man is also more or less reduced.

In addition to a reduction or entire degeneration of these parts in Cetacea,
the nose is shortened and the nostrils have a dorsal position, some distance back
from the apex of the snout : in the toothed forms they unite and open by a
single valvular aperture. In this Order a series of paired "nasal sacs" are
present under tlie skin, the function of which is not known.

270

COMPARATITE ANATOMY

but with the reduction of this sense, they lose their primar}^
function, often persisting merely as air-sinuses or even disap-
pearing entirely ^ (Pinnipedia).

The nasal glands of Mammals may be divided into two sets,
— numerous small, diffuse Botvmans glands, and a large Stensons
gland. The latter appears early in the embryo, and in many eases

undergoes reduction ; it is
situated in the lateral or
basal walls of the nasal
cavity, opening into the
vestibule of the nose, and
may extend into the maxil-
lary sinus.

The presence of an ex-
ternal nose (cf. p. 131), which
must be regarded as a de-
rivative of the outer nasal
chamber of Reptiles and
Birds, is very characteristic
of the olfactory organ of
certain Mammals, that of
Man being of a specialised
type not exactly comparable
to the so-called external
nose of other Mammals.
It is supported by an out-
ward extension of the nasal
bones and by the cartila-
ginous septum nasi which
arises from the ethmoid, by
the roofing lateral nasal
cartilages connected with
the septum, and by the
vomer, as well as by other secondarily independent cartilages
(alinasals), which were primarily continuous with the general
soHd cartilaginous wall, but become differentiated from the latter
in various ways in accordance with varied functional adaptations.
The outer nose contains a paired cavity (vestibule) continuous with
that of the olfactory chambers, and may be provided with a
complicated musculature, which in diving Mammals forms a
sphincter in connection with a special valvular apparatus for
closing the nostrils. An excessive development and increase in
the musculature, as well as an upward and backward shifting of
the nasal apertures in their relation to the skull, is seen in those

Fig. 199.— Transverse Vertical Section

THROUGH THE NaSAL CaVITY OF MaN.

a, h, Cj inferior, middle, and superior nasal
passage ; C.cr, cranial cavity ; HG, hard
palate ; /, //, ///, inferior (maxillary),
middle, and superior turbinal ; J, J,
position of vestigial Jacobson's organs,
which are situated nearer the floor of the
cavity than is indicated in the figure ; M,
maxilla ; Or, wall of orbit ; aS^, septum
nasi ; SL, ethmoidal labyrinth ; *, point
at which tlie nasolacrymal duct opens ;
t, entrance into the maxillary sinus (Cm).

^ The maxillary sinus is the most constant, and is typical for the Eutheria :
it usually extends into a number of neighbouring bones. In general, the pneu-
maticity of the skull is in direct proportion to the size of the animal : in Insecti-
vores and Bats the maxillary sinus is the only one present.

VOMERO-NASAL ORGAN 271

forms in which the external nose grows out to form a longer or
shorter trunk or proboscis, at the distal end of which the nostrils
open {e.g. Shrew, Mole, Pig,^ Tapir, Elephant). By means of its
abundant nerve-supply, the proboscis serves as a delicate organ
of touch and may even give rise to a prehensile apparatus
(Elephant). In the Ape Nasalis the peculiar and grotesque
external nose, with downwardly directed nostrils, cannot be
directly compared with the human external nose.

VOMERO-NASAL (JaCOBSON's) OrGAN.

By the term " Jacobson's organ " is understood a paired acces-
sory nasal cavity which in an early embryonic stage becomes
differentiated from the nasal chamber, and which is supplied by
the olfactory and trigeminal nerves. '

This cavity is first met with in Amphibians, but is wanting in
Proteus and Necturus. In the larvse of Anura and Myctodera a
small gutter-like medio-ventral outgi'owth of each nasal cavity
is formed, and in most Urodeles this later undergoes a relative
change of position, so as to be situated laterally (Fig. 200, a-d) :
at its blind end a gland is developed. The accessory nasal
chamber of Caecilians - (p. 264) is developed in a similar manner
(e), and a large gland is in connection with it.

The vomero-nasal organ of the Amniota is also developed in the
medio-ventral part of the nasal chamber, close to the septum nasi.
It loses its primary connection with the former, but in Hatteria,
Lacertilia, and Ophidia retains its median position, lying between
the floor of the nasal cavity and the roof of the mouth. It is lined
by an olfactory epithelium and communicates in front with the
mouth through the nasopalatine canal. A papilla usually extends
into its cavity from the floor (Figs. 194 and 260, f). These organs
are not present in Crocodiles, Chelonians, and Birds, but more or
less distinct indications of them have been described in the embryo
in some cases.

Amongst Mammals, Jacobson's organ is well marked in Mono-
tremes (Fig. 200, G), in which it is much more highly developed
than in most Lizards (except Monitors).^ It contains a well-marked,

^ The external nose may be further supported by a median pre-nasal bone
{e.g., Mole, Pig). In Chrysochloris it is capped by a horny shield, and in
Condylura its tlat disc is provided with numerous tactile appendages,

- A curious apparatus exists in the Gymnophiona in connection with the
nasal cavity and orbit. It consists of a retractile tentacle, a fibrous capsule with
muscles, and a large gland, opening near the snout. Its function is not thoroughly
understood, and the same is true of the tentacle-like "balancers" of larval
Urodela and Aglossa.

'■' In the Australian Bat, Miniopterus, it is even larger relatively than in
Monotremes.

g.m.

Fig. 200. — Sections of the Nose of Various Vertebrates.

A— D, Illustrating the various ontogenetic and phylogenetic stages of the Jacobson's organ of
Urodeles ; in A its position is median, and in D lateral. E, Gymnophiona, in which the
organ becomes separated from the main nasal cavity. F, Lacerta agilis. G, Ornithorhjaichus
(After Symington.) H, Placental Mammal ; I, the same, in longitudinal vertical section.

a, epiderm ; c.j, Jacobson's cartilage ; d.n, naso-lacrymal duct ; g.m, intermaxillary gland ; y.n,
nasal gland ; jc, Jacobson's organ ; mx, maxilla ; na, main nasal cavity ; li.o, olfactory'
nerve; ii.t, trigeminal nerve; o.d, dumb-bell shaped bone (prevomer) ; s/>, septum nasi.

EYE 273

turbinal-like ridge, supported by cartilage coDtinuous with that
enveloping the organ and covered with ciliated epithelium, and
numerous glands are present in the mucous membrane. In other
Mammals (h, i) it becomes more or less reduced, though often
well-marked, even in the adult, consisting of two tubes lying at
the base of the septum nasi, usually enclosed by separate para-
septal cartilages, which, as in Lizards and Monotremes, are
differentiations of the nasal septum (e.g. Marsupials, Edentates,
Insectivores, Rodents, Carnivores, Ungulates). A branch of the
olfactory nerve entere the tube posteriorly, and anteriorly the
cavity of the organ communicates with the mouth through the
incisive or naso-palatine canals. Vestiges of the organ exist even
in Man (Fig. 199>

The function of Jacobson's organ may be concerned with bring-
ing the food taken into the mouth under the direct control of the
olfactory nerve.

Eye.

As already mentioned, the first rudiment of the eye aiises as
a paired outgrowth from the primary fore-brain, known as the
prima')^ optic vesicle (Fig. 201, a). It, therefore, like the olfactory
lobe, represents a part of the brain, and in this respect differs
from the Invertebrate eye, which arises by a differentiation of the
cells of the superficial ectoderm.

At the point where the vesicle touches the ectoderm, the latter
becomes thickened, and the outer wall of the vesicle is asymmetri-
cally invaginated to form a double-walled cup, the secondary optic
vesicle (Fig. 201, B), at first open below at the slit-like choo^oid
fissure. The inner and outer walls of the cup then become
fused, the former giving rise in its deeper part to the sensory
epithelium of the retina, and the latter to the pigment epi-
theliitm, and also to the muscles of the iris, which are thus
of ectodermal origin (p. 275). As the optic vesicle grows out-
wards towards the outer skin of the embrj^o, the portion which
connects it with the brain becomes constricted and by degrees
loses its cavity, giving rise to a solid cord, the optic stalk. The
fibres of the optic nerve are first differentiated in the retinal
portion, and grow centripetally along the optic stalk towards the
brain ; centrifugal fibres also arise later.

In the adult brain, the optic nerve is seen to arise from the
diencephalon, its fibres extending upwards and backwards to the
optic lobes, and three more or less shai-ply-differentiated portions
of it may in most cases be distinguished ; these are spoken of,
from the proximal to the distal end respectively, as the optic tract,
chiasma, and neii^e.

274

COMPARATIVE ANATOMY

A chiasma, that is, a crossing of the fibres of the two optic
nerves, doubtless always occurs though not always freely exposed,
for it may retain a prijnitive position deeply embedded in the base
of the brain {e.g. Myxinoids, Dipnoans, and to a certain extent in
Petromyzon). In most Teleosts the optic nerves simply overlie
one another (Fig. 202, a), but in some of these Fishes (Clupea
(b), Engraulis), one nerve passes through a slit in the other, and

v/i

ATil

AB.^

Fig. 201, A.— Diagram showing the Mode of Formation of the Primary
Optic Vesicles [ABl).

VHy fore-brain ; V, V, ventricular cavity of the brain, which communicates with
the cavities of the primary optic vesicles at tf.

B. — Semidiagrammatic Figure of the Secondary Optic Vesicle, and of
THE Lens becoming separated off from the Ectoderm.

G, vitreous chamber of the eye, between the lens and retina, which later becomes
filled by the vitreous humour ; H, remains of the cavity of the primary optic
vesicle ; IB, inner layer of the secondary optic vesicle, from which the
retina arises ; f, point at which the latter is continuous with the outer layer
(AB), from which the pigment epithelium is formed ; L, lens, which arises
as a cup-shaped involution of the ectoderm {E) ; *, point of involution of
ectoderm to form the lens ; MM, mesodermic tissue, which at M^,M^, grows
in between the outer ectoderm and the lens as the latter becomes separated
off, and which gives rise to the cornea as well as to the iris

this condition of things is gradually carried still further in Reptiles,
until finally the fibres of the two nerves intercross in a very com-
plicated manner (c, d), giving rise to a sort of basket-work ; this is
finest and most delicate in Mammals, where its structure can only
be analysed by comparing a series of sections.

To return to the further development of the eye, the ectodermic
thickening mentioned above becomes separated from the ectoderm,
sinks more and more into the interior of the optic vesicle, and is
differentiated to form the crystalline lens (Fig. 201, b). It is usually

EYE

275

at first a hollow invagination, reminding one of a primitive sen-
sory organ of the Anamnia or of the primitive olfactory pit. On
the closure of the aperture of invagination, it forms a vesicle, the
thinner outer wall of which gives rise to the
so-called lens-epithelium, while the cells of
the thicker outer wall elongate to form the
transparent fibres of which the greater part
of the lens is composed.

The remaining space within the optic
vesicle becomes filled by tissue^ which ex-
tends through the so-called choroid fissure
(p. 273), and gives rise to the vitremts hotly or
hiLmour (Fig. 201, b). Blood-vessels also
extend into the vesicle in the same manner,
and others arise at its periphery, where a
definite vascular and pigmented membrane,
the choroid, is formed from the surrounding
mesoderm.

Internally to the lens, the choroid gives
rise to the ciliary folds, while more extern-
ally it passes in front of the lens to form
the iris, which retains in the centre a circular
or slit-like aperture, the pupil, through which
the rays of light pass (Fig. 203). The
amount of light admitted is regulated by
the dilator and constrictor (sphincter) muscles ^ig. 202.— Chiasma of
of the iris, which are able to increase or the Optic Nerves.
lessen the size of the pupil ; the iris thus
serves as a screen to regulate the amount of
light which enters the eye.

Not only are the size and form of the
pupil inconstant, but the lens is also capable chi, chiasma of the
of undergoing considerable change in relative bundle of nerves lying
position (e.g. Fishes, Amphibians, Snakes) or l^^''^s\\tev^l fibres,'
in form, becoming more flattened or more which do not cross,
convex, as the case may be (e.g. Mammals,
Birds, Lizards. Chelonians) : the former condition occurs when
distant, the latter when near objects are looked at. This
delicate accommodating apparatus in higher forms is regulated
by a ciliary muscle (tensor choroideo}) supplied by the oculomotor
nerve, which arises in a circle all round the eye from the point of
junction of the iris and sclerotic and is inserted along the peri-
pheral border of the iris.

Externally to the vascular layer of the choroid is a lymph-
sinus with pigmented walls (lamina fusca), and externally to this,
again, is a firm, fibrous, partly cartilaginous, or even ossified

^ By some eml)ryologists this tissue is said to be ectodermic, and not meso-
dermic in origin, except as regards the evanescent embryonic blood-vessels.

T 2

Semidiagrammatic.
A, most Teleostei ; B,
Herring ; U, Lacerta
agilis ; D, Agama ; E,
Mammal.

276

COMPARATIVE ANATOMY

layer, the sclerotic^ which is also surrounded by a lymph-space.
The latter passes internally into the sheath of the optic nerve,
which is continuous with the dura mater, and externally into the
cornea, the outer surface of which is covered over by an epithelial

layer continuous with the epi-

^z:c\

J'f^^

VK-

UK

derm — the conjunctiva. The
sclerotic and cornea together
form a firm outer support for
the eye, and thus, together with
the gelatinous mass of the vit-
reous humour, guarantee the
rigidity necessary lor the physio-
logical activity of the nerve
end-apparatus. Between the
cornea and iris there is a large
lymph-space, the aqueous cham-
ber, its contained fluid being
called the aqueaiis humoitr : ex-
tending around the chamber is
a venous plexus, which is bathed
by the aqueous humour.

The relative development of
the eye is affected by the ex-
ternal conditions,^ and is in
general proportional to the
rapidity of the movements per-
formed by the animal concerned
and to the relative development
of the mid-brain.

In all Vertebrates the eye-
ball is surrounded by a mem-
branous, sac-like investment
(the periorhita), which arises in
the region of the optic foramen
and radiates outwards towards
the skin, its distinctness being less marked the more completely
the orbit is surrounded by skeletal parts. As in most Vertebrates
the cavity of the orbit is more or less continuous with the temporal
and oral cavities, the orbital sac has a close relation to the muscles
of the jaws, certain portions of which may invade it and take on

* The adaptive modifications of the eye are very varied amongst Vertebrates.
It may reach a very high degree of perfection ; or may, on the other hand,
undergo more or less degeneration in those animals which live in caves or burrows,
or in the deep sea {e.g. Fishes — Amblyopsis speljeus, Troglichthys, Typhlo-
gobius ; Amphibians — Proteus, Spelerpes maculicauda, Typhlotriton, Typhlo-
molge, Gymnophiona ; Reptiles — Typhlops vermicularis, Rhineura floridana ;
Mammals — Notoryctes tj'phlops, Talpa). In the Cetacean Platanista gangetica
the eyes are extremely minute. The reduction of the eyes in many deep-sea
Fishes may be compensated for by the special development of tactile organs. (Cf.
also under Cyclostomes, p. 278).

Fig. 203. — Diagram of a Horizontal
Section through the Left Human
Eye, seen from above.

C, ciliary process ; Ch, choroid, with
its lamina fusca [Lf) and vascular
layer {GS) ; Cj, conjunctiva ; Co,
cornea ; CP, canal of Petit ; CaS',
venous plexus (canal of Schlemm) ;
Ov, vitreous chamber ; Fo, yellow
spot (fovea centralis) ; H, hyaloid
membrane ; HK, so-called posterior
chamber : /r, iris ; L, lens ; Zc,
ciliary ligament ; MD, posterior
elastic lamina (membrane of Des-
cemet) ; MF, blind-spot ; Op, optic
nerve ; OS, sheath of optic nerve ;
Rt, retina ; PE, pigment epithelium
of retina ; Sc, sclerotic ; VK, aqueous
chamber ; Z, Zonula ciliaris (zone of
Zinn).

EYE 277

functional relations to the optic apparatus (Amphibia, Sauropsida).
Only in the higher Mammals {e.g. Primates) is the orbit almost
completely surrounded by bone and thus separated from the
masticatory muscles, and in this case the periorbita is more or less
closely applied to or fused with the orbital periosteum.

The deep orbit, formed by the skull, serves as a further pro-
tection for the eye, in connection with which there are also certain
accessory structures, viz. : —

1. Eyelids (palpehw).

2. Olatididar organs.

8. Micscles for moving the eye-ball.

Thus the eye-ball is formed of a series of concentric layers,
which are called from within outwards — retina (sensitive and
nervous layer), choroid and iris (vascular layer), and sclerotic and
cornea (skeletal layer). The first corresponds with the nervous
substance of the brain, the second with the pia mater, and the
third with the dura mater. The interior of the eye contains
refractive media, viz., the lens and vitreo^cs Munoair. In addition
there are the above-mentioned accessory struct^ires : these, as well
as the retina, will be dealt with after a description of the eyes
of the various Glasses of Vertebrates has been given.

Before concluding this introductory section, it must be again
pointed out that the mode of development of the vertebrate eye
diffei's markedly from that of other sensory organs in not being
derived directly from the outer ectoderm, and in that the optic nerve
is not an ordinary peripheral nerve, but a conducting tract passing
between different parts of the central organ itself — that is, between
the retina and brain. On the other hand, it must be borne in
mind that the sensitive elements are developed in that part of the
retinal layer which is originally continuous with the outer border
of the ectoderm — that is, from the same layer as that which gives
rise to the other sensory organs and the central nervous system.

Amphioxus. — Paired eyes, comparable to those of the Craniata,
are wanting in the Acrania.

In addition to an unpaired pigment-spot in the front wall of the
cerebral vesicle, which is supposed to serve as a light-perceiving
organ, a series of simple cup-like structures, somewhat resembling
the eyes of Flat Worms, are situated on either side of the spinal
cord (Fig. 204). These structures are arranged in groups, corre-
sponding to the myomeres, and gradually becoming less numerous in
passing backwards to the tail. Each of these bodies is described
as consisting of a single optic cell with a nerve fibre, and is partly
surrounded by a pigment cell. More doreally, above the oral
region, somewhat similar, but pigmentless, bodies have been
observed.

278

COMPARATIVE ANATOMY

Cyclostomes. — The eye of Cyclostoines is at a very low stage
of developDient, find has evidently undergone partial degeneration :
this is indicated, not only by the structure of the retina, but also,
in Myxinoids, in which the choroid fissure persists, by the absence
of the lens, iris, and of a differentiated sclerotic and cornea as well
as of eye-muscles. Moreover, the eye in Myxinoids and in the

Eyes'

Dorsal zone oj " optic cells '

Anterior pigment-spot

Fig. 204. — A. — The Anterior Portion of Amphioxm lanceolatu«.
(Modified from H. Joseph.)

B. — One of the Eye-like Organs in A. (After H. Joseph.)

GK, capsule, with neuroglia elements ; K, nucleus of optic cell ; N, its nerve-
process, and t, its granular border ; Pig, pigment-cell, showing three layers ;
St, striated border of optic cell.

larval Petromyzon lies beneath the skin and subdermal connec-
tive tissue, but in the latter the skin covering the eyes becomes
thinned out, and thus the animal, which was blind, or nearly blind,
in the larval state, can see after undergoing metamorphosis : at the
same time the eye becomes more highly organised, though the
primary lumen in the lens (cf. p. 274) does not entirely disappear.

EYE

279

Fishes. — The eyes of all the true Fishes are, with few excep-
tions, of considerable size, and are formed on essentially the same
plan as that described in the introductory portion of this chapter.

The eye-ball is almost always surrounded by a gelatinous sub-
stance, penetrated by connective tissue fibres, and in many
Elasmobranchs it is articulated on its inner circumference with a
rod of cartilage connected distally with the lateral wall of the
skull. The sclerotic is usually extensively chondrified, and not
infrequently becomes calcified or ossified towards its junction with,
the cornea.

The lens of Fishes is globular, or nearly so, and possesses there-
fore a high refractive index. It touches the cornea and fills up the
greater part of the eye-
ball, so that only a small Or
space is left for the vitreous ; I ^p Ir
humour. It differs from a
that of other Vertebrates f
in the fact that, in the )<^^;;^f^"^^Kv^S

condition of rest, it is ac- ;5c. fej^^fliM''. ( .x^^,IS^-' )i

commodated for seeing llllfl vVj^^ -^^™

near objects. Fishes pos- T — "^^^m, T>t^

sess no ciliary muscle, and j\n^ ^^S"^!^! Cv /^/^"^Jl.

in many of them (most L5 -^ ^^^^^^^^>^ J^^^I^A

Teleostomi) accommoda- ^^^^^^^^K^^^^^

tion takes place by means t

of a process of the choroid,

the processits falciformis. '^Sr'' ^^w^- 05

This extends through the A^

embryonic choroid fissure "

into the vitreous humour Fio. 205.— Diagram of the Eye in a

towards the lens, around Typical Teleost.

which it expands to form Ag, argentea ; Co, cornea; Cp, campanula

the so-called campamda Halleri; Cv, vitreous body; Ir, iris; L,

EMeri, which is often 'ZL'^L^t O^^l^^, Os]

pigmented (Fig. 205). In sheath of optic nerve ; PE, pigment epi-

the interior of this Struc- thelium ; Pr, processus falciformis ; Rt,

tnrP am nprvp^ vpqspk retina; Sc, sclerotic, with cartilaginous

ture are nerves, vessels, ^^^ osseous (t) portions; Tp, tapetum;

and smooth muscle-fibres, VK, aqueous chamber,

and the latter exert an in-
fluence on the lens, drawing it towards the retina. The processus
falciformis is small in Ganoids and certain Teleosts, is apparently
absent in Cyclostomes and Dipnoans, and is probably represented
in Elasmobranchs, at any rate in the embryo : how accommodation
is effected in these Fishes is not known.

The iris of Fishes takes no part in accommodation, and only
appreciably reacts to light or electric stimuli in a few species living
in shallow water in which the eyes are directed upwards.

Externally to the choroid proper, and internally to the supra-

280

COMPARATIVE ANATOMY

choroideal lymph-space, is a silvery or greenish-gold iridescent
membrane, the anientca. It extends either over the whole interior
of the eye (Teleosts), or is limited to the iris (Elasmobranchs). A
second layer with a metallic lustre, the tapetitm lucidum, is present
in Elasmobranchs internally to the iridescent portion, and within
this again is the layer of the choroid known as the choriocapillaris.
No tapetiim appears to be present in Teleostoi or Pctromyzon.

Fig. 206.

-Longitudinal Vertical Section of the Eve of Dissomma.
( After A. Brauer. )

CA, choroid ; i'Vi, fibres of the argentea ; LK, lens cushion ; Dp, ligamentum
pectinatum ; M, smooth muscle ; o, optic nerv^e ; rr, accessory retina ; rr',
portion of accessory retina ; Sc, sclerotic.

The so-called " choroid gland" present in many Teleostei and
in Amia, consists of a network of blood-vessels (rete mirabile)
which has the form of a cushion, lying near the entrance of the
optic nerve, between the argentea and pigment epithelium of the
retina; it thus has nothing to do with a "gland," and is in relation
to the pseudobranch (cf under Respiratory and Vascular organs).

The eyes of certain deep-sea Teleosts are of particular interest,

EYE 281

as they are adapted in a remarkable manner both as regards position
and structure to the special conditions under which these Fishes
live in comparative darkness. In most Fishes, the laterally situated
eyes are capable of monocular vision only, while most deep-sea forms
possess binocular " telescopic " eyes, the axis of the two organs being
nearly parallel. The eye-ball, moreover, is much elongated (Fig 206),
so that there is a considerable distance between its inner wall and
the lens ; the latter is especially large, and the cornea is very
convex and forms a considerable part of the wall of the eye-ball ;
the iris is almost entirely wanting ; and finally, the retina is differ-
entiated into a main portion and a small accessory portion — usually
situated only on that part of the wall which is nearest to the
median line of the head. The eye-muscles are more or less reduced
and shifted in position, while the apparatus for accommodation, the
suspensory ligament and retractor of the lens, are well developed.
These modifications result in allowing as many rays of light as
possible to enter the eye and spread out over the retina.^

In the Flat-fishes (Pleuronectidae), which have acquired the
habit of swimming and lying on the bottom on one side, the eye of
the lower side gradually rotates so as to reach the upper surface.

In Dipnoans the eye is relatively small as compared with that
of other Fishes. In Protopterus the sclerotic is partly cartilaginous,
and the lens is globular and relatively large. As already mentioned,
there is apparently no processus falciformis,and a tape t u m, argen tea,
and " choroid gland " are wanting. On the whole, the eye is inter-
mediate in structure between that of Ganoids and that of Urodeles.

Amphibians. — The eyes of Amphibians are in general
relatively small, and do not exhibit any essential advance in
structure as compai-ed with those of Fishes : in certain respects
they show negative character as compared with the latter, for an
argen tea, a tapetum, a " choroid gland," and a processus falciformis
and campanula Halleri are wanting.

The eye-ball is nearly globular and the cornea moderately
convex ; the pupil is round, or occasionally three-cornered (Bom-
binator). The large lens is more convex on its inner than on its
outer surface, especially in Anurans, and the larvae usually have a
smaller lens-index than adults. Accommodation does not take place
by an alteration in form of the lens, but in some cases, apparently,
the lens can be shifted towards the cornea by the action of the
ciliary nuiscle.'- Ciliary processes can be recognised in Urodeles,
but are much more distinct in Anurans, and the iris has well-
developed smooth muscles. The sclerotic, as in Fishes, encloses

' In Periophtlialmiis and Boleophthalmus the structure of the eye is pecu-
liarly moclitied for vision in the air as well as in the water.

- A protractor of the lens, consisting of smooth fibres, occurs amongst
Urodeles and in the Frog, the fibres, however, taking a different course in the
two cases. It is not known whether this muscle is the homologueof the retractor
of the lens in Fishes.

282 COMPARATIVE ANATOMY

hyaline cartilage and is often pigmented : ossifications are not
known to occur.

Reptiles and Birds. — In the Sauropsida, more especially in
Birds, the eye-ball is much larger relatively to the head than in
the Amphibia. The sclerotic here, too, is in great part cartilagin-
ous, and in Lizards and Chelonians is provided with a ring of
delicate Jbony scleo'otic plates around the external portion. Many
fossil Reptiles and Amphibians possessed similar plates, as do also
existing Birds (Figs. 207, 208) ; in Birds horseshoe-shaped or ring-
shaped bony structures are also usually present
close to the entrance of the optic nerve.

The eyeball of Reptiles has a more or less
globular form, while that of Birds, more especially
nocturnal Birds of prey (Owls), is more elongated
and tubular, an external larger segment being
sharply marked off from an internal smaller one,
the outer portion being bounded externally by
Fig. 207. — Eye the very convex cornea and enclosing a large
ov Lacertamu- aqueous chamber: moreover, the whole eye is
THE ' Ring of I'^latively larger (Fig. 208). In all the Saurop-
BoNY ScLERo- sida, except Snakes, there is a complicated ciliary
TIC Plates. muscle composed of striped fibres, one portion of

which, extending towards the cornea, is known
in Crampton's muscle ; ciliary processes are well developed, especi-
ally in Birds. This muscle is also transversely striated in Reptiles,
and, especially in Chelonians, is always well developed, though not
to such an extreme degree as in Birds.

In Reptiles {e.g. Lizards) a tapetum may be developed, but
an argentea and " choroid gland " are never present ; all these
parts are wanting in Birds. A cone-like or cushion-like structure,
the so-called " pecten," which is to some extent homologous with
the processus falciformis of Fishes, is, however, present in Lizards,
but is more or less reduced in other Reptiles. It varies much in
its development in different forms, is very vascular and some-
times pigmented, and extends into the vitreous chamber through
the proximal end of the choroid fissure. In Birds,^ a somewhat
similar organ is very largely developed, and in some cases may
even extend as far as the capsule of the lens : it is four-cornered,
more or less folded, and pigmented, and consists mainly of a
closely-felted network of capillaries. From its form, it is better
described in Reptiles as a " cone " or " cushion " and in Birds as a
''fan " (Fig. 208). In both Reptiles and Birds this organ appears,
amongst other possible functions, to be important in connection
with the nutrition of the contents of the eyeball and of the retina :
it has nothing to do with accommodation.

The iris, regulated by striated muscle, is able to respond very

1 In Apteryx the pecten disappears during development.

EYE

283

quickly to visual impressions: it is often brightly coloured,

owing to the presence, not only of pigment, but also of

coloured fat globules. The pupil is as a rule round, but in many

Reptiles and in Owls has the form of a vertical slit. The form,

structure, and relative size of the lens varies considerably :

amongst Reptiles, its refractive index is lowest in Snakes, and

highest in Lizards (except

Geckos). In many Birds it has

a very peculiar form. The

mechanism of accommodation

in some Snakes resembles that

seen in Amphibians (p. 281),

while in the other Sauropsida

the ciliary muscle, acting on

the capsule of the lens, is able

to alter the curvature of the Ch

latter, as is also the case in

Mammals.

Mammals. — In Mammals
the eyeball, especially in Prim-
ates, is more completely enclosed
within the bony orbit than is
the case in most other Verte-
brates, and this may partially
account for the fact that, except
in Monotremes, the sclerotic no
longer shows traces of cartilage
or bone, but is entirely of a
fibrous character : in Whales it
is extremely tough. Excepting
in aquatic Mammals, in which
it is somewhat flattened, the
cornea is moderately convex, and
the whole eyeball is of a more
or less rounded form.

A tcqjctum lucidum, consisting either of cells or fibres, exists in
the choroid of numerous Mammals, and gives rise by interference
to a glistening appearance when seen in the dark (Carnivora,
Ruminants, Perissodactyla, &c.). In some Manmials there is a
differentiation of the capillary vessels (membrana choriocapillaris)
from the larger arterial trunks of the choroid : the veins are
situated externally to the arteries. Certain structures homologous
with the processus falciformis or pecten are present in Mammals in
the embryo only.

The ciliary muscle consists of smooth elements. The external
surface of the lens is less convex than the internal, which latter lies
in the so-called fossa patellaris of the vitreous humour. The pupil

Fig. 208.— Eye of an Owl.

Ch, choroid ; CM, ciliary muscle ; Co,
cornea ; Cv, vitreous chamber ; /r,
iris ; L, lens ; Op, OS, optic nerve
and sheath; P, "fan" (pecten); Bt,
retina ; Sc, sclerotic, with its bonj'
ring at t ; VK, aqueous chamber ;
VNy point of junction between
sclerotic and cornea. The dotted
line around the broadest portion of
the circumference, divides the eye
into an inner and an outer segment.

284 COMPARATIVE ANATOMY

is not always round, but may be transversely oval {e.g. Ungulates,
Kangaroos, Cetaceans), or slit-like and vertical {e.g. Cat).

Retina.

The fibres of the optic nerve, which pass into the eyeball at a
right or acute angle, cross one another at the point of entrance,
and are then distributed to the sensitive elements of the retina.
The latter is thus thickest at the point of entrance of the nerve,
which is known as the "blind spot" (Fig. 203), and gradually
decreases in thickness towards the ciliary processes until, at the
point of origin of the iris, it consists of a single layer of cells.

The retina is bounded on its outer periphery by a structureless
hyaline membrane {limitans externa)} while on its inner side it is
covered by the hyaloid membrane, which, strictly speaking, belongs
to the vitreous humour. The retina is quite transparent in the
fresh condition, and consists of two portions which are histo-
logically and physiologically quite distinct : they are, a suiJ][)orting
part and a nervous ^m?-^. The former is stretched as on a frame
between the limitans externa and hyaloid membrane.

The nervous elements are arranged in the following concentric
layers : —

I. Developed from the internal layer of the secondary o^Mc vesicle,

A. Cerebral layer.

1. Layer of nerve-fibres (of optic nerve).

2. Layer of ganglion-cells.

3. Inner reticular layer.

4. Granular layer (inner).

5. Outer reticular or subepithelial layer.

B. E2nthelial layer.

6. Layer of visual cells (outer granular layer with the rods

and cones).

II. Developed from the external layer of the secondary optic vesicle.

7. Pigment epithelium (retinal epithelium).

These layers are so arranged that the nerve-fibres lie next to
the vitreous humour, that is, internally, while the rods and cones
are situated towards the choroid, i.e. are external. Thus the terminal
membeis of the neuro-epithelium are turned away from the rays
of light falling upon the retina, and the rays must therefore pass
through all the other layers before they reach the rods and cones. "^

' The limitans externa encloses the entire retina externally in the embryo, but
later the rods and cones come to project tlirough it (cf. Fig. 209).

^ Fishes possess the longest, Amphibians the thickest rods, so that in the
latter there are only about 30,000 to a square millimetre, while in Man there are
from 250,000 to 1,000,000. In Fishes, Birds, and Mammals the rods (which are
the phyletically older structures) far exceed the cones in number, while in
Reptiles the reverse is the case : in Petromyzon and Elasmobranchs, for example,
there are no cones at all. In some Reptiles there are no rods, and in nocturnal

EYE

285

In the retina of all Vertebrates there is a specially modified
region of most acute vision. This is called the yellow-spot {fovea
centralis or macula hctea), and is due to the thinning-out of all the
layers except that of the neuro-epithelium, and even the rods
disappear, only the cones persisting (Fig. 203). A yellow spot
is wanting in most Fishes, in Urodeles, and in certain Mammals
(e.(j. Insectivores, Rodents), and is only slightly marked in the

Fibrous
basket-work.

Concentric \
supporting-cells*^^
(nucleated).

Concentric
supporting-cells
(non-nucleated).

Radial fibres.

Radial fibrous -
cone.

Rods and cones.
/ Meiiibraua
■— 1 limitans.

Nuclear layer of
roils and cones.

Outer reticular

laj-er.

( Sub-epithelial

\ ganglion-cell

Star-sliaped
ganglion-cell.

Bipolar \

ganglion-cell. /

Multipolar \
ganglion-cell. J

uner reticular
layer.

Centrifugal
nerve fibres.

Multipolar
ganglion-cell.

Layer of
nerve fibres.

Fig. 209. — Diagram of the Elements of the Retina. (Supporting elements
on the left, and nervous elements on the right.) (After Ph. Stohr.)

Anura and most Sauropsida, but in Birds there may be two
" yellow spots." Only in Mammals does the retina possess special
blood-vessels, and even in certain of these they may be reduced
or wanting. In other Vertebrates (except the Eel) the retina is
non-vascular, and is nourished by the vessels of the vitreous body,
processus falciformis, or " pecten."

animals the rods are more numerous than the cones. The cones of many Reptiles,
of Birds, and of Marsupials, are distinguished by the presence of brightly
coloured oil-globules.

286 COMPARATIVE ANATOMY

Accessory Organs in Connection with the Eye.

{a) Eye-Muscles.

The movement of the eye-ball is in general (except in Myxinoids,
cf. p. 278) effected by six muscles, four of which are known as the
Q^ecti (superior, inferior, external or posterior, and internal or anterior),
and two as the ohliqui (superior and inferior). The former, which
arise from the inner portion of the orbit, usually from the dural
sheath of the optic nerve, together circumscribe a pyramidal cavity,
the apex of which lies against the inner portion of the orbit, while
the base surrounds the equator of the eye-ball, where the muscles
are inserted into the sclerotic.

Both the oblique muscles usually arise from the anterior or nasal
side of the orbit, and as they respectively pass from this region
dorsally and ventrally in an equatorial direction round the eye-ball,
they constitute a sort of incomplete muscular ring.^

Besides these six muscles, others are usually present from the
Amphibia onwards. Of these, the retractor hulbi, which often consists
of several portions, is derived genetically from the posterior rectus,
and is supplied by the abducent nerve. In Amphibians, Reptiles,
and Birds, a portion of the striated masticatory musculature,
supplied by the trigeminal, extends into the fibrous walls of the per-
iorbita (p. 276), and in Rana, for example, gives rise to an elevator of
the eye-ball, a depressor of the lower eyelid, and a kind of extensor
of the lower wall of the orbital sac, the connection of which with
the masticatory muscles is plainly seen. In Anurans there is also a
more ventral, transverse layer, corresponding to the depressor of the
lower lid in Lizards and Birds, in which latter there is also a layer
in the orbital membrane corresponding to the depressor of the lower
orbital wall in Amphibians.

As the transversely striped periorbital muscles gradually be-
come of less importance in the Vertebrate series, the smooth
muscles are further developed. The latter are already indicated in
Teleosts ; but in Reptiles (especially Lizards and Chelonians) are
much more marked, and are continued into the eyelids. In
Mammals, the development of the smooth orbital musculature varies
considerably, and is less marked the more the orbit is enclosed by
bone (e.g. in Primates, in which it lies in the infra-orbital fissure).
It, like the upper and lower palpebral muscles, which are
specially important in Mammals, are supplied by the sympathetic.

^ A deviation from this arrangement is seen in Mammals, in which the
superior oblique has gradually come to arise from the inner part of the orbit, and
then passes forwards towards its anterior (internal) angle, where it becomes
tendinous, and passes through a fibro- cartilaginous pulley (trochlea) attached to
the upper border of the orbit, on the frontal bone. Hence it is sometimes called
the trochlear muscle. From this point it changes its direction, and becomes
reflected obliquely outwards and backwards to the globe of the eye.

EYELIDS 287

(h) Eyelids (Palpebr^).

In Fishes and other lower aquatic forms the upper and lower
eyelids are usually very rudimentary, having at most the form of
stiff folds of the skin, and in all other Vertebrates below the
Mammalia they never reach a very high stage of development :
even in those Mammals in which the facial muscles are not much
differentiated they are not greatly specialised. They are lined
on the inner surface by a continuation of the epiderm, the con-
junctiva, which is continued over the cornea (Fig. 208),^ and in
the Ichthyopsida are usually not sharply marked off from the
rest of the skin, and are capable at most of very slight movement :
the eyelids in these forms therefore only protect the eye to a
slight extent, and allow it comparatively little free movement.'-

In the Sauropsida they are in certain respects much more highly
differentiated. Occasionally (Chameleon) they are circular and are
movable by muscles, while in Geckos, Amphisbaenians, and Snakes
the two eyelids gi'ow together to form a transparent membrane
overlj'ing the eye, and this comes away with the rest of the outer
part of the skin when it is shed.

A levator of the upper lid, which latter is usually the better
differentiated of the two and which in many Reptiles and Birds
may be supported by a membrane-bone or fibro-cartilage, occurs in
Chelonians, Crocodiles, Birds, and Mammals. Lizards, Chelonians,
Birds, and many Mammals (e.g. Ungulates) possess a depressor of
the lower lid.

In Mammals, the eyelids, more particularly the upper one, are
extremely movable, and may be provided with hairs (eyelashes)
on their free margin. In their interior a hard body, the so-called
" lid-cartilage," enclosing the Meibomian glands, is developed
(Fig. 210, b), and they are closed by a circular muscle (orhicidaris
or sphincter oculi) which suiTounds the whole slit between the lids,
and is a derivative of the mimetic musculature.

The want or comparatively slight development of upper and
lower eyelids in Vertebrates below the Mammalia is compensated
for in certain forms, at any rate to a certain extent, by the
presence of a nictitating memhrane? This ''third eyelid" di^ev^
from the others in having nothing to do with the outer skin proper,
consisting simply of a reduplicature of the conjunctiva, and being

^ In Fishes and Amphibian larvge the conjunctiva retains essentially the
same structure as the epiderm from which it is derived, while in higher forms it
undergoes modifications, and the stratum corneum becomes less horny.

- Accessory lid-folds occur, e.g., in the Herring and Salmonidse.

' The structure known as a " nictitating membrane " in many Elasmobranchs
is not exactly comparable to the nictitating membrane of higher Vertebrates. It
is secondarily^ derived from the lower lid-fold, and is provided with several
muscles, supplied by the trigeminal (or facial) nerve, and derived from integu-
mentary muscles in the region of the spiracle.

288 COMPARATIVE ANATOMY

regulated by special muscles supplied by the abducent nerve, and
known in Reptiles and Birds as the quadratus (hursalis) and
pyramidalis, which are genetically related to the retractor bulbi.

The nictitating membrane, which may enclose a cartilage, is
situated within the lower eyelid, or it may lie more towards the
anterior angle of the eye. The former condition is seen, e.g., in
Anurans and the latter in Birds, in which a third eyelid is so largely
developed as to be capable of covering the whole freely exposed
portion of the eye-ball. In Mammals also it is situated in the
anterior angle of the eye, and in Primates becomes reduced to a
small, half-moon-shaped fold (plica semilunaris).

(c) Glands.

The glands in connection with the eye are : (1) the lacrimal,
(2) the Sarderian, or gland of the nictitating membrane, and (3)
the Meihomia7i glands.

The secretions of all these serve to keep the free surface of the
eye-ball moist, and to wash away foreign bodies. In Fishes the
outer medium appears to suffice for this purpose, but the first
attempt of a Vertebrate to exchange an aquatic for an aerial
existence necessitated the development of a secretory apparatus in
connection with the eye.

Thus in Urodeles a glandular organ is developed from the con-
junctival epithelium along the whole length of the lower eyelid ; in
Anurans and Reptiles this becomes more developed in the region
of the anterior, and in many Reptiles also of the posterior, angle
of the eye, the original connecting bridge gradually disappearing:
thus two glands are developed from the primitively single one,
each of which becomes further differentiated both histologically
and physiologically. From one is formed the Harderian gland}
which is situated at the anterior angle of the eye, surrounding
to a great or less extent the an tero- ventral portion of the eyeball,
while the other gives rise to the lacrymal gland (Fig. 210). The
latter retains throughout life its primitive position at the posterior
angle of the eye, and even in Birds lies in the region of the
lower eyelid, and is supplied by the second division of the tri-
geminal. In Mammals it becomes gradually further subdivided,
and extends into the region of the upper eyelid, so that its ducts
open above the eye into the upper conjunctival sac. Neverthe-
less, even in the Primates, in which it consists of two parts, more
or fewer ducts are present which open into the lower conjuctival
sac, and thus the primitive position of the lacrymal gland is
indicated.

The secretion usually passes by several ducts into the con-

* In Crocodiles, Snakes, and Hatteria the lacrymal gland is wanting, while
in Chelone it is extremely large.

GLANDS OF THE EYE

289

junctival sac, where it would accumulate were it not for the fact
that the movement of the lids drives it towards the anterior (inner)
angle of the eye, where the intncta lacrymalia are situated, often on
small papillae. These lead into short ducts communicating with
the so-called lacrymal sac which opens into the naso-lacrymal duct.
A well-differentiated Harderian gland, sometimes consisting of

Tff

Fig. 210. — A. Harderian Gland {H, H^) and Lacrymal Gland {Th) of

Anguis fragilis.

B, eyeball ; M, muscle of jaw.

Fig. 210.— B. Diagrammatic Transverse Vertical Section thbough the
Eye of a Mammal.

B, ej-eball ; Fo, Fo, upper and lower conjunctival sac ; H, IT, eyelashes ; LH,
LIT, outer skin of the eyelids, which at the free edges of the latter, at t,
becomes continuous with the conjunctiva ; Op, optic nerve ; T, the so-called
tarsal fibro-cartilages, in which the Meibomian glands (MD) lie embedded,
the latter opening at *.

Fig. 210.— C. Diagram of the Lacrymal Apparatus of Man.

Z>, naso-lacrymal duct ; S, lacrymal sac ; TD, lacrymal gland, divided up into
several portions; TB, T^i, upper and lower lacrymal canals; **, ducts of
the lacrymal gland ; ft, puncta lacrymalia.

histologically and physiologically different elements, is present
from the tailless Amphibia to the Mammalia, but is rudimentary
in the Primates.

Th.Q Meibomian glands (Fig. 210, b) are confined to the Mammalia,
and lie embedded in the substance of the eyelids in the form of
branched, tree-like tubes or clustered masses. They open on the
free edge of the lid, produce a fatty secretion, and originally

u

290

COMPARATIVE ANATOMY

correspond to true sebaceous glands developed in connection with
hair-rudiments, the hairs disappearing but the glands remaining.
They are wanting in some Mammals {e.g, Monotremes, Armadillo,
Manis, Dolphin, Seal, Elephant, Camel). Certain modified sweat-
glands knoAvn as the glands of Moll are also present within the
eyelids of Mammals, opening on the margins of the lids close to
the eyelashes.^

Auditory Organ.

Certain relations to the integumentary sense-organs of Fishes
and Amphibians can be traced in the organs of taste (p. 254), and the
same is true of the auditory organ, the function of which is con-
cerned with equilibration as well as with
hearing : in all three cases the sensory
epithelium is derived directly from the
ectoderm.

The first trace of the auditory organ
is seen as a thickening of the ectoderm
(auditory p. ate) on either side of the
primary hind-brain between the trigeminal
nerve and vagus group, which becomes
invaginated and separated off to form a
vesicle (Fig. 211), the epithelium of which
eventually undergoes differentiation into
elongated cells of sensory epithelium pro-
vided with hair-like processes, and sup-
porting cells (Fig. 212), as in that of
the integumentary sense-organs referred
to above. The sensory cells are in rela-
tion with nerve-fibres which arise from
the auditory ganglion.

After the vesicle of either side has
become separated off from the ectoderm,
it sinks deeper and deeper into the meso-
dermic tissue of the skull, loses its original
P3n:iform or rounded shape, and becomes
divided into a superior and an inferior
part, called respectively the utricuhcs and
saccuhis, at first connected with one another
by a wide utriculo-saccular canal (Fig. 213). From the former the
so-called semicircular canalshecome developed, while from the latter
the tube-like ductus endolymphaticus and the lagena {cochlea) are
formed.

^ In Cetaceans the whole lacrymal apparatus has undergone degeneration in
adaptation to the external conditions, and the nictitating membrane is vestigial :
well-developed lacrymal and Harderian glands are present in the embrj^o. A
greater or less reduction of the lacrymal gland may also occur in other Mammals,
e.g. Seal, Hippopotamus, Elephant, Otter, and Mole.

Fig. 211.— Head and An-
terior Portion of Body
OF A Chick. (In part
after MoMenhauer. )

A , eye ; LB, primitiv^e
auditory vesicle seen
through the wall of the

. head ; RG, olfactory pit ;
t, point at which the ex-
ternal auditory passage
begins to be formed ; /
to IV J first to fourth
visceral arches.

AUDITORY ORGAN

291

The whole of this complicated apparatus constitutes the internal
ear or memhranmis lalyrinth. It becomes surrounded secondarily
by mesodermic tissue, which is at first gelatinous and in close
contact with it. A process of absorption then takes place in the
innermost layers of the mesoderm, and thus a space is developed
which closely repeats the form of the membranous labyrinth, as
does also the mesoderm which encloses this space and which later

Fig. 212.— Isolated Elements of the Membranous Labyrimh of vakious
Vertebrates. (After G. Retzius. )

\, from the macula acustica communis of Myxine glutinosa ; B, from the macula
acustica neglecta of Baia clavata ; C, from the crista acustica of an ampulla
of Amhlystoma ; D, from the crista acustica of the anterior ampulla of
Bana esculenta.

'z, thread-like cells; hz, hair- cells with auditory hairs {h) ; n, nerve. On the
left side of D the auditory hair has become broken up into its constituent
fibres.

becomes chondrified, and often also ossified. A membranous and a
cartilaginous or ho7iy labyrinth can thus be distinguished, and
between them is a cavity {cavum perilympkaticum) tilled with a
lymph-like fluid (perilymph), and penetrated by connective tissue
and blood-vessels extending from its walls to the membranous
labyrinth. The cavity within the latter, which also contains a
fluid {endolym'ph), is spoken of as the cavum endolymphaticum.

In Amphioxus, an auditory organ is wanting. In all Craniates,
except Cyclostomes (p. 295), three semicircular canals are present,

u 2

292

COMPARATIVE ANATOMY

and these lie in planes roughly at right angles to one another.
They are distinguished as the anterior vertical, the posterior vertical,
and the horizontal (external) canal (Fig. 213). The first and last-
named arise from the portion of the utriculus known as the recessus
utriculi, and at its origin each has a vesicle-like swelling or
ampulla, enclosing sensory cells. The posterior canal also arises
with an ampulla from a prolongation of the utriculus. The other
end of the horizontal canal opens by a funnel-shaped enlargement

into the utriculus, while that of
the anterior and of the posterior
canal unite to form a common
tube, the so-called canal commis-
sure (sinus superior), Avhich also
opens into the utriculus.

Concretions composed mainly
of carbonate of lime are present
in the regions of the various
nerve end-plates of the auditory
organ in all Vertebrates. These
otoliths, which arise in the epi-
thelium lining the organ, and
are then set free into its cavi-
ties, present the greatest variety
both in form and size. The
largest and most massive ones
are seen in Teleosts. They
either consist of a single masS;
or separate otoliths are present
in different regions of the laby-
rinth.

The sensory epithelium, to
which the branches of the audi-
tory nerve are distributed, is
situated in the following parts of
the m embranous labyrinth : ( 1 ) the
three ampullae of the canals, in
each of which the auditory cells
are situated on a ridge (crista
acustica) projecting into the lumen
(Fig. 214); (2) the utriculus
and the recessus utriculi ; (3) the sacculus and lagena, or rudiment
of the cochlea ; (4) the rudimentary macula acustica neglecta, which
in Fishes, Birds, and Reptiles is situated on the floor of the
utriculus close to the sacculo- utricular canal, in Amphibians on the
inner side of the sacculus, and in Mammals undergoes a gradual
reduction and may even become obliterated.^ The several portions

^ In addition to these, there is a transitory macula ductus reunientis situated
in the region of the sacculo'Cochlear duct.

Fig. 213.— Semidiagrammatic Figure
OF THE Left Membranous Laby-
rinth OF a Vertebrate. As seen
from the outer side.

ass, apex of the sinus utriculi superior ;
ca, ce, cp, anterior, external, and
posterior semicircular canals ; aa, ae,
ap, the corresponding ampullee ; cus,
utriculo-saccular canal ; de, se, ductus
and saccus endolymphaticus, the
former arising from the sacculus at
t ; I, recessus sacculi (lagena) ; rec,
recessus utriculi ; s, sacculus ; sp,
sinus utriculi posterior ; ss, sinus
utriculi superior ; u, utriculus.

AUDITORY ORGAN

293

of the sensory plate or macula acustica, which are originally con-
tinuous, become disconnected from one another later, and from
Teleosts onwards are seen as separate maculse acusticae.

A number of the regions characterised by the possession of
this sensory epithelium are not concerned with the sense of
hearing : those of the ampullae, for instance, and probably those
situated in the utriculus, are to be looked upon as organs of
equilibration, serving for the orientation of the individual in space.
The fact that the above-mentioned parts of the membranous
labyrinth are phylogenetically very ancient structures, and also
that the organ in Invertebrates to which the function of hearing
was formerly ascribed has now been proved to be concerned mainly
with equilibration, indicates that the latter function was the primary

Fig. 2U.— Longitudinal Section of an Ampulla of Gobius. (After Hensen.)

a, base of semicircular canal ; h, point of opening of the ampulla into the alveus
communis ; c, epithelium on the wall of the ampulla ; rf, auditory hairs ; 7t,
nerve passing into the connective-tissue of the crista.

one in the vertebrate membranous labyrinth. Later, on the gradual
appearance of the cochlea, a division of labour took place, the
more ancient part of the labyrinth retaining its primitive function,
while the new portion by degrees became differentiated into
a specialised auditory organ serving for the analysis of sound
vibrations.

The higher we pass in the Vertebrate series, the greater share
does the mesoderm take in the formation of the auditory organ.
At first — that is, in Fishes — the auditory capsule lies close to the
surface, and is thus easily accessible to the waves of sound, which
are conducted partly through the operculum (when present), and
partly through the gill-cavities or spiracle. In the higher animals
the auditory organ gradually sinks further and further inwards

294

COMPARATIVE ANATOMY

from the surface, so that a new method for conducting the sound-
waves to the internal ear is necessary, and certain accessory

Fig. 215.— Diagram of the Entire Auditory Organ of Man.

External Ear.— Mae, external auditory meatus; M, M, pinna; Mt, tympanic
membrane ; 0, wall of meatus.

Middle Ear.—Ct, Ct, tympanic cavity ; M, fenestra rotunda (cochleae) ; 0\ wall
of tympanic cavity ; 0", wall of Eustachian tube ; SAp, sound conducting
apparatus, drawn in the form of a rod, representing the auditory ossicles ;
the point t corresponds to the stapes, inserted into the fenestra ovalis
(vestibuli) ; Th, Eustachian tube ; Tb\ its opening into the pharynx.

Internal Ear, with the greater part of the bony labyrinth [KL, KL^) removed. —
a, b, the two vertical canals, one of which {h) is shown cut through : the
horizontal canal is seen between 2 and S ; c, Co, commissure of the canals of
the membranous and bony labyrinths respectively ; Con, membranous cochlea,
whicli ends blindly at 4- ; Cou^, bony cochlea ; Cp, cavum perilymphaticum ;
Cr, canalis reuniens ; B.p, ductus perilymphaticus, which arises from the
scala tympani at (/, and opens at D.p^ ; S, sacculus ; S.e, D.e, saccus and
ductus endolymphaticus ; the latter bifurcates at 2, where its outline should
have been dotted, so as to indicate that it opens on the inner side of the
lab3'rinth ; So and (S*;, scala vestibuli and scala tympani, which at * pass
into one another at the cupula terminalis {Ct).

structures become developed (Fig. 215). In the first place, a canal,
developed in the position of the hyomandibular or spiracular cleft/

1 The cavity of the middle ear is doubtless derivable in the first instance
from the spiracular cleft which in Elasmobranchs is in close relation to the ear,
and thus seems to be particularly well adapted for conducting sound waves. In-
dications of the former relation of the tympanic cavity with the respiratory func-
tion are seen in the Frog, in which the cutaneous branch of the pulmonary artery
sends a large branch to its mucous membrane. The morphology of the tym-
panum itself is not clear, as it is developed secondarily and is apparently not strictly-
homologous in Anura, Sauropsida, and Mammajij!,,

AUDITORY ORGAN 295

takes on a close relation to the auditory apparatus, and gives rise
to a spacious chamber, the tympanic cavity or middle ear, communi-
cating with the pharynx by the EvjStachian hihe, and being closed
externally by a vibratory tympanic membrane, between w^iich and
the auditory capsule a sound-conducting apparatus (columella or
auditory ossicles, pp. 98, 133) extends. In higher forms still, the
tympanic membrane is situated more deeply at the base of an
external auditory meatus or passage, to which in Reptiles the first
indications of a muscular fold of the integument are added. Only
in Mammals, however, does the meatus play an important part, and
in them a typical pinna or auricula is developed, which, with the
meatus, constitutes what is known as the external ear.

Cyclostomes. — In Petromyzon there are only two (vertical)
semicircular canals, which unite together in a common section, the
so-called commissure ; in Myxine only one canal is present, but
this, as it possesses two ampullae, probably represents the two
united with one another (Fig. 216, a).

Fishes. — The auditory organ of all true Fishes (Fig. 216, A^-c)
follows the general plan given above, and the same may be said for
all the higher Vertebrates, Almost without exception we meet
with a division into a pars superior — represented by the utriculus
and semicircular caiuds, which remains essentially much in the
condition already described, and a pars inferior — constituted by
the saccidus and cochlea, which gradually becomes more differ-
entiated, and attains a higher and higher degree of development
and functional perfection. In Fishes, the lagena or cochlea consists
simply of a small knob-like appendage of the sacculus, which opens
freely into the main cavity of the latter by means of the saccule-
cochlear canal : it is absent in Chimaera. The utriculus and sacculus
also communicate with one another by the saccule-utricular canal.^
In Elasmobranchs the ductus endolymphaticus opens to the
exterior dorsally, and is thus in free communication with the sea-
water.' In certain Teleosts, in addition to a large otolith situated
in the sacculus and smaller ones in the lagena and recessus utriculi
respectively, calcareous masses may be present in outgrowths from
the sacculus.

In Chimaeroids, Ganoids, Teleosts, and Dipnoans the auditory
capsules are not completely surrounded by cartilage or bone, the
perilymphatic and cranial cavities only being separated by a fibrous
partition. As compared with the practically independent carti-

^ In Plagiostomes the utriculus and sacculus are not divided off from one
another, and the anterior and posterior canals do not unite in a sinus utriculi.
In Rays the canals are almost completely circular.

- The endolymphatic duct corresponds to the stalk of the auditory vesicle
connecting it with its superficial point of origin ; in Vertebrates other than
Elasmobranchs (except in Teleosts, in which it is wanting) it becomes separated
very early from the outer ectoderm.

ms s

Fig. 216. — Membranous Labyrinth of Various Fishks. (After G. Retzius.)
A, Myxine glutinosa, from the inner side.

aa, apy anterior and posterior ampulla ; cc, canalis communis ; de, ductus endo-
lymphaticus ; era, crista acustica of the anterior, and crp, of the posterior
ampulla ; mc, macula acustica communis ; ra, rp, anterior and posterior
branches of the auditory nerve ; sc, saccus communis ; se, sacciis endolym-
phaticus.

A\ Acipenser sturio, from the outer side ; B, Ghimara monstrosa, from the inner
side ; C, Perca Jlitviatilis, from the inner side.

aa, ae, ap, anterior, external, and posterior ampulla ; ac, auditory nerve ; res?,
apex of the sinus superior ; ca, ce, cp, anterior, external (horizontal) and
posterior semicircular canals ; cr, crista acustica of the ampullae ; cus,
utriculo-saccular canal ; de, ductus endolymphaticus, which in B opens
externally through the skin ha at ade ; I, lagena (cochlea) ; mn, macula
acustica neglecta ; ms, macula acustica of the sacculus ; mu, macula acustica
of the recessus utriculi ; o (in C), otoliths in the recessus utriculi, sacculus,
and lagena ; pi, papilla acustica of the lagena ; raa, rae, rap, ru, rs, rl, rn,
the various branches of the auditory nerve ; rec, recessus utriculi ; s, sac-
culus ; se, saccus endolymphaticus ; ss, sp, sinus utriculi superior and
posterior ; u, utriculiSi

AUDITORY ORGAN 297

laginous capsules of Cyclostomes, those of Gnathostomes in general
became gradually more and more drawn into the skull in the
course of development, and on the partial reduction of their
cartilage, certain of the cranial bones having primarily no relation
to them may take part in enclosing the labyrinth (many Teleosts ^).
The auditory organ of the Dipnoi most nearly resembles that of
Elasmobranchii, and more particularly that of Chimaera. In
Protopterus the swollen endolymphatic duct of either side gives
rise to a number of tube-like diverticula filled with otolithic
substance. These almost entirely cover the fourth ventricle of the
brain, and extend backwards as far as the sensory root of the first
spinal nerve.

Amphibians. — The membranous labyrinth of Amphibians re-
sembles that of Fishes and Dipnoans in many respects, but
important differences are seen — more particularly as regards the
lagena, which, especially in the Anura, becomes further constricted

ap

Fig. 217. — Right Membranous Labyrinth of BaiiT, escvlenta, from the inner
side. (After G. Retzius. )

au, aperture of utriculus ; ciis, utriculo-saccular canal ; I, lagena cochleae ; mu, m-s,
mn, macula acustica recessus utriculi, sacculi, and neglecta ; ph, pars basilaris
cochleae ; pl,pph, papilla acustica lagenfe and basilaris : run, rap, rs, m, rl, rh,
branches of auditory nerve to the anterior and posterior ampullae, sacculus,
macula neglecta, lagena, and pars basilaris. (Other letters as in Fig. 216,
A^— C.)

off from the sacculus and has close relations to the perilymphatic
system, reaching a higher stage of development. The sacculus
itself in Anura is considerably reduced, while in Urodeles it is
relatively larger than in Fishes.

^ In certain Teleosts (Siluroidei, Gymnotidae, Characinidae, Gyranarchidae,
Cobitidae, Cyprinoidae) the auditory organ comes into relation with the air-bladder
by means of a chain of bones (" Weberian ossicles") surrounded by a continuation
of the dura mater and derived from certain parts of the four anterior vertebrte
and corresponding pairs of ribs : it is possible that by this means the relative
fulness of the air-bladder can be appreciated by the Fish. Connections between
processes of the air-bladder and the internal ear are also met with in several other
Teleosts,

298

COMPARATIVE ANATOMY

The first indications of a pars hasilaris with its papilla hasilaris
lagence are already met with in certain Urodeles, but they lie
inside the upper part of the lagena, close to the sacculus.

In the Anura (Figs. 217 and 218) a higher condition is seen in
the presence of a peculiar small outgrowth of the thickened wall
of the lagena on which a definite region, supported by cartilage,
corresponds to the basilar membrane of higher types. The lagena
and pars hasilaris now open close together but independently of
one another in the posterior part of the sacculus. Thus, in addition
to those present in Fishes, a new patch of nerve-endings is added,
viz., the papilla acustica basilaris lagence.

The ductus endolymphaticus may give rise to large sac-like
enlargements containing calcareous matter which meet those of
the other side, either on the dorsal surface only, or on both dorsal
and ventral surfaces of the brain. The latter is the case in Anura,

Fig. 218.— Diagram showing the Relations of the Lagena and Pars
Basilaris in Anura (A) and Sauropsida (B). The outer wall of the
sacculus faces upwards, (After H. Spencer Harrison. )

D, ductus sacculo-cochlearis ; Lg, lagena ; MS, macula sacculi ; Ph, pars
basilaris with its cartilaginous framework ; Pph, papilla basilaris ; Ppl^
papilla lagense ; S, sacculus.

for instance, in which the sac extends as an unpaired structure
along the whole vertebral canal dorsally to the spinal cord, giving
rise to paired outgrowths extending through the intervertebral
foramina and forming the characteristic calcareous bodies situated
close to the spinal ganglia. These consist of numerous inter-
communicating tubes lined by pavement epithelium and plentifully
supplied with capillaries.

The perilymphatic cavity of Amphibians communicates medially
with the cranial cavity by a perilympliatic duct, and the same is
true of the Amniota.

A further advance in structure as compared with Fishes is seen
in the gradual differentiation of a middle ear. In the outer wall of
the auditory capsule is a membranous space, the fenestra vestibuli
(ovalis), which is plugged by a cartilaginous stapedial p>late, and
from the latter a rod-like cartilage or bone, the columella, usually
extends outwards towards the quadrate and paraquadrate (p. 98).

A tympanic cavity, with a tympanic membrane supported by a

AUDITORY ORGAN 299

ring of cartilage lying on the level of the skin, and a Eustachian
tube opening into the pharynx, are met with in most Anura,^ in
which also the columella is more perfect, consisting of a bony and
cartilaginous rod expanded distally to fit against the tympanic
membrane. The columella is wanting in certain Urodeles (e.g.
Triton). A membranous fenestra cocMecB {rotunda) in the outer
wall of the auditory capsule is present in many Amphibians and
in all higher Vertebrates in addition to the fenestra vestibuli. The
ear of the Gymnophiona essentially resembles that of the Urodela.

Reptiles and Birds. —In Chelonians, the auditory organ
shows many points of resemblance to that of Urodeles ; and in all
the Sauropsida the chief modifications are confined to the cochlea,
which gradually shows a higher condition of development in passing
from Chelonians and Snakes to Lizards and Crocodiles.

In Chelonians the lagena and pars basilaris are almost com-
pletely united in the cochlea,^ and, in contrast to the Anura, they
now communicate with the sacculus by a single aperture. The
cochlea grows out in the forai of a short canal (" ductus cochlearis),"
which in Crocodiles and Birds is considerably longer and slightly
coiled (Figs. 219 and 220). A more marked differentiation also
gradually takes place in the membrana basilaris and the papilla
acustica basilaris (Fig, 218, b). Both become more and more
elongated, and, at the same time, distinct indications of scalce
tympani and vestibuli are seen (cf. Fig. 223). In the Lacertilia the
most varied t}^es of auditory organ are met with, gradually leading
up to that seen in the Crocodilia. Thus there is a continuous and
an unbroken series from the lower forms to the higher.^

Thus the cochlea gradually becomes more independent of the
sacculus, which shows the greatest variety both as to form and size
in the different types. It is usually very small in Birds, for in-
stance, while in Lizards {e.g. Lacerta) it is of considerable size.
The aperture of communication between the utriculus and sacculus
persists, though it gradually becomes narrowed, as does also that
between the sacculus and cochlea. The connection between these
latter may be draw^n out to form a canal {canalis reuniens), and this
is particularly the case in Birds ; in Crocodiles an intermediate
condition between Birds and Lizards is seen. The membranous
labyrinth of Birds, however, is always specially characterised by the
peculiar arrangement of the anterior and posterior canals, which
are greatly arched and the position of their openings into the sinus
superior (canal commissure) is reversed. In lower t3^pes (Swimming
Birds) this peculiarity is less marked than in the higher forms.

' Except, e.g. the Pelobatidae,

^ At the same time even here a posterior portion (pars basilaris) can be
plainly distinguished from an anterior portion (lagena) by a ridge extending into
the lumen {e.y. Lacerta).

•^ The structure of the auditory organ of Hatteria shows many striking
peculiarities ; like that of Chamteleo, it occupies an isolated position.

300

COMPARATIVE ANATOMY

In Chelonians an external and an internal portion of the
tympanic cavity can be distinguished : the former is surrounded
by the quadrate, which is correspondingly hollowed out and covered
externally by the tympanic membrane. The two portions com-
municate by a canal through which the columella extends, and
which is bounded either entirely by the quadrate or partially by
soft parts also.

In many Reptiles the distal end of the ductus endolymphaticus
is situated close under the roof of the skull beneath the parieto-
occipital suture, and in the Ascalabota the duct even leaves the
cranial capsule, passes back between the muscles of the neck, and

mri

Fig. 219. — Right Membranous Labyrinth of A, Lacerta viridis, and B,
Alligator mississippiensia, from the outer side. (After G. Retzius.)

ade, aperture of the ductus endolymphaticus ; esc, sacculo-cochlear canal ; frt,
foramen recessus seal* tympani ; mh, membrana basilaris : sc, septum
cruciatum ; tv, tegmentum vasculosum. (Other letters as in Figs. 216 and
217.)

in the region of the pectoral arch becomes swollen to form a large
folded sac, from which finger-shaped processes extend to the
ventral surface of the vertebral column and to the sub-mucous
tissue of the pharynx. These processes may also branch out in a
labyrinthic manner into the orbit : they are filled with a white
semi-solid mass of otolithic substance, as is the endolymphatic
duct of all Vertebrates, at any rate in the embryo. In Birds, the
duct does not pass out of the cranial cavity.

A .tympanic membrane is present in Birds and all Reptiles
except Hatteria, Snakes, and Amphisbienians, in which the tym-
panum and Eustachian tube have undergone degeneration.

AUDITORY ORGAN

301

Especially in Crocodiles and Birds, in which the two Eustachian
canals open by a single median aperture into the pharynx/ the
tympanic cavity is complicated, and is continued into pneumatic
cavities in the neighbouring bones. The osseo -cartilaginous
•columella is well developed in most of the Sauropsida, and is not
distinct from the stapedial plate ; in Hatteria it is continuous
distally with the hyoid (p. 115).

In most Lizards the tympanic membrane is on a level with the
skin, but in certain forms {e.g, Ascalabota, Lacerta, Monitor) an
indication of the develop-
ment of an external audi-
tory passage is seen, the
tympanic membrane being
partially covered posteri-
orly by a small fold of
skin, usually enclosing the
anterior border of the di-
gastric muscle : in Croco-
diles there is a definite in-
tegumentary valve moved
by muscles (abductor of
the mandible, supplied by
the facial nerve) and en-
closing a dermal bone, and
in certain Birds also {e.g.
Owls) there is a movable
valve. The tympanic
membrane in Birds is situated some distance from the surface in
an external auditory passage, and is stretched on a ring formed
by several bones of the skull.

Mammals. — The auditory organ of Mammals reaches a much
higher stage of development, but in Monotremes it shows many
points of resemblance to that of Reptiles and Birds.

The cochlea now reaches its highest development, and forms
a long tube which becomes spirally coiled on itself (Figs. 221
and 222).^ In this, as well as in the more highly-specialised
histological structure of the cochlea, lies the characteristic peculi-
arity of the auditory organ of Mammals. The auditory nerve
forms the axis of the spiral.

In consequence of this development of the cochlea, the papilla
basilaris acustica, or, as it is called in Mammals, the organ of Cm^ti^

^ In Crocodiles this aperture leads into three canals, a median and a paired,
all of which communicate with the tympanic cavity in a complicated manner.

2 In Man it forms nearly three coils, and in other Mammals from one and a
half (Cetacea) up to as many as four or more. Thus in the Rabbit there are two
and a half, in the Ox three and a half, in the Pig almost four, and in the Cat
three coils. The cochlea, as well as the sacculus and all parts of the pars superior
of the membranous labyrinth, vary considerably both in form and arrangement in
the various types.

Fig. 220. — Right Membranous Labyrinth
OF Tiirdiis musicuf, from the inner side.
(After Ct. Retzius.) Letters as in previous
Figures.

302

COMPARATIVE ANATOMY

is drawn out to a considerable length, and the part of the wall of
the cochlea on which it lies is called the basilar membrane, while
the opposite wall is spoken of as the membrane of Beissner

Fig. 221. — Right Membranous Labyrinth of Rabbit {Lepns cuniculus). A,
from the inner, and B from the outer side. (After G. Retzius. )

C8Cj canalis reunions Henseni ; /, facial nerve ; lis, spiral ligament ; mh, basilar
membrane ; rh, basilar branch of the auditory nerve [ac) ; sus, sinus utri-
cularis sacculi. (Other letters as Figs. 216-220. )

{membrana vestibularis) (Fig. 223) : this is already represented in
Reptiles.

Except in Monotremes, the papilla acustica lagense has dis-
appeared : thus in other Mammals there remain only six patches of
nerve-endings.

AUDITORY ORGAN 303

The original aperture of communication between the pars
superior and pars inferior of the membranous labyrinth — that is,
between the sacculus and utriculus, is obliterated in Mammals, the
two parts being only indirectly connected with one another by
means of the ductus endolymphaticus. This bifurcates at its point
of insertion into the membranous labyrinth, one limb being con-
nected with the utriculus and the other with the sacculus (Fig.
215) ; w^hile its upper end perforates the inner wall of the carti-
laginous or bony auditory capsule, passes into the cranial cavity,
and terminates by an expanded extremity (saceits endolymphaticits)
in the dura mater. Osmosis can thus occur between the lymph
contained in the endolymphatic and epicerebral lymph -spaces
respectively.

The tympanic membrane becomes secondarily situated deep
down in the external auditory meatus, and an important difference
is thus seen as compared with the Amphibia and most Sauropsida.
The tympanic cavity and Eustachian tube are well developed, and
in place of a single bony columella there is a chain of three
auditory ossicles, articulating with one another and extending
between the tympanic membrane and the fenestra vestibuli. These
are the malleus, the incus with its orbicular apophysis, and the
stajws}

Two striated muscles are present in connection with the middle
ear. The phyletically older stapedius arises from the wall of the
tympanic cavity and is inserted into the stapes, serving to keep the
membrane of the fenestra vestibuli stretched. It is supplied by
the facial nerve and corresponds to the dorsal portion of the deep
constrictor inserted on the hyoid in Fishes, from which the
hinder belly of the biventer of Mammals also arises. A te^isor
tympani, supplied by the mandibular division of the trigeminal and
derived from the system of the adductor mandibulae (pars
pterygoidea),- also arises from the wall of the tympanic cavity, and
is inserted on to the manubrium of the malleus, serving to stretch
the tympanic membrane.

As already mentioned, the form of the membranous labj^inth
is repeated by its enclosing cartilaginous or bony capsule, which
forms, so to speak, a sort of cast around its individual parts. In
Mammals the skeletal labyrinth becomes ossified before any other
part of the skull, and is incompletely divided into two parts
enclosing the utriculus and sacculus respectively. With the latter

^ Cf. Fig. 95, in which the mode of development of these parts is shown.
There is often also a bony (interhyal) rudiment in the tendon of the stapedius
muscle,

^ In Man the tensor tj'mpani is from the first connected with the tensor veli
palatini muscle. In Ornithorhynchus it has a double origin, one part being
continuous with the pharyngeal muscles and the other arising independently. A
stapedius muscle is wanting in Ornithorhynchus and Echidna, and a tensor
tympani in Manis ; and in all these three animals the tympanic cavity is sub-
cfivided into an upper and a lower portion by a horizontal septum of connective
tissue.

304 COMPARATIVE ANATOMY

part is connected the hony cochlea, the axis of which lessens in size
from base to apex (Fig. 222), and round it a bony lamella {lamina
spiralis ossea) winds in a spiral manner; this extends into the
cavity of the coils of the cochlea without quite reaching as far as
the opposite wall, being continued outwards by two laterally-
diverging lamellae, mentioned above as the basilar membrane and
membrane of Reissner ; these lie at an angle to one another and
correspond to the inner walls of the membranous cochlea {chtctus
cochlearis or scala media), which is approximately triangular in
transverse section. The outer wall abuts against a portion of the
peripheral part of the bony cochlea. It is apparent therefore that
the scala media does not by any means fill up the lumen of the
bony cochlea, but that a cavity is left on either side of it, corre-

Fig. 222. — Bony Cochlea of Man. (After A. Ecker.)

A, axis; H, hamulus ; Lso, Lso^, lamina spiralis ossea, the free edge of which,

perforated by the fibres of the auditory nerve, is visible at f.

Fig. 223. — Diagrammatic Transverse Section of the Cochlea of a

Mammal,

B, membrana basilaris, on which the neuro-epithelium lies ; C, membrane of

Corti ; KS, bony cochlea ; L, limbus laminjB spiralis ; Lo, Lo^, the two
layers of the lamina spiralis ossea, between which at N the auditory nerve
(together with the ganglion, left of L) is seen ; Ls, liganientum spirale ; B,
Reissner's membrane ; Sm, scala media (membranous cochlea) ; St, scala
tympani ; Sv, scala vestibula.

spending to those already met with in the auditory organ of Birds
and known as the scala %estihuli and scala tympani (Figs. 215 and
223). Both of these are continuous with the perilymphatic space,
and, following the direction of the scala media, open into one
another at the blind end of the latter. The scala vestibuli is
shut off from the tympanic cavity by the membrane of the fenestra
vestibuli, to which the stapes is applied externally ; the scala tym-
pani is closed by the membrane of the fenestra cochleae.

On the floor of the bony cochlea, not far from the fenestra
cochleae, is an opening leading into a narrow ductus perilymphaticus
or aqueduct us cochlcce (Fig. 215).

The fibres of the auditory nerve running along the axis of the
bony cochlea extend in their course laterally outwards, between

AUDITORY ORGAN

305

the two plates of the lamina spii-alis ossea (Figs. 223, 224). On
the free border of the latter they pass out, and break up into
terminal fibrillge on the inner surface of the basilar membrane.
The fibrillse extend to the sensory or auditory cells, and these are
stretched as in a frame between the firm supporting and isolating
cells or bacilli. From the surface of the bacilli a resistant net-like
membrane {memhrana reticularis) extends laterally, and through
the meshes of the latter the hairs of the auditory-cells project.
The auditory cells are covered by a thick and firm membrane

Fig. 224.— The Organ of Corti. (After La vdowsky. )

B, B, basilar membrane ; Ba, Ba, bacilli, or supporting cells ; C, membrane of
Corti ; Lo, Lo^, the two plates of the lamina spiralis ossea ; Ls, ligamentum
spiiale, passing into the basilar membrane ; JiJz, membrana reticularis ; X,
auditor}^ nerve witli ganglion ; JV^ X'-, the nerve branching up into fibrillie
and passing to the auditory cells ((?, G) ; B, membrane of Reissner ; Sm,
scala media.

— the membrana tectoria s. Corti — which perhaps acts as a
damper, and which arises from the vestibular lip of the lamina
spiralis ossea. The whole extent of the basilar membrane consists
of clear, thread-like and very elastic fibres, of which about 16,000
to 20,000 can be made out in Man.

A true pinoia or auricula (Fig. 225), attached to the border of
the external auditory meatus and projecting freely from the
head, appears for the first time in Mammals. It is present
in Monotremes, and, more especially in Echidna, is in organic
connection with the upper end of the hyoid^ by means of the

^ This fact appears to indicate a probable genetic relation between the
auricula and the visceral arches.

306

COMPARATIVE ANATOMY

cartilaginous auditory passage, with which it is continuous. In its
formation and farther development, the dermal musculature
primarily plays the most important part.

In the higher Mammals the pinna and the cartilaginous part of

-::::Zj

Fig. 225. — The Pinna of Various Primates.

In A, the shaded portion (6) represents the zone of the auditory eminences of the
embryo, the unshaded that of the later-formed auditory fold. B, Man,
Baboon and Ox, drawn to the same scale and superposed : s', s", s, spina or
tip of the ear. C, Macacus rhesus, with upwardly directed tip ; and D, Cerco-
pithecus, with backwardly directed tip. E, Man : the muscles are indicated
as follows — m.a, attolens auriculae; m.a', antitragicus ; m.t, tragicus ;
m.t', inconstant muscle, extending from the tragicus to the margin of the
helix ; m.h', helicis major; m.h", helicis minor ; s, tip of the ear rolled over.

A-D, after Sehwalbe ; E, after Henle.

the external meatus (here also continuous with one another) arise
from a series of rounded eminences on the first and second visceral

AUDITORY ORGAN 307

arches, around the hyomandibular (spiracular) cleft, the lower part
of which closes up, while the upper part gives rise to the external
auditory meatus. These auricular eminences unite to form a
nearly continuous ring, on which are later formed the characteristic
protuberances known as the helix, antihelir, tragus, and antitragus.
The variations in form of the pinna which are seen in various
Mammals concern essentially the later formed portion (auditory
fold), which projects upwards and backwards from the head
(Fig. 225).

The muscles which move the ear as a whole, and which are
supplied by the facial nerve, include the following in the majority
of 5lammals : (a) attrahentes s. adductores, (b) levatores s. attolentes,
(c) ahditctores s. retrah^ntes, (d) depressores, and (e) rotatores. A
gradual reduction of these muscles is seen in the following series :
Artiodactyla and Perissodactyla, Canidse, Felidae, Prosimii, Primates,
the reduction being most marked in Man.^

^ The auditory fold may undergo marked reduction, e.g. in aquatic and sub-
terranean forms. Thus amongst the Pinnipedia, only the Otaridae possess an
"external ear." 1'he corresponding muscles become transformed into sphincters
for closing the auditory aperture. -

X 2

p. ORGANS OF NUTRITION.

ALIMENTARY CANAL AND ITS APPENDAGES.

The alimentary or enteric canal consists of a tube which
begins at the aperture of the mouih, passes through the body-
cavity (coelome), and ends at the vent or anus} The walls of
the canal consist of several layers, of which the mucous mem-
hrane, lining the cavity, and the muscular layer external to this,
extend throughout the canal. The mucous membrane consists of
a superficial epithelium, and a deeper connective tissue layer, the
outer part of which, or submucosa, forms a loose network con-
necting it with the muscula,r layer."^ The epithelium is derived
from the endoderm, with the exception of that lining the mouth
and anus {stomodceitm and 'proctodceum) which is ectodermic in origin
(p. 5). The connective tissue and muscular layers arise from the
splanchnic layer of mesoderm of the embryo; and the muscular
coat, consisting almost entirely of un striated fibres, supplied by
nerves from the sympathetic system, is, as a rule, divided into tw^o
layers, the inner being constituted by circular, and the outer by
longitudinal fibres. These serve for the contraction or peristalsis
of the wall of the gut, and thus fulfil the double function of
bringing the nutritive contents of the latter into the closest
possible relation with the whole epithelial surface, and at the
same time of removing from the body the substances which have
not been absorbed. Striated (voluntary) muscular fibres, supplied
by cerebral or spinal nerves, only occur at the anterior and posterior
ends of the canal.

An outer accessory sero^bs coat, the per itonettm, encloses the gut
externally in the region of the coelome. This is covered on its
free surface by pavement epithelium, and, dorsally to the alimentary
canal, is reflected round the entire body-cavity, converting the

1 The mouth of Amphioxus apparently corresponds to the first gill-cleft of the
left side of craniate embryos {i.e. to the left spiracle of Fishes). The mouth of
Craniates is probably a new acquisition {neostoma, cf. p. 203), which has arisen
by the confluence of'a pair of gill-clefts. The anus, which in many Vertebrates
arises directly from the blastopore, is phylogenetically older than the neostoma.

^ A layer of smooth muscular fibres may be present in the submucosa, which
also encloses lymphoid or adenoid tissue {solitary follicles, Peyer's patches).

ALIMENTARY CANAL AND ITS APPENDAGES 309

. 226. — Diagram of the Oral Cavity and Pharynx in a Fish (A),
AN Amphibian (B), a Reptile (C), and Man (D).
Ch, internal nostril ; D, alimentary canal ; K, gill-slits ; L, lung ; N, external
nostril ; O, oesophagus ; T, trachea ; the arrow marked A indicates the
respiratory passage, that marked B the nutritive passage ; f, the point where
the two passages cross one another.

310 COMPARATIVE ANATOMY

latter into a large lymph-sinus. A parietcd layer, lining the body-
cavity, and a visceral layer, reflected over the viscera, can thus be
distinguished in the peritoneum (Fig. 10). The region where one
passes into the other, which is thus primitively double, is called
the mesentery} and this serves not only to support the alimentary
canal from the dorsal body-wall, but also to conduct the blood-
vessels, lymph-vessels, and nerves. With the lengthening of the
alimentary canal during development, the mesentery may give rise
to a more or less complicated system of folds in which the viscera
are enveloped.

The most anterior section of the primitive alimentary tract
of the Ichthyopsida serves as a respiratory cavity as well as a
food-passage. A row of sac-like outgrowths, lying one behind the
other, are developed in the embryo from the mucous membrane
and eventually unite with the ectoderm, apertures being formed
to the exterior (Fig. 226, a). In the septa between the channels
thus formed, the visceral arches are situated (cf Fig. 63), and
along the septa certain vessels arise by means of which a continual
interchange of gases can take place between the blood and the
air contained in the water passing through the sacs. In this
manner the gills or hranchim arise. Even in the Amniofca,
although gills are not developed, that portion of the cavity of
the mouth and pharynx which lies behind the internal nostrils
serves as a common passage for air and food unless a proper
palate is formed (Fig. 226, B, c).

With the formation of a secondary palate (p. 132), the priuii-
tive mouth-cavity becomes divided into an upper respiratory,
and a lower nutritive jjortion — that is, into a nasal and a secondary
or definitive motith- cavity. The separation, however, is never a
complete one, the passage being common to both cavities for a
certain region (jpharynx), which in Mammals is partially separated
from the mouth by a muscular fold, the velnm palati, or free edge
of the soft palate (Fig. 226, d).^

The alimentary canal of Vertebrates is typically divisible into
the following principal sections (Fig. 227) : — mouth- or oral-cavity,
pharynx, gullet or wsophagics, stomach, and intestine, the last men-
tioned being usually differentiated into a small and a large
intestine. The small intestine is in most cases the longest section
of the alimentary tract ; the bile and pancreatic ducts open into
its anterior portion (duodenum).

The large intestine communicates with a cloaca, which also
receives the urinary and genital ducts, or it may open independently
to the exterior. The small intestine may be further differentiated

^ In Mursenoids, Dipnoans and Lepidosteus, a ventral mesentery is also present,
but in Lepidosteus it only extends for a short distance along the hinder part of
the gut.

2 A membranous velum palati exists in Crocodiles. A median, finger-like
process of the soft palate, the uvula, is well developed only in Man and some
Apes.

ALIMENTARY CANAL AND ITS APPENDAGES 311

into ditodemcm, jejunum and ileum, and the large intestine into
colon and reckom. A blind-gut or ccecicm is often present at the
junction of the large and small intestine. Between the stomach

Fig. 227. — Dl\gram of the Alimentary Canal of Man.

A, anus ; Ca, Ct, Cd, ascending, transverse, and descending portions of the colon :
DtZ, small intestine ; Gls, salivary glands; Gl.th, thyroid; GL thy, thymus;
Lb, liver ; Lg, lung ; Mg, stomach ; Oe, oesophagus ; Pa, pancreas, Ph,
pharynx ; Pv, vermiform appendix (caecum) ; B, rectum ; Vic, position of
ileocolic valve ; Z, diaphragm.

and duodenum, as well as between the ileum and large intestine,
there is as a rule a marked elevation of the muscular coat serving
as a sphincter (pyloric and ileo-colic valves). These serve not only

312 COMPARATIVE ANATOMY

to prevent the food from passing along the canal in the wrong
direction, but also to retain it within the same portion of the
canal for a certain time. There is also a sphincter muscle at the
anus.

In almost all cases the alimentary canal becomes more or less
coiled, and thus presents a greater surface for absorption. As a
general rule, it is relatively longer in herbivorous than in carni-
vorous animals. A considerable increase of surfiice also commonly
results from the elevation of the mucous membrane to form folds,
villi, and papillae.

Certain appendages are also present in connection with the
alimentary canal. These are all developed primarily from the
endoderm and are thus of epithelial origin : mesodermic elements
are added to them secondarily. Whether serving as glands or
not, they all arise in the same manner as glands.

Beginning from the mouth the following appendicular organs
of the alimentary canal may be distinguished (Fig. 227) : —

(1) Mucous and salivary glands.

(2) The thyroid.

(3) The thymus.

(4) The siuim-bladder or lungs.

(5) The liver.

(6) T\iQ pancreas.

In addition to these, gastric and intestinal glands are embedded
in the wall of the gut.

Oral Cavity.

In Amphioxus and Cyclostomes the aperture of the mouth is
surrounded by an oral hood or funnel supported by skeletal parts,
which, in the former and in Myxinoids, is edged with tentacles or
cirri: all other Vertebrates are provided with an upper and a
loiver jaw.'^

Definite lips provided with muscles first appear in Mammals,
and are very varied in form. The space between them and the
jaws is spoken of as the vestibtilum oris ; this may become extended
on either side to form cheek-pouches, which serve as food reservoirs
(many Monkeys and E-odents).^ The lips, together with the
cheeks and mobile tongue, are important in suction, as well as
in articulate speech (Man). Monotremes are the only Mammals
in which they are wanting : in them the jaws are covered by a

^ The mouth of the Lamprey serves as a suctorial organ for attaching the
animal to foreign objects. The larvsae of Lepidosteus, Polj^pterus, Lepidosiren,
Protopterus, and Anura are temporarily provided with suctorial organs.

2 Cheek-pouches, opening externally and lined by hair, occur amongst
Rodents (Geomyidaj).

TEETH

313

tough, hairless integument, and in Ornithorhynchus somewhat
resemble the beak of a Bird.

The chief organs of the oral cavity are the teeth^ the glands, and
the tongue.

Teeth.

The teeth are developed quite independently of the endo-
skeleton, and both ectoderm and mesoderm take part in their
formation. The first traces of the teeth are seen primarily in
the form of superficial papillae of the mucous membrane; but
secondarily, owing to w^ant of space, the epithelium of the mouth
grows inwards so as to give rise to a dental lamina which becomes

zir

Jfy 'pjlt

Fig. 228. — Diagram of the Development of a Tooth.

By, connective tissue follicle or sac surrounding the tooth ; DS, dentine ;
EM, epithelium of mouth ; Ma, enamel epithelium ; O, odontoblasts ; SK,
dental lamina ; ZK, dental papilla.

Fig. 229. — Semidiagrammatic Figure of a Longitudinal Section through

A Tooth.

PH^, aperture of the pulp-cavity {PH) ; ZB, dentine : ZC, cement ; ZS, enamel.

enlarged distally at certain points to form the so-called enamel-
organs. These, as they grow deeper into the mesoderm, become
bell-shaped, and enclose modified masses of connective tissue, the
dental papilloB ; the upper cells of the papillae, i.e. those next to
the enamel-organ, are known as odontoblasts (Fig. 228). The
epithelial and connective tissue germs come into the closest rela-

314 COMPARATIVE ANATOMY

tion with one another, and give rise respectively to the calcified
enamel with its " cuticula dentis," and to the dentine, of which the
teeth are composed : the dentine consists of calcified hard dentine
and vascular vaso-dentine. The enamel is the harder and contains
little organic matter, and the dentine (ivory) is permeated by a
system of fine canals into which delicate processes of the odonto-
blasts extend. A third, bone-like substance, the cement, is also
formed from the mesoderm round the bases of the teeth and
between the folds of enamel when these are present; it may
unite with the bones of the jaw.

The root of the tooth, embedded in the gums, is provided at
its lower end with an opening leading into the central pulp-cavity
(Fig. 229), containing the pulp of the tooth, which consists of cells,
blood-vessels, and nerves, and which renders the tooth sensitive.

The form of the teeth is largely dependent on that of the
jaws and on the mode of articulation of the latter, as well as on
the nature of the food. Functional adaptation may result in com-
plications in the relations of the enamel, dentine, and cement, so
as to produce a cutting or grinding surface of different degrees of
hardness, and these modifications are generally more marked in the
upper than in the lower jaw.

In most Vertebrates below Mammals all the teeth are essenti-
ally similar in form (homodont dentition) : in Mammals, on the
other hand, they become differentiated into distinct groups {hetero-
dont dentition), viz. into incisors, canines, and cheek-teeth or grinders
(^'premolars and molars).

A succession of teeth takes place throughout life in almost all
Vertebrates except Mammals, in which, owing to specialisation, a
reduction in the number of tooth-generations has taken place, so
that there are practically only two functional sets, the so-called
deciduous or milk-teeth and the successional teeth. This difference
is expressed by the terms polyphyodont and diphyodont.

Fishes and Amphibians. — The homology of the teeth and
their similarity with the dermal denticles of Elasmobranchs have
already been referred to (p. 39). The most primitive form of tooth is
that of a simple cone, but even amongst Plagios tomes, in which
the teeth are arranged in numerous parallel rows upon the carti-
laginous jaws, this form has already become modified in various
ways for seizing or crushing the food.

Of those Anamnia which possess a bony skull, four groups of
tooth-bearing bones may in general be distinguished, viz. (1) the
maxillary arch {yremaxilla and maxilla) ; (2) the palatal arch
{vomer, ^:>a/a^m6, pterygoid) ; (3) the unpaired parasphenoid ; and
(4) the mandibular arch (dentary and splenial)}

True teeth, with enamel, enamel-epithelium, and odontoblasts,

^ The teeth of Elasmobranchs are comparable to those of the palatal arch
and splenial. (For those of Holocephali, cf. p. 88.)

TEETH

315

are wanting in Cyclostomes, and are represented functionally by
a number of conical horny teeth, the morphological nature of
which has been variously interpreted. Amongst cartilaginious
Ganoids teeth are absent in the adult Sturgeon, though present
in the embryo. Amongst Teleostei they are wanting in the
Lophobranchii, and, except in very early stages, in Coregonus.

In bony Ganoids and Teleosts, teeth may be present on all the
bones bounding the oral cavity, as well as on the hyoid and the
branchial arches (" pharyngeal bones.") In the latter position, as
well as on the parasphenoid, they often form brush-like groups.
In form the teeth may be cylindrical, conical, hooked, or chisel-
shaped (Scarus, Sarginae) ; in some forms they give rise to a
definite pavement, are rounded in
form, and serve to crush the food ;
in others, again, they are delicate and
bristle-like (Chsetodon), or sabre-
shaped (Chauliodus).

In the Dipnoi (Fig. 71) the teeth,

which are wanting in enamel (though

an enamel organ is present in the

embryo Lepidosiren), are exceedingly

massive, presenting sharp edges and

points: they have probably arisen

by the concrescence of a number of

individual teeth. ^ More particularly

in Ceratodus, the origin of the vomer,

palatopterygoid, and dental plates of

the mandible from a fusion of the

bony basal portions of the teeth is Fig.230.— Skull of 5a^rafAo>,ei>s
• ] / J i 1 1, 1, atlenuatus. From the ventral

very evident, and the same has been ^^^^^ ^i^^^j^g ^j^e teeth on the
shown in the case of the Amphibia, parasphenoid.

In the Amphibia there is in
general a considerable diminution
as compared with Fishes ; and at
more uniform character is noticeable in their form through-
out (Fig. 231; A, b). They are conical, enlarged below, and
rest on a definite base, while above they become narrower and
slightly curved, ending either in a double (Myctodera, Anura), or
a single apex (Perennibranchiata, Derotremata, Gymnophiona) ; the
latter is the more primitive condition. The teeth lie deeply em-
bedded in the mucous membrane, and are present, as a rule, on the
premaxiila, maxilla, and mandible (except in Anura), as well as on
the vomer and palatine, but rarel}' on the parasphenoid {e.g.

^ There are no indications of a succession of teeth in the Dipnoi. In
Protopterus the teeth are covered by an epithelial horny layer during the torpid
period. The formation of complex teeth by concrescence is apparently not so
frequent amongst Vertebrates as was formerly supposed, and does not apply, e.g.
to those of various Elasraobranchs, of Labyrinthodonts, Ichthyosaurians, and
pjTobably also to the mammalim molars.

in the number
the same time

of teeth
a much

316

COMPARATIVE ANATOMY

Spelerpes, Plethodon, Batrachoseps, Fig. 230) ; in the larvae of Sala-
manders and in Proteus the splenial of the lower jaw is also
toothed. Horny teeth and horny jaios are present in larval
Anura (except Xenopus), and similar structures occur in Siren
lacertina. Teeth are altogether absent in the Bufonidse and in
Pipa.

The teeth of certain of the Stegocephali (Labyrinthodonta)
were extremely complicated, the enamel forming numerous
corrugated folds extending from the periphery towards the
centre.

Reptiles and Birds. — Corresponding with the greater firm-
ness of the skull in Reptiles, the teeth are usually strongly

RP

PJI

Fig. 231, A — Tooth of Frog, and B — of Salamandra atra.

31, maxilla ; FH, pulp-cavity ; EF, circular furrow ; S, apex, covered with
enamel ; ZK, crown ; ZS, base.

developed, and may be more highly differentiated than in Amphi-
bians. They are either situated upon a ledge on the inner
side of the lower jaw, with which their bases become fused
{pleurodont dentition — most Lacertilia); or they lie on the free
upper border of the jaw (acrodont dentition — Chameleon); or
finally, as in Crocodiles and numerous fossil Reptiles, they are
lodged in alveoli {thecodont dentition) (Fig. 232). Both upper
and lower jaws, and occasionally the palatine and pterygoid also,
are toothed (Lizards and Snakes) : and in Hatteria, vomerine
teeth may also be present. The teeth are usually conical, more or
less pointed (Fig. 232, b), and often long and recurved, for seizing
the prey {e.g. Snakes) : in Lizards the apex may be double, and
in many Reptiles {e.g. Uromastix spinipes, Agamse, and numerous

TEETH

317

fossil forms) a heterodont dentition appeai-s to be indicated.^ In
spite of the great modifications Avhich have taken place in the
palate of Crocodiles, their teeth, which have a conical form, show
the least amount of differentiation in the coui-se of their phylo-
genetic history. Almost all Reptiles are polyphyodont, but in some
cases certain of the teeth are not replaced {e.g. Agama colonorum),
and in others some of them undergo reduction {e.g. Typhlopidse).

In poisonous Snakes a varying number of maxillary teeth are
differentiated to form poison-fangs, which, like those of the lower
jaw of the poisonous Lizard, Heloderma, are longitudinally grooved
anteriorly. In the Viperine forms there are on each side a
number of poison-fangs arranged in rows ; the stronger ones project

A B

Fic. 232.— A, Diagrammatic Transverse Sections through the Jaws of
Reptiles, showing Pleurodont (a), Acrodont (&), and Thecodont (c)
Dentitions. B, a, Lower Jaw of Zootoca vivipaixi, and 6, of Amjuis
fragilis. (After Leydig.)

freely, while the lesser, reserve teeth lie within the gum (Fig.
233, a) ; only one of these teeth, however, is firmly fixed to the
maxilla at a time. Each fang is perforated by a poison-canal
formed by the meeting of the edges of the longitudinal groove,
and is incompletely suiTounded by the pulp-cavity, the latter
having the form of a half-ring in transverse section (Fig. 233,
B, c,) : the duct of the poison-gland passes into an aperture at
the base of the tooth which leads into the poison-canal, and the
latter opens by a slit at a short distance from the apex of the
tooth on the premaxilla (Fig. 233, a).^

^ The teeth of Hatteria are at first simple in form and their attachment is
acrodont ; the apparent heterodont condition in the adult is due to the fusion of
more than one generation of teeth to the bone, which embraces them basally and
grows down beyond the gums. In this animal, the vomerine teeth are in the
course of suppression ; they are usually absent, but may occur on one or both
vomers. Functionless teeth are present in the embryo which later disappear,
and the same is true of Crocodilus porosus and Iguana tuberculata.

2 A peculiar, broad, lancet-like tooth, originally paired, is present in
embryos of Lizards and some Snakes. It projects considerably beyond its
neighbours in the median line of the snout, and serves the young as a means of
breaking through the parchment-like egg-shell. This must not be confounded
with the analogous structure present in Rana opisthodon or with the horny
*'neb" in Hatteria, Crocodiles, Chelonians, Birds, and Monotremes, which is
purely of an epithelial nature.

318

COMPARATIVE ANATOMY

Chelonians, like existing Birds, are provided with horny sheaths
to the jaws instead of teeth. The presence of teeth in the embryo
of Trionyx, as well as of a vestigial dental lamina in embryos of
Chelone and certain Birds, proves, however, that this is only a
secondary condition. Moreover, in the Cretaceous Birds of
N, America {Odontornithes) teeth were present, and were either
situated in definite alveoli (Ichthyornis), or simply in grooves

Fig. 233. — Poison-Fangs of a Viperine Snake.

A, skull of Rattlesnake ; B, transverse section through about the middle of the
poison-fang of Vipera ammodytes ; C, transverse section through the poison-
fang of Vipera ammodytes near its distal end. (B and C after Leydig.)

GO, poison-canal ; Gz, poison-fang ; PH. pulp-cavity ; 7?z, reserve fangs.

(Hesperornis). The premaxillse were toothless, and seem to have
possessed a horny beak. The single-pointed teeth of Archreopteryx
were probably situated in alveoli.

Mammals.

In Mammals, a shortening of the jaws, which results in an
increase in their power and a diminution in the number of teeth,
has taken place along with a higher differentiation of the individual
tooth and a reduction in the number of tooth-generations. The
heterodont condition which characterises Mammals, as a whole,
has arisen from a more primitive homodont condition, and the
teeth have become adapted in various ways in correspondence with

TEETH 319

the method of taking in and biting or masticating various kinds
of food. The frequent presence of vestigial, functionless teeth
shows that a diminution in number has taken place in the coui-se
of time,^ and such reduction is more marked in the lower than in
the upper jaw. An increase in number, on the other hand, such
as is met with in Toothed Whales, is due to a secondary differentia-
tion, during ontogeny, of primarily multicuspidate teeth, and the
homodont dentition and conical form of the teeth in these
Mammals is therefore a specialised and not a primitive condition.

As already mentioned, the succession is nearly always limited
to two functional sets, the so-called deciduous or ??ii//j- teeth, and
the successional or permanent teeth, and in many cases even one of
these may be vestigial.^ The milk-teeth represent a historically
older generation than the successional teeth : they, however, show
numerous adaptations and modifications, and may even be retained
in the adult permanently {e.g. Marsupials) or for a considerable
time {e.g. Chrysochloris, certain Centetidse). Traces of a still earlier
set occasionally occur — most frequently and distinctly in the more
primitive orders {e.g. Marsupials, Insectivores, Rodents) : this may
be spoken of as a prelacteal dentition. As occasionally also certain
teeth appear which replace the corresponding " permanent " teeth,
indications of at least four sets can be recognised in Mammals.^

In each of the two functional sets, incisors, canities, and cheek-
teeth or grinders, can as a general rule be distinguished. The teeth
which replace the milk-grinders are distinguished as premolars ;
the molars are situated further back in the jaw and have no pre-
decessors."* The latter may therefore be considered as belonging
primarily to the milk-series; or more probably they arise from
germs, which, owing to an abbreviation of development, really
represent more than one set.

All the teeth are embedded in well-developed alveoli of the

^ The last molar of Man, or so-called *' wisdom-tooth," for instance, seems to
be gradually disappearing ; it appears last, is usually lost first, and often does not
reach the grinding surface.

'^ Thus in the Hedgehog, which presents an intermediate stage between the
diphyodont and monophyodont conditions, and also in the Mole and certain
Rodents, the milk-dentition is partially reduced. In Scalops and Condylura also
most of or all the milk-teeth become absorbed without cutting the gum, and a
similar condition is seen in the Pinnipedia, in which, however, they may not be shed
until shortly after birth. Thus the succession may take place in the fcetus,
so that the milk-teeth are functionless. This is also the case in certain Bats,
while in others the milk-teeth may be retained in order to hold the young fast to
the teats, and they thus form an interesting example of the retention of organs
o^Wng to a change of function.

^ Thus the polyphyodontism of the lower Vertebrates is replaced in certain
Reptiles by an oligophyodont condition, and this again, by the diphyodontism
of Mammals, which, by higher differentiation of the individual t^eth, may lead to
a monophj'odont condition.

■* It must, however, be remembered that in some cases the so-called premolars
have no predecessors, and therefore apparently belong to the milk-dentition.
Moreover, in Toothed Whales the persistent "milk-teeth" are said to include
representatives of an earlier and a later generation, and the same applies to
the Manatee.

320

COMPARATIVE ANATOMY

jaw-bones, the upper incisors being situated in the premaxillse, the
upper canines and cheek-teeth in the maxilla?, and the lower teeth
in the dentary. The canine, which corresponds to a specially

Fig. 234, A.— Teeth of Dog {Canis familiaris). The teeth of both jaws from
the side, and those of the upper jaw from below.

i, incisors ; c, canines ; pm, premolars ; m, molars.

m /,'m\

Fig. 234, B.— Teeth of Hedgehog {Erinaceus europceus).

differentiated premolar and is most characteristically developed
in Carnivora, is approximated to the incisors and forms a more or
less continuous series with them. The premolars follow behind

TEETH

321-

the canine, the space often existing between them being called the
diastema, and then come the molars. The primary arrangement
of the teeth is such that there is an alternation between those of
the upper and lower jaw : thus the teeth in one jaw do not usually
correspond in position with those of the other, but with the inter-
spaces between them.

In some cases the enamel-organ, with its dental papilla, persists
in all the teeth, which then continue to grow throughout life
{e.g. Lepus) ; in others this is true of the incisors only {e.g.
Elephant, numerous Rodents, Fig. 237) ; but more usually growth

Fig. 235.— Teeth of Sheep {Ovis aries).

(References as before, but the teeth of the lower instead of the upper jaw are
figured from the surface. )

ceases after a certain time, and the teeth then form definite /<27i^s
or roots, perforated by a small canal communicating with the reduced
pulp-cavity.

The incisors are usually chisel-shaped, while the canines, in
those cases where they are most characteristically developed
(Camivora), possess a pointed, conical form, and are more or less
curved. The cheek-teeth either have sharp, cutting crowns
{secodont, e.g. Carnivora), or may possess infoldings of the enamel,
or tubercles, the crowns being broad and more or less flat, and
adapted for grinding the food. If the tubercles are conical, the
tooth is described as hiinodont {e.g. Pig, Hippopotamus), and if
crescentic, selenodont {e.g. Horse, Ruminants) : in these cases the
relations of the enamel, dentine, and cement are such as to produce

322

COMPARATIVE ANATOMY

an uneven surface with wear, showing a characteristic pattern in
the different groups (cf. Figs. 234-236).

The relations and number of the tubercles, as well as the form
of the teeth in general, are of great importance in elucidating the
ancestral history of the Mammalia, and attempts have been made
to trace the evolution of the various forms of molar met with in
the Class. According to one view, the tuberculated molar has arisen
by the gradual modification of a single conical tooth, which has

tr

Fio. 236. — Teeth of a Catarrhine Monkey {N'asalis larvatm).
References as before.

produced lateral outgrowths or buds. Thus taking a simple conical
form as the most primitive type of mammalian tooth, we find that
certain extinct Mammals (e.g. Triconodon) possessed teeth with a
main cone and two lateral cusps. It has been supposed that the
more complicated forms have been derived from this triconodont
tooth — firstly by a rotation of the lateral cusps outwards in the
upper, and inwards in the lower one, thus forming a trihibercular
tooth, with three cusps arranged in a triangle ; and secondly by
the addition of accessory cusps (the first to appear being the
posterior heel or talon), as well as of connecting ridges.^

^ According to another hypothesis, the mammalian cheek-teeth were primarily
multitubercular, having originated by the fusion of a number of simple conical
teeth ; and certain facts in their development and the presence of multituber-
culate Mammals in the Triassic rocks indicate that in some cases, at any rate,
they may have become evolved in tliis manner.

TEETH

323

The Marsupials form a marked contrast to those Mammals re-
ferred to on p. 319 in which the milk-dentition has undergone re-
duction, for in them there is only one functional successional tooth,
the fourth premolar, all the others belonging to the milk series.
Bud-like enamel-germs occur, however, near all the persistent
teeth anterior to the third molar, and these must be looked upon
as rudiments of potential teeth and not as vestiges which have
become functionless.

In Omithorhynchus, in which there are indications that two
dentinal series are represented, the three multitubercular teeth
present on either side of the upper and lower jaws become replaced
functionally after a time by the development of homy masticatory

Fig. 237. — Outline of the Skull of Geomys, showing the Relations of the

Teeth (after V. Bailey).

I, incisors (which in Rodents correspond to the second incisors of other Mammals) ;

p^, premolars ; m, 1, 2 3, molars.

plates,^ and in Echidna they are wanting altogether, the dental
lamina undergoing reduction at an early stage. Adult Whalebone-
Whales and certain Edentates (Myrmecophaga, Manis) are toothless,
but vestiges of teeth occur in the embryo. In other Edentates the
teeth are wanting in enamel. Canines are absent in certain
Mammals (e.g. Rodents), and the incisors may also be wanting. In
the typical Ruminants functional incisors and canines are present
in the lower jaw only, though tooth- vestiges occur in the pre-
maxillary region of the embryo and occasionally upper canines persist
in the adult."^

^ Horny crushing plates are also present in the Sirenia, the existing forms of
which possess numerous teeth, while the extinct Rhytina was toothless.

2 In the Manatee, the dentition is very peculiar. Traces of incisors, canines,
and premolars occur in the embryo, but the adult possesses molars only, which
undergo a constant succession from behind throughout life as the anterior ones
are pressed forward and fall out ; there may be as manj^ as 8 — 10 functional at
the same time. In the Elephant there is also a similar succession of molars, but
it is here limited to six.

Y 2

324 COMPAKATIVE ANATOMY

Sexual differences in dentition exist in a number of Mammals.
Thus in Apes the canines and first premolars are more strongly
developed in the male than in the female. In the male Wild Boar,
Narwhal (Monodon), Dugong (Halicore), and Musk-deer a modifi-
cation of certain of the teeth (the canines or the incisors) to form
tushs occurs, and these serve as fighting weapons. In the Elephant
and Walrus tusks are present in both sexes : in the former they
correspond to incisors, and in the latter to canines. In Elephas
antiquus they were relatively enormous.

In describing the teeth of a Mammal it is convenient to make
use of a dental formula in which their number and arrangement
can be seen at a glance, the teeth of one side only being repre-
sented. Thus the adult dental formula of those animals, the teeth
of which are represented in Figs. 234 to 237, would be : —

Fig. 234, A Dog, l;!^^ =42

1 -4 • 3
,, 234,BHedgehog|;^/.|;|

:36

235. Sheep, ^J-^^ =32

236. Catarrhine Monkey, ^ ' ^ ' ^ ' ^ = 32

„ 237. Geomys, ^ .' ^ .' ^ .' ^

2 • 1 • 2 • 3

= 20

The most complete dentition is seen amongst Marsupials, the

dental formula of Myrmecobius being - — - — - — — = 52 — 54.

3 * 1 ' 3 • 5 or 6

. 3 • 1 * 4 • 3

The more typical arrangement is '^ — - — j — = 44.

Glands of the Mouth.

The glands of the mouth, like those of the orbit and integu-
ment, appear first in terrestrial Vertebrates, that is from Amphi-
bians onwards.^ They have the function of keeping moist the
mucous membrane which comes into contact with the outer air.
From being at first almost entirely unspecialised, and giving rise
simply to a slimy fluid, they become differentiated later into
structures the secretions of which take on a very important function
in relation to digestion ; they may also, as in the case of poisonous
Snakes and Lizards, constitute dangerous weapons of offence.

^A paired glandular organ (so-called "salivary gland") is present in
Petromyzon between the mouth and first pair of gill apertures, towards the
ventral side. It is surrounded by muscles, apparently produces a fatty secretion,
and opens into the mouth.

GLANDS OF THE MOUTH 325

With their increasing physiological importance a greater
morphological complication both as regards number and arrange-
ment takes place. Their histological character also undergoes
changes, so that the most varied forms of glands may be
recognised.

Amphibians. — With the exception of the Perennibranchiata
Derotremata, and Gymnophiona, a tubular gland becomes developed
in all Amphibia from the anterior portion of the roof of the mouth
(cf. Fig. 193), the main mass of which in Urodeles lies in the cavity
of the nasal septum or premaxilla {intermaxillary or intcrnasal
gland). In Anura its position is more anterior than in the Urodela,
and it is more largely developed ; but in both cases the ducts open
into the anterior part of the mouth. In Anura there is a second
gland {-pharyngeal gland) present in the region of the internal
nostrils, the secretion of which passes partly into the latter and
partly into the pharynx. Numerous gland-tubes are also present
in the tongue of Amphibians, and in the Gymnophiona oral glands
are abundant.

Reptiles. — The oral glands in Reptilia show an advance on
those of Amphibia in being separated into groups. Thus not
only is there ^'palatine gland, homologous with the intermaxillary
gland, but lingual'^ and sublingual, as well as upper and lower
labial glands are present. Chameleons and Snakes are distin-
guished by a remarkable richness in glands, which are most
specialised into definite groups in the latter.

In poisonous Snakes the poison- gland becomes differentiated
from a portion of the upper labial gland. It is tubular in structure,
is enclosed in a strong fibrous sheath, and is acted upon by powerful
muscles, so that its secretion can be poured with great force into
the duct and thence into the poison-fang (Figs. 233 and 238).
The sheath of the poison-gland is formed by a pocket-like enlarge-
ment of the zygomatic ligament, and is compressed by the muscles
of the jaws. When the Snake strikes, the lower jaw is depressed
to its fullest extent and the quadrate, pterygoid, palatine, and
transpalatine are pushed forwards by the contraction of the
posterior pterygo-sphenoid and pterygo-parietal muscles, thus
causing the maxilla to move on its articulation with the prefi'ontal
and to erect the fang. The upper jaw is again brought into its
position of rest by the contraction of the anterior pterygo-
sphenoidal and transverso-maxillo-pterygo-mandibular muscles.

The sublingual gland of a Mexican Lizard, Heloderma, is also
of poisonous nature. The secretion passes out through four ducts,
which perforate the bones of the lower jaw in front of the grooved
teeth (p. 317).

^ In Lacerta, Anguis, and Pseudopus there are numerous depressions of the
lingual epithelium lined by goblet cells, which, however, are not differentiated
into definite compound glands.

326

COMPARATIVE ANATOMY

In marine Chelonians and Crocodiles there are no large glands
united into groups connected with the mouth, but in Testudo
grseca there are well developed sublingual glands.

PterygosphenoidaZis Pterygo-

ant. parietalis

Retractor quaclrati

Maxilla

Fang

Quadrate

-' Pterygosphen-
oidalis post.

Mandible

Palatine '■ -j
Pterygoid

Pterygospkenoidalis post. —

Duct oj poison-gland
— Fang
~ Zygomatic ligament

Kr-

-'Poison-gland and sheath

"■^ Masticatory muscles
" Zygomatic ligament
— Mandible

Portion oJ trans.-max.-pter.-mand. muscle

Fig. 238. — Head of the Viper (after Katheriner).

A, from the left side : the integument, zygomatic ligament, poison-gland, jaw-
muscles, and palatine and pterygoid teeth are not indicated. B, left side
from below : the muscles of the jaw are cut through transversely, and part
of the sheath of the poison-gland is slit open.

Birds. — In Birds, and more especially in climbing Birds
(Scansores), a well-developed lingual gland is present opening on
the floor of the mouth, and another at the angle of the latter.

TONGUE 327

There is no doubt that the lingual glands are to a great extent
homologous with those of Lizards, but it is not known whether
the gland at the angle of the mouth corresponds with the posterior
upper labial gland of Reptiles — that is, to the poison-gland of
Snakes. The median palatine glands of Birds are not homologous
with those of Reptiles, and labial glands are wanting. The lingual
glands are supplied by the glossopharyngeal, the others by the
trigeminal.

Mammals. — Three larger sets of salivary glands which have
become secondarily separated from one another may be distin-
guished in connection with the mouth in Mammals : these are
called, according to their position, (1) parotid^ (2) siibmaxillary,
and (3) si(hlingual. Each of the two former opens into the
mouth by a well-defined duct, that of the sublingual having
several independent ducts. A special retrolingual portion usually
becomes differentiated from the sublingual gland and commu-
nicates with the submaxillary duct. They all have a tubular or
tubulo-alveolar structure.^

The parotid is usually situated at the base of the external
ear. The submaxillary is a compound mucous and serous gland,
consisting of elements which differ from one another histologically :
it lies beneath the mylohyoid muscle, close to which the retrolingual
gland is also situated : the latter is wanting in only a few Mammals
{e.g. Rabbit, Horse). The sublingual gland extends between the
tongue and the alveoli of the teeth, and is rarely absent {e.g.
Mouse, Mole, Shrew).

With the exception of the parotid, the homology of which is
not clear, all these glands, together with certain smaller and less
important ones (buccal, lingucd, palatine, and labial glands), are com-
parable to the onil glands of lower Vertebrates.^

Tongue.^

Fishes. — The tongue is, rudimentary in Fishes, and, as a rule,
is simply represented by a fold of mucous membrane covering the
basi-hyoid, which in all the higher Vertebrates serves as a point of
origin for many of the lingual muscles. Except in Cyclostomes,
where it has to do with the suctorial or boring apparatus, the

1 In some Mammals {e.g. Lepus) there is also an infraorbital or zygomatic
salivary gland, covered by the zygoma and extending into the orbit, its duct
opening anteriorly to that of the parotid. Salivary glands are wanting in the
Cetacea.

- The mucous glands are phylogenetically older than the serous glands, and
their essential function is merely to moisten the food. The serous glands have to
a greater or less extent become differentiated into an apparatus for producing a
secretion containing enzymes and acting chemically on the food.

^ For the papillae of the tongue, of. p. 254.

328

COMPARATIVE ANATOMY

tongue of Fishes is not capable of movement apart from the
visceral skeleton, and is wanting in a proper musculature. It is
provided with papillae and serves only as a tactile organ, or, when
provided with teeth {e.g. certain Teleosts, Fig. 69), as a pre-
hensile organ also. In Dipnoans the tongue is not more highly
differentiated than in many other Fishes.

Amphibians. — In Perennibranchiata and young larvae of
Myctodera the tongue is very similar to that of Fishes, and is
comparable only to a small posterior and median part of the
definitive tongue of the latter group : the larger, anterior,

glandular part is a new forma-
tion, and is not represented in
Fishes.

The definitive tongue is very
similar in many Urodeles and
Anurans, although various dif-
ferences are seen in its mode of
development in the two groups.
Thus in Anura the primitive
tongue persists for a much
shorter time, and is connected
on the floor of the mouth with
the more anterior, larger, second-
ary part in a different way; the muscles are earlier developed
and more numerous, and the glands appear very late. In general
the tongue is more highly differentiated in Anurans than in
Urodeles in consequence of functional adaptation connected with
catching the prey.^

The surface of the tongue is velvet-like, owing to the numerous
papillse, and its mobility varies greatly in the different forms. It
is usually attached only by the anterior end (Fig. 239) or by a

Fig. 239.— Figure showing the
Tongue of the Frog in Three
Different Positions.

Fig. 240. — Head of Spelerpes fuscus, with the Tongue Extended.

portion of its ventral surface : in other cases it is free all round,
and in Spelerpes (Fig. 240) is capable of being extended far out
of the mouth by means of a complicated mechanism.

Reptiles. — The tongue of Reptiles reaches a much higher
.•stage of development than that of Amphibians. As in them, its

^ The rapid movements of the Frog's tongue are effected by the genioglossus
•and hyoglossus muscles, the former acting as a protractor and the latter as a
retractor, while the intrinsic muscles are responsible for gripping the prey. In
Jbhe A^lossa {Pipa and Xenopus) the tongue has .undergone degeneration.

TONGUE 329

main part arises anteriorly to the primitive larval tongue from an
originally independent region lying between the lower jaw and
basihyoid, and known in the Amniota as the tuhercuhtiii impar.
To this, however, are added portions belonging to the region of the
median part of the hyoid and part of the first branchial arch, as
well as extensive lateral ridges belonging to the mandibular
region, and this fact accounts for the presence of an additional
lingual nerve — a branch of the third division of the trigeminal —
which is wanting in Amphibians. The tongue is provided with
numerous sensory organs, but no glands are present within it.
It is usually very mobile (least so in Chelonians and Crocodiles),
and part of it may be enclosed by a " sheath " (Fig. 241).
In form and relative size it is much more variable than in
Amphibians, and this is especially the case in Lizards, in which
the tongue is used for classificatory purposes (Vermilingtcia,
Crassilinguia, Brevilinguia, Fissilinguia) : in the Fissilinguia
and in Snakes it is forked at the apex. In the Chameleon it is
protrusible, as in Spelerpes amongst Amphibians, but the
mechanism is different in the two cases.

Birds. — Although the early development of the base of the
tongue -rudiment in connection with the hyoid and first branchial
arch is apparently similar in all Birds and resembles that seen in
Reptiles, many differences are noticeable in its subsequent develop-
ment in the various groups of Birds (e.g. Lamellirostres and
Fringillidse), and these concern the tuberculum impar and the
modification of its anterior region in connection with various
functional adaptations. Important differences are seen in the
muscles and nerves as compared with Reptiles. A sensory branch
of the trigeminal nerve is wanting, and is replaced functionally by
the strongly developed glossopharyngeal.

The tongue of Birds is in general poorly provided with muscles.
It usually possesses a horny covering and is provided with papillae
and pointed, recurved processes ; it may, as in many Reptiles, be
split up at its distal end, being either bifurcated (Trochilidse) or
having a brush-like form. In Woodpeckers (cf p. 123) the tongue
may be thrown far out from the mouth by means of a complicated
system of muscles, and it thus serves as a prehensile organ. The
tongue is relatively largest in predatory Birds (Rapaces) and
Parrots : in the latter it is soft and cushion-like, its size being due
not so much to the special development of muscles as to the
presence of fat, vessels, and glands.

Mammals. — The tongue reaches its most complete morpho-
logical and physiological development in Mammals, and undergoes
the most various modifications in form and function. It is as
a rule flat, band-like, and rounded anteriorly, is rich in glands^ and

^ A gland on the apex of the tongue (gland of Blandin or Nuhn) occurs in
Man, the Orang-outan, and Sheep.

330

COMPARATIVE ANATOMY

Fig. 241. — A, Tongue, Hyoid-Apparatus, and Bronchi of a Gecko {Phyllo-
dactylus europceus) ; B, Tongue of Lacerta ; C, of Monitor indicus ; D, of
Emys europcea ; E, of an Alligator.

B, bronchi ; HH and VH, anterior and posterior cornua of hyoid-apparatus ;
K, larynx ; L, glottis ; Lg, lung ; M, mandible ; T, trachea ; Th, thyroid ;
Z, tongue ; ZK, entoglossal bone ; ZS, sheath of tongue.

THYROID

331

is always extensile except in the Cetacea. The intrinsic muscu-
lature is highly developed, and may even extend backwards over
the sternum (Manis, Myrmecophaga). In the Ruminants, in which
upper incisors are wanting, it is very important in browsing. In
some cases {e.g. Felidse) its surface is horny.^ A fold, the so-called
suhlingua (plica fimbriata), is present on the lower surface of the
tongue, and is especially well marked in Lemurs ; in the Slender
Loris (Stenops) it is supported by cartilage.^ This structure has
been supposed to correspond to the last vestige of the tongue of
lower Vertebrates (Reptiles) which has been replaced by the more
highly-developed organ characteristic of Mammals, the latter
having arisen secondarily from the reduced sublingua.^

Tni

Thyroid.

In Amphioxus, as in Ascidians (Tunicata), a ciliated groove,
the e7idostyle, is present along the ventral border of the extensive
pharynx, and the cells lining this groove secrete a glutinous
substance in which the food particles be-
come entangled and by the action of the
cilia are carried onwards to the intestine.
This endostyle is without doubt homologous
with the thyroid of Craniates, the presence
of which is as characteristic of them as is
the notochord. In consequence, however, of
the different method of taking in food and
the presence of jaws, it has undergone a
change of function and never remains open
to the pharynx throughout life, but gives rise
to a so-called " ductless gland," the substance
formed by its "internal secretion" passing
into the lymph or blood.

The thyroid arises primarily as a median
ventral diverticulum of the pharynx which
extends along the region of the first four or
five visceral clefts, and in the course of de-
velopment may become subdivided into two
lobes. In addition to this unpaired diver-
ticulum, paired portions, situated more pos-
teriorly, are developed in Mammals : the
former, as in all Gnathostomes, arise from the basihyal region,

^ Horny papilla are present on the tongue of Ornithorhynchiis.

- The so-called "/yssa" of the Mammalian tongue consists partly of cartilage,
and partly of muscle, fat, and connective tissue : it corresponds to a vestige of
the lingual cartilage of lower Vertebrates, and undergoes various modifications.

^ According to another view, the sublingua is not a vestige, but a structure
appearing for the first time in certain Mammals which has been secondarily dif-
ferentiated from a ventral portion of the tongue proper : even then, however, its
cartilage may have an ancestral significance.

Fia. 242.— Thyroid
AND Thymus of
Lacerta agilis.

H, heart ; T, trachea ;
T7n, thymus ; Tr,
thyroid.

332

COMPARATIVE ANATOMY

just in front of which is the tuber-
culum impar of the tongue-rudiment.

In the Ammocoete, the simple
diverticuhim, which is lined by cili-
ated epithelium, opens into the
pharynx between the third and
fourth clefts (Fig. 260), but in the
adult Petromyzon the organ, as in
all Craniata, loses its connection with
the pharynx and undergoes consider-
able reduction and a change of
function, giving rise to numerous
closed follicular masses.^

In Elasmobranchs the thyroid is
unpaired and lies behind the man-
dibular symphysis, just in front of
the bifurcation of the ventral aorta ;
in adult Teleosts it is paired, and is
situated in the region of the first
branchial arch. In Dipnoans it lies
anteriorly to the muscles of the
visceral skeleton and shows an in-
dication of a division into right and
left lobes.

In the Urodela and Anura the
thyroid is situated close to the
anterior end of the pericardium : it
undergoes subdivision and forms
numerous vesicles lying posteriorly
to the second ceratobranchials in the
former, and on the ventral side of the
posterior cornua of the hyoid in the
latter.

In Lizards the thyroid is situated
close to the trachea, slightly behind
the middle of its course (Fig. 242),
and in Chelonians and Crocodiles it
Fig. 243 -T™us and Thyroid usually possesses right and left lobes

lying on the great vessels just after
Q they leave the heart. In Birds (Fig.

Tm, 243) the organ is paired, and has a
similar position.

The thyroid of Mammals (Fig.
227) consists of two lobes often connected by a median isthmus

OF A Young Stork.

B, bronchi ; H, heart
phagus ; 7\ trachea
thymus ; Tr, thyroid.

^ In Myxine the thyroid consists of rounded or oval capsules surrounded by
fat ; they are nearly all median and are arranged separately or in groups
between the gill-sacs on either side, the gullet above, and the ventral aorta
below.

THYMUS 333.

situated on the ventral side of the larynx and trachea, and if well
developed, constituting a " middle lobe."^

The function of the thyroid is not thoroughly understood, but
the organ is essential for the well-being of the individual, its
extirpation commonly resulting in various disturbances of the
mental and organic functions. It is extremely vascular, especially
in Mammals, and gives rise to a substance which may contain
iodine.

Thymus.

The thymus has always a paired, epithelial origin, and is thus
primarily of a glandular nature : its epithelial character is retained
throughout life, although the large cells of which it is orginally
composed undergo a marked subdivision into smaller elements.

In Cyclostomes, the presence of a thymus has not been proved.
In Elasmobranchs it arises on either side from the endodermic
epithelium lining the upper angles of the first five gill-clefts, near
the ganglia of the ninth and tenth cerebral nerves, as well as in
the neighbourhood of the spiracle : it appears that all the gill-
pouches originally took part in its formation, as is also indicated
in Teleosts, Caecilians, and even in Snakes.

In Teleostomes and Dipnoans, the thymus is similarly situated
dorsally to the branchial region, but certain modifications occur,
part of it undergoing resorption, while a subdivision into lobes or
a secondary fusion of originally distinct parts occurs. In adult
Urodeles and Anurans it lies behind and above the articulation of
the lower jaw.

In all the Amniota, the th3rmus is developed in connection
with the three or four anterior pharyngeal pouches. In
adult Snakes, and also in Lizards and Chelonians, it consists of
two or more separate lobes situated near the carotid arteries.
Young Crocodiles and Birds possess an elongated, band-like, lobed
thymus extending along the neck (Fig. 243).'^ In Mammals
usually only a small portion of the organ is present in the cervical
region, its greater part being situated in the thorax, just above
the sternum. In young animals it is usually very voluminous, and
increases in size up to sexual maturity, then gradually becoming
reduced, without however, losing its function, whatever that

^ Under the term parathyroids, or" accessory thyroids," are understood those
parts of the thyroid which may arise from its unpaired rudiment {ductus
thyreoglossus). The so-called " epithelial bodies" formed from the second to the
fifth pharyngeal pouches, as well as the ulttmobranchial (posfhranchial) body
arising from the most posterior cleft, have nothing to do with the thyroid,
although they may come into intimate relation with it as well as with the
thymus. The structure known as the '^carotid gland'' in Mammals, which is
situated at the bifurcation of the common carotid artery of either side, belongs
to a peculiar category of organs which have genetic relations to the sympathetic
system (cf. p. 247) : a similar structure is said to be present in Birds.

^ Muscular elements occur in the thymus of Amphibia and Sauropsida.

334

COMPARATIVE AKATOMY

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OESOPHAGUS, STOMACH, AND INTESTINE 335

function may be : at any rate it is not a " lymphoid organ," and
the origin of the leucocytes occurring in it is not known.^

(ESOPHAGUS, STOMACH, AND INTESTINE.

Ichthyopsida. — The oesophagus in the Anamnia is short, and
usually not distinctly marked off from the stomach, though excep-
tions to this rule often occur {e.g. many Teleostei, Siren lacertina).

The stomach is often defined as a widened section of the enteric
canal situated between the posterior end of the gullet and the
entrance of the bile duct. Such a dilatation can strictly only be
spoken of as a stomach when its epithelium possesses specific
characteristics and gives rise to gastric glands : in this sense
a stomach is wanting in Amphioxus, Cyclostomi, Holocephali,
certain Teleostei {e.g. Cyprinidse, certain Labridae, Gobiidse, and
Bleniidse, Syngnathus, Cobitis), and Dipnoi (Fig. 247). In many
Teleosts the bile-duct opens far forwards, so that the gastric
region is very short.

In other Fishes, as well as in all Amphibians, a true stomach
is present, and is usually externally recognisable as a more or
less dilated sac ; it may be curved on itself, so as to form a
U-shaped loop, the two (cardiac and pyloric) limbs of which lie
parallel to one another (Fig. 244). In general, its form is adapted
to that of the body : thus Rays and Anurans possess a far wider
stomach than do most other Fishes and Amphibians (cf Figs.
244-249). The stomach of Teleosts varies considerably in form.^

The intestine may be straight or nearly straight, or may be
more or less coiled, and in the former case a spiral fold or xalve
may be developed in Fishes, to increase the absorptive surface.

In the Lamprey a longitudinal fold or typhlosole, taking a slightly
spiral course, extends into the lumen of the intestine. In Elasmo-
branchs. Ganoids, and Dipnoans, the fold is more highly developed
and forms a well-marked spiral valve, the turns of which may lie
so close together as to almost fill the cavity of the intestine (Fig.
244).^ In the Ganoids it begins to undergo degeneration : thus
in Lepidosteus it is only present in the hinder part of the in-
testine (Fig. 245). Traces of a spiral valve can even be recognised
amongst the Teleostei (Cheirocentrus).

^ The so-called " Hassal's corpuscles " in the thymus arise secondarily from
groups of the small epithelial cells.

- In numerous Teleosts {e.g. Tinea vulgaris, Cobitis fossilis) outer longi-
tudinal and inner circular stHated fibres are present in both stomach and intestine
externally to the unstriated muscular coat. They grow backwards from the
oesophagus.

^ The arrangement and extent of the valve vary considerably ; it may begin
close behind the pylorus, or the valveless anterior part (" bursa entiana") may be
relatively longer. In some cases the valve is scroll-like, and not spiral {e.g.
Garcharias).

336

COMPARATIVE ANATOMY

-St

lr..-\

a.

My\

s<.-::.

sp.v

Fig. 245. — Alimentary Vis-
cera AND Swim-Bladder
OF Lepidosteus {in situ).
(After Balfour and Parker.)

a, anus; a.h. swim-bladder;
a.¥, its aperture into the
throat ; h.d^, aperture of
bile-duct into intestine ;
c, pyloric casca ; y. h, gall-
bladder ; hp.d, hepatic
duct ; Ir, liver ; py, pyloric
valve ; s, spleen ; sp. v,
spiral valve ; st, stomach.

^ In the Holocephali there
the rectum encloses numerous

Pyloric cmca are met with in Ganoids
(except Amia) and most Teleosts, and
consist of longer or shorter finger-
shaped processes of the small intestine,
situated posteriorly to the pylorus in
the region of the bile-duct (Figs. 245,
246). Their number varies from 1
(Polypterus and Ammodytes) to 191
(Scomber scomber) : in some fishes they
are bound together by connective tissue
so as to form a compact mass. The
Shark Lsemargus possesses a pair of
caeca opening into the anterior part of
the intestine.

In the narrow-bodied Gymnophiona
the intestine is only slightly coiled,
while in Anura it becomes considerably
folded on itself: its form in Sala-
manders is about mid-way between
these two extremes.

In the Cyclostomi, Holocephali,
Ganoidei, and most Teleostei, there is
a separate anus; in all other Fishes
as well as in the Amphibia the large
intestine opens into a cloaca, common
to it and to the urinogenital ducts.
The large intestine (rectum) is com-
paratively short and takes a straight
course ; in Amphibians, as well as to
some extent in certain Ganoids and
Teleosts, it is plainly marked off from
the small intestine by its size, and be-
tween the two there is often a circular
valve. In some cases the rectum is
considerably swollen and may even
exceed the stomach in capacity (Fig.
249). An outgrowth of the ventral
wall of the cloaca in Amphibia gives
rise to the urinary Uadder and repre-
sents the allantois (q.v.) of higher forms.
In Plagiostomes a finger-shaped
rectal gland (processus digitiforniis) ^
opens into the anterior part of the
rectum, and this perhaps corresponds
to the blind gut or ccecum of higher
forms. Traces of a caecum are seen in
certain Teleosts {e.g. Box). In the

is no processus digitiformis, but the thick wall of
gland-tubules.

(ESOPHAGUS, STOMACH, AND INTESTINE 337

► Fig. 246. — Alimentary
Canal of Perch.
A, anus ; Ap, pj^oric c^eca ;
ED, rectum ; J/, stomach,
with caecal process (t) and
short pyloric region (P—P) ;
MD, small intestine ; Oe,
oesophagus.

ap,

Fig. 247.
Fig. 247.— Alimentary Canal and Appkndages of Protopterus annectens.
(After W. N. Parker.)

abdominal pore; h.d, common bile duct, and h.d}, its aperture into the
intestine; h.ent, bursa entiana (anterior portion of intestine); cl, cloaca;
cl.c, cloacal cfecura ; c.m.a, cceliaco-mesenteric artery ; cy.d, cystic duct,
g.b, gall-bladder; h.d, hepatic ducts ; h.p.v, hepatic portal vein; k.d, base
of kidney duct; Ir, liver; m.a^, m a^, mesenteric arteries; ood, base of
oviduct ; oes, oesophagus ; py. v, pyloric valve ; re, rectum ; sp, spleen ;
sp.v, spiral valve; st , "stomach" ; v, vent. The pancreas is not seen, as it is
embedded in the walls of the stomach and anterior part of the intestine
on the dorsal and right side.

Z

338

COMPARATIVE ANATOMY

Oe

if

-M

\MD

-FD

-M

Fig. 248. Fig. 249.

Fig. 248. — Alimentary Canal of Birm lacertina.

ED, large intestine ; MD, small intestine ; Oe, oesophagus, marked off from the

stomach [M) by a constriction (t) ; P, pyloric region.

Fig. 249. — Alimentary Canal of Rana escuhnta.

A, opening of the rectum into the cloaca {CI) ; D, ileum ; Du, duodenum ; fj
boundary between small and large intestine ; HB, urinary bladder ; Oe,
oesophagus ; M, stomach ; Mz, spleen ; Py, pyloric region ; E, rectum.

Dipnoi a cloacal caecum is present (Fig. 247; cf. under Urino-
genital Organs).

In all Fishes in which a cloaca is wanting, the anus is
anterior to the urinogenital aperture.

(ESOPHAGUS, STOMACH, AND INTESTINE 339

Reptiles. — In correspondence with the more definitely differen-
tiated neck, the oesophagus of Reptiles is relatively longer than in
the animals as yet considered;' it is always plainly marked off
fi'om the much wider stomach, which is usually sac-like, or bent
upon itself, in which latter case it lies transversely (Chelonians).^
As regards external form, the stomach of Crocodiles is more
specialised than that of other Reptiles, approaching that of Birds.

Snakes, snake-like Lizards, and Amphisbsenians possess a
narrow, spindle-shaped stomach, which lies in the long axis of the
body ; in correspondence with the large size of the masses of food,
which are swallowed whole, it is capable of great distension. In
these the intestine is only slightly coiled : in other Lizards the
coils are more marked, and in fomis with broad bodies (e.g,
Chelonians, Crocodiles) the folding is carried still further.

The large intestine has a straight course, is often considerably
swollen, and opens into a cloaca. It may {e.g. certain Chelonians)
be as long as the small intestine and be bent on itself. [For the
urinary (allantoic) bladder present in many Reptiles, cf under
Foetal Membranes and Urinary Organs.]

In many Reptiles {e.g. most Lizards, Snakes), a small blind-
gut or caecum is present at the anterior portion of the large
intestine : it is generally asymmetrical.

Birds. — In correspondence with the kind of nutriment, the
mode of life, and the absence of teeth, certain modifications of the
oesophagus and stomach occur in Birds. In graminivorous Birds and
Birds of Prey either the whole gullet forms a dilated sac or else it
gives rise to a ventral outgrowth ; in both cases the enlargement
is known as the crop {ingluvies) (Fig. 250, a). This serves as a food
reservoir, and in some cases its walls are glandular.

The stomach, instead of remaining simple, generally becomes
divided externally into two portions, an anterior and a posterior.
The former, which on account of its richness in glands is called the
glandular stomach {proventricuhts), alone takes part in dissolving
the food; while the latter, which is lined by a keratinoid layer
consisting of a hardened glandular secretion, has simply the
mechanical function of grinding the food, in correlation with
which a very peculiar and thick muscular wall provided with two
tendinous discs is developed (Fig. 250, B and c). The degree
of development of this muscular stomach, or gizzard, is in
direct proportion to the consistency of the food. Gramini-
vorous Birds possess the strongest muscular layer and the thickest
keratinoid lining, while in the series of insectivorous Birds, up to
the Birds of Prey, this condition becomes gradually less marked,

^ The oesophagus of marine Chelonians, like that of many Birds, is lined by
horny papillae, and in the stomach a differentiation of distinct gland-zones is
seen, such as is already indicated in certain Fishes and is carried still further in
Mammals.

z 2

340

COMPARATIVE ANATOMY

and the division of labour is not so noticeable. Thus in the series
of existing Birds we can trace the course of the phylogenetic
differentiation of the organ.

The small intestine is usually of considerable length and
becomes folded on itself to a greater or less degree ; it varies,
however, in form, relative length, and diameter.

The straight large intestine opens into a cloaca, and differs as
to its relative diameter. The ca3cum is usually paired, and may

^

MD

DM

or '\-jy/M

dm:

MS

-JIM

C

Fig. 250, A. — Diagram of the (Esophagus and Stomach of a Bird.
DM, glandular stomach ; Ig, crop ; MD, duodenum ; MM, muscular stomach ;

Oe, Oe^, oesophagus.

Fig. 250, B. — Glandular Stomach and Gizzard of Fulica atra.
S, tendrinous disc. (Other letters as in A. )

Fig. 250, C. — Transverse Secpion through the lateral part of the Gizzard
OF Tetrao urogallus.
D8, glandular layer ; L, lumen ; MS, muscular layer ; BP, keratinoid
triturating layer.

be extremely long (Lamellirostres, Rasores, Ratitas). All kinds of
intermediate stages between this condition and an entire absence
of a caecum are to be met with. When largely developed, it must
have an important relation to digestion, as an increase of surface
of the mucous membrane is thus effected ; this increase may even
be carried further by each caecum being provided with a spiral fold
consisting of numerous turns, as in the Ostrich.

This so-called bu7'sa Fabricii is a structure peculiar to Birds,
and arises as a small, solid, epithelial outgrowth from the ecto-

(ESOPHAGUS, STOMACH, AND INTESTINE 341

dermal portion of the cloaca (proctodaeum), later becoming exca-
vated to form a vesicle. It is situated in the pelvic cavity between
the vertebral column and the posterior portion of the intestine,
opening into the outer section of the cloaca posteriorly to the
urinogenital ducts. It is probably present in all Birds, but becomes
atrophied more or less completely in the adult ; its physiological
function is unknown.

Mammals. — The oesophagus, like that of Birds, is sharply
marked off from the stomach, and its muscles consist for a greater
or less extent of striated fibres derived from those of the pharynx.

The stomach undergoes much more numerous modifications
than are met with in any other vertebrate Class. As a rule it
takes a more or less transverse position and has a sac-life form,
the cardiac portion, into which the oesophagus opens, and the
fundus, \fh.\Q\i lies towards the left side of the abdomen being usually
more swollen and having thinner walls than the 'pyloric portion,
which communicates with the duodenum. The gastric glands
have in general a different histological and physiological character
in the three regions of the stomach, so that three glandular zones
may be distinguished (Fig. 251; and cf p. 845)

According to the definition given on p. 835, a true stomach is
wanting in Monotremes (Fig. 251, a); and although the organ is
represented by a wide sac, it is entirely wanting in glands, and
is lined throughout by stratified epithelium: this condition is
doubtless secondary. Amongst Edentates, a similar peculiarity is
seen in Manis javanica — in which, however, some of the glands are
retained in a sac-like outgrowth from the greater curvature, the
rest of the stomach being lined by a horn-like layer.

In herbivorous Mammals the stomach is, as a rule, relatively
larger and more complicated than in carnivorous Mammals,
and it may become divided into two or more chambers. In
Bradypus, many Rodents (Muridge) and in the Horse distinct
cardiac and pyloric chambers can be recognised, and in herbivorous
Marsupials and Ungulates numerous intermediate forms between
a simple and an exceedingly complex stomach, such as occurs in
the typical Ruminants, are to be met with (Fig. 251). In the
latter (Fig. 252) the stomach is divided into four chambers, which
are called respectively rumen (paunch), reticulum, psalterium, and
ahomasum. The two first, which may be looked upon as parts of
one and the same chamber, simply serve as storage cavities, the
food returning from them into the mouth, once more to undergo
mastication. It then again passes down the gullet, and is con-
ducted along a groove to the psalterium, the edge of the groove
closing, and finally into the abomasum, the latter alone being pro-
vided with peptic (rennet) and pyloric glands, and serving as the
true digestive stomach : the other chambers are almost or entirely
glandless, and are lined by pavement epithelium.

342

COMPARATIVE ANATOMY

Fig. 251.— Diagrams of the Stomach in Various Mammals showing the
Different Regions. (After Oppel. )

A, Ornithorhynchus ; B, Kangaroo [Dorcopsis luctosa) ; C, Toothed Whale
{Ziphius) ; D, Porpoise ; E, Horse, F, Pig ; G, Hare ; H, Hamster

( Cricetus jfrumentariug ) .

The oesophageal region (lined by stratified epithelium) is indicated by transverse
lines ; the region of the cardiac glands by oblique lines ; that of the fundus-
glands by dots ; and that of the pyloric glands by crosses.

Z>. duodenum ; /(in H), fold bounding the a?sophageal region ; I — IV (in D), the
four chambers of the stomach ; I (in B), lymphoid tissue ; Oes, oesophagus ;
P, pylorus ; a;...a; (in B), boundary line between the oesophageal and cardiac
regions.

(ESOPHAGUS, STOMACH, AND INTESTINE 343

The psalterium is the latest to be differentiated both phylo-
genetically and ontogeneticallj, and is rudimentary in the Tragu-
lidae. In Camels the rumen gives rise to two masses of gland-
containing outgrowths, known as " water-cells " : the latter are
separated from one another by septa and provided with sphincter-
like muscles. In the Cetacea (Fig. 251, c and d) and Hippo-
potamus, the stomach is divided into several chambers, and
various other modifications in form and structure are met with

Fig. 252. — Stomach of Sheep. (From Oppel, after Carus and Otto.)
«?sophagus ; b, c, d, the three subdivisions of the rumen, marked off from one
another by the folds e and/; g, reticulum ; h, oesophageal groove ; ^, psalte-
rium ; k, aperture leading from the psalterium into the abomasum (?, m) ; n,
pyloric valve ; o, duodenum*

amongst Mammals. Thus in the Kangaroo, for instance (Fig.
251, b), the walls of the stomach are curiously folded, and in the
blood-sucking Bat, Desmodus, the pyloric region gives rise to a
caecum two-thirds as long as the whole intestine.

The small intestine is usually long, and varies more as to rela-
tive length and diameter in domesticated than in wild forms : its
first part, as in Birds, usually forms a duodenal loop.

The large intestine, which is made up of a varying number of
coils, usually reaches a great length, and its diameter is much
greater than that of the small intestine : these two portions are

344 COMPARATIVE ANATOMY

thus sharply marked off from one another, and the distinction
between them is rendered still more marked by the sacculations
of the anterior part of the large intestine. Only the posterior
portion of the latter, or rectum, which passes into the pelvic cavity,
corresponds to the large intestine of lower Vertebrates ; the re-
maining and far larger part occurs only in Mammals, and is called
the colon.

The caecum, which is almost always present, undergoes various
modifications both as to form and size. Thus in Edendates (Manis,
Bradypus), many Carnivora, Odontoceti, Insectivora, and Cheirop-
tera, it is very small or even entirely wanting, while in Herbivora
it may exceed the whole body in length. An inverse development
in size is usually noticeable between it and the rest of the large
intestine. In many cases (certain Rodents, Anthropoid Apes, and
Man) an arrest of a portion of the caecum takes place in the course
of individual development, so that little more than the distal end
(processics vermi/ormis) remains (Fig. 227). In Lepus the enor-
mous caecum is provided with a spiral valve, and in Hyrax, besides
a large sacculated caecum at the junction of the small and large
intestines, there is a pair of large, simple, conical caeca further
back.

Monotremes only amongst Mammals possess a distinct cloaca,
though in Marsupials and some Rodents and Insectivores (especi-
ally in the female) the anal and urinogenital apertures are sur-
rounded by a common sphincter. In other Mammals these
apertures become completely separated from one another.

HISTOLOGY OF THE MUCOUS MEMBRANE OF THE ALIMENTARY

CANAL.

The epithelium lining the alimentary canal of Vertebrates — with
the exception of that of the mouth and cloaca, w^hich is usually
stratified — consists primitively, that is, phylogenetically, of amoeboid
or ciliated cells. In some cases this is also true ontogenetically, and
in Amphioxus and Protopterus for instance, the ciliated epithelium
persists throughout life and in the Lam'prey until metamorphosis.
In the adult Petromyzon, as w^ell as in many Fishes and even Amphi-
bians, ciliated epithelium occurs constantly only in certain parts of
the gut, and in the higner Vertebrates cilia are only seen excep-
tionally after the embryonic period, so that, as a rule, only ordinary
columnar epithelium is present. A striated margin is observable
along the free border of the columnar cells, and may be looked
upon as a last indication of the earlier ciliated covering : in some
lower Vertebrates {e.g. Elasmobranchs, Proteus, Salamander larvae)
the individual cells are even capable of an active amoeboid move-
ment. In this active participation of the cells in the process of
absorption an inheritance from primitive Invertebrates can be

HISTOLOGY OF THE MUCOUS MEMBRANE 345

recognised {intracellular digestion) ; but, at the same time, extra-
celhUar digestion, in which no external change in the individual
cells can be seen, is always the more important in Vertebrates
and occurs exclusively in the higher types.

In Amphioxus, Cyclostomi, and Dipnoi, the whole of the
alimentary epithelium must be looked upon as secretory, each
individual cell acting as an independent gland. In other Fishes
and in Amphibians and Reptiles, a higher stage is reached, inas-
much as groups of cells in the stomach give rise to ttchular glands of
a simple nature. A further differentiation of the cells gradually
leads to the condition seen in the gastric glands of Mammals, in
which three kinds of glands can be distinguished, viz. cardiac.

B

E

Fig. 253. — Semidiagrammatic Figures of the Mucous Membrane of the
Intestine of Fishes, showing Intermediate Forms between Longitu-
dinal Folds and Round Crypts. (After Edinger. )

.4, Petromyzon, showing the spiral fold ; 5, an Elasmobranch ; C to E, various

Teleosts.

fundus, and pyloric glands, and in the fundus glands, which have
the greatest physiological importance, the cells become differenti-
ated into chief cells and parietal cells.

In the higher Vertebrates, more especially in Birds and
Mammals, the epithelium of the intestine also gives rise to tubular
intestinal glands {crypts or glands of Lieherkilhn) as well as
(in Mammals) to Brunner's glands in the duodenum, closely
connected phylogenetically with the pyloric glands of the stomach.
Mucus-secreting goblet cells are common throughout the alimentary
epithelium of Vertebrates, and the same is true of leucocytes : the
latter are especially abundant in the submucosa, from whence they
may wander into the lumen of the gut. The lymphoid tissue is
often aggregated into definite smaller or larger masses or follicles
(e.g. Peyers patches), and in some cases {e.g. Protopterus) is very
abundant.

In order to effect an increase of the absorptive surface, longi-

346 COMPARATIVE ANATOMY

tudinal folds of the mucous membrane are formed, and a special
development of such a fold, taking a spiral course, may result in
the formation of a spiral valve. Complications then arise by the
development of transverse folds between the longitudinal ones
(these are already seen in Elasmobranchs and many other Fishes) ;
and by still further modifications, crypts of varied form and depth
are produced, into which open the microscopic glands, when
present (cf Fig. 253). Finger-shaped outgrowths or villi of the
mucous membrane of the intestine are first plainly distinguish-
able in Amphibians (especially Anura) and are especially well
developed in Mammals.^

Appendicular Organs of the Alimentary Canal.

LIVER.

The liver, the form of which is always closely adapted to that
of the surrounding parts (Figs. 254 and 255), and which is typically
lobed, underlies to a greater or less extent the ventral side of the
intestinal tract, and is present in all the Craniata. It arises

Du

Fig. 254. — Liver of Rana escuhnta. From the ventral side.

Du, duodenum ; H, heart ; L, L^, LP-, the different lobes of the liver ; M,
stomach. A gall-bladder is present, but is not indicated.

in the form of a tubular gland,^ as an outgrowth from the
endodermic epithelium of the intestine close to the junction of the

^ The transverse folds of the mucous membrane occurring e.g. in Ratitse
and Mammalia in the small and large intestine are known respectively as 'plicm
circulares (valvulce conniventes) oxiAplicce semilunares.

2 Further differentiations occur, which gradually lead to a dendritic or net-
like arrangement of the gland-ductules, and this shows great variation in different
groups and in different stages of development.

LIYER

347

latter with the stomach. Both ontogenetically and phylogenetically
the liver is an older organ than the pancreas, which is developed
from the same endodermal matrix.

In Amphioxus a simple sac-like caecum (Fig. 258) arises from
the intestine just behind the pharynx, and this " hepatic caecum "
may probably be looked upon as the rudiment of a liver.

Fig. 255. — Viscera of Lacerta agilis.

Bl, urinary bladder ; Ci, postcaval ; ED, large intestine ; GB, gall-bladder ;
H, heart : L, liver ; Lg, Lg^, the two lungs ; M, stomach ; MD, small
intestine ; Oe, oesophagus ; Pn, pancreas ; Tr, trachea.

In the Anamnia, the liver is usually relatively larger than in
the Amniota, and Carnivores generally possess a larger liver than
Herbivores. In Myxinoids it departs least from the original tubular
type, from v/hichthat of the Amphibia and Reptilia, and more especi-
ally the Mammalia, diverges most widely. All these modifications
in its structure are traceable, in the first instance, to the vascular

ZiS

COMPARATIVE ANATOMY

system, which has a very specific arrangement in the liver and
shows a divergent mode of development in passing towards
Amphibians and Reptiles on the one hand, and Mammals on the
other.

The liver is connected with the body- wall by folds of the peri-
toneum, and the differences which are seen in its form are due to
the fact that it becomes more or less closely adapted to fit against
and between the neighbouring organs (stomach, intestine, &c.), and
as regards the formation of lobes, to its close relations with the
portal and hepatic veins and (in Mammals) to the diaphragm.

Fig. 256. — A, B, C, Various Modifications in the Arrangement of the

Bile-ducts.

c and s, cystic duct ; ch, common bile-duct ; D, duodenum ; h, hepatic duct ;
he, hepato-cystic duct ; he, hepato-enteric duct ; Vf, gall-bladder.

The number of lobes varies, and in Mammals (e.g. Carnivores)
there may be as many as six or seven.

The function of this large and vascular organ consists, in the
first instance, in the secretion of bile, but it has a further importance
in connection with the formation of glycogen, urea, &c. It is con-
nected with the lumen of the anterior part of the intestine by
means of one or more bile-ducts, an offshoot from which may
become enlarged and give rise to a receptacle, the gall-bladder.
The duct from this is known as the cgsHc duct : it is connected in
various ways with the system of heimtic ducts from the liver, and
thus may form a common hile-duct opening into the intestine^
(Figs. 256, 257).

PANCREAS.

The pancreas arises from the proximal portion of the small in-
testine, Dear the liver, in the form of several independent endo-

^ On the metamorphosis of the ammocoete-larva of the Lamprey, the single-
lobed liver, like the intestine, undergoes a peculiar modification and partial
degeneration. It is at first of a typical tubular structure, and possesses bile-
ducts and a gall-bladder ; the bile- capillaries, ducts, and gall-bladder gradually
disappear, and at the same time the blood-capillaries become enlarged and more
completely surround the cell-trabeculae, so that a change of function probably
occurs.

PANCREAS

349

dermic outgrowths. These are respectively dorsal and ventral in
position, and in most Craniates one dorsal and two ventral
rudiments can be recognised, of which one or more may undergo
reduction or fusion later, so that the number of ducts opening into
the intestine varies in the different groups: there may be only
one, or there may be several {e.g. Birds, Crocodiles, Emydse, and

Fig.

257.— Pancreas and Liver of Frog, to show
their ducts.

THE Arrangement of

Dcy, cystic ducts, which, together with the hepatic ducts {Dh), form a network
from which three collecting ducts arise, and these unite to form the common
bile-duct [Dc.) : the latter passes through the substance of the pancreas (P),
receiving further hepatic ducts {Dh^), and the pancreatic ducts {P^) ; at JJc^
it becomes free from the pancreas, and passes back to open into the
duodenum {Du) at Dc- ; G, gall-bladder ; L, L^, L^, the lobes of the liver
turned forwards ; Lhp, duodeno-hepatic omentum ; M^ stomach ; Py,
P3'lorus,

some Mammals). In some cases the ducts communicate with the
bile-duct ^ (Fig. 257).

Varying much in form and size, the pancreas early gives rise to
a band-shaped or more or less lobulated and very vascular organ,

^ The large digestive glands are said to appear ontogenetically in the
following order: 1, liver; 2, dorsal pancreas; 3, ventral pancreas. Phylo-
genetically the order is : 1, liver (Amphioxus, Cj'clostomata) ; 2, liver and dorsal
pancreas (Elasmobranchii) ; 3, liver, and dorsal -f ventral pancreas (most other
Vertebrates, including Amphibia, Sauropsida, and Mammalia).

350 COMPARATIVE ANATOMY

its greater part usually lying in the fold of the duodenum. In
some cases it remains embedded within the wall of the gut {e.g.
Protopterus, cf. Fig. 247). Amongst Teleosts it may be in part
surrounded by the liver : in part, however, it does not form a com-
pact gland, but has the form of scattered lobules extending
throughout the mesentery. It is unrepresented in Amphioxus.

In Petromyzon, a pancreas is developed in the embryo, and is
embedded in the wall and spiral fold of the gut and in the dorsal
portion of the liver. In Myxine and Bdellostoma also a glandular
organ can be recognised in the neighbourhood of the bile-duct,
into which its lobules open independently.^

1 The histological structure of this organ in Cyclostomes resembles that of
the peculiar " intertubular cell-masses" or "islets of Langerhans" present
amongst the ordinary pancreatic tubules of other Vertebrates. These are of
epithelial origin, but have no ducts : they probably pour their secretion into the
surrounding lymph-vessels and blood-vessels, and may be included under the
category of " glands with internal secretion" (cf, p. 247). It has been suggested
that the pancreas may represent phylogenetically two distinct glands, the more
primitive of which is alone present in Cyclostomes, while in other Vertebrates it
has been largely replaced by the pancreas proper.

G. ORGANS OF RESPIRATION.

The typical respiratory organs of Vertebrates are closely con-
nected with the enteric or alimentary canal both as regards
position and development, and are of two kinds, gills and lungs.
The former, as the phylogenetically older organs, are adapted for
aquatic respiration, and are connected with the pharynx in the
region of the visceral clefts : the latter always arise as sac-like
outgrowths of the pharynx, which grow backwards so as to lie
within the body-cavity.

Both gills and lungs may be developed in the same individual,
but are usually not functional at the same time. Both are sup-
plied with venous blood which becomes oxygenated while passing
through their capillaries.

The swim-hlctdder or air-hladder present in many Fishes, and
acting as a hydrostatic organ, arises in a similar manner to the
lungs — that is, as an outgrowth from the fore-part of the aliment-
ary tract : it usually receives oxygenated blood from the aorta, and
venous blood passes from it into the cardinal, hepatic, or hepatic
portal veins; but in some cases (e.g. Bony Ganoids and certain
Teleosts) it may act as an accessory respiratory organ.

In some cases also the oral and pharyngeal mucous membrane
{e.g. certain Amphibians), or the intestine {e.g. certain Siluroid
Fishes), may take part secondarily in respiration, and the integu-
ment may be very important in this respect {e.g. in Amphibians).

I. GILLS.

The gills arise in connection with a series of laterally- arranged
outgi'owths of the pharynx lying one behind the other, which be-
come open to the exterior. Passages or clefts separated by septa
are thus formed for the water entering by the mouth, and in order
that oxygen may become absorbed, leaf-like or thread-like vascular
processes, the internal gills or hranchice, become developed in the
region of each cleft. Apart from these, external gills sometimes
occur, and in certain cases, both kinds are developed in the same
animal.

In the development of the internal or enteric gills, the endo-
derm plays the chief part as the lining or covering layer, the

352 COMPARATIVE ANATOMY

ectoderm being limited to the outer parts of the branchial septa and
only being of secondary importance. The true external or integu-
mentary gills arise as projections of the surface, covered by ecto-
derm, at points between which the clefts are subsequently formed,
but it has recently been maintained that the endoderm takes part
in their formation also.

Fishes possess gills throughout life. Amongst Amphibians this
is only the case in the Perennibranchiata : all the others simply
pass through a gilled stage, and later nearly always breathe by
means of lungs.

Thus the study of this one Order furnishes us with an excellent
representation of the course of phylogenetic development through
which all the higher Vertebrates must have passed, and w^hich
is still indicated in them by the appearance in the embryo of
gill-clefts and gill-arches with a corresponding arrangement of
the blood-vessels : these occur throughout the entire series of the
Amniota — that is, in forms in which they no longer possess a
respiratory function.^

Amphioxus. — The small mouth leads from the cavity of the oral
hood into that of the pharynx, and is provided with a muscular
fold, the velum. The numerous (80 — 100 or more) gill-clefts,
which are arranged in pairs and supported by elastic cuticular
rods, extend backwards nearly to the middle of the body. At first
they open freely to the exterior, but at a later period of develop-
ment they become enclosed by a paired fold of the integument
which gives rise to an atrial or perihranchial chamher, opening by
a single pore situated somewhat behind the middle of the body
(for details, c£ Fig. 258).

The relative extent of the branchial apparatus is considerably
limited, even in the lowest Craniata, as compared with Amphioxus.

Cyclostomes. — In the Am mocoete -larva of the Lamprey the
oesophagus is continued directly backwards from the pharynx
(Fig. 259, a), and at the anterior end of the latter there is a
muscular velum, covered by the mucous membrane (Fig. 260).
The seven gill-sacs provided with leaf-like folds of mucous mem-
brane which are present in the Ammocoete, persist in the adult ;
but, with the formation of a suctorial mouth, the portion of the
oesophagus into which they open becomes closed posteriorly,
the gullet apparently growing forwards above the latter, and
joining the mouth-cavity at the velum. Thus two canals pass

^ Thus indications of five or six clefts are seen in the embryos of most
Reptiles, and of five in Birds and Mammals ; in many cases, however, they do not
become open to the exterior. Their order of disappearance is from behind for-
wards, and the most anterior (mandibulo-hyoid) cleft persists in a modified condi-
tion even in the adult, undergoing a change of function in connection with the
auditory organ (p. 294). Certain of the anterior arches persist in a modified
form (c/. under Skull and Larynx).

GILLS

353

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354

COMPARATIVE ANATOMY

backwards from the mouth, a ventral branchial or respiratory tube,
and dorsal oesophagus (Fig. 259, b). Inspiration as well as expira-

FiG. 259. — Diagram of a Longitudinal Section through the Head of
THE Larval (A) and Adult (B) Lamprey.

tion takes place through the gill-apertures when the animal is
attached by its suctorial mouth.

In Petromyzon and Bdellostoma ^ the individual branchial sacs,
which communicate directly with the pharynx, open freely to the
exterior : in Myxine this original condition becomes modified by
the outer parts of the gill-passages growing out into long tubes.

En JnfllHML Cli Ji

\ h \ o\c \

Fig. 260.— Longitudinal Section through the Head of a Larval Lamprey.

h, c, ventricles of the mid- and hind-brain ; Ch, notochord ; Ep, epiphysis ;
HH, ML^ hind-brain ; Jnf, infundibulum ; K, K, K, the three anterior gills ;
N, nasal sac ; o, subdural cavity ; P, papillae of mucous membrane ; R, spinal
cord ; Th, thyroid (hypobranchial furrow) ; V, velum ; *, communication
between the ventricle of the olfactory lobe and that of the telencephalon,

which unite to form a common duct on either side ; this opens far
behind the branchial apparatus on the ventral side of the body.

^ In Bdellostoma there are usually six or seven pairs of branchial sacs, and
behind these, on the left side, an (esophageo-cutaneoun duct opens directly into the
pharynx, as is also the case in Myxine. Bdellostoma bischoffi and B. stouti
possess eleven or twelve, and B. polytrema thirteen or fourteen pairs of gill-
pouches.

GILLS

355

Fishes. — In the true Fishes the gills come into close relation
with the visceral arches, and in Elasmobranchs they consist of
closely approximated transverse laminse, which are firmly attached
to both sides of the septa extending outwards from the arches and
separating the individual gill-sacs from one another, so that each
septum bears a half-gill, or hemihranch, on both its anterior and
posterior surface (Figs. 261, A, and 202). A gill, or holohranch,

Yui. 261. — Dissection of the head from the dorsal side of A, an Elasmobranch
{ZyijiLua maffeu.<), and B, a Teleost {Gadii.-* (tf/fefi nits'', to show the branchial
apparatus. In both figures the branchial arches on the left side are shown
cut through horizontally. (From R. Hertwig's Zooloffy.)

a.-*, external branchial apertures ; h, branchial arch ; hl^, hP, hemibranclis ;
h, branchial septum ; hm, hyomandibular ; is, internal branchial apertures
and gill-rakers ; m, oral cavitj' ; ma, maxilla ; o, cesophagus ; ojj, operculum ;
ops, opercular aperture ; pa, palatine : j/hi, inferior pharyngeal lK)ne ; Pq,
palatoquadrate, and a, its connection with the cranium anteriorly : prm,
premaxilla ; .•*, pectoral arch ; id; lower jaw ; z, tongue.

thus consists of the branchial septum and arch ^:>/v/.s- the posterior
hemibranch of the sac in front of it and the anterior hemibranch of
the following sac. The gill-sacs, of which there are commonly five
in Plagiostomes,^ open separately to the exterior, and a vestigial

^ There are six in Hexanchus and
Heptanchus in addition to the spiracle.

Chlamvdoselachus and seven in

A A'

356

COMPARATIVE ANATOMY

sacs.

gill-cleft known as the spiracle (p. 88), is nearly always present
more anteriorly, between the mandibular and hyoid arches. In
the Holocephali the spiracle is wanting, and there are only four
clefts and three holobranchs in addition to hemibranchs on the
hyoid and fourth branchial arch ; moreover, an opercular membrane
is present, covering the external branchial apertures and opening
by a slit posteriorly. In Chlaraydoselachus fringed folds from the
hyoid and interbranchial septa project over the clefts.

In Ganoids and Teleosts there are no longer chambered gill-
The septa on which the gill-laminse are borne become
greatly reduced, so that the apices of the
latter extend freely outw^ards ; the whole
branchial region is, moreover, covered over
by the operculum and branchiostegal
membrane (cf. pp. 89 and 94), and thus,
as in the Holocephali, "the gill-slits open
into a common branchial chamber, which
communicates with the exterior by a
single slit-like aperture on either side
(Figs. 261, B, and 262). A spiracle is
present in Acipenser, Polyodon, and Poly-
pterus amongst Ganoids.

The mechanism of respiration in
Teleosts is as follow^s. In inspiration, an
expansion of the oral cavity takes place
by the opercular apparatus being raised,
the branchiostegal membrane at the same
time moving inwards so as to close the
opercular slit. An elastic, valve-like fold
of the mucous membrane, enclosing
numerous smooth muscle-elements, is
present in the maxillary region projecting
downwards from the roof of the mouth,
and a similar fold arises from the floor
of the mouth in the mandibular region.
On the expansion of the oral cavity and
closure of the branchiostegal valve, the
pressure of the water causes the maxillary
and mandibular valves to open inwards,
and thus to admit the inspiratory current (Fig. 263, a). The
movements of expiration then follow by the contraction of the
opercular apparatus, which causes the water in the mouth to press
on the maxillary and mandibular valves and thus close them,
while the branchiostegal valve is opened (b). Thus the mechanism
of these valves is quite similar to that of the valves of the heart,
and the respiratory current is produced by the contraction of
the Walls of the mouth, which act like a pump.

Fig. 262. — Transverse
Section THRoroH a
HoLOBRANCH OF Zy<janm
(on the Right) and
Gadus (on the Left).
Slightly Enlarged.
(From R. Hertwig's
Zoology.)

a, afferent, and v, efferent
branchial vessels ; b,
branchial arch ; h/^, an-
terior, and hl'^, posterior
hemibranch of the gill;
'h, septum ; r, cartilagin-
ous gill-ray ; z, gill-
rakers.

GILLS

357

As a rule Teleosts possess only four holobi-anchs,^ and this holds
good for all Ganoids. A vestigial gill or pseiidobranch is present
on the anterior wall of the spiracle of many Elasmobi-anchs and
of cartilaginous Ganoids {maiidiindar pscudobraTwh) ; the posterior
hyoid hemibranch, which is functional in Acipenser and Lepi-
dosteus, becomes more or less reduced in other Ganoids and
Teleosts, forming the so-called opermdar psendobranch. Traces of

A B

Fig. 263. — Diagram illustrating the Mechanism of Respiration in
Teleosts. (After Dahlgren.)

A, phase of inspiration ; B, phase of expiration. In both figures the anterior
oral part {cav.oris) represents a vertical section, and the posterior pharyn-
geal part enclosing the gills [Kiemen) a horizontal section. The arrows
indicate the direction of the water-current and pressure, and those passing
through the walls of the oral cavity the expansion and contraction of the
opercular apparatus. In A, the maxillary and mandibular valves are open,
and the branchiostegal membrane closed : in B^ this condition is reversed.

a cleft, lying behind the functional branchial clefts, are found
in the embryos of certain Fishes and Amphibians. All these facts
indicate the presence of a more extensive branchial apparatus in
ancestral forms. (For the sieve-like gill-raker s^ cf.Figs. 261 and 262.)
In the Lophobranchii the gills are replaced by tufted processes,
and in many Teleostei certain accessory structures are developed
in the posterior region of the branchial chamber by a modification

^ They may be reduced to three, or two, and even these may be more or less
rudimentary.

A A»*

358 COMPARATIVE ANATOMY

of the branchial cavities and skeleton.^ These serve to retain
water and air, and thus the Fish is able to breathe for some
time out of the water (Anabas, Saccobranchus, Heterobranchus,
Clarias).

The Dipnoi, as their name implies, possess both gills and
lungs, the latter alone being functional in Protopterus and
Lepidosiren during the torpid period (p. 20). The internal gills
are covered by a small operculum. In Ceratodus there are four
holobranchs on the first four branchial arches as well as a " hyoid
hemibranch " (which perhaps belongs to the first branchial arch) ;

the gill-lamella3 extend round the
JO? clefts, so that the hemibranchs of

each cleft are continuous. In
Protopterus and Lepidosiren a re-
duction of these organs has taken
place, gills being absent in the
former genus, for example, on the
first and second branchial arches ;
there is, however, in addition, an
anterior hemibranch on the fifth
branchial arch.^

_ ^ In embryos of Elasmobranchs

Fig. 264.— External Gills of •, x-mi - /ux 4-

Larva of Gymnarchus nilo- and certain Teleosts (Heterotis,

iicus, 4 Days after Hatch- Gymnarchus, Fig. 264), long, vas-

iNG. (After J. S. Budgett.) ^ular, thread-like "external gills"

DS, yolk sac ; KB, external gills, arise from the endoderm of the

clefts and extend backwards over
the body. In the larval Polypterus and Calamichthys there is a
single true (integumentary) external gill on either side in the
hyoid region, which differs markedly from those just described,
and consists of a main stem giving off a double row of filaments
and supported at its base by cartilage (Fig. 265, h). In larvae of
Protopterus and Lepidosiren somewhat similar external gills are
present, but are four in number on either side and are situated on
the branchial arches (Fig. 265, a). Of these, vestiges of the three
upper ones persist in Protopterus even in the adult.

Amphibians. — In the embryos of Urodeles, indications of five
gill-clefts can usually be recognised, but the most anterior (hyo-
mandibular) cleft, as in other Amphibians, does not become open

^ Other parts may also become modified to serve as accessory respiratory
organs. Thus in Cobitis and Callichthys intestinal respiration takes place ; and
in Monopterus javanensis (which, like Protopterus and Lepidosiren amongst the
Dipnoi, passes through a torpid period in holes in the ground during the dry
season) buccal, pharyngeal, and intestinal respiration occur, and interesting
modifications of the blood vessels are seen.

^ Cf. Note on p. 97. There are five clefts in Ceratodus and Protopterus,
and four in Lepidosiren. It is probable that the hyobranchial cleft is closed in
Protopterus, as is the case in Lepidosiren; the spiracular cleft does not become
perforated in the embryo.

GILLS

359

to the exterior : they are covered over by an opercular-like fold of
the skin, thus leaving only a single aperture externally. In the
larvae, as well as in adult Perennibranchiates, there are three
external gill-tufts in connection with the three anterior branchial
arches, lying one over the other ; these extend backwards, pro-
jecting freely to the exterior, and are unsupported by cartilage.
Each consists of a main axis with secondary branches, so that the
gills have the form of tufts or delicately branched structures

Fig. 265. — Larv^ of (a) Profopferus annectem (17 Days after Hatchin«)
AND (b) Polypteriis laj^radei (\\ in. Long, x about 4). (After J. S.
Budgett.)

BF, pectoral fin ; HO, sucker, or cement organ ; KB, external gills (single in

Polypterus).

showing the most varied arrangements for increasing the respira-
toiy sui'face (cf Fig. 266). These external gills must not be
confused with the internal gills, which are wanting in all Urodeles.
They are acted on by a complicated system of muscles and are
covered by ciliated epithelium^ which serves to keep up a continual
current in the sunounding medium.

The gills are lost at metamorphosis in the Derotremata and
Myctodera. In larval Myctodera, as mentioned above, there are
five open clefts, all of which disappear at metamorphosis. In the

360

COMPARATIVE ANATOMY

Perennibranchiata, the most anterior (hyobranchial) cleft becomes
closed in Siren ; in Necturus, Proteus, and Typhlomolge, the most
posterior also disappears, while in the Derotremata {e.g. Am-
phiuma) only one remains, viz. that between the third and fourth
arches, and this not in all cases. In this respect, therefore, the
Myctodera are the most primitive, and there is every reason for
assuming that the Perennibranchiata, though retaining certain
larval characters, have been derived from caducibranchiate forms.

In Anuran larvae, the three pairs of external gills are less
complicated than those of Urodeles (cf Fig. 266), and are soon
replaced by internal gill-tufts situated in the three branchial
clefts.^ By the growth of opercular folds, which contain no
skeletal parts, the external respiratory aperture of either side
becomes gradually reduced in size, and the two branchial chambers

A B

D E

Fig. 266. — Diagram illustrating the DEVELorMEXT of the Amphibian
Gill. (Mainly after P. Clemens.)

A, rod-like, unbranched, primitive form, retained in the adult of certain Aniira
{e.g. Xenopus). B, form in which there is only a single series of branches
(Anura). C, feather-form, with two opposite series of branches (Derotremata :
this form is also primarily met with in the Gymnophiona and in embryos of
Myctodera). D, wedge-shaped, unbranched axis, on the lower border of
which the gill-filaments arise in rows (embryos of most Urodeles). K, leaf-
like, unbranched axis, on which the filaments increase in number, and are
arranged in four rows on the surface as well as on the edges (Axolotl- stage
of Amblystoma, Necturus). F, branched axis (Proteus, Siren).

usually open eventually by a single aperture, which is situated
either in the median ventral line (Bufo, Bombinator) or laterally
(Rana). The larvae of the Gymnophiona also possess external gills,
which in Epicrium glutinosum, for example, are feather-like.^

1 The so-called internal gills of Anura are said to correspond to a series of
outgrowths from the bases of the external gills, and thus not to be comparable
wdth the endodermal gills of Fishes.

2 The external gills of Amphibia present a great variety of form, often
resulting from adaptation. Thus, in the intra uterine larvte of the viviparous
Salamandra atra, they reach a length of 5-6 centimetres ; in Ciiicilia com-
pressicauda they consist of two large, flattened, vascular folds, which apparently
cover the body of the larva like a mantle ; and in Notodelphj's (Nototrema)
amongst Anurans, in which the larvai undergo development in the pouch on
the back of the mother, they are bell-shaped and stalked. In certain other
Batrachians in which there is no free larval stage, it appears that respiration
may take place before hatching by means of the broad and vascular tail (Hy lodes

SWIM-BLADDER 361

11. SWIM-BLADDER AND LUNGS.

1. The Swim-Bladder.

As already mentioned (p. 351), the lungs and swim-bladder are
developed in a similar manner, and only differ from one another in
the fact that the former always arise from the ventral side of the
pharynx, and the latter usually on the dorsal side. The various
attempts which have been made to trace the phylogenetic
connection between swim-bladder and lungs have so far not been
completely successful. It has been supposed that the dorsal origin
of the former has come about by a process of rotation : on the
other hand, it may be that the two organs are not strictly
homologous, each having arisen independently from a similar
outgrowth at a different point in the alimentary tube. In this
case, the so-called swim-bladder of Polypterus would be directly
comparable to the lungs of Dipnoans and higher forms rather
than to the smm-bladder of other Fishes.

The exact point of origin of the swim-bladder from the ali-
mentary canal varies,^ and its duct (ductus pve2tmaticus) msiy either
remain open throughout life, as in Ganoids and some Teleosts
(Physostomi), or it may later become reduced to a solid fibrous
cord or even entirely obliterated, as in other Teleosts (Physoclisti).
In the latter case there is no communication between the swim-
bladder and the external air, and the contained gas must therefore
be given off from the walls of the swim -bladder itself: this has
been shown to be the case amongst Physostomi also. In certain
of the latter, vascular " red-bodies " are present ; in others and in
the Physoclisti, gland-tubules are present amongst the vascular net-
works or " retia mirabilia," and the number, relative size, and
position of these gas-glands varies in different forms. Difiiision
of gas (probably mainly oxygen) into the bladder takes place in
certain thin, oval regions lined by a flat epithelium and provided
with regulating dilator and sphincter muscles.

A swim-bladder is wanting in Amphioxus, Cyclostomes, and
Elasmobranchs, and in other Fishes varies much in form and in its
relations to surrounding parts (alimentary canal, blood-vessels
skeleton). As a rule, it lies above the peritoneum on the dorsal
side of the body-cavity, between the vertebral column, aorta, and
kidne3^s on the one hand, and the alimentary canal on the other,
and is invested by the peritoneum on the ventral side only. It is

martinicencis) or folds of the Ixxly-wall (Raiia opisthodon). In Xenopus, the
])ranched tentacles at the angles of the mouth are said to have a respiratory
function, and, like the "balancers" of Urodeles (of. note on p. 271), may corre-
spond morphologically to the external gill of the first visceral arch.

^ In Erythrinus it arises laterallj', and in some Physostomi {e.g. Herring)
it opens further back, into the stomach.

362

COMPARATIVE ANATOMY

more or less sac-like in form, is only exceptionally paired, and

usually extends along the whole length of the body-cavity ; its

walls are composed of connective,
elastic, and muscular tissue.^ In
some Teleostei the swim-bladder
is transversely constricted so as
to form several successive divi-
sions ; in other cases it may give
rise to a more or less numerous
series of caRcal processes. Its
internal surface may be either
smooth or spongy (Fig. 267) ow-
ing to the formation of a mesh-
work of trabeculse, the structure
of which resembles that of the
lungs of Dipnoi and Amphibia,
and, as already stated, it has a
respiratory function in some
cases.
Attention has already been directed to the relations which

may exist between the swim -bladder and the auditory organ

(cf. p. 297).

Fig. 267. — Internal Surface of
THE Air-bladder of Lepidosteus,

SHOWING THE TRABECULE.

B, fibrous longitudinal band.

2. The Lungs.

The lungs arise at the hinder border of the branchial region of
the pharynx. Their phylogenetic history is not clear, and the
view that they arose primarily from visceral clefts does not seem
to be a very probable one.

On the first appearance of the lung-rudiments, the pharynx
becomes laterally compressed immediately above the fifth or sixth
arterial arch, and divided by a longitudinal horizontal fold into a
dorsal and a ventral portion, the latter of which gives rise to a
blind sac, opening anteriorly by a wide aperture into the form.er
and lined by endoderm (Fig. 268). A longitudinal vertical furrow
is then formed, dividing this primitive lung- sac into right and
left halves : the narrower proximal portions of these represent
the primitive bronchi, which communicate with the pharynx by
a short tube, the primitive windpipe or trachea. The proximal
end of the latter subsequently becomes differentiated to form a
larpoix, or organ of voice, which opens into the pharynx on its
ventral side by means of a slit-like aperture, the glottis. The
lungs are therefore phylogenetically older organs than the bronchi,

^ In Dactylopterus volitans, the two halves of the swim-bladder extend
beyond the general coelome into special cavities, and the organ is covered by
large bony plates. In the Gymnodonts (e.g. Diodon, Tetrodon), the whole
resophagus is capable of great distension.

LUNGS

363

trachea, and larynx, and this fact is supported by a study of their
comparative anatomy.

Hollow outgrowths and buds arise from the endoderm lining

PD

PI)

PD

JS

A'.-

A B C

Fig. 268.— a, B, C, Diagrams showing the Mode of Development of the

Lungs.

fc, bronchus ; PD, primitive alimentary tube ; S, S^, the lung-sacs, which are at
first unpaired ; t, trachea.

the primary central cavity (" intrapulmonary bronchus ") of each
lung: these extend into the surrounding vascular mesoderm, which

Fig. 269. — Diagrams illustrating the Budding of the Bronchi in the
Developing Lung of Emys. (After Fanny Moser.)

A, buds from the intrapulmonary bronchus are shown extending into the lung-
wall. B. The intrapulmonary bronchus, which still forms a narrow tube
extending through the whole length of the lung, has given off a number of
primar}' pulmonary vesicles extending into the thick wall of the latter. C.
B}^ a marked thinning-out of the lung-wall, the vesicles form chambers, of
which four dorsal [d) and- four ventral {v) are visible, each of which has
developed secondary buds (JV) ; the bronchus itself has become enlarged
and given rise to a terminal chamber (EK). D. Lung of adult. The
chambers have become enlarged and are separated from one another merely
by narrow septa, corresponding to the reduced lung-walls : the secondary
buds have given rise to buds of a third order, which form the lung-crypts,
and the terminal chamber {EK) is enlarged.

gives rise to muscular fibres and connective tissue, and thus a
branched system of cavities communicating with the bronchi is

364

COMPARATIVE ANATOMY

gradually formed (secondary and tertiary bronchi, &c.). The ends
of these branches are swollen, forming crypts known as infundihda,
which are made up of a number of alveoli and are surrounded by
blood-capillaries, through the thin walls of which the interchange
of respiratory gases takes place (Figs. 269 and 270). The bronchi

\\L,

JW

Fig. 270. — Diagram of the Embryonic Human Lung, x 50.
(After W. His.)
Aj), pulmonary artery ; lb, pulmonary vesicle undergoing division ; Ir, air-
passage ; M, middle lobe of the lung ; O, right anterior upper lobe with
its "eparterial" bronchus; 0^, left anterior lobe with its " hyparterial "
bronchus ; sp, oesophagus ; V, V^, right and left posterior lobe.

are lined by ciliated epithelium, the infundibula and alveoli by
pavement epithelium.

Thus a great increase in the respiratory surface is gradually
produced in the ascending series of Vertebrates and in the
individual development of the higher forms, in which the lungs
may also become secondarily divided up into lobes.

In the following account the air-tubes will be dealt with
separately from the lungs proper.

Air-Tubes and Larynx.

The walls of the air-tubes may consist, in addition to their
lining of ciliated epithelium, of connective tissue and elastic and
muscular fibres only, but as a general rule cartilaginous elements
are also formed, and these serve to keep the tubes permanently
open. From the Amphibia onwards the most anterior of these
cartilages, which support the larynx, become differentiated to form
a frame on which are stretched the structures by means of which
the voice is produced — the vocal cords : these cartilages are acted
upon by muscles. The relative length of the windpipe as a
general rule corresponds with that of the neck.

In certain Fishes there may be a more or less complicated
arrangement of the muscles derived from those of the gill-arches

AIR-TUBES AND LARYNX

365

(dilator, constrictor, protractor, and retractor) around the aper-
ture of the swim-bladder, though no cartilaginous elements are
formed (Fig. 271) : these muscles are inner ved by the vagus,
whether the aperture is dorsal (Lepidosteus, Amia) or ventral
(Polypterus). The same is true of the Dipnoi, in which there is
a tongue-shaped supporting plate composed of dense connective
tissue anteriorly to the glottis, which leads into a muscular
vestibule or " laryngo-tracheal " chamber communicating with the

Cav. or.

Fig. 271. — Median Longitudinal Section through Part of the Head
AND Trunk of Lepidosteus ossens.

Ao, aorta; Cav, cavity of the swim-bladder; Co, Co^, and **, constrictors of
the pharynx, "larynx," and swim-bladder; Da, dilator; G, cranial cavity ;
K, cushion-like deviation of the supporting elements of the laiynx ; L.B'j,
loose connective tissue between the swim-bladder and pharynx ; Perky
pericardium ; Ph^, 3, 4, intrapharyngeal branchial muscle ; Phar, pharynx ;
B.M, neural canal ; a^^, fibrous laryngeal supporting elements ; jr^S', vertebral
column.

lung: but it is doubtful whether this plate can be regarded as
the fii-st phylogenetic indication of the laryngeal skeleton of
higher forms, and whether the laryngeal muscles of the Amphibia
have been derived from those represented in the Dipnoi.

A comparison of these parts in Ganoids and Dipnoans indi-
cates the possibility of the former existence in the vertebrate
series of two larj^nges, a " dorsal " and a " ventral " (Fig. 272),
traces of the former of which can still be recognised in Lepidosiren
in addition to the ventral larvnx.

366

COMPARATIVE ANATOMY

Amphibians. — The vestibule, or laryngo-trachcal chartiber, com-
municates with the pharynx on the one hand and with the lungs on
the other, and is supported by definite cartilages ; it is provided
with intrinsic (dilator and constrictor) and extrinsic muscles, the
former derived from pharyngeal muscles, and the latter from
trunk-muscles, as in all the higher forms. A definite trachea is

Fig. 272. — Diagrajhivjatic Transverse Section illustrating the structure
AND Relations of the Dorsal and Ventral Larynx.

Aor, aorta : Cav.cr, cranial cavity ; Co, Co^, constrictor of the dorsal and ventral
larynx respectively ; Co.ph, constrictor of the pharynx ; i>, dorsal larynx ;
Dil, Dil^, dilator; KB, branchial arch; Kpf.D, lumen of pharynx; Mac,
mucous membrane of pharynx ; SE, SE^, supporting elements ; " V, ventral
larynx ; Vag, vagus nerve.

differentiated in Siren, Amphiuma, and the Gymnophiona only ;
it reaches a length of 4 to 5 or more centimetres, and its wall is
strengthened by a series of small, irregular cartilages, which usually
tend to unite into bands (Fig. 273) : only in the Gymnophiona,
however, do these bands begin to take on the form of half-rings,
and to surroimd the trachea more or less completely.

AIR-TUBES AND LARYNX

367

The primary skeletal parts are a pair of lateral cartilages,
situated in the walls of the vestibule on either side of the glottis ;
these appear to have arisen phylogenetically by a modification
of the vestige of a branchial arch (possibly the sixth), as is
indicated by the innervation of this region by a branch of the
vagus. The most primitive form of this lateral cartilage has
probably been retained in the caducibranchiate Ellipsoglossa. In
various other Urodeles it becomes further developed in various
ways, chiefly by its anterior section being differentiated into an
arytenoid cartilage, while its posterior section gives rise to
the crico-tracheal skeleton (Fig. 273). The latter gradually be-
comes more and more closely adapted to the walls of the air-
passages, and eventually extends along the whole length of the

Fig. 273. — Laryngeal and Tracheal Skeleton of Urodeles. A, Necturm
B, Siren lacertina ; C,' Amphiuma ; D, Salamandra maculosa.

a, the lateral cartilages (arytenoids) on either side of the glottis ; a', ridge for
muscles ; co, constrictor of the larnyx ; Kb, the more definite tracheal
cartilaginous tracts in Amphiuma and Salamandra ; K^^', fourth branchial
arch, from which the dilator {d) of the trachea and larynx arises : it is
inserted into an aponeurosis at H ; L, L', lungs; *, the representative of
the cricoid cartilage ; ft, cartilages of the trachea in Siren.

trachea, its elements passing more or less completely around the
windpipe, so that in some higher forms they give rise to com-
pletely closed tracheal rings.

The anterior end of the crico-tracheal skeleton in Urodeles
gives rise to a very simple ring-shaped cricoid cartilage, which,
like the rest of the larynx, is much more highly differentiated in
Anurans. In these the larynx is regulated by a well -developed
series of muscles, and is provided with vocal cords, the sound pro-
duced by which is often intensified by the presence of vocal sacs
developed from the floor of the mouth. The laryngo-tracheal
chamber lies between the posterior cornua of the hyoid (thyro-
hyals), with which it is connected by ligaments, and is supported

368

COMPARATIVE ANATOMY

by a thin arytenoid cartilage on either side of the glottis as well
as by a ring-shaped cricoid cartilage, from which delicate processes

Fig, 274. — Cartilaginous Skeleton of the Laryngotracheal Chamber of
Eana esculenta. A, from above ; B, from the side.

Ca, Ca, arytenoid cartilages; CM — C./"*, cricoid cartilage; P, plate-like ventral
part of the cricoid; Sp, pointed process of the cricoid; SB, glottis; ***,
three conical prominences of the arytenoids.

pass backwards to the roots of the lungs (Fig. 274). Vocal cords
are developed in the Anura only, each being attached to the inner
concave surface of the corresponding arytenoid.

The intrinsic laryngeal muscles of Amphibians, which are sup-

FiG. 275. — Larynx of Phyllodactylm europoMS. (A, skeleton, and B,
musculature of larynx.)

-4r, arytenoids ; Cc, cricoid ; D, dilator ; Oe, entoglossal ; S, anterior median
process of cricoid ; S^, sphincter ; T, trachea.

plied by a branch of the vagus and are important in respiration
(as well as in the production of the voice in Anura), include a

AIR-TUBES AND LARYNX

369

dilator and one or more constrictors of the glottis. A differentiation
of these into distinct pharyngeal and laryngeal muscles occui-s
only in higher forms.

Reptiles. — The cartilaginous rings of the trachea gradually
become more solid and complete in Reptiles, and as in other

bd h.

Fig. 276.— Section through the Syrinx of a Male Blackbird (Tardus
merula). (After V. Hacker.)

hd, brochidesmus ; B.I — III, 1st— 3rd bronchial ring; h, ventral cavity (part
of the anterior thoracic air-sac) ; l.e and l.i, external and internal labia ; M,
muscles ; m.t.e and m.t.i, membrana tympanifonnis externa and interna; si,
niembrana semilunaris ; S(, pessulus ; T, "'tympanum"; T.r, tracheal rings.

Amniota are well developed, though not always complete dorsally.
The length of the windpipe varies according to the length of the
neck and to the point at which it bifurcates into the two bronchi,
cartilages being present in these also, and gradually extending
along their intrapulmonary portions.

B B

370

COMPARATIVE ANATOMY

The cricoid is much more sharply differentiated than in
Amphibians, and may give off processes ; with it the arytenoids
are movably connected.^ Dilator and sphincter muscles (Fig. 275)
are present much as in Urodeles, and as in Mammals, are supplied
by two branches of the vagus — an anterior (corresponding to the
first branchial branch of Fishes) and a posterior (recurrent), the
homologue of that present in Urodeles and of the fourth branchial
branch.

Except in Snakes, in which considerable reductions of the
hyoid apparatus occur (p. 117), a close connection obtains between
the latter and the larynx ; in Crocodiles and Chelonians, for
instance, the larynx is firmly embedded in a shallow depression
on the dorsal surface of the basi-hyoid (Fig. 85). Moreover, the

Fig. 277.— Larynx of Male Duck.

external, and B, internal view,

Br, bronchus ; S, pessulus, from which a lateral outgrowth [S, between b and h)
extends into the tympanum, thus dividing its aperture into the trachea into
two portions (6, h) ; the aperture is further diminished by the circular fold
of mucous membrane, SF ; T, the "tympanum"; Tr, trachea; t, thin
region in pessulus.

larynx becomes shifted further forwards towards the nasal air-
passage. On the whole, however, no considerable advance of the
larynx as an organ of voice is seen as compared with that of
Amphibians.

Birds. — In Birds 'there are two larynges, an Vjfper (anterior)
and a loioev (posterior). The former lies in the usual position
behind the tongue on the floor of the pharynx, and is plainly
homologous with that of other Vertebrates, though it has become
reduced and is incapable of producing sound.

The lower larynx, or syrinx, is of much greater importance ; it is

^ In some Reptiles {e.g. certain Lizards and Chelonians) a structure is
present which to some extent recalls the epiglottis of Mammals.

AIR-TUBES AND LARYNX 371

usually situated at the junction of the trachea and bronchi, or more
seldom at the lower end of the trachea alone or on the bronchi
alone. It serves as the organ of voice, and appears lirst in, and
is restricted to, Birds. In the most usual form (hroiicho-tracheal
syrinoj), there is a movable connection between the most anterior
bronchial rings, with which a complicated system of muscles is con-
nected; these, by their contraction, cause a stretching or relaxing of
certain vibratory membranes. A bar of cartilage or bone, the
jpessuhcs, extends from the junction of the bronchi into the more or
less swollen "tympanum" at the base of the trachea^: this supports
a slight fold of the mucous membrane called the memhrana semi-
lunaris, while the membranous inner wall of each bronchus is known
as the memhrana tympaniformis interim : the external wall may
also give rise to a memhrana tymyaniformis exteriui. The tympanum,
which is strengthened by fused tracheal rings, attains a relatively
enormous development in some Water-Birds {e.g. the male Duck),
where it gives rise to a bony vesicle which serves as a resonance
cavity (Fig. 277).

All the muscles of the s}Tinx are derived from the sterno-
hyoid group, i.e. from the cervical continuation of the rectus-
system: this is indicated by their innervation from h3^^oglossal
and cervical elements (Fig. 278). They are thus derivatives of the
trunk-muscles, and are therefore fundamentally different from
the laryngeal muscles, which have arisen by a modification of the
pharyngeal muscles, i.e. are of visceral origin.-

The relative length of the trachea varies greatly in different
Birds, and its complete cartilaginous rings usually become calcified
or ossified.^

Mammals. — The larynx of Mammals is distinguished from
that of all other Vertebrates by the marked differentiation of the
muscles — the constrictors always exceeding the dilators in number,
— and by the constant presence of an epiglottis and a thyroid
cartilage.

The thyroid cartilage is derived from part of the fourth and
fifth visceral arches (cf. Fig. 279), and is closely connected with
the hyoid-apparatus : in Monotremes, in which alone it is paired,
this connection remains a close one. Apart from this fact, the

^ In the Chelonian Cinixys homeana there is a somewhat similar "tym-
panum," the vocal cords are wanting in the larynx, and the laryngeal muscles
are reduced.

- Many diflferences are seen amongst the various avian groups as regards the
syringeal muscles. In soYTft, both tracheo- bronchial and sterno-tracheal muscles
are M'anting, while in others there ma\- be as many as seven pairs. The syrinx
is simpler and more primitive in the female than in the male.

^ In some cases {e.g. Swan, Crane), the windpipe extends into the hollow
keel of the sternum, where it becomes more or less coiled, and then again passes
out close to its point of entrance and enters the body-cavity. In certain repre-
sentatives of the Stumidae it extends between the skin and the muscles of the
thorax, and there gives rise to numerous spiral coils.

B B*

g.c.s.

c.c(.s.

Fig. 278.— -Syrinx of Crow {Corvus corone) showing the Muscles, Nerves,
AND Vessels. (After V. Hacker.)

AlK-TUBES AND LARYNX

373

B.III, 3rd bronchial ring ; car, carotid artery; j, jugular vein; oJi, h^'oid (cut
through) ; thym, thymus ; thyr, thyroid.

Muscles and their insert iotw^ : — st.tr, sterno-trachealis ; tr.hr.d.h, trach. -bronch. -
dorsalis brevis (membrana tynipaniforinis interna) ; tr.hr.d.l, trach. -bronch. -
dorsali.s longus (dorsal end of B.I I) ; tr.hr.o, trach. -bronch. -obliquus (ventral
end of B.III); tr.hr.v, trach. -bronch. -ventralis (ventral end of B.II and
pessulus).

Xerve-s: — c, 1st cervical; c.a, cervicalis ascendens ; c.d.i, c.d.s, cervicalis
desceudens inferior et superior; g, glossopharyngeal; g.c.», anterior (superior)
cervical ganglion; (j.p, petrosal ganglion (nearer to g.c.s than shown on
figure) ; h' and h", 1st and 2nd root of hypoglossal ; p.c, cervical plexus ; r.c,
cervicalis ; s, cervical sympathetic ; i\ vagus.

larynx of Monotremes, and especially of Echidna, exhibits a much
more primitive condition than that of other Mammals. In these,
the thyroid is unpaired, its two halves uniting ventrally, though

Fig. 279. — Larynx of Echidna. (A, ventral, B, lateral, and C, dorsal view.)

AL, arytenoids, with an intercalary piece, S^ ; RK, cricoid, with a dorsal
intercalary piece, S ; SK, skeletal element which is partially subdivided
into two portions ventro-laterally ; of these the anterior (t) corresponds to
the greater coniu of the hyoid*^ of other Manmials (that is, to the 3rd
visceral arch), and tlie posterior (*) gives rise to the anterior thyroid element
( = 4th visceral arch); SK', the posterior thyroid element (=5th visceral
arch), with a basal piece or copula {c) ; Tr, trachea ; ZB, body of hyoid
( = copula of 2nd and 3rd visceral arches) ; ZH, lesser cornu ( = 2nd visceral
arch).

still showing traces of its primary paired nature. This shield-
shaped thyroid becomes separated from the hyoid, and surrounds
the lateral and ventral regions of the larynx, overlapping the
cricoid above,^ and serving as a point of origin and insertion for
important intrinsic and extrinsic muscles.

1 The cricoid may be complete or incomplete ventrally, and its dorsal portion
usually forms a broad plate with which the arytenoids are articulated (Figs. 280
and 281). The latter, growing out dorsally, may each give rise to a distinct
comiculate cartilage (cartilage of Santorini).

B B**

374 COMPARATIVE ANATOMY

The vocal cords extend between the thyroid and the arytenoids,
and the niucoiis membrane above them becomes evoluted laterally

Fig, 280. — Diagram to illustrate the Metamorphosis during Develop-
ment OF the First to Fifth Visceral Skeletal Arches (I — V) in

Man.

From the proximal end of the first arch (Meckel's cartilage) arise two of the
auditory ossicles, the malleus and incus {ml and in), p, pinna ; pr, mastoid
process of skull.

From the second arch (hj'oid) arise proximally the styloid process (p.s),
distally the anterior (lesser) cornu of the hyoid (c.a) and a portion of the basi-
hyoid {h.s). By far the greater portion of this arch becomes the stylo-hyoid
ligament {l.g). [Concerning the stapes {st) cf. p. 134.]

The third (first branchial) arch gives rise to the greater part of the body
(/>..s) of the hyoid, and the posterior or greater cornu of the hyoid {c.p).

The fourth (second branchial) arch gives rise to the anterior (upper), and
the fifth (third branchial) to the posterior (lower) segment of the thyroid
cartilage {th', fh"). The arytenoid cartilage (ar) is probably a derivative of the
fifth arch, cr, cricoid cartilage ; tc, cartilage triticea, connecting the hj'oid and
thj'roid ; fr, trachea.

to form the laryngeal pouches, one (more rarely both) of which may
undergo special development and in Anthropoids may reach such a
large size that it serves as a resonance cavity. In other Monkeys,

AIR-TUBES AND LARYNX 375

a median pouch projects between the epiglottis and thyroid
cartilage (or more rarely between the latter and the cricoid), and
may come to lie partially within the body of the hyoid, which {e.g.
in Mycetes, Fig. 281) ^ is swollen to form a large bony chamber.
The folds of mucous membrane bounding the laryngeal pouches
anteriorly are spoken of as false vocal cords ; these are not present
in all Mammals. True vocal cords may also be wanting, being
replaced functionally by a ridge of the mucous membrane which
is only slightly vibratory and niay be temporarily strengthened by
the contraction of the thyro-arytenoid muscle {e.g. Monotremes,
Cetaceans, Monkeys).

The epiglottis, which consists niainly of elastic fibro-cartilage,
has close relations to the soft palate, extending upwards from
the anterior border of the thyroid cartilage in front of the glotcis,
which it covers when pressed backwards : it is often, when at rest,
embraced more or less firmly by the soft palate in such a way that
its distal end lies in the passage of the posterior nostrils (naso-
phar}Tigeal chamber), so that respiration and feeding can go on
independently of one another.^

An interesting adaptation for the method of lactation is seen
in the larynx of Marsupial embryos, in which it and the epiglottis
become greatly elongated and are firmly embraced by the soft
palate, so that it cannot be moved from this position. Thus
respiration can go on freely while the milk is pumped down
the oesophagus on either side of the larynx by the action of the
compressor mammae muscle of the mother. In Cetacea (more
particularly the Odontoceti), a similar arrangement occurs, and is
here adapted for the aquatic life of the animal. In all Mammals
a similar position of the larynx is seen in the embryo if not in the
adult.

The length of the trachea is proportional to that of the neck :
when the latter is short, the two bronchi may arise close behind
the cricoid cartilage (Sirenia) : except in Bradypus, in which it
takes a peculiar course, it lies close to the ventral side of the
gullet. The cartilaginous rings are usually incomplete dorsally,
where their open ends are connected by a membrane enclosing
smooth muscle-fibres.^ In the bronchi, the rings are only excep-
tionally closed on the dorsal side, and are usually present for a
considerable distance along the intrapulmonary bronchi.

1 Outgrowths from various parts of the laryngeal mucous membrane may
occur in various Mammals {e.g. Cetacea, Ungulates).

2 The phylogenetic history of the epiglottis is unknown. It was probably
originally a paired structure, and the small cartilages of Wrisberg are apparently
derived from it.

•^ In the Cetacea they are incomplete ventrally, and in certain Marsupials
and Rwlents, as well as in Phoca and Lemur, they form complete rings.

376

COMPARATIVE ANATOMY

A

Fig. 281. — Larynges of Various Mammals.

A, Deer, seen from the left side ; B, Fox, longitudinal section ; C, Howling
Monkey {Mycetes tirsinus), from the left side ; D, Chimpanzee [Simia
troglodytes), from the ventral side,

Ca, arytenoid cartilage ; Or, ventral, and Cr\ dorsal plate of the cricoid ; Ct,
Ct^, thyroid cartilage ; Ctr, cartilaginous rings of the trachea ; Ep, epi-
glottis ; H, body of hyoid ; h, lesser, h}, greater conma of the hyoid ; Lt,
crico-thyroid ligament ; M, laryngeal pouchy which shows an enlargement
at t ; M.ge, genioglossus muscle ; Mth, thyro-hyoid ligament ; mii, submucous
tissue with muscles ; oh, uh, anterior and posterior cornua of the thyroid ;
pm, processus muscularis of the arytenoid ; S, mucous membrane of the
trachea and tongue ; Tr, trachea ; Z, tongue ; 1, 2, 3, resonance cavities.

LUNGS

377

A

1\

I

\

The Lungs.

Dipnoans. — In Ceratodus the lung is a wide, unpaired sac
without any trace of a dividing septum : in other Dipnoans it is
distinctly paired (as in Polyp terus, cf. p. 361) throughout the
greater ]mrt of its length, the anterior unpaired portion being
largely filled up by spongy trabeculse.

The lung extends through the whole length of the body-cavity,
and is covered by peritoneum on the ventral surface only; the
lining nmcous membmne forms bands and networks similar to
those seen in the swim-bladder of many Fishes (e.g. Lepidosteus,
Fig. 267), and smooth muscles are abundant.

Amphibians. — The lungs of Proteus and Necturus (Fig. 282),
as well as those of the Newt (Triton), though paired throughout,
remain at a lower stage than those of the
Dipnoi, their internal surface being per-
fectly smooth, and having, therefore, a
relatively much smaller superficial extent.
They consist of two delicate, elongated
sacs of unequal length and constricted
in the middle; in Proteus they extend
much further backwards than in Nec-
turus. A difference in length between
the two lungs is seen also in other Am-
phibia, such as Amphiuma and Siren, in
which the two cylindrical lungs lie near
together, close to the aorta. Their in-
ternal surface is raised into a network
enclosing numerous smooth muscle-fibres
and coiTesponding with the distribution
of the blood-vessels, the meshes being
much finer in Amphiuma, and still more
so in Menopoma, than in Siren.

With the further development of the
blood-vessels, they gradually encircle the
wall of the lung like the hoops of a
barrel, so that septum-like structures
arise, between which the thin pulmonary
walls project in the form of rounded
vesicular elevations (Fig. 283). Thus a
beginning is made of a differentiation in
the cavity of the lung into respiratory regions and air-passages
(intra-pulmonary bronchi) such as is seen in higher forms.

In many other Urodela, as well as in the Anura and Gymno-
phiona, the muscular and elastic walls of the lung become still
further complicated by the development of a respiratory network

Fio. 282.— Lungs of Pro-
teus (A) AND Necturus

(B).

The communication with the
pharynx is indicated by a
black spot anteriorly.

378

COMPARATIVE ANATOMY

of trabeculae, forming a system of alveoli lining the vesicles
(Fig. 284). The lungs are, as a rule, equal in size ; in form they
are cylindrical in Salamanders, and elliptical in Anurans. In the
long-bodied Gymnophiona the right lung alone is fully developed,
the left being only a few millimetres long.

The mechanism of respiration is briefly as follows. The floor of
the mouth (which is kept closed) is lowered, and thus air is
inspired through the nostrils, which are then closed by a special
mechanism, while the glottis is pressed forwards and opened. The
floor of the mouth is next raised so that air is forced into the

Fig. 283.—^, Lung of Salamandra
maculosa, showing the regular ar-
rangement of the blood-vessels
extending between the individual
vesicles ; this regularity is slightly
disturbed on the right side at f.
B, Lung of Bana temporaria, in
which the blood-vessels and vesicles
have a more irregular arrangement.
(After Fanny Moser. )

Fig. 284.— Diagram of a Simple
Lobular Lung (Frog). (From
Oppel, after Renault. )

A, alveoli; a, artery; e, epi-
thelium of lung ; I, vesicles
lined by the alveoli ; G, glottis ;
s.p.p, pleuroperitoneum ; -v,

lungs. After a pause, the glottis and nostrils are opened, and thus
expiration takes place. The mechanism is therefore that of a
force-pump.

In many Salamanders (e.g. Salamandrina perspicillata, Typhlo-
molge, Amblystomatina3, Desmognathinse, Plethodontinae) the lungs
undergo a more or less complete degeneration, even though all
traces of the gills disappear. The fact that the floor of the mouth
is continually raised and lowered, as in other Amphibians which
possess lungs, indicates that a bucco-pharyngeal respiration takes

LUNGS 379

place in addition to the cutaneous respiration common to most
Amphibians, in spite of the fact that the skin is especially
vascular in these forms. The walls of the mouth, pharynx, and
even oesophagus (Desmognathus fusca) are abundantly supplied
with capillaries, which may even extend between the epithelial
cells.

Reptiles. — In Reptiles, as in all other air-breathing Verte-
brates, the form of the lungs is to a great extent regulated by
that of the body. In the higher types, such as the Chelonia and
Crocodilia, their structure is much more complicated than in
Amphibia ; this complication finds expression in a very considerable
increase of the respiratory surface. With the exception of the
thin-walled lungs of many Lizards, which retain a more primitive

EK ■"

Fk;. 285.— Lung of (A), Emys lutaria (1-6 mm. in Length), and B, Anyuis
fragitisy both rendered Transparent. (After Fanny Moser. )

In A are shown the three large transverse septa (1 — 3), which separate the
lateral chambers, as well as the entrance of the extra-pulmonary bronchus
(EB), the first anterior chamber (1 DK), and the posterior chamber {EK).

condition, more like that seen in the Frog except for the presence
of transverse septa, we no longer meet with a large central cavity,
but the organ becomes penetrated by a branched system of
bronchi connected with a comparatively narrow central bronchus.

In the simplest condition the trachea opens into the lung
by two bronchial apertures, but as the alveolisation of the lung
increases, extrapulmonary portions of the bronchi are more
marked : they are at first short, but gradually become longer, and
each is continued into the corresponding lung as an intrapulmonary
bronchus, which may be provided with cartilaginous rings along a
considerable part of its course, and which is connected by means

380 COMPARATIVE ANATOMY

of numerous apertures with smaller secondary bronchi : these ma}^
again give rise to bronchi of a third order, and so on.

This high differentiation of the lung occurs even in certain
Lizards, only its posterior end retaining a considerable lumen
(Fig. 286), and is more marked in Chelonians and Crocodiles
(Figs. 287 and 288). In Snakes, on the other hand, the central
lumen (i.e. the main bronchus) remains more roomy, and, as
in Amphisbajnians, in correlation with the elongated form of the
body, the right lung only is as a rule fully developed, the left
remaining in a vestigial condition or even disappearing entirely.

The posterior end of the lung may be continued into a delicate
finger-shaped hollow process, in which the alveoli are little marked

Secondare/ bronchi in
anterior part of lung

Main bronchus — ^ —

Secondnry bronchi tcith
(dceoli

Ventrfd margin

Secondary bronchi icith alveoli<

Sicollen termination of main*
bronchuH

.-^^^ ^^^k Dorsal marqin

Fig. 286.— Lung of Varanus variiis. {After A. Malani.)

or entirely wanting (e.g. certain Ascalabota, Iguanidse, Varanidse,
and Testudinidse). In Chameleons, in which, as in most other
Lizards, only the anterior end of the lung is spongy, numerous
thin-walled processes are given off from its ventral side, which are
spindle-shaped, club-shaped, or lobular in form (Fig. 289).
These processes seem to foreshadow a condition which reaches its
highest development in Birds, but here have to do with the curious
habit of inflating the body possessed by these animals.

The walls of the reptilian lung gradually undergo a consider-
able thickening owing to the increase in the amount of connective
tissue, and the smooth muscles which are enclosed in them play an
important part in the mechanism of respiration. A result of this

LUNGS

381

thickening is that the branches of the intrapulmonary bronchus
form cornparatively narrow canals separated by mesodermic tissue.

Median border

Lateral border

Late^'cU bronchi and
their svMivinons

Fig. 287. — Thalassochelys careita. (After A. Malani.)

Moreovei*, a distinction of the bronchi into portions serving merely
for the passage of air and into respiratory portions becomes

Venind border

Secondary bronchi in
anterior part oflvng

Ventral second-
aiy bronchi ^^^
%rith alveoli \^'"

\

Svollen termina .-
Hon of main
bronchus

Dorsal border

Dorsal iecondary bronchi
■with alveoli

Fig. 288. — Lung of Alligator mississipiensis.

The bronchus enters the lung some distance from the anterior end.
(After A. Malani )

gradually more marked, and thus eventually leads to the condition
seen in Birds and Mammals. Both ontogenetically and phylo-

382

COMPARATIVE ANATOMY

genetically a process of centrifugal
budding of the endodermic epi-
thelium followed by a further
development of the mesodermic
portions gradually results in a
complicated structure of the lung
such as occurs in the higher forms.

Birds. — The respiratory ap-
paratus of Birds presents so many
remarkable peculiarities, both as
regards the structure of the lungs
and in the presence of air-sacs, that
it must be considered in some
detail. Physiologically, it reaches
the highest degree of perfection
amongst Vertebrates.

The comparatively small but
highly vascular lungs (Figs. 290
and 291) are closely applied to the
thoracic vert -"brse and heads of the

fi I ribs, and are but slightly elastic

/ and capable of very little disten-

sion. The lower surface of each
lung is closely invested by a thin
fibrous membrane, the fulmonary
J ^g V ; ( I f aponeurosis} into which are in-

m I^W / ^ "^ f serted a variable number of

muscular bands {costo-puhnonary
muscles) : these arise from the
vertebral ribs, and are supplied
by the intercostal nerves.

The main bronchus (mesobron-
chium) enters the lung at about
the middle of its ventral surface,
and gradually losing its carti-
laginous rings, extends to the
posterior end, giving off secondary
bronchi. Of these, eight are ven-
tral (entobronchia) and six to ten
dorsal (ectobronchia), and the
branches of small calibre which
arise v from them are perforated
by numerous close- set apertures
leading into small tubes — the so-called " lung-pipes " (para-
bronchia). These again, give oif short,. radially arranged bronchioli

•^ The pulmonary aponeurosis, as well as the oblique septum, is often spoken
of as a "diaphragm" (cf. p. 184).

J

Fig. 289. — Lungs of Ghamcelco
monachus.

T, trachea.

<h>-y <?"- s^ =

r.Ald.S.-

FiG. 290.— Abdominal Viscera and Air Sacs of a Duck after the Re-
moval OF THE Ventral Body-Wall. (From a drawing by H. Strasser.)

Aa, Va, innominate artery and vein with their branches ; Ap, puhnonary artery ;
C, C, cervical sacs; Cd, coracoid ; D, intestine; D.th.a, oblique septum;
F, furcula ; H, heart, enclosed within the pericardium ; Ifcd, coraco-furcular
ligament ; Lg, Lg^, lung ; l-ih, suspensory (falciform) ligament, and led, Ics,
right and left coronary ligament of the liver ; P, pectoralis major ; p,
axillary sac lying between the coracoid, scapula, and the anterior ribs, and com-
municating with the sub-bronchial air-sac ; pa, pv, pectoral artery and vein ;
r.Abd.S, l.Ahd.S, right and left abdominal (posterior) air-sac; rL,lL, right
and left lobes of liver ; S, subclavius muscle ; s, s, partition walls between
the anterior thoracic air-sacs and the unpaired sub-bronchial sac, lying in the

anterior part of the body-cavity

^ partition walls between the anterior

and posterior thoracic air-sacs ; T, trachea ; v, portion of anterior wall 'of
the body-cavity ; *, point of entrance of the bronchi into the lung ; t,
anterior thoracic air-sac ; ft, posteiior thoracic air- sac.

384 COMPARATIVE ANATOMY

which divide into finer and finer branches ("air-capillaries")
with which the blood-capillaries are interwoven. The delicate air-
passages communicate with one another, so that none of the
alveoli apparently end blindly (Fig. 294, a). The relative delicacy
of the bronchial network is proportionate to the power of flight.

When the ventral body-wall of a Bird is removed, the heart,
stomach, liver, and intestine are seen in the middle, and on either
side of them is a tightly- stretched fascia, the oblique septum, which
shuts them off from a paired lateral sub-pulw onary chamber
(Fig. 290). Other chambers are situated in the anterior thoracic
region, ventral to the lungs. Others, again, are seen in the region
of the heart and in the posterior part of the abdominal cavit}^
These chambers are occupied by the air-sacs with which certain
of the bronchi communicate.

The most posterior chamber on either side encloses an abdominal
(posterior) air-sac (Fig. 290). In Apteryx, this is completely closed
in by the oblique septum, but in other Birds it gives rise to a large,
distensible diverticulum which extends backwards ventrally to the
kidney, amongst the viscera. In front of this there are two air-
sacs lying above and externally to the oblique septum, and
constituting the main part of the sub-pulmonary chamber ; these
may be called the anterior and posterior thoraeic sacs. A transverse
dividing- wall separates these, at the level of the coeliac artery,
and a second septum shuts off the anterior thoracic sac from
another one lying in front of it. The posterior thoracic air- sac
presents the simplest and most constant relations, and never com-
municates with any of the neighbouring chambers, as is often the
case with the anterior thoracic sac. A pair of cervical air-sacs lies
on either side of the oesophagus above each bronchus, anterior
to the hilum of the lung, and below this a sub-bronchial sac is
situated, which is separated behind from the anterior thoracic sac
by the septum already referred to. This sac is usually unpaired,
that of either side fusing with its fellow to form an interclavicular
chamber, bounded by the furcula ^ ; it communicates with neigh-
bouring air-cavities which lie between the pericardium and ster-
num and in the axilla, outside the body-cavity {axillary sac).

The main bronchus {mesobronchium) as a rule opens directly
into the abdominal air-sac (Fig. 291). From it a large lateral
bronchus, is given off, which opens into the posterior thoracic sac
by one or two apertures. The first entobronchium radiates out
anteriorly to the hilum of the lung, and one of its branches
opens into the cervical sac. Almost without exception, a large
aperture or ostium is present on the wall of the third entobron-
chium, communicating with the anterior thoracic air-sac. A
branch of the second entobronchium opens externally to the
hilum of the lung into the sub-bronchial sac.

^ In some Birds {e.g. Ivhea, Vulture, Adjutant) a median septum is present
separating the two sub-bronciiial sacs.

LUNGS

Fig. 291.— Left Lung of the Duck, in situ. (From a drawing by H. Strasser.)
The main bronchus is cut open ; internally to it lies the pulmonary vein, and
externally the pulmonary artery.

a«, opening of the main bronchus into the abdominal sac ; b, opening of the outer
lateral branch of the mesobronchium into the posterior thoracic air-sac ;
&S second ostium of the latter, more towards the middle line (present in
Passeres) ; Br. Ws, thoracic vertebrae ; /, first entobronchium, and c, its
ostium communicating with the cervical air-sac ; i, a, e, its internal,
anterior, and external branches ; lie, Hi, internal and external branch of
the second entobronchium : the end of He opens into the sub-bronchial sac
at d ; III, third entobronchium, with the aperture for the anterior thoracic
air-sac; IV, fourth entobronchium; m.l.c, longus colli muscle ; JV, kidney;
Oe, cesophagus ; stv, stv, sections of ribs which are connected with the
sternum ; Tr, trachea ; v, v, ends of free vertebral ribs. The boundary of
the pulmonary aponeurosis is seen along the outer edge of the lung, and the
costopulmouary nuxscles are shown extending from it to the ribs.

The air-sacs arise from the embryonic pulmonary vesicles as
delicate-walled hollow processes, lined by pavement epithelium :
these grow rapidly, and soon exceed the lung proper in size,
extending amongst the viscera. Moreover, they are not confined
to the body-cavity, but in numerous places extend beyond it, passing

c c

586

COMPARATIVE ANATOMY

between the muscles, beneath the skin, and even into most
of the bones. The latter are thus rendered 7:>?im??ift/2c, and conse-
quently the buoyancy of the body is increased. The pneumati-
city of the bones is not, however, an essential peculiarity connected
with flight, for in many Birds which are extremely good fliers
(e.g. Larus, Sterna) the bones are hardly if at all pneumatic, while

in the cursorial Ratitse, on the other
hand, they are markedly so.^

The air-sacs, though not serving
to increase the actual respiratory
surface,^ must be looked upon as an
integral part of the respiratory ap-
paratus : by their means, a greater
quantity of air can rapidly pass in
and out through the lungs when the
body-cavity is expanded and con-
tracted during inspiration and expira-
tion respectively, especially through
the larger bronchi ; consequently
there is less need for the expansion
of the lung-parenchyma. Moreover,
as part of the inspired air passes
directly into the air-sacs and their
prolongations, the absorption of oxy-
gen can take place during expiration
as well as inspiration. The aeration
of the blood is thus very perfect and
its temperature correspondingly high.
Rhythmical respiratory move-
ments take place when the Bird is
^ at rest, the sternum being alternately

From the ventral raised and lowered. But during
flight, when the weight of the body
is supported by the wings, the
eparterial" bronchus of either sternum, as well as the coracoid and

:;''<!£ \orr^.h^;Xial" "bs, are relatively immovable, and
bronchi ; V, pulmonary vein. inspiration and expiration are efiected

by the raising and lowering of the
wings, which may take place from three to thirteen times in the

^ The bones of Archaeopteryx were solid, and those of the recently extinct
Moa of New Zealand were much less pneumatic than in existing Ratitse.

Pneumaticity of the bones is not a special peculiarity of Birds ; it occurred
amongst the gigantic fossil Dinosaurians, and the skull of Crocodiles is also
pneumatic. Amongst Mammals, frontal, maxillary, and sphenoidal sinuses
are present in Anthropoids, Ungulates, Elephants, and Marsupials for instance :
and all these communicate with one another, and also with the tympanic
cavity. They are in many cases developed in order to give a greater surface
for the attachment of muscles, and also to effect a saving of material and a
lightening of the skull.

2 The walls of the air-sacs are supplied with blood by small nutrient arteries
arising from the aorta, the veins communicating with the postcaval.

Fig. 292.— Diagram or thk Ar-
rangement OF THE Bronchi in
Mammals.
side.

A, pulmonary artery ; a,

LUNGS 387

second. Thus during flight the air passes in and out without any
special respiratory movements being made, and the bird can travel
rapidly and continuously through the air without getting out of
breath.

A further importance of the air-sacs consists in the resulting
enlargement of the anterior part of the body surrounded by the
pectoral arch. An extended development of the skeleton can thus
take place, giving an increase of surface for muscular attachment
without any considerable increase in weight. Everything, in fact,
combines to establish an organ of flight with a large wing-surface
and increased muscular power.

Mammals. — Though not directly derived from the reptilian
type of lung, a certain parallelism can be recognised between the
lung of Reptiles and that of the Echidna as regards, for instance,
the presence of large air-spaces; but the resemblance here is
probably only of a secondary nature.

The main bronchus extends T^

throughout the length of the lung
and gives oflf a double row of second-
ary bronchi on its dorsal and ventral
aspects respectively, the components
of the ventral system being larger
than those of the dorsal.^

The morphological importance of
the lobes into which the lungs are
usually more or less divided (Fig.
293) is secondary to that of the
branching of the bronchi, and does
not essentially affect the latter: the
furrows between the lobes frequently
disappear to a greater or less extent.
The right lung possesses in many Fig. 293.— Lung of Man. From
cases an accessory lobe anteriorly, *^^ ^^r^tv^l side,

and another posteriorly, the former -S*, sulcus for the subclavian artery ;
in connection with an apical bronchus f 0 tpchea ; Z, base of hmg ;
, , , ,, /,, ^ 1 1 >j\ -.1 t, incisura cordis ; 1, 2, 3, lobes

and the latter (" azygos lobe ) with of the right, and 2a, 3a, of the
an accessory bronchus arising ven- left lung.
trally from the main bronchus, and

this accessory bronchus may be present even if the azygos lobe is
undifferentiated.

The cartilages of the bronchi become more and more sparse

^ The most anterior of the bronchi may arise from the main bronchus — or
even from the trachea — anteriorly to the point at which the pulmonary artery
crosses the main bronchus, and thus has been distinguished as the " eparterial
bronchus" from the others, or " hj^arterial bronchi," which arise posteriorly to
this point (Fig. 292). As a general rule, an " eparterial bronchus" is present on
the right side only, and as the nature and meaning of this asymmetr}' are not
■clear, it is better to speak of the anterior secondary bronchi, whether "epar-
terial" or " hyparterial," merely as a^ica^ 6ro?^cA^.

c c 2

388

COMPARATIVE ANATOMY

and finally disappear as the latter divide up into finer and finer
branches. The ultimate bronchioles open into small terminal
vesicles, the sacculi ahcolares or " infundihula " (Fig. 294, b), which
are surrounded by a close network of capillaries, and the walls of

OT.

Fig. 294. — Diagram of the Structure of the Lung in A, Birds, and
B, Mammals. (The whole of the lung is not represented.)

A. -Br, main bronchus; Br^, secondary bronchi: LP, "lung-pipes" (para-

bronchia). The arrows indicate the ostia of the air-sacs {L).

B. Br, main bronchus ; Br^, ventral, and Br^, dorsal secondary bronchi ; J A ,

sacculi alveolares ("infundibula"), only a few of which are indicated.

which are swollen to form numerous alveoli, thus causing a con-
siderable increase in the respiratory surface of the vesicles, the
size of which varies in different Mammals.

CCELOME.

Serous Membranes. — In the Anamnia, the serous membrane
( plmcroperitoneum) lining the coelome is continuous throughout
(cf p. 184 and Fig. 10), except that the heart is enclosed in a
special pericardial chamber usually completely shut off from the
rest of the coelome ^ and enclosed by the pericardial memhranc.
In Reptiles an indication of a further subdivision is seen : thus in
Crocodiles and Chelonians a chamber in which the lungs are
situated is shut off from the rest of the abdominal cavity. In
Birds, this subdivision is still more marked, and finally in
Mammals, on account of the development of the diaphragm, the

^ In Elasmobranchs it communicates with the general body-cavity by peri-
cardio-peritoneal canals.

CCELOME

389

pleuroperitoneal cavity is divided into two main sections, an
SLiitenor 2>^eural and a ipostevior jyeritoneal chamber.

In each of these three serous membranes (pericardial, pleural,
and peritoneal), a parietal and a visceral layer can be distinguished,
the former lining the outer wall of the chamber in question, and
the latter being involuted so as to invest closely and to suspend
its contained organ or organs^

Towards the middle line, the parietal layer of the pleura of
either side is reflected so as to form a septum betw^een the right
and left thoracic cavities. This septum is called the mediastinum,
and the space between its two layers the mediastinal space:
through this, the aorta, oesophagus, and postcaval vein run, and in

TcT/S

Fig. 29.5. — Diagram of the Pleural and Pericardial Cavities of Mammals,

BASED ON THE RELATIONS IN THESE PaRTS IN MaN. (A, horizontal

section ; B, transverse section. )

Br, bronchi ; II, heart ; m, mediastinum ; L, lung ; P, parietal, and P^,
visceral layer of the pleura ; Pc, Ps^, parietal and visceral laj-^ers of the
pericardium ; R, ribs (wall of thorax) ; S, sternum ; Tr, trachea ; W,
vertebral column ; +t, points at which the parietal and visceral layers of
the pleura pass into one another at the hilum pulmonalis (Hi).

the region of the heart the mediastinum is reflected over the
parietal layer of the pericardium. There is a lymphatic fluid
between the two layers of all these membranes, which renders
the movements of the contained organs smooth and e^isy.

Abdominal pores. — By the term abdominal pore is under-
stood a perforation — usually paired — of the posterior end of the
wall of the peritoneal cavity which puts the coelome into direct
communication with the exterior.

In Cyclostomes a pair of pores opens into the urinogenital
sinus, serving to conduct the generative products to the exterior :
they probably do not correspond to the abdominal pores of other
forms, which normally do not have this function, and are better

^ In connection with the suspensory mesenteric structures in the peritoneal
cavity, mention must be made of the omentnl folds extending between the
abdominal viscera.

390

COMPARATIVE ANATOMY

desmbed as genital pores, comparable to the like-named pores of
Teleosts (cf. under Generative Organs).

Apart from the indirect connection of the coelome with the
exterior by means of the oviducts in the female, such as exists
in most Vertebrates, other connections are seen, for example, in
both male and female Elasmobranchs. These are either indirect,
through the nephostomes of the kidney (q.v.), or direct, through
the abdominal pores : in some cases, both these means of communi-
cation with the exterior exist in the same Elasmobranch, but this
is never the case in other Anamnia, so that they appear to be to a
great extent mutually exclusive.

In the Elasmobranchii the abdominal pores are usually paired
and are situated posteriorly to the cloaca (Figs. 296 and 297), and

^* a

--^ 6, h'

Fig. 296.

-Diagrammatic Horizontal Section through the Cloacal
Region of a Plagiostome. (After E. J. Bles.)

a, blind ectodermal invagination (cloacal pouch) ; h, ¥, cloacal papilla ; c, peri-
toneal cavity, which opens by the abdominal pore at c^ ; Cloake, cloaca ;
Darm, rectum ; ***, points along the transversely striped section of the
cloacal papilla at which the abdominal pore may break through, in which
case the distal part of the papilla is solid (Raja).

may be enclosed within its lips. They are wanting in the Noti-
danidse, Cestracionidae, and Rhinidae, and are not constantly
present in the Scylliidse and others, even in individuals of the
same species, and they may only appear at sexual maturity. In
Ganoids, they open between the urinogenital aperture and anus,
but are apparently wanting in Amia when sexually mature.
Amongst Teleosts, they are said to be present only in the
Salmonidae and Mormyrida3, right and left of the anus ; but even
in these, the pore of one or of both sides may be absent. In
Ceratodus the abdominal pores are paired, and open behind the

CCELOME

391

Ma. Kp Mfiii

UrMg

Ma. Kp Msn

C

Fig. 297. — Diagrams illustbating the Three Possible Ways in which the
Peritoneal Cavity may Commttnicate with the Exterior in Fishes.
(After E. J. Bles).

A. Connection by means of nepbostomes {Xphs) only (Cestracion, Rhina, certaiu

other Elasmobranchs before sexual maturity, larval Amia).

B. Connection by means of nephostomes {Xvhs) and abdominal pores (Por.ahd)

(certain adult Scylliidae and Spinacidae).

C. Connection by means of abdominal pores {Por.ahd) only (Carchariidae,

Lamnidae, Batoidei, Holocephali, adult Ganoids, certain Dipnoi and
Salmonid^, Mormyridfe).

Gly Clo, cloaca, Ma.Kp, Malpighian capsules of the ruesojftephros {Ms)i) ; Per.B^
peritoneal cavity ; Uv-Ggy mesonephric duct^

392 COMPARATIVE ANATOMY

cloaca, while in Protopterus a single, apparently blind, canal is
present on the same side of the ventral fin as the vent, sometimes
to the right and sometimes to the left of the middle line, either
within or without the sphincter of the cloaca.^

Abdominal pores are not known to occur in Amphibians, Birds,
and Mammals, but amongst Reptiles they are perhaps represented
by the peritoneal canals of the Chelonia and Crocodilia, which in
the former are in close relation with the penis or clitoris, and
usually end blindly, while in the latter they open into the cloaca.

The abdominal pores may possibly correspond to the remains
of segmental ducts. At any rate, they and the nephostomes,
whichever may be the older phylogenetically, come under the
same physiological category, inasmuch as both might serve to
remove the products of regressive metamorphosis from the coelome,
which to a large extent represents an excretory organ (cf Fig. 297).

^ Abdominal pores are apparently wanting in Lepidosiren.

H. ORGANS OF CIRCULATION.

(VASCULAR SYSTEM.)

In Amphioxus, the vessels are of a simpler type than in the
Craniata, and to a certain extent retain characters only seen in the
embryos of the latter. There is no heart, and the circulation of
the blood is effected by the peristaltic contraction from behind
forwards of the ventral blood-vessel (ventral aorta).

In the Craniata the vascular system, which arises from the
mesoderm, consists of a hollow central muscular organ, the heart,
which is connected with a series of closed tubes, the blood-vessels,
containing a coloured fluid, the blood : there is also another system
of vessels containing a colourless fluid, the lymph, which, however,
besides permfeating all the tissues, is present in various spaces or
sinuses in the body as well as in the lymph-vessels. The
lymphatic system is therefore not completely closed, the vessels
communicating with the sinuses on the one hand, and with the
blood-vessels on the other. The lymph-vessels coming from the
intestine are known as ladeals.

The blood, which serves to carry the absorbed food and oxygen
to, and the waste products from, all parts of the body, is kept in
constant circulation through the vessefe by the rhythmic contraction
of the heart, which acts both as a force-pump and a suction-pump.

All the blood-vessels which bring back the blood to the heart
are known as veins, while those which carry it from the heart
are called arteries : the latter usually contain oxygenated blood
of a bright red colour, the former impure, darker blood, rich in
carbon dioxide and other products of destructive metabolism ; but
this is by no means always the case. Many of the veins, and also
of the lymph- vessels, are provided with valves, which are adapted
to prevent the reflux of the blood : they usually have the form of
semilunar folds of the internal coat, and are so arranged that two
are placed opposite to one another. The arteries (and also certain
of the veins) divide up into smaller and smaller branches, eventually
giving rise to microscopic tubes called capillaries, the walls of
which consist of a single layer of epithelial cells, surrounded by
contractile structures analogous to the smooth muscle-fibres of the
larger vessels and consisting of branched muscle-cells which are

394 COMPARATIVE ANATOMY

under the control of the nervous system : the capillaries again
unite to form the factors of the veins. The walls both of veins
and arteries consist, in addition to the epithelium, of connective
and elastic tissue and of unstriated muscular fibres, and are much
thicker in the case of the arteries than in that of the veins, in
some of which the muscular elements may be altogether wanting.

Both blood and lymph consist of a colourless fluid, the plasma,
in which float numerous cells or corpuscles. The Uood-corpuscles are
of two kinds — colourless, nucleated, amoeboid cells, known as white
or colourless _ corpuscles or leucocytes, and far more numerous red
hlood-corpuscles or erythrocytes.^ The colour of these is due to
hcemoglohm, which readily enters into loose chemical combination
with oxygen, and they are the specific respiratory cells. They have
no longer, however, the characteristic structure of protoplasm, and
are always surrounded by a membrane. The lymph contains
leucocytes only ; these are similar to those of the blood, and are
sometimes also spoken of as phagocytes.^

The nuclei of the red corpuscles persist, and the whole cell is
biconvex, in all Vertebrates below Mammals ; and, even in these
nucleated red cells may be seen in the marrow of the bones (in
which more especially they are formed throughout life), in the
blood of the spleen, and often in that of the portal vein : in all
other parts of the body of Mammals they lose their nuclei.
In all Mammals, except the Camelidse, the red corpuscles are seen to
have the form of circular, biconcave discs; ^ in the last-mentioned
family and in all other Vertebrates except Cyclostomes they are
oval. They are largest in certain Urodeles, being in Amphiuma as
much as 75//. in their longest diameter; then come, in order, other
Urodeles and Dipnoans, Reptiles, Anurans, Fishes, Birds, and
Mammals, in the last-mentioned of which they are the smallest,
varying in different families from 2'ofi (Tragulidse) to 10//-.

The heart is enclosed within a serous membrane, the pericar-
dium (Fig. 295), which, as already mentioned, consists of parietal
and .visceral layers. In most Anamnia and in early embryos of
higher forms it is situated close to the head, but on the differ-
entiation of a neck, comes to lie relatively further back. It arises
either as a single (Cyclostomi, Elasmobranchii, Ganoidei,
Amphibia) or as a paired (Teleostei, Sauropsida, Mammalia)
tubular cavity in the splanchnic layer of the mesoderm along the
ventral region of the throat, close behind the gill-clefts, and the
part of the coelome around it gives rise to the pericardial cavity.
Its wall becomes differentiated into three layers, an outer

* In Amphioxus the blood contains no formed elements.

^ In addition to the leucocytes and erythrocytes, a third kind of corpuscle
occurs in the blood : these structures are known as hlood-plaies or thromhocytes.
Each has the form of a minute, flat disc, is colourless and amoeboid, and consists
of nucleated protoplasm. It is very possible that they are derivatives of the red
and white corpuscles. In coagulation of the blood they undergo characteristic
changes.

'^ They are said to be primarily cup-shaped.

VASCULAR SYSTEM

395

serous {pericardium), a middle muscular (myocardiitm), and an
inner epithelial (endocardium)} In this respect it essentially
corresponds with the larger vessels, in the walls of which, as already
mentioned, three layers can also be distinguished ; but in the heart
the muscular fibres are striated, and the boundaries between the
cells of which they are primarily composed may disappear, so that a
syncytium results.

By a study of its development we thus see that the heart
corresponds essentially to a strongly-developed blood-vessel, which
later becomes complicated by the formation of various folds and
swellings. The embryonic tubular heart,
which contracts peristaltically, undergoes a
division into two chambers, an atrium or
auricle, and a^ ventricle, between which
valvular structures arise from the endo-
cardial layer: these only allow the blood
to flow in a definite direction on the con-
traction of the walls of the heart, viz.,
from the atrium to the ventricle, and any
backward flow is thus prevented (Fig. 298).
The atrium, into which the blood enters,
represents primitively the venous portion
of the heart, the ventricle, from which
the blood flows out, corresponding to the
arterial portion. The venous end further
becomes differentiated to form another
chamber, the sinus venosus, which opens
into the atrium by a narrow aperture
provided with two valves; while the
arterial end gives rise distally to a truncus
arteriosus; the proximal muscular end of
this (con us arteriosus or pylangiiom) is pro-
vided with more or less numerous valves
arranged in longitudinal rows, and its distal

end (bulbus arteriosus or synangium) is continued forwards into
the arterial vessel (ventral aorta).

These four chambers of the heart now contract rhythmically
in the following order: sinus venosus, atrium, ventricle, conus
arteiiosus. The bulbus, which may be more or less swollen and
corresponds to the base of the ventral aorta, contains no striated
muscular fibres.

The ventral aorta gives off right and left a series of symmetrical
afferent branchial arteries (Figs. 299-301, 320 and 321), each of
which runs between two consecutive gill-clefts, branches out into
capillaries in the gills, when present, and then becomes continuous
with a corresponding efferent branchial artery. After the first pair

1 The epithelial lining of the heart is said by some embryologists to be derived
from the endoderm, by others from mesenchymatous cells.

Fig. 298. — Diagram show-
ing THE Primitive Re-
lations OF THE Differ-
ent Chambers of the
Heart.

A, atrium or auricle; Ba,
bulbus and Ca conus ar-
teriosus (together consti-
tuting the embryonic
truncus arteriosus) ; Sv,
sinus venosus, into which
the veins from the body
open : V, ventricle.

396

COMPARATIVE ANATOMY

of these has given off branches to the head (carotids), they all unite
above the clefts, usually forming a longitudinal trunk on either side :
these constitute the right and left roots of the dorsal aorta, which

Fig. 299. — Diagram of the Embryonic Vascular System from the ventral
SIDE. (The portal systems are not shown.)

A, atrium ; A, A, dorsal aorta ; Acd, caudal artery ; All, allantoic (hypogastric)

arteries ; Am, vitelline arteries ; B, truncus arteriosus ;

carotids

D, precaval veins (ductus Cuvieri), E, external iliac artery ; Ic, common
iliac ; KL, gill-clefts ; RA, RA, right and left roots of the aorta, which arise
from the branchial vessels Ah, by means of the collecting trunks, S, S^ ;
JSb, subclavian artery ; Sh^, subclavian vein; Si, sinus venosus ; V, ventricle;
VC, HG, anterior and posterior cardinal veins ; Vm, vitelline veins.

extends backwards along the ventral side of the vertebral axis
into the tail as a large unpaired trunk, giving off numerous
branches — including paired embryonic vitelline or omphalo-

VASCULAR SYSTEM

397

mesenteric arteries to the yolk-sac, and (except in Fishes) allantoic
arteries to the embryonic urinary bladder or allantois (p. 9).

Primarily, the blood becomes purified in the vessels which
branch out over the yolk-sac, from whence it is returned by the
vitelline or owphalo-rtiesenteric veins (Fig. 300). These join with
the allantoic veins and veins of the alimentary canal to form what

Fig. 300. — Diagram of the Circulation of the Yolk-Sac at the end of
THE Third Day of Incubation in the Chick. (After Balfour.)

A A, the second, third, and fourth aortic arches : the first has become obliterated
in its median portion, but is continued at its proximal end as the external
carotid, and at its distal end as the internal carotid ; Ao, dorsal aorta ;
D.C, ductus Cuvieri ; H, heart; L.Ofj left vitelline vein; L.Of.A, left
vitelline artery ; R Of, right vitelline vein ; R.Of.A, right vitelline artery ;
S.Ca. V, anterior cardinal or jugular vein ; S. T, sinus terminalis ; S. F, sinus
venosus ; F. Ca, posterior cardinal vein. The veins are marked in outline,
and the arteries are made black. The whole blastoderm has been removed
from the egg, and is supposed to be viewed from below.

eventually becomes the hepatic portnl vein, which divides up into
capillaries in the liver. The capillaries then unite to form the
hepatic veins, which open directly or indirectly into the sinus
venosus.

Into the sinus venosus there also opens on either side a pre-
caval vein or anterior vena cava (ductus Cuvieri), which receives an
anterior cardinal or jugular vein from the head, and a posterior

398 COMPARATIVE ANATOMY

Cardinal vein from the body generally (not including the alimentary
canal). The caudal vein, which lies directly beneath the caudal aorta,
is connected with the posterior cardinals, usually indirectly, through
the renal portal veins (cf. Figs. 325-329). The further development
of the embryonic vessels may take place in one of three ways.

(1) The embryo may leave the Qgg, and take on an aquatic
existence (Anamnia), making use of its branchial vessels for pur-
poses of respiration, the entire allantois (Amphibia) when present
giving rise to the bladder.

(2) In the Amniota, which from the first breathe by means of
lungs, a modification and reduction of the branchial vessels and
allantois takes place, and the latter may even disappear entirely.

(3) The embryo undergoes a longer intra-uterine existence,
the allantois coming into close connection with the walls of the
uterus by means of the chorionic villi: the allantoic vessels extend
into the wall of the uterus and come into more or less close relations
with the maternal vessels, thus serving for the respiration and
nutrition of the foetus. In this way a placenta and a placental
circulation arise (pp. 9 — 11).

On the appearance of pulmonary respiration, important changes
take place in the branchial vessels and heart. The formation of a
septum both in the atrium and in the ventricle leads to the presence
of two atria or auricles and two ventricles, the conus arteriosus
and sinus venosus becoming eventually more or less incorporated in
the right ventricle and auricle respectively. Thus a right (venous)
and a left (arterial) half can be distinguished ; and a new vessel,
the fidmonaTy artery, arising from the last arterial arch, becomes
connected with the right ventricle ; this conveys venous blood to the
lungs, while special vessels {pulmonary veins) return the oxygenated
blood from the lungs to the left auricle, from which it passes 'into
the left ventricle and so through the aorta into the general
circulation of the body.

The branchial vessels never become functional, as such, in any
period of development either in Sauropsida or Mammalia, but those
which persist give rise, as already mentioned, to important vascular
trunks of the head and neck (carotids), anterior extremities (sub-
clavians), and lungs (pulmonary arteries), and also to the roots of
the aorta, one or both of which may remain (cf. Fig. 301).

The primitive number of arterial arches is six, the first two of
which (belonging to the mandibular and hyoid arches respectively)
almost always disappear early (Fig. 301): in caducibranchiate
Amphibia (including Anura) and in Amniota, the fifth arch also
disappears. The third gives rise to the carotid arch ; the fourth of
both sides (Amphibia, Reptilia), or of one side (Aves, Mammalia),
to the aortic or systemic arch, and the sixth to the pulmonary arch.

From the Dipnoi onwards, the posterior cardinals become more
or less completely replaced functionally by a large unpaired vein,
the postcaval or posterior vena cava, which opens independently into
the right auricle.

Fig. 301.— Diagram of the Arterial Arches of Various Vertebrates,
FROM the ventral SIDE. (After Boas.)

A, embryonic condition ; B, Fish ; C, Urodele ; D, Reptile (Lizard) ; E, Bird ;
F, Mammal. The parts which disappear are dotted.

a, h, c, the vessels into which the ventral arterial trunk is divided in Reptiles,
Birds, and Mammals ; ao, dorsal aorta ; ca, carotid ; k and h, the two first
embryonic arches, which almost always disappear ; I, pulmonary artery ;
8 (in F), left subclavian artery ; st, and s (in B), ventral aorta ; 1' and 3',
first and third afferent branchial arteries ; 1" and 3", the corresponding
efferent branchial arteries ; 1—4, the four branchial arches ; 2 in D and
F, second arch of the left side ; 2' in D, E ind F^ second arch of the right
side.

400

COMPARATIVE ANATOMY

i.(t.

The Heart, together with the Origins of the Main

Vessels.

Pishes (including Cyclostomes). — The heart in Fishes is
situated in the anterior part of the body-cavity, close behind the
head. It consists of a ventricle, with a truncus arteriosus or merely
a bulbus (Cyclostomi, most Teleostei), and an atrium or auricle,
the latter receiving its blood from a sinus venosus, and being
laterally expanded to form the appendices aiiricidm (Figs. 302
and 303.)

In correspondence with the work which each portion has to
perform, the walls of the atrium are comparatively thin, while those
of the ventricle are much stronger, its muscles giving rise in the

interior to a muscular network in
which a series of larger trabeculse
can usually be recognised : this
holds good throughout the Crani-
ata.

Between the sinus venosus and
atrium, and also between atrium
and ventricle, membranous valves
are present ; there are primarily two
atrioventricular valves, but they
may become further subdivided.
Numerous valves, arranged in
rows, are present in the muscular
conus arteriosus (Fig. 303, a): these
are most numerous in Elasmo-
branchs and Ganoids. There is a
tendency, however, for the posterior
valves, or those which lie nearest
the ventricle, gradually to undergo
reduction (b). Only the most
anterior row persists between the
ventricle and bulbus in Cyclo-
stomes and most Teleosts (c), but
amongst the latter two rows are
retained in the vestigial conus in
Albula (Butirinus) and Tarpon.

The heart of all Fishes ex-
cept Dipnoans contains venous
blood only, which it forces through the afferent branchial arteries
(Figs. 302, 320, 321) into the capillaries of the gills, where it
becomes oxygenated, to pass thence into the efferent branchial
arteries, and so into the dorsal aorta.

The heart of the Dipnoi (Figs. 304 and 305), in correspondence
with the double mode of respiration (by lungs as well as by gills)

Fa.d,.

TCa.s.

Fig, 302. — Heakt oy AcaniMi vul-
garis, FROM THE DORSAL SIDE,
WITH THE ATRIUM CUT OPEN.

(After Rose. )

Co, tr, truDCus anteriosus ; D.C.d
and D.C.8, right and left pre-
cavals ; O.av, atrio- ventricular
aperture; V.a.d. and V.a.s, right
and left valve of the sinus ven-
osus ; la^4a, afferent branchial
arteries.

HEART

i<Ei.EY. c

401

reaches a higher stage of development, mid-way between that seen
in Elasmobranchs and in Amphibians, and nearly resembling that
of Urodeles. The atrium becomes divided into a left and right
chamber by a septum : this is also true of the ventricle to some extent,
owing to the presence of a cushion composed of muscular fibres
and fibro-cartilage arising from the margin of the sinu-atrial aper-
ture, extending into the atrium and ventricle, and acting as a valve,
ordinary atrioventricular valves being absent. The sinus venosus,
from the Dipnoi onwards, opens into the right atrium.

The conus arteriosus is twisted spirally on itself: in Ceratodus
it is provided with eight transverse rows of valves, and begins to

Fig. 303. — Diagrammatic Longitudinal Sections through the Hearts of
Various Fishes. (From Boas's Zoology.) A, Fish with well-developed
conus arteriosus {e.g. Elasmobranch) ; B, Amia ; C, Teleost. In B and C
the sinus venosus and atrium are not indicated.

a, atrium ; ft, bulbus arteriosus ; c, conus arteriosus ; k\ valves ; s, sinus venosus ;
J, ventral aorta ; r, ventricle.

be divided into two chambers. In Protopterus this division is
complete, so that two currents of blood, mainly arterial and
mainly venous respectively, pass out from the heart side by side.
The former comes from the pulmonary vein, from which it passes
into the left atrium, thence into the left portion of the ventricle, and
thence to the two anterior branchial arteries. The venous current,
on the other hand, passes from the right portion of the ventricle into
the third and fourth afferent branchial arteries and thence to the
corresponding gills, where it becomes purified ; it reaches t he aortic
roots by means of the efTerent branchial arteries. The paired jow^-

D D

402

COMPARATIVE ANATOMY

npost-cetr

l.posi.citr Luntxjar

ZJ7i:.

ptcl.'vezTV
l.post.ca>rd

Fit;. 304

Fid, 304 — Hkart of Protopteriis aniiectens. From the left side, part of the wall
of the left atrium being removed. (After Rose. )

Co, conus arteriosus ; Z-. FA and JR.Vh, left and right atria ; ^S. a, septum
atriorum ; Si.v, sinus venosus, within which tne pulmonary vein (L?;) extends
to open into the left auricle by a valvular aperture ; W, fibrous cushion
extending into the ventricle.

Fig. 305. — Ceratodus forsteri. Diagrammatic View of the Heart and Main
Blood Vessels as seen from the Ventral Surface. (From Parker and
Haswell's Zoology, after Baldwin Spencer.)

aff. 1, 2, 3, 4, afferent branchial arteries ; I hr, 2 hr, 3 br, 4 h', position of gills
c. a, conus arteriosus ; d.a, dorsal aorta; d.c, ductus Cuvieri ; epi.l, epi.2
epi.S, epiA, efferent branchial arteries ; hy.art, hj^oidean artery ; ?'.?•.(!, post
caval vein ; l.ant.car, left anterior caroiid artery ; l.aur, left auricle Lhr.v
left brachial vein ; Ljtig.v, left jugular vein; I. post. car, left posterior caro-
tid artery ; l.posf.card, left posterior cardinal vein : Lpnd.art, left pulmonary
artery ; I sc.v, left sub-scapular vein ; r.anf car, right anterior carotid artery ;
r.aur, right auricle ; r.hr.v, right brachial vein ; r.jug., right jugular vein ;
r.post.car, right posterior carotid ; r.^»^.ar/, right pulmonary artery ; r.sc.v,
right subscapular vein ; vent, ventricle.

monary artery like the corresponding vessel in Crossoptery-
gians, arises from the fourth efferent branchial in Ceratodus

HEART 403

(Fig. 305), and from the aortic root in Protopterus and Lepidosiren,
that of the right side bifurcating to supply the dorsal surface of the
lung, while that of the left side supplies the ventral
surface. : The two pulmonar}^ veins unite to form a median
trunk which becomes closely connected with the sinus venosus, so
as to appear sunk within its walls in the form of two valve-like
projections ^ (Fig. 304). Thus the blood once more becomes
purified before it passes into the left side of the ventricle. A 'post-
caval vein, present from the Dipnoi onwards, opens into the sinus
venosus posteriorly to the precavals, and with it the hepatic veins
communicate (Figs. 305 and 328).

Amphibians. — With the exception of the Gymnophiona,
the heart in all Amphibians lies far forwards, below the anterior
vertebras. A septum atriorum is present,- but in Urodela
and Gymnophiona it is more or less fenestrated (Fig. 307):
it never completely separates the cavities of the two atria,
and forms an arch over the atrioventricular aperture. There

FlO. 300. — DlAGR^VM SHOWING THE COURSK OF THE BlOOD THROUGH THE HeART

IX Urodela (A) and Anui-a (B).

A, right atrium ; A^, left atrium ; ^>, Iv, pulmonary veins; tr, conus arteriosus,
divided in Auura (B) into two portions, tr, tr^ : through tr venous bloo<l
passes into the pulmonary arteries, Ap^, Ap^, while through tr^ mixed blood
goes to the carotids, ci — ce, and to the roots of the aorta, ^^4 ; F, ventricle ;
V, r, pre- and postcavals (only one precaval is indicated).

are always two sinu-atrial valves and two atrio-ventricular
valves, the latter of which are connected with the walls of
the ventricle by coixls. The two pulmonary veins unite before
opening obliquely, as in all the higher Vertebrates, into the left
atrium, at which point there are no special valves: the two
precavals and the postcaval open independently into the sinus
venosus.

^ The largest of these, as well as the fibrous cushion described above, take
a greater part in separating the two auricles than does the septum proper.

- In the lungless Urodeles (cf. p. .378) the septum atriorum and pulmonary
vein ai-e wanting, though the pulmonary artery can still be recognised.

D D*

404

COMPARATIYE ANATOMY

Neither in Urodela nor Anura is there a septum ventriciilorum,
so that the blood passing out from the spongy ventricle must have a
mixed character (Fig. 306). The ventricle is usually short and
compressed, but is more elongated in Amphiuma, Proteus, and the
Gymnophiona. It is continued anteriorly into a conus arteriosus;
this has usually a slight spiral twist, and possesses a transverse
row of valves at either end, as well as a spiral fold, formed by a
fusion of valves and extending into its lumen, thus partially
dividing it into a dorsal cavnm pulmonale, and a ventral cavum
aorticum communicating with the carotid and systemic

Fig. 307. — Heart of Cryptohranchusjaponicuf. From the ventral side. (After
Rose.) The left atrium is cut open.

L.v, L.v^, the two pulmonarj^ veins, opening by a single aperture into the left
atrium ; L. Vh, B. Vh, left and right atria ; O.ar, atrio-ventricular aperture ;
P.d and F.s, left and right pulmonary arteries; S.n, septum atrioi'um,
perforated by numerous small apertures ; tr, truncus arteriosus ; V.c.d
V.cs, posterior cardinal veins; F.c.i, postcaval vein; V.j.d and V.j.t<,
jugular veins; V.s.d and V.s.s, subclavian veins; 1", 4", the four arterial
arches.

arches. This holds good, e.g., for Amblystoma, Salamandra,
Amphiuma, and Siren. In others (e.g. Necturus, Proteus,
Gymnophiona), the conus is more elongated, and reduction is
seen in the disappearance of the spiral fold and the presence of
only a single row of valves.

In Anura, the fold lying within the conus extends so far back
that no undivided portion of the cavity is left. The consequence
of this is that the blood passing into the hindermost pair of the
arterial arches — that from which the pulmonary arteries arise — is
mainly venous, while the others contain more or less mixed blood

HEART

405

(Fig. 306, b) ; for, owing to the spongy nature of the ventricle,
there is no time for its contained blood to get thoroughly mixed
before it is forced into the conus.^

As in the Dipnoi, four afferent branchial arteries arise on
either side in the Amphibia, which — taking as a type the larva
of Salamandra — have the following relations (cf. Fig. 301, c).
The three anterior arteries pass to a similar number of ex-
ternal gill-tufts, in which they break up into capillaries (Fig. 308).
From the latter three efferent vessels arise, which pass to the
dorsal side, and there unite on either side to form the aortic root.
The fourth afferent branchial artery, which is smaller than the
others, does not pass to a gill, but to the pulmonary artery, w^hich

Fig. 308.— The Arterial Arches of the Larva of a Salamander. From
the ventral side. Slightly diagrammatic. (After J. E. V. Boas. )

a,a, direct anastomoses between the second and third afferent and efferent
branchial arteries; Ao, dorsal aorta ;.'-e, external carotid; c«, internal
carotid ; RA, aortic roots ; tr, truncus arteriosus; 1 — 3, the three afferent
branchial arteries ; / — ///, the corresponding efferent arteries ; 4, the
fourth arterial arch, which becomes connected with the pulmonary artery
{Ap) ; t, net-like anastomoses between the external carotid and the first
afferent branchial artery, which gives rise later to the carotid labyrinth. The
arrows show the course which the blood takes.

arises from the efferent branchials. The pulmonary artery,
therefore contains far more arterial than venous blood, and thus
the lungs of the Salamander larva, like the swim-bladder of Fishes,
can only be of secondary importance in respiration.

The internal carotid arises from the first efferent branchial
artery, towards the middle line, the external carotid coming off
further outwards (Fig. 308). The latter, as it passes forwards,
becomes connected with the first afferent bi*anchial by net-like

^ Special vessels supplying the walls of the heart are apparently wanting in
the case of many Fishes and of Amphibians. In Elasmobranchs, coronary arteries
arise from the liypobranchial artery and corollary vein^ open into the sinus
venosus or auricle ; similar vessels have also been described in certain Ganoids
and Teleosts. Coronary veins occur in Cryptobranchus japonicus.

406 COMPARATIVE ANATOMY

anastomoses, which later, losing the character of a network or rete
mirabile, gives rise in the adult to a swelling, the carotid labyrinth
(so-called " carotid gland "), which consists simply of a muscular
vesicle with septa in its interior, and probably acts as an accessory
heart. Direct connections exist, however, between the second and
third afferent and efferent arteries,

Towards the end of the larval period, the second efferent bran-
chial artery increases considerably in relative size, and the fourth
arterial arch also becomes larger. By a reduction of the anasto-
mosis with the third arch, the fourth carries most of the blood for
the pulmonary artery, and the latter thus now contains more
venous than arterial blood. When branchial respiration ceases, the
anastomosis between each corresponding afferent and efferent bran-
chial artery no longer consists of capillaries, but a direct connection

Fio. 309. —Arterial Arches ok ax Adult Salamandra marnlom, shown
SPREAD OUT. (After J. E. V. Boas. )

cd, carotid labyrinth ; ce, external carotid ; ci, internal carotid ; co, tr, truncus
arteriosus; ^, oesophageal vessels; RA, root of the aorta; 1—4, the
foiir arterial arches. The fourth arterial arch, which gives rise to the
pulmonary artery {Ap), has increased considerably in size relatively, and
IS only connected by a delicate ductus Botalli (t) with the second and third
arches.

between them becomes established (Fig. 309). Finally, the con-
nection between the first and second arches disappears, the
former giving rise to the carotid and the latter forming the
large aortic root ; an anastomosis remains throughout life, how-
ever, between the fourth arch, which forms the pulmonary artery,
and the second and third arches. This is usually spoken of as the
Mms Botalli. The third- arch varies greatly in its development :
it may be present on one side only, or may even be entirely
wantmg {e.g. Triton). In the larvae of Anura there are also four
afferent branchial arteries present on either side, but these are
connected with the corresponding efferent vessels by capillaries

HEART

407

only, there being no direct anastomoses. The consequence of this
is that all the blood becomes oxygenated. In the adult Frog
the third arterial arch is entirely obliterated, and there is no ductus
Botalli : the other vessels resemble those of the Salamander.

Reptiles. ^As in all Amniota, the heart of Reptiles arises far
forwards in the neighbourhood of the gill-clefts, but on the forma-
tion of a neck it comes to lie relatively further back than in the

A^vc

Fiu. 310. — Heart of A, Lacerta murcUis, and B, a
Large Varanus, shown cut open ; C, Diagram
OF THE Reptilian Heart. Ventral view.

A,A^, atria ; Ao^ dorsal aorta ; Ap^ Ap^, pulmonary
arteries ; Asc, As, subclavians ; Oa, Co}, carotids ;
Ci, postcaval: /, jugular; RA, root of aorta;
tr, Trca, innominate ; Vp, pulmonary vein ; Vs, subclavian vein ; V, F\ ven-
tricles ; 1, 2, 1st and 2nd arterial arches ; +, *, right and left aortic arches.
In C, the pre- and postcavals are indicated by Fe, Fe, only one precaval being
represented. A fibrous cord connects the apex of the ventricle with the peri-
cardium in most Lizards.

Anamnia, and this is more especially the case in Amphisbsenians,
Snakes, and Crocodiles. The carotid arteries and jugular veins
are thus correspondingly elongated.

The sinus venosus, which even in the Amphibia — especially

408

COMPARATIVE ANATOMY

A n.s

Anura — shows indications of becoming sunk into the right atrium
is now usually no longer recognisable as a distinct chamber ex-
ternally, though it still persists, with the two typical valves
(cf. Figs. 311-313). It becomes partially divided into two portions
by a septum, on the left of which the left precaval opens :
this subdivision of the sinus, hardly indicated in Chelonians, is

well marked in Crocodiles, and is
complete in Birds and Mammals.
The pulmonary veins unite into a
single trunk before entering the left
atrium.

The atrial septum is solid and
unperforated ; and, as its margin
reaches much further posteriorly
towards the ventricle than in Am-
phibians, it not only completely
separates the tw^o atria, but also
divides the atrio-ventricular aperture
into two. The two primarily dorsal
and ventral atrio-ventricular valves,
moreover, become fused together and
then again subdivided in the plane
of the atrial septum, so that a right
and left valve are now seen.^

The principal advance in structure
as compared with the amphibian
heart is, how^ever, seen in the appear-
ance of a muscular ventricular se^pturtiy
which may be incomplete (Hatteria,
Lizards, Snakes, Chelonians), or com-
plete (Crocodiles). The higher Liz-
ards {e.g. Varanida?, Fig. 310, b) come
nearest the Crocodiles in this respect, but in no Reptiles except
the latter is there more than a periodic physiological separation
between the two halves of the ventricle. A complete septum
ventriculorum thus appears for the first time in Crocodiles (Fig.
314), in w^hich consequently the right ventricle contains unmixed
venous blood, and the left ventricle unmixed oxygenated blood,
although, as will be seen presently, an admixture takes place in the
systemic arteries. In Crocodiles the right atrio-ventricular aper-
ture is guarded by a large muscular flap on the right (outer)
side of the aperture.

The conus arteriosus now becomes practically absorbed into
the ventricular portion of the heart, and each aortic root may be

Fig. .311. —Heart of Cydodus
hoddatrtei. From the dorsal
side. (After Rose. )

An^ An.s^ innominate arteries ;
Ao.ahd, dorsal aorta ; D.C.d,
D.C.8, precaval veins; L.i',
pulmonary vein; P.s^ P.d,
pulmonary arteries ; Sp. i, spa-
tium intersepto-valvulare (cf.
Fig. 316); V.C.d, posterior
cardinal ; V.j.d, jugular, and
V.s.d, subclavian vein of the
right side; V.c.i, postcaval vein.

^ In Ascalabota {e.g. Tarentola mauritanka) and Chelonia the proximal part of
the aortico-pulmonary septum encloses a plate of hyaline cartilage (cartilage of
Bojanus), and cai'tilages may occur in the neighbourhood of the semilunar valves
of the aorta and pulmonary artery in other Reptiles {e.g. Crocodiles).

HEART

409

made up at its origin of two arches, anastomosing with one
another (Lacerta, Fig. 310, a), or of one only (certain Lizards,
Snakes, Chelonians, and Crocodiles, Figs. 310, B, 312), from which
the carotid artery arises directly. The left and right aortic arches

DC.

Fio. 312.

Fig. 313.

Fio. 312. — Heart of a Young CrocodUus iiiloticus. From the dorsal side.

(After Rose.)

Ad and As, right and left aortic arches ; A.m, mesenteric artery ; L. V.h,
B.V.h, left and right atria; S.d, S.s, subclavian arteries; Tr.cc, common
carotid ; V.c.r, coronary vein. Other letters as in Fig. 311.

Fig. 313. — Heart of Crocodilm nilotkus. From the right side. (After Rose.)
Part of the wall of the right atrium is removed.

O.a.t', atrio-ventricular apertm-e ; Ta.fZ and Fa.*-, the two sinu-auricular valves,
the wliite line between which is the margin of the septum sinus venosi. Other
letters as in Figs. 311 and 312.

cross at their base, so that the left arises on the right side, and
vice 'ccrscL The most posterior arterial arch gives rise to the
pulmonar)^ artery (cf Fig. 301, d).

The blood from the right ventricle passes into the pulmonary
artery as well as into the left aortic arch, and, according as the

410

COMPARATIVE ANATOMY

septum ventriculorum is complete or incomplete, is either entirely
venous (Crocodiles) or mixed (other Reptiles). Thus even in
Crocodiles, although unmixed venous blood passes to the lungs,
the systemic arteries contain mixed blood; moreover, a small
aperture of communication (the foramen of Panizza) exists
between the two aortic roots at their origin, just distal to the
valves (Fig. 314). The valves at the base of the main arterial

T.a.d-

— T.a.

R.p.s

'At.s

.Ao.d

•Ao..s

.S.ao

•F.P

— ^O.a.v.8

■S. V
'F.a

'Ao.8

A. COS

Fig. .314. — Diagram of the Heart of a Crocodile. V^entral view.
(After A. Greil.)

Ac.c.d and Acc.s, right and left collateral artery ; A.c.p, prevertebral carotid ;
A.coe, cceliac ; Ao.d, Ao.s, right and left aortic arches or roots ; Ao.dors,
dorsal aorta ; A.s.d and A.s.s, right and left subclavian; At.d and At.s,
right and left atrium ; d.A, so-called dorsal anastomosis of the two aortic
arches; F.P, foramen of Panizza; L.B.d, right ligamentum Botalli ;
O.a.v.d and O.a.iKS, right and left atrio-ventricular ostium ; P. A, root of
pulmonarj^ artery ; R.p.d and R.p.s, right and left pulmonary artei'y ;
S.ao, aortic septum ; S.ao.j), aortico-pulmonary septum ; ^S*. V, ventricular
septum ; T.a.d and 2\a.s, right and left innominate ; y.c^and V.s, right and
left ventricle.

stems have undergone a considerable reduction in Reptiles as
compared with lower forms ; there is only a single row at the
origin of each aorta and the pulmonary artery. Coronary vessels
are well developed.

HEART 411

Birds and Mammals. — In these Classes, the atrial and
ventricular septa are always complete, and there is no longer any
mixture of the arterial and venous blood. The muscular walls of
the ventricle are strongl}- developed and ver}^ compact. This is
particularly the case in the left
ventricle, on the inner wall of
which the pajpillary muscles are
very strong: the left ventricle is
partially surrounded by the right,
the cavity of the latter having
a semilunar tmnsverse section,
and its walls being much thinner
than those of the former (Fig.
815).

Both in Birds and Mammals fi^. 315. -Transverse Section
the blood from the head and body through the Ventricles of
passes by means of the precavals ^^^^^^ anerea.
and postcaval into the right S, septum ventriculorum ; Vd,
atrium,! as does also that from "ght, and Vg, left ventricle,

the walls of the heart through

the coronary veins,- and the sinus venosus — especially in
Mammals — is scarcely recognisable (Figs. 816-318) : the right
atrium is separated from the right ventricle by means of a well-
developed valve. In Birds this valve, like that of Crocodiles,
is very large and muscular, while in most Mammals it consists
of three membranous lappets (tricuspid valve) to which are
attached tendinous cords, arising from the papillary muscles.
In Birds and Monotremes the left atrio-ventricular aperture is
provided with a valve consisting of three membranous folds : in
other Mammals there are only two folds, and the valve is therefore
known as the hicitspid or mitral ^^ Three semilunar pocket-like valves

^ Considerable variations are seen in Birds in this respect ; all three veins
may open together, or separately ; or again, the left precaval may open inde-
pendentlj-, and the right precaval and postcaval together.

- These open into the base of the left precaval [coronary sinus), the rest of
which disappears in certain Mammals. Coronary arteries are also present, arising
from the base of the aorta.

^ Tliere are no chordie tendine^e in Monotremes, the heart of which in man3'
other respects resembles that of the Sauropsida. One of the lappets on either
side is attached to the septum ; on the left side there is one lateral lappet, and on
the right two. In Marsupials, the number of lappets in the " tricuspid'' valve
varies between two and five. As regards the papillary muscles, the following
maj' be taken a.s a type in Placental Mammals. In the right ventricle the two
lateral lappets are connected with three papillary muscles or groups of mnscles ;
of these the strongest is lateral, arising from the septum or from the outer wall
of the ventricle, the others being respectively ventral and dorsal, the latter of
which is the weakest of the three. The median lappet is usually connected by
chordie tendineae directly to the septum, at which point small papillary muscles
may be present. In the left N^entricle there is a ventral and a dorsal group of
papillary muscles, from which the chordae tendinew exclusively arise : the lateral
valve is also connected directly with the lateral wall of the ventricle by a few
tendinous cords.

412

COMPARATIVE ANATOMY

are also present at the origins of the pulmonary artery and aorta
both in Birds and Mammals.

As regards the origin of the great vessels, Birds are distinguished
from Mammals by the fact that in them the right, while in Mammals
the left, aortic arch persists (Fig. 301, E, f) ; the corresponding
arch of the other side in both cases takes part in forming the
subclavian artery. Thus both in Birds and Mammals there is only a
single aortic arch. As in Amphibians, the posterior arterial arch
gives rise to the pulmonary artery.

The pulmonary veins, two from each lung, open close together
into the left atrium (Fig. 317).

Amongst the more important points in the development of the

MK^

Fig. 316.— Heart of Goose {Atiser vulgaris), dissected from the kioht side.

(After Rose. )

The right atrium and ventricle are cut^open, and their walls reflected. Ao,
aorta ; L. Vi, limbus fossai ovalis (Vieussenii)— a ridge arising from the ventral
wall of the right atrium ; the space between this and the septum atriorum is
known as the spatium intersepto-valvulare (cf. Figs. 311 and 312). MK,
MK^, muscular right atrio- ventricular valve ; S.a, septum atriorum ; V.a.d^
V.a.s, the two sinu-auricular valves, situated at the entrance of the postcaval ;
V.cx, aperture of coronary vein ; V.c.s.d, right precaval.

heart may be mentioned the fact that in the embryo the two atria
communicate with one another by means of an aperture, the
f(yramen ovale, or by several apertures, formed secondarily in the
atrial septum, through which the venous blood from the right
atrium passes directly into the left atrium (Fig. 318). This
foramen becomes closed later by secondary growths, which are less
complicated in Birds and Monotremes than in Placental Mammals,
in which a secondary circular septum is found, and when the lungs

HEART

413

come into use, its position can still be recognised as a thin
area (fossa oralis) in the atrial septum, surrounded by a circular

Kc.

.Yc.s.d.

Fig. 317. — Heart of Oimithorht/nchus paradoxus. From the dorsal side.

(After Rose.)
Ao, aorta; L.v, pulmonary- veins; P.d, P.s, pulmonary arteries; R.V.h, right
atrium ; Spi, Spatium intersepto-valvulare ; V.c.c, coronary vein ; F.c.t,
postcaval ; V.c.s.s, coronary sinus ; V.c.s.s, V.c.s.d, preeaval veins.

ridge {annulus ovalis). Extending from this to the base of the
postcaval and right preeaval respectivel}^ are two more or less well-

A B Vcs

f.s.

..Ao.

FaTh

Fig. 318. — Heart oi Hi.max F(ETls (8th Month). A, from the right, and B,
from the left side. (After Rose.) The walls of the atriuui and ventricle are
partly removed in each figure.

A.o^ aorta ; D.B, ductus Botalli (ductus arteriosus) ; F.o.c, foramen ovale ; L. V,
left atrium ; L.v, pulmonary vein ; P, P.d, P.s, pulmonary artery ; Va.s,
left sinu-auricular valve, fused with the septum atriorum {Sa, V.a.f) ;
Va. Th, Thebesian valve, in direct connection with the Eustachian valve
(Va.E)', r.c.f, coronarj' vein ; r.c.?, postcaval ; T'c.;*, left preeaval.

marked folds, known as the postcaval {Eustachian) valve and the
valve of the coronary sinus (Thebesian valve) respectively (Fig.

414 COMPARATIVE ANATOMY

318, a); these represent the remains of the sinu-auricular valves,
and in the embryo aid in conducting the blood from the right
atrium into the left.^

Great variations are seen in the mode of origin of the carotids
and subclavians from the arch of the aorta in Mammals. For
example, there may be a hrachiocephalic or innonmiate trunk on

Fig. .319. — Five Different Modes of Origin of the (Ireat Vessels from
THE Arch of the Aorta in Mammals.

Ao, aortic arch ; r, carotids ; s, subclavians ; fh, the, brachiocephalic trunk.

either side (Fig. 319, a) ; an unpaired common brachiocephalic,
from which the carotid and subclavian of one or both sides arise
(b, c, e); or a common trunk of origin for the carotids, the
subclavians arising independently on either side of it (d).

Arterial System.

In Amphioxus, the ventral aorta, as in the Craniata, gives off
a series of afferent hranchial arteries, which are, however, in cor-
respondence with the greater number of gill-clefts, much more
numerous than in other Vertebrates. The afferent branchial vessels
pass up the primary gill-bars, and give off branches to the secondary
gill-bars (Fig. 258). and both series of branchial vessels open
dorsally, as efferent hranchial arteries, into a dorsal acn^ta on either
side. The latter unites with its fellow posteriorly to the pharynx
to form a median dorsal aorta, which gives off branches to the
intestine, &-c.

The essential relations of the carotid arteries, dorsal aorta, and
pulmonary arteries in the Craniata, as well as of the embryonic vitel-
line arteries, have already been dealt with. Two carotids (usually
described as anterior and posterior, or as internal and external) can
usually be recognised in Fishes, arising independently on either side
from the anterior afferent branchial arteries, but varying much in
their arrangement (Figs. 304, 320, 321) : there may also be a ventral
mandihdar or lingual artery. From the Amphibia onwards,
internal and external carotids are formed by the bifurcation of each
common carotid. In these higher types, the internal carotid passes

^ Ossifications may occur in the neighbourhood of the atrio-ventricular and
arterial apertures in various Ungulates.

ARTERIAL SYSTEM

415

entirely into the cranial cavity, and supplies the brain with blood,
while the external carotid goes to the external parts of the head
(face, tongue, and muscles of mastication).

In Fishes a series of nutrient branchial vessels arise in connec-
tion with a hypobranchial artery formed by the union of vessels
given off from the ventral ends of the afferent branchial arteries
(Figs. 320, 321, and cf note on p. 405).

The origin of the subclavian artery,^ which supplies the
anterior extremity, is very inconstant, being sometimes symme-
trical, sometimes asymmetrical. It arises either in connection
with the posterior afferent branchial vessels, or from the roots or
main trunk of the aorta. Extending outwards towards the free

tp.cx,. p.c.a.

ccf-bjci

TiUa.

Fig. 320. — Diagram of the Branchial Arterial System of Mtu-tehis
antarcficus. Left lateral view. (From Bridge, after T, J. Parker.)

The ventral aorta and afferent branchial vessels are in solid black ; the afferent
arteries and their branches have double contours. The branchial clefts have
fringed borders to indicate their heraibranchs, and the arches are in simple
outline.

a.c.a, anterior (ventral) carotid; a.d.a, anterior dorsal aorta; af.h.a, afferent
branchial; br.a, brachial; c.w.a, coeliaco-mesenteric; rf. a, dorsal aorta ; E,
eye; ep.a, epibranchial ; H, heart; h.h.a, hj^pobranchial ; hy.a, afferent
pseudobranchial or hyoidean ; md.a, mandibular ; op.a^ ophthalmic ; p.c.a,
posterior (dorsal) carotid ; sh.a, subclavian ; »p, spiracle ; v.a, ventral aorta ;
1 — 5, the hyobranchial and four succeeding branchial clef ts.

extremity, the subclavian passes into the brachial artery, which in
higher forms divides into an ulnar and a radial branch, and these
again subdivide to form the arteries of the manus. The
condition seen in lower forms and in certain embryonic stages of
higher types shows that these vessels arise from networks, which
occur more particularly along the larger nerve-trunks. Each
main vessel is therefore merely a more strongly developed portion
of a connected system of canals which, especially in the
axillary region, is seen to be formed originally from segmental
vessels connected by longitudinal anastomoses.

^ In the Amphibia the great cutaneous artery, arising from the subclavian,
extends backwards, anastomoses with the epigastric artery, and gives off
numerous branches to the skin.

416

COMPARATIVE ANATOMY

From the dorsal aorta arise ^;«ne^aZ {intercostal, lumhar), and
cceliac, mesenteric, and urinogeniial arteries, supplying the body-
walls and viscera respectively. These all vary greatly both in number
and relative size : thus, for instance, there is sometimes a single
coeliaco-mesenteric artery, sometimes a separate cceliac, and one
or more mesenteric arteries (Figs. 322 and 323).^ The renal
and genital arteries also vary in number and arrangement. All
the branches of the dorsal aorta, however, present primarily an
approximately metameric character, their number becoming more
or less reduced owing to a concentration of the vessels, which is

Lda.

afb.d.

u.cu.

Hj.b.a.

Fig. 321. — Branchial Arterial System of Cod [Gadiis morrhua). Lateral
view. (From Bridge, altered from T. J. Parker.)

af.h.a, 1st and 2nd afferent branchial ; cl.a, coeliac ; d.a, median dorsal aorta ;
ef.h.a, first afferent branchial; ex.c, " external " carotid ; H, heart; hy.a,
hyoidean ; Hy.h.a, hypobranchial (to heart and pelvic fins); hy.p.s,
spiracular pseudobranch ; in.c, "internal" carotid; l.d.a, left supra-
branchial; m.a, mesenteric; on, orbitonasal; oph.a, ophthalmic; r.d.a,
right siiprabranchial ; sb.a, subclavian; v. a, ventral aorta; 1 — o, hyo-
branchial and succeeding gill-clefts.

more marked in short-bodied than in long-bodied Vertebrates.
In other cases a reduction in the number of vessels takes place by
the formation of anastomoses, one branch taking to itself the
peripheral twigs of another branch, so that the main stem of the
latter disappears.^

^ The cceliac typically supplies the stomach, liver, and spleen ; one or more
anterior mesenteries the whole intestine with the exception of the rectum, as well
as the pancreas ; and a posterior mesenteric 'the rectum. In Hatteria a very
primitive arrangement of the branches of the aorta is retained.

'^ Special mention must be made of the collateral i^ertehral artery, which in
Urodeles arises on either side from the root of the aorta and extends backwards
along the vertebral column to the end of the tail (Fig. 322). At the base of the
transverse processes numerous branches extend from it into the vertebral canal,
while others pass along the ribs and reach as far as the skin ; it also communicates
with the aorta along its entire length.

ARTERIAL SYSTEM

417

The aorta is continued posteriorly into the caudal artery, which
usually lies within a coelomic canal enclosed by the ventral arches

Int. car.

Adv. ren.

Fig. 322. — Diagram of the Chief Arteries (red) and certain of the Veins
(blue) OF A Urodele. From the ventral side (modified from Bethge's figures
of the vessels in Scdamandra maculosa and Triton tceniahis).

of the vertebrje ; the degree of its development is naturally in
correspondence with the size of the tail. In cases where the
latter is rudimentary^ as in Anthropoids and Man for instance, the

E E

418

COMPARATIVE ANATOMY

V

(kc

Fig. 323. — The Arterial System of Emy-i europoea. Fiom the ventral side.

Ac, dorsal aorta ; Aj), pulmonary ; Br, Br, the two bronchi ; C, caudal aorta ;
CaCy common carotids, with tracheal and (esophageal branches {Tr, Oe) ;
Co, Co^, and Me, coeliaco-mesenteric, which here arises as a bundle of
separate vessels ; Cr, femoral ; d, d, small intestine ; E, epigastric ; e, large
intestine ; /s, sciatic ; w?, stomach ; MB, posterior mesenteric ; BA, aortic
arches ; Sc, subclavian ; Tr, trachea ; UG, urinogenital arteries ; Ver,
vertebral.

VENOUS SYSTEM 4 1 9

caudal aorta is spoken of as the median sacral artery, and the aorta
here appears to be directly continued, not by it, but by the
common iliac arteries^ which pass outwards into the pelvic
region (Fig. 322).

Each common iliac artery becomes divided into an inteimal
iliac, or hypogastric, supplying the viscera of the pelvis and
derived from the proximal portion of the embryonic allantoic
artery, and an external iliac, which is continued into t\\Q femoral or
crural and supplies the hinder extremity. In some cases {exj.
Sauropsida) the internal and external iliacs come off separately
from the aorta (Fig. 323), thus indicating the primary segmental
nature of the arteries supplying the embryonic extremity.

The arteries of the hind-limb, like those of the fore-limb, have
undergone considerable modifications in the course of phylogeny.
Thus it is highly probable that the femoral artery was not
originally the chief vessel of the posterior extremity, but that the
main flow of blood passed along the sciatic artery arising more
posteriorly from the aorta, as is still the case in the Amphibia
and Sauropsida and in certain embryonic stages in Mammalia.^

Venous System.

Ampbioxus. — The blood from the intestine passes into a
suhintestinal vein which extends forwards as the hqmtic portal vein
to the ventral sid^ of the rudimentary liver, where it divides
up into a capillary network (Fig. 324). From this it passes into
a hepatic vein on the dorsal side of the liver, and this vein becomes
continuous with the contractile ventral aorta. A series of seg-
mental veins in the body walls opening into anterior and
posterior cardinal veins, comparable to those of Fishes and com-
municating with the ventral aorta by precaval veins, have also
been described (Fig. 324).

Fishes. — Taking the Elasmobranchii more particularly into
consideration, a few of the more important facts as regards the
development of the veins must first be considered (cf. p. 397).

The first veins to appear in the embryo are the paired omphalo-
mesenteric veins, which bring back the blood from the surface of
the yolk and from the walls of the gut (Figs. 300, 326, I, II). The
vessels from the former region are known as vitelline veins, while
those from the latter give rise to suhintestinal veins (Fig. 326, III —
VII) running beneath the embryonic intestine, which primarily

' The tibialis antica and tibialis postica, like the radial and ulnar arteries of
the fore-arm, do not represent the chief vessels of the shank. At an earlier stage
all these were relativeh^ small branches, while the main stream passed along an
{nteitial interosseous and then a median artery in the fore-arm, and a peroneal
or branches of a primitive saphenous artery in the shank.

E E*

420

COMPARATIVE ANATOMY

«
9
9

@

®

§

e^'^"~ t

■-..

,^- D(9(Z

— • Val

Fig. 324. — Diagram of the Venous System of Amphioxns.
(After B. Zarnik.)

DCd, DCs, right and left precaval ; Ds, intestinal sinus ; Gg, genital vessels ;
LI, heptic lacunse ; Lv, hepatic vein ; PI, parietal lacunar ; Qv^, — Qv^,
transverse veins; Sv, vental aorta ("sinus venosus ") ; Vca, anterior
cardinal ; Vcd, caudal ; Vcp, posterior cardinal ; Vsi, subintestinal vein.

extends into the caudal region as the post-anal gut. On the
disappearance of the latter, the posterior part of the subintestinal

VENOUS SYSTEM

421

vessels gives rise to the caudal vein, which now lies directly
beneath the caudal aorta and loses its direct connection with the
anterior part (VIII — XII). As the liver is gradually developed,

E E'

422

COMPARATIVE ANATOMY

H

I n m m F

" H li jj

DC

Os{ \od (XoJ t\od l\od hxOdCpYM.cp.-

Caj VC3

Ca n

CI

Cp

\'"'\

Fig. 326.— Diagram of Stages in the Development of the Veins in Elasmo-
BRANCHS. (I — XI after Rabl, XII after F. Hochstetter. )

Ca, Gp, anterior and posterior cardinal veins ; Cdx\ caudal vein ; CI, region of
the cloaca ; DC, precaval vein or sinus ; D, D, vitelline veins ; H, sinus
venosus of heart : J, subintestinal vein ; Jr. V, interrenal vein ; Lh, hepatic
veins ; Npf, renal portal sj^stem ; Os, Od, left and right omphalo-mesenteric
veins ; She, subclavian vein ; VP, hepatic portal vein ; Vpo, capillaries of the
hepatic portal sj^steni ; *, hepatic sinus ; f, cardinal sinus.

the main trunk of the left omphalo-mesenteric vein breaks up into
hepatic capillaries, which again unite anteriorly, opening into the
proximal ends of both these veins. The latter thus give rise to

VENOUS SYSTEM

423

Ci»rd.atvt[Juff)

Suhel^

Fig. 327. — For description see next page.

E E

*«*

424 COMPARATIVE ANATOMY

Fig. 327. — Diagram of the Veins of an Elasmobranch. From the ventral

side.

Card.ant.{Jug)^ anterior cardinal (jugular) ; the inferior jugular is seen nearer
the middle line ; Card. V. S, cardinal sinus, communicating with its
fellow in the middle line ; Caud.v, caudal vein, which divides into two
renal portals, A, A^, at the posterior end of the kidneys (N) : from
these arise the advehent veins of the renal portal system {V.adv) ;
Duct. Guv, precaval sinus ; Gen. V, genital veins ; H, heart ; Leb, liver ;
L.V.S, hepatic sinus; Seit.V, lateral vein, into which open a venous
network in the region of the cloaca {Ven.Gl.B), one or more cutaneous
veins of the tail {Gut. V), and veins from the body- walls and pelvic fins {HEV) ;
Suhcl, subclavian ; V.port, hepatic portal vein, receiving its blood from
the intestine (ED), stomach {Mg), and oesophagus (Oes. V), and anastomosing
with the lateral vein posteriorly and with the cardinal sinus anteriorly ;
V.rev, revehent renal veins, from which the posterior cardinals {GV) arise ;
t, anastomosis between portal and systemic veins.

the hepatic veins, which open into the sinus venosus. New vessels
from the various parts of the alimentary canal (gastric, splenic,
and mesenteric veins) are gradually developed, the pre-caudal
portion of the subintestinal vein becoming of minor importance ;
all these vessels unite to form what is now known as the hepatic
portal vein, and thus pour their blood through the capillaries
of the liver (Figs. 325-329, and 331).

Anteriorly to the heart, a paired precaval vein (ductus Cuvieri)
is developed, and opens into the sinus venosus. This is formed,
on either side, by the confluence of an anterior and a posterior
cardinal vein, the former bringing back the blood from the head
(external and internal jugulars), and the latter from the body,
in which it runs on either side of the aorta, between the latter
and the kidneys. A paired inferior jugidar from the ventral part
of the head into which the nutrient branchial veins open, also
communicates with the precaval. A subclavian vein from the
pectoral fin also enters to the precaval sinus or proximal end of
the posterior cardinal vein.

The caudal vein usually bifurcates posteriorly to the cloaca, each
branch passing along the outer side of the corresponding kidney,
and giving off advehent vessels into the latter. These divide
up into capillaries, forming a QrMcd portal system, the capillaries
again uniting to form revehent veins which open into the posterior
cardinals. Thus the condition of the veins typical of adult
Fishes is reached, and a few of the more important modifications
must now be briefly referred to.

In Cyclostomes, Elasmobranchs, and Dipnoans, the anterior
part of the subintestinal vein still persists as a small vessel
running within the spiral valve of the intestine. In the Elasmo-
branchs, many of the veins (e.g., precavals, anterior and posterior
cardinals, inferior jugulars, hepatic and genital veins) enlarge
to form capacious sinuses, and a large lateral vein (Figs. 325, 327),
running in the body-walls, opens into each precaval or posterior
cardinal. This probably corresponds to the vein of the primary
lateral-fin folds (p. 137).

VENOUS SYSTEM

425

caucl

Fig. 328. — Diagram of the Venous System of Protopterus annectens. From
the ventral side. (After W. N. Parker. )

at, atrium ; B. V. pelvic vein ; ca, conus arteriosus ; Cp, postcaval ; Da, intes-
tine ; DC, DC, precaval veins ; G.B, gall-bladder ; G.G, bile-duct ; Je, Ji,
internal and external- jugular ; L, liver ; L.G, lymphoid organ in the walls of
the stomach, the blood from which passes into the hepatic portal veins
{Vpo, Vpo^) ; M, stomach; N, JS , kidneys; ces, cesophageal vein; Oiw,
ovarian veins ; p, pericardium; par.v, parietal veins from the body-walls;
V, ventricle ; V.rard, left posterior cardinal vein, which is connected by
anastomoses {ans) with the postcaval {Cp) in the region of the kidneys ;
V.caiid, caudal vein ; Vh, Vh, hepatic veins ; V.ren.port, renal portal vein ;
Vsbc, subclavian.

426 COMPARATIVE ANATOMY

A renal portal system is absent in Cyclostomes and certain
Elasmobranchs, and is inconstant and very variable amongst
Teleostomes : in many instances the caudal vein communicates
directly with one or with both posterior cardinals, and in the former
case the other cardinal shows a tendency to become reduced in size :
a similar reduction occurs in many of the forms described below.

The chief point of interest as regards the veins of Dipnoans
(Fig. 328) is the presence of a large unpaired postcaval vein,
derived in part from the posterior cardinal, and comparable to
that of the Amphibia and Amniota. A renal portal system is
present, and the blood from the kidneys is collected into two veins
having the relations of posterior cardinals. Only the left of these,
however, opens anteriorly into the corresponding precaval, the
right, which is much the larger of the two, passing along the
dorsal border of the liver to open independently into the sinus
venosus in the middle line. The renal portion of this vein is
evidently homologous with the corresponding part of the posterior
cardinal, the anterior portion of which can no longer be recognised.
Thus the postcaval is made up of a posterior or renal portion, and
of an independently developed anterior or hepatic portion.

In Ceratodus the posterior cardinal and postcaval are directly
continuous with the caudal vein, and the renal portal, receiving
branches from the posterior end of the body, arises from the iliac
vein, which also gives off a pelvic branch. The latter unites with
its fellow in the middle line to form a median abdominal vein, com-
parable to that of the Amphibia (cf. Fig. 329) and opening into
the sinus venosus. Afferent renal veins also enter the postcaval
and cardinal, the corresponding efferent vessels opening into the
left cardinal and caudal veins ; the renal portal system is thus
more complicated than in other Dipnoans. The two pulmonary
veins unite into a single trunk before opening into the left
atrium.

Amphibians. — A large postcaval vein arises in essentially the
same manner as in the Dipnoi, its renal section being formed by
the fusion of the two posterior cardinals in this region. The
hepatic portion apparently arises in part from the right omphalo-
mesenteric vein, and in part independently, while the hepatic
portal vein is developed from the left omphalo-mesenteric. The
postcaval receives blood from the kidneys and generative organs,
as well as indirectly from the posterior extremities, body-walls, and
tail (when present). The anterior part of both posterior cardinals
persists in Urodeles and in Bombinator as the paired azygos vein,
and this may exceptionally be present on one or both sides in
other Anarans. It communicates with the corresponding precaval
(Fig. 329).

A renal portal system is present, and is formed, as in Fishes,
by the bifurcation of the caudal vein, which is wanting in adult

VMh

Fi«;. 329. — Diagram of the Venous System of Salamandra maculosa. From

the ventral side.

Card.po-it (Azyyos), posterior cardinal or azygos ; Caiid. V, caudal vein, which
bifurcates at the posterior end of the kidneys [N, X) to form the renal portal
sj'stem {Nkr.Pft.Kr) ; Cut.m, left great cutaneous (musculo-cutaneous) vein ;
Cut.m^, the same on the right side (partly removed); Duct. Guv, precaval ;
D, D, alimentar}' canal, from which the hepatic portal vein ( V.port) arises ;
H, heart ; Jug. ext. and Jug. int. external and internal jiigulars ; Lg. V, longi-
tudinal vein of the intestine: L.Pf, hepatic portal system; LV, hepatic
vein; Subcf, subclavian vein; V.adv, V.rev, advehent and revehent renal
veins; V.Cava inf. pars anter, and V.Cava inf. pars. pO'it, anterior and
posterior sections of the postcaval ; V.iliaca, femoral vein, which divides into
an anterior (tf) and a posterior (t) branch : the latter opens into the renal
portal, and the former (pelvic vein) unites with its fellow to form the
abdominal vein {Ahd. V), and also receives vessels (*) from the cloaca,
bladder, and posterior part of the intestine.

428 COMPARATIVE ANATOMY

Anura ; into the renal portal open the veins from the hind-limb,
and vessels from the body- wall often also communicate with it.
The blood from the kidneys passes into the postcaval. Connecting
the right and left renal portals (or femorals) is a transverse ^^^/we
vein, from which, in the medio-ventral line of the body, an abdominal
or epigastric vein arises, as in Ceratodus : this is primitively paired,
and its paired condition may even be retained in the adult
posteriorly (cf Fig. 322) ; it corresponds genetically to the lateral
veins of Elasmobranchs. This vein extends forwards in the ventral
body- wall into the liver, in which it breaks up into capillaries,
becoming secondarily connected by anastomosis with the hepatic
portal vein. The abdominal, vein receives blood from the cloaca,
bladder, and body- walls. In Urodeles remains of the subintestinal
vein also open into the hepatic portal system.

The arrangement of the anterior cardinals (external and in-
ternal jugulars) is essentially similar to that seen in Fishes.

The venous blood from the integument of the body and
frequently that of the tail also, passes into a great cutaneous or
mi(jSculo-c%itaneous vein, which extends forwards along the longi-
tudinal muscles of the trunk, and then makes a wide curve
towards the fore-limb, Avhere it unites with the subclavian vein
(Figs. 322 and 329). Shortly before opening into the sinus venosus
the combined trunk receives a small vein from the laryngeal
mucous membrane.

Amniota. — The section of the right posterior cardinal vein in
the region of the embryonic kidney (mesonephros) gives rise to the
hinder part of the postcaval : the hepatic section of the latter arises
as in Amphibia. In the Sauropsida the anterior portions of both
posterior cardinals disappear to a greater or less extent, and are
replaced by vertebral veins, while in Mammals they may persist
and may form part of the azygos veins. An anastomosis is
formed between these latter, and eventually the anterior part of
the left disappears more or less completely, the blood from both
sides passing into the right azygos (hemiazygos), which opens into
the right precaval (Figs. 330 and 331). As the azygos and post-
caval both receive part of their blood from the lumbar and pelvic
regions, the former forms an important commimicating channel
between the latter and the precaval.

The anterior cardinal gives rise, as in lower Vertebrates, to the
jugular, which, as well as the subclavian and vertebral or azygos,
opens into the precavals. In Reptiles, Birds, Monotremes, and
Marsupials, as well as in many Rodents, Insectivores, Bats, and
Ungulates, both precavals persist throughout life ; in other
Mammals the main part of the left disappears, all the blood from
the head and anterior extremities passing into the right precaval.
The coronary veins open into the base of the left precaval (coronary
sinus. Fig. 317).

VENOUS SYSTEM

429

A renal portal system occurs in connection with the embryonic
kidney in all Sauropsida, and traces of it can also be recognised in
Mammalian embryos, being particularly well seen in those of
Echidna. In adult Reptiles and Birds other connections of the
vessels are set up, so that only indications of the renal portal
system are usually retained, and in Mammals are entirely wanting.

As in Fishes, the first veins to appear in the embryo are the

Vcpd.\

V.P.d:

h'Kc.Jt.S^

■Vr.s.

yUjntcoTn^ri
VJ.L

Fig. 330. — Diagram showing the Relations of the Posterior Cardinal
AND Postcaval Veins in A, the Rabbit, and B, Man. (After
Hochstetter.)

V.c.i, postcaval; V.c.p.d, V.c p.s, right and left posterior cardinals;
V.iLint.comm, common internal iliac vein ; V.il.s.c, common iliac vein ;
F././, lumbar vein ; V.r.d, V.r.s, renal veins.

vitelline or omphalo-mesenteric veins (Fig. 331, a), bringing back
the blood from the yolk-sac, and uniting into a single trunk before
opening into the sinus venosus. As the liver becomes developed
it surrounds this trunk, which sends advehent branches into it,
revehent branches returning the blood into the anterior section
and eventually giving rise to a right and a left hepatic vein.
Thus a portal circulation arises, and the main trunk of the vein.

Fig. 331, A, B, and C. — Diagram illustrating Three Stages in the
Development of the Hepatic Portal System.

Az, azygos ; Ci, postcaval ; DA, ductus venosus ; DC, DC, precavals ; H, heart ;
//, iliac vein ; L, liver ; Mes, mesenteric vein, which later gives rise to the
hepaticportal {V.port), receiving blood from the alimentar}' cajiaX {Mes, D) ;
-iV, kidney; Om, Om^, Oni^, the three sections of the vitelline or omphalo-
mesenteric vein (the first still shows its originally paired nature at ft : in
stage B, the second section of this vein which passes through the liver,
disappears, so that Om and Om' are only connected by capillaries : in stage C,
the first section {Om) has quite disappeared, and the umbilical vein [Umh)
has become developed ; Sv, sinus venosus ; Vad. advehent veins ; Vr, revehent
veins ; *, connection of the umbilical vein with the capillaries of the liver.

RETIA MIRABILIA 431

where it passes through the liver, gradually disappears. In the
meantime, the cceliac and mesenteric veins have appeared,
and all the blood from them, as well as from the vitelline veins,
now passes through a common trunk, the hepatic portal vein, into
the capillaries of the liver, whence it reaches the sinus venosus
through the hepatic veins. The portions of the vitelline veins
posterior to the liver also gi-adually disappear as the yolk-sac
becomes reduced.

In addition to these vessels, the umbilical vein must also be
mentioned. This vessel also is originally paired, and corresponds
phylogenetically to the lateral veins of Elasmobranchs and to the
abdominal or epigastric vein of Ceratodus and Amphibians. It
is situated originally in the body-walls, and comes into relation
with the allantois (p. 9), opening primarily into the sinus
venosus and eventually into the postcaval : as the allantois
increases in size it brings back the oxygenated blood from this
organ (?>., from the placenta in the higher Mammalia) to the
embrvo. The right umbilical vein, however, is early obliterated,
and the left comes into connection with the capillaries of the
liver, its main stem in this region disappearing (Fig. 331, b) ;
thus the blood from the allantois has to pass through the capil-
laries of the liver before reaching the heart. In the course of
development, however, a direct communication is formed between
the left umbilical vein and the revehent branches of the fused
vitelline veins, and this trunk is known as the ductus venosus
(Fig. 331, c) : the point at which it opens corresponds to that from
which the postcaval has in the meantime arisen, the hepatic veins
now appearing as factors of the latter. On the cessation of the
allantoic (oi placental) circulation, the ductus venosus becomes
degenerated into a fibrous cord, so that all the portal blood has to
pass through the capillaries of the liver.

The intra-abdominal portion of the umbilical vein persists
throughout life as the epigastric vein in Reptiles and in Echidna,
but disappears in Birds and in other Mammals.^

Retia Mirabilia.

By this term is understood the sudden breaking- up of an arte-
rial or venous vessel into a cluster of fine branches, which, by
anastomosing with one another, give rise to a capillary network ;
the elements of this network may again unite to form a single
vessel. The former condition may be described as a unipolar, the
latter as a bipolar rete mirabile. If it is made up of arteries or of

^ The mode of development of the veins of the extremities is essentially
similar in all the Amniota, and at first resembles that occurring in Urodela,
though later on considerable differences are seen, more especially as regards the
veins of the digits.

432 COMPARATIVE ANATOMY

veins only, it is called a rete mirabile simplex ; if of a combination
of both kinds of vessels, it is known as a rete mirahile dwpleo:.

The retia mirabilia serve to retard the flow of blood, and thus
cause a change in the conditions of diffusion. They are extremely
numerous throughout the Vertebrate series, and are found in the
most varied regions of the body, as, for instance, in the kidneys
(glomeruli) — where their above-mentioned function is most clearly
seen ; on the ophthalmic branches of the internal carotid ; on the
pseudobranchs and on the vessels of the swim-bladder in Fishes ; on
the portal vein ; and along the caudal portion of the vertebral
column in Lizards. In Mammals, they are best developed in
Edentates, but occur also in Cetacea, Pinnipedia, Rodents,
Marsupials, Lemurs, &c.

Lymphatic System.

The lymphatic system consists of branched lymph-vessels,
situated in the connective tissue of various parts of the body, and
of lymph-sinuses: it serves to collect the blood-plasma which has
passed through the walls of the capillaries and to pass it into the
veins. The lymph, as already mentioned, contains leucocytes.

It is probable that in all Craniates the lymphatic system is
primarily paired and symmetrical : the first trunks which appear
in the embryo have apparently a similar course to that of the main
veins. Thus paired vessels (cephalic ducts) extend backwards ^om
the head, and others (thoracic ducts) forwards along the trunk in
similar positions to the anterior and posterior cardinal veins
respectively. On either side the cephalic and thoracic ducts unite
and open into a vein in the region of the precaval. In the course
of development, however, anastomoses are formed between the
trunk of either side : thus portions of the main trunks may
become of minor importance and undergo reduction, so that
asymmetry results, as is sometimes also the case in the venous
system. Lymph-vessels are usually abundant in various parts
(e.g. under the skin, on the alimentary canal) and may form
complicated networks.

In Fishes, the lymph-vessels are in many respects not so
plainly differentiated from the venous system as in higher forms :
a lymph-sinus connected with a vein occurs on either side in the
scapular region, and into it lymphatic trunks from the head and
body open.i A large vessel extends along the spinal canal, and
a subvertebral lymph-sinus surrounds the aorta and communicates
with others in the mesentery which receive the vessels from the
abdominal viscera. Lymphatics are also present on the walls of
the heart and great blood-vessels, forming perivascular sheaths.

^ The lacunar spaces in Cyclostoines, generally regarded as belonging to the
lymphatic system, have been found to contain blood.

LYMPHATIC SYSTEM 433

In Urodeles, the single thoracic duct bifurcates near the heart,
thus indicating its primarily paired character ; it receives lymph-
vessels from the head, and each branch opens into the corres-
ponding subclavian vein. The arrangement of the lymphatics in
Urodeles, and still more in Anurans, becomes considerably
modified owing to the presence of lymph-hearts and of large
sinuses under the skin which communicate with those of the
peritoneal cavity {e.g. sub vertebral, perioesophageal) : the cephalic
and thoracic ducts disappear, and the only trunks which remain
are those connecting the lymph-hearts with the sinuses. Valves
occur in the lymphatics w here they communicate with the lymph-
hearts, sinuses, and veins.

In Reptiles there are two cephalic ducts which open into the
subclavian vein along with the thoracic duct, which is either
paired (Snakes, Chelonians, Crocodiles), or single, bifurcating
anteriorly.

Lymphatic vessels are less developed in Birds than in
Mammals : two thoracic ducts arise in the region between the
thyroid gland and coeliac artery, having close relations to the
aorta and precavals, with the latter of which they communicate at
various points: their connection wdth the longitudinal channels
along the posterior part of the aorta takes place later (Fig. 332).
As in Reptiles, valves are not abundant, and occur in only a few
of the vessels.

In Mammals both thoracic ducts may persist, or that of one side
may disappear. Thus in Man the duct is unpaired and usually
arises from a sinus (cisterna or receptactdum ehyli) in the lumbar
region : it receives the lymphatics from the legs and pelvic organs
as well as those from the intestine {lacteals) and opens anteriorly
into the left brachiocephalic vein, with which also communicate
lymph-vessels from the head, neck, and right side of the thorax.
The lymphatics, like the veins, are provided with numerous valves
in Mammals.

Rhythmically contractile lymph-hearts (of which little is known
in Fishes except that two are present, e.g., in the caudal region in
Silurus and one in the Eel), occur in Amphibians, Reptiles, and
embryos of Birds (Fig. 332): they are surrounded by striped muscle.
In Urodeles there are as many as 14-20 on either side of the trunk
and tail under the skin at the junction of the dorsal and ventral
trunk-muscles,^ while in Anura the number is reduced to two
pairs with numerous valves, the anterior of which is situated
between the transverse processes of the third and fourth vertebrae
and the posterior between the urostyle and pelvis. In Reptiles
posterior lymph-hearts only are present, situated at the junction
of trunk and tail on the transverse processes or ribs. In Bird
embryos they occur at the boundary between the sacral and coccy-

* In the walls of thetruncus arteriosus of Urodeles a lymph-sinus is included,
which has been described as the "central Ij'mph- heart."

F F

434

COMPARATIVE ANATOMY

L. precava

L. aortic arch

Hypogast. arch

R lymph-heart

yd- Coccyg. mes.

L. lymph-heart

Pudend.

Fig. 332.— Diagram of the two Thoracic Ducts and great Abdominal
AND Caudal Lymph-trunks of a Chick of 17 days' Incubation. (After
L. Sala.)

The arteries are left clear, the veins shaded, the lymphatics black, and the lymph-
hearts dotted.

geal vertebrae partly covered by the coccygeus dorsalis muscle :
they communicate with the coccygeal and pelvic veins.

LYMPHATIC SYSTEM 435

Aggregations of leucocytes giving rise to follicles are present
in the lymphoid or adenoid tissue which occurs beneath the
mucous membrane in various parts of the body {e.g. tonsils, ali-
mentary canal, bronchi, conjunctiva, urinogenital organs) : these cells,
owing to their power of amoeboid movement, have a tendency to
wander through the mucous membrane to the surface, and thus
doubtless to remove useless and harmful material {e.g. broken down
particles, inflammatory products, Bacteria): in some cases suc4i
leucocytes {pliagocijtes), instead of passing out from the surface,
possibly serve to carry material from one part of the body to
another.

Lymphoid tissue is very abundant in the body-cavity of Fishes
and Amphibians : it occurs in the alimentary canal and round the
blood-vessels, but is especially abundant in the neighbourhood of the
urinogenital organs. To the same category belong the so-called " fat-
bodies " {corfcn^a adiposa) of Amphibia and Reptilia, as well as the
mass of lymphoid tissue on the heart of the Sturgeon and possibly
also the "hibernating glands" of certain Rodents and Insectivores.

The agglomeration of a number of lymphoid follicles gives rise
to those structures which are known as " lymphatic glands " and
as " hfiemolymph "or " Uood- glands" which, in spite of certain charac-
teristic differences, have much in common as regards the structure
of their enclosing capsules and of the network of connective tissue-
trabeculse extending through them, and various intermediate
forms occur.^ The spleen belongs to the same category as these,
and is in a sense intermediate in structure between the more pri-
mitive ha^molymph gland and the more highly differentiated
lymphatic gland, the three forming an almost continuous series.
In the last-mentioned the lymph, and in the two former the blood,
passes into a non-tubular network, where it becomes filtered ; the
worn-out red corpuscles are thus retained and become taken up
and digested by the lining epithelial cells and the leucocytes in
the process of phagocytosis. All three of these organs, moreover,
like the bone-maiTow, have the common function of giving rise to
new lymph-corpuscles. The spleen, which is present in all or
almost all Vertebrates (? Cyclostomes), arises in the region of the
mesentery, and has usually close topographical relations to the
pancreas: it corresponds to a specially differentiated portion
of a tract of Ij^mphoid tissue primarily extending all along the
alimentary canal, and in Protopterus still remains enclosed
within the walls of the stomach (Fig. 247). In most other Verte-
brates it is situated outside the walls of the canal, but even then
may extend along the greater part of the latter {e.g. Siren).
In some cases, however, either the proximal or the distal portion
of it undergoes reduction, or only the middle part persists

^ Lymphatic glands are most numerous in Mammals, and occur along the
lymph-trunks ; haimolymph glands have been found in numerous Mammals aiid in
certain Birds and Fishes.

F F 2

436 COMPARATIA^E ANATOMY

(Hatteria) : thus it is often situated near the stomach, but may
be met with in other regions of the intestinal tract, as, for
instance, at the commencement of the rectum (Anura, Chelonia).
In some cases {e.g. Sharks) it is broken up into several portions,
and frequently a larger " main spleen " and several smaller
" accessory spleens " can be recognised.

The structures known as tonsils are most highly developed in
Mammals, in which they give rise to a paired organ lying on either
side of the fauces — that is, in the region where the mouth passes
into the pharynx, and usually also to a mass situated more
posteriorly on the walls of the pharynx itself (pharyngeal tonsils) ;
the latter are phylogenetically the older organs and are present in
Reptiles, Birds, and most Mammals, and tonsil-like organs also
occur in Amphibians on the roof and floor of the oral cavity.
The tonsils consist of a retiform (adenoid) connective tissue
ground-substance enclosing a number of lymph-corpuscles, which
are arranged in so-called follicles.

MODIFICATIONS FOR THE INTRA-UTERINE NUTRI-
TION OF THE EMBRYO: FCETAL MEMBRANES.

I. Anamnia.

In several Elasmobranchs the oviduct gives rise to glandular
villi which secrete a nutritive fluid, and in an Indian Ray (Ptero-
platea micrura) there are specially long glandular villiform pro-
cesses which extend in branches through the spiracles into the
pharynx of the embryos, of which there may be as many as
three in each oviduct. In certain viviparous Sharks (viz., Mustelus
Igevis and Carcharias) the walls of the vascular yolk-sac become
raised into folds or villi, which fit into corresponding depressions in
the walls of the oviduct, the latter becoming very vascular. A
kind of umhilical placenta is thus formed, by means of which an
interchange of nutritive, respiratory, and excretory matters can
take place between the maternal and foetal blood-vessels.

Amongst viviparous Teleosts various arrangements for
the nutrition of the embryo occur. In Zoarces viviparus and
in the Embiotocidese the embryos are retained in the hollow ovary,
the empty follicles (corpora lutea) of which give rise to extremely
vascular villi, from which a serous fluid containing blood- and lymph-
cells is extruded into the cavity of the ovary and thus surrounds
the masses of embryos : these swallow the fluid and digest
the contained cells. In other forms (e.g. viviparous Blennies and
Cyprinodonts), the embryos undergo development within the
vascular ovarian follicles, and are probably nourished by diffusion ;
in Anableps, villi are developed from the yolk-sac, and these
doubtless absorb the nutritive fluid from the walls of the ovary.

FCETAL MEMBRANES 437

In certain Amphibians which have no free larval existence,
interesting modifications occur for nourishing the young until the
larval stage is passed. Thus in the Alpine Salamander (Salaman-
dra atra), a large number of ova (40 — 60) pass into each oviduct,
just as in the allied S. maculosa, in which the young are born as
gilled larvae. Were this the case in S. atra, the young would be
carried away in the mountain streams and destroyed, and a
curious adaptive modification has arisen in this form, in which
only one embryo (that nearest the cloaca) in each oviduct
undergoes complete development, remaining within the body of
the parent until the gills are lost and complete metamorphosis
has taken place. The other eggs break down and form a food-
mass for the survivors after their own yolk is used up. Degenerative
changes, moreover, take place in the epithelium of the oviduct,
and masses of red blood-corpuscles pass into the lumen of the
latter, undergo degeneration, and become mixed with the
broken-down yolk-masses, the resulting broth being swallowed by
the surviving young. The long, feather-like external gills are
closely applied to the uterine mucous membrane, and so effect an
interchange of gases in respiration. After the young is born, the
uterine epithelium becomes regenerated, and thus a process occurs
which somewhat resembles that of the formation of a decidua in
placental Mammals.

II. Amniota.

In all the Amniota, as already mentioned (p. 9), foetal mem-
branes, known as the amnion and allantois are developed, and the
latter, or primary urinary bladder, represented only in rudiment
in the Amphibia, is of great importance in connection with respira-
tion, excretion, and (in the higher Mammals) nutrition in the
embryo.

A glance at Fig. 8 will show that owing to its mode of
development, the amnion^ consists primarily of two layers; an
inner, the amnion proper, and an outer or false amnion. The
latter comes to lie close to the vitelline membrane, and forms the
so-called serosa, or seroics membrane. As the allantois grows, it
extends into the space continuous with the coelome between the
true and false amnion, and may entirely surround the embryo.

Amongst Reptiles^ the eggs of the viviparous Lizard, Seps chal-
cides, are relatively poor in yolk, and this is compensated for by
the yolk-sac and allantois coming into close relation with the
walls of the oviduct, thus forming an ttrnMlical and an allantoic
placenta, one at each pole of the embryo ; the latter of these is
the more important. Both foetal and maternal parts of the

^ As the head enlarges and sinks downwards, it is at first surrounded by a
modification of the head fold (p. 9) consisting entirely of ectoderm and called the
proamnion : this is afterwards replaced by the true amnion.

438 ' COMPARATIVE ANATOMY

placenta become extremely vascular, and thus the necessary inter-
change of materials can take place between the blood of the em-
bryo and mother. In Trachydosaurus and Cyclodus, as well as in
the Chelonia, a kind of umbilical placenta is apparently also
formed.

The fact that a vascular yolk-sac (often known as the umbilical
vesicle) is present in all Mammals^ indicates that they are de-
scended from forms in which, as in the Sauropsida, the eggs were
rich in yolk, and which were viviparous. This condition is more-
over retained in the Monotremes, and even in Marsupials the
ova are relatively large as compared with those of the higher
Mammalia.

As the amount of yolk was gradually reduced in the course
of phylogenetic development, close relations were set up between
the foetal (allantoic) and maternal blood-vessels, the allantois
becoming closely applied to the serosa to form a chorion (Fig.
333); but that this condition was only very slowly evolved is
shown by the fact that, even at the present day. Mammals exist in
which it has not been reached. These (viz., Monotremes and most
Marsupials) are therefore known as Aplacentalia or Achoria, in
contradistinction to the higher Placentalia or Ghoriata. Moreover,
in the Rodentia, Insectivora, Cheiroptera, Carnivora, and Ungulata
more or less distinct indications of an timbilical placenta, formed
in connection with the yolk-sac, can still be observed, and at a
still earlier stage the ova are nourished by the uterine lymph.

In Monotremes and Marsupials, both the yolk-sac and allantois
take part in respiration ; in the former the two are of equal
importance, while amongst the latter the yolk-sac is solely (e.g.
Dasyurus) or mainly (e.g. Phalcolarctos) important in this respect.
In Perameles obesula a further approach towards the formation of
a true allantoic placenta is seen, the allantois giving rise to small
vascular villi. In most Marsupials the allantois serves merely as
a urinary reservoir, and in none of them does it possess any
important function as an organ of nutrition, the young being born
at a relatively early stage, when they become attached to the
teats of the mother, and are then nourished by means of milk
(cf. p. 375) : the form of the mouth and also that of the teats is
especially adapted to this end.

In the higher Mammals, the umbilical placenta has usually
only a very temporary importance, though in some cases (e.g.
Rodents) it probably takes some part in respiration and nutrition
during the whole uterine life. The allantois extends out from the
body of the embryo and becomes attached to the serous membrane
to form the chorion, from which numerous villi extend into the
uterine wall (Fig. 333). As both the latter and the allantois become
extremely vascular, the uterine and allantoic capillaries and
sinuses coming into close contact with one another, a complicated
allantoic placenta arises, consisting of maternal and fcetal parts

FCETAL MEMBRANES

439

(Fig. 9). Thus the embryo is supplied by diffusion with the
necessities for existence during its comparatively long intra-uterine
life.

Various forms of placenta are met with amongst the Placentalia.
The most primitive type is apparently that in which the allantois
becomes attached around the whole serosa, so that the resulting
chorion, from which the comparatively simple villi arise, are equally
distributed over the whole surface
(Fig. 333). This form is known
as a diffuse placenta, and is met
with in Manis, the Suidse, Hippo-
potamus, Tylopoda, Tragulidae,
Perissodacbyla, and Cetacea.

The next stage is characterised
by the chorionic villi becoming
more richly branched so as to
present a greater superficial ex-
tent, and at the same time con-
centrated into definite and more
or less numerous patches or
cotyledons. Thus a polycotyledcra-
ary placenta arises, such as is met
with in most Ruminants, some of
which, such as Cervus mexicanus
and the Giraffe, show an interesting
intermediate form of placenta be-
tween the diffuse and the cotyle-
donary.

The chorionic villi in these two
types of placenta, even when

branched, separate from the uterine mucous membrane at birth,
the latter not becoming torn away; they are therefore spoken of as
non-deciduate.

A further complication is seen in the forms of placenta known
as zonary, the dome- or hell-sharped, and the discoidal, in which the
connection between foetal and maternal parts becomes much more
close, the villi giving rise to a complicated system of branches
within the uterine mucous membrane (Fig. 334). Thus the latter
becomes to a greater or less extent torn away at birth, forming a
decidua, the placenta being therefore spoken of as deciduate. In
these cases, the placental part of the chorion does not extend all
round the embryo. In the zonary placenta only the two opposite
poles of the chorion are more or less free from vascular villi, and
this girdle-like form occure in the Camivora, as well as in the
Elephant, Hyrax, Orycteropus, and Halicore. In Lemurs and
Sloths, the placenta is dome- or bell-shaped, while in Myi*meco-
phaga, Dasypodidie (Armadilloes), and Primates (Fig. 9) it forais
a discoidal mass on the dorsal side of the embryo. In Eodentia,

Fig. 333. — Diagram of the Fcetal
Membranes of a Placental Mam-
mal. (From Boas' s ^ooZogry.)

al, allantois ; am, amnion ; b, yolk-
sac (umbilical vesiple) ; the outer-
most line represents the serous
membrane. The outer wall of the
allantois has united with the serous
membrane to form the chorion from
which villi arise.

440

COMPARATIVE ANATOMY

Insectivora, and Cheiroptera, though the placenta is also discoidal,
it has probably not arisen, like that just mentioned (metadiscoidal
form) from a diffuse type, but was originally restricted to a discoidal
area, owing to the umbilical vesicle occupying a large surface of the
chorion.^

From the above description it is evident that the differences
in the form of the placenta are mainly those of degree, and that

Chot^lon0e>faesss

Mu.tljpr?. C^/jillsren

Jiuiterl

Dccia/ua.

Cfhffrcone/) ithel

2fftt€>/7 c/es Chorion.

V.

mi^ftffl. BFtttfefa'sj

Fig. 334. — Diagram to illustrate the Relations of the Fcetal and

Maternal Vessels in the Human Placenta, Showing Chorionic and

Maternal Vessels {Gefdsse) and Capillaries, Villi {Zotten), and
Decidua. (After Keibel.)

the placenta gives little indication of the systematic position of the
animal in question.

In the course of development the embryo becomes more and
more folded off from the yolk-sac (Fig. 8), the stalk of which
and that of the allantois, enveloped by the base of the amnion,
together form the umUlical cord. At birth, the foetal membranes
are shed, the intra-abdominal portion of the allantois persisting as
a cord, the urachus.

^ The histological structure of the placenta and the various modifications
seen in the maternal mucous membrane cannot be described here ; it is, how-
ever, important to remember that there is no direct communication between the
maternal and foetal blood, and that the maternal capillaries usually enlarge
to form sinuses, the walls of which become invaginated by the villi ; thus
the latter are covered by an epithelium furnished by the maternal tissues (Fig.
334). In some cases [e.g. Mole) part of the maternal tissue is absorbed in the
uterus.

I. URINOGENITAL ORGANS.

a. GENERAL PART.

The first traces of the urinary and generative organs of Verte-
brates arise from the mesoderm, and are closely connected with one
another genetically as well as morphologically and physiologically.

I. URINARY ORGANS.

As a rule, the development of the urinary organs from their
first rudiments to their final form extends through a relatively
long period, and the higher the animal in question, the longer this
period is. Unlike other organs, they do not reach their permanent
form step by step, without a break, but consist of a series of
organs, each of which has a similar function (viz., that of filtration
and excretion) and corresponds to a definite ontogenetic stage,
after which it is replaced functionally by another organ of the
series, and may then in part take on physiological relations to
the generative organs. In consequence of this peculiarity, these
developmental stages of the excretory organs of Vertebrates can
also be recognised phylogenetically, and are represented by the
paired organs known respectively as the fore-kidney or head-
Icidney {prone'phros), mid-kidney {mesonephros), and hind-kidney
(metanep/iros).

The number of urinary organs constituting the series corres-
ponds in general to the position of the animal in the vertebrate
scale. Thus so far as is known at present, the excretory organs of
Amphioxus and apparently of Myxinoids correspond to a single
set ; in all the true Fishes and in Amphibians, there is a provi-
sional (pronephric) and a permanent (mesonephric) organ, while in
the Amniota both pronephros and mesonephros are provisional
and become replaced functionally by a third organ, the metane-
phros.

All these organs consist essentially of epithelial canals or
tubules which arise from or close to that part of the mesoderm
which primarily connects the segmented somites w^ith the lateral
plates (cf p. 9) and is therefore often know^n as the nephrotomc,
to distinguish it from the myotome above and the lateral plates of

Fig. 335. — Diagrams to illustrate the Development of the Chief
Products of the Mesoderm. (After J. W. van Wijhe. )

A and B, transverse sections through an embryo in which the myotomes are
beginning to become separated from the middle portions of the mesoderm,
which includes the nephrotomes, A being in tlie pronephric, and B in the
mesonephric regions. C, the same, shortly after this separation is completed,
and D, later stage, in which the myotomes have extended dorsally and ven-
trally so as to surround the bodj'^, and have nearly lost their cavities.

ao, aorta, above which is the notochord, and above this again the medullary cord ;
d, intestine ; kd, gonad ; Ih, general ccelome ; msc, portion of coelome connect-
ing general coelome with its portion in the myotome (myoco-le, 7Jiyc) ; nt,
nephrotome or mesonephric tubule ; p, pronephros ; pg, pronephric or
segmental duct ; sH, sclerotome, which gives rise to the skeletogenous layer
from which the vertebral column arises ; si, mesenchyme of the lateral plates.

URINOGENITAL ORGANS 443

mesoderm below, the coelome primarily extending upwards through
both nephrotome and myotome (cf. Fig. 335).

Pronephros.

As a rule, the rudiments of the pronephros can be recognised in
the anterior segments of the trunk, and its glandular canals,
extending in the transverse plane of the body, must be distin-
guished from the primary pronephric (archinephric or segmental)
duct (Figs. 335-337), which passes backwards to open into
or close behind the cloaca. Each canal communicates on the one
hand with the coelome by means of a ciliated, funnel-shaped
aperture or nephrostome, and on the other with the pronephric
duct. A projection of the inner wall of the coelome arises right
and left of the mesentery, projecting towards each canal, and into

Pro/iephnc capsules

Pronephric duct

Glomerulus

Nephrostome

Fig. 336. — Diagram of the Pronephros showing the Formation of the
Glomeruli, (After Felix.)

A broad strip of the ectoderm and the ventro-lateral portions of the mesodermic
segments are removed, and the pronephric duct and tubules are seen above
the dorsal side of the coelomic wall ( transversely^ lined).

this branches from the aorta extend, each giving rise to a rete
mirabile consisting of a coiled tuft of capillaries or glomerulus, by
means of which water is filtered out from the blood : several
glomeruli may unite to form a glomus (Fig. 337). In many
Vertebrates, the parietal and visceral layers of the peritoneum in
the anterior part of the coelome unite around the glomeruli
and nephrostomes so as to form coelomic pronephric chambers or
capsules more or less completely shut off from the rest of the
bod3r-cavity (Fig. 336).

In Myxinoids the glandular pronephric rudiment extends along
almost the whole length of the coelome, but in other Vertebrates

444

COMPARATIVE ANATOMY

{e.g. Elasrnobranchii and Amniota) it is more or less shortened
and consists of only a small number of canals : this is doubtless
due to a secondary reduction, which also affects the glomeruli.
The anterior part of the pronephric duct is formed by the

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fusion of the distal ends of the pronephric tubules : the middle
and posterior parts of the duct arise in various ways in different
Vertebrates, and are developed from before backwards.

It is very probable that the pronephros at one time extended

URINOGENITAL ORGANS 445

further back — possibly along the whole length of the trunk, and
then became reduced owing to the development of another set of
urinary tubules constituting the mcsonephros.

Mesonephros.

The mesonej^hros or Wolffian body has usually a much greater
extent than the pronephros : it is originally strictly segmental, in
correspondence with the mode of development of the mesodermic
somites. The tubules of which it is composed correspond with
those parts of the mesoderm described above as the nephrotomes ^
(p. 441), which become separated from the somites but retain their
connection with the general body-cavity, into which each of them
opens by a ciliated nephrostome,^ the other end becoming connected
with the pronephric duct, which thus now serves as a mesonephric
duct.

The tubules then increase in length, each becoming coiled into
an S-shape and differentiated into several portions, the middle one
expanding to form a vesicle (Bowman's capsule), into which a
glomerulus formed on a branch of the aorta becomes invaginated,
the whole constituting a Malpighian capsule (Fig. 337). It will be
noticed that the glomeruli are here formed in a different manner
to those of the pronephros, which arise on the wall of the general
bod3*-cavity.

The further development of the mesonephros varies greatly in
different Vertebrates : in many Fishes it serves exclusively as a
urinary organ, but in Plagiostomes and higher forms it also
takes on relations to the generative apparatus, giving rise to the
rete and vasa efferentia of the testis, as well as to part of the
epididymis or parorchis, and, in Amniota, to other more or
less vestigial organs of secondary importance (parovaimcm, paro-
sphoron, hydatid of Morgagni, paradidymis, cf Fig. 339). Never-
theless, it may still serve as the permanent urinary organ
(Elasmobranchs, Amphibians), or may more or less entirely dis-
appear as such (Amniota) ; in the latter case, a third series of
tubules is formed, giving rise to a metanephros, or hind-kidney,
with which is connected a metanephric duct or ureter.^

^ These, as already mentioned, are primarily hollow coelomic canals, but in
Sauropsida and Mammalia they are at first solid and become hollowed secondarily.

2 The nephrostomes may be wanting, and this is especially the case in the
higher types, owing to the early separation of the tubules from the coelome as
well as from the somites,

^ The fact that in certain Lizards and Mammals larger or smaller portions
of the mesonephros retain their urinary function for a time in post-embryonic
stages, indicates that some of the ancestors of the Amniota retained the meso-
nephros throughout life as a functional excretory organ, before the complete
differentiation of the metanephros. On the other hand, the mesonephros is so
much reduced in the embryos of certain Mammals (e.g. Mouse) that it cannot
have any importance as a urinary organ, the excretory function here being
probably performed by means of the allantoic vessels and possibly those of the
umbilical cord.

446

COMPARATIVE ANATOMY

AG

-'>H/f

Metanephros.

All the tubules of the mesonephros, as well as the coiled
portions of the tubules and the glomeruli of the metanephros,
arise essentially from the same matrix (the " nephrogenetic
blastema "), the latter organ becoming later differentiated than the
former: the straight, collecting portions of the metanephric tubules,
on the other hand, originate as outgrowths from the mesonephric

ducts, as do also the meta-
nephric ducts (ureters), and,
in Mammals, the pelvis of the
kidney. The definitive kidney
of the Amniota is therefore not
a new organ, but corresponds
to a well-developed and special-
ised posterior portion of the
mesonephros, this specialisa-
tion having come about owing
to a still further separation of
that part of the kidney duct
which retains its original func-
tion from that part which is
taken into the service of the
generative organs. Nephro-
stomes are wanting in the
hind-kidney. The posterior
end of the ureter loses its
connection with the meso-
nephric duct and opens inde-
pendently either into the
cloaca or into a urinary bladder
(Figs. 338, 339, and 357-364).
Thus it will be seen that
the distinction between the
pronephros and mesonephros is
much greater than that between the latter and the metanephros ;
all three organs, however, are derived from the same matrix, and
merely represent three generations of the same ancestral organ.

Fig. 338.— Diagram illustrating the
Genetic Relations between the
Mesonephros and Metanephros.
(Modified from Schreiner.)

AG, allantoic duct; Z), intestine; UK,
urinary tubules arising in the nephro-
genetic blastema ; Kl, cloaca ; NG,
metanephric duct ; **, outgrowths
from metanephric duct ; ft, outgrowths
from mesonephric duct ( UG).

The Male and Female Genital Ducts.

In certain lower Vertebrates (Elasmobranchs) a second duct,
known as the Milllerian duct, becomes differentiated from the
primary mesonephric duct, and takes on an important relation to
the female generative organs, serving as an oviduct for conveying
the sexual products to the exterior, and remaining in connection

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Fir.. 339.— For description see next page.

448

COMPARATIVE ANATOMY

Fig. 339.— a Series of Diagrammatic Figures illustrating the Account
OF THE Comparative Morphology of the Urinogenital OrgaiNS of the
Vertebrata given in the Following Pages.

A, the pronephros stage of the Anamnia ; B, a later stage of the same ; C, the
urinogenital apparatus of the male Amphibian ; D, the same of the female ;

E, pronephros stage of the Amniota, the mesonephros as yet rudimentary ;

F, urinogenital apparatus of the Amniota at a stage at which the sexes are
not differentiated ; G, urinogenital apparatus of male Amniota ; H, the same
of female Amniota.

al, rectum ; al', al", allantois or urinary bladder ; d, cloaca ; d.m, Miillerian duct,
which in Mammals becomes differentiated (Fig. H) into the Fallopian tube
(/), the uterus {ut), and the vagina {vg) ; d.ms, duct of the mesonephros,
which in male Amphibians and Elasmobranchs becomes (Fig. C) the urino-
genital, and in females (Fig. D) the urinary duct. In the male Amniota it
gives rise to the seminal duct (Fig. G), and in the female to Gartner's duct
(Fig. H) ; d.pn., duct of the pronephros ,: g.c, Cowper's glands ; g.g, gonads,
undifferentiated stage ; hy, and u.m (Fig. G), unstalked hydatids and uterus
masculinus (vestiges, in the male, of the Miillerian duct, d.m) ; hy.s, stalked
hydatid ; ms, the developing mesonephros ; ms.r, renal portion of meso-
nephros ; ms.s, part of the mesonephros becoming converted into the epidi-
dymis and parovarium ; ms.v, vestiges of the mesonephros (paradidymis and
paroophoron) ; ni.t, the definitive kidney or metaiiephros of the Amniota ; os,
abdominal aperture of oviduct ; ov, ovary ; p. a, abdominal pore ; p.g, penis,
clitoris ; p.n, pronephros ; su, urinogenital sinus ; ts^ testis ; wr, ureter ; v.s,
seminal vesicle, an outgrowth of the duct of the mesonephros ; f, rete and
vasa efferentia testis ; ft, a network homologous with these structures at
the hilum of the ovary.

Tabulated Resume of the Facts Pictorially Illustrated on Page 447.

AiiJiimiia.

Amniota.

£

C

2

^3 .

S S

eg «

2"^

Develops in all Anamnia, but
rarely persists as a permanent
excretory organ.

Still develops in the Amniota,
but as an excretory organ under-
goes entire degeneration in the
embryo.

i
t

o

p

'i
1

a

1

In Elasmobranchs and some
Amphibians appears to give
origin by subdivision to the
mesonephric (Wolffian) arid
Miillerian ducts. In other
Amphibia, becomes converted
into the mesonephric duct. Its
fate in other Anamnia is not yet
fully investigated.

Persists as the mesonephric
(Wolffian) duct, and contributes
to some extent in the formation
of the Miillerian duct.

I

Serves in all Anamnia as a
urinary gland. In Plagio-
stomes, Ganoids, Dipnoans,
and Amphibians, a certain por-
tion becomes related to the male
genital apparatus, the remaining
jortion persisting as a permanent
cidney.

Loses its renal function in all
Amniota (as a rule in the em-
bryo), and becomes vestigial,
except so far as it becomes an
accessory portion of the genital
apparatus in the male.

URTNOGENITAL ORGANS

Tabulated Resume — {Continved).

449

Anamnia.

Amniota.

1

A portion (except in Cyclo-
stomes, Teleosts and (?) Holo-
cephali) becomes related to the
testis and functional in the trans-
mission of the sperms, the distal
retaining its renal function.

The anterior end becomes the
rete and vasa efferentia testis,
the caput epididymis, and per-
haps also the stalked hydatid
of Morgagni : the posterior end
becomes the paradidymis
(Giralde's organ).

1

Persists as the kidney.

The greater part of the an-
terior portion becomes the par-
ovarium, the posterior the paro-
ophoron.

j5

Serves in Teleosts merely as a
urinary duct.

In Elasmobranchs, Ganoids,
Dipnoans, and Amphibians
serves as the urinogenital duct.

The anterior portion becomes
the corpus and cauda epididymis
and the posterior the spermi-
duct (vas deferens).

1

Serves exclusively as the duct
of the mesonephros, i.e., the
urinary duct.

The greater part, as a rule,
degenerates ; the anterior por-
tion may be retained in a vestigial
form in the region of the par-
ovarium. In certain cases it
may persist, as a whole, as
Gartner's canal. The posterior
end becomes the organ of Weber.

3-

'u

r
6

a;

1

In Elasmobranchs it degene-
rates in post-embryonic life,
but vestiges of its anterior por-
tion are retained. Its existence
in most other Fishes is doubt-
ful. In Dipnoans and Amphi-
bians it is retained, at any rate
for some time, for its whole
length, in a functionless and
often but little degenerate con-
dition.

The anterior portion becomes
the unstalked hydatid of Mor-
gagni, the posterior, in some
Mammals, the so-called "uterus
masculinus" (prostatic vesicle).
In exceptional cases the whole is
retained as Rathke's duct. In
Sauropsida the posterior part
usually disappears.

When present, gives rise to
the whole genital duct.

Gives rise to the whole genital
duct.

s

30

II

c5
"3
S

Probably unrepresented.

Arises in part (ureter and col-
lecting ducts) from the posterior
end of the mesonephric duct, and
in part (secreting elements) as a
caudal extension of the mesone-
phros.

G G

450 COMPARATIVE ANATOMY

with the coelome anteriorly by means of an aperture derived from
the anterior nephrostomes ; only vestiges of this duct are retained
in the male (Fig. 339). Its formation is probably due to a division
of physiological labour, the primary mesonephric duct having
originally served to carry to the exterior the generative cells of both
sexes as well as the secretion of the primary urinary tubules,
through the nephrostomes.

The remaining part of the mesonephric duct ( Wolffian duct) still
serves as a urinogenital duct in male Elasmobranchs and Amphi-
bians. In the Amniota, in which the mesonephros gives up its
function as a urinary organ, the Wolffian duct serves exclusively as
a spermiduct (vas deferens)} Its coiled anterior portion takes
part in the formation of the epididermis.

The Gonads ("Generative Glands").

The sexual cells, which give rise to the ova and spr.rmatozoa,
originate from the germinal epithelium, which corresponds to
a differentiation of part of the ccslomic or peritoneal epithelium
on the dorsal side of the body-cavity on either side of the
mesentery, and into which the adjacent mesodermic stroma
penetrates ; thus a pair of gonads or " sexual glands " is formed
(Fig. 337). Primitively the gonads had a segmental arrangement,
and extended throughout a greater number of body-segments.

The primitive germinal cells are at first all similar to one
another, but in the course of development a differentiation takes
place, resulting in the formation of a male or a female gonad,
i.e., a spermary (testis) or an ovary.

The mode of development of the ova and spermatozoa is
briefly as follows : —

Ova. — The cells of the germinal epithelium grow inwards
amongst the connective tissue stroma of the ovary, or into its
cavity when hollow, in the form of clustered masses which may
become separated off from the periphery: certain of these cells
increase in size more than the others, and give rise to the ova,
while the smaller cells form an investment of follicle round them,
and may serve as nutritive material. The investing cells multiply,
and in Mammals a cavity containing a fluid is formed in the
middle of each follicle (Fig. 340) : the main mass of the follicular

1 The mode of development of the Miillerian duct in the Amphibia, Saurop-
sida, and Mammalia has undergone secondary modifications, the details being
to some extent still under controversy ; but at any rate it appears to be certain
that in the Amniota its anterior end arises from a groove in the coelomic epithe-
lium, while its posterior part, closely connected with the mesonephric duct, is
formed by a gradual backward growth of an originally solid epithelial cord.
This later loses its connection with the coelomic epithelium, develops a lumen,
and breaks through into the cloaca.

URINOGENITAL ORGANS

451

cells which enclose the ovum project, as the discus proligerus, into
the cavity of the follicle. When ripe, the ovum, surrounded by a
vitelline membrane, comes to the surface of the ovary and breaks
through into the abdominal cavity ;i it then passes into the
coelomic aperture of the oviduct.

Spermatozoa. — In the male, as in the case of the female, primi-
tive germinal cells can be at first distinguished in the development

JPS

Fig. 340. — Section through a Portion of the Ovary of a MammaLj
showing the mode of development of the graafian follicles.

D, discus proligerus ; Ei, ripe ovum, with its germinal vesicle (K) and germinal
spot ; KE, germinal epithelium, ingrowths from which extend into the
stroma of the ovary to form the ovarian tubes (PS) : the stroma is pene-
trated by vessels {g, g) ; Lf, liquor folliculi ; Mp, zona pellucida, showing
radiated structure ; S, cavity between the follicular epithelium (tunica
granulosa, Mg) and the primitive ova ; Tf, theca folliculi ; U, U, primitive

of the generative elements. These give rise to a series of seminal
tubules (Fig. 363), containing larger and smaller cells ; the former
undergo division to form the active and relative minute sperms
or spermatozoa. The nucleus gives rise to the so-called " head " of
the sperm, while the surrounding protoplasm becomes differ-
entiated to form the motile " tail," the *' neck " arising from the
centrosome of the cell (p. 3).

1 In Mammals a certain amount of blood is poured out through the broken
ends of the vessels in the stroma of the ovary into the cavity of the follicle
vacated by the ovum ; around it a cellular investment is formed from the follicle-
cells, and further modifications take place, resulting in a body of yellow colour,
known as the corpus luteiim, th^ function of which may possibly be that of
a "gland with internal secretion" (cf. under Pancreas, Adrenals, &c).

G G 2

452 COMPARATIVE ANATOMY

h. SPECIAL PART.
Urinary Organs.

In Amphioxus a series (90 or more) of independent segmental
tubules are present on either side in the reduced section of the
coelome situated on the dorsal side of the pharynx, in close rela-
tion to the branchial blood-vessels.

Each tubule is partly glandular and partly ciliated, and gives
off several knobs provided with peculiar club-shaped cells project-
ing into the coelome and corresponding to closed nephrostomes :
its other end opens into the atrial or peribranchial chamber
(p. 352) by a single aperture.^ The segmental arrangement of
the tubules in the adult corresponds to that of the branchial
apparatus, and not to that of the myotomes. No tubules are
present posteriorly to the pharynx.

Cyclostomes. — In the Petromyzontida;, the excretory system
resembles that of the Amphibia much more closely than in the
Myxinoidei. In the former, rudiments of at least thirteen prone-
phric tubules gradually appear, but only the five most anterior of
these become functional. In Ammoccete larvae about 10 cm. in
length, the pronephros reaches its highest stage of development,
and by this stage a mesonephros has also arisen from the coelomic
epithelium, so that for some time both organs are functional
(Fig. 341). As the mesonephros slowly develops further, from
before backwards, the pronephros becomes gradually reduced, and
eventually also a reduction takes place of about two-fifths of
the entire mesonephros also.

Observations on the excretory organs of Myxinoids are still
incomplete, and it is uncertain whether both pronephros and
mesonephros are developed, or whether only one pair of these
organs is represented.

In none of the Cyclostomes does the kidney come into relation
with the generative organs, and its duct, which opens into the
urinogenital sinus, probably in all cases represents the unaltered
pronephric duct.

Elasmobranchs. — In these Fishes, the transitory pronephros
has a rudimentary character, and is very variable : it usually only
extends over 3-5 body-segments (Raia, Torpedo) — more rarely over
7-8 segments of the embryo, and apparently has no excretory
function. As already mentioned (p. 446) a differentiation of the
primary mesonephric duct into Wolffian (secondary mesonephric)

^ It is very doubtful whether the suggestion is justified that these epi-
braiichial tubules correspond to some extent to the pronephros of the Craniata,
and the peribranchial chamber to its duct.

Nephrostomes

Pronephros

Glomendus

Pronephric dxict

Glomeruli

Mesonephros

Fig. 341. — The Excretory System of a Petromyzon jluviatilis, 22 mm. in
Length, from the inner side. (After Wheeler.) About half the entire
length of the primary urinary duct is represented, and behind the pronephros
it is greatly coiled. Four pronephric nephrostomes and a folded glomerulus
are present, and between the pronephros and mesonephros is a portion
wanting in tubules.

45i COMPARATIVE ANATOMY

and Mullerian ducts, takes place, and a distinction between an
anterior, a middle, and a posterior section of the mesonephros may
be observed in Plagiostomes (Figs. 339, 349). In the male the
anterior portion {epididymis or parorchis) comes into connection
with the testis by means of small ducts, the vasa efferentia, and
in the female undergoes reduction : the middle portion usually
becomes considerably reduced, especially in the female, but its
ducts still communicate with the Wolffian duct, which is more or
less dilated at its posterior end and in the rnale serves mainly as
a vas deferens : the large posterior portion forms the essential part
of the kidney, and empties its secretion by means of special urinary
ducts, some of which unite with one another, into the urinogenital
sinus.

This differentiation of the hinder part of the mesonephros,
and the formation of special ducts in connection with it, seems,
in a sense, to foreshadow the condition which occurs in the
Amniota (p. 446).

In the Holocephali no sexual portion of the kidney can be
distinguished, and the anterior part in the male Callorhynchus is
much larger and more massive than the posterior section, which,
as in Plagiostomes, is provided with special ducts.

The kidney is very variable in form and size : its outer
border is usually notched, and this, together with the arrange-
ment of the nephrostomes in the embryo, points to the original
segmental arrangement of the organ. The segmental char-
acter, however, disappears later on ; in the adult the nephro-
stomes are much less numerous than the vertebrae of this region,
but their number and size vary much in different genera and even
in individuals, and they do not persist in all (e.^. Carcharias,
Mustelus, Echinorhynchus, Myliobates, Raia).

Teleostomes. — The larval pronephros of Ganoids consists of a
varied number of canals, the segmental arrangement of which has
so far not been accurately ascertained.^ In the definitive kidney
(mesonephros) a segmental arrangement is recognisable. In
Sturgeons, its connection with the coelome by means of nephro-
stomes takes place only after the individual canals have become
connected with the pronephric duct.

The pronephros in the majority of Teleosts has only a
temporary significance, and extends over from one to five
segments.^ The mesonephros constitutes the excretory organ of
the adult, and consists of a narrow band varying in size and
diameter in different regions, situated on the dorsal side of the
body-cavity, between the vertebral column and the swim-bladder.
Secondary fusions between the two kidneys often occur.

^ The nephrostomes of the pronephros open either directly into the coelome
or into a space shut off from the latter which encloses the more or less folded
glomus (p. 443).

2 It persists in Lepidogaster, Fierasfer, and Zoarces.

URINARY ORGANS 455

The urinary duct in both groups probably represents the prone-
phric duct, and may lie more or less freely, or be embedded in the
substance of the kidney. Posteriorly the two ducts usually fuse
together and become expanded to form a kind of urinary bladder,
as is also the case in Ganoids (cf. Figs. 350 and 351) ; in its forma-
tion, however, the ectoderm of the cloaca also takes part. The
bladder usually opens behind the anus — either independently,
or together with the genital ducts — by a simple pore, or on the
summit of a urinogenital papilla. Thus a differentiation of the
pronephric duct into a Wolffian and a Mullerian duct is not
known to occur in Teleostei, nor does the mesonephros come into
connection with the gonads : the early development both of the
pronephros and mesonephros exhibits special peculiarities.

A close examination of the organ, which appears to the naked
eye as the kidney in Teleosts and Ganoids, shows that a larger or
smaller portion of it — more particularly the anterior part — consists
of an adenoid or lymphoid tissue, comparable to the hsemolymph
glands described on p. 435.

Dipnoans. — The pronephros of the larva consists of several
tubules, and possesses two nephrostomes. The mesonephric
tubules have at first a strictly segmental arrangement, but later
are more numerous than the corresponding myotomes : they
possess no nephrostomes. The mesonephros forms the definitive
kidney ; it is relatively longer in Protopterus than in Ceratodus,
extending through a considerable portion of the body-cavity,
being narrow anteriorly, and gradually broadening out further back
(Fig. 352). In Lepidosiren and Protopterus the posterior instead of
the anterior end of the kidney comes into relation with the testis
and so may be spoken of as a " posterior epididymis." In
Protopterus this is fused with its fellow and covered with black
pigment. The kidneys are invested by lymphoid and adipose
tissue, especially on their lateral and posterior borders. In the
female, the mesonephric ducts communicate with the cloaca
independently, behind the genital papilla : in the male they open
dorsally into the base of the cloacal caecum (Fig. 247) on a single
(Protopterus) or paired (Lepidosiren) papilla. The cloacal caecum
is possibly comparable to the sperm-sac of Elasmobranchs, and to
the urinary bladder of Teleostomes..

Amphibians. — Although the primary pronephric rudiments
extend over a large number of segments, the number of actual
pronephric canals is usually limited to 12 or 13 in the Gymno-
phiona, three in the Anura, and two in the Urodela.^

Thus the most t}"pical condition of the pronephros amongst
Vertebrates is met with in the embryos of Gymnophiona, and this

^ The pronephros undergoes degeneration at the beginning of metamorphosis
in Urodeles and ^Anurans, and at a relatively earlier stage in the Gymnophiona.

456

COMPARATIVE ANATOMY

primitive condition is also seen in the mesonephros, which consists
of long, narrow, varicose bands, usually extending from the heart
to the anterior part of the cloaca, which latter is often much

Fig. 342. — Diagram of the Urikogenital System of (A) a Male and (B)
A Female Urodele ; founded on a Preparation of Triton toaniatus.
(After J. W. Spengel.)

a, collecting tubes of the mesonephros, which open into the Wolffian (urino-
genital) duct {Ig) ; in the female the latter serves simply as the urinary
duct {Ur), and the system of the vasa efferentia (testicular network) is
vestigial ; GN, anterior portion of kidney (epididymis of the male) ;
Ho, testis ; mg, mg^, Miillerian duct ; iV, posterior non- sexual portion of
kidney ; Ot, cuelomic aperture of Miillerian duet (oviduct, Od) ; Oc, ovary ;
Fe, vasa efferentia of testis which open into the longitudinal canal of tlie
mesonephros, t.

elongated. In the embryo they consist of definite masses which
are arranged metamerically, and in each of them a glomerulus, a
nephrostome, and an excretory duct can be distinguished (Fig.

URINARY ORGANS

457

CyAo

354). This condition sometimes persists in the anterior portion of
the kidney, bat o\ving to secondary processes of growth, as
many as twenty nephrostomes are later on met with in a single
body-segment. The number of nephrostomes in the entire kidney
nSay amount to a thousand or more. As regards the urinary duct
and the relations of the entire renal apparatus to the generative
organs, the Gymnophiona in all essen-
tial points resemble other Amphibia.
The kidneys of Urodela and
Anura are situated in the usual
position on the doi-sal side of the
body-cavity ; in the former they
are band-like and more extended
longitudinally than in the latter,
in which they are shorter and more
compact, and are confined to the
middle portion of the coelome.

In Urodeles they always consist
of a naiTow anterior, and a broader
and more compact posterior portion.
The latter, as in Elasmobranchs,
gives rise to the functional kidney,
while the former becomes connected
in the male with the generative
organs. Delicate vasa efferentia,
developed from the mesonephros, in
all cases pass out from the testis
(Figs. 342, 355) into the substance
of the kidney, and there open into
the mesonephric tubules ; they may
either enter the kidney direct, or
else open first into a longitudinal
collecting duct, from which fine
canals pass to the tubules. Thus
the seminal fluid passes through the
nephridia as well as through the
Wolffian duct, which serves as a
urinogenital duct.

In tJrodela and Anura of both sexes the Wolffian duct nearly
always opens separately on either side into the cloaca, receiving
first, in Urodeles, a number of ducts from the posterior part of the
kidney (cf Elasmobranchs). In Anura the Wolffian ducts
pass some distance independently along the body-cavity in corre-
spondence with the position of the kidneys, and a seminal vesicle
(not present, e.g., in Rana esculenta) may open into each (Fig. 344).
The urinary hladdfr, representing the entire allantois (cf p. 437)
opens into the cloaca ventrally, opposite to the urinogenital
apertures. In its simplest form it is finger-shaped {e.g. Siren,

Fig. 343.— Male Urinogenital
Organs of Raiia esculenta. From
the ventral side.

Ao, aorta ; Cv, postcaval vein ;
FK, corpus adiposum ; Ho^ testis ;
S, S', apertures into the cloaca
{Cl)oi C7r, urinogenital ( Wolffian)
ducts, which appear on the lateral
surface of the kidneys at t ; Vr,
revehent renal veins.

458

COMPARATIVE ANATOMY

-^ST

Fig. 344. — Kidney of Discoglossus pictus. From the ventral surface, showing

the nephrostomes [St). (After J. W. Spengel.)

Ur, urinogenital duct, enlarging at Ur^ to form a seminal vesicle.

Proteus),^ but it usually becomes swollen distally and is often
bilobed : in Alytes and Bombinator it forms a double sac.

^ In Amphiuma, it is relatively very long. In the Gymnophiona, the bladder
presents certain peculiar secondary modifications.

URINARY ORGANS 459

Slight indications of a segmental arrangement are found only
in the anterior sexual portion of the kidney of Urodeles ; in the
posterior part, and in the entire kidney of Anura, all traces of
segmentation have disappeared. In both cases, however, the
nephrostomes remain throughout life in great numbers on the
ventral surface of the kidney, which is covered over by the
peritoneum (Fig. 344). The nephrostomes are connected with the
urinary tubules in larval Anura, but later on they become
separated from them, and open into the efferent renal veins. In
consequence of this, the body-cavity of adult Anura is a closed
lymph-sinus, as in the Amniota ; the peritoneal fluid, which in the
larva was carried to the exterior and lost, is in the adult poured
into the general circulation, like the rest of the lymph.

Reptiles and Birds. — In the Sauropsida, as in the Mammalia,
the mesonephros, so far as it is retained in the adult, is usually
entirely distinct from the functional excretory apparatus ; this

Ft'

Fig. 345.— Excretory Apparatus of Monitor indicua.

The right kidney is shown in its natural position, while the left is turned on its
longitudinal axis, so that the ureter and collecting tubes are visible.
The urinary bladder is not represented.

N, N, kidneys ; SG, collecting tubes which open into the ureter ( Ur, Ur^) ; Ur^,
aperture of ureter into the cloaca.

consists of a metanephros, entirely wanting in nephrostomes
(cf p. 446).

The metanephros never extends so far along the body-cavity
as does the mesonephros ; as a rule it has the form of a small,
compact or lobulated organ, usually situated in the posterior
half of the body-cavity, or even entirely confined to the pelvic

460

COMPARATIVE ANATOMY

region : it has the latter position, for instance, in most Reptiles
(Figs. 345, 357, 358) and all Birds (Fig. 346). The posterior
end of the kidney, which is generally narrower than the rest,
in ay even extend under the root of the tail, as in Lacerta, in
which region the two organs are fused.

Thus according to the position of the kidneys, the ureters
(metanephric ducts) either do not extend freely along the

Ao

Fig. 346. — Male Ukinogenital Apparatus of Heron [Ardta cinerea).

Ao, aorta ; BF, bursa Fabricii, which opens into the cloaca at BF^ ; Ep, epi-
didymis ; Ho, testis ; N, kidney ; Ur, ureter, opening into the cloaca (Cc)
at Sr ; V, V, furrows on the ventral surface of the kidney in which veins lie
embedded ; Vd, vas deferens, which opens at Vd^ on a papilla in the cloaca.

body-cavity, or they may have a longer or shorter free course.
The latter is the case, for instance, in Crocodiles, and more
especially in Birds (Fig. 346) : in the latter the kidneys
are closely embedded within the pelvis, and their ventral
flattened surface is usually divided into lobes and is often

URINARY ORGANS

461

penetrated by deep furrows and clefts into which the veins
extend ; posteriorly they may fuse together in the middle line,
as in Lizards.

There is not always a perfect symmetry between the organ
of either side, and this is most marked in Snakes, in which the
greatly lobulated kidneys, like those of limbless Lizards, are
elongated, narrow, and band-like, in con-espondence with the form
of the body.

A urinary bladder, arising from the ventral wall of the cloaca, is
present in most Lizards and Chelonians ; it is more or less bilobed.
A bladder is wanting in Snakes, Crocodiles, and Birds, as well as in
Monitors and Amphisbsenians amongst Lizards. It is derived in
part from the stalk of the allantois and in part from the cloaca ^
(cf. p. 440).

Mammals. — The definitive kidneys of Mammals ^ are propor-
tionately small, and lie on the quadratus lumborum muscle and
ribs. They usually possess a convex
outer, and a concave inner border ; ,---—.-..

the latter is called the hihcm, and
at this point the ureters arise and
the blood-vessels enter. The ex-
panded proximal portion of the ureter
is divided up to form one or more
calyces into which small papilliform
processes of the pyramids project
(Fig. 347) ; on the summits of these
the urinary tubules open in varying

number. The calyces are continuous t^ o4^t tv t

., , ••! • ^1 1 1 FiCx. 347.— Diagrammatic LoNGi

with a large cavity in the widened

portion of the ureter called the pelvis.

and from this the ureter (metanephric q^^ calyces ; M, M, medullary

duct) passes freely backwards for some substance arranged in pyramids

distance to open into the bladder on
its dorsal side, sometimes nearer the
apex, sometimes towards the fundus.
The bladder communicates with the
itrino genital canal or urethra (cf under
Genital organs).

The kidney is greatly lobulated in the embryo ; this condition
may remain throughout life {e.g. in Cetacea, Pinnipedia, Probos-
cidea, and certain Ungulates, Carnivores, and Primates, cf. Fig.
348), or the lobes may become more or less completely united. In
the latter case the original division into lobes may still be recog-
nised to a greater or less extent internally. A section of the

^ The urinary bladder is said to be represented in embryo Birds by an
enlargement of the stalk of the allantois, and in Crocodiles by a ventral outgrowth
from the cloaca.

^ In the embryonic raesonephros, nephrostomes occur only in Echidna.

TUDixAL Section through the
Kidney of a Mammal.

{Pr) ; between the latter the
cortical substance extends in
the form of the columns of
Bertini {B, B) ; R, B, cortical
substance ; Pe, pelvis ; C/r,
ureter.

462

COMPARATIVE ANATOMY

kidney shows an inner layer, the medullary sithstance, arranged in
the form of wedges — the urinary pyramids, — and an outer layer,
or cortical substance, extending as the columns of Bertini between
the pyramids (Fig. 347).

The Malpighian capsules as well as the coiled portions of the
tubules, surrounded by a network of blood-capillaries, lie in the
cortical substance, while the straight portions of the tubules extend
through the pyramids, where they gradually anastomose to form
larger collecting tubes.

The urinary bladder in Monotremes and nearly all Marsupials
represents the whole allantois, and the ureters open at
its junction with the urinogenital canal (Fig. 359). In placental

.1^

Fig. 348. — A, Right Kidney of a Deer ; B, Kidneys {N) and Adrenal
Bodies {N.N) of the Human Embryo. Both from the ventral side.

Ur, ureters.

Mammals, on the other hand, the greater part of the bladder is a
new formation, which arises by a special differentiation of the
cloaca, the latter becoming divided into a dorsal and a ventral
portion by the formation of a septum. The former is continuous
with the rectum, and the latter gives rise to the urinogenital canal
and bladder, which is continuous distally with the stalk of the
allantois from which the urachus (p. 440) and the median ligament
of the bladder are formed. These two sections of the cloaca
gradually become further separated from one another by the
development of the jperinceicm, or space between the urinogenital

GENITAL ORGANS 463

and anal apertures. The bladder of the Eutheria is thus mainly,
though not entirely, of endodermal origin, and the base of the
allantoic stalk into which the ureters open becomes included in it.

Genital Organs.

In Amphioxus the gonads are developed in a part of the
reduced coelome situated on either side of the pharynx and
intestine between the outer body- wall and the atrial cavity. They
have a marked segmental arrangement, and each portion sheds its
products independently into the atrial cavity, whence they pass
out through the atrial pore (cf p. 352 and Fig. 258). The sexes
are separate, and are easily distinguishable from one another when
the sexual products are ripe, although the gonads have a similar
form in both sexes. The gonads, which are said to act also as
excretory organs, are suiTounded by blood-spaces which communi-
cate with the portal system (Fig. 324).

In Cyclostomes also, generative ducts are wanting ; the sper-
matozoa or ova are shed directly into the body-cavity, and pass
through the genital pores (p. 389) into the urinogenital sinus,
which in Lampreys is produced into a papilla on the apex of
which the urinogenital aperture is situated. The gonad is a long,
usually unpaired organ suspended, as in other Vertebrates, to the
dorsal wall of the body-cavity by a fold of peritoneum, the
mesoarium or mesorchium.^

Myxine is hermaphrodite, and the adult is either predominantly
male or female. The posterior part of the gonad represents a
testis, and the remainder an ovary, and either the one or the
other becomes mature in each individual.^

In the tnie Fishes the gonads are only exceptionally unpaired,
and even then this is only a secondary condition, due to the fusion
of the two organs or to the reduction of that of one side : as in all
other Vertebrates, they are originally paired. There is sometimes
a want of symmetry observable between the organ of the right and
left side respectively.

In the greater number of Elasmobranchs the ovaries are
paired : but in some cases {e.g. Scyllium, certain Rays) only that
of one side becomes developed. The oviducts, as already men-
tioned (p. 452) correspond to Mullerian ducts. Their anterior
portion has a common opening into the body-cavity, and further
back each is provided with an oxiducal or shell-gland. The anterior
part of the oviduct is always narrower and more delicate than the
posterior, which dilates to form a kind of uterus in which, in vivi-
parous forms, the embryo undergoes development. Posteriorly,
the oviducts open into the cloaca somewhat behind the aperture of

^ In Mj'xinoids the genital ridge is occasionally paired.
^ Sterile forms are also said to occur.

Fig. 349. — The Urinogenital Organs of Scyllium canicula. (From Parker's

Practical Zoology.)

A, male, and B, female. Only the anterior end of the gonad is represented in
each figure, and except that in B both kidneys are shown, the organs of the
right side only are drawn. In A the seminal vesicle and sperm-sac are dis-
sected away from the kidneys and displaced outwards, and the urinary ducts
inwards.

ab.p, depression into which the abdominal pore opens ; cl, cloaca ; els, clasper ;
ef.d, vasa efFerentia extending between the testis and the anterior section of
the mesonephros ; e.p, k', middle section of mesonephros ; k, posterior section
of mesonephros ; Ir, anterior portion of liver ; m.d, vestigial Miillerian duct
in the male ; oes, gullet ; ov, ovary ; ovd, oviduct (Miillerian duct) ; ovd^
common coelomic and ovd", cloacal aperture of oviducts ; r, rectum ; sh.gl,
shell-gland ; spd, Wolffian duct (vas deferens) ; sp.s, sperm-sac ; s.v, vesicula
seminalis, and s.v', its aperture into the urinogenital sinus ; ts, testis; 7i.g.8,
urinogenital sinus ; ur, ducts of posterior section of kidney, and ur' their
apertures into the cloaca ; u.s, urinary sinus of female.

GENITAL ORGANS 465

the ureters — either separately, or by a common aperture (Fig. 349).
Internal fertilisation takes place by means of the claspers of the
male ^ (p. 486).

In Mustelus antarcticus the uterus becomes divided into several
compartments, each containing an embryo surrounded by a mem-
brane apparently representing the horny egg-shell of other forms ;
in others {e.g. Acanthias vulgaris, Trygon pastinaca, Trygorhina
fasciata, Rhinobates vincentianus), a common horny investment
encloses several eggs or embryos.

The testis of Elasmobranchs is paired and symmetrical : it is
oval in Chimaeroids and more elongated and relatively larger in
Plagiostomes, in which the two organs may become partially fused.
As already mentioned (p. 454), vasa efFerentia, opening into a
longitudinal canal, connect each testis with the anterior end of the
corresponding mesonephros (epididymis) in Plagiostomes, the
coiled Wolffian duct serving mainly as a vas deferens and giving
rise to a dilated portion {vcsicukt scniinalis), which communicates
with the urinogenital sinus ; a more or less elongated csecal
sperm-sac also opens into the urinogenital sinus, which commu-
nicates with the cloaca at the apex of a papilla (Fig. 349). In
the Holocephali, the vas deferens forms a large, coiled mass
anteriorly, and is connected with the testis by mesenteric folds,
but in the absence of a mesonephric part of the epididymis it is
not clear how the sperms pass into it : they become aggregated
into spermatophores before entering the elongated and septate
seminal vesicles. Vestiges of the anterior end or even the
whole (Holocephali) of the Miillerian ducts can be recognised
in adult Elasmobranchs.

The ovaries and testes of most Teleosts, which usually
produce an enormous number of generative cells, closely corres-
pond with one another as regards position and the arrangement
of their ducts. Do7'sal and ventral folds of the peritoneum are
developed in connection with the elongated ovary, and these
in most cases meet along its outer side, so as to enclose a portion
of the coelome and thus convert the ovary into a hollow sac
C cystoarian " condition), which is blind anteriorly and on the
mner, folded walls of which the ova arise. The peritoneal folds
are eventually continued backwards to form the oviduct (Fig. 350),
which IS generally short, and as a rule fuses with its fellow to
form an unpaired canal : this opens either by a genital pore (p. 390)
between the rectum and the urinary aperture on a level with
the integument, or on a papilla, which may become elongated to
form a tube or ''ovipositor"; or the ducts niay communicate
with a urinogenital sinus. The testis is elongated, often lobulated

^ The shell-gland secretes the horny material for the egg-case or "purse,"
which in Plagiostomes is usualh' produced at its four angles into longer or shorter
tendril-like threads ; in Cestracion it has a spiral ridge, and in Callorhynchus it
is very large and expanded and covered on one side by hair-like processes. In
viviparous forms the egg-shell becomes more or less reduced.

H H

466 COMPARATIVE ANATOMY

in form, and consists of radially arranged tubules or of acini.
Its duct has similar relations to those seen in the female, arising,
however, not in the form of a simple tube, but as a network of
anastomosing canals, which usually open into the posterior end of
the kidney-duct.

Thus the ducts, both of the ovary and testis, correspond to folds
of the peritoneum enclosing a coelomic cavity continuous with that
of the gonads, and originate quite independently of the nephridial
system. The oviducts must therefore be distinguished from true
Miillerian ducts.

In some Teleosts, as in most other Fishes, the ovary is solid and
not enclosed in a coelomic sac, the ova being shed into the body-
cavity (" gymnoarian " condition). In the Smelt (Osmerus) and
in Mallotus the oviducts (" peritoneal funnels ") are not
continuous with the ovaries, but have open coelomic apertures
close to the latter, into which the ova pass (c£ Fig. 350, b) ; while,
in other Salmcnidse and, e.g., in the Mursenidse and Cobitis, these
peritoneal funnels are shorter, and may even be absent, the ova
then being shed into the urinogenital sinus through a paired or
single genital pore.^ It is uncertain whether the latter is the
primitive arrangement amongst Teleostei, or whether the
peritoneal funnels represent reduced oviducts. In the Eel, a
similar reduction of the gonoduct is seen in both sexes.

Hermaphroditism regularly occurs in certain Teleosts (e.g.
Serranus, Chrysophrys), and has been occasionally observed in some
others {e.g. Cod, Mackerel, Herring).

Amongst Ganoids the female organs of Lepidosteus are
formed on the same type as those of the Teleostei. In Amia (Fig.
350, B) and Acipenser each oviduct opens by a wide funnel into
the coelome, but in all Ganoids each oviduct is probably com-
parable to that of Teleosts, and not to a Mullerian duct. In the
male Lepidosteus, Amia, and Acipenser, a series of vasa efferentia
pass out from the testis and open into a longitudinal canal from
which ducts enter the kidney, and then either communicate with
the Malpighian capsules, or else pass directly into the mesonephric
duct, which therefore serves as a urinogenital duct (Fig. 351).
In Lepidosteus the latter dilates before uniting with its fellow to
open into the urinogenital sinus. In Polypterus, there is a special
tesfcis-duct comparable to that of Teleosts ; it is associated with a
network of cavities and opens into the posterior end of the kidney-
duct. Except in Lepidosteus, representatives of the oviducts of

^ Most Teleostei are oviparous, but viviparous forms occur (p. 436). Tlie
male Stickleback builds a nest for the protection of the young formed of a
hardened secretion (mucin) of the kidney, which thus undergoes a change of func-
tion at the breeding-season. In Syngnathus and Hippocampus the young are pro-
tected within a pouch on the abdomen of the male, and in the female Soleno-
stoma in a pouch between the ventral fins ; amongst Siluroids they are carried
within the pharynx in the male Arius, and attached to the soft ventral integu-
ment in the female Aspredo.

GENITAL ORGANS

467

the female are said to be present in the form of short ccelomic
funnels opening into the kidney-ducts.

In the Dipnoi (Fig. 352) the elongated gonads are invested
with lymphoid and adipose tissue, are closely attached to the

^zfdL-

oz^d," ^^

Z.C

oi/et

w^.a/7

Fig. 350. Fig. 351.

Fig. 350.— Female Urinogexital Organs of A, Lepidosteus, and B, Amia.
(A, after Balfour and Parker ; B, after Huxley.)

a, degenerate anterior portion of kidney ; hi, bladder ; kd, kidney ; ovd, oviduct ;
ovd', aperture of oviduct into bladder, and ovd'', its peritoneal aperture ; ovy,
ovary ; p, peritoneum ; u.g.ap, urinogenital aperture ; wr, kidney duct.

Fig. 351. — Male Urinogenital, Organs of Lepidosteus. (After Balfour and

Parker. )

hlj bladder; l.c, longitudinal canal; ts, testis; u.g.ap^ urinogenital aperture;
Mr, kidney duct ; v.ef, vasa eflferentia.

outer border of the kidneys, and when ripe become greatly
enlarged, so as to embrace the gut ventrally. The oviducts,
which correspond to Miillerian ducts, are long and coiled, re-
sembling those of the Amphibia : posteriorly they unite before

H H 2

RodLG

ovJ

ANG
Poab

Fig. 352. — Urinogenital Organs of Protopterus annecfens. A, Female ; B,
Young Male. (After W. N. Parker, modified after J. Graham Kerr.) The
peritoneum {Per) is removed on the right side in A, and on the left in B.

In both figures : Cp, postcaval vein, connected by transverse anastomoses [ans)
with the left posterior cardinal vein {V.card) ', Ig, ly, lympathic tissue in
connection with the kidneys ; NG, mesonephric duct ; N, N'^, kidneys ;
Ont, ccelomic aperture of Miillerian duct ; Poah, abdominal pore ; Mc,
rectum ; RD, cloacal csecum. The veins from the gonads are also indicated.

In A : ANG, apertures of kidney ducts into the cloaca {CI) ; Ov, Ov^, ovaries ;
ovd, ovd\ oviducts ; Pap, genital papilla in the cloaca formed by the fusion
of the base of the Miillerian ducts.

In B : Pap, urinogenital papilla ; Hod, testes, with their investment of
lymphoid and adipose tissue {LG) ; Hod.G, vesicular portion of testis, which
unites with its fellow posteriorly, dorsal to which point the posterior ends
of the kidneys (epididymes) are also fused together, and are not deeply
pigmented, like the rest of the kidney ; MG, Miillerian duct.

GENITAL ORGANS 469

opening into the cloaca, and each communicates anteriorly with
the body-cavity by a funnel-shaped aperture. The wall of the
oviduct secretes albumen round the eggs as they pass along it.

In Protopterus and Lepidosiren, the greater portion of the testis
is composed of numerous sperm-producing ampullae opening into
a longitudinal duct continuous with a cavity in a simplified
posterior prolongation of the gonad, in which no sperms are
formed, and which converges posteriorly, fusing with its fellow in
Protopterus : it serves merely as a duct and as a vesicula seminalis.
From the posterior end of this portion a testicular network arises
in Lepidosiren, from which about six vasa efferentia pass into the
Malpighian capsules of the mesonephros (" posterior epididymis,"
p. 455), while in Protopterus there is only a single large and
irregular vas efferens on either side. Thus the posterior end of
the mesonephric duct is urinogenital in function.

In Ceratodus the manner in which the sperms are conducted
from the testis to the exterior is not definitely known. The
Mlillerian ducts in this genlis are retained m the adult, and
possess a lumen throughout : in Protopterus they undergo further
reduction, and apparently the middle part disappears, the posterior
end fusing with its fellow and ending blindly at the base of the
urinogenital papilla, while in Lepidosiren only the anterior end
persists.

Thus a communication between the testis and kidney occurs
in Elasmobranchii, certain Ganoidei (Lepidosteus, Amia,
Acipenser), Dipnoi, Amphibia, and Amniota, and on the assump-
tion that the condition in Polypterus and Teleostei is a secondary
and not a primary one, an attempt to bring these latter into line
with the majority of Vertebrates in this respect is represented
diagrammatically in Fig. 3o8.

Amphibians. — The form of the gonads of Amphibia is
usually modified in correspondence with the shape of the body.
Thus in the Gymnophiona the ovary has the form of a long and
narrow band, and the testis consists of a long chain of small bodies
united together by a collecting duct (Fig. 354). Each individual
portion of the testis of Caecilians is made up of a double row of
rounded capsules in which the sperms are formed, and from which
they are passed into a collecting duct, which perforates each portion
of the organ. A transverse canal is given off from the free portion
of the collecting duct lying between every pair of testis-lobes ;
this passes towards the kidneys, and opens into a longitudinal
canal. From the latter the sperms pass through a second system
of transverse canals to the Malpighian capsules, and thence through
the urinary tubules into the urinogenital duct.

The male generative apparatus of all Urodela and certain
Anura (Bufonidse) corresponds in the main with that "seen in
Caecilians, except as regards the form of the gonads : thus the testis

470

COMPARATIVE ANATOMY

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GENITAL ORGANS

471

is either pointed at one or both ends (Fig. 342), or more or less
round or oval (Fig. 343, 355).

In Rana (Fig. 355), Bombinator, and Alytes, the vasa efferentia
of the testis gradually become more separated from the kidney.
In Rana temporaria, the efferent ducts are connected with a
longitudinal canal, from which the sperms pass through the
so-called ampullae {i.e. Malpighian capsules which have lost their
glomeruli) and the transverse canals to the urinogenital duct : in
Rana esculenta the efferent ducts open directly into the urino-

Fig. 354.— Diagram of a Portion of the Male Generative Apparatus in

THE GyMNOPHIONA.

Ho, testis ; HS, urinogenital duct ; K, testicular capsules ; M, Malpighian
capsules ; N, kidnej' ; Q, transverse canals connecting the collecting duct
with the longitudinal canal (L, L) ; Q^, second series of transverse canals ;
^', convoluted portion of urinary tubule ; Sg, collecting duct of testis ;
.S'7^, nephrostome.

genital duct, without becoming connected with the urinary
tubules, while in Bombinator the greater number of the posterior
canals end blindly, only the anterior ones being directl}^ connected
with the urinogenital duct.^

*■ MuUerian ducts are always present in a more or less ves-
tigial condition in the male, and lie along the outer border of
the kidneys in a similar position to those of the female. They
may or may not be provided with a lumen and apertures of com-

^ In Alytes the relations of the generative ducts require further investigation :
the eflferent ducts at the anterior end of the kidney are said to open into the
persistent Miillerian duct.

472

COMPARATIVE ANATOMY

munication with the body-cavity and cloaca. A vesicula seminalis
may be present on the urinogenital duct (p. 457).

Hermaphroditism occasionally occurs in the Anura ; only
one case (Triton tyeniatus) is known amongst Urodeles. A body
attached to the anterior end of the testis (" Bidder's organ ")
in various species of Toads contains cells quite similar to young
ova, and a similar body is present at the anterior end of the ovary,
the' cells in which are, however, incapable of ripening. In the
males of Pelobates, Bufo, and Rana temporaria, ova are at times
developed within the substance of the testis (hermaphrodite gland
or ovotestis), and one testis may even be replaced by a rudimentary

Fig. 355.

-Testis and Anterior End of Kidney of Raim esculenta.
(Semidiagrammatic. )

Ho, testis ; L, longitudinal canal of the testicular network, from which the inter-
renal network {G, C) arises ; N, kidney; q, q, transverse canals of the testi-
cular network, which give rise to blind processes at f 1 5 ^^^> urinogenital
duct.

ovary : in Rana, the Mlillerian duct may then be as well developed
as in the female.

The ovaries of Urodela are always formed on a common plan :
each consists of an elongated, closed tube, with a continuous lumen.
In Anura, on the contrary, the ovarian sac (Fig. 356) is divided up
into a longitudinal row of numerous (3 to 20) separate pockets or
chambers, on the walls of which the ova are developed and project
into the cavity, eventually breaking through into the coelome.
The oviducts open far forwards into the body-cavity by funnel-
shaped apertures ; they take a tolerably straight course along the
outer borders of the kidneys to the cloaca in young animals, but
become greatly convoluted and glandular later. A short distance

GENITAL ORGANS

473

from its termination, each oviduct in Anurans becomes dilated to
form a thin-walled sac, in which the eggs, covered by a gelatinous
coating from the glands in the wall of the middle part of the
oviduct, become collected before oviposition : after again narrowing,
it usually opens separately on a papilla on the dorsal wall of

Fig. 356.

Urinogenital Organs of a Female Rana esculenfa.
From the ventral side.

iV, kidneys ; Od, oviduct ; Ot, abdominal aperture of oviduct ; Or, left ovary
(that of the right side is removed) ; P, opening of oviduct into the cloaca ;
S, S^, apertures of urinary ducts into the cloaca, surrounded by longitudinal
folds (*), which are separated by a deep depression (f) ; Ut, the dilated
posterior end of the oviduct.

the cloaca : in the genera Bufo and Alytes the two oviducts
fuse posteriorly into an unpaired canal.^

^ In Epicrium glutinosum (Gymnophiona) the ripe eggs are very similar to
those of Sauropsida : they are exceptionally large (9 mm. long), are of an oval
shape, possess a large yolk, and the segmentation is merohlaHtic, and takes place
in the oviduct. After fertilisation they become coated with tough albumen in
the oviduct, and this is drawn out at the poles into chalazfe, by means of which
che eggs are connected together like the beads of a necklace. The eggs are laid
in the earth, the mother coiling herself round them.

474 COMPARATIVE ANATOMY

The cloaca in both sexes of Urodeles is provided with definite
integumentary lips, and its cavity is often subdivided by folds. In
the female, its side walls enclose numerous tubes, which, during the
breeding season, serve as reccptacula semiiiis} In the male the lips
and dorsal w^all of the cloaca include numerous glands, amongst
which special pelvic and abdominal portions may be recognised, and
which are especially well-developed in the breeding season. These
serve to secrete a protective jelly-like mass around the spermatozoa,
thus uniting them into packets, or spermatophores. It has been
observed in Newts that as the spermatophores are extruded from
the cloaca of the male, they become taken up by the female
between the lips of the cloaca, so that fertilisation of the eggs
takes place before, and not after oviposition, as is the case in
Anura.

Fat-bodies {corpora adiposa) are present in all Amphibia in
connection with the gonads : they are formed of adenoid tissue,
fat, and leucocytes, and contain numerous blood-vessels. These
bodies have probably an important physiological (nutritive) re-
lation to the gonads: after remaining for months without food,
throughout their winter sleep. Amphibians are able as soon as
spring arrives to produce thousands of oft'spring.

Reptiles and Birds. — The essential differences between the
urinogenital organs of the Anamnia and Amniota have already
been referred to (cf Fig. 839).

In the Sauropsida, as in other Vertebrates, the form of the
gonads is influenced by that of the body : thus in Chelonians they
are broad, while in Snakes and snake-like Lizards they are more
elongated, and as well as in other Lizards, are asymmetrical,
the organ of one side lying more or less in front of that of
the other. More room is thus obtained for the development of
the ovaries; and, in cases where the eggs are very large, the
organs of one side tend to disappear, as in certain Elasmobranchs :
in Birds, for instance, the left ovary only is completely developed
and functional. In Reptiles the ovaries are penetrated by a
highly vascular network of trabeculae, in the lymph-cavities of
which the formation of ovarian follicles takes place.

The oviducts (Fig. 357) possess wide, funnel-shaped., abdominal
apertures, and are usually much folded transversely ; the right is
often longer than the left. Their walls are provided with numerous
muscular elements and glands for the formation of the albumen
and egg-shell, and they increase in size in the breeding-season.
In Birds the right oviduct, as well as the right ovary, becomes
more or less completely degenerated, and the left is considerably
coiled.

Only slight remnants of the mesonephros and Wolffian duct

^ In Salamandra macula ta and S. atra the contained sperms may retain their
vitality for a year or two.

GENITAL ORGANS

475

persist in female Reptiles, and these undergo fatty degeneration.
They are arranged in a single asymmetrical row on either side,

Fig. 357. — Female Urinogenital Apparatus of Lacerta muralis.

B, urinarj- bladder ; B^, neck of the bladder (cut open) ; N', kidneys ; Od, oviducts,
which open into the cloaca at Od^ ; Ot, abdominal openings of oviducts ;
Ov, ovaries ; B, rectum ; B^, opening of rectum into the cloaca {CI) ; Ur^,
apertures of the ureters into the cloaca ; f, remains of mesonephros.

between the oviduct and vertebral column. The vestiges of the
Wolffian duct are more marked in female Snakes, Chelonians, and
in Geckos than in the majority of Lizards.

476

COMPARATIVE ANATOMY

-Vd

The testes of Sauropsida correspond in position with the
ovaries, and, like them, increase in size in the breeding-season ;
in Birds, one is generally larger than the other. They have an
oval, round, or pyriform shape (Figs. 346 and 358), and are made
up of greatly convoluted seminal tubules connected by fibrous

tissue. In Reptiles {e.g. Lacerta,
Anguis), " yellow bodies," which
correspond to adrenals (p. 495), lie
along the outer border of the testis,
and at this point transverse canals
pass out from the testis to the epidi-
dymis. The latter consists of greatly
convoluted canals, and from it arises
the vas deferens (Wolffian duct),
which either takes a straight course,
or is more or less coiled. In Birds
it opens by an independent aperture
into the cloaca, while in Lizards it
fuses with the ureter shortly before
entering the latter.

Remains of the Mullerian ducts
are present in the male, correspond-
ing in position with those of the
female. Their lumen is not continu-
ous throughout, but the abdominal
aperture may remain open (Emys
europsea), and exceptionally {e.g. in-
dividuals of Lacerta viridis) they
may be as well developed as in the
female.

Accessory genital glands are pre-

FiG. 358.— Male Urinogenital gent in most Reptiles, as in Am-

S F. ZytilT ^"'''" Phibians, but it is doubtful whether,

„ . , , , , or to what extent, they are compar-

^hTsTl^o'tttV ,n!;: /e^g^e f'^ *° t^ose of Mammals (p. 484).
of the Mullerian duct ; N, kid- In Lizards they are well-developed
ney; p, common aperture of on the dorsal and ventral walls
deLenfon'^'-'pa'pnla^o' 21 °f the cloaca, and their secretion
dorsal wall of the cloaca (CI) ; passes into the grooved penes in the
r, rectum; Vd, vas deferens; male. In Snakes similar Hands

t,» yellow body "(adrenal). ^^^ p^^^^j^^^ ^^^^ -^ ^^^^^^,^ 5^^^^_

diles odoriferous (" musk ") glands
open into the cloaca, and are said to be eversible. Accessory
genital glands are wanting in Chelonians : in those in which
so-called "anal vesicles" are present, they are hydrostatic in
function.

Lymphoid organs are present in many Reptiles, and probably
here also have a physiological relation to the generative organs

GENITAL ORGANS 477

(cf. p. 474). In many Lizards they are large and variously
coloured, and lie within the pelvic region ; in Snakes they extend
along almost the entire body-cavity.

Among Birds, true hermaphroditism has been observed very
exceptionally in the Chaffinch. In some cases the ovary
may undergo structural changes, and no longer produce ova, the
female then taking on certain secondary sexual characters of the
male.

Mammals. — In Mammals the generative apparatus no longer
extends along the entire body-cavity, as in the lower Vertebrates,
but is confined to the lumbar and pelvic regions. Moreover,
in correspondence with the close relations which usually take
place between mother and embryo (p. 438), there is a much greater
differentiation of the generative organs than occurs in lower types.
The transition is not, however, a sudden one, for in the lowest
Mammals, viz., the Monotremes and Marsupials (Figs. 359 and
360), these organs show many points of resemblance to those of
Reptiles and Birds.

In the oviparous Monotremes^ the left ovary is better
developed than the right, and each has the appearance of a
bunch of grapes : the cloaca persists, and the oviducts (Mullerian
ducts), which in other Mammals become more or less fused with one
another proximall}^, remain distinct throughout, and open into the
urinogenital canal anteriorly to the ureters and bladder.

In the higher Mammals the oviducts become distinctly differ-
entiated into three portions, — a Fallopian tube, a uterus, and a
vagina. The vagina opens to the exterior (Figs. 339, 360, and 361),
while the Fallopian tube communicates with the abdominal cavity
by a funnel-shaped aperture which is usually fimbriated and
ciliated.

In Marsupials the ovaries vary much in form, and the fusion of
the two oviducts is much less marked than in the higher Mammals :
in order to trace the gradual differentiation. of these parts, their
condition in Opossums (Didelphidge) w^ll first be considered.

A dilated portion of each oviduct (Fig. 360, a), giving rise to a
uterus, is plainly distinguishable from the rest, and its narrowed
posterior end comes into close contact with its fellow in the middle
line. At this point (f) each uterus communicates with the vagina
by a distinct os uteri. The vagina curves sharply outwards, and,
then backwards, opening close to its fellow into the elongated urino-
genital canal. The ureters, as in all other Marsupials in which the
vaginoe have a similar arrangement, pass between the curved
portions of the vaginae to the bladder.

From tlie condition of the female generative organs in

^ The eggs (usually one in Echidna and two in Ornithorhynchus), surrounded
by a thin shell of keratin, have only been found in the left oviduct, in which
their earl}- development takes place.

478

COMPARATIVE ANATOMY

Didelphys, that seen in other Marsupials may be derived. In
Phalangista vulpina and Phascolomys wombat (Fig. 360, B and c)

Fig. 359. — A, Diagrammatic Median Section to show the Relations of
THE Bladder, Urinogenital Canal, and Cloaca in both Sexes of
Echidna aculeata ; B, Diagram of the Anterior Part of the Urino-
genital Sinus from the Dorsal Side. (After F. Keibel.)

C.Dr, aperture of Cowper's glands into the urinogenital canal ; D, rectum ; Dr,
apertures from which project bundles of hair, with sebaceous and sweat
glands ; g. T, genital pouch ; K, bend in urethral canal ; L, lymphatic
tissue ; Mg, Mlillerian duct ; PT, preputial pouch and copulatory organ ;
S.R, canal of copulatory organ ; S.ug, urinogenital canal; U, ureter; U.P,
papilla of ureter ; Wg^ Wolffian duct ; x, aperture of preputial pouch into
cloaca ; Z, aperture of rectum into cloaca.

the anterior ends of the knee-shaped bends of the vaginae lie
closer together, and begin to extend backwards towards the urino-

trt' W jrt ^^

Fig. 360.— Female Generative Apparatus of Marsupials. A. Didelphys
dorsigera ( juv. ) ; B, Phalangista vulpina ; C, Phascolomi/s xcombat (After
A. Brass.)

B, urinary bladder ; CI, cloaca ; g, clitoris ; N, kidney ; Od, Fallopian tube, and
Ot {Fim), its abdominal opening ; Ov, ovary ; r, rectum, which opens
at r^ ; Ur, ureter ; Ut, uterus ; Ut^, openings of uteri into the vaginal
cfficum, VgB ; Vg^, apertures of vaginae into the urinogenital canal {Sug) ;
*, t, rectal glands ; t, bend between uterus and vagina.

480 COMPARATIVE ANATOMY

genital canal, the septum between them disappearing at the
same time. A ^aginal cmciwi is thus formed : this may
become more elongated, and finally extend backwards so as to
meet the anterior wall of the urinogenital canal, into which it
may in some cases eventually open by the formation of a so-called
third vagina, as, e.g., in certain species of Macropus and Halmaturus.
The anus and urinogenital apertures are surrounded by a common
sphincter.

In nearly all the placental Mammals (Monodelphia) the
posterior portions of the Miillerian ducts become fused to form an
unpaired vagina, and a definite cloaca exists only in the embryo ;
but even in the adult, the anus and urinogenital aperture
may in certain cases {e.g. amongst Rodents) be enclosed by a
common fold of the integument as in Marsupials, and a median
septum is sometimes present in the vagina distally, indicating its
primarily double nature. The uterine portions of the oviducts may
also fuse with one another to a greater or less extent, and thus
the most various forms of uteri result (Fig. 361, A to d), viz. —
literiLfi duplex, with two ora uterorum (most Rodents), uterus
hipartitus and uterus hicornis, double in part only and with one os
uteri (Carnivores, most Ungulates), and uterus simplex (Primates).
In the last-mentioned form the primitively paired condition of the
Mullerian ducts is seen only in the Fallopian tubes, which vary
much in form. The ureters, unlike those of Marsupials, always
pass to the outer side of the genital passage, the vagina being
single.

The urinogenital canal may, as in Marsupials, be of consider-
able length {e.g. amongst Rodents), and a fold of the mucous
membrane (hymen) ^ is often present where the vagina opens into
it. On the ventral wall of the urinogenital canal, the clitoris
(p. 487) is situated. In both male and female the space
between the urinogenital aperture and the anus is known as the
perinceum.

The ovaries of monodelphous Mammals are usually small, and
rounded or oval in shape, their surface being either smooth,
irregular, or furrowed. The point at which the nerves and vessels
enter is not covered by peritoneum, and is called the hitum.
Various differences are seen as regards the relations of the peri-
toneum to the ovary, and from a simple investment on the ventral
side, the organ hardly sinking into the peritoneum at all {e.g.
Rabbit, Cat), all stages occur up to its complete investment so
as to form an ovarial sac.

Remains of the mesonephros, known as the parovarmin, are
present in the neighbourhood of the ovary, oviduct, and uterus.
These usually consist of small csecal tubes, forming a network,
which are connected together by a collecting duct. In cases

1 A similar fold, closing the apertures of the oviducts in the immature
condition, is present in Elasmobranchs.

GENITAL ORGANS
Od

481

Fig. 361. — Various Forms of Uteri. A, B, C, D, diagrams showing the
diflFerent stages in the fusion of the Miillerian ducts : A, uterus duplex ; B,
uterus bipartitus ; C, uterus bicomis ; D, uterus simplex ; E, female urino-
genital apparatus of Mustdhia, containing embryos (* *) in the uterus ; F,
ditto of Hedgehog {Erinacetis).

B, urinary bladder ; Ce, cervix uteri ; iV, kidney ; Nn, adrenal ; Od, Fallopian
tube ; Ot, abdominal aperture of Fallopian tube ; r, rectum ; Sug, urino-
genital canal ; Ur, ureter ; Ut, uterus ; Vg, vagina ; t, t, accessory glands.

where the Wolffian ductr persists in the female, it passes from the
parovarium to the urinogenital canal, and is spoken of as Gartners
duct (Fig. 339, h).

A fold of the skin of the abdomen forming a pcntch or mar-
suinum is present in Echidna and to a greater or less extent in

1 1

482 COMPAHATIVE ANATOMY

Marsupials^ (p. 35) : in the former this serves to protect the egg,
and in the latter the young, which are born in a very unripe con-
dition, thus rendering possible a longer connection between the
mother and embryo during lactation. The aperture of the marsu-
pial pouch opens anteriorly or posteriorly, according to the mode of
life of the animal, and is provided with a sphincter muscle capable
of closing it. In Marsupial embryos the margins of the lips
become partially fused secondarily and temporarily to form a
suctorial mouth, by means of which the young, many of the organs
of which are still in a " larval " condition, become attached to the
teats (cf p. 438).

In male Mammals, the testes arise in the same relative position
as the ovaries of the female. The ovary, however, does not
become shifted further backwards than the pelvis in the course of
development ; but the testis may pass out of the abdomen through
an inguinal canal into a purse-like outgrowth of the integument in

Testis /-v^

MesorchiumZ^ZZ— ',n^% I /

V as deferens ^_ .\ 7 H/ y j

Peritoneum — \ I B L' /

Int. obi. VI. >^ ^^ --T-^/ /

Transversalis'-^-^^^^ ^^STw/ •

Ext. obi. TO. '^^St. i y^^

Integ V\^^:l-^ Cremaster-sac

Scrotum .>1 laff' Gubernaculum (Lig. scroti)

Fig. 362. — Diagkam of the parts concerned in the Descent of the
Testis. (After M. Weber. )

a, ligamentum testis ; b, ligamentum inguinale ; c, muscular conus inguinalis.

the inguinal region called the scrotal sac, ^vhich is lined by a
continuation of the peritoneum, the hmica vaginalis (p. 362). The
two scrotal sacs may remain separate, or unite to form a scrotum :
in Marsupials this is situated in front of, and in placental Mammals
behind the penis. If the inguinal canals remain widely open,
the testes may be withdrawn periodically into the abdomen (as e.g.
in Rodentia and Insectivora, in which they only descend at sexual
maturity) : this is effected by means of the cremaster muscle, a
more suitable name for which would be the levator s. retractor testis.
This muscle is a continuation of the fibres of the internal oblique
and transversalis, or of the latter only, and corresponds to the
" compressor mammae " of female Marsupials. When the inguinal
canals become reduced (as e.g. in Man) the testes remain perma-
nently in the scrotum. In many Mammals, however {e.g. Mono-

^ Traces of the marsupial folds occur in young stages of various male
Marsupials, and indications of a Marsupial apparatus have been described in
several of the higher Mammals,

GENITAL ORGANS

483

tremes, most Edentates, Hyrax, Elephant), the testes are retained
within the abdomen.

Originally, the descent of the testes did not occur until sexual
maturity in all cases, but in many Mammals {e.g. Marsupials,
Ungulates, Carnivora, Primates) the process has gradually become
shifted backwards ontogenetically to earlier periods, so that the
formation of the scrotum takes place independently in the embryo
in the form of the external genital folds, from which also, in the
human female, arise the " labia majora " of the viilva.^

The testes are smooth, and somewhat* oval in form; their
relative size varies in different Mammals, and they may become
periodically enlarged at the breeding season {e.g. Roaents and
Insectivores) ; they are covered by a fibrous investment (Fig. 363),

Fig. 363. — Diagrammatic Section of the Testis of a Mammal.

tunica albviginea of the testis, which gives rise to the trabeculae {t, t) and the
corpus Highmori (t) ; Cv, coni vasculosi, which are connected together by
the collecting duct, Vep ; Ho, testis ; L, L, coils of the seminal tubules ; NH,
epididymis ; Va, vas aberrans ; Frf, vas deferens ; Ve, vasa eflferentia (rete

testis).

from which processes (trabeculae) usually extend inwards. Thus
the seminal tubules become separated into definite, lobed masses,
and a sort of lattice-work is also formed (corpus Highmori) by
means of which the elements of the vasa efferentia pass to the
epididymis. In the latter the seminal tubules become rounded off
to form the so-called coni vascidosi, and these are connected together
by a collecting duct, the vas epididymitis. The vas deferens arises
from the last conus vasculosus, and gives rise towards its distal end,
shortly before it opens into the urinogenital sinus close to an
elevation, the colliculus semirudis, to glandular outgrowths {vesiculm
semiiudes), which may attain a relatively enormous size (Fig. 364).

^ " Labia majora " also occur in certain other Primates, but in most Monkeys
' ' labia minora " are alone present bounding the vulv-a, and these belong morpho-
logically to the clitoris and not to the scrotal folds.

I I 2

484 COMPARATIVE ANATOMY

From this point to its termination at the apex of the penis, the
seminal canal is spoken of as the ductus ejaculatorius.

In many Mammals vestiges of the Mtillerian ducts are pre-
sent in the male, and open into the urinogenital sinus. In Man,
only the posterior end of the ducts remain in the form of an
unpaired vesicle {uterus maculinus), which lies embedded within
an accessory genital gland, the prostate.

Accessory genital glands (Fig. 364), certain of which have
just been referred to, are present in all Mammals, but vary much in

ves.v.d.

gl.prost I S^cvi^v J'XitmL

^-^ rf ^^%^^^^^y^ ^'^

// ^ — gl.amp

gl.prost III uSu^^B

" ^ ^^jmM^B ves.ur

^^ H ^^ gl.coicp

'ijl.pra-p

Fig. 364. — Urinogenital Canal and Accessory Genital Glands of a Male
Mouse {Mus musculus) x 2. (After M. Rauther.)

c.c.pi corpus cavernosum of the penis ; gl.amp, ampullary glands : gl.cowp, bulbo-
urethral (Cowper's) glands ; g.p, apex of penis, seen through a slit made in
the prepuce ; gl.prrjep, prteputial gland ; gl.prost I, prost III, prostate — the
posterior and dorsal portion is hidden ; ur, ureter ; v.d, vas deferens ; ves.nr,
urinary bladder ; ureth, urethra ; ves.v.d, vesiculse seminales.

number, form, and relative size. They may be classified as
follows : —

I. Glands arising from the seminal duct. A. Ampullary glands,
situated in an ampulla-like enlargement of the seminal duct or
embedded in the latter : they are present, e.g., in the Shrew,
Ruminants, and certain Carnivores and Rodents ; in many of the

GENITAL ORGANS . 485

last-mentioned Order they are greatly developed. B. Vesiculce
seminales, opening into the ductus ejaculatorius together with
the seminal duct : they are usually large, saccular, and tubular
structures, and occur in certain Rodents, Insectivores, and Bats,
and in Sirenians, Elephants, Ungulates (especially Perissodactyles)
and Primates.

II. Glands arising from the urinogenital canal. A. Prostate glands,
fully developed in the male only, enclosed by smooth muscles, and
sometimes more or less distinctly subdivided into two or three
lobes. The prostate is wanting only in Monotremes, Marsupials,
Edentates, and Cetaceans. A median sac, opening into the
urinogenital canal, is surrounded by the prostate, and the degree
of its development exhibits considerable variation. It is usually
described as the vesicula prostatica or uterus masmclimcs, and in most
cases (not, e.g., in Lepus) corresponds to the fused bases of the
Mtillerian ducts : a more appropriate name would therefore be
vagina masculina. B. Urethral glands, present in both sexes in
the form of scattered glands, and more definite and localised
bulbo-urethral or Oowper's glands, known in the female as the
glands of Bartholini. These are phylogenetically the oldest
glandular appendages of the urinogenital canal. The scattered
glands in some cases are abundant, and in others (e.g. Canidae)
are wanting : Co\\^er's glands are, with few exceptions (e.g. Dog,
Bear, aquatic Mammals), of constant occurrence and position : they
are surrounded by striped muscle.

III. Glands of the external genital organs and inguinal region
(preputial, ingidnxil, and anal glands). These are epidermic
structures, and are derivable from sebaceous glands or sweat-glands
(cf p. 33).

There seems to be no doubt that the secretions of the prostate,
ampullary glands, and seminal vesicles have an important relation
to the vitality and fertilising power of the spermatozoa, and a high
degree of fertility is usually seen in those animals in which these
glands are most markedly developed.^ The secretions of the
preputial, inguinal, and anal glands, in addition to protecting the
surface of the skin around the urinogenital and anal apertures, is
doubtless of secondary sexual importance, owing to the production
of odoriferous substances.

^ In many Mammals {e.g. Rodents, Insectivores) these glands have also
another function. Their coagulated secretion forms a kind of "stopper" for
closing the vagina and thus ensuring fertilisation. In certain Bats the mucous
membrane lining the neck of the uterus becomes modified directly after copula-
tion, and together with the secretion of the accessory genital glands closes
the canal and protects the mass of speims until the following spring, when
fertilisation of the ova takes place : it has been shown that the spermatozoa may
thus retain their vitality within the uterus for eight months. In some cases the
secretion in the vagina originates from the accessory genital glands of the male.

486

COMPARATIVE ANATOMY

Copulatory Organs.

Various forms of copulatory organs, morphologically distinct
from one another, occur amongst Vertebrates.

In male Elasmobranchs, an apparatus consisting mainly of
a specially modified portion of each pelvic fin (clasper or " mix-
opterygmm ") serves this purpose (Fig. 365). It consists of a vary-
ing number of more or less calcified cartilages, covered by skin and
muscles, which are movable upon one another, and most of which

J ^5

Fig. 365. — Pelvic Arch with Skeleton of Pelvic Fin and Claspers of a
Male Chimcara monstrosa. (After Davidoff.) Ventral view,

B, pelvic arch with iliac process, Pril ; Mt, basipterygium ; Ra, Ra}, radii of fin ;
SB^ anterior clasper ; a—f, 1 — 3, various segments of the posterior clasper.

are derivatives of the main and lateral fin-rays : it is provided with
a channel along the inner side, and becomes separated from the fin
itself in varied degrees in different forms. Its apex is usually
naked, but may be provided with one or more movable spines,
which in the Spinacidse consist of a curious tissue ("chondroden tin").
These claspers are inserted, in a closed condition, into the cloaca
of the female, and thence into the oviducts ; they are then opened
out by means of special muscles, and the seminal fluid flows along
their channels into the distended oviducts. In connection with
this apparatus there is a gland (p. 19) surrounded by muscular
fibres, which is formed as an involution of the outer integument
and consists of branched tubes.

COPULATORY ORGANS

487

In addition to pelvic claspers essentially similar to those of
other Elasmobranchs (the distal part in Chimsera being subdivided
and covered with numerous dermal denticles) the Holocephali
possess a pair of curious anterior claspers (Fig. 365), which are
protruded from a shallow pouch situated in front of the pelvic
fins: each of these consists of a plate covered with dermal
denticles, and in Callorhynchus a gi'ooved structure is present in
addition. There is also "^a knob-like organ, usually known as the
frontal clasper, on the upper surface of the head (Fig. 66).

Amongst Amphibians, the very muscular cloaca in the
Gymnophiona can be extruded to a length of 5 centimetres, and
thus serves temporarily as a kind of copulatory organ.

Two kinds of copulator\' organs are found in Reptiles, the one
being seen in Lizards and Snakes, and the other in Chelonians and
Crocodiles. In the former there are two copulatory sacs or penes

A B

Fig. 366. — Copulatory Organs of Lacerta anil is. (After F. Lej-clig.) In A
thej- are shown everted, and in B their position in the retracted condition is
indicated b}- dotted lines extending backwards from the vent.

Ce, vent ; R, R^, penes ; SD, the so-called femoral pores (see p. 23) ; f, spiral
furrow.

lying outside the cloaca, under the skin at the root of the tail, and
these can be everted and protruded through the vent, and again
withdrawn by means of a muscle inserted into the blind end of
the sac (Fig. 366). In its everted condition, a spiral furrow
extends along each sac down which the seminal fluid passes.
These organs, which show no cavernous structure, are also repre-
sented in the female, in which, however, they are much smaller.

In Chelonians and Crocodiles the penis is single, and con-e-
sponds to a thickened portion of the anterior lip and ventral wall
of the cloaca (Figs. 367, 368, a). It consists of fibrous and caver-
nous (erectile) tissue and is protrusible, and definite protractor and
retractor muscles occur in Chelonia similar to those in Ratite Birds.
In the female it is represented by a smaller clitoris. The penis
bifurcates proximally, and its distal tongue-shaped portion ends
freely; a longitudinal groove extends along the upper surface, at

1 I*

488 COMPARATIVE ANATOMY

the proximal end of which the vasa deferentia open. In Crocodiles
the free portion is relatively longer and the groove deeper than in
Chelonians.

In many Birds a copulatory organ is present, formed on a
similar plan to that of Crocodiles. It is well developed amongst
the Ratitse and Lamellirostres, and in many other Birds can be
recognised in a rudimentary condition. In Struthio it resembles
that of the Crocodile, except that the distal free portion is longer;
it is grooved above, encloses cavernous tissues, and is supported by
a fibrous body, bifurcated at the base. In Drom?eus and Rhea there
is an aperture at the apex of the penis leading into an elongated
and curved blind sac, in which is a furrow, lined by cavernous
tissue, continuous with the groove on the dorsal side of the organ.
In the Duck and Swan the spiral penis is essentially similar to that
of Dromaeus and Rhea. The absence of the blind sac in the
Ostrich is probably a secondary modification. A clitoris is preseut
in the female of the above-mentioned Birds.

The penis of Monotremes may be best understood by
imagining a hypothetical form intermediate between it and that of

Fig. 367. — Transverse Section of the Cloaca of a Chelonian. Slightly
diagrammatic. (After Boas.)

f, fibrous body : r, seminal furrow, bounded by cavernous issue ; r, wall of

cloaca.

Crocodiles and Chelonians (Figs. 368, b). We must suppose that
a sac-like outgrowth into which the ureters and vasa deferentia open
has become developed from the ventral cloacal wall at the base
of the penis, the groove in which has become converted into a
canal. The Monotreme condition is reached by the sac elongating
to form a urinogenital canal, into the distal end of which the
urinary and genital ducts and the bladder open (c, d). The penis
consists of an unpaired fibrous body enclosing the seminal
canal, and is only loosely surrounded by the mucous membrane
of the cloaca, so that it can be protruded from and retracted into
a sheath. In Echidna, cavernous tissue is present in the apex
or glans; and in Ornithorhynchus the apex is bifurcated and
covered with soft spines, the seminal canal or urethra opening
in each case by numerous fine canals situated on papilla?. A
clitoris is present in the female of all Mammals.

In Marsupials (Fig. 369, a), the penis-sheath opens on to the
surface of the body, below the anus ; the opening of the urino-
genital canal into the cloaca has become closed, and is continuous

Fio. 368.— Diagrammatic Loxgitudixal Sections of the Posterior Part of
THE Rectum, the Cloaca, a>'d the Copulatory Organs of Various
Vertebrates. (After Boas.) The relative positions of the ureter and vas
deferens are indicated although not situated in the median line.

A, Crocodile ; B, hj'pothetical form between A and ,C ; C, Monotreme (penis

extruded) ; D, Jlonotreme (penis retracted).
hi, connective tissue ; hi, urinary bladder ; cl, cloaca ; d, rectum ; /, fibrous body

(corpus cavernosum) ; h, ureter ; ps, sheath of penis ; ps^, aperture of the

sheath ; r, seminal furrow or tube (penial urethra) ; s, vas deferens ; u,

urinogenital canal.

Fig. 369. — Continuation of Fig. 368. For description see next page.

COPULATORY ORGANS

Fig. 369.— Continuation of Fig. 368.

491

A, Marsupial (very diagrammatic, for comparison with Fig. 368, C ; the obliter-
ated opening of the urinogenital canal into the cloaca is indicated by dotted
lines) ; B, BodeiU, {Ccelogenys paca) ; C, Ape{Cercopithecws): in most placental
Mammals the apex of the penis is not pendulous ; D, Man ; E, Human
ftetus.

Additional letterings : a, anus ; al, stalk of allantois ; h, pelvic symphysis ; p,
genital prominence which gives rise to the penis or clitoris.

with the urethra of the penis. The fibrous body is paired, and
both it and the walls of the penial urethra are composed of
cavernous tissue. Both penis and clitoris are frequently bifurcated
at the apex.

Amongst the Eutheria the penis of Rodents (Fig. 369, b) and
Insectivores comes nearest to that of Marsupials. The paired
fibrous body {corijus cavernosum) bifurcates proximally to form two
crura, which are nearly always attached to the ischia. The opening

"'■'W ^^^'- Cc/i

CUPp.

Fig. 370. — A, Semidiagrammatic Figure of the Human Penis. (In transverse
section and from the side). B, Clitoris of a Monkey {Cebus capucinus).

A, albuginea penis; A'^, albuginea urethrae ; Ccp, corpus cavernosum; Ccu,
corpus spongiosum, which gives rise to the glans penis at Gp, and forms an
oval enlargement (bulbus) at B ; Cli, clitoris, with its ventral furrow (/?),
glans {Gc), and prepuce (Pp) ; rd, rd}, crura of the corpora cavernosa; S,
sulcus dorsalis penis ; Sp, septum between the two corpora cavernosa.

of the penis-sheath gradually becomes further separated from the
anus, and is situated more on the ventral side of the body (cf. B
and c), the penis itself lying horizontally along the abdomen.
In Primates the organ becomes more or less free from the body-
wall, and either its distal end (Apes, c), or the whole of it (Man, d),
is pendulous, and the sheath forms a double tube-like investment
over the glans — the foreskin or prepuce, enclosing sebaceous
(preputial) glands.

In the coui-se of development, the penis of Marsupials and

492 COMPARATIVE ANATOMY

Placental Mammals passes through stages which resemble succes-
sively those which are permanent in Crocodiles and Chelonians
and in Monotremes. It arises from a " genital prominence " on the
ventral wall of the cloaca. A channel passes along the border
facing the cloaca to the opening of the urinogenital sinus: this
condition is usually retained throughout life in the case of the
clitoris, while in the male (and occasionally in the female also ^)
the groove becomes closed to form a canal continuous with
the urinogenital canal or urethra, which is thus considerably
lengthened. In addition to the paired, erectile corpora cavernosa
there is a median corpses spongiosum or corpus cavernosiim itrcthrce
in connection with the penis (Fig. 370) : corpora cavernosa are also
present in the clitoris, and the corpus spongiosum is represented
by the so-called hulhi vestihuli of the vulva.

In many Mammals a bone {os penis) becomes developed in
the septum between the corpora cavernosa (e.g. many Monkeys,
Rodents, Bats, Carnivores, Whales, Lemurs, Apes). In some there
is also a bone or cartilage in the clitoris. The penis may bear
horny papillae and even calcified plates and spines {e.g. certain
Rodents), and the glans is provided with a special kind of tactile
corpuscles.

(For the glands of the external genital organs, cf p. 484).

Adrenal Bodies ^

The adrenals are so called owing to the fact that they are
usually situated in close proximity to the kidneys, right and left
of the vertebral column. Hence they are usually treated of in
connection with the urinogenital organs — a procedure which is
only justified by the difficulty of knowing in what other connection
to describe them : they have no organic or genetic relation to
these organs.

Both morphologically and developmentally the adrenals consist
of two distinct parts, one of which arises from the ccelomic
epithelium and the other in connection with the rudiments of the
sympathetic nervous system ; thus both mesoderm and ectoderm
take part in their formation.

The constituents formed from the splanchnopleuric coelomic
epithelium originate in the form of numerous epithelial buds or
ridges at the dorsal margin of the base of the mesentery, which

1 The formation of a complete canal in the clitoris (e </. in many Insectivores,
Rodents, and Lemurs) results in an entire separation of the urethra from the
urinogenital canal, so that the latter is no longer concerned with the passage of
the urine.

2 The term adrenal body is here used to include the two primarily distinct
interrenal and suprarenal organs as described in the text, so as to prevent the
confusion arising by the use of "adrenal" and "suprarenal" as synonymous
terms.

ADRENAL BODIES 493

may extend along the whole trunk and are primarily bilaterally
symmetrical : they constitute the bodies which in the lower
Vertebrates are known as the interrenal organ, and in higher forms
(Mammals) correspond to the so-called cortex of the adrenals.
The ectodermic portion arises at an early stage from the un-
differentiated rudiments of the sympathetic ganglia (or from
these and the chromaffin cells, cf p. 247), and in lower Verte-
brates the resulting structures are spoken of as the suprarenal
oi-gan, which corresponds to the medulla of the adrenal in Mammals.
The original distinction of these two components is plainly
retained in the lower Vertebrates throughout life, their union in
the higher types being of a secondary nature.

Amongst Cyclostomes, the interrenal organ consists in the
Lampreys of small clusters of epithelial cells along the cardinal
veins, into the walls of which they extend as far as the lining
epithelium ; they are most markedly aggregated in the fatty
tissue lying between these veins and the aorta, and are also present
along the renal vessels. The suprarenal organ also consists of
epithelial tissue the constituents of which have the characteristics
of chromaffin cells : it extends along the parietal arteries arising
from the aorta, following their dorsal and ventral branches in the
body-walls, and also occurs between the aorta and cardinal vein
of either side, extending into the walls of the latter. The groups
of these cells may give rise to clusters {e.g. on the sinus venosus)
resembling those of Elasmobranchs and Amphibians ; and in many
places elements of the interrenal organ may occur amongst them,
without, however, uniting with them. Both interrenal and
suprarenal extend into the tail, mainly along the caudal artery
and vein. In the head the suprarenal alone is present, and is
situated along the jugular and parietal vessels, while in the
neighbourhood of the pronephric rudiments the interrenal alone
is found.

In Elasmobranchs the two constituents also remain perfectly
distinct (Figs. 371 and 372). The suprarenals extend along the
whole body-cavity, and in Acanthias, Mustelus, and Galeus occur
regularly in every segment, while in others a fusion may take place
in many parts, resulting in an apparent reduction in number
(Rays). Their connection with the sympathetic ganglia is a very
intimate one. The fully-formed interrenal bodies consist of more
or less distinctly paired masses composed of cellular tubes or cords
and of blood-vessels, and may be connected with one another
in a longitudinal direction and also left and right. Fatty
particles enclosed mthin the cells give the organ a yellowish
colour.

In Teleosts the suprarenals and interrenals are usually inter-
spersed amongst one another, and lymphoid cells always occur in
their neighbourhood. The suprarenals are related to the walls of
the cardinal veins — especially the larger, right cardinal — and are

494

COMPARATIVE ANATOMY

Fig. 371. — Diagram of the Supra-
renal Organ of Torpedo mar-
morata. (After E. Gryiifeldt. )

a.o, anterior part of the aorta and
its branches ; a.ax, axillary (sub-
clavian) artery ; h.d, dorsal, and
v.d, ventral branches ; c.s, supra-
renal bodies.

-r

•c.s.

C.l.

Fig. 372. — Interrenal and Supra-
renal Organs of Squalid^e. A,
Mustelus icuvis ; B, Centrina vvl-
pecula. (After E. Grynfeldt. )

ao, aorta ; a.ax, axillary (subclavian)
artery ; h.e.s, outer margin of car-
dinal sinus ; c.ax, the so-called
"axillary body"; c.i, interrenal
bodies ; c.s, suprarenal bodies ; r,
kidney.

most numerous along its anterior portion, projecting into the
lumen of the vessel so as only to be covered by its lining
epithelium. Their connection with the sympathetic cannot, at any

ADRENAL BODIES 495

rate in most cases, be traced in the adult. The interrenals are of
varied size and of a whitish or yellowish colour: they occur, enclosed
in fibrous capsules, sometimes on the dorsal and sometimes on the
ventral surface of the kidney, usually at its middle or posterior
third, more or less embedded in its substance. They are usually
paired, but rarely symmetrical on either side.

Amongst Ganoids, the suprarenals of Sturgeons are situated
on the median side of the cardinal veins and on the revehent veins
of the mesonephros : like the interrenals, they show many points
of similarity to the corresponding organs in Teleosts.

Amongst Dipnoans, the suprarenals of Protopterus are closely
related to the intercostal arteries all along the trunk-region, and
have a paired and segmental aiTangement. On the anterior part
of the cardinal and postcaval vein chromaffin cells also occur.

In Amphibians, the interrenal and suprarenal organs become
united to form a compound adrenal body. In the Myctodera, the
main mass of the adrenal lies on the inner border of the kidney,
from which it is plainly distinguishable by its yellow colour. It
forms a band consisting of irregular lobes extending straight
backwards all along the mesonephros, being most strongly
developed at the middle part of the latter. Here, again, it is
intimately related as regards position with the revehent renal
veins, and often also with the postcaval and cardinal. In Anurans,
the adrenal is usually situated on the ventral side of the kidney,
and more or less towards its outer border, but reaching to its
posterior end, and appearing like an irregular, yellow streak, the
lobules of which follow closely the course of the revehent renal
veins, around which they form a branched network : frequently
certain of the lobules come into relation with the ventral wall of
the postcaval.

In the Amniota, the restriction of the adrenals to more
definite and localised regions is carried considerably further than
in the Amphibia. They consist of a more uniform and circum-
scribed mass on either side in which both interrenal and supra-
renal substances are included.

In the Sauropsida, the adrenals are met with in the form of
elongated yellow areas, with smooth or lobed borders, in the
immediate neighbourhood of the gonads. In Mammals, at a
certain period of development of which (especially in Man,
Fig. 348, b) they are relatively voluminous, the adrenals are situated
near the kidneys, and the name of the organ is due to this fact.
As in all other Vertebrates, these structures are closely related as
regards position to the great vascular trunks of the body-cavity.

Although the two components are united in the adrenal of
Amniota, in the Sauropsida the relation of the one to the other is
not of the regular and characteristic kind seen in Mammals, but
there seems to be an irregular mingling of both elements. In
Mammals, on the other hand, as already mentioned, the interrenal

496 COMPARATIVE ANATOMY

forms the cortical substance enclosing the suprarenal portion
or medulla, with its chromaffin tissue,^ and the whole organ
is richly supplied with pigment, lymph-vessels, and blood-
vessels, although it possesses no portal system, as in Amphibia and
Sauropsida. As regards form, there is considerable variation in
the Mammalian adrenals ; they may be compact and smooth, or
more or less lobed, and are usually asymmetrical in position.

Physiological experiments on the adrenals indicate that they
are of great functional importance, and the two chief theories with
regard to them are briefly as follows. The medullary portion is
said to be a gland with " internal secretion," and to give off into
the blood a substance {adrenalin) which reacts on the muscular
contraction of the heart and blood-vessels, and increases the blood-
pressure. On the other hand, there is some evidence that the
cortex takes part in the manufacture of a substance which is
passed directly or indirectly into the blood-stream, and which
destroys the useless or harmful products of destructive metabolism.
Future researches must show whether and to what extent the
theory of " internal secretion " or of " auto-intoxication " is
universally applicable.

^ So-called " accessory suprarenals," consisting of nests of chromaffin cells,
are found in all Vertebrates in relation with the sympathetic system and blood-

APPENDIX

BIBLIOGRAPHY

List uf somt of the more iinjwrtant periodicals referred to in the following payts, the
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498 APPENDIX

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Jahresberichte iiber die Fortschritte der Anatomic und Physiologic. Bonn. {Jahresber.

Anat. u. Physiol.)
Jenaische Zeitschrift filr Naturwissenschaft. Jena. {Jen. Zeitschv.)
Journal de 1' Anatomic et de la Physiologic normales et pathologiques de I'Homme et

des Animaux. Paris. {Joiirn. de C Anat. et FIn/xiol.)
Journal of Anatomy and Physiology, normal and pathological. Loudon. {Journ. Anat.

and Physiol.)
Journal of the College of Science, Imperial University, Japan, Tokyo. {Journ. Coll.

Sci. Japan.)
Journal of the Linnean Society : Zoology. London. {Jon rn. Linn. Soc.) #

Journal of Morphology. Boston. {Journ. Morphol.)
Memoires de I'Academie des Sciences de I'Institute de France. Paris. {Mem. Ac.

France.)
Memoires de I'Academie imperiale des Sciences de St. Petersbourg. {Mem. Ac. St.

Petersh.)
Memoirs of the Museum of Comparative Zoology at Harvard College, Cambridge, U.S.

{Mem. 3fus. Harvard.)
Mitteilungen aus der zoologischen Station zu Neapel. Berlin. {Mitteil. Zool. Stat.

Neapel.)
Morphologische Arbeiten. Jena. {Morph. Arh.)
Morphol ogisches Jahrbuch. Leipzig. {Morph. Jahrh.)
Mathematische und naturwissentschaftliche Mittheilungen aus den Sitzungsberichten

derKonigl. preuss. Akademie der Wissenschaften zu Berlin, {Mat. Ak. Berlin,

see also Sitz-her. Ak. Berlin.)
Niederlandische Archiv fiir Zoologie. {Mederl. Arch.)
Nova Acta Academise Csesareee Leopoldino-Carolinse (Verhandl, d. K. Leop.-CaroL

deutssh. Akad. d. Naturforscher). Halle. {]V. Acta Acad. Cas. Lcop. Car.)
'* Petrus Camper." Amsterdam. {Pet. Camp.)

Philosophical Transactions of the Royal Society of London. {Phil. Trans.)
Proceedings of the American Academy of Arts and Sciences. Boston. {P. Amev.

Ac.)
Proceedings of the Linnean Society of London. {P. Linn. Soc.)
Proceedings of the Linnean Society of New South Wales. Sydney. {P. Linn. Soc.

^\s.w.)

Proceedings of the Royal Society of Loudon. {P. Eoy. Soc.)

Proceediags of the Zoological Society of London. {P. Zool. Soc )

Quarterly Journal of Microscopical Science. London. {Q.J. M.S.)

Revue biologique du Nord de la France. Lille. {Rev. hiol. Nor d France.)

Sitzungsberichte du Konigl-Preuss, Akademie der Wissenschaften zu Berlin {Sitz.-ler.

Ak. Berlin, and Mt. Ak. Berlin.)
Sitzungsberichte der Gesellschaft fiir Morphologic und Physiologic in Miinchen. {Sitz.-

ber. Morph. Physiol. Miinchen.)
Sitzungsberichte der mathematisch-naturwissenschaf tlichen Classe der K. Akademie der

Wissenschaften. Wien. (Sitz.-her. Ak. Wien.)
Sitzungsberichte der K. Bohmischen Gesellschaft der Wissenschaften. Prag. {Sitz.-her.

Bohm-Ges.)
Sitzuugsberichte der Gesellschaft naturforschenden Freuude zu Berlin. {Sitz.-her.

Naturf. Berlin.)
Sitzungsberichte der physikalisch-medicinischen Gesellschaft zu Wiirzburg. {Sitz.-her.

Ges. Wiirzhury.)
Transactions of the Royal Dublin Society. {Tr. Dublin. Soc.)
Transactions of the Royal Irish Academy, Dublin. {Tr. Irish. Ac.)
Transactions of the Linnean Society of London : Zoology. {Tr. Linn. Soc.)

APPENDIX 499

Trausactious and Proceediugs of the New Zealaud lustitute. Wellingtou. {Tr. N. Z.

Inst.)
Transactions of the Royal Society of Edinburgh. {Tr. R. Soc. Edin.)
Transactions of the Zoological Society of London. {Tr. Zool. Soc.)
Yerhandlungen der Anatomischen Gesellschaft. (Verh. Anat. Ges., see Anat.

Anz.)
Yerhandlungen der naturforschenden Gesellschaft in Basel. {Verh. Ges. Basel.)
Yerhandlungen der k. Akademie der AYissenschaften zu Amsterdam. {Verh. Ak.

Amsterdam.)
Yerhandlungen der physikalisch-medicinischen Gesellschaft zu Wiirzbarg {Verh. Gen.

Wiirzhurg.)
Zeitschrift fiir wissenschaftliche Zoologie. Leipzig. {Zeitschr. f. xciss. Zool.)
Zoologischer Anzeiger. Leipzig. {Zool. Anz.)
Zoologische Jahrbiicher, Abtheilung fiir Anatomie und Ontogenie der Thiere. Jena.

{Zool. Jahrb.)
Zoologischer Jahresbericht, herausgegeben von der zoologischen Station zu Neapel.

Berlin. {Zool. Jahresher.)

GENERAL EMBRYOLOGY AND PHYLOGENY.

Bcir, 'K. E. von. Ueber die Entwicklungsgesch. d. Tiere. Konigsberg, 18:?8-37.
Balfour, F. M. Comp. Embryology. 2 vols, London, 1880-81. A Monograph on the

Development of Elasmobranch Fishes. London, 1878. The Works of (Memorial

Edition). 4 vols. Loudon, 1885.
Bischoff, Th. Entwicklungsgesch. der Saugetiere und des Menschen. Leipzig,

1842.
Bonnet, R. Entwicklungsgesch. der Haussjiugetiere. Berlin, 1891,
Dohrn, A. Ursprung der AYirbeltiere und das Princip des Functionwechsels. Ijcipzig,

1875 ; Studien zur Urgeschichte des Wirbeltierkorpers. Mittheil. Zool. Stat,

Neapel, from 1892.
Duval, M. Atlas d'embryologie. Paris, 1888.
Foster, M., and Balfour, F. M. Elements of Embryology, revised by Sedgwick and

Heape. London, 1883.
Haeckel, E. Naturliche Schupfvmgsgeschichte. Leipzig, New ed. 1902 ; Studien zur

Gastraeatheorie. Jena, 1887, and Jen. Zeitschr. YIII and IX, 1874 and 1875.

Anthropogenie. Leipzig, New ed. 1904.
Hertwig, O. Die Coelomtheorie. Jena, 1881 ; Lehrbuch der Entwicklungsgeschichte

des Menschen und der Wirbeltiere. Jena, New ed. 1906. (Eng. trans, of 3rd

ed. by E. L. Mark. London and New York, 1892).
Hertwig, O. (Edited by) Handbuch der vergl. u. experim. Entwicklungslehre der

Wirbeltiere. 6 vols. Jena, 1901 — 6.
His, W. L^nsere Korperform. Leipzig, 1878 ; Anatomie menschlicher Embryoneu (with

Atlas). Leipzig, 1880—1885.
Keibel, F. Gastrulation u. Keimblattbildung d. Wirbeltiere. Anat, Hefte. Ergebu,

19C0.
Kolliker, A. Entw.-Geschichte des Menschen und der huheren Thiere. 2ud ed. Leipzig,

1879.
Marshall, A. Milnes. Yertebrate Embrj'ology. London, 1893.
Minot, C. S. Human Embryology. New York, 1892 ; Lab. text-book of Embryology.

Philadelj)hia, 1903; Bibliography of Yertebrate Embryology. Boston,

1892.
Quain's Elements of Anatomy. London.

Rathke, H. Entw.-geschichte der Wirbeltiere. Leipzig, 1861.
Remak, R. Entvvick. der Wirbeltiere. Berlin, 1850—1855.
Romiti, G. Lezioni di embriogenia umana e comp. dei Vertebrati. Siena, 1881, 1882

1888.
Schultze, O. Grimdriss der Entwicklungsgesch. des Menschen. und der Saugetiere.

Bearbeitet unter zu Grundlegung der 2*« Auflage des Grundrisses der Entwick-
lungsgesch. von A. Kolliker. Leipzig, 1897,
Selenka, E, Siudien iiber die Entwicklungsgesch. der Tiere. Heft I-Y, Wiesbaden,

1883-1892,
Ziegler, H, E, Lehrbuch d. vergl. Entw.-gesch d. niederen Wirbeltiere, Jena, 1902,

GENERAL GOMPARATIYE ANATOMY, &c,

Beddard, F. E. Cf. numerous papers on the Anat. of Yerts. in P. Zool. Soc.
Boas, J. E. Y. Lehrbuch d. Zoologie. Jena, 1894. (Eng. trans, by Kirkaldy
and Pollard. London, 1896.)

K K 2

500 APPENDIX

Bronn's Klasseu und Ordnuugen des Tiereiches. Pisces, by Hubreclit and Sagetnehl ;

Amphibia and Reptilia, by C. K. Hoffmann ; Aves, by Selenka and Gadow ;

Mammalia by Giebel and Leche.
Briihl, C. B. Zootomie aller Thierklassen. 1876-86.
Cambriclge Natural History. Vol. YII. — Amphioxus and Fishes, by Herdman, Bridge,

and Boulenger ; Vol. VIII. — Amphibia and Reptiles, by Gadow ; Vol. IX. —

Birds, by Evans ; Vol. X. — Mammalia, by Beddard. London.
Cams, C. G. Lehrbuch der vergl. Zootomie. II. Bde. Leipzig, 1834.
Carus, C G. and Otto. Erlauterungstafelu znr vergl. Anatomie. VIII. Hefte. Leipzig,

1826-52.
Glaus, C. Lehrbuch der Zoologie. Marburg and Leipzig, 1883. (Eng. trans, by Sedg-
wick and Heathcote, 2nd vol., 1885).
Cuvier, G. Le9ons d'Anat. comparee, 2nd. ed. Paris, 1835-46, and in German,

Leipzig, 1809-10.
Ecker, A. Icones physiologicse. Leipzig, 1855-9.
Gaudry, J. Les enchaiiiemenls du monde animal dans les temps geologiques.

1878, &c.
Gegenbaur, C. Vergl. Anatomie. Bd. 1 and 2. Leipzig, 1898 and 1901.
Haeckel, E. Gen. Morphol. d. Organismen. Berlin, 1866. New ed. (1906) ; System.

Phylogenie der Wirbsltiare. Berlin, 1895.
Hertwig, R. Lehrbuch d. Zoologie, 3rd ed. (Eng. trans, by G. W. Field, New York.)

7th German ed. Jena, 1905.
Hertwig O. The Cell : Outlines of General Anatomy and Physiology. (Eng. trans, by

Campbell. London, 1895.) Allgem. Biologie. Jena, 1906.
Howes, G. B. Atlas of Practical Elementary Zootomy. London, 1902.
Huxley, T. H. Lectures on the Elements of Comp. Anat. London, 1864 ; Anat.

of Vertebrated Animals. London, 1871.
Kingsley, J. S. Text-book of Vert. Zoology. New York, 1899.

Leydig, F. Lehrbuch der Histologic des Menschen und der Thiere. Frankfurt, 1857.
Meckel, J. F. Sy.stem der vergl. Anatomie. VI Bde. Halle, 1821—33.
Milns-Edwards, H. Lemons sur la Physiol, et I'Anat. comp. de I'Homme et des

Animaux. XIV Bde. Paris, 1857-80.
Oppel, A. Lehrbuch d. vergl. mik. Anatomie d. Wirbeltiere (6 Parts). Jena, 1896

and onwards.
Owen, R. Anat, of Vertebrates. London (4th ed.) 1871.

Parker, T. J. A Course of Instruction in Zootomy (Vertebrates). London, 1884.
Parker, T. J., and Haswell. W. A. Text-book of Zoology. (2nd vol.) London, 1897.
Pouchet, G., and Beauregard, H. Traite d'Osteologie comparee. Paris, 1889.
Reports of the Scientific Results of the Voyage of H.M.S. Challenger.
Rolleston, G. Forms of Animal Life 2nd ed. by W. Hatchett Jackson. Oxford,

1888.
Sedgwick, A. Text-book of Zoology, Vol. II. London, 1905.
Siebold, v., and Stannius. Handbuch der Zootomie. 2nd ed. Berlin, 1854.
Steinmann and Doderlein. Elemente d. Palseontologie. 2 Aufl. Leipzig, 1904.
Vogt and Yung. Lehrbuch d. pract. vergleich. Anat. Braunschweig, 1883-94. (Also a

French edition.)
Wagner, R. Lehrbuch der Zootomie. II Bde. Leipzig, 1843-8. ; Icones zootomicae.

Handatlas zur vergl. Anatomie. Leipzig, 1841.
Wieder.sheim, R. Vergl. Anatomie der Wirb3ltiere. 6th ed. Jena, 1906. Eiufuhruug

in die vergl. Anat. der Wirb3ltiere. Jena, 1907.
Woodward, A. S. Outlines of Vert. Palaeontology. Cambridge, 1898.
Zittel, K. V. Handbuch der Piilaontologie. Miinchen u. Leipzig. (Eng. trans, by

Eastman. 2 vols. London).^'

PAPERS, MONOGRAPHS, &c., DEALING WITH INDIVIDUAL TYPES
AND GROUPS OF ANIMALS.

Amphioxus.

Benham, W. B. Pharyngeal bars of Amphioxus. Q.J.M.S. 1893.

Boeke, J. Struct, of light-perceiving cells, etc., of Amphioxus. Proc. Meet. K. Akad.

d. Wetensch. Amsterdam, 1902.
Boveri, T. Sehorgane d. Amphioxus. Zool. Jahrb. Suppl. VII (Festschr. Weismann's)

1904.
Hatschek, B. Entwick. des Amphioxus. Arb. lust. Wien, 1882.

Johnston, J. B. Cranial and spinal ganglia, etc. in Amphioxus. Biol. Bulletin, 1905.
Joseph, H. Eigentiim. Zellstructuren in Zentralnervensvst. v. Amphioxu.s. Anat.

Anz., 1904.

APPENDIX 501

Kowalewsky, A. Eutw. d. Amphioxus. Mem. Ac. St. Petersb., T. XI.

Langerhans, P. Aaat. des Amphioxus. Arch. f. mikr. Anat. Bfl. XII, and Morph.

Jahrb., 1876.
Laukester, E. Ray. Contribs. to the knowledge of Amphioxus. Q. J.M.S., 1890.
Lankester and Willey. D^v. of the atrial chamber of Amphioxus. Q.J.M.S. 1890.
Legros, A. C'av. buccale d. I'Amphioxus. Arch. d'Anat. Micr. 1897-S.
Milller, J. Branchiostoma lumbricum. Abh. Ak. Berliu, 1844.
Rohon, J. O. Amphioxus. Deuk. Ak. Wien. Bd. XLY.
Rolph, W. Amphioxus lanceolata.s. Morph. Jahrb. 1876.

Van Wijhe, J. W. Amphioxus. Anat. Anz. 1893, and Pet Camp. 1901 and 1906.
Willey, A. Dev. of Amphioxus. Q.J.M.S., 1891. Amphioxus and the Ancestry of

the Vertebrates. New York and London, 1894.

Cyclostomi axd Pi.scks.

Agassiz, A. Dev. of Lepidosteus. P. Amer. Ac. 1877 ; Young Stages of some Osseous

Fishes. Loc. cit., 1877, 1878, and 1884.
Agassiz, A., and Whitman, C. O. Dev. of Osseous Fishes, I. The Pelagis Stages of

Yoiing Fishes. Mem. Mus. Harvard, 1885.
Agassiz, L. Recherches sur les poissons fossiles. 1833-43.
Agassiz, L., and Yogt, C. Anat. des Salmones. Neufchiitel, 1845.
Ayers, H. Beitr. zur Anat. und Physiol, der Dipnoer. Jen. Zeitschr. 1884.
Balfour, F. M. (See p. 499.)
Balfour, F. 31., and W. N. Parker. Struct, and Dev. of Lepidosteus. Phil. Trans.

1882.
Bischoff, Th. Lepidosiren paradoxa. Leipzig, 1840.
Budgett, J. S. Anat. of Polypterus. Tr. Zool. Soc. 1901 ; Breeding Habits of some

W. African Fishes ; Ext. Features in Dev. of Protopterus : and Larva of Polyp-
terus. Tr. Zool. Soc. 1901 and 1902.
Cole, F. J., and Johnstone, J. Pleuronectes. Liverpool Marine Biol. Comtee. Memoirs.

London, 1901.
Cole, F. J. Notes on Myxiue. Anat. Anz. 1905.
Bujor, P. Contrib. a I'etude de la Metamorphose de rAmraocoetes. Rev.' Biol. Nord

France, T. IH. 1891.
Cuvier and Valenciennes. Hist. nat. des Poissons. XXII. vol., 1828-48.
Dean, B. Fishes, Living and Fossil. New York, 1895.
Dean, B. Larval Dev. of Amia. Zool. Jahresber. 1896, and Q.J.M.S. vol. 38 ; Dev. of

Bdellostoma, Q.J.M.S., vol. 40 and Festschr. z. 70 Geburtstag von C. von

Kupffer, 1899 ; Pakeospondylus, &c. Mem. N. York. Acad, of S^i. 1900 (cf. also

P. ZooLSoc.,1898).
Dollo, L. Phylog. d. Dipneustes. Bull. Soc. Beige de Geol., &c. 1895.
Emery, C. Fiera.sfer. Atti Ace. Lincei, 1879-80.
Felix, W. Entwick.-gesch. d. Salmonidje. Anat. Hefte Arb. 1897.
Fritsch. A. Fauna der Gaskohle und der Kalksteine der Performation Bohmens.

Bd. II. Dipnoi. Prag, 1888. Bd. III. Selachii, Actinopterygii. 1893.

Palaeoniscidae. 1894.
Furbinger, K. Morphol. d. Skelettes der Dipnoer (Semon's Fors^h. Reisen). Jena

Denkschr. 19(H.
Garman, 8. Chlamydo.5elachus auguineus. Bull. Mus. Harvard. Vol. XII.
Gotte, A. Entwicklungsgesch. des Flussneuuauges. Leipzig, 1890.
Gudger, E. W. Breeding and seg. of egg. of Siphonostoma. Proc. U. S. Nat. Mus.

1905.
GUnther, A. Ceratodus. Phil. Trans., 1871 ; Introduction to the Study of Fishes.

Edin., 1880.
Guitel, F. Lepadogaster. Arch. Zool. exp. Vol. VI.
Hasse, C. Das natiirl. System der Elasmobranchier. Jena, 1879. Besond. Teil, I. and

II. Lief. Jena, 1882. Ergdnzungsheft, 1885 ; Beitrag zur allgem. Stammes-

gesch. der Wirbeltiere. Jena, 1883.
Howes, G. B. Marsipobranchii. Trans. Biol. Soc, Liverpool, 1892.
Huxley, T. H. Ceratodus fosteri. P. Zool. Soc, 1876.
Hyrtl, J. Lepidosiren. Abh. Bohm. Ges., 1845.

.IuHd, Ch. I'Anat. de FAmmo^oetes. Bull, scientif. du Depart, du Norvl. 1887.
Keibel, F. Normeutafeln z. Entwi^kgesch. des Ceratodus. Jena, 1901.
Kerr, J. G. Dev. of Lepidosiren. Phil. Trans., 1900, and Q.J.M.S., Vols. 45 and 46

(dealing also with Protopterns) ; Embryol. of certain of the Ixjwer Fishes, &c.

Proc. R. Physical So?. Edin., 1906 ; Dev. of Polypterus. Loc. cit., 1907.
Kupffer, C. Die Entwick. von Petromyzon. Arch. f. mikr. Anat. 1890.
Langerhans, P. Petromyzon. Verb. Naturforsch. Ges. Freiburg, 1875.

502 APPENDIX

Leydig, Fr. Mik. Anat. iiud Entw.-gesch. der Koclien imd Haie. Leipzig, 1852 ; Anat.

Histol. Untersuch. liber Fische und Reptilien. Berlin, 1853.
Leydig, Fr. Anat. und Histol. der Chimaera. Arcli. f. Anat. und Physiol., 1851.
List, J. H. Entw.-gesch. d. Knochenfische (Labriden). Arb. Zool. Inst. z. Graz-Leipzig,

1887.
Marcusen, J. Mormyren. Mem. Ac. St. Petersb., T. VII.
Miiller, J. Bau und die Grenzeu der Ganoiden. Berlin, 1846; Vergl. Anat. der

Myxinoiden. 1833^3.
Nuel, J. P. Dev. du Petromyzon. Arch. Biol. Vol. I. 1881.
Owen, R. Lepidosiren annectens. Tr. Linn. Soc. XVIII.

Owsjannikow, P. Zur. Entwick. d. Flussneunauges. Bull. Ac. St. Petersb. 1889.
Parker, T. J. On the Skeleton of Regalecus argenteus. Tr. Zool. Soc. 1886 ; Notes

on Carcharodon rondeleetii. P, Zool. Soc, 1887.
Parker, W. N. Anat. and Physiol, of Protopterus, Tr. Irish Ac. Vol. XXX. p. 3.

1892.
Peters. Lepidosiren. Arch. f. Anat. u, Physiol. 1845.
Pollard, H. B. Polypterus. Zool. Jahrb. Bd. V. 1892.
Salensky, W. Entwick. des Sterlets. Verhdlg. der naturf. Gesellsch. zu Kasan,

1878-79 and Arch. Biol. 1881 .
Schaninsland, H. (See p. 503).

Schneider, A. Vergl. Anat. und Entw.-gesch. der Wirbeltiere. Berlin, 1879.'
Scott, W. B. Entw.-geschich. der Petrorayzonten. Morph. Jahrb. Bd. VII. ; Dev. of

Petromyzon. Journ. Morphol. 1887.
Semon, R. Zoolog. Forschungsreisen in Australieu nod dem malayischen Archipel. I.

Die aussere Entwick. des Ceratodus. Jena, 1893.
Shipley, A. E. On some points in in the Dev. of Petromyzon fluviatilis, Q.J. M.S.

1887.
SoUas, W. J., and I. B. J. Palfeospondylus. Phil. Trans., 1903.
Traquair, R. H. Palseospondylus Gunni, see Ann. and Mag. Nat. Hist., 1890, and

Proc. Roy. Phys. Soc. Edin. 1893.
Vogt, C. Embryol. des Salmones. Neuchatel, 1842.
Woodland, W. Anat. of Centrophorus. P. Zool. Soc. 1907.
Worthington, J. Myxinoids. Amer. Nat. 1905.
Wright, R., McMurrich, J., Macallum, A., Mackenzie, T. Contrib. to the Anat. of

Amiurus. Proc. Canad. Inst. Toronto. 18P4.

Amphibia.

Bayer, F. Skelet der Pelobatiden. Abhandl. Bohm. Ges. 1884.

Bles, E. J. Life-history of Xenopus. Trans. R. Soc. Edin., 1905.

Brachet, A. L'ontog. d. Amphibiens, kc. (Siredon and Rana). Arch, de Biol. 1902.

Brauer, A. Entw.-gesch. u. Anat. d. Blindwiihlen (Gymnophionen). Zool. Jahrb.

1897, 1899, and 1902.
Cope, E. D. Batrachia of North America. Bull U.S. Nat. Mus. Washington, 1889.
Credner, H. Stegocephalen und Saurier aus dem •Rothliegenden, &c. (Zeitschr. d.

deutschen Geolog. Gesellsch.) Leipzig 1881-93; Die Urvierfiissler [Eotetrapoda]

des Sachs. Rothliegenden. Naturw. Wochensch. 1891.
Duges, A. L'osteologie et la myologie des Batraciens. Paris, 1834.
Ecker, A., and Wiedersheim, R. Anat. des Frosches. III. Aufl. bearb. v. E. Gaupp,

Braunschweig, 1896. (Eng. trans, by Haslam, Oxford, 1899.)
Emerson, E. T. Anat. of Typhlomolge. Proc. Boston Soc. Nat. Hist., 1905.
Fatio, V. Faune des Vertebres de la Suisse. Hist. nat. des Reptiles et des Batraciens.

in. Geneve et Bale, 1872.
Fischer, J. G. Anat. Abhandl. iiber die Perenuibranch. und Derotremen. Hamburg,

1864.
Fritsch, A. (See p. 501).

Gotte, A. Entwicklungsgesch. der Unke. Leipzig, 1875.
Gronberg, G. Anat. der Pipa americaua. Zool. Jahrb. 1894.
Hoeven, J. van de. Anat. van den Cryptobranchus. Haarlem, 1862 ; Menobranchus,

Leyden, 1867.
Hyrtl, J. Cryptobranchus japonicus. Vindoboufe, 1865.
Ishikawa, C. Megalobatrachus. Proc. Nat. Hist. Tokyo, 1904.
Jacquet, M. M. Axolotl (Skel. and Muscles). Arch. Sci. med. de Bucarest, 1889.
Kammerer, P. Verwaudschaftverhalt. von Salamandra atra u. maculosa. Arch. Entwick

Mechanik, 1903.
Kingsbury, B. T. Rank of Necturus amongst Batrachia. Biol. Bull., 1905.
Klinckowstrom, A. v. Anat. der Pipa americana. Zool. Jahrb. 1894.
Leydig, F. Die Anuren Bntrachier der deutschen P'auna. Bonn, 1877. Molche der

wiirttemberg. Fauna. Berlin, 1867 and Arch. f. Naturgesch. Bd. XXIII.

APPENDIX 503

Marshall, A. Milnes. The Frog. 6th ed. Loudon, 1896.

Miiller, J. Anat. d. Amphibieu. Zeitschr. f. Physiol. 1832.

Osawa, G. Anat. d. Japan. Kiesensalamanders. Mitteil, a. d. med. Fak. d. K. Japan.

Univ. Tokio, 1902.
Riisconi, M. Hist. nat. dev. et metam. de la Salaiuandre terrestre. Pavia, 1854. (Also

Siren and Proteus.)
Rusconi, M., and Configliachi. Del Proteo anguineo. Pavia, 1818.
Sarasin, P. and F. Ergebn. naturwissenschaftl. Forschungen auf Ceylon in den Jahr< n

1884-6. II. Entwicklungsgesch. und Anat. der Ichthyophis glutinosus. WicS'

baden, 1887-90.
Schwalbe, G. Biol. u. Entw.-gesch. v, Salamandra atra u. maculosa. Zeitschr. f. Biol.,

1897.
Wi?dersheim, R. Anat. der Gymnophionen. Jena, 1879; Salamandrina perspicillata

und Geotriton fuscus. Genoa, 1875 ; Entwicklung.sgesch. von Proteu.s. Arch, f .

mikr. Anat. 1890; Entw. gesch. von Salamandra atra. Loc. n't. 1890.
Wilder, H. H. Contrib. to the Anat. of Sirtn lacertina. Zool. Jahrb. 1891 ; Skel. of

Xecturus. Mem. Boston Soc. Nat. Hist., 1903.
Z?ller, E. Ncotenie d. Tritonen. Jahresber. d. Yereins f. vaterland. Naturkunde in

Wurttemberg, 1899.

Reptilia.

Beddard, F. E. Syst. Arr. and Anat. of certain Squaraata. P. Zool. Soc., 1907.

Bemmelen, J. F. van. Halsgegend bei Reptilien. I. Amsterdam, 1888.

Bojanus, Anatome testudinis europaeae. Vilnae, 1819-21.

Cope, E. D. Cf. numerous papers in various American Journals.

Credner, H. (See p. 502.)

Dendy, A. Dev. of Sphenoda (Hatteria). Q.J.M.8., 1899.

Dumeril and Bibron. Erpetologie generale. Paris, 1834-54. .

Duvernoy. Serpens. Ann. Sci. Nat. T. XXX.

Gimther, A. Anat. of Hatteria. Phil. Trans., 1867.

Leydig, F. Die in Deutschland lebenden Arteu der Saurier. Tubingen, 1872; Einheim-

ischen Schlangen. Abh. Senckenb. Ges. Bd. XIII. 1883.
Marsh, O. C. Dinosaurs of North America. "Washington, 1896.
Owen, R. Descript. and Illust. Cat. of the Fos.sil Reptilia of South Africa.
Osborn, H. T. Cf. numerous papers on Fossil Reptiles in Amer. Nat., " Science,"

Mem. Amer. Mus. of Nat. Hist., &c.
Peter, K. Normentafein z. Entwick. -gesch. d. Zauneidechse. Jena, 1904
Rathke, H. Entwick.-geschich. der (1) Natter, (2) Schildkroten, (3) Crocodile. (Kouigs-

berg, 1837; Braunschweig, 1848 and 1866.)
Schauiusland, H. Entw.-ge.sch." der Hatteria. Arch. f. mikr. Anat., 1900; Entwick. -

gesch. u. Anat. d. Wirbeltiere (Sphenodon), Callorhynchus, Chamaleo,

" Zoologica." Stuttgart, 1903.
Siebenrock, F. Various papers on Reptiles in the Sitz.-ber. Ak. Wien from 1892

onwards.
Smalian, C. Anat. der Amphisbaniden. Zeitschr. f. wiss. Zool. 1885.
Thilenius, G. Eiablage u. Entwick. der Hatteria. Sitz.-ber. Ak. Berlin, 1899.
Voeltzkow, A. Entw.-gesch. d. Reptilien (Krokodile). Abh. Senkenb. Ges., 1899.
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Riitimeyeri. Abh. S^hweiz, Pal. Ges., 1878.
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N. F. IX. 2.

AVES.

Baur, G. W. K. Parker's Bemerk. lib. Archaeopteryx, 1864. Zool. Anz. 1886.
Dames, W. Archaeopteryx. Paliiontol. Abhdlg. Berlin, 1884. Cf. also Sitz.-ber. Ak.

Berlin, 1897.
Fiu-bringer, M. Untersuch. zur. Morphol. und Systematik der Yogel. Theil. I. and II.

Amsterdam, 1888. See also abstract in Biol. Centralbl. Bd. IX.— XYII.
Keibel, F., and Abraham K, Normentafein z. Entwick. -gesch. des Huhns. Jena, 1900.
Marsh, O. C. Odontornitbes. Washington, 1880.

Marshall, W. Der Bau der Yogel (Weber's Naturwiss. Bibliothek). Leipzig, 1895.
Menzbier, M. von. Osteol. der Pinguine. Bull. Soc. Moscou, 1887.
Milne- Ed wards, A. La faune ornithol. eteinte des iles Mascareignes et de Madpgafcar.

1866-79.
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504 APPENDIX

Parker, "W. K. Numerous papers on the Osteology of Birds in P. Zool. Soc. 1860-5,

1889, 1890; Tr. Zool. Soc. 1862, 1866, 1869,1891; P. Koy. Soc. 1887-8; Ann.

Nat. Hist. 1864, 1865; Ibis, 1888-9; Tr. Irish. Ac. 1890; Plncycl. Brit. 9th ed.

(Article "Birds").
Parker, T. J. Observations on the Anat. and Dev. of Apteryx. Phil. Trans., 1891.

Addit. Observations, kc. Loc. cit. 1892. See also under Skull.
Quatrefages, A. de. Les Moas et les cliasseurs de Moas. Ann. Sci. Nat. 1883.
Wagner, R., and Nitsch. In Naumann's " Naturgeschich . d. Yogel Deuischlands."

Mam.walia

Assheton, R. Dev. of Rabbit and Frog, Q.J. M.S. 1894.

Beneden, Van, and Gervais. Osteographie des Cetacees. Paris, 1868-80.

Blainville, H. Osteographie ou descrip. ieouograph. comp. des Mammiferes rec. et

fossiles. 4 Bde. Paris, 1839-64.
Brandt. Die fossile und subfossile Cetaceen Europas. Mem. Acad. St. Petersb. 1873.
Burmeister. Annales del Museo publico de Buenos-Aires, 1874-89.
Caldwell, \V. H. The Embryol. of Monotremataand Marsupialia. Phil. Trans. 1887.
Camerano, L. Ricerche intorno all' anat. di \\n feto di Otaria jubata. Mem. Ace.

Torino. Ser. II. Tom. XXXV. 1882.
Cuvier, G. Rech. sur les ossements fossiles. 4th etl. 1834-6.
Delage, Y. Histoire du Balaenoptera musculus. Arch. Zool. exp. 1885, 1887.
Duvernoy, G. L. Caract. anat. des grands singes. Arch. d. Museum. Tom. III.
EUenberger, W., and Baum, H. Systemat. und topogr. Anat. d. Hundes. Berlin, 1891.
Eschricht. Zool.-anat.-physiol. Untersuch. iiber die nord. Walthiere. Leipzig, 1849.
Pick, R. Vergl. anatom. Studien an eiuem Orang-Utang. Arch. f. Anat. 1895.
Fletcher, J. J. Catalogue of papers and works relating to Marsupialia and Monotre-

mata. P. Linn. Soc. N.S.W., Vol. IX.
Flower, W. W. Introd..to the Osteol. of the Mammalia. 3th ed. revised with Gadow.

London, 1885.
Flower and Lydekker. Mammals, living and extiur-t. Loudon, 1891.
Frank, L. Anat. der Haustiere. Stuttgart, 1871.
Gaudi-y, A. Animaux fosniles et Geologie de I'Attique. Paris, 1862-7 ; Die Vorfahren

der Silugetiere in Europa. Leipzig, 1891.
Giicomini, C. Annotaz. sulla Anat. del Negro. Torino, 1878-92.
Giildberg, G. La Dyssemetrie morph. et funct. chez I'homme et les vert. sup. Christi-

ania, 1897.
Giildberg, G.,and Nans2n, F. Dev. and structure of the Whale. Part I. On the dev.

of the Dolphin. Bei-gens Mus. Skrifter, V. 1894.
Gurlt. Handbuch der vergl. Anat. der Haussiiugetiere. Berlin, 1860.
Hepburn, D. Various papers on Mammalian Anatomy in Journ. Anat. and Physiol.,

1884, 1899, 1900 to 1904, etc.
Hoeven, J. van de. Stenops. Verh. Ak. Amsterdam. T. VIII.
Hubrecht, A. A. W. Descent of the Primates. New York, 1897.
Keibel, F. Studien zur Eutwicklungesch. des Schweines. Morph. Arb., 1893-5 ; Nor-

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Kowalewsky, W. Anchitherium Aurelianense. Mem. Ac. St. Petersb., 1873. — Osteol.

of the Hyopotamidae. Phil. Trans. 1873. — Versuch einer natiirl. Klassification

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Krause, W. Anat. der Kaninchens. Leipzig, 1884.
Kiickenthal, W. Vergl. anat. und eutwicklungsgesch. Untersuch. an Waltieren.

I. and II. Theil. Jena, 1889.— Ueber d. Entsteh. und Entwick. des Ssiugetier-

stammes. Biol. Centralbl. XII. Bd. No. 13. 1892.— Anat. u. Entw.-gesch. Unter-
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Leisering and Miiller. Handbuch d. vergl. Anat. der Haussilugetiere. 1855.
Leche, W. Zur Anat. der B9ckenregion bei Insectivora, &c. K. Schwed. Acad. d.

Wissensch. Bd. XX. 1882 ; Ueber die Siiugetiergattung Galeopithecus. Loc.

cit. 1885.
Marsh, O. C. Dino^erata. Washington, 1884. Numerous pip?rs in Amer. J. Sn.,

1874-92.
Meckel, J. F. Ornithorhynchi paradoxi descriptio anatomica. Leipzig, 1826.
Mivart, St. G. The Cat. London, 1881.
Muri^, J. Manatus, Globiocephalus, Otaria, Trichecus, Lemuridae. Tr. Zool. Soc.

Vols VII. and VIII.
O.sborn, H. F. Cf. numerous papers in Amer. Nat., Amer. Joui-n. of S3i., Annals N.Y.

Acad. Sci., Mem. Amer. Mus. Nat. Hi.st., &c.
Owen, R. Extinct Mammals of Australia. London, 1877. Monogr. of fossil Mesozoic

Mammalia. Palaentol. Society, 1871 ; Giraffe, Rhinoceros, Myrmecophaga, &c.

Tr. Zool. Soc. Vols. II.-IV.

APPENDIX 505

Parker, T. J., aud Scott, J. H. Ziphiiis. Tr. Zool. Soc. Vol. XII.
Parker, W. K. Mammaliau Descent. London, 1884.

Rapp. (1) Die Edentaten, (2) Die Cetaceen. Stuttgart and Tubingen, 1837.
Keighard, J., and Jennings, H. S. Anat. of the C'at. New York, 1901.
Robinson, A. Observ. upon the Dev. of the Seg. Cavity, the Archenteron, the Ger-
minal Layers, and the Amnion. Q.J. M.S., Vol. XXXIII.
Rutimeyer, L. Beitr. zur Kentniss der fossil. Pferde. Basel, 1863. Ueb^r die Her-

kunft unserer Saugetiere. Basel, 1867. Versuch einer natxu-1. Geschichte des

Rindes. Abh. Schweiz. pal. Ges. 1877 fg. Die naturl. Geschichte der Hirsche.

Loc. cit., 1880; Die eocane Saugetierwelt v. Egerkingen. Loc. cit., 1891.
Selenka, S. Menschenaffen (Entwick. u. Schadelbau). Wiesbaden, 1906.
Semon, R. Zoolog. Forschungsreise in Australien, etc. II. Band. Monotremen u. Mar-

supialier. (or) Beobacht. ub. d. Lebensweise u. Fortpflanz. der Monotremen;

(/>) Die Erabryonalhiillen der Jlonotremen u. Marsupialier ; {c) Zur Entwick-

lungsgesch. der Monotremen.
Sewertzolf, A. Beitrag. z. Ent. Ge.sch. d. Wirbeltiers^hiidels. Anat. Anz., 1897.
Strauss-Durckheim, H. Anat. du Chat. Paris, 1845.
Struthers, J. Anat. of the Humpback Whale. Edin., 1899.
Sweet, G. Anat. of Notoryctes. Proc. R. Soc, Victoria, 1904; Q.J. M.S. 1906 and

1907.
Vrolik, W. Anat. comp. de la Chimpanse. Amsterdam, 1841.
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Morph. Jahrb., 1888 ; Zoolog. Ergebnisse einer Reise in Niederliind. Ost-Indien.

Bd. II. Beitr. z. Anat. und Entwic-k. des Genus Manis. Leiden, 1891. Die

Saugetiere. Jena, 1904.
Wiedersheim, R. Der Bau des Menschen alsZeugois flirseiue Vergangenheit. II. Aufl.

Freiburg i. B. 1902. (Eng. trans, by Bernard, " The Structure of Man." London,

1896).
Wilder, B. G., and Gage, S. H. Animal Technology, as applied to the Domestic Cat.

New York and Chicago, 1882.
Wilson, J. T. Description of a young Ornithorhynchus. P. Linn. Soc. N.S.W.

Vol. IX.
Ziickerkandl, E. Anat. d. Chiromys. Denk. Ak. ^Vien, 1899.

WORKS DEALING WITH INDIVIDUAL SYSTEMS OF ORGANS.

A. INTEGUMENT. 1

(a) PisCE.«!.

Bottard, A. Les Poissons venimeux. Paris, 1889.

Burckhard, R. Luminous orgs, of Selachians. Ann. Nat. Hist , 1900.

Brandes, G. Leuchtorgane der Tiefseefische, etc. Zeits?hr. f. Naturw. Bd. 71.

Emery, C. Scopelus. Mitteil. Zool. Stat. Neapel. Bd. V.

Fritssh, G. Die aussere Haut und die Seitenorgane d. Malopterurus electricus.

Sitz.-ber. Ak. Berlin. 1886. Die elektrisch. Fische. Leipzig, 1887 and 1890.
Handrick, K. Nervensystem u. Leuchtorgan d. Argyropelecus. "Zoologiea," XIII.

Bd. Stuttgart, 1901.
Hirota, S. Dendritic Appendage of Urogen. papilla of a Siluroid. Journ. Coll. Sci.

Imp. LTniv., Japan, 1895.
Kapelin, W. Haut von Petromyzon. BnlL So?. Moscou, 1893.

Kwietniewski, C. Strut, istol. dell' integumento dei Selachii. Padova- Verona, 1905.
Langerhans, P. Petromyzon. Verhand. Naturf. Gesellsch. Freiburg, 1875.
Lankester, E. R. On Lepidosiren and Protopterus. Tr. Zx)l. Soc, 1896.
Lendenfeld, R. V. Die Leuchtorgane der Fische. Biol. Centralbl. 1887.
I^ydig, F. Hautdeoke und Sinnesorgane der Fische. Halle, 1879. Inlegument

brlinstiger Fische und Amphibien. Biol. Centralbl. 1892.
List, J. Wanderzellen im Epithel. Zool. Anz. 1885. See also Arch. f. mikr. Anat.

Bel. XXV.
Maurer, F. Epidermis u. ihre Abkommlinge. Leipzig, 1895.

Pirkar, W. N. Poison-organs of Trachinus. P. ZjoI. So?., 186^. Anat. Anz. 1888.
Porta, A. S iir appare^chio velenifero di alcuue pesci. Anat. Anz. 1905.
Retzius, G. Haut von Myxine. Biol. Untersucli. 1905.
Sacchi, M. Struttura del tegumento negli embrioni ed avannotti del Salmo lacustris.

Rend, del R. Istituto Lombardo. Vol. XX. fa.sc. XV.— XVI. Milano, 1887.

Struttura degli organi del veleno della Scorpena. Boll. Mus. Genova. 1895.

Atti. Soc, Ligust. Vol. VI.

1 Cf. also under Sensory Organs.

506 APPENDIX

Schulze, F. E. Epithel- und Driiseuzellen. Arch. f. inikr. Anat. Bd. III. Ueber
kutic. Bildungen und Verhornungen voii Epithelzelleu. Arch. f. mikr. Anat.
Bd. V.

Sshultze, M. Die kolbenformigen Gebilde in der Haiit von Petromyzon. Arcli. f.
Anat. u. Phys. 1861.

Wolff, G. Die Cuticula der Wirbeltierepidermis. Jen. Z?itschr. 1889.

(A) Amphibia.

Eberth. Untersuch. z. norm. u. path. Anat. d. Froschhaut. Leipzig, 1869.
Engelmann, W. Hautdriisen d, Frosches. Pfiiiger's Archiv f. Physiol. Bd. V. and

VI.
Esterly, C. O. Struct, and regen. of Poison-glands of Plethodon. Univ. of California

Publics. Zool.,1904.
Hcalbi, E. Sulla struttura miniita della Pelle degli Anfibi. Atti della R. Accad.

Peloritana in Messina, 1896-7.
Heidenhain, M. Die Hautdriisen der Amphibien. Sitz.-ber. Ges. Wurzburg, 1893.
Huber, O. Ueb?r Brunstwarzen bei Rana. Zeitschr. f. wiss. Zool. 1887.
Junius. P. tJi bar die Hautdriisen des Frosches. Arch. Anat. 1896.
Leydig, F. Hautdecke und Hautsinnesorgane der Urodelen. Morph. Jahrb. 1876.

Ueber die allgera. Bedeckungea der Amphibian. Arch. f. mikr. Anat. 1876.

Vase. Epithel. Loc. cit, Bd. LII., 1898. Zum Integument niederer Wirbeltiere

abermals. Biol. Centralbl. 1892. (See also /oc. caY., 1893.)
Maurer, F. Glatte Muskelzellen in der Cutis der Anureu, &c. Morph. Jahrb. 1894.

Epidermis u. ihre Abkommlinge. Leipzig, 1895.
Maurer, F. Vaskulasierungd. Epidermis bei Anuren Amphibien. Morph. .Jahrb. 1898.
Nicoglu, Ph. Die Hautdriisen der Amphibien. Zeitschr. f. wiss. Zool. 1893.
Paulicki. Die Haut des Axolotls. Arch, filr mikr. Anat. 1884.
Pfitzner, W. Die Epidermis der Amphibien. Morph. Jahrb. 1880. Di?" Leydig'schen

Schleimzellen in der Epidermis der Larve von Salamandra. luaug.-Diss. Kiel,

1879.
Sohuberg, A. Ueb. den Bau und die Function der Haftapparate des Laubfrosches. Arb.

Inst. Wiirzburg. Bd. X. 1891. Zur Kenntniss der Amphibiephaut. Zool.

Jahrb. Bd VI.
Schulz, P. Die Giftdrilsen der Krotan und Salamander. Arch. f. mikr. Anat. 1889.
Studnicka, F. K. Sogen, Cuticula u. die Bildung dersselben, etc. Sitz.-ber. Bohm.

Ges. 1897.
Tonkoff, W. Elast. Fasern in der Froschhaut. Arch. f. mikr. Anat. 1900.
Wiedersheim, R. Die Kopfdrilsen der geschwanzten Amphibien und die Gland, iuter-

max. der Anuren. Zeitschr. f . wiss. Zool. Bd. XXVII.
Weiss, O. Hautdriisen von Bufo. Arch. f. mik. Anat. 1898.
Zalesky. Ueber das Samandarin. Medic, chem. Untersuch., herausgegeb. von Hoppe-

Seyler. Berlin, 1866.

{c) Reptilia.

Batelli, A. Zur Kenntnis des Banes der Reptilienhaut. Arch. f. mikr. Anat. Bd.

XVII.
Braun, M. Zur Bedeutung der Kuticularborsten auf den Haftlappen der Geckotiden.

Arch, zpol.-zoot. Wiirzburg. Bd. IV.
Cartier, O. tJber den feineren Bau der Haut bei den Reptilien. Verb. Ges. AVilrzburg.

N.F. III,V.
Ficalbi, E. Ges. Ricerche istologiche sul Tegumeuto dei Serpenti. Atti Soc. Toscana.

1888. (A French abstract in Arch. Ital. Biol. 1888.) Sulla Istologia della Pelle

dei Rettili Cheloniani. Atti. d. R. Accadem. dei Fisiocritici. Vol. I. Siena,

1889.
Giacomini, E. Glandule cutanee dorsali dei Coccodrilli. Monit. Zool. Ital,, 1903.
Goppert, E. Phylogenese der Wirbeltierkralle. Morph. Jahrb. 1896. (^Deals also

with the Amphibia.)
Kerbert, C. Die Haut der Reptilien, etc. Arch. f. mikr. Anat. Bd. XIII.
Lvoff. Histologic der Haut der Reptilien. Bull. Soe. Moscou, 1885.
Oppenheimer, E. Eigentuml. Organe in d. Haut. einiger Reptilien. Morph. Arb.

Bd. V. 3 H. 1896.
Osawa, G. Structur des Integuments der Hatteria. Arch. f. mikr. Anat. 1893 et scq.
Strong, R. M. Dev. of the primitive feather. Bull. Mus. Harvard, 1902.
Thilenius, G. Farbenwechsel von Varanus, Uromastix, u. Agama. Morph. Arb. 1897.
Tornier, G. Eidechsenschwanz mit Saugscheibe. Biol. Centralbl. 1899. Farbenmuster

der Eidechsen u. Schlaugen. Sitz.-ber. Ak. Berlin, 1904; Farbenmuster u.

Korperform d. Schildkrolen, Sitz.-ber. Naturf. Berlin, 1904.

APPENDIX 507

(d) Ates.

Davies, H. K. Zur Eutwick. der Feder. Morph. Jahrb. Bd. XV. 1889.

Ficalbi, E. Architettura istologica di alcuni peli degli uccelli. Atti Soc. Toscaua

Vol. XI. 1890.
Gardiner, E. Zur Kenntniss des Epitrichiums imd der Bildung des VogelschnaheLs,

luaug. -Dissert. Leipzig, 1884.
Haecker, V. Ueber die Farben der Vogelfedern. Arch. f. mikr. Anat. 1890.
Haecker, V., and Meyer, G. Die blaue Farber der Vogelfedern. Zool. Jahrb. (Abth. f.

System., Geog., u. Biol.) 1901.
Hurst, C. H. Struct, and habits cf Archseopteryx. Stud. Owen Coll., Manchester, 1895.
Keibel, F. Ontog. u. Phylog. von Haar u. Feder. Anat. Hefte Ergebn. lf-95.
Kiister, E. Innervation u. Eutwick. d. Tastt'eder. Morph. Jahrb. 1905.
Lnnghetti, B. Glandula uropigetii-a. Arch. ital. di. Anat. e Embr. 1903 (cf. Anat. Anz.,

1902).
Meijere, T. C. H. de. Die Federu der Vogel. Morph. Jahrb. Kd. XXIII.
Nitsch. System der Pterylographie. Halle, 1840.
Orlandi, S. Glandula uropigetiea. Boll. Mus. Genova, 1902.
Studer, Th. Die Entwicklung der Federn. Inaug.-Diss. Bern, 1873. Beitrage ziir

Entwicklungsgesch. der Feder. Zeitschr. f. wi.ss. Zcol. Bd. XXX.

(f.) Mammalia.

Adachi, B. Hautpigmeut (Menschen u. Aflfen). Zeitsch. f. Morphol. u. Anthropol.

1903.
Backmund. K. Entw. d. Haare u. Sc-hweissdriisen d. Katze. Anat. Hefte. 1904.
Beard, J. Birth-period of Trichosurus. Zool. Jahrb. 1897.
Blaschko, A. Anatomie der Oberhaut. Arch. f. mikr. Anat. Bd. XXX.
Boas, J. E. V. Morphologic der Njigel, Krallen, Hufe, uud Klaueu der Siiugetiere.

Morph. Jahrb. 1884 and 1894.
Bonnet, R. Haarspiralen und Haarspindeln. Morph. Jal rb. Bd. XI. Eingeweide-
melanose. Verb. Gas. Wilrzburg. 1890. Hypotricho>is congenita universalis.
Anat. Hefte. 1892. Die Mammarorgane im Liclit der Outogenie und Phylogenie
Anat. Hefte. Ergebn. 1892, 1897.
Brandt, A. Phylog. d. Saugetierhaare. Biol. Centralbl. 1900.

Bresslau, E. Entw.-gesch. d. Mammarorgane bei d. Beuteltieren. Zeitsch. f. Morphol.
u. Anthropol. 1902. Maramarapparated. Siiugetiere. An it. Anz. 19<12. Entw.
der Beutels. Verb. Deutsch. Zool. Gesells?h. 1904.
Burckhardt, G. Emb. Hypermastie u. Hyperthelie. Anat. Hefte Arb. 1897.
Cirlsson,A. Marsupialregion bei d. Beuteltiereu. Zool. Jahrb. 1903.
CreightoD, C Development of the Mamma, etc. Journ. Anat. and Physiol.

Vol. XI.
Disselhorst, R. Halsanhiiuge beim Meuschcn u. den Ungulaten. Anat. Anz. 1906.
Dombrowski, R. v. Geweihe und Gehorne. Wieu, 1885.

Ebner, V. v. Wachstum u. Wechsel d. Haare. Sitz.-ber Ak. Wi_m, Bd. LXXIV.
Eckert, A. Schenkelmammre. Ber. Ges. Freiburg, 3<1 • ^•

Ecker, A. Abnorrae Behaarung des Menschen, etc. Gratul.-Schrift fiir v. Siebold
1878. Vertex coccygeus, Glabella coccygea, Foveola coccygea, etc. Arch. f.
Anthropo!ogie. Bd. XII.
Eggeling, H. Stellung d. Milchdriiseu zu d. librigen Hautdriisen. Semen's
Forschungsreisen. (Jena. Denkschr.) 1901 and 1905. Hautdriisen d. Monotremen.
Verb. Anat. Ges. Pavia, 1900. Schhifendriisen d. Elefanten. Biol. Centralbl.
1901. Entw. d. mensch. Milchclriise. Anat. Anz. 1904.
Emery, C. A^erhaltnis der Saugetierhaare zu schuppenart. Hautgebilden.- Anat. Anz.

1893.
Eschricht. Richtung der Haare am menschl. Korper. Arch. f. Anat. imd Physiol.

1837.
Evatt, E. J. Dev. and evol. of the " papillary ridges" and patterns on vola surface of

hand. Journ. Anat. and Physiol., Vol XLI.
Exner, A. Function der menschl. Haare. Biol. Centralbl. 1896.
Feiertag, F. Bildung der Haare. luaug.-Diss. Dorpat, 1875.
Fjelstrup, A. Bau der Haut bei Globiocephalus. Zool. Anz. 1888.
Frederic, J. Sinushaare d. Affeu, etc. Zeitsch. f. Morphol. u. Anthropol. 1905.
Galton, F. " Finger-prints." London, 1902.

Gegenbaur, C. Zitzen der Siiugetiere. Morph. Jahrb. Bd. I. 1876. Morphol. des
Nagels. Morph. Jahrb. Bd. X. 1885. Mammarorgane der Monotremen. Leipzig,
1886.
Graff, K. Bau der Hautdriisen der Hausiiugetiere und des Mens:hen. Inaug.-Diss.
Leipzig, 1879.

508 APPENDIX

Grote,"^ Gr. Glandulae anales des Kaninchens. Inaug.-Dissert. Konigsberg.

1891.
Haacke, W. Marsupial ovum, mammary pouch and the male milk glands of Echidna

hystrix. Proc. Roy. 8oc. 1885.
Hanibiu-ger, C'. Eutw. d. Mammarorgaue (Zitze von Pferd u. Esel) Anat. Anz.

1900).
Hennebsrg, B. Entw. d. Mammarorgane d. Ratte. Anat. Hefte. Arb. Bd. 13.
Hassling, Th. v. Brunstfeige der Gemse. Zeitschr. f. wiss. Zool. Bd. VI.
Huss, M. Entw. der Milchdriisen, etc. Jen. Zeitschr. Bd. VII.
Jakobis. Pathogeuese der Pigmentirungen uud Entfiirbungen der Haut. Centralbl. f.

allg. Pathol, u. pathol. Anat. 1890.
Kallius, E. Milchleiste bei einem menschl. embryo. Anat. Heft, 1897.
Kate, ten H. Pigmentflecken d. Neugeborenen. Globus. 1902.
Keibel, F. Ontogenie u. Phylogenie von Haar u. Feder. Anat. Hefte. 1895.
Kidd, \V Direction of hair in Animals and Man. London. 1903.
Klaatsch, H. Zur Morphol. der Saugetierzitzen. Morph. Jahrb. Bd. IX. 1883.

Morphol. der Tastballen der SJiugetiere. Morph. Jahrb. Bd. XIV. 1888.

Beziehungen zvi^ischen Mammartasehe uud Marsupium. Morph. Jahrb. 1892.

(See also 1893.) Marsupialrudimente bei Placentaliern. Morph. Jahrb. 1893.

Studien zur Geschichte der Mammarorgane. I. Die Taschen- und

Beutelbilduugen am' Driisenfeld der Monotremen, Jena (Semon's Zool.

Forschungsreise. 1895).
Kromayer. Oberhautpigment d. Siiugetiere. Arch. f. mikr. Anat. 1893. Einige epith.

Gebilde, etc. Dermatol. Zeitschr. 1894.
Klikenthal, W. Anpassung von Saugetieren an das Leben im "NVasser. Zool. Jahrb.

1890; Reste eiues Hautpanzers bei Zahuwalen. Anat. An z. 1890. Uutersuch.

an Sirenen. Semon's Forschungsreisen. 1897.
Leboucq, H. Morphologie de la main chez les Mamraiferes marins, etc. Arch. Biol.

T. IX. 1889.
Leichtenstern. * Uberzahlige Briiste. Arch. f. path. Anat. 1878.
Leydig, F. Die jiusseren Bedeckungen der Saugetiere. Arch. f. Anat. uud Physiol.

1859. Epidermoid. Orgaue im Integ. v. Saugetiere. Arch. f. mikr. Anat. 1898.
List, J. Die Herkimft des Pigmentes in der Oberhaut. Biol. Centralbl. X. Bd.
Lwoff, W. Histol. d. Haares. d. Borste, u. d. Feder. Bull. So3. Moscou. 1884.
Martin, C. T., and Tidsvyell, F. Femoral Gland of Ornithorhynchus, &c. Proc. Liun.

Soc. N.S.W. Vol IX.
Maurer, F. Haut-Sinnesorgane, Feder- und Haaranlagen., etc. Morph. Jahrb. 1892,

1893, and 1898.
Meijere, J. C. H. de. Die Haare der Saugetiere, Morph. Jahrb. 1894.
Michaelis, L. Milchsekretion. Arch. f. mikr. Anat. 1898.
Norner, C. Bau des Pferdehufes, Arch. f. mikr. Anat. 1886.
Pinkus, F. Nebsnapparat am Haarsystem d. Menschen : Haarscheiben. Dermatol.

Zeitschr., 1902, 1903.
Poulton, E. B. The structure of the Bill and Hairs of Ornithorhynchus. &c. Q.J.M.S.

1894.
Profe, O. Outog. u. Phylog. d. Mammarorgane. Anat. Hefte. 1898.
Rawitz, H. Getaceenhaut. Arch. f. mik. Anat. 1899.
Reh. Schuppen der Saugetiere. Jen. Zeitschr. 1894.
Rein, G. Eutw.-geschichte der Milchdrilse. Arch. f. mikr. Anat. 1882.
Schmidt, H. Normale Hyperthelie mensch. Embryonen, &c. Morph. Ai-b.

1897.
Schickele, G. Normale u. iiberzalige Milchdriisen. Zeitsch. f. Morphol. u. Anthropol.

1899
Schlaginhaufen, O. Hautleistensystem d. Primatenplanta. Morph. Jahrb. 1905 ;

Relief d. Planta d. Primaten u. d. Mensohenrassen. Korresp. Bl. Deutsche Anthr.

Ges. 1905.
Schultze, O. Die erste Anlage des Milchdriisenapparates. Anat. Anz. 1892 ; Milch-

driisenentwick. und Polymastia. Sitz.-ber. Ges. Wiirzburg. 1892. See also

Verb. Ges. Wiirzburg. 1893.
Schwalbe, G. Farbenwechsel winter weisser Tiere. Morph. Arb. II. Bd. ; Vermeint.

offenen Mammartaschen bei Huftieren. Morph. Art, 1898. Ballen, Linien, u.

Lei-sten der Hand. Strassburg. Med. Znitung, 1905.
Semon, R. Mammarorgane d. Monotremen. Morph. Jahrb. 1899.
Spuler, A. Regen. d. Haare. Verb. Anat. Ges. Tiibingen, 1899.
Stohr, P. Mensch. AYollhaare. Anat. Hefte. 1903.

Strahl, H. Entw. d. Mammarorgane b?i Menschen. Verb. Anat. G3S. Kiel, 1898.
Torri Silvio, G. Significato di un' appendice epiteliale dei follicoli piliferi nell' uomo.

Ricersche Lab. Anat. Roma e altri Lab. Biolog. 1896
Unger, E. Anat. u. physiol. d. Milchdriise. Anat. Hefte Arb. 1898.

APPENDIX 509

Walcleyer, W. Atlas der nieuschl. u. tierischen Haare, &c. Heiausg. v. J. Grimm,

Oflfenburg. Lahr, 1884.
"Walter, H. E. Transit, epith. structores assoc. with Mammary app, in Mau. Auat.

Anz. 1902.
Weber, M. Neue Hautsecrete bei Saugetieren. Arch. f. mikr. Anat. 1888. Bemerk.

iiber den Ursprung der Haare und iiber die Schuppen bei Siiugetieren. Auat.

Anz. 1893.
Wilder, H. H. Palms and Soles. Amer. Journ. Anat. 1902. Racial differences

in Palm and Sole C'onfig. Amer. Anthropologist, 1904.
Wilson, J. T. , and Martin, C. J. Anatomy of the Integ. Structures in the Muzzle

of Ornithorhynchus. P. Linn. Soc. N.S.W. * 1894. (a) Femoral gland, (ft)

Dumb-bell shaped bone, and (c) Nasal septum in Ornithorhynchus. Loc. cit.
Whipple, I. L. Ventral surface of Mammalian Chiridium, etc. Zeitschr. f. Morph. u.
Anthropol. 1904.

Cf. also Krause, W., in Hertwig's Handbuch d. vergl. u. exp. Entw.-lehre.
Jena, 1906.

B. EXOSKELETOX.

Agassiz, L. Poissons fossiles. Neuchatel, 1833-43.

Baur, G. Osteol. Notizen iib. fossile Keptilien. Zool. Anz. 1886.

Bienz, A. Dermatemys Mavii, eine osteol. Studie mit Beitr. z. Kenntnis vom Bau der

Schildkroten. Pwevue Suisse de Zool. III. Bd. 1895.
Burckhardt, R. In Hertwig's Handbuch d. vergl. u. exp. Entw.-lehre. Jena, 1906.
Collinge, W. E. On the presence of scales in Polyodon. Journ. Anat. and Physiol.

Vol. XXIX.
Credner, H. (See p. 502.)
Dollo. Numerous papers on fossil Reptiles in the Bull. Musee roy. d'hist. nat. de

Belgique from 1882 onwards (e.g. " Troisieme note sur les Dinosauriens de

Bernissart." 1883).
Fraas, O. Aetosaurus ferratus. Stuttgart, 1877.
Fritsch, A. Reptilieu und Fische der bohmischen Kreideformation. Prag, 1878; Fauna

der G'dskohle und derKalksteiae der Permformation Bohmens. Prag, 1879 — 85.
Goldi, E. Kopfskelet und Sshultergiirtel von Loricaria, Balistes, und Acipenser. Jen.

Zeitschr. 1884.
Gotte, A. Entw. d. Carapax d. Schildkroten. Zeitschr. f. wiss. Zool. 1899.
Haycraft, J. B. The Dev. of the Carapace of the Chelonia. Trans. Roy. Soc. Ediu.

1891.
Hertwig, O. Bau und Entwick. der Placoidschuppen und Zahne der Selachier. Jen.

Zeitschr. Bd. VIII. ; Hautskelet der Fische. Morph. Jahrb. 1876, 1879, and

1881.
Hofcr, B. Bau und die Eutwick. der Cycloid- und Ctenoid-schuppen. Sitz-ber.

Morphol. und Physiol. Miincheu. 1889.
Klaatsch, H. Morphol. der Fi8.;hsehuppen, &c. Morph. Jahrb. 1890. Herkunft der

Scleroblasten, Morph. Jahrb. 1894 ; Bedeutung der Hautsinnesorgane fiir die

Ausschaltung der Scleroblasten aus dem Ektoderm. Verb. Anat. Ges., 1895.
Markert, T. Flossenstrahlen v. Acanthias. Zool. Jahrb. 1896.
Marsh, O. C. Numerous papers in the American Journ. of Sciences and Arts.
Meyer, H. v. Numerous papers in " Palaeontographica," Stuttgart, e.y. Bd. VI.

Archegosaurus.
Nickerson, W. S. Dev. of Ssales of Lepidosteus. Bull. Mus. Harvard. Vol. XXIV.
Parker, G. H. Correl. Abnormalities in Carapace of Tortoise. Amer. Nat. 1901.
Rumer, F. Panzer d. Gurteltiere. Jen. Zeitschr. Bd. 27, and cf . Anat. Anz. 1893.
Riitimeyer, L. Bau von Schale und Schiidel bei leb. und foss. Schildkroten. Verb.

Ges. Basel, VI, 1.
Traquair, L. H. Silurian Fishes. Trans. Roy. Soc. Edin. 1899.
Tims, H. W. M. Scales in Some Teleost Fish, Q.J.M.S. 1906.
Wiedersheim, R. Anat. der Gymnophionen. Jena, 1879 ; Histologic der Dipnoer-

schuppen. Arch. f. mikr. Auat. 1990.

C. ENDOSKELETON.

1. VERTEBRAL COLUMN.

(a) Amphioxus, Ctclcstomi, and Pisces.

Boeke, J. Entw. d. Chorda dorsalis. Pet. Camp. 1902.

Calberla, E Die Entw. d. Medullarrohres nnd d. Chorda dorsalis d. Teleostier und
Petromyzonten. Morph. Jahrb. 1887.

510 APPENDIX

Cartier, O. Entw.-gtischiclite der Wirbelsilule. Zeitschr. f. vviss. Zool. 1870.

Clans, 0. Herkuuft der die Cliordascheide der Haie begrenzenden ilussereu Elastica.

Sitz.-ber. Ak. Wien. 1894.
Ebuer, Y. v. Ban der Chorda dorsalis der Cyclostomen. • Sitz.-ber. Ak. Wien. 1895 ;

Ueber den feineren Ban der Chorda dorsalis von Myxiue nebst weiteren Benierk.

iiber die Chorda von Ammocoetes. Loc. cit. 1895; 1. Ban der Chorda

dorsalis v. Acipenssr. Loc. cit. 1896; 2. Bau der Chorda dorsalis bei

Amphioxus ; Ueb. die Wirbel der Knochenfische und die Chorda dorsalis der

Fische u. Amphibien. Loc. cit. 1896.
Gegenbaur, C. *Skeletgewebe der Cyclostomen. Jen. Zeitschr. Bd. V. ; Entw. der

Wirbelsaule des Lepidosteus. Loc. cit. Bd. III.
Gotte, A. Vergl. Morj^hologie des Skeletsystems der Wirbeltiere. Arch. f. niikr. Auat,

1878.
Grassi, B. Entv^ick, der Wirbelsaule der Teleostier. Morph. Jahrb. 1882; Lo

Sviluppo della Colonna vert, ne' Pesci ossei. Atti Ace. Lincei. 1882 — 83.
Gadow, H., and Abbott, E. C. On the evolution of the Yert. Col. in Fishes. Phil. Trans.

1895.
Haase, C. Das natural. System d. Elasmobrauchier, etc. Jena, 1879 — 82. Allgeni,

Stanimesgesch. d. "Wirbeltiere. Jena, 1883.
Harrison, G. R. Tails in Man. Bull. Johns Hopkins Hospital, 1901.
Hasse, C. Die fossilen Wirbal. Morph. Jahrb. 1876, 1877, 1878; Entwick. der

Wirbelsaule der Elasmobrauchier, etc. Zeitsshr. f. wiss. Zool. 1892; Entwick.

und Bau der Wirbelsaule der Ganoiden. Loc. cit. 1893 ; Cyclostomen. Loc.

cit. 1893 ; Dipnoi. Loc. cit. Bd. LY.
Joseph, H. Achsenskelet des Amphioxus. Zeitsch. f. wiss. Zool. 1895.
Klaatsch, H. Yergl. Anatomie der Wirbelsaule. Morph. Jahrb. Bd. XIX, XX,

XXII, XXIII.
Kolliker, A. Beziehung der Chorda zur Bildung der Wirbel der Selachier. Yerb. Ges.

Wlirzburg. Bd. X.; Weitere Beobacht. iiber die Wirbel der Selachier. Abh.

Senckenb. Ges. Bd. Y. ; Ende der Wirbelsilule der lebenden Teleostier uiid

einiger Ganoiden. Gratul.-Schrift f. d. Univ. Basel. 1860.
Lvoff, B. Studien iiber die Chorda und die Chordascheide. Bull. Soe. Moscou. 1887 ;

Bau u. Entw. d. Chorda von Amphioxus. Mitth. Zool. Stat. Neapel. 1890;

Bildung der primiiren KeimbUitter und Entstehung der Chorda und des Mesoderms

bei d. Wirbeltieren. Bull. Soc. Moscou. 1894.
Mayer, P. (See under Fins).

Miiller, W. Bau der Chorda dorsalis. Jen. Zeitschr. 1871
Ketzius, G. Hintere Ende des Eiickenmarks bei Amphioxus, Myxine, u. Petroi)iyzun_

Biol. Untersuch. Stockholm. 1895.
Scheel, C. Entw.-gesch. der Teleostierwirbelsjiule. Morph. Jahrb. 1893.
Schmidt, L. Untersuch. z. Kenntnis des Wirbelbaues von Amia. Inaug. -Dissert.

_ Strassburg i/E. 1892.
Schmidt, Y. Schwanzende der Chorda dorsalis bei den Wirbelt:ereu. Anat. Hefte,

Bd. 11.
Schneider, A. Yergl. Anat. u. En tv*^. -ges eh. d. Wirbeltiere. Berlin, 1879.
Studnicka, F. K. Gewebe d. Chorda dorsalis u. Sitz.-ber. Bohm. Ges. 1^97.
Upsow, S. Wirbel d. Teleostien. Bull. Soc. Moscou, 1900.
Wiedersheim, R. Skelet und Nervensystem von Lepidosiren (Protopterus). Morphol.

Studien. Jena. 1880.

{!)) Amphibia, Reptilia, and Aves.

Albrccht, P. Processus odontoides des Atlas bei den urodelen Amphibien. Centralbl,

f. die medic. Wissenschaft. 1878; Proatlas. Zool. Anz., 1880; Rudimeut de

Proatlas sur uu exemplaire de Hatteria. Bull. Mus. roy. d'hist. nat. de Belgique.

1883.
Baur, G. Proatlas einer Schildkrote (Platypeltis). Anat. Anz., 1895; Osteolog.

Notizen iiber Reptilien. Zool. Anz. 1886, 1887 ; Morphogenie der Wirbelsaule

der Amnioteu. Biol. Centralbl. 1886.
Bergfeldt, A. Chordascheiden u. Hypochorda bei Alytes. Anat. Hefte. Arb. Bd. YII.
Blessig, E. Untersuch. ii die Halswirbelsaule der Lacerta. Inaug.-Dissert. Dorpat.

1885.
Claus, C. Beitr. z. vergl. Osteol. der Yertebraten Sitz.-ber. Ak. Wien.' 1876.
Cope, E. D. Extinct Batrachia from the Perm. Form, of Texas. Palseon. Bui. Nr. 29 :

Proc. Amer. Philos. Soc. 1878, 1880, :1886. Pal.-eon. Bui. Nr. 32; Amer. Nat.'

1880, 1882, 1884, 1885, 1886.
Dollo, L. Morphol. de la Colonne vertebrale. Travaux du Laboratoire de Wimereux,

1892.

APPENDIX 511

Field, H. H. Eiitw. d. Wiibelsilule d. Amphibieu, etc. Morph. Jahrb. 1895.
Fraisse, P. Auat. des Pleurodeles Waltlii. Arb. last. Wiirzburg. Bd. V. ; Eigeuthuml.

Structurverhaltuisse ira Schwanz erwachseuer Urodelen. Zool. Auz. 1880.
Gegenbaur, C Vergl. Anatomie der Wirbelsaule der Amphibien u. Reptilieu.

Leipzig. 1862. Becken d. Vogel. Jfen. Zeitschr. Bd. VI.
Gadow, H. Oq the Evolution of the vert, col, of Amphibia and Amuiota Phil.

Trans. 1896.
Gotte, A. Die Zusammensetzung der Wirbelsjiule bei den Reptihen. Zool. Anz. 1894

Wirbelbau bei den Keptilien, etc. Zeitschr. f . wiss. Zool. 1896.
Hasse, C. Anat. und. paliiontol. Ergebnisse. Leipzig, 1878 ; Entwick. der "Wirbelsjiule

von Triton. Zeitschr. f. wiss. Zool. 1892; Entwick. der Wirbelsaule der

ungeschwanzteu Amphibien. Loc. cit.
Hasse, C, and Schwarck. Ytrgl. Anat. d. Wirbelsaule, etc. Hasse, Anat. Studien,

Heft. I.
Hoffmann, C. K. Vergl. Anat. Wirbeltiere. Niederl. Arch, f . Zool. Bd. IV.
Howes, G. B., and Swinuertou, G. H. Dev. of the Skeleton of Sphenodon, etc. Tr. Zool.

Soc. 1901.
Marsh, O. C. Various articles on fossil Reptiles and Birds. Amer. Journ. of Sci.

and Arts. Vols. XV.— XXIII.
Mivart, St. G. Axial Skel. of the Urodela. P. Zool. Soc. 1870.
Miiller, E. Abstossuug u. Regeneration des Eidechsenschwanzes. Jahr. Ber. d, Vereius

fiir vaterliind. Naturkunde, Wiirttemburg. Stuttgart. 1896.
Murray, J. A. Vert. col. of certain primitive Urotlela. Anat. Anz. 1897.
Oort, E. D. van. Osteol. d. Vogelschwanzes. luaug.-Dissert. Bern. 1904.
Osawa, G. Anat. d. Hatteria. Arch. f. mikr. Anat. 1898.
Parker, AV. K. On the Morphol. of Birds. P. Roy. Soc. 1887.
Peter, K. Die "Wirbelsaule der Gymnophionen. Ber. G?s. Frieburg. Bd. IX., 1894;

Atlas der Amphibien. Anat. Aiiz. 1895.
Reiuhardt, A. Hypochorda v. Salamandra. Morph. Jahrb, 1904.

Ridewood, "W. G. On the dev. of the vert. col. in Pipaand Xenopus. Anat. Anz. 1887.
Schwink, F. Entwick. des mittleren Keimblattes und der Chorda dorsalis der Am-
phibien. Miinchen, 1889.
Siebenrock, F. Rumpfskeletes der Scincoiden, Anguiden und Gerrhosauriden. Annal.

d. K.K. Naturhistor. Hofmuseums. 1895,
Tornier, G. (See under Limb-skeleton.)
Vaillant, Leon. Sur la disposition des vertebres cervicales des Cheloniens. Ann. sci.

nat. zool. No. VII.
Zittel, K. Flugsanrier aus dem lithograph. Schiefer Bayerns. Palaeontographica.

Bd. XXIX.

(c) Mammalia,

Albrecht, P. Epiphysen und Amphiomphalie der Saugetierwirbelkorper. Zool, Anz.

1879; "Wirbelkorpergelenke zwischen dem Epistropheus, Atlas und Occipitale der

Saugetiere. Compt. rend, des iuternat. medic. Congresses. Kopenhagen, 1884 ;

L^eb. die Chorda dorsalis und sieben knocherne "Wirbelcentren im knorpel. Nasen-

septum eines erwachseneu Rindes. Biol. Centralbl. Bd. V.
Bardeen, C. R. Dav. of human skel. • and thoracic vertebrae. Amer. Jom-u. Auat.

1905.
Cornet, J. Pro- Atlas des Manmiiferes et de Hatteria. Bull. Ac. Belgique. 3me serie.

t. XV. 1888.
Cunningham, D, J. The Lumbar Curve in Man and the Apes. Tr. Irish Ac. 1886.
Ebner, V. v. Urwh-bel vind Umgliederung der "Wirbelsaule. Sitz.-ber. Ak. "Wien.

1888 ; Beziehungen der "Wirbel zu den Urwirbeln. Loc. cit. 1892.
Ecker, A. Steisshaar wirbel. Steissbeinglatze und Steissbeingriibchen, etc. Arch. f.

Anthropologic. Bd. XII.
Fol, H. Sur la queue de I'embryon humain. Comptes rendus. 1885.
Froriep, A. Z u- Entwick. -gesch. der "Wirbelsaule, insbesondere des Atlas und Epis-
tropheus und der Occipital-Region. Arch. f. Anat. u. Physiol. 1886.
Gerlach, L. Schwanzbildung bei einem menschl. Embryo. Morph. Jahrb. Bd. VI.
Gi'ix, E. Halswirbelsiiule d. Ungulateu. Inaug. -Dissert. Bern. 1900.
Ha.sse, C. und Schwarck. Studien zur vergl. Anat. der Wirbelsaule, etc. Hasse, Anat.

Studieu. Heft I.
Keibel, F. Entwicklungsgesch. der Chorda bei Siiugern. Arch, f . Anat. 1889 ; Schwanz

des menschl. Embryos. Arch. Anat. 1891. (See »dso Anat. Anz. 1891.)
Kohlbriigge, J. H. F. Schwanzbildung u. Steissdriise d, Menschen, etc. Natuurkund.

Tijdschr. v. Ned. Indie. 1897.
Kolliker, A. Cliordahohle und Bildung der Chorda beim Kaninchen. Sitz.-ber, Ges.

AViirzburir. 1883.

512 APPENDIX

Leboucq, H, Disparitiou de la corde. dorsale chez les Vertebres superieurs. Arch,

Biol. Vol. I. 1880.
Keichenbach, Stromer von E. Wirbel d. Landraubtiere. Zoologica. 1902.
Kodackner, G. Sangetierschwanz, etc. Inaug.-Dissert. Freiburg. 1898.
Rosenberg, C. Entwick. der Wirbelsiiule iind das Centrale Carpi des Meusclien.

Morph. Jahrb. 1876 ; Wirbelsaule der Myrmecophaga, Festschrift fiir C.

Gegenbaur. Leipzig. 1896.
Steinbach, E. Zahl der Caudalwirbel beira Menscheu. Inaug.-Dissert. Berlin. 1889,
Waldeyer, W. Die Kaudalanhilnge des Menschen, Sitz.-ber. Ak. Berlin. 1896.
AVeiss, A. Entw, d. Wirbelsaule d. Ratte, etc. Zeitschr. f, \viss. Zool. 1901.

Cf. also Schauinsland, H, in Hertwig's Handbuch d. vergl. u. exp. Entw.-gesch.
Jena. 1906).

2. RIBS AND STERNUM.

Baur, G. On the Morphol. of Ribs, Amer. Nat. 1887, and Journ. Morphol. 1889

Rippen mid rippentihnliche Gebilde. Anat. Anz. 1893.
Blanchard, R. La septieme cote cervicale de I'homme. Revue scieutif. 1885.
Bridge, T. W. Ribs in Polyodon. P. Zool. Soc. 1897.
Brush-Clinten, E. Cervical ribs. Bull. Johns Hopkins Hospital. 1901.
(laus, C. Vergl. Osteol. der Vertebraten. Sitz.-ber. Ak. Wien, 1876.
Dollo, L. Morphologic des Cotes. Bull. sci. France-Belg. 1892.
Eggeling, H. Papers on Morphol. of Sternum in Anat. Anz., 1903 and 1906, and Jen.

Denkschr. Bd. XL
Ehlers, E. Zoolog. Miscellen. I. Processus xiphoideus und seine Musculatur von

Manis. Abh. Ges. Gottingen. 1894.
Fick, A. E. Zur Entw.-gesch. der Rippen und Querfortsatze. Arch. f. Anat. und

Physiol. 1879.
Gegenbaur, C. Ueb. die epistern. Skelettheile und ihr Voi-kommeu bei den Sauge-

tieren und beim Menschen. Jen. Zeitschr. Bd. I.
Goppert, E, Amphibienrippen. Zool. Jahrb. 1895 ; Uutersuch. zur Morpholog. d.

Fischrippen. Loc.cit. Bd. XXIII. ; Morphol. der Amphibienrippen. Festschr.

fiir C. Gegenbaur. Leipzig, 1896; cf. also Morpli. Jahrb., 1897, and Verb.

deutsch. Zool. Ges. 1898.
Gotte, A. Vergl. Morphol. des Skeletsysbems der AVirbeltiere. Arch. f. niikr. Anat.

Bd. XIV. and XV.
Hasse, C, und Born, G. Morphol. der Rippen. Zool. Anz. 1879.
Haswell, AV. A. Elasmobranch Skeleton. P. Linn. Soc. N.S.W. 1884.
Hatschek. Rippen der Wirbelticre. Verh. Anat. Ges. Jena. 1889.
Hoffmann, C. K. (See p. 511.)
Howes, G. B. The Morphol. of the Sternum. Reprinted, with a Correction, from

" Nature," Vol. 43.
Leboucq, H. Rech. sar les variations anat. de la premiere cote, chez I'homme. Mtni.

couronnes et Mem. des savants etrangers, publics par I'Acad. royale des Sciences,

etc.,de Belgique. 1896.
Lindsay, B. On the avian Sternum. P. Zool. Soc. 1885.
Markowsky, J. Ossif. d. menschl. Brustbeins, etc. Polu. Arch. f. biol. u. med.

AVissensch. 1902. Asym. Bau d. Brustbeins. Loc. cit., 1905. (Cf. Anat.

Anz. 1905.)
Miiller, A. Vergl. Anat. der Wii-belsiiule. Midler's Archiv. 1853.
Parker, W. K. Structure and dev. of the bhoulder-girdle and sternum. Ray Soc.

1867.
Parker, T. J. Origin of the Sternum. Tr. N.Z. Inst. 1890; Presence of a Sternum in

Notidanus indicus. " Nature," Vol. 43, 1890-91.
Pilling, E. Halsrippen des Menschen. Inaug.-Dissert. Rostock. 1894.
Rabl, C. Theorie des Mesoderms. Morph, Jahrb. 1892.

Rathke, H. Bau und Entwick. des Brustbeins der Saurier. Konigsberg. 1853.
Ruge, G. Entwicklungsvorgange am Brustbeine und an der Sternoclavicular verbindung

des Menschen. Morph. Jahrb. 1880.
Schoene, G. Befest. d. Rippen a. d. "Wirbelsaule. Morph. Jahrb. 1902.
White, P. J. A Sternum in Hexanchus griseus. Anat. Anz. 1895.
Wiedersheim, R. (See under Limb-skeleton.)

3. SKULL.

(rt) Cfclostomi and Pisces.

Ahlbom, F. Segmentation des Wirbeltierkorpers. Zeitschr. f. wiss. Zool. 1884.
Allis, E. P., Jr. Papers on Skull, muscles, and nerves of (a) Amia, Journ. Morphol.

1897, 1898, and Anat., Anz., 1899. {h) Polypterus, Anat. Anz. 1900. (r) Scomber,

Journ. Morphol. 1903.

APPENDIX 513

Ayers, H., and Jacksou, S. M. ^el. and Muscles of Myxinoidei. Journ. Morphol.

1901.
Baur, G. MorphoUand Origin of the Ichihyopterygia. Amer. Nat. 1887.
Boeke, J. Entw.-gesch. d. Teleostier. Pet. Camp. 1904.
Braus, H. Entw. d. Muskulatur u. d. periph. Nervensy stems d. Selachier. Morpli. Jahrb.

1899.
Bridge, T. "W. Skull of Osteoglos-sum formosum. P. Zool. Soc. 1895 ; Lepidosiren,

etc. Tr. Linn. Soc. 1898.
Cole, F.J. Anat. of Skeleton of My xine. Trans. Roy. Soc. Edin. 1905.
Dolirn, A. Studien zur Urgeschichte des Wirbeltierkorpers. Mitth. Zool. Stat.

Neapel. Bd. III., Y., YI., IX., 1890. XY., 1904. Losung des Wirleltieikopf-

Problems. Auat. Anz. 1890.
Dollo, L. Le Champsosaure, a la vie fluviatile. Bull. Soc. beige de Geol., etc. 1891.

(See also numerous papers in previous vols., and in Bull. Mus. Eoy. Hist. Nat.

Belg. und Bull, scient. Giard.)
Foote, E. The Extra-branchial cartilages of Elasmobranchs. Anat. Anz. 1897.
Froriep, A. Ziu* Kopffrage. Anat. Anz. 1902.
Fiirbringer, K. Yisceralskelett d. Selachier. Morph. Jahrb. 1903. Morph. d.

Skelets d. Dipnoer. Semon's Forschungsreisen. 1904.
Gadow, H. Modifications of the first and seix)nd visceral arches with especial ref. to

the homologies of the auditory ossicles. Phil. Tran. 1888.
Gegenbaur, C. Yergl. Anat. d. Wirbeltiere. Kopskelet der Selachier. Leipzig, 1872 ;

Kopfskelet von Alecocephalus. Festgabe des Morph. .Jahrb. Leipzig, 1878 ;

Occipitalregion und die ihr benachbarten Wirbel der Fische. Festschrift zu A.

V. KoUikers 70. Geburtstag. Leipzig, 1887. Metamerie des Kopfes und die

Wirbeltheorie des Kopfskelets. Morph. Jahrb. 1888.
Gaupp, E. Numerous papers on the Skull, in Anat. Hefte, Ei^ebn., 1897 and 1904; Auat.

Anz. 1900, 1903 (Teleosts), and 1905. (Cf . also Entw. d. Kopfskeletts in Hertwig's

Handbuch d. vtrgl. u. exp. Entw.-gesch., 1906, with Bibliography.)
Haswell, AV. A. (See p. 512.)

Hatsehek. Metamerie des Amphioxus u. des Ammocoetes. Yerh. Anat. Ges. 1892.
Hoffmann, C. K. Zur Entwick.-gesch. des Selachierkopfes, et:. Anat. Anz. 1894.

Morph. Jahrb., 1896 and 1897.
Huxley, T. H. Craniofacial apparatus of Petromyzon. Journ. Anat. and Physiol.

Yol. X.
Jaquet, M. L'Anat. et I'histol. du Silurus. Arch. d. S^i. Med. Bukarest. 1898.

Chihijera, Callorhynchus, Spiuax, Protopterus, Ceratodus, and Axolotl. Loc.

cit. 1897 ; Squel. ceph. d'une " Carpe Dauphin." Bull. Soc. Sci. de Bucarest,

Koumanie. 1902.
Johnston, .J. B. Morph. of vert. Head, etc. Journ. Comp. Neurol, and Psychol. 1905.
Klein, v. Schadel der Knochenfische. Jahresb. des Yereins f . vaterland. Naturkunde

in AViirttemburg. 1884—1886.
Killian, G. Metamerie des Selachierkopfes. Yerh. Anat. Ges. 1891.
Kiipffer, C. Yergl. Entw.-gesch. d. Kranioten. I. Entwick. des Kopfes von Acipenser.

Miinchen u. Leipzig, 1893. II. Entwick. des Kopfes von Petromyzon, 1894.

III. Entwick. der Kopfnerven von Bdellostoma, 1900 ; von Petromyzon planeri.

1895. Entwicklungsgesch. des Kopfes. Anat. Hefte, Ergebn. 1895 ; Entw.

d. Kicmenskeletts von Ammococetes, etc. Yerh. Anat. Ges. 1895 ; Kopfentwick.

v.^ Bdellostoma. Sitz. b?r. Morphol. Physiol., Miinchen. 1899.
Lopy, \V. A. Structure and Dev. of the vertebrate Head. Journ. Morphol. 1895.
Marshall, A. Milnes. Segmental value of the cranial nerves. Journ. Anat. and

Physiol, Yol. XYI. Head-Cavities and Associated Nerves in Elasmobranchs.

Q.J.M.S. Yol. XXI.
Neal, H. Y. Seg. of Nervous Syst. in Squalus, etc. Bull. Mus. Harvard. 1898.
Parker, W. K. Skull in Sharks and Skates. Tr. Zool. Soc. Yol. X. ; Skeleton of

the Marsipobrauch Fishes. I., The Myxinoids. II., Petromyzon. Phil. Trans.

1883 ; Skull in the Sturgeons. Phil. Trans. 1882 ; Skull in Lepidosteus

osseus. Phil. Trans. 1882 ; Skull in the Salmon. Phil. Trans. 1873.
Parker, \V. K. and Bettany, G. T. The Morphology of the Skull. London, 1877.
Piatt, J. Morphology of the Yertebrate Head. Journ. Morphol., 1891, and Anat. Anz.,

1891.
Pollard, H. B. Suspension of the Jaws in Fish. Anat. Anz., 1894; Oral Cirri of

Siluroids and the Origin of the Head in Yertebrates. Zool. Jarhb., 1895.
Pouchet, D. Dm dev. du squelette des poissons os.seux. .Journ. de I'Anat. et Physiol.

1878.
Rabl, C. Metamerie des Wirbeltierkoj)fes. Yerh. Anat. Ges., 1892.
Ridewood, "W. G. Spiracle and associated Structui-es in Elasmobranchs. Anat. Anz.,

1896.
Rosenberg, E. Occipitalregion des Cranium und prox. Teil der Wirbelsiiide eiuiger

L L

514 APPENDIX

Selachier. Eiue Festscrift. Dorpat, 1884 ; Kopfskelet einiger Selachier. Sitz.
ber. der Dorpater Naturforsch. Gesellsch. 1886.

Sagpmehl, M. Vergl. Anat.der Fische, Morph. Jahrb. 1884,1885,1891.

Schaffer, J. Knorp. Skelett v. Aramo:*oetes, etc. Z Mtschr. f . wiss. Zool. 1896.

Schleip, W. Eatw. d. Kopfknocheu bei dem Lachs, etc. In aug. -Dissert. Freiburg.
1903.

SewertzoflF, A. Ent wick, der Occipitalregion der niederen Yertebraten, etc. Bull. Soc.
Moscou, 2, 1895; Entw.-gesch. d. Wirbeltierschiidels. Anat. Anz., 1897; Entw.-
gesch. d. Wirbslti3rkopfes. Bull. So3. Moscou, 1898; Entw. d. S.4aehierschadel,
etc. Festschr. z. 70. Geburtstag von C. v. Kupffer, 1899 ; Entw.-gesch. des Cera-
todus. Anat. Anz., 1902.

Stohr, Ph. Eutw.-ge.schichta des Kopfskelets der Teleostier. Festschrift zur Feier des
300-jahr. Bestehens der Universitat Wurzburg. Leipzig. 1882.

Vrolik, J. A. Die Verknocherung und die Knoi-hea des Schiidels der Teleo.stier.
Niederl. Arch. Yol. I.

Walther, J. Entwick. der Deckknochen am Kopfskelet des Heehtes. Jeu. Zeitschr.
XVI. N. F. IX.

White, P. J. Skeletal Elements between the Mandibular and Hyoid Ai-ches in Hexan-
chus and Ltemargus. Anat. Anz. 1905.

Wijhe, J. W. van. Das Yisceralskelet und die Nerven des Kopfes der Ganoiden und
von Ceratodus. Nisderl. Arch., 1882; Die Mefodermsegmente und die Ent-
wick. der Nerven des Selachierkopfes. K. Acad, der Wissensch. zu Amsterdam,
1882; Kopfregion der Cranioten b?im Amphioxus, uebst Bemerk. uber die
Wirb^ltheorie des Schadel.s. Anat. Anz., 1889; Entw. d. Kopfskeletts bei
Selachiern. Comptes rendus du Cougr. internat. de Zool. (Berne), 1904.

AYin.slow, G. M. Chondrocranium in the Ichtliyopsida. Stud. Tuft's Coll., 1898.

Wright, Kamsay R. Skull and Aud. Org. of Hypophthalmu.s. Trans. Roy. S02.
Canada. 1895.

(I>) Amphibia.

Boi'n, G. Nasenhohlen und Thriinennasengang der Amphibieu. Bresl. Habil.-Schrift.

Leipzig, 1877, an 1 Morph. Jarhb. 1870.
Buchs, G. LTrsprung d. Kopf.skeletts bei Necturus. Morph. Jahrb. 1902.
Cope, E. D. Structure and Affinities of the Amphiumidse. Amer. Philos. So3., 1886;

Hyoid and Otic Elements of the Skel. in the Batrachia. Jouru. Morph ol.

1888.
Corning, H. K. Entw.-vorgange am Kopfe d. Anuren. Morph. Jahrb. 1899.
Duges, A. Sur I'osteologie et la myologie des Batraciens. Paris. 1835.
Gi.upp, E. Bildung und Umbildung des Primordialcraniums von Rana. Yerh. Anat.

Ges., 1892 ; B?itr. z. Morphol. des Schadels. I. Primordialcranium und Kiefer-

region von Rana. II. Hyobranchialskelet der Anuren. Morph. Arb. II. and

III. Bd. ; III. Vergl. Anat. der Schliifengegeud, etc. 3Iorph. Arb. 1894.
Hertwig, O. Z ihnsystem der Amphibien, etc. Arch. f. niikr. Anat. 1874.
Huxley, T. H. Structure of the Skull and of the Heart of ]\Ienobranchus lateralis. P.

Zool. Soc. 1874.
King.sley, J. S. Head of Embryo Amphiuma. Anx*r. Nat., 1892.
Parker, W. K. Skull in the Urodelous Amphibia, pt. I. Phil. Trans., 1877 ; Skull in

the Amphibia Urodela. Tr. Linn. Soc. Yol. II. ; Skull in the Urodeles. Tr.

Zool. Soc, 1882 ; Skull of the Common Frog. Phil. Trans., 1871 ; Skull in the

BatrachiH, pts. II. and III. Pliil. Trans., 1881.
Peter, K. Schadel v. Ichthyophis. Morph. Jahrb. 1898.
Piatt, J. B. Dev. of Cart. Skull and Musculatine in Nectuius. Morph. Jahrb.

1898.
Reichert, C. B. Entw.-ge.schichte des Kopfes der nackten Amphibien. Konigsberg.

1838.
Rie.se, H. Anat. des Tylototriton. Zaol. Jahrb. 1891.
Schulze, F. E. Inneren Kiemen der Batrachierlarven, etc. I. II. Ab. Ak. Berlin,

1888 und 1892.
Spemann, H. Entw. d. Tuba Eustachii u. d. Kopf skelett v. Rana. Zool. Jahrb., 1898.
Stohr, Ph. Entw.-geschichte des Urodelenschadels. Wiirzburger Habil.-Schrift, 1879,

and Zeitschr. f. wiss. Zool. Bd. XXXIII. ; Entw.-geschichte des Anurenschadel.s.

Zeitschr. f. wiss. Zool. Bd. XXYl.
AY alter, F. Yisceralskelet und seine Muskulatur bei den cinheimischen Amphibien ur.d

Reptilien. Jen. Zeitschr. Bd. XXI. N. F. XYI.
AYieder.sheim, R. Kopfskelet der Urodeleu. Morph. Jalnb., 1877; Skelet von

Pleurodelts Waltlii. Morph. Studien. Heft 1.
\Yinslow, G. M. Chondrocranium in Ichthyopsida. Stud. Tuft's Coll., 1898.

APPENDIX 515

(c) Eeptilia.

Baur, G. Osteolog. Notizen uber Reptilien. Zool. Anz., 18^6. (See also other pa).ers

in Anat. Anz.)
Benmielen, J. F. von. Phylogenie der Schildkrciten , Compte-rendu d. seances du

troisieme Congres internat. de Zool., Leyde, 1896 ; Schadelbau von Dermochelys

coriacea. Festschrift fUr C. Gegenbaur. Leipzig, 1896.
Broom, E. Dev. of pter.-quad. arch in Lacertilia. Journ. Anat. and Phy&iol., 1903.

Mam. and Rep. vomerine boces. Proc. Linn. Soc., N.S.W., 1902.
Gaupp, E. Beitr. zur Morphol. d. Schiidels. III. Yergl. Anat. der Schlafengegend, etc.

Morph. Arb. 1894; Entw.-gesch. d. Eidechsenschtldel . Ber. Ges. Freiburg, U 97,

and Verb. Anat. Ges., 1898; Chroudrocranium v. Lacerta, etc. Anat. Hefte

Arb., Bd. XV.
Goppert, E. Bedeutixng d. Zuuge f. d. sekuudaren Gaumen u. d. Ductus uaso-

pharyngens. Morph. Jalirb., 1903.
Howes and Swinnerlon. (See p. 511.)

Kingsley, J. S. Bones of the Reptilian lower jaw. Amer. Nat., 1905.
Oppel, A. Vorderkojifsomiten und Kopfhohle von Auguis fragilis. Arch. f. mikr.

Anat. 1890.
Parker, W. K. Skull in the common Snake. Phil. Trans., 1878 ; Skull in the Lacertilia.

Phil. Trans., 1879 ; the Dev. of the Green Tiirtle. The Zoolojjy of the Vovage of

H.M.S. Challenger. Vol. I. ; Skull in the Crocodilia. Tr. Zool. Soc, 1853^; Skull

in the Chameleons. T. Zool. Soc, 1881.
Rabl, C Einige Probleme d. Morphologie. Anat. Alz., 1903.
Schauinslaud, H. (See p. 503.)

Sewertzoff, A. N. Entw.-gesch. v. Ascalobotes. Atat. Anz, 19CH).
Siebenrock, F. Kopfskelet der Scincoiden, Anguiden imd Gerrhosauriden. Annal. d.

K. K. Naturhi.stor. Hofmuseums. Wien, 1892; Schildkrctcn. Sitz.-ber. Ak.,
Wien, 1897, and Ann. K. K. Hofmus. Wien, 1898. (See ako p. 5! 3.)

(rf) AVES.

Lurje, M. Pneumatisation d. Taufceu.schadels. Anat. Hefte, 1906.

Parker, W. K. Skull of (a) the Ostiich tribe. Phil. Trans., 1886; (b) the common

Fowl. Phil. Trans., 1869 ; On (Egithognathous Birds. Parts I. and II. Tr. Zool.

Soc. Vols. IX. and X. ; Skull in the Woodpeckers and Wrynecks. Tr. Lion.

Soc. Vol.1. 1875 ;*Bird's Skull. Part II. Loc.cit. (See also various papers in

Monthly Micros. Journ., 1871-1873.)
Parker, T. J. Skeleton of Notornis mantelli. Trans. N.Y. Inst., 1881. (See also

1885.) On the classification and mutual arrangement of the Dinornithidae. Loc. cit.

Vol. XXV., 1892 ; On the cranial osteology, classification, and phylogeny of the

Dinornithidfe. Trans. Zool. Soc, 1895. (See also p. 504.)
Pycraft, W. P. Osteol. of Birds. P. Zool. Soc, 1888, etc.
Strasser, H. Pneumatisation d. Taubenschiidels. Anat. Anz., 1905.
Suschkin, P. P. Schiidel v. Tinunnculus. Mem. Soc. Imp. Nat. Moscou, 1^99.
Toukoff, W. Entw.-gesch. d. Hiibnersch-idel. Anat. Anz , 1900.

{e) Mammalia

Allen. H. Ethmoid bone in Mammalia. Bnll. Mus. Harvard. Vol. X., 3.

Bardelebeu, C. v. Uuterkiefer, etc. Sitz.-ber. Ak. Berlin, 1905.

Bemmelen, J. F. van. Papers on the Monotreme Skull. K. Ak. d. Weteusch.,

Amsterdam, 1899, 1900, 1901; Zool. Anz., 1900; and Semon's Forschungsreistn,

1901.
Bertelli, D. Condotte mentale medicino, etc. Arch, di Anat. e di Emb., 1903.
Bolk, L. Occipitalregiou d. Primord.-C'raniums beim Meuschen. Pet. Can}p., 1903.
Broom, R. Mammalian preuasal cartilage. Proc. Linn. Soc, N.S.W., 1895.
Decker, F. Primordialschiidel einiger Sciugetiere. Zeitschr. f. wiss. Zool., 1884.
Dubois, E. Pithecanthropus erectus. Batavia, 1894. (See also Anat. Anz., Bd. XII.)
Dursy, E. Entw.-gesch. des Kopfes des Menschen und der hoheren Wirbeltiere.

Tubingen. 1869.
Ficalbi, E. 0.ssificazione delle Capsule periotiche ueH'uomo e negli altri Mammiferi.

Atti d. R. Accaa. Med. di Roma, 188G-87.
Fischer, E. Primordialcrauium von Talpa, etc Anat. Hefte, Arb., 1901 ; Entw.-gesch.

des Affenschiidels. Zeitschr. f. Morphol. u. Anthropol., 1903.
Froriep, A. Wirbeltheorie des Kopfskelets. Anat. Anz., 1887; Oc. Urwirbcl, etc.

Anat. Anz., 1905.
Fuchs, H. Herkunft u. Entw. d. Gehorknochelchen bei Kaninchen-embryonen. Arch.

Anat., 1905.

L L 2

516 APPENDIX

Gaupp, E. Siiuger- u. Menschenschadel. Korresp. Bl. Deutsch. Anthropol. Ges., 1903 ;

Ala temporalis, etc. Auat. Hefte, 1902; Nicht-Homologie d. Unterkiefers i. d.

Wirbeltierreibe. Auat. Anz., 1905.
Hallmann. Vergl. Auat. des Schlafeubeius, 1837.

Hartlaub, C Beitr. z. Keuntn. der Manatus-Arten. Zool Jalarb., 1886.
His, W. Nasen- u. Gaumenbildung beim menscb. Embryo. Abb. K. Sjicbs. Ges. d.

Wissenscb.,1901.
Howes, G. B. Mammaliau hyoid. Brit. Assoc. Adv. Sci., 1895.
Hrdlicka, A. Parietal boue, etc. Bull. Amer. Mus. Nat. Hist., N. York, 1903.
Jacoby, M. Zur Kenutnis des menschl. PrimordiakTauiums. Arch. f. mikr. Auat.,

1894.
Joseph, G. Kopfskelet des Mensaben uud der Wirbeltiere. Breslau, 1873.
Kampen, P. N. van. Tympanalgegeud d. Siiugetierschadel. Morph. Jahrb., 1905.
Kjellberg, K. Eutw.-gesch. d. Kiefergelenks. Morpb. Jahrb., 1904.
Laukester, E. R. Lateral horns of the Giraffe. P. Zool. Soc, 1897.
Mihalkovics, V. von. (See under Olfactory Organ.)
Osborn, H. F. See numerous papers iu Bulletin of the American Museum of Natural

History, etc.
Parker, W. K. Skull in the Pig. Phil. Trans., 1874 ; Skull in the Mammalia. Part II.

Edentata. Phil. Trans., 1884 ; Part III., Insectivora. Loc. cit., 1885.
Parsons, F. G. Joints of Mammals, etc. Jouru. Anat. and Physiol. Vol. 34.
Paulli, S. Pneumatizittit d. Schjidels bei d. SJiugetiere. Morph. Jahrb., 1899.
Kabl, C^ Entw. d. Gesichtes. Leipzig, 1902.
Kiitimeyer, L. Natiirlich. Geschicbte des Rinde/!, etc. Neue Denkschr. d. allg. schweiz.

Gesellsch. f. d. ges. Naturvv., 1866; Die Rinde der Tertiarepoche, etc. Abb.

schweiz. palajon. Gesellsch., 1877 ; Schwein und Hau.srind. Verb. Ges. Basel,

1882; Ge.sch. d. Hirschfamilie. Verli. Ges. Basel, 1877.
Salensky, W. Eatw.-geschich. der kuorpel. Gehorknochelcheu bei Siiugetieren. Morph,

Jahrb., Bd. VI.
Schwalbe, G. Stirnuaht bei d. Primaten. Zeitsch. f. Morph. u. Anthropol., 1904.'
Schwink, F. Zwischenkiefer bei Saugetieren. Miinchen, 1888.
Selenka, E. Menschenaffeu. Wiesbaden, 1898.
Spondli, H. Primordialschjidel der Siiugetiere und des Menschen. Inaug.-Dii>S3rt.

Zurich, 1846.
Stehlin, H. G. Postembr. Schadelmetamorphosen bei Wiederkiiueru. Basel, 1893.
Thyug, F. W. Squamosal bone in Tetrapodous Verts. Stud. Tuft's Coll., 1906.
Toldt, C. Ossicula Mentalia, etc. Sitz.-ber. Ak. AVien, 19013.
Wilson, J. T. Skel. of the Snout iu fcetns of Monotremes. Proc. Linn. Soc, N.S.W.,

1901.

4. PECTORAL ARCH.

Broom, R. Papers on Shoulder Girdle of Marsupials. Journ. Anat. and Physiol., 1897.

Trans. Roy. Soc. Edin., 1889. Journ. Linn. Soc, 1902.
Dohrn, A. Studien zur Urgescli. des Wirbeltierkorpers. VI. Die paarigen vmd un-

paaren Flosseu der Selachier. Mittheil. Zool. Stat. Neapel. 1886.
Duges. L'osteologie et la myologie des Batraciens a leurs differens ages. Mem. Ac.

France. 1835.
Emery, C, et Simoni, L. La ceinture scapulaire des Cypriuoides. Arch. ital. Biol.

1886; Beziehungen des Cheiropterygium zum Ichthyopterygium. Zool. Anz.

1887.
Gegenbaur, C. Vergl. Anat. der Wirbeltiere ; Schultergiirtel, Carpus und Tarsus, und

Brustflosse. Leipzig. 1864-65 ; Clavicula und Cleithrum. Morph. Jahrb. 1895.
Goldi, E. A. Kopfskelet und Schultergiirtel von Loricaria, Balistes, uud Acipenser.

Jena. 188i.
Gotte, A. Brustbein u. Schultergiirtel. Arch. f. mik. Anat. 1877.
Haller, B. Schultergiirtel d. Teleostier. Arch. f. mik. Anat. 1905.
Hoffmann, C. K. Vergl. Auat. der AVirbeltiere. Niederl. Arch., 1879; Morphol. des

Schultirgiirtels imd des Brustbeiues bei Reptilien, Vogeln, Saugetieren und dem

Menschen. Loc. cit.
Howes, G. B. Pectoral fiu-skeleton of the living Batoid fishes and of Squaloraja, etc.

P. Zool. Soc. 1890; Mammalian Coracoid. Jouru. Anat. and Physiol. 1887;

Coracoid of the Terrestrial Vertebrata. P. Zool. Soc. 1893.
Parker, W. K. Structure and development of the Shoulder Girdle and Steruum in the

Vertebrata. Ray Society. 1868.
Sabatier, A. Comparaison des ceintures et des membres ant. et post, dans la Serie des

Vertebres. Montpellier. 1880.
Siebenrock, F. Verbindungsweise des Schultergiirtel s mit. d. Schiidel bei d. Teleostieiu.

Anuales d. K. K. Naturhist. Hofmuseums, Wien, 1901.

APPENDIX 517

SwinnertoD, H. H. Pectoral skel. of Teleosteans. Q.J.M.S. 1906.

8wirski, G. Entwick. des Schultergiirtels unci des Skelefcs der Brustflosse des Hechtes.

Inaug.-Dissert. Dorpat. 1880.
Szily, A. V. Histogen. Untersuch. (Ectodermal origin of bones). Anat. Hefte. 1907.
Wiedersheim, R. Gliedmassenskelet der Wirbeltiere mifc besonderer Beriicksicht.

des Schulter- und Beckengiirtels bei Fischen, Amphibien und Reptilien. Jena,

1892. (See also pp. 503 and 510.)

5. PELVIC ARCH.

Baur, G. Pelvis of the Testudinata, etc. Journ. Morphol. 1891.

Bolk, L. Beziehungen zwischen Skelet, Musculatur und Nerven der Extremitaten.
Morph. Jahrb. 1894.

Bunge, A. Entwick. des Beckengiirtels der Amphibien, Reptilien und Yogel. Inaug.-
Dissert. Dorpat. 1880.

Dohrn, A. (See p. 516.)

Gegenbaur, C. Ausscliluss des Schambeines von der Pfanne des Hiiftgelenkes. Morph.
Jahrb. 1876; Becken der Vogel. Jen. Zeitschr. 1871.

Gorski. Becken der Saurier. Inaug.-Dissert. Dorpat. 1852.

Hoffmann, C. K. Becken der Amphibien und Reptilien. Niederl. Arch. Bd. III.

Howes, G. B. Mammalian Pelvis, with especial Ref . to the young of Orcithorhynchus.
Journ. Anat. and Physiol., Vol. XXVII.

Huxley, T. H. Pelvis in the Mammalia, etc. P. Roy. Soc. 1879.

Johnson, A. On the Dev. of the pelvic Girdle and Skeleton of the Hind-Limb in the
Chick. Stud. Morphol. Lab. Cam. 1884.

Leche, W. Pfannenknochen (Os acetabuli). Int. Monatsschr. Anat. 1884; Morphol.
der Beutelknochen. Verb, d Biolog. Vereins zu Stockholm. 1891, Cf.
numerous articles on Fossil Reptiles in Amer. .J. Sci. (and see p. 504).

Lubsen, N. J- Sclerozonentheorie. Pet. Camp. Dl. II.,Afl. 1.

Mehnert, E. Entwick. des Os pelvis der Vogel. Morph. Jahrb. 1888 ; Entwick. des
Beckengiirtels bei einigen Saugetieren. Loc. cit. 1889 ; Entwick. des Becken-
giirtels der Emys. Loc. cit. 1790; Entwick. des Os hypoischium (Os cloacae),
Os epipubis und Lig. medianum pelvis bei den Eidechsen. Loc. cit. 1891.

0.sborn, H. F. Evidence for a Common Dinosaur-Avian Stem in the Permian. Amer.
Nat., Vol. XXXIV.

Sabatier, M. A. Compar. d. ceintures thoraciques et pelv. dans la Serie des Vertebres.
Acad. Sci. et Lettr. de Montp?llier. 1880.

Wiedersheim, R. Becken der Fische. Morph. Jahrb. 1881 ; Phylogenie der Beutel-
knochen. Zeitschr. f. wiss. Zool. 1892. (And see above.)

Woodward, A. S. Palaeontol. Contribs. to Selachian Morphology. P. Zool. Soc. 1888.

6. FINS AND LIMBS (free portion).

Balfour, F. M. Dev. of the skel. of the paired fins of Elasmobranchs, etc. P. Zool. Soc.

1881. (Cf. also p. 499.)
Bardeleben, K, Os intermedium tarsi der Saugetiere. Zool. Anz. 1883; Morphol. des

Hand- und Fuss-skelets. Sitz.-ber. d. Jen. Gesellsch. Medic, u. Naturwissensch.

1885; Neue Bestandtheile der Hand- und Fusswurzel der Saugetiere, sowie das

Vorkommen von Rud. " iiberzahliger " Finger und Zehen beim Menschen. Jen.

Zeitschr. 1886 ; Bones and Muscles of the Mammalian Hand and Foot. P. Zool.

Soc. 1894 ; Hand und Fuss. Verb. Anat. Ges. 1894.
Baur, G. Tarsus der Vogel und Dinosaurier. Morph. Jahrb. 1883 ; Archipterygium

und Entwick. des Cheiropterygium. Zool. Anz. 1885 ; Carpus und Tarsus der

Reptilien. Loc. cit. 1885 ; Astragalus und das Intermedium tarsi der Saugetiere.

Moiph. Jahrb. 1885 ; Carpus und Tarsus der Wirbeltiere. Zool. Anz. 1885 ;

Der iilteste Tarsus ( Archegosaurus) . Loc. cit. 1886; Die zwei Centralia im

Carpus von Sphenodon und die Wirbel von Sphenodon und Gecko. Zool. Anz.

1886 ; Kanale im Humerus der Amnioten. Morph. Jahrb. 1887 ; Morphol. des

Carpus und Tarsus der Vertebrateu. I. Theil : Batrachia. Jena, 1888; Morphol.

des Carpus der Saugetiere. Anat. Anz. 1889.
Bemmelen, Van. Herkunft der Extremitaten und Zungenmuskulatur bei den Eidechsen.

Anat. Anz. 1889.
Boa<i, J. E. V, a) Metatarsus der Wiederkiiuer. b) Ein Fall von vollstandiger Ausbil-

dung des 2 und 5 Metacarpale beim Rind. Morph. Jahrb. 1890.
Bolk, L. Segmentaldifferenzierung d. mensch. Rumpfes, etc. Morph. Jahrb. 1898.
Born, G Sechste Zehe der Anuren. Morph. Jahrb. 1876; Anlage der vorderen Extre-

mitiit bei Embryonen von Anguis. Zool. Anz. 1883 ; Skelet des Fer.senh6ckers von

Rana, etc. Sitz. d. Schles. Geseilsch. f. vaterland. Kultur. 1899; Carpus und

518 APPENDIX

Tarsus der Saurier. Morph. Jahrb. 1876 ; Naclitrlige zum Carpus und Tarsus.

Morph. Jahrb. 1880.
Boyer, E. JR.. The mesoderm in Teleosts, etc. Bull. Mus. Harvard. 1892.
Brandt, E. Griffelbeine (Ossa calamiformia) der Wiederkiiuer. Zool. Anz. 1888.
Braus, H. Papers on the Extremities of Selachians, Ceratodus, etc. Vcrh. Anat. Ges.

189S; Simon's Fors^hung.sreisen, 1900; Verh. Ges. Wiirzburg, 1901. Jen.

Denkschr. XI. Festschrift Haeckel, 1904. (Cf. also Hertwig's Handbuch d. vergl.

u. exp. Entw.-lehre. Jena, 1906.)
Bridge, T. W. Mesial fins of Ganoids and Teleosts. Journ. Linn. Soc. 1896.
Bunge, A. Nachweisbaikeit eines biserialen Archipterygiums bei Selachieru und Dip-

nosrn. Jen. Zeitschr. Bd. VIII.
Carlsson, A. Weichen Telle des sogeu. Praspollex und Praehallux. Biolog. Forenin-

geus. Forhandliugar (Yerh. d. Biolog. Vereins), Stockholm, 1890; Weichen Telle

der sog. ilberziihligen Strahleu an Hand und Fuss. K. Svenska. Vet.-Akad.

Handlingar, Stockholm, 1891.
Cope, E. D. Palaeozoic and Mesozoic Fishes. Journ. Acad. Nat. Sci. Philad. Vol. IX.
Cuenod, A. L'articulation du coude. lut. Monatsschr. Anat. 1888.
Davidoff, M. v. Vergl. Anat. der hinteren Gliedmassen der Fische. Morph. Jahrb.

Bd. V.,VI.,IX.
Dean, B. The Fin-fold origin of the paired limbs in the light of the Ptychopterygia

of Palaeozoic Sharks. Anat. Anz. 1896.
Doderleiu, L. Skelet von Pleuracanthus. Zool. Anz. 1889.
Dohrn, A. (Seep. 516.)

Dollo, L. Nageoire caudale des Ichthyosaures. Bull. Soc. beige Geol. 1892.
Ducret, E. Developpement des membres pairs et impairs des poissons teleosteens.

Inaug. -Dissert Lausanne. 1894.
Eisler, P. Homologie der Extremitaten. Abh. Ges. Halle. 1895.
Emery, C. Beziehungen des Cheiropterygiums zutn Ichthyopterygium. Zool. Anz.,

1887; Hand- und Fuss-skelets. Anat. Anz., 1890; S-'-heletro della mano degli

Anfibi Anuri. Atti Aec. Lincei, 1892 ; Morfologia dei Membri dei Mammiferi.

Mem. R. Accad. d. Sci. dell'Istit. di Bologna, 1892 ; Morfol. dei Membri degli

Anfibi. Ricerche Lab. Anat. Roma e altri, 18.94 ; Morfol. del Tarso dei Mammiferi,

Atti Ace. Lincei, 1895 ; Hand- u. Fuss-skelets der Marsupialien u. Echidna.

Semon's Forschungsreisen, etc., 1897, 1902; Crossopterygium. Anat. Anz., 1897.
Ewart, J. C. T>^v. of the Skel. of the Limbs of the Horse. Journ. of Comp. Path, and

Therapeutics, 1894. (Cf. also Journ. Anat. and Physiol., 1894.)
Fiebiger, J. Bauchflossen d. Gobii. Anat. Anz., 1905. -
Fischer, E. Carpus u. Tarsus v. Hyrax. Jen. Zeitschr., 1903.
Goodrich, E. S. Dermal Fin-rays of Fishes. Q.J.M.S., 1905; Dev., struct, and origin

of Fins of Fishes. Q.J.M.S., 1903. (Cf. also Loc. cit., 1902.)
Fraas, E. Fund von Ichthyosaurus in Wiirttemberg. Neues Jahrb. f. Mineralog., etc.,

1892.
Fritsch, A. Brustflosse von Xenacanlhus. Zool. Anz., 1888.
Fiirbringer, M. Knochen und Muskeln der Extremitaten bei den schlangeniihu.

Sauriern. Leipzig, 1870 ; Morphol. und Systematik der Vogel. Amsterdam,

1888; Nervencanale im Humerus der Amnioten. Morph. Jahrb. 1886; Morph.

Streitfragen. Loc. cit., 1902.
Gegenbaur, C. Skelet der Gliedmassen der Wirbeltiere, etc. Jen. Zeitschr., 1870;

Skelet der Hintergliedmassen bei den Manncheu der Selachier und der Chimaren.

Loc. cit. ; Drehung des Humerus. Jen. Zeitschr. Bd. IV. ; Vergl. Anat. der

Wirbeltiere. Leipzig, 1864-5; Archipterygium. Jen. Zeitschr., 1872; Glied-
massen der Wirbeltiere, Morph. Jahrb., 1876 ; Polydactylie als Atavismus.

Morph. Jahrb., 1880; Gliedraassenskelet der Enahosaurier. Jen. Zeitschr., 1870;

Polydactylie. Morph. Jahrb., 1888; Das Flossenskelet der Crossopterygier und

das Archipterygium der Fische. IjOc. cit., 1894.
Goodsir. On the Morphol. Constitution of Limbs. Edinburgh New Philos. Journ.,

1857.
Gotte, A. Entwick. und Regeneration des Gliedmassenskelets der Molche. Leipzig,

1879.
Guitel, F. Developpement des Nageoires paires der Cyclopterus. Arch. Zool. exp.

Vol. IV.
Harrison, R. G. Entwick. der nicht knorpelig vorgebildeten Skeletteile in den Flossen

der Teleostier. Arch. f. mikr. Anat., 1893 ; Dev. of the Fins of Teleosts. Johns

Hopkins Univ. Circulars, 1894; Entwick. der unpaaren u. paarigen Flossen der

Teleostier. Arch. f. mikr. Anat., 1895.
Haswell, W. A. (See p. 512.)
Hoffman, C. K. (See p. 511.)
Holl, M. Entwick. der Sfcellung der Gliedmassen des Menscheu. Sitz.-ber. Ak. Wien.

1891.

APPENDIX 519

Howes, G. B. Skel. and Affinities of the Paired Fins of Ceratodus, etc. P. Zool. Soc.
1887 ; Pedal skeleton of the Dorkiug Fowl, with remarks on hexadactylism, and
phalangeal variation. Jouru. Anat. and Physiol. 1892 ; Variation and dev. of
the vertebral and limb skel. of the Amphibia. P. Zool. Soc. 1893 (and see
p. 511).

Howes, G. B., and Davies, A. M. Morphol. and Genesis of Supernumerary Phalanges,
etc. P. Zool. Soc. 1888.

Howes, • G. B., and Ridewood, R. Carpus and Tarsus of the Anura. P. Zool. Soc.
1888.

Humphry. Observations on the limbs of vertebrate animals, etc. 1860.

.Jordan, P. Entwickl. der vorderen Extremitat der Anuren Batrachier. Inaug. -Dissert.
Leipzig. 1888.

.Tungers3n, H. F. E. Structure of the Hand in Pipa and Xenopus. Ann. Nat. Hist.
1891.

Kehrer, G. Carpus und Tarsus der Amphibien, Reptilien und Sixuger. Ber. Ges.
Freiburg. 1886.

Klaat^ch, H. Brustflosse der Crossopterygier. Festschr. fiir C. Gegeubanr. Leipzig.
1896.

KoUmann, J. Handskelet und Hyp?rdactylie. Verb. Anat. Ges. 1888.

Kiikenthal, W. Hand der Cetaceen. Anat. Anz. 1888 (Cf. also Anat. Anz. Jahrg. IV.,
v., and X., and also Vergl.-anat. und Entwickl. -gesch. Untersuch. an
Waltieren. Jena, 1889, und 1893) ; Anpa.ssung von Siiugetierea an das Leben
im Wasser. Zool. .Jahrb. 1890; Carpus des Weisswals. Morph. Jahrb. 1892;
Entwick. des Handskelets des Krokodils. Loc. cit.

Lazarus, S. P. Morphol. des Fuss-skelets. Morph. Jahrb. 1896.

Leboucq, H. Morph. de Vespertilio. Bruxelles. 1899. Orgauogeni?d. Pinnipedes. Exp.
Antarct. beige. Anvers. 1904 ; Dev. du premier metatarsien et de son articulation
tarsieune chez I'homme. Extr. d'aunal. de la soc. de Med. de Gand. 1882;
Morphol. du carpe chez les mammifens. Bull, de I'Acad. r. de med. de
Belgique: 3. ser. t. XVIII. ; Morphol. du carpe chez les mammiferes. Arch,
de Biol. 1884; Morphol. du carpe et du tarse. Anat. Anz. 1886; De I'os
central du carpe chez les mammiferes. Bull Ac. Belgique. 1882 ; L^i nageoire
pectorale des Cetaces. Anat. Anz. 1887 ; L'Apophyse styloide du 3e Metacarpien
chez rhomme. Annal. de la So3. de Med. de Gand. 1887; Morphol. de la main
chez les Pinnipedes. Studies from the Mus. of Zool. in the Univ. Coll. Dundee.
1888 (Cf. also Anat. Anz. 1888) ; Morphol. de la main chez les Mammiferes
marins (des Cetaces). Arch, de Biol. 1889.

Leighton, V. L. Development of the wing of Sterna wilsouii. Amer. Nat.
1894.

Leuthardt, F. Reduction der Fingerzahl bei Ungulaten. Inaug. -Dissert. Jena, 1890.

Levdig, F. Bui der Zehen bei Batrachiern und die Bedeutung des Fersenhockers.
Morph. Jahrb. 1876.

Markert, F. Die Flossenstrahlen von Acanthias. Zool. Jahrb. 1896.

Mirsh, O. C Various papers on Fossil Reptiles in Amer, .J. Sci. and Arts, Vol.
XVI.— XXIII. The following are of special interest : — 1) Limbs of Sauranodon
(Vol. XIX.); 2) Wings of Pterodactyles (Vol. XXIII.); 3) Polydactyle horses
( Vol. XVII.) ; 4) Affinities and classific. of the Dinosaurian Reptiles (Vol. L.) ;
5) Restoration of some European Diuosaurs {loc. cit.) ; Recent polydactyle horses.
Amer. J. Sci. 1892.

Martins, Ch. Comparasoa des membres pelviens et thoraciques chez I'homme et chez
les mammiferes. Mem. de I'Acad. d. Montpelli?r. 1857; Ost. comp. des articula-
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Mayer, P. Die unpaaren Flossen der Selachier. Mittheil. Zool. Stat. Xeapel.
1885.

Mehnert, E. Kanogenesis, etc. Morph. Arb. 1897.

Mivart, G. Fins of Elasmobranchs, etc. Tr. Zool. Soc. 1879.

Mollier, S. Entwick. der paarigen Flossen des Stors. Anat. Anz. 1896; Extremitaten
der Wirbeltiere. I. Ichthyopterygium. II. Cheiropterygium. III. Entwickl. der
paarigen Flossen des Stors. Anat. Hefte. Bd. III. 1893. Bd.V. and VII. Entwick.
der fiinfzehigen Extremitat. Sitz. ber. Morph. Physiol. Miinchen. 1894.

Niemiec, J. Les ventouses dans le regne animal. Dissert. Genf. 1885.

Norsa, E. Morfol. dei Membri anteriori degli uccelli : Ricerche Lab. Anat. Roma e
altri Lab. Biologici. Vol. IV.

Parker, W. K. Morphology of Birds. Proc. Roy. Soc. 1887 ; Structure and Dev. of the
Wing in the Common Fowl. Phil. Trans. 1888 ; Opisthocomus. Tr. Zool. Soc.
1S91 (see also p. 504).

Paterson, A. M. The position of the mammalian limb, etc. Stud, in Anat. from the .
Anat. Dept. of Owens College. 1891 ; Fate of the Muscle Plate, and Dev. of the
Spinal Nerves and Limb Plexuses in Birds and Mammals. Q.J.M.S. 1888,

520 APPENDIX

Pee, P. vau. Dev. des Extremites chez Amphiuma u. Necturus. Compt, rend, de

I'Assoc. des Anatomisfces, Liege, 1903, and Anat. Anz., 1903-4.
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Ann. Sci. Nat. 1896.
Pfitzner, W. Die kleino Zehe. Arch, f, Anat. u. Physiol. 1890; Menschl. Carpus.

Verh. Auat. Ges. 1893 ; Doppelbildung der filnften Zehe, etc. Morph. Arb. 1895.
Prentiss, ('. W. Polydactylism in Man and Domest. Animals, etc. Bull. Mus. Harvard.

1903.
Rabl, C. Ursprung d. Extremitaten. Zeitschr. f. wi.ss. Zool. 1901 ; Einige Probleme d.

Morpliol. Anat. Anz. 1903.
Rautenfeld, E. V. Das Skelet der hinteren Gliedmassen von Ganoiden und Teleostiern.

Inaug.-Dissert. Dorpat. 1882.
Rosenberg, E. Entwick. des Extremitatenskelets bei einigen durch Reduct. ihrer Glied-
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Entwickl. der WirbelsJiule und das C'entrale Carpi des Menschen. Morph. Jahrb.

1876; Entwicklnngsstadien des Handskelets der Emys. Morph. Jahrb. 1891.
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Arch, fur Anat. n. Physiol. 1883.
Riige, G. Entw.-Gesch. d. Skelettes d. vord. Ext. v. Spinax. Morph. Jahrb. 1902.
Ryder, J. A. Extra terminal phalanges in the Cetacea. Amer. Nat. 1885.
Salensky, W. Dev, de I'lchthyopterygie des Ganoides et Dipnoi Jes. Ann. Mus. Zool.

Ak. Imp. Sci. St. Petersbourg, 1898.
Schaffer, J. Bau d. Zehen bei Fledermjiusen, etc. Zeitschr, f. wiss. Zool. 1905.
Schlosser, M. Modificationen des Extremitaten-Skelets bei deu einzelnen Sriugetier-

stammen. Biol. Centralbl. Bd. IX,
Schneider, A. Dipnoi. Zool. Beitnige. Bd. II. Breslau, 1887.
Schumanu, A. Hinter-extremitJiten v. Dipus. Morph. Jahrb. 1904.
Semon, R. Entw. d. Flossen d. Ceratodus. Forschungsreisen. Jen. Deuksshr. Bd. IV.

1898. (See also Morphol. Jahrb. 1899.)
Sewertzoff, A. N. Eutw. d. pentadaktylen Ext. d. Wirbeltiere. Anat. Anz. 1904.
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1889; Homologie der Gliedmassen der SJiugetiere imd des Menschen. Biol.

Ceutralbl. 1893. (Cf. also Anat. Hefte, Arb., Bd. VIII., and Biol. Centralbl.

1897.)
Storm, R. Adhesive Disk of Echeneis. Ann. Nat, Hist, 1888.
Strasser, H. Entwick. der Extremitatenkuorpel bei Salamandern und Tiitanen.

Morph. Jahrb. 1879.
Stromer, E. Foramen en tepicondy loideum u. Trochanter tertius d. Saugetiere. Morph.

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Stuckens, M. La ventouse abdominale du Liparis. Bull. Ac. Belgique. 1884.
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Berlin, 1889.
Symington, J. Cetacean. flipper, etc. Journ. Anat. and Physiol. 1906.
Thacher, J. K. Median and Paired Fins, etc. Trans. Connecticut Acad. 1878 ;

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1896; Die " iiberzahligen " Carpuselemente menschl. Embryonen. Anat, Anz,

1894; Entwicklungsgesch. der Sesambeiue der menschl. Hand. Morph. Arb.

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Thilo, O. Umbildungen an den Gliedmassen der Fische. Morph. Jahrb. 1876.
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Journ. Anat. and Physiol. 1886.
Tornier, G. Hyperdaktylie, etc. Arch. Entwick. Mechanik. 1896 ; Schwanzregenera-

tion bei Eidechsen, etc. Sitz.-ber. Naturf. Berlin, 1897; Ueberzahlige

Bildungen, etc. Ver. Internat. Zool. Kongr. Berlin, 1901 ; Exp. Erzeugen

ilberzahl. u. Zwilliugsbildungen. Zool. Anz. 1901 ; Schweinehinterfiiss mit

fiinf Zehen. Arch. Entwick. Mechanik. 1902; Vorderfuss-Hyperdaktylie b?i

C^rvus-Arten. Morph. Jahrb. 1903; Siiugetier-Praehallux, etc. Arch. f.

Naturgesch. 1891 ; Entstehen der Gelenkformen, Arch. f. Entw.-Mechanik

1894-95.
Traquair, R. H. Cladodus. Trans. Geol. Soc. Glasgow, 1897.
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Geneve, 1889.
Vogt, Ch. Verknocherung des Hohlhandbandes und andere Sesambeine der Siiuger, etc.

Inaug.-Dissert. Tilbiugen, 1894.
Wiedersheim, R, Die altesten Formen des Carpus und Tarsus der heutigen Amphibien.

Morphol. Jahrb. Bd, II., 1876, and Bd. III. ; Vermehrung des Os centrale ini

Carpus und Tarsus des Axolotl. Morph, Jahrb, Bd. VI,

APPENDIX 521

Zehnter, L. Entwick. von Cypsehis. Inaug. -Dissert. Bern, 1890.
Zwick, W. Amphibiengliedmassen. Zool. Arb. Tiibingen. Bil. II.

D. MUSCLES.

Albrecht, P. Morphol. des M. omo-hyoideus und der ventralen, inneren interbranch.-

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iind Fussmuskeln der Saugetiere, besonders die des Praepollex (Praehallux) und

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Fischel, A. Entwick. der ventralen Rumpf- und der Extremitjiten-muskulaturd. Vogel

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522 APPENDIX

Gadow, H. Bauchmiiskeln tier Krokodile, Eidechsen und Schildkroten. Morph

Jahrb. 1881 ; Vergl. Anat. der Mnskeln des Beckeiis und der hinteren C41ied-

massen der Ratiten. Jena, 1880 ; Myologi? d. hinteren Extremitjit der Reptilien.

Morph. Jahrb. 1881.
Gegenbaur, C. M. omo-hyoideus uud seine S^hliifselbeinverbindung. Morpli. Jalirb.

1876.
Gehring, K. Achselbogen d. Menschen, et?. Morph. Jahrb. 1903.
Giglio-Tos, E. Diaframnia degli Anfibianuri e quello dei Mammiferi. Atti Ace.

Torino, 1894.
Gossnitz, W. von. Diaphragmafrage. Semon'.s Forschun^reisen. 1901 ; Linkseitige

Zwerchfelldefekb. Jen. Zeits:h. 1903.
Grenacher. Muskulatur der Cjclostomen und Leptoeardier. Zit^chr. f. wiss. Zool.

Bd.XVII.
Haak, K. Muskulatur bei d. Katze, Hasen u. Kaninchen. Inaug.-Di.ssert. Bern,

1903.
Hair, M. Muscular Fibres of the Alligator. Journ. Auat. and Physiol. Vol. II.
Harrison, R. G. Metamerism of the Dorsal and the Ventral Longitudinal Muscles of

Teleosts. Johns Hopkins Univ. Circulars. 1904.
Hatta, S. Relation of met. seg. of Mesoblast in Petromyzon, etc. Annot. Zool. Japan,

1901.
Haughton, M. Muscular Anatomy of the Crocodile. P. Irish Ac, Vol. IX., and Ann.

Nat. Hist., Vol. XVI. ; Muscular Anatomy of the Alligator. Ann. Nat. Hi.st

Vol. I.
Hellen, E. v. d. Anat. d. Zwerchfells. Zeitsch. f. Morphol. u. Anthropol. 1903.
Hepburn, D. Comp. Anat. of the Muscles and Nerves of the Sup. and Inf. Extremities

of the Anthropoid Apes. Journ. Anat. and Physiol. 1892.
Hochstetter, F. Scbeidenwandbildung zwis^hen Pleura- u. Peritonealhohle bei einiger

Saurien. Morph. Jahrb. 1899.
Hogge, A. Muscles du perinea et du diaph. pelv. Ann. Maladies des org. gen. urin.

1904.
Holl, M. Homologie und Phylogenese der Miiskelu des Beckenausganges des Menschen.

Anat. Anz., 1896, and Anat. Heffce, ICrgebn., 1901.
Humphry. Various papers in Journ. Anat. and Physiol., 1873, etc.
Juge, M. Nerves cerebraux et la Musculature ceph. du Silurus. Rev. Suisse de Zool.

1898.
Kaestner, L. Entwick. der Rumpf- und Schwanzmuskulatur bei Wirbeltieren (Selachier,

etc.). Arch. f. Anat. und PJiysiol. 1892; Entwick. der Extremitaten- und

Bauchmuskulatur bei den anuren Amphibien. Arch. Anat. 1893.
Killian, G. Vergl. Anatomie und Entwickhmgsgesch. der Ohrmuskeln. Anat. Anz.

1890.
Kohlbriigge, J. H. F Anat. des Genus Hylobates (Muskelu und Nerven). Zool. Ergebn.

einer Reise in Niederl. Ost-Indien. 1890 ; Muskelu u. periph. Nerven d. Primaten,

etc. Verb. Ak. Amsterdam. 1897 ; Homotypie des Hals u. Rumpfes, etc. Arch.

Anat. 1898.
Lartsshneider, T. Steissbeinmnskeln des Menschen. Denk. Ak. Wien. 1895 ; Vergl.

Anat. des Diaphragma pelvis. Sitz.-ber. Ak. Wien. 1895.
L^che, W. Morphol. der Beckenregion der Insectivora. Morph. Jahrb. 1880; Anat,

der Beckenregion bei Ins?ctivora, etc. K. Schwed. Acad, der Wissensch. 1882.
Le Double, A- F. Variations du Systerae musculaire de I'homme, etc. Paris, 1897.
Livini, F. M. rectus abd. et M. supracostalis. Arch. Ital. Anat. e Embr. 1905.
Macalisfcer, A. Homologies of the flexor muscles of th? vert3brate limbs. Jouvn. Anat.

and Physiol. Vol. II.
Mall, P. Dev. of the human diaphragm. Proc. Assoc. Amer. Anatomists. 1900. (See

also Journ. of Morphol. 1897.)
Marion, G. E. Mandib. and phar. Muscles of Acanthias and Raja. Contrib. Biol. Lab.

Tuft's Coll., No. 43, and Amer. Nat., 1905.
Maurer, F. Aufbau und Entwick. der ventralen Rumpf muskulatur bei den urodeleu

Amphibien, etc. Morph. Jahrb. 1892; Elemente der Rumpfmuskulatur bei

Cyclostomen Tind hoheren Wirbeltieren. Morph. Jahrb. 1894 ; Untersuch. z.

vergl. Muskellehre d. Wirbeltieren. Die Mm. serrati postici d. Siiugetiere, etc.

Jena, 1905. (See also Festschr. f. C. Gegenbaur. 1896. Morph. Jahrb. 1898.

Anat. Hefte Ergebn. 1889 ; and Hertwig's Handbuch d. vergl. u. exp. Eutw.-

lehre. Jena, 1906.)
Michaelis, P. Myologie des Cynocephalus, Simla, u. Troglodytes. Arch. f. Anat. u

Physiol. 1903.
Mivart, St. G. Myology of Menopoma, Menobranchus, and Chamaeleon. P. Zool. Soc.

1869 and 1870.
N^al, H. V. Dsv. of Hypoglossvis Musculature in Petromyzon and Squalus. Arch.

Anz. 1897.

APPENDIX 523

Pirdi, F. Psoas minor, Ilio-psoas, u. Qviadratus lumbonim. Atti Soc. Toscana. 1902.

Patersoa. Fate of the Muscle-plate and dev. of Spinal Nerves, etc. Q.J. M.S. 1888.

Perrin, A. Myologie comjjaree : Menibre posterieur chez uu certain nonibre de
Batraciens et des Sauriens. Bull. 8ci. France, Belg. 1893.

Ravu, E. Eutwick. des Diaphragmas und d?r benachb. Organe bei den "Wirbeltieren.
Arch. f. Anat. und Physiol. 1889. (Cf. also Biol. Centralbl. Bd. VII.)

Raiser, E. Skelettmuskulatur von Hirsch, Reh, S^haf, u. Ziege. Inaug. -Dissert.
Bern, 1903.

Rex, H. Muskulatur der Mundspalte der Affen. Morph. Jahrb, 1887.

Riidinger, N. Muskeln der vorderen E.xtremitjit der \'ogel und Reptilien. Hamburg,
1868.

Huge, G. Entwick.-Vorgange an der Muskulatur des menschl. Fusses. Morph.
Jahrb. 1878 ; Vergl. Anat. der tieferen Muskeln in der Fuss-sohle. Lnc.
cit. ; Extensorengruppe am Unterschenkel und Fuss des Menschen und der
Siiugetiere. Loc. cit. ; Gesichtsmuskulatur der Halbatt'en. Morph. Jalirb. 1885 :
Gesichtsmnskulatur der Priniaten. Leipzig, 1887 ; Ge.sichtsmu.skeln eines
jungen Gorilla. Morph. Jahrb. 1887; Anatomisches uber den Rumpf der
Hylobatiden (Skelet, Muskulatur, Abschnitte des Gefjiss- und Nervensy stems,
serose Hohlen). Zool. Ergebn. eiaer Rei.re in Niederl. Ost-Indien. Leiden,
1890 ; INIetamere Verkiirzung des Kumpfes bei Saugetieren. Der M. rectus
thoraco-abdorainalis der Primaten. Morph. Jahrb. 1892; Ventrale Rumpf-
muskulatur der anuren Amphibien. Morph. Jahrb. 1894 ; Hautmuskulatiu- der
Monotremen und ihre Beziehungen zu dem Marsupial- u. Mammarapparate.
Semon's Zoolog. Fors^hungsreisen in Australien und dem Malayischen
Archipel. Bd. II. .Jenaisehe Denkschr. Y. ; Hautrumpfmuskel d. Saugetiere.

~~ ~ Morph. Jahrb. 1905.

Saar, G. von. Brustmuskelu u. Deltamuskel. Arch. Anat. 1903.

Schneider, A. Vergl. Anat. und Entwicklungsgesch. der Wirbeltiere. Be.lin, 1879.

Seydel, O. Zwischensehnea uud metamere Aufbau des M. obliqnus thoraco-
abdominalis ext. der Saugetiere. Morph. Jahrb. 1892.

Shufeldt, L. W. Myology of the Raven. London, 1890.

Sioli. Baush- und Zwischenrippenmuskulatur der Wirb3lti?re. Inaug.-Di.ssrrt.
Halle, 1875.

Smalian, V. Anat. d. Amphisbseniden. Zeit^ch. f. wiss. Zool. Bd. XLII.

Testut, L. Les anomalies musculaires chez Thomrae, etc. Paris, 1884.

Tiesing, B. Augen-, Kiefer- und Kiemenmuskulatur der Haie u. Rochen. Jen. Zeitschr.
Bd. XXX. N.F. XXIII.

Tobler, L. Achselbogeu d. Menschen, etc. Morph. Jahrb. 1902.

LTskow, N. Eutwick. des Zwerchfells, des Pericardiums und des Coeloms. Arch. f.
mikr. Anat. 1883.

Vetter, B. Vergl. Anat. der Kiemen- und Kiefermuskulatur der Fisjhe. Jen. Zeitschr.
Bd. VIII. und XII. N.F. I. Bd.

Westling, Ch. Echidna. Svenska Vet. Akad. Handl. 1889.

^Vijhe, .J. "NV. van. Mesodermsegmente und Eutwick. der Nerven des Selachierkopfes.
Verb. Ak. Amsterdam. 1883.

"VVilsou, J. T. Myology of Notoryctes typhlops. Trans. Roy. Soc. S. Australia. 1894.

Wiudle, B. C. A. Flexors of the digits of the hand. .loum. Anat. and Physiol. 1889 ;
Pectoral Group of Muscles. Tr. Irish Ac. 1889 ; Muscles of Mammals, etc.
Jouru. Anat. and Physiol. 1898.

Windle, aud Parsons, F. G. Myology of the Terrestrial Carnivora. P. Ziol. Soc. 1897,
1898; Edentata. Loc. cit. 1899 ; Ungnlata. Loc. cit. 1901.

E. ELECTRIC ORGANS.

Babu bin. Pseudoelektrische Oi-gane. Medic. Centr.-Blatt, No. 35; Entwick. der

elekt Organe und Bedeutung der mot. Endplatten. Medi-. Centr.-Blatt. 1870;

Entwick., Ban, und physiol. Verhjiltnisse der elektrischen und pseudo-elektrischen

Organe. Arch. f. Anat. n. Physiol. 1876; Zitterwelse und Mormyrus. Arch. f.

Anat. und Physiol. 1877.
Ballowitz, E. Bau des elekt. Organes von Torpedo, etc. Arch. f. mikr. Anat. 1893 ;

Bau des elekt. Organs des gewohnlichen Rochen. Anat. Hefte Arb. Heft

XXIII. (Bd. VIL, H. 3.)
Ballowitz, R Anat. des Zitteraales (Gymuotus), etc. Arch. f. mik. Anat. 1897; Nerv-

endigungen i.d. elektr. Organ des Malopterurus. Anat. Anz. 1898.
Boll, F. Physiol, von Torpedo. Arch, f Anat. und Physiol. 1873; (1) Siructur der

elektr. Flatten von Torpedo. (2) Structur der elektr. Flatten von Malopterurus.

Arch. f. mikr. Anat. 1874 ; Anat. und Physiol, von Torpedo. Monatsbericht der

Berliner Akad. 1875 ; Elektr. Flatten von Torpedo. Arch. f. Anat. und Physiol.

1876.

524 APPENDIX

Ciaccio, G. V. Intorno all' intima tessitura dell' organo elletrico della torpedine.
Acad, delle scienze dell' istitiito di Bologua. 1874; and in German — Moleschott's
Untersuch. Bd. XI.

Crevatin, F. Sogen. Stabchennetz im elektr. Organ d. Zitterroclien. Anat. Anz.
1898.

Du Bois-Reymond, E. Gesammelte Abhand. zur allgeni. Miiskel- nnd Nerven-pliy.'-ik.
Bd. II. ; Bericht iiber die von Professor G. Fritsch in Aegypten angestellten
neuen Untersuch. an elektr. Fischen. Monatsschr. d. Berl. Akad. 1881.

Ewart, J. C. Electric Organ, of the Skate. Phil. Trans. 1888 and 1892.

Fritsch, G. Fortsetzung der Untersuch. an elektr. Fischen. Erabryol. von Torpedo.
Sitz.-ber. Ak. Berlin. 1883; Die electr. Fische. Abt. I. Malopterurus. Leipzig,
1887; Abt. II. Die Torpedineen. Leipzig, 1890 (see also other papers in Sitz.-
ber. Ak. Berlin, e.c/. Gymnarchus).

Gotch, F. Electromotive Properties of the Electrical Organ of Torpedo. Phil. Trans.
1887, 1888.

Hartmann, R. Elektr. Organe der Fische. Arch. f. Anat. und Physiol. 1861.

Iwanfoff, N. Der mikrosk. Bau des electr. Organs von Torpedo. Bull. Soc. Moscou.
1894 ; Schwanzorgan von Raja. Loc. cit. 1895.

Retzius, G. Endigung d. Nerven im Elektr. Organ v. Raja. Biol. Untersuch. 1898.

Sachs, C. Gymnotus electricus. Arch. f. Anat. u. Physiol. 1877.

Sanctis, L. de. Embryogenie des Organs elect, de la torpille et des organs pseudo-
electriques de la raie. Journ. de Zool. p. Gervais, II.

Sanderson, J. B., and Gotch, F. Electrical Organ of the Skate. Journ. Physiol. 1889.

Schultze, M. Elektr. Organe der Fische. Abb. Ges. Halle. 1858 and 1859.

Schultze, O. Feinere Bau der ele]<tr. Organe d. Fische. Festschr. f. J. Rosenthal.
Leipzig, 1906.

F. NERVOUS SYSTEM.

(rt) CENTRAL NERVOUS SYSTEM.

{(() Cyclostomi and Pisces.

Ahlborn, F. Gehirn der Petromyzonten. Zeitsch. f. wiss. Zool. 1883.

Auerbach, L. Lobi optici der Telostier und die Vierhiigel der hoher organisirten

Gehirne. Morph. Jahrb. 1888.
Beard, T. History of a Transient Nervous Apparatus in certain Ichthyopsida. Zool.

Jahrb. 1896.
Beauregard, H. Encephale et nerfs craniens du Ceratodus forsteri. Journ. de I'anat.

et physiol. Paris, 1881.
Bellonci, J. Ursprung des Nerv. opticus und fein. Bau des Tectum opticura der

Knochenfische. Zeitschr. f. wiss. Zool. 1880; Centrale Endigung des Nervus

opticus bei den Vertebraten. Loc. cit. 1888.
Bethe, A. Allgem. Anat. u. Physiol, d. Nervensystenis. Leipzig, 1903.
Bing, R., and Burckhardt, R. Centralnervensystem v. Ceratodus. Journ. de I'Anat. et

de la Physiol. Paris, 1881.
Boeke, J. Papers i-elating to the infundibulum, hypophysis, etc. Anat. Anz. 1901 and

1902 ; K. Akad. d. Wetensch., Amsterdam. 1902. Pet. Camp. 1904.
Borchert, M. Centralnervensystem von Torpedo. Neurobiol. Arb. Jena, 1903.
Burckhardt, R. Centralnervensystem von Protopterus aunectens. Berlin, 1892 ;

Vergl. Anat. des Vorderhirus bei Fischen. Anat. Anz. 1894 ; Bauplan des

Wirbeltiergehirns. Morph. Arb. 1894.
Busch, W. De Selachiorum et Ganoideorum encephalo. Berlin, 1848.
Calberla, E. Eatwick. des Medullarrohrs und der Chorda dorsalis der Teleostier uud

Petromyzonten. Morph. Jahrb. 1877.
Dendy, A. Ciliated Grooves in Brain of Ammocoete. P. Roy. Soc. 1902.
Dohrn, A. Stud, zur Urgesch. des Wirbeltierkorpers. Mitt. Zool. Stat. Neapel. 1881,

1882,1884,1888,1890,1891.
Ecker, A. Anat. Beschreib. des Gehirns vom karpfenartigen Nilhecht. 1854.
Edinger, L. Vergl. Anatomie des Gehirns. Abh. Senkenb. Ges. 1888-91 ; Bau der

nervosen Centralorgane des Menschen u. der Tiere, 5. Aufl. Leipzig, 1.^96 ;

Herkunft d. Hirnmautels, etc. Berlin. Klin. Wocheuschr. 1905.
Fritsch, G. Feinere Bau des Fischgehirns. Berlin, 1878 ; Einige bemerkenswerthe

Elemente des Centralnervensystems von Lophius. Arch. f. mikr. Anat. 1886.
Froriep, A. Ganglionleiste d. Kopfes u. d. Rumpfes, etc. Arch. Anat. 1901.
Fusari, R. Feinere Anatomie des Gehirns der Teleostier. Internat. Monatsschr. Anat.

1887.
Gierse, A. Gehirn u. Kopf nerven v. Cyclothone. Morph. Jahrb. 1904.

APPENDIX • 525

Gotte, A. Entw.-geschichte der "Wirbeltiere. III. Entwick. des ceutralaerveusystem
der Teleostier. Arch, f . mikr. Anat. 1877 ; Entstehung und Homologieen d.
Hirnanhangs. Zool. Anz. 1883.

Gorouowitsch, X. Gehirn imd Crauialnerven von Acipenser. Morph. Jahrb. 1888.

Gottsche, M. Yergl. Anat. des Gehirns der Grjitenfische. Arch. Anat. und Physiol.
1835.

Gregory, E. H., Jr. Entw.-gesch. d. Knochenfische. Anat. Hefte Arb. 1902.

Haller, B. Centralnervensystem und Riickenmark von Orthagoriscus. Morph. Jahrb.
1891 ; Riickenmark der Telostier. Loc. cit. 1895 ; Hypophyse und Infundi-
bularorgane. Loc. cit. 1897 ; Ban d. Wirbeltiergehims (Salmo u. ScyUium).
Loc. cit. 1898.

Handrick, K. Nervensystem u. Leuchtorgane v. Argyropelecus. Zoologica. Stuttgart,
1901.

Herrick, J. Central gustatory paths in brains of Bony Fishes. Journ. Comp. Neurol,
and Psychol. 1905.

Hill, C. Primary segments of Vert. Head. Anat. Anz. 1899.

His, W. Eroff nungsrede zur YI, Yersammlung der Anat. Gesellsch. zu Wien. 3892.

Hoffmann, C. K. Outogenie der Knochenfische. Yerh. Ak. Amsterdam, 1882, und
Arch. f. mikr. Anat., 1883.

Holm, J. F. Finer Anat. of Nervous syst. of Myxine. Morph. Jahrb. 1901.

Johnston, J. B. Braiu of Acipenser. Bull. Zool. Lab. Ann Arbor Univ., Michigan,
1897, Anat. Anz., 1898, and Zool. Jahrb., 1901; Gehirn u. Cranialnerveu v.
Anamnier. Anat. Hefte Ergebn. 1901. Brain of Petromyzon. Journ. Comp.
Neurol. 1902.

Kingsbury, F. Str. and Morph. of Oblongata in Fishes. Journ. Comp. Neurol. 1897.

Kupffer, C. Entwick. der Knochenfische. Arch. f. mikr. Anat. 1868 ; Die Deutung des
Hirnauhanges. Sitz. Ber. Morph. und Physiol. Miinchen, 1894 ; Entwicklungs-
gesch. des Kopfes bei Acipenser. Loc. cit. 1891 ; Yergl. Entw.-gesch. der
Crauioten. 1. Heft. Kopf von Acipenser. 2. Heft. Kopf von Ammoccetes.
Miinchen und Leipzig. 1893; Hirnanhange. Sitz. Ber. Morph. Physiol. Munchen,
1894.

Lenhossek, M. v. Spinalganglien und Riickenmark von Pristiurusembryonen.
Anat. Anz. 1892. (Cf. also p. 529.)

Locy, W. A. Metameric Segmentation in the Med. Folds and Embryonic Rim. Anat.
Anz. 1894.

Lundborg, H. Entwick. der Hypophysis und des Saccus vasculosus bei Knochenfischen
und Amphibien . Zool. Jahrb. 1894.

Mayer, F. Zentralnervensystem v. Ammoco^tes. Anat, Anz. 1897.

Mayer, P. Studien uber das Gehu-n der Knochenfische. Zeitschr. f. wiss. Zool. 1881.

Miklucho-Maclai, v. Yergl. Neurologic der Wirbeltiere. Das Gehirn der Selachier.
Leipzig, 1870.

Miiller, W. Entwick. und Ban der Hypophysis und des Processus infundibuli
cerebri. Jen. Zeitschr. 1871.

Neal, H. Y. Segmentation of the Nervous System in Squalus. Anat. Anz. 1876.

Neumayer, L. Histol. Unters. iiber den feineren Bau des Centralnervensystems von
Esox. Arch. f. mikr. Anat. 1895. (Cf. also Sitz.-ber. Morphol. Physiol.
Miinchen, 1903.

Osborn, H. F. The Origin of the Corpus callosum, etc. Parts I. and II. Morph. Jahrb.
1888.

Parker, T. J. Notes from the Otago University Museum. Nomenclature of the Brain
and its Cavities. Nature. 1886; Carcharodon rondelettii. P. Zool. Soc. 1887.

Pendaschenko, D. Eatw. d. Mittelhirns d. Knochenfische. Arch. f. mik. Anat. 1901.

Piatt, J, Contrib. to the Morphol. of the Yertebrate Head. Journ. Morph. 1891.

Rabl-Riickhard, H. Die gegenseitigen Yerhiiltnisse der Chorda, Hypophysis, etc., bei
Haifischembryonen, etc. Morph. Jahrb. 1880; Deutung und Entwick. des
Gehirns der Knochenfische. Arch. f. Anat. u. Physiol. 1892 ; Entwick. des
Knochenfischgehirns (Zirbel). Sitz.-ber., Naturf. Berlin. 1882; Deutung dts
Gehirns der Knochenfische. Biol. Centralbl. 1883 ; Grosshim der Knochen-
fische und seine Anhangsgebilde. Arch. f. Anat. u. Physiol. 1883; Gehirn der
Knochenfische. Deutsch. medic. AVochenschrift. Berlin, 1884 ; Onto- und
phylogenetischen Entwick. des Torus longitudinalis im Mittelhirn der Knochen-
fische. Anat. Anz. 1887 ; Lobus olfactorius impar der Selachier. Anat. Anz.
1893; Yorderhirn der Cranioten. Anat. Anz. 1894.

Reichenheim, M. Kenntnis des elekt. Centralorganes von Torpedo. Arch. f. Anat. u.
Physiol. 1873.

Sagemehl, M. Yergl. Anatomie der Fische (Hirnhaute der Knochenfische). Morph.
Jahrb. 1883.

Sargent, P. E. Torus longitudinalis of Teleost brain, etc. Mark Anniv. Yol. 1903 ;
Optic reflex apparatus of Yerts., etc. Bull. Mus. Harvard. 1904.

526 • APPENDIX

Schaper, A. Morphol. vmd histol. Eutwi^klung ties Kleinhirus der Teleostier. Anat.

Anz. 1894; and Morph. Jabrb. 1894; Friihesto Diffeienz.-vorgauge ini

Centralaer^eu system. Arch. Eatwick. Mechanik. 1897; Finer Struct, of

Selachian's Cerebellum. Jouru. Corap. Neurol. 1898; Morphol. d. Kleinhirns.

Verb. Anat. Ges. 1899.
Scott, W. B. Eutw.-gesch. der Petromyzonten. Morph. Jabrb. 1881.
Sedgwick, A. Inadequacy of the Cell Theory, and Early Dev. of Nerves in Elasmo-

branchii. Stud. Moipli. Lab. Camb. 1896.
Serres. Anat. comp. du cerveau dans'les quatres classes des animaux vertebres, Paris,

1821—182(1.
Shipley, A. E. Dev. of Petrorayzon. Q. J.M.S. 1887.

Stannius, H. Ban des Gehirns des Stors. Arch. f. Anat. u. Physiol. 1835, 1846.
Sterzi, G. Meningi spinale dei Pesci. Monit. Zool. Ital. 1899 (cf. also Atti d,

R. Istit. Veneto d. Scienze, 1900-1) ; Regione iufundib. e dell' Ipofisi, etc.

Arch. Ital. di Anat. e di Embr., 1904, and Atti Accad. Sci. Veneto-Trentiuo-

Istriana, 1904.
Stieda, L. Centralnervensystem der Knochenfische. Zsitschr. f. wiss. Zool. Bd.

XVIII. ; Deutung der einzelneu Telle des Fischgehirns. Loc. cit. Bd. XXIII. ;

Bau des Ruckeumarkes der Rocheu nnd der Haie. Loc. cit. Bd. XXIII. (Cf.

also Festschr. z. 70. Geburtstag v. C. v. Kupffer. Jena, 1899.)
Strasser, H. Hiillen d. Gehirns u. Riickenmarkes, etc. Compt. rend, de I'Assoc. d.

Anatomistes. Lyon, 1901.
Studnicka, F. K. Anat. und Entwicklungsgesch. des Vorderhirns der Cranioten. I.

Abt. Sitz.-ber. Bohm. Ges. 1895 ; Die termiuale Partie des Riickenmarkes.

Loc. cit. 1895; Bau d. Ependyms. Anat. Hefte. 1900; Erste Anlage d.

Grosshirnhemispharen. Sit.-ber. Bohm. Ges. 1901 ; Vergl. Histol. u. Histogenese

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Waldschmidt, J. Anat. des Centralnervensystems und des Geruchsorganes von

Polypterus. Anat. Anz. 1887.
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(A) Amphibia.

Burckhardt, R. Hirn und Geruchsorgan von Triton und Ichthyophis. Zeitschr. f . wiss.

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Fish, P. A. Central Nervous System of Desmognathus. Joiu-n. Morphol. 1895.
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1904.
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Brains of Amphibians and Reptiles. Zool. Anz. 1886; Internal Structure of

the Amphibian Brain. Journ. Morphol. 1888.
Piatt, J. Ontog. Differenzirung des Ektoderms bei Necturus. Arch. f. mikr. Anat.

1894.
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APPENDIX 52?

Wlassak, R. Kleiuhira des FrossKes (Histol. Structur, Faserverlauf). Arch. Physiol

1888.
Zimmermauu. Metamerie des "Wirbeltierkopfes. Verb. Anat. Ges. 1891.

(c) Reptilia.

Edinger, L. Yergl. Anat. des Gehirns. Abh. Senckeub. G^s. 1898. (Cf. also p. 524,

and Anat. Anz., 1893.)
Gaupp, E. Anlage der Hypophyse bei Saurien. Arch, f . mikx-. Auat. Bd. 42.
Haller, B. Wirbeltiergehirn (Emys). Morph. Jahrb. 1900.
Herrick, C. L. Brain of the Alligator. Jouru. of the Ciueinn^ti So3. of Nat. Hist.

Vol. XII. (Cf. also papers in Journ. of Comp. Neurology, Tol. I., and in Anat.

Anz., 1891 and 1892.)
HoflFmann, C. H. Eutw.-gesch. der Reptilien. Morph. Jahrb. 1885.
Koppen, M. Anat. des Eidechsengehirns. Morphol. Arb. I. Bd.
Levi, G. Orig. filogtn. della formazione ammonica. Arch. Ital. Anat. Embr. 1904.
Orr. Embryol. of the Lizard. Journ. Morphol. 1887.
Osborn, H. F. (See p. 526.)
Rabl-Riickhard, H. Centralnervensystem des Alligators. Zeitschr. f. wiss. ZoA. Bd.

XXX. ; Fornixrudiment bei Reptilien. Zool. Anz. 1881 ; Gebirn der Riesen-

schlange. Zeitsclir. f. wiss. Zool. 1894.
Retzius, G. Biol. Untersuch. 189S.
Salvi, G. Regioue ipofisaria, etc. Studi Sassaresi, 1901, and Arch. Ital. Anat. Eml-r.

1902.
Steiner, J. Centralnervensystem der griinen Eidechse, nebst weitereu Untersach. iiber

das des Haifisches. Sitz.-ber. Ak. Berlin. 18S6.
Sterzi, A. J. Midolla spinaledei rettili. Atti Soc. Toscana. 1904.
Stieda, L. Centralnervensystems der Amphibien und Reptilien. Zsitschr. f. wiss.

Zool. Bd. XXV.
Straderini, R. Lobi dell' ipofisi nel Gongylus. Arch. Ital. Anat. Embr. 1903.
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Vorderhiru d. Geeko. Anat. Heft^. 1906.

(d) AVES.

Brandis, F. Gebirn der Vogel. Arch, f . mikr. Anat. 1893-95.

Braun, M. Entwick. des AVellenpapageies, etc. "Wiirzburg. 1881, and Verb. Ges.

Wiirzburg. Bd. XV., IV.
Bumm, A. Grossbirn der Vogel. Zeitschr. f . wiss. Zool. 1883.
Edinger, L., and Wallenberg, A. Gebirn d. Taube. Anat. Anz. 1899.
Imhof, G. Lumbalmark b. Vogeln. Arch. f. mikr. A-nat. 1905.
Kamon, K. Entw.-gesch. d. Gehirns d. Hiihncbens. Anat. Hefte. 1903.
Koelliker, A. Nebenkerne im Marke d. Vogel u. Reptilien. Zeitschr. f. wiss. Zool.

1902.
Locy, A. Access, optic vesicles in Chick. Anat. Anz. 1897.
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Stieda, L. Centralnervensystem der Vogel u. Saugetiere. Zeitschr. f. wiss. Zool. Bd.

XIX.
Streeter, G. L. Spinal Cord of Ostrich. Amer. Journ. Auat. 1903.

{e) M.VMMAI.IA.

Bechterew, W. Leitungsbahuen iiu Gehiru u. Riickenmark. II. Aufl. Leipzig, 1898 ;

Bewusstsein u. Hirnlokalisation. Leipzig, 1898.
Beddard, E. E. Brain in the L^miu-s (also Dendrolagus and Gulo). P. Zool. Soc.

1895.
Benham, AV. B. Cerebral Convolutions of the Chimpanzee kuown as " Sally," etc.

Q.J.M.S. 1894.
Bertelli, D. II solco intermediario ant. del midoUo spinale umano. Atti Soc. Toscana.

1890.
BiscbofF, Th. Grosshirnwindungen des Menschen. Miinchen, 1868. (Cf. also papers

on the Brain of the Chimpanzee, Orang and Gorilla in Sitz.-ber. Miinchener Acad.

1874, 1876 and 1877.;
Bolk, L. Beitr. z. Affenanat. (Gebirn). Pet. C^aujp., 1901, and Morphol. Jahrb., 1902;

Cerebellum d. Saugetiere. Pet. C.imp., 1904-5, and Monatsschr. f. Psych, u.

Neurologic, 1902.
Bradhy, C. Mam. Cerebellar fissures. Journ. Anat. and Physiol. Vols. XXXVII. and

XXXVIII. ; Dev. of hind-brain of Pig. Loc. cit. Vol. XL.

528 APPENDIX

Brauu, M. Entw.-Vorgange am Scliwanzende bei eiuigeu SJiugetiereu, etc. Arch. f.

Auat. u. Physiol. 1882.
Broca, P. Le cerveau du Gorille. Eevue d'Anthropologie. 1878 ; Auat. comp. des

circonvolutions cerebrale.s. Loc. cit. 1878; Les centres olfactifs. Loc. cit.

1879.
Cajal, Ramon y. Bau des Centralnervensystems. Arch. f. Anat. 1893.
Chiaragi, G. Org. nerv. che va dalla reg. del chiasma, etc. Mouit. Zool. Ital. 1895.
Cunningham, D. J. Surface Anatomy of the Cerebral Hemispheres, with a chapter

upon Cranial-cerebral Topography by Victor Horoley. Tr. Irish Ac. 1892.
Cutore, C. Estremo caudule del midolio spinale. Arch. Ital. Anat. Embr. 1905.
Dejerine, J. Anat. d. Centres nerveux. Paris, 1895.
Dennstedt, A. Sinus durai matris. Anat. Hefte. 1904.
Dexter, F. Morphol. of the Medulla Oblongata of the Rabbit. Arch. Anat. 1896 ;

Bau. d. Centralnervensystems d. Ungulaten. Morph. Jahrb. 1904.
Dorello, P. Solchi e circonvoluzione del cervello del maiale. Ric. Lab. Anat. Roma e

altri Lab. Biol. 1903; Corp. call., etc., nel Sus scrofa. Loc. cit. 1903.
Dursy, E. Entw.-geschichte des Kopfes des Mensshen und der hdheren Wirbeltiere.

Tubingen, 1869.
Eberstaller, O. Das Stirnhirn. Ein Beitrag zur Anat. der Oberfljlche des Grosshirns.

Wien und Leipzig, 1890.
Ecker, A. Entw.-gesch. der Furchen und Windungen der Grosshirnhemisphiiren im

Fotus des Menschen. Arch. f. Anthrop. Bd. III. ; Hiruwindungen des Menschen.

Braunschweig, 1869, 1885.
Edinger, L. (See p. 524.)
Familiant, v. Vergleichung der Hirnfurchen bei den Carnivoren und den Primateu,

etc. Inaug.-Dissert. Bern, 1885.
Figueiredo-Rodrigues, J. A. Riickenmark d. Orang-Utan. Arch. f. mik. Anat. 1901.
Flechsig, P. Leitungsbahnen im Gehirn und Riickenmark des Menschen. Leipzig,

1878.
Ganser, S. Gehirn des Maulwurfs. Morph. Jahrb. 1882.
Giacomini, C. Sul cervello d'un Chimpanse. Atti Ace. Torino. 1889.
Gierke, H. Stiitzsubstanz des Centralnervensystems. Arch. f. mikr. Anat. 1885,

1896.
Giesa, E. Bestandteile d. weissen Substanz d. mensch. Riickenmarks. Dissert, (in

Russian). St. Petersburg, 1898.
Goldstein, K. Entw. d. mensch. Gehirnes. Arch. Anat. 1903; Vorder u. Zwischen-

hirn einiger Knochenfische. Arch, f . mikr. Anat. 1905.
Golgi, C. Feinere Bau des Riickenmarkes. Anat. Anz. 1890 ; Fein. Bau d. central.

u. periph. Nervensystems. Jena, 1894.
Gronberg, G. Ontog. d. Gehirns d. Erinaceus. Zool. Jahrb. 1891.
Guldberg, G. Morphol. der Insula Reilii. Anat. Anz. 1887.
Haller,B. Wirbeltiergehirn. Morph. Jahrb. 1900.
Hammer, E. Lowengehirn. Int. JNIonatssshr. Anat. 1902.

Henle, J. Handbiich d. Nervenlehre d. Menschen. 2. Aufl. Braunschweig, 1879.
His, W. Menschliche Riickenmark und Nervenwurzeln. Abb. d. math.-phys.

CI. der K. Sachs. Ges, d. Wissensch. Leipzig, 1886 ; Neuroblasten und deren

Entstehung im embryonalen Mark. Loc. cit. 1888 ; Geschichte des Gehirns

sowie der centraleu und periphereu Nervenbahnen beim meuschlichen Embryo.

Loc. cit. 1888 ; Formentwicklung des menschl. Vorderhirns. Loc. cit. 1889 ;

Entwick. des meuschlichen Rautenhirns, etc. Loc. cit. 1890 ; Histogenese und

Zusammenhang der Nervenelemente. Referat i. d. anatom. Section des internat.

medizin. Congresses z. Berlin, 1890 (see also Arch, fiir Anat. und Physiol. 1890) ;

Das froutale Ende des Gehirurohres. Arch. f. Anat. und Physiol. 1893 :

Vorschliige zur Eintheiluug des Gehirns. Loc. cit. ; Vorstufen der Gehirn- und

Kopfbilduug bei Wirbeltiereu. Loc. cit. 1894 ; Ent. d. mensch. Gehirns. Leipzig,

19U4.
Hoche, A. Pyraraidenbahn, etc. Arch, f . Psychiatric. 1897 ; Blutversorgung d. Riicken-

marksubstanz. Zeitschr. -f. Morphol. u. Anthropol. 1899.
Hochstetter, F. Entw.-gesch. d. Gehirns. Biblioth. Med. 1898.
Holl, M. Var. papers on Brain of Verts, in Arch, f . Anat. u. Physiol. 1899, 1902 and

1903.
Jelgersma, G. Bau des Saugetiergehirns. Morph. Jahrb. 1889.
Kingsbury, B. F. Brain of Necturus maculatus. Jouru. Comp. Neurol. 1895.
Koiliker, A. Feinere Bau des verljingerten Markes. Anat. Anz. 1891.
Kiikenthal, W. Vergl. anatom. und entwick. -geschicht. Untersucli. an AValtieren.

Jena, 1889.
Kiikenthal, W., and Ziehen, T. Grosshirnfurchen der Primateu. Jen. Zeitschr. Bd.

29. (Cf. also Anat. Anz. Bd. XI.)
Krueg, J. Furchung der Grosshirnrinde der Ungulaten. Zeitschr. f. wiss. Zool. 1879 ;

APPENDIX 529

Furcheu auf tier Grosshimrihde der zouoplacentalen Saugetiere. Zeitschr. f.

wiss. Zool. 1881.
Lecbe, W. Eigeuart. Saugetiergehirn (Insectivoren). Anat. Anz. 1905.
Leuhossek, M. v. Pjrainidenbahnen im Eiickenmark einiger Saugetiere. Anat. Anz.

1889; Neuroglia des menschlichen Riickeninarkes. Verb. Anat. Ges.

1891; Feinere Ban des Xervensystems, etc. Fortschritte der Medizin. 1892;

Histol. des Nervensy.stenis und der 8innesorgaue. Wiesbaden, 1894.
Leuret et Gratiolet. Anat. comp. du systeme nerveux Paris, 1839 — 1857.
Lotbringer, S. Hypophyse einiger Saugetiere and des Menscbeu. Arch. f. mikr. Anat.

1886.
Lugaro, E. Circouvoluzioni cerebrali e cerebellari. Riv. di Patliologia nervosa el

meutale. 1897.
Luys, J. Le systeme nerveux cerebrospinal. Paris, 1865 ; Iconograpbie pbotograpbique

des centres nerveux. Paris, 1872.
Marcband, F. Entwi; kl. des Balkens iiu meuscbl. Gehirn. Arcb. f. mikr. Anat. 1891.

(cf. also L. Blumenau. Loc. cit.) ; Morpbol. des Stirnlappens und der In.sel der

Antbropomorphen. Arb. a. d. patbol. Inst. z. Marburg. 1893.
Martin, P. Entwickl. des Gebirnbalkeus bei der Katze. Anat. Anz. 1893.
Mibalcovics, Y. v. Entw.-gescbicbte des Geliirns. Leipzig, 1877.
Neumayer, L. Eutw.-gesch. d. Gehims d. Saugetiere. Festscbr. z. 70. Geburtstag von C

V. Kupffer. 1899. (Cf. Sitz.-ber. Mori^bol. Pbysiol. Miincben, 1899).
Nusbaum, J. Eatw.-gescbicbte der Hypophysis cerebri bei Saugern. Anat. Auz.

1896.
Panscb, A. Auordnung der Furcben und Windungen auf deu Grossbirnhemispbaren des

Menschen und der AfFen. Arcbiv. fiir Anthropol. Bd. III. ; Morpbol. des Gro.'s-

birns der Saugetiere. Morph. Jabrb. 1879.
Quanjer, A. A. Insula Reilii, etc. Pet. Camp. Dl. II. Afl. I.
Rawitz, B. Centralnerveusystem d. Cetaceeu. Arch. Anat., 1903, and Anat. Anz.,

1903.
Reicbert, C. B. Bau des menschl. Gehirns. Leipzig, 1859 und 1861.
Retzius, G. Ein dem Saccus vasculosus entsprecb. Gebilde am Gehirn des Menschen

und der Saugetiere. Biol. Untereucb. 1895 ; Das Menscbengebim. Mit Atlas.

Stockholm, 1896.
Salvi, G. Menengi cerebrale. Atti Soc. Toscana, 1897, 1898, and Mouit. Zool. Ital.

1898.
Salzer, H. Entw. d. Hypopby.se. Arch. f. mikr. Anat. 1897.
Schwalbe, G. Der Arachnoidalraum, etc. Medic. Centralbl. 1869.
Smith, G. E. Brain of a Fa^tal Ornitborhyncbus. Q.J.M.S. 1896, 1898 ; Comp.

Anat. of the Cerebrum of Notoryctes. Tr. R. Soc, S. Australia, XIX. (See abo

Anat. Anz., 1895; P. Liuu. Soc. X.S.W. Vol. IX.; Journ. Anat. and Physiol.,

1897 ; Deutsch. Arch. f. Klin. Med., 1902 ; Compt.-rend. de TA-ssoc. des Anato-

mistes, 1903; Zeitsch. f. Morpbol. u. Anthropol., 1904; Anat. Anz., 1903, 1904,

and Records of Egyptian Gov, School of Med., Cairo, 1904.)
Symington, T. Cerebral commissures in the Marsupialia and Monotremata. Journ. Anat.

and Pbysiol. 1892.
Tiedeiuann, Fr. Anat. und Bildungsgesch. des Gehirns im Fotu> des Menschen. Niirii-

berg, 1816.
Turner, W. Convolutions of the Brain, etc. Journ. Anat. and Physiol. 1890.
Waldeyer, M. Das Gorilla-Riickenmark. Abb. Ak. Berlin. 1888'; Das Gibbonhiru.

luternat. Beitr. z. wissensch, Medizin. 1891.
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1896.
Wilder, B. G. (See p. 526.)

Sperino, G. Cervelo del Gibbone. Giomale d. R. Accad. d. Med. Torino, 1898.
Sterzi, G. Meuingi midoUari, etc. Arch. Ital. Anat. Embryol., 1902, and Monit. Zool.

Ital., 1902.
Weber, A. Dev. de I'hypophyse chez les Chiropteres. Bibliog. anat. 1898.
Zander, W. Dura mater, etc. Festsch. z. 70. Geburtstag v. C. v. Kupffer. Jena, 1899.
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1890 ; Grosshirnfurchen des Hylobates und Semnopitbecus-Gehirnes. Loc. cit.

1896 ; Centralnervensystem d.Monotremen u. Marsupialer. Semou's For:>cburg--

reisen. Jena, 1897. Gehirn bei d. Halbaffen u. Galeopitbecus. Anat. Anz. 1903.
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Klubs, Wien. 1898-9 ; KoUateralfurche und Riecbstrahlung. Arb. Xeurol. Inst.

Wiener Univ. 1904 ; Morpbol. d. Affengehirns. Zeitschr. f . Morpbol. u. Anthropol.

1904.

(Cf. also Kupffer, K. v., and Ziehen, T, in Hertwig's Handbncb d. vergl.

u. exp. Entw.-lehre. Jena, 1906.)

M M

530 APPENDIX

{b) PINEAL APPARATUS.

Ahlborn, F. Bedeutuug der Zirbeldriise. Zeitschr. f. wiss. Zool. 1884.
Beard, J. The Parietal Eye of the Cyclostome Fishes. Q.J.M.S. 1888.
Beraueck, E. Parietalauge der Reptilieu. Jen. Zeitschr. 1887 ; Nerf parietal et la

Morphol. du troisieme oeil des Vertebres. Anat. Anz. 1892 ; L'individualite de

I'oeil parietal. Loc. cit. 1893 ; Coutrib, a I'einbryogenie de la glande pineale des

Arapiiibiens. Revue Suisse de Zool. et Anuale du Musee d'hi&t. nat. de Geneve.

1893.
Carriere, J, Parietalorgan. Biol. Centralbl. 1889.
Cattie, J. Epiphyse der Fische. Arch, de Biol. 1882.

Dendy, A. Djv. of the parietal eye, etc., in Sphenodoa. Q.J.M.S. 1899 ; Parietal sense-
organs in Geotria. Q.J.M.S. 1907.
Dexter, F. Dev. of paraphysis in Fowl. Amer. Jouru. Auat. 1902.
Duvdl et Kdt. Des yeux piueaux multiples chsz I'Orvet. Soc. de Biol. 1889.
Ehl3rs, E. Epiphyse am Gehirn der Plagiostomen. Zeitschr. f. wiss. Zool. Bd.

XXX.
Eycleshymer. Paraphysis and Epiphy is in Amblystoma. Anat. Anz. 1892.
Favaro, G. Fibre nerv. piepia. e pia. d. Mammiferi. Arch. Anat. Embr. 1904.
Flescb, M. Zirbel bei deu Saugetieren. Auat. Auz. 1888.
Fran-'otti. Developpement tie I'epiphyse. Arch, de Biol. 1888.
Giupp, E. Z rbel, P iriet dorgan, u. Paraphysi*. Anat. Hefte, Ergebu. 1897.
Graaf, H. de. Keunis van den Boiiw en de Oatwikkeling der Epiphyse bei Amphibieu

eu Reptilidn. Leiden, 188(); Auat. und Entwiekl. der Epipiiyse bei Amphibieu

und Reptilieu. Zool. Anz. 1886.
Grieb, A, Org. puietUe der Podarcis. Monifc. Zool. Ital. 1901.
Hiuitsch, R. Pineal Eye of Anguis. Pro:;. Biol. Soc. Liverpool. 1889.
Hill, 0. D3V. of the Epiphysis in Corej^onus. Jouru. Morphol. 1891 ; Epiphysis of

Teleosts and Amia. Loc. cit. 1894.
Julin, Ch. L'Epiphys3 (Glaade pineale) des Vei-tebres. Bull, scientif. du Nord.

Paris, 18t^7.
Kingsbury, B. F. Eacephalic Evagin. in Ganoids. Jouru. Comp. Neurol. 1897.
Kliuckowstrom, A. de. Dev. de I'ceil pineal, I'epiphyse et le nerf parietal chez Iguani.

Anat. Auz. 1893 (cf. aLo Zojlog. Jahrb. 1893) ; Zirbel und Foramen parietale

b.i Calli.'hlhys. Lvc. cit.
Leydig, F. Parietalorgau der Ampbibien und Reptilieu. Abb. Senckenb. Ges.

1890. (Cf. also Biol. Centralbl., 1889, and Arch. f. mik. Anat., 1897.)
Locy, W. A. Pineal Eye. Anat. Anz. 1894. (Cf. ako Juuru. Morphol. Vol.

vm.)

Minot, C. S. Morphol. of the pineal region. Amer. Jouru. Anat. 1901.
Owsiannikow, Ph. Das dritte Auge von Petromyzon, etc. Mem. Ac. St. Petersb.

1888.
Peytoureau, A. La glande pineale et le troisieme oeil. Paris, 1887.
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Ret/iius, G. Baude^ sog. P irietalauges von Amraocoetes. Biol. Uutarsuch. 1895.
Ritter, W. E. Parietal Eye in some Lizards from the western United States. Bull.

Mus. Harvard. 1891 ; Parapineal Organ in Pnrynoioma corouata. Anat. Auz.

1894.
Saott, W. B. Parietal Eye of Cyclostome P'ishes. Journ. Morphol. 1887,
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1894.
Spencer, W. Baldwiu. Presence and Structure of the Pineal Eye in Lacertilia.

Q.J.M.S. 1886.
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f. Anat. und Phys^iol. 1888.
Studnirka, F. C. Les orgaues parietaux de Petromyzon planeri. Travail du lab.

de zool. et d'anat. comp., a I'uuiversite de Boheme de Prague. 1893 ; Morphol.

der Parietalorgane der Cranioten. Zool. Centralbl. 1894 ; Auat. der sogen.

Paraphyse des Wirbeltierhirus. Sitz. ber. Bohm. Ges. 1895; Parietalorg. von

Petromyzon. Loc. cit. 1899 ; Parietalorg. u. sog. Paraphysis, etc. Verb. Anat.

G^s. 1900; Die Parietalorgane, in Oppel's Lehrb. d. vergl. mik. Anat. Jena,

1904.
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APPENDIX 531

(c) PERIPHERAL XERV.OUS SYSTEM.

Ailolplii, H. Variation der Spinalnerven und der Wirbelsaule anurer Amphibien.

Morph. Jahrb. XIX, Bd. ; Extremitiitenplexus u. Sacrum bei Triton. Loc. cit.

1898.
\llis, E. P. Various papers on cranial neives, etc., of Fishes in Jouru. of Morphol., 1897

and 1903 ; Q.J. M.S., 1902; and Anat. Anz., 1899 and 1903.
Anderson, O. A. Sympath. Nervensvstems der urodelen Amphibien. Zool. Jahrb.

1892.
Beard, J. The system of Branchial Sense Organs and their assoc. Ganglia in

Ichthyopsida, etc. Q.J. M.S. 1885 ; The Ciliary ganglion and the Ganglion of

the ophthalmicus profundus in Sharks. Anat. Anz. 1887 ; Dev. of the Peripheral

Nervotis-System in Vertebrates. Q.J.M.S. 1888. (Cf. also Anat. Anz. 1888.)
Beck, W. Austritt des Xerv. hypoglossus und N. c rvicalis primus aus dem Central-
organ beim Menschen, etc. Anat. Hefte. 1895.
Bemmelen, van. Halsgegend der Reptilien. Zool. Anz. 1887.
Beraneck, E. Le dev. des nerfs craniaux chez les Lezards. Recueil zoolog. suisse.

T. I.
Bolk, L. Neurol, d. unt, Ext. d. Primaten. Morpb. Jahrb. 1897; Mensch Rurapf. u.

Extrem. Loc. cit. 1898-9 ; Plexus cervico-brachialis d. Primaten. Pet. Camp

1902 (andcf. p. 517).
Bowers, M. A. Distrib. of cranial nerves of Spelerpes. Proc. Amer. Acad. Arts and

Sci. 1901.
Brauer, A. Entw. u. Anat. d. Gymuophionen. Zool. Jahrb. Suppl. VII. (Weismann's

Festschi-.). 1904.
Braus, H. Vaiious papers on Nerves of Ichthyopsida, in Jtu. Zeitschr., 1898; Morph.

Jahrb., 1898, 1899; Semou's Forschungsreisen, 1900; Verb. Anat. Ges. Jena,

1904 ; Protok. Xaturhist.-Med. Verein, Heidelberg, 1904 ; and Anat. Anz., 1905.
Broek, A. J. P. van den. Sympath. nervous syst. in Monotremes. Proc. Acad

Amsterdam. 1905.
Carlssou, A. Anat. d. Schwimmvogel. Abb. d. K. Schwed. Vet. Acad. Bd. III.

Gliedmassenreste der Schlangen. Konigl. Schwed. Acad. Bd. II.
Chevrel, O. L'Anat. du Systeme nerveux grand sympathique des Elasmobranches et

des Poissons osseux. Poitiers, 1889.
Chiarugi, G. Lo sviluppo dei Nervi vago, accessorio, ipoglosso e primi cervicali nei

Sauropsidi e nei Mammiferi. Atti Soc. Tofecana. 1889; Osservaz. intorno alle

prime fasi di sviluppo dei Nervi encefalici dei Mammiferi, etc. Monitore zool.

italiano. Firenze, 1891 ; Stud, dello sviluppo dei Nervi encefalici nei Mammiferi

in confrouto con altri Vertebrati. Public, del R. Istit. di Studi super pract. e di

perfez. in Firenze. 1894 and 1897.
Coggia, A. Alcuni dtrivati delF ectoderma nei capo dei Selaci. Ricerche Lab. Anat.

Roma e altri Lab. Biol. 1895.
Coghill, G. E. Fifth Nerve in Amphibia. Contrib. Anat. Lab. Brown Univ., 1901, and

Journ. Comp. Neurol., 1901.
Cole, F.J. Cranial Nerves of Chimjera, with a discussion of the lateral line system and

of the Morphol. of the Chorda tympani. Tr. R. Soc. Edin. 1896; Struct, and

Morphol. of Cranial Nerves and lat. sense-organs of Fish*; (Gadus). Tr. Linn.

Soc. 1898. (Cf . also Ti-ans. Liverpool Biol. Soc, 1898, and Anat. Anz., 1898, 1899,

and 1906.)
Cords, E. Kopfnervensystem d. Vogei. Anat. Hefte. 1904.
Cuneo, B. Neris craniens. Traite d'Anat. hum. 1899.
Davidoff, M. V. (See p. 518.)
Deyl, T. Vergl. Anat. des Sehnerven. Bull. Internat. de I'Acad. des Sci. del'Empereur

Francois Joseph I. Prag, 1895.
Dixon, A. F. Dev. of the Branches of the fifth cranial Nerve in Man. Tr. Dublin. Soc,

1896 (cf. also Journ. Med. Sci. Dublin, 1905) ; On the coiu-se of the taste-fibres.

Edinbuigh Med. Journ. 1897 ; Sensory distrib. of facial Nerve in Man. Joum.

Anat. and Physiol. 1899.
Dohrn, A. (See p. 524.)

Dogiel, A. S. Periph. Nervensyst. v. Amphioxus. Anat. Hefte, Arb. 1903.
Eisler, P. Plexus lumbosacralis des Meuschen. Abb. Ges. HaUe. 1892.
d'Erchia, F. Gang, ciliare. Mouit, Zool. Ital. 1895.

Ewart, J. C, and Cole, F. J. Nerves of Elasmobranchs. P. Roy. Soc. Edin. 1895.
Fischer, J. Nerv. -sympath. (Katze, Ziege, etc.). Inaug.-Diszert. Ziirich, 1905.
Fischer, J. G. Gehirnnerven der Saurier. Hamburg, 1852. (And see p. 526.)
Freud, S. Spinalganglieu und Riickenmark des Petromvzon. Sitz.-ber. Ak, Wieu

1878.
Frohse, F. Oberflach. Nerven des Kopfes. Berlin, Prag, 1895.

M M 2

5'32 APPENDIX

Froriej), A. Gangliou ties Hypoglossus uud Wirbelaulageu iu der Occipitalregioa.

Arch. f. Anat. uud Phys. 1882 ; Aulagen vou Siauesorgaueu am Facialis, Glosso-

pharyngeus uud Vagus, etc. Arch. f. Auat. 1885; Homologon der Chorda

tyinpani bei uiederea Wirbeltieren. Auat. Auz. 1887.
Froriep, A., und Beck, W. Vorkommen dorsaler Hypoglossuswurzelu mit Gauglieu iu

der Reihi der Saugetiere. Auat. Auz. 1895.
Fiirbriuger, M. Umbildungeu des Nervenplexus. Morph. Jahrb. 1879 ; Die spino

occipitaleu Nerveu der Selachier viud Holocephalen uud ihre vergl. Morphol.

Festschrift f. C. Gegeubaur. Leipzig, 1896; Morphol. Streitfrageu. Morphol.

Jahrb. 1902 ; Brust-schulterapparates, etc. Jeu. ZeitSDhr. 1902 (aud sse p. 521).
Gadow, H. (See p. 522.)
Gaskell, W. H. Struct., distiib. aud fuuction of the nerves which iunervate the visceral

aud vascular system. Journ. Physiol. 1886; Relation between the Struct.,

Fuuction, Distrib. aud Origin of the Cranial Nerves, together with a Theory

of the Origin of the Nervous System of Vertebrata. Jouru. Physiol. Vol. X.
Gegenbaur, C. Kopf nerveu vou Hexaachu.s, etc. Jeu. Zeitschr. Bd. VI. (Aud see

p. 513.)
Giglio-Tos, E. Orgaui branch, e lat. di senso uell' iiomo. Monit. Zool. Ital. 1902

(Cf. also Auat. Auz. 1902.)
Goronowitsch, N. Eutwickl. de sogeu. " Ganglieuleisteu ' im Kopfe der Vogclem-

bryouen. Morph. Jahrb. 1893; Trig. fac. Komplex v. Lota. Gegeubxiu-'s

Festschrift. Leipzig, 1897. (And cf. p. 525.)
Haller, B. Urspruug des Nervus vagus bei den Knochenfischeu. Verh. Deutsch.

Zool. Ges. Bd. V.
Harrison, R. G. Histogenese d. periph. Nerveusyst. bai Salmo. Arch. f. mikr. Anat.

1901. Eatw. d. periph. Nerveu. Sitz.-ber. Niederrhein. Ges. f. Natur- u.

Heilkuude. Bonn, 1904.
Hawkes, O. A. M. Cr. and sp. Nerves of Chlamydoselachus. P. Zool. Soc. 1907.
Herrick, C. J. Cranial Nerves of Bony Fishes. Jouru. Comp. Neurol. 1898 and

1901. Arch. f. Neurol, u. Psychopath. 1899.
His, W. (Jr.) Eutwickl. des Sympathicus bei Wirbeltieren. Verh. Auat. Ges.

1892; Anfauge des periph. Nerveusystems. Arch. f. Auat. u. Physiol. 1879;

Entwick. der ersteu Nervenbahnsu beim meuschl. Embryo. Arch. f. Auat.

uud Physiol. 1888.
Hoffmann, C. K. Eutwickl. -gesch. des Selachierkopfes. Anat. Auz. 1894 ; Entw.-

gesch. d. Selachii. Morph. Jahrb. 1899 ; Eutw. -gesch. d. Sympathicus.

Verh. Ak. Amsterdam. 1900, 1902.
Iversen, M. Die dorsaleu Wurzelu des Nervus Hypoglossus. Ber. Ges. Freiburg. 1886.
Jaquet, M. Sympath. cervical. Arch. d. Ssienc. Med. Bucarest, 1900 ; Auat. u.

Histol. V. Silurus. Bull. Soc. Sci. Bucarest, Roumanie, 1901.
Jhering, H. v. Periph. Nerveusystem der Wirbeltiere, etc. Leipzig, 1878.
Johnson, A., aud L. Sheldon. Dev. of the Cranial Nerves iu the Newt. P. Roy. Soc.

1886.
Johnston, J. B. Hind-brain and Cr. Nerves of Acipenser. Anat. Auz. 1898 ;

Morphol. of Vert. Head, etc. Journ. Comp. Neurol, and Psychol. 1905 ; Crauial

Nerve-compoueuts of Petromyzon. Morph. Jahrb. 1905; Nervous System

of Vertebrates. Philadelphia, 1906.
Julin, CJi. Systeme uerveux grand sympath. de I'Ammocoetes. Auat. Anz. 1887:

La valeur morphol. du nerf lateral du Petromyzon. Bull. Ac. Belgique. 1887.
Key, A., und G. Retzius. Anat. des Nerveusystems uud des Bindegewebes. Stock-
holm, 1875 uud 1876.
Klinkhardt, W. Entw. -gesch. d. Kopfganglien u. Sinuesliuieu der Selachier. Jeu.

Zeitschr. 1905.
Kohn, A. Sogeu. Carotiddrllse. Arch. f. mikr. Anat. 1900; Chromaffine Zellen, etc.

Med. Wocheuschr. Prag, 1902; Paraganglien. Arch. f. mikr. Auat. 1903.
KoUiker, A. Feiuere Ban imd Fuuctiouen des sympath. Nerveusystems. Sitz.-
ber. Ges. Wiirzburg. 1894.
Konige, E. Geschichte der Anatomie der Hirnuerveu. luaug. -Dissert. Freiburg i.

Br. 1897.
Kose, W. " Carotiddruse " u. " Chromaffineu Zellen" bei Vogel. Auat. Auz. 1902.
Krause, W. Doppeluatur des Ganglion ciliarc. Morph. Jahrb. 1881.
Kupffer, C. v. Eutwick. vou Petromyzon. Sitz.-bjr der K. Bayer. Acad, der

Wisseusch. Miiuchen, 1888 ; Entwick. der Kopfnerven der Vertebrateu. Verh.

Auat. Ges. 1891; Entw.-gesch. d. Kopfes. Auat. Hefte, Ergebu 1895.
(See also pp. 525 and 536.)
Lee, St. Kenntuis des Olfactorius. Ber. Ges. Freiburg. 1893.
Lennhossek, M. V. Spinalgauglieu desFrosches. Arcli. f. mikr. Auat. 1886.
Lo:-y, W. A. Dev. of Olf. Nerve. Auat. Auz. 1899 ; Newly recog. Nerve in cou. with
fore-brain of Selachians. Auat. Auz. 1905.

APPENmX 533

Lubosch, W. Ursprimg u. Phylogen. d. Nerv. access. Willisii. Arch. f. mikr. Anat.

1899.
Lugaro, E. INIoJificazioui delle cellule uervose uei diversi stati fuuziouali. Sperinientale,

Auuo XLIX. 8ezioue Biol. Fireuze, 1895.
Marchaud. Gland, carotica u. Neljenuieren. Intemat. Beitr. z. wiss. Med. (Festschr.

z. 70. Gebiirtstag v. K. Vircbow.) Bd. I.
Marsball, A. M Dev. of tbe Nerves in Birds. Joum. Anat. and Physiol. Vol. XI. ;
Dev. of the Cranial Xerves in the Chick. Q.J.M.S. Vol. XVIII. ; Morphol. of
the Vert. Olf. Organ. Loc. cit. Vol. XIX.; Head-Cavities and Associated
Nervea in Elasmobranchs. Loc. cit. Vol. XXI.; Segmental A'ame of the
Cranial Xerves. Joiiru. Anat. and Physiol. Vol. XVI.
Mar-shall, A. M., and W. B. Spencer. Cranial Xerves of Scyllium. Q.J.M.S. Vol. XXI.
Mitrophanow, P. Etude em bryog. sur Its Selaciens. Arch. Zool. exp. Vol. I.
Neal, H. V. Dev. of the ventral Xerves in Selachii. Mark Anniv. Vol. 1903. (And

see p. 525.)
Onwli, A. Entwick. des. syuipatb. Xerven.systenis. Arch. f. mikr. Anat. 1885-86 ;

Neurol. Untersuch. an Selachieru. Intemat. Monatsschr. Anat. 1886.
Owsiannikow, P. Svnipath. Nervensvstem der Flussneunaugtn, etc. Bull. Ac. St.

Petersb. 1883.
Pannett, R. C. Pelvic plexus, etc. (Mustelus). Phil. Trans. 1901.
P-arker, G. H. Optic Chiasma in Teleosts, etc. Bull. Mus. Harvard. 1903.
Paterson, A. M. Fate of the Muscle-Plate and the Dev. of the Spinal Nerves and
Limb Plexuses in Birds and Mammals. Q.J. M.S. 1887 ; Origin and Distrib, of
the Nerves to the Lower Limb. Journ. Anat. and Physiol. 1894 ; Dev. of the
Synipath. Nerve Syst. in Mammals. Phil. Trans. 1890.
Pinkus, F. Hirnnerven des Protopterus. Morph. Arb. 1894. (Cf. also Anat. Anz.

1894.)
Plessen, J. von, und .J. Rabinovicz. Kopfnervern von Salaraandra. Miinchen, 1891.
Popowsky, T. Entwick. -geschichte des N. Facialis beim Mensclien. Morph. Jahrb.

1895.
Rabl,K. Gebiet des Nerv us facialis. Anat. Anz. 1887.
Retzius, G. Ganglion ciliare. Anat. Anz. 1894.
Robinson, A. Formation and Struct, of the Optic Nerve and its Relation to the Optic

Stalk. Journ. Anat. and Physiol. Vol. XXX.
Rotgans, J. Halsgedelte d. laatste vier Hersenzeueweu. Meppel, 1886.
Ruge, G. Verschieb. in den Eudgebieteu der Nervendes Plexus lumbalisder Primaten.
Morph. Jahrb. 1894 ; Periph. Gebiet d. N. facialis b-i AVirbeltieren. Festschr.
f . C. Gegeubaur. Leipzig, 1890.
Schaper, A. Ilistol. d. Glamlula carotica. Arcb. f. mikr. Anat. 1892; Glandulse

parathyreoidefe. Loc. cit. 1895.
Schneider, H. Augenmuskelnerveu der Ganoiden. Jen. Zeitschr. Bd. XV.
Schultze, O. Entw. d. periph. Nervensystems. Anat. Anz., Bd. 25, and Verb. Anat.
Ges., 1904: Histogencse d. Nervensystems. Arch. f. mikr. Anat. 1905. (See
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Schumacher. Kehlkopfnerven b. Lama, etc. Anat. Anz. 1906.
Schwalb?, G. Ganglion oculomotorii. Jen. Zeitschr. Bd. VIII.

Sherrington, C S. Experiments in exam, of the Peripheral Distiib. of the Fibres of
the Posterior Roots of some Spinal Nerves. Phi!. Trans. 1893 ; Arrangement
of some Jlotor Fibres in the lumbo-sacral plexus. Journ. Physiol. 1892.
Stannius, H. Peripliere Nervensystem der Fische. Rostock, 1849.
Steinach, E. Motorische Function hinterer Spiualnervenwurzeln. Arch. f. d. ges.

Physiol. 1895 and 1898.
Stras.ser, H. Alte und neue Probleme der entwicklungsgesch. Forschung auf dem

Gebiete des Nervensystems. Anat. Hefte, Ergebu. 1891.
Streeter, G. L. Dev. of cran. and spin. Nerves in oc. region of human embryo. Amer.

Journ. Anat. 1904.
Strong, O. S. Cranial Nerves of Amphibia. Journ. Morphol. 1895.
Vincent, S. Carotid Gland of Mammalia, etc. Anat. Anz. 1900.
AVatkin>on, G. B. Cranial Nerves of Varanus. Morph. Jahrb. 1906.
Westling, C. Periph. Nervensystews (Pithecus and Ornithorhynchus). Abh. d. K.

Schwed. Vet. Acad. Bd. IX.
Wijhe, J. W. van. Das Visceralskelet und die Nerven des Kopfes der Ganoiden und
von Ceratodus. Niederl. Arch. f. Zool. Bd. V.; Somiten und Nerven im
Kopfe von Vogel- und R'-ptilien-Embryonen. Zool. Anz. 1886 (and see
p. 523).

(Cf. also Ziehen, T., and Neumayt-r, L., in Hertwig's Handbuch d. vergl. u.
exp. Entw.-lehre. Jena, 1906.)

534 APPET^BTX

G. SENSORY ORGANS.

Integumbntauy Sensory Orgaxs and Organs of Taste.

Allis, E. P. Lat. line iu Amia. Journ. Morphol. 1889; in Polypt-rus. Anat. Anz.

1900; in Mnstelus. Q.J.M.S. 1902; in Muraenidae. Intern. Monatsschr. Anat.

1903; in Fishes. Loc. cit. 1904; Polyodon. Zool. Jahrb. 1903 ; Scomber, Journ.

Morphol., 1903.
Ayers, H. Struct, and Dev. of the Nasal Rays in Condylura. Biol. Oenti-albl. 1885.
Bath, W. Geschmacksorgane v. ('rocodilus. Zool. Anz. 1905; v. Vogel. Sitz.-ber.

Naturf. Berlin. 1905.
Beard, J. Segmental S?nse organs of the Lateral Line, and th? Morphol. of the Yert.

Aud. Org. Zool. Anz. 1884 ; Cranial Gangli* and segmental Sense Organs of

Fishes. Zool. Anz. 1885 ; Tlie System of branchial Sense Organs and their

asso3. Ganglia in Ichthyopsida, etc. Q.J.M.S. 1895.
Blaue, J. Ban der Nasenschleimhaut bei Fischen und Amphibien, etc. Arch. f. Anat.

u. Physiol. 1884.
Boll, F. Lorenzi'sdien Ampullen der Selachier. Arch. f. mikr. Anat, 1868 ;

Savi'schen Bliischen von Torpedo. Arch. f. Anat. imd Phys. 1875.
Botezat, E. Geschmacksorgane, etc., iu Schnabel d. Vogel. Biol. Centralbl. 1904.
Brock, J. Terminalkorperchen-ahnliche Organe in der Haut von Knoclientischen.

Intern. Monatsschr. Anat. 1887.
Coggi, A. Vesicole di Savi e gli organi della linea laterale nelle Torpedini. Rend.

Ace. Liucei. 1891 ; Sviluppo delle Ampolle di Lorenzini. Loc. cit.
Cole, F. T. Sensory and Ampullary (*anals of Chimjera. Anat. Anz. 1896 (and

see p. 531).
Collinge, AY. E. Sensory and Ampullary (^anals of Chimaira. P. Zool. Soc. 1895 ;

Sensory Canal System of Fishes (Physostomi). P. Zool. Soc. 1895; Sen.sory

Canal System of Fishes (Ganoidei). Q.J.M.S. Yol. XXXYL ; Morphol. of the

Sensory Canal System in some Fossil Fishes. Proc. Birmingham Philos. Soc.

1893; Lateral Canal System of (1) Lepidosteus osseus, (2) Polypterus. Proc.

Birmingham Philos. Soc. 1893.
Crevatin, F. Terminazione nervosi u. corio d. Congiuntiva e d. pelle d. Polpastrelli,

etc. Mem. Ac. Sci., Bologna. 1902.
Dogiel, A. S. Yarious papers in Arch, f . mikr. Anat.
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Eberth, C. J., and Bunge, R. Endigungen der NTerven in der Haut des Frosches.

Anat. Hefte. 1892.
Eimer, Th. Die Schnauze des Maulwurfs, etc. Arch. f. mikr. Anat. 1871 ; Nervenendi-

guugen iu der Haut der Kuhzitze. Arch. f. mikr. Anat. 1872.
Ewart, J. C, and Mitchell, J. C. Lateral Sense Organs of Elasmobranchs (Laemargus

and Skate). Tr. Roy. Soc. Edin. 1892.
Fritsch, G. Aeussere Haut und Seitenorgane des Zitterwelses (Malopterurus). Sitz.-
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also p. 524.)
Garman, S. Lateral Canal system of the Selachia and Holosephala. Bull. Mus.

Harvard. 1888.
Gmelin. Morphol. der Papilla vallata und foliata. Arch. f. mikr Anat. 1892.
Grandry. Les corpuscles de Pacini. Journ. de I'anat. et physiol. 1869.
Guitel, F. La ligne laterale dvi Lophius. Arch. Zool. exp. 1891 ; Les boutons nerveux

bucco-pharyngiens de Baudroie (Lophius). Loc. cit.
Harrison, R. G. Entw. d. Sinnesorgane d. Seitenlinie bei d. Amphibien. Avoh.. f. mikr.

Anat. 1903.
Hermann, F. Feinere Bau des Geschmacksorganes. Sitz.-ber. d. K. Bayer. Acad.

1888.
Hesse, Fr. Tastkugeln des Entenschnabels. Arch. f. Anat. u. Physiol. 1882.
Hoffmann, C. K. Ontogenie der Knochenfische. Arch. f. mikr. Anat. 1883.
Holl, M. Anat. der Mundhohle von Rana. Sitz.-ber. Ak. Wien. 1887.
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Anat. u. Physiol. 1861.
Key, A*., and Retzius, G. Anat. des Nervensystems. Arch. f. mikr. Anat. 1873.
Kingsbury, B. F. Lateral Line system of Sense Organs, in some American Amphibia,

and Comp. with Dipnoans. P. Amer. Micros. Soc. 1895.
Kolliker, A. Stiftchenzelleu in der Epidermis von Froschlarven. Zool. Anz. 1885;

Histolog. Studien an Batrachierlarven. Zeitchr. f. wiss. Zool. 1886.
KoUmann, A. Tastapparat der Hand der menschl. Rassen und der Affen in seiner

Entwick. und Gliederung. Hamburg u. Leipzig, 1883.

APPENDIX 535

Krause, W. Nervenendigungen inuerhalb der terminalen Korperchen. Ari^-h. f. mikr,

Anat. 1881.
Leuhossek, M. v. EaJknospeu der Barbe und des Aales. Beitr. z. Hist, des Nerveu-

.systems uad der Sianesorgaue. Wiesbaden. 1894 ; Geschmacksknospen in deu

blattformigeu Papillen der Kaninchenzunge. Yerh. Ges. Wiirzburg. 1893.
Leydig, F. Anat. uud Histol. der Chimaera. Arch. f.Auat. und Physiol. 1851; Hist.

von Polypterus. Zeitschr. f . wiss. Zool. Bd. V.; Organe eines sechsten Sinnes,

etc. Nova acta Acad. Caes. Leopold. Carol, germ. nat. curios. Bd. 34. 1868;

Schwanzflosse, Tastkorpercheu und Endorgane der Xerven bei Batrachiern. Arch.

f. mikr. Auat. 1876; Siuuesorgane der Sell langen. Arch. f. mikr. Anat. 1872;

Die Haut einheimischer Ophidier. Arch. f. raikr. Anat. 1873. (See also

pp. 502 and 505).
Malbrauc, M. Seitenlinie und ihren Sinnesorganen bei Amphibian. Zeitschr. f. wiss.

Zool. 1875.
Maurer, F. Haut-Sinnesorgane, Feder- und Haaranlagen und deren gegenseitig? Bizie-

hungen. 3Iorph. Jahrb. 1892 ; Epidermis und ihre AbkomTnlinge. Leipzig,

1895.
Mayer, S. Bau der Sinneshaare. Arch. f. mikr. Anat. 1890.
Merkel, Fr. Endigungen der sensibleu Nerveu \a der Haut. Nachr. Gas. Gottiugen.

1875; Tastzelleu und Tastkorpercheu bei den Haustieren und beim Menschen.

Ai-ch. f. mikr. Anat. 1875 ; Endigungen der sensibleu Nerven in der Haut der

AVirbeltiere. Rostock, 1880 ; Tastzallen der Enta. Arch. f. mikr. Anat. 1878.
Minckert, W. Lorenzi'scheu Ampillen. Auat. Anz. 1901.
Mitrophanow, P. Eatwickluugsgeseh. u:id Inuervation derXervenhiijel der Urodelen-

larveu. Biol. Centralbl. 1887.
Miiller, H. Nervosa Follikelapparat der Zitterrochen und die sogen. Schleimcanjile der

Knorpelfische. Yerh. Ges. Wiirzburg. 1852.
Nagel, A. Geruchs- und Geschmackssinn und ihre Organe, etc. Bibliotheca Zool.

Stuttgart, 1894.
Parker, G. H. Function of lat. line. Bull. Bureau of Fisheries. 1905 ; Sens? of hearing

in Fishes. Amer. Nat. 1903.
Pfitzner, W. Nervenendigungen in Epithel. Morph. Jahrb. 1881.
Pinkus, F. Hautsinnesorgane neben d. mensch. Haare (Haarscheiben), etc. Arch. f.

mikr. An It. 1904. (Cf. Yerh. Physiol. Ges. Berlin, 1902-3 ; Arch. f. Der.natol.,

1902 : and Dermatol. Zaitschr., 1902-3.)
Pollard, H. B. Lat. Lin 3 System in Siliiroids. Zool. Jahrb. 1892.
Rauber. Yater Pacini'scher Korperchen am Menschen und Saugetiere. ZdoI. Anz.

188^ ; Yater'sche Korper-^hen der Bander und Periostnerven. Neustadt a. H.

1865; Yorkommen unci Bedeutung der Yater'schen Korperchen. Miinchen, 1867.
Retzius, G. Nervendigungen in d. papillen d. Zuugeu d. Amphibien. Biol. Untersuch.

1905. (See also p. 529.)
Ritter, W. E. On the Eyes, the integumentary sense-papill?e, and the integument of

Typhlogobius. Bull. Mus. Harvard. 1893.
Sarasin, P. and F. EntwicklungsgesA. von Ichthyophis. Zool. Anz. 1887. (See also

p. 503.)
Schultzo, M. Kolbanformigen Gebilde in der Haut von Petromyzon, etc. Arch, fiir

Anat. und Phy.siol. 1861.
S-dudze, F. E. Becherformigen Organe der Fische. Zeitschr. f. wiss. Zool. 1863 ;

Epithel- und Driisenzellen. Arch. f. raikr. Anat. 1867 ; Nervenendigung in den

sog. Sohleimcanalen der FisAe, etc. Arch. f. Anat. und Physiol. 1861 ; Sinne.s-

organe der Seitenlinie bei Fischen und Amphibien. Arch. f. mikr. Anat. 1870 ;

Geschmacksorgane der Froschlarven. Loc. cit.
Sjhwalbe, G. Epithel der Papillse vallatre. Arch. f. mikr. Anat. 1867 ; Geschmacks-

organe der Saugetiere und des Menschen. Arch. f. mikr. Anat. 1868 ; Papillae

fungiformes der Saugetiere. Centralbl. f. d. medecin. Wissens"h. 1868.
Solger, B. Seitenorgane der Kuochenfische. Centralbl. f. d. medecin. Wissensch. 1877;

Anat. der Saitenorgane der Fis.die. I. Seitenorgane von Chimaera. II. Seiten-
organe der Selachier. Arch. f. mikr. Anat. Bd. XYII. III. Seitenorgane der

Kuochenfische. Loc. cit. Bd. XYII. und XYIII. ; Feinere Bau der Seitenorgane

der Fi^che. Sitz.-ber. naturf. Ges. z. Halle. 1887.
Stihr, H. Papilla foliata beim Kaninchen. Anat. Anz. 1902 ; Papillae fungiformes,

etc. Zeitschr. f. Morphol. u. Anthropol. 1901 ; Ausdehnung d. Papilla foliata,

etc. Arch. Eutwi -k. Mechanik. 1903.
Wilson, H. Y.,aa I Mittocks, J. E. Lafc. sensory Anlage in Salmon. Anat. Anz. 1897.
Zander, R. Yerbreitungsgebiet d. Gefiihls- u. Geschmacksnerven i. d, Zungenschleitq-

haut, Anat. Anz. 1897.

536 APPENDIX

Olfactory Okgax.

I3:illo\vitz, E. C4erucli.sorgan cl. Cyclostomata. Sitz.-ber.Ak. Berlin. 1904.
Bancroft, J. R. Nasal organs of Pipa americana. Bull. Essex Institute. 1897.
Beard, J. Nose and Jacobson's Organ. Zool. Jahrb. 1889. (See al.«^o p. 534.)
Berliner, K. Geruchsorgan d. Selachier. Arch. f. mikr. Anat. 1902.
Blaue, J. Ban der Naseusjhleirahaut bei Fischen und Amphibien. Zool. Anz. 1882 ;

Bau der Nasenschleimhaut bei Fischen und Amphibien, etc. Arch. f. Anat. u.

Physiol. 1884.
Born, G. Nasenhohlen und Thriinennasengang der Amphibien. Morph, Jahrb,

1876 ; Nasenhohlen und Thriinennasengang der amnioten Wirbeltiere. Morph.

Jahrb. 1879 und 1882.
Broom, R. Org. of Jacobson in: — Miuiopterus, P. Linn. Soc. N.S.W., 1895;

Marsupials and the Horse, lor. n't., 1896 ; Monotremata, Journ. Anat. and

Physiol., Vol. 30. (Cf. also Journ. Anat. and Physiol., Vol. 32, 1898, and Trans.

Roy. Soc, Edin., 1898.)
Bruun, A. v. Merabrana limitans olfactoria. Med. Centralbl. 1874; Riechepithel.

Arch. f. mikr. Anat. 1875 ; Riechepithel und sein Verhalten zum Nervus

olfactorius. Arch. f. mikr. Anat. 1879; Eudigung der 01 f actor iusfasern im

Jakobson'schen Organe des Schafes. Arch. f. mikr. Anat. 1892.
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Wilson, J. T. Anat. and Relations of the Dumb-bell-shaped Bone in Ornitho-

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540 APPENDIX

Virchow, H. Fischaugen. Verh. Ges. Wiirzburg. 1881 ; Die Gefasse im Auge nndin der

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Hoffmann, C. K. Beziehung dei* ersteu Kiementasche zu der Anlage der Tuba Ens.

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van het Gehoororgaan en de Morphologische Betukenis van bet Gehoorbeentje

bij de Reptilien. Verh. Ak. Amsterdam. 1889.
Howes, G. B. A Tympanum in the Genus Raja. Journ. Anat. and Physiol. Vol.

XVII.
Huxley, J. H. On the Repres. of the Malleus and the Incus of the Mammalia in the

other Vertebrata. P. Zool. Soc. 1869.
Iwanzoff, N. Anat. der Knochelchen des mittleren Ohres bei Amphibieu u. Reptilien.

Anat. Anz. 1894.
Kastschenko, N. Das Schicksal der emb. Schlundspalten bei Saugetiereu. Arch. f.

mikr. Anat. 1887.

APPENDIX 541

Keibel, F. Eutw. d. Labyrinth- Auhauges. Aiiat. Auz. 1^99.

Killian, G. Vergl. Anat. und Entwickliingsgesch. tier Ohrmuskelu. Auat. Auz.

1890 ; Ohrmuskelu cles Krokodiles, uebst vorlauf. Bermerk. uber die Homol.

des Musculus stapedius und des Stapes. Jen. Z?itschr. Bd. XXIV. N.F,

XVII.
Kiugsbury, B. F. Columella and Nervus facialis in Urodela. Journ. Comp. Neurjl.

1903.
Kiugsley, J. 8., and Ruddick, AV. H. Ossicula Auditus, etc. Amer. Nat. 1899, aud

Stud. Tuft's Coll., 1900.
Krause, G. Columella d. Vogel, etc. Berlin, 1901.
Krause, R. Eutwickluugsgesch. der hiiutigen Bogengjinge. Arch. f. mikr. Anat. 1890 ;

Eudiguugeu des Xervus acusticus im Gehororgan. Verb. Annt. Ges. 1896;

Entw. d. Aquaeductus vestibuli. Auat. Anz. 1901.
Kuhn. Hautige Labyrinth der Knochenfische. Arch. f. mikr. Anat. 1877; H:iutig*

Labyrinth der Aniphibieu. Lot. cit. 1880; Hautige Labyrinth der Reptilieu.

Lot: cit. 1882.
Lavdowsky, M. Acust. Endapparat der 8iiugeti?rp. Arch. f. mikr. Auat. 18 77.
Lenhossek, M. v. Histol. de.'? Nervensystams und der Sinuesorgane. Wiesbaden, 1894.

(Cf. also Xervenendignngen in den Macula? und Cristaeacustioae. Anat. Hcfte,

Bd. III., Heft 2.)
Xusbaum, J. Anatom. Verhiiltuis zwischen dem Gehororgan u. der Schwimmbl:ue

bei den Cyprinoiden. Zool. Anz. 1881.
Osawa, G. (See p. 537.)
Parker, G. H. (See p. 535.)

Peter. Ohrtrompeten der Stiugetiere und ilu-e Anhiinge. Arch. f. miki*. Auat. 1^94.
Poll, C. Entwick. der Gehorblase bei den Wirb?ltieren. Arch. f. roikr. Anat. Btl.

XLYIII.
Retzius, G. Gehorlabyriuth der Knochenfisohe. Stockholm, 1872 ; Gehorlabyrinth

bei den Kuorpelfischen. Arch. f. Anat. u. Physiol. 1878; Gehororgan der AVir-

beltiere. Arch. f. Anat. u. Physiol. 1880; Gehororgan der "NVirbeltiere. I.

Gehororgan der Fische und Amphibieu. Stockholm, 1881. II. Gehororgau d:r

Reptiheu, Vogel u. Siiugetiere. Stockholm, 1884; Peripheris:he Endigun^s-

weise der Gehornerven. Biol. Uutersuch. 1881; Gehororgan von Polyptera.s

und Calamoichthys. Loc. cit.
Ridewood, X. G. Air-bladder aud Ear of British Clupeoid Fishes. Journ. Anat. and

Physiol. Vol. XXVI.
Rothig, P., and Brugsch, T. Entw. d. Labyrinths beim Huhu. Arch. f. mikr. Anat.

1901.
Ruge, G. Kuorpelskelett d. iius. Ohres d. ^lonotremen, etc. 3Iorph. Jahrb. 1897.
Sagemehl, M. Vergl. Anat. der Fische, III. Morph. Jahrb. 1884.
Sakaki, Y. Ohrmnscheln d. Ainu. Mitteil. Med. Fak. d. K. Japan. Univ. Tokio.

1902.
Sarasin, P. and F. (See p. 535.)
Schaefer, K. L. F'uuction und Fuuctiousentwick. der Bogengjinge. Zeitschr. f.

Psychol, uud Physiol, der Sinuesorgane. 1894.
Scjhauiusland, H. (Seep. 503.)

Schmidt. Ohrmuschel verschied. Siiugetiere. Alt«nburg, 1902.
Schwalbe, G. Circulatiousverhaltms.se in der Gehorschnecke. Festschr. z-i C:irl Lmlwig's

70. Geburtstag. Leipzig, 1886 ; Das Darwin'sche Spitzohr beim meuschlichen

Embryo. Auat. Anz. 1889; luwiefern i.st die menschl. Ohrmuschel eiu

rud. Organ? Arch. f. Auat. u. Physiol. 1889; Gehorgangwulst der Vogel.

Arch. f. Anat. n. Physiol. 1890; Anthropologie des Ohres. " Internat.

Beitriige zur wissensch. Medicin." Festschr. Rudolf Virchow gewidmet zur

Volleuduug seines 70. Lebeusjahres. 1891 ; Auricularhocker bei Reptilieu, etc.

Auat. Anz. 1891 ; Aussere Ohr. Haudb. d. Auat. d. Meuschen (BardelebenV 1897.
Sidoriak, S. Entw.-geseh. d. endulymph. app. d. Fische. Anat. Anz. 1898.
Steifensand. Gehororgan der "Wirbeltiere. Arch. f. Anat. u. Physiol. 1830.
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Zeitschr. f. wiss. Zool. Bd. XXXIII.
Tataroff, D. Muskehi der Ohrmuschel und eiuigj Besonderheiten dc-s Ohrkuorpels.

Arch. f. Anat. und Pbysiol. 1887.
Thompson, d'Arcy ^V . Auditory labyrinth of Orthagoriscus. Dundee, 1889.
Trolt.sth, V. Anat. des iiusseren u. mittleren Ohres, etj. Lehrbuch der Ohrenheilkunde.

7. Aufl. Leipzig, 1881.
TuUberg, T. Lal>yrinth d. Fische, etc. Bihang t. Svenska. Vet. Akad. Handl. Bd.

XXVIII. Stockholm.
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542 APPENDIX

Versluys, J. Mittlere u. Aussere Ohrsphare d. Lacertilia u. Rhyuchocephalia. Zool.

Jahrb. 1898.
Villy, F. Dev. of the Ear and Accessory Organs in the Common Frog. Q.J.M.S.,

1890, and Stud. Owens Coll., Manchester, 1890.
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(See also p. 514.)

H. ALIMENTARY ORGANS.
Teeth.

Adloff, P. Entw.-gesch. d. Nagetiergebisses. Jen. Zeitsclir. Bd. 32. Zahnsyst. v. Sus,

Anat. Anz. 1901 ; Entstehung. d. heut. Saugetierzahuformeu. Zeitschr. f.

Morphol. u. Anthropol. 1903.
Baarrl, J. Nature of the Teeth of the Marsipobranch Fishes. Zool. Jahrb. 1889.
Burckhardt, R. Gebiss der Sauropsiden. Morph. Arb. 189<5.
Carlson, U. Zahnentwick. bei einigen Knochenfischen. Zool. Jahrb. 1894 ; Schmelz-

leiste bei Sterna. Anat. Anz. 1896 ; Zahnersatz bei Agama. Anat. Anz. 1896 ;

Zahnentw. d. diprot. Beuteltiere. Zool. Jahrb. 1899.
Chigi, A, Dentatura dell' Hemicentetes. Monit. Zool. Stat. 1896 ; Becco dei

pappagalli. Anat. Anz. 1902.
Cope, E. D. Mechanical Origin of the Sectorial Teeth of tlie Carnivora. Proc. Amer,

Assoc. Adv. Sci. 1887; Tritubercular Molar in Human Dentition. Jouru,

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Sachs. Ges. d. Wissensch. 1893.
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der Zahnheilkunde, Heft 3—4. Wien, 1890.
Fleischmann, A. Grundform der Baekzahue bei Saugetieren und die Homol. der einzel-

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Flower, W. H. Lectures on Odontology. Brit. Med. Journ. 1871.
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Harrison, H. S. Dev. and Succession of Teeth in Hatteria. Q.J. M.S., 1901, and

Anat. Anz., 1901.
Hertwig, O. Zahnsystem der Amphibieu, etc. Arch, f . mikr. Anat. 1874.
Huxley, T. H. Cranial and dental characters of the Canidaj. P. Zool. Soc. 1880.
Jacoby, M. Die Hornzahne der Cyclostomen, etc. Arch. f. mikr. Anat. 1891.
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SJrenen. Semon's Forschuugsreisen. 1897 (see also p. 504).
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Morphol. des Zahnsystems der lusectivoren. Anat. Anz. 1897 ; Milcligebiss

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Human Molar Cusps. Anat. Anz. 1892; Rise of the Mammalia in North

America. Stud. Biol. Lab., Columbia Coll. Vol. I. ; History of the Cusps of

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Pondrelli, M. Callo embrionale dei Sauropsidi. iVnat. Anz. 1903.
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1888.
Romer, O. Zahnhistol. Studien. Strassburg, 1899.
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APPENDIX 543

Schmelziosen Zahnrudimente des Menschen. Verhdl. d. deutsch. odontolog.
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Elephas und Zahnentwick. der Crocodile, Schwanzmolche und Chlamydoselachus
Morph. Arb. 1894,1895.

Rosenberg, E. Umformungen an den Incisivi der zweiten Zahngeneration beim Men-
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Kyder, J. A. Evolution and Homologies of the Incisors of the Horse. P. Ac. Phliad.
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Saheidt, P. Morphol. und Ontog. des Gebisses der Hauskatze. Morph. Jahrb. 1894.

Schlosser, M. Stammesgeschichte der Hufthiere, et<;. Morph. Jahrb. 1886 ; Deutung
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Schwalbe, G. Theorien der Dentition. Verb. Anat. Ges. 1894.

Selenka, E. Rassen u. Zahuwechsel des Orang-Utan. Sitz.-ber. Ak. Berlin. 1896.

Bemon, R. Zahnentwick. d. Ceratodus. Zool. Forschvmgsreisen. (Jen. Denkschr. IV.)
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Sluiter, C. P. Eizahn und die Eischwiele eiuiger Reptilien. Morph. Jahrb. 1893.

Stach, J . Ersatzgebiss, etc., bei d, Saugetiere. Bull. Acad. Sci. de Cracovie. 1904.

8ternfeld, B. Structurdes Hechtzahnes, etc. Arch. f. mikr. Anat. 1882.

Storrie, J. Tooth of a species of Mastodonsaurus, etc. Trans. Cardiff Naturalists* Soc.
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Studnicka, E. K. Eiuige Modif. d. Epithel.-gewebes, etc. Sitz.-ber. Bohm. Ges. 1899.

Terra, M. Odontographie d. Menschenrasseu. Berlin, 1905.

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Tims, H. AV. M. Origin ot the Mammalian Teeth, Trans. Odont. Soc. of Great Britain.
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Tomes, C. Manual of Dental Anatomy, Human and Comp. London, 1882; Dev. of
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Topiuard, P. L'evolutiou des molaires et premolaires chez les Primates et en partic.
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Treuenfels, P. Zahne von Myliobatis. Inaug. -Dissert. Basel, 1896.

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Wilson, J. T., and Hill, J. P. Dev. and Succession of the Teeth in Perameles, etc.
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Woodward, M. F. Milk-Dentition of Pi'oca\-ia (Hyrax) and of the Rabbit, etc. P. Zool.
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Zograff, N. Zahne der Kuorpelgauoiden. Biol. Centralbl. 1887.

Zuckerkandl, E. Anat. der Mundhohle, mit besonderer Beriicksichtigung der Zahne.
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GLAXD.S OF THE OkAL CaVITY.

Born, G. Xasenhohlen und Thriiuennasengang der Amphibien. Morph. Jahrb.

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19(X).

oU APPENDIX

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Reichel, P. Morphol. der Muadhohlendiiisen der Wirbeltiere. Morph. Jahrb. 1882.
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Gegenbaur, C. Unterzunge des Menschen und der Silugetiere. Morph. Jahrb. 1884;

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Antipa, G. Beziehungen der Thymus zu den sog. Kiemenspaltenorgauen bei Sela-

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APPEXmX 545

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X X

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APPENDIX 547

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N N 2

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550 . APPENDIX

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(c) Reptii.ia.

Butler, G. W. Complete or Partial Suppression of the r. Lung in the Amphisbaenidse
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Cope, E. D. Lungs of the Ophidia. Proc. Amer. Philos. Soc. Vol. XXXIII.

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Osawa,G. (See p. 547.)

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{(l) AVES.

Bar, M. Anat. und Physiol, der Atemwerkzeuge bei den Vogeln. Tiibinger Zool.

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Beddard, F. E. (See Papers in P. Zool. Soc.)
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Siena. 1904.
Fischer, G. Bronchialbaum d. Vogel. (luaug. -Dissert., Leipzig.) Zoologica. 1905.
Hacker, V. TJntere Kehlkopf, etc. Anat. Anz. 1898. Gesang d. Vogel. .Tena,

1900.
Huxley, T. H. Respiratory organs of Apteryx. P. Zool. Soc. 1882.
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A. L. C. nat. cur. t. XIX. 1842.
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APPENDIX 551

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Setterwall, G. C. Syrinx hos Polymyoda Passeres. Lund. 1901.

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Zumstein. Bronchialbaum d. Sanger u. Vogel. Sitz.-ber. ges. Z. Beford. d. ges.

Natiirw. z. Mirburg. 1900.

(e) Mammalia.

Aeby, Cb. Bronchialbaum der Saugetiere und des Menschen. Leipzig, 1880.
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Berlin. 1885
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Howes, G. B. Anat. of the Porpoise. Journ. Anat. and Physiol. Vol. XIV. ; Rabbit

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Tijdscbr. voor Ned. Indie. 1895.
Lornnberg, E. Kehlsack b. Rentier. Anat. Anz. 1902.
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Biol. XCVII.
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Dundee, 1889.
Zumstein, .J. (1) Bronchialbaum des Menschen und einiger Saugetiere. (2) Corrosions-

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CcELOME, Abdominal Pores, and Serous Membranes.

Anderson, J. Cloacal bladders and perit. canals in Chelonia. Journ. Linn. Soc.

1876.
Ayers, H. Pori abdominales. Morph. Jahrb. 1885.
Beddard, F. E. Systematic position of Monitor. Anat. Anz. 1888.
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552 APPENDIX

Bracliet, A. Dev. cle la Cavite hepatoenterique chez les Amphibiens. Anat. Anz.

1896; L'Evol. de la portion ceph. d. C'avites pleurales, etc. Journ. de I'Anat. et

Physiol. 1897.
Bridge, T. AY. Pori abdominale.s of Yertebrata. Journ. Anat. aud Physiol., Vol. XIV.
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1904; Eutw. d. Mesenterien, etc., bei den Luugenfischen. Semou's Forschungs-

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P. Zool. Soc. 1889 and 1892.
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and Physiol. Vol. XXXVI.
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Klaatsch, H. Morphol. der Mesenterialbildungen am Darmcanal der Wirbeltiere.

Morph. Jahrb. 1892. Cf. also Bd. XX.
Mall, F P. Dev. of human Coelome. Journ. Morphol. 1897.

Mathes, P. Morphol. der Mesenterialbildungen bei Amphibien. Morph. Jahrb. 1895.
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Grenzlinien der Pleurasacke und Lagerung des Herzens bei Primaten, etc.

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Morph. Jahrb. 1891.
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K. ORGANS OF CIRCULATION.

(a) Heart and Blood Vessels.

Allen, W. F. Bl. vase. syst. of the Lori.^ata. Proc. Acad. Sci. Washington. 1905.
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P. Zool. Soo. 1907 (Cf. also numerous papers on the Vascular syst. etc. of Reptiles.

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Beddard, F. E., and Mitchell, P. C. He^rt of the Alligator. P. Zool. Soc. 1895.
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Dohrn, A. (See p. 513.)

APPENDIX 553

Ebner, V. v. Klappenartige Yorrichtungen i. d. Arterien d.S.'hwellkorper. Yerh. Anat.

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Eisler, P. Gefass- und peripb. Xerveusystem des Gorilla. Halle a/S, 1890.
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Biologia. 1889.
Fritscb, G. Zur vergl. Auat. des Aniphibienberzeus. Arch. Physiol. 1896.
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Gompertz, C Herz und Blutkreislauf bei uackten Amphibien. Arch. Physiol., 1884.
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1905.
Grafe, E. Urnieren-Pfortader beim Hiihnerembryo. Inaug. -Dissert. Bonn, 1904.
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Greil, A. Anat. u. Eutw.-gesch. d. Herzens u. d. Trunciis art. d. "Wirbeltiere. Yerh.

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Grosser, O., and Brez'na, E. Entwiek. tier Yenen des Kopes u. Halses bei Reptilien.

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Hochstetter, F. Einfluss der Eat wick, der bleibenden Nieren auf die Lage des

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Saugetiereu. Loc cit., 1887 ; Yergl. Anat. und Entwicklungsgesch. des

Yenensysteras der Amphibien und Fische. Morph. Jahrb., 1888. (For the

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Amsterdam, 1894.
Hoffmann, M. Gehirn- u. Riickenmarksvenen . Zeitschr. f. Morphol. u. Anthropol.,

1901.
Holbrock, A. T. Origin of the Endocardium in Bony Fishes. Bull. Mus. Harvard.

1894.
Howes, G. B. Azygos Yeins in the Anurous Amphibia. P. Zool. Soc, 1888 ; Intestinal

Canal of th<' Ichthyopsida, with especial reference to its Arterial Supply and the

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Hubrecht, A. A. W. The Placentation of Erinaceus with Remarks on the Phylogeny

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Hyrtl, J. Bau des Gefasssystem von Hypochthon. Med. Jahrb. d. osterr. Staates.

1844 ; Yergl. Gefasslehre. Med. Jahrb. d. osterr. Staates. Bd. XXIY ; Arterielle

Gefasssystem der Edentaten. Denk. Ak. AYien. Bd. YI. ; Arterielle Gefasssystem

der Monotremen. L.oc. cit. Bd. Y. ; Arterielle Gefasssystem der Rochen. Loc. cit.

Bd. XY. 1858 ; Kopfarterien der Haifische. Loc. cit.A^ll. (And see p. 502.)
Huxley, T. H. Structure of the Skull and Heart of Menobranchus. P. Zool. Soc, 1874.
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Kellicott, ^X. E. (See p. 549.)
Lang, A. Tropho?oeltheorie, etc. Jena, 1903. Centralteile d. Blut-Gefasssystems d.

Tiere. Yiertelj. Sshr. d. Naturf. Ges. Ziirich, 1902.
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Mackay, J. Y. Dev. of the Branchial Arterial Arches in Birds, etc. Phil. Trans.,

1888.
MacCallum, J. B. Muse, architect, and growth of ventricles of heart. .Johns Hopkins
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554 APPENDIX

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Mall, J. P. Blut- und Lymphwege im Diinndarm des Hundes. Abh. d. K. Sachs.

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Marshall, A.M., and Bles, E. J. Dev. of the Blood- Vessels in the Frog. Stud. Owens

Coll., 1890.
Masslow, G. Morph. u. Entw. d. Blntelemente. Arch. f. mikr. Anat., 1897.
Maurer, F, Kiemen und ilire Gefasse bai anuren and urodelen Amphibien, etc. Morph.

Jahrb. Bd. XIV.
Mayer, P. Entwick. des Herzens und der grossen Gefassstamme bei den Selachiern.

Mittheil. Zool. Stat. Neapel. 1887 ; Kreislaufsorganen der Selachier. Loc. cit.

1888.
Mehnert, E. Ursprung und Entw. d. Haenao*asalg3webes (Gefasshofsichel) bai Emys

und Struthio. Morph. Arb. 1896.
Miller, A. M. Dev. of postcaval in Birds. Amer. Journ. Anat. 1903.
Miller, W. S. Blood- and lymph-vessels of the lung of Necturus. Amer. Journ, Anat.

1905.
Muller, E. Morphol. d. Gefasssystems. Anat. Hefte, 1903, 1904.
Miiller, J. Gefasssytem der Fische. Abh. Ak. Berlin, 1839 ; Vergleich. der Blntgefass-

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Nicolai. Verlauf und die Vertheilung dar Venen bei Vogelu, Amphibien, und Fischen,

etc. Isis, 1826.
Owen, R. Heart in the Perennibranchiata. Tr. Zool. Soc. 1835.
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Parker, G. H., and Tozier, C. H. Thoracic derivatives of the postcardinal veins in

Swine. Bull. Mus. Harvard. 1898.
Parker, G. H., and Davies, F. K. Blood-vessels of the heart in Carcharias, Raja, and

Amia. Proc. Soc. Nat. Hist. Boston. 1899.
Parker, T. J. Blood-vessels of Mustelus, etc. Phil. Trans. 1886 Venous system of

the Skate. Trans. N. Z. Instit. 1880.
Parker, W. N, Persistence of the Left Posterior Cardinal Vein in the Frog, with

Remarks on the Homologies of the Veins in the Dipnoi. P Zool. Soc. 1889.
Pitzoruo, M. Arterie subclavia ed ascellarie (Chelonia). Mem. Soc. Sci. Nat. Toscana.

1905.
Popowsky, T. Phylogenesis des Arteriensystems der unteren Extremitiiten bei den

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Venensystems bei Selachiern. Festschr. zum 70. Geburtstag R. Leuckart's.

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Aorta au.sgeheu. Arch. f. Anat. u. Phys. 1843; Aortenwurzeln der Saurier.

Denk. Ak. Wien. 1857; Bildung der Pfortader und der Lebervenen bei Sauge-

tieren. Meckel's An-hiv. 1830; Bau und die Entwickl. des Venensystems der

Wirbeltiere. Bericht ilber das naturhist. Seminar der Univ. Konigsberg. 1838.
Rex, H. Morphol. der Hirnvenen der Elasmobrancliier. Morph. Jahrb. 1892; Mor-
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Anat. des Herzens der Wirbeltiere. Loc. cit. 1890.
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Jena, 1906.
Ruge, G. Varietaten im Gebiete der Arteria femoralis des Meuschen. Morph. Jahrb.

1894; Gefasslehre d. Menschen. Loc. cit. 1883.
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nat. Ser. 5. Tom. XIX; Le coeur dans la serie des Vertrebres. Montpellier et

Paris, 1873.
Salzer, H. Entwick. der Kopfvenen der Meerschweinchen. Morph. Jahrb. 1895.
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1900.
Senior, H. D. Heart of Tarpon atlanticus. Biol. Bulletin, 1907.
Silvester, C. F. Blood. -vase. syst. of Tile-fish. Bull. Bureau of Fisheries for 1904.

Washington. 1904.
Sobotta, J. Entw. d. Blutes, d. Herzens, u. d. Gefassstamme d. Salmouiden, etc. Anat.

Hefte. 1902.

APPENDIX 555

Spencer, W. B. Ceratodus. The Blood- Vessels. Macleay Memorial Vol. 1892.
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Arch. f. d. ges. Physiol. 19! '3.
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Arch. f. Anat. und Physiol. 1889.
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1898 ; Anat. Hefte, Arb. 1901, 1902 ; Anat. Anz. 1903 ; Morph. Jahrb. 1902.
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1830.
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Inaug. -Dissert. Dorpat. 1886.
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Anat. Hefte. 1892.
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Arb. Bd. 4. Cf. also Bd. V.

{b) Lymphatic Systeimc.

Allen, W. F. Lymphatics of Scorpaenichthys. Proc. Acad. Sci. Washington. 1906.
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Kollman, J. Lymphknotchen, Tonsillen, u. Milz. Arch. Anat 1900.
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Lymphherzen der Chelonier. Abh. Ak. Berlin. 1839 ; Lymphgefasse der

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Anat. Bd. XLV.

556 APPENDIX

Nushaum. J. Sac. lymph, pai'avertebralis, etc., bei Knochenfisehen. Anat. Anz. 1903.
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1906.
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Anat. Roma e altri Lab. Biol. 1900.
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1880 ; Vaisseaux lympliat. chez rhorume et les vertebres. Paris, 1886.
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Arch. f. mikr. Anat. 1889 ; Entwick. des adenoiden Gewebes, der Zimgenbalge

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L. URINOGENITAL ORGANS.

General.

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APPENDIX 557

Fiiibringer, M. Vergl. Anat untlEntw.-geschichte der Excretionsorgaue der Vertebraten.

Morph. Jahrb. 1878.
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Jen. Zeitschr. 1887 ; Bauplan des Urogeuitalsystems der Wirbeltiere. (Ich-

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(a) AMPHlOXrS, CiXXOSTOMATA.

Boveri, T. Xiere des Amphioxus. Miineh. Medicin. Wochensclirift. 1890; Nieren-

canalcheu des Amphioxus, etc. Zool. Jahrb. 1892.
Cunningham, J. T. Structure and Dev. of the reproductive Elements in Myxine.

Q.J.M.S. 1887.
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558 APPENDIX

Price. Ontog. d. Bdellostoma. Sitz.-ber. bayer. Ak. d. Wisseusch. Miinchen. 1896.

(,Cf. also Verb. Aoat. Ges. Berlin, 1896, and Zool. Jahrb., 1897.)
Riickert, J. Entstehung der Execretionsorgane b3i Sslachiern. Arch. f. Anat. uud

Physiol. 1888.
Scott, W. B. Entwick. der Petromyzonten. Morph. Jahrb. 1881.
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Auffassung des Urogenitalsystem der Wirbeltiere. Festschrift f. C. Gegeubaur.

Leipzig, 1896.
Speugel, J. W. Excretionsorgane voQ Myxine. Auat. Auz. 1897.

(b) Elasmobbanchii.

Bolau, H. Paaruug uud Fortpflanzung der Scylliumarteu. Zeitschr. f. wiss. Zool.

1882.
Borcea, I. Urogen. syst. d. Elasmobranchier. Arch. Zool. exp. 1906.
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Biol. 1897.
Guitel, F. Nephrostonies et canaux seg. de Selacieus. Arch. Zool. exp. 1900;

Entonnoirs seg. du rein des Selacieus. Loc, cit. 1898.
H Liber, O. Kopulationsglieder der Selachier. Zeitsch. f. wiss. Zool. 1901.
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Anz. 1898; Appendices geni tales in Greenland Shark and other Selachians.

Danish Ingolf. Exped. Copenhagen, 1899.
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1895.
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of Scymuus. Loc. cit. 1882; Vesiculse Seminales and the Sperm atop bores of

Mustelus. Proc. Austr. Assoc. Adv. Sci. 1892 ; On some embryos of Callorhyn-
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Vol. XXXIV.
Petri, K. Copulatiousorgane der Plagiostomen. Zeitschr. f. wiss. Zool. Bd. XXX.
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Amsterdam, 1898, aud Tijdschr. d. Nederl. Dierkuud Vereeniging. 1899.
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Physiol. 1888.
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Anat. Anz. 1888. (See also p. 557.)

(f) Teleostomi and Dipnoi.

Balfour, F. M. Nature of the organ in adult Teleosteans and Ganoids which is usually

regarded as the Head-Kidney. Q.J. M.S. 1882. (See also p. 449.)
Beard, T. Pronephros of Lepido^teus. Auat. Auz. 1894.
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Fullarton, T. H. Dev. of the Plaice. Rep. Fishery Board of Scotland. 1895.
Grosglik, S. Morphol. der Kopfuiere der Fische. Zool. Auz. 1885; Persisteuz der

Kopfuiere der Teleostier. Zool. Anz. 1886.
Guitel, F. Orifices gen. -ur. deBleuuius. Arch. Zool. exp. 1893; Rein de Lepadogaster

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Howes, G.B. Hermaphrodite Genitalia of the Codfish, etc. Jouru. Linn. Soc. 1891;

Arrangement of Living Fishes, as based upon the Study of their Reproductive

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Hyrtl, J. Urogenitalorgane der Fische. Denkschr. Ak. Wien. 1850 ; Zusammenhaug

der Geschlechts- uud Haruwerkzeuge bei den Ganoiden. Loc. cit. 1854.
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APPENDIX 559

Juugersen, H. Eutwickl. iler Gkschlechtsorgaue bei den Knochenfischen. Arb. Inst.

Wiirzburg. 1889; Embryonalniere cles Stors. Zool. Adz. 1893 (cf. also

Saertryk af Videusk. Meddel. fra den naturb. Foren i. Kbhn. 1893) ; Em-
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Kerr, J. G. Male uricogen. org. of Lepidosiren and Protopterus. P. Zool. Soc. 1901.

(See also Proc. Camb. Philos. Soc. Vol. XI.)
MacLeod, J. L'appareil reproducteur des poissons o:seux. Bull. Ac. Belgique. 50

Ann., Ser. III., Tom. I ; La structure et le dev. de l'appareil reproducteur fenielle

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Arch. f. mikr. Auat. 1885.
Rathke, H. GesLhlechtstheile der Fische. Xeueste Schrift. d. natuif. Gesellsch. z.

Danzig. Bd. I. Halle, 1824. (Also in Beitrage zur Geschichte der Thierwelt. II.

Halle, 1824 ; Anat. der Fische. Arch. f. Anat. und Physiol. 1836.
Stuhlmaun, F. Ovarium der Aalmutter (Zoarces viviparus Cuv.) " Abhandl. aus dem

Gebiete d. Xaturwissenschaften." Bd. X. Festschrift zur Feier 50 jjihr. Bestehens

des Xaturwissensch. Vereics, Hamburg, 1887.
Syrski. Reprod.-Organe des Aals. Sitz.-ber. Ak. Wien. Bd. LXIX.

{d) Amphibia.

Bles, E. J. Commimication between perit. cav. and renal vein.'', etc., in the Frog. Proc.

Cambridge Philosoph. Soc. 1898.
Brauer, A. Exkretionsorgaue d. Gynmopbioneu. Zool. Anz. 1900.
Cole, F. J. A case of Hermaphroditism in Kana temp. Auat. Anz. 1895.
Constant int SCO, C. J. Triton (with Miill. d. serving as vas def.). Bull. Soc. de Sci,

Bucarest-Roumanie. 1899.
Dauen, J. Rud. Driise b. weibl. Triton. Morphol. Arb. 1897.
Field, H. H. Dev. of the Pronephros and Segmental Duct in Amphibia. IJull. Mus.

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Zool. Gesellsch. 1892; Vornierenkapsel, veutraleMuskulatur, und Extremitaten-
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Morph. Arb. 1894. (Cf. also Anat. Anz. 1894.)
Filatow, D. B. Eatw.-gesch. d. Eskretionssyst. b. d. Amphibien. Bull. Nat. Moscou.

1904 (and Anat. Anz. 1904).
Fraukel. Ausfuhrwege d. Harnsamenniere d. Frcsches. Zeitsch. f. -wiss. Zool. 1897.
Friedmanu, F. Rud. Eier im Hoden von Rana. Arch. f. mikr. Auat. 1898.
Filrbringer, M. Entw. der Amphibienniere. Heidelberg, 1877. Morph. Jahrb. 1878.
Gemmill, J. F. Entst. d. Miiller'schen Ganges in Amphibien. Arch. Anat. 1897.
Giles, A. E. Dev. of the Fat-bodies in the Frog, etc. Q..J.M.S. 1888, and Stud.

Owens Coll., Manchester, 1890.
Giglio-Tos, E. Corpi grassi degli Anfibi. Atti Ace. Torino. 1894-95 ; Origine dei

Corpi grassi negli Anfibi. Loc. cit. 1895-96.
Hall, R. W. Dev. of Mesonephros and Miill. ducts iu Amphibia. Bull. Mus. Harvard.

1904.
Heidenhain, R. Mikr. Anat. und Physiol, der Nieren. Arch. f. mikr. Anat. Bd. X.

Topographie und Histol. der Cloake und ihrer driisigen Adnexa bei deu einheim-

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Kammerer, P. Salainandra atra u. maculosa. Arch. f. Entwick. Mechanik. 1903.
Kingbury, B. F. Spermatheca and Methods of Fertilisation in some American Newts

and Salamanders. Proc. Amer. Micr. Soc. 1.^95.
Kuappe, E. Bidder'.sche Organ, etc. Morph. Jahib. 1886.
Lebrun, H. L'appareil genital femelle de quelqurs Batraciens indigenes. "La Cellule."

1891.
Levi, E. Pronephros nella Salamandrina perspic. Arch. Ital. Anat. e Embr. 1903.
MacBride. Dev. of the Oviduct in the Frog. Q.J. M.S. 1892.
Marshall, A. M. Certain Abnormal conditions of the Reproductive Organs in the Frog.

Journ. Anat. and Physiol. 1885.
Marshall, A. M.. and Bles, E. J. Dev. of the Ki'ineys and Fat-bodies in the Frog.

Stud. Owens Coll., Vol. II. 1890.
Meyer, F. Urogenital systems der Selachier und Amphibien. Sitz.-ber. naturf. Ges.

Leipzig. 1875.
Mollier, S. Entstehuug des Vomierensystems bei Amphibien. Arch. f. Anat. u.

Phy.siol. 1890.
Miihlen, A. von zur. Urogen. App. d. Urodelen. Inaug. -Dissert. Dorpat. 1894.
Miiller, J. Wolflf'sche Korper bei den Embryonen der Frosche und Kroten. Meckel's

Arch. f. Anat. u. Phys. 1829.
Nussbaum, M. Endigung der Wimpertrichter in der Niere der Anuren. Zool. Anz.

1880; Bau und die Thcitigkeit der Driisen. Arch. f. mikr. Anat. 1886.

560 APPENDIX

Habl, H. Eutvv. d. Miill. Gauges bei Salamaudra. Auat. Hefte. 1903; and Arch. f.

niikr. Anat. 1904.
R.issner, H. Samenatleitendeu AVege bei Rana fuscau. e.'cuknta. Arch. f. miki-

Anat. 1898.
Sampson, L. V. Unusual modes of breeding and dev. amongst. Anura. Amer. Nat.

1900.
Schvvalbe, E. Biol. u. Entw.-gesch. v. Salamaudra afcra u. maculosa. Zeitschr. f. Biol.

1897.
Selenka, E. Emb. Excretiousapparut des kiemenlosen Hylodes. Sitz.-ber. Ak. Berlin.

1882.
Semen, R. Morphol. Bedeutung der Urnitre in ihrem Verhaltnis zur. Vorniere uud

Nebenuiere uud ilber ihre Verbindung mit dem Genitalsystem. Anat. Anz. 1890.
. (See also p. 557.)
Spengel, J. W. Urogenitalsystem der Amphibien. Arb. Inst. Wiirzburg. 1876; Urogen.

syst. d. Amphibien. Arb. In t. Wiirzburg. 1876.
St. George, La Valette. Zwitterbilduug beim kleiutu \Va.s.seru;olch. Arch. f. mikr.

Anat. 1895.
Witticb, von. Harn- und Geschlechtswerkzeuge der Amphibien. Zeitschr. f. wiss. Zool.

1853.
Wiedersheim, E. (See p. 503).
Wilson, G. Dev. of the Miillerian Ducts in the Axolotl. Auat. Auz. 1894 ; Dev. of the

Miilleriau Duct of Amphibiaus. Tr. R. Soc. Ediu. Vol. XXXVIII.
Zeller, E. Befruchtung bei Urodeleu. Zeitschr. f. wiss. Zool. 1890.

(c) Reptilia.

Bertelli, D. Pieghe d. Reni primitivi. Atti. Soc. Toscana. 1897.

Boas, J. E. V. Morphol. der Begattungsorgane der amnioten Wirbeltiere. Morph.

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Brandes, G., and Schoenichen, W. Brutpflege d. schwanzlosen Batrachien. Ab. ges.

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Braun, M. Urogenitalsystem der einheimischen Reptilien. Arb. lust. Wurzburg. 1877.
Gadovv, H. On the Cloaca and the Cop. Org.s. of the Amuiota. Phil. Trans.

Vol. CLXXVIII.
Giacomini, E. Ovidutto dei Sauropsidi. Mouit. Zool. Ital. 1893.
Gregory, E. R. Dev. of Excr. syst. in Turtles. Zool. Jahrb. 1900.
Hoffmann, C. K. Entw.-geschichte der Urogenitalorgane bei deu Reptilien. Zeitschr.

f. wiss. Zool. 1889.
Howes, G. B. Vestigial structures of the reproductive apparatus in the male of the

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Milauo, 1887.
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AViedersheim, R. Entwick. des Urogenitalapparates boi Krokodilen uud Schildkroteu.

Arch. f. mikr. Anat. 1890.
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(/) AVES.

Balfour, F, M., and Sedgwick, A. A Head-kidney in the Embryo Chick, etc. Q.J.M.S.

1879.
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abstract in German.) Tijdsclir. d. Ned. Dierk. Vereen. 1894.
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"im Hiihnerei. Mem. Ac. St. Petersb., 1880.
Felix, W. Aulage des Excretioussystems des Hiihnchens. Festschrift z. SOjahr.

Doctor-eTubiliium von Njigeli und Kolliker, Zurich. 1891.
Gas.ser, L. Entwick. des Urogeuitalsystems der Hiihner-Embryoueu. Sitz.-ber d.

GeselLsch. z. Beford. der gesammteu Naturwisseusch. Marburg. 1879 ;

Entstehuug der Cloakenoffnung bei Huhnerembryonen. Arch. f. Anat. u.

Phy.siol. 1880: Ob?re Eude des Wolff'schen Gauges. Sitz.-ber. der Gesellsch.

zu"Bef(>rd. der gesammt. Naturwis^'. zu Marbin-g. 1878.
Hoffmann, C. K. Etude sur le develoi)pement de I'appareil urogenital des oiseaiix.

Verb. Ak. Amsterdam. 1892.

APPENDIX 561

Kowalevski, R. Urogenitalanlage bei Hiihuerembryonen. Warschau. 1875.
Kupffer, C. Entwick. des Ham- und Geschlechtssj-stems. Arch. f. mikr. Anat. 1866.
Miiller, J. Erectile maanliche Geschlechtsorgane der straussenart. Vogel, etc.

Berlin, 1836
Romiti, W. Bilduiig des Wolff'schen Ganges beim Hiihnchen. Centralbl. f. d. med.

"NVissensch. 1873 ; Bau uud Entwick. des Eierstockes iiud des Wolff'schen Ganges.

Arch. f. raikr. Anat. 1873.
Sedgwick, A. Dev. of the Kidney in its Relation to the Wolffian Body in the Chick.

Q.J. M.S. 1880; Dev. of the Structure known as the Glomerulus of the

Head-kidney in the Chick. Loc. cit. 1880; Dev. of the Wolffian Duct and

Anterior Wolffian Tubules in the Chick. Loc. cit. 1881.
Tichomiroff, A. Androgynie bei den Vogeln. Anat. Anz. 1888.

{g) Mammalia.

Amann, J. A., ]r. Morphol. der Miiller'sehen Gange und iiber accessorische Tuben-

ostieu. Arch. f.Gynakol. 1892.
Arudt, R. Anat. und Entwick. -gesch. des Ruthenknochens. Inaug. -Dissert. Erlangen.

1889.
Beauregard et Boulart. Les appareils genito-urinaires des Balaenides. Tome XVIII.
Beddard, F. E. Ovary of Echidna. Proc. R. Physical Soc. Edin. 1885.
Belling, K. Makrosk. u. mickrosk. Anat. d. Vagina u.d. Uterus. Arch. f. mikr. Anat.

1906.
Bischoff, Th. Die ausseren weibl. Geschlechtsorgane des Menschen und der Affen.

Ab. Bayer. Ak. XIII.
Bonnet, R. Ektodermale Entstehung des Wolff'schen Ganges bei den Saugetieren.
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Arb. Bd. IX.
Brass, A. Weibl. Urogenitalsy stems der Marsupialier. Inaug.-Dissert. Leipzig. 1880.
Braus, H. Glandula bulbo-urethralis d. Menschen. Anat. Anz. 1900.
Broek, A. G. P. van der. Gen. cords of Phalangista. K. Akad. b. Wetensch. 1904 ;
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Cattaneo, G. Organi riproduttori femminili dell' Halniaturus. Milano. 1882 ; Con-
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Clark, J. G. Blood-vessels of human ovary. Johns Hopkins Hosp. Rep. 1900.
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Courant. Pniputialdriisen d. Kaminchens. Arch. f. mikr. Anat. 1903.
Fischer, E. Anat. d. weibl. Urogen.-organe b. Orang-utan. Morph. Arb. 1898.
Fletcher, J. J. Dii-ect communication between the median vaginal cul-de-sac and the
urogenital canal in certain Species of Kangaroos. P. Linn. Soc. N.S.W.
1881 ; Anat. of the Urogenital Organs in Females of certain species of Kangaroos.
P. Linn. Soc. N.S.W. 1882 and 1883.
Forster, A. Aeussere mjinnl. Geschlechtsorgane d. Menschen. Zeitsch. f. Morphol. u.

Anthropol. 1903. •
Fraukl, O. Descensus testiculorum. Sitz.-ber. Ak. Wien. 1900.

Gerhardt, U. Entw. d. bleibenden Niere. Arch. f. mikr. Anat. 1901 ; Kopulations-
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Gilbert, Th. Os priapi der Sanger. Morph. Jahrb. 1892.
Grosser, O. Physiol, bindegeweb. Atresie d. Genitalkanales v. Vf sperugo, etc. Anat.

Anz. 1903.
Grosz, S. Access. Geschlechtsdriisen d. Insektivoren u. Nager. Arcli. f. mikr. Anat.

1905.
Hasse, C. Wanderung des menschl. Eies. Zeitschr. f. Geburtshilfe und Gjnakol.

Bd. XXII.
Haycraft, J. B. Devel. of the Kidney in the Rabbit. Int^rnat. Monatsschr. Anat. 1895.
Hensen, V. Befrucht. und Entwickl. des Meerschweinchens und Kaninchens. Arch.

f. Anat. u. Physiol. 1875.
Hill, J. B. Female Urogen. orgs, in Marsupialia, etc. Proc. Linn. Soc. N.S.W. 1899.
Janosik, J. Vorniere u. Vornierengang. Bull, internat. de I'Acad. d. Sci. d. Eoheme.

190i.
Jarisch, A. Schlagadern des menschlichen Hodens. Ber. d. naturw. Vereins zu

Innsbruck. 1889.
Katz, O. Der Bauchdecke und der mit ihr verkniipften Organe bei den Beuteltieren.
Zeitschr. f. wiss. Zool. 1882.

O O

562 APPENDIX

Keibel, F. Entwicklungsgesch. der Harnblase. Anat. Anz. 1891 ; Harnblase und

Allantois des Meerschweinchens, nebst einer Bemerkung iiber die Entstehung

des Nierenganges (Ureters) bei Saugern. Anat. Anz. 1893 ; Entw.-geschichte

des menschl. Urogenitalapparates. Arch. Anat. 1896 (ef. also Arch. Anat.

1888, and Verh. anat. Ges. 1895) ; Urogen.-apparat. v. Echidna. Jen. Denkschr.

1904.
Kempe, H. A. E. Genitaalstreng-epitheel v. de Wille Bat., etc. Proefschrift, Leiden,

1903.
Kip, M. J. van Erp Taalman. Ontwik. van de Miiller'sche Gang bij Zoogdieren (with

abstract in German). Tijdschr. d. Ned. Dierk. Vereen. 1863.
Klaatsch, H. Descensus testiculorum. Morph. Jahrb. 1890.
Kohlbrugge, J. H. F. Umgestalt. d. Uterus d. Affen nach der Geburt. Zeitschr. f.

Morphol. u. Anthropol.' 1901.
Langhaus, P. Accessorischen Driisen der Geschlechtsorgane. Virch, Archiv. Bd.LXI.
Lataste. Bouchon vaginal des Rongeurs. Zool. Anz. 1883. (Cf. also Actes de la

Societe scientif. du Chili. 1893.)
Lister, J. J., and Fletcher, J. J. Median portion of the Vaginal Apparatus in the

Macropodidae. P. Zool. Soc. 1881.
MacCallnm, J. B. Wolffian body of higher animals. Amer. Journ. Anat. 1902.
Marshall, F. H. A. Cop. org. in Sheep. Anat. Anz. 1901 ; Dev. of corpus luteum.

Q.J.M.S. 1906.
Messing, W. Testikel der Saugetiere. Inaug. -Dissert. Dorpat, 1877.
Meyer, H. Entwick. der Urniereu beim Menschen. Arch, f . mikr. Anat. 1890.
Miller, G. S., Jr. Introitus vaginae of certain Muridae. Proc. Boston Soc. Nat.

Hist. Vol. XXVI.
Moran, H. Transf. epitheliales de la muqueuse du vagin de quelques Rongeurs.

Journal de I'Anat. et Physiol. T. XXV.
Miiller, C. Prostata d. Haussaugetiere, etc. Anat. Hefte. 1904.

Miiller, P. Porenfeld (Area cribrosa) der Nieren des Menschen und einiger Haussauge-
tiere. Arch, f . Anat. u. Phys. 1883.
Nagel, W. Entwick. des Urogenitalsystems des Menschen. Arch. f. mikr. Anat.

1889 ; Entwick. des Uterus und der Vagina beim Menschen. Loc. cit. 1891 ;

Entwick. der Urethra und des Dammes beim Menschen. Loc. cit. 1892.
Oudemans, J. Access. Geschlechtsdriisen d. Saugetiere. Haarlem, 1892.
Palhn. Prostata u. Samenblasen. Arch. Anat. 1901.
Pfliiger, E. Eierstocke der Saugetiere und des Menschen. Leipzig, 1863.
Pinner, O. Uebertritt des Eies aus dem Ovarium in die Tube beim Saugetiere.

Arch, f . Anat. und Phys. 1880.
Pohlmann, A. G. Miill. duct and Cryptorchism. Amer. Med. 1904 ; Dev. relations

of Kidney and Ureter. Johns Hopkins Bull. 1905; Abnormalities in Kidney

and Ureter. Loc. cit.
Poulton, E. B. The structures in con. with the Ovarium Ovum of Marsupialia and

Monotremata. Q.J.M.S. 1884.
Rauther, M. Genitalapparat einiger Nager u. Insectivoren, etc. Jen. Zeitschr. 1903 ;

Chiropteren. Anat. Anz. 1903.
Rautmann, H. Glandula vestib. major. Arch. Anat. 1904.
Retterer, M. Ed. Le dev. du penis et du squelette du gland chez certaines rongeurs.

Comptes Rendus hebdom. des Seances et Mem. de la Soc. du Biol. 1887.
Rielander, A. Paroophoron. Habilitat.-Schrift. Marburg, 1904.
Robinson, A. Position and peritoneal Relations of the Mammalian Ovary. Journ.

Anat. and Physiol., 1887, and Stud. Owens Coll., Manchester, 1890.
Roth, M. Urnierenreste beim Menschen. Festschr. zur Feier des 300jahi'. Bestehens

der Univ. Wiirzburg. 1882.
Selenka, E. Entwicklungsgesch. der Tiere. 4. Heft : Das Opossum. Wiesbaden,

1886.
Sellheim, H. Kastration u. Knochenwachstum. Beitr. z. Geburtshilfe u. Gyna-

kologie. 1899.
Sobotta, J. Corpus luteum b. Kaninchen, etc. Anat. Hefte. Bd. VIII. ; Corpus

luteum d. Saugetiere. Anat. Hefte, Ergebn. 1898 and 1901. (See also Sitz.-

ber. Ges. Wiirzburg, 1904.)
Steinach, E. Vergl. Physiol, d. Mannl. Geschlechtsorgane. Arch. f. d. ges. Physiol.

1894.
Studnicka, F. K. Ausfiihrapp. u. Anhangsdriisen d. mannl. Geschl.-organe. Oppel's

Lehrb. d. vergl. mikr. Anat. 1904.
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(Cf. also Felix, W., and Biihler, A., in Hertwig's Handbuch d. vergl. u. exp.
Entw.-lehre. Jena, 1906.)

APPENDIX 563

FcETAL Membranes, Placenta, etc.

Assheton, R. Ungulate placenta. Phil. Trans. 1906; Placenta of Elephant. Q.J.M.S.

1906.
Beneden, E. van, and Julin, C. La formation des annexes fcetales chez le.s

Mammiferes. Arch. Biol. 1884.
Bonnet, R. Uterinmilch und ihre Bedeutung fiir die Frucht. Stuttgart, 1882 ;
Embryotrophe. Deutsche Med. Wochen.schr. 1899 and 1902; Emb. d. Hundes.

Anat. Hefte. 1902 ; Syncytien, etc., in der Placenta. Monatsschr . f , Geburtshilfo
u. Gynakologie. 1903.
Bumm, E. Entwick. des miitterlichen Elreislaufes in der menschlichen Placenta.

Arch. f. Gynakologie. 1892.
Caldwell, W. H. Arrangement of the Embryonic Membranes in Marsupial animals.

Q.J.M.S. 1884; Embryology of Monotremata and Marsupialia. Phil. Trans.
1887.
Duval, M. Le Placenta des Rongeurs. Paris, 1889-93.
Frommel, R. Entwick. der Placenta von Myotus. Wiesbaden, 1888.
Giacomini, E. Contributo alia migliore conoscenza degli annessi fetali nei Rettili.

Recez. del sacco vitellino e dell' allantoide nella cavita abdominale. Monitore

zool. Italiano. Firenze, 1893.
Godet, R. La structure intime du Placenta du Lapin. Inaug.-Diss. Bern, 1877.
Heiuricius, G. Entwick. und Structur der Placenta beim Hunde. Arch. f. mikr.

Anat. 1889.
Hill, J. P. Placenta of Perameles. P. Linn. Soc. N.S.W. 1895.
Hill, J. P., and Martin, C. J. Platypus embryo from the intra-uterine egg. P. Linn,

Soc. N.S.W. Vol. IX.
Hoohstetter, F. Entw.-gesch. d. Emys lutaria. Denk. Ak. Wien. 1907.
Hofmeier, M. Anat. u. Entwick. d. menschl. Placenta. Zeitschr. f. Geburtshilfe u.

Gynakologie. Bd. XXX.
Hubrecht, A. A. W. Spolia Nemoris (includes Lemurs and Edentates). Q.J.M.S.

1894 ; Placentation of the Shrew. Loc. cit. 1893 ; Phylogenese des Amnions

u. d. Bedeutung des Trophoblastes. Verb. Ak. Amsterdam. 1895. (See also

p. 553.)
Jenkinson, J. W. Placenta in L^gulates. P. Zool. Soc. 1906.
Keibel, Fr. Entwicklungsgesch. der menschlichen Placenta. Anat. Anz. 1899.
Kollmann, J. Placenta b. d. Makaken. Anat. Anz. 1900.
Kolster, R. Embryotrophe placent. Sauger. Anat. Hefte, Arb. 1902, 1903.
Langhans, T. Menschliche Placenta. Arch. f. Anat. u. Phys. 1877.
Maximow, A. Feinere Bau d. Kaninchen-placeuta. Arch. f. Mikr. Anat. 1897.
Minot, C. S. Uterus and Embryo. I. Rabbit. II. Man. Journ. Morphol. 1889 ;

Placenta des Kaninchens. Biol. Ceutralbl. 1890; A theory of the struct, of the

placenta. Anat. Anz. 1891 ; Uterus and Embryo. Journ. Morphol. 1889.
Mitsukuri, K. Foetal Membranes of the Chelonia. Journ. Coll. Sci. Japan. 1890.
Osbom, H. F. Fcetal Membranes of the Marsupials. Journ. Morphol. 1887. (See

also Q.J.M.S., 1883.)
Parker, T. J. Foetal Membranes of Mustelus. Tr. N. Z. Inst. 1889.
Popoff, D. Dottersack-Gefasse d. Huhnes. Wiesbaden, 1894.
Robinson, A. Nutritive Importance of the Yolk-Sac. Journ. Anat. and Physiol.

1892 ; Seg. cavity, etc. Q.J.M.S. Vol. XXXIII.
Ryder, J. A. Origin of the Amnion. Amer. Nat. 1886. (Cf. loc. cit. 1887.)
Schauinsland, H., and Strahl, H. In Hertwig's HanSbuchd. vergl, u. exp. Entw.-lehre.

Jena, 1906.
Selenka, E. Entstehung der Placenta des Menschen. Biol. Centralbl. 1891 ;

Atypische Placentation eines SchwanzafFen. Ann. d. Jardin hot. de Buiten-

zorg. Suppl. II.
Semon, R. Entstehung u. Bedeutung der emb. Hiillen u. Anhangsorgane d. Wirbeltiere.

Compte-rendu des Seances du troisieme Congres internat. de Zool. Leyde, 1895.
Strahl, H. See numerous papers on the Placenta in Arch. f. Anat. und Physiol, and

Marburger Sitzungsber. from 1888 onwards; Uterus post-partum. Anat.

Hefte, Arb. 1894: Puerperale Uterus der Hiindin. Loc. cit. 1895. (See also

Abh. Senckenb. Ges. 1899 and 1904).
Tafani. Condizioni uteroplacentari della vita fetale. Firenze, 1886.
Turner, W. Structure of the human placenta. Journ. Anat. and Physiol, 1873 ; On

the placenta, with special ref. to the theory of evolution. Loc. cit. 1877;

Lectures on the anatomy of the placenta. Edinburgh, 1876.
Waldeyer, W. Bau der Menschen- und Aflfenplacenta. Arch. f. mikr. Anat. 1890;

Placentarkreislauf des Menschen. Sitz.-ber. Ak. Berlin. 1887.
Wiedersheim, R. Entw.-gesch. von Salamandra atra. Arch. f. mikr. Anat. 1890.

0 0 2

564 APPENDIX

Wilson, J. T. Youug specimen of Ornithorhynchus, P. Linn. Soc. N.S.W. Vol. IX.
Wood-Mason, J., and Alcock, A. Uterine villiform papillae of Pteropiatea and their
relation to the embryo. P. Roy. Soc. 1891.

Adrenal Bodies.

Aichel, O. Entw.-gesch. u. Stammesgesch. d. Nebennieren, etc. Arch, f . mikr. Anat.

1900.
Alexander, C. Die Nebenniere und ihre Beziehungen zum Nervensystem. Ziegler's

Beitr. d. path. Anat. u. allg. Path. 1891.
Ciaccio, C. Capsule reuale. Anat. Anz. 1903.

Collinge, W. E. So-called suprarenal bodies in Cyclostoma. Anat. Anz. 1896.
Diamare, V. Corp. surrenali, etc., dei teleostei. Boll. Soc. di Naturalisti in Napoli.

1895; Org. surrenale degli Elasmobranchi, etc. Mem. di mat. e di fis. d. Soc.

Ital. d. Sci. S. 3,T. X.
Felicine, L. Blutgefasssyst. u. d. Zellen d. Nebenniere. Arch. f. mik. Anat. 1902.
Flint, J. M. Reticulum of the adrenal. Anat. Anz., 1899, and Johns Hopkins Hosp.

Reports, 1900.
Giacomini, E. Numerous papers on the suprarenals and interrenals in Atti d. R. Accad.

d. Fisiocritici, 1897, 1898; Monit. Zool. Ital., 1902, 1904; R. Accad. d. Sci. dell'

Istit. di Bologna, 1903-4-5 ; and Atti Ace. IJucei, 1906 ; Capsule surrenali degli

Anfibi, etc. Siena, 1902.
Grynfeltt, E. Org. surrenaux des Plagiostomes. Bull. Sci. France, Belgique. 1903.
Hultgren, E. O., and Anderson, O. A. Nebennieren. Schwed. Gesellsch. d. Aertze.

Gekronte Preisschr. L ipzig, 1899.
Kohn, A. Nebennieren. Med. Wochenschr., Prag, 1898; Anat. Anz., 1899; and Arch.

f. mik. Anat., 1898.
Kose, W. " Carotisdriise " u. "chromafSne Gewebe" bei Vogeln. Anat. Anz.

1902, 1904.
Moore, B., and Vincent, S. Comp. Chemistry of the suprarenal capsules. P. Roy. Soc.

1897, 1898.
Petit, A. Les capsules surrenales. Theses presentees a la faculte des sciences, etc.

Paris, 1896. (See also Bull. Soc. Zool. France. XX.)
Pfaundler, M. Anat. der Nebenniere. Sitz.-ber. Ak. Wien. 1892.
Poll, H. Entw.-gesch. d. Nebenniere. Anat. Anz., 1904, and in Hertwig's Handb. d.

vergl. u. exp. Entw.-lehre. Jena, 1906.
Roud, A. Dev. de la capsule surrenale de la souris. Bull. Soc. Vaudoise d. Sci. Nat.

1903.
Srdniko, O. V. Nebennieren bei Anuren. Anat. Anz. 1900 ; Knochenfische, etc.

Arch. f. mikr. Anat. 1903.
Stilling, H. Anat. der Nebennieren. Virchow's Arch. 1887.
Vincent, Swale. Suprarenal capsules in the lower Vertebrates. Proc. Birmingham

Nat. Hist, and Philosoph. Soc. 1896 ; Morphol. and physiol. of the suprarenal

capsules in Fishes. Anat. Anz. 1897 ; Suprarenal bodies in Fishes, and their

relation to the so-called head-kidney. Tr. Zool. Soc. 1897. (Cf. also P. Roy.

Soc. 1897 ; Proc. Physiol. Soc. 1897; Anat. Anz. 1897, 1900; Journ. Physiol.

1898 ; Journ. Anat. and Physiol. 1903; "Lancet," 1906.)
Weldon, W. F. R. Head Kidney of Bdellostoma, etc. Stud. Morph. Lab. Cam.

1884 ; On the suprarenal Bodies of Vertebrata. Q. J.M.S. 1885.
Wiesel, J. Kompensations-Hypertrophie d. access. Nebennieren bei d. Ratte. Cen-

tralbl. f. Physiol. 1899; Entw. d. Nebenniere d. Schweines. Anat. Hefte,

Arb. Bd. XVI.

INDEX

Amphioxus : — Segmentation of ovum, 5 ;
development of body-cavity, 8 ; in-
tegument, 17; notochord, 47 ; branchial
skeleton, 84 ; lateral or metapleural
folds, 137 ; muscles, 179 ; central
nervous system, 195,204; nerves, 238;
sense-organs, 250, 259; " eyes," 277 ;
oral hood and mouth, 312 ; endostyle,
331 ; alimentary canal, 335 ; histology
of mucous membrane, 344 ; liver, 346 ;
gills, 352 ; blood-vessels and corpuscles,
393, 414, 419 ; urinary organs, 452 ;
genital organs, 463

Cyclostomes : — Segmentation of ovum,
5 ; integument, 18 ; vertebral column,
48 ; skull and branchial skeleton, 84 ;
median hns, 136 ; muscles, 179, 187 ;
brain-membranes, 195 ; spinal cord,
198 ; brain, 204 ; spinal nerves, 233 ;
cerebral neives, 234 ; sympathetic,
248 ; sense-organs of the integument,
250 ; olfactory organ, 259 ; eye, 278 ;
retina, 284 ; auditory organ, 295 ;
oral hood and mouth, 312 ; horn}'^ teeth,
315 ; tongue, 327 ; thyroid, 332 ; ali-
mentary canal, 335 ; histology of
mucous membrane, 344 ; liver, 346 ;
pancreas, 348 ; gills, 352 ; genital pores,
389 ; blood corpuscles, 394 ; heart and
main vessels, 400; arteries, 414 ; veins,
419 ; urinary organs, 452 ; genital
organs, 463 ; adrenal bodies, 493

Fishes -.—Segmentation of ovum, 5 ; in-
tegument, 18 ; exoskeleton, 39 ; ver-
tebral column, 48 ; ribs, 63 ; skull and
visceral arches, 87 ; unpaired fins, 136 ;
paired fins, 137 ; pectoral arch, 140 ;
pelvic arch, 145 ; paired fins, 155 ;
parietal muscles, 179 ; muscles of the
appendages, 185 ; visceral muscles,
187 ; electric organs, 190 ; brain mem-
branes, 195 ; spinal cord, 198 ; brain,
208 ; spinal nerves, 233 ; cerebral nerves,
234 ; sympathetic, 248 ; sense-organs
of the integument, 250; olfactory or-
gan, 261 ; eye, 279 ; retina, 284 ; eye-
muscles and eyelids, 286, 287; auditory
organ, 295 ; relations of auditory organ

with air-bladder, 297; teeth, 314;
tongue, 327 ; thyroid, 332 ; thymus,
333 ; alimentar}' canal, 335 ; histology
of mucous membrane, 344 ; liver, 346 ;
pancreas, 348 ; gills, 355 ; swim-bladder,
361 ; lungs, 377 ; coelome, 388 ; ab-
dominal and genital pores, 390 ; blood-
corpuscles, 394 ; heart and main ves-
sels, 400 ; arteries, 414 ; veins, 419 ;
retia mirabilia, 431 ; lymphatic system,
432; spleen, 435; nutrition of embryo,
436; urinary organs, 452; genital
organs, 463 ; claspers, 486 ; s^renal
bodies, 493

Amphibians : — Segmentation of ovum,
5 ; integument, 20 ; exoskeleton, 42 ;
vertebral column, 54 ; ribs, 66 ; sternum,
70 ; skull and visceral arches, 97 ; un-
paired fins, 137 ; pectoral arch, 141 ;
pelvic arch, 146 ; limbs, 161 ; in-
tegumentary muscles, 176 ; parietal
muscles, 181 ; muscles of the append-
ages, 185 ; visceral muscles, 188 ; brain-
membranes, 195 ; spinal cord, 198 ;
brain, 215 ; spinal nerves, 233 ; cere-
bral nerves, 234 ; sympathetic, 248 ;
sense-organs of the integument, 250 ;
tactile cells, 255 ; olfactory organ,
263 ; Jacobson's organ, 271 ; eye, 281 ;
retina, 284 ; eye-muscles and eyelids,
286, 287; glands of the eye, 288;
auditory organ, 297 ; teeth, 314 ; glands
of the mouth, 325 ; tongue, 328 ;
thyroid, 332; thymus, 333 ; alimentary
canal, 335 ; histology of mucous mem-
brane, 344 ; liver, 346 ; pancreas, 348 ;
gills, 358 ; air-tubes and larynx, 366 ;
lungs, 377 ; coelome, 388 ; blood cor-
puscles, 394 ; heart and main vessels,
403 ; arteries, 414 ; veins, 426 ; lym-
phatic system, 432 ; spleen, 435 ; nu-
trition of embryo, 437 ; urinary organs,
455 ; genital organs, 469 ; copula-
tory organs, 487 ; adrenal bodies, 495

Reptiles : — Segmentation of ovum, 5 ;
amnion, 9 ; integument, 23 ; exo-
skeleton, 42 ; vertebral column, 57 ;
ribs, 67 ; sternum, 72 ; episternum,

566

INDEX

44, 72 ; skull and visceral arches, 108 ;
median fins, 137 ; pectoral arch, 142 ;
pelvic arch, 149 ; limbs, 163 ; in-
tegumentary muscles, 176 ; parietal
muscles, 181; "diaphragm," 184;
muscles of the appendages, 185 ; vis-
ceral muscles, 189 ; brain-membranes,
195 ; spinal cord, 198 ; brain, 220 ;
spinal nerves, 233 ; cerebral nerves,
234 ; sympathetic, 248 ; end-buds, 254 ;
tactile cells, 255 ; Pacinian corpuscles,
257 ; olfactory organ, 265 ; Jacobson's
organ, 271 ; eye, 282 ; retina, 284 ;
eye-muscles and eyelids, 286, 287 ;
glands of the eye 288 ; auditory organ,
299 ; teeth, 316 ; glands of the mouth,
325 ; tongue, 328 ; thyroid, 332 ;
thymus, 333 ; alimentary canal, 339 ;
histology of mucous membrane, 345 ;
liver, 346 ; pancreas, 348 ; air- tubes
and larynx, 369 ; lungs, 379 ; calome,
388 ; abdominal pores, 392 ; blood -
corpuscles, 394 ; heart and main
vessels, 407 ; arteries, 414 ; veins,
428 ; retia mirabilia, 431 ; lymphatic
system, 432 ; spleen 435 ; foetal mem-
branes, 437 ; urinary organs, 459 ;
genital organs, 474 ; copulatory organs,

487 ; adrenal bodies, 495

Birds : — Segmentation of ovum, 5,
amnion, 9 ; integument, 25 ; vertebral
column, 59 ; ribs, 69 ; sternum, 72 ;
skull and visceral arches, 120 ; pectoral
arch, 143 ; pelvic arch, 152 ; limbs,
165 ; integumentary muscles, 176 ;
parietal muscles, 182; "diaphragm,"
184 ; muscles of the appendages, 185 ;
brain-membranes, 195 ; spinal cord,
198 ; brain, 221 ; spinal nerves, 233 ;
cerebral nerves, 234 ; sympathetic,
248 ; taste-buds, 254 ; tactile cells,
255 ; Pacinian corpuscles, 257 ; olfac-
tory organ, 266 ; eye, 282 ; retina,
284 ; eye-muscles and eyelids, 286,
287 ; glands of the eye, 288 ; auditory
organ, 299 ; teeth, 316 ; glands of the
mouth, 326 ; tongue, 329 ; thyroid,
332 ; thymu^s, 333 ; alimentary canal,
339 ; histology of mucous membrane,
345 ; liver, 346 ; pancreas, 348 ; air-
tubes and larnyx, 370 ; lungs and air-
sacs, 382 ; cffilome, 388 ; blood cor-
puscles, 394 ; circulation in embryo,
397 ; heart and main vessels, 411 ;
arteries, 414 ; veins, 428 ; lymphatic
system, 432 ; urinary orgsms, 459 ;
genital organs, 474 ; copulatory organs,

488 ; adrenal bodies, 495

Mammals :— Segmentation of ovum, 5 ;
amnion, 9 ; integument, 28 ; mammary
glands, 34 ; exoskeleton, 44 ; vertebral
column, 60 ; ribs, 69 ; sternum, 72 ;
episternum, 44, 72 ; skull and visceral

arches, 123 ; horns, 130 ; median fins,
137; pectoral arch, 143; pelvic arch, 154
limbs, 168 ; integumentary muscles
178; parietal muscles, 183 ; diaphragm
184 ; muscles of the appendages, 185
visceral muscles, 189; brain membranes
195 ; spinal cord, 198 ; brain, 225 :
spinal nerves, 233; cerebral nerves
234 ; sympathetic, 248 ; taste-buds!
255 ; tactile cells, 255 ; Pacinian cor
puscles, 257 ; olfactory organ, 267
Jacobson's organ, 271; eye, 283;
retina, 284 ; eye-muscles and eyelids,
286, 287; glands of the eye, 288 ^
auditory organ, 301 ; histology of
cochlea, 304; lips, 312; teeth, 318
glands of the mouth, 327 ; tongue
329 ; thyroid, 332 ; thymus, 333 ; ali
mentary canal, 341 ; histology of
mucous membrane, 345 ; liver, 346
pancreas, 348 ; air-tubes and larynx
371 ; lungs, 387 ; ccelome, 388 ; blood
corpuscles, 394 ; heart and main vessels
411; arteries, 414; veins, 428; retia
mirabilia, 431 ; lymphatic system, 432
spleen, 435 ; tonsils, 436 ; fatal mem
branes, 438 ; urinary organs, 461
genital organs, 477 ; accessory genital
glands, 484 ; copulatory organs, 488
adrenal bodies, 495

Abdominal pores, 389

Abomasum, 341

Acetabular bone, 153, 154

Acetabulum, 150

Aehromatin, 3

Acrania, 14

Acrocoracoid process, 143

Acrodont dentition, 316

Acromion, 144

Actinotrichia, 137

Adenoid tissue, 435

Adrenal bodies, 492-496

Adrenalin, 496

Air-bladder, 351

Air-sacs of Birds, 382

Air-tubes, 364

Alimentary canal, 308-346
appendages of, 346-350
mucous membrane of, 344

Allan tois, 9, 336, 431, 437

Allostoses, 79

Alveoli, 316

of lung, 364

Amnion, 9, 437

Amniota, 9

Amphicoelous vertebrae, 52

Amphistylic skull, 88

Ampulla of ear, 292

Ampullary tubes, 253

Anal vesicles, 476

Anarania, 9

INDEX

567

Anapophyses, 62

Annulus tympanicus, 105

Anosmatic, 269

Antibiachiuni, 160

Antlers, 130

Anus, 308

Aortic arches, 398-414

Aponeuroses, 175

Aponeurosis, pulmonary, 382

Appendages, 13

Appendices aiiriculse, 400

Apteria, 28

Aqueduct of Sylvius, 204

Aqueductus cochleoB, 304

Arachnoid, 196

Arachnoid fluid, 198

Archenteron, 6

Arches, neural and haemal, 47

pleural, 63
Archinephric duct, 443
Archipterygium, 137
Arcualia, 47
Argentea, 280
Arrectores pilorum, of Mammals, 33

plumarum, of Birds, 25
Arterial arches, 398-414
Arteries, 393-399, 414-419
Artiodactyle foot, 171
Arytenoid cartilages, 367
Asterospondylic vertebra, 51
Astragalus, 161
Atlas of Reptiles, 57, 58 ; of Birds, 59 ;

of Mammals, 62
Atrial chamber of Amphioxus, 352
Auditory capsule, 77

organ, 290-307 : development of,
290 ; relation to air-bladder, 297

ossicles, 133, 295, 303
Auricula, 295
Auto-intoxication, 496
Autostoses, 80
Autostylic skull, 88
Axis, 58, 59, 62
Axis fibre, 193

B

"Balancers" of Urodeles, 271, 361

Baleen, 31

Barbicels, 27

Barbs, 26

Barbules, 26

Basalia, 155

Basal plate, 77

Basal processes of vertebral column, 49

Basidorsals, 49

Basihj'al, 81

Basilar membrane, 298

Basipterygia, 136, 138, 155

Basiventrals, 50

Bicuspid valve, 411

Bidders organ, 472

Bile-duct, 348

Biserial fin, 137, 157

Blastocoele, 4

Blastoderm, 5
Blastomeres, 4
Blastopore, 6, 308
Blastosphere, 4
Blastula, 4
Blood, 394

corpuscles, 394

vessels, 393
Bodies of vertebrae, 47
Body -axis, 13
Body-cavity, 8, 12, 76, 388
Bones, cartilage, 45

dermal, 41, 42

epineural, 65

epipleural, 65

intermuscular, 65, 42, 67, 172

investing, 45

membrane, 45

replacing, 45
Bowman's capsule, 445
Brachial swelling, 198
Brachium, 160
Brain, development, 195

membranes, 195

general structure, 199

pallium, 200

convolutions, 201, 225

optic vesicles, 201

epiphysis, 202, 215

hypophysis, 202

ventricles, 195, 203

saccus vasculosus, 203, 210
Brain-case, 75
Branchiie, 310, 351
Branchial arches, 75, 81, 100
Branchial basket of Cyclostomes, 86
Branchial clefts, 75, 352-359
Branchiostegal membrane and raysj 90,

95
Breast-bone, 12
Bronchi, 362-387
Bronchioli, 382
Bulbi vestibuli, 492
Bulbus arteriosus, 395
Bulla tympani, 130
Bunodont teetli, 321
Burr, 130
Bursa entiana, 335

Fabricii, 340

Civicum, 311, 336, 339
Calamus, 26
Calcaneal process, 165
Calcaneum, 161
Calcareous bodies, 298
Calyces of ureter, 461
Campanula Halleri, 279
Camptotrichia, 137
Canalis reuuiens, 299
Cannon-bone, 171
Capillaries, 393
Capitulum, 68, 69
Carapace, 43

568

INDEX

Carotid labyrinth, 406
Carpalia, 161
Carponietacarpus, 167
Carpus, 160
Cartilage bones, 45
Cauda equina, 198
Cavum aorticum, 404

endolymphaticum, 291
perilymphaticum, 291
pulmonale, 404
Cement of teeth, 314
Centra, 47

chordal and perichordal, 47
Central canal, 195
Central nervous system, 12, 195
Centrale, 161
Centrosome, 3
Ceratotrichia, 137
Cerebellum, 200
Cerebral flexure, 204
hemispheres, 200
nerves, 234
vesicles, 199
Cerebro spinal cavity, 12
Cheek -pouches., 312
Cheiropterygium, 159
Chevron bones, 58, 62
Chiasma optic, 273
trochleare, 237
Choanse, 259
Chondrocranium, 76, 87
Chondrodentin, 486
Chorda dorsalis, 45
Chordae tendinese, 411
Choriocapillaris, 280, 283
Chorion, 438
Choroid, 275
fissure, 273
"gland," 280
plexus, 197, 205
Chromatin, 3

Chromatophores, 17, 22, 25
Cilia, 17, 18, 20, 195, 259, 263, 332, 344,

443, 359, 364, 443
Ciliary folds and muscles, 275
Circulation (foetal), 395
Cisterna chyli, 433
Claspers, 156, 465, 486
Classification of Vertebrates, 13, 14
Clavicle, 71, 141
Claws, 17, 22, 25, 31, 167
Cleithrum, 142
Clitoris, 480, 487
Cloaca, 310, 336, 339, 341, 344
Coccyx, 63
Cochlea, 290, 299, 301 ; histology of,

304, 305
Coelome, 8, 12, 338

cranial, 76
Coenogenetic, 1
Colliculus seminalis, 483
Colon, 311, 344

Columella auris, 98, 105, 111, 115, 295
Commissures of brain, 201
" Cone "or " Cushion " of Reptiles, 282

Coni vasculosi, 483

Conjunctiva, 276, 287

Constriction of notochord, 50, 51, 52

Conas arteriosus, 395

Copula, 81

Copulatorj'' organs, 486-492

Coracoid, 70, 141

process in Mammals, 143
Coraco-sternum, 70
Corium, 17
Cornea, 276
Corona radiata, 226
Coronary vessels, 405, 410, 428
Corpora adiposa, 435, 474

bigemina, 200, 228

cavernosa, 491
Corpus callosum, 201, 226

Highmori, 483

luteum, 451

spongiosum, 492

sterni, 73

striatum, 200, 228
Corpuscles, of blood, 394
Costae fluctuantes, 69
Crampton's muscle, 282
Cranial rib of Dipnoi, 95
Craniata, 14
Cranium, 75
Cribriform plate, 129
Cricoid cartilage, 367
Crista acustica, 292
Crop, 339

Crura cerebri, 200, 229
Crus, 160
Ctenoid scales, 41
Cuboid, 170
Cuticvda dentis, 314
Cutis, 17
Cycloid scales, 41
Cvclospondylic vetebra, 51
Cystic duct, 348

D

Decidua, 439

Delamination, 6

Dental formula, 324

lamina and papillae, 313

Denticles, dermal, 39

Dentine, 39, 314

Dentition, milk, 319

Dermal fin-rays, 136
skeleton, 39

Derm, 17

Dermotrichia, 137

Deuteroplasm, 3

Development : — General, 4 ; feathers, 25 ;
hairs, 29 ; teats, 35 ; dermal skeleton,
40 ; vertebral column, 45 ; tail of
fishes, 52; ribs, 63; sternum, 70;
skull, 76; horns, 130; limbs, 136;
muscles, 173 ; electric organs, 191 ;
brain, 195, 199 ; central nervous
system, 195 ; nerves, 232 ; sympathetic,
247 ; sensory organs, 249 ; olfactory
organ, 258 ; eye, 273 ; glands of eye.

INDEX

569

288 ; auditory organ, 290 ; alimentary
canal, 308 ; teeth, 313 ; thyroid, 331 ;
thymus, 333 ; alimentary glands, 324,
346, 348 ; gills, 351 ; lungs, 362, air-
bladder, 361 ; air-sacs, 385 ; heart,
395 ; placenta, 438 ; urinogenital
organs, 441 ; genital cells, 450; ad-
renal bodies, 492
Diaphragm, 184, 382
Diastema, 321
Dieneephalon, 199

Digestion, intra- and extra-cellular, 345
Digitigrade, 171
Digits, 160
Diphvcercal tail, 52
Diphyodont, 314
Discoid segmentation, 5
Discus proligerus, 450
Dorsal fissure, 198
Duct, hepatic, 348, 349

naso-lacr} mal, 265, 289
nasopalatine of Myxinoids, 260
pancreatic, 349
saliv^ary, 324
urinogenital, 446
Ductus, Botalli, 406
Cuvieri, 397, 424
endol^mphaticus, 290, 295
ejaculatorius, 484
perilymphaticus, 298,. 304
pneumaticus, 361
venosus, 431
Duodenum, 311
Dura mater, 196
vertebralis, 195

Ear, 290-307

Ectoderm, 5

Egg-cell, 2

Electric lobes in Rays, 211

organs, 190
Embryonic area, 9
Enamel, 39, 314
Enamel organs, 313
End-buds, 254
Endocardium, 395
Endochondral bone, 45, 80
Endocranium, 197
Endoderm, 5
Endolymph, 291
Endorachis, 195
Endoskeleton, 44
Endostyle, 331
Ensiform process, 73
Enteric canal, 308
Enteroccele, 8
Entoglossal, 82
Ependj-me, 193
Epiblast (see Ectoderm)
Epilxjly, 6
Epicoracoid, 71, 144
Epiderm, 6, 17
Epididymis, 445, 454, 476, 483

Epidural space, 196
Epiglottis, 370, 371
Epiphyses of vertebrae, 60
Epiphysis of brain, 202, 215
Epipubic process, 146
Epipubis, 147, 149, 151, 155
Episternum, 44, 72
Erythrocytes, 394
Ethmoidal region of skull, 78
Eustachian aperture and tube, 105, 130,
295

valve, 413
Exoskeleton, 39

External auditory meatus, 130, 295
Extrabranchials, 88
Eye, 273-285

glands in connection with, 277, 288

muscles of, 277, 286
Eyelashes, 287
Eyelids, 277, 287
Eyes, rudimentary, 276

Fallopian tube, 477

Falx, 197

" Fan " of Birds, 282

Fascia;, 175

Fat-bodies, 435, 474

Fauces, 436

Feathers, 25

Femoral pores of Lizards, 23, 487

Femur, 160

Fenestra oralis, 98, 111, 129, 298

rotunda, 111, 129
Fertilization of ovum, 3
Fibula, 160
Fibulare, 161
Filoplumes, 27
Filum terminale, 198
Fin-rays, 136, 137, 155-159
Fins, 13

unpaired, 136 ; paired, 137
Fissure of Rolando, 228
Flocculi, 225, 229
Flumina pilorum, 31
Foetal membranes, 436
Fontanelles, 87
Food-yolk, 3, 5
Foramen, condylar, 129

lacerum anterius, 129

lacerum posterius, 129

lacrymal, 131

of Monro, 204

opticum, 129

ovale, of skull, 129 ; of heart, 412

Panizzae, 410

rotundum, 129
Fornix, 201, 220, 226
Fossa ovalis, 413

patellaris, 283
Fovea centralis, 285
Frontal clasper, 487
Fundus, 341
Furcula, 143

570

INDEX

G

Gall-bladder, 348

Ganglia, habenulse, 201
of cerebral nerves, 235
sympathetic, 247

Ganoid scales, 41

Gartner's duct, 481

Gasserian ganglion, 235

Gastrula, 5

Genital cells, development of, 450
ducts, 446
organs, 463-485

Geniculate ganglion, 235

Genital pores, 390, 463

Germinal epithelium, 450
layers, 5
membrane, 4
spot, 3
vesicle, 3

Gill-arches and clefts, 75, 81, 100

Gills, 310, 351

external, 352, 358
spiracular, 88, 356

Gill-rakers, 357

Gill-sacs, 6, 352

Gizzard, 339

Glands : — ampullary, 484 ; anal, 485 ;
Blandin's, 329 ; Bowman's 270 ; Brun-
ner's, 345; carotid, 333, 406; cho-
roid, 280 ; Cowper's, 485 ; digestive,
345 ; gastric, 335 ; Harderian, 28S ;
infraorbital, 327 ; inguinal, 485 ; in-
tegumentary, 17, 33 ; intermaxillary
or internasal, 325 ; labial, 325 ; lac-
rymal, 288 ; lingual, 325, 326 ; lymph-
atic, 435 ; mammary, 34 ; Meibomian,
288; mucus, 22; multicellular in
skin, 20, 21 ; " musk," 476 ; (of Croco-
diles), 23 ; nasal (external of Birds),
267 ; of Bartholini, 485 ; of claspers,
19 ; of eye, 288 ; of Lieberkiihn, 345 ;
of Moll, 290 ; of mouth, 324-327 ; of
olfactory mucous membrane, 264 ;
oviducal, 463 ; palatine, 325 ; parotid,
327 ; pharyngeal, 325 ; poison, 19, 22,
325 ; preputial, 485, 491 ; prostate,
484, 485; rectal, 336; "salivary" of
Petromyzon, 324 ; sebaceous, 29, 33 ;
Stenson's, 270 ; sublingual, 325, 327 ;
submaxillary, 327 ; sweat, 33 ; unicel-
lular, 20 ; urethral, 485 ; uropygial, 25

Glandules pterygopodii, 19, 486

Glans penis, 488, 492

Glenoid cavity, 71, 141

Glomerulus, 443

Glomus, 443

Glossohyal, 82

Glottis, 362

Goblet-cells, 18, 265

Gonads, 450

Giandry's corpuscles, 255

Granular-cells in Lamprey, 19

Gut, post-anal, 9

Gullet, 310

G^'mnoarian condition of ovary, 466
Gymnokrotaphic type of skull, 113
Gyri, 201, 225

H

Hasmal spines, 50
Haemoglobin, 394
Hsemolymph glands, 435
Hairs, 17, 28, 257
Hallux, 169

Hassal's corpuscles, 335
Head, 12

cavity, 76

fold, 9
Heart, 400-414
Helix, 307
Hemibranch, 355
Heredity, 1
Hermaphrodite structures, 463, 466, 472,

477
Heterocercal tail, 53
Heterocoelous vertebra, 59
Heterodont dentition, 314
Hibernating glands, 435
Hilum, 461
Hippocampal commissure, 226

lobe, 214
Hippocampus, 215

major, 227
"Histology, 2

Holoblastic segmentation, 5
Holobranch, 355
Homocercal tail, 53
Homodont dentition, 314
Hoofs, 171
Horns, 31, 130
Humerus, 160
Humour, aqueous, 276

vitreous, 275
Hyaloid membrane, 284
Hyaloplasm, 3
Hymen, 480
Hyoid arch, 81
Hyomandibular, 82
Hyostylic, 88
Hypapophysis, 59, 62
Hypermastism, 37
Hyperthelism, 37
Hypoblast (see endoderm)
Hypochorda, 47
Hypoischiatic process, 146
Hypophj^sis, of brain, 202
Hyporachis, 27
Hypural bones, 53

I

Ichthyopsida, 14
Ichthyopterygium, 159
Heo-colic valve, 311
Ileum, 311
Ilium, 146
Impregnation, 3
Incus, 133, 303

INDEX

571

Infraventrals, 50
Infiindibula (of lung), 364, 388
Infundibulura, 202
Ingluvies, 339
Inguinal canal, 482
Innominate trunk, 414
Integument, 17-38

sense-organs of, 25, 250-258
Integumentary muscles, 175
Intercalary- pieces of vertebrae, 49
Intercentra, 55, 57, 58
Interclavicle (see prosternum)
Interdorsals, 49
Intermaxillary sinus, 98
Intermedium, 161

Intermuscular bones, 42, 65, 67, 172
Interneural plate, 49
Interorbital septum, 77, 112
Interrenal bodies, 492-496
Interventrals, 50
Intervertebral discs, 57, 59
Intestine, small and large, 310, 335
Iridocytes, 20
Iris, 275

Ischial callosities, of Apes, 31
Ischiopubic foramen, 149
Ischium, 148
Islets of Langerhans, 350
Iter, 204

Jacobson, anastomosis of, 238, 245

organs of, 271
Jaws, 312
Jejunum, 311
Jugular plate in Ganoids, 89

Karyokinesis, 3
Kidney, 441-463

K

Labia majora and minora, 483

Labial cartilages, 82

Labyrinth, bonj- and membranous, 291

Lacrymal glands, 288

Lacteals, 393, 433

Lagena, 290

Lamina cribrosa, 95, 129

fusca, 275

perpend icularis, 129

terminalis, 201

spiralis ossea, 304
Laminae papjTace^e, 131
Lanugo, 31

Larjnigeal pouches, 374
Laryngotracheal chamber, 365, 366
Larynx, 362, 364
Lateral fin-folds, 137, 424
Lateral folds, of embryo, 9
Lateral line, sensory organs of, 253
Lateral malleolus, 168

Lateral plates, of mesoderm, 9

Lens, crystalline, 274

Leptotrichia, 42, 137

Leucocytes, 18, 394

Leydig's cells in Urodeles, 20

Limbs, 13, 137

Limitans externa, 284

Linea alba, 70

Ligaments, intervertebral, 47

Lips, 312

Liquor amnii, 9

Liver, 6, 346

Lobi inferiores, 210

Lumbo-sacral enlargement, 198

Lung-pipes, 382

Lungs, 6, 351, 361, 362, 377-388

Lymph, 393

hearts, 433

sinuses, 393, 432

vessels, 393, 432
Lymphatic glands, 435

system, 432
Lymphoid substance in relation with
urinogenital organs : — of Teleostei,
Ganoidei, Dipnoi, 455 ; of Amphibia,
Reptilia, 435, 474, 476
Lymphoid tissue, 435
Lyssa, 331

M

Microsmatic, 269
Macula acustica, 292

lutea, 285
Malleus, 133, 303
Malpighian capsule, 445
Mammary glands, 34

pockets, 35

pouch, 35
Mandibular arch, 80
Manubrium of sternum, 73
Manus, 160
Marsupial bones, 155

pouch, 35, 481
Maturation, 3
Meatus, internal auditory, 129

external auditory, 130, 295
Meckel's cartilage, 81
Mediastinum, 389
Medulla oblongata, 200
Medullary cord, 195

groove, 195

sheath, 193
Membrana reticularis, 305

semilunaris, 371

tympaniformis, 371

tympani (see tympanic membrane)

vestibularis, 302
Membrane bones, 45
Membrane of Corti, 305.

of Reissner, 302
Membranous labyrinth, 290
Meninges, 195
Menisci of vertebr*, 57, 59
Meroblastic segmentation, 5
Mesencephalon, 199

572

INDEX

Mesentery, 310

Mesoblast (see mesoderm)

Mesoderm, 5

Mesodermic somites, 9

Mesonephrie duct, 446-450

Mesonephros, 441, 445

Mesopterygium, 140, 156

Mesorchium, 463

Mesovarium, 463

Metacarpus, 160

Metamerism of head and body, 75, 181,

233, 237
Metanephric duct, 445
Metanephros, 441, 446
Metapleural folds, 137
Metapophyses, 62
Metapterygium, 140, 155
Metatarsus, 160
Metencephalon, 199
Microsmatic, 269
Milk-dentition, 319
Mitosis, 3
Mitral valve, 411
Mixopterygium, 156, 465, 486
Morphology, 2
Morula, 4
Mouth, 308

Mucous membrane, 344
Miillerian duct, 446
Muscular system, 173-189
Muscles : — cranial, 187 ; integumentary,
175 ; involuntary, 173 ; of diaphragm,
184 ; of extremities, 185 ; of eye, 286 ;
of feather-sacs, 25 ; of iris, 275 ; of
trunk, 179 ; parietal, 173 ; visceral,
173, 187 ; voluntary, 173
Muscles, arrectores pili, 33

ciliary, 275

Crampton's, 282

cremaster, 482

intercostal 182

levatores costarum, 182

masseter, 189

mylohj'oid, 188

obliquus externus and profundus,
181, 182

pauniculus carnosus, 178

papillary, 411

pectoralis, 182

platysma myoides, 178

psoas, 182

pyramidalis, 183

rectus, 181

sphincter colli, 178

stapedius, 189, 303

sternohyoid, 173, 181, 183

tensor tympani, 303

trochlear, 286
Musk -gland of Crocodiles, 23
Myelencephalon, 200
Myelin, 193
Myocardium, 395.
Myocommas, 63, 179
Myomeres, 179
Myotomes, 9, 442

N.
Nails, 31, 32
Nares (see Nostrils)
Nasal capsules, 78
Naso-lacrymal duct, 265, 289
Naso-palatine canal, 273

duct of Myxine, 260
Navicular, 170
Neck, 12
Neocranium, 87
Neostoma, 203, 308
Nephrostomes, 392, 443
Nephrotome, 441
Nerve-cells, 193

eminences, 250

fibres, 193

lateral, 245

phrenic, 185

plexuses, 233

sacs of Teleostomes, 253
Nerves : — cerebral, 234-247 ; olfactory,
258 ; optic, 273 ; oculumotor, trochlear
and abducent, 237 ; trigeminal, 240 ;
facial, 242 ; auditory, 244 ; glosso-
pharyngeal, 244 ; vagus, 245 ; spinal
accessory, 245 ; hypoglossal, 246
Nerves, spinal, 233

spino-occipital, 231, 246
Nervous system, 193-249

central, 195-230

peripheral, 230-247

sympathetic, 247-249
Nervus terminalis, 236
Neural, arches, 48

plate, 49

ridge, 232

spines, 49

tube, 11
Neuraxis, 193
Neurenteric canal, 195
Neurilemma, 193
Neurocranium, 75, 76
Neuroglia, 193
Neuropore, 204
Nictitating membrane, 287
Nose (external), 270
Nostrils, 259, 261, 263, 265, 266, 270
Notochord, 6, 11, 45
Nuchal plates, of Reptiles, 43
Nuclear membrane, 3
Nucleolus, 3
Nucleus, 3
Nucleus pulposus, 60
Nutrition, organs of, 308

O

Obturator foramen, 145, 149
Occipital condyles, 97, 111, 122, 127
Odontoblasts, 313
Odontoid process, 57
OEsophageo-cutaneous duct, 354
(Esophagus, 310, 335
Olecranon, 169

INDEX

573

Olfactory capsules, 78

lobes, 200

organ, 258

scrolls, 267

tract and bulb, 210
Omentum, folds of, 389
Omosternum, 70
Ontogeny, 1
Oosperm, 4
Opercular bones, 90
Operculum, 88, 356
Opisthocoelous vertebrfe, 52
Optic capsules, 77

lobes, 200

thalami, 201

vesicles, 201
Oral cavity, 310, 312
Orbital ring, 94, 103
Organ of Corti, 301
Organs, 2
Organ-systems, 2
Oro-nasal groove, 261
Os clitoridis, 492

cloacEe, 150

penis, 492

uteri, 477
Ossification, centres of, 80
Osteocranium, 79
Otic bones, 133, 295, 303
Otoliths, 292
Ovarian follicle, 450
Ovary, 450
Oviducal gland, 463
Oviduct, 446
Ovipositor, 465
Ovotestis, 472
Ovum, 2

Pacinian corpuscles, 257
Paheocranium, 85
Pal;eontolog3% 1
Paheostoma, 203
Palate, 112, 118, 122, 132, 310
Palatopterygoid, 81
Palatoquadrate, 81
Palingenetic, 1
Pallium, 200
Palpebral, 277
Pancreas, 6, 348
Panniculus adiposus, 33

carnosus, 178
Papilla foliata, 255
Papillary muscles, 411
Parachordal cartilages, 76
Paradidymis, 445
Paraphysis, 200
Parapineal organ, 208
Parasternal elements, 42
Parathyroids, 333
Parietal foramen, 103, 112, 114,116

organ, 202, 221
Paroophoron, 445
Parorchis, 445, 454

Parotoids, 21
Parovarium, 445, 480
Pars acetabularis, 151

basilaris, 298
Patella, 172

Pearl- organs of Cj^rinoids, 252
Pecten, 282
Pectineal process, 152
Pectoral arch, 140
Pelvic arch, 145

plates, 145
Pelvis of kidney, 461
Penis, 487-491
Peuna, 27

Pentadactyle limb, 159
Pericardium, 388, 394
Perichondral bone, 45
Perichondrium, 41
Peridural space, 196
Perilymph, 291

Perimeningeal space and tissue, 195
Perinjeum, 462, 480
Periorbita, 276
Perissodactyle foot, 171
Peristalsis, 308
Peritoneal canals, of Reptiles, 392

funnels, 466
Peritoneum, 308
Pes, 160
Pessulus, 371
Peyer's patches, 308, 345
Phagocytes, 394, 435
Phalanges, 160

Pharyngeal teeth of Teleosts, 83, 95, 315
Pharynx, 310
Phosphorescent organs 19
Phylogeny, 1
Physiology, 2
Physoclisti, 14, 361
Physostomi, 14, 361
Pia mater, 196
Pigment of skin, 17, 20
Pineal organ, 103, 201, 208, 228
Pinna, 295, 305
Pisiform, 163
Pituitary body, 6, 202

sac, 261

space, 77
Placenta, allantoic, 11, 438-440

umbilical, 436-438
Placodes, 236
Placoid organs, 39
Plantigrade, 171
Plastron, 43

Platj'basic type of skull, 77
Pleura, 389
Pleurocentra, 57
Pleurodont dentition, 316
Pleuronectidje, asymmetry of head, 94,

281
Plexus (or telse) choroidei, 197, 205
Plica circulares, 346

fimbriata, 331

semilunaris, 288, 346
Pluma, 26

574

INDEX

Pneumatic bones, 386

Poison-fangs, 317

Poison -organs of Fishes, 19

Polar cells, 3

Pollex, 169

Polymastism and polythelism, 37

Polyphyodont, 314

Pons Varolii, 203, 229

Postcentra, 50

Precentra, 50

Prehallux, 162, 170

Prelacteal dentition, 319

PrepoUex, 170

Prepubic process, 146

Prepuce, 491

Primitive meninx, 195

streak, 6
Pro-amnion, 437
Pro-atlas, 57, 62
Processus digitiformis, 336

falciformis, 279

vermiformis, 344
Procoelous vertebrae, 56
Procoracoid, 71, 141
Proctodseum, 6, 308
Promontory, of sacrum, 62
Pronation, 169
Pronephric duct, 443
Pronephros, 441, 443
Pronucleus, male and female, 3
Propterygium, 140, 155
Proscapula, 142
Prostate, 484
Prosternum, 44, 72
Protocercal tail, 53
Protovertebrae, 9
Proventriculus, 339
Psalter ium, 341
Pseudobranch, 357
Pseudo-electric fishes, 190

-turbinal, 266
Pterygiophores, 136
Pter^'gopodium (see Clasper)
Pterylffi, 28
Pubis, 148

Puncta lacrymab'a, 289
Pupil, 275

Pygal plates of Reptiles, 43
Pygostyle, 60
Pylangium, 395
Pyloric caeca, 336

valve, 311

Quadrate cartilage, 81

R

Rachis, 27

Radiale, 161

Radii of fins, 155-159

Radius, 160

Rami communicantes, 247

Receptacula seminis, 474

Receptaculum chyli, 433

Rectrices, 27

Rectum, 311, 336

Remiges, 27

Reissner's fibres, 199

Reproduction of tail in Lizards, 58

Respiratory organs, 351

Rete testis, 445

Retia mirabilia, 361, 406, 431

Reticulum, 341

Retina, 273, 284

Ribs, 12, 63

abdominal, 42, 43, 67, 70

caudal, of Hatter ia, 67

cranial, of Dipnoi, 95

false and true, 67
Rolando, fissure of, 228
Rostrum, 88
Rumen, 341
Ruminant stomach, 341

S

Sacculi alveolares, 388

Sacculus, 290

tSaccus vasculosus, 203, 210

Sacrum, 56, 60, 62

Sauropsida, 14

8avi's vesicles, of Torpedo, 253

Scala media, 304

tympani, 299

vestibuli, 98, 299
Scales, 18, 20, 22, 23, 25, 31, 39
Scapula, 71, 141
Scapus, 27
Schizocoele, 8
Sclerotic, 276

plates, 282
Scrotal sacs, 482
Secodont teeth, 321
Segmental duct, 443
Segmentation cavity, 4

nucleus, 3

of head, 75, 237

of oosperm, 4
Selenodont teeth, 321
Semicircular canals, 290
Sense-capsules, 76

Sense-organs of integument, 25, 250-258
Sensory organs, 249-307
Septum atriorum, 401, 403

lucidum, 204

oblique, 384

transversum, 185

ventricular, 408
Serosa, 437

Sesamoids, 170, 172, 175
Sexual reproduction, 3
Shell-gland, 463

INDEX

575

Sinus-feathers, 28

hairs, 30

rhoraboidalis, 198

venosus, 395
Skeletogenous layer of vertebral column,

45
Skin, 17
Skull, 74

bones of, 82
Slime-sacs, in Myxinoids, 19
Sole-horn, 32
Somatopleure, 8
Spermarj% 450
Spermatophores, 474
Spermatozoon, 3, 451
Sperm-cell, 3

sac, 465
Sphenoidal fissure, 129
Sphincter oculi, 287
Spina scapulae, 144
Spinal cord, 195, 198
Spines, neural and haemal, 49, 50

of Porcupine, 31
Spino-occipital nerv^es, 231
Spiracle, 88, 105, 294, 356
Spiracular cartilages, 88
Spiral valve of intestine, 335
Splanchnocranium, 75, 80
Splanchnopleure, 8
Spleen, 435
Spongioplasm, 3
Spots, blind and yellow, of retina, 284,

285
Stapedial plate, 98, 298
Stapedius muscle, 303
Stapes, 133, 303

Stegokrotaphic type of skull, 113
Sternebrte, 73
Sternum, 12, 67, 70
Stomach, 310, 335
Stomoda?um, 6, 308
Stratum corneum, 17

germativum, 17

Malpighii, 17
Subarachnoid space, 197
Subdural space, 196
Sublingua, 331
Submucosa, 308
Subnotochordal rod, 47
Suctorial mouth, 352, 359
Sulci, 201, 225
Superior vermis, 229
Supination, 169
Supraclavicle, 141
Supracondyloid foramen, 168
Supradorsals, 49
Suprarenal bodies, 492-496
Suprascapula, 71
Supratemporal arcade, 100
Suspensorium, 81
Swim-bladder, 351, 361
Sylvian aqueduct, 204
Sylvian fissure, 228
Sympathetic nervous system, 247

Symphysis pubis and ischii, 154
Symplectic, 82
Synangium, 395
Syrinx, 369, 370

Tactile cells and corpuscles, 255
Tail, 13
Tail-fold, 9

Tapetum lucidum, 280
Tarsalia, 161
Tarsometatarsus, 168
Tarsus, 160
Taste, organs of,' 254
Teat«, 35

Tectospondylic vertebra, 51
Tectum synoticum, 83
Teeth, 40, 94, 96, 112-119, 123, 135,
313-324

horny, 86, 105, 315, 316
Telencephalon, 199
Tendons, 175
Tensor choroidese, 275
Tentorium, 197
Testis, 450
Thebesian valve, 413
Thecodont dentition, 316
Thoracic duct, 432
Thread-cells in Myxinoids, 19
Thrombocytes, 394
Thymus, 6

Thyro-hyal ligament, 135
Thyroid, 6, 331

cartilage, 135, 371
Tibia, 160
Tibiale, 161 •
Tibiotai-sus, 168
Tissues, 2
Tongue, 327

muscles of, 188
Tonsils, 436
Tori, 32

Tortoiseshell, 25
Trachea, 362
Tragus, 307

Truncus arteriosus, 395
Trabecular cranii, 76
Transverse process of vertebra, 49
Triconodont tooth, 322
Tricuspid valve, 411
Tritubercular tooth, 322
Trochanters, 168
Tropibasic type of skull, 77
Trunk, 12

Tuber acusticum, 242
Tuberculum, 68, 69

impar, 329
Tuber ischii, 154
Tubules of kidney, 445, 446
Tunica vaginalis, 482
Turbinals, 264, 265, 266, 268
Tusks, 324

576

INDEX

Tympanic membrane and cavity, 105,

111, 130, 295
Tympanum of syrinx, 371
Typhlosole, 335

U

Ulna, 160
Ulnare, 161
Umbilical cord, 440

vesicle, 438
Uncinate processes, 69
Unguli grade, 171
Uniserial fin, 156
Urachus, 440, 462
Ureter, 445
Urethra, 484, 492
Urinary bladder, 336, 458, 461, 462

of Fishes, 455

organs, 441-463
Urinogenital organs, 441-485
Uropygium, 25
Urostyle, 53, 54
Uterus, 463, 477

masculinus, 484, 485
Utriculus, 290
Uvula, 310

V

Vertebrates, classification of, 13, 14

gradual development in time of, 16
Vertebrarterial canal, 62, 68, 69

foramen, 59
Vesicula prostatica, 485

seminalis, 457, 465, 469, 472, 483
Vestibulum oris, 312
Vestigial limbs, 162, 164
Vexillum, 27
Vibrissa?, 30, 257
Villi, of intestine, 346

of placenta, 11, 439
Viscera, 12
Visceral arches, 80, 100

tube, 11
Vitelline membrane, 3
Vitello-intestinal duct, 9
Vitellus, 3

Vitreous body or humour, 275
Vocal cords, 364, 367

sacs of Anura, 367
Vomero-nasal organ, 271
Vulva, 483

W

Weberian ossicles, 297
Wolffian body, 445
duct, 450

Vagina, 477
Vaginal cajcum, 480
Valve of Vieussens, 203
Valvula cerebelli, 211, 214, 215
Valvular conniventes, 346
Variability, 1
Vascular system, 393-432
Vas deferens, 450

epididymitis, 483
Vas efferentia, 445, 454
Veins, 393-399, 419, 431
Velum, 352

palati, 310

transversum, 200
Vent, 308
Ventral fissure, 198
Ventricles of brain, 195, 203
Vermis, 229
Vertebrae, 47
Vertebral column, 12, 45

theory of skull, 74

Xiphisternum, 71
Xiphoid process, 73

Yellow bodies, 476
Yolk, 3, 5
Yolk-sac, 9

Z

Zygantra, 58
Zygapophyses, 51
Zygokrotaphic type of skull, 113
Zygomatic arch or bar, 100, 106,

132
Zygosphenes, 58

K. CLAY AND

LTD., BREAD ST. HILL, E.G., AND BUNGAY, SUFFOLK.

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