STRUCTURE OF
THE VERTEBRATES

J.
Structure

of the Vertebrates

MALCOLM E. LITTLE

^Assistant Professor of Biology
<£Mew York University

Ray Long & Richard R. Smith, Inc.

New York 1932

Copyright, May, 1932, by

Ray Long & Richard R. Smith, Inc.

All rights reserved.

PRINTED IN THE UNITED STATES OF AMERICA
BY STRATFORD PRESS, INC., NEW YORK

To the Memory of
George Otis Schoonhoven

ACKNOWLEDGMENTS

Free and unacknowledged use has been made of the many
excellent texts and original papers on the subject. Few con-
clusions, even minor ones, have been contributed by a single
worker; and all of the older results have now become common
property, a part of our scientific heritage. Further, complete
acknowledgment in the text would be confusing to more ad-
vanced students than those for whom this text is designed.

The author owes thanks to his colleagues who helped plan the
course, and who assisted in re-shaping the outline and content
of this book. I am particularly grateful to Dr. H. A. Charipper
who read Parts I and II, and to ^Ir. R. T. Kempton who read
the entire manuscript. Dr. H. E. AVood, 2nd., read Part III,
and Mr. Everett Lyne the entire book.

The drawings for Chapter XV were made by Dr. Florence
D. Wood. Mr. Anthony Barbanera illustrated Chapters YIII,
XI, XII, XVII, and XVIII, and made some drawings for other
chapters.

In the preparation of the manuscript for publication, and in
reading the proofs, the author has had the assistance of Miss
Lillian Fuellhart, Miss Charlotte Rowe, and Miss Katharine
Twichell; and of his assistants, Robert H. Ringewald, Thomas
G. lerardi, James G. Meyers, and Herman Littman. Dr. John
J. Schoonhoven edited the glossary.

]\Iany of the drawings are from the publications of, or mate-
rial in, the American Museum of Natural History. Several of
the birds were copied from paintings; many of the natural history
and skeletal drawings were made from specimens. The author
is grateful for the permission to work at the ^luseum.

Although the manuscript was carefully edited by men who
have had long experience in teaching anatomy, numerous minor
errors have undoubtedly escaped their attention. Wherever there
was a disagreement as to method of treatment or the inclusion
of material it was necessary for the author to depend upon his
own judgment. For any errors of fact, and for the general poli-
cies, the author takes entire responsibility.

PREFACE

This text is designed primarily for use in a half-year course in
Comparative Anatomy. To attain this end it was necessary to
reduce the amount of detail that is included in the usual text
and to arrange the material for flexibility. Additional material
for those who desire it will be found in separate sections. The
author makes no apology for the omission of anatomical minutiae.
Such technicalities usually do not have wide application, and
often so confuse the student that the central theme is lost.

An attempt has been made to simplify the descriptions of
development from the generalized to the specialized, yet, at the
same time, to emphasize the essentials of anatomy. For this
reason enough natural history is included to make clear the com-
parison of structures and to show the evolutionary implications
of anatomy.

The result of a desire for simplification and flexibility is a book
in three parts, each to a certain extent independent of the others.
Chapters are numbered consecutively, and each is divided into
sections, so that the instructor may omit certain chapters or sec-
tions without destroying the continuity of the book.

Vertebrate Zoology, part i, considers the systematic rela-
tionships of the groups of vertebrates. A study of the compara-
tive anatomy of organs {systemic anatomy) presupposes this
knowledge of the vertebrates, for it is impossible to compare un-
known entities. In addition, the author is sufficiently attached to
the "Old School" to believe that a student, whether pre-profes-
sional or general, should know the major groups of animals as he
knows the major periods of history.

Each vertebrate class is included in a separate chapter. At the
beginning of each chapter is a general discussion, and this is fol-
lowed by discussions of the more important orders. Those orders
which have a direct bearing upon the evolution of man are in-

X PREFACE

eluded first; the others are added for those instructors who can
give more time to natural history. At the end of each chapter
is included a brief discussion of the embryology of the class,
placed in a distinct section, so that it may be conveniently
omitted. This omission will probably be desirable in curricula
where a course in embryology follows comparative anatomy.

Comparative Morphology, part ii, is a study of the compara-
tive development and form of the organic systems. Reference is
made to fossil evidence in places where it does not entail a knowl-
edge of stratigraphy. Emphasis is placed upon the dogfish and
the mammal, as most half-year courses are limited to a laboratory
study of these two forms. It is assumed that most of Part II
will be covered in the course. The chapter on ''Mechanics of
Development" is included, as this phase of anatomy has gained
unusual prominence in anatomical research during the past few
years.

Evolution of the Vertebrates, part hi, takes up the palaeon-
tological evidence and some other points of interest which are
necessarily omitted in the first two parts, and includes a chapter
on Geographical Distribution. This material may be assigned
as collateral reading, or omitted. The chapter entitled ''Adaptive
Radiations of the Vertebrates" is designed as a summary of the
courses which vertebrate evolution has followed. It is felt that
a review of the lines of evolution tends to coordinate the evi-
dence which has been gained in a study of individual systems.

It is my hope that the book in part meets the objections of my
students, both pre-medical and general, to the books previously
assigned to them; and that it complies with the following re-
quests : (1 ) that unnecessary detail be omitted, particularly those
details which apply to small groups and have little comparative
significance; (2) that ordinal relationships rather than specific
comparisons be given, so that laboratory work can be correlated
more easily; (3) that generic names be omitted wherever possible
and explained when used, as few students are taxonomists; and
(4) that less be left to the imagination when comparing gen-
eralized mammalian and human structures.

PREFACE xi

Compliance with the above requests might seem to be limiting
the individual initiative of the student, and putting class emphasis
upon the middle group of students rather than the selected few.
The defense for this is: (1) the belief that the interested and
brilliant student will discover enough for himself to stimulate
his imagination; (2) the pragmatic statement that the average
student goes to professional school with little idea as to the
bearing of comparative anatomy upon human structures; and
(3) the fact that this method has proved its efficiency with our
students at New York University.

Although most scientific terms are defined when first used, a
glossary is appended for convenience. In it both the derivations
of the words and their definitions are given. A second appendix
gives a more complete classification of the vertebrates than is
included in the body of the text. A generic name which is used
in the text will be found defined in the glossary. Then by refer-
ring to the classification, its relationships and also all the other
members of the group can be located.

Instead of footnotes, cross references are made in the text.
This arrangement appears to be less distracting to the reader
and will also, it is hoped, bring the chapters into closer unity.

Malcolm E. Little.
New York University,
:\Iay, 1932.

CONTENTS
PART I. VERTEBRATE ZOOLOGY

PAGE

Preface ix

CHAPTER

I. The Chord ATE Phylum 3

II. The Early Chordates H

III. Cyclostomes 29

IV. Fishes 35

V. Amphibia 48

VI. Reptilia 56

VII. AvES 71

VIII. :^Iammalia 76

PART II. CO:^IPARATIVE MORPHOLOGY

IX. Tissues of the Body 99

X. Integumentary Structures 107

XL Supporting Structures 125

XII. Muscular System 175

XIII. Digestive System 192

XIV. Respiratory System 205

XV. Vascular System 221

XVI. The Urinogenital System 250

XVII. Nervous System 271

XVIII. Organs of Special Sense 300

XIX. ]\Iechanics of Development 313

XIU

40240

CHAPTER I

, THE CHORD ATE PHYLUM

In classifying living organisms it was found necessary to divide
them into Plant and Anirnal kingdoms. Each of these, in turn,
was divided into large groups, or phyla, each phylum separated
from some other by only a few fundamental structural charac-
teristics. On this basis all animals with a vertebral column were
placed in the phylum Vertebrata. However, as research in natural
history and embryology continued, several primitive groups of
animals were discovered which lacked vertebrae but which were
evidently closely allied to the vertebrates and not to any other
known phylum. As a result the phylum now includes the verte-
brates and these primitive forms, and is called the Phylum
Chordata.

Considered either as to range in size or complexity of struc-
ture, the Chordates form the most diverse phylum of animals
which we know. Its members range in size from animals barely
visible to the naked eye to the whale and the extinct dinosaur;
in structure from worm-like, swimming creatures to the elephant
and the bird; and in physiological organization and function
from stationary animals with a degenerate nervous system (their
only apparent activity being the digestion of passively acquired
food, and the reproduction of their kind) up to the anthropoid
apes and man.

The chordates have only three characteristics which are not
found in any other group, and as two of these disappear in the
adults of some vertebrates, we must state that embryologically
all chordates have (1) a notochord which gives the group its
name; (2) a dorsal hollow nerve cord; and (3) gill slits in the
pharynx. Thus the phylum is linked by fundamental develop-
mental characters, not by structures which are acquired during
growth and later development. Man, for example, is as surely a

3

4 STRUCTURE OF THE VERTEBRATES

chordate as a fish with persistent notochord and gills, although
the notochord is crowded out of existence during late foetal life
or infancy, and the gill slits are present for only a short period.

Notochord. The notochord is an elastic rod of tissue acting
as a supporting structure of the body. It is the only dorsal sup-
port of the primitive chordates, but in the vertebrates it becomes
surrounded by the cartilage or bone which forms the vertebrae,
and it is completely lost in the adults of the higher groups.

The notochord arises from endodermal cells which lose their
nuclei, and it then becomes an apparently homogeneous, non-
cellular mass of tissue surrounded by sheaths of connective
tissue. Its position is dorsal to the primitive gut and ventral
to the nerve cord.

Nerve Cord. The vertebrate brain and nerve cord, which
together form the central nervous system, are hollow and dorsal
in position. In both characteristics they differ from all other
animals, the invertebrates having a solid nerve cord, ventral in
position.

The chordate nervous system arises from ectodermal tissue
as a groove along the dorsal surface of the developing embryo.
The significance of this dorsal position will be apparent to the
student if he will consider the great development of the brain
in the higher vertebrates, and its position in relation to the
digestive tract.

Pharyngeal Gill Slits. All chordate embryos develop gill slits
in the pharynx, that region of the digestive tract between the
mouth cavity and the esophagus. These gill slits are retained
by the adults in all the primitive chordates; and, aniong the
vertebrates, in the fish and some of the salamanders. In frogs
they are found in the tadpoles but are lost in the adults. In
higher vertebrates the gill slits disappear in the embryo, except
for the first slit which remains as the cavity of the middle ear.

The slits arise in the embryo as paired, endodermal outpocket-
ings from the pharynx, extending laterally toward the sides of
the animal. As each pouch approaches the ectoderm the latter
forms a depression at the point of contact. The membrane
formed at this point by the two layers breaks, forming a tube
connecting the pharynx with the outside.

STRUCTURE OF THE VERTEBRATES 5

The student should distinguish between gills and gill slits.
Gills are respiratory organs which in primitive animals are con-
tained within the gill slits. However, other types of gills exist;
and in three groups of vertebrates (reptiles, birds and mammals)
gills are never present, even in the embryo, although slits are
always formed.

Other Characteristics. In addition to the aboA^e three charac-
teristics, which may be called the diagnostic structures of the
chordates, tliere are others which are common to the vertebrates
and other phyla. Among the 'more important of these are the
following.

1. Metamerism, or segmentation, is the division of the animal
into definite somites or segments, the condition arising in the
mesoderm as a serial repetition of parts. Among invertebrates
segmentation can be studied in the earthworm and the insect;
and can easily be seen in the lower groups of vertebrates. It is,
however, obscured in the more specialized forms. This apparent
loss is due to the development of fins or legs with their large
muscles which cover the fundamental structure.

2. Coelomic Cavity, or body cavity, is found in several inver-
tebrate groups and in the chordates. It arises as a splitting of
the early mesodermal segment, the inner layer of mesoderm
lying next the digestive tract, the outer layer in contact with
the body wall. The coelom is between.

3. Bilateral Symmetry is found in the chordates and in most
groups of the invertebrates. If a median plane is drawn from
anterior to posterior (from head to tail) either half of the animal
will be a mirror image of the other. Due to growth of organs
complete symmetry is soon lost in tlie developing embryo; but
in most vertebrates the skeleton, muscles and nervous system
keep their symmetry th.roughout life.

4. Cephalization is the tendency of the nervous system, the
sensory and coordinating mechanism, to concentrate in the
anterior or head end. Within tlie chordate phylum there is a
progressive tendency toward a concentration and shortening of
the central nervous system.

5. Blood Flow, or the direction of circulation, in the vertebrate
is the opposite of that of the invertebrate. The chordate heart is

6 STRUCTURE OF THE VERTEBRATES

ventral in position, and blood from the heart is forced dorsally
and then posteriorly, the large veins returning the blood being
ventrally located.

Nomenclature. Scientific terms can best be explained when
they first appear in the text. In addition, a glossary is appended
to assist the student when these definitions have been forgotten.
A few terms, however, which involve evolutionary principles, are
defined here before the classification of the phylum is given.

1. Primitive, when used to app'ly to organisms, refers to that
which is racially ancient. Technically, any animal which is liv-
ing today is "recent"; but if it retains many of the character-
istics which were typical of its early ancestors, it is referred to
as a primitive animal.

2. Specialized animals are those which, during the course of
evolution, have become adapted to life in a limited environment;
or those which are specialized for limited activity. The entire
animal may be spoken of as specialized, or one may refer to
specialized individual structures. Therefore an animal may be
primitive, and yet specialized in many of its characteristics.

3. Generalized animals or characters are those which are
capable of modification, those which are not specialized toward
one particular function. ]\Ian's hand is more generalized than
his foot.

It is clear that any one of these terms may be applied to an
individual structure, or to an animal as a representative of a
group. If the latter, the entire sum of characters must be con-
sidered. Consequently, a generalized animal may have many
specialized characteristics; and a primitive animal may be
either generalized or specialized. Further, each of the three terms
is used as a limiting adjective, and any statement must be
applied to a specific group and considered in relation to other
similar races. We may speak of a primitive man, though he
would not be a primitive vertebrate; or, a primitive vertebrate
would not be primitive in relation to the lower chordates. Limit-
ing the term thus is necessary due to the very nature of the
evolutionary process. For, during millions of years of life, it is
inconceivable that any large group of animals could have existed
without changes and specializations; and these changes would

STRUCTURE OF THE VERTEBRATES 7

proceed in different ways and at different rates in isolated races
of the group.

4. Adaptive Radiations refers to the specializations which
adapt different divisions of a group to various environments.
The whale and monkey are both mammals though adapted to
entirely different conditions. This tendency of an ancestral stock
to radiate into many evolutionary lines is frequently called
divergence. Any group which gives rise to another is ancestral
to those which follow. However, the process of divergence can-
not be carried on indefinitely for races, like individuals, die.
They may become so specialized that they cannot survive the
slightest change of environment, and it is therefore apparent
that the possibilities of divergence are greatest in a general-
ized race.

5. Degenerate conditions are merely a phase of specialization.
It implies a loss of functions which once were present. If an
animal during its embryonic life has a well developed nervous
system, and then during the course of its development loses much
of the function of its coordinating structures, we think of it as
a degenerate adult. In racial history the same applies. A Boa
constrictor, with only diminutive structures in place of hind legs,
is a specialized reptile with degenerate legs.

6. Homology. The above necessitates a consideration of homol-
ogy, for before one can say that a snake has degenerate legs
it must be admitted that his racial ancestors had functional legs.
The proof of this lies in the fact that the tiny appendages of a
Boa are developmentally similar to those of a lizard, and the
fossil history confirms the embryological evidence. As a result
the evolutionist thinks of them as homologous structures.
Homology does not necessarily imply any functional relation-
ship, but a developmental similarity; that is, two structures are
homologous if they evolved from the same organ in a racial
ancestor. In this way the wing of a bird and the hand of man
are homologous, though functionally different. Their racial and
embryological history show that they arose from a similar primi-
tive structure.

7. Analogy. Analogous structures are those which serve a
similar function, for example the wing of a bird and the wing of
a butterfly. They are structurally, racially and embryological ly

8 STRUCTURE OF THE VERTEBRATES

different, though both are used for flying. But the wings of the
bat and the bird are not only analogous but skeletally homol-
ogous. They are functionally similar and evolved from the
same ancestral vertebrate limb.

Classification of Chordates. Like all other phyla, chordates
are classified according to the Linnean sj^stem. The heterogeneous
assemblage included in a phylum must be divided into smaller
and more coherent groups according to their fvmdamental struc-
tures. The simplest method of classification is to split the phylum
into the necessary number of Classes; each class into Orders,
an order being a fairly large group of animals with numerous
similarities of development and structure; the order is in turn
divided into Families; a family into Genera, a genus being a
coherent small group, and the genus includes a varying number
of Species. The species is the smallest group that can be accu-
rately set apart from others.

It is often impossible to carry out a scheme as simple as the
above and maintain accuracy. Particularly is this true in a
diverse group like the chordates. It is necessary to set up Sub-
phyla or Sub-classes which may include only a few forms. An
illustration is a Sub-phylum of chordates which contains less
than a dozen species. These few species are so closely related
that they form only one family; but as a group they differ so
widely in structure from all other chordates that they must be
given an independent position equivalent to that of the verte-
brates, which contains six classes and several hundred orders.
In technical works it is often necessary to include numerous
other divisions in an attempt to keep equivalent groups in a
proper balance. As these divisions often become confusing even
to the technically trained they have been omitted as far as pos-
sible, for they are of more academic than practical interest.

The following are the major divisions of the phylum with the
classes of each. Under the separate chapter headings a more
detailed classification will be given, but in each case the author
has taken the liberty of excluding any order not considered
necessary for the continuity of the discussion.

STRUCTURE OF THE VERTEBRATES 9

Phj'lum. Chord ATA

Sub-phylum I. Hemichordata. Primitive animals with a poorly
developed notochord located in the anterior end. The type
order is composed of worm-like animals, the genus
Balanoglossus being the usual form for study. The other
two orders are aberrant and difficult for the beginner.
Sub-ph3dum II. Urochordata. A degenerate group in which
the embryos have a well developed notochord in the tail
region (iiro, tail). One order remains free-swimming and
retains the tail in the adult. The other two orders lose the
tail after the larval period. Embryology is necessary to
show the affinities of most of the group.
Sub-phylum III. Cephalochordata. A small group, closely
allied to the vertebrates, and often included with them as
the Euchordata ("complete" notochord). Amphioxus is
the type form, and is the simplest expression of verte-
brate characteristics known to naturalists. The notochord
extends from tip of head to end of tail.
Sub-phylum IV. Vertebrata or Craniata. Animals with verte-
brae surrounding, incompletely or completely, the noto-
chord. A cranium, or brain case, is also typical. The
sub-phylum is very large, and is divided into the follow-
ing classes.

Class 1. Cyclostoviata are round-mouthed animals, as the
name indicates. The cyclostomes have no jaws,
no appendages, and very incomplete vertebrae.
The larva closely resembles Amphioxus.
Class 2. Pisces, or fish, have jaws, vertebrae completely
surrounding the notochord, and fins as ap-
pendages. All known fish differ widel}" from the
cyclostomes.
Class 3. Amphibia include the frogs and salamanders^
animals with a ''double" life. The larva is a
water-living form with gills and many other fish-
like characteristics, and usually metamorphoses
into a lung-breathing, land-living animal.
Class 4. Reptilia. The reptiles are the turtles, lizards,
snakes and allis;ators. Thev are covered with

10 STRUCTURE OF THE VERTEBRATES

scales of ectodermal tissue, and agree with the
following two classes in having fluid-filled
embryonic membranes surrounding the embryo;
and in being lung-breathing animals which never
develop gills.

Class 5. Aves or birds are vertebrates with feathers.
Structurally they can be described as warm-
blooded reptiles with feathers.

Class 6. Mammalia, the mammals, are w^arm-blooded ani-
mals with milk glands, and are usually covered
externally with hair. All mammals have some
hair, though it may be reduced to a few whiskers.

The classification of man illustrates the method used, and
how any species can be identified with the use of a key to the
particular phylum.
Classification of Man:
Phylum — Chordata

Sub-phylum — ^Vertebrata
Class — Mammalia

Order — Primates (Monkeys, Apes and Man)

Family — Hominidae (Extinct and living genera
of men)

Genus — Homo (A few fossil types and all
living men)

Species — sapiens (all living men.)
For proper identification both the genus and species are
always given, the genus capitalized, and written Homo sapiens.
The specific name may be given to other, widely different,
species; but by agreement there can be only one genus with
the name.

A species is often divided into varieties or races. Living men
are all included in one species, though divided into four races:
Caucasian or white; ^Mongolian, the yellow, brown and red;
Ethiopian or African dark-skinned people; and the Negrito, or
blacks of the Pacific islands. Similar racial divisions may be
used whenever a species shows variations in different parts of its
range of habitat.

CHAPTER II

THE EARLY CHORDATES

The cephalochordates and the vertebrates form a closely
linked, progressive series; but the hemichordates and urochor-
dates differ so widely from the others that they are included
here merely to show the trial and error method on the part of
nature in developing the fundamental characteristics of the
vertebrate form.

Hemichordata. Considered as a whole the Hemichordates
form a rather artificial group, the lower forms being included
with the chordates because of certain affinities with Balanoglos-
sus. The latter has unmistakable chordate form. The student
should not consider this inclusion as a feeling of insecurity on
the part of naturalists, but as the natural result of adaptive
radiations. The very existence of these intermediate forms with
doubtful affinities indicates the development of the higher
chordates from worm-like animals whose descendants evolved
in many directions.

Balanoglossiis is a burrowing animal, varying in size from a
few centimeters to a meter or more in length. These chordates
are often called the Acorn Worms, though their worm-like char-
acteristics are all external. The burrowing proboscis (or ''acorn")
is a tough, distensible organ with muscular and connective tissue
bands running in both circular and longitudinal directions. At
the posterior end of the proboscis is the collar, with the mouth
within its ventral lip. The mouth empties almost immediately
into the pharynx, in which are located the gill slits and gills.
The intestine is a straight tube leading to the posterior anus.
The nerve cord with its small anterior ganglion, or brain, is
located on the dorsal side. The notochord is poorly developed.

11

Myotomes
Gonad

Amphioxus (Cephalochordata)

Notochord

Nerve cord

Endostyle
Pharynx

Adult Tunicate
(Urochordata)

Balanoglossus (Hemichordata)

Fig. 1. Types of primitive Chordata. The Hemichordata are most

primitive; the two diagrams of the Tunicate show the degenerative

speciaHzation of the adult; and at the top is Amphioxus, which is almost

vertebrate in structure.

STRUCTURE OF THE VERTEBRATES 13

Urochordata. The Urochordates, or Tunicates, are a large
group showing intimate connections with each other, and dis-
tinct relationship to the Cephalochordates. The larger forms
are familiar objects in shallow water along the sea coast, and
are there known as "Sea Squirts". Until their embryology was
studied the affinities of these animals were unknown, due to the
degeneration which they undergo in metamorphosis from larva
to adult.

The larva of a typical tunicate has clearly defined chordate
characteristics. The mouth opens into a relatively large pharynx
with a few gill slits. Within the pharynx there is a median, ven-
tral groove, the endostyle, which collects food particles and
passes them into the short intestine. The intestine becomes
spiralled early in development. The nerve cord shows marked
advances over the hemichordate condition. The anterior ganglion
is well developed and often called a brain. Connected with it is
a light-sensitive organ, the so-called eye, and an otocyst which
functions as a balancing organ. The notochord is also well
developed, though it extends no farther anteriorly than the
posterior end of the pharynx.

The larva leads a free-living existence for a short time and
then undergoes rapid metamorphosis into the adult form. At
this time it settles to the bottom and becomes attached to a
stone, wharf pile, or other object. The nerve cord rapidly degen-
erates; the pharynx increases in size with a multiplication of
gill slits; and the tail, with the entire notochord, is absorbed, the
animal feeding on its own tissues during the metamorphosing
process. Synchronously with the degenerative changes a tunic,
or cover, is secreted, surrounding the animal as a lifeless mantle
of animal cellulose. This bears the same relationship to the
animal as does the shell to a snail. The tunic has two openings,
one an incurrent pore, the other excurrent. The animal is then
a completely stationary organism carrying on nutritive and
reproductive processes.

The tunicates, with the exception of the hemichordates, are
perhaps the most ancient line of the phylum. The larvae show
their group relationships, but during their evolution the adults
have become specialized in a degenerative manner.

I

p.

STRUCTURE OF THE VERTEBRATES 15

Cephalochordata. The cephalochordates are small, lance-
shaped animals living on sandy shoals of the ocean, near the
coasts of Europe, Asia and North America. The type form is
Amphioxus, its "double pointed" structure being an adaptation
to burrowing in the sand. The anterior end is braced by the
extension of the notochord into the rostrum which projects in
front of the mouth. The animals are active during the night, but
in the day burrow into the sand with only the anterior end pro-
jecting above the surface. In this position they wait for cur-
rents of water to wash food into their mouths.

External Anatomy. The adult Amphioxus varies in length
from four to six centimeters, about two inches. The anterior
end can be determined by the pointed rostrum, and the ventral
mouth a few millimeters from the front. The latter is surrounded
by a circle of buccal cirri. In cross section the animal is roughly
triangular in shape, the base being the ventral side.

In a preserved animal the oral hood and funnel can be seen
within the circle of cirri. The structures are formed by a fold
of tissue extending downward and backward from the rostrum,
so that a cross section through the hood would be circular. At
the bottom of the funnel is the mouth, which can be studied only
microscopically. The "fins" of the animal are simple in struc-
ture; a dorsal fin extends from the base of the rostrum to the
posterior end of the animal, where it becomes wider and swings
around the posterior tip to form the caudal fin, and the nar-
rower ventral fin. With the fins may be included the metapleural
folds, which will be discussed later. These folds are paired,
extending along the lateral ventral sides of the animal to a point
just anterior to the ventral fm, where the two folds meet. Exam-
ination will show that at the confluence of the folds there is an
opening, the atriopore. The anus can be located posterior to
the atriopore, on the left side of the ventral fin.

The animal is covered with a thin skin, composed of an
ectodermal layer one cell thick, and a dermis, which is a loose,
gelatinous, mesodermal connective tissue. A non-cellular cuticle
is secreted by the ectodermal cells, acting as a transparent pro-
tective layer on the outside. Through the thin skin can be seen
the distinctly metameric, V-shaped muscle segments, separated
from each other by thin sheets of connective tissue, the

16 STRUCTURE OF THE VERTEBRATES

myosepta. These structures are important, for the muscle fibers
are attached to them, and they bind the myotomes to each
other and to the notochord. If the animal is thoroughly adult
the metameric gonads, or reproductive organs, will be visible.
The sexes cannot be distinguished except microscopically. In
sections the female ovary shows the spherical eggs or ova, and
the male testis has very minute sperms.

Digestive Tract and Accessory Structures. Beginning at the
anterior end, the first nutritive structures are the buccal cirri.
In the living animal these are constantly in motion, and in
addition to a sensory function, probably assist in carrying a
stream of water with suspended food particles into the funnel,
and thus into the mouth. Within the funnel are two structures
which are supposed to have a sensory function: (1) a ciliated
groove (Groove of Hatschek) extending the length of the fun-
nel, slightly to the right of the dorsal midline; and (2) the
"wheel organ", patches of thickened, ciliated epithelium sur-
rounding the mouth and extending anteriorly like spatulate
fingers. The latter probably assists in passing the water toward
the mouth and pharynx.

The mouth is guarded by a velum, or curtain, hanging from
the dorsal wall of the rounded opening. The mouth opens
directly into the pharynx, the largest single organ of the entire
animal. As the water which is forced through the mouth be-
comes more or less still in the enlarged cavity, the food par-
ticles tend to drop to the ventral floor where they are caught
in a groove along the mid-ventral line, the endostyle. This
groove is lined with cilia and mucous secreting cells, so that
the food is caught in the mucus and passed anteriorly toward
the mouth. At the level of the mouth the endostyle splits into
two grooves, the peripharyngeal grooves, which pass dorsally
around the opening and unite in the dorsal-median line as the
epipharyngeal groove. In this groove the cilia beat backward,
carrying the mucus with its attached food particles to the open-
ing of the stomach-intestine, a straight tube in which digestion
and absorption take place. The intestine opens to the outside
at the anus.

Immediately posterior to the pharynx there is a single ventral

STRUCTURE OF THE VERTEBRATES 17

outpocketing of the stomach-intestine, the hepatic caecum. This
blind pouch grows forward on the left side of the pharynx, and
assists digestion. From its method of origin it has been identi-
fied with the liver of the vertebrates.

Within the pharynx are the gill slits and their gills. In the
larva these are metameric, paired openings; but in the adult
metamerism is soon lost, and new gills are added until there
may be more than a hundred pairs. The gills serve as respira-
tory organs, the water which carries food particles also carrying
dissolved oxygen. The course of the water is, therefore, im-
portant. It is forced through the mouth into the pharynx. There
a portion of its food is dropped, and the oxygenated water is
carried out through the gill slits, supplying oxygen to the vascu-
lar gills and taking up the carbon dioxide. In the adult an
atrium is present, a specialized external covering which protects
the gills (page 27).

Supporting Structures. The main supporting element of
Amphioxus is the notochord. This rod of spongy tissue is sur-
rounded by two concentric sheaths of connective tissue, the
notochordal sheaths, which give strengih to the notochord.
Fibers pass out from these sheaths between the muscles, sup-
porting them and giving points of attachment to the muscle
fibers. Any contraction of the latter exerts a pull on the
notochord and affects the entire animal.

The cartilage of Amphioxus is a very soft tissue having few
of the characteristics of true cartilage. These cartilages can be
seen in the dorsal fin as the segmental fin rays. A horseshoe-
shaped buccal cartilage surrounds the funnel, the open side at
the bottom. From this cartilage a small ray extends out into
each cirrus. The gill slits are also supported by cartilaginous
gill bars which lie between the slits. The illustrations give the
details of these structures.

Muscular System. The muscles are of the simplest nature.
The myotomes, or muscle bands, are V-shaped with the apex
pointed forward. The muscle fibers run longitudinally, groups
of fibers being caught into bundles by connective tissue sheaths,
and a large number of bundles forming a myotome. Between
each myotome is the myoseptum which is intimately connected

Gill slits

^Efferent

Gill Arteries

Pharynx

Dorsal Aorta
Intestine

Afferent Arteries ^Ventral Aorta

Vascular System of Amphioxus

Nerve cord
Notochord
Endostyle
Pharynx
Coelom

Hepatic
caecum

Hepati
caecum

^Sub-intestinal
vein

Gonad

Hypobranchial
Typical Cross section Groove

Nerve cord of
Amphioxus

The Cardinal Veins

Fig 3 Internal Anatomy of Amphioxus. The cross section shows the

relative position of the structures. The top diagram gives the relationship

between the digestive and vascular systems.

STRUCTURE OF THE VERTEBRATES 19

with the notochordal sheaths. Each fiber is attached at either end
to the myoseptum, so that the myotomes are functionally con-
tinuous although broken into definite metameres.

Any muscular contraction exerts a pull on the anterior-pos-
terior axis, so that the range of movement is limited to a side
to side motion. Forward movement of the animal is caused by
the larger size of the anterior end. The tail region is more
flexible, and an alternate contraction of the muscles of either
side causes the tail to act as a propeller both in swimming
and in burrowing.

Vascular System. The blood system of Amphioxus is the per-
fect starting point for a study of the vertebrate system. A glance
at the diagrammatic drawing will assist in understanding the
description. Recall that the vascular system has the functions
of (1) taking up absorbed food materials from the digestive
tract; (2) carrying waste products from the cells to the organs
of excretion (and respiration) ; (3) taking up oxygen from the
gills; and (4) distributing food and oxygen to the tissues of the
entire body. Further, Amphioxus, like the vertebrates, has a
closed system. The blood vessels are continuous, and the blood
remains within the lining of the vessels; therefore, when an
artery divides into capillaries, these same capillaries re-collect
into veins without any definite break. Consequently it is impos-
sible to make any definite distinction between arterial and
venous capillaries.

Amphioxus has a slightly enlarged, pulsating ventral aorta,
homologous with the vertebrate heart. From this point the aorta
continues anteriorly, giving off a pair of afferent branchial
arteries at alternate gill supports. These arteries break into
smaller vessels in the gills, and then collect into efferent bran-
chial arteries. Lateral branches pass into the secondary gill
supports, with the result that there is an efferent artery for
each gill. Passing through the gills the blood has been aerated.
The efferent branchials enter the paired dorsal aortae, one on
either side of the notochord. Branches go forward into the head
region, the two dorsal aortae extending backward and fusing
to form a single vessel at the posterior end of the pharynx. The
aorta then continues al^Aft- the body and into the tail, giving off
metameric vessels to the muscles and the digestive tract.

20 STRUCTURE OF THE VERTEBRATES

The capillaries draining the intestine and muscles collect into
veins, and these fuse to form a ventral subintestinal vein. At
the hepatic caecum this vein breaks into smaller vessels which
spread around the caecum and re-form into a continuation of
the ventral vein. This is the equivalent of a hepatic portal sys-
tem and a hepatic vein. The latter is continuous with the ven-
tral aorta.

A minute group of vessels, draining the gonads and part of
the head region, has been described and is considered the
homologue of the cardinal system of vessels found in the higher
groups.

Observe that the blood flows forward in the ventral vessels
and backward in the dorsal. This is the typical vertebrate sys-
tem. The student should work out for himself the relative
amounts of food, oxygen and waste matter contained in the
blood of each region of the body.

Excretory System. The organs of excretion are paired
nephridia, each of which has a head in the coelomic cavity
and a short nephric tubule which embryologically empties to
the outside of the body. This is slightly modified in the adult
by the development of a protective atrium (see page 27). In
their metameric arrangement the nephridia may be considered
as primitive structures; but the individual nephridium is a highly
specialized organ.

Reproductive System. The gonads (ovaries and testes) are
metameric, very numerous, and lie along the ventral half of
each side of the animal. As in the vertebrate the gonads arise
in the dorsal mesodermal tissues, outside the coelomic cavity
and covered by the peritoneum, the lining of the body cavity.
As the atrial folds develop, the gonads, each surrounded by a
part of the coelomic cavity and peritoneum, are pushed ven-
trally. Therefore, as the sperm and ova ripen they break
through the covering membranes and fall into the atrial cavity,
from which they pass to the outside through the atriopore and
fertilization takes place externally. This is a specialization in
Amphioxus, in correlation with the development of the atrium
(page 27), and differs from the condition found in the verte-
brates. In the most primitive vertebrates the ova and sperm
fall directly into the coelomic cavity; in all others the sperms

STRUCTURE OF THE VERTEBRATES 21

have tubes which conduct them to the outside, while the s^Derms
retain the primitive condition.

Nervous System. Amphioxus has no brain, the anterior part
of the nerve cord being even smaller than that lying farther
posteriorly. This is considered a specialized condition, for the
tunicate has a better developed brain than the cephalochordates.
At the extreme anterior end of the nerve cord are two so-called
cranial nerves, and between them a light-sensitive, pigmented
eye spot. Posterior to this region are the metameric spinal
nerves. Each has two roots arising from the spinal cord, a
dorsal root and a ventral root. The dorsal is a mixed nerve,
being partly sensory and partly motor, while the ventral is
entirely motor. Differing from the vertebrate, all the nerve
cells are contained within the cord; and the roots do not unite
into a single spinal nerve, but each retains its independence
and ramifies into the tissues of the body.

Embryology of Amphioxus. Amphioxus in its development is
properly considered the basis for all vertebrate embryology. Its
study has cleared many obscure points due to its perfectly gen-
eralized developmental stages, and the leisurely manner of organ
formation in the early embryo.

The ova are transparent objects on the verge of human vision.
Both eggs and sperms are thrown into the water about dusk
during the breeding season, where fertilization takes place. The
ovum has a small amount of yolk material slightly concentrated
at the ventral pole. This is of more than passing significance
in cleavage, as it is a general rule that cells loaded with inert
material cleave more slowly than others.

About an hour after fertilization the egg divides, the first
cleavage being through the dorso-ventral axis. The second is in
the same direction, at right angles to the first. The third
cleavage is horizontal, perpendicular to the first two, and
slightly asymmetrical, the four dorsal cells being slightly smaller
than the four ventral. Due to the use of cellular materials to
supply energy for cleavage, and the natural tendency of a
liquid to assume a spherical shape, a cavity has appeared in the
center of the eight-celled embryo, called the segmentation
cavity.

22

STRUCTURE OF THE VERTEBRATES

After the fourth or fifth cleavage there is a slightly more
rapid rate of cleavage in the dorsal hemisphere, and these cells
are distinctly smaller. The segmentation cavity has also en-
larged, so that at about the 256-cell stage the embryo is a
hollow sphere, the blastula. The process of gastrulation begins
immediately after, and may be illustrated by pushing a finger
into a rubber ball until a cup-shaped, two-layered structure
results. In Amphioxus gastrulation is caused by two synchronous
processes: (1) the more rapid growth of the dorsal cells causes
this hemisphere to become larger than the ventral, and the
blastula assumes a hemispherical shape; and (2) the ventral
cells invaginate, or push inward, until they come in contact
with the dorsal layer. The earlier segmentation cavity is prac-
tically obliterated, and a new cavity is formed inside the cup.
Thus the early gastrula is two-layered, the outer layer of cells
being the ectoderm, the inner layer (derived from the ventral
hemisphere) being the endoderm. The inner cavity, surrounded
by endoderm, is the archenteron or primitive gut. Continued
growth of the gastrula causes it to elongate, and the lips of the
cup to narrow toward a central point. The opening left after

Polar body

Uncleaved
egg

2 Cells

4 Cells

8 Cells ^^ ^^^^^ Blastula

Fig. 4. Cleavage of Amphioxus from egg cell to blastula.

STRUCTURE OF THE VERTEBRATES

23

Segmentation
cavity.

Beginning of Gastrulation

ctoderm
Endoderm
Archenteron

Segmentation
cavity

Blastopore
Completion of Gastrulation

Fig. 5. Sagittal Sections of Amphioxus showing gastrulation. The ventral

(endodermal) pole invaginates synchronously with the overgrowth of the

dorsal (ectodermal) pole.

gastrulation is complete is the blastopore which eventually gives
rise to the anus.

Organogenesis, or organ formation, begins almost at once. The
development of the ectodermal dorsal nerve cord, the endodermal
notochord, and the mesodermal tissues are practically syn-
chronous. They are considered separately for convenience, and
constant reference should be made to the drawings.

The nerve cord begins as a plate of cells (the neural or
medullary plate) along the dorsal, median line of the embryo.
The lateral margins of the plate soon break away from the
ectodermal cells covering the remainder of the embryo. The
ectoderm along either side of the plate becomes elevated, form-
ing neural folds, and these folds grow toward each other along
the dorsal midline. When the epidermal covering of the embryo
is about complete the neural plate sinks in the middle to form
a neural groove, and the continued infolding of the groove and
the upgrowth of the edges of the plate soon develop the dorsal

24 STRUCTURE OF THE VERTEBRATES

neural tube which becomes the nerve cord of the animal.

The notochord begins its development slightly later than the
nerve cord. It is formed from the dorsal cells of the archenteron.
These endodermal cells develop a groove, the curve being exactly
opposite to that of the nerve cord, and the lateral margins grow
downward until they meet. The notochord becomes a continuous
loose rod of cells beneath the neural groove.

The mesoderm arises from the primitive endoderm as paired,
lateral pouches from the gut. It should be kept in mind that
though the nerve cord and notochord are continuous structures,
the mesodermal pouches are a series of discontinuous outgrowths,
or enterocoels (literally ^'gut pouches"). Thus metamerism, or
segmentation, has its origin in the mesoderm. Only three pairs
of pouches arise directly from the archenteron, the others de-
veloping by the growth and division of the third (most posterior)
primitive pouch.

Soon the lining of the gut fuses along the dorsal border cut-
ting the notochord off as a separate structure ; and following this
the mesodermal somites lose their connection with the enteron.
The embryo is now an ellipsoid organism completely enclosed by
ectoderm, for as the neural folds closed along the dorsal mid-
line they also overgrew the blastopore. Therefore, for a short
time, the neural canal is continuous with the enteric canal.
Later the posterior end of the nerve cord closes, and the blasto-
pore breaks through the ectoderm to form the anus, or posterior
opening of the enteron.

A coelomic cavity develops through the enlargement of the
original enterocoel cavities. As the metameres grow downward
the ventral portion becomes thin-walled, and the dorsal part of
each thickens into a mass of tissue. The dorsal segment is the
epimere which gives rise to the muscle tissues and the connec-
tive tissues; the ventral hypomere forms the smooth muscle of
the digestive tract, the heart, the mesenteries, and the lining of
the coelomic cavity. These relationships can be studied in the
illustrations. Observe that as the hypomere pushes ventrally
the median layer of cells is pushed against the gut, which has
separated from the notochord. During this process the dorsal
epimere has grown around the notochord and also ventrally,
forcing its way between the ectoderm and the lateral layer of

Nerve cord
ndoderm
Archenteron.

Fig. A

Fig. B

Nerve cord
Notochord
Mesoderm,

'Ectoderm

Endoderm
rchenteron-

Nerve cord-

' 'otochord^

Epimere.

Fig. C

Fig. K

Longitudinal Section Through Stage "C

Dissected Embryo at Stage "C"

Fig. 6. Organogenesis in Amphioxus. Diagrams A to I are cross sections
to show the development of the continuous nerve cord and notochord,
and the discontinuous enterocoels. Drawing J is a horizontal section.
Drawing K has the outer ectoderm dissected away to show the relationship
between the archenteron and the enterocoels.

26 STRUCTURE OF THE VERTEBRATES

the hypomere. Eventually, both the epimere and the hypomere
reach the raid-ventral line, each meeting its mate which de-
veloped on the other side. The coelomic cavity is now almost
complete. Further development consists in strengthening the
dorsal mesentery which supports the gut; the absorption of most
of the ventral mesentery; and the disappearance of the septa
which up to this time have divided the coelomic cavity into
separate pockets. When complete the coelom is a continuous
cavity from anterior to posterior, with no separation of the two
sides at the bottom, but divided dorsally by the mesentery.

Gill Slits develop in the anterior, pharyngeal part of the
enteron. These begin as paired outpocketings of the endoderm,
the bubble-like structures growing laterally until they reach the
ectoderm. The ectoderm pits in at these points, fuses with the
endoderm, and then breaks through to form the gill slit which
connects the pharynx with the outside. At first the slits are
metameric, but during development metamerism is lost in these
structures.

The mouth is formed by the anterior wall of the gut coming
in contact with the ectoderm and breaking through. In
Amphioxus the mouth opening is at first asymmetrical, but it
later moves into its normal terminal position. The heart, or ven-
tral aorta, is at first a thin tube of mesodermal cells lying ventral
to the gut, but later it is surrounded on either side by the hypo-
mere. The latter develops into the cardiac muscle which gives
the heart its strength and rhythmic beat.

Importance of Amphioxus. More time has been given to the
structure and development of Amphioxus than the student may
feel is necessary, but the group illustrates perfectly the funda-
mental plan of the entire chordate group. Its embryology is so
generalized that it clears many doubtful points in the develop-
ment of the true vertebrates, and the developmental approach
to comparative anatomy needs no defense.

The importance of the group lies in the fact that in no other
animals are the essential chordate characteristics shown as
simply and with so few complications. Particularly is this true
of the young animal. The adult has numerous specializations,
and the student should review the meaning of "generalized" and

STRUCTURE OF THE VERTEBRATES 27

"primitive" in relation to Amphioxus. The group is primitive
because it is fundamentally similar to the earliest ancestors of
the phylum. The young Amphioxus is generalized because its
characters apply to the chordates as a whole. However, the
group has many specializations, or characters which have been
added to serve a particular function. The student has but to
consider the enormous length of time since the origin of the first
Amphioxi to understand this. The constant mutations which
occur in all animals; constantly changing environmental condi-
tions which place new hazards upon existence; and the destruc-
tion of the least adapted types due to natural selection, explain
why a group rarely exists for long periods without adding spe-
cializations which adapt it for particular conditions.

Specializations of Amphioxus. An incision in the ventral
body wall of a frog or other vertebrate would open the coelomic
cavity; but a similar incision in the body of Amphioxus would
open an ectoderm lined cavity, the atrium, which serves to pro-
tect the gills of the burrowing animal. This cavity is closed at
the anterior end and opens posteriorly through the atriopore.
Therefore, water entering the mouth of the adult passes through
the gills into the atrium, and out through the atriopore. In this
way the circulation of water is not hindered by the surrounding
sand. Its development is illustrated by the drawings. The atrial
folds of the larva begin in the anterior dorsal region and pass
posteriorly and ventrally until they nearly meet anterior to the
anus. These continuous folds then grow ventrally, being covered
on both sides by ectoderm. Eventually they meet along the mid-
ventral line, fuse for their length, and develop the new cavity.
The metapleural folds are simply longitudinal folds of skin
along the lateral walls of the atrial walls.

The coelomic cavity of the pharyngeal region is also highly
specialized. This is due to (1) the increased size of the
atrium, and (2) more especially to the great number of gills.
The body of a vertebrate can be compared to a room with its
inner layer of plaster (the endoderm), the outer layer of brick
(the ectoderm), and the cavity between (the coelomic cavity).
If the windows are compared with the gill slits, it will readily
be seen that every window destroys a certain portion of the wall

28 STRUCTURE OF THE VERTEBRATES

cavity. Continue the multiplication of windows, and eventually
the internal cavity remains only in the region above the win-
dows, as narrow strips between them, and ventral to the inner
cavity. As the gill slits of Amphioxus are very long and close
together the coelomic cavity of the adult is left (1) as a ven-
tral tube; (2) two dorsal tubes separated by the mesentery;
and (3) long, very narrow tubes, between the primary gills,
which connect the dorsal and ventral portions. During develop-
ment each of these primary gills is divided into two by the
downgrowth of a septum which naturally does not carry any
of the coelomic cavity with it. Therefore, only every alternate
gill support has a tube of coelom.

The rostrum and the anterior extension of the notochord are
also considered as specializations. This is based on the fact that
in all vertebrates the notochord extends no farther than the
middle of the brain, and in Amphioxus the rostrum is a later
growth. Unfortunately the notochord of the more primitive chor-
dates offers no solution, for in the tunicates the notochord is
limited to the tail; and in the more primitive hemichordates the
notochord is rudimentary and found in the anterior region.

The asymmetry of the mouth opening is correlated with a
bilateral slippage during development. One half of the embryo
pushes forward about half the distance of one body segment,
and though the mouth finally corrects its position, the myotomes
and the gonads maintain this asymmetry throughout life. This
accounts for the fact that when cross sections of Amphioxus are
studied a pair of gonads is never cut through the same region,
one section always being larger than the other.

CHAPTER III

CYCLOSTOMES

Cyclostomes are the most primitive vertebrates, in many re-
spects intermediate between the Amphioxi and the fish. They
are popularly called "round mouthed eels", the name being
derived from a superficial eel-like structure, not from any
anatomical similarity. The animals inhabit both fresh and salt
water and are very widely distributed.
The characteristics of the group are:

1. No paired appendages. The cyclostomes entirely lack the
two pairs of appendages typical of the vertebrates. The student
should not confuse the leglessness of snakes, some fish, and other
animals with the inherent absence of paired fins in this class of
vertebrates. In the higher legless forms there are always
embryonic limb buds, which may remain as rudiments in the
adult. The cyclostomes have no embryonic evidence of ever
having had fins.

2. Round mouth. The word cyclostome means ''round mouth",
in reference to the round oral hood, somewhat similar to that
of Amphioxus. A rasping "tongue" scrapes the food into fine
fragments, while the sucking oral hood attaches the animal to
its prey.

3. No jaws. Lacking jaws, the oral hood remains permanently
open, the two characters being interdependent. The absence of
jaws is a very primitive characteristic, as is shown by both
embryological and fossil evidence.

4. Gill structure. One of the orders of living cyclostomes has
seven gill openings, the other has as many as fourteen. As five
gill slits is the typical number of the higher vertebrates, and
the most primitive fish known has seven, the cyclostome condi-
tion is considered primitive. The internal and external openings

29

30 STRUCTURE OF THE VERTEBRATES

of the gill pouches are round, agreeing with the structure of the
primitive fossils, but differing from the fish.

5. Nervous system. The brain is primitive compared with
that of the fish, although highly developed compared with
Amphioxus. The olfactory lobes arp highly developed, but there
is no distinct cerebrum or cerebellum. The other organs of spe-
cial sense are poorly developed. The lampreys have two semi-
circular canals in the inner ear, agreeing with the fossil forms;
the other order (myxinoids) have a single inner ear canal,
which is considered by some anatomists to be a degenerative
specialization. Most of the cyclostomes are blind, or nearly so,
the eyes appearing to be degenerate and not primitive.

6. Supporting structures. The notochord is covered with
sheaths, as in Amphioxus, and is the main support of the body.
The vertebrae, which appear first in this class, are minute
cartilages which could serve no possible function. A chondro-
cranium, or primitive cartilaginous "skull" has developed. It is
an open trough without a roof, in which the brain rests.

7. No bone. The living orders of cyclostomes lack bone en-
tirely. The "teeth" of the oral hood are ectodermal horny struc-
tures, and have no resemblance nor relationship with the teeth
of higher vertebrates.

8. Nasal opening. There is a single, dorsal, nasal opening
connecting the medium with the olfactory sacs. As the tube
extends past the olfactory region it is usually called the naso-
pituitary opening.

Classification of the Cyclostomes. The living cyclostomes
are divided into two orders which vary widely in development
and structure. The fossil forms include three orders which are
grouped together as the Sub-class Ostracodermi. From the
embryological evidence (Dean) and the fossil evidence (Stensio)
each of the living orders may be considered as a sub-class,
equivalent to the Ostracodermi.

With the cyclostomes is often included a group of very small,
extinct, animals known from a single genus (Palaeospondylus).
If it is an adult rather than a larval stage, it gives evidence
of the early segmentation of the notochordal supporting
structures.

STRUCTURE OF THE VERTEBRATES

31

Petromyzontea, or lampreys, include Petromyzon marinus,
the form ordinarily used in laboratories for dissection. The
lampreys have a complete cartilaginous branchial basket and
seven gill pouches. During development a longitudinal constric-
tion separates the dorsal gullet from the ventral respiratory

,Naso-pituitary
opening

Petromyzon

Ectodermal
teeth

Anus

Naso-pituitary groove

Nerve cord

Mouth

Oral Hood
Petromyzon

Heart'

Mating Brook Lampreys

Bdellostoma (Myxinoid Cyclostome)

Fig. 7. Cyclostomes. Bdellostoma belongs to the more primitive order,

the Myxinoidea. The section of the lamprey larva (Petromyzontia) shows

the pinching off of the endostyle to form the thyroid.

32 STRUCTURE OF THE VERTEBRATES

tube. The naso-pituitary pouch ends blindly under the brain.
There are two semicircular canals in the inner ear. The eyes
are better developed than those of the following order.

Myxinoidei, the hag-fishes, are ecto-parasitic and a serious
menace to fish. They are limited to the Pacific Ocean. This
order has a reduced branchial basket and fourteen gill open-
ings. These may open directly to the outside, or unite as a
single ventral outlet. The oral hood of some forms is surrounded
by fleshy barbels. The eyes are degenerate, and there is a single
semicircular canal in the ear.

Ostracodermi. These are fossil forms which have clear-cut
cyclostome affinities (Stensio, 1927 and 1931). They lack jaws
and teeth, and have no paired appendages. Sections of the ani-

A. Thelodus B. Birkema

Fig. 8. Ostracoderms ; extinct cyclostomes with dermal plates. Birkenia

is a free-swimming type with a few dorsal spines. Thelodus has numerous

dermal denticles and is specialized for bottom living.

mals show the following agreement with the living cyclostomes:
(1) round gill openings; (2) the structure of the brain and
spinal nerves; (3) the arrangement of the cranial circulatory
system; (4) two semicircular canals in the inner ear; (5) a single
dorsal nostril; and (6) an unconstricted notochord. The ap-
parently well developed eyes, the round mouth, and the lack
of jaws can be determined from a study of the external anatomy.
The ostracoderms differ from the living cyclostomes in hav-
ing dermal denticles, scales, or plates covering the body. The
more primitive genera had small bony denticles without a
broad base; the more specialized groups developed a broad base
to the denticle, and the most specialized forms had plates cov-
ering the body.

Development of Petromyzon. The egg is approximately a
millimeter in diameter, and the early stages of development

STRUCTURE OF THE VERTEBRATES 33

are sufficiently like those of Amphioxiis to be understood with-
out repetition. The embryo hatches as a larva which shows rela-
tionship with the primitive chordates.

The larva is strikingly like Amphioxus in body shape and
in structure, though (1) the reduced number of gill slits, (2)
the more advanced brain, and (3) the chondrocranium are
typically vertebrate. Even in the early larva the ventral hepatic
outpocketing, which remains in Amphioxus as a caecum, has
become a glandular liver. The similarities are in the oral hood,
the myotomes, and the pharyngeal structures. The food getting
structures of the pharynx are a ventral endostyle and a dorsal
epipharyngeal groove. Both are ciliated and secrete mucus.
After three or four years the larva metamorphoses into the adult
tj^pe. The dorsal part of the pharynx, including the upper
groove, pinches off as an esophagus, leaving the pharynx as a
blind sac. The endostyle also closes, becomes at first a blind
pouch with a slight pharyngeal connection, and then the con-
nection is lost as the pocket becomes glandular. The endostyle
is then the thyroid gland. This is one of the clearest homologies
in vertebrate anatomy, the thyroid of higher types always aris-
ing as a median, ventral outpocketing of the pharj-nx.

Specializations of Cyclostomes. The larva of the lamprey
shows such definite relationships with Amphioxus, and the fossil
forms are fundamentally so similar to the living representatives,
that most anatomists consider the class as very primitive,
although highly specialized, vertebrates. Other anatomists con-
sider them as completely degenerate vertebrates. The evident
degeneracy of the myxinoids as regards the branchial basket,
the eyes, and the fins is used as evidence for the latter theory.
The research on the ostracoderms, on the other hand, gives
weight to the older theory.

The fact that all known ostracoderms had some form of
skin, or dermal, bone indicates that these structures may have
once existed in all cyclostomes and have been lost in the living
orders. According to this theory the ostracoderms lost their
free-swimming ability with the development of armor and be-
came extinct.

34 STRUCTURE OF THE VERTEBRATES

Although the living cyclostomes are far removed from the
fish the lines of evolution evidently having split far back m
the 'history of the vertebrates, certain of the ostracoderms are
sufficiently generalized to have been the ancestors of the more
highly developed fish.

CHAPTER IV
FISHES

The Class Pisces, or fishes, is the largest class in number of
genera. With a few exceptions they are completely aquatic in
habit, and are distributed throughout the world in both fresh
and salt water. In their anatomical characteristics they show
a wide variation, as would be expected from their wide geograph-
ical distribution, and the enormous length of their racial his-
tory. The group may be defined as aquatic vertebrates with
fins, gills, and dermal scales. The fish evolved from a cyclostome
type of ancestor at such an early period in vertebrate history
that they may almost be considered as parallel lines of develop-
ment. The following list of characters will help to show their
advance over the cyclostomes, and give an idea of their funda-
mental structure.

1. Fins. The fish have two pairs of appendages, pectoral and
pelvic, in addition to the dorsal, ventral and caudal fins. The
fins are characteristic in having the distal (outer) border of
dermal rays, entirely unlike the feet and toes of more advanced
vertebrates. In several highly specialized recent fishes the fins
have been lost. Such cases are degenerative specializations.

2. Jaws. Like all higher vertebrates, the fish have developed
jaws for holding, biting, or crushing food. The jaws are modified
gill arches.

3. Scales. Dermal scales are found in the fish and set them
apart from the cyclostomes. Such scales arise in the mesodermal
tissues of the skin (the dermis) and are developmentally dif-
ferent from epidermal structures. Fish scales vary from minute
bony dermal denticles in the most primitive group of fish to
heavy bony plates, or flexible dermal scales in the typical food
fish. In a number of specialized forms the scales are almost, or

35

36 STRUCTURE OF THE VERTEBRATES

quite, absent. This degeneration of the scales often accompanies
a loss of fins, and is correlated with mud-living habits.

4. Gills. The typical fish has internal gills located in the gill
slits, serving a respiratory function. In two groups of fish,
including the order which is considered ancestral to the higher
vertebrates, the larvae have external gills similar to those of
the amphibia.

5. Vascular System. The heart, a twisted, S-shaped structure,
has one auricle and one ventricle. The circulatory system is
built around the gills, essentially like that of Amphioxus,
although with many modifications,

6. Nervous System. The brain is very simple in character. The
cerebral hemispheres are functionless, but the cerebellum is
well developed. The eyes and organs of balance are typical of
vertebrates from fish up. The latter structure is the only "ear"
of the fish, with the result that none of these animals hear as
it is understood by man.

7. Skeleton. The primitive fish have only a cartilage skeleton,
but in the higher fish this is replaced by bone. The vertebrae
are advanced over the cyclostome condition in having the sepa-
rate cartilages fused to form a solid vertebra surrounding the
endodermal notochord. In all fish the notochord is constricted
by the vertebrae, and in the higher forms is entirely obliterated
in the adult.

The class is divided into three Sub-classes.

I. The Elasmobranchii, or "plate gilled" fish, which includes
the sharks, skates and rays, and are the most primitive fish
still existing.

II. The Teleostomi, or fish with a complete, terminal mouth.
It includes practically all the food and game fish — all the fish
known to most students, with the exception of the elasmobranchs.

III. The Dipnoi or lung fish. The name, "double breathing",
refers to the fact that they have gills and a lung-like swim
bladder. At present there are only three genera in the tropical
regions of three continents.

Elasmobranchii. The elasmobranchs are characterized by a
skeleton made entirely of cartilage; and although the group

STRUCTURE OF THE VERTEBRATES 37

includes the largest known fish, some reaching a length of sixty
feet, even these animals lack bone in the skeleton. There are,
however, bony scales as outgrowths of the skin. These are the
forerunners of the dermal or skin plates of the higher groups.
The teeth, which are found first in the elasmobranchs, are con-
tinuous with the scales, and are modified dermal denticles. The
mouth is ventral in position.

The sub-class is divided into several extinct orders, and two
recent orders. Of the former, only the type genus of one order
will be discussed in this text. This is Cladoselache, an ani-
mal that gives an indication as to the evolution of the fins. One
of the living orders (Holocephali) may be considered an end
group and far from the line of evolution of the higher types.
The remaining order, the Selachii, are divided into two sub-
orders, one including the sharks, the other the skates and rays.

The elasmobranchs, like the higher fish and the reptiles, have
two methods of reproduction. The eggs are large yolked and fer-
tilized internally. However, some of the selachians secrete a
horny shell around the egg and discharge them into the water.
These elasmobranchs are oviparous, or egg laying. Other mem-
bers of the order retain the eggs in the oviducts until the embryo
is capable of independent existence, at which time the young
fish is discharged into the water. This condition is called ovo-
viviparous to distinguish it from the viviparous development of
the mammals. In ovo-viviparous animals the young depends
upon the yolk for its nourishment, and grows by drawing on
the food stored in the egg. The mammalian development will be
discussed under the heading Mammalia.

The skates and rays are a specialized group, and are of inter-
est to the naturalist as showing the adaptive radiations of the
order. An almost complete series can be made showing the
course of evolution from the sharks. Beginning with a shark-
like skate, these animals have become flattened dorso-ventrally
and proportionately widened. In this process the pectoral fins
have become fused to the body w^all, and the tail has degen-
erated until it is only a whip-like structure. Specimens of the
ray have been captured measuring more than twenty-five feet
across. These fish are an interesting offshoot of a primitive
stock.

Prihiitive Shark

Primitive Shark
(Chlamydoselachus)

Fig 9 Adaptive Radiations of the Elasmobranchs. The types illustrated

are all recent forms. The arrows indicate the two mam Imes of evolution

and the probable course of speciahzation.

STRUCTURE OF THE VERTEBRATES 39

The sharks are the most primitive fish known, some of them
having six or seven gills. The most anterior slit has become
modified in the elasmobranchs and a few higher fish into a
small opening, the spiracle. This structure is of importance in
the evolution of the land vertebrates. With the exception of
these two families, the fish have five gills.

The dogfish, a small shark, is an ''epitome of vertebrate
anatomy", and is usually used in courses of comparative anat-
omy for dissection.

1. External Anatomy. The dogfish has a fusiform body adapt-
ing it for rapid locomotion in the water. A pointed rostrum
projects in front of the mouth; and at the end of the tapering
body the vertebral column turns up to form an asymmetrical,
heterocercal, tail. The pelvic fins are in the primitive position
immediately anterior to the anus; and, with the pectoral fins,
are used as balancing organs in swimming. The body is covered
with dermal denticles which have a wide basal plate with a
spine which projects through the skin to the outside. The cloaca
is the outlet of the digestive and urino-genital ducts.

2. Internal Anatomy. The chondrocranium is roofed and en-
closes the brain. The jaws are not fused to the chondrocranium
but are suspended by ligaments and cartilages. The pharynx
is large. As the food is swallowed whole the stomach carries on
a very large part of the protein digestion, and the absorptive
small intestine is highly coiled and fused to form a spiral valve.
The oviducts, into which the eggs pass from the ovary, are large
and vascular. As the eggs remain in the oviducts until the
young have absorbed the yolk mass, the vascular supply un-
doubtedly assists in respiration and the removal of waste
products.

Teleostomi. The name refers to the fact that the mouth of
these fish, like that of the higher vertebrates, has the jaws en-
cased in bone, with the upper jaw fused to the skull. This
makes the upper jaw immovable and is a definite advance
over the elasmobranch condition. In this group the dermal
denticles have developed into dermal plates or scales, and in
the head region the plates cover the chondrocranium as the
dermal bones of the skull. The sub-class is very large and

40 STRUCTURE OF THE VERTEBRATES

ordinal differences will be considered in the following discussion
of group characteristics.

Order Chondrostei, the most primitive of the teleostome fish,
include the sturgeons and the spoonbill fish. The latter are
found in the Mississippi valley and in China. The former are
found in the fresh and salt waters of the northern hemisphere.
Caviar is the egg roe of the sturgeon, and comes from Russia,
the Great Lakes region of America, and Alaska.

The sturgeon resembles the shark in many respects. The
rostrum projects in front of a ventral mouth; the tail is
heterocercal ; the digestive tract has a spiral valve; the skeleton
is cartilage; and, in the type genus, the spiracle is retained. The
bony plates cover the entire head, and the ordinal name refers
to the fact that there is a perfect cartilaginous chondrocranium
encased in bone. Posterior to the head the plates are arranged
in five rows, one dorsal, and two on either side. Between them,
and on the lateral-ventral regions, are many dermal denticles.
There are no teeth in the recent forms, though the fossil ances-
tors had a proper supply.

This group of fish developed a swim bladder, a structure of
great phylogenetic interest. The bladder arises in the animal as
a dorsal outpocketing of the end of the pharynx, and the con-
nection with the digestive tract is maintained throughout life.
It can be inflated or deflated, serving as a hydrostatic organ by
lessening the specific gravity of the fish; and in poorly aerated
water it has a respiratory function.

Order Holostei. The holosteans are fish with an entire bony
skeleton, but with sufficiently typical characters to set them
apart from the following order, although the two merge almost
imperceptibly. The most primitive recent form is the Gar-pike
(Lepidosteus, not related to the pike or pickerel) which has
bony scales covering the body in a tile-like manner. The
rhomboidal scales fit edge to edge, and do not overlap as in
most fish. The teeth are well developed; the tail is partly
heterocercal; the spiracle is only a pit; and the digestive tract
retains many primitive characteristics. The swim bladder re-
tains its connection with the gut and has a vascular supply.
The Bowfin has over-lapping scales, and in all other structures
shows the line of evolution toward the following order.

Siad (Teleostei

Salmon (Teleostei)

Amia (Holostei)

Gar-pike (Holostei)

Sturgeon (Chondrostei)

Fig. 10 Types of Bony Fish. The Sturgeon is the most primitive living

bony fish. The drawmgs show the change of the tail from heterocercal to

homocercal, and the shift of the pelvic fins to an anterior position

42

STRUCTURE OF THE VERTEBRATES

Order Teleostei form the largest group of the fish, and express
the characteristics which one thinks of as "fishy". They include
all the common food and game fish, from the more primitive
Tuna to the highly specialized globe-fish and the sea horse.
Though they lie entirely out of the line of evolution of the higher

■ Notochord

Fig. 11. Tail Skeleton of the Stur-
geon. The notochord extends almost
to the tip of the tail and the radial
cartilages are practically equal on
dorsal and ventral sides. A typical
heterocercal tail.

Notochord

Fig. 12. Tail Skeleton of the Gar-
pike. The notochord has shortened,
and the ventral radials have elon-
gated, giving a homocercal appear-
ance.

Fig. 13. Tail Skeleton of a Teleost
Fish. The notochord has been oblit-
erated and the caudal bones are al-
most homocercal. Other teleost fishes
carry the modification further.

groups, there is no more perfect order for a study of adaptive
radiations and specializations. Technically they are divided into
thirty or more orders, and numerous sub-orders; and there are
more than two hundred families.

The scales typically overlap and have lost the heavy bony
structure which is found in the more primitive forms. The tail

Stickle back

Fig. 14. Adaptive Radiations of the Teleostei. The primitive teleosts
resembled the Salmon, Fig. 10, and evolution has proceeded in many direc-
tions. Several of the modifications are illustrated.

44

STRUCTURE OF THE VERTEBRATES

is normally homocercal, although it may assume weird propor-
tions in some families. The mouth is terminal and teeth are pres-
ent except in a few families. In the most primitive family the
swim bladder retains a small connection with the gut. In the
others the connection is lost and the organ serves only a hydro-
static function.

Order Crossopterygii, which today is limited to a very few
genera in the Nile, Niger and Congo rivers, was once a widely
distributed and flourishing group. It is of particular interest to
the anatomist and evolutionist as it is now agreed that the

cm

ipiracle

Polypterus larva with
External Gills

Polypterus (Crossopterygii)

Fig. 15. A Recent Crossopterygian Fish. The larva has the external gill

and assumes the foot position typical of the amphibia. The order is

characterized by the flesh}' lobe of the pectoral fin.

amphibia arose from this order of fishes. In fact, by some anat-
omists, it is considered the stem form from which all the Tele-
ostomi evolved. The name means ''fringe fin", although "lobe
fin" is more descriptive of the fin condition. Instead of a fin sup-
ported by three parallel cartilages or bones, these animals have
appendages with a fleshy lobe surrounded by a fringe of dermal
rays. The recent forms have a rather degenerate skeleton in the
lobe, but their fossil ancestors had three large bones and several
small ones, which are clearly homologized with the humerus,
radius and ulna, and carpels of the land animals.

The head plates strongly resemble those of the earliest am-
phibia. The primitive crossopterygians had bony plates on their

STRUCTURE OF THE VERTEBRATES 45

bodies, although the recent forms have overlapping scales. The
spiracle is retained. The swim bladder is a ventral outpocketing
of the gut, and may be either one or two lobed. The bladder is
highly vascular, the blood supply coming from the last (sixth)
gill artery in much the same way that the lungs of the higher
forms is supplied.

Aside from the fossil and anatomical evidence, the larva of
the recent forms shows relationship with the amphibia. Unlike
most fish, the crossopterygian hatches as a larval form with
external gills in addition to the normal internal. In this they
resemble the amphibian tadpole. Equally as interesting is the
fact that the larva settles to the muddy bottom with its front
fins drawn under the body, using them as a support, not merely
as a balancing organ as does the typical fish. After metamorphos-
ing from the larva to the adult type, the fish makes constant use
of its respiratory swim bladder, for its normal habitat is in the
muddy rivers and lakes which tend to become de-oxygenated
and unhealthy for gill breathing animals. Due to its importance
in the scheme of human evolution, frequent reference will be
made to this order in the later chapters.

Dipnoi. This small group has been given equal rank with the
two great sub-classes, although the living representatives are
limited to three genera. This is on account of their wide diver-
gence anatomically, and their ancient lineage as a widely dis-
tributed, definite, easily defined division of vertebrates. All re-
cent forms live in tropical regions which have seasonal periods
of rain and drought, one in Africa, one in Australia, and one in
South America. Most anatomists agree that they are a group
which has paralleled the amphibia in evolution; a trial, and it
appears now largely an error, on the part of nature in evolving
land animals.

From earliest times these animals were highly specialized as to
teeth and fins, two characters which effectively prevented further
evolution toward land life. The eggs hatch as larvae with ex-
ternal gills. During larval life the lungs develop from a ventral,
paired, highly vascular swim bladder. These lungs are more
lobulated, larger, and more efficient than those of most amphibia.
This is an adaptation to the dry seasons during which the

46

STRUCTURE OF THE VERTEBRATES

animals burrow into the mud and aestivate, a condition physi-
ologically similar and seasonally opposite to hibernation. As the
water in the lakes recedes the fish burrows into the ground,
secretes a mucous capsule around itself, and remains there un-
til the return of wet weather. Normally this is not more than six
or seven months, although specimens have been kept in the
laboratory encased in dry mud for more than eighteen months
without any marked loss in weight. On being broken out of the
capsule and placed in water, respiration begins in a few seconds,

Lepidosiren larva

Lepidosiren (Dipnoi)

Fig. 16. Recent Lung-fish. Ceratodiis is more primitive. Lepidosiren has
degenerate fins and an elongated body. The specimen illustrated has the
vascular outgrowths which appear on the pelvic fins of the male during

breeding time.

and the fish is apparently none the worse for the experience.
These modifications have gone so far in the South American
form that the animal is incapable of supplying itself with suffi-
cient oxygen through its gills, and drowns when kept immersed
in water. During the breeding season the male of this genus
(Lepidosiren) develops accessory gills on its pelvic fins, and is
thus able to remain under water and guard the eggs.

STRUCTURE OF THE VERTEBRATES 47

Evolution of the Fish. The student should keep in mind that
no living group of animals can be the direct ancestors of an-
other. From this standpoint '4iigher" and ''lower" are rather
meaningless. The terms are based on the human concept of
superiority, and to a large extent refer to the evolution of the
nervous system; although we may say that '^higher" animals are
those which have evolved farthest from the fundamental, primi-
tive characteristics.

We may also logically speak of the fish ancestors of men, not
meaning any living group, but some common ancestral type
which evolved in several directions. One group of fish became
better adapted for certain aquatic conditions, another for land
conditions; or, stated differently, a group which was generalized
had a far better chance of developing in several directions than
did a more specialized type.

All the above conditions are abundantly illustrated by the
fish. The earliest ancestor was an amphioxus-like chordate. From
this animal numerous varieties developed, the recent Amphioxi
having remained most nearly like the ancestral form. Another
variation from the original ancestor became more cyclostome in
structure, and from this group new variations appeared. Those
which most resemble the ancient stem form are the Cyclostomes,
while other groups which added new^ characters enabling them
to cope better with environmental conditions evolved into primi-
tive fish.

All the earliest known fossil fish belong to the elasmobranchs,
and there again we see evolution proceeding in many directions.
The generalized sharks were not only capable of surviving in
more varied environments, but were more liable to develop new
mutations having a survival value.

Some of the adaptations have been mentioned. Those which
tended toward respiration and locomotion on land, as did the
specializations of the Crossopterygii, naturally opened up an
enormous area as a habitation for the vertebrates. They were
poor fish but excellent ancestors; and therefore Darwin's '^sur-
vival of the fittest" has been modified to '^survival of the best
adapted for a particular environment".

CHAPTER V

AMPHIBIA

The Class Amphibia consists of those four-footed animals which
have a double life, one adapted for water living in the larval
stage, and an adult phase when they take on the characteristics
of land animals. The newts, salamanders, frogs and toads are
representative of the class. The living examples make a very
homogeneous group, although the earliest forms resembled the
Crossopterygii on the one hand, and the reptiles on the other.
They are in every respect more closely allied to the fish than to
the reptiles. The most important difference from other land
animals lies in their embryology, the eggs always being laid in
water or moist places, where they hatch as gilled larvae.

The first amphibia varied from their fish ancestors in several
respects, most largely in the development of four walking ap-
pendages with toes. The larva retained the external gills and an
internal anatomy so like their ancestors that the tadpole is es-
sentially a fish in structure and mode of life. At metamorphosis
the early amphibia evidently lost their gills and breathed with
lungs, but the fossil evidence indicates that they remained mud-
living animals. The bones of the skull and the dermal covering
of the body were essentially similar to those of the fossil Cross-
opterygii. The spiracle, which is found in the ancestral fish, is
modified in the amphibia into the middle ear. As can be seen
in a frog, the middle ear is a slightly enlarged cavity lying next
the internal ear, and covered on the outside by a skin membrane
(the tympanum or ear drum). The wide inner part opens directly
into the pharynx.

The recent amphibia can be identified by their smooth skin,
usually well supplied with mucous glands; and, except in forms
with degenerate legs, by four front toes and five on the hind legs.

4S

STRUCTURE OF THE VERTEBRATES

49

Their nervous system shows definite advances over the fish type,
except for the cerebellum which is poorly developed in recent
amphibia. The vascular system has several important structures
added which in later evolution became of supreme importance.
However, the system is poorly adapted for land life, and the
steady disappearance of species as soon as more efficient land

Primitive Extinct Amphibian
(Stegocephalia)

Fig. 17. Types of Amphibia. The extinct Stegocephaha gave rise to the

recent orders.

50 STRUCTURE OF THE VERTEBRATES

structures developed shows that the class was poorly adapted
for land existence, and could not stand the competition with the
reptiles which evolved from them.

Order Stegocephalia. For simplicity these fossil amphibia
have been classified as one order. The Stegocephalia vary widely
in structure, the earliest known form (Archegosaurus) having a
skull with several bones found nowhere else but in the fish. Dur-
ing the years when there was no competition on land the am-
phibia multiplied and evolved in many directions. They were
well encased in an armor of bone, strong enough to protect them
from each other and from any predaceous fish; but too heavy to
permit of great activity. The evidence indicates that the primi-
tive amphibian foot had six toes, which soon became reduced to
the typical five of the land vertebrates. Some of the Stegocephalia
have a further reduction on the pectoral foot, like that of recent
forms.

Order Apoda. These are legless amphibia. Their burrowing
habits are their only defense against more active animals and
came with the loss of appendages. In fundamental structure they
are the most primitive of living amphibia. Their skull bones are
less modified than those of other orders; they have minute
dermal scales beneath the skin; and their development is an
important link in comparative embryology. The order is rather
widely distributed in the sub-tropical regions of the earth.

Order Urodela. The urodeles, as the name implies, are the
tailed amphibia, and in body shape are more nearly like the-
Stegocephalia than any other recent order. They may be com-
pletely aquatic, mud-living, or intermediate in their habits. None
survive in very dry regions, nor in the tropics.

The largest living amphibian is Cryptobranchus of China and
Japan which attains a length of more than six feet. A closely
related species of the same genus inhabits the INIississippi valley.
Other large forms live in the southeastern part of the United
States. These are very specialized, particularly as to gills and
appendages.

]\Iost urodeles are small and lack gills in the adult. These are
the newts and salamanders which can be found in streams and
muddy places in most parts of the Northern Hemisphere. Some
of the genera are highly modified, one or two lacking both gills

STRUCTURE OF THE VERTEBRATES

51

and lungs and breathing entirely through the skin. Others re-
tain the external gills throughout life.

Certain of the urodeles remain in the larval condition, reach-
ing sexual maturity without metamorphosis. This permanent

Bufo, a toad
(Anura)

Cryptobranchus (Urodela)

Fig. 18. Tj^pes of Amphibia. Cryptobranchus and Amphiuma are the
largest living American amphibians. The frogs and toads are liighly

specialized forms.

52 STRUCTURE OF THE VERTEBRATES

larval condition is called paedogenesis and was discovered in the
Axolotl of Mexico. The animals had been classified as a separate
genus and family, but when the environment was changed
the young metamorphosed into a well known salamander (Am-
bly stoma). Many students of the subject today interpret the
retention of external gills in other forms as paedogenesis, al-
though experiment has so far failed to confirm the theory.

Order Axura. The tailless amphibia are the frogs and toads,
and are distributed throughout the world. They are in many
respects the most specialized living amphibia. j\Iost of the genera
are capable of living their adult life away from water. This is
particularly true of the toads, whose warty skin, containing
relatively few mucous glands, adapts them for life in semi-arid
regions. But although the adult may live in her burrow on a dry
hillside, the eggs are invariably laid in water where they hatch
as tadpoles. In the smaller toads the larval life is very short
and they will hatch into tiny toads in a few weeks, giving rise
to the myth that it rains toads. Other myths relating to their
longevity are also without foundation. They may live for long
periods without food or water, but to believe that they are found
embedded in solid rock demands credulity rarely found in a
scientist.

Evolution of the Amphibia. Almost from the beginning of
amphibian history two lines of evolution are apparent. Some
families retained rather generalized amphibian characteristics,
and probably gave rise to the amphibia of today; other families
became highly specialized with enormous heads, or other struc-
tures which eventually rendered them unfit for survival in a
changing environment; and others assumed characteristics which
appear reptilian. Some of these pro-reptilian forms were un-
doubted amphibia (Eryops, for example) for the skeletons of
gilled larvae have been discovered.

Embryology of the Frog. The frog has a very generalized
development, more primitive than the recent fish. For this reason
it is selected for discussion in most texts, as it logically follows
the developmental processes of Amphioxus. The Apoda have a
development which verges toward the reptilian and, as was men-

STRUCTURE OF THE VERTEBRATES 53

tioned, is used to illustrate an intermediate stage between the
amphibia and the higher vertebrates.

The student is undoubtedly familiar with the processes of
maturation of the egg and sperm. The mature eggs of frogs and
toads are fertilized by the male at the time of deposition, before
the thin coating of gelatinous material swells by the addition of
water. The toad lays its eggs in a string, the frog and salamander
eggs are laid in masses. The frog egg is spherical, about one
millimeter in diameter, and has more inert yolk material than is
found in Amphioxus. These yolk granules are also more concen-
trated at the ventral pole. The plans of the first two cleavages
are dorso-ventral, and that of the third is transverse. This is
similar to the first three cleavages of Amphioxus, with the slight
exception that the third is more asymmetrical, the dorsal four
cells of the frog being decidedly smaller than the ventral. The
general principle that a cell loaded with inert matter cleaves
more slowly than one with relatively more protoplasm has been
stated; therefore in the frog cleavages after the fourth proceed
more rapidly in the dorsal region of the embryo. The result is a
blastula with the segmentation cavity entirely in the upper half
of the egg.

Gastrulation in Amphioxus was described as due to both in-
vagination and the overgrowth of the rapidly dividing dorsal
cells. In the frog invagination is largely suppressed, and the
latter process greatly increased. This overgrowth is more rapid at
the anterior end. The student can visualize the process if he will
think of the equator of the egg remaining static while the
rapidly dividing upper cells flow ventrally as a double layer,
more rapidly on one end than the other. As this two-layered
structure approaches the posterior pole the opening tends to
constrict; and when gastrulation is complete, only a narrow
aperture is left as the blastopore. From this it is clear that most
of the endodermal lining of the archenteron has been derived
from the dorsal pole cells. The remainder of the endoderm de-
veloped from the invaginating ventral pole cells. During gas-
trulation the segmentation cavity was practically crowded out
of existence.

At the completion of gastrulation the process of elongation
begins, and then the dorsal neural groove is formed. The hollow

54 STRUCTURE OF THE VERTEBRATES

nerve cord develops by the dorsal and median growth of the
neural folds; but the frog differs from Amphioxus, and agrees
with all other vertebrates, in having the folds meet at the mid-
dorsal line before the neural plate breaks away from the epi-
dermal cells.

2 Cells h Cells 8 Cells

Cleavage Stages of Frog's Egg

16 Cells

32 Cells

Blastula Beginning of Gastrulation

Longitudinal Sections of Early Embryo

Blastopore!

Longitudinal Sections : Completion of Gastrulation

Late Larval Stage

Fig. 19. Embryology of the Frog. The diagrams show the holoblastic

cleavage and gastrulation. Gastrulation is almost entirely an overgrowth

of the dorsal pole cells.

STRUCTURE OF THE VERTEBRATES 55

The stages in the development of the notochord are telescoped.
Instead of infolding as in Amphioxus, the endodermal cells mul-
tiply rapidly along the dorsal line of the archenteron and split
off to form the continuous rod of notochordal tissue. The 7nes-
oderm also goes through a compressed development. Unlike the
metameric enterocoels of Amphioxus, the frog mesoderm arises
as a proliferation of cells on either side of the notochord, the
continuous band of tissue soon dividing into metameric blocks.
A cavity develops within each mesodermal mass, homologous
with the enterocoelic cavities of Amphioxus. Later development
of the organs is sufficiently similar to that of Amphioxus not to
need further discussion in a text on anatomy.

As the embryo grows the nervous system and the organs of
special sense develop. The mouth breaks through, and the em-
bryo, now a young larva, ceases to depend upon its yolk mass
for food and begins eating. Up to this time respiration has been
through the epidermis to the blood of the already functioning
vascular system, but as the larva develops in mass gills develop
in correlation with changes in the branchial arches. These gills
are external and three in number. In time the external gills are
lost, internal ones having developed within the gill slits, and the
frog tadpole pursues a fish-like existence in the water. Hind legs
make their appearance, getting larger as the tadpole grows older.

The larval stage varies in length in different species, lasting
from six weeks in the wood frog to as much as three years in the
bull frog. Temperature causes a variation in time, heat speed-
ing metamorphic processes. When the tadpole approaches meta-
morphosis it stops eating; the front legs, which have been en-
closed in an atrium analogous with that of Amphioxus, break
through; the gills are lost and the animal makes more use of its
lungs; and the tail is absorbed and used for food. When meta-
morphosis is complete the young frog hops out on land, weighing
approximately half as much as did the tadpole. From this time
until maturity development is only a matter of growth and dif-
ferentiation of certain organs, as in the land vertebrates.

CHAPTER VI

REPTILIA

The reptiles are represented today by the turtles, alligators,
lizards, snakes, and a small group of New Zealand reptiles. These
are divided into five orders. The numerous fossil orders and the
recent reptiles form four distinct sub-classes. Three of these are
represented today. Most of the reptiles disappeared after the
mammals arose and the reptiles ceased to rule the land areas of
the earth. Reptiles can be identified in the field by their ecto-
dermal scales which give the animal a rough feeling when the
hand is drawn from the tail toward the head. There is no truth
in the belief that snakes and lizards are slimy, their skins being
remarkable for the lack of integumentary glands.

Anatomically the living reptiles vary widely from each other,
and from other living groups. The great gulf which separates
amphibia from reptiles is based upon their embryology. All
reptiles lay large yolked eggs (which may be retained in the
oviducts until time of hatching from the egg membranes) and
the young begins life as a small adult. There is never a larval
period; and though the reptiles, like all other vertebrates, have
gill slits during the embryonic period, there is never a sign of
gills. Embryonic respiration takes place through the shell, and
the blood is oxygenated through a specialized breathing structure,
the allantois, which is an outpocketing of the posterior end of the
primitive gut and disappears at the time of hatching. Reptiles
also develop an amnion, an enveloping, fluid-filled, protective
embryonic membrane. This is in correlation with their land life,
the surrounding fluid acting as a buffer against any chance blow
and preventing drying of the embryo. These membranes are also
found in the birds and mammals, with the result that all three
of the higher classes of vertebrates are included together as the
Ajnniotes.

56

STRUCTURE OF THE VERTEBRATES 57

Sufficient identification of the class Reptilia is: (1) the cover-
ing of ectodermal scales; (2) large, yolk laden eggs; (3) a vari-
able temperature, depending to a large extent on the environment ;
and (4) the allantois and amnion. Other differences will become
clear in later discussions.

For a study of mammal evolution, only the first three of the
following orders are necessary. The others will assist in under-
standing the general scheme of vertebrate evolution. The first
order is usually studied in anatomy courses as representative of
the earliest reptiles. The other two are extinct ancestral stocks
from which the mammals evolved.

Order Chelonia. The turtles, tortoises and terrapins are the
most primitive, and at the same time the most superficially spe-
cialized of living reptiles. They are easily identified by their
bony covering, forming a dorsal and a ventral ''shell". This is
composed of dermal plates which become fused with the ribs, and
serves as a protection to otherwise inefficient, lethargic animals.

The skull shows relationship with the earliest reptiles, and this
is corroborated by the simple development of their vascular and
nervous systems. The group, in their present form, is very an-
cient; and during their history they have become adapted to al-
most every environment. Some inhabit dry, sandy regions, their
burrows giving them the popular name of "gophers"; others, like
the Box Tortoise, are found in more or less moist regions; the
small pond turtles and the terrapins are largely aquatic; and
the Sea Turtle is so highly modified for life in the ocean that it
rarely, or never, leaves the water except at breeding time. In
this the reptiles reverse the breeding habits of the amphibia. No
matter how specialized a turtle is for aquatic life, it always goes
to the land to lay its eggs. The degree of specialization for water
life can, within limits, be determined by the cross section of its
shell: the deeper the shell in proportion to its width, the greater
its adaptation for land; and conversely, the flatter the shell, the
more modified for water.

In size the turtles vary through equal limits. The largest living
specimen is the Galapagos tortoise which is found only on the
barren islands off the coast of South America. But even this huge
chelonian is dwarfed by some of the fossil specimens which have
been found in India and other regions.

58

STRUCTURE OF THE VERTEBRATES

The breeding habits are rather consistent, all turtles laying
their eggs in the sand and depending on the sun's heat to hatch
them. There is no evidence of care of the young, the newly
hatched animal having to depend upon its own resources for food
and protection.

2^^

Seymouria (Primitive Extinct Reptile)
Reconsitruction

i

Sea Turtle

Fig. 20. Anapsid Reptiles. Sej-mouria (at top) represents the most primi-
tive reptiles. The turtles and tortoises are specialized representatives of
the Sub-class Anapsida,

STRUCTURE OF THE VERTEBRATES

59

:^Iammal-like Reptiles. These animals are all extinct and
known only as fossils. Two orders are recognized, which are
classified together as the sub-class Synapsida; and from the
standpoint of human evolution are the most interesting of the

Order
Theromorpha

Reconstructions of two
Extinct mammal-like
reptiles

Fig 21 At the top are extinct mammal-like reptiles (Sub-class Synap-
sida) which gave rise to the Class Mammalia. The other three are recent

Diapsid reptiles.

60 STRUCTURE OF THE VERTEBRATES

reptiles. They branched directly from the early type, of which
the turtles are the only living representatives. Some of these
synapsid reptiles carried their legs straighter under the body,
not in the typical reptilian position, and the skull assumed
mammal-like proportions. Skulls of these orders are illustrated
on page 136.

Order Theromorpha. The Theromorpha were reptiles of more
or less ''mammal form". It is the more primitive of the two
orders of synapsid reptiles.

Order Therapsida. The mammal-like characters which showed
their beginning in the former order were carried to their logical
conclusion in the Therapsida. In size they varied from that of
a rat to animals as large as a bear. The legs were held in the
mammal position, with the elbows against the body instead of
at right angles ; and the skull was almost completely mammalian.
By comparison with existing orders of reptiles and mammals,
they are much nearer the latter than the former.

Evolution of the Reptiles. The first reptiles had a body
form resembling a shortened lizard and came directly from the
amphibian stock. With their adaptations for land life they soon
migrated widely and began their own adaptive radiations as
the masters of the earth. These early ancestral reptiles and the
turtles together form a sub-class, the Anapsida. From this primi-
tive stock arose three lines of evolution, giving rise to the three
other sub-classes of the Reptilia.

1. The Synapsida, or mammal-like reptiles, are known from
slightly earlier times than the following groups, although it is
probable that the three lines arose almost synchronously.

2. The Parapsida include the snakes and lizards of the present.
Several fossil orders are known. The snakes are the most spe-
cialized living reptiles.

3. The Diapsida include the Crocodilia (crocodiles and alli-
gators), Rhynchocephalia, and the extinct pterodactyls and
dinosaurs. In the great number of forms which have evolved, this
is the largest of the reptilian groups, and the dominant animals
during the Age of Reptiles. The birds (Class Aves) evolved
from a primitive diapsid stock.

Order Rhynchocephalia. The order is represented today by

STRUCTURE OF THE VERTEBRATES 61

one genus (Sphenodon) found in New Zealand. It is an ideally
generalized reptile, having been described as a ''living fossil".
In its skeletal structure and body shape it is the last remaining
specimen of a lost race.

Order Crocodilia. This order includes the alligators of the
United States and China; the crocodiles, which are distributed
throughout the warmer regions of the earth; the caimans of
Central and South America; and the gavials of southern Asia.
These animals are carnivorous in their habits, and in certain
regions are dangerous to man. This is particularly true in the
Ganges River.

This is also an ancient order, related to the dinosaurs. The
skull and bony skeleton are generalized; the back is well sup-
plied with dermal plates which are hidden under the ectodermal
scales; and the soft anatomy is typically reptilian, though
highly modified. The brain is the best developed among the rep-
tiles, and the heart is four-chambered.

Their breeding habits are much like those of the turtles, the
eggs being laid in nests near the water. The American alligators
build large nests of sticks or mud, and after the eggs are laid
they are covered by the mother and left for the atmospheric
heat to hatch them. They grow slowly after hatching, and have
a long span of life. This characteristic of gr-owth, coupled with
the value of their skin for leather, has rapidly depleted their
ranks.

Order Lacertilia. The lizards, which are included in this
order, are so nearly related to the snakes that they are often
classified together. Most of the lizards have four legs with five
toes on each foot, although some genera have degenerate legs or
lack them altogether. A legless lizard can be distinguished from
a snake by its eyelids, which are lacking in the snakes, and the
small ventral scales. The smaller lizards are popularly confused
with the salamanders, but their scales and their rapidity of
motion make identification easy.

Lizards are either oviparous or ovo-viviparous (page 37).
AA^ith one exception they are non-poisonous, the genus Heloderma
(Gila Monster) of Mexico and the southwestern part of the
United States having poison glands. There is wide disagreement
as to the violence of their poison, but it is doubtful if their

62

STRUCTURE OF THE VERTEBRATES

bite would be fatal to man. The ability to change color is an-
other characteristic of several genera. The best example of this
is the chameleon of Europe, Asia and Africa. The small green
lizard (Anolis) of the southern United States has similar, al-
though less marked, ability to change under differing environ-

Garter

--^^\ Rattle Snake

Fig. 22. Types of Lizards and Snakes (Sub-class Parapsida). The lizards
show the specialization of the order toward leglessness, bony armor, pro-
tective poison, and the running modification.

STRUCTURE OF THE VERTEBRATES 63

mental conditions. The mechanism is similar to that of the tree
frogs, depending upon the shrinkage or expansion of the pigment
cells.

In size the lizards range from a few inches to several feet; a
species recently captured in the Malayan Islands measuring more
than twelve feet. They are carnivorous in diet, the smaller ones
depending upon insects, the larger eating small mammals or birds,
particularly the eggs of the latter.

Order Ophidia. The snakes are legless reptiles. Most genera
have even lost the girdles, although the boas and pythons have
minute vestiges of hind legs. They are all elongated in body
shape, and their skeleton and internal organs are highly spe-
cialized for the condition. The long ribs are attached to the
ventral plates. Locomotion is attained by the motion of the ribs
and by lateral coiling, bracing, and pushing forward. There are
no records of snakes moving by dorso-ventral coiling.

The skull is the most specialized among the reptiles. The bones
of the mouth and skull are so arranged that the jaws can be
stretched downward and laterally to enormous limits. A small
boa, with a transverse head measurement of a centimeter, is able
to swallow a grown mouse. The soft anatomy is almost as modi-
fied. Usually there is a single, highly elongated lung, and the
digestive glands and tract are built on the same long lines. The
skin has few glands, but most snakes have anal glands which
secrete fluids of such distinctive odors that a number of genera
or families can be identified by the scent. Like the lizards, the
snakes are both oviparous and ovo-viviparous, the latter having
given rise to the myth that snakes swallow their young in times
of danger.

The poison glands are highly developed in a number of snakes.
In correlation with this there are modifications of certain teeth,
the fangs. These may have simply a groove down one face, or the
edges of the groove may grow over to form a hollow tube. The
glands lie in close contact, and usually open into the cavity of
the tooth. The result is a natural hypodermic syringe, the poison
being forced under the skin. There is no sure rule for telling
poisonous from non-poisonous snakes except by close inspection,
and this is often inconvenient in the field. In the United States
the only poisonous snakes of any importance are the rattle-

64 STRUCTURE OF THE VERTEBRATES

snakes (Crotalus) ; the ''copper head" and the "cotton mouth
moccasin", the last two belonging to the same genus (Agkis-
trodon) .

The smallest snakes are only a few inches in length, the largest
more than thirty feet. The latter are the python of the Old
World, and the anaconda of America. Both are tropical in range,
and not poisonous. They throw coils around their prey and con-
strict until the animal is dead.

Embryology of the Reptiles. Fertilization in the reptiles is
always internal, and takes place before the membranes and shell
are laid around the yolk. As the egg approaches maturity in the
ovary yolk granules develop, and when the egg is mature it is
large and heavily loaded with yolk. This inert material is cen-
tered at the ventral pole, with the active protoplasm only a small
drop located at the dorsal side. It is apparent that the pro-
toplasm cannot exert sufficient force to cleave the entire mass,
and the result is that the cleavages are limited to the drop of
protoplasm. This type of cleavage is called merohlastic, as op-
posed to the holoblastic cleavage of eggs which divide entirely,
as illustrated by Amphioxus and the frog.

The first and second cleavages cut the protoplasm, or germinal
disc, at right angles to each other. But the protoplasm is so thin
that transverse cleavage is impossible, and the third cleavage is
in the same direction as the first two. The result is that the eight
cells are in one horizontal plane. Future divisions continue to
cut the disc, until a blastula is formed as a flat plate of cells
spread out over the yolk. The student can visualize this and
later processes by breaking a fertilized hen's egg and looking
for a white spot about three millimeters in diameter on top of the
yolk. If the slightest bit of red appears it shows that the blood
vessels have developed and the embryo is fairly well advanced.
The average kitchen is an excellent embryological laboratory if
a hand lens is available.

Gastrulation is due to the ventral growth of cells at one point
on the blastula. Comparing Amphioxus and the frog, it will be
seen that gastrulation in the latter proceeds almost entirely by
the overgrowth of dorsal pole cells, invagination of the ventral
cells playing a minor part. The reptile and bird have carried the

STRUCTURE OF THE VERTEBRATES

65

process to the extreme limit, the ventral yolk being non-cellular
and serving only as a supply of food.

At the beginning of gastrulation the blastula is a plate of
cells, with an inner thicker portion which gives rise to the em-

FiG. 23. Adaptive Radiations of the Reptilia. A group of extinct reptiles
showing a few of the major specializations.

66 STRUCTURE OF THE VERTEBRATES

bryo, and an outer ring of cells which are beginning to spread
around the yolk as extra-embryonic tissue. At the edge of the
inner disc a ventral inpocketing of the marginal cells begins,
and by continued growth folds under and spreads anteriorly
and laterally, paralleling the original blastula cells. The point of
inpocketing is homologous with the blastopore.

Thus the gastrula is composed of two parallel, flat plates of
cells, the original plate being the ectoderm, the ventral layer
next the yolk being the endoderm. The slight cavity between the

Blastodisc-

First

Second

Fourth

Top View of Egg Long. Section

of Egg

... .^ /Ectoderm

Segmentation cavity / 4 rchenteron
^/ Blastopore ,^^^ /.Endoderm

Yolk

Cleavage of Blastodisc

Extra-embryonic
membranes Embryo

/Amniotic
fold

I

Fig. 24. Early Embryology of the Chick. The egg has a large yolk with a
dorsal drop of protoplasm. Cleavage is meroblastic. The longitudinal sec-
tions (bottom, left) show gastriilation. The diagrams at the right show the
overgrowth of the extra-embryonic membranes, the embryo limited to a
small area at the top.

endoderm and the yolk is the archenteron. It is spread out over
the yolk, and only takes shape with the development of the
embryo.

The neural groove appears as a faint line extending anteriorly
from the blastopore. The median line of cells sinks while the
lateral neural ridges rise toward the dorsal median line, much
as in the frog. Synchronously with the above, the notochord
makes its appearance as a proliferation of endodermal cells,
forming a continuous structure ventral to, and parallel with, the
neural groove.

STRUCTURE OF THE VERTEBRATES 67

The mesoderm follows immediately after the beginning of the
notochord. It arises as two groups of somites on either side of the
notochord, and grows lateralh'. As the two sheets of mesoderm
spread between the ectoderm and endoderm the outer portion
splits (recall the enterocoel of Amphioxus, and the cavity in the
frog mesoderm). As growth proceeds the upper layer of meso-
derm follows the ectoderm, and the lower adheres to the en-
doderm.

The embryo is now taking shape, and as differentiation of
organs proceeds the peripheral cells divide, carrying embryonic
tissues around the yolk. Eventually these membranes of em-
bryonic origin completely surround the yolk mass, the inner
membrane composed of endoderm and one layer of mesoderm
being the yolk sac. The outer membrane is mesoderm and ecto-
derm. However, although these membranes are of embryonic
origin, only the original region forms the embryo, the outlying
tissues being extra-embryonic and are lost at hatching time.

The embryo grows more rapidly at the anterior end, and as
the neural groove closes and enlarges to form the brain it lifts
up from the surrounding tissues. The posterior end then lifts off
and the sides of the embryo bend downward and constrict ven-
trally. These processes lift the embryo from the extra-embryonic
membranes, so that the embryo is raised above the still spherical
yolk. Recall that the ectoderm, mesoderm and endoderm of the
embryo are continuous with the membranes which spread around
the yolk mass, and the archenteron is still open ventrally above
the yolk sac. Anterior and posterior growth accomplishes two
results: (1) the two ends of the archenteron are pulled out so
that a distinct gut is formed; and (2) the original point of con-
nection between the embryo and yolk is left as a circular body
stalk. The connection between the gut and the yolk sac continues
until the time of hatching, but as the body stalk increases only
slightly in diameter and the embryo grows rapidly, the con-
nection is relatively very slight after the first few days.

Amnion and Allantois. These two distinctive structures of
the amniotes (reptiles, birds and mammals) have begun their de-
velopment at this time. Recall that the extra-embryonic tissues,
continuous with those of the embrvo's bodv, form two sheaths

68 STRUCTURE OF THE VERTEBRATES

around the yolk with a cavity between them. The cavity was
formed by the splitting of the mesoderm, the portion lying
within the embryonic body being the coelomic cavity and the
larger outer cavity the extra-embryonic coelom. The two are
connected through the body stalk. The inner sheath or mem-
brane surrounds the yolk as the yolk sac, the endodermal layer
lying next the yolk w^ith the visceral layer of mesoderm next the
cavity. The outer membrane is composed of mesoderm and ecto-
derm, the latter being external.

Ectoderm-

Fig. A

Fls. B

llantois
Yolk sac

Tig. 25. Organogenesis in the Chick. Figures A— E are cross sections

through the embryonic region, the yolk omitted. Fig. F shows a total

embryo, with a hemi-section of the extra-embryonic membranes.

The amnion slightly precedes the allantois in development. It
is formed by two synchronous processes: (1) the embryo sinks
as the yolk materials below it are absorbed and the weight of
the embryo increases; and (2) the extra-embryonic tissue grows
dorsally and medially over the embryo. The anterior region of
the amnion develops more rapidly and the embryo sinks head
foremost. The result is that the amniotic fold progresses posteri-
orly as a crescentic, double membrane. The posterior fold grows
more slowly than the anterior and lateral folds. Fusion eventu-

STRUCTURE OF THE VERTEBRATES 69

ally takes place near the posterior end, and is due to a steady
narrowing of the circular opening caused by the growth of the
folds toward a common point. When fusion occurs there are two
encircling membranes which have developed from the original
outer membrane of ectoderm and mesoderm. (1) The inner is
the amnion, lined with ectoderm which is continuous with that
of the embryonic body, and completely surrounds the embryo
except at the body stalk. Fluids diffuse into the amniotic cavity,
and the embryo continues its development in a fluid medium.
(2) The outer membrane is the chorion and lies next the porous
shell. At this stage the embryo is surrounded by the amnion;
the yolk sac is ventral; and the chorion surrounds the entire
structure.

The allantois begins its development before the completion of
amnion formation. It develops as an outpocketing of the poste-
rior end of the gut and grows outward toward the chorion be-
tween the amnion and the yolk sac, lying in the extra-embryonic
coelom. At this stage the gut is covered by a layer of mesoderm,
and the allantoic outpocketing naturally is covered by a meso-
dermal layer. The allantois becomes enlarged at the outer end,
and as it reaches the chorion (lying against the shell) it spreads
until it almost fills the extra-embryonic coelom.

Blood vessels pass outward from the embryo with the allantois
and come into contact with the porous shell. Respiration takes
place by osmosis through the thin membranes of the shell into
the blood vessels of the chorio-allantoic membrane. The circu-
lating blood transports the carbon dioxide from the embryo to
the distal capillaries, where waste products are given up and
oxygen is received. During embryonic life the aerated blood goes
to the embryo through the allantoic veins and is mixed with the
unaerated blood of the embryo before reaching the heart. The
complete separation of aerated and unaerated bloods does not
take place until after the animal hatches and the lungs become
functional.

Later Growth. After the nervous system and the vascular
systems are laid down, the organs of excretion begin their devel-
opment. The mesoderm gives rise to the muscles, connective tis-
sues and bone, and the glands begin their development. It is a

70 STRUCTURE OF THE VERTEBRATES

differentiation of the primitive germinal layers into tissues and
organs, and their individual histories are traced in Part II.

Continued growth increases the size of the embryo and de-
creases the amount of yolk. At the time of the completion of the
amnion the embryo is a small structure on top of a large yolk
mass; but by the time of hatching their relative sizes have been
reversed and the yolk sac is a small mass on the ventral side of
the young animal. In hatching the shell and surrounding mem-
branes are broken and the latter are sloughed off in a short time.

Laboratory Study. The great majority of laboratories find it
impractical to get reptiles for a study of embryology and use
the chick instead. The bird is essentially similar to the reptile in
its developmental processes, and due to the ease with which fer-
tilized hen's eggs are obtained and hatched in the laboratory,
they make ideal material for a study of meroblastic cleavage
and the principles of amniote development. For this reason the
illustrations used show chick development.

CHAPTER VII

AVES

The birds (Aves) are feathered vertebrates; or, as Huxley ex-
pressed it, ''glorified reptiles". The class evolved from the diap-
sid reptiles, but are far more specialized than any living rep-
tilian group. In fact, it is the most coherent class of vertebrates;
and when considered as an entire class, is the highest major
group of the vertebrates. ]\Ian and certain other mammals sur-
pass them in the specialization of the nervous system, although
the average of the birds is almost as high as that of the lower
mammals.

Birds are warm blooded and have a constant temperature; the
skeleton is light and strong, a modification in correlation with
their flying habits; the heart is completely four-chambered like
the mammals; and the fore limbs are highly modified from the
primitive arm of the land vertebrates.

Little is known about the evolution of the flying mechanism
in birds. Two fossils from the lithographic limestone of Germany
tell something as to the evolution of the birds from the reptiles,
but it is still in doubt as to how flying originated. Various the-
ories have been advanced. The fossils from Germany are called
Archaeopteryx, or "ancient wing", and these fossils indicate that
the early birds were not powerful fliers. Study of the illustration
will show a number of interesting features. The skull is heavy
and reptilian, not light as in living birds; there are conical teeth,
much like those of a reptile; the digits of the wing are well devel-
oped with claws at the end; and the tail is long like that of a
lizard, but with a row of feathers on either side. Evidently this
animal was hardly more than a feathered reptile.

Two later fossil birds are known, which are more distinctly
bird-like in their anatomy. The tail, wing, and hind leg had be-
come modified to the avian condition before the teeth were lost

71

72 STRUCTURE OF THE VERTEBRATES

in the course of evolution. This fact is of interest as no known
living bird has any sign of teeth, the bill being made of thick-
ened epidermis like that of the claws. The so-called "egg tooth"
of the newly hatched bird is an epidermal growth of the bill
and is lost soon after the embryo pecks its wa}^ out of the shell.

The birds of today are divided into two great groups showing
two distinct lines of evolution: (1) the flying birds which have
a keel (carina) on the breast bone, and (2) the running birds
with degenerate wrings and a smooth breast bone. The latter un-
doubtedly evolved from the former, but for ease of discussion
will be considered first.

Ratite birds are those with smooth breast bones, including
several extinct species and the living ostrich, rhea, cassowary,
emu, and the kiwi. The roc of Sinbad the Sailor belonged to
this group, and was an enormous bird which laid an equally
enormous egg. As the skeleton and egg only were known, it was
easy for the layman to draw the conclusion that it would have
great powers of flight, despite the fact that it was a non-flying
species.

The ostrich is a native of Africa, but the animal has been
naturalized in many of the warmer regions of the earth, the
plumes from the farm animals being of better quality than those
taken from wild birds. It stands about eight feet high, with small
wings and powerful legs. The egg has the content of about two
dozen hens' eggs, and is the largest single cell known.

The rhea is South American in distribution. The emu and
cassowary inhabit Australia, and like the former, occupy open
grassy plains. The Kiwi or Apteryx (without wings) is the most
specialized of the ratites, and is confined to New Zealand. The
wings are small boned and are not visible externally. For their
size, these birds lay huge eggs, each being approximately one-
fourth of the body weight.

Carinate birds are world-wude in distribution. In size they
range from the smallest humming bird to the albatross and con-
dor. In flying ability they vary from the non-flying penguins to
the great speeds attained by migratory birds. Modern airplanes
have demonstrated speeds of more than a hundred miles an
hour in some birds, but whether this could be maintained for
long periods is not proved.

STRUCTURE OF THE VERTEBRATES

73

During their evolution the orders of birds have developed a
wide number of adaptive radiations. Some are typically wading
animals with long legs and beaks, as in the cranes, storks and
flamingoes; others are adapted for swimming, as the ducks,
swans and penguins ; the carnivorous birds are adapted for tear-

Flying Specialization
(Carinatae)

Penguin
Aquatic Specialization
(Carinatae)

A large extinct Ratite

Fig. 26. Adaptive Radiations of the Birds. Archaeopteryx was a reptile-
like bird. Recent evolution has proceeded along two major lines, repre-
sented by the Hawk and Ki\vi.

I Hummingbird

Hen Ostrich

Moa (Extinct)

Aepyomis
(Extinct)

Fig. 27. Specializations of the Birds. The beak is modified for pecking,
tearing, or catching fish. The legs and claws are adapted for correlated func-
tions: walking, grasping, or wading. The eggs of different birds (at bottom)
show the comparative sizes.

STRUCTURE OF THE VERTEBRATES 75

ing flesh with both claws and bill, for example the owls, hawks,
eagles and vultures; the sand-piper, secretary bird, and the
common fowl, while well equipped for flying, can run swiftly
along the ground; and the opposite of the last adaptation is
found in the humming birds, which have almost no ability for
ground locomotion, but are able to remain on the wing, poised
in one place before a flower, for a sustained period.

It has been often stated that anatomically a "bird is a bird",
but this applies only to the soft anatomy. For the many modi-
fications, and the numerous orders, the student is referred to
books of natural history. As the birds are far removed from the
line of human evolution a full discussion cannot be attempted.

Embryology of the Bird. Avian development is more spe-
cialized than that of the reptile, but is the same in all essential
details. This similarity was discussed under reptilian embry-
ology. Most birds sit upon the eggs during the entire time of
incubation, only a few genera which live in dry s^ndy regions
depending at all upon the heat of the sun. The body tempera-
ture is higher than that of the typical mammal, being about 103
degrees Fahrenheit, and this temperature must be maintained
for the proper development of the embryos.

When the egg is laid development has already begun, but
cleavage is suppressed when the egg cools. Embryos may re-
main in this arrested condition for several weeks, and when heat
is again applied begin growth. In this way all the eggs, which
were laid at different times, begin development together and
hatch at about the same time.

CHAPTER VIII
MAjNIMALIA

Mammals can be defined as animals with hair and milk glands.
Either part of the definition will serve, for only mammals se-
crete milk for their young; and although some mammals are
almost lacking in hair, even the whale has a few whiskers. One
extinct reptile has been described as having hair, but this is
probably a hair-like structure, rather than true hair. However,
it is not beyond the realm of possibility that some therapsid rep-
tiles had hair between the scales. In any event, the above defini-
tion will serve to identify the living mammals.

INIammals include a heterogeneous assemblage of animals.
Some lay eggs like a reptile, others give birth to very immature
young, and others, which make the majority group, give birth
to young in a well developed condition. The last group includes
the mice, porcupines, cats, dogs, seals, whales, elephants, pigs,
monkeys and men — in fact, practically all the mammals which
any student outside of Australia would know. The mammals
are divided into three sub-classes and a number of orders.

Sub-class Prototheria. This group includes one order and a
few genera. They are the most primitive of living mammals and
are restricted in their distribution to Australia, New Zealand,
and the adjacent islands. The animals have the body covered
with hair, and there are milk glands. The latter have no teats,
the secretory ducts opening separately into depressions on the
mother's abdomen, and the young lick the milk off.

The order is called the Monotreiuata (one opening) in refer-
ence to the cloaca into which the digestive and urinogenital
openings empty. This structure is typical of the amphibia, rep-
tiles and birds, and is not found elsewhere among the mammals.

76

STRUCTURE OF THE VERTEBRATES 77

The monotremes lay small, reptile-like eggs which are hatched
in a nest built by the parents. The young are very immature at
the time of hatching, and are kept in the nest for some time.
The brain is better developed than that of any reptile, but very
primitive compared with other mammals. The body tempera-
ture is not constant, varying widely with the environment. The
skeleton is very reptilian in character, the legs being held later-
ally at right angles to the body. These characters have given
rise to the theory that they arose from the reptilian stem as a
separate line of evolution, and that they are not as closely re-
lated to the other mammals as are the Therapsida. The mono-
tremes have been described as a * 'museum of reptilian charac-
teristics".

Ornithorhynchus, or the Duck-billed Platypus, is the best
known, although rarely seen in captivity, and it has never been
successfully kept in zoos in America or Europe. It is a water-
living mammal, inhabiting the ponds and quiet streams of Xew
Zealand. The nests are dug into the bank above the water level,
the opening being protected by the water which fills the tunnel.

The more widely spread Echidnas live in drier regions and,
due to their spiny covering, are erroneously called hedge-hogs.
They are also called spiny ant -caters, and though many do live
on ants, the term is misleading for the name was long before
given to an entirely different group. Specimens have been kept
for long periods in zoological gardens.

Sub-class Metatheria. One order, the Marsupialia, comprises
the entire sub-class. INIarsupial refers to the marsupium or
breeding pouch, which is simply a deep fold of skin on the lower
abdomen. The teats are within the pouch, and immediately after
birth the immature young climb into the pouch and remain
tliere until able to maintain independent existence.

AVith the exception of the opossums of the American conti-
nent, and obscure South American forms, the order is limited
to Australia and the nearby territory. The American opossum
is perhaps the most generalized mammal in existence, its skele-
ton having remained practically unchanged since the early his-
tory of the race. It has survived in competition with the higher
mammals by remaining in hiding except at night. For the causes

Kangaroo
(Marsupialia)

Fig. 28. Primitive Living Mammals. At top are shown the two famihes
of egg-laying mammals. Below are types of Marsupialia.

STRUCTURE OF THE VERTEBRATES 79

of preservation of the marsupials in Australia, the student may
refer to Chapter XX.

Among the better known Australian marsupials are the kan-
garoos. The student need not be reminded of their small fore
feet, their large hind legs and their powerful tails which assist
the animals in jumping. The largest stand more than six feet
high. The wombat is a herbivorous marsupial resembling a
ground-hog. Other herbivorous species are no larger than rats,
and much resemble these rodents in external appearance. Other
marsupials are carnivorous, the better known being the small
Tasmanian Devil and the Tasmanian Wolf. The latter is about
the size of the gray wolf, and but for its fundamental marsupial
structure could be mistaken for it. However, no matter how
closely one of these animals may resemble a higher (placental)
animal, it is structurally and developmentally a marsupial.

Development. The marsupials have small eggs which are al-
most without yolk, and of microscopic size. Fertilization is
internal, as in all amniotes. The fertilized eggs are retained in
the modified posterior part of the oviducts. This portion of tlie
ducts becomes the uteri (and later a single uterus) in the two
higher groups of mammals. The uterine wall secretes a nutrient
fluid, the so-called ''uterine milk", upon which the embryos live
by absorption. As there is little or no yolk, and no connection
with the mother, the young are born in a very immature con-
dition.

The new born marsupial is merely an embryo with a few well
developed, specialized characters. At birth an opossum, \vhich
when adult is about the size of a house cat, is not larger than a
bean. It weighs approximately four grains; or, to state it more
graphically, 1750 of them weigh a pound. A few reflexes are pres-
ent, including a negative geotropism, and the young climb up
the side of the mother and into the pouch with the aid of front
claws which are later shed. There they attach themselves to the
nipples, which are long and about the diameter of broom straws.
The epithelium of the lips fuses to the nipple and they remain
firmly attached for a week or more. By that time they are able
to see, and soon begin climbing out of the pouch on to the
mother's back.

80 STRUCTURE OF THE VERTEBRATES

As with most transitional groups, there are many characters
which, by comparison with the higher types, seem very ineffi-
cient; but when compared with lower forms their selective value
is apparent. This is true of the development of the marsupials.
They managed to survive the Age of Reptiles due to higher intel-
ligence and some care of the young; but when their more highly
developed descendants arose they all but disappeared. And the
majority of those which survived were in a region protected from
the higher mammals. There is a single exception to the above
description of development, but this will be mentioned in the
following chapter.

Sub-class EuTHERiA. This group is also known as the Placen-
talia on account of the placenta, an embryological development

Shrew (Insectivora)

Fig. 29. Order Insectivora. The living representatives of the most primi-
tive placental order are externally specialized. The Order Primates rose
directly from the Insectivora.

of the allantois which attaches the young to the wall of the
mother's uterus. Their distribution is world-wide except for
Australasia, where only the flying mammals were found until
others were naturalized by white men. Various types of placen-
tals were mentioned at the beginning of the chapter.

The eggs are microscopic in size and almost lacking in yolk.
Growth depends upon food and oxygen being absorbed from the
blood stream of the mother, and the young are born in a fairly
well developed condition. The following grouping of orders is
simplified. Those which bear upon human evolution, or which

STRUCTURE OF THE VERTEBRATES

81

are used for dissection in most courses in anatomy, are included
first.

Order Insectivora. This order was the first to appear in the
evolution of the placentals, and is still the most primitive order.
Externally they have numerous specializations, for example: the
spines of the European hedge-hog, the jumping habits of the
African shrews, and the blindness and digging claws of the
moles. Their teeth are very generalized and adapted for insect
eating. One family (often set up as a separate order) appears

Cebus (American Primate)

American Monkey (Primates)
a thumbless, arboreal form

Fig. 30. Order Primates. The Lemurs (top) are the most primitive mem-
bers of the order. The American monkej^s are an early branch from the

parent stem.

about mid-way between the insectivores and the most primi-
tive primates in several characters.

Order Primates. The primates are the lemurs, monkeys, apes
and man; and a few less well known families. They evolved
from the insectivores and are therefore one of the oldest orders
of mammals. Most of them are arboreal in habit, and all of
them show unmistakable specializations for tree living condi-
tions. Man is the least adapted for this ancestral type of life.
Tl>e order is divided into the following sub-orders.

82 STRUCTURE OF THE VERTEBRATES

Sub-order Lemuroidea. The lemurs were so named for their
nocturnal and ''ghostly" habits. The most primitive members
almost merge into the insectivores. Fossil lemurs were widely
distributed, but of the fifty existing species thirty-six are limi-
ted to Madagascar, the others being found in tropical Africa
and the Orient. The different species indicate the course of evo-
lution from the most primitive to the monkeys.

Sub-order Anthropoidea. This sub-order carries the speciali-
zation of the nervous system and other body characters from
the primitive condition to man. In this group the teats are al-
ways in the pectoral region; the embryological structures of
nutrition (placenta) are similar; the skeleton shows the course
of evolution toward the upright position; and the nervous sys-
tem shows the steady advance from the lower monkeys to the
human.

The South American monkeys are an early off-shoot from
the original stem, including the marmosets and the typical or-
gan grinder's monkey. They can be identified by the distance
between the nostrils, the septum being wide and the apertures
directed outward. Also, many of them are able to hang by their
prehensile tails.

The Old World monkeys are more man-like, the nostrils be-
ing close together and separated by a narrow septum. The tail
is never prehensile. The teeth are thirty-two in number, the
dental formula being the same as in the higher families. The fam-
ily includes both the tailed monkeys and the apes, and members
are found in Africa, Southern Europe and Asia. The apes are
tlie most specialized of the primates, their dog-like faces and
vivid coloration making them unpleasantly ugly. These apes
and baboons have little relationship to the groups which gave
rise to the human family.

The simian family includes the gibbons, which walk in an
almost upright position; the orang outans of the Malayan
regions; and the chimpanzees and gorillas of Africa. The last
two genera are the nearest relatives of man among living ani-
mals. The muscular system and the nervous system are almost
identical with those of man. In judging similarities between
groups the young should be studied, as older animals tend to

Asiatic Monkey (Primates)

li:^^^Baboon ^^^k^^^'M
a highly specialized pnmate

Chimpanzee

Gorilla
Fig. 31. Old World Primates.

84

STRUCTURE OF THE VERTEBRATES

develop their own characteristics, making them appear more
divergent than they are in reality.

The Hominidae include among existing animals only the races
of man. All existing men are classified as one genus and one
species, Homo sapiens. Several genera of fossil men are known,
and these connect the present representatives of the genus Homo
with the earlier anthropoids. And even among the living repre-
sentatives of the higher anthropoids the relationships are so
close that there is a tendency on the part of recent investigators
to include man and the gorilla in one family.

Fox
<Carnivora)

Fig. 32. Carnivora. Three types of land living carnivores (Fissipedia) .

STRUCTURE OF THE VERTEBRATES

85

Walrus (Carnivora)

Seal (Carnivora)

Fig. 33. Carnivora. Aquatic or partially aquatic carnivores (Pinnipedia).

A comparison of the Sea Lion and Seal shows the latter's increased

specialization for water life.

Order Carnivora. The carnivores, or flesh eaters, are distin-
guished by the enlargement of the canine or ''eye" teeth. The
posterior teeth (premolars and molars) are usually modified for
cutting, and the animal is able to tear and cut his food. The
order is divided into two large groups. (A) The land carnivores
include the dog family, the cats (lions, tigers, pumas, and the
smaller cats), the hyenas, the bears, and a few others which
will be unknown to most students. (B) The carnivores modified
for swimming form the second group: the sea-elephant, sea-
lion, walrus, and the Atlantic seals. The last are the most highly
modified for aquatic life.

Order Rodextia. The rodents are gnawing animals and have
the two median incisor teeth of both jaws highly developed. The
posterior teeth are modified for grinding. The animals were orig-
inally herbivorous, although the rats and others are omniv-
orous. In this order are the mice, rats, guinea-pigs, and the
large South American capybara which is the size of a hog. By
some anatomists the rabbits are included in another order, due

86 STRUCTURE OF THE VERTEBRATES

to the presence of the lateral incisors in addition to the median,
and a history which indicates early separation from the typical
rodent stock. The rabbit is a favorite laboratory animal for
dissection.

Order Cheiroptera. The bats, which form this order, are fly-
ing mammals. The name literally means ''hand wing", the digits
being enormously elongated with a leathery membrane stretched
between them. Their dentition and internal anatomy show them
to be closely related to the insectivores. The existing members-

Rabbit (Rodentia)

Fig. 34. Rodentia. Capybara is the largest living rodent. The rabbit is

more technically placed in a different order, Lagomorpha. The porcupine

is a true rodent with long ectodermal spines.

are mostly insect eaters, although some are fruit eating, and a
few (the vampire bats) cut the skin of other animals and drink
the blood. This genus of South American bats has sharp canines
and incisors, a gullet which permits only liquid food, and an
almost straight digestive tract forming a "stomach-intestine".
The distribution of the order is world wide, their flying habits
having permitted them to invade Australia and other insular
regions.

Order Edentata. The edentates are not all entirely toothless,
a few having small teeth, and some have teeth in the young

STRUCTURE OF THE VERTEBRATES

87

which are later lost. This order is heterogeneous and should be
divided into three orders. Simplicity of classification is the only
excuse for grouping them. Included here are the South American
ant eaters, sloths, armadillos; the genus IVIanis of Africa and the
Orient; and the Aard varks, or Gape ant eaters, of Africa.

The armadillos are unique among living mammals in possess-
ing a heavy bony covering. This is arranged as scutes, or plates,
placed transversely to the axis of the body. The species are clas-
sified by the number of plates. When frightened the animal rolls
up, presenting a heavy defensive armor to any predaceous
animal.

(Chiroptera)
Flying Adaptation

Rlanis (Edentata)
a scale covered mammal

Armadillo (Edentata)

Fig. 35. Specializations of Mammalia.

Manis is of particular interest to the anatomist on account of
the covering of ectodermal scales. These scales are reptile-like
in character and cover the entire body from head to tail. Be-
tween the scales are the hairs, usually in groups of three, one
group under each scale. Due to the evidence regarding the evo-
lution of the hair, it is thought that this represents the condition
found in the earlier mammals.

Order Artiodactyla. The even-toed, herbivorous animals in-
clude the camels, sheep, cattle, deer, antelopes, pigs and hippo-
potami. These animals can be identified by the fact that the
weight is carried on the third and fourth digits, with the axis

Mountain Sheep
(Artiodactyla)

Elk
(Artiodactyla)

Hippopotamus
(Artiodactyla)

Fig. 36. Order Artiodactyla. The order of even-toed placentals has become
specialized toward size, a heavy protective skin, or defensive horns or

antlers.

STRUCTURE OF THE VERTEBRATES 89

of the leg passing between them. This order and the following
were formerly classified together as the ungulata, but their evo-
lutionary history shows them to have been separate groups since
very early in the history of mammals.

Order Perissodactyla. These are the odd-toed herbivorous
mammals. The axis of the leg passes through the third digit,
with the weight resting upon it. The rhinoceros, horse, zebra,

Tapir (Perissodactyla)

Rhinoceros (Perissodactyla)

Fig. 37. Order Perissodactyla. The order includes the horse, ass, zebra and

the above forms.

tapir and ass are members of the order. The horse is a very
highly specialized member of the group, the number of digits
having been reduced to one in the adult. There is, perhaps, no
evolutionary history better worked out than that of tlie horse.
Order Proboscidea. The proboscidea are animals with a great
elongation of the nose forming a proboscis, and the living mem-
bers of the order are usually included in one genus, Elephas.
The evidence warrants the separation of the African elephants
as a different genus from the Asiatic elephant. The elephants

90 STRUCTURE OF THE VERTEBRATES

are the largest land animals, the legs being modified into straight
columns with little flexion at the joints, a specialization in corre-
lation with the great weight of the body. The Indian elephant
is more easily tamed, although progress has been made in train-
ing the African species for work. Most circus elephants are of the
Indian variety. Judging by their brain capacity the elephant is
a rather low order of mammal, and experiments indicate that
stories of their intelligence are highly colored. Several extinct
families of proboscideans are known, including the Mastodon
and INIammoth, the latter a true elephant more highly specialized
than the recent forms.

Order Sirenia. The manatees and dugongs are herbivorous
aquatic mammals. Most of them inhabit the sea, although they

Manatee (Sirenia)
Fig. 38. Order Sirenia, aquatic, herbivorous placentals.

may go farther up rivers than the brackish waters; and some
are apparently limited to fresh water. The manatees are found
in the Atlantic Ocean and its tributaries on the shores of Africa
and the warmer parts of America. The sirenia gave rise to the
mermaid myth, for when a dark head rises above the water,
and the mother is seen holding her young in her flippers, the
illusion is sufficient to mislead the average seaman. And the illu-
sion is complete when the animal dives and the flukes of the
tail are seen.

Order Cetacea. The cetacea are the whales and their allies.
They are the largest living animals, and from the standpoint
of mass are probably the largest which have ever existed. The
herbivorous dinosaurs were longer but not so large. Some whales
reach a length of ninety feet or more, while the smaller por-
poises are only a few feet long. The young are born with the

STRUCTURE OF THE VERTEBRATES

91

swimming instinct well developed, but when speed is required
the young are carried on the front flipper of the mother. The
teats of some species are partly enclosed by a fold of skin so
that the young can be suckled while the mother is swimming
slowly.

The skeleton is highly modified, the digits being enclosed in
skin, forming a flipper. The number of phalanges is always in-
creased. The pelvic limbs are lost, but a vestigial pelvis is

Fig. B

Fig. 39. Order Cetacea. Aquatic, carnivorous placentals including the
porpoises and whales. They are the largest known mammals.

present, and femori have been found in several specimens. The
skull is modified, with the nostrils on top of the head, and the
frontals reduced. Some of the whales have teeth which are small,
very numerous, and similar from front to back; others have
only a few, or entirely lack teeth. The last have an epidermal
strainer in the mouth, the so-called ''whale bone", which as-
sists the animal in catching its food. This consists of enormous
numbers of small fish and squid which are taken into the mouth
with the water and held by the strainer as the water is ejected.
The whales are rapidly disappearing because of modern whal-

92 STRUCTURE OF THE VERTEBRATES

ing apparatus, and the increasing demand for oil from the thick
layer of subcutaneous fat. Ambergris, a cholesterol substance
used in the manufacture of perfume, is a secretion of the intes-
tinal tract. A great deal of mythology has grown up around the
whale. The ''blowing" of a whale is the condensation of the
breath in cold air, not a jet of water as usually depicted in
story books ; and the belief that it is a fish goes back to biblical
times. Even as late as the nineteenth century natural histories
called it a fish, a great convenience, as the flesh of the porpoise
was a royal dish and with this classification could be eaten
during Lent.

Embryology of the Mammals. The Monotremes, with their
large yolked eggs, have developmental processes essentially like
those of reptiles. The two higher groups, however, have lost the
large amount of yolk in the egg, and specializations of funda-
mental importance have developed. The following description of
early embryology applies generally in its essentials, but each
order has its own modifications which come properly in a study
of comparative embryology. The later development of marsu-
pials has been mentioned, and the present discussion is limited
to the placental, using the rabbit as a type.

The minute egg is fertilized in the "oviduct", the upper end
of the uterine tube, and passes into the uterus. It soon becomes
attached to the uterine wall. The early cleavages are holoblastic,
and the cleaving ovum forms a hollow, elliptical organism with
a small knot of cells at the dorsal pole. This hollow sphere is
not a blastula, but the placental homologue of the yolk sac, the
knot of cells at the top being the equivalent of the embryonic
disc. The embryo, having formed itS' own yolk sac, proceeds to
develop in a typically amniote manner.

The dorsal embryonic cells, or embryonic knot, begins divi-
sion and forms a flat plate of cells, the true blastula of the
placental mammal. Gastrulation is by an invagination near one
edge of the blastula, much like that of the reptile. The amniotic
folds grow over the embryo, the inner layer forming the amnion
enclosing the amniotic cavity; and the outer layer, encircling
the amnion and yolk sac, is the chorion. The latter is in con-
tact with the wall of the uterus. The allantois develops as an

STRUCTURE OF THE VERTEBRATES

93

outpocketing of the hind gut and grows outward toward the
chorion, lying between the amnion and yolk sac.

Up to this time the more fundamental processes of the pla-
cental have parallelled those of the reptile; but the later develop-
ment of nutritive processes differs materially from those of other

Embryonic knot

Neural groove

Placenta

Allantois

Yolksac

Early Embryo

Umhilical cord
Later Embryo

\terine ivall

Placental Attachment

Tig. 40. Embrj^ology of the Mammal. Diagrams D to H inclusive are
cross sections to show the development of ectoderm, endoderm and 3'olk
sac from the embryonic knot of cells. Drawing I and J are longitudinal
sections to show the development of the embryo, the umbilical cord,
and placenta. Drawing K shows the attachment of the embryonic placenta
to the maternal uterine wall, and the placental villi extending into the
blood sinus of the uterus.

94 STRUCTURE OF THE VERTEBRATES

vertebrates. The chorion of meroblastic embryos lies in contact
with a porous shell, oxygen being absorbed by the capillaries
of the allantois from diffused air, and food from the yolk gran-
ules. The essentials of nutrition are supplied the placental em-
bryo by the development of a placenta.

Hair-like villi develop on the chorion. These processes secrete
an enzyme which eats into the maternal tissue to form numer-
ous cavities which eventually fuse to form a relatively large
sinus in the thickened wall of the uterus. When the allantois
reaches the chorion the growth of the villi is accelerated in this
region and the allantoic tissue grows into the villi. Sj^nchro-
nously the villi of the other parts of the chorion degenerate. As
the allantoic villi burrow into the maternal sinus numerous veins
and arteries are cut, so that the sinus fills with maternal blood.
The placenta is the mass of tissue formed by the chorion and
allantois, with its villi and blood vessels. In the rodents and
most primates the placenta is discoidal in shape, and is con-
nected with the embryo by the body stalk.

Blood vessels pass downward from the embryo to the pla-
centa and there break into capillaries and enter the villi. By
osmosis through the membranes of the placenta waste products
are given out into the maternal blood sinus, and oxygen and
food are taken up from the blood entering the sinus from the
cut arteries. The blood is constantly being drained from the
sinus by the veins with the result that the supply is always
fresh.

The connection between the placenta and the body of the
embryo is the elongated body stalk, known as the umbilical
cord. After birth the cord and placenta slough off and are lost.
The point of attachment of the embryo remains as the umbi-
licus or navel.

The sub-class Placentalia received its name from the placenta,
which is typical of the group. The structure is not found in any
other mammal, with the exception of a primitive homologue
developed by one marsupial (Perameles). There are several
types of placentae, apparently equally efficient. (1) The discoid
placenta is found in the rodents, the higiier primates, and in
other orders; (2) the diffuse placenta, spread evenly over the
chorion, is found in the horse, pig, and lemurs; (3) a cotyle-

STRUCTURE OF THE VERTEBRATES 95

denary placenta, consisting of rounded masses of villi spaced
at intervals over the chorion, develops in the cow, sheep and
other herbivorous animals; (4) an intermediate type, combin-
ing types (2) and [S) is present in a few herbivores; and (5)
a zonary placenta develops in the carnivores, consisting of a
median band around the chorion.

With its efficient feeding mechanism the placental mammal
is born in a fairly well developed condition. The degree of de-
velopment varies with different orders and even the genera
within an order. In general terms, the more primitive mammals
are born in a less mature condition, but even there one must dis-
tinguish between total development and the specialization of
certain characters. The newborn rat, for example, is able to
crawl although its eyes are not open; a newborn monkey has its
eyes open but has very limited body coordinations. The length
of the embryonic period is fairly correlated with size at birth,
although the body size of the parents is a contributing factor.
Further, the length of this period of gestation and the rapidity
of post-natal development are correlated with the total life
span of the race. The poor development of the human at birth,
and the prolonged period of development have been variously
interpreted.

I

PART II

COMPARATIVE MORPHOLOGY

Anatomy can be studied either by taking type forms of the dif-
ferent classes and completing one before beginning the next; or
by studying all of the types comparatively, one system of or-
gans at a time. The former is systematic anatomy and was the
method used in the first part of this text. The latter is systemic
anatomy, and presupposes some knowledge of the animals to
be studied. The following chapters on morphology, or form, con-
sider the organs and systems of organs in relation to their devel-
opment and completed structure.

Each system is traced from the simpler to the more complex,
both from the standpoint of individual development (ontogeny)
and racial development (phylogeny). Sufficient information has
been given about natural history and embryology to show that
each organism is an individual, and that each group is more or
less definitely separated from others; but the student has un-
doubtedly realized that there are underlying relationships be-
tween groups, just as there is a relationship between the organs
of the body. Thus, from the standpoint of evolution, the classes
and orders of the vertebrates are but units in a coordinated
whole.

The fundamental unity of development that is found in the
races of vertebrates is also true of the organs of the body. In
the preceding embryological discussions, and in the chapters
which follow, each system has been considered as a unit. But,
although this is convenient for study, independence of organs
is never true functionally for each system is dependent upon
every other; and the individual is not a group of discrete or-
ganic units, but a coordinated whole.

The stress which is generally placed upon laboratory dissec-
tions applies particularly to the study of comparative mor-
phology. The anatomical facts should be gained at first hand
from a study of the animals themselves, and the best that a
book can do is to correlate these more or less isolated details into
a coherent scheme. A text, however, cannot replace the knowl-
edge gained in the laboratory.

CHAPTER IX

TISSUES OF THE BODY

The tissues and organs of the body illustrate perfectly the prin-
ciple of unity of structure in the individual organism. We speak
of the body as being made of tissues, each specialized for cer-
tain particular functions. Various tissues make up the organs
and systems of organs, and the systems make the individual. It
is therefore well to understand the fundamental tissues before
considering organs.

Ordinarily five types of tissues are recognized: (A) epithelial
or covering tissues which cover the body and line all cavities;
(B) connective tissues which include all the supporting elements
of the body; (C) fluid tissues, the blood and lymph, which are
often included with the connective tissues; (D) muscular tis-
sues, those which are specialized for contractility; and (E) the
nerve tissues, the most highly specialized cells of the body, func-
tioning as carriers of stimuli. These five types are treated sep-
arately below.

A. Epithelial tissues are perhaps the most diverse in struc-
ture and function of any found in the body. Most epithelial
cells retain the ability to reproduce themselves and replace
those cells which are lost. Epithelia include the outer layers of
the skin; the lining cells of the blood vessels and digestive tract;
and the secretory cells in the glands of the body. In fact, all cells
are included which act as a cover to other tissues. These cells
differ widely in origin and structure. The epithelium covering
the body is ectodermal; that which lines the digestive tract and
the glands derived from it are endodermal; and those lining
the blood vessels, the coelomic cavity, and other spaces, are
mesodermal in origin. Structurally several types are recognized.

1. Squamous tissues have flat polygonal cells which are very

99

100

STRUCTURE OF THE VERTEBRATES

thin, and lie against each other in a tile-like manner. Those in
the skin lose the power of division.

2. Cuboidal epithelium, as the name implies, is composed of
cells which are about as deep as they are in width.

3. Columnar cells are long, one of the smaller faces lying in
contact with the opening. This type is naturally the thickest of
those epithelia which are only one cell deep.

Any surface may be covered by epithelium which is a single
layer in thickness, or by stratified epithelium composed of sev-
eral layers of cells. The former is simple and is easily classified
by its type of cells. In stratified epithelia it is not uncommon to
have a base of columnar cells with several layers of cuboidal
cells above them. These tissues vary in thickness and in type

Surface
view

Cross section
Squamous cells

Cuboidal (section)

Columnar (section)
Fig. 41. Epithelial Tissues.

Stratified (section)

of cells composing the strata. Also, epithelial cells may change
their shape when covering a distensible membrane. The squa-
mous cells of the arteries become thinner under force of blood
pressure, and the cuboidal cells of the bladder become squamous
w4ien the organ is enlarged.

B. Connective Tissues. These are mesodermal tissues which
support the other tissues. They are the most abundant tissues
of the body, ramifying through every gland and muscle. It is
customary to divide them into (1) connective tissues proper
and (2) cartilage and bone.

1. Embryonic tissues are of little importance in a study of
the adults of any group, except in pathology. The healing proc-

STRUCTURE OF THE VERTEBRATES

101

ess in a cut or wound depends upon other connective tissues re-
verting to the embryonic condition, filling the opening, and then
re-developing into the original type.

2. Reticular connective tissue fills every available space in
the body. A section of a gland or muscle stained for connective
tissue will demonstrate this. In either a gland or muscle the
reticular fibers surround every cell, interlacing to form a firm
network throughout the structure. These fibers are usually con-
tinuous with the following, thus binding the tissues and organs
strongly together.

3. White fibrous tissues form the loose areolar tissues found
under the skin and most of the tendons and ligaments of the
body. The fibers of the latter lie parallel, are strong and tough,
and allow of little stretching.

Reticular

White fibrous

Fig. 42. True Connective Tissues.

Yellow elastic

4. Yellow Elastic fibers are less abundant than the preced-
ing, and are usually mixed with other types. The fibers split and
fuse again, and are elastic in nature. They are found in the
walls of arteries, in the connective tissues under the skin, and in
other places where they automatically pull a structure back
into shape after displacement. Most mammals have a strong
bundle of these fibers in the dorsal part of the neck.

5. Cartilage is a strong, firm connective tissue. The cells of
cartilage are essentially like those of the true connective tissues,
but instead of developing fibers as do the others, they secrete
a non-cellular, translucent substance around themselves. As
more of this substance is formed the cells are pushed farther
apart until the cells lie isolated from each other in the surround-
ing matrix.

The above description applies to the basic structure of all
cartilage, and to the adult condition found in hyaline (clear)

102

STRUCTURE OF THE VERTEBRATES

cartilage, which is simplest and most abundant. In the discs be-
tween the vertebrae and other parts of the body the simple carti-
lage is strengthened by white connective tissue fibers, and is
known as fibrous cartilage. Elastic cartilage, containing elastic
fibers, is found in the larynx and other isolated parts of the
vertebrate.

6. Bone is the hardest of the connective tissues. The bone
cells develop fibers which finely interlace, and then salts of cal-
cium and potassium are laid down in the spaces between the
fibers. So intimately are the organic and inorganic elements mixed
that either may be destroyed without affecting in the slightest

Matrix

Bone cells

Hyaline cartilage

Pcnosteiim

' ,.y Haversian
system

.M

-f^-K"

.45=.

Bone section
(Low power)

Fig. 43. Cartilage and Bone.

Canaliculi^!^^A^W^^^'-q>.

canal '^'s^^^---:.^.^-/ 'N

Two Haversian Systems
(High power)

the appearance of the minutest structure of the tissue. The cells
lie in spaces (lacunae) in the matrix, with microscopic canals
(canaliculi) radiating in all directions and joining the canals
from other cells. This r akes a protoplasmic connection through-
out the entire bone. The blood supply is secured from small
arteries and veins which pass longitudinally through the mat-
rix, a pair (artery and vein) lying in a Haversian canal.

A cross section of a long bone shows that the matrix is laid
down as concentric layers or lamellae. The lamellae in contact
with the membrane surrounding the bone extend completely
around the shaft; but within these few concentric layers, the
lamellae surround the Haversian canals, forming Haversian sys-
tems. Each canal is surrounded by concentric lamellae, each
system being microscopic in size. The cell spaces lie between
the lamellae. A more complete discussion is given in Chapter
XI, page 172.

Althou2;h the bones are structurallv similar, there are two de-

STRUCTURE OF THE VERTEBRATES

103

velopmental tj'pes in the liigher vertebrates. The oldest, from
the standpoint of evolution, are the dermal bones which are
homologous with the dermal denticles of the dogfish, and in all
cases develop in the mesodermal layers of the skin. The others
are replacement bones which originate in cartilage. The carti-
lage is not transformed but is first destroyed, and bone is laid
down by bone building cells. This process is very gradual, much
skeletal cartilage being left in the new-born human, and the
replacement in the long bones is not complete until growth has
ended. Especially in the lower vertebrates, large regions are
never completely ossified.

C. Fluid Tissues. The blood of the vertebrate is composed
of a fluid medium with cells, or corpuscles, floating in it as dis-
crete units. The fluid is the plasma. As most of the corpuscles
arise in the long bones, these tissues are often included as fluid
connective tissues.

Blood corpuscles are of two main types: (1) red cells or
erythrocytes, containing the haemoglobin which has an affinity'
for oxygen; and (2) the white cells, or leucocytes, which are
ameboid in nature and assist in freeing the body of foreign par-
ticles. A more detailed description is given in the chapter on
the vascular system.

D. Muscle Tissues are highly specialized for a contractile
function. In the higher vertebrates, at least, they lose the ability

Smooth

Striated
Fig. 44. Muscle Tissues.

Cardiac

104

STRUCTURE OF THE VERTEBRATES

in the adult to divide and reproduce themselves, although cut
ends of fibers may be regenerated. The "growth" of a muscle
as a result of exercise is not true growth in the sense that the
number of fibers is increased, but depends upon the individual
cells adding fluids. The muscle increases in size without an in-
crease of the elements of which it is made. The three types of
muscle tissue differ functionally and structurally.

1. Striated or skeletal muscle forms the greater part of the
bulk of the body. Each individual fiber is very long, some be-
ing tw^elve or fifteen centimeters in length, and is syncytial in
structure. Under the microscope these fibers appear transversely
striped, light and dark bands of protoplasm alternating. The
fiber is surrounded by a very thin sheath of connective tissue;
and in the protoplasm are the numerous nuclei. These muscles
are voluntary in function, that is, they are under the control of
the central nervous system, and can be contracted at will.

2. Smooth or visceral muscles are those which are involun-
tary in function. They contract slowly when stimulated and are
not under the direct control of the brain. The cells are small,
spindle-shaped, and each has a single nucleus.

3. Cardiac muscle is found in the heart and combines the
functions of striated and smooth. It is involuntary in action,
but responds rapidly to stimuli. Cardiac muscle is usually de-
scribed as a syncytium, the protoplasm extending continuously
throughout the mass. Microscopically the tissue appears divided
into irregular cells which are striated, each with a single nucleus.
Physiologically the heart is a compact unit, nerve impulses trav-
eling rapidly over its entire area.

E. Nerve Tissues are ectodermal in origin and are discussed
fully in the proper chapter. The individual nerve cell has a cell

Fig. 45. A typical Nerve cell (Neurone)

STRUCTURE OF THE VERTEBRATES

105

body with processes at either end. One set of fibers (the den-
drites) receive impulses which are passed through the cell body
and out to nerve cells or other tissues through the long axone.
These tissues are the coordinating mechanism of the body.
The processes may be re-developed, but the destroyed cells can-
not be replaced.

The student cannot be told too often that the organism is a
unified mechanism and depends upon the proper coordination
and function of all its parts. Proper function depends upon
proper balance between tissues and organs. This interdependence

^ Epithelium
Muscle

tMJbnnective
tissues "^^

Secretory
epithelium

Wall of Artery

Salivary Gland

Fig. 46. Organ Formation, showing different tissues combined in a single

organ.

of the tissues can be illustrated with any organ of the body.
We speak of muscle tissues, but a muscle is an organ, not a tis-
sue. The muscle tissues are the contractile elements, but for
their proper growth and function they must be bound securely
into a functional unit by connective tissues; the impulses which
cause them to contract must pass over the nerve fibers which
innervate them; and for food and oxygen they depend upon
the fluid tissues to transport these substances to them. And the
thin epithelial walls of the capillaries permit the plasma to pass
out to the muscle fibers.

Similarly, every organ of the body is made of several types
of tissue. The blood vessels are composed of muscular and con-
nective tissues, lined by epithelia, and controlled by nerve stim-

106 STRUCTURE OF THE VERTEBRATES

uli. In the same manner the individual is a group of organs and
systems which must function as a whole.

In studying the following chapters the student will do well
to keep function in mind, and remember that the individual
is the sum of all its parts. Without this, anatomy becomes a
mass of labelled structures, each unrelated to any other.

CHAPTER X

INTEGUMENTARY STRUCTURES

The integument, or covering, of the vertebrate consists of the
skin and its derivatives. Amphioxus has the simplest possible
skin structure, consisting of a thin layer of mesodermal con-
nective tissue, with an ectodermal layer of epidermis only one
cell thick. The latter cells secrete a thin cuticle of non-cellular
material which covers the animal externally. The vertebrate
skin is based on the same principle although it is thicker, lacks
the cuticle, and is complicated by other structures which grow
from either layer.

The skin is essentially protective in function. The cyclostomes
secrete a protective layer of mucus, the fish have protective
scales which arise from the skin, and the reptiles, birds and
mammals have a dry integument w^iich is impervious to patho-
genic bacteria.

The ectodermal part of the skin is the epidermis and lies upon
the dermis (corium) which develops from the epimere of the
mesodermal somite. The dermis is a fibrous structure resembling
the typical connective tissues, and is usually a close-packed
structure well supplied with nerves and blood vessels. It is con-
nected with the underlying muscles by connective tissue fibers
which freely interlace with the reticular fibers. There is a layer
of subcutaneous connective tissue lying between the well de-
fined skin and the muscles. This permits some movement of the
skin without affecting the muscles, and explains the ease with
which most vertebrates are skinned. In addition, the subcutane-
ous tissues are normally loaded with droplets of fat, which
assists in heat regulation and supplies a store of food for the
animal. This becomes of great importance in animals which
hibernate or aestivate.

The epidermis is several layers thick. Lying next the dermis
is a growth layer, the stratum germinatiinim (stratum Malpi-

107

108 STRUCTURE OF TPIE VERTEBRATES

ghii) which is in constant cellular division. Above this is a pig-
mented area, not well defined in most vertebrates, and then a
various number of layers considered together as the stratum
corneum. The cuboidal cells of the stratum germinativum be-
come flattened in the corneous layer, the outer ones dead. Due
to continual mitoses the skin would become progressively thicker
if the outer dead cells were not discarded. In the amphibia and
reptiles this is accomplished by periodic ecdyses or moults, de-
pending to a large extent upon the amount of food and the con-
sequent more rapid growth. In the birds and mammals the outer
cells are shed as small flakes. Dandruff and the minute rolls of
epidermis removed with a rough towel are illustrations of the
mechanism. Death in these cells is in large part due to lack of
nutrition, nerves and blood vessels being lacking in the. epi-
dermis.

In the making of leather the epidermis is lost. The remaining
thick mat of dermal fibers is softened, cured and used in manu-
facture. Practically all commercial leather is from the mammal,
though alligator, snake and lizard skins are widely used for
ornamental leathers; and ostrich skin is frequently used for
bill-folds and other small articles. The dermis retains prac-
tically all of the original structure, the scales of the reptile or
the feather pits of the bird.

A. Dermal Derivatives of the Skin

Structures derived from the skin mesoderm appeared first
in the most primitive fish. The primitive condition has been
retained by the sharks (Elasmobranch fish) where they are
found as isolated dermal denticles. These minute bony plates
cover the body, varying slightly in size, and continue over the
lips where they have become modified into teeth. They are,
therefore, homologous with the teeth of all vertebrates. Embry-
ologically dermal denticles and teeth are composed of both
mesoderm and ectoderm, the basal plate and body of the den-
ticle being mesoderm, while the outer covering, or enamel, is of
ectodermal origin. The student should study the diagrams of
the denticles and compare them with the drawings of developing
teeth on page 142.

I

Enamel
Dentin f

^'^'^^f^^^^ermimtivum Epidermis
-_£-^: CV;^?^^^:^-^ '-;->:'lDermis Basal plate

" i^-vr-; ^^-%^Cy^''>^^ tissue
Skin Section'"" "^^^
(Generalized)

-Fat cell

Dermis

Dermal Denticle

Stratum germinativxim
Stratum corneum \

Papilla.

-^l

EnameL
^^i^^ Dentine. Jif

"- y-T^^J^ermis

Developmental Stages of a Dermal Denticle

Section of a Bony Scale ( Lepidosteus)

Epidermis's
Scale.

Surface View of Bony Scale
(Lepidosteus)

Dermis'=^~'

Longitudinal Section through
skin and scales of a Teleost

Teleost Scales
(Surface View)

Fig. 47. Skin and Dermal Scales of the Fish.

no STRUCTURE OF THE VERTEBRATES

In the chondrostean fish (the sturgeon and its relatives) the
denticles are fused into plates which are relatively large, cover-
ing the head with a close-set group of dermal bones, and the
body with bony plates. In this and the higher groups the ecto-
dermal part of the dermal plates is lost. The head plates and
the body plates are continuous in these fish, and the former are
not fused to the underlying chondrocranium. The teeth in the
bony fish and the land vertebrates have become well differenti-
ated from the dermal plates, and retain the enamel covering.

In the Holostean fish (page 40) there is a more marked dif-
ference between the head and body plates. The former fuse to
the skull bones which developed from the chondrocranium, form-
ing a definite skull derived from both dermal and replacement
bone. This same relationship is carried through all the higher
classes of vertebrates. The Teleostei (page 42) are essentially
like the Holostei, though the dermal scales lose the heavy bony
characteristics of the earlier fish, and overlap as flexible scales.

The primitive Crossopterygii (page 44), which gave rise to
the Amphibia, had large heavy scales practically continuous
with the head plates. Naturally this primitive relationship was
carried over to the Stegocephalia. These extinct amphibia had a
completely ossified skull, a ventral group of dermal plates, and
a dorsal covering. The recent amphibia have lost all signs of
body plates, except for very minute ones buried in the skin of
the Apoda (page 50).

The amphibian line which branched toward the reptilian line
of ancestry retained the body plates. These were a protective
mechanism and were of selective value in the constant struggle
for food. Practically all the early reptiles had these dermal
plates, which, by analogy with the recent forms, we presume
were embedded in the skin and covered by the ectodermal layer.
Two lines of evolution, as far as the dermal structures are con-
cerned, are apparent. The large, sluggish reptiles tended to keep
the plates, for in these animals they are of protective value;
the smaller, more active animals tended to lose them, for the
race in life is to the swift or well protected. And the student
should remember that racial survival depends upon individuals
remaining alive. Among the recent reptiles the snakes and liz-
ards lack the plates, the other three orders retain them.

STRUCTURE OF THE VERTEBRATES

111

There is no evidence that the body plates were carried over
to the mammals, although the bones of the top of the head and
face are homologous structures. In certain small whales there
are calcified plates on the dorsal surface; and the armadillos
(page 87) are well armored with bony shields. Most anatomists,
however, consider these cases as secondary developments, not as
homologous structures. The evidence is too complicated for a
discussion here.

B. Ectodermal Derivatives of the Skin

With the exception of the dermal denticles of the elasmo-
branchs, there is a clean-cut distinction between dermal and
epidermal derivatives of the skin. The difference is based upon

.Single secretory cells

land duct

M^^OjiMM^

Lamprey Skin
(Long. Section)

Secretory
epithelium

Frog Skin (Section)
Simple aciniform gland

Branching
Aciniform

Simple tubular

Fig. 48. Gland Types. The skin drawings (top) show the relationships

with the dermis and muscles. The diagrams (bottom) show the branching

of simple glands to form more complicated structures.

112 STRUCTURE OF THE VERTEBRATES

origin. Functionally all the ectodermal structures depend upon
the dermis for support, and as the blood vessels lie in the der-
mal layers the nutritive supply is also from these tissues.

Phylogenetically, the earliest ectodermal structures are glands.
In the three most primitive classes (cyclostomes, fish and am-
phibia) there are no other structures of importance. In the
cyclostomes and fish these are mucous glands which keep the
skin clear of accumulated dirt, and are to a certain extent pro-
tective. In the amphibia the same functions are present, and in
addition most amphibia depend upon the mucous to keep the
skin moist, which is necessary for the skin's respiratory function.

All glands of the vertebrates are based upon three simple
types: (1) single secreting cells; (2) tubular glands which are
straight or coiled impocketings of the ectoderm; and (3) acini-
form glands which are flask-shaped, the neck being the duct to
the outside and deeper part the acinus. Either of the last two
types may be branched, (1) as simple branches, or (2) into
highly complex structures.

Functionally all glands go back to the individual, secreting,
epithelial cell. These cells are modified for secretory activity;
and the most complex gland is merely a collection of cells spe-
cialized for the same function. The duct serves as an outlet,
the lining cells being flattened and not secretory.

Reptiles. The reptile skin is almost free of glands. A few
buccal, or mouth, glands are present; and most reptiles have
scent glands in the anal region. In a number of turtles and
snakes the odor is so distinctive that the animal can be de-
tected and identified without seeing it.

Ectodermal scale _^^ Dermal plate

,." Epidermis

Papilla Alligator Skin (Long. Section)

tizardSkin (Long. Section) with dermal scale embedded in

the skin.

Fig. 49. Sections through Reptile Scales. Reptiles are characterized by
ectodermal scales, although in some forms these are supported by

dermal plates.

STRUCTURE OF THE VERTEBRATES 113

Ectodermal scales are an identifying character of the reptiles.
The lizard skin is typical of the primitive type. In this group
the scales are usually sharply pointed at the posterior tip, over-
lapping the one behind. Under each scale is a mesodermal
papilla, and so intimately connected are the two that the epi-
dermis may be removed without destroying the appearance of
the skin. In the turtles the scales are wide and flat, but are
essentially like those of the lizard. The snakes have dorsal scales
like the lizard, but the ventral scales, or scutes, are wide with a
rib attached at either side.

Claws arose first in the reptiles as modified areas of epi-
dermis on the tips of the toes. They are present in all reptiles
with feet. The development of claws is discussed under the
mammal (page 115).

Structures of particular interest to the evolutionist are sen-
sory organs in the skin of lizards. Under each scale there is a
minute invagination with a nerve ending at its base, and from
the slight pit a hair-like sensory thread projects. The evidence
indicates that this is the reptilian homologue of the mammalian
hair.

Birds. Like the reptiles, the birds are almost devoid of skin
glands. Some have scent glands around the anus, and most of
them have oil glands at the base of the tail feathers. Scales and
claws were carried over to the bird, the legs of most birds be-
ing encased in typical reptilian scales, with their toes terminated
by claws. The horny beak is also an epidermal derivative.

Feathers are modified scales, although highly different in the
full grown condition. There are two major types of feather: (1)
down feathers, found in the young and scattered under the larger
feathers of the adult; and (2) contour feathers. The latter are
the typical feathers, the flat vanes spreading on either side of
the central quill. These vary in size and structure from the hair-
like filoplumes to the large quills of the wing and tail.

Mammals. The mammalian skin and its derivatives are so
highly modified that only a few of the latter can be thought of
as directly inherited from the reptiles. The claws, nails and
hoofs; the scales of the primitive orders; and the pigment are

114

STRUCTURE OF THE VERTEBRATES

inherited. The hair has a possible homologue, but the skin glands
are unlike anything in the ancestral groups. In addition, most
orders have some epidermal modification which arose inde-
pendently.

""Papilla

Developmental Stages of Feather

Scales

Beak

Head of Pigeon

Homy beak

Mouth
cavity

Tongue

-Mandible

Cross Section of
Bird's Beak

Fig. 50. Integumentary Structures of the Bird. The scales are homologous

with the feathers. The horny beak is a modification that has appeared

in other toothless forms.

Scales

Tail of White Rat

Body Scales of Manis^

Fig. 51. Scales of the Mammal. Scales are usually limited to the tail
region in the Mammalia. Manis (Fig, 35) is covered with overlapping

ectodermal scales.

STRUCTURE OF THE VERTEBRATES

115

1. Scales. The earliest mammals were midoubtedly supplied
with scales. Among the primitive orders today there are many
illustrations of their survival, particularly in the tail region which
is most liable to retain primitive characters. A casual examina-
tion of the tail of a rat or mouse will demonstrate the condition.
Manis, an edentate (page 87), is completely covered with large
scales. The hairs, in groups of three, grow out from the under
side of the scale, in the same relative position as the sensory
threads of the lizard. Scales are absent in the human, such
freaks as Barnum's "Alligator Boy" being diseased conditions
and not reversions to a primitive type.

Monkey

Man

Echidna Carnivore

Fig. 52. Sections of Claws and Nails, showing the forward growth of the
ball of the finger, and the shortening of the ventral plate of the claw.

2. Claws, Nails, and Hoofs. The primitive ending of the
mammal's digits is the claw, similar in origin and structure to
that of the reptile. Claws are retained by the insectivores, ro-
dents, carnivores and others. During the course of evolution
modification has proceeded in two directions: (1) in the her-
bivorous, running animals the claws have become rounded
hoofs, and (2) in the primates the claws have gradually become
flattened into nails, which reach their culmination in the human.

3. Pigment. Although the pigment of the lower vertebrates
is more often located in the dermal layer as pigmented con-
nective tissue cells, pigment is extensively scattered through the
epidermis of reptiles and birds. In the mammals the pigment
layer is apparently limited to the epidermis of the skin. The
pigment granules are located within the cells and give the skin

116 STRUCTURE OF THE VERTEBRATES

and its derivatives their color. The baboons have patches of
brilliant scarlet or blue (the combination giving a purplish
color). In most mammals the pigment is a dark brown, the
amount and position giving the varying shades. In the human
hair red is not a true pigment but a soluble dye. A person, there-
fore, might be genetically red headed, with the dye obscured
by a layer of brownish pigment.

Apparently too wide conclusions have been drawn as to skin
pigment and climate. In general terms the lighter races live in
colder climates. But the Eskimos are darker than their white
neighbors to the south; the Congo pigmies are lighter than the
taller tribes in the mountains ; and the white Sicilians are darker
than some of the Negro tribes. It would appear, therefore, that
it is inherited character, and the lighter races have migrated
away from the points of greatest actinic energy. (Page 329 dis-
cusses the effects of natural selection).

Albinism is a complete lack of pigment and differs genetically
from white coat color. Melanism is an excess of pigment. Pig-
mentation in the human is a multiple factor in heredity, and
inbreeding never gives a simple ratio.

4. Hair. Hair is one of the distinctive characteristics of the
mammals, and from the evidence as to its phylogeny one may
conclude that it was first a sensory rather than a protective
structure. The long vibrissae, or lip hairs, help corroborate the
belief. The hairs originate in the ectoderm of the embryo. A
knot of cells pushes downward toward the dermis, the end
forming a hollow vesicle. As development proceeds the cord of
cells connecting the vesicle with the outer skin becomes tubular,
thus making the hair follicle. The vesicle settles against the
mesodermal papilla and invaginates around it, in this way com-
ing into contact with a nervous and vascular supply. The hair
grows by a proliferation of ectodermal cells from the bottom of
the follicle, and as growth proceeds the hair is pushed up the fol-
licle to the outside.

Most mammals are covered with hair except for a few areas
— as the glans penis, lips, and soles of the feet. Cetacea are an
exception to the rule, and the elephant, rhinoceros, and others
have very sparse hair. The distribution of hair is typical for
the species or race; in many species there are sex differences;

STRUCTURE OF THE VERTEBRATES

117

and there are definite regional differences. Hair may be defini-
tive in length or continue growing. The latter is usually more
marked in the male than the female. There is no definite
moulting of hair, though the covering tends to be thicker in win-
ter than in summer in animals of cold climates. Hair is con-
stantly being replaced in the individual, unless the growth cells
at the base are killed, and as one falls it is soon replaced by a
new one growing from the bottom of the follicle.

Each hair has a central medulla with concentric layers of
cells around it. The young cells at the base of the hair are
cuboidal, but as the hair is pushed upward the cells flatten and

/ctodermal
inpocketing^

.Stratum Stratum

comeum [germinatwum

Fig. 53. Development of Hair. The four stages represent progressive
embrj^onic growth, and are not found in the same specimen.

die. Those on the outside, the cuticular layer, overlap the more
proximal ones and give the hair a scaly appearance. Many
species, like the human, have smooth hair. Others have elongated
cuticular scales and have a felting qualit}^ like the wool of
sheep. Some animals (rabbits and hares for example) have long

118

STRUCTURE OF THE VERTEBRATES

hairs with an under layer of hairs which are used for making
felt.

5. Glands. The glands of mammals are apparently new de-
velopments within the class. Anal or other scent glands may be
present, but the principal integumentary glands are (1) sweat
and (2) sebaceous, or oil. The sweat glands are simple tubes
with the inner end highly coiled. The secretion assists in heat
regulation by the evaporation of water on the skin, and they are
to a certain extent excretory. Salts and a slight amount of nitro-
gen wastes are thrown out with the sweat. Dogs and some other
animals lack the sweat glands, the heat regulatory function

Stratum comeum
Germiruitivum

Dermis

y^Ectodermal inpocketing^^^

Duct,

Development at
4 to 5 months
of foetal life

Development^^
at 7 months
,of foetal life

Definitive
Structure

Fig. 54. Development of the Sweat Glands. (Adapted from Kollman).

being taken over by the vascular tongue and mouth, and the
correlated loss of heat from the lungs.

The sebaceous glands empty into the follicles in which the
hairs are located and keep the hair and skin oiled. These glands
differ from most secretory epithelium cells in that the cell is
destroyed when the secretion is thrown out. Consequently the
gland repairs itself by cellular divisions in the layer next the
basal membrane which bounds the epithelial covering.

Mavimary, or milk, glands are diagnostic of the mammals.
They are probably modified sweat glands. Although present in
both male and female they are rarely functional in the male. In
the female they are highly functional at the end of pregnancy.
Both periodic and sex differences in function are controlled by

Amputated

limb bud Two Months

ositionoS Embryo

definitive gland

Milk line Epidermis
Limh bud

Four Months Embryo

'Human Embryo

of six weeks showing anlage

of the mammary glands.

Duct-*-

Seven Months Embryo

Fig. 55. Development of Mammary Glands. The embryo (left) shows the
continuous milk line which degenerates in man, except in the pectoral
region. Other mammals develop a series of mammary glands on either
side. Super-numerary nipples appear in man as an anomaly. The cross
sections through the milk line show three stages in the development of the
glands and nipple. Compare with Fig. 54. (After Kollman, Tourneux

and Basch.)

B Marsupial
before lactation

iP Seven months
Human Embryo

C Marsupial
Lactating

F Cow, showing
teat or pseudo-
nipple.

Fig. 56. Diagrammatic drawings of Mammary Glands and Nipples. A
comparison of drawing D (Fig. 56) with the Seven Months Human
Embryo in Fig. 55 will explain the relationships of structures. (After

Weber) .

120 STRUCTURE OF THE VERTEBRATES

glands of internal secretion. Milk glands are normally found on
the ventral surface of the body, varying widely as to number
and position.

The monotremes (page 76) have the glands opening into
slight depressions on the posterior abdominal wall. The young
lap the milk, much as a kitten licks milk from a saucer. The
marsupials (page 77) have developed teats to which the young
become attached after birth. In the opossum there is a single
gland duct leading into each teat.

The placentals usually have several ducts leading into a teat.
In the more primitive type, as in the primates, the ducts empty
directly to the outside at the end of the teat. In others, as the
cow, there is an inpocketing of ectoderm at tlie nipple, forming
a slight reservoir in the teat with the milk ducts opening proxi-
mally into it. The increased number of ducts in the placental
gland furnishes a more abundant supply of milk to the more
highly developed young.

The primitive placentals had a double row of mammae along
the abdomen, but in the more specialized orders tliey are more
limited in number and located either pectorally or inguinally.
In the embryos of these animals there are usually several pair
which do not develop; but accessory, functionless, nipples are
frequently found in both sexes of the human and other placentals.

6. Horns. Horns and antlers, which are found in the artio-
dactyls (page 87), are distinct in development and structure.
Horns have a permanent bony core with an epidermal covering,
and the entire structure is permanent throughout life. The Long

^Frontal bone

Fig. 57. Growth and Structure of Horns. Horns are permanent, with a
bony core and an ectodermal cover.

STRUCTURE OF THE VERTEBRATES

121

Horn steers of the western plains were almost an end product
in horn development, some of them measming eleven feet from
tip to tip. After death the core shrinks and the epidermal cover
is easily removed. The prong-horn antelope (Antilocapra) is an
exception. This animal has a tj'pical bony core in the horn, but
the ectodermal cover is periodically shed.

Fig. 58. Prong-horn Antelope (Antilocapra). An
American artiodactyl with horns apparently inter-
mediate between true horns and antlers. The bony
core is permanent, but the ectodermal cover is shed
annually. The diagram shows the horn at time of
shedding, with the skin surrounding the core.

Antlers, on the other hand, are shed each year and replaced
with larger ones until the deer, moose or elk approaches senility.
The antlers are lost in the early spring and the new ones begin
their development soon after. Bone building cells congregate at
the stub of the shed antler and growth begins. The food supply
is gained from the vessels of the skin which is carried out over
the growing antler. When full size is reached the skin constricts
at the base of the antler, dies, and is rubbed off. The antler,
consequently, is bone, and logically has its place with the skeletal
svstem.

Fig. 59. Growth and Structure of Antlers. During the growth season the
antler is covered with skin which is shed when full size has been reached.
The antlers are shed annually, and the skin covers the growth base on
the frontal bone (D). The growth process repeats itself during the spring
and earlv summer.

122 STRUCTURE OF THE VERTEBRATES

7. Other Structures. Under this heading are listed a few of the
epidermal structures which are limited in distribution. (1) The
horn of the rhinoceros has no core, and is composed of epidermal,
hair-like fibers. (2) The whales which lack teeth have an ecto-
dermal strainer in the mouth (page 91). The horny material is
tough and flexible and was of commercial value in a past gener-
ation. (3) The ischial callosities on the rump of many Old World
primates are callous-like thickenings of the epidermis. These
are genetic characters, and differ from the acquired callouses on
the hands and feet of humans.

End of tail

Rhinoceros Horn Rattle of Rattle-snake

Fig. 60. Other Ectodermal Structures of the Integument.

Located in the skin as ectodermal derivatives are a number
of nerve receptors and organs of special sense. Perhaps the most
prominent of these is the lateral line system of the fish and
amphibia. The structure and relationships of the organs is dis-
cussed in Chapter XVIII, under Organs of Special Sense.

C. Human Skin

The skin of man makes an excellent summary of the skin and
its derivatives. The illustration gives the relationships of the two
primary layers of the integument, and the position and structure
of its outgrowths. The dermis is fairly thick and is of a loose
fibrous structure. The stratum germinativum has 'a waved ap-
pearance, being several cell layers in thickness. Immediately
above it is the pigment layer, the number of granules varying
in different races and parts of the body. The thick outer layer is
the stratum corneum. This last may be very thin, as on the lips,
glans, etc.

STRUCTURE OF THE VERTEBRATES 123

Friction ridges are found on the soles and palms. These are
evidently remains of the scaly covering of the ancestral types.
Following the work of Galton the study of finger prints has be-
come an independent science. As no two hands entirely agree in
configuration, but fall into definite groups, indexing is easy and
identification is absolute. These ridges apparently change little
throughout life, and if all hospitals adopted the methods of the
more progressive ones in making finger or foot prints of the
mother and newborn child before leaving the operating room,
the too frequent litigations over mixed babies in hospitals would
end immediately.

The sweat glands pass well into the dermis before coiling, and
are distributed over the entire body without relationship to the
hair follicles. The sebaceous glands, on the other hand, evidently
developed in correlation with hair. In the human these are quite
large. If the follicle of the hair is closed, the secretion continues
and the gland may hypertrophy. The result is a spherical mass
under the skin, the larger ones being called wens.

Technically speaking there are no "pores" in the skin. The
only openings are the follicles and the sweat glands, and these
end blindly in the dermis. There is no possibility of a respiratory
function, although some excretion takes place. Therefore, the
alleged cases of individuals dying from sealing the pores were
caused by lead poisoning or other extraneous conditions. The
fallacy is kept alive by soap manufacturers and the mothers of
boys.

Hair distribution has been mentioned in relation to the sex
differences and regional distribution. Related to the latter is pat-
tern distribution on the body. It is common observation that the
hair of the head grows away from the crown, and a secondary
crown or "cow^ lick" is not uncommon. The body hair has similar
patterns. In general terms, the direction of hair growth is the
same as the scale direction of the reptile: (1) from anterior to
posterior, and (2) from dorsal to ventral. On the human back
the pattern lines are antero-posterior along the spinal column,
but dorso-posterior on the sides. This is similar to the pattern
of the other primates.

Embryologically hair arises in groups of three or four, again
indicating a scale pattern as found in ]\Ianis. During later em-

124 STRUCTURE OF THE VERTEBRATES

bryonic life the foetal body is covered with a deciduous lanugo,
a soft thick coat of hair. As it cannot be interpreted in terms of
function, the only explanation offered is that it is a hang-over
from our hair ancestors. The pattern of the lanugo is the same
as that of the anthropoids.

CHAPTER XI

SUPPORTING STRUCTURES

The vertebrate supporting structures include the endodermal
notochord, and tlie mesodermal cartilage and bone. "With these
should be included the connective tissue sheaths of the latter and
the ligaments which tie them together. These are necessary for
proper function in protection and locomotion, and will be dis-
cussed under the leverage of muscles.

The original function of the skeleton was leverage, the struc-
tures serving for the attachment of muscles. With the develop-
ment of the skull and vertebral column came a protective func-
tion as the bones surrounded the nervous system ; and in the land
vertebrates the marrow of the long bones forms most of the
corpuscles of the blood.

The vertebrate has an internal skeleton. The external skeleton
of the insects and Crustacea is perhaps more efficient so far as
leverage is concerned, the muscular pull being more direct; but
what tlie vertebrate lost in leverage was more than counter-
balanced by flexibility of body parts.

The skeleton of the vertebrate is divided into two regions:
(1) the axial skeleton including the skull and vertebral column,
both of which develop around the notochord; and (2) the appen-
dicular skeleton, including the skeletal girdles and the appen-
dages. These two regions are development ally independent, but
where both exist they are functionally dependent upon each
other.

A. The Skull

As was mentioned in the first part of the book, the vertebrate
skull has a double origin, (1) from the primitive chondro cranium
(cartilage skull) and (2) from the dermal plates or bones. The

125

126 STRUCTURE OF THE VERTEBRATES

chondrocraniiim is phylogenetically and ontogenetically older^
and will be discussed first.

The embryonic chondrocraniiim of the higher vertebrates has
its counterpart in that of the cyclostome as an open trough in
which the brain lies, with a cartilage bridge over the nerve cord
at the posterior margin. The cupped hands with the tips of the
thumbs touching will assist in visualizing the structure. In addi-
tion to the original trough there are three sensory capsules which
develop synchronously. The most anterior capsule surrounds the
olfactory nerves; the middle one is the eye, or optic, capsule; and
the most posterior is the ear, or otic capsule which encloses the
organs of equilibrium, and the true ear of the higher vertebrates.
Olfactory and otic capsules form an integral part of the chon-
drocranium, but the eye capsule always remains free, leaving the
eye movable.

The development of the chondrocranium is from two pairs of
ventral cartilages, and three pairs of capsules, each arising as a
separate cartilage center. The ventral cartilages lie on either side
of the notochord, the posterior pair being the parachordal (be-
side the notochord), and the anterior are the prechordal. The
three pairs of capsules arise slightly later. As the ventral carti-
lages grow, they fuse ventrally, surrounding the notochord, but
leaving a ventral hole immediately anterior to the notochord.
This opening remains in the higher forms as the pit for the pitui-
tary gland.

The lateral margins of the basal cartilages then grow dorsally.
In the meantime the olfactory capsule has surrounded the olfac-
tory center, and the otic has surrounded the ectodermal ear
structures. These soon fuse with the chordal cartilages to form a
coherent unit. Cartilages then grow from either otic capsule to-
ward the dorsal-median line and join to make the posterior
^'bridge" or sijnotic tectum, and the chondrocranium is complete.
The posterior opening through which the nerve cord passes is the
foramen magnum, and numerous smaller foramina are caused
by the passage of nerves and blood vessels from the brain
through the chondrocranium to the body tissues.

The dogfish chondrocranium is in many ways specialized. In
addition to the simple trough a roof has developed, completely
enclosing the brain except for the foramina. Also, a rostrum has

^: --^Prechordal ^

'"-■^ — ^ -Optic capsule

Olfactory capsule

Lens
■Retina

Otic capsule

Synotic tectum'
Notochord-

A. Chondrocranial
Cartilages

B. Early Chondrocranium

Fig. 61. Development of the Chondrocranium. In (A) the cartilages are

shown lying on either side of the notochord, and surrounding the organs

of special sense; (B) shows the fusion of the cartilages to form the

primitive chondrocranial trough.

Brain

Parachordal

Otic
cap^ide

Notochord

Fig. 62. Cro.ss section of (A), Fig.
61, through the otic region.

Otic capstde

Foramen magnum

Fig. 63. Posterior View of (B),
Fig. 61. The narrow synotic tec-
tum is the only roof of the chon-
drocranium in the higher verte-
brates.

128

STRUCTURE OF THE VERTEBRATES

developed, supporting the anterior projection of the head; and
there are a few minor specialized" structures. The great advance
which the early elasmobranchs made was in the gill, or visceral,
arches. The first gill arch developed into jaws (upper and lower),
and the second into the hyoid apparatus which functions in the
dogfish as a suspensory cartilage for the jaws and a support for
the throat and tongue. The suspensory cartilage is the hyomandih-
ular. It is attached to the chondrocranium at the otic capsule,
and passes laterally and slightly ventrally to the upper jaw. The
spiracle of the dogfish is posterior to it. The upper jaw cartilage
is the ptery go-quadrate, and the lower jaw is MeckeVs cartilage.

^Rostrum

Olfactory capsule
Ptery go-qua(^r ate
Meckel's cartilage

Otic capsule

Myomandihidar

Branchial arches

Fig. 64. Chondrocranium and Branchial Arches of the Dogfish. The jaws
and hyoid apparatus are shown in position.

To summarize, the jaw apparatus of the elasmobranch is made
up of three major cartilages: (1) the suspensory hyomandibular,
(2) the ptery go-quadrate of the upper jaw, and (3) aNIeckel's
cartilage of the lower.

The chondrostean skull (page 40) is useful in showing the
probable evolution of the higher skull. The chondrocranium is
like that of the dogfish, but the cartilages have become en-
cased in bone. The upper jaw became immovably' attached to the
skull. The lower jaw was covered with the bony plates, except at
the point of articulation with the upper jaw. The articulation re-
mained between the pterygo-quadrate and jNIeckel's cartilages.

In the higher vertebrates the skull, as such, takes form. The
chondrocranium becomes ossified, or replaced by bone tissue, and
the dermal roofing bones fuse with the replacement bones of the

STRUCTURE OF THE VERTEBRATES

129

chondrocranium. The student should again be reminded that the
typical chondrocranium is like that described in the first para-
graph, with the result that the roof of the skull is dermal in
origin.

Ossification of the Chondrocranium. Bone replacement begins
in a number of centers, arranged in transverse series. Due to this
arrangement of the bones the skull was once considered as modi-
fied vertebrae, but embryology has dispelled this theory. Begin-
ning posteriorly there are four series of bones.

Ptery go-quadrate
MeckeVs cartilage
Orbit

Operculum

Rostrum

Fig. 65. The Chondrocranium and Dermal Skull of the Sturgeon (Order

Chondrostei). (A) is the unossified chondrocranium. (B) shows the external

appearance of the skull with the dermal bones overlying and completely

covering the chondrocranium.

1. The cartilage surrounding the foramen magnum ossifies as
four bones, the occipital complex. The ventral is the basioccipi-
tal, the lateral are the paired exoccipitals, and the dorsal is the
supraoccipital.

2. In front of the occipital bones is the posterior sphenoid
complex composed of three bones: the ventral basisphenoid, and
the lateral paired alisphenoids. Recall the chondrocranium shape,
and it will be clear that the alisphenoids form the sides of the
skull.

130

STRUCTURE OF THE VERTEBRATES

3. The anterior sphenoid complex is in contact with the former
group, and like it has three bones: the unpaired presphenoid and
the lateral orhito sphenoids. The latter are at the level of the
optic capsule and form part of the orbit, or cavity, in which the
eye rests.

4. The ethmoid group is most anterior, though covered by
dermal bones. These bones are developed in part from the an-
terior half of the prechordal cartilages, and partly from the

Olf. cap.

Foramen
7nagnmn

Sijnotic tectum

For. mag.

Fig. 66, Chondrocraniiim of the
Pig. Centers of ossification appear
in the chondrocraniiim soon after
the first strands of bone appear in
the dermal fibers. (After Mead).

Fig. 67. Ossification of the Chon-
drocraniiim (Diagrammatic). The
major regions are heavily outHned.

olfactory capsule. The ethmoid bones of the lower vertebrates
are not as separate as in the mammals. In the latter group there
is a ventral ethmoid; a median mesethmoid, forming a septum
between the two nostrils; and the paired ectethmoids.

The otic capsule ossifies as five separate centers. In the lower
vertebrates these are often separate. The lower mammals also
have these otic bones defined, but there is a tendency toward
fusion in the higher groups. In man the five embryonic bones
form a single petrosal bone which fuses with a dermal bone to
make the temporal.

STRUCTURE OF THE VERTEBRATES 131

Jaws. In the bony fish, amphibia, reptiles and birds the
pterygo-quadrate cartilage ossifies as the pterygoid and the
quadrate bones. The latter is the ossified posterior end and there-
fore articulates with the lower jaw. The posterior end of Meckel's
cartilage ossifies as the articidar bone, being in contact with the
quadrate. The anterior portion of this cartilage disappears with
the growth and development of the dermal covering.

In the primitive reptiles there were seven bones on either side
of the lower jaw, one replacement bone (articular) and six dermal
bones. A comparative study shows that the dentary, the most
anterior of the dermal bones, pushed farther back during the
course of evolution, occupying more and more of the jaw. In the
Theromorpha (page 59) the posterior elements, including the
articular, are small; and in the Therapsida the dentary makes
up practically all of the jaw, with the posterior bones a nipple-
like structure at the end. The point of articulation is, however,
between the quadrate and the articular. In one species of the
therapsids there is a secondary point of articulation between the
dentary and the squamosal, a dermal bone of the skull. The
mammals have completely lost the old articulation. The lower
jaw is composed of one bone, the dentary, and this articulates
directly with the squamosal.

Dermal Bones of the Skidl. As one would expect from their
origin from dermal denticles and dermal plates, the covering
bones of the skull have proceeded from the complex to the simple.
The reverse has been true in the more fundamental systems.
Therefore, clarity may be gained by describing the dermal bones
of a simple mammal first, and comparing afterward. The opos-
sum, a marsupial (page 78) is sufficiently generalized to serve
the purpose.

The top of the skull, is covered by four pairs of bones, the
longitudinal suture, or point of contact between two bones, being
the mid-dorsal line. From posterior to anterior these are: (1)
the dermal supraoccipital, completely fused with the replacement
occipital and indistinguishable from it except in the embryo;
(2) the parietals, covering the posterior part of the brain case
and extending over the sides of the skull; (3) the frontals, the
longest of the dorsal bones, bordering and extending into the

132

STRUCTURE OF THE VERTEBRATES

orbits, or eye sockets; and (4) the nasals, which roof the nostrils
or anterior nares.

On the side of the face are: (1) the squamosal, in contact with
the parietal and occipital bones; and forming the temporal re-
gion and the bulla of the ear, and a part of the zygomatic arch,
to which jaw muscles are attached; (2) the jugal which com-
pletes the arch below the orbit; (3) the maxilla, a large bone
of the face, carrying most of the teeth and extending across the
roof of the mouth to form the hard palate; (4) the premaxilla,
which completes the skull anteriorly, bounds the ventral margin

Premaxilla-'..^yX

IX

Mfl^i//a-^^^ \

1/

NfUiol-..^ J

\\

^^-^^K

^

Jugal-^
Fronf.nl~-l^

I7\

^

n

T\

ParietajJlj'

^

ii

il

SQuain.osn.l\\^^^^^ j J

^

^

Or^ipiioZ— -^^^v;;>'^^

o

Fig. 68. Op

ossiim S
View.

kuU,

Dorsal

Maxilla

Palatine^

Sphenoid

Fig. 69. Opossum Skull, Ventral
View.

of the nostrils, and bears. the front teeth; and (5) the lacrimal,
a small bone at the anterior margin of the orbit which is punc-
tured by the duct from the lacrimal gland.

The ventral side of the skull shows few dermal bones except
in the anterior region. The roof of the mammalian mouth is bony,
and covered by the maxillae and the more posterior palatine
bones. The formation of a hard palate has carried the internal
opening of the nares back to the naso-pharynx. This is found
only in the mammals, the mammal-like reptiles, and a few other
reptiles. In the amphibia and most reptiles the nares open di-
rectly into the mouth cavity.

STRUCTURE OF THE VERTEBRATES 133

The loss of dermal bones is more easily studied with pictures
than description. The fish has been largely omitted on account of
the complexity of the skull, the teleost particularly having little
relation to other forms. The primitive fish had a more typical
skull, which was complicated, however, by the bony operculum
covering the gills. This gill cover is found in all the fish above
the elasmobranchs, and one or two of the opercular bones were
carried over to the earliest amphibia.

Premaxilln

Nasal

Maxilla

Lacrimal

Frontal
Prefrontal
Postfrontal

Orbit
Jugal

Parietal
Squamosal
uad. -ju
Quadrat

Occipital

A. Capitosaurus
(Stegocephalia)

B. Seymouria
(Extinct Reptile)

Fig, 70. Primitive Skulls, Dorsal Views. (A) Capitosaurus (Order

Stegocephalia) a primitive extinct amphibian. (B) Seymouria, a primitive,

extinct, anapsid reptile.

The recent amphibia have a very specialized skull structure.
The typical primitive form is illustrated by the Stegocephalia
(page 50). The earliest amphibian (Archegosaurus) was ex-
tremely fish-like, resembling the fossil Crossopterygian in struc-
ture. The later members of this primitive order diverged in two
directions, (1) toward the unarmored amphibia and (2) toward
the reptilian form. Until recent research definitely placed the
latter as amphibia thej- were classified as Pro-reptilia.

The reptiles continued the loss of bones, the elements of the
skull becoming reduced from 76 to 26 in the mammal. The reduc-
tion in number was gradual, only two pairs of bones being found

134 STRUCTURE OF THE VERTEBRATES

in the mammal-like reptiles which are unknown in the mammals:
(1) the post-frontals and (2) the post-orbitals. The posterior
jaw elements have been transferred into the skull as ear ossicles.

Evolution of the Skull. The evolution of the mammal skull
is intimately correlated with the shifts of the dermal bones out-
lined above. In addition to the chance mutation of the bones,
there are ontogenetic influences which depend upon the modifica-
tion of other structures. The development of the brain dictates
the size of the calvarium, or brain case. Pathological conditions
in the human demonstrate that the skull takes the size of a

Dentary

'Articular,

Turtle (Living Anapsid Reptile)

Seymouria (Extinct Anapsid)

Fig. 71. Skulls of Anapsid Reptiles. Side View. (A) Turtle, (B) Seymouria.
Observe the lack of teeth and the greater depth of the turtle skull.

degenerate brain, and an increase in the amount of cerebral
fluids causes a hypertrophy of the calvarium. In other words,
although bone is the hardest tissue of the body, developmentally
it is the most easily modified. A second influence is the shift in
the muscles of the head and face. There is evidence that changed
size and position of muscles directly influence the shape and
position of bony elements. It is equally true that the reverse is
true, and that bone changes cause the increase or decrease in the
length of muscles.

The definitive vertebrate skull was laid down in the Stego-
cephalia. The brain case was very small, with a huge roof of
dermal bones. The muscles of the jaw, neck and face were at-
tached to the under side of the roof, and the skull was shallow

STRUCTURE OF THE VERTEBRATES

135

dorso-ventrall}' and very wide. The majority of the skull space
was taken by the lateral projections, and the huge mouth.

In the primitive reptile (Seymouria) the skull had deepened
and shortened. At the posterior end there was an otic notch. The
dermal roof was unbroken except for the anterior nares and the
orbits. This is the type skull of the sub-class Anapsida (without
an apse, or arch). The turtle is the living representative of this
early group (page 60).

The next development in the skull was the breaking through
of an opening (fenestra) in the roof, through which the temporal
muscles of the jaw passed to the upper surface of the dermal
bones. The brain case was enlarged, though still occupying a

Fenestra

Dentarp'

Surangular

Fig. 72. Skull of Alligator, a Diapsid Reptile. The face is greatly elon-
gated, and two pairs of fenestrae have developed in the temporal region.

small part of the total mass of the skull. This sub-class was
called the Synapsida, through the mistaken belief that the
opening was a fusion of two fenestrae. As mentioned (page 60)
the mammal-like reptiles carried this development to its logical
conclusion. The dinosaurs and Crocodilia were an offshoot from
the early Anapsid stem, and with a few less well known orders
form a sub-class, the Diapsida, which have two fenestrae on
either side.

The Synapsida were the ancestors of the mammals. A study
of the drawings will show that the fenestra became larger, leav-
ing a lateral border of dermal bones, with the jaw muscles gain-
ing a more prominent footing on the upper surface of the cal-
varium. The brain had enlarged, occupying more room in the
skull, and the face had shortened materially. In the Therapsida

136

STRUCTURE OF THE VERTEBRATES

the skull had practically assumed the mammal form. The ventral
border of the skull below the fenestra had become a distinct
zygomatic arch to which the outer jaw muscles (masseter mus-
cles) were attached, while the temporal muscles had their origin
on the side of the calvarium. The latter pass through the zygo-
matic opening to the jaw.

Fenestra

Position of
zygoma

Quadrate
■Articular

Surangular

Fenestra

Squamosal

Articular
urangular

Angular

Fig. 73. Evolution of
the Mammal Skull. (A)
Primitive mammal - lilve
reptile (Order Thero-
morpha) ; (B) Special-
ized mammal-like reptile
(Order Therapsida) ; and
(C) a mammal (Opos-
sum). Observe the en-
largement of the lateral
fenestra, the growth of
the brain case, and the
loss of the posterior
lower jaw bones.

•Sqiia7nosal

STRUCTURE OF THE VERTEBRATES

137

The general characteristics of the mammal skull have been
discussed. The evolution of the human skull shape depended
upon the shortening of the face and the great growth of the
brain. These two movements automatically carried the calvari^-m
upward and forward, until in the human a line drawn from the
foramen magnum to the chin is at right angles to the vertebral

C. Chimpanzee

F. Recent Man

'*^W^^^

B. Old World Monkey

E. Neanderthal Man

A. Lemur

D, Pithecanthropus

Fig. 74. Skulls of Primates. Note the increase in size of the brain case
(calvarium) and the shortening of the face. The calvarium is stippled.

138 STRUCTURE OF THE VERTEBRATES

column. In other words, the mouth and face have been sub-
merged under the calvarium. A comparison of the opossum,
lemur, monkey, anthropoid ape, a primitive man, and the present
man will demonstrate this line of evolution.

The Human Skull. The early embryology of the human fol-
lows the stages outlined under the development of the chon-
drocranium. As bone cells begin the replacement of cartilage, fine
strands of bone can be seen in the deeper layers of the skin.
These bone strands begin at definite centers, one for each bone
of the skull. As the bony area spreads, the dermal bones come in

Frontal

Parietal

Parietal

Occipital

Top View

^^^— ■^\ ^'^^^^^^^ ^Occipital

Squamosal
ympanicring
2ygorm

Side View

Fig. 75. Skull of a new born Human Infant.

contact at their rounded margin, and naturally leave spaces at
the junction of four rounded corners. These spaces are the jon-
tanelles. The "soft spot" on top of the young human is the fon-
tanelle formed by the juncture of the parietals and frontals.

In structure the human skull is only a shortened, dorsally en-
larged mammal skull. As in other mammals, spirals of bone have
grown into the nasal cavities, arising from the ethmoids and the
dermal bones. These serve the function of moistening and warm-
ing the air, and on them are located the olfactory sense endings.
The olfactory nerves pass back toward the brain as a number
of small nerves, each piercing the ethmoid bone which forms the
anterior of the calvarium. This sieve-like bone is the cribriform
plate. The nerve foramina are packed into a smaller area than in

STRUCTURE OF THE VERTEBRATES

139

the lower forms, due to the antero-posterior shortening. One
other structure is of phylogenetic interest. The pituitary opening
at the end of the notochord was described. From the dogfish to
man this pit becomes closed ventrally to form a pit in the floor
of the skull. In the mammal this pit is bounded anteriorly and
posteriorly by sharp ridges of bone and is the sella turcica
(Turk's saddle) in which the pituitary gland rests.

Teeth of Vertebrates. The vertebrate teeth are homologous
with the dermal denticles of the elasmobranchs. In this group the

Fig 76 Types of Tooth Attachment. (A) Acrodont, attached on biting

margin of jaw; (B) Pleurodont. attached on mner side of jaw; and (C)

Thecodont, the teeth sunk into cavities or thecae.

Tooth

Tooth

Dentary

Tooth

A. Acrodont

B. Pleurodont

C. Thecodont

Fig. 77. Cross Sections showing Tooth Attachment,
teeth are continuous with the denticles of the jaws, and are em-
bedded in the skin without any contact with the jaw cartilages.
The teeth are all alike and can be replaced indefinitely.

With the development of bony jaws the teeth came in contact
with the bones, and fused to the tooth-bearing bones either (1)
on biting rims of the jaws, or (2) along the inner sides of the
bones. In the amphibia and primitive reptiles the teeth are all
similar, although some may be slightly longer than the others.

The Crocodilia retain the tooth similarity, the homodont con-
dition, but show advance in having the teeth sink into the jaw
bones, fitting into sockets. The mammal-like reptiles also have
teeth in sockets, and other advances. In these reptiles the teeth

140 STRUCTURE OF THE VERTEBRATES

become heterodont, the front being differentiated from the back.
In the higher (Therapsid) types there are front incisors, long
canines, and primitii^e molars posteriorly. The evidence indicates
that the number of replacing teeth was also reduced.

The mammals show the evolution of the teeth from the simple
chfferentiated condition to the highly specialized condition found
in many orders. The primitive mammals had molars with three
points or cusps. In time accessory cusps developed, and so stable
and definite are these cusps that the evolution of the different
orders can be traced by their tooth structure.

The mammals have, two sets of teeth, the baby or milk teeth,
and the permanent teeth which replace the first. The posterior
molars are an exception to this rule. Those which are replaced
are: (1) the incisors which are borne on the pre-maxillary bones
of the upper jaw, and the homologous teeth of the lower; (2) the
canines, the first tooth on the maxilla of the upper jaw, and the
lower tooth which fits in front of it; and (3) the premolars,
posterior to the canines, and having more than one cusp in the
normal condition. (The Cetacea and some other animals are ex-
ceptions to this rule.) Posterior to the premolars are the per-
manent molars, which are not replaced after the original erup-
tion of the tooth. The difference between premolars and molars
often cannot be determined without this knowledge of develop-
ment.

Dental Formula refers to the written formula expressing the
number of different teeth in any particular species. In these
formulae I represents incisor; C, the canines; P equals the pre-
molars, and M the molars. One side of the jaw is given, so that
the formula is multiplied by two. The upper teeth are placed

2 1 2

above a line, the lower below it, thus: I — -^;C — -ir; P — -o,*

3

]\/[ . This is the human formula, and when the numbers are

o

added and doubled, it gives the normal thirty-two teeth of the
species.

3 14 3

The primitive dental formula of the placental is: _; _; _; -

o i 4 o

— 44 teeth in all. More specialized animals have many varia-

\

STRUCTURE OF THE VERTEBRATES

141

tions of this formula, a general reduction in number being typ-
ical. The carnivores tend to lose the molars, and the rodents
usually have no canines and anterior premolars. The loss of
these teeth leaves a wide diastema, or space, between the anterior
and posterior teeth. The Cetacea, which have teeth, always have
an increased number.

A. Man

B. Rabbit

Fig. 78. Mammal Jaws, showing position of Teeth. Observe the wide
space (diastema) between the incisors and premolars of the rabbit.

Development of Teeth. As one would suspect, the verte-
brate teeth and the dermal denticles have may points of devel-
opment in common. (See page 109.) The ectoderm covering the
body swings over to line the lips, gums and part of the mouth
cavity, and it is from the gum ectoderm that the dental vesicles
develop. From the embryonic ectoderm a cord of cells pushes
downward, the inner end enlarging to form a hollow vesicle. As
this ectodermal vesicle meets the knot of mesodermal cells which
forms a papilla, the vesicle invaginates to form a double cup.
The cells of both mesoderm and ectoderm then begin secreting
an inorganic deposit of hard material. The mesodermal body of
the tooth is the dentine, the ectodermal cover is the enamel. The
latter is the hardest and most completely inorganic substance
of the body. As the tooth takes form it is pushed upward un-
til it breaks through the gum tissues. The papilla, which con-
tains nerves and blood vessels, forms the pulp cavity inside the
tooth. The tooth proper has neither nerves nor blood vessels,
the pain from tooth decay or a dentist's drill being transmitted
to the pulp cavity.

The majority of mammals have teeth with closed pulp cavi-
ties, and these teeth do not grow after eruption is completed un-
less the top of the tooth is worn off and the dentine forming

142

STRUCTURE OF THE VERTEBRATES

cells are released. In such cases the tooth developing cells begin
forming bone on top of the pulp cavity. And conversely, if den-
tine begins to fill the cavity, bone destroying cells normally re-
sorb it as rapidl}^ as it is developed.

Other mammals have certain teeth with an open pulp cavity —
that is, the base remains wide open. Under these conditions the

•Jctodermal inpocketing

Secondary
vesicle

Fig. 79. Development of Teeth (Man). Note the inpocketing of ectoderm
(A) to form a dental vesicle which meets the mesodermal papilla; the
development of a secondary vesicle and the beginning of tooth develop-
ment (B) ; and the growth up to the time of eruption (C) and (D).

dentine forming cells may continue to secrete without filling the
inner cavity, and in that way killing the tooth. As dentine is
laid down on the inner side of an open cavity, the tooth is
pushed upward and Gontinues growing throughout life. The in-
cisors of rodents and several other animals continue their growth,

STRUCTURE OF THE VERTEBRATES

143

but are normally worn down by w^ear against hard substances.
AYhen removed from their barren grazing lands the incisors of
llamas have been known to grow so long that the animals died
as a result. The canine tusks of the pig and some carnivores are

Enamel-
Dentine

Fig. 80. Structure of In-
cisor Teeth. Note the
structure of the open
pulp cavity which per-
mits the continued up-
ward growth of the
tooth.

A. Closed Pulp Cavity

B. Open Pulp Cavity

similar in structure. The tusks of the elephant are the second
incisors of the upper jaw, lack enamel, and are the largest teetli
known. Certain extinct elephants had tusks eleven feet long,
weighing more than 250 pounds each.

B. Visceral Skeleton

The visceral, or branchial, skeleton is technically an entity,
although it forms an important part of the skull and can be
considered with the axial skeleton of the vertebrate. There is
no proved homologue of the branchial arches below the elasmo-
branch fishes. The numerous gill bars of Amphioxus afford no
clue as to the origin of vertebrate gill supports, while those of
the jawless cyclostomes are too highly modified to be of much
assistance. The latter group today has an intricate branchial
basket which has little relationship. with the gill arches of the
other vertebrates.

In the most primitive sharks there are nine visceral arches
in the embryo, with rudiments of several more; but when the
number of gills was reduced to the typical five, the number of
arches was also reduced to seven. The anterior two of these be-
came modified into jaws and supporting structures of the throat

144 STRUCTURE OF THE VERTEBRATES

and tongue, the posterior five remaining as the branchial, or gill,
arches.

The evidence for the homology between the jaws and the
visceral skeleton is based upon several factors: (1) in the very
early embryo of the dogfish the first visceral cartilage is clearly
divided into two major portions, but these form a wide angle;
(2) the second (hyoid) arch lies anterior to the spiracle which
carries rudimentary gills; (3) the muscles of the jaw and hyoid
arise with the branchial muscles from the hypomere as the
visceral muscles; and (4) the innervation of these muscles
completes the proof of homology. The muscles and nerves are

TT^.

^:£^j^F^^

Meckel's cartilage «

'^n 'm '^ IV " V VI VII

Fig. 81. Chondrocranium and Visceral Arches (Diagrammatic). Arches I

and II form the jaws and hyoid apparatus; Arches III to VII form the

branchial cartilages.

undoubtedly the strongest evidence. The illustration of the chon-
drocranium and visceral arches of the dogfish shows the rela-
tionships of these cartilages to the chondrocranium.

The enclosure of the upper and lower jaws by dermal bone
has been discussed. AVith this development of the dermal skull
the hyomandibular of the fish became a practically functionless
structure, lying in close contact with the spiracle. This is the
condition in the Crossopterygian fishes, the hyomandibular
bone being very small.

The amphibia evolved an entirely new relationship between
these structures. The outer (distal) end of the spiracle enlarges
and engulfs the hyomandibular into its cavity, and a membrane
o-f skin closes the outer opening of the cavity. In this way the
middle car of the amphibian is developed. The skin membrane

STRUCTURE OF THE VERTEBRATES

145

is the tympanum or ear drum, and the opening into the pharynx-
is the Eustachian tube. Recall the position of the hyomandibu-
lar, its proximal end in contact with the otic capsule. Its posi-
tion in the amphibia is unchanged, except for its inclusion
within the middle ear. Therefore the hyomandibular of the fish
becomes the columella of the amphibia, and is the ear ossicle
which assists in transmitting sound waves from the tympanum
to the inner ear which contains the organs of equilibrium and
the rudimentary hearing apparatus.

Brat
Cavity

Pharynx — Spiracle j ^Pharynx

PharynxA;

'Eustachian'
tube

A- Dogfish

B. Froff

C. Mammal

Fig. 82. Cross sections of the Pharynx (Diagrammatic). The spiracle (A)

develops into the middle ear and Eustachian tube (B), closed by the ear

drum. The Mammal (C) shows the deep position of the ear drum and

the elongated Eustachian tube.

In the reptiles and birds the same conditions exist in regard
to the three replacement bones under discussion: (1) the articu-
lar, (2) the quadrate, and (3) the columella. In the mammal-
like reptiles the replacement bones of the jaws become smaller,
and an accessory point of articulation develops between the der-
mal dentary and squamosal.

The embryo of the mammal demonstrates the final evolution
of the three jaw cartilages. The hyomandibular cartilage re-
mains as an ossicle of the middle ear, and is called the stapes
in the mammal. The minute end of the pterygo-quadrate car-
tilage ossifies in close contact with the hyomandibular and is
also included within the middle ear cavity as a second ossicle,
the incus. The third mammalian ossicle, the malleus is a deriva-
tive of the ossified ]Meckel's cartilage, and is homologous with
the articular bone of the amphibia and reptiles.

146

STRUCTURE OF THE VERTEBRATES

Those relationships can be ascertained by staining the car-
tilages of the early embryo and clearing the tissues. It can be
then demonstrated that the IMeckel's cartilage passes from the
lower jaw into the skull, and its posterior end is in contact with
the quadrate cartilage. As growth continues the jaw part of the
cartilage degenerates, and the posterior end ossifies as an ear
ossicle. The relationships of the three cartilages is unchanged,

Hyomandihular-

Columella
(Stapes)

■Squamosal
uadrate
Articular

A. Elasmobranch

B. Reptile

'Squamosal
uadrate
rticular.

Angular
Surangular

tympanic
ring

r ossicles
Squamosal

Dentary

C. Mammal-like Reptile

D. Mammal

Fig. 83. Jaw Articulation and the Development of the Ear Ossicles. The
Dogfish (A) has the three jaw cartilages. The typical reptile (B) has the
skull encased in dermal bone, cartilage bones forming the jaw articulation,
the hyomandibular being an ear ossicle; the mammal-like reptile (C)
shows the back growth of the dentary; and the mammal (D) has the
articulation between dermal bones, the quadrate and articular having
passed into the middle ear as the incus and malleus.

STRUCTURE OF THE VERTEBRATES

147

but their position has shifted. The homology is confirmed not
only by the embryology of the cartilages, but by the muscles
attached to the stapes and their innervation. The latter proofs
are too complex for this discussion.

Pte')-y go-quadrate

T ympanic ting

Fig. 84. Mammal Embryo with Unossified Jaw Cartilages. The proximal

ends of the pterygo-quadrate and Meckel's cartilages lie within the

tympanic ring and ossify as ear bones.

The posterior five arches, the branchial arches of the fish,
eventually become transformed into the cartilages of the larynx.
The primitive amphibia which retain gills naturally retain the
branchial arches in reduced number, the others forming discon-
nected throat cartilages. The larynx is completed in the mam-
mals and will be discussed under the respiratory system. The
following chart shows the homologies in tabular form.

Elasmobraxchs

Amphibia
Reptiles

Mammals

Hyoman-
dibular

Jaw Suspension

Columella, or Stapes
(Ear Ossicle)

Stapes
(Ear Ossicle)

Pterygo-
quadrate

Upper Jaw

Quadrate
(Jaw articulation)

Incus
(Ear Ossicle)

Meckel's
Cartilage

Lower Jaw

Articular
(Jaw articulation)

Malleus
(Ear Ossicle)

Branchial
Cartilages

Gill supports

Partly or entirely

lost
Throat supports

Laryngeal
Cartilages

148 STRUCTURE OF THE VERTEBRATES

C. Vertebrae

The vertebral column is part of the axial skeleton. The indi-
vidual vertebrae develop from mesodermal tissues which are laid
down around the endodermal notochord. The latter becomes
constricted in the elasmobranch fishes, and completely obliter-
ated in the adults of the higher vertebrates.

Cyclostomes have the most primitive vertebrae of any class.
Each vertebra is composed of six minute cartilages, three on
either side, which have no connection with each other. The verte-
brae are separated from each other by the myoseptum. It is
difficult to ascribe any function to these discrete elements.

The Elasmobranchs have well developed, cartilaginous verte-
brae which are biconcave and constrict the notochord in the

J

Vertebra^ ^.'oiochord. ^^<-^^r<T7x ^'^^^^^rc^ ^Notochord

V^

A. Cyclostome B. Dogfish C. Dogfish (sagittal section)

Fig. 85. Cartilaginous Vertebrae. The Cyclostome vertebrae are isolated
metameric cartilages; the Dogfish (B) has centra surrounding the noto-
chord and a neural arch enclosing the nerve cord; (C) shows the centra
constricting the notochord, leaving large discs of notochordal tissue between.

middle of each vertebra. The notochord is, however, continu-
ous, being shaped in the adult like a string of beads — a narrow
neck through the center of each vertebra, and a lens-shaped
area fitting into the cavities of the two contiguous vertebrae. The
circular part of the vertebra which surrounds the notochord is
the centrum, and is evidently the most ancient part of the verte-
bra. Evidence of this is found in a small fossil (Paleospondylus).
The Elasmobranchs have also developed processes for pro-
tection of the nerve cord and for the attachment of muscles. On
the dorsal side of each vertebra is an arch made of a pair of
processes, through which the nerve cord passes. This is the
neural arch of the vertebrae, and is found in all classes above the
cyclostomes. In the Elasmobranch there are inter-neural car-
tilages which, with the neural arch, completely enclose the neural
canal except where it is pierced by spinal nerves. On the lateral-

STRUCTURE OF THE VERTEBRATES 149

ventral portion of the centrum is another pair of transverse
processes serving as points of muscular attachment. In the caudal
region of the dogfish these processes grow toward the ventral
median line, fuse, and form a ventral haemal arch through
which passes the aorta.

The vertebrae of the living Elasmobranchs are more spe-
cialized than those of many fossil fish and amphibia, and the

Intercentrum

Hntercentrum

A. Primitive Stegocephalian
(Cricotiis)

B. Stegocephalian
(Eryops)

Neural
arch

C. Tail Vertebrae of Cat

D. Lumbar Vertebrae of Mammal

Fig. 86. Comparative Anatomy of Bony Vertebrae. Primitive vertebrae
consist of two centra in each body segment (A) ; the intercentra are pushed
ventrally (B) in many amphibia and reptiles; in the tail region of some
mammals (C) the intercentra are left as chevron bones, and are lost
completely in mammals without a tail.

150 STRUCTURE OF THE VERTEBRATES

tail vertebrae of some recent fish. Considering these facts, and
the embryology of the vertebrae, it becomes evident that the
primitive animals had two vertebrae to each body segment.
Wherever the condition is found the anterior vertebra has a
neural arch and is called the neurocentrum; and the posterior
is the inter centrutn. This structure of the vertebrae was car-
ried over from the early Crossopterygii to the Stegocephalian
amphibian.

The more primitive fossil amphibia have the two alternating
vertebrae, but the later fossils show a forward growth of the
neurocentrum, with the intercentrum being pushed ventrally
to form a triangular element. The. mammal-like reptiles carried
this destruction of the intercentrum a step farther, the inter-
centrum being very small. In the mammals the intercentrum is
entirely crowded out in the body region, but remains in the
caudal region of many species as the small chevron bones.

With the anterior growth of the neurocentrum, and its en-
largement, the position of the vertebrae has changed. Originally
the vertebrae lay between the myosepta, but the forward growth
carried each into the more anterior body segment so that the
myoseptum crosses its middle portion. In this way the vertebral
number has been reduced to half, although metamerism has not
been lost. Observe, however, that each body segment contains
two halves of vertebrae, the posterior half of one, and the ante-
rior half of another.

Above the fish the vertebrae displace the notochordal tissue
completely, and articulate against each other. This is usually a
ball and socket joint, a rounded projection of one vertebra fitting
into a concavity of the adjacent one. The concavity may be
either on the anterior or posterior face. The mammals have an-
other modification, both faces of the centrum being fiat. Move-
ment is assisted by a disc of cartilage lying between the two
faces.

Accessory processes are found in most groups of the verte-
brates. In the reptiles these are rather simple. The mammal
vertebrae are more complex. From the neural arch develop ante-
rior and posterior zygapophyses, the anterior articulating with
the posterior ones of the vertebra in front of it. This permits
flexibility, but gives added strength.

STRUCTURE OF THE VERTEBRATES

151

Vertebral Regions. The vertebrae of the land vertebrates
can be divided into five more or less distinct regions: (1) the
cervical, or neck; (2) the thoracic, bearing the pleural ribs; (3)
the lumbar, lying in the lower abdominal region; (4) the sacral,
to which the pelvic girdle attaches; and (5) the caudal, or tail,
vertebrae.

Cervical vertebrae. In the lower forms these are only slightly
modified from the more posterior ones. The higher reptiles show
definite differentiation, connecting the primitive type and the

O ft^

Sacrum

Coccyx

Fig. 87. Vertebral Regions. The spinal column of man with vertebrae
from the different body regions.

152 STRUCTURE OF THE VERTEBRATES

mammal. Typically there are seven vertebrae in the mammal,
although this number is reduced in some whales, and increased
in the edentates and one or two isolated species. However, the
giraffe, man, and most others have the same number, no matter
how long the neck. All but the first two of these vertebrae bear
cervical ribs which are fused by two points to the centrum, and
through the canal thus formed runs the vertebral artery. In
abnormal cases the cervical ribs (particularly the last two)
may be long and movable.

The first cervical is highly modified to form the atlas which
bears the head. The centrum of this vertebra fuses with the sec-
ond vertebra in the mammal, and the atlas is left as a bony
ring. The second cervical is the axis, the head rotating at the
joint between the atlas and axis. The centrum of the first forms
the odontoid process of the axis. The posterior five cervicals are
very similar in structure.

Thoracic vertebrae are distinguished by the articular facets
for the ribs. The spines of the neural processes are rather long,
like those of the cervical ribs, and the muscles and tendons of
the occipital region are attached to them. The ribs attached to
these vertebrae meet the breast bone, or sternum.

Lumbar vertebrae of the mammal are heavy and have a stout
neural spine. The transverse processes are directed toward the
head, and there are no facets for rib attachment. In the lower
vertebrates these lumbar vertebrae are hardly distinguishable
from those of the thoracic region.

Sacral vertebrae have the pelvic girdle attached to them. The
amphibia have only one vertebra which bears a modified rib
for the girdle attachment, while the reptiles have two with
sacral ribs. The mammal retains the reptilian number of true
sacral vertebrae; but the ribs and the transverse processes of
two, three or four other vertebrae become indistinguishably
fused to form a heavy sacrum to which the pelvis becomes at-
tached. Tlie sacrum of the bird is very large, due to the fusion
of seven or more vertebrae.

Caudal vertebrae are the simplest in structure. In most mam-
mals only the first two or three bear any sign of transverse proc-
esses, and the more posterior ones consist of only a small cen-
trum, the size of this element being reduced from anterior to

STRUCTURE OF THE VERTEBRATES

153

posterior. The caudal vertebrae of the human are fused to form
a terminal process, the coccyx.

Embryology of the Vertebrae. The developmental history
of the vertebrae varies in the different classes of vertebrates.

[nlage of intercentrum

of neurocentrum

A. Early Stage

' Notochord

\

B. Later Stage
nlage of neurocentrum^

A'. Cross section of "A'

B'. Cross section of "B"

Intercentrum

Jeurocentruml

Fig. 88. Embryology of the Vertebrae. (A) and (B) show the isolated
cartilages forming rings around the notochord, equivalent to the inter-
centrum and neurocentrum. Drawings (C) and (D) show the intercentra
being crowded ventrally during development, and lost in the definitive

structure (E).

The following is a generalized description. The vertebrae first
appear as eight minute cartilages in each body segment, forming
two incomplete rings around the notochord. As these cartilages
grow they fuse into two rings, thus parallelling the fossil history.
The more anterior ring of each segment enlarges, developing a
neural arch, and pushes anteriorly across the myoseptum. In

154 STRUCTURE OF THE VERTEBRATES

this way the embryonic tissues of the ring in front of the neuro-
centrum (the posterior ring of the next segment) are crowded
downward. This posterior ring of the segment is the homologiie
of the intercentrum, and in many mammals these ossify as the
small chevron bones of the caudal region. The intercentra in the
more anterior regions of the vertebral column are completely
crowded out.

All vertebrates above the cyclostomes begin with similar car-
tilaginous vertebral blocks, but there is a differential develop-
ment in different classes and even the regions of the body. The
most primitive vertebrae ossified as two independent rings in
each segment, and this condition is retained in the tail region
of many fish. Others carried the growth of the neurocentrum
further before complete ossification, as was shown above, while
the human and some others have carried the process so far that
the intercentral tissues are completely obliterated before ossi-
fication is completed.

D. Ribs and Sternum

The most primitive ribs are very small cartilages found in
the elasmobranch fishes. The ribs develop in the myoseptum, and
are in contact with the transverse processes of the vertebrae.
The Crossopterygians retain these myoseptal ribs, but have an
accessory pair with a different origin. The second pair is typical
of the bony fishes and is the so-called "fish rib". Apparently
it was not carried over to the amphibia, as the ribs of all the
land vertebrates originate in the connective tissues between the
muscle segments.

In the primitive amphibia the ribs articulated with two verte-
brae, being in contact with the vertebral column at the point
of articulation between the neurocentrum and the intercentrum.
But, as the neurocentrum pushed anteriorly, the rib came to lie
in contact with the middle of a single vertebra.

The amphibia and reptiles have movable ribs on all vertebrae
from the third cervical to the last sacral. In the mammals the
movable ribs are reduced to twelve, in the thoracic region. How-
ever, both the cervical and sacral vertebrae have small ribs
fused to the centra.

Neural arck

k t/^^X

Neural arc

Centrum — • )

True rib^^

A. Dogfish

Fish rib

B. Crossopterygian

C. Amphibian

Fig. 89. Structure of Ribs. Evohition of the true (myoseptal) ribs; and
the position of the ''fish" ribs of the Crossopterygii and Teleostei.

Sternum

Fig. 90. Thorax of Man. The

relationship between the ribs, the

vertebrae and the sternum is

shown.

Centrum

Sternum

Fig. 91. Cross Section of the

Thorax of Man. Compare with

Figures 89 and 90.

156 STRUCTURE OF THE VERTEBRATES

E. Origin of the Appendages

The pectoral and pelvic girdles together form the appendicu-
lar skeleton. With the girdle proper, which is the internal bracing
structure, is included the appendage — either fin, leg, arm, or
wing.

There are several theories to account for the origin of the
paired fins from the finless, jawless ancestors of the fish, but
the most acceptable to most anatomists is the fin fold theory.
This theory requires an ancestral type with a continuous fin
along either side of the body, beginning posterior to the gills
and extending posteriorly and ventrally to a point posterior to
the anus. At this point they fuse and continue to the end of the
animal as the ventral and caudal fins. This animal is supposed
to have developed metameric cartilage supports to give strength
and rigidity to the long fin fold. Then, during evolution, the
middle and most posterior regions of the paired fins disappeared,
leaving wide pectoral and pelvic fins. In similar manner the
continuous dorsal fin became reduced to two dorsal fins, and the
upper portion of the caudal. The evidence from both palaeon-
tology and embryolog}" supports this theory.

The most ancient and primitive sharks are known only as
fossils. Of these Cladoselache is the type form. Its notochord
was continuous and its vertebrae only slightly better developed
than those of the cyclostomes. Its most interesting feature was
the appendages, the pelvic fin being more primitive than the
pectoral. Each fin had a wide base and was half-moon shaped.
The pelvic was supported by metameric, disconnected cartilages;
while the pectoral had similar cartilaginous rays and a longi-
tudinal series of small cartilages at their base. Eventually this
basal series shortened to form the three basal cartilages found
in the living sharks.

The embryological evidence is equally clear. In the elasmo-
branch embryo the first sign of the appendages is a double row
of limb buds in the pectoral and pelvic regions, (page 158). Each
limb arises from six or more segments. As these muscle buds
grow each carries a spinal nerve with it, the entire structure
extending distally as a wide, rounded embryonic limb. As the
limb takes shape the base constricts and soon the definitive fin

Fig. 92. Hj^pothetical Stages in the Development of Fins. The shortening
of the single dorsal and paired lateral fins to form the discontinuous dorsal
and lateral (pectoral and pelvic) fins according to the fin-fold theory.

(After Dean).

B. Ventral View

Fig. 93. Cladoselache, Side and Ventral Views. A primitive, extinct
Elasmobranch with fins having wide bases and numerous radial cartilages.

(After Dean).

158 STRUCTURE OF THE VERTEBRATES

is formed. The limb development of the higher vertebrates is
modified and less diagrammatic, but the origin from several seg-
ments is the cause of the large plexus of nerves in the limb of
all vertebrates.

F. Pectoral Girdle

The recent sharks have a simple girdle and limb supports.
The girdle is a pair of cartilages held tightly together in the mid-

Myotomes

imb buds

Fig. 94. Embrj^ology of Fins. Note the numerous myotomes which
collectively form a single appendage.

ventral line, and loosely anchored to the vertebrae with con-
nective tissue. About the middle, on the posterior side, is a notch
for the articulation of the fin cartilages. The region ventral to
this notch is the coracoid, and in the higher forms is a separate
bone. The dorsal portion is the scapula.

The fin is supported by three cartilages, the "pro-'', "meao-"
and metapterygium. Distal to these is a row of small cartilages.
The dorsal and ventral fin muscles are attached to the cartilage
supports. Distally the fin ends in a fleshless flap supported by
dermal fin rays.

The most primitive crossopterygians known already had a
pectoral fin showing evidence of its kinship with that of the
land animals. The middle pterj^gial cartilage was largest, with
the two others smaller and separated from the girdle. The more

STRUCTURE OF THE VERTEBRATES

159

specialized members of this order had a bony skeleton which is
clearly a pre-amphibian arrangement. The middle bone was
large, forming the main support of the fleshy lobe (page 44),
and distally were two other bones. These three bones are homolo-
gized with the proximal humerus and the radius and ulna of the
forearm. Distal to these were small bones which evidently gave
rise to the carpals of the wrist joint, and the metacarpals and

Shoulder girdle

A. Dogfish

B. Primitive Extinct
Crossopterygian

Fig. 95. Stages in the Evolution of the Pectoral Fin. Compare the Dogfish
fin with that of Sauripterus (Order Crossopterygii) . The latter has all the
elements of a form ancestral to the amphibia. (Sauripterus after Hussakoff).

bones of the digits. Two genera of these fish have been care-
fully studied, Sauripterus and Eusthenopteron, the latter being
illustrated.

The girdle of these fish had become ossified as two bones, the
ventral coracoid and the dorsal scapula, which had kept their
primitive positions. In addition to these replacement or cartilage
bones, three small dermal bones had become attached to the
scapula at its dorsal margin. These formed an attachment be-
tween the girdle and the head. Another pair of bones was added

160

STRUCTURE OF THE VERTEBRATES

to the girdle of this animal, the clavicle, parallelling the cora-
coid on its anterior face, and also meeting its mate at the mid-
ventral line. It is unknown if this bone is of cartilage or dermal
origin in the fish. In the frog it is of mixed origin, and in the
higher forms the dermal part is apparently lost.

Ulna

D

Fig. 96. (A) Shoulder Girdle and Appendage of Eryops (Order Stegoce-
phalia), an extinct amphibian with a primitive girdle and a heavy, short
humerus. (B) The foot-print of Thinopus, the earhest record of land

animals.

The crossopterygian girdle was carried over to the earliest
amphibia almost in its entirety. The ventral clavicles and cora-
coids met in the midline; and the scapulae were connected with
the head by small dermal bones. The latter were soon lost, for
the connection was a liability under conditions where flexibility
between head and shoulder were required.

STRUCTURE OF THE VERTEBRATES 161

In the first known amphibia the bones of the arm and hand
were specialized for land life and had assumed their typical
tetrapod (four footed) form. The humerus was short, the radius
was on the preaxial (thumb) border of the arm, and the ulna
was on the postaxial border. The carpals were numerous. Little
is known regarding the original number of digits. The oldest
evidence is a foot-print made by an early amphibian (Thino-
pus), which might indicate a foot with three toes. It is more
probable that the print was made by some five-toed animal
walking on the margin of its foot. The crossopterygian larva
rests on the outer (postaxial) border of its fins (page 44), and
the young frog also holds the foot in this position. Therefore,
as the earliest skeletons had the typical five toes, it is believed
that Thinopus was simply an amphibian which had not become
flat footed.

The first reptiles had lost the dermal bones, and had the
"complete" girdle: (1) dorsal scapulae not connected with the
head, and (2) coracoids and clavicles meeting (3) the median
ventral sternum. The heavy coracoids held the humeri at right
angles to the body, with the forearms bent forward. The recent
reptiles are similar in structure. When quiet the body of the
animal rests upon the ground, but when running the legs are
pulled under the body as it is lifted from the ground.

This complete girdle was carried over to the monotremes with
almost no changes, and is a large part of the evidence that these
animals arose as a separate line of evolution. The mammal-
like reptiles had a marked reduction of the coracoid bones, with
the result that the legs were drawn under the body into the run-
ning position, and in this respect were more like the higher mam-
mals than are the monotremes. The evolution of the separate
bones is traced below.

1. Coracoid. The mammal-like reptiles had greatly reduced
coracoids which still met the sternum. The embryo of the living
marsupials shows a similar condition, with cartilaginous cora-
coids meeting in the mid-line. But as development continues
these cartilages recede from the sternal position and become at-
tached to the scapula as coracoid processes. In the placentals
the coracoid cartilages never meet the mid-line, and develop as
processes of the scapula.

Ueithrum
/Clavicle

A. Dogfish

Scapula-
Coracoid-

B. Reptile (primitive)

Scapula

Clavicle

Glenoid'
fossa

Coracdd

Inter-
Clavicle

Uernum
D. Monotreme

Acromion process
Glenoid fossa^

Scapula

Clavicle
Sternum

E. Man

Fig. 97. Comparative Anatomy of the Shoulder Girdle. The Dogfish has
a single pair of cartilages meeting ventrall3^ The primitive reptile (B)
and the Monotreme (D) have a complete girdle. The Placental Mammal
(E) shows the reduction of the corycoid bone to a process of the scapula,
and the development of the scapular spine. Diagrams (A), (B), (C) and
(D) are ventral views; (E) is an anterior view.

STRUCTURE OF THE VERTEBRATES 163

2. Scapula. In the reptiles the scapula is a flat, rather nar-
row bone. The monotreme scapula is no wider, but the anterior
border is turned outward. The embryonic scapula of the marsu-
pials is similar, and then develops an anterior extension of the
blade of the bone, which leaves the upturned margin as the spine
of the scapula. The spine divides the lateral face of the bone
into two fossae in which shoulder muscles are attached. The
placental scapula is similar, although the development is less
diagrammatic.

3. Clavicle. The clavicle is the most recent and most vari-
able bone of the shoulder girdle. It may be small or entirely
absent. In the placentals there is a marked correlation between
its size and the function of the pectoral limbs. In general terms,
the bone is small or absent in running animals; while the
climbing types retain a well developed clavicle. When the legs
move in a single, antero-posterior, plane the clavicle is a liabil-
ity; but when the limbs are moved in many directions the
clavicle has selective value for muscular attachments, and for
bracing the shoulder.

G. Pelvic Girdle

The evolution of the pelvic girdle is more obscure than that
of the pectoral, and in the fish and amphibia is much simpler
in structure. In the fish and earlier amphibia the anterior limbs
carried a greater functional burden. In the fish the pectoral fin
is the major balancer of the body, and in the primitive amphibia
the head is raised from the ground on the fore limbs as the
animal wriggles through the mud. These facts probably explain
the greater selection exerted upon the anterior appendages.

Cladoselache gives the earliest clue to the development of
the pelvic region, the fin being supported by isolated cartilages
with no body supports. In the sharks the anterior rays have be-
come fused into a relatively large cartilage on either side, the
two meeting ventrally. Further evolution is indicated by the
chondrostean fish, the ventral portion of each cartilage con-
stricting off to form a paired ventral segment. In the living
crossopterygian (Polypterus) these two segments have fused to

164

STRUCTURE OF THE VERTEBRATES

form a small plate. The longer dorsal portions of the cartilages
strongly resemble the jemori of the land vertebrates.

A wide gap exists, however, between any known fish and the
amphibia; and the anatomist is tempted to say that the true
pelvis developed from the simple elasmobranch type after the
evolution of the tetrapods. The primitive living urodeles have
a flat ventral cartilage with a pair of posterior centers of ossi-
fication, and in addition, a pair of dorsal bones which are at-
tached to the sacral vertebrae. The latter are the ilia. In certain
frogs there is an additional pair of ventral ossifications. These

Acetabulum.

Primitive Extinct Amphibian
(Stegocephalia)

Human Embryo

Fig. 98. Evolution of the Pelvis. The primitive pelvis (A) consisted of
three separate bones forming a soUd plate. The higher animals develop
an obturator foramen in the pubis and ischium. The mammal embryo has
the sutures between the three bones, although the bones ankylose during

development.

four ventral bones are the homologues of the anterior pubes and
the posterior ischia of the higher animals. But the fact that the
pelvic girdle of the Stegocephalia is clearly divided by sutures,
renders any evidence from comparative anatomy doubtful.

The reptiles have a pelvis typical of all higher vertebrates.
The acetabulum is a lateral depression in which the femur
articulates, and is the center from which the three pelvic bones
radiate. The dorsal ilia attach to two sacral vertebrae, and
each pubis and ischium meets its mate in the mid-ventral line.
Thus, including the sacrum, a complete circular girdle is formed
which is decidedly stronger than the pectoral girdle, and con-
versely is less flexible.

STRUCTURE OF THE VERTEBRATES

165

Further evolution is merely modification of this primitive plan.
The pubic bones have developed along two main lines: (1) the
primitive reptilian type with the pubic union, or syiyiphysis, in
an anterior position; and (2) the bird-like modification with
the pubes growing posteriorly, parallelling the ischia. The mam-
mal-like reptiles and mammals retained the primitive condition.

Homology of Parts. The girdles evidently arose independ-
ently of each other, and homology of parts is consequently dif-
ficult. The appendages, on the other hand, arise embryologically
in the same manner and all the evidence points to a complete
homology of parts.

Region

Pectoral

Pelvic

Upper limb

Humerus

Femur

Lower limb

Radius (preaxial)
Ulna fpostaxial)

Tibia (preaxial)
Fibula (postaxial)

Wrist— Ankle

Carpals

Tarsals

Palm

Metacarpals

Metatarsals

Digits

Phalanges

Phalanges

The primitive tetrapod limbs are thought of as being held in
a straight line at right angles to the vertebral column. Each
limb had a joint between the upper and lower limb, the elbow
of the pectoral limb, and the knee of the pelvic. The front leg
was then rotated forward and under the body, and the elbow
was forced posteriorly as the arm was flexed. Exactly the oppo-
site occurred in the hind leg. The foot was carried posteriorly
under the tail, and the knee joint was forced anteriorly. The
limb position indicates the function. The front legs primitively
act as a support of the head, and the pelvic limbs push the
body along. The development of the pelvis and its appendages
is in correlation with the greater function of me limbs as a loco-

166 STRUCTURE OF THE VERTEBRATES

motor apparatus when the vertebrates left the water and
muddy flats and became true land living animals.

H. Specialization of the Limbs

Modifications of the appendages begin with the fish, the two
main lines of evolution having been traced, that is, the typical
balancing fin, and the crossopterygian type. The amphibia arose
as five-toed animals with short legs, and there are few speciali-
zations in the class except the loss of digits and entire limbs.
Thus, in the recent amphibia, we have species without append-
ages, others lacking the hind legs, one with minute front and
hind legs, and some lacking some of the digits. The typical
amphibian has five digits on the pelvic limb, and four on the
pectoral.

Among the reptiles the specializations become more pro-
nounced. The snakes and some lizards have no limbs, the pythons
having minute stubs on the degenerate pelvic girdle; and others
lack certain digits. Among the fossil reptiles there are many
adaptive radiations, many of which are parallelled in the mam-
mals and may be considered under separate headings as func-
tional adaptations. The student should keep in mind that these
similar evolutionary changes are separate lines of specialization,
and that there is no genetic relationship between the modifica-
tions in different classes. The typical five toes of the fossil
amphibia and reptiles were inherited by the mammals, and any
specializations for particular function have arisen independently.

1. Aquatic. Numerous orders of reptiles and mammals show
varying adaptations for water life. Among the reptiles, the
turtles (page 58) show many marked changes of the append-
ages. In addition to the shift in shape of the carapace (shell)
the appendages become webbed, and in the sea-turtle the feet
become flippers. The aquatic life was carried to its furthest ex-
tent in the extinct ichthyosaurs (fish-like lizards). These rep-
tiles had flippers with the digits completely enclosed by the skin
covering, the body was fusiform in shape, the tail was short
with a terminal flattening, and the animals were ovo-viviparous
(page 37) . Apparently they lived their entire lives in the water.

The mammals, recent and fossil, show similar adaptations.

STRUCTURE OF THE VERTEBRATES

167

The carnivores show almost every degree of aquatic specializa-
tion. Beginning with animals like the otter which spends a large
part of its time in the water but has typical five-toed feet, a
further degree of change is found in the Alaskan sea otter
which spends most of its time floating on its back. The Pinni-
pedia (water living carnivores) form a separate sub-order and
carrv the modifications still further. The sea-lion and walrus

Fig. A
Dolphin

Fig. B

Ichthyosaur

OOP o^

Fig. 99. Aquatic Adaptations of the Pectoral Appendages. The number

of phalanges is increased, and in the more specialized forms the arm and

hand form a flipper.

have flippers, both pairs of which can be used for locomotion on
land. In the Atlantic seal the pelvic flippers are enclosed by
skin for most of their length, and are functionless on land. The
Cetacea and Sirenia (page 90) have carried modifications for
water life to the ultimate extent. The front flippers have an in-
creased number of digits, and the pelvic appendages have dis-
appeared except in rare instances where a small femur is found.
Although these animals are externally modified, they are

168

STRUCTURE OF THE VERTEBRATES

fundamentally like other members of the class to which they
belong. All come to the surface for air, and the mammals feed
their young on milk.

2. Bipedal. Bipedal animals are found in all the three amni-
ote classes. The pectoral appendages are small or highly modi-
fied, and the pelvic limbs are large. The weight of the body is
thrown backward for balance. All living birds are completely
bipedal. The same modification was found in many extinct rep-
tiles, although the bird was not descended from any known

A. Plantigi-ade

B. Digitigiade

C. Unguligrade

Fig. 100. "Walking and Running Modifications of the Appendages. The

most marked specializations for running occur in the mammals. In this

Class three major tj'pes are recognized, typical of mammals which walk

(plantigrade), jump (digitigrade) or run (unguligrade).

group of this type. Both herbivorous and carnivorous reptiles
were of this type, the latter being more frequent. Among the
mammals the specialization has occurred in several unrelated
groups. The marsupial kangaroos are highly modified for bi-
pedal life; and man has similar locomotion, although there is
less modification of the pectoral limb. The kangaroo has become
adapted for a hopping motion. ^Man descended from an arboreal
(tree living) group and has retained completely functional hands.
3. Cursorial. The running modifications have developed most
prominently in animals living on dry plains where a pre-

STRUCTURE OF THE VERTEBRATES 169

mium was placed upon speed. The limbs move in a single antero-
posterior plane, and with this digital modifications have ap-
peared. An animal on its toes runs more rapidly than one
with its feet held flat. All grades are found in the placental
mammals. Man, the bears, and many others walk in the planti-
grade or flat-footed position, with the heel touching the ground.
The carnivores and others have feet adapted for springing, with
the weight resting on the ball of the foot, and the heel off the
ground. The Artiodactyla and Perissodactyla (page 89) have
carried the specialization to its furthest extent. These animals
walk on the tips of the digits, only the terminal phalanx touch-
ing. This is the ultimate in ease of speedy motion. The evolu-
tion of the horse will be found in any biology text.

4. Arboreal. Tree-living modifications are of many types.
The more primitive adaptation has bowed legs for holding the
limb or tree trunk, with sharp claws. Among the primates, which
have rounded nails, the specializations have been toward swing-
ing from limb to limb, rather than a slow climbing. The primate
condition has been termed brachiation. Long, well developed
arms and fingers are typical of this group. The extreme speciali-
zation is found in two genera, one African and one South Amer-
ican, where the thumb is vestigial or lacking, and the other four
fingers are very long. Mutations of the thumb are not infre-
quent, and the selective value of a small thumb is apparent. In
jumping from limb to limb the thumb would often interfere with
a proper landing; and a sudden fall from a tree-top, or an in-
fected thumb from injury, are not conducive to longevity.

5. Flying. Under flying adaptations may be considered true
flight and the volplaning modifications of several groups. Among
the latter are the flying squirrels which have a fold of the body
skin extending from the front to hind legs. The animal is en-
abled to descend at a flat angle and the jumping distance is
greatly increased. True flight is found in all the amniote groups.
The fossil pterodactyls ("wing fingers") had greatly elongated
digits with a leathery flap connecting them. A similar modifica-
tion has appeared among the placentals in the bats (page 86).
This order has the digits elongated, with a thin membrane con-
necting them with the hind legs. The bats have speed, endurance
and remarkable coordinations. Flight modifications in the birds

170

STRUCTURE OF THE VERTEBRATES

are of an entirely different nature. The digits sliow a great de-
generation, with a fusion of the carpals. AVing expanse is gained
with the feathers.

There are other distinct modifications, for example digging or
burrowing, and there are variations of the above. Specializations
of the appendages are more frequent and more easily observed
than in any other part of the body. Environment places a high

Fig. D Bat

Fig. 101. Flying Adaptations. Flying Reptiles and Mammals (A and D)

have elongated digits with membranous flaps stretched between them to

give surface resistance. Increased area is given birds by the feathers.

STRUCTURE OF THE VERTEBRATES 171

selective value upon locomotion, and at the same time degen-
erative changes are not necessarily fatal.

Due to the interdependence of the skeletal and muscular sys-
tems the bones may be modified by mutations in other tissues.
An inherent degeneration of the muscles would undoubtedly af-
fect the bones. The degeneration of the embryonic coracoid of
the marsupial can be interpreted in this way, for there are no
muscles attached to them. Conversely, the degeneration of a
bone will cause developmental abnormalities in the muscles. The
thumbless monkeys have all the thumb muscles, although they
are vestigial and almost microscopic in some specimens. The
interdependence of tissues is discussed in Chapter XIX.

I. Development of the Skeleton

The two developmental types of bone (dermal and replace-
ment) have been defined. Either may give rise to the two ana-
tomical types, long and flat bones. In the higher vertebrates few
of the long bones are derived from dermal structures, these be-
ing limited to the dermal ribs and a part of the clavicle in a
number of amphibia and reptiles. Flat bones include the body
plates of the lower vertebrates, and the bones composing the
skull of all bony vertebrates. Thus, the supra-occipital is flat,
and composed in part of both replacement and dermal bone.

In addition to the above, bone is classified by histological
structure into compact, or hard bone, and cancellous, or spongy.
If a long bone is hemisected it will be seen that the head and
the inner part of the shaft are made of coarse fibers which
divide and anastomose, forming a spongy substance with many
air spaces between the fibers. The heavy part of the shaft is a
solid, compact structure. The flat bones are formed of two
layers of compact bone, with an inner layer of spongy bone.
The following classification will make this clear.

Classification of Bone
Embryological Anatomical Histological

1. DennalzziIIII~ \ Long^_ ^Compact

2. RoploppTYiPTif. % Flat ^ Cancellous

172 STRUCTURE OF THE VERTEBRATES

Dermal bone is limited in its distribution in the mammal, and
its origin in the dermal fibers of the skin has been described.
The student should recall that the osteoblasts deposit the bony
substance around fibers which radiate from definite centers of
ossification, growth in size being due to growth of the distal ends
of the fibers and peripheral deposition of bone.

The locomotor elements of the skeleton, with a few minor ex-
ceptions, are replacement bone; and with the exception of the
dermal bones which have been mentioned, the entire embryonic
skeleton is laid down in cartilage which is gradually replaced
by bone. However, it is a matter of common observation that
growth continues long after embryonic life is ended, and that
changes in bony configuration continue throughout life.

Bone cells are of two types: (1) osteoblasts, or bone building
cells which secrete the organic fibers and the inorganic salts
which give hardness to the bone; and (2) osteoclasts, or bone
destroyers. Both types are at work, even after the limit of
growth has been reached. The healing of a broken bone and the
smoothing of the jawbone after a tooth extraction are illustra-
tions which the student has observed.

The processes of growth can be demonstrated in a long bone
of the arm. The embryonic cartilage is surrounded by a con-
nective tissue sheath, the periosteum (called the perichondrium
when surrounding cartilage). At about the middle of the shaft
of the bone the periosteum breaks and blood vessels migrate
into the cartilage, the osteoclasts destroying the cartilage tissue.
From this central cavity the destruction proceeds toward either
end of the bone, the cartilage being destroyed unevenly, so that
finger-like processes are left projecting into the original cavity.
Osteoblasts then begin secreting bone which fills the original
cavit}^, and also the spaces at either side. As destruction pro-
ceeds, the cavities are filled with bone.

Secondary centers of ossification begin at either end of the
cartilage producing the epiphi/ses which form the ends of each
long bone. As the shaft continues to grow it soon approaches
the terminal epiphyses. The skeleton of a young animal shows
this condition, the epiphyses being separated from the shaft by
a thin line of cartilage. Growth takes place in this cartilage
area. Length of the long bone is determined by the time of

STRUCTURE OF THE VERTEBRATES 173

fusion between the two, for when complete fusion occurs the
growth stops. In the human tliis takes place, usually, between
fifteen and eighteen years, taller individuals continuing to grow
for a longer time than the short ones. Other animals may con-
tinue growth throughout life.

There is also a growth in diameter. This takes place in the
periosteum, which lays down concentric layers of bony tissue,
the outer layer being the younger. And as the bone grows out-
ward, osteoclasts destroy the bone at the center, with the result
that the inner marrow cavity becomes larger.

The growth and structure of bone, however, are not so simple
as the above description. A cross section of a long bone will
show that the outer layers are concentric, extending entirely
around the shaft, but the major portion is composed of micro-
scopic groups of concentric .layers, each built around a canal in
which lie an artery and vein. These small canals are the
Haversian canals, and the canal and the concentric rings sur-
rounding it form a Haversian system. As the osteoclasts eat
out the peripheral layers, blood vessels with osteoblasts migrate
in and lay down a long tube of bone. Other, smaller, layers are
formed within the original one, until a new Haversian system is
formed. In this case the inner ring is the youngest. Old systems
are constantly being destroyed and new ones built. This can be
demonstrated experimentally by feeding young animals a stain
which colors the bone which is being laid down at the time of
feeding. By alternating the stained rings with unstained, the
order of destruction and rebuilding can be determined.

After the fusion of the epiphyses there is little further growth
in size, the changes being internal. However, in pathological con-
ditions bone may grow or be destroyed. A broken bone releases
bone cells and growth proceeds rapidly. Diseases affecting the
pituitary gland, which lies in the sella turcica of the skull, will
cause bone growth later in life. This usually affects the jaw and
the cartilaginous joints of the fingers.

Destruction is shown in old age by the thinning of the skull
bones, and the degeneration of the angles of the jaw bones. The
jaw of an old man is usually small and thin.

The size of the bones is influenced by glandular secretions,
and these organs are also a factor in determining height. Proper

174 STRUCTURE OF THE VERTEBRATES

nutrition and otlier environmental factors also influence height,
particularly in species which have a definite size limit. Those
which continue growth throughout life are less influenced by
periods of semi-starvation. Malformations of bone may be due
to injuries of the periosteum, or to improper feeding. Rickets,
which causes bowed legs and other deformities, is caused by a
lack of bone developing vitamines. There are, however, racial
and individual differences in susceptibility to the condition.

CHAPTER XII

MUSCULAR SYSTEM

Muscle tissues are of mesodermal origin and are specialized for
contractility. In all groups of animals above the coelenterates
it is these tissues which are concerned with locomotion. In the
vertebrates a study of the muscular system is usually limited to
the striated skeletal muscles, for by common consent the smooth
muscles are considered with the digestive tract, and the cardiac
muscles with the vascular system.

A. Development and Classification of Muscles

The mesoderm arises as metameric blocks on either side of
the notochord. Each of these segments soon becomes differenti-
ated into epimere, an intermediate cell mass or nephrotome, and
the hypomere (illustrations on page 68). From the hypomere
arise the cardiac and smooth muscles, and the striated muscles
of the gills; and also the mesenteries and peritoneum. The cav-
ity which forms within the hypomere becomes the coelomic cav-
ity of the body.

The epimere gives rise to the dermis of the skin, the con-
nective tissues and bone, and the skeletal muscles. The last de-
velop from the central portion of the epimere (the myotome)
which grows ventrally between the peritoneum and the ectoderm
of the embryo. When each myotome meets its mate from the
opposite side the metameric structure of the body is completed.
The myotomes are heavier in the dorsal region than in the
ventral. Slightly later in development the myotomes are divided
into dorsal and ventral regions by an axial line from anterior
to posterior which parallels the ventral border of the notochord.
The muscles which develop from the myotomes above this line
are the epaxial group, the ventral muscles being the hypaxial

175

176 STRUCTURE OF THE VERTEBRATES

group. Reference to Amphioxus will make the relationships
clear. The embryos of all vertebrates are similar in this general
plan, although the adults vary widely in the degree of speciali-
zation from this primitive condition.

The striated muscles are classified both by their origin and
their position in the body. In the more primitive vertebrates the
topographic arrangement of the muscles will usually indicate
their origin and homology, but in the higher groups the position
of the muscles has become more highly specialized. Therefore
the anatomist must depend upon embryology and comparative
anatomy to determine the correct relationships of the individual
groups. Four major groups of skeletal muscles are usually
recognized.

1. Axial muscles are the original metameric muscles which
arise from the epimere and lie along the axis of the body. They
are primitively metameric, although the metamerism is largely
lost or obscured in the higher classes.

2. Branchiomeric muscles arise from the hypomere in con-
nection with the branchial, or gill, cartilages. After the disap-
pearance of the gills and the modification of the cartilages, these
muscles are retained in the jaw and throat.

3. Appendicular muscles arise from the primitive axial group
and grow distall}^ on to the appendages. They are so highly
modified that they are classified as a separate major group of
muscles. Embryologically they can be traced to the epimere and
are classified on the basis of their topographic arrangement.

4. Integumentary muscles arise from all three of the above
groups. They become attached to the skin of the animal and
form a distinct functional group.

B. NOMEXCLATURE AND FUNCTION

There is no system of the body in which a knowledge of
homologies between individual structures is as incomplete as in
the muscles. The major topographic divisions can easily be
followed from class to class, but homologies between individual
muscles are not always certain. Homologies between reptiles and
mammals are particularly difficult. This is caused by the fact
that each muscle is not always clearly defined, for in the course

STRUCTURE OF THE VERTEBRATES 177

of evolution a single muscle may divide to form several; and
conversely, several separate muscles may fuse to form a single
one. The three bases of homology are (1) comparative anatomy,
(2) embryology, and (3) innervation. The last is, perhaps, the
soundest basis of judgment.

The names of muscles are also confusing to students. The
earliest study of anatomy was human, and the muscles were
named accorchng to their shape, position or function in man.
These names rarely apply to the muscles of other animals. The
more recent method of naming muscles by their points of attach-
ment is an advance in the direction of simplicity. However, as
the skeletal structure of the vertebrates has changed, the muscu-
lar attachments have changed with them; and a muscle which
can be accurately homologized will have different names in the
various classes of vertebrates. The result is that scientific accu-
racy is incompatible with simplicity of nomenclature, and the
subject of homologies has been left largely in the hands of spe-
cialists in the field.

Structurally a muscle consists of muscle fibers bound together
and attached at either end with connective tissues. Each fiber
is surrounded with a microscopically thin sheet of tissue, and
groups of fibers are bound into bundles. Several of these bundles
make a muscle, the entire structure being surrounded by a strong
covering of connective tissues. These fibers completely interlace,
and in this way bind the muscle into a strong functional unit.

A muscle is usually attached by a broad base known as the
origin. It then widens to form, the belly of the muscle, and from
this point narrows almost to a point at its insertion on a bone.
The insertion is defined as the point of attachment on the part
which is to be moved. Frequently the function of a muscle
changes with the position of the animal, because of the change
of leverage. The rectus abdominis, which extends from the pubic
symphysis to the lower border of the ribs, illustrates the point.
In man the contraction of this muscle tends to pull the ribs,
and therefore the upper part of the body, downward; but in a
rabbit, or a man when he is suspended by his arms, the muscle
pulls the pelvis upward. Either end might be called the inser-
tion. Structurally, however, the muscle arises on the pelvis as a
thick, rounded mass, and spreads out on the ribs as a thin

178 STRUCTURE OF THE VERTEBRATES

aponeurosis, or sheet of connective tissue. Similar changes in
function are found in numerous other muscles. The important
thing to understand is the usual or major function of a muscle
or group of muscles.

The tendons and ligaments which attach the muscles to other
structures (a bone or other muscle) are important in their func-
tion. All muscle fibers are attached by connective tissue strands
to the periosteum of the bone or to other connective tissues. As
a long muscle approaches its point of insertion the amount of
connective tissue increases as the number of fibers decreases;
until near its insertion only a band or cord of tendinous tissue
is present. The fibers of the tendon become interwoven with
those of the periosteum, in that way firmly attaching the muscle.

During the course of evolution the tendency has been for
muscle groups to divide into smaller units, thus increasing the
total number of separate muscles. Consequently, several muscles
may be inseparable at the point of origin. In tracing the his-
tory of a muscle the progressive stages of the splitting can be
found, varying from a single muscle which inserts by two or
more tendons to those which are separate throughout their en-
tire length. What appears to be the opposite condition is also
found, a single muscle arising from several heads, or points of
origin.

Although the muscles furnish the power in any movement,
the skeleton gives form and rigidity to the body, flexibility
being gained by the joints between the individual bones. The
contraction of muscle results in various movements of the bones,
acting upon them as any power acts upon a lever. A number
of motions are possible, their range being limited by the struc-
ture of the joints and the articulations between bones. The more
important skeletal movements follow.

1. Flexion is the formation of an angle between two bones.
When the arm is bent at the elbow it is flexion between the
upper and lower arm. Similarly the hand may be flexed in
relation to the forearm, or the fingers flexed toward the palm.

2. Extension refers to straightening an angle already formed.
After a part is flexed the extensors pull it back into the original
position.

STRUCTURE OF THE VERTEBRATES 179

3. Abduction is drawing an entire unit of structure from its
median axis. When the arm is hanging at the side, and is then
drawn upward and outward, it is abducted from the axis of the
body. The thumb is abducted when it is drawn away from the
axis of the arm ; that is, away from the first finger.

4. Adduction is the opposite of abduction, the function of
drawing a structure back into a position parallel with the axis.

5. Circumduction consists in swinging a unit of structure so
that its distal end describes a circle. Holding the arm out from
the body and moving it so that the fingers describe a circle ful-
fils the conditions.

6. Rotation, on the other hand, is turning a structure while
its axis remains unchanged. The head may be turned without
shifting the body axis; or, if the arm is held out as above
described, it may be rotated slightly at the joint between the
humerus and scapula.

7. Pronation is limited to the lower arm and leg. Hold the
arm out with the palm up. Pronation turns the hand through
180 degrees so that the thumb is toward the median ventral line
of the body.

8. Supination is the opposite of pronation, turning the thumb
outward from the ventral line. Supination and pronation differ
from rotation in that, in the former, the bones of the arm are
twisted upon each other, and not rotated at a joint.

9. Elevation is raising a structure. The jaw is elevated when
the mouth is closed, and the ribs are elevated when the chest
is expanded.

10. Depression is lowering an elevated structure. Muscular
contraction which depresses the lower jaw opens the mouth.

11. Dilation is enlarging a circular opening. The reflexive
opening of the pupil of the eye is dilation.

12. Constriction is closing an opening. The muscles which
cause constriction of openings are usually sphincter muscles, and
are found in the so-called valves of the digestive tract.

Any movement of a skeletal part involves leverage. The levers
of the vertebrate body fall into the three classes which are rec-
ognized in physics. The following symbols are used in the
diagrams:

180

STRUCTURE OF THE VERTEBRATES

1. L — Lever.

2. W = Weight, the object to be moved through the arc of
a circle by the force exerted on the lever.

3. F = Fulcrum, the point upon which the lever moves.

4. P zn Power, the force which moves the lever upon its ful-
crum, and thus shifts the position of the weight. It is clear that
the position of the fulcrum in relation to the weight ana power
will materially influence the speed and force with which the
weight is moved.

M.

W : F •• ?

A. 1 St Order

W.

M.

F : P : w

B. '2^^^ Order

F : W
C. 3'"*' Order

1

In Figure I it will be seen that when a weight is held in
the hand, and the forearm is flexed, the leverage is like that of
the second order (B). AVhen (Diagram II) the back of the hand
is pressed against an object as the weight to be moved, the ex-
tension of the arm observes the conditions of the first order lever
(A). In Diagram III the ball of the foot is the fulcrum, the
tibia and fibula support the weight of the body, and the calf
muscles act as the power. This is like the third lever (C —
F:AV:P), and is powerful although the range of movement is
limited and the motion is slow.

STRUCTURE OF THE VERTEBRATES

181

Diagram III will show how muscle levers may be changed
by shifting the function and therefore the fulcrum. If the heel
is raised and the floor is tapped with the toe, the calf muscles
function as in the first lever, W:F:P. In this case the lever has
remained the same, while the fulcrum has been shifted. Other
cases involve a shift in the lever itself. Thus, a monkey may
stand on the ground and pull the limb of a tree downward; or,
he may hold a limb and pull his body upward. Essentially the
same muscles are brought into play although the weights and
levers have been changed.

]\Iuscle groups are named for their more prominent function,
although at times their action may be different. For example,
the muscles on the palmar (ventral) side of the forearm are
called flexors. Those on the back (dorsal) side are the exterisors
because their major function is to straighten an angle caused by
the contraction of the flexor group. But when the hand is flexed
at the wrist, the extensors may draw it into the position of a
straight angle and then continue until the hand is in a position
of dorsal flexion.

C. Axial IMuscles

Amphioxus has the simplest possible arrangement of the axial
muscles. Each segment is approximately equal to every other,
and is a V-shaped band extending from the dorsal to the ventral

Myotomes^

—Myoseptum

A. Amphioxus

B. Cyclostx)me

C. Dogfish

Fig. 102. Axial Muscles of Primitive Chordates (Diagrammatic). Note
the bending and elongation of the myotomes in the Cyclostome and

Dogfish.

182 STRUCTURE OF THE VERTEBRATES

side. The dorsal portion is thick, and thins out ventrally. Sep-
arating each myotome from its neighbor is a myoseptum, a band
of connective tissue, uniting adjacent muscle bands with each
other and with the notochordal sheaths. The muscle fibers ex-
tend longitudinally, being attached with connective tissues to
the myosepta. Therefore, when one side contracts the animal
forms the arc of a circle. As the anterior end of the animal is
more rigid than the caudal end, the alternate contraction of the
two sides sets up a propeller-like motion and sends the animal
forward.

The cyclostomes have more complex myotomes. The middle
portion of the ''V" is retained in place, the apex being at the
axial line, but the dorsal and ventral portions are bent forward,
giving a sigma shape to each myotome. Contractions thus tend
to give a dorso-ventral as well as a lateral motion.

The myotomes of the dogfish are more complicated by the
further bending and elongation of the dorsal part of the epaxial
group. Each myotome is attached to several vertebrae, losing in
part the simple metameric structure. The contraction of the
myotome affects more than one segment of the body, giving
greater flexibility to vertebral movements.

Future evolution, particularly in the land vertebrates, is in
correlation with (1) the lack of a supporting water environ-
ment which diffuses shocks, and (2) the flexibility required by
land life where locomotion is supplied by the legs rather than
axial movements of the body. There is a progressive elongation
and splitting of the myotomes, attaching them to a greater num-
ber of vertebrae, and giving a nicety of individual function im-
possible in the simpler forms.

The embryology of the mammal demonstrates many of these
changes. The first myotomes are simple metameric structures,
broken at the axial line. The epaxial group elongate, split, and
become largely longitudinal in position. A great number of
muscles result, some remaining as short muscles of small diam-
eter, others fusing to form long muscles extending more than
half the length of the vertebral column. These back muscles are
classified into several distinct groups. In the anterior region are
short occipital muscles which are attached to the head and
cervical vertebrae. These are structurally continuous with the

STRUCTURE OF THE VERTEBRATES

183

short vertebral muscles which attach to the spines of the verte-
brae. Lateral to these in the anterior half of the body are the long
muscles of the back, the longissimiis dorsi, composed of a num-
ber of slips. In the posterior half of the back, in addition to the
short spinal muscles, are two major groups: (1) the midtifidus
and (2) the sacro-spinalis, which pass from the sacrum and
pelvic girdle to the transverse processes of the vertebrae, and
give leverage to the back. This is, usually, the strongest muscle
group of the body.

Fig. 103. Axial Muscles of the Human Embryo. The primitive myotomes
bend, elongate and fuse to form the long axial muscles of the mammal.

The caudal muscles are continuations of the axial muscles.
In the reptiles the body decreases in size toward the tip of the
tail without any definite break in continuit}' at the pelvis. In
the mammals the tail is less prominent, and does not serve as a
locomotor or balancing organ as in the reptiles; and as the tail
degenerates these muscles become less and less prominent. A
functional exception exists in the South American monkeys, and
other mammals with prehensile tails, for in these the caudal
muscles are well developed to the tip; but in these animals the
tail is approximately the same size throughout its length, and is

184

STRUCTURE OF THE VERTEBRATES

clearly delimited from the body. In the higher anthropoids and
man, in which the tail is vestigial, the caudal muscles are left
as a small group of anal muscles, giving added support to the
perineum in the upright walking position.

Certain of the epaxial muscles migrate to the ventral side of
the vertebral transverse processes and pass posteriorly to the

Occipital mmcles
Irr— Spines of vertebrae

Spinal muscles

Ribs

Costal muscles

Sacro-spinales

Fig. 104. Back Muscles of Man. The superficial mviscles are shown dis-
sected away. The left side of the figure shows muscles which are deeper
in position than those on the right side.

pelvis and femur. This is the powerful ilio-psoas group, its homo-
logue beginning in the urodele amphibia.

During the course of evolution the hypaxial region remains
more primitive. The ventral elements of the myotome form the
costal, or rib, muscles which are distinctly metameric. In most
vertebrates they are limited to the thoracic region of the body.
In the abdominal region the hypaxial muscles form the layers
of the body wall and the rectus abdominis. The abdominal

STRUCTURE OF THE VERTEBRATES

185

muscles are in three la^-ers, (1) an inner transverse group with
the fibers running dorso-ventrally ; (2) the middle oblique fibers
which are directed in an antero-ventral direction; and (3) the
external oblique fibers which pass from the epaxial line in a
postero-ventral direction. The fibers from either side meet a con-
nective tissue septum, the linea alba in the midline. The rectus
abdominis muscle is another part of the abdominal group. The
fibers are longitudinal, arising on the pubic bones and extending

Spine of vertebra

paxial muscles

Hypaxial

{abdominal)

Mtiscles

Rectus abdominis
Linea alba

Fig. 105. Cross Section of a Mammal through the Lumbar region. The
skin and viscera are removed, with the muscle groups in position.

anteriorly to the ribs, where they insert in a wide band. The
hypaxial caudal muscles, like those of the epaxial region, undergo
degeneration during their evolution, and are left in man as a
few small muscles in the anal region.

D. Branchiomeric IMuscles

These striated muscles which develop from the hypomere
were originally attached to the gill bars. As the anterior two
pairs of visceral arches developed into the jaws and hyoid ap-

186 STRUCTURE OF THE VERTEBRATES

paratus, the muscles attached to them shifted their position and
function. Those of the posterior five arches remained as ele-
vators and depressors of the gill arches.

In the urodele amphibians there is a further degeneration of
the arches, with correlated changes of function in the muscula-
ture. The anterior muscles become attached to the throat car-
tilages and the cranium. In the mammals the final evolutionary
stages are found. The visceral muscles control the function of
the jaw, the hyoid, and the larynx. Most of these muscles are
very small, although those of the jaw are heavy, and those
which pass posteriorly from the larynx to the sternum are fairly
long and slim. If it were not for their embryological origin, and
innervation from cranial nerves, they could be considered with
the ventral axial group.

E. Appendicular Muscles

The origin of the appendicular muscles is intimately con-
nected with the origin of the limbs. The entire appendage arises
embryologically as two rows of metameric buds, one epaxial
and one hypaxial, which grow distally from the mesodermal
somites. (See page 158.) This diagrammatic metamerism in
origin is found only in the fish. In more specialized vertebrates
there is some fusion of the separate cell masses, until in the
mammals each appendage arises as a mass of cells which comes
from several segments and grows distally to form the limb.
The complete homology between the limbs of different verte-
brates is shown by the number of segments included and the
spinal nerves which grow out into the appendage.

The appendicular muscles begin in all vertebrates as dorsal
and ventral groups, and no matter how much torsion or spe-
cialization of the limb takes place, this relationship is main-
tained. After the full development of the limb the muscles fall
into two divisions: (1) the extrinsic muscles which have their
origin on the girdles, or other bones, and pass out to the limb
where they insert; and (2) the intrinsic muscles which lie en-
tirely on the appendage. The latter, even including the muscles
of the digits, retain the primitive dorsal and ventral relation-
ships.

STRUCTURE OF THE VERTEBRATES

187

1. Pectoral limb. The extrinsic muscles of the pectoral limb
are most primitive in the fish. They are fan-shaped groups of
fibers, the apex inserting on the basal cartilages. In the urodeles
the dorsal group becomes divided into several distinct muscles
which can be homologized with those of the mammal: (a) an
anterior sheet, the trapezius, arising on the spines of the verte-
brae; (b) a larger, more posterior, muscle, the latissimus dorsi,
which draws the forelimb posteriorly and slightly dorsally; (c)
the deltoid, which lies between the two former muscles, and is
an abductor of the arm; and (d) one or two other dorsal muscles

Myotome.

Fig. 106. Head and Shoul-
der of Necturus (Order
Urodela). Note the meta-
meric structure of the axial
muscles, the simple arrange-
ment of the extrinsic mus-
cles of the arm, and the
modification of the bran-
chial musculature.

which are not clearly homologized. The ventral, pectoralis,
muscles are almost undivided in the urodeles. During the course
of evolution other muscles are added to the anterior group, and
the pectoralis muscle is divided into two or three parts. All the
muscles change their shape and relative size, but their position
and function are very constant throughout the different classes.
The intrinsic muscles of the anterior limb undergo much
greater specialization. In the dogfish these muscles arise on the
basal cartilages and pass out to the distal cartilages as a group
of separate slips or bands. With the development of tetrapod
limbs in the urodeles the muscles take a more definite shape
with a division of function. The ventral biceps, which flexes the
forearm, is well developed and remains in the higher classes as

188

STRUCTURE OF THE VERTEBRATES

a single muscle. The dorsal muscle group is a large undivided
muscle in the lower forms. It becomes variously divided, the
larger median part becoming the triceps of the mammal. The
forearm of the urodele has the extensor, flexor, and pronator
muscles developed. The pronator muscle remains relatively un-
changed. The first two groups, however, become greatly divided,
accounting for the highly developed use of the hands and fingers.
The opposability of the thumb, which was supposed to be lim-
ited to man, has been shown to be a gradual evolution within

I

Fig. 107. Arm and Shoulder of Man, Ventral View. The ventral adductor

and flexor muscles of the shoulder and upper arm are in normal position.

The forearm is pronated to show the dorsal extensors and the ventral

flexors of the hand.

the primates, and correlated with structural changes. In the
lower primates the long adductors and abductors are tendinous
slips from the extensor and flexor series. In the higher primates
there is a progressive splitting off of the thumb muscles until in
man there is complete independence of the thumb from the
other digits. The short digital muscles are too complicated for
an elementary discussion.

2. Pelvic limb. The extrinsic muscles of the posterior ap-
pendage, like the skeleton, are usually more modified in the
land animals than are those of the anterior limb. Beginning with

Trapeziu

dorsi

-Abdominal

Glutea
muscles

Tensor fasciae latae
Vastus externns

Serratus

Latissimus

■Rectus femons

Biceps femoris

Fig. 108. Musculature of the Cat and Man. Side View. A comparison

demonstrates the fundamental similarity, although the size and shape

of the muscles have changed.

190 STRUCTURE OF THE VERTEBRATES

the first urodeles the pelvic limbs were used for propulsion. When
the knee was drawn toward the anterior part of the body, the
torsion placed the dorsal muscles of the thigh in an anterior
position. With this change the origins of the pelvic extrinsic
muscles became compressed into narrower limits. This is par-
ticularly true of man and the higher anthropoids, where a more
or less upright walking position has been assumed. In the mam-
mals the rectus femoris on the anterior of the thigh; the ad-
ductor magnus which lies laterally; and the large gluteus group
which form the buttocks, can be compared in position with the
three dorsal muscles of the pectoral limb. In the lumbar region
a large muscle mass migrates into a sub-skeletal position, with
origins on the ventral side of the transverse processes of the
vertebrae. This is a very constant group in all land vertebrates,
and is the ilio-psoas group of the mammals. They insert on the
femur near its head.

The intrinsic pelvic muscles are also more modified than the
pectoral, particularly the more distal ones. In most vertebrates
the foot is held in a permanent position of dorso-flexion. There-
fore the dorsal (extensor) group increase the flexion of the foot
against the shank; and conversely, the ventral (morphologically
the flexor) gastrocnemius and other calf muscles pull the foot
into a straight line with the limb. Reference should be made to
the discussion of the evolution of the leg position. The calf
muscles are usually much more powerful than the dorsal group.
In the normal walking position of the vertebrate the push of the
pelvic limb is gained by straightening the three angles — that be-
tween body and thigh, thigh and shank, and the shank and foot.
The last gives the final push forward, particularly when the
animal is in rapid motion.

F. Integumentary Muscles

The muscle slips which become attached to the skin arise
from all three of the skeletal muscle groups. The integumentary
muscles are of small importance when they occur in the reptiles,
but they become quite prominent in most of the orders of mam-
mals. A dog or horse can shake his skin without movement of
the skeletal muscles. In the armadillo, which is covered with

STRUCTURE OF THE VERTEBRATES 191

dermal plates, these muscles are very large. During the evolu-
tion of the primates the integumentary muscles of the body re-
gion become of increasingly less importance, until in man they
have all but disappeared.

The panniculus carnosiis is the major integumentary muscle
of the body, and in most mammals is distributed over the entire
back and sides of the animal. The panniculus of the lemurs is
large, enveloping the body. In the lower monkeys it becomes re-
stricted to a band on either side of the body, centering in the
axillary and inguinal regions ; and in the anthropoids it is further
reduced to small slips which spread out from the axilla to the
skin. The appearance of the panniculus in man is only occasional,
and it is then limited to thin bands in the axilla or to elongated
slips along the sternum.

The integumentary muscles of the neck and face, however, do
not undergo the same degeneration. The alligator and some other
reptiles have a well developed, transverse band of muscles in
the neck, the sphincter colli. In the mammals this muscle divides
to form two distinct groups: a neck muscle which retains the
original name; and a larger portion, the platysma, which spreads
over the face and moves the skin, scalp and ears. The mimetic
muscles of the higher primates are derived from the platysma.
These become progressively differentiated in the anthropoids and
man. In the latter the muscles attached to the ears and scalp
have degenerated, the ability to move either of these structures
appearing only as an anomaly. The derivatives of the platysma
which control the eyes, lips and facial expression, on the other
hand, are more developed in man than in other mammals. As
with other muscle functions, it is incorrect to say that frowning
or smiling is limited to any one genus. The chimpanzee and
gorilla have every facial expression known to man, although in
many cases the coordinations are less well developed.

CHAPTER XIII
DIGESTIVE SYSTEM

Due to the fact that the resph^atory organs, with a few minor
exceptions, develop from the digestive tract, the two systems are
frequently classified together as the digestive and respiratory
system. Functionally, however, the two are widely different, and
are technically separate systems. The intimate connection be-
tween them is most apparent in the more primitive classes. In
Amphioxus, the cyclostomes and the fish the oxygen used in
metabolism is dissolved in the water, and both food and oxygen
enter through the mouth. The excess water passes out from the
pharynx through the gill slits and the food is swallowed. Actually,
a limited connection is maintained in all vertebrates, the pas-
sages of the digestive and respiratory tracts meeting in the
pharynx.

The digestive system includes the functions of food getting,
digestion, absorption and defecation. The other nutritive proc-
esses include distribution of digested materials by the blood-
vascular system, metabolism within the cells, and the excretion
of waste products. Distribution and excretion are discussed in
later chapters.

The teeth, which function in securing and crushing food ma-
terials, were included with the skull, for they belong anatomically
with the skeletal system. The specializations of the teeth should
be reviewed, for there is a close correlation between tooth struc-
ture, the anatomy of the digestive tract, and the type of food
taken by the animal.

A. Development

The origin of the digestive tract begins with gastrulation,
when the archenteron (primitive gut) is formed. Around this en-

192

STRUCTURE OF THE VERTEBRATES

193

dodermal tube the mesodermal smooth muscles and mesenteries
are added. The digestive tract in its completed form consists of
the endodermal lining (the epithelium) and the contracting and
strengthening tissues.

The blastopore, formed at the time of gastrulation, is posterior
and forms the anus. The mouth later breaks through at the an-
terior end. The early embryonic tract is a straight tube opening
to the exterior at either end. There is an infolding of ectoderm at
both mouth and anus, with the result that the anal opening and

Gill potwhes

Foregut

l^-Liver

Fig. B

Fig. 109. Development of the Digestive Tract in Man. (A) The early
embryo in sagittal section showing the elongation of the tract and its
continuity with the yolk sac. (B) A later stage, the tract dissected out.

a part of the mouth cavity are lined with ectoderm. The latter
is of particular interest in the development of teeth, which are
covered with a layer of enamel secreted by ectodermal cells.

The primitive tube becomes lengthened and modified into more
or less sharply defined regions. In the lower vertebrates the
mouth cavity, the pharynx and the esophagus are practically
continuous. There is more separation between the regions in the
reptiles, particularly the alligators and allied members of the
order; and in the mammals the three regions are clearly de-
limited. The stomach is developmentally a pouch-like enlarge-

194 STRUCTURE OF THE VERTEBRATES

ment of the digestive tube. In most vertebrates a constrictor
muscle surrounds the posterior end of the esophagus; and, ex-
cept in the cyclostomes, the outlet of the stomach is guarded by
a strong pyloric sphincter. The intestine is the longest portion of
the tube. In this region the digestion of foods is continued and
the absorption of nutrient materials takes place. The intestine is
itself divided into regions. The stomach contents empty into the
small intestine. Its anterior portion is the short duodenum into
which the liver and pancreatic ducts empty, and is often classi-
fied as a separate part of the intestinal tract. In the higher ver-
tebrates the small intestine joins a heavy-walled large intestine
or colon which enters the rectum, an enlarged portion of the in-
testinal tract. In the fish, amphibia, reptiles, birds and mono-
tremes the anus empties into the cloaca, an enlarged sac-like
structure receiving the products of the digestive and urinogenital
tracts. In the higher mammals the cloaca is divided so that the
anus and urinogenital ducts empty separately.

The digestive glands develop as outpocketings from the gut.
As in other glands, these vary from single secretory cells to large
compound glands supported by masses of connective tissue. Of
the latter, the pancreas and liver are the most important.

1. The pancreas develops as one or several dorsal outpocket-
ings from the duodenal region of the intestine. In more primitive
animals it is fairly compact, but in the higher vertebrates the
pancreas is a large, diffuse glandular structure. The secretory
acini drain into numerous ducts which fuse to form one, two, or
three large ducts which carry the pancreatic juices to the in-

Stomachy

pUlslits^ ^Stomach

intestine

Phat-ynx

Tancreas

Fig. A Amphioxus Fig:. B Human Embryo

Fig. 110. Development of the Digestive Glands. (A) The stomach-intes-
tine of Amphioxus with the hepatic caecum. (B) The human tract, show-
ing the endodermal outpocketings which give rise to the liver and pancreas.

STRUCTURE OF THE VERTEBRATES 195

testine. The pancreas also functions as a gland of internal secre-
tion (Chapter XIX).

2. The liver is a ventral outgrowth, almost opposite the pan-
creatic ducts. The homologous hepatic caecum of Amphioxus is
similar in development, but does not become highly glandular.
The embryonic hepatic caecum of the vertebrates grows an-
teriorly, the distal end dividing to form a mass of tubular
glands. The liver becomes attached to the diaphragm anterior
to the stomach, but the large duct, which is the original out-
pocketing, empties into the duodenum. A comparison with Am-
phioxus will explain the relationships in the vertebrate.

The spleen is frequently considered with the digestive system
although it is mesodermal in origin, and functionally a part of
the vascular system. It has no duct leading from it. Its function
is (1) a blood forming organ in the lower vertebrates, and (2) a
blood corpuscle destrojung organ in the higher.

B. Development of Mesodermal Visceral Structures

The student should refer to the diagrams of the embryology
of Amphioxus and the reptile to understand the relationships of
the digestive organs in the body, and the origin of their muscular
walls and mesenteries. Recall that as the hypomere of the meso-
dermal somite grows distally from the notochord it divides inta
two layers, (1) the somatopleure (the outer or body layer) and
(2) the inner visceral layer, or splanchnopleure. The cavity be-
tween is the coelomic cavity. As the splanchnopleure pughes in
toward the median line the archenteron is separated from the
notochord and forced ventrally. The right and left sides of the
splanchnopleure enclose the gut and thicken to form the heavy
wall of smooth muscle with its peritoneal covering. Dorsally the
layers join to form the thin, strong mesentery which suspends
the gut from the dorsal wall of the body cavity. The thin layer
of mesodermal epithelium lining the coelomic cavity and cover-
ing the viscera is the peritoneum.

It is evident that any outgrowth from the digestive tract will
push the mesodermal tissues out with it and become surrounded
by connective tissues and peritoneum. Therefore, although struc-
tures appear to lie in the coelomic cavity, they are technically

196

STRUCTURE OF THE VERTEBRATES

outside the peritoneum. Glands are usually suspended by long
mesenteries; the kidneys are rarely completely surrounded by
the peritoneum on the dorsal side; and the urinary bladder may
have but slight contact with the peritoneal lining.

^erve cord

Mesente

Mesentery

f-Intestine

Muscle

Peritoneum ^.^^^^

Peritoneum -^^

Fig:. B

ndoderm

Fig. C

Gut

Fig. 111. Development of the Mesenteries and Peritoneum. (A) Shows
the embryo in cross section showing the relationship between the perito-
neum, mesentery and smooth muscles of the gut. (B) The same structures
at a later stage, with glands developing in a retro-peritoneal position. The
extensions of the mesentery are shown in cross section (C) and in side

view (D).

When the mesentery develops in the embryo it is connected
ventrally as well as dorsally; but except for a few isolated re-
gions the ventral portion is soon lost. The dorsal mesentery (or
simply "mesentery") becomes very complex. The lengthening of
the intestine stretches it along the ventral margin so that it be-

STRUCTURE OF THE VERTEBRATES

19:

comes thrown into wide folds or ruffles, while the attachment
along the dorsal median line remains only the length of the
animal's body. The growth of glands further complicates its
structure. Each, as it grows out, pulls the mesentery- with it, so
that the adult has numerous small supporting mesenteric folds.
In the mammals a ventral pocket of the mesentery grows pos-
teriorly from the stomach, forming an apron which covers the
viscera. This is the fat laden omentum.

Fig. 112. Mesentery of
a Cat. As the intestine
grows and coils the ven-
tral margin of the mesen-
tery is thrown into folds.

C. Modifications of the Digestive Tract

The digestive tract as a whole has undergone so many modi-
fications during the course of evolution that recent animals give
slight evidence of the evolutionary history. The functions of the
different regions of the tract remain very constant, but the struc-
tural variations are almost limitless. Within a family or order
the specializations are usually not great; but there is little pro-
gressive evolution from class to class.

There is an excellent, although not perfect, correlation between
the food habits of the animal and the length and shape of the
intestine. The herbivores have a complex stomach, a long in-
testine, and usually a large caecum. In the carnivores the
stomach and intestine are simpler in structure, the tract being
decidedly shorter. This correlation is also found in a single
order (for example the bats), in which the fruit eating varieties
have longer intestines than the carnivorous ones. The blood
eating bats have the shortest intestine known among the mam-
mals. Even in a species there may be variations in correlation

198

STRUCTURE OF THE VERTEBRATES

with diet, the Eskimos having a shorter intestine than the white
races.

1. IMouTH AND Pharynx. In the fish and primitive amphibia
the mouth cavity and the pharynx are continuous, the Latter
being the larger of the two regions. With the disappearance of

Gill slits,

Eicstachian
me

Mouth

^^^^^^ Heart

Fig. A Dogfish

Eustachian
tube

Mouth'",

Fig. C

Mouth
cavity

Alligator

Esophagus

Fig. 113. Comparative Anatomy of the Mouth Cavity and Phar3'nx. In
the Dogfish the cavity of the mouth and pharynx is continuous. A slight
separation occurs in the lizard, and in the alligator folds of tissue

separate the two.

the gill slits in the adult, which occurs in the more specialized
amphibia and all the amniotes, the pharynx tends to be relatively
smaller, and in the higher amniotes is separated from the mouth
by a fold of tissue. In the mammals this is the soft palate.
The mouth is distinguished by the jaws with teeth. In the fish

STRUCTURE OF THE VERTEBRATES 199

the skin covering the jaws forms the anterior margin of the
mouth, but in the amphibia lips develop as folds of tiss^ie an-
terior to the gums of the jaws. The lips and cheeks are well de-
veloped in most reptiles, the Chelonia being an exception, and
are highly specialized in the mammals. The birds lost the lips
with the loss of teeth and the development of the beak.

The tongue, a ventral muscular organ, is found in most fish
as a flat thickening above the hyoid apparatus. In the higher
classes of vertebrates it becomes more specialized as to size and
distensibility. The frog's tongue which is attached at the anterior
margin and can be turned outward as a trap for insects, and the
forked tongue of many reptiles are well known specializations.
Particularly in the mammal, the tongue assists in swallowing
and contains most of the taste buds.

The glands of the mouth are limited to simple mucous cells in
the fish. INIore complex glands appear in the land vertebrates and
have the double function of moistening the lips, mouth and food,
thus assisting in getting and swallowing food; and secreting
various enzymes. The glands are classified by their position as
(1) labial, (2) buccal, (3) lingual (tongue) or sublingual, (4)
mandibular or (5) submaxillary. The secretion varies widely in
composition. The salivary secretion of the mammals is either
serous or mucous. The poison of reptiles is a toxic secretion from
specialized mouth glands, as is the gelatinous material used in
the building of many birds' nests.

The gill slits are located in the pharynx, which is large in the
fish. In the land vertebrates the posterior slits disappear, the
most anterior remaining as the Eustachian tube and middle ear.
This opens into the pharynx.

In mammals and some reptiles the pharynx is the common
cavity for swallowing and breathing. The nasal cavities have been
cut off from the mouth and enter the pharynx. The opening to the
lungs is at the posterior end of the pharynx, and the mechanism
which controls breathing and swallowing is located in this re-
gion.

2. Esophagus and Stomach. The esophagus is a straight tube
leading from the pharynx to the stomach. Its length varies with
neck length rather than any functional adaptation. The diameter
tends to be larger in animals which swallow their food whole,

200

STRUCTURE OF THE VERTEBRATES

as the reptiles and carnivorous mammals. The sphincter which
separates the esophagus from the stomach is poorly developed
in amphibia and reptiles, making regurgitation of food a simple
matter; but in mammals the opening is small and the constrictor
muscles are more completely under the control of the nervous
system.

The stomach breaks the larger masses of food, and begins
the digestion of proteins. The acids and digestive enzymes de-

A. Rat

B. Pig

C. Dog

D. Sheep

K Human

Fig. 114. Structure of the Mammalian Stomach. The stomach is struc-
turally simple in most mammals. Herbivorous mammals show more ex-
treme modification. In (D), the sheep, the arrows show the course of
food in a cud-chewing animal.

stroy the connective tissues which bind together the cells of
muscles, and hasten the stomach and intestinal digestion. In
correlation with this function the walls are generally thick and
muscular, churning the mass until it is mixed with the digestive
fluids.

Anatomically, the simplest stomach is a slight enlargement,
and in the cyclostomes is continuous with the intestine. This
stomach-intestine is sufficient to digest the finely ground particles
of flesh which are swallowed. In the vertebrates generally the

STRUCTURE OF THE VERTEBRATES

201

carnivorous ones have a simpler stomach than those with herbiv-
orous habits. Man is omnivorous in habit, but has a simple,
pouch-like stomach, the anterior border being much shorter than
the posterior. The cud chewing herbivores have a stomach di-
vided into four distinct regions, the anterior acting as a reservoir
for the unchewed herbs which are later regurgitated and chewed
while the animal is at rest. In most vertebrates the stomach is
terminated by the strong pyloric sphincter.

3. Intestine. The cyclostomes have a straight intestinal tube,
not divided into anatomical regions. The dogfish intestine con-

B.

Fig. 115. Digestive Tract of a Dogfish. (A) Shows the entire tract. (B)
Shows the structure of the spiral vahe which greatly increases the absorp-
tive surface of the small intestine.

sists of a short duodenum, a greatly enlarged spiral valve, and a
short rectal region. The valve arises as a spiral infolding of the
gut wall, strongly supported by connective tissues and covered
with the digestive and absorbing epithelium. The structure gives
a greatly increased functional surface. The spiral valve was car-
ried over in modified form to the more primitive bony fish, in-
cluding the Crossopterygii and a few of the Teleostei. In the
higher vertebrates a coiling of the intestine gives the same func-
tional result as the spiral value.

The mammalian intestine is distinctly divided into a thin-

202

STRUCTURE OF THE VERTEBRATES

Availed small intestine and a thick-walled large intestine or colon.
The small intestine is not continuous with the large, but enters
it at approximately a right angle some distance from its anterior,
blind end. This blind pouch, the caecum, is large in the herbi-
vores and a functional part of the intestine, for its bacterial con-

Small

intestine-^

Fig. 116. Caecum of a Rabbit. Digestive caeca are found in many groups
of vertebrates. In the mammals it usually is found at the ileo-colic junc-
ture. In some herbivores the caecum contains cehulose-breaking bacteria.

Small
intestine

'Vermiform
appendix

A. 2 months Foetus

ermiform
apperidix

B. Newborn

C. 4 Years

Fig. 117. Development of the Vermiform Appendix in Man. The digestive
caecum degenerates in man, and is loft as a small appendix in the adult.

(After Kollman).

tent assists in breaking down the cellulose of plants. The caecum
of the human is short and terminated by a small vermiform ap-
pendage. The evidence from embryology and the comparative
anatomy of the primates indicates that the appendix is the ves-
tigial remains of a once large caecum.

The rectum is the terminal enlarged portion of the intestine,

STRUCTURE OF THE VERTEBRATES 203

and is a reservoir for the feces. The rectum terminates at the
anus which is closed by the sphincter ani.

4. Cloaca. The cloaca develops from the enlarged part of the
hind gut from which the allantois outpockets, and is partly lined
by inpocketed ectoderm. With a few exceptions (as in some
cyclostomes and fish) a cloaca is present in all vertebrate groups
except the marsupials and placentals. In the last two sub-classes
the cloaca is present in the embryo until a horizontal septum
cuts the dorsal from the ventral part and separates the anus from
the urinogenital openings.

E,n,er.ns V^^r,

Allantois

Tail gut

Fig. A
Adult Reptile

Fig. B ^^S. C

Mammal (embryo)

Fig. 118. Structure of the Cloaca. The cloaca remains undivided in the
majority of vertebrates, (A). In the mammals (B and C) a horizontal
constriction separates the dorsal digestive tract from the ventral urinary

bladder.

D. The Human Digestive Tract
The human mouth cavity and esophagus are typically mam-
malian. The stomach is a simple sac slightly divided into two re-
gions: the cardiac or larger portion; and the pyloric region
which lies toward the right side and empties into the duodenum.
The small intestine averages about twenty-two feet in length,
although variations of from sixteen to thirty-one feet have been
noted. It is divided into the duodenum, about a foot long; the
jejunum, the anterior two-fifths of the remainder and the ileum.

204 STRUCTURE OF THE VERTEBRATES

the posterior three-fifths. The small intestine enters the colon
at the ileo-colic valve.

The colon is nearly five feet in length, the ileo-colic valve
which constricts the entrance of the ileum being about three
inches from its blind end. The vermiform appendix aver-
ages a few inches in length. Its lumen, which opens into the end
of the caecum, varies widely. At times the point of division be-
tween appendix and caecum is hardly determinable. The juncture
of the ileum and colon, and the position of the appendix, is nor-
mally in the lower right side. From this point the colon passes
anteriorly as the ascending colon ; bends and lies across the body
as the transverse colon; and on the left side near the spleen it
bends posteriorly at the splenic flexure, and passes posteriorly
as the descending colon. In the posterior part of the body cavity
the colon makes a curved turn, the sigmoid flexure, and then
enters the rectum. The latter is slightly less than six inches in
length.

i

CHAPTER XIV
RESPIRATORY SYSTEM

Respiration is the process of taking in oxygen for metabolic
processes, and giving off the excess carbon dioxide from the
blood. In all vertebrates respiration is divided into (1) external
respiration, the mechanism for getting oxygen from the surround-
ing medium; and (2) internal respiration, the passage of oxygen
from the blood across the membrane of each cell of the body.
Anatomically, respiration is usually limited to the organs which
take the oxygen from the animal's environment.

Any moist, vascular membrane may serve as an organ of
respiration. In the vertebrates there are a number of such res-
piratory membranes in isolated groups, but the most widely dis-
tributed organs are gills in the water-living groups, and lungs in
the land vertebrates. Some amphibia, however, lack both lung^
and gills, and other groups have various organs modified for
respiration. Gills are the most primitive respiratory organs of
vertebrates.

A. Gills

1. Origin of the Gill Slits. An outline of the development of
gill slits was given on page 4. The embryonic pharynx is rela-
tively much larger than that of the adult, the difference being
much more marked in the higher groups of vertebrates. In Am-
phioxus there is a great number of the endodermal pharyngeal
pockets, each of which becomes secondarily divided into two. In
the cyclostomes the number is reduced to fourteen for the primi-
tive group, and seven for the lampreys. The latter number of gill
slits was carried over to the primitive sharks, and in several
living genera seven or six are found. In the dogfish the number
is reduced to the definitive vertebrate number, five, with the

205

206

STRUCTURE OF THE VERTEBRATES

first anterior pouch forming the spiracle. This structure in the
dogfish carries a rudimentary gill on its posterior surface. In
most of the higher fish the spiracle is completely lost, although
it is retained by the crossopterygian fish, and from them in-
herited by the amphibia as the Eustachian tube and middle ear.
The embryos of all vertebrates develop the gill pouches and,
with doubtful exceptions, these break through to the outside of

GUI slit

Nerve cord
Notochord

Gill slit

Nerve cord
Notochord-
Pharynx
Gill pouch

Fig. 119. Embryology of the Gill Slits (Diagrammatic). Drawings (A),

(B), and (C) are in horizontal section. Cross sections of similar stages are

placed immediately below. The gill pouches begin as outpocketings of the

pharynx and meet invaginations of the ectoderm.

the animal as gill slits. Gills develop only in the cyclostomes,
fish and amphibia, and are never found in amniote embryos. The
septum between each gill pouch is supported by a gill bar, or
branchial cartilage. These branchial cartilages become variously
modified in the vertebrates. (See Visceral Skeleton, page 143.)
A horizontal section through the human embryonic pharynx
illustrates the typical vertebrate condition. The five paired
branchial evaginations extend laterally from the pharynx. With
the exception of the first, these pouches are never functional;
but from each pouch develops a proliferation of cells which be-

STRUCTURE OF THE VERTEBRATES

207

comes differentiated into a glandular structure. These glands,
which will be discussed in Chapter XIX, are ductless and func-
tion as glands of internal secretion. The derivatives of the gill
pouches are:

I, the paired Eustachian tubes;

II, the palatine tonsils, which retain their position in the
pharynx throughout life;

III, the thymus gland, the two halves of which unite and mi-
grate posteriorly, the adult gland being located near the bifur-
cation of the trachea;

Thyroid

ustachian

^

4

Tonsil-^

Pharynx n TY'

Parathyroid^

\'

ni l^

/^j

)j^

•IV r^

Th/yrrms^/ {
Parathyroid

^

. ?f

Post-branchial^
body

\ ('

A Ventral View

B. Longitudinal Section

Fig. 120. Derivatives of the Human Pharj^nx. The ventral view (A) shows

the branchial pouches, the thyroid and lungs. The diagrammatic horizontal

section shows the glandular derivatives of pouches I to V.

Ill and IV, develop two pairs of small glands, the parathy-
roids, which lie against the thyroid cartilage of the larynx em-
bedded in thyroid tissue ; and

V, the postbranchial bodies.

The development of the middle ear and Eustachian tube from
the first pouch is a direct change of function of a structure. The
pharyngeal glands are derivatives of the pouches, not modifica-
tions of the structures themselves.

2. Internal Gills. Internal gills develop within the gill slits
of the early chordates, the fish, and the larvae of most amphibia.
In several amphibian groups they are carried over to the adult.

Cyclostome

B. Shark

perctdum

C. Teleost

Fig. 121. External Appearance of the Gill Region of Cyclostomes and

Fish. In fish above the Elasmobranchii the gills are covered with a bony

operculum. Compare Fig. 122.

Operculum

A. Hexanchus
(primitive shark)

B. Teleost

Fig.

122. Horizontal Sections of the Pharynx. (A) Primitive Elasmo-
branch, showing the six gill slits, and (B) a typical Teleost with five
gill slits covered by an operculum. In the Elasmobranch the septa extend

to the outside.

STRUCTURE OF THE VERTEBRATES

209

These internal gills ma}- be covered entirely by endoderm, or
by both endoderm and ectoderm.

In Amphioxus, it will be recalled, the afferent branchials break
into capillaries in the gills, and as the blood stream slows down
the exchange of gases takes place. The capillaries then re-collect
into efferent branchials which pass into the dorsal aortae. This
is essentially the same condition which prevails throughout the
gill-breathing fish and amphibia. In the amniotes the relation-
ships of the branchial arteries are the same, although they do not
break into capillaries.

In the dogfish the gill is a paired structure, developing along
the inner surface of the gill bar. The two halves, or hemi-branchs,
of each gill lie in adjacent gill slits. Therefore the gills which lie
in a gill opening belong to different branchial cartilages: one
half belongs to the anterior bar, the other half to the next
posterior gill support.

This relationship of the hemi-branchs is carried over to the
higher groups. The greatest change which occurs in the more
specialized fish is the modification of the septa separating the
gills from each other. The septa of the dogfish extend from the
pharynx to the outside of the animal, and the septum is cov-
ered externally by the body skin. Each gill slit, in other words.

A. Dogfish

Gilt-
support

Septum-
Afferent
branchial
artery

Efferent
artery

Skeletal arch

B. Sturgeon
(Chondrostei)

C. Gar pike
(Holostei)

D Typical Teleost

Fig. 123. Cross Sections of Gills, showing reduction of the septum.

210 STRUCTURE OF THE VERTEBRATES

is a simple opening through the body wall, connecting the
pharynx with the exterior. In the Chondrostei (page 40) a bony
covering, the operculum, shields the gill slits. In this group there
is a degeneration of the distal part of the septum, so that the
body covering of skin is no longer present in the region. The
reduction of the septa proceeds further in the Holostei, and in
the Teleostei the septa are reduced to a small fleshy membrane
along the gill bar. This modification obscures the primitive and
embryological origin of the gill slits. It gives, however, greater
function to the gills, for they are protected by the operculum
and the gill filaments are in freer contact with the water.

3. External Gills. External gills make their first appearance
in the Crossopterygian and Dipnoan fish. In the larvae of these
animals the gills develop on the outer side of the branchial septa.
Their position is shown in the drawing on page 46. As the in-
ternal gills develop both types of gill are present for a time,
and then the external disappear. Almost the same condition is
found in the amphibia, the external gills being wholly ectodermal
and appearing before the gill pouches break through. They are
very prominent structures in the early larvae. In Urodeles with
persistent gills both sets remain; but in the salamanders and
anurans the external gills disappear when the internal gills and
operculum develop. The operculum is a fleshy membrane not
homologous with that of the fish. At metamorphosis the lungs
replace the gills. Exceptions to this rule, due to accessory respira-
tory structures, will be discussed later.

The vascular supply of the external gills comes from the
branchial arches by branches which pass out into the gills. With
the disappearance of the gills the blood follows the more primi-
tive course, circling through the gill support from the ventral
to the dorsal aorta. The branchial arteries vary considerably in
the different groups of amphibia, in correlation with the reduc-
tion in number of gills and the branchial arches. The second
and fifth branchial pouches are rarely functional in amphibia.

B. Lungs

Lungs are the functional respiratory organs of land animals.
Their internal position keeps the epithelium moist. Like the gill

STRUCTURE OF THE VERTEBRATES 211

slits they are a derivative of the digestive tract. In the fish,
however, the connection between the two systems is closer. Water,
the environmental medium, is drawn into the mouth with the
food, and the dissolved oxygen of the water is taken up as it
passes over the gills in its course to the outside. With the
development of lungs a certain degree of separation begins be-
tween the digestive and respiratory tracts, although the environ-
mental oxygen is drawn into the pharynx and passed into the
respiratory tissues.

1. Evolution of the Lungs. The first homologue of the lungs
appears in the chondrostean fishes. In the sturgeon (page 41)
a dorsal outpocketing of the gut develops in the region between
the pharynx and the esophagus. The distal portion of the diver-
ticulum enlarges to form a swim-bladder, its duct maintaining a
free connection with the gut throughout life. The organ is slightly
vascular, the supply coming from the sixth branchial artery. In
Amia (a holostean fish) the swim-bladder is highly vascular,
and the connection with the gut remains in the adult.

From the simple, slightly vascular condition, the swim-bladder
evolved in two directions: (1) toward the condition found in the
teleost fishes, where the organ is non-vascular, loses its connec-
tion with the gut, and remains as a dorsal sac solely hydrostatic
in function; and (2) toward a ventral position, the duct open,
and the function respiratory. The latter condition is found in the
Crossopterygii and Dipnoi which have a lung-like swim-bladder.
Polypterus (a recent crossopterygian fish) has a swim-bladder
which develops as a ventral outpocketing of the gut. The struc-
ture becomes bi-lobed and highly vascular, the blood supply
coming from the sixth branchial as in the other fish. During the
dry periods, or when the river waters become muddy, the fish
gulps air and carries on respiration with the bladder. The Dipnoi
(page 45) parallel the amphibia in development and have a
swim-bladder w^hich is functionally a lung. In the South Amer-
ican genus the structure is so specialized that the animal is
unable to survive if kept submerged in water.

When the amphibia evolved the lung relationships of the
crossopterygian ancestors were retained. The lung develops as a
ventral outpocketing of the gut at the posterior end of the
pharynx, and soon divides into two distinct lobes. The opening

A. Sturgeon (Chondrostei)

B. Gar pike (Holostei)

Closed duct~\

C. Typical Teleost

Esophagus

Vascidar-
J bi-lobed
^ bladder

I

D. Crossopterygian

E. Dipnoan

Fig. 124. Comparative Anatomy of the Swim Bladder. Observe the evolu-
tion toward a closed hydrostatic bladder in the Teleostei, and toward a
ventral lung in the Dipnoi. (Adapted from Dean).

STRUCTURE OF THE VERTEBRATES 213

from the pharynx is the larynx; the undivided upper portion of
the tube is the trachea; and the two tubes into which the
trachea divides are the bronchi. Urodele lungs are simple ex-
pansions of each bronchus, and are less lobulated than those of
the dipnoan fish. During the larval period of development all
amphibia have the gill and lung combination for respiration, and
this double system is retained by many urodeles.

The homology of the swim-bladder is shown by (1) the com-
parative anatomy and function of the two structures; (2) by the
mode of embryological development, which is practically iden-
tical in the crossopterygian and dipnoan fish and the land verte-
brates; and (3) the vascular supply. In all cases the supply is
from the sixth aortic artery. It is only in the mammals that
there is a complete separation of the pulmonary arteries, which
arise as branches of the sixth branchial, and the ventral aorta.

2. Comparative Anatomy of the Lungs. Beginning with the
unlobulated, elongated lungs of the primitive urodeles, the lungs
evolved toward more complex divisions. Salamanders have the
anterior part of the lungs divided into shallow pockets or alveoli,
giving the lungs a honeycomb appearance. Each alveolus opens
wide into the central cavity, but the low septa add to the ab-
sorptive area of the epithelium.

Frogs show a definite advance over the salamanders. The lungs
are more rounded in correlation with their body shape, and the
alveoli are deeper and cover the entire inner surface of the
lungs. The toads, which live in drier areas, have lungs of the
frog pattern; but the dividing septa close partly over the alveoli,
giving them an almost spherical shape with a circular opening
into the central lung cavity.

Sphenodon (a New Zealand primitive reptile) and some turtles
have lungs which are no better developed than those of a frog.
A definite advance in lung structure appears in the lizards. Dur-
ing development the alveoli move further from the central
cavity, and are connected with it by small ducts, homologous
with the bronchioles of the birds and mammals.

Alligators and crocodiles have the best developed lungs among
the reptiles. As the lungs develop each bronchus divides into
several branches, thus cutting the lung into several lobes, each
with its own cavity. From the air cavity of each lobe numerous

214

STRUCTURE OF THE VERTEBRATES

bronchioles are given off, each ending in a spherical alveolus.

A progressive degeneration of one lung is found in the lizards
and snakes. In the primitive condition the lungs are bilaterally
symmetrical; but in the elongated lizards one lung is definitely
smaller than the other, and in the snakes one is either vestigial
or lacking.

Although the lungs of flying birds are not so greatly lobulated

Fig. 125. Comparative Anatomy of the Lungs. There is a steady increase

in number of bronchi and alveoH. Each division of the Alhgator kmg is

equivalent to a complete lizard lung.

as those of the mammals, their lungs are specialized for more
efficienc}^ The organ is not of unusual size, but greater efficiency
in gas exchange is effected by three pairs of air sacs. These sacs
are continuations of bronchial ducts, which expand after pass-
ing through the lungs to form membranous sacs in the pleural,
abdominal, and wing regions. As the air is drawn into the lungs
it passes across the alveoli and into the sacs, greatly decreasing
the specific gravity of the animal. The ramifications of the sacs

STRUCTURE OF THE VERTEBRATES

215

into the humeri can be demonstrated by cutting the humerus of
a pigeon and blowing smoke into the trachea. In addition to
lightening the animal, the mechanism tends to lessen the total
amount of air left in the lungs. Therefore the air, which is
normally only slightly depleted of its oxygen content, passes
across the vascular ducts at inspiration and expiration as it is
forced into and from the air sacs.

]\Iammals have complexly lobulated lungs, with innumerable
bronchioles and terminal alveoli. The structures have evolved
so far that there is no sign of a central air cavity in any of the
lobes. The idea of structure can best be gained by a description
of the development of mammalian lungs. The human lungs be-
gin in embryos of about 3.5 millimeters as a ventral groove along

'■^Lung
bud

■Esophagus

-Secondary
bronchi

Fig. 126. Embryology of the Human Lung. The secondary bronchi divide
minutely, each ending in an alveolus. (Adapted from His).

the esophagus. The posterior portion of the groove soon becomes
separated from the floor of the gut and forms a blind tube open-
ing into the pharynx. This opening is retained as the glottis.
The posterior end divides into two lobes, the distal ends be-
coming vesicular. At this stage there is a trachea and two
bronchi. The latter continue to divide, the early divisions forming
the larger lobes of the lung. The bronchi continue to divide into
smaller and smaller tubes until microscopic bronchioles are
formed. Each bronchiole terminates in a spheroidal alveolus. The
endodermal organ is enveloped and bound together by meso-
dermal tissues. Capillaries from the pulmonary arteries ramify
around the smaller tubes.

3. INIechaxism of Breathixg. Most amphibian tadpoles take
the air into the mouth and force it into the trachea by com-

216 STRUCTURE OF THE VERTEBRATES

pressing the mouth and raising the tongue. This method is only
slightly modified in the primitive lung breathing urodeles; but
in the frogs the air is drawn into the mouth cavity through
the nares which open immediately behind the teeth, and the air
is forced through the glottis with the floor of the mouth. Rep-
tiles, with the exception of the Chelonia, and Crocodilia, depend
almost entirely upon the ribs as the respiratory mechanism. The
contraction of the costal muscles forces the air from the lungs;
and as the muscles relax the glottis opens and atmospheric pres-
sure fills the lungs again. The crocodiles and alligators develop
a muscular diaphragm which separates the pleural cavities from
the abdominal cavity and is an aid in respiration.

The diaphragm is highly developed in the mammals. It is a
dome-shaped band of muscle, the convex side being toward the
pleural cavities. As the costal muscles contract, pulling the ribs
downward and inward, the diaphragm relaxes and the air is
forced from the lungs. The reverse process greatly increases the
size of the chest, and air rushes in when the glottis is opened.
In the human, at least, diaphragmatic breathing has largely
replaced costal breathing.

4. Development of the Larynx. The larynx is the supporting
mechanism of the glottis and upper end of the trachea. During
evolution the development of the lar3^nx has been correlated with
the development of cartilages supporting the trachea and bron-
chi. The cartilages of the trachea are incomplete dorsally. Those
surrounding the lower portion where the trachea bifurcates, and
also those surrounding the bronchi, are complete annular car-
tilages. The tracheal and laryngeal cartilages differ in origin, the
latter being modified from the visceral (branchial) cartilages
of the embryo. It will be recalled that the first and second
visceral arches form the jaws and hyoid apparatus in all verte-
brates above the cyclostomes, the posterior (branchial) cartilages
supporting the gills. In the dipnoan and crossopterygian fish,
where the swim bladder is modified into a respiratory organ, the
opening of the trachea is unsupported; but with the reduction of
the number of gill openings and gill cartilages in the amphibia,
the larynx begins its development. The seventh visceral (fifth
branchial) cartilages are the first to become modified. These
form the arytenoid cartilages on either side of the glottis.

STRUCTURE OF THE VERTEBRATES

217

The next advance is in the reptiles. Tlie arytenoids are of in-
creased size, and the sixth visceral arches form an epiglottis.
The paired cartilages migrate toward the ventral side of the
glottis and unite to form the epiglottis which curves over the
opening of the glottis. In addition, the third visceral (first
branchial) cartilages fuse with the hyoid bone, forming the
greater horns of the hyoid, and bringing the structure into close
contact with the larynx. Another modification is the fusion of
the anterior rings of the trachea to form a supporting cricoid
cartilage of the larynx.

Epiglottis,

Arytenoid
Glottis

Vocal cords

Trachea

Hyoid-

Thyroids
cartilage

Arytenoid-
Cricoid

A. Necturus (Amphibia) B. Tortoise (Reptilia) C. Opossum (Mammalia)

Fig. 127. Development of the Larynx.

In the mammals the remaining cartilages of the branchial
skeleton are brought into the larynx. The second and third
(fourth and fifth visceral) cartilages unite across the ventral
side of the larynx to form the thyroid cartilage. In the mono-
tremes this cartilage is clearly separated into two parts. A simi-
lar division is found in the embryos of the higher mammals, but
in these groups a complete fusion takes place during development.

The homologies of the first two visceral arches were given in
the section on the Visceral Skeleton, but are included here for
clarity. The jaws and cartilaginous supports for the gills as-
sumed their definitive condition in the elasmobranch fishes.
Therefore, the posterior five visceral cartilages are called the
branchial cartilages, and Visceral III is the same as Bran-
chial I.

218

STRUCTURE OF THE VERTEBRATES

Visceral
Cartilage

Urodeles with
Persistent Gills

Other
Amphibia

Reptiles

Mammals

I

Articular
Quadrate

Articular
Quadrate

Articular
Quadrate

Malleus
Incus

II

Columella
Hyoid

Columella
Hyoid

Columella
Hyoid

Stapes
Hyoid

III

Functionless

Functionless

Unites with
hyoid

Unites with
hyoid

IV

Branchial

Functionless

Functionless

Thyroid
Cartilage

V

Branchial

Functionless

Functionless

Thyroid
Cartilage

VI

Branchial

Functionless

Epiglottis

Epiglottis

VII

Arytenoid
Cartilages

Arytenoid

Arytenoid

Arytenoid

Vocal cords are found first in the anuran amphibia. These are
membranous bands of tissue attached to the arytenoid car-
tilages, and partially closing the glottis on either side. The con-
traction or relaxation of these membranes by movements of the
larynx give a limited change in pitch to the voice. This primitive

Glottis

Vocal-
cords

C. Glottis

B. Dorsal View

Fig. 128. Structure of the Human Larynx. (A) Shows the thjToid car-
tilage attached to the hyoid; (B) is a dorsal view with the esophagus
removed; and (C) shows the appearance of the glottis as seen through

the mouth.

STRUCTURE OF THE VERTEBRATES 219

voice mechanism is only slightly changed in the reptiles. In the
mammals, however, with the development of the thyroid car-
tilages, the vocal cords stretch from the arytenoids to the thyroid
cartilages. This increased length, correlated with the greatly de-
veloped muscular and nervous control of the larynx, gives the
mammals a fuller control of pitch and tone. The principle of
voice is based on the fact that a taut membrane vibrates more
rapidly and with a higher pitch than one less tightly stretched.
Differences in normal voice pitch in the mammals are due to the
length of the membranes. Under equal tension a long cord will
vibrate more slowly than a short one. The deeper voice of the
male is caused by the growth of the larynx in a dorso-ventral
direction at puberty. Length of larynx and depth of voice in a
species are definitely correlated with the prominence of the
Adam's apple.

The voice box of the birds is an entirely different structure,
being located at the bifurcation of the trachea. This syrinx is a
remarkably efficient organ as is demonstrated by parrots, mock-
ing birds, and other birds which have ability for mimicry and
a wide variation in pitch.

C. Other Respiratory Structures

In addition to gills and lungs, a number of animals have mod-
ifications of other structures which have a respiratory function.
The only one of wide distribution is the skin which serves as a
respiratory organ in most urodeles and many anura. In water
living urodeles the cutaneous artery along the side of the body
is larger than the pulmonary artery. Even the frog, when at rest
and in a moist location, can secure sufficient oxygen through the
skin. The most highly developed case of skin breathing is in some
salamanders which lack both gills and lungs in the adult condi-
tion, and carry on their entire respiration through the skin.
Cutaneous breathing is necessarily limited to those animals
which have a very thin, highly vascular skin, and live in wet
regions. For these reasons it is not found in any group above the
amphibia.

An isolated, but interesting, modification is found in the South
American dipnoans. As was mentioned on page 46, the pelvic

220 STRUCTURE OF THE VERTEBRATES

appendages of the male become vascular and swollen dm'ing the
breeding season and the annuals are able to stay submerged for
long periods while guarding the egg masses.

The sea turtle, although a lung breathing amniote, has two
specializations which aid in respiration. Both the pharynx and
cloaca are vascular and have thin epithelial coverings, and by
keeping a constant stream of water passing into and out of
these openings the animals can remain immersed for long periods
without damage. Only a small amount of oxygen is needed, for
the turtle at rest has a low metabolic rate and a minimum of
respiratory function suffices.

There are no accessory respiratory structures in the birds and
mammals, both of which have a heavy, dead skin. This is also
true of the completely aquatic forms. Porpoises and whales come
to the surface frequently for air. Some of the latter have a
pharyngeal air chamber w^hich gives a reserve supply to the
animal while under w^ater, but respiration takes place in the
lungs.

I

I

CHAPTER XV

VASCULAR SYSTEM

The blood-vascular system is the distributing mechanism of the
body. Food materials are absorbed by the epithelium of the diges-
tive tract, and oxygen is diffused through the lining of the lungs,
and these pass into the blood. In the more complex animals
the means of distribution are very important, for substances
necessary for metabolism cannot be absorbed directly by the
individual cells of the organism. Therefore an efficient system
of distribution has been vital in the evolution of the higher
groups.

The chordates, and particularly the vertebrates, have a closed
system. The epithelial lining of the vessels is continuous. The
arteries divide to form capillaries, and the arterial capillaries
become venous capillaries without any break. The blood, there-
fore, courses through unbroken channels. A certain amount of
the vascular fluids seep through the capillary walls and sur-
round the cells. These fluids are collected by the lymph vessels
(page 243) which empty into, and have a lining continuous with,
the veins.

The typical chordate system is found in Amphioxus. The
ventral, pulsating artery [ventral aorta) functions as a heart
and forces the blood forward along the ventral vessel lying
under the pharynx. Paired branchial arteries pass from the aorta
to the gills on either side, where each breaks into capillaries. The
capillaries from each gill collect to form an efferent branchial
artery, and these in turn form two dorsal aortae, one on either
side of the notochord. At the posterior end of the pharynx these
fuse to form a single median aorta lying ventral to the notochord.
Arteries are given off to the muscles and viscera, supplying them
with food and oxygen, taking up the waste products of metabo-

221

222 STRUCTURE OF THE VERTEBRATES

Anterior mesenteric

Coeliac Renal portal

Post mesenteric,^^ j

Fig. 129. Generalized Vascular System of the Elasmobranch. Only the
larger vessels are shown, and the connections between arteries and veins

are omitted.

lism, and collecting food materials from the intestine. Veins are
formed from the capillaries and in Amphioxus there are two
venous systems: (1) the subintestinal vein, which is homologous
with the lateral body veins and the veins entering the liver
(hepatic portal) of the higher vertebrates; and (2) the cardinal
veins, a small group of vessels which drain the gonads and
nephridia. The cardinal veins enter the subintestinal vessel an-
terior to its passage around and through the hepatic caecum.
The combined vessel is continuous with the pulsating portion of
the ventral aorta, completing the vascular system of the animal.
(See the diagram of the blood system on page 19.)

A. Vascular System of the Dogfish

The heart of the dogfish is a muscular organ consisting of
four chambers in a series, lying along the antero-posterior axis
of the body. It is enclosed in a pericardial sac, a division of the
primitive coelomic cavity. As the most anterior chamber and the
most posterior are found only in the embryos of the higher
classes of vertebrates, the heart is usuallv considered as consist-

STRUCTURE OF THE VERTEBRATES

223

ing of only two chambers, one auricle and one ventricle. The
ventricle is thick-walled and muscular, and is the pumping region
of the heart. Anterior to the ventricle is the muscular conus
arteriosus which continues anteriorly as the ventral aorta. The
most posterior chamber is the sinus venosus which receives the
blood from the veins and empties it into the thin-walled auricle,
from which it passes to the ventricle.

yeniral' aorta

Coronary artery

Fig. 130. Branchial Vessels of the Dogfish. Only the gross relationships of
the left side are shown. The vessels break into capillaries in the gills.

The six pairs of afferent branchial arteries of the embryo (see
page 235) are reduced to three in the adult. Beginning pos-
teriorly, the fifth and sixth rise close together and in Squalus are
fused for a short distance from their point of origin. The fourth
embryonic arch passes into the gill arch as the second afferent
artery of the adult. The second and third unite to form the first
afferent artery, and then separate to supply the two anterior gill
arches. The first afferent arch is not complete in the elasmo-
branchs, and forms the internal and external carotids. In Squalus
these vessels appear to rise as branches from the first efferent
arter}^

224 STRUCTURE OF THE VERTEBRATES

Afferent

Embryonic

Artery

Adult Structure in Squalus

I

Carotids

II
III

First afferent artery

IV

Second afferent artery

V
VI

Third afferent artery

After the afferent branchials break into capillaries in the gills
they re-collect as efferent branchial arteries. The four pairs of
efferent branchials of Squalus form complete loops around each
gill pouch, and then enter the paired dorsal aortae which forms a
single vessel posterior to the pharynx.

From the first efferent branchials a pair of common carotid
arteries passes forward, and these divide to form the internal
and external carotids. These vessels supply the head and brain.
Posterior to the carotids branches pass from the aortae to the
mouth and esophagus; and posterior to these the large suh-
clavians go to the pectoral fins. The hypobranchial arteries sup-
ply the muscles of the gills, and from them the coronary arteries
pass to the heart and pericardium.

The body muscles are supplied b}" the small, metameric parietal
arteries. The larger branches of the dorsal aorta, in order from
anterior to posterior, are: (1) coeliac which divides into several
branches, and goes to the liver, stomach, duodenum, pancreas,
and parts of the ileum; (2) the anterior mesenteric artery to the
left side of the intestine; (3) the gastro-splcnic to the spleen and
parts of the pancreas and stomach; (4) the posterior mesenteric
supplies the rectal gland, rectum and cloaca; (5) the renal arter-
ies to the kidneys; and (6) the iliac arteries which supply the
pelvic fins. The dorsal aorta is continued into the tail as the
caudal artery. In the female the oviducal arteries arise directly
from the dorsal aorta.

Blood is returned to the heart through the veins, which will

STRUCTURE OF THE VERTEBRATES 225

be traced from points of origin to the heart. Blood in the muscles
of the body is collected by the paired lateral veins and a single
cutaneous vein. These empty directly into the heart. The lateral
veins are joined by the subclavian and iliac veins from the
appendages. The head region is drained by the anterior cardinals
and the jugular veins. These may unite before entering the heart,
or enter separately.

The caudal vein drains the tail region, and at the level of the
cloaca splits into two vessels which enter the kidneys and there
break into capillaries. These two vessels are the renal portal
veins. Observe that the blood passing to the excretory organs is
both arterial and venous in origin. The nitrogenous wastes are
taken from the arterial blood. The venous blood from the renal
portal resorbs sugars from the kidney tubules, and collects the
blood carried in by the metameric and renal arteries. It is here
also that the tonus of the blood is maintained. The capillaries
from the kidneys collect into a number of small veins along the
median sides of the kidneys, and these form the large paired
posterior cardinal vessels which pass anteriorly, close to the
dorsal median line, to empty into the heart.

The digestive tract and mesenteries are drained by the hepatic
portal system. Food materials are taken from the absorptive
cells of the intestine and taken to the liver. There the hepatic
portal vein breaks into capillaries and the sugars are to a large
extent deposited as glycogen. The capillaries in the liver re-form
as several vessels which unite to form the hepatic veins which
enter the heart.

The large vessels carrying blood to the heart are classified
as two groups: (1) the systemic veins which enter the heart
directly, and (2) the portal veins which break into capillaries
before entering the heart. In the lower groups of vertebrates the
veins do not enter the auricle, but into a sinus venosus, the thin-
walled posterior chamber. The sinus is extended laterally in
the Elasmobranchs as the ducts of Cuvier. The sinus venosus is
present in the embryos of all vertebrate classes, but grows pro-
gressively smaller as a definitive structure until it disappears
entirely in the birds and mammals.

226 STRUCTURE OF THE VERTEBRATES

B. Embryonic Circulation of the Amniotes

The embryonic development of the vascular system closely
parallels the phylogenetic history. Therefore the embryonic cir-
culation of the higher groups gives a generalized idea of the adult
circulation of the lower forms. The following discussion is based
upon the embryology of the chick which is essentially like that
of the other amniotes, although the placentals have numerous
minor modifications in correlation with placental nutrition. The
student will recall that the reptile, bird and monotreme embryos
develop on large yolked eggs with meroblastic cleavage ; but that
the marsupials and placentals have small ova w^iich cleave
holoblastically. The latter develop a yolk sac upon which the
embryo grows in a typically amniote manner.

In large-yolked eggs the outline of the embryo is laid out on
the blastodisc before the blood vessels begin development. The
first sign of the vascular system is a group of rounded blood
islands in the extra-embryonic mesoderm, each of which soon
begins to branch. As lumina (internal cavities) appear, the
branching arms fuse and the larger vessels begin to grow in
toward the embryo, until a series of vessels is formed on either
side of the embryo, AVithin the embryo a synchronous growth of
vessels has formed a pair of ventral veins which grow outward.
These unite with the extra-embryonic series of vessels and form
the vitelline veins, one on either side, which carry food materials
from the yolk to the embryo.

A continued growth of the vitelline veins carries them an-
teriorly to the region of the pharynx, where they turn dorsally
and join the developing dorsal aortae. The dorsal aortae are at
first paired throughout their entire length, and continue poste-
riorly into the caudal end of the embryo. Immediately poste-
rior to the vitelline veins a group of capilliform vessels is given
from each aorta. These pass outward to the yolk and make
contact with the capillaries of the veins, thus completing the
earliest circulatory system. The small arteries to the yolk later
fuse to form a pair of large yolk sac arteries. At this time the
fundamental outline of the circulatory system has developed: (1)
a pair of incoming veins from the yolk to embryo; (2) an en-
larged region in the anterior portion of these vessels, destined

Carotid

Branchial arterie

Vitelline vein'

■Artery

Fig. 131. Embryonic Circulation of the Chick. At the top is an early

stage, showing the entire blastodisc. The lower drawing shows the torsion

of the heart and the development of the branchial arteries.

228 STRUCTURE OF THE VERTEBRATES

to become the heart; (3) the first aortic loop connecting the ven-
tral vessels with the dorsal; (4) the paired dorsal aortae; and
(5) branches from the aortae completing the circuit as they pass
outward from embryo to yolk.

Modifications rapidly appear in the embryo. The vitelline
veins begin a fusion to form the heart and a single ventral
aorta; and the aortae fuse from the posterior end of the pharynx
to the tail, forming a single dorsal aorta. Synchronously with
this the carotid arteries grow anteriorly from the first aortic loop
into the head region, and small arteries grow into the tissues from
the dorsal aorta. The blood carried out to the body is returned
to the developing heart by the cardinal veins. There are two
anterior cardinals draining the head and brain, and two post-
cardinals which carry blood from the posterior tissues and the
developing kidneys.

In the meantime the aortic loops have increased in number.
The primitive gill outpocketings have appeared along the pharyn-
geal wall ; and the aortic, or branchial, loops pass between them.
At this stage the chick embryo is essentially like the dogfish
adult. In the latter the branchial arches break into capillaries
in the gills and form afferent and efferent branchial arches. In
the amniotes the arches remain as continuous vessels which
undergo various modifications.

Amniote embryos of this stage develop an allantois (page 69)
from the posterior end of the gut. Veins and arteries are carried
with it, and these eventually lie in contact with the chorion.
Thus a vascular, moist membrane is next to the porous shell, and
acts as a respiratory structure for the growing embryo. In
placentals the allantois induces the development of a placenta
(page 93) and the allantoic vessels have the same function as
they do in the chick. In time the allantoic veins fuse with the
vitelline veins as the omphalomesenteric veins which pass
through the liver to the heart. The right omphalomesenteric dis-
appears and the left remains as the hepatic portal vein which
enters the liver and breaks into capillaries. The anterior ends of
the original pair remain as the two hepatic veins.

At this stage the system of the amniote embryo does not differ
essentially from that of the adult dogfish, with the exception of
the allantoic vessels. Future changes in the embryo can best be

STRUCTURE OF THE VERTEBRATES 229

treated in connection with the comparative study of the differ-
ent regions of the system, and the developmental changes will be
discussed in connection with the comparative anatomy of the
vessels.

C. Development of the Vascular System

The student at this time should have a concept of the general
structure of the vascular system. It is clear that the entire group
of structures included in the system is a unified whole, both
anatomically and functionally. For convenience the system can
be divided into its integral parts and each treated separately.
It should be borne in mind that development is synchronous, the
entire system growing and developing as a unit in correlation
with the growth and differentiation of the other organs.

1. Comparative Anatomy of the Heart. Amphioxus has a
straight, muscular, pulsating ventral aorta. In the cyclostomes
the heart is S-shaped, with a single dorsal auricle and a heavy-
walled ventricle. The blood from the systemic veins empties
into a sinus venosus, from which it passes to the auricle, then
to the ventricle and is forced into the ventral aorta. In the larva
of the lamprey (page 31) the coelomic cavity surrounding the
heart is partly cut off from the body coelom, and in the adult the
separation is completed. This coelomic pocket is the pericardial
cavity and is found in all the vertebrates.

The dogfish heart is essentially similar, with the addition of a
conns arteriosus containing numerous cup-shaped semi-lunar
valves to prevent the back flow of the blood from the ventral
aorta. This muscular conus is gradually lost in the higher fish
and the semi-lunar valves reduced to a typical three.

The amphibia make a definite advance in heart structure with
the separation of the auricle into two distinct chambers, the
dividing septum passing between the pulmonary veins which
enter the auricle directly, and the sinus venosus which receives
the body veins. A single auriculo-ventricular opening is present.
The left auricle consequently contains aerated blood, and the
right is filled with unaerated; but there is necessarily an ad-
mixture of blood as it enters the single ventricle. In this respect
the amphibian heart is less efficient than that of the fish. The

230 STRUCTURE OF THE VERTEBRATES

conus arteriosus is muscular and is usually guarded by several
rows of valves.

The reptiles as a group differ from the amphibia in several
important points: (1) the sinus venosus is small and usually not
visible on the outside of the heart; (2) the septum of the auricle
passes through the auriculo-ventricular opening, cutting the
valves into two groups; and (3) the ventricle is always more or

Ventral aorta
Ventricle

uricle

Auricular septum

A. Dogfish

Auricular septum

B. Crossopterygian

Pulmonary

artery

Ventricular septum

Aorta

C. Amphibian

D. Reptile

E. Mammal

Fig. 132. Comparative Anatomy of the Heart (Diagrammatic). The twist-
ing of the heart has been ehminated, the diagrams showing the hearts in
their primitive position with the ventricle and the auricle posterior. Xote
the growth of the septum, with the eventual separation of the ventral aorta.

less divided by a septum into two chambers. The degree of sep-
aration in the ventricle varies widely in the different orders. In
the turtles and lizards the ventricular septum is a semicircular
band of tissue incompletely dividing the chamber. Blood from
the right auricle is directed by flaps of the valves into the right
(pulmonary) cavity, the aerated blood entering the left chamber
of the ventricle. As the heart contracts the opening in the septum

STRUCTURE OF THE VERTEBRATES

231

narrows and forces the unaerated blood of the pulmonary cham-
ber into the pulmonary artery, and the aerated blood of the left
chamber into the two loops (radices) of the ventral aorta. In the
alligator the septum is complete, making a four-chambered heart,
but there is an opening between the right and left ventral aortae
which permits a slight admixture of bloods.

Pulmonary vei

Auricles.

Chordae tendinae

■Aorta

Pulmonary artery
Semilunar valves

Left ventricle
Papillary muscles

Cava-]— s=i

Tricuspid valve
Right ventricle

Fig. 133. Structure of the Mammal Heart. A portion is removed to show

the internal structure. The ventricular septum curves toward the right,

separating the semi-lunar valves of the aorta and pulmonary artery.

The mammal heart is structurally and functionally completely
four-chambered. The sinus venosus is left as a vestigial band of
tissue within the right auricle. The septum is complete, dividing
the heart into left auricle and ventricle, and right auricle and
ventricle. The auriculo-ventricular opening is guarded by two
sets of valves, each valve being a flap of tissue attached to the
inner periphery of the opening, and the loose edge held by
strands of connective tissue (the chordae tendinae) which are
attached to the wall of the ventricles. The left (mitral) valve
has two cusps while the right {tricuspid) has three. The ventral
aorta consists of a single loop (radix) and is completely sep-
arated from the pulmonary. Therefore the blood is never mixed,
except in pathological conditions. The heart of the birds is es-
sentiallv like that of the mammals.

232 STRUCTURE OF THE VERTEBRATES

2. Development of the Mammal Heart. The heart begins as
^ fusion of the two vitelline veins, and is a single straight
tube. As the heart region enlarges and becomes more muscular
it assumes an S-shape, the posterior (auricular) end moving
dorsally. The ventricular portion becomes heavily muscled and
forces the blood into the ventral aorta as it is received 'from the
auricles. Further growth of the ventricle forces it posteriorly, so
that the heart soon has the appearance of the auricles being
anterior and the ventricle posterior in position. If the student
understands the twisting and growth of the heart, the primitive
morphological relationships w411 be clear.

As the septa in the heart develop the ventricular septum com-
pletely cuts this chamber into two; but the auricular septum is
left incomplete until after birth, this opening (the foramen ovale)
between the two auricles being of functional benefit in embryonic
circulation, for the blood is aerated in the placenta rather than
in the lungs. If the foramen does not close soon after birth the
blood is mixed as it enters the aorta and a cyanotic condition
results in the individual. The so-called "blue babies" are persons
affected by this anomaly.

3. Aorta and Aortic Arches. The ventral aorta is the vessel
from the heart passing out of the pericardial cavity along the
ventral side of the pharynx. In the early embryonic condition of
all vertebrates the aortic, or branchial, arches pass dorsally to
the paired dorsal aortae. The typical number of arches is six,
although the cyclostomes and most primitive elasmobranch fishes
have a greater number. Beginning with this primitive condition
there are many modifications in the groups of vertebrates, but
the ontogenetic and phylogenetic histories of the classes are
closely parallel.

The elasmobranchs retain the most primitive condition. The
major changes are: (1) the first arch forms the carotid arteries,
and (2) the more posterior arches break into capillaries, thus
forming the afferent and efferent arteries from the original vessel
Most sharks (see Squalus, page 223) are further modified by the
fusion of vessels, so that only three leave the ventral aorta in
the adult condition.

The urodele amphibians show a reduction in the number of
arches, strongly resembling the modifications found in the dip-

I

Right
Auricle

Right ventricle

Left ventricle

Fig. 134. Embryology of the Human Heart. Drawings (A) to (D) are

shown in lateral-ventral view to show the bending and constriction of the

organ Drawing (E) is in ventral view. (F) Is the definitive structure.

234 STRUCTURE OF THE VERTEBRATES

noan fish. The first and second arches are lost, the base of the
third serving as the common carotid. The portion of each dorsal
aorta between the third and fourth arches is greatly reduced,
as is also the fifth arch. Large branches from the sixth arches
have grown posteriorly to enter the lungs as the pulmonary
arteries, and the dorsal segment of each sixth is greatly reduced.
Therefore, most of the blood entering the sixth branchials is
carried to the lungs, a small amount passing to the dorsal aorta
in an unaerated condition. The pharyngeal portion of the dorsal
aorta is paired, the two radices being formed by the fourth
branchial arches.

The anuran amphibia are mor€ effectively specialized for
land life. On either side the connection between the third and
fourth arches has completely disappeared; the fifth arch is lost;
and there is no connection between the pulmonary artery and
the dorsal aorta. However, the pulmonary arteries remain as
branches of the single ventral aorta, causing a further mixture
of blood as it leaves the heart. The separation of aerated and
unaerated blood is due to the anterior position of the former in
the ventricle, and as the contraction of the heart forces it into
the ventral aorta the first blood takes the line of least resistance
into the large ventral vessel and the two radices of the aorta.
When the unaerated blood leaves the heart, pressure has de-
veloped in the aortae and the pulmonary vessels are filled.

The typical reptile has a separation of the pulmonary arteries
from the aorta, the separation accompanying the development
of the ventricular septum. The pulmonary artery is cut from
the ventral aorta by a longitudinal constriction, so that the
two vessels leave the heart independently. Due to the structure
of the ventricular septum the pulmonary vessel carries mostly
unaerated blood.

In most reptiles the right radix of the aorta is larger and
filled with almost unmixed blood. It is from this vessel that the
carotids arise. The left radix carries partially mixed blood which
is emptied into the dorsal aorta and carried to the tissues. The
alligator has the most complete separation of blood of any
reptile, the small opening between the pulmonary and aortic
vessels permitting of only slight mixture.

In the birds the separation of aerated and unaerated blood is

Dorsal aorta ^

N\ \N W N\ N\

Yeniral aorta

A. Embryonic
^Carotid^

Afferent-
Efferent-

Subclavia

-Dorsal aorta-
B. Elasmobranch C. Urodele

ulmonary
Subclavian

'arotid

Radix of
dorsal aorta

Pulmonary

D. Anura

Radix of
dorsal aorta

Subclavia

Carotids

Carotids

Subc

VI

Pulmonary

■Aorta

orta

Subclavian

E. Reptile (primitive) F. Mammal (embryo) G. Mammal (adult)

Fig. 135. Comparative Anatomy of the Aortic Arches. Drawing (A) is

the generalized embryonic condition; (B) shows the division of each

arch to form an afferent and efferent branchial; (C) to (G) show the

reduction of the arches in the different groups of vertebrates.

236 STRUCTURE OF THE VERTEBRATES

complete. The left radix, which is less functional than the right
in reptiles, has disappeared; the right remaining as the aorta
of the birds. The base of the left fom^th branchial (the left radix
of the reptiles) remains as the left subclavian artery, supplying
the wing and shoulder.

Mammals are essentially like the birds in the modification of
the aortic arches, with the exception that the left radix remains
as the aorta. The left subclavian, like that of the more primitive
vertebrates, is a branch from the aorta, and the right subclavian
is connected with the aorta by the base of the fourth branchial
arch (the right radix of the reptiles). Following is a summary
of the degeneration of the aortic arches of the mammal. Recall
that in the embryo the ventral aorta is bifurcated from the
level of the third arch to its anterior end. Each half of the
anterior ventral aorta extends to the head as the external carotid,
and the anterior portion of the dorsal aortae form the internal
carotids. The common carotids are formed by the divided region
of the ventral aorta between the third and fourth arches.

Arch I, disappears.

Arch II, disappears.

Arch III, connects the common carotid with the internal
carotid.

Arch IV, forms: (1) the leit gives rise to the "aorta"; and
(2) the base of the right connects the aorta with the subclavian.

Arch V, disappears.

Arch VI, forms the pulmonary artery. The dorsal half disap-
pears, destroying the connection between the pulmonary and
the aorta.

4. The Cardiac Cycle of the Mammal. The foetal circulation
of the mammal differs widely from that of the adult. The lungs
are non-functional, respiration taking place in the placenta. Con-
sequently, aerated blood comes into the heart through the veins,
mixed with unaerated. Blood is carried back to the placenta
through branches of the dorsal aorta, with the result that blood
within the embr^^o is always mixed.

Blood is prevented from going to the unexpanded lungs by
three mechanisms: (T) as the blood pours into the foetal auricles,
most of that which falls into the right auricle passes through the
opening in the auricular septum (foramen ovale) and goes into

STRUCTURE OF THE VERTEBRATES 237

the left ventricle; (2) the mammal embryo retains the primitive
connection between the pulmonary artery and the dorsal aorta
(the distal portion of the sixth arch) and this tends to shunt
the blood away from the lungs; and (3) the pressure within the
collapsed lungs prevents the blood from entering. The connecting
vessel between the pulmonary artery and the aorta is called the
ductus arteriosus (or ductus Botalli). It begins to degenerate
soon after birth, but may remain open for some months in the
longer lived animals. Breathing is initiated in the newborn mam-
mal by a hypertension of carbon dioxide in the blood, and the
ductus arteriosus degenerates in correlation with the function
of the lungs.

. The cardiac cycle depends upon the alternate contraction and
relaxation of the heart and the flow of blood in one direction
through the arteries and veins. The latter is made possible by the
system of valves which direct the flow through the heart. These
are: (1) the auriculo-ventricular valves separating the auricles
from the ventricles; and (2) the semi-lunar valves at the base
of the aorta and the pulmonary artery.

Assume that the heart muscles are completely relaxed. The
blood then pours into the auricles from the systemic veins and
passes directly into the ventricles. As the ventricles fill, a wave
of contraction begins in the auricles, in the node of tissue formed
by the primitive sinus venosus. This wave spreads downward,
continuing into the ventricles. The swirl of blood in the ventricles
has thrown the valves outward, and the ventricular contraction
forces them upward and together, preventing the flow of blood
backwards into the auricles. The continued contraction of the
ventricles forces the blood into the aorta and pulmonary artery.
Relaxation occurs in the same order as does the contractile wave,
and as the pressure in the ventricle decreases the semi-lunar
valves are forced open by the column of blood in the arteries.
The fluid is unable to flow backward into the ventricle, and
is forced into the smaller arteries by a peristaltic wave of con-
traction in the larger vessels which carries it into the capillaries.
The heart completes its relaxation, and the cycle begins anew.
The closure of the valves is due to a differential pressure
within the regions of the heart and arteries. The sounds of the
heart are made by the snap of the valves as they close, the

238 STRUCTUKE OF THE VERTEBRATES

larger auriculo-ventricular valves making a slightly deeper note.
Any weakness or hardening of the valves may cause them to
close imperfectly and permit a backflow, the regm'gitating blood
causing a murmur as it rushes past the valves.

5. The Aorta and Its Branches. In their fundamental plan
the branches of the aorta remain remarkably constant in the
classes of vertebrates. The coronary arteries supply the heart. In
the dogfish they arise from the efferent loops around the third
gill pouch. In the amniotes they arise directly from the proximal
portion of the ventral aorta. The origin of the carotid arteries,
which are next in order from anterior to posterior, was discussed
with the development of the branchial arteries.

The subclavian arteries supply the pectoral appendages. The
vessel receives other names as it passes distally, the changes
being based upon human terminology. It is the axillary artery
as it passes the arm-pit; the brachial in the upper arm; and when
it divides at the elbow the two main branches are the radial
and ulnar. In the classes which have both radices of the aorta
present, the subclavians are given off as branches from each
side. The condition in the mammal and bird has been dis-
cussed.

Posterior to the subclavians the blood vessels can be divided
into two groups: (1) the parietal and (2) the visceral. The
parietal arteries are primitively metameric, although complete
metamerism may be lost in the higher animals. In the thoracic
region the parietal arteries pass to the muscles of the back and
ribs, the latter being the costal arteries. Posterior to these are
the lumbar and sacral arteries. The large iliac arteries, which
supply the pelvis and pelvic limbs, are branches from the aorta.
In most vertebrates the dorsal aorta is continued into the tail
as the caudal artery.

The visceral vessels are: (1) the coeliac axis which divides
into several large vessels supplying the liver, pancreas, stomach
and upper parts of the intestine; (2) the anterior mesenteric
which is more widely distributed in the mammal than in the dog-
fish, passing to most of the small intestine in the former class;
(3) the posterior rnesenteric which supplies the posterior part of
the digestive tract, including the colon and rectum.

The amniotes have several visceral arteries arising directly

STRUCTURE OF THE VERTEBRATES 239

from the dorsal aorta which are absent in the lower groups, or
arise as branches from other vessels. The genital arteries are
paired and supply the reproductive glands. The adrenal arteries
go to the adrenal glands which are located near the kidneys;
and the renal arteries go to the kidneys. The latter are present
in the dogfish, but arise as very small, metameric vessels. In
the amniotes these are collected into a few vessels, or a single
pair as in the mammals.

6. The Systemic Veins. The systemic veins are those which
enter directly into the heart without breaking into capillaries.
Of these systemic vessels, the anterior cardinals remain rela-
tively unchanged. In the higher vertebrates they are known as
the internal and external jugular veins. The internal pair drains
the brain and upper portion of the head, the external pair
drains the superficial musculature. In the mammal there are
frequent anastomoses between the two veins, and their inde-
pendent origin can be less clearly seen.

The lateral veins of the dogfish, which drain the sides of the
body and the paired fins, disappear in the urodeles. However, the
abdominal vein arises as a pair of veins which fuse, and this
median vessel is considered homologous with the laterals. The
alligators have two lateral veins. The other reptiles have an
abdominal vein similar to that of the urodeles. In the mammals
the lateral (or abdominal) veins practically disappear, and the
subclavian veins join the jugulars before entering the heart.

The postcardinal veins are very prominent in the dogfish,
passing anteriorly from the kidneys, and joining the anterior
cardinals before entering the heart. The further evolution of
these veins is intimately correlated with the development of the
postcava, or posterior vena cava, and these two veins must be
considered together. The postcava makes its first appearance in
the dipnoan fish. In the urodele amphibia it is a large vessel
passing from the anterior level of the kidneys to the right
auricle. At the kidneys the vessel divides and passes to each
kidney, taking over much of tlie function of the cardinals. The
latter are small vessels which appear to be branches of the
postcava, and pass anteriorly to enter the ducts of Cuvier,
draining the intercostal muscles and thorax. Embryological de-
velopment shows that the postcava grows posteriorly from the

240 STRUCTURE OF THE VERTEBRATES

auricle and joins the cardinals. Therefore the posterior part of
the postcava is morphologically the older cardinal vessels.

In the reptiles the cardinals lose their posterior connection
with the postcava, and are small paired vessels draining the
thoracic region. The postcava drains the posterior part of the
body (the abdominal veins having disappeared) and the cardinal
vessels have further degenerated. The left vein loses its connec-
tion with the jugulars, and develops one or more transverse con-
nections with the right cardinal. The left postcardinal is called
the hemiazygos; and the right half, which empties into the
jugular, is the azygos. As in the reptiles, it drains the costal
muscles.

The embryonic development of the veins of the mammal will
review these changes. The embryo has two postcardinals carry-
ing the blood from the primitive kidneys (mesonephroi) to the
ducts of Cuvier. Subcardinal vessels develop on the median sides
of the kidneys, taking over the smaller vessels of the post-
cardinals. The postcava grows posteriorly and joins the sub-
cardinals, taking its definitive form. In the meantime the post-
cardinals have begun to degenerate in the kidney region, and
the cross connections have developed. The left cardinal then de-
generates along its anterior course, the blood from both cardinals
emptying through the right azygos into the jugular.

7. The Renal Portal System. A portal vein is one which
breaks into capillaries before reaching the heart. In the dogfish
the caudal vein divides and passes to either kidney, the lateral
veins making a connection with the two renal portal vessels.
Blood from the tail, therefore, may pass to the heart either
through the kidneys, or around the kidneys into the lateral
veins.

The vessels entering the kidneys break into capillaries around
the tubules, with the result that both venous and arterial blood
is received. The renal portal system is present, almost fully
developed, in the amphibia; but with the degeneration of the
cardinals in the reptiles the portal veins become less important.
In the mammals the kidneys are supplied by large arteries, and
the renal portal veins disappear.

8. The Hepatic Portal SYSTE^L It has been mentioned that
the paired omphalo-mesenteric vessels pass anteriorly around the

STRUCTURE OF THE VERTEBRATES 241

liver to the heart. As the liver tubules develop, the right omphalo-
mesenteric is interrupted and breaks into capillaries. This con-
dition persists for a short time, and then the left vein breaks into
capillaries and the sj'stem is completed.

The earliest homologue of the hepatic portal is found in
Amphioxus, where the subintestinal vein breaks into small ves-
sels which re-unite before entering the ventral aorta. In the
elasmobranch fishes the system is fully developed, and with
minor changes remains in all the classes of vertebrates. Blood
from the liver is carried to the heart by the two hepatic veins,
which belong with the systemic vessels.

9. The Human Blood System. Blood leaves the heart through
the aorta, which turns dorsally from right to left. A few inches
from its base is given off the innominate artery which gives rise
to the right common carotid and the right subclavian. Next is
given off the left common carotid, and then the left subclavian.
The development of the aortic arches will make the relationship
clear. The external branch of the carotids passes to the muscles
of the neck and face; the internal branch goes to the brain and
its coverings. At the base of the brain the two internal carotids
anastomose forming a circle of vessels.

The aorta passes posteriorly along the ventral side of the verte-
brae, giving off the metameric parietal arteries to the ribs and
body muscles and several esophageal branches. Next are the
phrenic arteries to the diaphragm which separates the pleural
cavities from the abdominal cavity. The large coeliac axis is
next, going to the viscera, and then in order: (1) the superior
mesenteric; (2) the paired suprarenal (adrenal) vessels; (3) the
paired renals; (4) the paired spermatic (genital) arteries to the
gonads; (5) the inferior mesenteric to the colon and rectum; and
(6) the sacral artery. Immediately posterior to the last the aorta
divides to form the two large iliac arteries which supply the
pelvic limbs. The numerous branches of the latter need not be
discussed.

The external jugular vein, after making connections with the
internal jugular, enters the subclavian. The internal jugular
fuses with the subclavian to form the innominate. The conditions
are approximately the same on both sides. The two innominate

242

STRUCTURE OF THE VERTEBRATES

veins then coalesce and form the superior vena cava which enters
the right auricle.

The metameric costal veins are a part of the azygos, which
empties into the superior vena cava. The posterior part of the
body is drained by the inferior vena cava (postcava). The ex-
ternal and internal iliac veins fuse to form the paired common
iliacs, which unite at the level of the sacrum to form the vena
cava. This large vessel receives blood from the renal veins,
which are joined by the spermatics. Next in order are the

Carotid

Subclavian
'Aorta

Hepatic portal

Coeliac

■Ant. mesenteric

Renal

Post, mesenterii

Fig. 136. Vascular System of Man (Diagrammatic),

STRUCTURE OF THE VERTEBRATES 243

suprarenals and the phrenic veins. The vena cava then passes
through the liver and empties into the auricle.

The hepatic portal system is composed of the following major
branches: (1) the superior mesenteric; (2) splenic; (3) inferior
mesenteric; (4) the gastric; and (5) the pyloric vein. The com-
pleted hepatic portal vessel is short, and as it reaches the liver
it divides into right and left branches. The right is the larger
vessel. In the liver the vessels break into capillaries and sinuses.
The blood from the liver is collected into several small veins,
and the two large hepatic veins which enter the vena cava before
it passes through the diaphragm.

The heart consists of the four chambers typical of the mam-
mals, the auricles being anterior and the ventricles posterior. It
is roughly triangular in shape, the apex pointing downward, and
located usually in the fifth intercostal space; that is, between
the fifth and sixth rib cartilages. About one-third of the heart
lies to the right of the median line, the axis of the organ passing
from right to left. The aortic arch usually lies behind the
sternum. There is considerable range in the size and shape of
the heart, both being correlated with the general shape of the
body, the tall thin individual tending to have a slender heart,
while the broad type tends to have a shorter rounder one.

D. Lymphatic System

Although the veins and arteries of the vertebrate form a
closed system, a certain amount of the blood fluid passes through
the capillary walls, or is forced through their inter-cellular
spaces. This fluid is the lymph and lacks most of the corpuscles,
or cells, of the blood, and some of the coagulating elements. The
lymph bathes each cell of the body and carries food m.aterials
and free oxygen. This fluid is collected from the tissues by a
definite system of vessels, called the lymphatics, and is returned
by them to the veins. Failure of the lymphatic system to func-
tion properly causes an accumulation of fluid in the tissues or
body cavities, and the affected region becomes swollen or edema-
tous. There are many other causes of edema (infections, heart
trouble, and others) but during embryonic life lymphatic edema
is perhaps most frequent.

244 STRUCTURE OF THE VERTEBRATES

The lymphatic sj^stem also functions in collecting the fats
absorbed by the digestive tract. Each villus which projects into
the lumen of the intestine is supplied with a minute lymphatic
duct in addition to the vein and artery. The fats pass through
the lining epithelium into the lacteals, or Ij^mphatic ducts of
the villi, and through them into the main lymphatic stream. The
whitish, fat-ladened fluid is known as chyle and the lymphatics
from the intestine are frequently called chyle ducts. As a result
of these two functions of the lymphatic system, the lymph enter-
ing the veins has a high content of fats and waste products, and
a low content of oxygen, carbohydrates and proteins.

Both in phylogeny and ontogeny there is a reduction in the
extent of the lymphatic system. In the primitive vertebrate and
in the embryo there are large irregular lymph sinuses, and these
become reduced to a definite system of vessels, the system oc-
cupying relatively less space in the more specialized animal.

In the cyclostomes and fish the lymphatic capillaries collect
into definite vessels which empty into large sinuses connecting
with each other. The largest is below the vertebral column, lying
around the dorsal aorta. Others are in the ventral body wall and
underneath the skin. A single lymphatic vessel drains each of
the sinuses, and these efferent ducts fuse to form four main
ducts which empty into the veins. The anterior two join the
veins at the junction of the jugulars with the subclavians, and
the posterior two enter either the caudal or the iliac veins. At
the point where the lymphatic vessel leaves the sinus there is
a pulsating lymph heart forcing the lymph toward the veins.
Each is protected by valves w^iich keep the fluid flowing in the
same direction.

The lymph hearts are retained by the amphibia, particularly
the urodeles. In the anura (frogs and toads) the hearts are
definitely smaller and the anterior hearts disappear. Almost the
same condition is found in the lower reptiles. The crocodiles and
alligators retain the two posterior hearts, but the large dorsal
sinuses are reduced to thoracic lymph ducts. The birds lose the
lymph hearts entirely in the adults, although there are both
anterior and posterior connections with the venous system. In
both reptiles and birds there is an increase in the number of the
smaller lymphatic vessels, and the system takes on the appear-

STRUCTURE OF THE VERTEBRATES 245

ance of a definite group of vessels leading from the inter-cellular
spaces and digestive tract to the veins.

The mammalian lymphatic system shows the following
changes: (1) the terminal capillaries ramify more fully through
the tissues; (2) there are no definite sinuses with the exception
of minute ones in several glands; (3) the posterior openings into
the veins are lost, the entire body lymph entering the veins
through the two anterior connections with the subclavians; (4)
the area drained by the right lymphatic duct is limited to the
head and thoracic region, while the left duct drains the left
anterior half and the entire posterior part of the body; and (5)
lymph nodes develop.

The lymph nodes of the mammal are ovoid masses of glandu-
lar connective tissue, each divided into a number of independent
lobes or follicles. The lobe has a center of blood capillaries sur-
rounded by small lymphatics. The connective tissue is character-
ized by rather close-packed small cells with relatively large
nuclei, without any definite arrangement. The largest lymph
nodes are in the neck, the axilla (arm pit), and groin. The lymph
nodes have a double function, (1) the formation of certain white
blood cells and (2) filtration of the blood. In many types of in-
fection the lymph nodes swell due to the concentration of bacteria
at these points. Similar masses of lymphatic tissue appear in
other parts of the body: in the mesenteries; the intestine where
they are called Peyer's patches; the thymus; and the adenoidal
tissues of the naso-pharynx.

Developmext. The mammalian lymphatics begin develop-
ment as seA^eral outpocketings from the veins at the juncture of
the jugulars and subclavians. As these grow anteriorly and
posteriorly lymph hearts appear, and from the latter many
branches grow out into the tissues. The branches continue divid-
ing until the entire system of vessels is formed, each h'mphatic
capillary ending blindly in the tissues. There is little evidence
to support the theory that the lymphatic vessels have a double
origin, with the capillaries arising as enlarged inter-cellular
spaces independent of the outpocketings from the veins. The
terminations of the lymphatic capillaries are closed, the lining
cells of all the vessels are similar and continuous, and the em-

246 STRUCTURE OF THE VERTEBRATES

bryological evidence supports the idea that the vessels arise from
definite centers of growth.

As the lymph hearts of the embryo degenerate the lymph
nodes develop, and all signs of sinuses disappear. The vessels and
nodes are relatively more prominent in the young than in the
adult. The chyle ducts enter the left lymphatic duct.

E. Structure of the Blood

The blood is a fluid tissue (page 103) acting as the distributing
medium of the body. It is composed of discrete cells or corpuscles
floating in a fluid medium. The latter is the plasma, and is com-
posed of a clear straw-colored serum and fibrinogen. The fibrino-
gen when properly activated is the clotting element, precipitating
and leaving the clear serum, thus coagulating the blood. Lymph
is the serous portion of the fluid with less fibrinogen than is
found in the normal plasma. The corpuscles of the blood are of
two types, (1) red cells or erythrocytes, and (2) white cells or
leucocytes.

Leucocytes. The white cells of the blood are very similar
throughout the vertebrates, although the relative proportion of
the various cell types varies widely. The leucocytes are divided
into two major groups: (1) agranulocytes which have a clear
cytoplasm lacking granules; and (2) granulocytes which have
the cytoplasm loaded with granules. The non-granular cells are
the lymphocytes and the large mononuclear leucocytes. The
granular cells include the mast cells of the blood and the poly-
morphonuclear cells.

1. Lymphocytes are small cells which arise in the lymph tis-
sues, particularly the lymph nodes of the mammals. They have
a single nucleus almost as large as the cell. In the human these
form about twenty-five per cent of the leucocytes.

2. Large mononuclear cells have a single, often crescentic
nucleus, and are the largest of the white corpuscles.

3. Mast cells have granules taking a basic stain, and are con-
sidered by many investigators as degenerate cells. They form
less than one percent of the leucocytes in normal blood.

4. Polymorphonuclear leucocytes have nuclei which assume
many shapes, often giving the appearance of being divided

STRUCTURE OF THE VERTEBRATES

247

into several distinct nuclei. They are ameboid in appearance and
are actively phagocytic (that is, they ingest foreign particles).
These cells form about seventy-four per cent of all leucocytes
in the human. The great majority show a neutral staining
reaction, but a few take an acid stain. The granulocytes are
classified according to their staining reactions, and the size of
the granules.

A- Lymphocyte

C. Neutiophile

B. Mononuclear

D. Mast E. Acidophile M^

F. Platelets

Fig. 137. Mammalian Leucocytes and Blood Platelets.

The proportion of leucocyte types is of clinical value, since the
blood count indicates different physiological conditions. Most of
the white cells are phagocytic, and therefore have a definite func-
tion in preventing infections. Pus is made of dead white cells
and serum. In the human there are approximately 8,000 leuco-
cytes in each cubic millimeter of blood.

Erythrocytes. The red cells contain haemoglobin which has
a strong affinity for oxygen, the combination of the two being
an unstable chemical compound. Due to this instability the
oxygen is liberated in the capillaries and passed to the tissues.
Internal respiration depends upon the amount of oxygen carried
to the cells, and insufficient oxvgenation may be caused either

248

STRUCTURE OF THE VERTEBRATES

by a lack of haemoglobin in each cell, or by a lessened number
of cells in the blood. In the normal human there is an average of
four and a lialf to five million erythrocytes in a cubic millimeter
of blood. There is a slight sex difference, the female averaging
less than the male.

Erythrocytes vary markedly in structure in the different
vertebrate classes. With the exception of mammals all verte-
brates have nucleated red cells, and most of these are elliptical
in shape. The largest known are found in the urodele amphibia.
Although essentially the same shape in the reptiles and birds
they grow progressively smaller and more numerous.

B. Amphibian

C. Lizard

CD

D. Camel

E. Bird

Fig. 138. Eiythrocyte Types. Diagram (D) shows the enucleated, oval
erythrocyte of the camel.

Fig. 139. Development of Mammalian Erythrocytes. Diagrams illustrating
the changas which occur in the marrow of the long bones; a surface view
of a red cell with fragmented nucleus; and a side view of an erythrocyte.

All normal adult mammals have red cells without nuclei. These
cells, as in all bony vertebrates, arise in the marrow of the long
bones. In their early state these cells have nuclei, and are car-
ried into the blood stream of the embryo as nucleated cells. But
before birth (or shortly after in some species) the erythrocytes
begin losing their nuclei before entering the blood. The nucleus
of the maturing red cell is thrown out and disintegrates, the

STRUCTURE OF THE VERTEBRATES 249

enucleated cell passing into the capillary. The functional life of
the cell is evidently ver}^ short.

The erythrocytes are disc-shaped and bi-concave, and their
size varies in different groups of mammals. Microscopic examina-
tion of the blood can often determine the genus from which it
was taken, even in a dried specimen. The usual circular shape
of the mammalian erythrocytes has an exception in the camel
and its relatives. In these animals the red cells are elliptical in
shape, although they agree with all other mammals in lacking
nuclei. In pathological conditions any mammal may throw
nucleated cells into the blood stream but these are rarely, if
ever, numerous.

CHAPTER XVI

THE URINOGENITAL SYSTEM

The urinary and genital systems are considered as a unit in
vertebrate anatomy because of their close association. Physio-
logically they are entirely different; but certain structures are
common to both systems, and during the course of evolution
some excretory (urinary) ducts became a part of the reproduc-
tive system. In the lower chordates there is no connection be-
tween the two systems, but in the cyclostomes an association
begins and becomes progressively closer in the more specialized
classes.

The essential structures of excretion and reproduction are (1)
nephridia which collect wastes from the body and pass them to
the outside, and (2) gonads in which the reproductive cells (ova
and spermatozoa) develop. With the exception of the lower
chordates there are always ducts for the conduction of w^astes
and reproductive cells to the outside. In the higher groups struc-
tures may develop (1) for the storage of wastes; (2) to permit
the fertilization of the ova while they are still in the maternal
body; and (3) for the retention of the fertilized ova during their
development.

The nephridium is the functional unit of excretion. There is
no evidence of homology between the nephridia of invertebrates
and those of the vertebrates, although there is a functional
similarity. In the primitive condition each is an independent
structure composed of a head within the coelomic cavity, a
short tubule for the conduction of wastes, and an excurrent pore
opening on the outside of the animal. The head has a funnel-
shaped opening, the nephrostome, surrounded by cilia which beat
in toward the lumen of the tubule. The nephridia are paired,
metameric structures. This completely primitive condition is not
found in any of the living chordates, but is present in some in-

250

STRUCTURE OF THE VERTEBRATES 251

vertebrates. Amphioxus has retained the primitive metamerism
and position of the nephridia, but the coelomic head differs from
that found in the higher invertebrates and lower vertebrates.
Instead of a nephrostome the head is terminated by numerous
minute closed knobs. The tubules are short and each opens
directly to the outside.

The gonads of the vertebrates are paired mesodermal glands,
primitively located in the anterior part of the body, dorsal to
the peritoneum. The female gonad is the ovary, that of the male
is the testis. The sexes are separate in the majority of vertebrates,
an individual developing only ova or spermatozoa. The cyclo-
stomes and some of the teleost fishes, however, develop eggs and
sperms in the same animal. The condition is called hermaphrodit-
ism. Although isolated cases have been described in the higher
groups, most of the specimens are aberrant males or females
which have some of the characters of the opposite sex.

A. Organs of Excretion

The earliest vertebrate nephridia do not open independently
to the outside but enter a pair of pronephric ducts. The tubules
of the nephridia begin in the mesodermal somite at the junction
of the epimere and hypomere, in the region called the nephrotome
(see illustration on page 68). The tubules on either side grow
posteriorly and unite to form a pronephric duct. At the end of
the pronephric region the duct continues its growth toward the
posterior outlet. As the coelomic cavity develops the first verte-
brate kidney, or pronephros, makes its appearance in the ante-
rior part of the animal. The pronephridia are metameric and few
in number. Nephrostomes grow toward the coelomic cavity and
break through the peritoneum to enter the cavity. The two
pronephric ducts parallel each other to a point near the anus,
where they fuse to form a slight enlargement and leave the body
through a single opening.

The excretion of wastes from the blood into the coelomic cavity
is secured by small metameric arteries. These paired renal (kid-
ney) arteries pass laterally and ventrally to the peritoneum
where they coil and form a glomendus, a knot of thin-walled
capillaries. The glomerulus puslies into the coelomic cavity but

Urinary
papilla

^Developing mesonephros
""Pronephros "^Proriephric duct

Nerve cord

Dorsal aori
Renxilarterir^ / \

Cardinal vein

Xotochord
Myotome

Glomerulus

Pronephric
duct

Nephrostome

Fig. 140. Pronephros. Top, tadpole with body wall and viscera removed
to show the position of the pronephros and pronephric ducts. The meta-
meric mesonephros is shown bcsiinning its development. Bottom, a diagram-
matic cross section of a dogfish embryo, showing the structure and position

of the pronephros.

STRUCTURE OF THE VERTEBRATES 253

remains covered by the peritoneum, the waste materials being
forced into the cavity by osmosis and blood pressure.

It will be seen that there is no direct connection in the
pronephros between the artery and the nephridium. A physio-
logical unity is attained by a slight longitudinal constriction of
the cavity, forming a dorsal pocket which remains open to the
main body of the coelom. Wastes passing into this pocket are
collected by the open nephrostomes, and then enter the pro-
nephric ducts to be conveyed out of the body. The pronephros is
present in the embryos of all vertebrates. It functions in the
young of the cyclostomes and is partially retained by the adult.
It is also functional in those fish which pass through a larval
stage (for example the crossopterygians and dipnoans) and in
the tadpoles of the amphibia. It probably does not function in
the embryos of amniotes. The pronephros is soon replaced by the
niesonephros.

Mesonephros. The mesonephros develops posterior to the
pronephros and is serially homologous with it. The mesonephriclia
are rapidly surrounded by connective tissues, and due to a
process of budding of the primary tubules to form others the
metameric character is lost. The mesonephric tubules do not
develop a new pair of ducts but enter the pronephric ducts, and
when the pronephridia degenerate it is called the mesonephric
duct.

The most important advance is the development of a connec-
tion between the glomerulus and the excretory tubule. The renal
arteries do not grow in toward the peritoneum but parallel w^ith
it, and when the glomerulus forms it is not in contact with the
coelomic cavity. The glomerulus is met by a branch of the
mesonephric tubule. This accessory tubule begins as an outpock-
eting from the original tubule at a point about halfway between
the nephrostome and the entrance to the main duct. The branch
grows dorsally and its distal end enlarges into a vesicle which
presses against the glomerulus. The vesicle then invaginates,
grows around the knot of blood vessels and practically encloses
it, forming a Bowman's capsule. In this way the waste products
are passed directly into the kidney tubules and not into the
coelomic cavity; but any products of excretion which may be
in the cavity are eliminated by the nephrostomes. The glomerulus

Nerve cor<
Notochord

Dorsal aorta
Reimlartei

Glomerulus

Pronephric
duct

Boivman's
capsule

Nephrostomy

\

Heart

/M'esonephros

Developing
metayiephros

Mesonephric
duct

L.hro \ Mesonephros. At top, diao^rammatic cross section of amphibian

!" 1 T x"" u ^^'T^"-''^ °^ mesonephros. At bottom, embrvo of man

^vltll body wall and viscera remo^-ed to show mesonephros', with the

metanephric outpocketings from the mesonephric ducts

STRUCTURE OF THE VERTEBRATES

255

and the Bowman's capsule together are known as a renal or
Malpighian corpuscle. In amniote embryos the nephrostomes
rarely, if ever, break through into the coelomic cavity.

The mesonephros is the functional kidney of the cyclostomes,
fish and amphibia. It appears as a large body in amniote embryos
and was first described by the anatomist Wolff. As a result of
his discovery the mammalian mesonephros is known as the
Wolffian body, and the mesonephric duct as the Wolffian duct.
In the amniotes the mesonephros is merely a transitory embryonic

Draining
tubule

Fia. 142. Development of the
Metanephros. The metanephric
out pocketing is shown in contact
with the nephrotome. Compare
Fig. 118 for the earher stages.

Bowman^s
capsule

Glomerulus

Fig. 142-a. The Development of
Collecting Tubules. The pelvis
divides, the tubules joining the
nephrogenic tubules (right).
(From preparations by R. T.
Kempton).

structure. A part of it, however, remains in the male in connec-
tion with the reproductive system. The metanephros is the func-
tional kidney of the amniotes.

]\Ietanephros. The metanephros and its draining ducts have
a double origin. The renal corpuscles and the outer ends of the
tubules arise from the primitive nephric anlagcn, the nephro-
tomes, and are serially homologous with the mesonephric and
pronephric kidneys which arise more anteriorly. The ureters and
collecting tubvles which drain the kidneys arise as paired out-
pocketings from the posterior ends of the mesonephric ducts. Im-
mediately anterior to the fusion of the mesonephric ducts an

256

STRUCTURE OF THE VERTEBRATES

evagination arises on either side and grows anteriorly toward
the developing metanephros. The anterior end of each outpocket-
ing enlarges and branches. Due to continued division the tubes
become smaller and greatly multiplied until the terminal
branches become the collecting tubules. These grow^ into and
are surrounded by the nephric tissues. The tubules of the renal
corpuscles have been developing synchronously, each renal

raining
tubule

'Boivman's capsule
■ -Loop of Henle

A. Single Tubule

Cortex-
Medulla

Entrance
of Ureter

B. Gross Anatomy of
Urinary Organs

Fig. 143. Structure of the Kidney. (A) A single tubule with the branching
collecting tubule. (B) The gross anatomy of the kidney and bladder in long

section.

tubule developing from a small vesicle which invaginates at one
end to form a Bowman's capsule, and elongates at its other end
to form a kidney tubule. The tubules from the Bowman's cap-
sules unite with the collecting tubules, and the connection is
completed.

The amniote kidney is therefore composed of an outer cortex
from the nephrotome, and an inner medulla which arises from
the branching ureter. The collecting tubules join to form larger
tubes, and these empty into the pelvis of the kidney. The latter

STRUCTURE OF THE VERTEBRATES 257

is an enlargement formed at the original point of branching.
The pelvis narrows into the m^eter which passes backward to
its outlet from the animal's body.

The major differences between the metanephros and the
mesonephros are: (1) the loss of the nephrostome; (2) the com-
plete loss of metamerism in its origin; (3) the loss of a meta-
meric blood supply and the development of paired renal arteries
which leave the dorsal aorta and reach the pelvis before divid-
ing; and (4) the development of new draining ducts and col-
lecting tubules.

Urin.\ry Bladder. The urinary bladder is homologous with
the allantois. The latter develops as a ventral outpocketing from
the posterior gut, and its proximal portion is incorporated into
the cloaca. An allantoic bladder develops first in the amphibia;
but in the embryos of these animals the body wall is con-
tinuous (enclosing the yolk sac and allantois) and the allantois
is limited in size. In the adult amphibian this bladder is a
posterior pocket from the cloaca and the urinary products fall
into it as they pass from the mesonephric ducts.

In the reptiles and birds the allantois grows out as the em-
bryonic breathing and excretory organ. AVhen the body stalk
closes after hatching, the proximal end of the allantois remains
as a distinct urinary bladder which has the same relative posi-
tion as in the amphibia. In the birds (with a few exceptions)
the bladder is merely a division of the cloaca, known as the
urodeum. The structure of the cloaca and bladder of the mono-
treme mammals is similar in all essentials.

The marsupials and placentals develop an allantois which is
similar in development to that of the reptiles and birds; and,
as in the other amniotes, it is the proximal end of this out-
pocketing which develops into the urinary bladder. In both
groups during embryonic life the cloaca is present with its
allantoic pocket, the anus and urinogenital ducts opening into
it; but as development proceeds a constriction cuts the ventral
bladder from the more dorsal rectum. See diagram on page 255.
The ureters enter the bladder. The urethra which carries wastes
from the bladder to the outside is joined by the genital ducts
of the male. In the female the urinary and genital outlets are
separate in all vertebrates above the cyclostomes.

258 STRUCTURE OF THE VERTEBRATES

B. Development of the Urinogenital System

The simplest urinogenital system among the vertebrates is
found in the cyclostomes. In this class the mesonephros, and the
few retained pronephric tubules, are drained by the mesonephric
ducts which pass posteriorly and unite to form a urinogenital
sinus which has a genital pore on either side. Consequently the
coelomic cavity is connected with the outside of the body by the
pores which open from the coelom to the sinus, and then to
the outside through the urinogenital duct. The reproductive
cells as they mature in the gonad break through the closely
adherent peritoneum and fall into the coelomic cavity. With the
movement of the animal they pass posteriorly, through the geni-
tal pores into the sinus, and out into the water with the urinary
wastes. Fertilization is external. The system is structurally
identical in the two sexes, the difference lying in the type of
reproductive cells which are produced; and, although the same
animal may produce both sperms and eggs, in most cases these
mature at different times and self fertilization does not occur.

Dogfish. The embryo of the dogfish shows the fundamental
vertebrate plan of the urinogenital system. There are two im-
portant changes from the system as found in the cyclostomes:
(1) the genital pores lose their connection with the sinus, and
remain as apparently functionless abdominal pores connecting
the coelomic cavity with the cloaca; and (2) the pronephric
ducts split longitudinally to form two ducts, (A) the meso-
nephric ducts and (B) a new pair of ducts, the Milllerian or
oviducts. Each oviduct has an anterior ostium opening into
the coelomic cavity, and a posterior outlet into the cloaca. The
mesonephric tubules, as in the more primitive condition, drain
into the mesonephric duct. This undifferentiated stage is found
in the embryos of both sexes.

As development proceeds the female shows no material change.
The Wolffian (mesonephric) ducts continue to drain the kid-
nej'S; they fuse posteriorly, and open to the outside through a
urinary papilla which projects into the cloaca. The oviducts
develop wide ostia in the anterior part of the coelom, and in
Squalus these fuse in the adult to form a single opening. As the

I

5 Pronephros

Gonad

Mesonephros

Mullerian duct

Mesonephric duct

'Urinogenital sinus
[0 ^U.G.pore
G. papilla

A. Cyclostome*

R Undifferentiated
Dogfish Embryo

Ostium

vary

viduct

Epididymis
Testis

Mesonephros

Mesonephros

—Mesonephric duct

C. Female Dogfish

D. Male Dogfish

Fig. 144. Urinogenital System of Anamniotes. The Cyclostome condition
is shown in (A), the arrows indicating the course of the reproductive cells
from the gonads to the urinogenital pores. Drawing (B) is the undifferen-
tiated stage in the Fish and Amphibia, with the definitive structure of the
female (C) and the male (D).

260 STRUCTURE OF THE VERTEBRATES

eggs break from the ovaries they enter the ostia and pass into
the oviducts. The dogfish has internal fertilization and the eggs
remain in the oviducts until the embryos are capable of inde-
pendent life. This is true of most sharks. In the skates, rays, and
most teleosts the eggs are thrown into the water and fertilized
externally.

The male dogfish undergoes more radical changes during
development. The testes remain in the primitive anterior posi-
tion, and the mesonephric tubules of the anterior (sexual) por-
tion of the kidney grow through the mesonephros and enter the
gonad. The Wolffian ducts, therefore, function as both urinary
and genital outlets. The papilla through which the united ducts
leave the body is called the urinogenital papilla. The IMlillerian
ducts degenerate, and disappear entirely except for a small,
vestigial mass of tissue.

The system as described for the dogfish is typical of that
of the fish and amphibia. In the vertebrates above the Elasmo-
branchs the oviducts arise parallel to the Wolffian ducts, but
from the same anlage. The essential changes during develop-
ment are the same. In those fish which retain their eggs during
development (the ovo-viviparous condition) the male develops
specialized intromittent organs for placing the sperms in the
oviducts, so that internal fertilization may take place. A few
amphibia have internal fertilization but deposit their eggs in
an early stage of cleavage; but in this group the sperms are
usually deposited in capsules, and the female has a specialized
mechanism for taking up the sperm capsules.

Amniotes. Amniote embryos develop oviducts and meso-
nephric ducts similar to those of the fish and amphibia. A
slightly generalized drawing of the undifferentiated embryonic
stage would apply equally to any vertebrate above the cyclo-
stomes. Amniote embryos add, however, the metanephric kid-
ney and the mesonephros gradually degenerates. In both sexes
the Wolffian ducts lose their excretory function.

The female amniote loses the mesonephros and the meso-
nephric ducts, a small fragment of tissue remaining in the
mesentery as a vestigial body. The IMlillerian ducts remain as
the oviducts and undergo various modifications in correlation

STRUCTURE OF THE VERTEBRATES

261

with the reproductive habits of the animal, which may be (1)
oviparous or egg laying; (2) ovo-viviparous, the retention of
large-yolked eggs until the embryo develops; or (3) viviparous.
In the last case the oviducts are specialized as uteri. The
urinogenital system then consists of two practically independent
systems: (1) the ovaries and oviducts; and (2) the kidney,
ureter, allantoic bladder, and urethra.

The male amniote loses the Miillerian ducts during develop-
ment, a small posterior fragment remaining as the prostate

Ovary
^^Ostium

Testis

Vestigial
mesonephros

Oviduct

(Mesonephros)

Vas deferens-
(Wolffian)

Metanephros Metanephros
Ureter ^Ureter-

■Bladder

Female

Bladder
Prostate

Fig. 145. Structure of the Amniote Urinogenital System. Diagram (B), Fig.
144, shows the undifferentiated stage with both Miillerian and Wolffian

ducts.

gland which secretes a part of the seminal fluid. The meta-
nephros carries on the excretory function and the posterior por-
tion of the mesonephros disappears. The anterior portion of the
mesonephros, which functions in the fish and amphibia as a
sexual kidney, and the "Wolffian duct remain in the amniotes
as the efferent tubules and ducts of the testis. As in the anam-
niotes, the tubules burrow through the anterior part of the meso-
nephros into the testis, and these structures remain in the male
after the mesonephros degenerates. The mass of tubules in

262 STRUCTURE OF THE VERTEBRATES

connection with the testis is the epididymis, and the Wolffian
ducts are the vasa deferentia {ims deferens is the singular).

All amniotes have internal fertilization, and a number of in-
tromittent structures have developed. In the turtles, crocodilia,
a few birds, and all mammals the copulatory organ is the penis
which is homologous throughout these groups. The testes remain
in the primitive position in the reptiles, birds, monotremes and
a few placentals; the descent of the testes is described in the
following section.

C. Reproductive Structures of the Mammals

The above description is a generalized discussion of the
amniote system and applies particularly to the reptiles, birds
and monotremes. The last, although mammals, are typically
reptilian in their reproductive organs, both sexes retaining the
cloaca, and the female having oviducts with separate openings
on either side of the urinary papilla. In the marsupials and
placentals the cloaca disappears and other genital structures
make their appearance.

Female. With the division of the cloaca and the separation of
the digestive and urinogenital tracts, the female mammals de-
velop labia, or lips, guarding the external orifice of the urino-
genital openings. These, particularly in the higher mammals,
are double, consisting of outer and inner labia. Dorsally, within
the labia, is the opening of the vagina which develops largely as
a fusion of the posterior ends of the Miillerian ducts. Ventral to
this is the single urethral opening; and at the ventral union
of the inner labia is a small clitoris which is homologous with
the penis of the male.

The greatest modification occurs in the evolution of the
uterus, which retains the minute ova during development of the
embryos. The union of the oviducts is very incomplete in the
marsupials, with the result that the vagina is paired or separated
by a tissue septum, which causes each half to persist as an
almost independent unit. At the anterior end of each hemi-
vagina there is a marked constriction with a cervical opening
into the uterus. Sperms deposited in the vaginae during coitus

Ostium

Uterus

Oviduct

Cloaca

Paired-
vagina

A- Monotreme

B. Marsupial
(Opossum)

Uterus

Vagina

C. Rodent
DUPLEX UTERUS

Vagina

D. Carnivore
BIPARTITE UTEEUS

E. Artiodactyl
BICORNUATE UTERUS

F. Human
SniPLEX UTERUS

Fig 146. Comparative Anatomy of the Oviducts and Uterus. Observe the
loss of the cloaca and the fusion of the oviducts to form a vagma and

uterus.

264 STRUCTURE OF THE VERTEBRATES

pass through the cervical opening into the uteri and fertilize
the ova in their anterior ends.

The vaginal septum disappears in the placentals. However, in
the more primitive orders (insectivores and rodents) the two
uteri are distinct and each has a cervical opening into the
vagina. This is a duplex uterus. Further fusion of the two ducts
forms a bipartite uterus (found in carnivores and certain fami-
lies of other orders) with a single cervical opening, but the two
uteri united only at their posterior ends. The bicornuate uterus
occurs in horses, sheep, cows, and most other herbivorous ani-
mals, and is functionally a single uterus. The majority of
animals of this type have only one embryo developing at a time
and it occupies a median position in the uterus. Displacement
occurs when twins develop. ]\Iammals with duplex or bipartite
uteri more frequently have a number of offspring at a birth.

The simplex uterus is a further step in the evolution of the
organ, and is characteristic of the anthropoids and man. The
uterus has a single median cavity, with the small oviducts
(Fallopian tubes of human anatomy) passing from the lateral
ovaries to the uterus. The uterus is roughly triangular in shape,
the apex being at the cervix.

]\Iale. In the monotremes the seminal fluid containing the
spermatozoa from the testes is thrown into a common urinogeni-
tal sinus and conducted through the grooved penis. The latter
lies within the cloaca, but is protruded during coitus.

Two paired erectile bodies (the cavernous bodies) are added to
the penis in the marsupials, and the lateral walls of the groove
grow medially to form a tube. The tube is in the unpaired
median body of the penis, and is functionally a continuation of
the urethra. A cross section of the organ appears as a single
ventral body containing the urethra, and paired erectile bodies.
The penis with its muscles is attached to the ischial bones, and
distally is enclosed within a sheath of cutaneous tissue. The
urethra opens through the glans, an enlargement of the median
body.

The position of the penis shifts in the upper groups of mam-
mals. In the monotremes it lies within the cloaca and closes
the anal opening as it protrudes. With the disappearance of the

Ostiun.

Fallopian
tube

Fig. 147. Sagittal Section of Human Female.

266

STRUCTURE OF THE VERTEBRATES

cloaca in the marsupials, the penis becomes enclosed within its
own sheath, and projects posteriorly from its attachments on
the pubic and ischial bones. The organ of the placentals is
structurally the same, although it projects anteriorly from its

Ureter

Inguinal opening

Prepuce
Glan^

Epididymi

Fig. 148. Sagittal Section of Human Male.

origin on the pelvis. In the majority of the placentals the
organ is enclosed within a preputial sheath and is protruded
when the erectile bodies are engorged with blood. The penis is
pendant (not enclosed within a sheath) in the bats and primates.

STRUCTURE OF THE VERTEBRATES

267

Descent of the testes. The primitive position of the testes is
within the bod}''; but in the marsupials and the majority of
placentals the testes descend into a scrotal sac which is homolo-
gous with the external labia of the female. The testes do not
descend in the elephants, whales and sirenia, but this is ev-
idently a specialization. In other mammals the sperm cells will
not mature when kept in the body cavity. Descent of the testes
is seasonal in many rodents, the glands remaining within the
body except during the mating season ; in some the testes may be
withdrawn at will from the scrotum; while in others the connec-
tion between the scrotum and the body cavity is lost and the
testes remain in position after descent.

The gonads arise in a retro-peritoneal position and are cov-
ered by the somatic layer of the peritoneum. At the posterior end

Coelomic'
cavity

Testis

Peritoneum

GubemcLCulum

Coelomic
cavity

Vas deferens-
Scrotw.

Testis

Tunic of
testis

Fig. 149. Descent of the Testes in the Mammal. As the testes descend from

their body position, a pocket of the coelomic cavity is drawn into the

scrotum. In some mammals the connection with the body cavity remains

open as in (B) ; the connection is lost in man. (C).

of the embryonic gland there is a gubernacular ligament which
passes from the testis to the scrotum. As growth proceeds the
testes are drawn posteriorly, and the peritoneal covering is
carried with them as the tunic of the testes. Shortly before (or
after) birth in the human the testes pass through the openings
of the body wall into the scrotum. The inguinal canals through
which the glands pass are located in the groin, and the testes
leave the body anterior and ventral to the pubic bones, passing
between the pelvis and the skin. A pocket of the coelomic cavity
is carried into the scrotum with them, and the somatic mesoderm

268 STRUCTURE OF THE VERTEBRATES

comes in contact with the scrotal wall and develops the mus-
cular lining of the sac. Each vas deferens passes from the
epididymis over the pubic bone, through the inguinal canal, and
around the urinary bladder to its entrance into the urethra.

The semen develops as a fluid medium for the sperms. These
cells are motile and live best in an alkaline medium. Part of
the fluid is secreted by the tubules of the testis and epididymis,
but the major portion is secreted by several sets of glands
located around the urethra. The prostatic fluid is strongly alka-
line and neutralizes the urethral passage. Near the prostate are
the paired seminal vesicles, and more distally, the small Cow-
per's glands. Small lubricating glands, which have no connection
with the seminal fluid, are found near the glans.

D. Differentiation of Sex in the Human

Except for the fundamental difference in sex as determined
by the chromosomes at fertilization, much of sex differentiation
is controlled by the secretion of the ductless glands. Sex char-
acters are divided into the primary characters, those structures
wliich are directly related to the reproductive function; and the
secondary characters, which develop later and are simply mani-
festations of sex difference. The growth or degeneration of the
different internal reproductive structures has been discussed.
The following is limited to the external organs.

In the early, undifferentiated, human embr^^o the cloaca is
present and the external orifice is bounded by nearly circular
folds. Near the ventral margin of the fold appears a genital
tubercle resembling a small papilla. Synchronously with the
growth of this tubercle the anal opening is separated from the
urinogenital region. The tubercle grows larger and the lateral
folds become more prominent. At this stage the sexes are still
externally undifferentiated.

From this time on the sexes rapidly take on their typical
characteristics. In the male the tubercle enlarges and is joined by
two small inner folds. The latter develop into the erectile or
cavernous bodies of the penis. The tubercle forms the urethral
body and the glans. The larger, outer, genital folds push ven-
trally, unite, and form the scrotum into which the testes descend.

Amputated leg

Umbilical cord

Genital-

papilla

Anal fold-

Fig. 150. Undifferentiated Stages of the External Genitalia in Man.
(Adapted from Otis).

Glans pe??;<?.-„s, 'yi^^
Penis — — --^'' --^^"j^

Scrotal-^^M- ^- ' / \ V
iold ^X , / %

Urethral groove
/ Labia majora-.

Labia minora-
Glans chtondi^-

Vestibule—'\^ _y^

■Anus,

Ami.s,

/^->^^ ^'^" " ^ penis

f\

'Raphe
Scrotum

Vestibide

(i:

-Anus

Anus-

§-.

Fig. 151. Differentiation of Sex in Man. Diagrams (A) and (B) show the

development in the male; diagrams (C) and (D) are the female. Compare

undifferentiated stages in Fig. 150. (Adapted from Otis).

270

STRUCTURE OF THE VERTEBRATES

The female is less differentiated from the embryonic condition.
The tubercle remains as the small ventral clitoris; the primary
genital fold forms the outer labia (labia majora) ; and the inner
folds develop into the inner labia (labia minora). The vaginal
and urethral openings are enclosed by the paired labia.

Embryonic Structure

Definitive Structure

Undifferentiated

Male

Female

Primary genital fold

Scrotum

Labia majora

Inner genital fold

Erectile bodies

Labia minora

Genital tubercle

Urethral body

Clitoris

These changes occur at an early stage in embryonic life,
and further differentiation is a matter of slow growth until the
time of puberty. At birth the sexes are practically identical in
body form, but at the end of one or two years the child begins
to assume the pectoral and pelvic shape characteristic of the sex.

I

I

CHAPTER XVII

NERVOUS SYSTEM

The nervous system is composed of cells specialized for the
transmission of impulses, and acts as the coordinating mecha-
nism of the body. Stimuli from one region are transmitted to other
regions, and through the function of the nerves the body is made
to respond as a unified whole. So closely are the muscles cor-
related with the nerves that the two are physiologically grouped
as the neuro-muscular system. Even the involuntary bodily re-
actions are regulated by the nervous system.

A. Development

Nerve tissues are derived from the ectoderm along the dorsal
portion of the embryo. The nervous sj^stem begins as a neural
groove anterior to the blastopore (page 23). The lateral ridges
of the groove grow medially until a tube is formed. The develop-
ment progresses more rapidly at the anterior end, with the result
that the brain becomes specialized into regions before the posterior
end of the groove has closed to form a tube. As the neural tube

Neural groove

Neural crest Somatopleure
'Ectoderm

Epimere ' Endokerm
Notochord Splanchnopleure

Fig. 152. Cross Section of Chick Embryo.
271

272

STRUCTURE OF THE VERTEBRATES

separates from the covering ectoderm, small metameric masses
of ectodermal tissue develop on either side of the tube along
its dorsal periphery. These are the neural crests which give rise
to the sensory cells of the metameric spinal nerves. This is the
developmental beginning of the division of the nervous system
into (1) the central nervous system composed of the hrain and
spinal cord; and (2) the peripheral nervous system. The latter
includes the nerves and ganglia which develop from the central
system and connect it with the organs of the body.

Fig. A

Fig. B

Fig. 153. Development of the Brain (Chick).

The brain and spinal cord are continuous, structurally and
functionally, and the separation of the two is entirely arbitrary
(the foramen magnum being accepted as the dividing line). The
brain grows rapidly, two constrictions dividing it into three
major regions: (1) the forebrain (prosencephalon); (2) the
midbrain (mesencephalon) ; and (3) the hindbrain (rhomben-
cephalon). From the forebrain arise the olfactory lobes, the cere-
bral hemispheres, and the diencephalon. The midbrain develops
into the optic lobes; and the hindbrain gives rise to the cere-
bellum and medulla. The individual regions are discussed in
later paragraphs.

STRUCTURE OF THE VERTEBRATES

273

Prosencephalon

Mesencephalon

Rhombencephalon

fRhinencephalon (olfactory lobes)
Telencephalon (cerebrum and ventral
regions)
Diencephalon

Optic lobes

r IMetencephalon (cerebellum)
(^ Myelencephalon (medulla)

As the brain develops in the early embryo flexures, or bends,
appear. They are more prominent in the higher vertebrates and
are retained throughout life, while in the lower vertebrates they
are weak in the embryo and almost entirely disappear in the

A. Primary
(Apical)

B. Secondary
(Pontal)

Fig. 154. Flexures of the Brain (Chick),

C. Tertiary
(Nuchal)

adult. The first, primary, flexure occurs in the midbrain, and the
forebrain is bent ventrally until it lies at right angles to the
cord. The second flexure is at the end of the medulla {nuchal
flexure) and this bends the brain until the forebrain almost
touches the cord, its anterior end pointing posteriorly. The third
ipontal) flexure through the cerebellar region bends the brain
backward toward its original position. In the fish and am-
phibia the third flexure carries the brain back into line with the
antero-posterior axis of the body. In the amniotes the first and
second flexures are progressively stronger; and in the birds and
mammals the forebrain remains at right angles to the axis of
the body.

The neurocoel is the cavity formed in the central nervous
system when the groove closes. It is a typical characteristic of

274

STRUCTURE OF THE VERTEBRATES

the chordates. The cavity remains small in the spinal cord, but
as the brain grows the neurocoel expands and becomes en-
larged into ventricles. Each region carries a ventricular cavity
with it, but the ventricles of the brain are connected with each
other and with the neurocoel of the cord. The ventricles are
fluid filled and help supply the inner surfaces of the brain with
oxygen and food.

The brain and cord are enclosed by connective tissue cover-
ings, the meninges of the central nervous system. In the embryo
this is a single meninx, closely adherent to the nerve cord,
which carries blood vessels to the brain. The space between the
meninx and the periosteum of the skull and neural arches

I

Fig. 155. Comparative Anatomy of the Meninges. The single dura spinalis

of the dogfish (A) becomes double in the reptiles. A third membrane, the

arachnoid, appears in the mammal (C).

is filled with fluid, and the two membranes are connected by
fibers which pass freely between the two. This simple covering
is retained by the fishes throughout life. Beginning with the
amphibia there is an increasing complexity of the meninges. The
meninx separates to form (1) a thin covering of the brain, the
pia mater; and (2) a heavier outer layer, the dura spinalis. In
the mammals the pia divides, a microscopically thin arachnoid
layer being added as a meninx. The dura spinalis becomes the
dura mater. This membrane lies against the periosteum of
the bone and the two eventually (about the eighth year in the
human) become inseparably united. The spaces between the
membranes is always filled with fluid which is protective and
nutrient. The meninges pass out over the spinal and cranial
nerves, enclosing them as a sheath.

STRUCTURE OF THE VERTEBRATES

275

B. Units and Structure and Function

The brain and spinal cord are composed of nerve cells and
their fibers, and an ectodermal supporting tissue called neuroglia.
The latter makes up the vast majority of the bulk of the central
system. The nerve cell is the neurone, the anatomical unit of
the nervous system.

A neurone is composed of (1) a cell body with its nucleus
and cytoplasmic inclusions; (2) filamentous dendrites which re-
ceive the stimuli; and (3) a long axone which carries impulses

Fig. 156. Types of Nerve Cells. Motor Cell (A) from the ventral horn
of the cord; Sensory Cell (B) ; and Brain Cells, (C) and (D).

from the cell body and (4) ends in a highly branched terminal
arborization. Neurofibrils pass from the dendrites through the
cell body and out to the end of the axone. The cell body is the
center of metabolism. The dendrites, axone, and terminal arbor-
ization carry on the specialized function of the nerve tissues.

Neurones are divided by their structure into two large groups:
multipolar cells which comprise the great majority, and uni-
polar sensory cells. The former vary enormously in structure,
from the rather simple type found in the nerve cord to the
highly complex cells of the cerebral region. In this type the

276 STRUCTURE OF THE VERTEBRATES

dendrites are short and enter the cell body at several points,
or poles, and pass out through the single axone. In the unipolar
cells the dendrites are very long and the axone short. The
relative length of the processes in the two types becomes clear
if position and function are considered. The cell bodies lie in
or near the spinal cord, and are connected with all the regions
of the body through their neurofibrils. The sensory cells have
long dendrites in contact with the periphery of the body, and
short axones to the cord. The motor cells, lying in the cord, have
short dendrites and long axones. The cells of the brain may
vary in the relative length of dendrites and axone.

The axones give off collateral branches during their course.
Axones and their collaterals wliich pass from and to the cord
are usually covered with a fatty myelin sheath. The lipoid sub-
stance is enclosed in a delicate connective tissue sheath which is
broken at irregular nodes. The sheath between the nodes is
continuous and formed of two concentric tubes with the space
between filled with the myelin substance. The cell bodies are
bare. The unmyelinated fibers are those of the association
neurones which do not leave the brain or cord, and the fibers
of the sympathetic system.

Unmyelinated tissues have a pale, translucent appearance
when cut and are known as the gray substance of the nervous
system. The myelinated fibers have a waxy white appearance
and form the white substance. A trans-section of the brain or
cord will demonstrate the two types of neural substance.

Cell bodies are usually collected into definite masses. Any
group of cells which is separated from other groups is a gang-
lion; and the groups of nerve cells which develop from the
neural crests are therefore the spinal ganglia. If, however, cell
groups are continuous or barely separated from one another, as
in the brain and cord, they are termed nerve nuclei.

The functional unit of the nervous system is made by three
or more neurones forming a reflex arc. External stimuli are
usually received by specialized sensory cells, the receptors of
sensations. The impulse is carried from the receptor to the den-
drite of a sensory neurone, and is transmitted through the spinal
ganglion into the cord. Small association neurones take the
stimulus from the dorsal region of the cord to the motor cells

STRUCTURE OF THE VERTEBRATES 277

located in the ventral portion of the cord. Dendrites receive
the stimulus and a motor impulse is carried out over the axone
to a muscle, gland, or other effector. This is the simplest type of
nerve reaction. It is doubtful if a two-neurone arc is possible;
and in the normal animal an arc as simple as the one described
rarely occurs. Every sensory stimulus reaching the cord is car-
ried to the brain, and numerous pathways are usually stimulated.
The anatomical connection between two neurones is the
synapse. There is no evidence that the neurofibrils are in actual
contact, the experimental evidence indicating that there is a
definite space between. The gap apparently causes the impulse
to travel in one direction, for experiments have shown that nerve
fibers carry impulses in either direction with equal facility.

C. The Cord axd Spinal Nerves

The spinal cord is more primitive than the brain and will be
described first on account of its simplicity. In dissection the
spinal cord of the primitive vertebrate appears as a tapering
structure with two enlargements; one in the neck, the cervical
enlargement, and a posterior one near the pelvic limbs, the
sacral enlargement. Except in the most primitive types, the
cord is divided both dorsally and ventrally by deep fissures. In
transverse section the cord of the higher vertebrates is approx-
imately circular, and almost divided into two halves. The fis-
sures are caused by the unequal growth of the embryonic tube,
the cord enlarging laterally and not dorso-ventrally. In the
center is the neurocoel, surrounded by the tissue and fibers
connecting the right and left sides. The peripheral region of the
cord is white substance; and centrally is an H-shaped region of
gray substance, the neurocoel being in the center of the cross
bar.

In the primitive vertebrate the spinal nerves leave the cord as
paired, metameric structures. Each has a dorsal sensory root
with its dorsal ganglion near the cord, and a ventral motor root.
In Amphioxus and some cyclostomes (Petromyzon and others)
these roots do not unite, but pass independently to the tissues of
the body. In all other vertebrates the roots coalesce to form a
single spinal nerve.

278

STRUCTURE OF THE VERTEBRATES

The cord tends to shorten in the reptiles, and in the most
primitive mammals barely reaches the sacrum. The tendency
of the nerve tissues to become concentrated in the anterior re-
gion is described as cephalization. This centralization of func-
tion is most prominent in the higher mammals. The fish, amphib-
ia and reptiles show a progressive development away from the

Sacrum

B. Human
Embryo

C. Adult
Human

A Turtle

Fig. 157. Spinal Cords. Illustrating the shortening of the cord during evolu-
tion and during individual development.

spinal type of animal. The extinct dinosaurs, however, had a
highly decentralized nervous system. The spinal centers were
more important than the brain, and the selective value of this
is apparent when one considers the great size of the animals. It
has been estimated that in the largest dinosaurs it would re-
quire approximately five seconds for an impulse to travel from
the tip of the tail to the brain, and back to the tail.

STRUCTURE OF THE VERTEBRATES

279

With the shortening of the cord the spinal nerves become
telescoped, and plexuses of nerves leave the two enlargements.
This is most prominent in man and the anthropoids. In these
animals the cord has shortened until it reaches only to the mid-
dle of the lumbar region, and is anchored to the pelvis with a
connective tissue filimi terminale. From the cervical enlargement
arise the nerves supplying the pectoral girdle; and from the
sacral (lumbar) enlargement comes a great plexus of nerves, the
Cauda equina of human anatomy. The nerves of the thoracic

Ascending tracts
Gray substance

Dorsal root

Dorsal
ganglion

Fig. 158. Typical Section of the Spinal Cord of the Mammal.

region retain, to a large degree, the primitive metameric arrange-
ment.

The white substance of the cord is composed of the long
ascending fibers which pass from the spinal nerves up the cord
to the brain; and the descending motor fibers from the brain to
the motor cells of the cord. Recall that each impulse reaching the
cord through a sensory fiber is transferred to the brain through
a myelinated collateral or another sensory fiber; and it becomes
evident that the number of long ascending fibers increases at
each spinal nerve. Similarly, the motor fibers from the brain
decrease in numbers at each lower level due to the fibers w^hich

280

STRUCTURE OF THE VERTEBRATES

end at the spinal motor nerve cells. An examination of the cord
sectioned through the two enlargements will show that there
is more white than gray substance in the cervical region, and
that the arrangement is exactly reversed in the sacral en-
largement.

In a general way, the fibers of the cord which carry similar

Brain

Dorsal root

Dorsal ganglion

Sensory neurone

Fig. 159. Sections of the Cord at Different Levels. Sensory fibers are added

at each spinal nerve, and motor fibers decrease in number at each more

posterior level. Observe that the sensory 'tracts are concentrated in the

dorsal region, and the motor tracts are mainly in the ventral half.

STRUCTURE OF THE VERTEBRATES 281

tj'pes of impulse are grouped into tracts. The sensory fibers
tend to be concentrated in the dorsal part of the cord, and the
motor fibers in the ventral half. The division of the tracts is
carried even further, particularly in regard to the sensory
fibers. Fibers carrying specialized impulses (as tactile, pain, or
kinesthetic sensations) are congregated into smaller tracts on

A. Cervical

B. Thoracic

C. Lunibo-sacral

D. Sacral

EiG. 160. Sections of the Cord at Four Levels, showing the increase of
fibers in the cervical region. The sacral enlargement controls the pelvis

and pelvic limbs.

either side of the cord. The triangular region bounded dorsally
by the periphery of the cord, and laterally by the entrance of
the dorsal roots of the spinal nerves, is filled with the sensory
fibers from the muscles carrying kinesthetic sensations. These
fibers convey sensations of muscle position to the brain, and
their loss causes a lack of coordination in the affected part of
the body. The tactile and pain sensory fibers are located

282 STRUCTURE OF THE VERTEBRATES

laterally, between the dorsal and ventral roots. In this same
region, but more central in position, are a number of motor
tracts. The ventral segment of the cord is almost entirely motor.

A destruction of the motor tracts causes a loss of voluntary
action, for the connections between the brain and the motor
nerves of the cord would then be lost. The reflex pathways would
not be affected; but reflex movements w^ould be exaggerated for
the coordinating influence of the brain would be lost. If, how-
ever, the motor cells of the cord are destroyed, both voluntary
and involuntary reactions would cease in the regions supplied
by affected nerves. Motor impulses from the brain would con-
tinue to pass down the cord but could not be relayed to the
effectors. As a result of any such destruction of cells the mus-
cles, including those of the arteries, lose their power to react;
and a degenerative condition follows. Infantile paralysis is
caused by the destruction of ventral motor nerve cells.

Thus far the fibers of the cord and spinal nerves have been
classified simply as motor or sensory ; but each of these types can
be divided into somatic fibers which pass to or from the muscles
and skin, and visceral fibers to or from the internal organs.
Each spinal nerve therefore contains four types of fibers: vis-
ceral and somatic motor, and visceral and somatic sensory. The
visceral fibers form the connection between the central nervous
system and the sympathetic ganglia.

D. Sympathetic System

In addition to the central nervous system and the cranial and
spinal nerves which arise from it, there are a number of vis-
ceral ganglia which are not affected by voluntary motor im-
pulses. The nomenclature of this vegetative nervous system has
never been standardized. The simpler terminology uses autonom-
ic to describe this entire system of ganglia and nerves which
are involuntary in action. The ganglia of the thoracic region
form the sympathetic part of the system. Those ganglia which
lie in the cervical and lumbar regions are the parasympathetic.
The classification is based upon position and the physiological
reactions of the ganglia.

Lying immediately ventral to the centra of the vertebrae, and

STRUCTURE OF THE VERTEBRATES

283

to either side of the midline, is a metameric group of autonomic
vertebral ganglia. These are connected in series with each other,
and through the rami communicantes with the spinal nerves.
Each ramus communicans is composed of the visceral motor and

Muscle

Hair

Sweat gland

Capillary

Tig. 161. Diagrammatic Cross Section of the Body. The pathways of the
somatic and visceral nerves are shown. The dotted lines indicate the auto-
nomic motor fibers from the vertebral ganglia to the integumentary region

of the body.

the visceral sensory fibers of the spinal nerve. In addition, un-
myelinated motor fibers pass from the ganglia to the somatic
regions of the body, forming the gray ramus. These fibers pass
to the skin with the somatic fibers of the cord and innervate the

284 STRUCTURE OF THE VERTEBRATES

sweat glands, the muscles of the cutaneous capillaries, and the
minute muscles of the hairs. Other fibers from the vertebral
ganglia extend ventrally into the mesentery to the small pre-
vertebral ganglia, and take impulses to and from the glands
and smooth muscles of the viscera.

Although the autonomic system is not under "voluntary" con-
trol, it is intimately connected with the central nervous system
and influenced by it. Any pain sensation in the deeper viscera
is transmitted to the brain; and conversely, somatic sensory
sensations may be transferred over to the visceral motor tracts.
Motor stimuli from the brain may be sent to the autonomic
ganglia in response to pain, visual or auditory sensations; and
in turn cause cutaneous or visceral reactions. The lungs, heart
and upper viscera are innervated by autonomic and also by
cranial nerve fibers. Functionally the latter may be termed
parasympathetic fibers, although they are antagonistic in their
action to those of the autonomic ganglia. Cranial motor fibers
in the viscera are usually depressors of action while the autonom-
ic impulses are accelerators, and any imbalance between the
two types of stimuli tends to cause a physiological upset in the
organs affected.

Let us summarize the reactions. A sensation received in the
skin through a receptor will be transferred over to a somatic
sensory fiber to the cord. At this point it will be sent in two
directions: (1) directly to a spinal motor nerve, and (2) to the
brain. The reflex stimuli will go immediately from the cord to
the nearby effectors; and following them, voluntary impulses
will travel from the brain to the motor nerves. The time differ-
ence between reflex and voluntary movement is measurable;
and in an animal such as the giraffe, with short legs and a long
neck, a sensation received in the hind foot would return as a
motor impulse from the cord before the sensory stimulus had
reached the brain.

A sensory impulse may be, and usually is, transferred to the
visceral motor fibers of the cord and sent to the vertebral
ganglia. From this point motor impulses would be sent out to
the deeper viscera and the somatic tissues innervated by these
autonomic fibers. Pain sensations from the viscera travel to the
vertebral ganglia, and from them over two pathways: (1)

STRUCTURE OF THE VERTEBRATES 285

over myelinated sensory fibers to the cord and thence to the
brain where the sensation is registered; and (2) over unmyeli-
nated motor fibers from the ganglion to the cutaneous regions.
The latter cause blanching of the skin and profuse sweating.
The autonomic reactions are influenced by the sensitivity of the
nervous system, and to a certain degree by the conditioning
(experience) of the individual. The voluntary response follow-
ing the registration upon the brain is almost entirely controlled
by training.

The study of nerve pathways differs from most anatomical
systems in that knowledge of structure depends very largely
upon physiological reactions. The function of a muscle is in-
dicated by its attachments; but a study of the cord and its
nerve tracts tells almost nothing as to regional function, and the
pathways have been traced through the degeneration of fibers
which follows injuries to the cord and brain.

E. The Brain

The brain of the lamprey (Cyclostomata) most nearly re-
sembles the embryonic brain with its three simple divisions. But
in this most primitive group there are clear indications of the
division of the brain into more complex regions. From the fore-
brain come two pairs of outpocketing, anterior evaginations to
form olfactory lobes, and dorsal cerebral hemispheres ; and the
hindbrain has a slight constriction which separates the anterior
cerebellum from the medulla.

The embryos of the higher vertebrates develop a more com-
plex structure: (1) both the fore- and hindbrains are distinctly
divided into two regions; (2) complex evaginations appear in
both these regions, with folds and creases which increase the
surface area without greatly increasing the bulk of the entire
structure; and (3) certain areas increase in thickness due to the
growth of nerve nuclei and the invasion of nerve fibers.

The forebrain is divided into an anterior telencephalon and a
narrow, thin-walled diencephalon. The former undergoes the
greatest differentiation. Paired anterior outpocketings develop
into the olfactory lobes and nerves, w^ith the lamina terminalis
of the forebrain between them. Laterally grow out two optic

D. Mammal

Midbrain

(^corpora quadrigemina)

Fig. 162. Vertebrate Brains in Dorsal View.

Epiphysis

lidbrain (Optic lobes)

^Cerebellum

A. Dogfish

^y} Diencephalon

erebellum

C. Reptile Diencephalon^^
Cerebnim

Olfactory lobe

forpora quadngemina

Cerebellum
Medulla

'^MEM^

D. Mammal

Diencephalon |^;^

Fig. 163. Hemisected Vertebrate Brains. The olfactory and cerebral regions

are lined; the diencephalon is black; the midbrain is cross lined, and the

hindbrain is stippled.

288 STRUCTURE OF THE VERTEBRATES

vesicles which form the optic stalks and the retina of each eye;
and dorsally there are two pockets, the cerebral hemispheres, the
ventricle of each connected with the original median ventricle
of the forebrain by a wide joramen of Munro.

The forebrain was probably given over entirely to the ol-
factory function in the primitive ancestors of the vertebrates,
for in the cyclostomes this function occupies practically all of
this region; but in the elasmobranch fishes two lateral nuclei
develop, the corpora striata. The dorsal covering, or pallium, is
non-nervous, and functions only as a roof for the ventricle. Dur-
ing evolution the olfactory function becomes progressively less
important and the development of the cerebral hemispheres be-
comes more and more prominent.

Cerebral Hemispheres. The telencephalon of the dogfish has
the nuclei enclosed by the peripheral nerve fibers and non-nerv-
ous tissues. The amphibia keep the same general relationship
with two advances, (1) a material increase in the size of the
hemispheres, and (2) the invasion of the ventricular side of the
hemispheres by nerve cells from the lateral nuclei. Both tenden-
cies increase in the turtles and other lower reptiles. The nerve
cells cover the inner surface of the pallium and collect along the
median line as two distinct nuclei. The dorsal and median
growth of the hemispheres has pushed them close together, so
that the telencephalon is divided by a deep fissure resembling
the fissure of the spinal cord. The cerebral pallia so far described
are grouped together as archipallia, the primitive pallium with
non-nervous tissue on the outside.

The neopallium is found first in the crocodilia. In these ani-
mals the nerve cells of the ventricular side migrate through the
pallium to the outer surface and form a primitive cerebral cortex.
A more extensive cortex is found in the birds and monotremes.
In both groups the cerebral hemispheres are large, covered with
a thick cortex, but perfectly smooth. Further progress is found
in the marsupials. The cortical layer of cells multiplies more
rapidly than the pallium grows, and the cortex becomes slightly
creased and folded into convolutions. The evolution of the mam-
malian brain is an advance along the two lines indicated, growth
in absolute size of the hemispheres, and multiplication of cortical
cells.

A. Dogfish

ArchipaUium-

"^Ventral nuclei

Cerebral nuclei

Cortex

C. Alligator

B. Frog

Neopallium
Cerebral nuclei

Ventral nuclei

Cortex

D. Monotreme

Convoluted cortex

E. Primitive Placental

Fig. 164. Cross Sections through the Cerebral Hemispheres. Observe the

development of cerebral nuclei in the Frog (B), and a cerebral cortex in the

Alligator (C). The cortex increases in size in the Monotreme, and becomes

convoluted in higher mammals.

290 STRUCTURE OF THE VERTEBRATES

The development of the neopallium causes several important
changes. One of the first forebrain centers to become separated
from the olfactory function is the hippocainpus, located as an
important nucleus on the side of the telencephalon in the lower
groups including the reptiles. In the mammals this region be-
comes infolded and covered by the cerebral hemispheres. The
corpus striatum enlarges and is also pushed deeper into the brain
tissue, along with smaller nuclei. With the continued growth
of the cortex in the higher mammals the shallow creases multiply
in number, and certain ones become deep fissures separating
the cerebrum into fairly well-marked lobes. The four major
lobes are the frontal forming the dorsal and anterior portion;
the parietal posterior to the former; and the occipital as the
most posterior portion of the cortex. The outer ventral region
is the temporal lobe. The names are most applicable to the
human, in which the lobes correspond generally to the skull
bone of the same region.

DiENCEPHALON. The diencephalon is a relatively small region
between the telencephalon and the midbrain. The ventral region
is invaded by the thalamic nuclei, important points of transfer
for impulses from the brain to the cord. The diencephalon con-
tains the large third ventricle, the older anatomists having num-
bered the right and left ventricles of the cerebral hemispheres
one and two. The dorsal wall of the diencephalon is a thin mem-
brane on which lies the choroid plexus of blood vessels. In most
vertebrates the plexus sinks into the ventricular space, osmosis
giving a constant supply of nutrient fluid to the ventricular
spaces. This is the most important blood plexus of the brain.

From the diencephalon arise two evaginations, one dorsal and
one ventral, which play an important part in growth and de-
velopment. The dorsal epiphysis is a glandular tube in the cyclo-
stomes and fish. From this structure, or from nerve cells which
grow out with it, a third (or parietal) eye developed in the
earliest amphibia. A dorsal eye was of selective value to mud-
living animals with wide flat heads, and the structure was car-
ried over to the primitive reptiles. Living amphibia have little
evidence of the structure, but a number of reptile embryos de-
velop a dorsal optic vesicle which soon degenerates. The more
primitive reptilian groups develop not only an optic vesicle, but

STRUCTURE OF THE VERTEBRATES 291

a primitive retina and lens. The eye extends to the surface
through a median parietal foramen. Its highest development is
seen in the most primitive living reptile (Sphenodon of New
Zealand), but there is no proof that it is capable of visual func-
tion. The mammals show no evidence of a dorsal eye, the organ
being glandular in structure and deeply hidden between the
cerebral hemispheres. This pineal gland is discussed further in
Chapter XIX.

The ventral outpocketing is the infiindibuliim. As it grows
ventrally (in the vertebrates above the cyclostomes) it is met
by an invagination of ectoderm from the stomodeal region.
The stalk connecting the latter with the mouth is eventually
cut off by the developing bones of the skull, and the combined
organ becomes the pituitary gland which rests in the sella
turcica of the skull. In the cyclostomes there is an infundibular
evagination from the brain, but the anterior pouch (known in
the embryology of the higher vertebrates as Rathke's pouch)
remains separate, and is the naso-pituitary sac. In the lamprey
the sac ends blindly, its tip lying immediately below the infun-
dibulum.

Midbrain. The midbrain persists in a less modified condition
than any primary part of the brain. In all classes except the
mammals this region becomes the thick-walled, bi-lobed optic
lobes. These lobes are the center of visual sensations. The optic
vesicle arises from the forebrain, but the nerves which develop
from the retina grow backward into the midbrain and carry light
stimuli to the nuclei of the optic lobes.

The lobes are secondarily divided in the mammals by a
transverse fissure, and the four bodies are called the corpora
quadrigemina. The nuclei relay optic and auditory sensations
from the organs of special sense to the cortex, the major centers
of sight and hearing in the mammals being located in the
cerebrum.

The ventricle of the midbrain is fairl}^ large in the dogfish and
widely connected with the third ventricle of the forebrain; but
in the higher groups the growth of nuclei and the heavy bands
of fibers which pass ventrally as the cerebral nuclei increase,
gradually crowd out the ventricular space. In the mammals this

292

STRUCTURE OF THE VERTEBRATES

portion of the ventricle is a small tube, the iter or aqueduct oj
Sylvius.

jMetencephalon. The meteneephalon is the anterior portion
of the hindbrain. From its dorsal region develops the cerebellum,
the major seat of the nerves influencing equilibrium and coordi-
nation in the vertebrate.

The cerebellum is the first brain region to develop a cortical
zone. In the primitive animals it is relatively small, with few
fibers on the ventral side. In recent amphibia the cerebellum
has undergone an interesting degenerative specialization, the

Corpora quadrigemina

Optic lobes

Cerebellum

Medulla

B. Lizard

C. Mammal

A. Dogfish

Fig. 165. Midbrains and Hindbrains of Dogfish, Reptile and Mammal. The
midbrain of the mammal is divided transversely to form the corpora

quadrigemina.

region being very small, with a consequent loss of well coordi-
nated movements. The higher reptiles and the birds have the
central region of the cerebellum well developed, with lateral
outgrowths (the flocculi) which increase the functional size of
the structure.

The mammals add two lobes to the cerebellum. The original
lobe becomes transversely creased and is known as the vermis,
with the lateral lobes on either side. The flocculi remain, but are
relatively less important than in the birds. The ventral part of
the cerebellum is a great mass of transverse myelinated fibers,
the pons Varolii, or more usually simply pons.

STRUCTURE OF THE VERTEBRATES 293

INIyelencephalon. The posterior part of the hindbrain is the
medulla oblongata which is continuous with, and most resem-
bles, the spinal cord. The ventricle of the medulla is continuous
with that of the cerebellum, and is the large fourth ventricle
of mammalian anatomy. The roof is thin and covered with a
plexus of nutrient vessels which usually dip into the ventricular
space.

The medulla is roughly triangular in shape, the ventral region
being filled with the large tracts of nerve fibers which pass from
the cord to the upper centers of the brain. In external view the
most prominent are the pyramidal tracts, which are large
bundles on the ventral surface. Other tracts and nuclei occupy
the lateral expansions, and these nuclei are relay centers for
impulses to the cerebellum and cerebrum. The posterior limit
of the medulla is an artificial division from the cord, determined
by the foramen magnum.

Commissures. A commissure is any large group of fibers
crossing from one side of the brain to the other. Every impulse
entering the body crosses to the opposite side before reaching
the upper centers of the brain. Thus, the left half of the brain
influences the right side, and the right influences the left. This
is due to the decussation, or crossing, of the fibers. The fibers
may cross at any level of the brain or cord, but the decussations
in the cord are isolated and few in number. More generally the
fibers from receptors pass up the cord on the original side of
entrance, and cross in the brain stem.

The pons of the cerebellum is a large commissure, and others
have been mentioned. In the cerebral region there are several
commissures connecting one side with the other. The most an-
cient in a phylogenetic sense is the anterior commissure which
lies in front of the third ventricle in the lamina terminalis. The
amphibia develop a commissure dorsal to this one, the pallial
commissure, connecting the hippocampal regions. Another de-
velops in this region in the higher mammals, the corpus callosum.
This large body of fibers lies dorsal to the ventricle and in
longitudinal sections gives the appearance of a strong band of
connective tissue. The corpus callosum has not been described
in the monotremes; it is very weak in the marsupials; and in-
creases in size in mammals with a well developed cerebral cortex.

294 STRUCTURE OF THE VERTEBRATES

The commissure is an accurate indication of the number of cere-
bral cells, for each sends a fiber posteriorly and most of these
cross in the cerebral commissure. The posterior commissure,
crossing at the junction of the diencephalon and the midbrain,
is also very primitive. The pons, connecting the two sides of the
cerebellum, has been mentioned.

Exceptions must be made to the impression that all impulses
eventually cross from one side of the central nervous system to
the other. The incomplete crossing of optic fibers in the higher
animals is discussed on page 308. Also, it will be understood
that simple motor reflexes do not decussate, for a sensory stim-
ulus reaching the cord is transferred to a motor cell on the
same side; but the sensory fiber which carries the impulse to
the brain when it enters the cord, crosses to the opposite side
before reaching the higher centers. In the same way motor
impulses from, the brain cross before reaching the ventral motor
cells of the cord, and then pass out to the body. Therefore,
destruction of fibers in the cord would be more liable to affect
the same side of the body, whereas a destruction of brain
tissue would affect the opposite side.

F. Cranial Nerves

The cranial nerves are those which pass out to the somatic
regions of the body through foramina of the skull. There is
evidence of two cranial nerves in Amphioxus, and it is thought
that these are homologous with the first two (the olfactory and
optic) nerves of the higher vertebrates. Synchronously with the
development of a chondrocranium in the cyclostomes the num-
ber of cranial nerves increases to eight; and, as cephalization
proceeds and the cranial cavity increases in size, the number of
cranial nerves is increased to ten in the fishes and amphibia,
and to twelve in the amniotes.

The above evidence, and the structure of the nerves them-
selves, have given rise to the theory that the cranial nerves pos-
terior to the first two are modified spinal nerves, and that the
brain is metameric in nature. According to this theory there
are at least eight neuromeres, the first three corresponding to
the three primary vesicles described above. The third brain

STRUCTURE OF THE VERTEBRATES 295

pouch would then form the cerebellum, and the posterior five
neuromeres would form the medulla.

Two other factors give evidence to the validity of the theory.
(1) The appearance of muscle segments in the eye and ear
regions indicates the former extension of metamerism in the
higher vertebrates. (2) The structure of the cranial nerves is
further evidence. Although most of the nerves are highly spe-
cialized, some being entirely sensory and others entirely motor,
most of them are mixed like the spinal nerves; and some arise
from the brain as several roots.

The cranial nerves are known by names which describe their
function or location, and are also designated by numbers.
These numbers are usually written Nerve I, Nerve II, etc. As
the cranial nerves are very important, and vary widely in func-
tion, each will be treated separately.

Nerve I, Olfactory. The olfactory nerve is sensory and con-
nects the olfactory lobe of the brain with the epithelium of the
nasal sacs. It is usually considered that the epithelium of the
sac gives rise to the receptors and their axones which extend
posteriorly toward the brain. In many vertebrates, including the
fish, reptiles and mammals, these axones are very short and
make connection with the long dendrites extending outward
from the olfactory lobe. In this case the nerve is really an ol-
factory tract, as is found in the elasmobranchs. In other verte-
brates the sensory axones are long and pass toward the brain,
forming an olfactory nerve. These axones meet the short den-
drites of the brain nerves within the olfactory lobe.

Terminal Nerve. The terminal nerves lie mediad to the
olfactory, and should be designated as Nerve I, but they were
discovered after the more prominent nerves were numbered.
Their function is unknown. They are found in most elasmo-
branchs and dipnoans, and in the embrj'os of many higher
vertebrates, including man and other mammals. Apparently they
are sensory nerves of the rostrum, whose function has been
replaced in the higher groups by Nerves V and VII.

Nerve II, Optic. The optic nerve is formed by the axones of
association neurones which connect the receptors of the retina
(sensory layer of the eye) with the brain. The retina arises as
an evagination from the primitive forebrain, and the nerves

296 STRUCTURE OF THE VERTEBRATES

form a tract connecting this ganglionic mass with the higher
centers. In the lower forms the optic axones pass through the
diencephalon to the midbrain, but in the higher groups some
of the fibers make connection in the thalamic nuclei of the
diencephalon.

There is a complete decussation of the optic fibers in lower
vertebrates, forming a complete optic chiasma, but in the mam-
mals half the fibers cross to the opposite side while half do not
cross. These relationships will be considered more fully in the
chapter on organs of special sense.

Nerves III, IV, VI; Oculomotor, Trochlear, Abducens.
These are the motor nerves controlling the muscles of the eye.
It is now known that each carries a small bundle of sensory
fibers, transmitting proprioceptive muscle sensations to the brain.

The eye muscles arise from three small metameric muscle
bundles, the most anterior (innervated by Nerve III) splitting
to form four of the muscles. The second and third myotomes
form one muscle each, with the result that each has a cranial
nerve, while the oculomotor supplies the remaining four. The
six eye muscles are arranged in two groups; (1) the rectus
muscles, four in number, having their insertions equally spaced
around the equator of the eye, and arising together on the inner
surface of the eye socket; and (2) the two oblique muscles, one
dorsal and one ventral, which pass from the eye to the anterior
(median in the human) surface of the socket. The rectus muscles
are the dorsal and ventral, and the antenor and posterior. In
the human these are called the superior and inferior, and in-
ternal and external recti.

Nerve III, oculomotor, innervates the muscles arising from
the first myotome: the dorsal and ventral recti; the anterior
(internal) rectus; and the ventral oblique.

Nerve IV, trochlear, goes to the dorsal (superior) oblique. As
these muscles, when they contract synchronously, pull the eyes
medially and upward, the alternative name patheticus is often
given them. The trochlear leaves the skull through the most
dorsal of the nerve foramina.

Nerve VI, abducens, innervates the posterior (external) rectus.
The fourth and sixth nerves are the smallest of the cranial
nerves and are frequently lost in dissection.

STRUCTURE OF THE VERTEBRATES 297

Nerve V. Trigeminal. The trigeminal is one of the largest
of the cranial nerves, arising on the anterior part of the medulla.
In the elasmobranchs there are fom- branches, each bearing a
ganglion near its exit from the brain. The ganglia are distinct
in the embryos of amphibia and amniotes, but soon fuse to
form a single large Gasserian ganglion. Nerve V, with Nerve VII
(Facial), supplies most of the face, the two parallelling each
other throughout most of their courses.

Near the ganglion the nerve splits to form three branches:
(1) the ophthabnic, a purely sensory branch, which derives its
name from the fact that it passes through the orbit; (2) the
maxillary, also sensory, to the face and maxillary region; and
(3) the mandibular, a mixed nerve, to the lower jaw and adjacent
regions.

The ophthalmic branch is most prominent in the fish and
urodele amphibia, in which groups it is associated with the
lateral line system of sense organs (page 301). It is smaller in
the anura and amniotes. This branch receives autonomic fibers,
the exact relationships of which are not understood.

The maxillary nerve is larger in the groups lacking the
lateral line, innervating the face and upper teeth.

The mandibular branch goes to the teeth and muscles of the
lower jaw. It is a mixed nerve, receiving autonomic fibers; and,
as it goes to the branchiomeric musculature (page 185), it may
be considered a visceral nerve. In some reptiles and the mam-
mals a small branch goes to the tongue.

Nerve VII, Facial. The facial is a mixed nerve, parallelling
the fifth in much of its course. It arises from the medulla near
the origin of the fifth. A sensory branch goes with the fifth to
the tip of the tongue. The motor portion innervates the muscles
of the neck and face.

Nerve VIII, Auditory. The auditory nerve is pureh^ sensory,
the fibers being distributed to the ear. In the fish it is a single
nerve, the ear being only a balancing organ; but in the higher
vertebrates the nerve has two branches, one to the organ of
equilibrium and one to the hearing portion of the ear. The
eighth nerve rises near the seventh, the ganglia of the two being
fused in the higher classes.

298 STRUCTURE OF THE VERTEBRATES

Nerve IX, Glossopharyngeal. The name is descriptive of
the mammalian condition, the nerve being distributed to the
muscles of the tongue and pharynx. In gilled vertebrates the
nerve supplies the first gill slit, dividing into anterior and pos-
terior branches. The anterior branch goes to the oral cavity and
the hyoid muscles; the posterior branch goes to the muscles of
the first gill arch, with a small branch to the taste buds of the
primitive tongue. With the modification of the gill arches in the
higher vertebrates, the nerve supplies the homologous muscles
in the hyoid (which supports the tongue), some laryngeal
muscles, the pharyngeal region, and some of the taste buds.

Nerve X, Vagus. The vagus is the most widely distributed
of the cranial nerves, supplying hypomeric (visceral) muscles.
Vertebrates with gills have two major branches, (1) the lateral
and (2) the hranchio -intestinal. The former is a sensory branch
in connection with the lateral line system. It persists throughout
life in fish and urodele amphibia, but disappears at metamor-
phosis in the anura. The lateral branch is missing in the am-
niotes.

The branchio-intestinal nerve of gill bearing animals sends
a branch to each of the gill slits posterior to the first. The main
trunk passes posteriorly and innervates the heart, stomach, part
of the intestine, and the swim bladder of the three groups which
have one. There is a degeneration of the branchial nerves in
the amniotes, the laryngeal and pharyngeal branches of the
vagus apparently being homologous structures. The intestinal
branch goes to the heart, stomach, parts of the intestine, and
the lungs. The last is contributory evidence in the undoubted
homology of swim bladder and lungs.

The vagus has numerous connections with the autonomic
system, and the relationships are very close due to the inner-
vation of organs by both types of fibers — cranial and autonomic.
The relationships between the two parts of the nervous system
were discussed under the sympathetic nervous system.

Nerve XI, Spinal Accessory. The accessory nerve is evi-
dently a modified spinal nerve. Its origin is on both the posterior
medulla and the spinal cord, the spinal roots fusing to form a
nerve which passes into the cranium, joins the roots from the
medulla, and then passes out through the foramen to the ster-

STRUCTURE OF THE VERTEBRATES 299

nomastoid and cleidomastoid muscles, which unite to form one
in the human. The nerve also innervates the trapezius.

Nerve XII, Hypoglossal. Like the accessory, the hypoglossal
is motor. It has several roots on the cord which pass into the
cranium and out to the retractors and muscles on the ventral
side of the tongue and jaw.

The relationships of the cranial nerves and their evident
homology with spinal nerves, have an interesting bearing on the
metamerism of the face and head. The long discarded theory of
Owen (1846) that the skull is metameric has no foundations in
either evolution or embryology; but the metameric origin of the
jaws and muscles of the face is now well established. First
developed the hypocranial structures of the cyclostomes, and
the metameric muscles of the eye. Following this the anterior
gill arches became modified into jaws and hyoid apparatus,
and the posterior gill clefts were pushed closer under the chon-
drocranium, with the inclusion of their nerves as the ninth and
tenth cranial nerves. When the gill clefts disappeared in the
amniotes, and the cartilages and muscles became incorporated
into the laryngeal and neck region, two other spinal nerves came
into the skull, making the twelve of the higher vertebrates.
These nerves are all highly specialized, but their relationships
correspond perfectly w^ith the other evidence regarding the
evolution of the skull.

CHAPTER XVIII
ORGANS OF SPECIAL SENSE

Organs of special sense are the receptor's of the body, receiving
stimuli and passing them on to the sensory neurones. Through
the latter, connection is made with the ganglia and nuclei of the
sympathetic and central nervous systems. As the essential ele-
ment of the receptor is the nerve ending, the sense organs are
ectodermal in origin; but the majority become surrounded by
complex accessory structures derived from the mesodermal con-
nective tissue.

Receptors are located in the deeper tissues of the body (striated
and smooth muscles, mesenteries and cavities of the joints)
and in the outer covering. The latter are more numerous and in-
clude the more complex sense organs. These may be either in
the epidermis or embedded in the mesodermal supporting layers
of the skin. There is, during the course of evolution, a general
tendency toward assuming a deeper position.

A direct correlation exists between the diverse stimuli re-
ceived and a specialization of the receptors to become specific
for certain types of stimuli. Thus the vertebrates have proprio-
ceptive organs which respond to pressure; tactile cells or cor-
puscles; pain corpuscles; and the more specialized organs which
receive chemical, or light and sound wave stimuli. The latter
tend to be collected into close packed groups, while the skin
and visceral endings are usually isolated corpuscles and are
known as the nerve end apparatus.

Nerve endings. The simplest, and perhaps most primitive, type
is the free nerve termination. The sensory fiber loses its myelin
sheath and breaks into fine fibrils which ramify into the muscle
or skin. This type is typical of striated muscle endings. Free
terminations in the skin are more typical of the water living
vertebrates than of the land living animals.

300

i

STRUCTURE OF THE VERTEBRATES 301

End organs are more complex. The simplest type is the tactile
corpuscle, the nerve ending at a tactile cell surrounded by a
connective tissue cup. In other types the nerve may end in a
minute plexus resembling a glomerulus and surrounded by a
sheath. The largest corpuscles are those described by Pacini
(Pacinian corpuscles) in which the nerve core is surrounded by
concentric layers of connective tissue. These are easily demon-
strated in sections of the mesentery, or the pancreas of the cat.
They are large enough to be located without a lens.

A. Lateral Line Organs

In addition to the isolated cutaneous sense organs, the cyclo-
stomes, fish, and gill-bearing amphibia have epithelial sense
organs arranged in definite lines upon the head and body. The
distribution of these lines varies greatly, although there is a
basic arrangement of the primitive lines which is added to or
modified in many ways. These lines radiate from the region
of the ear. Two are facial, one above and one below the eye,
converging toward the snout; a third is mandibular; and the
fourth, which gives the name to the group of organs, lies along
the axis of the body. This last follows generally the horizontal
line separating the epaxial from the hypaxial regions of the
muscle segments. The lateral line is parallelled by the lateral
branch of the vagus nerve.

During development the organs appear as conical patches of
modified epithelium, the base of the cone resting upon the der-
mal tissues. The central cells are sensory, each having a flagel-
lum-like bristle projecting from the epidermis, and are sur-
rounded by the supporting cells of the cone. The fibrils from
the lateral nerve pass through the supporting cells and ramify
around the sensory group.

In the cyclostomes and amphibia each cone sinks into a pit
which is not directly connected with other depressions. In the
fish these pits are modified to form connecting grooves or canals.
In one group of the cartilaginous fishes the canals remain open
in the adult; in the others the canals sink deeper and the lateral
ridges close over to form canals which open to the outside through
pores located above the sensory patches. The position of the

302 STRUCTURE OF THE VERTEBRATES

canals and pores in relation to the scales differs widely. In a few
fish the cranial canals run through the skull bones.

Many investigators have attempted to homologize the lateral
line organs with other organs of special sense. Patches of lateral
line receptors are thought to have migrated into the olfactory
pits and mouth, and to have inpocketed to form the ear. How-
ever, as the system appears almost completely developed in
anuran larvae, and then as completely disappears, including
the lateral nerve; and, as the organs appear synchronously in
the vertebrates, it seems more logical to the author to assume
that these structures developed independently.

B. Taste Buds

The taste buds, in their individual structure, more nearly re-
semble the lateral line organs than any other sensory organs.
The taste bud consists of a cone of supporting cells enclosing
an inner cone of taste receptors. The latter lie in a depression
in the supporting cells, connected with the outside through a
taste pore. The taste buds are found in the oral cavity and
pharynx. In the mammals they are most prominent on the
tongue, where groups of taste buds are collected on relatively
large papillae.

Taste is one of the chemical senses, the substance with a
detectable taste being necessarily in solution. Most ''taste" as
recognized by the human is olfactory, taste being limited to
elementary sensations of bitter, sweet, sour, salt, etc. As sweet
and bitter can be more easily detected in different parts of the
mouth, it is reasonable to conclude that there is a specialization
of function in the different buds.

C. Olfactory Sense

The olfactory sense, like taste, is chemical. The odors detected
by water living animals are in solution, and in land living verte-
brates the gases become dissolved in the layer of moisture
covering the epithelium. The sensory epithelium is located in
olfactory pits, the axones of the receptors passing posteriorly
to the dendrites of the olfactorv lobe.

A

STRUCTURE OF THE VERTEBRATES 303

The olfactory epithelium arises on the dorsal side of the
embryonic head, and the region soon pits in to form olfactory
sacs. The deeper layer of cells develops short dendrites which
push between the supporting cells toward the lining of the sac;
and axones which grow backward toward the olfactory lobe,
thus forming the olfactory nerve.

The cyclostomes develop a single naso-pituitary pouch. The
paired olfactory sacs and nerves lie near the dorsal region of
the tube. The pituitary portion of the tube extends posteriorly
to the level of the hypophysis of the brain, ending blindly in
the lampreys, but opening into the mouth in the myxinoid
cyclostomes.

The olfactory pits are paired in the fish and all higher verte-
brates, with the pituitary pouch arising separately from the
stomodeal region. As the chondrocranium develops the sacs are
pushed anteriorly, and ventrally in fish with a rostrum. The
sacs end blindly in the fish, and serve only an olfactory function.

In the amphibia the sacs have an origin similar to that of the
fish, but the invaginations push ventrally toward the mouth
cavity, and break through to form the internal nares, or choanae.
The olfactory epithelium lies near the outer end of the tubes,
the external nares, although accessory patches of sensory epi-
thelium are near the internal opening. (This internal organ
degenerates in higher vertebrates, and is degenerate and non-
functional in the human).

The conditions in the early reptiles are hardly changed, with
the exception of the development of lateral projections into the
nasal passage. These conchae are homologous with the more
highly developed turbinals of the mammals. The Crocodilia
have more highly developed conchae, and in this order of rep-
tiles the maxillary and palatine bones grow medially to form a
hard palate, thus elongating the nasal passage so that the in-
ternal nares are at the back of the mouth near the pharynx.

There is a great increase in the sensory area in the mammals,
although the olfactory lobes are relatively much smaller than
in more primitive vertebrates, particularly the fish. The sensory
epithelium is accommodated by the growth of turbinals, bony
scrolls projecting into the nasal passage. The more anterior
naso- and maxillo-turhinals, arising from the nasal and maxillary

304 STRUCTURE OF THE VERTEBRATES

bones, are ordinarily not covered with olfactory epithelium,
but with serous cells which tend to warm and moisten the air.
The posterior ethnio-tiirbinals push in between those from the
dermal bones, and carry the olfactory receptors.

The external nares are separated by a cartilage projecting
anteriorly from the mesethmoid. Laterally the nares are enclosed
by fleshy bands of tissue, which may be prolonged into a snout
or, in the tapir and elephant, a greatly elongated proboscis.

Water living mammals tend to have a poorly developed
sense of smell. The toothed whales lack even the olfactory nerves
and the foramina of the cribriform plate. The primates also have
a small sensory area, and in comparison with the rodents,
carnivores, and most other orders of mammals an inefficient
sense of smell.

Development in the Human. In the embryo of five weeks'
development the mouth is wide, the nose is broad and flat, and
the nostrils are widely separated and connected with the oral
cavity by grooves. The median nasal region rapidly becomes
elevated as the lateral masses of tissue grow medially, forcing
the nares closer together until they are separated by a narrow
septum and the passages look downward. The upper lip is
formed by this median growth, completely cutting the nares
from the mouth. The line of fusion is left in the adult as the
median line of the upper lip. A failure of these tissues to coalesce
causes hare lip, a common defect of the human. Less frequently
the maxillary bones fail to meet in the mid-line and cleft palate
results.

D. The Eyes

The vertebrate eye differs from that of the invertebrate in
the inversion of layers of the retinal cells. That is, the eye
receptors of the vertebrate lie nearest the supporting cover,
with the association neurones between the sensory layer and
the source of light. The invertebrates have the direct method
of receiving light rays directly against the sensory cells, the
association neurones being deeper and next the supporting layers.
But, after the vertebrate eye developed, there has been little

STRUCTURE OF THE VERTEBRATES 305

change in the organ itself. The evolutionary changes have
been in position, the optic nerves, and the protective structures
around the eye.

Embryology. The eyes begin their development as lateral
pouches from the forebrain before the posterior cord is closed.
As each evagination continues to grow laterally the distal end
enlarges into an optic vesicle which soon begins to inpocket and

^ptic vesicle

Retina,

Optic cup Ophl

Lens

Fig. 166. Development of the Eye. (A) Section through the developing
optic vesicles; (B) the invagination of the optic cup, and the epidermis to
form the lens; (C) a later stage showing the lens; and (D) the

embryonic eye.

306 STRUCTURE OF THE VERTEBRATES

form a double layered optic cup. The cell layer lining the cavity
of the optic cup (which will be spoken of as the inner layer)
develops into the retina which contains the light sensitive cells.
The outer, surrounding, layer of cells is thinner than the retinal
layer, and develops into a pigmented coat protecting the retina.

Synchronously with the development of the optic cup, the
covering ectoderm of the embryo thickens over this region,
pushes inward, and becomes the lens of the eye, lying within
the circular lip of the cup. As the lens pushes inward, the skin
ectoderm closes over to form a continuous layer over the eye.
In the completed structure this thin layer forms the epithelium
of (1) the conjunctiva which surrounds the eye as folds of
tissue, and (2) the cornea, the transparent part of the mesoderm
which completely surrounds the eye ball. The continuity of this
epithelium and the epidermis is demonstrated whenever a reptile
sheds its skin, for the eye covering is moulted as an integral
part of the shed epidermis.

The inner layer of the cup (that which receives light rays as
they pass through the cornea and lens) becomes rapidly modi-
fied into sensory retinal, cells and association nerve cells. The
axones of the latter grow centrally toward the optic stalk which
connects the eye with the brain, and through the stalk to the
visual centers of the central nervous system. In this way the
stalk is filled and transformed into the optic nerves, which
become a nerve tract connecting the retinal ganglion with the
central nuclei.

The original optic cup is surrounded by the cartilaginous
optic capsule of the chondrocranium. This never fuses to the
skull as do the other capsules, and thus permits free movement
of the eye. The larger proximal portion of the capsule becomes
thickened into the sclerotic protective membranes of the eye.
The circular region over the lens forms the transparent cornea,
covered externally by the ectodermal epithelium.

Structure of the Eye. The eye consists of (1) ectodermal
retina, pigmented layer, lens, and outer epithelium; and (2)
mesodermal iris which is pigmented, sclerotic protective mem-
branes, and cornea. As the lens is attached along its equator
to the eye membranes, the spherical hollow of the eye is divided
into two cavities which are filled with a heavy fluid.

STRUCTURE OF THE VERTEBRATES 307

1. Retina. The retina consists of three rather distinct layers
of cells: (1) an outer receptor layer (in contact with the pig-
mented layer) ; (2) a middle group of association neurones with
short processes; and (3) an inner layer (next the cavity) of
association cells with long axones which pass to the brain. Due
to their shape the sensory cells are known as rod and cone cells.
These are partially embedded in the pigmented layer. Stimuli
are received by the rods and cones, transferred to the short
association cells, and then passed to the inner association layer.
Thus, a ray of light entering the eye passes through two layers
of retinal cells before reaching the receptors. The axones of
the inner layer of association neurones converge toward a cen-
tral point where they enter the optic stalk. Near the exit of the
optic nerve the axones are close packed and the area occupied
by the nerve is not sensory. This is the blind spot of the eye.
The area surrounding the blind spot is packed with sensory
cells, and this region (the macula lutea) is extremely sensitive.
Between the nerve cells of the retina are ectodermal supporting
cells, resembling the similar neuroglia of the brain.

The pigmented layer, which lies in contact with the meso-
dermal covering, is very thin and becomes an integral part of
the retina. The cells expand or shrink, assisting other eye
structures in protecting the sensory cells from too intense light.

2. Choroid layer. The choroid layer is the inner portion of
the optic capsule, lying in contact with the pigmented layer of
the retina. It corresponds functionally with the pia mater of
the brain. It is highly vascular and is the major nutrient source
of the eye. The choroid is in close contact with the sclerotic
layer throughout most of its area, but on the distal surface the
two separate. The outer sheath is continued over the eye as the
cornea; the choroid forms (1) the circular iris, and (2) the
ciliary process. The latter arises at the point where the iris
separates from the cornea, and is the muscular base to which
the lens is attached. The ciliary muscles move the eye forward
and backward and slightly adjusts its shape. The iris continues
over the lens as a curtain with a central opening, the pupil
of the eye. The change in pupillary size is caused by the con-
striction or dilation of the iris by its sphincter muscles. The
iris is normally pigmented, giving color to the eye. The dark

308 STRUCTURE OF THE VERTEBRATES

cavity of the eye gives the black appearance to the pupil. The
pink eyes of albinos are caused by the lack of pigment in the
iris, which permits the minute blood vessels to show and give
their color to the eye.

3. Sclerotic layer. The sclerotic, or hard, layer is continuous
over the entire eye, and forms both the proximal tough mem-
branous covering and the cornea. The latter is very similar in
all vertebrates. The sclerotic coat varies more widely in struc-
ture. In a number of reptiles and some birds the sclera becomes
ossified into bony plates; but in all cases the eye remains freely
movable.

4. Cavities. The attachment of the lens to the ciliary processes
of the choroid layer separates the cavity of the eye into two
distinct regions. The distal chamber between the cornea and
the lens is filled with a refracting fluid, the aqueous humor.
The larger space between the lens and retina is filled with a
semi-solid vitreous body or humor. The origin of the vitreous
body is in dispute; its function is evidently to maintain the
almost spherical shape of the eye.

5. Function. Light rays passing through the cornea strike the
lens, and its biconvex shape concentrates them on the retina.
Each point of light and shade that is reflected to the eye is
focussed separately so that the retina receives a mosaic picture.
These stimuli are transmitted over the optic nerve and coordi-
nated in the brain. If the rays come to a focus in front of or
behind the retina the point of light becomes circular and the
image is blurred. This is near-sightedness or far-sightedness.

Accommodation to distance is cared for by the movement of
the lens and the slight ability to vary the shape of the focussing
structure so that its convexity becomes greater or less. Light
accommodation is adjusted by several structures. The function
of the pigment layer has been mentioned. The major part of
adjustment is cared for by the constriction of the iris. In the
normal eye this reflex action admits only sufficient light to make
a clear picture and prevent the blurring from very intense
radiation.

Bifocal vision appears first in the mammals. In the lower
groups the eyes are placed on either side of the head, and there
is a complete optic chiasma. In the mammals, however, decus-

STRUCTURE OF THE VERTEBRATES 309

sation is incomplete. The axones from the two sides of the
retina divide, so that the outer (posterior) half from the left
eye and the inner (anterior) half from the right eye pass to the
left side of the brain. The condition is similar in the right optic
nerve. The complete nerve from the eye meets its mate from
the other side just anterior to the pituitary body, and forms an
incomplete chiasma.

Accessory Structures. The eyelids form protective covers for
the eyes. The fish either lack or have very poorly developed
dorsal and ventral lids; but a few sharks and teleosts have a
third lid, the nictitating membrane, which arises on the anterior
margin of the eye and moves posteriorly. The anura and most
reptiles have all three lids developed; but in the mammals the
membranous third lid becomes small, and in the human is left
only as a small fold on the inner angle of the eye. The mam-
malian lids are fringed with stiff hairs, the lashes of the eye.
The epidermal covering of the lids is continued over the inner
surface as the conjunctiva. The lids and the surrounding skin
are well supplied with muscles. The eyes are bathed by fluids
from the lacrimal glands, the excess fluid passing to the nasal
passages through the lacrimal ducts.

E. Auditory Organs

The ears undergo marked progressive changes in the different
classes of vertebrates. Primitively the organ is concerned only
with a sense of balance or equilibrium. This is true of the
cyclostomes and fish, in which the ear is limited to the inner ear,
consisting of the semicircular canals.

In all vertebrates the auditory apparatus arises as a thicken-
ing of the ectodermal covering of the embryo which sinks in as
a hollow vesicle, a pore connecting it with the outside. The
connection with the exterior remains open in the adults of some
elasmobranchs, but closes in all the other groups. The cavity
of the inner ear is filled with an endolymphatic fluid, and the
cavity in which it rests is similarly filled with perilymph.

The most primitive ear is found in the lower cyclostomes.
these animals having a single canal, flattened on the bottom
and rounded dorsally. The tube is lined by epithelial cells with

310

STRUCTURE OF THE VERTEBRATES

sensory processes, and the movement of fluid within stimulates
the cells and indicates body position. The canals are paired in
the lamprey and its relatives.

The definitive inner ear of the vertebrates is established in
the elasmobranchs. To the two dorso-ventral canals a third
horizontal one has been added, all of which empty into an en-

FiG. 167. Development of the Semicircular Canals (Mammal). (A) Shows

the utriciilus and endolymphatic duct; in (B) the first (transverse) canal

has developed and the cochlea is elongating; (C) and (D) show further

progress in development.

larged central cavity, the utricidus. From the ventral side of
the utriculus develops a rounded sacculus which is of great
morphological importance in the evolution of the higher verte-
brates. Each canal has an enlarged ampulla at one entrance to
the utriculus. The ampullae are at the ventral ends of the two
vertical canals, and at the anterior entrance of the horizontal

STRUCTURE OF THE VERTEBRATES 311

canal. The ampullae are characterized by large sensory areas
with unusually long sensory processes on the cells.

The ear of the dogfish functions in the same way as do those
of the cyclostomes. The endolymphatic fluid contains minute
crystals of calcium salts (which in some fish are collected into
a large otolith) and as the animal changes position the crystals
are washed against the sensory hairs. This gives the impression
of falling in that direction. The three canals of the typical
vertebrate give a complete record of movement in any direction;
and the dizziness which accompanies a whirling motion is due
to centrifugal force throwing the fluid away from the center,
and the impression made upon the brliin of falling in all di-
rections at once.

The amphibia develop a cochlea, a coiled outgrowth from
the sacculus, which receives sound waves. The cochlea has its
beginning in the fish where a small lagena, or pouch, projects
from the ventral posterior portion of the sacculus. With the
progressive growth of the cochlea the powers of hearing become
greatly increased. In the mammal the cochlea is so highly coiled
that it becomes spiralled, and contains the specialized organ of
Corti. The function of this organ is speculative, for the birds,
which lack the structure, perceive tones and pitch.

The inner ear is closely connected with the eighth, auditory,
nerve which spreads over it and enlarges into ganglionated
areas. The nerve is the essential organ of hearing, conveying the
impressions to the brain, and the associated structures are the
specialized receptors.

Middle Ear. The 77iiddle ear first becomes a definite structure
in the anuran amphibia. It will be recalled that the first gill
pouch develops into a spiracle in elasmobranchs and crossop-
terygian fish. In the anura and amniotes the pouch does not
break through to the outside, but enlarges distally to form a
middle ear cavity, the proximal portion remaining as the Eu-
stachian tube. The external surface is covered by the tympanic
membrane, a fusion of the branchial pouch and the body cov-
ering.

In the anura, reptiles, and birds, only the columella, or stapes,
is present, the embryonic hyomandibular having been taken
into the cavity. When the jaw articulation between the articular

312 STRUCTURE OF THE VERTEBRATES

and quadrate is lost in the mammals, these bones are incor-
porated into the middle ear as the malleus and incus. The stapes
is in contact with the inner ear, passing through a foramen in
the bone and resting against a thin membrane of the utriculus.
The malleus is in contact with the tympanic membrane with
the incus lying between. The three assist in transmitting vibra-
tions of the tympanum to the inner ear.

External Ear. The external ear is often limited to the conch,
or shell-like funnel, which projects outward in most mammals.
It may also include the external auditory meatus, the canal
from the outside to the tympanic membrane.

The anura and lower 'reptiles have a tympanum flush with
the surface of the body. In other reptiles the membrane sinks
below the surface; and in the crocodiles and birds the meatus
is partially covered by flaps of tissue. The conch, a mechanism
for concentrating the sound waves, appears first in mammals.
It is supported by cartilages, and in the primitive condition
rises to a dorsal point. Darwin's point, on the upper rim of the
human ear, is a remnant of this primitive structure. The conch
varies widely in size and shape. It is particularly large in bats.
It tends to be small in animals adapted for water life, and has
been secondarily lost in the Atlantic seals, sirenia and whales.

CHAPTER XIX

MECHANICS OF DEVELOPMENT

Biologically considered, development includes the growth of
the individual from the fertilization of the egg to the time of
death. Birth is a poor developmental landmark, for there is a
wide variation in the degree of development at the time of birth
or hatching. The amphibian hatches as an immature larva,
while the reptile is usually highly organized and is anatomically
a miniature adult. Similarly, the degree of differentiation in
different groups of birds or mammals is extreme. Compared
with a newborn calf, a marsupial at birth is an early embryo
with a few specializations. And comparing man with his nearest
relatives, the anthropoid apes, similar although less pronounced
differences exist.

By carrying developmental processes over into postnatal
growth it is found that the races of man grow and develop at
different rates of speed. The changes in the individual are, how-
ever, continuous. Birth causes a complete shift in environment,
with the attendant changes in the anatomy of the nutritive
mechanisms, for the individual passes from a parasitic life to
one of partial independence. But, although birth is one of the
developmental landmarks, there are probably as great physio-
logical changes at puberty as at birth. Therefore, a considera-
tion of anatomy and the functions of structures necessarily de-
pends upon the developmental changes which occur in the
individual or the race.

The regulators of development fall naturally into three
groups: (1) the differentiation which exists in the protoplasm
of the egg; (2) the organizators which are developed by tissues
or organs, and which influence the growth processes of other
structures; and (3) the definite organs of internal secretion
which develop during embryonic life and perhaps play the

313

314 STRUCTURE OF THE VERTEBRATES

greatest role in postnatal growth and differentiation. There
naturally is a great overlap in the influence of these regulators.
Each tissue probably influences others throughout life, and the
internally secreting (endocrine) glands exert an influence at a
very early stage of development.

The differentiation which exists in the egg varies widely in
the classes of vertebrates. In all there appears to be an antero-
posterior specialization; and in some there is evidence that
there is a transverse and dorso-ventral differentiation. The frog
is an illustration of this highly regulative type of egg. In others
the protoplasm is more generalized in nature, with less localiza-
tion of developmental potentialities.

In the latter group the first cleavage apparently adds nothing
to the specialization of parts already present. This is indicated
by the so-called identical twinning, two individuals developing
from a single fertilized ovum. It is well established that in
Amphioxus each of the first four, or eight, cells has the poten-
tiality of forming a complete animal. This has been proved
true of some mammals, identical twinning being known to
occur in a number of groups. The armadillos offer the best
illustration of this process in the mammals. One species regularly
develops four embryos from a single egg, and another species
has eight young at a time when only one egg is given off from
the ovary. In both cases the embryos are attached to a single
placenta.

In all vertebrates, however, this generalized condition of the
protoplasm disappears after a few cleavages; and with the
formation of the primary germ layers each region becomes
specialized and limited in its ability to develop various tissues.
A biological determinism has taken place within the cells. But,
although there is an apparent specialization controlling the
fate of each group of cells, many internal and external factors
may influence future growth. The external influences, such as
mutilations, temperature and food supply, may be disregarded
in this discussion. The internal influences, the organizators of
the body, are more relevant to a discussion of anatomy.

Only the surface has been touched in the study of these
organizing substances. Working upon the frog it has been found
(Brachet and others) that the anterior region exerts a more

STRUCTURE OF THE VERTEBRATES 315

powerful influence than the posterior portion. The removal of a
fragment from the posterior part of an early embryo may affect
the immediate surroundings; but the removal of a similar bit
of cells from the anterior region will carry its influence through-
out wdde areas. As organs begin their development the organiz-
ing influence becomes more specific, both in regard to the extent
of the area affected, and the reaction initiated in the neighbor-
ing structures. One or two cases will illustrate the principle
involved.

During the development of the eye, the lens and the optic cup
develop sjmchronously; but if the epidermal covering over
the optic vesicle is removed, and epidermis from any other
region of the animal's body is grafted in its place, a lens will
develop in the normal position. The organizing substance which
causes lens development is apparently not limited to the cup
itself, but is present in the optic region of the brain. The con-
verse of this experiment is the transfer of tissues carrying the
more dominant organizator. The pelvic anlagen of early am-
phibian larvae can be removed and grafted against the vertebral
column in a more anterior position. These anterior vertebrae
will then develop sacral structures.

From the above it will be seen that there are two influences
in the development of the organism as a unified whole: (1) the
inherent potentiality of cells and tissues to develop along spe-
cific lines; and (2) the modifying influences of one tissue upon
another. The nerves always arise from ectoderm, and the central
nervous system from a limited region of the ectodermal cover-
ing; but the position and structure of the nerves are influenced
by the mesodermal somites. Thus, in the growth of organs there
is a definite interdependence between tissues, causing an orderly
growth of the organism and its structures.

The third regulator of development, the ductless glands, do
not function until the embryo is well formed. On the other hand,
there is an imperfect line of demarcation between the organizing
substances given off by tissues and the endocrine organs. The
latter are usually limited to those glands which secrete specific
hormones, and throw their secretions directly into the blood
stream. These internally secreting glands develop synchronously
with the other organs, but it is unknown when, during the course

316 STRUCTURE OF THE VERTEBRATES

of development, they begin functioning. These glands develop
from ectoderm, endoderm, or mesoderm; and in one case from
two germ layers. In all cases the glandular, secreting, portion of
the organ is supported by mesodermal connective tissues. The
position, structure and function of the glands is equally diverse.
Most of the endocrine organs have been mentioned previously
in connection with the structural system with which they
develop or are associated; but they are reviewed at this point
as they may be considered together as a functional system of
structures, although there may be no anatomical connection
between them.

1. Pineal. The pineal gland arises as the epiphysis of the
diencephalon, and is connected with the parietal eye in develop-
ment. In its original condition in the fish, and in the mammals,
it is solely glandular as far as its histological picture is con-
cerned. Its function is not known.

2. Pituitary. As the stomodeal inpocketing meets the hypoph-
ysis of the diencephalon, the former spreads over the evagina-
tion from the brain and gives rise to the pituitary gland. The
gland is divided into three portions, the anterior and posterior
lobes being most important. The anterior part secretes a hor-
mone (or hormones) which, among other effects, influences the
growth of bone. An over secretion causes large heavy bones to
be formed. If this occurs before the epiphyses are ossified,
gigantism usually results, the degree depending upon the amount
of secretion. The results differ if the over function occurs later
in life. Those animals with a low secretion of the pituitary tend
to be small boned; and, due to some correlation with the mecha-
nism of carbohydrate metabolism, usually have an accumulation
of fat.

Recent research has shown that the pituitary secretion, either
independently or through some other gland, has a direct influence
upon general differentiation and the development of sex. The
metabolic influences are too complex to be considered here.

3. Thyroid. The thyroid gland is considered as homologous
with the endostyle of Amphioxus. The development of the
thyroid of the cyclostome from the endostyle of the larva has
been described. In other vertebrates the gland develops as a
median ventral outpocketing of the pharynx about the level of

STRUCTURE OF THE VERTEBRATES 317

the second gill pouch, and the gland soon loses its connection
with the pharyngeal cavity.

The thyroid was one of the first endocrine glands brought to
the attention of research workers; for goiter, which is very
prevalent in certain regions, is a pathological condition of the
organ. The gland lies across the thyroid cartilage of the larynx
and is composed of secreting vesicles surrounded by connective
tissue. The thyroid secretion influences differentiation. In the
mammal the lack of sufficient secretion results in a form of
dwarfism (cretinism) which keeps the individual more or less
infantile throughout life. Although the condition is corrected
by feeding thyroid extract, there is evidence that the effect is
caused by the influence of the thyroid upon the pituitary, and
not a direct influence upon the tissues. The first experiments
relating to differentiation were upon frog tadpoles (Guder-
natsch). Feeding tadpoles bits of thyroid caused rapid meta-
morphosis. Larvae of bull frogs which normally would have
remained as tadpoles for two years can be metamorphosed into
small frogs in ten or twelve days. The thyroid gland also acts
as a metabolic accelerator. Effects upon metabolism are greater
when mammalian thyroid tissue is used, for the amphibia ap-
pear to lack, to a large extent, the essential radical in the
-thyroxin molecule.

4. Parathyroids. The parathyroid glands received their name
from their position in juxtaposition with the thyroid gland. They
arise as proliferations of cells from the gill pouches, and consist
of four discrete structures. They secondarily become partially
embedded in the thyroid, but there is no relationship between
the two types of glandular tissue. The recent extraction of the
effective hormone and the experiments which have followed,
show that these glands influence calcium metabolism in the
body. So far as the mammals are concerned, the proper secretion
of the glands is not only necessary for the development of bone
and teeth, but for the function of the muscles as well. In some
animals accessory parathyroid tissues are present and carry on
the function of the glands in extirpation experiments.

5. Thymus. The thymus arises as a paired gland from the
branchial pouches. The two halves usually unite, and in the
mammal the gland migrates posteriorly to a position near the

318 STRUCTURE OF THE VERTEBRATES

bifurcation of the trachea. It is closely associated with the other
lymphoid tissues of the body, and in the mammal a hypertrophy
(overgrowth) of the thymus is correlated with an increased
amount of other lymph tissues. In the human its absolute weight
is greatest at about the time of puberty, although relatively it
is largest at birth. Removal of the gland has so far had little
effect upon the experimental animal, probably because of func-
tional as well as structural similarity between the thymus and
the lymph nodes. Clinical observations show that when the
thymus is hypertrophied general development proceeds slowly.
Feeding experiments on tadpoles (Gudernatsch) indicate the
correctness of the observations on the human. The effects are
the exact opposite of thyroid feeding, for thymus inhibits meta-
morphosis past the normal time of differentiation.

6. Pancreas. The pancreas is a duct gland emptying into the
duodenum, with small "islands" of endocrine tissue scattered
throughout its mass. The extract of the endocrine portion (the
islands of Langerhans) is known as insulin. This hormone in-
fluences the absorption of carbohydrates by the cells, and thus
the general carbohydrate metabolism. Diabetes is the patho-
logical effect of improper function.

7. Adrenal glands. The adrenal glands of the mammal lie
near the kidneys, and have a mesodermal cortex and an ecto-
dermal medulla. The functions of the two are entirely different,
and they may be considered as composites of two different
glands. In the fish the mesodermal and ectodermal glands are
separate. The ectodermal portion lies as two strips of chromaffin
tissue in close contact with the sympathetic system of ganglia.
The mesodermal glands are bands of inter-renal tissue parallel-
ling the mesonephros. In the reptiles the two portions of the
gland have come into close contact. In the mammals the ecto-
dermal gland is completely surrounded by the mesodermal por-
tion, forming a single structure.

The function of the cortex has not been solved, although
extracts from it have been isolated which correct the effects of
Addison's disease. In the mammal the removal of the cortical
portion is followed by death. Clinical evidence indicates that
the glands are related to anatomical development. A peculiar
condition characterized by almost complete hairlessness and

STRUCTURE OF THE VERTEBRATES 319

premature senility is always accompanied by degenerated cor-
tical tissue.

The functions of the medullary portion of the adrenals is
better understood. Adrenalin, the active substance of its secre-
tion, has been isolated and widely used. The effects are upon
the activity of the animal rather than upon growth and bodily
form, the hormone activating the muscles and autonomic nerv-
ous systems.

8. Sex glands. The testis and ovary, like the pancreas, are
both duct and ductless in their structure and function. The
primitive function of the gonads is the production of reproduc-
tive cells; but in the land vertebrates (and probably in the
more primitive vertebrates) certain cells take on an endocrine
function. This function in the vertebrates is usually assigned to
the interstitial tissue which lies between the tubules of the
testis and the follicles of the ovary. In the latter organ, glandu-
lar bodies are developed after ovulation (corpora lutea) which
secrete a hormone of great physiological importance. It influ-
ences the fixation of the fertilized ovum on the uterine wall, and
the retention of the embryo in the uterus. In the rodents, at
least, the degeneration of the corpora lutea is largely responsi-
ble for the termination of pregnancy; and their proper function
helps initiate function in the mammary glands.

Knowledge of the anatomical effects of the gonads has been
gained through castration and grafting experiments. The effects
of gonad removal are more prominent in the male, the individual
remaining in an undifferentiated stage with some female char-
acteristics. If castration occurs in youth it affects the shape of
the male pelvis, thorax and larynx.

Removal of the sex glands of either sex during early youth
causes a delayed fusion of the epiphyses of the long bones with
the shaft. As a result a cartilage growth area remains after the
usual time of ankylosis. This permits continued growth of the
long bones, and such individuals have unusually long appendages.
Congenital, or very early, castration may result in apparent
gigantism, but the height is in the limbs, not in the body.

Inter-relationships of the glands. It is always difficult to prove
a specific, independent action for a gland secretion. This is
particularly true for the hormones concerned with bodily growth

pineal
Pituitary

Parathyroids
^^— ^^ -Thymus

Fig. 168. Position of the Endocrine Glands in Man.

STRUCTURE OF THE VERTEBRATES 321

and differentiation, for the same result is often obtained by the
operative removal of different glands. Infantilism, for example,
results from the removal of either the thyroid or the pituitary;
and thyroidless animals may be brought to sexual maturity by
the injection of the extract of the pituitary. In like manner
the removal of the sex glands affects the structure and function
of the pituitary, and the excision of the pituitary prevents the
full development of the gonads.

So frequently and so intimately is the pituitary linked with
the glands causing morphological changes that it is often called
the "master gland". It affects sex development and bodily size,
the effects appearing together in one individual, or independ-
ently of each other; and, conversely, other glands are necessary
for the proper function of the pituitary.

From our present knowledge it seems justified to speak of the
balance of glandular function; and any imbalance existing in
the group will cause widespread morphological and physio-
logical upsets. The subject is still in its infancy, and far-reach-
ing conclusions are not warranted by the evidence.

Conclusion. The three mentioned regulators of development
are not independent or mutually exclusive. The original pattern
within the egg may be modified in its expression by the organ-
izators, and both may be shifted by the function of the ductless
glands; but under normal circumstances all work interdepend-
ently and synchronously. The endocrines are, perhaps, most
readily affected by external, environmental causes; but in the
final analysis all are but the expression of the inherited tenden-
cies, the genetic pattern. The frequent statements regarding the
''effects of the endocrines upon evolution" are but another way
of saying, "the effects of heredity upon the glands, and thus
upon evolution".

Gland complexes are as definitely inherited as are e^'e color or
hair conditions. A recent discovery (MacDowell) proves that a
single point mutation may affect the pituitary so that infantil-
ism results; and although the individual may be brought to
normality by gland injections, the genetic mutation remains
unaffected and is passed on in a Mendelian manner. Therefore
the endocrines, like all other known characters, are the expres-
sion of heredity modified by environment.

I

PART III
EVOLUTION OF THE VERTEBRATES

The Greek philosophers and scientists, under the influence of
Empedocles and Aristotle, realized that species of plants and
animals were not immutable, and that special creation could not
account for the innumerable changes which have occurred among
living organisms. But after the destruction of Greek civilization
in Greece and Alexandria science became static, or retrogressed,
under the influence of tradition and authority. AYith the revival
of Greek culture evolution again crept into the philosophical
literature; but it was not until Descartes carried the mechanistic
theory to its logical conclusion that any coherent effort was made
to controvert the prevailing idea that each species was created
as an unchanging unit.

So rock-bound had the dogma of special creation become that
Buffon, one of the first revivers of the evolutionary theory,
found it necessary in the middle of the eighteenth century to
qualify his scientific speculations with the statement that all
divine creations were immutable. Toward the end of the century
the grandfather of Charles Darwin, Erasmus Darwin, conjec-
tured that ''one and the same kind of living filament is and has
been the cause of all organic life".

A few years after Darwin, Lmnarclz in 1809 published his
Philosophy of Zoology, a work which has gained permanence
as the first clear statement of the theory. Unfortunately the in-
fluence of the dead Linnaeus and the contemporary Cuvier were
sufiicient to obscure the results of the great naturalist's observa-
tions; and it was not until fifty years later that the theory
of evolution was placed upon a firm foundation.

Although during the years between 1809 and 1859 a number
of papers had been published which gave hints of a workable
theory as to the cause of evolution, the essential facts of evolu-
tion remained unnoticed until 1858 when two papers were read
before the Royal Society in London. One was by Wallace who
had been working in the East Indies, and the other by Charles
Darwin who for twenty years had carefully collected data in
England and during a long scientific voyage.

Briefly, Darwin's ideas can be stated as the theory of natural
selection of the variations which always occur in all species. Be-
cause experimental research in physiology and genetics was still
undreamed of, he tentatively accepted the Lamarckian theory
of the inheritance of acquired characters which come about due
to environment and the use and disuse of structures. He ac-
counted for this by saying that each cell of the body sends
fragments of itself, or "genes", to form the germ cells. When
such variations appear, as it is known that they do, whatever
their cause, certain ones will be saved; because, (1) far more
individuals are born than can survive and reproduce; (2) there
is a constant struggle for existence due to shortage of food and
space; and (3) there will be a selection by nature, natural
selection, of those animals which are most fit for existence.

Bases of Evidence

Although evidence for organic evolution has been accumulat-
ing since the first observations on natural history were preserved,
it was not until Darwin began correlating the facts of different
sciences that the evidence was placed upon a sound and co-
ordinated basis. Early research in biology was limited to a
study of nature, to the naming and cataloguing of specimens.
This led to living organisms being grouped into phyla and classes,
and the outcome was speculation as to the relationships of
genera. From these random speculations and hypotheses grew
the theory of evolution, which is now supported by all the
biological and geological sciences.

Arranged in approximate chronological order, the sciences
which have contributed to the theory are: (1) Comparative
Anatomy; (2) Geographical Distribution, the study of organisms
in relation to their distribution on the earth's surface; (3)
Palaeontology, the studv of fossils; (4) Embryology; (5) Genet-
ics, the study of heredity; and (6) Physiology, the function of
cells and tissues.

The student has studied the theoretical background of evolu-
tion in General Biology. The following chapters briefly discuss
geographical distribution in relation to the vertebrates; the
geological record, and the adaptive radiations of the vertebrates.

I

CHAPTER XX

GEOGRAPHICAL DISTRIBUTION

It has long been known that the life on two islands separated
by a narrow strip of water might differ more than the plants
and animals of northern Em'ope and Canada. Geographical dis-
tribution attempts to explain these similarities and differences,
and depends upon geography and geology for the solution. The
general term distribution is divided into time distribution, or
palaeontology, which studies the relative periods in history when
certain groups have lived; and geographical distribution which is
limited to living organisms and their relative position on the
earth's surface. The latter is frequently divided into vertical
distribution, life from the depths of the ocean to the tops of the
mountains; and horizontal distribution, the spatial relationships
on the surface of the earth. The present discussion will confine
itself largely to the horizontal distribution of vertebrates and
its causes.

Geographical distribution depends largely upon two factors:
(1) the migration of animal groups, and (2) the paths which
they have followed. Migration would tend to bring animals into
new environmental conditions, with the elimination of those least
adapted to the particular conditions ; and in time, with the slow
shifting of the earth's surface, new species would evolve. Nat-
urally the greatest similarit}^ in animals would be found under
conditions where free migration is possible.

There are several causes for the migration of animals. Passive
migrations (when animals are carried by winds, water currents,
or floating masses of debris) are of slight importance except
among the fish and birds. A too swift current which sweeps fish
from their original location would start a new center for their
multiplication and spread; and a storm may carry birds miles
away from their natural habitat. Some sea birds have in this

325

326 STRUCTURE OF THE VERTEBRATES

way been carried miles inland to fresh water lakes. Many cases
are known where mammals have been transported across straits
and large bodies of water on floating ice, logs, or tiny floating
islands of matted roots and earth which have broken from their
anchorage on the shore. Under these conditions a single pregnant
female would start a new colony, provided the climate and
other conditions were suitable.

An inherent tendency toward roaming activity would cause
a marginal extension of range. The localized area occupied by
a specific group is gradually enlarged at the periphery. An
extension of the principle is a longer migration of a group. Al-
though there is a tendency among higher vertebrates to return
to the same location, which might militate against permanent
residence in the new environment, it is not unusual for animals
or groups of animals to wander off and become lost.

According to Darwin the greatest cause of migration is the
struggle for existence. Survival of one group means death for
another, because the food supply of a region is soon exhausted.
Only a perfect balance between plants and animals, and a com-
plete limitation of the birth rate, could prevent this. As neither
has ever been found in nature, it follows that there is a constant
warfare between species, and individuals of the same species,
for a proper supply of food and a place to live. Certain individ-
uals would undoubtedly wander and start' new colonies in new
regions.

Migration pathways are equally important, and always a
group would find obstacles in the way. Such natural obstacles are
barriers. The effectiveness of barriers depends upon the habits
and specializations of the animals involved. Mountain ranges
would not stop birds in their flight, but dry rocky slopes would
effectively prevent the migration of reptiles and amphibia. On
the other hand, mountains might aid migration if the range lay
north and south. To illustrate, urodele amphibia are unadapted
for extreme heat; but in southerly migrations these animals
would ascend the slopes where the mean temperature is lower,
and would go to higher altitudes as they moved to lower latitudes.
These conditions are known in such ranges as the Rockies, where
the urodeles are found in the higher valleys of INIexico, many
miles south of their normal range. The same mountain range

STRUCTURE OF THE VERTEBRATES 327

effectively stops east and west migrations, for the urodele can-
not go over the mountains. Most groups of animals find high
or rocky mountains insuperable barriers.

Bodies of water are other effective barriers. Few mammals
will swim far in salt water, although fresh water is not so effec-
tive in stopping them. Recently several deer were found four
miles off the coast of New Jersey in an exhausted condition, and
when the tide or current is with them these animals have been
known to swim twenty miles in the ocean. However, studies of
the mammals on islands indicate that only a few miles of salt
water acts as a complete barrier.

Deserts, or a lack of water, are as difficult to cross as a super-
abundance of water. Only animals highly adapted to drought
can stand the desert. Reptiles succeed better than either amphib-
ia or mammals, although certain toads and mammals have
become specialized for life in very dry regions.

Other barriers of physiological nature are less well under-
stood. Throughout a large area of central Europe there is a
complete lack of urodeles, correlated with a high lime content
in the soil. Some mammals seem to avoid areas which lack a
supply of salt, and alkalis in the water supply prevent spread
into other regions. It is unknown how far the chemical nature of
the soil and water has affected migration; but as the soil dic-
tates the plant life, herbivorous animals would naturally seek
plant food to which they were formerly adapted, and in this
indirect way the soil would affect animal life.

Pathways from one region to another must be present or all
islands would be limited to birds and flying mammals. This we
know is not true. America is completely cut off from Asia, yet
the animal life of North America is very similar to that of
northern Europe and Asia. Evidently a land bridge at one time
existed between Alaska and Siberia. The long stretch of the
Aleutian Islands which leaves a relatively short space of open
water between Alaska and Kamchatka would permit the migra-
tion of human beings in canoes; and in view of the fact that
most of the islands are volcanic, and the region is in a condition
of geological flux, the existence of a land bridge in recent times
is more than mere speculation. A rise or fall of a few hundred
feet in the land surface would make a land bridge; and the ev-

328 STRUCTURE OF THE VERTEBRATES

idence is that it opened and closed regularly during the Tertiary
period. The existence of other connections between continents is
more hypothetical, except for the island groups in the Pacific.
Australasia is undoubtedly a drowned area, the high lands and
peaks being left as the large and small islands.

With the shifting of the earth's surface barriers and bridges
may rise or disappear. The repeated shifting of the coast in
Italy and other regions during historical times illustrates the
(geologically speaking) rapid changes which occur; and the fos-
silized sea shells on the rim of the Grand Canyon bear testimony
to the great upheavals which have formerly taken place.

Any change of the earth's surface may be a gradual shifting
of the land's weight, or a sudden revolutionary process. A small
area or an entire region may sink, leaving a fault line to mark
the clean-cut division between the two. Such fault lines are more
prominent in volcanic regions where violent earthquakes are
more or less frequent; but the most stable lands of the earth
show sign of faults where one part of the land sank far below
the other.

Australasia was cut from the continent of Asia by a series of
faults. The general course of the major fault was determined by
Wallace and is known as Wallace's Line; but more recent re-
search has shown that the division is not so regular as was
thought half a century ago. However, Bali and Lombok, two
islands less than twenty miles apart, have faunas more different
than those of Europe and Canada, and even than those of
Eastern Asia and the Mississippi Valley.

The sinking of the lands of the southern Pacific occurred after
the marsupials had become widely distributed and before the
placentals had arisen. The result is that in Australasia no
placentals exist except those carried into the region by man, and
a few bats and rodents (and the latter may have gone with the
early Malay settlers). The other mammals are monotremes and
marsupials. The latter, safe from competition with the more
active placentals, evolved in many directions. Almost all the
groups of placentals are parallelled in external form. Some have
the gnawing incisors of the rodents; others are wolf-like; others
resemble insectivores ; and others resemble nothing known among

STRUCTURE OF THE VERTEBRATES 329

the higher group. It is one of the finest examples of adaptive
radiations known among the mammals.

The opposite conditions are found among the marsupials which
exist in America. INIost of them failed to meet the competition
with animals with better brains, and placentae which better
equipped the young for the struggle of life. Those which remained
were the highly generalized, nocturnal opossums which survived
by hiding from their enemies. They eked out a precarious ex-
istence while the placentals conquered the earth. The American
opossum is skeletally almost identical with the Cretaceous mar-
supials; more generalized than any mammal known to Australia.

Human migrations have been less controlled by natural bar-
riers, but human enemies have been a telling influence. The
Eskimos have been a thorn in the flesh of anatomists who contend
that pigmentation and climate are definitely correlated. These
arctic dwellers are a yellow-skinned race with apparently no
physical adaptation for their environment. They are a Mon-
golia race, but not of the warlike Tartar type. When they mi-
grated across the Alaskan chain of islands and settled in the
north, they evidently attempted to find a southern home, for
relics of their civilization are found along the Pacific coast. War-
like Indians drove them back and they remained in the frozen
north because they could not fight their way out, not because
they particularly liked the neighborhood.

The same evidence could be adduced as to the location of
the pigmies in the valleys of the Amazon and the Congo. Small,
low thyroid, unwarlike, they were never a match for the taller
tribes of the plains. And when forced into an unfit environment,
climate and disease soon selected a race which was adapted
for survival. In this way barriers, both natural and human,
have largely dictated the course of racial and cultural history.

CHAPTER XXI

THE GEOLOGICAL RECORD

Assuming that the earth began its development as a central
core which grew by the accretion of matter from space — and
millions of meteors still reach our atmosphere every day — the
force of gravity would be sufficient to hold an atmosphere when
the sphere reached a diameter of slightly more than three thou-
sand miles. As soon as gases and water had collected around the
earth, winds and rains would be present, and the erosion of the
surface would begin.

With an increase in diameter there would be an increase of
pressure at the center, and heat would be generated, probably
sufficient to transform the core into the state of a ''solid gas".
If then there were any reduction of pressure, these minerals
would become liquid. The crust of the earth is reasonably firm
and rigid, but fissures and weak lines develop which permit the
potentially molten interior to push upward toward the surface.
It follows that volcanic or igneous rocks would then be formed
as part of the crust, due to the cooling and crystallization of the
laval outcrops. Following the laws of crystallization, the more
rapid the cooling process the smaller the crystals, and igneous
rocks can be found which form a progressive series from those
which resemble glass, to others composed of huge crystals. All
volcanic flows do not reach the surface and erupt at a volcanic
peak. Some are columnar masses, others are vertical planes which
open and fill a crack; but the majority of such flows intrude be-
tween layers of rock already formed, and are left as great hori-
zontal sheets of igneous rocks. It follows that all types of
volcanic intrusion will vary in size from minute lines to great
sheets which cover miles of territory.

The sedimentary rocks are the ones which interest the evolu-
tionist. The meteoric rocks were the original mass, and they

330

STRUCTURE OF THE VERTEBRATES 331

were cut and metamorphosed by igneous intrusions before the
accumulation of an atomosphere; but as soon as winds and rains
appeared these original rocks were decomposed and washed or
blown into the valleys. Synchronously the accumulated moisture
would fill the depressions and form lakes and seas, which would
be gradually filled by the sediment of sand and mud. It is in
these deposits that the remains of plants and animals are caught
and buried. Rocks are formed from the sedimentary layers as a
result of pressure and cementation. Layers of sand become
sandstone, mud becomes shale, and deposited calcium carbonates
are turned into limestone. The last are particularly interesting,
for it is generally conceded that limestone deposits show the
presence of living organisms. The carbon dioxide of metabolism
unites with the calcium oxide nearly always found in water, and
calcium carbonate is precipitated. Therefore, the fact that highly
altered limestones are among the earliest known rocks, indicates
the presence of some form of life much older than any rec-
ognizable fossils.

Sedimentary rocks are usually laid down in strata. And
change in composition of the sediment would cause a distinct
line of demarcation. Even so slight a change as the fallen
leaves of autumn would distinguish those strata from the clearer
ones of spring. When the strata are undisturbed the older ones
are at the bottom, and this condition is found in many places
of the earth. But fortunately for the palaeontologist, the move-
ments of the earth's surface have tilted the strata, or turned
them on end, so that miles of strata can be studied which would
never be found if each stratum had remained in place. The earth
movements are so vast that the relative position is rarely dis-
turbed, and the sequence of history can be traced. A more dis-
turbing problem for the palaeontologist is presented when a
region shows that the ocean bottom has risen above the surface,
been eroded, and then sunk again with more recent strata above.
In such cases the line of erosion (unconformity) is usually dis-
tinct, and the gap can be partially filled from a knowledge of
other undisturbed strata.

By a study of the rocks a surprisingly clear record of the
earth's story has been deciphered. After the fundamental posi-
tion of the strata had been determined and the fossil life care-

332 STRUCTURE OF THE VERTEBRATES

fully studied, any new region discovered could be quickly and
accurately identified by the remains of living organisms found
embedded.

In the very nature of things the record is, and probably al-
ways will be, incomplete. Each year new data are added as new
fossils are found and studied; but at best the record can be
compared to an ancient parchment, its writing partially erased
and re-written upon until the process had been repeated several
times.

A. The Preservation of Fossils

Fossils are the remains, impressions, or petrifactions of living
organisms embedded in rock by natural means. This definition
has to be slightly qualified or amplified; for the twigs, beads
and bits of pottery which are placed in the heavily charged
w^aters of Yellowstone Park or Auvergne, France, would not be
considered fossils, whereas the mammoths embedded in the ice of
Siberia would be.

It is not necessary that the remains be exceedingly old nor
"petrified". A smaller percentage than is usually thought are
actually turned to stone. An unusual case of such preservation
is the Siberian mammoth, some of which are so well preserved
in the strata of ice that one authenticated case of its flesh being
eaten is on record, and travellers would have one believe that
it is of frequent occurrence. The statement is not unbelievable.
These extinct elephants existed in such enormous numbers that
the ivory of their tusks is an article of commerce.

In most of the skeletal remains of mammals there has been
no replacement of bone by other substances, although the spaces
and cavities have been filled with soluble carbonates or silicates.
Microscopic sections show the minutest canaliculi. Similarly
the shells of mollusks, and other hard structures, are often simply
enclosed by the rock.

Impressions are also fossils. Fossil leaves and plants are
usually formed by the darkening of the rock by the contained
carbon. The imprint of the outline and veins was formed when
a thin film of mud was laid on the leaf, and eventually pressed
into stone. Jelly-fish, although about ninety-eight per cent

STRUCTURE OF THE VERTEBRATES 333

water, are preserved as impressions in the sand, every structure
being sometimes shown with perfect accuracy. And in this class
of fossils would be placed the footprints of long extinct reptiles.

Petrifaction is the slow infiltration of water bearing dissolved
mineral matter which gradually replaces the structure of the
plant or animal. It is not difficult to understand how the cell
spaces of a tree might be filled with silica as the protoplasm
oozed away; and later, as the walls decayed, these in turn be
replaced with quartz, so that thinly ground sections show the
perfect woody structure. It is more difficult to understand how
animal tissues were so perfectly preserved. Some crinoids (Phy-
lum Echinodermata) have been silicified, and sections show the
delicate ampullae in position, as well outlined as in the living
animal.

Conditions for Fossilization. The calcareous shells of sea
living animals are most abundant, for these simply remained
buried in the sand or mud of the sea. If that particular region
of the coast were sinking, new layers of sand, mud, or limestone
would be laid above and the shell surrounded by potential rock.
The shell would be preserved; or, more frequently, the lime dis-
solved away and the space filled with mineral matter. This
latter makes a natural cast of the shell which is as perfect as
the preserved animal.

Fish also would be preserved by falling to the bottom and
being quickly covered before the body had disintegrated or been
eaten. The layer of mud and water would prevent much of the
bacterial action, and the imprint would be saved indefinitely.
The essential is quick burial to prevent separation of parts, and
the packing down of the superimposed stratum of earth.

Land animals are much less frequently fossilized, for condi-
tions are not so propitious. Most animals at death are eaten by
others, or the winds and rain scatter the bones so that they
decay before being preserved. The destruction by carnivorous
animals is veiy important. jNIammalian fossils show the grooves
made by the gnawing teeth of rodents; and a herbivorous dino-
saur (American Museum of Natural History) has vertebrae with
grooves which exactly fit the tooth spacing of a carnivorous
dinosaur found close by. Birds, as would be expected, are the
rarest of fossils, most of those found being water living varieties.

334 STRUCTURE OF THE VERTEBRATES

The great number of animals found demonstrates the great
length of time involved, and the abundance of material which
might have been preserved. Water living amphibia and reptiles
would sink into the mud. Some animals were covered by sudden
landslides. Animals which roamed dry regions with constantly
blowing sand would be buried, as illustrated by a group of
primitive pug-like animals, the mother and her litter being
almost in place. If the region were very arid the animal might
be mummified before being covered. A dinosaur (Trachodon)
has been found with the leathery skin preserved almost intact.
The quickest death would be falling into quicksand or deep
bogs of peat. A similar hazard to animal life were pits of as-
phalt, a particularly fine example being found in California where
a great mass of bones indicates that many animals became
mired in the sticky material and were buried. The evidence
shows that carnivorous animals lost their lives as they at-
tempted to eat others which had been caught in the thick natural
oil. When pits of asphalt or sinks of quicksand can be located
the bones of extinct animals are found enormously concentrated.
Another rich source is the bottom of a cave in which animals
lived, the packed down dung often disclosing layer after layer
of animal remains, giving a chronological history of life in the
caves.

Fossils are frequently well preserved. It is a fallacy, en-
couraged by those who do not understand the methods of
palaeontology, that ''animals are reconstructed from a single
fragment". AVhen the complete skeleton of an animal is known
it is true that a larger bone from a similar animal indicates a
larger individual; but reconstructions from incomplete animals
are made by comparison with known species, and while minor
discrepancies may later come to light the method is accurate
and scientific. And the readiness with which the palaeontologist
admits any error preserves the science from bigotry and tradi-
tion.

Microscopic Anatomy. The preservation of bone and wood
and invertebrates has been mentioned. Other structures are
equally well preserved as to microscopic structure. The histology
of fish scales is studied as accurately in fossils as in living ani-
mals; the joint and vertebral tendons of some dinosaurs are

STRUCTURE OF THE VERTEBRATES 335

shown in all their relationships; and even striated muscle has
been found. The last was described in a fossil shark (Cladose-
lache) by Dean. This could hardly be petrifaction for the animal
was buried in mud, and the discovery evidences the marked dif-
ference in the chemical composition between the light and dark
bands of the tissue. Such illustrations could be multiplied many
times, despite the fact that palaeontological histology has hardly
been touched.

In conclusion it may be said that the anatomy of many fossils
is almost as well known as though the animals were living. The
attachment of muscles leaves a roughness on the bone, and in
this way the musculature can be "dissected" with great ac-
curacy. Casts of the calvaria show the gross anatomy of the
brain, even to the convolutions; and with these facts in hand
the activity and the habits of the animal are more than specu-
lation, although one has to be guarded in conclusions regarding
the analogy between living animals and the dead.

B. Geological Time Table

The known sedimentary rocks have been divided into great
eras. Each of these divisions, which covered enormous lengths
of time, is divided into periods, and each of these into minor
subdivisions. Thus, by locating the stratum in which it occurs,
the relative age of the fossil is known. There is a remarkable
similarity in the grade of life found in the same period, no
matter where on the earth's surface it is found. There are ex-
ceptions to this rule, as illustrated by the distribution of mam-
mals in Australia, but in such cases other animals and the
sequence of strata make identification accurate.

The scientist is not as much concerned over the number of
years involved as he is in the position of the strata, but many
attempts have been made to give approximate dates. The
biologists made the effort to determine the number of years
which would be required for certain evolutionary processes to
take place, but recent genetic research has shown that rays
which are present in light will greatly increase the rate of muta-
tions in the germplasm. Stratigraphers, by studying seasonable
depositions of minute strata, gave an idea of the time required

336

STRUCTURE OF THE VERTEBRATES

for the great strata and periods. IMore recently physicists have
used the degeneration of radio-active minerals as evidence re-
garding the time elapsed since particular periods were laid
down. All the evidence agrees that the earliest fossil bearing
rocks are billions of years old. Estimates vary widely, but com-
pared with the entire time involved a few million years is no
more than a day in the life of a geologist.

Major Eras of Time

Era

Dominant Life

Cenozoic

Mammals, including man

Mesozoic

Reptiles. Beginning of mammals

Palaeozoic

Earliest reptiles

Amphibia

Fish

All invertebrate phyla

Proterozoic

Marine Invertebrates
(Arthropods, Sponges, Protozoa)

Archeozoic

Indirect evidence of Life
(Marbles and graphites, with "fossil algae".)

These eras are arranged according to their relative age, the
most ancient at the bottom. The Cenozoic includes time up to
the present. Man evolved from the anthropoid stem in the late
Cenozoic, probably half a million years ago. Estimates as to
the age of the Archeozoic vary from one to several billion years.

The Archeozoic and Proterozoic, which probably represent
more time than the other three combined, are only interesting
to the vertebrate anatomist in that they may in time give the
clue to the origin of the chordates. The vertebrates were not
autochthonous, but trace their history to the most ancient forms
of life. Evidences of their existence, however, begin in the
Palaeozoic era; no remains or impressions of the earlier soft
bodied chordates having been found. It is more than possible
that such remains were preserved, for a very small, cyclostome-

STRUCTURE OF THE VERTEBRATES

337

like fossil (Palaeospondylus) is known from the middle of the
Palaeozoic.

The following tables, beginning with the oldest vertebrate era,
are divided into Periods or major divisions; while the last table,
covering the Cenozoic, is further divided into Epochs. The
Cenozoic is the age of the rise of mammals and is of most
interest in the evolution of man. In these tables dominant life
refers to the plants and animals which are (1) most abundant;
or (2) had reached the highest degree of specialization, with
particular reference to the vertebrate line. It is not inferred
that the invertebrate phyla were not represented.

Palaeozoic Time

Periods

Beginning of New Lines

Dominant Life

Permian

Reptile radiation

Amphibia

Pennsylvanian

Earliest Reptiles

Amphibia

Mississippian

Higher Amphibia

Amphibia

Devonian

Amphibia (?)
(Thinopus)

Fish

(Ray-finned,

Dipnoi

Crossopterj^gii)

Fish

Silurian

Sharks

Higher invertebrates
(Most abundant)

Ordovician

Armored "Cyclostomes"
(Ostracoderms)

Molluscs
(Cephalopods)

Cambrian

(?) Chordates
Invertebrates

Trilobites
Brachiopods

338 STRUCTURE OF THE VERTEBRATES

Mesozoic Time

Periods

Beginning of New Lines

Dominant Life

Cretaceous

Placentals

(Insectivores)

Marsupials

(Opossums)

Reptiles

Jurassic

Sub-2^1arsupials
Birds

Reptiles

Triassic

First Mammals
Dinosaurs
Pterosaurs

Reptiles

Cenozoic Time

Periods

Epochs

Beginning of New Lines

Dominant Life

Quarternary

Pleistocene

Homo sapiens

Man

Pliocene

Early Man
Most Genera of mammals

Proboscidia

Miocene

Sub-families of existing
mammals

Artiodactyla

Tertiary

Oligocene

Establishment of existing
families

Rhinoceroses

Eocene

Rodents
Perissodactyls

Perissodactyls

Paleocene

Carnivores

Primates

Archaic Mammals

A new line is considered as beginning when fossils of un-
doubted relationships have been found in n particular period.
Evidently the group had more primitive ancestry, and as re-
search continues in the fossil bearing beds the origin of lines is
being steadily pushed farther back in history. The sharks, for
example, are listed as having arisen in the Silurian Period al-
though their ancestry must go back into the Ordovician; for
their rise to dominance is synchronous with that of the armored
fish (Antiarchs and Arthrodires) . These are undoubted fish, al-
though gill-arch jaws are not present.

CHAPTER XXII

ADAPTIVE RADIATIONS OF THE VERTEBRATES

It is significant that no biological science has offered evidence
which in any way controverts the theory of evolution (stated
many years before controlled conditions were used in experi-
mental research) and that each has contributed positive evidence
for the theory. From these rather diverse sciences an evolutionary
tree has been worked out. Undoubtedly the major portion of the
evidence as to the course of evolution has come from palaeontol-
ogy, the other sources of evidence confirming the fossil story.

]\Iany phylogenetic trees, particularly in the older books, are
shown as a large main stem with small branches on either side.
This pine tree analogy does not fit the evidence. The semi-
vining "Wandering Jew" is more illustrative. From an original
stem are given off many shoots which take root and grow. Each
in turn divides, and as the new branches grow the older stems
tend to die or become unproductive and fail to develop new
buds. The plant spreads in all directions, rapidly in some places,
more slowly in others, depending upon the viability of the shoot
and the environmental conditions. In time only the terminal
shoots are left, with a few old stems to show the lines of
growth.

Similar have been the adaptive radiations of the vertebrates,
and as a result many of the ancestral stems are imperfectly
known. Eventually, perhaps, palaeontologists will find the "miss-
ing links" which complete the story; but as long as hiatuses ex-
ist there will be a fertile field for speculation. It is not true that
a knowledge of ancestral groups is lacking, but that there are
few completely generalized individual specimens. This does not
interfere with the weight of evidence, for the student is con-
stantly amazed at the great number of specimens which have
been found and prepared for study.

It is incorrect to speak of any living group as being an-

339

340 STRUCTURE OF THE VERTEBRATES

cestral to another, except in the inexact sense that one may be
much nearer the common stem. A distant cousin could not be an
ancestor, and it is well to keep in mind that in actual number
of years each evolutionary line is as old as any other. Some of
the lines have remained much more like the early stock than
others, and may be used as representatives of the common stem.
Amphioxus is an animal of this type. The present members of
the cephalochordate group have many specializations which
have been added as adaptations to changing conditions; but in
fundamental anatomy and embryological development they are
remarkably generalized in nature, and form a perfect beginning
for the family tree of the vertebrates.

Only fairly generalized animals will exist through violent
environmental changes. Many specializations fit a group for
only one type of life, and here lies one of the reasons for the
great number of races which have evolved and then passed
completely from the earth. But when the dominant animals of
the time become extinct, or of no great importance, some ap-
parently insignificant race survives and gives rise to some new
dominant type. To this class belong the mammal-like reptiles.
Small, rapidly moving, seemingly unfit for existence when in
competition with the dinosaurs, they were the parent stem of
the mammals which survived a revolution of the earth's surface
and became the dominant life of the following era.

The diagrams (pp. 337-8) show the major groups of verte-
brates, and the lines from which they came. Some of the lines
are left unconnected, to indicate that the earliest ancestors are
not known, and the period in which they arose is undetermined.
This is more true of the earlier lines, both for the reason that
they were less likely to be preserved, and the older the stratum
the more alteration there would be in the fossil bearing rocks.

A. Origin of the Vertebrates

In the first two chapters is an outline of the Chordate theory
of vertebrate evolution. This was included there in view of the
fact that the pre-vertebrates are of undoubted chordate rela-
tionships, and because the author feels that this is by far the
best established hypothesis of vertebrate evolution.

STRUCTURE OF THE VERTEBRATES 341

The Hemichordata and Urochordata of the present day are
highly specialized, but they show the three diagnostic characters
of the vertebrates (dorsal hollow nerve cord, notochord, and
pharyngeal gill slits) in a simple, yet progressive, condition. The
theory does not, however, account for their evolution from the
invertebrates; and it is admitted that no clear evidence exists as
to the parental stem of the phylum.

The greatest difficulty to be overcome in deriving vertebrates
from invertebrates is the position of the body and the internal
organs. The invertebrate has a ventral, solid nerve cord, and the
currents of the large blood vessels are exactly opposite those
of the vertebrate. Also, the retinal cell layers of the vertebrate
are inverted, while those of the invertebrate have the sensory
cells toward the source of light — except in the Cephalopods
(Phylum ]Mollusca). The chordate theory presupposes that the
first chordates came from a race in which definite eyes were not
well developed, or were lacking, and that the vertebrate eye is
a new development. The adherents of this theory have not been
entirely fair in stating that other theories derive a vertebrate
''by turning an invertebrate inside out and upside down."

The oldest theory of vertebrate origins traced their ancestry
to the Annelids (earthworm group), and found many points
of comparison. (1) "Dorsal" and ''ventral" in any given animal
are arbitrary terms, the mouth being considered ventral, and
an annelid might easily reverse his position. The nerve cord
and blood vessels would then be in the proper position; and (2)
a new mouth is believed to have developed, leaving the old
mouth as a vestigial structure — the neuropore found in Amphi-
oxus. (3) A new anus developed anterior to the old, which is
supported by the fact that a post-anal gut is found in the caudal
end of many vertebrate embryos. (4) The notochord evolved
from strands of connective tissue which are found along the
ventral aorta of the annelid, a point which is not corroborated
by the endodermal origin of the chordate structure. (5) The gill
slits are homologized with the external openings of the anterior
nephridia, and the fact that no nephridia develop in the pharyn-
geal region of the vertebrates is used as evidence. However, with
all the apparent evidence, most embryologists and comparative

342 STRUCTURE OF THE VERTEBRATES

anatomists accept the chordate theory rather than the annelid
theory of vertebrate development.

Another theory traces the vertebrates back to the Xemertines,
a group of worms somewhat related to the flatworms. In this
case it is not necessary to turn the animal over, for the nerve
cord has two lateral branches and a small dorsal one. The verte-
brate would be evolved by the great overgrowth of the dorsal
branch with the modification of the lateral branches into the
paired vagi. It has been shown that the lateral nerves of this
worm have two branches, and these are supposed to be homolo-
gous with the intestinal and lateral branches of the tenth cranial
nerve. Unfortunately the other organs do not show as much
similarity as does the nervous system.

Patten and others have developed the theory that the verte-
brates have come from an Arachnid line (spiders, scorpions,
the extinct euripterids, etc.). The Horseshoe crab is the closest
living relative of the extinct line which is used as the basis for
the theory. The theory automatically discards the primitive
chordates, leaving Amphioxus and the cyclostomes as degenerate
animals of no phylogenetic importance; for the vertebrates are
supposed to have arisen as armored cyclostomes with appendages
directly from an arachnid line. The theory gains strength from
recent research on the Ostracoderms, or armored cyclostomes.
Casts of this fossil group show resemblances to the extinct
arachnids which cannot be easily dismissed. Patten derives the
vertebrate jaws from the anterior appendages of the arachnid.
The objections to the theory are numerous. There are no known
cases of a group so highly specialized giving rise to another
highly specialized one. Further objections are: (1) the reversal
of layers of the eyes; (2) the dorsal chitinous armor of the
arachnid remains dorsal when the animal turns over; (3) the
arachnid shell becomes bony without intermediate evolution;
(4) the arachnid appendages arise from a single somite, the
vertebrate appendages from several; and (5) the general em-
bryological history of the two groups is entirely different.

Gaskell, in England, working on crustacean materials, at-
tempted to overcome the difficulties of turning the animal over,
and developed the theory that a new gut evolved. There
are two major points in his theory. (1) The primitive gut was

STRUCTURE OF THE VERTEBRATES 343

enclosed by the lateral growth of the flat nerve cord and left
as the neurocoel of the vertebrate. (2) As it was necessary to
get rid of the numerous appendages along the body, and as they
were found to secrete small amounts of digestive enzymes, he
conceived of them fusing along the bottom and thus forming
the intestinal tract.

With all the evidence stated these theories do not appear as
fanciful as a brief summary' would indicate. However, when the
three diagnostic characters of the chordates are found in such
primitive condition in living chordates, it seems to the author
unreasonable to search for the ancestral line among specialized
phyla which have no embryological similarity with the verte-
brates. As stated previously, these early ancestors would hardly
be preserved as fossils; but, like other worms of the Cambrian
Period, would be known to us only as burrows in the primeval
sands.

B. Adapted Radiations of the Early Chordates

It is believed that the chordate stock had reached an Amphi-
oxus-like form in the Cambrian Period. This would give three
main evolutionary lines: (1) the most primitive which has sur-
vived as the two widely different groups of Hemichordates; (2)
the ancient, but now highly specialized, Urochordates (Tuni-
cates) ; and (3) the line with a complete notochord which has
survived as Amphioxus and the vertebrates. The last apparently
gave rise in the Cambrian to two distinct lines: (A) the cyclo-
stomes and the armored ostracoderms, the latter showing more
relationships with the cyclostomes than with the other verte-
brates, and both being more closely similar to Amphioxus than to
the fish; and (B) the Elasmobranchs. There is also much ev-
idence that the bony fish had arisen before the end of the
Cambrian.

The earliest sharks known have a reduced number of gill arches,
and the jaws are already formed from the first pair of branchial
cartilages. Cladoselache, which has been described, supplies in-
valuable evidence as to the fin-fold theory. The most generalized
sharks still living have seven gill slits, and others six. These
groups show the beginnings of the heterocercal (upturned) tail.

344 STRUCTURE OF THE VERTEBRATES

The shark-like structure of the sturgeon (Chondrostei) makes
it and its relatives a good starting point for the development of
the Holostei and the Teleostei. The two former have always
been rather limited in their distribution. The teleosts (most of
the recent fish) have evolved in every conceivable direction.
Some forms are eel-like; others have become flattened dorso-
ventrally into wide, slow moving fish; others have flattened
laterally until they are almost transparent; a few are so short-
ened that they consist of hardly more than a head and a very
short body; the soles and flounders undergo a torsion during
development; and many of the deep-sea fish are strangely dis-
proportioned. The teleosts represent the extreme specializations
toward ''ichthyization", and represent fish-like characters as
we know them today.

Early in the Silurian the lobe-finned fish (Crossopterygii)
developed from the primitive group. These and the Dipnoi
parallelled each other, and became ''de-ichthyized" in character
as the swim-bladder evolved into a functional lung. In their
early history the crossopterygians were more generalized, and
one small group developed the rudiments of a humerus, radius
and ulna. The dipnoans, on the other hand, were specialized
from earliest known time. The teeth had lost the generalized
character necessary for further evolution, and the fins had a
median line of bones with rays on either side. The latter could
not have evolved into a vertebrate hand and arm, and special-
ization apparently could proceed only in the direction w^hich
it has taken — toward degeneration.

C. Evolution of the Amphibia

The evidence for the evolution of the amphibia from the
crossopterygian fish has been summarized under separate chap-
ter headings. This wuU be recalled as (1) the retention of the
spiracle, which became the middle ear; (2) the structure of the
pectoral fin; (3) the swim-bladder with its ventral "trachea"
and its bi-lobed, vascular lungs; (4) the blood supply of the
lungs coming from the sixth aortic arch; (5) external gills in
the larva, and the general similarity between the tadpoles of

STRUCTURE OF THE VERTEBRATES 345

the Crossopterygii and amphibia; and (6) the marked similar-
ity between the positions of the head bones in the two groups.

In view of the extraordinary similarity between an amphibian
tadpole and a fish larva, both structurally and functionally,
the embryological transition from fish to amphibian is simpler
than that from amphibian to reptile. The most difficult point to
understand is the development of feet and toes, and this has
been partly bridged by the discovery of crossopterygian fossils
{Sauripterus and Eusthenopteron). The most fish-like amphib-
ian yet discovered [Eogyrinus) is elongate; covered with bony
plates; the head bones have the crossopterygian pattern; and
there is a large cleithrum, a typical bone of the fish, connecting
the shoulder girdle with the skull. The vertebrae are of the
primitive type with two distinct rings to each segment.

The fossil Stegocephalia (the first amphibia) evolved a num-
ber of specializations. The line leading toward the living amphib-
ia is soon lost, the legless Apoda most resembling the Stego-
cephalia in skull type and in having minute scales beneath the
skin. Others of the Stegocephalia became reptilian in body
shape, and until larval forms were discovered these were known
as Pro-reptilia.

The fossils supply evidence as to the evolution of the skeleton
from amphibia to reptiles, but this gives us no clue as to the
evolution of the amniote type of development. The gap is partly
filled by the type of egg cleavage, gastrulation, and early de-
velopment of the Apoda. The egg is relatively large and yolk-
laden and cleavage is partly meroblastic, the dorsal blastoderm
being a layer of columnar cells. Gastrulation is almost reptilian,
and is an essential link in understanding the process in reptiles
and birds. A small area on the edge of the blastoderm folds
under and the inturned tissue grows forward as the endodermal
layer. It will be recalled that gastrulation in the chick begins
as a bit within the blastodermic disc.

Although this does not account for the origin of the amnion,
it is not difficult to understand how the yolk mass became greater
and the blastoderm relatively smaller. This being admitted,
the overgrowth of amniotic folds is not the enormous transition
that might at first be supposed; for, not only is the embryo
spread out as a sheet with its archenteron open to the yolk

346 STRUCTURE OF THE VERTEBRATES

sac, but the allantoic bladder is well developed in the amphib-
ia. Under the conditions of a large yolk the ventral body wall
would not interfere with the continued growth of the allantois,
and its enlargement into a breathing organ would be possible.
It is probable that the first reptiles laid their eggs in moist
places with the yolk surrounded by a gelatinous capsule. It is
certain that the typical embryological membranes were de-
veloped before the recent reptilian sub-classes were separated
from the original stock.

D. Radiations of the Reptiles

Seymoiiria, of the Lower Permian Period, an era when the
amphibia were still the dominant type of vertebrate life, is the
most primitive reptile known. The skull is wide and flat, the
brain cavity very small in proportion to the total size, the poste-
rior margin is indented by an otic notch across which the
tympanum was stretched, and the dorsal bones retain the
primitive arrangement. In the vertebrae both an intercentrum
and neurocentrum are present, the former being relatively large,
the latter having short dorsal arches. The ribs articulated with
both inter- and neurocentrum, and become progressively smaller,
extending on past the sacrum to the tail vertebrae. A single
sacral rib is enlarged for the attachment of the pelvis.

The more primitive characters of the first reptiles were soon
lost. The skull became deeper, the ribs were less numerous, the
intercentrum was smaller, and the typical two sacral ribs be-
came attached to the ilium. These reptiles were lacking fenestrae
in the dermal covering of the skull, openings through which the
muscles of the jaw became attached to the dorsal surface of the
skull roof. This group forms the stem of reptile evolution. The
turtles are the. only living representatives of the group, the Sub-
class Anapsida.

Two other sub-classes sprang from the early anapsid reptiles,
the Parapsida and the Synapsida. The former have a single pair
of dorsal fenestrae, and a deep notch on the margin of the skull
between the maxilla and the quadrate. The lizards and snakes
are the living representatives of the parapsid reptiles.

Synapsid reptiles had a single lateral fenestra in the dermal

STRUCTURE OF THE VERTEBRATES 347

roof of the skull. This group evolved in two major directions.
The smaller group evolved toward the mammal-like reptiles, and
eventually gave rise to the Class ^Mammalia. The other line
developed a second pair of fenestrae and form the reptilian sub-
class Diapsida.

From the diapsid reptiles came the great reptilian groups of
the Age of Reptiles: (1) the dinosaurs which dominated the
earth for millions of years. Some were the largest land animals
known, some were marvellously armored, and others were bi-
pedal, swift, carnivorous and powerful. (2) The Pterosaurs were
carnivorous flying reptiles which became extinct with the de-
struction of the dinosaurs. Also included in this sub-class are
(3) Ichthyosaurs, ovo- viviparous animals highly specialized
for water life; (4) the living Sphenoclon of New Zealand,, sole
representative of a once large group; and (5) the Crocodilia, dis-
tributed throughout the tropical and semi-tropical regions of
the earth, which with their semi-aquatic habits survived in large
numbers.

The birds, an offshoot of the diapsid stock, have left few fos-
sil remains. Archaeopteryx, the first known bird, lived in the
Jurassic. Toothed, water living birds have been found in the
Upper Jurassic and Cretaceous; but the immediate ancestors of
the birds are almost unknown.

There are two problems which face the student of vertebrate
distribution in the transition from the ]Mesozoic to the Cenozoic
—the destruction of the great reptiles and the survival of the
mammals. The latter is more easily understood. Small, gener-
alized, active, slinking mammals could avoid the larger reptiles
during the many changes of the earth's surface; and their gen-
eralized structure would have adapted them for new environ-
ments. There are several theories for the practical extinction of
the larger reptiles. "Racial senescence" is used to express the
tremendous specialization of most of them, with their size, or
armor, or bony excrescences; but it is doubtful if a race does
become senile and die without other cause. Certain lizards to-
day, although smaller, have the same weird growths. Another
theory (Schuchert) accounts for the disappearance of the dino-
saurs as due to climatic changes. The late Cretaceous was a
period of elevation in America, at least, with the Rocky INIoun-

348 STRUCTURE OF THE VERTEBRATES

tains. in process of formation accompanied by the regression of
the great inland sea which covered the Great Plains. The wide
marshes which bordered the sea were the home of many herbivo-
rous dinosaurs and their carnivorous enemies. The destruction
of the herbivores would cut off the food supply of the carnivores.
Botanical evidence indicates that the late Cretaceous was char-
acterized by a much cooler climate than had formerly existed,
and this may have been a contributing factor. In any event,
with the disappearance of the larger and more dangerous rep-
tiles, the conflict between the reptiles and the mammals became
less unequal and the better coordinated mammals won.

E. From Reptile to Mammal

Fossil remains give the story of the evolution of the mammals
from mammal-like reptiles. Unfortunately one is limited to the
skeletal remains in drawing conclusions from this group, for the
physiological changes were vastly more important, and here one
can depend only upon analogy with living groups.

A higher basal metabolism is well correlated with the increase
of brain structure, and the calvaria of the ancestral reptile
group shows that their brains had kept pace with their other
mammalian characters. Equally good analogies are found in
the structure of the appendages and girdles. The typical reptile,
with large coracoids and his humerus and femur almost at right
angles to his body, moves rapidly for only a short time ; and the
reptile therefore gorges himself at one meal and lies down for
days or weeks of rest. But with a higher metabolic rate more
frequent eating is necessary, and this is correlated in all forms
with a running, active type of body. The Therapsid (mammal-
like) reptiles had a skeleton indicative of swift, prolonged mo-
tion. The body was slender, the coracoids were small, and the
elbows and knees were drawn in towards the vertebral axis of
the body.

Further evidence is from analogy with the higher reptiles and
more primitive mammals. It is evident from living mammals
(armadillos and Manis) that hair and scales exist at the same
time. Hairs probably arose as sensory structures, and surely
have small function in an animal like Manis; but in the higher

STRUCTURE OF THE VERTEBRATES 349

forms they assist in maintaining a constant temperature. Fur-
ther aid in temperature control would be supplied by the dia-
phragm, a structure which is well developed in the Alligator
and tj^pical of the mammals.

The embryological processes are further evidence of the kin-
ship between reptiles and mammals. The monotremes lay large
yolked eggs with meroblastic development like that of the rep-
tile. The young are hatched by the body heat of the mother.
But in the marsupials and placentals the egg is microscopic
and holoblastic. These eggs are structurally very dissimilar
from those of a reptile; and yet, as the egg develops, a yolk
sac is formed and the embryo assumes the spread-out condition
of the reptile with the archenteron connected with the yolk sac.
The heart begins its development from paired vitelline veins,
and many other developmental similarities exist which can be
accounted for in no other way except that it is a repetition of
a reptilian scheme.

But so far removed from the other mammals does the ancestry
of the monotremes appear, that the mammals are thought to
have arisen as two separate lines from forms which are called
^'reptiles" rather than mammals. Aside from much other evi-
dence, the bony structure makes this more than plausible. The
shoulder girdle of the monotreme is more reptilian than that of
the higher Therapsida. The monotreme skull and ear ossicles,
however, are mammalian.

The evolution of the face has been outlined. The therapsid
reptiles had an enlarged calvarium; a shortened, deepened skull;
e^^es which were focussed forward and not outward; and a het-
erodont dentition. The face and skull were mammalian with the
exception of a few reptilian bones.

The embryology of the ear ossicles was discussed in the sec-
tion on the visceral skeleton. The skull of the Therapsida sup-
plies the anatomical evidence. The lower jaw of the primitive
reptiles is formed of six bones, the dentary being the most
anterior. With the rise of the mammal-like reptiles the dentary
becomes the largest, and the other bones are pushed far to the
proximal end, or are lost. In the higher therapsid types the
articular, angular and surangular form a small group posteri-
orlv. The articulation of the jaws remains between the quadrate

350 STRUCTURE OF THE VERTEBRATES

and the articular, both cartilage bones. In at least one known
form, a second point of articulation has developed between the
dentary and the squamosal. Both are functional, thus disposing
of the old argument that the transition animal "could neither
eat nor hear".

Until comparatively recent research showed the embryologi-
cal mechanism for the evolution of the vertebrae, this was an-
other point which depended entirely upon palaeontology for
its proof. It has been stated that in the primitive condition there
are two vertebrae in each body segment (an inter centrum and
a neurocentrum) with the rib developing in the myoseptum.
This condition is found in the earliest amphibia. The intercen-
trum grows progressively smaller, until in the mammal it is
present only as small chevron bones in the tail region. In tail-
less animals they are entirely lacking.

Embryologically the vertebrae begin as eight centers of carti-
lage in each segment, forming two incomplete rings and cor-
responding to the isolated condition of the vertebrae of the
cyclostomes. Soon two rings are formed in the notochordal
sheaths, the anterior ring of each segment developing a neural
arch. These neurocentra push anteriorly, crowding the soft tissues
of the intercentra downward, until each neurocentrum articulates
with the next anterior one. In the Therapsida the intercentra
are left as small triangular bones between the neurocentra. In
the mammals they are pushed out completely except when left
as chevron bones.

Detailed research has deciphered the history of each bone of
the body in its evolution from fish to mammal; the changes
and the homologies of the muscles are known; and even the
minutiae of embryology have been studied and found to fit per-
fectly into the picture. At best these facts can be but briefly
stated in an elementary text, and the student is referred to
more technical books and papers.

F. INIammals

Fossils have been found in the Triassic Period which appear
to be mammals or an intermediate stage. They are probably the
forerunners of the mammals which are known from the Juras-

STRUCTURE OF THE VERTEBRATES 351

sic, contemporaries of the dinosaurs. The earliest mammals were
about the size of mice. When the first Jurassic mammalian jaw
(Amphitherium) was discovered more than a century ago Cuvier
called it a small opossum, and little evidence has since appeared
to change the classification. In the Cretaceous appear numerous
definite marsupials, roughly contemporary with the early In-
sectivora.

In the earliest strata of the Paleocene there is a fullblown
mammalian fauna. These were all small, generalized, and dom-
inantly placental. From the insectivorous stock had developed
two other of the living orders, the carnivores and the primates.
The latter were apparently herbivorous or omnivorous. Tlie pri-
mates of the Paleocene were lemuroid in structure, and from
this group during the succeeding periods evolved the lemurs,
monkeys, anthropoids and man.

During Eocene time most of the land mammals of the present
time had their beginning. On the plains were the four-toed (and
later three-toed) ancestors of the horse. The leathery armadillos
had attained fair size; and the Artiodactyla, the even-toed ungu-
lates, had begun their radiations. The archaic carnivores had
attained larger size, these creodonts cUsappearing in the Oli-
gocene Period, their smaller and more generalized relatives sur-
viving as the present Carnivora. In this period arose the largest
of the early mammals, the herbivorous, horned, Titanotheres.

The following succession of strata show the rise of hundreds
of genera, and a number of orders. Eleven of the known orders
completely disappeared in the struggle for existence, although
sixteen orders have survived to the present. These surviving
orders have become adapted to nearly every type of environ-
ment. For their adaptive radiations the student is referred to
Scott's A History of the Land Mammals in the Western Hem-
isphere.

G. Evolution of Man

The first primates appeared in the Paleocene Period, are
lemuroid in structure, and appeared almost synchronously in
Europe and America. All were adapted for arboreal life. The
orbits were large, indicating large and (by analogy with the

352 STRUCTURE OF THE VERTEBRATES

present primitive stock) nocturnal eyes. The brain case is not
highly expanded, the cerebellar region being relatively greatest,
and has not grown forward over the long primitive face. The
teeth are almost like those of the insectivores.

The Old World monkeys appear first in the Oligocene strata
of Egypt and evidently gave rise to the higher primates. They
soon became adapted for sitting upright in a tree; the brain
enlarged; the dental formula was typical for the recent group
(two premolars and three molars) ; and the foreleg became dis-
tinctly arm-like. The adaptive radiations which followed led
to the development of many specialized forms: (1) completely
arboreal types lacking thumbs, the specialization appearing
independently in both Africa and South America; (2) long-
tailed forms with short faces; (3) the baboons and mandrills
with long, dog-like heads and brilliant coloration, the most
aberrant of the primates; and (4) the genera which are more
like the human in structure.

The anthropoid stem appears to have split from the ancestral
type in the Oligocene period. Dryopithecus, found in Europe,
is sufficiently generalized in its structure that it could easily
have been the stem from which the anthropoids and man
evolved. The skeleton shows that the line leading toward the
anthropoids and man was becoming less arboreal during early
Miocene times.

A skull found in Southern Africa in recent years adds fur-
ther evidence to the relationships between the African anthro-
poids and man. This primate, Australopithecus, is evidently a
young, highly developed anthropoid, but an animal which has
more human characteristics than either the chimpanzee or the
gorilla. It probably lies near the common ancestral stem, but
with the group which eventually gave rise to man.

The most famous human fossil known is that discovered in
Java by Dubois, Pithecanthropus erectus. Since its discovery in
1891 it has been extensively studied and shown to be definitely
human, of a very primitive type. The skull cap shows a cranial
capacity of only a few ounces less than that of the minimum
for the normal human, and although the brain was extremely
primitive it had evolved more than halfway between the upper
limit for the gorilla and the lower limit for the recent human.

STRUCTURE OF THE VERTEBRATES 353

The brain was nearly fifty per cent larger than that of the max-
imum for the gorilla, but smaller than that of even the Austral-
ian bushman. If, on the other hand, the brain of the white
race is used as the standard for comparison, with an average
brain weight of forty-eight ounces. Pithecanthropus lies about
lialfway between the gorilla and man.

The other physical characteristics of the "apeman of Java"
are equally primitive. The femur shows that the race walked
w^ith knees bent, but that they stood in an erect position, not
ivith the knuckles on the ground as do the higher anthropoids.
The face and teeth protruded, the facial angle being more primi-
tive than that of the bushman. With this projecting face there
were heavy ridges over the eyes and a receding forehead, giving
the race an ape-like appearance. Pithecanthropus is considered
a specialized branch from the human stem, not the ancestral
stock of the present humans.

Another recent find is the skull of Sinanthropus from China.
It is the remains of a distinct human, although considered as
too far removed from the present races of man to be included in
the same genus. The Piltdown man, which is known from frag-
ments of a skull and a jaw found in England in 1913, has a
larger brain case than Sinanthropus, but shows highly special-
ized characters. The jaw is powerful, with large canines, and
slopes directly back from the base of the teeth instead of having
a protruding point as in the typical human. These characteris-
tics are distinctly ape-like. It appears from the evidence that
Sinanthropus lies nearer the human stem than does the Piltdown
man (Eoanthropus), the latter belonging to a specialized race
which has entirely disappeared.

In the middle of the last century the fragments of a primi-
tive skeleton were discovered in Germany and placed in the
same genus with the present race but in a different species. It
was named from the valley in which it was found. Homo nean-
derthalensis. Since that time bones and even entire skeletons
have been found in different parts of Europe and Asia Elinor,
showing that the race extended from Germany to Spain, and
eastward to the Sea of Galilee. Associated with the fossils in
caves and other excavations have been found crude chipped

354 STRUCTURE OF THE VERTEBRATES

stone implements, evidently knives and scrapers, the earliest
record of man's cultm^al beginning.

The Neanderthal man was short in stature, measuring about
five feet four inches in height, and walked with knees slightly
bent. The head was large but did not extend much above the
level of the eyes, the forehead being receding. The orbital ridges
were heavy as in other primitive men, the bridge of the nose
was flat, and the jaws protruded in front. The lower jaw had
a slight chin which was not as prominent as that of the present
race. He most resembled the bushmen of Australia, man's most
primitive living relative. With the rise of the present species,
Homo sapiens, the men of the earliest stone age were probably
forced to take refuge in marshes, mountains and other inacces-
sible places. It is interesting that in several such spots in
Europe, untroubled by the ravages of invasion and immigra-
tion, groups still live who have heavier orbital ridges and more
protruding jaws than are found in their neighbors.

AVith the rise of the Cro-Magnon men, a tall race with large
heads, stone work became an art. No longer were they limited
to crude implements of flint, but awls, knives, spears and arrows
were made of finely chipped flint and polished bone. From this
time on culture grew. Close-grained rock was hammered and
polished to make axes and other implements, and fragments
of pottery jaws are found with their remains. From very early
times the walls of the caves were decorated with finely drawn
pictures of animals of the chase, many of which have long since
become extinct. At first these drawings were in black and white,
and then colors were added. In the later drawings the artists
show a knowledge of perspective. Sea shells, bones and teeth
were fashioned into ornaments for the body, and domestic and
hunting implements were decorated with beautifully sculptured
designs.

Later development is not the province of anatom}^ The Cro-
INIagnon men were of the same species as the present race, and
there is no evidence that man has evolved either physically or
mentally since the beginning of these early cave dwellers. The
time element involved, however, is so short that there is no
reason to believe that the evolution of man has ceased; but as
to the course which it may follow, it is impossible to prophesy.

APPENDIX I

BOOKS FOR REFERENCE

The following list makes no pretensions of being a bibliog-
raphy of vertebrate anatomy. The student who wishes to go to
the original sources should have the advice of his instructor.
The references included will be found in most college libraries,
and can be read without a great expenditure of time.

PART I

Parker and Haswell: Text-book of Zoology, Volume II.

A book of general reference for the classification of the Chordata.
There is an excellent chapter on the structure and development of
Amphioxus.

Cambridge Natural History.

One of the best references for natural history, with minor discussions
of anatomical details. The several volumes on the vertebrates cover
the subject thoroughh'.

Newman: Vertebrate Zoology.

A brief, but inclusive, treatment of natural history, with sufficient
anatomical detail to show the evolutionary implications of the subject.

Willey: Amphioxus and the Ancestry of the Vertebrates.

A specialized discussion of the early chordates and their relationship
with the vertebrates.

McEwEN : Vertebrate Embryology.

A textbook covering the details of comparative embryology, dis-
cussed systematically, and beginning with Amphioxus.

Jenkinson : Vertebrate Embryology.

A systemic treatment of comparative embryology, more technical
than the former reference. It is particularly valuable for the section
on mammalian development.

PART II

Parker and Haswell: Text-book of Zoology. Volume II.

The book includes a thorough description of the anatomy of verte-
brate types, with sections on the comparative anatomy of each class.
Although not treated in a systemic manner, it is one of the best
references for the details of anatomy.

355

356 APPENDIX I

Arey: Developjnental Anatomy.

An embryological treatment of mammalian anatomy, with discussions

of chick anatomy. One of the best references for the organogenesis

of systems.
Davison: Anatomy of the Cat.

A brief treatment of the anatomy of the cat, with discussions of

general mammalian anatom3\
Schafer: Textbook of Histology.

A condensed text covering microscopic anatomj'. It is also valuable

for discussion of technique at the end of each chapter.
Broom : The Origin of the Human Skeleton.

A technical book dealing with the vertebrate skeleton, excellent for

a study of fossil amphibians and reptiles.
Jayxe: Mammalian Anatomy.

A reference book on the skeleton.
Bailey: A Text-hook Histology.

A general reference for histology, the chapter on the nervous system

being particularly valuable.
Vincent: Internal Secretions and the Ductless Glands.

One of the few books giving the comparative anatomy and evolution

of the endocrine organs.
Schafer: The Endocrine Organs.

A more recent work in two volumes on the function of the glands

of internal secretion.

PART III

XoRDENSKioLD : The History of Bi(

An accurate and recent book, more technical in treatment than the

following.
Locy: Biology and Hs Makers.

A well written volume, giving biographical data and the contributions

of those who are included.
Schuchert: Historical Geology {Vol. II of Pirsson and Schuchert).

One of the best works on palaeontology for the student.
Scott: A History of the Land Mammals in the Western Hemisphere.

A book which is rather limited in scope, and in many places tech-
nical in treatment. It does not include recent research, but is well

illustrated.
Morgan: Evolution and Genetics.

A brief discussion of the bearing of genetics upon evolution.
Plunkett: Outlines of Modern Biology.

One of the best discussions of the physiological aspects of evolution.

APPENDIX II

CLASSIFICATION OF THE CHORD ATA

The following classification includes the living classes and
orders of the Chordates, and the fossil orders which are con-
sidered most important. All fossil groups are indicated with an
asterisk (*). In an effort to indicate equivalence of phylo-
genetic rank it is often necessary to group animals into Super-
classes, Super-orders, or Super-families. Division or Grade are
also used to show such relative positions. Although this is un-
doubtedly the more scientific procedure, there is frequently a
lack of agreement among taxonomists; therefore, for simplicity,
these divisions are used as little as possible in this scheme of
classification.

With the exception of the Order Primates, which is classified
to family, only a few of the more important families of the
vertebrates are included. The basis of selection is the need of
the student in his laboratory work, rather than scientific im-
portance.

PHYLUM CHORD ATA

Sub-phylum I. HEMICHORDATA

Class 1. Enteropneusta [Balanoglossiis, and other

"Acorn Worms").
Class 2. Pterobranchia (Two little known genera).
Sub-phylum II. UROCHORDATA

Class 1. TuxiCATA (Tunicates).
Sub-phylum HI. CEPHALOCHORDATA

(Contains only a few genera, including Amphioxus lan-
ceolatus).

357

358

APPENDIX II

Sub-phylum IV. VERTEBRATA or CRANIATA
Class 1. Cyclostomata

Sub-class 1. CYCLOSTOMI

Order 1. Myxinoidea {Myxine, Bdellostoma)
Order 2. Petromyzontea {Petromyzon marinus, the
prev; Lampcfra, and others.)
OSTRACODERMI*
Anaspida *
osteostraci *

1am-

Sub-class 2.

Order 1.

Order 2.

Order 3. Heterostraci *
Class 2. Pisces

Sub-class 1. ELASMOBRANCHII

Order 1. Cladoselachii * {Cladoselache '^)

Order 2. Pleuropterygii *

Order 3. Acanthodii *

Order 4. Selachii (Sqiialus, Mustelus, and many others.
Includes several hundreds of genera of sharks,
skates, and raj^s; both living and fossil).

Order 5. Holocephali (an aberrant group, often given
the rank of a sub-class.)
Sub-class 2. TELEOSTOMI

Order 1. Chondrostei {Acipenser, Scaphyrhynchus, Poly-
odon and numerous fossils, including most prim-
itive Actinopterygii.)

Order 2. Holostei {Lepidosteiis, Amia, and many fossils).

Order 3. Teleostei (a heterogeneous assemblage, tech-
nically divided into several orders.)

Order 4. Crossopterygii {Eusthenopteron*, Sauripterus*,
Polypterus, Calamoichthys.)
Sub-class 3. DIPNOI

Order 1. Sirenoidei {Neoceratodus, Protopterus, Lepidosi-
rcn, and some closely allied fossils.)
PLACODERMI *
Antiarchi *
Arthrodira *

Sub-class 4.
Order 1.
Order 2.

Class Amphibia

Order 1. Stegoceph.\lia *

{Archaegosaurus *, and others.)
Order 2. Urodela (Inckides those with persistent gills,
Necturus, Siren; Crypt obranchus; and the sala-
manders and newts, Amhlystoma, Triturus.)
Order 3. Anura (The frogs and toads; Rana, Bnfo.)
Order 4. Gymnophiona (The caecelians, or legless am-
phibia.)

Class Reptilia

Sub-class 1. ANAPSIDA

Order 1. Cotylos.^uria * (Seymouria*)
Order 2. Chelonia (Turtles, Chelone,
the land tortoises.)
Sub-class 2. SYNAPSIDA*
Order 1. Theromorpha*
Order 2. Therapsida * (Cynognathus*, Ictidopsis*)

Chrysemys; and

APPENDIX II 359

Sub-class 3. DIAPSIDA

Order 1. Rhynchocephalia (Sphenodon)

Order 2. Crocodilia (Alligator, Crocodilus)

Order 3. Dinosauria *

Order 4. Pterosauria *
Sub-class 4. PARAPSIDA

Order 1. Lacertilia (Lizards)

Order 2. Ophidia (Snakes)

Order 3. Ichthyosauria * (Aquatic reptiles).

Order Plesiosauria (Probably a separate sub-class, be-
tween the Synapsida and Parapsida).

Class AvES

Sub-class 1. ARCHAEORXITHES * (Archaeopteryx) *
Sub-class 2. NEORNITHES

(The birds with shortened tail and bird-like
wings are divided into the fossil tooth-bearing
birds, and the recent birds which lack teeth.
The latter include the (1) Ratitae, with more
or less degenerate wings and a smooth ster-
num; and (2) the Carinatae, or flying birds,
with a keel (carina) on the sternum. There are
twenty-eight orders of the Neornithes.)
Class Mammalia

Sub-class 1. PROTOTHERIA

Order 1. Monotremata (Ormthorhynchiis, Echidna.)
Sub-class 2. METATHERIA

Order 1. Marsupialia (The American opossum, Didel-
phys; and the AustraHan marsupials, kangaroos,
wombat, etc).
Sub-class 3. EUTHERIA

Order 1. Insectivora (The moles, shrews, etc.).
Order 2. Primates

Sub-order 1. Lemuroidea (Lemur, Tarsius, Chiro-

mys).
Sub-order 2. Anthropoidea

Family 1. Hapalidae (Hapale, the marmoset)
Family 2. Cebidae (South American monkeys)
Family 3. Cercopithecidae (Old World monkej^s;
Macacus, Cercopithecus and the ba-
boons, CynocepJialus).
Famil}' 4. Simiidae (The anthropoid apes; Gib-
bons, Hylobates; orangs, Simia; chim-
panzee, Pan; gorilla. Gorilla; Austra-
lopithecus *)
Family 5. Hominidae (Pithecanthropus^ ; Eoan-
thropus*; Sinanthropus* ; and Homo).
Order 3. Chiroptera (The bats)
Order 4. Dermoptera (One genus, Galeopithecus) .
Order 5. Tubulidextata (Edentates; the Aard varks).
Order 6. Pholidota (An edentate, Manis).
Order 7. Xenarthra (Ant-eaters, Armadillos; sloths).

360 APPENDIX II

Order 8. Rodentia (Rat, capybara, beaver; for the rab-
bit see Lagomorpha).
Order 9. Lagomorpha (Rodent-like animals, with two

pairs of incisors; Lepus, the rabbit).
Order 10. Carnivora

Sub-order 1. Fissipedia (Canis, Felis, and other car-
nivores adapted for land life.)
Sub-order 2. Pinnipedia (Walrus, Sea-lion, Seals).
Order 11. Hyracoidea (Hyrax).
Order 12. Cetacea (The whales and porpoises).
Order 13. Sirenia (The diigong and manatee).
Order 14. Proboscidea (The elephants, Mastodon*, Mam-
moth *).
Order 15. Perissodactyla (Horses, Eohippus* Merychip-
pus* Mcsohippus,* Equus; Zebras; Asses;
Rhinoceros).
Order 16. Artiodactyla (Cattle, Bos; Bison; Deer; Hip-
popotamus).

APPENDIX III

GLOSSARY

An alphabetical list of technical terms and systematic names,
with their derivation and meaning. The following abbreviations
are used: G., Greek; L., Latin; F., French.

A, or AN (G. without), a prefix combined with any terms; Anura; acen-
trous.

AB, (L.), away from; abduct; aboral.

ABDOMEN (L.), the body region containing the viscera; in mammals Hmited
to the region between the diaphragm and the pelvis.

ABDUCENS (L. ab, plus duco, to lead), Cranial Nerve VI.

ABDUCT (L. ahduco, lead away from), to carry away from a given point.

AcANTHiAs (G. acantha, a horn), a genus of sharks; also the specific name
of Squalus acanthias, the American dogfish.

ACENTROUS (G. a, plus kentron, a center), usually in reference to a verte-
bra lacking a centrum ; e. g., the atlas.

ACETABULUM (L. a cup), the socket in which the head of the femur artic-
ulates.

AciPENSER (L. acipenser, a sturgeon), a genus of chondrostean fish, the
most primitive living bony fish.

ACOUSTIC (G. akoustikos, related to hearing), pertaining to the auditory
structures or function; acoustic nerve.

ACROMION (G. acros, top, plus omos, shoulder), the prolongation of the
spine of the scapula.

ACUTE (L. acuo, to sharpen) ; in anatomy, sharp when applied to a proc-
ess or angle; in medicine, having a short severe course.

AD (L. toward, upon), a prefix; adduct, adrenal.

ADDUCT (L. ad, plus duco, to lead), to carry toward; adductor.

ADIPOSE (L. adeps, fat) an adjective referring to fat, as adipose tissue.

ADRENAL (L. ad, upon, near; plus regies, kidney) ductless glands near the
kidney's.

ADRENALIN, the active hormone secreted by the adrenal medulla.

AFFERENT (L. ad plus jcw, to bear) a nerve, blood vessel, or other struc-
ture leading toward a given position.

ALBINO (L. albus, white) an individual genetically lacking pigment in
the skin and its derivatives.

ALi (L. ala, a wing) a prefix, as in alisphenoid.

ALLANTois (G. allas, allanto, sausage, plus eidos, form) an outpocketing of
the archenteron, posterior to the yolk sac. The embryonic respira-
tory organ of the reptiles and birds.

361

362 APPENDIX III

ALVEOLUS (L. a small cavity) a small cavity or pocket, as the socket of a
tooth, or the air pocket of the lung. Plural, alveoli; adjective,
alveolar.

Amblystoma (G. amhlys, blunt plus stoma, mouth) a genus of urodele
amphibians; an important form in experimental anatomy.

Amia (G. amia, a variety of tuna fish) a genus of the Holostei; a fish
more resembling the teleosts than other genera of the order. The
name is a misapplication of terms.

Ammocetes (G. ammo, sand, plus coetes, sunk in) the larval stage of
the lamprey (a cyclostome) formerly thought to belong to a different
family.

AMNION (G. amnion, a foetal membrane) a membrane enclosing the em-
bryo in the reptiles, birds and mammals.

Amniota, or AMNioTES (see amnion) a group of the vertebrates which in-
cludes those which develop an amnion and allantois.

Amniotic fluid, the fluid secreted by the embryo which fills the amni-
otic cavity.

Amphibia (G. amphi, double, plus hios, life) a class of vertebrates which
hatch as gill-breathing larvae, and usually metamorphose into lung
breathing adults.

AMPHicoELous (G. amphi, plus koilos, hollow) a structure concave at
both ends, applied to vertebrae.

Amphioxus (G. amphi, plus oxys, sharp) a vertebrate-like lower chor-
date, of the Sub-phylum Cephalochordata.

amphiplatyan (G. amphi, plus platys, flat) flat on both ends, applied
to vertebrae; the mammalian type.

Amphiuma (G. amphi, plus pneuma, breath) a genus of urodele am-
phibia with rudimentary legs, and retaining gill slits.

ampulla (L. a flask) a bladder-shaped enlargement, as the ampullae of
the semicircular canals.

analogous (G. ana, according to, plus logos, proportion) similar in func-
tion, but not necessarily alike in genetic relationship; opposed to
homologous. Two structures may be analogous, homologous, or
both. The wings of a bird and those of a bat are both analogous
and homologous.

Anamnia (G. an, plus amnion) vertebrates which do not develop an
amnion during embryonic development; the cyclostomes, fish, and
amphibia.

anastomosis (G. several openings) a union, or running together, as of
two or more veins or other structures.

ankylosis (G. ankylos, bent) a union or knitting together of two or
more bones or parts of bones.

ANTE (L. before) in front of, anterior.

ANTHRO (G. anthropos, man), a prefix referring to human; anthropology;
Pithecanthropus.

Anthropoidea (G. anthropos, plus eidos, form), a sub-order of primates
including the higher apes and man.

Anura (G. an, plus oura, tail), an order of tailless amphibia, the frogs
and toads.

anus (L. a ring), the posterior opening of the intestinal canal.

.\ORTA (G. aeiro, to lift), the large artery leaving the heart.

Apoda (G. a, plus pons, foot), see Gymnophiona.

APPENDIX III 363

APONEUROSIS (G. apo, from, plus neuron, a sinew), a broad flat fascia or

sheet of tissue, attaching a muscle.
APSIS (G. an arch), a combining form; synapse, Anapsida, Diapsida.
AQUA (L. water), a combining form; aqueous, acjueduct.
ARACH (G. a spider's web).

ARBOREAL (L. arhor, a tree), pertaining to trees, as tree-living.
ARACHNOID (G. arach, plus eidos, form), the very thin, middle covering of

the brain, between the pia and dura mater.
Archaeopteryx (G. archos, ancient, plus pteron, wing), a fossil bird of

the Jurassic Period, with teeth and a reptilian tail.
Archegosaurus (G. archos, plus sauros, a lizard), a primitive genus of

extinct stegocephahan amphibia.
ARCHENTERON (G. archos, plus entcron, gut), the embryonic digestive tract,

formed by gastrulation.
Archeozoic (G. archos, ancient, plus zoon, animal), the most ancient

sedimentary rocks, followed in succession by the Proterozoic.
ARCUALiA (G. arcus, a bow), the cartilaginous anlagen of the vertebrae,

eight arcualia appearing in each segment.
.ARTHROS (G. a joint), a combining form; arthritis, Arthrodira.
Artiodactyla (G. artios, even, plus daktylos, finger, toe), an order of

herbivorous mammals with the axis of the leg between the second

and third digits; cows, deer, sheep, pigs, and hippopotami.
ARYTENOID (G. arytaiua, a funnel), a pair of laryngeal cartilages lying

posterior to the thyroid cartilage.
ATLAS (G. tlao, to bear), the first cervical vertebra.
ATRIUM (L. entrance chamber), an outer cavity, as the atrium (auricle)

of the heart; or the specialized outer cavity of Amphioxus or the

tadpole.
ATROPHY (G. a, without, plus tre-pho, nourish), a wasting or withering of

the body or any of its parts.
AURICLE (L. diminutive of auris, ear), the pinna of the ear, or the receiving

chamber of the heart.
Australopithecus (L. auster, south, plus G. pithckos, ape), a fossil

anthropoid found in Africa, with human characteristics.
AUTO (G. self), a prefix; autonomic, autointoxication, autostyhc.
AvES (L. birds), a class of vertebrates with feathers,
AXILLA (L. diminutive of axis), the armpit.
axis (L.), a line around which parts are symmetrically arranged; as the

antero-posterior axis of the body.
AXONE (G. axon, axis), the efferent fiber of a nerve cell.
AZYGOS (G. a, without, plus zygon, a yoke), a mammalian vein homolo-
gous with the cardinals. Azygons (adjective), occurring singly.

B.\LANOGLossus (G. balanos, acorn, plus glossa, tongue), a genus of marine,

worm-like, Hemichordates; the "acorn worms".
BASi (L. base), a prefix, basidorsal, basioccipital.
Bdellostoma (G. bdellion, a variety of plant, plus stoma, mouth), a genus

of myxinoid cyclostomes from the Pacific.
Bi (L. two), a prefix, biceps, bi-lobed.

BIOS (G. life); a combining form; biology, amphibia, biogenetic.
BLAST (G. blasteo, to sprout), a combining form denoting formation or

development.

364 APPENDIX III

BLASTOCOEL (G. blastos, a sprout, plus koilos, cavity), the cavity of the
blastula, the segmentation cavity.

BLASTODERM (G. blastos, plus derma, skin), the germinal membrane giving
rise to the embryo; found in vertebrate eggs with meroblastic cleav-
age; blastoderm is in distinction to extra-embryonic tissue.

BLASTULA ((j. dim. of blastos), a hollow sphere of cells formed by early
cleavages; in the amniotes generally, a flat plate of cells.

BODY STALK, an embryonic stalk connecting the embryo with the yolk
sac; its external cell layers are continuous with those of embryo and
amnion.

BRACHIAL (L. brachium, arm), an adjective referring to the arm or its
structure.

BR.'^CHY (G. short), a prefix; brachycephahc, brachydactylous.

BRANCHIA (G. branchia, gills), a combining form in hemibranch, branchial,
Branchiostoma; branchia may be used as a noun.

BRONCHIOLE (G. dim. of bronchos, windpipe), a minute bronchial tube.

BRONCHUS (G. bronchos, windpipe), one of the larger divisions of the
trachea.

BUCCA (L. cheek or mouth) ; as buccal cavity.

BULLA (L. a bubble), a hollow bony growth, as the tympanic bulla.

BURSA (L. purse or wallet), a pouch or sac, as the bursae of a joint.

CAECUM (L. caecus, blind), a pouch open at only one end; particularly

the pouch at the juncture of the small with the large intestine.
CALVARiUM (L. skull), the brain case.
Cambrian (L. Cambria, for Northern Wales), the oldest geological period

of the Paleozoic Era.
CANINE (L. canis, dog), the first mammalian tooth posterior to the incisors;

they are unusually long in the carnivores.
CAPILLARY (L. capillus, a hair), any hair-like structure; particularly the

smallest blood vessels lying between the arterioles and venules.
Capitosaurus (L. caput, head, plus G. sauros, a lizard), a genus of extinct

stegocephalian amphibia.
C.-^RBONiFEROUS (from carbon, or coal measures), a general name for the

upper periods of the Paleozoic Era, in which most of the coal beds are

found.
cardia (G. kardia, heart), pertaining to the heart; cardiac, pericardium.
CARINA (L. keel), a projecting, keel-like structure, as the carina of a

dogfish chondrocranium, or the keel of a bird sternum.
Carinatae (L. carina, a keel), a sub-class of birds having a keel on the

sternum, the flying birds. See Ratitae.
Carnivora (L. caro, (earn), flesh, plus voro, to devour), an order of

flesh-eating mammals with large canine teeth; dogs (Canidae), cats

(Felidae), bears (Ursidae), etc.
CAUDA (L. tail), usually in adjectival form; caudal, caudad; or, cauda

equina.
Cenozoic (G. kainos, new, plus zoon, animal), the most recent of great

geological eras, following the Mesozoic.
CENTRUM (L. center), a heavy central region of a vertebra, from which

spring the spinous and transverse processes.
cephale (G. kephale, head), a combining form; cephalic, cephalad.

APPENDIX III 365

CEPHALizATioN (G. kephale) , the tendency of nerve tissues to collect in
the anterior part of the body.

CERCOS (G. kerkos, tail), as suffix in heterocercal, homocercal.

CERATO (G. keras, horn), as prefix in ceratohyal, etc.

Ceratodus (G. keras, plus oclous, tooth), an extinct genus of lung fishes
(Dipnoi) ; sometimes Neoceratodus.

cerebellum (L. dim. of cerebrum, brain), the anterior development from
the hindbrain.

CEREBRUM (L. cerebrum, brain), the dorsal growth of the telencephalon;
the higher centers of the brain.

CERVIX (L. neck), a neck-like opening, the cervix of the uterus; as adjec-
tive, cervical, applying to neck.

Cetacea (G. ketos, whale), an order of mammals, including the porpoises
and whales.

Chelonia (G. chelone, tortoise), an order of reptiles; turtles, etc.

CHiASMA (G. a cross mark), usually in reference to the optic chiasma or
crossing of the nerve fibers from the retina.

Chimeroids (G. chimaira, monster), an order (Holocephali) of elasmo-
branch fishes. They are very aberrant relatives of the sharks.

Chiroptera (G. cheir, hand, plus pteron, wing), an order of flying mam-
mals, the bats.

Chlamydoselache (G. chlamys, a mantle, plus selachos, shark), a genus
of primitive, six-gilled sharks from the coast of Japan.

choana (G. a funnel), the internal and external openings of the nares.

chondrocranium (G. chondros, cartilage, plus kranion, skull), the car-
tilaginous skull of cyclostomes and elasmobranchs; and the embryonic
skull of higher embryos, around which the dermal bones are laid
down. The chondrocranium ossifies in groups above the Chondrostei.

chondros (G. cartilage), a combining form; perichondrium; Chondrostei.

Chondrostei (G. chondros, plus osteon, bone), an order of bony fishes,
including the sturgeon and spoon-bill (Polyodon), in which the
chondrocranium remains unossified. The most primitive living bony
fish.

CHORDA (L. a cord or string), a combining form; notochord, Chordata.

Chord.ata (L. chorda), a phylum of animals, including the vertebrates and
three other sub-phyla.

CHORION (G. membrane), the outer embryonic membrane, developed syn-
chronously w^ith the amnion; and until fusion is completed, continu-
ous with it.

CHYME (G. juice, fluid), a suffix, as in mesenchyme, referring to embryonic
cells before tissue formation has taken place.

CILIARY (L. cilium, eyelid), near, or associated with, the eye or eyelashes.

Cladoselache (G. dado, branch or twig, plus selachos, shark), a genus of
extinct sharks with fins attached to the body by broad bases; an
important link in the fin-fold theory.

CLAST (G. klao, to break), a suffix as in osteoclast, the bone destroying
cells.

CLITORIS (G. kleio, to close), a structure of the female genitalia, homolo-
gous with the urethral body of the penis.

CLO.^CA (L. cloaca, a sewer), the common opening of the digestive and
urinogenital openings; found in many fish and amphibia, reptiles,
birds, and monotreme mammals.

366 APPENDIX III

COELOM (G. koilos, a cavity), the cavity formed between the somatopleure

and splanchnopleure of the hypomere; the body cavity.
COMMA (G. to cut off), a suffix referring to a dividing sheath; myocomma,
CONDYLUS (L. knuckle), a condyle or projection.
CONJUNCTIVA (L. conjunctus, to join together), the ectodermal epithelium

connecting the outer covering of the eyelids with the covering of

the cornea.
CORACOID (G. korakoeides, Hke a crow's beak), particularly the ventral pair

of bones in the pectoral girdle, lost in the higher mammals except

for the coracoid process of the scapula.
CORIUM (L. leather), the dermal portion of the skin.
CORNEA (L. corncus, horny), the outer coat of the eyeball, derived from

ectoderm and mesoderm.
coRNUA (L. horn); as, cornu (horn) of hyoid or uterus; as adjective,

cornuate.
CORPUS (L. body), corpora, plural; corpora quadrigemina, corpus luteum.
CORPUS LUTEUM (L. corpus, plus luteo, yellow), cellular bodies formed on

the ovaries after the rupture of a follicle.
CORTEX (L. bark); anatomically, the outer portion; cerebral, renal, or

adrenal cortex.
COSTA (L. rib) ; costal cartilages, intercostal spaces.
CRANIAL (G. kranion, skull), the skull, specifically the brain case or cal-

varium; cranial nerves. As a noun, cranium.
Cretaceous (L. creta, chalk), the most recent period of the Mesozoic Era.

following the Jurassic; described first in the chalk measures of

England.
cretin (F. cretin, an idiot), a type of infantilism, partially corrected by

thyroid feeding.
cribriform (L. cribrum, a sieve), the sieve-like ethmoid plate through

which the olfactory nerves pass.
Crocodilia (G. krokodeilos, crocodile), an order of diapsid reptiles in-
cluding the crocodile, alligator, cayman and gavial.
Cro-Magnon (towm in France), a race of recent men (Homo sapiens),

the cave dwellers of France and Spain.
CROssoi (G. fringe), a combining form.
Crossopterygii (G. crossoi, plus ptcron, a wing or fin), an order of bony

fishes from which the amphibia evolved.
crypto (G. hidden), cryptorchid, Cryptobranchus.
Cryptobranchus (G. kryptos, hidden, plus bronchia, gills), a genus of

American amphibia; the largest living amphibian (of Japan) is

sometimes included in the same genus.
cutis (L. skin) ; cutaneous, of or pertaining to the skin.
CYCLOS (G. a circle), a combining form; cycloid, cyclostome.
Cyclostomata (G. kyklos, round, plus stoma, mouth), a class of verte-
brates lacking jaws; the most primitive vertebrates.
Cynognathus (G. kynikos, doglike, plus gnathos, jaw), a genus of ex-
tinct therapsid, or mammal-like, reptiles.
cyst (G. kijstis, a bladder), a membranous sac or vesicle; the cystic lobe

of the liver.
CYTO (G. kytos, a hollow space), a combining form; eytology. leucocyte.

dactyl (G. daktylos, a finger), a combining form; pterodactyl, brachy-
dactyl.

APPENDIX III 367

DEFERO (L. to cany away), as vas deferens.

DELPHiN (L. dolphin), in Delphinus, a cetacean.

DELPHOS (G. uterus), a combining form; Didelphys, the opossum; Mono-
delphia, a variant name for Placentalia.

DELTOID (G. fourth letter of alphabet), shaped like the letter delta, tri-
angular.

DEXDRON (G. tree), a stem in dendrite, dendritic,

DENS (L. tooth) ; dental, dentary, dentine.

DERMA (G. skin); dermal, dermis; relating to skin.

DERMATOME (G. derma, plus toma, a cut), the portion of the epimere
giving rise to the skin.

DEVONIAN (from Devon, England, where the strata were described), a
Paleozoic period lying between the Silurian and Mississippian.

Di (G. two) ; digastric, dimorphic, didelphic, diapsid.

DiA (G. dia, between, through) ; diaphragm, diapedesis.

DIAPHRAGM (G. dia, through, plus phragnymi, enclose), a dividing mem-
brane; the diaphragm of the ear; or, a muscular partition between the
thoracic and abdominal cavities.

DIASTEMA (G. interval), a wide space betw^een teeth, usuallj' caused by
developmental suppression of the intervening teeth.

DIGIT (L. digitus, a finger), a finger or toe; digital.

DIMORPHISM (G. di, plus morphe, form), existing under two distinct forms.

DiNOSAURiA (G. deinos, terrible, plus sauros, lizard), an order of extinct
diapsid reptiles.

DiPLos (G. double), diploblastic, diploid.

Dipnoi (G. di, plus pneo, to breathe), a sub-class of fishes; the lung
fishes with gills and lungs.

DORSAL (L. dorsum, back), pertaining to the back of an animal.

Duco (L. to lead), a combining form; adduct, abduct, aqueduct.

DUODENUM (L. duodeni, from duodecim, tw'elve), the part of the intestine
immediately following the stomach, in reference to its length in the
human.

DURA (L. hard, tough), the heavy external membrane surrounding the
central nervous system; dura spinalis, dura mater.

Echidna (G. echinos, a hedgehog, or G. echidna, an adder), a genus of

monotremes of Australasia, the spiny anteater.
ECT, ECTO (G. outer, outside), a prefix; ectoplasm, ectoderm.
ECTODERM (G. ektos, plus derma, skin), the outer cell layer of an embryo.
Edentata (L. e, without, plus dens, a tooth), an order of placentals with

degenerate teeth. The order is an artificial group, and is technically

divided into three distinct orders.
effector (L. ejficio, to carry out, to perform), a structure or organ

transforming motor impulses into motor action.
EFFERO (L. to carry away from), a combining form; an efferent nerve or

blood vessel leading away from a given point; efferent branchials; vasa

efferentia.
ELASMO (G. elasmos, a plate), a combining form; fiat, plate-like.
Elasmobranchii (G. elasmos, plus branchia, gills), a sub-class of fishes

including the sharks, skates and rays. Their skeleton is entirely of

cartilage. Several extinct orders are known.
enamel (L. smaltum, enamel), the ectodermal covering of the teeth, the

hardest substance of the body.

368 APPENDIX III

END, ENDO (G. within, inside), a prefix.

ENDOCRINE (G. eiido, plus krino, to separate), pertaining to the ductless or
internally secreting glands.

ENDODERM (G. endo, plus derma, skin), the inner primary germ layer
of cells; the lining of the archenteron, formed at gastrulation.

ENDOSTYLE (G. eudo, plus stylos, a column), the ventral pharyngeal
groove of Amphioxus and the lamprey larva, homologous with the
thjToid gland.

ENTERON (G. intestine), the primitive gut; a combining form, archenteron,
enteritis.

Eocene (G. eos, dawn, plus kainos, new), an early geological period of
the Cenozoic Era. The Paleocene now includes the oldest strata of
the Cenozoic.

EP, epi (G. upon, over), a prefix; epiphysis, epidermis.

EPAXiAL (G. ep, plus axis), above the axis of the body; particularly the
dorsal half of the myotome.

EPIDERMIS (G. epi, plus derma, skin) the outer ectodermal layer of the skin.

EPIDIDYMIS (G. epi, plus didymos, testicle) the mass of efferent tubules of
the testes, homologous with the mesonephric tubules; the vasa
efferentia.

EPIMERE (G. epi, plus meros, part), the upper part of the primitive meso-
dermal somite, separated horn the hypomere by the nephrotome; it
gives rise to the mj^otome, dermatome, and sclerotome.

EPIPHYSIS (G. epi, plus phyo, to grow), a dorsal growth or outpocketing;
the epiphysis of the diencephalon (pineal) ; or the center of ossifica-
tion at the end of a long bone.

ERYTHRO (G. red) ; combining form in erythrocyte, Erythrogaster, ery-
thema .

ESOPHAGUS (G. oiso, will bear, plus phagein, to eat) the food tube, lead-
ing from the phar^-nx to the stomach.

ETHMOID (G. ethmos, sieve, plus eidos, like), the anterior group of re-
placement bones in the chondrocranium, including the cribriform
plate.

Eustachian (Eustachio, an Italian anatomist, 1534-1574) ; the Eustachian
tube, connecting the pharynx with the middle ear; a modified gill
sht.

ev.\gination (L. e, out from, plus vagina, a sheath), an outpocketing from
a hollow structure.

EX (L, out, outside), external, excretory, exophthalmic.

EXCRETA (L. ex, plus cemo, to separate) waste materials excreted by the
body; distinguished from secretions, having a bodily function; and
feces, waste products which have not been digested.

EXCRETORY (L. sce exci'cta), any gland or duct concerned with excretion.

FALCIFORM (L. falx, a sickle), curved like a sickle; as, falciform cartilage

or ligament.
Fallopian (Fallopio, Italian anatomist, 152?-1562), Fallopian tube, in

human anatomy the duct connecting the ovary with the uterus;

developmentally the upper end of the Miillerian or oviduct.
fascia (L. a band), a sheet of connective tissue investing an organ or

attaching a muscle.

APPENDIX III 369

FENESTRA (L. window), any window-like opening; the diapsid reptiles

have two fenestrae in the roof of the skull.
FERO (L. to carry or bear), suffix in afferent, efferent.

FiLUM (L. a thread), combining form; filiform, filoplume, filum terminale.
FLOCCULUS (L. floccus, a piece of wool), the lateral outgrowths on either

side of the cerebellum.
FOLLICLE (L. jolliculus, diminutive of jollis, a bag), any minute cavity,

sac or tube; as, the follicle from which a hair grows.
FONTANEL (L. diminutive of jons, a fountain), an unossified area in the

embr^-onic or infant skull; the space where four dermal bones have

not met.
FORAMEN (L. an opening, from joro, to bore through), an opening through

a membrane or bone; the foramen ovale in the embryonic auricular

septum; or, the foramina of the skull.
FOSSA (L. fossus, dug out), a shallow pit or depression; the fossae of the

scapula; the temporal fossa; or the endoh^mphatic fossa.
FRONTAL (L, irons, the brow), pertaining to the forehead.
FUNDUS (L. bottom),' the bottom or basal part; the rounded bottom of

a hollow organ; the fundus of the stomach.

GANGLION (G. a tumor), a group of nerve cells, set off to themselves, serv-
ing as a center of nervous influence.
GANOID (G. ganos, bright, plus eidos, like), a shiny substance found in

the scales of certain fish; the ganoid fishes include widely different

forms, the Chondrostei, Holostei, and Crossopterj-gii.
GASTER (G. stomach), as combining form (gastro, gastero) referring to the

stomach; gastric, mesogaster, etc.
GASTRULA (L. diminutive of gaster), an embryo at the stage when two

germ layers are present, endoderm and ectoderm, formed by gastrula-

tion.
GENITAL (L. genitalis, from gigno, to beget) pertaining to the reproductive

organs; genital papilla,
GENUS (L. origin), a division in classification, a more coherent group

than a famih-, but more diverse than a species. Generic names are

capitalized : Homo sapiens.
GERM (L. germen, a sprig; germino, to sprout), a germ cell; in combining

forms, a primary source.
GERMiNATivuM (L. germhio) , the basal ectodermal layer of the skin in

which growth proceeds.
CLANS (L. acorn), the rounded end of the clitoris or penis, derived from

the genital papilla.
GLOMERULUS (L. a ball of yarn), an agglomeration of vessels forming a

rounded mass; specifically, the coiled vessel of a renal corpuscle.
GLOSSA (G. tongue), a combining form; glossopharyngeal, Balanoglossus,

hypoglossal.
GLOTTIS (G. glotta, tongue), the opening from the pharynx into the

trachea.
GNATHOS (G. jaw), a combining form; agnathous, Gnathostome.
Gnathostomata (G. gnathos, plus stoma, mouth), vertebrates with jaws;

distinguished from the Agnathostomata or cyclostomes.
G0N.\D (G. gonos, seed), a reproductive gland, either testis or ovary.

370 APPENDIX III

Graafian (de Graaf, a Dutch anatomist, 17th Centur}'), specifically the

graafian follicle which surrounds the ovum in the ovary.
GRADUS (L. gradus, step, from gradior, to walk) ; as plantigrade, digitigrade,

unguligrade.
GUBERNACULUM (L. a rudder), the short ligament in the mammalian

embryo, attached to both testis and scrotum.
Gymnophigna (G. gymnos, naked, plus ienai, to go), an order of legless

amphibia, primitive in skull structure, and having minute dermal

scales in the skin. Synonymous with Apoda.

HAEMO (G. haima, blood), a combining form; variant of hemo.
HAEMOGLOBIN (G. haiiiia, plus L. globus, a globe), a blood protein con-
taining iron, with an affinity for oxygen.
Hatteria; see Sphenodon.
Heloderma (G. helo, a nail head, plus derma, skin), a genus of lizards

of the Southwest U. S. and Mexico; the only poisonous lizards

known.
HEMi (G.-L. half), a prefix; hemibranch, hemisection.
Hemichordata (G. hemi, plus chorda, a string), a sub-phylum of chor-

dates with a small anterior homologue of the notochord; includes

Balanoglossus and other forms.
HEPAR (G. liver), a combining form; hepatic, heparin.
HERBIVOROUS (L. herba, grass, plus voro, to devour), pertaining to animals

which eat only vegetable matter.
HERM.\PHR0DiTE (G. Hcrmaphroditus, the son of Hermes and Aphrodite)

an individual combining both sexes; hermaphroditic animals are

rarely self fertilizing.
HETERO (G. different), a prefix; heterodont, heterocercal.
HETEROCERCAL (G. hetcro, plus kerkos, tail), fish with the tail upturned

at the posterior extremity, tj'pical of the sharks.
HETERODONT (G. hetcro, plus odous, tooth), having teeth of different

shapes; i.e., incisors, canines, premolars, molars.
HOLO (G. holos, whole, entire), a prefix, as holoblastic.
HOLOBLASTic (G. Jiolos, plus blastos, a germ), a type of cleavage in which

the entire cell divides; opposed to meroblastic.
HoLocEPHALi (see Chimera).
HoLOSTEi (G. holos, plus osteon, bone), an order of bony fishes including

Amia and the gar-pikes, structurally intermediate between Chondrostei

and Teleostei. The name is in reference to the ossification of the chon-

drocranium, and its fusion with the dermal bones.
HOMO (G. homos, the same), a prefix opposed to hetero. See heterodont

and heterocercal.
Homo (L. man), a genus of the higher primates including all recent races

of man. The only existing species is H. sapiens. See Neanderthal and

Cro-Magnon.
HOMOLOGUE (G. homologeo, to agree), a structure which agrees with an-
other in genetic origin. Adjective, homologous. See analog}'.
HORMONE (G. hormao, to excite), "a chemical messenger" secreted by an

organ and influencing another.
HYAL (G. hyalos, glassy), clear or translucent; as hyaline cartilage.
HYOID (G. y, plus eidos, form, like the letter upsilon), derived from or

pertaining to the hyoid bone — developmentally the second visceral

arch.

APPENDIX III 371

HYP, HYPO (G. hypo, under, below), a prefix in hypoglossal, hypophysis,
hypo-secretion.

HYPAXiAL (G. hypo, plus axis), see epaxial.

HYPER (G. over, above), a prefix, hyperpharyngeai, hyper-secretion.

HYPERTROPHY (G. hypcr, plus trcpho, nourish), an overgrowth of an organ
or structure. See atrophy.

HYPOMERE (G. hypo, plus meros, a part), the ventral part of the primi-
tive mesodermal somite, giving rise to the somatopleure and splanch-
nopleure. See epimere.

HYPOPHYSIS (G. hypo, plus phyo, to grow), a ventral outpocketing; spe-
cifically, the ventral evagination of the diencephalon which meets
Rathke's pouch from the stomodeum, the two developing into the
pituitary gland.

Hyracoidea (G. hyrax, a shrew-mouse), an order of mammals, the ''con-
ies", including the type genus Hyrax.

ICHTHY (G. ichthys, a fish), a combining form, as ichthyology, Pter-

ichthys.
IcHTHYOPSiDA (G. ichthys, plus opsis, appearance), vertebrates which

resemble fish, the fishes and amphibia.
IcHTHYOSAURS (G. ichthys, plus saurus, lizard), an order of extinct, aquatic

reptiles.
IGNEOUS (L; ignis, fire), formed through the action of heat; igneous rocks

are consolidated from a molten state. Those which solidify at the

surface are volcanic.
ILEUM (G. eilo, to twist), the posterior portion of the small intestine,

meeting the large intestine at the ileo-coHc juncture.
ILIUM (L. flank), a bone of the pelvis.
ixciDO (L. to cut into), incisure, incisor.

IXFUNDIBULUM (L. a funnel), the stalk by which the pituitary is at-
tached to the brain.
INGUINAL (L. iiiguin, groin), pertaining to the groin; the inguinal region,

or inguinal canal. The mammalian testicles descend through the

inguinal "canals to the scrotum.
INTEGUMENT (L. iutego, to cover), the skin of a vertebrate and its

derivatives.
INTER (L. between), prefix; intercostal, intervertebral.
INTRA (L. within), prefix, intracellular.

1NV.\GINATI0N (L. in, in to, plus vagina, a sheath), an inpocketing or fold-
ing in of a structure; as, the invagination of the vegetal pole of a

blastula to form the gastrula.
ISO (G. isos, equal), a prefix implying equal degree; as isotonic, isogamy.
ITER (L. a passage) ; specifically, the ventricle of the midbrain, which

in the mammals is a narrow tube between larger ventricles.

JEJUNUM (L. jejunus, hungry-), that portion of the small intestine of the

mammal, extending from the duodenum to the ileum.
JOINT (L. junctus, from jungo, to join), an articulation between two

bones, the structure of the joint limiting the motion between the

parts.
JUGAL (L. jugum, a yoke), a bone of the skull assisting in the formation

of the zygomatic arch.

372 APPENDIX III

JUGULAR (L. jugulum, the collar bone), the major vein of the neck,
draining the head region, and homologous with the anterior cardinal.

Jurassic (from Jura Mts. between France and Switzerland), the middle
period of the Mesozoic Era, between the Triassic and Cretaceous.

LABIA (L. labium, a hp), a Hp or lip-hke structure; the labia of the
cerebral hemispheres, over the corpus callosum; the structures sur-
rounding the mammalian female genitalia — the labia majora and
minora, homologous with the scrotum and erectile bodies of the male.

Lacertilia (L. lacerta, Hzard), an order (or sub-order in some classifica-
tions) of reptiles including lizards.

LACRiMA (L. a tear), a combining form, denoting tears or structures as-
sociated with them; lacrimal glands, lacrimal bone, lacrimal duct.

LACUNA (L. lacus, a basin or lake) ; specifically, the spaces in cartilage or
bone containing the individual cells.

LAMPREY (L. lambo, lick, plus petra, rock), See Petromyzontia.

LARYNGE.AL (L. larynx, a gullet), adjectival form of larynx; laryngeal
cartilages.

L.ARYNX (L. gullet), the cartilaginous and bony structures surrounding
the glottis, containing the vocal cords.

LATissiMUS (L. broadest), as latissimus dorsi, a muscle of the back.

LATUS (L. side or broad), a stem and combining form; lateral, lateralis,
along the side.

LEPis (G. a scale), combining form; Cheirolepis, Lepidosteus.

Lepidosiren (G. lepis, plus siren, a mermaid), a South American genus of
dipnoan fish, the most modified of the lung fishes.

Lepidosteus (G. lepis, plus osteon, bone), a genus of Holostean fish with
bony plates arranged in a tile-like manner; the gar-pike and its
relatives.

levator (L. levo, to raise), a muscle which raises a structure; the Levator
scapuli; the levators of the jaw; or costal levators.

LiNEA (L. a line) ; in anatomy, usually a connective tissue line separating
two muscle groups; linea alba.

lingual (L. lingua, tongue), pertaining to the tongue; lingual muscles;
lingual glands.

LiTHOS (G. stone), a combining form; otohth; lithographic limestone.

lumbar (L. lumbus, loin), the body region between the thoracic and
sacral regions.

LUNA (L. moon), a combining form; lunatum; semilunar valves.

LUTEA (L. claj'-yellow), a descriptive word; macula luteum; corpora
lutea.

LYMPH (L. lympha, clear water), blood fluids which have passed through
the capillary walls and into the lymphatic vessels; it lacks the
erythrocytes, and clots more slowly than blood.

LYMPHOCYTE (L. lympha), leucocytes which develop in the lymph nodes,
the smallest of the white cells.

MACRO (G. makros, long, large), a prefix, opposed to micro; as, macro-
scopic anatomy.

MALLEUS (L. a hammer), descriptive of the shape in the mammalian ear
ossicle; develops from the posterior end of Meckel's cartilage; homolo-
gous with the articular bone.

APPENDIX III 373

Malpighian (from Malpighi, an Italian anatomist, 1628-1694) ; Mal-

pighian (renal) corpuscle.
MAMMA (L. breast), the breast of a mammal; a combining form, mam-
mary glands, mammary artery.
Mammalia (L. viavima), a class of vertebrates with mammary glands

and hair.
MAN.\TEE (Spanish manati, a sea cow), a genus of aquatic mammals,

Order Sirenia.
MANDIBLE (L. mandibula, jaw), the lower jaw of a vertebrate.
Manis (L. manes, good), a genus of Old World edentates; the scaly

ant-eater; a mammal covered with large horny scales.
Marsupialia (L. marsupium), a sub-class and order of mammals which

lack a placenta, and have an abdominal pouch in which the immature

young remain for some time after birth.
MARSUPIUM (L. a pouch), a combining form, marsipobranch, marsupial.
MASSETER (G. mascter, a chew), one of the larger muscles of the jaw.
MASTOID (G. mast OS, breast, plus eidos, form), a rounded protuberance,

as the mastoid process of the skull.
MAXIMUS (L. greatest).
MEATUS (L. meatus, a passage, from meo, to go), any conspicuous

passage, as the external meatus of the ear.
Meckel's cartilage (from Meckel, a German anatomist, 1781-1833), the

cartilage of the lower jaw of the vertebrates; above the elasmobranchs

the cartilage is surrounded by dermal bones; its posterior end ossifies

as the articular bone. See malleus.
medius (L. middle) ; as, median, toward the middle.
meninges (G. meninx, a membrane) ; plural of meninx.
meninx (G. a membrane), a connective tissue covering of the central

nervous system; pia mater, arachnoid, and dura mater in the

mammals.
mero (G. meros, part), a combining form; epimere, h3Tomere.
MEROBLASTic (G. meros, plus hlastos, germ), a type of cleavage in which

only the protoplasm divides, leaving the yolk mass uncleaved; typical

of large yolked eggs.
MES, MESO (G. mesos, middle), a prefix; mesorchium, mesoderm.
MESENTERY (G. mesos, plus cnteron, intestine), the band of tissue which

suspends the viscera, developing from the hypomere, and continuous

with the peritoneum.
MESODERM (G. mesos, plus derma, skin), the middle layer of the three

primary germ layers, lying between the ectoderm and endoderm.
MESOMERE (G. mesos, plus meros, a part), the small portion of the meso-
dermal somite connecting the epimere with the hyomere; equivalent

of nephrotome.
MESONEPHROS (G. mesos, plus nephros, kidney), the middle kidney of

the vertebrate; it develops posterior to the pronephros, and at a

later date; and anterior to the metanephros; the functional kidney of

the anamniotes.
MET, META (G. over, after, on the farther side of), a combining word. See

following definitions.
METANEPHROS (G. meta, plus nephros, kidney), the functional kidney ot

amniotes, partly serially homologous with the more anterior kidneys.

See mesonephros.

374 APPENDIX III

METAPLEURAL (G. meta, plus pleura, side), specifically the folds of tissue
along the sides of Amphioxus; metapleural folds.

MINIMUS (G. least), used as a limiting adjective.

MINOR (L. less), descriptive adjective; as pectoralis minor or p. major.

Miocene (G. meion, less, plus kainos, recent), a geological period of
Tertiary (Cenozoic) times, between the Oligocene and Pliocene.

MississiPPiAN (from Mississippi Valley where it was first described), a
period of late Paleozoic time; the lower strata of the rocks formerly
included as the Carboniferous Period.

MITRAL (G. mitra, a turban; a divided bishop's hat), relating to the
bicuspid valves in the left auriculo-ventricular opening of the mam-
malian heart.

MOLAR (L. molaris, belonging to a mill), the posterior, permanent, teeth
of a mammal.

MONO (G. single), a combining form.

MoNOTREMATA (G. moiio, single, plus tremato, an opening), a sub-class of
mammals from Australasia; egg laying mammals with a cloaca, or
single opening for the anus and urinogenital outlets.

MORPH (G. morphe, form), a combining word.

MORPHOLOGY (G. morphe) , the study of structure or form; more fre-
quently u^ed in the sense of developmental anatomy.

MuLLERiAN (from Miiller, a German anatomist, 1801-1858), specifically
the Miillerian ducts of the mammal embryo which give rise to the
uteri; same as oviduct.

MusTELUS (L. mustela, weasel), a genus of sharks; the smooth dogfish.

MYELON (G. marrow), a combining form referring to marrow or a marrow-
like substance; the myelin sheath of neurones; myelocytes.

MYo (G. mys, muscle), a combining form; myology, the study of muscles;
myocomma; myotome.

MYOCOMMA (G. mys, plus komma, a segment), a connective tissue band
separating two myotomes.

MYOTOME (G. mys, plus tomes, a cut), a muscle segment or somite.

Myxine (G. myxa, slime), a genus of myxinoid cyclostomes.

Myxinoidea ((^. myxa, plus eidos, like), a sub-class of cyclostomes, in
many respects more primitive than the lampreys; the "sHmy eels".

MYzoN (G. sucker), a combining form; Petromyzon, the lamprey or
"stone sucker".

NARis (L. nostril), singular of nares; the opening of the air passages, both

internal and external.
Neanderthal (a valley in Prussia), an extinct race of man; H. neander-

thalensis.
Necturus (G. nektos, to swim, plus oura, tail), a genus of urodele

amphibia with external gills throughout life; a typical form in labora-
tory dissections.
neo (G. neos, new), a combining form referring to recent development.
neop.\llium (G. neos, phis palli, a mantle or cover), the cover of the

cerebral hemispheres which has a cortical region; opposite of archi-

pallium.
NEPHROS (G. kidney), a combining form; mesonephros; nephric tubule.
nephrostome (G. nephros, plus stoma, mouth), the ciliated opening of a

pronephric or mesonephric tubule into the coelomic cavity.

APPENDIX III 375

NEPHROTOME (G. Tiephros, plus toma, a cut), the anlage of the kidney

tubules; the mesomere.
NEUROCOEL (G. neuTon, a nerve, pkis koilos, hollow), the cavity of the

nerve cord; homologous, and continuous, with the ventricles of the

brain.
NEURON (G. a nerve), a combining form; neurone; neurology.
NiCTiT.ATiNG (L. nicititatus, from nicto, to wink), the third eyelid; moving

from the anterior (median) margin toward the posterior; usually

small in the mammal.
NOTO (G. iiotos, the back), a combining form; notochord.
NOTOCHORD (G. Tiotos, plus chordc, a string), the endodermal rod of tissue

ventral to the nerve cord of the Chordata ; it becomes surrounded (or

obliterated) by the mesodermal vertebrae.
NUCHA (L. nucha, nape of neck), a combining form in reference to the

neck region; nuchal ligament; nuchal flexure of the brain.

OBLONGATA (L. oh, before, plus longus, long), specifically the medulla

oblongata.
OBTURATOR (L. ohturo, to close), a structure that closes off a passage;

specifically, the structures which close the obturator foramen of the

pelvis.
OCCIPUT (L. back of head); as adjective, the occipital region; or occipital

bones, which surround the foramen magnum.
ocuLO (L. oculus, eye), a combining form; oculomotor; oculist.
ODON (G. odous, tooth), a prefix referring to teeth; as a suffix, don, or

donl; odontoblast; homodont; thecodont.
OLFACTORY (L. olfactorium, a smelling bottle), pertaining to the sense of

smell; olfactory nerves; olfactory capsule.
Oligocene (G. oligos, few or small, plus kainos, new), a geological period

of Cenozoic time, following the Eocene.
OMENTUM (L. oment, a fold), an extension of the mesenteries which folds

over the viscera.
OMO (G. shoulder), a combining form; omohyoid.
ONTO (G. being), a combining form; ontogeny; ontogenetic.
ONTOGENY (G. OTito, being, plus genesis, origin), the development of the

individual; opposed to phylogeny, the development of the race.
opercular (L. operciil, a cover), a protective cover; specifically, the bony

covering of the gills in fish above the elasmobranchs.
Ophidia (G. ophis, a serpent), a sub-order (or order) of reptiles, including

the snakes.
ophthalmic (G. opJithalmos, eye), of or pertaining to the eye.
oral (L. OS, mouth), pertaining to the mouth.
ORBicuLO (L. orhis, a circle), a circular structure; specifically muscular;

orbicularis oris of the mouth.
ORBIT (L. orbis, circle), specifically the eye socket; orbital ridges.
Ordovician (from Ordo vices, a Celtic tribe of Wales) ; a geological period

of the early Paleozoic, between the Silurian and Devonian.
Ornithorhynchus (G. amis, a bird, plus rhis, nose), a genus of mono-

treme mammals; the ''Duckbill Platypus".
orthogenesis (G. orthos, straight, plus genesis, beginning), the theory

which considers evolution to have progressed in a straight line; rather
than as a fluctuating, branching development.

376 APPENDIX III

ORTHOS (G. straight), a combining form; orthogenesis, Orthoptera, ortho-
pedic.

osTEo (G. osteon, bone), a combining form; osteoblast; osteoclast;
osteology;

OSTEOBLAST (G. osteou, plus blastos, a germ), a bone building cell; opposed
to osteoclast (G. klao, to break), a bone destroying cell.

OSTIUM (L. ostium, a door), specifically the opening of the oviduct into
the body cavity.

OsTRACODERMi (G. ostrac, shell, plus derma, skin), an extinct order of
armored vertebrates; they were formerly classified with the fish, but
are now generally considered as being more related to the cyclo-
stomes.

OTIC (G. otikos, ear), pertaining to the ear; otic capsule.

OTOLITH (G. otikos, ear, plus lithos, stone), a concretion in the utriculus
of the inner ear of some vertebrates, which stimulates impulses by
hitting the sensory hairs.

OVARY (L. ovum, egg), the female gonad; the organ in which the repro-
ductive cells develop.

OVIDUCT (L. ovum, egg, plus ductus, leading), the duct receiving the ova
and conducting them to the outside or to the uterus.

OVUM (L. egg) ; plural ova; the female reproductive cell.

oxYS (G. sharp), a combining form; Amphioxus.

PALATAL (L. palatum, palate), relating to the palate; either hard palate,

or soft palate.
Paleocexe (G. palaios, ancient, plus kainos, new), the most ancient of

the Cenozoic strata; the first geological period following the Meso-

zoic.
PALLIUM (L. jpalli, a mantle), a covering over a cavity; specifically, the

roof of the cerebral hemispheres. See neopallium.
PANCREAS (G. -pas, all, plus kreas, flesh), a digestive gland emptying into

the duodenum, with "islands" of endocrine tissue which secrete

insulin.
PAPILLA (L. a nipple), any small, nipple-like structure; the papillae of the

tongue; or, the mesodermal dental papilla which forms the dentine

of the tooth.
PARA (G. para, besides), a combining form.
PARIETAL (L. paries, wall); the parietal bones of the skull; the parietal

foramen.
PECTORAL (L. pectoralis, breast), relating to the upper thoracic region; pec-
toral muscles.
PELVIC (L. pelvis, a basin), pertaining to a pelvis; the bony girdle of the

posterior limb ; the enlarged part of the ureter attached to the kidney.
PENIS (L.) the male copulatory organ, homologous with the clitoris of the

female.
Pennsylvanian (from Pennsylvania, where first described), one of the

later geological periods of Paleozoic times; the upper strata of the

Carboniferous. See Mississippian.
PERI (G. around), a prefix; peripharyngeal grooves; pericardium.
PERICARDIAL (G. pcri, plus kurdia, heart), a membranous sac containing

fluid surrounding the heart; morphologically a part of the body

cavity.

APPENDIX III 377

PERINEUM (G. perineon (?)), the pelvic region including the digestive and

urinogenital outlets.
PERIPHERAL (G. pei'i, ai'ound, plus phero, to bear), that which lies near the

margin, away from the center; the peripheral nerves, in distinction

from the central nervous system.
Perissodactyl.\ (G. perissos, odd, plus daktylos, digit), an order of

herbivorous mammals, including the horse, rhinoceros, zebra, etc.
PERITONEAL (G. peri, plus teino, to stretch), the membrane covering the

intestines and mesenteries, and lining the body cavities.
Permian (from Perm, a province of Russia), a geological period including

the most recent strata of the Paleozoic Era; it is followed in suc-
cession by the Triassic Period of the Mesozoic.
PETRO (G. petros, rock), a combining form; petrosal, Petromyzon.
Petromyzon (G. petros, plus myzon, sucker), a genus of lampreys. See

Petromyzontia.
Petromyzontia (from Petromyzon, the type genus), a sub-class of cyclo-

stomes, including the lamprej's.
PHAG (G. to eat), a combining form; that which eats, as phagocj'te.
pharyngeal (G. pharynx, throat), the region of the digestive tract be-
tween the mouth cavity and the esophagus; the region from which

the gill pouches develop.
PHYLOGENY (G. pliyloii, tribe, plus genesis, beginning), the history of a

race; opposed to ontogeny; phylogenetic is the adjective.
PHYLUM (G. phylon, tribe), a large group of animals or plants; a major

division of a kingdom; the Phylum Chordata.
PHYSOS (G. a bubble, from phyo, to grow), a combining form; epiphysis;

hj'pophysis.
pia (L. tender), the more deHcate membrane covering the brain, carrying

blood vessels; pia mater.
pineal (L. pinea, a pine cone), an endocrine gland on the dorsal side of

the diencephalon. See epiphysis.
pinna (L. feather), a wing or fin; specifically, the external ear.
Pisces (L. plural of piscis, fish), a class of vertebrates, the fish.
Pithecanthropus (G. pithekos, ape, plus anthropos, man), a genus of

extinct, ape-hke men; the fossilized remains having been found in

Java. P. erectus.
Pituitary (L. pituitarius, from pituiia, phlegm), an endocrine gland of

the brain, derived partly from the diencephalon and partly from a

stomodeal invagination.
PLACENTA (L. a cake, in reference to its discoidal shape), an embryonic
organ of the placental mammals, forming the embryonic attachment
to the uterine wall.
PLANTIGRADE (L. plantn, sole of foot, plus gradior, to walk), referring to
animals which walk with the sole of the foot on the ground; humans
or bears.
Pleistocene (G. pleistos, much, plus kainos, new), the most recent

geological period, the uppermost strata of the Cenozoic.
Plesiosauria (G. pleistos, much, plus sauros, lizard), an order of extinct,

aquatic reptiles.
pleura (G. side), a combining form; metapleural; pleural cavities.
plexus (L, interweaving), a group of interlacing or anastomosing nerves
or blood vessels.

378 APPENDIX III

Pliocene (G. pleion, more, plus kainos, new), a Cenozoic geological period
above the Miocene; the Quarternary (Pleistocene) follows it.

PNEUMA (G. air), a combining form, referring to breathing in anatomy;
Dipnoi, pneumogastric.

PODA (G. pons, foot), a combining form; Apoda, Tetrapoda.

POLY (G. polys, many), a prefix; polydactyl; polyembryony.

PoLYPTERUS (G. polys, plus plerou, fin or wing), a genus of recent crossop-
terygian fish found in Africa.

PONS (L. a bridge), specifically the pons Varolii, a large commissure of the
brain.

PORTAL (L. porta, a gate), veins which break into capillaries before enter-
ing the heart; renal portal; hepatic portal,

POST (L. after, behind), a prefix; posterior; post cardinal.

PRE (L. before), prefix; precava; premolar.

PREPUCE (L. praeputium, from pre, plus G. posthion, penis) the foreskin
of the penis covering the glans; specifically that of the primates.

Primates (L. primus, first), an order of mammals including the lemurs,
monkeys, anthropoid apes and man.

PRO (L. before, in front of), a prefix; procoelous, pronate.

Proboscidea (G. pro, before, plus bosko, to feed), an order of mammals
with a proboscis or prolonged snout, including the African and Indian
elephants.

proctodeum (G. proktos, anus, plus daio, to divide), the region of the
anus lined by invaginated ectoderm.

pronephros (G. pro, plus nephros, kidnejO, the first embr3^onic kidney,
and the most anterior one; it disappears in the adults of all verte-
brates except the cyclostomes.

PROSTATE (G. prostates, one in the front rank), a seminal gland of the
higher vertebrates (specifically the mammal) located near the exit
of the urethra from the bladder; the homologue of the Miillerian
ducts.

Proterozoic (G. protos, first, plus zoon, animal), a Pre-cambrian Era,
later than the Archeozoic, covering an enormous length of time.

Pterodactyla (G. pteron, wing, plus daktylos, finger), a group of extinct
flying reptiles with membranes stretched between the fingers.

pteron (G. wing or fin), a combining form; pterygoid, Polypterus.

PTERYGO-QUADRATE (G. pterou, plus L. quadratiis, square), the cartilage of
the upper jaw in vertebrates; its posterior end ossifies as the quad-
rate bone ; homologue of the incus.

pubic (L. pubes, grown up), the lower hypogastric region, covered with
hair at maturity; pubic symphysis.

pulmonary (L. pulmon, lung), relating to the lungs; pulmonary arteries
or veins.

pylorus (G. pyloros, gate keeper), the ventral portion of the stomach;
pyloric sphincter, the constrictor muscle between the stomach and
duodenum.

QUADRATE (L. Quadratus, square), the bone of the upper jaw articulating
with the articular of the lower jaw. See pterj^go-quadrate.

RADIAL (L. radius, a ray), pertaining to a ray, radial cartilage; radius, a
bone of the forearm on the pre-axial side.

APPENDIX III 379

RAMUS (L. branch), a branch or outgrowth; as. the ramus of the mandi-
ble; the aortic rami of the amphibia and birds; the ramus com-
municans of the peripheral nerves.

RAPHE (G. rhaphe, a seam), a seam-like appearance of an organ; the
median line of the body; the dividing line of the scrotum.

Ratitae (L. ratitiis, like a raft), a sub-class of birds lacking a carina on
the sternum; the non-flying birds including the ostrich, emu, kiwi,
etc.

RECTUM (L. rectus, straight), the lower, enlarged portion of the intestine.

RENAL (L. rencs, kidneys), pertaining to the kidnej^s; renal corpuscles;
renal portal.

Reptilia (L. reptilis, from repo, to creep), a class of vertebrates, covered
with ectodermal scales, including turtles, lizards, snakes, dinosaurs, etc.

RESPIRATORY (L. re, back, plus spero, to breathe), relating to breathing; ob-
taining oxygen from the surrounding medium.

RETINA (L. rete, a net), the cell laj^er of the eye containing the receptors
of light impulses.

RETRO (L. backward), that which is back or behind another structure;
retro-peritoneal.

RHiNAL (G. rhis, nose), relating to the nose or snout; rhinencephalon ;
rhinocoel; rhinitis.

Rhynchocephalia (G. rhynchos, snout, plus kephale, head), an order of
reptiles from New Zealand; the most primitive liA'ing reptiles.

RoDENTiA (L. rodo, to gnaw), an order of mammals with two gnawing
incisors in each jaw; includes rats, beavers, capybara, etc. Rabbits are
usually included in a separate order (Lagomorpha) as they have
four incisors and other divergent characters.

ROSTRUM (L. beak), a projecting snout, or any projecting process; spe-
cifically, the cartilage supporting the snout of the dogfish.

Ruminants (L. rumen, throat), an artificial grouping of mammals, in-
cluding the herbivorous land mammals which chew a cud.

SACCULUS (L. diminutive of saccus, a sac), a small, pouch-Hke structure;

the sacculus of the ear.
SACRUM (L. sacrum, from sacer, sacred; the part offered in sacrifice), the

lower end of the vertebral column, attached to the pelvic girdle;

sacral, pertaining to the sacrum.
SAGITTAL (L. sagitta, an arrow), a median plane along the antero-posterior

axis; a longitudinal section may be to either side of the median line.
Salamandrina (G. salamandra, salamander), a group of urodele am-
phibia, including the salamanders which lose their gills in the adult

stage, and are generally land living.
SALIVARY (L. saliva, spit), pertaining to the glands which secrete saliva.
Sauropsida (G. sauros, lizard, plus opsis, appearance), a group term which

includes the reptiles and birds.
SAURUS (G. sauros, a lizard), a combining form, referring to lizard, or

lizard-like; Ichthyosaurs ; Dinosaurs, etc.
SCLERA (G. skleros, hard), a combining form, implying that which is

hard, or which gives rise to hard structures; sclerotic, sclerotome.
SCLEROTOME (G. sklcros, hard, plus toma, a cut), that part of the epimere

which gives rise to the vertebral column and connective tissues.

380 APPENDIX III

SCROTUM (L. scrotum, pouch), the pouch in which the mammahan testes
he; the scrotum is homologous with the labia majora of the female.

SEDIMENTARY (L. sedimentum, settling), in geology, referring to the rocks
which have been deposited in water or land depressions. The original
materials may be carbonates, mud, sand, or gravel which are
cemented or pressed into rock formations. They are the fossil bearing
rocks.

Selachii (G. selachos, a shark), a group of elasmobranch fish, including
the sharks.

SELLA (L. a seat or saddle), sella turcica, the depression of the skull in
which the pituitary rests.

SEMI (L. half), combining form; semilunar, semipermeable, semispinahs.

SEMILUNAR (L. semi, plus luna, moon), half moon-shaped ; specifically the
valves guarding the orifices of the arteries leaving the heart.

SEMINAL (L. semen, seed), pertaining to the fluid medium of spermatozoa,
secreted by the testicular tubules and the glands associated with the
ducts; semen is the fluid.

SEPTUM (L. sepes, a fence), a dividing partition; the septum between
the auricles; myosepta.

Silurian (from Silures, a tribe in England), a geological period of the
Paleozoic, above the Ordovician, and below the Devonian.

Sinanthropus (G. Sinai, Chinese, plus anthropos, man), a genus of ex-
tinct men, a skull of which was discovered near Peiping.

SINUS (L, a bend or curve), in anatomy, a cavity in a bone or an en-
largement of a blood vessel; the sinuses of the head and face are
cavities in the bones, as the frontal or maxillary sinuses; also,
lymphatic sinuses; sinus venosus.

Sirenia (G. seiren, a mermaid), an order of herbivorous aquatic mammals,
including the dugong and manatee.

SOMA (G. body), a combining form; somatic, somatopleure.

soMATOPLEURE (G. soma, plus plewa, side), the laj^er of the hypomere
lying next the ectoderm; it gives rise to the somatic peritoneum and
a part of the mesenteries.

SOMITE (G. soma, body), a body segment; a metamere.

Sphenodon (G. sphen, a wedge, plus odoiis, tooth), a genus and only
living representative of the order Rhynchocephalia, the most primitive
recent reptile.

SPHENOID (G. sphen, a wedge, plus eidos, form), a wedge-shaped bone or
process; specifically, the sphenoid bones of the skull, developing from
the chondrocranium.

SPIRACLE (L. spiraculum, an air hole), the modified first gill slit of the
sharks; the structure was carried over to the Crossopterj'gii, and to
the tetrapods as the middle ear and Eustachian tube.

SPLANCH (G. splanchnon, viscera), a combining form; splanchnic, splan-
chnopleure.

SPLANCH NOPLEURE (G. splancknon, plus pleura, side), the layer of the hypo-
mere in contact with the endoderm; it encloses the archenteron and
gives rise to the smooth muscle, mesentery, and visceral peritoneum.
The anterior part develops the heart and branchial (visceral) muscle.
Squalus (L. fish), a genus of small sharks; Squalus acanthias is the spiny
dogfish.

APPENDIX III 381

SQUAMA (L. a scale), a combining form; squamous epithelium.

Squamata (L. squama), an order of reptiles including the lizards and

snakes; it is sometimes divided into Lacertilia and Ophidia.
STAPES (L. a stirrup), one of the ear ossicles; the name is descriptive of

the shape in the mammal. The stapes is homologous with the

columella, and originally the hyomandibular.
STEGO (G. cover), a combining form; Stegosaur, covered with plates.
Stegocephalia (G. stego, plus kcphalc, head), an extinct order of am-
phibia; the group from which the more recent ones developed.
STERNO (G. breast), referring to the breast. The sternum is the unpaired

bone in the median ventral line to which the ribs arc attached.
STOMA (G. mouth), a combining form; stomodcum, nephrostome.
STOMODEUM (G. stoiiia, plus daio, divide), the ectoderm-lined region

of the mouth cavity.
STRATUM (L. spread out, from sterno, to spread). In anatomy, a layer or

sheet of tissue, as stratum Malpighii. In geology, a layer or sheet of

sedimentary rock. Stratified refers to a series of layers, one above the

other.
STRIA (L. a furrow), used alone or as a combining form; corpora striata;

striated muscle.
SUB (L. under), a prefix impMng under the following stem; subintestinal,

subcla"\'ian.
SULCUS (L. suico, to plow), a groove or furrow of the brain.
SUPER (L. over), a prefix; superior.

SUPRA (L. above), a prefix; supraorbital; supraoccipital.
SYM, SYN (G. together), a prefix; synotic, sympathetic; syncytium.
SYMPHYSIS (G. sym, plus phijo, to grow), a union between two parts;

pubic symphysis.
SYNOTIC (G. syn, plus otikos, ear), the cartilaginous connection between

the otic capsules in the chondrocranium, bounding the foramen

magnum.
SYRINX (G. pipe), the voice box of the birds, located at the bifurcation

of the trachea.

TELEO (G. teleos, whole), a prefix; Teleostei.

TEMPORAL (L. tempus, temple), the region of the skull dorsal and posterior

to the eye.
TENDON (L. teiido, to stretch), a connective tissue band attaching a

muscle.
TENTORIUM (L. tcntori, spread like a tent), a partition of the dura mater

pushing between the cerebellum and the cerebral hemispheres.
TESTES (L. testis), the male reproductive glands; the testis is the gland;

the testicle is the gland with its accessory structures.
THECODONT (G. tJieke, a case, plus odous, tooth) ; referring to teeth which

are embedded in sockets.
Therapsida (G. theros, a wild beast, plus apsis, an arch), an order of

extinct mammal-like reptiles with heterodont dentition.
Theromorpha (G. theros, beast, plus morphos, form), an order of synapsid

reptiles, less mammal-like than the Therapsida.
THORAX (L.), the anterior part of the body enclosed by the ribs, a term

of mammalian anatomy.

382 APPENDIX III

THYMUS (G. thymon, thyme), a gland of lymphoid tissue, arising as
proliferations of cells from the gill pouches, and migrating poste-
riorly (in the mammal).

THYROID (G. thyreos, a shield), an endocrine gland arising as a median
evagination from the floor of the pharynx; homologous with the
endostyle.

TOMA (G. a section), a suffix, meaning a cut or segment; sclerotome;
mj'otome.

TRABECULA (L. a Small beam), a supporting band of connective tissue;
also, the finger-hke cartilaginous processes during the development
of bone.

TRACHEA (G. trachys, rough), the windpipe from the glottis to the bifurca-
tion.

TREMA (G. an opening), a combining form; monotreme, post trematic,
pretrematic.

TRi (L. three), a prefix; trituberculate, tricuspid.

Triassic (G. trias, from treis, three), a geological period of the Mesozoic,
the lowest strata, followed by the Jurassic and Cretaceous.

TUBER (L. a swelling), a rounded swelling or prominence; tuber cinerium.

TUNIC (L. tunica, a tunic), a covering or investing sheet, the tunic of the
testis; also as a combining form, Tunicata.

TuNiCATA (L. tunica), a sub-phjdum of chordates, the Urochordata. The
tunicates have a notochord in the tail region during embryonic life,
then become sessile and develop a tunic.

TURCICA (L. Turcus, Turk), in sella turcica, the Turk's saddle; see sella.

TYMPANIC (G. tympanon, a drum), a drum-like membrane; specifically,
the ear drum, or tympanum.

ULNA (L. ulna, elbow), the post-axial bone of the forearm, against which
fits the humerus; its proximal end is the olecranon process.

UMBILICAL (L. umbilicus, navel), pertaining to the umbilicus; the umbilical
cord connects the embryo with the placenta.

Ungulata (L. ungula, a hoof), an artificial grouping of mammals, includ-
ing those which walk on the tips of their toes (the orders Artiodactjda
and Perissodactj'la).

unguligrade (L. ungula, a hoof, plus gradior, to walk), a foot position,
with the sole of the foot entirely off the ground, and standing on the
tips of the toes.

UNI (L. one), a prefix; unilateral, uniform.

URA (G. oura, tail), a combining form; Urodela, Anura.

URETER (G. ouron, urine), the metanephric duct leading from the kidney
to the urinary bladder.

URETHRA (G. ouron, urine), the duct leading from the bladder to the
outside; in the male it is joined by the vasa deferentia.

URINOGENITAL (L. wina, urine, plus genitalis, genital), the organs as-
sociated with the excretory and reproductive functions.

Urodela (G. oura, plus delos, evident), an order of amphibia with a
well developed tail, including salamanders, newts, and the gilled
amphibia.

UTERUS (L. the womb), a mammalian structure homologous with the
oviducts, in which the embryos develop.

APPENDIX III 383

UTRicuLus (L. a little bag), the cavity of the inner ear, from which the
semicircular canals arise.

VAGINA (L. a sheath), in mammalian anatomy the outer part of the
reproductive canal; it is homologous with the ends of the oviducts; it
is connected with the uterus through the cervical opening.

VAGUS (L. wandering), the tenth cranial nerve.

VAS (L. a duct), specifically the ducts connected with the testes; the
vasa efferentia form the epidid3'mis; the paired vasa deferentia con-
duct the sperms to the urethra.

VEXA (L. a vein), as vena cava; venous; sinus venosus.

VENTRICLE (L. diminutive of venter, belly) ; specifically the cavities of the
brain; or the muscular chambers of the heart.

VERMIS (L. a worm), as the vermis of the cerebellum; vermiform ap-
pendix.

VERTEBRA (L. a joint), the single structural units of the vertebral column.

VILLI (L. villus, shaggy hair), the hair-like processes lining the intestine.

VISCERA (L. viscus, an internal organ), the internal organs, including the
digestive tract and the glands.

VITELLINE (L. vitellus, yolk), specifically the paired embryonic veins
which form the heart.

VITREOUS (L. glassy), in vitreous humor of the eye.

"Wolffian (from Wolff, a German anatomist, 1733-1794), the mesonephric
ducts of the amniotes from which develop the vasa deferentia. Also,
the mesonephros, or Wolffian body.

zooN (G. animal), a combining form; spermatozoa; zoologj'; Cenozoic.

ZYG, ZYGO (G. zygon, yoke), combined in zygomatic arch, zygote, etc.

zygomatic arch (G. zygon), the cheek bone of the mammals and mam-
mal-like reptiles; homologous with the ventral margin of the skull,
and formed by the enlargement of the lateral fenestra of the skull.

INDEX

Abducens nerve, 296

Acetabulum, 164

Adaptive Radiations, Def., 7

Adrenal glands, 318

Age of Reptiles, 80

Albinism, 116

Allantois, 56, 67, 68, 92, 228

Alligator, 61

Dermal plates, 112

Skull, 135

Vascular system, 234
Amblystoma, 49, 52
Amnion, 56, 67, 345
Amphibia, 48

Cleavage, 345

Dermal plates, 110

Evolution, 344

Glands, 111

Muscles, 187

Xares, 132

Ribs, 155

Skull, 133

Vascular system, 235

Vertebrae, 149
Amphioxus, 14 ff.

Embryology, 21

Muscles, 181

Skin, 107

Spinal nerves, 277

Vascular system, 221
Analogy, Def., 7
Anapsida, Skull, 134
Annelid Theory, 341
Anthropoidea, 82
Anthropoids, Evolution, 352
Antilocapra, 121
Antlers, 121
Anura, 52

Aorta, Branches of, 38
Aortic Arches, 232
Apoda, 50

Dermal plates, 110

Aponeurosis, 178
Appendages

Aquatic, 166

Bipedal, 168

Cursorial, 168

Flying, 169

Homology, 165

Origin, 156
Appendicular Skeleton 125
Appendix, vermiform, 202, 204
Apteryx, 72

Aquatic adaptation, 166
Arachnid Theory, 342
Arboreal adaptation, 169
Archaeopteryx, 71

Evolution, 347

Wing, 170
Archegosaurus, 50

Skull 133
Archenteron

Amphioxus, 22

Frog, 53

Reptile, 66
Archipallium. 288
Armadillo. 87
Artiodactyla, 87
Atrium, Amphioxus, 27
Auditory nerve, 297
Auditory organs, 309
Aves, 71
Axial skeleton, 125

Balanoglossus, 11
Barriers, geographical, 326
Bats, 86

Wing, 170
Bifocal vision, 308
Bilateral symmetry, Def., 5
Bipedal adaptations, 168
Birds, 71

Embryology, 75
Integument, 114

385

386

INDEX

Bladder, urinary, 257
Blastopore

Amphioxus, 23

Frog, 53

Reptile, 66
Blastiila

Amniote, 64

Amphioxus, 22

Frog, 53

Reptile, 64
Blood, 103

Islands, 226

Structure, 246
Body stalk. 67
Bone

Cells, 172

Malformations, 173

Structure, 173

Tissue, 102

Types, 171
Bowman's capsule, 253
Brain

Comparative Anatomy, 285

Development, 272

Flexures, 273
Branchial arches, 143, 147

Derivatives, 232

Dogfish, 223
Branchial arteries, 221, 232 ff.

Evolution, 232 ff.
Branchial skeleton, 206, 216 ff.
Breathing, 215
Bronchioles, 214
Buffon, 323

Caecum, digestive, 202
Calvarium, 134
Carina, 72
Carinates, 72
Carnivora, 84, 85
Cartilage, 101
Cauda equina, 279
Cephalization. 5, 278
Cephalochordata, 15
Cerebellum, 292
Cerebral Hemispheres, 288
Cetacea, 90

Appendages, 167
Cheiroptera, 86
Chelonia, 57
Chevron bones. 150

Chondrocranium

Cyclostome, 30

Development, 125

Ossification, 128
Chondrostei, 40, 344

Dermal plates, 110

Skull, 128, 129
Chordata, Def., 3

Classification, 8
Choroid layer, 307
Cladoselache, 37, 343

Appendages, 156

Pelvic fin, 163
Classification, 10
Clavicle, 163
Claws

Mammal, 115

Reptile, 113
CKtoris, 262
Cloaca, 76, 194, 203
Cochlea, 311
Coelomic cavity

Amphioxus, 24

Vertebrate, 195
Colon, 202
Commissures, 293
Connective tissues, 100
Coracoid, 158, 161
Cornea, 306
Corneum, stratum, 108
Corpora quadrigemina, 291
Corpus callosum, 293
Corpus striatum, 288, 290
Cranial nerves, 294
Cretinism, 317
Cribriform plate, 138
Crocodiha, 60, 61, 135
Cro-Magnon Man, 354
Crossopterygii, 44, 344, 345
Dermal plates, 110
Fin, 159
Gills, 210
Larva, 161
Ribs, 154
Skull, 48
Cryptobranchus, 50, 51
Cursorial adaptations, 168
Cyclostomes, 29
Brain. 285

Chondrocranium, 126
Glands, 111, 112
Intestine, 201

INDEX

387

Cyclostomes — Continued
Muscles, 181
Spinal nerves, 277
Urinogenital system, 258, 259
"Vertebrae, 148

Darwin, Charles, 323
Decussation, 293
Degenerate, Def., 7
Dental formula, 140
Dermal bone. 103, 110

of skull, 131
Dermal denticles. 35, 108
Dermal plates. 109 ff.

Ostracodermi. 33

Teleostomi, 39
Dermis, 107

Development, regulators, 313
Diaphragm, 216
Diapsida, 60
Diencephalon, 290
Digestive glands, 318

Development, 194
Digestive system

Amphioxus, 16

Development, 193

Human, 203
Digestive tract, modifications of, 196
Digitigrade. 168, 169
Dipnoi, 45. 344

Gills, 210
Dogfish

Brain, 286

Chondrocranium, 126

Dermal denticles, 108. 109

Digestive tract, 198, 201

Fin, 159

Gills, 209

Muscles. 183

Urinogenital system, 258

Vascular system, 222

Vertebrae, 148
Dura mater, 274

Ear, external. 312
Ear, inner. 309

Cyclostome. 32
Ear, middle, 144, 311
Ear ossicles, 145 ff., 311, 349

Homologies. 147
Echidna, 77, 78
Edentates, 86

Elasmobranchii, 36
Elephant, 89

Tusks, 143
Embryology

Amphioxus, 21

Bird, 75

Cyclostome, 32

Frog. 52

Marsupial, 79

Placental. 92

Reptile, 64
Enamel, 108
End organs, 301
Endocrine glands, 316, 320
Endolymph, 309
Endostyle

Amphioxus, 16

Tunicate, 13
Enterocoels, 24
Eogyrinus, 345
Epaxial muscles, 175
Epidermis, 107
Epididymis, 262
Epimere

Amphioxus, 24

Development, 175
Epiphysis

Bone. 172

Brain. 290
Epithelial tissues, 99
Erythrocytes, 247
Esophagus

Development, 193

Structure, 199
Eustachian tube, 145, 206, 207, 311
Eutheria, 80
Excretory organs, 251
Extensors. 181
External gills

Crossopterygii, 44, 45

Dipnoi. 45. 46

Tadpole, 55
Eye, 304

Cavities, 308

Embryology. 305

Function. 308

Lids, 309

Muscles. 296

Structure, 306

Facial nerve, 297
Feathers, 114

INDEX

Fenestra, of skull, 135
Filiim terminale, 279
Finfold theory, 156
Fins, origin, 156
Fishes, 35
Flexors, 181
Flexures of brain, 273
Flying adaptations, 169
Fontanelles, 138
Fossils

Microscopic anatomy, 334

Preservation, 332
Friction ridges, 123
Frog embryology, 52

Gastrula

Amniote, 64

Araphioxus, 22

Frog, 53

Placental, 92
Generalized, Def., 6
Genitalia

Development, 268

Female, 262

Male, 264
Geological Periods, 336
Geological time table, 335
Gila monster, 61
Gill pouches, derivatives of, 207
Gill slits, 4

Amphioxus, 17

Origin, 205

Primary, 28

Reptile, 56
Gills

External, 210

Internal, 207

Septum, 208
Glands

Digestive, 194

Endocrine, 316 ff.

Genital, 268

Mammalian, 118

Salivary, 199
Glands, types. 111, 112
Glans, 264
Glomerulus, 251
Glossopharyngeal nerve, 298
Gonads, 251

Function, 319
Gubernaculum, 267

Hair

Distribution, 123

Structure, 116
Hard palate, 132, 303
Haversian system, 102, 173
Heart

Cardiac cycle, 236

Comparative Anatomy, 229

Dogfish, 222

Embryology, 232

Valves, 229
Hemichordata, 11, 343
Heterocercal tail, 39, 40, 42
Heterodont dentition, 139
Holoblastic cleavage, 64
HolocephaH, 37
Holostei, 40, 344

Dermal bone, 110
Hominidae, 84
Homocercal tail, 42
Homodont dentition, 139
Homology, Def., 7
Hoofs, 115
Horns, 120

Human skull, 137, 138
Hyomandibular, 128
Hypaxial muscle, 175
Hypoglossal nerve, 299
Hypomere, 24, 175, 195

Ichthyosaurs, 166, 347
Igneous rocks, 330
Infundibulum, 291
Inguinal canal, 267
Insectivora, 81
Intestines, 201

Jaws

Articulation, 146
Elasmobranch, 128
Evolution, 131
Origin of, 144

Kangaroo, 79
Kidney, 256

Lacertilia, 61
Lacteals, 244
Lamarck, 323
Land bridge, 327
Lanugo, 124

INDEX

389

LarjTix, 213, 216

Lateral Line Organs, 301

Lemuroidea, 82

Leucocytes, 246

Limb buds, 156

Liver, 194, 195

Lizards, 61

Lungs

Anatomy, 213

Development, 215

Evolution, 211
Lymph Hearts, 244
Lymph nodes, 245
Lymphatic system, 243

Development, 245

Malpighian corpuscle, 255
Malpighian layer, 107
Mammal-like reptiles, 59

Skull, 135

Teeth, 139
Mammals, 76

Embryology, 92

Evolution, 348, 350
Mammary glands, 118
Man, Evolution, 351
Manis, 87, 348

Integument, 115
Marsupials, 77

Embryology, 79

Radiations, 328
Meckel's cartilage, 128
Medulla, 293
Meninges, 274
Meroblastic cleavage, 64
^Mesenteries, 195

Mesoderm, Frog development, 55
Mesonephros, 253
Metamerism, Def., 5
Metanephros, 255
Metatheria, 77
Metencephalon, 292
Midbrain, 291
Middle ear, 48, 144, 311
Monkeys, 81, 82, 83
Monotremcs, 76, 78

Mammary glands, 120

Pectoral girdle, 162
Mouth

Development, 193

Structure, 198
Miillerian Ducts, 258

Muscles

Abdominal, 184 ff.

Adductor magnus, 190

Appendicular, 186

Attachment of, 177

Axial, 183

Biceps, 187

Branchiomeric, 185

Caudal, 183

Classification, 175

Costal, 184

Deltoid, 187

Development, 175

Epaxiai, 175

Extensors, 181

Extrinsic, 186

Eye, 296

Flexors, 181

Gastrocnemius, 190

Gluteus, 190

Gross structure, 176

Hypaxial, 175

Ilio-psoas, 184, 190

Integumentary, 190

Intrinsic, 186

Latissimus dorsi, 187

Leverage, 180

Longissimus dorsi, 183

Metameric arrangement, 182

Mimetic, 191

Multifidus, 183

Nomenclature, 176

Panniculus carnosus, 191

Pectorahs, 187

Pelvic, 188

Platysma, 191

Rectus abdominis, 185

Rectus femoris, 190

Sacro-spinalis, 183

Sphincter colli, 191

Trapezius, 187

Triceps, 188
Muscle tissues, 103
Myelencephalon, 293
Myotomes, 17, 175, 182
Myxinoidei, 32

Nails, 115

Nares, 303

Naso-pituitary sac, 30, 32, 303

Neanderthal man, 137, 353

Necturus, muscles, 187

390

INDEX

Neopallium, 288
Nephridia, 250
Nephrotome, 175, 251
Nerve cells, 275
Nerve tissues, 104
Nerve tracts, 281
Nervous system

Brain, 285

Development, 271

Sympathetic, 282
Neural groove, 271

Amphioxus, 22

Frog, 53

Reptile, 66
Neurocoel, 274
Neurone, 275
Notochord, Def., 4

Oculomotor nerve, 296
Olfactory nerve, 295
Olfactory organs, 302, 304
Omentum, 197
Operculum, 133, 208
Ophidia, 63
Opossum skull, 131
Optic capsule, 126, 127, 306
Optic chiasma, 296, 308
Optic lobes, 291
Optic nerve, 295
Optic vesicle, 305
Organ formation, 105
Organizators, 314
Origin of vertebrates, 340
Ornithorhynchus, 77, 78
Osteoblasts, 172
Osteoclasts, 172
Ostracodermi, 30, 32
Ostrich, 72
Ovary, 260, 319
Oviparous, 37, 61
Ovo-viviparous, 37, 61, 260
Owen, 299

Paedogenesis, 52
Palaeospondylus, 30, 148, 337
Pallium, 288
Pancreas, 194, 318
Parapsida, 60

Parathyroid glands, 207, 317
Parietal eye, 290
Pectoral girdle, 158
Pelvic girdle, 163, 164

Penis, 262, 264
Perameles, 95
Pericardium, 229
Periosteum, 172
Perissodactj'la, 89
Peritoneum, 195
Petrifaction, 333
Petromyzon, 31, 32
Pharynx, structure, 198
Pia mater, 274
Pigment, 115
Piltdown man, 353

Pineal gland, 291, 316

Pithecanthropus, 137, 352

Pituitary gland, 139, 291, 316

Placenta, 80, 93

Placentals, embryology, 80, 92

Plantigrade, 169

Polypterus, 44

Porpoise, 90

Primates, 81

Primitive, Def., 6

Proboscidea, 89

Pronephros, 251

Prototheria, 76

Pterodactyl, wing, 170

Pterygo-quadrate, 128

Pyloric sphincter, 201

Ramus communicans, 283
Ratites, 72

Rattlesnake, 62, 63, 122
Rays, 37, 38
Receptors, 300
Rectum, 202
Renal portal vein, 240
Replacement bone, 110
Reptiles

Age of, 347

Embrj-ology, 64

Evolution, 346
Reptilia, 56
Respiration, Def., 205
Retina, 306, 307
Rhinoceros, 89, 122
Rhynchocephalia, 60
Ribs, 154
Rodentia, 85

Sacculus, 310
Salivary glands, 199
Sauripterus, 159, 345

INDEX

391

Scales

Dermal, 110
Mammal, 115
Reptile, 113
Scapula, 158, 163
Sclerotic layer, 308
Scrotum, 267

Sebaceous glands, 117, 118, 330
Sedimentary Rocks, 330
Segmentation cavity, 21
Sella turcica, 138
Semen, 267

Semicircular canals, 309
Sex differentiation, 268
Sex glands, 319
Seymouria, 58, 134, 346
Sinanthropus, 353
Sirenia, 90
Skates. 37, 38
Skeleton, development, 171
Skin, 107

Human, 122
Skull

Dermal bones, 131
Development, 125
Evolution, 134
Fenestrae, 135
Reptile, 346
Snakes, 62, 63
Somatopleure, 195
Specialized, Def., 6
Sphenodon, 61

Spinal accessory nerve, 298
Spinal cord, structure, 277
Spinal nerves, 277
Spiracle, 39, 144
Spiral valve, 39, 201
Splanchnopleure, 195
Spleen, 195
Stegocephalia, 50, 345
Dermal plates, 110
Pelvis, 164
Skull, 134
Vertebrae, 149
Sternum, 154
Stomach
Development, 193
Modifications, 199
Strata, formation, 331
Sweat glands, 118, 123
Swim bladder, 40, 45, 211
Sylvius, aqueduct of, 292

Sympathetic system, 287
Synapsida, 60
Synotic tectum, 126
Systematic, Def., 97
Systemic, Def., 97

Taste buds, 302
Teeth, 139 ff.
Teleostei, 42, 344
Teleostomi, 39
Tendons, 178
Terminal nerve, 295
Testes

Descent, 267

Function, 319
Therapsida, 59

Jaws, 131

Skull, 136
Theromorpha, 59, 60

Jaws, 131

Skull, 136
Thinopus, 160, 161
Thorax, 155
Thvmus, 207, 317
Thyroid, 33, 207, 316
Time table, geological, 335
Tongue, 199
Trigeminal nerve, 297
Trochlear nerve, 296
Turbinal bones, 303
Tunicates, 13
Turtles, 57

Skull, 134
Twinning, 314

Unguligrade, 169

Ureter, 255

Urethra, 257

Urinogenital svstem, development,

258
Urochordata, 13, 343
Urodela, 50
Uterus, 262
Utriculus, 310

Vagina, 262
Vagus nerve, 298
Valves, of heart, 231
Vascular system

Amphioxus, 19

Development, 229

Dogfish, 222

392

INDEX

Vascular system — Continued

Embryonic, 226

Heart, 229

Human, 242

Lymphatics, 243

Ventral aorta, 221
Vas deferens, 262
Veins

Hepatic portal, 240

Renal portal, 240

Systemic, 239

Vitelline, 226
Ventricles, brain, 274
Vertebrae, 148 ff, 350

Vertebral ganglia, 283
Visceral skeleton, 143
Vitelline veins, 226
Viviparous, 37
Vocal cords, 218

Wallace, 323

Line, 328
Whales, 90

Appendages, 167

Teeth, 122
Wolffian body, 255

Zj'gomatic arch, 136