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Marine Biological Laboratory Library
Woods Hole, Mass.
Presented by
the estate of
Dr. Herbert W. Rand
Jan. 9, 1964
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COMPARATIVE ANATOMY
OF
VERTEBRATES
"
MACMILLAN AND CO., LIMITED
LONDON . tOMBAY . CALCUTTA
MELBOURNE
THE 'MACMILLAN COMPANY
NEW YORK . BOSTON . CHICAGO
ATLANTA . SAN FRANCISCO
THE MACMILLAN CO. OF CANADA, LTD.
TORONTO
COMPARATIVE ANATOMY
VERTEBRATES
ADAPTED FROM THE GERMAN OF
DR. ROBERT WIEDERSHEIM
PROFESSOR OF ANATOMY, AND DIRECTOR OF THE INSTITUTE OF HUMAN AND COMPARATIVE. ANATOMV
IN THE UNIVERSITY OF FREIBURG-IN-BADEN
BY
W. N. PARKER, PH.D.
PROFESSOR OF ZOOLOGY AT THE UNIVERSITY COLLEGE OF SOUTH WALES AND MONMOUTHSHIRE
IN THE UNIVERSITY OF WALES
THIRD EDITION
(FOUNDED ON THE SIXTH GERMAN EDITION)
IBRARY =
*
WITH THREE HUNDRED AND SEVENTY-TWO FIGURES
AND A BIBLIOGRAPHY
MACMILLAN AND CO., LIMITED
ST. MARTIN'S STREET, LONDON
1907
All rights reserved
RICHARD CLAY ANL> SONS, LIMITED,
BREAD STREET HILL, E.C., AND
BUNGAY, SUFFOLK.
PREFACE
THE developmental history of this book has been somewhat
complicated. The first German edition appeared in 1882, under
the title of Lehrbuch der vergleichendc Anatomic der Wirbeltiere>
and a second edition followed in 1886. In 1884, a short
Grwidriss was published, which, after passing through four
editions and gradually increasing in size, replaced the Lehrbuch
under the title of Vergleichende Anatomic der Wirbeltiere, in 1902.
A further edition appeared in 1906, and this was followed by a
shorter Einfuhrung in 1907 : the latter was written to meet the
requirements of beginners, and it contains no bibliography.
The first and second English editions (1886 and 1897) were
based respectively on the first and third editions of the Grundriss,
and considerable modifications in detail were introduced. The
present (third) edition, which has been almost entirely re-written,
was prepared from the German editions of 1906 and 1907, and I
am much indebted to Professor Wiedersheim for allowing me to
make such alterations as seemed desirable in the interests of
English students, and for the pleasure of his collaboration last
summer, when I had the advantage of discussing various points
with him personally.
The general plan of the original has been retained throughout,
but I found it advisable to extend some portions and to abridge
others, besides making various minor modifications.
After re-editing by Professor Wiedersheim, the bibliography
given in the German original is inserted entire, except that the
titles and references have been abbreviated and slight rearrange-
ments made in order to save space : I have also ventured to
introduce a few additions. Though rather extensive for a work of
vi PREFACE
the kind, the list must not be regarded as anything approaching a
complete one of the more important papers relating to Vertebrate
Comparative Anatomy ; but I trust that such a list of references
in an easily accessible form will be found useful to advanced
students.
My thanks are due to Mr. T. H. Burlend, Demonstrator and
Assistant Lecturer in Zoology at this College, for preparing the
index.
W. N. PARKER.
UNIVERSITY COLLEGE, CARDIFF,
August, 1907.
CONTENTS
Preface v
INTRODUCTION 1
I. On the Meaning and Scope of Comparative Anatomy 1
II. Development and Structural Plan of the Vertebrate Body ... 2
III. Classified List of the Principal Vertebrate Groups 14
IV. Table showing the Gradual Development of the Vertebrata in
Time 16
SPECIAL PART.
A. INTEGUMENT 17
of Amphioxus 17
of Fishes 18
of Amphibians • 20
of Reptiles 23
of Birds 25
of Mammals 28
Mammary Glands 34
B. SKELETON 39
1. EXOSKELETON 39
2. ENDOSKELETON 44
I. VERTEBRAL COLUMN 45
of Fishes 48
of Amphibians 54
of Reptiles 57
of Birds 59
of Mammals • . . 60
II. 'RIBS 63
of Fishes 63
of Amphibians 66
of Reptiles 67
of Birds • .... 69
of Mammals 69
8249
viii CONTENTS
PAGE
III. STERNUM 70
V. SKULL 74
General part 74
a. Brain -case 7(i
/*. Visceral Skeleton 80
c. Bones of the Skull 82
Special part 84
A. The Skull of Fishes . . . . • 84
i:. ,, of Amphibians ... ... 97
c. ,, of Reptiles . . 108
D. ,, of Birds . . 120
E. ,, of Mammals 123
VI. APPENDICULAR SKELETON 135
a. Unpaired Fins 136
b. Paired Fins or Limbs 137
Pectoral Arch . . • 140
of Fishes 140
of Amphibians 141
of Reptiles 142
of Birds 143
of Mammals 143
Pelvic Arch 145
of Fishes . . 145
of Amphibians 146
of Reptiles . .... 149
of Birds i;,2
of Mammals 154
Paired Fins of Fishes 155
Paired Limbs of the higher Vertebrata 159
of Amphibians • 161
of Reptiles . 163
of Birds 165
of Mammals 168
C. MUSCULAR SYSTEM . 173
INTEliUMENTARY MUSCLES . .... 175
MUSCLES OF THE TRUNK 17'.)
of Amphioxus and Fishes 17(.t
of Amphibians . 181
of Reptiles 181
of Birds ... . .182
of Mammals . 183
MUSCLKS OF TIII: HI \I-IIKAI;M . 184
CONTENTS ix
i' \<.i<:
MUSCLES (IF THE APPENDAGES 185
BYE-MUSCLES . . 186
VISCERAL MUSCLES .... 187
<>f Fishes .... ... 187
of Amphibians 188
of Amniota 189
D. ELECTRIC ORGANS ... . . 190
E. NERVOUS SYSTEM AND SENSORY ORGANS . . 193
T. CENTRAL NERVOUS SYSTEM . . ... 195
Membranes of the brain and spinal cord 195
1. SPINAL CORD . . 198
2. BRAIN (general description and development) 199
of Cyclostomes 204
of Elasmobranehs . 208
of Ganoids ... 211
ofTeleosts 211
of Dipnoans 214
of Amphibians .... 215
of Reptiles ... 220
of Birds .221
of Mammals 225
II. PERIPHERAL NERVOUS SYSTEM . . 230
1. SPINAL NERVES .... 233
2. CEREBRAL NERVES . 234
Sympathetic 247
III. SENSORY ORGANS (general description and development) . 249
SENSE-ORGANS OF THE INTEGUMENT 250
<(. Nerve-Eminences 250
I. End-buds and gustatory organs . . 254
c. Tactile Cells and Corpuscles 255
'/. Club-shaped or lamellar Corpuscles 257
OLFACTORY ORGAN (general description and development) . . 258
of Cyclostomes 259
of Fishes .... . . 261
of Amphibians ... ... 263
of Reptiles . . 265
of Birds .... ... 266
of Mammals 267
Vomero-nasal (Jacobson's) Organ 271
EYE (general description and development) .... . 273
of Amphioxus ... 277
of Cyclostomes .... 278
of Fishes . 279
x CONTENTS
PAGE
EYE (continued)—
of Amphibians 281
of Reptiles and Birds 282
of Mammals 283
Retina 284
Accessory Organs in Connection with the Eye 286
a. Eye-Muscles 286
6. Eyelids 287
c. Glands 288
AUDITORY ORGAN (general description and development) . . . 290
of Cyclostomes 295
of Fishes .... 295
of Amphibians 297
of Reptiles and Birds 299
of Mammals 301
F. ORGANS OF NUTRITION 308
ALIMENTARY CANAL AND ITS APPENDAGES (general
description) 308
I. ORAL CAVITY 312
Teeth (general description) 313
of Fishes and Amphibians 314
of Reptiles and Birds 316
of Mammals ... 318
Glands of the Mouth 324
of Amphibians 325
of Reptiles 325
of Birds . . 326
• of Mammals 327
Tongue 327
THYROID 331
THYMUS • . . . 333
II. CKSOPHAGUS, STOMACH, AND INTESTINE 335
of Ichthyopsida 335
of Reptiles 339
of Birds 339
of Mammals 341
Histology of the Mucous Membrane of the Alimentary Canal 344
LIVER 346
PANCREAS ... 348
G. ORGANS OF RESPIRATION . . 351
I. GILLS 351
of Amphioxus 352
CONTENTS xi
PAOE
GILLS (continued)
of Cyclostomes • ". . 352
of Fishes 355
of Amphibians . '. . . 358
II. SWIM-BLADDER AND LUNGS 361
1. SWIM-BLADDER 361
2. LUNGS • -' . 362
Air-Tubes and Larynx • 364
of Amphibians 366
of Reptiles 369
of Birds ' . , 370
of Mammals ...'.. '371
The Lungs 377
of Dipnoans 377
of Amphibians :...... 377
of Reptiles 379
of Birds 382
of Mammals ; . 387
CCELOME 388
SEROUS MEMBRANES • . . 388
ABDOMINAL PORES - 389
H. ORGANS OF CIRCULATION ... 393
General Description and Development • 393 '
Heart, together with origin of Main Vessels 400
of Fishes • . . 400
of Amphibians 403
of Reptiles 407
of Birds and Mammals 411
Arterial System 414
Venous System 419
of Amphioxus 419
of Fishes 419
of Amphibians 426
of Amniota 428
Retia Mirabilia 431
Lymphatic System 432
MODIFICATIONS FOR THE INTER-UTERINE NUTRITION
OF THE EMBRYO : FCETAL MEMBRANES 436
1. Anamnia 436
2. Amniota 437
xii CONTENTS
PAGE
I. URINOGENITAL ORGANS (general description and develop-
ment) 441
Pronephros 443
Mesonephros . 445
Metanephrous 446
Male and Female Generative Ducts 446
Gonads 450
URINARY ORGANS ... ... 452
of Amphioxus .... 452
of Fishes . 452
of Amphibians 455
of Repti]es and Birds 459
of Mammals . . . • 461
GENITAL ORGANS ... 463
of Amphioxus . 463
of Fishes . 463
of Amphibians . ... 469
of Reptiles and Birds 474
of Mammals . 477
AITESSORY UENITAL GLANDS OF MAMMALS . ... 484
COPULATORY ORUANS .... . 486
ADRENAL BODIES 4!'L'
APPENDIX (Bibliography) 497
INDEX 565
ERRATA
PAGE
5 5th line from bottom, for "walls," read " wall."
13 Fig. 11, insert " gonad (ovary)" below " kidnc\'."
33 ,, 25, for " N and G nerves," read " JV, nerves ; G, blood-vessels."
40 8th line from top, for "disappear," i-««l "disappears."
40 llth ,, ,, /or " ossifications," read "ossification."
.').'> Fig. 42, transpose A and B.
72 Line 6 from bottom, dfkte " the first " and " cervical.1'
73 ,, 10 .. for "are," read ~ is."
73 ,, 3 ,, for " proximal," read " anterior. "
73 ,, 2 ,, for " distal," rend " posterior.''
92 Fig. 69, insert "0" before "orbital ring," and " */. , symplcclic."
96 ,, 71, ,, "A", external gills."
112 3rd line from top, /or " system/' rratl " septum."
119 Fig. 86, for " condyles," read " condyle."
145 10th line from bottom, delete " B."
151 4th ,, top, for " show," read "shows."
155 18th ,, ,, for " stright," read " straight."
164 Fig. 129, for " 1st," read "5th."
17P 17th line from top, for "and thus effect.'1 ,-< nil " thus effecting."
298 Fig. 218A, for " Pr," read " Ph."
304 ,, 223, for " vestibula," read " vestibnli."
315 3rd line from top, for " cartilaginious," read "cartilaginous."
340 Fig. 250n, for " tendrinous," read "tendinous."
43s 20th line from bottom, for " Phalcolarctos," ,-, ad " Phascolarctos."
445 6th and 7th lines from bottom, for " parosphoi'on," read " paroophoron."
450 13th line from top, for " epididennis." nail " epididymis."
^
but also to understand, tne meaning 01 numerous
B
xii CONTENTS
I1 Ac.K
I. URINOGENITAL ORGANS (general description and develop-
ment) 441
Proiie[)hros 443
Mesonephros 445
Mctanephrous 44fi
Male and female Generative Ducts 44(5
(ronsuls
COMPARATIVE ANATOMY
INTRODUCTION.
I. ON THE MEANING AND SCOPE OF COMPARATIVE ANATOMY.
A KNOWLEDGE of the natural relationships and ancestral history
of animals can only be gained by a comparative study of their
parts (Comparative Anatomy) and of their mode of develop-
ment (Embryology or Ontogeny). In addition to existing
animals, fossil forms must also be taken into consideration (Pa-
laeontology), and by combining the results obtained under these
three heads, it is possible to make an attempt to trace out the
development of the various races or groups in time (Phylogeny).
As the different phases of development of the race may be repeated
to a greater or less extent in those of the individual, the depart-
ments of Phylogeny and Ontogeny help to complete one another.
It must, however, be borne in mind that in many cases the
phases of development are not repeated accurately in the individual
—that is, are not palinyendic, — but that '; falsifications " of the re-
cord, acquired by adaptation, very commonly occur along with
them, resulting in cccnogenetic modifications in which the original
relations are either no longer to be recognised at all, or are more
or less obscured. In this connection, two important factors must
be taken into consideration, viz., heredity and variability. The
former is conservative, and tends to the retention of ancestral
characters, while the latter, under the influence of change in
external conditions, results in modifications of structure which are
not fixed and unalterable, but are in a state of constant change.
The resulting adaptations so far as they are useful to the
organism concerned, are transmitted to future generations, and
thus in the course of time gradually lead to still further modifica-
tions. Thus heredity and adaptation are parallel factors, and a
conception of the full meaning of this fact helps us not only to
gain an insight into the blood-relationships of animals in general,
but also to understand the meaning of numerous degenerated and
P,
2 COMPARATIVE ANATOMY
rudimentary or vestigial organs and parts in the adult organism
which would otherwise remain totally inexplicable.
Histology is a subdivision of anatomy which concerns the
structural elements — the building-stones of the organism, and the
combination of these to form tissues. Various combinations of
the tissues give rise to organs, and the organs, again, combine to
form systems of organs.
The structural elements consist primarily of cells and second-
arily of cells and fibres often enclosed in an intercellular substance
or matrix', and the different tissues may be divided into four
principal groups :—
1. Epithelium, and its derivative, glandular tissue.
2. Supporting tissue (connective tissue, adipose tissue, cartilage,
bone).
3. Muscular tissue.
4. Nervous tissue.
In accordance with the functions they perform, epithelium and
supporting tissue may be described as passive, and muscular and
nervous tissue as active.
By an organ we understand an apparatus which performs a
definite function : as, for instance, the liver, which secretes bile ;
the gills and lungs, in which an exchange of gases is effected with
the surrounding medium ; and the heart, which pumps blood
through the body.
The organ-systems, which will be treated of in order in this
book, are as follows : — 1. The outer covering of the body, or inte-
gument; 2. The skeleton; 3. The muscles, together with electric
organs ; 4. The nervous system and sense-organs ; 5. The organs of
nutrition, respiration, circulation, excretion, and reproduction.
The closely-allied branches of science denned above are in-
cluded under the term Morphology, as opposed to Physiology
which concerns the functions of organs, apart from their morpho-
logical relations. The combined results obtained from these two
fields of study throw light on the organisation of animals in
general — that is, on Zoology in its widest sense.
II. DEVELOPMENT AND STRUCTURAL PLAN OF THE
VERTEBRATE BODY.
The structural elements described in the preceding section as
the building-stones of the organism, i.e. the cells, all arise from a
single primitive cell, the egg-cell or ovum. This forms the
starting-point for the entire animal-body, and a general account of
its structure and subsequent development must therefore be given
here.
The ovum consists of a rounded, nucleated, protoplasmic body
INTRODUCTION 3
(Fig. I), consisting of the vitellus, in the interior of which is the
germinal vesicle, enclosing one or more germinal spots: the
membrane which covers the vitellus is spoken of as the vitelline
membrane. Since the ovum corresponds to a single cell, we may
speak of the vitellus as the protoplasm of the egg-cell, the
germinal vesicle as its nucleus, and the germinal spot as its
nucleolus : the vitellus, however, consists of two different sub-
stances — protoplasm and deuteroplasm (yolk} — in varying propor-
tion and relative distribution in different animals.
The nucleus is enclosed by a delicate nuclear membrane, and is
made up of two chief constituents — the spongioplasm or chromatin,
and the hyaloplasm or achromatin. A
small particle, the ccntrosome, is also
present in the protoplasm of the cell,
and takes an important part in the
process of cell-division. An outer
limiting membrane, corresponding to
the vitelline membrane, is not an in-
tegral part of the cell, but may be
differentiated from the peripheral pro-
toplasm ^IC{- 1- — DIAGRAM OF THE
In the process of sexual reproduc-
tion, which occurs in all Vertebrates, A vitellus; KB, germinal
the fusion with the ovum of the vesicle ; KF> Serminal 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 (karyokinesis or mitosis) similar to that
which occurs in tissue-cells, except that the resulting daughter-
cells, in addition to the reduction in their chromatin, are of
different sizes, two small evanescent polar-cells (Fig. 2) being suc-
cessively thrown off from the larger ovum. A similar process also
occurs in the development of the sperm-cell, except that there is
no difference in size between the products of division. The
portion of the original nucleus remaining in the ovum or sperm
is then known respectively as the female (or maternal) and male (or
paternal) pronucleus.
A sperm-cell then makes its way into the ovum, and its pro-
nucleus unites with the female pronucleus to form the segmentation
nucleus. This process, which is known as impregnation or
fertilisation, thus consists in a material intermingling of the
generative substances of both sexes, or more accurately of the
sperm-nucleus and egg- nucleus. The essential cause of inherit-
ance can thus be traced to the molecular structure of the nuclei of
B 2
COMPARATIVE ANATOMY
is
the
both male and female germinal cells. This structure
morphological expression of the characters of the species.
After fertilisation has taken place development begins. The
segmentation nucleus divides into two equal parts, each of which
forms a new centre for the division of the oosperm, as it must
now be called, into two halves or Uastomcres. This division, the
beginning of the process of segmentation, takes place by the
formation of a furrow round the egg which becomes deeper and
deeper until the division is complete. (Fig. 2, A).
The first stage in the process of segmentation is thus com-
pleted ; the second takes place in exactly the same way, and
FIG. 2. —DIAGRAMS OF THE SEGMENTATION OF THE OOSPERM.
A, first stage (two segments) : JfK, polar cells. B, second stage (four segments).
C, further stage. D, morula stage.
results in a division of the oosperm into four parts ; and by a similar
process are formed eight, then sixteen, then thirty-two blastomeres,
and so on, the cells becoming smaller and smaller, and each being
provided with a nucleus. (Fig. 2, c — D). In short, out of the
original oosperm a mass of cells is formed which represents the
building-material of the animal body, and which, from its likeness
in appearance to a mulberry, is spoken of as a morula.
In the interior of the morula a cavity (segmentation cavity or
llastoccele) filled with fluid is formed, and the morula is now spoken
of as the blastosphere or blastula (Fig. 3). The peripheral cells
enclosing this cavity form the germinal membrane or blasto-
INTRODUCTION
TIT)
FIG. 3. — BLASTOSPHERE.
BD, blastoderm ; FH, segmentation
cavity.
derm. Consisting primarily of a single layer of cells, the blasto-
derm later on becomes two-layered, and then three-layered. From
the relative position of these, they are spoken of respectively as
the outer, middle, and inner
germinal layers, or as ecto-
derm (epiblast), mesoderm
(mesoUast*), and endoderm
(hypoblasf).
The mode of distribution
of the yolk-particles in the
ovum, and an increase in their
amount, result in certain
modifications of the primitive
form of segmentation as de-
scribed above. Yolk is an
inert substance, and its pre-
sence tends to hinder or even
entirely to prevent segmenta-
tion in those parts of the
ovum in which it is abundant.
When the whole ovum undergoes division, the segmentation is
known as entire or lioloblastic ; when division is restricted to part
of the ovum only, the segmentation is said to be partial or mcro-
blastic l (Fig. 4).
The question as to the origin of the
germinal layers, on account of its im-
portant signification, is one of the most
burning problems in morphology, and as
yet we cannot arrive at any full and
satisfactory conclusion on the subject.
It may, however, be stated that in all
Vertebrates the blastosphere passes — or
did so in earlier times — into a stage
called the gastrula, which is retained
in an unmodified form only in the lowest
Vertebrate (Amphioxus, cf. p. 14). The
gastrula is derived primitively from the
blastula by the walls of the latter
(Fig. 3) becoming pushed in or invaginated at one part, thus
giving rise to a double-walled sac (Fig. 5). The outer wall
then represents the ectoderm, which serves as an organ of
protection and sensation, while the inner, or endoderm, encloses
1 In holoblastic segmentation the resulting cells are approximately equal in
the Lancelet and in Mammals (with the exception of Monotremes and some
Marsupials) ; and unequal in the Cyclostomes, Sturgeon, Lepidosteus, Dipnoans,
and nearly all Amphibians, the segmentation sometimes approaching the mero-
blastic type. In Elasmobranchs, Teleosts, Reptiles, Birds, and Monotremes
the segmentation is from the first meroblastic and discoid, i.e.., restricted to the,
upper pole of the ovum (Fig. 4).
B*
No,
FIG. 4. —DIAGRAM OF A MERO-
BLASTIC OOSPERM WITH
DISCOID SEGMENTATION.
Bla, blastoderm ; Do, yolk.
6 COMPARATIVE ANATOMY
a central space, the primitive digestive cavity (arckenterpn). The
opening of the latter to the exterior, where the two germinal
layers are continuous, represents the primitive mouth or Nastopore,
which is represented to a certain extent by the primitive streak of
higher forms.
From the ectoderm arise the epiderm and its derivatives, the
entire nervous system, the sensory cells, the lens and certain muscles
of the eye, the oral and anal involutions (stomodceum and procto-
dceuiii), and the oral portion of the pituitary body attached to the
brain. In an early stage the endoderm gives rise to an axial rod,
the notochord (Figs. 6 and 7), and eventually to the epithelium of the
Z'At-
I o
FIG. 5.— GASTBULA.
Blp, blastopore ; Ekt, ectoderm ; Ent, endoderm ; U, archenteron.
greater part of the alimentary canal (Figs. 6 and 10) and its
glands, including the thyroid, thymus, liver and pancreas, as well
as to the epithelial parts of the gill-sacs and lungs.
Though we may look upon the ectoderm and endoderm — that
is, both the primary germinal layers — as arising primitively in the
manner above described, various modifications occur, depending
largely on the type of segmentation, and known as overgrowth
(epiboly\ dclamination, and partial delamination. The middle layer
or mesoderm is a secondary formation, and is phylogenetically
younger than the other two germinal layers ; both as regards the
origin of its cells and histologically, it is of a compound nature,
and thus forms a marked contrast to the two germinal layers proper.
One of its first and most important functions is the formation of
blood-cells ; later it gives rise to the heart, vessels, and to nearly
all the supporting and connecting substances (connective tissue,
adipose tissue, cartilage, bone), serous membranes (peritoneum,
INTRODUCTION
K
Ekt
Fi<;. 6, A AND B. — DIAGRAMMATIC TRANSVERSE SKCTIONS THROUGH A DEVELOMM;
VERTEBRATE EMBRYO.
A, aorta; Chl, (Fig. B), the notochord now constructed oft' from the endoderm ;
Go, Co'l, ca'lome ; J), alimentary canal ; Ekf, ectoderm ; Ent, endoderm,
showing in Fig. A the thickening (Gh) which will form the notochord ; H,
remains of the upper part of the cu'lomu in the interior of the mesodei inic
somites; M<><1, central nervous system (medullary <-<>rd) : — in Fig. A it is
shown still connected with the ectoderm, from which il has become constricted
off in Fig. B; So P, somatic, and $pP, splanchnic mesoderm ; U YOLK-SAC. A AND I'., IN F>ONCITI-I»IN AI, SKI Ti<>, y<> k-sa • : A\ liody "t embryo; M,
medullary cord : />}>, ccclonu' ; ", somatopleuve ; '», splanchnopleure ; t, vitello-
jntestinal duct.
INTRODUCTION
11
allantois. The latter becomes attached to a definite region of
the uterine wall, and from it vascular processes or villi arise, so
that the foetal and maternal blood-vessels come into very close
relations with one another. Thus an allantoic placenta is
Pa
PC(CLf)
Dv
FIG. 9.— DIAGRAMMATIC .SECTION THROUGH THE HUMAN GRAVID UTERUS.
A, aorta ; A, A, A, the cavity of the amnion filled with fluid : in the interior of the
amnion is seen the embryo suspended by the twisted umbilical cord ; Al,
allantoic (umbilical) arteries ; Chi, chorion Izeve ; D, the remains of the
yolk-sack (umbilical vesicle) ; Dr, decidua reflexa ; Dv, decidua vera, which
at Pu passes into the uterine portion of the placenta ; H, heart ; Pf, f<
=
o
«
ACRANIA (CEPHALOCHORDATA).
Lancelet (Amphioxus).
GRANT ATA.
/ o. A N A M N I A.
Class I. CYCLOSTOMATA (Suctorial Fishes).
Order 1. Myxinoidei (Hag Fishes — Myxine, Bdello-
stoina).
,, 2. Petromyzontes (Lampreys).
Class II. PISCES (True Fishes).
Sub-class 1. ELASMOBBANCHII.
Order 1. Pluyioxtomi (Sharks— Selachii, and Rays
— Batoidei).
,, 2. Holot-ephali (Chimzera, Callorhynchus).
Sub-class 2. TELEOSTOMF.
( Order 1. Cro8sopterygii(Polypterus,Cala.michihys).} •£
,, ± Chrondrostei (Acipenser, Polyodon).
3. Holoxtt-i (Lepidosteus, Amia).
4. Te.lv.oxli i. ^
Sub-order a. Physostomi (Carp, Pike, Salmon,
Herring, Eel, Silnroids).
., /*. Anocanthini (Cod, Flat-fishes).
,, c. A canthopteri (Perch, Stickleback,
Gurnard, Blenny).
., d. Pharynyoynatlii (Wrasse).
,, e. Plectognuthi (Q'runk- and File-
tishes).
,, /. Lophobranchii (Pipe-fish, Sea-
horse).
Sub-class 3. DIPNOI.
Order 1. Monopneumona (Ceratodus).
,, 2. Dijmeumona (Protopterus, Lepidosiren).
Class III. AMPHIBIA.
Order 1. Urodela.
a. Perennibraiichiatu (Proteus, Siren, Necturus).
{Derotremata (Amphiuina,
Menopoma)
Myctodera (Salamandra, Tri-
ton, Amblystoma),
2. Anuru (Frogs and Toads).
., 3. Gymnophiona (Limbless Csecilians).
/ /3. A M N i o T A.
Class IV. REPTILIA.
Order 1. Rhynchocephali (Hatteria).
,, 2. Lacertilia (Lizards).
,, 3. O/thidia (Snakes).
,, 4. Chelonin (Turtles and Tortoises).
,, 5. Grocodilia (Crocodiles and Alligators).
(.'lass V. AVES.
a. Jfiititu (Cursorial Birds— Ostrich, Rhea, Emu, &c. ).
I. Carinatte (Birds of Flight).
Class VI. MAMMALIA.
Sub-class 1. PROTOTHERIA or ORNITHODELPHIA (the Ovi-
parous Monotremes -- Ornithorhynchus,
Echidna).
,, 2. METATHERIA or DIDELPHIA (Marsupials-
Kangaroo, Phalanger, Opossum. ).
,, 3. ElTTIIKKIA 01' MitNOUELPHIA.
INTRODUCTION
15
s
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Order 1. EdtuJatn (Sloth, Anteater).
2. Sirenia (Dugong, Manatee).
3. Cetacea (Porpoise, Whale).
4. Ungnlata (Rhinoceros, Horse, Ruminants.)
5. Hyracoidea (Hyrax).
6. Proboscidea (Elephant).
7. fiodentia (Rabbit, Mouse, Beaver, Cavies).
S. Cheiroptera (Bats).
9. Insectivora (Shrew, Mole, Hedgehog).
10. Camivora (Bear, Dog, Cat, Seal).
11. Prosimii (Lemurs).
12. Primate* (Monkeys and Man).
16
COMPARATIVE ANATOMY
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SPECIAL PART.
A. INTEGUMENT.
THE skin consists of a superficial ectodermal layer, and a
deeper mesodermal layer. The former is called the epiderm
(scarf-skin) and the latter the derm (corium, cutis).
In the epiderm, which consists of cells only, two parts may in
gene'ral be distinguished : — an external layer, composed of flattened
and hardened cells (stratum corneum, horny layer}, and a deeper, of
more columnar, formative cells (stratum Malpighii s. germativum,
mucous layer). The latter serves for the regeneration of the
horny layer, the superficial part of which is continually scaling
off, as well as for the formation of such horny structures as hairs,
bristles, nails, claws, and hoofs, and of the integumentary glands.
The peripheral sensory end-organs also arise by a differentiation
of this layer of cells.
The derm, which is usually thicker and tougher than the
epiderm, is made up principally of connective tissue and smooth
muscular fibres : it is usually not sharply marked off from the
subcutaneous connective tissue, which commonly encloses more or
less fat. Externally, the derm may give rise to numerous papillae
projecting into the epiderm, especially in higher forms. Apart
from the horny and glandular structures extending into it from
the epiderm, the derm encloses vessels, nerves, and often bony
structures also.
Pigment cells (chromatophores) and free pigment occur in both
layers of the skin : they correspond to modified connective tissue
cells, and in them a temporary shifting of the contained pigment
may occur, this process being under the control of the nervous
system.
In Amphioxus, the epiderm differs from that of all the Craniata
in the fact that it consists of a single layer of cells : its surface is
covered with cilia in the larval (gastrula) stage, and this must
undoubtedly be considered as an inheritance from Invertebrate
ancestors.
C
18
COMPARATIVE ANATOMY
Fishes.
The character of the epiderm varies greatly in the different
groups (Fig. 12). The striated cuticnlar border (present e.g. in
Cyclostomes, Teleosts, and Dipnoans) possibly indicates the
former possession of cilia.1 Cornification of the superficial layer
occurs, especially in Teleosts, over those parts of the scales (Fig. 13)
which are not overlapped by their fellows. Numerous lymph-
-JTo
Co
FIG. 12.— DIAGRAMMATIC TRANSVERSE SECTION ILLUSTRATING THE STRUCTURE OP
THE SKIN IN FISHES.
B, B, goblet-cells opening on the surface ; Co, derm ; CS, cuticular margin ;
Ep, epiderm ; F, subcutaneous fat ; G, vessels which pass upwards in the
vertical connective tissue bundles (8) of the derm ; Ko, goblet-cells ; Ko,
granular cells ; W, horizontal connective-tissue bundles.
cells (leucocytes) are found in the epiderm which have wandered
out of the derm, and some of them contain pigment. The derm
consists mainly of horizontal and vertical layers of connective
tissue, and encloses the other structures already referred to (Figs.
12 and 13).
Various kinds of mucus-secreting cells are formed in the
epiderm, and in addition to the relatively larger or smaller goblet
1 Cilia are occasionally present in very early stages in Teleosts.
INTEGUMENT
19
cells commonly present may be mentioned the granular cells in
the Lamprey (the nature of which is not understood), and in
Myxinoids the numerous slime-sacs, formed as invaginations of the
epiderm and containing peculiar thread-cells.
Special aggregations of gland-cells occur in relation with the
copulatory organs or claspers of male Elasmobranchs (glandulce
2rterygopodii), and on the operculum and dorsal fin-rays of certain
Acanthopteri (e.g. Trachinus, Thalassophryne, Synanceia), in
which latter they constitute a poison-apparatus serving for offence
defence, and consist of modified epidermic cells enclosed in
or
grooves of the spines of the operculum and dorsal fins. Most
Scales.
Mucus cells.
Pocket enclosing
scale.
Epiderm.
Muscles.
FIG. 13. — LONGITUDINAL SECTION OF SKIN OF YOUNG TROUT (15 CM. LONG),
FROM THE TAIL.
™
g
of these poison-fishes are inhabitants of the temperate and
warmer seas : in fresh-water forms (e.g. Perca, Cottus), the
apparatus has apparently undergone partial or total degeneration.
Poison-organs are also said to occur in a number of other Teleosts
(e.g. in connection with the dorsal and pectoral spines of many
Siluridte) and in certain Elasmobranchs ; a closer examination will
probably prove their existence in many other Fishes.
Phosphorescent organs, formerly known from their appearance
as " accessory eyes/' occur on various parts of the head, body, and
tail of several families of deep-sea Teleosts (e.g. Stomiatida?,
Halosauridae, Anomalopidse), and in certain species of Elasmo-
branchs belonging to the family Spinacidse. Their arrangement,
distribution, and structure is very varied in different forms. The
luminous part consists of gland-cells, supplied by the trigeminal,
c 2
20 COMPARATIVE ANATOMY
facial, and spinal nerves ; and a number of accessory parts may be
present, e.g. a pigment- layer, a reflecting apparatus, a vitreous
body, and structures resembling a lens and an iris. These organs
probably serve to attract prey or to help their possessors in seek-
ing food in the darkness of the deep sea.1
In addition to very numerous goblet-cells, the epiderm of the
South African Dipnoan, Protopterus, gives rise to cup-shaped
muUiccllidar glands, resembling those of the Amphibia. During
the dry season, this animal (like its South American ally,
Lepidosiren 2), buries itself in the river-bottom : its integumentary
glands then produce a varnish-like secretion and an enclosing
cocoon or capsule, by means of which it is protected during the
torpid period.
Pigment-cells, which, as already mentioned, are under the
control of the nervous system, and are able to cause a change of
colour, are present sometimes in both layers of the integument,
sometimes in one only. Colour may also be produced by reflecting
bodies consisting of excretory products (guanin) and known as
iridocytes.
The presence of scales (see under Exoskeleton) may affect the
epiderm when they project from the surface, and in some cases it
may disappear so that the scales become superficial (e.g. in Elas-
mobranchs, Ganoids, and some Teleosts).
Amphibians.
The epiderm of Amphibians differs markedly from that of
Fishes, inasmuch as nearly all the special forms of cells so
characteristic of the latter are wanting. Both epiderm and derm,
moreover, differ in the larva and in the adult. The epiderm at
first consists of a single layer of cells, and then of two layers, the
superficial one being provided with a ciliated or a striated cuticular
border,3 and remaining throughout the larval period as a covering
layer (Fig. 14). The deeper layer, on the other hand, undergoes
various modifications : it becomes stratified, and replaces the super-
ficial cells as they are lost. Slime-secreting goblet-cells, such as are
characteristic of the epiderm of fishes, are typically wanting,4 and
leucocytes are not abundant. Unicellular glands, known in
Urodeles as Leydiys cells, are, however, abundant in the larva ;
1 For the electric organs of Malopterurus, which are said to be epidermic in
origin, cf. under Electric Or/jans.
2 In the breeding season the posterior extremities of the male Lepidosiren are
provided with numerous long vascular papillae.
3 Cilia occur abundantly in Salamander larvae over parts of the head and
body, and their distribution is related to that of the integumentary sense-organs ;
they are also found in very young Anuran larva?.
4 In older Urodele larvte, after the epiderm has become thickened, numerous
goblet-like cells can be recognised and probably represent the Leydig's cells
described above.
INTEGUMENT
21
and cells with thread-like and vacuolated contents have also been
described in Anuran larvse. Later on, immediately before meta-
morphosis, numerous muliicdlular glands of alveolar structure
FIG. 14. — SKIN OF LARVA OF SALAMANDER (Salamandra maculosa).
a, stratum corneum ; It, stratum Malpighii ; Co, derm ; OS, striated border ;
Ep, epiderm ; LZ, Leydig's cells (unicellular mucus glands).
(cf. p. 20) appear in adaptation for terrestrial life. These have
nothing to do with the unicellular larval glands : their great
abundance is very characteristic of the Amphibian skin (Figs. 15
and 16). As regards their distribution, they may be scattered singly
throughout the skin, or arranged in groups — in Anurans chiefly
FIG. 15. — SEMIDIAGRAMMATIC SECTION THROUGH THE SKIN OF ADULT
SALAMANDER (S. mactilom).
Co, derm, in the connective tissue stroma of which (B) the various sized integii-
mentary glands (A, C, D, D) lie embedded ; E, epithelium of glands ; Ep,
epiderm ; Ml, the muscular, and Pr the connective-tissue layer of the
glands ; M, the same, seen from the surface ; Mm, subcutaneous layer of
muscles, through which vessels (G) extend into the derm; Pi, Piy, pigment
cells in the derm ; S, secretion of glands.
along the back, in Urodeles (and Toads) at the junction of head and
trunk (" parotoids") or laterally along the body and in the caudal
region (e.g. Spelerpes, Plethodon). These aggregated glands vary
COMPARATIVE ANATOMY
not only in relative size, but also in the structure of their cells
and in function. Mucus-glands, and much larger ^>ois0?i-(/Zfmrfs
(the secretion of which is granular), can usually be distinguished,
the latter serving as a passive means of defence ; but intermediate
forms may be recognised. Smooth muscle-cells are very numerous
in the derm, certain of them surrounding these glands, and form-
ing constrictors and dilators.
In the Anura, the blood-vessels are not always confined to the
derm, but in connection with the respiratory function of the skin
extend far into the epiderm before metamorphosis, during which
process the capillary loops in the epiderm increase markedly,
decreasing again subsequently. This may be explained by the
fact that during metamorphosis the gills are no longer functional
Mucus glands. Pigment. Sensory organs.
I
Superficial and deep layer
of epiderm.
.
•'mi ,^^r~. .
- • ^ _--r^' *-*;=•*.' ~-^:^~~*f'~:~- ;.:.:"-•-
DI I'm.
FIG. 16. — SECTION' THROUGH THE SKIN OF A Triton alpestris IN THE BREEDING
SEASON.
and pulmonary respiration alone is apparently insufficient ; an
additional vicarious arrangement is therefore temporarily neces-
sary.
The coloration of the skin, which may undergo change, is due
to chromatophore* of different tints in the derm. The derm is
similar to that of Fishes, and is, moreover, characterised by an
abundance of blood-vessels and nerves, as well as of smooth
muscle-fibres. Calcifications and even ossifications (e.g. in Cera-
tophrys dorsata) may occur in the derm, and in the Gymnophiona
definite dermal scales are present.
A stratified stratum corneum becomes developed even in
perennibranchiate types, and may become more pronounced in
adaptation to a terrestrial existence in caducibrarichiate forms at
metamorphosis. The cornification is especially marked along the
back, and may result in the formation of warts and papillae ;
occasionally claw-like structures are developed on the digits
(Xenopus, Onychodactylus). The horny layer of the epiderm is
shed periodically, either in pieces or entire.
INTEGUMENT
23
Reptiles.
In adaptation from the first to a terrestrial in place of an
aquatic existence, the skin of Reptiles is dry and more or less
pneumatic. Integumentary glands are practically wanting. The
" femoral pores " of Lizards, which were formerly looked upon as
glands, are now known to be merely subcutaneous, branched,
tube-like cavities lined by cornified cells which project from the
pores in the form of solid cones, and possibly serve as clasping
organs during copulation : it is doubtful whether these structures
originated from glands in the first instance. In the Crocodilia a
row of about twenty small, gland-like sacs are present under the
skin along the back from the neck to the base of the tail, at the
boundary between the first and the second rows of scutes.
Nothing is known as to the function of these, or of the evaginable
FIG. 17. — DIAGRAMMATIC SECTIONS THROUGH VARIOUS KINDS OF EPIDERMIC
SCALES OF REPTILES. (From Boas's Zoology.)
A, rounded scales ; B, shields; C, imbricating scales; D, the same, with bony
scutes in the underlying derm ; h. horny layer, and s, Malpighian layer of
the epiderm ; /, derm ; o, bony scutes.
and odoriferous " musk-gland" on the lower jaw of Crocodiles and
the imaginations of the integument on the margins of the carapace
in Chelonians.
A further characteristic difference between the skin of
Reptiles and that of most Amphibians is seen in the presence of
scales (Fig. 17). Horny epidermic scales and dermal bony
structures, both of which may be present in the same animal,
must be distinguished from one another (D). In all cases, the first
traces of the scales are due to the formation of dermal papilla?,
which may or may not become calcified or ossified. In the former
case, the resulting bony scale or scute still remains covered by the
more or less horny epiderm (e.g. Anguis, Scincidse). As a general
rule, the epidermal cornification is much more marked than the
ossification.
COMPARATIVE ANATOMY
The dermal papilla is thus always the primary part of the
scale which causes the elevation of the epiderm (Fig. 18). At
Horny papilla.
Malpigltian layer Horny layer
Papilla1. of eviderm. of epiderm.
^•'VW'
Derm.
_„
Subcutaneous conntrtire
tissue with vessels.
,•
FIG. IS.— SECTION OF SKIN OF A YOUNG TORTOISE (Testudo yra'ca), FROM THE
NECK.
the same time, a marked proliferation of the epiderm occurs, at
first uniformly and later in different degrees on the upper and
lower surfaces of scales when they overlap one another (Fig. 19).
Loose connective Epitrk-hous
tissue layer. lai/er.
Horny layer (scale).
Piyment In;/' r.
Muscles.
FIG. 19. — SECTION OF SKIN OF LIZARD (Laccrla ayi/i*).
As in Amphibians, a periodic casting of the superficial part of
the many-layered horny epiderm occurs, either in shreds, or
INTEGUMENT 25
entire (e.g. Snakes, Anguis). Most Lizards simply creep out of
their cast skin, as out of a sack ; while in Snakes it becomes
turned inside out while being shed.
The horny substance may undergo a variety of modifications,
and may give rise to such structures as ridges, prickles, warts,
claws, shields (e.g. the " tortoiseshell " of Chelonians l), and
rattles (Rattlesnakes); or it may develop bunches of cuticular
hair-like bodies, such as those found on the toe-discs of Geckos.
In the derm, a superficial and a deeper layer may be dis-
tinguished. The latter is composed mainly of strong bundles of
connective tissue fibres which as a rule cross one another at right
angles, as in Fishes and Amphibians. The superficial or sub-
epidermic layer is looser in structure, and in addition to con-
nective tissue fibres, encloses smooth muscles and chromatophores
(Fig. 19), the degree of development of the latter differing greatly :
several rows of them may be present, e.g. in the Chameleon. The
power of changing colour, so characteristic of the last-named, is,
however, possessed to a greater or less extent by many other
Reptiles.
Birds.
The skin of Birds is characterised by giving rise to feathers, as
well as by the relatively thin epiderm and derm, the connective
tissue fibres of the latter being irregularly felted. A uropygial
gland, peculiar to Birds, and situated at the base of the rudi-
mentary tail (uropygium) is present in nearly all, being wanting
only in a few groups (e.g. Ratitse) : its secretion serves to oil the
feathers, and it is especially well developed in Water-Birds. A
gland is also present in the neighbourhood of the auditory passage
in certain Gallinaceo?, but otherwise integumentary glands are
wanting in Birds. Characteristic of the derm is its richness in
sensory organs (tactile corpuscles) and muscle-fibres, most of which
latter are inserted into the feather-sacs and serve to erect the
feathers (arrectores plumarum). Epidermic scales are present
on the feet.
The feather is foreshadowed in the reptilian epidermic scale,
of which it is merely a further modification. That scales and
feathers are homologous structures is, at any rate to a certain
extent, indicated by their mode of development, which is briefly as
follows.
In the region where a feather is to be formed, the dermal tissue
becomes slightly raised up towards the thickened epiderm (Fig.
20, A), and thus gives rise to a vascular papilla. As this papilla
grows out to form an elongated cone, the feather-germ (B), its
The individual epidermic shields of Chelonians are independent of the
underlying bony plates (Fig. 33), and do not correspond with them in arrange'
ment.
•26
COMPARATIVE ANATOMY
vascular base gradually sinks deeper and deeper into the derm,
and thus becomes surrounded by a sort of pocket- -the feather-
follicle. The horny, as well as the Malpighian layer of the
epiderm extends into the base of the follicle, and thence on to the
feather-germ, the interior of which is throughout filled by cells of
the derm which give rise to the pulp.
As the feather-germ keeps on growing, the cells of the
Malpighian layer begin to proliferate rapidly, giving rise to a
series of radial folds arranged along a central axis and extending
inward towards the pulp (Fig. 21, A). These folds, between
which the nutritive pulp extends, then become cornified and
separated from above downwards from the surrounding cells (B) ;
and, on a gradual drying of the central pulp-substance, give rise
to a tuft of horny rays (C), which are, however, at first bound
together by the enclosing stratum corneum, which forms a sheath
around them. Most Birds are hatched when the feathers are in
IB P c
FIG. 20. — Two EARLY STAGES IN THE DEVELOPMENT OF THE FEATHEK
(SEMIDIAGRAMMATIC).
B, blood-vessel ; C, derm ; E, proliferating epiderm ; F, rudiment of follicle ;
h, horny, and m, Malpighian layer of epiderm ; P, pulp of the papilla.
this stage of development, and they thus appear as if covered with
brush-like hairs.
By the shedding of the investing horny sheath, the rays or
barbs — on which smaller secondary rays or barbulcs become
developed — become free, and thus an embryonic down-feather
(pluma) is formed. The whole feather-germ, however, does not
become divided up in this manner : its lower portion, embedded in
the skin, forms the quill (calamus), the interior of which contains
a peculiar flaky and air-containing horny substance, the dried
remains of the pulp.
Thus the earlier stages of development of the feather and
reptilian scale are very similar, but during later stages the
feather becomes adaptively specialised. The warm-blooded but
still flightless ancestors of Birds probably possessed a covering of
down-feathers which served as a protection against the cold, and
which only later become adapted in connection with flight.
In many Birds the feathers retain throughout life the essential
characters of down, with more or less differentiation (e.g. Ratitas,
and more especially the Cassowaries) ; but in most cases the
INTEGUMENT
27
down1 becomes covered or replaced by the more complicated
definitive contour-feathers, the proximal barbs of which usually
still retain their down-like character. A contour-feather (penna)
at first closely resembles a down-feather, but in the course of
further growth, two adjacent rays become enlarged to form, with
the relatively longer or shorter quill, a main axis or stem (scapus):
the part distal to the quill, to which the barbs are attached in
a double row opposite one another is called the shaft (rackis).
At the base of the quill is a small aperture, into which the
vascular papilla extends ; and a second very small aperture is
present at the junction of quill and shaft on the inner surface.
•St
A
FIG. 21. — THREE STAGES IN THE DEVELOPMENT OF THE EMBRYONIC DOWN
DIAGRAMMATIC (after Davies). A, B, IN TRANSVERSE, and C IN LONGI-
TUDINAL SECTION.
F, dried remains of pulp; F.S, follicle; F.Sp, quill; P, pulp, with its ex-
tensions towards the feather sheath at t in A, which separate the developing
barbs (St) : these have become free in C.
The barbs together constitute the vane (vesdllum), and the
barbules arise obliquely, in a double row on each barb, so as to have
relations to the latter similar to those of the barbs to the shaft.
On the barbules a double series of barbicels are developed, certain
of which may bear minute booklets which interlock with one
another, and so connect the barbs together into a continuous
sheet : this is particularly the case in the row of large wing-
feathers (remiges) on the fore-arm and manus and of tail-feathers
(rcctriccs) on the rump or uropygium.
In many Birds each quill of the ordinary feathers of the body
bears two vexilla, which may be equal in size (Cassowary) ; but
usually one, the aftershaft (hyporachis), is smaller than the main
shaft.
1 Various modifications of the down feathers occur (e.g. floplnmex, which
by some zoologists are supposed to represent the last remains of a primitive
feather-covering from which both down-feathers and contour-feathers have
become differentiated).
28 COMPARATIVE ANATOMY
The contour feathers are generally not distributed irregularly
over the body, but are arranged in definite feather- tracts (ptcrylce)
separated by down-covered spaces (apteria), having a more or less
different arrangement in the various groups.1
A periodic casting of feathers, or moulting, takes place in all
Birds, and corresponds to the similar process of the casting of the
horny epidermic layer in Amphibians and Reptiles : the papilla per-
sists, and in connection with it the replacing feather is developed.
The feather-covering of Birds must have been acquired in very
early geological periods, for Archasopteryx, found in the Jurassic
strata of Bavaria, possessed well-formed feathers with a delicate
shaft and vane (Fig. 49).
The colours of feathers are due in part to the presence of
various pigments (viz., red, yellow, orange, black, and brown), and
in part to the phenomenon of interference, which may produce
white, grey, blue, and metallic or iridescent tints.
Mammals.
The integument of Mammals is characterised by the presence
of hairs, and the question as to how far scales, feathers, and hairs
are comparable to one another is an interesting one. No inter-
mediate forms are known, but there is no doubt that the
feather is much nearer to the scale than is the hair. The study
of their development, however, shows that the origin both of hair
and feather may be traced in the first instance to similar scale-
like structures, in spite of their very different final form. Thus
phylogenetically both are closely related to the horny scales of
Reptiles.
The development of hairs, as well as their grouping and distri-
bution, indicates certain topographical relations to scales, and
also that they first arose in relation with a primitive scaly coat.
Secondarily they appeared on or behind the scales, which were
gradually reduced as the hairs underwent increasing differentia-
tion. Hairs, which, like feathers, are arranged in groups, are not,
however, individually homologous with scales, but arise from parts
of a scale-area, while the feather possibly corresponds to an
entire scale. There can be little doubt that the earliest
Mammals, which arose from primitive Reptiles, possessed an
extensive scaly covering in addition to a sparse coat of hairs.
In development, the first essential indication of the hair is
seen in the epiderm, which may or may not become raised up at
1 In some Birds bristle-like feathers occur on the head, and the foot-scales
or shields may bear feathers of a peculiar form. In insectivorous and nocturnal
forms, tactile or sinus-feathers are present around the eye and ear and at the base
of the beak, analogous to the sinus-hairs of Mammals (q.v.).
INTEGUMENT
29
the point in question 1 (Fig. 22). This thickening of the epiderm
grows downwards in the form of a papilla (hair-germ) and
is surrounded by the cells of the derm, so that, as in the case
of the feather, it comes to lie within a kind of pocket, the hair
follicle. The originally uniform mass of cells of the hair-germ is
later differentiated into a peripheral and a central portion : the
latter (bulb-cone, C) gives rise later (D) to the hair-shaft with its
medulla and pith, and to the cortex, as well as to the cuticle of
the shaft and the so-called inner root-sheath ; from the former
arises the outer root-sheath. The origin of both sheaths, as well
as the sebaceous glands, can be traced to the Malpighian layer
(cf. Fig. 23.) The base of the hair-shaft which fills up the bottom
of the follicle is broadened out to form the hair-bulb, which extends
A
B
D
FIG. 22. — DIAGRAM OF FOUR STAGES IN THE DEVELOPMENT OF THE HAIR
(founded on Stohr's figures).
A, hair germ ; B, hair-cone ; C, bulb-cone, showing formation of bulb, papilla, and
hair-cone, which latter is becoming cornified at the apex. D, later stage in
which the hair is further differentiated, but has not yet reached the surface.
round the highly vascular hair-papilla like a cap. The hair
usually breaks through the skin obliquely, the direction differing
in different parts of the body.
Thus the more or less cylindrical hair-shaft consists of three
parts— medulla, cortex, and cuticle : the medulla is the most
important part of the hair, and on its structure mainly depend
the differences seen in the hair of individual species.
The colour of the hair is due to three causes : firstly, to a
greater or less accumulation of pigment in the cortical layer;
secondly, to air contained in the intercellular spaces of the
medulla ; and lastly to the nature of the surface of the hair, i.e.
whether it is rough or smooth.
The mode of formation of new hairs in post-embryonic stages is
not thoroughly understood : when the hair is shed, it is not known
1 This hair-rudiment at first more or less resembles the rudiment of an
integumentary sense-organ of a Fish or gilled Amphibian ; and this fact has led to
the expression of a view that the origin of hairs may be traced phylogenetically
to such sensor v organs of the lower Vertebrates.
30
COMPARATIVE ANATOMY
whether the old papilla remains, or whether a new one is formed.
The former method seems to occur in most instances, although not
ip;,/A
.
FIG. 23.— LONGITUDINAL SECTION THROUGH A HAIR. (Diagrammatic.)
Ap, arrectores pili ; Co, derm ; F, outer longitudinal layer, and F', inner trans-
verse layer of connective tissue fibres of follicle ; Ft, adipose tissue ; GH,
hyaline layer, which lies between the inner and outer hair-sheaths, i.e.,
between the root-sheath and the follicle ; HBD, sebaceous glands ; HP,
hair-papilla, containing vessels ; M, medulla ; 0, cuticle of shaft ; It. cortex ;
Sc, stratum corneum ; Sch, hair shaft ; SM, stratum Malpighii ; WS, WS1,
external and internal root-sheath — the latter reaches above only as far as the
sebaceous ducts, and is not continuous with the epiderm.
infrequently new hairs are formed throughout life direct from the
epiderm, as in the embryo. From a primary hair-germ an entire
group of hairs may be formed by subsequent division.
The special tactile hairs (mbrissce, sinus-hairs) present on parts
INTEGUMENT 31
of the face are usually much longer and stronger than the
others, and are provided with striped muscle-fibres. They are the
first to appear in the embryo, and the last to be retained in those
forms which have lost their hairy covering in connection with an
aquatic life (e.g. Cetacea). Between the outer and inner layers of
their follicles are blood-spaces and cavernous tissue, and they are
well supplied with branches of the trigeminal _ nerve. The
ordinary hairs are also well innervated, especially in the case of
nocturnal animals, and aie sensory as well as protective in
function. Other modifications of the hairs are seen, e.g. in the
eye-lashes, the long tail-hairs of most Ungulates, and various other
forms of bristles : spines, such as are characteristic of the Hedge-
hog and Porcupine, are merely especially strongly developed
bristles.
Hairs, like feathers, are arranged in definite tracts (flumina
pilorum), and the fur often consists of finer and coarser elements.
A richer hairy covering (lanugo] is often met with in the
embryonic condition than in the adult (e.g. in the human foetus) ;
and this fact, together with the occasional appearance of abnorm-
ally hairy individuals, indicates that at one time Man was dis-
tinguished by a far more abundant clothing of hair than at the
present day.
Hairs are most scanty in the Cetacea and Sirenia, in the
former of which they are often limited to a ;few bristles (sinus-
hairs) in the region of the lips (Toothed Whales) or chin (Whale-
bone-Whales), or may be entirely wanting except in embryonic
stages. In the Sirenia, apart from the persistent hairs, a thick
coat of fine hairs is present in the embryo, and modified traces of
these can be recognised in the epiderm of the adult.
The hairy coat may be shed and renewed periodically (e.g. in
the case of Mammals exhibiting differences in their summer and
winter fur), or the shedding and renewal may take place con-
stantly, and so result in no marked change of coat.
Epidermic scales may also occur in Mammals, but are rarely
present on parts which are well covered with hair. They are large
and well marked in Manis, covering the dorsal surface of the head
and body, the sides of the latter and the whole tail, and are
present on the tail of various Rodents (e.g. Beaver, Anomalurus,
Muridse), Insectivores, Anteaters,1 and Marsupials. Other epidermic
structures formed as thickenings of the horny layer also play an
important part in Mammals : such are, claws, nails, hoofs, the horn-
sheaths of Ruminants, the so-called whalebone (baleen) of the
Mystacoceti, the palatal plates of Sirenia, the thickened regions of
the epiderm in Cetaceans and Pachyderms, the ischial callosities of
certain Apes, and the nasal horns of the Rhinoceros, the last-
mentioned of which consist of numerous hair-like horny fibres.
According to the form taken by the horny covering of the
1 Vestiges of horny scales also occur in Armadilloes.
32
COMPARATIVE ANATOMY
distal ends of the digits, the Mammalia may be subdivided into
Unguiculata and Ungulata, the former group including those with
claws or nails, and the latter those with hoofs. But no hard and
fast line can be drawn between these structures, which in all cases
are derivable from a simple form of claw, like that of Reptiles and
Birds. The terminations of the digits are without a horny
covering in Cetacea, though rudiments are present in the embryo
of Toothed Whales ; while among the Sirenia the Manatee possesses
vestigial and variable nails.
The horny nail-plate is situated on the dorsal side of the
digit, while ventrally is the softer sole-horn, which is continuous
proximally with the pads or tori on which the foot partially or
D
FIG. 24. — DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH THE DISTAL ENDS
OF THE DIGITS OF — A, ECHIDNA ; B, AN UNGUICULATE MAMMAL ; C, MAN ;
AND Z>, HORSE (after Gegenbaur and Boas).
1 — 3, phalanges ; b, torus ; N, nail-plate ; S, sole-horn ; W, bed of claw or nail.
entirely rests when on the ground. The essential relations and
chief modifications and reductions of these parts in various
Mammals are illustrated in Fig. 24. Tori are present in most
Mammals, and have a definite arrangement on the palms and soles
(apical, interdigital, and proximal), and in them the dermal
papillae are either irregular or are definitely grouped, and may
give rise to a series of concentric lines and arches.1
When pigment is present (e.g. on the snout, external genitals,
and teats), it is chiefly situated in cells of the Malpighian layer
1 Compare those of Man, which yield the characteristic "finger-prints."
INTEGUMENT
33
into which it wanders from the derm, which may also contain
pigment.
In the derm, as may be seen by a glance at Fig. 25, an outer
papillary and an inner reticular portion may be distinguished.
The papilla? of the former are accurately adapted to the over-lying
epiderm : some of them contain blood- and lymph-capillaries, and
others, nerves with tactile corpuscles. The latter, on the other hand,
becomes lost without any
sharp boundary line in the
subdermal connective tissue
and in the more or less
w e 1 1-d e v e 1 o p e d fatty lay er
(panniculus adiposus). The
panniculus may be very
largely developed in aquatic
Mammals — e.g. in the Ce-
tacea, in which it serves to
preserve the heat of the
body, and at the same time
to reduce the specific gravity
of the animal.
As in Birds, the con-
nective tissue fibres of the
derm are irregularly felted.
Most of the smooth muscle-
fibres are inserted into the
hair-follicles (arrectores pilo-
rum, Fig. 23), but may #0, derm ; F, subcutaneous fat ; GP, vascular
FIG. 25. — SECTION THROUGH THE HUMAN
SKIN.
papilla ; H, hair with sebaceous glands
( D) ; N and G, nerves ; NP, sensory -
papillae ; Sc, stratum corneum ; SD, sweat-
glands, with their ducts (SDl);SM, stratum
Malpighii.
occur independently of hairs,
e.g., in the scrotum and
teats.
In the great abundance
of integumentary glands,
Mammals differ greatly from Reptiles and Birds, and more nearly
resemble Amphibians. They may be present in all parts of the
skin, and differ greatly with regard to the consistency, composition,
colour, and odour of their secretions. Those which serve for the ex-
cretion of products of destructive metabolism in general, and for the
formation of odoriferous substances, are either tubular or alveolar
in structure. The former, which were probably derived from those
of ancestral Amphibians, possess a muscular investment, have
mostly the form of the characteristically coiled sweat-glands, and
are rarely entirely wanting (e.g. Cetacea) : the latter, which are a
new acquisition and are known as sebaceous glands, appear to be
not only functionally, but also ontogenetically and phylogenetic-
ally closely connected with the hairs (Figs. 23 and 25).
Various modifications of both kinds are met with, and they
are often arranged in groups. Thus the iiasolabial glands of
D
34
COMPARATIVE ANATOMY
cattle, the lateral glands of the Shrew, and the dorsal glands of
Hyrax resemble sweat-glands ; while the preputial and Meibo-
mian glands, the inguinal glands of certain Rodents, and the facial
glands of Bats, are largely, at any rate, modified sebaceous glands.1
Another important modification of the integumentary glands
is seen in the characteristic mammary ylands, to the possession of
A B
g.m
d.
FIG. 26. — A, VENTRAL VIEW OF A BROODING FEMALE OF Echidna hystrix. B,
DISSECTION OF THK VENTRAL INTEGUMENT FROM THE DORSAL (INNER)
SIDE. (After W. Haacke.)
cl, cloaca ; t, T, the two tufts of hair in the lateral folds of the marsupial pouch
(b.m.) from which the secretion flows. On either side of the pouch, which
is surrounded by strong muscles, a group of mammary glands (y.m.) opens.
which the Class owes its name, and which secrete milk for the
nourishment of the young. Nothing is known of their phylogeny
in the ancestors of Mammals, but in all cases they correspond to
1 Amongst many other modifications of these glands of both types may be
mentioned the anal glands (especially well developed, e.ij., in Manis and the
Skunk) ; the perineal or prescrotal glands of Viverra ; the caudal gland of
the Fox and Wolf ; the suborbital or ant-orbital glands situated in the cavity of
the lacrymal bone in Cervidse ; and the interdigital glands of many Ruminants.
The preputial glands of the Beaver and Musk-deer also deserve special mention.
A peculiar tubular, femoral or spur-gland is present at the knee in Echidna
and dorsal to the hip-joint in Ornithorhynchus, near the vertebral column. It
opens by means of a long duct on to the tarsal spur, and, though present in the
embryo of both sexes, undergoes reduction in the female.
g. m.
INTEGUMENT
35
modified integumentary glands which have a certain similarity to
sweat glands, and like these, are probably specialised forms of
primitive tubular glands. The sebaceous glands arise much later
ontogenetically, but it is interesting to note that in the develop-
ment of the mammary area, traces of early stages of hairs may be
observed : these disappear later, but their sebaceous glands become
connected with the mammary ducts.
Potentially, therefore, mammary organs may be developed in
any part of the skin, but as a matter of fact, they are limited to
the ventral side in adaptation to the
method of suckling the young. — — „
Amongst the oviparous Mono-
tremes, a marsupial poucli appears in
the embryo of Echidna as an infold-
ing of the abdominal wall (Fig. 26).
This pouch, which serves to shelter
the egg and young, becomes tempor-
arily enlarged in the breeding season
as the offspring increases in size,
and has the form of a deep sac ex-
tending backwards and provided
with closing muscles. On its lateral
walls are a pair of depressions, the
so-called mammary pockets, which also
arise periodically. A bunch of hairs
is present in each pocket, the follicles
of which open along with the mam-
mary ducts Oil these two glandular
areas, which are sharply marked off
from the rest of the pouch. The
mammary glands themselves, which
are almost equally developed in the
two sexes, consist in Echidna of long, coiled, and much branched
tubes, the blind ends of which are swollen. Both they and the
mammary pockets are acted upon by a strong integumentary com-
pressor muscle, the presence of which is all the more necessary
as Monotremes possess no teats. The manner in which the young
take in the milk is uncertain : it has been supposed that the milk
drops from the two bunches of hairs and is then licked off; or
it may be that a temporary teat is formed by suction.1
_ Amongst Marsupials, in which teats are present, the pouch is
evidently homologous with that of Echidna, but it reaches a higher
1 Nothing definite is known as to the manner in which Ornithorhynchus cares
for its young. The eggs are laid in burrows in the earth, and it appears that
110 marsupial pouch is formed at any time ; possibly it may have gradually
disappeared as the animal acquired an aquatic habit. The sieve-like apertures
of the mammary gland are distributed over two spindle-shaped areas on the
ventral body-wall which are covered by the fur and surrounded by integumentary
muscles.
D 2
FIG. 27. — RUDIMENT OF THE
MARSUPIUM IN Diddphys
marsupialis, RECONSTRUCTED
FROM TRANSVERSE SECTIONS.
(After Bresslau. )
M, marsupial pockets ; Z, teats
or mammary pockets ; f,
lateral walls of the marsu-
pial pockets which fuse to
». • form the walls of the pouch.
36
COMPARATIVE ANATOMY
stage of development. A solid ring-like ridge of the epiderm is
formed around each teat (or mammary pocket from which the teat
arises), and later on the areas thus enclosed sink inwards so as to
give rise to a row of hollows or marsupial pockets, which together
constitute what has been called the marsupial line, (Fig. 27). By
a fusion of the adjacent lateral walls of the pockets arise the
marsupial folds which form the pouch (Fig. 28). The lowest
Marsupials show no trace of a pouch, and their ancestors probably
never possessed one.
Marsupial pockets persist permanently in various degrees of
perfection in a number of the Eutheria ( e.g. Manida?, Murida?,
FIG. 28.— POUCH OF Thylacinus, AFTER REMOVAL OF THE SKIN.
(After Cunningham.)
r, compressor mammas ( = cremaster of male), passing over which are seen blood-
vessels and the genito-crural nerve ; /, lymphatic glands ; s, sphincter
marsupii ; z, teats.
CervidaB, Carnivora). In Mice they can be recognised until the
beginning of lactation, and are then evaginated and thus lengthen
the teats.
The teats may become developed in one of two ways. Either
the skin surrounding the mammary pocket (Fig. 29, A) becomes
raised up to form a circular rampart, and thus gives rise to a teat
perforated by a canal, into the base of which the ducts of the
gland open (B) ; or the gland surface itself becomes elevated into
INTEGUMENT
37
a papilla, while the surrounding skin remains almost on a level
with the rest of the integument (c). In the latter case, therefore,
there is an evagination of the mammary pocket, and the teats
may be described as secondary or true (Marsupials, Rodents,
Lemurs, Monkeys, and
Man), and in the former
as primary or pseudo-
teats (Carnivores,
Pigs, Horses, and Ru-
minants). The latter
condition is already in-
dicated in certain Mar-
supials (e.g. Phalangista
vulpina).
The teats- are often
situated in two nearly
parallel rows along the
ventral side of the
thorax and abdomen
which slightly converge
towards the inguinal
region (e.g. Carnivores,
Pigs) : in other cases
they may be restricted
either to the inguinal
(Ungulates and Ceta-
ceans) or to the thoracic
region (Sloths, Manis,
rjlephants, oirenia, JTIG 09. — DIAGRAMMATIC REPRESENTATIONS OF
many Lemurs, Cheirop- THE EARLY DEVELOPMENT OF THE LEADING
" TYPES OF MAMMARY GLANDS. (Modified from
ueeenbaur.)
B
G
tera and Primates):
, .,' . , .
while m others, again,
they mav be axillary or
i j • • i
abdominal, or they may
OCClir in various com-
binations of all these
> first or undifferentiated (mammary pocket)
stage ; B, stage of the pseudo- (primary) teat ;
^ |tage 'of thfee true (secondary) teat ; d, mam-
mary canal ; f.y, glandular area; gl, mammary
glands ; v, rim (or rampart) of the glandular area.
regions.
The number of teats varies greatly : there may be as few as
one pair, or as many as eleven pairs (Centetes) : in general, their
number corresponds to that of the young produced at one
birth.
Not infrequently, supernumerary or accessory mammae and
teats can be recognised (e.g. in Sheep and Cattle), and there may
be indications in the foetus of a greater number of teats than that
which occurs in the adult: thus in the embryo of Whalebone-
Whales there are eight pairs, while the adult possesses only a
single pair on either side of the vulva. Cases of such a hyper-
maslism and hyperthclism are also well known in the human
38 COMPARATIVE ANATOMY
subject, as frequently in men as in women. The accessory
mammas and teats are usually anterior (above) or posterior to (below)
the normal ones, and thus form with them two converging rows
from the axillary to the inguinal region, just as in many other
Mammals and in the human embryo, at a certain stage of which
four pairs of additional rudiments of mammary organs can always
be recognised. There is thus an indication of normal hyper-
mastism and hyperthelism in human ontogeny, which is paralleled
in those numerous mammals in which a " mammary ridge " or
"line" is formed, a structure which is probably comparable to the
" marsupial line " forming the rudiment of the pouch in Mar-
supials.
In the male, the mammary apparatus becomes absorbed,
though frequently at birth and at puberty milk is produced in
the human subject. Male goats and castrated sheep have also
been known to give milk.
B. SKELETON.
1. EXOSKELETON.
THE hard exoskeleton, consisting of bone or of other calcified
tissues, must be distinguished from the horny exoskeletal parts
already described. Thus it will be remembered that the term
" scale " is sometimes used for a horny epidermal structure, and
sometimes for a bony dermal one (pp. 20 and 23).
In Cyclostomes, scales are entirely wanting.
Elasmobranchs. — The integument of most Elasmobranch
Fishes encloses certain hard structures each consisting of a basal
FIG. 30.— PLACOID SCALES FROM THE SKIN OF AN ELASMOBRANCH.
(Semi-diagrammatic. )
S, basal or socket-plates in the dermal connective tissue (By) ; X,
denticles.
plate or socket bearing a pointed spine or denticle (Fig. 30), differ-
ing considerably in form and relative size in the various members
of the Order, and known as a placoid scale. These placoid organs
are continually being formed anew throughout life, and are pro-
tective in function. The basal plate is rhomboid or rounded in
form and consists of bony tissue, while the denticle itself is
composed of dentine covered over with enamel. In many Rays,
there is a relatively small number of these placoids, while in most
Sharks and Dogfishes they are much more numerous and close-
set. In the Electric Rays they are wanting. The primary part
40
COMPARATIVE ANATOMY
is the enamel, which is formed as an excretion of the epidermic
cells (Fig. 31), while the later formed mesodermal dentinal and
bony portions become closely connected with the enamel secondarily.
Thus the enamel is the first and originally the only hard substance
of the placoid organ.
In the Holocephali (Chimsera, Callorhynchus) a double row
of placoids is developed along parts of the dorsal region in the
embryo, but disappear in later stages : in the adult these organs
apart from the spine on the anterior margin of the dorsal fin, occur
only on the claspers and frontal organ of the male.1
Teleostomes. — In these Fishes, ossifications in the derm to
form bony scales takes place independently of any stimulus from
FIG. 31.— VERTICAL SECTION THROUGH THE SKIN OF AN EMBRYO SHARK.
(From Gegenbaur's Comp. Anatomy.}
C, derm ; c, c, c, d, layers of the derm ; E, epiderm ; e, its layer of columnar
cells ; o, enamel layer ; p, dermal papilla.
the epiderm. Thus the denticle, which in Elasmobranchs is the
primary cause of the development of the basal plate, gradually
disappears in ontogeny, and the latter is the only part of the
placoid organ which remains, its independence being retained
in the formation of bony skeletal substance in higher Vertebrates.
In Lepidosteus, denticles are still developed in the skin but
are quite transitory, and this primitive method of starting the
formation of bony tissue is again met with in the Vertebrate series
in connection with certain parts of the Amphibian skull ; here
certain bones (vomer, palatine, pterygoid, &c.) which originally
served as supports for oral teeth, persist even if the teeth dis"-
appear, as they have become an integral part of the facial
skeleton.
1 In addition to the ordinary placoid scales, larger or smaller spines of a
similar structure may become developed in connection with the dorsal tin, around
the first cartilaginous ray (e.g. Acanthias, Trygon, Chimera). In the Saw-
fish (Pristis), there is a double row of large denticles on the long rostrum.
EXOSKELETON 41
It is therefore evident that the first bony hard substances
of the Vertebrata arise in connection with the external skin and
oral mucous membrane, and that the bony integumentary skeleton,
or exoskeleton, is therefore phylogenetically older than the bony
internal skeleton or cndoskeleton. The latter owes its origin
to a gradual extension of the exoskeleton from the surface to
deeper parts, where it takes on relations to the cartilaginous endo-
skeleton. An independent ossification of the perichondrium, or
membrane which invests the cartilage, may also take place, so
that bone and cartilage now combine in the formation of the
skeletal framework and thus further complications arise. To
the original dermal ossification is added a formation of bone in
the perichondrium, and finally even a secondary endochondral
ossification may occur, replacing the cartilage : the former is most
marked in the Anamnia, the latter in the Amniota, and the result
of both is usually the subjection of the cartilaginous tissue in
the struggle of the tissues in the organism.
In most Ganoids, thick plates, usually rhombic in form, are
present in the skin ; in the bony Ganoids (Polypterus, Lepidosteus) l
these cover the entire body, their margins being in apposition.
These ganoid scales correspond to the deeper part of the placoid
basal plates. Their surface is dense and smooth, owing to the
presence of a layer of ganoin, of mesodermal origin, and formerly
erroneously described as enamel. The exoskeleton was largely
developed amongst fossil Ganoids.
The scales of Teleosts are usually thin, and of the form known
as cycloid or ctenoid ; in the former the whole margin is smooth,
while in the latter the posterior margin is toothed and comb-like,
but various intermediate stages occur. The scales are arranged in
FIG. 32.— DIAGRAMMATIC LONGITUDINAL, SECTION THROUGH THE SKIN OF A
TELEOSTEAN, TO SHOW THE RELATION OF THE BONY SCALES. (From Boas's
Zoology.}
I, derm ; o, epiderm ; s, scale.
oblique rows and are situated directly beneath the epiderm, the
individual scales not touching one another. Secondarily, they
usually come to lie within definite pockets or sacs, and to overlap
one another like tiles on a roof (Figs. 13 and 32). The surface
of the scales may be sculptured.
In the developing scale, a superficial dense portion, formed
1 In Amia the scales have a "cycloid" form, and in the adult Polyoclon
they are absent.
42 COMPARATIVE ANATOMY
from cells and corresponding to the dentinal layer of the basal
plate, can be distinguished from a deeper part composed of several
layers of connective tissue : each of these becomes independently
ossified in a typical manner.
Numerous other forms of the dermal skeleton are met with in
Teleosts. In some of these Fishes (e.g. Plectognathi, Lopho-
branchii, certain Siluroids),1 as in many of the earliest Paleozoic
Vertebrates (Ostracodermi), bony scutes are developed and form a
strong cuirass. In others, again (e.g. many Siluroids and Eels), the
scales may be reduced or absent. The bony dermal fin-rays or
" leptotrichia " of Teleostomes possibly correspond to modified
scales.
Dipnoans. — In the Dipnoi, as in the Teleostomi, the scales
are not directly derivable from the Elasmobranch placoids. In
form, in their overlapping arrangement, and in their situation in
pockets of the derm, they resemble the cycloid scales of Teleosts ;
but this similarity must have come about independently in the
two cases, that is, must be due to convergence.
Amphibians. — Recent Amphibians have retained only very
slight traces of such a dense integumentary bony armour as was
present in the fossil Stegocephali. Amongst these, specially strong
dermal plates were formed in the region of the shoulder-girdle, and
very commonly most of the body was covered with scales. A series
of oblique and bilaterally symmetrical rows of scales covered the
entire ventral surface between the shoulder- and hip-girdles, a
further differentiation of which results in the scales no longer
overlapping, but forming short parallel rods, which correspond
to the so-called " abdominal ribs " (" parasternal elements ") of
certain Reptiles. As examples of exoskeletal structures in exist-
ing forms may be mentioned the bony plates in the skin of the
back of certain Anura (Ceratophrys dorsata and Brachycephalus
ephippium), as well as the scales lying between the ring-like folds
of the limbless Amphibia (Gymnophiona). The latter resemble
in many respects the scales of Fishes, and may be derived from
such a scaly covering as that of the Permian Salamander Dis-
cosaurus.
Reptiles. — The dermal skeleton was very highly developed
amongst fossil Reptiles, e.g. certain Dinosaurs, such as the Jurassic
Stegosauridae, in which enormous bony plates and spines covered
with horn, sometimes as much as sixty-three centimetres long,
were present in the dorsal region. Teleosaurus and Aetosaurus
(Crocodilia), as well as some of the gigantic Cretaceous Dinosaurs,
(Ceratopsidoe), possessed a strong exoskeleton.
Amongst existing Reptiles a series of well-developed "abdo-
' In these Siluroids, the bony plates may bear sockets in which denticles,
consisting of dentine and enamel, are implanted. But although denticles are
retained in these cases, they do not contribute to the formation of the basal plates,
as in Elasmobranchs.
EXOSKELETON
43
minal ribs" (cf. p. 42) are present in Hatteria in the rectus
abdominis muscle, each consisting of a median and of a paired
lateral bar, and being considerably more numerous than the body
metameres. In Crocodiles similar bars are present, their number
corresponding with that of the ribs: they no longer reach the
middle line, and with the exception of the first, each consists, on
either side, of two firmly united portions. Evidently these structures
have here begun to undergo reduction.
Crocodiles, many Lizards (Anguis, Cyclodus, Scincus), and more
especially Chelonians, exhibit a well-developed dermal skeleton,
the scutes composing which cover the body more or less completely.
In the last-mentioned group there is a dorsal and ventral shield
(carapace and plastron} consisting of numerous more or less closely
united pieces, and completely encircling the body (Fig. 33). The
FIG. 33. — A, CARAPACE, and B, PLASTRON OF A YOUNG Testudo c/rceca;
C, PLASTRON OF Chelone midas.
G, costal plates ; E, entoplastron ; Ep, epiplastron ; Hp, hypoplastron ; Hy,
hyoplastron ; M, marginal plates ; N, neural plates ; Np, nuchal plate ;
Py, pygal plates ; R, ribs ; Xi, xiphiplastron. ( V indicates the anterior,
and If the posterior margin.)
plastron, the larger posterior portion of which probably corresp
to greatly modified abdominal ribs, arises entirely by ossification
of the derm ; while parts of the carapace have a close relation to the
endoskeleton (neural arches and ribs), which early in development
became broadened out into plates. At the same time, the inter-
costal muscles disappear completely, and the muscles of the back
undergo partial reduction, while the articulations of the vertebrae
and ribs disappear. The nuchal, pygal, and marginal plates (cf.
Fig. 33) are entirely independent of the endoskeleton — that is, are
purely exoskeletal bones ; while the costal and neural plates
correspond to much thickened periosteal bones developed around
the cartilaginous ribs and neural spines respectively : though they
are subcutaneous in position, they have nothing to do with the
skin genetically.
44 COMPARATIVE ANATOMY
Reference has already been made to the dermal bones amongst
fossil Fishes, Amphibians, and Reptiles, and certain of these in
the antero-ventral region of the trunk are of special interest, as
they are represented in certain Reptiles by a bone known as the
episternum, which underlies the sternum. Though always arising
as a paired structure, the episternum, which is present, e.g., in
Palaeohatteria, most Lizards (Fig. 56) and Crocodiles, forms in the
adult an unpaired plate of varied form. It is wanting in
Chamaeleo, Anguis, Ophidia, and Chelonia.1
Birds. — Reduced abdominal ribs occur in the primitive
Archa^opteryx, otherwise no fossil or existing Birds possess a
dermal exoskeleton, and no independent elements corresponding
to an episternum can be recognised even in the embryo: they
have evidently long ago disappeared.
Mammals." — Armadillos are the only Mammals possessing a
bony exoskeleton,3 which consists of a series of five movable
transverse bony scutes covering the head, neck, and body, and of
smaller plates on the tail and limbs. Sparse hairs occur between
these plates. It is very doubtful whether this exoskeleton has
been derived from that of Reptiles: more probably it, like the
horny exoskeleton of Manis (p. 31), has arisen secondarily, and in
consequence of its development the hairs have become reduced.
In Glyptodon, a large fossil member of this group, the dermal
plates were firmly united together to form a shield which covered
the whole body.
2. ENDOSKELETON.
Under the term exoskeleton are included the bony parts
which, as a rule, remain throughout life in connection with the
integument : the endosMcton consists mainly of cartilaginous and
bony parts, all of which have a deeper position. The cartilaginous
portions, which in their entirety constitute the primordial endo-
sMcton, have undoubtedly from the first arisen in this position,
and for a long period formed, together with the notochord,
the entire internal skeleton, as they practically do at the present
day in Elasmobranchs as well as in Cyclostomes. As already
1 Unless the element of the plastron marked E in Fig. 33 is to be interpreted
as such.
2 A certain part of the anterior end of the sternum, as well as certain carti-
laginous and bony elements in the region of the sterno-clavicular articulation, are
sometimes said to correspond to the last remains of the dermal episternum of
lower forms ; but as further proofs are required before such a homology can be
definitely accepted, the term proxttrnum has been proposed to include the
elements in question (cf. under .Sternum).
3 It is possible that the peculiar horny tubercle in the region of the dorsal
fin in certain Cetaceans may represent the last vestiges of a dermal bony arma-
ture such as was present in the extinct Zeuglodon.
VERTEBRAL COLUMN 45
mentioned (p. 41), bone may be developed in connection with
this primordial skeleton and may arise in two different ways. It
may be formed in parts which are not superficial ; or bony elements
may become associated with the cartilaginous skeleton which are
derived phylogenetically from the exoskeleton but which in course
of time have taken up a deeper position : these become secondarily
connected with the bones which have arisen independently in this
position. In order to determine as to which of these two
categories any particular bone belongs, an appeal to comparative
Embryology is necessary.
The relations of a bone to the cartilaginous skeleton may be of
such a nature that it merely becomes applied to the outside of the
cartilage, when it may be described as an investing lone. A bone
may, however, originate in the perichondrium or membrane which
covers the cartilage, and then, in the course of phylogenetic
development, may invade and replace the cartilage : in other
words, perichondral bone may become endochondral.1 The cartilage
beneath investing bones may gradually disappear in the course of
time, and occasionally a bone which was originally perichondral
may attain apparent independence by the loss of the cartilage
around which it was formed in the first instance.
I. VERTEBRAL COLUMN.
An elastic rod, the notochord, or chorda dorsalis, lying in the
longitudinal axis of the embryo between the neural and visceral
tubes (cf. p. 6), is the first part of the endoskeleton to be formed,
and is the primitive forerunner of the vertebral column. It is
developed as a ridge of the primary endoderm, from which it
becomes constricted off, and is therefore of epithelial origin. In
the large parenchyma-like cells of which it is composed vacuoles
soon appear, and eventually only the walls of the cells persist in
the greater part of the notochord ; these become flattened by
mutual pressure, so that they appear like a meshwork of pith-cells
(Fig. 34, A, B). At the periphery, however, the cells retain their
protoplasm, becoming flattened and arranged like an epithelium.
Around the notochord two homogeneous, cuticular sheaths are
successively developed from its cells. The primary sheath is first
secreted by the peripheral notochordal cells : the thicker secondary
sheath, which has a similar origin from the so-called " notochordal
epithelium," appears later.
From the surrounding mesoderm a skdetogenous layer is
developed : this not only surrounds the notochord, but extends
dorsally to it as well as ventrally. Thus a continuous tube of
embryonic connective tissue is formed enclosing the spinal cord,
1 The unsatisfactory terms "membrane-bones" and " cartilage " bones are
usually used in describing the investing and replacing bones.
46
COMPARATIVE ANATOMY
B
sk.l
sh.1
Ir.p
nc.
sk.l
FIG. 34. — DIAGRAMS ILLUSTRATING THE DEVELOPMENT OF THE NOTOCHORDAL
SHEATHS AND VERTEBRAL COLUMN.
A. — Early stage, showing notochordal cells (nc) and primary sheath (.s/t1), 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 (sh2) is derived :
paired dorsal and ventral cartilages, or arcualia (d.a, v.a) have arisen in the
skeletogenous layer (Cyclostomes, Cartilaginous Ganoids).
C. — Cartilage cells have passed through the primary sheath, and are invading
the secondary sheath (Elasmobranchii, Dipnoi).
D. — The cartilages are growing round the notochord, outside its sheaths, which
gradually become reduced (Bony Ganoids, Teleostei, Amphibia, Amniota).
(A — D represent the caudal region.)
E. — A later stage in the development of a pre-caudal vertebra. The notochord
(nc) has become constricted, and the cartilages have united into a single
mass and have given rise to a centrum (<•), neural arch (n.a), neural spine
(n.sp), transverse processes (tr.p), and articular processes (art).
and only broken through at the points of exit of the spinal
nerves. This stage is often known as the membranous stage, and
in it no indication is seen of the metameric segmentation which
VERTEBRAL COLUMN 47
occurs later in the vertebral axis. The cause of this segmentation
is to be traced primarily to the muscular system ; and it is evident,
on mechanical grounds, that the segmentation of the vertebral
column must alternate with that of the muscular segments or
myotomes. Small paired and segmentally arranged masses of
cartilage later appear in the skeletogenous tissue dorsally and
ventrally to the notochord, and these represent the rudiments of
the vertebra} (Fig. 34, B, E). This is the beginning of the second
or cartilaginous stage of the vertebral column ; and now ossification
may occur (bony stage). Those parts of the fibrous tissue which
do not become consolidated in this manner give rise to the ligaments
of the vertebral column.
Two different modes of development of the vertebral column
from the above-mentioned dorsal and ventral cartilages or arcualia
may be observed. In Elasmobranchs and Dipnoans the secondary
notochordal sheath undergoes a fibrillar degeneration, and becomes
invaded by cartilage-cells from the arcualia which break through
the primary sheath at these points (Fig. 34, c) and gradually extend
so as to surround the notochord, thus forming a cartilaginous
sheath which may undergo segmentation to form a series of
vertebral bodies or centra ; the arcualia at the same time extend
dorsally and ventrally respectively to form the vertebral (neural
and hcemal) arches. In other cases (e.g. Bony Ganoids, Teleosts,
Amphibians, and Amniota), the arcualia extend at their bases
round the notochord so as completely to surround it without
penetrating the primary and secondary sheaths (D). The centra
which are then formed by segmentation of this perichordal
cartilage may be described as perichordal centra, to distinguish
them from the chordal centra of Elasmobranchs and Dipnoans.
During these differentiations of the skeletogenous tissue, the
notochord suffers a very different fate in the various Vertebrate
groups ; it may increase in size and persist as a regular cylindrical
rod, or it may become metamerically constricted by the formation
of the vertebral bodies, and even entirely disappear. In all Verte-
brates above Elasmobranchs, the embryonic vertebral column is
relatively shorter than the notochord around which it is formed,
and thus there has been a phylogenetic reduction in length of the
axial skeleton.1
Amphioxus. — The notochord of Amphioxus exhibits many
primitive as well as special characters. It extends along the
whole length of the animal, whereas in the Craniata it always
ends anteriorly below the brain just behind the pituitary body.
1 An ephemeral structure, the subnotocliordal rod or hypocJiorda, occurring in
embryos of Fishes and Amphibians, may be briefly referred to in this place. It
arises as a longitudinal furrow or ridge of the endoderm in the head and trunk,
on the dorsal side of the gut, with which it may for a time remain in connection,
but eventually becomes constricted off as a rod lying beneath the notochord. It
soon undergoes degeneration, but traces of it may persist as an elastic band. It
seems probable that this structure is a vestige of the epipharyngeal groove of
Amphioxiis.
48
COMPARATIVE ANATOMY
A delicate primary sheath is present and is surrounded by con-
nective tissue which is continuous with that enclosing the neural
canal and separating the muscular segments or myotomes.
Cyclostomes. — In these, as in all the true Fishes, only two
regions can be distinguished in the vertebral axis, a trunk- or pre-
caudal region, and a caudal region. An advance on the primitive
condition in Amphioxus is seen in the development of a thick
secondary sheath and, at any rate in the caudal region, of
cartilaginous elements : in the adult Petromyzon these are present
all along the notochord in the form of rudimentary neural arches,
which, however, do not meet above the spinal cord (cf. Fig. 34, B),
and of which there are two pairs to each muscular segment
Tc
FIG. 35. — PORTION OF THE VERTEISRAL COLUMN OF Polyodon. Side view.
FIG. 36. — TRANSVERSE SECTION OF THE VERTEBRAL COLUMN OF Acipenser
ruthenus (in the anterior part of the body).
Ao, aorta; C, notochord ; Ee, primary, and Cs, secondary sheath of the noto-
chord ; EL, longitudinal elastic band ; Fo, median ingrowths of the lo wet-
arches enclosing the aorta ventrally ; Tc, intercalary pieces (inter-dorsal and
inter-ventral); M, spinal cord; Ob, upper arch (basi-dorsal) ; P, pia
mater ; P,s, neural spine ; SS, skeletogenous layer ; Ub, lower arch (basi-
ventral ; Z, "basal stiimps" of the lower arches.
FIG. 37.— PORTION OF THE VERTEBRAL COLUMN OF Protopterus. Side view.
C, notochord ; DF, neural spine ; FS, fin-ray ; FT, interspinous bone.
or myotome (cf. Elasmobranchs). In the tail haemal arches, enclos-
ing the caudal aorta and vein, are also present, and fusion of the
cartilaginous elements occurs.
Fishes. — To the condition found in Cyclostomes, that seen in
the Cartilaginous Ganoids, Holocepliali, and Dipnoi, is directly
comparable : the notochord is persistent, no centra being formed,
as was also the case in the most primitive Palaeozoic Elasmo-
branchs, and thus the metameric character of the skeletal axis
is only seen in the arches (Figs. 35, 36, and 37). In the
Holocephali and Dipnoi, however, the thick secondary sheath
encloses cartilage cells amongst its fibres. In Chimasra narrow
VERTEBRAL COLUMN 49
calcified rings arc also developed in the sheath : these are con-
siderably more numerous than the arches. The latter remain
cartilaginous in the Cartilaginous Ganoids and Holocephali, but
become densely ossified in the Dipnoi (Fig. 37). The upper arches
may be completed above by neural spines. In the caudal region
the hcemal arches usually enclose the caudal aorta and vein com-
pletely ; further forwards the cartilages do not meet in the middle
line below, thus only surrounding the coelome to a slight extent, and
consequently the lower arches end on either side in a laterally-
directed cartilaginous projection, the transverse process or " basal
stump."
The relations of the arches in Plagiostomes, Bony Ganoids, and
Tcleosts are similar to those described above, but their structure
is more complicated in all Fishes than is there indicated.
The upper arches, which in many Fishes are not closed in
dorsal ly,1 consist on either side of several distinct elements, which
06 Je
asg^spsppigsiEg^^^'ltSJSIS^'"1
. ,
Jr ° °
FK;. 38. — PORTION OF THE VERTEBRAL COLUMN OF Scymnm.
Ic, intercalary pieces (interdorsals) ; Ob, neural arches (basidorsals) ; WK,
centra. The apertures for the roots of spinal nerves are shown.
are most plainly distinguishable in Cartilaginous Fishes. One of
these (directly above the centrum where such is developed) may
be described as the basidorsal or neural plate, and is usually
perforated by the foramen for the motor root of a spinal nerve.
Intercalated between successive basidorsals is another cartilage,
the interdorsal (intercalary piece, or interneural plate}, through
which the sensory root of a spinal nerve, situated anteriorly to the
corresponding dorsal root, usually passes : both these cartilages
may meet above, so as to complete the arch (Fig. 38). In some
cases, more than one intercalary piece is present on either side,
and frequently another series of cartilages becomes segmented off
from the basidorsals and interdorsals respectively to form the
keystone of the arch : these give rise in the median line to more
or less marked neural spines. The bases of this series of upper
intercalary pieces or supradorsals fit in alternately between the
1 A longitudinal elastic ligament is constantly present in this region (Figs.
36 and 42) and also in relation with the ventral arches.
E
50
COMPARATIVE ANATOMY
basidorsals and interdorsals, and thus may be twice as numerous
as the centra (Fig. 3D).1 The lower arches consist of basivcntral
cartilages, between which are sometimes intercalated a series of
interventrals (Fig. 35), and which, in the tail, are produced into
JicKmal spines: these may be formed of distinct infra/central
elements.
In Dipnoans the interdorsals and interventrals are fused or
wanting, and in Bony Ganoids and Teleosts the various elements
usually become united in the fully-formed vertebra. Distinct fused
/crfcr 'i.n.p np^^
JL_J^
A/jfe
M '.''•"
'"••:•/
• ••?"•• \
n.a
n.a,
nlc
n.a
n.a
h.a
Wiim-h-Bp
FIG. 39. — PORTIONS OF THE VERTEBRAL COLUMN OF Scyllhun
(From Parker's Practical Zooloyy.)
A and B, from the trunk ; C and /), from the middle of the tail ; A and C, two
vertebrae in longitudinal section ; B and D, single vertebrae viewed from one
end ; b, calcined portion of centrum ; c, centrum ; for, foramen for dorsal,
and for', for ventral root of spinal nerve; h.a, haemal arch (basi-ventral) ;
Ji.c, ha-mal canal; h.*p, haemal spine; i.it.p, intercalary piece (interdorsal,
or interneural plate) ; n.a, neural arch ; n.c, neural canal; n.}), neural plate
(basi- dorsal) ; n.*p, neural spine ; nt<-, intervertebral substance (remains of
notochord) ; r, proximal portion of rib ; tr.pr, transverse process (basal
stump).
pairs of supradorsals, however, persist and remain unossified in
Lepidosteus (Fig. 42), and in the caudal region of Amia the basi-
dorsals and basiventrals remain separate from the interdorsals and
interventrals, thus giving rise to double vertebral bodies (pre-
centra, bearing the arches, and archless postcentra}. A somewhat
similar condition is seen in the Jurassic Eurycormus and other
fossil Ganoids. From what has been said above, it will be seen
that the number of arch elements does not necessarily correspond
1 It was mentioned on p. 48 that in the Lamprey there are two pairs of
arcualia to each myotome : it is possible that they correspond to alternating
basidorsals and interdorsals.
VERTEBRAL COLUMN
with that of the centra, or the number of the latter with that of
the myotomes.
Articular processes (zygapophyses) are usually present on the
neural arches of Bony Fishes.
In Plagiostomes, the cartilage which has invaded the sheath of
the notochord is segmented into definite vertebral centra, which
become partially calcified in various ways. The calcification (Fig.
40) may in each centrum take the form of a double cone, con-
stricted in the middle, as in Scymnus and Acanthias (cyclo-
spondylic form) ; concentric layers may be added to this, as in the
Rays (tectospondylic} ; or longitudinal plates may be formed radiating
outwards from the double cone, as in Scyllium (aster ospondylic).
The dorsal and ventral arches usually extend round the centrum
• •ex m •
ha
na -
ha-
ha-
A B C
FIG. 40. —DIAGRAMMATIC TRANSVERSE SECTION THROUGH THE MIDDLE OF A
CvCLOSPONDYLIC (A), A TKCTOSPONDYLIC (B), AND AN ASTEROSPONDYLIC
VERTEBRA (C). (From Zittel, .after Hasse. )
<1, middle portion of the calcined double cone ; d\ additional concentric calcified
layers ; d", double cone with radia-ting calcified layers ; ex.m, external elastic
membrane; h.a, haemal arch ; n.a, neural arch ; n.c, notochordal cavity.
so as to enclose it, and in the tail there may be two or more sets of
vertebral elements to each body segment.1
In Bony Ganoids and Telcosts, there is a tendency towards a
reduction of the cartilage ; that which forms the centra is entirely
outside the notochordal sheaths, and the vertebras become more or
less densely ossified.
In the course of development of the centra in all cases, the
notochord becomes constricted by the growth of the cartilage at
regular intervals while the latter undergoes segmentation into
centra. Each point of constriction corresponds to the middle of a
centrum, i.e., it is intravertebral in position, and the notochord
may here disappear entirely ; intervertebrally it remains expanded
1 In Rays and Chima?roids the anterior vertebral elements become fused
into a single mass, on which a definite condyle is formed for articulation with the
skull ; amongst Sharks and in Dipnoans also, a concrescence of the anterior vertebral
elements with one another and with the skull may occur.
E 2
52
COMPARATIVE ANATOMY
XnKn1 Li
and so persists as a kind of connecting or packing substance
between contiguous centra, which are consequently of a deeply
biconcave or amphicalous form (Fig. 41).
One of the Bony Ganoids, Lepidosteus, forms a marked excep-
tion to other Fishes as regards its
vertebral column, inasmuch as de-
finite articulations are formed be-
tween the centra (Fig. 42). A con-
cavity is formed at the hinder end of
each centrum which articulates with
a convexity on the vertebra next be-
hind (opisthoccelous form). The noto-
chord (except in the caudal region)
entirely disappears in the adult; in
the larva it is seen to be expanded
intravertebrally and constricted in-
FIG. 41. -PORTION OF THE VERTE- tervertebrallv, a condition of things
BRAL COLUMN OF A YOUNG ... . »
DOGFISH (Scyllwm caiw-iila). whlch appears again in the higher
(After Cartier.) types, as, for instance, in Reptiles.
C, notochord; FK, the fibro-car- In a sti11 earlier larval stage, how-
tilaginous mass lying between ever, the constrictions are intra-
the cartilaginous zones which vertebral, as in other Fishes,
is undergoing calcification ; FTM i i ^ r .1.1 i
Kn, oute?, and Kn', inner, ^he skeleton of the posterior end
zone of cartilage ; Li, inverte- of the tail in Fishes requires special
bral ligament. notice, and the condition in Amphi-
oxus, Cyclostomes, and Dipnoans,
may be taken as a starting-point. In these, the notochord extends
straight backwards to the hinder end of the body and is sur-
f.S.
i.e.
A
-li.tt.
Fia. 42. — PORTION OF THE VERTEBRAL COLUMN OF Lepidoxtvus. (After Balfour
and Parker.)
A, vertebra from anterior surface ; B, two vertebra from the side, en, anterior
convex face, and en', posterior concave face of centrum ; h.n, transverse
process; i.c, intercalary cartilages (fused supra-interdorsals) ; i.x, inter-
spinous bone ; /, /, longitudinal ligament ; n.a, upper arch (basi -dorsal).
rounded quite symmetrically by the tail-fin, and the tail is
therefore spoken of as diphyccreal : this condition is also met
VERTEBRAL COLUMN
53
with in certain Palaeozoic Fishes. In most other Fishes the
ventral part of the tail-fin with its supporting skeleton, as a
result of unequal growth, is more strongly developed than the dorsal
part, and the vertebral column becomes bent up dorsally, giving
rise to a Itcterocercal tail. This form of tail may be recognised
externally in most Elasmobranchs, Ganoids, and numerous fossil
Fishes, or may be masked by a more or less symmetrical tail-tin,
as in Lepidosteus, Arriia, and more particularly in most Teleosts,
FIG. 43A. — TAIL OF Lvpidostvm.
n
Fir;. 43fi. — CAUDAL END OF VERTEBRAL COLUMN OF SALMON. (From Boas's
Zoology. )
h, centrum ; h', urostyle ; n, haemal arch ; n', hypural bone ; o", neural arch ;
t, neural spine.
in which the heterocercal character is only visible internally,
and the tail is described as homocercal (cf. Figs. 43, A and !>}.
The posterior end of the vertebral column is then frequently
represented by a rod-like urostyJc, and in Teleosts one or more
wedge-shaped hypnral bones (enlarged hannal arches) generally
occur directly -beneath it.1
1 The diphycercal character of the tail in Dipnoi and curtain Teleostuini is
probably not primitive (protocercal), but has been acquired secondarily.
54 COMPARATIVE ANATOMY
As a rule Elasmobranchs and Ganoids possess a greater number
of vertebras (up to nearly 400) than Teleosts, in which we seldom
meet with more than 70 : the Eel, however, possesses more than
200, while amongst the Plectognathi there may be as few as 15.
The tendency towards a fusion of the various components such
as occurs in the ossified vertebra? of Bony Ganoids and Teleosts is
also seen in the Amphibia and Amniota, the notochord being of
less importance and the vertebra? becoming more consolidated and
secondarily modified in various directions. Thus the homology of
the different elements of which they are composed can only be
traced by a study of their development ; but even in the adult,
parts are frequently present which recall the primarily composite
nature of the vertebrae, as will be seen in the following pages.
Amphibians. — Amongst Amphibians, the vertebral column is
more or less distinctly differentiated into cervical, thoraco-lumbar,
sacral, and caudal regions, and these regions can be recognised,
except in certain modified types, in all the higher Vertebrates. On
account of the absence of extremities in Ca?cilians, there is no sacral
region, and in Anura, the caudal portion is modified to form a
urostyle (Fig. 45).
The notochord of Urodele larvae, like that of most Fishes,
undergoes intravertebral constrictions, while intervertebrally it
remains thicker, and accordingly appears expanded. Thus the
centra here also are ampliiccelous. In the course of their develop-
ment, a gradual reduction of the cartilage may be observed, and
the bone, originally perichondral in origin, becomes correspond-
ingly independent (Fig. 44). The cartilage is more and more
limited on the one hand to the arches (Fig. 52), and on the other
to the intervertebral regions round the notochord, extending to a
greater or less degree into the anterior and posterior ends of the
individual bony vertebral bodies, thus constricting or even entirely
obliterating the notochord in these regions. The bony centra are
formed from the bases of the arches, which, before ossification,
only rest on the notochord and do not enclose it. Finally a
differentiation, as well as a resorption, extending inwards from the
periphery, occurs in these cartilaginous parts : in the interior of
each an articular cavity is formed, so that in the vertebra? of many
higher Urodeles an anterior convexity and a posterior concavity
may be distinguished, both covered with cartilage ; they are,
therefore, opisthocoelous (Fig. 44).
In the development of the vertebral column of Urodeles we
can thus distinguish three stages: — (1) A connection of the indi-
vidual vertebra? by means of the intervertebrally expanded
notochord ; (2) a connection by means of intervertebral masses
of cart i Inge; and finally (3) an articular connection. These
three different stages of development find a complete parallel
in the phylogeny of tailed Amphibians, inasmuch as many of
VERTEBRAL COLUMN
55
the Stegocephali of the Carboniferous period, as well as the
Perennibranchiata, Derotremata, and many Myctodera, possess
simple biconcave bony centra without differentiation of definite
articulations.1
Thus the bony parts of the vertebrae of Urodeles are not
formed from the cartilage .surrounding the notochord, but in
-Liyf
11
FKI. 44. — LONCUTUDINAL SECTION THROUGH THK VERTEBRAL CENTRA of VARIOUS
URODELES. A, Ra/nodon xiltericns ; B, Amblystoma tigrinum ; C, GyrinophUus
porphyriticus (1, II, III, the three anterior vertebrae) ; L), Salamandrina
perspicillata.
Ch, notochord ; CK, intravertebral cartilage and fat-cells ; Gp, concave posterior
face, and Gk, convex anterior face of centrum with articular socket and head ;
Jvk, invertebral cartilage ; K, superficial bone of centrum ; Liyt, intervertebral
ligament ; Mh, marrow cavity ; If, transverse process ; S, intravertebral
constriction of notochord in Amblystoma, without cartilage and fat-cells ;
**, intervertebral cartilage.
connective tissue, there being only an intervertebral cartilaginous
zone, extending into the ends of the centra. In the Anura, on the
1 In certain of the Stegocephali incomplete hoops of bone, the intercentra,
and pleurocentra, twice as numerous as the arches, surrounded the persistent
notochord (cf. the caudal region in Amia and Elasmobranchs, p. 50),
56
COMPARATIVE ANATOMY
other hand, as in Elasmobranchs, Teleosts, Bony Ganoids, and the
higher Vertebrates, the vertebrae are preformed in cartilage, and
true articulations are always formed between them : as a rule, but
by no means always, the convexity is posterior and the concavity
anterior (procodous form). A further difference is seen in the
relations of the notochord, which persists
intravertebrally longer than interverte-
brally, in this respect resembling Lepi-
dosteus and Reptiles.
The configuration of the caudal region
of the vertebral column must also be
remarked upon, as it differs in tailed and
tailless Amphibians. The long caudal
portion of the vertebral column in Anuran
larvae, which is very similar to that of
Urodeles, undergoes during metamorphosis
a gradual retrogressive modification, and
the vertebras of its proximal end become
fused and co-ossified to form a long, un-
segmented, dagger-like bone, the urostvle
(Fig. 45). '
Haemal arches are present in the
caudal region of Urodeles only. The
neural spines, as well as the transverse
processes, which in Urodeles are as a rule
bifurcated at the base and are present
from the second vertebra onwards, show
the greatest variety as regards shape and
size, differing in the several regions of the
FIG. 45. - VERTEBRAL body. The transverse processes of the
COLUMN OF Discoylossu* single sacral vertebra, which give attach-
pictus- ment to the pelvis, are particularly
Ob, upper arch of first ver- strongly developed, especially in the An-
tebra; Pa, articular ura (Fig. 45), in which the number of
s
neural spine ; Pt, Articular processes (zygapophyses) are
transverse processes of well developed in all Vertebrates from
trunk vertebra; Ptc, Uroc[eles onwards, and consist of two
t ran verse processes of ...
caudal vertebra (uro- pairs of projections arising from the an-
style, Oc) ; R, ribs ; Sy, terior and posterior edges respectively of
condylar facets of first th { h Thir surfaces are
vertebra ; »S W , sacral
vertebra. covered with cartilage and overlap one
another from vertebra to vertebra, and
in some Urodeles the neural spines also articulate with one
another: thus a well-articulated and mobile, chain-like vertebral
column results.
The first, or ci'rcic.td vertebra becomes differentiated from the
others, and consists of a comparatively simple ring which articulat es
VERTEBRAL COLUMN 57
by means of lateral facets with the two condyles of the skull, and
also, in Urodeles, with the base of the latter by means of a projec-
tion, of varying size and form, the so-called " odontoid " process ;
thus a freer movement between the skull and vertebral column is
rendered possible. This vertebra, however, is not homologous with
the first vertebra (i.e. the atlas) of the higher Vertebrates, as is
demonstrated by a study of its development, which shows that the
real atlas, with the exception of the part which forms the " odon-
toid," loses its individuality as a separate mass, and becomes
united with the occipital region of the skull.
The number of vertebras present in Urodeles is inconstant, and
varies greatly : it may reach to nearly 100 (Siren), and in Cascilians
may be very much greater (up to 275).
Reptiles. — In many fossil Reptiles (Theromorpha, Ichthyo-
sauria, &c.) the centra were biconcave, and this primitive form,
with an intervertebrally expanded notochord, is retained in the
Ascalabota amongst existing forms : the Rhynchocephali also
possess amphicoelous vertebrae, but intercentral fibro-cartilagin-
ous discs occur in their existing representative, Hatteria. A
primitive character of the Rhynchocephalian vertebral column is
seen in the retention throughout of the primary components of the
centra as distinct elements, wedge-shaped intercentra being inter-
calated between the centra proper or pleurocentra : in the majority
of Lacertilia intercentra also occur, but are usually only recognis-
able in the neck and tail ; in Chelonians a few intercentra are
present in the neck region. A pair of elements interposed between
the upper part of the first vertebra (atlas) and the skull in
Crocodiles, usually known as the " pro-atlas " (Fig. 46), which is re-
presented also in Hatteria, Chameleons, and many fossil forms, corre-
sponds to a disconnected pair of " supra-dorsal " elements (p. 49).
In the majority of Reptiles, the notochord remains expanded
longer in the intravertebral regions than intervertebrally, but in
the adult it becomes entirely aborted and replaced by bony tissue.
This stronger and more solid ossification of the whole skeleton
forms a characteristic difference between the Ichthyopsida on the
one hand and the Amniota on the other. As a rule the centra
of Reptiles are of the procoelous type and become definitely articu-
lated with one another : the forms with intervertebral remains of
the notochord and those with fibro-cartilaginous intervertebral discs,
(e.g. Crocodiles) form an exception to this rule. In Crocodiles the
vertebras are mostly procoelous, an exception being seen in the two
sacrals and first caudal. In Chelonians there is great variation in
the form of the individual centra of the cervical vertebras — even in
the same individual procoelous, opisthocoelous, biconcave, and even
biconvex centra, with intervertebral discs, may occur ; while the
thoracic and lumbar vertebra- have flattened faces, and are firmly
united with one another by cartilage, and also with the
(p. 43).
58
COMPARATIVE ANATOMY
What has been said as to the classification of the vertebrae into
different regions in Urodeles, as well as to the presence of the
various processes, usually applies here also to a still greater extent.
Except in limbless form, there are always several cervical vertebra}
instead of a single one, and also typically at least two sacral
vertebrae. The two first cervical vertebra become differentiated
to form an atlas — bearing a single occipital facet and usually
formed of three pieces, and an axis — with an odontoid bone
belonging morphologically to the centrum of the atlas (cf. p. 57).
The neural spines vary in size, and transverse processes arise
from the centra themselves or close to them. Lower arches, or
chevron bones, corresponding to the intercentra, are present in the
tail in Lizards, Crocodiles, and some Chelonians : and besides
Po
FIG. 46. — ANTERIOR PORTION OF THE VERTEBRAL COLUMN OF A YOUNG
CROCODILE.
atlas; Ep, axis, articulating with the atlas at h ; /«, intervertebral disc; o,
" pro-atlas " ; Ob, neural arch ; Po, odontoid bone ; />*•, neural spine ; Pt,
transverse process, arising from the base of the arch and articulating with
the rib at t ; Rl, ^2, ^, ribs ; u, ventral element, and s, arch of atlas ; WK,
centrum.
these, median inferior processes of the centra themselves are seen
in many of the vertebras of Lizards, Crocodiles, and Snakes :
in the last mentioned paired processes partly enclose the caudal
vessels. The arches in Snakes, Lizards, and usually in Chekmians,
become united with the centra by synostosis, while in Crocodiles
they remain, at any rate for a long time, separated from them by
sutures (Fig. 46).
In Snakes, Hatteria, and some Lizards (Iguana) extra articular
processes (zygosphenes and zygantrob) are developed on the neural
arches ; and in the caudal region of Hatteria and Lizards an
unossified septum remains in the middle of each centrum (which
really corresponds to two primary vertebral elements), so that the
tail easily breaks off' at these points. When this happens the tail
grows again, but true vertebras are not formed.
VERTEBRAL COLUMN
59
The greatest number of vertebrae is seen in Snakes, in which
there may be over 400.
Birds. — The vertebral column of Birds has many points of
resemblance with that of Reptiles both phylogenetically and onto-
genetically. In both groups the notochord usually eventually
disappears entirely, and the whole skeleton becomes strongly
ossified. Archffiopteryx, as well as the Cretaceous Ichthyornis,
possessed biconcave vertebrae, but in existing adult Birds this
character never occurs except in the free caudal vertebrae.
Cervical, thoracic, lumbar, sacral, and caudal regions can be distin-
guished. The arches always become united into a single mass
with the corresponding centra, not remaining separated from them
by sutures, as is the case in certain Reptiles : even the ligament
which keeps the odontoid process of the axis in its place may
FIG. 47, A.— ATLAS AND Axis (from the left side), and B, THIRD CERVICAL
VERTEBRA (ANTERIOR FACE) OF WOODPECKER (Picux viridis).
A, Ob, A, arch and centrum of atlas ; Po, odontoid process ; Pa, neural -spine of
axis ; Pt, transverse process ; WK, centrum of axis, and Su, its saddle-
shaped articular surface for the third vertebra ; t, condylar facet.
B, Ft, vertebrarterial foramen ; Ob, neural arch ; Pa, articular process ; Pt, Pi,
the two bars of the transverse process, shown on one side ankylosed with
the cervical rib (R) ; Pxi, median inferior process (hypapophysis).
become ossified. Fibro-cartilaginous discs or menisci, perforated
in the centre by a ligament, are present between the centra.
In the cervical region, which is extremely flexible and often
very long, the centra are in nearly all cases connected by means of
saddle-shaped (heteroccelous) synovial articulations ; the upper part
of each bifurcated transverse process arises from the arch, the
lower from the centrum, and these may unite with the correspond-
ing forked rib, the vertebral artery and vein extending through
the foramen thus formed (Fig. 47). The ring-like atlas, with its
single facet for the occipital condyle, is relatively small, and the
odontoid is fused with the axis. In the thoracic and lumbar
regions more or fewer of the vertebrae usually become immovably
united together.
The sacral region in Bird-embryos, like that in existing adult
Reptiles, consists of two vertebrae only, the transverse processes of
60
COMPARATIVE ANATOMY
which ossify separately and correspond to fused ribs, as in
Amphibia and Amniota. During further development, however,
a number of other (secondary sacral) vertebra? (thoracic, lumbar,
and caudal), with their rudimentary ribs, become fused with the
too primary ones (Fig. 48), so that the entire number of vertebra?
in the sacrum may be as many as twenty-three. In Archa?opteryx
the sacrum was much shorter than in
existing Birds, and fewer vertebra? were
united with it.
In existing Birds the actual caudal
region always exhibits a more or less
rudimentary character, and in its posterior
portion the vertebra? usually fuse together
to form a flattened bone, the pyc/osty/e,
which supports the tail quills (Fig. 132).
In the Batita? there is never more than
an insignificant pygostyle (Struthio), and
all the caudal vertebra? may remain dis-
tinct. That the latter is the more original
condition in Birds is shown by a study
of their development as well as by the
condition of the tail in Archa?opteryx, in
which it was supported by numerous
elongated free vertebra? (Fig. 49). It
must, however, be borne in mind that
the pygostyle may be made up of from
six to ten fused vertebra?, and in the
sacrum even a greater number may be
included, so that as many as twenty or
more caudal vertebra? may be represented.
Mammals. — The notochord here per-
sists longer intervertebrally than intra-
vertebrally, but it disappears entirely by
the time the adult condition is reached.
A jelly-like pulpy mass, the nucleus pul-
posus, persists, however, throughout life
in the centre of the fibre-cartilaginous
menisci which are developed between the centra. The whole
vertebral column is preformed in cartilage, and the arches
develop in continuity with the centra but become ossified from
separate centres, as do also the various processes ; these separate
ossifications are no longer recognisable in the adult. The
presence of bony discs or epipliyscs on the flattened ends of the
centra which unite with the latter comparatively late, is very
characteristic of Mammals; they are, however, absent or only
imperfectly developed in Monotremes and in existing Sirenians.
True articulations between the centra ;iiv usually only formed
• Hi the atlas and anterior face of the axis: well-developed 'articular
FIG. 48. — PELVIS OF OWL
(Strix bubo). Ventral
view.
It, ilium ; /.y, ischium ; P,
pubis ; H, last two pairs
of ribs ; W, position of
the primary sacral verte-
br;f : between ft and //,
and behind W, are seen
the secondary sacral ver-
tebra^, fused with the pri-
mary (IF); f foramen
between ilium and ubis.
VERTEBRAL COLUMN
61
processes (zygapophyses) are present on the neural arches.1 The
cervical region is usually the most movable, and the rcnlm
may hen; possess articulations and have a,n opisthoccelous form
FIG. 49. — Archtcopteryx lithorjraj)hica. From the Solenhofen slates (Jurassic).
After Dames, from the specimen in the Berlin Museum.
c, carpus; d, clavicle; co, coracoid; h, humerus; r, radius; .sr, scapula; u, ulna'
/ — ///, digits of manus; / — IV, digits of pes.
(Ungulata). In some cases, on the other hand, the cervical
vertebra may become firmly fused with one another (e.g. Dasypus,
Talpa, Cetacea).
1 In certain Edentata (e.gr. Myrmecophaga, Dasypus) extra articular processes
are present besides the ordinary zygapophyses on the posterior thoracic and
lumbar vertebra-.
62 COMPARATIVE ANATOMY
The atlas1 and axis essentially resemble those of Birds,
except that the condylar facet on the former is paired ; in
many Marsupials the ventral part of the axis may consist merely
of a fibrous band. The differentiation of the vertebral column
into regions characterised by difference of form is much more
sharply marked than in any other Vertebrates. There are as
a rule seven cervical vertebrae ; amongst the Edentata, however,
Bradypus possesses eight to nine, and Tamandua bivittata, eight,
while in Choloepus (and also in the Manatee) there are only six.
The transverse processes are simple except in the cervical
region and arise from the base of the arch. In the neck, they are
united with the vestiges of the cervical ribs, and in nearly all cases
enclose a vertebrarterial canal, as in Birds (p. 59): in Monotremes
these rib- vestiges remain distinct at any rate for a long time. In
the thoracic region the transverse processes are tipped with
cartilage on the ventral side of their distal ends for articula-
tion with the tubercle of the rib (q.v.). In the lumbar and
sacral regions they arise from the centra, and contain fused rib-
elements.
The number of thoraco-lumbar vertebrae varies greatly in
different Mammals ; there may be as few as fourteen (Armadillo)
or as many as thirty (Hyrax). In Ungulates the number is con-
stantly nineteen. In the lumbar vertebra? the transverse processes
are especially long, and other processes (anapophyses, metapophyses,
hypapophyses) may be characteristically present in this region.
Thus, as in Amphibians, Reptiles and Birds, the pelvis is con-
nected with the sacrum by means of vestigial ribs. As in the two
last-mentioned groups, there are not more than two primary sacral
vertebra?, but except in Ornithorhynchus and most Marsupials a
few caudal vertebrae become later included in the sacrum and are
usually more or less closely united with it by synostosis. The
various processes of the sacral vertebrae are more or less reduced.
In Anthropoids, and still more markedly in Man, the first sacral
vertebra is plainly marked off from the last lumbar by the forma-
tion of the so-called promontory. A sacrum is wanting in the
Cetacea and Sirenia, in correspondence with the absence of hind-
limbs : the horizontal tail-fin in these forms is not supported by
hard parts.
The caudal vertebra? vary extremely in their development, and
excepting in most long-tailed Mammals — such as Kangaroos,
Sirenians, Cetaceans, and certain Apes — no longer develop lower
arches. When present these " chevron bones " are intervertebral
in position.2 The greatest number of caudal vertebra? is found in
1 A nodule of bone in the atlanto-occipital ligament of the Hedgehog may
represent the vestige of a "pro-atlas" (p. 57).
' The question as to homology of the chevron bones, as well as of certain
bony elements present in some Mammals beneath the intervertebral discs in the
tail (e.g. Dasypus, Erinaceus) and lumbar region (e.q. Talpa), requires further
investigation : it is doubtful to what extent they represent the lower arches or the
intercontra of other Vertebrates, or are structures peculiar to Mammals.
RIBS 63
Manis macrura (about fifty), and the caudal region is most reduced
in the higher Primates, in which it forms a stump-like coccyx
consisting of at most five to six vestigial vertebrae, all fused
together, and these may even (e.g. in Man) unite with the sacrum.
In the human embryo of 4-6 mm. in length, a distinct tail
is present, consisting of all the characteristic parts ; it gradually
undergoes reduction, and what is left no longer projects
externally.
II. RIBS.
Some doubt still exists as to whether the ribs are to be
considered as primitively independent skeletal structures, arising
in the intermuscular septa or myocommas, or as parts of certain
processes of the vertebrae which have become segmented off from
the latter, as is plainly seen to be the case, for example, in embryos
of Hatteria. Their relations to the axial skeleton, whether
primary or secondary, are of the very closest kind.
The ribs are situated in the septa between the great lateral
muscles of the body, and present much variation in the various
vertebrate Classes: they may be short and stump-like and almost
horizontal in position, or may grow ventralwards as delicate rod-
like structures, so as to encircle the body-cavity more or less
completely. Primitively, ribs may be present all along the
vertebral column, but, especially in the higher types, they become
reduced in certain regions.
A careful study of the ribs, in which their relations to the soft
parts (muscles) is taken into consideration, shows that they are
not completely homologous throughout the vertebrate series,
and that those of most Fishes are not exactly morphologically
comparable to those of Elasmobranchii, Amphibia, and Amniota.
Fishes. — Two kinds of ribs, situated at different levels, may
be distinguished amongst Fishes — dorsal ribs and ventral ribs (or
plcural arches} : the former extend into the transverse septa which
separate the epaxial or dorso-lateral from the hypaxial or ventro-
lateral muscles, while the latter are situated internally to the
muscles, just outside the peritoneum, but never more than
partially encircle the ccelome (Fig. 50). Both kinds of ribs are
usually considered as corresponding to prolongations of the transverse
processses (basal stumps) of the vertebral axis, from which they
have become segmented off but with which they remain closely
connected : another view as to their primary origin has been
stated above. The ventral ribs appear to be phylogenetically older
structures than the dorsal ribs, which can only have originated
after the differentiation of the intermuscular septa in which they
are situated.
Towards the caudal region, the ventral ribs, together with the
corresponding transverse processes, gradually take on the form of
RIBS
65
haemal arches, which in Teleosts, as in Elasmobranchs, arc
developed from the transverse processes alone (Fig. 50, B).
The dorsal ribs take no part in the formation of the hsemal arches :
towards the posterior part of the trunk they become rudimentary,
but may sometimes still be recognised in the tail as lateral
processes at the bases of the h.Temal arches.
In most Ganoids and in Dipnoans (Fig. 50, A, c) ventral ribs only
are present. In Crossopterygians (Polypterus, Figs. 50, E, anil
51) larger dorsal and smaller ventral ribs occur, so that there are
-JV
FIG. 51. — ANTERIOR END OF THE VERTEBRAL COLUMN OF POLYPTEIIUS. From
the ventral side.
Ps, parasphenoid ; WK, centra ; / — I7", first five pairs of dorsal ribs ; ft, ventral
ribs.
two pairs of ribs to each body-segment. Dorsal ribs can also be
recognised in certain Tclcosts (Salmonidse, Clupeoidei) in addition
to ventral ribs, and like these, are always preformed in cartilage.1
In many forms, the ventral ribs may undergo reduction, and in
Elasmdbrancks they are wanting, while dorsal ribs are usually
present.2 In Chima3roids and many Rays, as is also the case in
Cyclostomes, a fibrous band extends outwards from the vertebral
1 This fact alone is sufficient to distinguish them from the intermuscular
bones often present in this region in Teleosts. In addition to these epicetitni/
intermuscular bones, others — the epineurals and epipleurals — are situated moi'o
dorsally and more ventrally respectively, and all of them are merely ossifications
in the septa.
! The hfemal arches of these Fishes, as well as of Ganoids, Dipnoans, and
Amphibians, apparently contain components corresponding to ventrals rib.
F
66
COMPARATIVE ANATOMY
axis in the position usually occupied by dorsal ribs : thus these
forms are ribless, and also in certain Teleosts and Ganoids the ribs
are wanting (e.g. Lophobranchii) or quite vestigial (Polyodon).
Amphibians. — The ribs in the Amphibia correspond to the
dorsal ribs of Fishes, and are alwaj's connected with transverse
processes or at any rate with the vestiges of the basal stumps
(Fig. 50, F). The latter arc originally situated, as in Fishes,
towards the ventral side of the vertebral axis, nnd in the tail give
B
FIG. 52.— A, VERTEBRA FROM ANTERIOR PART OF TAIL OF LARVA (43 MM.)
OF Nc<-tiiru$\ B, SACRAL VERTEBRA FROM LARVA (43 MM.) OF Nedum*;
C, FOURTH TRUNK-VERTEBRA FROM NEWLY-BORN LARVA OF Salanwiuli-n
maculosa. (After Goppert. )
Art. n/i., vertebral artery ; B, cartilage of basal stump ; B1. vestige of same in
larva of Salamander; B'*, bony bar which replaces the same functionally ;
Ch, notochord ; DRX, dorsal bar of ril> : .//. ilium ; N, neural arch ; /.'. rib ;
RT, A'7'1, ventral and dorsal rib-bearing portions of vertebra ; .s'/-1, lateral
process of hamial arch (//).
rise to haemal arches (Necturus, Salamander-larvae). In connection,
apparently, with the more dorsal position of the horizontal inter-
muscular septum in which they are situated, the transverse
processes, even in Salamanders, tend to arise more from the neural
arches than from the centra, and this upward displacement is carried
still further in the Gymnophiona and Anura. In TJrocleles (Fig.
52) the cartilaginous, rib-bearing basal stump is in close connection
with the centrum, but gives off secondarily an upwardly directed
RIBS 67
process which becomes connected with the neural arch and on
further development may serve as the chief point of attachment
for the rib. The proximal part of the primitive basal stump is
correspondingly reduced, and, with rare exceptions, is no longer
developed : in its' place is formed a bony bar, arising from the
centrum, and generally not preformed in cartilage.
The ribs of the Urodela and Gymnophiona are bifurcated
at their proximal ends, the ventral bar corresponding to the
primary rib-rudiment, while the dorsal bar is a secondary
structure formed in order to give the rib a firmer connection with
the vertebra : in Urodeles it becomes connected with the rib-
bearing portion of the vertebra, and in the Gymnophiona with the
neural arch itself.1
The ribs of Amphibians are never very highly developed :
they are only slightly curved and do not encircle the body-cavity
to any extent. In Anurans they are not bifurcated and are
very short and stump-like (Fig. 45), usually becoming fused
with the transverse processes : they have doubtless undergone
reduction.
In many Urodeles the ribs are limited to the trunk, but
occasionally one or more pairs occur in the anterior part of the
tail, where the basal stumps have already extended ventralwards
to form the hasmal arches.
Finally, reference must be made to the cartilaginous " abdominal
ribs " (cf. p. 42) developed in the ventral intermuscular septa in
many Amphibians (Necturus, Menopoma, Bombinator).
Reptiles. — As already mentioned, the ribs of the Amniota are
comparable to those of the Amphibia, but they grow further
ventralwards and so encircle the body-cavity to a greater or less
extent. Ribs may also be present in the tail : in Hatteria, for
instance, there are seven or more pairs of caudal ribs.
The dorsal (proximal) section of the rib may also become
segmented from the distal (ventral) portion,2 and the former is
plainly homologous with the Urodele-rib. As a rule a certain
number of the ribs unite together ventrally to form a sternum
(cf. p. 72 and Fig. 56) : these are usually distinguished as " true "
ribs from the others, or " false " ribs.
The ribs of Snakes show the least amount of differentiation ;
for, without giving rise to a sternum, they extend along the
whole trunk from the third vertebra to the anal region, having
a similar form and size throughout. In Lizards, in which a dorsal,
unforked. bony portion and a ventral, cartilaginous portion can be
distinguished, three or four ribs reach the sternum, and are not
always completely segmented off from it. The proximal ends of
1 According to another view, the bifurcated amphibian rib is originally a
double structure, the dorsal bar of the fork originating independently and only
uniting secondarily with the ventral bar.
- An intermediate section also occurs in Crocodiles and man}' Lizards.
F 2
68
COMPARATIVE ANATOMY
the ribs of Hattcria are broadened out and articulate both with
the centra and arches, thus indicating a differentiation into a
capitulum and a tuberculum (cf. p 69.).
In Chelonians the cervical ribs unite with the vertebrae more
or less completely, and in the region of the trunk the ribs become
broadened out to form the costal plates of the carapace (p. 43).
FIG. 53. — SKELETON OF THE TRUNK OF A FALCON.
Ca, coracoid, which articulates with the sternum (St) at t ; Cr, keel of sternum ;
Fu(Cl), furcula (clavicles) ; G, glenoitl cavity for humerus ; S, scapula :
Ir, vertebral, and Sp, sternal, portion of rib ; Un, uncinate process.
Their proximal unbifurcated ends are attached between the centra,
at the junction of centrum and arch. There is no sternum.
The proximal ends of the cervical ribs in the Crocodilia are
bifurcated, in correspondence with the double transverse processes
in this region, and thus a vertebrarterial canal is formed (cf. p. 59).
Further back, the ribs increase in length, and become segmented
into two or three articulated portions. In passing from before
backwards, their point of origin becomes gradually shifted, so that
while the anterior thoracic ribs are attached to the centra, the
RIBS 69
posterior ones arise entirely from the transverse processes, which
increase in size correspondingly. Eight or nine ribs reach the
sternum, and from the eighteenth vertebra backwards the trans-
verse processes no longer bear ribs, but only short cartilaginous
apophyses.
Flat, curved cartilages, or uncinates, are present in connection
with the ribs in the Crocodilia as well as in Hatteria.
Birds. — The ribs of Birds exhibit a much more marked
segmentation into vertebral and sternal portions, both of which
become ossified, and this evidently stands in relation to their
more active respiration. Bony uncinates, comparable to those
mentioned above, arise from, and usually become ankylosed to,
the vertebral portions in nearly all Birds, and overlap the ribs
next behind them (Fig. 53). The whole costal apparatus is
usually rendered still firmer by the
fusion of many of the trunk vertebrae
(p. 60), by the individual ribs often
being very broad, as well as by the
form and arrangement of the sternum
and pectoral arch. The last three or
four cervical vertebra? may bear com-
paratively large and movable ribs.
The number of ribs which articulate
with the sternum varies between two
(Dinornis elephantopus) and nine
(Cygnus). The delicate ribs of Archse-
'. .„ FIG. 54.— COSTAL ARCH OF
opteryx (v ig.49; more nearly resembled MAN.
those of Lizards.
__ . m1 . , ., . (Ja, capitulum ; Co, neck, Cp,
Mammals.— 1 he Cervical ribs in bony vertebral, and Kn,
nearly all cases unite completely with cartilaginous sternal por-
the vertebras, and a vertebrarterial tion of "J ; p*> neural
, . , f , nil i spine ; ft, transverse
canal is thus rormed. 1 he last cervical process; St, sternum ; Tb,
rib may be well developed and may tuberculum; f-F/if, centrum
articulate with the corresponding of vertebra-
vertebra (e.g. Choloepus hofraanni).1 The seventh cervical rib
is also long in Bradypus, and the eighth and ninth ribs do
not reach the sternum ; they may therefore be counted as
cervical. There is considerable variation with regard to the
number of ribs which reach the sternum (e.g. in Manatus
2-3, in Cebus and Ateles 10): and in some cases the sternal,
as well as the vertebral ribs may become ossified. In the
vertebral portion a capitulum, a neck, and a tuberculum. may be
distinguished (Fig. 54). The capitulum usually articulates
with its own centrum as well as with that next in front,
in the region of the epiphysis ; the tuberculum articulates with
1 As amongst Reptiles, the ventral cartilaginous portions of some of the
anterior " false ribs " are connected with those in front, while the posterior ribs
end freely in the body-wall (" cost;e fluctuantes").
70 COMPARATIVE ANATOMY
the cartilaginous facet on the trans.verse process. In the " false "
ribs, these characters become to a greater or less extent lost in
passing from before backwards, so that the posterior ribs have a
more rudimentary character. As already mentioned (p. 62),
vestiges of ribs are present in the lumbar and sacral regions, and
unite with the corresponding transverse processes. There are
usually thirteen pairs of ribs, but their number may vary between
nine (Hyperoodon) and twenty-four (Choice pus). These facts
indicate that there has been a gradual phylogenetic reduction in
the number of ribs, and the occasional presence of supernumerary
ribs is to be explained as a reversion.1
III. STERNUM.
Never present in Fishes, the sternum appears for the first time
in Amphibians in the form of a small variously-shaped plate of
cartilage situated in the middle line of the chest (Fig. 55). It
arises as a paired cartilaginous plate 2 derived in the first in-
stance from chondrifications in an intermuscular septum on the
median border (linea alba) of the rectus abdominis muscle, and
therefore may be looked upon as comparable to a pair of
" abdominal ribs." Such cartilaginous structures must have been
present in greater numbers in the ancestors of existing Urodeles
(cf. p. 67). In many tailless Batrachians (£.#., Ranidas) the ventral
portion of the pectoral arch is continued forwards in the middle
line, from where the two clavicles meet, as a slender rod, the
omosternum (Fig. 55, D) : this has a similar origin, and the
proximal portion both of it and of the sternum becomes ossified.
Thus the sternum and omosternum of Amphibians are not to be
considered as corresponding to differentiations of the pectoral arch
(coraco-sternum), a view which is often held, but as consisting of
skeletal parts which primarily belong to the body-wall, and only
secondarily come into connection with the limb-skeleton.
In most Urodeles and certain Anurans (e.g. Pipa, Discoglossus,
Bombinator, Alytes), this cartilaginous sternal plate is inserted into
the grooved median margins of the two overlapping coracoids (Fig.
55, B, c). In Rana, on the other hand (D), in which the two
1 A primitive and a secondary type of thorax may be distinguished. The
former is the more usual, and occurs in most Mammals even up to the lower
Apes : it is characterised by an elongated form, and by the dorse-ventral
diameter being much greater than the transverse diameter. The latter occurs in
anthropoid Apes and Man, in which the dorso-ventral diameter has, both
ontogenetically and phylogenetically, become considerably reduced relatively :
the broad thorax is thus more cask-like in form, and may often even be flattened
dorso-ventrally. A somewhat similar modification is seen amongst insectivorous
Bats.
- It is unpaired from the first in Triton and Rana, but this is probably due
to an abbreviation of development.
Km. 55. — PECTORAL ARCH OF VARIOUS AMPHIBIANS. From the ventral side.
A — Urodele (diagrammatic) ; B — Axolotl (Amblystoma) ; C — Bomlnnnin,-
iy news ; U, J'nnn ewii/cnta.
C, coracoirl ; Cl, procoracoid ; C71 (Cl in U), clavicle ; EC, Co1, epicoracoid ;
Ep, omostermun ; Fe, fenestra between procoracoid and coracoid bars ; /\n,
cartilaginous xiphisternum ; t, Pf, O, glenoid cavity for the humerus ;
S, scapula ; SS, supraacapula ; St, Sfl, sternum. *, j (in B) indicate nerve-
apertures.
72
COMPARATIVE ANATOMY
halves of the pectoral arch are much more closely connected in the
middle line, by far the greater part of the sternum lies entirely
posterior to the coracoids, which do not overlap one another. In
the Perennibranchiata and Derotremata the sternum is much
simpler than in other Amphibians, and in Proteus and Amphiuma
it is entirely wanting.
In the Amniota, the form of the sternum, like that of the
pectoral arch, depends largely on the nature and function of the
forelimbs. It is usually considered as arising primarily by a
number of ribs running together ventrally so as to form a con-
tinuous cartilaginous longitudinal tract on either side. By the
more or less complete fusion of these two tracts, an unpaired sternal
FIG. 56. — PECTORAL ARCH AND STERNUM OF A GECKO (Hemidactyhis
n rriicoaus). From the ventral side.
o, b, c, membranous fenestne in the coracoid ; Co. coracoid ; Co1, cartilaginous
epicoracoid ; Cl, clavicle ; Ep, episternum ; G, glenoid cavity for the
humerus ; R, ribs ; 8, scapula ; Si, cartilaginous cornua to which the last
pair of ribs is attached ; SS, suprascapula ; St, sternum.
band or plate is formed, from which the ribs are secondarily
segmented off by the formation of articulations, and beneath which
a dermal episternum is present in some cases (p. 44). The main
part of the anterior end of the sternum of Mammals is formed by
the median union of the first two or three pairs of cervical ribs,
and beneath and in front of this region in Monotremes is a large
T-shaped bone, the prosternum (" episternum "), the lateral parts of
which come into relation with the clavicles (Fig. 103). l
The sternum may become calcified (Reptiles), or converted
into true bone (Birds, Mammals). In Reptiles,'2 Birds, and Mono-
1 Cf. note on p. 44.
- In (Snakes ami Chelonians there is no trace of a sternum.
STERNUM
73
tremes, the coracoids, as in Amphibians, come into direct connec-
tion with the lateral edges of the sternum (Figs. 53, 56, and 103),
and in other Mammals, the clavicles, when present, are connected
with it directly or indirectly.
The sternum is greatly developed in Birds, and consists of a
broad, more or less fenestrated plate, provided in the vast majority
of Carinata3 with a projecting keel, which forms an additional
surface for the origin of the wing-muscles (Fig. 53). In contrast
to these, the cursorial Ratitse are characterised by a broad, more
or less arched, shield-like sternum without a keel. In some
flightless Carinatse, however, the keel is rudimentary or even
absent, and the vestige of a keel may occur, though not constantly,
FIG. 57. — A, STERNUM OF Fox; B, OF WALKUS ; AND C, OF MAN.
From the ventral side.
C, body ; J/Z>, manubrium ; Pe, xiphoid process ; B, ribs.
in certain Ratitse. The presence or absence of a keel is not, there-
fore, a constant character separating these two groups of Birds
from one another.1
A far greater number of ribs are as a rule concerned in the
formation of the sternum of Mammals than is the case in Reptiles
and Birds. Consisting at first of a simple cartilaginous plate, it
later becomes segmented into definite bony portions (stcrnclircK)
the number of which may correspond to the affixed ribs (Fig. 57,
A, B) : in other cases as, for instance, amongst Primates (c), the
individual bony segments may run together to form a long plate
(corpus sterni}. Its proximal end forms a more or less distinct
manubrium, and the distal end a partly cartilaginous xiphoid or
ensiform
1 A keel was also present in the Pterosauria, and may be developed
wherever a larger surface fur the origin of the pectoral muscles is required (e.g.
Cheiroptera).
74 COMPARATIVE ANATOMY
IV. SKULL.
General Part.
The question as to the primary origin of the skull in the
Craniata has always taken a foremost place amongst the morpho-
logical problems relating to the structure of Vertebrates; and
the first point which requires elucidation relates to the nature of
the head — whether it is a structure sui generis, or whether its
parts are due to modifications and further developments of parts
present in the trunk.
Until past the middle of the present century the theory which
held the field was the " vertebral theory " of Goethe and Oken,
according to which the skull consisted of a number of modified
vertebras (" cranial vertebrae "). On this theory, therefore, the
skull was regarded as a special modification of the anterior part
of the vertebral column, and a large number of facts were brought
forward in support of it : even when morphological science had
made further considerable advances, there still seemed to be a
certain amount of justification for this view.
The arguments in support of the vertebral theory of the skull
may be briefly stated as follows. As in the vertebral column, a
cartilaginous and a bony stage may be distinguished in the skull,
ontogenetically as well as phylogenetically. There is thus an
important correspondence between these two parts of the cranio-
spinal axis, and this is further emphasised by the fact that the
notochord always extends for a certain distance into the base of
the skull, so that the latter is developed on the same skeletogenous
basis as, and in direct continuation of, the vertebral axis. More-
over, the cranial cavity, enclosing the brain, may evidently be
considered as a continuation of the neural canal.
For a long time it was not recognised that as this theory
depended on giving an exact account merely of the skeletogenous
elements taking part in the formation of the skull, it could
not possibly lead to a true interpretation of the origin of the
vertebrate head. Any such attempt meant " putting the cart
before the horse," by looking upon the last acquisition of the head
—its skeleton — as the leading point for future researches.
Although it gradually became evident that, except occasion-
ally in the hinder (occipital) region, no trace of segmentation
of the cartilaginous elements can be recognised in the head
of any existing Craniata, it still seemed to be an open question
whether such a segmentation may not have occurred in early
phylogenetic stages and have gradually become suppressed owing
to deep-seated physiological and morphological modifications.1
1 It is still by no means clear whether or not the sense-capsules were primi-
tively independent of the rest of the axial part of the skull, and it is quite
conceivable that the part of the latter anterior to the vagus foramen consisted
originally of independent skeletal portions which only secondarily became
connected with one another.
SKULL 75
The original segmentation of the head — i.e., the segmentation of
the mesoderm into somites — may have more or less closely resembled
that seen in Amphioxus ; but it must be borne in mind that there
is no direct connection between the Acrania and Craniata, and that
there must have been a whole series of intermediate forms. As a
matter of fact, only vestiges of the primary metamerism of the
head have persisted, and are more or less plainly indicated onto-
genetically by the ganglia, nerves, gill-arches, and myomeres. It
is nevertheless certain that the structural plan of the head, like
that of the trunk, is based on a condition of metamerism, although
it is doubtful how many primary segments are included, and
whether segmentation is not limited to the post-auditory region
(chordal or " spinal " portion) of the skull, and does not concern
the more anterior (prechordal or " prespinal ") portion.
In any case, however, the metameric character is much more
plainly seen in the post-auditory (occipital) region than in the
more anterior part of the head, in which the primary relations are
no longer recognisable owing to parts having become reduced,
displaced, fused, lost, or functionally changed in connection with
the modifications resulting from the development of the brain,
skull, the olfactory, optic, and auditory organs, and the oral muscles.
A reduction, fusion, or loss of cephalic myotomes has also occurred
in the post-auditory parts, the occipital region being of a very varied
and fluctuating nature, and it may even include spinal elements.
It is therefore evidently impossible in this place to give more than
the briefest sketch of the problem under consideration before
making a detailed study of the parts composing the head.
The portion of the skull which is situated along the main axis
in continuation of the vertebral column and which encloses the
brain, is known as the brain-case or cranium (neurocranium), and
is primarily composed of cartilage. A series of cartilaginous
arches arise in serial order on the ventral side of the brain- case ;
these encircle the anterior part of the alimentary tract like hoops,
incomplete dorsally, and are distinguished as the visceral portion
of the skull {splanchnocranium}. This bears an important relation
to branchial respiration, as between each consecutive pair of
arches a passage (gill-cleft) lined by endoderm, is present,
communicating between the pharynx and the exterior, and
through which the water passes in branchiate forms : the fore-
most visceral arch, which bounds the aperture of the mouth,
becomes modified to form the skeleton of the jaws. The arches,
therefore, serve primarily as gill-supports. Ossification may occur
in connection with the cranial and visceral portions later.
Before the cartilaginous skeleton begins to be formed in the
embryo, the greater part of the head consists of a mesodermic
formative tissue, which gives rise to a membranous capsule around
the brain and in which the rudiments of the individual cerebral
nerves can be plainly distinguished. The paired olfactory, optic,
and auditory organs also appear at a very early stage ; and these,
76
COMPARATIVE ANATOMY
in the course of further development, become situated in bays
or cavities within the head and enclosed by definite sense-capsules,
which take on close relations with the cranium, and thus are of
extreme importance in modifying the configuration of the skeletal
structures which are formed around them later.
The relations of the visceral to the cranial skeleton, and those
of both to the primary metamerism of the head, must be taken
into consideration. Both cranial and visceral regions must have
been originally segmented, and each myotome at one time included
a ventral portion (lateral plate of the mesoderm) which enclosed a
corresponding section of the cranial coelome, or t; head-cavity."
Later, however, the visceral region became relatively shifted to a
greater or less degree, especially in the anterior part of the head,
V-----A'
C
FIG. 58. — FIRST CARTILAGINOUS RUDIMENTS OF THE SKULL.
C, notochord ; X, A, O, the three sense-capsules (olfactory, optic, and auditory) ;
PE, parachordal elements ; PR, primary pituitary space ; Tr, trabecuke
cranii.
so that its segments no longer corresponded to those of the cranial
region, which is in general more conservative as regards its
metamerism. Thus we find that the segmentation of the nervous,
muscular, and visceral parts of the head do not correspond with
one another.
a. Brain Case (Neurocranium).
The first cartilaginous rudiments of the primordial skull or
chondrocranium are seen in the form of an anterior and a posterior
pair of bars — the trabeculce cranii and the imrachordal cartilages
SKULL
77
(Figs. 58 and 59), which may be continuous with one another.
They lie along the base of the brain, the parachordals embracing
the anterior end of the notochord. The parachordals soon unite,
more or less completely, to form a basal plate, which grows round
the notochord dorsally and ventrally, and thus early forms a solid
support for the hinder part of the brain. The trabeculaB project
forwards and enclose a space, which, as the pituitary body extends
from the brain through its posterior part, may be spoken of as the
primitive pituitary s/tace (anterior basicranial fontanelle). In the
parachordal region, an anterior auditory or otic, and a posterior
occipital portion, may be recognised on either side. The occipital
region, as already mentioned, may show indications of segmentation
Occipital arch
Cornu tra'jecv.lce
Notochord
Auditory capsule
Otic process
Palatoquadrate
Articular process
FKI. 59. — NEUROCRANIUM AND PALATOQUADRATE OF LARVAL AMBLYSTOMA,
9 MM. IN LENGTH, SEEN OI'.LK.H'KLY FROM THE LEFT SIDE AND AP.OVE.
: ABOUT 35. (From copy by Fr. Ziegler of a model by Ph. St, labial cartilage; ff/.fy', ligaments supporting the jaws from
the cranium ; Lj, MeckePs cartilage; Nv. 2, optic foramen ; Nv. 5, foramen
for trigeminal and facial nerves; olf.cp, olfactory capsule; or, orbit; r,
rostral cartilage ; up.j, palatoquadrate. (The spiracular cartilage is not
indicated.)
connected with one another by longitudinal bars, the whole forming
a delicate cartilaginous basket-work : the last bar is connected
with a cartilage in the walls of the pericardium. This basket-
work has a very superficial position.
In Myxine the branchial skeleton is rudimentary, and amongst
other peculiarities, the long nasal passage is surrounded by
cartilaginous rings and communicates with the pharynx by a
naso-palatine duct.
No fossil Cyclostomes are known, but Palceospondylus gunni
SKULL
87
from the Old Red Sandstone of Caithness possibly shows affinities
with this group.
In Elasmobranchs the skull presents the simplest conditions
and most easily comprehensible relations, so that it may be taken
as the starting-point for the study of the skull of all other Verte-
brates. It consists of a simple cartilaginous and fibrous capsule
more or less movably articulated with the vertebral column, the
chondrocranium here reaching its highest development (Figs. 65
and 66), while from the Elasmobranchs onwards it undergoes, on
the whole, a gradual reduction owing to the increasing import-
ance of the osteocranium. The skull may become more
or less calcified, but true bones are never formed. The fibrous
V.JO
r
FIG. 66. — SKULL OF Chinuxra monstrosa, LATERAL VIEW. (From
Parker and Haswell's Zoology, after Hubrecht. )
a.s.c, position of anterior semicircular canal ; rh.y, ceratohyal ; ep.hy, epi-
hyal ; fr.cl, frontal clasper ; h.n.c, position of horizontal semicircular canal ;
i.o.s, interorbital septum; tb. 1, Ib. 2, Ib. 3, labial' cartilages ; Mck.C,
mandible ; No. 2, optic foramen ; Nv. 10, vagus foramen ; olf.cp, olfactory
capsule; op.r, opercular rays; pal.qn, palatoquadrate ; ph.hy, pliaryngo-
hyal ; p.x.c, position of posterior semicircular canal; qu, quadrate region;
r, rostrum.
portions (fontanelles) are most marked in the prefrontal region,
except in the tropibasic skull of the Holocephali, in which there is
no prefrontal fontanelle and the interorbital region consists of a
thin membranous septum between the large eyes (Fig. 66).
As in all Vertebrates above Cyclostomes, an assimilation of
vertebral elements has taken in the occipital region, so that the
nerves belonging to the vagus-group perforate the skull ; the part
of the skull situated posteriorly to these foramina has therefore
been described as a " neocranium " (cf. p. 85).
The nasal region is often elongated to form a cut-water or
88 COMPARATIVE ANATOMY
rostrum, at the proximal end of which the olfactory sacs are
situated, their cavities being separated from the cranial cavity by a
membrane. Behind them are the deep orbital hollows, which are
bounded posteriorly by the strongly projecting auditory capsules.
Labial cartilages (cf. p. 82) are present in connection with the lips,
nostrils, and jaws.
The palatoquadrate meets with its fellow in the middle line and
is usually connected with the basis cranii by ligaments (Fig. 65). A
process may be present on it which articulates at some point with
the trabecular region. In the Chimseroids (Fig. 66) it becomes
immovably fused with the cranium, whence their name of
Holocephali. In the Sharks and Rays the palatoquadrate is not
directly united to the skull, but is suspended from it by the
hyomandibular (p. 82, and Fig. 65). In this case the skull may be
described as hyostylic, to distinguish it from autostylic skulls, in
which the hyoid takes no part in the suspensorium. In Notidanus,
both mandibular and hyoid arches are independently connected
with the skull, which is therefore spoken of as ampliistylic. A
vestigial cleft, the spiracle, is situated in front of the hyomandi-
bular, and leads into the pharynx ; on its anterior wall may be
found remnants of the embryonic spiracular gill, beneath which
are one or more spiracular cartilages which probably represent gill-
rays (cf. below).1
In Plagiostomes the palatoquadrate and lower jaw are provided
with numerous teeth, arranged in rows ; in the Holocephali the
teeth have the form of strong and sharp-edged plates.
The branchial skeleton is relatively smaller in the Holocephali
than in other Elasmobranchs, in which it is always richly developed,
and owing to secondary segmentation and also to fusion of its
parts, exhibits characteristic modifications. On the outer circum-
ference of each branchial arch, as well as on the hyomandibular
and hyoid, radially-arranged cartilaginous rays are developed, which
serve as supports for the gill-sacs (Fig. 65). Externally to these
rays rod-like " extra-branchial " cartilages are present : these
correspond to the displaced uppermost and lowermost gill-rays.
In Plagiostomes the gill-slits open freely on to the surface of
the body, but in the Holocephali a fold of skin, the gill-cover or
opcrculum arising from the hinder border of the hyomandibular,
overlies them. In the frilled Shark (Chlamydoselachus) there is
an indication of an operculum.
Amongst Ganoids, the lowest condition is met with in
those forms in which the hyaline primordial skull is still retained,
immovably fixed to the vertebral axis, part of which becomes
secondarily assimilated to it. These forms are spoken of as
Cartilaginous Ganoids. The presence of definite bones, however,
1 A small basimandibular element has been described in Laanargus, and
mandibular rays can be recognised in the primitive Pleuracanthidtt from the
Permian formation.
SKULL
89
divides them sharply off from Elasmobranchs, and shows that
their skull has reached a much higher stage of development.
These bones have the form of richly sculptured plates and shields,
and are developed partly from the mucous membrane lining the
mouth and covering the visceral skeleton, and partly from the skin
covering the roof of the skull, where the arrangement of the bones
(e.g. frontals and parietals) typical of higher forms can to some
extent be recognised. A narrow parasphenoid forms a roof to the
oral cavity. The operculurn is more pronounced than in the
Holocephali, and is also supported by bones (cf. p. DO). The
whole palato-mandibular apparatus — which is comparatively small,
M(g
FIG. 67. — CRANIAL SKELETON OK STURGEON (Acipemer) AFTER REMOVAL OF THE
EXOSKELETAI. PARTS.
Ar, articular; C, notochorcl ; Cop, basal elements of the visceral skeleton ; De,
dentary ; GK, auditory capsule ; Hm, hyomandibular ; hy, hyoid ; 7 to V,
first to fifth branchial arches, with their segments — the double pharyngo-
branchial (a), the epibranchial (b), the ceratobranchial (c), and the hypo-
branchial (d) ; Ih, interhyal; //, optic foramen ; Md, mandible ; Na, nasal
cavity ; Ob, neural arches ; Orb, orbit ; PF, AF, postorbital and antorbital
processes; PQ, palatoquadrate ; P8,'Ps', Ps", parasphenoid; Psp, neural
spines ; Qu, quadrate ; JR, rostrum ; Ri, ribs ; /S?jJV, apertures for spinal
nerves ; Sy, symplectic ; WS. vertebral column ; x, vagus foramen ; *, pro-
minent ridge on the basis cranii.
bears no teeth, and in relation with which bones are formed — is
connected very loosely with the skull by means of a hyoman-
dibular and symplectic, as well as by ligaments (Fig. 67).
The dermal skeleton attains a much more considerable develop-
ment in the Bony Ganoids (Crossopterygii and Holostei), and
gives rise to a dense armour composed of numerous bones lying
on the roof and extending into all parts of the skull and jaws
(Fig. 68, A and B) : amongst these may be noted a median
(Amia) or paired (Polypterus) jugular plate between the rami of
the mandible. In addition to the investing bones, replacing bones
are present in the occipital, obic, orbitotemporal, and ethmoidal
regions. Investing and replacing bones very similar to those
90
COMPARATIVE ANATOMY
of Teleosts (p. 92) also occur in connection with the palatoquad-
rate and the entire visceral skeleton, including Meckel's cartilage
and the branchial arches.
Though still largely re-
tained, especially in Amia,
the cartilage thus becomes
relatively reduced as com-
pared with the cartilagin-
ous Ganoids (Fig. 68, B).
At the posterior end of
the trabeculge, which only
remain separated from one
another by a narrow slit,
a lateral basipterygoid
process arises in Lepidos-
teus for articulation with
the palatoquadrate arch,
which is thus connected
with the skull not only
indirectly, through the
hyomandibular, but also
directly.
The opercular bones are
more highly developed than
in cartilaginous Ganoids,
and may include an oper-
culum, a preoperculum, a
suboperculum, and an in-
teroperculurn as well as
branchiostegal rays : these
in part correspond to in-
vesting bones of the carti-
laginous byoid rays. A
symplectic, an interopercu-
lum, and branchiostegal
rays are wanting in Poly-
pterus.
The branchial skeleton
in Ganoids consists of four
or five more or less strong-
ly ossified and segmented
gill-arches, decreasing in
size antero - posteriorly
(Fig. 67) ; in Bony Ganoids
the surface which looks towards the throat is beset with teeth.
The Ganoidei are of special interest, as they, with the Elasmo-
branchii, constitute almost the entire Fish-fauna through the
Silurian, Devonian, and Carboniferous periods, and as the Teleostei,
Fin. 68A. — SKULL OF Polypter-u* Iiirkir FROM
THE DORSAL SIDE.
a, b, c, (I, supraoccipital shields. The two
arrows pointing downwards under the
spiracular shields show the position of
the openings of the spiracles on to the
outer surface of the skull. F, frontal ;
M, maxilla; N, nasal; Xa, external
nostril ; Op, operculum ; Or!>, orbit ; 1\
parietal ; Pm.c, prenmxilla : PO, pre-
operculum; Sb, Sl>', anterior and posterior
suborbital ; SO, suboperculum ; Sj), pre-
spiracular bones.
SKULL
91
which appear later, are doubtless derived from them. They show,
moreover, a connection with the Dipnoi and with the oldest
Amphibia from the Carboniferous and Trias (Stegocephali).
In the Teleosts, the skull (Figs. 69 and 70) presents a large
amount of variation ; its ground-plan, however, may always be
derived from that of the Bony Ganoids, as is best seen by a com-
sp.eth-(-
op.o-
Ofl.O-
occ.-\-
Pa sPr fr
Ptf
Sb
Sf> Na
O.t
f/a
FIG. 68B. — SKULL OF POLYPTERITS. A, LATERAL, AND B AND C DORSAL VIEWS,
THE LATTER AFTER REMOVAL OF THE DERMAL BONES, THE CARTILAGE
DOTTED. (From Traquair.)
An, angular ; Ar, articular ; D, dentary ; E, mesethmoid ; f.m, foramen magnum ;
Fr, frontal ; l.e, lateral ethmoid ; MX, maxilla ; Na, Na', nasal and accessory
nasals ; occ, occipital : ol, nasal aperture ; op, operculum ; op.o, opisthotic ;
O.t, "os terminals"; Pa, parietal; Pmx, premaxilla ; P.t, posttemporal ;
Ptf, postparietal ; Qu, quadrate ; S.b, S.b', suborbitals ; S.Op, sub-operculum ;
Sp, splenial; *}>.eth, "sphenethmoid," in the orbitosphenoid and alisphenoid
region, resembling the like-named bone of Anura (q.v.) ; sp.o, sphenotic ; Spr,
prespiracular ossicles ; S.t, supratemporals ; Y, preoperculum (cheek plate);
Y', Y", smaller cheek plates ; z, postspiracular ossicles ; z', prespiracular
ossicles.
parison of the Siluroids with Amia. On the other hand, no
relations with the Amphibia are observable, and we must consider
the whole group of the bony Fishes as a side branch of the piscine
phylum.
Much of the cartilaginous primordial skull persists in many
92
COMPARATIVE ANATOMY
Teleostei (Fig. 70), and in this respect such forms as Argyropelecus
and Cyclothone acclinidens deserve special mention. The cranial
cavity may reach between the eyes as far as the ethmoidal
region, or may become reduced to a narrow cartilaginous and
fibrous interorbital septum.
In addition to the general account of the various investing and
replacing bones of the skull on pp. 82-84, the following points may
Sphot pat' soec
dent
FIG. 69. — CRANIAL SKELETON OF THE SALMON. From the left side.
art, articular ; branchiost, branchiostegal rays ; dent, dentary ; epiot, epiotic ; tth,
supraethmoid ; fr, frontal; hyom, hyomandibular ; into}), interoperculum ;
•'".'/, jugal ; mpt, mesopterygoid ; mt/>t, metapterygoid ; mx, maxilla ; nas,
nasal ; orbital ring ; op, operculum ; jjal, palatine ; par, parietal ;
P.iiix, premaxilla ; jtra'p, preoperculum ; pt, YJterygoid ; pier, pterotic
(squamosal) ; Quail, quadrate ; socc, snpraoccipital ; sphot, sphenotic ; sultoj),
suboperculum ; Zitnge, tongue.
be mentioned, and the reader is referred to Figs. 69 and 70 for
further details.
As in Ganoids, the chief roofing bones of the skull are the
parietals and frontals, the former of which may be separated from
one another by a process of the supraoccipital. Laterally to the
frontal is a sphenotic, which extends backwards to the pterotic
(squamosal, cf. p. 83). Supratemporals and jugular plates are
never present.
Forming the lateral walls of the skull in the orbital region is
SKULL
93
A
' /,••'•'-•" ^f*y,'~
, ' *^z»z-- I
- / / f fe- ^r
p'roo'f flsjvh alsjtli
N.olf orl.sph '
socc . /
enioT
* i
B
mf. eocene
-Col.verf
I a son
Of!C
FIG. 70. — A. CRANIAL SKELETON OF SALMON AFTER REMOVAL OF THE JAWS
AND ORBITAL AND OPERCULAR BONES. From the right side.
B. The same in longitudinal section. The cartilaginous parts are dotted in
both figures.
alsph, alisphenoid ; basocc, basioccipital ; basph, basisphenoid ; Col. vert, point
of connection of the skull with the vertebral column ; ekteth, ectoethmoid ;
epiof, epiotic ; exocc, exoccipital ; fr, frontal ; N.olf, canal for the olfactory
nerve ; opiafh, opisthotic ; orbsph, orbitosphenoid ; pfero, pterotic (squamosal) ;
prool, prootic ; jwph, parasphenoid ; socc, supraoccipital ; sy>hot, sphenotic ;
co, vomer.
an ossified zone, the anterior and posterior parts of which are
usually known respectively as the orbitosphenoid and alisphenoid.
On the base of the skull is a basisphenoid, ventrally to which
is a parasphenoid, developed in the mucous membrane of the
mouth. More anteriorly is a vomer, and laterally the palato-
94 COMPARATIVE ANATOMY
quadrate bar, which remains separate from its fellow and is con-
nected with the skull-base anteriorly. In connection with the
anterior part of this bar the palatine (investing and replacing
bone) is formed, and with the posterior part a quadrate.
Between these, bony elements are developed which are known as
pterygoids, of which may be distinguished a replacing meta-
pterygoid, an entopterygoid, and an investing mesopterygoid or
ectopterygoid (cf. pp. 82-84). These bones are already represented
in Bony Ganoids, and form, together with the base of the skull,
the roof of the oral cavity.
The olfactory sacs are sunk in the ethmoid cartilage, in which
region supraethmoid and lateral ethmoid (ectethmoid) bones are
developed.
In the auditory region, as in Bony Ganoids, are a prootic, an
epiotic, and an opisthotic, the most important of which is the
prootic. The opisthotic usually does not form an actual part of
the auditory capsule, with which, however, as already mentioned,
other bones (pterotic, sphenotic) may come into relation.
In the occipital region, with which vertebral elements are
assimilated, are exoccipitals, which largely or entirely surround the
occipital foramen, and a basioccipital, as in Bony Ganoids, as well
as a very variable supraoccipital, which is wanting in the last-
mentioned group (Fig. 68, B). Where the basioccipital is in contact
with the vertebral column, it presents a concavity containing
notochordal tissue.1
Forming the margin of the upper jaw are a premaxilla and a
maxilla. These play an important part in all Vertebrates from
the Bony Ganoids onwards, but in Teleosts more particularly they
show considerable variation with regard to their relative develop-
ment, form, and arrangement, and in many cases the maxilla takes
no part in bounding the actual gape of the mouth, and does not
form a continuous bar with the premaxilla. Of the bones in relation
with the oral cavity, the vomer, the parasphenoid, the premaxilla,
arid the maxilla may bear teeth. The maxilla, however, is
edentulous except in the Physostomi.2
Besides the above-mentioned bones in connection with the
jaws, the cranial capsule of Teleosts is surrounded by other out-
works consisting of bony plates and bars. These arise as true
dermal bones in the region of the eyes (orbital ring), and in the
gill-covers (opercular bones) : the latter are similar in number and
name to those of many Bony Ganoids (p. 90). A large number of
1 A curious asymmetry is seen in the head of adult Pleuronectidfe. When
hatched, these Fishes are quite symmetrical, but later on the eye of one side
becomes rotated, so that eventually both eyes are situated 011 the same side ; in
consequence of this, the skull also becomes asymmetrical. In many Teleosts a
canal, lying in the axis of the base of the skull, encloses the eye-muscles, and
opens on either side into the orbits.
2 The tactile barbules present on the head of many Fishes, (e.g., Siluroids)
are supported by skeletal parts (cf. p. 82.)
SKULL 95
branchiostegal rays are developed in the ventral parts of the
opercular fold or branchiostegal membrane (Fig. G9).
Anteriorly, the opercular apparatus lies against a bony chain
consisting of three pieces — the hyomandibular, symplectic, and
quadrate — which serves as a suspensorial apparatus for the lower
jaw (Fig. 69). The latter consists of Meckel's cartilage and of
several bony elements, the largest of which is the toothed dentary :
the others are the articular, angular, and coronary. The last two,
however, may be wanting. The articular is developed in the
articular portion of Meckel's cartilage, which latter is ensheathed
by the dentary and angular.
The hyoid arch is usually followed by four branchial arches and
a rudimentary fifth which forms the " inferior pharyngeal bone."
The dorsal segments of these arches become fused together to
form the "superior pharyngeal bone," which, like the inferior
pharyngeal, usually bears teeth.
The skull of Dipnoans is in a sense intermediate between
that of ChimaBroids and Teleostomes on the one hand, and that of
Amphibians (more especially Urodeles) on the other. In various
respects, however, it presents special characters, such as the marked
metameric segmentation of the occipital region and the relations
of certain of the investing bones.
The chondrocranium is retained almost entirely in the most
primitive existing representative of this group — Ceratodus, and to
a large extent in the other two genera : the only perichondral
bones being a pair of exoccipitals (Fig. 71). The occipital region
is firmly connected with the vertebral axis, and the two or
three anterior vertebral elements which are united with the skull
may possess more or less distinct neural arches and spines (e.g.
Protopterus) : the vagus nerve passes through a space between
the auditory capsule and first neural arch.1 A large " cranial
rib " articulates with the hinder part of the skull on either
side, in a position corresponding to the third occipital neural
arch.
The cranial cavity extends forwards between the orbits to the
ethmoidal region (platybasic type), and its front wall (lamina
cribrosa} is largely cartilaginous. The cartilaginous nasal capsules
are lattice-like, and as in all Vertebrates higher in the scale, each
nasal cavity communicates with the mouth by internal nostrils :
the external nostrils are covered by the upper lip.2
The ethmo-nasal region is covered by a median dermal supra-
ethmoid, postero-laterally to which is a supraorbital bony lamella
(" dermal lateral ethmoid "), and articulating with it posteriorly
in the median line in Ceratodus is another unpaired bony lamella
1 In the embryo of Ceratodus it has been shown that there are five myotomes
anterior to this point.
! There are two so-called "labial cartilages," one of which arises from the
trabecular region, passing behind and to the outer side of the external nostril, and
the other probably belongs to the nasal skeleton.
90
COMPARATIVE ANATOMY
in
(" scleroparietal "). The last mentioned element is wanting
the other two genera, in which an unpaired frontoparietal covers
the roof and part of the side walls of the chondrocranium, on the
ventral side of which is a large parasphenoid.
The squamosal is closely applied to the solid palatoquadrate
cartilage, which becomes fused with the cranium (autostylic type),
FIG. 71.— SKULL, WITH THE PECTORAL ARCH AND Fix, OF PROTOPTERUS.
A, splenial ; AF, antorbital process (the labial cartilage in this region is not in-
dicated) ; a, b, SL. teeth ; B, co, fibrous bands ; Z>, angular, FP, fronto-
parietal ; Ht, membranous fontanelle, perforated by the optic foramen (II) ;
Hy, ceratohyoid ; KR, cranial rib; Kn, coraco-scapular cartilage; LK,
clavicle ; MK, supraclavicle ; NK, fenestrated cartilaginous nasal capsule ;
Ob, auditory capsule ; Occ, exoccipital, with the hypoglossal foramina ; Op,
operculum ; Op', interoperculum, overlying cartilaginous vestiges of hyoid
rays ; PQ, palatopterygoid, which converges towards its fellow at PQ' ; SE,
dermal supraethmoid ; SK, supraorbital (dermal lateral ethmoid) ; Sq,
squamosal, overlying the quadrate cartilage ; TV, palatoquadrate cartilage ;
W, W, vertebral elements with their neural spines (P*p) united with the
skull ; x, facet on the pectoral arch for articulation with the basal^ segment
(b) of the fin ; **, vestigial lateral rays on the basal segment of the fin ; 1—3,
the three following segments : ft, projections of Meckel's cartilage ; I— V,
branchial arches : I and II are segmented (concerning the bar arising from I
anteriorly, cf. note on p. 97).
and in connection with which a palatopterygoid bone is present.
A premaxillo-maxillary arch is wanting.
The strong lower jaw is ossified by an angular and a splenial,
and in Ceratodus a dentary is also present. Meckel's cartilage
extends freely for a short distance anteriorly.
The teeth, which are sharp and blade-like, are borne on the
palatopterygoid and mandible ; small " vomerine " teeth are also
present, though there is no actual vomer.
SKULL 97
The hyoicl arch consists on either side of a large ceratohyal,
and in Ceratodus a small hyomandibular and hypohyal, as well as
a median basihyal, are also present. The five branchial arches l
are comparatively small and weak, and some, or even all of them,
may be entirety unsegmented (Lepidosiren) .
The Dipnoi constitute a very ancient group, which must have
diverged from the main piscine stem at a very early period, for
they occur in the Trias and Carboniferous, and even extend into
the Devonian and possibly into the Silurian.
Amphibians.
Urodela. — The skull of tailed Amphibians is distinguished
from that of bony Fishes in general principally by negative
characters, — on the one hand by the presence of less cartilage in the
adult, and on the other by a reduction in the number of bones
(Fig. 72). In brief, its structure is in many respects simpler, and
becomes modified in adaptation to the different mode of life.
Moreover, no nerve-apertures are present in the occipital region
behind that for the vagus ; but as this region extends to a slight
extent posteriorly to the vagus foramen, it appears that a reduction
has here taken place. The occipital part of the skull has the form
of a neural arch, united with the auditory capsules above and
broadening out below where it abuts against the notochord, form-
ing a basal plate primarily including vertebral elements, on the
posterior surface of which are two occipital condyles, as in all other
Amphibians (Figs. 60 and 72). An exoccipital bone is developed
on either side.
The platybasic cranium is not laterally compressed in the
orbital region, and the brain, flanked by the cartilaginous and
bony cranial walls, extends between the orbits as far as the olfac-
tory capsules, at which point the cranial cavity is closed by a mem-
branous (Triton) or cartilaginous (Salamandra) ethmoid region
(lamina cribrosa), perforated by the olfactory nerves, or in certain
cases by special modifications of the frontal bones (Proteus, Sala-
mandra perspicillata). The anterior part of the lateral cranial
walls may be ossified as an orbitosphenoid. The well-developed
auditory capsules are connected with one another dorsally by a
narrow cartilaginous bar (tectitm synoticum) — all that remains
of such an extensive cartilaginous roof as is seen in Elasmo-
branchs : this is retained in all the higher Vertebrates. In the
ossification of the capsules the prootics take the chief part, and
1 In Protopterus, a delicate cartilaginous rod arises from the first branchial
arch (Fig. 71), concerning the homology of which opinions differ. It may
represent the first branchial arch (and in this case the number of branchial
arches is six) ; or it may belong to the hyoid arch, thus indicating that the latter
is primarily double ; or, again, it may possibly correspond to a branchiostegal ray.
H
98 COMPARATIVE ANATOMY
later unite with the exoccipitals. A new and important modifica-
tion as compared with Fishes is the presence of an aperture, the
fenestra ovalis, s. vestibnli, 011 the outer and lower side of each
capsule, and corresponding to part of the original space between
the capsule and the parachordal cartilage. This fenestra is closed
by a cartilaginous plug, the stapcclial plate, which is connected with
the quadrate and paraquadrate (see p. 82) by ligament, or by a
cartilage or bone (coluinelln a/iris), the two structures probably
together corresponding phylogenetically to the upper section of the
hyoid arch (hyomandibular^ though this homology can no longer
be traced ontogenetically. The olfactory capsules are well
developed and arise in part independently and partly in connection
with the converging trabecula?. In Necturus and Proteus they are
delicate and fenestrated, and united with the cranium by connec-
tive tissues only.
The snout is limited anteriorly by the toothed premaxillas,
which usually more or less completely enclose a cavity (inter-
maxillary or internasal sinus) containing a gland. Each external
nostril is bounded by the nasal process of the premaxilla, the
nasal, and the toothed maxilla, and a small investing bone, the
septomaxillary, is also present between the maxilla and nasal in
relation with the nostril. The premaxilla? and maxilla form the
upper boundary of the gape. Between the nasal and maxilla is a
prefrontal, and medially to this a frontal, followed behind by a
parietal, which partly covers the auditory capsules.
Forming the greater part of the skeletal roof of the oral cavity
and strengthening the skull-base is a large and broad parasphenoid
(Fig. 72), which, as in Fishes, is sometimes provided with teeth.
It extends forwards from the occipital region to the olfactory cap-
sules, closing over the basicranial fbntanelle, and ventral to it is
the paired and toothed vomero-palatine bar, the two elements
comprising each of which become fused in adult Urodeles, but vary
much in form and arrangement. The vomerine part of this bar
is situated beneath the olfactory capsule and is in contact with the
premaxilla and maxilla, thus helping to strengthen this region, at
the posterior part of which is the internal nostril, situated much
more posteriorly than in Dipnoans. Internally to the suspen-
sorium is a pterygoid bone, a process of which extends forward
towards the maxilla.
The suspensorium is much more simple than that of Fishes
(Figs. 72 and 73). It consists of the palatuquadrate only, with
a quadrate ossification, and has usually four typical processes con-
necting it with surrounding parts (pedicle or basal process, otic,
ascending, and pterygoid processes). The quadrate l becomes fused
.secondarily with the skull, and on its outer surface is an investing
1 In Tylototriton i'tiTnco*ti* the quadrate sends forwards a process which
connects it with the maxilla, and thus forms a lower zygomatic arch or infra-
temporal arcade.
SKULL
99
M
7±\. — SKULL OF"A YOUM; AXOLOTL
(.\ini>ly*stomQ). Ventral view.
CoccOsp
Fin. 7-i;.— SKULL OF Salamandrajatra
(ADULT). Dorsal view.
Cl
FIG. 7'2c. — SKULL OF .W^/»«//'//" ntf.i (ADULT). Ventral view.
posterior part of " alisphenoid " region ; Bp, cartilaginous basal plate between
the auditory capsules ; Can, nasal cavity ; Cocc, occipital condyles ; /',
frontal ; Fl, foramen for the olfactory nerve ; Fov, fenestra ovalis, closed
on one side by the stapedial plate (St) ; IN, internasal plate, which extends
laterally to form processes (TF and AF) bounding the internal nostrils
(C'/t) ; Lyt, ligament between the stapes and suspensorium ; M, maxilla ;
N, nasal ; Na, external nostrils ; NK, nasal capsule ; OB, auditory capsule
and exoccipital ; On, orbitosphenoid ; Osp, tectum synoticum ; P, parietal ;
Pf, prefrontal, perforated ac D for the lacrymal duct ; PI, palatine ; Pmx,
premaxilla ; Pot, otic process, PED, pedicle, and Pa, ascending process of
the quadrate ; Pp, palatine process of maxilla ; PS parasphenoid ; Pt, bony
pterygoid ; Ptc, cartilaginous pterygoid ; Qu, quadrate ; Jit, point of entrance
nt the ophthalmic branch of the fifth nerve into the nasal capsule; Squ,
paraquadrate (" squamosal'') ; TV, trabecula ; To, vomer ; Vop, vomeropala-
tine ; Z, tongue-like outgrowth of the internasal plate, which forms a roof
for the internasal cavity ; II, optic, V, trigeminal, and VII, facial foramina.
H 2
100
COMPARATIVE ANATOMY
bone, the paraquadrate (Gaupp), usually described as a squamosal.
The quadrate, exoccipital, prootic, orbitosphenoid, and columella
arise in the perichondrium and are replacing bones, while all the
others are investing bones.
The temporal region is either uncovered by skeletal parts,
or an upper zygomatic bar (supratemporal arcade) is formed by
processes of the paraquadrate and frontal respectively, and indi-
cates a reduction of a more marked development of bone in this
region such as occurred in the Stegocephali.
In connection with the lower jaw are usually developed a
replacing articular at the proximal end of Meckel's cartilage, and
investing splenial and dentary bones. The rest of the visceral
skeleton of Urodeles undergoes various modifications in the
gu.
mk,
FIG. 73. — SKULL AND VISCERAL AKCHES OF Ah Koroma. From the side.
I, mandible ; II, hyoid ; III- VI, branchial arches ; qii, quadrate, covering which
is the paraquadrate (" squamosal") ; ar, articular; ml', Meckel's cartilage
enclosed by the dentary bone.
different types. We may consider the ground-form, as exhibited
in the larva, to consist of five pairs of bars in addition to the
mandibular arch (Fig. 73), in which latter the palatoquadrate and
Meckel's cartilage chondrify independently. The anterior bar, or
hyoid, consists of two pieces (Fig. 74. A), as do also the two first
branchial arches. The third and fourth branchial arches are much
smaller, and even vestigial in Salamanders. All these bars are
connected with a single or double basal piece. At the close of
larval life, that is, when the gills are lost, the two hinder pairs of
arches disappear entirely, while the two anterior pairs undergo
changes as regards form and position,1 and may become more or
less densely ossified (Fig. 74, B — D).
Gymnophiona. — In contrast to the extensive and compact
chondrocranium of most Urodela and of Anura, that of
the limbless Amphibians consists of delicate cartilaginous rods
1 In the genus Spelerpes, which possesses a sling-like tongue, the dorsal
segment of the first branchial arch grows out into a long cartilaginous filament,
which extends far back under the dorsal integument (Fig. 74, D).
SKULL
101
separated by wide spaces ; and even where connected sheets
of cartilage are present, they are very delicate and thin. At the
same time the skull nearly resembles that of Urodeles (more
Cp
K, !»-. l-l V
Cp Rad.I
Fit;. 7-4. — HYOIJRANOHIAL APPARATUS OF UKODELKS. A, Axolotl (Sirndon. stage
of Amblystoma) ; B, Salamandra macvlosa, ; C, Triton cristatus ; D, Spelerpes
fuscus.
Cp, Cps, O.th, basihyobranchial or copula ; G.th, thyroid gland ; Hpbr. I and II,
first and second hypobranchial ; HpH, Bad I, hypohyal ; Kebr. I — IV, first
to fourth ceratobranchial ; Keif, ceratohyal ("anterior cornu " of hyoid in
Caducibranchs — the "posterior cornu " being made up of Hpbr I and II and
Kebr I). Rad. II arises in Salamandra secondarily during metamorphosis.
especially perennibranchiate forms) in spite of a considerable
reduction in its parts — especially in the occipital, auditory, and
orbital regions, as well as of the peculiar and characteristic
II*
102
A
COMPARATIVE ANATOMY
B A*.
Co
n
- 1
•£*
& ^::
ii
in
IV
Fio. 7">. — A, DORSAL, B, VENTRAL, C, LATERAL VIEW OK SKULL OF Sip
Jy<, external nostril ; tiny, angular ; Car, carotid foramen : Ch, internal nostril ;
Co, occipital condyle : deitf, dentary ; JJK, apertures for ducts of tentacular
gland ; E, ethmoid region ; F, frontal ; M, maxilla ; Xji>; naso-preniaxilla ;
Orl>, orbit ; Pal, palatine ; Po, petroso-occipital ; Pj>. Pp', palatine process
of the naso-premaxilla ami of the maxilla ; /'*, parasphenoid, united
posteriorly with the auditory and occipital elements ; J't, pterygoid ; Qn,
quadrate; O<-, subocular or palatopterygoid arch;
C.2J)'»-<>(f, inferior prenasal cartilage; Cr.x. a, subnasal crest ; Eth, spheneth-
moid ; F II — lr, foramina for cerebral nerves ; Foss.cond, condyloid fossa, in
which are the foramina for the IXth and Xth cerebral nerves ; Fr.ji'n;
frontoparietal ; I. max, premaxilla ; J/o.c. maxilla ; Nn, nasal ; Occ.lat, ex-
occipital ; Pal, palatine ; Para, parasphenoid ; Pr. front, frontal process of
maxilla ; Proof, prootic ; Pr.-.t/< i'//yoid
cart.
Ill
V, VII
Aiunthi-".
Prtmaxilla
Septonmx'Ma
Nasal
Max if /a
Artie pro-
cess of
quadrate
Pterygoid
Frontoparietal
f- Columella
Quadratojugal
Paraquadrate
Aud. 1-npxule
Exoccipital
FIG. 78.— SKULL UK A YOUNG Raua dmporaria, 2 CM. IN LENGTH, JUST AFTER
METAMORPHOSIS, KROM THE DOKSAL SIDE. THE INVESTING BONES ARE
REMOVED ON THE LEFT SIDE (x ABT. 11.) After Gaupp, from a model by
Fr. Ziegler.)
Cartilage — blue ; replacing bones — ymy ; investing bones — yellow.
The frontal and parietal of either side are as a rule fused,
thus giving rise to a frontoparietal. The maxillary bar grows
backwards much further than in Urodeles, and becomes connected
with the suspensorium by means of a small intermediate bone, the
quadratojugal, or quadratomaxilla (Figs. 77 and 78). There is
thus a lower sygomatic arch (cf. p. 98) ; an upper zygoma like
that of many Urodeles is never developed, and consequently
the temporal region is uncovered by skeletal parts. The palato-
quadrate is united anteriorly with the cartilaginous nasal capsule
SKULL
107
this is characteristic for the Anura (except Ranodon) as compared
with the Urodela. (For the relations of the bones bounding the
mouth-ca\dty, cf. Figs. 77, A and B).1
,'l'r.anthy.
fom.ttrmS.
B
^~ Pr. ant.
Corn,
Fia. 79. — A. HYOBRANOHIAL .SKELETON OF A LARVAL Kana ttmporaria, 29 MM.
IN LENGTH, FROM THE DORSAL SIDE. B. THE SAME OF A LARVA, 15 MM. IN
LENGTH, AT THE END OF METAMORPHOSIS, AFTER DISAPPEARANCE OF THE
TAIL. C. HVOID CARTILAGE OF A YOUNG FROG, 2 CM. IN LENGTH, FROM
THE VENTRAL SIDE.
(All these figures are from wax models after (Jaupp. )
A and B (in part), Brunch I — IV, branchial arches ; Com. term. I — ///, terminal
commissures of same; Cop, basal plate (copula); Hy, hyoid ; Pr.aut.hy,
Pr.lat.hy, Pr.jiont.hy, anterior, lateral, and posterior processes of the hyoid ;
S-pic. I — IV, cartilaginous processes.
B (in part) and C. Corp.cart.hy, body of hyoid cartilage; Corn princ, anterior
cornu ; Alan, " manubrium " ; Pr.al, alary process; Pr.ant, anterior pro-
cess; Pr.poxt .lat , post ero- lateral process; Pr.thyr.post.med, thyroid or
postero-medial process (posterior cornu.)
The bones of the lower jaw are a dentary and an angular (or
angulosplenial). At the distal end of Meckel's cartilage a small
1 A septomaxillary, helj>ing to close the nasal fenestra on the outer side, is
present, as in Urodeles (p. 98),
108 COMPARATIVE ANATOMY
portion (" lower labial cartilage " of larva) is bent inwards towards
the median line and unites with its fellow in a symphysis, forming
in the adult the mentomandibular (p. 84).
There is a much greater reduction of the branchial skeleton at
the close of larval life than inUrodeles. In the larva, representatives
of the hyoid and of four branchial arches can be recognised, but
these are all united together and form a continuous structure (Fig.
79, A). The greater part of the broad basal parts of this
apparatus, as well as the four branchial arches, disappear during
metamorphosis. The hyoid cartilage of the adult -(B, c) is
formed partly from the remains of the hyobranchial cartilages of
the larva and partly by new outgrowths from it.
Reptiles.
The skull in Reptiles is extremely complex and varied as
regards its bones and their relations. Although differing markedly
in many important respects from the cranial skeleton of Am-
phibians, the ground-form of the latter is distinctly recognisable,
especially in the primitive Hatteria and in Lizards. On the other
hand, numerous points of similarity are seen in the skull, as
well as many other parts, of Reptiles and Birds, which are,
therefore, included together under the term Sauropsida.
In spite, however, of the similarity of plan of the amphibian
and reptilian skull, it must be borne in mind that no recent
Amphibian lies on the direct line of descent of the Reptiles,
though certain fossil Amphibians (Stegocephali) and Reptiles, as
Avell as the existing Hatteria, help to bridge over the space between
the two Classes.
In order not to cause confusion by reference to the multi-
farious details which present themselves in dealing with the
reptilian skull, it will be as well to consider first its more
important characteristics, many of which are common to the
Amniota in general, before treating specially of the various Orders.
In this general description, the lacertilian skull will be chiefly
referred to as a typical form (Figs. 80 and 82).
Apart from its naso-ethmoidal region, the chondrocranium
plays no important part in Reptiles subsequently to the embryonic
period, and it no longer forms such a complete structure as, c.y.,
in Anura, but is considerably reduced and frequently largely
fenestrated (Fig. 80). This want of completeness, however, is
later partly compensated for by the investing bones, and as the
ossification is very considerable, a firm and solid skull results.
The cranium includes three more vertebral elements than in
Amphibia (p. 97), so that the foramina for the three roots of the
compound hypoglossal nerve perforate the skull. In all Amniota
SKULL
109
the cranio-vertebral boundary is in a similar relative position, in
spite of differences of form in this region. The cerebro-nasal
axis, which is horizontal in Amphibians, becomes more or less bent
Sup. olf. cart.
Dorsal nasal fenestra
Lat. nasal fenestra
Orb. nas.Jissurt
Spheneth. carl
Max. proc.*~—
Pituitary fenestra
Asc. proc. s
of pal. quad.
Columella.
Optic fenestra
FICJ. 80. — SKULL OF AN EMBRYO Lacerta agilis, 47 MM. IN LENGTH. A,
DORSAL, AND B, VENTRAL VIE\V. C, LOWER JAW AND HYOBRANCHIAL
SKELETON, FROM THE VENTRAL SIDE.
(After Gaupp, from a wax model by Ziegler x ^.) Cartilage, blue ; 1'eplacing bones,
gray ; investing bones, ytUow.
downwards in front of the interorbital region, and thus causes
various modifications as regards the relations of the nasal and
cranial cavities (cf. Fig. 90).
110
COMPARATIVE AN ATOM Y
The cranial bones (Figs. 80-86) are much more numerous and
varied in form than in recent Amphibia. The solid base of the
skull is formed by bones developed on a cartilaginous foundation,
viz., of a basioccipital and a basisphenoid, on which latter there
may be a liasiptcrygold process on either side for articulation with
the pterygoid bone. An alisphenoid ossification may be present ;
presphenoids and orbitosphenoids are usually wanting. The
Pr, ma -
.In rial '
Transpal.
/•.>/, paraquadrate ("quadrato-
jugal"); (Jit, quadrate; Ts,
transpalatine.
and transpalatine : the vomer is paired. The palatines and ptery-
goids are very firmly attached to the base of the skull, and both
these bones, as well as the paired premaxilke and maxillae, take
part in the formation of the hard palate.1 Thus the internal
nostrils open far back, beneath the basioccipital, from which alone
the occipital condyle is formed. The exoccipitals meet above the
foramen magnum, thus shutting out the supraoccipital. Teeth are
present in sockets on the premaxillae, maxillae, and dentaries.
A series of air-passages extends into the bones from the
1 In the pre-Cretaceous Crocodiles the pterygoids did not form palatal
plates.
,**
120
COMPARATIVE ANATOMY
tympanic cavity, and the Eustachian canals open into the pharynx
by a median aperture behind the internal nostrils.
The hyobranchial skeleton is much reduced, and consists of a
body with a single pair of cornua : it is not known whether these
belong to the hyoid or to the first branchial arch
[Birds.
As already mentioned, the skull of Birds is formed on a similar
plan to that of Reptiles — more particularly of Lizards, but it
exhibits certain special characteristics (Figs. 87 and 88).
In correspondence with the higher type of brain, with its
well developed cerebral hemispheres, the brain-case is relatively
Interorl}. septum
Prefont (lac.)
Squamosal ^
aritt
Dent.
And. caps
Pteryr/.
Supra- Ang. " Paraiph.
Ang. § «»
5 *"*
<&•>»
FIG. ST.— -SKULI, OF AX EMBRYO CHICK 65 MM. IN LENGTH. From the right
side. (From a model b\- W. Toukoflf, x i). Cartilage, l/>n : investing bones,
yellmo.
Prentax.
large, and correlative modifications occur, especially in the occipital
and auditor}- regions. The relatively large size of the eyes,
moreover, has resulted in a limitation of the cranial cavity in an
anterior direction and in the expansion of the brain laterally and
SKULL
121
rtff'i -r.
^
;JPV
^p'V:" fc
i?ur^
ec:'(p \ y«?
Atf
//
^?^
,- «/>
Fio. 88. — SKULL OF A WILD DUCK (Anas boschas). A, from above ; B, from
below ; C, from the side. (From a preparation by W. K. Parker. )
ag, angular : als, alisphenoid ; a.p.f, anterior palatine foramen ; ar, articular ;
6.0, basioccipital ; b.py, basipterygoid ; b.t>, basisphenoid ; b.t, basitemporal ;
d, dentary ; e.n, external nostril; e.o exoccipital ; eth, ethmoid; e.n,
Eustachian aperture ; f.m, foramen magnum ; fr, frontal ; i.c, foramen for
internal carotid artery ; j, jugal ; fc, lacrymal ; mx, maxilla ; mx.p,
maxillopalatine process ; n, nasal ; n.px, nasal process of the premaxilla ; p.
parietal ; py, pterygoid ; pi, palatine ; p.n, internal nostrils ; px, presphenoid ;
px, premaxilla ; q, quadrate; q.j, quadrate- jugal ; s.o, supraoccipital ; $q,
squamosal ; ty, tympanic cavity ; v, vomer ; //, foramen for optic nerve ;
V, for trigeminal ; IX, X, for glossopharyngeal and vagus ; XII, for hypo-
glossal.
122 COMPARATIVE ANATOMY
posteriorly. There is thus a well-developed interorbital septum,
and the tropibasic type of skull reaches its extreme. The cranial
cavity has become further enlarged at the cost of parts formerly
situated extracranially than is the case amongst Reptiles.
The bones show a tendency to run together by the obliteration
of the sutures originally present between them : they are usually
delicate and spongy (" pneumatic "), thus contrasting greatly with
those of Reptiles.1 Only in the ethmoidal region does the cartilage
persist throughout life to any extent.
The unpaired occipital condyle no longer lies at the posterior
boundary of the skull, but becomes relatively shifted forward along
the base, so that the axis of the latter lies at an angle with that of
the vertebral column.
The basis cranii is formed by a basioccipital and a basisphenoid,
from which latter a bony rostrum, the remains of the anterior part
of the parasphenoid. extends forwards : near the base of this,
basipterygoid processes, articulating with the pterygoids, may be
present. The posterior part of the parasphenoid persists as a
large and primarily paired plate, the lasitemporal, which underlies
the basisphenoid and part of the basioccipital.
The interorbital septum is thin, as in Lizards, but is more solid
and less membranous than in the latter : it becomes ossified
anteriorly by a mesethmoid and posteriorly by a presphenoid.
Orbitosphenoids and alisphenoids are also developed. The auditory
capsules, which are more drawn in to the cranial cavity than in
Reptiles, ossify by three centres (prootic, epiotic, and opisthotic)
which later become fused with one another and with neighbouring
bones, and the relations of the tympanic cavity, auditory fenestrge
and columella, including the stapes and extracolumella, are very
similar to those of Reptiles. The two Eustachian tubes open
together in the middle line.
The quadrate is movable upon the skull as is also the whole
maxillopalatine apparatus, the delicate palatopterygoid bar, which
is always more or less separated from its fellow in the middle line,
sliding on the rostrum of the basisphenoid, and so allowing the
beak to be raised or lowered to a greater or less extent : thus
a complete bony palate is never present. This mobility 'of the
upper jaw is most marked in Parrots, in which the frontonasal
joint forms a regular hinge.
The vomers, which may be absent, usually unite with one
another and with the palatines to a greater or less degree.2 The
1 It should, however, be remembered that the development of air spaces
within the bones of the skull is hinted at in Crocodiles as well as in certain fossil
Reptiles.
2 The differences in details as regards the arrangement of the bones of the
palate are important for purposes of classification, as are also the mode of con-
nection of the lacrymal with surrounding parts, including the small bones (like
those in Lizards) which may be present in the neighbourhood of the lacrymal
(supraorbital, infraorbital, lacry mo-palatine).
SKULL
123
(basibrcmchial)
posterior nostrils are always situated between the vomers and
palatines. The two premaxillse, on the form of which depends
that of the beak, are fused, and a maxillopalatine process arises
from the maxilla anteriorly. The maxilla and quadrate are
connected by a delicate jugal and quadratojugal, and a squamosal
is present. Other investing bones are the nasals, frontals, parietals,
and lacrymals or prefrontals.
Teeth were present in Jurassic and Cretaceous Birds (Archae-
opteryx, Hesperornis, Ichthyornis), but were no longer developed
from the Tertiary period on-
wards, their place being taken
functionally by horny sheaths
covering the bones of the jaws,
and thus forming a beak, much
as in Chelonians.
In Meckel's cartilage, two
replacing bones are formed, viz.,
an articular and a mentoman-
dibular: the investing bones
are a dentary, splenial, coronary,
supra-angular, and an angular,
and their relations are essenti-
ally similar to those seen in
Reptiles : they, however, become
fused in the adult, and the two
rami of the mandible unite
distally by synostosis.
The hyobranchial skeleton
(Fig. 89) is greatly reduced. FIO. 89. -HYOBRANCHIAL SKELETON OF
The median body consists of an FOWL. (After Gegenbaur ; lettering
entoglossal (basihyoid) passing after Kallius. )
anteriorly into a primarily paired
paraglossal, which extends into the tongue, and posteriorly into a
urohyal (basibranchial). The single pair of cornua belongs to
the first branchial arch, and may, as in the Woodpecker, give rise
to long, jointed rods extending far over the skull. The columella
is the only part of the hyoid which persists, and even in the
embryo there is no trace of a second branchial arch.
branchial nrcli
Mammals.
In Mammals, the skull of which in many respects indicates an
origin of the Order from reptile-like ancestors, there is a much
closer connection between the cranial and visceral regions than is
the case in the Vertebrates already described. In the fully-
developed skull both maxillary and palatopterygoid regions are
closely united to the cranium, so that the facial and cranial portions
124
COMPARATIVE ANATOMY
are firmly united with one another. The higher we pass in
the Mammalian series, the more does the former come to lie below
instead of in front of the latter, the facial skeleton becoming
proportionately small as contrasted with the large cranial portion
B
FIG. 90.\. — LONGITUDINAL VERTICAL SECTIONS THROUGH THE SKULLS OF— A,
Salamandra maculosa. B, Tvxtndo i/wcu, AND C, Cornt-f corone, TO SHOW THE
RELATIONS BETWEEN THE CRANIAL AND NASAL PORTIONS.
of the skull, and the reduction of the angle between the basi-
cranial and vertebral axes being carried still further than in Birds
(cf. Fig. 90).
The base of the skull is mainly preformed in cartilage (Fig. 91),
and is but little interrupted except for the passage of vessels and
nerves. It consists of basioccipital, basisphenoidal and ethmoidal
SKULL
125
regions, continuous with one another and with the nasal septum.
Side walls are also partly formed by the chondrocranium, but are
FIG. 9i)B. — -LONGITUDINAL VERTICAL SECTIONS THROUGH THE SKULLS OF A, DEER,
B, BABOON, AND C, MAN, TO SHOW THE RELATIONS BETWEEN THE CRANIAL
AND NASAL PORTIONS.
considerably fenestrated. The occipital region includes the
equivalents of four vertebra.
126
COMPARATIVE ANATOMY
Apart from a median cartilaginous bridge connecting the
anterior orbital region with the nasal capsules, and corresponding
to the interorbital septum of the Sauropsida (by the ossification of
which a presphenoid may arise), these capsules are connected
Kasal cups.
Sejituiii nasi.
Orbitonas. fissure
Ala orbitalis.
Ala temp, (alisph.)
Malleus
Incus.
Parotic crest
And. caps.
Jug. for.
— Sup. orb. Jissurt
Carotid for.
Parietal
Int. aud. meatua
Jug. for.
X Endolymph.for.
flvpofflostal for
Tect. synot.
Fit;, ill, A. — SKULL OK AN EMBRYO MOLE (4'2'3 MM. ix LENGTH FROM NOSE TO
BASE OK TAIL). FROM AN ENLARGED MODEL, x ^. A, dorsal, and B, ventral
view. The investing bones are removed on the left side. (After E. Fischer.)
with the cerebral part of the chondrocranium merely by thin bars
on either side (sphenethmoid cartilage) : the Mammalian skull is
therefore of the tropibasic type l (p. 77).
The anterior part of the basis cranii is formed by the ossifica-
tion of the cartilage, which either gives rise to a distinct pre-
1 Certain facts in the development of the skull in Apes indicate that the
Primates diverged very early from the common mammalian stem, many primi-
tive characters being present which are no longer recognisable in other "lower"
Mammals.
SKULL
127
sphenoid, as already mentioned, or may be due to a union of the
basal parts of the two orbitosphenoids. Alisphenoids, as well as
a basisphenoid, a basioccipital, a supraoccipital, and exoccipitals
(often with paroccipital or paramastoid processes) are always present,
the paired condyle x being furnished by the exoccipitals (Fig. 92).
A'arial fenestra
Preitiax
Basal fenestra
Paraseptal cart.
Nasal caps.
Vomer
Splteneth. cart.
Orbitonas. fissure
Meckel's cart.
__ Ala temp, (alisplt.)
Malleus
Carotid for.
Max. ...
Jugal —
Pal — 0-
0
1 P teri/g. " (parasph. ) ~
9
Tympanic
Squamosal
Fen. cochlea
Jug. for. -
Mypoglostal for.
for. may. Tect. synot.
FlG. 91, IJ.
The enlargement of the cranial cavity in correspondence with
the increased size of the brain affects the form of the skull in
various respects. Thus the supraoccipital becomes shifted
1 The presence of two condyles appears at first sight to form an important
difference to Reptiles, and this is the more remarkable as the occipital region has a
similar primary constitution in both groups and differs from that of Amphibians.
But in the case of the Sauropsida there axe four points of connection between the
occipital and the vertebral column. The single condyle is usually formed of
three parts (p. Ill), the median or axial of which articulates parti}' with the
centrum proper of the atlas and partly with the odontoid process, with
which it is connected by ligament. In Mammals, the lateral articulations
are alone developed, and in the Mole embryo there is a single continuous
articulation between the skull and vertebral column.
128
COMPAPxATIVE ANATOMY
J'ai
C.occ
Occ.las.
C.occ*
Jm
tSr/.oc
For.m
C.occ
SKULL 129
Fi<;. 92. —SKULL OF GREYHOUND. A, from above; B, from the side; C, from
below ; and D, in longitudinal section.
B.occ, Occ.bas, basioccipital ; Cav.gl, glenoid cavity for the lower jaw; Cho,
posterior narial passage ; C.occ, occipital condyles (exoccipitals) ; Eth, lamina
perpendicular-is of the ethmoid; Eth', cribriform plate ; F, frontal; For.m,
foramen magnum ; Jg, jugal ; Jm, premaxilla ; L, lacrymal, surrounding
the lacrjnnal canal ; M, maxilla, with the infraorbital foramen (Finf) ; Maud,
external auditory meatus ; Md, mandible ; N, nasal ; P, parietal ; Pal (P in
C), palatine ; Pet, petrous portion of periotic ; Pjt, zygomatic process of the
squamosal ; Pt, pterygoid ; Sph, alisphenoid ; Sph1, basisphenoid ; Sph",
presphenoid ; Sq, squamosal; Sq.occ, supraoccipital ; T, tympanic; Vo,
vomer.
relatively backwards and the auditory region downwards to a
varied extent, so that the squamosal (as is also the case in Birds)
now usually helps to a greater or less extent to complete the walls
of the brain-case dorsally to the displaced auditory capsule.
Moreover, the course taken by the facial and auditory nerves
through the skull-walls has become altered.1
In adaptation to the characteristic high development of the
olfactory organs amongst Mammals, the ethmoidal portion of the
skull is specially developed for enclosing the nasal cavities. The
ethmoid is formed from the anterior part of the chondrocranium,
which is continued forwards as the olfactory chamber, divided into
right and left halves by a cartilaginous septum (mesethmoid), and
separated from the cranial cavity by the cribriform plate (lamina
cribrosa), which, however, is not directly homologous with that of
lower types (p. 97) : this has a more or less oblique or vertical
position, according to the form and relations of the cerebral hemi-
spheres and olfactory lobes. The posterior part of the mesethmoid
becomes ossified as the lamina perpendicularis, and lateral ethmoids
are present at the sides of the nasal region ; the vomer, which
is unpaired in the adult, arises as a paired perichondral bone
ventrally to the nasal septum,2 and the latter is thus in part bony.
The auditory capsules are ossified from prootic, epiotic, and
opisthotic centres, which early unite together to form the periotic
OTpetromastoid bone. The denser internal (petrous) portion of this
bone, which corresponds mainly to the prootic, encloses the essential
part of the organ of hearing, and a fenestra ovalis and fenestra
rotunda are present on its outer surface : the more spongy mastoid
1 Considerable differences exist amongst Mammals as regards the number
and arrangement of the nerve apertures. Thus amongst Carnivores, for example,
the following foramina are distinct from one another : opticum (II), foramen
lacerum anterius or uphenoidal fissure (III, IV, V1, VI), rotundum (V'2), ovale
(V3), meatus auditorius interims (VII, VIII), foramen lacerum posterius (IX, X,
XI), and the condylar foramina (XII). In the lower Mammals (e.g. Monotremes,
Marsupials, and certain Insectivores), the optic foramen and sphenoidal fissure are
not separate from one another, or, in some cases (Echidna, certain Insectivores,
Dasypus, Lemurs), from the foramen rotundum. The cribriform plate of the
ethmoid has numerous perforations for the olfactory nerve in all Mammals
but Ornithorhynchus.
- In Ornithorhynchus a small dumb-bell shaped bone (prevomer) is present
between the diverging premaxillaj.
K
130 COMPARATIVE ANATOMY
(opisthotic) portion reaches the surface of the skull between the
exoccipital and the lympanic bone, the homology of which is open
to doubt, but which possibly corresponds to the paraquadrate or
quadratojugal. The tympanic overlies the petrous portion of the
periotic, and gives attachment to the tympanic membrane : in
the Placentalia it forms the tubular external auditory passage or
meatus below which it may expand into a bulla tympani, which
encloses the tympanic cavity and communicates with the pharynx
by means of the Eustachian tube. The " temporal bone " of
human anatomy represents the fused periotic, tympanic, and
squamosal, the two last of which are investing bones.
The cranial cavity is roofed in by frontals, parietals, and a
supraoccipital : a primarily paired interparietal, not preformed in
cartilage, may remain distinct or may unite with the supraoccipital
or frontals. These roofing elements, like many of the other cranial
bones, are united by sutures which usually persist, at any rate for
a long time. Many of the bones are more or less spongy internally,
and may contain definite air-sinuses (e.g. in Ungulates).
The parietals and frontals vary much as regards form and
relative size in the different orders. In Primates, amongst many
others, the parietals are well developed, while in Insectivores they
are small : in toothed Whales they become separated from one
another by a large bone formed by the fusion of the supra-
occipital and interparietal, which reaches to the frontal (Fig. 94).
In many Mammals there is a large parietal and supraoccipital
crest in correlation with the strongly-developed muscles of the jaws
and neck. The frontals, which, like many of the neighbouring
bones, may become united together, extend downwards towards the
orbital region and cribriform plate, and thus take part in forming
the walls of the cranium and orbit.
Most of the true Ruminants are provided with horns or antlers
projecting from the frontal bones, the formation of which is to be
traced primarily to the integument (Fig. 93).
In the Camcornia (Bovina?, Antelopina3, Caprinse, Ovina?)
bony processes arise from the frontals, which become hollow and
are enveloped by horn formed from the epiderm. They are usually
present in both sexes, but in Tragelaphus, Neotragus, and others
are absent in the female. In the Ccrvidw a solid integumentary
bone is developed and becomes united with the frontal, growing
out to form the antler. After attaining its full development,
the investing skin dries up owing to the development of the
" burr " at its base ; this constricts the vessels, and the antler,
being deprived of nutriment, falls off periodically at the close of
the breeding season. In the young animal the antlers are simple,
but year by year they become more or less complicated and
branched. They are confined to the male except in the case of the
Reindeer. Amongst Giraffes, in addition to a short median " frontal
horn " present in many of the sub-species, both sexes possess small
SKULL
131
lateral horns covered with hair : these are usually described as
separate ossifications which become united with the frontals ; but
it has recently been shown that they originate in connection with
the fibrous osteogenetic tissue of the parietal bones.
Dorsally and laterally to the cartilaginous olfactory capsules
investing bones arise, viz., the variously-shaped nasals and the
FIG. 93. — EARLY STAGES IN THE DEVELOPMENT OF ANTLERS (A, B) AND HORNS
(C, 1), E). (After M. Weber.)
Cor, derm ; Ep, epiclerm ; #$, horny sheath ; HZ, bony process of the frontal,
with the epiphysis-like " os oornu" (00} at its apex : the latter is comparable
to the beam of the antler, and the former to the pedicle : in E the two are
already fused (HZ +00) ; ^?, zone of resorption, at which point the antler
is shed ; SZ, process of the frontal still covered with the integument ; &21,
the same after loss of the integument.
lacrymals, each of the latter perforated by a lacryrnal foramen ; in
this region also are the lateral plates of the ethmoid (lamina;
papyracece). The scroll-like turbinals which are usually well-
developed within the olfactory chambers will be described later.
Cartilage persists in the adult only in the nasal septum, in the
form of the alinasal and aliscptal cartilages.1 A septomaxillary
1 An external nose is peculiar to certain Mammals (e.g. Man). Representa-
tives of the cartilages mentioned above are present amongst other Amiiiota and
in Reptiles, in which, however, they do not extend anteriorly to the rest of the
K 2
132 COMPARATIVE ANATOMY
(cf. p. 82) can be recognised in embryos of Echidna close behind
the external nostrils : it unites later with the premaxilla, forming
its extra-nasal process, which in other Mammals possibly has a
similar independent origin.
The premaxillse, which may become fused, still take an
important part in enclosing the nasal cavities, and in the Dugong
(Halicore) they are very large and are bent downwards in corre-
lation with the large pair of incisor tusks. The maxillae form the
larger part of the facial skeleton, and are also important in
contributing to the walls of the nasal chambers and orbits. Each
maxilla is connected by means of a jugal (malar) with a process of
the squamosal, instead of with the quadrate, as in the Amphibia
and Sauropsida ; thus a zygomatic arch is formed from these
three bones. The orbit and temporal fossa are marked off from one
another in varying degrees : they are continuous, e.g. in Rodents,
Insectivores, and Carnivores, while in Perissodactyles, Ruminants,
and especially Primates, they are more or less completely separated
from one another by a process of the frontal meeting the jugal.
As regards the structure of the hard palate, Mammals agree
essentially with Crocodiles, and more or less complete palatine
plates are formed by the premaxillse, maxillse, and palatines ; but
the small " pterygoids " l (except, e.g. in Anteaters and some
Cetaceans) do not take part in its formation : in Echidna the
pterygoids form part of the basis cranii. The palate is very long
in Echidna and in certain Edentata and Cetacea, and often (e.g.
Marsupialia) presents unossified vacuities.
The general form of the skull differs very greatly amongst
Mammals. It is sometimes short and broad, sometimes elongated —
especially in the region of the snout (e.g. Myrmecophaga, Cetacea).
Amongst the Cetacea (Fig. 94), the facial bones are of so great a
relative length that the skull may be one-third as long as the
whole animal (e.g. Balasna) ; the external nostrils are situated far
back, and there are numerous other secondary modifications apart
from those seen in the lower jaw, which is not used for purposes
of mastication and in certain respects shows traces of degeneration.
The genesis of the lower jaw is briefly as follows (Fig. 95). In
the embryo, the proximal end of Meckel's cartilage is differentiated
into two portions, corresponding to the articular and suspensorial
skull, and are entirely covered by bones, the most important of which in this
respect is the median nasal process of the premaxilla (Figs. 7"2, 80, 82, and 83).
This prenasal process is present only in the Monotremes amongst Mammals, and
with its fellow forms the transitory o.s caruncidcB. On the loss of this ascending
process of the premaxilla, a freer development of the cartilaginous skeleton is
rendered possible ; and under the influence of muscles, certain parts of it become
separated off to form the independent cartilages of the external nose (cf. under
Olfactory Organ).
1 True pterygoid bones, corresponding to those of the Sauropsida, are
apparently only known to occur in Monotremes. The so-called pterygoid (or
internal lamina of the pterygoid process of the basisphenoid) of other Mammals
has been shown to correspond to a posterior part of the parasphenoid.
SKULL
133
parts of the jaw in the lower Vertebrates : these become ossi-
fied and enclosed within the tympanic cavity situated within the
tympanic bone externally to the periotic. They thus represent
Fio. 94. — A, SKULL OF DELPHINUS. (From M. Weber, after Boas.) B, SKULL
OF POSTAL Balcena japonica. (From M. Weber, after Eschricht. )
C, occipital condyle ; Fr, frontal ; Ju, jugal ; L, lacrymal : MX, maxilla ; n,
external nostril ; JVrt, nasal ; oe, exoccipital ; 0*, supraoccipital ; Pa, parietal ;
Pal, palatine ; Ft, pterygoid ; Px, premaxilla ; Sq, squamosal ; Ty, tym-
panic and bulla tympani.
the articular and quadrate, and are known as the malleus and incus
respectively. Having undergone a change of function, they form,
together with a third element — the usually stirrup-shaped stapes,
a connected and articulated chain of auditory ossicles extending
134
COMPARATIVE ANATOMY
between the fenestra ovalis and the tympanic membrane, and
serving to conduct sound vibrations to the inner ear.1 An investing
bone, the dentary, is developed around the main part of Meckel's
cartilage, distal to the malleus ; the cartilage itself may undergo
partial ossification, but gradually disappears, the dentary forming
the bony mandible, which develops a new articulation with the
e.n
or.c
(Modified from
FIG. 95. — SKULL OF EMBRYO OF ARMADILLO ( Tatusia hybrida).
a drawing by W. K. Parker.)
a.ty, tympanic annulus ; au, auditory capsule ; b.hy, basihyal ; c.hy, ceratohyal ;
cr, cricoid ; d, dentary ; e.hy, epihyal ; e.n, external nostril ; eo, exoccipital :
/, frontal ; h.hy, liypohyal ; i, jugal ; in, incus; lc, lacrymal ; ink, Meckel's
cartilage; ml, malleus; mx, maxilla; n, nasal; oc.c, occipital condyle ;
2>, parietal ; pa, palatine ; px, premaxilla ; so, supraoccipital ; st, stapes ;
s.t, ethmoturbinal ; nt.m, stapedius muscle; sq, squamosal; (h, thyroid;
tr, trachea ; //, optic foramen ; V1, V~, foramina through which the first
and second divisions of the trigeminal pass out from the orbit.
squamosal, characteristic of, and confined to, the Mammalia, all
other Craniata possessing the more primitive quadrato-mandibular
articulation. The two rami of the lower jaw may remain distinct
at the symphysis, or many unite with one another (e.g. Bats,
1 There is some doubt as to how far it is justifiable to consider the tympano-
eustachian cavity as homologous with the spiracle of Fishes, and the tympanic
cavity and membrane of Amphibia, Sauropsida, and Mammalia as homologous
with one another.
The stirrup form of the stapes is due to its being perforated by an artery
(as in the case of the stapedial plate of the Gymnophiona), which in
certain cases persists in the adult. The stapes, however, is not perforated in
Monotremes and certain Marsupials and Edentates. The homology of this
element is by no means clear, but there are reasons for considering it to correspond
to the stapedial plate of the Sauropsida and to the whole columella of Amphibia ;
it is possible that all these structures are derivatives of the hyomandibular of
Fishes.
APPENDICULAR SKELETON 135
Perissodactyles, Primates) ; and on each a condylar, a coronoid,
and often an angular process (Marsupials, Rodents, Insectivores)
may be distinguished.1 Teeth, which are only exceptionally
wanting (e.g. Echidna, certain Edentates), are confined to the pre-
maxilla, maxilla, and mandible. They present marked differences
in number, form, and size ; together with the muscles, they are
the cause of considerable modifications in the form of the jaws
and their articulation and may indirectly influence the entire
skull, in the study of which the law of correlation must always
be borne in mind.
The hyoid arch (Fig. 95) is connected proximally with the
base of the auditory capsule and sometimes becomes more
or less ossified, but the greater part is usually reduced to a
fibrous band, and may be quite rudimentary ; its dorsal end forms
the styloid process of the periotic, and its ventral end the lesser
(anterior) cornu of the so-called hyoid bone of the adult. The
body of this bone represents the basal parts of the hyoid and first
branchial arch, the greater (posterior) cornua belonging to the
latter. The hyoid apparatus is connected with the larynx by a
membrane, the thyro-hyal ligament, and the thyroid cartilage of
the larynx arises in the blastema of the second and third branchial
arches.
V. APPENDICULAR SKELETON
The problem of the evolution and morphology of the fins and
limbs of Vertebrates is one which, in point of interest and im-
portance, is comparable to that relating to the head. During the
last thirty years it has been attacked vigorously both from the
embryological and the paleeontological sides, and has given rise to
so many speculations— often of a very contradictory nature — that
only the barest outline of some of the more important results
obtained can be given in the course of the present chapter.
The fins or limbs, which are distinguished from the axial organs
(head, neck, and body) as appcndicular organs, serve mainly for
locomotion, and may be divided into two groups, the unpaired
and the paired (pectoral and pelvic). They arise independently of
the axial skeleton.2
1 Two or more small bones ("ossa mentalia") occur in Man between the
distal ends of the mandibular rami, with which they unite, taking part in the
formation of the mental prominence.
'2 Numerous and varied modifications of the tins occur amongst Wishes to
form, e.g. organs for protection, support, attachment, offence, defence, or for
alluring prey.
136
COMPARATIVE ANATOMY
A. Unpaired Fins.
The unpaired, or median fins, which are mainly characteristic
of Fishes, arise in the embryo as a ridge of the integument (ecto-
derm and mesoderm) extending along the median dorsal line from
the anterior part of the trunk backwards to the tail, around the
apex of which it is continued forwards for some distance along the
ventral side : thus a dorsal, caudal, and ventral portion can be
distinguished. In the course of further development, these
portions either remain continuous, or else certain parts undergo
reduction, so that the ridge only persists in certain regions, where
it forms independent dorsal, caudal, and ventral or anal fins (Fig.
BF An
FIG. 96. — DIAGRAM SHOWING (A) THE UNDIFFERENTIATED CONDITION OF THE
PAIRED AND UNPAIRED FINS IN THE EMBRYO, AND (B) THE MANNER IN
WHICH THE PERMANENT FINS ARE FORMED FROM THE CONTINUOUS FOLDS.
AF, anal fin ; An, anus ; BF, pelvic fin ; Bi'F, pectoral fin ; Z>, dorsal fin-fold ;
RF, FF, dorsal fins ; SF, tail-fin ; S, S, lateral folds, which unite together
at S1 to form the ventral fold.
96, A, B) : in these regions muscles and skeletal parts become
developed in Fishes.
These skeletal parts consist of supporting rays of two kinds.
In the base of the fin cartilaginous radii, or pteryyiophores, usually
segmented (typically into three portions), are formed ; these may
unite proximally to form one or more basipterygia, and in bony
Fishes they become extensively ossified : they frequently come
into secondary connection with the vertebral column. Except in
Cyclostomes, the peripheral part of the fin is supported by dermal
rays, which may consist of numerous delicate horny fibres (Elas-
mobranchs), or of bony rods, entire or jointed, often cleft at the
base, and articulating with the pterygiophores, and not preformed
APPENDICULAR SKELETON 137
in cartilage (Teleostomes) : recent researches indicate that the
latter are ectodermal in origin.1
Median fins are also present in the Amphibia, in which they
may persist throughout life (e.g. Perennibranchiata), or only occur
in the larval stage ; occasionally also they become specially devel-
oped during the breeding season (e.g. Newt). They have the
form of a continuous integumentary fold extending round the tail
and along the back for a greater or less distance, but enclose no
skeletal elements.
Amongst Reptiles one or more median fins were present in
Ichthyosaurus, and these are comparable to the dorsal fins occur-
ring in the Cetacea amongst Mammals : in both cases they, like
the horizontal tail fin of these forms, must be looked upon as
structures acquired secondarily in connection with an aquatic
existence.
B. Paired Fins or Limbs.
As regards the origin of the paired fins, there is much difference
of opinion. According to one view, they correspond to modified
gill-arches and rays, the former giving rise to the pectoral and
pelvic arches or girdles, and the latter to the free portion of each
fin, one of the rays becoming enlarged so that the others are
attached in a row on either side of it, instead of to the arch. This
would result in a biserial form of fin, the " archipterygium " of
Gegenbaur, such as is most nearly retained in Ceratodus (Fig. 118),
and is also indicated in many Elasmobranchs. The fact that the
branchial arches are situated in the pharyngeal wall and the limb
arches in the body- wall, alone forms an important objection to this
theory.
Another view, which seems to be the more likely one, is
as follows. It is highly probable that primitive Vertebrates at
one time possessed, in addition to the median fins, a pair of con-
tinuous lateral fin-folds, traces of which, beginning with a prolifera-
tion of the mesoderm, can still be recognised in young embryos of
Elasmobranchs (Fig. 97) and to a less extent in those of other
Fishes and of Amphibians, and which, though never continuous, are
indicated by muscle-buds on the intermediate myotomes. They
extended backwards along the sides of the body from just behind
the head, gradually converging towards the anal region, where
they became continuous with the ventral part of the median fin-
fold (Fig. 96, A), and in this respect resembled the persistent lateral
or metaplcural folds present in the adult Amphioxus, though it is
1 The dermal fin rays or dermotrichia are classified by Goodrich as follows :—
1. Horny ceratotrichia in Elasmobranchs ; 2. Bony leptotrichia in Teleostomes ;
3. Horny actinotrichia occurring in the embryo and in the margins of the fins of
adult Teleostomes, in addition to (2) ; 4. Fibrous, calcified,.or horny camjdot ricliia
in Dipnoans : it is doubtful whether the last-mentioned correspond to (1) or to (2).
138
COMPARATIVE ANATOMY
m. ~
doubtful how far this comparison is justifiable. As is usually the
case in the median fins, certain parts of these lateral folds have
undergone reduction, only the anterior and posterior portions
remaining to form respectively the pectoral arid pelvic fins, which
must therefore be looked upon as
the localised remains of a con-
tinuous lateral fin-fold on either
side of the body.1
Into these paired folds extend
metameric processes of the myo-
meres, which undergo further
development in those regions
which will give rise to the pectoral
and pelvic fins, and disappear in
the intermediate region. More or
fewer spinal nerves pass into the
fins, and finally also cartilaginous
supports (pterygiophores), as in
the case of the median fins. These
radii appear first of all at the base
of the fin, gradually extending
centrifugally into the latter, and
also, becoming fused, centripetally
into the body- wall (Fig. 98).2 An
articulation is then formed second-
arily between the fused basal part
of the skeleton situated in the free
portion of the fin (basipterygium)
and that which extends into the
lateral body-wall and serves as a
support for the limb proper : this
constitutes the limb-arch or girdle,
The arch may remain compara-
tively small and not extend far
dorsally ; but when the extremity is destined to perform more im-
portant movements in locomotion or to give a more definite
1 The essential part of this conception as to the origin of the paired
extremities is due to Thacher, Mivart, Balfour, Haswell, and Dohrn, and a
somewhat similar idea was put forward by Goodsir as early as 1856. The
Palffiozoic Cladoselache is very suggestive in this respect.
' Thus phylogenetically both anterior and posterior extremities can be traced
to a metameric ground-plan. At the same time it must be borne in mind that the
above account is not altogether borne out oiitogenetically. The muscle-buds are
not strictly metameric, as they fuse together before coming into connection with
the skeletal parts, with which they do not always correspond numerically and
which appear to consist at first of a single unsegmented basipterygium : in other
words, the radii arise secondarily. Moreover, it is held by some embryologists
that oiitogenetically the girdle is the primary part of the extremity from which
the free portion grows out secondarily, and a similar axifugal growth can be
recognised in the median tins. All this, however, may only mean that recapitula-
tion is incomplete, and the arguments against the lateral fin-theory are still not
conclusive.
FIG. 97. — TRANSVERSE SECTION
THROUGH THE EMBRYO OF A
SHARK (Pristiurus melanosto-
mns), 9 MM. LONG, SHOWING
THE MODE OF ORIGIN OF THE
PECTORAL LIMB-BUDS.
ap, limb-buds ; ch, notochord ; co,
ccelome ; m, myomeres, which
are extending ventrally ; my,
spinal cord ; re', re", rudiment
of kidney tubule and duct.
APPENDICULAR SKELETON
139
support to the body, in addition to meeting with its fellow
ventrally, the arch may extend upwards so as to come into
connection with the axial skeleton, thus forming an almost
complete girdle around the body. The parts of the limb-skeleton
rd.
FIG. 98. — A, B, C. DIAGRAM OF THREE SUCCESSIVE STAGES IN THE DEVELOP-
MENT OF THE PELVIC FIN OF A SHARK.
cl, cloacal aperture ; fo, obturator foramen ; rd, primitive radii, which in A are
beginning to fuse into a basal plate (7«). In B this fusion has taken place
on both sides, and at * the proximal ends of the two basals are approximat-
ing to form the arch. In C the process is completed, and at f an articulation
has been formed between the arch and the free portion of the fin. On the
left side in C the radii are becoming secondarily segmented.
may become ossified later. The pelvic fin of Fishes as a rule
remains at a simpler and more embryonic stage than the pec-
toral fin.
The paired extremities are not connected with any particular
body-segments, but vary greatly as to their relative positions and
the nerves which supply them.
140
COMPARATIVE ANATOMY
Pectoral Arch.
Fishes. — Paired fins and arches are wanting in Cyclostomes.
In Elasmobranchs the pectoral arch consists of a comparatively
simple cartilaginous bar (Fig. 99), situated just behind the
branchial apparatus, the two halves of which are united ventrally
by cartilage or fibrous tissue,1 and in embryos of Teleostomes it
has at first a similar structure. Later, however, in the last-named
FIG. 99. — PECTORAL ARCH AND Fix OF Heptanckus.
n, l>, the main fin-ray, lying in the axis of the metapterygium (3ft) ; t, single ray on
the other side of the axis (indication of a biserial type) ; F8, horny rays,
cut through ; Pr, Ms, Mf, the three basal elements of the fin (pro-, meso-,
and metapterygium) ; Ra, fin-rays ; SB, SB1, pectoral arch, with a nerve
aperture at NL.
Order, bony structures originating from the integument are
developed in this region ; so that a secondary or bony pectoral arch
may be distinguished from a primary or cartilaginous one, the
latter becoming less marked in proportion to the development of
the former (Fig. 100).
In all Fishes the free extremity, or fin, is connected with the
hinder and outer circumference of the (primary) arch, convex
1 In Heptanchus there is a small ventral element which has been compared
to a "sternum."
PECTORAL ARCH
141
articulations being formed on the arch which fit into concave
facets on the fin. The point of attachment of the extremity may
be taken as separating the arch into an upper dorsal and a lower
ventral section. The former, which may exceptionally be con-
nected with the vertebral
column (viz., Raiidre), cor-
responds to the scapula, and
the latter to the coracoid plus
procoracoid of the higher
Vertebrata.
In Teleosts and bony
Ganoids the secondary arch,
consisting of a row of bones,
forms the principal support
of the fin in the adult, the
main element being a large
clavicle. The arch becomes
secondarily connected with
the skull. (For further
details, cf. Fig. 100.) In
Dipnoans, clavicles and
supra-clavicles invest the
cartilaginous arch (Fig. 71). FR; m_LEFT PECTOKAL ARCH AND FlN
OF THE TROUT. (From the outer side.)
Amphibians. — In this
Class the pectoral arch shows
no direct connection with
that of Fishes, but is similar
in fundamental plan to that
of all the higher Vertebrates.
It always consists on
either side of a cartilagin-
ous or bony dorsal plate
(scapula and suprascapula),
which curves round the side
of the body and is con-
tinuous with two ventral
plates — an anterior (procoracoid} and a posterior (coracoid} (Figs.
101 and 102). The ventral part of the arch becomes connected with
the sternal apparatus. The humerus articulates with a concave
glenoid facet at the junction of the scapula and coracoid. The
two coracoid plates either overlap one another in the mid-
ventral line (Urodeles, Fig. 55, A, B and certain Anura — e.g.
Hyla, Bombinator, Fig. 55, c), or else their free edges come into
apposition and unite (other Anura, e.g. Rana, Fig. 55, D). In
Anurans the procoracoid has a more transverse position than
in Urodeles, and comes into connection with the coracoid in the
mid-ventral line, thus giving rise to a fenestra between the two.
Co(Cl)
D, Dl, D2, 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 ; Ml, metapterygium ;
Ra, Rn, the second and third, and 4,
the fourth basal element of the fin ;
Ha1, the second cartilaginous row of
radii ; 8 and Co(Cl), bony scapula and
coracoid, which have become developed
in the cartilage Kn.
142
COMPARATIVE ANATOMY
E
The whole arch is, moreover, more strongly ossified, the procoracoid
being covered by an investing bone — the clavicle, which may
more or less completely replace it. This integumentary bone
corresponds to the part of the secondary
arch which first appears in Ganoids : in the
Stegoeephali there was a well-developed
clavicle connected with the episternum (see
p. 44) and peripherally with another bony
rod (cleithrum), which also occurred in the
FIG.IOI.-DIAGRAMOFTHE fossil RePtile Pareiasaurus
GROUND-TYPE OF PEC- Reptiles. — As in Amphibians, the most
TOKAL ARCH MET WITH IN essential parts of the pectoral arch of
ReRt!les. are the scap«lau and coracoid-
arising in connection with a continuous
;1 cartiiagincras bar or plate, as is well seen in
scapula. Lizards (Fig. 56). A procoracoid may also
be formed, and in Chelonians a bone
usually described as the procoracoid is strongly developed, but
is firmly united with the pillar-like scapula, the two being
separated from the coracoid by a suture ; hence the bone in
ri
FIG. 102. — PECTORAL ARCH OF THE RIGHT SIDE OF Salamandra maculosa,
considerably magnified, and flattened out.
a, b, bony processes extending into the procoracoid and coracoid respectively ;
Cl, procoracoid ; Co, coracoid ; G, glenoid cavity, surrounded by a rim of
cartilage (L); S, scapula (ossified); SS, suprascapula.
question is sometimes spoken of as a proscapida. In other recent
Reptiles the procoracoid is much reduced or even absent.
Traces of the relations of the procoracoid to the clavicle can
still be seen in some cases, but the latter, when present, arises
mainly from a connective-tissue blastema unconnected with a
procoracoid (Fig. 56). Nevertheless a primary and a secondary
part of the pectoral arch can also be recognised in Reptiles, the
PECTORAL ARCH 143
former represented by the more constant elements, while the latter
tends to become reduced and may even entirely disappear.
Clavicles are absent in Chelonians, and are either wanting or rudi-
mentary in Crocodiles and Chameleons.
On the loss of the extremities (certain Skinks, Amphisbsenians,
Snakes), the primary shoulder-girdle becomes reduced or even
entirely lost, the reduction beginning with the sternum.
The shifting backwards of the pectoral arch, which is already
to some extent seen in Amphibians as compared with Fishes, is
still more marked in Reptiles, in which it is situated some distance
from the head; this is especially seen in Chelonians and many
fossil forms, and reaches its maximum in Birds.
In Lizards, unossified spaces are left in the coracoid, giving
rise to fenestras closed over by fibrous membrane. A main fenestra
(cf. Fig. 56, a, dorsal to which a bony process, the vestigial pro-
coracoid, can be seen) maybe distinguished from accessory fenestrse
of varied form and number, and is typical of all Lizards : it
arises in the primary arch and corresponds to that occurring in
Amphibians (Fig. 55).
Birds. — In Birds, the scapula consists of a thin and narrow
plate of bone often extending far backwards, the strong coracoid
being bent at an acute angle and united by ligament with it in
typical Carinate Birds (Fig. 53). In the Ratitre the relatively
small scapula and coracoid are ankylosed with one another.
The lower end of the latter bone is firmly articulated in a groove
on the anterior edge of the sternum, while its upper end takes
part with the scapula in forming the glenoid cavity, beyond
which it is produced in the Carinatas and in Archeopteryx to form
an acrocorcicoid process.
In Struthio the broad coracoid is fenestrated, and its anterior
part may be looked upon as a procoracoid: in other Ratitas the
latter is considerably reduced, and may be represented merely by
a ligament ; in Carinatas it can often no longer be recognised.
In almost all Flying Birds the clavicle, a purely dermal bone,
is well developed, and becomes united with its fellow to form a
furcula (Fig. 53). Amongst the Cursorial Birds, the Emu and
Cassowary possess vestigial clavicles : in the others they are
wanting, and they have also undergone more or less complete
reduction in some Carinate Birds (e.g. certain Parrakeets and
Owls).
Mammals. — In Monotremes the pectoral arch retains primi-
tive characters, and in them only amongst Mammals does the
coracoid extend ventrally to reach the sternum (Fig. 103) ; in all
other members of this Class it characteristically becomes reduced,1
and simply forms a prominent process on the scapula (coracoid
1 In early stages of certain Marsupials (e.g. Trichosaurus), and possibly in
all, the coracoid is well developed and articulates with the sternum, but it sub-
sequently undergoes reduction.
144
COMPARATIVE ANATOMY
process), which is ossified from a separate centre, apparently repre-
senting an epicoracoid, while the coracoid proper may be occa-
sionally indicated by a small centre of ossification on the glenoid
margin of the scapula.
Thus the scapula becomes freer from the rest of the skeleton,
and it alone serves to support the extremity ; it becomes at the
same time greatly broadened, and gives rise on its outer side,
in connection with the highly differentiated mu.scles of the limb,
to a strong ridge (spina scapulae), which extends downwards to
form the so-called acromion. The distal end of the clavicle usually
SC.
St.
FIG. 103. — PECTORAL, ARCH AND STERNUM OF OrnithorJiynchus paradoxus.
c1, c1, c3, first, second, and third ribs; d, clavicle; e.c, epicoracoid; e*1 and es^,
prosternum (episternum) ; m.c, coracoid (metacoracoid) ; m.s, manubrium
stern i ; sc, scapula ; st, sternebra.
becomes connected with the acromion, its proximal end articulat-
ing with the anterior edge of the sternum.
In those Mammals in which the fore-limbs are capable of very
varied and free movements (Lemurs, certain Marsupials, many
Rodents and Insectivores. Bats, and Primates) the clavicles are
strongly developed.1 In others (e.g. Ungulates, Cetaceans, Carni-
vores, most Edentates, Rodents, Marsupials) they may be en-
tirely wanting or only vestigial, and in the latter case their rela-
tions to the scapula become altered.
1 The clavicle is primarily independent of the coraco-scapular portion of the
pectoral arch. Its original dermal character is retained in Monotremes, but in
all other Mammals it is developed on a cartilaginous basis.
PELVIC ARCH
145
Pelvic Arch.
Fishes. — In Cartilaginous Ganoids, indications of a pelvis
are seen, but are very variable, even in individuals of the same
species. They consist of two calcified or ossified pelvic plates,
which correspond to portions segmented off from the basal
cartilage (basipterygium) of the fin. In some cases even this
segmentation does not take place, and thus the pelvis remains
FIG. 104. — SIMPLE FORMS OF PELVIS AMONUST FISHES AND AMPHIBIANS.
A, Pleuracanthus — the pelvis is here not differentiated from the proximal end
(tf) of the basipterygium ; B, Scapliirhynclms cataphrati H-S ; 0, Polypttrnx
bichir ; D, Neil tint* (Menobranchus). Ap, apophysis of the basipterygium ;
Sas1, basipterygium ; Fo, obturator foramen ; P, pelvis ; Had, radii.
undifferentiated. This simple condition is also met with in the
ancient forms Pleuracanthus and Xenacanthus (Fig. 104, A, B).
In Polypterus the pelvis shows some advance on that of
Sturgeons. Owing, doubtless, to the necessity of a firmer connec-
tion of the fin with the body-wall, the two pelvic plates become
united together in the mid-ventral line (Fig. 104, c). In spite,
however, of the rudimentary character of the pelvis of Polypterus,
the essential form of that of the Dipnoi and Amphibia (D) is
already sketched out (for Teleosts, cf. p. 159).
The pelvis of Elasmobranchs consists of a tranverse bar extend-
ing between the two basipterygia, from which it has become
146
COMPARATIVE ANATOMY
segmented off secondarily (Fig. 98) : it is perforated by nerves,
and gives rise on either side to an iliac process (most marked in the
Holocephali) extend-
Cep ing upwards into the
lateral walls of the
body (Fig. 105). A
prepubic process is also
present, and there is
apparently also an in-
dication of a median
epipubic process (cf.
in fret}. The whole
pelvic plate essentially
corresponds, more or
FIG. 105.— DIAGRAM OF THE ELASMOBKANCH PELVIS. less completely, with
From the ventral side.
the
of
ischiopubis
Bus, Pro, Rad, basipterygium, propterygium, and higher forms.
radii of the fin ; BP, pelvic plate (ischiopubis) ; Jn the Dipnoi the
Oep, epipubic process ; Fol. obturator foramen ; , -i
cartilaginous
7, iliac process; PP, prepubic process;
region of the ischiopubic symphysis.
fly,
narrow
pelvic plate (Fig. 106)
is provided with a long
and delicate anterior median, a short posterior median process,
and two pairs of lateral processes. Of the latter the anterior
(prepubic processes) are much
longer in Protopterus than in
Ceratodus, and each is embedded
intermuscular septum ;
n an
with the posterior process the
skeleton of the fin is articu-
lated by means of an inter-
mediate piece. The anterior
unpaired process may be looked
upon as an epipubic process,
corresponding with that of Am-
phibia and Amniota (q. v.}. The
posterior or hypoischiatic process
bears a ridge for the attachment
of muscles.
Amphibians. — It will be
seen by a glance at Fig. 104 D, Fu! 1()(i_pELVIS()F Protopfeni,<. From
that the ventral portion of the
pel vie arch of Necturus is formed
on the same plan as the pelvic
plate Of the Dipnoi and CrOSSO-
.. , , • ii TT 11 j
pterygll, but 111 all Lrodela and
Amniota it is perforated by the
obturator nerve. Like the pelvis
of all Vertebrates, it has a paired origin, and in Proteus and
the ventral side.
prepubic process, which may become
forked at its distal end ; b, process to
which the pelvic fin (HE) is attached ;
c, epipubic process; Gfr, ridge tor
attachment of muscles ; M, myotomes ;
Ml, intermuscular septa.
PELVIC ARCH
147
ft -\-\-(Crp)
Sllfa)
PP
JP
i ''1i:tli('.'|l/''/'"fl'lini '(
C :>:,' '<''[';>
I , ).!, ;;!Y.
•V:-W
« r !'r ' I US'SSSiG
oV/tf— ii' ijiij i .v^vT'v.'v
^ ' ' ... '- •%.•.•;"':••"
~" — •('=*•<•
fHSy)
--J
Ac, acetabulum ; 6V (*S'y), muscular ridge
on the ventral side of the ischiopubis ;
Fo, Fol, obturator foramen ; J, J1,
ilium ; JP, JP1, ventral pelvic plate
(ischiopubis) ; Lalb, linea alba ; My,
intermuscular septa ; PP, prepubis ; Sy,
S3'inphysis, in which region a strong
tendinous area (SH) exists in Amphiunm,
the pubic regions only coming together in
the middle line at * ;*' (in A), ossified
region of the ischium ; ** (in C and D),
secondary bifurcation of the epipubis ;
z, outgrowth from this bifurcation ; t,
(in C), hypoischiatic process, present in
the l)erotremata and Nee turns ; ft
(Cep), Ep, epipubis.
Fm. 107.— PELVIS OK (A Proteus-. (B) Amphiuma; (C) Cryp'olirmwlius; AND
(D) Salamandra mwnloxa. From the ventral side.
L 2
148
COMPARATIVE ANATOMY
Amphiuma this is indicated by the fact that its anterior epipubic
process is paired throughout life (Fig. 107, A, B). In the
Derotrema and Myctodera, the anterior end of the median epi-
pubic process is bifurcated (c, D).
As already indicated, the ischiopubic plate is phylogenetically
the oldest part of the pelvis, and various modifications as regards
the degree of its fusion into a median unpaired plate and of its
ossification occur amongst Amphibians ; the typical triradiate
arrangement of the pelvic bones (ilium, ischinm, and pulis\ such
B
C
FIG. 108. —PELVIS OF ANURA. A, Xenopus, from below; B, the same from
the front ; C, Eana i xculenta, from the right side.
Ac, acetabulum ; Cep, epipubic cartilage ; /, ilium ; P (in Xenopus), the
proximal end of the ilium, which is separated from its fellow and from
the pubis by a + -shaped zone of cartilage, f, * ; /*, ischium ; P, pubis (/" in
Kaiia, pubic end of ilium).
as is further differentiated in certain Stegocephali and in Reptiles,
is already sketched out.
One of the most characteristic differences between the pelvis
of Fishes and that of Amphibians is seen in the marked develop-
ment of the iliac region in the latter group. The ilium, like the
scapula, extends upwards in the lateral walls of the body ; and in
Proteus and Amphiuma, owing to the reduction of the limbs in
these forms, does not reach the vertebral column (Fig. 107, A, B).
In all other Amphibia, as in the Amniota, it comes into connection
with the sacrum, owing to the necessity for the hind-limb to act
as a support for the body in terrestrial animals.
The pelvis of the Anura differs from that of Urodela in the
following characteristics. In correspondence with their mode of
progression, the ilium of each side becomes extended so as to form
PELVIC ARCH
149
a long rod (Fig. 108, c) ; and the ischiopubic plate, which in
Urodeles lies in the plane of the abdominal walls, becomes closely
pressed together in the middle line and gives rise to a well-marked
ventral keel : it is not perforated by the obturator nerve. The
pubic region, moreover, though often calcified, is independently
ossified only in the case of Xenopus (Fig. 108, A, B).
Reptiles. — The chief characteristics of the Reptilian pelvis as
compared with that of Amphibians consist in: (1) a much more
marked differentiation of the pubis, which is more distinctly
separated from the ischium by an ischiopubic foramen ; (2) the
greater development of the ilium, which is sometimes broadened
out at its vertebral end ; and (3) the more intense and solid
ossification of the arch as a whole.
Points of connection with the pelvis of Amphibians are seen
in certain fossil forms (e.g. Palseohatteria, Plesiosauria), and also
PP
Fo*.
FIG. 109. — PELVIC AKCH OF Hatltrin. (After Credner.) From the
ventral side.
Cep, epipubic cartilage ; Fol, obturator foramen ; /, ilium ; /*•, ischium ; P,
pubis ; i>i>, prepubis ; *, hypoischiatic process, which becomes segmented
off from the pelvis in other Reptiles, t, t, ischiopubic foramina.
in Hatteria and the Chelonia. In the Plesiosauria and Hatteria
(Fig. 109) the pubes are not very widely separated from the
ischia, so that the ischiopubic foramina are not so extensive as in
many other Reptiles.
From this condition that seen in the Chelonia, more especially
in Macrochelys and Chelydra, maybe easily derived (Fig. 110, A).
In both cases the epipubis and prepubis are strongly marked. In
other respects there is great variation in the form of the pelvis in
Chelonians, but the obturator and ischiopubic foramina are never
distinct from one another (Fig. 110).
The pelvis of the typical Lacertilia (Fig. Ill) is characterised by
150
COMPARATIVE ANATOMY
a lightness of build. The rod-like pubis and ischium are separated
from one another by a large ischiopubic foramen, and between them
in the middle line is a longitudinal fibre-cartilaginous ligament,
continuous anteriorly with the plug-like epipubic cartilage and
posteriorly with the hypoischium or os doacce (absent in Chame-
leons). This tract represents the remnant of the median ends of
.Is
FIG. 110. — PELVIC ARCH OF VARIOUS CHKLOMANS. From the ventral side.
A, J\fr.icrorliff/i/n (after G. Baur) ; B, S}iharslralis. Lateral view. (After Marsh.)
a, acetabulum ; if, ilium ; in, ischium ; p, pectineal process from the pars
acetabularis ; p*, pubis.
(dorsally) only in Rhea. A process given off from the posterior end
of the pubis in the Emu, and extending forwards, may represent an
epipubis.
In Archseopteryx, all the elements of the pelvis were
independent and relatively small, the ilium coming into relation
FIG. 114.— RIGHT HALF OF THE
HUMAN PELVIS. LATERAL VIEW.
Fo, obturator foramen ; the three
bones— ilium (//), ischium (/.$), and
pubis (P) — are shown distinct from
one another in the acetabulum.
J—
Fi<;. 115. — DIAGRAM SHOWING THE
RELATIONS OF THE PARS ACE-
TAPULAKIS in Viverra ciretta.
A, acetabular bone ; Ar, aoetabuluni ;
-/, ilium ; •/•<, ischium ; P, pubis.
with about six vertebras only, and the pubis and ischium being
less backwardly directed than in recent Birds.
154
COMPARATIVE ANATOMY
Mammals. — The ilium and ischium of Mammals, like those
of the Anura and Sauropsida, are respectively preacetabular and
postacetabular in position, and the elements of the pelvis remain
separated for a long time by cartilage, but later become fused
(Fig. 114). The pubis always takes less part in the formation of
the acetabulum than do the other two bones, and may be more or
less entirely shut out from it by an ossification of the pars aceta-
bularis, which subsequently unites with either the ilium, ischium,
or pubis (Fig. 115). This acctabular bone is especially well
Tlll.il).
B
FIG. 116. — PELVIS OF A, Echidna hystrix (ADULT), AND B, Didelphya azarce
5 '5 CM. IN LENGTH). From the ventral side.
Ep, epipubis (marsupial bone) ; Fobt, obturator foramen ; J, ilium ; J.y,
ischium ; LIJ and Lyf, ligament between the pubis and epipubis ; P, pubis ;
Sy, ischiopubic symphysis ; Tub.il.p, iliopectineal tubercle; **, cartila-
ginous apophysis at the anterior end of the epipubis.
In Fig. A, GH, articulation between the pubis and epipubis ; Tb, cartilaginous
tuber ischii ; Z, process on the anterior border of the pubis ; t*, t, tt, ilio-
and ischio-pubic sutures.
In Fig B, b, b1, cartilaginous base of the epipubis, continuous with the inter-
pubic cartilage at t ; *, *t, ischio-pubic and ischio-iliac sutures.
developed in the Mole, in which it shuts the ilium, as well as the
pubis, out of the acetabulum ; in Monotremes the acetabulum is
perforated. The angle between the axes of the ilium and sacrum
is largest in Ornithorhynchus, and most acute in Rodents ; the
ilium is connected with a varied number of vertebrae in the
different forms.
The original type with both pubic and ischiatic symphyses,
indicating an elongated form of pelvis, is seen in Monotremes,
Marsupials (Fig. 116), many Rodents, Insectivores, and Ungulates.
In many other Insectivores, in Carnivores, and more particularly
FINS 155
in the Primates, the ischia no longer meet below, and the broaden-
ing of the ilia seen in the higher forms of the last named Order
culminates in Man. The greatest amount of variety in the form
of the pelvis in any one order with typical appendages is seen in
Irisectivores, in some of which (e.g. Mole, Shrew), as well as in
most Bats, there is no symphysis pubis, so that the relatively small
pelvic cavity is not enclosed ventrally by bone. The obturator
foramen is always surrounded by bone.
In the Cetacea, in which hind limbs are wanting, paired
vestiges of the ischiopubic region of the pelvis are present : they
are unconnected with one another and with the vertebral column.
In the Sirenia.a paired bony rod (Manatus) or plate (Halicore) repre-
sents the last vestige of an ilium, in which an ischium is included
in the latter genus.
In Monotremes and Marsupials of both sexes, two strong so-
called "marsupial bones" (Fig. 116) arise from the anterior
border of the pubes, right and left of the middle line, and extend
forward in a stright or oblique direction embedded in the body-
walls, serving for the attachment of muscles. They form an
integral part of the pelvis, and in the embryo are seen to be in
direct connection with its cartilaginous symphysis (Fig. 116, B) ;
but later on definite articulations are formed between them and
the pubes (A). It is not improbable that these structures are
the homologues of the epipubis of lower Vertebrates, which has
been retained in non-placental Mammals in order to serve as a
support for the abdominal walls in connection with the marsupial
pouch.
PAIRED FINS OF FISHES.
Fishes.
The development of the extremities has already been alluded
to (p. 137). The pelvic fin usually retains a simpler and more
primitive form than the pectoral fin.
Elasniobmnchs. — The cartilaginous skeleton of the fins is the
most richly segmented in these Fishes. There are usually two main
elements (basalia)in the pelvic fin which articulate with the arch and
with which a variable number of segmented rays are connected, the
latter passing towards the periphery of the fin (Fig. 117). Both
the larger, posterior lasipterygium or rnetaptcryyiuiii, and the
smaller, inconstant propterygium must be looked upon as originat-
ing phylogenetically by a fusion of the proximal ends of the primary
cartilaginous rays of the fin ; and the form and relations of these
main elements vary according to the degree in which such a fusion
156
COMPARATIVE ANATOMY
has taken place.1 This is also true as regards the pectoral fin, in
which an additional basal piece, or mesopterygium, is usually present
(Fig. 99) : there may even be four basalia. These complications
arise in connection with the greater importance of the pectoral
fin as an organ of locomotion. The distal portions of both fins
are supported by horny fibres (cf. note on p. 137). With the
a,
Raff
FIG. 117. — RIGHT PELVIC FIN OF
Heptanckus. From the ventral
side.
BP, pelvic plate ; Fol, f, nerve-for-
amina ; Pr, propterygium ; 1'ml,
radii, which show secondary seg-
mentation ; S /'«(!,, basi- or meta-
pterygium.
FIG. 118. — PECTORAL Fix OF Cera-
todus fosteri.
a, 1>, the two first segments of the
main axial ray ; FS, dermal rays,
shown only on one side ; f, f,
lateral rays.
exception of one — or at most of very few — all the rays are
situated on the same side of the basalia (uniscrial type).
In Rays, the propterygium of the pectoral fin, and usually also the
metapterygium, are strongly developed, the former extending far
forwards so as to be connected with the skull by ligament, and in
some cases even uniting with its fellow in front of the skull.
Dipnoans. — The cartilaginous pectoral and pelvic fins are here
1 In male Elasmobranchii a number of pieces of cartilage are connected witli
the distal snd of the basipterygium of the pelvic iin as a support for the
copulatory organs or claspers (q. n. ) : these may become more or less calcified.
FINS
157
also essentially similar to one another, the latter being rather
the simpler of the two. From a segmented main ray or axis a
number of segmented secondary rays arise on either side in
Ceratodus: these are not, however, strictly symmetrical (Fig. 118).
Beyond them dermal rays are present (p. 137). A proximal
(basal) segment of the axis, which bears no rays, articulates with
the arch. In Protopterus and Lepidosiren the fins, with their
skeleton, have undergone a marked reduction, so that little more
than the segmented axis remains.
Thus the fins of Dipnoans differ from those of most Elasmo-
branchs (as well as of Teleostomes) in being formed on a biserial
type, indications of which are, however, as already stated,
FKJ. 119.— RIGHT PELVIC FIN OF A Y/orxo Pol yodon folium.
From the dorsal side.
F8, bony dermal rays ; M, metapterygium ; Pru, uncinate ("iliac") processes;
Ra, Ha}, radii of the first and second orders.
seen in the embryos and adults of certain Elasmobranchs.
Physiologically, the Dipnoan fin, like that of the young Polypterus,
serves not merely as a swimming organ, but also to support the
body when the animal is resting on the bottom, as do the limbs of
a Urodele.
Ganoids. — The skeleton of the fin is much simpler and the
primary rays much fewer in number in Ganoids than in Elasmo-
branchs. This is, however, compensated for by the formation of
secondary dermal bony structures, as in the case of the pectoral
arch and skull : these arise on either side of the fin and may or
may not be segmented: they are always more strongly developed on
the anterior than on the posterior border of the fin. The most
158
COMPARATIVE ANATOMY
anterior or marginal ray comes into close connection with the
cartilage of the primary fin-skeleton (Sturgeons) or entirely
replaces it (Amia).
In the pelvic fin of cartilaginous Ganoids (Fig. 119) more or
fewer of the radii are connected proximally with a segmented
basale, which is perforated by nerves, and from which a very
primitive pelvic plate may in some cases become differentiated
(Fig. 104, B). It is important to bear in mind that the distinction
between an axis and secondary rays cannot, therefore, be strictly
recognised, as the basale corresponds to a number of fused radii,
and is perhaps not comparable to the metapterygium of Elasmo-
A
FIG. 120. — LEFT PECTORAL FIN OF A, Polyodon, AND B, Amia.
a — ff, radii which do not reach the arch and are connected with the most
posterior ray (IV. in A, ///. in B) ; 7 — IV, cartilaginous radii connected
with the arch (»S') ; KS, bony dermal rays.
branchs : but it is doubtful whether this character is primitive or
secondary.
The primitive relations have to a certain extent disappeared in
the pectoral fin of cartilaginous Ganoids, which, however, also
consists of a varied number of rays. Of these, four reach the arch
in Polyodon (Fig. 120, A), and five in Acipenser.
In the pectoral fin of Amia (Fig. 120, B) two large converging-
marginal rays articulate with the shoulder-girdle, and only one
intermediate ray reaches the arch : this condition may be compared
with that seen in the highly-developed pectoral fin of Polypterus
(Fig. 121), which is flanked on either side by a strong, ossified ray,
between which is an intermediate region. The fin, therefore,
resembles that of an Elasinobranch Avith its propterygium, meso-
pterygium, and metapterygium.1
1 Even if it should be proved that the intermediate region (MS) no longer
arises in the embryo by a fusion of separate rays, it is possible that this was the
ease phylogenetically.
LIMBS
159
Oss
FIG. 121. — PECTORAI, FIN OF
Polypterus.
The form of the pelvic fin in Polypterus and other bony
Ganoids may be easily derived from that seen in the cartilaginous
representatives of this order, and it
may be assumed that the basale is -F
due to the concrescence of a larger
number of separate radii, which are,
therefore, much less numerous than
in the Sturgeons (Fig. 104). Bony
rays support the distal part of both
pairs of fins (p. 137).
Tcleosts. — A still further reduc-
tion has taken place in the primitive
skeleton of the paired fins in Tele-
osts, there being at most only a few
radials articulating with the arch
(Tig. 100), and even these (especi-
ally in the case of the pelvic fin, in
which the arch is usually considered
to be undifferentiated), may be want-
ing. The main part of each fin is
supported by bony rays, as in Ganoids.
The skeleton of the fins of Siluroids,
Cyprinoids, and Gymnotidse conies
nearest to that of Ganoids.
FS, bony dermal rays ; Nf, nerve
foramina ; 0**, centre of ossi-
fication in MS ; Pr, Mt, bony
marginal rays, which meet at f,
so that the intermediate region
(MS) does not reach the arch ;
Ra, JRa1, radii.
Though it is possible to derive
the skeleton of the fin of all the Orders of Fishes from a single
ground -type, it is a far more difficult task to trace the connection
of the latter with the extremities of Amphibia and Amniota.
Between these two types of extremity there seems to be a wide
gap, in consequence of the different conditions of life existing
between aquatic and terrestrial Vertebrates. We do not know
how the pentadactyle limb of an air-breathing Vertebrate (cluiro-
pterygium), adapted for progression upon land, has been derived
from the fin (icMhyopterygimn\ only fitted for use in the water,
and Paleontology has so far furnished no solution to this problem.
There is, however, a certain amount of probability in the view
that the cheiropterygium has arisen from such an ichthyopterygium
as that seen in cartilaginous Fishes, although it is quite un-
certain as to how far the individual parts are comparable to one
another (Fig. 122), and how the fin, which is practically a single-
jointed lever, amply sufficient for the movement of the body in a
fluid medium, became gradually transformed into a many-jointed
system of levers.
PAIRED LIMBS OF THE HIGHER
VERTEBRATA.
160
COMPARATIVE ANATOMY
As the function of the limb is now no longer simply to propel
the body, but also to lift it up from the ground, the firmly con-
nected elements of its skeleton are placed at an angle to one
another (elbow and knee, in which the angle is directed backwards
and forwards respectively), definite articulations being formed
between them in a proximo-distal direction. The fore-limb serves
in typical cases mainly for pulling and the hind-limb for pushing
the body along the ground, and on this fact depend the various
differences between the two as regards their relation as a whole to
the trunk and of their various parts
to one another. Instead of project-
ing horizontally outwards, the limb
extends downwards, and thus the
angle between it and the median
plane of the trunk is gradually re-
duced, until in Mammals eventually,
the longitudinal axis of the limb,
when at rest, is parallel with the
median plane of the body. In the
higher types this is more particularly
the case as regards the posterior
THE extremities, the anterior limbs under-
THE going the most varied adaptative
modifications, and giving rise to
prehensile or to flying organs — or,
as in aquatic Mammals, becoming
once more converted into paddles.
The fore-limbs and hind-limbs of all
Vertebrates above Fishes may, how-
ever, be reduced to a single ground- type.
A division into four principal sections can always be recog-
nised: in the case of the fore- limb these are spoken of as upper
arm (Irachium), fore-arm (antibrachiuni), wrist (carpus), and hand
(mantis') ; and in the hind-limb as thigh (femur}, shank (cms),
ankle (tarsus), and foot (pcs) (Fig. 123). The bone of the upper
arm (humerus'), like that of the thigh (femur) is always unpaired,
but two bones are present in the fore-arm and shank. The former
are called radius and ulna, and the latter tibia and fibula. The
hand and foot are also respectively divisible into two sections, a
proximal metacarpus and metatarsus, and a distal series of
phalanges, which form the skeleton of the fingers and toes (digits).
Both manus and pes are made up of several series of cylin-
drical bones. There are never more than five complete series,
which — except as regards number — present essentially similar
primary relations throughout the higher Vertebrates. The
skeleton of the carpus and tarsus, each of which typically consists
of a series of small cartilages or bones, shows much variation ; but
the following arrangement may be taken as typical (Fig. 123).
FIG. 122. — DIAGRAMMATIC FIGURES
TO SHOW THE RELATIONS OF
ANTERIOR EXTREMITY TO
TRUNK IN FISHES (A), AND THE
HIGHER VERTEBRATES (B).
Mt, metapterygium ; I'd, radialia
in A, radius in B ; S, pectoral
arch : Ul, ulna ; proximally to Ul
and Ed is the humerus.
arrangement
LIMBS
161
Fe-
FL
Round a centra /e, which may be double, is arranged a series of
other elements, of which three are proximal, and a varying number
(four to six) distal. The proximal, in correspondence with their
relations to the bones of the fore-arm and shank respectively, are
spoken of as radiale or tili't«l<', ulnare or fibula re, and intermedium ;
while the distal are called carpalia or tarsalia (in the narrower
sense). They are counted
beginning from the pre-
axial (radial or tibial) side
of the limb.
Amphibians. — The
anterior and posterior
extremities of Urodela
are formed essentially on
the ground-plan described
above, but more or fewer
of the carpals or tarsals
may undergo fusion. In
them, as in Anura, there
are five digits in the
hind-limb, and usually
only four in the fore-
limb. In the Anura the
radius and ulna become
united, and a separate
intermedium is not re-
cognisable; the proximal
row of the tarsus, more-
over, consists of only two
cylindrical bones, one of
which (astragalus) corre- Fl(1
sponds to a tibiale, and
the other (cakaneum) to %, + digits ^/V-.^femur; Fi^, fibula;
a fibulare (Fig. 124).
In the distal row of
the carpus four separate
elements are formed in
Anura, but this number may become reduced owing to secondary
fusion ; in rare cases a fifth carpal may also be present. Tarsalia
// and /// are the most constant elements, but even these may
undergo fusion, and tarsalia /Fand Fare generally represented
by a ligament.1
In Anura the metat:\rsals and phalanges, between which the
web of the foot is stretched, are, like the proximal tarsals, very
long and slender. The femur, as well as the fused bones of the
1 Very different views are held with regard to the homologies of the
individual carpals and tarsals in Amphibians, and the older numbering and
nomenclature are therefore provisionally retained here.
M
^
1-23. — HIND LIMB OF A URODKLE (Spderpes
fuscus).
, meta-
tarsals (7 — V) : Ph, phalanges ; T, tibia ;
Ta, tarsus, consisting of — c, centrale ; f,
fibulare ; i, intermedium ; t, tibiale ; and
1 — 5, distal tarsalia.
162
COMPARATIVE ANATOMY
shank, are also exceedingly long, in correlation with the mode of
progression of these animals. The skeleton of the extremities
is more strongly ossified in Anurans than in Urodeles, in which
many of the elements remain cartilaginous.
Traces of an extra element (" prehallux"') occur on the tibial
side of the tarsus, and in both Urodeles and Anurans indications
B
. Twicde
; Tui'xiile
Tui-xule I
Centrals
Groove be-
t if I 1,1 ,'llllill.1
Hill/ I'/llll
Ulna
UlHfl,:
Car pal t
III—
Fir;. 124. — A, RIUHT FORE-ARM AND HAND, AND B, RICHT FOOT, OF
I'tina escitlenta. From the dorsal side, x 2. After E. Glaupp.
of an additional pre-axial ray in the manus are occasionally met
with. The number of phalanges in the individual digits varies in
different Amphibians.
Vestiges of the extremities can be recognised externally in
embryos of the limbless Gymnophiona.
LIMBS
163
Reptiles. — In existing Reptiles as a general rule the
body is only slightly raised from the ground in locomotion., but
in some the limbs serve as more highly organised organs of
support, and in certain of the Dinosauria the hind limbs were the
main organs of progression. The fore limbs in such cases tend to
take on other functions, and in the Hying Pterosauria the fifth
finger was produced into a long, jointed rod which supported a
wing-like expansion of the integument.
Chelonians, and more particularly Hatteria, come nearest to
the Urodeles in the structure of the carpus.1 Five digits are
usually present in Reptiles in both manus and pes, and traces also
of the former possession of an extra ray both on the radial and
7
Fie. 125.— CARPUS OF A, Hatteria jjtmrfata, AND B, Emydnru krf/t'ti.
(After Baur.)
c1, radial cent rale ; c2, ulnar centrale ; i, intermedium ; p, ulnar sesamoid
(pisiform) ; /?, radius ; r, radiale ; U, ulna; •«, ulnare ; 1—5, carpalia ; /--T,
metacarpals.
ulnar side ("pisiform") can usually be recognised (Figs. 125-
130). The tibia and fibula always remain separate.
In Lizards and Crocodiles the carpus and tarsus diverge more
from the primitive form. In the latter, which, like Anurans,
possess no trace of an intermedium, the proximal row of the
carpus consists of two hour-glass-shaped bones — a larger radiale,
and a smaller ulnare (Fig. 128). The centrale, as in An ura, comes
to be situated in the distal row, which, like the fourth and fifth
digits, is much reduced.
In Ichthyosaurus and Plesiosaurus the limbs were modified to
1 In Hatteria and certain Chelonians, as well as in the extinct Protcrosaurus,
a double centrale is present in the carpus, and more or less distinct traces of a
double condition of this element are seen in certain other Chelonians. Indica-
tions even of a third centrale occur in Hatteria.
M 2
164
COMPARATIVE ANATOMY
n>
v
FIG. 126. — RIGHT CARPUS OF Emy*
From above.
i, intermedium ; 7?, radius ; r.c, fused
radiale and centrale ; U, ulna ; n,
ulnare ; t and *, elements on the
radial and ulnar side respectively,
indications of additional radial and
ulnar (pisiform1! rays ; 1--5, the
carpalia, of which 4 and 5 are
fused ; / — V, metacarpals.
FIG. 127. — LEFT CARPUS OK Lm-i rln
agilis. From above.
c, centrale ; i, intermedium ; li,
radius ; r, radiale, formed by the
fusion of two elements, one of
which corresponds to a prepollex ;
U, ulna ; u, ulnare ; t, pisiform ;
1 — 5, carpalia; / — V, the meta-
carpals.
form paddles: the radius and ulna were very short, and there were
numerous phalanges l (cf. Cetacea), additional rays being present
in the former genus.
Amongst the snake-like kinds of Lizards, various degrees of
JT
u
FIG. 128.— RIGHT CARPUS OF A YOUNG
Afliynfor lui-iitx. From above.
C, centrale ; R, radius ; r, radiale ;
U, ulna ; 11, ulnare ; t, pisiform ;
1 to 5, the five carpalia, as yet un-
ossified, of which 1 and 2, as well
as 3, 4, and 5, have become fused ;
7 — V, metacarpals.
FIG. 129.— RIGHT TARSUS OF Emy«
enropwa. From above.
F, fibula ; (i\f.f.f, the fused inter-
medium^), fibulare, tibiale, and
centrale ; Phl, phalanx of 1st digit ;
T, tibia ; 1 — 4, distal tarsals ; / — I',
metatarsals.
reduction of the extremities occur, and in such forms as Anguis
and Amphisba^na they have practically disappeared entirely, as in
1 An indication of this condition is seen in the embryo Crocodile.
LIMBS
165
most Snakes. In certain of the latter, howevei, traces of the
hind limbs exist (e.g. Python).
The tibia gradually becomes of relatively greater size than the
fibula in the reptilian series. The tarsus always undergoes a
marked reduction, especially in its proximal portion, and gradually
leads to the type seen in Birds. Thus in Chelonians and Lizards
(Figs. 129 and 130) the proximal tarsals may all run together into
a single mass, which in the former corresponds to the tibiale,
intermedium, fibulare, and centrale. In Lizards a centrale can
no longer be recognised, even in the embryo, and there is no distinct
trace of an intermedium. In the distal row three or four separate
tarsals are developed, but these may unite with one another
to a greater or less extent, and there is an increasing tendency
J7T
FIG. 130. — RIGHT TARSUS OF LacerUt
II fit/ it. From above.
F, fibula; T, tibia; f.f.i.c, fused
tibiale, intermedium, fibulare, and
centrale ; t, trace of a 6th ray
present in Geckos ; 3 — 5, distal
tarsals ; / — V, metatarsals.
FIG. 131.— RIGHT TARSUS OF CROCO-
DILE. From above.
F, fibula ; f, fibulare (calcaneum) ;
T, tibia ; t,i,c, astragalus, corre-
sponding to fused tibiale, inter-
medium, and centrale ; 1, 2, 3.
fused 1st— 3rd distal tarsals ; 4, 4th
distal tarsal ; / — IV, metatarsals ;
F?, ath tarsal and metatarsal.
for the movement of the foot to take place by means of an inter -
tarsal articulation, as in the Dinosauria and also in Birds.
In Crocodiles (Fig. 131) there are two bones in the proximal
row of the tarsus, one of which corresponds to a tibiale, inter-
medium, and centrale, the other to a fibulare. The former is
spoken of as the astragalus, the latter as the calcaneum, and on it
a definite heel (calcaneal process) is seen for the first time in the
animal series. The distal row consists originally of four small
cartilages, but these later undergo a partial fusion.
Birds. — In consequence of the fore limb of Birds having
become adapted for Might, the manus loses its primitive character
and undergoes reduction, while the humerus and the bones
of the fore arm — more particularly the ulna, as well as the
entire pectoral arch and sternum, are extraordinarily developed,
166
COMPARATIVE ANATOMY
the wings in good fliers being considerably longer than the legs,
which alone bear the entire weight of the body when on the
ground (Fig. 132). In Cursorial Birds (Ratitse), however, the wing
has undergone regressive changes in connection with their habits,
ScJi.
-MF
- Z'
FlG. 132. -SKELKTiiN OF THK LtMB.S AND TAIL OV A C'ARINATE BlKU. (The
skeleton of the body is indicated by dotted lines.)
F, digits ; Fl, fibula ; H W, carpus ; MF, tarsometatarsus ; MH, carpometa-
carpus ; OA, humerus ; OS, femur ; Py, pygostyle ; #, coracoid ; Rd, ulna ;
Sch, scapula ; >SV, sternum, with its keel (Cr) ; T, tibiotarsus ; £//, radius ;
~], :., digits.
and in the extinct New Zealand Moa (J)inornis) no trace of it
has been found : in Penguins it serves as a paddle.
The relation of the superficial surface of the wings to the
weight of the body is far from constant, and depends largely on
the relative power of flight ; on the whole, the wings are relatively
largest in small, light Birds than in large, heavy ones.
LIMBS
167
In the carpus, at least seven elements are recognisable. In
the proximal row is an intermeclio-radiale and a centro-ulnare,
each of which consists of two parts in the embryo. In the distal
row there are also two elements, one of which (carpale 2 + 3) is
evidently primarily double : the other corresponds to carpale 4.
In early stages four distinct metacarpals can be seen, and these
-Klaue
Klaue
B
FIG. 133.— CARPUS OF EMBRYO OF Sterna wilaoni. A, STAGK AT WHICH
OSSIFICATION BEGINS ; B, STAGE IMMEDIATELY T.EFORE HATCHING. (After
V. L. Leighton. )
r, distal carpals ; Ktaite, claw ; Had, radius ; rad, intermedio-radiale : Ufn,
ulna ; -tiln, centro-ulnare ; II — V, metacarpals (in B, / V and V have become
fused).
seem to correspond to the 2nd-5th rather than to the lst-4th : the
5th metacarpal soon fuses with the 4th (Fig, 133).
The distal carpals become fused with the corresponding meta-
carpals, thus forming a carpomeiaearpus (Figs. 132, 133), and in
the adult only the two proximal elements remain separate as a
radiale and an ulnare. The three metacarpals themselves become
united proximally, and the second ('///) and third (IV) distally:
they only bear a limited number of phalanges at their free ends.
Claws were present on the terminal phalanges of all three
digits in Archaeopteryx (Fig. 49). In certain recent adult Birds
(e.g, Chauna) the first digit (//) bears a claw, and more rarely
168 COMPARATIVE ANATOMY
(Ratitse) the second (HI], and even the third (IV") also (e.g.
Struthio). Claws may be present in the young only (e.g. Opistho-
comus, Sterna, Fig. 133).
The tarsus is still more reduced in Birds than in Reptiles, and
consists in the embryo of three elements, two small proximal and
a broader distal, which in many cases (e.g. Penguin) consists
primarily of four distinct pieces. The former (tibiale and fibulare)
unite later with the distal end of the tibia, thus forming a tibiotarsus,
while the latter, which corresponds to tarsalia / to V, becomes
included in the base of the metatarsus. Thus the foot of adult
Birds no longer possesses any distinct tarsal elements, though, as
in Chelonians and Lizards, it really moves by an intertarsal
articulation. Of the original five metatarsals, the fifth soon dis-
appears, while the second, third, and fourth become united with
one another and with the distal element of the tarsus to form a
single bone, the tarsometatarsus (Fig. 132), grooves at the ends
of which indicate its compound nature, which is especially well
seen in Penguins. The first metatarsal remains to a greater
or less extent independent.
The number of toes varies between two (Struthio) and four :
that of the phalanges is normally 2, 3, 4, 5, reckoning from the
first to the fourth digit. The tibia, even from the first, greatly
exceeds the splint-like fibula in size, and the two bones become
united distally.1
Mammals. — In Mammals the anterior extremity either
remains in the condition of a simple organ of locomotion, serving
for progression on land, or it may give rise to a digging or a
prehensile organ ; or, again, may become modified in adaptation to
an aerial (Bats) or aquatic (Pinnipedia, Cetacea, Sirenia) mode
of life.
The humerus, which may possess a supracondyloid foramen near
its distal end, is variously modified as regards form and relative
length and the presence of ridges and elevations for the insertion
of muscles ; and the same is true as regards the femur and its
ridges or trochanters.
The tibia is the more important bone of the shank, and
the fibula often becomes united with it to a greater or less
extent distally and sometimes proximally also, usually taking
no part in the knee joint. The two shank-bones lie parallel,
and are at most very slightly movable on one another (e.g.
climbing Marsupials). The fibula never disappears entirely,
but in some cases (Bats, Ruminants) only its distal end is
recognisable as the lateral (external) malleolns.
The radius and ulna are connected with the humerus by a
hinge-joint at the elbow, only allowing movement in one plane,
and primarily their relations to one another are similar to those
of the tibia and fibula. This is the case in Monotremes and in
1 For the pneumatic character of Birds' bones, of, under Air-sacs,
LIMBS 169
all Mammals in which the radius is fixed in a position of pronation
(vide infra). In certain Mammals --more particularly the
Primates, in which the fore limb is prehensile, the bones of the
forearm, instead of being firmly connected together, articulate
with one another, the distal end of the radius being capable ot
rotation round the ulna. When the two bones lie parallel and the
wrist is not bent, the palmar surface of the manus looks inwards,
and when rotated on one another towards the body, backwards : the
former position is spoken of as that of supination, the latter that
of pronation. Indications of these movements are seen even in
climbing Marsupials.1 The radius is the more important in
supporting the hand, while the ulna forms the chief connection
with the humerus. The ulna extends proximally beyond the
elbow joint as the olecranon, on which the extensor muscles are
inserted. The ulna may undergo more or less reduction and fusion
with the radius, so that in some cases only the olecranon is
distinguishable.
In addition to the power of rotation of the forearm, the
Prosimii and Primates proper are characterised by a higher
differentiation of the first finger (pollex), which becomes more
independent and is capable, not only of abduction and adduction,
but also of being brought into opposition with the palm of the
hand to a greater or less extent. As regards the pes, the hallux
even in Marsupials may be opposable, but never as markedly so
as in Lemurs and Monkeys, which are often spoken of as
Quadrumana.2
A brief account of the mammalian carpus and tarsus must
suffice in this place, as considerable differences exist in the various
groups, and there is no consensus of opinion as regards the
homologies of the various components.
The carpus and tarsus most nearly correspond with those of
Urocleles, Hatteria and Chelonians. Primarily the centrale can be
1 The rotation of the radius on the ulna has doubtless come about largely
owing to the gradual increased differentiation of the muscles during phylogeny ;
but this does not sufficiently account for the different relative positions, of the
two bones of the fore-arm and shank respectively. The tibia lies on the inner
side of the shank, while the corresponding bone of the fore-arm, the radius,
owing to secondary shifting, is external when in the position of supination. The
reason of this cannot be due to a rotation of the distal end of the humerus, for
even in Amphibians the same conditions are plainly seen. The crossing of radius
and ulna has rather resulted in consequence of the manus becoming rotated in
a contrary direction to that of the limb as a whole as it extends inwards towards
the body in order to act as a support for the latter. Consequent!}', the originally
parallel position of the two bones of the forearm is not retained, as it is in the
case of those of the shank, in which the rotation follows the same direction as
that of the entire limb.
2 In the Marmosets (Arctopithecini) the thumb is not opposable, and the
opposable hallux is the only digit which bears a flat nail, all the others having
claws. In Ateles the pollex is vestigial and possesses only a single small phalanx,
while in Colobus it may even be wanting. In consequence of the erect position
of Man, and of the foot being used merely as an organ of support and loco-
motion, the prehensile character of the pes has become lost,
170 COMPARATIVE ANATOMY
recognised as a typical element in all pentadactyle Mammals ; but
as a rule it later becomes fused with one, or even with two, of the
neighbouring carpals — generally with the radiale, less frequently
with carpal e 2 or 3. Occasionally indications of a second centrale
are seen, which usually fuses with the intermedium (Homo).
Similar fusion and shifting in relative position may also occur
in other carpals and tarsals (e.g. radiale and intermedium).
The " pisiform " corresponds to an additional ulnar ray, and not
to a sesamoid.
In the tarsus the centrale (navicular) is retained, and is usually
situated on the inner (tibial) border : it may be primarily double.
The astragalus possibly corresponds to the tibiale and intermedium,
and the calcaneum to the fibulare, while the ciiboid represents
tarsalia 4 and 5.
Traces of a " prepollex " and " prehallux " are present in all
pentadacyle Mammals, especially in lower forms, in which they may
each consist of two or more elements : in the higher Mammals
there is never more than one such bone, which usually becomes
fused with its neighbours.1
There are typically five complete digits on each foot, but this
number may be reduced, the disappearance taking place in the
following order, — 1, 5, 2, 4 : thus in the Horse the third is the only
complete digit remaining (Fig. 134). The number of phalanges
is similar in both hand and foot : in the first digit there are only
two, while in the others there are three. An exception to this
rule is seen in Cetacea, in which the phalanges are numerous.
The short humerus is enclosed in the body-wall in Toothed
Whales, which possess five digits, the fourth of which commonly
bifurcates in the embryo ; Whalebone-Whales possess only four
digits.
It is interesting to trace the reduction which has taken place
in the feet of the true Ungulates in the course of time. This
order has been undoubtedly derived from that of the Carnivora,
the fossil Condylarthra from the American Eocene and the tri-
tubercular Creodonta from the Cretaceous forming connecting
links between the two. In the Eocene, the Ungulata vera
diverged into two groups, the Perissodactyla (Tapir, Rhinoceros,
Horse) and Artiodactyla (Pigs, Hippopotami, Ruminants). In
Fig. 134 sketches of the stages in the phylogenetic development
of the fore-foot of the Horse are given, showing how it has been
gradually derived from a tetradactyle form : the embryo passes
through these stages in the course <>f its development. While in
this case the third digit becomes greatly enlarged relatively
1 Different views have been expressed as to the morphological nature of the
prepollex and prehallux, which m consequence of functional adaptation may
undergo further development in some Mammals (e.g. Talpa). It is not possible
in all cases to make a satisfactory comparison between individual elements of
the carpus and tarsus, or tohomologise the so-called "accessory elements."
LIMBS
171
(verissodactylc form},1 and eventually is the only complete one
remaining, in cloven-footed Ungulates the third and fourth digits
8
c
FIG. 134.— FORE-FOOT OF ANCESTRAL FORMS OK THK HORSE. 1. OROHIPPUS
(Eocene). 2. MESOHIPPUS (Upper Eocene). 3. MIOHIPPUS (Miocene).
4. PROTOHIPPUS (Upper Pliocene). 5. PLIOHIPPUS (Uppermost Pliocene).
6. EQUUS.
are both functional and equally strongly developed (artipdactyU
form, Fig. 135) ; their metacarpals may be united with one
another and with the vestiges of
the proximal ends of the second
and fifth to form a " cannon-
bone," while the other digits are
gradually reduced. A similar re-
duction takes place in the hind-
foot, and is here as a rule more
rapid.
The Protungulata must origin- ?
ally have been pentadactyle and
plantigrade (i.e. the whole foot
rested on the ground) or semi-
plantigrade, with ungual phal-
anges but little broadened. On
the gradual elongation and
straightening out of the limbs
and unequal development of the
digits, they become digit-igrade
(as in most Carnivora), and
eventually unguli grade, only the J t/l\.
hoofs at the extremity of the
distal phalanges bearing the
weight of the body.'
The Tylopoda, as well as Ele- OFF
phants (Subungulata) have not F[(, 135.— SKELETON OF THE LEFT
reached the unguligrade stage: FORE-LIMB OF A, PIG ; B, HYO-
they are practically digitigrade, a MOSCHUS ; C , TRAOULUS ; D, ROE-
J . i. j i VUCK ; E, SHEEP ; F, CAMEL.
large integumentary pad or sole (From Bell aftei. , ,.m.0(L ,
(t-f. Fig. 24), from which the small
" hoofs" project, bearing the main weight of the body (Fig. 13(i).
Some of the many other adaptive modifications of the limbs in
1 The Tapir has four digits on the fore-foot and three on the hind-foot ; the
Rhinoceros has three on ea.ch foot,
172
COMPARATIVE ANATOMY
Mammals must also be briefly mentioned. In Bats, the phalanges
are greatly elongated to support the wing-membrane ; the fore
limbs are modified for digging in certain Mammals (e.g. Echidna,
Mole) ; and in the Cetacea (cf. p. 170) and Sirenia the digits are
not free, and servre as supports
for the fin-like paddles. Hind
limbs are absent in the two last-
mentioned orders (cf. p. 155),
but indications of them can be
seen even externally in very
young embryos of the Porpoise.
In the leaping Jerboa (Dipus),
the metatarsals are much elon-
gated, and may even become
ankylosed, as in Birds.
A bony knee-cap or patella,
such as occurs in certain Lizards
(e.g. Varanus) and in Birds, is
present in most Mammals, being
wanting only in Cetacea, Sirenia,
Cheiroptera, and some Marsu-
pialia. It has no genetic con-
nection with the bones of the
136. -LONGITUDINAL SECTION thigh and shank, and so is in no
OUGH THE M.ANUS OF THE LLAMA 1 i - , i , , ,
(Am-hema). (After M. Weber.) waJ comparable with the oleo-
cranon of the ulna, as was
1, metacarpal ; 2, 3, 4, phalanges; 5, so- fnrmpr]v *,irmriopr] T> i•
also the phalanges are more vertical), individual joints of the digits,
which has arisen m the tendon
of the quadriceps femoris muscle in consequence of the friction
between this tendon and the condyle of the femur.
THROU
C. MUSCULAR SYSTEM.
THE muscles, commonly spoken of as " flesh," may be divided
into two groups, according to the histological character of their
elements, which consist of cells elongated to form contractile
fibres : namely, into those with smooth and those with transversely-
striated fibres. The former are phylogenetically the older, and are
to be looked upon as the precursors of the latter. The action of
both in causing movements is dependent on the nervous system, a
nerve entering each muscle at a definite point.
The smooth or involuntary muscle-fibres preponderate in the
viscera, derm, and vessels, and are not under the control of the
will ; the striated muscles occur chiefly in the body-walls and
organs of locomotion, and are almost without exception under the
control of the will (voluntary muscles).1 The following general
statements refer exclusively to the latter kind of muscles, which
may, according to their mode of development, be arranged in the
following groups :—
a. Muscles of the trunk, including the
coracohyoid of Fishes ( = sterno-
hyoid) and its derivatives in
I. Parietal muscles de-
rived from the meso-
dermic somites.
higher Vertebrates : these repre-
sent the oldest and most primitive
part of the muscular system.
b. Muscles of the diaphragm.
c. Muscles of the extremities.
d. Eye-muscles.
II. Visceral muscles, de- (Cranial muscles, with the exception
rived from the lateral -j of those included under a and d
plates of the mesoderm. [ above.
In its simplest form an origin, a belly, and an insertion, may be
distinguished in each muscle. The muscles of the trunk are as a
1 Exceptions are seen in the muscles characteristic of the heart, and in
those of the alimentary canal in the Tench. More or less of the anterior part
of the digestive canal may contain striated fibres in various Vertebrates.
174
COMPARATIVE ANATOMY
A
MUSCULAR SYSTEM 175
FIG. 137.— A— C. (After P. Buffa.)
A, Diagram showing the various phases in the movement of the scutes in Snakes.
a and b, two consecutive scutes ; c, the intervening integument ; d, fixed
point at free margin of scute ; e, distance along which the scute a is moved ;
A, resting stage; B, stage in which n is raised and in which there is the
greatest forward extension of the skin (c), while the free margin of the scute
catches against the ground ; 6', stage in which the scute a again takes on a
horizontal position, the skin (<•) shows the greatest backward extension, and
the scute l> is moved forwards along the distance e.
15, Semi-diagrammatic figure of a longitudinal section through the ventral and
lateral parts of the skin of Tropidonotiis natrix, and of the costo-cutaneous
muscles in connection with the rib. c, rib ; c.c.i, r.r.,s, inferior and superior
costo-cutaneous muscle ; m.c.i, intrinsic musculature of the skin ; .s.?.i-/i i-
11 Mlllllllllll'll
gland
cloficce
— — C/oara
A, ventral view of male Or-
niihorhynchus ; B, ventral
view of male Echidna ; C,
lateral view of the head
and neck of Echidna, all
the muscles shown in which
are supplied by the facial
nerve.
Sphincter colli
Fid. 138. — A— C, THE INTEGUMENTARY MUSCLES OF MONOTREMES. (After Huge.
N
178
COMPARATIVE ANATOMY
ment in Mammals, and exhibits numerous modifications in passing
from Monotremes to Man. In lower forms (Monotremes, Fig. 138,
as well as, e.g. Dasypus, Centetes, Erinaceus, Pinnipedia, &c.),
it extends over the trunk and limbs (panniculus carnosus), while
in Primates it becomes reduced, and confined essentially to the
neck (jplatysma myoidcs) and head (mimetic muscles) : these
muscles are closely related genetically, and are all supplied by the
facial nerve. Two layers can be distinguished in the platysma
(Figs. 138 and 139), the more superficial of which has an oblique
or longitudinal direction, while the deeper layer (sphincter colli] is
circular : the two layers together correspond to the sphincter colli
MJevatwlqbii
M.orMo auric. M. helms
M.oiit.ocuH \ M.auric.sup /
\. M.maiicli-
bulo-miricnl
Via. 139. — SUPERFICIAL FACIAL MUSCLES OF Lepilcmin- •mnxtefiint.t. The deep
layer is recognisable on the neck. (After Huge.)
of the Sauropsida. They are continued on to the head, and there
give rise to a number of new muscles which are mainly grouped
around the eye, mouth, nose, and ear (Fig. 139). These mimetic
muscles are most highly differentiated in Man, but at the same
time reduction or tendinous transformation of certain of them
takes place, and some disappear entirely.
The action of the integumentary muscles is very varied in
different Vertebrates. It may serve to roll up the body into a ball
(e.g. Hedgehog, Armadillo), or aid in the movements of the limbs
and tail in swimming (e.g. Ornithorhynchus), or serve to erect the
MUSCULAR SYSTEM
179
integumentary spines (e.g. Echidna) ; or may cause local movements
(" twitching ") of the skin (many Mammals).
Parietal Muscles.
A. Muscles of the Trunk.
In Amphioxus the body muscles are made up of a series (60 or
more) of lateral muscular segments or myomercs separated by
> -shaped connective-tissue septa or myocommas, between which
the fibres run longitudinally. The myomeres have an alternating
Mo
RM
D
Fi<;. 140.— THE MUSCULATURE OF LARVAL AMKLYSTOMA (AXOLOTL).
the side.
From
CV>, external ceratohyoid muscle ; Oph, cervical origin of the constrictor of the
pharynx ; Cu, cucullaris ; I), dorsal, and V, ventral portion of caudal
muscles ; Dg, digastric ; Z>.s, dorsalis scapulre ; LI, lateral line ; Lf, latissiinus
dorsi ; Lr, levator arcuum branchialium ; ttt> levator branchiarum ; Ma,
masseter ; Mr, myocommas between the myomeres of the dorsal portion of
the lateral muscles ; Mh\ mylohyoid (posterior portion) ; 0, superficial
layer of the external oblique muscle, arising from the lateral line, and ex-
tending to the fascia, F; at * a piece of this layer is removed, exposing the
deeper layer of this muscle (Oh) ; at Re the oblique fibres of the latter pass
into longitudinal fibres, indicating the beginning of the differentiation of a
rcctus abdominis ; at Re1 the rectus-system is seen passing to the visceral
skeleton ; J^h, procoraco-humeralis ; RM, dorsal portion of lateral muscles of
the trunk ; SS, suprascapula ; T, temporal muscle ; Th, thymus.
arrangement on the two sides. On the ventral region of the
anterior two-thirds of the body is a thin transverse sheet of fibres.
In Fishes the myomeres and myocommas, arising exclusively
from the mesodermic somites (p. 9), have a zigzag arrangement on
N 2
180
COMPARATIVE ANATOMY
either side of the body, each of the former consisting, in its simplest
condition, of dorsal and ventral portions, separated from one another
FIG. 141. — THE MUSCULATURE OF LARVAL AMBLYSTOMA (AXOLOTL). Ventral
Add, adductor arcuum branchialium ; C, constrictor arcuum branchialium ; Cbb,
coracobrachialis brevis ; Ce, Ci, Cil, external and internal ceratohyoid : the
former is inserted on to the hyoid (Hy) ; do, cloaca; Cph, portion of the
constrictor of the pharynx, arising from the posterior branchial arch ; Dp,
depressores branchiarum ; Gh, geniohyoid ; La, linea alba ; Mh, J/A1,
anterior and posterior portions of the mylohyoid, which is cut through in
the middle line, and removed on the left side, so as to show the proper
visceral musculature ; 0, superficial layer of the external oblique, passing
into the fascia, which is shown cut through at F ; Oh, second layer of
the same muscle ; Ph, claviculo-lmmeralis ; Spc, supracoracoideus ; Re, rectus
abdominis, passing into the visceral musculature (sternohyoid) at Re1, and
into the pectoralis major at P.
MUSCULAR SYSTEM 181
by a connective tissue septum extending from the axial skeleton
to the integument at the region of the " lateral line " l (cf. Fig. 140).
The myomeres meet together in the mid-dorsal and mid-ventral
lines, and constitute the great lateral muscles of the trunk.
This primitive metameric arrangement of the lateral muscles of
the trunk forms a characteristic feature in Vertebrates, and stands
in close relation with the segmentation of the axial skeleton and
spinal nerves, the number of vertebral segments and pairs of
nerves corresponding primitively to that of the myomeres.
The lateral muscles largely retain their primitive relations in
Fishes, but on the ventral side of the trunk, where they enclose
the body-cavity, certain differentiations occur which indicate the
formation of the recti and obliqui abdominis of higher types. The
dorsal portions of these parietal muscles, as well as the ventral
portions in the caudal region, retain a more primitive condition.
Amphibians. — In Urodeles (Figs. 140 and 141) primary and
secondary ventral trunk-muscles can be distinguished, and both of
these groups, like the dorsal muscles, are segmented. The former
consists of internal obliques, arising directly from the muscle-plate
of the somite, and of external obliques developed from the ventral
border of the myomeres ; the obliqui towards the ventral middle
line are connected with the rectus abdominis.
The secondary muscles arise by delamination from the primary,
and give rise to a superficial external cb/iquc, a superficial rectus, a
transversalis, and a sulvcrtcbralis. These, however, only attain im-
portance in caducibranchiate forms, in which they become marked
during metamorphosis, and the primary musculature then under-
goes more or less reduction. Thus various conditions of the ventral
musculature are found amongst Urodeles.
In the broad-bodied Anura, on the other hand, both primary
and secondary muscles present a marked uniformity and relative
simplicity ; in the adult they give rise to a segmented rectus, in
part passing into a sternohyoid, a non-segmented obliquus externus,
and a transversalis, as well as to a cutaneus abdominis derived
from the external oblique. No trace of an internal oblique can be
seen in the adult.
Reptiles. — In Reptiles, the lateral muscles of the trunk attain
a much higher grade of development. This is to be accounted for
by the more perfect condition of the skeleton, more especially of
the ribs and pectoral arch. The ribs and intercostal muscles now
play an important part in respiration, and changes, necessitated by
the higher development of the lungs, are thus brought about.
1 This septum is not present in Myxinoids, and is absent in Petromyzon and
Lepidosteus posteriorly to the gills.
182 COMPARATIVE ANATOMY
The ventral muscles of Reptiles represent the primary as well
as. the secondary muscles of Amphibians, though differing in their
further development, in consequence of which and of the course
taken by the nerves, relations of the parts are seen which lead up
to the condition occurring in Mammals. The primitive segmen-
tation may be retained or more or less completely lost, in which
latter case the muscles in question run together to form broad
plates.
The distinction between thoracic and abdominal regions becomes
gradually more plainly marked, and, in addition to the four
muscular layers present in Amphibians, well-marked external and
internal intercostal muscles are present : these are homologous with
the primary abdominal muscles of the last-named Order, as are also
the obliquus profundus (belonging to the system of the internal
intercostals) and the median, deep rectus abdominis. A transvcrsus
is present except in Snakes. A subvertebralis extends from rib to
rib, but is wanting in the lumbar region. A quadratus lumborum
(lumbar portion of the intercostalis) appears first in Reptiles, and
from it a psoas major and psoas minor may become differentiated.
The rectus muscle, which in Amphibia extends anteriorly to
the pectoral arch and is in part continuous with the neck muscles,
is in Reptiles interrupted at the sternum, so that pre- and post-
sternal portions can be distinguished. The rectus abdominis is
always well developed, and may consist of a segmented median and
of unsegmented lateral portions : it is not strictly comparable to
that of Urodeles, and the pyramidalis does not correspond to the
like-named muscle of Mammals.
While no important differentiation is noticeable in the dorsal
portion of the lateral body-muscles in Urodeles, a marked sub-
division of these muscles is seen in Reptiles. In them may be
distinguished a longissimus, an iliocostalis, interspinalcs, scmispinalcs,
iiiidtifidi splenii, and levatores costarum, together with the scaleni,
which belong to the last-mentioned group.
The muscles of the main part of the tail retain primitive
relations similar to those seen in Fishes : at the root of the tail
and in the cloacal region, however, new muscles become differ-
entiated, viz., the ilio-,ischio-, -Andpubo-catula/is and muscles of the
anus (already indicated in Anura) and generative organs.
Birds. — In Birds the primitive character of the trunk-muscles
has disappeared far more than in Reptiles. This is mainly to be
accounted for by the excessive development of the muscles of the
anterior extremity — the pectoralis major l more particularly — and
the corresponding backward extension of the breast-bone.
External and internal oblique muscles are both present in the
1 The relative size of the pectoralis major does not always correspond to
the power of flight. It is very compact in Carinatas, and contains elements
corresponding to the pectorales major and minor of Man.
MUSCULAR SYSTEM 183
abdominal region, but only slightly developed : this is more par-
ticularly true of the internal oblique, which appears to be under-
going degeneration. No trace of a transversalis can be distinguished
in the abdominal region, but, on the other hand, a distinct, paired,
unsegmented rectus is present, reduced anteriorly and posteriorly.
External and internal intercostals are well developed, and a
triang-ularis stcrni (last trace of the transversus) appears for the
first time on the inner surface of the sternal ends of the ribs.
The dorsal portion of the trunk-musculature is only slightly
developed in the region of the body, though very strongly marked
in the neck.
All these modifications in Birds seem to be accounted for by
the specialisation of the mechanisms for flight and respiration, to
assist which the greatest possible number of muscles are brought
into play, and thereby influence the whole organism : an essential
difference is thus brought about between Birds and Reptiles.
Mammals. — In general, there is a reduction of the ventral
musculature in Mammals. Three lateral abdominal muscles are
always present, an external and internal oblique and a transversalis.
In many cases, more particularly in Tupaia and in Lemurs, the
external oblique possesses tendinous intersections, thus indicating-
its primitive segmental character ; but in general all these muscles
consist of broad, uniform sheets. Towards the middle line, they
pass into strong aponeuroses which ensheath the rectus abdommis
The latter consists of a single band on either side and possesses a
varying number of myocommas ; it is no longer connected with the
axial muscles of the neck belonging to the same system (sterno-
hyoid, sternothyroid, &c.) as is the case in Urodeles, for the sternum
is always interposed between them, as in the Sauropsida. It, how-
ever, may occasionally (e.g. in lower Primates) reach as far forwards
as the region of the first rib : in higher forms it becomes more pi-
less shortened, the greatest loss of myomeres being seen in
Anthropoids and Man, in connection with the development and
relations of the great adductor (pectoralis major) of the fore limb.
In Monotremes and Marsupials, the strong pyramidaiis muscle
lies on the ventral side of the rectus abdominis. It arises from
the inner border of the marsupial bones (p. 155) and may
extend forwards as far as the sternum. In the higher Mammals,
in which marsupial bones are wanting, the pyramidaiis usually
becomes greatly reduced or entirely lost. Traces of it are, how-
ever, commonly to be met with even in the Primates, arising from
the anterior border of the pubis, right and left of the middle line.1
The external and internal oblique muscles are represented in
the thoracic region in Mammals, as in the Sauropsida, in the form
1 A sphincter marsupii muscle is developed in connection with the marsupixim
(Figs. 28 and 138).
184 COMPARATIVE ANATOMY
of external and internal intercostals.1 The subvertebral muscle is
represented by a longus colli and rccti capitis antici. What
has been said above with regard to the quadratus lumborum
and to the differentiation of the dorsal portion of the trunk-
muscles in Reptiles applies essentially also to Mammals, in which
also the metamerism of the dorsal body-wall is retained longer
than that of the ventral.
In the caudal musculature, flexors, extensors, and abductors
may be distinguished, and their degree of development is pro-
portional to that of the tail : in Man, for example, they become
reduced, and some of them (the pubo- and ilio-coccygeus) have
undergone a change of function, giving rise to the levator ani or
" pelvic diaphragm," consisting morphologically and phylogenetic-
ally of three portions (pubic, ischiatic, and iliac).2
B. Muscles of the Diaphragm.
The formation of a diaphragm results from a gradual sub-
division of the coelome (pleuroperitoneal cavity) into pleuro-
pericardial and abdominal portions, and the differentiation of the
serous membranes which line these (pleura, pej'icardiiiin, peritoneum^)
can only be understood in connection with the complicated
development of the primitive urinogenital folds, liver, lungs, and
great veins, and so cannot be dealt with in this place.
From the Sauropsida onwards, a more or less distinct
separation of the pleural and peritoneal cavities is seen. In
Chelonians and Lizards a partition is present between these
chambers, but this is complete only in Crocodiles and Birds.
Subperitoneal muscular elements are present which connect it
with the vertebral column and ribs, but the innervation of these
precludes any homology with the diaphragmatic muscles of
Mammals.3 It is here therefore only a case of analogy ; and it must
be remembered that in the Sauropsida the pericardium lies in the
general peritoneal cavity.
The very variable serrati postici superior and inferior are peculiar to
Mammals above Moiiotremes. They do not form a single layer, but are indepen-
dent of one another, and are derived respectively from the external and internal
intercostals.
2 It is doubtful how far the external sphincter of the anus, the muscles in
connection with the external generative organs, and the transi'trsu-f pcrittci pro-
f a mlus are derivable from the original sphincter cloacre of the Amphibia and
Sauropsida. In Mammals the pubo-coccygeux (or the pubic portion of the levator
ani), as well as the Mjilniu'tur ani c.i-fi-.niHx and bnlbo- and ischio-cavernosi, are
considered to represent separate portions of the integumentary muscle which
primarily extended over the greater part of the trunk.
3 Amongst the Amphibia (Rana) fibres from the transversus which extend on
to the gullet have been compared to a diaphragm, but the relations are here
quite different to those of the muscles of the mammalian diaphragm, in the forma-
tion of which the rectus abdominis plays an important part. In Birds, two
entirely different structures have been described as diaphragms (cf. under
Air-sacs).
MUSCULAR SYSTEM 185
A complete diaphragm dividing the coelome into thoracic and
abdominal cavities occurs only in the Mammalia. It is dome-
shaped and muscular, its muscles arising from the vertebral column,
ribs, and sternum. The diaphragm is of great importance in
respiration, as it allows of a lengthening of the thoracic cavity in
a longitudinal direction. It is supplied by paired phrenic nerves
arising from one or more of the cervical nerves (usually the 4th or
4th and 5th, but varying from the 3rd to the 8th) ; and is per-
forated by the oesophagus, aorta, postcaval and azygos veins,
thoracic duct, &c. In most cases it consists of a central tendon
from which muscular fibres radiate to the periphery and form
dorsally two strong " pillars " of the diaphragm. In some Mammals
(e.g. Echidna, PhocEena) the diaphragm is entirely muscular : in
the higher Primates the central tendon unites secondarily with
the pericardium.
The nerve-supply of the diaphragm indicates a polymeric
origin from the ventral portions of several myomeres. In the
course of development, it, like the pericardium, becomes shifted
backwards. The first rudiment of the diaphragm (" septum
transversum ") is composed of connective tissue into which the
musculature extends secondarily, and is situated ventrally on
either side of the median line: eventually it becomes closed in
laterally and posteriorly.1 It is important to note that in the
innervation, as well as the grouping of the muscles, a costo-
sternal and a lumbar portion can be recognised in the mammalian
diaphragm.
Although in many respects the mode of evolution of the
mammalian diaphragm still requires elucidation, it is at any rate
certain that a close connection exists between its development and
that of the thorax and the changed respiratory conditions. The
diaphragm acts as an important respiratory muscle, and also aids
the abdominal muscles in the compression of the abdomen.
c. Muscles of the Appendages.
All the muscles of the appendages of Vertebrates are primarily
to be looked upon as derivatives of the ventral muscles of the
trunk, i.e., of the myomeres. This is indicated, apart from the
nerve-supply, by their mode of development in numerous Anamnia,
although in the Amniota the primitive mode of formation is not
clearly recognisable owing to an abbreviation of development.
Two principal groups of appendicular muscles may always be
distinguished : one lying in the region of the pectoral and pelvic
arches, dorsally and ventrally, the other in the free extremity. In
1 ri-
This mode of formation can be recognised not infrequently in those terato-
logical cases in which the costal and lumbar portions of the diaphragm do not
become united.
186 COMPARATIVE ANATOMY
Fishes the latter group consist essentially of elevators, adductors, and
depressors of the fins, and these again may become differentiated
into several layers. From the Amphibia onwards, in correspondence
with the more highly differentiated organs of locomotion, consider-
able complication is seen, and there is a much more marked separa-
tion into individual muscles corresponding with the different sections
of the extremity. Thus elevators, depressors, rotators, flexors, extensors,
protractors, retractors, abductors, and adductors are present in con-
nection with the pectoral and pelvic arches, the upper arm and
thigh, forearm and shank, and hand and foot : the digits are
also moved by a highly-differentiated musculature. The number
of muscles gradually increases in passing from the Urodela through
the Sauropsida to the Mammalia, and greatly influences the form
of the skeleton.
The most important muscles of the shoulder, the origin of
which from the trunk gradually becomes broader in the higher
forms, are the cucullaris, the stcrnocleidomastoideus (belonging
morphologically to the cucullaris, and, like it, supplied by the
spinal accessory nerve), the rhomboidci, and the Icvator anguli
scapulae : these act as rotators, protractors, and retractors of the
scapula.
The muscles connected with the pelvic arch cannot all be
looked upon as the serial homologues of those of the more movable
shoulder, for in many respects the different mechanical relations
of the hind limb have caused modifications in the muscles. Thus,
representatives of the Icvator anguli scapukv, rhomboideus, and
sc.rratus magnus are not present.
A much greater similarity — especially marked in Urodeles—
exists between the muscles of the free portions of the fore and
hind limbs. In correspondence with the fact that the angle formed
by the upper and middle sections points in opposite directions in
the pectoral and pelvic limbs, the extensor muscles of the former
are on its posterior border, and those of the latter on its anterior
border, while the flexors have the converse arrangement. From
the latter the pronators have arisen : these are more specialised
in the fore limb than in the hind limb. The supinaturs originated
from the extensors.
A very varied differentiation of the individual layers of muscle
takes place in different Vertebrates in connection with the shank
and foot, as well as the fore-arm and hand. The degree of differ-
entiation of the muscles in question in general corresponds to the
functional specialisations of the foot and hand, and is most marked
in the hand of Primates, more especially of Man.
D. The Eye-Muscles.
(These will be dealt with in connection with the organ of
vision.)
MUSCULAR SYSTEM 137
Visceral Muscles.
Fishes. — The visceral muscles of Fishes l have been most
satisfactorily investigated in Elasmobranchs, and are classified by
Fiirbringer as follows :—
A. Cranial or cerebral muscles (consisting originally of trans-
verse or circular fibres) supplied by the Vth, VIIth, IXth,
and Xth cerebral nerves.
1 . Constrictor arcuum visceralium, incl. constrictor superficialis
dorsalis et ventralis.
Innervation,
Levator labii superioris ~\
,, maxilke ,, V.
, , ] >alpebne nictitantis 2 J
,, rostri ^
,, hyomandibularis VT1
Depressor rostri j
,, mandibularis et hyomandibularis J
Interbranchiales IX, X.
Trapezius X.
2. Arcuales dorsales IX, X.
3. Adductores, iitd. adductor mandibulse ... V.
and abductores arcuum branchialium . . IX, X.
It. S/rinal muscles, originally longitiid- ^ c • • -,
• i j j- -j j 1-1 .a. j.1. f opino-occipital 6 and
inal, and divided, like the other > - ,
11 I spinal nerves,
trunk-muscles, into myomeres. J
(«) Epibranchial spinal muscles, dorsal to visceral skeleton.
4. Subspinalis Spino-occipital nerves.
T Spino - occipital nerves,
5. Interbasales as well as the first
spinal nerve.
(b} Hypobranchial spinal muscles, ventral to visceral skeleton.
r ri • , ( Spinal nerves, and partly
b. Uoraco - arcuales. me/, coraco- \ r, , , J
•i i-i i -j the last one or more
branchiales, coraco - hyoideusX c ,, • • ., ,
and coraco-mandibularis . / °f the spmo-occipital
^ nerves.
In the Ganoiclei, Dipnoi, Teleostei, Amphibia, and Amniota
there are no epibranchial spinal muscles, and the hypobranchial
muscles have a very different form from those of Elasmobranchs :
in Teleosts, for instance, they are much simplified. In Amphibians,
as already mentioned, the rectus system of the trunk is only
In Cyclostomes there is a remarkable transformation of the cranio-visceral
musculature in correspondence with their peculiar cranial skeleton (suctorial
apparatus) and branchial basket. It is covered over secondarily by the trunk
muscles.
This muscle lias nothing to do with the other eye-muscles.
1 These are spinal nerves emerging from the occipital region of the skull (cf,
under Nervous Sj'stem).
188 COMPARATIVE ANATOMY
partially interrupted by the sternum and pectoral arch, and is
continuous with the sternohyoid. These different conditions of
the muscles result from the varied adaptations of the visceral
skeleton and respiratory organs.1
Amphibians. — It is to be expected, a priori, that the muscu-
lature of the visceral skeleton should be more highly developed in
branchiate than in air-breathing Amphibians ; in the former,
more primitive relations are met with, while in the latter a greater
modification, or rather reduction, of these muscles takes place.
The muscles of the hyoid and branchial arches may be divided
into three groups — a dorsal (levatorcs arcuum), a middle (muscles
of the external gills and the external ccratohyoid\ and a ventral
(internal ccratohyoid, subarcuales, and interbranchialis 3 or 4). The
nerve-supply of the dorsal group is strictly branchiomeric ; in the
middle, and still more in the ventral group, this condition is not
retained.
Between the two rami of the lower jaw is situated a muscle
with transverse fibres (the myloliyoid or intermandibular muscle),
supplied by the third division of the trigeminal and the facial
nerve ; this represents the last remnants of the ventral superficial
constrictor muscle of Fishes. As elevator of the floor of the
mouth, it stands in important relation to respiration and deglutition,
and is retained throughout the rest of the Vertebrata up to Man
(Figs. 140, 141).
A continuation of the trunk-musculature (the omo-, stcrno-, and
gcnio-hyoid), provided with tendinous intersections, lies above the
mylohyoid (Fig. 141). These muscles, which serve to pull the
visceral skeleton forwards and backwards, are supplied by the first
and second spinal nerves.
In contrast to Fishes, there is in Amphibians a definite differ-
entiation into muscles of the tongue, that is, into a Jii/oglossns and
a genioglossus ; these also must be considered as originating from
the anterior end of the ventral muscles of the trunk ; they are
present in all Vertebrates from the Amphibia onwards, and are
supplied by the hypoglossal — that is, the first (or second, Anura)
spinal nerve.
In the Perennibranchiata and in Salamander larvae the muscles
of the hyoid and of the visceral arches may, by analogy with Fishes,
be divided into a ventral and a dorsal group ; the latter disappears
in adult Salamanders and Anurans, only the ventral persisting.
Their function is to raise and depress the branchial arches, as well
as to draw them forwards and backwards. To these may be added,
in branchiate forms, levators and adductors of the external gills
(Figs. 140 and 141). They are innervated by the vagus and
glossopharyngeal.
1 The visceral muscles of Polypterus are of especial interest, as they present
an intermediate condition between those of Elasmobranchs and Urodeles.
MUSCULAR SYSTEM 189
The jaw-muscles may be divided into a depressor (digastric, or
biventer mandibulcc, which here has only a single belly, Fig. 140),
supplied by the facial nerve, and into several elevators of the lower
jaw (masseter, temporal, and pterygoid muscles), supplied by the
third division of the trigeminal. The last-mentioned muscles may
be derived from the adductor of the mandible of Elasmobranchs,
and the biventer from the portion of the superficial constrictor of
Fishes which passes to the lower jaw : it arises from the same
matrix as the platysma, and serves to open the mouth.
Amniota. — With the simplification of the visceral skeleton in
Amniota there is a considerable reduction of the musculature
belonging to it. All muscles connected with branchial respiration
are of course wanting, and the ventral trunk-muscles, as mentioned
above, are always interrupted in their forward extension by the
sternum and pectoral arch. At the same time, the muscles along
the neck and on the floor of the mouth met with in Amphibia are
present here also ; they are the mylo-, sterno-, omo-, and genio-
hyoid, as well as the hyoglossus and genioglossus. To these may
be also added a sternothyroid, and a thyrohyoid continued for-
wards from it.
The stylohyoid, styloglossus, and stylopharyngeus of Mam-
mals, arising from the styloid process and stylohyoid ligament and
undergoing numerous variations, are peculiar to Mammals. They
are supplied partly by the facial nerve, partly by the glossopharyn-
geal, and act as retractors of the tongue and levators of the pharynx
and hyoid.1
The muscles of the jaws resemble those of Amphibia, although,
especially in the case of the pterygoids, they are much more sharply
differentiated into superficial and deep or external and internal
portions, and may become subdivided secondarily (e.g. in the region
of the temporal muscle) : they are throughout more strongly
developed.2
1 For the tensor tympani and stapedius muscles, cf. under Auditory Organ.
The latter muscle, together with the stylohyoid, is possibly derived from the dorsal
portion of the deep constrictor layer of Fishes which passes to the hyoid, but
more probably corresponds to the ventral portion of this muscle.
- An anterior belly of the biventer appears in Mammals in consequence of a
shifting of the superficial layer of the mylohyoid, the fibres of which are originally
transverse. Its connection with the tendon of the posterior belly is therefore
secondary, as are also the relations of the mylohyoid to the hyoid bone.
D. ELECTRIC ORGANS.
ELECTRIC organs arc present in some Fishes, being most
strongly developed in certain Rays (Torpedinidte. e.g. Torpedo,
Hypnos) found in the Atlantic Ocean and various southern seas,
in a South American Eel (Gymnotus ckdricus) and in an African
Siluroid (Malopterurus electricus). Gymnotus possesses by far the
strongest electric power, next to it comes Malopterurus, and then
Torpedo. The electric batteries of these three Fishes are situated
in different parts of the body : in
Torpedo they have the form of a
broad mass, extending throughout
the substance of that part of the
body lying between the gill-sacs
and the propterygium on either
side of the head (Fig. 142); in
Gymnotus they lie in the ventral
region of the enormously long tail
(Fig. 143), that is, in the position
usually occupied by the ventral por-
tion of the great lateral muscles ;
and finally, in Malopterurus, the
electric organ extends between the
skin and muscles round almost the
entire circumference of the body,
thus enclosing the Fish like a
mantle : it is especially strongly
developed along the sides, but is
separated by the branchial apparatus
Fio. 142. -- Torpedo marmorata, into dorsal and ventral portions.
WITH THE ELECTRIC ORGANS (E) The electric power of tiloso
EXPOSED. „. , , . , ,, , ,
Wishes which were formerly known
Au, eye ; KK, gill clefts ; s, skull ; as « pseudo-electric " has now been
fully demonstrated, though it is
much feebler than in the forms described above. To this category
belong, e.g. all the Rays, excluding Torpedo, and the various
species of Mormyrus and Gymnarchus (both belonging to the
Teleostei). In all these the electric organs lie on either side of the
end of the tail and have a metameric arrangement like that of the
caudal muscles ; in the Mormyridse, for example, there is on either
ELECTRTC ORGANS
191
A
side an upper and lower row of electric organs. In addition to the
Fishes here referred to, electric organs have also been described
in other Teleosts (e.g. Astroscopus).
With the possible exception of Malopterurus, in which the
electric apparatus is said to be derived from the epiderm, the
electric organs of Fishes
consist of metamor-
phosed striated muscu-
lar fibres, and the nerve-
endings belonging to
them are the homo-
logues of the motor end-
plates which are ordin-
arily found on muscles.
As regards the struc-
ture of the electric
organs, the same essen-
tial arrangements are
met with in all. The
framework is formed of
fibrous tissue enclosing
numerous cells, which,
running partly longi-
tudinally, partly trans-
versely through the
organ, gives rfse to
numerous polygonal or
more or less rounded
chambers or compart-
ments. These latter are
arranged in rows, either FIG. 143, A and B.— THE ELECTRIC ORGAN OF
along the longitudinal Gynmo/ux elu-trim*. (B, from a preparation by
axis of the body (Gym-
notus, Malopterurus) or A, anus ; DM, DM1, dorsal portions of the great
Ff
lateral muscles, seen partly in transverse, partly
in longitudinal, section ; E, the electric organ,
seen in transverse section at E (B), and from
the side at E1 ; Ft, fin ; H, skin ; LH, posterior
end of body-cavity ; Sep, median longitudinal
fibrous septum between the left and right
electric organ and lateral trunk-muscles ; VJ\I,
VM1, ventral portions of the great lateral
muscles, seen partly in transverse, partly in
longitudinal, section ; WS, vertebral column
from the side, showing the spinal nerves, and
in transverse section.
iii a dorso-ventral direc-
tion (Torpedo), forming
definite prismatic col-
umns (Fig. 144). The
compartments are filled
with a homogeneous
fluid or semi-fluid sub-
stance, the nature of
which is not thoroughly
understood. It is known
to correspond to modified muscle-substance, and it contains
numerous large, round and oval nuclei, as well as certain highly
refracting bodies.
Numerous vessels and nerves ramify in the connective tissue
192
COMPARATIVE ANATOMY
lying between these compartments, the nerves being enclosed in
thick sheaths, and having a great variety of origin according to
the species of Fish under consideration. In Torpedo, in which
the electric organs probably arise in connection with the great
adductor muscle of the mandible and the constrictor of the gill-
arches, the nerves arise from the " electric lobe " of the medulla
oblongata, a single branch coming also from the trigeminal ; in all
" pseudo-electric " Fishes, as well as in Gymnotus, in which over
two hundred nerves pass to the electric organ, they arise from the
spinal cord, and most probably are in the closest relation with the
ventral cornua, which are particularly well developed in the last-
named Fish/ It is remarkable that the electric nerves of Malop-
terurus arise on either side from a single enormous lens-shaped
nerve-cell, which, situated in the neighbourhood of the second
spinal nerve, is continued into a very large
primitive-fibre, which passes towards the end of
the tail, dividing as it goes. The latter is invested,
by a thick sheath.
It is stated that in all electric Fishes the side
of the electric plate on which the nerve branches
out is negative at the moment of discharge, while
the opposite side is positive. Thus the different
arrangement of the parts in Gymnotus and
Malopterurus renders it clear that the electric
shock must pass in different directions in these
Fishes: in Malopterurus it passes from the head
to the tail, and in Gymnotus in bhe contrary direction, while in
Torpedo the discharge passes from below upwards.
Experiments have shown that all electric Fishes are proof
against the electric current, with the limitation that muscles and
nerves — even the electric nerves themselves — separated out from
the body are capable of being excited by the current. ' The last
and most important question with regard to the electric Fishes
naturally concerns the mechanism whereby the electric plates
become temporarily charged with electricity. The reply to this
question, although probably not so difficult a one as that relating
to the mechanism of muscular contraction, is still far from being
answered" (Du Bois-Reymond). The only thing that can be stated
with certainty is, that the electromotive force is under the control
of the will.
i
FIG. 144.— ELEC-
TRIC PRISMS OF
Torpedo «<"/•-
morata. (Semi-
diagrammatic. )
E. NERVOUS SYSTEM.
The nervous system has the important function of placing the
organism in communication with its surroundings, stimuli received
by the sensory organs being transformed into nerve-impulses which
are conducted along the afferent or sensory nerve-tracks to the
central organ of the system. In the latter these stimuli are
transformed, or new ones are originated, and they travel along the
efferent or motor nerve-tracks to muscular elements, thus causing
their contraction, or to glands, causing them to secrete. The
intimate connection between muscle and nerve has already been
referred to.
It was pointed out in the Introduction that the nervous system
arises from the ectoderm. The parts of it which first become
differentiated histologically are the nerve-cells (ganglion-cells},
from which nerve-fibres arise later and serve as the conductors
of nervous impulses. The most important constituent of the
nerve-fibre is a central neuraxis or axis-fibre, and in those nerve -
fibres which are spoken of as medullated this is surrounded by a
highly refractile, fat-like substance (myelin), which forms the
medullary sheath. In certain (non-medullated) nerve-fibres this
sheath is wanting, but the two kinds of fibres are not sharply
marked off from one another, either locally or genetically : a fibre
may be medullated in one part of its course, and non-medullated
in another. Externally each nerve-fibre is enclosed by a delicate
sheath, the neiirilemma.
Part of the ectodermic tissue which forms the nervous system
of the embryo does not become transformed into nervous tissue,
but gives rise to an epithelial layer (ependyme) and also to a
supporting, connecting, or isolating framework — the neuroglia,
which plays a very important part in the central nervous system ;
externally, investing membranes as well as blood- and lymph- vessels
are formed from the mesoderm. As compared with the central
organs, the peripheral tracks are comparatively poorly supplied with
blood.
The nervous system thus consists of central and peripheral
portions (Fig. 14.5). The central part (brain and spinal cord) is
o
194
COMPARATIVE ANATOMY
XT
Fie. 14.1. — THE ENTIRE NERVOUS SYSTEM OF THE FROG.
From the ventral side.
(After A. Ecker.)
/', facial nerve ; G, ganglion of the vagus ; He, cerebral hemispheres ; 7 to A', first
to tenth eerebral nerves; Lop, optic lobes; M, spinal cord; Ml to J/ln,
spinal nerves, which are connected at SM by branches (ranii communicantcs)
with the ganglia (SI toSlO) of the sympathetic (>sr) ; Ar, nasal sac ; Ni, sciatic
nerve ; No, femoral nei^ve ; o, eye ; Va to Ve, the different branches of the tri-
geminal ; Vy, Gasserian ganglion ; Fx, connection of the sympathetic with the
< iasserian ganglion ; A'l to A'4, the different brandies of the vagus. Some of the
fibres of the sympathetic should be shown accompanying the vagus peripherally.
NERVOUS SYSTEM 195
the first to arise, and is torn KM I MS a direct product of the ectoderm ;
the peripheral portion (<:crcbra/, .y>in«/, and sympathetic wr>rx, and
their ganglia) becomes established later.
1. THE CENTRAL NERVOUS SYSTEM.
The first indication of the central nervous system is a longi-
tudinal furrow (medullary groove, Fig. 6, A) which appears on the
dorsal side of the embryo, and which gradually becomes converted
into a tube by the meeting of its edges ; this tube, consisting
originally of epithelial cells like the ectoderm from which it arises,
then becomes separated from the latter, and gives rise to the
hollow medullary cord ] (Fig. 6, B), in which nerve-cells and fibres
become differentiated ; it comprises a more expanded anterior,
and a longer and more slender posterior section. From the former
arises the brain, from the latter the spinal cord.
In an early stage of development the lumen of the medullary
cord is primitively continuous posteriorly with that of the primary
intestine by the ncurenteric canal, but this connection soon dis-
appears. The cord consists of a cylindrical or more or less flattened
tube, the cavity of which expands in front to form the ventricles of
the brain, and is lined by ciliated epithelium. With the thickening
of the walls of the tube, this cavity becomes greatly reduced, and
in the spinal cord is spoken of as the central canal.
Some of the cells in the brain and spinal cord serve as sensory
centres, others as motor-centres, new centres being added
which complicate the originally simple reflex circuit, and various
other modifications gradually occur in the course of development of
the head.
Membranes of the brain and spinal cord (meninges).
In Amphioxus, the central nervous system is enclosed by an
undirFerentiated investment of connective tissue. In the lowest
Craniata, a differentiation takes place into a primitive meninx,
which closely invests the spinal cord, and a second membrane
(endorachis), which lines the vertebral canal : the latter, formerly
known as the "dura vertebralis," is comparable merely to the
perichondrium or periosteum, and has nothing to do with the
meninges proper. The blood-vessels supplying the spinal cord are
contained in the primitive meninx, the space and tissue directly
external to which may be spoken of as the perimeningeal space and
tissue. This condition is retained in Fishes (Fig. 146, A).
A further process of differentiation takes place in Urodeles
and is more marked in Anurans, reaching a higher stage in Reptiles
and a still higher one in Birds. This process consists in the
1 The cord is at first solid in certain Fishes (e.y. Petromyzon, Lepidosteus,
Amia, Teleostei, Lepidosiren), its cavity appearing later.
O 2
190
COMPARATIVE ANATOMY
appearance of a lymph-space in the primitive meningeal membrane,
dividing it into an outer dura mater spinalis and an inner primitive
pia mater. There is thus a pi /•//e recognised (<./< .>-n.v), from which the paraphysisis not always distinguishable
(cf. Fig. 165). The latter apparently represents a glandular organ, recalling that
connected with the infiindibulum : whether it also includes the vestige of a sensory
apparatus, like the parietal and pineal organs, is doubtful (cf. pp. '20'2 and •2
BRAIN
201
embryonic epithelial character : this is usually regarded as being-
due to regressive metamorphosis, the cause of which, however, is
difficult to explain. The relative distribution of the gray and white
matter differs in various pares of the brain.
Connecting the two lateral halves of the brain are certain
transverse bands of nerve-fibres or commissures. In addition to a
small superior or habenular commissure in the pallium (Fig. 150),
an anterior commissure is present in the posterior region of the
secondary fore-brain, a middle in the primary fore-brain (in
Mammals only), and a posterior in the anterior part of the mid-
brain. In addition to these, others may be developed in the pallial
region (e.g. anterior and posterior pallial commissures, cf. Fig. 165) ;
and amongst Mammals those known as the corpus callosum and
fornix are of great importance.
Mesencephalon
Diencephalon
Pineal- undPftrietal-
/ organ,
,'Zirb
Veuan. tmnsversam
'piihelMe QuErlhlL
Fit;. 150. — DIAGRAMMATIC LONGITUDINAL SECTION THROUGH PART OF THE
EMBRYONIC BRAIN.
Zirbefpolster, pineal cushion.
The outer surface of the hemispheres is more or less smooth,
except amongst the Mammalia, in which fissures (sulci) and convolu-
tions (gyri) may be present. These consist of folds of the entire
pallium or cortex, and they cause a greater or less increase of the
superficial area.
From the primary fore-brain, the ventricle of which is walled
in anteriorly by the lamina terminalis, the following structures also
arise (Fig. 149) : — the optic thai ami, formed as thickenings of its
basal walls, and the ganglia- haJicntt/cc on the posterior lateral
margin of the dorsal region, with the superior conuiiixxurc between
them; the primary optic vesicles, arising as paired ventro-lateral
outgrowths, from which the optic nerves and retina with its pigment
epithelium are derived later; the pineal apparatus, developed as
202
COMPARATIVE ANATOMY
outgrowths of the roof; and finally, the infundibulutn, formed as an
extension of the floor, together with part of the pituitary body
(hypophysis). Another portion of the pituitary body is derived from
the epithelium of the primary oral involution (stomodseum).1
The pineal apparatus consists of the cpiphysis or pineal organ
proper, which persists in a more or less vestigial condition in
all Vertebrates, and of a more anterior outgrowth which may be
called the parietal organ, arising from the epiphysis or inde-
pendently from the roof of the diencephalon, and becoming
atrophied in the majority of Vertebrates. Each of these structures
H
n~
"13 J.
-H
FIG. 151. — DIAGRAM ILLUSTRATING THE STRUCTURE OF THE HYPOPHYSIS OF
VERTEURATKS. A , PETROMYZON ; B, PISCES; G, AMPHIBIA; D, SAUROP-
SIDA ; E, MAMMALIA. (After Sterzi.)
H, " chromophilous " portion, and H1, " chromophobic " portion of hypophysis;
P, infundibular process ; S, saccus vasculosus.
represents a vestigial sensory organ, and may retain to a greater
or less extent the characters of a median eye, which in some cases
has probably a light-perceiving function. Certain facts indicate
that these organs may have been paired primitively or that the
two convspond to members of a pair ; but further researches are
desirable on this point, as well as on the relation of the two organs
1 Possibly the endodermie epithelium nt' the primary fore-gut may also take
part in its format ion.
BRAIN 203
to one another and the nature of certain accessory vesicles
in this region found in certain forms (e.g. Anguis). Both pineal
and parietal organs are in the embryo connected with the brain by
a special nerve or tract which grows out from the organ and
becomes connected with the brain secondarily (cf. Figs. 165 and
168).
As already stated, a nervous and an epithelial portion are to be
distinguished in the hypophysis, the former originating from the
infundibulum, the latter from the epithelium of the stomodteum
(Fig. 151). In Cyclostomes the nervous portion consists of a thin-
walled sac arising from the infundibulum (infundibular sac or
process), which in all the true Fishes is in part thrown into folds
by the invasion of numerous vessels. Thus arises the so-called
saccus vasculosus, the development of which shows great variation
amongst the different groups of Fishes. In the higher Vertebrates
the infundibular process undergoes various modifications, espe-
cially as regards the saccus vasculosus,1 only traces of which may
still be recognisable (e.g. Mammals).
Both the primary and the secondary fore-brain are situated in
the prechordal region of the skull, all the other divisions of the
brain lying in its chordal portion (p. 75). The mid-brain and
medulla oblongata undergo fewer modifications than the fore-brain,
though each optic lobe becomes subdivided into an anterior and a
posterior lobe in Mammals ; only the anterior part of the thin roof
of the medulla (valve of Vieussens) is nervous, while its floor
becomes greatly thickened, and in Mammals gives rise anteriorly
to a transverse band of fibres (pons Varolii). It is important to
note that the greater number of the cerebral nerves arise from the
medulla oblongata. The cerebellum may be more or less distinctly
folded and subdivided into median and lateral lobes.
In the course of further development, the walls of the cerebral
vesicles become more and more thickened, so that their cavities,
transformed into the ventricles of the brain, undergo a gradual
reduction (Fig. 152).
A series of unpaired ventricles (tdocoele, diaccele, mesoccde,
metaccele, and myeloccde) lying in the longitudinal axis of the brain,
as well as paired ventricles, can be distinguished. When cerebral
hemispheres are more or less distinctly developed, the teloccele
gives rise to paired cavities extending into them and known as the
lateral ventricles (= ventricles 1 and 2); each of these communi-
cates with the diacoele or third ventricle (which extends into the
1 Various hypotheses have been put forward with regard to the primary
nature of the hypophysis : it may represent a sensory organ, or may correspond
to the primitive mouth ("palseostoma") of the Protovertebrata, which is to a
greater or less extent represented by the combined unpaired nasal and pituitary
passage of Cyclostomes (see under Olfactory Organ, and Fig. 190) : on the latter
hypothesis the mouth of existing Vertebrates is a "neostoma." It is very
probable, especially in the higher Vertebrates, that the epithelial part of the
hypophysis lias an important function as a ductless gland, which gives off its
secretion into neighbouring blood- and lymph-capillaries.
204
COMPARATIVE ANATOMY
infundibulum) by means of an opening, the foramen of Monro, and
may be continued into the corresponding olfactory lobe as an
olfactory ventricle or rhinoccelc. Each optic lobe also usually con-
tains an optic ventricle, or optoccele, communicating with the
mesocoele (iter or aqueduct of Sylvius). There
may be a distinct metacoele in the cere-
bellum opening into the myelocoele or fourth
ventricle.1
All five cerebral vesicles lie at first in the
same horizontal plane, but in the course of
development a cerebral flexure takes place,
the axis of the vesicles becoming bent down-
wards, so that at a certain stage the mesen-
cephalon forms the apparent apex of the
brain. In Mammals, the parts of the brain
become still further folded on one another,
so that a parietal, a Varolian, and a cervical
bend may be distinguished (Fig. 153): this
process is connected with the further develop-
ment of the skull and the rapid longitudinal
growth of the brain.
In Fishes and Amphibians the cerebral
flexure later becomes practically obliterated,
but it persists more or less markedly in the
Cc, central canal of the higher types, more particularly in Mammals,
spinal cord (R); HH, T 4-U l , r«i 4.1.
cerebellum ; MH, mid- In the latter Class> moreover, the original
brain, which encloses relation of the parts becomes still further
the iter (Aq), communi- complicated by the large development of the
eating between the 1,11 • i_ i.- i i i i
third and fourth ven- cerebral hemispheres, which grow backwards,
tricles; Nil, medulla and thus gradually overlie all the other
oblongata, with the parts of the brain: this condition of things
-fit
'. — i?
n;. 1.V2. — DIAGRAM
THE YENTKICLES OF TUE
VERTEBRATE BRAIN.
OF
each lateral
communicates with the
men of Monro (FM).
hem'i- attains its greatest perfection in Man.
spheres, with the lateral
ventricles (,SF) ; Zff, Amphioxus. — The conical and enlarged
anterior end of "the spinal cord of the Lancelet
ventricle contains a widened portion of the central
canal which must be looked upon as a
ventricle. This opens freely to the exterior
dorsal ly by a neuropore, which represents
the last indication of the primitive connec-
tion of the central nervous system with the outer skin. It is
impossible to say with any degree of certainty to what extent
this " brain " of Amphioxus corresponds to parts of the Craniate
brain.
Cyclostomes. — The brain of these forms remains in many
1 A so-called "fifth" ventricle, lying between the corpus callosum and fornix,
is found in Mammals, but morphologically it has nothing to do with the ventricles
proper, and simply represents a spaee between the thin internal walls (npjitiun
I ii<-iil a in) of the two hemispheres.
BRAIN
205
VH ZH
MH
respects in ;in embryonic condition : a dorsal connection of its two
halves by nervous elements is very incomplete, narrow bridges of
nervous substance only occurring in the primitive hind-brain, the
posterior portion of the mid-brain, the posterior commissure, and
the ganglion habenuhe of the right side. The main part of the
roof consists of membrane and vessels.
In the larval Petromyzon or Ammocoete, very primitive condi-
tions are met with (Fig. 154), and, as is also the
case in the adult, the individual vesicles lie in
an almost horizontal direction one behind the
other ; the telencephalon consists of a median
part and of small paired hemispheres con-
tinuous anteriorly with the larger, rounded
olfactory lobes. The median portion of the
teloccele is continued transversely outwards
into each hemisphere, in which it gives rise
to a lateral ventricle : this is continued for-
wards for a short distance into the base of the
olfactory lobe, as well as backwards into the
hemisphere. The roof (pallium) of the median
portion of the ventricle is non-nervous, and
consists of a single layer of epithelial cells,
which, together with the pia mater, has been
removed in the preparation represented in
Fig. 154, A. The mid-brain and elongated
medulla oblongata are relatively broad, and
the cerebellum is represented by a mere narrow
ledge overhanging the fourth ventricle anteri-
orly. The roof of the mesoccele is formed
mainly by a layer of epithelial cells, and, like
that of the third and fourth ventricles, is
covered by a thickened and vascular portion
of the pia mater, or choroid plexus.
The brain of Myxinoids (Fig. 155) shows
many peculiarities, and the morphology of its
parts requires further investigation. Its sub-
divisions are broader and more closely approxi-
mated than in the Lamprey, and its right
and left halves are more plainly marked off
from one another by a continuous longitudinal dorsal furrow. No
pallium is recognisable, at any rate in the adult. The ventricles
have undergone reduction, and present individual variations : in
the ventral region of the fore-brain there is a small isolated cavity
which probably represents the vestige of a third ventricle. The
broad olfactory lobes are separated from the telencephalon by a
transverse furrow. The diencephalon is not visible from the
dorsal side, but ventrally there is a distinct infundibular process.
The mid-brain is the most prominent division : the mesoccele ends
li
Fia. 153. — DIAGRAM
TO ILLUSTRATE THE
CEREBRAL FLEX-
URE OF A MAMMAL.
HH, metencephalon ;
MH, mesenceph-
alon, which at SB
forms the most
projecting portion
of the brain, re-
presenting the so-
called " parietal
bend " ; NH, my-
eleneephalon, f orm -
ing the "cervical
bend" (NB): the
" Varolian bend "
(BB) arises on the
ventral circumfer-
ence, at the junc-
tion between HH
and NH; R, spinal
cord ; VH, telen-
cephalon ; ZH,
diencephalon, with
the pituitary body
(H) at its base.
206
COMPARATIVE ANATOMY
- L.ol.
NH IV Z.ff
lilt Mil
'
-Lot
B
IT/ r/
L.ol
FIG. 154. — BRAIN OF LARVAL LAMPREY. A, from above ; B, from below;
C, from the side.
, ganglion habenulae ; Op, pineal organ ; HH, cerebellum ; Hyp, hypophysis ;
L.ol, olfactory lobe ; ^f^n the
medulla.1
In Petromyzon the pineal apparatus is represented by t\\"
vesicles, which arc probably the displaced members of a pair, each
connected with the dorsal surface of the diencephalon (ganglion
habenulre) and lying one above the other just beneath the roof of
the skull : the integument immediately above these vesicles is
pigmentless. The cells on the ventral side of the larger dorsal
vesicle (epipkysis OY pineal oryan — probably representing the organ
of the right side) are arranged radially and contain pigment, form-
Tz&zzm&z
" PeUucn'n -
%##?,
"Rctiiin" -i&£f£W-
'4 ' , tff
" PilJi'fiiJn" ~
" Retiii» '
Parapineal
Fin. 156. — TRANSVERSE SECTION OF PINEAL APPARATUS OF Petromy~on
(After Studnicka. )
The connective tissue roof of the skull (.«) is seen above, the roof of the third
ventricle (di) below.
ing a kind of retina, but they show signs of degeneration ; the
lower vesicle or parapineal (left) or^cm, which probably corresponds
to the parietal organ of Lizards (q.v.) is simpler and more variable,
and is without pigment. The pineal stalk, arising just in front of
the posterior commissure, passes directly into the dorsal vesicle,
and the ventral vesicle is also connected with the roof of the
diencephalon (habenular ganglion) by a stalk. In Myxinoids the
pineal apparatus is evidently much degenerated, and nothing is
known of an epiphysis proper.2
Elasmobranchs. — The brain of these Fishes, like that of
Cyclostomes, is in many respects of a specialised form, character-
The homology of the parts as given above has recently been questioned :
it is possible that the division described as the telencephalon corresponds to the
diencephalon, and that the " cerebellum " belongs to the mid-brain.
- For the hypophysis of Cyclostomes, see under Olfactory Organ.
BRAIN
•209
istic or, and confined to, the group, though the particular regions
are much more highly developed than in Cyclostomes : the pallium
FIG. 157.— BRAIN OF Scylltiun <-«)ii<-nla. A, dorsal ; B, ventral ; C, lateral view.
6.0, olfactory bulb; ep, base of epiphysis ; /. 6, telencephalon ;//•, fourth ventricle ;
h.li, cerebellum ; h.p, hypophysis; if, lobi inferiores ; m.b, mid-brain (optic
lobes) ; m.d, medulla oblongata ; *<-, saccus vasculosus ; th, diencephalon ; to,
olfactory tract (very short in Scy Ilium). The epithelial and vascular roof of
the third and of the fourth ventricle has been removed, ii-x, cerebral nerves
(the ventral vagus roots are omitted in B).
is almost exclusively nervous. According to its external form, two
main types can be distinguished. Thus in Spinax, Scymnus, Noti-
210
COMPARATIVE ANATOMY
melcL
danus, and the Holocephali, it is very narrow and elongated, while in
the rest of the Plagiostomi the individual parts are more closely
compressed and approximated together (Fig. 157). In almost all
Sharks the telencephalon is relatively much larger than any of the
other parts. The olfactory lobes arise from the anterior or antero-
lateral ends of the telencephalon, and in some Elasmobranchs
remain in close connection with it : in others, in which the
olfactory capsules are situated further
forwards, they become drawn out into
long olfactory tracts, each arising from
a basal olfactory tubercle and continuous
anteriorly with an olfactory bulb from
which the olfactory nerves arise.
A division of the telencephalon into
paired halves is hardly indicated at all
in Rays, and only slightly so in the
commoner Dogfishes (e.g. Scyllium,
Acanthias), in which, however, lateral
and olfactory ventricles are present
(Fig. 158). Only in Scymnus, and to
some extent in the Notidanidre, is there
a distinct separation of the pallium
into two hemispheres. In Rays there
is only a small unpaired telocoele, the
telencephalon consisting of a practic-
Tf ni -7 ally solid mass, and the olfactory lobes
bio. Io8. — BRAIN or (Jlieilo- •> ' . .. J. . ,
scyllium, (From Parker and are also solid ; in the Myliobatidse
there is no trace at all of a telocoele.
The narrow diencephalon is roofed
over by a choroid plexus, and the tube-
veiiLricies removeu so as to i -i ' • i • / • m i \
show the relations of the llke epiphysis (wanting in Torpedo)
cavities (semi-diagrammatic), may reach such a length as to extend
r, dilatation from which the beyond the anterior end of the brain
metacoele is given off; dia, for a considerable distance, and pass
diaccele— the reference line distally into or beyond the roof of the
points to the opening leading i 11 . inrliontinn pan hp CJPPTI nf a
• , fl i * f* I'l 1 • V oK-llll . 11O lllLUL/dulUil Cfill Ut; otJtJll Ul tl
iter(mesocoele), into which the parietal organ. A pair of small lobes
optocu?les (opt) open; met a, —the lobi infcriorcs — are present on
, • i ^ -\ • , utiG iiiiiiiicti on 1 uiii , nnci ti sa ecus /L''(tscfit~
of teloccele ; /-A,' rhinoccele. losus or infundibular gland, surrounded
by a blood sinus, is present on the
sides and floor of the infundibulum, with the ventricle in which it
communicates and with which a pituitary body is connected
posteriorly.
The cerebellum is a.lwa\*s very large, overlapping the optic
lobes and medulla oblongata to a greater or less extent : it is
divided into several lobes lying one behind the other, and
usually contains a metacoele opening into the fourth ventricle.
Haswell's Zoology. )
Viewed from the dorsal side,
and the roofs of the various
ventricles removed so as to
Cf)
BRAIN
211
olf.l
c-.h
prs
In Sharks, especially in Scymnus and the Notidanidse, the
medulla oblongata is elongated and cylindrical, while in Rays it is
more compressed and triangular ; at its anterior end are a number
of elevations corresponding to origins of the nerves arising from
the gray matter of the floor of the fourth ventricle in this region.
In electric Rays a pair of electric lobes (p. 192) are present at this
point, and these enclose a mass of giant nerve-cells.
Ganoids. — The pallium covering the median teloccele consists
mainly or entirely of epithelial and connective tissue elements, much
as in Cyclostomes ; and the telencephalon,
which may be produced dorso-laterally into
lobes (Fig. 159), gives rise anteriorly to
cerebral hemispheres containing lateral ven-
tricles and continuous with the olfactory lobes.
The well-developed diencephalon has a
marked ventral flexure, and from its roof
arises a strong pineal peduncle, the distal
end of which extends into a hollow in the
cranial roof, but undergoes atrophy in Amia,
becoming completely separated off from the
brain.1 Well-marked lobi inferiores are pre-
sent, and the hypophysis and saccus vascu-
losus are voluminous : the latter consists
largely of glandular tubules which open into
the mfundibulum, as in Elasmobranchs.
The optic lobes are well-marked in most
Ganoids. The large cerebellum gives rise to
a mlvuhi cerebelli (cf. Fig. 161) extending
forwards into the ventricle of the mid-brain.
Except that only the median wall of the
pallium is epithelial, the brain of Amia on
the whole most nearly approaches that of the
Teleosts in structure.
opt. I
cbl
7TL.O
FIG. 159. — BRAIN OF
Lepidosteus. Dorsal
view. (After Balfour
and Parker.)
rW, cerebellum ; c.h, cere-
bral hemispheres ; ill,
diencephalon ; m.o,
medulla oblongata ;
olf. /, olfactory lobes ;
opt. I, optic lobes ; prs,
lobes of telencephalon.
Teleosts. — As is the case in many other
Fishes, the brain in most Teleosts by no
means fills the cranial cavity, and it is separ-
ated from the roof of the skull by a greater or less amount of a fat-
like tissue and lymph : it never attains to so large a relative size
as does that of Elasmobranchs. Its form varies greatly, more by
far than in any other Vertebrate group, and only the following-
essential points can be mentioned here.
1 In Polypterus and Calamichthys the pineal body gives rise to a peculiar
and extremely large epithelial vesicle, and the hypophysis communicates with
the mouth-cavity by a hollow duct, even in the adult. The brain of these
forms presents other special characters, and requires further investigation. In
Devonian (ianoids, as well as in the Placoderms, there was a parietal foramen
(p. 103).
p 2
212
COMPARATIVE ANATOMY
The pallium is entirely epithelial in structure (Figs. 160 — 162) :
it presents no median involution dividing the anterior part of the
A
7
L.ol.
W
M
, the epithelium (ependyme), lining the walls
of the ventricles ; Gp, pineal body, with a cavity (Gj)1) in its interior ; ff, //',
hypophysis ; J, infundibulum ; Li, lobi inferiores ; Sv, saccus vasculosus ;
TVo, roof of the optic lobes ; Tl, torus longitudinalis ; fr, pathetic nerve ; Val,
valvula cerebelli ; V.cm, common ventricle of the secondary fore-brain
(teloctele) ; V.t, third ventricle ; t, point at which the epithelial roof of the
secondary fore-brain (pallium, Pa) becomes continuous with the lining of the
anterior wall of the pineal tube ; above /is seen an outgrowth which represents
a rudimentary parietal organ.
The diencephalon is very small, and is depressed between the
telencephalon and mid-brain. The epiphysis (Figs. 160, 161) is
plainly distinguishable, but it usually does not pass into the roof
of the skull ; an outgrowth arising from the roof of the brain in
front of the epiphysis represents the parietal organ, but this
214
COMPARATIVE ANATOMY
becomes constricted off from the brain and disappears during
development.1 Lobi inferiores, as well as a hypophysis and a
glandular saccus vasculosus are present, but these vary much in
the degree of their development. The saccus vasculosus here too,
opens by several apertures into the infundibulum and is surrounded
by a blood-sinus.
The mid-brain and cerebellum are extremely large relatively ;
the latter is bent upon itself, overlies the medulla oblongata behind,
and is prolonged in front into the ventricle
of the mid-brain as a valvula cerebelli
(Fig. 161), as in Ganoids.
The Teleostean brain is a further
specialisation of the type seen in Ganoids,
and has no direct connection with that of
Cyclostomes or Elasmobranchs.
Dipnoans. — Both as regards external
and internal structure, certain points of
resemblance may be seen between the
brain of Dipnoans and that of Elasmo-
branchs and Ganoids, but in other respects
it is specialised. In Ceratodus, there is a
considerable space between the walls of
the cranium and the brain except in the
region of the large olfactory lobes. The
telencephalon is well developed, and the
thin pallium, which is mainly nervous, is
involuted along the median longitudinal
line so as to completely separate the two
hemispheres from one another dorsally in
Protopterus and Lepidosiren: in Ceratodus
they are partly united together dorsally and
posteriorly by a narrow bridge formed by
the choroid plexus. Olfactory lobes arise
from the telencephalon anteriorly, and contain ventricles : in
Ceratodus they overlie the hemispheres. Postero-laterally each
hemisphere gives rise to a distinct hippocampal lobe. The pineal
body has a long stalk, and its distal vesicle perforates the carti-
laginous roof of the skull : in the embryo Ceratodus it even
reaches as far as the integument. The complicated choroid plexus
in this region gives rise to a large vesicle over which the pineal
stalk extends. Lobi inferiores are present.
The well-marked mid-brain is indistinctly paired in Ceratodus,
but is unpaired in Protopterus and Lepidosiren. The cerebellum
1 A parietal foramen is present in the embryo in several Teleosts (e.y.
Cottus, Salmo), and in some others (e.y. Callichthys) persists in the adult without
a corresponding development of the pineal organ. In certain deep-sea forms (e.g.
Argyropelecus, Cyclothone), however, the latter is comparatively complicated,
and, as in Petromyzon, consists of two vesicles, showing regressive characters.
FIG. 162. - TRANSVERSE
SECTION THROUGH THE
FORE-PAKT OF THE TELE-
OSTEAN BRAIN.
G.»t, corpora striata ; Ep,
ependyme ; fr, frontal
bone, underneath which
the pineal tube, Gp, is
visible in transverse sec-
tion, and below this the
perimeningeal tissue, Pm;
Pa, the pallium, formed
of a simple epithelial
layer ; T, T, olfactory
tracts ; V.cm, teloccele.
BRAIN
215
\
is relatively much smaller than in Elasmobranchs and Teleosts : it
gives rise to a valvula cerebelli, and a complicated choroid plexus
roofs over the fourth ventricle.
Amphibians. — The cerebral hemispheres of the Amphibia are
distinguished from those of the Dipnoi by a higher development
of the pallium, which, however, is differentiated even in the latter
group into an external layer of nerve fibres and an internal cellular
layer (central gray matter). The basal
ganglia (corpora striata) are less marked,
except in the Gymnophiona, and merely
form a more or less prominent thickening
of the wall of each hemisphere projecting
into the lateral ventricle. A hippocampal
lobe is not distinctly developed, but a
hippocampus is represented by elevations
of the central gray matter, which are con-
nected right and left by a small anterior
pallial commissure just above the anterior
commissure (Fig. 164, ]>)
The Amphibian brain docs not, how-
ever, lead directly towards that of Reptiles.
Although the telencephalon is more highly
differentiated than in lower forms, the cli-
encephalon and mesencephalon are sim-
pler than in Fishes ; and, on the whole,
the brain of Amphibians is less com-
plicated than that of any other Verte-
brates, except Lampreys.
In Urodeles the individual parts are
more elongated and separated from one
another than in Anurans, and the dien-
cephalon is therefore more freely exposed.
The hemispheres are almost cylindrical,
and the olfactory lobes are distinct from
one another, while in the Anura they are
fused for a short distance anteriorly
(Fig. 164). The diencephalon and optic
lobes are much broader in Anurans than
Urodeles. The cerebellum consists
- G.UI
Fid. 163. — BKAIN OF Cera-
tod'iis foxteri. Dorsal
view. (From Parker
and Haswell's Zoology. )
and, auditory nerve ; chl,
cerebellum ; fac, facial
nerve ; ijf, glossopharyn-
geal ; med, medulla ob-
longata ; mes, mesen-
cephalon ; or, oculomotor
nerve ; opt, optic nerve ;
•}iroi, cerebral hemis-
pheres ; rh, olfactory
lobes ; ,•,„,„ -^jfl&^^^jjfo E.n. ,•„,p
. Nerve
Epithelial roof of •jg£f3fa8. — SKKTCH UK THE PINEAL APPAKATUS OF HATTERIA. (After Dendy. )
formation of a definite cortex, containing the characteristic pyra-
midal cells such as are present in all the higher Vertebrates. It
appears that the first differentiation of a cortex phylogenetically
was connected with the olfactory sense : while in Fishes, for
example, the olfactory tracts terminate in the corpora striata, most
of their fibres extend into a definite region of the pallium from
Reptiles onwards. Thus an " olfactory cortex " is formed, to which
other centres are gradually added in the ascending series of
Vertebrates.
The pallial commissures (Fig. 165), like those of Amphibians,
are not large relatively, but in addition to an anterior pallial or
hippocampal commissure, traces are present of a so-called "fornix"
(posterior pallial commissure, p. 201); the hippocampal lobes
BRAIN 221
with their choroid plexuses are much more distinct in many cases
(e.g. Hatteria, Chelonia, Crocodilia).
The olfactory lobes may be closely applied to the hemispheres
(e.g. Anguis, Amphisbaena, Typhlops), or may consist of a well-
marked olfactory tract, passing anteriorly into an olfactory bulb
from which the nerves of smell arise (e.g. Hatteria, Lacerta,
Crocodilus). Olfactory ventricles are usually present.
The diencephalon is always depressed, and is hardly, or not at
all, visible from the dorsal side. A distinct hypophysis and in-
fundibulum as well as an epiphysis are present, and in most
Lizards the parietal organ (cf. p. 202) retains more or less distinctly,
even in the adult, the structure of a median eye.1
This parietal eye (Fig. 168) is situated in the parietal foramen
of the skull, and is in close connection with the more posteriorly
situated pineal organ, though in the embryo the nerve which
supplies it is seen to arise independently from the brain, in front
of the pineal outgrowth. It has the form of a vesicle, the dorsal
wall of which may become thickened to form a transparent lens-
like body, while the rest of the wall consists of several layers and
forms a pigmented "retina," with which the more or less rudi-
mentary nerve is continuous. The vesicle is surrounded by a
vascular connective tissue capsule, and in many cases the integu-
ment immediately overlying it is pigmentless and transparent,
forming a kind of cornea. Traces of a vitreous body have also
been observed.2 Various degrees of reduction of the " retina "
and other parts as they occur, c.t/. in Hatteria, are seen amongst
Lizards (e.g. Lacerta, Anguis), and the organ may be recognised in
a simpler form in embi'yo Snakes.
As in all the Amniota, two chief divisions can usually be recog-
nised in the infundibulum of Reptiles : a dorsal vascular and
glandular body, corresponding to the saccus vasculosus of the
Anamnia, and a more ventral infundibular portion, the glandular
character of which is still retained to some extent in the Sauropsida,
but there is no opening into the ventricular cavity.
In the mid-brain the two well-marked optic lobes in some cases
show indications of a further subdivision into four ; from them the
optic tracts pass downwards and forwards to the chiasma. The
cerebellum is relatively small, except in the Crocodilia (Fig.
167), in which it consists of a thicker median lobe, and of two
lateral portions. The medulla oblongata has a marked ventral
flexure.
Birds. — The avian brain (Fig. 169) is of a very peculiar type :
it has few points of resemblance to that of Mammals, and is very
different from that of Reptiles, though especially as regards its
1 A parietal organ is wanting in Gecko, Ameida and Tejus, and there is no
pineal organ in the Crocodile.
! The parapliysis gradually extends beneath the epiphysial outgrowth, and
forms a sort of cushion under the parietal eye.
222
COMPARATIVE ANATOMY
individual sections it is more or less comparable to that of certain
of the latter (e.g. Chelonia). The basal ganglia (corpora striata) of
Hyp\
Tr.opt J/if
FIG. 169. — BRAIN OF PIGEON. A, dorsal; B, ventral ; and C, lateral view.
HH, cerebellum ; Hyp, hypophysis ; luf, infundibuluin ; Z.o/, olfactory lobes;
Med, spinal cord ; MH, optic lobes ; Nil, medulla oblongata ; Tr.ojtt, optic
tract; VH, cerebral hemispheres; I-X1I, cerebral nerves; 1, 2, first and
second spinal nerves.
the hemispheres reach a relatively larger size in Birds than in
any other Vertebrates. An advance on Reptiles is seen in the
BRAIN
223
connections of the pallial cortex in various directions, and in- many
Birds indications of cortical centres can already be recognised.
In the well-developed hemispheres frontal, parietal, and temporal
regions can be recognised : their surface is perfectly smooth, and
f-P-
\
•• I lu
h.l.
FIG. 170. — BRAIN OF RABBIT. A, dorsal ; B, ventral ; and C, lateral view.
6.0, olfactory bulb ; rb' superior verniis, and <•!>", lateral lobe of cerebellum ; r/-,
crura cerebri ; fp, pineal body ; f.b, cerebral hemispheres ; f.p, pallial fissure ;
h. b, cerebellum ; h.l, hippocampal lobe ; lip, hypophysis; m.b, optic lobes;
•iii.tl, medulla oblongata ; ji.r, pons Varolii ; r.f, rhinal fissure ; tr.o, olfactory
tract; i-xii, cerebral nerves.
the lateral ventricles are not extensive. The different parts of the
brain overlie one another much more markedly than in any Reptile,
and the hemispheres are much larger relatively, covering over the
diencephalon and part of the mid-brain. The olfactory lobes are
224
COMPARATIVE ANATOMY
poorly developed.1 The distal enlarged end of the pineal body
extends as far as the dura mater, and the structure of the internal
B
H PO vim
FIG. 171. — BRAIN OF DOG (POINTER). A, dorsal; B, ventral; and C,
lateral view.
Bo, Bo1, arcuate fissures ; B.ol, olfactory bulb ; Cr.ee, crura cerebri ; Fi.p, pallial
fissure ; FS, Sylvian fissure ; HH, lateral lobe, and HH1, flocculus of cere-
bellum ; Hyp, hypophysis ; LH, hippocampal lobe ; Med, spinal cord ; XH,
medulla oblongata ; Po, pons Varolii ; fiF, rhinal fissure; 8c, sulcus cruci-
atus ; TO, olfactory tract ; VH, cerebral hemisplieres ; Wu, superior vermis ;
I-XII, cerebral nerves.
part of the organ resembles that of a tubular gland, penetrated by
fibrous tissue and blood-vessels. There is no trace of a parietal
organ.
1 The toothed Birds of the Cretaceous period, with Hesperornis at their
head, possessed a very lowly organised, reptilian form of brain, with small
hemispheres and large olfactory lobes.
THE BRAIN
225
The cerebellum consists of a well-developed and folded median
lobe, and of two lateral portions (flocculi), which vary much both
in form and size. Posteriorly it completely covers the fourth
ventricle. The two optic lobes, in which, as in certain Reptiles, a
subdivision is indicated, are separated from one another and
pressed downwards, so as to lie at the sides of the brain in the
angle between the hemispheres, cerebellum, and medulla oblongata,
and they are connected by a broad commissure. The ventral side
of the short medulla shows a marked flexure, bending upwards to
the spinal cord.
Mammals. — The brain in embryo Mammalia is very similar
to that of the Sauropsida, but the subsequent differentiation
of its parts, and more particularly that of the pallium, gives it a
epi 7tip.com
\mid.com
c.ytt
cbl
artl.com
rnecl
c.mam
Fio. 17'2. — LONGITUDINAL SECTION OF BRAIN OF ROCK WALLABY (Petrogale
(From Parker and Hasvvell's Zoology.)
ant.com, anterior commissure ; cbl, cerebellum ; i-.inntn, corpus mammillare ; c.qit,
optic lobes ; cnir, orura cerebri ; epi, epiphysis, with the posterior com-
missure immediately behind it; f.mon, position of foramen of Monro ;
hip.i-otn, hippocampal commissure, consisting here of two layers, continuous
at a posterior bend, the splenium, somewhat divergent in front where the
septum lucidum extends between them ; hypo, hypophysis ; med, medulla
oblongata ; mtiL<-om, middle commissure ; olf, olfactory lobe ; opt, optic
chiasma ; <•<•«?. 3, third ventricle.
very special character. The cerebral cortex becomes much more
highly developed and in many Mammals is more or less markedly
convoluted1 (Figs. 171 and 173, B). In others, again, the
surface of the hemispheres remains smooth (Fig. 170), but a
subdivision into lobes (frontal, parietal, occipital, and temporal), as
well as certain fissures (e.g. rhinal, hippocampal, callosal) can
always be recognised to a greater or less extent, and the
hemispheres are relatively so large as to cover over the more
posterior parts of the brain ; in some of the lower forms, the mid-
brain can still be seen from above, while in the higher types
1 It is only possible to homologise the -main, sulc-i to a greater or less extent
amongst the various types of convolution seen in the Mammalian brain (cf. p. '228).
Q
2-_>6
COMPARATIVE ANATOMY
(Primates) even part of the cerebellum is hidden (Figs. 173, A
and B), although this is to a greater extent the case in some of
the lower Apes, with smooth hemispheres (e.g. Hapale, Chrysothrix),
than in Man. No satisfactory explanation has so far been given
for the different degrees of convolution seen amongst Mammals:
as a general rule, the brain in lower and smaller types (except, e.g.
in Echidna) is less convoluted than in higher and larger ones.
The number of fibres radiating from the cortex (corona radiata)
is very small in lower types (e.g. Rodents), and largest in Man. A
complex network of fibres in the cortex itself connects its various
parts together, and other strong bundles extend through the
MH
Jiff
FIG. 173A. — HUMAN BRAIN. Median longitudinal vertical section.
(Mainly after Reichert. )
corpus callosum ; G, fornix, which extends antero-ventrally to the lamina
terminalis (Col), in the upper part of which is seen the anterior commissure
(Ca), and between the latter and the optic thalami (To) the foramen of
Monro (FM) ; H, pituitary body ; ////, cerebellum ; MH, corpora bigemina,
with the iter (Aq], anterior to which is seen the posterior commissure (Cp) ;
NH, medulla oblongata, with the pons Varolii (P) ; -ff, spinal cord; T,
infundibulum ; Teh, tela choroidea ; To, optic thalamus (diencephalon), with
the middle commissure (Cm); VH, cerebral hemisphere; Z, pineal bod}';
/, olfactory nerve ; //, optic nerve.
hemispheres connecting individual regions of the pallium with one
another. The commissures between the hemispheres known as
the corpus callosum and fornix (Fig. 173, A) are also much more
highly developed than in other Vertebrates. The former is an
important structure in the higher Mammalia, its development
corresponding to that of the pallium: ifc extends upwards and then
backwards from the region of the lamina terminalis in the form of
a thin plate, and reaches its highest development in Primates.
A corpus callosum is apparently wanting in Monotremes and
Marsupials, in which a hippocampal commissure is present in
the position of the body of the fornix, just above the anterior
THE BRAIN
227
commissure (Fig. 172, and cf. pp. 201 and 220), and the brain of
these forms remains at a comparatively low stage of development :
the anterior (basal) commissure is comparatively large, whereas
in the Eutheria its relative size is in inverse proportion to
that of the more important corpus callosum. In Edentates the
brain is also of a low type, and the same is true of that of Rodents,
Insectivores, and Bats, though a considerable advance is here
seen as compared with Marsupials. A large middle commissure
connects the two optic thalami.
In addition to the lobes mentioned above, a central lole of the
hemispheres is present in Primates, and increases in development
in passing from the Gibbon, Orang, Chimpanzee, and Gorilla, up
to Man. But there is no justification for the statement that " the
human brain is only an enlarged anthropoid brain," for in the
JUL
FIG. 173B.— CONVOLUTIONS OF THE HUMAN BRAIN. (After A. Ecker.)
a,l,r, superior, middle, and inferior frontal gyri ; em, the calloso-marginal sulcus
on the dorsal surface ; FS, Sylvian fissure ; HH, cerebellum ; Lj\ frontal
lobe ; Lo, occipital lobe ; Lp, parietal lobe ; NH, medulla oblongata ; Po,
parieto-occipital fissure ; P, Jn, superior and inferior parietal gyri separated
from one another by the interparietal fissure (/) ; P,, spinal cord ; T, tem-
poral lobe ; A', # I, anterior and posterior central convolutions, separated from
one another by the fissure of Rolando (A'); 1 to 3, superior, middle, and
inferior temporal convolutions.
former a number of entirely new regions have been acquired,
especially as regards the frontal, temporal, and central lobes, which
have consequently undergone extension.
In correspondence with the division of the hemispheres into
lobes, there is a differentiation of the lateral ventricles,1 so that an
anterior, a posterior, and an inferior cornu can be distinguished
in each ; the inferior cornu extends into the temporal portion,
which corresponds to the hippocampal lobe of Reptiles, and an
eminence on its floor, the hippocampus major, 2 formed as an
1 The ventricles are lined by epithelium (epeudyme), which, strengthened by
connective tissue layers derived from the pia mater (tc/a1 choroidecK), also forms
the roofs of the third and fourth ventricles and extends into the lateral ventricles
as plexus choroi(l< i.
"* The hippocampal system has important relations to the olfactory centre.
The (/tji-ux dt'-nfatiift (fascia dent at a) and i\\ejiml>ria arise in close connection with
the hippocampus, the fimbria having intimate relations to the fornix.
228 COMPARATIVE ANATOMY
involution of the median wall of the hemisphere, is much more
marked than in lower forms : the line of involution is known as
the hippocampal fissure.
The central olfactory apparatus (rkincnccphalon) in its entirety
is represented by the olfactory bulb, peduncle, and tubercle, the
piriform lobe, and the hippocampus, and is separated from the
pallium by the rhinal fissure (Figs. 170, 171). This fissure is in
close relation to the splenial (cattosal) fissure, which bounds the
supracallosal gyrus dorsally, extending more or less parallel to the
corpus callosum. The Sylvian fossa or fissure is also a typical
fissure : it is situated at about the middle of the rhinal fissure,
and in the higher Mammals is overlapped by the pallium so that
the fossa is converted into a fissure. In Carnivores, Cetaceans, and
Ungulates, three gyri arch over the Sylvian fissure, one above the
other, and are separated by the so-called arcuate fissures (Fig. 171).
The upper of these, bounded above by the longitudinal pall in I
fissure, is spoken of as the marginal gyrus. Along the lateral sur-
face of the hemisphere, the cruciate sulcus (the homologue of the
central sulcus or fissure of Rolando of Primates) extends upwards
to the pallial fissure. Characteristic of the brains of all Apes except-
ing those with smooth hemispheres is the parieto-occipital sulcus,
between the parietal and occipital lobes : in Man, the lateral parts
of this fissure are more or less indistinct (cf. 173, B, in which other
gyri and sulci of the human brain are shown).
The corpus striatum is surrounded and perforated by fibres
passing down from the pallium (anterior limb of the internal
capsule of Primates). Unlike the corresponding structure in
other Vertebrates, the corpus striatum of Mammals becomes
gradually more deeply situated, and is comparatively small as com-
pared with the rest of the brain.
The olfactory lobes usually extend forwards freely from the base
of the telencephalon, and each may retain throughout life a
prolongation of the lateral ventricle (e.g. Perissodactyles) ; in
some cases (e.g. numerous aquatic forms and Primates) they are
completely covered by the frontal lobes. The degree of their
development is in proportion to that of the olfactory sense, and
they may even be entirely reduced (cf. under Olfactory Organ).
The pineal body is displaced downwards by the hemispheres
and lies against the anterior lobes of the mid-brain, not reaching
to the roof of the skull and brain-membranes. Its bifurcated
peduncle connects it with the roof of the diencephalon and
contains nervous substance : its distal end has the form of a
rounded or oval sac, consisting of compact epithelial tissue and
containing concretions. A parietal organ is wanting. Traces of
the saccus vasculosus and lobi inferiores still occur, even in Man,
in connection with the infundibulum.
The mid-brain (corpora bigemina) is of smaller relative size
than in other Vertebrates. A transverse furrow across the solid
THE BRAIN
229
optic lobes subdivides them into an anterior larger and a posterior
smaller pair of lobes (cf. p. 221).
The division of the large cerebellum into a median and two
lateral portions, already indicated in Reptiles, is carried to a still
further extent in Mammals. The median portion gives rise to the
so-called superior veruiis, while the lateral parts form the lateral
lobes and ftocculi (Figs. 170, 171). In Carnivores, certain
Edentates, Pigs, and Lemurs, the verm is is relatively large as
compared with the lateral portions ; while in Cetaceans, Elephants,
Apes, and Man the latter are more highly developed and the
median lobe reduced. The two lateral lobes of the cerebellum are
connected by a large commissure, the pons Varolii : this extends
round the medulla oblongata ventrally, and is more largely
developed the higher we pass in the mammalian series. Other
bands of nerve-fibres connecting the cerebellum with various parts
of the brain are spoken of as the anterior, middle, and posterior
Tfv
LCar
Jftf
Cacb
Fio. 174. — DIAGRAM OK THE CHIKK SYSTEMS OK FIBRES OF THE MAMMALIAN*
(HUMAN) BRAIN. (From a drawing by A. Ecker.)
Cac, crura cerebelli ad corpora bigemina ; Cacb, crura medulla ad cerebellum ;
Cap, crura cerebelli ad poiitem ; CC, crura (pedunculi) cerebri ; CS, corpus
striatum ; HH, cerebellum ; HM, hemisphere ; L, lemniscus : P, pons ;
Th, optic thalamus.
peduncles of the cerebellum, the relations of which, and of the
crura cerebri, are indicated in Fig. 174.
A study of the brain-casts in certain North American Eocene
forms is very instructive from an evolutional point of view, and
shows that the brain, and more especially the fore-brain, in these
animals was of extremely small size relatively (Fig. 175). The
brain of Dinoceras mirabile might easily be mistaken for that of
a Lizard, and was so small that it could easily be drawn through
the greater part of the neural canal : in the Cretaceous Dinosaurian
Triceratops, the brain was apparently still smaller relatively. The
olfactory nerves were extremely well developed in these forms.
In connection with the importance of the brain in modelling
the form of the skull, it may be mentioned that in many
Mammals, including Man, the outer surface of the skull in various
230
COMPARATIVE ANATOMY
parts shows a kind of relief of certain underlying portions of the
brain. In some cases, only the larger divisions of the brain
(cerebrum and cerebellum) are thus indicated externally: in others,
a relief of the convolutions is also seen, and in Mustela and Lutra. for
example, it is almost complete on the lateral portions of the skull
FIG. 175. — CASTS OF THE BRAINS OF VARIOUS EOCENE MAMMALS.
(After Marsh.)
Skull, with brain indicated, of A, Tillotherium fodium ; B, Rronfotha-iiuii in O
w oi-S
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240
COMPARATIVE ANATOMY
The trigeminal, facial, glossopharyngeal, and vagus nerves are
usually described as branchial or branch iomeric nerves, i.e. they are
primarily related to gill-clefts. A typical branchial nerve has a
ganglion near its origin from the brain and divides into (1) a
dorsal (somatic sensory) branch to the skin, (2) a palatine (visceral
sensory) branch to the oral mucous membrane, and (3) a branch
associated with the epibranchial ganglion (p. 236) which bifurcates
IV
G.L.IX.X.XI
FIG. 181. — PERIPHERAL NERVES OF A HUMAN EMBRYO OF 4 WEEKS (6'99 MM.
IN LENGTH), RECONSTRUCTED FROM SECTIONS. (After G. S. Streeter. )
1, 2, 3, mamlilmlar, hyoid, and 1st branchial ridges; ///, oculomotor; IV,
trochlear ; ln, T7'2, Vs, the three main branches of the trigeminal ; Fniot,
motor portion of trigeminal ; (JV, ganglion of trigeminal ; VII, facial ; VIII,
auditory ganglion ; IX, glossopharyngeal with the petrosal ganglion
(Gy.petr) ; A', vagus with the ganglion nodosum ((Jg.nod) and the anterior
(superior) laryngeal nerve (Lar.siit))) ; G. L, ganglion ic ridge of the IXth, Xth,
and Xlth nerves ; XII, hypoglossal. The abducent (VI) is not visible.
over the corresponding branchial cleft into a prebranchial (visceral
sensory) and a, postbranchial (visceral motor) nerve.
Trigeminal Nerve. — This is one of the largest of the cerebral
nerves. It arises from the ventro-lateral region of the anterior
part of the medulla oblongata (pons Varolii of Mammals) by a large
sensory and a smaller (ventral) motor root, has a large primarily
CEREBRAL NERVES 241
double intra-cranial or extra-cranial Gasserian ganglion at the origin
of the former, and in Fishes (Fig. 179), divides into two main
branches, an ophthalmic (including a superficial and a deep or
profundus portion), and a maxillo-mandibular : in most terrestrial
forms (Figs. 180 and 181) the maxillary and mandibular nerves
arise separately. From the presence of these three characteristic
branches, often known as the first, second, and third divisions of the
trigeminal, its name is derived. It passes out from the skull some-
times through a single aperture, and sometimes by two or three dis-
tinct ones. On the supposition that the mouth corresponds to a fused
pair of branchial clefts, the ophthalmic would correspond to the dorsal
branch of a branchial nerve, and the maxillary and mandibular to
the prebranchial and postbranchial branches ; the palatine branch
may be represented by a branch going to the roof of the mouth.
The superficial branch of the first division is usually distinct in
Fishes, in many of which, however, and in higher forms, it may be
united with the deep branch. In Amphibians its homology has not
been clearly made out.1 It passes dorsally over the eye-ball, crosses
the superficial ophthalmic branch of the facial, with which it may
become secondarily connected, and is distributed to the skin
anterior to and above the orbits. The deep branch passes below
the superior and anterior recti and superior oblique muscles, and
supplies the integument of the snout, the eyelids and conjunctiva,
the mucous membrane of the nose, and the lacrymal glands (e.g.
in Mammals). A connection of the profundus with the ciliary
ganglion arises secondarily.
The second division of the trigeminal, like the superficial
and deep ophthalmic, is purely sensory. On it is a sphenopalatine
ganglion derived from the sympathetic, and it is connected with
the facial. It extends first along the floor of the orbit, supplying
the lacrymal and Harderian glands when present, the conjunctiva,
the mucous membrane of the nose, and the roof of the mouth ; it
then passes to the upper jaw, supplying the teeth ; and finally,
as the infraorbital branch, perforates the skull to reach the integu-
ment in the region of the upper jaw, snout, and upper lip.
The third division of the trigeminal is of a mixed nature ; its
motor portion, which has the character of a visceral nerve, supplies
certain masticatory muscles and some of the muscles of the palate
and floor of the mouth. The sensory portion extends along the
rami of the lower jaw and divides into two main parts, a lingual
and a mandibular proper, the former of which is not well differenti-
ated in the Anamnia and Sauropsida. The lingual or gustatory
nerve innervates the mucous membrane of the mouth and tongue,
containing gustatory fibres from the chorda tympana (cf. under
facial nerve).
The special mandibular branch, which may pass through the
inferior dental canal of the mandible, supplies the teeth and
1 It possibly corresponds to the ramus frontalis of Mammals.
R
242 COMPARATIVE ANATOMY
mucous membrane of the lower jaw, and then gives off one or
more branches to the integument of the latter and of the lower
lip : in Mammals, a smaller branch passes upwards in front of the
ear to the temporal region, supplying the adjacent skin and the
pinna of the ear. Two ganglia, the submaxillary and the otic (Fig.
180), derived from the sympathetic, are connected with its sensory
portion, the former being situated close beneath the exit of the
nerve from the skull, the latter on the lingual nerve at the point
where it passes into the tongue. The otic ganglion is connected
with the glossopharyngeal nerve, but it is doubtful whether the
gustatory fibres in connection with the lingual ganglion are
derived from this nerve or from the facial.
Facial Nerve. — This, which is also a mixed nerve, presents
important differences in branchiate and pulmonate forms respec-
tively. In many Fishes (e.g. Cyclostomes, Elasmobranchs, many
Teleosts, Dipnoans) and in perennibranchiate Urodeles, it possesses
two distinct ganglia at its origin in connection with the sensory and
mixed portions respectively. In other Fishes (e.g. Chimrera, Polyp-
terus, Lepidosteus, certain Teleosts) and more especially in Anurans,
the facial nerve comes into such close connection with the trigeminal
that the ganglia in question are no longer distinguishable from the
Gasserian ganglion, and such complications arise that the original re-
lations of many of the components of the two nerves are no longer
traceable and cannot be analysed by dissection. Another (the gcnicu-
late) ganglion of the facial nerve is retained in all Vertebrates.
In aquatic branchiate Vertebrates the facial nerve consists of
the following main branches (Fig. 179):
I. A system of sensory branches for the supply of the in-
tegumentary sense-organs of the head (q.v.). These branches,
together with the auditory nerve and the lateral line branches of
the glossopharyngeal and vagus (p. 245), arise from the same centre
in the medulla oblongata (tuber acusticum), each originally possessing
its own ganglion, and together forming a primitive acustico-lateral
sensory nervous system, arising, like the sensory organs which they
supply, direct from the ectoderm. The following branches may be
distinguished : — (a) a superficial ophthalmic, running parallel to the
like-named branch of the trigeminal and sometimes (e.g. in
Chimsera) becoming very closely connected with its deep portion :
(6) a buccal, close to the maxillary portion of the trigeminal, and,
giving off near its origin an otic branch ; and (c) an external
mandibular, in the region of the hyomandibular nerve, dividing
into an anterior and a posterior branch and frequently anastomos-
ing with the mandibular branch of the trigeminal.
II. — A sensory (a) palatine * which may anastomose with the
1 There can be no doubt that the palatine branch of the facial in the Anamnia,
comparable to the visceral or pharyngeal branches of the glossopharyngeal and
vagus, corresponds to the greater superficial petrosal of Mammals, which is
a purely sensory nerve : the motor fibres which are said to arise from it probably
belong to the vagus.
CEREBRAL NERVES
243
maxillary branch of the trigeminal and which innervates the
mucous membrane of the pharynx, and (b) chorda tympani,1 going
to the mucous membrane of the floor of the pharynx. These two
nerves correspond to the " portio intermedia " of the facial of
Mammals (Fig. 182), and are closely related at their origin with
the geniculate ganglion. The chorda tympani corresponds to the
/'. -1,11,1 VII
in nil.
FIG. 182. — DIAGRAM SHOWING THE RELATIONS OF THE PORTIO INTERMEDIA OP
THE FACIAL NERVE IN MAX. (After A. F. Dixon ; slightly modified. )
I, II, III, the three branches of the trigeminal ; *, geniculate ganglion of the
facial ; t, sphenopalatine ganglion, in the neighbourhood of //; Ch.fi/, chorda
tympani; C.t, tympanic cavity, outlined; G'. ti-iij, (iasserian ganglion of
the trigeminal ; P.int.mVII, intermediate (sensory) portion of the facial;
P. mot. VII, motor portion of the facial (hyomandibular) ; H.lintj, lingual
branch of ///; Jl. ma ml, mandibular branch of /// ; li.yxil, palatine (greater
superficial petrosal) branch of facial. The motor portion of trigeminal ///
is not indicated.
prebranchial and the hyomandibular to the postbranchial branch,
but from Amphibians onwards the chorda tympani becomes post-
spiracular in position.
III. A main post-spiracular hyomandibular trunk, extending
along the hyoid arch, and essentially motor, except for the com-
ponents which give rise to the sensory external mandibular and
a few twigs supplying the mucous membrane of the spiracle,
1 The chorda tympani ("alveolar" branch of the facial) passes internally to
the lower jaw in Elasmobranchii, Ganoidei, Perennibranehiata, and Anura. In
other Amphibians, as in Reptiles, it passes into the bony lower jaw.
244 COMPARATIVE ANATOMY
anterior wall of the pharynx, floor of the mouth, and the skin. Its
motor fibres supply visceral muscles in connection with the
mandibular and hyoid arches.
In correspondence with the change from an aquatic to a terres-
trial mode of life, the integumentary sense-organs in caduci-
branchiate Urodela, Anura, and in Amniota, become more or
less completely lost, and the corresponding branches of the facial
nerve are reduced. The parts which persist, in addition to the
large motor hyomandibular, are the palatine and the chorda
tympani (cf. Figs. 179-182).
In the Amniota the chorda tympani has a very different
position from that seen in the Anamnia, and becomes character-
istically related to the tympanic cavity ; in Birds it is absent, and
is replaced functionally by the glossopharyngeal. From the Amphi-
bia and Reptilia onwards, a gradual development of the facial
muscles leads to the characteristic mimetic muscles of Mammals
and more especially of Primates, which are supplied by the hyoman-
dibular nerve. The complicated networks of this nerve, however,
appears late phylogenetically, and are wanting even in certain
embryonic stages in Man. In addition to the mimetic muscles, the
hyomandibular nerve in Mammals supplies the platysma, the stylo-
hyoid, the posterior belly of the digastic, and the stapedius.
Auditory Nerve. — This large nerve, which has a ganglion at
its origin, arises in close connection with the facial, and comes'
under the same category as the sensory portion of the latter nerve,
inasmuch as it is probable that the auditory organ is a modified
portion of the lateral line organs. Soon after its origin it divides
into a vcstilular and a cochlea/1 branch. The latter passes to the
lagena or cochlea of the ear, while the former supplies the rest of
the auditory labyrinth.
Vagus group. — This group includes the glossopharyngeal,
vagus, and spinal accessory, which stand in the closest relation to
one another, and, owing to the fact that the head in this region has
undergone fewer phylogenetic modifications, are less specialised
than the cerebral nerves already described. These nerves all
consist of both afferent and efferent fibres, the former being con-
nected with ganglia (the petrosal of IX, and the jugular and
cervical of X).
In Fishes and perennibranchiate Amphibians the glosso-
pharyngeal leaves the skull through a special foramen, and not
along with the vagus, as in other Vertebrates. In branchiate forms,
in addition to a palatine branch, it is distributed to the region of
the first (hyobranchial) gill-cleft, over which it bifurcates into a
smaller prebranchial and a larger postbranchial branch (Fig. 179).
In other Vertebrates it is distributed to the pharynx and tongue.
and as a rule anastomoses with the vagus and also with the geni-
CEREBRAL NERVES 245
culate ganglion or palatine branch of the facial and the otic gang-
lion of the third division of the trigeminal (Jacobsons anastomosis},
a continuation of this branch extending forwards, close to the
palatine branch of the facial.1 In the higher Vertebrates, the
large lingual branch forms a gustatory nerve supplying the tongue,
tonsils, and epiglottis : this nerve is apparently already indicated
in Dipnoans.
The vagus has a very wide distribution, and is not limited to
the head but extends into the trunk. It includes a sensory lateral
line branch, pharyngeal ( = palatine), and branchial branches ; the
last-mentioned fork over the second and following gill-clefts and
supply the mucous membrane and muscles of the branchial appar-
atus in branchiate forms. Its visceral branch supplies the larynx,
heart, swim-bladder or lungs, and a considerable portion of the
digestive tract (gullet, stomach, and more or less of the intestine).
In pulmonate Vertebrates a reduction takes place of the motor
components of the branchial nerves along with the corresponding
muscles (Fig. 180).
The origin of both glossopharyngeal and vagus by numerous
roots, and the fact that they give off branches in the region of the
pharynx and visceral arches in which a metameric arrangement
can be recognised, indicates that they correspond originally to a
number of separate nerves.
The lateral branch of the vagus, as already mentioned (p. 242),
does not belong originally to this nerve, but to the lateral nervous
system of the head, having a similar central origin to that of the
acustico-facial group, with which it may even be directly con-
nected by a commissure outside the auditory capsule (Protopterus).
There is a special ganglion at its origin from the medulla
(Fig. 179), and its exit from the skull by the same foramen as the
vagus is evidently secondary. After giving off a supratemporal
branch, it extends along the trunk to the apex of the tail, and may
subdivide into several branches, some of which may be situated
directly under the skin and others (like the main lateral nerve of
Elasmobranchs and Dipnoans), beneath the lateral muscles close
to the vertebral column. All these branches supply the sensory
organs belonging to the lateral line system.2
The so-called spinal accessory (accessorius Willisii) is a true
cerebral nerve, and can already be recognised in Elasmobranchs,
in which it is included in the vagus, from the posterior roots
1 It is possible that the lateral line fibres which may be associated with the
glossopharyngeal, and even with the trigeminal, are always derived from the
vagus and facial.
- Certain nerves present in Teleostomes and formerly described under the
term " ramns lateralis trigemini," may be included under the. term "ramus
lateralis accefssoriiix." They form a sensory system of nerves, provided with
ganglia, which are formed typically from somatic sensory fibres derived from the
Vth, Vllth, IXth, and Xth cerebral nerves and a varied number of spinal nerves.
Branches pass to some of, or even all, the fins, and supply sensory end-buds
(q.v.). The so-called lateral nerve of Petromyzon belongs to this system.
246 COMPARATIVE ANATOMY
of which it arises : it is therefore primitively a cerebral and not a
spinal nerve. It presents certain characteristic peculiarities in
the Amphibia, Sauropsida, and Mammalia respectively, so that
a direct comparison of the nerve in these groups is impossible.1
Owing to secondary differentiations, the accessory of Mammals
takes on a very different character from that of the Sauropsida :
only that part of it in the former arising from the spinal cord can
properly be described as the accessory, while its cerebral portion
must be included under the vagus group. In the Sauropsida, the
nerve is better described as the spinal portion of the vagus. In
Mammals, the accessory contains viscero-motor elements from
the dorsal roots of the 5th to 7th spinal nerves, and extending
along the course of the vagus gives off branches to the larynx and
to the trapezius and sternocleidomastoid muscles.
Spino-occipital and Hypoglossal Nerves. — Under the
term " spino- occipital nerves " is understood a group of nerve-
roots in the occipital region and anterior trunk-myotomes which
are in close relation to the hypoglossal. As most of their com-
ponents are bound up in the vagus-group, they were formerly
erroneously described as " ventral roots of the vagus."
In Cyclostomes they have either not been assimilated by the
cranium (cf. p. 85) or are not even differentiated from the cerebral
nerves, so that in this case they cannot be spoken of as spino-
occipital. In Plagiostomes, in which, as in Amphibians, vertebral
elements are fused with the occipital region of the skull, a series of
intracranial spinal nerves can be recognised which maybe described
as " occipital," a reduction in which, from before backwards, can
already be observed. In the Holocephali, owing to a still greater
assimilation of vertebral elements to the skull, three additional
spinal nerves later became intracranial, while the number of
occipital nerves is reduced to two.2 The relative number of these
two series varies in Ganoidei, Dipnoi, and Amniota, the occipital
nerves having entirely disappeared in the Teleostei.
In Fishes the first spinal nerve, which corresponds to the hypo-
glossal of higher forms, supplies the muscles of the trunk, the floor
1 The evolution of the spinal accessory in the higher Vertebrates must have
taken place somewhat as follows. Beginning with the Amphibia, in which the
vagus group does not extend into the spinal cord, the accessory in the primitive
Amniota must have possessed the following characters : — close connection with the
vagus, extension backwards at least as far as the first cervical segment, origin
from a lateral collection of cells of the ventral cornu, and course on the ventral
side of the dorsal cornu of the gray substance. From this primitive form the
nerve must have developed along two different lines in the Sauropsida and Mam-
malia respective!}', in both of which, however, in contrast to the Amphibia, it thus
forms a kind of connecting link between the cerebral and the spinal nerves, this
region including in the Sauropsida at most three, in Mammalia seven, segments.
'• These additional nerves have been described as " occipito-spinal " to dis-
tinguish them from the "occipital" nerves : each series constitutes a sub-section
• >f tlnj spino-occipital group. In Amphibians (except Ichthyophis) the occipital
nerves are no longer recognisable, even in the embryo,
CEREBRAL NERVES 247
of the mouth, and the skin of the back, and also sends twigs to the
brachial plexus. In higher Vertebrates the hypoglossal becomes
gradually more differentiated from the other cervical nerves, and
innervates the intrinsic muscles of the tongue, takes up cervical
elements, and with them gives rise to the so-called n.inius descendcns
and the ansa hypoglossi, from which arise branches to the sterno-
hyoid and other muscles.
In the Gymnophiona, Urodela, and Aglossa amongst the Anura,
the first spinal nerve perforates the first vertebra : in other Anurans
this nerve has disappeared, though occasionally recognisable in the
embryo, and the nerve which arises behind the vagus and emerges
between the first and second vertebrae in reality corresponds to the
second spinal nerve (hypoglossal, cf. Figs. 145 and 164).
From the Sauropsida onwards, the hypoglossal, which arises
postero-ventrally to the vago-accessory group, leaves' the skull
through one or more apertures : it has three roots,1 which corre-
spond to three spino-occipital nerves of the Anamnia.
Dorsal roots may be present temporarily or permanently in con-
nection with the hypoglossal of Sauropsida and Mammalia, and may
be provided with ganglia, as in the case of the accessory and of the
spino-occipital nerves of many Fishes. A reduction of dorsal roots
may also take place further backwards : in many Mammals, including
Man, that of the first cervical nerve (and even of the second in e.g.
the Orang) may be reduced or entirely wanting.
Sympathetic.
The sympathetic system is a derivative of the spinal system, with
which it remains throughout life in close connection by means of
raini communicantes (Fig. 145). It is distributed mainly to the
alimentary tract, the vascular system, and the glandular organs
of the body.
The sympathetic ganglia are derived from the developing spinal
ganglia, and, like these, show originally a segmental arrangement.
They contain typical ganglion-cells,2 and usually become united
together secondarily by longitudinal commissures, thus giving rise
to a chain-like paired sympathetic cord lying on either side of the
vertebral column and aorta. From its ganglia nerves pass off
to the above-mentioned organs, and form plexuses. Numerous
peripheral ganglia, derived from the others, are also present in
the plexuses.
1 Other, more anterior elements occur in the embryo in Sauropsida.
- A special small form of cell occurs in the embryonic sympathetic ganglia,
and may extend beyond them to a greater or less degree into other parts. Thus
these chromaffin edit are found, e.ff., in the pancreas (islets of Langerhans),
coccygeal gland, hypophysis (Fig. 151), and suprarenals (medullary substance, q.v. ) :
—in fact, in all " glands with internal secretion."
248 COMPARATIVE ANATOMY
The sympathetic, accompanying the arterial trunks, extends
along the vertebral column and passes anteriorly into the
skull, where it comes into relation with a series of the cerebral
nerves (cf. pp. 237, 241 and Fig. 180) similar to those which it
forms further back with the spinal nerves.
The original segmental character frequently disappears later on,
and this is especially the case in those regions where marked
modifications of the earlier metameric arrangement of the body
have taken place — viz., in the neck and certain regions of the trunk,
especially towards the tail : thus in Mammals there are never more
than three cervical ganglia.
Nothing is known of a sympathetic in Amphioxus. In
Petromyzon typical ganglion-cells occur more or less sparsely along
the dorsal and ventral spinal nerves in the lateral walls of the body.
Nests of cells are present more frequently in the region where the
parietal veins open into the cardinal veins than alongside the aorta,
and they also occur along the caudal vein and its branches : these
ganglia are connected with the suprarenal organ (fj.v.}. The
sympathetic extends into the head.
In Elasmobranclis the sympathetic reaches a higher stage of
development, and it has been shown that the ganglia first appear
after the dorsal and ventral roots have united to form the spinal
nerve-trunks, just at their point of union, each ganglion containing
from the first both afferent and efferent elements. Except in
its most anterior embryonic segment, in which the ciliary ganglion
represents a part of this system, the head is without sympathetic
ganglia. A sympathetic cord, connecting the ganglia, is not de-
veloped in Elasmobranchs, although some of the individual ganglia
may become united together, while others disappear at an early
stage.
A cranial portion of the sympathetic exists in Tdcosts, arising
from the trigemino-facial system of nerves and possessing three
ganglia : in the trunk, too, there is a well-developed cord of ganglia,
frequently connected with its fellow by transverse commissures, the
two cords gradually converging antero-posteriorly. A similar con-
dition has been found to occur in the Dipnoi (Protopterus), in which
the delicate longitudinal sympathetic cords, with occasional ganglia,
extend along the aorta and notochord : nothing is known of
their connection with the cerebral nerves.
In Amphibians (Fig. 145), the sympathetic reaches a high stage
of development. It ends anteriorly in the ciliary ganglion, extends
along the aorta through the trunk and caudal regions as a
ganglionated cord, and has numerous anastomoses with the spinal
and cerebral nerves ; it is intimately related with the suprarenal
and abdominal veins (postcaval and revehent renal' veins).
In the Sauropsida the cervical portion of the sympathetic is
usually double, one part running within the vertebrarterial canal
alongside the vertebral artery, In all other Vertebrates the whole
SENSORY ORGANS 249
curd lies along the ventral and lateral region of the vertebral
column : it is generally situated close to the latter, overlying the
vertebral end of the ribs.
In Mammals, the cervical portion of the cord may have an
independent course from the vagus, or it may be more or less
closely applied to the latter nerve, the anterior cervical ganglion of
the sympathetic and the vagus-ganglion forming a single mass ; the
posterior cervical ganglion commonly fuses with the first thoracic.
From the anterior cervical ganglion the sympathetic passes into
the skull along with the internal carotid artery, and its cranial
portion takes on relations to the cerebral nerves — more particularly
the Vth, IXth, and Xth, as in other Vertebrates. Numerous
branches also pass from the anterior cervical ganglion to the hypo-
glossal, the anterior cervical nerves, and to the pharynx, larynx, &c.
III. SENSORY ORGANS.
The specific elements of the sensory organs originate, like the
nervous system in general, from the ectoderm ; the peripheral
terminations of the sensory nerves are thus always to be found in
relation with cells of ectodermic origin, which become secondarily
connected by means of nerve-fibres with the central nervous
system.1
The sensory apparatus was primarily situated on a level with
the. epiderm, and served to receive sensory impressions of but
slightly specialised kinds ; but in the course of phylogeny parts of
it passed inwards beneath the epiderm, certain of these becoming
differentiated into organs of a higher physiological order, viz.,
those connected with smell, sight, hearing, and taste. These are
situated in the head, and except the last mentioned, become
enclosed in definite mesodermic sense-capsules (p. 77) ; they must
be distinguished from the simpler integumentary sense-organs, which
are concerned with the senses of touch, pressure, and temperature.
In addition to free nerve-endings in the skin, various specific
forms of sensory cells occur, and these may be surrounded by
supporting or isolating cells, both kinds, however, being ectodermic.
The mesoderm may also take part in the formation of the sensory
organs, giving rise not only to the above-mentioned sense-capsules,
but also to various protective coverings and canals as well as to
contractile and nutritive elements (muscles, blood- and lymph-
channels).
In the sensory organs of the integument of branchiate Verte-
brates, as well as in all the higher sensory organs, the surrounding
1 The vertebrate eye forms an exception to the other sense-organs in that it
arises from a part of the ectoderm which has been involuted, to form the medullary
tube.
•250
COMPARATIVE ANATOMY
medium is always moist, and in both cases, rod-, club-, or pear-
shaped sensory cells are met with.
In those animals which in the course of development give up an
aquatic life and come on land (most Amphibians), the external
layers of the epiderm dry up, and the integumentary sense-organs
pass further inwards from the surface, undergoing at the same
time changes of form. Thus from Reptiles onwards other kinds
of sense-organs are met with in the skin.
SENSE-ORGANS OF THE INTEGUMENT.
a. Nerve-eminences.
In Amphioxus certain rod -shaped or pear-shaped cells can be
recognised in the epiderm, especially in the anterior part of the
animal ; each of these is provided distally with a hair-like process
and proximally is in connection with a nerve. The cells are
distributed irregularly, but in the neighbourhood of the mouth
and cirri they form groups.
stz sz stz
£0
FIG. 183. — VERTICAL SECTION THROUGH THE SKIN AND A LATERAL LINE ORGAN
OK THE LARVA OF Triton tceniatus, 3 CM. IN LENGTH. (After F. Manrer. )
BO, blood-vessel ; Ep, epiderm ; SZ, sensory cells; St.Z, supporting cells.
It is doubtful whether these structures in Amphioxus are
directly comparable to the integumentary sense-organs of Fishes
and Amphibians, but it is important to note that each of the latter
always arises in the first instance from a single cell which forms a
group by division. 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 thorn 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. 18o and 184).
In Dipnoi, perennibranchiate Amphibia, and amphibian larvae
these organs retain their peripheral free position, on a level with
SENSE-ORGANS OF THE INTEGUMENT
251
A
AE JE S7.
.wz
FIG. 184, A.— LONGITUDINAL VERTICAL SECTION OF THE SKIN AND A LATERAL
LINE ORGAN OF Triton rrixtninx 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; StZ, . supporting
ctil 1 1:
13. —ISOLATED SUPPORTING (StZ) AND SENSORY (SZ) CELLS FROM A LATERAL
LINE ORGAN OF TRITON.
C. — VERTICAL SECTION THROUGH THE LATERAL CANAL OF Amia talva.
(After Allis ; slightly modified.)
i, lateral nerve, and A7, branches to sensory organs ; Oe, apertures of the lateral
canal to the exterior ; &, scales ; SO, sensory organs in the lateral canal,
252
COMPARATIVE ANATOMY
the cpidcrm,1 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 Chimcera monstrosa. (After F. J. Cole.)
innervation is indicated by the different kinds of shading.
The
(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 ( R ) + sub- rostral (SS).
(•2.) Infra-orbital canal (buccal + otic of facial — dotted) = orbital (Or) +
sub-orbital (iS'O) + portion of angular (A) + nasal (N).
(3.) Hyomandibular or operculo-mandibular canal (external mandibular of
facial — black) = remainder of angular (^4) + oral (O) + jugular (J).
(4.) Lateral canal (lateral line branch of vagus — oblique shading) — lateral
(L) + occipital (Oc) + aural (^4 «) + post-aural (PAn).
The small dots on the snout represent the apertures of ampullary 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-orbital, infra- orbital, and hyomandiljidar tracts
1 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 l (Figs. 185 and 186). These struc-
tures are thus often spoken of as segmented sensory organs, or organs
of the lateral line 2 : 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 vesicles of Torpedo, the nerve-sacs or
pit-organs of Teleostomes, and the ampullary tules of Elasmo-
branchs correspond to modified nerve-eminences. These are
FIG. 186. — DISTRIBUTION OF THE LATERAL SENSE-ORCJANS IN A SALAMANDER
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 Torpedo they become completely separated from the
epiderm, while in other Elasmobranchs they are tubular, each
tube giving rise to one or more swellings or ampullae 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
1 There are several lateral lines in Polypterus and various other Fishes, in
Proteus, and in all amphibian larvte.
' The lateral canal system of Polyodon comes nearest to that of Elasmo-
branchs, and in Acipenser it shows certain resemblance to that of Bony 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
Pteraspidae : 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.
b. End-buds and gustatory organ*.
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 foimer 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
papillae 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.1
1 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 fungi form papilla^ as well as on the papilla
foliata.1
Thus the specific integumentary sense-organs of aquatic \Vrti--
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.2
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 " Grand ry's corpuscles " 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 papillae are present in Monotremes, three in Marsupials,
and a variable number in the Eutheria. Foliate papilla? are especially well
developed in Rodents, but in many Mammals are little marked or wanting. The
relative functional importance of the different kinds of papilla- s-aries in the
course of the individual life in Man. In Cetaceans (e.g. Dolphin) only vestiges
of the gustatory organs are retained.
2 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 nerve-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; Ar, nerve, which
loses its medullary sheath
at X1.
FIG. 187c. — A TACTILE CORPUSCLE (END-BULB,
OR KRAUSE'S CORPUSCLE) FROM THE MARGIN"
OF THE CONJUNCTIVA OF MAN. (After
Dogiel.)
/', nucleated fibrous investment ; n, medullated
nerve fibre, the axis-fibre of which passes
into a closely coiled terminal skein.
H
FIG. 187B. — DERMAL PAPILLA
KROM THE HUMAN FINGER
ENCLOSING A TACTILE COR-
ITSCLE (MEISSNBR'S COR-
PUSCLE). (After La wdowski.)
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.
FK;. 187o. — TRANSVERSE SECTION-
THROUGH A TACTILE CORFU SIM:
(GRANDRYS' CORPUSCLE) FROM
THE BEAK OF A DUCK. (After
Carriere. )
n, nerve, entering the capsule A',
its sheath (8) 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 )i\
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 vibrissre, 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 modified 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-surface : 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-urea" 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.
A B
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 lamella^ : 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 fascia1, 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. Such free 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 plate is present.
The primary origin of the ollactory 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
12- \
as- -
centripetally towards the olfactory lobes and pass into the fore-brain,
when they become connected Avith the olfactory centre. The
individual olfactory cell and fibre thus form an organic unit — a
primitive condition such as occurs 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 ncuro-epitlieliiim in Vertebrates, as the nerve arises in connec-
tion with the cell itself, Avith Avhich it remains continuous (primary
sensory ccll~) : 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, SAvollen 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,
Avhich 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
Avith the cavity of the mouth as Avell as
Avith the exterior. In consequence of this,
anterior or external nostrils, and posterior or
internal nostrils (clwanm} can be distin-
guished : as a free passage is thus formed
through Avhich 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.1 From the Amphibia onwards FIG. ^.-EPITHELIUM OF
, ° , , ,, THE OLFACTORY Mucous
glandular elements are present, the secre-
tion of Avhich 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 fibre-cartilaginous 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
front of the cranial cavity, arid opens on the dorsal surface of the
head by a chimney-like tube, Avhich in Myxine is long and is
1 The mode of formation of the primitive choante 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
iiaso-oral funnel, and the development of the choaiia; in the higher Vertebrates is
accompanied by a secondary perforation of the primarily blind nasal sac.
s 2
MEMBRANE. A,ofPetro-
myzon planer i ; 15, of
Salamandra atra.
E, interstitial epithelial
cells ; It, 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
HO Hyj>
B
Ch
FIG. 190. — A, B, C, MEDIAN LONGITUDINAL SECTION THROUGH THE HEAD OF A
LARVA OF Petromyzon plancri 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 Kupffer
and Dohrn.)
Ch, notochord ; Ohio,*, optic chiasma ; ftp, pineal body ; ////, hind-brain ; /////>,
pituitary sac ; Jfi-f, infundibulum ; MB, stomodseum ; M H, mid-brain ; OL,
UL, upper and lower margins of oral funnel ; /,'O, olfactory sac ; VET,
position of endodermic part of gut shown in C (VOD) opening into the
stomodseum ; 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-palatine auct.
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 (stomodseum), and between it and the mouth is
another ectodermal invagination, the pituitary sac (Fig. 190). In
the course of development the olfactory and pituitary irivaginations
become sunk in a common pit, which, owing to the growth of the
large oral funnel, gradually becomes shifted to the dorsal side. On
the farther 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 KO
OL
VOD
FKJ. 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 dorsally.
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
C
uso
FIG. 191. — A, VENTRAL VIEW OF THE HEAD OK A DOGFISH (Scyl/iitm cnni<:ul«).
HSO, integumentary sense-organs ; M, month ; N, nostril.
B, LATERAL VIEW OF THE HEAD OF A PIKE (Exo.c Inciux). Ay, eye ; a and l>, the
anterior and posterior openings of the external nostrils, and +, fold of skin
separating them.
FIG. 191. — C, LATERAL VIEW OF THE HEAD OF Mura'tin ln/l»cn into the oral cavity rather further back.2
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
1 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 representatives 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 witli 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
J\r
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
From the dorsal adult> but in Urodeles, this
position is attained secondarily.
A F, antorbital process ; F, frontal ; They are opened and closed by
olfactory sac ; 01, olfactory musc}es
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 perennibranchiatc Amphibians by
FIG. 19'2.— OLFACTORY ORGAN OF
side* ma>'"latli*'
nerve ; P, process of the parietal ;
Pmz, premaxilla ; PP, palatoptery-
goid.
FIG. 193.— TRANSVERSE SECTION THROUGH THE OLFACTORY CAVITIES OF
Plethedou ylutiuonus.
C, cartilaginous, and Sl, fibrous portion of the turbinal, which causes the
olfactory epithelium (E) to project far into the nasal cavity ; F, frontal ;
ID, intermaxillary gland, shut oft' from the cavity of the mouth by the oral
mucous membrane ( J/.S') , A', maxillary cavity ; M, maxilla : N, main nasal
cavity ; /'/', 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 arc for
the most part enclosed by the vomer, or vomero-palatine.
A nctso-lacrymal 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.
Reptiles. — Owing to the growth of the brain and facial region
and to the formation of a secondary palate (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. 194;. The latter alone
is provided with sensory cells, the former
being lined by ordinary stratified epi- AN, IN, outer and inner
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 lur-
binal, 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 Amphisbtenidse. 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 others into the internal nostrils (Ophidia), which, as in
Amphibians, are usually situated on the anterior part of the roof
of the mouth.
FIG. 194. — DIAGRAM OF THE
OLFACTORY ORGAN OF A
LIZARD. (Longitudinal
vertical section. )
nasal chambers ; t, 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
0]ir prominence, which may be spoken of as a
pseudo-turbinal, and which possibly corre-
sponds to the upper turbinal of Birds.
Birds. — In all Birds, as in Lizards, there
is an outer chamber lined by stratified epi-
thelium, and an olfactory chamber proper,
situated above the former. In addition to
FIG. 195.— TRANSVERSE a turbinal corresponding to that of Reptiles
SECTION THRO UGH THE and usually known as the middle turbinal,
RIGHT NASAL CAVITY th j so-called upper turbinal (Fig. 195) :
OF A SHRIKE (Lnnuix . -i i ji -11
minor). the former is comparable to the maxillo-
turbinal and the latter to the naso-turbinal
n, upper, and o. lower „ ,, , . ,
nasal passage; LK, of Mammals (q.v.). A special projection
air-chamber, which composed of undifferentiated epithelium and
extends int.. a hollow grated in the outer chamber may be dis-
ot the upper turbinal; . . , , ., , , 7 . ; rm
OAf, MM, upper and tmguished as the vcstibular turbinal. Inere
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
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- f
geminal, and corresponds to
the lateral nasal gland of
Lizards.
nr
cr.
OK.
Fm. 196.\. — LATERAL VIEW OF THE NASAL
CHAMBER IN THE HUMAN EMBRYO.
7, inferior (maxillary), //, middle, and
///, superior turbinal ; cr, base of
skull ; n, tip of nose ; ox, Eustachian
aperture ; pi, hard palate ; t, super-
numerary ridge (ectoturbinal) which
occurs in the embryo.
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
vcgion 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,
(\ \ ir '«
ft \M
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,
— OS.
V. i.
—r. ii.
Fit;. 196u. — SAGITTAL SECTION THROUGH THE
L.« NASAL AND BUCCAL CAVITIES OF THE
HUMAN HEAD.
/. inferior (maxillary), //, middle, and ///,
superior turbinal ; lie, entrance to mouth ;
lij, tongue ; o-s, aperture of Eustachian
tube ; m', frontal sinus ; *«.", sphenoidal
sinus; r.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 Avails 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 ethmoturbinals project forwards between the
nasotnrlinals and maxillpturbina,ls\ 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 (endoturbinals) are peculiar
to Mammals, as are also certain accessory folds situated laterally
to them and also belonging to the ethmoid (Fig. 198) : these may
c
D
E
J
Fi<;. 197. — -VARIOUS FORMS OF THE MAXILLOTURBINAL BONE IN MAMMALS.
A, double coil ; _/>, transition from latter to single coil, E, F ; C, transition from
double coil to the dendritic form D. (After Zuckerkandl.)
be described as the postero-lateral or ectoturbinals to distinguish
them from the endoturbinals 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 endoturbinals 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 Lemur:. In Ungulates, Elephants, and Edentates, further
complications have arisen, and the number of endoturbinals has
considerably increased secondarily (/.//. to nine in Orycteropus). In
Primates (Figs. 196 and 199), on the other hand, reduction has
occurred from the condition found in Lemurs.1
The ectoturbinals differ so greatly in the individual orders and
even species that they cannot be reduced to a common type. In
Marsupials, Insect! vores, 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
,* Eiidnt ii rhinitis
Ectoturbinals --\\f
~~* Endoturlindls
Septum nasi
FIG. 198.— DIAGRAMMATIC TRANSVERSE SECTION THROUGH THE NASAL CAVITY
OF A MAMMAL, TO SHOW THE RELATIONS OF THE ENDO- AND ECTO-
TURBIXALS. (Modified from Paulli.)
lobes, &c., cf. pp. 200, 228), we may distinguish between Mammals
which are macrosmatic (the majority of the mammalian orders),
microsmatic (c.y. Seals, Whalebone-Whales, Primates), and tnws-
matic (most Toothed Whales).
Except in Monotremes, the nasal chamber communicates
with neighbouring cavities, such as the maxillary, frontal, and
sphenoidal sinuses (Figs. 196fi 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 ;
1 Primates possess one to three ethmoturbinals, but traces of as many as five
have been recognised in the embryo. The nasotnrbinal 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 the skin, the function of which is not known.
270
COMPARATIVE ANATOMY
but with the reduction of this sense, they lose tleir primary
function, often persisting merely as air-sinuses or wen disap-
pearing entirely l (Pinnipedia).
The nasal glands of Mammals may be divided into two sets,
—numerous small, diffuse Bowman s glands, and a large Stcnson's
aland. The latter appears early in the embryo, and in many cases
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
FIG. 199.— TRANSVERSE VERTICAL SECTION Man being of a specialised
THROUGH THE NASAL CAVITY OF MAN. type not exactly comparable
a, I, c, inferior, middle, and superior nasal to tne SO-Called external
passage ; C.cr, cranial cavity ; HG, hard nose of other Mammals,
palate ; /, //, ///, inferior (maxillary), ft jg supported by an out-
middle, and superior turbmal ; J, J, , . fj , ,
position of vestigial Jacobson's organs, ward extension 01 the nasal
which are situated nearer the floor of the bones and by the cartila-
cavity than is indicated in the figure; M, ginous septum nasi which
maxilla; Or, wall of orbit; S, septum & ff ,, ., ,
nasi ; SL, ethmoidal labyrinth ; *, point anses n'om the ethmoid, by
at which the nasolacrymal duct opens ; the roofing lateral nasal
t, entrance into the maxillary sinus (C.m). 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
solid 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
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 Insect i-
vores and Bats the maxillary sinus is the only one present.
VOMERO-NA8AL ORGAN 271
forms in which the external nose grows out to form a longer or
shorter tru ik or proboscis, at the distal end of which the nostrils
open (e.fj. Shrew, Mole, Pig,1 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 Ncisulis the peculiar and grotesque
external nose, with downwardly directed nostrils, cannot be
direct!}7 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 Iarva3 of Anura and Myctodera a
small gutter-like medio-ventral outgrowth 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 Csecilians 2 (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)/5 It contains a well-marked,
1 The external nose may be further supported by a median pre-nasal bone
(e.y., Mole, Pig). In Chrysochloris it is capped by a horny shield, and in
Condylura its Hat 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 witli
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.
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, Ornithorhynchus
(After Symington.) H, Placental Mammal ; I, the same, in longitudinal vertical section.
u, epiderm ; '•../, Jacobson's cartilage ; (Ln, naso-lacrymal duct ; y.m, intermaxillary gland ; g.n,
nasal gland ; jc, Jacobson's organ ; mx, maxilla ; na, main nasal cavity ; n.o, olfactory
nerve; ;/./, trigeminal nerve; o.d, dumb-bell shaped bone (prevomer) ; sp, septum nasi.
EYE 273
turbinal-like ridge, supported by cartilage continuous 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-
sept al cartilages, which, as in Lizards and Monotremes, are
differentiations of the nasal septum (e.rj. Marsupials, Edentates,
Insectivores, Rodents, Carnivores, Ungulates). A branch of the
olfactory nerve enters 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
c)
olfactory nerve.
EYE.
As already mentioned, the first rudiment of the eye arises as
a paired outgrowth from the primary fore-brain, known as the
primary 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 choroid
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-
thelium, 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 embryo, 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 sharply-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 nerve.
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 primitive 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
r/i
jf
A B
FIG. 201, A. — DIAGRAM SHOWING THE MODE OF FORMATION OF THE PRIMARY
OPTIC VESICLES (AR1).
VH, fore-brain ; V,V, ventricular cavity of the brain, which communicates with
the cavities of the primary optic vesicles at tt-
B.— SEMIDIAORAMMATIC FIGURE OF THE SECONDARY OPTIC VESICLE, AND OF
THE LENS BECOMING SEPARATED OFF FROM THE ECTODERM.
C, 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 ; t, 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, mesoderrnic tissue, winch at J/1, M1, 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 l which ex-
tends through the so-called choroid fissure
(p. 273), and gives rise to the vitreous body or
hu'inour (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 Ce <0
the dilator and constrictor (sphincter) muscles plt, 202.— CHIASMA OF
of the iris, which are able to increase or THE OPTIC NERVES.
lessen the size of the pupil ; the iris thus Semi diagrammatic.
i , i c A, most leleo.stei ; B,
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
in form, becoming more flattened or more
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 choroidecu) 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
1 By some embryologists this tissue is said to be ectodermic, and not meso-
dermic in origin, except as regards the evanescent embryonic blood-vessels.
T 2
Herring ; 0, Lacerta
agilis ; D, Agama ; E,
Mammal.
S, Sl, lateral fibres,
which do not cross.
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-
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 for the physio-
logical activity of the nerve
end-apparatus. Between the
cornea and iris there is a large
lymph-space, the aqueous cham-
Ix-r, its contained fluid being
called the aqueous humour : ex-
tending around the chamber is
a venous plexus, which is bathed
by the aqueous humour.
The relative development of
FIG. 203. — DIAGRAM OF A HORIZONTAL
SECTION THROUGH THE LKFT HUMAN
EYE, seen from above.
C, ciliary process ; Ch, choroid, with
its lamina fusca (L/) and vascular t, j affected by the ex-
layer (GS) ; Of, conjunctiva; Go, f ,. . J ,
cornea; CP, canal of Petit; Cti, teriial conditions,1 and IS 111
venous plexus (canal of Schlemm) ; 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-
Cv, vitreous chamber ; Fo, yellow
spot (fovea centralis) ; H, hyaloid
membrane ; HK, so-called posterior
chamber : Ir, iris ; L, lens ; L<-,
ciliary ligament ; MD, posterior
elastic lamina (membrane of Des-
cemet) ; MF, blind-spot ; Op, optic
nerve ; OS, sheath of optic nerve ; ball is surrounded by a mem-
tina ; PE, pigment epithelium branouS) saC-like investment
(the periorbiia), which arises in
the region of the optic foramen
of retina ; Sc, sclerotic ; FA', aqueous
chamber ; Z, Zonula ciliaris (zone of
Zinn).
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
1 The adaptive modifications of the eye are very varied amongst Vertebrates.
1 1 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.y. Fixhe* — Amblyopsis spelieus, Troglichthys, Typhlo-
gobius ; Amphibian* — Proteus, Spelerpes maculicauda, Typhlotriton, Typhlo-
molge, (jymnophiona ; Reptile* — Typhlops vermicularis, Rhineura floridana ;
Mammal* — Notoryctes typhlops, 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 orgaris. (Cf.
also under Cyclostomes, p. 278).
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 (palpcbroe).
2. Glandular organs.
3. Muscles 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 vitreous humour. In addition
there are the above-mentioned accessory structures: these, as well
as the retina, will be dealt with after a description of the eyes
of the various Classes 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
differs 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 Avail of the
cerebral vesicle, which is supposed to serve as a light-perceiving
organ, a scries 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 dorsally, 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 development, and 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
B
Pig
— St
FIG. 204. — A. — THE ANTERIOR PORTION OF Amphioxnx lanceolafit*.
( Modified from H. Joseph. )
B. — ONE OF THE EYE-LIKE ORGANS IN A. (After H. Joseph.)
G'K, capsule, with neuroglia elements ; A', nucleus of optic cell ; A7, its nerve-
process, and f, its granular border ; Piy, 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
space is left for the vitreous
humour. It differs from
that of other Vertebrates
in the fact that, in the
condition of rest, it is ac-
commodated for seeing
near objects. Fishes pos- T
sess no ciliary muscle, and /\^
in many of them (most L$ .9....
Teleostomi) accommoda-
tion takes place by means
of a process of the choroid,
the processus falciformis.
This extends through the
embryonic choroid fissure
into the vitreous humour
towards the lens, around
which it expands to form
the so-called campanula
HaUcri, which is often
pigmented (Fig. 205). In
the interior of this struc-
ture are nerves, vessels,
and smooth muscle-fibres,
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-
FKJ. :205. — DIAGRAM OF THE EYE IN A
TYPICAL TKLEOST.
Ay, argentea ; Co, cornea ; Cp, campanula
Halleri ; CV, vitreous body ; Ir, iris ; L,
lens ; L-s, lamina suprachoroidea ; Lr,
lamina vasculosa ; Op, optic nerve ; OS,
sheath of optic nerve ; PE, pigment epi-
thelium ; Pr, processus falciformis ; Ri ,
retina ; Sc, sclerotic, with cartilaginous
and osseous (I) portions; Tp, tapetum ;
VK, aqueous chamber.
280
COMPARx\TIVE ANATOMY
choroideal lymph-space, is a silvery or greenish-gold iridescent
membrane, the arf/entca. 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 tapctum luciduw, is present
in Elasmobranchs internally to the iridescent portion, and within
this again is the layer of the choroid known as the choriocapillaris.
No tapetum appears to be present in Teleostei or Petromyzon.
--IT'
Fi<;. 206. — LONGITUDINAL VERTICAL SECTION OF THE EYE OF DLSSOMMA.
(After A. Brauer. )
Ch, choroid ; Fa, fibres of the argentea ; LK, lens-cushion ; L]>, ligamentum
pectinatum ; M, smooth muscle ; o, optic nerve ; rr, accessory retina ; rr',
portion of accessory retina ; .SV, 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. Inmost 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.1
In the Flat-fishes (Pleuronectidse), 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 falciform is, and a tapetum, argentea,
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 compared with those of Fishes : in certain respects
they show negative characters as compared with the latter, for an
argentea, 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 muscle.'2 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
1 In Periophthalmus and Boleophthalmus the structure of the eye is pecu-
liarly modified 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 homologue of the retractor
of 1 he 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 bony sclerotic 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 oft' from an internal smaller one,
the outer portion being bounded externally by
Fi<;. -207. — EYK the very convex cornea and enclosing a large
owLacertamu- aqueous chamber: moreover, the whole eye is
•
SHOWINC ,• , , /T7V cessus 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.
(«) 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
recti (superior, inferior, external or posterior, and internal or anterior),
and two as the cMiqui (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.1
Besides these six muscles, others are usually present from the
Amphibia onwards. Of these, the retractor lulbi, 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.
1 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
(&) EYELIDS
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. 203 ),1 and in
the Ichthyupsida 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.2
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, Amphisbsenians, and Snakes
the two e}Telids grow together to form a transparent membrane
overlying 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 Meibornian glands, is developed
(Fig. 210, B), and they are closed by a circular muscle (orbicufaris
or sphincter oculi) which surrounds 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 membrane? This " third eyelid " differs
from the others in having nothing to do with the outer skin proper,
consisting simply of a reduplicature of the conjunctiva, and being
1 In Fishes and Amphibian larva; 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 corneuni becomes less horny.
2 Accessory lid-folds occur, e.g. , in the Herring and Salmonida;.
:: 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 quadralus (bursalis) 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 lacrymal,
(2) the Narderian, or gland of the nictitating membrane, and (3)
the Meibomian (/lands.
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,1
which is situated at the anterior angle of the eye, surrounding
to a great or less extent the antero- 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-
1 In Crocodiles, Snakes, and Hatteria the lacrymal gland is wanting, while
in Chelone it is extremely large.
GLANDH OF THE EYE
289
jiinctival 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 puncta lacnjinalia are situated, often on
small papilhe. 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
Fie. 210. — A. HARDERIAN GLAND (H, II1) AND LACRYMAL GLAND (Th) OF
Any uin fragilis.
B, eyeball ; M, muscle of jaw.
Fio. 210. — B. DIAGRAMMATIC TRANSVERSE VERTICAL SECTION THROUGH THE
EYK OF A MAMMAL.
B, eyeball ; Fo, Fo, upper and lower conjunctival sac ; H, H, eyelashes ; LH,
LII, 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.
D, naso-lacrymal duct ; 8, lacrymal sac ; TD, lacrymal gland, divided up into
several portions: TR, TR1, 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.
The 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 known as the glands of Moll are also present within the
eyelids of Mammals, opening on the margins of the lids close to
the eyelashes.1
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 plate) 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.
A, eye; LB, primitive After the vesicle of either side has
tagh^: wat of "he Become separated off from the ectoderm,
head ; HG, olfactory pit ; it sinks deeper and deeper into the meso-
dermic tissue of the skull, loses its original
pynfoim or rounded shape, and becomes
divided into a superior and an inferior
par^ called respectively the utriculus and
sacculus, at first connected with one another
by a wide utriculo-saccular canal (Fig. 213). From the former the
so-called semicircular canals become developed, while from the latter
the tube-like ductus endolymphaticus and the lagena (cochlea) are
formed.
1 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 embryo. A
greater or less reduction of the lacrymal gland may also occur in other Mammals,
t.tj. Seal, Hippopotamus, Elephant, Otter, and Mole.
FIG. 211.— HEAD AND AN-
TERIOR PORTION OF BODY
OF A CHICK. (In part
after Moldenhauer. )
t, point at which the ex-
to IV, first to fourth
visceral arches.
AUDITORY ORGAN
291
The whole of this complicated apparatus constitutes the internal
ear or membranous labyrinth. 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
Fio. 212. — ISOLATED ELEMENTS OF THE MEMBRANOUS LABYRINTH OF VARIOUS
VERTEBRATES. (After G. Retzius. )
A, from the macula acustica communis of Myxine, ylntinoxa ; B, from the macula
acustica neglecta of Raia clavata ; C, from the crista acustica of an ampulla
of Amblyxtwna ; I), from the crista acustica of the anterior ampulla of
liana esatlenta.
t~, 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 bony labyrinth can thus be distinguished, and
between them is a cavity (cavum perilymphaticum) filled 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 (endolympli), 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
se
ass
and these lie in planes roughly at right angles to one another.
They are distinguished as the anterior vcrf/m/, 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), which 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,
otoliths are present
regions of the laby-
rinth.
The sensory epithelium, to
which the branches of the audi-
tory nerve are distributed, is
utriculo-saccular canal ; de~, se, ductus Situated in the following parts of
and saccus endolymphaticus, the the membranous labyrinth : (1) the
former arising from the sacculus at three ampulla? of the canals, ill
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 neglccta, 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.1 The several portions
1 In addition to these, there is a transitory Manila tlm-hi* rtuni< ////'.< situated
in the region of the sacculo-cochlear duct.
RINTH OF A VERTEBRATE.
from the outer side.
As seen
ass, apex of the sinus utriculi superior;
ce, cp, anterior, external, and
ca,
posterior semicircular canals ;; aa, ae,
op, the corresponding ampullae : CMS.
t ; I, recessus sacculi (lagena) ; rec,
rtJCCSSUS 11 1 1*1 CIlll ', 6'j SUC/CUlllS £ ^?2»
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 maculae acusticse.
A number of the regions characterised by the possession of
this sensory epithelium are not concerned with the sense of
hearing: those of the ampulla', 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
Fio. 214. — LONGITUDINAL SECTION OF AN AMPULLA OF Gomus. (After Hensen.)
a, base of semicircular canal ; b, point of opening of the ampulla into the alveus
communis ; c, epithelium on the wall of the ampulla ; d, auditory hairs ; n,
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 meat/us; M, M, pinna; Mt, tympanic
membrane ; O, wall of nieatus.
Middle Ear. — Ct, Of, tympanic cavity; M, fenestra rotunda (cochlea?) ; O1, 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) ; Tb, Eustachian tube ; Tb', its opening into the pharynx.
Internal Ear, with the greater part of the bony labyrinth (KL, KL1) removed.—
'a, b, the two vertical canals, one of which (b) 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,
which ends blindly at - ; Con\ bony cochlea ; Gp, cavum perilymphaticum ;
Cr, canalis reunions ; D.p, ductus perilymphaticus, which arises from the
scala tympani at d, and opens at 2).pl ; .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
labyrinth ; .S'y and >SV, scala vestibuli and scala tympani, which at * pass
into one another at the cupula terminalis (6V).
structures become developed (Fig. 215). In the first place, a canal,
developed in the position of the hyomandibular or spiracular cleft,1
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 Mammalia. .
AUDITORY ORGAN 295
takes on a close relation to the auditory apparatus, and gives rise
to a spacious chamber, the tympanic cavity or middle car, communi-
cating with the pharynx by the Eustachian title, and being closed
externally by a vibratory tympanic membrane, between which 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 canals, which remains essentially much in the
condition already described, and a pars inferior — constituted by
the sacculus 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 sacculo-
cochlear canal : it is absent in Chimaera. The utriculus and sacculus
also communicate with one another by the sacculo-utricular canal.1
In Elasrnobranchs the ductus endolymphaticus opens to the
exterior dorsally, and is thus in free communication with the sea-
water.2 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 Chima3roids, 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-
1 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.
adc
~Jui
ns
rs
ms
FIG. 216. — MEMBRANOUS LABYRINTH OF VARIOUS FISHES. (After G. Ret/Jus.)
A, Afyxine glutinosa, from the inner side.
raa, ctp, anterior and posterior ampulla ; cc, canalis communis ; de, cluctus endo-
lymphaticus ; era, crista acustica of the anterior, and crp, of the posterior
ampulla ; me, macula acustica communis ; ra, rp, anterior and posterior
branches of the auditory nerve ; xc, saccus communis ; se, saccus endolym-
phaticus.
A1, Acipenser sturio, from the outer side ; B, Ckimcera monxtroxa, from the inner
side ; C, Perca JluviatUis, from the inner side.
aa, ae, ap, anterior, external, and posterior ampulla ; ac, auditory nerve ; ana,
apex of the sinus superior ; era, ce, cp, anterior, external (horizontal) and
posterior semicircular canals ; cr, crista acustica of the ampullae ; o/*,
utriculo-saccular canal ; ony 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 (ductus
cochlea/ins or seal a 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-
JCff
Fit;. 222. — BONY COCHLEA OF MAN. (After A. Ecker.)
A, axis; H, Immulus ; Lso, Lso1, 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 ; XS, bony cochlea ; L, limbus lamina? spiralis ; Lo, Lo1, 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 ; K,
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 ^estibuli 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 cochlea?.
On the floor of the bony cochlea, not far from the fenestra
cochlea?, is an opening leading into a narrow ductus perilymphaticus
or agueductus cochlea? (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 spiralis ossea (Figs. 223, 224). On
the free border of the latter they pass out, and break up into
terminal fibrill» on the inner surface of the basilar membrane.
The fibrillaB 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 (mcmbrana reticulnris) 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
Fio. 224. — THE ORGAN OF CORTI. (After La vdowsky.)
B, B, basilar membrane ; Ba, Ba, bacilli, or supporting cells; 6', membrane of
Corti ; Lo, Lol, the two plates of the lamina spiralis ossea ; Ls, ligamentum
spirale, passing into the basilar membrane ; Mr., meinbrana reticularis ; X,
auditory nerve with ganglion ; JV1, JV-, the nerve branching up into fibrilla-
and passing to the auditory cells (G, (V) ; A', membrane of Reissner ; X«<,
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 pinna 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 hyoid1 by means of the
1 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
s.
FIG. 225. — THE PINNA OF VAKIOUS PRIMATES.
In A, the shaded portion (b) 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 auricula?; 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 Schwalbe ; 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, antihelioc, tragus, arid 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 Mammals : (a) attrahentes s. adductores, (b) levatores s. attolentes,
(c) abductores s. retrakcntes, (d) depressores, and (e) rotatores. A
gradual reduction of these muscles is seen in the following series :
Artiodactyla and Perissodactyla, Canidte, Felidoe, Prosimii, Primates,
the reduction being most marked in Man.1
1 The auditory fold may undergo marked reduction, e.g. in aquatic and sub-
terranean forms. Thus amongst the Pinnipedia, only the Otarida; possess an
"external ear." The corresponding muscles become transformed into sphincters
for closing the auditory aperture.
x 2
F. ORGANS OF NUTRITION.
ALIMENTARY CANAL AND ITS APPENDAGES.
The alimentary or enteric canal consists of a tube which
begins at the aperture of the mouth, passes through the body-
cavity (coelome), and ends at the vent or anus.1 The walls of
the canal consist of several layers, of which the mucous mem-
brane, 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 muscular layer.'2 The epithelium is derived
from the endoderm, with the exception of that lining the mouth
and anus (stomodccum and proctodoeum) 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 unstriated fibres, supplied by
nerves from the sympathetic system, is, as a rule, divided into two
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 serous coat, the peritoneum, 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 (neoxtoma, 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.
2 A layer of smooth muscular fibres may be present in the submucosa, which
also encloses lymphoid or adenoid tissue (solitary follicles, Peyer's jMtches).
ALIMENTARY CANAL AND ITS APPENDAGES 309
. -J26. — DIAGRAM OF THE ORAL CAVITY AND PHARYNX IN A FISH (A),
AN AMPHIBIAN (13), A REPTILE (C), AND MAN (D).
Ch, internal nostril ; D, alimentary canal ; A', gill-slits ; L, lung ; N, external
nostril ; 0, 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 parietal 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 branchiw arise. Even in the Amniota,
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 primi-
tive mouth-cavity becomes divided into an upper respiratory,
and a lower nutritive portion — that is, into a nasal and a secondary
or definitive mouth-cavity. The separation, however, is never a
complete one, the passage being common to both cavities for a
certain region (pharynx'}, which in Mammals is partially separated
from the mouth by a muscular fold, the velum palati, or free edge
of the soft palate (Fig. 226, D).2
The alimentary canal of Vertebrates is typically divisible into
the following principal sections (Fig. 227) : — mouth- or oral-cavity,
pharynx, gullet or oesophagus, 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
1 In Mura'noids, Dipnoans and Lepidosteus, a rent rat 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 duodenum, jejunum and ileum, and the large intestine into
colon and rectum. A blind-gut or ccecum is often present at the
junction of the large and small intestine. Between the stomach
Gls
Gl.th
Gl.thy
Fi<;. 227. — DIAGRAM or THE ALIMENTARY CANAL OF MAN.
A, anus ; Ca, Ct, Cd, ascending, transverse, and descending portions of the colon :
Dd, 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) ; R, rectum ; Vic, position of
ileo-colic 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 surface 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 swim-bladder or lungs.
(5) The liver.
(6) The 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
lower 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 vestibulum oris ; this may become extended
on either side to form cheek-pouches, which serve as food reservoirs
(many Monkeys and Rodents).2 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
1 The mouth of the Lamprey serves as a suctorial organ for attaching the
animal to foreign objects. The larva; of Lepidosteus, 1'olypterus, Lepidosiren,
Protopterus, and Anura are temporarily provided with suctorial organs.
2 Cheek -pouches, opening externally anil lined by hair, occur amongst
Rodents (Geomyidse).
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 papilhe of the mucous membrane; but
secondarily, owing to want of space, the epithelium of the mouth
grows inwards so as to give rise to a dental lamina which becomes
— EM
ZC
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 ; 0, odontoblasts ; SK,
dental lamina ; ZK, dental papilla.
FIG. 229. — SEMIDIAGRAMMATIC FIGURE OF A LONGITUDINAL SECTION THROUGH
A TOOTH.
PH1, 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 papilla ; the upper cells of the papillse, 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 snccessional teeth. This difference
is expressed by the terms polypliyodont 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 Plagiostomes, 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 (premaxilla and maxilla} ; (2) the palatal arch
(vomer, palatine, pterygoid); (3) the unpaired parasphenoid ; and
(4) the mandibular arch (dentary and splenial).1
True teeth, with enamel, enamel-epithelium, and odontoblasts,
1 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 arid 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 (Cha?todon), 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.1 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
very evident, and the same has been
shown in the case of the Amphibia.
In the Amphibia there is in
general a considerable diminution in the number of teeth
as compared with Fishes ; and at the same time a much
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
premaxilla, maxilla, and mandible (except in Anura), as well as on
the vomer and palatine, but rarely on the parasphenoid (e.g.
1 There are 110 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 Elasmobranchs, of Labyrinthodonts, Ichthyosaurians, and
probably also to the mammalian molars.
teeU,
parasphenoid.
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 jaws are present in larval
Anura (except Xenopus), and similar structures occur in Siren
lacertina. Teeth are altogether absent in the Bufonidae 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
ZK
RF
zs
B
Fi<;. 231, A — TOOTH OF FROG, and B— OF Salnmii«lra atra.
M, maxilla ; PH, pulp-cavity ; RF, 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, Agamae, and numerous
TEETH
317
fossil forms) a heterodont dentition appears to be indicated.1 In
spite of the great modifications which have taken place in the
palate of Crocodiles, their teeth, which have a conical form, show
the least amount of differentiation in the course 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
Fi = 52 — 54.
3 ' 1 ' 3 ' 5 or 6
q . 1 . A . Q
The more typical arrangement is '— = 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.1 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.
JA 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 internasal
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 (pharyngcal 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 * arid suUingual, 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 prefrontal
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).
1 In Lacerta, Anguis, and Pseiulopus 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
gneca there are well developed sublingiial glands.
Pterygosphenoidalis Pterygo-
ant. parietalis
i
Retractor quadrati
Maxilla
Fang
Quadrate
Pterygosphen-
oidalis post.
Mandible
Transveo-maxillo
pteryyo-mandibula
B
Maxilla
Palatine L -j
Pterygoid • -
— Duct oj poison-gland
— Fang
- Zygomatic ligament
-Poison-gland and sheath
x \\X\\iYVJf \ A^\.l'\l I' xM.
Ptert/gosphcnoidalis post. »
•-" Masticatory muscles
~~ Ziigomatic ligament
— Mandible
Portion oj trans.-inax.-pter.-nw.nd. 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) submaxillary,
and (3) sublingual. 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.1
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, lingual, palatine, and labial glands), are com-
parable to the oral glands of lower Vertebrates.2
Tongue.3
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.
2 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.
3 For the papillae of the tongue, cf. 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 larvse 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
Fir,. 239.— FIGURE SHOWING THE siinrtpr t:rnp anfi ;„ mrmpotpr]
TONGUE OF THE FROG IN THREE sfiortei June, ;an<; 1& connecte
DIFFERENT POSITIONS. on the rloor 01 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.1
The surface of the tongue is velvet-like, owing to the numerous
papillae, and its mobility varies greatly in the different forms. It
is usually attached only by the anterior end (Fig. 239) or by a
i, --•
FIG. 240. — HEAD OF Spelerpesfusctis, 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
1 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 ai~e responsible for gripping the prey. In
the Aglossa (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 tnberculum 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 ( Vermilinguia,
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
Fringillidas), 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 (Trochilida?,) 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 glands1 and
1 A gland on the apex of the tongue (gland of Blandin or Nuhn) occurs in
Man. the Orang-outan, and Sheep.
330
COMPARATIVE ANATOMY
A
FIG. 241. — A, TONGUE, HYOID-APPARATOS, AND BRONCHI OF A GECKO (Phyllo-
dactylus europceus) ; B, TONGUE OF Lacerta; C, OF Monitor indicus ; D, OF
Emyn europcea ; E, OF AN ALLIGATOR.
R, bronchi ; HH and VH, anterior and posterior cornua of hjToid-apparatus ;
A', larynx ; L, glottis ; Ly, lung ; M, mandible; 7', 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. Felidas) its surface is horny.1 A fold, the so-called
sublingua (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.'3 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.3
THYROID.
In Amphioxus, as in Ascidians (Tunicata), a ciliated groove,
the endostyle, 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 FIO. 242.— THYROID
five visceral clefts, and in the course of de- AND THYMUS
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,
1 Horny papilla are present on the tongue of Ornithorhynchus.
- The so-called " lysxa" 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.
3 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.
Lacerta ay His.
, heart ; T, trachea ;
Tm, 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
diverticulum, 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.1
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
usually possesses right and left lobes
FIG. 243 — THYM us AND THYROID J " & , . c,
OF A YOUNG STORK. lying on the great vessels just alter
they leave the heart. In Birds (Fig.
B. bronchi ; H. heart ; Oe, ceso- _.••;, • j 11
phagus ; T\ trachea ; Tm, 243) the organ is paired, and has a
thymus ; Tr, thyroid. similar position.
The thyroid of Mammals (Fig.
227) consists of two lobes often connected by a median isthmus
1 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." l
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, CaBcilians, 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 thymus 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).2 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
1 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 ultimobranchial (postbranchial) 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.
2 Muscular elements occur in the thymus of Amphibia and Sauropsida.
334
COMPARATIVE ANATOMY
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CESOPHAGUS, 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.1
(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. Cyprinidfe, certain Labridse, Gobiidas, and
BleniidaB, 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.2
The intestine may be straight or nearly straight, or may be
more or less coiled, and in the former case a spiral fold or valve
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).3 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).
1 The so-called "Hassal's corpuscles" in the thymus arise secondarily from
groups of the small epithelial cells.
- In numerous Teleosts (e.y. Tinea vulgaris, Cobitis fossilis) outer longi-
tudinal and inner circular striated fibres are present in both stomach and intestine
externally to the unstriated muscular coat. They grow backwards from the
oesophagus.
3 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.
Carcharias).
336
COMPARATIVE ANATOMY
lr—-\
Pyloric cmca are met with in Ganoids
(except Araia) 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 Laemargus possesses a pair of
cseca 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 bladder and repre-
sents the allantois (q.v.) of higher forms.
In Plagiostomes a finger-shaped
rectal gland (processus digitiformis) x
opens into the anterior part of the
rectum, and this perhaps corresponds
to the blind gut or ccecum of higher
forms. Traces of a ca?cum are seen in
certain Teleosts (e.g. Box). In the
1 In the Holocephali there is no processus digitiformis, but the thick wall of
the rectum encloses numerous glaud-tubules.
I f/\r
sp.v
FIG. 245. — ALIMENTARY VIS-
CERA AND SWIM-BLADDER
OF Lepidosteus (in situ).
(After Balfour and Parker.)
a, anus ; a. ?>. swim-bladder ;
a. ft1, its aperture into the
throat ; b.d1, aperture of
bile-duct into intestine ;
c, pyloric cieca ; y.l>, gall-
bladder ; hp.d, hepatic
duct ; />•, liver ; pi/, pyloric
valve ; «, spleen ; sp.-v,
spiral valve ; at, stomach.
(ESOPHAGUS, STOMACH, AND INTESTINE 337
r
A
»FiG. 246. — ALIMENTARY
CANAL OF PERCH.
. , anus; Ap, p}'loric cajca ;
ED, rectum ; M, stomach,
with cajcal process (t) and
short pyloric region (P — P) ;
MD, small intestine ; Oe,
oesophagus.
FIG. 247.
FIG. 247. — ALIMENTARY CANAL AND APPENDAGES OF Protopterus annectenx.
(After W. N. Parker.)
up, abdominal pore; b.d, common bile duct, and b.d1, its aperture into the
intestine; b.ent, bursa entiana (anterior portion of intestine); cl, cloaca;
d.c, cloacal c;vcum ; c.m.a, coeliaco-mesenteric artery; cy.d, cystic duct,
boundary between small and large intestine ; HB, urinary bladder ; Oe,
oesophagus ; M, stomach ; Mz, spleen ; Py, pyloric region ; R, rectum.
Dipnoi a cloacal csecum 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
from the much wider stomach, which is usually sac-like, or bent
upon itself, in which latter case it lies transversely (Chelonians).1
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 forms 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 csecum 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 (inglumcs) (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 (proventricuhis), 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,
1 The oesophagus of marine Chelonians, like that of many Birds, is lined by
horny papilla?, 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 caecum is usually paired, and may
Oe
Oe—\
o.L.
C
FIG. 250, A. — DIAGRAM OF THE (ESOPHAGUS AND STOMACH OF A BIRD.
DM, glandular stomach ; Ig, crop ; MD, duodenum ; MM, muscular stomach ;
Oe, Oe1, oesophagus.
FIG, 250, B. — GLANDULAR STOMACH AND GIZZARD OF Fulica atra.
S, tendrinous disc. (Other letters as in A. )
FIG. 250, C. —TRANSVERSE SECTION THROUGH THE LATERAL PART OF THE GIZZARD
OF Tetrao iiroyallus.
DS, glandular layer ; L, lumen ; MS, muscular layer ; RP, keratinoid
triturating layer.
be extremely long (Lamellirostres, Rasores, Ratitae). All kinds of
intermediate stages between this condition and an entire absence
of a csecum 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 tarns, as in the Ostrich.
This so-called bursa 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,vrhich 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. 345)
According to the definition given on p. 335, 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 (Murida?) 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, psaltcrium, and
abomasuin. 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 abomasuin, 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
Oes,
H
Fir;. 251.— DIAGRAMS OF THE STOMACH IN VARIOUS MAMMALS SHOWING THE
DIFFERENT REGIONS. (After Oppel.)
A, ORNITHORHYNCHUS ; B, KANGAROO (Dorcojms litctom) ; C, TOOTHED WHALE
(Ziphiit*) ; D, PORPOISE ; E, HORSE, F, PIG ; G, HARE ; H, HAMSTER
( Cricet us frumentarius).
The resophageal 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.
/>. duodenum ; /(in H), fold bounding the cesophageal region ; 7 — IV (in D), the
four chambers of the stomach ; I (in B), lymphoid tissue ; Oes, cesophagus ;
P, pylorus ; x...x (in B), boundary line between the cesophageal and cardiac
regions.
(ESOPHAGUS, STOMACH, AND INTESTINE 343
The psalterium is the latest to be differentiated both phylo-
genetically and ontogenetically, and is rudimentary in the Tragu-
lidse. 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 Cams and Otto.)
a, oesophagus ; b, c, d, the three subdivisions of the rumen, marked off from one
another by the folds e and/; g, reticulum ; h, cesophageal groove ; i, psalte-
rium ; k, aperture leading from the psalterium into the abomasum (I, 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
(processus vermiformis) 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, which 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 Lamprey until metamorphosis.
In the adult Petromyzon, as well 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 larva?)
the individual cells are even capable of an active amosboid 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-
cellular 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 tubular 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,
A
:
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.)
A, Petromyzon, showing the spiral fold ; B, 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 Lieberkiihn} as well as
(in Mammals) to Br miner'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.1
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
J)u
FIG. 254. — LIVER OF Rana esculenta. From the ventral side.
Dti, duodenum ; H, heart ; L, Ll, L-, the different lobes of the liver ; M,
stomach. A gall-bladder is present, but is not indicated.
in the form of a tubular gland,2 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 Ratitre
and Mammalia in the small and large intestine are known respectively as plicae
circulares (vafriifw conniventes) and plicae semilunares.
- 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.
LIVER
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 csecum "
may probably be looked upon as the rudiment of a liver.
Qe
MD
EJJ-
Fio. 255. —VISCERA OF Lacerta agilis.
Bl, urinary bladder ; Ci, postcaval ; ED, large intestine ; GB, gall-bladder ;
If, heart : L, liver ; Lc3 ; G, gall-bladder ; L, L*, L'*, the lobes of the liver
turned forwards ; Lhp, duodeno-hepatic omentum ; M, stomach ; Py,
pylorus.
some Mammals). In some cases the ducts communicate with the
bile-duct1 (Fig. 257).
Varying much in form and size, the pancreas early gives rise to
a band-shaped or more or less tabulated and very vascular organ,
1 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, C3'clostomata) ; 2, liver and dorsal
pancreas (Elasmobranchii) ; 3, liver, and dorsal + 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 (i'.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
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-bladder or air-bladder 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.y. 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
outgrowths 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 branchial, 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 which
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.1
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 peribranchial chamber, opening by
a single pore situated somewhat behind the middle of the body
(for details, cf. Fig. 258).
The relative extent of the branchial apparatus is considerably
limited, even in the lowest Craniata, as compared with Amphioxus.
Cyclostomes. — In the Ammocoete-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 Ammoccete, 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
1 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, the}' 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 (cf. under (Skull and Larynx).
GILLS
353
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354
COMPARATIVE ANATOMY
backwards from the mouth, a ventral branchial or respiratory
and dorsal oesophagus (Fig. 259, B). Inspiration as well as expira-
\\\\\\\
B
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 l 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,
JnfHHML
Cli A'
FIG. 260. — LONGITUDINAL SECTION THROUGH THE HEAD OF A LARVAL LAMPREY.
l>, c, ventricles of the mid- and hind-brain ; Ch, notochord ; Ep, epiphysis ;
HH, ML^ hind-brain ; Juf, infundibulum ; K, K, K, the three anterior gills ;
N, nasal sac ; o, subdural cavity ; P, papillae of imicous membrane ; R, spinal
cord ; Th, thyroid (hypobranchial furrow) ; V, velum ; *, communication
between the ventricle of the olfactory lobe and that of the telencephalou,
which unite to form a common duct on either side ; this opens far
behind the branchial apparatus on the ventral side of the body.
1 In Bdellostoma there are usually six or seven pairs of branchial sacs, and
behind these, on the left side, an <*\, to show the branchial
apparatus. In both figures the branchial arches on the left side are shown
cut through horizontally. (From R. Hertwig's Zooloyy. )
a*, external branchial apertures ; 1>, branchial arch ; II1, bP, hemibranchs ;
/i, branchial septum; lim, hyomandibular ; is, internal branchial apertures
and gill-rakers : m, oral cavity ; inn, maxilla ; o, cesophagus ; op, operculum ;
opt, opercular aperture ; pa, palatine ; phi, inferior pharyngeal bone ; Pq,
palatoquadrate, and «., its connection with the cranium anteriorly ; prm,
premaxilla ; s, pectoral arch ; «/.', lower jaw ; ;, tongue.
thus consists of the branchial septum and arch phis 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,1 open separately to the exterior, and a vestigial
1 There are six in Hexanchus and Chlamydoselachus and seven in
Heptanchus in addition to the spiracle.
A A
*
35G
COMPARATIVE ANATOMY
sacs.
gill-cleft known as the spiro.dc (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 Chlamydoselachus fringed folds from the
hyoid and interb ranch ial septa project over the clefts.
In Ganoids and Teleosts there are no longer chambered gill-
The septa on which the gill-lamina3 are borne become
greatly reduced, so that the apices of the
latter extend freely outwards ; 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 follows. 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* THROUGH A
HOLOBRANCH OF Zl/'J« ii't
(ox THE RIGHT) AND
(Indus (ON THE LEFT).
SLIGHTLY ENLARGED.
(From R. Hert\vig's
Zooloi/y. )
a, afferent, and i; efferent
branchial vessels ; b,
branchial arch : lif1, an-
terior, and W-, posterior
heniibranch of the gill;
/(, septum ; r, cartilagin-
ous gill-ray ; z, gill-
rakers.
GILLS
357
As a rule Teleosts possess only four holobranchs,1 and this holds
good for all Ganoids. A vestigial gill or pscudobranch is present
on the anterior wall of the spiracle of many Elasmobranchs and
of cartilaginous Ganoids (mandibular pscudobranch) ; 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 opercular pseudobranch. Traces of
A B
FIG. 263.— DIAGRAM ILLUSTRATING THE MECHANISM or 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 pharyu-
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-rakers, 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
1 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.1 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 Ceratodns 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-lamellae extend round the
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.2
In embryos of Elasmobranchs
FIG. 264. — EXTERNAL GILLS OF , -mi /rr
LARVA OF Gymnarchw nilo- and certain leleosts (MeterotlS,
tifiis, 4 DAYS AFTER HATCH- Gymnarchus, Fig. 264), long, vas-
ING. (After J.S.Budgett.) cnlar,; 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, b). In larvse of
Protopterus and Lepidosiren somewhat similar external gills are
present, but are four in number 011 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
1 Other parts may also become modified to serve as accessory respiratory
organs. Thus in Cobitis and Callichthys intestinal respiration takes place ; and
in Monopteriis 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.
2 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
larvje, 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
KTi
FIG. 265.— LARVAE OF (a) Protopteru* annecteii* (17 DAYS AFTER HATCHING)
AND (b) Polypterua Itipradei (1^ 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-
tory suiface (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 surrounding 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 Typhlornolge, 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.1 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
C
D E
FIG. 266. — DIAGRAM ILLUSTRATING THE DEVELOPMENT OF THE AMPHIBIAN
GILL. (Mainly after P. Clemens.)
A, rod-like, unbranched, primitive form, retained in the adult of certain Anura
(e.tj. 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 glutmosum, for example, are feather-like.2
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
with the endodermal gills of Fishes.
'• The external gills of Amphibia present a great variety ,of form, often
resulting from adaptation. Thus, in the intra uterine larva1 of the viviparous
Salamandra atra, they reach a length of 5-6 centimetres ; in Ca^cilia com-
pressicauda they consist of two large, flattened, vascular folds, which apparently
cover the body of the larva like a mantle ; and in Notodelphys (Nototrema)
amongst Anurans, in which the larva; 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 (Hylodes
SWIM-BLADDER 361
II. 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 swim-bladder of other Fishes.
The exact point of origin of the swim-bladder from the ali-
mentary canal varies,1 and its duct (ducfus pncumaticns') may 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 (Physodisti].
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. Diffusion
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 g'vim-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
kidneys 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 body-wall (Rana opisthodon). In Xeuopus, the
branched tentacles at the angles of the mouth are said to have a respiratory
function, and, like the "balancers" of Urodeles (cf. note on p. 271), may corre-
spond morphologically to the external gill of the first visceral arch.
1 In Erythrinus it arises laterally, 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.1 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 caecal 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
FIG. 267. -- INTERNAL SURFACE OF of which resembles that of the
THE AIR-BLADDER OF LEPIDOSTEUS, iunffs of Dipnoi and Amphibia,
SHOWING THE TRABECUL^. ,& i j J V 1
and, as already stated, it has a
B, fibrous longitudinal band. 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).
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 former
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
Ifiryn.?, 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 phylogenctically older organs than the bronchi,
1 In Dactylopterus volitans, the two halves of the swim-bladder extend
beyond the general cielome into special cavities, and the organ is covered by
large bony plates. In the Gymnodonts (e.g. Dioclon, Tetrodon), the whole
oesophagus 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
JS-
PD
B
FIG. 268. — A, B, C, DIAGRAMS SHOWING THE MODE OF DEVELOPMENT OF THE
LUNGS.
b, bronchus ; PD, primitive alimentary tube ; 8, S1, 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
primary pulmonary vesicles extending into the thick wall of the latter. C.
T$y a marked thinning-out of the lung-wall, the vesicles form chambers, of
which four dorsal (d) and four ventral (y) are visible, each of which has
developed secondary buds (N) ; 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 infundibula,
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
o
FIG. 270. — DIAGRAM OF THE EMBRYONIC HUMAN LUNG, :: 50.
(After W. His.)
Af>, pulmonary artery ; Ib, pulmonary vesicle undergoing division ; Ir, air-
passage ; M, middle lobe of the lung ; O, right anterior upper lobe with
its " eparterial" bronchus; O1, left anterior lobe with its " hyparterial "
bronchus ; sp, oesophagus ; F, Tn, 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 innerved 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
Ao.
€av. or.
FIG. '271. — MEDIAN LONGITUDINAL .SECTION* THROUGH PART OF THE HEAD
AND TRUNK OF Lepidoxleus osseus.
Ao, aorta; Car, cavity of the swim-bladder; Co, Co2, and **, constrictors of
the pharynx, " larj*nx," and swim-bladder ; Da, dilator ; G, cranial cavity ;
K, cushion-like elevation of the supporting elements of the larynx ; L.B'j,
loose connective tissue between the swim-bladder and pharynx ; Peric,
pericardium ; PA1, 3, 4, intrapharyngeal branchial muscle ; Phar, pharynx ;
I\.M, neural canal ; SS, fibrous laryngeal supporting elements ; WS, vertebral
column.
lung; but it is doubtful whether this plate can be regarded as
the first 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 larynges, a " dorsal ". and a " ventral " (Fig. 272),
traces of the former of which can still be recognised in Lepidosiren
in addition to the ventral larynx.
366
COMPARATIVE ANATOMY
Amphibians. — The vestibule, or laryngo-trachcal chamber, 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. — DIAGRAMMATIC TRANSVERSE SECTION ILLUSTRATING THE STRUCTURE
AND RELATIONS or THE DORSAL AND VENTRAL LARYNX.
A or, aorta : Cav.cr, cranial cavity ; Co, Co1, constrictor of the dorsal and ventral
larynx respectively : Co.ph, constrictor of the pharynx ; D, dorsal larynx ;
Dil, Dil1, dilator; KB. branchial arch; Kpf.D, lumen of pharynx: Jfuc,
mucous membrane of pharynx ; SE, SE1, siipporting elements ; F, ventral
larynx ; Va.p, pleuroperitoneum ; /•,
vein.
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, Amblystomatinae, Desmognathinse, Plethodontinse) 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
_- Eli
FIG. 285.— LUNG OF (A), Emy* lutaria (T6 MM. IN LENGTH), AND B, Anyuis
fragiiis, 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
(EH), 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 may
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 Amphisbaenians, 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
Kecomlftrii bronchi in
a, it' riin' part of I ,' mi
/'• ,lt ,'lll 1,111 I'llill
Xii--l>roncliial sao.
LUNGS
385
-m.l.c
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.
an, 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 ;
bl, 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 ; He, Hi, internal and external branch of
the second entobronchium : the end of He opens into the sub-bronchial sac
at d ; ///, third entobronchium, with the aperture for the anterior thoracic
air-sac; IV, fourth entobronchium ; m.l.c, longus colli muscle ; N, kidney;
Oe, oesophagus ; stv, stv, sections of ribs which are connected with the
sternum ; 7V, trachea ; r, v, ends of free vertebral ribs. The boundary of
the pulmonary aponeurosis is seen along the outer edge of the lung, and the
costopulmonary muscles 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 o
386
COMPARATIVE ANATOMY
a
a
between the muscles, beneath the skin, and even into most
of the bones. The latter are thus rendered pneumatic, 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.1
The air-sacs, though not serving
to increase the actual respiratory
surface,2 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
raised and lowered. But during
flight, when the weight of the body
is supported by the wings, the
sternum, as well as the coracoid and
ribs, are relatively immovable, and
inspiration and expiration are effected
by the raising and lowering of the
wings, which may take place from three to thirteen times in the
1 The bones of Archsopteryx 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 splienoidal 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 che aorta, the veins communicating with the postcaval.
FIG. 292. — DIAGRAM OF THE AR-
RANGEMENT OF THE BRONCHI IN
MAMMALS. From the ventral
side.
A, pulmonary artery; a, a,
"eparterial" bronchus of either
bronchi ; V, pulmonary vein.
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 trie 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
throughout the length of the lung
and gives off 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.1
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.
cases an accessory lobe anteriorly,
and another posteriorly, the former S, sulcus for the subclavian artery ;
in connection with an apical bronchus t^in^^cor^a ^T^l ^lobe's
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
1 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 " hyparterial 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 asymmetry are not
clear, it is better to speak of the anterior secondary bronchi, whether " epar-
•terial" or "hyparterial," merely as apical bronchi.
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 alvcolarcs or " infundibula " (Fig. 294, B), which
are surrounded by a close network of capillaries, and the walls of
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; Srl, secondary bi'onchi : LP, "lung-pipes"' (para-
bronchia). The arrows indicate the ostia of the air-sacs (L).
B. Br, main bronchus ; J3rl, ventral, and Br2, dorsal secondary bronchi ; JA ,
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 Anarnnia, the serous membrane
( pleuroperitoneum') lining the ccelome 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 l and enclosed by the pericardial membrane.
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
1 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
anterior pleural and a posterior peritoneal chamber.
In each of these three serous membranes (pericardia!, 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.1
Towards the middle line, the parietal layer of the pleura of
either side is reflected so as to form a septum between the right
and left thoracic cavities. This septum is called the mediastinum,
and the space between its two layers the mcdiastinal space:
through this, the aorta, oesophagus, and postcaval vein run, and in
in -4
FIG. 295.— 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 ; H, heart ; m, mediastinum ; L, lung ; P, parietal, and Pl,
visceral layer of the pleura ; PC, Ps1, parietal and visceral layers of the
pericardium ; R, ribs (wall of thorax) ; S, sternum ; Tr, trachea ; W,
vertebral column ; It, 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 easy.
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 ccelome 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
1 In connection with the suspensory mesenteric structures in the peritoneal
cavity, mention must be made of the oinvittdl folds extending between the
abdominal viscera.
390
COMPARATIVE ANATOMY
described as genital pores, comparable to the like-named pores of
Teleosts (cf. under Generative Organs).
Apart from the indirect connection of the ccelome 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
j 7 if
'
FIG. 296. — DIAGRAMMATIC HORIZONTAL SKCTION THROUGH THE CLOACAL
REGION OF A PLAGIOSTOME. (After E. J. Bles. )
a, blind ecbodermal invagination (cloacal pouch) ; h, It1, cloacal papilla ; c, peri-
toneal cavity, which opens by the abdominal pore at c1 ; Gloake, cloaca ;
Darin, 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-
danidoe, Cestracionidae, and Rhinidaa, 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
Salmonidse and Mormyridse, 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
Ma.Kp
391
Ur.Qg
Ma.Kp Msn
B
Ur.Gg
Ma. Kp Msn
C
FIG. 297. — DIAGRAMS ILLUSTRATING THE THREE POSSIBLE WAYS IN WHICH THE
PERITONEAL CAVITY MAY COMMUNICATE WITH THE EXTERIOR IN FISHES.
(After E. J. Bles).
A. Connection by means of nephostomes (Xphs) only (Cestracion, Rhina, certain
other Elasmobranchs before sexual maturity, larval Amia).
B. Connection by means of nephostomes (X-ohn) and abdominal pores (Por.abd)
(certain adult Scylliida? and Spinacidse).
C. Connection by means of abdominal pores (Por.abd) only (Carchariidae,
Lamnida), Batoidei, Holocephali, adult Ganoids, certain Dipnoi and
SalmonidEe, Mormyridae).
Cl, Clo, cloaca, Ma.Kp, Malpighian capsules of the raesonephros (Man) ;
peritoneal cavity ; Ur.Gg, 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.1
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 ccelome,
which to a large extent represents an excretory organ (cf. Fig. 297).
1 Abdominal pores are apparently wanting in Lepiclosiren.
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 m 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 permeating 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 coining from the
intestine are known as lacteal s.
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 vessels 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 ot
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 blood-corpuscles are
of two kinds — colourless, nucleated, amoeboid cells, known as white
or colourless corpuscles or leucocytes, and far more numerous red
Hood-corpuscles or erythrocytes.1 The colour of these is due to
haemoglobin, 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 Camelidee, the red corpuscles are seen to
have the form of circular, biconcave discs;3 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/i 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 2'5/A (Tragulidse) to lOyu,.
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
1 In Amphioxus the blood contains no formed elements.
2 In addition to the leucocytes and erythrocytes, a third kind of corpuscle
occurs in the blood : these structures are known as Uood-plates or thromhocytes.
Each has the form of a minute, flat disc, is colourless and ama-boid, 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.
3 They are said to be primarily cup-shaped.
VASCULAR SYSTEM
395
—-So,
serous (pericardium}, a middle muscular (myocardium}, and an
inner epithelial (endocardium}.1 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 FIG. 298.— DIAGRAM SHOW-
the blood flows out, corresponding to the ING THE PRIMITIVE RE-
arterial portion. The venous end further
becomes differentiated to form another
chamber, the sinus vcnosus, 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 (conus arteriosus or pylangium} 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
arteriosus. 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.
A
Sir
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
Fm. 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 ; c, c1, 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 Ab, by means of the collecting trunks, »!?, Sl ;
>/», subclavian artery ; SI*1, subclavian vein; Si, sinus venosus ; V, ventricle;
VG, HO, anterior and posterior cardinal veins ; Yin, 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 mtelline or omphalo-
VASCULAR SYSTEM
397
mcscntcric 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-mesenteric veins (Fig. 300). These join with
the allantoic veins and veins of the alimentary canal to form what
Co.
L.of.A
SX,
FIG. 300. — DIAGRAM OF THE CIRCULATION OF THE YOLK-SAC AT THE END OF
THE THIRD DAY OF INCUBATION IN THE CHICK. (After Balf our. )
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 ; //, heart; L.Of, left vitelline vein; L.Of.A, left
vitelline artery ; H Of, right vitelline vein ; R.Of.A, right vitelline artery ;
S.Ca. V, anterior cardinal or jugular vein ; S. T, sinus terminalis ; S. V, sinus
venosus ; V.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 hematic portal vein, which divides up into
capillaries in the liver. The capillaries then unite to form the
licpaiic, wins, 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 rein 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 egg, 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 placenta!
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 pulmonary 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.
B
C
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, b, 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 alwaj's disappear ; I, pulmonary artery ;
s (in F), left subclavian artery ; st, and s (in B), ventral aorta ;"l' and 3',
first and third aS'erent 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 and F, second arch of the right
side.
400
COMPARATIVE ANATOMY
in.
THE HEART, TOGETHER WITH THE ORIGINS OF THE MAIN
VESSELS.
Fishes (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 auriculae (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 trabeculaj
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)
O.av.
D.C.s
Fct.x. Fa.d.
FIG. 302. — HEAKT OF Acanthis vnl-
yaris, FROM THE DORSAL SIDE,
WITH THE ATRIUM CUT OPEN.
(After Rose.)
Co, tr, truncus anteriosus ; D.C.d
and D.C.s, 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
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 fibre-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
A
B
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 ; (7, Teleost. In B and 0
the sinus venosus and atrium are not indicated,
a, atrium ; //, bulbus arteriosus ; c, conus arteriosus ; k, valves ; ,y, 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 the aortic-
roots by means of the efferent branchial arteries. The paired pul-
D D
402
COMPARATIVE ANATOMY
LJ7K—
— l.posl.c£i>rd>
•Si-v.
FIG. 304.
FIG. 305.
Fit;. 304 — HEART OF Protoji/t nt* nnnecttns. From the left side, part of the wall
of the left atrium being removed. (After Rose.)
Co, conus arteriosus ; L.Vh and R.Vh, left and right atria; S.a, septum
atriorum ; Si. ?>, sinus venosus, within which the pulmonary vein (Lv) extends
to open into the left auricle by a valvular aperture ; W, fibrous cushion
extending into the ventricle.
FIG. 305. — Ceratodtisforsteri. DIAGRAMMATIC VIKW OF THE HEART AND MAIN
BLOOD VESSELS AS SEEN FROM THE VENTRAL SURFACE. (From Parker and
HaswelFs Zoology, after Baldwin Spencer. )
off". 1, 2, 3, 4, afferent branchial arteries ; 1 br, 2 br, 3 br, 4 br, position of gills ;
c.a, conus arteriosus ; d.a, dorsal aorta; d.c, ductus Cuvieri ; epi.l, epi.2,
epi.3, epiA, efferent branchial arteries ; hy.art, hyoidean artery ; i.v.c, post-
caval vein ; I. ant. car, left anterior caroiid artery ; l.aur, left auricle f.hr.r,
left brachial vein ; f.jtii/.r, left jugular vein; l.post.car, left posterior caro-
tid artery ; l.y>oxt.card, left posterior cardinal vein : J.pul.art, left pulmonary
artery; I sc. r, left sub-scapular vein ; r.mit car, right anterior carotid artery ;
r.rtur, right auricle ; r.ln-.r, right brachial vein ; r.jug., right jugular vein ;
i:i>oxt.<-ar, right posterior carotid ; r.piif.nrt, right pulmonary artery ; r.sr.r,
right subscapular vein ; mi/, ventricle.
monary artery like the corresponding vessel in Crossoptf ry-
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 pulmonary 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 l (Fig. 304). Thus the blood once more becomes
purified before it passes into the left side of the ventricle. A post-
caved 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,'2 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
A
FIG. 306. — DIAGRAM SHOWING THE COURSE OF THE BLOOD THROUGH THE HEART
ix UrodfJa (A) AND Anura (B).
A, right atrium ; A1, left atrium ; ir, fi; pulmonary veins; tr, conus arteriosus,
divided in Anura (B) into two portions, tr, tr1 : through tr -venous blood
passes into the pulmonary arteries, Aj>}, Aj>1, while through tr1 mixed blood
goes to the carotids, ci — <'e, and to the roots of the aorta, Ft A ; V, ventricle ;
11, v, 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 cords. 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.
1 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 are wanting, though the pulmonary artery can still be recognised.
D D*
404
COMPARATIVE ANATOMY
Neither in Vrodela nor Anura is there a .septum ventriculorum,
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 t \vist, 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 caw.m pidmoncde, and a ventral caw in
aortictim communicating with the carotid and systemic
/. ft.
y.a
j. a
Try/.
FIG. 307. — HEART OF Cryptdbranchiis japonicus. From the ventral side, i After
Rose.) The left atrium is cut open.
L.i, L.i-1, the t\vo pulmonary veins, opening by a single aperture into the left
atrium ; L. Vh, P. Vh, left and right atria ; O.ar, atrio-ventricular aperture ;
P.d and P.*, left and right pulmonaiy arteries ; 8.n, septum atriorum,
perforated by numerous small apertures; tr, truncus arteriosus; V.c.d
r.t.x, posterior cardinal veins; r.*1. /, postcaval vein; V.j.d and !"../.«,
jugular veins; V.*.d and !".>.•>-, 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 conns.1
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 (cr. 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, which
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 ; <-i, internal
carotid ; It A, aortic roots ; /*'. truncus arteriosus ; 1 — 3, the three afferent
branchial arteries ; 7 — 777, 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 branchial by net-like
1 Special vessels supplying the walls of the heart are apparent!}' wanting in
the case of many Fishes and of Amphibians. In Elasmobranchs, coroiinrt/ nrferies
arise from the hypobranchial artery and coronary veins 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 rcte
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
FIG. 309. — ARTERIAL ARCHES OF AX ADULT f>rtlamainli-« ,n\VN
SPREAD OCT. (After J. E. V. Boas.)
"/, carotid labyrinth ; ce, external carotid ; <-i, internal carotid ; co, tr, truneus
arteriosus ; n-, (esophageal vessels; KA, root of the aorta; 1 — 4, the
four 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 (f) 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
ductus liotfdli. The third arch varies greatly in its development :
it may be present on one side only, or may even be entirely
wanting (e.g. Triton). In the larv;c 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 thi»
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
FIG. 310. — HEART OF A, Lacerta muralis, AND B, A
LARGE Vnranus, SHOWN CUT OPEN ; C, DIAGRAM
OF THE REPTILIAN HEART. Ventral view.
A, A1, atria ; Ao, dorsal aorta ; Ap, Apl, pulmonary
arteries ; Asc, As, subclavians ; Ca, Ca1, carotids ;
Gi, postcaval : ./, jugular; RA, root of aorta;
tr, Trca, innominate ; Vp, pulmonary vein ; Vs, subclavian vein ; T', V1, ven-
tricles ; .7, 2, 1st and 2nd arterial arches ; t, *, right and left aortic arches.
In C, the pre- and postcavals are indicated by Ve, Ve, only one precaval being
represented. A fibrous cord connects the apex of the ventricle with the peri-
cardium in most Lizards.
RA
RA
Anamnia, and this is more especially the case in Amphisbeenians,
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
this
A n.s
I.
A n
..Vf.il
c.d
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 :
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 two 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.1
The principal advance in structure
as compared with the amphibian
heart is, however, seen in the appear-
ance of a muscular ventricular septum,
which may be incomplete (Hatteria,
Lizards, Snakes, Chelonians), or com-
plete (Crocodiles). The higher Liz-
ards (c.y. Yaranidjp, 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 twro halves of the ventricle. A complete septum
ventriculorum thus appears for the first time in Crocodiles (Fig.
314), in which 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
1 In Ascalabota (< .;/. Tojanus), and cartilages may occur in the neighbourhood of the semilunar valves
of the aorta and pulmonary artery in other Reptiles (e.g. Crocodiles).
I. V....
Fii;. 311. —HEART OF Cydodu*
boddatrtei. From the dorsal
side. (After Rose. )
An, A /i.*, innominate arteries ;
Ao.rtlxt, dorsal aorta ; D.C.', spa-
tium intersepto-valvulare (cf.
Fig. 316); V.C.d, posterior
cardinal; V.j.d, jugular, and
V.x.d, subclavian vein of the
right side; V.c. i, postcaval vein.
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
J).C.S
•m^m
A.
FIG. 31-2.
FIG. 313.
FIG. 31'2. — HEART OF A YOUNG L'rovodilun itiloticiis. From the dorsal side.
(After Rose.)
A<1 and ^4*, right and left aortic arches; A.m, mesenteric artery; L.V.li,
K. V.h, left and right atria; S.d, S.s, subclavian arteries; Tr.<:c, common
carotid ; V.c.c, coronary vein. Other letters as in Fig. 311.
FIG. 313.— HEART OF Crowlilux nil ot If us. From the right side. (After Rose.)
Part of the wall of the right atrium is removed.
O.a.v, atrio- ventricular aperture ; Va.d and Va.x, the two sinu-auricular valves,
the white line between which is the margin of the septum sinus venosi. Other
letters as iri Figs. 311 and 312.
cross at their base, so that the left arises on the right side, and
vice versa. The most posterior arterial arch gives rise to the
pulmonary 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
A.c.p A.c.c.s
,,,.,M //•--
A-i-d^\\ I /
T.a.d
— T.a.s
-R.p.Z
FIG. 3 1 4. — DIAGRAM OF THE HEART OF A CROCODILE.
(After A. Greil.)
Ventral view.
Ac.c.d and Ac.c.s, right and left collateral artery ; A.c.p, prevertebraHcarotid :
A.cce, cceliac ; Ao.d, Ao.s, right and left aortic arches or roots ; Ao.dorx,
dorsal aorta; A.s.d and A.*.*, right and left subclavian ; At.d and Af.*,
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.v.s, right and left atrio-ventricular ostium ; P. A, root of
pulmonary artery; E.p.d and ft. p.*, right and left pulmonary artery;
(S'.ao. aortic septum ; S.ao.p, aortico-pulmonary septum ; S. V, ventricular
septum ; T.a.d and T.a.s, right and left innominate ; r.rfand Tr.*, 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
Birds and Mammals. — In these Classes, the atrial and
ventricular septa arc always complete, and there is no longer any
muscular walls of
This
mixture of the arterial and venous blood.
the ventricle are strongly developed and very compact.
particularly the case in the left
ventricle, on the inner wall of
which the -papillary muscles are
very strong : the left ventricle is
partially surrounded by the right,
the cavity of the latter having
a semilunar transverse section,
and its walls being much thinner
than those of the former (Fig.
315).
Both in Birds and Mammals
the blood from the head and body
passes by means of the precavals
and postcaval into the right
is
FIG. 315.— TRANSVERSE SECTION
THROUGH THE VENTRICLES OF
Grit* r/'ii('i-ea.
S,
atrium1 as does also that from
septum vent riculor urn ; TV,
right, and Vg, left ventricle.
venosus — especially in
the walls of the heart through
the coronary veins,2 and the sinus
Mammals — is scarcely recognisable (Figs. 316-318) : 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 bicuspid or mitral.3 Three semilunar pocket-like valves
1 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-
pendently, and the right precaval and postcaval together.
'• These open into the base of the left precaval (roronary Ktnitx), the rest of
\vhieh disappears in certain Mammals. Coronary arteries are also present, arising
from the base of the aorta.
3 There are no chorda3 tendinere in Monotremes, the heart of which in many
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-
may be taken as a type in Placental Mammals. In the right ventricle the two
lateral lappets are connected with three papillary muscles or groups of muscles ;
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
chord* tendinea? directly to the septum, at which point small papillary muscles
may be present. In the left ventricle there is a ventral and a dorsal group of
papillary muscles, from which the chorda? teudinese 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
A a.
MR.
-M.Kl
FK;. 316. — HEART OF GOOSE (An^a- cxli/rii-i*), DISSECTED FKO.M THE KICIIT SIDE.
(After Rose.)
The right atrium and ventricle are cut open, and their walls reflected. Ao,
aorta ; L. Vi, limbus fossa- 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,
J/AT1, muscular right atrio-ventricular valve ; S.«, septum atriorum : ]r.a.-..-•, V.c.s.d, precaval veins.
ridge (annulus ovalis}. Extending from this to the base of the
postcaval and right precaval respectively are two more or less well-
B
Vc.s.
Va.s
Va.Th.1
Fi<;. 318. — HEART or HUMAN F, 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 branchial 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 branchial arteries, into a dorsal aorta 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
mandibular 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
1 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,1 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
ji.c.a
c.m a.
FIG. 320. — DIAGRAM OF THE BRANCHIAL ARTERIAL SYSTEM OF Mut-tehis
antarcticus. 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 hemibranchs, and the arches are in simple
outline.
a. CM, anterior (ventral) carotid ; a.d.a, anterior dorsal aorta ; af.k.a, afferent
branchial; br.n, brachial ; c.m. a, cceliaco-mesenteric ; d. n, dorsal aorta ; E,
eye; ep.a, epibranchial ; H, heart; h.b.a, hypobranchial; hy.a, afferent
pseudobranchial or hyoidean ; md.a, mandibular ; ojt.a, ophthalmic ; p.c.a,
posterior (dorsal) carotid ; sb.a, subclavian ; .s/j, spiracle ; ?>.a, ventral aorta ;
1 — 5, the hyobranchial and four succeeding branchial clefts.
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.
1 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 parietal (intercostal, lumbar}, and
cceliac, mesenteric, and urinogenital 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 creliac, and one
or more mesenteric arteries (Figs. 322 and 323).1 The renal
and yenital 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
I. d.a.
exc.
sb.a.
in.c
H
Hba.
(/a/.
FIG. 321.— BRANCHIAL ARTERIAL SYSTEM OF COD (Gadits morrhua). Lateral
view. (From Bridge, altered from T. J. Parker.)
af.b.a, 1st and 2nd afferent branchial ; d.a, cceliac; d.a, median dorsal aorta;
ef.b.a, first afferent branchial; t.r.c, " external :> carotid ; H, heart; h>/.ral arlrry, 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 ccelomic canal enclosed by the ventral arches
Kit!!,, art.
E.'t. car.— .
Car. lab
-4,-f. n.r/iff:. __
E.ct. manil.
Ling.
X"
, Plt"i:
K.I I. ,}-,'ii.
Int. j).fniatu»\.
of the vertebra? ; 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
FIG. 3-23. — THE ARTERIAL SYSTEM OF Emys europrea. Fiom the ventral side.
Ac, dorsal aorta ; Ap, pulmonary ; Br, Br, the two bronchi ; C, caudal aorta ;
(.'«'•, common carotids, with tracheal and cesophageal branches (Tr, Oe) ;
Co, Co1, and Me, eoeliaco-mesenteric, which here arises as a bundle of
separate vessels ; Or, femoral ; d, d, small intestine ; E, epigastric ; e, large
intestine ;/s, sciatic ; in, stomach : MB, posterior mesenteric ; RA, aortic
arches ; Sc, subclavian ; Tr, trachea ; UG, urinogenital arteries ; Ver,
vertebral.
VENOUS SYSTEM 419
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 internal
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 the femoral or
crural and supplies the hinder extremity. In some cases (e.cj.
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.1
VENOUS SYSTEM.
Amphioxus. — The blood from the intestine passes into a
sulintestinal vein which extends forwards as the Italic portal vein
to the ventral side 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 subintestinal veins (Fig. 326, III-
VII) running beneath the embryonic intestine, which primarily
1 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 relatively small branches, while the main stream passed along an
internal interosseous and then a median artery in the fore-arm, and a peroneal
or branches of a primitive sctphenous artery in the shank,
E E*
420
COMPARATIVE ANATOMY
DOs
Vnl
FIG. 324. — DIAGRAM OF THE VENOUS SYSTEM OF Amphioxus.
(After B. Zarnik.)
DCd, DCs, right and left precaval ; Z>.s, intestinal sinus ; Gg, genital vesselsj
LI, heptic lacunae ; Lv, hepatic vein ; PI, parietal lacunas ; Qvl, — Qv~,
transverse veins; Si', rental 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
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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,
422
COMPARATIVE ANATOMY
II
Os
i ii m IE Y
1
1
2 ? * V IT •*-
*\ A\ A A A
"'l
&-/ Lft?
-a (//)
\
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J1\
1 Jr~
oT-
,7
cz
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1
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-"1
k
-Cdv
Fi<;. 3'26. — DIAGRAM OK STACKS IN THE DEVELOPMENT OF THE VEINS IN ELASMO-
BRANCHS. (I— XI after Rabl, XII after F. Hochstetter. )
Ca, C'p, anterior and posterior cardinal veins ; Cdv, caudal vein ; C7, 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 ; Lb, hepatic
veins ; Npf, renal portal system ; O«, Od, left and right omphalo-mesenteric
veins ; She, subclavian vein; VP, hepatic portal vein ; Vpo, capillaries of the
hepatic portal system ; ":i, 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
Card. ant.(Jug
Sulcl.
tfett.V-
-SeitJ.
-Ven.Cl.S
FIG. 3'27. — 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.i, caudal vein, which divides into two
renal portals, A, A1, 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.CLB), one or more cutaneous
veins of the tail (Cut. V), and veins from the body-walls and pelvic fins (HEV) ;
Sitbcl, 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 (CV) 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 jugular from the' ventral part
of the head into which the nutrient branchial veins open, also
communicates with the precaval. A suMavian 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 renal 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
4-25
^r
Cp-
vh^---
,7e
— Vibe
'BV
-V.ca.iul
FIG. 328. — DIAGRAM OF THE VEXOUS SYSTEM OF Profojif) ;
A, kidney ; Out, Oml, Our, 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 Our are only connected by capillaries : in stage C,
the first section (Om) has quite disappeared, and the umbilical vein (Umb)
has become developed ; tir, sinus venosus ; Varl. 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 gradually 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 (i.e., from the placenta in the higher Mammalia) to the
embryo. 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 (or 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.1
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
1 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 arb seen, more especially as regards the
veins of the digits.
432 COMPARATIVE ANATOMY
veins only, it is called a rcte mirabile simple? ; if of a combination
of both kinds of vessels, it is known as a rcte miralilc duple./.
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-vessel*,
situated in the connective tissue of various parts of the body, ami
of I y m pi i -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 from
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.1 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.
1 The liwmnar spaces in Cyclostomes, generally regarded as belonging to the
lymphatic system, have been found to contain blood.
LYMPHATIC SYSTEM 433
In Urocleles, 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 tinder the skin which communicate with those of the
peritoneal cavity (e.g. sub vertebral, pericesophageal) : 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 where 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 cceliac artery, having close relations to the
aorta and precavals, with the latter of which they communicate at
various points: their connection with 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 (cistcrna or reccptaculum chyli) in the lumbar
region : it receives the lymphatics from the legs and pelvic organs
as well as those from the intestine (lactcals) 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,1 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 andcoccy-
1 In the walls of thetruncus artoriosus of Urodeles a lymph-sinus is included,
which has been described as the "central lymph-heart."
F F
434
COMPARATIVE ANATOMY
R. precaval
Aorta
R. Aortic arch
R. i'ii S S
C ft * ^J ^3
!J3 a> w 3
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Hill's
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 mesonephros.
Mesonephros.
The mesonephros or Wolffian lody 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 l
(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,2 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
body -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 (parovarium, 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.3
1 These, as already mentioned, are primarily hollow coelomic canals, but in
Sauropsida and Mammalia they are at first solid and become hollowed secondarily.
'• 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 ccelome as
well as from the somites.
3 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
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
The posterior
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.
Kl
hind-kidney.
FIG. 338.— DIAGRAM ILLUSTRATING THE ^ -? •i*'
GENETIC RELATIONS BETWEEN THE
MESONEPHROS AND METANEPHROS.
(Modified from Schreiner. )
AG, allantoic duct; D, intestine; HK,
urinary tubules arising in the nephro-
genetic blastema ; Kl, cloaca ; NG,
metanephric duct ; **, outgrowths
from metanephric duct ; tf, outgrowths
from mesonephric duct ( UO).
THE MALE AND FEMALE GENITAL DUCTS.
In certain lower Vertebrates (Elasmobranchs) a second duct,
known as the Miillerian 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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stomes, Ganoids, Dipnoans,
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genital apparatus, the remaining
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kidney.
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.
URINOGENITAL ORGANS
449
TABULATED RESUME — (Continued).
o
03
, urinogenital papilla ; Nod, 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 ampullse 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
Mtillerian ducts in this genus are retained in 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 Lepiclosiren 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. 3o3.
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 Ca3cilians 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
Coscilians, 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 efferent™
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
glomeruh) and the transverse canals to the urinogenital duct : in
Rana esculenta the efferent ducts open directly into the urino-
-ns
FIG. 354. — DIAGRAM OF A PORTION OF THE MALE GENERATIVE APPARATUS IN
THE GYMNOPHIONA.
Ho, testis ; H8, urinogenital duct ; A', testicular capsules ; M, Malpighian
capsules ; N, kidney ; Q, transverse canals connecting the collecting duct
with the longitudinal canal (//, L) ; Ql, second series of transverse canals;
X, convoluted portion of urinary tubule ; Sy, collecting duct of testis ;
ST, 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 directly connected
with the urinogenital duct.1
? Mlillerian 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-
1 In Alytes the relations of the generative ducts require further investigation :
the efferent ducts at the anterior end of the kidney are said to open into the
persistent Miillerian duct.
472
COMPARATIVE ANATOMY
nmnication with the body-cavity and cloaca. A vesicula seminalis
may be present on the urinogenital duct (p. 457).
Hermaphroditism occasionally occurs in the Annra ; only
one case (Triton taeniatus) 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 Ran a 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
Vr
Fi<;. 355. — TESTIS AND ANTERIOR END OF KIDNEY OF Rana esculent a.
(Semidiagrammatic. )
Ho, testis ; L, longitudinal canal of the testicular network, from which the iiiter-
renal network (C, C) arises ; .Y, kidney ; q, q, transverse canals of the testi-
cular network, which give rise to blind processes at f 1 5 Ur, urinogenital
duct.
ovary : in Rana, the Miillerian 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
Ot
Od-—
FIG. 356.— URINOGENITAL ORGANS OF A FEMALE Hmin /, vestige _ T . 1,1 n i i i
of the Miillerian duct'; X, kid- In Lizards they are well-developed
ney ; p, common aperture of on the dorsal and ventral walls
the ureter (Ur, £V) and yas Qf the cloaca and their secretion
delerens on a papilla on the , , ,
dorsal wall of the cloaca (Cl) ; passes into the grooved penes in the
r, rectum ; Vd, vas deferens ; male. In Snakes similar glands
t," yellow body" (adrenal). are present) and in mature Croco-
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 Monotremes1 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 (Miillerian
ducts), which in other Mammals become more or less fused with one
another proximally, 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 (Didelphidee) will 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 (j-) 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
vagina? have a similar arrangement, pass between the curved
portions of the vagina? to the bladder.
From the condition of the female generative organs in
1 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 early development takes place.
478
COMPARATIVE ANATOMY
Didelphys, that seen in other Marsupials may be derived. In
Phalangista'vulpina and Phascolomys wombat (Fig. 360, P. and c)
A
B
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, Miillerian duct ; PT, preputial pouch and copulatory organ ;
S. ft, canal of copulatory organ ; S.tty, 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 vaginte lie
closer together, and begin to extend backwards towards the urino-
Jff.
Fim
*•>! I rW -,
luSSfe^,,.
Fio. 360. — FEMALE (GENERATIVE APPARATUS OF MARSUPIALS. A. Dihy*
dorsigem (juv. ); B, Phrt/>tnt//*fa viifjiina ; C, Phaacolomyfi wombnt (After
A. Brass.)
B, urinary bladder ; CV, cloaca ; rn moiixtroxa. (After Davidoff.) Ventral view.
B, pelvic arch with iliac process, Pril ; Mt, basipterygium ; Ret, I\al, radii of fin ;
SB, anterior clasper ; o,—f, 1 — 3, various segments of the posterior clasper.
arc 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("chondrodeutin").
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.
COPULATOKY ORGANS
4S7
In addition to pelvic claspers essentially similar to those of
other Elasmobraiichs (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 grooved 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 copulatory 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
FIG. 360. — COPULATORY ORGANS OF Lactrta ni/ilia. (After F. Leydig. ) In A
they are shown everted, and in B their position in the retracted condition is
indicated by dotted lines extending backwards from the vent.
Ce, vent ; A', Kl, 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 corre-
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 proxiinally, 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 Ratitae 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 Dromseus 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 Dromseus and Rhea. The absence of the blind sac in the
Ostrich is probably a secondary modification. A clitoris is present
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.)
"it—
;'. 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 papillae. 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
A
B
-fis
FIG. 368.— DIAGRAMMATIC LONGITUDINAL SECTIONS OF THE POSTERIOR PART OF
THE RECTUM, THE CLOACA, AND 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, hypothetical form between A and C ; C, Mono/rune (penis
extruded) ; T), Monotreme (penis retracted).
'*/, connective tissue ; bl, urinary bladder ; d, cloaca ; d, rectum ; /, fibrous body
(corpus cavernosum) ; h, ureter ; p*, sheath of penis ; pa1, aperture of the
sheath ; r, seminal furrow or tube (penial urethra) ; x, vas deferens : //,
urinogenital canal.
A
Fie;. 3U9. — Continuation of Fig. 308. For description see next page.
COPULATORY ORGANS
491
FIG. 369. — CONTINUATION OP FIG. 368.
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, Rodent, (Ccelogenys paca) ; C, Ape(Cercopithecus): in most placental
Mammals the apex of the penis is not pendulous ; D, Man ; E, Human
foetus.
Additional letterings : a, anus ; at, stalk of allantois ; b, 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 (corpus cavernosum) bifurcates proximally to form two
crura, which are nearly always attached to the ischia. The opening
CU
FIG. 370. — A, SEMIDIAGRAMMATIC FIGURE OF THE HUMAN PENIS. (In transverse
section and from the side). B, CLITORIS OF A MONKEY (Cebu-i capucinus).
A, albuginea penis; A1, albuginea urethra?; Ccp, corpus cavernosum; Ccu,
corpus spongiosum, which gives rise to the glans penis at Gp, and forms an
oval enlargement (bulbus) at B ; Gli, clitoris, with its ventral furrow (K),
glans (Gc), and prepuce (Pp) ; rd, rd1, crura of the corpora cavernosa ; *SY,
sulcus dorsalis penis ; tip, 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 course 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 l)
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 corpus spongiosum or corpvs cavernosum urcthrcv
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 bulbi vestibuli 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 2
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 coelomic
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 ">.
Keibel, F. Gastrulatiou u. Keimblattbildung d. Wirbeltiere. Auat. Hefte. Ergebu.
1900.
Kb'lliker, A. Eutw.-Geschichte des Menschen uud der huhereu Thiere. 2nd ed. Leipzig,
1879.
Marshall, A. Millies. Yertebrate Embryology. London, 1893.
Minot, ('. S. Human Embryology. New York, 1892 ; Lab. text-book of Embryology.
Philadelphia, 1903; Bibliography of Yertebrate Embryology. Boston,
1892.
Quaiii's Elements of Anatomy. Londou.
Rathke, H. Entw.-geschichte der Wirbeltiere. Leipzig, 1861.
Remak, R, Eutwick. der Wirbfltiere. Berliu, 1850—1855.
Romiti, G. Lezioui di embriogenia umaua e comp. dei Yertebrati. Siena, 1881, 188^J
1888.
Schultze, O. Gruudriss der Eutwickluugsgesch. des Meuscheu. uud der Saugetiere.
Bearbeitet unter zu Gruudleguug der 2te Auflage des Gruudrisses der Eutwick-
luugsgesch. von A. Kolliker. Leipzig, 1897.
Scleuka, E. Siudieu liber die Entwicklungsgesch.der Tiere. Heft I-Y. Wiesbaden,
1883—1892.
Ziegler, H. E. Lehrbuch d. vergl. Eutw.-gesch d.niederen Wirbeltiere. Jena, 1902.
GENERAL COMl'ARATIYE ANATOMY, &c.
Beddard, F. E. Of. numerous papers on the Anat. of Verts, 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
Brouu's Klasseu und Orduuugen dcs Tiereiches. Pisces, by Hubrecht and Sagemehl ;
Amphibia and Keptilia, by C. K. Hoffmann ; Aves, by Seleuka and Gadow ;
Mammalia by Giebel and Leche.
Bruhl, C. B. Zootomie aller Thierklassen. 1876-86.
Cambridge Natta-ul History. Vol. YII. — Amphioxus arid Fishes, by Herdmau, Bridge,
and Bouleuger ; Vol. VIII.— Amphibia and Reptiles, by Gadow ; Vol. IX.—
Birds, by Evans ; Vol. X. — Mammalia, by Beddard. London.
Cams, C. G. Lehrbuch der vergl. Zootumie. II. Bde. Leipzig, 1S34.
Cams, C'. G. and Otto. Erlauterunffstafeln zur vergl. Anatomie. VIII. Hefte Leipzig
1826-52.
Glaus, C. Lehrbuch der Zoologie. Marburg and Leipzig. 1&S3. (Eug. trans, by Sedg-
wick and Heathcote, 2nd vol., 1885).
Cuvier, G. Lemons d'Anat. comparee, 2nd. ed. Paris, 1835-46, and in German,
Leipzig, 1809-10.
Ecker, A. Icones physiologicse. Leipzig, 1855-9.
Gaudry, J. Les enchainements dn moude animal dans les temps geologiuues.
-|Qr*op .tOOl
18*8, &c.
Gegeubaur, C. Vergl. Anatomie. Bd. 1 and 2. Leipzig, 1898 and 1901.
Haeckel, E. Gen. Morphol. d. Orgauismen. Berlin, 1866. New ed. (1906) ; System.
Phylogenie der Wirbeltiere. Berlin, ]*!>.">.
Hertwig, H. 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. (Eug. trans, by
Campbell. London, 1895.) Allgem. Biobgie. Jena, 1906.
Howes, G. B. Atlas of Practical Elementary Zootorny. London, 1902.
Huxley, T. H. Lectures on the Elements of Comp. Auat. London, 1864 ; Auat.
of Yertebrated Animals. London. 1871.
Kiugsley, J. S. Text-book of Vert. Zoology. New York, 1899.
Leydig, F. Lehrbuch der Histologie d:s Meuscheu uud der Thiere. Frankfurt, 1857.
Meckel.J. F. System der vergl. Anatomie. VI Bde. Halle, 1821— 33.
Milne-Edwards, H. Lemons sin- la Physiol. et 1'Aiiat. conip. de 1'Homme et des
Auiaiaux. XIV Bde. Paris, 1857-80.
Oppel, A. Lehrbuch d. vergl. mile. Anatomie d. Wirbeltiere (6 Parts). Jeua, 1896
and onwards.
Owen, R. Anat. of Vertebrates. London (4th ed.) 1871.
Parker, T. J. A Course of Instruction in Zootomy (Vertebrates). London, lNK4.
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. Chiil/i-m/cr.
Rollestou, G. Forms of Animal Lite 2nd ed. by W. Hatchett Jackson. Oxford.
1888.
Sedgwick, A. Text-book of Zoology, Vol. II. London, 1905.
Siebold, v., and Stauuius. Handbuch der Zootomie. 2nd ed. Berlin, 1854.
Steiumann and Doderleiu. Elemente d. Paheoutologie. 2 Aufl. Leipzig, 1904.
Vogt and Yung. Lehrbuch d. pract. vergleich. Auat. Braunschweig. lS.Sj-94. (Also a
French edition.)
Wagner. R. Lehrbuch der Zootomie. II Bde. Leipzig, 1.M3-N. : Icones zootomicae.
Handatlas zur vergl. Auatomie. Leipzig, 1841.
Wiedersheim, J;. Vergl. Anatomie der Wirbeltiere. 6th ed. Jena, 1 906. Eiufuhrung
iu die vergl. Auat. der Wirb .dtiere. Jeua, 1907.
Woodward. A. S. Outlines of Vert. Paleontology. Cambridge, 1M"^.
Zittel, K. v. Han-U)uch der P;il;iontologie. Munchen u. Leipzig. (Eug. trans, liy
Eastmau. 2 vo!s. London i. '
PAPERS, M(.)N()GRAPHS, &c., DEALING WITH INDIVIDUAL TYPES
AND GUOUI'S <>K ANIMALS.
AMPHIOXUS.
Beuham, W. B. Pharyngeal bars of AmpLioxus. Q.J.M.S. 1893.
Boeke, J. Struct, of light-perceiving cells, etc., of Amphioxus. 1'roc. Meet. K. Akad.
d. Wetensch. Amsterdam, 1902.
Boveri, T. Sehorgane d. Amphioxus. Zool. Jahrb. Suppl. VII (Festschr. Weismann's)
1904.
Hatschek, B. Eutwick. des Amphioxus. Arb. lust. Wi<-n, 1SS2.
Johnston, J. B. Crani:il and spinal ganglia, etc. in Amphioxus. liinl. linlletin. 1!M).">.
Joseph, H. Eigentiim. Zellstructuren in Zentralnervensyst. v. Amphiosus. Air.it.
Auz., 1904.'
APPENDIX 501
Kowalewsky, A. Eutw. d. Ainphioxiis. Mem. Ac. St. Petersb., T. XI.
Langerhans, P. Auat. ties Amphioxus. Arch. f. mikr. Anat. Bil. XII, and Mnrph.
Jalirb., 1876.
Lankester, E. Kay. Oontribs. to the knowledge of Amphioxus. <,>..!. M.S., 1890.
Laiikester ami Willc-y. Div. of the atrial chamber of Amphioxus. <.,».J.M.S. 1890.
Legros, A. ('av. buccale tl. I'Ainphidxus. Arch. il'Auat. Micr. 1897-^.
Muller, ,T. Brauchiostoma luml>rk',m>. Abh. Ak. Berlin, 1844.
Kohou, J. O. Amphioxus. Denk. Ak. AVicn. Bd. XLV.
Rolph, W. Amphioxus laureolat'is. Morph. Jahrb. 1876.
Van Wijhe, J. W. Amphioxus. Anal. Anz. 1893, and Pet. Camp. 1901 and 1906.
Willey, A. Dev. of Amphioxus. Q.J.M.S., 1891. Amphioxus ami the Ancestry of
tlii- Vertebrates. New York and London, 1894.
( 'Yfl.DSTIIlII AND PlSCKS.
Ao-assiz, A. Dev. of L'.-pidosteus. P. Atner. 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. Tlu' Pelagis Stages of
Young Fishes. Mem. Mns. Harvard, 1885.
Agassiz, L. Recherches sur les poissons fossiles. 1833-43.
Agassiz, L., and Vogt,C. Anat. des Salmones. Nenfchatel, 1S45.
Ayers, H. Beitr. zur Anat. lind Physiol. der Dipnoer. Jen. Zeitschr. 1884.
Balfour, F. M. (See p. 499.)
Balfour, F. M., and W. N. Parker. Struct, and Dev. of Lepidosteus. Phil. Trans.
1882.
Bischoff, Th. Lepidosiren paradoxa. Leipzig, 1840.
Budgett, J. S. Anat. of Polypterns. Tr. Zool. Soc. 1901 ; Breeding Habits of some
W. African Fishes ; Ext. Features in Dev. of Protopterus ; ami Larva of Polyp-
terus. Tr. Zool. Soc. 1901 and 1902.
C'ole, 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 1'etude de la Metamorphose de 1'Ammocoetes. Rev.' Biol. Nord
France, T. III. 1891.
Cuvier and Valenciennes. Hist. nat. des Poissous. 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. ?.. 70 Geburtstag von O. von
Kupffer, 1899; Paheospondylus. &c. Mem. N. York. Acad. of SH. 1900 (cf. also
P. Zool. Soc., 1898).
Dollo, L. Phylog. d. Dipuenstes. Bull. Soc. Beige de Geol., &c. 1895.
Emery, ('. Fierasfer. Atti Ace. Liucei, 1879-80.
Felix, W. Entwick.-gesch. d. Salmonidse. Anafc. Hefte Arb. 1897.
Fritsch, A. Fauna der Gaskohle nud tier Kalksteine der Pel-formation Btilimens.
Bd. II. Dipnoi. Prag, 1888. Bd. III. Selachii, Actinopterygii. 1893.
Palaeouiscidae. 1894.
Fuvbiuger, K. Morphol. d. Skelettes der Dipnoer (Semon's Forseh. Reisen). Jena
Denkschr. 19iU.
Garman, S. Chlamydoselachus auguineus. Bull. Mus. Harvard. Vol. XII.
Gotte, A. Entwicklungsgesch. des Flnssneuuauges. Leipzig, 1890.
Gudger, E. W. Breeding and seg. of egg. of Siphonostoma. Proc. U. S. Nat. Mus.
1905.'
Gtiuther, A. " C'eratodus. " 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. Erg;inzuugsheft, 1885 ; Beitrag zur allgem. Stammes-
gesch. der Wirbeltiere. Jena, 1883.
Howes, G. B. Marsipobranchii. Trans. Biol. Soc., Liverpool, ]S92.
Huxley, T. H. C'eratodus fosteri. P. Zool. Soc., 1S77.
Kupffer, C'. Die Entwick. von Petromyzon. Arch. f. mikr. Anat, 1890.
Laugerhans, P. Petromyzon. Verh. Natnrforsch. Ges. Freiburg, 1875.
502 APPENDIX
Leydig, Fr. Mik. Anat. uucl Etitw.-gesch. cler Rochen uncVHaie. Leipzig, 1852 ; Anat.
Histol. Uutersuch. liber Fische und Reptilien. Berlin, 1853.
Leydig, Fr. Anat. imd Histol. der Chimaera. Arch, f. Anat. uud Physiol., 1851.
List, J. H. Eutw.-gesch. d. Knocheofische (Labridem. Arb. Zool.Inst. /. Graz-Leipzig,
1887.
Marcuseu, .T. Mormyren. Mem. Ac. St. 1'i-tersb., T. VII.
Miiller, J. Bau und die Grenzeu der Gauoideu. Berlin, 1840; Vergl. Anat. der
Myxinoiden. 1833^3.
Nuel, J. P. Dev. du Petromyzon. Arcb. Biol. Vol. I. 1881.
Owen, R. Lepidosiren anuecteus. Tr. Linu. Soc. XVIII.
Owsjanuikow, P. Zur. Entwick. d. Flussueunauges. Bull. Ac. St. Petersb. 1889.
Parker, T. J. On tbe Skeleton of Regalecus argenteus. Tr. Zool. SOL-. 1886 ; Notes
on Carcbarodon rondeleetii. P. Zool. Soc., 1887.
Parker, \V. X. Anat. and Physiol. of Protopterus. Tr. Irish Ac. Vol. XXX. p. 3.
1 *92.
Peters. Lepidosiren. Arch. f. Anat. u. Physiol. 1845.
Pollard, H. B. Polypterus. Zool. Jahrb. Bd. V. 1*92.
Salensky, W. Entwick. des Sterlets. Verbdlg. der naturf. Gesellscb. zu Kasau.
l*7*-79aud Arcb. Biol. 1881.
Schaninsland, H. (Seep. 503 1.
Schneider, A. Vergl. Anat. und Entw.-gesch. der Wirbeltiere. Berlin, 1879.'
Scott, W. B. Entw.-geschich. der Petromyzonten. Morph. Jahrb. Bd. VII. ; Dev. of
Petromyzon. Journ. Morpbol. 1887.
Semon, R. Zoolog. Forschungsreiseu in Australieu mid dem malayischen Arcbipel. I.
Die iiussere Entwick. desCeratodus. Jena, 1893.
Shipley, A. E. On some points in in tbe Dev. of IVtrmnyzon fluviatilis. Q.J.M>.
1887.
Sollas, W. J., and I. V,. J. Palseospondylus. Phil. Trans.. 1903.
Traquair, R. H. Palseospondylus Guuni, see Ann. aud Mag. Nat. Hist., 1890. ;md
Proc. Roy. Phys. Soc. Eiliu. W.\.
Vogt, C. Embryol. des Salmoues. Xeuchatel, 1842.
Woodland, W. Anat. of Centrophorus. P. Zool. Soc. 1907.
Worthington, J. Myxiuoids. Amer. Nat. 1905.
Wright, R., McMurrich, J., Macallum, A., Mackenzie. T. Oontrib. to the Anat. of
Amiurus. Prof. Canad. Inst. Toronto. 1^4.
AMPHIBIA.
Bayer, F. Skelet der Pelobatidt-n. Abhaudl. Bohm. Ges. 1884.
Bles, E. J. Life-history of Xeuopus. Trans. R. Soc. Ediu.. ]90."i.
Brachet, A. L'outog. d. Ampbibiens, cV'c. (Siredou and Raua). Arcb. de Biol. 1902.
Brauer, A. Entw.-gesch. u. Anat. d. Blindwiihleu (Gymnophiouen). Zool. Jahrb.
1897, 1899, and 1902.
Cope, E. D. Batrachia of North America. Bull U.S. Nat. Mus. Washington, 1889.
< Yedner, H. Stegocephaleu uud Saurier aus dem Rothliegenden, &c. (Zsitschr. d.
deutscheu Geolog. Gesellsch.) Leipzig- 1881-93; Die Urvierfi'issler [Eotetrapoda]
des Sachs. Rothliegenden. Naturw. Wochensch. 1MM.
Dugi-s, A. L'osteologie et la myologie des Batraciens. Paris, 1V>).
Ecker, A., and Wiedersheim, R. Anat. des Frosches. III. Ann. bearb. v. E. Gaupp,
Braunschweig, 1896. (Bng. trans, by Haslam, Oxford, 1899. i
Emerson, E. T. Anat. of Typhlomolge. Proc. Boston Soc. Nat. Hist., 1905.
Fatio, V. Faune des Vertebres .
Gronberg, G. Auat. der Pipa americaua. Zool. Jahrb. 1894.
Hoeven, J. van de. Auat. van den Cryptobranchus. Haarlem, 1862 : Menobranchus,
Lcydeu, 1867.
Hyrtl, J. Cryptobranchus japonicns. Vindoboure, 1865.
Ishikawa, ('. Megalobatrachus. Proc. Nat. Hist. Tokyo, 1904.
Jacqtiet, M. M. Axolotl (Skel. and Muscles). Arch. Sci. med. de Bucarest, 1889.
Kammerer, P. Verwandschaftverhalt. von Sulamandra atra u. maoulosa. Arcb. Entwick
Mechauik, 1903.
Kingsbury, B. T. Rank of Necturus amongst Batrachia. Biol. Bull., 1905.
Klinckowstroni, A. v. Auat. der Pipa americaua. Zool. Jahrb. 1S94.
Leydig, F. Die Anuren Batrachier der deutscbeu Fauna. Bonn, 1877. Molche der
\viirttemberg. Fauna. Berlin, 1867 and Arch. f. Naturgescb, Bd. XXIII.
APPENDIX 503
Marshall, A. Millies. The Frog. 6th ed. London, 1896.
Miiller.J. Anat. d. Amphibien. Zditschr. f. Physiol. 1832.
Osawa, G. Auat. d. Japan. Riesensalamanders. Mitteil. a. d. med. Fak. d. K. Japan.
Univ. Tokio, 1902.
Rusconi, M. Hist nat. dev. et inetam. de la Salamandre terrestre. Pavia, 1854. (Also
Siren and Proteus.)
Ruscoui, M., and Coufigliachi. Del Proteo anguineo. Pavia, 1818.
Sarasin, P. and F. Ergebu. uaturwissenscliaftl. Forschungen auf Ceylon in den Jahn n
1884-6. II. Eutwicklungsgesch. und Anat. der Ichthyophis glutiuosus. Wies-
baden, 1887-90.
Schwalbe, G. Biol. u. Eiitvv.-gesch. v. Salamaudra atra u. maculosa. Zeitschr. f. Biol.,
1897.
Wiedersheim, R. Anat. der Gymuophioueu. Jena, 1879 ; Salarnaudrina perspicillata
imd Geotritou fuscus. Genoa, 1875 ; Eutwicklungsgesch. von Proteus. Arch, f .
mikr. Anat. 1890; Entvv. gesch. von Salamandra atra. Loc. cit. 1890.
Wilder, H. H. Coutrib. to the Auat. of Sirtn lacertina. Zool. Jahrb. 1891 ; Skel. of
Necturus. Mem. Boston Soc. Nat. Hist., 1903.
Z?ller, E. Neotenie d. Tritoueu. Jahresber. d. Yereius f. vaterliind. Naturkunde in
Wurttemberg, 1899.
REPTILIA.
Beddard, F. E. Syst. Arr. and Auat. of certain Squamata. P. Zool. Soc., 1907.
Bemmelen, J. F. van. Halsgegeud bei Reptilieu. I. Amsterdam, 1888.
Bojanus. Anatome testudinis europaeae. Viluae, 1819-21.
Cope, E. D Cf. numerous papers in various American Journals.
Credner, H. (See p. 502.)
Dendy, A. Dev. of Spheuoda (Hatteria). Q.J.M.S., 1899.
Dumiril and Bibron. Erpetologie generate. Paris, 1834-54.
Duveruoy. Serpeus. Ann. Sci. Nat. T. XXX.
Giinther, A. Anat. of Hatteri i. Phil. Trani., 1867.
Leydig, F. Die in Deutschland lebenden Arteu der Saurier. Tubingen, 1872; Eiuheim-
ischeu Schlaugen. Abh. Seuckeub. Ges. Bd.XIII. 1883.
Marsh, O. C. Dinosaurs of North America. Washington, 1896.
Owen, R. Descript. and Illust. Cat. of the Fossil Reptilia of South Africa.
Osboru, H. T. Cf. numerous papers 011 Fossil Reptiles iu Arner. Nat., " Science,"
Mem. Amer. Mus. of Nat. Hist., &c
Peter, K. Normentafeln z. Eutwick. -gesch. d. Zauueidechse. Jena, 1904
Rathke, H. Eutwick.-geschich. der (1) Natter, (2) Schildkroten, (3) Crocodile. (Ko'nigs-
berg, 1837; Braunschweig, 1848 and I860.;
Schauiiisland, H. Entw.-gesch. der Hatteria. Arch. f. mikr. Auat., 1900; Entwick.-
gesch. u. Anat. d. Wirbeltiere (Sphenodon), Callorhynchus, Chamaleo,
" Zoologica." Stuttgart, 1903.
Siebenrock, F. Various papers ou Reptiles in the Sitz.-ber. Ak. Wien from 1892
onwards.
Smalian, C. Anat. der Amphisbaniden. Zeitschr. f. wiss. Zool. 1885.
Thileuius, G. Eiablage u. Eutwick. der Hatteria. Sitz.-ber. Ak. Berlin, 1899.
Voeltzkow, A. Eutw. -gesch. d. Reptilien (Krokodile). Abh. Senkeub. Ges., 1899.
Wiedersheim, R. Phyllodactylus europaeus, &c. Morph. Jahrb. 1876. Labyriiithodon
Rutimeyeri. Abh. Schweiz, Pal. Ges., 1878.
Zittel, K. Flugsaurier a. d. lithogr. Schiefer Bayerns. Palaeoutographica (Stuttgart)
N. F. IX. 2.
AYES.
Baur, G. W. K. Parker's Bemerk. iib. Archaeopteryx, 1864. Zool. Anz. 1886.
Dames, W. Archaeopteryx. Palaoutol. Abhdlg. Berlin, 1884. Cf. also Sitz.-ber. Ak.
Berlin. 1897.
Fiirbringer, M. Uutersuch. zur. Morphol. und Systematik der Vogel. Theil. I. and II.
Amsterdam, 1888. See also abstract in Biol. Ceutralbl. Bd. IX.— XVII.
Keibel, F., and Abraham K. Norineutafelu z. Eutwick. -gesch. des Huhus. Jena, 1900.
Marsh, O. C. Odontoruithes. Washington, 1880.
Marshall, W. Der Bau der Vogel (Weber's Naturwiss. Bibliothek). Leipzig, 1895.
Menzbier, M. von. Osteol. der Piuguiue Bull Soc. 'Moscou, 1887.
Milne-Edwards, A. La fauue ornithol. eteiute des iles Mascareigues et de Madagascar.
1866-79.
Oweu, R. Aves, in Todd's Cyclopaedia I. On the anat. of the southern Aptevyx. Tr.
Zool Soc. II., III. Archaeopteryx. Phil. Trans., 1863.
504 APPENDIX
I'arkor, W. K. Numerous papers ou the Osteology of Birds in P. Zool. Soc. 1860-5,
1889, 1890; Tr. Zool. Soc. 186:2, 1866, 1869,1891 ; P. Hoy. Soc. 1887-8; Aim.
Nat. Hist. 1864, 1865; Iliis, l,v<8-9; Tr. Irish. Ac. 1890; Kiieycl. Brit. 9th c«l.
(Article "Binls").
Parker, T. J. Observations on the Anat. and Dev. of Apteryx. Phil. Trans., 1891.
Addit. Observations, &c. Loc. cit. 1892. See also under Skull.
Quatrefages, A. de. Les Moas et les chasseurs de Moas. Ann. Sci. Nat. 1883.
Wagner, R., and Nitseh. In Naumanu's " Naturgescliieh. d. Yogel Deuisehlands."
MAMMALIA
Assheton, R. Dev. of Rabbit aud Frog, Q.J.M.S. 1894.
Beneden, Van, and Gervais. Osteographie des Cetacees. Paris, 1868-80.
Blainville, H. Osteographie ou descrip. icouograph. comp. des Mammiferes rec. ct
fossiles. 4Bde. Paris, 1839-64.
Brandt. Die fossile und subfosaile Cetaceen Europas. Mem. Ac-ad. St. Petersh. 1873.
Burmeister. Auuales del Museo publico de Buenos-Aires, 1^74 S<).
Caldwell, W. H. The Embryol. of Monotremata aud Marsupialit. Phil. Trans. 1887.
( 'amerano, L. Ricerche iutornc all' auat. di un feto di Otaria jubata. Mem. Ace.
Torino, Ser. II. Tom. XXXV. 1882.
Cuvier, G. Rech. sur les ossements fossiles. 4th ed. 1831-6.
Delage, Y. Hi.stoire du Balaenoptera inusculus. Arcli. Zool. exp. 188,5, 1887.
Duvernoy, G. L. Caract. anat. des grands singes. Arch. d. Museum. Tom. III.
Ellenberger, W., and Baurn, H. Sy.stemat. und topogr. Anat. d. Huudes. Berlin, 1891.
Eschricht. Zool.-anat.-physiol. Untersuch. Tiber die nord. Walthiere. Leipzig, I,s4!i.
Fick, R. Vergl. auatom. 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. "\V. Introd. to the Osteol. of the Mammalh. 3th ed. revised with Gadow.
London, 1885.
Flower and Lydekker. Mammals, living and extinct. London, 1891.
Frank, L. Auat. der Haustiere. Stuttgart, 1871.
Gautlry, A. Auimaux fosdles et Geologie de 1'Attique. Paris, 1862-7 ; Die Vorfahren
der Saugetiere in Europa. Leipzig, 1891.
Giicomini, C. Annotaz. sulla Anat. del Negro. Torino, 1878-92.
Giildberg, G. La Dysseruetrie morph. et fuuct. chez I'lioinme et les vert. sup. ( 'hristi-
ania, 1897.
Giildberg, G-., and Nansen, F. Dev. and structure of the Whale. Part 1. On the dev.
of the Dolphin. Bergens Mus. Skrifter, V. 1894.
Gurlt. Haudbuch der vergl. Auat. der Haussiiugetiere. Berlin, 1860.
Hepburn, D. Vai'ious papers on Mammalian Anatomy in Journ. Auat. and Physiol.,
1884, 1899, 1900 to 1904, etc,
Hoeven, J. van de. Stenops. Verb. Ak. Amsterdam. T. VIII.
Hubrecht, A. A. W. Descent of the Primates. N--\v York, 1897.
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506 APPENDIX
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Beriiissart." 1883).
Fraas, O. Ae'tosaurus ferratus. Stuttgart, 1877.
Fritsch, A. Reptilieu uud Fische der bohmischeu Kreideformation. Frag, 1.^78 ; Fauna
der Gaskohle und der Kalksteiue der Permformation Bohmeus. Frag, 1879 — 85.
Groldi, E. Kopfskelet uud Schultergiirtel von Loricaria, Balistes, uud Acipenser. -Ten.
Zeitschr. 1884.
Gotte, A. Entw. d. Carapax d. Schildkroten. Zeitschr. f. wiss. Zool. 1899.
Haycraf t, J. B. The Dev. of the Carapace of the Chelonia. Trans. Roy. Soe. Ediu.
1891.
Hertwig, O. Bau uud Eutvvick. der Placoidschuppen uud Zaline der Selachier. Jen.
Zeitschr. Bel. VIII. ; Hautskelet der Fische. Morph. Jahrb. 1876, 1879, and
1881.
Hofcr, B. Bau uud die Eutwick. der Cycloid- und Ctenoid-scliUppen. Sitz-ber.
Morphol. und Physiol. Miincheu. 1889.
Klaatsch, H. Morpbol. der Fisuhsehuppen, &e. Morph. Jahrb. 1890. Herkuuft der
Seleroblasten, Morph. Jahrb. 1894 ; Bedeutuug der H-iutsiuuesorgaue fiir die
Ausschaltung der Seleroblasten aus dem Ektoderm. Verh. Anat. Ges., 1M>">.
Markert, T. Flossensbrahlen v. Acanthias. Zool. Jahrb. 1896.
Marsh, O. C. Numei-ous papers in the American Jouru. of Sciences aud Arts
Meyer, H. v. Numerous papers in " I'alaeoutographica," Stuttgart, u.y. Bd. VI.
Archegosaurus.
Nickerson, W. S. Dev. of Scales of L.-pidosteus. Bull. Mus. Harvard. Vol. XXIV.
Parker, G. H. Correl. Abnormalities in Carapace of Tortoise. Amer. Nat. I'.KJl.
Romer, F. Panzer d. Gurteltierc. Jeu. Zeitschr. Bd. 21, and cf. Auat. Auz. IMi:;.
Riitimeyer, L. Bau von Schale und Schiidel bei leb. uud foss. Schildkruten. Vi-rh.
C4es. Basel, VI, 1.
Traquair, L. H. Silurian Fishes. Trans. Roy. Soc. Edin. 1S99.
Tims, H. W. M. Scales iu Some Teleost Fish. Q..T.M.S. 1906.
Wiedersheiin, R. Anat. der Gyuiuophioueu. Jena, 1879 ; Histoloyie tier Dipuoer-
schuppeu. Arch. f. mikr. Auat. 1990.
C. ENDOSKELETON.
1. VERTEBRAL COLUMN.
(«) AMI'IIIOXI.'S, Cv(.-l.cif.']'()-MI, AND PltC'KS.
Boeke, J. Entw. d. Chorda dorsalis. Pet. Camp. 190i'.
Calberla, E Die Entw. d. Medullarr«>hres uud d. Chorda dorsalis d. Tclrostirr uud
Petromyzouten. Morph. Jahrb. 1887.
510 APPENDIX
Cartier, O. Entw.-geschichte der Wirbelsaule. Zeitschr. f. wiss. Zool. 1.87~>.
Glaus, C. Herkunft der die Chordascheide der Haie begrenzenden aussereu Elastica.
Sitz.-ber. Ak. Wieu. 1894.
Ebner, Y. v. Bau der Chorda dorsalis der Cyclostomen. Sitz.-ber. Ak. "\Vien. 1895 ;
Ueber den feiuereii Bau der Chorda dorsalis vou Myxiiie uebst weiteren Bemerk.
iiber die Chorda vou Ammocretes. Loc. cit. 1895; 1. Ban der Chorda
dorsalis v. Acipenser. Loc. cit. 1896; 2. Bau der Chorda dorsalis bei
Amphioxus ; Ueb. die Wirbel der Kuochenfische und die Chorda dorsalis der
Fist-he u. Anrphibien. Loc . cit. 1896.
G-egenbaur, C. Skeletgewebe der Cyclostomen. Jen. Zeitschr. Btl. Y. ; Entw. der
Wirbelsiiule des Lepidosteus. Loc. cit. Bd. III.
Gotte, A. Yergl. Morphologic des Skeletsystenis der Wirbeltiere. Arch, f. mikr. Auat.
1878.
Grassi, B. Entwick. 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. Allgem.
Starnmesgesch. d. "Wirbeltiere. Jena, 1883.
Harrison, G. R. Tails in Man. Bull. Johns Hopkins Hospital, 1901.
Hasse, C. Die fossilen Wirb?l. Morph. Jahrb. 1876, 1877, 1878 ; Entwick. der
Wirbelsaule der Elasmobrauchier, etc. Zeitschr. f. wiss. Zool. 1892; Entwick.
uud Bau der Wirbelsiiule der Ganoiden. Loc. cit. 1893 ; Cyclostomen. Loc.
cit. 1893 ; Dipnoi. Loc. cit. Bd. LY.
Joseph, H. Achsenskelet des Amphioxus. Zeitsch. f. wiss. Zool. 1895.
Khntsch, H. Yergl. Anatomie der Wirbelsaule. Morph. Jahrb. Bd. XIX, XX.
XXII, XXIII.
Kulliker, A. Beziehuug der Chorda zur Bilduug der Wirbel der Selachier. Yerh. Ges.
Wiirzburg. Bd. X.; Weitere Beobacht. iiber die Wirbel der Selachier. Abb.
Senckenb. Ges. Bd. Y. ; Ende der Wirbelsiiule der lebeuden Teleostier und
eiuiger Ganoiden. Gratul.-Schrift f. d. Univ. Basel. 1860.
Lvoff, B. Studieu iiber die Chorda uud die Chordascheide. Bull. So?. Moscou. 1887 ;
Bau u. Entw. d. Chorda von Amphioxus. Mittli. Zool. Stat. Neapel. 1890;
Bildung der primaren Keimbliitter und Eutstehung 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. 1S71
Retzius, G. Hiutere Ende des Riickenmarks bei Amphioxus, Myxine, u. Petromyzou.
Biol. ITntersuch. Stockholm. 1895.
Scheel, C. Entw.-gesch. der Teleostierwirbelsiiule. Morph. Jahrb. 1893.
Schmidt, L. Uutersuch. z. Keuutnis des Wirbelbaues vou Auiia. Inaug. -Dissert.
Strassbnrg i/E. 1892.
Schmidt, Y. Schwanzende der Chorda dorsalis bei den Wirbeltieren. Aiiat. Hefti1.
Bd. II.
Schneider, A. Vergl. Auat. u. Entw.-gesch. d. Wirbeltiere. Berlin, 1879.
Studnicka, F. K. Gewebe d. Chorda dorsalis u. Sitz.-ber. Biihm. Ges. 1M»7.
Uf-sow, S. Wirbel d. Teleostieu. Bull. Soe. Moscou, 1900.
AViedersheim, R. Skelet und Nervensystem vou Lepidosiren (Protopterus). Morphol.
Studien. Jena. 188n.
dn AMPHIBIA, REPTII.IA, AND AVES.
Albr,jdit, P. Processus oclontoides des Atlas bei den urodelen Amphibieu. Ceutralbl.
f. die medic. Wissenschaft. 1878; Proatlas. Zool. Anz., 1880; Rudiment de
Proatlas stir uu exemplaire de Hatteria. Bull. Mus. rov. d'hist. uat. de Belgique.
1883.
I'.anr. G. Proatlas eiuer Schildkrote ( Platypeltis). Auat. Auz., 1895; Osteolog.
Notizeu iiber Reptilieu. Zool. Anz. 1886, 1887 ; Morphogeuie der AVirbelsiiule
der Amnioten. Biol. Ceutralbl. l>Mi.
Bergfeldt, A. Chordascheiden u. Hypochorda bei Alytes. Auat. Hefte. Arb. Bd. YII.
Blessig, E. Untersuch. ii die Halswirbelsiiule der Lacerta. luaug. -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. Palason. Bui. Nr. 29 :
Proc. Amer. Philos. Soc. 1*78, 1880, ;1 886. Palreou. Bui. Nr. 32; Amer. Nat.'
1880, 1882, 1884, 1885, 1886.
Dollo, I.. Morphol. de la Colonue vertebrale. Travaux du Luboratoire de Wimcreux,
1892.
APPENDIX 511
Field, JI. H. Entw. d. Wirbelsaule d. Amphibien, etc. Morph. Jahrb. 1895.
Fraisse, P. Aiiat. des Pleurodeles Waltlii. Arb. lust. Wiirzburg. Bd. V. : Eigeuthuml.
Structurverhiiltuisse irn Schwanz erwachseuer Urodeleu. Zool. Anz. 1880.
Gegenbaur, C. Vergl. Auatomie der Wirbelsaule der Amphibien u. Reptilieu.
Leipzig. 1862. Beckeii d. Vogel. Jen. Zeitschr. Bd. VI.
Gadow, H. On the Evolution of the vert. col. of Amphibia and Amuiota, Phil.
Trans. 1896.
C46tte, A. Die Zusammensetzung der Wirbelsiiule bei den Reptilien. Zool. Auz. 1894
Wirbelbau bei den Reptilien, etc. Zeitschr. f. wiss. Zool. 1896.
Hasse, C. Anat. und.palaontol. Ergebuisse. Leipzig, 1878 ; Eutwick. der Wirbelsaule
von Triton. Zeitschr. f. wiss. Zool. 1892 ; Eutwick. der Wirbelsaule der
uugeschwanzteu Amphibien. Loc. c/t.
Hasse, C., and Schwarck. Vergl. 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 Swinuerton, G. H. Dev. of the Skeleton of Sphenodon, etc. Tr. Zool.
Soc. 1901.
Marsh, O. C. Various articles on fossil Reptiles and Birds. Amer. Jouru. of Sci.
and Arts. Vols. XV.— XXIII.
Mivart, St. G. Axial Ske!. of the Urodela. P. Zool. Soc. 1870.
Miiller, E. Abstossung u. Regeneration des Eidechsenschwauzes. Jahr. Ber. d. Vereins
fur vaterlaud. Naturkunde, Wiirtteniburg. Stuttgart. 1896.
Murray, J. A. Vert. col. of certain primitive Urodela. Auat. Anz. 1897.
Oort, E. D. van. Osteol. d. Vogelsehwauzes. Inaug.-Dissert. Bern. 1904.
Osawa, G. Auat. d. Hatteria. Arch. f. niikr. Auat. 1898.
Parker, W. K. On the Moiphol. of Birds. P. Roy. Soc. 1887.
Peter, K. Die Wirbelsaule der Gymuophioueu. Ber. Ges. Frieburg. Bd. IX., 1894;
Atlas der Amphibien. Auat. Auz. 1895.
Reiuhardt, A. Hypochorda v. Salamandra. Morph. Jahrb. 1904.
Ridewood, W. G. On the dev. of the vert. col. in Pipaand Xeuopus. Auat. Auz. 1887.
Sell wink, F. Eutwick. des mittleren Keimblattes und der Chorda dorsalis der Am-
phibien. Miiucheu, 1889.
Siebenrock, F. Runipfskeletes der Sciucoideu, Anguiden und Gerrhosaurideu. Auual.
d. K.K. Naturhistor. Hofmuseuins. 1895.
Toruier, G. (See under Limb-skeleton.)
Vaillaut, Leou. Sur la disposition des vertebres cervicales des Chelonieus. Ann. sci.
nat. zool. No. VII.
Zittel, K. Flugsanrier aus dem lithograph. Schiefer Bayerus. Palaeoutographica.
Bd. XXIX.
(V) MAMMALIA.
Albrecht, P. Epiphyseu und Amphiomphalie der Saugetierwirbelkorper. Zool. An/.
1879; Wirbelkorpergelenke zwischen dem Epistropheus, Atlas und Occipitale der
Saugetiere. Cornpt. rend, des iuteruat. medic. Congresses. Kopenhageii, 1884 ;
Ueb. die Chorda dorsalis und sieben knocherne Wirbelceutren iin kuorpel. Naseu-
septuni eiues erwachseueu Riudes. Biol. Centralbl. Bd. V.
Bardeen, C. R. D?v. of human skel. aud thoracic vertebrae. Amer. Jouru. Auat.
1905.
Cornet, J. Pro- Atlas des Mammiferes 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. Urwirbel imd Umgliederuug der Wirbelsaule. Sitz.-ber. Ak. Wien.
1888; Beziehuugen der Wirbel zu den Urwirbelu. Loc. cit. 1892.
Ecker, A. Steisshaarwirbel, Steissbeiuglatze und Steissbeiugriibchen, etc. Arch. f.
Anthropologie. Bd. XII.
Fol, H. Sur la queue de 1'embi-you humaiu. Comptes rendus. 1885.
Froriep, A. Zur Entwick. -gesch. der Wirbelsaule, insbesondere des Atlas uud Epis-
tropheus uud der Oncipital-Regiou. Arch. f. Auat. u. Physiol. 1886.
Gerlach, L. Schwanzbilduug bei einem meuschl. Embryo. Morph. Jahrb. Bd. VI.
Grix, E. Halswirbelsaule d. Uugulateu. Inaug.-Dissert. Beru. 1900.
Hasse, C. und Schwarck. Studieu zur vergl. Anat. der Wirbelsaule, etc. Hasse, Anat.
Studien. Heft I.
Keibel,F. Entwicklungsgesch. der Chorda bei Siiugern. Arch. f. Anat. 1889; Schwauz
des meuschl. Embryos. Arch. Anat. 1891. (See ulso Anat. Anz. 1891. )
Kohlbriigge, J. H. F. Schwauzbilduug u. Steissdriise d. Menschen, etc. Natuurkuud.
Tijdschr. v. Ned. Indie. 1897.
Kc'illiker, A. Chordahohle und Bildung der Chorda beim Kaniucheu. Sitz.-ber. Ges.
512 APPENDIX
Leboucq, H. Disparitiou de la cordu dorsale cliez les Vertebras superieurs Arch
Biol. Vol. I. 1880.
Eeicheubach, Stromer von E. "\Virbel d. Landraubtiere. Zoologica. 1902.
Kodackner, G. Siiugetierschwanz, etc. Inaug.-Dissert. Freiburg. 1S98.
Rosenberg, C. Eutwick. der "\Virbelsiiule und das Central e Carpi dts Meusehen.
Morph. Jahrb. 1876; Wirbi-lsaule der Myrmecophaga. Festschrift fiir C.
Gegenbaur. Leipzig. 1896.
Steiubach, E. Zahl der Caudalwirbel beiui Menschen. Inaug. -Dissert. Berlin. 1889.
AValdeyer, AT. Die Kaudalauhange cles Meusehen. Sitz.-ber. Ak. Berlin. 1896.
AVeiss, A. Entw. d. AVirbelsaule d. Ratte, etc. Zeitschr. f. wiss. Zool. 1901.
Cf. also Schauinsland, H, in Hertwig's Haudbuch d. vergl. u. exp. Entw -gesch
Jena. 1906).
2. RIBS AND STERNUM.
Baur, G. Oil the Morphol. of Ribs. Amer. Nat. 1887, and Journ. Morphol. 1SS9
Rippeu und rippeuiihiiliche Gebilde. Anat. Auz. 1893.
Blanchard, R. La septieme cote cervieale de 1'honime. Revue scieutif. Lv>">.
Bridge, T. W. Ribs in Polyodou. P. Zool. Soc. 1897.
Brush-Clinton, E. Cervical ribs. Bull. Johns Hopkins Hospital. 1901.
Claus. ( '. Vergl. Osteol. der Vertebrateu. Sitz.-ber. Ak. AVien. 1876.
Dollo, L. Morphologic des Cotes. Bull. sci. France-Belg. 1892.
Eggeliug, H. Papers 011 Morphol. of Sternum in Auat. Anz., 1903 and 1900, and Jen.
Deukschr. Bd. XL
Ehlers, E. Xoolog. Miscellen. I. Proc?ssus xiphoideus uiid seine Musculatur von
Maiiis. Abh. Ges. Gottingen. 1894.
Fick, A. E. Zur Entw. -gesch. der Rippeu und Querfortsiitzt-. Arch, f. Anat und
Physiol. 1879.
Gegeiibaur, C. Ueb. die epistern. Skelettheile uiid ihr Vorkomnicu bei den Siiuge-
tiereu uud beim Menscheu. Jen. Zeitsclir. Bd. I.
Goppert, E. Amphibienrippen. Zool. Jahrb. 1895 ; UutemuOi. /ur Morpholog. d.
Fischrippen. Loc.cit. Bd. XXIII. ; Morphol. der Amphibienrippeu. Festschr.
fiir C. Gegeubaur. Leipzig, 1S9G; cf. also Morph. JaLrb., 1897, and Verh.
deutsch. Zool. Ges. 1898.
Gutte, A. Vergl. Morphol. des Skeletsysfcems drr AVuiieltiere. Anh. f. mikr. \uat.
Bd. XIV. and XV.
Hasse, C., und Born, G. Morphol. der Rippeu. Zool. Anz. 1*79.
Haswell, W. A. Elasmobrauch Skeleton. P. Liuu. Soc. N.S.AV. 1>M.
Hatschek. Rippeu der Wirbelticre. Verb. Anat. (Jes. Jena. 18Mi.
Hoffmann, C. K. (Seep. 511.)
Howes, G. B. The Morphol. of the Steruinu. Reprinted, with a Correction from
"Nature," Vol.43.
Leboucq, H. Rech. snr les variations auat. dc la premiere cote, c-hfx, riiomme. Mem.
couronues et Mem. des savants etraugers, publics par 1'Acad. roya.le des Sciences,
etc.,de Belgique. 1896.
Lindsay, B. On the avian Sternum. P. Zool. Soc. ls>">.
Markowsky, J. Ossif. d. uieuschl. Brustbeius, etc. Polu. Ai\-h. f. biol. u. med.
Wisseuscb. 1902. Asj'iu. Ban d. ]!rustl>eiiis. Loc. cit., l!tn"> i('f. Auat.
Auz. 1905.)
Miiller, A. Vergl. Auat. der \Virbelsiiule. Miiller's Archiv. 1853.
Parker, W. K. Structure and dev. of the shoulder-girdle and sternum. Ray Soc.
1867.
Parker, T. J. Origin of the Sternum. Tr. N.Z. List. 181)0; Presence of a Sternum iu
Notidauus indicus. " Nature," Vol. 43, I.^KI !i|
Pilling, E. Halsrippen des Menscheu. Inaug.-Dissert. Rostock. 1^94.
Rabl, C. Theorie des Mesoderms. Morph, Jahrb. 189i'.
Rathke, H. Ban uud Eutwick. des Brustbeius der Saurier. Koiiigsberg. 1853.
Ruge, G. Eutwickluugsvorgange am Brustbeiue und an der Sternoclavinularverbindung
des Menscheu. Morph. Jahrb. 1880.
Sclioene, G. Befest. d. Kippen a. d. Wirbelsaule. Morph. Jahrb. 1902.
White, P. J. A Sternum in Hexanchus griseus. Anat. Anz. 1.^95.
Wiedersheim, R. (See under Limb-skeleton.)
3. SKULL.
((/) CVCLOSTOMI AND PlSCE.S.
Ablboru, V. Segmeuttition des Wirbeltierkorpers. Zeitschr. f. wiss. Zool. 1^84.
Allis, K. P., J)-. 1'apers on Skull, muscles, and nerves of («i Amia, Journ. Morphol.
IM»7, 1*9*. and Anat,. Anz.. 1S99. (/>) Polypterus, Anat. An/.. I'.mo. (,• i Snunber,
Jouru. Morphol. 1903.
APPENDIX 513
Ayers, H., and Jaeksuu, S. M. Skel. and Muscles of Myxiuoidei. Jouru. Morphol
1901.
Baur, G. Morphol. and Origin of the Ichrhyopterygia. Arner. Nat. ixsy.
Boeke, J. Entw.-gesch. d. Teleostier. Pet. C'amp. 1904.
Braus, H. Eutw. d. Muskulatur u. d. periph. Xerveusystems d. Selachier. Morph. Jain l>
1899.
Bridge, T. "W. Skull of Osteoglossum forniosuui. P. Zool. Soc. 1895; Lepidosiren,
etc. Tr. Liuu. Soc. 1898.
Cole, F. J. Auat. of Skeleton of Myxine. Trans. Roy. Soc. Ediu. 1905.
Dohru, A. Studieu zur Urgesehiehte des Wirbeltierkorpers. Mitth. Zool. Stat.
Neapel. Bd. III., V., VI., IX., 1890. XV., 1904. Losung des Wirleltieikopf-
Probleins. Auat. Auz. 1MMI.
Dollo, L. Ls Champsosaure, a la vie fluviatile. Bull. Soc. beige de Geol., etc. 1891.
(See also numerous papers in previous vols., and iu Bull. Mus. Koy. Hist. Xat.
Belg. und Bull, scieut. Giard.)
Foote, E. The Extra-branchial cartilages of Elasmobranchs. Auat. ADZ. 1897.
Froriep, A. Zur Kopffrage. Auat. Auz. 1902.
Fiirbrioger, K. Visceralskelett d. Selachier. Morph. Jahrb. 1903. Morph. d.
Skelets d. Dipnoer. Semou's Forschungsreisen. 1904.
Gadow, H. Modifications of the first and second visceral arches with especial lef. to
the homologies of the auditory ossicles. Phil. Tran. 1888.
Gfgeubaur, C. Vergl. Auat. d. Wirbeltiere. Kopskelet der Selachier. Leipzig, 187:.';
Kopfskelet von Alecocephalus. Festgabe des Morph. Jahrb. Leipzig, 1878 ;
Occipitalregion und die ihr benachbarteu "NVirbel der Fist-he. Festschrift zu A.
v. Kollikers 70. Geburtstag. Leipzig, 1887. Metamerie des Kopfes und die
Wirbeltheorie des Kopfskelets. Morph. Jahrb. 1888.
Gaupp, E. Numerous papers on the Skull, iu Auat. Hefte, Ergebu., 1897 and 1904; Auat.
Auz. 1900, 1903 (Teleosts), and 1905. (Cf . also Eutw. d. Kopfskeletts iu Hertwig's
Haudbuch d. virgl. u. exp. Eutw.-gesch., 1906, with Bibliography.)
Haswell, W. A. (See p. 512.)
Hatschek. Metamerie des Ainphioxus u. des Ainmoccetes. Verh. Auat. Ges. 1892.
Hoffmann, C. K. Zur Entwick.-gesch. des Selachierkopfes, et-'. Auat. Auz. 1894.
Morph. Jahrb , 1896 and 1897.
Huxley, T. H. Crauiofacial apparatus of Petromyzou. Jouru. Auat. and Physiol.
Vol. X.
J;iquet, M. L'Auat. et 1'histol. du Silurus. Arch. d. S.-i. Mud. Bukarest. 1898.
Chim;era, Callorhyuchus, Spiuax, Protopterus, Ceratodus, and Axolotl. Loc.
cit. 1897 ; iSquel. ceph. d'une "' Carpe Dauphin." Bull. Soc. Sci. de Bucarest,
Itoumauie. 1902.
Johnston, J. B. Morph. of vert. Head, etc. Journ. Comp. Neurol. aud Psychol. 1905.
Klciu, v. Schiidel der Kuochenfische. Jahresb. des Vereius f . vaterlaud. Naturkunde
iu AViirttemburg. 1884—1886.
Killiau, G. Metamerie des Sslachierkopfes. Verh. Auat. Ges. 1891.
Knpffer, C. Vergl. Entw.-gesch. d. Kranioten. I. Entwick. des Kopfes von Acipeuser.
Miiucheu u. Leipzig, 1893. II. Entwick. des Kopfes von Petromyzou, 1894.
III. Eutwick. der Kopfnerven von Bdellostoiua, 1900; von Petromyzou planeri.
1895. Eutwickluugsgesch. des Kopfes. Anat. Hefte, Ergebu. 1895 ; Entw.
d. Kitmeuskeletts von Amiuocoostes, etc. Verh. Auat. Ges. 1895 ; Kopfeutwick.
v. Bdellostoma. Sitz. b?r. Morphol. Physiol., Miinchen. 1899.
Locy, W. A. Structure and Dev. of the vertebrate Head. Jouru. Morphol. 1895.
Marshall, A. Milnes. Segmental value of the cranial nerves. Jouru. Anat. and
Physiol. Vol. XVI. Head-Cavities and Associated Nerves in Elasmobranchs.
Q.J.M.S. Vol. XXI.
Neal, H. V. Seg. of Nervous Syst. iu Squalus, etc. Bull. Mus. Harvard. 1898.
Parker, W. K. -Skull in Sharks aud Skates. Tr. Zool. Soc. Vol. X. ; Skeleton of
the Marsipobranch Fishes. I., The Myxinoids. II., Petromyzou. Phil. Trans.
1883 ; Skull iu the Sturgeons. Phil. Trans. 1882 ; Skull in Lepidosteus
osseus. Phil. Trans. 1882 ; Skull in the Salmon. Phil. Trans. 1873.
Parker, W. K. and Bettany, G. T. The Morphology of the Skull. London, 1877.
Platt, J. Morphology of the Vertebrate Head. Jouru. Morphol., 1891, and Anat. Auz.,
1891.
Pollard, H. B. Suspension of the Jaws iu Fish. Anat. Anz., 1894; Oral Cirri of
Siluroids and the Origin of the Head in Vertebrates. Zool. Jarhb., 1895.
Pouchet, D. Du dev. du squelettu des poissons osseux. Journ. de 1'Auat. et Physiol.
1878.
Rabl, C. Metamerie des Wirbeltierkopfes. Verh. Auat. Ges., 1892.
Kidewood, W. G. Spiracle and associated Structures iu Elasmobranchs. Auat. An/..,
1896.
Rosenberg, E, Occipitalregiou des Cranium uiid prox. Teil der Wirbelsiiule einiger
L L
514 APPENDIX
Selachier. Erne Festscrift. Dorpat, 1884 ; Kopfskelet einiger Selachier. Sitz.
her. der Dorpater Xaturforsch. Gesellsch. 1886.
Sagemehl, M. Vergl. Anat. der Fische. Morph. Jahrb. 1884,1885,1891.
Schaffer, J. Knorp. Skelett v. Ammo?cetes, etc. Zeitschr. f. wiss. Zool. 1896.
Schleip, W. Eatw. d. Kopfknocheii bei dem Lachs, etc. luaug. -Dissert. Freiburg.
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Sewertzoff, A. Entwick. der Occipitalregion der niederen Vertebraten, etc. Bull. Soc.
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Stcihr, Ph. Entw.-geschichts des Kopfskelets der Teleostier. Festschrift zur Feier des
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Wright, Ramsay R. Skull aud Aud. Org. of Hypophthalmus. Trans. Roy. Soc.
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(/') AMPHIBIA.
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APPENDIX 515
(e) KEPTIUA.
Baur, G. Ostt'olog. Notizeu liber Reptilieu. Zool. Anz., 1886. (See also other papers
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Bemmelen, J. F. von. Phylogeuie tier Schildkroteu. Couipte-reudu d. seances du
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Goppert, E. Bedeutung d. Zuuge f. d. sekuudiiren Gaumeu u. d. Ductus uaso-
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in the Chameleons. T. Zool. Soc., 1881.
Eabl, C. Einige Problems d. Morphologic. Auat. Anz., 19U3.
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Sewertzoff, A. N. Entw.-gesch. v. Ascalobotes. Auat. Anz , 1900.
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Lurje, M. Pneumatisation d. Taubeuseluidels. Auat. Hefte, 1906.
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Dinornithidse. Trans. Zool. Soc., 1895. (See also p. 504.)
Pycraft, W. P. Osteol. of Birds. P. Zool. Soc., 1888, etc.
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(e) MAMMALIA
Allen. H. Ethmoid boue in Mammalia. Bull. Mus. Harvard. Vol. X.. 3.
Bardelebeu, C. v. Uuterkiefer, etc. Sitz.-ber. Ak. Berlin, llin.V
Bemineleu, J. F. van. Papers on the Mouotreme Skull. K. Ak. d. Weteusch.,
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Bolk, L. Occipitalregion d. Primord.-Craniums beim Meuschen. Pet. Camp., 1903.
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Fuchs, H. Herkuuft u. Entw. d. Geborknochelcben bei Kauiucheu-embryoueu. Arch.
Auat., 1905.
L L 2
516 APPENDIX
Gaupp .E. Sauger- u. Meuscheuschadel. Korresp. Bl. Deutsch. Authropol. Ges., 1903 :
Ala temporalis, etc. Anat. Hefte, 1902 ; Nicht-Homologie d. Unterkiefers i. d.
Wirbeltierreihe. Anat. Auz., 1905.
Hallmaun. Vergl. Aiiat. des Schlafenbeins, 1837.
Hartlaub, ('. Beitr. z. Keuutu. der Manatus-Arten. Zool Jahrb., 1886.
His, VV. Nasen- u. Gaumeubildung beiiu meusph. Embryo. Abb. K. Sachs. Ges. d.
"\Vissensch. ,1901.
Howes, G. B. Mammalian hyoid. Brit. Assoc. Adv. Sei., 1895.
Hrdlicka, A. Parietal boue, etc. Bull. Anu-r. Mus. Nat. Hist., N. York, 1903.
Jacoby, M. Zur Kenutnis des menscbl. Primordialcraniums. Arcb. f. mikr. Auat,.
1894.
Joseph, G. Kopfskelet des Meusaheu uud der Wirbeltiere. Breslau, 1873.
Kampeu, P. N. van. Tympanalgegend d. Saugetiersebadel. Morpb. Jahrb., 1905.
Kjellberg, K. Eutw.-gesc-h. d. Kiefergeleuks. Morpb. Jabrb., 1904.
Laukester, E. R. Lateral boms of the Giraffe. P. Zool. Soc., 1897.
Mibalkovics, V. von. (See under Olfactory Organ.)
Osborn, H. F. See numerous papers in 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.
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Parsons, F. G. Joints of Mammals, etc. Journ. Auat. and Phy.siol. Vol. 34.
Paulli, S. Pneuir.atizitat d. S.-liadrls liei d. Sangetiere. Morpb. Jabrb., 1899.
Rabl, ('. Eutw. d. Gesicbtes. Leipzig, 190i'.
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Scbwalbe, G. Stirnuaht bei d. Primateu. Zeitsch. f. Morph. u. Autbropol., 1904.
Schwiuk, F. Zwiscbeukiefer bei Saugetiereu. Miincben, 1888.
Seleuka, E. Meuscheuaffen. Wiesbaden, iMt^.
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4. PECTORAL ARCH.
Broom, R. Papers on Shoulder Girdle of Marsupials. Jouru. Auat, and Pbysiol., 1897.
Traus. Roy. Soc. Edin., 1889. Journ. Liuu. Soc., 19d2.
Dobm,A. Studien zur Urgescb. des Wirbeltierkorpers. VI. Die paarigcu und uu-
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1887.
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Sdiult . -rgiirtels mid des Brustbeines bei Keptilien, Vi'igelu, Saugetiereu und dem
Meuschen. Li,<-. <-it.
Howes, (i. li. Pectoral fiu-skeleton of the living Batoid fislirs and of Squalor aj a, etc.
P /ool. Soc. 1M90: Mammalian Coracoid. Jouru. Auat, and Pbysiol. Ls^7 :
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1'arkcr, \V. K. Structure and development of the Shoulder Girdle and Sternum in tl:c
N'crtebrata. Ray Society. Lsj V A"T TX
Dean, B. The Fiu-fold origin of the paired limbs in th- light of the Ptychopterygia
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Emery C Beziehungen des Cheiropterygiums zum Ichthyopterygium. Zool. Auz.,
1887- Hand- und Fuss-skelets. Auat. Anz.,lN»0; S .-heletro delta mauo degli
\iifibi Aniu-i. Atti Ace. Liucei, 1892 ; Morfologia d-i Membri dei Mammifen.
Mem. Pt. Accad. d. Sci. dell'Istit. di Bologna, 1892; Morfol. dei Membri degli
Anfibi Ricerche Lab. Auat. Roma e altri, 1894 ; Morfol. d<4 Tarso dei Mammifen,
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Ewart, J. C. D^v. of the Skel. of the Limbs of the Hors?. Jouru. of Comp. Path, and
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Fiebiger, J. Bauchflosseu d. Gobii. Auat. Anz., 1905. .
Fischer, E. Carpus u. Tarsus v. Hyrax. Jen. Zeitschr., 1903.
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' 1892.
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Fiirbriuger M Knocben uud Muskeln der Extremitaten bei den schlangenann.
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Gegenbaur, C. Skelet der Gliedmassen der Wirbeltiere, etc. Jen. Zeitschr., IbiO;
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Gb'tte, A Eutwick. und Regeneration des Gliedmassenskelets der Molche. Leipzig,
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Harrison R G Eutwick. der nicht knorpelig vorgebildeten Skeletteile m den * lessen
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Hoffman, C. K. (See p. 511.)
Holl M. Eutwick. dor Stellung der Gliedroassea des Meuschen. Sitz.-ber. Ak. \\ it
1891.
APPENDIX 519
Howes, G. B. Skel. and Affinities of the Paired Fins of Ceratodus, etc. P. Zool. Soc.
1887 ; Pedal skeleton of the Dorking Fowl, with remarks oil hexadactylism, and
phalangeal variation. Jouru. Anat. and Physiol. 1892; Variation aud 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. Soe. lsv\
Howes, G. B., and Ridewood, R. Carpus and Tarsus of the Auura. P. Zool. Soc.
1888.
Humphry. Observations on the limbs of vertebrate animals, etc. 1860.
Jordan, P. Entwickl. der vorderen Extremitat der Auuren Batrachier. Inaiig. -Dissert.
Leipzig. 1888.
Juugers?n, H. F. E. Structure of the Hand in Pipa and Xeuopus. Ann. Nat. Hist.
1891.
Kehrer, G. Carpus und Tarsus der Amphibien, Reptilieu imd Sauger. Ber. Ges.
Freiburg. 1886.
Klaatsch, H. Brustflosse der Crossopterygier. Festschr. fiir C. Gegeubanr. Leipzig.
1896.
Kollraaiin, J. Haudskelet und Hyptrdactylie. Verh. Anat. Ges. 1888.
Kiikenthal, W. Hand der Cetaceen. Anat. Anz. 1888 (Cf. also Auat. Auz. Jahrg. IV.,
V., and X., and also Vergl.-anat. uud Eutwickl.-gesch. Uutersuch. an
Waltieren. Jena, 1889, und 1893); Aupassung von Saugetieieu an das Lebeu
im Wasser. Zool. Jahrb. 1890; Carpus des Weisswals. Morph. Jahrb. 1.C92 ;
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Lazarus, S. P. Morphol. des Fuss-skelets. Morph. Jahrb. 1896.
Leboncq, H. Morph. de Vespertilio. Bruxelles. 1899. Organogenic d. Pinnipedes. Exp.
Antarct. beige. Aiivers. 1904 ; Dev. du premier metatarsien et de son articulation
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Belgique: 3. ser. t. XVIII. ; Morphol. du carpe chez les mammiferes. Arch,
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chez 1'homme. Anual.delaSoc.de Med. de Gaud. 1887; Morphol. de la main
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Leighton, V. L. Development of the wing of Sterna wilsonii. Amer. Nat.
1894.
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Leydig, F. Ban der Zehen bei Batrachieru und die Bedetitung des Ferseuhockers.
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Markert, F. Die Flossenstrahleu von Acanthias. Zool. Jahrb. 1896.
Marsh, 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 aud classific. of the Dinosaurian Reptiler, (Vol. L.) ;
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Amer. J. Sci. 1892.
M irtins, Ch. Compara'son des membres pelviens et fchoraciques chez 1'homme et chez
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Mayer, P. Die unpaareu Flossen der Selachier. Mittheil. Zool. Stat. Neapel.
1885.
Mehnert, E. Kauogenes-is, etc. Morph. Arb. 1897.
Mivart, G. Fins of Elasmobranehs, etc. Tr. Zool. Soc. 1879.
Mollier, S. Entwick. der paarigen Flossen des Stors. Anat. Anz. 1896; Extremitaten
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paarigeu Flossen des Stors. Anat, Hefte. Bd. III. 1893. Bd.V. and VII. Entwick.
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Niemiec, J. Les ventouses dans le ri-gue animal. Dissert. Genf . 1885.
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Parker, W. K. Morphology of Birds. Proc. Roy. Soc. 1887 ; Structure aud Dev. of the
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1891 (see also p. 504).
Paterson, A. M. The position of the mammalian limb, et?. Stud, in Anat. from the
Anat. Dept. of Owens College. 1891 ; F;ite of the Muscle Plate, and Dev. of ^the
Spinal Nerves and Limb Plexuses in Birds and Mammals. Q.J.M.S. 1888,
520 APPENDIX
IVc, P. van. Dev. des Extivniitcs cliez Amphiuma u. Necturus. Compt. reml. de
1'Assoc. des Anatomistes, Liege, 1PU3. and Anat. Auz., 1903-4.
Perriu, A. Carpe ties Auoures. Bull. Sci. France, Belg. Affiuites zool. du 1'Hatteria.
Ann. S:-i. N-,it. 1896.
Pfitzuer, W. Die kleine Zehe. Arch, f. Anat. u. Physiol. 1890; Menschl. Carpus.
Verh. Auat. Ges. 1893 ; Doppelbilduug iler fiinften Zehe, etc. Morpli. Arb. 1*95.
Prentiss, ('. W. Polydactylism in .Alan and Domest. Aumvds, etc. Bull. Mus. Harvard.
1903.
Rabl, C. Ursprung d. Extreniitat;-n. Zeitsdir. f. wiss. Zool. 1901 ; Eiuige Probleme d.
Morphol. Anat. Anz. 1903.
Rautenfeld, E. V. Das Skelet der hintereii Gliedumsseu vou Gauoideu und Teleostiern.
Inaug. -Dissert. Dorpat. 1882.
Rosenberg, E. Eutwick. ties Extremitatenskelets bei einigen durch Redact, ihrer Glied-
massen cliaracterisirten Wirheltieren. Zeitsihr. f. wiss. Zool., Bd. XXIII. ;
Entwickl. der Wirbelsaule und das Centrale Carpi cles Menscheu. Morph. Jahrb.
1876; Entwicklnngsstadieu des Haudskelets der Emys. Morpli. Jahrb. 1891.
Roux, W. Morphol. der fimctionellen Anpassuug, etc. (Schwanzflosse des Delphins).
Arch, fiir Anat. u. Physiol. 1883.
R'ige, G. Entw.-Gesch. d. Skelettes d. vord. Ext. v. Spiuax. Morph. Jahrb. 1902.
Ryder, J. A. Extra terminal phalanges in the Cetacea, Amer. Nat. 1885.
Salensky, W. Dev. de LTchthyopterygie des Ganoides et Dipnoi les. Ann. Mus. Zool.
Ak. Imp. Sci. St. Petersbourg, 1898.
Schaffer, J. Bau d. Zehen bei Fledermausen, etc. Zeitsc-hr. f. wiss. Zool. 1905.
Schlosser, M. Modificationeu des Extremitaten-Skelets bei deu einzelnen S'iugetier-
stammen. Biol. C'entralbl. Bd. IX.
Schneider, A. Dipnoi. Zool. Beitnige. Bd. II. Breslau, 1887.
Schumann, A. Hinter-extremitaten v. Dipus. Morph. Jahrb. 1904.
Semon, R. Eutw. d. Flossen d. C'eratodus. Fors^hungsreisen. Jen. Deuks;hr. Bd. I\".
1898. (See also Morphol. Jahrb. 1899.)
Sewertzoff, A. N. Entw. d. pentadaktylenExt. d. "Wirbeltiere. Anat. Auz. 1904.
SHeda, L. Talus uud das Os trigonum Bardeleben's beim Menschen. Auat. Anz.
1889; Homologie der Gliedmassen der Saugetiere und des Menschen. Biol.
Centralbl. 1893. (Cf. also Anat, Hefte, Arb., Bd. VIII., and Biol. Central!)!.
1897.)
Storm, R. Adhesive Disk of Echeueis. Ann. Xat. Hist. 1888.
Strasser, H. Eutwick. der Extremitiiteukuorpel bei Salamandern und Tiitaneii.
Morph. Jahrb. . 1879.
Stromer, E. Foramen entepicoudyloideuin u. Trochauter tertiusd. Saugetiere. Morph.
Jahrb. 1902.
Stuckcns, M. La ventouse abdominale du Liparis. Bull. Ac. Belgique. 1884.
Studer, Th. Die Forschungsreise S.M.S. Gazelle. Herausgeg. von dem hydro-
graph. Amt der Admiralitat. III. Theil : Extremitaten-Entwicklung des Pinguin.
Berlin, 1889.
Symington, J. Cetacsan flipper, etc. Journ. Anat. and Physiol. 1906.
Thacher, J. K. Median and Paired Fins, etc. Trans. Connecticut Acad. 1878;
Ventral Fins of Ganoids. Loc . cit.
Tuilenius. (}. Access. Elemente am ni?nscbl. Carpus u. Tars'is. Morphol. Arl).
1896; Die" iiberzahligen " Carpuselemeute meuschl. Embryonen. Auat. Auz.
1894; Eutwicklungsgesch. der Sesambeiue der menschl. Hand. Morph. Arb.
1895; Os intermedium antebrachii des Menscheu. Loc. cit.
Thilo, O. Umbilduugen an den Gliedmassen der Fische. Morph. Jahrb. 1876.
Thompson, D'A. W. Hind-Limb of Ichthyosaurus, and Morphol. of Vertebrate Limbs.
Jourii. Anat. and Physiol. 1886.
Tornier, G. Hyperdaktylie, etc. Arch. Entwick. Mechauik. 1896 ; Schwanzregenera-
tion bei Eidechsen, etc. Sitz.-ber. Naturf. Berlin, 1897 ; Ueberz;ihlige
Bildungen, etc. Ver. Internat. Zool. Kougr. Berlin, 1901 ; Exp. Erzeugeu
iiberziihl. u. Zwilliugsbildungen. Zool. Anz. 1901 ; Schweinehinterfuss in it
fiiaf Zehen. Arch. Entwick. Mechauik. 1902; Vorderfuss-Hyperdaktylie bn
Cervus-Arten. Morph. Jahrb. 1903 ; Saugetier-Praehallux, etc. Arch. f.
Naturgesch. 1891 ; Entstehen d?r Gelenkformen. Arch. f. Entw.-Mechanik.
1894-95.
Traquair, R, H. Cladodus. Trans. Geol. Soc. Glasgow, 1897.
Tschau, A. L'Extremite anterieure de? Oiseaux et dss R-ptiles. Iuaug.-Dis.sert.
Geneve, 1889.
Vogt, Ch. Verknricherung des Hohlhandbandes uud audere Sesambeiue der Sfinger. etc.
Inaug. -Dissert. Tubingen, 1894.
Wiedersheim, R. Die altesten Formen des Cm-pus uud Tarsus der heutigen Amphibien.
Morphol. Jahrb. Bd. II., 1876, and Bd. III. ; Vermehruug des Os centrale im
Carpus uud Tarsus des Axolotl. Morph. Jahrb. Bd. VI.
APPENDIX 521
Zehnter, L. Eutwick. von Cypselus. Iiiaug. -Dissert. Bern, 1890.
Zwick, "\V. Amphibiengliedmassen. Zool. Arb. Tiibiugen. Bd. II.
D. MUSCLES.
Albrecht, P. Morphol. des M. omo-hyoideus und der ventralen, inneren intcrln-;mch.-
Muskulatur. Inaug.-Dissert. Kiel, 1876.
Allis, E. P. Cranial Muscles and 1st spinal nerves in Amia. Journ. Morphol. 1897.
Bardeleben, C. Muskel und Fascie. Jen. Zeitschr. Bd. VI. N.F. VIII. ; Haud-
und Fussmuskeln der Saugetiere, besonders die des Praepollex (Praehallux) imd
Post-minimus. Anat. Auz. 1890. (Cf. also p. 517.)
Berfcelli, D. Morfol. del Muscolo Diaframma nei Manimiferi. Arch, per le Sci.
mediche. 1895 (cf. also Atti Soc. Toscaua. Vol. XV.) ; Reui primitivi lie
Rettili,etc. Atti Soc. Toscana. 1897; Diaframma oruitieo. Monit. Zool. Ital.
1898; Emb. e Auat. comp. sul diaframma, etc. Arch. Ital. Anat. e Embr.
1905.
Bishoff, Th. Auat. des Hylobates leuciscus. Miiucheu, 1870.
Blum, F. Die Schwanzmusculatur des Menscheu. Anat. Hefte Arb. 1894.
Bovero, A. Musculns Cutaneo-mucosus Labii. Accad. R. de Scienze di Torino. 1902.
Brachet, A. Le develop, du Diaphragms et du Foie chez le lapiii. Journ. de 1'Auat.
et Physiol. 1895.
Bradley, C. O. Muscles of Mastication, etc., in Lacertilia. Zool. Jahrb. 1903.
Braus, H. Muse. u. periph. Nervensyst. d. Selachier. Morph. Jahrb. 1898-9; Muse.
u. Nerveu d. Ceratodusflosse. Semon's Forsclmngsreiseu. 1900.
Bromau, J. Eutw. d. Zwerchfells beim Mensehen. Anat. Auz. 1902.
Brooks, H. Morphol. of the Extensor Muscles. Stud. Mus. Zool. in Univ. Coll.,
Dundee. 1889.
Bruner, H. L. Muskelapparat zum Schliesseu und Oeffuen der Nasanlocher bei deu
Salamaudriden. Auat. Auz. 1896. (Cf. also Arch. Auat. 1896.)
Buffer, P. Muse. cut. dei Serpenti, etc. Accad. Veneto-Trentino-Istriana. 1904-5.
Buri, R. O. Auat. d. Fliigels voii Micropus, etc. Jen. Zeitschr. 1902.
Cadiat, M. Developpemeut de la partie cephalothoracique de 1'Embryon, etc. Journ.
de 1'Auat. et Physiol. 1878.
Carlson, A. Gliedmassenreste bei Schlangen. Kouigl. Schwed. Acad. Bd. XI.
Chappuis. Morphol. Stelluug der kleiuen, hiutern Kopfmuskeln. luaug.-Dissert.
Bern, 1876.
Corning, H. K. Eutw. d. Kopf u. Extrem.-Muskulatur bn Reptilieu. Morph. Jahrb.
1899.
Davidoff, M. v. (See p. 518.)
De Man. Myol. eu Neural. Stud, over Amphibieu en Vogels. Leiden, 1873.
Driiner, L. Zuugenbein-, Kiemenbogen- u. Kehlkopf muskelu d. Urodelen. Zool. Jahrb.
1904; Muse. d. Visceralskel. d. Urodelen. Auat. Auz. 1903.
Duges, A. L'osteologie et la myologie des batraciens .
Smaliau, 0. Anat. d. Amphisbpeniden. Zeitfch. f. \\iss. Zool. Bd. XI.II.
Testut, L. Les anomalies musculaires chez 1'homme, etc. Paris, 1881.
Tiesing, B. Augen-, Kiefer- uud Kiemeumuskulatur der Haie u. Rochen. Jen. Zeitschr.
Bd. XXX. N.F. XXIII.
Tobler, L. Achselbogeu d. Menscheu, etc. Morph. Jahrb. 1902.
Uskow, N. Entwick. des Zwerchfells, des Pericardiums und des Coeloms. Arch. t.
mikr. Anat. 1883.
Vetter, B. Vergl. Anat. der Kiemen- und Kieferm'uskulatur der Fisjhe. Jen. Zeitschr.
Bd. VIII. uud XII. N.F. I. Bd.
Wcstliug, Cb. Echidna, Svenska Vet. Akad. Handl. 1889.
Wijhe, J. W. van. Me.sodermsegmente und Eutwick. der Nerveu des Selachierkopfes.
Verb. Ak. Amsterdam. 1883.
Wilson, J. T. Myology of Notoryctes typhlops. Trans. Roy. Soc. S. Australia, 1894.
Wiudle, B. ('. A. Flexor.-; of the digits of the hand. Jouru. Anat. and Physiol. 1889 ;
Pectoral Group of Muscles. Tr. Irish Ac. 1889 ; Muscles of Mammals, etc.
Jouru. Anat, and Physio! . 1898.
Wiudle, and Parsons, F. G. Myology of the Terrestrial Carnivora. P. Ziol. Soc. 1897,
1898; Edentata. Loc. cit. 1899 ; TJnjjulata. Loc. cit. 1901.
E. ELECTRIC ORGANS.
BOiu-hin. Pseudoelektrisi-he Organe. Medic. Ceiitr.-Blalt, No. :<~> ; Entwick. der
elekt Organe und Bedeutuug der mot, Eudplatten. Medi-. (Vntr.-I51att. 1870 ;
Eutwick., Ban, und physiol. Verhiiltuisse der elektrischeu und pseudo-elektrischen
Organe. Arch. f. Anat. u. Physiol. 1876; Zitterwelse uud Mormyrus. Arch. f.
Anat. uud Physiol. 1877.
Ballowitz, E. Bau des elekt. Organes von Torpedo, etc. Arch. f. mikr. Anat. 1893 ;
Ban des elekt. Organs des gewo'hulichen Ro:-heu. Auat. Hefte Arb. Heft
XXIII. (Bd. VII., H. 3.)
]>dlo',vitz, R Anat. des Zitteraales (Gymnotus), etc. Arch. f. inik. Anat. 1897; Nerv-
endiguugeu i.d. elektr. Organ des Malopterurus. Anat. Anz. 1898.
Boll, F. Physiol. von Torpedo. Arch, f An it. und Physiol. 1873; (1) Siructur der
el?ktr. Platten von Torpedo. (2) Structur der elektr. Platteu von Malopterurus.
Arch. f. mikr. Auat. 1874 ; Auat. uud Physiol. von Torpedo. Monatsbericht der
Berliner Akad. 1875 ; Elektr. Platten von Torpedo. Arch. f. Auat. und Physiol.
1876.
524 APPENDIX
Ciaccio, G. V. lutoruo all' intima tessitura dell' orgauo elletrico della torpedine.
Acad. delle scienze dell' istituto di Bologna. 1874; ami in German— Moleschott's
IJntersuch. Bd. XI.
('rcvatiu, F. Sogeu. Stabchenuetz im rlcktr. Orgau d. Zitterrochen. Anat. Anz.
1898.
Du Bois-Reymoud, E. Gesammelte Abhand. zur allgem. Muskel- nud Nerven-physik.
Bd. II. ; Bericht iiber die von Professor G. Fritsch iu Aegypteii angestellteu
neuen Uutersuch. an elektr. Fischen. Monatsschr. d. Berl. Akad. Issl.
Ewart, J. C. Electric Organ of the Skate. Phil. Trans. 1888 and 1892.
Fritsch, G. Fortsetzimg der Untersuch. an elektr. Fischen. Embryol. von Torpedo.
Sitz.-ber. Ak. Berlin. 1883; Die electr. Fiscbe. Abt. I. Malopterurus. Leipzig,
1887; Abt. II. Die Torpedineeu. Leipzig, 1890 (see also other papers iu Sitz.-
h.-r. Ak. Berlin, e.y. Gyinuarchus).
Gotch, F. Electromotive Properties of the Electrical Organ of Torpedo. Phil. Trans.
1887, 1888.
Hartmaun, R. Elektr. Orgaue der Fische. Arch. f. Anat. und Physiol. 1861.
Iwanzoff, N. Der mikrosk. Bau des electr. Organs von Torpedo. Bull. So;-. Moscou.
1894; Schwanzorgan von Raja. Lor. cit. 1S05.
Retzius, G. Endigung d. Nerven im Elektr. Organ v. Raja. Biol. Untersuch. 189s.
Sachs, C. Gymnotus electricus. Arch. f. Anat. 11. 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. Jouru. Physiol. 1889.
Schultze, M. Elektr. Orgaue der Fische. Abh. Ges. Halle. 1858 and 1859.
Sehultze, O. Feinere Bau der elektr. Orgaue d. Fische. Festschr. f. J. Rosenthal.
Leipzig, 1906.
F. NERVOUS SYSTEM.
(«) CENTRAL NERVOUS SYSTEM.
(a) CYCLOSTOMI AND PISCES.
Ahlborn, F. Gehirn der Petromyzouten. Zeitsch. f. wiss. Zool. 1883.
Auerbach, L. Lobi optici der Telostier und die Vierhiigel der holier organisirten
Gehirne. Morph. .Tahrb. 1888.
Beard, T. History of a Transient Nervous Apparatus in certain Ichthyorsida. Zool.
.Tahrb. 1896.
Beauregard, H. Eucephale et nerfs crauiens du Ceratodus forsteri. Jouru. de 1'anat.
et physiol. Paris, 1881.
Bellonci, J. Ursprung des Nerv. opticus und fein. Bau des Tectum opticum der
Kuochenfiscbe. Zeitschr. f. w is.?. Zool. 1880; C'entrale Endigung des Nervus
opticus bei den Yertebraten. Loc. cit. If-'f-'S.
Bethe, A. Allgem. Anat. 11. Physiol. d. Nervensystems. Leipzig, 1903.
Bing, R., and Burckhardt, R. Centraluerveiisystem v. C'eratodus. Journ. de 1'Anat. ct
de la Physiol. Paris, 1881.
Boeke, J. Papers relating to the infundibulum, hypophysis, etc. Anat. Auz. 1901 and
1902; K. Akad. d. Weteusch., Amsterdam. 1902. Pet. Camp. 1904.
Borchert, M. Centralnervensystem von Torpedo. Neurobiol. Arb. Jena, 1903.
Burckhardt, R. Ceutralnervensystem vou Protopterus aunecttns. Berlin, 1F92;
Vergl. Anat. des Vorderhirus bei Fischeu. Anat. Anz. 1894; Bauplan des
"\Virbeltiergehirns. Morph. Arb. 1894.
Busch, \V. De Selachiorum et Ganoideorum encephalo. Berlin, 1848.
Calb?rla, E. Entwick. des Medullarrohrs und der Chorda dorsalis der Teleostier und
Petromyzonten. Morph. Jahrb. 1877.
Dendy, A. Ciliated Grooves in Brain of Ammocoete. P. Roy. Soc. 1902.
Dohrii, A. Stud, zur Urgesch. des Wirbeltierkorpers. Mitt. Zool. Stat. Neapel. 1881,
1882, 1884, 1888, 1890, 1891.
Ecker, A. Anat. Beschreib. des Gehirns vom karpfeuartigeu Nilhecht. 1854.
Ediuger, L. Vergl. Anatomic des Gehirns. Abb. Seukenb. Ges. 1888-91 ; Bau der
uervosen Centralorgane des Menschen u. der Tiere. 5. Aufl. Leipzig, 1896 ;
Herkunft d. Hirnmautels, etc. Berlin. Klin. AVocheuschr. 1905.
Fritsch, G. Feinere Bau des Fischgehirns. Berlin, 1878 ; Eiuige bemerkeuswerthe
Elemeute des Centralnerveusystems vou Lophius. Arch. f. rnikr. Anat. 1886.
Froriep, A. Ganglionleiste d. Kopfes u. d. Rumpies, etc. Arch. Anat. 1901.
Fusari, R. Feinere Anatomie des Gehirns der Teleostier. Internat. Monatsschr. Auat.
1887.
Gierse, A. Glehiru u. Kopfuerveu v. Cyclothoue. Morph. Jahrb. 1904.
APPENDIX 525
Gotte, A. Eutw.-geschicbte tier "\Virbeltiere. III. Eutwick. des centraluerveusystem
der Teleostier. Arch, f. mikr. Auat. 1877 ; Eiitstehung uud Homologieeu d.
Hiruanhaugs. Zool. Ariz. 1883.
Goronowitsch, N. Gehiru und Crauialnerveu von Acipenser. Morph. Jahrb. 1888.
Gottsche, M. Vergl. Auat. des Gehirns der Gratenfische. Arch. Aiiat. uud Physiol.
1835.
Gregory, E. H., Jr. Entw.-gesch. d. Knoehenfische. Auat. Hefte Arb. 1901*.
Haller, B. Centralnervensysfcem uud Riickenmark von Orthagoriscus. Morpb. Jabrb.
1891 ; Riickenmark der Telostier. Loc. cit. 1895 ; Hypophyse uud Infuudi-
bularorgane. Loc. cit. 1897 ; Ban d. Wirbeltiergehirns (Saliuo u. Scyllium).
Loc. cit. 1898.
Handrick, K. Nerveusystem u. Leucbtorgaue v. Argyropelecus. Zoologica. Stuttgart,
1901.
Herrick, J. Central gustatory patlis in braius of Bony Fishes. Jouru. C'omp. Neurol.
and Psycho!. 1905.
Hill, C. Primary segments of Vert. Head. Auat. Anz. 1899.
His, W. Eroff uungsrede zur VI. Versaimnluug der Anat. Gesellscb. zu Wieu. 1892.
Hoffmann, C. K. Ontogenie der Kuocbenfische. Verb. Ak. Amsterdam, 1882, und
Arch. f. mikr. Auat., 1883.
Holm, J. F. Finer Auat. of Nervous syst. of Myxine. Morph.. Jahrb. 1901.
Johnston, J. B. Brain of Acipeuser. Bull. Zool. Lab. Ann Arbor Viiiv., Michigan,
1897, Auat. Anz., 1898, and Zool. Jahrb., 1901; Gehiru u. Cranialnerveu v.
Auamnier. Anat. Hefte Ergebn. 1901. Brain of Petromyzon. Jouru. C'omp.
Neurol. 1902.
Kiugsbury, F. Str. and Morpb. of Oblougata iri Fishes. Jouru. C'omp. Neuroi. 1897.
Kupffer, C. Entwick. der Knochenfisehe. Arch. f. mikr. Anat. 1868 ; Die Deutung des
Hiruauhauges. Sitz. Ber. Morpb. und Pbysiol. Miinchen, 1894 ; Eutwicklungs-
gesch. des Kopfes bei Acipenser. Loc. cit. 1891 ; Vergl. Eutvv.-gesch. der
Grauioteu. 1. Heft. Kopf von Acipenser. 2. Heft. Kopf vou Ammoctetes.
Miinchen und Leipzig. 1893; Hirnanhange. Sitz. Ber. Morph. Physiol. Miinchen,
1894.
Lenhossek, M. v. Spinalganglien uud Eiiekeumark vou Pristiurusembryonen.
Anat, Auz. 1892. (Of. also p. 529. )
Locy, W. A. Metanieric Segmentation in the Med. Folds and Embryonic Kim. Auat.
Anz. 1894.
Lundborg, H. Entwick. der Hypophysis uud des Saccus vasculosus bei Knocbeufiscben
uud Amphibien. Zool. Jahrb. 1894.
Mayer, F. Zeutralnervensystem v. Ammoctetes. Anat. Anz. 1897.
Mayer, P. Studieu uber das Gehirn der Kuocheufische. Zeitschr. f. wiss. Zool. 1881.
Miklucho-Maclai, v. Vergl. Neurologie der Wirbeltiere. Das Gehiru der Selachier.
Leipzig, 1870.
Muller, W. Eutwick. und Ban der Hypophysis und des Processus infuudibuli
cerebri. Jen. Zeitschr. 1871.
Neal, H. V. Segmentation of the Nervous System in Sqtialus. Anat. Auz. 1870'.
Neumayer, L. Histol. Unters..uber den feinereu Bau des Ceutraluerveusy stems vou
Esox. Arch. f. mikr. Aiiat. 1895. (C'f. also Sitz.-ber. Morphol. Physiol.
Miincheu, 1903.
Osboru, H. F. The Origiu 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 ; Carcharodou rondelettii. P. Zool. Soc. 1887.
Pendaschenko, D. Eiitw. d. Mittelhirns d. Knochenfisehe. Arch. f. mik. Auat. 1901.
Platt, J. Contrib. to the Morphol. of the Vertebrate Head. Journ. Morph. 1891.
Rabl-Riickhard, H. Die gegenseitigeu Verhaltuisse der Chorda, Hypophysis, etc., bei
Haifischerubryonen, etc. Morph. Jabrb. 1880; Deutuug uud Eutwick. des
Gehirris der Kuocheufische. Arch. f. Anat. u. Physiol. 1892 ; Entwick. des
Knocbenfischgehirus (Zirbel). Sitz.-ber., Naturf. Berlin. 1882; Deutuug dts
Gehirns der Kuocheufische. Biol. C'entralbl. 1883 ; Grosshirn der Knochen-
fisehe und seine Anhaugsgebilde. Arch. f. Anat. u. Physiol. 1883; Gehim der
Kuochenfische. Deutsch. medic. Wochenschrift. Berlin, 1884 ; Onto- und
phylogenetischen Entwick. des Torus longitudiualis im Mittelhirn der Kiiochen-
fiscbe. Auat. Auz. 1887 ; Lobus olfactorius impar der Selacliier. Anat. An/.
1893; Vorderhiru der Crauioten. Auat. Anz. 1894.
Reichenheim, M. Keuutnis des elekt. Centralorganes vou Torpedo. Arch. f. Auat. u.
Pbysiol. 1873.
Sageuiehl, M. Vergl. Auatouiie der Fisc-he (Hiruhiiute der Knocheufische). Morph.
Jahrb. 1883.
Sargent, P. E. Torus longitudinalis of Teleost brain, etc. Mark Auuiv. Vol. 1903 ;
Optic reflex apparatus of Verts., etc. Bull. Mus. Harvard. 1904.
526 APPENDIX
Schaper, A. Morphol. und histol. Eutwickluug des Kleinhirns der Teleostier. Anat.
Auz. 1894; and Morph. Jahrb. 1894; Friiheste Differenz.-vorgange im
Centraluer ceii system. Arch. Eutwick. Mechanik. 1897; Finer Struct, of
Selachian's Cerebellum. Jouru. Comp. Neiirol. 1898; Morphol. d. Klemhirus
Verb. Anat. Ges. 1899.
Scott, W. B. Eutw.-gesch. der Petromyzonten. Morph. Jahrb. 1881.
Sedgwick, A. Inadequacy of the Cell Theory, and Early Dev. of Nerves ill Elasrno-
branchii. Stud. Morph. Lab. Camb. 1896.
Serres. Anat. comp. du cerveau dans les quatres classes des auimaux vertebres. Paris,
1821—1826.
Shipley, A. E. Dev. of Petromyzou. Q.J.M.S. 18*7.
Stannius, H. Ban des Gehirns des Stors. Arch, f . Auat. u. Physiol. 1835, 1846.
Sterzi, G. Meuiugi spiuale del Pesci. Monit. Zool. Ital. 1899 (cf. also Atti d.
K. Istit. Yeueto d. Scieuze, 1900-1); Regioue iufundib. e dell' Ipofisi, etc.
Arch. Ital. di Auat. e di Einbr., 1904, and Atti Accad. S;i. Veneto-Treutiuo-
Istriaua, 1904.
Stieda, L. Centralnervensystem der Knocheufische. Zeitschr. f. wiss. Zool. Bd.
XVIII. ; Deutimg der eiuzelueu Teile des Fischgchirns. Loc. cit. Bd. XXIII. ;
Ban des Ruckenmarkes der Rocheu imd der Haie. Loc. cit. Bd. XXIII. (Cf.
also Festschr. z. 70. Geburtstag v. C. v. Kupffer. Jena, 1899.)
Strasser, H. Hiillen d. Gehirns u. Riickeumarkes, etc. Compt. rend, de 1'Assoc. d.
Anatomistes. Lyon, 1901.
Studnicka, F. K. Auat. uud Eutwickluugsgesch. des Yorderhirus der Crauioteu. I.
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(c) REPTILIA.
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(e) MAMMALIA.
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528 APPENDIX
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.M M
530 APPENDIX
(6) PINEAL APPARATUS.
Ahlborn, F. Bedeutuug d-r Zirbeldriise. Zeitschr. f. wiss. Zool. 1884.
Beard, J. The Parietd Eye of the Cyclostome Fishes. Q.J.M.S. 1888.
Beraueck, E. Pariet.dauge der Reptilieu. Jen. Zeitschr. 1887 ; Xerf parietal et la
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Carriere, J. Parietalorgan. Biol. Centralbl. 1889.
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Hitter, W. E. Parietal Eye iu some Lizards from the western Uuited States. Bull.
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APPENDIX 531
(c) PERIPHERAL NERVOUS SYSTEM.
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the ophthalmicus profimdus in Sharks. Anat. Anz. 1 S87 ; Dev. of the Peiipheral
Nervous-System in Vertebrates. Q. J.M.S. 1888. (Cf. also Anat. Anz. 1888.)
Beck, W. Austritt div; Norv. hypoglossus und N. c rviralis primus a us deni Central-
organ beini Menscheu, etc. Anat. Hefte. 1895.
Bemmeleu, van. Halsgegend der Keptilien. Zool. Anz. 1887.
Beraueck, E. Le dev. des nerfs crauiaux chez les Lezards. Hecueil zoolog. suisse.
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Bolk, L. Neurol. d. uut. Ext. d. Primaten. Morph. Jahrb. 1897; Meusch Rumpf. u.
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Bowers, M. A. Distrib. of cranial nerves of Spelerpes. Proc. Amer. Acad. Arts and
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Braus, H. Various papers 011 Nerves of Ichthyopsida, in Jtn. Zeitsdir., 1898; Morph.
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Carlsson, A. Auat. d. Sclnvimnivogel. Abb. d. K. Scb.wed. Vet. Acad. Bd. III.
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Chevrel, O. L'Aiiat. du Systeme uerveux grand sympatbiqne des Elasmobranches et
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Coggia, A. Alcuni derivati dell' ectoderma uei capo dei Selaci. Ricerche Lab. Auat.
Roma e altri Lab. Biol. 1895.
Coghill, G. E. Fifth Nerve iu Amphibia. Coutrib. Anat. Lab. Brown Univ., 1901, aud
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Cole, F. J. Cranial Nerves of Chimera, 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 hit. sense-organs of Fishes (Gadus). Tr. Linn.
Soc. 1898. (Cf. also Trans. Liverpool Biol. Soc., 1898, and Anat. Auz., 1898, 1899,
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Cords, E. Kopfnerveusystem d. Vugel. Anat. Hefte. 1904.
Cuneo, B. Neri's cranieus. Traite d'Auat. hum. 1899.
Davidoff, M. v. (See p. 518.)
Deyl, T. Vergl. Auat. des Sehnerveu. Bull. Internat. de 1'Acad. des Sci. del'Empereur
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Dixou, A. F. Dev. of the Branches of the fifth cranial Nerve in Man. Tr. Dublin. Soc.
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Edinburgh Med. Journ. 1897 ; Sensory distrib. of facial Nerve in Man. Jouru.
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Dohrn, A. (See p. 524.)
Dogiel, A. S. Periph. Nervensyst. v. Amphioxus. Anat. Hefte, Arb. 1903.
Eisler, P. Plexus lumbosacralis des Meuscheu. Abh. Ges. Halle. 1892.
d'Erchia, F. Gang, ciliare. Mouit. Zool. Ital. 1895.
Ewart, J. C., and Cole, F. J. Nerves of Elasmobrauchs. P. Roy. Soc. Edin. 1895.
Fischer, J. Nerv.-sympath. (Katze, Ziege, etc.). Inaug.-Diszert. Zurich, 1905.
Fischer, J. G. Gehirnuerveu der Saurier. Hamburg, 1852. (And see p. 526.)
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1878.
Frohse, F. Oberuach. Nerveu des Kopfes. Berlin, Prug, 1895.
M M 2
532 APPENDIX
Fruriep, A. Gangliou des Hypoglossus uud Wirbelaulageu in der Oecipitalregion.
Arch. f. Anat. uiul Phys. 1882 ; Aulagen vou Sinuesorgaueu am F'acialis, (ilosso-
pharyngeus uud Vagus, etc. Arch. f. Anat. 1885; Homologon der Chorda
tympani bei uiedereu Wirbeltieren. Aiiat. Auz. LSS7.
Froriep, A., uud Bsck, W. Vorkommen dorsiler Hypoglossiisvvurzelu luit Gauglieu iu
der Reih^ dcr Saugetiere. Auat. Auz. 1895.
Fiirbriuger, M. Umbilduugeu des Xerveuplexus. Morph. Jahrb. 1879 ; Die spiuo
occipitaleu Xerveu der Selachier uud Holocephaleu und ihre vergl. Morphol.
Festschrift f. ('. Gegeubiur. Leipzig, 1896: Morpliol. iStreitfragen. Morphol.
Jahrb. 1902 ; Brust-scliulterapparates, etc. Jen. Zeirschr. 1902 (ami sae p. 521 1.
Gadow, H. (See p. oil', i
Gaskell, "\V. H. Struct., distrib. aud function of the nerves which innervate the visceral
and vascular system. Jouru. Pliysiol. 1886 ; Relation between the Struct.,
Function, Distrib. and Origin of the Cranial Nerves, together with a Theory
of the Origin of the Xervous System of Vertebrata. Journ. Physiol. Vol. X.
Gegenbaur, C'. Kopfnerven vou Hexauchus, etc. Jen. Zeitschr. Bel. VI. (Aud see
p. 513.)
Giglio-Tcw, E. Organ! branch, e lat. di senso n-jH' uonio. Monit. Zool. Ital. ]9(L'
(Cf. also Auat. Auz. 1&02.)
Gorouovvitsch, X. Eutwickl. de sogen. " Ganglienleisten ' im Kopfe der Vogclem-
bryouen. Morph. Jahrb. 1893; Trig. fac. Komplex v. Lota. G^geub.uir's
Festschrift. Leipzig, 1*97. (And cf. p. 525.)
Haller. B. Ursprung des Xervus vagus bei den Knocheufischeu. Verb. Deutsch.
Zool. Ges. Bd. V.
Harrisou, R. G. Histogeuese d. periph. Xervensyst. b3i Salmo. Arch. f. inikr. Auat.
1901. Eut\v. d. periph. Xerveu. Sitz.-ber. Xiederrheiu. Ges. f. Xatur- u.
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Hawkes, O. A. M. Cr. and sp. Xerves of Ohlamydoselachus. P. Zool. Soc. 1907.
Herrick, C. J. Cranial Xerves of Bony Fishes. Journ. Comp. Xeurol. 1898 and
1901. Arch. f. Xeurol. u. Psychopath. 1899.
His, W. (Jr.) Entwk-kl. des Sympathicus bei Wirbeltieren. Verb. Auat. Ges.
189.2 ; Anfauge des periph. Xerveusystems. Arch. f. Auat. u. Physiol. 1879;
Eutwick. der ersteu Nervenbahnen beim meuschl. Embryo. Arch. f. Auat.
uud Physiol. 1888.
Hoffmann, C. K. Entwickl.-gesch. des Selachier kopfes. Anat. Auz. 1894 ; Eiitw.-
gesch. d. Selachii. Morph. Jahrb. 1899 ; Entw.-gesch. d. Sympathicus.
Verli. Ak. Amsterdam. 1900, 1902.
Iverseu, M. Die dorsalen Wnrzelu des Nervus Hypoglossus. Ber. Ges. Freiburg. 1886.
Jaquet, M. Sympath. cervical. Arch. d. S^ieuc. Mod. Bucarest, 1900; Auat. u.
Histol. v. Silurus. Bull. Soc. S:-i. Buctrest, Roumanie, 1901.
Jlu-ring, H. v. Periph. Xerveusystem der Wirbeltiere, etc. Leipzig, 187'x.
.Idhusou, A., and L. Sheldon.' Dev. of the Crauial Xerves iu the Xewt. P. Roy. Soc.
1886.
Johnston, J. B. Hind-brain and Cr. Xerves of Acipeuser. Air.it. Auz. 1898 ;
Morphol. of Vert. Head, etc. Jouru. Corup. Xeurol. and Psychol. 1905 ; Cranial
Nerve-components of Petromyzon. Morph. Jahrb. 19n.j ; Nervous System
of Vertebrates. Philadelphia, 1906.
Juliu, Ch. Systeme uerveux grand sympath. de 1'Ammoecetes. Anat. Auz. 1887:
La valeur morphol. du uerf lateral du Petromyzou. Bull. Ac. Belgique. 1887.
Key, A., uud G. Retzius. Auat. des Xervensystems uud des Biudegewebes. Stock-
holm, 1875 uud ]s7ii.
Kliukhardt, W. Entw.-gi-s -h. d. Kopfgauglieu u. Sinnesliuien der Selachier. Jen.
Zeitschr. 1905.
Kohn, A. Sogen. C'arotiddriise. Arch. f. inikr. Auat. 1900; Chromaffine Zellen, etc.
Med. Wocheuschr. Prag, 1902; Paragauglieu. Arch. f. inikr. Auat. 1903.
Kiilliker, A. Feiuere Bui uud Fuuctionen des symjiath. X'ervensystems. Sitz.-
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Kriuige, E. Geschichte der Anatomie der Hiruuerveu. Inaug. -Dissert. Freiburg i.
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K'ise, W. " Carntiddruse " u. " ChromaflBnen Zellen" bei Vogel, Auat. Anz. 1I.MI2.
Kranse, AV. Doppelnatur des Ganglion ciliaiv. Morph. Jahrlj. 1881.
Kupffer, C. v. Eutwick. vou Petromyzou. Sitz.-b^r der K. Bayer. Acad. der
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i See also pp. 525 and 536.)
L'.-e. St. Kenntnisdes Olfactorius. Ber. (ies. Freiburg. 1893.
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\V. A. I >ev. of ( )lf. Xerve. Air.it. Auz. IMMI : Newly recog. Nerve in con. with
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AFPEXPTX ").T.",
Luhosch, AV. Ursprung u. Phylogen. d. Nerv. access. AYillisii. Arch f. mikr. Auat.
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Lugaro, K. Moditicazioui dellc cellule uervose uei diversi stati fim/.ionali. Sperimeiitale,
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Marshall, A. M Dcv. of the Nerves in Birds. Journ. Auat. and Physiol. ATol. XI. ;
Dev. of the Cranial Nerves iu the Chick. Q.J.M.S. Vol. XVIII. ; Morphol. of
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Marshall, A. M., and AV. B. Spencer. < Iranial Nerves of Scylliiiin. Q.J.M.S. Vol. XXI.
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Ouodi, A. Entwick. des. sympath. Nervensystems. Arch. f. mikr. Auat. 1885-86 ;
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Strong, O. S. Cranial Nerves of Amphibia. Journ. Morphol. 1M»-V
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AVestling, C. Periph. Nervensystews (Pitliecus and Omithorhynchus). Abb. d. K.
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(Cf. also Zieheu, T., and Neumayer, L., in Hertwig's Handbucb d. vergl. n.
exp. Entw.-lehre. Jena, 190(i. i
534 APPENDIX
U. SENSORY ORGANS.
INTEUUMESTAKY SKNSOKY OKGAXS AND ORGANS OF TASTK.
Allis, E. P. Lat. Hue iu Amia. Journ. Morphol. 18*9; iu Polyptarus: Auat. Auz.
1900; iu Mustelus. Q.J.M.S. 1902; iu Mursenidse. Intern. Mouatsschr. Anat.
1903; iu Fishes. Loc. cit. 1904; Polyoilon. Zool. Jahrb. 1903; Scomber, Journ.
Morphol., 1903.
Ayers, H. Struct, ami Dev. of the Nasal Kays iu C'omlylura. Biol. Ceutralbl. 1885.
Bath, W. Geschmacksorgauc v. Crocodilus. Zool. Anz. 1905; v. Yogel. Sitz.-ber.
Naturf. Berlin. 1905.
Beard, -1. Seguif.ntal S -use organs of the Lateral Line, ami the Morphol. of the Vert.
Aud. Org. Zool. Anz. 1884; Cranial Gaugli t ami segmental Sense Organs of
Fisb.38. Zool. Anz. 18^5 ; Tin1 System of branchial Sense Organs and their
asso3. Ganglia in Ichthyopsida, etc. Q.J.M.S. 1895.
' Blaup, J. Ban der Nasenschleimbaut bei Fischen nud Amphibian, etc. Arch. f. Anat.
u. Physiol. 1884.
Boll, F. LorenziVheu Ampnlleu der Sclaehier. Arch. f. mikr. Auat. 1868 ;
Savi'scheu Bhischen YOU Torpedo. Arch. f. Auat. und Phys. 1875.
Botezat, E. Geschmacksorgaue, etc., iu Schuabel d. Yogel. Biol. Centralbl. 1904.
Brock, J. Termmalkurperchen-uhuliche Organs in dev Hant von Kuocheutischeu.
Intern. Mouatsschr. Auat. 1887.
Coggi, A. VesL'ole di Savi e gli orgaui della liuea laterale uelle Torpediui. Reud.
Ace. Liucei. 1891; Sviluppo delle Ampolle di Lorenziui. Loc. cit.
Cole, F. T. Sensory ami Ampullary Canals of Chimsera. Auat. Anz. 1896 (and
see p. 531).
Colliuge, AV. E. Sensory and Ampullary Cauals of Chimaira. P. Zool. Soc. 1895;
Sensory Canal System of Fishes (Physostomi). P. Zool. Soc. 1895; Sensory
dual System of Fishes (Gauoidei). Q.J.M.S. Vol. XXXYI. ; Morphol. of the
S?nsory Canal System in some Fossil Fishes. Proc. Birmingham Philos. Soc.
1893; Lat?ral Canal System of (1) Lepidosteus osseus. (2) Polypterns. Proe.
Birmingham Philos. Soc. 1893.
Crevatin, F. Termiuazione uc-rvosi u. corio d. Cougiuntiva e d. pelle cl. Polpastrelli,
etc. Mem. Ac. Sci., Bologua. 1902.
Dogiel, A. S. Yarious papers iu Arch. f. mikr. Auat.
Eberth, C. J. Normaleu uud path. Auat. der Froschhaut. Leipzig, 1869.
Eberth, C. J., aud Buuge, K. Endiguugen der Nerveu in der H uit d<-s Frosches.
Anat. Hefte. 1892.
Eimer, Th. Die Sehnanze des Maulwurfs, etc. Arch. f. mikr. Auat. 1871: Nervenendi-
gungeu iu der Haut der Knlizitze. Arch. f. mikr. Anat. 1872.
Ewart, J. C., and Mitchell, J. C. Lateral Sense Orgaii.s of Elasmohranchs (Laemargus
and Skate). Tr. Roy. Soc. Ediu. 1892.
Fritsch, G. Aeussere Haut und Seitenorgane des Zitterwelses (Malopterurus). Sitz.-
b'er. Ak. Berlin. 1886; Angelapparat von Lopbins. Loc. cit. 1884. (See
also p. 524.)
Garman, S. Lateral Canal system of the Selachia and Holo?ephala. Bull. Mus.
Harvard. 1888.
Gmelin. Morphol. der Papilla vallata und foliata. Arch. f. mikr Auat. 1892.
Grandry. Les corpuscles de Pacini. Journ. de 1'anat. et physiol. 1869.
Guitel, F. La ligne laterale du Lophius. Arch. Zool. exp. 1891 ; Les boutons nerveux
bucco-pbaryngiens de Baudroie (Lophius). Loc. cit.
Harrison, R. G. Entw. d. Sinne-organe cl. Seitenlinie bei d. Amphibien. Arch. f. mikr.
Auat. 1903.
Hermann, F. Feinere Ban des Geschmacksorgaues. Sitz.-ber. d. K. Bayer. Acad.
1888.
Hesse, Fr. Tastkugeln des Entenscbnabels. Arch. f. Anat. u. Physiol. 1882.
Hoffmann, C. K. Ontogenie der Knodieufische. Arch. f. mikr. Anat. iss;1,.
Holl,M. Anat. der Mundhohle von Rana. Sitz.-ber. Ak. Wien. 1887.
Key, A. Endigungsweise der Ges^hmacksnerven in der Zuuge ?u bei Batrachiern. Arch.
f. mikr. Auat. 1876; Sinnesorgane der Sell Ian gen. Arch. f. mikr. Anat. 1872;
Die Haut eiuheimischer Ophidier. Arch. f. mikr. Anat. 1873. (See also
pp. 502 and 505).
Malbrauc, M. Seitenlinie und ihreu Sinnesorganen bei Amphibien. Zeitschr. f. wiss.
Zool. 1875.
Maurer, F. Haut-Sinneaorgane, Feder- und Haaranlagen uud dereu gegenseitig^ Bj'zie-
hungen. Morph. Jahrb. 1892; Epidermis uud ihre Abkommliiige. Leipzig,
1895.
Mayer, S. Bau der Sinneshaare. Arch. f. mikr. Auat. 1890.
Merkel, Fr. Endigungen der sensibleu Nerven in der Haut. Nadir. Ges. Gottiugen.
1875; Tastzellen uud Tastkorperchen bei den Haustieivu und beim Menschen.
Arch. f. mikr. Anat. 1875 ; Endigungen der sensibleu Nerven in der Haut der
Wirbeltiere. Rostock, 1880 ; Tastz?llen der Ent?. Arch. f. mikr. Auat. 1878.
Minckert, W. Loreuzi'scheu Ampullen. Auat. Auz. 1901.
Mitrophauow, P. Entwicklungsgesch. u:id Inuervation der Nervenhii j'el der Urodeleu-
larveu. Biol. Central!)!. 1887.
Miiller, H. Nervose Follikelapparat tier Zitterrochen uud die sogen. Schleimcaruile der
Kuorpelfische. Verb. Ges. Wiirzburg. 1852.
Nagel, A. Geruchs- und Geschmaekssinn und ihre Orgaue, etc. Bibliotheca Zool.
Stuttgart, 1894.
Parker, G. H. Function of hit. Hue. Bull. Bureau of Fisheries. 1905; Sens? of hearing
in Fishes. Aiuer. Nat. 1903.
Pfitzner, W. Nerveneudiguugeu iuEpithel. Morph. Jahrb. 1881.
Pinkus, F. Hautsinnesorgane uebeu d. meusch. Haare (Haavsehtiiben), etc. Arch. f.
mikr. Anat. 1904. (Cf. Verb. Physiol. Ges. Berlin, 1902-3 ; Arch. f. Deraiatol.,
1902: and Dermatol. Zeitschr., 1902-3.)
Pollard, H. B. Lit. Line System in Siluroids. Zool. Jahrb. 1892.
Ka'-iber. Vater Pacini'scher Korperchen am Meuseheu uud Saugetiere. Zool. Auz.
1880; Vater'sche Korperchen der Bander uud Periostnerven. Neustadt a. H.
1865; Vorkommen und Bedeutung der Vater'schen Korperchen. Miinchen, 1867.
Retzius, G. Nerveiidiguugen in d. papillen d. Zungeu d. Amphibien. Biol. Untersuch.
1905. (See also p. 529.)
Ritter, W. E. Ou the Eyes, the integumentary ssnse-papillpe, and the integument of
Typhlogobius. Bull. Mus. Harvard. 1893.
Sarasiu, P. and F. Eutwickluugsges^h. YOU Ichthyophis. Zool. Auz. 1887. (See also
p. 503.)
S^hultze, M. Kolbsnformigen Gebilde in der Haut von Petrouiyzon, etc. Arch, fur
Anat. uud Physiol. 1861.
S^hulze, F. E. Beeherformigen Orgaue der Fische. Zeitschr. f. wiss. Zool. 1863 ;
Epithel- uud Driisenzellen. Arch. f. mikr. Anat. 1867 ; Nervenendigung in den
sog. Sohleimcaualeu der Fisshe, etc. Arcli. f. Auat. und Physiol. 1861 ; Sinues-
orgaiie der Seiteulinie bei Fischen und Amphibien. Arch. f. mikr. Auat. 1870 ;
Gesehmaeksorgane der Froschlarven. Loc. cit.
S.-hwalbe, G. Epithel der Papilhe vallatae. Arch. f. mikr. Anat. 1867 ; Geschmacks-
organe der Saugetiere und des Menschen. Arch. f. mikr. Anat. 1868 ; Papilhe
fongiformes der Saugetiere. Centralbl. f. d. medecin. Wissens°h. 1868.
Solger, B. Seitenorgane der Kuocheufische. Ceutralbl. f. d. medecin. Wissensch. 1877;
Anat. der Seitenorgane der F1s?he. I. Seitenorgane von Chimsera. II. Seifceu-
organe der Selachier. Arch. f. mikr. Anat, Bd. XVII. III. Seitenorgane der
Kuocheufische. Loc. cit. Bd. XVII. uud XVIII. ; Feiuere Bau tier Seiteuorgaue
der Fische. Sitz. -her. uaturf. Ges. z. Halle. 1887.
Stihr, H. Papilla foliata beim Kauiuchen. Auat. Auz. 1902 ; Papilla? fuugiformes,
etc. Zeitschr. f. Morphol. u. Anthropol. 1901 ; Ausdehnung d. Papilla foliata,
etc. Arch. Eutwi_-k. Mechanik. 1903.
Wilson, H. V., and Mattocks, J. E. Lab. sensory Anlage in Salmon. Anat. Anz. 1897.
Zander, R. Verbreitungsgebiet d. Geftihls- u. Geschruacksnerven i. d. Zungenschleim-
liaut, Auat. Auz. 1897.
r>36 APPENDIX
OLFACTORY OKGAX.
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540 APPENDIX
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(b) AMPHIBIA AND REPTILIA.
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K. Akad. vau Weteusehap. te Amsterdam. 1888.
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APPENDIX 54?
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Heron-Royer et van Bainbecke. La vestibule de la bouclie chez les tetards des Batraciens
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Osawa, G. Eingeweiden der Hatk-ria. Arch. f. mik. Auat. 1M»7.
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N N 2
548 APPENDIX
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J.— RESPIRATORY ORGANS (INCLUDING SWIM-BLADDER).
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APPENDIX f> 10
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(/>) AMPHIBIA.
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:>:>0 APPENDIX
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(c) REPTII.IA.
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r»r,2 APPENDIX
Brachet, A. Dev. de la Cavite hepatoenterique chez les Amphibiens. Auat. Anz.
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:-):-> 4 APPENDIX
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APPENDIX r,:,r,
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r,5G APPENDIX
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L. TTRINOGEXITAL ORGAN'S.
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O O
562 APPENDIX
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APPENDIX 563
FCETAT, MEMBRANES, PLACENTA, ETC.
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1906.
Beneden, E. van, arid Juliu, C. La formation des annexes ftetales chez les
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Bonnet, R. Uteriuuiilch und ihre Bedeutuug fiir die Frucht. Stuttgart, 1882 ;
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Duval, M. Le Placenta des Rougeurs. Paris, 1889-93.
Frommel, R. Entwick. der Placenta von Myotus. Wiesbaden, 1888.
Giacomiui, E. Contribute alia migliore couosceuza degli annessi fetali nei Rettili.
Recez. del sacco vitelliuo e dell' allautoide uella eavita abdominale. Monitore
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p. 553.)
Jenkiuson, J. W. Placenta in Ungulates. P. Zool. Soc. 1906.
Keibel, Fr. Entwioklungsgesch. der meuschlichen Placenta. Anat. Anz. 1899.
Kollmann, J. Placenta b. d. Makaken. Auat. Anz. .1900.
Kolster, R. Embryotrophe placent. Siiuger. Anat. Hefte, Arb. 1902, 1903.
Langhans, T. Meuschliche Placenta. Arch. f. Auat. u. Phys. 1877.
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placenta. " Anat. Anz. 1891 ; Uterus and Embryo. Journ. Morphol. 1889.
Mitsukuri, K. Foetal Membranes of the Ghelonia. Journ. Coll. Sci. Japan. 1890.
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also Q.J.M.S., 1883.)
Parker, T. J. Foetal Membranes of Mustelus. Tr. N. Z. Inst. 1889.
Popoff, D. Dottersack-Gefasse d. Hulmes. Wiesbaden, 1894.
Robinson, A. Nutritive Importance of the Yolk-Sac. Jouru. Anat. and Physiol.
1892 ; Seg. cavity, etc. Q.J.M.S. Vol. XXXIII.
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Jena, 1906.
Selenka, E. Entstehung der Placenta des Meuschen. Biol. Centralbl. 1891 ;
Atypische Placeutation eines Schwanzaffeu. Ann. d. Jardiu hot. de Buiten-
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Semon, R. Entstehuug u. Bedeutung der erub. Hiilleu u. Auhangsorgane d. Wirbeltiere.
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Tafani. Condizioui uteroplacentari della vita fetale. Firenze, 1886.
Turner, W. Structure of the human placenta. Jouru. 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, 187G.
Waldeyer, W. Bau der Menschen- und Affenplaceuta. Arch. f. mikr. Anat. 1890;
Placentarkreislauf des Menschen. Sitz.-ber. Ak. Berlin. 1887.
Wiedersheim, R. Eutw.-gesch. von Salamandra atra. Arch. f. mikr. Anat. 1890.
O O 2
564 APPENDIX
Wilson, J. T. Young specimen of Oruithorhyuchus. P. Linn. Soc. N.S.W. Vol. IX.
Wood-Mason, J., and Alcock, A. Uterine villiform papillae of 'Pteroplatea and their
relation to the embryo. P. Roy. Soc. 1891.
ADRENAL BODIES.
Aichel, O. Eutw.-gesch. u. Starnmesgesch. d. Nebenniereu, 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.
Colliuge, "W. E. So-called suprarenal bodies in Cyclostoma. Anat. Anz. 1896.
Diamare, V. Corp. surrenali, etc., dei teleostei. Boll. Soc. di Naturalist! in Napoli.
1895; Org. surrenale degli Elasmobranchi, etc. Mem. di mat. e di fis. d. Soc.
Ital. d. Sci. S. 3,T. X.
Feliciiie, L. Blutgefasssyst. u. d. Zelleu d. Nebenniere. Arch. f. mik. Auat. 1902.
Flint, J. M. Reticulum of the adrenal. Auat. Anz., 1899, and Johns Hopkins Hosp.
Reports, 1900.
Giacomini, E. Numerous papers on the suprarenals and iuterreuals iu Atti d. R. Accad.
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Istit. di Bologna, 1903-4-5 ; and Atti Ace. Liucei, 1906 ; Capsule surrenali degli
Anfibi, etc. Siena, 1902.
Grynfeltt, E. Org. surreuaux des Plagiostomes. Bull. Sci. France, Belgique. 1903.
Hultgreu, E. O., and Anderson, O. A. Nebeuniereu. Schwed. Gesellscli. d. Aertze.
Gekronte Preisschr. L.ipzig, 1899.
Kohu, A. Nebenuieren. Med. Wochenschr., Prag, 1898; Anat. Auz., 1899; and Arch.
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Kose, W. " Carotisdriise" u. "chromaffine Gewebe" bei Vogeln. Anat. Auz.
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. "VVien. 1892.
Poll, II. Entw.-gesch. d. Nebeuniere. Auat. Auz., 1904, and in Hertwig's Handb. d.
vergl. u. exp. Entw.-lehre. Jena, 1906.
Roucl, A. Dev. de la capsule surrenale de la souris. Bull. Soc. Vaudoise d. Sci. Nat.
1903.
Srduiko, O. V. Nebenniereu bei Anuren. Anat. Anz. 1900 ; Kuochenfische, etc.
Arch. f. mikr. Auat. 1903.
Stilling, H. Auat. der Nebenuieren. 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. Auz. 1897 ; Suprarenal bodies iu Fishes, and their
relation to the so-called head-kidney. Tr. Zool. Soc. 1897. (Cf. also P. Roy.
Soc. 1897; Proc. Physiol. Soc. 1897; Auat. Anz. 1897, 1900; Jouru. Physiol.
1898 ; Jouru. 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. Nebenniereu bei d. Ratte. Cen-
tral bl. f. Physiol. 1899; Eiitw. d. Nebeuuiere 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, 233;
sense-organs, 250, 259; "eyes, "277;
oral hood and mouth, 312 ; endostyle,
331 ; alimentary canal, 335 ; histology
of mucous membrane, 344 ; liver, 34(3 ;
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 fins, 136 ; muscles, 179, 187 ;
brain-membranes, 195 ; spinal cord,
198 ; brain, 204 ; spinal nerves, 233 ;
cerebral nerves, 234 ; sympathetic,
248 ; sense-organs of the integument,
250 ; olfactory organ, 259 ; eye, 278 ;
retina, 284 ; auditory organ, 295 ;
oral hood and mouth, 312 ; horny 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 ; alimentary 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 ; adrenal
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
nruscles, 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 ; cctlome,
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*, 333 ; alimentary canal,
339 ; histology of mucous membrane,
345 ; liver, 346 ; pancreas, 348 ; air-
tubes and larnyx, 370 ; lungs and air-
sacs, 382 ; coelome, 388 ; blood cor-
puscles, 394 ; circulation in embryo,
397 ; heart and main vessels, 411 ;
arteries, 414 ; veins, 428 ; lymphatic
system, 432 ; urinary organs, 459 ;
genital organs, 474 ; copulatoiy 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 ; crelome, 388 ; blood-
corpuscles, 394; heart and main vessels,
411 ; arteries, 414 ; veins, 428 ; retia
mirabilia, 431 ; lymphatic sj'stem, 432 ;
spleen, 435 ; tonsils, 436 ; foetal 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
Achromatin, 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
Ananmia, 9
INDEX
567
Anapophyses, 62
Annul ua tympanicus, 105
Anosmatic, 269
Antibrachium, 160
Antlers, 130
Anus, 308
Aortic arches, 398-414
Aponeuroses, 175
Aponeurosis, pulmonary, 382
Appendages, 13
Appendices auriculas, 400
Apteria, 28
Aqueduct of Sylvius, 204
Aqueductus cochleas, 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
Avtiodactyle 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
Basal i a, 155
Basal plate, 77
Basal processes of vertebral column, 49
Basidorsals, 49
Basihyal, 81
Basilar membrane, 298
Basipterygia, 136, 138, 155
Basiventrals, 50
Bicuspid valve, 411
Bidder's organ, 472
Bile-duct, 348
Biserial fin, 137, 157
Blastoccele, 4
Blastoderm, 5
Blastomeres, 4
Blastopore, 6, 308
Blastosphere, 4
Blastula, 4
Blood, 394
corpuscles, 394
vessels, 393
Bodies of vertebra?, 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
Branchiae, 310, 351
Branchial arches, 75, 81, 100
Branchial basket of Cyclostomes, 86
Branchial clefts, 75, 352-359
Branchiostegal membrane and rays, 90,
95
Breast-bone, 12
Bronchi, 362-387
Bronchioli, 382
Bulbi vestibuli, 492
Bulbus arteriosus, 395
Bulla tympani, 130
Bunodout teeth, 321
Burr, 130
Bursa entiana, 335
Fabricii, 340
C
Caecum, 311, 336, 339
Calamus, 26
Calcaneal process, 165
Calcaneum, 161
Calcareoiis bodies, 298
Calyces of ureter, 461
Campanula Halleri, 279
Camptotrichia, 137
Canalis reuniens, 299
Cannon-bone, 171
Capillaries, 393
Capitulum, 68, 69
Carapace, 43
568
INDEX
Carotid labyrinth, 406
Carpalia, 161
Carpometacarpus, 167
Carpus, 160
Cartilage bones, 45
Cauda equiua, 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
Cent rale, 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
trochlearc, 237
Choanae, 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 (fatal), 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
Ccelome, 8, 12, 338
cranial, 76
Crenogenetic, 1
Colliculus seminalis, 483
Colon, 311, 344
Columella aims, 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
Conns arteriosus, 395
Copula, 81
Copulatory 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
Costa? 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
Cuticula dentis, 314
Cutis, 17
Cycloid scales, 41
Cyclospondylic vetebra, 51
Cystic duct, 348
1)
Decidua, 4.S!)
Delam ination, 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, 7";
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
Diencephalon, 199
Digestion, intra- and extra-cellular, 345
Digiti grade, 171
Digits, 160
Diphycercal tail, 52
Diphyodont, 314
Discoid segmentation, 5
Discus proligerus, 450
Dorsal fissure, 198
Duct, hepatic, 348, 349
naso-lacrymal, 265, 289
naso-palatine of Myxinoids, 260
pancreatic, 349
salivary, 324
urinogenital, 446
Ductus, Botalli, 406
Cuvieri, 397, 424
endolymphaticus, 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
Ependyme, 193
Epiblast (see Ectoderm)
Epiboly, 6
Epicoracoid, 71, 144
Epiderm, 6, 17
Epididymis, 445, 454, 476, 483
Epidural space, 196
Epiglottis, 370, 371
Epiphyses of vertebra, 60
Epiphysis of brain, 202, 215
Epipubic process, 146
Epipubis, 147, 149, 151, 155
Episternum, 44, 72
Erytlirocytes, 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, 2SS
muscles of, 277, 286
Eyelashes, 287
Eyelids, 277, 287
Eyes, rudimentary, 276
F
Fallopian tube, 477
Falx, 197
"Fan" of Birds, 2S2
Fascia', 175
Fat-bodies, 435, 474
Fauces, 436
Feathers, 25
Femoral pores of Lizards, 23, 487
Femur, 160
Fenestra ovalis, 98, 111, 129, 298
rotunda, 111, 129
Fertilization of ovum, 3
Fibula, 160
Fibulare, 161
Filoplumes, 27
Filum terminals, 198
Fin-rays, 136, 137, 155-159
Fins, 13
unpaired, 136; paired, 137
Fissure of Rolando, 228
Flocculi, 225, 229
Flumina pilorum, 31
Fivtal membranes, 436
Fontanelles, S7
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
Panizza?, 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, habenula*, 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, 35S
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 Lieberkiilm, 34-5 ;
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
Glandular pterygopodii, 19, 486
Glaus 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
Gymnoarian condition of ovary, 466
Gymnokrotaphic type of skull, 113
Gyri, 201, 225
H
Hamial spines, 50
Hemoglobin, 394
Hamolymph 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
Heteroccelous vertebras, 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
Hypapophj'sis, 59, 62
Hypermastism, 37
Hyperthelism, 37
Hypoblast (see endoderm)
Hypochorda, 47
Hypoischiatic process, 146
Hypophysis, of brain, 202
Hyporachis, 27
Hypural bones, 53
Ichthyopsida, 14
Ichthyopterygium, 159
Ileo-colic valve, 311
lleum, 311
Ilium, 146
Impregnation, 3
Incus, 133, 303
INDEX
571
Infraventrals, 50
Infundibula (of lung), 364, 388
Infundibulum, 2U2
Ingluvies, 339
Inguinal canal, 482
Innominate trunk, 414
Integument, 17-38
sense-organs of, 25, 250-258
Integumentary muscles, 175
Intercalary pieces of vertebra, 49
Intel-centra, 55, 57, 58
Interclavicle (see prosternum)
Interdorsals, 49
lutermaxillary sinus, 98
Intermedium, 161
Intermuscular bones, 42, 65, 67, 172
Interneural plate, 49
Interorbital septum, 77, 112
Intel-renal 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, bony and membranous, 291
Lacrymal glands, 288
Lacteals, 393, 433
Lagena, 290
Lamina cribrosa, 95, 129
fusca, 275
perpeiidicularis, 129
terminalis, 201
spiralis ossea, 304
Lamime papyracere, 131
Lanugo, 31
Laryngeal pouches, 374
Laryngo-tracheal 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
Li mi tans 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
Maudibular 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 vertebrfe, 57, 59
Meroblastic segmentation, 5
Mesencephalon, 199
572
INDEX
Mesentery, 310
Mesoblast (see mesoderm)
Mesoclerm, 5
Mesodermic somites, 9
Mesonephric 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
Mierosmatic, 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
mylohyoid, 188
obliquus externus and profundus,
181, 182
panniculus 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
Neurilemrna, 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
0
Obturator foramen, 145, 149
Occipital condyles, 97, HI, 122, 127
Odontoblasts, 313
Odontoid process, 57
CEsophageo-cutaneous duct, 354
(Esophagus, 310, 335
Olecranon, 169
INDEX
573
Olfactory capsules, 78
lobes, 200
organ, 258
scrolls, 267
tract and bulb, 210
Omontum, folds of, 389
Omostemum, 70
Ontogeny, 1
Oosperm, 4
Opercular bones, 90
Operculum, 88, 356
Opisthoccelous vertebra?, 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
cloaca?, 150
penis, 492
uteri, 477
Ossification, centres of, 80
Osteocranium, 79
Otic bones, 133, 295, 303
Otoliths, 292
Ovarian follicle, 4.~>()
Ovary, 450
Oviducal gland, 463
Oviduct, 446
Ovipositor, 465
Ovotestis, 472
Ovum, 2
Pacinian corpuscles, 257
Pala?ocranium, 85
Paleontology, 1
Pal;eostoma, 203
Palate, 112, 118, 122, 132, 310
Palatopterygoid, 81
Palatoquadrate, 81
Palingenetic, 1
Pallium, 200
Palpebra;, 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
LIBRAE
Parotoids, 21
Parovarium, 445, 480
Pars acetabularis, 151
basilaris, 298
Patella, 172
Pearl- organs of Cyprinoids, 252
Pecten, 282
Pectineal process, 152
Pectoral arch, 140
Pelvic arch, 145
plates, 145
Pelvis of kidney, 461
Penis, 487-491
Penna, 27
Pentadactyle limb, 159
Pericardium, 388, 394
Perichomkal bone, 45
Perichondrium, 41
Peridural space, 196
Perilymph, 291
Perimeningeal space and tissue, 195
Perina?um, 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
Platybasic type of skull, 77
Pleura, 389
Pleurocentra, 57
Pleurodont dentition, 316
Pleuronectida?, asymmetry of head, 94,
281
Plexus (or tela?) 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
Prepollex, 170
Prepubic process, 146
Prepuce, 491
Primitive nieninx, 195
streak, 6
Pro-amnion, 437
Pro-atlas, 57, 62
Processus digitiformis, 336
falciformis, 279
vermiformis, 344
Procoelous vertebrae, 56
Procoracoid, 71, 141
Proctodifium, 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
Protovertebrse, 9
Proventriculus, 339
Psalterium, 341
Pseudobranch, 357
Pseudo-electric fishes, 190
-turbinal, 266
Pterygiophores, 136
Pterygopodium (see Clasper)
Pteryhe, 28
Pubis, 148
Puncta lacrymalia, 289
Pupil, 275
Pygal plates of Reptiles, 43
Pygostyle, 60
Pylangium, 395
Pylori c cteca, 336
valve, 311
Q
Quadrate cartilage, 81
R
Rachis, 27
Radiale, 161
Radii of fins, 155-159
Radius, 160
Rami communicantes, 247
Receptacula seminis, 474
Receplaculum 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 Hatteria, 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
Saccus vasculosus, 203, 210
Sacrum, 56, 60, 62
Sauropsida, 14
Savi'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
Sccodont 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
traiisversum, 185
ventricular, 408
Serosa, 437
Sesamoids, 170, 172, 175
Sexual reproduction, 3
Shell-gland, 463
INDEX
575
Sinus-feathers, 28
hairs, 30
rhomboidalis, 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
Spermary, 450
Spermatophores, 474
Spermatozoon, 3, 451
Sperm-cell, 3
sac, 465
Sphenoidal fissure, 129
Sphincter oculi, 287
Spina scapula?, 144
Spinal cord, 195, 198
Spines, neural and h;emal, 49, 50
of Porcupine, 31
Spino-occipital nerves, 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
Sternebrffi, 73
Sternum, 12, 67, 70
Stomach, 310, 335
Stomodseum, 6, 308
Stratum corneum, 17
germativum, 17
Malpighii, 17
Subarauhnoid 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
Supi*adorsals, 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
T
Tactile cells and corpuscles, 255
Tail, 13
Tail-fold, 9
Tapetum lucidum, 280
Tarsalia, 161
Tarsometatarsus, 168
Tarsus, 160
Taste, organs of, 254
Teats, 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 choroidetB, 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
Tibiotarsus, 168
Tissues, 2
Tongue, 327
Tiiuscles of, 188
Tonsils, 436
Tori, 32
Tortoiseshell, 25
Trachea, 362
Tragus, 307
Truncus arteriosus, 395
Trabeculaj 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
Ulna, 160
Ulnare, 161
Umbilical cord, 440
vesicle, 438
Uncinate processes, 69
Unguligrade, 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
Vagina, 477
Vaginal cajcum, 480
Valve of Vieussens, 203
Valvula cerebelli, 211, 214, 215
Valvulaj 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
trans versum, 200
Vent, 308
Ventral fissure, 198
Ventricles of brain, 195, 203
Vermis, 229
Vertebra?, 47
Vertebral column, 12, 45
theory of skull, 74
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
Vibrissre, 30, 257
Villi, of intestine, 346
of placenta, 11, 439
Viscera, 12
Visceral arches, 80, 100
tube, 11
Vitelline membrane, 3
Vitello-intestiiial 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
WolfHan body, 445
duct, 450
X
Xiphisteriuini, 71
Xiphoid process, 73
Vellovv bodies, 476
Yolk, 3, 5
Yolk-sac, 9
Z
Zygantra, 58
Zygapophyses, 51
Zygokrotaphic type of skull, 113
Zygomatic arch or bar, 100, 106, 113,
132
Zygosphenes, 58
H. CLAY AND SONS, LTD., BREAD ST. HILL, B.C., AND BUNGAY, SUFFOLK.