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COMPARATIVE ANATOMY
OF VERTEBRATES

KINGSLEY

COMPARATIVE ANATOMY

OF

VERTEBRATES

BY

ae oe KINGSLEY

10LOGY IN TUFTS COLLEGE

WITH 346 ILLUSTRATIONS
LARGELY FROM ORIGINAL SOURCES

PHILADELPHIA
P. BLAKISTON’S SON & CO.
1012 WALNUT STREET
1912

CopYRIGHT, 1912, BY P. Biaxiston’s Son & Co.

THE+MAPLE-PRESS-YORK-PA

PREFACE.

Vertebrate anatomy is everywhere taught by the laboratory method.
The student studies and dissects representatives of several classes, thus
gaining an autoptic knowledge of the various organs and their positions
in these forms. These facts do not constitute a science until they are
properly compared and correlated with each other and with the condi-
tions in other animals. It is the purpose of the author to present a
volume of moderate size which may serve as a framework around which
these facts can be grouped so that their bearings may be readily recog-
nized and a broad conception of vertebrate structure may be obtained.

In order that this may be realized, embryology is made the basis,
the various structures being traced from the undifferentiated egg into
the adult condition. This renders it easy to compare the embryonic
stages of the higher vertebrates with the adults of the lower and to
recognize the resemblances and differences between organs in the
separate classes. There has been no attempt to describe the structure
of any species in detail, but rather to outline the general morphology
of all vertebrates. To aid in the discrimination of the broader features
and the more minor details, two sizes of type have been used, the
larger for matter to be mastered by the student, the smaller for details
and modifications in the separate classes to which reference may need
to be made.

Considerable space has been given to the skull, as there is no
feature of vertebrate anatomy which lends itself more readily to
comparative study of the greatest value to the beginning student,
while the same specimens can be used in the laboratory year after year.
The skull also has a special interest since nowhere else is there the same
chance of tracing modifications in all groups since the first appearance
of vertebrates on the earth. To aid in this, extinct as well as recent
species have been included.

It was the desire of the author to adopt the nomenclature of the
German Anatomical Society (‘BNA’), but this was often found im-
practicable. The BNA was based solely upon human anatomy and
it fails utterly in many respects when the attempt is made to transfer
its terms to other groups. The single example of ‘transverse process’

Vv

vi PREFACE.

is sufficient to illustrate this. To the writer another objection is that
the BNA strives to do away with all personal names. These, it would
seem, have a great value as they are indications of the history of
anatomical discovery and memorials of the great anatomists of the
past. Dorsal and ventral are used instead of the anterior and pos-
terior of human anatomy, while anterior indicates toward the head,
posterior toward the tail, these terms being readily applied to all ver-
tebrates, man only excepted. Cephalad and caudad, adopted by
some, lead to occasional peculiar phrases. The German word
‘anlage’ has been adopted bodily, and seems to call for no defense.
It implies the indifferent embryonic material from which a part or an
organ develops.

The illustrations have been drawn or redrawn expressly for this
work. Some of them are original, some based on figures in special
papers. Practically none have ever appeared in any text-book in the
English language. In selecting the objects to be figured especial
pains has been taken to avoid the forms usually studied in our
laboratories, thus relieving the student of the temptation of copying
the figure, instead of drawing from nature. Especial thanks are due
to Professor C. F. W. McClure, who allowed me to draw at will from
the splendid collection: which he has built up at Princeton. These
figures are indicated by the word ‘Princeton’ followed by the num-

ber of the preparation in the museum of the University.
Turrs COLLEGE, Mass.

CONTENTS

Introduction . .
Introductory embryology . .
Histology. an

Epithelial tissues
Nervous tissues.
Muscular tissues
Connective tissues

Comparative morphology of vertebrates
Integument
Skeleton .

Dermal skeleton.
Endoskeleton
Vertebral column
Ribs
Sternum
Episternum
Skull

Skull of cyclostomes . . ig.

Skull of elasmobranchs
Skull of teleostomes
Skull of amphibia
Skull of reptiles
Skull of birds... . .
Skull of mammals :
Appendicular skeleton .
Median appendages
Paired appendages.
Shoulder girdle
Pelvic girdle
Free appendages
Coelom (body cavities) :
Muscular system
Parietal muscles
Visceral muscles
Dermal muscles
Diaphragm ...-.. .
Electrical organs
Nervous system. . .

Central nervous system. .....
Spinalcord ........,

viil

Brain: « « 4 = & a wes
Brain of cyclostomes
Brain of elasmobranchs
Brain of teleostomes .
Brain of dipnoi. . .
Brain of amphibia
Brain of reptiles
Brain of birds. . .

CONTENTS.

Brain of mammals. . .

Peripheral nervous system
Spinal nerves. .
Sympathetic system . . .
Cranial nerves

Sensory organs j

Nerve-end apparatus.

Lateral line organs *

Auditory organs. .

Organs of taste . .

Olfactory organs

Eyes... .

Digestive organs

Oral cavity

Teeth .
Tongue ee
Oral glands. . .

Pharynx

Csophagus

Stomach .

Intestine . .

Liver

Pancreas. .

Respiratory organs

Gills (branchiz)

Pharyngeal derivatives. . .

Swim bladder

Lungs and air ducts
Air ducts
Lungs .

Accessory respiratory structures .

Organs of circulation
Blood and lymph .
Blood-vascular system .

Embryonic circulation
‘Heart . .
Arteries...
Veins

CONTENTS.

Heat. 62.6 es jo «*%

Arteries: ¢ 2 2 oe eB ees
Veins a

Feetal circulation .. . :

Circulation in the separate classes . . .
Lymphaticsysem ..........
Urogenital system. ..........
Excretory organs ...... ie mane Pe 2
Reproductive organs .........
Reproductive ducts ........

Excretory organs of the separate groups .
Reproductive organs of the separate groups
Copulatory organs ........
Hermaphroditism .........

Foetal envelopes... 2... .....04.
Adrenal organs... . eae (ee am
Bibliography... 2.0 2. ee eee
Definition of systematic names ..... ..
INDEM cae ee a wa Oe Goa ae GR

ix
PaGE
280
281

282
284

. 289
. 293

. 294

. 302

397

. 308

319
321
326

- 331

342

- 346
. 348
. 352°

354

. 382
» 385

INTRODUCTION.

Any animal or any plant may be studied from several different
points of view, four of which are concerned/the(it) present volume.
We may study its structure, ascertaining the parts of which it is com-
posed and the way in which these parts are related to each other. This
is the field of Anatomy. If we go into the more minute structure, for
which the microscope has to be used, we are entering the special
anatomical field of Histology. When two or more different animals
are compared in points of structure, their resemblances and differences
being traced, the study is called Comparative Anatomy, and it is only
through such comparisons that we are able to arrive at the true meanings
of structure. Then it is of interest to see the way in which thestructure
comes into existence in development from the comparatively simple egg
from which it arises—the province of Embryology or Ontogeny. Anat-
omy and ontogeny together give us a knowledge of the form and how
it has arisen and they are frequently grouped as Morphology. But mor-
phology merely deals with the parts of a machine and these are usually
studied in the dead organism; fully to appreciate the mechanism we
should know how the parts and the whole perform their work, the
study of function or Physiology.

In view of the foregoing the present volume is to be regarded as
rather a comparative morphology of vertebrates, with here and there
hints at the physiological side. Farther, there is an adaptation of
the organism to the conditions in which it has to live, and the inter-
actions of this environment upon the animal have to be considered, at
least to a slight extent.

Zoologists divide all animals into two great groups, the Protozoa,
in which the organism consists of a single cell, and the Metazoa, in
which the body is composed of many cells, which vary according to
the functions they have to perform. Of the Metazoa there are several
divisions—Porifera (sponges), Coelenterata (sea anemones, jelly fish),
Echinoderma (starfish, sea urchins), Platodes (flatworms), Rotifera,
Ccoelhelminthes (ordinary worms), Mollusca, Arthropoda (crabs,
insects), and Chordata.

2 INTRODUCTION.

The Chordata are bilaterally symmetrical animals with metameric
bodies, which agree in several features not found in the other groups.
These are (1) a central nervous system, entirely on one side of the di-
gestive tract; (2) the presence of gill slits in the young if not in the
adult; (3) an unsegmented axial rod, the notochord, between the
digestive tract and the nervous system. All of these features will be
described later.

There are three or four divisions of Chordata, the uncertainty
depending upon the position to be accorded the Enteropneusta. These
are worm-like animals, occurring in the sea and represented on our
shores by Balanoglossus. What has been described as a notochord is
a pocket from the digestive tract, lying in a curious proboscis above
the mouth.

The next division, the Tunicata, includes the (marine) ‘sea-squirts.’
They were long regarded as molluscs, but the discovery that the young
have true gill slits, a nervous system on one side of the alimentary
canal, and, above all, a notochord, placed them in the present associa-
tion. Their young (larve) are tadpole-like, the notochord is confined
to the tail, but later the tadpole features are lost and with them the
tail and notochord, and the adult is a sac-like animal with no re-
semblances to its former state, or to its allies.

The third division, the Leptocardii, embraces Amphioxus and
a few othermarine, fish-like animals. They were long classed as fishes,
but are far more simple than any true fish. The body is markedly
segmented, the gill slits are very numerous and the excretory organs
open separately to the exterior and are vermian in character. Stomach,
vertebre and heart are lacking and the brain and sense organs are
very rudimentary, while jaws and paired appendages are absent.

The last class, the Vertebrata, are most nearly related to the Lepto-
cardii, but differ in many important respects. Thus there is always
a skull and vertebral column; the brain is larger than the spinal cord;
there are always nose, eyes and ears; a heart is present and the excre-
tory organs open into a common duct on either side, with an external
opening near the anus.

Most of the characteristics of a vertebrate may be seen from the
accompanying diagram. The body is bilaterally symmetrical, with
anterior and posterior ends, dorsal and ventral sides well differentiated.
There is no external segmentation, since the muscles are not directly
attached to the skin, but a metameric arrangement of parts is notice-

INTRODUCTION. 3

able in muscles, skeleton, nerves, blood-vessels, and, to a less extent,
in the excretory organs. ‘There is no cuticular skeleton but the outer
layer of the skin may be cornified or the deeper layer may give rise
to ossifications (scales of fishes, etc.).

There is an internal axial skeleton, consisting of the notochord,
around which are developed rings of denser material, constituting a
backbone or vertebral column, while in front a skull encloses the brain
and organs of special sense, and gives support to the primitive respira-
tory organs (gills), which are always connected with the digestive tract.
Typically there are two kinds of appendages, each with an internal
skeleton. ‘These are the unpaired or median fins, dorsal and ventral,
which occur only in the Ichthyopsida, and the paired appendages,
of which there are two pairs, anterior and posterior in position.

Fic. 1.—Diagram of a vertebrate. u, anus; b, brain; c, coelom; da, dorsal aorta; df,
dorsal fin; g, gonad; gd, genital duct; h, heart; 2, intestine; /, liver; m, mouth; 1, notochord;
p, pancreas; pc, pericardium; pf, pectoral fin; ph, pharynx, with gill clefts; s, stomach,
s¢, spinal cord; sp, spleen; u, ureter; va, ventral aorta; vc, vertebral column, rf, ventral fin.

The central nervous system consists of brain and spinal cord which
lie dorsal to the notochord, and are usually protected by arches arising
from the vertebre and by the roof of the skull. Eyes and ears are the
highest of the sense organs. The alimentary canal always has a
liver connected with it, and a portion of the canal just behind the mouth
is developed into a pharynx, from which, in the young of all, gill clefts
extend through to or toward the exterior. In the terrestrial vertebrates
these gill clefts are later replaced by lungs which develop front the
hinder part of the pharyngeal region.

The blood, which always contains two kinds of corpuscles, flows
through a closed system of vessels. A heart, ventral to the digestive
tract and lying in a special cavity, the pericardium, is always present

4 INTRODUCTION.

The heart consists of two successive chambers, an auricle (atrium)
and a ventricle, and in forms which respire by means of gills, contains
only venous blood. With aerial respiration both chambers may become
divided into arterial and venous halves. A dorsal aorta, lying above
the alimentary canal, is always present.

The sexes are usually separate. The reproductive and excretory
systems are closely related, giving rise to a urogenital system. ‘The
excretory ducts usually carry off the reproductive products (eggs and
sperm). The urogenital ducts empty near the anus. Reproduction
is strictly sexual; parthenogenesis and reproduction by budding do not
occur and alternation of generations is unknown. The viscera are
enclosed in a large body cavity (ccelom) which in the adult does not
extend into the head. Each viscus is supported by a fold (mesentery)
of the lining membrane of the cavity.

For details of the classification of vertebrates reference must be
made to special text-books of zoology, but as some of the larger groups
must be referred to frequently, so these with a slight definition and one
or two examples are given here.

Serres I. CYCLOSTOMATA.

These are eel-like in form, breathe by gills, have but one nostril,
a circular mouth, incapable of closing, for no jaws are present.
The skeleton is poorly developed and there are no paired appendages.
—Lampreys and hagfishes.

Series II. GNATHOSTOMATA.

This includes all other vertebrates. They have usually two pairs
of appendages, true jaws and a well developed skeleton.

GRADE J. ICHTHYOPSIDA.

Fish-like, breathe, at least while young, by gills, have paired ap-
pendages, in the shape of legs or fins. In development there are never
formed those structures to be described later as amnion and allantois.

Class I. Pisces.

Fishes respire permanently by gills developed in gill slits in the
sides of the pharynx, have median and paired fins unless the latter be
lost by degeneration.

INTRODUCTION. 5

Sub-class I, Elasmobranchit.

Fishes with cartilaginous skeleton, mouth usually on the lower
side of the head, the gills usually opening separately on the neck, and the
tail with the upper lobe the larger (heterocercal). Sharks and skates.
The Holocephali differ in having the gill slits covered with a fold of
skin, so that but a single external opening appears.

Sub-class II. Ganoidea.

Intermediate between elasmobranchs and _ teleosts.—Garpike,
sturgeon.

Sub-class III. Teleostet.

Fishes with bony skeleton, mouth with true jaws at the tip of the
snout, gill openings concealed by an operculum or gill-cover supported
by bone. ‘Tail with upper and lower lobes equal.—All common fishes.

Sub-class IV. Dipnoi.

The lung fishes are tropical forms in which the air bladder func-
tions as a lung, the gill openings are covered with an operculum, and
the tail is very primitive (diphycercal).

Class II. Amphibia.

Ichthyopsida with legs replacing the paired fins, lungs present and

replacing the gills in the adult, nostrils connecting with the mouth.
Sub-class I. Stegocephali.

Extinct amphibians with well developed tail.

Sub-class II. Urodela.
Amphibia with well developed tail, gills sometimes retained through
hfe.—Salamanders, Tritons, newts, efts.
Sub-class III. Anura.
Tailless as adults, the young a tadpole with external gills.—Frogs
and toads.
Sub-class IV. Gymnophiona.

Blind, burrowing, legless amphibians occurring in the tropics.—
Cecilians.

6 INTRODUCTION.

GRADE II. AMNIOTA.

Vertebrates in which there are never fins, never functional gills,
the respiration being by lungs. In development the embryo becomes
covered by an embryonic envelope called the amnion, while a second
_ outgrowth from the hinder end of the digestive tract is concerned in
the embryonic nutrition and is called the allantois.

Class I. Sauropsida.

Body, at least in part, with scales, eggs large.

Sub-class I. Reptilia.

Cold-blooded vertebrates, the whole body covered by scales or
horny plates. The living forms are turtles, lizards, snakes and alli-
gators (crocodiles) and a New Zealand species Sphenodon. The
fossil forms are more numerous and include Theromorphs, Plesiosaurs,
Ichthyosaurs, Dinosaurs, and Pterodactyls.

Sub-class II. Aves.

The birds are recognized by their warm blood and their feathers.

Class II. Mammalia.

The mammals are as sharply marked by their hair as are the birds
by their feathers. They have warm blood; except the monotremes
they bring forth living young which are nourished by milk secreted
by glands (mamme) in the mother.

There are a few other terms of convenience which may be defined
here as they will save much circumlocution. The term Teleostomes is
applied to ganoids and teleosts, from the fact that they have true jaws.
The amphibia and the amniotes are frequently united as Tetrapoda,
from their possessing feet, in contrast to the fishes with fins.

The geological history of these groups is important; their first
appearance and their geological range is indicated in the accompanying
table of the geological periods.

INTRODUCTORY EMBRYOLOGY.

The structure of an adult vertebrate can be fully appreciated and the
bearing of the facts recognized only by a knowledge of the develop-
ment of the parts concerned. It would often appear, for example,
that certain organs in different groups were exact equivalents of each

INTRODUCTION.

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Ostracoderms
Palaeospondylus
Elasmobranchs
Ganoids
Teleosts
Arthrodira
Dipnoi
Stegocephals
Gymnophiona
Urodela

Anura
Theromorphs
Plesiosaurs
Chelonia
Ichthyosaurs
Rhynchocephals
Dinosaurs
Squamata
Crocodiles
Pterodactyls
Birds

Monotremes

Marsupials

Edentata

Insectivores

Chiroptera

Rodentia

Ungulata

Sirenia

Cetacea

Carnivores

Primates

Table showing the geological distribution of the various groups of vertebrates.

8 INTRODUCTION.

other—duplicates in function and details of structure—while a knowl-
edge of their development may show that they have had entirely

different origins and different histories, and hence cannot be identical;
they are examples of what the evolutionist calls convergent evolution.
in which they occur. Farther, the development affords a framework
around which the details of organization may be arranged in a logical
following pages are based on embryology. Not only are the histories
of the separate organs traced before an account is given of the adult
form the earlier stages before the organs are outlined.
r ; The enormously complicated
2 3 : ;
body of every vertebrate is derived
from a comparatively simple special-
ovum, must be fertilized by a still
more specialized cell, the spermato-
Fic. 2.—Successive stages in the seg- this fertilization the egg goes through
mentation of an amphibian egg. 1-7,
planes. of changes which bring it contin-
ually nearer the adult condition. The phases of this differ with
is subject to modifications in the several groups, for an account of which
reference must be had to embryological text-books.
the segmentation or cleavage of the egg, in which it divides again and
again, until the single-celled egg is converted into a large number of cells
modified accordingly as the egg is large or small, as it contains varying
amounts of nourishment—deutoplasm or food yolk stored up for the
development; the description given here follows the simplest conditions.
As a result of segmentation the egg is converted into a spherical
cavity because it is formed during segmentation. It also has the
name archiceele as it is the first or oldest space to appear in the

Such cases are apt to lead one astray as to the relations of the forms
manner, thus aiding in their remembrance. For these reasons the
conditions, but this introductory chapter gives in the most generalized
ized cell, the egg or ovum. This

4 LEDS

| FX

Cy

ce zoon, derived from the male. After
Results of the corresponding cleavage an orderly but very gradual series
different animals; here only a generalized account will be given, which
The Segmentation of the Egg.—The first steps of the process are
or blastomeres (fig. 2). The character of this segmentation is
growing embryo. These same variations also affect the later stages of
mass of cells in which a cavity appears, called the segmentation
embryo. This stage of the embryo is called the blastula (fig. 3).

EMBRYOLOGY. 9

Its cells at first show but little differentiation except in size. Next
follow processes which are to differentiate the cells into layers, charac-
terized by both position and fate.

Gastrulation.—In the simplest form this differentiation is brought
about by an inversion of one-half of the blastula into the other, thus
more or less completely obliterating the segmentation cavity, much as
one may push one side of a rubber ball into the other, forming a double-
walled cup (fig. 4). This stage is called the gastrula, and the process
of inpushing is invagination. With this the first appearance of the
structures of the adult is seen. The outer wall of the cup is turnedto
the external world and thus act as a skin for the embryo. This layer
is ‘called the ectoderm. The opening or mouth into the cup is the

°
Fic. 3.—Diagram of a typical Fic. 4.—Diagram of a gastrula.
blastula with central segmentation a, archenteron; b, blastopore; ec, ecto-
cavity. derm; en, entoderm; sc, segmentation
cavity.

blastopore. The inside of the cup is well fitted for the digestion of
food as it can be held together there and the digestive fluids are less
liable to waste. Hence the cavity is called the archenteron (primitive
stomach), and the layer of cells which line it is the entoderm. That
these comparisons are more than analogies of position is shown by
their fates, the ectoderm forming part of the skin of the adult, the
entoderm the lining of the digestive tract. Between ectoderm and
entoderm are the remains of the segmentation cavity, filled with an
albuminous fluid. It will be convenient later to speak of the line where
ectoderm and entoderm meet at the blastopore as the ect-ental line.

Closure of the Blastopore.—Next, the blastopore closes, the
process beginning at what will be the head end of the embryo and pro-

Io INTRODUCTION.

ceeding gradually backward. Usually the closure is complete, but
occasionally the hinder part remains open and forms the anus. Where
it closes completely the vent is subsequently formed in the line of closure.
This union of the two lips of the blastopore in closing marks the middle
line of the back of the future animal, and is called at first the primitive
groove, the region on either side of it being known as the primitive
streak, terms of importance in understanding the gastrulation of the
higher vertebrates.

Mesoderm.—With the closure of the blastopore the embryo elon-
gates and the archenteron is converted into a tube. Next, from the
region of closure and from the entodermal tissue, a fold of cells grows
in on either side between ectoderm and entoderm, thus farther en-
croaching on the segmentation cavity. These cells form the middle

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Fic. 5. Fic. 6.

Fic. 5.—Stereogram of the anterior end of a developing amphibian, showing the out
lining of the mesothelium, nervous system and notochord. 4, anterior end; a7, archenteron;
c, cceelom; ch, notochordal cells; ec, ectoderm; mp, mesodermal pouch; ng, primitive groove;
np, neural plate; mr, neural folds; sc, segmentation cavity; so, somatic wall of celom; sp,
splanchnic wall of ccelom.

Fic. 6.—Stereogram of the anterior end of a vertebrate, showing the relation of the
coelomic pouches; ¢, ccelom; d, digestive tract; e, ectoderm; nc, nervous system; m, notochord;
sc, segmentation cavity; so, somatic and sp, splanchnic walls.

layer ormesoderm. Inside this fold is a space, connected at first with
the archenteron, but soon the cavity of each side is cut off by a growing
together of the opening into the archenteron and is henceforth known
as a coelom’ or body cavity. Each coelomic space has two walls, one
toward the ectoderm, the somatic layer, the one toward the entoderm
being the splanchnic layer (figs. 5 and 6).

The mesoderm arising in this way and bounding the ccelom is
called mesothelium to distinguish it from another kind—the mesen-

1 4 ceelom formed in this way is an enteroccele. Usually the coelomic walls arise as a
solid mass of cells from the corresponding region, which later splits internally, forming a
schizoceele. The two are readily compared.

EMBRYOLOGY. II

chyme—which also comes to lie in the segmentation cavity. This
mesenchyme arises as separate cells, coming largely from the mesothe-
lium, and to a less extent from the entoderm (see p. 16). Whether
any arises from the ectoderm is disputed.

The Germ Layers.—Ectoderm, entoderm and the two types of
mesoderm are called the germ layers, because in the animals first
studied they were arranged like layers one on the other. Each plays
its part in the formation of the adult and gives rise to its peculiar.
structures.

The ectoderm forms the outer layer of the skin, hair, claws, feathers,
the outer layer of scales, enamel of teeth, and the essential or character-
istic part of all sensory and nervous structures.

The entoderm gives rise to the lining of the digestive tract, and the
various outgrowths—gills, lungs, liver, pancreas, etc.—connected with
it. The notochord is also entodermal and possibly the lining of the
blood-vessels is derived from this layer.

The mesothelium produces the lining of the ccelomic cavities
—pericardial, pleural, peritoneal—the reproductive and excretory
organs and the voluntary muscles and those of the heart.

_The mesenchyme develops the deeper layer (corium) of the skin
and of scales, the dentine of teeth, involuntary muscles (except those of
the heart) connective tissue, ligaments, cartilage, bone, and the corpus-
cles of blood and lymph.

In the development of the embryo several processes of differen-
tiation occur simultaneously, but in the written account one has to
follow another. Hence it must be understood that the modifications
described here may be taking place at the same time.

The Central Nervous System.—During the closure of the blasto-
pore the ectoderm in front and to either side of the blastoporal lips
becomes thickened, the cells elongating at right angles to the surface
and becoming cylindrical or fusiform. These cells form the neural or
medullary plate (fig. 5, mp), sharply marked off from the surrounding
cells, which are more flattened, and which eventually are concerned in
the formation of the outer layer (epidermis) of the skin. The neural
plate is to develop into the brain and the spinal cord, and it is to be
noted that later it extends around the hinder end of the blastopore.
After it is outlined the plate is rolled into a tube, its front end and lateral
margins rising up, forming neural folds (nr), between which is the
medullary groove. Eventually the folds meet and fuse above so that

12 INTRODUCTION.

the tube results (fig. 6, 2c), the cavity of which persists throughout life
as the cavities (ventricles) of the brain and the central canal of the
spinal cord. From the cells of the walls of the canal the nervous tissue
arises.

This process of infolding progresses from in front backward. For
a time, in some vertebrates, a small opening, the anterior neuropore,
persists at the anterior end. The infolding extends back to the poste-
rior end of the neural plate so that, as will readily be understood, the
whole limits of the blastopore are included in the floor of the neural
canal. Occasionally the closure of the neural folds is completed before
that of the blastopore so that for a short time a short tube, the neuren-
teric canal (fig. 7), connects the archenteron with the neural canal.
Soon after the closure of the neural tube the fused tissue splits horizont-
ally, separating the nervous sys-
tem from the rest of the ectoderm.

SST ee
[AC] SURE EPR Per EE eo mIorl Bejan Z
ss iairareanat mee Its subsequent history will be

aa TM od int
efelefofofoe

Qe,

traced in the section of the Ner-
¥ vous System. :
he. The Notochord.—Immediately
beneath the neural plate is an axial
strip of entoderm (fig. 5, ch),
Fic. 7.—Schematic section of the hinder bounded on either side of the out-
end of an amphibian embryo, showing the growing mesothelium. When the

relations of the neurenteric canal. ac, alimen- .
tary canal; ec, ectoderm (black); , notochord; latter separates (p. 10) this band

sa aaa aa rics ag ?, is momentarily rejoined to the rest

of the entoderm butis still recogniz-
able from its different cells. It soon rolls into arod (a tube in some
amphibians and birds), is cut off from the rest (fig. 6, m) and lies
between the digestive tract and the nervous system where it forms an
axis around which the skull and vertebral column develop later.

The Digestive Tract.—After the separation of the notochord, the
entoderm forms a tube, closed in front and usually behind as well.
The anterior end of the tube abuts against the ectoderm of the ventral
side of the embryo. Later the ectoderm grows in at the point of con-
tact, carrying the entoderm before it and forming a pocket, the stomo-
deum, which gives rise to the cavity of the mouth. (In some the
stomodeal ingrowth is at first solid, the pocket being formed later by
splitting). Eventually the ectoderm and entoderm fuse at the bottom
of the cup, and then the fused area breaks through, placing the archen-

EMBRYOLOGY. 13

teron in connexion with the exterior. A similar, but less well defined
proctodeum (fig. 7, ») arises at the hinder end of the digestive tract.
Thus the anterior and posterior ends of the alimentary canal are
ectodermal, the middle region entodermal, in origin.

Metamerism.—In the adult, various parts, essentially like each
other, are repeated one after another—are metameric. The list
includes, among others, muscles, nerves, blood-vessels, vertebrz, ribs,
etc. There is much evidence to show that metamerism had its origin
in the mesothelial structures and has been secondarily impressed on
other systems.

Fic. 8.—Stereogram of a later stage than fig. 6, showing the segmentation of the meso-
thelium. The approach of the walls of the ccelom (c), dorsal and ventral to the alimentary
canal, to form the mesenteries is shown. al, alimentary canal; em, epimere; fb, forebrain;
hb, hind brain; hm, hypomere; m, myotome; mb, midbrain; mm, mesomere; mc, metaccele;
myc, myoccele; m, nervous system; 7c, notochord; s, stomodeal region; so, sp, somatic and
splanchnic layers; st, sclerotome.

The mesothelial coelomic pouches, as left above, are near the dorsal
side of the embryo. With growth they gradually extend downward
on either side and tend to enclose the whole archenteron, and upward
on either side of the notochord and spinal cord (fig. 8). The fates of
the different parts of the mesothelial walls warrants the recognition of
three horizontal regions or zones in the walls of each coelom. These
are a dorsal muscle-plate zone (epimere, em), a lower or lateral-plate
zone (hypomere, fm), and a middle-plate zone (mesomere, mm)
between them. All three of these occur in the trunk, but only the
epimere is well developed in the anterior part of the head.

14 INTRODUCTION.

A series of vertical constrictions begins at the dorsal margin of each
cceelomic pouch and cuts down through epimere and mesomere, so
that the whole may be compared to a glove with a large number of
fingers extending from its upper surface, each finger being hollow, and
all of the cavities connecting with that in the hypomere (palm). This
process begins at front and gradually extends backward. Viewed
from above in the transparent embryo, each of these fingers appears
like a square box and early students thought that they gave rise to the
vertebre, and so they were called protovertebre. Next, the dorsal
part of each of these fingers is cut off from the rest, along the line
between mesomere and epimere, thus forming a series of hollow cubes,
known as myotomes, each with a part of the ccelom in its interior, the
myoccele. After the separation from the rest each myotome grows
upward and to a greater extent downward, insinuating itself between
the ectoderm and the somatic wall of the hypomere (fig. 9, in the
direction of the arrows). From these myotomes the body (somatic)
musculature arises.

From the medial mesomeral part of the fingers arises the mesen-
chyme that gives origin to the vertebre while the rest furnishes the
material for the excretory organs. From their origin both of these
are metameric. at first, the skeletogenous parts being called scler-
otomes, the excretory parts, nephrotomes (fig. 8, mm, st). The
history of both will be followed in their proper places.

The Cclom.—The parts of the cclom in the myotomes soon
disappears, that in the nephrotomes, of inconsiderable size, forms
the lumina of the excretory ducts. That in the hypomere (fig. 9, c)
forms the large body cavity (peritoneal cavity) surrounding the
chief viscera, and the smaller one (pericardial) around the heart.
In surrounding the archenteron the walls of the two coelomic cavities,
which at first are separate, tend to meet above and below the entoderm,
so that there is in both regions a thin membrane supporting the digest-
ive tract above and below. Such supports are collectively called
mesenteries. Usually that below (v mes) largely disappears, but the
dorsal (d mes) one persists more or less completely. At first these
mesenteries are merely double membranes of mesothelium, but soon
mesenchyme grows in between them and extends around the digestive
tract, so that mesothelium and entoderm are bound together by the

- invading tissue. In a similar way the somatic wall of the ccelom is
bound to the muscles arising from the myotomes and these in turn to

EMBRYOLOGY. I5

the ectoderm by the mesenchyme. In this way the ccelom comes to
have two thick walls. That on the outer side, consisting of ectoderm,
muscles and peritoneal lining, is called the somatopleure (so), that of
peritoneum and digestive wall is the splanchnopleure (sf).

For convenience the different mesenterial structures have separate names,
As the digestive tract becomes coiled, the different parts of it are connected by
similar membranes which are called omenta (om). The dorsal mesentery is sub
divided into regions supporting the different portions of the digestive tract.

Fic. 9.—Diagrammatic transverse section of a vertebrate to illustrate mesenteries,
omentum and downward growth of the myotomes. al, alimentary tract; ao, aorta; c,
ceelom; ec, ectoderm; dmes, dorsal mesentery; my, myotome; mc, notochord; neph, nephro-
tome; 0, omentum; sc, spinal cord; so, sp, somatic and splanchnic layers of mesothelium;
umes, ventral mesentery.

Thus there is a mesogaster for the stomach, a mesentery proper for most of the
intestine, and mesocolon and mesorectum for colon and rectum respectively.
On the ventral side there is a mesohepar, bounding the liver to the ventral body
wall. In the same way the omenta are distributed into hepato-duodenal, gas-
tro-hepatic (small omentum), etc., while in mammals there is a great omentum,
a double fold of mesogaster and mesocolon which connects the stomach with the
transverse colon. .

Similar folds are formed in connection with other organs. Thus the heart

16 INTRODUCTION.

for a time is bound to the pericardial walls by dorsal and ventral mesocardia ;
there is, in mammals, a mediastinum between the two pleural cavities, connect-
ing the pericardium to the body wall, while frequently the ovaries and the testes
project into the ccelom, carrying the peritoneum with them, thus giving rise
to a mesOvarium or a mesorchium, according to the sex.

The Mesenchyme has two chief places of origin. One is from
the splanchnic wall of the segments of the mesomere, each of which
is the centre of rapid cell proliferation and forms the sclerotome (fig.
8, st), since some cells arising from it are concerned in the formation
of the axial skeleton. These cells pass in to surround the notochord,
and upward on either side of the central nervous system and downward
beside the alimentary canal, thus forming a partition between the
two sides of the body. A second source of the mesenchymatous cells
is from the somatic wall of each myotome, all of the cells of which are
transformed into this layer, and lie immediately beneath the ectoderm.
Thus there is a complete envelope of mesenchyme around the whole
body. From these and from other sources the mesenchyme extends
everywhere in the remains of the segmentation cavity—between the
muscles and around the various viscera—forming a framework in
which the products of all the other layers are enveloped (fig. 30). This
mesenchymatous framework has great importance in the development
of the skeleton and its general plan will be described in connection with
the skeletal structures.

HISTOLOGY.

In the gastrula the cells differ from each other chiefly in position,
and -the same is true even when the germ layers are first differentiated.
As development goes on the differences between the various groups of
cells increase, each group becoming more specialized for some one
purpose and losing the power to do more than the one kind of work.
For community of work cells of the same kind become associated to-
gether, the result being tissues. A tissue then is a connected mass of
cells similar in appearance and function, together with a varying
amount of intercellular substance, usually formed by the cells them-
selves. The study of the minute structure of animals and especially
of the tissues is the province of histology.

There are many kinds of tissues, only a few of which need mention
here, but all may be grouped under four great heads: epithelial

HISTOLOGY. 17

nervous, muscular and connective tissues; the members of each group
having certain fundamental points in common.

Epithelial Tissues.

Epithelia are the covering tissues, and occur on any free surface,
internal or external, of the body. Both comparative anatomy and
embryology shew them to be the primitive tissues, for there are many
lower animals which are made up entirely of epithelia, while in the
vertebrates the embryo consists solely of epithelia until the mesenchyme
appears. Epithelia may come from any of the germ layers, in rare
cases (synovial cavities) even from mesenchyme.

Cc

iS

Fic. ro.—Epithelia: A, cubical; B, squamous; C, cylindrical; D, stratified cylindrical,
ciliated at EZ; F, stratified squamous.

The character of epithelium varies according to the character of
the work it has to perform. That on the outside of the body is largely
protective, hence it is often thickened and strengthened in different
ways to afford resistance against external injuries. In other places,
as glands, it has to elaborate and to allow the passage outward of
material from within. In the body cavity and in the blood-vessels
it has merely to form the thinnest of coverings, while in the case of
sensory structures it is modified (sensory epithelium) to receive the
stimuli from without.

The usual classification of epithelia is based on the shapes and
arrangements of the cells. Thus in cubical epithelium (fig. 10, A)
the cells are about as high as broad; in columnar (C) their height

2

18 INTRODUCTION.

exceeds their diameter; while in squamous epithelium, the cells are
thin and flat, covering the largest amount of surface with the least
amount of material (B). Sometimes the epithelial cells are in a single
layer, forming simple epithelium (A, B, C);-in other places there are
several layers—the epithelium is stratified (D, E, F).

Frequently epithelia, usually of the columnar variety, are called upon
to move fluids slowly; then the free surface is covered with minute
vibratile hairs or cilia (EZ) which create currents. In glandular
epithelium the cells, usually cubical or columnar, are specialized for
the elaboration of secretions to be used by the animal or of waste prod-
ucts (excretions) to be voided from the body.

Fic. 11.—Different types of glands; A, to D, tubular; E, F, acinous; A, simple; B, coiled;
Ce, branched.

Glands.—The chief kinds of glands may be mentioned here. All have for
their function the extraction and elaboration of certain products from the blood,
consequently they have a good blood supply. Glands may be unicellular or multi-
cellular according as they consist of isolated cells or of-many cells. In unicellular
glands (abundant in the digestive tract) each cell passes its own secretion directly
to the place where it is to be used (fig. 19, 2).

Multicellular glands occur where a large amount of secretion is necessary in a
limited space, hence they are not on the surface but at some deeper point, and their
product is conveyed to the desired place bya duct. Multicellular glands are of two
structural kinds. In the tubular gland the whole is approximately of the same
diameter throughout, with little differentiation of gland and duct. It may be
simple (A) or coiled (B) or branched (C, D), these modifications serving to in-
crease the secreting surface. In acinous glands (D, E) there is a marked’ differ-
ence between gland and duct, the glandular part forming an enlargement (acinus)
on the end of the duct. Both simple and compound acinous glands are common.

Still another type of gland, the ductless or ‘internal secretion’ gland occurs.
In this there is no duct, the secretion elaborated by the cells passing by osmose into
the blood-vessels. These secretions, collectively known as hormones, have
recently acquired great prominence from their influence on different organs.

HISTOLOGY. Ig

Nervous Tissues.

Nervous tissue has for its function the correlation of the animal with
its environment. In order to accomplish this it must provide for the
recognition of stimuli from without, the inauguration of other impulses
within itself and the transfer of both to other parts. The essential
constituent of the tissue is the nerve cell, ganglion cell or neuron,
to which are added others of a supportive (glia cells) or nutritive
character. As the parts to be connected by the nervous tissue are often
remote from each other the neuron is not compact like most other cells,
but gives off long processes from the central mass, these processes differ-
ing in their terminations. Some end in places where they can only

Fic. 12.—Various kinds of nerve cells. A, multipolar cells; B, portion of nerve fibre
with sheaths; C, unipolar cell; D, pyramidal cell; a, axon; c, collateral; d, dendrites; cb,
cell body; m, medullary sheath; 2, nucleus of cell of Schwann’s sheath; s, sheath of Schwann;
t, telodendron. .

receive stimuli, others where the stimuli can only cause parts to act.
Thus the processes are physiologically divisible into afferent and
efferent tracts, the body of the cell being the place for the regulation and
correlation of the impulses, and apparently in many cells for the inau-
guration of new impulses.

A nerve cell (fig. 12) is uni-, bi- or multipolar accordingly as it has
one, two or more of these processes. In the case of unipolar cells (C)
the single process sooner or later divides, so that the cell in reality is at
least bipolar. At the ends the processes may either break up in minute
twigs (dendrites, ¢) or may end, as in muscles and sense organs, in
special end organs. The part connecting the efferent termination and
the central cell body is called the axon (a). Axons and cell bodies are
gray in color, but usually the axons are surrounded by a medullary
sheath (m) of a peculiar white substance (myelin) rich in fat, which

20 INTRODUCTION.

apparently acts as an insulator, preventing nervous impulses from pass-
ing from one axon to another. This sheath does not continue over the
dendrites. Frequently the dendrites of two neurons interlace for the
transference of stimuli from one to the other, but the present opinion is
that, at least in vertebrates, there is no actual continuity of substance
between neurons, only an interlacing of terminal twigs. The medullary
sheath is not cellular, but frequently fibres may be surrounded by a
sheath of Schwann (s), with scattered nuclei. This has been re-
garded as mesenchymatous, but recent researches tend to show that
it is ectodermal, its cells coming from the nervous system.

Nervous tissue consists of these neurons plus connective tissue and
glia cells. A nerve, as found in dissection, consists of numbers of
axons, bound together by a connective-tissue envelope (perineureum).
The myelin gives these nerves a white color. In the brain and spinal
cord there are tracts of medullated fibres (white matter) while the
parts with abundant nerve cells are gray. When such gray matter is
aggregated in the course of a nerve, it causes an enlargement called a
ganglion. Interlacing among the neurons in brain and spinal cord is
the neuroglia, which is also derived from the ectoderm, and acts as a
support but has no nervous functions. Certain of these glia cells
develop many branches (mossy cells) which twine among nerve cells,
axons, and dendrites.

Muscular Tissues.

While several kinds of cells have the power of changing shape,
those composing muscular tissue. possess it in a marked degree, acting
quickly and with force, so that these tissues are preeminently the tissues
of motion. The cells become elongate and develop on their interior a
large amount of contractile substance (myofibrille), which on stimula-
tion, contracts, shortening the cell. In the vertebrates, muscular tissue
always arises from the mesoderm, yet two types are recognized, differing
markedly in origin, appearance and physiological action.

The smooth or involuntary muscles arise from the mesenchyme.
They consist of long and spindle-shaped cells (fig. 13, A), each with a
single nucleus, the protoplasm traversed by numerous myofibrille,
which appear like fine longitudinal lines. In the vertebrates the
smooth muscle is not under control of the will; it contracts slowly.

In contrast to the smooth is the striped or voluntary muscular tis-

HISTOLOGY. 21

sue, which arises from a modification of the mesothelium. Except in
the case of the muscles of the heart, the striped tissue is under control of
the will; it usually occurs in larger masses than does the smooth, and
is capable of rapid contraction. It differs structurally from smooth
muscle. Instead of distinct, uninucleate cells there are long cylindrical
elements (fig. 13, B), the primitive fibres, each with several nuclei in the
interior in lower vertebrates, on its periphery in the higher. Most of
the protoplasm of the fibre has been altered to minute contractile
fibrille, each crossed by lighter and darker bands, and as these come
opposite each other in the different fibrille, they give the fibre its
characteristic cross-banded appearance.

|

|

Iz ma
|Z

St

Fic. 13.—A, smooth muscle cell; B, striped muscle.

The primitive fibres rarely branch at their extremities. Each is
surrounded by a structureless envelope, the sarcolemma, while num-
bers of fibres are bound into bundles and muscles by connective
tissue (perimysium) which carries nerves and blood-vessels. At the
ends of the bundles the perimysium continues into the tendons which
attach the muscles to other parts.

The heart muscle also arises from the mesothelium, is cross-banded,
but is removed from control of the will. The cells are usually short
(usually with a single nucleus); they branch, the branches connecting
adjacent muscle cells.

Connective Tissues.

The tissues grouped here arise from the mesenchyme and are
distinguished from all other tissues by the great amount of intercellular
substance produced by the cells themselves. This substance or matrix
varies in character and determines the variety of tissue. Frequently it
is dense and hence the connective tissues may give the body support,
and in fact they are sometimes called supportive tissues.

22 INTRODUCTION.

In the earliest phase, known as embryonic connective tissue
(fig..14, A), the cells are scattered, with long radiating processes, and
between the cells a thin gelatinous matter. It is by increase of this
intercellular substance by taking up water that many embryos gain so in
size without taking food. The embryonic connective tissue may de-
velop in various directions.

Fic. 14.—Connective tissues. A, embryonic, from Amblystoma; B, expanded and con-
tracted pigment cells from Amblystoma; C, fibrous, from tendon.

Thus some of the cells may contain pigment granules, forming
pigment cells (B), or oil globules may be deposited in them to such an
extent that the cells become spherical, while the intercellular substance
is reduced, thus affording fat or adipose tissue. Most common of
the connective tissues is fibrous tissue (white or non-elastic tissue) in
which the cells are branched or spindle-shaped while the matrix is
filled with fine fibrille of considerable strength and little elasticity.

These fibrille are parallel to each other in tendons (C), which have to
convey strains in one direction; or they may be interlaced confusedly, the
tissue then forming sheets or membranes. Occasionally, as between
the skin and the muscles, the fibrous tissue may be loose (areolar
tissue). In elastic tissue fibres of another kind are mingled among
the non-elastic fibrils. These are yellow and elastic, and when abun-
dant give an elastic character to the whole.

_ In cartilage and bone the matrix is more solid and is abundant.
These are the skeleton-building tissues. In cartilage the matrix is

HISTOLOGY. 23

firm and consists of a peculiar substance called chondrin. When the
chondrin is nearly pure it is milky in appearance (hyaline cartilage,
fig. 15), but it may be invaded by numerous strands of fibrous or
elastic tissue, resulting in fibrous or elastic cartilage. Cartilage in-
creases in size by additions to the exterior and also by divisions of
its cells and by increase in the amount of matrix. Externally it is

Fic. 15.—Hyaline cartilage.

bounded by ‘an envelope of connective tissue (perichondrium) which
bears blood-vessels and may give attachment to muscles, etc.

|< Bone may arise directly from embryonic connective or fibrous tissue,
or by the ossification of cartilage. In either case the result is a strong
matrix composed of calcium phosphate and carbonate in a ground

Fic. 16.—A, Stereogram of bone; B, cross-section of bone, more enlarged; ¢, canaliculi;
bl, bone lamelle; h, Haversian canal; /, lacuna.

substance of organic matter (ossein). Minute tubes: (Haversian
canals), bearing blood-vessels, etc., run through the matrix (fig. 16),
and parallel to these canals or to the external surface of the bone are
the cells arranged in layers. The space occupied by a cell is called’
a lacuna, from which minute tubules or canaliculi penetrate the matrix.
There are small spaces in many bones occupied by the red marrow,

24 INTRODUCTION.

which is especially noticeable as one of the places of formation of red
blood-corpuscles. Externally every bone is covered by a layer of
fibrous connective tissue, the periosteum.

The dentine of teeth and placoid scales is closely allied to bone,
the chief difference in density, the bone-forming cells (odontoblasts)
not being enclosed in the matrix, while the canaliculi (here called den-
tinal canals) are parallel to each other.

Blood is sometimes regarded as a connective tissue, the corpuscles
being the cells and the fluid part (plasma) the matrix. It is here dealt
with in connection with the circulatory system.

COMPARATIVE MORPHOLOGY OF
VERTEBRATES.

THE INTEGUMENT.

The integument is the covering of the body, the term including
the skin (cutis) and all structures derived from it. From its position
it is a protective coat. It comes into relation with the external
world and is modified in various ways, becoming hardened to ward
against mechanical injury, developing sensory structures to give in-
formation of untoward conditions and being impervious so as to prevent
loss of the body fluids or the entrance of others from without. Natur-
ally the habitat, aquatic or terrestrial, has great influence in the
character of the modifications.

In all vertebrates the integument consists of two layers, an outer
epidermis which consists of the ectoderm after the separation of the
nervous system, and a deeper layer, the corium (derma) of mesenchyme,
derived from the somatic wall of the myotomes, into which other struc-
tures (nerves, blood-vessels, etc.) extend. Strictly speaking the
bony scales of fishes are integumental, but on account of their close
relations to the skeleton they are best treated in that connexion.

In the epidermis, again, two layers are always present. At the base,
next to the corium is the Malpighian layer (stratum germinativum),
the cells of which are nourished by the fluids of the corium. Hence
they can grow and divide, the new cells thus formed gradually passing
to the outside where they form the second layer, the stratum corneum,
the outer cells of which are usually worn away as fast as new ones are
added from below. Occasionally these outer cells come off in large
sheets, as when a salamander or-a snake sloughs its ‘skin.’ In the
development of the epidermis of the terrestrial vertebrates the first
layer of cells budded from the Malpighian stratum form a continuous
sheet which is later shed as a whole. This is the periderm (fig. 17),
the older name of epitrichium being inappropriate, since the layer is
found in reptiles and birds where no hair occurs.

25

26 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

The Malpighian layer alone is concerned in the formation of the
glands connected with the skin, and the corresponding part of the
ectoderm contributes to the sensory structures like the nose and ear.
The corneum, on the other hand, is concerned in the formation of
protective structures like hair, nails, claws, feathers, and other cuticular
outgrowths. The epidermis is generally thicker in terrestrial than in
aquatic vertebrates, and in the latter, being constantly moist, shows
less of the horny consistency, than occurs in animals which live in the
air.

The corium lies immediately beneath the epidermis and is less
sharply separated from the deeper tissues by a looser layer of connective
tissue (subcutis, tela subjunctiva) in which fat is frequently exten-
sively developed. The corium is largely composed of fibrous connec-
tive tissue, intermingled with elastic tissue, blood-vessels, nerves,
smooth muscle fibres, etc. It is usually thin in the lower vertebrates,

Fic. 17.—Section of developing scales of lizard, Sceleporus. c, papilla of corium;
e, outer layer of epidermis which later becomes cornified; /, fibrous layer of skin; m, Mal-
pighian layer; ~, periderm; #s, tela subjunctiva.

but is much thicker in most mammals, and forms the whole of ordinary
leather. Pigment cells may occur in both epidermis and corium. These
are mesenchyme cells, loaded with pigment, which are frequently
under control of the nervous (sympathetic) system, and can be altered
in shape (chromatophores), thus producing color changes, which, as
in the chameleons, may be very marked.

Horny scales, produced by a cornification of the epidermis, are found
in all groups of terrestrial vertebrates, but they are rare in amphibians
and mammals. The development is best seen in reptiles (fig. 17).
By a multiplication of the cells of both corium and epidermis in defi-
nite regions the skin becomes divided into thicker areas, separated
by thinner lines, each area corresponding to a future scale, which arises
by the conversion of the stratum corneum into horny material. In
snakes and lizards these scales, together with all of the stratum corneum
(even the covering of thé eye) is periodically molted, the separation tak-

INTEGUMENT. 27

ing place at the surface of the stratum Malpighii. . In turtles and
alligators there is a gradual wearing away of the surface.

Closely allied to scales are claws, hoofs and nails (fig. 18). A
claw may be regarded as a cap of the tip of a digit, formed by two scales
one dorsal (unguis), the other ventral (subunguis). Of these the
unguis is the more important. It grows continually from a root, and in
mammals is forced forward over its bed. In the claw (B) the unguis
is curved both transversely and longitudinally, the subunguis forming
its lower surface. In the human nail (A) it is nearly flat in both direc-
tions and the subunguis is reduced to a narrow plate just beneath the

Fic. 18.—Diagrams of (A) nails, (B) claws, and (C) hoofs, based on Boas. ¢, unmodified
epidermis; 7, unguis; s, subunguis.

tip of the nail. In the hoof (C) the unguis is rolled around the tip of. the
toe, while the subunguis forms the ‘sole’ inside it. The ‘frog’ is
the reduced ball of the toe which projects into the hoof from behind.

The integument presents many different conditions in the separate
groups of vertebrates, and so details are best given under the special
heads.

FISHES.—The aquatic life renders the epidermis of fishes soft and
cornifications of it are comparatively rare, among them the peculiar
‘pearl organs’ which appear in the skin of some teleosts at the breed-
ing season. Glands, on the other hand, are abundant. These are
unicellular and multicellular mucus glands of different shapes in the
epidermis, the secretion of which furnishes the slime on the surface.
Some elasmobranchs and a number of teleosts have poison glands,
usually in close relation to the spines of the fins. The elasmobranchs
also have large glands in the ‘claspers’ of the males, but their purpose
is not well understood.

28 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

Possibly the most striking of the epidermal organs are the luminous
organs or photophores, which are most common in elasmobranchs and
teleosts from the deep seas, where sunlight does not exist. They are
apparently modified glands, and the development is known in Porich-

Fic. 19.—Section of skin of Protopterus. c, corium; e, epidermis; g, multicellular gland;
u, unicellular gland.

thys. There is an involution of cells of the Malpighian layer into the
corium, where they become cut off from their point of origin, and are
differentiated into a deeper glandular layer and an outer rounded body,
the lens (fig. 21). Around this the corium forms a reflecting layer

Fic. 20.—A, head of Noturus flavus; B, section of poison gland of Schilbeodes miurus
(after Reed). ¢, epidermis; p, pore of poison gland, pg; s, spine of pectoral fin.

enclosed in a pigment coat. The glandular layer is the seat of light
production. In other photophores either reflector or pigment may be
lacking, but in their highest development they so resemble an eye that
at first they were described as such.

In the myxinoids the skin contains numerous thread cells in pockets which may
extend into the underlying muscles. Each thread cell contains a long thread, which

INTEGUMENT. 29

is discharged upon stimulation, the threads forming a network in which the mucus
secreted by the ordinary gland cells is entangled. .

The corium is.thin and consists of horizontal bands of fibrous tissue, crossed at
intervals by vertical strands. Fat is common in the tela subcutanea, and in some
fishes this layer contains numerous crystals of guanin which gives it a silvery
appearance. ‘This guanin forms the base of ‘essence of pearl’ from which artificial
pearls are made. The scales of fishes, although formed in the skin, are con-
sidered in connection with the skeleton.

Fic. 21.—Section of luminous organ (photophore) of Porichthys, after Greene. e,
epidermis with mucous cells; gi, glandular layer of photophore; /, lens; r, reflector sur-
rounded by pigment.

AMPHIBIA.—The amphibia are remarkable in that the epidermis
of the larvee is ciliated in the early stages, and is two cells in thickness
from the first. The skin, in the larve and the aquatic species, con-
tains numerous mucus glands and some for the production of poison,
some of the latter being prominent like the ‘parotid glands’ on the
neck of the anura and the gland on the back near the base of the tail.

The corium is thin, and in the frogs is separated from the underlying parts by
large lymph spaces which render the skinning of these animals so easy. As the
amphibia respire largely by the skin (there are several lungless salamanders) the
corium is richly supplied with blood-vessels, and at the time of the metamorphosis
of the anura these penetrate even into the epidermis, as at that time the lungs are
not yet functional and the gills are absorbed. The stratum corneum is shed
periodically, either as a whole (urodeles) or in patches. The warts of toads are in
part: cornifications of the epidermis, and a similar hardening of the skin on the
ends of the toes of some results in claws. In the males-of an African frog
(Astylosternus) the skin has the granules of the surface developed, at the breed-

30 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

ing season into hair-like structures, supplied with nerves and apparently
sensory in character.

REPTILES.—All living reptiles are characterized by the extensive
development of horny scales and frequently of bony plates in the skin,
but some of the fossil groups (ichthyosaurs, pterodactyls, some dino-
saurs, possibly plesiosaurs) had a naked skin. Correlated with this
cornification of the epidermis, glands are rare. Some turtles have scent
glands beneath the lower jaw and along the line between carapace and
plastron; snakes and crocodilians have them connected with the cloaca,
while the latter have others, of unknown function, between the first and
second rows of plates along the back, as well as protrusible musk
glands on the lower jaw. These latter are not true glands as they
produce no secretion but cast out the lining cells.

The corium presents two layers, the outer rich in chromatophores, but, aside
from some snakes and lizards, the color changes are not remarkable. The femoral
pores of lizards are not connected with glands but with branching tubes filled with
cast cells. Claws are common on the toes.

BIRDS have both layers of the skin very thin, the epidermis develop-
ing both scales and feathers. Correlated with this extensive develop-

Fic. 22.—Diagram of base of contour feather. a, aftershaft; b, barbs; bl, barbules;
h, hooks on-ends of barbules; Ju, lower umbilicus; g, quill; s, shaft; «, umbilicus; v, vane.
A, portion of a barb showing the barbules and hooks.
ment of cornified structures is a striking’paucity of glands. There are
none in the ostriches, but others have the familar oil (uropygial) glands
at the base of the tail, the secretion of which is used in dressing the
feathers. The only other dermal glands in birds are modified sebaceous

INTEGUMENT. ai

glands near the ear in some rasores. The scales on the legs and the
claws on the feet and occasionally on the wings, are derivatives from
reptilian ancestors. The feathers are also derived from scales, but are
greatly modified.

Feathers.—There are several kinds of feathers but all may be
grouped under three heads: hair feathers (filoplumes), down feathers
(plumulz), and contour feathers (plume). The latter have all of the
feather features (fig. 22) and in the typical form consist of shaft and
vane. The basal part of the shaft is the hollow quill, in which is a

Fic. 23.—Feather tracts of Geococcyx californianus, after Shufeldt.

small amount of loose pith. In the region of the vane the shaft, here
called rhachis, is solid, and running the length of its lower surface is a
groove, the umbilicus. The vane consists of lateral branches (barbs)
on either side, which have, in turn, smaller side branches (barbules),
these with small hooks at their sides and tips (B). Interlocking of
these hooks gives firmness and continuity to the whole vane. In down
feathers the barbs arise directly from the end of the quill, and as hooks
are lacking, the barbs do not interlock and no vane is formed. Hair
feathers are merely long and slender shafts with no barbs, the simplest,
if not the most primitive kind of feather. It is still a question as to

32 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

the primitive type. Tle oldest fossil bird, Archaeopteryx, had well
developed contour feathers.

Except in the ostriches, penguins, and toucans, feathers are not
distributed everywhere on the surface of the body, but are gathered in
feather tracts (pteryle), separated by apteria in which no contour
feathers and but few down or hair feathers occur. These vary in their
arrangement in different groups of birds and are of systematic im-
portance (fig. 23).

Complicated as they are, feathers are probably derived from scales, and the
section of lizard skin (fig. 17) might well represent an early stage in the develop-
ment of a feather. A down feather begins as a thickening of the corium, pushing
the epidermis before it. By continued growth this forms a long, finger-like papilla,

Fic. 24.—Stereogram of developing down feather. bv, blood-vessels entering pulp;
c, corium; ep, epidermis; f, feather follicle; p, pulp (mesenchyme) of developing feather;
per, periderm; r, rods of epidermis, which later dry, separate, and form the down.

projecting from the skin. The corium extends into the outgrowth, carrying blood-
vessels with it, while an annular pit, the beginning of the feather follicle, forms
around the base of the papilla. Next, the corium or pulp of the distal part of the
papilla forms several longitudinal ridges (fig. 24) which gradually increase in
height, growing into the epidermis and pressing the Malpighian layer above them
against the periderm. As a result the stratum corneum is divided distally into a
number of slender rods arising from the base (quill), which at last are only held
together by the periderm. Then the pulp retracts, carrying with it the Mal-
pighian layer. With the blood supply removed, the epidermal parts dry rapidly,
the periderm ruptures, allowing the rods to separate to form the down.

A contour feather has much the same development, differing in details, for an
account Of which reference must be made to special papers. The ridges of the
corium are no longer longitudinal, but beginning on the dorsal side of the papilla,
run obliquely outward and downward (fig. 25) until they meet below. Thus

INTEGUMENT. 33

there are formed a series of rods set at an acute angle to the undivided dorsal strip,
the future shaft. When set free, as before, by the rupture of the periderm, these
rods straighten out, forming the vane. In the region of the shaft there are two
longitudinal ridges on the ventral side. These gradually roll together, thickening
and strengthening the shaft, the groove between them forming the umbilicus. As
will be understood, the dorsal and ventral sides of the feather were the outside and
inside of the stratum corneum of the papilla.

The corium is thin and consists of irregularly interlaced fibres; it is rich in sense
(tactile) organs and smooth muscle fibres, which are largely used in elevating the
feathers. The colors of feathers depend in part upon pigment—red, yellow, orange,
brown, and black—deposited in them, but the iridescent colors are due to interfer-
ence spectra.

Fic. 25.—Stereogram of part of developing contour feather; compare with fig. 24. b
developing barbs; pc, pith cavity; per, periderm; s, rhachis.

MAMMALS have a skin relatively thicker than have other verte-
brates, both layers contributing to the thickness and the whole rather
loosely attached to the lower tissues. There are numerous glands, and
the hair, abundant in all orders except the whales and sirenians, is
found in no other class. Other cuticular structures as horn and claws
(p. 27) are widely distributed and scales occur in several forms.

The corium is thick and composed of irregularly interlaced fibres with muscles,
blood-vessels, etc. Its outer surface is frequently thrown into papille or ridges,
especially on the palms and soles, these carrying the epidermis with them. In the
thick epidermis several strata may usually be recognized: at the base a thick
Malpighian layer; then a thin stratum lucidum in which distinct cells cannot be
recognized; and on the outside the stratum corneum. One or more others are
sometimes present. As will readily be understood a cell passes through all of
these layers before it is worn from the surface of the skin.

Hair.—The epidermis takes the initiative in the formation of hair.

It thickens in spots, the thickenings pushing into the corium and each

being cupped at the tip, blood-vessels extending into the cup. The basal

cells of the ingrowth, thus richly nourished, proliferate rapidly and the
3

34 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

new cells thus formed are forced outward, forming the hair. While
this is going on the ingrowth splits around the hair, forming the follicle,
while another ingrowth of the Malpighian layer forms the sebaceous
gland which oils the hair.

A section through a hair and its follicle gives the following layers (fig. 26).
Around all is the connective-tissue envelope, formed from the corium; next inside
is the outer root sheath formed of the Malpighian layer and extending to the cavity
of the follicle. Around the root of the hair is the inner root sheath, two cells in
thickness, the layers being known as Henle’s and Huzxley’s layers. These do not
extend outside the follicle. In the hair itself there is a cortical layer surrounding
the central medulla, the hair not being hollow.

ep
Jayay a
2S

(oro)

THES
eK Tey
eaves

Fic. 26.—Diagram of structure of hair. 0, blood-vessels; ct, cuticle of hair; cx, cortex g,
gland; h, hair; he, Henle’s layer; hf, hair follicle; hx, Huxley’s layer; m, medulla; p, papilla;
sg, stratum germinativum of epidermis.

‘<7

15%

Hair differs greatly in size, the spines of the porcupines forming one extreme, the
prenatal hair (lanugo) of man the other. Hair is shed at intervals. The old hair
ceases to grow, separates from its base, and later is pushed out when the root begins
again to proliferate. There are smooth muscle fibres connected with the roots of
the hairs, their function being to raise the hair from its usual inclined position under
influence of the sympathetic system. ‘There are also usually nerves distributed
to the base of the hairs, making them to some extent sense organs, a condition
which reaches its greatest development in the facial hairs (vibrisse) of carnivores
and the hairs on the wings of bats.

Scales occur in several orders, being usually best developed on the
tail and feet. They may be rounded, quadrangular or hexagonal, the
square scales being arranged in rings around the part, the others in
quincunx. ‘These are closely similar to the cuticular scales of reptiles
(p. 26). Recent investigations tend to show that there is a close rela-

INTEGUMENT. 35

tion between scales and hairs, since in the mammals with scales the
hairs are usually arranged in groups of three or five behind each scale
(fig. 27); while in those without scales the hairs are frequently grouped
in the same manner. The illustration (fig. 28) is interesting as
showing the arrangement in man and the possible relation to ancestral

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Fic. 27.—A, arrangement of the two kinds of hair in Ornithorhynchus; B, Arrangement
of hair in Ptilocerus lori, with the probable relation of the hair to the ancestral scales;
both after Meijere.

Fic. 28.—Arrangement of the hairs in groups of threes and fives in the human embryo,
with the probable ancestral arrangement of the scales; after Stohr.

scales. The statement is also made that at first the hairs are arranged
in longitudinal rows and that the grouping comes later.

The mammalian skin is usually rich in glands which are of two
types, tubular and acinous (p. 18). To the first belong the sweat glands,
which extend from the Malpighian layer, where they arise, down through

36 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

the corium and then are coiled in order to obtain greater length. The
acinous glands are represented by the sebaceous glands in connection
with each hair (fig. 26, g), and by the scent glands in the anal or in-
guinal region of many carnivores, rodents and edentates. Others —
may occur in very diverse regions as on the face (bats, deer), in
the occipital (camel) or temporal region (elephant) or on the legs
(swine).

The mammary or milk glands are now known to be modified tubu-
lar glands possibly derived from sweat glands. In the monotremes the
simplest condition is found, numbers of glands opening into a pair of
sacs in the sides of the marsupium, or pouch where the young are kept,

Fic. 29.—Scheme of different kinds of nipples, based on figures by Weber. Single
line, ordinary integument, double line, that of primary mammary pocket. A, primitive
condition, found in Echidna; B, human nipple; D, Didelphys before lactation; C, same at
lactation; Z, embryonic, F adult condition in cow. Band C are true nipples, F a pseudo-
nipple (teat).
on the ventral side of the body. In the marsupials there is a slight nip-

-ple developed from the bottom of the pocket. In the higher groups of
mammals the first appearance of the milk glands is the formation of a
‘milk line,’ a ridge on either side of the body from in front back to the
inguinal region. This is soon divided into ‘milk points’ from each of
which there is an ingrowth of epidermis into the corium, the interme-
diate parts of the line disappearing. Each of the points may develop
into a definitive mamma, but not all of them come to full development,
for the number in the adult is less then in the embryo, varying from a
single pair to eleven in Centetes, the number roughly corresponding to

'

SKELETON. a7

the number of young at a birth. This method of formation explains
the varying position of the mamme and also the occasional occurrence of
more than the normal number (polymastism) in man and other mam-
mals. Each gland is provided with a nipple and of these there are two
kinds (fig. 29). In the one the whole surface on which the lacteal
ducts empty becomes elevated, in the other the region around the
openings of the ducts becomes drawn out into a tube with the ducts
opening at the bottom (ungulates).

THE SKELETON.

The term skeleton as used here is applied to any of the harder parts
of the body, developed from the mesoderm and serving for support,

Fic. 30.—Diagram of the skeletogenous tissue in the caudal region of a vertebrate.
bu, blood-vessels; epmu, epaxial muscles; hs, horizontal partition; hymy, hypaxial muscles;
msd, msv, dorsal and ventral median septa; mys, myosepta; x, spinal cord; mc, notochord.
for the attachment of muscles, for protection and the like. This ex-
cludes any purely epidermal hard parts, and these have been included
with the integument.

As the skeleton can only develop where there is mesenchyme, the
distribution of the chief skeletogenous parts, sometimes called the
membranous skeleton, may be given here, continuing the account from
page 16. First is the corium, immediately beneath the epidermis,

38 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

forming an envelope around the internal structures. This connects
in the middle line, above and below, with a longitudinal partition which
separates the muscle masses of the two sides. This partition splits
to pass on either side of the central nervous system and the notochord,
and, just beneath the peritoneum, around the viscera. From the
median partition sheets of mesenchyme (myosepta) pass vertically
between the myotomes to the dermal layer, they being, like the myotomes,
metameric. Then there is a horizontal sheet on either side which lies
between the epaxial and hypaxial muscles (p. 127). Not all parts of
this membranous skeleton develop hard structures, but these are most
apt to arise at the intersection of the various planes.

The skeletal structures are divided into the dermal, arising in the
outer mesenchymatous envelope, and the endoskeleton, formed in
the other parts and lying deeper in the body. The dermal skeleton
includes the scales of fishes, the dermal armor of many reptiles and
fossil amphibians and the bony scales in the skin of crocodilians and
some mammals. In the strict sense the so-called membrane bones of
the skull and the cleithrum of fishes and the clavicle and episternum
of higher vertebrates should be included here, since they apparently
have been derived from dermal ossifications, but convenience of treat-
ment necessitates their consideration with the endoskeleton, with which
they are intimately associated.

It is a question whether the dermal or the endoskeleton is the older. The most
primitive of the living species, the cyclostomes, have no exoskeleton, but have
cartilage developed to some extent. In development, also, cartilage always ap-
pears before there is a trace of the exoskeleton. On the other hand, some of the
oldest fishes known have a well developed dermal armor, while the best preserved
ostracoderms show no trace of an internal skeleton. The external skeleton has
probably arisen as a means of protection, the internal as a result of muscular or
other strains.

Bones are connected (articulated) with each other in different ways-
They may be so articulated that one can move on the other (diar-
throsis) or there may be no motion possible (synarthrosis), each with
several varieties. Of the immovable joints there may be sutures,
where the two bones are connected by the interlocking of saw tooth-like
projections, or the two may be united by bony growth (anchylosed)
so that the line between the two disappears. In those cases of diar-
throdial joints where there is much motion there is usually a closed sac,
lined by a synovial membrane between the two bones. This mem-
brane secretes a fluid which lubricates the surfaces.

SKELETON. 39

Cartilages and bones are covered on their outer surfaces by an
envelope of connective tissue, called respectively perichondrium or
periosteum. These membranes form the means by which muscles
are attached to the bones and by which blood-vessels obtain entrance to
them. The periosteum is also a seat of bone formation.

DERMAL SKELETON.

When present, the dermal skeleton arises by a marked prolifera-
tion of cells at definite points in the corium. These cellsbecome
specialized (scleroblasts, odontoblasts or osteoblasts) for the
deposition of salts of lime plus a varying amount of organic matter
(ossein). Upon limy plates formed in this way other parts, also
calcareous, may be laid down by the basal surface of the epidermis,
so that the whole dermal element may be in part mesenchymatous,
in part ectodermal in origin.

Fic. 31.—Cross-sections of developing scale of Acanthias. c, stratum corneum; d, dentine
of scale; ce, enamel organ; m, stratum Malpighii; , pulp.

It is generally thought that the primitive dermal skeleton resembled
that of existing sharks, and that from the hypertrophy or fusion of
such scales the so-called membrane bones have arisen. Then the
scales of other vertebrates are to be traced back to an elasmobranch
ancestry, while teeth are thought to be modified scales. Hence the
structure and development of the elasmobranch scale should be
understood.

At regular intervals in the skin of a shark there is a multiplication of
cells of the corium, each aggregation forming a small papilla which
projects above the surrounding corium, carrying with it the basal layer
of the epidermis. The surface cells of the papilla and the region
around it becomes converted into osteoblasts which secrete calcic

40 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

salts on their outer ends, thus forming a small plate of dentine (p. 24)
with a central spine into which the papilla extends. The overlying
epidermal cells form an enamel organ, the lower surface of which
secretes an even harder layer of enamel’ upon the dentine base, this
being thickest on the tip of the spine. The mesenchyme in the papilla
is the so-called pulp. With continued growth the spine projects through
the epidermis, giving the skin of the shark its characteristic rough
(shagreen) condition. This is the placoid type of scale.

FISHES.—In the adult elasmobranchs the scales may be large and remote from
each other (skates) or small and closely set. In the torpedo scales are lacking,

while in the chimzroids they occur only on the claspers, on the frontal horn, and
as extreme forms, in a great spine in front of the dorsal fin.

Fic. 32.—Ventral armor of Stegocephals (after Credner-Zittel). A, Branchiosaurus; B,
detail of same; C, detail of Archegosaurus; D, of Petrobates.

A few ganoids lack scales (Polyodon), while the sturgeon have minute granules
and five rows of large plates along the sides. Ama has scales of the cycloid type,
soon to be described. With these exceptions the ganoids have ganoid scales, which
are rhomboid in outline and joined to each other like parquetry. They consist of
two layers, the lower apparently homologous with the dentine of sharks, except that
it is formed in, not on, the corium. The outer layer of ganoin is formed by the
corium and consequently cannot be enamel as once was thought.

A few teleosts are scaleless (some eels), but elsewhere scales are formed in
pockets in the corium (fig. 181). At first they lie side by side, but with growth they
overlap like shingles. There is only one layer of bone mixed with a large amount
of ossein. In cycloid scales the element is circular and is marked with concentric
and radiating lines. The ctenoid scales differ in having the posterior edge of

1 There is some question whether this layer is really enamel; the usual statement as to
its nature is followed here.

SKELETON. 41

each scale truncate and this edge and the surface toothed. Cycloid and ctenoid
scales intergrade and both kinds may occur on the same fish (many gobiids).

AMPHIBIA.—A dermal skeleton occurs in the recent amphibians only as rows
of plates in the cutaneous rings on the bodies of the cecilians and in the skin of the
back of a few exotic toads. In some fossil stegocephalans there was a ventral
armor and in others one protecting the whole body. The ventral exoskeleton,
sometimes of scales or plates, sometimes long bars, is arranged in oblique rows,
and is interesting as probably being the source of the gastralia found in many
reptiles (infra). Episternum and clavicle were possibly dermal in these forms,
but they will be described in connection with the shoulder ‘girdle. Apparently
certain of the gastralia of these fossils were modified into comb-like organs which
have been thought to have sexual significance.

REPTILES.—The dermal skeleton is best developed in the turtles of living
reptiles, though here it is closely associated with the endoskeleton. The dermal
plates form a box for the protection of the body. This consists of a dorsal carapace
and a ventral plastron, united to varying extents and each consisting of a number
of elements. In the carapace there is a middle line of neural plates (fused with

Fic. 33.—Section through developing vertebra, rib and exoskeleton of Chelone imbricata,
after Gitte. ¢, cutis; cs, primitive vertebral body, ef, epidermis; m, external oblique muscle;
p, peri ichondrium; r, rib; sp, spinal process.

the vertebre), marginal plates around the margin, and costal plates, fused to
the ribs, between neurals and marginals. The plastron (fig. 34) usually consists
of nine plates, wholly dermal, the names shown in the figure. ‘The three posterior
pairs are regarded as the same as the gastralia of other reptiles, the anterior pair as
the clavicles, while the unpaired entoplastron is supposed to be homologous with
the episternum of other tetrapoda.

Some of the extinct crocodilia were armored with closely applied scales and
these have been retained in the existing species in a reduced condition. They
also have well developed gastralia. These are of rods dermal bone in the ventral
body wall between the true ribs and the pelvis, and so closely resemble ribs that
they were called ‘abdominal ribs.’ They do not meet in the middle line; each,
except the first, consists of two distinct parts, and the pairs correspond to the
somites in number. In Sphenodon (fig. 35) the gastralia are more numerous than
the somites.

In a few lizards there are dermal scales, while the extinct stegosaurs had

42 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

dermal ossicles, sometimes of great size (plates a yard across, spines half a yard long)
in the dorsal region.

BIRDS.—Recent birds lack all dermal ossifications, but Archeopteryx had
gastralia.

Mammals rarely have dermal bones. They are known in the extinct zeuglo-
dont whales and in several fossil edentates, but in the living species they occur

Fic. 35.—Ventral ends of ribs (7)
plastron; ef, epiplastron; hpp, hypoplastron; and gastralia (g) of Sphenodon.
hyp, hyoplastron; xp, xiphiplastron.

only in the armadillos where they form a complete armor above, the plates
arranged in transverse rows, some of which are movable on each other. In the
extinct glyptodons they formed an inflexible case. It is uncertain whether these
are a new acquisition in the edentates or have been inherited from non-mammalian
ancestors.

THE ENDOSKELETON.

The endoskeleton may pass through three stages in its develop-
ment, including the membranous stage. From this it may pass through
a cartilage stage before becoming bone, or it may in part develop
directly into bone from membrane, or, lastly, it may never pass beyond
the cartilage stage. Thus only the membranous stage is constant.
These differences in development are of great importance in tracing
homologies between bones in different groups, but the distinction be-
tween bones developing directly from membrane (membrane bones)

SKELETON. 43

and those passing through a cartilage stage (cartilage bones) can only
be recognized by following the ontogeny of the element in question.

As stated above, there is much evidence to show that the membrane bones are
dermal bones which have sunk to a deeper position and have become secondarily
"associated with the endoskeleton. This is especially evident in the skulls of some
of the lower ganoids. Ossification of cartilage takes place in two ways. In
ectochondrostosis the deposit of lime salts begins on the deeper surface of the
perichondrium and gradually invades the cartilage. In entochondrostosis the
cartilage becomes broken down in the interior, some of the cells becoming modified
into osteoblasts, and from these as centres of ossification, the process of bone forma-
‘tion extends in all directions. In ectochondrostosis at least, the centres of ossifica-
tion may have been derived, phylogenetically, from elements of the dermal skeleton.
In ossification the bone is developed in layers, between which the osteoblasts are
arranged. In the elasmobranchs the skeleton is frequently strengthened by
deposits of lime, but this calcified cartilage differs from bone in that the deposits
of lime take the form of polygonal plates and there are no lacune.

OOO

Fic. 36.—Diagram of growth of bone. A, from an animal recently fed with madder
causing a layer of bong (black) colored by the dye; B, later, no madder fed for some time,
a deposit of colorless bone on outside of colored layer, internal layer thinner; C, still later,
outer layer thicker, inner layer absorbed.

Many bones increase in length by the addition of epiphyses at the ends. These
are separate ossifications which only unite with the main bone at the time the adult
condition is reached. The increase in diameter has some interesting features. In
animals fed with madder, the bone formed during the feeding is colored. In this
way it is found that the new bone (fig. 36, A) is laid down on the outside of the
other, and that with growth (B and C), the ‘marrow cavity’ on the inside is in-
creased in size by the resorption of the bone already formed.

For convenience of treatment the endoskeleton is divided into axial
and appendicular portions, the axial consisting of the vertebral column
(backbone) and the skull, together with the ribs and sternum which are
closely associated with the vertebre. The appendicular skeleton in-
cludes the framework of the limbs and fins and the girdles to which
they may be attached.

Axial Skeleton.

Both the skull and the vertebral column surround and protect the
brain and spinal cord, and in this way the skull is an enlarged and

44 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

specialized portion of a continuous axis, but it is not possible to carry
the comparison into details. ' The idea of Oken that the skull is a com-
plex of three or four vertebre has long been overthrown... The skull
differs markedly from the vertebral column in the presence of numerous
membrane bones.

VERTEBRAL COLUMN.

The notochord (p. 12) is the foundation around which the verte-
bre and the posterior part of the skull are developed. It is a cylin-

Fic. 37.—Section of developing vertebra of 45 mm. Amblystoma. c, cartilage of inter
centrum; cs’, outer chorda sheath; cs’, inner chorda sheath; dm, dura mater; e, epithelioid
layer of notochord (elastica interna); end, endorhachis, torn from wall of vertebral canal;
np, neurapophysis (ossified); ms, neural spine of preceding vertebra; nt, notochord; sc,
spinal cord sd, subdural space.

drical rod of entodermal origin, without segmentation,’ extending from
the infundibulum (see brain) to the posterior end of the body. Its cells
become vacuolated and at length most of the protoplasm, together
with the nuclei, migrate to the surface of the cord, where they appear
like an epithelium, which, together with its basal membrane, is called
the internal elastic membrane (elastica interna, fig. 37, e).

* Segmental undulations occur in the notochords of some mammals, but their significance
is not clear.

SKELETON. 45

Next, mesenchymatous cells, derived from the sclerotomes, form
a notochordal sheath, bounded externally by an elastica externa.
The mode of formation and the history of the sheath vary in different
groups, for accounts of which reference must be made to special
papers. Other skeletogenous tissue extends outward from the sheath
toward the periphery, as described on a previous page (p. 38, fig. 30)
from which the ribs of all vertebrates are developed, the cyclostomes
passing but little beyond this membranous condition in the trunk
region.

With the appearance of cartilage segmentation is introduced into
the skeleton. As cartilage is firm and comparatively unyielding, in

Fic. 38. Fic. 39.

Fic. 38.—Two caudal vertebre of alligator. c¢, centrum; ha, hemapophysis; hs,
hemal spine; na, neurapophysis; ns, neural spine; poz, prz, post- and prezygapophyses;
t, transverse process. The arrow passes through the neural arch.

Fic. 39.—Diagrams of (A and B) fish vertebre and (C) vertebra from higher groups.
b, basal stumps; ¢, capitular head of rib; ct, centrum; d, diapophysis; /7, fish rib; ha, hemal
arch; ma, neural arch; ~, parapophysis; 7, rib; ¢, tubercular head.

order that the trunk may bend, the cartilage becomes divided into
separate blocks, which, in order that they may be moved by the muscles
connected with them, must alternate with the myotomes. Hence the
metamerism of the vertebral column is the result of that of the muscular
system.

A typical vertebra, whether of cartilage or bone, consists of several
parts, the names of which are necessary for the understanding of the
following account. Surrounding the notochord is the body or centrum,
developed from the notochordal sheath or from tissue surrounding it.
A neural arch, enclosing the spinal cord, extends dorsally from the
centrum. It consists of a plate on either side (neurapophysis), the

46 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

arch being completed by a neural spine as a keystone. Ventral
to the centrum is a similar hemal arch, composed, in like man-
ner, of hemapophyses and hemal spine, and enclosing, in the
caudal region, the caudal artery and vein, farther forward, the
coelom and viscera. This type of vertebra is common in many
fishes, and in the tails of some higher forms. In the lowest fishes
it is simplified by the omission of parts, while in the higher verte-
brates other structures are added. Among these are articular proc-
esses (Zygapophyses) on the anterior and posterior faces of the neural
arch (distinguished by position as pre- and post-zygapophyses)
which lock the successive vertebre together and strengthen the column
without interfering with its flexibility (fig. 38).

Fic. 40.—Diagrammatic sagittal sections of (A) amphiccelous; (B), proccelus; (C), opistho
ceelous; and (D), amphplatyan vertebre; the head supposed to be at the left.

In all vertebrates above fishes most of the vertebre bear transverse
processes (pleurapophyses), extending laterally on either side. Of
these there are two kinds, a parapophysis arising from the centrum,
and a diapophysis projecting from the neural arch. The ribs articu-
late with the ends of these, as will be explained later. The distinctions
are the most marked in the lower vertebrates, but careful comparisons
show them in the mammals. Other processes, of less frequent occur-
rence, will be mentioned below in connection with the groups in which
they occur.

The ends of the centra, where they articulate with each other,
may take five different shapes. They may be hollow at both ends
(amphiccelous); they may fit together with a ball and socket joint,
the hollow being sometimes in front (proccelous), sometimes behind
(opisthocelous). In the mammals flat or amphyplatyan conditions
are common, while in birds saddle-shaped ends occur (figs. 40, 49).

In the history of vertebra both comparative anatomy and embryology agree
that the process of vertebral formation began with the arches and extended thence

SKELETON. 47

to the sheath of the notochord. In what must be considered the most primitive
condition the arches extend no further than the sheath and nothing comparable to
a centrum is found, even when ossification occurs. In the formation of centra two
methods of extension of cartilage to the chordal region are known. In the elasmo-
branchs immigrating cells from the arches break through the elastica externa and
distribute themselves through the sheath, converting

it into cartilages. In other vertebrates (fig. 43) the dL
immigrating cells extend around the elastica externa j
so that the sheath eventually comes to lie inside the
centrum.

In many fishes and fossil amphibians
another element, the intercalare, enters into _
the composition of the neural arch on either ae
side. The intercalaria lie above and behind Fr

1G. 41.—Trunk vertebre
the neurapophyses and may expand dorsally of Rhynchobatus, after Dumé-
s th: t th h s 1 t d b th b Til. h, hemal process; i, in-
o that the arch is completed by them above. tercalary plate; il, ligament;
The dorsal root of the spinal nerve usually %, neural process; 7, rib; s,

‘s spinous process.

passes through the intercalare, the ventral
through the neurapophysis, but both roots may pass between them.
Similar intercalaria may occur in the hemal arch. In the trunk region
there may be separate elements of the centra; in each somite a trans-
verse cartilage (hypocentrum) across the under side of the neural
sheath, and a pleurocentrum on either side, behind the hypocentrum

(fig. 42).

Fic. 42.—Stegocephalan vertebre, after Zittel and Woodward. A, phyllospondylous;
B, rhachitomous of Chelydrosaurus; C, Callopterus; D, embolomerous of Eurycormus; hs,
hypocentrum arcuale; hp, hypocentrum pleurale; np, neurapophysis; 7s, neural spine.

Comparisons of different adult vertebre show that these vertebral elements
may combine in different ways, though they have not been recognized in the on-
togeny of the higher forms. Apparently the phyllospondylous vertebra of some
stegocephals (fig. 42) are formed of hypocentrum and neural arch, both contribu-
ting to the hollow transverse process. In others hemal arch and hypocentrum
unite, while the pleurocentra meet and fuse above the notochord. Expansion of
these makes the vertebral column look like a series of interposed triangles (fig. 42 C).

48 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

This is the rhachitomous or temnospondylous vertebra. Still farther expansion
of hypo- and pleurocentra causes the former to unite with the neural arch, while
the two pleurocentra meet below the notochord (fig. 42 D), the result being two rings
in each somite, the embolomerous vertebra, which occurs in some stegocephali,
some fossil ganoids and in the tail of Amia. Lastly these two rings (often called
centrum and intercentrum) may fuse, giving the typical centrum.

The neural and hemal spines which complete the arches are formed by seg-
mental chondrifications of the interspinous ligament which runs the length of the
body above and between the halves of the neural arches.

The vertebre are outlined at an early stage of the embryo and their
number is not subsequently increased. Consequently increase in

Fic. 43.—Earlier and later stages of development of a vertebra of Amblystoma. cc,
cartilage in centre of vertebra; ¢7, elastica interna; 7, incisure cutting through ic, intercentral
cartilage; , notochord; ms, notochordal sheath; v, vertebra (bone) black.

length of the vertebral column can only occur by growth of the vertebrx
themselves. When first formed each centrum encircles the notochord
and prevents its increase in diameter at this point, while between the
centra it can expand. Asa result the notochord soon resembles a string
of beads (moniliform) with intervertebral enlargements. Then, as
additions are made to the centra to increase their length, the new parts
must form around the intervertebral enlargements and in this way the
ends of the centra become cup-shaped and the amphiccelous condition
(fig. 43, I) is produced. In some urodeles this stage is followed by

SKELETON. 49

the deposition of cartilage in the cups (fig. 43, II) producing inter-
vertebral constrictions of the cord. As this progresses absorption of the
cartilage begins between the ends of the vertebra (ic) and continues
in such a way that the result is a ball of cartilage attached to the hinder
vertebra and a corresponding cup in the one in front; in other words.
an opisthoccelous condition. “

Several regions may be differentiated in the vertebral column, these
being the most numerous in the higher groups of vertebrates. These
are (x) the cervical, in the neck, with great reduction or even absence
of ribs; (2) the thoracic, following the cervical, with distinct ribs; (3)

Fic. 44.—Section through atlas Fic. 45.—Proatlas, atlas
(at) and axis (ax) of fowl, cut sur- and axisofalligator. a, atlas;
faces lined. e, epistropheus; /, facet e, epistropheus (axis); p, pro-
for articulation with skull; /, trans- atlas; 7, rib of third vertebra;
verse ligament. ra, re, ribs of altas and epis-

tropheus.

lumbar, without ribs; (4) sacral, including one or more vertebre with
which the.pelvis is connected; (5) caudal, the tail, behind the sacrum.
_sometimes the ribs extend back to the sacrum so that thoracic and
lumbar cannot be distinguished, all being then grouped as dorsal.
Then in the fishes and some higher vertebrates (snakes, whales, etc.)
sacral vertebra are not differentiated, and in the fishes there is no line
between cervicals and dorsals, so that only trunk or abdominal, and
caudal regions can be distinguished, the line being drawn (fishes) at the
point where hemal arches are transformed into ribs.

One or two of the anterior vertebra are modified in the higher
(tetrapodous) vertebrates and have received names. The first, which
immediately adjoins the skull, is the atlas. It bears on its anterior face
an articular surface which receives the one or two condyles of the cra-
nium. Inthe amniotes the second vertebra, the axis or epistropheus is
also specialized. On the anterior face of its centrum is a pivot (the

4

5° COMPARATIVE MORPHOLOGY OF VERTEBRATES.

dens or odontoid process) on which the atlas turns. Development
shows that this dens is the centrum of the atlas which has separated
from its own vertebra and has fused to that of the axis.

In a few reptiles and possibly some mammals a so-called proatlas occurs as a
plate or pair of plates (fig. 45) of bone between the atlas and the skull, in the posi-
tion of a neural arch. It is not certain whether this is the remains of a vertebra
which once occupied this position, or is a new formation. Nor has it been settled
whether the atlas of the amphibians is homologous with that of mammals.

In cyclostomes, fishes and aquatic urodeles the posterior end of the
vertebral column is concerned in the formation of the caudal fin,
which presents three modifications. The most primitive is the diphy-

Fic. 46.—Tails of fishes. A, young Amia; skeleton (homocercal); B, diphycercal; C,
heterocercal; D, homocercal; hk, hypurals; ”, notochord; s, spinal cord.

cercal tail in which the vertebral column runs straight to the end of the
body, the fin being developed symmetrically above and below it. This
is found in the young of all fishes and in the adult cyclostomes, dipnoans,
many crossopterygians and urodeles. In the heterocercal tail, which
occurs in elasmobranchs and ganoids, the axis bends abruptly upward
near the tip, and while retaining the caudal fin of the diphycercal stage,
has a second, smaller lobe developed below, giving the whole an unsym-
metrical appearance. In the homocercal tail, which occurs in Amia
and all teleosts since the cretaceous, there is the same upward bend to
the vertebral column, but symmetry is restored externally by the re-
duction of the neural arches and the development and fusion of the
hzmals into larger plates (hypurals), while the lower lobe of the tail
grows out to equal the other.

SKELETON. 51

CYCLOSTOMES have a persistent notochord, increasing in size with the
growth of the animal, and lacking constrictions since no centra are developed.
In the myxinoids there are neurapophyses and intercalaria developed in the caudal
region; in the lampreys they occur in the trunk as well.

FISHES.—In the elasmobranchs the typical vertebra are developed in cartilage,
with intercalaria in connection with the arches. Usually the centra undergo more
or less calcification (p. 43), the lime being either deposited in concentric rings
around the notochord (cyclospondylous vertebrz) or in radiating plates (astero-
spondylous). In the trunk region each centrum often bears a pair of transverse
processes with short ribs at their extremities. In a few forms (skates, etc.)
embolomerism (p. 48) occurs in the tail, and in the holocephali the centra are
replaced by numerous rings of cartilage. In skates and in Chimera there is a true
joint between the skull and the column, but in the sharks the anterior vertebra are
fused together and to the skull.

Fic. 47. Fic. 48.
Fic. 47.—Diagrammiatic sections of elasmobranch vertebre. A, B, cyclospondylous;
C, asterospondylous.
Fic. 48.—Cross-section of teleost vertebra; bone, black; cartilage, dotted.

The ganoids vary greatly in vertebral characters, some of the Chondrostei
having only cartilage and some of the fossil forms lacked centra. On the other
hand, nearly the whole vertebra is ossified in A mia and Lepidosteus, the latter having
opisthoceele vertebrae, a condition not reappearing until the amphibians, as all
other fishes in which centra are developed have amphiccelous vertebre.

As the name implies, ossification of vertebrae and other parts is common in
teleosts. The arches are almost always ossified, while the centra may be, or those
parts directly connected with the arches may remain cartilaginous while the rest
ossifies (fig. 48), so that the section presents a radiate figure as in the asterospondy-
lous sharks. Some teleosts have zygapophyses and a few genera have transverse
processes on some of the vertebre.

The dipnoans, so far as ossification of the vertebra is concerned, are ona par
with the cartilaginous ganoids. There are no centra and the notochord grows
throughout life.

AMPHIBIA, except the legless forms, have caudal, sacral, trunk, and a single
cervical vertebra, the sacrals being single except in a family of extinct anurans.
Zygapophyses and both kinds of transverse processes may be present.

52 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

The stegocephals had the greatest range of vertebral structure, rhachitomous,
embolomerous, and amphiccelous types occurring, the first two even in the same
individual. Phyllospondylous vertebre (fig. 42) are found only in the fossil
Branchiosauride.

The cacilians have a very large number (up to 275) of amphiccelous vertebra
in correlation with the snake-like body form. The perennibranchs, derotremes and
many salamandrina are amphiccelous; the rest of thé urodeles are opisthoccelous.

The anura, as a rule, have proccelous vertebre, but a few genera have them
opisthoccele. All recent species have eight presacral vertebre, but there were nine
in the tertiary forms. A striking feature is the
fusion, in the adult, of all of the caudal verte-
bre into the well-known rod, the coccyx or
urostyle.

REPTILES always have the vertebre ossi-
fied, although remnants of the notochord may
persist in the centra, of which all types, amphi-,
pro-, opisthoccelous and flat occur in the group.
In lizards, snakes and dinosaurs the articulation
between the successive vertebre is strengthened
by zygantra and zygosphenes, a cavity on one
vertebra into which a projection from the next

Fic. 49. Fic. 50.

Fic. 49.—Cervical vertebra of a bird showing the saddle-shaped articular surface (af)
on the centrum, c; cr, cervical rib; mc, neural canal; ms, neural spine; poz, prz, post- and
prezygapophyses.

Fic. 50.—Central view of synsacrum and pelvis of hawk (Bufeo). il, ilium; is,
ischium; ~, pubis; ~p, pectineal process; s, sacral ribs.

fits. In the existing species there are never more than two sacral vertebre, but
the pterosaurs had from three to seven, while in the dinosaurs there might be ten,
all being co-ossified when there were more than three.

Little is known of the theriomorph backbone, except that some had persistent
notochords, others amphiccelous centra. In the plesiosaurs they were flat, while
in the turtles the dorsals are fused and the neural spines are united with the neural
plates (p. 41). The other centra vary. Those of the rhynchocephais and most
dinosaurs are flat, while snakes and lizards, except the geckos have them proccelous.
In the earliest crocodiles they were amphiccelous, while later they are proccelous or
flat, and in the pterodactyls they are proccelous in front, amphiccelous in the tail.

BIRDS usually have saddle-shaped ends to the centra (the atlas proccelous).

SKELETON. 53

Several of the dorsals are usually fused for strength, but the first presacral is free.
A characteristic feature is the synsacrum, foreshadowed in the dinosaurs. As the
bird stands on two feet and holds the body obliquely, several of the dorsal and caudal
vertebra (up to 20) fuse with the sacrals into a common mass, a large proportion
also uniting with the pelvis. The true sacrals (three in ostriches, two elsewhere) lie
just behind the pits occupied by the kidneys and may be recognized by their lower
articulation to the pelvis. A few of the caudals behind the synsacrum are free, as
all were in Archeopteryx, but the others in recent birds are united into an upturned
bone, the pygostyle.

MAMMALS, except whales where the sacrum is lacking, have all the five verte-
bral regions differentiated. With four exceptions the cervicals are seven in number
(Manatus australis and Cholepus hofmanni, six; Bradypus torquotus, eight; B.
tridactylus, nine). The dorsals (thoracics plus lumbars) vary between fourteen in
armadillos and thirty in Hyrax, but usually are nineteen or twenty, the number
of thoracics usually increasing at the expense of the lumbars. There are primi-
tively two sacrals, but others may unite until they amount to nine or ten in some
edentates. Usually the centra are amphiplatyan, but in the cervicals of ungulates
opisthoceele vertebre are common. It is to be noted that the ‘transverse proc-
esses’ of the cervical vertebre are, as in birds, composed in part of reduced ribs, as
will be shown below.

Riss.

Two different structures are included under the common name of
rib, both connected at one end with a vertebra, the other supporting the
body walls around the viscera. In following forward the hemal arches
in the skeleton of a bony fish (fig. 39, A, B) it is seen that when the

Fic. 51.—Vertebre and ribs of (J) anterior and (7Z) posterior trunk region of Polypterus,
after Gegenbaur. #, pleural rib; 4, hemapophysial rib.
‘body cavity is reached the arch becomes incomplete below, the
two hzemapophyses separating and coming to lie just beneath the
peritoneum in the walls of the ccelom. Above, it is either united
directly to the centrum or is jointed to a small process of it. More
careful study shows that this fish rib (hemapophysial rib) lies in
the intersection of a myoseptum with the median partition of the
skeletogenous tissue (p. 38) and is medial to the hypaxial muscles.
In the higher vertebrates the rib is formed in the intersection of

54 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

the myosepta with the horizontal plate, and thus is lateral to the
hypaxial muscles and between them and the epaxial series. This
is the true or pleural rib. Any vertebra may bear ribs of either
kind (including hzemal arches) and the two kinds frequently coexist on
the same vertebra in the trunk of salmonids, clupeids and Polypterus,
and in the caudal region of urodeles and some reptiles. Their possible
occurrence in all parts of the body is explained by the existence of the
myosepta and other skeletogenous structures in all regions.

The hemapophysial ribs end freely below, never being connected with a sternum.
In some aberrant fishes they are lacking, while in the ostariophysi they play a part
in the ‘Weberian apparatus’ connecting the swim bladder with the ear (see ear).
The teleosts have, in addition, numerous rib-like structures which are not preformed
in cartilage (epineurals, epimerals, epipleurals) which are formed in the epaxial
or hypaxial regions or in the horizontal partition. ,

Fic. 52.—Front and side views of cervical vertebra of fowl, showing the cervical rib.
c, centrum; cs, spinal canal; d, diapophysis; p, parapophysis; 7, rib; va, vertebrarterial
canal; the arrow in the side view passes through the canal.

The typical rib (it is not certain whether this is the primitive form)
has two heads for articulation with the vertebra, a capitular head
connecting with the parapophysis, a tubercular head joining the
diapophysis. Between the two heads and the centrum is a space, the
vertebrarterial canal, through which the vertebral artery passes
(fig. 39, C.) The true ribs, which are preformed in cartilage, have
various extents in the different regions of the body. In the thoracic
region, where they have the greatest extension, the ribs have to allow
for changes in size of the contained cavity, and hence parts of them
are frequently left unossified, or at least they are jointed, the two parts
being called vertebral and sternal ribs.

In the cervical region the true ribs are usually greatly reduced and are lacking
in the turtles. In many reptiles they clearly show their nature, being short,
bicipital and with their heads articulated to dia- and parapophyses (fig. 45). In
the birds they may be recognized (fig. 52), their distal ends being bent inward to

SKELETON. 55

protect the carotid arteries. In the mammals they form the distal part of the
“transverse process’ of human anatomy, the vertebrarterial canal and the develop-
ment revealing their true nature.

The dorsal ribs are very short in amphibians, never extending far
from the backbone. They are bicipital in most forms, except the
anura where they form small projections on the ends of the transverse

Fic. 53.—Skeleton of trunk of common goose, Anser domesticus. c, cuneiform; ca,
carina; co, coracoid; f, furcula (clavicle); fe, femur; #, humerus; 7/, ilium; zs, ischium; mc,
metacarpals; , pubis; ph, phalanges; 7, radius; s, scaphoid; sc, scapula; sr, sternal rib; st,
sternum; #, uncinate process; u/, ulna; vr, vertebral rib; 2, 3, 4, digits.

processes. In the amphibia the vertebral artery is ventral to the par-
apophysis. In all other vertebrates with a sternum at least a part of the
dorsal ribs reach that structure, encircling the viscera like the hoops of a
barrel. Those ribs which do not reach the sternum are called false

ribs. In most reptiles and some birds.most of the thoracic ribs bear an
uncinate process directed upward.and backward (fig. 53), overlapping

56 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

the rib behind and strengthening the thorax. In the chelonia the ribs
are confined to the dorsal side of the body and are fused to the costal
plates (dermal skeleton) to form the carapace. Single- and double-
headed ribs often occur in the same individual of various groups, and
in the mammals the capitular head, instead of articulating with a
distinct parapophysis, may rest in a socket formed by two successive
vertebre.

Fic. 54.—Sacral vertebre, ribs and pelvis of Trionyx, obliquely from below. f, head and
trochanter of femur; #, ilium; zs, ischium; ~, pubis; sv, sacral ribs; sv, sacral vertebra.

The pelvis is never directly united to the sacrum, but sacral ribs
intervene. These are distinct in the reptiles (fig. 54), but are fused to
the transverse processes in other groups.

THE STERNUM (BREASTBONE).

The sternum includes the skeletal parts on the ventral side of the
body, which are closely connected with the shoulder girdle and, except
in the amphibia, with the ribs. The fact that it occurs only in verte-
brates with legs (it is lacking in snakes and cecilians) shows that it has
arisen in adaptation to terrestrial locomotion. In man it consists of
three parts, a manubrium in front, a middle piece (gladiolus), and a
xiphoid (ensiform) process behind, and these terms have been car-
ried into other groups.

In development the sternum arises in mammals by the formation of a longi-
tudinal bar of cartilage in the linea alba on either side, ventral (medial) to the ends

SKELETON. 57

of the ribs, eventually connecting them together (fig. 55). With continued growth
these bars of the two sides meet and fuse in the median line, forming a median plate,
the sternum. Later this separates from the ribs, and with the appearance of bone,
becomes a series of separate elements, the sternebre (fig. 57), alternating with the
ribs; by fusion of sternebre the parts in man arise.

In the amphibia the short ribs never extend to the sternum, but skeletal parts
occur near the mid-ventral line in a few forms, which may be ventral ribs as they
participate in the formation of the sternum. Nothing is known of a true sternum
in the stegocephals. In the urodeles it is a short cartilaginous plate, lying mostly
behind the girdle, with its sides grooved to receive the medial ends of the coracoids.

Fic. 55.—Development of sternum in 30 mm. human embryo, after Ruge. cl, lower end
of clavicle; 7, ribs; s, two halves of sternum; ss, suprasternalia.

In the toads and their allies (arcifera) it has hardly passed beyond the urodele
condition, but the hinder angles are produced into long processes. In the frogs
(firmisternia) it consists of a slender thread between the medial ends of the girdles
(epicoracoids), but in front it expands into an omosternum, ossified behind; while
behind the girdle it forms a broad xiphisternum, the anterior part of which is bone.

In the lizards the sternum is a large rhomboid plate, largely cartilag-
inous, sometimes perforated with two foramina and joined by a vary-
ing number of ribs (fig. 56). In the crocodilia there is an anterior
rhombic plate, joined by two pairs of ribs and followed by a second,
so-called abdominal sternum, connected with from five to seven pairs
of ribs. Ichthyosaurs, plesiosaurs and snakes have no sternum,
while it was imperfectly ossified in theriomorphs and dinosaurs.

In the birds (fig. 53) the sternum is ossified and at most is con-
nected with eight pairs of ribs. Behind it may be rounded, perforated,

58 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

notched, or prolonged into one or two long processes. In the ostriches
the ventral surface is smooth and this was formerly used as a character
separating these birds as a group of ratites, in contrast to all other
birds (carinate) which either use their wings in flight or in swimming
(penguins) and in which there is a necessity for strong wing muscles.
For the attachment of these the ventral surface of the sternum is de-
‘veloped into a strong projecting keel (carina). It is to be noted that
a similar keel is developed in the bats and pterodactyls.

Fic. 56.—Sternum, etc., of Fic. 57.—Sternum of guinea
Iguana tuberculata, after Blan- pig. sr, sternal rib; st, sterne-
chard. c, coracoid; cl, clavicle; bre; ur, vertebral rib, x, xiphi-
e, episternum; #, humerus; pc, sternum. j

procoracoid; x, xiphisternum.

In the mammals the number of ribs connected with the sternum
is greater than in the lower classes. The sternebre may remain dis-
tinct throughout life (fig. 57) or, as in man, they may fuse into fewer
elements, the xiphoid process being unconnected with the ribs. In the
edentates and rodents elements known as ossa suprasternalia and pro-
sternum occasionally occur in front of the sternum, the significance of
which is unknown. It is possible that traces of them persist in the
higher orders, even in man (fig. 55).

SKELETON. 59
EPISTERNUM (INTERCLAVICLE).

In stegocephals and the oldest rhynchocephals there is a median
bone on the ventral surface, called the episternum (fig. 58). It
is rhomboid in front and may have a long posterior process, the medial
ends of the clavicles lying ventral to the broad anterior end. This
element is regarded as homologous with a T-shaped membrane bone
which occupies a similar position in lizards (fig. 56) and crocodilians,
where it acts as a brace between the shoulders. It arises by two centres

Fic. 58.-— Shoulder girdles of (A) Melanerpeton and (B) diagram of Branchiosaurus, after
Credner, the determination of elements after Woodward. cl, clavicle; co, coracoid; e,
episternum; s, scapula.

of ossification in membrane and hence cannot be the same as the su-
prasternalia of mammals. An episternum also occurs in theriomorphs,
pythonomorphs, ichthyosaurs, and plesiosaurs, and possibly the
entoplastron of the chelonians (fig. 34, p. 42) is the same structure.
It has not been recognized in birds, but it reappears in the monotremes
among mammals (fig. 113), with nearly the same relations as in the
lacertilians.

THE SKULL.

The skull is a complex structure and the last word concerning its
composition has yet to be said. A century ago Oken pointed out that
a series of parts could be distinguished in the mammalian skull, each
of which somewhat resemble a vertebra in its general relations, and
thus laid a foundation for a ‘vertebral theory of the skull’ which was
farther developed by Owen. Huxley showed that these were superficial
resemblances, that the three or four vertebre they would recognize were
nothing of the sort, and that the skull shows no real metamerism
except in the occipital region and in the visceral arches. :

In its development the skull, like the rest of the skeleton, passes
through two, and in the bony vertebrates, three stages: membranous,

60 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

cartilaginous and osseous, and in the early stages there is no trace of seg-
mentation or of vertebre, the Okenian segments only appearing with
the appearance of bone. The skull may be divided into two portions,
a cranium, composed of a case for the brain, and sense capsules en-
closing the organs of special sense (ears, eyes and nose); and a visceral
skeleton, more or less intimately related to the anterior end of the
digestive tract. ,

Development of the Skull.

Little is known in detail of the development of themembranous
skull save that it envelops the brain and sense organs, extends into
the visceral region, and that it affords the substance in which the second,
or cartilaginous, skull is formed.

Fic. 59.—Early chondrocranium of Acanthias, after Sewertzoff. (The brain in outline.)
als, alisphenoid cartilage; ch, anterior end of notochord; #, hyoid arch; ma, mandibular
arch, not yet divided into pterygoquadrate and Meckelian; oc, otic capsule; ¢, trabecula;
1-5, branchial arches. :

The cartilaginous envelope of the brain and sense organs is called
the chondrocranium. The notochord extends forward beneath
the brain as far as the infundibulum and a horizontal cartilage plate
forms on either side of it. These parachordal plates extend later-
ally as far as the ears, forward as far as the end of the notochord and
back to the exit of the tenth nerve. A little later a cartilaginous otic
capsule forms around each ear and joins the parachordals, thus form-
ing a trough in which the posterior part of the brain lies, its floor formed
of parachordals and notochord (basilar plate) and its sides of the
sense capsules. .

From this posterior part two cartilages extend forward on either
side, forming a somewhat similar trough for the anterior part of the

SKELETON. 61

brain;. the lower of these, the trabecule cranii, join the anterior
margin of the basal plate while the dorsal bars, the ale temporales
or alisphenoid cartilages are eventually connected with the anterior
wall of the otic capsules. In most vertebrates the trabecule and
alisphenoids develop as a continuum, but in some elasmobranchs they
are at first distinct (fig. 59). The two
trabeculae unite in front to form a
median ethmoid plate beneath the
olfactory lobes, beyond which they
diverge as two horns, the cornua tra-
beculz, ventral to the nasal organs.
The floor of the trough is formed by
the ethmoid plate in front, while behind
it is usually of membrane, but in the
elasmobranchs cartilage gradually ex-
tends from one trabecula to the other,
closing last below the infundibulum
and hypophysis, these lying for a time
in an opening (fenestra, later fossa
hypophyseos), and after the closure,
in a pocket in the floor of the chon-
drocranium, one of the cranial land- -
marks, the sella turcica. eh TH

as Fic. 60.—Early (platybasic) chon-
In the elasmobranchs and amphibians drocranium of ‘an_ elasmobranch,

the trabecule are widely separated until they ‘t:aightened out. Compare with fig.
othe cthmota slate: a condition corel 59- als, alisphenoid; cir, cornua tra-
reach the ethmoid plate, a condition correla. pecule; ep, ethmoid plate; fhyp fenes-
ted with the anterior extension of the brain. tra pepe: oc, otic papsules ov,
ee i : occipital vertebre; , notochord; fc,
This is the platybasic chondrocranium. In pargehonial plate ds uabecdle, ’
the other classes the brain does not extend
so far forward and the two trabecule meet just in front of the hypophysis (fig.
62) to continue forward as a trabecula communis to the ethmoid region. The
trabecula communis is usually compressed between the eyes to a vertical interor-
bital septum. This represents the tropibasic chondrocranium.

In the more primitive vertebrates the trough is converted into a
tube around the brain by the extension of cartilages between the ali-
sphenoid cartilages and the otic capsules of the two sides dorsal to the
brain. This roof or tegmen cranii is usually incomplete, having one
or more gaps or fontanelles, closed only by membrane. In the higher
vertebrates the cartilage roof is at most restricted to a mere arch, the

synotic tectum, between the otic capsules of the two sides. Later

ep ot

62 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

a pair of nasal capsules develop around the olfactory organs. These
are usually fenestrated and become united to the cornua, alisphenoids,
and ethmoid plate. In a similar way a sclera (sclerotic coat) forms

Fic. 61.—Diagram of early elasmobranch chondrocranium in side view, the brain out-
lined behind. al, alisphenoid plate; bp, basal plate; ge, gill clefts; k, hyoid; hm, hyomandib-
ular; J, upper labials; /, lower labials; nc, nasal capsule; oc, otic capsule; ov, occipital
vertebre; ptgg, pterygoquadrate; s/, suspensory ligaments; sp, spiracle; é7, trabecule; v,
vertebre; [-V II, visceral arches; 1-5 branchial arches.

around each eye, but since the eye must move, this sense capsule never
unites with the rest of the cranium. Behind the otic capsules a vary-
ing number of (four in some sharks and most teleosts, in others three,

BE ee A = aop
Fic. 62.—Ventral view of (tropibasic) cranium of Lacerta agilis after Gaupp. op,
antorbital plate; bpt, basipterygoid process; c, entrance to nasal conch; col, columella;
Sh, fenestra hypophyseos; fpo, post-optic foramen; na, nasal capsule; nf, notochord; of,
optic foramen; pa, prominence of posterior ampulla; pf, pterygoid; g, articular process of

quadrate; ¢c, trabecula communis; tmg, tenia marginalis; é, trabecula; VII, XII
seventh and twelfth nerves.

in amphibia two) occipital vertebrz are developed, which later fuse
with the rest of the chondrocranium. They alternate with myotomes
and nerves in this region as do the vertebre of the vertebral column.

SKELETON. 63

The cartilaginous visceral skeleton arises in the pharyngeal region
which is weakened by the presence of the gill clefts. It consists of a
series of pairs of bars, the visceral arches (fig. 61, J-VIJ), lying in
the septa between the clefts, the bars of a pair being connected below the
pharynx. Each bar, at first, is a continuous structure, but to allow of
changes of size in the pharynx, each becomes divided into separate parts,
while the arches become connected in the mid-ventral line by unpaired
elements, the copula. The two anterior arches are specialized and
have received special names, the first being the mandibular, the second
the hyoid arch, the others, in the region of the functional gills, being
called collectively gill or branchial arches. The number of these
last varies with the number of gill clefts, there being seven in the primi-
tive sharks, a smaller number in the higher groups, in which, with the
loss of branchial respiration, their form and functions may be altered.
At first all are clearly in the head region, but by the unequal growth of
cranium and pharynx the gill arches are carried back. All of the
arches are cartilaginous at first.

The mandibular arch lies in the region of the fifth nerve, behind the
mouth and between it and the first visceral cleft or pocket, the spiracle
or Eustachian tube. The arch is divided into dorsal and ventral
halves (fig. 61, J), known respectively as the pterygoquadrate (pala-
toquadrate pigq), and Meckelian cartilages (m). In the elasmo-
branchs and chondrostei the pterygoquadrate forms the upper jaw,
lying parallel to and joined to the cranium by ligaments or (chimeroids)
by continuous growth. With the appearance of bone a new upper jawis
formed, as described below, and the pterygoquadrate becomes more or
less reduced, and ossifies as two or more bones with greatly modified
functions. Meckel’s cartilage is the lower jaw of the lower vertebrates,
while in the higher it forms the axis around which the membrane bones
of the definitive jaw are arranged.

The hyoid arch lies between the spiracle and the first true gill cleft,
in the region of the seventh nerve. It divides into an upper element
the hyomandibular cartilage (fig. 61, hm), and a ventral portion, the
hyoid proper, which may subdivide into several parts (infra). In the
lower elasmobranchs the hyomandibular and the rest of the hyoid arch
are closely connected, but in the higher fishes the hyomandibular be-
comes more separated from the ventral portion and tends to intervene
between the mandibular arch and the cranium, becoming a suspensor
of the jaws (fig. 63). Still higher it loses its suspensorial functions,

64. -COMPARATIVE MORPHOLOGY OF VERTEBRATES.

becomes greatly reduced, and apparently is subsidiary to the sense of
hearing (see auditory ossicles), or it may be lost, the matter not
being decided. The hyoid proper becomes more or less intimately con-
nected with the arches behind and is largely concerned in affording a
support for the tongue.

The branchial arches are all similar to each other in the lower
vertebrates, but with the loss of branchial respiration in the higher

Fic. 63.—Ventral view of cranium and visceral arches of skate (Raia) after Gegenbaur.
cp, copula; h, hyoid; hm, hyomandibular; Ja, upper labials; mk, Meckelian cartilage;
nc, nasal capsule; pg, pterygoquadrate; 7, rostrum.

groups, they tend to become reduced, the reduction beginning behind.
Some may entirely disappear, others give rise to the laryngeal cartilages
(see respiration) and the first may fuse with the hyoid. The first arch
is in the region of the ninth nerve; the others in that supplied by the
tenth.

SKELETON. 65

The elements of the branchial arches have the names, beginning above, pharyn-
gobranchial, epibranchial, ceratobranchial and hypobranchial, the copul#
being the basibranchials. The elements of the hyoid are correspondingly, epi-,
cerato-, and hypohyal. These parts lie in the medial ends of the gill septa, medial
to the aortic arches.

Other cartilages, which seem to be of less morphological importance, occur in
the same region. Among these are the labial cartilages (fig. 67, J), usually two
above and one below, which lie in front (outside) of the cartilages of the mandibular
arch of sharks, and in a modified form as high as some of the ganoids. By some they
are regarded as remnants of visceral arches of the preoral region. In the branchial

2
3
\4
“5
6

Fic. 64.—Branchial arches of (A) Heptanchus; (B), Chlamydoselache; and (C) Cestracion;
A and C after Gegenbaur, B after Garman. , ceratobranchial; e, epibranchial; h, hyoid;
hb, hyobranchial; he, hyoid copula; cbr, cardiobranchial (posterior copula); ~, pharyngo-
branchial; 1-7, branchial arches.

region of the elasmobranchs a variable number of extrabranchial cartilages may
occur, small bars external and parallel to the upper and lower ends of the gill arches.

The foregoing sketch of the chondrocranium is based on conditions in the
gnathostomes, and ignores the peculiarities of the cyclostomes which are summar-
ized below.

In the elasmobranchs and cyclostomes the skull is cartilaginous
throughout life, or at most is calcified cartilage, without sharp division
into separate elements. In the higher vertebrates the cartilage is sup-
plemented or almost entirely replaced by bone which may be of the
two kinds, cartilage bone and membrane bone (p. 42), the distinctions
between which must constantly be kept in mind in tracing homol-
ogies in the different classes. The membrane bones are usually
derivatives of the deeper or dentinal layer of scales or teeth which have
fused together (fig. 65) and have sunk to a deeper position, coming

5

66 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

into close connection with the elements derived from the cartilage
skull, in some cases replacing considerable of it. The cartilage bones
arise by an ossification of the cartilage. Even in the sturgeons the
chondrocranium is complete, the membrane bones being superficial
and not intimately connected with the deeper parts.

The names of the bones are largely based on the term-
inology of human anatomy. In many cases what appears
as a single bone in the human skull is represented by
several bones in the young and in the lower vertebrates.
In these cases the bones in the lower forms are usually

Fic. 65.—Vomer of given names which indicate their relation to the human
25mm. Amblystomalarva, bones or to the part or region in which they occur.
after Hertwig, showing .
the bone developed by the Dermal bones are apparently the older, phylogenetically,
fusion of the bases of but for convenience the cartilage bones are considered
teeth. first.

The chondrocranium shows several centres of ossification, but only
those giving rise to distinct bones are considered here.‘ The bones of

Fic. 66.—Ventral view of schematic skull, chondrocranium dotted, cartilage bones
with lines and dots. basioc, basioccipital; basisph, basisphenoid; als, alisphenoid; exoc,
exoccipital; ors, orbitosphenoid; presph, presphenoid; premax, premaxilla; qu, quadrate:
quju, quadratojugal; squamos, squamosal; zygom, zygomatic; other names in full. ,

the cartilaginous brain case may be arranged in four groups, beginning

behind and called respectively occipitalia, sphenoidalia and ethmoi-

? Basi- and presphenoid, for example, arise each from two centres, but in all vertebrates
the resulting bones are unpaired.

SKELETON. 67

dalia, there being two sets of sphenoidalia. The occipitalia arise in the
occipital vertebre and in the basilar plate. Of these there are four
(figs. 66, 67): A supraoccipital above, an exoccipital on either side,
and a basioccipital below, the latter extending forward into the basilar
plate. These four form a ring around a central opening, the foramen
magnum, through which the spinal cord connects with the brain.

Just in front of the basioccipital the basilar plate ossifies to form the
basisphenoid, which extends forward to the sella turcica, and there is
succeeded by the presphenoid, arising from the trabecule, and ex-
tending forward to the ethmoid plate. On either side a bone, the
alisphenoid, ossifies in the cartilage of the same name, and comes into
close relation with the basisphenoid. Father in front a second element,
the orbitosphenoid, arises in the alisphenoid cartilage and comes into
relation to the presphenoid. The alisphenoid bone is just in front of
the otic capsule, but there is always a large gap (sphenoidal fissure,
foramen lacerum anterior) between it and the orbitosphenoid, through
which the third, fourth, and sixth and the ophthalmic branch of the
fifth nerve pass, the rest of the fifth nerve passing through the alisphe-
noid bone. The optic nerve usually perforates the orbitosphenoid,
but may pass through a notch in its margin.

The ethmoid plate may ossify into a median mesethmoid bone
bounded on either side by an ectethmoid and in some there may be
added other bones included among the ‘turbinal bones.’ The ol-
factory nerves pass on either side of the mesethmoid, the ectethmoids
(below) in the mammals developing as perforated plates (cribiform
plate).

A series of otic or petrosal bones is developed in each otic cap-
sule. The most constant of these are a prootic in front, an opisthotic
behind, the two usually meeting below (fig. 66), and between them,
above, an epiotic, concerning which more evidence is needed. In
the teleosts and some other forms the lateral wall of the otic capsule
may develop in addition a sphenotic in front and a pterotic behind,
the latter overlying the horizontal semicircular canal of the ear. In the
higher groups the various otic bones fuse in the adult to a single petro-
sal bone, which is wedged in between the lateral parts of the basi-
occipital and basisphenoid. :

In the stegocephals, reptiles and birds the sclera often gives rise to
a ring of sclerotic bones (fig. 67), which, however, never unite with
the other bones of the skull. The nasal capsules often develop a

68 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

lateral ethmoid on the upper wall, and turbinals on the medial and
lateral walls.

To place these bones in the terms of human anatomy: the four occipitalia fuse
to form the single occipital of man; the six sphenoidalia similarly unite to form
the single sphenoid, the alisphenoids forming the greater wings, the orbitosphenoids
the lesser wings, while the ethmoidalia fuse to the ethmoid.

In all bony vertebrates the cranial walls are completed dorsally
by membrane bones, which in the lower fishes overly the tegmen cranii,
while in the higher groups they replace it, the cartilage failing to develop

Fic. 67.—Dorsal view of schematic skull, the chondrocranium dotted, cartilage bones
with lines and dots. premax, premaxilla; pref, prefrontal; postfr, postfrontal; postor,
postorbital; sqwamos, squamosal; guju, quadratojugal; qu, quadrate; inp, interparietal;
exoc,"exoccipital; supratem, supratemporal; other names in full.
in the roof. The number of these elements varies between wide limits,
the following being the most constant.

Beginning in front (fig. 67), there are, on either side of the median
line a pair of nasal bones covering the olfactory region; a pair of
frontals between the orbits; a pair of parietals at the level of the
otic capsules, between which there is frequently a parietal foramen
for the connexion of the parietal eye with the brain; and an inter-
parietal, arising from paired centres, between the parietals and the
supraoccipital.

In the higher vertebrates (where the interparietal frequently fuses

SKELETON. 69

with the supraoccipital) these are practically all of the membrane
bones in the cranial roof of the adult. In the lower groups there are
several other bones, some of which may appear in the development of
the higher forms. Thus lateral to each parietal there may be a su-
pratemporal; behind the orbit a postfrontal may articulate with the
frontal, and lateral to this, and forming the rest of the posterior wall of
the orbit a postorbital. Occasionally the superior (or medial) wall of
the orbit is formed by one or more supraorbital bones, which, when
present, exclude the frontal from the orbit. The orbit may be bounded
in front by a prefrontal bone, adjoining the antero-lateral margin of
the frontal, and lateral to this there is usually a lacrimal bone. Less
constant are an intertemporal bone dorsal (medial) to the alisphenoid,
a pair of postparietal bones between parietals and interparietals and
a so-called ‘epiotic’ above each otic capsule, which, since it is not a
cartilage bone and has no relation to the true epiotic, is better called
the tabulare.

In the ichthyopsida, and to a less extent in the sauropsida the
basilar plate and trabecule may fail to ossify. In these cases the floor
of the cranium (roof of the mouth) is formed by a membrane bone,
the parasphenoid, which lies ventral to the cartilage in the sphenoid
region. Farther forward, in the nasal region, are an additional pair
of membrane bones, the vomers. Both vomers and parasphenoids
frequently bear teeth and their origin by fusion of the bases of teeth
is clearly seen in developing amphibia (fig. 65).

Some think the parasphenoid the homologue of the mammalian vomer, calling
the vomers of the non-mammals prevomers, their representatives being sought in
the ‘dumb-bell bone’ of the monotremes. More evidence is needed on these
points.

With the appearance of bone the mandibular arch undergoes the
greatest modifications of all the visceral arches. Its pterygoquadrate
half loses its function as the upper jaw and becomes more closely
connected with the cranium in front, its median portion disappearing,
even as cartilage, and being replaced by a pair of membrane bones,
the palatines (fig. 66), which lie between the pre- or parasphenoid and
the vomers. The rest of the arch ossifies as two bones on either side,
an anterior pterygoid and a posterior quadrate, which now becomes
the suspensor of the lower jaw. In the teleosts and reptiles there are
a series of pterygoid bones.

A second arch of membrane bones develops outside of the pterygo-

7° COMPARATIVE MORPHOLOGY OF VERTEBRATES.

quadrate to form the functional upper jaw (figs. 66, 67) in all bony
vertebrates. In its fullest development it consists of bones on either
side, beginning behind with a squamosal, which overlies the quadrate,
and followed by a quadratojugal, a zygomatic (malar or jugal),
and a maxillary, which joins the premaxillary, the latter forming the

Fic. 68.—Dorsal and ventral views of skull of young Sphenodon, after Howes and
Swinnerton. Explanation of letters used in figures of skulls (figs. 68 to 105) unless other-
wise stated. an, angulare; ao, antorbital; ap, antorbital process; ar, articulare; as, ali-
sphenoid; 6, basale; bb, basibranchial; bh, basihyal; bo, basioccipital; bs, basisphenoid;
cb, ceratobranchial; ch, ceratohyal; cl, columella; co, coronoid; cp, copula; cr, cranial rib;
cr. eth, cribiform plate of ethmoid; d, dentary; de, dermal ethmoid; dee, dermal ectethmoid;
eb, epibranchial; ee, ectethmoid; eh, epihyal; enp, entopterygoid; eo, exoccipital; ep,
ectoptery goid; epo, epiotic; eth, ethmoid; ethpp; perpendicular plate of ethmoid; es, extra-
scapular; exb, extrabranchial; /, frontal; /p, frontoparietal; g, goniale; k, hyoid; hb, hypohyal;
hm, hyomandibular; hr, hyoid rays; 7, incus; if, infratemporal fossa; io, interoperculum; if,
interparietal; j, jugal; /, lacrimal; Ja, labial; md, mandibular; me, mesethmoid; mk, Meck-
elian; ml, malleus; mm, mentomeckelian; mpt, mspt, mesopterygoid; mtp, metapterygoid;
mx, maxillary; mxp, maxillopalatine; mxt, maxilloturbinal; n, nasal; na, neural arch; nc,
nasal capsule; 70, notochord; 9, occipital; oc, occipital condyle; 00, opisthotic; op, operculare;
os, orbitosphenoid; of, otic bones; p, parietal; pd, predentary; pe, petrosal; pf, postfrontal;
pl, palatine; pm, premaxillary; po, preoperculare; poo, postorbital; pg, pterygoquadrate;
prf, prefrontal; pro, preorbital; prot, prootic; prs, presphenoid; ps, parasphenoid; pt, ptery-
goid; ptc, pterygoid cartilage; pio, pterotic; g, quadrate; gj, quadratojugal; 7, rostral; rm,
rostrum, sa, suprangulare; sbo, suborbital; sc, sagittal crest; scl, sclerotic; se, sphenethmoid;
sf, supratemporal fossa; sh, stylohyal; so, supraoccipital; sop, subopereulare; sor, supra-
orbital; sp, sphenoid; spht, sphenoturbinal; spi, splenial; spo, sphenotic; spt, supratemporal;
sq, squamosal; ssc, suprascapular; st, stapes; sy, symplectic; ¢, temporal; ér, transversum;
tu, turbinal; ty, tympanic; v, vomer; vp, vomeropalatine.

tip of the jaw and meeting its fellow of the opposite side. Of these
only the maxillary and premaxillary bear teeth.

In the lower vertebrates the roof of the skull is continuous, its only
openings being those for the nares and the orbits. In the higher

SKELETON. 71

groups vacuities or fossz appear in the postero-lateral parts, these being
bounded by bars or arcades of bone. At most there may be three of
these fosse. The more lateral of these, the infratemporal fossa
(fig. 68), is bounded laterally by the zygomatic and quadratojugal,
while on the inner side it is separated from the supratemporal fossa
by a -squamoso-postorbital arcade. The posttemporal fossa lies
between parietal, supratemporal and occipital bones. Occasionally
only the infratemporal fossa is present, or, by disappearance of the inter-
vening arcade, infra~ and supratemporal fosse may unite in a single
temporal fossa. Lastly, by the breaking down of the zygomatic-
postorbital bar, the temporal fossa and the orbit may unite.

One or another of these bones may disappear in some groups, either by fusion
or by complete dropping out. Occasionally they may obtain different connexions
and relations as in the case of the quadrate in mammals (see ear bones) so that the
homologies are traced with difficulty. The complexity is increased by the fusion of
membrane bones and cartilage bones and by the union of cranial bones with those
of the visceral arches.

In the lower jaw there are no such extensive modifications as in the
upper. At most Meckel’s cartilage gives rise by ossification to two
bones in either half. Behind, at the articulation of the jaw with the
quadrate, there is an artic-
ular bone, while at the
tip, at either side of the
union (symphysis) of the
two halves of the jaw, there
is rarely a mento-Meck- Fc. 69.—Reconstruction of developing jaw of Scele-

lian hone. ‘ihe ast ot porus, cartilage dotted; letters as in fig. 68.
Meckel’s cartilage forms an axis around which the membrane bones
which form the definitive jaw are arranged. These are, at most, as
follows: (1) a dentary which surrounds the Meckelian in front and
usually bears teeth; (2) a splenial on the inner side, behind the
dentary and frequently bearing teeth; (3) an angulare on the lower
side, usually extending back to the hind end of the jaw; (4) a supran-
gulare on the outer posterior part of the jaw; (5) a coronoid on the
upper side, affording attachment for the muscles which close the
jaws; and (6) a goniale (dermarticulare) on the medial and ven-
tral sides of the articulare, with which it usually fuses. This whole
series is present in few vertebrates, dentary, splenial and angulare
being the most constant.

72 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

TABLE OF THE PRINCIPAL CRANIAL BONES.

¢ Chondrocranium

Cranium
Membrane bones
( Maxillary arch
Mandibular arch J
Visceral
skeleton

Hyoid arch

Gill arches

Notochord
parachordals

Trabecule

Ethmoid
plate

Basi-, ex-, and supra-.

occipitals

Basi- and ali-, pre- and
orbitosphenoids

Mes- and ectethmoids

{ Pro-, epi-, opisth-, pter-

Otic es sphenatics (petro-

Sense cap- { Optic
Nasal

sules

Lateral line

Membrane bones
Pterygoquadrate
cartilage
Membrane

Meckel’s cartilage

Membrane

Gilt arches

sal)

(Sclerotics)
f Lateral ethmoid, tur-
\ binals

Parietals, frontals, na-
sals, pre- and post-
frontals, supra- and
postorbitals

Lacrimals, infraorbitals
Premaxillary, mazxilla-
ry, zygomatic, quad-
tatojugal, squamosal
Pterygoid (ect-, ent-,
epi-, mesopterygoids),
quadrate (incus)
Palatines, vomers
Articulare (malleus),
mento-Meckelian

ue

Dentary, splenial, coro-
noid, angulare (tym-
panic), suprangulare,
goniale

Hyomandibulare (sta-
pes) symplectic, inter-
hyal, epi-, cerato-,
hypo-, and _basihyal
(corpus, copula) (col-
umella) (lesser cornua)
Pharyngo-, epi-, cerato-,
hypo-, basi-, hypohyal,
| (copula, greater cornua)

_——"

SKELETON. 73

In the hyoid and branchial arches ossification occurs to a greater or
less extent, the resulting cartilage bones having the same names as the
corresponding cartilages. There are never any membrane bones in
this region. In the teleosts the hyomandibular ossifies as two bones, a
dorsal hyomandibular and a lower symplectic which connects with the
quadrate. There is, however, a considerable amount of union between
the various arches in the adults of all tetrapoda, where the branchial
respiration is lost and the arches have to assume other functions than
the support of gills. os
The mode of suspension of the K 7
jaws varies. In a few elasmobranchs L AN a
the pterygoquadrate articulates di- \ : Mf Me [
rectly with the cranium (amphistylic); ; Ct:
in others it is suspended by ligament |
and by the interposition of the
hyomandibular between the otic cap-
sule and the hinder end of the jaw
(hyostylic); while in all groups above
the fishes the pterygoquadrate is more |
or less completely fused with the N
cranium (autostylic).

The ear bones or ossicula audi-
tus are best treated together here, }
although their consideration requires — yg. 49 Diagram of the middle
the mention of structures not yet de- eat of a lizard, after Versluys. a,

3 * articulare; ¢c, columella; ec, extracolu-
scribed. The ear bones occur only mella; h, hyoid; ie, inner ear; mpt,
in the tetrapoda; they present several ee eae tee
modifications not readily homologized s, stapes; #, tympanic cavity; tm,
with each other, though they all have Ee eee
the same function of conveying sound waves across the tympanum
to the inner ear. In all there is an opening, the fenestra vestibuli
(f. ovale) in the lateral wall of the otic capsule, which is occu-
pied by a movable bone, the stapes, of uncertain homologies, but
probably representing the hyomandibular of the fishes, which otherwise
is lacking in all tetrapoda. This view is the more probable since in
some vertebrates the stapes is connected developmentally with the rest
of the hyoid arch.

In urodeles and cecilians a slender process extends from the quad-
rate across the poorly developed tympanic cavity to articulate with the

74 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

stapes (fig. 82). In the anurans there is no connection of quadrate
with stapes, but there is a slender rod, the columella, extending from
the tympanic membrane to the stapes. This columella arises behind
the tympanic cavity but with growth is included in it, so that in the
adult it appears to run directly through it. In the sauropsida the
relations are much as in the anura, but when ossification sets in, the
columella may form several elements. In development the columella
in these forms is directly connected with the hyoid arch.

In the mammals there is
a chain of three bones to carry
the sound waves across the
tympanic cavity. In the
fenestra vestibuliis the stapes,
which connects with an incus
and lastly comes the malleus,
which has two long processes,
a manubrium which is in-
serted in the tympanic mem-
brane, and a processus an-

extends into the petrotym-
2 panic (Glaserian) fissure

_ of the temporal bone. That
ae these parts are not to be com-

Fic. 71.—Diagram of ear bones of embryo pig, pared to the columella of the

the tympanic cavity laid open. gg, goniale; 7, sauropsida and anura_ is
incus; 4j, lower jaw; m, malleus; mk, Meckel’s
cartilage; mm, manubrium of malleus; s, stapes; shown by the fact that they

sp squamostl: zygomatic. Theoutinesof tbe invade the tympanic cavity
dotted. from in front and that they
are in front of the chorda

tympani nerve,-the columella of the non-mammals lying behind it.
The homologies of these parts seem clear. In development the
malleus is the posterior end of Meckel’s cartilage, being in the position of
the articulare of lower groups. It articulatés with the incus, which in
turn at first articulates with the wall of the otic capsule, as well as with
the stapes, and thus corresponds with the quadrate. The stapes is
apparently the same throughout the whole of the tetrapoda. It is to be
noted that many paleontologists deny the homologies recognized here,
think that in the mammals the quadrate has been lost in the glenoid

terior (Folian process) which —

SKELETON. 75

' fossa, and find the malleus and incus in the columella. For this they
have no evidence except comparisons with certain theriomorph reptiles.
The literature, which is extensive, should be consulted for details.

The Skull in the Different Classes.

CYCLOSTOMES have only the cartilage skull, and this can be homologized
only in part with that of other vertebrates; indeed the skulls of the two groups of
cyclostomes are not easily compared. The peculiarities are in part due to the
development of a suctorial mouth with its necessary framework. The chondro-
cranium of the Ammoccete stage of Petromyzon
is readily understood. Parachordals, otic capsules
and trabeculae (fig. 72) are normal, but a pair of
ventral horns are problematical. Their position
in front of and below the otic capsule renders
doubtful the interpretation of hyoid or quadrate
sometimes given them.

The adult Petromyzon has a typical brain
trough, roofed by a slender synotic tectum and
fibrous tissue and closed in front by the unpaired
nasal capsule, bound to the rest by fibrous tissue.
The cranium is continued forward by a large plate
(mesethmoid?) lying dorsal to the mouth, this
part being roofed by two ‘dorsal cartilages,’ the
anterior articulating with the annular cartilage Hie 4 — Barly -Gidadios
supporting the mouth. A subocular bar extends cranium of Ammoceete stage
‘forward from each otic region and an elongate of Pelromyzon, after Schneider.
3 i h, hyoid; nc, notochord; oc, otic
lingual cartilage extends from the mouth back capsule; ¢r, trabecule. :
to the gill region. Several other elements occur,
the names and positions of which may be seen from the figures.

The myxinoid skull, the development of which is unknown, is readily inter-
preted so far as basilar plate, trabecule and otic capsules are concerned. The large
nasal capsule is continued forward by a latticed framework for the naso-hypo-
physial canal and a bar (pterygoquadrate) joins the trabecula of either side and in
front is continued in a cornual cartilage. The lingual cartilage is enormous (is it
the lower jaw as has been suggested ?), is divided into three segments and bears a
dental plate with teeth at its tip. There are cartilage axes to the tentacles around
the mouth.

The branchial skeleton of the lampreys consists of a gill basket of continuous
cartilage with fenestre for the gills and above and below them as well. It cannot
be homologized with the branchial skeleton of other vertebrates as it lies imme-
diately beneath the skin and is lateral to gill pouches and aortic arches. It is more
easily compared to the extrabranchials (p. 65) of. the elasmobranchs. The
branchial apparatus of the myxinoids is reduced, consisting of two true gill arches,
in front of which is another arch, usually interpreted as a hyoid.

76 COMPARATIVE MORPHOLOGY OF VEGTEBRATES.

ELASMOBRANCHS have a nearly typical chondrocranium which is never
divided into separate elements and is never ossified. The floor is complete, the hypo-
physis resting in a sella turcica. Above there is an anterior fontanelle, closed by
membrane and a posterior fontanelle may occur. The occipital region typically

“tf

le

Idm ly eb

Fic. 73.—Ventral and lateral views of the skull of lamprey (Petromyzon marinus), after
Parker. ad, anterior dorsal cartilage, bb, branchial basket; gc, gill cleft; Jc, labial carti-

lage; Jdm, lateral distal mandibular; /g, lingual cartilage; nc, nasal capsule; oc, otic capsule;
on, optic nerve; pc, pericardial cartilage; pd, posterior dorsal cartilage.

ec ds Pg ho Br’ bre b

Fic. 74.—Side view of cranium of Bdellostoma, after Ayers and Jackson. 5, basal

plate; br, branchial basket; c, cornual cartilage; d, dental plate; #, hyoid; J, lateral labial

cartilage; x, nasal tube; nc, notochord; 9, otic capsule, oc, olfactory capsule; pg, pterygo-
quadrate bar; sp, suprapharyngeal plate.

articulates with the vertebral column by a pair of prominences, the, occipital con-
dyles, but in most species this joint is not functional, the skull being immovably
united to the backbone. In front the snout is supported by rostral cartilages,
usually three in number, but these are frequently fused to a single mass,

i

SKELETON. 77

The pterygoquadrate and the Meckelian cartilages bear teeth and form the
functional jaws. Most species are hyostylic (p. 73), the pterygoquadrate being
supported in front of the orbit by a ethmopalatine ligament on either side; behind
by ligament and by the hyomandibular. The Notidanids are amphistylic, the
hyomandibular being connected with the rest of the hyoid and not acting as a
suspensor of the jaws, but the pterygoquadrate bears a strong process which ar-
ticulates with the postorbital process of the cranium. A third condition is found
in the holocephalans where the pterygoquadrate, free in the young, becomes auto-
stylic by fusion with the cranium.

The variations in the branchial skeleton (figs. 63, 64) are readily reducible to
the typical conditions. In living elasmobranchs the number of gill arches is five,

Fic. 75.—Skull of Squatina, after Gegenbaur. h, hyoid; km, hyomandibular; 1” .1', labial
cartilages; m, Meckel’s cartilage; pg, pterygoquadrate; 7, rostrum.

except in Hexanchus and Chlamydoselache (six) and Heptanchus (seven). Hyoid
and branchial arches bear numerous branchial rays which support the gills and
the gill septa, while smaller cartilages on the inner surface of each arch extend into
the gill strainers.

TELEOSTOMES show a wide range of structure of skull, yet the series so inter-
grade that no sharp lines can be drawn. The chondrocranium persists to a consid-
erable extent, and numerous membrane bones are present, supplementing those of
cartilaginous origin. With few exceptions cartilage bones (the four occipitals, orbito-
and alisphenoids and prootics are the most constant) are developed, while the inner
wall of the otic capsule disappears, so that the cavity is connected with that for the
brain. Even more characteristic is the presence of skeletal structures supporting
the opercular fold that covers the external openings of the gill slits. This is in part
of membrane bones, in part of cartilage or cartilage bones. There are two parts
to the opercular fold, a gill cover or operculum above and a branchiostegal
membrane below. The latter is supported by branchiostegal rays, comparable
to the hyoid branchial rays of the elasmobranchs, while the operculum contains
membrane bones, there being, at most, four of these: a preoperculum in front,
and behind this in a row from above downward, operculare, suboperculum and
interoperculum. The preoperculum overlies hyomandibular, symplectic and
quadrate, and it is possible that the opercular bones have been developed in con-

78 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

nexion with the hyomandibular rays of the elasmobranchs. There are five
branchial arches, the last more or less reduced. Often they bear teeth on their
inner surfaces, thus acting as accessory chewing organs.

Fic. 76.—Side view of skull of mackerel (Scomber) after Allis. For letters see fig. 68,

The chondrostei, the most shark-like of the GANorpDs, have no cranial cartilage
bones. They are also primitive in the great development of the rostral cartilage
(enormous in Polyodon), which gives the mouth its ventral position, and in the

Fic. 77.—Chondrocranium of Polypterus, after Budgett. a, afferent artery to external
gills; b\-‘, branchials; e, efferent artery from external gills; /b, labial cartilage; 2, 5, 7,
nerve exits; other letters as in fig. 69.

‘ extension of the cranial cavity into the ethmoid region. They have a few bones in
the visceral skeleton, while there are numerous membrane bones in the roof of
the skull, a few of them readily homologized with those of other vertebrates.

SKELETON. 19

In other ganoids (holosteans and crossopterygians) the skull is much like that
of the teleosts, differing in the extension forward of the cranial cavity. There are

Fic. 78.—Median section of skull of mackerel (Scomber) after Allis. For letters see fig. 68.

one (Amia) or two (Polypterus) gular bones developed between the rami of the
lower jaw, and in Polypterus parietals, frontals and nasals fuse with age, and there
are numerous small bones in the cranial roof, developed along the lateral line

canals. Amia has several splenials in
the lower jaw.

TELEOSTS (fig. 76-80) have a consider-
able range of skull structure. In the lower
groups like siluroids and cyprinids, the
chondrocranium is largely persistent and
the cranial cavity extends into the eth-
moid region as in the higher ganoids.
In other teleosts the trabecule are ap-
proximate between the orbits (tropibasic)
and develop a thin interorbital septum
which limits the anterior ends of the
cranial cavity. The cartilage bones are
more numerous. All four occipitalia are
present, the occipital condyle being formed
by basi- and exoccipitals. Basi-, ali-, and
orbitophenoids occur, and besides ecteth-
moids a pair of mesethmoid ossifications.
In the otic capsule there are usually
pterotic and sphenotic ossifications.

The cranial roof is largely formed by 4

the frontals and parietals, the latter fre-
quently separated by a strong process of
the supraoccipital. Several of the car-
tilage bones are visible from above. The
roof of the mouth is formed by the large
parasphenoid andthe vomers. Premaxil-
faries (rarely lacking) and mazxillaries

Fic. 79.—Dorsal view of skull of mack-

form the upper jaw, both usually bearing erel, Scomber, after Allis; letters as in fig. 68

80 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

teeth, but occasionally, by overdevelopment of the premazxillary, the maxillary is
excluded from the margin of the jaw.

Instead of the single pterygoid of higher vertebrates there are three bones, an
entopterygoid adjoining the palatine, a mesopterygoid (ectopterygoid) which ex-
tends back to the quadrate, and a metapterygoid above the quadrate. When the
hyomandibular cartilage ossifies it forms a hyomandibular bone from its upper por-
tion and a symplectic (an element not known outside the teleostomes), which sup-
ports the quadrate. A small bone, the interhyal, intervenes between the hyoman-
dibular and the rest of the hyoid. The hyoid copula consists of several elements,
the anterior, which supports the tongue being called the entoglossal, the posterior,
which connects with the branchial arches, the urohyal. The fifth gill arch consists
of a single element on either side, the hypopharyngeal bone, which usually bears

Fic. 80.—Pterygoids, suspensorium and operculum of mackerel (Scomber) after Allis.
For letters see fig. 68.

teeth, the two sides being fused in the plectognaths, forming a pharyngeal jaw.
The upper elements of the other arches are frequently expanded, bear teeth, and
are called epipharyngeal bones.

Dipnor.—In the three existing genera the skull is comparatively uniform, but
the fossils, beginning in the Devonian, have a wide range of structure. In the
former the cavity of the chondrocranium extends to the ethmoid region and the nasal
capsules have a second opening, corresponding to the inner nares (choanz) inside
the oral cavity. The pterygoid is fused with the cranium (autostylic) and there are
one (Protopterus) or two (Ceratodus) labial cartilages connected with the nasal
capsules. In Ceratodus there are no cranial cartilage bones, but in the other
genera a plate composed of fused ex- and supraoccipitals occurs.

The membrane bones are few, but their homologies are not always certain.
The roof is largely formed by an unpaired bone in the position of frontals and
parietals, in front of which is a median bone (supraethmoid or fused nasals) above
the nasal capsules. In Ceratodus a bone of uncertain homology occurs on either
side of the fronto-parietal, but it is lacking in the others, unless it be represented in
Protopterus by a pair of bones which abut against the supraethmoid and overlap

SKELETON. 81

the fronto-parietals. The otic capsule and quadrate are covered by a squamosal,
and the roof of the mouth is formed by a large parasphenoid, in front of which are
a pair of palatines. In advance of these last are a pair of large teeth resting directly
on cartilage, their bases representing the greatly reduced vomers. The lower jaw
has three bones on either side, a small dentary, a larger angulare, and an enormous
splenial, which alone bears teeth.

In Ceratodus there is a hyomandibular fused to the cranium behind the exit of
the seventh nerve, but elsewhere there is only the hyoid. The operculum has one
or two elements (operculare and interoperculum) the free edges of which bear

Ras 4 7

Fic. 81.—Skull of Lepidosiren, after Bridge. on, angulare; ap, antorbital process; ch,
ceratohyal; cr, cranial rib; de, dermal ethmoid; dee, dermal ectethmoid; eo, exoccipital; fp,
frontoparietal; #r, hyoidean ribs; mk, Meckel’s cartilage; na, first neural arch; nc, nasa
capsule; msp, neural spine; pg, pterygoquadrate; sc, sagittal crest of frontoparietal: sp
splenial; sg, squamosal; 1-10, nerve exits.

cartilaginous rays, and the gill arches are five in Ceratodus, six in the other genera.
A peculiar feature of Protopterus and Lepidosiren is the so-called head rib, a slender
cartilage bone articulated with the chondrocranium below the occipital plate, and
extending backward and downward across the shoulder girdle.

In those extinct Dipnoi which are united with the recent genera to form the order
Sirenoidea, the skull is much as in the existing forms, except for the more numer-
ous bones. In the Arthrodira (formerly called placoderms) the cranium is hinged
to a large plate which covers the anterior part of the trunk, and the skull is roofed
with a few large plates, some of which may be homologized with those of the siren-
oids, the others not being readily compared with the bones of other vertebrates.
The suggestion has been made that the problematic fossil Paleospondylus resembles,
in its skull, the larve of the dipnoans, the adults of which were common in the same
seas.

6

82 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

AMPHIBIA.—Several points distinguish the amphibian from other skulls. The
chondrocranium is platybasic (p. 61); except for a small synotic tectum frequently
present, it is not roofed by cartilage; the otic capsule has a fenestra vestibuli occu-
pied by the stapes, a development connected with the power of hearing (p. 73);
there are two occipital condyles; and the quadrate is immovably united to the
cranium by two processes, an otic process, joining the otic capsule, and an ‘ascend-
ing process’ which reaches the upper margin of the trabecula, and which, in many
reptiles, often ossifies as the epipterygoid bone.

Fic. 82.—Chondrocranium of Amphiuma, lateral and dorsal views. aop, antorbital
process; ap, ascending process of quadrate (epipterygoid); ct, cornua trabeculz; de,
foramen, for ductus endolymphaticus; ep, ethmoid plate; fo, fenestra vestibuli; , Meckel’s
cartilage; n, notochord; oc, olfactory capsule; ov, occipital vertebre; , parachordal; g,
quadrate; s, stapes; t, trabecula; 2-8, nerve exists.

The cartilage cranial bones are few. Usually only exoccipitals are developed
in the hinder region, while the rule is a single petrosal (prootic), but occasionally
epi-, opisth-, and pterotic occur. There is but a single pterygoid, while basi-, pre-,
and alisphenoids are not ossified. The membrane bones in existing amphibians
have separated from the integument and have sunk to a deeper position than in
fishes, but in the stegocephals the presence of grooves for the lateral line system
would indicate a close connexion between skin and bones. In the latter group the
membrane bones are numerous, but in existing species they are noticeably reduced.
Except in stegocephals and the cecilians there are large vacuities in both floor and
roof of the skull. The lower jaw also has a reduced number of bones, there being
at most five including the articulare and the mento-Meckelian.

The most primitive conditions occur in the stegocephals, where, as the name

SKELETON. 8 3

indicates, the dorsal surface is covered, leaving only gaps for the eyes and nostrils.
In general the account of the skull given on page 67 ff will apply to these forms, and
so far as the dorsal surface is concerned little more needs to be said, aside from the
fact that the supratemporal is sometimes transversely divided, that an interparietal
foramen occurs (indicating the existence of a parietal eye), that the bones called
supraoccipital may be interparietal, and that the sclerotics are common. The
floor of the cranium is formed by a large parasphenoid, bordered in front by a pair
of (usually toothed) palatines, in front of which are the vomers. Of the carti-
laginous parts almost nothing is known; a few, clearly larval forms have well
developed branchial arches preserved.

Fic. 83.—Skull of a stegocephalan (Capitosaurus) after Zittell. Letters as in fig. 68.

Of the GyMNopPHIONES (cecilians) the cartilage skull is known only in Ichthy-
ophis, its peculiarities are the reduced parachordals, an ethmoidal nasal septum,
a stapes, perforated as in mammals, and alisphenoid and trabecular cartilages more
distinct than in most amphibia. Most noticeable of the cartilage bones is the eth-
moid, while otics and exoccipitals are fused as are quadrate and pterygoid. The
membrane bones form a complete roof to the skull, recalling the stegocephals, but
the number of bones is smaller, squamosal, supratemporal, jugal and quadrato-
jugal being absent, while a large prefrontal and a larger postfrontal (usually called
squamosal) occur. In the roof of the mouth maxillary and palatine are fused, the
vomers distinct, while the united .parasphenoid and basioccipital form a large os
basale. In the lower jaw there are only dentary and angulare, the latter being
produced behind the articulare in a remarkable way.

In the cartilage skull of the URODELEs (fig. 82) the pterygoid does not usually
reach the anterior part of the skull but projects as a process from the quadrate,
which bears, besides the two processes already mentioned (p. 82), a palatobasal

84 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

process joining the otic capsule in front of the otic process. Cartilage bones are
few; supra- and basioccipital, alisphenoid and ethmoids are lacking; the otics fuse
to a single petrosal; an orbitosphenoid occurs and quadrate and pterygoid are
‘continuous.

The roof of the adult skull is chiefly formed of parietals, frontals and nasals,
the latter being frequently separated by processes of the premaxillaries. Each

Fic. 84.—Skull of Amblystoma punctatum, after Wiedersheim. Letters as in fig. 68.

frontal has a ventral process which limits the cranial cavity in front; there is usually
a prefrontal and a septomaxillary may be developed on the postero-lateral part of
the nasal capsule. A supratemporal is always lacking, the squamosal extending
up to the parietal. The upper jaw is composed of premaxillaries and (except some
perennibranchs, fig. 85) maxillaries; a jugal is always absent and the quadratojugal,

Fic. 85.—Skull of Proteus, after Wiedersheim. For letters see fig. 68.

formed in the larva, fuses with the squamosal. In the roof of the mouth are the
large parasphenoid, frequently with teeth, and a pair of vomero-palatines, the
choane lying behind the vomerine portion, which is farther back than in
the dipnoi.

In the lower jaw Meckel’s cartilage persists, its hinder end forming the articulare,
while in front it is surrounded by the dentary and splenial, each bearing teeth. In
the larve the branchial skeleton is nearly typical, there being a hyoid and four gill
arches. In the adult, with the loss of aquatic respiration, the posterior arches are

SKELETON. 85.

reduced or even disappear, those remaining being connected by a one or two-jointed
copula.

The chondrocranium of the larval ANURA (Rana, fig. 86) differs considerably
from that of other amphibia as well as from the adult conditions. Like all amphib-
ians it is platybasic. The pterygoquadrate has, besides the normal otic and
epipterygoid processes, a cranio-quadrate process connected with the nasal region;
in front of which is the articulation of the lower jaw. In front of the cornua, are a
pair of suprarostral cartilages and a similar pair of infrarostrals lie in front of the

Fic. 86.—Chondrocranium of tadpole of Rana before the metamorphosis; after Gaupp.
¢.,ant, anterior canal; cls, superior labial cartilage; cfr, cornu trabeculz; car, foramen for
carotid; ext. c, external canal; fe, ethmoid fenestra; m, Meckel’s cartilage; pc, posterior
canal; po, otic process of quadrate; pr. as.’ ascending process of quadrate; g, quad-
rate; #m, tectum medialis; ttm, tenia tecti marginalis; tsyn, tectum synoticum; J-V, nerves
and nerve exits.

Meckelian, from which they are apparently derived. These four rostrals form a
ring around the suctorial mouth and recall the labial cartilages of the elasmobranchs
and the annular cartilage of the cyclostome mouth.

At the time of metamorphosis the changes are great, and as the result is more
like the chondrocranium of other amphibia, the larval condition must be regarded
as adaptive rather than ancestral. The suprarostrals disappear and the jaw
shifts the hinge back to the normal position, this being accompanied by the elon-
gation of Meckel’s cartilage, an absorption of the ascending process and a folding
of the pterygoquadrate bar. At the same time a pterygoid grows out in front to
join an antorbital process from the cranium. A stapes develops and connects

86 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

with the columella, which meets the tympanic membrane. This membrane is
stretched on a cartilaginous tympanic annulus, derived from the pterygoquadrate.
(Annulus and columella are lacking in those genera, Bombinator, etc., which have no
tympanum). There is no connexion between stapes and quadrate.

The chondrocranium largely persists, the only constant cartilage bones being
the exoccipitals and prootics. A supraoccipital rarely occurs and basioccipital and

ttmea

Fic. 87.—Chondrocranium of a frog after metamorphosis, from Gaupp. ov, fenestra
ovalis; m, Meckel’s cartilage; mg, metapterygoid; nc, nasal capsule; pigq, pterygoquadrate;
tnas, tectum nasalis; tsyn, tectum synoticum; ttmed, tenia tecti medialis.

basisphenoid are unknown. In the ethmoid region, except in the aglossa, there is
a peculiar bone, the sphenethmoid, which arises as two bones on either side.
These fuse, forming a ring (‘os en ceinture’) around the olfactory nerves and the
anterior end of the brain.

The frontals and parietals of a side are fused and often the fronto-parietals are
continuous across the middle line. They may extend to the nasals or there may

Fic. 88.—Dorsal and ee views of male toad, Bufo a americanus. For letters see
fig

be a gap between, leaving the sphenethmoid visible from above. A large squamosal
extends above the quadrate, from the otic region to the angle of the jaw. The upper
jaw consists of premaxillary and maxillary, and, except in the aglossa, of quadrato-
jugal. The pterygoid cartilage persists, but is overlaid by a membrane bone, also
called the pterygoid. Slender palatines, transverse to the axis of the skull, are
lacking only in the aglossa, while small vomers are almost always present. The

SKELETON. 87

floor of the cranium is completed by a J-shaped parasphenoid, which extends to
the premaxillaries in the aglossa, elsewhere’ only to the sphenethmoid.

In the lower jaw there is a mento-Meckelian in front, followed by dentary and
angulare, Meckel’s cartilage persisting through life. The larval branchial and
hyoid arches are typical, there being four gill arches. With the loss of gills the
posterior arches disappear, and the broad hyoid plate of the adult has four processes
which are new formations.

REPTILES.—The skull of existing reptiles is very different from that of amphib-
ians, but that of many theriomorphs is strikingly like that of the stegocephalans.
The principal differences alluded to in the first sentence have arisen by reduction
and disappearance of bones appearing in the more primitive types, but aside from
these there is little except the parasphenoid to separate the two groups.

i

Fic. 89.—Chondrocranium of Sphenodon, stage ‘R,’ after Howes and Swinnerton.
ep, epipterygoid; es, ethmosphenoidal plate; ex, extranasal cartilage; exp, extranasal process;
h, hyoid; mk, Meckel’s cartilage; nc, nasal capsule; oc, otic capsule; pt, pterygoid; g, quad-
rate; sb, subnasal process; 1-5, exits of nerves.

The chondrocranium is known in but a few forms and these agree with other
amniotes in being tropibasic, except in snakes and amphisbeenans (see fig. 62).
In the adults cartilage largely disappears, except in the ethmoid region, more
persisting in Sphenodon (fig. 89) and the lizards than elsewhere. All four occipitalia
are ossified, but some may not participate in framing the foramen magnum, the
basioccipital being excluded in many chelonians, the supraoccipital in snakes,
crocodiles and theriomorphs. There is but a single occipital condyle (except in a
few theriomorphs), which is borne on the basioccipital as in the crocodiles, or on
this and the exoccipitals as in chelonians and squamata. Basi- and presphenoids
are present, orbito- and alisphenoids are but slightly ossified and the ethmoid region
is largely cartilaginous. Pro-, epi- and opisthotics are present, the epiotic fusing
with the supraoccipital, while the opisthotic in all recent. forms except the turtles
unites with the exoccipita! in the adult.

In all except the squamata, in which it is movable (streptostylic), the quad-

88 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

rate is firmly united to the squamosal and sometimes to other bones (monimostylic).
The pterygoids extend forward to the palatines. In the squamata and the ichthyo-
saurs pterygoids and palatines are widely separated in the middle line, but else-
where they are closely approximate, the pterygoids even meeting the basisphenoid.
In all except chelonians, some dinosaurs and the typhlophida an ectopterygoid
(os transversum) extends from pterygoid to maxilla, while in plesiosaurs and
most lizards (kionocraniate) ossification of the ascending process of the quadrate
forms an epipterygoid bone between the pterygoid and the parietal.

Membrane bones are more numerous than in the amphibians. In many
theriomorphs there is a supratemporal fossa between parietal and supratemporal
bones and the same is true of plesiosaurs, ichthyosaurs and chelonians. In the
rhynchocephals, dinosaurs, pterodactyls and crocodiles there is in addition, an
infratemporal fossa, bounded laterally by an arcade in which squamosal, quad-
ratojugal and zygomatic participate in varying degrees. In the lizards the two
unite in a single temporal fossa by the disappearance of the upper arcade, and
lastly, in the snakes the lower arcade is lost and the fossa becomes a gap in the
side of the skull.

Parietals and frontals are usually paired, a parietal foramen being common;
pre-and postfrontals usually occur, sometimes excluding the frontal from the orbit.
Lacrimals are common and the margins of the upper jaw are formed in front by
premaxilla and maxillary, the latter connected with the squamosal, sometimes
by jugal and quadratojugal, or the jugal may drop out, or lastly the jaw may end
with the maxillary. Several membrane bones may aid in the formation of the
roof of the mouth. There is a small parasphenoid in ichthyosaurs, plesiosaurs,
many squamata, some rhynchocephals, and rarely in turtles. It is usually asso-
ciated with the basisphenoid and in ophidia it forms the base of the interorbital
septum. The vomers are paired except in the chelonia, and only in Sphenodon
of recent species do they bear teeth, and here but one on each bone. The maxil-
laries usually have broad palatal processes extending toward the middle line, causing
the choanz to open farther back, and in some, these, together with the palatines
and pterygoids, form a false palate, ventral to the nasal passages, so that, as in the
crocodiles, the choane are carried far back in the mouth. In some dinosaurs
there is a rostral bone in front of the premaxille.

The two halves of the lower jaw are united by ligament in most rhynchocephals,
snakes and pythonomorphs; by suture in crocodiles, rhynchocephals and lizards;
while they are fused in turtles and pterosaurs. All of the bones mentioned on
page 71 may occur in the lower jaw, usually with distinct sutures, while in croco-
diles, theriomorphs and some dinosaurs there are gaps or vacuities in its walls.
In many dinosaurs there is a predentary bone at the tip of the jaw. Except in
the chelonia and a few isolated forms, both jaws bear teeth, which may be restricted
to maxillaries and premaxillaries, or may also occur on palatines, vomers and
pterygoids. In their fixation three types are found: acrodont, when fused to the
margin of the bone; pleurodont, when fastened to the side of the bone; and the-
codont, when implanted in sockets.

The hyoid apparatus is much modified, but is adequately known only in recent

SKELETON. 89

species. The branchial arches are usually better developed than the hyoid proper,
which is cartilaginous in most snakes and is lacking in the crocodiles. In the
chelonia (fig. 93) two branchial arches are usually present.

The THERIOMORPHS (fig. go) have a short, broad skull with parietal foramen;
and that of the cotylosaurs was much like that of the stegocephals. In the more
differentiated groups the skull recalls that of mammals, especially in the partici-
pation of the squamosal in the hinge of the jaw. Lacrimals are occasionally
absent, sclerotics sometimes present. The palatal region is known in a few forms.
The pterygoids may meet only in front, leaving a vacuity between it and the
basisphenoid, or they may meet that bone. The choanz are in front of the pal-
atines but (theriodonts) may be displaced backward by palatine processes of the
mazxillaries.

All four occipitalia are developed; the occipital condyle is tripartite, being formed
by basi- and exoccipitals, but in Cynognathus the recession of the basioccipital
results in a dicondylic condition. The greatest variations occur in the temporal

Fic. 90.—Skull of Procolophon, after Woodward. For letters see fig. 68.

region. In the lower cotylosaurs the cranial roof is without fosse (Broom doubts
the infratemporal fossa of Procolophon). In other theromorphs quadratojugal
and supratemporal are lacking, the squamosal meeting the parietal. Placodus
has only the supratemporal fossa, but in the majority the upper arcade has dis-
appeared, leaving a large temporal vacuity, much as in mammals.

Little is known of the lower jaw. The bones are sometimes discrete, sometimes
extensively fused. The teeth are thecodont, and in the theriodonts are differentiated
into incisors, canines and molars, but in the anomodonts teeth are absent, or at most
there are a pair of large incisors in the upper jaw.

In the PLesrosaurs and their allies the skull is about a twelfth of the total length.
There is a parietal foramen between the parietals, which have a process for artic-
ulation with the squamosal, the supratemporal being absent. The large pre-
frontals intervene between the frontals and the orbits; lacrimals and usually
nasals are absent. The large temporal fossa in bounded externally by the jugal
which extends back to the quadrate. The choanz are in front of the palatines; an
os transversum is present and there is frequently a parasphenoid in the inter-
pterygoid vacuity. All have a subtemporal vacuity and there is another in the

go COMPARATIVE MORPHOLOGY OF VERTEBRATES.

Fic. 91.—Skull of Plesiosaurus macrocephalus, after Andrews. ang, angulare; art,
articulare; ch, choana; jr, frontal; orb, orbit; pa, parietal; pal, palatine; pas, parasphenoid;
po+pof, postorbital and postfrontal; sf, supratemporal fossa; ¢, transversum. Other
letters as in fig. 68.

ws

siamo tee ere

S) Wy

Fic. 92.—Dorsal and ventral views of’ the skull of turtle, “Piles $s, exoccipital;
m, maxillary; p, (behind naris), preorbital; pno, postorbital; s, supraoccipital; vise
letters as in fig. 68.

SKELETON. gI

plesiosaurs in the angle between palatine and transversum. The usual bones are
frequently distinct in the lower jaw. ;
In the CHELonzans the cranial cavity extends forward between the eyes and the
mesethmoidal cartilage largely persists in the adult. Although the bones are
comparatively few, the skull is primitive and can only be derived from that of the
cotylosaurs. The bones are firmly united, but the sutures are evident. The
basioccipital is usually excluded from the foramen magnum, and it and the ex-
occipitals participate in the tripartite occipital condyle. The supraoccipital is
often prolonged into an occipital spine and is fused with the epiotics. The basi-
sphenoid is present, but pre-, ali- and orbitosphenoids are not ossified, a descending
plate of the parietal taking the place of the alisphenoid. The pterygoids meet the

Fic. 93.—Hyoid apparatus of Tryonyx. 51, b?, first and second branchial arches; bh,
basihyal (copula); , reduced hyoid; cartilage dotted.

basisphenoid and may extend to the basioccipital. No ectopterygoid is present.
The monimostylic quadrate is large and expanded laterally to support the tympanic
membrane, and notched or perforate behind for the columella.

In the most primitive chelonians a complete false roof is formed by the expanded
postfrontals, parietals and squamosals. In most of the species the recession of the
parietals and squamosals causes a large gap, bounded in front by postfrontal and
jugal and exposing the otic bones. Laterally this gap is limited by an arcade of
squamosal and quadratojugal, but the latter may be reduced or (Cistudo) absent.
In front of the. frontals are a pair of bones, which bound the single naris behind.
These occupy the position of lacrimals, nasals and prefrontals, and are called by
the latter name. The premazxillaries are usually fused; the maxilla have broad
palatal processes and trenchant margins. They, together with the zygomatics,
form the lower border of the orbit.

The vomer is a single vertical plate separating the two choanez. ‘The palatines,
which bound the choanz behind are broad and are firmly united to pterygoids
and basisphenoid. A parasphenoid is known only in Dermochelys. In the lower

g2 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

jaw the bones are often fused, the two halves being united. Again the bones may
be distinct, the splenial being the least constant element. The hyoid apparatus
consists of a cartilaginous copula and two pairs of cornua which do not reach the
cranium.

IcatHyosaurs have a short temporal region but elongate nasals and pre-max-
illaries form a long rostrum. There is a large supratemporal fossa and enor-

Fic. 94.—Dorsal (A), posterior (B), ventral (C), and lateral (D) views of the skull
of Ichthyosaurus longifrons, after Woodward. mar, naris; pas, parasphenoid; pmx, pre
maxilla; ptf, postfrontal; pfo, postorbital. Other letters as in fig. 68.

mous orbits, bounded above by pre- and postfrontals, below by an elongate jugal,
and containing a sclerotic ring. The nares are just in front of the orbits and the
parietal foramen is at the junction of frontals and parietals. All four occipitalia
bound the foramen magnum; the basisphenoid is short, the presphenoid long; and
the pterygoids are separated in front by the vomers, leaving large pterygoid vacui-

Fic. 95.—Side and posterior views of skull of young Sphenodon, after Howes and Swinner-
ton. Compare with fig. 69. Cartilage dotted; letters as in fig. 68.

ties. The choanz are far forward. Teeth (sometimes absent) occur in grooves.
The lower jaw has five or six distinct bones, and a rib-like hyoid has been found in
some species.

The only living RHYNCHOCEPHALIAN is Sphenodon (Hatteria) of New Zea-
land. It is lizard-like, but its skull (figs. 68, 95) differs in the three temporal
fosse, the infratemporal arcade being osseous as in no lizard. Then the quadrate
is anchylosed to pterygoid, squamosal and quadratojugal. Premaxille, maxille and

SKELETON. 93

palatines bear teeth; an epipterygoid is present and the lower margin of the orbit
is formed by the maxillary. In the extinct genera the jugal may bound the orbit
below (Palaeohatteria), and the vomer may bear teeth.

Dinosaurs have both supra- and infratemporal fosse and frequently a pre-
orbital vacuity as well. The rostral and predentary bones have been mentioned
(p. 88). The palatal region recalls that of Sphenodon, except that the teeth, in
grooves or sockets, never occur on the palatines. There are such variations in the
skulls that few general statements can be made.

Statements that will apply to all Squamara are few. Except in chameleons
the quadrate is movable, a quadratojugal is lacking, the boundary of the infra-
temporal fossa being completed by ligament. The external nares are separate, there

Fic. 96.—Skull of Gerrhonotus imbricatus, after Siebenrock. For letters see fig. 68.

are large vacuities in the floor of the skull and the choane are forward. An
ectopterygoid occurs except in the typhlopids and all four occipitalia bound the
foramen magnum.

The chondrocranium of the Lizarps (fig. 62), while much like the general type of
tropibasic, is very light and is fenestrated to an extent not seen in the ichthyopsids.
Among the peculiarities of the adult skull are the fusion of exoccipital and opisthotic
to form a ‘parotic process’ which, together with the squamosal, supports the quad-
rate. There is a looseness of connexion of the front of the skull with the occipito-
sphenoidal portion, these parts moving on each other. The hyoid apparatus
bears two cornua which either end freely in the neck or may reach the parotic
process.

In the PyrHonomorpHs the striking features are the large supratemporal fosse,
the quadrate recalling that of chelonians; and the joint in the lower jaw, between
dentary and angular regions.

94 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

The Opnip1a (snakes) lack parotic process, parietal foramen, temporal arcades
and epipterygoid, and have the squamosal excluded from the cranial wall. The
attachment of the visceral skeleton to the cranium is loose, the pterygoid being
connected to the other parts by a long bar, consisting of squamosal and quadrate
behind and by transversum and palatine in front, features related to the great
distensibility of the jaws. In the poisonous serpents the poison fangs are either
permanently erect, or they fold back when the mouth is closed. In the latter the
fangs are supported on the maxillaries, which are moved by a rod formed of quad-
rate, pterygoid and ectopterygoid. In the lower jaw distensibility is provided for
by the elastic ligament connecting the two halves in front. Some species have
remnants of the hyoid apparatus, but occasionally all are lost in the adult.

Fic. 97.—Skull of snake, Tropidonotus, after W. K. Parker.

When the whole series of Crocoprria, recent and extinct, is considered the
range of variation in the skull is considerable. In all, supra- and infratemporal
fosse are present, the quadrate is immovable, there is more or less of a secondary
palate, no parietal foramen, and the thecodont teeth are confined to the margins
of the jaws. In the complete series the gradual change of position of the choane
can be traced from the oldest in which they are beside the vomers; then in the meso-
suchia the palatines meet in the middle line, carrying the choane back as a single
opening: behind these bones; while in the recent species the pterygoids have also
met, so that the choanz are between them and the basisphenoid.

Among the recent species the basioccipital is excluded from the foramen mag-
num, pre- and orbitosphenoids are imperfectly ossified, the nasals are long and the

SKELETON. 95

premaxillaries short so that the nares are far in front; parietals and usually the
frontals are fused in the middle line. There are vacuities in both walls of the
lower jaw, which is also pneumatic.

Although there is no relation between the two, the skull of the Prerosaurs is
very bird-like in its length and in having its axis at right angles to that of the body,
while the elongate premazxille form a bird-like beak. The sutures between the
bones are largely obliterated in the adult and the brain cavity recalls that of birds.
The resemblances are heightened in some by the lack of teeth, in others they are
in‘sockets. Both supra- and infratemporal fosse are present, as well as a large
preorbital vacuity, sometimes united with the naris. Squamosal and quadrate
are inclined forward so that the hinge of the jaw is often beneath the orbit. There
is no parietal foramen and all of the bones of the jaw are fused, including those of
the two halves.

Fic. 98.—Skull of Caiman latirostris, based on a figure by Reynolds; the irregularitesi of
the surface omitted. Letters as in fig. 68.
CKo@ebir eB)

AVES.—The skull of birds is similar in many respects to that of lizards. The
chondrocranium arises as two distinct parts, pre- and perichordal, which, on account
of the great head flexure, are at an angle of 100° to each other, later increased to
160°, which persists through life. ‘There are three (or four?) occipital vertebre be-
hind the ear, the last being the most prominent, and there is a small synotic tectum.
From the first the otic capsules are continuous with the basal plate and the fenestra
vestibuli is formed later by resorption of the cartilage. The trabecule are at first
distinct from each other as well as from the perichordal part; later they fuse in
front of the hypophysis to give rise to the base of the interorbital septum. In
Tinnunculus the ethmoid plate arises early as an intertrabecular mass, from which,
later, the dorsal part of the interorbital septum arises as a backward growth of
cartilage. Large alisphenoid cartilages are connected with the otic capsules.
The nasal capsules are complicated and later give rise to several centres of ossi-
fication. The quadrate is free from the rest (streptostylic) and its pterygoid process,
the homologue of the pterygoid cartilage in other groups, is greatly reduced. The
other visceral arches are much as in the adult (injra).

96 COMPARATIVE VERTEBRATES OF MORPHOLOGY.

The bones are lighter than those of reptiles and are often pneumatic, that is,
are penetrated with canals connected with the respiratory system. The brain
cavity is larger than in reptiles; sutures between the bones largely disappear in the
adult, and the single occipital condyle (mostly basioccipital) is on the floor of the
skull so that the axis of the skull is at right angles to that of the body. There is
only a single temporal fossa, bounded laterally by an arcade of jugal and quad-
ratojugal, connecting quadrate and maxillary. There is a preorbital vacuity;
and the nares may have the posterior margin rounded (holorhinal) or slit-like
(schizorhinal), The premaxillaries are fused and sclerotic bones are common.

A peculiarity of the ventral surface is the union of the anterior part of the

Fic. 99.—Earlier and later stages of skull of bird (Tinnunculus) after Suschkin. al,
alisphenoid cartilage; ai, foramen for internal ophthalmic artery; b, basal plate; bt,
basipterygoid; ec, external semicircular canal; hm, ‘hyomandibular;’ iorb, interorbital
plate; :t, intertrabecula; mc, middle concha of nose; ov, occipital vertebre; pc, posterior
semicircular canal; sorb, supraorbital; str, supratrabecula; ér, trabecula.
parasphenoid to the basisphenoid to form a ‘rostrum sphenoidale’ which projects
forward in the middle line. The rest of the parasphenoid forms a ‘basitemporal
plate’ below the basisphenoid and basioccipital. Dorsal to the rostrum is a
small presphenoid (sometimes lacking in the adult) to which the orbitosphenoids are
attached as ale, while the alisphenoids become similar wings to the basisphenoid.
Ectethmoids are connected with the mesethmoid; they are sometimes large, appear-
ing (‘prefrontals’) on the top of the skull. Epi- and ectopterygoids are lacking.
The pterygoids, here membrane bones, extend from the quadrates to the palatines,
and the two either slide along the rostrum or the vomers intervene. This, together
with the hinging of the front part of the skull upon the rest, forms a mechanism by
which the upper jaw is raised when the mouth is opened, the temporal arcade aiding
in the motion. The vomers may be paired; usually they form a thin vertical plate
between the anterior ends of the pterygoids; occasionally they disappear. The
choanz are between the palatines and vomers. Some birds have an ‘os uncin-

SKELETON. 97
atum,’ a small bone connecting the lacrimal with the palatine or jugal bar. All
of the bones enumerated on page 71 may appear in the development of the lower
jaw.

Teeth occur only in a few fossil birds, where they are implanted in sockets;
several species are known to have a dental ridge in the embryo (see Development
of Teeth). The hyoid apparatus (fig. ror)
consists of a pair of cornua (first branchials)
sometimes extremely long, connected by the
hyoid copula (os entoglossum), behind which
is a second copula (urohyal) while in front of
the entoglossum is a ‘paraglossal’ element
with a pair of small cornua.

The palatal structures have considerable
importance in classification. All living birds
can be arranged in two groups. In the ‘dro-
mzognathous’ group the palatines and ptery-

Fic. roo.—Ventral view of skull Fic. 1o1.—Hyoid of hen, after Parker.
of a duck; letters‘as in fig. 68. e, entoglossal; ~, paraglossal; u, urohyal;

III, posterior cornua.

goids do not articulate with the rostrum, the vomers usually intervening. In
the ‘euornithes’ the articulation occurs. The latter are subdivided into the
desmognathous forms where the vomer is small or wanting, and the maxillo-
palatines meet in the miiddle line; the schizognathous in which the maxillo-
palatines do not meet the vomer or each other; the egithognathous, like the
last except that the vomer is broad and truncate; and the saurognathous
with delicate, rod-like vomers and mazxillopalatines scarcely extending inwards
from the mazxillaries.

7

98 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

The chondrocranium of the MAMMALS has several peculiarities. There are
four occipital vertebre, the last only with a complete vertebral character, all event-
ually fusing with the synotic tectum. The dorsal part of the otic capsule chondrifies
first, owing to the late development of the cochlear part of the ear in the lower half;
and the capsules themselves have their axes inclined, so that the exit of the seventh
nerve is on the anterior rather than on the lateral face. The trabeculz soon join
the basal plate and from their sellar part an alary process is given off on either side

EEE

Fic. 102.—Chondrocranium of a pig, after Mead. as, alisphenoid; ¢l, posterior clinoid
process; cr, fenestra cribrosa; end, foramen for endolymph duct: fm, foramen magnum; h,
fossa hypophyseos; /sr, lateral superior recess: os, orbitosphenoid; I, parietal lamina; sm,
septum nasi; é#, tectum nasi; 2-12, exits of nerves. -

which extends upward to join an alisphenoid (ala temporalis) which chondrifies
separately, but soon joins the otic capsule above, leaving between them the foramen
ovale for the third branch of the fifth nerve, the other branches passing forward
over the ala and then between it and the orbitosphenoid (ala orbitalis) through
the sphenoidal fissure (foramen lacerum anterior). The ala orbitalis joins the
trabecula by two processes, bar and processes sometimes forming a reduced inter-
orbital septum. Later a marginal band (tenia marginalis) extends back from

SKELETON. 99

the orbitosphenoid to a cartilage plate developed on the otic capsule. The ethmoid
parts are complicated, consisting of the two nasal capsules, the septum between
them, and, on the inside, coiled turbinal cartilages to support the olfactory membrane.

Some of the visceral arches have been mentioned in speaking of the ear bones
(p. 74). The pterygoid cartilage is apparently lacking, and there is nothing that
can be interpreted as a quadrate except the incus. Meckel’s cartilage extends for-
ward from the incus to the tip of the jaw. In the procartilage stage the hyoid is
continuous with the stapes; later it joins the otic capsule behind the fenestra ves-
tibuli, while ventrally it joins its fellow and is connected with the first branch-
ial arch by a median cartilage, probably the copula.

In the adult the so-called facial bones are more closely related to the cranium
than in the lower groups, and distinct bones are fewer than in lower vertebrates,
the reduction being due in part to actual loss, in part to the fusion of elements

Fic. 103.—Diagram of the bones of the mammalian skull, altered from Flower. Cartilage
bones dotted, membrane bones lined; 2-12, nerve exits,

which elsewhere remain distinct. The obliteration of sutures has gone farther in
the monotremes and some of the carnivores and apes than elsewhere. Connected
with the loss of bones is the absence of the supratemporal arcade, but the infra-
temporal bar consisting of processes from the squamosal and zygomatic (malar)
is always present, bounding the single temporal fossa. This may be separated
from the orbit by a bar formed by zygomatic and frontal, or the bar may be in-
complete or absent so that orbit and fossa are one.

Usually the bones fuse in such a way that the complexes named on page 66 are
readily recognized. The occipitalia are usually united into a single occipital bone,
though the sutures between them may persist for some time. The basioccipital
forms the so-called basilar process, while the exoccipitals bear the two occipital
condyles for articulation with the atlas. The exoccipitals may also bear strong,
ventrally directed, paramastoid processes (paroccipital). The membranous
interparietal is sometimes distinct, sometimes fused to the supraoccipital, though it
may unite with the parietals.

100 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

The sphenoidalia form the sphenoid bone of humananatomy, Basi-and pre-
sphenoid form a ‘body’ from which two pairs of ‘wings’ arise, the alisphenoids
being the greater, the orbitosphenoids the lesser wings. A pair of pterygoid pro-
cesses are given off from the ventral side of the body and a part of these in some cases
persist as distinct pterygoid bones, but apparently are not homologous with the
elements of the same name in the lower vertebrates since they are membrane bones.”
The equivalents of the pterygoids of the non-mammals occur in the monotremes.
A second pair of membrane bones, the intertemporals, also belong to the sphenoid
complex, fusing at an early date with the dorsal margin of the alisphenoids.

The ethmoid complex consists of a mesethmoid which ossifies in the septum
between the nasal organs, and an ectethmoid in the outer wall of each nasal capsule.
Mes- and ectethmoids are distinct for a time, the olfactory nerve passing between
them. Later bony strands passing between the nerve fibres unite them, producing
perforated cribiform plate, characteristic of the mammals. The part of the
mesethmoid projecting above the cribiform plates is the cristi galli, below them is

Fic. 104.—Median section of skull of young Erinaceus, after Parker. For letters see
fig. 68.

the perpendicular plate. Two other centres in the lateral wall of each capsule
give rise to coiled bones (inferior and sphenoidal turbinal) on which the olfactory
membrane is spread, while two other turbinals (superior and middle) arise from
the ectethmoid. A few mammals have in addition, a prenasal bone, developed
in the septum in front of the mesethmoid.

The temporal complex consists of squamosal, otic bones and tympanic. On
the ventral side of the squamosal is the glenoid fossa for the articulation of the
lower jaw; in front the bone gives off a zygomatic process for articulation with a
similar process of the zygomatic (malar) bone, the two forming the arcade bounding
the temporal fossa. The tympanic (apparently the angulare of the lower vertebrates)
curves below the auditory meatus, joining the squamosal on either side. In many
forms it expands to form a large capsule, the auditory bulla. The otic bones
(it is said that there are six centres of ossification in the otic capsule) unite early
to form a single petrosal bone, which, in turn (cetaceans excepted) fuses with squa-
mosal to form the temporal bone. Later, the posterior part of the otic region expands
to form the mastoid process, while the upper part of the hyoid, fused to the cap-
sule, forms a styloid process.

On account of the great size of the brain some parts of the skull are changed in

SKELETON. IOI

position. Thus the petrosal, instead of forming part of the side wall, is carried to
the floor of the brain cavity and the squamosal forms part of the lateral wall.
The roof of the brain cavity is largely formed by parietals and frontals. (In some
whales, denticetes, the supraoccipital and interparietal extend to the frontal, pre-
venting the parietals from meeting.) The frontals may be distinct or they may
fuse. In many ungulates they bear horns or antlers. In cattle, antelopes, sheep
and goats (cavicornia) a strong bony process or horn core is developed on each
frontal, and this is covered by a cornified epidermis and persists through life. The
antlers of the deer differ from horns. Each year there is an outgrowth of bony
material, covered by a richly vascular skin, from each frontal bone. This grows
with remarkable rapidity, and when its full extent is reached, the skin (‘velvet’) is
lost, leaving the core alone. After about a year resorption takes place at the base
so that the antler is soongost, to be replaced by a similar but larger one in a few
weeks.

The nasals lie above and behind the nares. The margin of the upper jaw is
formed by premaxillaries followed by the maxillaries which ossify from several
centres, difficult to homologize with distinct bones in the lower vertebrates. The
inferior turbinals fuse to the inner surfaces of the maxillaries. Premaxillaries and
maxillaries may fuse or they may remain distinct. They have broad palatine
“processes on the oral surface, these meeting in the middle line and forming the
anterior part of the hard palate, with frequently one or two incisive foramina
for the passage of the nasopalatine nerve between them. The choanz are usually
behind the palatine bones which form the rest of the hard palate, but in some eden-
tates‘and whales the pterygoids form part of the partition between the narial pas-
sages and the mouth cavity.

The ingrowth of the hard palate has forced the vomer from the roof of the mouth
to a position just ventral to the anterior part of the cartilage of the nasal septum.
In the monotremes there is a ‘dumb-bell
bone’ in front of the vomer (p. 69). A
lacrimal bone always occurs at the inner
side of the orbit and the zygomatic forms
the external wall of that cavity.

The lower jaw articulates directly with
the squamosal without the intervention of
a quadrate (see ear bones, p. 74). Its
halves may unite in front by ligament or

z i Fic. 105.—Hyoid of rhinoceros (Ate-
by complete anchylosis, It is usually Jodus). ac, anterior cornu; b, body; ¢,

described as consisting of a pair of den- ceratohyal; e, epihyal: pc, posterior cornu

: 7 ]).
taries, but there are several centres of ossi- (thyrohyal)

fication and a splenial and possibly a coronoid may be recognized. The angulare
is apparently the tympanic, while the articulare of lower vertebrates is the malleus.
A remarkable feature in development is an enormous cartilage at the posterior
angle of the jaw, the dorsal side of which forms the condyle for articulation with
the glenoid fossa.

The hyoid apparatus varies. As described above, the hyoid is connected above |
with the otic region, below with the first branchial. The part connected with the

‘

102 COMPARATIVE MORPHOLOGY OF VERTEBRATES,

otic capsule forms the styloid process (p. 100), while the rest may ossify as epi-,
cerato-, and hypohyals, or a part may change to a stylohyal ligament, connecting
the ventral parts with the skull. The hyoid of the adult consists of the copula
forming the body, a part of the hyoid the anterior cornua, while the first branchial
arch (of which at most but one or two ‘thyrohyal’ elements are formed) give rise
to the posterior cornua. These are connected by ligament with the greatly modified
posterior branchial arches, described in connection with the larynx (see respiratory
organs).

Appendicular Skeleton.

The appendages fall in two categories, the median or azygos
(median fins) found only in aquatic vertebrates and the paired appen-
dages, which (cyclostomes excepted) are found in every class, although
here and there individual species or genera may lack them. Both
kinds have an internal skeleton. Opinions differ as to the origin of
these appendages. The two most prominent views are given below.

Fic. 106.—Diagram of the origin of median and paired appendages from lateral fin folds.

According to one view the two types have no relation to each other. The
paired appendages are derived from gill septa, all traces of which are otherwise
lost from these somites. The girdles which support the appendages are modified
gill arches, while the skeleton of the appendage itself is derived from the radialia
which support the gills, one radial forming an axis, the adjacent radials being
arranged on either side of this, and carried outward from the arch by the growth
of the septum to form the body of the appendage (fig 122). A somewhat similar
view is that the appendage itself is a modification of an external gill, such as is
found in larval amphibians.

Another view supposes an ancestor with two pairs of longitudinal folds running
the length of the body behind the head, each fold supported by a series of skeletal
rods (fig. 106). With farther development the upper folds on either side migrated

SKELETON. 103

dorsally until the two met and fused in the middle line of the back, thus producing
a continuous dorsal fin. The ventral folds migrated downward in the same way,
eventually meeting behind the vent, but that opening prevented their meeting
farther forward. From the fused part behind the vent the anal and the lower part
of the caudal fins were formed, while the paired appendages are differentiations of
the preanal parts of the ventral longitudinal folds.

It may be said that in development there is no such double origin of the dorsal
fin. In several sharks the paired fins arise from continuous folds, while in the
Japanese gold fish the anal fins are frequently paired and the caudal has a double
condition below, such as would result from the failure of folds to unite in this region.
In criticism of the gill-arch theory it may be said that the supports of the paired
appendages arise outside of the body musculature, while the visceral arches (p. 65)
are internal.

Toe MEDIAN APPENDAGES.
®

The median or azygos appendages always have the form of fins,
and may be dorsal, terminal (caudal) or ventral (anal) in position.
Primitively, and in many species through life, they are continuous, but
usually gaps occur during development so that the fins of the adult are
separated by intervals from each other. They occur in practically all
fishes, in larval and tailed amphibians, and in isolated groups like the
ichthyosaurs and whales. In amphibians and higher groups the
median fins have no skeleton, but elsewhere it is of cartilage, bone,
or a horny substance (elastoidin), the latter being the most constant
and occurring in connection with either of the others.

The simplest skeleton consists of a metameric series of cartilage or
osseous bars, each usually divided into a deeper basale and a more
distal radiale, the former frequently articulating or alternating with
the spinous processes of the vertebre, while the latter support the fin
proper. The elastoidin elements consist of a number of slender rods
(actinotrichia), outnumbering the somites, and arising from the
corium, immediately below the epidermis. Frequently they are
united into bundles (soft fin rays) and may replace the radialia.

PAIRED APPENDAGES.

The paired appendages are not, as the gill-arch theory would demand, derived
from a single somite, but a varying number of segments participate in their forma-
tion. Apparently the simplest fin known is that of the extinct shark, Cladoselache
(fig. 107), in which it is a rounded lobe supported by @ tumber of rods, like the
radialia in a median fin. These are attached prdximally to a few larger plates, the

104 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

basalia, the basalia of the two sides being unconnected with each other. Greater
growth of the basalia would result in some of them meeting and fusing in the mid-
dle line, thus forming a bar across the ventral side of the body, giving additional
support to the fin. . Then to compensate for the rigidity, the basals become jointed
on either side, leaving the medial bar
with an articular surface on either side
for the reduced basalia. The ventral
muscles of the fin would find firm at-
tachment to the bar, while the need for
a similar attachment for the dorsal
results in an extension of the bar dor-
sally above the articulation of the
limb, thus producing the typical girdle.
The derivation of the fin of any fish
from that of Cladoselache is easily
imagined, but no satisfactory compari-
son of the fin with the leg has yet been
made.

In the appendicular skeleton
the internal supports or girdles
and the skeleton of the free ap-
pendage are to be recognized.
Each girdle is an inverted arch
crossing the ventral side of the
body and extending up on either
side above the articulation of
the limb. The girdles, as well
as the skeleton of the free ap-
pendage, are always laid down
in cartilage, and in the latter,
aside from the actinotrichia, no
parts of other than cartilaginous
orginoccur. Inthe girdles mem
brane bones may be added as
Fic. 107.—Ventral surface of Cladoselache, will appear below.

Bice Jaceeel In its typical state each girdle
consists of three elements, one dorsal and two ventral, meeting at the
point of attachment of the free appendage, all contributing to the
socket (glenoid fossa, acetabulum) which receives the basal element of
the skeleton of the limb. The limbs themselves are much alike in
their general structure, as may be seen from the adjacent diagram.

SKELETON. 105

The Shoulder Girdle.

FISHES.--The pectoral or shoulder girdle in the elasmobranchs is
more or less U-shaped, the bottom of the arch crossing the ventral
surface between the skin and the peritoneal membrane, this ventral
portion being known as the coracoid region, which is limited dorsally

Fic. 108.—Diagram of girdles and appendages from the posterior side; upper letters,
fore limb; lower, hind limb. a, acetabulum; ¢, carpus; co, coracoid, f, femur; /7, fibula;
g, glenoid fossa; h, humerus; #, ilium; is, ischium; mc, mt, metacarpals, metatarsals; 9,
pubis; pc, procoracoid; ph'—*, phalanges; 7, radius; s, scapula; «, ulna; 1-5 digits.
by the point of attachment (glenoid fossa) of the fin. Dorsal to the
fossa is the scapular region. Not infrequently the dorsal part of

the scapular region is segmented off as a separate suprascapula.

Fic. 109.—Pectoral girdle and cartilaginous fin skeleton of Scyllium. c, coracojd region;
gl; glenoid surface; ms, mesopterygium; mt, metapterygium; p, propterygium; 7, radialia:
s, scapular region.

The girdle is usually free from the axial skeleton, but in the skates
(raiz) the suprascapula articulates with the adjacent vertebre.

In the simpler teleostomes (some ganoids, dipnoans) the cartilagin-
ous girdle is reinforced by membrane bones derived from the skin.

106 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

Of these there are at least two on either side, a pair of clavicles which
overlie the coracoid region and meet in the middle line, and lateral to
each clavicle and extending to or above the glenoid fossa, a second
bone, the cleithrum. In some ganoids (Polypterus, fig. 110) the.
cleithra extend toward the middle line, and a little higher in the scale,
meet and take the strains. This assumption of stress by the membrane
bones results, in the higher forms, in the separation of the two halves
of the cartilaginous girdle.

In the higher ganoids and teleosts the cleithrum has increased
greatly, usurping the function of the clavicles, which have consequently

Fic. 110.—Pectoral girdles of (A) Acipenser and (B) Polypterus, after Gegenbaur. ct,
cleithrum; cv, clavicula; dr, dermal rays; g, glenoid surface.

disappeared. Dorsal to the cleithra other membrane bones frequently
occur. There may be one or two supracleithra (post- or supra-
temporals, fig. 79) which connect the girdle with the skull, and
occasionally others as postclavicle, infraclavicle, etc. As a result
of the great development of the cleithra the cartilaginous girdle has been
reduced, but it usually has at least two ossifications on either side, a
scapula dorsal to the glenoid fossa and a coracoid in the ventral region,
these contributing to the support of the appendage.
AMPHIBIA.—In the stegocephals the cartilage has not been
preserved and the bones are variously interpreted (fig. 58). The bone
meeting the episternum is the clavicle, and lateral to this is an equally
slender bone, usually called scapula, but by some the cleithrum. A

SKELETON. 107

large round element is called the coracoid. In the recent amphibians
we are on firmer ground. The halves of the girdle develop separately,
and the cleithrum is lacking. In urodeles the coracoid region has two
processes diverging from the glenoid fossa, an anteriorly directed pro-
coracoid and a coracoid proper, directed toward its fellow of the
Opposite side, the two meeting the sternum behind and overlapping in
front. Ossification sets in in the neighborhood of the glenoid fossa,
the resulting bone being called the scapula, although it invades the
coracoid region, the cartilage dorsal to it being the suprascapula.

In the toads and allied anura (arcifera) the halves of the girdle
overlap as in the urodeles, but the procoracoids extend toward the
middle line, each being joined to its coracoid by longitudinal cartilage
plate, the epicoracoid, leaving a gap between them. With the ap-
pearance of bone, scapula and coracoid ossify, while a clavicle of mem-

Fic. 111.—Arciferous pirdle of Ceratophrys ornatus. cl, clavicle; co, coracoid; e, epicora-
coid; #, head of humerus; s, scapula; ss, suprascapula; cartilage dotted.

branous origin overlies the procoracoid cartilage. In the frogs (firmi-
sternia) the relations are much the same, except that the epicoracoids,
instead of overlapping, abut against each other, and the clavicles nearly
or quite replace the procoracoid, while sternum and omosternum join
the girdle in front and behind. Girdles are lacking in the gymnophiones.
REPTILES.—With the development of a considerable neck in the
reptiles the pectoral girdle is removed farther from the head; it shows
considerable differences in the various groups. In the fossil rhyn-
» chocephals it is much as in the stegocephals, except that the scapula
is large. In the turtles it occupies a peculiar position, being inside
the carapace, 7.¢., internal to the ribs; but this is explained by the de-
velopment; the girdle arises in front of the ribs and later sinks to the
definitive position. Scapula, procoracoid and coracoid are well
developed, the medial ends of the latter two being connected by a cartil-
aginous epicoracoid. Elsewhere in the reptiles the procoracoid tends
to reduction, the clavicle taking its place, though it is retained in the
lizards in a reduced condition (fig. 112). The clavicle in turn is

108 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

lost in chameleons and crocodiles, and if present in the chelonians, it is
represented by the epiplastron (p. 41), an element of the carapace.
The girdles are greatly reduced in the limbless lizards and have van-
ished in the ophidians.

In the BIRDS (fig. 53) the scapula is a sword-shaped bar overlying
the ribs, while the coracoid extends from its junction with the scapula
at the glenoid fossa to the anterior end of the sternum. ‘The clavicles
of the two sides are united at their medial or ventral ends to form the
well-known furcula (wishbone) which may articulate with the sternum
between the two coracoids, or, with diminishing powers of flight, may
end freely below.

FIG. 112. FIG. 113

Fic. 112.—Sternum and pectoral girdle of Amblyrhynchus, after Steindacher. c,
coracoid; cl, clavicle; e, epicoracoid; es, episternum; , humerus; m, mesocoracoid; ms,
mesoscapula; ~, procoracoid; sc, scapula; s, sternum.

Fic. 113.—Shoulder girdle of Ornithorhynchus. cl, clavicle; co, coracoid; e, epister-

num; g, glenoid fossa; pc, procoracoid; s, scapula; st, sternum. r 0

MAMMALS.—The shoulder girdle of the monotremes is strikingly
like that of lizards, the coracoids acting as a brace between sternum and
glenoid fossa, while the resemblance is strengthened by the presence of
the episternum. This same large development of the coracoids occurs
in the young of some marsupials, but in the adults, as in the rest of the
mammals, the coracoid is greatly reduced, persisting only as a small
projection, the coracoid process, anchylosed to the ventral end of the

SKELETON. 10g

scapula, where it often forms a part of the glenoid fossa. The scapula
is always well developed, and in the placental mammals bears a strong
crest (spina scapulz) on its external surface, terminating ventrally in
an acromion process. The clavicle varies with the freedom of motion
of the limb. Thus in rodents, insectivores, bats, some marsupials and
the higher primates it forms a strong brace between shoulder and ster-
num. In ungulates, whales, and a few carnivores it has ‘entirely dis-
appeared, while in other mammals it persists as a rudiment without
functional value. In development two small elements frequently
intervene between the clavicles and the sternum (fig. 55). They
are preformed in cartilage but eventually fuse with the sternum.
Their homology is very uncertain. They have been called episternalia,
suprasternalia, etc.

The Pelvic Girdle (Pelvis).

In its broader features the pelvis (¢f. fig. 108) is much like the
shoulder girdle, and in its full development, may be compared, part by
part, with the anterior arch. ‘Thus the acetabulum or socket where the
appendage is attached, is comparable to the glenoid fossa. Dorsal
to this is the ilium in the position of the scapula, while ventral and-
medial to the acetabulum are, on either side, an os pubis in front, an
ischium behind, with a gap (ischio-pubic fenestra) between them, just
as between coracoid and procoracoid. An important landmark is
the point of passage of the obturator nerve through the pelvis. This
may have its own (obturator) foramen, though the pubic portion or
the foramen may unite with the fenestra, the condition in the mammals
where the common opening is called the obturator foramen.

The phylogenetic history of the pelvis is more clearly indicated than
is that of the pectoral girdle, for in many fossils, as well as in the
sturgeon, there is little advance over Cladoselache (p. 104). The
basalia of a side have fused to a single basal, often perforated for the
obturator nerve, and bearing the radialia on its distal surface. The
basalia of the two sides have not met, but there is frequently between
them a pair of small cartilage plates, possibly the homologues of the
epipubis of the tetrapoda (imfra). There is no acetabular joint. In
the other ganoids and in teleosts there is little advance, aside from ossifi-
cation of parts, while no epipubic elements occur. A noticeable
feature in many acanthopterygians is the forward migration of the pelvic

IIo COMPARATIVE MORPHOLOGY OF VERTEBRATES.

fins so that they come to lie in front of the pectorals (the old group
of ‘jugulares’).

The elasmobranchs have a true girdle, but without separate ele-
ments as it does not pass beyond the cartilage stage. It consists of a
continuous ischio-pubic bar, extending from one acetabulum to the
other, and usually prolonged dorsally above the acetabulum by an
iliac process.

In all fishes the pelvic girdle is free from the vertebral column, but
in the tetrapoda, where the limbs have to support the poy weight, the
girdle becomes connected with the sacrum by 4
the intervention of one or more sacral ribs (p.
56). In the interpretation of some of the
pelvic elements there is some uncertainty.

In the stegocephals ischium and ilium and
usually pubis were distinct bones with appar-
ently considerable cartilage between them. In

Fic. 114. Fic. 115.

Fic. 114.—Pelvis of Discosaurus, after Credner. il, ilium; is, ischium; p, pubis.

Fic. 115.—Ventral view of pelvis and ypsiloid cartilage of Cryptobranchus, after Wieder
sheim. a, acetabulum; 7, ilium; zs, ischium; 0, obturator foramen; ~, conjoined pubes;
y, ypsiloid cartilage.

the urodeles the two ischio-pubic cartilages are usually united in the
median line, but the,ossifications vary in extent, the pubic region lagging
behind the ischium and being at times indistinguishably fused with it.
In some cases there is, as in Neciurus, an extension of the median
cartilage forward in an epipubic process, and frequently a pectineal
process from the antero-lateral of each pubis. An interesting feature
is furnished by the ypsiloid cartilage (fig. 115) formed independently
of the pubis and extending forward in the linea alba through two or
three somites. This occurs only in salamanders with functional lungs,
where it furnishes attachment for muscles connected with respiration.

In the anura all three pelvic bones are present, and all participate in
the formation of the acetabulum. Correlated with the leaping habits

SKELETON. IIt

the ilium is very long and the ischio-pubis is strongly compressed,
obturator foramen and ischio-pubic fenestra being absent.

Omitting the extinct rhynchocephals, whose pelvis resembles that of
the stegocephals, the reptiles have the pelvic bones more solid and dis-
tinct than do the ichthyopsida; the ilium is strong, with its dorsal end
frequently expanded; the ischio-pubic fenestra is large; and ischium
and pubis are united to their fellows directly, or by the intervention of
the epipubic cartilage, or its modification, the ligamentum medium
pelvis. Asa rule all three bones meet in the acetabulum and there
are large prepubic processes, though these are small in the lizards and
are lacking in crocodiles.

Fic. 116. Fic. 117.
Fic. 116.—Pelvis of snapping turtle (Chelydra) from below. e, epipubis; f, femur;
h, hypoischium; /, ligamentum medium pelvis; p, pubis; pp, pectineal process.
Fic. 117.—Pelvis of Iguana tuberculata, after Blanchard. a, acetabulum; e, epipubic
cartilage; f, femur; i, ilium; is, ischium; of, obturator foramen; ~, pubis; p, prepubis;
s,! s?, first and second sacral vertebra.

Many theriomorphs have the pelvic bones fused much as in mam-
mals. In Sphenodon and turtles the epipubic cartilage bounds the
fenestra on the median side, and Sphenodon and the plesiosaurs have
a separate obturator foramen, but the two are merged in the chelonians.
Most lizards have slender pubic bones, perforated by the foramen, and
the part of the epipubis between the fenestre reduced to a ligament,
while the posterior part of this, behind the ischium, may ossify as a
distinct bone (os cloace or hypoischium). In the footless lizards the
pelvis is reduced, being represented in the amphisbenans by rudiments
of ischium and pubis, while all traces of the pelvis are lost in snakes,
except the boas and some opoterodonts. The obturator foramen is
very large in the crocodiles, the result of the oblique position of the

II2 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

pubes, which do not unite with each other; each is tipped with car-
tilage (? separate epipubes). All three bones meet at the acetabulm
which is perforate in recent species. The lower end of the ilium sepa-
rates as a distinct bone (pars acetabularis).

The pelvis of the dinosaurs has the same great extension of the ilium
forward and back as is seen in the birds and a corresponding in-
crease of the sacrum (p. 53), the result of the partially upright position.

Fic. 118.—Pelvis and hind limb of Camptosaurus, after Marsh. /f, femur; fb, fibula; 7,
~ ilium; is, ischium; p, pubes; pp, postpubis; ¢, tibia; I-IV, digits.

The ischia are greatly elongate and are directed backward, being fre-
quently united below. The pubic bones are remarkable in being
directed forward and downward and in having strong postpubic
processes which are parallel to the ischium. Frequently the ilium
gives off an iliac spine near the acetabulum.

The pterodactyls had the same elongate ilium as the dinosaurs, the
ischium being fused to it so as to exclude the pubis from the acetab-
ulum, the latter’ being usually loosely articulated to the ischium and.

Ha pubis is sometimes regarded as a prepubis, the ischium being called an ischio-
pubis.

SKELETON, 11g

meeting its fellow in the median line below. The pelvic opening was
very small. The pelvic bones of the ichthyosaurs were weak, long and
slender, and apparently were imbedded in the muscles.

In recent birds (figs. 50, 53) the pelvic bones arefused. The ilium
is greatly elongate and usually fused with the synsacrum (p. 53); ischium

Fic. 119. Fic. 120,

Fic. 119.—Development of pelvis of chick, after Miss Johnson. A, chick of 6 days.
B, older; C, 20 days; cartilage dotted, bone white. a, acetabulum; 2/, ilium; zs, ischium;
in, ischiadic nerves; on, obturator nerve; p, pubis; pp, pectineal process.

Fic. 120.—Pelvis of Galeopithecus, after Leche. ab, acetabular bone; i, ischium; 7,
ilium; p, pubis; cartilage dotted.

and pubis directed backward. The pubes, lying in the position of the
postpubes of the dinosaurs, never meet below except in the ostriches.
In the embryo (fig. 119) they are at first directed forward and only
attain the final position later. A pec-
tineal process arises from the aceta-
bular region and extends forward, simu-
lating the dinosaur pubis.

" Tn the mammals, obturator foramen
and ischio-pubic fenestra are united,
the opening being bounded on the
medial side by processes from ischium yg. r21—Teeft side of pelvis of
and pubis. All three bones may meet in sora arpa at es on
the acetabulum, but more often the ex- base Gon ae abitirator foes:
tension of ilium and ischium excludes ™¢?i?) 0S Pubis; sv, sacral vertebra.
the pubis from the fossa. A peculiarity is the common occurrence of an
additional bone in the formation of theacetabulum (acetabular or coty-
loid bone). This lies between ilium and pubic bone and may fuse with
any ofthe elements. In marsupials and monotremes the interpubic car-

8

II4 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

tilage persists for some time, or through life, but elsewhere it disap-
pears and the elements unite by symphysis. The same groups of non-
placental mammals are characterized by the presence of marsupial
bones (fig. 121). These are preformed in cartilage and extend for-
ward from either pubis in the ventral abdominal wall. Their homol-
ogy is very uncertain; but they are not the ypsiloid of the urodeles.

VZLLLLLL ES LLM
E

we

sss *

_ Fic. 122.—Diagrams illustrating theories of origin of appendages. 4A, B, C, origin of
biserial appendage (C) from gill arch (A); D, biserial appendage (archipterygium); Z, F,
evolution of elasmobranch fin; G, dotted lines indicate parts involved in origin of leg from
fin; H, dotted parts show another view of origin of elements of leg.

The Free Appendages.

These are of two kinds, the paired fins (ichthyopterygia) of the
fishes and the legs or their modifications (chiropterygia) found in all
classes of tetrapoda. The formér is merely a mechanism for altering
the position of the body in the water, and requires a small amount of
flexibility, being moved as a whole. The assumption of terestrial.
habits necessitates the support of the body above the ground and its
propulsion. Hence the chiropterygium must have a firmer skeleton,
with at the same time joints for motion and intrinsic muscles to move the
parts on each other. The chiropterygium was undoubtedly derived
from the fish fin, but the problem of how the change was made has not
been solved. Only paleontology can give the answer.

There are two views as to the origin of the chiropterygium, both based upon the
loss of certain ‘parts and the persistence of others in a modified form. One view
assumes the persistence of a basal as the framework (humerus or radius) of the

SKELETON. 11g.

upper limb. Two proximal radials as that of the next limb segment, while the
skeleton of ankle and foot is derived from a corresponding number of distal radials
on the anterior side of the fin. The ‘archipterygial theory’ of Gegenbaur assumes
an appendage like that of Ceratodus (the ‘archipterygium’) as the type from
which all legs and other fins have been derived, by a
shortening of the axis and a loss of radials, chiefly on
the preaxial side. The two views are illustrated in the
adjacent sketches. No known facts of either embry-
ology or paleontology throw any certain light on the
matter.

Cladoselache (fig. 107) and the lower ganoids
have what is apparently the most primitive type
of fin with a large number of basalia which
support a large number of radialia. From these,
as we go upward in the scale, there is a reduction
in the number of basalia, either by disappear-
ance or fusion, while the other parts are variously
modified. Thus in recent elasmobranchs the
characteristic number of basalia is three in the
pectoral, two in the pelvic fin. These are known,
from in front backward as the pro-, meso-, and
metapterygium, the middle one being absent

Fic. 123. Fic. 124.

Fic. 123.—Pelvic fin and part of girdle of Ceratodus, after Davidoff.a, axial skeleton
of fin; pil, iliac process; pim, processus impar; 7, radialia.
Fic. 124.—Skeleton of pectoral fin of Xenacanthus, after Fritsch.

from the hind limb. The numerous radials are jointed transversely
(fig. 109), permitting more flexibility, and these may be arranged
entirely on one side of the basalia (uniserial), or the metapterygium
may be prolonged as an axis, and while most of the radialia are
on the preaxial side, some may occur on the postaxial side (biserial)
as seen in the carboniferous shark, Xenacanthus (fig. 124). In the
recent species the skeleton of the fin is continued by actinotrichia.

116 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

In the male elasmobranchs the pelvic fin is divided into two lobes,
the medial, the so-called clasper (mixipterygium) being the longer and
narrower. ‘Thisis used in copulation and is supported by thespecialized
terminal radialia of the metapterygium.

In other ganoids and in teleosts the skeletal parts are more or less
ossified, the basalia more numerous than in the higher elasmobranchs
and are shortened and more closely associated with the girdles, while
the numerous radii form most of the
skeleton of the fin itself. It is not un-
common for the anterior element of the
pectoral fin to form a strong defensive
spine, not infrequently connected with a
poison gland. In some teleosts, e.g.,
eels, the pelvic fin may be lacking.
The fins of the dipnoi are easily under-
stood by comparison with a biserial fin
like that of Xenacanthus (fig. 124). The
axial part has been elongated and in
Ceratodus it bears biscerial radialia,
while in Protopterus and Lepidosiren
only the axis persists.

Fic. 125.—Cartilage skeleton of i imiti
digildenpinisiau ifepe ton linet Embryology tells little as to the primitive

larval Polypterus, after Budgett, buf, condition of the ichthyopterygium, for in the
foramina for blood-vessels; c, cora- procartilage stage the condensation of mesen-

coid; cf, coracoid foramen; mes,
mesopte eyesiines met, metapte ae chyme for the skeleton of the fin forms a con-

pro, protopteryigum; 7, developing tinuum which later becomes broken into the
radialia; s, scapula. separate parts (fig. 125).

The legs (chiropterygia) of all tetrapoda are essentially alike (fig.
108). Each consists of several regions, comparable in detail with each
other. The proximal is the upper arm (brachium) or thigh (femur)
containing a single bone, the humerus or femur in the fore and hind
limb respectively. The next region, the forearm (antebrachium)
or shank (crus), contains two bones, a radius or tibia on the preaxial
and an ulna or fibula on the postaxial side. Next follows the podium,
the hand (manus) in front, the foot (pes) behind, each consisting of
three portions. The basal podial region, the wrist (carpus) or ankle
(tarsus) consists of several small bones; the second division (metapo-
dium) is the palm (metacarpus) or instep (metatarsus) and lastly
come the fingers or toes (digits), each digit consisting of several bones,

SKELETON. 117

the phalanges. These separate parts are included in the accompany-
ing table, in which the terms given to the separate elements of the wrist
and ankle of man are included.

Fore Lime (arm) Hinp Luvs (LEG)
Upper arm (Branchium) Humerus =Femur Thigh
2 Radius = Tibia i Shank
Fore arm (Antebrachium { Ulna =Fibula { Crus)
( Naviculare Radiale = Tibiale Astragalus
(Scaphoid) (Talus)
Lunatum Intermedium = Intermedium
Triquetrum Ulnare = Tibiale Calcaneus
Centrale!+? =Centrale'+? Naviculare
; Pisiforme pedis Basi
Basi- (Scaphoid) :
podium )° Multangulum podium
Wrist : : Ankle
majus Carpale!=Tarsale! Cuneiform? (Parsus)
(Carpus) (Trapezium) a
Multangulum
minus Carpale? = Tarsale? Cuneiform?
(Trapezoides)
Capitatum Carpale*® = Tarsale® Cuneiform?
, (em 4
Hamatum ciel nenempuiges Cuboides
Carpale’=Tarsale®
Palm (Meta
(Metapo- Metacarpale!—5 = Metatarsale!—* or Instep
, dium)
dium)
Fingers (Phalanges) Digits'—° = Digits!—* (Phalanges) Toes

The basal podial region, which is nearly typical in some reptiles,
urodeles and man, consists of three rows of bones, a proximal of three
bones, a radiale or tibiale on the anterior side, an ulnare or fibulare
on the other, and an intermedium between them. The distal row
consists of five carpales or tarsales, numbered from the anterior side.
The third row is composed of one or two centrales between the other
rows. The metapodials and the digits, also numbered from one to
five, have, in some cases special names, the thumb (digit I) being
the pollex, the corresponding great toe being the hallux, the fifth
digit being called minimus.

From this typical condition all forms—legs, arms, wings—are
derived by modification, fusion and disappearance of parts. The
more distal a part the more variable it is; reduction takes place on the
margins of the appendage, the axial portions being the last to disappear.
Occasionally in various groups (amphibia, mammals) there occur what

118 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

are interpreted as rudimentary additional digits—prehallux, prepol-
lex, postminimus—but their statusisuncertain. Therearealsocertain
membrane bones developed in the appendages including the patella
(knee-pan) in some reptiles, birds and many mammals, in the tendon
that passes overghe knee-joint, the fabellz in the angle of the knee of
a few mammals, ‘and the pisiforme in the carpus of man and some other
mammals.

In the ancestral limb, as exemplified in the urodeles, the basal
joint was directed horizontally at right angles to the axis of the body, but
higher in the scale it approaches the sagittal plane and in such a way
that the angles of the fore and hind limbs open in opposite directions.

nie a Besides there is frequently a torsion of the bones

ae Affi of the forearm (fig. 127) or shank on each other.

®. wil “a ‘g The lower amphibians have nearly typical legs,
L although, as in Siren and Amphiuma, they may be

Ss, <\ greatly reduced, while in some stegocephals and

Fic. 126. Fic. 127.

Fic. 126.—Tarsus of Geotriton, after Wiedersheim, showing the arrangement of the
metapodial elements. c¢, centrale; f, fibulare; F, fibula, 7, intermedium; #, tibiale; T
tibia; 1-5, tarsales. .

Fic. 127.—Torsion in developing human arm, after Braus. u, 7, ulna and radius;
dotted line, course of radial nerve.

the gymnophiones they are entirely lacking. In the anura the radius
and ulna or tibia and fibula are frequently fused and the tarsals
elongated.

The most marked feature of the reptilian limb is the occurrence of
an intratarsal joint, the motion of the foot upon the leg being largely
between the two rows of tarsal bones, instead of between tarsus and the
bones of the shank (fig. 128). There is also a greater range of form
than in the amphibia. Limbs are lacking in snakes and some lizards;
on the other hand there is a great increase in the number of phalanges,
correlated with a shortening of the proximal bones in the plesiosaurs,
which reaches its extreme in the ichthyosaurs where there may be a
hundred phalanges in a digit. The wings of the pterodactyls are re-

SKELETON, 119

markable for the great development of the fifth digit (elongation of the
phalanges) as a support for the wing; the other digits are more normal.

Fic. 128.—Hind leg of snapping turtle (Chelydra) showing intratarsal joint at 7 h,
humerus, 7, radius; #4, ulna; /-V, digits.

The wings of birds (fig. 55) are even more modified. Until the
carpus is reached the structure is approximately normal, but the carpal
bones are greatly reduced by fusion, while the metacarpals and digits,
extensively modified, number only
three. Developmentshows thatthe
first digit is entirely lost and that the
fifth metacarpal, which is present in
the embryo, fuses early with the
fourth, so that the digital formula
is JJ, III, IV. There is also an ex-
tensive fusion of the bones of the
tarsus and pes. The ankle-joint is
markedly intratarsal, the basal row
of tarsal bones fusing with the tibia
(the fibula is reduced) to form a
‘tibiotarsus,’ while the tarsales
have united in the same way with
the fused metatarsals, forming a
‘tarso-metatarsus’ (fig. 129).
The toes are rarely more than four
in number, the first apparently lack-
ing, and as a rule the number of
phalanges increases from two in Pe erg am al ap
digit II to five in digit V. Many 1, tarsometatarsus; #, tibiotarsus; II-V,
birds have the toes reduced to “8's
three and in the true ostriches to two.

In the mammals the limbs, especially the fore limbs, exhibit a con-
siderable range of modification. Thus in the primates the skeleton is

I20 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

nearly typical, but there is a marked power of rotation of the foot and
especially of the hand by the motion of the lower end of the radius
around the ulna. There also the appendages may form grasping
organs, both features being found to a less extent in several lower groups.
In the bats digits II to V are greatly elongated (either metacarpals or
phalanges may be lengthened) to support the wing, the first digit remain-
ing normal. In the whales and sirenians the basal parts of the fore
limb are greatly shortened, while there is a multiplication of the pha-
langes, recalling that of the plesiosaurs. The hind limb is entirely
lacking in the sirenians and some of the whales; in other whales there
are two vestigial bones (?femur and tibia) imbedded in the muscles of
the trunk.

The mammalian humerus is frequently perforated by a (supra- or
entepicondylar) foramen passing through the inner lower end, a
feature found elsewhere only in some theriomorphs. In many un-
gulates the ulna is reduced and may be fused with the radius; elsewhere
it is well developed. Even where reduced it always bears on its prox-
imal end a strong olecranon process, extending beyond the elbow-
joint for the attachment of the extensor muscles of the lower limb.
The femur bears a varying number (up to three) of prominences or
trochanters for the attachment of muscles. The fibula resembles the
ulna in its tendency to reduction. The patella (p. 118) at the knee-
joint is analogous to the olecranon process, though it never joins the
other bones.

The details of the modification of the feet cannot be described here.
The ankle-joint is never intratarsal but always between tarsal and
crural bones. ‘There is considerable variety in the extent to which the
bones of the feet rest upon the ground. In the plantigrade foot, as in
the bear and man, the sole of the foot includes the metapodial bones? in
the digitigrade forms, like the dog and cat, the sole includes only the
distal phalanges, while in unguligrades (cow, horse) the weight of the
body is supported on the hoofs (p. 27) developed on the upper (ante-
rior) surface of the distal phalanges. There is frequently a reduction
of the digits, reaching its extreme in the horse where only digit III
persists in a functional condition.

THE C@LOM (BODY CAVITIES).

‘The ccoelom includes all of the primitive cavities, right and left,
enveloped by the mesothelium (p. 10). With the division of the

C@LOM. I21I

walls into epimere, mesomere and hypomere, the ccelom undergoes a
corresponding division. That portion in the epimere is divided into
a series of cavities in the myotomes (myoceeles), which are eventually
obliterated (p. 126); the portions in the mesomere persist only as the
lumina of the excretory organs and their ducts, described under the
urogenital system; while that part of the original ccelom in the hypomere
gives rise to all of the permanent body cavities of the adult.

The hypomeres gradually descend between the ectoderm and the
entoderm (fig. 130) until their lower margins meet, ventral to the diges-
. tive tract. In this way the latter
becomes surrounded by a pair of
cavities, the splanchnoceeles or body
cavities of the adult. Each is
bounded by epithelium, the tunica
serosa, in which an outer or somatic
wall is turned toward the ectoderm,
while the inner or splanchnic wall
adjoins the alimentary canal. Later,
when the muscle plates extend down-
ward (fig. 135), they unite ectoderm
and serosa into the outer body wall,
the somatopleure, while the invasion
of mesenchyme unites the splanchnic , ee . henoties

serosa with the entoderm into a similar hypo-, and mesomeres, the walls of the
ceelom, dm, vm, to form the dorsal

splanchnopleure. and ventral mesenteries. 4, alimen-
Mesenteries.—As has just been ‘ty canal; ¢, ectoderm; so, sp, soma-

stated the walls of the two coelomic ie ea ee
cavities meet below the digestive tract, thus forming a double membrane
running lengthwise of the body and binding the alimentary canal to the
ventral body wall. This membrane is called the ventral mesentery.
In a similar way the splanchnic walls meet above the digestive tract
forming a dorsal mesentery. ‘These mesenteries are eventually more
than double serosal walls, since mesenchyme comes in between, uniting.
them and affording a tissue through which blood-vessels, lymphatic
vessels and nerves can reach the digestive organs.

For convenience of reference different parts of these mesenteries have received
special names, according to the organs supported. The ventral mesentery
usually almost entirely disappears, only a small portion persisting in the region
of the liver, the mesohepar, which, in the ichthyopsida may carry blood-vessels

122 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

to that organ. The dorsal mesentery is usually more complete, but it is
interrupted in various groups. Its regions are called mesogaster, mesentery
proper, mesocolon, mesorectum, etc., accordingly as they support stomach,
small intestine, colon and rectum. Except in the cyclostomes the alimentary
canal is bent on itself and the bends are connected by similar membranes, here
called omenta. These also have special names. Thus the gastrohepatic
omentum (small omentum) connects stomach and liver; then there are gastro-
splenic, doudeno-hepatic omenta, etc., while in mammals there is a great
omentum, a double fold of mesogaster and mesocolon which connects the stomach

YY

AUT AAN \S
Wy ee
AAR ,

RR
SSA = es y
$o AVY _ Gs

Fic. 131.—Diagrammatic section of a vertebrate to show the relation of the body walls,
etc. av, aorta; c, coelom; e, ectoderm; ep, epaxial muscles; g, gonads; ha, hemal arch; hp,
hypaxial muscles; 7, intestine; mes, mesentery; x, nephridium; 0, omentum; 7, rib; so,
somatopleure; sp, splanchnopleure; v, vertebra.

with the colon. This forms a large sac, the bursa omentalis, which opens into
the rest of the body cavity by a small foramen of Winslow (foramen epiploicum)
near the hinder end of the liver.

Homologous structures are formed in connection with other organs. Thus
in the formation of the heart there are formed temporary membranes, the meso-
cardia, connecting it with the walls of the pericardium; while in the mammals a
mediastinum, between the two pleural cavities binds the pericardium to the ventral
body wall. Frequently the reproductive organs project so far into the body cavity
that the serosa meets behind them, forming similar supports, mesovaria for the
ovaries, mesorchia for the testes.

The primitive body cavity extends from a point just behind the head
back to the vent. It soon becomes divided into two cavities. Just
in front of the liver a pair of blood-vessels, the Cuverian ducts, enter
the heart from the sides. These arise in the ventral body wall but soon
ascend, carrying the serosa before them. In this way they form a

CCELOM. ; 123

transverse partition, the septum transversum, attached to the anterior
wall of the liver, which cuts off an anterior pericardial cavity, con-
taining the heart, from the posterior part (metaccele) of the body cavity.
In many lower vertebrates the septum is not complete, but one or more
openings (pericardio-peritoneal canals) connect the pericardium
with the metaccele.

In the mammals a second partition, the diaphragm (p. 135), cuts
off another pair of (pleural) cavities from the metaccele. Traces of
similar structures occur as low as the amphibia; their homology with
the mammalian diaphragm is not always certain, but in some cases the

Fic. 132.—Diagram showing the relations of the ccelomic cavities (black) in A, fishes,
B, amphibians and sauropsida; and C, in mammals; H, heart in pericardial coelom;
L, liver; P, lungs in C in pleural ccelom; S, septum transversum; D, diaphragm.
parts concerned have the same nerve supply. The development of the
diaphragm is very complicated and can be stated here only in outline.
It involves in part the septum transversum, in part is a new formation.
At first a part of the metaccele extends forward, dorsal to the pericardial
cavity and alimentary canal, and into this the lungs protrude as they
are developed. Then a pair of muscular folds arise from the dorsal
surface of the metaccele, posterior to the lungs; these grow downward
until they meet the septum adjacent to the attachment of the liver,
cutting off a pair of pleural cavities containing the lungs, from the
rest of the metaccele, now known as the peritoneal cavity. With
increase of the lungs in size the pleural cavities increase, insinuating
themselves laterally beween the pericardium and the body wall, and
eventually reaching the ventral side, where the two are separated by
their two walls, the ventral mediastinum. From the original folds
the dorsal muscles of the diaphragm are derived; the ventral come
from the rectus muscles of the ventral abdominal wall. The dia-

124 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

phragm undergoes many shiftings of position before reaching its final
place.

The tunica serosa lining the various divisions of the splanchnoccele has special
names in each. Thus the pericardial and pleural cavities are lined by peri-
cardium and pleura respectively, that portion of the pericardium covering the
heart being sometimes called the epicardium. The metaccele or peritoneal cavity
is lined by the peritoneum. ;

The metaccele is not always cut off completely from the external
world. In the lower vertebrates the urinary ducts frequently open into
the body cavity by the nephrostomes (fig. 133), and in these and even
in the mammals the oviducts of the female connect the cavity with the

Cc l

} °LLZELLZZZZZ ZIZZO
Fic. 133.—Diagram of possible connection of ccelom with the exterior, modified from
Bles. c, coelom; ci, cloaca; g, glomerulus of kidney; 7, intestine; x, nephrostome; fa,
porus abdominalis.
exterior. In many fishes there are pori abdominales leading from the
metaccele to the outside near the vent. These may be single or paired
and are found in cyclostomes, many elasmobranchs and teleosts,
ganoids, and dipnoi. None are known in amphibia, birds or mammals,
but in turtles and crocodilians so-called peritoneal canals occur,
usually ending blindly in chelonians, but emptying into the cloaca
in the crocodiles. These may be homologous with the abdominal
pores, but only the development can settle the question. In some
fishes the pores serve for the escape of the genital products; in other
animals their function is uncertain.

THE MUSCULAR SYSTEM.

Practically all motion in vertebrates is caused by muscles arising from
the mesoderm. While other cells may have a certain power of chang-
ing shape, the muscle cells possess this in a marked degree, and so
that they may cause the greatest amount of motion in the parts to which
they are attached, they are very long, stimulation causing them to
contract in length and at the same time to increase in diameter.

MUSCULAR SYSTEM. 125

There are two kinds of muscles which differ in origin, histological
appearance, physiological action and distribution. The smooth
muscles, the appearance of which has been described (p. 20), arise
from the mesenchyme and are not under control of the will, but are

~ ~innervated by the sympathetic nervous system. ‘Their action is much
slower than that of the other type. They are found in the skin, in the
walls of blood-vessels and of the alimentary canal, and in the urogenital
system. Occasionally they occur as isolated fibres, but frequently
they form sheets or bands, sometimes of considerable thickness.

In the alimentary tract they are arranged in two layers in the straight
parts of the tube, an outer layer of fibres which run longitudinally,
and inside this a layer of circular muscles. In enlargements of the
tube this regularity is interrupted and the course of the fibres is more
irregular. The circular muscles, by their contraction, lessen the diam-
eter of the canal, at the same time causing it to elongate, while the
longitudinal fibres shorten it and cause it to increase in diameter. In
the blood-vessels there are only circular fibres, the snares of the
lumen being caused by the internal blood pressure.

The striped muscles are derived from the walls of the pans and
hence are of mesothelial origin. Excepting those of the heart (to be
mentioned below) and some of those at the anterior end of the alimentary
canal, they are under control of the will and are supplied by the motor
nerves of the brain and spinal cord. They are also able to contract
more rapidly than the smooth muscles. The striped muscles make up
the great mass of the musculature—the ‘flesh’—of the body. They
occur in the body walls, organs of locomotion, the head, diaphragm
and the anterior part of the digestive canal.

The voluntary muscles are derived in part from the somites (myo-
tomes), in part from the lateral plates, the latter furnishing the vis-
ceral muscles, including those of the head (except the eye muscles and
the sternohyoid and its derivatives in the higher vertebrates) and those
of the heart. The heart muscles, the development of which is traced in
the account of the circulatory system, differ from the other striped
muscles in the uninucleate condition of their short and usually branched
cells, while, physiologically, they are involuntary in character.

THE PARIETAL MUSCLES.

After the myotomes are cut off from the rest of the ccelomic walls
(p. 14) each consists of a closed sac, containing a part of the colom

126 COMPARATIVE MORPHOLGY OF VERTEBRATES.

(myoceele) and an inner (splanchnic) and an outer (somatic) wall
The cells of the splanchnic wall rapidly increase in number and size,
thus tending to obliterate the myoccele. At the same time they be-
come rearranged, so that, instead of forming a cubical or columnar
epithelium, they have their long axis parallel to the long axis of the body

TOC CE S

Fic. 134.—Myotomes of Amblystoma developing into muscle fibres. ec, ectoderm; my,
myoccele; ms, mesenchyme; so, somatic layer which will form corium.

(fig. 134), each becoming multinucleate. Gradually the mass of the
protoplasm becomes converted into contractile substance and the cell
is converted into a muscle fibre, the nuclei being in the interior in the
lower vertebrates, on the surface of the fibres in the mammals. In this
way the splanchnic wall of each myotome is converted into a muscle;

Fic. 135.—Diagram of descending myotomes. c, ccelom; g, gonad; m, splanchnic wall
of myotome developing into muscles; mc, myoccele; p, peritoneum; pd, pronephric duct;
so, somatic wall of myotome; v, ventral border of myotome.

hence there are as many pairs of these primitive muscles as there were
of myotomes. The somatic wall of the myotome does not participate
in the muscle formation, but is gradually changed into mesenchyme
and eventually gives rise to the corium of the skin. Mesenchyme also
invades the spaces between the successive myotomes, develops into

‘

MUSCULAR SYSTEM. 127

fibrous connective tissue, and forms the ligamentous connections
(myosepta, myocommata) between the muscles of a side. This
primitive condition is readily recognized in the trunk and tail of the
lower vertebrates, and even in the adults of the more modified birds
and mammals the original segmentation can be traced in the inter-
costal and rectus abdominis muscles. At first the myotomes lie
at about the level of the notochord and spinal cord, but with growth
they extend upward and to a greater extent downward, insinuating
themselves between the skin and the walls of the ccelom and thus

Fic. 136.—Head of embryo dogfish (Acanthias) seen as a transparent object, showing
the preotic mesodermal somites, with dotted outlines, as a, 1, 2, and 3. 5'-d‘, gill clefts,
the fifth not yet open; e, eye; oc, otic capsule; p, epiphysial outgrowth; s, spiracle; V, tri
geminal, VII, facial-acustic; IX, glossopharyngeal; X, vagus nerves.

forming part of the somatopleure. The downward growth continues
until the muscles of the two sides all but meet in the mid-ventral line,
the intervening space being occupied by connective tissue, the linea
alba of the adult.

In the fishes the trunk and tail muscles formed in this way become
divided horizontally into dorsal and ventral portions, the epaxial and
hypaxial muscles, the line of division which follows more or less
closely the lateral line, being marked by a partition of connective tissue
already mentioned (figs. 30,131). These plates of muscle do not retain
their flat ends in the adult, but oné end becomes conical and fits into a
corresponding hollow in the next plate. In the tail of the amphibia

128 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

epaxial and hypaxial muscles are clearly recognizable, but farther
forward the hypaxials are greatly reduced, and in the amniotes the
reduction is carried so far that the hypaxial muscles, greatly modified,
can only be recognized in the cervical and pelvic regions.

In the head the developmental conditions are more complicated
than in the trunk, our information being most complete with regard to
the ichthyopsida. Here, in the region which develops into the head,
ten coelomic pouches are developed (in amniotes the number is appar-
ently twelve). These are known by number, except that the most anter-
ior, which was not known when the numbers were applied is called A.
These ccelomic cavities (also known as head cavities) differ from the
myotomes farther back in having no undivi-
ded portion of the coelom below, correspond-
ing to the hypomeral zone, a difference possi-
bly due to the existence of visceral clefts in
this region (fig. 136).

Four of these cavities lie in front of the ear.
Of these A disappears completely, its cells
joining the mesenchyme, while the other three

Fic. 137.—Diagram of give rise to the ‘eye muscles’ which move

the eye muscles of the right * : :
eye, teen fern die tostial. ooo eye Dall. Without going into all of the

side. er, external rectus; ifr, details, 1, which lies in front of the mouth,
ablinue: Te iniemtl recta, gives rise to the superior oblique muscle; 2,
so, superior oblique; s7,supe- which lies in the region of the jaws, forms four
rior rectus; JZZ, coulomotor; a 7 ;

IV, trochlearis; VI,abducens muscles, the inferior oblique and three of
RSENS? the rectus muscles (in some forms also a
retractor bulbi), while 2, in the hyoid region, develops the external
rectus. This method of origin explains the distribution of the eye-
muscle nerves to be described later, each nerve supplying only the
derivatives of a single myotome. Several of the other head myo-
tomes disappear in development, while the posterior form the so-called
hypoglossal musculature (fig. 138).

In the above account there is given merely the origin of the con-
tractile tissue of the muscles. To this other parts of connective tissue
are added. Mesenchyme cells invade the masses of muscle fibres,
forming envelopes (perimysium) which bind the fibres into bundles
(fasciculi) which, in turn, are united by other envelopes, the fascia.
These connective-tissue envelopes are extended beyond the contractile
tissue and form the cords or tendons by which the muscle is attached to

MUSCULAR SYSTEM. 129

other parts. One point of attachment, the origin, is fixed, that
to the part to be moved iscalledtheinsertion. Tendons may belong and
slender, allowing the muscle to lie in or near the trunk, while the part
to be moved is in the appendage. Again they may form broad flat
sheets (aponeuroses), and these may occur not only at the ends but in
the middle of a muscle. Not infrequently parts of tendons may ossify,
as in the patella or in the ‘drum-stick’ of the turkey. Small rounded
ossifications of this kind are called sesamoid bones. In a few cases
the parietal muscles are without attachment, but form rings which are
used to diminish the size of an opening (sphincter muscles).

Muscles vary greatly in shape. They are usually short and flat in the trunk,
prismatic or spindle-shaped in the appendages. They may be simple or they
may have several ‘heads’ or points of origin (biceps, triceps, etc.), or several
points of insertion as in pinnate or serrate muscles. Again, there may be two
or more contractile portions (bellies) in a muscle, separated by a tendon or
aponeurosis. 5

Usually muscles are arranged in antagonistic groups, the action of one being
the opposite of its antagonist. Thus there are flexors to bend a limb, extensors to
straighten it; elevators to close the jaw, depressors to open it; sphincters working
against dilators, etc.

Only a few points in the progressive modifications of the primitive
musculature described above can be mentioned here, partly from lack of
space, partly from deficient knowledge. ‘There are great difficulties in
tracing exact homologies through the different groups of vertebrates,
on account of their very different functions in the separate classes and
their great variability, even in the same family. The best test of
homology is nerve supply, every muscle derived from any one myotome
being innervated by branches of the nerve originally connected with
the segment, as is beautifully illustrated in the case of the eye muscles
as mentioned above. Next in importance are origin and insertion of
the muscles, while the work done by the muscles is of little value.
Differentiations from the primitive condition may take place in various
ways. A single muscle may split into layers or it may divide longi-
tudinally into two or more distinct muscles, or transversely into two
successive portions. On the other hand, two muscles, different in
origin, may fuse, while with loss of function of a part, its muscles may
degenerate or entirely disappear. Muscles may wander far from their
point of ontogenetic origin and become connected with parts widely
remote, acondition strikingly illustrated in the facial muscles of the

9

130 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

higher mammals, where nerve supply still shows the original history.

In the ichthyopsida the trunk muscles clearly show their myotomic
origin, but even here there are tendencies to division and specialization.
The ventral muscles on either side of the body cavity of the amphibia
(fig. 140) are divided into a lateral oblique and a medial rectus sys-
tem, the rectus muscles of the two sides being separated by the linea
alba already referred to. The rectus muscles, in turn, become divided
into successive groups, a rectus abdominis in the abdominal region,
extending from the pelvis to the sternum; a sternohyoid from the

Fic. 138.—Diagram of muscle segments in head of embryo vertebrate, based upon a
shark, after Neal. The anterior myotomes tend to divide into dorsal and ventral moieties;
persistent myotomes lined, transient with broken lines; central nervous system dotted,
nerves black. a, premandibular somite; ab, abducens; nerve, hyp, hypoglossal musculature;
hymn, hypoglossal nerves; om, oculomotor nerve; s#, spiracle; 1-6, first six somites (4, 5, 6, _
functional in Petromyzon); J-VIII neuromeres.

sternum to the hyoid bone, and a geniohyoid from the hyoid to the tip
of the lower jaw. The oblique region is also divided into three layers
(obliques and transversus) characterized by the direction of the fibres.
In the higher vertebrates, with the appearance of well developed ribs,
the oblique muscles furnish the two layers of intercostal muscles,
extending from rib to rib, and in front of the ribs they form the scalene
muscles, extending from the ribs along the side of the neck, and the
sternocleidomastoid, from the breast bone and clavicle to the skull.
In the non-placental mammals a strong pyramidalis muscle extends,
ventral to the rectus, from the inner side of the marsupial bones to the
sternum, but disappears with these bones.

The dorsal muscles are more conservative, undergo less modifica-
tion than those just mentioned, and always show, more or less clearly,
their metameric nature. They become connected with various parts
of the vertebre and with the ribs, and are correspondingly divided into

MUSCULAR SYSTEM. I31

different groups. Thus the spinales connect the spinous processes, the
transversales the transverse processes of the successive vertebrae,
while the transverso-spinales extend from the transverse process of
one vertebra to the spinous process of the next. In the higher verte-
brates the anterior spinalis, connecting the first vertebra with the skull,
is divided into several rectus capitis muscles. The longissimus
dorsi group extends from the pelvis to the head, lying on either side in
the angle between spinous and transverse processes. It may be differen-
tiated into separate muscles—a longissimus dorsi proper in the lumbar
region, an ileo-costalis inserted on the dorsal part of the ribs, anda
longissimus capitis along the side of the neck to the temporal region
of the skull.

The muscles which move the appendages are divided into the
intrinsic, which are located in the limb itself and have their origin
either from the bones of the limb or from the supporting girdle, and the
extrinsic, which have their origin on the trunk and are inserted on the
girdle or the base of the limb. The latter move the limb as a whole,

my my

“ev Se

Fic. 139.—Budding of muscles of appendage from myotomes in Pristiurus, after Rabl
b, muscle buds; my, myotomes.

while the intrinsic bend the limb on itself. As would be expected from
the motions of the fins, the intrinsic muscles are hardly noticeable in the
fishes, the various movements being accomplished by the extrinsic
group. These latter are divided into protractors which draw the
member forward; retractors which pull it back against the body;
levators which lift it and depressors which pull it down.

In those vertebrates which are sufficiently known the first traces of the develop-
ment of the musculature of the appendages are the appearance of two buds (fig.
139) from the ventral border of a varying number of myotomes in the region of the
developing limb. These buds proliferate cords of cells which soon lose their
distinctness and form a blastema from which the intrinsic muscles arise, the

definitive muscles being innervated by as many spinal nerves as there are contribut-
ing myotomes. The extrinsic muscles arise directly from the myotomes.

132 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

With the development of the paired appendages into organs for the
support of the body (tetrapoda) the skeleton of the leg is converted into
a series of levers, and the intrinsic muscles are correspondingly dif-
ferentiated and developed. Details cannot be given here as there are
so many modifications, but they may be grouped as flexors, which
bend the limb or its parts; extensors which straighten it, and rotators
which turn it on its axis. These undergo the most modification in the
peripheral regions, the muscles of the upper arm and thigh being more
constant in character and position. Even more constant are the ex-
trinsic muscles, which may be grouped as in fishes. Most prominent

Mi

LEE |

LEZ iy iy
Wy, i
UYU YE?

Fic. 140.—Superficial muscles of anterior part of Salamandra maculata, after Fiir-
bringer. a, anconeus; bi, humero-branchialis inferior (biceps); bs, levator scapul; cuc,
cucularis; dir, dorso-trachealis; dg, digastric; ds, dorsalis scapule; eo, external oblique;
id, latissimus dorsi; m, petro-tympano-maxillaris (masseter); mh, mylohyoid; pc, pectoralis;
ph, procoraco-humeralis; ra, rectus abdominis; spc, supracoracoid.

of the levators of the fore limb are the trapezius and levator scapule
muscles, while the pectoralis and serratus anterior act as depressors;
the sternocleidomastoid and the levator scapule anterior act as
protractors, the pectoralis minor and the latissimus dorsi are their
antagonists. In the pelvic region the extrinsic muscles are less dif-
ferentiated in function. The pectineus and adductors act as pro-
tractors, the pyriformis counteracts them; the limb is drawn toward
the middle line by a pubofemoralis, while the gluteus muscle acts as
a retractor and elevator. ;

THE VISCERAL MUSCLES.

In the gill-bearing vertebrates a special system of muscles is devel-
oped in connection with the visceral arches, which have to open and
close the visceral clefts, including the mouth. With the loss of the gills
some of these muscles are lost while others become changed in function,
several retaining their connection with the hyoid. These visceral
muscles may be divided into two sets according as they are derived

MUSCULAR SYSTEM. 133

from muscles which originally ran in a transverse (circular) or in a
longitudinal direction.

To the first category belong the epibranchial muscles, the sub-
spinales and interbasales, which lie in the dorsal part of the branchial
region, while the coraco-arcuales are in the ventral or hypobranchial
half. The most anterior of the circular group are those which open
(digastric or depressor mandibule) or close (adductors) the mouth,
and the mylohyoid which extends between the two rami of the lower

Fic. 141.—Dorsal and ventral head muscles of the skate (Raia), after Marion; the dorsal
muscles more deeply dissected on the left side, the ventral on the right. aml, lateral man-
dibular adductors; emm, medial mandibular adductors; csd, csv, dorsal and ventral con-
strictors; cm, coraco-mandibularis; chy, coraco hyoideus; chm, coraco-hyomandibularis;
cbr, coraco-brachialis; cac, common coraco-arcual; intbr, interbranchials; l/s, superior
labial levators; /mz, levator of lower jaw; /4m, hyomandibular levator; /r, levator of rostrum;
tr, trapezius; VII, seventh nerve; dm, depressor mandibule (digastric).

jaw. Usually there are several adductors, known as masseter, tem-
poralis, pterygoideus, accordingly as they have their origin from
different parts of the skull. The longitudinal muscles are largely con-
fined to small slips which pass from one arch to the next. In the
amphibians these various muscles undergo considerable differentia-
tion, while in the amniotes this is in part carried farther, in part is re-
duced on account of the loss of branchial respiration and the degenera-
tion of the parts connected with it. Hence the most noticeable of the
visceral muscles are those connected with opening and closing the
mouth.

134 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

THE DERMAL MUSCLES.

The muscles already mentioned are connected with the skeleton,
but in the higher vertebrates a dermal musculature appears in which
the muscles are inserted in the skin, although they are derived from the
skeletal muscles. This system is poorly developed in the amphibia,
and increases in the reptiles and birds, where it serves to move the
scales, scutes and feathers. It is especially noticeable in the snakes,
where it is largely concerned in the movement of the scutes in
creeping.

The system acquires its greatest development in mammals. In the
marsupials, for instance, there is an extensive dermal musculature, the
panniculus carnosus, covering a large part of the body and the ap-
pendages. It is by means of this that various mammals twitch the

Fic. 142.—Principal dermal muscles of head of man. a, as, auriculares anterior and
superior; 7, frontalis; m, masseter; oc, occipitalis; 00, orbicularis oris; of, orbicularis pal-
pebrarum; pm, platysma myoides; s, sternocleidomastoid; ¢, trapezius.

skin to dislodge insects, etc., while armadillos and hedgehogs roll them-
selves into a ball by means of a part of the layer. In the primates the
dermal muscles are restricted to the neck (platysma myoides) and the
head, all parts of them being supplied by the facial nerve belonging
primitively to the hyoid region. The platysma extends forward from
the neck and by growth and division gives rise to the muscles of ex-
pression—the orbiculares which close the lips and eyelids, the muscles

MUSCULAR SYSTEM. 135

which lift lips, nose and lids and those which move the ears—muscles
which as a whole have their highest development in man (fig. 142).

THE DIAPHRAGM.

The diaphragm is a transverse voluntary muscle which crosses the
body cavity of the mammals just behind the pericardium and lungs.
Its muscles are in part derived from those of the back, in part from the
rectus muscles of the lower surface. Various attempts have been made
to recognize similar muscles in the lower vertebrates, in some cases
with considerable success. Its development is outlined in the section
on the ccelom (p. 123). The diaphragm is dome-shaped and is attached
to the vertebral column and to the ribs. It is traversed by the
cesophagus and the large arterial and venous trunks. In some verte-
brates the muscular portion is confined to the margin, the centre being
membranous; in others the muscle fibres extend across it. Contrac-
tion of the muscles flatten it, thus enlarging the pleural cavities and
drawing air into the lungs, thus aidingin respiration. Itis supplied by
the phrenic nerve.

ELECTRICAL ORGANS.

It is well known that the contraction of a muscle causes the dis-
charge of a minute amount of electrical energy, so it is not surprising
that in certain cases muscles are modified into electrical organs. ‘The
known cases occur only in elasmobranchs and teleosts. The discharge
is weak in most species, but is strong in Torpedo and Gymnotus. In all
but Malapterurus the electrical organs are clearly modified muscles, situ-
ated in the head in Torpedo and Astroscopus, in the trunk of Gymnotus,
and in the tail of Raia, the nerve supply being correspondingly varied.
Thus in Torpedo the seventh, ninth and tenth cranial nerves are con-
cerned, while in Gymnotus and the skates the supply is from the spinal
nerves. Malapterurus is peculiar in that the organ is in the integu-
ment and has been supposed by some to arise from modified glands.
It is more probable that here as elsewhere it is derived from the muscles,
as the organ is under control of the will; the development has yet to be
studied. This diversity of origin clearly shows that the electrical
functions have been separately acquired in the different species.

The organs are composed of a large number of electrical plates

136 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

(electroplaxes) arranged at right angles to the axis of the primitive
muscle, each derived, where the history has been traced (Torpedo,
Raia), from a primitive muscle cell. In the typical condition each
plate consists of an outer electric layer, differentiated into a nervous side
and a so-called nutritive side, with a middle striated layer between them,
the latter in a few cases being weakly developed or absent. Nervous
stimulation is always by motor roots leading to the nervous layer, the
connexion corresponding to the nerve-end of a muscle cell. Numbers

Fic. 143.—Head of Astroscopus y-grecum, after Dahlgren and Silvester. The dotted
line on right shows extent of electric organ, on theleft the eye-muscles, and nerves as forced
out of place by the electric organ. ab, abducens; cil, ciliary nerve; e, eye; en, electric nerve;
n, naris; olf, olfactory nerve; om, oculomotor; of, optic nerve; re, rif, rint, rs, external, in-
ferior, internal and superior rectus muscles; rp, palatine nerve; so, superior oblique muscle;
tf, trigeminal-facial nerve.

of these electroplaxes are included in a connective-tissue compartment
with a gelatinous substance between them and all with their nervous
layer turned in the same direction.

In Torpedo the organ apparently is derived from part of the jaw
muscles and the prisms of plates are arranged vertically. In Astro-
scopus (fig. 143) it is supposed that the tissue comes from one of the eye
muscles, while in Gymnotus the ventral trunk muscles are concerned
and the columns of electroplaxes are horizontal. In the same fish the
discharge is always in the same direction, e.g., in Torpedo from below
upward.

NERVOUS SYSTEM. 137

THE NERVOUS SYSTEM.

Nervous and sensory structures are closely related to each other,
and their distinction in the higher animals is the result of differentiation
among cells which were originally both nervous and sensory in character,
and it is in this broader sense that the term nervous structures is used
in these introductory paragraphs.

The nervous system primarily has to inform the animal of the con-
ditions, good and bad, in the environment, to correlate this information
and to regulate the motions so that advantage may be had of this knowl-
edge. These facts have determined several features of the nervous
system. ‘Thus they have determined its origin in the ectoderm, the
outer layer of the body, which comes into relation with the external
world. Since this information has to be carried to internal parts, con-
ducting tracts or nerves have arisen, while the correlating function
has been localized in the body of the cells where incoming and out-
going tracts meet.

Most important of the primitive functions was the determination
of the character of the food, which would lead to the greater aggregation
of the nervous tissue around the mouth. As we have seen (p. 11)
the anlage of the central nervous system of the vertebrates occupies
such a position around the blastopore, or mouth of the gastrula, in the
form of the neural plate. As the external surface of the body is most
exposed to injury, the nervous structures, with the closure of the blasto-
pore, have been protected by removal to a deeper position, through the
rolling of the plate into a tube. The closure of the blastopore brings
the two halves of the plate into close association with each other, making
it a bilateral structure. With bilaterality comes the tendency of one
end of the animal to take the lead, resulting in the concentration of
nervous and sensory structures at the anterior end, which first comes
in contact with foreign objects. In this way a brain has been special-
ized apart from the rest of the nervous system.

With the appearance of metamerism in the mesothelium and the
development of muscles from the myotomes there results a serial
repetition of motor nerves going to these, since each muscle must have
its own nerve supply, while sensory nerves are the result of the sinking
of the neural plate to a deeper position, as the sensory organs must be
largely in the skin.

138 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

The close association of sensory and motor nerves in the trunk region of verte-
brates is not yet satisfactorily explained. The fact that in Amphioxus the two
kinds of nerves are independent of each other throughout their course shows that
the vertebrate condition is not primitive.

The infolding of the nervous plate has been described (p. 11)
and with that stage the present account begins. As the plate is broad-
est in front, the result is a larger anterior portion of the tube, the brain,
while the rest of the tube gives rise to the spinal cord. Brain and
cord constitute the central nervous system, while. the nerves arising
from the brain and cord form the peripheral nervous system.

CENTRAL NERVOUS SYSTEM.

The two halves of the neural plate are separated by a median band
of non-nervous tissue, hence, when it is rolled into a tube, the mid-
ventral line—the floor plate—is thinner and differs from the side walls.
With the closure of the tube (fig. 144, A) a similar roof plate appears,

Fic. 144.—A, diagram of early spinal cord; B, later, showing increase in size and con-
sequent ventral fissure. c, central canal; e, ectoderm; /, floor plate; g, anlage of spinal
ganglion; mc, neural crest; 7, roof plate; s, sulcus of Monro; v, ventral fissure.

while the lumen of the tube, the central canal, is oval in section, its
side walls, consisting of embryonic nervous tissue, being thicker than
roof or floor.

From the method of its formation it will be seen that the inner surface of the
neural tube is homologous with the outer surface of the general epidermis of the
body. The account given above is not exact for cyclostomes, teleosts and some
ganoids, where the neural plate forms a keel extending below the surface, in

which a central canal appears later, so that the final result is closely similar to
the typical condition.

The Spinal Cord.

From this simple tube the spinal cord of the adult is developed by
several modifications. The cells of the side walls proliferate rapidly,

NERVOUS SYSTEM. 139

while those of roof and floor plates do not. As a result the sides soon
extend downward on either side beyond the floor plate, thus forming a
longitudinal groove, the anterior or ventral fissure of the cord, ex-
tending its whole length (fig. 144, B). The roof plate, on the other
hand, is at first carried upward by the growth, thus increasing the ver-
tical diameter of the central canal. Then the dorsal portion of the
tube closes up—the exact steps are uncertain—and later the tissue
along the line of closure is invaded by connective tissue and
blood-vessels, the result being the dorsal or posterior fissure of the
cord.

Besides the increase in the number of cells, the sides of the cord are
modified in other ways. Those cells which line the cavity—floor, roof
and sides—retain their epithelial character, never develop nervous struc-
tures, and are known as the ependyma. ‘The remaining cells become
differentiated in two directions. Some develop processes which sur-
round and support the others, these forming the neuroglia (‘glia’),
while the others form the true nervous tissue—ganglion or nerve cells.
In the primitive condition the primitive nerve cells have no connexion
with distant points and hence cannot function. These connexions are
established by protoplasmic outgrowths from each cell, these forming
the fibres (dendrites or axons). Some of these extend directly out-
ward from the cord as nerves (see below), but others run for a greater
or less distance on the external surface of the cord, and since these
have medullary sheaths (p. 20) and are consequently white, these
tracts constitute the white matter of the cord, in contrast to the gray
matter formed by the cell bodies and neuroglia.

In sections of the adult cord the gray matter has something of the
shape of the letter H,, its uprights forming the anterior and posterior
horns or cornua, while the cross-bar extends above and below the
central canal, from one side to the other. Physiological phenomena
and matters of nerve origin lead to the recognition of a lateral cornu
on either side, in the lateral prominence of gray matter. Since both
dorsal and ventral cornua approach the surface of the cord to connect
with the nerve roots described below, they divide the white matter into
three tracts on either side, known as the anterior, lateral and pos-
terior columns of the cord, each subdivided in the higher vertebrates
into several bundles. As this white matter is composed of nerve fibres,
it follows that these columns are the tracts by which nervous impulses
are carried to and from the brain, the anterior columns leading from,

I40 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

the dorsal to the brain (ascending and descending tracts), while’
impulses travel in both directions in the lateral columns. oe
In fishes the cord tapers regularly to the tip, but with the develop-
ment of legs in the terrestrial vertebrates the cord is considerably en-
larged where the nerves to the limbs are given off, the enlargements
bearing some ratio to the size of the limb. In the early stages the

spinal nerves leave the cord at right angles to its axis. With growth

Fic. 145.—Diagram of spinal cord and nerve roots; gray matter shaded. 4A, JL, P;
anterior, lateral and posterior columns; A, anterior pyramidal tract; B, column of Burdach,
cc, central canal; ca, cl, cp, anterior, lateral and posterior cornua; Dr, dorsal root; Fa, Fp,
anterior and posterior fissures; G, column of Goll; g, ganglion of dorsal root; /c, lateral
cerebellar tract; /p, lateral pyramidal tract; sm, spinal nerve.

the angle changes since the peripheral parts increase more in length
than does the cord. The result is that the posterior nerves are very
oblique and in the hinder part of the spinal canal they form a bundle
of parallel nerves, the cauda equina. Another result of the unequal
growth may be the drawing out of the hinder end of the cord into a
slender non-nervous thread, the filum terminale.

The Brain.

‘The spinal cord throws light on the extremely complex brain.
Here, as in the cord, there is primitively a tubular structure, with roof,
floor and sides, and with nerves connected with it which recall those
of thecord. In its development, as stated above, the brain from the first
is larger than the cord. It early has three enlargements separated by
two constrictions, the third enlargement passing gradually into the cord.
These enlargements are called, from in front backward, fore-brain,

BRAIN. I4I

mid-brain and hind-brain, the constriction between mid- and hind-
brains being the isthmus. In the sides, as in the cord, two zones
may be recognized, dorsal and ventral, separated internally by a
groove, the sulcus of Monro, which lies at about the middle of the
tube. At the extreme anterior end a small region, the optic recess,
is wedged in between the two zones on either side, the end of the tube

Fic. 146.—Diagrams of (1) primitive brain. (2) an intermediate stage, and (3) with
the definitive parts. (Compare 3 with fig. 147). AQ, aqueduct; AC, anterior commissure;
C, cerebral region; CB, cerebellum; CS, corpus striatum; HC, habenular commissure;
I, infundibulum; LT, lamina terminalis; MO, medulla oblongata; O, olfactory region;
P, epiphysial region; PC, posterior commissure; RO, optic recess; ST, subthalamica; T,
tegmentum; TH, thalamus. Dorsal zone plain, ventral zone dotted.

just above the recesses being the lamina terminalis. The most
marked modifications in converting the primitive into the adult brain
take place in the dorsal zone.

Tn the fore-brain the anterior part of the dorsal zone on either side
forms an outgrowth which rapidly increases in size, the two eventually
forming a pair of hollow vesicles, the cerebral hemispheres (telen-
cephalon, prosencephalon) which extend far beyond the lamina
terminalis. In the wall of each hemisphere may be recognized a basal
ganglionic portion, the corpus striatum, while the rest of the wall
is the pallium or mantle. An olfactory lobe (rhinencephalon)
grows out from the lower anterior part of each hemisphere to meet

I42 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

the olfactory epithelium (see sense organs), and into this a portion of
the cavity (ventricle) of the hemisphere may extend (fig. 146).

Considerable differences exist in the olfactory lobes. In some cases they are
directly continuous with the hemispheres, but they may be prolonged, each having
two portions, a narrower stalk, the tractus olfactorius, and a distal enlargement,
the bulbus olfactorius. The true olfactory nerve takes its origin from the end
of the bulb or its homologue (for details see cranial nerves).

The ventral zone and posterior part of the dorsal zone of the fore-
brain, after the differentiation of the telencephalon, forms the thala-
mencephalon (’twixt-brain, diencephalon). Its sides, the optic

Fic. 147.—Half of model of brain of embryo pig, 15 mm. long. (Compare with fig.
146, 3.) ¢, cerebrum; cb, cerebellum; cs, corpus striatum; i, infundibulum; is, isthmus;
fi, interventricular foramen; m, mesencephalon; mo, medulla oblongata; ¢, thalamus.
thalami, remain without marked modification, but its floor and roof
form median outgrowths, corpus pineale, epiphysis, etc., above, in-
fundibulum below—to which reference will be made again later.

In the mid-brain there is little modification except a thickening of
the walls forming a pair of prominences, the optic lobes, or corpora
bigemina (in mammals two pairs of lobes, corpora quadrigemina)
on the dorsal surface. The mid-brain of the adult is also called the
mesencephalon (fig. 146).

In the hind-brain the great modifications occur again in the dorsal
zone. Its dorsal portion extends itself upward and backward as a
broad lobe which tends to arch over the rest of the hind-brain. This
outgrowth forms the cerebellum or metencephalon, while the
remainder of the hind-brain constitutes the medulla oblongata or
myelencephalon (fig. 146).

Thus there arise in the adult brain five regions—telencephalon,
thalamencephalon, mesencephalon, metencephalon and myelencephalon
—derived from the primitive three. These usually retain in the interior

BRAIN. 143

the cavity of the primitive three (continuation of the central canal of
the spinal cord), but modified in different ways. The cavity in the
primitive fore-brain is divided with the outgrowth of the hemispheres
into three chambers known at ventricles, a pair of cerebral ventricles
in the hemispheres and a third ventricle in the thalamencephalon.
The paired ventricles are connected with the third by a pair of narrower
passages, the foramina of Monro (for. interventriculares). In the
higher vertebrates the cavity of the mid-brain becomes reduced to a
narrow tube, the aqueduct (or iter), but in the lower classes (fig. 156)
this expands dorsally into a cavity, the epiccele, in the upper part of the
optic lobes. The aqueduct terminates behind in the fourth ventricle
which lies in the hind-brain, extending forward beneath the cerebellum
and gradually diminishing in the medulla to the central canal of the
spinal cord. Sometimes there is a prolongation of the fourth ventricle
into the cerebellum (metaceele, fig. 156).

Fic. 148.—Median section of brain of pig 15.5 mm. long, showing flexures of the brain
C, principal flexure; cs, corpus striatum; CP, chorioid plexus of fourth ventricle; h, hypo-
physis; 7, infundibulum; M, mid-brain; N, nuchal flexure; P, pontal flexure; RO, optic
recess; 7’, ’twixt-brain.

So far the brain has been treated as if it were a continuation of the
spinal cord in a straight line. In reality, by unequal growth in dorsal
and ventral zones, it becomes flexed in the vertical plane. In the lower
vertebrates, these flexures never attain great prominence and largely
disappear in the adult. They are more developed in the higher groups
and persist throughout life. Most constant is the primary flexure
in the mid-brain, by which the derivatives of the fore-brain are bent
downward at a right angle (or more) to the axis of the rest. Second
to appear is the nuchal flexure in the hinder part of the medulla ob-

144 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

longata, which also bends in the same direction. The pontal flexure,
beneath the cerebellum, bends in the opposite direction and thus
tends to counteract the other two. Nuchal and pontal flexures are at
best but weakly developed in the ichthyopsida and all are practically
obliterated in the adult, but in the amniotes they are increasingly
developed and persist through life (fig. 148).

The brain, like the spinal cord, is composed of nerve cells (gray
matter) and fibres (white matter), but their arrangement is exceedingly
complicated and but the slightest outline of their distribution can be
attempted here, in connection with the general account of the regions
of the brain.

Fic. 149.—Cross-section of medulla of Acanthias embryo, 60 mm. long, showing the
greatly broadened roof plate and, below, a bit of the meninx of the nervous system. c,
cartilage of basal plate; e, ependyma; mp, meninx primitiva; pc, perichondrium (endo-
rhachis); 7, roof plate.

The myelencephalon is most nearly like the spinal cord of any part
of the brain. It is triangular in outline, viewed from above, and is
widest anteriorly, due in part to the separation of the side walls by the
great development of the roof plate over the fourth ventricle. Blood-
vessels press against the roof, carrying parts of it before them into the
ventricle, thus forming the chorioid plexus of the fourth ventricle, a
means of introducing nourishment into the brain. (Usually in dis-
sections this roof is torn away, leaving a triangular or rhomboid opening
into the fourth ventricle—fossa rhomboidea). The floor plate in
this region is obliterated by the development of numerous nerve centres
—‘nuclei’ or ganglia—in the walls, some closely connected with the

BRAIN. 145

fibre tracts soon to be mentioned, some with nerves arising from this
region.

Most noticeable of these ganglia are the olivary bodies (oliva) near the roots of
the hypoglossal or first spinal nerves; the nuclei of the cuneate and slender funiculi
connected with the posterior columns of the cord; the eminentia medialis in the
floor of the fourth ventricle, connected with the anterior and lateral columns; and
the tuber acusticum, an enlargement connected with the eighth nerve; its anterior
end in the ichthyopsida is specialized as the lobe of the lateral line system.

The cerebellum is developed from the dorsal zones and the roof
plate, the latter invaded by nerve cells from the sides. In front it dips
deeply into the fourth ventricle, its anterior portion being vertical and
together with part of the roof of the isthmus, forming the valve of

Fic. 150.—Diagrammatic longitudinal section of brain. ac, anterior commissure in

lamina terminalis; ag, aqueduct; c, cerebrum; cb, cerebellum; cp, chorioid plexus; cs, corpus
striatum; cv, cerebellar ventricle; kh, hypophysis; hc, habenular commissure; 7p, inferior
chorioid plexus; m, mesencephalon; ml, myelencephalon; , pinealis; pa, paraphysis; pc,
posterior commissure; pe, parietal eye; v, valve of Vieussens; vt, velum transversum with
aberrant commissure.
Vieussens (velum medullare anterius,fig.150). In the ichthyopsida
and lower reptiles there is no special differentiation of parts in the cere-
bellum, but in the higher reptiles and in the birds a central portion, the
vermis, and a pair of lateral lobes, the flocculi (fig. 161) occur. In the
mammals the cerebellum is still farther enlarged, chiefly by the develop-
ment of large cerebellar hemispheres between vermis and flocculi, the
latter being forced by them to the lower side of the cerebellum. In the
walls of each hemisphere, besides others, there is a large nerve centre,
the nucleus dentatus, connected with the posterior peduncle of the
cerebellum to be mentioned shortly, and with the fibres which go
farther forward in the brain.

The mesencephalon is relatively largest in the lower vertebrates,
less conspicuous and tending to be covered by cerebrum and cerebellum
in the higher groups. On its dorsal surface are the two optic lobes
(transversely divided in the mammals) each connected with an optic

Io

146 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

tract leading to the eye of the opposite side. In the lower groups the
lobes contain an epiccele (p. 143), but in the higher they are solid, the
cavity being reduced to the aqueduct. The floor of the mid-brain is
‘formed of large fibte tracts (see below), the floor plate having been
invaded by their fibres.

In the thalamencephalon (’twixt-brain) the lateral walls are thick-
ened, the dorsal zones developing a nerve centre, the optic thalamus,

IN (ely Te

g

ESSE

l
0

AiG

Fic. 151.—Parietal, eye of Anguis fragilis, after Nowikoff. ct, connective tissue cells
around nerve; gc, ganglion cells; /, lens; , nerve fibres; pm, parietal nerve; fc, pigment cells;
r, retinal cells; vb, vitreous body.

on either side. These are ganglionic and are closely related to the
corpora striata. Frequently the thalami of the two sides touch or
even unite above, forming’ the so-called soft commissure (commis-
sura mollis, fig. 152)—really not commissural in character. Still
more dorsal is a small habenular ganglion on either side, in front
of the pinealis to be described in a moment.

Under the head of epiphysial structures are several parts devel-
oped in the roof plate of the primitive fore-brain. At the junction of
cerebral hemispheres and twixt-brain (fig. 150) there is an internal epi-
thelial fold, the velum transversum, depending from the cerebral
roof. In front of this an outgrowth, the paraphysis, arises on the top
of the brain in nearly all vertebrates. It is non-nervous and apparently

-is an extra-ventricular chorioid plexus with secretory functions. The
other epiphysial structures belong to the ’twixt-brain and consist of a
parietal organ and a pinealis. Both arise from the roof between the

BRAIN. 147

habenular ganglion and the posterior commissure, at the boundary
between ’twixt- and mid-brains, sometimes as two distinct structures,
sometimes as the result of division of a single outgrowth of the roof.
The anterior of these is the parietal organ or eye; the other the
pinealis or epiphysis proper. The two vary in development in dif-
ferent vertebrates, the parietal eye being well-marked only in cyclos-
tomes, Amia, teleosts and most lizards (fig. 151), while the pinealis
is almost invariably present.

In its fullest development in lizards and Sphenodon the parietal
organ extends as a slender stalk, hollow at first, through the parietal
foramen of the skull, expanding beneath the skin to a vesicle, above
which the integument is usually thin and transparent, forming a physi-
ological cornea. The distal wall of the vesicle is thickened in the
middle, forming a lens, while the cells of the proximal side elongate,
each becoming differentiated into a distal, rod-like end and a proximal
portion which contains the nucleus and is connected with a nerve fibre.
Pigment is deposited between these cells so that the whole forms a
retina. An important point, to be better appreciated after the con-
sideration of the paired eyes, is the fact that these parietal eyes are
like those of most invertebrates in having no inversion of the retina.
How far these eyes are actually functional is not settled. Even in
Sphenodon, where it is best developed, experiments have resulted in no
decided reactions.

In other vertebrates the parietal organ does not pass outside the
skull, and’ even may not appear transitorily in development. The
pinealis to some extent may take its place and often shows, as in certain
lizards, traces of a visual structure. In the anura its tip approaches
the skin and later is cut off from the brain by the development of the
skull, forming the so-called frontal organ, visible from the exterior.
Pineal and parietal organs differ in their nerve supply, the parietal being
connected with the superior commissure of the ’twixt-brain, the
pinealis and its derivatives with the posterior commissure. In the
higher vertebrates the epiphysial structures are completely covered by
the backward growth of the cerebrum. ‘The large parietal foramina
in many extinct reptiles would seem to indicate that they had well
developed parietal or pineal organs. The roof of the brain in this
region, behind the lamina terminalis, also gives rise to a chorioid plexus
like that of the fourth ventricle, a part of which invades the third
ventricle and another portion, the inferior plexus, sends branches

148 COMPARATIVE MORPHOLOGY OF VERTEBRATES,

through the foramina interventriculares into the ventricles of the hemi-
spheres, thus providing for a blood supply on the interior of these
structures (fig. 150).

The floor of the diencephalon remains thinner behind the optic
recess, a portion of it becoming funnel-shaped and pushing out from the
ventral surface toward the roof of the mouth. This is the infundib-
ulum which meets an ectodermal diverticulum, the hypophysis.
This arises, in the cyclostomes from the ectoderm between the nostril
and the mouth; in other vertebrates from the roof of the oral cavity.
It retains its connection with the parent epithelium for a time, the point

‘of ingrowth being known as Rathke’s pocket. Later the stalk dis-
appears and the infundibulum and hypophysis, closely associated, lie
just beneath the brain in the sella turcica on the floor of the skull
(p. 61). In the hypophysis (pituitary body) two parts are distin-
guished, rich in blood- and lymph-vessels and forming a gland of internal
secretion whose action is connected with the fat-storing powers of the
animal. The infundibulum may be a simple pit, as in most vertebrates,
or its lateral walls may become enlarged and folded, blood-vessels
lying in the folds, and the whole forming the so-called saccus vascu-
losus. The paired eyes are also connected with the ’twixt-brain, both
in origin and in the adult; they are described with the other sense
organs.

The cerebrum (telencephalon) consists of a pair of hemispheres,
separated in front by an intercerebral fissure, slight in fishes, well
marked in other vertebrates. Each hemisphere typically contains a
ventricle, the walls of which are formed by the corpus striatum below
and elsewhere by a thinner portion, the pallium or mantle. To the
roof belong the paraphysis and the inferior chorioid plexus, already men-
tioned. In some vertebrates, like the teleosts, the whole of the pallium
remains thin and epithelial throughout life; elsewhere it is invaded to a
greater or less extent by nervous matter. In the amphibia and reptiles,
where the olfactory lobes are merged in the hemispheres, the medial
wall of each hemisphere as far back as the interventricular foramen is
called the septum, while the part above the foramen, together with the
posterior dorsal and lateral walls, is to be regarded as homologous with
a region, long recognized only in mammals, the hippocampus, connected
with the olfactory sense. In the mammals a new element, the neopal-
lium, appears in the cerebrum. In the lower groups it is on the outer
wall, behind the olfactory tract, and, increasing in extent in the higher

1B 1 aceRa' ~aten Hsliuintin, - °

BRAIN. 149

groups, forces the hippocampus to the medial side of the hemisphere.
Other modifications are better understood after a consideration of
the commissures of the brain.

The amount of gray matter in the pallium is evidently correlated with the mental
powers of the animal, being greatest in the mammals. Here the nerve cells form
a layer (cortex) on the surface of the neopallium. Increase in the number of
these cells can be accommodated to some extent by increase in the size of the
cerebrum, but the extent of this increase is limited, and in the higher mammals the
amount of surface is increased by folding, so that the cerebrum is marked extern-
ally by numerous fissures or sulci separating convolutions or gyri, as will be
mentioned in the paragraphs on the mammalian brain.

_ In order that the two sides of the body may work in harmony it is
necessary that the right and left side of the central nervous system be

te coe al

Fic. 152.—Medial plane of brain of Ornithorhynchus, after G. Elliot Smith. ac, anterior

commissure; bo, bulbus olfactorius; cm, commissura mollis; cl, cerebellum; e, epiphysis;

fd, fasciculus dentatus; fi, interventricular foramen; h, hypophysis; kc, habenular commis-

sure; /p, lobus pyriformis; mc, corpus mamillare; md, medulla oblongata; 2, nodulus; ol,

olfactory lobes; of, olfactory tubercle; pal, pallium; pc, posterior commissure; fc, tuber
cinereum; v, velum medullare; vmo, motor root of fifth nerve; vm, maxillary of fifth.

connected. This is accomplished in the spinal cord by nerve fibres
which pass above and below the central canal from one side to the other.
In the brain these commissures are more localized. Then there are
longitudinal fibre tracts in the brain, some of which are continuous with
the columns of the cord already mentioned. Only a few of these connex-
ions, which are more numerous in the higher than in the lower verte-
brates, can be mentioned here.

Most constant and important of the commissures are the following:

150 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

In the lamina terminalis, a little below the interventricular foramen,
an anterior commissure, connecting the two hemispheres; a poster-
ior commissure in the roof at the junction of di- and mesencephalon;
and a superior or habenular commissure associated with the habenular
ganglia and lying between the epiphysial structures and the velum
transversum. In the amphibia, with the differentiation of the hip-
pocampal region, a dorsal or hippocampal commissure appears in
the lamina terminalis, just dorsal to the anterior commissure, connecting
the hippocampi of the two sides. This persists, with slight modifica-
tions, through the sauropsida and monotremes, but in the higher
mammals it is subdivided into the hippocampal commissure proper and
a more anterior portion, the corpus callosum. This corpus callosum
is only in part the result of the division, but is more largely formed by
new fibres, anterior to the hippocampal portion, connecting the neopal-
lium of the two sides. The result is a broad band (the largest com-
missure in the brain of man) which invades the intercerebral fissure
from behind. In the lower vertebrates a few fibres pass downward from
either side of the cerebellum beneath the fibre tracts of the medullary
region and so to the other side of the cerebellum. In the mammals
these are greatly increased in number, forming-a marked projection on
the lower surface, the pons (Varolii), the prominence of which is in-
creased by the great development of ‘nuclei’ in the medullary floor.

The longitudinal tracts are more numerous and more complex.
As will be recalled, there are dorsal, lateral and ventral columns in
the spinal cord. These extend into the medulla oblongata and there
pursue different courses.

Some of the fibres of the dorsal columns end in connection with the nuclei of
the medulla (p. 144), while others unite with fibres from the lateral column and with
some from the oliva to form an enlargement, the corpus restiforme, and then
bend upward (posterior peduncle) to enter the cerebellum. Other fibres from
the lateral column, together with some from the dentate nucleus, enter the cere-
bellum farther in front as the anterior peduncle, those from the dentate nucleus
pass forward to the roof of the mid-brain, some terminating in the optic lobes,
others continuing to the cerebrum. In this forward course, after leaving the cere-
bellum, the fibres cross (decussate), those from the right side passing to the left
side of the brain farther forward and vice versa. In the dorsal region of the medulla
there is a short tractus solitarius (fasciculus communis) derived from fibres
from the seventh to tenth nerves and extending-no farther forward than the seventh.

In the higher vertebrates there are the crossed and the direct pyramidal tracts
on the ventral side of the medulla, the direct being continuations of part of the ven-
tral columns, the crossed of the deeper lateral columns. In the medulla these en-

BRAIN. 151

large and become somewhat pyramidal, the enlargement being due in part to the
decussation of the crossed tracts. The tracts pass forward from the decussation
and in the mid-brain region they diverge to pass the hypophysial structures farther
in front, the diverging portions being called the crura cerebri. The fibres of the
crura enter the corpora striata and in the mammals, the cerebral cortex.

The direct pyramidal tracts have no decussation in the medullary region, but
pass to the hemisphere of the same side; the fibres, however, do cross in the spinal
cord. Recently attention has been called to Reissner’s fibres which occur in all
vertebrates, but are relatively largest in fishes. They arise from the roof of the
mid-brain, descend to the aqueduct and pass through the fourth ventricle and into
the central canal to terminate at various points in the region of the spinal nerves.
It has been suggested that they afford a short cut for visual reflexes. Another
supposition is that they regulate the flexion of the body.

Of the numerous longitudinal tracts in the anterior part of the brain
the fornix must be mentioned. It appears first in the amphibia and
is well developed in the mammals. Its fibres are connected in front
with the hippocampus, pass downward through the lamina terminalis
to the floor of the third ventricle, where they produce a marked swelling
(corpus albicans) on either side of the ventral surface of the dien-
cephalon. They ascend from this point to the optic thalami. The
passage of the tracts of the fornix through the lamina terminalis and
the forward growth of the corpus callosum stretch the lamina into a
thin triangular area, the septum pellucidum, and at the same time the
callosum causes the lamina to split, the enclosed cavity being called
the ‘fifth ventricle’ though it has no relation, physical or mor-
phological, with the true ventricles of the brain.

ENVELOPES (MENINGES) OF THE CENTRAL NERVOUS
SYSTEM.

Both brain and spinal cord are surrounded by envelopes (meninges)
of connective tissue which support and protect them, and also, by
carrying blood-vessels, provide for their nourishment. These meninges
become more complicated with ascent in the vertebrate series. The
canal of the vertebral column and the cavity of the skull are lined with
a layer of connective tissue, the endorhachis, which is really the perios-
teum or perichondrium of the skeletal parts and hence not a true meninx.
In the fishes (fig. 149) there is a single envelope, the meninx primi-
tiva, which bears the blood-vessels and lies close upon the spinal cord.
Between it and the endorhachis is a perimeningeal space, somewhat
broken by strands of tissue passing from meninx to endorhachis, and

152 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

filled, like all meningeal spaces, with an albumen-containing cerebro-
spinal fluid.

From the urodeles upward there is an increasing division of the
meninx primitiva into two layers, a pia mater bearing the blood-
vessels and lying close to the cord, and a dura spinalis, separated from
the pia by a subdural space, the perimeningeal space now being known
as the peridural. In the mammals the pia becomes invaded by cavities
separating a delicate arachnoid membrane from its outer surface, so
that there is another space, the subarachnoid, in these forms.

There may be slight differences in the region of the brain in the
higher groups where the dura presses against and finally unites with the
endorhachis, forming the dura mater of human anatomy, thus obliterat-
ing the subdural space. In the mammals and to
a less extent in birds the dura mater forms two
strong folds. One of these is longitudinal and
presses in between the two cerebral hemispheres as
a firm membrane, the falx cerebri. The other
fold, the tentorium, is transverse, and is inserted
between cerebrum and cerebellum. It is occasion-
ally ossified and united to the skull.

THE BRAIN IN THE SEPARATE CLASSES.

CYCLOSTOMES.—The brain is very different in the
two classes of cyclostomes. All parts lie in the same hori-
zontal plane, the flexures having disappeared, and the
whole presents a primitive, almost embryonic appearance.
In the lampreys the somewhat slender brain is elongate
and its roof is largely epithelial, this extending to the mid-
brain, of which only the hinder part is nervous in the middle
line. The small cerebral hemispheres are largely com-

High 5g; —eaia of posed of the corpora striata and the dorsal part of the
Bdellostoma (Princeton, pallium is purely epithelial, the ventricles being well de-
2204). 0, skeleton of veloped and extending into the olfactory lobes. The
olfactory organ, the: _. i
brain behind this; Vx, Ptic lobes and the medulla are relatively broad, but
nerves. the cerebellum is reduced to an inconspicuous fold in front

of the fossa rhomboidea.

Authors do not agree regarding the interpretation of some parts of the myxinoid
brain. The whole is much broader and shorter than in the other class and is
marked dorsally by a groove running the whole length. According to Retzius, the
*twixt-brain of Myzxine is invisible from above and the cerebellum is large, com-
pletely covering the fossa rhomboidea. The cavities are greatly reduced, the

BRAIN. 153

aqueduct ending blindly in the mid-brain, in front of which is only the third ven-
tricle, completely cut off from the rest. The brain of Bdellostoma (fig. 153)
differs from this in several respects.

ELASMOBRANCHS (figs. 154, 167) usu-
ally have the brain somewhat compact, but in
a fewit is long and slender. The more strik-
ing features are the slight development of the
intercerebral fissure, the large hemispheres be-
ing lateral expansions just in front of the dien-
cephalon. The optic lobes are large and the
large cerebellum overlaps both lobes and the
fossa rhomboidea. The olfactory lobes arise
from the antero-lateral angle of each hemi-
sphere; their length varies between wide limits.
The epithelial roof of the ’twixt-brain is wide
and bearsa pinealis which often reaches the roof
of the skull, but the parietal organ is lacking.
The hypophysis and infundibulum are pro-
vided with large inferior lobes and a well devel-
oped saccus vasculosus. The cerebellum hasa
longitudinal groove and usually one or more
transverse grooves, dividing the upper surface
into paired lobes. The medulla differs in the
sharks and the skates, being very short in the
latter, much longer in the former. In both the
corpora restiformia are large folds on either
side of the cerebellum, in front of and lateral _.
to the fossa rhomboidea.

In most elasmobranchs the ventricular sys-
tem is well developed, but in some the paired
and third ventricles are not well separated,
while in the Myliobatide there is no cavity in
the cerebrum. There is a large epiccele ex-
tending upward from the aqueduct into the
optic lobes and a similar cavity usually enters
the cerebellum. ;

TELEOSTOMES.—There is a wide range Fic. 154.—Brain of Heptanchus,
of form in the brain of ganoids and teleosts. It after Gegenbaur. bo, bulbus olfac-
: F 3 torius; c, cerebrum; cb, cerebellum;
is usually small in proportion to the size of the gm eminentia teretes; i, infundibu-
animal and is noticeable for the small size of lum; m, mesencephalon; oo, olfactory
the telencephalon and the usually non-nervous eles peg er peed
character of the pallium, which in the teleosts is nial nerves.
purely epithelial. Consequently the cerebrum
consists largely of the corpora striata and the intercerebral fissure is slightly de-
veloped. The paired ventricles are small, but they extend into the olfactory lobes.
The ’twixt-brain, at a lower level than the rest, has a large infundibulum, saccus

154 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

Ey

: beta)
Fic. 155.—Dorsal and side views of brain of buffalo fish (Carpiodes tumidus) after

Herrick. c¢, cerebrum; cl, cerebellum; cs, corpus striatum; #, hypophysis; , infundibulum;
ol, olfactory lobes; , pallium; vl, vagus lobes; JJ-X, nerves.

(~~ ------- = ane ty

ag sv Ii h

Fic. 156.—Sagittal section of brain of trout, after Rabl-Riickhard. ag, aqueduct; bo,
bulbus olfactorius; ca, ch, ci, cp, anterior, horizontal, inferior and posterior commissures;
cc, central canal; cl, cerebellum; #, hypophysis; 7, infundibulum; oc, optic chiasma; 9,

pallium; #7, pinealis; sv, saccus vasculosus; #, torus longitudinalis; fo, tectum of optic
lobes; v*, v* ventricles; vc, valvula cerebelli.

BRAIN. . 155

vasculosus and inferior lobes. On its roofisa large pinealis which reaches the skull in
afew ganoids. The parietal organ appears in the embryo and soon degenerates; the
paraphysis is usually well developed. The optic lobes are large and are usually
divided into two hemispheres by a median groove, but this occasionally is scarcely
noticeable. The cerebellum is large, much larger than appears from the surface,
since a considerable part, the valvula, projects into the ventricle of the mid-brain.
In the cerebellar region there is sometimes an enormous development of the lobes
of the vagus (fig. 155).

The brain of Polypterus differs from that of other ganoids in several respects.
There is no differentiation of cerebral hemispheres; the optic lobes and the cerebel-
lum are moderate, the latter being thin in the median line and the valvula smaller.
The medulla oblongata has thin walls and the ventricle is large. The brain has a
primitive appearance, but it shows little resemblance to those of the amphibia or
of the dipnoi.

DIPNOI.—The brains of Lepidosiren and Protopierus differ considerably from
that of Ceratodus. In all the cerebrum is larger than the optic lobes and the

Gr

Fic. 157.—Brain of Protopterus, after Burckhardt. cb, cerebellum; e, epiphysial
structures; h, hypophysis; z, infundibulum; m, mid brain; se, saccus endolymphaticus; s?,
spinal nerves; ¢, cerebrum; 1-12, cranial nerves.

olfactory bulb is separated from the cerebrum by a long olfactory tract. In Cer-
atodus the hemispheres are united above by a part of the chorioid plexus, while
internally they are separated from the diencephalon by a well marked velum. The
pinealis is long and rests upon a large ‘zirbelpolster’ developed as an outgrowth
of the roof of the third ventricle in front of the superior commissure. The optic
lobes are separated into two hemispheres, while the cerebellum is scarcely more than
a transverse plate and is, together with the fossa rhomboidea, covered with a com-
plicated chorioid plexus. In Protopterus (fig. 157) the elongate hemispheres are
parallel, the pinealis and its ‘polster’ are smaller and the mid-brain has but a
single rounded lobe.

AMPHIBIA.—The parts of the amphibian brain are more distinct from each
other than is usual in vertebrates, and, except in the gymnophiones, the flexures
have largely disappeared in the adult. There is a deep intercerebral fissure
between the hemispheres, but in the anura the two halves of the cerebrum are
connected by a transverse band just behind the olfactory lobes. The telencephalon
is relatively larger than in fishes, the increase being due to the invasion of the pal-
lium by nervous matter, while the corpora striata are relatively smaller than in other
ichthyopsida. In the pallium the inner part is largely composed of nerve cells,
the outer layer consisting of nerve fibres.

The diencephalon, broad in the anura, narrower in the urodeles and czcilians,

156 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

is visible from above. The infundibulum and hypophysis are well developed but
the saccus vasculosus and inferior lobes are smaller than in fishes. In the gymno-
phiones, owing to the pontal flexure the hypophysis is carried back beneath the
medulla oblongata. Both paraphysis and pinealis are present, the latter not reach-

Fic. 158.—Dorsal and ventral views and sagittal section of brain of Desmognathus, after
Fish. a, anterior commissure and rudimentary corpus callosum; c, cerebrum; cl, cere-
bellum; e, epiphysis; 4, hypophysis; 7, infundibulum; oc, optic chiasma; ol, optic lobes; ,
paraphysis; pc, posterior commissure; cp, chorioidplexuses; sc, superior commissure; J-X,
nerves.
ing the cranial roof except in the anura, the conditions in this group having already
been mentioned (p. 147). The cerebellum is very small, a mere transverse fold
on the anterior border of the fossa rhomboidea. The gymnophione brain is notice-

able for the pontal flexure already alluded to, which carries the hemispheres so far

BRAIN. 157

back that they almost touch the sides of the medulla, and for the double roots of the
olfactory nerves.

REPTILES.—There is considerable range in the brain of the reptiles, all show-
ing an advance over the amphibians in having the cerebrum larger than the optic
lobes; in having, in the pallium, besides the basal layer of gray matter, a distinct
cortical layer of nerve cells; the well developed hippocampus; while the olfactory

lobes may either be sessile upon the hemispheres or differentiated into tracts and
bulbs.

Fic. 159. Fic. 160.
Fic. 159.—Brain of Iguana tuberculata (Princeton, 2293). Compare fig. 172.
Fic. 160.—Side and dorsal views of brain of young alligator, after Herrick. ¢, cere-
brum; cl, cerebellum; e, epiphysial structures; #, hypophysis; 7, infundibulum; ol, olfactory
lobes; IJ~XTITI, cranial nerves.

The greater size of the cerebrum and the large optic lobes result in covering the
diencephalon so that it is scarcely visible from above (figs. 159, 160). Infundibulum
and hypophysis are well developed, but the sacci vasculosi are rudimentary and the
inferior lobes are inconspicuous. The epiphysial structures reach their highest
development in this group. In most species the parietal organ is rudimentary, but
in many lizards and especially in Sphenodon it penetrates the roof of the skull and

I 58 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

forms a well-developed eye (fig. 151), lying just beneath the skin and connected
with the brain by more or less rudimentary nerves. In some the pinealis also
shows eye-like features.

The optic lobes are distinct from each other. The cerebellum is usually
small (fig. 159), but in the crocodilia (fig. 151), it attains considerable size. In
all reptiles there is a thicker central portion and thinner lateral parts, an approach
to the differentiation into vermis and flocculi found in birds. There are no
special features in the medulla calling for notice.

AVES.—The bird’s brain (fig. 161) is short, broad and highly specialized. The
smooth cerebral hemispheres are large, their size being due more to the enormous
corpora striata than to enlargement of the pallium, which is comparatively thin,
while the olfactory lobes are very slightly developed, in correlation with the deficient
powers of smell. The large cerebellum extends forward between the hinder ends

Fic. 161.—Brain of golden eagle, Aquila chrysetos, after Herrick. c, cerebrum; cl, cere-
bellum; /, flocculus; mo, medulla oblongata; ol, optic lobes; on, optic nerve.

of the cerebrum, thus forcing the optic lobes into a lateral position and completely
covering the ’twixt-brain. The epiphysial structures are large but rudimentary in
character, the pinealis extending up in the angle between cerebrum and cere-
bellum. Below, the hypophysis completely hides the infundibulum. The large
cerebellum has its median portion transversely furrowed, this constituting the
vermis, while the smaller lateral lobes, which vary in extent, form the flocculi.
The myelencephalon is very short and the fossa rhomboidea is covered by the
cerebellum.

MAMMALS.—The brain in the mammals becomes exceedingly complex. Only
the most important features and those of general occurrence will be noted here.
Most marked are the large size of the cerebellum and the still greater development
of the cerebrum, correlated with the great increase in mental powers. The cere-

BRAIN. 159

brum covers the di- and mesencephalon, and in the primates even the whole of the
cerebellum from above. This increase of the cerebrum is largely an increase of
the nervous matter of the pallium, a portion—the neopallium—developing on the
lateral side of each hemisphere between the hippocampus and the basal structures
(pyriform lobes). This increase in cerebrum is limited in forward and backward
growth by the limitations of skull development. Hence it overlaps the olfactory

Fic. 162.—Ventral surface of brain of Ornithorhynchus, after G. Elliot Smith.’ bo,
bulbus olfactorius; c1, first cervical nerve; cl, cerebellum; cm, corpus mamillare; f, floc-
culus; /p, lobus pyriformis; op, olfactory peduncle; rf, rhinal fissure; fc, tuber cinereum; fo,
olfactory tubercle; ¢V, tuberculum quinti; Vm, Vmd, Vmx, motor root and maxillaris and
mandibularis roots of trigeminal nerve; J-XJI, cranial nerves. See also fig. 152.

lobes in front, so that they appear to rise from its ventral surface, while behind it
extends backward, then turns downward and lastly extends forward along the sides
of the mid- and ’twixt-brains, even overlapping a part of the cerebrum itself. In
this way the cerebrum becomes marked off into a series of regions called the frontal
lobes in front, the parietal above, the occipital behind, while the reflexed ventral
portion of either side makes a temporal lobe.

This folding and overgrowth causes grooves or fissures in the surface of the
cerebrum, the most constant being a rhinal fissure between olfactory and frontal
lobes, a Sylvian fissure between the temporal lobe and the lower surface of the

160 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

cerebrum against which it is folded. In the bottom of the Sylvian fissure is a part
of the side wall of the cerebrum which has received the name of insula (island of
Riel), while a hippocampal fissure causes the hippocampus to appear as a pro-
nounced swelling on the floor of each ventricle. In the lower mammals these are
the only fissures present, the rest of the cerebral surface being smooth. In the
higher mammals other grooves (sulci) separating convolutions (gyri) appear.
These convolutions increase the extent of cerebral surface and as a consequence
they permit of more cortical gray matter upon which mentality depends. The
number of gyri increases in the primates and reaches its extreme in man. The
folding of the cerebrum also affects the cavities of the cerebrum as well as the course
of the fibre tracts, especially of the fornix which becomes greatly bent on itself.
In the ventricles distinct regions or ‘horns’ are recognized, an anterior cornu in

Fic. 163. Fic. 164.

Fic. 163.—Brain of Chrysothryx sciureus, after Weber. f, frontal lobe; z, interparietal
fissure; 0, occipital lobe; ~, parietal lobe; s, Sylvian fissure; ¢, temporal lobe; ¢s, sulcus
temporalis.

Fic. 164.—Brain of Manis javanica, after Weber. ch, cerebellar hemispheres; h,
hippocampal lobe; 0, olfactory lobe; ps, presylvian fissure; s, Sylvian fissure; ss, sulcus
sagittalis; v, vermis; 1, optic nerve.

the frontal lobe, a posterior in the occipital lobe and an inferior cornu in the
temporal lobe. Associated with the cortical gray matter are nerve fibres (compara-
tively few in the lower, extremely numerous in the higher mammals) which form
a corona radiata and connect the cortex with the more posterior regions of the
brain. In the non-placental mammals the anterior commissure is very large,
forming the chief association tract between the two hemispheres, but in the higher
groups the corpus callosum becomes greatly developed and largely replaces it.
The diencephalon is greatly reduced, the hypophysis and infundibulum being
small, the latter showing traces of the saccus vasculosus and inferior lobes so
prominent in the lower vertebrates. The parietal organ is lacking, but the pinealis
is relatively large. It is separated from the roof of the skull by the occipital
lobes of the cerebrum. It is connected with the roof of the brain by two bands
or peduncles and its cavity contains a quantity of so-called ‘brain sand.’ A
transverse groove divides the optic lobes so that they consist of four lobes (corpora

quadrigemina).

SPINAL NERVES. 161

The cerebellum is divided into a median vermis and a pair of lateral portions,
each consisting of a large cerebellar hemisphere, ventral (morphologically lateral)
to which is a flocculus (fig. 162), homologous to that of the sauropsida. The
surface of the hemispheres is convoluted and this results in the arrangement of
the white and gray matter in such a way that they have a markedly dendritic ap-
pearance (arbor vita, fig. 152) when seen in longitudinal section. The pons,
‘characteristic of the mammalian brain, has already been mentioned (p. 150).

THE PERIPHERAL NERVOUS SYSTEM.
The Spinal Nerves.

The spinal nerves are metameric structures, connected with the
spinal cord by two separate portions or roots which differ greatly from
each other in development, structure and function. At the time of
the closure of the neural tube a band of cells occurs on either side of the
neural plate at the junction of neural and epidermal areas. With the
closure of the tube these form two bands, the neural crests, one on
either side of the dorsal surface of the cord (fig. 144). By unequal
growth each crest soon develops a series of metameric enlargements, the
portions of the crest between these gradually disappearing, while the en-
largements form the ganglia of the dorsal roots of the nerves. Each of
its cells, like those of the cord, sends out processes, one of which grows
medially and enters the cord in the region of the posterior cornu, while
the other extends peripherally to the skin or viscera, these processes
constituting the dorsal root of the nerve, the ganglion forming an
enlargement upon it, near its connection with the cord. The other or
ventral root is formed by fibres which grow out in a similar way from
cells in the ventral horn of the cord itself and leave it between the an-
terior and lateral columns, to extend to the muscles, glands, etc. As the
ganglion cells are inside the cord, there is no ganglion on the ventral
root. Except in the cyclostomes the dorsal and ventral roots unite
soon after leaving the cord, the combined trunk being a typical spinal
nerve (figs. 145, 166).

Physiologically the roots differ in that the dorsal roots are mainly
composed of sensory fibres, while the ventral roots contain only motor
fibres. That is, on stimulation of the parts to which they are distrib-
uted the dorsal roots and their fibres carry nervous impulses to the cord
—they are afferent—while the impulses in the ventral roots are carried
in the opposite direction by efferent fibres. In their case stimulation
arises in the central nervous system and the impulse is carried outward

Il

162 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

to the parts to which the fibres are distributed, causing these to act—
muscles to contract, glands to secrete, etc. Hence the ventral roots are
called motor roots. Their fibres are without sensory functions, while
sensory fibres are equally unable to cause action in any peripheral
part (Bell’s law).

After a longer or shorter course, each spinal nerve, formed by the
union of dorsal and ventral roots, divides into three branches, each of
which receives both sensory and motor fibres. These are known as

iti ll

S

| Pee

Fic. 165.—A, diagram of collector nerve; B, of a nerve plexus, after Braus; C, branchial

plexus of Salamandra maculata, after Fiirbringer.
the ramus dorsalis, ramus ventralis and ramus visceralis or in-
testinalis. The first goes to the skin and muscles of the dorsal region;
the second to those of the sides and ventral parts of the body; while the
visceral branch descends to the roof of the ccelom, near the insertion of
the mesentery, where it connects with the sympathetic nervous system
to be described below (fig. 166).

Recent physiological and histological analysis shows the existence
of two groups of nervous elements in both sensory and motor nerves.
There are somatic sensory and motor fibres, distributed to the skin
and most of the external sense organs and to the voluntary muscles, and

SYMPATHETIC SYSTEM. 163

there are also visceral fibres of both kinds, supplying the viscera
(alimentary canal, excretory and reproductive organs) and the circula-
tory system. The dorsal and ventral rami contain mostly somatic
fibres with a few of the visceral type, while the visceral rami are com-
posed of visceral fibres alone. The farther subdivision of these nerves
will be considered later.

To the statement that the dorsal roots are purely sensory the exception must be
made that in the lower vertebrates some of the visceral motor fibres, arising in
the neighborhood of the lateral cornu, pass out from the cord through the dorsal
root. In the mammals they are said to leave by the ventral roots like all other
motor fibres.

In the regions of the appendages the spinal nerves usually form
networks or plexuses, branches of a varying number of ventral rami
interlacing in a complicated manner before entering the appendage.
Plexuses are poorly developed in the fishes, but here many spinal nerves
are united before entering a limb by means of a longitudinal ‘col-
lector’ nerve, there being no exchange of fibres such as occurs in a
plexus. In the amphibia there are two plexuses, a cervico-brachial
near the fore limb, and a lumbo-sacral for the hind limb. In the
higher groups there may be four plexuses: cervical, brachial, lum-
bar and sacral, the positions of which are indicated by their names.

The Sympathetic System.

The function of the sympathetic system is the control of the viscera,
various glands, the smooth muscles, and through the latter, of the size
of the blood-vessels and the supply of blood to the various parts.
The system is connected with the spinal nerves by the visceral rami
(rami communicantes) already mentioned. As has just been said,
these visceral rami contain both motor and sensory fibres. As these
rami extend downward in their development, they carry with them
ganglion cells derived from the ganglia of the dorsal roots of the
spinal nerves, and these give rise to the sympathetic ganglia. Of
these there are three groups. Nearest to the spinal nerves on either
side are a series of the sympathetic trunk (chain ganglia), usually
connected with each other by a longitudinal sympathetic trunk.
Nerves run from these chain ganglia to the prevertebral ganglia,
some of which, like the cardiac, pelvic, hypogastric and solar
(plexuses) are of considerable size. From these nerves go to the

164 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

peripheral ganglia, situated at various points along the viscera, some at
some distance from the sympathetic centres.

In the sympathetic system four kinds of nervous elements are to be
distinguished. The original trunk that grows out (the ramus visceralis)
consists of motor and sensory fibres. The latter arise from ganglion
cells in the ganglia of the dorsal roots. The motor fibres have their
cell bodies in the cord at about the level of the lateral cornu, and pass

visceral motor

a : H somatic motor
<q "Peg f De _ wtsceral sensory

sbeveecineeadeeee somatic sensory”

{)
db, edo gna
PF NL KAngw Spe

Fic. 166.—Diagram of the relations of the sympathetic system, based on Huber. ‘The
character of the different fibres is shown by conventional lines. bv, blood-vessel; cg, chain
ganglion; d, dorsal ramus; dr, dorsal root; g, gland; gr, gray ramus; pg, peripheral gangiion;
pug, prevertebral ganglion; st, sympathetic trunk; v, ventral ramus; vi, visceral ramus; vr,
ventral root; wr, white ramus.

out, in the lower vertebrates by the dorsal, in the mammals by the
ventral root. In the sympathetic system itself there are sensory and
motor (excitatory) cells, derived from the ganglion cells carried down
by the growing nerves. These develop their dendrites and axons,
and some of these run up the rami communicantes to the dorsal and
ventral rami, and follow along them to the peripheral glands and
blood-vessels of the body. Others grow into the various viscera. These
purely sympathetic fibers are not medullated and hence are gray in

CRANIAL NERVES. 165

color, and form gray rami communicantes for a part of their course to
the spinal nerves.

The sympathetic system is best developed in the trunk, but it
extends forward into the head, where a series of sympathetic ganglia
(ciliary, sphenopalatine, etc.) is connected with the cranial nerves
as far forward as the fifth. The sympathetic trunk in this region is
usually closely connected with other nerves, but occasionally (Vidian
nerve from the sphenopalatine to the facial ganglion, Jacobson’s
commissure from the seventh to the ninth, fig. 170) it is distinct.

A few words may be added to this general account. In the elas-
mobranchs there is no sympathetic trunk, this first appearing in the
teleosts. The system is more highly developed in the aquatic than
in the terrestrial urodeles or in the anura. In the sauropsida the
trunk is usually double on either side in the neck region, one branch
running through the vertebrarterial canal of the vertebra. In the
mammals the cervical part of the trunk is usually closely associated
with the pneumogastric nerve. In the development certain ganglion
cells migrate from the developing sympathetic system and pass to various
parts of the body, being usually closely associated with the glands of
so-called internal secretion—hypophysis, carotid gland, suprarenals,
etc. They possess a peculiar affinity for chromic salts and are
known as chromaffine cells. Little is known of their function.

The Cranial Nerves.

The nerves which arise from the brain and pass out through the
foramina in the skull are known as the cranial nerves. While in a
general way they resemble the spinal nerves, they have been specialized
and modified in many respects in correspondence with the specialization
of the head itself, some consisting of sensory fibres alone, some of only
motor fibres, while others are mixed, that is, contain both kinds of
fibres. There can also be recognized somatic and visceral nerves as in
the trunk, while the somatic sensory fibres may be arranged in dif-
ferent groups, differing in their connexions inside the brain and in the
sense organs to which they are distributed. Thus six different kinds of
fibres may occur in the cranial nerves, as follows:

1. The somatic motor, which go to the muscles derived from the
myotomes of the head.

2. The visceral motor, distributed to the muscles of the gill region

166 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

y

Fic. 167.—Brain and cranial nerves of Carcharias littoralis (Princeton 310), natural size
bo, olfactory bulb; br’-*, branchial nerves; cr, corpus restiforme; d, diencephalon; e, ep:
physis; ec, external canal of ear; ev, external rectus; eo, external oblique; hm, hyomandibula
nerve; /, lateralis nerve; md, mandibularis nerve; ms, mesencephalon, also, maxillari
superior; 00, olfactory organ; os, ophthalmicus superficialis nerve; of, olfactory tract: pa
palatine nerve; pc, posterior canal; po, post-trematic branch; pr, pretrematic branch; «
superior oblique; sr, superior rectus, ¢, telencephalon; #, utriculus; v, visceral branch of X
I-X, cranial nerves.

CRANIAL NERVES. 167

and their homologues in the higher vertebrates, arising from the lateral
plate region of the embryo.

3. The visceral sensory nerves are connected inside the brain
with the communis tract of the medulla (fascicularis solitarius of human
anatomy), while they terminate in special taste organs, usually within
the mouth, but in many teleostomes distributed on the sides of the body
as well.

4. The general cutaneous sensory nerves, corresponding to the
somatic sensory of the trunk. Internally they are connected with the
dorsal horns of the spinal cord and the homologous parts of the myelen-
cephalon, while distally they terminate either as free nerves or in special
tactile organs in the skin.

5. The acustico-lateralis nerves, the centre of which is in the
cerebellum and in the tuberculum acusticum of the myelencephalon.
Distally the fibres terminate in peculiar collections of sense cells known
as sense hillocks or neuromasts occurring in the inner ear and in the
lateral line organs of the ichthyopsida.

6. The nerves of special sense (olfactory and optic).

The first four of these groups occur in the spinal nerves; the last two
are confined to the head. While each spinal nerve contains all four
components, the same is not true of most of the cranial nerves, some
having but a single kind of fibre. On this and other accounts it is
necessary to review each nerve in some detail. In the lower verte-
brates (ichthyopsida) there are ten of these cranial nerves; in the am-
niotes there are twelve. These are known by both name and number.

I. The Olfactory Nerve differs considerably in the various groups
of vertebrates. The term strictly includes only the fibres extending
between the olfactory lobe of the brain and the olfactory epithelium,
the fibres terminating in the rhinencephalon by dendrites which, in-
terlacing with dendrites of cerebral neurons, form oval bodies, the
glomeruli. The olfactory nerve differs from all others in that it arises
from cells of the epidermis. In some vertebrates (elasmobranchs,
some teleosts, ganoids, snakes, some lizards, fig. 168, A), the nerve proper
is very short, while the olfactory lobe is developed into an elongate
structure in which separate regions may be distinguished, a part of the
lobe remaining in connexion with the cerebrum, next a narrower stalk,
the tractus, and lastly a larger bulbus olfactorius, containing the
glomeruli, close to the nasal organ. In other vertebrates (some
teleosts, amphibia, some lizards, turtles, fig. 168, B) the nerve is more

168 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

elongate and the lobe is not differentiated into bulb and tract. The
nerve in all forms consists solely of special sensory fibres, and its ap-
parent origin from either the tip or the ventral surface of the cerebrum

ED

Fe SE
B

Fic. 168.—Diagrams of the different kinds of olfactory bulb, tract, and nerve. 4a, ol-
factory bulb; g, glomeruli; ol, olfactory lobe; on, olfactory nerve; to, olfactory tract.

Fic..169.—Brain and olfactory and (nf) terminalis nerves of Raia, after Locy.

is to be explained by the varying development of the hemispheres as
given above (p. 159).

In some fishes (several elasmobranchs, dipnoi, Amia) a small
nerve arises dorsally (some elasmobranchs) or ventrally from the cere-
brum, has a distinct ganglion and, following somewhat closely the

CRANIAL NERVES. 169

olfactory nerve, is distributed to the olfactory epithelium. Apparently
the same nerve occurs in-the human embryo and it may be looked for
elsewhere. It is called the terminalis nerve and probably belongs to
the general cutaneous system.

II. The Optic Nerve arises from the floor of the diencephalon and
extends to the eye where it spreads over the inner surface of the retina.
Together with the olfactory nerve it is usually stated to differ from the
other cranial nerves in being an outgrowth from the brain. In its
history, which is closely connected with that of the eye, there is first
formed the optic stalk with the optic vesicle at its tip (see eye for details).
The stalk grows out from the recessus opticus and hence is clearly
dorsal in position. Soon after the involution of the optic cup, nerve
cells are proliferated from the distal surface of the retina, which pass
through the chorioid fissure and along the groove on the ventral side of
the optic stalk. These fibres and not the cells of the stalk form the
definitive optic nerve of the adult, and the cells from which they arise
form the optic ganglion, which, to a certain extent, is comparable to the
ganglion of a dorsal root. This view also lessens the differences be-
tween the optic and other cranial nerves, a view which was natural
before the history of the nerve was known and when it was thought
that the stalk itself was transformed into the nerve.

The nerve fibres, in their centripetal growth, do not stop on reaching
the diencephalon, but continue across its ventral surface and become
connected with the opposite side of the brain. There is thus a crossing
or chiasma of the optic nerves, that from the left eye going to the right
side of the brain and vice versa. In most vertebrates the chiasma is
plainly seen from the surface, but in cyclostomes and dipnoans it may
occur in the substance of the brain itself. In the lower vertebrates the
chiasma is complete and the nerves from the two sides may simply over-
lap or they may interlace with varying degrees of complexity. In the
mammals, on the other hand, the chiasma can be analyzed only by
microscopic methods, so intimately are the fibres interwoven, while here
some of the fibres (‘lateral fibres’), instead of crossing, enter the cor-
responding side of the brain. The internal connections of the optic
nerves are not with the ’twixt-brain, but the fibres, after passing the
chiasma, grow dorsally and posteriorly and become connected with the
dorsal part of the mid-brain, hence called the optic lobes.

There has been described in the embryo elasmobranch, under the name thal-
amic nerve a small strand arising between the di- and mesencephalon. It disap-

170 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

pears without leaving a trace, unless it contribute to the ciliary ganglion. Its status
as a nerve is very uncertain. J

The Eye Muscle Nerves (fig. 137).—The III (oculomotorius),
IV (trochlearis) and VI (abducens) nerves are distributed to the
muscles which control the movements of the eye and hence are treated
together. The oculomotor supplies the superior, inferior and internal
rectus and inferior oblique muscles; the trochlearis goes to the superior
oblique, while the abducens innervates the external rectus muscle.

lateralis
Tom ySCeTal motor
TOOT visceral Sensory
mE general cutancus

Fic. 170.—Diagram of branches and components of the fifth or trigeminal nerve in a
shark. gg, Gasserian ganglion; 7, Jacobson’s commissure, connecting with glossopharyn-
geal; md, mandibularis nerve; mx, maxillaris nerve; op, os, ophthalmicus profundus and
superficialis nerves.

These peculiarities of distribution are explained by the development
of the muscles (p. 128), the derivatives of each somite having a common
nerve supply. The oculomotor nerve springs from the ventral surface
of the mid-brain, the fourth from the dorsal surface at the hinder margin
of the mesencephalon, while the sixth comes from the ventral surface
of the myelencephalon. Inside the brain the trochlearis is traced to
its nucleus in a ventral position.

In the majority of vertebrates these nerves are readily traced from
the brain to the muscles they supply, but not infrequently the abducens
(lacking in Petromyzon) is united proximally with the fifth nerve,
while in a few forms the trochlearis has not been recognized, and it is
said that in the adult Bdellostoma all eye-muscle nerves are lacking.
The ciliary ganglion is closely associated with the oculomotor nerve.
All three eye-muscle nerves belong to the somatic motor group.

V. The Trigeminal, one of the largest of the cranial nerves, arises

CRANIAL NERVES. 171

from the anterio-lateral angle of the myencephalon, and its fibres pass
almost immediately into the semilunar (Gasserian) ganglion,
which may lie either within or without the skull. In the higher verte-
brates the nerve divides beyond the ganglion into three main trunks—
ophthalmic, maxillary, and mandibular—whence the name. In
the lower vertebrates the maxillary and mandibular pursue a common
course for some distance before separating.

In the fishes the ophthalmic is represented by two branches, an
ophthalmicus superficialis (not to be confused with the similarly
named branch of the seventh with which it is closely associated) and
an ophthalmicus profundus, which passes between the eye muscles
on its way to the tip of the head. Both are purely sensory, in most
vertebrates general cutaneous, but in the teleosts they, together with
the maxillary, supply also the taste organs (visceral sensory) of the
surface of the head. The superficialis innervates the skin above and
in front of the eye; the profundus goes to the eyelids, conjunctiva,
snout and the mucous membrane of the nose, passing through the
ciliary ganglion in its course. In the urodeles, where the maxillaris
is reduced, the profundus supplies its region.

The maxillaris, with components similar to those of the ophthalmic,
runs beneath the eye, passing the sphenopalatine ganglion (p. 165)
in its course, supplying much the same territory as the ophthalmic and,
in addition, the roof of the palate and the teeth of the upper jaw.

The mandibularis ramus is a mixed nerve. The motor components
(visceral) innervate the muscles of the jaws, some muscles of the floor
of the mouth and in mammals the tensor tympani muscle. The sensory
component (general cutaneous) divides into two parts, the lingualis
going to the tongue and the mandibularis to the skin of the lower
jaw, chin and lower lip, and to the lower teeth. In mammals there is
a weak auricularis superficialis nerve arising from the mandibularis
and going to the temporal region and to the conch of the ear.

Two ganglia of the sympathetic system are associated with the trigeminal: the
otic ganglion near the exit from the skull and the submaxillary where the lingualis
bends to enter the tongue. From the otic a trunk (Jacobson’s commissure, p. 165)
runs back to connect with the ninth nerve.

The fifth nerve is usually compared to a post-otic nerve (vide infra) in that the
mouth is regarded as the homologue of a pair of gill clefts, the maxillary being
the pre- and the mandibularis the post-trematic nerves (see nerve IX, below).
The homologies of the ophthalmicus are less certain. Some facts seem to point

172 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

to the fifth being a compound nerve, this branch being the remnant of a somite
otherwise lost. Others would view the ophthalmic as the representative of the
dorsal ramus of a spinal nerve.

VII. The Facial Nerve, the hindmost of the preotic nerves, differs
greatly in the branchiate and the pulmonate vertebrates. In all there
is a close association with the eighth nerve, and in the anura, some
teleosts and ganoids, and in the holocephals the fifth and the seventh
are so closely related that their ganglia are fused.

Fic. 171.—Dagram of seventh (facial) nerve; for components see fig. 170. 6, buc-
calis nerve; ct, chorda tympani; gg, geniculate ganglion; h, hyoid nerve; hm, hyomandib-
ular nerve; /g, lateralis ganglion; mxe, maxillaris externus nerve; os, ophthalmicus
superficialis nerve; pal, palatine nerve; sp, spiracle.

In the aquatic ichthyopsida the several roots by which the seventh
nerve leaves the medulla unite in a compound ganglion, the upper
element being the ganglion of the lateralis component, the lower the
geniculate, the true ganglion of the seventh nerve. Beyond the
ganglion the nerve divides into five trunks, as follows:

A. The ophthalmicus superficialis, which runs forward near
the dorsal surface of the head; B. the buccalis, which courses nearly
parallel to the maxillaris of the trigeminal and is often bound up with it;
C.themandibularis externus, which usually divides into two branches,
usually follows much the same course as the mandibularis trigemini,
and supplies the lower jaw and the spiracular and hyoid region; D.
the palatinus which goes to the mucous membrane of the oral cavity,
E. the hyoideus (usually united with the mandibularis for some distance
as a hyomandibular nerve), which goes ventrally and supplies the
mucosa of the mouth and the muscles of the hyoid region. In cases,
like many elasmobranchs, where a spiracle is present, the hyomandib-

CRANIAL NERVES. 173

ularis passes behind it and hence is a post-trematic ramus. In some
cases a small twig bends down from the palatine and represents the
pretrematic branch.

Three of these—ophthalmicus superficialis, buccalis and mandib-
ularis externus—belong to the lateralis system, which is unrepre-
sented inthe spinal nerves. This has its own ganglion, which may unite
with geniculate or semilunar, and it supplies the lateral line system of
the head (see sense organs). The superficial ophthalmic innervates the
supraorbital line of these organs, and in the elasmobranchs, breaks
up distally to go to the modified organs (ampulle of Savi and Loren-
zini) at the tip of the snout. In the same way the buccalis supplies
the infraorbital line and the mandibularis externus those of the lower
jaw and the hyoid and spiracular regions.

As there are no myotomic muscles in the region supplied by the
facial nerve, there are no somatic motor components The general
cutaneous elements run in the hyoideus to the skin of the hyoid region,
but in other vertebrates this territory is supplied by branches of the fifth,
which spread to the operculum and to the dorsal surface of the head.
The visceral motor components occur in the hyoid nerve and, in the
aquatic forms, they supply the muscles of the hyoid region and the
posterior belly of the depressor mandibule. In the mammals, with
the development of the muscles of expression (p. 134), the same
branch (known in human anatomy as the main branch of the facial) has
a much greater extension, the result of the migration of the muscles
from the hyoid region to their definitive position.

The geniculate ganglion belongs to the visceral sensory system,
the fibres of which run in the hyoid and palatine nerves to reach the
sense organs in the oral cavity and in the spiracular gill when this is
present. In those fishes where there are taste, organs on the outer
surface of the body there is a ‘nerve of Weber’ (ramus lateralis ac-
cessorius) which goes to the dorsal surface of the head and then to back,
fins and tail, wherever the gustatory organs occur. It is frequently
accompanied by fibres of the tenth nerve. In the mammals the visce-
ral sensory fibres occur in the main trunk of the seventh, in the great
superficial petrosal and in the chorda tympani nerves. This last is a
post-trematic nerve which passes through the middle ear and thence on
the medial side of the lower jaw to join the lingualis.

In the adult anura and in the amniotes, where gills and lateral line
organs are lacking, the facialis undergoes a corresponding reduction,

174 COMPARATIVE MORPHOLOGY OF VERTEBRATES

the lateralis nerves being lost, while, as stated above, the motor por-
tions are increased in the mammals, in correlation with the greater
development of the facial muscles.

Fic. 172.—Ventral view of brain and cranial nerves of Iguana, after Fischer. J-—XII,
cranial nerves; 1-3, first three cervical nerves; gp, petrosal ganglion; 7, Jacobson’s commis-
sure; #, hypoglossal; , nasalis ramus of V; 7/, ramus frontalis of V; sy, sympathetic.

VIII. The Acustic (Auditory) Nerve is closely associated with the
seventh, but microscopic analysis shows that it has its own roots and
ganglion. It is purely sensory, its branches going to the sensory areas
of the inner ear. Its connections inside the brain and the development
of the ear itself (see sense organs) show that the nerve belongs to the
lateralis system, the ear being a group of modified lateral line organs.
Beyond the ganglion the nerve divides into a vestibular branch,
supplying the utriculus and semicircular canals, and a cochlear branch,
going to the lagena and to its homologue in the mammals, the cochlea.

CRANIAL NERVES. 175

IX. The Glossopharyngeal, the first of the post-otic nerves, has its
typical development in the branchiate vertebrates. Its roots, both
motor and sensory, pass into the petrosal ganglion, beyond which a
dorsal famus is given off to the top of the head, while the main trunk,
passing outward and backward, leaves the skull, either by its own fora-
men (most branchiates) or together with the tenth nerve (anura and
amniotes). It divides, just above the first gill cleft, into pre- and post-
trematic nerves, which run in the anterior and posterior walls of the
cleft to the ventral wall of the pharynx, the pretrematic giving off a
nerve to the mucous membrane of the palate. Ninth and tenth nerves

Fic. 173.—Diagram of ninth (glossopharyngeal) and tenth (vagus) nerves of a shark;
for components see fig. 170. d, dorsal ramus; g, gastric nerve; #, to heart; 7, Jacobson’s
commissure; /, lateralis nerve; /g, lateralis ganglion; po, pr, post- and pretrematic branches;
sp, spiracle; st, to supratemporal lateral line organs.

are usually closely associated (their ganglia may fuse), while ninth and
fifth are frequently connected by Jacobson’s commissure and the pala-
tine branch may connect with the geniculate ganglion. The glos--
sopharyngeal may contain three kinds of components, the somatic
motor fibres being absent because of the failure of themyotomic muscles
to develop. The general cutaneous is usually represented by the dorsal
ramus alone, but in Petromyzon its fibres reach the ventral skin through
the post-trematic nerve. The visceral sensory fibres reach the taste
organs by way of the pharyngeal and lingual nerves, while the visceral
motor elements go to the muscles of the gill region by way of the post-
trematic ramus.

X. The Vagus is a complex of nerves, each similar to the ninth,
with the addition, in the branchiates, of lateralis components, and sup-
plying in them the remaining gill clefts. Numerous rootlets pass from

176 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

the medulla to the ganglion jugularis, beyond which the dorsal rami
arise and then the main trunk runs backward, giving off as many
branchial nerves as’ there are gill clefts, each with an epibranchial
ganglion and each dividing into pre- and post-trematic rami. To
this extent the tenth is a polymeric nerve with coalesced proximal
portions.

Near the last cleft the main trunk divides into two nerves. One of
these, the ramus lateralis, continues back, just beneath the skin, to
innervate the lateral line organs of the trunk and tail. The other, the

Fic. 174.—Diagram of cranial nerves of a cat, the lower jaw reflected, after Mivart.
II~XII, cranial nerves; ct, chorda tympani; d, dentary nerve; g, Gasserian ganglion; 7a,
infraorbital nerve; /, lingual nerve; /z, /s, laryngeus inferior and superior; md, mandibularis
nerve; mx, maxillaris nerve; 0, ophthalmic nerve; ¢, tongue.
ramus intestinalis, goes inward and backward to supply the cesoph-
agus, stomach, heart and other viscera (in air-breathing vertebrates the
lungs also, whence the name pneumogastric nerve). In the dorsal
rami and the branchial nerves the components are about the same as in
the ninth nerve. The most caudal of the motor roots of the vagus
furnish visceral motor fibres which go to some of the muscles connected
with the pectoral arch and appendages, while others pass, by way of the
intestinalis, to the viscera. In the same way visceral sensory fibres go
through the same nerve to the taste buds of the pharynx, and in the

SENSORY ORGANS. 177

higher vertebrates the same components occur in the pharyngeal,
laryngeal, cesophageal, and gastric branches of the intestinalis. The
distribution of the vagus shows that the parts supplied are to be re-
garded as morphologically derived from the head, though (heart,
lungs and stomach) they may be far removed from it in the adult.

Although details have been mentioned, some differences between
air- and water-breathing vertebrates may be summarized. The lateral
line organs are associated with an aquatic life, occurring in the branchiate
forms, even of the amphibia. With the assumption of a pulmonate
respiration the lateral line organs are lost and with them go the lateralis
elements of the seventh and tenth nerves. In the amniotes neither the
organs nor the nerves appear, even in development. Also the loss of
gills, and the closure of the clefts results in a modification of the nerves
of the ventral regions.

XI. The Accessory Nerve appears as a distinct nerve in the am-
niotes, though traces of it appear in the ichthyopsida where the poster-
ior roots of the vagus furnish fibres which go to muscles in the pectoral
region. In the amniotes the number of these roots is increased (up to
seven in mammals), the additions being made to the posterior end of the
series. The fibres run forward between the dorsal and ventral roots of
the cervical nerves and unite to form a trunk, distinct from the vagus,
which bends back to supply muscles connected with the pectoral arch.
The components of this accessory nerve belong to the visceral motor
system, and the explanation of muscles connected with locomotion being
supplied by visceral nerves is not easy.

XII. The Hypoglossal Nerve of the adult contains only somatic
motor fibres, but in the young of several forms, both amniote and
ichthyopsidan, two or more ganglionated roots are formed which soon
disappear. The roots of the nerve lie at the junction of brain and
spinal cord, and hence the nerve lies outside the skull in the lower, in-
side it in the higher forms. The nerve contributes to the innervation
of the tongue, the trunk and the brachial plexus in the lower vertebrates,
while in the higher groups it is more restricted to the tongue and the
sternohyoid muscle.

THE SENSORY ORGANS.

The sensory organs are to receive information from without and
to transform it into stimuli to be carried by the-nerves to the ganglia,
usually those of the central nervous system. ‘This information varies

12

178 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

in character and the organs consequently differ in structure according
to the impressions they are to receive.

With very few exceptions the characteristic portions of the organs,
the sensory cells, arise from the ectoderm, but accessory parts, chiefly
of mesodermal origin, may be so abundant as to form the bulk of the
organ. In some cases the organs may remain in connexion with the
surface of the body (the parent ectoderm) throughout life, but frequently
they sink to a deeper position and become surrounded with a protective
sense capsule, while those connected with the sympathetic system may
be scattered throughout almost the entire body.

Fic. 175.—Free nerve termina- Fic. 176.—Sensory cells, after Fiirbringer.

tions in the skin of Salamandra, a, crista cell of ear; b, rod cell of eye; c, ol-
freely after Retzius. factory cell.

The recipient structures may be of two kinds. In the one (fg.
175) the ends of the nerve receive the impressions from without, often
aided by various accessory structures. In the other there are specialized
sense cells (fig. 176), the peripheral ends of which bear different kinds
of cuticular percipient parts—hairs, bristles, rods, cones, etc.—while
the basal ends of the cells are connected with the terminations of nerve
cells which act as the conducting elements. The distinction between
the two is one of convenience rather than one of physiological or mor-
phological importance, for the ‘nerves’ of the first are in reality but
the prolongations of sensory cells.

Nerve-end Apparatus.

In many cases—skin, alimentary tract, muscles, etc_—the ends of
the sensory nerves lose their medullary sheath and break up into fine
fibrilla which terminate, without special accessory structures, among the
cells of the tissue to which they are distributed (free nerve termina-
tions). On the other hand, there are numerous end organs, espe-

SENSORY ORGANS. . 179

cially among the terrestrial vertebrates, in which accessory parts are
present. For details of these reference must be made to histological
text-books; only a mention of some of the kinds can be made here.

In the simple tactile corpuscle the nerve terminates with a cup
in which is seated a lenticular tactile cell (fig. 177, A). Somewhat
allied are Grandry’s (Merkel’s) corpuscles in which two or more
tactile cells are enclosed in a connective tissue sheath, while the nerve,
losing its medullary sheath as it reaches the capsule, expands into
plates which are inserted between each two tactile cells (fig. 177, B).

y
oS

Fic. 177.—A, tactile corpuscle; Fic. Saeee corpuscle.
B, Grandry’ S$ corpuscle.

In another series of sensory structures the end of the nerve is club-
shaped and is surrounded by a connective-tissue sheath, either simple
(cylindrical corpuscles), or in Pacini’s (Vater’s, fig. 178) and
Herbst’s corpuscles, the sheath is formed of layers of cells, recalling
the coats of an onion, while immediately around the club is a layer
of cubical cells. Still another variant is found in Krausse’s (corpus-
culum bulboideum) and Meissner’s corpuscles, where the nerve,
on entering the corpuscle, breaks up into numerous branches which
surround an axial core of large cells.

It is impossible at present to state with certainty the function of
each of these and other nerve-end apparatuses and to say which are
connected with the different senses—tactile, pressure, pain, heat and
cold, muscular, etc.—which are commonly confused under the term
‘touch.’

Lateral Line Organs.

The lateral line organs occur only in the ichthyopsida and here
only during the branchiate stages. They arise as thickenings of the
ectoderm on either side of the head in the neighborhood of the ear.
From here the thickenings extend in definite lines which determine the

180 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

series of organs in the adult. At points on these lines the sensory
areas are developed by the differentiation of two kind of cells, the
supporting cells which extend through the epidermis from the corium
to the free surface, and the sensory cells which reach from the surface
only part way to the base. The latter are pear-shaped and bear
cuticular hairs or bristles on their free ends (fig. 179), while the deeper
ends are embraced by the non-medullated fibrils of the lateralis system
of nerves, which follow the lines of organs, and in development keep
pace with their extension. These sensory areas are the nerve hillocks
or neuromasts already referred to.

Fic. 179. Fic. 180.

Fic. 179.—Sense organ of lateral line of Diemyctylus (aquatic form) freely after Kings-
bury, C, cone cells; s, spindle cells.

Fic. 180.—Developing lateral line organ on one side of head of A mia, showing method
of closure of grooves to canals, after Allis. a7, anterior naris; io, so, infra- and supraorbital
lines; px, posterior naris.

In the cyclostomes and aquatic amphibia each sensory patch
sinks into a separate pit (fig. 179), but in all other itchhyopsida the
lines of organs sink in the same way, the patches being connected by
grooves. In Chimera these grooves remain open, but in all others they
are closed except at certain points where pores connect the canals
formed by the closed grooves with the exterior. In this way the sensory
areas come to lie in canals beneath the surface, water obtaining access
to them through the pores. In many teleosts (fig. 181) the pores
pass through notches or openings in the scales, while on the head the
canals themselves frequently run through some of the cranial bones.

Of considerable morphological importance, especially in connection with the
morphology of the ear, are the facts that the sensory areas multiply by elongation,
followed by division, and that the pores themselves increase in the same way

SENSORY ORGANS, 181

(fig. 180); the pore elongates and then its margins meet in the middle, thus
producing two pores. ‘There has been much discussion as to the development of
the lateralis nerves, especially that of the trunk, some thinking that it increases by
additions from the ectoderm of the skin. It appears more probable that all of its
material is derived from the nerve and that there are no additions from other sources.

Fic. 181.—Stereogram of lateral line organs of a fish. c, lateral line canal; /m, lateralis
nerve; ~, pores connecting with the exterior; 5, scales in skin; so, sense organs of lateral line.

Fic. 182.—Head of pollack, showing lateral line canals and nerves of the lateralis system,
after Cole. Lateralis nerves black, canals and brain dotted. 6, buccalis ramus of VII
nerve; dl, dorsal ramus of lateralis of X nerve; #, hyomandibularis nerve; hm, hyomandib-
ular line of organs; io, infraorbital line; /, lateral line canal; , nares; 0, olfactory lobe; of,
operculum; os, ophthalmicus superficialis nerve; soc, commissure connecting lines of the
two sides; so, supraorbital line of organs; sf, supratemporal part of lateral line; v/,
ventral ramus of lateralis of X nerve; x, visceralis part of X nerve.

The distribution of these organs and their canals varies considerably.
The most constant lines are the following (fig. 182): A supraorbital
line running forward from the region of the ear, above the eye, to the

182 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

tip of the snout and innervated by the ophthalmicus superficialis
branch of the seventh nerve; an infraorbital line running in the same
way beneath the eye and supplied by the buccalis nerve; a hyomandib-
ular line extending along the lower jaw (and the operculum when
present), and innervated by the mandibularis externus; and lastly
the lateral line proper (sometimes double) which runs back on either
side to the tail and is supplied by the lateralis of the tenth nerve.
Frequently the systems of the two sides are connected by a supra-
temporal line extending across the hinder part of the skull, from one
side to the other.

The lateral line organs appear in the larve of all amphibia, but on
the assumption of a terrestrial life they sink beneath the skin and
usually degenerate, all traces of them and the lateralis nerves being lost
in the adult. In a few cases (Triton, Amblystoma, etc.) they are said
not to be entirely lost, but to reappear at the surface when the animals
return to the water for oviposition. Various functions have been assigned
to the lateral line organs. Since they contain much mucus they were
long called slime organs. Then they were recognized as sensory and a
‘sixth sense’ was attributed to them. Recently it has been made very
probable that they are to recognize vibrations of a slow rate in the
water and thus, among other things, to determine currents, etc.

Closely allied to the lateral line organs in nerve supply are the
ampullz of Savi and Lorenzini which occur on the head of elasmo-
branchs. Each consists of a long tube, opening by a pore at the surface
of the skin and ending with a chambered enlargement, the ampulla,
at the deeper end. The tube is filled with a crystal mucus and the
ampulla is embraced by fibres of the lateralis nerve. The organs
have been supposed to be connected with a pressure sense. The
statement is made that when they are removed the fish is unable to
sink; this may throw some light on their functions. .

The Auditory Organs.

Both in character of innervation and in certain peculiarities of
development the sensory parts of the vertebrate ears are closely related
to the lateral line organs. In their most complete expression three
parts are recognized in the auditory organs, the outer, middle and
inner ears. Of these the last is the essential portion and occurs in all
vertebrates, the middle ear first appearing as such in the amphibia

AUDITORY ORGANS. 183

and the outer ear, more or less completely developed, is found only in
‘ the amniotes.

The Inner Ear arises as a circular area of thickened ectoderm on
either side of the head, between the seventh and ninth nerves (fig. 136).
This soon becomes cup-shaped and then the cup closes in to form an
auditory vesicle (fig. 183), the cavity of which is connected with the
exterior by a slender tube, the endolymph duct, the result of incomplete

Fic. 183.—Diagram of developing human labyrinth from 6 to 30 mm. long, after
Streeter. @, ampulla; c, cochlear region and cochlea; au, ampullo-utricular region; d,
endolymph duct; e, endolymph region; sc, semicircular canal; se, endolymph sac; s, sac-
culus: #, utriculus; us, utriculo-saccular canal; v, vestibule.

closure. As one portion of the medial wall of the vesicle develops an
area of sensory epithelium like that of the lateral line system, this
stage may be compared to an isolated canal organ with a single pore,

In the amphibia and some of the ganoids, where there is a two-layered ectoderm
from the early stages, there is never an open auditory cup. The lower, so-called
nervous layer of the ectoderm is alone concerned in the formation of the auditory
vesicle, while the outer layer extends as an unbroken sheet across the cup. In
the elasmobranchs the endolymph duct opens to the exterior throughout life, the
external pores being recognizable on the top of the head. Elsewhere they later lose
their external openings, and the distal end of each usually expands into an enlarge-
ment, the sacculus endolymphaticus; but in the amphibia the ducts of the two
sides may unite dorsal to the brain, while other parts may branch and grow in a
root-like manner, in the canal of the spinal cord, sending diverticula (frog) into the
so-called calcareous glands, which surround the basal parts of the spinal nerves.

The next stage in the auditory vesicle is its differentiation by a
constriction into two chambers, an upper vestibulum or utriculus

184 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

‘

and a lower sacculus (fig. 183), the two connected by a narrow sac-
culo-utricular canal. The sensory area becomes divided between.
the two, but the endolymph duct is connected with the sacculus alone.
The anterior, posterior and lateral walls of the utriculus now produce
flattened outgrowths, the lateral in the horizontal, the others in vertical
planes, and parts of the sensory areas extend into each. Next, the
walls of these diverticula become pinched together so that each pocket

Fic. 184.—Diagram of the membranous labyrinth of a vertebrate, the sensory areas
dotted. ac, anterior semicircular canal; ap, ampulle; ca, criste acustice in the ampulle;
de, ductus endolymphaticus; hc, horizontal (external) canal; 1, lagena; ml, mn, ms, mu,
macule of lagena (neglecta, sacculi and utriculi); pc, posterior semicircular canal; s,
sacculus; se, saccus endolymphaticus; suc, sacculo-utricuar canal; u, utriculus.

is converted into a tube or canal, open at either end into the utriculus,
and hence approximately semicircular in outline. In one end of each
of these semicircular canals there is a patch of sensory epithelium
and the wall expands around this into an ampulla, the ampull of the
anterior and external canals being side by side, that of the posterior
canal at its lower end.

In the lower ichthyopsida there is little differentiation in the sac-
culus, but in the higher a pocket, the lagena, is given off from its poster-
ior side, a portion of the sensory epithelium extending into it. With in-

AUDITORY ORGANS. 185

creasing powers of hearing the lagena becomes greatly elongate, until
in the mammals it acquires a peculiar development and is known as the
scala media, the structure and relations of which are described below.

In the cyclostomes utriculus and sacculus are not differentiated. In the
myxinoids there is but a single semicircular canal, with, however, an ampulla at
either end. In the lampreys there are two canals, both in the vertical plane, and
each with an ampulla at its lower end.

.
: |
. —_ \ Ly

Fic. 185. Fic. 186,

Fic. 185.—Labyrinth of human embryo, 30 mm. long, after Streeter. a, ampulla; ac,
anterior canal; c, cochlea; cr, crus; de, endolymph canal; nc, cochlear nerve; s, sacculus; se,
endolymph sac; 4, utriculus; v, vestibular nerve.

Fic. 186.—Section through one of the coils of cochlea of guinea pig, after Schneider.
Bone lined; /s, spiral ligament; 7, Reissner’s membrane; sg, spiral ganglion; sm, st, sv,
scale media (ductus cochlearis), tympani and vestibuli.

These parts of the internal ear form the membranous labyrinth.
With the formation of canals, lagena, etc., the sensory epithclium
divides into separate areas (fig. 184), some of which (macule
acusticz) have sensory cells with short hairs or bristles, while others
(cristee acustice), characteristic of the ampulle, have cells with longer
hairs. The membranous labyrinth is filled with a fluid, the endo-
lymph, in which are solid particles, the otoliths. These are usually
microscopic crystals of calcium carbonate which give the endolymph

186 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

a milky appearance, but in the teleosts the lime is aggregated into one
or more ‘ear stones’ of considerable size.

With the appearance of cartilage the membranous labyrinth be-
comes enclosed in a protecting otic capsule (p. 60), which usually
follows pretty closely the divisions and canals of the epithelial parts,
thus forming the skeletal labyrinth, separated from the membranous
labyrinth by a slight gap filled with fluid (the perilymph). When
ossification occurs the skeletal labyrinth is converted into the several
otic bones. Sometimes the perilymph space is separated from the
brain cavity by membrane alone, but usually firmer structures inter-
vene, interrupted only by foramina for the passage of nerves and blood-
vessels, for the endolymph duct and for a similar perilymph duct
which extends downward. On the other hand, in all vertebrates in
which the middle ear is developed the lateral part of the skeletal wall
has two openings into the middle ear. The lower of these (fig. 188),
the fenestra tympani (f. rotunda), is closed by membrane. In the
upper (fenestra ovale or vestibuli) the membrane supports a small
bone, the stapes (p. 73).

One part of this compound skeletal and membranous labyrinth of
the mammals becomes very complicated. The lagena becomes greatly
elongated and -in order to accommodate its length it is coiled in a
spiral, its sides reaching the walls of the skeletal labyrinth on either
side. In this way the perilymph space is divided into two spiral tubes
(fig. 186), called scale, from their resemblance to spiral stairways.
The upper of these is the scala vestibuli, the lower the scala tympani,
while the scala media is formed by the lagena. This whole part of
the inner ear is the cochlea, so-called from its resemblance to a spiral
shell.

The sense organ of the scala media is very specialized and is known
as the organ of Corti (fig. 187). In general it may be said that the
scala diminishes in width from base to apex of the cochlea, and is accom-
panied in its coils by a branch (cochlear) of the acustic nerve. The
sensory structures consist of hair cells and Deiter’s cells, regularly
arranged, and a series of pillar cells, inclined to each other like the
rafters of a roof, in an A-like manner (fig. 187). As the A’s diminish
in width from base to apex of the cochlea, this part has been thought
to play a part in the recognition of pitch. There is also a cuticular
structure, the membrana tectoria, which extends from the medial wall out
over the hair cells, and this may be the intermediate organ of stimulation

AUDITORY ORGANS. 187

and may have to do with the recognition of sound waves of different
rapidity. It has recently been shown that the membrana tectoria is
connected with the hairs of the hair cells. The fact that in birds, where
pitch is certainly recognized, there.isno organ of Corti, renders
all speculation doubtful.

Fic. 187.—Organ of Corti of guinea pig, after Schneider. d, Deiter’s cells; 4c, Henson’s
cells; th, inner hair cells; 7p, inner pillar cells; /s, limbus spiralis; mf, membrana tectoria; 7,
nerve fibres; oh, outer hair cells; op, bates pillar cells; si, inner sulcus; st, scala tympani; t,
tunnel; én, tunnel nerve.

The Middle Ear or tympanum first appears in the anura. It con-
sists of a cavity (cavum tympani) in front of and below the otic
capsule, connected by a slender duct, the Eustachian tube, with the
pharynx. Externally it is separated from the outer world by a thin
partition, the tympanic membrane, from which a chain of bones, the
ossicula auditus (p. 73), extends across the cavity to the fenestra ovale,
and serves to transmit the sound waves to the inner ear. The tympanic
cavity is the homologue of the spiracular cleft of the elasmobranchs
(see respiration), which never breaks through. The tympanic mem-
brane, covered externally with ectoderm, on the inner surface with
entoderm, represents the imperforate wall of the cleft, while the Eusta-
chian tube is the narrowed internal end of the spiracle. The chain of
ear bones has already been described. It is to be noted that the
chain consists of columella and stapes in anura and sauropsida,
while in the mammals columella is replaced by incus and malleus.
In the urodeles and gymnophiones, where no tympanic cavity is devel-
oped, the quadrate articulates with the stapes.

The External Ear.—In the anura and in many reptiles the tym-
panic membrane is flush with the surface of the head, but in other rep-
tiles and in birds it is at the bottom of a canal, the external auditory
meatus, the simplest expression of an external ear. In the mammals

188 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

(whales, sirenians and some seals are exceptions) an external conch is
developed behind the meatus to collect the sound waves and to direct
them to the inner parts. In some birds the feathers are arranged
around the meatus so as to play the same part. The conch is strength-
ened by cartilage and is moved by muscles (fig. 142). There is evi-
dence which points to the conch being homologous with either the
operculum of fishes or with the first external gill of amphibians.

Fic. 188.—Diagram of mammalian ear. a, ampulla of semicircular canals; an, acustic
nerve; cn, cochlear nerve; em, external auditory meatus; ev, Eustachian tube; ft, fenestra
tympani; 7, incus; m, malleus; p, perilymph space (black); pd, perilymph duct; ph, pharynx;
5, Stapes; sc, sacculus; sm, st, sv, scala media, tympani et vestibuli; sg, spiral ganglion; ¢,
tympanic cavity; ¢m, tympanic membrane; x, utriculus; v, vestibular nerve.

Functions.—The vertebrate ear is primarily an organ of equi-
libration by which the animal recognizes all changes of position.
Though the purposes of the various parts are not accurately known,
the following conclusions seem warranted. Every movement of the
head affects the endolymph and the contained otoliths, causing them
to move (by gravity or by momentum, or by both) over the criste
acustice in the ampullz and thus to stimulate the sense cells and nerves.
The position of the semicircular canals in approximately the three
dimensions of space would seem to afford a means for the recognition
of the directions and amounts of the components of any motion. The
macule, and especially that of the lagena, are probably concerned in the
recognition of sound. In the fishes the lagena is poorly developed,

OLFACTORY ORGANS. 189

and while some fishes have been proved to hear, others have given
negative results. With the terrestrial vertebrates the sound percipient
functions of the ear are beyond a doubt, while they still retain their
equilibrational use. The sound waves strike the tympanic membrane,
are carried across the middle ear by the auditory ossicles, and set the
perilymph in motion and thus affect the parts of the membranous
labyrinth.

Organs of Taste.

The sense of taste is resident in groups of cells known as taste buds.
These differ morphologically from the lateral line organs in having
each sensory cell extend the depth of the bud, ending at the basal mem-
brane, while the majority of the supporting cells are on the outer side of
the bud. Each sense cell bears a short, bristle-like percipient struc-
ture on its free end, while the basal end is embraced by the fibrillz of the
nerve. According to the accounts of the development the taste buds
are derived from the entoderm, the only case apparently established
for the origin of sense organs except from the ectoderm. In the higher
vertebrates the organs are restricted to the cavity of the mouth where
(mammals) they occur on the tongue, especially on and near the cir-
cumvallate papilla, on the soft palate and on the epiglottis. In the
fishes the distribution is much wider, for they are found in the pharynx,
on the gills, and in many species on the surface of the body, even upon
the tail. The barbels about the mouth of many forms are richly
supplied with these organs.

The taste organs are supplied by different nerves. Apparently
those of mammals are supplied by the chorda tympani and the lingual
branch of the ninth nerve. In the fishes those of the pharyngeal
region are supplied by the post-trematic branches of the glossopharyn-
geal and vagus; those of the mouth by the palatine and mandibular
branch of the seventh; while those on the head of teleostomes are
supplied by the ophthalmic and maxillary branches of the fifth; and
those of the trunk by the nerve of Weber (p. 173), formed by fibres
from the seventh and sometimes of the tenth nerves.

Olfactory Organs.

While the senses of smell and taste are closely associated physiolog-
ically, being what might be called the chemical senses, the organs con-

Igo COMPARATIVE MORPHOLOGY OF VERTEBRATES.

cerned differ considerably in structure and relations. The olfactory
epithelium is always restricted to one or two patches at the anterior end
of the head and differs from the taste buds in histological structure.
Both sensory and supporting cells of the olfactory organs are variously
constituted. The supporting cells are the stouter, some being ciliated,
some muciparous at their free ends. The sense cells (fig. 176, C) are
thread-like or rod-like, being greatly expanded around the spherical
nucleus, while the basal end of each contracts to a nerve fibre which
extends back to the olfactory tract (p. 168), where the dendrites, inter-
lacing with those of the olfactory lobe, form the glomeruli. In the
higher vertebrates a third kind of cells, the basal cells; occur at the base
of the olfactory epithelium.

The olfactory epithelium arises as part
of the surface ectoderm of the top of the
head, but with growth it changes its position.
For protection it sinks beneath the surface as
an olfactory sac, connected with the external
world by (usually) a pair of openings, the ex-
ternal nares. The growth of the dorsal side
of the head carries the nares toward the tip
of the snout and, in the elasmobranchs, to
the ventral side of the head.

The accessory parts of the olfactory

Hit, die Neaoea gr Lee the skeletal nasal capsules (p.
cecilian (Epicrium), after Sara- 62), which are always present; in the tetra-
Tuten aa dee . podous forms glands to keep the epithelium
lateral cavity; mp, middle pas- moist, and the organ of Jacobson. The in-
sage; os, olfactory sac. é P

volution of the nasal sacs necessitates some
mechanism for bringing the external medium (water or air) to the sen-
sory cells. These will be described in connection with the several
groups below. The organ of Jacobson is a kind of accessory
olfactory organ, first appearing in the amphibia, supplied by the first
and fifth nerves and apparently serving to test the character of the food
while in the mouth. The position of the organ near the internal
nostrils lends probability to this view of the function.

'* The cyclostomes differ markedly from the other vertebrates in their olfactory
organs. The unpaired area of olfactory epithelium develops in the region of the
anterior neuropore (p. 12) and becomes involved with the involution for the
hypophysis (fig. 190) so that there is but a single external opening, serving for both

OLFACTORY ORGANS. Igi

olfactory organ and hypophysis. Hence cyclostomes, having but a single nostril,
are called monorhinal, in comparison with all other vertebrates which have two
nostrils (amphirhinal). The median opening or naris of the cyclostomes connects
with a naro-hypophysial duct, on the upper, posterior wall of which is the ol-
factory sac, formed of pairs of lateral folds (fig. 191) covered with the olfactory

Fic. 190.—Longitudinal section of head of 19 day Petromyzon embrgyo. ch, optic
chiasma; ep, epiphysial outgrowth; h; hypophysial ingrowth; mes, mesenteron; n, nasal
eee nc, notochord; oc, oral cavity; op, oral plate; sc, canal of spinal cord; th,

yreoid.

epithelium and supplied by a pair of olfactory nerves. The lower part of the duct,
now purely hypophysial, descends to the hypophysis on the ventral] side of the brain,
where it either ends blindly (petromyzons) or opens into the dorsal part of the oral
cavity (myxinoids). In the latter group the olfactory organ is surrounded by a
complicated nasal capsule of enormous size (fig. 153).

Fic. 191 Fic. 192.
Fic. 191.—Nario-hypophysial region of Petromyzon, from above. , cartilage of nasal
capsule; hd, hypophysial duct; of, folds of olfactory membrane; on, olfactory nerve.
Fic. 192.—Head of Murena, after Jordan and Evermann, showing double nostrils.

All other vertebrates have paired olfactory areas and paired nostrils
(nares) are developed in connection with them, and they have at no
time any relation to the hypophysis. ‘The mechanism for bringing the
water or air to be tested to the olfactory surface differs accordingly as
the animals are air or water breathers. In all fishes, with the exception
of the dipnoi, the sensory surface is at the bottom of a pit with no
connection with the alimentary canal. In the elasmobranchs, in

1g2 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

order that water may flow more readily through this pit, a fold is’
developed on one side of each naris, which practically divides it into
two. In many teleosts there is an actual division of each primitive
nostril into two, which may be at some distance from each other,

Fic. 193.—Section through the nasal labyrinth of Polypterus. The nerve runs through
the centre.

Fic. 194. : Fic. 195.

Fic. 194.—Head of chick of 54 days, showing development of oro-nasal canal
after Keibel. cf, chorioid fissure; /, thickening for lacrimal duct; 2, nasal pit; on, oro-
nasal groove.

Fic. 195.—Model of mouth of Echidna embryo, after Seydel, showing method of in-
growth of palatal folds (pf) to cut off secondary nasal passages. ch, primitive choanz; et,
egg tooth; 7, opening of Jacobson’s organ.

an

often at the ends of prominent tubes (fig. 192). Inside the nasal
capsule the olfactory epithelium is variously folded in order to increase
the sensory surface, often forming a labyrinth of considerable complex-

ity (fig. 193).

OLFACTORY ORGANS. 5 193

In air-breathing vertebrates, beginning with the dipnoi, a means is
developed for drawing air over the sensory surface, the first traces of
which are seen in the elasmobranchs. These frequently have an
oro-nasal groove, leading from each naris to the angle of the mouth.
In some species this groove is practically converted into a tube by the
meeting of the walls below. Beginning with the dipnoi and continuing
with the amphibia and amniotes (fig. 194) a similar groove is formed
on either side before the formation of skeletal parts. This closes in,
the edges of each groove uniting, so that a tube or duct is formed, lead-
ing from the naris into the oral cavity, where an internal naris or
choana occurs. Later maxillary and premaxillary bones arise ventral
to the narial passage, so that the ducts appear to run through the skull.
The position of the choane varies considerably, being just inside the
jaws in the amphibia and lower reptiles, farther back in the higher
reptiles and the birds and mammals, the nasal passages being cut off
from the roof of the primitive mouth by the ingrowth of the palatal
processes of the maxillary bones and higher, by. similar extensions of
the palatines, and in some cases, of the pterygoids (fig. 195).

Incomplete closure of the oronasal groove results in the deformity known as
‘hare-lip’ externally, while ‘cleft palate’ is the result of failure of palatines, and
sometimes of maxillaries to meet below the nasal passages.

Fic. 196. Fic. 197.
Fic. 196.—Section through the nasal region of Siren, after Seydel. cn, nasal cavity,
jg; Jacobson’s gland; jo, organ of Jacobson; v, vomer.
Fic. 197.—Section of nose of Chelonia cauana, after Gegenbaur. c, concha; ch, choana;
i, inner olfactory groove; ”, projection of naris between dotted lines.

In the dipnoi the olfactory membrane forms a few large folds on
the dorsal side of the respiratory duct formed from the oronasal tube.
In the amphibia the sensory surface has a similar position on the upper
medial surface (fig. 196), with frequently a lateral pocket lined with
sensory epithelium, the beginnings of an organ of Jacobson. In the
same group glands (inner and outer Jacobson’s glands) occur for

13

194 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

moistening the olfactory epithelium. Usually there is little complica-
tion of the olfactory surface, but in a few urodeles (Plethodon) there
is a projection from the lateral wall, the first indication of the conche
which acquire such development in the higher groups. There is fre-
quently a differentiation of the nasal passage into a ventral respiratory
duct lined with ordinary and a more dorsal olfactory duct lined with
sensory epithelium. In the lower urodeles the diverticulum repre-
senting the organ of Jacobson is on the medial side of the nasal cavity;
a little higher it is ventral, while in the highest urodeles it has rotated
to the lateral side. It may be noted that some of the amphibia have
smooth muscles to close the external nares.

Aside from the varying position of the choane the changes from
amphibia to reptiles in the olfactory organs are comparatively slight.

Fic. 198.—Longitudinal section of nasal region of alligator, after Gegenbaur. c, concha;
ms, maxillary sinus; 1, naris; p, pseudoconcha.

The olfactory region becomes more distinct from the respiratory tract
and the latter shows a tendency to be differentiated into an anterior.
atrium or vestibule, a middle area connected with the olfactory region,
and a posterior naso-pharyngeal duct between the basis cranii and
the roof of the mouth. This latter duct varies in length accordingly
as the choane are anterior or posterior in position, the extreme being
reached in the crocodiles, where by ingrowth of palatines and ptery-
goids, the internal nares are carried back nearly to the hinder end of
the skull. A single concha, supported by bone, is developed in the
lateral wall of the reptilian nose. It is weak in the turtles (fig. 197),
but is larger elsewhere, and in the crocodiles (fig. 198) it becomes
divided in front, while a ‘pseudoconch’ (its homology with the supe-
rior concha of birds is uncertain) is developed above and behind the
true concha. Jacobson’s organ occurs only in the squamata, where
it forms a simple pocket in the primitive position, ventral and medial
to the nasal cavity, near the nasal septum.

OLFACTORY ORGANS. 1g5

There are three folds developed on the wall of each nasal cavity in
birds, an anterior and inferior concha vestibuli, a middle and a superior
fold, the middle supported by the maxillo-turbinal, the superior by the
naso-turbinal bones. The vestibular conch lacks olfactory epithelium
at all times, while it disappears from the middle one after hatching,

Fic. 199.—Olfactory region of hen in longitudinal and transverse section, after Gegen-
baur. c, middle concha; ch, choana; 7, inferior (anterior) concha; 0, connection of air
cavity of head; p, septum of nose; s, superior concha.

leaving the upper conch as the sole seat of smell in the adult, which
corresponds with the limited sense of smell in these animals. Jacob-
son’s organ is never developed in the adult, though traces of it appear
in the embryos.

With the great increase of the sense of smell in the mammals the

FIG. 200. Fic, 201.

Fic. 200.—Model of the nasal cavity of a rabbit embryo, 134 mm. head length, after
Peter. ch, choana; ét, first ethmoturbinal; 7, organ of Jacobson; 0j, opening of same; mt,
maxilloturbinal; 2, nasoturbinal.

Fic. 201.—Nasal cavity of Erinaceus, after Paulli, showing the foldings of the maxillo-
turbinals (m#) and the nasoturbinals (nt). :

nasal labyrinth undergoes a corresponding complication, and is farther
characterized by the great length of the naso-pharyngeal duct, and by
the position of the olfactory area below a part of the brain cavity. The
folds of the labyrinth may be supported by processes, more or less com-
plicated, of three bones or cartilages, the ethmo-turbinals, the naso-

196 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

turbinals and the maxillo-turbinals (fig. 200), the purpose of these
folds being to increase the amount of sensory surface, while the skeletal
supports keep the folds from touching each other. With diminution
of the powers of smell the folds are correspondingly reduced, even to
a loss of the turbination of the bones concerned.

The maxillo-turbinals and naso-turbinals arise from the lateral wall
of the nasal cavity (the former as a distinct turbinal bone), the ethmo-
turbinals as outgrowths from the ethmoid bone,
appearing first at the upper hinder part of the
septal wall and extending to the lateral wall.
The result is that the ethmo-turbinal tends to
insinuate itself between the hinder ends of the
other two (figs. 200, 201). Each of these may be
subdivided, with corresponding subdivision of
the epithelial covering, and in the case of the
ethmo-turbinals the subdivisions may be of
varying heights (fig. 202), the ecto- and ento-

Fic. 202--Section trbinals. The nasoturbinals often disappear in
through the nasal cavity the adult, while the epithelium of the maxillo-
er pt ey turbinals is not sensory in character, this part
eo Ob Ae nese being apparently to warm and

moisten the air in its passage to the lungs.

The homologies of the various parts of the nasal labyrinth in dif-
ferent amniotes are thus stated (Peter).

I. Concha of the anterior epthelium: concha vestibuli (birds).

II. Conche of the primitive sensory epithelium:

1. Arising from the lateral wall (conche laterales).
A. Anterior: Be a
a. Primary, ventral: concha of reptiles; middle concha
of birds; maxillo-turbinals of mammals.
b. Secondary, dorsal: Upper or posterior of birds; naso-
turbinals of mammals (? pseudoconch of crocodiles).
B. Arising from the posterior part: conche obtecte of mam-
mals.
2. Arising from the primitively median wall: ethmo-turbinals
of mammals, numbered from in front backward.

Jacobson’s organ (vomero-nasal organ) is laid down in the embryo
of most mammals as a groove or pocket on the lower medial side of
each nasal cavity, opening in rodents and in man near the duct of

OLFACTORY ORGANS. 197

Stenson’s gland; in other mammals, so far as known, its duct becomes
cut off from the nasal cavity and opens into the naso-palatal canal. Its
medial wall is covered with sensory epithelium, supplied by a branch
of the olfactory nerve. In the primates the organ is more or less de-
generate in the adult.

There are two kinds of glands in the nasal cavity, the smaller and
scattered Bowman’s glands and the larger Stenson’s gland lying in
the lateral ventral wall and opening into the vestibule. There are
usually several sinuses in the bones of the skull, connected with the

Fic. 203.—Lateral wall of nasal cavity of man, after Corning. cg, crista galli; ci, cm,
cs, inferior, middle and superior conch; fpm, foramen palatinum majus; fsp, sphenopala-
tine foramen; ic, incisive canal; osm, opening of maxillary sinus; sf, frontal sinus; ss,
sphenoidal sinus.

nasal cavities by foramina. Chief of these are the maxillary sinuses
(antra of Highmore), the frontal and sphenoidal sinuses in the
corresponding bones, the relations of which may be seen in fig. 203.
Others may occur in other bones of the face.

Mammals are characterized by an external fleshy nose, supported
by the nasal bones and by cartilages, developed in part from the eth-
moid cartilage of the embryo, in part from paired cartilages, a new
acquisition of the mammals. Beyond these skeletal parts is the fleshy
portion which may form a proboscis of considerable size (swine,
elephant shrew, elephant).

198 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

‘In most mammals the sense of smell is well developed, but is comparatively
slight in the seals, whalebone whales and in the primates, while it is completely lost
in most of the toothed whales where even the olfactory nerve may disappear.

The Eyes.

The sensory part of the eyes comes from the ectoderm of the neural
plate, and in several embryos the regions which are thus destined may
be recognized on its dorsal surface
before it is infolded to form the vesi-
cles of the brain. The accessory parts
of the eye are derived in part from the
general ectoderm, in part from meso-
derm of both kinds. :

As the neural plate closes up to form
\ \ the brain (p. 11), the optic areas begin
~_, to grow outward from the fore-brain
Mg toward the sides of the head, each form-
ieee ing at first a hollow outgrowth, the optic
vesicle, connected with the brain by a

Fic. 204.—Stereogram of de-
veloping eye. cf, chorioid fissure; fb, hollow optic stalk. The next phase is
cut wall of fore-brain; /, anlage o ? . F bs e
lens; oc, optic cup; os, optic stalk; , the involution or invagination of the

layer for pigmented epithelium; 7,

retinal layer. distal side of the vesicle so that it is

converted into a double walled optic
cup (fig. 204). There thus results a differentiation of parts in the
optic outgrowth and a partial obliteration of the cavity of the vesicle.
The distal wall which forms the inside of the cup is called the retinal

ES
Ay

Bec ooHuue!

Fic. 205.—Sections of successive stages in the development of the lens of the eye from the
first thickening of the ectoderm (ec) to the complete separation ot the lens, /.

layer; the outer wall the pigment layer, in anticipation of their de-
velopment into the corresponding parts of the adult.
The involution of the retina is not easily described, but may be

EYES. 199

understood from figure 204. It occurs on the lower distal side so that’
the cup is not complete but is interrupted by a deep notch, the chorioid .
fissure, below, and this is extended as a groove on the ventral side of

the optic stalk. Later the fissure closes (fig. 194), but not until

some of the changes described below have occurred.

Opposite the distal part of each optic vesicle the ectoderm of the
side of the head thickens, then becomes invaginated (fig. 205), the
mouth of the invagination closes, and the hollow ball thus formed is
cut off from the rest of the ectoderm and sinks into the mouth of the
optic cup, where it forms the lens of the eye. From the first the cells
of the two sides of the lens differ in size, those of the outer wall being
cubical, those of the other being elongate, while the cavity is a narrow
cleft. Later the cavity is obliterated, while the lens is increased in
size by the addition of new cells, like the coats of an onion, by budding
from the equatorial zone of the lens.

Go

Fic. 206.—Mammalian retina; above the general appearance, below the diagrammatic
relations; the lens toward the left. c¢, cone; cc, cone cell; g, ganglion cells; 7g, inner granular
layer; im, inner molecular layer; m, basal membrane; n/, nerve fibres; og, outer granular
layer; om, outer molecular layer; 7, rod; rc, rod cell.

The Retina consists of several layers which constitute the ganglion
and the sensory cells, the latter being on the outer surface, 7.e., that
which is turned away from the lens. Each sensory cell bears on its
outer end the percipient structure, rod or cone, which has given these
the name of rod and cone cells. These rods and cones project
through the basal membrane which encloses the retina into the
pigment layer to be described shortly. The bodies of the cells with
their nuclei are inside the basal membrane, where they form the so-

200 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

called outer granular (nuclear) layer, separated by an outer
‘molecular’ (reticular) layer of interlacing dendrites from the
inner granular layer. This is ganglionic in character and is con-
nected by the inner molecular layer with the rest of the ganglionic layer
which lines the inside of the retinal cup.

In order to understand the latter layer and the relations of the optic
nerve, an account of the development is necessary. At first the retinal
layer is comparatively thin, but it increases in thickness, in part by a
multiplication of cells, in part by their increase in length and the devel-
opment of the dendrites of the molecular layers. Each cell of the inner
layer (the one turned toward the lens) also develops an axon which
runs over the free surface of the cells to the chorioid fissure, passes
through this and along the ventral groove of the optic stalk to the
diencephalon.

As will readily be understood, it is these fibres and not the optic
stalk which form the optic nerve (p. 169). When the chorioid fissure
closes, the nerve appears to leave through the centre of the retina,
and as this part contains no sense cells, the point of exit constitutes
the ‘blind spot’ of physiological works. Besides the cells already
mentioned the retina contains supporting or radial cells, like other
sense organs or like the brain itself (meuroglia). These extend through
from the nerve fibres to the basal membrane. Either rods or cones
may be absent in isolated groups of vertebrates. Usually there is a
spot, the macula lutea (yellow spot) or fovea centralis at the centre
of the retina where vision is most distinct. Here the rod and cone cells
are shorter and more crowded than elsewhere.

Here may be mentioned a point of morphological importance. It will be
recalled (p. 138) that the ependymal surface of the brain corresponds to the external
surface of the ectoderm of the rest of the body. Therefore, as a glance at fig. 204
will show, the rods and cones are on the primitively outer and the ganglion cells and
nerve fibres are on the deeper surface of the ectoderm. Hence rods and cones
correspond to the percipient cuticular structures of other sensory organs like the
lateral line, taste buds and the like. Before it can affect the sensory cells the light
has to traverse the whole of the retina and then the nervous impulses have to
pass back through the same layers to reach the optic nerve. This constitutes an
‘inverted eye’ and, with the exception of a few molluscs, it is unknown, except in
the vertebrates: A comparison with the parietal eye of reptiles (fig. 151) is very
instructive.

The cavity between lens and retina is filled with a semisolid vitre-
ous body, the origin of which is in dispute. In mammals blood-vessels

EYES. 201

and mesenchymatous cells enter the optic cup through the chorioid
fissure before its closure. Some suppose that the vitreous body arises
from a modification of these cells, some regard it as an exudate from the
blood-vessels, and others think it a retinal secretion. The fact that
the blood-vessels mentioned do not occur in birds is of interest in this
connection. In mammals, when the chorioid fissure closes, the vessels
appear to enter through the centre of the optic nerve (central retinal
artery and vein—fig. 207). In the early stages the retinal artery

an
(Ao 004 ay

nO 5 yo

Fic. 207.—Diagrammatic section of half a mammalian eye. a+, anterior chambcr;
ca, ciliary arteries; c, eyelash (cilium); cj, conjunctiva; co, cornea; cp, ciliary process; cr,
central retinal artery and vein; cs, conjunctival sac; ct, chorioid tunic; d, dura of optic
nerve; 7, iris; on, optic nerve; os, ora serrata; pc, posterior chamber; pe, pigmented epithel-
ium; 7, retina; sc, sclera; tg, tarsal gland; vv, vorticose vein; cc, zonula zinii.

divides inside the cup, one branch (hyaloid artery) going through the
vitreous body to the neighborhood of the lens, the other being distrib-
uted over the inner surface of the retina. Later the hyaloid artery
disappears, while retinal arteries are rare except in mammals.

The outer wall of the optic cup forms the pigmented epithelium
of the eye, developing a large amount of black pigment which eventu-
ally surrounds and isolates the rods and cones, so that each can be
affected only by the light which falls directly upon it. As will readily be
understood the side of the pigment layer away from the retina corre-
sponds to the deeper surface of the skin and so comes into relation with
the connective tissue. From this is developed the envelopes of the
eye—tunica vasculosa, sclera, etc.

Surrounding the retina and pigmented epithelium and extending
forward over the lateral parts of the lens is the tunica vasculosa,
in which two parts are recognized, the iris and the chorioid. The

202 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

whole is richly vascular, and the chorioid, supplied by the ciliary
arteries which enter at the side, is the chief source of nourishment
for the rod and cone cells. To the vascular part certain other portions
are added in some groups. Thus just outside the blood-vessels there
may be a large lymph space, and outside of this, in most fishes and
some amphibia and turtles, there is an argenteal layer containing
calcic crystals which give the layer a whitish appearance. On the
other hand, the side toward the retina frequently develops ‘a somewhat
similar tapetum lucidum, with a metallic lustre, which reflects
light strongly and is the cause of the apparent shining at night of the
eyes of many selachians and some other fishes and carnivore mammals.
In a few teleostomes (those with a pseudobranch) there is a so-called
chorioid gland just oustide the vascular layer, near the entrance of
the optic nerve. It partakes of the nature of a rete mirabile.

The chorioid extends as far forward as does the retina, when its
anterior edge is produced into a circular ciliary process, which is best
developed in the amniotes, though appearing here and there in the
ichthyopsida. This process is muscular (ciliary muscles) at its base and
is connected at its margin with the delicate capsule surrounding the
lens by a double fenestrated membrane, the zonula ciliaris (Zinnii).
By the action of the muscles the lens is moved toward or away from the
retina, while variations in tension may slightly alter its shape, thus
changing {ts focal point (accommodation of the eye).

Beyond the ciliary process the vascular tunic continues in front
of the lens as the iris, a circular curtain with a central opening, the
pupil. Pigment in the posterior layer (uvea) of the iris renders it
opaque, while in many fishes the outer surface is silvery owing to the
continuation of the argentea into this region. The rest of the iris is
muscular, the muscles increasing in extent from the lower to the
higher forms. They are arranged in two groups. The circular muscles
(sphincter pupillz), by their contraction, diminish the size of the pupil;
the radial (dilator pupillz) are antagonistic and effect an enlarge-
ment of the opening in the iris. In the sauropsida these iridial muscles
are cross banded, in amphibia and mammals of the smooth variety.

Surrounding all of the structures of the eye so far described is the
sense capsule, which differs from all other sense capsules (p. 62) in
not being connected with the rest of the skull, as a result of its neces-
sity for movement. In the capsule two parts are distinguished, the
sclera which covers the proximal side of the eye, and the cornea,

EYES. 203

perfectly transparent, through which light passes to reach the lens.
The cornea, covered externally by the conjunctiva, the modified
epidermis of the front of the eye, consists of connective tissue; the
sclera is usually white. In most of the lower vertebrates and in the mono-
tremes it is partly or wholly cartilaginous, but in other mammls and in
the lampreys it consists of fibrous tissue. In the stegocephals and in
many reptiles and birds portions of the sclera ossify as a ring of scle-
rotic bones (p. 67).

Sclerotic bones are lacking in snakes, plesiosaurs and crocodiles. In the
sturgeon and many teleosts two or more dermal bones develop upon the sclera, but
neither these nor the calcifications to be found in some sharks‘and teleosts are to
be confused with true sclerotics.

Between cornea and lens is a cavity which is partially divided by
the iris into anterior and posterior chambers which connect with
each other through the pupil. These are filled with a refracting fluid,
the aqueous humor.

The parts so far described form the eye-ball (bulbus oculi) which
is more or less freely movable in its socket in the side of the head. It
is moved by the six muscles (p. 128) which are constantly present.
Others may occur here and there. Thus in the amphibia a distinct
muscle (retractor bulbi) is developed from the external rectus to
pull the ball back into the socket, while portions of the jaw muscles
may be set apart as elevators and depressors of the ball. In the elasmo-
branchs a cartilaginous rod, the optic pedicel, extends from the ball
to the skull. This is replaced in the teleosts by a fibrous band, the
tenaculum, but its equivalent is not found in the higher groups.

Among the accessory parts of the eye are the lids, of which there
may be three, the upper and the lower lids so familiar in the higher
vertebrates and the third lid, the nictitating membrane, a transparent
sheet which may be drawn horizontally across the front of the eye
from the inner (anterior) angle of the eye or from beneath the lower lid.
All three lids are folds of the skin. The upper and lower are poorly
developed in the ichthyopsida, but appear in the amniotes. They
are lined on the side next the eye by a continuation of the conjunctiva,
which continues beyond the edge of the lid as the epidermis. The
nictitating membrane appears in some sharks, again in the amphibia,
and receives its highest development in the sauropsida, while in the
mammals it is reduced to a rudimentary fold, the plica semilunaris,
at the inner angle of the eye. :

204 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

There are no glands connected with the eyes in cyclostomes or
fishes, but in the urodeles a series of glands is developed from the con-
junctival lining of the lower lid. In the amphibia they show little
differentiation, but in all sauropsida (glands are lacking from a few
reptiles—crocodile tears are non-existent) they become divided into
two groups. One becomes aggregated near the inner angle and forms
what is known as Harder’s glands (glandula membrana nictitans);
the others migrate toward the outer angle of the eye and constitute the
true lacrimal or tear gland. In the mammals the migration continues
until the gland comes to lie beneath the upper lid, where it shows its
multiple nature by the numbers of ducts by which it pours its secretions
into the conjunctival sac. In most mammals Harder’s gland degener-
ates. The tears secreted by the glands pass over the conjunctiva and are
collected at the inner angle of the eye, where they are drained by the
lacrimal duct into the cavity of the nose. This duct is formed as a
thickening of the epidermis which later becomes perforated. It
follows the course of an earlier groove (fig. 194) leading from the orbit
to the nasal invagination and which was formerly thought to form the
duct.

The eyes of the cyclostomes are degenerate. In the larval (Ammoccetes) stage
of Petromyzon the eye is buried under a thick skin, but this thins out in the adult.
In the myxinoids the lens and eye muscles are lacking, and iris, cornea and sclera
are not differentiated. ;

Fishes have eyes with a very flattened cornea, a spherical lens and very long
retinal rods. A peculiar feature in many fishes is the falciform process, a vascular
and muscular structure which enters the retinal cup through the chorioid fissure and
extends to the lens where it bears an expansion, the campanula Halleri. The
whole is supposed to act as a means of accommodation, there being no ciliary
muscles. In most fishes the eyes are so placed on the sides of the head that there
must be monocular vision. In the flat fishes (Heterosomata) one of the eyes mi-
grates during development, so that both eyes come to lie on one side of the head.

Most sauropsida are characterized by the development of a process from the
inner retinal surface which reaches its extreme in the pecten of the birds. In the
reptiles it is a small conical process arising from the point of entrance of the optic
nerve, but in the birds this expands distally into a quadrangular plate, folded like a
fan, to which various functions have been ascribed. It has been recently shown
to be rich in sense cells. The shape of the eye of the bird is peculiar, but is not
easily described. It consists of a hemispherical posterior part, followed by a
conical portion, and this surmounted by a hemispherical corneal region, the whole
being somewhat telescopic in shape. The whole is very large in proportion to the
size of the animal.

The pecten is said to be outlined in the foetal stages of some mammals. The

DIGESTIVE ORGANS. 205

pupil of the mammals is not always circular, but is a vertical slit in the cats, a
horizontal opening in the whales, many ungulates, etc. During development the
lids fuse for a time, separating in some cases, only after birth. The edges of the
lids are fringed with short hairs, the eye-lashes or cilia, and internal to these are
the ducts of sebaceous glands (tarsal or Meiobomian glands), the glands them-
selves being in the substance of the lids.) The whales have an enormously thick
sclera which, here as elsewhere, appears as a continuation of the dural sheath of
the optic nerve.

THE DIGESTIVE ORGANS.

Few articles of food, as they come to a vertebrate, are in shape to be
taken immediately into the organism and to be used, without modifica-
tion, as a source of energy or as material for the construction of new
tissue or the repair of old. They have to be altered so that they are
soluble and so able to pass by osmosis into the blood-vessels (proteids,
carbohydrates), or they must be broken up (hydrocarbons) so as to be
taken up by the absorbtive vessels (lacteals) of the lymphatic system.
These changes in the food, which are the result of the action of the
secretions of the digestive glands, constitute the process of digestion.
The digestive tract or alimentary canal, where these changes take
place, also has to provide for the passage of the digested food into the
blood-vessels, to be carried by them to all parts of the body. It is there-
fore richly supplied with blood- and lymph-vessels.

The alimentary canal , which is complete (z.e., has both mouth and
vent), is largely entodermal in origin, but small portions at either end
are derived from the ectoderm. ‘The entodermal portion, the mesen-
teron, consists of the wall of the archenteron (p. 12) after the separa-
tion of the notochord, the mesothelium, and a few less prominent struc-
tures. The ectodermal parts are a stomodeum at the cephalic end and
a proctodeum behind.

In the early stages of all vertebrates the mouth is lacking, the ce-
phalic end of the archenteron abutting directly against the ectoderm of
the ventral side of the head, so that an oral (pharyngeal) plate is
formed, consisting of both ectoderm and entoderm. Next this plate is
pushed inward, either as a pocket (fig. 190) or as a solid plug, carrying
the entoderm before it. This ingrowth constitutes the stomodeum,
and the site of its ingrowth forms the mouth opening of the adult.
Later the oral plate breaks through, placing the stomodeum and
mesenteron in communication.

206

COMPARATIVE MORPHOLOGY OF VERTEBRATES.

In the majority of vertebrates the blastopore closes behind, so that

Fic. 208.—Reconstruc-
tion of‘ alimentary canal of
human embryo, after His.
al, allantois stalk; cl, cloaca;
g, glottis; hk, hyoid arch; k,
liver; /u, lung; md, mx, man-
dibular and maxillary arches;
n, nasal pit; 0, omphalomesa-
raic vein; s, stomach; v, vis-
ceral arches; vi, vitelline
stalk; w, Wolffian body.

the anus is a new formation, although it arises
in the line of closure. In the amniotes this
opening is preceded by the formation of a
pocket, the proctodeum, similar to the
stomodeum, and opening later into the
mesenteron in the same way. In the adult it
is impossible to find any lines separating the
three regions, stomodeum, mesenteron and
proctodeum.

The proctodeum lies wholly behind the entrance
of the urogenital ducts into the cloaca. The ecto-
derm of the stomodeum extends inward as far as the
posterior teeth, following the outline of the jaws. On
the dorsal side of the oral cavity two pits persist for
some time, the limits of ectoderm and entoderm pass-
ing between them. ‘The posterior of these, Seessel’s
pocket, is of unknown significance. The other,
Rathke’s pocket (fig. 253), lies just in front of the
oral plate. It marks the point of invagination of the
hypophysis (p. 148) and remains open for a time as
the hypophysial duct (fig. 148).

In some teleosts, where the stomodeal ingrowth is
slight, the mouth appears at first as a pair of per-
forations in the oral plate, these later coalescing to
form the permanent mouth. This condition lends
plausibility to the view that the vertebrate’ mouth has
arisen from the coalescence of a pair of gill clefts.

Except in the higher mammals the ento-
dermal part of the alimentary canal contains a
large amount of food yolk in the early stages.
In the sauropsida this is so abundant that the
whole cannot be contained in the body walls,
and hence it causes ‘the ventral side of the
canal to protrude as a yolk-sac, which is
gradually absorbed with the digestion and re-
moval of the yolk by the blood-vessels.

The first differentiation in the mesenteron
is the development of a ventral diverticulum,
the anlage of the liver, which arises just caudal

to the head. This divides the alimentary canal into pre- and post-

DIGESTIVE ORGANS. 207

hepatic portions (fig. 209). From the anterior of these is formed
part of the cavity of the mouth with the salivary glands, the pharynx,
cesophagus, stomach, and duodenum; the post-hepatic portion gives
rise to large and small intestines, rectum and cloaca, as well as to the
urinary bladder. Of these parts the pharynx will be considered in
connection with the respiratory organs, the bladder with the urogenital
system. Mouth and pharynx belong primitively to the head, but by
unequal growth the pharynx may be carried apparently to some
distance behind the brain and other characteristically cephalic
structures.

Fic. 209.—Diagrams of the alimentary canal in embryos of 6 and 8 days of Gymnarchus
niloticus, after Assheton. ab, air bladder; b, early diverticulum for air bladder; gb, gall
bladder; /, liver; pa, pancreas; b, posterior part of air bladder; pc, pyloric ceca; ph,
pharynx; s, stomach.

In the following account stress is laid upon the epithelial lining
(entoderm), the characteristic tissue of the digestive tract, but it must
not be forgotten that the wall contains other tissues of mesenchymatous
origin. That part of the canal which runs through the body cavity
has the following layers. The lining epithelium is supported by a
layer of connective tissue, containing the capillary absorbtive vessels;
outside of this are two layers of smooth (involuntary) muscles, the inner
with the fibres running in a circular, the other in a longitudinal direc-
tion. By the action of these antagonistic muscles the peristalsis or
movement of the digestive tract is effected, by which the food undergoing
digestion is churned and thoroughly mixed with the digestive fluids,
and all parts of it are brought into contact with the absorbtive surfaces.
The outer surface of stomach, intestine and associated glands is covered
with the serous coat, the lining of the peritoneal cavity, but this is
lacking from those parts (pharynx, cesophagus, etc.) which are outside
the region of the ccelom.

208 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

THE ORAL CAVITY.

The cavity of the mouth is limited anteriorly by the line of the stom-
odeal involution and extends back to the pharynx. It is lined in part
by ectoderm, in part by entoderm, the line between the two, as stated
above, not being recognizable in the adult. In the amphibia the lining
is ciliated, the cilia extending back to the stomach. In the cyclostomes
the oral cavity is funnel-shaped, with a circular or quadrangular open-
ing supported by a cartilaginous ring and has the name of oral hood.
It is permanently open, there being no jaws capable of closure (see
skeleton, p. 73) thus furnishing a marked contrast to all other verte-
brates in which there are jaws and which are consequently known
as gnathostomes. (Development gives little support to the view that
the cyclostome tongue is the homologue of the lower jaw of the
gnathostomes.)

In development the mouth arises on the ventral side of the head,
some distance from the anterior end of the body. This position
is retained throughout life in most elasmobranchs and in the sturgeons;
but elsewhere, by the development of the bony upper jaw in front of
the pterygoquadrate (p. 69) and the concomitant extension of Meckel’s
cartilage, the mouth opening is gradually transferred to the anterior
end and becomes terminal.

In most lower gnathostomes (the holocephali and other isolated
forms are exceptions) the mouth opening is bounded by folds of epithe-
lium which meet when the mouth is closed. Usually these folds are
soft and are supported below by connective tissue, but in birds, turtles
and monotremes they are cornified. It is only in the mammals that
true lips occur. These are fleshy folds around the mouth and their
development in this group is correlated with the presence of the dermal
facial muscles (p. 134), by which they are moved. With the develop-
ment of lips there is formed a space between lips and teeth, the vesti-
bule of the mouth, which sometimes (e. g., some rodents) forms cheek
pouches, lined with hair, of considerable size.

Teeth.

The primitive function of the teeth was apparently to hold the prey
taken into the mouth and this is their sole use in many forms. In
other species they have become efficient organs for the comminution of
food, either by cutting or by crushing it.

DIGESTIVE ORGANS. 209

There are two types of teeth, much alike in function, but differing
markedly in structure and development and without genetic relation-
ships. The typical vertebrate teeth are comparable to placoid scales;
they arise as a calcareous secretion at the junction of ectoderm and
mesenchyme and are a product of both layers. The other type contains
purely cuticular teeth, formed by a cornification of the epithelium and
have their analogues in many invertebrates.

True Teeth.—The ability to form scales is characteristic of the
skin of many vertebrates. The primitive type of these scales is the
placoid (p. 40), consisting of a basal portion of dentine capped with
enamel and the apex projecting through the integument as a spine.
When invaginated to form the stomodeum the skin retains this capacity
of forming hard structures and hence any portion of the stomodeal
walls may secrete scale-like plates. In fact, in the teeth of some
elasmobranchs (Raia, Mustelus, Trygon, etc.) the placoid scale canbe
recognized with scarcely a modification. In the ichthyopsida teeth
may form anywhere in the oral cavity where there are skeletal parts
—cartilage or bone—to support them: “ Thus they may occur, not only
on the margins of the jaws, but on vomers, palatines and parasphenoid,
and in some teleosts:on. the tongue, where they are attached to the
hyoid. In the amniotes (some squamata excepted) teeth occur only
on the margins of the jaws. Teeth are lacking, here and there, in
various families of vertebrates as well as from all turtles and living
birds, but some extinct birds had teeth. In the embryos of both |
chelonians and aves the dental ridge is formed (vide infra), but it soon
completely disappears.

In the development of a tooth, as of a placoid scale, there is
first a thickening of the ectoderm, the basal layer of which pushes into
the cutis, and at the same time the mesenchyme cells of the latter layer
multiply beneath the centre of the ectodermal ingrowth, pushing it
outward, so that the basal layer forms a cup with the opening toward
the deeper tissues (fig. 210). The mesenchyme within the cup forms
enamelorgan. With farther development the outer cells of the papilla
are converted into odontoblasts, so-called from their function of form-
ing a bone-like substance, the dentine or ivory of the tooth. This,
in accordance with the method of its formation by secretion from the ends
of the odontoblasts, has a prismatic structure. The basal surface of
the enamel organ secretes a denser substance, the enamel, which lies like

14

210 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

a cap, firmly united to the top and sides of the dentine. By continued
additions to the deeper portions of the dentine the tooth is gradually
forced up through the epithelium so that its tip or crown comes into
position for use (eruption of the tooth).

Fic. 210.—Section of developing tooth germ of Amblystoma. e, epidermis; eo, enamel
organ; m, Malpighian layer; me, mesenchyme; #, pulp of tooth.

In the lower vertebrates there may be a separate invagination of the ectoderm
for each tooth, but in the mammals there is a continuous ingrowth, the dental ridge
(fig. 211) along the margin of the jaw. Later this becomes differentiated into
separate enamel organs and dental papille, the separate teeth developing much as

f hon
aaa Grae ape ~

Fic. 211.—Model of ectodermal parts of jaw of human embryo 40 mm. long, after Rése,
showing the dental ridge with the enamel organs for the first teeth.

in other groups. From the posterior side of this dental ridge there arises a continu-
ous projection, the dental shelf (fig. 212) which later gives rise to the enamel
organs for the second or permanent dentition (infra).

The dental papilla persists throughout life as the pulp of the tooth,
continuing to occupy the space (pulp cavity) in which it first appeared.

DIGESTIVE ORGANS. 211

Nerves (branches of the trigeminal) and blood-vessels enter the cavity
through the base of the tooth. Usually, when the tooth is fully formed,
the odontoblasts cease to act, but exceptionally, even in mammals
(tusks of elephants, incisors of rodents) they function through life and
the tooth continues to grow. In the mammals an additional layer of
modified bone, the cement, is formed around the root of the tooth
and may extend on to the crown.

Just as the scales are arranged in quincunx on the surface of the
body, so are the teeth in the mouths of skates and some other elasmo-

Fic. 212. Fic. 213.

Fic. 212.—Diagram of germs of milk and permanent dentitions in a mammal, based on
Rose. b, basal layer of e, ectoderm; dr, dental ridge; ds, dental shelf; eo, enamel organ of
milk tooth; m, mesenchyme; #, pulp of milk tooth; pg, germ of permanent tooth.

Fic. 213.—Diagrammatic section of incisor tooth. c, cement; d, dentine; e, enamel;
?, pulp cavity.

branchs, where they form a tessellated pavement above and below, the
teeth being flattened and used for crushing the molluscs on which these
animals feed. More commonly the teeth are flattened in the antero-
posterior direction and have sharp cutting edges. In such cases, as
a rule, only the anterior row of teeth is functional, the others lying folded
down behind, ready to come into use when one of the first row is lost.

Most vertebrates have a succession of teeth (polyphyodont dentition)
and the elasmobranchs show how this has come about. The second
arises on the (morphologically) posterior side of the first and so on.
In the non-mammalian classes the number of such dentitions is in-
definite (polyphyodont), but in the great majority of mammals there
are two, the first or milk dentition and the second or permanent
dentition (diphyodont condition).

212 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

In a few mammals only one dentition has been retained (monophyodont) ; among
these may be mentioned the monotremes, sirenians and cetacea. In the marsupial
Myrrecobius, where the permanent dentition is greatly reduced, and in some of the
insectivores and rodents, a prelacteal dentition has been observed in the embryo,
while Rése has described traces of a prelacteal and a post-permanent dentition in
man. Ina number of mammals (guinea pigs, many bats, etc.) the milk dentition is
lost before birth.

Fic. 214.—Jaws of a six month lion, after Weber. Milk teeth white, permanent dotted.
4, incisors; ¢, canines; m, molars; p, premolars.

Only a few fishes (adult Acipenser, Coregonus, etc.) lack teeth, while
in most they extend to the lining bones of the mouth and in some to
the hyoid and branchial arches (pharyngeal bones). Usually they are
conical, but they may be flattened and pavement-like or even form
large plates, apparently by the coalescence of numbers of primitive
teeth (dipnoi). In the amphibians the teeth are not so widely distrib-
uted in the mouth, occurring on the margins of the jaws and on the
palatines and vomers, rarely on the parasphenoid, while they are
entirely lacking in Bufo and Pipa.

* Among the reptiles the turtles and some of the pterodactyls are
toothless; most of the others have the teeth confined to the margin of
the jaws, though they occur on the palatines and pterygoids in the
snakes and lizards, and rarely (Sphenodon) on the vomer. While the
conical shape prevails, the teeth present a great variety of forms, some of
the theriomorphs closely simulating the mammals in their heterodont
dentition. The teeth may be anchylosed to the summit of the jaws

DIGESTIVE ORGANS. 213

(acrodont); applied to their inner side (pleurodont, fig. 97, d); or
have their roots implanted in grooves or sockets or alveoli (thecodont).
Mention must also be made of the poison fangs of certain serpents.
These are specialized teeth borne on the maxillary bones and are either
permanently erect (proteroglypha) or the bone may turn, as on a pivot,
so that when the mouth is closed the teeth lie along the roof of the
mouth, but when it is opened, they are brought into position for striking
the prey (vipers, rattlesnakes—solenoglypha). Correlated with the
fixed or movable condition is a modification in the teeth themselves. In
the proteroglypha a groove runs along the anterior side of the fang by

Fic. 215.—Poison gland and fang of rattlesnake, Crotalus horridus. (Princeton 1404)
p, poison gland; J, labial glands.

which the poison is conducted from the poison gland into the wound.
In the solenoglypha the groove is rolled into a tube with openings near
the base and apex of the tooth (fig. 215). In these solenoglyphous
snakes only a pair of fangs are functional at a time, but there are
reserve teeth which can come into use on the loss of the first.

The greatest variation is found in the teeth of mammals, the heter-
odont dentition being the rule. Four kinds of teeth are recognized.
These are the incisors in the premaxillary bones, followed by a single
canine at the anterior end of each maxillary bone. This resembles
the incisors and differs from the other maxillary teeth in its conical shape
and single root. Behind the canines come the premolars (the bicus-
pids of the dentists) which have two roots and complicated crowns and
appear in both milk and permanent dentitions.. Lastly are the molars,
like the premolars in form, with several roots, but appearing only in the
permanent dentition. The corresponding teeth in the lower jaw have
the same names.

In a few mammals, like the whales, all of the teeth are of a simple
conical shape, but in the majority the crown of the molars is marked
by projections—cones, tubercles, crests, etc.—which are variously

214 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

arranged. When the teeth are adapted for cutting they are called
secodont (cats, fig. 214); for crushing, bunodont (man); when mark-
ed by transverse ridges, lophodont (elephants) ; when there are longitu-
dinal crests, more or less crescentic in outline, they are selenodont
(horse, fig. 216).

In the triconodont tooth there are three prominences in the crown
arranged in a straight line, parallel to the axis of the jaw. The middle
and more prominent of these in the upper jaw is the protocone, with
a smaller paracone in front and a metacone behind. In the lower
jaw the corresponding terms are proto-, para-, and metaconid. In

Fic. 216.—A, triconodont tooth of Dromatherium; B, tritubercular tooth of Spalaco-
therium; C, interlocking of upper (dark) and lower (light) tritubercular molar teeth (after
Osborn); D, molar of Erinaceus; E, of horse (selenodont type); c, cingulum; m, metacone
(metaconid); p, paracone (paraconid); pr, protocone (protoconid); ¢, talon.

a tritubercular tooth the three cones are arranged in a triangle, in
such a way that they alternate in the two jaws, the protocone being on
the inner side, the protoconid on the outer. ‘Tritubercular teeth may
have a lower projection (talon) on the hinder side. When this devel-
ops into a prominent tubercle (hypocone, hypoconid) the tooth
becomes quadritubercular. Then crests or lophs may develop,
connecting the cones, so that the crown becomes ridged rather than
tubercular.

In the homodont dentition the number of teeth may be very large, varying
from 100 to 200. With the heterodont dentition the number is smaller, the full
dentition in the placental mammals including 44 teeth. From this number reduc-
tions may occur by the loss of teeth of any kind. The number of teeth and of
those of each kind is important in systematic work, and a dental formula has been
devised to express this. As the number of teeth in the two sides of each jaw is the
same, only one side is represented in the formula, while the teeth of the upper and
lower jaws are represented as fractions. The number of incisors, canines, pre-
molars and molars of man are represented by

oo FT 2.3 : I
1 c=,pm-. ue that of the opossum by is, c-,pm§, m4.

4

DIGESTIVE ORGANS. 215

Not infrequently the enamel is lacking from the teeth of mammals, as in whales,
dugongs and edentates, or it may be restricted to one side of a tooth,’ as in the
incisors of rodents. Sexual differentiations occasionally occur in mammals, certain
teeth (usually canines or incisors, more rarely premolars) being better developed
in the males than in the females of the same species.

There are two views as to the way in which the complicated molars of the mam-
mals have arisen. Both start with the conical tooth as the primitive condition.
One theory is that the fusion of such simple teeth is sufficient to account for the
multiplication of roots and tubercles in all of their varying forms (figs. 217, 218).

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Fic. 217.—Diagram of the relation of the human teeth to the primitive dentition, after
Rose.

The other hypothesis is that parts have been developed on the primitive cone,
giving, first, the triconodont shape. Next these three cones have been shifted to
the tritubercular position; and later other parts—hypocone, lophs, etc.—have
been added and these have been modified in different directions. Each view has
much in its favor. Embryology is not at all decisive, while paleontology favors the
latter view.

Epidermal Teeth occur in cyclostomes and in larval amphibia and
in embryonic monotremes. In the cyclostomes they are cones of
cornified epithelium covering an underlying core of the integument; they
are differently arranged in the lampreys and myxinoids. In the latter
they are few, there being a single tooth on the ‘palate’ and two chev-
ron-shaped rows on the tongue. In the lampreys nearly the whole
inner surface of the oral hood is lined with these teeth of varying shape,

216 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

and there are a varying number upon the tongue. These teeth are
used as a means of fastening the animals to their prey, and those of the
myxinoid tongue are used for boring into the fishes on which these
animals feed.

In the larval anura (the larval Siren is said to resemble them) the
edges of the jaws are armed with cornified papilla, serving as teeth, the
arrangement of which varies in different genera. They are frequently
aggregated in dental plates, used in scraping the alge from submerged
objects. They are not related to the teeth of cyclostomes.

In the embryo monotreme teeth are formed as in
other mammals, of a multituberculate type, with a
normal enamel organ (fig. 219), but these are lost
before birth. During their eruption the adjacent epi-
dermis becomes cornified, gradually extends beneath

op) tr ch
“py ee

Fic. 218. Fic. 2109.

Fic. 218.—Teeth of Chlamydoselache (after-Rése), showing a triconodont tooth arising
from the fusion of three simple teeth.

Fic. 219.—Diagram of development of teeth in Ornithorhynchus, after Thomas and
Poulton. a, tooth covered with enamel organ, beneath oral epithelium; b, just before
eruption; c, tooth erupted; d, edges of epithelium cornified; e, horny plate formed, contains
the tooth; /, tooth lost, plate separated from its surroundings.

each tooth and after the loss of the true tooth this forms a horny
plate, used, like those of many birds, in holding and crushing the
food.

In this connection mention may be made of the baleen or ‘ whale-
bone’ of the balenid whales. This takes the form of large plates of
horny material, attached in series to the margins of the upper jaw, so
that with their fringed ends and edges they serve as strainers to extract
the plankton (minute floating life) from the sea. This baleen is formed
by the agglutination of enormously developed cornified papille.

Egg Teeth.—In the embryos of certain lizards and snakes one of the median
teeth of the first dentition of the premaxillary region projects from the mouth and
is used for the rupture of the egg shell, thus allowing the escape of the young.
In the turtles, Sphenodon, crocodiles, birds, and monotremes an egg tooth is formed
on the upper surface of the beak which is used for the same purpose. However, it
differs greatly as it is but a thickening, often calcified, of the epidermis (Fig. 195).

DIGESTIVE ORGANS. 217
The Tongue.

The tongue as it occurs in its more primitive condition in the fishes
is merely a fleshy fold developed from the floor of the mouth between the
hyoid and mandibular arches, the hyoid frequently extending into and
supporting it. It is incapable of motion, except as moved by the sup-
porting skeleton, for it lacks intrinsic muscles. It is sensory, having
both tactile and gustatory functions. It is often papillose, and in a
few teleosts it bears teeth (p. 209).

The tongue in the cyclostomes is considerably different. Here it
is thick and fleshy and is supported by a cartilaginous skeleton (p. 75)
and is moved by appropriate protractor and retractor muscles at the
base, developed from the postotic myotomes and innervated by the
hypoglossal nerve. With its terminal armament of epidermal teeth
it serves as the boring organ with which the myxinoids obtain entrance
into their prey, while in the lampreys it serves as a rasping organ and
also as part of the sucking apparatus.

In the amphibia there is a greater range of structure. In a few
anura (aglossa) the tongue is practically absent; in the perennibranchs
it is scarcely more advanced than in the fishes, but elsewhere it contains
intrinsic muscles.and is extremely mobile. It consists of a small basal
portion corresponding to the tongue of the fish, to which is added a
large glandular part arising between the copula and the lower jaw.
This secretes the slime, so useful in capturing the prey. In the anura
the tongue is attached at the margin of the jaw, its free end, when at
rest, being folded back on the floor of the mouth. In urodeles the base
of attachment is more extensive and embraces the anterior margin of
the tongue and part of the ventral surface as well. The supporting
skeleton (fig. 85) consists of the median portion (copula) with usually
two pairs of cornua, largely formed from the ventral ends of the hyoid
and first branchial arches (see p. 64).

The reptilian tongue includes not only the parts found in the am-
phibia (the fold above the basihyal), but also a median growth, the
tuberculum impar, arising between the basihyal and the lower jaw,
and also a pair of lateral folds lying above the first visceral arch
(Lacerta). In the turtles and crocodiles the tongue lies on the floor
of the mouth and isnot protrusible. In the squamata it can be extended
from the mouth, and in snakes and many lizards there is a sheath into
which it is withdrawn. In many snakes the tongue is two-pointed at the

218 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

tip; in the lizards its shape varies greatly, the differences being used in
classifying these animals. In the reptiles (fig. 220) with retractile
tongue the hyoid apparatus extends into
the tongue, its. unpaired anterior portion
being called the os entoglossum (copula
or basihyal), while the two cornua (usually
hyoid and first branchial) afford attachment
for the retractor muscles. In addition to
the usual lingual nerve (glossopharyngeal)
the tongue also receives a lingual twig from
the mandibular branch of the fifth nerve.
In birds the tongue has lost the lateral
parts of the reptilian tongue and with this
the trigeminal branch. It contains no in-
trinsicmuscles. Initsformitvaries greatly,
but usually it is slender and is covered with
retrorse papilla. Its skeleton is also re-
Fic, 220.—Hyoid peer of duced (fig. ror) and consists of an os en-
Heloderma, after Cope. _}, first toglossum, bearing in front a pair of ele-
branchial; c, copula; h, hyoid. :
ments (paraglosse@) and on the sides a
pair of cornua (first branchials) and in the median line behind, a
urohyal portion. This skeleton has a marked development in the
woodpeckers, where the cornua curve around the base of the skull.

Zee :
Fic. 221.—Two stages in developing tongue and pharyngeal floor of man, after His.
¢, copula (basihyal element); cs, cervical sinus; ep, epiglottis; g, glottis; h, hyoid arch; m,
mandibular arch; mth, median anlage of thyreoid; ¢, tuberculum impar; tg, tongue.

and over its dorsal side to the neighborhood of the nostril, a condi-
tion correlated with the use of the tongue in these animals.

DIGESTIVE ORGANS. 219

In the whales the tongue has little power of motion, but elsewhere
in the mammals it is very mobile, reaching the extreme in the ant-eaters.
This mobility is largely due to the extensive intrinsic musculature.
The tongue is developed from the tuberculum impar, which furnishes
the larger anterior part (fig. 221), the rest arising from the fleshy ridges
abovethehyoid arch. In the adult the line between these parts is largely
obliterated, but it lies near the line of circumvallate papille (p. 189)
and the foramen cecum, a blind tube connected with the development
of the thyreoid gland. Arising in this
way from the tubercle and the lateral supra-
hyoid parts, the tongue of the amphibia is

Fic. 222. Fic. 223.

Fic. 222.—Ventral and side views of tongue of Stenops gracilis, after Weber. 1,
lateral margin of sublingua; m, plica mediana.

Fic. 223.—Section through lyssa of late dog embryo, after Nussbaum. ¢, cartilage of
lyssa, cl, capsule of lyssa; m, muscles of tongue; ml, longitudinal and transverse muscles
of lyssa; s, septum of tongue.

unrepresented in that of most mammals, unless it be in the sublingua,
a fleshy fold developed beneath the functional tongue in the marsupials
and lemurs (fig. 222). Traces of this are to be found in other mam-
mals, even in man, as folds (plice fimbriatz) beneath the tongue.
In some cases (Stenops) this sublingua is supported by a cartilage
which is regarded as an entoglossal part. Others think that the
tongue of the lower vertebrates is represented in the mammalian
tongue and regard the lyssa as the os entoglossum. The lyssa is
a vermiform structure of cartilage, muscle and connective tissue (fig.
223) lying ventral to the septum of the tongue.

The tongue varies considerably in shape in the different mammalian
‘orders, but the differences are of little morphological importance.

220 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

The dorsal surface is usually covered with a soft epithelium, developed
into papille of varying shapes, some being sensory in character, and
some are occasionally (monotremes, felids) cornified.

The skeleton of the mammalian tongue (hyoid apparatus) varies
considerably. In its most complete development it consists of a body
(copula of the hyoid and first branchial) in the median line, which bears
two pairs of cornua. The anterior pair (lesser horns of human
anatomy) are usually elongate, and consist of a series of ossicles (p. 101)
connecting the body with the otic region of .the skull. . The second
pair (greater cornua of man) are occasionally absent. In man the
greater part of the anterior cornua is represented by. the stylohyoid
ligament, the proximal portion being fused to the skull as the styloid
process.

Oral Glands.

In the cyclostomes there is a large, so-called ‘salivary gland’ of
unknown function, opening into the mouth on either side below the
tongue. With this exception, glands are lacking from the mouths of
aquatic ichthyopsida. With the assumption of pulmonate respiration
and more terrestrial habits, the mouth is no longer constantly bathed
with water and so glands appear,
increasing in number and com-
plexity in the higher forms. The
secretion of these glands aids in
moistening the food, and not in-
frequently it is adhesive and is
used in capturing the prey. In the
mammals true salivary glands ap-
pear. The saliva secreted by them

: contains not only mucus, but also a
ee ce et ee ae digestive ferment (ptyalin) which
d, tooth; h, hyoid cartilage; /, labial glands; changes starch into sugar. The
m, muscles; si, sublingual gland; ¢, tongue. :

names of the various oral glands
(labial, buccal, lingual, retrolingual, etc.) are roughly indicative of
their position.

In the terrestrial amphibia, snakes (fig. 215) and lizards there are
labial glands, opening at the bases of the teeth, and an intermaxil-
lary or internasal gland in the septum between the nasal cavities, as
well as palatal glands near the choanz (the internasal gland is lacking

DIGESTIVE ORGANS. 221

in the cecilians). Many reptiles also have a sublingual gland on either
side (fig. 224). In many snakes a pair of the labial glands are greatly
developed and have migrated into the zygomatic ligament, where they
have become modified into the well-known poison glands (fig. 215),
the ducts of which connect with the poison fangs
(p. 213). In the only known poisonous lizards
(Heloderma) the sublingual glands furnish the
poison. Oral glands are poorly developed in
the sea turtles and the crocodilians.

Birds lack the labial and internasal glands,
but they have numerous other glands opening
separately into the roof of the mouth (fig. 225)
as well as anterior and posterior sublinguals and
frequently an ‘angle gland’ at the angle of the
mouth, which may be the last remnant of the
labial glands of the other Sauropsida.

Besides numerous smaller glands (labials,
buccals, linguals, palatines) imbedded in the
mucous membrane and opening separately into

the mammalian mouth, the salivary glands,
though absent from the cetacea, form a distin-
guishing feature of the group. These salivary
glands are usually in the neighborhood of the
mouth, but one or more of them may be carried
back into the neck (fig. 226), but in all cases the
homologies are decided by the openings of the

Fic. 225.—Palatal sur-
face of hen, after Heid-
rich. ch, anterior end of
choana; gs, openings of
sphenopterygoid glands;
in; infundibular opening;

lp, mp, openings of lat-

eral and medial palatine
glands; m, opening of
gl. maxillaris monosto-

matica.

ducts. The salivary glands include the sub-
maxillary and sublingual of the lower groups, and in addition the
parotid gland, apparently a development within the class. The sub-
maxillary normally lies in the lower jaw beneath the mylohyoid
muscle, and its duct (Wharton’s duct) opens near the lower incisor
teeth. Near this is frequently a retrolingual gland, its duct open-
ing near the former. The sublingual gland occurs between the tongue
and the alveolar margin of the lower jaw and usually empties by
numerous duct. The parotid gland has its normal position near the
ear and its ducts (Stenon’s duct) pours the secretion out near the
molars of the upper jaw. Other oral glands are occasionally present,
like the molar glands of ungulates and the orbital glands of dogs,
both of which have ducts leading into the mouth.

222 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

PHARYNX.

The pharynx is the division of the alimentary canal intervening
between the cavity of the mouth and the cesophagus and is characterized
by being at once alimentary and respiratory. From its walls are devel-
oped the gill clefts and lungs as well as a number of derivatives of
these, and it also receives the internal openings of the nasal passages.
Hence it is best described in connection with the respiratory system.

Fic. 226.—Salivary glands of fruit bat, Pteropus conspicillatus (Princeton, 2065).
, pd, parotid gland and duct; 7/, rid, retrolingual gland and duct; sm, smd, submaxillary
gland and duct.

THE CSOPHAGUS.

That part of the digestive tract between the pharynx and the
entrance of the bile duct (fig. 209) develops into cesophagus, stomach
and that part of the intestine known as the duodenum. Stomach and
duodenum are separated by the pyloric valve described below, but it
is difficult to draw a clear line between cesophagus and stomach. In
general it may be said that the cesophagus is the tract immediately
succeeding the pharynx, lying in front of the body cavity and thus
lacking a serous coat; that it is smaller than the stomach, and that
there are no digestive glands in its walls; but all of these statements
have exceptions.

DIGESTIVE ORGANS. 223

The cesophagus varies in length with the length of the neck of the
animal, being short in the ichthyopsida, longer in the reptiles, and
reaching its extreme in the birds. In some its internal lining epithelium
is smooth, but more commonly it bears longitudinal folds, while in the
chelonians it is provided with cornified papille pointing backward.
Outside of the epithelium its walls contain muscles, those at the
cephalic end being striped and these may extend back, in some in-
stances, even on to the stomach. They are apparently derivatives
of the pharyngeal region. Usually the cesophagus is of the same di-
ameter throughout, but frequently in birds it has a marked dilatation,
the ingluvies or crop. This may be an expansion of one side of the
tube, or, as in pigeons, it may consist of a median and a pair of lateral
chambers. The extreme of development of the crop occurs in. Opis-
thocomus, where the organ is extremely muscular and has numerous
longitudinal folds.

The crop, which is usually supported by the furcula, may be either
a reservoir for food, or it may be a glandular organ, its secretions
serving to moisten the food or even to initiate its digestion. In the
pigeons at the breeding season the secretion is a milky fluid and is
used in feeding the young.

THE STOMACH.

The stomach is apparently a new acquisition in the vertebrates,
possibly arising as a place for the storage of food. This view is sup-
ported by several facts. In the embryo vertebrate and in the adult of
Amphioxus the duct from the liver immediately follows the pharynx,
opening just behind the last gill cleft; while the innervation from the
tenth nerve shows that both stomach and cesophagus are parts of the
pharynx greatly drawn out (fig. 209).

The pylorus, which limits the stomach behind, is a fold of the
lining mucous membrane projecting into the interior and reinforced
by a circular (sphincter) muscle, which by its contraction, closes the
tube so that no food can pass from the stomach until it is properly
acted upon by the gastric fluids. The anterior end of the stomach is
not so well marked. Usually it is differentiated from the cesophagus
by its greater diameter, but in some of the fishes (fig. 227, a) there
is no distinction in size. The stomach lies in the coelom and hence is
covered externally by the serous membrane (peritoneum), but the

224 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

cesophagus usually extends a short distance into the body cavity and
then its lower end has the same coat.

The true stomach is characterized by the presence of glands, de-
veloped from the mucous layer and emptying into the lumen. Of
these glands there are at most (mammals) three kinds: cardiac, near
the entrance of the cesophagus, which secrete an albuminoid fluid;

Pm

is)

Fic. 227.—Different shapes of stomachs, mostly after Nuhn (Keibel). a, Belone; 6,
Proteus; c, Tropidonotus natrix; d, Gobius; e, shark; f, Phoca vitulina; g, Polypterus; h,
Fulica atra; i, Testudo greca; k, land tortoise; 1, rabbit; m, pig; 2, owl; 0, crocodile; p,
Delphinus; q, Halmaturus.
pyloric, near the pylorus, which form mucus; and the most character-
istic, the fundus glands, which secrete a digestive ferment, pepsin.
(For the structure of these glands reference should be made to histological
text-books.) Tested by glands, many vertebrates (dipnoi, cyprinoids)
lack a true stomach, while the sturgeons have the gastric glands extend-
ing into the cesophagus. On the other hand, a part of the enlargement
called the stomach in mammals often includes a part of the cesophagus
(fig. 228, A, Z).

DIGESTIVE ORGANS. 225

The shape of the stomach is to some extent dependent upon the
shape of the body. In the elongate species it lies in the axis of the
trunk, especially in the lower vertebrates (fig. 227, a), but with increase
in the body width it becomes more transverse. This involves a bending
and a torsion of the tube, always to the right, and results in two faces
or ‘curvatures,’ a lesser or anterior, and a greater or posterior, the
greater curvature often expanding into a so-called fundus region.
The end of the stomach which connects with the cesophagus is nearest
the heart and hence is called the cardiac end.

In the fishes the stomach may be either straight or saccular, often assuming the
form of a blind sac (fig. 227, g). The line between cesophagus and stomach is not
well marked, as the cesophageal folds may continue into the stomach. The teleosts
exhibit the greatest variety in shape, in correlation to the differences in food. All
gastric glands are lacking in the cyprinoids, while Amia has both cardiac and pyloric
glands, and, like many teleosts, the stomach is ciliated. In the amphibians and
reptiles the distinctions between cesophagus and stomach are more marked, most
in the crocodiles. In the amphibians the ciliation of the mouth is continued into
the stomach.

In the birds there is a differentiation of the gastric region into two
regions, an anterior glandular stomach or proventriculus, and a pos-
terior muscular gizzard. The proventricular glands secrete a diges-
tive fluid, and the food, mixed with this, is passed on to the gizzard.
The walls of the latter have their muscles developed into a pair of discs
with tendinous centres, while the glands of the gizzard form a secretion
which hardens into a horny (keratoid) lining, sometimes developing
into tubercular structures, of great use in grinding the food, thus in part
making good the absence of teeth. In the grain-eating birds small
pebbles are taken into the gizzard and are used in triturating the food.
(In the fossil pterodactyls small clusters of stones are sometimes found
in such a position as to lead to the supposition that these reptiles also
had'a gizzard.) The gizzard is best developed in the grain-eating
birds and is weakest in the birds of prey. In one species of pigeon
part of the wall of the gizzard is ossified.

The mammalian stomach shows the greatest range of form (figs.
227, 228) and the greatest development of different kind of glands.
It may be a simple sac or it may be subdivided into a series of chambers.
It may be almost wholly cesophageal in character (Ornithorhynchus,
fig. 228, A). Occasionally the cardiac glands may be absent. It
may be a simple sac, longitudinal or transverse in position, or it may be

15

COMPARATIVE MORPHOLOGY OF VERTEBRATES.

226

Fic. 228.—Outlines of the stomachs of various mammals (various authors), after
Oppel, to show the distribution of the different glandular regions. Horizontal lines,
cesophageal; oblique, cardiac; dots, fundus; crosses, pyloric; A, Ornithorhynchus; B,
gray rat; C, tapir; D, seal; E, whale (Lagenorhynchus); F, mouse; G, dog; H, kangaroo

(Macropus).

Fic. 229.—Diagram of ruminant stomach, the dotted line showing the course of the
food. u, abomasum; oe, esophagus; , pylorus; ¢s, psalterium (omasus, manyplies); ¢r,
teticulum (honeycomb); ru, rumen (paunch).

DIGESTIVE ORGANS. 227

divided into chambers, the division reaching its extreme in the rumi-
nants (fig. 229) and the cetacea (fig. 228, E) where four compart-
ments can be recognized. In the ruminants two of these, the rumen
or paunch and the reticulum or honey-comb are expansions of the «
‘cesophagus and serve as reservoirs for food before its complete mastica-
tion, after which it follows the course of the dotted lines to the
psalterium, omasus or manyplies and the abomasus or rennet
stomach for gastric digestion.

INTESTINE.

The remainder of the pre-hepatic portion of the alimentary canal,
the duodenum, extending from the pylorus to the entrance of the bile
duct, is considered as part of the intestine. It is especially noticeable

Fic. 230.—Digestive tube of garpike, Lepidosteus (after Gegenbaur). i, small intestine;
oe, cesophagus; pc, pyloric ceca; pg, pylorus; 7, rectum; s, stomach; sv, spiral valve.

in many ganoids and teleosts (figs. 230, 233) where it may bear from
one to two hundred blind digestive tubes, the pyloric ceca. The
same region in a few elasmobranchs may have a pair of these ceca or
(Galeus) it may be expanded into a pouch (‘bursa Entiana’).

The post-hepatic intestine is the seat of most of the digestive pro-
cesses and of absorption of the products of digestion. Here the food,
coming from the stomach, is mixed with the bile from the-liver and
with the pancreatic juice and with the secretions of numerous small
glands in the intestinal wall. The increase of surface needed for ade-
quate digestion and absorption is provided in several ways. There
may be an elongation of the tube which results in its becoming coiled in
the body cavity; the mucous lining may develop folds, both longitudinal
and circular; or the folds may break up into numerous minute, finger-
like processes (villi) which give the surface a velvety appearance. The
food undergoing digestion is moved back and forth (peristaltic mo-
tion) by the antagonistic action of the muscles of the intestinal wall
(p. 207), bringing all of it in contact with the absorbtive surface.

228 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

The length of the intestine is roughly related to the food, being
longer in the plant-eating than in the carnivorous species. This is
strikingly shown in the frogs, where the tadpole (larva) has a very
long intestine, correlated with the vegetable food, while the adult
flesh-eating frog has a canal hardly longer than that of the tadpole of
half the size.

In the intestine there are two divisions, an anterior small intestine
and a posterior large intestine, terms adapted from the digestive tract
of man, though not always appropriate in the lower groups. The
line between the two may be marked externally by the development of

Fic. 231.—Spiral valve of Raia, after Mayer.

one or two blind pouches or ceca at their junction or by a circular
fold or a pair of internal folds of the lining, constituting an ileo-colic
(ileo-czecal) valve, both valve and ceca coexisting in many cases.
Both large and small intestines may be subdivided, chiefly by differ-
ences in their walls. Thus in the small intestine
there may be recognized in different groups a
jejunum, a spiral valve region and an ileum,
while the large intestine may furnish a colon, a
rectum and a cloaca.

In the cyclostomes but two regions occur,
the intestine and the rectum, differentiated ex-
ternally by the larger size of the latter. In the
petromyzonts there is an internal fold of the in-

Fic. 232.—Diagram i i : :
of spiral valve of Carcha- testine which pursues a slightly spiral course,

ios constituting a spiral valve, a structure which

reaches its highest development in the elasmobranchs.

In the elasmobranchs the intestine is nearly straight, but its dif-
ferentiation has proceeded farther. At the junction of small and large
intestine is a dorsal blind sac, the rectal gland. Its function is un-

DIGESTIVE ORGANS. 229

known, but it apparently corresponds to the ceca of the higher groups.
In the ‘small’ intestine is the spiral valve which has two forms, both
leading to increase of surface. In most species a fold, carrying blood-
and lymph-vessels, arises in a spiral line from the wall of the tube, and
its free edge projects into the lumen like a spiral stairway (fig. 231).
In a few forms (Carchariide, Galeocerdo) the
line of origin of the fold is straight and its free
margin is coiled like a roll of paper (fig. 232).
In the large intestine rectum and cloaca are
recognized, the cloaca being that part which
receives the ends of the excretory and repro-
ductive ducts and thus is both digestive and
urogenital in character.

Ganoids and dipnoi (figs. 230, 233) also have the
intestine nearly straight and a spiral valve, least
developed in Lepidosteus. In the teleosts the canal
may be straight (fig. 227) or may make more or fewer
coils, the predaceous species being simplest, while in
the mullet (Mugil) there may be 13 or 14 turns. In ‘
the teleosts the line between small and large intestine \
is often marked by an ileo-colic valve and a few species
have a cecum or rectal gland. A spiral valve rarely q
occurs in teleosts and a cloaca is never found. Ina { ]
few teleosts, in correlation with the translation of the |, @
ventral fins, the anus may lie in front of the pectoral
girdle.

The intestine is straight in the cacilians, has a
few coils in the perennibranchs and more in the sala- (
manders, while the anura have a greatly convoluted Br
intestine. (Reference has already been made to the Fic. 233.—Digestive tract
differences between the intestines of the larval and pated (Stenostomus chr ysops

ae —FPrinceton 296). 6d, bile
adult frogs (p. 228). The line between small and duct; gd, gall bladder; J, liv-
large intestine is frequently marked in the amphi- ¢t; /, large intestine: pc,
bians by an ileo-colic valve and in a few forms bye Gees; Sh small ip
(Rana, Salamandra) there is a rudimentary cecum.
The rectum is larger than the rest of the intestine and a cloaca is always present
in the amphibia.

The reptiles have the intestine coiled (nearly straight in amphisbenans) and
usually of about the same diameter throughout. Small and large intestine are
separated by an ileo-colic valve, and except in crocodiles a caecum is usually present,
while a cloaca constantly occurs. The spirally twisted coprolites of the ichthyo-
saurs have been supposed to indicate the existence of a spiral valve, but since in
other groups the feces are formed in the rectum, this is not conclusive.

230 COMPARATIVE MORPHOLOGY OF VEGTEBRATES.

The intestine is longer in the birds than in the reptiles, but there is considerable
difference in the group in this respect. The great increase comes in the colon which
is coiled in different ways, which may be reduced to seven plans or combinations
of loops and spirals (fig. 234). In a few forms (woodpeckers, parrots, etc.), there

Ivegey any

Fic. 234.—Types of coiling of the intestines of birds, after Gadow. A, isoccelous;
B, anticcelous; C, antipericcelous; D, isopericcelous; E, cycloccelous; F, plagioccelous; G,
telogyrous; p, pylorus. :

is no cecum, but usually the junction of large and small intestine is marked by one
or two ceca (fig. 235). In some cases these ceca are lined with villi, or portions
may be ciliated, while the very large caecum of the ostrich is spirally coiled. Many
birds have a pocket, the bursa Fabricii, of unknown functions, developed from the

Fic, 235.—Alimentary canal of Chauna, after Mitchell. c, ceca; /, large intestine; p
Econ: pv, portal vein; rv, rectal vein; s, small intestine; v, remnant of vitelline
uct.

dorsal part of the cloaca. It arises from the ectodermal (proctodeal) portion and
extends forward, dorsal to the rectum (fig. 236). In some cases it degenerates in
the adult.

The limits of large and small intestine in the mammals are usually marked by an
ileo-colic valve and a single cecum, but there are two ceca in some edentates,
while some edentates, bats, carnivorous mammals and many whales lack either
cecum or valve. The cecum is larger in the herbivorous forms and frequently

DIGESTIVE ORGANS. 231

there is a relation between the development of cecum and stomach. The cecum
becomes enormous in certain rodents and marsupials (sometimes longer than the
body) and plays an important part in digestion, being sometimes lobulated or
furnished with internal folds, those of the rabbits being arranged in a spiral manner.
In man and the anthropoids and some other forms, as is well known, the distal part
of the caecum degenerates to a rudiment, the vermiform appendix, which tends to
become obliterated with increasing age.

,

‘ag
Fic. 236. FIG. 237.

Fic. 236.—Diagrammatic longitudinal section of the cloacal region of a duck embryo
at the twenty-second day. of incubation, after Poindyer. ag, anal groove; c, cloaca; cp,
cloacal plate; f, bursa Fabricii; p, phallus, with cecal duct; sp, stercoral pouch of rectum.

Fic. 237.—Semidiagrammatic course of intestine of new-born deer Cervus canadensis,
after Weber. c, cecum; d, duodenum; co, colon; 7, jejunum; m, mesentery.

Both small intestine and colon are at first straight, but with growth they become
longer, involving convolutions varying in pattern and extent in different groups,
the patterns of the colon being of some systematic value. The full history has
been worked out only for man, two stages being represented in figure 238. The
genus Hyrax is remarkable for a pair of cecal diverticula arising from the colon
(fig. 239). In the monotremes the rectum terminates in a cloaca as in the saurop-
sida, and the same condition occurs in the young of all higher mammalia. In the;
later stages, however, the urogenital and digestive openings become separated by
the formation of a perineal fold between the two.

THE LIVER (HEPAR).

The liver, the largest gland in the body, has several functions. It
secretes the bile (gall) and forms several internal products such as
glycogen, urea and uric acid, of great importance in the animal economy.

232 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

i

Fic. 238.—Scheme of alimentary canal and mesenteries in human embryos, 30 and 50
mm. long, after Klaatsch. c, cecum; co, colon; d, duodenum; k, kidney; 7, rectum; rd,
recto-duodenal ligament; r/, recto-lienal ligament; rrd, recto-duodenal recess; s, stomach;
so, spleen.

Fic. 239.—Alimentary canal of Hyrax capensis after Flower. c, cecum; d, blind diverticula
of colon; 7, ileum; 7, rectum; s, stomach; si, small intestine.

DIGESTIVE ORGANS. 233

The bile is passed to the intestine by the bile duct (choledochal or
hepatic duct), but the other products are carried away by the blood
(internal secretion).

The anlage of the liver is a ventral diverticulum irom: the archenteron
(p. 206), which grows forward from its point of origin, branches again
and again, the ultimate branches forming the glandular part of the
organ, the proximal parts of the outgrowth giving rise to the bile duct
(ocasionally multiple) which empties into the intestine. As a result of
this method of formation the liver is to be regarded as a compound
tubular gland, the lumens of the tubules forming the gall capillaries
which eventually empty into the duct. This tubular condition is
readily recognized in the ichthyopsida, but itis masked in the amniotes
and especially in the mammals, in part by the anastomosis of the
tubules, in part by the interrelation of the bile and blood-vessels.

With development the liver grows cephalad from its point of origin,
but this forward growth is limited by the presence of the blood-vessels
which develop the sinus venosus and the hepatic veins and also contrib-
ute to the septum transversum (hepatic
veins—see circulation), and so its later
increase must cause it to grow in the op-
posite direction. As it increases in size
there is an immigration of mesenchyme
between the lobules and with these the

Fic. 240. Fic. 241.
Fic. 240.—Diagram of two types of bile ducts. 6, gall bladder; ch, choledochar duct;
h, hepatic ducts; 7, intestine.
Fic. 241 —Liver and pancreas of American ostrich (Rhea) after Gegenbaur. d, duo-
denum; dh, bile ducts; /, liver; oe, oesophagus; p, pancreas; pd, pancreatic duct; s, stomach.

blood-vessels enter. At the same time the liver grows away from the
alimentary canal, carrying the peritoneum before it so that it receives
an outer serous coat.

Usually the bile duct (when there are several ducts only one is con-
cerned) forms a lateral diverticulum, the gall bladder, which serves as
a reservoir for the bile. This is usually placed on the dorsal side of the

234 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

liver, but it may be immersed in the substance of the gland. In some
cases, even in mammals, the gall bladder may be lacking. When a
gall bladder is present, three regions may be recognized in: the bile
ducts. Those parts which lead from the liver to the connexion with
the bladder are called hepatic ducts; these are met by the cystic duct
leading from the bladder, and the common duct, formed by the two and
which empties into the intestine is the choledochal duct (fig. 240).
The shape of the gland is in part determined by the shape of the body,
being long in elongate species, sometimes consisting of two consecutive
lobes. Another modifying factor is the shape and size of the adjacent
organs, stomach, etc. Usually the liver is divided into right and left
halves, these corresponding to the first division of the anlage, but these
halves are hardly indicated in some of the teleosts. Frequently, and
especially in mammals, the halves become subdivided into lobes of
varying size, which are arranged in various ways. The liver is rela-
tively larger in the ichthyopsida than in the amniotes, but the cyclo-
stomes have a small liver, that of the myxinoids being in two parts. It
is larger, too, in the flesh-eating than in the herbivorous species. The
blood supply, chiefly through the portal vein and to a less extent by the
hepatic. artery (see circulation) is very large. The color of the gland
_is very variable, especially in teleosts, where it may be brown, yellow,
purple, green and even vermilion.

THE PANCREAS.

The second largest of the digestive glands, the pancreas, secretes
digestive ferments of great strength (trypsin, steapsin, amylopsin),
which digest both proteids and carbohydrates. In some respects it
resembles the salivary glands and so compensates in part for the.
absence of them in the lower vertebrates (p. 220). The pancreas
arises by diverticula from the wall of the intestine close to the liver.
There are usually three of these diverticula, one dorsal and two ventral,
the ventral soon uniting (fig. 242), but in the sharks there is only a
single dorsal, diverticulum, while in the sturgeon there are two dorsal and
twoventral. Inageneral way these develop much like the liver, the distal
portions of the divisions forming the glands, which are of the acinous
type; the proximal portions form the ducts. Of these ducts all may
persist; all but one may disappear, while in the lampreys all may be
lost. In many mammals two ducts persist, the ventral forming the

RESPIRATORY ORGANS. 235

main pancreatic duct (Wirsung’s duct), the dorsal, the accessory
or Santorini’s duct. The ducts may remain distinct; they may unite
before entering the intestine or one of them may unite with the bile
duct.

For a long time it was supposed that a pancreas was lacking in
certain vertebrates (some teleosts, dipnoi, cyclostomes), but recent
studies have shown its presence in many of these. In the case of some

Fic. 242.—Diagram of developing pancreas of cat, after Thyng. c, ductus coledo-
chus; d, duodenum; dp, dorsal pancreas; dd, its duct; i, small intestine; s, stomach; vp,
ventral pancreas.

teleosts it occurs as a slender tube in the mesentery; in the dipnoi it is
outside of the muscles in the intestinal wall, while in the cyclostomes
it is partly concealed at the insertion of the spiral valve, partly (myxi-
noids) inthe liver. In these forms, owing to the complete disappearance
of the duct it becomes a gland of internal secretion. The pancreas
may be elongate, compact, or sometimes extremely lobulated. Usually
(fig. 241) it lies in a loop of the duodenum. From certain peculiarities
of structure the queston has arisen as to whether two distinct structures
are included in the pancreas.

: THE RESPIRATORY ORGANS.

The respiratory organs have for their purpose the exchange of gases
between the blood and the surrounding medium—water or air—
carbonic dioxide being given off and oxygen being absorbed by the
circulating fluid. In order that the exchange be readily effected it is
necessary that the organs be richly vascular, that the walls between the
blood and the surrounding medium be extremely thin so as to permit
rapid osmosis, and that the osmotic surface be as great as possible.
Further, there must be an adequate mechanism for passing the oxygen-
containing medium over the respiratory surfaces.

2 36 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

In the vertebrates the organs of respiration are developed in more
or less intimate connection with the cephalic portion of the digestive
tract, just behind the cavity of the mouth. This part of the alimentary

ABNER
ae

Via

Fic. 243.—Pharyngeal region of a
young Acanthias embryo. 8b, blood-
vessels; c, coelomic cavities of gill arches;
g, developing gills; gc, gill clefts; h,
hypophysis; m, mouth; x, notochord; a,
oculomotor nerve; oe, cesophagus; 2,
peritoneal cavity; s, spiracular cleft;
I-III, first to third head cavities.

canal, which thus serves for the pass-
age of food and for the performance
of respiratory functions is called the
pharynx. The organs themselves
may take the form of gills or branchiz,
adapted for aquatic respiration, or of
lungs (pulmones) fitted for breathing
air. In this connection must be con-
sidered the cases of certain fishes,
amphibia, and turtles whererespiration

is effected in part by the skin, the

pharyngeal epithelium, or the diges-
tive tract. There are also a number
of other structures—air bladder, thy-
mus and thyreoid glands, etc., which
are derived from the pharynx, though
they are without respiratory functions.

GILLS OR BRANCHIA.

The typical gills or branchie are
developed on the walls of some of the
visceral clefts (gill or branchial
clefts) which are formed in the sides
of the pharynx. These clefts arise as
paired pouches or grooves of the en-
toderm of the pharynx (fig. 208).
They extend laterally, pushing aside
the mesoderm, until they reach the
ectoderm, ectoderm and entoderm
then fusing to a plate. This in most
cases becomes perforated, so that the

cavity of the pharynx is connected with the exterior by a series of
openings (fig. 243), the clefts developing in succession from the

cephalic end backward.

These visceral pouches develop in all vertebrates, but in the mam-
mals only a few or even none of them break through to the exterior. In

RESPIRATORY ORGANS. oA7

the adult amniotes the pouches may completely disappear without
leaving a trace, aside from the Eustachian tube (p. 187) and the thymus
glands to be mentioned below. The number of clefts or pouches
varies between considerable limits. The largest number in any true
vertebrate (there are more in Amphioxus and the enteropneusts) is
fourteen pairs in some specimens of Bdellosioma. In other cyclo-
stomes there are seven, eight to seven in the notidanid sharks, six in
other elasmobranchs, five or six in teleostomes, amphibia and reptiles
and five in mammals. In this numbering the oral cleft is not included,
although there is some evidence that the mouth has arisen by the
coalesence of a pair of gill clefts (p. 206).

The serial repetition of the visceral clefts does not strictly correspond to the
other segmentation of the body, their number and position being at variance with
those of the myotomes. There is a branchiomerism or serial repetition of the
gill clefts, apparently distinct from the true metamerism of the head. The ap-
pearance of these clefts or pouches and the relation of aortic and branchial arches
in the amniotes, where gills are never developed, can best be explained by the
assumption that these forms have descended from branchiate ancestors.

Between each two successive gill clefts there is an interbranchial
septum, covered externally with ectoderm, internally with entoderm,
and with an axis of mesoderm, the latter in the earlier stages carrying
with it a diverticulum of the ccelom (fig. 243, c). Later blood-vessels
(aortic arches) and skeletal elements (visceral arches, p. 63), are devel-
oped in each septum, the visceral arches appearing on the splanchnic
side of the ccelom and hence not being comparable to ribs or girdles.

In the cyclostomes and fishes the gills are developed from the an-
terior and posterior walls of the typical interbranchial septa. They
were long regarded as of entodermal origin, but in recent years con-
siderable doubt has been thrown on this, at least for the fishes, and
there is some evidence for their ectodermal origin. The matter cannot
yet be regarded as settled. These gills are either filamentous or la-
mellate outgrowths of epithelium, each carrying a loop of a blood-
vessel. Thus each typical cleft is bounded in front and behind by gill
plates or filaments (fig. 246), those on a side constituting a demibranch,
the two demibranchs of a septum constituting a gill, while a cleft is
bounded by demibranchs belonging to two gills. In the young
elasmobranchs and in the young of a few teleosts (before birth) the
gill filaments protrude from the clefts as long threads, but later they
are withdrawn.

238 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

In the cyclostomes and notidanid sharks the first cleft (between the
mandibular and hyoid arches) bears gills like the rest, but elsewhere
it differs. In most elasmobranchs and in a few ganoids (Acipenser,

Fic. 244.—Diagram of relations of cesophagus and respiratory tracts in (A) Myxine and
Ammoceetes, and (B) Petromyzon, 6, bronchus; ve, esophagus; #, thyreoid gland.

Polyodon, Polypterus) it becomes reduced in size in the adult, the
closure beginning ventrally (fig. 136) so that the persistent part of the
opening is on the upper side of the head. This opening is called the
spiracle. In other vertebrates, including the
chimeroid sharks and many true sharks, the
spiracle is closed in the adult, but in the anura and
the amniotes its inner portion persists as the
Eustachian tube and the tympanic cavity of the
ear (p. 187).

Usually the series of gills begins with the
demibranch on the caudal side of the hyoid arch,
while none ever appears on the caudal side of the
last cleft. In the teleosts the series of gills is still
further reduced, the reduction reaching its ex-
treme in Amphipnous, where there are no demi-
branchs on the first and fourth branchial arches
and only one on the second.

In the cyclostomes the gill clefts occur at a consider-
able distance behind the mouth, partly the result of the
eee great development of the lingual apparatus. In the larve

FIG. 245.—Gill of Petromyzon (Ammoceetes) the seven gill clefts are
pouches and blood-vessels nearly typical, the gill extending inward nearly to the
. rr eal oe ee pharyngeal wall, each cleft having a short efferent duct
ee eo, external gill leading to the exterior, and the esophagus beginning at
opening; h, heart; oe, the hinder end of the pharynx (fig. 244, A). In the meta-
fee liet garip crea morphosis to the adult the cesophagus grows forward,

dorsal to the gill clefts, to the cephalic end of the
pharynx, thus cutting off a ventral respiratory tube, the so-called bronchus (fig.
244, B). At the same time the gill-bearing region of each cleft becomes separated

RESPIRATORY ORGANS. 239

from the bronchus by the development of a short afferent duct, while the demi-
branchs come to lie in oval pouches (much as in Mywxine, fig. 245), in allusion to
which the cyclostomes are sometimes called marsipobranchs (pouched gills).

In the myxinoids the tract between the mouth opening and the pharynx is
more elongated and the pharyngeal region (fig. 244, A) is not differentiated into
cesophagus and bronchus, as in the adult lampreys. In Mywine there are six pairs
of gills; in Bdellostoma the number ranges from seven to fourteen, varying even on
the: two sides of our Pacific species, B. dombeyi. In the petromyzons and in

Fic. 246.—Diagram of gill clefts in (A) elasmobranchs and (B) teleosts. A’ and B’,
a single gill of each. a, artery; br, branchial ray; d, demibranchs; gc, gill chamber; gr,
gill raker; 0, operculum; ve, cesophagus; 00, opercular opening; s, spiracle; v, veins.

Bdellostoma the efferent ducts of the gill pouches open separately to the exterior; in
Mysine (fig. 245) they unite into a common duct on either side, the left also receiv-
ing an oesophago-cutaneous duct, behind the last gill. This duct, which leads
from the cesophagus to the exterior, resembles a gill cleft, but lacks gills. A similar
duct occurs in the same position in Bdellostoma.

In the fishes there are two types of gills and associated structures.
In the elasmobranchs (the chimeroids excepted) the interbranchial
septum is greatly developed (fig. 246, A’), extending some distance
laterally beyond the gill folds so that the distal part of the cleft forms an
excurrent canal. This prolongation of the septum extends to the ex-
terior and then turns backward, thus protecting the delicate gills from

240 FOMPARATIVE MORPHOLOGY OF VERTEBRATES.

injury (fig. 246, A). In other fishes the posterior margin of the hyoid
septum grows back as a broad fold over the clefts behind, thus forming
a gill cover or operculum (fig. 246, B, 0), enclosing an extrabran-
chial or atrial chamber into which all of the clefts empty and which
in turn opens to the exterior by a single slit (00) behind the operculum.
This opercular opening is usually broad, but it is reduced to a circular
opening on either side in a few teleosts, while in the symbranchii the
openings of the two sides are united to a single one in the mid-ventral
line. Correlated with this protection of the gills by the operculum is
the reduction of the interbranchial septum (fig. 246, B’), which forms
only a slender bar, from which the demibranchs project far into the
gill chamber.

Fic. 247.—Head of Chlamydoselache, after Garman; /, opercular fold.

Usually the two opercular folds are continuous beneath the pharynx,
which points to the beginnings of an operculum in the shark, Chlamy-
doselache (fig. 247). In the chimzroids the operculum is farther
developed and is supported by cartilaginous rays. In the teleostomes
two parts may be recognized in the operculum, the operculum or gill
cover proper, supported by a series of large bones (p. 77), and a more
ventral part, the branchiostegal membrane, which is very flexible
and has a skeleton of slender (branchiostegal) rays, connected with
the hyoid.

In the sea horses and pipe fishes (lophobranchs) the gills form small rounded
tufts. In the labyrinthine fishes there is a complicated bony structure in the bran-
chial chamber, covered by a folded membrane which is used in aerial respiration.
In the young crossopterygians (Polypterus, Calamoichthys) bipinnate external gills
persist for some time. In Amphipnous, just referred to, a sac opening between
the hyoid and the first branchial arch is developed on either side of the head.
Its walls are very vascular thin vessels being connected with both the branchial
arteries and the dorsal aorta.

The gills are so placed that there can be an almost continuous stream
of water over them, thus bringing the oxygen needed by the blood. As
a rule, this water is drawn in through the mouth by the enlargement of
the oral cavity, and by its contraction is forced out through the clefts.

RESPIRATORY ORGANS. 241

In the myxinoids the cesophago-cutaneous duct is supposed to act as the
incurrent opening when these animals burrow into fishes. In the lam-
preys the water is said to pass both in and out through the gill clefts
when these animals are attached to some object. In at least some of
the elasmobranchs water passes in through the spiracle which regularly
opens and closes.

Many, if not all of the teleosts have breathing valves. There are two pairs of
these, an anterior pair attached to the margins of the jaws, which permit the ingress
of the water but prevent its outflow. The other pair is formed by the branchiostegal
membrane, which closes the opercular opening and only allows the water to pass
out. The action of both pairs can be easily seen from fig. 248.

Fic. 248.—Breathing valves of teleosts, after Dahlgren. A, schematic figure, the
anterior half in the vertical, the posterior in the horizontal plane; B, mouth of sunfish
(Eupomotis); b, branchiostegal valve; mn, mx, mandibular and maxillary valves; v, oral
valves.

In certain fishes with an operculum (Acipenser, Lepidosteus, many teleosts) a
gill is developed as a series of lamella on the inner surface of the operculum. This
opercular gill has respiratory functions. The pseudobranchs are homologous
with the true gills. They are developed in some elasmobranchs as vertical folds on
the anterior wall of the spiracular cleft, occurring in some cases, even where the
spiracle is closed externally. They, however, receive arterial blood and so cannot
be respiratory in function. The blood, still arterial in character, passes from them
to the chorioid coat of the eye and in some cases to the brain. From their position
they must be interpreted as the demibranch of the posterior side of the mandibular
arch.

Pseudobranchs are common in teleosts, usually lying on the medial side of the
hyomandibular bone. When free, they are gill-like in appearance, but in some
species (fig. 249) they are covered by muscles and connective tissue, when they
have a blood-red, glandular appearance. Pseudobranchs also occur in Lepidosteus,
most sturgeons and Ceratodus; they are lacking in Amia and Protopterus. Polyp
terus and Polyodon have opercular gills.

16

242 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

In the amphibia the gill clefts are formed in the same way as in the
fishes, but the first and fifth never break through, and all are usually
closed in the adult, the exceptions being in the perennibranchs and
derotremes where from one to three clefts remain open through life. In
the urodeles and cecilians there is a reduced operculum -which never
becomes prominent, being merely a fold of the integument in front of
the gill area. In the larval anura it is well developed, though skeletal

Fic. 249.—Dissection of pseudobranchs (gs) and cephalic circle in pike (Esox), after
Maurer. cc, cephalic circle e, vessels to eyes; g, gills; , vessels to palate and nose;
I-IV, efferent branchial arteries.

supports are lacking, as in all amphibia. Before the time of metamor-
phosis it grows backward over thegills, gill clefts, and the anlagen of
the fore limbs, and fuses with the sides of the body behind the latter.
In this way these parts are enclosed in an extrabranchial or atrial
chamber, the chambers of the two sides being in communication below.
During larval life the branchial chambers usually communicate with
the exterior by a single excurrent pore, usually on the left side, but in
the larval aglossa right and left excurrent pores are found.

The gills of the amphibia are certainly of ectodermal origin (cf. p.
237). First to appear are the external gills, covered with ciliated epi-

RESPIRATORY ORGANS. 243

thelium. ‘Three pairs of these usually arise, before the gill clefts break
through, on the outer surface of the third, fourth and fifth arches, and
they are supplied by the corresponding (aortic) arches of the blood
system. They are without any skeletal support and are of varying form
—-pectinate, bipinnate, dendritic, etc. (fig. 250)—and in one species

Fic. 250.—External gills of young Amphiuma, partially covered by opercular fold.

of cecilians, where but a single pair occurs, they are large leaf-like
lobes. When the gill clefts break through there is an ingrowth of ecto-
derm into each cleft, from which (except in perennibranchs) gill fila-
ments are developed on the sides of the septa, so that for a time there
may be both external and internal gills (fig. 251, right side). In the

Fic. 251.—Diagram of the relations of external and internal gills in the anuran tad-
pole, after Maurer. ab, eb, afferent and efferent branchial arteries; #, heart; 0, ear cavity;
ph; pharynx; ra, radix aorte.

perennibranchs the external gills persist through life (they are said to
be absorbed and reformed in Siren), but in other urodeles and in cecil-
ians they are absorbed at the time of metamorphosis. In the anura
(fig. 251), as the operculum grows back over the clefts, the external
gills, which are so prominent in the earlier stages, become folded into
the extrabranchial chamber, where they are gradually reduced, while

244 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

those belonging to the cleft become the functional organs, the water
taken in through the mouth passing over them in its way to the exterior
via the extrabranchial chamber. Then, with the completion of the
metamorphosis, the lungs become functional, the gill clefts are closed
and the gills absorbed, the legs are developed and the anterior pair
released from the extrabranchial chamber, the tail is absorbed, and the
tadpole (larva) becomes the adult.

Fic. 252.—Cast of oropharyngeal region of pig embryo, 17 mm. long, after Fox. al/;
alveo-lingual fold; ctm, cervical cord of thymus; dp, dp”, dorsal apex of first and second
pharyngeal pouches; dptm, dorsal plate of thymus; /, filiform appendix of second pouch;
tlr, lateral thyreoid; stf, sulcus tubo-tympanicus; ¢m, thymus; vf, vestibular fold of mouth.

Little is known of the gills in the stegocephals, but the presence of well developed
branchial arches in the larve of some species (p. 83) would imply the existence
of functional gills. ‘

For some time it was thought that the fish gills were of entodermal origin, and
those of the amphibia were derived from the ectoderm. Hence the conclusion was
that the two had no genetic connexion, the gills of the amphibia being a new
acquisition, developed within the group or arising from the external gills of some
form like Polypterus. Lately the doubts thrown upon the entodermal origin of the
gills of fishes (p. 237) render it possible that all vertebrate gills are homologous.

Gills are never developed in the amniotes, but in the embryos the
paired visceral pouches are formed (figs. 208, 252)—five in the saurop-
sida, four in mammals—in the same way as in the fish-like forms.
Few, if any, of them break through to the exterior, although their
position is indicated by grooves on the outside of the neck. The proc-
ess of obliteration of these external grooves is interesting. The ante-
rior arches enlarge and slide back over the posterior, so that at least
the external branchial grooves lie in the wall of a pocket, the cervical
sinus, on either side of the neck (fig. 253). Later a process of the
anterior (hyoid) arch extends over and closes the sinus, a process re-

RESPIRATORY ORGANS. 245

calling the history in the anura. Internally the entodermal branchial
pouches, with the exception of the first, disappear, but the first persists
as the tympanic cavity and Eustachian tube described in connexion
with the ear.

Fic. 253.—Head of human embryo with pharyngeal floor removed, after Hertwig.
Cut surfaces lined. Compare with fig. 221. cs, cervical sinus; e, eye; h, hyoid arch; hd,
hypophysial duct (Rathke’s pocket); /, lung; /g, lacrimal duct; 2, naris; md, mandible;
on, oronasal groove; ér, trachea. ,

Pharyngeal Derivatives.

Several structures arise in the pharyngeal region—some developed
from gill clefts, some from other parts—which, while not respiratory
in character, naturally come for mention here.

Among these are the thymus glands. These arise from the ento-
dermal epithelium at the dorsal angle of a varying number of visceral
clefts (elasmobranchs, clefts 2-6 and possibly the spiracle; teleosts and
cecilians, 2-6; urodeles, 1-5, 1 and 2 degenerating; anura, 1 and 2,
the latter only persisting; amniotes 3 and 4).

The organ which results has varying positions and shapes in the
different groups. It becomes richly vascular, and by the intrusion of
connective tissue, assumes an acinous form. In Myxine a number of
lobules behind the gill region have been regarded as a thymus, but now
areinterpreted as pronenephric. Insome cases (fishes, etc.) the thymus
retains its primitive position dorsal to the gill clefts (usually above the

246 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

fourth in teleostomes), and it maintains its branchiomeric character in
snakes and gymnophiones. It maylieaboveand behind theangle of the
jaw (most amphibians), close to the carotid arteries (most sauropsida),
sometimes extending along the neck (crocodiles and birds). In the
young mammals the thymus (sold in the markets as ‘throat sweet-
breads’), which arises from a single pair of clefts, is largely behind the
sternum, extending forward along the neck. Later it gradually grows
smaller, the extreme development being reached in man between the
fourteenth and sixteenth years, but retaining its functional structure

Fic. 254.—Schemes of the origin of several pharyngeal derivatives in (A) Raia, (B)
anuran and (C) chick, after Verdun. cd, carotid gland; e, epithelial body; gr, gill remnants;
p, postbranchial body; ¢m, thymus; #7, thyreoid; I-VI, gill pouches or clefts.

until middle life. The function of the thymus glands is as yet unknown;
though leucocytes are abundant in them, they are not lymphoidal in
character.

Other structures arising in the pharynx, either from the gill clefts
or from the pharyngeal walls, are the ‘ epithelial bodies,’ post-branchial
bodies, suprapericardial bodies, gill remnants, etc., concerning which
little is known. The carotid glands of the same region are referred to
elsewhere.

The thyreoid gland cannot be dismissed in such a summary
manner. This is a ductless gland in the pharyngeal region of all
vertebrates, ventral to the alimentary tract. In the lower vertebrates
it arises as an unpaired pocket in the floor of the pharynx (fig. 254),
this retaining its connexion with the parent tube in the ammoccete
stage of the lamprey (fig. 190), but at the time of metamorphosis it
loses its duct (as is early the case in all other vertebrates) and eventu-

RESPIRATORY ORGANS. 247

ally becomes follicular. In most vertebrates, the anlage, after separa-
tion, forms a network of epithelial tubes before becoming follicular.
Usually it exhibits a marked bilaterality, and in amphibia and birds
it becomes divided into two glands.

In the elasmobranchs the thyreoid lies between the end of the ventral
aorta and the symphysis of the lower jaw; in teleosts the groups of
follicles lie around the ventral aorta, extending out on the anterior aortic
arches. In the urodeles the gland lies just behind the second arch and
in the anura on the hinder margin of the thyreoid process of the hyoid
plate. In reptiles it is ventral to the trachea (at about its middle in
lizards, nearer its division in other groups), while in the birds the two
glands occur at the base of the bronchi. In the mammals it is usually
near the larynx, and while generally two-lobed, it is here and there
(monotremes, some marsupials, lemurs, etc.) paired.

Like the other ductless glands, the thyreoid supplies the blood with
substances necessary to the well-being of the organism, in the case of
mammals at least, an iodine-containing albumen. Degeneration or
extirpation of the thyreoid result in cerebral trouble. In the ancestral
vertebrate the thyreoid apparently had to do with some part of the
digestive work, as is shown by its late connexion with the pharynx
in the ammoceete.

In the pharynx and at the entrance of the mouth into the pharyn-
geal cavity (isthmus of the fauces) occur certain lymphatic structures
called tonsils, concerning which our knowledge is yet very deficient.
One account says they arise from inwandering epithelial cells, the
other maintains that they are formed from the sub-epithelial meso-
derm. Two different groups of organs are included under this name,
the true tonsils at the isthmus of the fauces, and the pharyngeal
tonsils. The latter may be represented by lymphoid structures in the
floor or roof of the pharynx of urodeles and anura. They are well
developed in reptiles and birds, occurring in the latter behind the
choane. In mammals they are inconstant structures. The true
tonsils of mammals lie one on either side of the isthmus. Both
types of tonsils consist of an adenoid ground substance containing
numerous lymph cells, and become follicular after birth.

THE SWIM BLADDER.

While the air or swim bladder (pneumatocyst) is not respiratory,
it is included here from its possible connexion with the lungs. It

248 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

occurs only in teleostomes, and while found in most species (frequently
absent from bottom-feeding forms—pleuronectids, etc.), it is lacking
here and there, even among species classed as physostomous (Lori-
caria, etc.). In the young of a few sharks (e.g., Scyllium) there is a
pouch on the dorsal side of the cesophagus which suggests the possible
origin of the organ. ’

The swim bladder lies dorsal to the alimentary tract, outside of the
peritoneum (which frequently covers only its ventral surface) immedi-

Fic. 255.—Air bladder of Megalops cyprinoides, after de Beaufort. uw, anus; }, air
bladder; d, pneumatic dust leading from the cesophagus; /, ligament; p, anterior part of
bladder extending to skull.

ately below the vertebre and excretory organs (mesonephroi). In
some instances it extends the whole length of the body cavity and
(clupeids) may even send diverticula into the head. In other species
it may be much shorter. In development it arises as a diverticulum of
the alimentary canal (fig. 209), and in the ganoids and one group of
teleosts (physostomi) it is connected with the digestive tract throughout

E
5 d
sat SB Ze ee
oe oe oe

Fic. 256.—Swim-bladders of physostomous fishes; A, pickerel (Esox); B, carp (Cypri-
nus); and C, eel (Anguilla) after Tracy. 6, swim-bladder; d, duct; g, red gland; oe,
cesophagus.

life by the pneumatic duct. This usually empties into the cesophagus,
but it may connect with the stomach. In most teleosts, however, the
duct becomes closed at an early date and the bladder loses its connex-
ion with the digestive tract (physoclisti).

The swim bladder is usually unpaired (paired in most ganoids) and
may be simple or divided into two (rarely three) connecting sacs (fig.
256). It is usually regular in outline, but diverticula of all kinds are

RESPIRATORY ORGANS. 249

common, the form being most varied in the physoclistous species. In-
ternally the walls may be smooth and the cavity simple, or it may be sub-
divided by septa (fig. 257), or, as in Amia and Lepidosteus, it may be
alveolar, recalling the condition in the lungs of higher vertebrates.
The walls sometimes contain striated muscle, and in some siluroids and
cyprinoids they are more or less calcified, partly by the inclusion of
processes from the vertebre.

tre

Fic. 257.—Ventral view of opened air bladder and Weberian apparatus of Macrones,
combined from Bridge and Haddon. 4, atrial cavity; ac, anterior chamber of air bladder,
the arrows showing the connexion with the posterior chamber; de, endolymph duct; s,
sacculus; sc, scaphium; sk, subvertebral keel; tra, trc, anterior and crescentic processes of
tripos; #, utriculus.

The blood supply is arterial, coming from either the aorta or the
ceeliac axis, in some instances different portions receiving blood from
both. In the walls the arteries break up into networks of minute
vessels (‘rete mirabile’), these frequently making ‘red spots’ on the
inner surface. From the retia the blood passes to the body veins, (post-
cardinal, hepaticorvertebral). In the ganoids and phystomous species,
especially those with a wide pneumatic duct, the gases contained in the
swim bladder may be obtained directly from the air or water, but in the
physoclists this is impossible and the red spots may be the place of its

250 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

secretion and possibly of its absorption, the probability being increased
by the greater abundance of the spots in species with closed ducts.

While the pneumatic duct usually connects with the dorsal side of the alimentary
canal, it enters the left side in Erythrinus, and in the mid-ventral line in Polypterus
and in Calamoichthys. In Polypterus the bladder arises from the ventral side and
there are paired swim bladders, the right being the longer. The blood in this genus
comes from the efferent branchial arteries and hence is arterial.

The swim bladder is supposed to have hydrostatic functions, aiding
in the recognition of differences of pressure due to changes in depth.
In the clupeids the air bladder sends a diverticulum into the head,
there giving a branch to each ear. In some physostomes (siluroids,
cyprinids, gymnonoti) parts of the anterior vertebre are modified into a
chain of bones—the Weberian apparatus—adapted to convey dif-
ferences of bladder pressure to the internal ears. One pair of bones
is connected with the dorsal wall of the air bladder, a second with a
diverticulum (sinus impar) of the internal ear, while others are in-
tercalated between these extremes (fig. 257). Changes in the distention
of the bladder are thus conveyed to the inner ear and probably affect
the sense organs.

LUNGS AND AIR DUCTS.

Lungs arise as a diverticulum from the ventral side of the pharynx,
immediately behind the last gill pouch. The diverticulum divides
almost as soon as outlined into right and left halves, each the anlage of
the corresponding lung. As development proceeds, the two grow in’a
caudal direction into the trunk, carrying the peritoneum with them as
they protrude into the ccelom, so that they eventually have an entodermal
lining,. derived from the epithelium of the pharynx; an outer serous
layer of peritoneum, with mesenchyme carrying blood- and lymph-
vessels, nerve and smooth-muscle fibres between the two. In this
development two parts are differentiated, the lungs, the actual seat of
the exchange of gases, and the air ducts leading from the pharynx to
them. The ducts may consist of an anterior unpaired portion, the
wind-pipe or trachea, connecting with the pharynx, and usually divid-
ing at its lower or posterior end into two tubes, the bronchi, leading to
the two lungs. In most air-breathing vertebrates the anterior part of
the trachea is specialized and forms a larynx. In addition to these
parts, the mechanism by which air is drawn into and expelled from the
lungs forms a part of the respiratory apparatus.

RESPIRATORY ORGANS. 251

THE AIR DUCTS.

The opening from the pharynx into the air ducts is known as the
glottis, usually an elongate slit capable of being closed and opened
by appropriate muscles. This is immediately succeeded by the ducts,
which, except in the dipnoi, are more or less differentiated into regions

and have skeletal supports in their walls.

In the dipnoi the glottis is either in the mid-ventral line (Protopierus) or a little
to one side (Lepidosiren, Ceratodus) and the air duct passes up on the right side
of the cesophagus to reach the lungs which are dorsal to the alimentary canal. The
tube is without skeletal supports and connects directly with both lungs without any

division into bronchi.

Larynx.—The beginnings of the larynx are seen in the amphibia,
where in the lower types (Necturus) a pair of cartilages are developed on

the sides of the glottis, in the position of
a reduced visceral arch, each cartilage
extending posteriorly a short distance
along the air ducts. In other genera of
urodeles the anterior end of each lateral
cartilage separates from the rest as an
arytenoid, the first of the laryngeal carti-
lages, imbedded in the walls of the glottis.
The rest of the lateral cartilages may
remain entire (fig. 258) or they may
separate into a number of pieces, extend-
ing along the lateral walls of the trachea
and bronchi. Usually the anterior pair
of these pieces fuse in the mid-ventral line,
thus forming the second (cricoid) ele-
ment of the pharyngeal framework.
These parts are moved by antagonistic
muscles. One set of these, extending to

Fic. 258.—Trachea, etc., of
Amphiuma, after Wilder. a,
arytenoid cartilages; b*, fourth bran-
chial arch; dir, dilatator trachee,
muscle; hp, hyopharyngeus mus-
ae trachea with cartilages in its
walls.

the persistent branchial arches, serves as dilatators of the glottis; the
others, connected with the laryngeal cartilages themselves, constrict
the opening. In the anura the cricoid is converted into a ring, with
the arytenoid hinged within and anterior to it, the whole larynx moving
anteriorly to a position between the hinder processes of the hyoid plate.
Inside of the ‘short larynx thus framed by these cartilages are a pair
of folds of the laryngeal lining, the vocal cords, extending parallel to

gi

252 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

the margins of the glottis. These may be tightened or relaxed, and
by their vibration of their edges under influence of the breath the
voice is produced.

The larynx is scarcely more developed in reptiles. The cricoid is usually an
incomplete ring, to which the arytenoids are attached, and the whole is placed just
ventral to the median part of the hyoid, with which it is closely associated (fig. 259).
In several reptiles there is a fold of the mucous membrane just in front of the glottis
which is supposed to represent the beginnings of an epiglottis (infra), while in
geckos and chameleons a pair of folds, running dorso-ventrally in the larynx,
serve as vocal cords. The larynx is also rudimentary in the birds, its place as a
vocal organ being taken by the syrinx to be described below, in connexion with the
trachea. The arytenoids are frequently ossified in birds.

Fic. 259. Fic. 260.

Fic. 259.—Laryngeal apparatus of Chelone, after Goppert. a, arytenoid; b'~, first and
second branchial arches; cr, cricoid; d, dilator laryngis muscle; g, glottis; , hyoid; he,
hyoid cornua; sph, sphincter laryngis; tr, trachea; cartilage dotted, bone black.

Fic. 260.—Ventral and side views of monotreme larynx, after Gegenbaur. c, cri-
coid; h, hyoid; th, thyreoid; ér, trachea. :

In the mammals the larynx reaches its highest development. Its
framework is formed by the arytenoid and cricoid cartilages, homol-
ogous with those of the lower groups, and in addition, a thyreoid
cartilage (or cartilages) on the dorsal side anterior to the arytenoids
and cricoids. The origin of the thyreoid is best seen in the monotremes
where the hyoid apparatus enters into close relations with the larynx
(fig. 260), while the second and third branchial cartilages form two
plates, the lateral elements of the thyreoid on either side, the median

RESPIRATORY ORGANS. 253

element of the hyoid forming a copula. In the higher mammals the
association of hyoid and larynx is not so intimate, even in the embryo,
but the thyreoid shows its double origin in its development.

_ In the higher mammals the thyreoid cartilage forms a half ring on
the ventral side of the anterior end of the larynx, its anterior dorsal
angles being produced into cornua connected by ligament with the
hyoid (fig. 261). Dorsal to the thyreoid is the glottis with the aryte-
noids in its walls. Posterior to it is the ring-shaped cricoid, following
which is the trachea. Anterior to the glottis is a fold of the mucous

Fic. 261.—Dorsal and side views of larynx of opossum, Didelphys vir ginianus (Prince-
ton 1739) cartilages dotted. a, arytenoid; ¢, cricoid; e, epiglottis; g, glottis; 4, hyoid;
t, trachea; th, thyreoid.

membrane of the pharynx, the epiglottis, supported by an internal car-
tilage (possibly the fourth branchial arch) which articulates with the
anterior margin of the thyreoid. The epiglottis usually stands erect,
leaving the glottis open for respiration, but during deglutition it folds
back over the glottis, thus preventing the entrance of food into the
trachea.

Internally the cavity of the larynx bears a vocal cord on either side.
These are folds of the mucous membrane, extending from the thyreoid
to the arytenoids, and by movements of these latter cartilages they can
be tightened or relaxed, thus altering the pitch of the note caused by
their vibration. Anterior to these cords is a pocket, the laryngeal
ventricle (sinus of Morgagni) on either side, small in most mammals,
but developed in the anthropoid apes to large vocal sacs (in some
there is a median vocal sac in addition), which act as resonators,
adding to the strength of the voice.

254 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

In the.whales and young marsupials the larynx is prolonged so that it projects
into the choana behind the soft palate. In the whales (fig. 262) this is an adaptation
to the manner of taking food from the water and breathing at the same time. In
the young marsupials the milk is forced into the mouth by the muscles of the mam-
me of the mother and this arrangement prevents strangulation.

Trachea.—In the tetrapoda the trachea is strengthened by the
formation of cartilage in its walls, the beginnings of which are seen
in the urodeles where the fifth branchial arch gives rise to these ele-
ments (p. 251). Their arrangement varies considerably in the urodeles
_and cecilians, being sometimes scattered pieces,
sometimes regularly arranged and even united in
the lateral walls (fig. 258). Corresponding to the
posterior position of the lungs the trachea is long
in these groups, but in the anura it can scarcely
be said to exist, the lungs succeeding almost
immediately to the larynx.

In the reptiles the trachea varies in length,
being shortest in lizards (except amphisbznas),
longer in snakes, tortoises and crocodiles, divid-

Fic. 262.—Larynx of ing into bronchi at varying distances from the
peas a faltes lungs. It is frequently bentin turtles. In many
showing the prolongation reptiles the cartilage rings of the trachea are in-
Po ee complete, but in Sphenodon, lizards and some
eae cricoid; snakes some cartilages (usually the more anter-

' ior) form complete rings, the others being com-
pleted dorsally by membrane. In snakes the successive rings are
often united, especially on the sides.

The trachea is greatly elongate in birds in correlation with the
length of the neck and the position of the lungs within the thorax.
The rings, which are usually complete, are frequently ossified. The
trachea is occasionally (male ducks, etc.) widened in the middle and
in various groups becomes greatly convoluted so that its length from the
glottis to the lungs exceeds that of the neck. In some these convolu-
tions occur beneath the integument of the thorax; in some between the
sternum and the muscles; and in the cranes and swans within the
keel of the sternum.

The larynx is never the organ of voice in the birds, its place being
taken by a‘somewhat similar structure, the syrinx, at the division of
the trachea into the bronchi. The sound-producing elements are

RESPIRATORY ORGANS. 255

membranes which vibrate by the passage of air, as do the vocal
cords of mammals. Most common is the broncho-tracheal syrinx,
in which the last rings of the trachea are united to form a reso-
nating chamber, the tympanum, while folds of membrane, internal
and external tympanic membranes (not to be confused with the simi-
larly named structure in the ear, p. 187), extend into the cavity from the
median and lateral wall of each bronchus. In some cases there is also
an internal skeletal element (pessulus) which bears a semilunar mem-
brane on its lower surface. In many birds this type of syrinx is
often asymmetrical (fig. 263) and is ex-
panded into a (usually) bony resonat-
ing vesicle. In the tracheal type of
syrinx the lateral port ons of the last
tracheal rings disappear and the mem-
brane which closes the gap forms the
vibratile part. In the bronchial syrinx
the membranes occur between two suc-
cessive rings of each bronchus, each ring
being concave toward its fellow. By a
shortening of the bronchial wall these
membranes are forced as folds into the
tube. In all types of syrinx there are
muscles attached to trachea and bronchi,
which, by moving these parts, alter
the tension of the folds, thus changing
the note.

In the mammals the trachea is elon- :
gate (shortest in the whales and sire- ae oe ee
nians, dividing in the latter immedia- 915). 8, bronchi; p, pessulus; ¢, tra-
tely behind the cricoid into the two ‘°/% tympanum.
bronchi), and the cartilage rings are usually incomplete dorsally,
the gaps being closed by membrane. This structure allows the tube to
remain open under ordinary conditions and yet allows it to give when
food is passing down the cesophagus, just dorsal to it. In the cetacea
and sirenia the tracheal cartilages are sometimes spirally arranged.

Lungs.

The morphology of the lungs may be understood by following their
development in the mammals and then describing their modifications

2 56 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

in the various classes of vertebrates. As stated above the lungs arise as
a diverticulum (fig. 264, A) on the ventral side of the pharynx which
quickly divides into two sacs, the anlagen of the two lungs. ‘These are
gradually pushed posteriorly toward the body cavity, still retaining
their connexion with the pharynx by the air duct, and each consisting
of an enlarged terminal vesicle connected by a slender portion (the
beginning of the primary bronchus) with the undivided tracheal portion.
With continued growth each terminal vesicle divides again and again,
the result being a number of rounded vesicles connected with the pri-
mary bronchi by slender tubes, the secondary bronchi (fig. 264, B).

Fic, 264. Fic. 265.

Fic. 264.—Two stages in the development of the lung of the pig, ventral views, after
Flint. A, pig 5 mm. long; B, 18.5 mm. long. 26, gill pouch; d, /, v, dorsal, lateral and
ventral bronchi; oe, cesophagus; ¢, trachea.

Fic. 265.—Scheme of mammalian lung structure. ad, alveolar duct; b, bronchus;
fi, bronchiole; 7, infundibulum lined with alveoli.

By a continuation of this process tertiary and other bronchi are out-
lined, and also slender tubes, the bronchioles, to be described later,
which connect the terminal vesicles with the ultimate bronchi. Next,
the inner wall of each vesicle becomes divided into small chambers, the
alveoli, the whole vesicle now being known as an infundibulum.
The result of these many divisions is an enormous amount of internal
respiratory surface without great increase in the size of the whole
organ. It is to be noticed that in this subdivision the entodermal li-
ning takes the initiative, the outer (serous) surface showing but slight
signs of the internal modifications.

Each infundibulum has its own duct which, when smooth internally,
is called a bronchiole, when lined with alveoli, an alveolar duct.

RESPIRATORY ORGANS. 257

The alveoli of infundibulum and duct are lined with squamous
epithelium, and in the walls is an extensive network of capillary blood-
vessels. The lining cells of the bronchioles are cubical and those of the
bronchi ciliated columnar. There are no skeletal elements in the bron-
chioles, but the bronchi have small cartilages. in the walls, these ex-
hibiting a tendency in the larger tubes to approximate the _vings or
semi-rings of the trachea.

In their backward growth into the ccelomic region the lungs either
insinuate themselves dorsal to the lining of the dorsal side of the
body cavity (dipnoi and a few scattered forms) so that only their ventral
surface has a serous coat; or they grow out as free structures, covered
on all sides by the ccelomic epithelium, and are bound to the dorsal wall
by a mesenterial-like fold of varying extent. This outer coat of epithe-
lium has received the name of pleura, the term being extended in the:
case of the mammals to include the
whole lining of the pleural cavity,
separated from the rest of the ccelom
by the diaphragm (p. 135).

DIPNOI.—In Ceratodus there is a single
lung sac; Protopterus and Lepodosiren have Cc
paired lungs, the two being united in front at
the entrance of the air-duct. In all three the
inner surface is divided more or less regu- é;
larly into groups of alveoli, separated by A B
more prominent partitions. The pulmonary
arteries arise from the last efferent branchial
artery of either side, and hence the blood Hid, 266 —Ditiereut aypes of ane
supply, under normal conditions, is arterial phibian lungs. A, Necturus, without
and the lungs cannot act as respiratory @lveoli; B, alveoli in the proximal por-
organs. In times of drought (Protopterus) Benge frog, atyeolt roughstt:
or of foul water (Ceratodus) the gills no longer function and the pulmonary arteries
bring venous blood to the lungs.

AMPHIBIA.—In the lower urodeles the two lungs are elongate (the
left the longer) and are united at their bases, true bronchi being absent.
Internally they may be entirely smooth as in Necturus, or there may be
alveoli in the basal portion (fig. 266), the whole representing a terminal
vesicle either connected directly with the trachea (A) or by the interven-
tion of an alveolar duct (B). In the cecilians the left lung is very short;
the other elongates, with alveoli developed throughout. In the frogs
(fig. 266, C) the two lungs are distinct, and their walls are divided into

17

2 58 COMPARATIVE MORPHOLOGY OF VERTEBRATES,

a series of sacs or infundibula lined with alveoli. The infundibula
open into a central chamber, which, since it is ciliated and has numerous
glands in its walls, may be compared toa bronchiole. In the toads and
aglossa the alveoli are more extensively developed in correlation with
the more terrestrial habits.

Tt has recently been shown that a number of terrestrial urodeles are lungless in
all stages of development, and that no traces of larynx or trachea occur, even after
the gills are absorbed. In these species there is a great development of capillaries
in the skin and in the walls of the mouth and pharynx, the respiratory functions
being transferred to these parts. In the frogs the skin is also respiratory and it is
largely supplied by the cutaneous arteries which arise from the same arch as the
pulmonary arteries.

In the amphibia the air ducts enter the anterior end of the lungs,
but in the amniotes the lungs extend anteriorly to the entrance of the
bronchi which is on the medial side. This change is in part the result
of the transfer of the heart into the thorax, the position of the pulmonary
arteries forcing the bronchi toward the centre of the lungs. In the
amniotes, also, the ducts are characterized by the presence of cartilage
in their walls, so that they are true bronchi. These bronchi may also
extend inside of the lungs, often dividing into secondary and tertiary
bronchi inside them.

REPTILES.—In many reptiles (snakes, amphisbenans, many
skinks) the lungs are asymmetrical (left usually larger in snakes, right in
lizards) and exceptionally one may be absent in snakes. The internal
structure shows considerable variation. The simplest conditions are
found in the snakes and in Sphenodon (fig. 267), where the lungs consist
of a single sac lined with infundibula in the basal portion (snakes) or
throughout (Sphenodon). In the lizards (fig. 268) one or more par-
titions or septa extend from the distal wall of the lung nearly to the en-
trance of the bronchus, thus dividing the lung into chambers lined with
alveoli; while a part of the bronchus may extend (main bronchus,
fig. 268, B) to’the extremity of the lung. In the chameleons the septa
do not reach the distal wall so that the chambers communicate here as
well as at the proximal side, the result being that the bronchus enters a
cavity, the atrium, which connects with the chambers separated by the
septa, and these in turn open into a terminal vesicle, a condition recall-
ing the parabronchi of the birds, soon to be described. This resem-
blance is heightened by the development in these same lizards of long,
thin-walled sacs from the posterior part of the lung which extend among

RESPIRATORY ORGANS. 250

the viscera, even into the pelvic region. These air sacs, which are
used to inflate the body, foreshadow the similarly named structures in
birds. In the higher lizards (Varanus, fig. 268, B) and the turtles and
crocodiles there is no atrium, the bronchus, on entering the lung,
breaking up into several tubes. As these connect with smaller tubes
which lead to the infundibula, the whole lung has a spongy texture.

Fic. 267. Fic. 268.
Fic. 267.—Lungs of Sphenodon, after Gegenbaur; the left lung opened to show the
alveoli.;
Fic. 268.—A, left lung of Iguana; B, right lung of Varanus, after Meckel. 0, bronchus
c, connection between dorsal and ventral chambers; cb, chief bronchus; d, dorsal chamber;
1b, lateral bronchi; s, septa; sb, secondary bronchus; v, ventral chamber.

BIRDS.—In the birds the lungs are closely united to the ribs and
vertebral column and hence undergo less considerable changes of
shape than those of other groups. Each bronchus enters the meso-
ventral surface of the lung, immediately expanding into a sac, the
atrium or ventricle, and then continues as a main trunk, the meso-
bronchus, near the ventral side of the organ (fig. 269). In this course
it gives rise to the secondary bronchi (usually eight lateral ectobronchi
and from five to six dorsal entobronchi) and these in turn connect with
very numerous small tubes, the lung pipes or parabronchi. These
run approximately parallel to each other and connect with another
bronchus at the other end. Each parabronchus bears a number of
elongate diverticula radiately arranged (fig. 270), these having a nar-
rower basal portion and being branched and lobulated distally. The

260 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

Fic. 269. Fic. 270.

Fic. 269.—Diagram of structure of bird’s lung. a, connexions of bronchi with air
sacs; b, bronchus; e, entobronchi; ec, ectobronchi; i, infundibula; m, mesobronchus; ,
parabronchi.

Fic. 270.—A, lung pipes of bird from a corrosion preparation; B, section of lung pipe
with radiating infundibula, after Schulze. .

Fic, 271.—Diagram of the relations of the chief air sacs in a bird, lung tissue shaded.
a, axillary sac; cb, abdominal sac; ai, anterior intermediate sac; 6, bronchus; pb, pre-
bronchial sac; pi, posterior intermediate; sb, subbronchial sac; ¢, trachea.

RESPIRATORY ORGANS. 261

parabronchi are to be compared to bronchioles, the diverticula to
infundibula. ~

The mesobronchus and usually four other bronchi do not stop at
the lung wall, but are continued as thin walled vesicles, the air sacs,

Fic. 272.—Air sacs of pigeon, after Bruno Muller. c, c*, intertransverse canal;
da‘, da”, axillary diverticulum and its ventral outgrowth; dc, diverticulum costale; dfa,
dfp, divert. femorale anterior et posterior, dot, divert. cesophago-tracheale; ds, div. sub-
scapulare; dst, div. sternale; pc, preacetabular canal; sad, sas, saccus abdominalis dexter et
sinister; sc, saccus cervicalis; sia, sip, saccus intermedius, anterior et posterior.

structures peculiar to birds (and in a slight extent to chameleons) and
occurring in all recent species. Each sac (figs. 271, 272) has received
several names. The sub-bronchial, anterior to the furcula, is usually
unpaired. The cervical, lateral to the first, lies at the base of the

262 COMPARATIVE MORPHOLOGY OF VERTEBRATES,

neck, and gives off a branch which forms an axillary sac in the axillary
region. Other sacs lie in the abdomen, lateral to the viscera, and are
called the anterior intermediate, posterior intermediate and
abdominal, the latter extending into the pelvis. From these air sacs
slender diverticula; not shown in the figures, extend among the viscera
and into certain of the bones. The pelvis, humerus, coracoid, sternum
and ribs most frequently contain prolongations of the air sacs—are
pneumatic—less frequently the femur, furcula and scapula.

The functions of the air sacs are not certainly known. The fact that the walls
are supplied with blood by branches from the aorta negatives the idea that they are
respiratory. It has been suggested that they are concerned with the maintenance
of the equilibrium of the body during flight and that they also lessen the specific
gravity of the body. More plausible is the view that by the motion of the parts
about them they aid in the inspiration and expiration of air, especially during flight,
thus allowing the thoracic framework to remain rigid as an attachment of the
muscles, and at the same time causing the air to pass twice over the respiratory
surfaces of the lungs. The bones of the fossil bird Archeeopteryx were not pneu-
matic but those of some of the dinosaurian reptiles were.

MAMMALS.—The general structure of the mammalian lung was
outlined above (p. 256). The external shape is largely due to the
position in the pleural cavity, where it has to fit itself around the peri-
cardium, while it is flattened or truncate behind as a result of the
presence of the diaphragm. Ina number of mammals (cetacea, sirenia,
horse, rhinoceros, Hyrax, etc.) both lungs are undivided, but usually
one or both are subdivided into lobes (the larger number in the right
lung), there being as many as five or six lobes in some species. In-
ternally there is a main bronchus from which dorsal and ventral
secondary bronchi arise, the ventral being the stronger. The bronchi
are supported and kept open by cartilages, rings in the larger, scattered
pieces in the smaller trunks. Frequently the bronchi are grouped as
eparterial and hyparterial (fig. 264), accordingly as they lie above
or below the pulmonary artery, but the distinction has little morpho-
logical value. Eparterial bronchi may be lacking or there may be
one or two in each lung.

The phylogenetic history of the lungs is uncertain, one view being that they
have arisen from the air bladder of the fishes, the other being that they are modified
gill pouches, which, instead of growing laterally and fusing with the ectoderm,
have extended caudally and have encroached upon the ccelom. In favor of the
former view are the double condition of the bladder in some ganoids, with alveolar
walls like those of the lungs of higher vertebrates, and the peculiarities of the pneu-

RESPIRATORY ORGANS. 263

matic duct and the blood supply in Polypterus. On the other hand the dorsal
position of the opening of the duct into the cesophagus and the arterial supply from
the aorta in fishes are difficult to reconcile with the conditions obtaining in the
tetrapoda.. Favoring the gill-pouch theory are the following facts. The lungs are
paired outgrowths from the pharynx immediately behind the last gill cleft; the
blood supply can readily be derived from the branchiate condition; while the skeletal
supports of the larynx have the appearance of rudimentary visceral arches, and the
muscles of the region are modified from those of the gill arches.

The mechanisms by which air is caused to enter the lungs (in-
spiration) or is expelled from them (expiration) differ considerably
in the various classes. In the amphibia air is drawn into the mouth via
the nares by depressing the floor of the oral cavity. Then, the nares
being closed by small muscles, the contraction of the mylohyoid muscle
forces the air into the lungs. Expiration is affected in part by the
elasticity of the lungs, in part by the muscles of the body wall. In
most reptiles the position of the ribs is altered by the action of the
intercostal muscles, thus altering the size of the pleuro-peritoneal
cavity, to accommodate which air is drawn into and expelled from the
lungs. It is difficult to understand how inspiration is effected in the
chelonia, but transverse muscles run ventral to the lungs, and these by
their contraction, expel the air. In the birds the lungs are attached to
the ribs and vertebrae, so that any motion of the latter necessitates a
change in shape and size of the lungs. In addition the air sacs, as
noted above, may play a part in the movement of the air.

In the mammals the ribs are hinged at an oblique angle to the verte-
bral column, the angle being changed accordingly as the intercostal
muscles are contracted or relaxed, and thus the size of ‘the thoracic
cavity is increased or dimininshed. Then the diaphragm (p. 135)
also plays an important part in this alteration in size. This transverse
muscle forms a complete partition between pleural and peritoneal
cavities, projecting into the former like a dome when relaxed. When
it contracts it flattens, thus increasing the size of the pleural cavity
and drawing air in through the trachea. The abdominal muscles
also have their effect. Expiration is caused in part by the action of the
intercostal and abdominal muscles, in part by the elastic tissue and
smooth muscles in the lungs themselves.

ACCESSORY RESPIRATORY STRUCTURES.

Allusion has already been made to the pharyngeal and dermal
respiration of the amphibia (p. 258). There are several fishes in which

264 COMPARATIVE -MORPHOLOGY OF VERTEBRATES.

the hinder part of the alimentary tract is also respiratory. ‘Thus in
Cobitis water is drawn in and expelled from the anus, and the posterior
half of the digestive canal is richly vascular and is the seat of consider-
able respiration.

Before hatching or birth the lungs of the amniotes are unable to
function, while a certaim amount of oxygen is necessary for the devel-
opment and the carbon dioxide formed must be carried away. This
respiratory function is assumed by the allantois. The allantois is
a ventral diverticulum from the hinder part of the alimentary canal,
which during foetal or embryonic life, acquires a relatively enormous
development. It extends beyond the body limits and ‘in reptiles and
birds comes into close relations with the porous egg shell, while in the
mammals it plays an important part in the formation of the placenta.
In all these the allantois is extremely vascular, developing a rich net-
work of blood-vessels close to the shell (sauropsida and monotremes)
or to the walls of the maternal uterus, (mammals) which serves for
the rather limited exchange of gases necessary for the young. After
free life begins the allantois is either absorbed (sauropsida) or is lost with
the rest of the placenta (mammals), only the basal part persisting as the
urinary bladder, described in connection with the urogenital system.

ORGANS OF CIRCULATION.

The functions of the circulation are two-fold: to carry food and
oxygen to the tissues and organs of the body and to remove the waste
from them. In addition it has been made probable that every activity
of the body results in the formation of peculiar substances—activators—
which have fixed and definite effects upon the various organs. These
activators pass into the blood and form the stimulus which may cause
other organs or cells, remote from the place where the activator is formed,
toact. This subject is a new one and much may be expected from it in
the future.

The structures concerned in the circulation are two fluids, the blood
and the lymph; and the vessels (vascular system) in which the fluids
circulate, certain parts of the vessels being specialized (hearts) for the
propulsion of the blogd and lymph. A blood heart occurs in all verte-
brates in connexion with the blood circulation; most vertebrates have
lymph hearts in connexion with the lymph vessels, but in the higher
groups the flow of the lymph is due to the blood pressure and also to the
motion of the parts through which the lymph vessels course.

CIRCULATORY ORGANS. 26 5

BLOOD AND LYMPH.

The two circulating fluids, blood and lymph, are much alike.
Each consists of a fluid portion, the plasma, in which float numer-
ous solid particles, the corpuscles. The plasma is colorless or
slightly yellow and can be separated by clotting into a solid part,
fibrin, and a fluid, the serum, which is, under ordinary circum-
stances, incapable of clotting again. The lymph plasma contains
-less of the fibrin-forming substances (fibrinogen) than does the blood
plasma. The composition of the plasma is very complex. Besides
water it contains proteids, extractives, salts, and a number of less-
known substances, internal secretions, enzymes, etc. The plasma
can also absorb a considerable amount of carbon dioxide. It serves
to carry nourishment to the tissues and takes away from them the
waste of metabolism.

The corpuscles are of three kinds, erythrocytes, leucocytes and
blood plates. Only the leucocytes occur in the lymph while the
blood contains all three.

The erythrocytes, or red corpuscles give the blood its color.
They have fixed outlines and are flattened oval discs in the non-
mammals and the camels, circular biconcave discs in the other mam-
mals, and in all except the mammals they are nucleated throughout
their existence. They owe their color to an iron-containing proteid,
hemoglobin, which readily combines with oxygen and carbon dioxide
and as readily gives up these gases in places where they are scanty.
This renders the erythrocytes the respiratory elements of the blood.

It has recently been stated that the erythrocytes of the mammals are hat-
shaped, (hollow cones) while inside the blood-vessels and that they assume the
biconcave shape after leaving them. This account has been disputed.

The size of the erythrocytes varies in different vertebrates, being the largest
in the amphibia (Amphiuma) and smallest in the vertebrates (musk deer). A
few measurements are giving here in microns (0.001 mm.). Where two dimen-
sions are given they are the length and breadth of the oval corpuscles. Musk
deer, 2.54; man, 7.74; hen, 7x12p; carp, gxi5#; frog, 16x25; Necturus,
31x58.54; Amphiuma, ?x75H.

In the higher vertebrates the red corpuscles arise by division of giant cells
(erythroblasts) in the red bone marrow, but in the young and at times of great

depletion of the blood new red corpuscles may be formed in the spleen and the
liver. At first all are nucleated but in the mammals the nucleus is soon lost.

The leucocytes or white corpuscles (divided accordingly as they

266 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

occur in blood or lymph into leucocytes and lymphocytes) are very
variable in shape (amoeboid) and may be uni- or polynucleate. By
their amoeboid motions they are able to pass through the endothelial
walls of the capillaries and to pass among the cells of the different
tissues, hence they are often called wandering cells. They have the
power of ingesting foreign bodies which renders them of value in
combating pathogenic organisms; and they also aid in the absorbtion
of fats and peptones. :

The blood plates are very little known. Their size is less than
that of the red corpuscles and they rapidly degenerate when drawn
from the vessels. They are circular or elliptical in outline.

THE BLOOD-VASCULAR SYSTEM.

The blood-vessels include the arteries, which carry the blood from
the heart to all parts of the body; the veins, which bring it back, and the

Fic. 273.-—Embryonic circulation of snapping turtle, Chelydra, showing relations of
allantois, after Agassiz and Clarke. a, right auricle; al. allantois; av, allantoic vessels; c,
caudal vein; da, dorsal aorta; h, hypogastric artery; j, jugular; /, liver; oa, ov, omphalo-
mesenteric artery and vein; pc, post-cardinal; sc, subcardinal vein; uv, umbilical vein; Wy
Wolffian body; y, yolk sac.

capillaries which connect the ends of the arteries and veins, for the
system is closed, and there is a complete circulation.
Since all transfer of gases and nourishment takes place through the
‘capillaries, these vessels have extremely thin walls, consisting of a
single layer of squamous epithelium, the so-called intima. Usually, as

CIRCULATORY ORGANS. 267

the name implies, the capillaries are very small in diameter, but atten-
tion has recently been called to the sinusoids, vessels with similar walls
but larger in diameter, which are noticeable in some developing organs,
especially the liver. Here also must be mentioned the retia mirabilia,
places where an artery or vein suddenly breaks up into a network of
small vessels (often capillary) which unite again, as in the glomeruli of
the kidney, to form a vessel as large as before. In the lymph nodes
there are similar networks of the lymph vessels.

Arteries and veins (fig. 274) are larger than the capillaries and they
have their walls strengthened outside of the intima by layers of smooth

a

Fic. 274.—Diagram of artery or vein. At the left the intima alone; covered in the middle
by the muscularis, and at the right with the adventitia added.

muscle fibres (muscle wall) and connective tissue, mostly elastic (ad-
ventitial wall). Since the arteries are subjected to greater pressure
than the veins their walls are relatively much thicker, but in other re-
spects the two are much alike, except that valves to prevent the back-
flow of the blood, may occur in the veins, especially those that are
vertical in the normal position of the animal (legs).

It has been suggested, with much plausibility, that the main blood-vessels are
the remnants of the segmentation cavity, which elsewhere has been obliterated by
the increase of the mesoderm. As will be recalled (p. 121) the mesothelium grows
toward the middle line above and below the digestive tract, thus tending to narrow
the segmentation cavity in these regions into two longitudinal tubes. ‘The epimeral
part of the mesothelium divides into somites, and of course the segmentation cavity
extends between these, and as these somites grow downward, these lateral exten-
‘sions of the segmentation cavity are carried ventrally, so that at last they form a
series of pairs of transverse vessels connecting the longitudinal trunks, thus forming
the vessels of the somatic wall. Other tubes, connecting the dorsal and ventral
trunks, would form between the two walls of the mesentery and between the
splanchnic mesoderm and the entoderm, thus outlining the vessels of the alimentary
tract.

Even more speculative is the suggestion that the original circulation was lymph-
oidal and that the blood circulation is a specialization of a part of this, the definitive
lymph vessels being the unmodified part of the primitive system of vessels.

268 COMPARATIVE MORPHOLOGY OF VERTEBRATES.
@

An appreciation of this probable ancestral condition makes the
actual structures more easily understood. In development much of
this phylogenetic history has been lost, while other parts have been
masked by the development of additional vessels. Many vessels,
which theoretically should arise as spaces between other tissues, are
actually formed as solid cords of cells, which are later canalized and
converted into tubes. Again, separate vessels of the embryo may fuse
during development into a single vessel of the adult.

The chief features of the theoretically primitive condition may be
summarized here (fig. 275). A dorsal tube carries the blood toward
the tail. From this transverse vessels—right and left, somatic and
splanchnic—arise, which connect with two ventral longitudinal tubes,

Fic. 275.—Diagram of the primitive vertebrate circulation. @, anus; al, alimentary
canal; av, abdominal vein; ca, cv, caudal artery and vein; da, dorsal aorta; h, heart; ic,
intercostal (somatic) transverse vessels; iv, intestinal vessels; m, mouth; sz, subintestinal
vein; va, ventral aorta.

one in the wall of the alimentary tract and extending forward to its junc-
tion with the second which runs in the ventral body wall, a single tube
coursing from the point of union to the anterior end of the body.
In Amphioxus various parts of this system develop muscular walls and
act as pumping organs. In the vertebrates, so far as the blood system
is concerned, there is a single pumping organ, the heart (the portal
heart of the myxinoids may be ignored in this general statement).
The heart arises in the ventral tube beneath the pharynx and anterior
to the junction of the two tubes. It marks the line of division of the
transverse tubes into ascending and descending, those in front of the
heart carrying the blood upward while those behind return it to the
ventral vessels which carry it forward. The transverse vessels are not
continuous, but capillaries intervene between their dorsal and ventral
moieties.

The Embryonic Circulation.

In all vertebrates a series of blood-vessels is laid down in the early
stages, forming a framework around which the rest of the circulation is

CIRCULATORY ORGANS. 2 69

arranged. Hence these parts are first described, the additions and
modifications being taken up later.

Tue HEarr.

The heart, the central organ for the propulsion of the blood, lies in a
sac, the pericardium, a part of the coelom, which is ventral to the
pharynx or cesophagus and is partially filled with a serum, the per-

Fic. 276. Fic. 277.

Fic. 276.—Diagram of the formation of the heart tube, showing the descending meso-
thelial plates from above. c, ccelom; cd, first appearance of the Cuvierian ducts; h, grooves
to form heart and ventral aorta; /, liver; m, mouth; ma, mandibular artery; om, omphalo-
mesenteric veins; so, sp, somatic and splanchnic walls of ccelom.

Fic. 277.—Early stage of the heart; the descending plates of fig. 276 have met, forming
the heart and ventral aorta. c, peritoneal ccelom; ~, pericardial coelom; ppc, pericardio-
peritoneal canals; other letters as in fig. 276.

icardial fluid. In the heart we have to consider its epithelial lining
(endocardium), its muscular walls (myocardium) and its covering
epithelium and connective tissue (epicardium).

The development of the heart is simplest in the vertebrates with
relatively small yolk. It is more modified in the elasmobranchs,
where the head is early completed below, and is most modified in the
large yolked eggs of the sauropsida and in the mammals where the yolk
sac is large, though 'the yolk is small. The following account is based
upon the development in the amphibia:

From just behind the point where the first or spiracular gill cleft
is to form, backward to the region just in front of the anlage of the

270 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

iver the hypomeral portions (lateral plates) of the coelomic walls grow
ventrally beneath the alimentary canal, in much the same way as
farther back (p. 121). In these descending plates splanchnic, mesen-
terial and somatic walls, as well as the ccelomic cavity can be recognized.
As they descend, cells which have received the name of vascular
cells appear between the ccelomic walls and the entoderm. The
origin of these has been in dispute, but the present evidence favors
their origin from the mesothelium. Some of these vascular cells are
more dorsal and aid in the formation of the dorsal blood-vessels, while
the ventral (fig. 278, A) contribute to the heart and the ventral trunks.

elofo[eTslelsleleal TIS

Wr

sesso

Fic. 278.—Diagrammatic cross sections of developing heart. Compare with figs.
276 and 277. In A the descending mesothelial plates have nearly met, a numbez of
vascular cells between them. In B the plates have met ventrally, forming the ventral
mesocardium; most of vascular cells utilized in forming the endocardium. InC the plates
have met dorsally, with the resulting dorsal mesocardium; the ventral mesocardium has
disappeared, placing the two ccelomic cavities, now the pericardium, in communication.
c, coelom; ec, ectoderm; en, entoderm; end, endocardium; m, edges of descending meso-
thelium; ~, pericardium; v, vascular cells.

The descent of the lateral plates continues until their lower edges
meet just dorsal to the ventral ectoderm and the ventral parts of the
mesenterial regions of the two sides fuse to a vertical plate, the ventral
mesocardium (fig. 278, B), above which is a groove in which the
ventral vascular cells lie. Next, the edges of the plates crowd in above
the groove and meet to form a dorsal mesocardium, the result being
that groove is converted into a tube. The mesocardia disappear early,
the ventral usually being lost before the dorsal is formed (fig. 278,
C). The walls of the tube, which are to form the muscular and epicar-
dial walls of the heart, are called the myoepicardial mantle.’ The
vascular cells, which are enclosed in this mantle, gradually arrange
themselves as a continuous sheet, the endocardium, which lines the
future heart.

With the disappearance of the mesocardia the ccelomic spaces on
the two sides communicate with each other so that the myoepicardial
mantle lies free on all sides in a ccelomic sac, being bound to the walls
only at the two ends. ‘This cavity or sac is the pericardial cavity,

1 The fact that the heart muscles arise from this layer—mesothelial and yet not myotomic
—partly explains the differences between cardiac and other muscle.

ca

CIRCULATORY ORGANS. 271

the extent of which is decreased by the fusion laterally of the somatic
and splanchnic walls (fig. 277).

In front of and behind this tube the descending lateral plates are
kept from meeting in the middle line by the projections for the mouth
and liver (fig. 276). Vascular cells, however, are formed in these
regions and these furnish the lining of tubes on either side, arising
in the edges of the lateral plates. These tubes consequently diverge
from the myoepicardium in front and behind and form the first stages of
the vessels connected with the heart, the anterior pair giving rise to the
mandibular arteries, the posterior to the omphalomesenteric veins.
At about the same time a transverse tube appears on either side, which
connects with the heart tube, just in front of the division into omphalo-
mesenterics (fig. 276). These transverse vessels continue laterally
between the lateral plate and the ectoderm, forming the venous trunks
known as the ducts of Cuvier (trunci transversi), the other rela-
tions of which will be described later. The ccelom on either side of
the heart is restricted behind by the ridge formed by the Cuvierian
ducts (fig. 277); with growth this interruption grows larger, the result
being a transverse partition, the septum transversum, which bounds
the pericardial cavity behind and separates it from the rest of the ccelom,
the peritoneal cavity. At first this septum is incomplete, and in the
elasmobranchs it never closes dorsally to the omphalomesenterics, but
leaves two openings, the pericardio-peritoneal canals (fig. 277).
Elsewhere the pericardial and peritoneal cavities are entirely separate
in the adult.

In teleosts and amniotes, where the early embryo is closely appressed to the
very large yolk sac, the development of the heart is modified. At first the pharynx
is not complete below but communicates ventrally with the yolk. Hence the two
hypomeres are prevented, for a time, from meeting ventrally. Each, however, is
accompanied by its vascular cells; its edge becomes grooved and the grooves are
rolled into a pair of tubes, lined with endocardium, so that for a time the anlage of
the heart consists of two vessels, each connected in front and behind with its own
mandibular artery and omphalomesenteric vein, and is surrounded with its
pericardial sac. Later the two tubes approach and fuse, with the formation of
mesocardia as before: these latter soon disappearing, leaving the whole much as
in the small yolked forms.

In the early stages the pericardium is relatively large, but it doés
not keep pace with the growth of the other parts, until finally in the adult
it is only large enough to accommodate the changes in size and shape

272 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

of the heart, due to its alternating enlargement (diastole) and contrac-
tion (systole).

While the mesocardia are present the cardiac tube is a straight
canal, lying in the pericardial sac and connected with its walls in front
and behind. With their disappearance the tube increases in length
more rapidly than the pericardium, the result beng the flexure of the
tube on itself, something like the letter «©, the flexures being largely
in the vertical plane. At the middle point of the flexure the tube re-
mains small, forming the atrio-ventricular canal, but in front of
and behind this the walls become thickened and the lumen enlarged.
The posterior and dorsal of the chambers thus formed becomes the
atrium (auricle), the ventral and anterior the ventricle of the heart.

The atrium is bounded posteriorly by a constriction, behind which
the tube expands into another chamber, the sinus venosus, which
extends back to the posterior wall of the pericardium and receives the
ducts of Cuvier and the omphalomesenteric veins. The ventricle, also,
does not reach the anterior wall of the pericardium, but the anterior
part of the heart tube forms a smaller trunk, the truncus arteriosus,
while from the pericardium to the mandibular arteries is an arterial
vessel, the ventral aorta.

Muscles, as stated above, are developed in the wall of the heart,
but to an unequal extent in the different parts, being scanty in the
sinus venosus, and most abundant in the ventricle. Folds or valves of
the endocardium appear in places at an early date and are so arranged
that they permit the blood to flow forward but prevent any backflow.
In the base of the truncus these valves take the form of pockets on the
walls, there being several (3-5) rows with several valves in a row in the
elasmobranchs (fig. 287, A) and ganoids. This valvular part of the
truncus is called the conus arteriosus. In other vertebrates the conus
is reduced to a single row of valves.

Valves also occur in the atrio-ventricular canal (fig. 279) but here
the pocket-like condition is impossible. The folds extend from the
canal into the ventricle and are prevented from folding back into the
atrium, under the heavy ventricular pressure, by ligaments—chorde
tendineze—which extend from the edges of the valves to the opposite
wall of the ventricle, and are kept taut during systole by short muscles
(columne carnea) atthebase. Othervalves, moresimplein character,
occur around the opening from the sinus into the atrium and, in some
vertebrates, where the hepatic veins empty into the sinus,

CIRCULATORY ORGANS. 273

In many fishes the conus arteriosus is followed by a strongly muscu-
lar region, the bulbus arteriosus (fig. 287, B) which has muscles like
those of the heart (p. 125), while the truncus in front of this has smooth
muscles, like the rest of the blood-vessels. Hence conus and bulbus
are to be regarded as a part of the heart, while the region in front is a
part of ventral aorta to be described below.

When first formed, the heart lies
close behind the mandibular artery (first
aortic arch to be described below), but
as other vessels are formed it is forced
farther back into a position, in the lower
vertebrates, ventral to and a little behind
the pharynx, but in the adult tetrapoda
it is carried back, as a result of unequal
growth even into the thorax, the extreme
of migration being seen in the giraffe
and the long-necked birds. ;

Although all of the blood of the body Fic. 279.—Diagrammatic cross
passes through the heart at short inter- section of heart showing atrio-

on acs ‘ ventricular valves; a@, atrium; ct,
vals, this is not sufficient for the nourish- chorda tendinea; m, muscula pap-
ment of that organ. Therefore its mus- rare a a hy Hone
cles are usually supplied with blood
through coronary arteries which arise from the aortic arches and
run back along the truncus arteriosus to reach the atrium and ventricle.

THe ARTERIES.

Aorta and Aortic Arches.—The ventral aorta is the trunkin front
of the pericardium, extending from the truncus arteriosus to the mandib-
ular artery (first aortic arch). It runs, not through a cavity, but be-
tween muscles and through connective tissue. The mandibular arter-
ies continue dorsally on either side of the pharynx until they reach its
dorsal surface. With development, the ventral aorta elongates and at
the same time other aortic arches arise between the mandibular arteries
and the pericardium, these extending dorsally until they meet the back-
ward prolongations of the first, thus forming a pair of longitudinal
tubes, dorsal to the alimentary tract, the radices aorte.

The number of pairs of aortic arches varies with the number of gill
clefts, the vessels coursing between the clefts. The number of arches

18

274 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

is greatest in the myxinoids, where the number of clefts varies (p. 239);
seven or eight in the notidanid sharks; and, as recent investigations
tend to show, probably six in the embryos of all other vertebrates. The
history of these arches differs greatly in the different classes (fig. 280),
there usually being a reduction in number by the more or less complete

Fic. 280.—Modifications of the aortic arches in different vertebrates, after Boas.
A, primitive scheme; B, dipnoan; C,'urodele; D, frog; E, snake; F, lizard; G, bird; H,
mammal. ¢, cceliac artery; da, dorsal aorta; db, ductus Botallii; ec, ic, external and internal
carotids; p, pulmonary artery; s, subclavian; va, ventral aorta. Vessels carrying venous
blood black, those which disappear, dotted.

abortion of one or more pairs as well as a modification of those that per-
sist, accompanying changes in the respiratory system.

With the development of gills (ichthyopsida) each aortic arch be-
comes divided into two portions, an afferent branchial artery convey
ing blood from the ventral aorta to the gills and an efferent branchial
artery (sometimes called a branchial vein) carrying it from the gills

CIRCULATORY ORGANS. 275

to the radix aorte (fig. 281). These two vessels parallel each other for
a part of their course and are connected with each other by numerous
capillary loops which run through the gill filaments. In passing
through the gills the blood loses its carbon dioxide and takes up oxygen,
and thus becomes converted from venous to arterial blood. In the am-
niotes afferent and efferent branchial arteries are never differentiated, the
aortic arches being continuous from ventral aorta to the radices aorte.

The first of these arches (the mandibular arteries) never forms
afferent and efferent portions since no gills are ever developed in their
region. From each half of this arch an artery, the external carotid,

da

TT TC OTTO

Fic. 281.—Scheme of branchial circulation in elasmobranchs. a, atrium; aa, afferent
branchial arteries; av, abdominal vein; c, gill clefts; cc, common carotid; da, dorsal aorta;
ea, efferent branchial arteries; hv, hepatic vein; ic, internal carotid; ec, external carotid
artery; 7, jugular vein; /, liver; pc, postcardinal vein; sc, subclavian vein; sv, sinus venosus;
tr, truncus arteriosus.

extends forward to supply the lower and a part of a upper jaw, while an
internal carotid artery forms an extension forward of each radix and
supplies the brain and face. Later their relations are such that the
carotids appear to arise from the first of the functional arches.

The radices aorte of the two sides meet and fuse behind the last
aortic arch, forming a single tube, the dorsal aorta, which runs in the
middle line, dorsal to the alimentary tract, to the end of the body. The
fusion may also extend forward from the last aortic arch, involving the
whole of the radices.

From the dorsal aorta segmental arteries extend laterally between
the somites, these forming the upper halves of the transverse somatic
vessels alluded to on page 268. To these the name of intercos-
tal arteries, derived from human anatomy, is given. Ventral to them
the aorta also gives off other arteries (nephridial arteries) to the excre-

276 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

tory organs. Other arteries, arising from the dorsal aorta, run ventrally
into the mesenterial structures and supply the alimentary canal and
other viscera. Two pairs of these, the.omphalomesenteric (omphalo-
mesaraic) and the hypogastric arteries, may be mentioned at present.
The first of these arise in the trunk region, pass on either side of the
intestine, and finally empty on the lower side of the body into the om-

Fic. 282.—Diagram of the circulation in an early stage of a small yolked vertebrate
(amphibian). , anus; ca, cv, caudal artery and vein; da, dorsal aorta; de, Cuverian duct;
ec, external carotid; h, heart; ha, hypogastric artery; i, intestine; ic, internal carotid; 7,
inferior jugular; j, superior jugular; J, liver; m, mouth; oma, omv, omphalomesenteric
artery and vein; pc, postcardinal vein; si, subintestinal vein; 1-6, aortic arches.

phalomesenteric veins, soon to be described. The hypogastric arteries
arise from the dorsal aorta at the junction of trunk and tail and pass on
either side of the intestine, to meet posterior continuations of the
omphalomesenteric veins, here known as the subintestinal veins.
Behind the origin of the hypogastric arteries the dorsal aorta is called
the caudal artery (figs. 275, 282).

VEINS.

Behind the pericardium the edges of the descending lateral plates
(p. 270) are kept from meeting by the anlage of the liver (figs. 276,
277). The edges of the plates become grooved just as in front and each
groove becomes rolled into a tube, lined with vascular cells, so that two
vessels, the omphalomesenteric veins, extend backward from the heart,
around the liver, to meet the omphalomesenteric arteries already de-
scribed. Behind the connection of the omphalomesenteric arteries and
veins the pair of vessels continue back, ventral to the alimentary canal
as the subintestinal veins, until just behind the anus they fuse into a
median tube, the caudal vein, which extends the length of the tail.

The two subintestinal veins soon fuse to a single median vessel (fig.
283, B) save for a loop around the anus connecting it with the caudal
vein. The right omphalomesenteric vein disappears except for a short
distance between the sinus venosus and the liver, leaving the left as the

CIRCULATORY ORGANS. 277

trunk connecting the posterior parts with the heart, this passing along
the left side of the liver (fig. 283, B).

Portal Circulation.—As the liver develops from the simple sac it is ©
at first, into the compound tubular condition (p. 233), the left omphalo-
mesenteric breaks up into a sort of rete mirabile of sinusoids, which
ramify among the liver tubules, finally connecting with both omphalo-
mesenterics on the anterior side of the liver (fig. 283, B). As the liver
increases in size the network of sinusoids increases in complexity,
supplying all of the tubules. For a time the left omphalomesenteric

lo

Fic. 283.—Three stages in the development of the hepatic portal system. A, primitive;
B, liver tubules beginning to develop, right omphalomesenteric interrupted; C, definitive
condition, liver not indicated. dc, Cuverian ducts, hp, hepatic portal vein; hv, hepatic
vein; /, liver; lo, ro, left and right omphalomesenteric veins; sz, subintestinal veins; sv, sinus
venosus.

retains its primitive importance on the side of the liver and is known
as the ductus venosus (Arantii), but soon this preeminence is lost
and all blood coming from behind passes through the network of cap-
illaries in the liver before it enters the heart (fig. 283, C). Such a
capillary circulation occurring in the course of a vein is known as a
portal system, and this one occurring in the liver is the hepatic portal
circulation. It consists of the vessels bringing the blood to the liver
(portal vein)—a part of the original omphalomesenteric—the capil-
lary vessels and the bases of both omphalomesenterics, now known as
the hepatic veins, which convey the blood from the liver to the heart,

In eggs with a large yolk (elasmobranchs, sauropsida) the presence of ‘this
large food supply exercises a modifying influence on these ventral veins (fig. 284).
From the junction of the omphalomesenteric and the subintestinal. veins a pair of
large vitelline veins run out into the yolk sac, over the yolk, and play a large part
‘in the transfer of material to the growing embryo. The distal ‘parts of these veins
follow the margin of the yolk sac, forming:a tube (sinus terminalis) into which

278 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

smaller veins empty. Blood is brought to the yolk by the omphalomesenteric
arteries, which are also distributed to the yolk sac, dividing up distally into a net-
work of capillaries connecting distally with the vitelline veins. By these the blood
is carried to the liver and through the portal circulation to the heart. In the mam-
mals a similar vitelline circulation is developed, but as the yolk sac contains no
yolk, it is of minor importance.

In the amniotes an outgrowth, the allantois (p. 318), arises as a diverticulum
from the hinder end of the alimentary canal, increases in extent, growing downward
and carrying the ventral body wall before it. Branches of the hypogastric arteries,
known as the allantoic arteries, extend into it and are connected by capillaries

da pc ca
; —
; Ba tha c
dc
oma an
UG ys \

h. a

y
J Vi x Bi st y
Fic. 284.—Diagram of embryonic circulation in a large-yolked vertebrate; compare
with fig. 282. aa, aortic arches; al, allantois; an, anus; ca, cv, caudal artery and vein; da,
dorsal aorta; dc, Cuverian duct; h, heart; ha, hypogastric (allantoic) artery; z, jugular vein;

1, liver; oma, omv, omphalomesenteric artery and vein; pc, postcardinal vein; sz, subintes-
tinal vein; st, sinus terminalis; va, ventral aorta; y, yolk; ys, yolk stalk.

with umbilical veins which arise from the subintestinal vein behind the vitelline
veins. There thus is formed an allantoic circulation which is both respiratory
and nutritive in character. In the reptiles both of the umbilical veins persist
through the fcetal life (only one shown in fig. 273), but in birds and mammals one
aborts, leaving the other as the efferent vessel of the allantois. With the end of
foetal life (at hatching or at birth) both the vitelline and the allantoic circulations
disappear, leaving only inconspicuous rudiments.

The entrance of the Cuverian ducts into the heart was mentioned
on page 271. These ducts are a pair of transverse vessels which enter
the sinus venosus, one from either side, and, together with the hepatic
veins, mark the posterior limit of the heart. Each develops outside
of the somatic wall of the hypomere and extends dorsally until it reaches
the level of the top of the ccelom (fig. 282). In this course, in the
fishes, each receives an inferior jugular vein which comes from the
head, bringing back blood from the muscles of the lateral and ventral
branchial regions. At its dorsal end each Cuverian duct divides into

CIRCULATORY ORGANS. 279

the two cardinal veins, an anterior cardinal (superior jugular) and
a postcardinal vein (fig. 285), which belong to the dorsal half of
the body. The superior jugular comes from the head, dorsal to the
gill clefts and brings blood from the more: dorsal regions. Since the
inferior jugulars are found only in fishes and salamanders, the anterior
cardinal is usually called simply the jugular and that usage will be
followed here.

The postcardinals are closely related in development to the nephric
system, and keep pace with its development backward, so that they
eventually reach the loop which the caudal and subintestinal vein

Fic. 285.—Developing anterior veins of Scyllium embryo, 26 mm. long; after Grosser.
b 1-5, veins of the visceral arches; cd, Cuverian duct; /, vein of hyoid arch; 2), inferior jugu-
lar; m, vein of mandibular arch; os, orbital sinus; sv, segmental veins; vca, vcp, pre- and
post-cavas; JIJ—X, cranial nerves; 2-8, spinal nerves.

makes in passing around the anus. They run just above the dorsal
side of the ccelom and dorsal to the nephridial arteries (p. 275). They
are preeminently the blood-drainage system of the early excretory
organs and they retain that function throughout life in the lower
vertebrates.

Closely associated with the postcardinals are the subcardinals.
As the mesonephroi (see Excretory Organs) reach the hinder end of the
coelom, the caudal vein loses its primitive connection with the subintesti-
nal vein and becomes connected with a pair of vessels, the subcardinal
veins, which develop between the mesonephroi and ventral to the nephrid-
ial arteries (fig. 286, B). The blood from the tail now goes through
the subcardinals and from them into the excretory organs, passing
through a system of capillaries, to be gathered again in the postcardinals

280 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

and by them to be returned to the heart. Here, then, there is
another portal system (p. 277), the first renal-portal system,
which may be modified later as will be described below.

\

liom ~pe-| 5 A
pn bk Bias? ‘
Be EE Ae
Z 4 mh
2 ab 4G
ZA ~ 4
, mr ae
Si Z ge
E Avei ;
Alla pe
ca
ca

Fic. 286.—Scheme of development of the principal veins. uw, anus; az, azygos major;
¢, coronary vein; ca, caudal vein; cd, Cuvierian duct; e7, external iliac; g, gonads; ge,
genital (spermatic, ovarian) vein; h, hepatic veins; ht, heart; 7, ischiadic; 7, jugular; h,
left innominate; mn, mtn, meso- and metanephroi; om, omphalomesenterics; p, postcava;
pe, postcardinal; pn, pronephros; pr, precava; 7, renal; 77, right innominate; s, subclavian;
se, subcardinal; sz, subintestinal; s¢c, superior intercostal.

In A the early condition with paired omphalomesenterics and subintestinals, the post-
cardinals extending back as far as the pronephroi. B, mesonephroi developed and with
them the subcardinals and the beginning of the postcava; one omphalomesenteric lost and
subintestinals and caudals beginning to fuse; the intestinal vessels omitted in the later
figures. C, postcava has joined sinus and postcardinals have reached caudals; D, amniote,
appearance of metanephroi (true ‘idneys) with obsolescence of mesonephroi; the post-
cardinals lose connexion with caudal, their place being taken by the backward extension
of the subcardinals; formation of cross connexions between jugulars and between post-
cardinals of the two sides. £, breaking up of postcardinals and disappearance of left
Cuvierian duct, the other being called the precava.

Postcaval elements crosslined, subcardinal, dotted, other veins black.

The Definitive Circulation.

It is impossible here to follow in detail the development of all parts
of the circulatory system, or even to mention all of the vessels in all
of the groups. All that can be attempted is an account of the more
important parts and their modifications, with here and there references
to their history which will render their peculiarities more intelligible.
Most of the major trunks are now known to appear at first as lines
of vascular cells, similar to and arising in the same way as those de-
scribed in connexion with the heart (p. 271), and it seems possible that

CIRCULATORY ORGANS. 281

the intima of all of the blood-vessels is in genetic relations to such lines
of cells. It should be remembered that the vascular system is ex-
tremely variable, even within the limits of the species.

THE HEART.

The heart, as it was left on page 273, was a venous or branchial
heart, in that all of the blood which enters it is venous blood and is
all pumped directly to the gills to lose its carbon dioxide and to take
up oxygen, before being distributed to the various parts of the body.

pany
A B Cc E Y F
Qa a | [sea | afta sa aja
¢ s Ss XS L wv PC 7
OE Yo? WM HOLE “RE

Fic. 287.—Different stages in the differentiation of the parts of the heart. A, elasmo.
branch; B, teleosts; C, amphibia; D, lower reptiles; Z, alligator; F, birds and mammals-
a, atrium; ao, aorta; b, bulbus arteriosus; c, conus; cd, Cuvierian duct; h, hepatic veins; pa,
pulmonary artery; pc, pre- and postcaval veins; pv, pulmonary vein; pa, pulmonary artery;
5, sinus venosus; sa, septum atriorum.

In its course through the body it passes but once through the heart in
order to make the complete circuit. Such, in general, is the heart in
the cyclostomes and fishes (fig. 287, A, B).

When, however, lungs are formed (dipnoi and amphibia) to share in
the respiratory processes, the heart begins to divide into arterial or
systemic, and venous or respiratory halves. This division is brought
about by the formation of a septum or partition in the atrium, partially
or completely dividing the chamber, the pulmonary vein (infra) open-
ing into the left half, which thus becomes arterial, while the sinus,
with its veins, is connected with the right alone (fig. 287, C).

Still higher in the scale the partition or septum extends through the
atrio-ventricular canal, dividing its valves into two groups (tricuspid
valves on the right side, mitral on the left) and partially dividing the
ventricle (most reptiles fig. 287, D). In the crocodilia (fig. 287, E)

282 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

the division of the ventricle is completed by the extension of the
septum to the anterior end, but there is an opening (foramen
Pannizz) between the two sides of the aortic trunk, so that some
admixture of arterial and venous blood can occur. In the birds
and mammals (fig. 287, F) there is complete internal separation
of the two sides of the heart, though externally it shows but slight
signs of the division. Asa result of this division blood must pass twice
through the heart (once through the venous, once through the arterial
half) in order to make a complete circuit of the body. Venous blood
enters the right atrium, passes to the right ventricle, by which it is
forced to the lungs (pulmonary or respiratory circulation). Re-
turning to the heart by the pulmonary veins, it passes through the left
atrium and ventricle and thence through the systemic circulation, by
which all parts of the body are supplied. Details of the modifications
of the heart in the different classes of vertebrates are given at the end of
‘this chapter.

Aortic ARCHES.

As was said above, the typical number of aortic arches is six pairs,
this number being but rarely exceeded. In all groups except cyclos-
tomes and fishes they undergo considerable modification, and in the
fishes they are frequently more or less reduced in correlation with the
reduction of the gills (p. 238). The modifications may be outlined as
they occur in the successive pairs of arches. .

In many fishes and all tetrapoda the first arch on either side dis-
appears beyond the point where the external carotid arises, while,
correlated with the reduction of the spiracular gill, the second pair of
arches is partially or completely lost in the adult. The third pair is
always persistent and through them flows the blood for the internal
carotids and, in the fishes, gymnophiona and a few urodeles (fig. 280,
C) and reptiles, (EZ) blood for the radices aorte as well. In all other
tetrapoda the radix disappears between the third and fourth arches (fig.
280, D) and consequently here the third arch is purely carotid in char-
acter. When this occurs the portion of the ventral aorta between the
third and fourth arches carries blood for the carotids alone and hence
forms a common carotid trunk, usually divided into right and left
common carotid arteries.

The fourth pair of arches are the systemic trunks in all tetrapoda,

CIRCULATORY ORGANS. 283

carrying blood from the ventral to the dorsal aortz, while the fifth, re-
duced in size, perform a similar function in a few urodeles (fig. 280, C),
but elsewhere they entirely disappear. The fourth arches show a dif-
ferentiation between the two sides in many reptiles. That on the left
side becomes separated from the rest of the ventral aorta (fig. 280, E, F)
and has its own trunk connecting with the right side of the partially
divided ventricle, and, as will be understood from the relations of the
heart (p. 281), it may carry a mixture of arterial and venous blood.
From the dorsal side, this blood of the left fourth arch is largely dis-
tributed to the digestive tract, the cceliac axis arising from its radix,
while the part connecting it with the dorsal aorta is reduced in size.
The right arch and the carotids are connected with the left side of the

Fic. 288.—Aortic arches of amniotes, after Hochstetter. A, Varanus; B, snake; C,
alligator; D, bird; Z, mammal. 4, basilar artery; cc, common carotid; ch, ce, internal and
external carotids; da, dorsal aorta; p, pulmonary; s, subclavian.

heart and hence are purely arterial, the arch forming the main trunk
connecting the heart with the dorsal aorta. In the birds (fig. 280, G)
the radix of the left side of the adult disappears distal to the origin of
the subclavian artery, so that this arch supplies only the fore limb of
that side, while the right arch is purely aortic in character. In the
mammals (fig. 280, H) these relations are exactly reversed, the right
arch being subclavian, the left supplying the dorsal aorta and the
subclavian of that side.

With the development of lungs (dipnoi, tetrapoda) a pair of pul-
monary arteries are developed from the sixth pair of arches on the
ventral side of the pharynx. These grow back into the lungs, while the
rest of the arch, dorsal to their origin, becomes reduced to a small vessel
the ductus arteriosus (d. Botallii) in some urodeles, and persists
occasionally vestigially in higher vertebrates. Elsewhere it entirely dis-
appears. Inthe dipnoi and amphibia, where the ventricle remains

284 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

undivided, the pulmonary arteries are connected with the same trunk
(ventral aorta) as are the other aortic arches (fig. 280,C, D). In the
amniotes (E, F, G, H) with partial or complete division of the ventricle,
the truncus and the ventral aorta are divided in such a manner that
derivatives of the sixth arch are connected with the right side of the heart,
while the rest of the ventral aorta, save for the exception noted in the
reptiles above, receives its blood from the left side of the heart.

In connexion with the almost complete obliteration of the fifth arch, and in
most pulmonate vertebrates, the separation of the sixth from the rest, it is interesting
to note that in the lower vertebrates (elasmobranchs) there is already a differentia-
tion of these two arches from the rest of the series (fig. 281).

ARTERIES.

The dorsal aorta arises from the fusion of two primitive trunks
running approximately parallel to the notochord, and extends as a me-
dian vessel, usually lying just dorsal to the origin of the mesentery,
from the point of union of the radices back nearly to the posterior end
of the body.

In human anatomy the different parts of the aortic vessels have names different
from those adopted here. The persistent portion of the ventral aorta is called the
ascending aorta, the persistent fourth arch is the arch of the aorta, and the
adjacent part of the dorsal aorta is the descending aorta. The rest of the dorsal
aorta is divided into the thoracic and abdominal aorta, accordingly as they lie
in the regions of the corresponding cavities. These terms are inapplicable in
comparative anatomy.

The arteries arising from the dorsal aorta may be grouped under the
‘two categories, visceral and somatic (p. 268). To the former belong
the vessels running through the mesenterial-like structures (mesen-
teries, omenta, mesorchium, etc.) to supply the digestive tract and the
excretory and reproductive organs. In the primitive condition those
going to the alimentary canal are numerous but they do not show a meta-
meric character. In the majority of vertebrates they become united
into a smaller number of main trunks from which branches go to the
various regions. ‘The principal of these trunks are the following:
There is usually present a ceeliac artery, arising from the radix or
from the dorsal aorta near it, and dividing in the mesogaster into
gastric, splenic and hepatic arteries, distributed to stomach, spleen
and liver. The superior mesenteric artery is connected in develop-

CIRCULATORY ORGANS. 285

ment with the omphalomesenteric arteries (p. 276) and goes to the ante-
rior part of the intestine; while frequently an inferior mesenteric artery
is distributed to the posterior part of the digestive tract. The superior
mesenteric may fuse with the coeliac to form the cceliac axis while not
infrequently other mesenteric arteries may be developed.

The hypogastric arteries, already mentioned, need further notice.
These primitively connect the dorsal aorta with the subintestinal vein
in the neighborhood of the anus, and later give off vessels tq the region
of the rectum. When, as in all classes, from the amphibia upward, a
urinary bladder is developed from the rectal (cloacal) region, the

Fic. 289.—Diagram of vertebrate circulation based on a urodele. Arteries cross-
lined; veins black except the pulmonary vein, white. av, abdominal vein; a, cceliac artery;
ca, cv, caudal artery and vein; d, dorsal aorta; ec, external carotid; g, gonad; h, hepatic
vein; ha, hepatic artery; hy, hypogastric artery; ic, internal carotid; i, iliac artery and vein;
j, jugular; /v, liver; m, mv, mesenteric artery and vein; pa, pulmonary artery; ped, post-
cardinal; pcv, postcava; pu, hepatic portal vein; 7, rectal artery; ra, renal advehent vein;
sc, subclavian artery and vein.

hypogastrics form its blood supply, these vessels being the vesical
arteries. In the amniotes the distal end of the anlage of the bladder
forms a foetal structure known as the allantois, described in another
section (p. 318), and parts of the vesical arteries are carried out as
allantoic arteries (fig. 273), into the new formation. Since these
pass through the umbilicus, they are also known as the umbilical
arteries. Later, when the umbilicus disappears, the allantoic arteries
are lost and only the rectal and vesical arteries remain of the hypo-
gastric trunks.

The arteries going to the excretory and reproductive organs are
paired and, in the more primitive vertebrates show a marked metamer-
ism. They are best described in details along with the urogenital
structures in a subsequent section. It may be mentioned here that the
metamerism is well shown in the nephridial or renal arteries going to

286 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

the pro- and mesonephroi, while there is usually but a single pair of
renal arteries to supply the metanephroi (true kidneys) of the amni-
otes. The arteries to the gonads may be included under the single
head of genital arteries, though they are usually subdivided into the
spermatic and ovarian arteries according to the sex. Like the neph-
ridial, the genital arteries are
more numerous in the lower
and are reduced in number in
the higher forms.

The somatic arteries are
more numerous and are meta-
merically arranged. In _ the

da

FIG. 290. Fic. 291.

Fic. 290.—Diagram of early relations of vertebral arteries in an amniote. av, vertebral
artery; da, dorsal aorta; ec, ic, external and internal carotids; pa, pulmonary artery; 7a,
radix aorte; sa, subclavian.

Fic. 291.—A, side view of developing anterior arteries of Lacerta, after van Bemmeln;
the vertebral artery not developed hehind; B, ventral view of the relations of the arteries at
the base of the vertebrate brain. av, vertebral artery; b, basilar artery; cw, circle of Willis;
da, dorsal aorta; ec, ic, external and internal carotids; pa, pulmonary artery; ra, radix
aorte; sa, segmental arteries; sc, subclavian; 2-6, aortic arches.

early stages they are given off in pairs from the radices and the
dorsal aorta, an artery on either side, extending laterally between
each two successive myotomes (fig. 275). Many of these remain
in a slightly modified condition and are called intercostal arteries
(including lumbar and sacral arteries, etc., according to position).
These usually become connected on either side (fig. 290), near their

CIRCULATORY ORGANS. 287

origin, by a longitudinal vessel, the vertebral artery, which, in the
higher vertebrates, runs through the vertebraterial canal (p. 54) of
. the vertebre. ;

In the region of the aortic roots, after the formation of the vertebral
artery, all of the segmental.arteries except the last of the series lose
their connexion with the radix and
henceforth are supplied by way of the
posterior segmental and the vertebral
(fig. 291). Anteriorly the vertebral
arteries pass to the ventral side of the
spinal cord (or medulla oblongata)
dividing there into two branches, one
of which, joining its fellow of the
opposite side, runs back beneath the
spinal cord as a spinal artery, while
the anterior branches unite in the same fic, 292. Dike of anata of
way to forma basilar artery, running blood supply of vertebrate appendage.

v, abdominal vein; da, dorsal aorta;
forward beneath the medulla (fig. 291, si, subintestinal vein; so, somatic (seg-
B). At-the point just behind the ™en%)) vascular arch.
hypophysis the basilar divides, one-half passing on either side of
that structure and receiving the internal carotid of that side. The
trunks thus formed unite in front in the region of the optic chiasma.
There is thus formed an arterial ring, the circle of Willis, round
the hypophysis.

Fic. 293.—Three stages in the development of the arteries of the forelimb of the white
mouse, after Géppert. A, 8 days; B, 9 days; C,,10 days; a, aorta; b, brachial plexus.
(The vessels are extremely variable, not agreeing even on the two sides of a single
individual.)

As the limbs grow out, segmental arteries, corresponding in number
to the somites concerned in the appendages, grow out into the member.
Distally these arteries become connected with each other and with the
veins of the limb by a network of small vessels. By enlargement of

288 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

parts of these main trunks and of the connecting network, and the
partial or complete atrophy of other portions the definitive circulation
of the limb is established. This explains the numerous variations in ,
the blood supply of the limbs, both in the distal parts and in the origin
of the main trunks, which may arise from the dorsal aorta or from the
radices as far forward as the third aortic arch.

The main trunk of the fore limb may have different names in differ-
ent parts of its course. It is the subclavian artery as it leaves the
dorsal aorta, the axillary as it enters the limb, and the brachial in the
upper arm. It divides near the elbow into radial and ulnar arteries,
which run near the corresponding bones into the podium.

There are some additional elements of complexity in the develop-
ment of the arteries of the hind leg. As in front several somatic vessels
are concerned and there is the same formation of a capillary network.
Two of the arteries attain special prominence. In front is the epigas-
tric artery, which descends from the aorta to the ventral side of the
body and runs forward to supply the lower portion of the myotomes,
becoming connected at first with the epigastric veins, although later
they may anastomose with the hinder ends of the cutaneous arteries
(infra). When the hind limb grows out, the epigastric sends a branch,
the external iliac or femoral artery, into its anterior side. As the
leg increases in size this may surpass the parent epigastric in size, the
latter now appearing as a side branch.

The second pair of somatic arteries are the sciatic (ischiadic)
arteries. These descend into the posterior side of the leg, the name
changing at the angle of the knee to popliteal artery, and farther
down it divides into peroneal and anterior and posterior tibial
arteries, the peroneal supplying the calf of the leg, the others continuing
into the foot.

The arrangement of vessels thus outlined is characteristic of the
lower tetrapoda where the femoral artery is small. It is also character-
istic of the embryos of the mammals, but in the latter, before birth, the
femoral artery grows down, joins the popliteal, and thus becomes the
chief supply of the limb. These trunks and the hypogastric do not
always remain distinct, but may fuse in different ways at the base.
Epigastric and hypogastric arteries are distinct in many reptiles and in
birds, but elsewhere they fuse to form the common iliac artery, so
called since the proximal portion of the femoral is often called the
external, the hypogastric the internal iliac artery. The sciatic, too,

CIRCULATORY ORGANS. 289

may remain distinct or it may fuse with the others at the base, and
then its independent portion appears as a branch of the common
iliac artery.

The dorsal aorta, which continues into the tail, is called the caudal
artery behind the point where the sciatics (common iliacs) arise.

A cutaneus artery, arising from either the subclavian or the
pulmonary artery of either side (both conditions occur in the amphibia),
runs backward in the skin of the trunk, and may extend back and unite
with the epigastric artery. When, as in the amphibia, these arise
from the pulmonary they contain venous blood and the skin acts as
a subsidiary respiratory organ (p. 258).

VEINS.

The position and development of the chief longitudinal venous ©
trunks have already been outlined. Both these and other veins yet
to be mentioned frequently undergo shiftings of position and other
modifications during growth, but before describing these changes some
other vessels must be described.

With the development of the limbs corresponding veins arise (fig.
294), a subclavian vein for each fore limb, a common iliac for the
hind leg, these bringing the blood from the appendage to the trunk.
In the young each subclavian empties into the postcardinal of the same
side, but in the adult the opening may shift to the Cuvierian duct.
The common iliac vein likewise empties into a vein, the epigastric
or lateral abdominal, which runs forward in the body wall to connect
with either the postcardinal or the duct of Cuvier (fig. 294, A). This
condition obtains throughout life in some elasmobranchs, but higher
in the scale the iliac vein, while retaining its connexion with the
epigastric, grows toward the middle line and joins the postcardinal of
the same side, a condition which is permanent in amphibia and reptiles
(fig. 294, B, C), where blood coming from the hind limb has two
routes to the heart.

The epigastric veins of the two sides may fuse in the median line
in front (amphibia, some reptiles, birds), forming an anterior ab-
dominal vein (fig. 294, C) which reaches the heart by passing through
the remains of the ventral mesentery (ligamentum teres) to the liver
and thence forward. A similar anterior abdominal vein has been
described in Echidna but is unknown elsewhere in the mammals.

19

290 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

In the fishes the vessels of the appendages are but slightly developed,
there being a subclavian vein entering the Cuvierian duct, and occa-
sionally a brachial vein which may empty into the sinus venosus. In
the amphibia a cutaneus magnus vein (fig. 302), coming from the
skin of the trunk, may enter the subclavian, while in all tetrapoda the
subclavian, after leaving the limb, receives a superficial cephalic and an
axillary vein, the latter changing its name in the appendage to the

Fic. 294.—Relations and modifications of the post- and subcardinal, abdominal and
postcaval veins in different stages ot the amphibia. In A the veins (i) from the hind limb
return directly to the heart by the lateral abdominal veins (72), while the blood from the
tail (c) passes by way of the subcardinals (sc) through the mesonephroi to the postcardinals
(pc). In B the lateral abdominals have united in front to form the anterior abdominal
vein (aa); the iliacs have sent a branch to the postcardinals, which have grown back to join
the caudals, while the subcardinals have lost their connexion with the caudal and have
acquired one with the postcava (p), a backward growth from the sinus venosus. In C the
postcardinals have been interrupted, the posterior half of each now forming an advehent
vein while the subcardinals, as in B, form the revehent veins (7).

brachial vein. In the hind limb the common iliac vein is formed by
the union of the femoral and sciatic (ischiadic) veins, as well as the
hypogastric (internal iliac) vein already referred to.

In the classes above fishes (dipnoi, amphibia and amniotes) a new
vein, the postcava (vena cava inferior) appears. This arises in
part from scattered spaces, in part as a diverticulum of the sinus
venosus and the hepatic veins, and grows backward, dorsal to the liver,
until it meets and fuses with the right subcardinal vein (fig. 295), a

CIRCULATORY ORGANS. 2gI

portion of which now forms a new trunk, carrying blood from the
posterior part of the body to the heart (figs. 294, 295).

With the appearance of the postcava changes are introduced in the
embryonic renal portal circulation ( p. 280) which may be summarized
as follows: The subcardinals lose their connexion with the caudal vein
and become connected with each other by transverse vessels (interrenal
veins) while parts of the postcardinals adjacent to the nephridial
organs separate from the parts in front, while they grow backward

Fic. 295.—Development of postcaval system in birds (A, B, sparrow; C, D, chick),
schematized after A. M. Miller. In A the postcardinals have extended nearly to the
pelvic region and the subcardinals are appearing as isolated spaces. In B the
subcardinal spaces are uniting and the capillary system connecting with the postcardinals
is developing, while the postcava is arising. In C the postcava has united with the
subcardinal of the right side. az, ischiadic artery; aie, external iliac artery; au, umbilical
(hypogastric) artery; da, dorsal aorta; m, mesonephric veins; om, omphalomesenteric
artery; ~, postcava and its anlagen; sc, subcardinal and its elements; vei, external iliac
vein; vi, ischiadic vein. F

and connect with the caudal vein (fig. 295). These posterior parts
of the postcardinals now become the advehent veins of a second
renal portal system, bringing blood from the tail and hind limbs to the
excretory organs (mesonephroi). The subcardinals of the two sides
usually fuse in the middle line, a process initiated by the appearance
of the interrenal veins, and now act as a revehent vessel, carrying
blood from the excretory organs to the postcava and the anterior

292 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

portion of the postcardinals which have joined the anterior ends of the
subcardinals (fig. 294, C). The changes in the postcardinals and the
renal portal system of mammals will be described below.

In Ceratodus (dipnoi, fig. 296, A) there are some differences from the above
account. Thus the anterior portion of the right postcardinal (not shown in the
figure) loses its connexion with the vessels behind and acts as a vertebral vein,
taking the blood from the intercostal veins of that side back to the heart. The

Fic. 296.—A, venous system of Ceratodus, dorsal view, after Spencer; B, of a urodele,
ventral view. ab, abdominal vein; av, ven advehentes; b, brachial; c, caudal; cd, Cuvierian
duct; ej, external jugular; #, heart; hp, hepatic portal; 27, inferior jugular; 7, jugular; i,
iliac; J, liver; /c, lateral cutaneus; m, mesonephros; p, postcava; pc, postcardinal; r, vene
revehentes; s, subclavian; /, testes.

caudal and the subcardinals form a continuous trunk, the revehent vessels forming
side branches. The posterior portions of the postcardinals grow back into’ the
tail as paired vessels, forming no connexion with the caudal vein. In Protopterus
the vertebral vein is lacking, the subcardinals are not fused behind while the
advehent veins are connected with the caudal.

The development of lungs brings about the appearance of one or
more pairs of pulmonary veins which bring the (arterial) blood from
these organs to the heart. These arise as an outgrowth from the

CIRCULATORY ORGANS. 293

left atrial portion of the heart, dividing farther back to reach the two
lungs. At no time do the pulmonary veins connect with the sinus
venosus, but they always empty into the left atrium (fig. 285).

The Fetal Circulation.

Some features of the foetal circulation of the amniotes have already
been alluded to, but the whole may be summarized here. In
the amniotes, with the development of a large yolk sac and of the
allantois, the vessels on the ventral side of the body become corre-
spondingly modified. The processes involved may be readily under-
stood from a comparison of figs. 282 and 284. The yolk sac is to be
regarded as a diverticulum of the intestine while the allantois. is a
similar outgrowth from the urinary bladder, itself a process of the ali-
mentary canal. These outgrowths naturally carry with them the blood-
vessels distributed to the parts from which they arise. Hence the
omphalomesenteric artery and the vitelline veins (derivatives of the
omphalomesenteric veins) extend out ever the yolk, increasing in
number as well as in extent of their branches as the yolk sac spreads
over the yolk.

In the same way the hypogastric arteries are carried out with the
allantois, these portions being called the allantoic or umbilical
arteries, the blood being carried back to the trunk by a single allan-
toic vein. These two kinds of vessels—arteries and veins—are con-
nected in the distal part of the allantois by a rich network of capillary
vessels. It is by these that the allantois is able (p. 264) to act in the
sauropsida as an organ of respiration. In the mammals, by means of
osmosis through the placenta, it is not only respiratory, exchanging
gases with the uterine walls (there is no exchange of blood with the
mother), but they serve as recipients of nourishment by the passage
of plasma from the maternal tissues.

From the foregoing statements it will be seen that in the sauropsida
five vessels—three arteries and two veins—pass out through the um-
bilicus to the foetal adnexa, but in the mammals, where the yolk is
wanting and the yolk sac reduced and transitory in character, the
omphalomesenteric artery and the vitelline vein disappear early, leav-
ing but three vessels in the umbilical cord. In the elasmobranchs,
where there is a large yolk sac but no allantois, only the yolk sac cir-
culation is found.

2904 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

Circulation in the Separate Classes.

CYCLOSTOMES present marked differences in the circulation of the two
groups, the petromyzons being nearly normal, the myxinoids decidedly aberrant.
The aortic arches vary in number with the number of gill pouches (p. 239). In the
myxinoids the common carotid is connected with all of the efferent branchials by a

ac

“VAT

pe

"pe
sv hv \ P
&
Fic. 297.—Oblique ventral view of venous system of Petromyzon, drawn from a corro-
sion preparation (Princeton, 669); ac, precardinal; c, caudal; gs, genital sinus; hv, hepatic
vein; 7j, inferior jugular; pc, postcardinal; sv, sinus venosus; va, ventral aorta.

trunk running parallel to the body axis, just dorsal to the gill pouches. The inter-
segmental arteries of the dorsal region are irregular, sometimes alternating, some-
times appearing in pairs on the two sides of the median line. In the myxinoids
(fig. 297) the subcardinals are united behind, the postcardinals in front, these
latter uniting with the single inferior jugular of the left side to form the unpaired
Cuverian duct, the presence of which renders the sinus venosus asymmetrical and

Fic. 298.—Anterior arterial vessels of the tile fish (Lopholatilus), after Silvester. a,
auricle; ab, to air bladder; am, to angle of mouth; c, cceliac axis; d, dorsal arteries; da,
dorsal aorta; ec, external carotid; g, genital artery; gs, gastrosplenic; h, hyoid artery; ha,
hepatic; /, lingual; dg, left gastric;.m, mesenteric; mh, middle hypobranchial; 0, ophthalmic;
pa, parietal; po, postorbital; ps, pseudobranch; rg, right genital; so, supraorbital; v, ven-
tricle; va, ventral aorta,

forces the hepatic veins to empty into the right side. The hepatic portal receives
a vein from the head, and then passes back to a contractile portal heart, just before
it enters the liver.

FISHES.—In the fishes, the dipnoi excepted, the circulation corresponds rather
closely in’ its main features with the primitive condition described above. The

CIRCULATORY ORGANS. 295

heart is purely venous and the only peculiarities to be mentioned are the following:
In the elasmobranchs and ganoids the valves of the conus are arranged in several
(3-8) rows, but in the teleosts (Butyrinus excepted) they are reduced to a single row,
apparently corresponding to the first of the lower forms. In the latter group the
bulbus is especially well developed. The aortic arches correspond in number to
the functional gill slits—six or seven in the notidanid sharks, five in other elasmo-
branchs and at most four in ganoids and teleosts. Paired inferior jugulars are
usually present, but they are lacking in Polypterus, while in Lepidosteus and many
teleosts they are united into a single trunk emptying directly into the sinus venosus.

Epigastric veins are usually present and paired but are absent from many bony
fishes.

FIG. 299.—Anterior venous system and heart of Lopholatilus, after Silvester. @, auricle
ab, veins from air bladder; b, bulbus; bv, brachial vein; c, cerebral vein; cd, Cuvierian duct;
cv, caudal vein; d, dorsal branches of parietal veins; f, facial vein; g, gastric veins; hp,
hepatic portal; hv, hepatic veins; 7j, inferior jugular; im, 7s, veins from intestine and spleen;
1, liver; pc, postcardinal; pd, postcloacal; per, peritoneal; ph, pharyngeal; po, postorbital;
re, anterior revehentes; s, sinus venosus; s¢, veins from stomach and intestine; th, thyreoid;
tm, thymus; v, ventricle; va, ventral aorta; vf, vein from ventral fin; w, outline of Wolffian
body.

DIPNOI.—In this group the atrium, in correlation with the development of
lungs, becomes partially divided as described above. No true atrio-ventricular
valves occur, their place being taken by a strong ridge which, in systole, closes the
canal and at the same time partially divides the ventricle into arterial and venous
halves. The conus has eight rows of valves and in Ceratodus the truncus shows the
beginning of a division (completed in Protopterus) separating the arterial from the
venous arches. For veins, see fig. 206.

AMPHIBIA.—In the amphibia the division of the atrium by a septum atriorum
into right (venous) and left (arterial) halves is carried farther. This septum is
fenestrate in urodeles and gymnophiones, entire in anura, but in none is it carried
clear to the atrio-ventricular wall. In systole the edge of the septum is forced for-
ward, completely separating the two atria. No corresponding septum is developed
in the ventricle, but numerous muscular bands extending through its cavity tend
to prevent the mingling of arterial and venous blood. In Proteus, Cryptobranchus
and the cecilians the bulbus is simple but in the other urodeles and the anura a
spiral septum (possibly representing fused valves) is developed in it, separating it

296 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

into two tubes. This is continued in the anterior part of the truncus by a horizontal
septum (short in urodeles, longer in anura) separating aortic and pulmonary trunks,
the former subdivided in a similar way a little farther forward into carotid and aortic
portions.

In the early larve of the amphibia each fully developed aortic arch except the
last extends into the gills, but as the branchie begin to be absorbed, a small vessel
connecting the afferent and efferent arteries at the base of each gill enlarges and

Fic. 300.—Heart and adjacent parts of Protopterus, after Rése a, atrium; aoe,
cesophageal artery; J, air bladder (lung); c, conus; h, hepatic vein; ji, is, superior and
inferior jugular veins; oe, cesophagus; pa, pulmonary artery; pc, postcardinal vein; ph,
pharyngeal artery; s, sinus venosus; sc, subclavian vein; 1-4, afferent branchial (aortic)
arteries.

becomes the path of the main blood stream and a part of the arch of the adult (fig.
304). Of these four arches—3, 4, 5, and 6 of the primitive scheme—the fifth is
lost in the adults of all except a few urodeles and cecilians. The fourth connects
with the dorsal aorta and the sixth with the pulmonary arteries. These last, which
often have a ductus Botallii, are noticeable for the large cutaneus arteries—anterior
and posterior—which arise from them and which play an important part in respira-

CIRCULATORY ORGANS. 297

tion. Connected with the carotid arteries are the carotid glands (fig. 304). In
| the larval stage each consists of a network of blood-vessels—a rete mirabile—
| between the afferent branchial and the carotid artery, but in the adult this degener-

5 maxsup
mn tens
in tju q

NBZN/

/

|
7

|

SS
era j
$
™

;

|

ify
j
Py]

Fic. 3or. Fic. 302.

ates into a small muscular organ containing sympathetic cells (p. 165), at the
base of the carotid.
The postcava is well developed and the epigastric veins unite to form an anterior

298 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

FIG. 303.

Fics. 301, 302, 303.—Circulatory system of Desmognathus fuscus after Miss Seelye;
fig. 301, superficial vessels; fig. 302, deeper vessels; fig. 303, vessels of the dorsal body
wall; all from the ventral surface. aames, mesenteric arteries; acut, cutaneus artery; aduo,
duodenal artery; aepig, epigastric artery; ag, artery to anal gland; agas, gastric arteries;
ahep, hepatic artery; az, iliac artery; azntcom, communis intestinal artery; ao, aorta; aoc,
ocular artery; aph, pharyngeal artery; an, anus; apul, pulmonary artery; asc, subclavian
artery; asp, splenic artery; b/, urinary bladder; ca, caudal artery; cutmag, cutaneus major
vein; cutp, cutaneus parva vein; cv, caudal vein; ec, external carotid; extmax, external
maxillary; zc, internal carotid; ilv, iliac vein; intcom, common intestinal; intjug, internal
jugular; intv, intestinal vein; ling, lingual; liv, liver; maxinf, maxsup, inferior and superior
maxillaries; mm, mesonephros; @, cesophagus; pc, postcava; 7, rectum; sfl, spleen; st,
stomach; sv, sinus venosus; vabd, abdominal vein; veut, cutaneus vein; vhp, hepatic vein;
vert, vertebral artery; umes, mesenteric vein; vp, portal vein; vra, vena renalis advehentis:
vsp, splenic vein; vves, vein from bladder. -

CIRCULATORY ORGANS. 299

abdominal vein (fig. 294), while the blood from the hind limbs may return to the
heart through either the anterior abdominal or the renal portal system.

In the lungless salamanders (p. 258) the heart and blood-vessels show corre-
sponding modifications. There is no septum atriorum and the pulmonary arteries
and veins fail to develop. The cutaneus arteries and the smaller vessels supplying
the pharyngeal region are greatly enlarged, respiration taking place through the
skin and the mucous membrane of the throat.

The action of the anuran heart may be out-
lined here. The two atria contract at the same
time, forcing arterial and venous blood into the
ventricle, but it is kept from mixing by the mus-
cular bands already alluded to. At the systole
of the ventricle the venous blood, which is near-
est the truncus, is first forced forward. This
takes the most direct course through the wide
and shorter pulmonary arteries, which are prac-

Fic. 304. Fic. 305.

Fic. 304.—Diagram of the aortic arches in amphibia. Arterial blood cross lined,
venous black. The gill circulation omitted, its course indicated by arrows; the permanent
circulation after the absorption of gills shown. cg, carotid gland; da, dorsal aorta; d,
ductus Botalli; pa, pulmonary artery; va, ventral aorta; 3-6, aortic arches.

Fic. 305.—Heart of snapping turtle, Chelydra serpentina (Princeton, 479). aa, aortic
arch; ¢, coeliac artery; da, dorsal aorta; db, Botall’s duct; ec, ic, external and internal
carotids; Ja, left auricle; ~, pulmonary artery; va, right auricle; sc, subclavian artery; v,
ventricle; m, mesenteric artery.

tically empty at the time. The next portion of the blood, containing both arterial
and venous, follows the next easiest course through the aortic arches, while the
last to leave the ventricle, consisting of pure arterial blood, can only go into the
carotids, where the resistance is greater on account of the small size of the vessels
and the obstacles presented by the carotid glands. :
REPTILES,.—In the reptiles the division of the heart (fig. 287) is carried still
farther and the sinus venosus tends to be merged in the right atrium. The atrial
septum is complete and is continued forward as a ventricular septum, partially

300 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

(Sphenodon, turtles, squamata) or completely (crocodiles) separating the two
ventricles. The peculiar relations of the aortic arches have been mentioned (p.
283). Correlated with the differences between the aortic (fourth) arches of the
two sides in the majority of reptiles are certain features in the origin of the arteries.
Thus both of the subclavian arteries (lacking in snakes) arise from the right radix,
while the left gives rise to the coeliac artery. In many reptiles the anterior parts of
the postcardinals are replaced by vertebral veins. The renal portal system is
developed in the embryo and persists (much as in the amphibia) to a greater or less
extent in the adult. Usually paired anterior abdominal veins are present.

BIRDS.—The peculiarities of the heart and aortic arches were mentioned on
page 283. Birds have the same reduction of the postcardinals as is found in reptiles.
The renal portal system is formed in the embryo, but the only blood received by
the adult kidney comes through renal arteries like those of mammals. The iliac
veins extend to the postcava and lose all connexion with the anterior abdominal
veins. The paired epigastric veins persist only in front.

MAMMALS.—In the mammals the four chambers of the heart are completely.
separated and the sinus venosus has been completely merged in the right atriumt
The persistent left fourth aortic arch forms the sole connexion between the hear.

Fic. 306.—Modifications of the origin of the carotid and subclavian arteries in
mammals.

and the dorsal aorta and from it arise the carotid and subclavian arteries, the
arrangement of these representing almost every possible condition (fig. 306). In
the lower groups (¢.g., rodents) both Cuvierian ducts persist, but in the higher orders
a cross connexion (the innominate vein) arises between the trunks formed from
the jugulars and subclavian veins of the two sides (fig. 308) so that the blood from
the left side of the head, neck and fore limb joins that of the left side in a common
trunk, the precava (anterior vena cava) which enters the right atrium. With
this development the left Cuvierian duct, as such, disappears.

The renal portal system has but a transitory existence in the embryo (best
developed in the monotremes) and early disappears with the degeneration of the
Wolffian bodies (mesonephroi). As these organs disappear a part of the capillary
system of the Wolffian bodies enlarges and forms a main trunk connecting the
postcava with the posterior parts of the postcardinal veins (fig. 307, C) which bring
the blood from the tail, the iliacs and the permanent kidneys. With farther develop-
ment (D, E) the left postcardinal is largely lost (except the part connecting with the
suprarenal and gonad of that side) and all the blood from the posterior part of the
body is returned by the right postcardinal and the postcava, which appear (fig.
308, A) as if they arose from a union of the iliac veins. Correlated with these
changes in the venous system and the impossibility of venous blood entering the
excretory organs, there is developed a renal artery from the aorta for each of the
permanent kidneys.

CIRCULATORY ORGANS. 301

Fic. 307.—Development of posterior veins of rabbit, after Hochstetter. Cand D repre-
sent only the hinder part of the whole shown in A toC. In B the veins for the postcaval-
subcardinal system have tapped the postcardinal veins, which in C have lost their connec-
tion with the anterior part and empty now through the postcava exclusively. In E the
left posterior postcardinal is entirely lost. 7, ischiadic vein; ie, external iliac; z, jugular;
mt, metanephros (kidney); , postcava; pc, postcardinal; s, subclavian; sc, subcardinal:
sr, suprarenal; u, ureter.

NE

4

Fic. 308.—Development of the anterior veins of a mammal. 4A, earlier stage, to be
compared with fig. 307 C; B, definitive condition of adult. wu, azygos; ¢c, coronary; e, 7,
external and internal jugular; ha, hemiazygos; 7, iliac; im, innominate; p, postcava; pc,
postcardinal; pre, precava (superior vena cava); sé, superior intercostal veins.

302 COMPARATIVE MORPHOLOGY OF VFEREBRATES.

The anterior parts of both postcardinals have separated from the posterior por
tion and receive only blood coming from the intercostal veins (fig. 308). A
cross vessel now connects the posterior parts of the postcardinals of the two sides,
after which the left vessel separates into two portions. The anterior of these
(fig. 308, B) connects with the heart by way of the jugular and innominate vein and
forms the superior intercostal vein of human anatomy. The rest of the left

_ postcardinal is now known as the hemiazygos vein and it returns blood from the
trunk by way of a cross connexion and the anterior part of the right postcardinal
(now called the azygos vein), to the precava and so to the heart.

THE LYMPHATIC SYSTEM.

The lymphatic system consists of (1) a series of lymph vessels which
penetrate all parts of the body; (2) of pulsating portions of these vessels,
the lymph hearts; and (3) peculiar aggregates of connective tissue,
leucocytes and lymph vessels which are grouped under the general
head of lymph glands.

There are different views as to the morphology of the blood and lymph systems.
According to one (Marcus) the lymph vessels were primitively connected with the
coelom and: have only secondarily come into relations with the blood-vascular
system. Others think that both blood and lymph vessels have arisen from
extraccelomic spaces, from which, by modification and specialization, the two
systems have been differentiated. The fact that in many invertebrates there is
but a single system, best compared with the lymph system of the vertebrates, and
that, even in the crustacea, lymphatic and blood systems are but partially differ-
entiated, is of interest in this connexion.

The lymph vessels are, in part, capillary in character with walls of
endothelium alone. The larger ducts and the still larger sinuses are
strengthened by smooth muscle fibres and by elastic and fibrous tissue.
The capillaries have numerous anastomoses, but the vessels are said
to terminate blindly, while, at least in the higher vertebrates, some may
connect with the ccelom by minute openings (stomata) in the peritoneal
lining. The larger vessels have valves at intervals to prevent back-
flow of the lymph, these often giving the vessels a lobulated appearance.
Proximally the vessels open at two or more points into the veins. The
fluid portion of the lymph is derived in part by osmose from the walls of
the blood capillaries, in part from the alimentary canal.

The development of the lymph vessels has been traced mainly
in birds and mammals (chiefly in the latter), with fewer observations on
amphibia and other classes. Many points remain to be worked out,
there being considerable differences in the various accounts. Appar-
ently the process in its main features is as follows:

CIRCULATORY SYSTEM. 303

Near the junction of pre- and postcardinals on either side numerous
small diverticula are given off from the lateral side of these veins (fig.
309, A). These diverticula unite with each other, forming small
tubes parallel to the parent vessels and united to them for a time at
numerous points where the budding took place. Later these connex-
ions are lost and the tubes are separated from the veins (fig. 309, B)
forming an anterior cephalic duct, extending forward, parallel to the
jugular vein; an ulnar lymphatic duct destined to grow into the fore
limb; and, a little later, a thoracic duct grows back, parallel to the

Fic. 309.—Early development of the lymph vessels in the cat, after McClure and
Huntington. A, in 6.5 mm. embryo; B, in 10.5 mm. embryo; C, definitive stage; D,
diagram of developing diverticula of chick which are to form lymph heart, based on Sala.
ac, anterior cardinal vein; ¢ *-*, coccygeal veins; cd, Cuverian duct; cv, cephalic vein; dls,
dorsal veno-lymphatic sinus; ej, 7j, external and internal jugulars; pre, precava; th, thoracic
duct; ul, primitive ulnar lymphatic; uva, anlage of ulnar vein; vls, ventral veno-lymphatic
sinus; 1-7, segmental vessels; lymphatic-forming tissue stippled.

postcardinal vein. All of these vessels are united near their point of
origin by a large sinus, the jugular lymph sac (fig. 309, C). Later
the lymph sac reestablishes communication at one or two points in the
subclavian-jugular region with the vein.

The conditions at the posterior part of the body are less certainly
known (fig. 309, D). In this region a cistern of chyle (a mesenterial
lymph sac) and a posterior lymph sac develop in close connexion
with the postcava in the region of the nephridial organs, and it is pos-
sible that a portion of the thoracic duct grows forward from the cis-
tern of chyle, while other vessels grow into other regions. Later the
primitive trunks thus outlined give off branches which gradually ex-

304 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

tend into all parts of the body, but of their development little is known.
Anastomoses occur between the vessels of the two sides of the body
and not infrequently the thoracic duct of one side shows more or less
degeneration, resulting in a lack of symmetry in the adult.

Not enough is known of the distribution of the lymphatic trunks to
render broad generalizations possible, but it may be said that the sys-
tem is most extensively developed in the subcutaneous tissue, in the
corresponding envelopes (meninges) of the central nervous system,
in the intermuscular connective tissue, in the walls
of the alimentary canal, and, as a network, in close
connexion with the blood-vessels of the body.

The lymph hearts are enlarged and contrac-
tile portions of the lymph vessels, provided with
valves to prevent backflow of the fluid (fig. 310).
Usually these contract by means of the intrinsic
muscles of the walls, but in some urodeles (Am-
blystoma) there is an unpaired lymph heart beneath
the truncus arteriosus which enlarges and con-
tracts with the systole and diastole of the blood

heart. Is

As was intimated above there is a constant S a
osmosis of fluid from the blood capillaries into — Fic. 310.—Scheme
the surrounding tissues. This finally passes into °% oes eae ee
the distal capillaries of the lymph system, while +,atrium; /, lymph ves-
i Z : sels; Js, lymph sinus; v,
in the walls of the alimentary canal there are, in ventricle; vs, venous
addition, the results of the digestive processes ‘us of caudal vein.
added to the fluid in the lymph vessels. As this latter portion has a
milky appearance, due to the contained fat, it is called chyle and
the lymphatics which contain it are called lacteals and chyle
ducts. All of these additions to the contents of the lymph vessels
make a current in the larger lymph trunks, and finally the whole
of the lymph is returned to the veins by the several connexions already
mentioned. In addition to the propelling force of the lymph hearts and
the pressure due to absorption and osmosis, the lymph is also carried
along by the motions of the parts in which the vessels ramify, their
pressure being supplemented by the action of the valves.

In those fishes which have been accurately studied the lymph system is well
developed and opens into the veins in the cardiac and caudal regions. The vessels
are especially developed in the tail, where (myxinoids, teleosts) lymph hearts occur.

CIRCULATORY ORGANS. 305

There is also a large lymph sinus in the scapular region into which the trunks from
head and body empty. Frequently there is also a large caudal sinus (physostomes)
connected with a lymph heart (fig. 310) which forces the lymph into the caudal
vein.

The urodeles have the thoracic ducts united behind but separate in front, a
cephalic trunk emptying into each, and each duct opening into the corresponding
subclavian vein, while a series of from fourteen to twenty lymph hearts occur in
connexion with the trunk accompanying the lateral line. The anura are noticeable
for the complete disappearance of the thoracic ducts, their place being taken by a

PUY

SDT pe
SS area TD

Fic. 311.—Deeper anterior lymphatics (stippled) of Scorpenichthys, after Allen. a,
auricle; abs, abdominal sinus; 6, brachial sinus; br, brain; cs, cephalic sinus; d, dorsal trunk;
Jm, facialis-mandibularis vein; hs, hyoid sinus; 7j, inferior jugular vein; ips, inner pectoral
fin sinus; j, jugular vein; /, lateral trunk; om, orbito-nasal vein; p, pericardial sinus; p/,
profundus facialis lateral trunk; pv, profundus ventral trunk; sf, superficial lateral trunk;
ssl, superior spinal longitudinal trunk; v, ventricle; va, ventral aorta; v/s, ventral fin sinus;
vp, ventral pericardial sinus; vt, ventral abdominal trunk.

pair of trunks between the dorsal myotomes and those of the lateral body wall.
They have also enormous subcutaneous lymph spaces, separated from each other
by narrow partitions. It is the presence of these large spaces that makes the skin-
ning of a frog such an easy matter. Two pairs of lymph hearts are present, one
pair in the neighborhood of the extremity of the urostyle, the other between the
transverse processes of the third and fourth vertebrae. In the cecilians there is a
pair of lymph hearts for each segment of the trunk.

Reptiles have two cephalic lymph trunks and one (lizards) or two thoracic

20

306 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

ducts, the one of the lizards being divided in front so as to empty into either sub-
clavian vein. There is a single lymph heart at the junction of trunk and tail. In
the birds both thoracic ducts occur and there is a pair of lymph hearts present in
the young in the position occupied by the single heart of the reptiles.

In the mammals the primitively paired thoracic ducts are sometime retained
throughout life, but usually only one persists. This begins at the cistern of chyle
in the lumbar region and empties into the left brachiocephalic vein near the entrance
of the single cephalic duct. The thoracic duct receives the lymph vessels from the
limbs and those (lacteals) from the alimentary canal. In those cases where there

Fic. 312.—Early lymph system (black) of 10 mm. rabbit embryo, after F. T. Lewis.
af, anterior tibial; c, caudal; fz, primitive fibular; ¢7, 77, external and internal jugular; em,
external mammary; pc, postcardinal; u/, primitive ulnar veins.

is but a single thoracic duct in front, its representative on the right side is a much
smaller vessel connected with the right side of the venous system. No lymph
hearts are known in the mammals. The jugular lymph sacs of the embryo have
been regarded as such, but the absence of valves and muscles in the walls renders
such an interpretation doubtful.

Lymph Glands.—In connexion with the lymph vessels are numer-
ous structures included under the heads of lymph glands, lymph
nodules and blood-lymph glands. These are most abundant in the
walls of the ccelom (mesenteries) and of the digestive tract, although
they may be found at remote points. They consist of aggregates of

UROGENITAL SYSTEM. 307

adenoid tissue (reticular connective tissue crowded with leucocytes).
Well-known among these structures are the so-called fat bodies (cor-
pora adiposa) connected with the gonads of the amphibia, and the
‘hibernating glands’ of some rodents and insectivores, which con-
sist of richly vascularized masses of fat. In the lymph nodules this
adenoid tissue is enmeshed in a rete mirabile of lymph vessels. In
the blood-lymph glands there is a somewhat similar relation to blood-
vessels as well, for the details of which reference must be made to
histological text-books. These lymph structures, which occur at
various points of the body, are apparently places for the formation of
leucocytes (lymphocytes).

The spleen, attached to the mesentery near the stomach and pan-
creas, js intermediate in some respects between lymph and blood-
lymph glands and is the largest lymph structure in vertebrates. It
is developed in the walls of the alimentary canal and is said to have
an entodermal origin. Later it separates from the stomach and
assumes its definitive position. It serves, apparently, as a place for
the disintegration of the red blood corpuscles in addition to functioning
as a leucocyte-forming organ.

The tonsils (p. 247) belong to the category of adenoid glands.
There are two kinds of these, the pharyngeal and the palatine tonsils,
the latter occurring between the inner ends of the Eustachian tubes of
amniotes, the palatine (best developed in mammals) are paired struc-
tures on either side of the pillars of the fauces. Other tonsil-like
structures occur at different points of the floor and roof of the mouth
of the tetrapoda.

THE UROGENITAL SYSTEM.

In several phyla of the animal kingdom there is an intimate relation
between the reproductive and excretory organs, the ducts of the latter
serving either to carry the products of the gonads directly to the ex-
terior or acting as brood organs where a portion of the development
of the egg takes place. This close association of the two systems is
especially marked in most vertebrates and hence this section is headed
Urogenital System, because of the difficulty of treating the two com-
ponents separately.

The urinary or excretory organs have for their purpose the elimina-
tion of the nitrogenous waste (and occasionally other products) from

308 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

the system. They are paired organs which consist of glandular por-
tions, the nephridia (kidneys), and their ducts. The reproductive
organs include the gonads or sexual ‘glands,’ which (ovaries) pro-
duce the eggs or (testes) the spermatozoa, and the passages by which
these products are carried to the external world. To these are fre-
quently to be added accessory reproductive structures by which, in
certain cases, the sperm is transferred to the female.

Fic. 313.—Urogenital organs of Emys europea, after Bojanus. 6, urinary bladder;
g, opening of vas deferens into the urogenital sinus; k, kidney; ¢, testis; u, ureter; vd, vas
deferens.

THE EXCRETORY ORGANS.

The nephridia consist of a series of excretory tubules, specialized
in different ways, and of the ducts into which the tubules empty. As
the function of the nephridia is the elimination of the nitrogenous
waste (uric acid, urea, etc.) which accumulates in the blood, they have
an abundant blood supply, entirely derived, in the younger stages of
all vertebrates and in the adults of the higher groups from the dorsal
aorta, while in the later developmental stages and in the adults of most
anamniotes the aortic blood is supplemented by blood coming from
the tail and hind limbs by way of the caudal and iliac veins (fig. 303).

In its extreme development one of the excretory tubules may con-
sist of the following parts (fig. 314): At the proximal end the tubule
opens into the ccelom (metaccele) by a ciliated funnel, the nephro-

UROGENITAL SYSTEM. 309

stome; the cilia, which may continue for some distance along the
inside of the tubule, serving to create a current which carries the
coelomic fluid into the tubule and thence outward. Farther along
the tubule expands into a Malpighian or renal corpuscle (fig. 315).
This consists of a vesicle (Bowman’s capsule), one side of which

Fic. 314.—Diagram of conventionalized excretory tubule. , ascending limb of
Henle’s loop; b, Bowman’s capsule of Malpighian body; c'—c?, first and second con-
voluted tubules; ct, collecting tubule; d, descending limb of Henle’s loop; g, glomerulus
of Malpighian body; with artery and vein; 4, Henle’s loop; n, nephrostome opening into
coelom; x, entrance of other tubules into collecting duct.

projects into the other, nearly filling the cavity. This inturned portion
is the glomerulus. It consists of a network of capillary blood-vessels,
supplied by an artery and drained by a vein. Beyond the connexion
of the Malpighian body the tubule becomes contorted or convoluted
and its cells are strongly glandular in character. This first convoluted
tubule is succeeded by a nearly straight tract, folded once on itself into

Fic. 315.—Diagram of renal (Malpighian) corpuscle. a, artery; b, Bowman’s capsule;
. gi, glomerulus; ”, nephrostome; ¢, nephridial tubule; v, vein.

the descending and ascending limbs of Henle’s loop. Next follows
the second convoluted tubule, which passes by means of a short
connecting tubule into a non-glandular collecting tubule into which
several other systems of excretory tubules enter, and which leads
more or less directly into the urinary duct which conveys the waste
from the excretory organ to the exterior.

310 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

One or another of these typical parts may be lacking in certain
groups. Thus in the amniotes the nephrostomes are never formed,
though ‘they occur in most ichthyopsida. In the pronephros the
Malpighian corpuscle is rudimentary or lacking at all stages while
there is no differentiation of convoluted tubules and Henle’s loop.

The function of the various parts of the nephridial tubule is in
outline as follows: Theoretically it would appear that in the primitive
condition the nitrogenous waste, which is elaborated in the liver,
collected in the ccelom and, together with the coelomic fluid, was
passed outward through the nephrostomes and the tubules, which
acted merely as ducts. Higher in the scale the parts become more
differentiated and specialized. The renal corpuscles form a filtering
apparatus by which water is passed from the blood-vessels of the glom-
erulus into the tubules near their beginning, and this serves to carry
out the urea, uric acid, etc., secreted by the glandular portions of the
walls of the tubules (convoluted tubules, ascending limb of Henle’s
loop).

In development there may be three successive series of nephridial
structures, the higher number occurring only in the amniotes. These
are known as the pronephros (head kidney), mesonephros (Wolffian
body), and the metanephros (permanent kidney of the amniotes).
All three dre closely related in development and structure but are
distinguished by differences in origin and in the final details. Three
views are held as to their relations one to another. According to one
they are parts of an originally continuous excretory organ (holone-
phros) which extended the length of the body cavity. This has be-
come broken up into the separate parts which differ merely in time
of development and function, with minor modifications in details. A
second view is that they are three separate organs, while a third regards
them as superimposed structures which occasionally overlap (birds,
gymnophiona) and thus are not, strictly speaking, homologous but
rather homodynamous. The first view has the most in its support,
but for convenience the three structures are kept distinct here. All
arise from the mesomeric somites or from the Wolffian ridge which
appears on either side of the median line where the mesomeres separate
from the rest of the wall of the body cavity, the mesomeric cells furnish-
ing the nephrogenous tissue from which the definitive organs develop.

Pronephros.—The pronephros is the first to appear in develop-
ment. As will be recalled (p. 14) the mesomere, like the epimere,

UROGENITAL SYSTEM. gir

becomes segmented, and later, when the epimere separates to form
the myotome, the dorsal end of each mesomere becomes closed, the

whole then forming a sac, opening below into the ventral, undivided ©
coelom (metaccele). A varying number of these nephrotomes (as
they are called) lying a little behind the head are concerned in the

Fic. 316.—Scheme of origin of pronephric tubules after Felix. A, earlier, B, later
stage. c, ccelom; d, pronephric tubule and duct; e, epimere; h, hypomere; m, mesomere
(lined); 2, nephrostome; my, myotome; so, sp, somato- and splanchnopleure.

formation of the pronephros (two in most urodeles and amniotes;
three in lampreys, anura, some sharks and some amniotes; four or
five in some sharks and Lepidosteus; seven or eight in skates; eight to
eleven in Amia; and a dozen in some cecilians; while it is claimed that
the whole series of nephridial tubules of Bdellostoma is pronephric).
The somatic wall of these nephrotomes (fig. 316) grow out toward

Fic. 317.—Reconstruction from longitudinal sections of pronephros of Hypogecphis
(cecilian), after Brauer. Pronephric duct (fd) and primary pronephric tubules light;
the rest of the somites (nephrotomes) black; glomeruli between tubules 2-8. The three
trunk somites in front of 1 develop no tubules.

the ectoderm, thus forming slender pronephric tubules (or solid cords
which later become canalized), the proximal end of each communica-
ting freely with the metaccele by way of the cavity of the nephrotome,
the opening of the latter into the metaccele being the nephrostome.
As will be understood, these tubules, like the nephrotomes, are meta-
meric in character, equalling the somites in number. ‘The distal ends

312 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

grow outward until they are just beneath the ectoderm, when they
bend toward the posterior end of the body, the anterior tubules fusing
. with those behind. From the junction a tube, the pronephric or
archinephric duct, gradually grows backward just beneath the
ectoderm (figs. 317, 318) until it reaches the posterior end of the meta-
coele, when it fuses with the hinder end of the digestive tract (cloaca)
or with the ectoderm in the vicinity of the anus. An opening now
breaks through, thus putting the ccelom indirectly in communication
with the outer world.

At first the pronephric duct lies closely below the ectoderm and is
almost equally near the lining of the metaccele. As the myotomes
grow downward they come to lie between the ducts and the ectoderm
so that eventually the ducts are just beneath the lining of the definitive
body cavity. |

There has been considerable dispute as to the origin of the cells which form the
pronephric duct. They were long thought to be solely of mesothelial character,
arising by proliferation from the tube itself. Then it was noticed that the back-
ward-growing tube fused at its tip with the ectoderm and it was thought that there
was an actual contribution of ectodermal cells at this point. This view received
considerable support from its agreement with certain theoretical views. The
matter is not yet decided. The writer is convinced, from the study of perfectly
preserved material in which cell boundaries are clearly shown, that in the sharks
(Acanthias) which were thought most strongly to support the view of ectodermal
contribution, that the whole duct is of mesothelial origin.

In the teleosts the dorsal end of the nephrotome grows out to form the pro-
nephric tubule, to which both somatic and splanchnic walls thus contribute. In
the amphibia the nephrotome is not distinctly separated from the lateral plates
(hypomere) and the pronephric tubules are formed from the common area.

The pronephros is functional for a time in the embryos of some
lower vertebrates; in other groups it is a rudimentary and transitory
structure, save for its participation in the oviducts and the ostium
tube abdominale (see below). When functional it takes the nitro-
genous waste from the body cavity, while its filtering apparatus con-
sists either of separate glomeruli (one for each tubule) or the glomeruli
of the separate somites may run together, forming a glomus. These
glomeruli or the glomus of the pronephros do not project into a Bow-
man’s capsule, but lie immediately above the dorsal wall of the ccelom,
between the mesentery and the nephrostomes (fig. 318), pushing the
epithelium before them. Later, as in the cecilians, they and the
nephrostomes may be enclosed in a cavity cut off from the ccelom,

UROGENITAL SYSTEM. 313

-so that the whole resembles a renal corpuscle, but is different in origin.
In either case the exuding fluid passes into the metaccele from which
it is drawn by the cilia of the nephrostomes and passed into the tubules.

The blood is brought to the glomus or glomeruli by short segmental
arteries arising from the dorsal aorta (fig. 318) and, after passing
through the capillaries, it is carried away by the postcardinal veins
of the corresponding side to the heart, these veins keeping pace in
their backward development with the development of the nephridial
tubules.

Fre. 318:==Stereogram of developing pro- and mesonephros. a, aorta; g, glomus or
glomerulus; m, mesentery; mt, mesonephric tubule; , notochord; nc, cavity of (nt) nephro-
tome; ms, nephrostome; pc, postcardinal vein; pd, pronephric duct; pf, pronephric tubule;
ptm, peritoneal membrane.

There is much that goes to show that the pronephros formerly had a much
greater extension than at present, including a larger number of somites. It has,
however, been replaced in the adults of all vertebrates (with the possible exception
of Bdellostoma) by the mesonephros, and later, in the amniotes, by the metanephros
as described below.

Mesonephros.—The mesonephros or Wolffian body is the second
excretory organ to arise. It arises after the pronephros and its
duct are formed, by the development of a series of mesonephric tubules,
which grow out from the nephrotomes behind those concerned in the
formation of the pronephros. These tubules extend laterally until
they meet and fuse with the pronephric duct, which now acts as the
excretory canal of the new gland. In some cases the point of origin

314 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

of the mesonephric tubules is clearly dorsal to that of the pronephric
tubules (fig. 318), and in some cases (birds, cecilians) pro- and meso-
nephric tubules have been described as arising from the same nephro-
tome, one above the other. In most ichthyopsida the opening of the
nephrotome into the metaccele forms the nephrostome, but in the
amniotes this opening is closed before the tubules are formed and
consequently nephrostomes are lacking in the latter group.

__ FIG. 319.—Stereogram of mesonephros. a, aorta; cv, postcardinal vein; g, genital
ridge; gl, glomerulus; m, mesentery; mc, Malpighian corpuscle; mt, mesonephric tubules;
my, tayotome; n, nephrostome; nc, notochord; #, peritoneal lining; w, Wolffian duct.

Segmental arteries grow out from the aorta to the splanchnic wall
of each nephrotome, forming there a network of capillaries at a higher
level than the pronephric glomeruli (fig. 319). The glomerulus thus
formed presses the wall before it, while the rest of the nephrotome
closes around it as a Bowman’s capsule, the whole forming a Mal-
pighian body (in some rodents the glomeruli are rudimentary or absent).
In most ichthyopsida the Malpighian body is connected on one side
with the metaccele by the nephrostome, and on the other with the
mesonephric tubule.

UROGENITAL SYSTEM. 315

Thus at first the mesonephros is a metameric structure, extending
over a much larger number of somites than does the pronephros and
reaching nearly to the posterior limits of the metaccele. As the devel-
opment of the embryo proceeds the number of tubules increases by
budding in a manner not readily described (fig. 320). These tubules
unite with the distal ends of those first formed, so that the distal part
of these form collecting tubules. Each of these secondary tubules
forms its own Malpighian body and all of the tubules elongate, be-
come convoluted, and the mesonephros loses its primitive metameric
character.

Fic. 320.—Reconstruction of three somites of the Wolffian body (mesonephros) of
Hypogeophis, after Brauer. a, aorta; m'—m?, primary and secondary Malpighian bodies;
n'-n?, corresponding nephrostomes; s, tertiary segments of mesonephros; ¢'~i?, primary
and secondary mesonephric tubules; w, Wolffian duct.

At the same time changes are introduced into the mesonephric
circulation. The veins emerging from the renal corpuscles extend
out into the region of the tubules, each breaking up there into a second
system of capillaries which envelop the tubules before returning the
blood to the postcardinal vein. The subcardinal vein (p. 279) brings
the blood from the caudal region (and usually from the hind limbs)
to the Wolffian body and this is also returned via the postcardinals to
the heart. (For details of the modifications of the mesonephric circu-
lation see pages 290-292.)

The Mesonephric Ducts.—The conditions in the elasmobranchs
have been regarded as very primitive. In them (and to some extent
in some of the amphibia), when the mesonephros develops, the pro-
nephric duct divides longitudinally from its hinder end as far forward
as the anterior end of the Wolffian body. Of the two ducts thus
formed (fig. 321, A), one, the Wolffian (Leydig’s) duct, remains
connected with the tubules of the mesonephros and forms its excretory
canal. The other, the Miillerian duct, is similarly related to the

316 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

pronephros and its derivatives, and in the female forms the tube
(oviduct) by which the eggs are carried to the exterior. In other
amphibia and in the amniotes the pronephric duct does not divide,
but remains solely in the service of the mesonephros and -forms the
Wolffian duct, while the oviduct arises in another manner, to be de-
scribed in connexion with the reproductive organs. In the teleosts
also there is no division of the pronephric duct.

Fic. 321.—Diagrams of urogenital structures in (A) indifferent and in female elas-
mobranchs and amphibians; (B) male elasmobranchs and amphibians; (C) male amniote
(mammal); (D) female amniote (mammal). 5, urinary bladder; ¢, cloaca; e, epididymis;
k, kidney (metanephros); f, Fallopian tube; g, gonad; h, ‘stalked hydatid’ ; /, longitudinal
tubule; m, Miillerian duct (oviduct), rudimentary in B andC 3 mn, mesonephros; 0, ovary;
ot, ostium tube abdominale; pd, paradidymis; po, paroéphoron; pu, parovarium; r, rectum;
#, testis; u, uterus; wa, urethra; ur, ureter; va, vas aberrans; vd, vas deferens; ve, vasa effer-
entia; w, Wolffian duct, urinary in A, urogenital in B, genital in C and rudimentary in D.

Metanephros.—The mesonephros is functional in the embryos of
all vertebrates and throughout life in the ichthyopsida. It also func-
tions for a short time after birth in certain reptiles (lizards) and in the
lowest mammals (Echidna, opossum). It becomes replaced in the
adults of all amniotes by the mesonephroi, the only structures to which
the name kidneys is strictly applicable. Each metanephros arises
behind the mesonephros of the same side. From the dorsal hinder
end of the Wolffian duct, near its entrance into the cloaca, a tube, the

UROGENITAL SYSTEM.

317

Fic. 322.—Profile reconstructions of lizard (Lacerta agilis) (A) 16 mm. long; (B) 20mm.
long; and (C) human embryo 115 mm. long, after Schreiner. a, allantois stalk; c, cloaca;
cc, cranial collecting tubule; cd, caudal collecting tubule; &, permanent kidney (metanephros);
mct, median collecting tubule; mf, metanephric (nephrogenous) tisssue; mtb, mesonephric
tubules; pct, primary collecting tubule; gu, Wolffian duct (primitive ureter); 7, rectum;

s, secondary collecting tubule; u, ureter; cm, u and pu, common portion of primitive and
permanent ureters.

FIG. 323.—Models of two stages in the development of tubules of kidney (metanephros)
of man, after Stoerk. 6, Bowman’s capsule; c, collecting tubule; cm, connecting tubule;
cv, convoluted tubule; #, Henle’s loop; i, intercalary tubule; /, lower arch; m, middle piece.

318 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

ureter (fig. 322, 8) grows forward, parallel to the parent duct, into .
the tissue posterior and dorsal to the mesonephros. This nephro-
genous tissue is apparently serially homologous with that from which
the mesonephric tubules have arisen, but all traces of metamerism
have disappeared from it. In this nephrogenous tissue the anterior
end of the ureter gives off a varying number of branches (fig. 322),
each of which expands at its tip, thus forming a primary renal vesicle,
and a little later the place where the branches and the ureter unite
expands, the enlargement forming the pelvis of the definitive kidney.
The cells of the nephrogenous tissue form a number of aggregates
around each primary vesicle; each aggregate soon becomes hollow,
and develops into an S-shaped tubule (fig. 323, left), one end of which
joins the primary renal vesicle, while a glomerulus arises at the other
end, but no nephrostomes are formed. Later there is a great mul-
tiplication of these tubules and an extension of the capillary system
of the glomeruli around them, much as in the mesonephros. The
differentiation of each tubule into convoluted, collecting and Henle’s
regions occurs early (fig. 323, right). A

Urinary Bladder.—At or near the hinder ends of the excretory
ducts there is frequently a reservoir for the urine, the urinary bladder
or urocyst. Of these there may be three kinds. In most fishes the
bladder arises by a fusion of the hinder-ends of the Wolffian ducts
plus a part derived from the hinder end of the digestive tract (cloaca),
the Wolffian ducts emptying into it and the whole opening to the
exterior, usually dorsal and posterior to the anus. In the dipnoi there
is a diverticulum from the dorsal wall of the cloaca, anterior to the
openings of the Wolffian ducts. This is usually called the urinary
bladder (fig. 325, D), but it may be homologous with the rectal gland
of the elasmobranchs.

The third type, the allantoic bladder, occurs in all tetrapoda.
This arises as a ventral diverticulum from the cloaca. In the amphibia
the whole of the outgrowth forms the bladder and its walls are sup-
plied by the hypogastric arteries. In the amniotes the proximal
portion alone is converted into the urinary bladder, while the more
distal portion, in the embryo becomes the respiratory organ of the
growing young, the allantois. This part extends far beyond the body
wall, carrying with it branches of the hypogastric arteries (allantoic
arteries), and in the mammals forms a part of the placenta. The
allantois becomes reduced in the later stages and at the beginning of

UROGENITAL SYSTEM. 319

free life is entirely absorbed or is lost with the placenta. In the
amphibia the urine finds its way into the urinary bladder via the cloaca,
as the urinary ducts (Wolffian ducts) do not open into it. In those
amniotes in which a bladder is present the ureters open into it, and
the urine is conveyed to the exterior by a single tube, the urethra.
In many sauropsida there is no urinary bladder, though the allantois
is formed in development.

There is great difficulty in comparing the excretory system of the vertebrates
with anything known in the invertebrates. In general the nephridial tubules may
be compared with those of the annelids, Both have nephrostomes opening into
the ceelom, convoluted tubules, enveloped in a network of capillary blood-vessels,
but in the annelid each tubule opens separately to the exterior in the somite behind
that in which the nephrostome lies, while in the vertebrate the series of tubules
empty into acommon duct, Whenit was thought (p 312) that the ectoderm con-
tributed to the pronephric duct, the homologies appeared easy. The duct was
originally a groove on the outer surface into which the separate tubules opened.
Then the groove was rolled into a tube which continued backward to the vicinity
of the anus By the downgrowth of the myotomes the duct became cut off from
its primitive position and came to lie just outside the peritoneal lining When,
however, it is considered that in all probability the pronephric duct is formed solely
from the mesoderm the homology falls to the ground and an explanation is still a
desideratum.

THE REPRODUCTIVE ORGANS.

The tissue which is to form the ovaries and testes early forms a
pair of genital ridges, one on either side of the mesentery and between
it and the Wolffian ridge (fig. 319). At one time it was thought that
the anlage of the gonad was segmental in character and ‘ gonotomes,’
comparable to nephrotomes and myotomes, were described. It has
since been shown that no metamerism exists and that the primary
germ cells, which alone characterize the gonads, arise in several groups
of vertebrates (possibly in all) from the entoderm, which is never
metameric. At about the time of the differentiation of the somites
they migrate through the developing mesoderm to their definitive posi-
tion in the epithelium of the genital ridges, the primitive or primordial
ova (whether to form eggs or sperm) being recognizable from their
size and their reaction to microscopic stains (fig. 324, 0). In the
adults of many vertebrates the gonads at maturity project far into
the coelom and are often suspended by a fold of peritoneum which is
called a mesorchium in the male, a mesoarium in the female.

320 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

Ovaries.—In the ovarian epithelium the primitive ova multiply,
and the products, accompanied by some of the epithelial cells, sink
into the deeper stroma of connective tissue, thus forming ovarial cords
each containing a number of ova. Then the cords break up and each
egg becomes surrounded with a layer of epithelial cells, the whole
forming a Graafian follicle, the follicle cells supplying nourishment
to the contained ovum. In the higher vertebrates there is a great
increase in the number of follicle cells, which become arranged in
several layers. Then a split arises in the follicle, the cavity becoming

i
)
3

an

oe
pa

ic

CH
&

CS
a)

i

cigs

OO

Fic. 324.—Section of genital ridge of chick of five days’ incubation, after Semon. ¢, epithel-
ium of ridge (coelomic wall); c, medullary cords; 0, primordial ova.

filled with a follicular liquor, while the ovum, surrounded by several
layers of cells, adheres to one side of the cavity, this part being called
the discus proligerus.

When the eggs have attained their full size and the proper time
has arrived some of the follicles migrate to the surface of the ovary,
then the follicles rupture and the contained ova escape into the ccelom.
Their history from this point will be outlined in connection with the
genital ducts. Each ruptured follicle (at least in elasmobranchs,
amphibians and amniotes leaves a scar on the surface of the ovary—
the corpus luteum—characterized by the presence of peculiar (‘lutein’)
cells.

Testes.—In the gonads of the male (testes) there is a somewhat
similar insinking of the primordial ova and epithelial cells into the
stroma of the genital ridge, but, instead of breaking up into separate
follicles, each sexual cord develops a lumen and becomes converted

UROGENITAL SYSTEM. . 321

into a seminiferous tubule, in the walls of which both the epithelial
cells and the primordial ova are recognizable, as well as a third kind
of cell, called Sertoli’s cell, concerning which accounts are some-
what at variance, some regarding them as derivatives of the epithelial
cells, others as coming from the primitive germ cells. They play no
part in the actual formation of the spermatozoa, but act rather as
nutritive or ‘nurse cells’ for the developing spermatozoa. For the
differentiation of the germ cells into spermatozoa reference must be
made to the text-books of embryology and histology. In most verte-
brates the testes continue in the position where they first appear, but
in most mammals they eventually descend to a position outside of the
body cavity and are enclosed in a special pouch, the scrotum. This
descent of the testes is described in connection with the reproductive,
organs of the mammals, below.

THE REPRODUCTIVE DUCTS.

The reproductive products formed in the gonads have to be carried
to the exterior, either as spermatozoa, or as eggs or young in different
stages of development, the ducts in the male being called vasa defer-
entia, those of the female being oviducts. The’ former are usually
the Wolffian ducts, the latter may be either the Miillerian ducts or
tubes developed for the special purpose, or lastly, the abdominal pores.

Male Ducts.—In elasmobranchs, amphibia and amniotes the
Wolffian ducts (fig. 321) serve as the outlet for the sperm. While
the seminiferous tubules are developing, there occurs a proliferation
of cells from the wall of the Bowman’s capsules in the anterior end
of the mesonephros. These medullary cords extend through the
adjacent connective tissue and into the genital ridge where they.come
into close connexion with the developing seminiferous tubules (fig.
324). When the latter acquire their lumen the medullary cords also
become canalized, so that both form a continuous transverse tubule
(vas efferens) leading from the genital cells to the Malpighian cor-
puscles, and thence by the mesonephric tubules to the Wolffian duct
(fig. 325, A). These vasa efferentia become connected by a longi-
tudinal canal before entering the Wolffian body, while usually there
is another longitudinal canal connecting them in the body of the testis
(fig. 321, B). Usually this connexion of testis and Wolffian body
takes place at the anterior end of the mesonephros, but in some dipnoi

21

322 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

the posterior end of the mesonephros alone is involved. This is fre-
quently accompanied by a degeneration of the glomeruli of the tubules
concerned, so that this part of the mesonephros loses its excretory
character and becomes subsidiary to reproduction. With this forma-
tion of vasa efferentia the sperm never enters the coelom except as this
is represented in the cavities of the mesonephric tubules.

As a farther result the anterior end of the Wolffian duct becomes
purely reproductive in the male and is usually greatly coiled, this
portion being called the epididymis. In the amniotes, where the
hinder portion of the mesonephros is supplanted by the true kidney
(metanephros), the whole Wolffian duct is a sperm duct (vas deferens)
in the male, while in the female it largely or completely degenerates.
In the amphibia and elasmobranchs the hinder end of the duct is both
reproductive and excretory in the male; in the female it is purely
excretory.

In the ichthyopsida, other than elasmobranchs and amphibia, the
sperm is carried to the exterior in other ways, and there is no connexion
of the testes with the excretory organs. In the cyclostomes the sperm
escapes from the testes into the coelom and then is passed to the exterior
by way of the abdominal pores (p. 124) which in the lampreys open
into a cavity (sinus urogenitalis) which also receives the hinder ends
of the Wolffian ducts. In the myxinoids the pores are united and
open to the exterior behind the anus and between it and the urinary
openings.

The conditions found in the sturgeons (fig. 325, A) and in Polyp-
terus give a possible explanation to the aberrant structures of the tele-
osts. In the first group can be made out the vasa efferentia and the
two longitudinal canals connecting them, these extending the whole
length of the testis. In Polypterus (fig. 325, C) the connexion between
the testis and mesonephros is confined to the hinder portion of
organs, the anterior vasa efferentia and the longitudinal canal
disappearing in front, the longitudinal testicular canal taking the
sperm from the anterior end of the testis and carrying it farther back
for passage through the mesonephros. Here the anterior end of the
Wolffian duct is purely excretory. A farther concentration of the
efferent functions to the last vas efferens would give, with a few other
modifications, the conditions of the teleosts (fig. 325, B). In all of
this group there is no connexion of testes with mesonephroi. The
seminiferous tubules are connected by a longitudinal canal (apparently

UROGENITAL SYSTEM. 323

the longitudinal testicular canal of other vertebrates) which runs in
the membrane (mesorchium) supporting the testis, back to the external
opening, which is either directly to the exterior between the urinary
opening and the anus (fig. 328) or into a urogenital sinus (fig. 321, B).
This view is farther supported by the relations in the dipnoi. In
Ceratodus there are numerous vasa efferentia which extend from the
testis into the mesonephros. In Lepidosiren the efferent ductules are

Fic. 325.—Diagrams of urogenital organs of male fishes, after Goodrich. A, Acipenser
(Lepidosteus and Amia similar, but lack the oviduct); B, teleosts; C, Polypterus; D, Pro-
topterus; E, urogenital openings of female salmon. uw, anus; apf, abdominal pore; cb,
cloacal (‘urinary’) bladder; e, vasa efferentia; gp, genital pore (papilla); m, mesonephros;
md, Miillerian(?) duct; 7, rectum; rc, renal corpuscle; s, urogenital sinus; ¢, testis; u, up,
urinary pore; wgp, urogenital pore; v, vas deferens; w, Wolffian duct.

fewer in number and they arise from a posterior degenerate portion
of the testis, while in Protopterus (fig. 325, D) there is but a single
vas efferens on either side and this passes through the posterior end
of the Wolffian body.

Oviducts.—In the elasmobranchs the Miillerian duct, which, as
described above, arises by a splitting of the pronephric duct, serves
as the oviduct. After separation from the Wolffian duct this opens
in front into the coelom by means of the pronephric tubules and their

324 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

nephrostomes. Then these flow together, forming a large opening,
the ostium tubz abdominale, on either side (in elasmobranchs the
ostia of the two sides are usually united ventral to the liver) through
which the eggs, which pass from the ovary into the ccelom are carried
into the oviduct.

In some amphibia (Salamandra) the pronephric tubules and neph-
rostomes take a part in the formation of the ostium tube and the
beginning of the oviduct, while in Amblystoma the ostium develops
in close connection with the pronephric nephrostomes. Here, as in
all other tetrapoda, the rest of the oviduct arises by the formation of a
groove of the peritoneal membrane close beside the Wolffian duct.
This becomes rolled into a tube, the Miillerian duct. In the amniotes
the anterior, end of the groove does not close, but remains open as the
ostium tube (fig. 321, A). :

Usually the condition in the elasmobranchs has been regarded as
the primitive one, a supposition which renders it difficult to homologize
the Miillerian ducts (oviducts) of elasmobranchs with those of other
forms. Still, when the adult conditions are considered—similar ostia,
similarity of position, and of external openings—it is hardly possible
to believe them as merely analogous, as examples of convergence.
The facts in the amphibia, referred to in the preceding paragraph
are additional evidence of homology. If, however, it be assumed
that the more common type of development, by the infolding of cce-
lomic epithelium, be the primitive condition, the difficulties are less,
though not entirely solved. Then, if it be that the homologous tissue
in the elasmobranchs was at first included in the tissue of the pro-
nephric duct and that the splitting is a secondary operation to separate
parts which elsewhere are always distinct, the similarities are more
apparent.

In the females, as in the males, of cyclostomes and teleosts the
reproductive ducts are not easily brought into harmony with those
of other vertebrates, and an answer to all questions cannot be had until
the development of the parts has been studied in more forms, and
especially the ganoids and dipnoi. In the cyclostomes the eggs are
shed from the ovaries into the ccelom and are thence passed outward
through the abdominal pores.

In the teleosts there are several conditions. The ovaries may be
simple and solid bands or saccular in character with an internal lumen
(fig. 326, E). In the first the eggs pass into the coelom and thence

UROGENITAL SYSTEM. 325

to the exterior by abdominal pores or by oviducts of varying lengths
(fig. 326, F). Concerning the nature of these ducts there is uncer-
tainty. They may be true Miillerian ducts or new formations within
the group. The fact that similar tubes occur, with permanently open
ostia in both sexes of the sturgeons (fig. 325), and that these open

Fic. 326.—Diagrams of urogenital organs of female fishes, after Goodrich. A, Pro-
topterus; B, Polypterus; C, Amia; D, Lepidosteus; E, most teleosts; F, salmonid. ap, ab-
dominal pore; cb, cloacal bladder; cl, cloaca; f, funnel of oviduct; gp, genital pore or papilla;
m, mesonephros; 0, ovary; od, oviduct; 7, rectum; s, urogenital sinus; up, urinary pore,
(papilla); «gp, urogenital pore (papilla); w, Wolffian ducts.

behind into the Wolffian ducts, lends probability to the view that the
ducts of the ordinary teleosts are Miillerian in character, but greatly
modified.

The saccular condition of the ovaries appears to arise in two ways.
In the one the primitively free edge of the ovary bends laterally and
fuses with the coelomic wall, thus enclosing a cavity, the parovarial

326 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

canal, closed in front. In the other type a groove of the covering
epithelium forms on the surface of the ovary. This closes over and
sinks inward, forming what is termed as an entovarial canal. Either
canal may extend backward to the hinder end of the body cavity, thus
forming an oviduct, or the oviduct may be formed from both kinds of
canals, one in front, the other behind. From this it would appear
that the ovary originally extended back to the hinder end of the coelom
(as it does in Cyclopterus) or that the par- or entovarial canal had
united with a Miillerian duct which has otherwise been entirely lost.
The oviducts thus formed usually unite before opening to the exterior,
either directly or via a urogenital sinus. The oviducts in the dipnoi
(fig. 326, A) are much like those of the selachians, emptying inde-
pendently into the cloaca. They persist, though of small size, in the
males (fig. 325, D).

EXCRETORY ORGANS IN THE SEPARATE GROUPS.

CYCLOSTOMES.—In the lampreys the pronephros extends over thirteen
somites, but only the anterior five form complete tubules, the remainder, however,
join the pronephric duct. The pronephros is best developed in the Ammoccete,
zo mm. long, and in this stage the mesonephros is also developed and both are
functional. With increase in size there is a degeneration of the mesonephric tubules
in front and a formation of new ones behind, the definitive organ extending over
about two-fifths of the body length. Each pronephros projects into the ccelom as
a band supported by a fold of the peritoneal membrane. The two pronephric ducts
unite a little in front of the hinder end, forming a urogenital sinus into which the
abdominal pores empty, and which, in turn, opens at the tip of a urogenital papilla
just behind the anus.

In the myxinoids the nephridial tubules develop as a continuous series, the
organ in the earliest stage known extending over somites 11-80. Later the organ
becomes divided into two parts by the degeneration of the intermediate tubules.
The anterior part projects into the body cavity and is provided with nephrostomes,
while the posterior part, reaching through some twenty or thirty somites, has its
tubules strictly segmental, each with a Malphigian body. This is the functional
excretory organ.

ELASMOBRANCHS.—The pronephros is never furictional as an excretory
organ. The Wolffian bodies of the two sides are somewhat influenced in form by the
other viscera, and are sometimes asymmetrical. Usually the nephrostomes are closed
in the adult, but they persist in several genera, among them Acanthias, while they are
lacking in Scyllium and Raia. The anterior end of each mesonephros is narrowed
and serves as the connexion with the testes in the male, while the anterior end of
the Wolffian duct forms a much-coiled epidymis in the same sex. A urinary blad-
der is formed by the union of the ducts of the two sides. In the female the blad-

UROGENITAL SYSTEM. 327

der opens to the exterior at the tip of a genital papilla, but in the male it connects
with a urogenital sinus, into which a pair of reservoirs of sperm empty. The duct
from the urogenital sinus opens into the cloaca at the tip of a urogenital papilla.
In Chimera the anterior end of the mesonephros lacks Malphigian bodies and forms
a large (Leydig’s) gland, the secretion of which may possibly be used in dissolving
the spermatophores (fig. 331).

GANOIDS.—In Polypterus the pronephric tubules are two in number, belonging
to the second and fifth post-otic somites; in Lepidosteus there are five or six; sturgeon
six; and Amia eight to eleven. The large size of the pronephros in Polypterus is
due to the extensive coiling of the anterior end of the duct. In the sturgeon a part
of the excretory organ is separated from the rest but it is not certain that this is
really a pronephros.

The mesonephros is markedly segmental, the glands of the two sides being en-
larged and united behind in the sturgeon. Nephrostomes are late in appearance, not
being formed until after the tubules have joined the duct. The urinary bladder
differs from that of teleosts in that the Miillerian ducts enter it.

TELEOSTS (fig. 327) have a pronephros which extends over from one to five
somites. It is usually transitory in character, but it persists through life in several
species and functions during the larval stages in many more. The mesonephros
varies considerably in shape. Where there is an air bladder this covers some or all
of the ventral surface of the mesonephroi. Frequently the organs of the two sides
are united behind, while lobes may extend forward from the main mass, or back
into the tail. The duct is sometimes visible from bélow, sometimes it is immersed
in the mass of the organ. There is no sexual part to the mesonephros and there
are no nephrostomes in the adult. The urinary ducts of the two sides unite behind
and from the united portion and from the ventral wall of the cloaca the urinary
bladder is formed. Later the opening of the bladder separates from the cloaca and
usually comes to lie behind the anus, sometimes united with the sexual openings.

DIPNOI.—In Ceratodus there are two pronephric tubules, that of the third
somite being complete, that of the fourth rudimentary. The glomerulus lies beside
the open nephrostome. The mesonephros is at first strongly metameric. There
are no nephrostomes in the adult and none appear at any time in Lepidosiren. The
adult mesonephros'is widest behind, but the relations of the efferent ductules of the
male are differently arranged in the separate genera, as mentioned above.

AMPHIBIA.—The pronephros (developing from two somites in the urodeles,
three in anura and twelve or more in gymnophiones) retains its functions in uro-
deles and anura until the metamorphosis, when its tubules degenerate. At first
the mesonephros consists of a tubule with nephrostome and renal corpuscle for each
somite, but in the adult this metamerism is lost, except at the anterior end, by the
development of secondary tubules, each complete like the original ones, the nephro-
stomes sometimes amounting to over a thousand on the ventral surface of each
Wolffian body. In the adult anura the nephrostomes lose their connexion with
the excretory system and join branches of the renal arteries, thus placing the cceelom
in connexion with the circulatory system.

In the urodeles the mesonephroi form a pair of ridges on the dorsal wall of the
coelom, but they occasionally project as folds. Their length is somewhat propor-

328 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

tional to the total body length. The anterior end of each loses its excretory char-
acter and in the male becomes accessory to reproduction, as described above (p.
522). In the anura the organs are more compact and the differentiated anterior
end is lacking, though the efferent ductules of the testes pass through the organ.
The cecilians (fig. 334) resemble the urodeles, except in having the mesonephroi , , |
more lobulated, the result of aggregates of tubules around the collecting tubules.

Fic. 327.-—Urinary organs of teleosts, after Haller. A, pronephros and ducts of young
Salmo fario; B, excretory organs of adult perch, Perca fluviatilis; C, of carp, Cyprinus carpio;
uw, aorta; cv, caudal vein; d, urinary duct; m, mn, mesonephros; ped, pcs, right and left
postcardinal veins; p, pn, pronephros; 7, rectum; u, urinary bladder; w, wd, Wolffian duct.

The Wolffian ducts are excretory in both sexes and are also reproductive in the
male. The ducts of the two sides open separately into the cloaca, with, usually in
the male, an enlargement, the seminal vesicle, which in the breeding season serves
as a reservoir for the sperm. ‘The urinary bladder differs from that of the ichthy-
opsida in being ventral to the cloaca; it is of the allantoic type (p. 318). It is very

UROGENITAL SYSTEM. 329
4

long in the cecilians (fig. 334) and Amphiuma, saccular in most urodeles, and bifid
at the tip in most anura, being even divided into two sacs, connected only at the
opening into the cloaca in some species.

SAUROPSIDA.—In reptiles and birds, as in all amniotes, the pronephros is
rudimentary at all stages and never functions as
an excretory organ. The mesonephros takes its
place in foetal life, and in some it continues to
function for some time after hatching, but in all it
is eventually replaced by the metanephros, though
its degenerate remains persist in the reptiles (better
preserved in the female) forming the so-called
‘golden yellow body.’ Another part is retained
in the male as a part of the efferent ductules of
the testes, somewhat as in mammals.

The metanephros (fig. 328) never has the ex-
tent of the mesonephros of the ichthyopsida, but
it is usually restricted to the posterior half of the
body cavity, often to the pelvic region. It is usu-
ally small and compact (snakes form an exception)
or somewhat lobulated, in the snakes the lobulation
sometimes being so extensive that the lobules’ are
only connected by the ureter. In the lizards the

Fic. 328. Fic. 329.

Fic. 328.—Urogenital organs of Monitor, after Gegenbaur. d, opening of digestive
tract into cloaca; e, epididymis; k, kidney; p, papille of urogenital system; 7, rectum;
t, testes; u, ureter; od, vas deferens.

Fic. 329.—Urogenital organs in pig embryo 67 mm. long, after Klaatsch. a, allantois;
&, gonad; ms, mt, meso- and metanephroi; sv, adrenal.

organs of the two sides may be connected behind. In the birds there are usually
three lobes in each mesonephros, these lying in cavities in the pelvis between
the sacral vertebra and the transverse processes.

330 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

The Wolffian ducts persist only as the ducts of the testes (vasa deferentia) and
the ureters take their place as carriers of the nitrogénous waste. These latter
tubes open separately into the cloaca. An (allantoic) urinary bladder is found
only in lizards and turtles (fig. 313). The urine is semisolid and consists largely
of uric acid.

MAMMALS.—In the mammals but two tubules are outlined in the
pronephros and these never become functional. The pronephric duct
is formed as a solid cord on the surface of the nephrotomic segments
which later becomes canalized. Of the fate of the pronephros
nothing certain is known. The mesonephros (fig. 329), on the other
hand, is an important structure in foetal
life, and in the monotremes and mar-
supials it continues to function in the im-
mature stages. Later it largely disap-
pears in all, with the exception of the
parts concerned in the formation of the
efferent ductules of the testes and some
inconsiderable remnants in both sexes.
Only in Echidna are nephrostomes
formed and in some rodents there is no
formation of glomeruli.

The peculiar development of the
mammalian metanephros (p. 316) results
in the kidney of the young stages having
a lobulated appearance, the lobules cor-

responding to the ducts given off from
aed cicee De the end of the ureter, so that each has
densis (Princeton, 2234). a, aorta; itsownduct. This condition is retained
#, ureter; v, postcava. A

in the adult elephants, some ungulates,
carnivores (fig. 330) and primates, and especially in the aquatic
species (whales, seals), the lobules being most numerous in some of
the whales. In all other forms the ducts fuse later and the lobules
unite into a compact mass lying in the lumbar region near the last
rib. Each kidney has a peculiar shape (giving rise to the adjec
tive reniform), convex on the lateral, concave on the medial sur-
face, the latter being called the hilum and receiving the excretory
duct (ureter) and the blood-vessels of the organ (hepatic artery
and vein). Just inside the hilum is a cavity, the pelvis of the
kidney, into which one or several papille project, each bearing the

UROGENITAL SYSTEM. 331

openings of numerous collecting tubules (p. 309). In section the
substance of the kidney shows two different textures, recognizable
to the naked eye. There is an quter cortical and an inner medul-
lary substance, the two interlocking as a series of pyramids. These
different appearances are due to the fact that the cortex contains the
renal corpuscles and convoluted tubules, while the medulla is com-
posed of the straight tubules of Henle’s loops and of the collecting
system. ~

The ureters are free for most of their course from the kidney to the
urinary bladder, into which they enter instead of the cloaca. The
bladder, in the monotremes and marsupials, is solely allantoic in
nature, but in the placental mammals a portion of the cloaca is also
included in it. From the bladder a single tube, the urethra, leads to
the exterior. The mammalian urine contains urea instead of uric
acid, a resemblance to the amphibia and a contrast to the sauropsida.

REPRODUCTIVE ORGANS OF THE SEPARATE GROUPS.

CYCLOSTOMES.—The gonads, which are usually unpaired, are supported by
a fold of the peritoneal membrane (mesorchium or mesovarium, p. 122). The eggs
and sperm escape into the ccelom and are carried thence by way of the abdominal
pores. The myxinoids have hermaphroditic gonads, the anterior part being female,
the posterior testicular; but one sex predominates. Nansen believes that the sexes
alternate in function (proterandric hermaphroditism). The eggs of the petromy-
zonts are small, those of the myxinoids are larger and are enclosed in a horny shell,
with anchoring hooks at either end.

ELASMOBRANCHS,—In the elasmobranchs, as in all other vertebrates, the
gonads are at first paired and symmetrical, though occasionally one side or the other
may be reduced or become degenerate or those of the two sides may fuse. Thus in
some skates only the left gonad may be functional. Elsewhere in the group they are
paired and lie far forward, attached to the dorsal wall of the coelom. The Miillerian
ducts of the two sides in the female meet in front in a common opening (ostium
tube), the derivative of the pronephric nephrostomes. This receives the eggs,
which pass from the ovaries into the ceelom. The different parts of the duct are
specialized, the upper part serving as a shell gland, forming the capsule for the
eggs. This is horny and in most species is provided with tendril prolongations at
the four corners, by which the eggs (‘skate barrows’) are attached to submerged
objects. Some species of both sharks and skates are viviparous. In these the
lower part of the Miillerian duct (oviduct) serves as a kind of uterus. In some
species the lining of this uterus is covered by vascular villi, by which nourishment
and oxygen are conveyed to the growing young which escapes in approximately the
perfect shape. The eggs of elasmobranchs are very large, those of some
species exceeding even those of the ostrich in size.

332 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

The testes, supported by mesorchia, lie at various levels in the celom. The
relations of their ducts to the mesonephros are typical (p. 521). The vasa deferentia
of the two side unite just before entrance into the cloaca to form a urogenital sinus,
with which an oval sperm sac is connected on either side. In Chimera the genital
portion of the mesonephros (fig. 331) is
widely separated from the functional por-
tion, the two being connected by the
Wolffian duct. In the male the Miillerian
duct is rudimentary and frequently is with-
out a lumen.

GANOIDS.—Nothing is known of the
development of the sexual organs of the
ganoids, except as to the origin of the germ
cells in two species. In most species the
ovary is band-like and the oviducts open by
broad funnels into the ccelom, but in Lepi-
dosteus the ovary is saccular, the eggs pass-
ing into the central cavity, the duct being
apparently a sterile, backward prolongation
of the ovary. In the male the testes are
frequently lobulated and a system of effer-
ent ductules, connected by a longitudinal
canal, pass from the testes into the meso-
nephros (fig. 325) and thence separately
or by a single tubule into the Wolffian
duct. In the males of all but Lepidosteus
there are short tubes with funnels, appar-
ently the homologues of the oviducts of the
females.

TELEOSTS—In; some’ of the lower. Fw age Tesis she, enterion cfd
et of mesonephbros 0. 1mera, aiter Par-
teleosts (salmonids, etc.) the elongate ovary jer and Burland. bv, blood-vessel;

is solid and the eggs pass from it into the cv/, longitudinal tubule; m, Miillerian
ccelom and are carried thence to the exterior ‘ucts ms, anterior end of mesonephros
fe (Leydig’s gland); spd, sperm duct; ve,
by short peritoneal funnels (fig. 332), or the yet, vasa efferentia; vs, seminal vesicle.
tubes and funnels may be absent and
the eggs then pass out by abdominal pores. In most teleosts, however, each ovary
is a closed sac (like that of Lepidosteus, fig. 326) continued behind by a slender
oviduct. The ducts of the two sides may open separately, but usually their hinder
ends are united and open by a single genital pore between the anus and the rectum
In some instances (fig. 325, E), the urinary and genital pores are on a urogenital
papilla. In the male the elongate testes are either simple or lobulated. Internally
each consists of radial chambers of varying shape which are connected with a
complicated system of tubules which lead to a vas deferens running back to open
into the hinder end of the Wolffian duct, or separately to the exterior (fig. 333, go).
In most teleosts the number of eggs produced in a season is very large, sometimes
numbering millions. Usually, after passing from the oviducts, they are left to the

UROGENITAL SYSTEM. 333

mercy of the water, but a number of species (Embiotocids, Gambusia, several
Cyprinodonts, etc.) are viviparous, the development of the eggs taking place in the
ovary, which sometimes provides nourishment for the growing young. In the
lophobranchs the eggs are received in a pouch between the ventral fins of the male
and are incubated there. Other peculiar breeding habits are known.

IG

CAMA I i

Fic. 332.—Relations of oviducts and pori abdominales in Coregonus, after Weber.
a, anus; 7, intestine; ”, nephridial opening; 0, ovary; p, pore of right side; 7, opening of
oviduct. :

DIPNOI.—In the dipnoi more normal conditions occur. There are oviducts
with inner ostia, resembling in structure, at least, the Miillerian ducts, and especially
those of the amphibia, like them secreting a gelatinous substance around the eggs.
These same ducts are also retained in the male Ceratodus and to a less extent in
the other genera (Lepidosiren and Protopterus), The gonads are long and are cov-

aE Caen
MM MyM Ay wy SUMAN UNE UMD VAM MN My cv
3 S y >, Cre, = se

ee

i
WPVM pIAAAAN Aan

Fic. 333.—Hinder part of urogenital organs of male pike, Esox lucius, after Goodrich
a, anus; ab, air bladder; ao, aorta; d, Wolffian duct; c, cardinal vein; g, genital duct; go,
genital opening; 7, intestine; pc, postcardinal vein; ud, urinary bladder; wo, urinary opening-

ered on the ventral side with lymphoid tissue. The testes in Protoplerus and
Lepidosiren contain numerous alveoli lined with sperm-forming cells. The sperm is
carried into a longitudinal tubule (fig. 325) and from thence by one (Protopterus)
or several efferent ductules to the Malpighian bodies of the posterior end
of the mesonephros, the epididymis thus being posterior in position. In Ceratodus,
which is imperfectly known, the ductules are more numerous and the epididymis
is anterior.

AMPHIBIA.—The amphibians are the most typical of the anamnia, the elasmo-

334 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

branchs excepted. The gonads are roughly correlated in form to the shape of the
body, being shortest in the anura, longest in the cecilians and urodeles. The
ovaries are saccular (a single long sac in urodeles, a number of short ones in anura)
and the eggs pass into the cavity and then break into the celom. The oviducts

Fic. 334.—Male
urogenital organs of
Epicrium, after
Spengel. a, anus; 5,
urinary bladder; ci,
cloaca; f, fat bodies;
m, Miillerian ducts;
mg, glandular part
of same; #, testes; ¢I,
longitudinal __ testi-
cular canal; w,
Wolffian body.

are Miillerian ducts with ostia far forward. In the adults
they are greatly coiled and are glandular, their walls se-
creting the gelatinous substance which envelops the eggs.
Usually the oviducts of the two sides open separately into
the cloaca, but the two unite behind in Bufo.

The testes have both the longitudinal and the testicular
canals connecting the efferent ductules. In the gymno-
phiona (fig. 334) the testes resemble a string of beads, each
bead consisting of a number of seminiferous sacs, the
string being united by the testicular canal. The efferent
ducts pass through the mesonephros, sometimes utilizing
the nephridial tubules, sometimes pursuing a separate course,
the two conditions being found in different species of frog
(Rana) in Europe. Our species have not been studied in
this respect.

The cloaca of the urodeles has a glandular lining and in
the females it contains tubules which act as reservoirs of
sperm. In the male the glands secrete a substance binding
the spermatozoa together In many urodeles fertilization
is internal, though thereis no intermittent organ save the
somewhat protrusible cloacal opening.

There are many interesting accessory reproductive rela-
tions among the amphibia. Thus the cecilians and Am-
phiuma lay their eggs in long strings in the soil and the
female incubates them. The male often takes charge of the
eggs. In Pipa each egg undergoes development in a pit in
the skin of the back of the female and in Nototrema and
Opisthodelphys (South America tree-toads) there is a large
pocket in the skin of the back, opening near the coccyx,
where the eggs are carried until partially (Nototrema) ‘or
entirely developed. Salamandra maculosa and S. atra bring
forth living young, the former being born with gills, the latter
in the perfect condition. Oviposition usually occurs in the
spring in colder climates (in the autumn with Cryptobranchus
of America) and as the drain on the system is very consider-
able immediately after hibernation, the substance of the
fat body, which always is closely connected with the gonads,
is utilized at this time.

‘SAUROPSIDA.—The birds and reptiles agree in the broader features of the

amniote urogenital system as outlined in the general account above.

There is a

general correlation between the shape of the body and that of the gonads, and often

there is a lack of symmetry between the organs of the two sides

Thus in snakes

UROGENITAL SYSTEM. 335

and lizards the gonad of one side is in advance of the other, while in forms with
large eggs there is a marked tendency for one ovary to degenerate (right in birds)
the other alone being functional.

The oviducts, which are Miillerian ducts, are modified in accordance with the
peculiarites of the eggs. The upper portion is usually much coiled and glandular,
this part of the tube secreting the white, while parts farther toward the external
opening form the shell membrane and the shell. The walls are also somewhat
muscular, the muscles acting like constrictors to force the eggs along. The

Fic. 335.—Model of cloacal region of human embryo, 6.5 mm. long, after Keibel
uw, allantois; c, cloaca; cm, cloacal membrane; k, outgrowth to form kidney and uréter;
r, rectum; “, where bladder will develop; wd, Wolffian duct.

mesonephros and the Wolffian duct are largely degenerate in the female, being
represented by rudiments between the oviduct and the vertebral column, best
developed in turtles and snakes.

The testes (figs. 313, 328) are short, round or oval in outline, and in birds one
is usually the larger, though both increase in size at the breeding’season. The
Wolffian duct is solely reproductive (vas deferens), and its anterior, greatly coiled
end, together with the vasa efferentia form the epididymis. Traces of the Miillerian
duct persist in the male sauropsida. There are several accessory reproductive
glands in the reptiles but little is known of their function.

MAMMALS.—In considering the urogenital structures of the mam-
mals the following parts are to be kept in mind: They are composed of

336 COMPARATIVE MORFHOLOGY OF VERTEBRATES.

the embryonic excretory organs (mesonephroi) and their (Wolffian)
ducts; the permanent kidneys (metanephroi) and the ureter; the gonads;
the Miillerian ducts; the cloaca and the anlagen of the external genitalia,
which arise in the anterior or ventral wall of the urogenital sinus.

In the embryonic stages the Wolffian and Miillerian ducts and the
ureters open into the cloaca (fig. 335). Then a part of the latter, with
the openings of these ducts, is cut off to form the allantois, a portion
of which becomes the urinary bladder, this part receiving the ureters

ene

Fic. 336.—Model of pelvic region of human embryo 25 mm. long, after Keibel. (Com-
pare with fig. 335.) u, anal opening; /, lateral ligament of uterus; m, Miillerian duct; a,
ovary; pu, primitive ureter (Wolffian duct); 7, rectum; s, symphysis pubis; sg, septum of
genital protuberance; sug, urogenital sinus; «, ureter; ub, urinary bladder; ur, recto-uterine
excavation.

(except in monotremes) while the Wolffian and Miillerian ducts open
into the basal part of the allantoic outgrowth which is separated from
the bladder by a narrower stalk which becomes the urethra. This
part, into which the two pairs of ducts and the urethra empty, forms
the urogenital sinus (fig. 336, svg). With the formation of the per-
manent kidneys the mesonephros largely disappears (see p. 341) and
the same fate extends to one or the other pair of ducts, the Miillerian
largely disappearing in the male, the Wolffian in the female. The
parts which persist are more specialized than in any other group of
vertebrates, this being in part due to the fact that usually a large part

UROGENITAL SYSTEM. 337

of the development of the young is passed inside the body of the
-- mother.

In their early stages the gonads arise anteriorly to the permanent
kidneys and they retain this position in the adult monotremes (fig.
337). In all others they are gradually carried farther posterior in the
abdominal cavity, so that they lie on the caudal side of the kidneys.

Fic. 337.—Urogenital organs of male Ornithorhynchus, after Gegenbaur. 6, bladder;
ep, epididymis; k, kidney opened, showing ends of collecting tubules; sr, adrenal; sug
urogenital sinus; ¢, testis; wr, ureter; vd, vas deferens.

This transfer of position is effected by a rather complicated apparatus,
only the broader features of which can be outlined here. In the early
stages the membranes supporting the gonads (mesorchia, mesoaria)
are attached to the medial side of the double fold of the serous mem-

brane around the mesonephros. When the latter organ degenerates

the fold becomes the broad ligament of the female, while another
22

338 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

fold continues down the genital ducts forming the ligament of the
ovary or testis. In the male broad ligament and ligamentum testis
together form the gubernaculum. Unequal growth of body and
these ligaments draws the gonads (except in the monotremes) farther
back into the pelvic region.

There is some variation in the ovaries. In the monotremes the left is larger
(cf. birds) and it is interesting to note that eggs have been found only in the left
oviduct. There is also some variation in shape in the marsupials. Elsewhere
the ovaries are relatively small (sometimes increasing in size at the breeding season),
rounded or oval and with the surface smooth or furrowed.

In male whales, elephants, some edentates, etc., the testes remain
permanently in the abdominal cavity. In all others a descent of the
testes occurs. By the same relative difference of growth of body
and gubernaculum the testes are drawn out of the abdomen into a
pouch (scrotum)—really a part of the body wall into which a part
of the ccelom (bursa inguinalis) extends. The wall of this is formed
in part from the genital folds (see copulatory organs) which surround
the genital prominence. This scrotum is in front of the penis in the
marsupials, behind it in all placentals. When the canal connecting
the cavity of the bursa with the rest of the ccelom remains open (mar-
supials, insectivores, rodents, bats, etc.) the descent is temporary, the
testes being withdrawn into the ccelom at the close of the breeding
season by a ‘cremaster muscle.’ In other mammals the descent
is permanent, though in some species it does not occur until the time
of sexual maturity.

In the oviducts (Miillerian ducts) two regions can be recognized
in monotremes (figs. 338, 339, A), three in all other forms. The two
are the Fallopian tube, which opens into the body cavity by a broad,
fringed ostium tube, and second the uterus, in which the egg is retained
for a part of its development. In the other mammals Fallopian tube
and uterus are retained, the latter being specialized for the longer
development of the young, and the third region is added—the vagina,
which receives the copulatory organ of the male. The vagina opens
into the urogenital sinus (fig. 339, B), but in the monotremes the
vagina is lacking and the uterus and the sinus are directly connected.
In the marsupials a vagina is developed for each Miillerian duct, and
in some there is a peculiar fusion of the ducts distal to the vagine so
that a cecal pocket results, and in a few this pocket also connects with
the urogenital sinus, thus forming a third vagina (fig. 339, B).

UROGENITAL SYSTEM. 339

In the placental mammals the posterior (vaginal) ends of the two
Miillerian ducts fuse in the median line, thus forming a single vagina.
In some the two uteri remain distinct, each having its own opening
(os uteri) into the vagina. This forms the uterus duplex (figs. 339, B,
340, II), found in most rodents. In carnivores, ruminants, horse and

Fic. 338.—Female genitalia of Echidna, after Owen. a, openings of ureters into,
ug, urogenital sinus; 6, bladder, a bristle passing into urogenital sinus; c, cloaca; d, opening
of rectum into cloaca; 0, ovary, od, oviduct, the lower part uterine, 7, rectum; u, ureters.

pig the fusion has been carried farther so that there is.a single os uteri
and the two uteri are almost completely separated (uterus bipartitus,
fig. 340, ZZ) or the fusion is carried farther, the result being the
uterus bicornis (fig. 339, C) in which the double nature is still shown
by the two pouches at the upper (anterior) end. Lastly, in the pri-
mates, the fusion of the two primitive uteri is complete, the result
being the uterus simplex (figs. 339, D; 340, IJJ-VI), in which the

340 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

double nature is shown only by the separate openings of the two
Fallopian tubes.

In the monotremes the primitive relation of urogenital sinus and
rectum—both emptying into the cloaca (figs. 338, 339, A)—persists
through life, the result being a single external opening for the digestive

Fic. 339.—Uteri of (A) Ornithorhynchus; (B) Halmaturus; (C) sheep and (D) Inuus,
after Gegenbaur. 8, bladder; bo, bursa ovarica; c, cornua uteri; cl, cloaca; /, ligament of
ovary; 0, ovary; od, oviduct (Fallopian tube); pv, processus vaginalis; sus, sug, urogenital
sinus; “, uterus; ur, ureter; v, vagina; vc, vaginal canals.

tract and the urogenital ducts, whence the name monotreme. In
all other mammals the cloaca becomes divided by a partition, the
perineum, between the urogenital and the rectal portions, there thus
being formed two external openings. However, in certain mammals,
as in marsupials and some rodents, both may be enclosed in a common

UROGENITAL SYSTEM. 341

fold of integument (fig. 341) and in the former group may be provided
with a common sphincter muscle. ,

The testes are relatively small and the outer surface is smooth as
the result of the development around them of a fibrous envelope, the
tunica albuginea. This sends inward partitions (trabeculae) which
separate groups of séminiferous tubules into’ lobules. From the

Fic. 340.—Modifications of female urogenital structures in J, monotreme; JJ,
Orycteropus (uterus duplex); JI, many monodelphs (uterus bipartitus); 7V, most mono-
delphs; V, Bradypus; VI, Dasypus; b, bladder; c, urinary canal, cu, urogenital sinus; g,
genital sinus; 0, oviduct, u, uterus; v, vagina.

lobules the sperm is carried outward by numbers of small tubules, the
homologues of the efferent ductules of the lower vertebrates, and
like them connected together by vessels which correspond to the longi-
tudinal canals. The ductules empty into the anterior end of the
Wolffian duct, the upper end of which is greatly coiled, the coiled por-
tion and the ductules forming the epididymis. From the entrance
of the ductules to its entrance into the urogenital sinus or canal the
duct is called the vas deferens. From this point the urogenital canal
is provided with muscular walls and forms an ejaculatory duct.

In the female the Wolffian duct and the mesonephros are largely lost in the
adult, the mesonephros forming a small collection of tubules near the anterior
end of the ovary which are known as the parovarium. In the male the Miillerian

342 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

‘duct is also largely lost, the lower portion sometimes persisting as a small blind
tubule imbedded in the prostate gland and known as the uterus masculinus.

In the testes between the tubules are smal] aggregates of cells known as inter-
stitial cells, which have recently been shown to be glands with internal secretion.
In man their products, which pass into the blood, apparently cause the assumption
of the secondary male characters—growth of hair on the face, change of voice, etc.
—at the time of puberty. There would also seem to be some analogous structure
in the ovary governing the development of female characteristics and controlling
some of the features of. menstruation.

There are a number of accessory glands connected with the genital ducts, these
being usually better developed in the male than in the female. Only the more

Fic. 341.-—Diagram of male genitalia of beaver, Castor canadensis, after-Weber. a,
anus; ag, anal gland; }, urinary bladder; gv, gland of vas deferens; oa, opening of anal gland;
op, os penis; p, prostate; pp, preputial gland; r, rectum; u, ureter; vd, vas deferens.

prominent are mentioned here. The seminal vesicles (present in some rodents,
bats, insectivores and in ungulates and primates) are a pair of tubular or saccular
glands opening into the vasa deferentia just before their entrance into the urogenital
‘canal. The prostate glands, which occur in all placental mammals with the
exceptions of edentates and whales, are connected with the urogenital canal.
Farther along the canal are Cowper’s glands which occur in almost all mammals
as scattered bodies or aggregated into larger masses, and surrounded by smooth
muscle.

Concerning the functions of these glands considerable uncertainty exists.
From the fact that removal of the prostate and the seminal vesicle in rats prevented
fertilization, and the further fact that the secretion of the seminal vesicles increases
the activity of the spermatozoa, it seems probable that they are of great importance
in connexion with fertilization. Then it has been shown that in some instances
the coagulation of the secretion of these glands closes the vagina after copulation
has occurred, thus preventing the exit of the sperm.

COPULATORY ORGANS.

In many vertebrates the eggs are fertilized after passing from the
oviducts. This is the case with the cyclostomes, most fishes, with
the exception of the elasmobranchs, and with many amphibians. In

UROGENITAL SYSTEM. 343

other groups fertilization is internal. In some cases the transfer of
the sperm from the male to the female is effected. by the apposition
of the cloace of the two sexes, but in others copulatory organs of an
intromittent character occur. These are formed on several plans and
are not homologous throughout.

Fic. 342.—Hemipenes of Crotalus horridus, after J. Miller. One hemipenis is
exserted, the other retracted but laid open. cl, cloaca; g, seminal groove; #, hemipenis;
r, rectum, rp, retractor muscle of hemipenis; u, ureter; vd, vas deferens (Wolffian duct).

In the male elasmobranchs the posterior or inner side of the
pelvic fins are specialized for this purpose. The metapterygium
(p. 116) and the basalia connected with it are more or less completely
separated from the rest and form the so-called clasper (‘mixip-
terygium’). Each of these is grooved along its medial surface and

344 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

when the two are inserted in the cloaca the grooves unite to form a
tube for the passage of the sperm. There is a large gland in the
clasper but its relation to copulation and fertilization is unknown.

In the snakes and lizards a second kind of structures occurs. In
the young there are developed behind the vent a pair of sacs presenting
the appearance of appendages. With farther growth these two
hemipenes are withdrawn into a sac opening into the hinder side of

Fic. 343.—Cloacal region of adult turtle (Emys lutaria), after von Moller. The
rectum and cloaca have been laid open from the dorsal surface and the urogenital sinus
exposed. From the opening of the sinus into the cloaca a seminal groove extends along
the ventral cloacal surface and can be cut off by a pair of folds (plice urorectales) from the
cloacal cavity. av, anal vesicle; 6, urinary bladder; 0, opening of anal vesicle into cloaca;
p, penis, exserted; pu, plice urorectales; 7, rectum; sg, seminal groove; ug, urogenital
groove.

the cloaca. Each hemipenis bears a spiral groove for the passage of
the sperm. At the time of copulation these are everted through the
anus (fig. 342).

In all other aminotes the copulatory organs are formed from the
same anlage. The lower anterior wall of the cloaca is largely con-
cerned in this, the anterior cloacal lip being produced into a genital
prominence (fig. 336) which can be traced in many forms as the
clitoris of the female and the glans penis of the male. In the embryos
of the higher mammals it is surrounded by a pair of integumental

UROGENITAL SYSTEM. 345

folds which develop into the labia of the genital opening in the female
while in the male they furnish a part of the scrotal envelope.

The most primitive type of the cloacal penis is found in the chel-
onians (fig. 343) and crocodiles, and slightly more developed in the

Fic. 344.—Ventral cloacal wall and penis of Rhea (schematized), after Boas. 5,
blind sac; f, corpus fibrosum; g, seminal groove; g’, its continuation along blind sac; 0,
opening of blind sac. Mucous membrane dotted, seminal groove black.

ostriches and some of the aquatic birds. In these the ventral or
anterior wall of the cloaca and its lip beconie specialized by the develop-
ment in it of a longitudinal band of fibrous tissue, covered on the
cloacal side by cavernous tissue (containing large spaces, which on

Fic. 345.—Diagrams of male urogenitalia in J, monotreme; JJ, marsupials; and,JIT;
monodelphs, after Weber. a, anus; 0, bladder; c, cloaca; cc, corpus cavernosus urethra,
cp, corp. cav. penis; cd, Cowper’s gland; p, perinzeum; pg, prostate gland; 7, rectum; s,
symphysis pubis; ¢, testis; u, ureter; v, vas deferens; vg, vesicular gland; vm, ventral muscles.

being filled with blood render the whole firm and enlarged—erectile
tissue). The cavernous tissue is marked by a longitudinal groove
through which the seminal fluid from the urogenital sinus runs. Be-
sides the enlargement caused by the filling of the cavernous tissue with

346 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

blood, the whole structure, the distal end of which is free, can be
protruded from the cloaca and retracted by suitable muscles (fig. 344).

In the monotremes (fig. 345, I) the penis is still cloacal in position
and the urogenital sinus still communicates with the cloacal cavity.
But the advance is made that the groove of the sauropsida has been
converted into a tube which carries the urine as well as the sperm.
The whole structure can be protruded and retracted again into a
sheath formed from the loose mucous membrane of the cloaca. In
the other mammals the connection of the urogenital ducts with the
alimentary tract is lost and the cloaca disappears. In the lower
mammals (figs. 341, 345, IJ) the retractile condition is retained but
in the higher the organ is permanently external (fig. 345, JI). In
the marsupials the tip of the penis is frequently bifurcate, corresponding
to the two vaginz of the female. In many rodents (fig. 341, of),
bats, many carnivores, whales and some of the primates a penis bone
is developed in the middle line of the intromittent organ.

HERMAPHRODITISM.

Individuals of either sex which have assumed some of the external
or secondary sexual characters of the other sex are sometimes spoken
of as hermaphrodites, especially in the case of mammals if the cepu-
latory organs be concerned. This is not true hermaphroditism, which
consists in having both ovarian and testicular organs or tissues in the
same individual and as a consequence the ability to produce both eggs
and spermatozoa. There may be both kinds of tissue in the different
parts of the same gonad, or the two may be intermingled (ovotestis)
or the gonads of the two sides of the body may be of different sexes.
Both ovaries and testes may be functional at the same time, or one
may be functional at one time and the other at another (proterandric
hermaphroditism).

There is an enormous literature dealing with the problem of the
determination of sex. Almost every conceivable possibility has been
invoked to account for fact that one individual is: male and another
female—chance, multiple impregnation, difference in age of parents
or of eggs and spermatozoon, matters of temperature and nutrition,
etc. Within the last few years there has been a strong tendency to
regard the matter as determined at the time of impregnation of the
egg and to depend upon differences in chromosomes.

UROGENITAL SYSTEM. 347

In the formation and maturation of spermatozoa and eggs a peculiar substance in
the nucleus—chromatin— becomes aggregated in small bodies called chromosomes,
the number of which in the mature genital products is half of that occurring in
the other cells of the body. In most species the number in the body cells is always
even and is therefore exactly divisible, but it was found that in certain insects there
were differences between the sexes, the male having an odd, the female an even
number. When the reduction division occurs, by which the chromosomes are
divided between the mature eggs or the spermatozoa (for details see cytological
works), the eggs would all have the same number of chromosomes while the
spermatozoa would be dimorphic, some having an odd and some an even
number of chromosomes, In other cases there is frequently one or more
chromosomes (idiochromosomes) which differ from the rest, and these are dis-
tributed in the same way at the reduction division. At the fertilization of the
egg there is an addition of the chromosomes of the spermatozoa to those of the
egg, consequently some of the eggs will have the odd number and some the
even number of chromosomes, this being perpetuated in all of the cells of the
resulting organism until the next reduction division. It would thus follow that
sex was determined at the time of fertilization of the egg. But this is difficult
to reconcile with the existence of hermaphroditism.

Another view, which better accords with the facts, is that sex is a matter of
Mendelian inheritance, the females in some instances being heterozygous, the
males homozygous; or these relations may be reversed In the first condition
the element of ‘femaleness’ dominates over the recessive ‘maleness’, In such
cases it seems reasonable to suppose that the hermaphrodites are really
heterozygous females in which the normally recessive ‘maleness’ has become
equally potent with the female, while under ordinary conditions the matter of
sex is dependent upon the character of the chromosomes combined with the
Mendelian inheritance,

Among the cyclostomes there are occasional specimens of lam-
preys which have been regarded as hermaphroditic, but in the myx-
inoids this is the regular occurrence, the anterior end of the gonad
is male and the posterior female. One or the other of these is func-
tional, the animal being predominantly either male or female, and
some individuals are regarded as sterile. Nansen regards this as a
case of proterandric hermaphroditism. In the teleosts several species,
of Serranus are regularly hermaphroditic as is Chrysophrys aurata,
while in several other species it is an occasional occurrence. Triton
teniatus is the only urodele in which it is reported, but in the anura it
is more common. ‘Thus it is frequent in the frogs and occasional in
other genera. In the toads (Bufo) there is frequently a ‘Bidder’s
organ’ in front of the gonads which contains immature ova in the
male. Among the birds the phenomenon has been reported in the
chaffinch. (The assumption of male plumage by female birds at the

348 COMPARATIVE MOGPHOLOGY OF VERTEBRATES.

close of sexual life is not a case of hermaphroditism.) Among the
mammals the cases are extremely rare, but cases, apparently well .
authenticated, have been reported in the goat, pig and man.

NUTRITION AND RESPIRATION OF THE EMBRYO—FETAL
ENVELOPES.

In all vertebrates except the mammals there is enough nourish-
ment stored in the egg to carry the young through its development
up to the point where it hatches and shifts for itself. In the cyclo
stomes, dipnoi and amphibia this nourishment (food-yolk or deuto-
plasm) is soon enclosed in the body wall. In ganoids and teleosts,
where it is relatively larger in amount, it forms for a time a projecting
mass enclosed in a yolk sac, and this condition reaches its extreme in
the elasmobranchs and sauropsida. The yolk sac, in the fishes, is an
extension of the intestine and the body wall and is richly supplied by
vitelline arteries and veins which are derivatives of the omphalo-
mesenteric vessels (p. 276). In the sauropsida, owing to the develop-
ment of the amnion and the consequent separation of the non-
embryonic somatopleure from the yolk, the yolk sac is composed of
the splanchnopleure alone, but it has homologous blood-vessels. In
the mammals (monotremes excepted) the yolk is greatly reduced and
the yolk sac (here often called the umbilical vesicle) is vestigial in
character.

The vitelline vessels take the yolk and carry it into the body where
it is utilized in building the embryo, all of it being eventually metabo-
lized and used by the cells. The rich supply of capillary vessels in the
sac also forms an efficient respiratory apparatus. In the viviparous
sharks villi are developed on the oviducal lining and these afford a
means of exchange of gases with the embryo, and for getting rid of the
nitrogenous waste. It is a question how far there is a transfer of food
by the same means. In some species of Mustelus and Carcharias
the villi fit into depressions in the yolk sac, thus forming an analogue
to the placenta of the mammals— a vitelline placenta—though formed
in a greatly different manner.

The viviparous teleosts have saccular ovaries and the development
of the egg takes place in the cavity, the walls of which at the breeding
season become villous. In the viviparous Salamandra aira only one
egg develops and this leaves the mother in the adult shape. The
other eggs degenerate and are used as food by the one. There is also

UROGENITAL SYSTEM. 349

a modification of the lining of the oviduct in this species which allows
some blood to escape and this gives additional nourishment.
In the amniotes the yolk sac reappears and there are in addition

Fic. 346.—Diagrams of the development of amnion and allantois. Upper figure earlier,
transverse section; lower later, longitudinal. «@, amnion; al, alimentary canal; am. cav,
amniotic cavity; ch, beginning of chorion; s, serosa; so, somatopleure; ys, yolk stalk.

350 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

two other embryonic structures which are peculiarly characteristic,
the allantois and the amnion, to which reference has been made before.

The amnion arises as a fold of the somatic wall of the ccelom in
front of and on either side of the embryo. These folds extend upward
and then inward until they finally meet above the embryo, thus en-
closing it in an amniotic cavity. The folds fuse in the middle line and
then the two sides break through so that above the wall of the amniotic
cavity—the true amnion—there is a second cavity directly continuous
with the ccelom, and this is bounded externally by the rest of the
amniotic fold, this part being called the serosa or false amnion. This
lies immediately beneath the vitelline membrane of the egg or its
equivalent, to which many different names have been given.

Little is known as to the phylogeny of the amnion, a structure without parallel
in the animal kingdom except in the scorpions, where one is formed in the same
way. Of course there is no genetic connexion between the two. It has been sug-
gested that in both groups there is a tendency for the embryo to sink into the yolk
and that the amnion is to prevent its being completely covered with this substance.

The homologue of the allantois is found in the urinary bladder of
theamphibia. Itisan outgrowth from the hinder end of the alimentary
tract and consists of a lining of entoderm, covered externally with the
splanchnic layer of the mesoderm—is purely splanchnopleuric—and
projects into the ceelom. In its outgrowth it carries with it branches
of the hypogastric blood-vessels, now known as the allantoic arteries
and veins (usually but a single vein). As it develops, the distal
end of the allantois swells into a large vesicle, connected with the
digestive tract by a slender stalk. The vesicle extends into the ccelom
between the amnion and serosa and soon fuses with the serosa. The
terminal sac flattens and gradually extends until it encloses the whole
embryo and amniotic sac.

In the sauropsida the allantois (and serosa) comes eventually to
lie just beneath the shell, and as the latter is porous and the allantois
is very vascular, the latter is in position to act as the respiratory
apparatus of the growing young. The cavity of the allantois, con-
nected by its stalk with the cloacal region, serves as the reservoir for
the urine.

While the embryo is increasing in other respects, the side walls of
the body gradually close in ventral to the embryo until they reach the
stalks of the yolk sac and the allantois. In this way these structures
come to be connected with the body by a narrow cord, called in mam-

UROGENITAL SYSTEM. 351

mals the umbilical or navel cord, in which the blood-vessels run. In
the mammals there are several variations from the above account of the
development of the allantois, but they can be reconciled with the
typical condition in the sauropsida. There are also several othet
variations and the relations of allantois to the other structures is more
complicated, but details and the many modifications must be ignored
here, only an outline of the broader features being given.

In the mammals there is the same fusion of allantois and serosa as
in the sauropsida, the fused area here being called the chorion. On
arrival in the uterus by way of the Fallopian tube, the egg becomes
implanted in the uterine wall, and a little later, with the development
of the chorion, villi are formed on the outer surface of the egg. These
are invaded by the chorionic blood-vessels and they branch and extend
into depressions or crypts in the walls of the uterus. The latter become
very vascular, the blood spaces of the maternal tissue enveloping the
villi with only the thinnest of walls between the vessels of the mother
and those of the young. (There is never any actual connexion between
the blood-vessel of parent and embryo and so blood corpuscles cannot
pass from one to the other. All that takes place is largely of the nature
of osmosis—solutions of gases, of nourishing substances and of nitrog-
enous waste passing from one to the other. There is difficulty in
explaining the passage of proteids and fats.) This structure, consisting
of the allantoic derivatives of the embryo and the mucous lining of the
uterus, is known as the placenta.

In the monotremes and in most marsupials no placenta is formed,
but it has been recently shown that a true placenta occurs in a few of the
latter group. In other mammals a placenta always occurs, the struc-
tures presenting many forms, but these.may be grouped under a few
heads. (It must be borne in mind that this classification is purely
morphological and does not necessarily imply close relations of the
species included or identity of method of formation.)

In many mammals, at the time of birth, the maternal and embryonic
parts of the placenta simply separate, only the latter passing away
with the young. These are called non-deciduate placente. In
the others the union of the foetal and the maternal tissues is so intimate
that the inner surface of the uterus is included in the afterbirth. These
form the deciduate type. The non-deciduata include two divisions.
In the diffuse placente (edentates, whales, perissodactyls, many
artiodactyls) the villi are distributed over the entire surface of the

352 COMPARATIVE MORPHOLOGY OF VERTEBRATES.

chorion. In the cotyledonary placenta the villi are grouped in small
areas (cotyledons) with spaces of naked chorion between them. This
form is characteristic of the ruminants. The deciduate type includes
the zonary and the discoidal forms. In the zonary placenta (eden-
tates, sirenians, elephants, hyracoids and carnivores) the villi form a
girdle around the placental sac, the ends of the chorion being free from
them. In the discoidal forms (insectivores, rodents, bats, edentates,
primates) the villi are restricted to one side of the chorion.

ADRENAL ORGANS.

Under this heading are included two sets of structures, interrenals
and suprarenals, of uncertain morphology and function. The names
are given in allusion to the fact that they are usually closely associated
in position with the nephridial structures, though they have no other
relation to them. The two differ in structure and probably in function
and are very distinct in the lower vertebrates but in amphibia and
amniotes they are united in a common structure, the interrenals forming
the cortex, the suprarenals the medulla of the mammalian adrenals.

The interrenals arise from the coelomic epithelium but it is as yet
uncertain as to the details, some thinking that they are connected with
the pronephros, others with the mesonephric structures, while still others
regard them as distinct in origin. They are at first either isolated
clusters of cells or longer bands of cells near the dorsal margin of the
mesentery, sometimes bilaterally symmetrical and in the lower verte-
brates extending through the length of the ccelom.

The suprarenals find their anlage in the sympathetic ganglia, from
which certain cells early separate. Among these are peculiar cells
which are called chromaffin cells (chromaphile or pheochrome
cells) because of their staining brown or yellow with chromic acid
salts. These usually are closely associated with the blood-vessels,
either the dorsal branches of the segmental arteries or the postcardinal
veins.

In the fishes the two organs are separate, the suprarenals often
being more or less metameric in character, and in close relations to
the vessels of the mesonephros. The interrenals form more compact
organs between the nephridia of the two sides. In all tetrapoda the
two organs are more closely associated, the tissues of the two being
mixed in the adults of the amphibia and reptiles, while in the mammals

UROGENITAL SYSTEM, 353

the interrenal tissue is on the outer side of the adrenal organ, the su-
prarenal forming the inner portion. In the amphibia the adrenals are
closely connected with the mesonephroi, being attached to their
inner margins (urodeles) or to the ventral surface (anura). In the
reptiles they are lobulated structures near the gonads. In the mammals
they are more compact (often called suprarenals) and are placed at the
anterior end of the kidneys, often unsymmetrically.

Both organs are regarded as glands of internal secretion, their
product being passed directly into the blood. The secretion of the
medullary portion (suprarenal) of the mammals is adrenalin, an acti-
vator or hormone, which by its action on the muscular system causes
an increase in the blood pressure. Even less is known of the function
of the interrenal. Certain observations render it probable that the
secretion of this is of value in destroying certain products of metabolism
which otherwise might be injurious to the organism.

23

BIBLIOGRAPHY. :

Tn this list of books and articles dealing with vertebrate morphology there have been in-
cluded only such titles as are likely to be accessible in the majority of the laboratories of
the country. Hence citations are largely from the periodicals and society publications of
America and England and from the leading journals of the Continent. The student who
wishes to go farther into any subject will find additional references in the papers quoted
here and also in the works of Wiedersheim, Gegenbaur, Hertwig and others, while the cur-
rent papers are listed in the Anatomischer and Zoologischer Anzeigers. For economy of
space the titles have been abbreviated, but in such a way as to indicate something of the
character and conténts of the work.

JOURNALS AND TRANSACTIONS.

Academy of Natural Sciences, Philadelphia, Proceedings.
American Naturalist.

American Journal of Anatomy..

American Academy of Arts and Sciences, Proceedings.
Anatomical Record.

Anatomischer Anzeiger.

Anatomische Hefte.

Archiv fiir Anatomie und Physiologie, Anatomische Abtheilung.
Archiv fiir mikroscopische Anatomie.

Biological Bulletin.

Boston Society of Natural History, Memoirs and Proceedings.
Ergebnisse der Anatomie und Entwicklungsgeschichte.
Jenaische Zeitschrift fiir Naturwissenschaften.

Journal of Anatomy and Physiology.

Journal of Comparative Neurology.

Journal of Morphology.

Mittheilungen aus der zoologischen Station zu Neapel.
Morphologische Arbeiten.

Morphologisches Jahrbuch.

Museum of Comparative Zoology. Bulletin.

Quarterly Journal of Microscopical Science.

Royal Society of London, Philosophical Transactions.
Zeitschrift fiir wissenschaftliche Zoologie.

Zoologischer Anzeiger.

Zoologischer Jahrbiicher, Abteilung fiir Anatomie und Entwicklungsgeschichte.
Zoological Society of London, Proceedings and Transactions.

TEXT-BOOKS, MANUALS AND GENERAL WORKS.

Balfour: Treatise on comparative embryology. 2 vols., London, 1880-82.

Barker: Anatomical Terminology. Philadelphia, 1907. (Contains nomenclature of
Basel Commission—‘BNA.”).

Béhm und Davidoff: Histology, trans. by Huber, Philadelphia.

Bronn’s Klassen und Ordnungen des Thierreichs.

Works of John Samuel Budgett. Cambridge, 1907. (Mostly teleosts, dipnoi and am-
phibia.) ;

359

356 BIBLIOGRAPHY.

Cambridge Natural History. 10 vols., London, 1895-1909.

Choronshitzky: Entstehung der Milz, Leber, Gallenblase, Pankreas, und Pfortader-
system bei verschiededen Wirbelthiere. Anat. Hefte, 13, 1910.

Dahlgren and Kepner: Principles of animal histology. N. Y., 1908.

Dohrn: Studien zur Urgeschichte des Wirbelthierkérpers. Mitth. zool. Sta. Neapel,
317, 1881-1904.

Festschrift zu 7osten Geburtstage Rudolf Leuckarts. Leipzig, 1892.

Gegenbaur: Vergleichende Anatomie der Wirbelthiere. 2 vols. Leipzig, 1898-1901.

Handbuch der vergleichend. und experim. Entwicklungslehre (edited by O. Hertwig).
3 vols., Jena, 1901-1906.

Hertwig: Lehrbuch der Entwicklungsgeschichte des Menschen und der Wirbelthiere,
gth edition, Jena, r9z0. (An earlier edition, trans. by Mark. London, 1892.)

Hill: Primary segments of vertebrate head. Zool. Jahrb., 13, 1899.

Hubrecht: Die Sdugetiereontogenese. Jena, 1909.

Huxley: Manual of the anatomy of vertebrated animals. N. Y., 1872.

Huxley: Scientific memoirs. 5 vols. London, 1898-1903.

Keibel and others: Normentafeln zur Entwicklungsgeschichte: Pig (Keibel); hen
(Keibel and Abraham); Ceratodus (Semon); rabbit (Taylor); Lacerta (Peter);
deer (Keibel); Tarsius (Hubrecht); man (Keibel und Elze); Acanthias (Scammon).

Kingsley: Text-book of vertebrate zoology. N. Y., 1899.

Lankester: A treatise on zoology. London, 1900—(g9 vols. published).

Minot: Human embryology. N. Y., 1892.

Minot: Bibliography of vertebrate embryology. Memoirs Boston Socy. Nat. Hist., 4,
1892.

McMurrich: Development of the human body. Philadelphia, 1904.

Oppel: Lehrbuch der vergleichenden mikroscopischen Anatomie der Wirbelthiere.
Jena, 1896—Incomplete, 6 pts. published.

Owen: Anatomy of vertebrates. 3 vols. London, 1866-68.

Parker: Course of instruction in zootomy. London, 1895.

Parker and Haswell: Text-book of Zoology. 2 vols. London, 1897.
Schneider: Lehrbuch der vergleichenden Histologie. Jena, 1902.

Stannius: Handbuch der Zootomie, zweiter Theil, Wirbelthiere. (Only fishes, amphibia
and reptiles published) Berlin, 1856. (Invaluable as summary of older work.)

Stéhr: Text-book of histology, trans. by Lewis. Philadelphia, 1910.

Wiedersheim: Vergleichende Anatomie der Wirbeltiere; 7th edition, Jena, 1909.
(Abridgment trans. by Parker, London, 1908.)

Wilder: History of the human body. N. Y., 1909.
Woodward: Outlines of vertebrate paleontology. Cambridge, 1898.
Ziegler: Lehrbuch der Entwicklungsgeschichte der niederen Wirbeltiere. Jena, rgo02.

Zittell: Handbuch der Palaontologie. 5 vols. Miinchen, 1880-93. (A translation and
adaptation by Eastman. 2 vols. London.)

MONOGRAPHS ON SINGLE SPECIES AND GROUPS.

Cyclostomes.
Ayers and Jackson: Morphology of Myxinoids. Bull. Cincinnati Univ., 1, 1900. Skele-
ton and muscles. Jour. Morph., 17, gor.
Cole: Papers on anatomy of Myxine: Trans. Roy. Socy. Edinburg, 1905-12.
Dean: Development of Bdellostoma. Quar. Jour. Micr. Sci., 40, 1897.

Howes: Affinities, interrelationships and systematic position of marsipobranchs, Trans
Biol. Socy. Liverpool, 6, 1892.

Scott, W. B.: Entwicklungsgeschichte der Petromyzonten. Morphol. Jahrb., 7, 1882.
Scott, W. B.: Development of Petromyzon. Jour. Morph., 1, 1887.
Shipley. A.: Development of Petromyzon. Quar. Jour. Micr. Sci., 27, 1887.

GENERAL. 357

Fishes.

Allis: Cranial muscles and nerves of Amia. Jour. Morph., 12, 1897; of Scomber. same, 18,
1903.
Ayers: Anatomie und Physiologie der Dipnoer. Jena. Zeitschr., 18, 1884.

Balfour: Monograph on the development of Elasmobranch fishes. London, 1878. (Ext
Jour. Anat. and Physiol., 1876-78.)

Balfour and Parker: Structure and development of Lepidosteus. Phil. Trans., 1882.
Dean: Fishes, living and fossil. N. Y., 1895.

Dean: Development of garpike and sturgeon. Jour. Morph., 11, 1895.

Dean: Chimeroid fishes and their development. Carnegie Inst., 1906.

Garman, S.: Chlamydoselachus, a living cladodont shark. Bull. Mus. Comp. Zool., 12
Gunther, A.: Ceratodus. Philos. Trans. Royal Soc’y., 1871.

Gunther: Introduction to the study of fishes. Edinburgh, 1880.

Kellicott: Development of vascular and respiratory systems of Ceratodus. Mem. N. Y.
Acad. Sci., 2, 1905.

Kerr: Developniedt of Lepidosiren. Quar. Jour. Micros. Sci., 45 and 46.

Locy: Contribution to structure and development of vertebrate head (Acanthias). Jour.
Morphol., 9, 1891.

Miller: Muskeln und nerven der Brustflossen und Kérperwande bei Acanthias. Anat.
Hefte, 43, 1911.

Parker: Anatomy and physiology of Protopterus. Trans. R. Irish Acad., 30, 1892.

Pollard: Anatomy and position of Polypterus. Zool. Jahrbuch, Abth. Anat., 5, 1892.

Rauther: Panzerwelse. Zool. Jahrb. Abt. Anat., 31, 1911.

Wiedersheim: Skelet und Nervensystem von Lepidosiren. Jena. Zeitsch., 14, 1892.

Wilson: Embryology of sea bass (Serranus). Bull. U. S. Fish Comm., 9, 1891.

Wright et al.: Anatomy of Amiurus. Proceed. Canad. Inst., 2, 1884.

Amphibia.

Brauer: Entwicklungsgeschichte und Anatomie der Syminopliones: Zool. Jahrbuch,
Abth. Anat., ro, 12, 16, 1897-1902.

Cope: Refrain of North America. Bull. U. S. National susie 34, 1889.
Emerson: Anatomy of Typhlomolge. Proc. Boston Socy. Nat. Hist., 32, 1905.

Fischer, J. G.: Anatomische Abhandlung tiber Perennibranchiaten und Derotremen.
Hamburg, 1864.

Gaupp: Anatomie des Frosches. 3 vols. Braunschweig, 1896-1904. Contains extensive
bibliography.

Gétte: Entwicklungsgeschichte der Unke. Leipzig, 1875.

Holmes: Biology of frog. N. Y., 1906.

Hoffmann: Amphibien, in Bronn’s Klassen und Ordnung das Thierreiches.

Huxley: Article Amphibia in Encyclop. Brit., oth edit., vol. 1.

Kingsley: Systematic position of Cacilians. Tufts Coll. Studies, 1, 1902.

Klinkowstrom: Anatomie der Pipa. Zool. Jahrbuch, Abth. Anat., 7, 1894.

Miller and others: Papers on anat. of Necturus (Lung, circulation, brain). Bull. Univ.
Wisc., 33, Ig00.

Platt: Development of cartilaginous skull and branchial and hypoglossal musculature of
Necturus. Morph. Jahrbuch, 25, 1902.

Reese: Anat. of Cryptobranchus. Am. Nat., 40, 1906.

Sarasin, P. and F.: Entwicklungsgeschichte und Anatomie der ceylonischen Blindwithle
Ichthyophis. Wiesbaden, 1887-1890.

Seelye: Circulatory and respiratory systems of Desmognathus. Proc. Boson Socy. Nat.
Hist., 32, 1906.

Smith: Development of Cryptobranchus. Jour. Morph., 23, 1912.

358 BIBLIOGRAPHY.

Wiedersheim: Anatomie der Gymnophionen. Jena, 1879.
Wilder: Anatomy of Siren lacertina. Zool. Jahrbuch, Abth. Anat., 4, 1891.

Reptiles.

Beddard: Visceral anat. of Lacertilia. Proc. Zool. Socy. London, 1888.

Clark: Embryology of turtle, in Agassiz, Contrib. to Nat. Hist. of U. S., 2, 1857.

Clarke: Embryology of alligator. Jour. Morph., 5, 1891.

Coe and Kunkel: Anatomy of Aniella. Trans. Conn. Acad. Arts and Sci., 12, r1g06.

Cope: Crocodilians, lizards, and snakes of North America. Rept. U. S. Nat. Mus. for
1889, 1900.

Hay: Fossil turtles of North America. Carnegie Inst., 19038.

Giinther: Anatomy of Hatteria [Sphenodon]. Philos. Trans., 1867.

Hoffmann: Reptilia, in Bronn’s Klassen u. Ordnungen, 3 vols., 1890.

Martin and Moale: How to dissect a chelonian. N. Y., 1895.

Orr: Embryology of lizard. Jour. Morph., 1, 1887.

Reese: Embryology of alligator. Smithsonian Misc. Coll., 51, 56, 1908-10.

Schauinsland: Entwicklungsgeschichte der Hatteria [Sphenodon]. Arch. f. mikr. Anat.,
57, 1900.

Birds.

Beddard: Structure and classification of birds. London, 1898.

Coues: Osteology and myology of Colymbus torquatus. Mem. Bost. Socy., Nat. Hist.,
I, 1866.

Coues: Key to North American birds. Boston, 1887.

Duval: Atlas d’embryologie. Paris, 1888.

Foster and Balfour: Elements of embryology. London, 1896.

Gadow: Aves, in Bronn’s Klassen und Ordnungen d. Thierreichs.

Lillie: Development of the chick. N. Y., 1908. (Contains large bibliography.)
Marshall: Anatomy of Phenoptilus. Proc. Am. Philos. Socy., 44, 1905.

Parker: Morphology of duck and auk tribes. Cunningham Mem. R. Irish Acad., 6, 1890.

Mammals.

Caldwell: Embryology of Monotremata and Marsupialia. Part I. Phil. Trans., 178, 1887.

Carlsson: Anatomie von Notoryctes. Zool. Jahrb., Abth. Anat., 20, 1904.

Ellenberger und Baum: Vergleich. Anatomie der Haustiere. Berlin, 1g00.

Gegenbaur: Lehrbuch d. Anatomie der Menschen. Leipzig, 1898. (New Edition by
Firbringer in course of publication.)

Flower and Lyddeker: Mammals, living and extinct. Londgn, 1891.

Gray: Anatomy, descriptive and surgical. Philadelphia.

Hertwig: Entwicklungsgeschichte des Menschen. Jena, 8th edit., 1906. (3rd edition
translated by Mark. N. Y., 1892.)

Hubrecht: Descent of the primates. N. Y., 1897.

Kingsley: Origin of the mammals. Science, 14, 1gor.

Lewis: Gross anatomy of a 12 mm. pig. Am. Jour. Anat., 2, 1903.

Leisering und Miiller: Vergleich. Anatomie der Haussaugetiere. 1885.

McMurrich: Development of the human body. Philadelphia, 1904.

Minot: Laboratory text-book of embryology. Philadelphia, 1911.

Mivart: The cat. London, 1881. ,

Newman and Patterson: Development of the nine-banded armadillo (Tatusia). Jour.
Morph., 21, rgto.

Osborn: Origin of mammals. Amer. Nat., 32, 1898.

INTEGUMENT. 359

Osborn: Age of mammals. N. Y., rgr0.

Piersol: Human Anatomy, Philadelphia, 1907.

Reighard and Jennings: Anatomy of the cat. N. Y., 1gor.

Weber: Anatomisches iiber Cetaceen. Morph. Jahrbuch, 13, 1888.

Weber: Die Sdugethiere. Jena, 1904.

gs ia Der Bau des Menschen als Zeugniss fiir seine Vergangenheit. Tiibingen,

1908.

Wilder and Gage: Animal Technology as applied to the cat. N. Y., 1892.

Wilson, J. T., and Hill, J. P.: Development of Ornithorhynchus. Philos. Trans., B
199, 1907.

Thyng: Anatomy of a 7.8 mm. pig embryo. Anat. Record, 5, 1911.

His: Anatomie menschlicher Embryonen. Leipzig, 1880-84.

Morris: Text-book of anatomy. Philadelphia.

CLOM, ABDOMINAL PORES.

Ayers: Pori abdominales. Morph. Jahrb., 10, 1885.

Bles: On the openings in the wall of the body cavity of vertebrates. Proc. Roy. Soc.
London, 62, 1897.

Broman: Entwicklung und Bedeutung der Mesenterien und Kérperhéhlen bei, Wirbeltieren.
Ergebnisse, 15, 1905.

Broman: Entwicklung d. Pericardium und Zwergfells. Ergebnisse, 20, 1911.

Gegenbaur: Pori abdominales. Morph. Jahrb., 10, 1885.

Klaatsch: Morphologie der Mesenterialbildungen am Darmcanal der Wirbeltieren. Morph.
Jahrb., 18, 1892. .

Mall: Development of the human cceelom. Jour. Morph., 12, 1897.

Mathes: Morphologie der Mesenterialbildungen bei Amphibien. Morph. Jahrb., 23,
1895.

Weber: Abdominalporen der Salmoniden nebst Bemerkungen iiber Geschlechtsorgane.
Morph. Jahrb., 12, 1887.

INTEGUMENT.

Batelli: Bau der Reptilienhaut. Arch. mikr. Anat., 17, 1879.

Boas: V. Wirbelthierekralle. Morph. Jahrbuch, 23, 1895.

Boas: Morphologie der Nagel, Krallen, Hufe und Klauen. Morph. Jahrb., 11, 1884.
See also 21, 1894.

Bonnet: Die Mammarorgane (Summary). Ergebnisse, 2, 1892; 7, 1898.

Bonnet: Haarspiral und Haarspindel. Morph. Jahrb., 11, 1886.

Bowen: Epitrichium of human epidermis. Anat. Anz., 4, 1889.

Brauer: Leuchtorgane der Knochenfische. Verhandl. zool. Gesellsch., 13, 1904.

Burckhardt: Luminous organs of selachians. Ann. and Mag. Nat. Hist., VII, 6, 1900.

Davies: Entwickelung der Feder, u. s. w. Morph. Jahrb., 15, 1889.

Drasch: Giftdriisen des Salamanders. Verhandl. anat. Gessellsch., 6, 1892.

Esterly: Poison glands of Plethodon. Pubs. Univ. Cal., Zool., 1, 1904.

Géppert: Phylogenie der Wirbelthierkralle. Morph. Jahrb., 25, 1898.

Green: Phosphorescent organs in Porichthys. Jour. Morphol., 15, 1900.

Henneberg: Entwicklung der Mammarorgane der Ratte. Anat. Hefte, 13, 1899.

Japha: Haare der Waltiere. Zool. Jahrb., Abt. Anat., 32, 1911.

Jeffries: Epidermal system of birds. Proc. Boston Socy. Nat. Hist., 22, 1883.

Jones: Development of the nestling feather. Oberlin Lab. Bulletin, 13, 1907.

Kerbert: Haut der Reptilien. Archiv mikr. Anat., 13, 1876.

Keibel: Ontogenie und Phylogenie von Haare und Feder. Ergebnisse, 5, 1895.

360 BIBLIOGRAPHY.

Kerr: Development of skin and derivatives in Lepidosiren. Quar. Jour. Micros., Sci., 46,
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Krause: Entwickelung der Epidermis und ihrer Nebenorgane. Handbuch der En-
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Leydig: Vascularisiertes Epithel. Archiv mikr. Anat., 52, 1898.

Maurer: Vascularisierung der Epidermis bei Anuren. Morph. Jahrb., 26, 1898.

Maurer: Haut-Sinnesorgane, Feder, und Haaranlagen, Morph. Jahrb., 18, 1892; 29, 1893
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de Meijere: Die Haare der Sdugetiere und ihre Anordnung. Morph. Jahrb. 21, 1894.
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Muhse: Cutaneous glands of toad. Am. Jour. Anat., 9, 1909.

Nicoglu: Hautdriisen der Amphibien. Zeitschr. wiss. Zool., 56, 1893.

O’Donoghue: Growth changes of mammary apparatus of Dasyurus. Quar. Jour. Micr
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Parker: ee of melanophore pigment, especially in lizards. Jour. Exp. Zool., 3,
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Parker =e Starrall: Color changes in skin of Anolis. Proc. Am. Acad. A. and S., 40,
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Poulton: Bill and hair of Ornithorhynchus. Quarterly Jour. Micro. Sci., 36, 1894.

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Pycraft: Interlocking of feathers. Nat. Science, 3, 1893.

Reed: Poison glands of catfishes. Amer. Nat., 41, 1907.

Reh: Schuppen der Saugetiere. Jena: Zeitsch., 29, 1895.

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Sawadsky: Larval Haftapparates bei Acipenser. Anat. Anz., 40, IgII.

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Schulz: Giftdriisen der Kréte und Salamander. Archiv mikr. Anat., 34, 1889.

Schwalbe: Farbenwechsel winterweisser Thiere. Morph. Arbeiten, 2, 1893.

Semon: Mammarorgane der Monotremen. Morph. Jahrb., 28, 1899.

Spencer and Sweet: Hairs of Monotremes. Quar. Jour. Mic. Sci., 41, 1899.

Stéhr: Entwicklung des menschlichen Wollhaares. Anat. Hefte, 23, 1903.

Strong: Development of the primitive feather. Bull. Mus. Comp. Zool., 40, rIg02.

Wallace: Axillary gland of Batrachus. Jour. Morphol., 8, 1893.

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Wilder: Palms and soles. Amer. Jour. Anat., 1, 1902.

Zeitschmann: (Odoriferous glands of Cervide). Zeits. wiss. Zool., 74, 1903.

DERMAL SKELETON.

Boldt: Ruckenschild der Ceratophrys. Zool. Jahrb. Abt. Anat., 32, 191r.

Hase: Schuppenkleid der Teleostier. Jena. Zeitsch., 42, 1907.

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SKELETON. 361

SKELETON—GENERAL.

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Gaupp: Schlafendgegend am Wirbeltiereschidel. Morph. Arbeit., 4, 1894.

Gaupp: Alte Probleme und neuere Arbeiten tiber Wirbeltiereschidel. Ergebnisse, 10,
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Gaupp: Fragen an d. Lehre vom Kopfskelett. Anat. Anz:, Erganzungshefte, 29, 1896.

Gaupp: Unterkiefer der Wirbeltiere. Anat. Anz., 39, 1911.

Gaupp: Saugerpterygoid und Echidnapterygoid, u.s. w. Anat. Hefte., 42, 1910.

Gegenbaur: Skelett der Gliedmassen der Wirbelthiere und der Hintergliedmassen der
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Gegenbaur: tiber das Archipterygium. Jena. Zeitsch., 7, 1872.
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Gétte: Morphologie des Skelettsystems der Wirbeltiere. (Girdle and breastbone) Arch.
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Hertwig: Zahnsystem der Amphibien und seine Bedeutung fiir die Genese des Skeletts der
Mundhéhle. Arch. mikr. Anat., 11, 1874.

Howes: Morphology of the sternum. Nature, 43, 1891.

Kingsley: The ossicula auditus. Tufts Coll. Studies, 1, rgoo.

Parker and Bettany: Morphology of the skull. London, 1877.

Parker: Structure and development of the shoulder girdle and sternum. Ray Society,
1867.

Parker and Bettany: Morphology of the skull. London, 1877.

Reynolds: Vertebrate skeleton. Cambridge (England), 1897.

Thacher: Median and paired fins. Trans. Conn. Acad., 3, 1877.

Thyng: Squamosal bone in Tetrapoda. Tufts Coll. Studies, 2, 1906.

Woodward: Vertebrate paleontology. Cambridge, 1898.

Wiedersheim: Gliedmassenskelett der Wirbelthiere. Jena, 1892.

Winslow: Chondrocranium in the ichthyopsida. Tufts Coll. Studies, 1, 1898.

Zittell: Handbuch der Palaontologie. Miinchen, 1884-93. (A translation of an abridg-
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Fishes.

Adams: Skull of Anarrhichthys. Kansas Univ. Sci. Bull., 4, 1908.

Allis: Cranium of Amia. Jour. Morph., 2, 1889; 14, 1898.

Allis: Jaw bones and breathing valves of Polypterus. Anat. Anz., 18, 1900.

Allis: Skull of Scomber. Jour. Morph., 18, 1903.

Balfour: Development of skeleton of paired fins of elasmobranchs. Proc. Zool. Socy.
London, 1881.

Bridge: Ribs in Polyodon. Proc. Zool. Socy. London, 1897.

Brohmer: Kopf eines embryos von Chlamydoselachus u. d. Segmentirung des Selachier-
schidels. Jena. Zeitsch., 44, 1908.

362 BIBLIOGRAPHY.

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1892.

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Fiirbringer: Visceralskelett der Selachier. Morph. Jahrb., 31, 1903.

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Gegenbaur: Kopfskelett von Alepocephalus. Morph. Jahrb., 4, Suppl., 1878.

Gegenbaur: Clavicula und Cleithrum. Morph. Jahrb., 23, 1895.

Gegenbaur: Flossenskelett der Crossopterygier und das Archipterygium der Fische.
Morph. Jahrb., 22, 1894.

Gegenbaur: Uber das Archipterygium. Jena. Zeitsch., 7, 1872.

Goodrich: Pelvic girdle and fin of Eusthenopteron. Quar. Jour. Micr. Sci., 45, rgor.

Goodrich: Dermal finrays of fishes. Quar. Jour. Micr. Sci., 47, 1904.

Goodrich: rates tial structure and origin of fins of fishes. Quar. Jour. Micr. Sci.,
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Géppert: Morphologie der Fischrippen. Morph. Jahrb., 23, 1896.

Grassi: Entwicklung der Wirbelsiule der Teleostier. Morph. Jahrb., 8, 1882.

Harrison: Entwickelung der unpaaren und paarigen Flossen der Teleostier. Arch. mikr.
Anat., 46, 1895. See also 42, 1893.

Hasse: Entwicklung der Wirbelsaiule der Elasmobranchier. Zeitsch. wiss. Zool., 55, 1892;
Ganoiden, 57, 1893; Cyclostomen, 57, 1893.

Hay: Vertebral column of Amia. Field Columbian Museum, Zool. Publications, 1, 1895
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Mayer: Unpaaren Flossen der Selachier. Naples Mittheilungen, 6, 1885.

Parker: Structure and development of the skull in sharks and skates. Trans. Zool. Socy.
London, Io, 1878.

Parker: Skeleton of Marsipobranchs. Phil. Trans., 1883.

Parker: Structure and development of skull in Lepidosteus. Phil. Trans., 1882; of stur-
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Pollard: Suspension of jaws in fishes. Anat. Anz., 10, 1894.
Pollard: Oral cirri of siluroids and origin of head. Zool. Jahrb., Anat. Abth., 8, 1895.
Sagemehl: (several papers on skulls of fishes). Morph. Jahrb., 9, 10, 17, 1884-1891.

Shufeldt: Osteology of Amia (based on Francque). Rept. U. S. Fish Commis. for 1883
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Starks: Several papers. Proc. U. S. Nat. Mus., 21-27, 1898-1904.

Thacher: Median and paired fins. Trans. Conn. Acad., 3, 1878.
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Walther: Entwicklung der Deckknochen am Kopfskelett des Hechtes. Jena. Zeitsch., 16,
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SKELETON. 363

Wright: Skull and auditory organ of Hypophthalmus. Trans. Roy. Soc. Canada, 4, 1885.
Ziegler: Heroindest der Selachier und die Flossenstrahlen der Knochenfische. Zool. Anz.,
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Amphibia.

Cope: Hyoid and otic elements in batrachia. Jour. Morph., 2, 1888.

Credner: Stegocephalen und Saurier. Zeitschr. deutsch. geolog. Gesellsch., 1881-1893.

Eggeling: Aufbau der Skeletteile in Gliedmassen. Jena, 1911.

Field: Entwickelung der Wirbelsiule der Amphibien. Morph. Jahrb., 22, 189s.

a aa of vertebral column of amphibia and amniotes. Phil. Trans., 187, B,
1896.

Gaupp: Primordial-cranium von Rana. Morph. Arbeiten, 2, 1893.

Géppert: Amphibienrippen. Morph. Jahrb., 22, 1895.

Kingsbury and Reed: Columella auris in amphibia. Jour. Morph., 20, 1909.

Murray: Vertebral column of certain urodeles. Anat. Anz., 13, 1897.

Parker: Structure and development of skull in urodeles. Phil. Trans., 1877.

Parker: Morphology of skull in Amphibia urodela. Trans. Linn. Socy., Zool., 2, 1879.

Parker: Structure and develop. of skull of common frog. Phil. Trans., 1871.

Parker: Struct. and dev. skull in Batrachia, Pt. II and III. Phil. Trans., 1881

Parker: Struct. and dev. skull in urodeles. Trans. Zool. Socy. London, 11, 1882.

Peter: Schddel von Ichthyophis. Morph. Jahrb., 25, 1898.

Platt: Development of cartilaginous skull of Necturus. Morph. Jahrb., 25, 1897.

Shimada: Wirbelsdule und Hiillen des Riickenmark von Cryptobranchus japonicus. Anat.
Hefte, 44, 1911.

Terry: Nasal skeleton of Amblystoma. ‘Trans. St. Louis Acad. Sci., 16. 1906.
Whipple: Ypsiloid apparatus of urodeles. Biol. Bull., 10, 1906.

Wiedersheim: Kopfskelett der Urodelen. Morph. Jahrb., 3, 1877.

Wilder: Skeletal system of Necturus. Memoirs Boston Socy. Nat. Hist., 5, 1903.

Reptilia.

Baur: Osteologische Notizen iiber Reptilien. Zool. Anz., 9 and 10, 1886-87.

Baur: Pelvis of the Testudinata. Jour. Morph., 4, 1891.

Baur: Morphologie des Carpus und Tarsus der Reptilien. Zool. Anz., 8, 1885.

Cope: Osteology of lacertilia. Proc. Am. Phil. Socy., 30, 1892.

Corning: Neugliederung der Wirbelsaule bei Reptilien. Morph. Jahrb., 17, 1891.

Cope: Degenerate scapular and pelvic arches in lacertilia. Jour. Morph., 7, 1892.

Fiirbinger: Knochen und Muskeln der Extremitaten bei schlangenahnlichen Saurien.
Leipzig, 1870.

Gaupp: Chondrocraniun von Lacerta. Anat. Hefte, 15, 1900.

Gétte: Wirbelbau bei Reptilien. Zeitsch. wiss. Zool., 50, 1892.

Gétte: Entwicklung des Carapax der Schildkréten. Zeit. wiss. Zool., 66, 1899.

Hay: Fossil turtles of North America. Carnegie Inst., 1908,

Howes and Swinnerton: Development of skeleton of Sphenodon. Trans. Zool. Socy.
London, 16, 1901.

Kingsley: Reptilian lower jaw. Amer. Nat., 39, 1905.

Marsh: Numerous papers on fossil reptiles. Am. Jour. Sci., 16-50.

Moodie: Reptilian epiphyses. Am. Jour. Anat., 7, 1908.

Neit: Schidel d. Dermochelys. Zool. Jahrb., Abt. Anat., 33, 1912.

Ogushi: Skelett der japanischen Trionyx. Morph, Jahrb., 43, 1911.

Parker: Skull in the common snake. Phil. Trans., 1878.

Parker: Skull in the lacertilia. Phil. Trans., 1879.

364 BIBLIOGRAPHY.

Parker: Skull in the crocodile. Trans. Zool. Socy., 19.

Versluys: Mittlere und Aussere Ohrsphare der Lacertilia. Zool. Jahrb., Abth. Anat., 12,
1898.

Versluys: Columella auris bei Lacertilien. Zool. Jahrb., Abth. Anat., 19, 1903.

Birds.

Gegenbaur: Becken der Vogel. Jena. Zeitsch., 6, 18, 1871.

Leighton: Development of wing of Sterna. Tufts Coll. Studies, 1, 1894.

Lindsay: The avian sternum. Proc. Zool. Socy. London, 1885.

Marsh: Odontornithes. U. S. Geol. Survey, 1880.

Mehnert: Entwicklung des Os pelvis der Vogel. Morph. Jahrb., 13, 1888.

Morse: Carpus and tarsus of birds. Ann. N. Y. Lyceum Nat. Hist., 10, 1872.

Osborn: Evidence for a dinosaur-avian stem in the Permian., Am. Nat., 34.

Parker: Skull of the common fowl: Phil. Trans., 1869.

Pycraft: Osteology of birds. Proc. Zool. Socy. London, 1898.

rake Hompholeny of duck and auk tribes. Royal Irish Acad., Cunningham memoir,
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Parker: Structure and development of wing in common fowl. Phil. Trans., 179, 1888.

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Siegelbauer: Entwicklung der Végelextremitét. Zeit. wiss. Zool., 97, 1911.
Mammals.

Allen: Ethmoid bone in mammals. Bull. Mus. Comp. Zool., 10, 1883.

Bardeen: Development of human skeleton. Am. Jour. Anat., 4, 1905.

Bardeen: Development of thoracic vertebra in man. Jour. Anat., 4, 1905.

Baur: Morphologie des Carpus der Saéuger. Anat. Anz., 4, 1889.

Brush: Cervical ribs. J. Hopkins Hosp. Bull., 12, r90r.

Boas: Metatarsus der Wiederkiuer. Morph. Jahrb., 16, 1890.

Collinge: Skull of dog. London, 1896.

Dilg: Morphologie des Schadels bei Manatus. Morph. Jahrb., 39, 1909.

Eggeting: Clavicula, Preclavicula, Halsrippen und Manubrium sterni. Anat. Anz., 29,
1900.

Fischer: Primordialcranium von Talpa. Anat. Hefte, 17, 1901.

Flower: Osteology of the mammalia. London, 1885.

Frits: Entwicklung der Wirbelsdule von Echidna. Morph. Jahrb., 39, 1909.

Froriep: Entwicklung der Wirbelsiule. Arch. Anat. und Phys., 1886.

Gaupp: Neue Deutungen a. d. Gebiet der Lehre des Saugetierschadels. Anat. Anz., 27,
1905.

Gegenbaur: Episternal Skelettteile. Jena. Zeitsch., 1, 1864.

Holder: Osteology of right whale. Bull. Am. Mis. Nat. Hist., 1, 1883.

Howes: Mammalian hyoid. Jour. Anat. and Phys., 30, 1897.

Kiikenthal: Hand der Cetaceen. Anat. Anz., 3, 1888; Morph. Jahrb., 19, 1892.

Mead: Chondrocranium of pig. Am. Jour. Anat., 9, 1909.

Olmstead: Primordialcranium eines Hundeembryo. Anat. Hefte, 43, 1911.

Parker: Skull of pig. Phil. Trans., 1874.

Parker: Structure and devel. skull in mammalia, Pt. 2, Insectivora. Pt. 3, Edentates.
Phil. Trans., 1885.

Stromer: Foramen entepicondyloideum und Trochanter tertius. Morph. Jahrb., 29, 1902.
Van Kampen: Tympanalgegend des Saugetierschidels. Morph. Jahrb., 34, 1905.
Voit: Primordialcranium des Kaninchens. Anat. Hefte, 38, 1909.

MUSCULAR SYSTEM. 365

Weiss: Entwicklung der Wirbelsaule der weissen Ratte. Zeitsch. wiss., Zool., 69, IgoL.
Whitehead and Waddell: Development of human sternum. Am. Jour. Anat., 12, 1911.

MUSCULAR SYSTEM.

Allis: Cranial muscles of Amia. Jour. Morph., 12, 1897.

Ayers and Jackson: Myology of myxinoids. Jour. Morph., 17, 1gor.

Blum: Schwanzmuskulatur des Menschen. Anat. Hefte, 4, 1894.

Braus: Entwicklung der Musculatur und periph. Nervensystem der Selachier. Morph.
Jahrb., 26, 27, 1898-9.

Bruner: Smooth facial muscles of Amphibia. Morph. Jahrb., 29, 1901.

Byrnes: Develop. limb muscles in amphibia. Jour. Morph., 14, 1897.

Charnock: Muscles of mastication and movements of skull in lacertilia. Zool. Jahrb.,
Anat, Abth., 18, 1903.

Corning: Entwicklung Kopf- und Extremitdten-Muskulatur bei Reptilien. Morph.
Jahrb., 28, 1899.
Corning: Vergl. Anat. der Augenmuskulatur. Morph. Jahrb., 29, 1900.

Davidoff: Vergl. Anat. der hinteren Gliedmassen der Fische. Morph. Jahrb., 5-6,
1879-80.

Driiner: Zungenbein-, Kiemenbogen- und Kehlkopfmuskein der Urodelen. Zool. Jahrb.
Abth. Anat., 15, 1901; 19, 1904.

Edgeworth: Development of head muscles,in Gallus. Quar. Jour. Micr. Sci., 51, 1907.

Edgeworth: Morphology of the cranial muscles of some vertebrates. Quar. Jour. Micr.

” Sci., 56, r91z.

Fewkes: Myology of Echidna. Bull. Essex Inst., 9, 1877.

Fiirbringer: Muskeln der schlangenahnlichen Saurien. Leipzig, 1870.

Fiirbringer: Vergl. Anat. der Schultermuskeln. Urodeles, Jena. Zeitsch., 7, 1873; Anura,
1. c., 8, 1874; Birds, 1. c., 36, 1902; Reptiles. Morph. Jahrb.,1, 1876.

Fiirbringer: Muskulatur des Kopf der Cyclostomen. Jena. Zeitsch., 9, 1875.

Firbringer: Muskulatur des Végelfliigels. Morph. Jahrb., 6, 1885.

Gadow: Bauchmuskeln der Reptilien. Morph. Jahrb., 7, 1881.

Gadow: Myologie der Extremititen der Reptilien. Morph. Jahrb., 7, 1881.

Humphrey: Several papers on muscles of sharks, dipnoi and urodeles. Jour. Anat. and
Phys., 1873.

Humphrey: Muscles of Lepidosiren (Protopterus). Jour. Anat. and Phys., 6, 1872.
Muscles of Ceratodus, same vol.

Lamb: Eye-muscles in Acanthias. Am. Jour. Anat., 1, 1902.

MacDowell: Myology-of Anthropopithecus. Am. Jour. Anat., 10, 1910.

McMurrich: Phylogeny of forearm flexors. Am. Jour. Anat., z, 1903; of palmar mus-
culature, same vol.

Mall: Development of human diaphragm. Jour. Morph., 12, 1897.

Mall: Development of human diaphragm. Johns Hopkins Hosp. Bull., 12, rgor.

Marion: Mandibular and branchial muscles of elasmobranchs. Am. Nat., 39, 1905;
Tufts College Studies, 2, 1905.

Maurer: Ventral Rumpfmuskulatur der Urodelen. Morph. Jahrb., 17, 1892.

Mivart: Myology of Menopoma, Menobranchus, Chameleon. Proc. Zool. Socy. London,
1869-70.

Neal: Development of hypoglossal musculature in Petromyzon and Squalus. Anat. Anz.,
13, 1897.

Ribbing: Armmuskulatur der Amphibien, Reptilien und Saugetiere. Zool. Jahrb. Abth.
Anat., II, 1907.

Riige: Geschichtsmuskeln der Halbaffen. Morph. Jahrb., 11, 1885.

Schufeldt: Myology of the raven. London, 1890.

Shufeldt: Anatomy (mostly muscles) of Geococcyx. Proc. Zool. Socy. London, 1886.

366 BIBLIOGRAPHY.

Uskow: Entwicklung des Zwergfells, u. s. w. Arch. mikr. Anat., 32, 1883.
Wilder: Appendicular muscles of Necturus. Zool. Jahrb., Suppl. 15, 2 Bd., rgz2.

ELECTRICAL ORGANS.

Ballowitz: Anatomie des Zitteraales. Arch. mikr. Anat., 50, 1897:

Ballowitz: Elektrischen Organe von Torpedo. Arch. mikr. Anat., 42, 1893 (Large
bibliography.) :

Dahlgren and Silvester: Electric organ of Astroscopus. Anat. Anz., 29, 1906.

Ewart: Development of electric organ in skate. Phil: Trans., 179B, 1889.

NERVOUS SYSTEM.

Barker: The nervous system and its constituent neurones. N. Y. , 1899.

Edinger: Vorlesungen iiber den Bau der nervésen Centralorgane des Menschen und der
Tiere. 7th edit., 2 vols., 1904-8.

Johnston: Nervous system of vertebrates. Philadelphia, 1906.

Johnston: Morphology of vertebrate head from viewpoint of functional divisions of
nervous system. Jour. Comp. Neurol., 15, 1905.

Johnston: Central nervous system of Vertebrates. Ergebnisse und Fortschritte der
Zoologie, 2, 1910.

BRAIN AND SPINAL CORD.

Barnes: Development of posterior fissure of spinal cord. Proc. Am. Acad. A. and Sci.,
1883-4.

Dejerine: Anatomie des centres nerveux. Paris, 1895.

Herrick: Morphology of forebrain in amphibia and reptiles. Jour. Comp. Neurol., 20,
IgIo.

Hill: Primary segments of vertebrate head. Zool. Jahrb., 13, 1899. ;

His: Allgemein. Morphologie des Gehirn. Arch. Anat. und Phys., Abth. Anat., 1892.

Johnston: Morphology of forebrain vesicle in vertebrates. Jour. Comp. Neurol., 19, 1909.

Johnston: Morphology of vert. head from point of division of nervous system. Jour.
Comp. Neurol. 15, 1905.

Johnston: Gehirn und Cranialnerven der Anamnier. Ergebnisse, 11, 1901.

McClure: Segmentation of primitive brain. Jour. Morph., 4, 1890.

gs Origin of cerebral cortex and homology of optic lobe layers. Jour. Morph.,
4, 1890.

Osborn: Origin of corpus callosum. Morph. Jahrb., 12, 1886.

Smith: Origin of corpus callosum. Trans. Linn. Socy., 7, 1897.

Tilney: Hypophysis cerebri. Memoirs Wistar Inst., 2, 1911.

Cyclostomes and Fishes.

Ayers and Worthington: Finer anatomy of brain of Bdellostoma. Am. Jour. Anat., 8,
1908.

Bing und Burckhardt: Zentralnervensystem von Ceratodus. Anat. Anz., 25, 1904.

Burckhardt: Centralnervensystem von Protopterus. Berlin, 1892.

Chandler: Lymphoid structure above myelencephalon of Lepidosteus. Univ. Calif. Pub.
Zool., 9, IQII.

Cole: Cranial nerves of Chimera. Trans. Roy. Socy. Edinb., 38, 1896.

Dammerman: Der Saccus vasculosus der Fische ein Tieforgane. Zeit. wiss. Zool., 96,
1910.

Franz: Das Mormyriden (brain). Zool. Jahrb., Abt. Anat., 32. 1911.

Franz: Kleinhirn der Knochenfische. Zoo). Jahrb., Abt. Anat., 32, 1911.

NERVOUS SYSTEM. 367

Goronowitsch: Gehirn und Cranialnerven von Acipenser. Morph. Jahrb., 13, 1888.

Haller: Bau der Wirbeltiergehirns. I, Salmo und Scyllium. Morph. Jahrb., 26, 1898.

Herrick: Brains of some American fresh water fishes. Jour. Comp. Neurol., 1, 1891.

Herrick: Brain of certain ganoids. Jour. Comp. Neurol., 1, 1891.

Johnston: Brain of Acipenser. Zool. Jahrb., 15, 1901.

Johnston: Brain of Petromyzon. Jour. Comp. Neurol., 12, 1902.

Johnston: Telencephalon of selachians. Jour. Comp. Neurol., 21, 1911.

sear at Olfactory lobes, forebrain and habenular tracts of Acipenser. Zool. Bull., 1,
1898.

Johnston: Telencephalon of selachians. Jour. Comp. Neurol., 21, 1911.

Johnston: Telencephalon of ganoids and teleosts. Jour. Comp. Neurol., 21. r91z

Kappers: Teleost and selachian brain. Jour. Comp. Neurol., 16, 1906.

Kingsbury. Oblongata in fishes. Jour. Comp. Neurol. 7, 1897.

Less Sontnbahien to structure and development of vertebrate head. Jour. Morph., 11,
1895.

Mayer: Gehirn der Knochenfische. Arch. Anat. und Phys., 1882.

Mayser: Gehirn der Knochenfische (Cyprinoids). Zeit. wiss. Zool., 36, 1881.

Neal: Segmentation of nervous system in Acanthias. Bull. Mus. Comp. Zool., 31, 1898.

Nicholls: Reissner’s Fibre. Anat. Anz., 40, 1912.

Sargent: Reissner’s fibre. Bull. Mus. Comp. Zool., 45, 1904.

Sargent: Torus longitudinalis of teleost brain. Mark Anniv. Vol., 1904.

ea: Centralnervensystem und Geruchsorgane von Polyterus. Anat. Anz., 2,
1887.

Worthington: Brain and cranial nerves of Bdellostoma. Quar. Jour. Micr. Sci., 49,
“1905.

Amphibia.

Burekhacd Hirn und Geruchsorgan von Triton und Ichthyophis. Zeitsch. wiss. Zool.,

52, 1891.

Fish: Central nervous system of Desmognathus. Jour. Morph., 5, 1895.

Fischer: Amphibiorum nudorum neurologie specimen primus. Berlin, 1843.

Gage: Brain of Diemyctylus compared with Amia and Petromyzon. Wilder quarter-
century book, 1893.

Griggs: Early development of central nervous system in Amblystoma. Jour. Morph.,
21, 1910. |

Kingsbury: Brain of Necturus. Jour. Comp. Neurol., 5, 1895.

Kingsley and Thyng: Hypophysis in Amblystoma. Tufts Coll. Studies, 1, 1904.

Osborn: Brain of Amphiuma. Proc. Acad. Nat. Sci., Philadelphia, 1883.

Osborn: Internal structure of amphibian brain. Jour. Morph., 2, 1888.

Waldschmidt: Nervensystem der Gymnophionen. Jena. Zeitsch., 20, 1886.

Reptilia.

Gisi: Gehirn von Hatteria [Sphenodon]. Zool. Jahrb., Abth. Anat., 25, 1907.

Haller: Bau des Wirbeltiergehirns. II, Emys. Morph. Jahrb., 28, 1900.

Herrick: Brain of certain reptiles. Jour. Comp. Neurol., 1, 1891; see also vol. 3.
Herrick: Brain of alligator. Jour. Cincinnati Socy. Nat. Hist., 12, 1890.

Humphrey: Brain of Chelydra. Jour. Comp. Neurol., 4, 1894.

Képpen: Anatomie des Eidechsensgehirn. Morph. Arbeiten, r.

Rabl-Riickhard: Centralnervensystem des Alligator. Zeitsch. wiss. Zool., 30.
Rabl-Riickhard: Gehirn des Riesenschlange. Zeitsch. wiss. Zool., 58, 1894.

368 BIBLIOGRAPHY.

Birds.

Bumm: Grosshirn der Vogel. Zeitsch. wiss. Zool., 38, 1893.

Kamon: Entwicklung des Gehirns des Hiinchens. Anat. Hefte, 30, 1906.
Streeter: Spinal cord of ostrich. Am. Jour. Anat., 3, 1903.

Turner: Avian brain. Jour. Comp. Neurol., 1, 1891.

Mammals.

Bechterew: Leitungsbahnen im Gehirn und Riickenmark. Leipzig, 1898,

Gronberg: Untersuchungen an Gehirn von Erinaceus. Zool. Jahrb., 15, 1891.

Haller: Bau des Gehirn von Mus und Echidna. Morph. Jahrb., 28, 900.

Herrick: Brain of rodents. Bull. Denison Univ., 6, 1891.

Landau: Das Katzenshirns. Morph. Jahrb., 38, 1908.

Sabin: Atlas of medulla and mid brain. Baltimore, rgz0.

Smith: Brain of foetal Ornithorhynchus. Quar. Jour. Micr. Sci., 39, 1896.

Smith: Morphology of brain in mammals. Trans. Linn. Socy. London, Zool., 8, 1903.

Symington: Commissures in marsupialia and monotremes. Jour. Anat, and Phys., 27,
1892.

EPIPHYSIAL STRUCTURES.

Beard: Parietal eye of cyclostomes. Quar. Jour. Micr. Sci., 29, 1888.

Dendy: Devel. parietal eye, etc., in Sphenodon. Quar. Jour. Micr. Sci., 42, 1899.

Dexter: Development of paraphysis in fowl. Am. Jour. Anat., 2, 1902.

Eycleshymer: Paraphysis and epiphysis in Amblystoma. Anat. Anz., 7, 1892.

Gaupp: Zirbel, Parietalorgan und Paraphysis. Ergebnisse, 7, 1897.

Hill: Develop. of epiphysis in Coregonus. Jour. Morph., 5, 1891; of teleosts and Amia,
idem, 9, 1894. 7

Kingsbury: Encephalic evaginations in ganoids. Jour. Comp. Neurol., 7, 1897.

Minot: Morphology of pineal region based on Acanthias. Am. Jour. Anat., 1, 190r.

Nowikoff: Parietalauge von Saurien. Zeit. wiss. Zool., 96, 1910.

Reese: Develop. of paraphysis and epiphysis in alligator. Smithson. Misc. Coll.,
54, Igro.

Ritter: Parietal eye in some lizards. Bull. Mus. Comp. Zool., 20, 1891.

Spencer: Pineal eye in Lacertilia. Quar. Micr. Sci., 27, 1886.

Warren: Pineal region in Necturus. Am. Jour. Anat., 5, 1905.

Warren: Pineal region in reptiles. Am. Jour. Anat., 11, 1911.

PERIPHERAL NERVES.

Allis: Cranial muscles and nerves of Amia. Jour. Morph., 12, 1897.
Allis: Cranial nerves in Scomber. Jour. Morph., 18, 1903.

Beard: Branchial sense organs and associated ganglia in ichthyopsida. Quar. Jour.
Micr. Sci., 25, 1885.

Bowers: Cranial nerves of Spelerpes. Proc. Amer. Acad., 36, 1901.

Braus: Innervation der paarigen Extremitaten bei Selachiern und Dipnoer. Jena.
Zeitsch., 31, 1898.

Brook: Bau des sympathet. Nervensystems der Saugetiere. Jour. Morph., 37, 1907;
38, 1908. ;

Brookover: Olfactory nerve, terminalis nerve and preoptic sympathetic in Amia. Jour.
Comp. Neurol., 20, 1910; olfact. and terminalis in Amiurus. Idem, 21, 1911.

Carpenter: Develop. oculomotor and abducens nerves and ciliary ganglion in chick.
Bull. Mus. Comp. Zool., 48, 1906.

SENSE ORGANS. 369

Coghill: Cranial nerves of Amblystoma. Jour. Comp. Neurol., 12, 1902.
Cole: Cranial nerves of Chimera. Trans.Roy. Socy., Edinburg, 38, 1896.
Cole: Cranial nerves of Gadus. Trans. Linn. Socy. London, Zool., 7, 1898.

Fischer: Anat. Abhandl. tiber Perrennibranchiaten und Derotremen. Hamburg, 1854.
See also Amphib. Nudorum, etc., under Brain.

Gegenbaur: Kopfnerven von Hexanchus. Jena. Zeitsch., 6, 1871.

Hammersten: Innervation der Bauchflossen bei Teleostiern. Morph. Jahrb., 42, r9rr.

Herrick: Cranial nerves of Menidia. Jour. Comp. Neurol., 9, 1899.

Herrick: Cranial nerves of siluroids. Jour. Comp. Neurol., 11, 1901.

Herrick: Criteria of homology in peripheral nervous system. Jour. Comp. Neurol., 19,
Igo09.

Herrick: Peripheral nervous system of bony fishes. Bull. U. S. Fish Commiss. for 1898.

Herrick: Nervus terminalis in frog. Jour. Comp. Neurol., 19, 1909.

Huber: Sympathetic nervous system. Jour. Comp. Neurol., 7, 1897.

Huber: Minute anat. of sympathetic ganglia. Jour. Morph., 16, 1899.

Johnston: Cranial nerves of Petromyzonts. Jour. Comp. Neurol., 18, 1908.

Johnston: Cranial nerve components of Petromyzon. Morph. Jahrb., 34, 1905.

Kunz: Development of sympathetic in turtles. Am. Jour. Anat., 11, 1911; mammals and
birds. Jour. Comp. Neurol., 20, 1910; of amphibia, idem, 21, 1911; Evolution symp.
syst. in vertebrates, idem, 21, IgII.

Kupffer: Entwicklungsgeschichte des Kopfes. Ergebnisse, 1895.

Kupffer: Development of cranial nerves. Jour. Comp. Neurol., 1, 1891.

Landacre: Cranial ganglia in Amiurus. Jour. Comp. Neurol., 20, 1910.

Landacre: Epibranchial placodes of Lepidosteus and their relation to the cerebral ganglia.
Jour. Comp. Neurol., 22, 1912. ;

ory, Nex cranial nerve in selachians. Mark Anniv. Vol., 1903. See also Anat. Anz.,
26, 1905.

Lubosch: Nervus accesorius Willisii. Arch. mikr. Anat., 54, 1899.

Mayhoff: ‘Monomorphe’ Chiasma opticum der Pleuronectiden. Zool. Anz., 39, 1912.

McKebben: Nervous terminalis in Amphibia. Jour. Comp. Neurol., 21, 1911.

Neal: Development of ventral nerves in selachii. Mark Anniv. Vol., 1903.

Norris: Cranial nerves of Amphiuma. Jour. Comp. Neurol., 18, 1908.

Parker: Optic chiasma in teleosts. Bull. Mus. Comp. Zool., 40, 1903.

Pinkus: Hirnnerven des Protopterus. Morph. Arbeiten, 4, 1894.

Prentiss: Development of hypoglossal ganglion in pig. Jour. Comp. Neurol., 20, 1910.

Punnett: Pelvic plexus and nervus collector in Mustelus. Phil. Trans. 192, B, 1900,

Sheldon: Nervus terminalis in carp. Jour. Comp. Neurol., 19, 1909.

Stannius: Peripherische Nervensystem der Fische. Rostock, 1849.

Streeter: Development of cranial and spinal nerves in occipital region of man. Am. Jour.

’ Anat., 4, 1904.
Strong: Cranial nerves of amphibia. Jour. Morph., ro, 1895.
SENSE ORGANS.

Okajima: Sinnesorgane von Onychodactylus. Zeit. wiss. Zool., 94, 1909.
Osawa: Sinnesorgane der Hatteria [Sphenodon]. Arch. mikr. Anat., 52, 1898.
Schwalbe: Lehrbuch der Anatomie der Sinnesorgane. Erlangen, 1883.

Dermal and Lateral Line Organs.

Allis: Lateral line system in Amia. Jour. Morph., 2, 1889.

Allis: Lateral sensory canals of Mustelus. Quar. Jour. Micr. Sci., 45, 1902.

Allis: Lateral canals of Polyodon. Zool. Jahrb., Abth. Anat., 17, 1903.

Ayers and Worthington: Skin end organs of trigeminal and lateralis nerves of Bdellos-
stoma. Am. Jour. Anat., 7, 1907.

24

370° BIBLIOGRAPHY.

Beard: Segmental sense organs and associated ganglia in ichthyopsida. Quar. Jour. Micr.
Sci., 25, 1885.

Boll: Savi’schen Blaschen von Torpedo. Arch. Anat. und Phys., 1875.

Clapp: Lateral line system of Batrachus. Jour. Morph., 15, 1899.

Collinge: Sensory canal system of ganoids. Quar. Jour. Micr. Sci., 36, 1894.

Ewart and Mitchell: Lateral sense organs of Elasmobranchs. Trans. Roy. Soc. Edinburg,
38, 1892. (See Garman, Bull. Mus. Comp. Zool., 17, 1888.)

Ewart: Sensory canal of Lemargus. Trans. R. Socy., Edinb., 37, 1893.

Harrison: Entwicklung der Sinnesorgane der Seitenlinie der Amphibien. Arch, mikr.
Anat., 63, 1903.

Kingsbury: Lateral line system of American amphibia and comparisons with dipnoi.
Proc. Amer. Micros. Socy., 17, 1895.

Maurer: Die Epidermis and ihre Abkémmlinge. Leipzig, 1895.

Moodie: Lateral line system in extinct amphibia. Jour. Morph., 19, 1908.

Morrill: Pectoral appendages of Prionotus. Jour. Morph., 11, 1895.

Munkert: Lorenzini’schen Ampullen. Anat. Anz., 19, 1901.

Parker: Function of lateral line system in fishes. Bull. Bureau of Fisheries, 24, 1905.

Pollard: Lateral line system in siluroids. Zool. Jahrb., 5, 1893.

Ritter: Eyes, integumental sense papilla and skin of Typhlogobius. Bull. Mus. Comp.
Zool., 24, 1893.

Taste.

Herrick: Phylogeny and morphol. position of terminal buds of fishes. Jour. Comp.
Neurol., 13, 1903.

Herrick: Organs and sense of taste in fishes. Bull. U. S. Fish Comm., 22, 1903.

Herrick: Terminal buds of fishes. Jour. Comp. Neurol., 13, 1903.

Schwalbe: Geschmacksorgane der Saugetiere. Arch. mikr. Anat., 4, 1868

Tuckerman: Gustatory organs of mammals. Jour. Morph., 2, 1888; 4, 1890: 7, 1892.

Smell.

Anton: Jacobson’schen Organ und Nasenhohle der Cryptobranchiaten. Morph. Jahrb.,
38, 1908.

Bawden: Nose and Jacobson’s organ with reference to amphibia. Jour. Comp. Neurol.,
4, 1894.

Berliner: Entwicklung des Geruchsorgane der Selachier. Arch. mikr. Anat., 60, 1902.

Blaue: Nasenschleimhaut bei Fischen und Amphibien. Arch. Anat. und Phys., Anat.
Abth., 1884.

Born: Nasenhéhle und Thranennasengang der Amphibien. Morph. Jahrb., 2, 1876; der
Amnioten, ibid., 5, 1879; 7, 1882.

Disse: Riechschleimhaut und Riechnerv bei Wirbeltiere. Ergebnisse, 11, rgor.

Dogiel: Geruchsorgane bei Ganoiden, Knochenfische, und Amphibien. Arch. mikr.
Anat., 29, 1887.

Fischer: Nasenhéhle und Tranengange der Amphisbeenen. Arch. mikr. Anat., 55, 1900.

Frets: Entwicklung der Nase bei Affen, Singern und Menschen. Morph. Jahrb., 44,
IgI2.

Gaupp: Nervenversorgen der Mund- und Nasenhdhlendriisen der Wirbeltiere. Morph.
Jahrb., 14, 1888.

Gegenbaur: Nasenmuscheln der Végel. Jena. Zeitsch., 7, 1873.

Holm: Develop. Olfactory organ in Teleosts. Morph. Jahrb., 21, 1894.
McCallum: Nasal region in Eutenia. Proc. Canadian Inst., 1, 1883.

Peter: Entwicklung d. Geruchsorgane. Ergebnisse, 20, 1911.

Read: Olfactory apparatus in dog, cat and man. Am. Jour. Anat., 8, 1908.
Schaeffer: Lateral wall of cavum nasi in man. Jour. Morph., 21, rgro.

SENSE ORGANS. 371

Sey Nasenhéhle und Jacobson’sche Organ der Amphibien. Morph. Jahrb., 23,
1895.
Strong: Olfactory organ and smell in birds. Jour. Morph., 22, rg11.

Wilder: Nasengegend von Menopoma [Cryptobranchus] und Amphiuma. Zool.
Jahrb., Abth. Anat., 5, 1892. :

Wilder: Lateral nasal glands of Amphiuma. Jour. Morph., 20, 1909.
Zukerkandl: Jacobson’sche Organs. Ergebnisse, 18, 1910.
Zukerkandl: Jacobsonsorgane und Riechlappen der Amphibien. Anat. Hefte, 41, rgro.

Eyes.

Bage: Retina of lateral eyes of Sphenodon. Quar. Jour. Mic. Sci., 57, 1912.

Berger: Sehorgane der Fische. Morph. Jahrb., 8, 1882.

Brauer: Augen der Tiefseefische. Verhandl. deutsch. zool. Gesellsch., 1g02.

Carriere: Sehorgane der Thiere. Miinchen, 1885.

Corning: Anatomie der Augenmuskulatur. Morph. Jahrb., 29, 1900.

Eggeling: Augenlider der Saugetiere. Jena. Zeitsch., 39, 1904.

Eigenmann: Eyes of blind vertebrates. Biol. Bull., 2, 1900; 5, 1903.

Eigenmann: Eyes of Amblyopside. Arch. Entw. Mechan., 7, 1899.

Eycleshymer: Development of optic vesicles in amphibia. Jour. Morph., 8, 1893.

Eycleshymer: Development of optic vesicles in Amphibia. Jour. Morph., 8, 1890.

Jelgersma: Ursprung des Wirbeltierauges. Morph. Jahrb., 35, 1906.

Lamb: Development of eye muscles of Acanthias. Am. Jour. Anat., 1, gor.

Locy: Optic vesicles of elasmobranchs and their relations to other structures. Jour.
Morph., 9, 1894.

Mall: Histogenesis of retina. Jour. Morph., 8, 1893.

Peters: Harder’schen Driise. Arch. mikr. Anat., 36, 1890.

ahh Ha und Entwicklung der Linse. Zeitsch. wiss. Zool., 63, 1898; 65, 1898; 67,
1899.

Robinson: Formation and structure of optic nerve and its relation to optic stalk. Jour.
Anat. and Phys., 30, 1896.

Schaeffer: Develop. nasolacrimal passages in man. Am. Jour. Anat., 13, 1912.
Studnicka: Sehnerven der Wirbeltiere. Jena. Zeitsch., 31, 1897.

Weysse: Histogenesis of retina. Am. Nat., 40, 1906.

Williams: Migration of eye in Pseudopleuronecetes. Bull. Mus. Comp. Zool., 40, 1902.

Ears.

Ayers: Vertebrate cephalogenesis (large bibliography). Jour. Morph., 6, 1892.

Ayers: Relations of hair cells of ear. Jour. Morph., 8, 1893.

Bridge and Haddon: Air bladder and Weberian ossicles of siluroids. Phil. Trans., 184,
1893.

Driiner: Anatomie und Entwicklung des Mittelohres beim Menschen und Maus. Anat.
Anz., 24, 1904.

Gaupp: Schalleitenden Apparatus bei Wirbeltiere. Ergebnisse, 8, 1898.

Kingsbury: Columella auris and nervus facialis. Jour. Comp. Neurol., 13, 1903.

Kingsley: Ossicula auditus. Tufts College Studies, 1, 1g00.

Mall: Development Eustachian tube, middle ear, etc., of chick. Studies Biol. Lab.
Johns Hopkins, 4, 1887.

Norris:. Development of auditory vesicle in Amblystoma. Jour. Morph., 7, 1892.

Okajima: Entwicklung d. Gehororganes von Hydnobius. Anat. Hefte, 45, 1911.

Parker: Hearing and allied senses in fishes. Bull. U. S. Fish. Comm. for 1g02, 1903.

' See also Am. Nat., 37, 1903.

Streeter: Development of labyrinth and acoustic and facial nerves in human embryo.

Am. Jour. Anat., 6, 1907.

372 BIBLIOGRAPHY.

Versluys: Mittlere und aussere Ohrsphire der Lacertilier. Zool. Jahrb. Abth. Anat., r2,
1898.

Willy: Development of ear and accessory organs in frog. Quar. Jour. Micros. Sci., 30,
1890.

ALIMENTARY CANAL.

Teeth.

Beard: Teeth of marsipobranchs. Zool. Jahrb., 3, 1889.

Burckhardt: Gebiss der Sauropsiden. Morph. Arbeiten, 5, 1895.

Cope: Tritubercular molar in human dentition. Jour. Morph., 2, 1888.

Harrison: Development and succession of teeth in Hatteria [Sphenodon]. Quart. Jour.
Micr. Sci., 44, Igor.

Hertwig: Zahnsystem der Amphibien und seine Bedeutung fiir den Genese des Skelettes
der Mundhéhle. Arch. mikr. Anat., 9, 1874.

Kiikenthal: Urspring und Entwicklung der Saugetierzahne. Jena. Zeitsch., 26, 1892.

Laaser: Entw. der Zahnleiste der Selachier. Anat. Anz., 17, 1900.

Leche: Entwicklung des Zahnsystem der Sduger. Morph. Jahrb., 19, 1892.

Oppel: Verdauungsapparat. Ergebnisse, 13, 1903: 14, 1904: 16, 1906.

Se eee of mammalian molars to and from tritubercular type. Am. Nat., 22,
1888.

Osborn: Succession of teeth in mammals. Am. Nat., 27, 1893.

Osborn: Trituberculy. Am. Nat., 31, 1897.

Poulton: Teeth and horny plates of Ornithorhynchus. Quar. Jour. Micr. Sci., 29, 1888.

Rése: Entwicklung der Zahne des Menschen. Arch. mikr. Anat., 38, 1891.

Rése: Zahnleiste und Eischwiele der Sauropsiden. Anat. Anz., 7, 1892.

Rése: Phylogenese des Siugetiergebisses. Biol. Centralblatt., 12, 1892.

Ryder: Mechanical genesis of tooth forms. Proc. Acad. Nat. Sci., Philadelphia, 1878.

Tomes: Manual of Dental Anatomy. Philadelphia, 1898.

de Terra: Vergl. Anatomie menschlichen Gebisses und der Zahne der Vertebraten.
Jena, rozr.

Warren: Teeth of Petromyzon and Myxine. Quar. Jour. Micr. Sci., 45, 1902.

Wilson: Tooth development of Ornithorhynchus. Quar. Jour. Micr. Sci., 51, 1907.

Mouth and Tongue.

Flint: Submaxillary gland. Am. Jour. Anat., 2, 1903.

Gegenbaur: Unterzunge der Saugethiere. Morph. Jahrb., 9, 1884.

Gegenbaur: Phylogenese der Zunge. Morph. Jahrb., 11, 1886; 21, 1894.

Gegenbaur: Gaumenfalten des Menschen. Morph. Jahrb., 4, 1878.

Hammar: Entwicklung der Zunge und Speicheldriisen. Anat. Anz., 19, 1901.

Heidrich: Mund und Schlundkopfhéhle der Vogel und ihre Driisen. Morph. Jahrb., 37,
1907.

Kallius: Entwicklung der Zunge. Anat. Hefte, 16, 1901; 28, 1905.

Kallius: Entwicklung der Zunge. Anat. Hefte, 41, 1910.

Maurer: Blutgefisse im Epithel. Morph. Jahrb., 25, 1887.

Oeder: Munddriisen und Zahnleiste der Anuren. Jena. Zeitsch., 41, 1906.

Pawlowsky: Giftdriisen einiger Scorpeniden. Zool. Jahrb., Abt. Anat., 31, 1911.

Poulton: Tongue of Perameles. Quar. Jour. Micr. Sci., 23, 1883.

Reichel: Mundhéhidriisen der Wirbeltiere. Morph. Jahrb., 7, 1882.

Wiedersheim: Kopfdriisen der Amphibien. Zeit. wiss. Zool., 28, 1877.

Thyreoid Glands, Etc.

Erdheim: Kiemenderivate bei Ratte, Kaninchen und Igel. Anat. Anz., 29, 1906.

ALIMENTARY CANAL. 373

Greil: Kiemendarmderivate von Ceratodus. Anat. Anz. Erginz. Hefte, 29, 1906.

Ferguson: Thyreoid in Elasmobranchs. Am. Jour. Anat., 11, 1911.

Gudernatsch: Thyreoid of Teleosts. Jour. Morph., 21, 1911.

Hammar: Elasmobranch Thymus. Zool. Jahrb., Abt. Anat., 32, IgII.

Johnstone: Thymus in marsupials. Jour. Linn. Socy., London, Zool., 26, 1898.

Kastschenko: Schicksal d. embryon. Schlundspalten bei Saugetieren. Arch. mikr.
Anat., 30, 1887.

Marcus: Schlundspaltgebiet der Gymnophionen. Arch. mikr. Anat., 71, 1908.

eae Schilddriise, Thymus und Kiemenreste der Amphibien. Morph. Jahrb., 13,
1887.

Norris: Ventraler Kiemenreste and Corpus propericardiale of the frog. Anat. Anz., 21,
1g02.

Platt: oe aaa of Thyroid and suprapericardial bodies in Necturus. Anat. Anz.,
II, 1896.

Rabl: Anlage der ultimobranchialen Kérper bei Vogel. Arch. mikr. Anat., 70, 1907.

Schaffer: Schilddriise von Myxine. Anat. Anz., 28, 1906.

Séderlund und Bachman: Studien tiber Thymusinvolution. Arch. mikr. Anat., 73, 1909.

Stockard: Development of thyreoid in Bdellostoma. Anat. Anz., 29, 1906.

Zuckerkandl: Entwicklung der Schilddriise und Thymus bei der Ratte. Anat. Hefte, 21,

1903.
Digestive Tract.

Boas: Magen der Cameliden. Morph. Jahrb., 16, 1890.

Brachet: Développement du foie et pancreas de ’Ammoccetes. Anat. Anz., 13, 1897.

Bensley: Pancreas of guinea pig. Am. Jour. Anat., 12, 1911.

Braun: Pancreas bei Alytes. Morph. Jahrb., 36, 1906.

Claypole: Enteron of lamprey. Proc. Am. Micros. Socy., 1894.

Choronshitzky: Entstehung der Milz, Leber, Gallenblase, Bauchspeicheldriise und
Pfortadersystem bei verschieden. Wirbeltiere. Anat. Hefte, 13, 1900.

Eggeling: Dunndarmrelief und Ernahrung bei Knochenfischen. Jena. Zeitsch., 43, 1907.

Géppert: Entwicklung des Pancreas bei Knochenfischen. Morph. Jahrb., 20, 1893.

Gadow: Verdauungssystems der Végel. Jena. Zeitsch., 13, 1879.

Helbling: Darm einiger Selachier. Anat. Anz., 22, 1903.

Helly: Pancreasentwicklung der Saugetiere. Arch. mikr. Anat., 67, Igor.

Howes: Intestinal canal of Ichthyopsida. Jour. Linn. Socy. London, Zool., 23, 1890.

Johnston: Limit between ectoderm and entoderm in mouth of amphibia. Am. Jour.
Anat., 10, Igto. :

Jungklaus: Magen der Cetaceen. Jena. Zeitsch., 32, 1898.

Kerr: Development of alimentary tract in Lepidosiren. Quar. Jour. Micr. Sci., 54, 1910.

Killian: Bursa und Tonsilla pharyngea. Morph. Jahrb., 14, 1888.

Kingsbury: Enteron of Necturus. Proc. Am. Micros., 1894.

Lewis and Thyng: Intestinal diverticula in embryos of pig, rabbitand man. Am. Jour.
Anat., 7, 1908.

Mayr: Entwicklung des Pancreas bei Selachier. Anat. Hefte, 8, 1897.

Mayer: Spiraldarm der Selachier. Neap. Mittheil., 12, 1897.

Oppel: Verdauungsapparat. Ergebnisse, 7, 1897.

Osawa: Eingeweiden der Hatteria [Sphenodon]. Arch. mikr. Anat., 49, 1897.

Parker: Spiral valve in Raia. Trans. Zool. Socy. London, 11, 1880.

Piper: Entwicklung von Magen, Duodenum, Schwimmblase, Leber, Pancreas und Milz
bei Amia. Arch. Anat. und Physiol., 1902.

Pohlman: Development of cloaca in human embryos. Am. Jour, Anat., 12, IgII.

Rex: Morphologie der Saugerleber. Morph. Jahrb., 14, 1888.

374 BIBLIOGRAPHY.

Riickert: Entwicklung des Spiraldarmes bei Selachiern. Arch. f. Entwick. mechan.
4, 1896. .

Segeratsrale: Teleostierleber. Anat. Hefte, 41, 1910.

Stieda: Bau und Entwicklung der Bursa Fabricii. Zeit. wiss. Zool., 34, 1880.

Stohr: Entwicklung der Hypochorda und dorsal Pancreas bei Rana. Morph. Jahrb., 23,
18g5.

Teichmann: Kropf der Tauben. Arch. mikr. Anat., 34, 1889.

Thyng: Pancreas in embryos of pig, rabbit, catand man. Am. Jour. Anat., 7, 1908.

Volker: Entwicklung des Pancreas bei den Amnioten. Arch. mikr. Anat., 59, Igor:

RESPIRATORY ORGANS.

General.

Clemenz: Aussere Kiemen der Wirbeltiere. Anat. Hefte, 5, 1904.

ris et der Amphibien und Reptilien. Morph. Jahrb., 22, 1904; 26, 1898;
28, 1899.

Gotte: Ursprung der Lunge. Zool. Jahrb., Anat. Abth., 21, 1905.

Miller: Structure of the lung. Jour. Morph., 8, 1893.

Moser: Entwicklungsgeschichte der -Wirbeltierlunge. Arch. mikr. Anat., 60, 1902.

Oppel: Athmungsapparat. Ergebnisse, 13, 1903; 14, 1904; 16, 1906.

. Schmidt: Kehlhiigel der Amnioten. Morph. Jahrb., 43, rg1r.

Spengel: Schwimmblasen, Lungen und Kiementaschen der Wirbeltiere. Zool. Jahrb.

Suppl., 7, 1904.
Cyclostomes and Fishes.

Babak: Darmathmung der Cobiten. Biol. Centralb., 27, 1907.

Beaufort: Schwimmblase der Malacopterygii. Morph. Jahrb., 39, 1909.

Braus: Embryonal Kiemenapparat von Heptanchus. Anat. Anz., 29, 1906.

aa and Haddon: Air-bladder and Weberian ossicles of Siluride. Phil. Trans., 184,
1893.

Corning: Wundernetzbildes in Schwimmblase der Teleostier. Morph. Jahrb., 14, 1888.

Dahlgren: Breathing valves of teleosts. Zool. Bull., 2, 1898.

Dohrn: ‘Urgeschichte, u.s.w. Spritzlochkieme der Selachier, Opercularkieme d.
Ganoiden, Pseudobranchie der Teleostier. Neapel. Mitth., 7, 1886.

Greil: Homologie der Anamnierkiemen. Anat. Anz., 28, 1906.

Jaeger: Physiologie der Schwimmblase. Biol. Centralbl., 24, 1904.

Kellicott: Develop. vasc. and respiratory systems of Ceratodus. Mem. N. Y. Acad.
Sci., 2, 1904.

Mauer: Pseudobranchien der Knochenfisches. Morph. Jahrb., 9, 1883.

Moroff: Entwicklung der Kiemen der Knochenfischen. Arch. mikr. Anat., 60, 1902.

Moser: Entwicklung der Schwimmblase. Arch. mikr. Anat., 63, 1904.

Muller: Entwicklung und Bedeutung der Pseudobranchie bei Lepidosteus. Arch. mikr.
Anat., 49, 1897.

Nusbaum: Gasdriise in Schwimmblase. Anat. Anz., 31, 1907.

Rand: Functions of spiracle in skate. Am. Nat., 41, 1907. See also Darbyshire, Jour.
Linn. Socy., Zool., 30, 1907.

Stockard: Development of mouth and gills in Bdellostoma. Am. Jour. Anat., 5, 1906.

Thilo: Luftsacke bei Kugelfische. Anat. Anz., 16, 1899.

Wiedersheim: Ein Kehlkopf bei Ganoiden und Dipnoern. Zool. Jahrb. Suppl. 7,
1904.

Zograff: Labyrinthine apparatus of labyrinthine fishes. Quar. Jour. Micr. Sci., 28, 1889.

Amphibia.

Bruner: Smooth facial muscles of anura and salamandrina (respiratory mechanism).
Morph. Jahrb., 29, 1901.

CIRCULATION. 375

e
Greil: Anlage der Lungen und Ultimobranchialkérper. Anat. Hefte, 29, 1905.
Fox: Tympano-Eustachian passage in toad. Proc. Acad. Nat. Sci., Phila., gor.
Martens: Entwicklung der Kehlkopfknorpel bei Anuren. Anat. Hefte, 9, 1897.
Ochsner: Lung of Necturus. Bull. Univ. Wisconsin, 33, 1900.

Seelyee: Circulatory and respiratory systems of Desmognathus. Proc. Boston Socy.,
Nat. Hist., 32, 1906.

Whipple: Ypsiloid apparatus of Urodeles. Biol. Bull., 10, 1906.

Wilder: Phylogenesis of larynx. Anat. Anz., 7, 1892.

Whipple: Naso-labial groove of salamanders. Biol. Bull., rz, 1906.

Wilder: Amphibian larynx. Zool. Jahrb. Abth. Anat., 9, 1896.

Wilder: Lungless salamanders. Anat. Anz., 9, 1894; 12, 1896.

Wilder: Pharyngeo-cesophageal lung of Desmognathus. Am. Nat., 35, 1901.

Sauropsida.

Cope: Lungs of ophidia. Proc. Am. Phil. Socy., 33, 1904.
Gage: Aquatic respiration in soft-shelled turtles. Am. Nat., 20, 1886.

Hacker: Unter Kehlkopf der Singvogel. Anat. Anz., 14, 1898.

Heidrich: Mund-Schlundhéhle der Vogel. Morph. Jahrb., 37, 1907.
Huxley: Respiratory organs of Apteryx. Proc. Zool. Socy. London, 1882.
Milani: Reptilienlungen. Zool. Jahrb. Abth. Anat., 8, 1894; 10, 1897.
Miller: Air-sacs of pigeon. Smithsonian Misc. Coll., 50, 1907.

Sappey: Recherches sur l’apparaeil respiratoire des oiseaux. Paris, 1847.
Strasser: Luftsacke der Vézel. Morph. Jahrb., 3, 1877.

Mammals.

Bremer: Lungs of opossum. Am. Jour. Anat., 3, 1904.

Dubois: Morphologie des Larynx. Anat. Anz., 1, 1886.

Fox: Pharyngeal pouches and their derivatives. Am. Jour. Anat., 8, 1908.

G6éppert: Herkunft des Wrisberg’schen Knorpels. Morph. Jahrb., 21, 1894.

His: Bildungsgeschichte der Lungen bei mensch. Embryonen. Arch. Anat. und Phys.,
1887. *

Justesen: Entwicklung und Verzweigung des Bronchialbaumes der Saugetierlunge. Arch.

‘mikr. Anat., 56, 1g00.

Mall: Branchial clefts and thymus of dog. Johns Hopkins Studies Biol. Lab., 4, 1888.

Shaeffer: Sinus maxillaris in Man. Am. Jour. Anat., 10, 1gro0.

Schaeffer: Lateral walls of cavum nasi in man. Jour. Morph., 21, 1gto.

Symington: The marsupial larynx. Jour. Anat. and Physiol., 33, 1898; 35, 1899.

CIRCULATION.
General.
Allis: Pseudobranchial and carotid arteries in gnathostomes. Zool. Jahrb., Abth. Anat., 27,
1908. om

Ayers: Morphology of the carotids. Bull. Mus. Comp. Zool., 17, 1889.

Boas: Arterienbogen der Wirbelthiere. Morph. Jahrb., 13, 1887.

Broman: Entwicklung, ‘Wanderung’ und Variation der Bauchaortenzweige bei Wirbel-
tieren. Ergebnisse, 16, 1906. :

Greil: Anatomie und Entwicklung des Herzens und Truncus arteriosus der Wirbelthiere.
Morph. Jahrb., 31, 1903.

Greil: Entwicklung des Truncus arteriosus der Anamnier. Verhandl. Anat. Gesellsch.,
17, 1903.

Grosser: Kopfvenensystem der Wirbeltiere. Verh. Anat. Gesellsch., 21, 1907.

Hochstetter: Vergl. Anat. und Entwicklung des Venensystem der Amphibien und Fische,
Morph. Jahrb.. 13, 1888.

376 BIBLIOGRAPHY.

Hochstetter: Entwicklungsgeschichte des Gefasssystem. Ergebnisse, 1, 1892.
Howell: Life history of the formed elements of the blood. Jour. Morph., 4, 1890.
Lewis: Development of the vena cava inferior. Am. Jour. Anat., 1, 1902.
Lewis: Sinusoids. Anat. Anz., 25, 1904.

Rése: Vergl. Anat. des Herzens der Wirbeltiere. Morph. Jahrb., 16, 18g.
Weidenreich: Die roten Blutkérperchen. Ergebnisse, 13, 1903.

Weidenreich: Morphologie der Blutzellen. Anat. Record, 4, 1910.

Wright: Histogenesis of blood platelets. Jour. Morph., 21, 1910.

Weidenreich: Blut und Blutbildenden und -zerstérenden Organe. Arch. mikr. Anat.
65-72, 1904-8.

Fishes.

Allen: Blood-vascular system of Loricati. Proc. Washington Acad. Sci., 7, 1905.

Allis: Pseudobranchial and carotid arteries in Polypterus and Amiurus. Anat. Anz., 33,
1908; in Esox, Salmo, Gadus and Amia, 1. u., 41, 1912.

Allen: Subcutaneous vessels in head of Polyodon and Lepidosteus. Proc. Washington
Acad. Sci., 9, 1907.

Carazzi: Sistema arteriosa di Squalidi. Anat. Anz., 36, 1905.

Danforth: Heart and arteries of Polyodon. Jour. Morph., 23, 1912.

Hoffmann: Entwicklung des Herzens und Blutgefasse bei Selachiern. Morph. Jahrb.,
19, 1893. Venensystem, idem, 20, 1893.

Holbrook: Origin of endocardium in bony fishes. Bull. Mus. Comp. Zool., 25, 1894.

Jackson: Vascular system of Bdellostoma. Jour. Cincinnati Socy. Nat. Hist., 20, 1901.

Allen: Subcutaneous vessels in tail of Lepidosteus. Am. Jour. Anat., 8, 1908.

Kellicott: Development of vascular and respiratory systems of Ceratodus. Mem. N. Y.
Acad. Sci., 2, 1905.

Mayer: Entwicklung des Herzens u. d. grossen Gefassstamme bei Selachier. Mittheil.
zool. Sta. Neapel, 7, 1887; see also 8, 1888.

Parker and Davis: Blood-vessels of heart of Carcharias, Raia and Amia. Proc. Boston
Socy. Nat. Hist., 29, 1899.

Parker: Blood-vessels of heart of Orthagoriscus. Anat. Anz., 17, 1900.

Rand: Posterior connections of lateral vein in skates. Am. Nat., 39, 1905.

Rex: Hirnvenen der Elasmobranchier. Morph. Jahrb., 17, 1891.

Senior: Conus arteriosus in Tarpon and Megalops. Biol. Bull., 12, 1907.

Senior: Development of heart in shad. Am. Jour. Anat., 9, 1909.

Silvester: Blood-vascular system of Lopholatilus. Bull. Bureau of Fisheries, 24, 1904.

Sobotta: Entwicklung des Blut, Herzens und grossen Gefassstamme der Salmoniden.
Anat. Hefte, 19, 1902.

Amphibia.
Bethge: Blutgefasssystem von Salamandra, Triton und Spelerpes. Zeit. wiss. Zool., 63,
1898.
Bruner: Heart of lungless salamanders. Jour. Morph., 16, 1900.
Huxley: Skull and heart of Menobranchus [Necturus]. Proc. Zool. Socy. London, 1874.
Hopkins: Heart of lungless salamanders. Am. Nat., 30, 1896.

Marshall and Bles: Development of blood-vessels in frog. Studies Biol. Lab. Owens’
College, 2, 1890.

Maurer: Kiemen und ihre Gefasse bei Amphibien. Morph. Jahrb., 14, 1888.
Miller: Blood- and lymph-vessels of lung of Necturus. Am. Jour. Anat. +3 4, 1905.

Parker: Persistence of left postcardinal vein in frog; homologies of veins in Dipnoi. Proc.
Zool. Socy. London, 1889.

Rabl: Bildung des Herzens der Amphibien. Morph. Jahrb., 12, 1887.

Rex: Hirnvenen der Amphibien. Morph. Jahrb., 19, 1892.

CIRCULALION. 279

Romeiser: Abnormal venous system in Necturus. Am. Nat., 39, 1906.
Santhoff and van Vorhis: Vascular system of Necturus. Bull. Univ. Wisc., 33,1900.
Seelye: Circulatory and respiratory systems of Desmognathus. Proc. Boston Socy. Nat.
Hist., 32, 1906
Sauropsida.

Bruner: Cephalic veins and sinuses of reptiles. Am. Jour. Anat., 7, 1907.
Davenport: Carotids and Botall’s duct of alligator. Bull. Mus. Comp. Zool., 24, 1893.
Evans: Earliest blood-vessels in anterior limbs of birds. Am. Jour. Anat., 9, 1909.

Grosser and Brezina: Entwicklung der Venen des Kopfes und Halses der Reptilien.
Morph. Jahrb., 23, 189s.

Hochstetter: Entwicklungsgeschichte des Venensystems der Amnioten. Reptilien. Morph.
Jahrb., 19, 1892.
Hochstetter: Arterien des Darmcanals der Saurier. Morph. Jahrb., 16, 1898.

aan Development of branchial arches in birds. Trans. Roy. Socy. London, 179,
1888.

Miller: Development of postcava in birds. Am. Jour. Anat., 2, 1903.
Strémsten: Anat. and develop. venous system of Chelonia. Am. Jour. Anat., 4, 1905.

Mammals.

Beddard: Azygos veins in mammals. Proc. Zool. Socy. London, 1907.

Born: Entwicklungsgeschichte des Sdugetierherzens. Arch. mikr. Anat., 33, 1889.

Davis: Chief veins in early pig embryos. Am. Jour. Anat., 10, Igro.

Dexter: Vitelline veins of cat. Am. Jour. Anat., 1, 1902.

Géppert: Entwicklung von Varietiten im Arteriensystem der weissen Maus. Morph.
Jahrb., 40, 1909.

Hochstetter: Entwicklungsgeschichte des Venensystems der Amnioten. Mammalia.
Morph. Jahrb., 20, 1893.

Hochstetter: Venensystem der Edentaten. Morph. Jahrb., 25, 1897.

Lewis: Development of vena cava inferior. Am. Jour. Anat., 1, 1902.

Lewis: Development of veins in limbs of rabbit. Am. Jour. Anat., 5, 1905.

McClure: Abnormalities in postcava of cat. Am. Nat., 34, 1900.

McClure: Anatomy and development of venous system of Didelphys. Am. Jour. Anat.,
2, 1903; 5, 1905. :

Minot: Veins of Wolffian body of pig. Proc. Boston Socy. Nat. Hist., 28, 1898.

Parker and Tozier: Thoracic derivatives of postcardinals in swine. Bull. Mus. Comp.
Zool., 31, 1898.

Reagan: Fifth aortic arch in mammals. Am. Jour. Anat., 12, 1912.

Rose: Entwicklung des Sdugetierherzens. Morph. Jahrb., 15, 1890.

Salzer: Entwicklung der Kopfvenen des Meerschweinnchens. Morph. Jahrb., 23, 1895

Sicher: Entwicklung der Kopfarterien von Talpa. Morph, Jahrb., 44, 1912

Tandler: Anatomie der Kopfarterien bei Mammalia. Anat. Hefte, 18, 1901.

Lymphatics.

Allen: Lymphatics of Scorpznichthys. Proc. Washington Acad. Sci., 8, 1906. Am. Jour.
Anat., I1, IgIr.

Allen: Subcutaneous vessels in tail of Lepidosteus. Anat. Record, 3, 1908.

Baetjer: Mesenteric lymph sac in pig. Anat. Record, 2, 1908.

Budge: Lymphherzen bei Hihnerembryonen, Arch. Anat. u. Physiol., 1887.

Helly: Hamolymphdriisen. Ergebnisse, 12, 1902.

Hopkins: Lymphatics and enteric epitheilum of Amia. Wilder Quarter Century Book,
1893.

Hoyer und Udziela: Lymphgefisssystem von Salamanderlarven. Morph. Jahrb
44, 1912.

378 BIBLIOGRAPHY.

Huntington: Anatomy and development of systematic lymphatic vessels of cat. Memoirs
Wistar Inst., 1, 1911.

Huntington and McClure: Numerous papers on lymph system of mammals in Am. Jour.
Anat. and Anat. Record.

Killian: Bursa und tonsilla pharyngea. Morph. Jahrb., 14, 1888.

Knower: Development of lymph hearts and lymph sacs in frog. Anat. Record, 2,
rgo8.

Lewis: Development of lymphatics in rabbit. Am. Jour. Anat., 5, 1905.

McClure: Development of lymphatics in cat. Anat. Anz., 32, 1908.

Marcus: Intersegmentale Lymphherzen der Gymnophionen. Morph. Jahrb., 38, 1908.

Maurer: Anlage der Milz und lymphat. Zellen bei Amphibien. Morph. Jahrb., 16, 1890.

Meyer: Hemolymph glands of sheep. Anat. Record, 2, 1908.

Miller: Development of jugular lymph sac of birds. Am. Jour. Anat., 12, 1912.

Miiller: Lymphherzenz Chelonier. Abhandl. Berlin Acad., 1839.

Sabin: Origin of lymphatic system in pig. Am. Jour. Anat., 1, 1902; 3, 1904; 4, I905.

Sabin: Lymphatic system in human embryos. Am. Jour. Anat., 9, 1909.

Sabin: Recent articles on development of lymph system. Anat. Record, 5, 1911
(Bibliography.) :

Stéhr: Lymphknoten des Darmes. Arch. mikr. Anat., 33, 1889.

Stéhr: Entwicklung von Darmlymphknétchen, u. s. w. Arch. mikr. Anat., 51, 1898.

Tonkoff: Entwicklung der Milz bei Végeln. Anat. Anz., 16, 1899.

Tonkoff: Entwicklung der Milz bei Amnioten. Arch. mikr. Anat., 56, 1900.

Tonkoff: Entwicklung der Milz bei Tropidonotus. Anat. Anz., 23, 1903.

Weliky: Vielzahlige Lymphherzens bei Salamandra. Zool. Anz., 7, 1884.

UROGENITAL ORGANS.
General.

Bardeleben: Spermatogenese bei Menschen. Jena. Zeitsch., 31, 1898.

Born: Entwicklung der Geschlechtsdriise. Ergebnisse, 4, 1895.

Disselhorst: Harnleiter der Wirbeltiere. Anat. Hefte, 4, 1894.

Felix: Entwicklungsgeschichte des Excretionsystemes, von Riickert (1888) bis 1904
Ergebnisse, 13, 1903.

Fiirbringer: Excretionsorgane der Vertebraten. Morph. Jahrb., 4, 1878.

Gerhardt: Kopulationsorgans der Wirbeltiere. Ergebnisse und Fortschritt der Zoologie,
I, 1909.

Hoffmann: Entwicklung der Urogenitalorgane bei Anamnia. Zeits. wiss. Zool., 44,
1886.

Montgomery: Morphology of excretory organs of metazoa. Proc. Am. Philos. Socy.,
47; 1908.

Peter: Bau und Entwicklung der Niere. Jena, 1909.

Riickert: Entwicklung der Exkretionsorgane. Ergebnisse, 1, 1892.

Semon: Bauplan der Urogenitalsystem der Wirbeltiere, u. s. w. Jena. Zeitsch., 26,
1891.

Semper: Urogenitalsystem der Plagiostomen. Arbeit. 4. d. zool. zoot. Inst. Wiirz-
burg, 2 1875. °

Taussig: Development of the hymen. Am. Jour. Anat., 8, 1908.

Wijhe: Mesodermsegmente des Rumpfes und Entwicklung des Exkretionsystemes.
Arch, mikr. Anat., 33, 1889.

Cyclostomes and Fishes.

Allen: Origin of sex-cells of Amia and Lepidosteus. Jour. Morph., 22, ry11.
Dodds: Segregation of germ-cells of Lophius. Jour. Morph., 21, 1910.

Emery: Kopfniere der Teeostier. Biol. Centralb., 1, 1881-2.

UROGENITAL ORGANS. 379

Balfour and Parker: Lepidosteus. Phil. Trans., 1882.
Haller: Phylogenese des Nierenorganes der Knochenfische. Jena. Zeitsch., 43, 1908.

Kerr: Male genito-urinary organs of Lepidosiren and Protopterus. Proc. Zool. Socy.
London, igor.

Krall: Mannliche Beckenflosse von Hexanchus. Morph. Jahrb., 37, 1908.

a, Entwicklung der Vorniere und Urniere bei Myxine. Zool. Jahrb., Abth. Anat., 10,
1897.

Miller: Urogenitalsystem des Amphioxus und der Cyclostomen. Jena. Zeitsch., 9, 1875.

Price: Development of excretory organs of Bdellostoma. Am. Jour. Anat., 4, 1904.

Rabl: Entwicklung des Urogenitalsystems der Selachier. Morph. Jahrb., 24, 1896.

eae Entstehung der Exkretionsorgane bei Selachiern. Arch. Anat. u. Physiol.,
1888.

Schreiner: Generationsorgane von Myxine. Biol. Centralbl., 24, 1904.
Wheeler: Development of urogenital organs of Lamprey. Zool. Jahrb. Abth. Anat., 13,
1899.

Wijhe: Entwicklung des Exkretionssystemes und andere Organe bei Selachiern. Anat.
Anz., 2, 1888.

Woods: Origin and migration of germ-cells in Acanthias. Am. Jour. Anat., 1, 1902.
Amphibia.
Field: Development of pronephros and segmental duct in amphibia. Bull. Mus. Comp.
Zool., 21, 1891.

Field: Morphologie der Harnblase bei Amphibien. Morph. Arbeiten, 4, 1894.

Hall: Development of mesonephros and Miillerian ducts in amphibia. Bull. Mus. Comp.
Zool., 45, 1904.

King: Bidder’s organ in Bufo. Jour. Morph., 19, 1908.

King: Anomalies in genital organs of Bufo. Am. Jour. Anat., 10, 1910.

Marshall and Bles: Development of kidneys and fat bodies in frog. Studies Biol. Lab.
Owens Coll., 2, 1890.

Mollendorf: Entwicklung der Darmarterien und Vornieren Glomerulus bei Bombinator.
Morph. Jahrb., 43, 1911.

pe Bauplan der Urogenitalsystems, dargelegt an Ichthyophis. Jena. Zeitschr., 26,
1891.

Spengel: Urogenitalsystem der Amphibien. Arbeit. zool. zoot. Inst. Wiirzburg, 3, 1836.

Sauropsida.

Boas: Begattungsorgane der Amnioten. Morph. Jahrb., 17, 1891.

Cope: Hemipenes of the sauria. Proc. Acad. Nat. Sci., Philadelphia, 1896.

Fleck: Entwicklung des Urogenitalsystem beim Gecko. Anat. Hefte, 41, 1910.

Gasser: Entstehung der Kloacaléffnung der Hiihnerembryonen. Arch. Anat. und Physiol.,
1880.

Gregory: Development of excretory system in turtles. Zool. Jahrb., Abth. Anat., 13,
Igo0.

Hoffmann: Entwicklung der Urogenitalorgane bei Reptilien. Zeit. wiss. Zool., 48, 1889.

Rabl: Entwicklung der Vorniere bei Vogel. Arch. mikr. Anat., 72, 1908. '

Schreiner: Entwicklung der Amniotenniere. Zeitsch. wiss. Zool., 71, 1902.

Wiedersheim: Entwicklung des Urogenitalapparates bei Krokodilien und Schildkréten.
Arch. mikr. Anat., 36, 1890.

Mammals.

Beiling: Anatomie der Vagina und Uterus der Saugetiere. Arch. mikr. Anat., 67, 1906.
Broek: Urgenital-apparates der Beutler. Morph. Jahrb., 41, 1910.
Boas: Begattungsorgane der Amnioten. Morph. Jahrb., 17, 1891.

380 BIBLIOGRAPHY.

Bremer: Morphology of tubules of testis and epididymis. Am. Jour. Anat., 11, rgrz.

Broek: Mannlichen Geschlechtsorgane der Beuteltiere. Morph. Jahrb., 41, 1910.

Cole: Intromittent sac of male guinea pig. Jour. Anat. and Physiol., 32, 1897.

Courant: Preputialdriise des Kaninchens. Arch. mikr. Anat., 62, 1903.

Daudt: Urogenitalapparates der Cetaceen. Jena. Zeitschr., 32, 1898.

Gerhardt: Entwicklung der bleibenden Niere. Arch. mikr. Anat., 57, Igor.

Gudernatsch: Hermaphroditismus verus in man. Am. Jour. Anat. 11, 1911.

Gilbert: Os Priapi der Sduger. Morph. Jahrb., 8, 1892.

Gerhardt: Kopulationsorgane der Sdugetiere. Jena. Zeitsch., 39, 1904.

Kaudern: Mannl. Geschlectsorgane von Insectivoren und Lemuriden. Zool. Jahrb.,
Abt. Anat., 31, IgIo.

Keibel: Entwicklung der Harnblase. Anat. Anz., 6, 1891.

Keibel: Entwicklung des menschlichen Urogenitalapparates. Arch. Anat. und Physiol.
Anat. Abth., 1896.

Klaatsch. Descensus testiculorum. Morph. Jahrb., 16, 1890.

MacCallum. Wolffian body of higher animals. Am. Jour. Anat., 1, 1892.

Montgomery: Human cells of Sertoli. Biol. Bulletin, 21, 1911.

Miiller: Prostate der Haussdugetiere. Anat. Hefte, 26, 1904.

Poulton: Structures connected with ovarial ovum of marsupials and monotremes. Quart.
Jour. Micros. Sci., 24, 1884.

Robinson: Position and peritoneal relations of mammalian ovum. Jour. Anat. and
Physiol., 1887.

Schreiner: Entwicklung der Amniotenniere. Zeitsch. wiss. Zool., 71, 1902.

Sobotta: Entstehung des Corpus luteum. Ergebnisse, 8, 1898; 11, 1901.

Weber: Entwicklung des uropoetisches Apparats. bei Sdugern. Morph. Arbeiten, 7, 1897.

SUPRARENALS.

Aichel: Entwicklungsgeschichte und Stammesgeschichte der Nebennieren. Arch. mikr.
Anat., 56, 1900.

Collinge and Vincent: So-called suprarenals in cyclostomes. Anat. Anz., 12, 18096.

Flint: The Adrenal. Johns Hopkins Hospital Report, 1900.

Kohn: Nebennieren der Selachier. Arch. mikr. Anat., 53, 1898.

Kunz: Develop. adrenals in turtle. Am. Jour. Anat., 13, 1912.

Srdinko: Nebennieren bei Anuren. Anat. Anz., 18, I900.

Srdinko: Nebennieren der Knochenfischen. Arch. mikr. Anat., 71, 1908.

Vincent: Discussion of suprarenals. Jour. Anat. and Phys., 38, 1903.

Weldon: Suprarenals of vertebrates. Quar. Jour. Micr. Sci., 24, 1884; 25, 1885.

FQ:TAL ENVELOPES, PLACENTA, ETC.

Corning: Erste Anlage der Allantois bei Reptilien. Morph. Jahrb., 23, 1895.
Hill: Placentation of Perameles. Quar. Jour. Micr. Sci., 40, 1898.

Hubrecht: Placentation of Erinaceus. Quar. Jour. Micr. Sci., 30, 1889. Of Sorex.
idem, 35, 1893. Spolia Nemoris (lemurs and edentates), idem, 36, 1904.

Minot: Uterus and embryo. Jour. Morph., 2, 1889.
Minot: Theory of the structure of the placenta. Anat. Anz., 6, 1891.
Osborn: Foetal membranes of marsupials. Jour. Morph., 1, 1887.

Robinson: Segmentation cavity, archenteron, germ layers and amnion of mammals.
Quar. Jour. Micr. Sci., 33, 1892.

Turner: Lectures on the anatomy of the placenta. Edinburgh, 1876.

Van Beneden et Julin: Formation des annexes fcetales chez les Mammiferes. Archives
de Biol., 5, 1884.

DEFINITIONS OF SYSTEMATIC NAMES.

Acanthias, genus of sharks including com-
mon dogfish,

Acipenser, genus of ganoids; sturgeon.

Aglossa, tongueless toads from Africa and
South America.

Allantoidea, the higher vertebrates with
allantois; reptiles, birds and mammals.
Amblystoma, genus of tailed amphibians,

largely American.

Amia, genus of ganoid fishes peculiar to
America.

Ammoceetes, the larval stage of the lam-
preys.

Amniotes, division of vertebrates with
amnion and allantois in development;
reptiles, birds and mammal.

Amphibia, class of vertebrates, young with
gills, adults with lungs; frogs, toads and
salamanders.

Amphioxus, fish-like form without verte-
bre, type of group of Leptocardii.

Amphipnous, eel-like fishes from India.

Amphisbeenans, legless lizards.

Amphiuma, genus of tailed amphibians
with rudimentary legs and gill slits;
southern U. S.

Anallantoidea, vertebrates
allantois; ichthyopsida.

Anamunia, vertebrates without an amnion;
ichthyopsida.

Anguis, genus of footless lizards.

Anser, genus of birds including geese.

Anthropoids; sub order of primates includ-
ing the higher apes and man.

Anura, order containing the tailless amphib-
ians; frogs and toads.

Aquila, genus of birds including eagles.

Archzopteryx, a fossil bird with teeth and
a reptilian tail

Archegosaurus, genus of extinct stego-
cephal amphibians

Arcifera, group including toads and tree
toads.

Arthrodira, order of extinct dipnoi (lung-
fishes) some very large.

Artiodactyla, ungulate mammals with even
number of toes; cattle, sheep, deer.

Astroscopus, genus of electric fishes;
marine.

Atelodus, genus of two-toed rhinoceros.

Aves, the class of birds.

without an

Bdellostoma, genus of myxinoids; hag
fishes of the Pacific.

381

Belone, genus of fishes; bony gars.

Bombinator, genus of European toads,
unke,

Bradypus, genus of edentate sloths,

Branchiosaurus, genus of extinct stego-
cephal amphibia.

Bufo, genus of amphibians, toads.

Buteo, genus of raptorial birds, hawks,

Butyrinus, genus of herring-like fishes.

Cacilians, a group of legless tropical am-
phibians,

Caiman, genus of crocodiles,

Calamoichthys, genus of ganoid fishes from
Africa,

Callopterus, genus of extinct ganoid fishes.

Camptosaurus, genus of extinct dinosaur
reptiles.

Capitosaurus, genus of extinct stegocepha-
lous amphibia.

Carcharias, genus of sharks; sand shark.

Carinate, birds with a keel to the sternum,
includes all living birds except ostriches.

Carnivores, order of flesh-eating mammals;
cats, dogs, bears, weasels, seals.

Castor, genus of rodents, beaver.

Ceratodus, genus of dipnoi (lung-fishes)
from Australia.

Ceratophrys, genus of So. American toads.

Cervus, genus of Ungulates, common deer.

Cestracion, genus of sharks from the
Pacific.

Cetacea, order of mammals, whales.

Chauna, genus of So. America crane-like
birds; hooded screamers.

Chelonia, order of reptile turtles.

Chelone, genus of turtles, greec turtle.

Chelydra, genus of turtles, snapping turtle.

Chelydrosaurus, genus of extinct stego-
cephalous amphibia.

Chimera, genus of peculiar deep-water
sharks.

Chimeroids, order of shark-like fishes;
Holocephaili.

Chiroptera, order of mammal bats.

Chlamydoselache, genus of primitive deep-
sea sharks from Japan.

Cholcepus, genus of edentates, sloths.

Chondrostei, order of ganoid fishes, stur-
geon.

Chrysophrys, genus of fishes; sea bream of
Europe.

Chrysothrix, a genus of So. American
monkeys.

382

Cistudo, genus of chelonia; box turtles.

Cladoselache, genus of extinct sharks.

Clupeide, family of fishes including herring,
shad, alewives and menhaden.

Cobitis, genus of fishes; loaches.

Coregonus, genus of fresh-water fishes;
white fish.

Crocodilia, order of reptiles including the
alligator.

Crotalus, genus of snakes, rattlesnakes.

Cryptobranchus, genus of tailed amphibians
with permanent gill slits; hellbender of
No. America.

Cyclostomes; class of vertebrates without
jaws, including lampreys and hag fishes.

Cynognathus, genus of extinct theromorph
reptiles.

Cyprinids, family of freshwater fishes,
carp, minnows.

Delphinus, genus of whales; dolphins.

Derotremes, tailed amphibia with perma-
nent gill slits.

Desmognathus, genus of salamanders.

Didelphys, genus of marsupials, opossums.

Diemyctylus, genus of small spotted sala-
manders,

Dinosaurs, extinct terrestrial reptiles, some
of enormous size.

Dipnoi, sub-class of fishes with gills and
lungs, lung-fishes.
Discosaurus, genus

amphibians.
Dromatherium, genus of extinct, primitive
mammals.

of stegocephalous

Echidna, genus of monotremes, spiny ant-
eaters of Australia.

Edentates, order of mammals including
sloths, armadillos, etc.

Elasmobranchs, a sub-class of vertebrates
including the sharks and skates.

Embiotocids, family of fishes from the
Pacific which bear living young; surf
perches.

Epicrium, genus of cecilians.

Erinaceus, genus of insectivorous mammals;
hedgehogs.

Erythrinus, genus of tropical fishes.

Euornithes, a name given to all recent birds.

Eupomatus, fresh-water sunfish.

Eurycormus, genus of fossil ganoid fishes.

Firmisternia, anurous amphibia with the
halves of the sternum united to each
other; frogs.

Fulica, genus of water bird; coots,

Galeocerdo,
sharks,
Galeopithecus, a flying mammal from Asia

of uncertain position.
Galeus, genus of sharks; dogfish.

genus of selachians; tiger

DEFINITIONS OF SYSTEMATIC

NAMES.

Gallus, genus of birds including the com-
mon fowl.

Gambusia, genus of fishes; top-minnow.

Ganoids, subclass of fishes intermediate
between sharks and bony fishes; stur-
geon, garpike, etc.

Geococcyx, a genus of cuckoos.

Geotrition, a genus of European salaman-
ders.

Gerrhonotus, genus of lizards.

Glyptodon, genus of edentates allied to
armadillos.

Gnathostomes, vertebrates which have jaws;
includes all except cyclostomes.

Gobiids, family of small fishes, mostly
marine; gobies.

Gymnophiona, order of amphibia without
tail or legs; tropical; cecilians.

Gymnotus, electric eel of So. America.

Halmaturus, genus of kangaroos.

Hatteria, another name for Sphenodon.

Heloderma, poisonous lizard from Arizona;
Gila monster.

Heptanchus, primitive shark with seven
gill slits.

Hexanchus, primitive shark with six gill
slits.

Holocephali, order of shark-like fishes;
Chimera.

Hypogeophis, genus of Cecilians.

Hyracoidea, order of mammals including
Hyrax. -

Ichthyophis, genus of cacilians from Ceylon.

Ichthyopsida, group of vertebrates which
have gills; fishes, amphibia.

Ichthyosaurs, extinct aquatic reptiles.

Iguana, genus of tropical American lizards.

Insectivores, order of small mammals;
moles, shrews, etc.

Inuus, genus of macaques including the
Barbary ape.

Lacerta, genus including the common
lizards of Europe.

Lacertilia, sub-order of reptiles including
all lizards.

Lagenorhynchus, a genus of dolphins.

Lepidosiren, genus of lung fishes (dipnoi)
from South America,

Lepidosteus, genus of ganoid fishes, gar-
pike.

Lopholatilus, genus of teleosts from Gulf
Stream; tile fish.

Macropus, genus of marsupials; kangaroos.

Mammals, class of vertebrates, with hair,
nourishing the young with milk.

Manatus, genus of sirenians, manatees.

Manis, genus of old-world edentates; scaly
ant-eaters.

DEFINITIONS OF SYSTEMATIC NAMES.

Marsupialia, subclass of mammals with
pouch for young, opossums, kangaroos,
etc.

Megalops, genus of fishes including the tar
pon. :

Melanerpeton, genus of extinct stegocephal
amphibians.

Monodelphia, subclass of mammals, in-
cluding all except monotremes and
marsupials.

Monotremata, subclass of mammals with
cloaca; includes duckbill and Echidna
of Australia. ;

Morones, genus of catfishes.

Mugil, genus of fishes, mullets.

Mustelus, genus of small sharks; dogfish.

Myrmecobius, genus of Australian mar-
supials.

Myxine, genus of cyclostomes; hag fishes.

sa a “group of Cylostomes; hag

shes.

Necturus, genus of aquatic amphibians
with tail and external gills, central U. S.

Dotidanids, sub-order of sharks with more
than five gill clefts.

Nototrema, genus of South American toads
with dorsal brood sac.

Ophidia, sub-order of reptiles; snakes.

Opisthocomus, South American bird, type
of a separate sub-order.

Opisthodelphys, genus of tropical American
tree-toads. :

Ornithorhynchus, genus of monotremes;
duckbill of Australia,

Ostariophysi, bony fishes with Weberian
apparatus,

Ostracoderms, a group of extinct verte-
brates of very uncertain position.

Paleohatteria, a fossil reptile allied to
Sphenodon,

Palzospondylus, a problematical fossil
vertebrate from Scotland.

Perennibranchs, ‘tailed amphibia which
retain the gills through life.

Perissodactyls, sub order of mammals with
odd number of toes; horses, rhinoceros,

tapirs.

Petrobates, genus of extinct theromorph
reptiles.

Petromyzonts, subclass of cyclostomes,
lampreys.

Phoca, genus of carnivores including com-
mon seals,

Physoclisti, fishes in which the air-bladder
is closed.

Physostomi, group of fishes in which the air-
bladder has a duct; mostly fresh water.

Pipa, tongueless toad from South America.

Pisces, the class of fishes.

383

Placentalia, all mammals (except marsupials
and monotremes) in which a placenta
occurs. ;

Placodus, genus of extinct. theriomorph
reptiles

Plesiosaurs, order of extinct, long-necked
swimming reptiles

Polyodon, genus of ganoid fishes, paddle
fish.

Polypterus, genus of ganoids from Africa.

Porichthys, genus of fishes from Pacific;
midshipman.

Primates, highest order of mammals,
including monkeys, apes and man.

Pristiurus, genus of European dogfish.

Proboscidea, order of mammals, including
elephants.

Procolophon, genus of extinct theromorph
reptiles.

Proteus, genus of tailed amphibians from
caves of Austria, allied to Necturus.

Protopterus, genus of dipnoi from Africa.

Psittacus, genus of parrots.

Pterodactyls, extinct flying reptiles.

Pterosaurs, extinct flying reptiles, ptero-
dactyls,

Pythonomorphs, a group of extinct swim-
ming reptiles.

Raia, genus of elasmobranchs, including
the skates.

Rana, genus of amphibia, frogs.

Ratite, birds without keel to sternum,
ostriches.

Rhea, three-toed South American, ostrich.

Rhynchobatus, genus of tropical skates.

Rhynchocephalia, order of lizard-like rep-
tiles; Sphenodon of New Zealand only
living species.

Rodentia, order of mammals with gnawing
teeth, rats, rabbits, beaver.

Ruminants, group of ungulate mammals
which chew the cud.

Salamandra, genus of tailed amphibia from
Europe.

Salamandrina, order of tailed amphibians
without gills.

Salmonids, family of fishes including trout
and salmon.

Sauropsida, class of vertebrates including
reptiles and birds.

Sceleporus, genus of lizards of eastern
United States.

Scomber, genus of fishes; mackerel.

Scorpenichthys, genus of sculpins.

Selachii, order of elasmobranchs; sharks.

Serranide, family of marine, perch-like
fishes.

Siluroids, order of fishes containing the
cat-fishes,

Siren, genus of tailed amphibian from U.S.
with external gills.

384

Sirenia, order of marine mammals;
manatees and dugongs

Sirenoidea; order of lung-fishes, containing
the living species.

Spalacotherium, genus of extinct mammals.

Sphenodon, genus of lizard-like reptiles
from New Zealand; order Rhyncho-
cephalia. Z

Squamata, order of reptiles including snakes

and lizards.

Stegocephala, order of extinct amphibians.

Stegosaurs, family of extinct dinosaur
reptiles, some very large.

Stenops, genus of lemurs.

Stenostomus, genus of fishes; scup.

Teleostomes, fishes with true jaw, includes
ganoids and teleosts.

Teleosts, order of fishes with bony skeleton,
including all common fishes.

Testudo, genus of land turtles.

Testudinata, turtles, same as a Chelonia.

Tetrapoda, term to include amphibia,
reptiles, birds, and mammals, which
have feet in place of fins.

Theromorpha, extinct reptiles forming the
lowest order of the class.

Tinnunculus, genus of hawk-like birds;
kestrel.

DEFINITIONS OF SYSTEMATIC NAMES.

Torpedo, genus of skates with remarkable
electric powers.

Trionyx, genus of fresh-water turtles.

Triton, genus of tailed amphibian, aquatic,
European.

Tropidonotus, genus of snakes, including
our water snake.

Trygon, genus of skates, string-rays.

Typhlopide, family of peculiar’ tropical
serpents,

Ungulates, order of mammals which walk
on the tips of the toes; horse, cattle, deer,
antelope, etc.

Urodeles, order of tailed amphibia.

Varanus, genus of lizards from Africa.

Xenarthra, sub-order of American edentates,
ant-éaters and armadillos.

Xenopus, genus of tongueless toads from
Africa,

Zeuglodon a genus of extinct whales
(Cetacea).
Ziphius, genus of toothed whales.

Abdominal aorta, 284
pores, 124, 322
ribs, 41
sternum, 57
vein, 289
vertebra, 49
Abducens nerve, 170
Abomasum, 227
Accessory nerve, 177
Acetabular bone, 112, 113
Acetabulum, 104, 109
Acinous glands, 18
Acrodont, 88, 213
dentition, 213
Acromion process, 109
Actinotrichia, 103
Activators, 264, 353
Acustico-lateralis nerves, 167
Acustic nerve, 174
Adductor muscles, 132
Adenoid tissue, 307
Adipose tissue, 22
Adrenalin, 353
Adrenal organs, 352
Advehent vein, 291
AEgithognathous, 97
Afferent branchial artery, 274
duct of gills, 239
nerve root, 161
Air-bladder, 247
ducts, 251
sacs, 261
‘Ala orbitalis, 98
temporalis, 61, 98
Alimentary canal, 205
Alisphenoid, 67
cartilage, 61
Allantoic arteries, 278, 285, 293
bladder, 318
veins, 350
Allantois, 264, 278, 350
Alveolar ducts, 256
Alveoli of jaws, 213
of lung, 256
Amnion, 350
Amniotes, copulatory organs, 344
development of heart. 271
Amniotic cavity, 350
Amphibia, brain, 155
circulation, 295
dermal skeleton, 41
excretory organs, 327 .
gills, 242

INDEX.

Amphibia, girdles, 106, 110
glands, 29
intestine, 229
larynx, 251 ;
lateral line organs, 180
limbs, 118
lungless, 258
lungs, 257
reproductive organs, 333
skin, 29
skull, 82
teeth, 212
thymus, 246
thyreoid, 247
tongue, 217
vertebral column, 51
Amphiccelous, 46
Amphiplatyan, 46
Amphiohinal, 191
Amphistylic, 73
Ampulle of ear, 184
of Lorenzini, 182
of Savi, 182
Amylopsin, 234
Anchylosis, 38
Angulare, 71
Anlage, vi.
Antibrachium, 116
Anterior abdominal vein, 289
cardinal vein, 279
cephalic duct, 303
chamber of eye, 203
cornua, 139
process, 74
vena cava, 300
Anthers, ror
Antrum of Highmore, 197
Aorta, 273, 284
Aortic arches, 273
arches, modifications of, 282
Aponeurosis, 129
Appendages, 102, 114
Appendicular skeleton, 102
Apteria, 32
Aqueduct, 143
Aqueous humor, 203
Arachnoid membrane, 152
Arbor vite, 161
Arcades, 11
Archenteron, 9
Archiccele, 8
Archinepteric duct, 312
Archipterygium, 115

385

386

Areolar tissue, 22
Argential layer of eye, 202
Arterial ring, 287
Arteries, 266, 284
afferent branchial, 274
allantoic, 278, 285, 293, 350
axillary, 288
basilar, 287
brachial, 288
branchial, 274
carotid, 275
caudal, 276
central retinal, 201
ciliary, 202
coeliac, 284
common carotid, 282
coronary, 273
cutaneus, 289
development of, 273
efferent branchial, 274
epigastric, 288
external iliac, 288
femoral, 288
gastric, 284
genital, 286
hepatic, 284
hyaloid, 201
hypogastric, 276, 285
iliac, 288
intercostal, 275, 286
ischiadic, 288
lumbar, 286
mandibular, 271
mesenteric, 284
nephridial, 275, 285
omphalomesaraic, 276
omphalomesenteric, 276
ovarian, 286
peroneal, 288
popliteal, 288
pulmonary, 283
radial, 288
renal, 286, 300
sacral, 286
sciatic, 288
somatic, 284
spermatic, 286
spinal, 287
splenic, 284
subclavian, 288
tibial, 288
ulnar, 288
umbilical, 285
vertebral, 287
vesical, 285
visceral, 284
vitelline, 293
Articular bone, 71
Articular process, 46
Articulare, 74
Articulations, 38
Arytenoid cartilage, 251
Ascending aorta, 284

INDEX.

Ascending process, 82
tracts, 140
Asterospondylous, 51
Astragalus, 117
Atlas, 49
Atrial chamber of gills, 240
Atrioventricular canal, 272
Atrium ‘of heart, 272
lungs, 258
of nose, 194
Auditory, bulla, 100
meatus, 187
nerves, 174
organs, 182
vesicle, 183
Auricle of heart, 272
Auricularis superficialis nerve, 171
Autostylic, 73
Axial skeleton, 43
Axillary artery, 288
vein, 290
Axis, 49
Axon, 19,139
Azygos appendages, 103.
Azygos vein, 302

Baleen, 216
Barbs, 31
Barbules, 31
Basalia, 103
Basibranchial, 65
Basilar artery, 287
plate, 60
Basioccipital, 67
Basisphenoid, 67
Basitemporal plate, 96
Bicuspids, 213
Bidder’s organ, 347
Bile, 231
duct, 233
Birds, see also Amniotes, Sauropsida.
air-sacs, 261
brain, 158
circulation, 300
gill pouches, 244
girdles of, 108, 113
intestine, 230
limbs, 119
lungs, 259
scales, 31
skin, 30
skull, 95
stomach, 225
thymus, 246
thyreoid, 247
tongue of, 218
vertebral column of, 52
Biserial fins, 115
Bladder, air, 247
allantoic, 318
swim, 247
urinary, 318
Blastomeres, 8

e

Blastopore, 9
Blastula, 8
Blood, 24, 265
Blood circulation, embryonic, 268
phylogeny of, 267
primitive, 268
Blood corpuscles, 265
-lymph glands, 307
plasma, 265
plates, 266
vascular system, 266
vessels, structure of, 267
Body cavity, 120
Bone, 23
development of, 43
of ear, 73
Botall’s duct, 283
Bowman’s capsule, 309, 314
glands, 197
Brachial artery, 288
plexus, 163
vein, 290
Brachium, 116
Brain, 140
flexures of, 143
sand, 160
ventricles of, 143
Branchiz, 236
Branchial arteries, 274
arches, 63.
clefts, 236
vein, 274
Branchiomerism, 237
Branchiostegal membrane, 77, 240
rays, 77, 240
Breathing valves of teleosts, 241
Breast bone, 56
Broad ligament, 337
Bronchi, 250, 256
Bronchioles, 256
Bronchus of lampreys, 238
Buccal glands, 221
Buccalis nerve, 172
Bulbus arteriosus, 273
oculi, 203
olfactorius, 142, 167
Bunodont, 214
Bursa Entiana, 227
inguinalis, 338
omentalis, 122

Ceca, intestinal, 228
pyloric, 227
Calcaneus, 117

INDEX.

Carapace, 41
Cardiac glands, 224
plexus, 163
Cardinal veins, 279
Carotid arteries, 275
Carotid glands, 246, 297
Carpale, 117 .
Carpus, 116
Cartilage, 22
bones, 43, 66
calcified, 43
lingual, 75
rostral, 76
Cauda equina, rfo
Caudal artery, 276
vein, 276
vertebre, 49
Cavum tympani, 187
Cement, 211
Central canal of nervous system, 138
Centrale, 117
Central nervous system, II, 137
Centrum, 45
Cephalic vein, 290
Ceratobranchial, 65
Cerebellar hemispheres, 161
Cerebellum, 142, 145
Cerebral hemispheres, 141
Cerebrospinal fluid, 152
Cerebrum, 148
Cervical plexus, 163
sinus, 244
vertebrae, 49
Chain ganglia, 163
Chambers of eye, 203
Chiasma, 169
Chiropterygia, 114
Choane, 80, 193
Choledochar duct, 233
Chondrin, 23
Chondrocranium, 60
Chorda tympani, 173
Chorde tendinie, 272
Chordata, «
Chorioid coat, 202
fissure, 199
gland, 202
plexus, 144, 147
Chorion, 351
Chromaffine cells, 352
Chromaphile cells, 352
Chromatophores, 26 ©
Chyle, 304
Chyle ducts, 304

387

388

Circulation, hepatic-portal, 277
portal, 277
pulmonary, 282
renal-portal, 280
respiratory, 282
systemic, 282

Circulatory organs, 264

Cistern of chyle, 303

Claspers, 27, 116, 343

Clavicles, 106

Claws, 27

Cleft palate, 193

Cleithrum, 106

Cloaca, 228

Coccyx, 52

Cochlea, 186

Cochlear nerve, 174, 186

Cceliac artery, 284
axis, 285

Ceelom, 10, 14, 120

Collecting tubule, 309

Collector nerves, 163

Colon, 228

Columella auris, 74

Columnz carnea, 272

Columnar epithelium, 17

Columns of cord, 139

Commissura mollis, 146

Common carotid artery, 282
iliac vein, 289

Concha of ear, 188
of nose, 194

Cones of eye, 199

Conjunctiva, 203

Connective tissues, 21

Contour feathers, 31

Conus arteriosus, 272

Convoluted tubule, 309

Convolutions of brain, 149, 160

Copulz, 63

Copulatory organs, 342

Coraco-arcual muscles, 133

Coracoid bone, 107
process, 108
region, 105

Corium, 25

Cornea, 202

Cornua of cord, 139
trabeculz, 61

Cornua radiata, 160

Coronary arteries, 273

Coronoid bone, 71

Corpora adiposa, 307
bigemina, 142
quadrigemina, 142, 160

Corpus albicans, 151
callosum, 150
luteum, 320
restiforme, 150
striatum, 141

Corpusculum bulboideum, 179

Cortex of cerebrum, 149

Corti’s organ,. 186

INDEX.

Cotyloid bone, 113
Cowper’s glands, 342
Cranial bones, table of, 72
nerves, 165
Cranio-quadrate process, 82
Cranium, 60
Cremaster muscle, 338
Cribriform plate, 67, 100
Cricoid cartilage, 251
Criste acustice, 185
Crista galli, 100
Crop, 223
Crura cerebri, 151
Crus, 116
Ctenoid scales, 40
Cubical epithelium, 17
Cuboides, 117
Cuneiform, 117
Cutaneus artery, 289
magnus vein, 290
Cutis, 25
Cuverian ducts, 271, 278
Cycloid scales, 40
Cyclospondylous, 51
Cyclostomes, brain, 152
circulation, 294
ear, 185
excretory organs, 321
eyes, 204
gills, 238
intestine, 228
lateral line organs, 180
mouth, 208
nasal organs, 190
reproductive organs, 331
skull, 75
teeth, 215
thymus, 245
thyreoid, 246
tongue, 217
vertebral column, 51
Cylindrical corpusle, 179
Cystic duct, 234

Decussation of fibres, 150
Deiter’s cells, 186
Demibranch, 237
Dendtites, 19, 139
Dens, 50
Dental formula, 214
papilla, 209
ridge, 210
shelf, 210
Dentary bone, 71
Dentinal canals, 24
Dentine, 24, 209
Dentitions, 211
Depressor mandibule, 133
muscles, 131
_ Derma, 25
Dermal muscles, 134
skeleton, 38, 39
Dermarticulare, 71

Descending aorta, 284
tracts, 40
Desmognathous, 97
Deutoplasm, 8
Diaphragm, 123, 135
Diapophysis, 46
Diarthrosis, 38
Diastole, 272
Diencephalon, 142
Digastric muscle, 133
Digestive tract, 12, 205
Digitigrade, 120
Digits, 116
Dilator pupillz, 202
Diphycercal, 50
Diphyodont dentition, 211
Dipnoi, brain, 154
circulation, 292, 295
excretory organs, 327
lungs, 257
reproductive organs, 333
skull of, 80
Discus proligerus, 320
Dorsal aorta, 275
fissure of cord, 139
nerve root, 161
vertebra, 49
Down feathers, 31
Dromzognathous, 97
Ductless glands, 18
Ductus arantii, 277
arteriosus, 283
Botalii, 283
Cuverii, 271, 278
venosus, 277
Dumb-bell bone, 69, ror
Duodenum, 227
Dura spinalis, 152

Ear, 182
bones, 73
external, 187
functions of, 188
inner, 183
middle, 187
stones, 186
Ect-ental line, 9
Ectethmoid, 67
Ectobronchus, 259
Ectochondrostosis, 43
Ectoderm, 9
Ectopterygoid, 80, 88
Ectoturbinals, 196
Efferent branchial artery, 274

INDEX.

Elasmobranchs, girdles of, 105
intestine, 228
reproductive organs, 331
skull, 76

Elastica externa, 45
interna, 44

Elastic tissue, 22

Electrical organs, 135

Electric plates, 135

Electroplax, 136

Embolomerous, 48

Embryology, 1, 6

Embryonic tissue, 22

Eminentia medialis, 145

Enamel, 40
organ, 40, 209

Endocardium, 269

Endolymph, 185
duct, 183
sac, 183

End organs, 178

Endorhachis, 151

Endoskeleton, 38, 42

Ensiform process, 56

Entepicondylar foramen, 120

Enteroceele, 10

Enteropneusta, 2

Entobronchus, 259

Entochondrostosis, 43

Entoderm, 9

Entoglossal, 80, 97, 218

Entoplastron, 42, 159

Entopterygoid, 80

Entoturbinals, 196

Entovarial canal, 326

Envelopes of nervous system, 151

Eparterial bronchi, 262
Epaxial muscles, 127
Ependyma, 139
Epibranchial cartilage, 65
Epibranchial ganglia, 176
muscles, 133
Epicardium, 124, 269
Epiceele, 143
Epicoracoid, 107
Epidermis, 25
Epididymis, 322
Epigastric artery, 288
vein, 289

; Epiglottis, 252, 253

Epimerals, 54
Epimere, 13

Epineurals, 54
Epiotic, 67, 69

389

39°

Episternum, 59
Epistropheus, 49
Epithelial bodies, 246
pigmented, of eye, 201
Epithelium, 17
Epitrichium, 25
Erectile tissue, 345
Erythrocytes, 265
Essence of pearl, 29
Ethmoidalia, 67
Ethmoid bone, 68
plate, 61
Ethmopalatine ligament, 77
Ethmo-turbinals, 195
Eustachian tube, 187, 237
Excitatory cells, 164
Excretory organs, 307
development of, 310
Exoccipital, 67
Extensor muscles, 132
External carotid artery, 275
ear, 187
gills, development of, 242
iliac artery, 288
Extrabranchial cartilages, 65
chamber, 240
Extrinsic muscles, 131
Eyelashes, 205
Eyelids, 203
Eye muscles, 128, 203
Eye-muscle nerves, 170
Eye, parietal, 147
Eyes, 198

Fabelle, 118
Facialis nerve, 172
Falciform process, 204
Fallopian tube, 338
False amnion, 350
Tib, 55
Falx cerebri, 152
Fascia, 128
Fasciculi, 128
Fasciculus communis, 150
Fat, 22
bodies, 307
Fauces, isthmus of, 247
Feather tracts, 32
Feathers, 31
Femoral artery, 288
pores, 30
vein, 290
Fenestra hypophyseos, 61
ovale, 73, 186
rotunda, 186
tympani, 186
vestibuli, 73, 186
Fibrous tissue, 22
Fibula, 116
Fibulare, 117
Fifth ventricle, 151
Filoplumes, 31
Filum terminale, 140

INDEX.

Fins, 102
anal, 103
biserial, 115
caudal, 103
dorsal, 103
paired, 114
uniserial, 115
Fishes, circulation, 294
eyes, 204
fins, 115
gills, 238
girdles, rro
glands, 27
intestine, 229
lateral line organs, 180
scales, 40
skin, 27
skull, 77
tails of, 50
teeth, 212
thymus, 245
thyreoid, 247
tongue, 217
vertebral column, 51
Fissures of brain, 149, 159
Flexor muscles, 132
Flocculi, 145
Flexures of brain, 143
Floor plate, 138
Feetal circulation, 293
envelopes, 348
Folian process, 74
Fontanelles, 61
Forebrain, 140
Foramen caecum, 219
epiploicum, 122
incisorum, rox
interventriculares, 143
lacerum anterior, 67
magnum, 67
of Monro, 143
of Panniza, 282
of Winslow, 122
Fornix, 151
Fossa hypophyseos, 61
rhomboidea, 144
Fosse of skull, 71
Fovea centralis, 200
Free appendages, 114
nerve terminations, 178
Frontal bones, 68
lobes, 159
organs, 147
Fundus glands, 224
Furcula, 108

Gall, 231

bladder, 233

capillaries, 233
Ganglia of dorsal roots, 161
Ganglion, 20

cell, 19

of retina, 200

INDEX. 391

Ganoids, excretory organs, 327 Glands, tarsal, 205

reproductive ducts, 322 tear, 204

scales, 40 _ uropygial, 30

skull of, 78 Glandula membrana nictitans, 204
Ganoin, 40° Glandular epithelium, 18
Gasserian ganglion, 171 Glaserian fissure, 74
Gastralia, 41 Glenoid fossa, 100, 104
Gastric artery, 284 Glia, 139
Gastrula, 9 cells, 19, 20
Gastrulation, 9 . Glomerulus, 309
Geniculate ganglion, 172 Glomeruli of olfactory nerve, 167
General cutaneus nerves, 167 Glomus, 312
Geniohyoid muscle, 130 Glossopharyngeal nerve, 175
Genital artery, 286 Glottis, 251

prominence, 344 Gluteus muscle, 132
Geological distribution, 7 Gonads, 308, 319

Germ layers, 11

Gill arches, 63
basket, 75
clefts, 236
cover, 77, 240

Goniale, 71

Gonotomes, 319
Graafian follicle, 320
Grandry’s corpuscle, 179
Gray matter, 20

pouches, 239 of cord, 139
remnants, 246 Great omentum, 122
Gills, 236 Guanin, 29
Girdles, 104 Gubernaculum, 338

Gizzard, 225
Gladiolus, 56

Gular bones, 79
Gyri, 149, 160

Glands, 18
of amphibia, 29 Habenular ganglion, 146
of birds, 30 Hemopophysial ribs, 53
buccal, 221 Hemal spine, 46

cardiac, 224

Hemapophysis, 46

carotid, 297 Hair, 33

chorioid, 202 Hair cells, 186
Cowper’s, 342 Hallux, 117
excretory, 307 Hamatum, 117

of fishes, 27 Harder’s glands, 204

fundus, 224
Harder’s, 204

hibernating, 307

Hare-lip, 193
Haversian canals, 23
Head cavities, 128

intermaxillary, 220 kidney, 310
internasal, 220 rib, 81

labial, 22 Heart, 281

lacrimal, 204° branchial, 281
lingual, 221 development, 269
of mammals, 35 division of, 281
mammary, 36 muscles, 125
meiobomian, 205 portal, 294
milk, 36 structure, 269
molar, 221 venous, 281

oral, 220 Hemiazygos vein, 302

orbital, 221

palatal, 220 221

parotid, 221

Hemipenes, 344 _
Hemispheres, cerebellar, 161
Hemispheres, cerebral, 141

392

Hibernating glands, 307
Highmore, antrum of, 197
Hilum of kidney, 330
Hippocampus, 148
Hindbrain, 141
Histology, 1, 16
Holonephros, 310
Holorhinal, 96
Homocercal, 50
Honey comb, 227
Hoofs, 27
Hormones, 18, 353
Horns, ror
Humerus, 116
Humors of eye, 203
Hyoid apparatus, 220
Hyoid arch, 63
Hyoideus nerve, 172
Hyomandibular bone, 73

cartilage, 63

nerve, 172
Hyoplastron, 42
Hyostylic, 73
Hyparterial bronchi, 262
Hypaxial muscles, 127
Hypobranchial, 65
Hypocentrum, 47
Hypocone, 214
Hypoconid, 214

Hypogastric artery, 276, 285

plexus, 163

vein, 290
Hypoglossal muscles, 128

nerve, 177
Hypoischium, rrz
Hypomere, 13
Hypopharyngeals, 80
Hypophysial duct, 191
Hypophysis, 148
Hypoplastron, 42
Hypurals, so

Ichthyopterygia, 114
Tleum, 228
Tleo-czecal valve, 228
Tleo-colic valve, 228
Tleocostal muscle, 131
Tliac artery, 288
vein, 289
Tlium, 109
Incisive foramina, ror
Incisors, 213
Incus, 74
Inferior jugular vein,.27S
mesenteric artery, 285
oblique muscle, 128
turbinal, 100
Infraclavicle, 106
Infratemporal fossa, 71
Infundibulum, 148, 256
Ingluvies, 223 ‘
Inner ear, 183
Innominate vein, 300

INDEX.

Insertion of muscles, 129
Insula 160
Integument, 25
Interbranchial septum, 237
Intercalare, 47
Intercellular substance, 21
Intercentrum, 48
Intercerebral fissure, 148
Interclavicle, 59
Intercostal arteries, 275, 286
muscles, 130
Interhyal, 80
Intermaxillary glands, 220
Intermedium, 117
Internal iliac artery, 288
vein, 290
Internal secretion, 18
Internasal gland, 220
Interoperculum, 77
Interorbital septum, 61
Interparietal bone, 68
Interrenal organs, 352
veins, 291
Interspinous ligament, 48
Interstitial cells, 342
Intertemporal bones, 100
Intestine, 227
Intratarsal joint, 118
Intrinsic muscles, 131
Invagination, 9
Inverted eye, 200
Involuntary muscles, 20, 125
Iris, 202
Ischiadic artery, 288
vein, 290
Ischio-pubic fenestra, 109
Ischio-pubis, 112
Ischium, 109
Island of Riel, 160
Isthmus, 141
Iter, 143
Ivory, 209

Jacobson’s commissure, 165, 171
gland, 194
organ, 190, 196
Jaws, 63
Jejunum, 228
Jugal bone, 70
Jugular ganglion, 176
lymph sac, 303
vein, 279

Kidney, 310
development of, 316 ~
Krausse’s corpuscle, 179

Labial cartilages, 65
glands, 220, 221

Labyrinth of ear, 185
nasal, 192

Lacrimal bone, 69
duct, 204 oe

Lacrimal gland, 204
Lacteals, 304
Lacune, 23
Lagena, 184
Lamina terminalis, 141
Laryngeal cartilages, 64
Laryngeal ventricle, 253
Larynx, 250, 251
Lateral abdominal vein, 289
column of cord, 139
commu, 139
ethmoid, 67
line lobe, 145
line organs, 173, 179
plate, 13
Lateralis nervous system, 173
Latissimus dorsi, 132
Legs, 116
Lens of eye, 199
Leptocardii, 2
Leucocytes, 265
Levator muscles, 131
scapulz, 132
Leydig’s duct, 315
Lids of eye, 203
Ligament, interspinous, 48
of ovary, 338
of testis, 338
Ligamentum medium pelvis, 111
teres, 289
Linea alba, 127
Lingual glands, 221
Lingualis nerve, 171
Lips, 208
Liver, 231
Longissimus capitis muscle, 131
dorsi muscle, 131
Lophodont, 214
Lophs, 214
Lorenzini’s ampullz, 182
Lower jaw, 71
Lumbar artery, 286
plexus, 163
vertebre, 49
Luminous organs, 28
Lunatum, 117
Lungs, 250, 255
Lungs, phylogeny of, 262
Lung pipes, 259
Lungless salamanders, 299
Lutein cells, 320
Lymph, 265
glands, 302, 306
hearts, 302, 304

nadulec 2A

INDEX.

Macule acustice, 185
Malar bone, 70
Male ducts, 321
Malleus, 74
Malpighian corpuscle, 309, 314
Malpighian layer, 25
Mammals, brain, 158
circulation, 300
dermal skeleton, 41
excretory organs, 328
foetal envelopes, 348
gill pouches, 237, 244
girdles of, 108, 113
glands of, 35
intestine, 230
larynx, 254
limbs, 119
lungs, 262
reproductive organs, 335
salivary glands, 221
skin of, 33
skull of, 98
stomach, 225
teeth, 213
thymus, 246
thyreoid, 247
tongue, 219
vertebral column of, 53
Mammary glands, 36
Mandibular arch, 63
arteries, 271
nerve, I71
Mantle of cerebrum, 141
Manubrium, 56
mallei, 74
Manus, 116
Manyplies, 227
Marsupial bones, 114
Masseter muscle, 133
Mastoid process, 100
Matrix, 21
Mazxillaris externus nerve, 172
Mazillary bone, 70
nerve, 171
Mazxillo-turbinals, 196
Meatus, external auditory, 187
Meckelian cartilage, 63, 71
Mediastinum, 16, 122
Medullary cords, 321
groove, IT
plate, 11
sheath, 19
Medulla oblongata, 142
Meibomian glands, 205

Maicenar’c cearnucecla tan

393

394

Merkel’s corpuscle, 179
Mesencephalon, 142, 145
Mesenchyme, ro, 16
Mesenteric arteries, 284
Mesenteries, 14, 121
Mesenteron, 205
Mesethmoid, 67
Mesobronchus, 259
Mesocardia, 16, 122, 270
Mesocolon, 122
Mesoderm, 10
Mesogaster, 15, 122
Mesohepar, 15, 121
Mesomere, 13
Mesonephric tubules, 313

ducts, 313, 315
Mesonephros, 310, 313
Mesopterygium, 115
Mesopterygoid, 80
Mesorchium, 16, 122, 319
Mesothelium, ro
Mesorectum, 15, 122
Mesovaria, 16, 122, 319
Metacarpals, 117
Metacarpus, 116
Metaccecle, 123, 143
Metacone, 214
Metaconid, 214
Metamerism, 13
Metanephros, 310
Metapodium, 116
Metapterygium, 115
Metapterygoid, 80
Metatarsale, 117
Metatarsus, 116
Metazoa, 1
Metencephalon, 142
Midbrain, 141, 145
Middle ear, 187

plate, 13

turbinal, 100
Milk dentition, 211

glands, 36

line, 36

points, 36
Minimus, 117
Mitral valve, 281
Mixipterygium, 116, 343
Molar gland, 221
Molars, 213
Monimostylic, 88
Monophyodont dentition, 212
Monorhinal, 191
Monro, foramen of, 143

sulcus of, 141
Morphology, 1
Mossy cells, 20
Motor-nerve root, 162
Mouth, 208
Miterian duct, 315
Multangulum, 117
Multicellular glands, 18
Muscle plate, 13
Muscles of appendages, development of, 131

INDEX.

Muscles of, dermal, 134
visceral, 132
Muscular system, 124
tissue, 20
Myelencephalon, 142
Myocardium, 269
Myoceele, 14, 121, 126
Myocommata, 127
Myoepicardial mantle, 270
Myofibrillz, 20
Mylohyoid muscle, 133
Myosepta, 38, 127
Myotomes, 14, 121, 127

Nails, 27
Nares, external, 190

internal, 193
Naro-hypophysial duct, 191
Nasal bones, 68

capsules, 190
Naso-palatal canal, 197
Naso-pharyngeal duct, 194
Naso-turbinals, 195
Navel cord, 351
Naviculare, 117

pedis, 117
Nepopallium, 148
Nephridia, 308
Nephridial arteries, 275, 285
Nephrotomes, 14, 308, 311
Nerve cell, 19
Nerve-end apparatus, 178
Nerve of Weber, 173, 189
Nervous system, 137

central, 138

development of, 137

tissue, 19
Neural arch, 45

crest, 161

folds, 11

plate, 11

spine, 46
Neurapophysis, 45
Neurenteric canal, 12
Neuroglia, 20, 139
Neuromasts, 167
Neuron, 19
Neuropore, 12
Nictitating membrane, 203
Non-elastic tissue, 22
Nose, 197
Notochord, 12, 44

sheath of, 45
Nuchal flexure of brain, 143
Nuclei of brain, 144
Nucleus dentatus, 145

Oblique muscles, 130
of eye, 128
Obturator foramen, 109

Occipitalia, 66
Occipital bone, 68
lobes, 159

Occipital vertebra, 62
Oculomotor nerve, 170
Odontoblasts, 39, 209
Odontoid process, 50
Csophago-cutaneus duct, 239
Csophagus, 222
Olecranon process, 120
Olfactory bulb, 142, 167

duct, 194

lobe, 141

nerve, 167

organs, 189

sac, 190

tract, 142
Oliva, 145
Olivary bodies, 145
Omasum, 227
Omentum, 15, 122
Omosternum, 57
Omphalomesaraic artery, 276

vein, 271
Omphalomesenteric artery, 276

vein, 271
Ontogeny, r
Operculare, 77
Opercular gill, 241
Operculum, 77, 240
Ophthalmic nerve, 171
Ophthalmicus profundus nerve, 171

superficialis nerve of seventh, 172

of fifth, 171

Opisthoc:elous, 46
Opisthotic, 67
Optic capsule, 202

chiasma, 169

cup, 198

ganglion, 169

lobes, 142, 169

nerve, 169, 200

pedicel, 203

recess, 141

stalk, 198

thalami, 142, 146

tract, 146

vesicle, 198
Oral cavity, 208

glands, 220

hood, 208

plate, 205
Orbicular muscles, 134
Orbital gland, 221
Orbitosphenoid, 67
Organ of Corti, 186
of Jacobson, 190, 196

INDEX. 395

Ossein, 23, 39
Ossicula auditus, 73, 187
Ossification, 43
Osteoblasts, 39
Ostium tube abdominale, 324
Otic bones, 67
capsule, 60, 67
ganglion, 171
Otoliths, 185
Ovarial cords, 320
Ovarian artery, 286
Ovaries, 308, 320
Oviduct, 316, 321, 323
Ovotestis, 346
Ovum, 8

Pacini’s corpuscle, 179
Paired appendages, 103

fins, 114
Palatal glands, 220, 221
Palatine bones, 69

nerve, 172
Palatoquadrate cartilage, 63
Pallium, 141, 148
Pancreas, 234
Pancreatic duct, 235
Panniculus carnosus, 134
Parabronchi, 259
Parachordal plates, 60
Paracone, 214
Paraconid, 214
Paraglossz, 218
Paraglossal, 97
Paramastoid process, 99
Paraphysis, 146
Parapophysis, 46
Parasphenoid, 69
Parietal bones, 68

eye, 147

foramen, 68

lobes, 159

muscles, 125

organ, 147
Paroccipital process, 99
Parotic process, 93
Parotid gland, 221

of Amphibia, 29

Parovarial canal, 325
Parovarium, 341
Patella, 118, 120
Paunch, 227
Pearl organs, 27
Pecten of eye, 204
Pectineal process, 110, 113

= 1

Dan ntin wean eeeennt i on

396

Pepsin, 224
Pericardial cavity, 14, 123
fluid, 269 -
Pericardio-peritoneal canals, 123, 271
Pericardium, 124, 269
Perichondrium, 39
Periderm, 25
Peridural space, 152
Perilymph, 186
duct, 186
Perimeningeal space, 151
Perimysium, 21, 128
Perineurium, 20
Periosteum, 24, 39
Peripheral nervous system, 161
Peristalsis, 207, 227
Peritoneal canals, 124
cavity, 14, 123
Peritoneum, 124
Permanent dentition, 211
Peroneal artery, 288
Perpendicular plate of ethmoid, 100
Pes, 116
Pessulus, 255
Petrosal bones, 67
ganglion, 175
Petrotympanic fissure, 74
Phzochrome cells, 352
Phalanges, 116
Pharyngeal bones, 80
derivatives, 245
plate, 205
tonsils, 247
Pharyngobranchial, 65
Pharynx, 207, 222, 236
Phosphorescent organs, 28
Photophores, 28
Phyllospondylous, 47
Physiology, r
Physoclistous, 248
Physostomous, 248
Pia mater, 152
Pigment cells, 22, 26
layer of eye, 198
Pigmented epithelium of eye 2o0r
Pillar cells, 186
Pinealis, 147
Pisiforme, 117
Pituitary body, 148
Placenta, 318, 351
vitelline, 348
Placoid scale, 40
Plantigrade, 120
Plasma, blood, 265
Plastron, 41
Platybasic skull, 61
Platysma myoides, 134
Pleura, 124
Pleural cavities, 123
rib, 53
Pleurapophysis, 46
Pleurocentrum, 47
Pleurodont, 88

INDEX.

Pleurodont dentition, 213
Plexus, chorioid, 144
Plexuses, 163
Plica fimbriata, 219
semilunaris, 203
Plume, 31
Plumule, 31
Pneumatic bones, 96
duct, 248
Pneumatocyst, 247
Pneumogastric nerve, 176
Podium, 116
Poison glands, 27, 221
Pollex, 117
Polymastism, 37
Polyphyodont denition, 211
Pons (Varolii), 150
Pontal flexure of brain, 144
Popliteal artery, 288
Pori abdominales, 124, 322
Portal circulation, 277
heart, 294
vein, 277
Postbranchial bodies, 246
Postcardinal vein, 279
Postcava, 290
Postclavicle, 106
Posterior chamber of eye, 203
column of cord, 139
cornua, 139
fissure of cord, 139
horns of cord, 139
lymph sac, 303
Postfrontal bone, 69
Posthepatic digestive tract, 206
Postminimus, 118
Postorbital bone, 69
Postotic nerves, 175
Postparietal bone, 69
Postpermanent dentition, 212
Postpubic process, 112
Post-temporal bone, 106
fossa, 71
Post-trematic nerves, 175
Postzygapophysis, 46
Precava, 300
Predentary bone, 88
Prefrontal bones, 69
Prefrontals of birds, 96
Prehallux, 118
Prehepatic digestive tract, 206
Prelacteal dentition, 212
Premaxillary bone, 70
Premolars, 213
Prenasal bone, 100
Preoperculum, 77
Prepollex, 118
Prepubic process, 111
Presphenoid, 67
Presternalia, 109
Pretrematic nerves, 175
Prevertebral ganglia, 163
Prevomer, 69

Prezygapophysis, 46
Primitive groove, ro
ova, 319
streak, 10
Primordial ova, 314
Process, articular, 46
transverse, 46
Proccelous, 46
Procoracoid, 107
Proctodeum, 13, 206
Pronephric duct, 312
tubules, 311
Pronephros, 310
Prootic, 67
Propterygium, 115
Prosencephalon, 141
Prostate glands, 342
Prosternum, 58

Proterandric hermaphroditism, 346

Proteroglypha, 213
Protocone, 214
Protoconid, 214
Protovertebrz, 14
Protractor muscles, 131
Proventriculus, 225
Psalterium, 227
Pseudobranch, 241
Pseudoconch, 194
Pterotic, 67
Pterygoid bones, 69, 80
muscle, 133
Pterygoquadrate, 63, 69
Pteryle, 32
Ptyalin, 220
Pubofemoralis muscle, 132
Pubis, 109 ;
Pulmonary arteries, 283
circulation, 282
veins, 292
Pulmones, 236
Pulp of tooth, 210
Pupil, 202
Pygostyle, 53
Pyloric ceca, 227
gland, 224
Pylorus, 223
Pyramidalis muscle, 130
Pyramidal tracts, 150
Pyriform lobes, 159
Pyriformis muscle, 132

Quadrate bone, 69, 74
Quadratogugal bone, 70
Quadritubercular, 214 -

INDEX. ; 397

Ramus intestinalis, 162
of tenth nerve, 176
lateralis accessorius, 173
of tenth nerve, 176
ventralis, 162
visceralis, 162
Rathke’s pocket, 148, 206
Rectal gland, 228
Rectum, 228
Rectus abdominis muscle, 130°
capitis muscles, 131
muscles, 128, 130
; of eye, 128
Red spots, 249
Reissner’s fibres, 151
Renal artery, 286, 300
corpuscle, 309
portal system, 280
Rennet, 227
Respiration, accessory structures, 263
mechanism of, 263
Respiratory circulation, 282
duct, 194
organs, 235
Reproductive ducts, 321
organs, 308, 319
Reptiles, see also Sauropsida, Amniotes.
aortic arches, 283
brain, 157
circulation, 299
_ copulatory organs, 344
dermal skeleton, 41
gill pouches, 244
girdles, 107, 111
glands, 30
intestine, 229
larynx, 251
limbs, 118
lungs, 258
scales, 30
skull, 87
thymus, 246
thyreoid, 247
tongue, 217
vertebral column, 52
Rete mirabile, 249, 267
Reticulum, 227
Retina, 199
Retinal artery, 201
ganglion, 200
layer of eye, 198
vein, 201
Retractor bulbi, 128, 203
muscles, 131 ;

398 INDEX.

Rostral bone, 88 Septum pellucidum, 151
cartilage, 76, 85 transversum, 123, 271
Rostrum sphenoidale, 96 Serosa, 350
Rotator muscles, 132 Serous coat of digestive tract, 207
Rumen, 227 Serratus anterior muscle, 132
Sertoli’s cells, 321
Sacculo-ventricular canal, 184 Serum, 265
Sacculus endolymphaticus, 183 Sesamoid bones, 129
of ear, 184 Sheath of Schwann, 20
Saccus vasculosus, 148 Shoulder girdle, 105 ©
Sacral artery, 286 Sex, determination of, 347
plexus, 163 Sexual cords, 320
vertebra, 49 organs, 308
Sacrum, 49 Sight, organs of, 198
Salivary glands, 220 Sinus, cervical, 244
Santorini’s duct, 235 frontal, 197
Sarcolemma, 21 impar, 250
Saurognathous, 97 maxillary, 197
Sauropsida, eyes, 204 of Morgagni, 253
excretory organs, 327 sphenoidal, 197
foetal envelopes, 348 terminalis, 277
reproductive organs, 334 urogenitalis, 322
teeth, 212 venosus, 272
Savi’s ampullz, 182 Sinusoids, 267
Scala media, 185 Sixth sense, 182
Scale of ear, 186 Skeletal labyrinth, 186
Scalene muscles, 130 Skeleton, 37
Scales, 26, 39 appendicular, ro2
ctenoid, 40 axial, 43
cycloid, 40 dermal, 38
development of, 39 membranous, 37
ganoid, 40 visceral, 63
of mammals, 34 Skin, 25
placoid, 40 of mammals, 33
Scaphoid, 117 Skull, 59
Scapular region, 105 development of, 59
Schizoceele, 10 table of bones of, 72
Schizorhinal, 96 Small omentum, 122
Sciatic artery, 288 Smell, organs of, 189
vein, 290 Smooth muscles, 20 124
Sclera, 62, 202 Soft commissure, 146
Scleroblasts, 39 Solar plexus, 163
Sclerotic bones, 67, 203 Solenoglypha, 213
coat, 62 Somatic layer, 10
Sclerotomes, 14 motor nerves, 165
Scrotum, 321, 338 nerves, 162
Secodont, 214 wall, 121
Seessel’s pocket, 206 Somatopleure, 15, 121
Segmentation cavity, 8 Spermatic artery, 286
of egg, 8 Spermatozoon, 8
Selenodont, 214 Sphenethmoid, 86
Sella turcica, 61 Sphenoid bone, 68
Semicircular canals, 184 Sphenoidalia, 67
Semilunar fold, 203 Sphenoidal fissure, 67
ganglion, 171 turbinal, roo
Seminal vesicles, 342 Sphenopalatine ganglion, 165, 171
Seminiferous tubule, 321 Sphenotic, 67
Sense hillocks, 167 ganglion, 165
Sensory epithelium, 17 Sphincter muscles, 129
nerve root, 162 pupillz, 202
organs, 177 Spina scapula, 10g
Septum, interorbital, 61 Spinal artery, 287

of cerebrum, 148 cord, 138

Spinal muscles, 131
nerves, 161
Spiracle, 238
Spiral valve, 228
Splanchnic layer, ro
wall, 121
Splanchnoceele, r2r
Splanchnopleure, 15, 221
Spleen, 307
Splenial bone, 71
Splenic artery, 284
Squamosal bone, 70
Squamous epithelium, 18
Squatina, genus of sharks.
Stapes, 73, 186
Steapsin, 234
Stenon’s duct, 221
Stenson’s gland, 197
Sternal rib, 54
Sternebre, 57
Sternocleidomastoid muscle, 130
Sternohyoid muscle, 130
Sternum, 56
abdominal, 57
Stomach, 223
Stomata of lymph system, 302
Stomodeum, 12, 205
Stratified epithelium, 18
Stratum corneum, 25
germinativum, 25
Streptostylic, 87
Striped muscles, 20, 125
Styloid process, 100
Subarachnoid space, 152
Subcardinal veins, 279
Subclavian artery, 288
vein, 289
Subcutis, 26
Subdural space, 152
Subintestinal vein, 276
Sublingua, 219
Sublingual gland, 221
Submaxillary ganglion, 171
gland, 221
Suboperculum, 77
Subspinal muscles, 133
Subunguis, 27
Sulci, 160
Sulcus of Monro, 141
Superficial petrosal nerve, 173
Superior intercostal vein, 302
jugular vein, 279
mesenteric artery, 284
oblique muscle, 128

INDEX,

Suprascapula, 105, 107
Suprasternalia, 58 |
Supratemporal bone, 69, 106
fossa, 71
Suspensor, 63, 69, 73
Sutures, 38
Sweetbreads, 246
Swim bladder, 247
Sylvian fissure, 155
Sympathetic ganglia, 165
system, 163
trunk, 163
Symplectic, 73, 80
Synarthrosis, 38
Synotic tectum, 61
Synovial membrane, 38
Synsacrum, 53
Syrinx, 254
Systemic circulation, 282
Systole, 272

Tabulare, 69
Tactile corpuscles, 179
Tenia marginalis, 98
Tails of fishes, 50“
Talon, 214
Talus, 117
Tapetum lucidum, 202
Tarsale, 117
Tarsal glands, 205
Tarso-metatarsus, 119
Tarsus, 116°
Taste buds, 189
organs of, 189
Tear gland, 204
Tectorial membrane, 186
Teeth, 208
development of, 209
epidermal, 215
phylogeny, 215
Tegmen cranii, 61
Tela subjunctiva, 26
Telencephalon, 141, 148
brain, 153
breathing valves, 241
excretory organs, 327
girdles, 105
skull, 77
reproductive organs, 332
Temnospondylous, 48
Temporal fossa, 71
lobes, 159
Temporalis muscle, 133
Tenaculum, 203

399

400

Thoracic aorta, 284
duct, 303
vertebra, 49

Thread cells, 29

Thymus glands, 245

Thyreoid cartilage, 252
gland, 246

Thyrohyals, 102

Tibia, 116

Tibiale, 117

Tibial artery, 288

Tibio-tarsus, 119

Tissue, 16

Tongue, 217

Tonsils, 247, 307

Trabecula cranii, 61
communis, 61

Trachea, 250, 254

Tractus olfactorius, 142, 167
solitarius, 150

Transverse bone, 88
muscles, 130
process, 46, 55

Transverso-spinal muscles, 131

Trapezium, 117
Trapezius muscle, 132
Trapezoides, 117
Triconodont, 214
Tricuspid valve, 281
Trigeminal nerve, 170
Triquetrum, 117
Tritubercular, 214
Trochanter, 120
Trochlearis nerve, 170
Tropibasic skull, 61
Truncus arteriosus, 272

transversus, 271
Trypsin, 234
Tuber acusticum, 145
Tubular glands, 18
Tubercular head of rib, 54
Tuberculum impar, 217
Tunica albuginea, 341

serosa, I2I

vasculosa of eye, 201
Tunicata, «
Turbinal bones, 67, 100, 195
Turtles, armor of, 41
’T wixt-brain, 142, 148
Tympanic annulus, 82

bone, 100

membrane, 187
Tympanum of ear, 187

of syrinx, 255

Ulna, 116
Ulnare, 117
Ulnar artery, 288
lymph duct, 303
Umbilical artery, 285,
cord, 351
veins, 278
vesicle, 348 :

INDEX.

Umbilicus of feather, 31
Unguis, 27
Unguligrade, 120
Uncinate bone, 97
Unicellular glands, 18
Uniserial fin, 115 ©
Upper jaw, 70
Ureter, 318
Urethra, 319, 331
Urinary bladder, 318
organs, 307
Urocyst, 318
Urogenital sinus, 322
system, 307
Urohyal, 80, 97, 218
Uropygial glands, 30
Urostyle, 52
Uterus, 338
masculinus, 342
Utriculus, 183
Uvea, 202

Vagina, 338

Vagus nerve, 175

Valve, ileocecal, 228
ileo-colic, 228
of Vieussens, 145
spiral, 228

Vas efferens, 321

Vasa deferentia, 321

Vascular cells, 270

Vater’s corpuscle, 179

Veins, 266, 276, 289
abdominal, 289
advehent, 291
allantoic, 350
anterior abdominal, 289

cardinal, 279

axillary, 290
azygos, 302
brachial, 290
branchial, 274
caudal, 276
central retinal, 201
cephalic, 290
common iliac, 289
cutaneus magnus, 290
epigastric, 289
femoral, 290
hemiazygos, 302
hepatic, 277
hypogastric, 290
iliac, 289
inferior jugular, 278
innominate, 300
internal iliac, 290
interrenal, 291
ischiadic, 290
jugular, 279
lateral abdominal, 289
omphalomesaraic, 271
omphalomesenteric, 271
portal, 277

Veins, postcard 279
pulmonary, 292
Ferhat 2Q1
sciatic, 290
subcardinal, 279
subclavian, 289
subintestinal, 276
superior intercostal, 302
jugular, 279
umbilical, 278
vitelline, 277
vertebral, 292

Velum medullare anterius, 145

transversum, 146
Vena cava, anterior 300

inferior, 290
Ventral aorta, 273

nerve root, 161
Ventricles, cornua of, 160

fifth, 151

laryngeal, 253

of brain, 12, 142

of heart, 272

of lungs, 259
Vermis, 145
Vertebra, development of, 48
Vertebre, 45

occipital, 62
Vertebral artery, 287

column, 44

rib, 54

vein, 292
Vertebraterial canal, 54
Vertebrata, 2
Vesical arteries, 285
Vestibular nerve, 174
Vestibule of mouth, 208

of nose, 194
Vestibulum, 183
Vidian nerve, 165
Vieussens, valve of, 145
Villi, 227
Visceral arches, 63

clefts, 236

INDEX.

Visceral motor nerves, 165
muscles, 132
nerves, 163
pouches, 236
sensory nerves, 167
skeleton, 63
Vitelline veins, 277
Vitreous body, 200
Vocal cords, 251, 253
$acs, 253
Voluntary muscles, 20, 125
Vomer, 69
Vomero-nasal organ, 196

Weberian apparatus, 54, 250
Weber’s nerve, 189
Whalebone, 216
Wharton’s duct, 221
White matter, 20

of cord, 139

tissue, 22
Willis, circle of, 287
Winslow’s foramen, 122
Wirsung’s duct, 235
Wishbone, 108
Wolffian body, 310, 313

duct, 315

ridge, 311

Xiphioid process, 56
Xiphisternum, 57

Yellow spot, 200
Yolk, 8

sac, 206, 277, 348
Ypsiloid cartilage, 110

Zones of nervous system, 141
Zonula ciliaris, 202

Zinnii, 202
Zygantra, 52
Zygapophysis, 46
Zygomatic bone, 70
Zygosphenes, 52

401

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