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PRACTICAL ANATOMY
OF THE RABBIT

AN ELEMENTARY
LABORATORY TEXTBOOK IN
MAMMALIAN ANATOMY

By
Baas BENSLEY,Ph-D:

Professor of Zoology in the University of Toronto

THIRD EDITION

TORONTO:
UNIVERSITY OF TORONTO PRESS
1921

COPYRIGHT, CANADA, 1921
BY
UNIVERSITY OF TORONTO PRESS

PREFACE TO THE THIRD EDITION.

The present edition of the ‘Anatomy of the Rabbit”’ appears
in the same form as previously adopted for the second, but with
some minor revisions and modifications. The necessity of re-
printing the text after a comparatively brief interval of use, and
the thorough revision in passing from the first to the second editions
are together accountable for the fact that the number of corrections
is not greater. Realizing that the ultimate value of a manual ot
dissection depends upon the combined experience of as great a
variety as possible of instructors and students using it, the author
has endeavoured ait all stages of revision to incorporate such new
ideas as have been received, and is appreciative of the interest
taken in the matter by an increasing number of individuals.

The chief features of the text may be summarized as follows:
The practical convenience of the rabbit is recognized as material
for elementary anatomical study in the same way as in other
fields of biological study and investigation. Though the principal
design of the book is to direct the student in an orderly study of the

tructure of a mammal, points of physiological interest have been
included so far as seemed advisable within the limits of a small
manual of anatomichl oyilook. The setting of gross, anatomy, In
respect of microscopic anatomy, embryology; and. the foundations of
evolutionary development in general, has been carefully con-
sidered. While the poirits to which reference is made in the dis-
section are given a restricted description, there is no suggestion to
the student of inadequate and superficial treatment, such as is
common and perhaps necessary in manuals where the study of a
number of types is presupposed as part of the course of instruction.

The book has been planned in part to serve the purpose of
those zoological students who seek to obtain knowledge of a
grade of organization sufficiently near that of the human body
as a foundation for comparative studies, but more especially for
premedical and medical students who by making a preliminary
practical study of a convenient and easily obtained mammal
may obtain thereby a knowledge of the foundations of human

4 PREFACE

structure, together with experience of the laboratory practice upon
which first-hand knowledge of gross anatomy depends. It is with
reference to the latter group that the plan of nomenclature has
been rigidly selected in accordance with the newer and more
uniform terminology now happily established in the texibooks of
human anatomy. The student will be warned against terminology
as an end in itself, as against other short cuts to knowledge, and
if properly directed will place the emphasis upon praciice and
initiative.

B. A. BENSLEY.

University of Toronto,
December Ist, 1920.

CONTENTS.

Page
[ENF RONDICC THON) Sac rae acho ete toc io i en cic oe ere en ee a 7
Part I. A GENERAL CONSIDERATION OF THE
STRUCTURE OF THE RABBIT.
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PHS SIE TONS ORMTHE MOSTERTOR (CEMB:,. = 0: au: Gaia ccs, aS ae eeies sod Aer 165
Part III. DISSECTION OF THE RABBIT.
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APPENDIX. DIRECTIONS FOR THE PRESERVATION OF MATERIAL........... 283

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INTRODUCTION.

As a laboratory exercise the anatomical study of an animal is
chiefly a matter of applying a certain practical method of exposi-
tion, the student’s attention being concentrated on those facts
which can be made out by direct observation. This method is
educative in the technical sense because it involves accurate
discernment of detail, and, as a means of obtaining first-hand
information, it is the foundation of laboratory practice.

In studying the structure of any organism, however, it is to be
considered that the final object is not simply to determine in what
its structure consists, i.e., its anatomy in a restricted sense, but
also to understand what structure signifies, either as functional
mechanism, or as the product of racial or evolutionary factors.
While it is conceivable that a single organism, either as individual
or species, may be considered by itself, a very superficial study
suffices to show that the structure and function of no living organism
can be interpreted apart from the general arrangements of organized
nature, and more especially from the corresponding features of
those organisms most nearly allied in point of resemblance.

This being the case, it becomes a more or less practical question
in comparative study combined with dissection, or other form of
laboratory practice, what the proper procedure should be. So far
as the present book is concerned, it is expected that the study of the
type will begin with the examination of the prepared skeleton
(part II). This will be followed by dissection (part III), in which
the order by sections will be found to be of less importance than
that of details in any particular region. The general matter of
part I is purely accessory, and though necessarily incomplete in
many ways, is designed to afford a comprehensive view of the
various factors upon which mammalian structure depends.

Regional sections of the foetus as figured in part I, or frozen
sections of the adult animal, are a useful adjunct, since they can
be used either for points of general organization, or, being sub-
stantially correct for two dimensions, can be used to remove some
erroneous impressions of the position of organs incidental to their
displacement in dissection.

PART I.

A GENERAL CONSIDERATION OF THE STRUCTURE |
OF THE RABBIT,

DIVISIONS AND METHODS.

Biology, the science or study of living organisms, includes
several related sciences, the chief of which are Anatomy, the study
of organized structure; Physiology, the study of function; and
Embryology, the study of development. Anatomy, or Com-
parative Anatomy, the latter referring to the comparative study
of organisms, and Embryology are also considered either as
divisions, or as practical methods, of Morphology, the general
science of the evolution of form.

The term “Anatomy” was originally applied to the dissection or
study of the human body, and is still considered as referring more
especially to the latter. Even in the early stages of biological
science, however, the use of the term was extended to organisms
generally; and afterwards, chiefly as a result of the introduction
of the microscope as a new method of examining structure, it
attained its present comprehensiveness as a term applying to the
study of structure generally.

It has been found convenient, especially in human anatomy, to
distinguish as Gross Anatomy, the study of that kind of structure
which is displayed by dissection, or is revealed by oe -eye
appearances, and as Microscopic Anatomy, the study of finer
structure through the application of the microscope; or, again, to
distinguish as Special or Descriptive Anatomy, the study of the
particular features of the organs of the body, and as General
Anatomy, the study of its more fundamental composition. General
Anatomy is practically equivalent to Histology, the latter con-
sidering the body from the point of view of the structure and
arrangement of its cells and tissues.

These distinctions are of interest in the present case chiefly as
defining more exactly the practical method and the kind of structure

IO ANATOMY OF THE RABBIT.

to be considered. Thus, dissection is to be recognized as a method
of displaying structure of a gross and special kind. It consists in
the orderly exposure and displacement of organs with the object
of observing their features and relations’ to surrounding parts.
~The plan is essentially one of analysis, since conceptions of structure
are based on the recognition of differences, the latter being estimated
by various features, such as form, color, texture, or position. On
the other hand, because of the class of structure with which it deals,
dissection is also to be recognized as a preliminary method in
comparison with various others involving the use of the microscope.

THE INTERPRETATION OF STRUCTURE.

Gross structure is, in a sense, only the outward expression of the
finer microscopic structure underlying it, the latter being the true
basis of the body. This refers not so much to the individual
features of the organs as to the relation existing between their
appearance as gross objects and their tissue composition. Since
this relation is more fully discussed below under the head of general
anatomy, it need only be mentioned here as an element in the
interpretation of structure as viewed from the gross standpoint.
All animal structure, however, may be considered from two points
of view—physiological and morphological.

The physiological aspect of structure concerns the functions
or activities of the living organism and of its individual parts. The
contraction of a skeletal muscle is a change in the axial relations of
living protoplasm, but the form and connections of the muscle are
such that the contraction results in movement of one bone upon
another. The excretion of urine on the part of the kidneys is the
final stage of a process which rids the body of soluble waste nitro-
genous materials by discharging them into a system of tubes
connected with the outside of the body. What is important in
these, as in a multitude of analogous cases, is that structure and
function are intimately related, and in point of interpretation,
serve to explain one another.

The morphological aspect of structure concerns various
features of form and arrangement which, although they have been
developed on a basis of utility, cannot be explained directly on that

THE INTERPRETATION OF STRUCTURE. II

basis, because the factors controlling them lie outside of the body
of the individual, and are such as have operated only through a long
series of gradually changing conditions in the evolution of its tpye
As applied to a particular animal, the morphological method con-
sists in explaining its adult structure by reference either to its
embryonic development or to the equivalent conditions in lower
existing, or perhaps fossil, forms. A recognized principle of
embryology is that known as ihe Law of Recapitulation It is
based on the general observation that the definitive structure of an
organism is attained through a series of embryonic stages, in which
it not only develops from a simple or ground type to a more com-
plex condition but also reflects in passing the features of lower, and
presumably its own ancestral, forms. That of comparative anatomy
depends on the comparison of higher, specialized animals with
lower, or generalized ones, the latter being assumed. in one feature
or another, to have remained in a backward or primitive state of
specialization, and therefore to reflect in such features a low grade
of structure of a kind possessed by the ancestors of existing higher
forms. These relations form a basis for the comparison of the
embryonic development of organisms with the evolution or history
of the groups which they represent, the former being distinguished
as ontogeny, the latter as phylogeny. The interpretation of the
adult structure of an organism is a matter of distinguishing its
more general features from its more special ones, the former being
in all cases those to which the ontogenetic and phylogenetic prin-
ciples are especially applicable.

How such conditions affecting the present form of an organism
have come about, may be explained by reference to ancestry. The
sum of characters, apart from influences of accident, are the result
of development of the primordial cell which constitutes the fertilized
egg. Such features as are impressed upon the animal during
growth or maturity are in this respect negligible, and the importance
of the egg-cell is in no way diminished by the fact that in the
majority of mammals it undergoes its early development within
the maternal body. The succession of generations, or continuity
of life, carries onward the structure of the body, and as fossil
organisms reveal, has maintained this process for countless millions
of years.

2 ANATOMY OF THE RABBIT.

With succession has also come modification, the evidence of
which lies not only, geologically speaking, in the relative times of
appearance of life forms on the earth, but also in the fact that
succession leads from primitive to specialized animals. revealing
in a large way the same kinds of differences observable among those
living at the present day. That the entire skeleton of a mammal
is patterned upon the primitive skeleton of the fossil amphibia
of the Carboniferous and Permian is evident from a comparison of
the components part for part, but it is equally evident from com-
parative anatomy that the viviparous condition of a higher mammal
is founded upon an oviparous condition in lower forms even if no
fossil evidence is forthcoming. That a mammal as an air-breathing
vertebrate should develop gill structures in the embryonic con-
dition, though circumstances never come about by which such
structures are used, is in itself an important fact bearing on adult
structure, but such a condition also shows to what extent a living
animal carries ancestral features, whether functionally modified
or not.

All characters of animals have thus an evolutionary basis, the
general nature of which is easily understood although the process
by which they have been developed is still a matter of uncertainty.
In comparison with one another, animals present certain resem-
blances and differences—diagnostic features, which are used as a
basis for classifying them into major and minor groups. In many
cases characters of resemblance have been shown to be secondary,
and are hence described as convergent. In some of these the
resemblances are of a gross type, and the structures are described
as analogous; in other cases they are exact or homoplastic.
As a rule, however, characters of resemblance are broad marks of
affinity, comparable to those seen on a small scale in human’
families, or in human races, and determined as in the latter cases
by heredity. The chief basis of comparison of animals with one
another is the general assumption that structures which are similar
or identical are homogenous—of common origin On the other
hand, their differences are chiefly marks of divergence in evolution.
Although it is conceivable that many of the internal features of
animals are the result of a general progressive development, more

THE INTERPRETATION OF STRUCTURE. 12

conspicuous in comparison with those of primitive types, the
majority of their differences are such as have resulted from adaptive
modifications of structure, by which they have become differently
adjusted to the particular conditions of their accepted habitats.
Adaptation is one great factor in modifying animal form, produc-
ing first divergences, as between one type and its contemporaries;
although such features may afterwards become settled in particular
groups, and thus appear for these as primitive, general, or
group-characters. Adaptation, in other words, is not a matter
of present conditions only, of fixed environment, or an environment
of a general or special kind. The rabbit as a gnawing animal or
rodent, for example, is also an air-breathing, walking vertebrate,
and shares these larger and also more ancient features with many
other vertebrates of otherwise different kinds.

It is customary to include under the term specialization all
those features in which an organism may be shown to be more
highly modified in comparison with another type. If the latter is
an ancestral type, or a lower form exhibiting ancestral features,
its more primitive features are said to be prototypal, because they
indicate the form from which the higher modification has been
derived. Such comparisons not only reveal the fact that different
animals are specialized in different degrees, but also show that a
given form may be greatly specialized in some respects and primitive
in others.

Moreover, it is to be considered that animals are at the present
time, as they have been in the past, more or less changeable, or
plastic types. Some of the most interesting features which they
exhibit depend on the circumstance that the adjustment of structure
which is rendered necessary by the opposing effects of heredity and
specialization is not exact or immediate. Thus, it is not difficult
to find in any specialized animal, in addition to those organs which
are functional or in full development, others which are retro-
gressive in character and reduced in size. It is also to be assumed
although difficult of proof among living forms, that there are also
organs which are sub-functional or progressive.

14 ANATOMY OF THE RABBIT.

ZOOLOGICAL POSITION.

It will be evident from the foregoing statement that every
specialized animal possesses in its organization a vast assemblage of
features which, if referred to their proper categories, are found to
represent many grades of morphological value. In so far as the
adult structure of a particular form is concerned, it is possible to
consider them anatomically without discrimination; but, on the
other hand, if they are to be explained, it is necessary to proceed
on a basis of function, embryonic development, or evolution. The
study of an animal as a type or representative of a group, however,
concerns only in a general way the features common to its various
members, since the majority of features present in any animal are
of minor importance, and as such are significant chiefly as indicating
the developments which may take place inside the group. The
question of what an animal is actually representative is a matter
of comparison with other forms, in other words, of its zoological
position. This isexpressed through the medium of classification,
the latter being arranged to indicate, so far as is possible, the relation-
ships of organisms to one another. In this connection the following
statement of the zoological position of the rabbit may be found
useful; and it may also be considered as illustrating, through the
comparison of this animal with allied forms, some of the more
general characters of animals as outlined above.

The domestic rabbit is represented by several races, of which
the common variously-colored forms, long-haired Angoras, Lop-
Ear Rabbits, and Belgian ‘‘Hares’’ are more familiar. They are
all descendants of the wild rabbit (Oryctelagus cuniculus, Lepvs
cuniculus) of Europe. The !atter is thought to have belonged
originally to the countries bordering the western portion of the
Mediterranean, but its distribution has been greatly extended
northward and to other continents through human agency.

The family Leporidae contains a large number of closely
related species formerly included in the single genus Lepus. They
are variously known as hares and rabbits, but the latter designation
is considered to apply more exactly to the European rabbit and
its domesticated races, the others, with one or two exceptions,
being more properly described as hares. The more familiar species

ZOOLOGICAL POSITION. 15

include the North American Cotton-Tail (Sylvilagus floridanus,
Lepus sylvaticus), and the Prairie Hare or Jack-Rabbit (Lepus
campestris) ; the European Common Rabbit (Oryctolagus cuni-
culus), and Hare (Lepus europeus). The two European species
differ in several well marked features, which form the basis of-the
accepted distinctions between hares and rabbits. The rabbit is
distinguished by its shorter ears and less elongated hind limbs;
also by its burrowing habits, and by the circumstance that the
young are born in a blind and naked condition. The hare is more
nearly a running or cursorial type, and is distinguished by its longer
ears—which, moreover, are tipped with black—longer hind limbs
and prominent eyes. Unlike the rabbit it does not burrow, but
inhabits only an open “‘form,’’ and the young when born are clothed
with hair and able to see.

The various species constituting this family are distinguished by
several features, including the imperfect development of the
clavicle, longer ears and limbs, and the presence of a distinct
although greatly reduced tail, from the Picas or Tailless Hares
(Ochodontide) of the mountainous districts of Central Asia and of
North America (Rocky Mountains). The two families are allied,
however, in the possession of a common feature, namely, the
presence in the upper jaw of a second pair of incisor teeth. This
feature distinguishes the sub-order Duplicidentata from that. of
the Simplicidentata, the latter containing the majority of rodents
and embracing all forms with a single pair of upper incisors.

_ The mammalian order Rodentia, to which the family belongs,
contains a very large assemblage of forms—the Squirrels, Marmots,
Cavies, Beavers, Mice, and Porcupines being among the more
familiar. This order is distinguished by the modification of the
anterior incisors in both upper and lower jaws to form chisel-like
cutting organs, the teeth having their enamel layer disposed
chiefly if not wholly on their front surfaces, so that they remain ina
permanently sharp condition. This modification is associated with
an extensive obliteration of intermediate teeth, comprising posterior
incisors, canines, and anterior premolars; also with elaboration
and often great complexity of the remaining premolar and molar
teeth, the lower jaw, and, indeed, the parts of the skull generally.
Characteristic of these animals is the extension, both forward and

16 ANATOMY OF THE RABBIT.

backward, of the jaw-musculature. The articulation of the lower
jaw exhibits an elongated articular process fitting into a corres-
ponding longitudinal fossa on the skull, the jaw being able to move
forward and backward in addition to vertically and from side to
side. The teeth are further arcuate in shape, and are provided
with open roots, so that their growth is not limited, as it is in the
majority of mammals. “The rodents are in many particulars
primitive types. For example, they tend to retain the five-toed
(pentadactyl), plantigrade foot, characteristic of primitive mam-
malia and, indeed, of terrestrial vertebrates, and exhibit also un-
elaborated cerebral hemispheres in the brain. In other respects,
however, as in the rodent characters above-mentioned and in the
elaboration of the intestine, especially the caecum, they exhibit
the characters of highly specialized herbivores.

Like all higher or placental mammalia (Placentalia), the
rabbit is viviparous, the young being retained through a period of
gestation in the maternal uterus, to the wall of which they are
attached by a vascular connection, the placenta. In this feature
the placental mammalia differ from the marsupial mammalia
(Marsupialia) of Australia and South America, the latter being
viviparous, but, with one exception, without placenta; also from
the egg-laying mammalia (Monotremata) of Australia, the
latter being oviparous, like the majority of the lower, reptilian
forms. These three sub-classes of mammals are united, however,
by the common features of the class Mammalia. They are
warm-blooded animals, provided with a complete double cir-
culation, and with a hairy investment for the surface of the
body. In all, the young are nourished for a time after birth
through the secretion of modified cutaneous, milk-producing,
or mammary glands.

Many of the more general features of the rabbit are such as are
not recognized by group designation, but yet are shared with other
terrestrial vertebrates, including mammals, reptiles, birds, and, in
part, amphibians. This refers to the development of the lungs
and associated respiratory tracts, both the true réspiratory tracts
and the accessory respiratory passages traversing the skull; further
the loss of the branchial or fish-type of respiration and the new
disposition of the branchial structures; the development of a tri-

ZOOLOGICAL POSITION. 17

segmented type of limb with a full complement of muscles, and
originally a pentadactyl, plantigrade foot, for support of the body
and for locomotion. The regional differentiation of the vertebral
column, especially the mobility of the neck, the free occipital
articulation, and the definition of the sacrum, the latter associated
with the elaboration of the pelvic girdle, are all features of general
signifiicance in the terrestrial vertebrates.

Finally, the rabbit agrees with other members of the phylum
Chordata in the possession of a ground-plan underlying the most
general features of its organs, and the position, arrangement, and
plan of development of its organ-systems. All Vertebrata or
back-boned animals possess an axial skeleton formed by the
segmenied vertebral column. In a very comprehensive way they
possess as chordates a still more fundamental axial support, the
notochord, the latter being an embryonic structure except in the
lowest chordates. In a more restricted sense, as Craniota, they
possess an organized head region with differentiated brain, special
sense organs, and enclosing primary skull. They possess a series
of branchial (branchiomeric) structures, appearing either in the
adult condition, as in fishes, or as part of the underlying plan in the
embryonic condition; and they add to their general features in
the arrangement of the organ-systems the further feature of
transverse segmentation (metamerism) of a considerable portion
of the body.

These facts may be set down in tabular form, as indicated below.
A similar plan can be constructed for any group of organisms, but
whether it constitutes a natural or an artificial classification depends
on whether or not it is based upon an actual study of the affinities
of the organisms concerned. <A natural classification should show
at a glance not only what the relative importance of any particular
character may be, but also how it stands in the scale of specializa-
tion. For example the placental stage of vertebrate develop-
ment, 7.e. the development of the placenta itself in the highest
stage of vertebrate evolution, is the culmination of a series of
arrangements for the care of eggs and young, and further the
adherence of human structure in the vast majority of features to
type of higher mammals is similarly expressed by the fact that man
is also a placental mammal.

18 ANATOMY OF THE RABBIT.

Phylum CHORDATA. Animals with notochord and gills.
+ Protozoa, ANNULATA, MOLLUSCA, etc., invertebrate phyla.

Sub-Phylum CRANIOTA. Chordates with organized head region.

+ AcRANIA. Lancelets. Also Ascidians and worm-like Chor-
dates, sometimes separately classified.

VERTEBRATA.
Class MAMMALIA. Warm-blooded craniotes, with hair coat.

Young nourished from mammary glands.

+ Pisces, AMPHIBIA, REPTILIA, AVEs, lower vertebrate classes.

Sub-Class PLACENTALIA. Mammals with placenta.
+ MARSUPIALIA. Viviparous mammals without placenta.

{ MONOTREMATA. Oviparous mammals.

Order RODENTIA. Gnawing placentals, with chisel-like incisors.
{+ CARNIVORA, UNGULATA, PRIMATES, etc. Various placental
orders otherwise adapted.

Sub-Order DUPLICIDENTATA. Rodents with two pairs of

upper incisors.
7 SIMPLICIDENTATA. One pair ot upper incissors.

Squirrels, Gophers, etc..
Family LEPORIDAE. Hares and Rabbits.
{ OCHODONTIDAE. Picas).

tEquivalent groups.

GENERAL ANATOMY. 19

GENERAL ANATOMY.

Although in every respect a continuous structure, the body is
differentiated into a large number of parts, or organs, the latter
being more or less individual in form, composition, or function.
Organs are arranged for the most part in systems, each of which is
concerned with some general or fundamental function, to which
several organs may contribute.

In a more general way the body may be considered as an
assemblage of tissues. The latter are layers or aggregations of
similarly differentiated cells. They are of several different kinds
and are variously associated.in the formation of organs. Being
structures of an intermediate position they may be considered either
as organ components or as products of specialized cells.

As a body-unit a cell
consists of a small mass of
living protoplasm, contain- a Hie fe.

ing a central body, the nuc- Kant

leus, and surrounded or en- S/

closed on its free border by ok

a cell-membrane. The = beste
nucleus is a highly organized = |: 2,
body, having an important s"

function in the reproduction L chr.

of the cell and also in its Ors =~
if aA)

general activity or metabol- OTT
ism. It contains a charac-

oa Fic. 1. Developing ovum of the_ rabbit.
teristic formed material, From a section of the ovary: chr., chromatin;

‘ n.m., nuclear membrane; p.f., cells of the primary
chromatin, and frequently ovarian follicle; z.p., zona pellucida; pr., proto-
. : plasm.

also a minute spherical body,
the nucleolus. The chief features of a typical cell are illustrated
in the accompanying figure (1) of the developing ovum, the latter
being a single cell, noteworthy for its large size, and also one in
which the external form is not greatly modified, as it is in the
majority of the cells of the body. Its enclosing membrane, the
zona pellucida, by which in its natural position in the ovary it is
separated from the surrounding follicular cells, is considered to
belong in part to the latter.

As fundamental living matter, protoplasm possesses certain
properties on which the functions of the body ultimately depend.

20 ANATOMY OF THE RABBIT.

Considered collectively, these functions are not so well illustrated
in the higher or multicellular organisms, in which particular
functions are assigned to particular cells, as in the lower unicellular
organisms, in which all functions are discharged by a single cell.
In simple or protozoan animals the protoplasm is seen to be capable
of ingesting food-materials, of discharging waste, of changing its
form, and of reacting in one way or another to stimuli arising out-
side of the body. Moreover, the protozoan cell is capable of giving
rise to new cells by division of its substance into two parts, which
process originates in the nucleus, and is associated at some stage
usually at least, with union or conjugation of parent cells.

All the cells of the body of a multicellular organism are products
of a single cell, the fertilized egg, but the larter .s a product of
fusion of two primary elements, the spermatozoon of the male
parent and the ovum of the female. The fertilized egg does not
exhibit the functions of a one-celled body, but possesses the poten-
tial of these functions, and the latter appear, to a large extent
individually, in the differentiation of its division-products into
specialized tissue-elements.

It is in this way that the body of a multicellular animal must be
founded with reference to the same elementary functions of life
as those appearing in one-celled organisms. But the repeated
division of the fertilized egg, in development toward the adult
condition, gives rise by division of labor to a great variety of
cells, each kind of which may be regarded as representing a minor
aspect of some major function.

The Tissues.

The primary tissues of the body are of four kinds—epithelial,
connective, muscular, and nervous. To these—the fixed .-
tissues—are to be added the fluid substances, blood and lymph, in
which the cell elements, the red and white corpuscles, or in the latter
case the white elements alone, are suspended in a fluid medium.

The following survey of the principal features of the tissues will
serve to make clear the extent to which the gross appearance of organs
depends upon tissue composition, though the account itself is in
no way intended as a guide to the microscopic structure of the rabbit,
which is more properly part of the subject matter of histology.

EPITHELIAL TISSUES. Pall

Epithelial Tissues.

Epithelial tissues are distinguished chiefly as surface invest-
ments, such as those of the exterior of the body, the interior of the
alimentary canal, the lungs, the respiratory and accessory respira-
tory tracts, and the ducts of the urinogenital organs. In all
epithelia the cellular feature is a prominent one, and it is largely
for this reason that as lin‘ng membranes they are not conspicuous
in gross structure. They are noteworthy, however, for their
products, the hairs and the various
kinds of secreting organs or glands.

The epithelium of the skin (Fig. 2)
is known as the epidermis or scarf-skin.

It is stratified, i.e., composed of
several layers deep of cells, of which the
deeper are formative, growing cells,
while those at the surface are flattened
squamous cells, and are successively
discarded. The several layers combined
produce but a thin membrane. It
extends over the entire surface of the
body and connects at certain points
with the epithelia of the internal sur-
faces. It is supported by a thick
resistant layer of connective tissue which
forms the true skin or corium.

2

The epithelium of the internal sur- Fic. 2. From a section of the

upper lip of a four-day-old rabbit.

iicesmiomimlce tien chiel. portion. of ithe “2) “ercctor muscles of the han;

c, connective tissue of corium; e,

mucous membranes. In the greater epidermis; f, hair follicle; g, seba-

ceous gland; p, papilla; s, hair shaft.

portion of the alimentary tract the
epithelial layer is simple or one layered and is associated with a thin
layer of smooth muscle to form a mucous tunic (Fig. 16, t.ms.).
The coating of hairs on the surface of the body, the presence
of which is a notable mammalian feature, is a protective invest-
ment arising from the epidermis. A hair is produced by the
modification of the central portion of an ingrowth of the epidermis,
termed the hair follicle (Fig. 2, f). The latter contains at its
base a small elevation of the underlying vascular connective tissue,
the hair papilla, through which the structure is nourished. On

22 ANATOMY OF THE RABBIT.

the general surface of the body the hair follicles are arranged in
groups (Fig. 5), and on the lips certain large follicles are set apart
for the production of the greatly enlarged sensory hairs or vibrissae.
Connected with the hair follicles are thin strands of smooth muscle,
the arrectores pilorum (Fig. 2, a,a’). They are placed in the
broad angles formed by the inclined follicles with the corium and
their contraction throws the hair into a more nearly erect position.

Epithelial glands are ingrowths
of the general layer, the cells of which
become greatly modified as secreting
structures. The lumen or cavity
of the gland, in most cases greatly
complicated through the division of
the gland substance, is connected
with the general surface by a duct
which serves to carry away its
secretion. In some cases the con-
nection of a gland with the epithelial
surface is embryonic, and in the
adult condition the gland is found
separated from the epithelium from
which it was originally formed. This
condition is represented by the
thyreoid and thymus glands of the
neck and thorax respectively. In
other cases the secreting element is

EG esha,

Diagram of a multi-

cellular gland: al., alveolar type; d.,

duct; ep., general epithelium; t.,
tubular type. B, The pancreatic duct
and associated portion of the duo-
denum in the rabbit.

a single cell, the latter thus repre-
senting a unicellular gland lying
directly in the general layer. The

mucus-secreting goblet cells of the
intestinal wall are structures of this nature. Typical epithelial
glands are accordingly external secreting glands in the sense that
the secretion is discharged and utilized on a free surface. There
are many structures, such as the suprarenal and pituitary bodies,
commonly described as internal secreting glands, the products of
which as hormones or chemical bodies of various kinds have a
modifying effect upon functions in outlying parts of the body.
They are not of necessity epithelial in origin and their resemblance
to ordinary glands is usually only a matter of superficial appearance.

EPITHELIAL TISSUES. 22

The majority of multicellular glands conform to one of two
types, namely, the tubular gland, in which the secreting portions are
of uniform calibre, and the acinous or alveolar gland, in which the
secreting portions are sacculated (Fig. 3, A). Both types occur in
simple, little branched and greatly branched conditions (Figs. 3, B, 4).

Cutaneous glands of two types are cqgqmmonly present in
mammals in association with the hairs, namely, sudoriferous or
sweat-glands, which are glands of the tubular type, and sebaceous
glands, which are of the acinous type. In the rabbit, glands are
absent from the general surface, but are found in special situations,
as, for example, in connection with the hair follicles of the lips,
the internal surface of the ear, and the
external genital organs. The inguinal
- glands comprise both tubular and acinous
portions. The mammary glands of the
female are greatly modified cutaneous
glands of an acinous type.

Apart from the mucus-secreting cells of
the general epithelium, the glands of the
alimentary canal comprise the important Fic. 4.. From a section of

the parotid salivary gland of

but less elaborated glands of the wall,such — therabbit. A. Duct system in

relation to body of gland: g,

as the gastric glands of the wall of the acini of gland body; d, inter-
lobular duct; B. Three indivi-
stomach; and the greatly elaborated, out- dualacini, highly magnified.
standing glands which lie beyond the wall
and are connected with the interior of the canal only through their
ducts. The latter comprise the oral glands, the liver, and the
pancreas. The oral glands include chiefly the submaxillary,
parotid, sublingual, and infraorbital glands—conspicuous
structures in the dissection of the surrounding portions of the head
and neck. Their ducts communicate with the cavity of the mouth.
The secretion of the oral glands has important mechanical

functions in moistening the comminuted food in preparation for
swallowing. It also contains ferments or enzymes, chiefly
ptyalin, which is capable of transforming starch in soluble materials,
though the digestive action is probably not exercised to a great
extent. Pancreatic secretion, on the other hand, has little or no
mechanical action, but its enzymes are of the greatest importance
in digestion.

24 ANATOMY OF THE RABBIT.

There is a close association between the epithelia of the surface
of the body and the nervous tissues, arising from the circumstance
that they are derivatives of a common embryonic layer, the ecto-
derm. In the adult we may distinguish as sensory epithelia
special aggregations of cells lying either in a deep or superficial
position, and associated more or less closely with the central
nervous system. They comprise the olfactory epithelium of the
nasal cavity, some of the cells of which are true nerve cells, the
gustatory epibhelumo of the tongue, and the auditory epithelium
of the membranous labyrinth
of the ear. The retina— the
nervous portion of the eye—is a
modified portion of the central
nervous system.

As linings of surfaces, the
ordinary epithelia may be dis-
tinguished from certain special
coverings of internal spaces, the
endothelia and mesothelia. The
latter consist microscopically of
—ts, thin pavement-like cells. They
differ from epithelia in origin, being
formed, not in connection with
originally free surfaces, but in

a ase AE niet eS relation to spaces of the mesoderm
of the side of the body of an adult rabbit. or intermediate layer of the body.

X about 10; showing the grouping of the a . riches
hair-follicles: co., corium; ep., epithelium; Endothelia form the linings of

m.c.m., cut aneus maximus muscle; (ESA

subcutaneous tissue. blood vessels and lymph canals,
while mesothelia are the chief,

layers of the smooth, moist serous membranes which line the

peritoneal, pleural, and pericardial cavities.

2. Connective Tissues.

The connective tissues form the supporting elements of the
body. As ordinary connective tissues they serve to connect
organs or parts of organs together, and as Skeletal tissues they
provide the rigid framework or skeleton from which all soft parts
of the body are suspended. They are distinguished by the presence
of two main components—the cell basis, and the intercellular

CONNECTIVE TISSUES. 25

substance or matrix. The cellular portion is formative, and is
much more conspicuous in the embryonic than in the adult condi-
tion. All connective tissues are products of an embryonic tissue,
the mesenchyme (Fig. 23, ms.), which consists of branched cells
connected together by their outstanding processes. Through the
activity of the cells there is formed an intercellular material con-
sisting either of a homogeneous matrix, or more frequently a matrix
containing formed elements of a supportive nature.

A. Ordinary Connective Tissues.

In the adult condition the ordinary connective tissues, with
few exceptions, consist of the cell basis with two kinds of fibrous
elements, the white, and yellow
fibres (Fig. 6). White fibres are
elements of great strength. They
are fine, unbranched fibres, which
do not occur independently, but
are joined in a parallel fashion
with one another, forming in this
way fibre bundles of larger or
smaller size. The yellow fibres are
of greater diameter. They branch
and communicate, but are not
associated to form bundles. They
also differ from white fibres in
being highly elastic. The tissue Beat ao be ara RTA Sry an Oe
roducedeinnthis way is\ known aS Guctieed teumens Ge. comective
forous connective tissue; Lt oe co tiie mae Of white. Shes
occurs in several forms according
to the relative concentration of the two kinds of fibres or the ad-
mixture of other materials.

The commonest kind of fibrous tissue in the adult is that
described as areolar. It is characteristic of the subcutaneous
tissue (Fig. 5) which connects the skin with the body; but areolar
tissue occurs also in various positions where it has a similar function

of joining structures loosely together. Subcutaneous connective
tissue is a white material, the peculiar appearance and properties
of which are due to the fact that the two kinds of fibrous elements
are arranged in a loose felt-like network (Fig. 6). When stretched

26 ANATOMY OF THE RABBIT.

it is found to yield up to a certain point, beyond which it is tough
and resistant. It tends to regain its original shape when the
tension is removed.

Fibrous connective tissue may be greatly modified through the
concentration of either one of the fibrous elements. Concentration
of white fibres is most common. This con-
dition is illustrated in the thick connective
tissue layer forming the true skin or corium
(Fig. 5), but is more conspicuous in the
glistening white tendons (Figs. 7,34) by which
muscles are attached to bone surfaces, in the
ligaments uniting bones with one another,
and in the thin, broad aponeuroses which
serve for muscular attachment. The struc-
tures known anatomically as fasciae are
special sheets of connective tissue covering
chiefly individual muscles or muscle groups.
Conceniration of yellow fibres occurs in the
dorsal ligament of the neck (ligamentum
muchae) where greater elasticity is required.
The ligament is not so conspicuous in the
rabbit as in larger mammals where the

Fic. 7: Ftom a section Yellow coloration is very noticeable.
of the tendon of origin of Fat or adipose tissue is a soft form of

the biceps muscle: m.f., E : E
muscle fibres ending on the - connective tissue in which the cells are greatly

tendon; t., fibrous con-
nective: Aseuevot the ten: enlareedsand icontamatatty. material athe
form of globules. It tends to occur in certain
situations and in particular association with the bloodvessels, but
otherwise is found in situations where areolar connective tissue -
might be expected to occur. Special fat masses, sometimes
distinguishable by their darker coloration, occur at the side of the
neck in the rabbit. In the foetus (cf. Plate VI) large masses of
vascular connective tissue are found in this situation, and are
probably similar in origin to the storing or hibernating glands of
certain other mammals. !
Coloration or pigmentation of certain portions of the body,
especially of the skin and hairs, and of the retina, ciliary body, and
iris of the eye, is due partly to the presence of special connective

CONNECTIVE TISSUES. 27

tissue cells, chromatophores, and partly to the presence of
pigment granules in epithelium. The absence of pigmentation in
animals belonging to species normally colored—albinism—is
indicated by the white coloration of the hair and by the pink
coloration of the eyes, the

latter being due to the cir- Lerecertie
cumstance that the _ blood- (BS Ge)
vessels of the vascular tunic eo. ee
are not concealed by pigment. eee eee: ——— -
es 2 <a
Se By 1
B. Skeletal Tissues. Le iS)

The skeletal tissues are
solid forms of connective
tissue which, on account of
their more permanent shape, are better adapted to form a support
for the body. They are of two kinds—cartilage and bone.

Simple or hyaline cartilage (Fig. 8) is a semi-solid and some-
what resilient material of a bluish or pearly coloration. It consists
of a homogeneous matrix in which the cells are imbedded. The
cells are distributed singly,

Fic. 8. Hyaline cartilage: c.c., cartilage cell;
1., lacuna; m., matrix.

or more often in groups of SS
¢ ae ae en eee |
two to four, each group being i < eee
: : 0 Te IG ONS NG a eos
contained in a small oval (Op Oe Raw
; NS ee
th rtil : | ER Mize = ,
space, the ca tilage lacuna ge hee k Coe ae huce
The size of the spaces, and (Bk See ee Cea l
eee . OF ne LL,
also their distance apart, 1s ne AG Si 7s ie ht
° pet Mie tt 0 eee a ag ae ey - Lh 9
subject to great variation. IC j Qrx Se
The addition of fibrous ele- ees =O --K lac.
° aire bone Or — a =
ments, usually of white fibres, EEE
5 “ ——— =
produces a modification
Fic. 9. Intramembranous bone; froma ground

known as fibrocartilage. section of the rabbit’s femur: c.l., circumferential
A ‘ lamellae; h.c., Haversian canal; h.l., Haversian
While fibro-cartilage occurs lamellae; i.1., interstitial lamellae; lac., lacuna.
in certain situations, as in the
symphysis of the pelvis, or in connection with the interarticular
menisci and at the capsular margins of the joints.
In the adult skeleton cartilage is present only in small amount.
It forms the articular surfaces of joints, the ventral portions or

28 ANATOMY OF THE RABBIT.

costal cartilages of the ribs, and a portion of the nasal septum;
it is also found uniting the basal bones of the skull. In the embryo,
however, it forms the entire skeleton, with the exception of a small
portion which, as described below, is formed of membrane bone.
In the course of development, except in the situations indicated,
the cartilage is replaced by bone.

Bone is a compact, resistant, but yet
somewhat elastic tissue, possessing much
greater strength than cartilage, and therefore
forming a more perfect skeletal support. As
indicated below, its appearance as a tissue
differs somewhat according to its mode of
formation. The more typical structure (intra-
membranous bone) is illustrated in the
accompanying figure (9) of a ground trans-
verse section of the dried shaft of the femur.
| Its dry weight consists of about one-third

a animal matter and two-thirds mineral matter,

| the latter being chiefly calcium phosphate.

| The bone materials are deposited in layers
| or lamellae, which are comparable to highly
| modified white fibres of fibrous connective
| tissue. The lamellae enclose between them
ZAC. greatly branched spaces, the lacunae, in
which in the natural condition the bone-cells
are accommodated. On account of certain
Bn differences in development, the lamellae are
ie) disposed in different ways. Some are dis-

Fic. 10. Divided femur : 5 ees
of the rabbit; c.t., can. posed concentrically around longitudinal

cellous tissue; e., distal : 5 é
epiphysis; e.l., epiphysial spaces, the Haversian canals, forming in

line; m.c., marrow-cavity; 3 5

s., shaft. this way the so-called Haversian systems.
The canals are occupied in the natural con-
dition by bloodvessels. Others of them, the interstitial lamellae,
are disposed in a somewhat parallel fashion between the Haversian
systems; while a third series, the circumferential lamellae,
are disposed in a parallel fashion with respect to the periphery or
the internal surface of the bone. In the natural condition the bone
is enclosed except on its articular surfaces by a layer of connective

CONNECTIVE TISSUES. 29

tissue, the periosteum. During the period of growth this mem-
brane contains large numbers of bone-forming cells, the osteo-
blasts, through the activity of which the deposition of the bone
lamellae is accomplished.

It is only in a few cases that the bones of the skeleton may be
looked upon as solid structures—as a rule they consist of a fairly
thin shell of hard or compact bone surrounding a central mass of
spongy or cancellous bone. This arrangement is one of great
mechanical strength, combined with lightness, and at the same time
provision is made in the interior of the bone for bloodvessels and
marrow spaces. Thus in a long bone (Fig. 10) the central portion
or Shaft consists of a cylinder of compact bone surrounding an
extensive space, the Marrow-cavity. The ends or extremities
consist each of a thin shell of compact bone continuing that of the
shaft and surrounding a mass of cancellous tissue. In the short,
flat, or irregular bones of the skeleton no continuous marrow cavity
is formed.

In point of origin bones are of two chief kinds—membrane
or derm bones, and cartilage or replacing bones, but a third
group is represented by the tendon or sesamoid bones which are
developed in the tendons of muscles. The membrane bones are
few in number. They comprise the roofing and facial bones of the
skull, most of which are distinguished by their flattened, tabulate
character. They are formed in connective tissue membranes, and
although they sometimes contain cartilage they are not formed on
a cartilage basis. Moreover, they are elements lying in a super-
ficial position with respect to the skeleton proper. They owe this
condition, and also their designation as derm bones, to the circum-
stance that they represent surface plates which in lower vertebrates
are associated with the skin.

Cartilage bones, on the other hand, are the characteristic
elements of the skeleton. They are known as replacing bones
because they are formed on a cartilage basis, the latter, as indicated
above, representing the primary cartilage skeleton of the embryo.
In the course of development, except in certain regions where the
cartilage persists throughout life, the cartilage material is replaced
by bone, which tends to surround and also invade it.

30 ANATOMY OF THE RABBIT.

The way in which bones are formed on a cartilage basis serves to
explain many peculiarities of the adult skeleton. In the embryonic
condition the cartilage rudiments are associated to form a com-
plete but primitive skeleton. In many cases the replacement of
these elements by bone is not direct, certain readjustments being
necessary, both for purposes of growth and on account of the much
more special functional requirements of the adult skeleton.

In the embryonic condition the cartilage rudiments are en-
closed by a connective tissue sheath, equivalent to the periosteum
of a bone, but described as the perichondrium. The osteoblasts
of this layer are concerned*with the formation of bone material,
both in the interior (endochondral bone) and on the surface
(intramembranous bone). The formation of endochrondral

bone is preceded by certain changes
tr. ma. which take place in the interior of the
cartilage. In the latter, in certaifi
J areas, known as the centres of ossifi-
cation, the matrix becomes partly
trnmi. dissolved, the cells enlarged and
ultimately broken down. These
changes are associated with a deposi-
tion of calcareous material, or calci-
Fic. 11. Outline sketch of the prox- > : 3

imal end of the femur of a young fication, by which the portion of the

animal: c.f., principal epiphysis for the a : 6
head of the femur. The accessory Cartilage undergoing transformation
fers (erat ond chia. (er ty cee is temporarily strengthened. Into
at this, “area? the -active ‘cells, .onaerne
perichondrium are carried through the agency of vascular in-
growths, the periosteal buds, and the result of their presence is
the deposition of bone material in association with the remaining
portions of the matrix. This condition is partly illustrated in the
distal epiphysis of the humerus shown in Fig. 13, A, the figure being

from a vertical section of the elbow-joint of a four-day-old rabbit.

In the long bones the formation of the first or main centres of
ossification takes place in the shaft, and there are formed afterwards
accessory or epiphysial centres for the extremities. A divided
extremity, such as the proximal end of the femur (Fig. 11), may
possess several such centres—a principal one for the chief epiphysis
or actual extremity of the bone and several subsidiary centres for

CONNECTIVE TISSUES. an

its outstanding processes. In the shaft the formation of endochon-
dral bone is of short duration. Through the activity of the osteo-
blasts lying directly in the perichondrium, or later the periosteum,
a process of formation of intramembranous bone goes on during
the whole period of growth, and the result of the peripheral deposi-
tion of bone lamellae is, as described above, that the transverse
diameter of the bone is greatly increased. The enlargement of the
marrow cavity, with which this is associated, is produced by the
absorption of bone from the interior.

In young animals both the epiphysial centres and the masses
of cartilage in which they are formed are sharply marked off from
the body of the bone (cf. Fig. 13). This is largely because the
formation of the epiphysial centres tends to lag behind that of the
main centres, and thus the cartilage
extremities of the bones are evident long
after the formation of the shaft is under
way. In the epiphysial centres the bone
formation is endochondral. The bone
masses which they form are distinguished
as epiphyses. During the period of
growth they are connected with the: body
of the bone by plates of epiphysial cartilage, Pie, re, ie eeelaoe

: . : : : tion of the skull ina three-day-
into which the surrounding perichondrium ¢id rabbit. bo, _ basioccipital

: : : : bone; ch, occipital portion of
extendssas an ossitication ridge. In-this — ‘ckoniroceanium: co, occipital

region bone formation takes place, with ferdvi' £0) <xoccipital im
the result that the whole structure is ° >
greatly increased in length.

After the period of growth, the duration of which differs in
different bones, the epiphyses become firmly co-ossified with the
body of the bone, although the lines of junction or epiphysial lines
may be still visible. Thus in the distal extremities of the radius
and ulna, in the proximal extremities of the fibula, or in the bodies
of the lumbar vertebrae, the epiphysial lines appear even in old
animals. In the foregoing figure (10) of the divided femur it will be
seen that the position of the epiphysial lines is indicated by bands
of compact tissue. Finally, in thoroughly macerated bones of
young animals, the epiphyses are usually found to be readily
separable from the bones.

a2 ANATOMY OF THE RABBIT.

In a comparison of the adult skeleton with the more primitive
embryonic skeleton, several differences in the arrangement of the
elements are evident. Thus many bones, notwithstanding their
possession of several centres of ossification, are to be looked upon as
individual either in the cartilage or in the bone condition. In other
cases, as in the basal portion of the skull, separate bone elements
are produced in a mass of cartilage primarily continuous. These
either remain distinct throughout life, or, as in the occipital region,
(Fig. 12), become fused together to form compound or composite
bones. In still other cases, as in the vertebrae, the apparently
single elements of the adult condition are the products not only of
originally distinct bones, but also of primarily separate cartilage
masses.

Fic. 13. Vertical sections of elbow and knee of four-day-old rabbit. A
elbow; c, capsule; eb, endochondral bone in the distal epiphysis of the humerus;
ea, extensor muscles of the forearm; em, extensors of the hand; fa, flexors of the
forearm; fm, flexors of the hand; h, humerus; ol, olecranon; r, radius; sc,
synovial cavity; u, ulna. B, knee; a, anterior cruciate ligament; c, capsule;
f, femur; lp, patellar ligament; p, posterior cruciate ligament; pv, popliteal
vessels; t, tibia; x,x, anterior and posterior ligaments of the lateral meniscus;
x’,x’, anterior and posterior ligaments of the medial mehiscus.

The bones of the skeleton are united or articulated with one
another by connective tissue in the form of ligaments, by cartilage,
or in some cases by both together, i.e., by fibrocartilage. Liga-
mentous union, distinguished as syndesmosis, is the most general
type of articulation. Cartilage union or synchondrosis occurs
in certain situations, as in the basal region of the skull. Union by
fibrocartilage or symphysis is characteristic of the articulation
of the two sides of the pelvis (symphysis pubis).

5

MUSCULAR TISSUES. a3

The articulations of bones are of two types—immovable
articulations or synarthroses, and movable articulations, diar-
throses, or joints. In the former, motion is either absent or at
least greatly restricted. In the latter, it is definitely provided for
through the presence of joint-structures. Thus in a joint (Fig. 13)
the apposed surfaces of the bones are accurately modelled in
relation to one another, and are moreover covered by a layer
of cartilage, the latter forming a joint cushion. Between the two
surfaces is a space, the cavity of the joint, containing a viscid
material, the synovia, which serves for lubrication. The space is
enclosed by a connective tissue capsule. The strength of the joint
depends largely on the enclosing capsule, but it is
usually greatly increased by the presence of
accessory ligaments. In the more complex joints,
such as that of the knee (Fig. 13 B), interarticular
cartilages (menisci) are enclosed between the bone
surfaces, and the latter are connected directly
by short ligamentous cords. The various liga-
ments of a joint permit free motion of the bones,
but only up to a certain point. :

Several differences are observable in joints
according to the form of the apposed surface and
the kind of motion provided for. Thus in the i
ball-and-socket joint or enarthrosis, exemplified Gari tone

: : section of the mus-
by those of the shoulder and hip, a bone is able to cular tunic of the
move in various directions about its base of
attachment, although actually, in the limbs, this motion is almost
restricted to an anteroposterior direction. In the ginglymus or
hinge-joint, as exemplified by the distal articulations of the limb,
motion is restricted to a single plane. The gliding joint or arth-
rodia is one in which a slight degree of motion is made possible by
one surface slipping over the other; it is exemplified in the accessory

articulations of the vertebral arches.

3. Muscular Tissues.

Muscular tissues are the active portions of the individual
muscles of the skeleton and of the muscle coats of visceral organs.
Their chief feature consists in the elongation of the cells to form

34 ANATOMY OF THE RABBIT.

fibres. These fibres may be considered to possess the contractile
properties of protoplasm, but with the contraction limited to one
direction. Except ina few cases the fibres are arranged in a parallel
fashion, so that the line of contraction of the muscle or muscle
layer is the same as that of each of its fibres. The result of con-
traction in both is the shortening of the longitudinal axis and the
increase of the transverse axis. Muscles are important structures
in the production of heat. The latter is liberated not only in
action but also when the muscle is in repose.

Two chief types of muscle fibres occur in the bobdy—the smooth
or unstriated fibres, which are characteristic of the involuntary
muscles or muscle coats of the visceral organs
or of the skin, and the striated fibres which com-
pose the individual or voluntary muscles of the
skeleton. Smooth fibres (Fig. 14) are elongated,
spindle-like cells, the substance of which is longitud-
inally striated, but possesses no transverse mark-
ings. The single nucleus of the cell occupies a
central position. The muscles which they form
are distinguished as involuntary because their
operation is not under the control of the will, their
connections being with the sympathetic nervous
system. The striated fibres (Fig. 15) are very
Fic. 15. Striated much larger, cylindrical structures, the substance
(skeleta)) muscle of of which possesses characteristic transverse stria-

tions. Each fibre is enclosed by a loosely attached
membrane, the sarcolemma, on the inner surface of which many
nuclei occur.* The presence of these nuclei indicates that the
fibre is not a single cell but a syncytium, 7.e., an association of ©
cells unseparated by cell boundaries. The muscles formed by such
fibres are under the control of the will, their connections being
directly with the central nervous system. They comprise not
only the typical muscles of the skeleton, but also the special muscles
connecting the skeleton with the skin.

*The position of the nuclei is characteristic of the so-called white muscles.
In the semitendinosus of the rabbit, which is a red muscle, the nuclei occur
between the fibril bundles of the interior of the fibre. Red fibres contract more
slowly but are more resistant to fatigue. The proportion of red and white
fibres varies in different muscles.

MUSCULAR TISSUES. 35

The muscular substance of the heart differs both from striated
and smooth muscle in being composed of branched anastomosing
fibres, which apparently form a continuous network. Like striated
muscle, it possesses characteristic transverse markings, but, like
involuntary muscle, it is under the control of the sympathetic
nervous system.

As gross structures the voluntary muscles are functional units,
each of which has a particular action according to the movement
permitted by the parts of the skeleton to which it is attached.
They present a longitudinal striation which is roughly referred to as
the direction of the fibres, and which is of great value in identifi-
cation. The striation is due to
the circumstance that the fibres
are arranged in parallel groups
or muscle bundles, each of which
is surrounded and_ separated
from the adjacent bundles by a
connective tissue enclosure, the
perimysium.

A muscle is typically spindle
shaped, consisting of a middle
fleshy portion, termed the belly
of the muscle, and of tapering Fic. 16. From a section of the pyloric end

(antrum pyloricum) of the stomach of the

ends which provide for attach- rabbit: m.m., muscularis mucosae; t.m.c.,

circular layer of the muscular tunic; t.m.l.,

Meme mine attachme%¢nt-is effect="" qoneitadinal layer of the tuscular. tunic:
edebyearstrons band? of fibrous). ‘ay Ce Snbmucosa,
connective tissue, the muscle

tendon (Figs. 7, 34). Some muscles, such as those of the abdominal
wall, are disposed in the form of flattened sheets, the ends of which
are attached by broad, thin sheets of connective tissue, the apon-
euroses. In unipennate muscles the fibres are attached obliquely
to the side of the tendon, or in bipennate muscles to both sides,
like the vane of a feather. In the so-called biceps, triceps and
quadriceps muscles of the limbs, the origin is divided into two,
three or four portions. The recognition of origin and insertion
(p. 63) depends on usual but not invariable relations. The exact
effect of muscle contraction depends as a rule on the relative
positions of the parts and on the synchronous action of other
muscles. A muscle like that forming the diaphragm does not

36 ANATOMY OF THE RABBIT.

possess an insertion after the fashion of ordinary muscles; and in
some cases, as in the intrinsic muscle of the tongue or the so-called
orbicular or sphincter muscles, both origin and insertion may be
absent.

Involuntary muscle is distinguished by its white or greyish
coloration and by its smooth or homogeneous appearance. It forms
characteristic layers in connection with visceral organs or with the
skin, and is thus much less individual than the voluntary muscles
in its relations to particular parts. It forms the muscular portion
(muscularis mucosae) of the mucous tunic of the alimentary
canal, and also a separate muscular tunic lying in the outer
portion of its wall (Fig. 16.)
In the muscular tunic the
fibres are arranged in both
circular and _ longitudinal
directions. Involuntary mus-
cle also forms a small con-
stituent of many organs, such
as certain glands, in which
contractility is not a chief
function. It forms a large
constituent of the wall of the
mes Wi che ea or fe iene ore urinogenital tubes, particular-
dendrites; c.g., chromatophile granules; nr., ly the bladder and the uterus.
aie In association with elastic
connective tissue it is an important constituent of the walls of the
bloodvessels.

Although there is an underlying community of structure in the
walls of the bloodvessels, the two chief types of vessels, arteries and
veins, present conspicuous differences, both in functional behaviour,
and in their appearance in the dead animal. The differences are
largely the result of differences in the relative amounts of the above-
mentioned constituents. The arteries are thick-walled, elastic
tubes, which, under the force of blood from the heart, first become
greatly expanded, and then gradually contract, so that the blood
is forced into the smaller capillary vessels. The veins on the other
hand are thinner-walled, less elastic vessels, through which the
blood is forced largely through the pressure from behind. In the

NERVvVOusS TISSUES. 207

dead animal the arteries appear white, flat or collapsed, and empty.
The veins on the other hand appear large and dark on account of
their distension with blood.

4. Nervous Tissues.

Nervous tissues form the basis of the central nervous system
and of the outlying nerves and ganglia. They comprise two kinds
of elements—nerve cells and nerve fibres. In the central nervous
system these elements are imbedded in a mass of neutral tissue, the
neuroglia.

Nerve cells are characteristic of the central
nervous system and of the spina! and sympathe-
tic ganglia. They differ greatly in form, but
typically each consists of a cell-body (Fig. 17)
bearing two kinds of processes—a fibre-process,
the neuraxis or neurite, and a series of branch-
ed protoplasmic processes, the dendrites. The
cell-body is distinguished by the presence in its
interior of granular masses, the chromatophile
or tigroid bodies. The latter extend into the
dendrites, but not into the neuraxis. The
dendrites may be greatly elaborated, and may be Pee che PerD samt ot
present to a considerable number. The neuraxis two, medullated nerve-

fibres from the sciatic

is a nerve fibre process. Since it continues as of the rabbit: a.c., axial

cord; m.s., myelin
the central portion or axial cord of a nerve fibre, pices luce oer
it may traverse a relatively enormous distance _nevzilemma; nr. node
on its way to a peripheral organ.

A nerve fibre consists of a central core, the axial cord, enclosed,
except in the case of those of the olfactory nerve, by certain mem-
branes. Two kinds of fibres are distinguished—medullated fibres,
and non-medullated fibres. The former are characteristic of
the peripheral nerves. In these (Fig. 18) the axial cord is sur-
rounded by a comparatively thick membrane of fatty material, the
medullary or myelin sheath. The latter is continuous except
at certain points, the nodes of Ranvier, where the axial cord
appears free except for an external investment of the whole fibre,
the neurilemma.

\

‘. Ae

38 ANATOMY OF THE RABBIT.

In the non-medullated nerve fibres the myelin sheath is lacking.
This type of fibre is chiefly distributed in connection with the
sympathetic system.

A nerve is an association of nerve fibres, the latter being
disposed in a parallel fashion and united together into bundles of
larger or smaller size by connective tissue, which also forms a
general peripheral investment, the epineurium. The dead-white
coloration of a nerve is due to the fatty materials of the myelin
sheaths, but nerves are commonly found imbedded in a fatty con-
nective tissue which is associated with the epineurium and is also of
white coloration.

Nerve fibres, and also
nerves, are distinguished
functionally as afferent
and efferent. They are
organs of conducuon,
which carry impulses
either from the peri-
pheral parts of the body
to the central nervous
system, or in the oppo-
a site direction. Sensory

fn.a nerves are afferent, while
Fic. 19. Section of the spinal cord of the rabbit: motor nerves aro efier-
c., central canal; f.m.a., anterior median fissure; s.m.p., ;
posterior median sulcus; f.a., f.l., and f.p., anterior, ent. Nerves, however,
lateral, and posterior funiculi of the cord; c.g.a. and -
c.g.p., anterior and posterior grey columns (horns of usually contain both
grey matter); r.a., and r.p., anterior and posterior nerve a
roots; s.a., white substance. afferent and efferent
fibres and are hence
described as mixed. In the distribution of afferent and efferent
fibres chere is a marked difference between the external or somatic
portions of the body and the internal or visceral portions. Con-
sequently, both somatic and visceral kinds of afferent and efferent
fibres are conveniently distinguished.

On account of certain differences in coloration, the cellular and
white fibrous constituents of the central nervous system produce
characteristic patterns where either one is concentrated. Thus the
cellular material is greyish, and is hence distinguished as the grey
substance, while white fibrous material produces when concen-

BLooD AND LYMPH. 39

trated an opaque white appearance similar to that seen in the
peripheral nerves, and is hence described as white substance.
In the spinal cord (Fig. 19) the grey substance is disposed as a
central core, the white substance as a peripheral investment.
A similar relation is found in the basal portion of the brain, but the
characteristic pattern in the cerebral hemispheres and in the cere-
bellum is one in which the grey substance forms a peripheral,
investing, or cortical layer.

5. Blood and Lymph.

Blood (Fig. 20) is fundamentally a cellular material, but owing
to the fact that the cells or corpuscles are contained in a fluid
medium, the plasma, it does not take on the features of the ordinary
tissues of the body. It is for this reason also that, notwithstanding
its important function, the appearances pre-
sented by the blood in dissection especially of
preserved animals are negligible. The cellular Fa
materials consist of (a) erythrocytes or red oe
blood cells, microscopic circular discs of fairly
definite though not rigid contour, containing
in the adult condition no nucleus. They have a Borneo sthe BEI
yellow color, which is due to the presence of —' ‘°°? Profle:
haemoglobin, and which gives to blood its deep red color when seen
in bulk. The haemoglobin is the specific carrier of oxygen which it
combines chemically. Arterial, oxygenated blood is bright red,
while venous blood is dark red or bluish. The number of red blood
cells is relatively somewhat greater in the rabbit than in man,
there being over six millions contained in each cubic millimetre.
The cells are formed in the vascular area of the embryo, later in
the spleen and liver, and finally in the marrow of bones; (b)
leucocytes and lymphocytes, also termed white or colorless
blood cells. They are amoeboid, nucleated cells, present in much

O =
ey &

>»
—
=3

smaller numbers in blood, and in lymphatic vessels, and are formed
in the lymph glands and spleen. They are capable of passing
through the walls of the smaller vessels, and occur more or less
throughout the tissues, where they have the function of carrying
materials or of ridding the bedy of injuricus substances and bac-

40 ANATOMY OF THE RABBIT.

teria; (c) platelets, minute nucleate amoeboid cells found in the
blood stream.

In all multicellular animals the tissues are removed more or less
extensively from the surfaces of absorption and excretion. Blood
and lymph are the media through which tissue metabolism is
maintained, and the vascular system is the mechanical means by
which the continuous circulation of fluid is brought about. The
composition of the blood varies from time to time according to the
individual functions performed. Oxygen and food materials are
carried to the tissues. Carbon dioxide and waste materials of
other kinds are carried to organs from which they can be excreted.
Blood, however, has been shown to vary in composition in different
species of animals, and to be chemically homologous in related ones;
while it possesses immunizing properties towards bacterial diseases,
also_differing in different species and individuals.

TERMINOLOGY.

In special or descriptive anatomy it is necessary to employ an
extensive system of terminology in order that the various structures
of the body may be individually designated, classified, and referred
to their respective positions. The terms used for this purpose may
be classified into four groups, as follows: (1) general terms—
those included in the names of parts, but applicable in themselves
to similar structures (arteries, nerves, etc.) in various parts of the
body; (2) specific terms or names of parts; (3) regional
terms—those defining areas (topographic); and (4) terms of
orientation.

Although few in number, the terms of orientation may be
regarded as the most generally useful terms of descriptive anatomy. —
This is because they are based on very general relations of the body
and are therefore of wide application. For this reason they are
here selected for definition to the exclusion of others of a more
restricting or individualizing kind.

In all vertebrates we may recognize a longitudinal axis,
corresponding, in general, to the line formed by the vertebral
column. In the usual or prone position of the body this axis is
horizontal. The uppermost surface is then described as dorsal,

TERMINOLOGY. 4I

the lowermost surface as ventral, the sides of the body as lateral.
Any position forward, with respect to the long axis, is anterior
in comparison with any position backward, which is posterior.

In relation to the long axis it is convenient to recognize a
median vertical plane of section, which is one dividing the body
into right and left halves; also transversal planes, which are
planes situated at right angles to the median plane and to the long
axis, and sagittal planes. The latter are vertical planes parallel
to, and also including (as midsagittal), the median vertical plane.

The median vertical plane is the plane of bilateral symmetry,
each half of the body as thus defined being in a general way the
reverse counterpart of the other. Structures situated in part in the
median plane are unpaired, and are described as median, while
structures situated wholly outside of the plane are paired, right
and left, or dextral and sinistral. In relation to the median plane
and to the sides of the body, structures are described as medial
when nearer the former, and as lateral when nearer the sides of the
body. The term intermediate is applied especially to a position
between medial and lateral, but this restriction is perhaps not
justifiable.

In considering the extent of bilateral symmetry, it is necessary
to bear in mind that, although a fundamental feature in verte-
brates, it is not perfectly retained in the adult condition: Symmetry
is destroyed by the migration of an unpaired structure from a
median to a lateral position, as is seen, for example, in the case of the
abdominal portion of the alimentary canal; or, again, by the reduc-
tion or disappearance of structures belonging to one side of the
body, as, for example, in the case of the mammalian aortic arch.

Referring to centre and circumference, either in the body as a
whole, or in particular parts, the terms deep and superficial,
central and peripheral, or internal and external may be
applied. It may be observed, however, that the terms internal and
external are sometimes used in the sense of medial and lateral, both
in descriptive language and in the names of parts.

In comparison with the terms medial (medialis) and median
(medianus) the term middle (medius) may be used to designate
the position of a structure lying between two others, the latter being
otherwise designated, for example, as anterior and posterior, or one
_ in the median plane.

42 ANATOMY OF THE RABBIT.

The limbs being more or less independent structures, it is proper
to apply to them certain terms otherwise applicable to the main
portion of the body. The chief terms not properly applied else-
where are proximal, meaning nearer the centre or base ofsattach-
ment, and distal, toward the extremity. In the middle segment
of the fore limb the respective positions of the bones (radius and
ulna) are indicated as radial and ulnar. The terms tibial and
fibular are also applicable, although with less reason, to the
corresponding segment of the hind limb. The dorsal and ventral
surfaces of the fore foot are described respectively as dorsal and
volar, those of the hind foot as dorsal and plantar.

In determining the identity of structures in a quadrupedal
mammal, considerable difficulty may at first arise from the fact
that descriptive terms, such as those just defined, are frequently
included in the names of parts, the latter being, at the same time,
terms applied in the first instance to the human body, in which the
recognized relatioris are somewhat differenc. In comparison with
that of a quadrupedal vertebrate, the human body occupies a
vertical or erect positioa, and is to be considered as having been
rotated upward through ninety degrees on the posterior limbs.
The latter accordingly occupy for the most part their original
position, and the human arm largely reassumes this position when
allowed to hang freely at the side of the body. As in all cases, the
face retains its forward direction. Thus the terms anterior and
posterior as used in human anatomy mean ventral and dorsal,
provided they refer to parts of the body, such as the entire trunk
region, which have been affected by rotation. The terms superior
and inferior as applied to man are similarly anterior and posterior
as applied to a lower form. Since it is unwise to change the form
of the official terms of human anatomy, it becomes necessary to
interpret all such terms when used for a quadrupedal mammal
according to the relations exhibited by man. The human terms
may in most cases be translated into terms acceptable for com-
parative anatomy by reading ventral for anterior, dorsal for
posterior, cranial or oral for superior, and caudal or aboral for
inferior. The exceptions then apply to those parts of the body
unaffected by rotation.

THE GROUND PLAN OF ORGAN SYSTEMS. 43

On the other hand in ordinary description of organs and their
position, where it is not a matter of the official names of parts, little
advantage is to be gained from adherence to this principle. The
terms anterior and posterior apply with much greater force to a
lower vertebrate, than to man, while the terms superior and
inferior are only of interest in the latter. In this case the rule here
followed is to use the terms anterior and posterior for descriptive
purposes without reference to the human relation. The same
remark applies to the terms of direction, viz., upward, downward,
forward, and backward.

In this connection it may be pointed out that the custom has
become more or less general in comparative anatomy of employ-
ing the termination ad with words otherwise signifying position
alone, in order to indicate position or course toward, e.g., dorsad =
dorsalward. In the present case this form is used only for course,
position being indicated by the adverbial termination ly, e.g.,
dorsally.

Reference may also be made here to the fact that the human
structures to which identifying names are applied frequently fail in
one way or another to correspond to structures in a lower form.
Composite structures to which individualizing names are applied,
for example, may be represented by independent parts. Also,
structures which are similar in form or function may be convergent.
Finally, although it is essential to endeavour to apply all terms as
accurately as possible, it will be remembered that a terminology
primarily arranged for one type cannot be exactly applied to
another without considerable qualification.

THE GENERAL FEATURES AND GROUND PLAN OF
THE ORGAN SYSTEMS.

It has already been stated as a general principle that the
structure of an organism is the expression of an underlying plan and
pattern, in the elaboration of which embryonic development and
ancestry play a very large part. How, asa matter of interpretation,
the comparative method is applied, may be demonstrated by
reference to any part of the body of an animal; and in the following
pages will be found, under the head of the respective systems, a

44 ANATOMY OF THE RABBIT.

preliminary statement of how certain outstanding features of the
rabbit may be explained, as to the nature of their origin, and what
grades of organization they may be presumed to illustrate. It will
be recognized that the mammals are in most respects less specialized
than man, and must accordingly show in their grades of organization
various stages through which man must be assumed to have passed.

CLASSIFICATION OF THE ORGAN SYSTEMS.

The term organ-system is employed in descriptive anatomy
to designate a group of organs which cooperate in.a general function.
In many respects the systems represent primitive functions, and
it is therefore largely on account of the independent elaboration
of these that the systems may be recognized also on a structural
basis as groups of organs allied in origin and development. The
exact number of systems recognized depends on certain arbitrary
distinctions, the following being those usually distinguished:

(1) The integumentary system, comprising the skin, and its
appendages, namely, the hairs and the general cutaneous,
mammary, and inguinal glands.

(2) The skeletal system, comprising the cartilage and bone

elements of the skeleton, with their connections.
‘ (3) The muscular system, comprising all contractile structures
of the body. Since, however, the involuntary muscles are arranged
as muscle layers in connection with visceral organs, the muscular
system is usually considered as including only the individual or
voluntary muscles of the skeleton and skin.

(4) The nervous system, comprising the central nervous
system (the brain and spinal cord) and the peripheral nervous
system, the latter consisting of the paired cranial and spinal nerves
with their associated ganglia. A special portion of the peripheral
nervous system is set apart as the sympathetic nervous system.
The latter consists of a pair of ganglionated sympathetic trunks
lying along the ventral surface of theverteb ral column, and of two
series of ganglia, prevertebral and peripheral, connecting the
trunks with the visceral organs.

(5) The digestive system, comprising the digestive tube and
its outstanding glandular appendages—the oral glands, the liver,
and the pancreas.

CLASSIFICATION OF THE ORGAN SYSTEMS. 45

(6) The respiratory system, comprising the lungs, and
respiratory passages, namely, the bronchi, the trachea and the
larynx. With this system may also be included the accessory
respiratory passages formed by the nasal fossae.

(7) The vascular system, comprising the organs of circulation,

Fic. 21. Schematic representation of the chief organ-systems of a generalized
vertebrate as seen in a transverse section of the abdominal region:

Integument—int.

Skeleton—yv., vertebral body; a.v., vertebral arch; c.v., vertebral canal.

Muscular system—s.m., skeletal muscle; v.m., visceral muscle.

, Nervous system—m.s., spinal cord, with the central canal, and the dorsal (pos-
terior) and ventral (anterior) roots of the spinal nerves; g.r.p., ganglion of the
posterior root; r.c., ramus communicans to sympathetic trunk; r.m.a. and r.m.p.,
anterior and posterior rami of a spinal nerve; t.s., sympathetic trunk.

Digestive system—i. intestine.

Vascular system—ao., aorta.

Urinogenital system—k., kidney; go., gonad (ovary or testis).

Serous cavity—c.p., general coelom, pleuroperitoneal, or peritoneal cavity;
p.v. and p.p., visceral and parietal parts of the serous tunic—visceral and parietal
peritoneum; mes., mesentery.

namely, the heart, arteries, capillary vessels, and veins. The
lymph-conducting canals are also portions of the circulatory
system, but since they are largely independent of the bloodvessels,
they are usually considered as forming with their associated lymph
glands a separate lymphatic system.

s

46 ANATOMY OF THE RABBIT.

(8) The urinogenital system, comprising the reproductive
and excretory organs, together with their common ducts—the
urethra of the male and the vestibulum of the female—and the
associated bulbourethral gland. The reproductive organs com-
prise, in the male, the central organs or testes, and the deferent
ducts, both of which are paired, the unpaired seminal vesicle,
and the paired prostatic and paraprostatic glands. In the
female, the reproductive organs comprise the paired ovaries,
uterine tubes and uteri, together with the unpaired vagina.
The excretory organs of both sexes comprise the paired kidneys
and ureters and the unpaired
urinary bladder.

Certain organs of the body
are not included in this classifi-
cation: (1) The thymus and
thyreoid glands are connected
with the digestive tube in the
embryonic condition, and_ for
this reason are sometimes in-
cluded with the digestive system,
although in the adult they occur
as independent structures. (2)
The suprarenal body is situa-
ted close to the kidney of either

ee te Oa SE Be pen gee side, but is independent of the
mm.):m., mandibular; h., hyoid; 1 and 2, first latter, both in the adult condi-

and second branchial arches; a.l., antérior
limb-bud; me., metameres; p.l., posterior +jon and in point of develop-

limb-bud. (After Minot and Taylor, in

Keibels Normentafeln, V.; Fig. 12.) ment. (3) The special (olfac-
tory, optic and auditory) sense-

organs of the head are highly elaborated structures, the relations

of which are partly with the central nervous system.

GENERAL ORGANIZATION.

In the rabbit, as in all vertebrates, the general plan of organiza-
tion involves three chief features, as follows:

(1) Axial orientation. The axial line of the body is indicated
by the chief portion of the skeleton, and the important organ-

GENERAL ORGANIZATION. 47

systems are grouped in a characteristic fashion about it (Figs. 21, 23).
(2) Metamerism. A large portion of the body, mainly
dorsolateral in position, is arranged on a segmented plan, in which
parts are repeated serially and longitudinally around and to either
side of the original axis. Metamerism does not appear to any
extent on the surface of the adult body, but becomes evident in

Fic. 23. Transverse section of a rabbit-embryo of about 10} days,
showing the arrangement of the organ-systems: ao., aorta; ch., not-
ochord; coe., coelomic cavity; d.m., dorsal mesoderm (myotomic
and sclerotomic divisions); e., primitive alimentary canal (enteron);
ect., ectoderm; |.b., limb-bud; ms., mesenchyme; my., external portion
of a myotome; n, nephrotome of embryonic kidney; intermediate mass
of mesoderm; sp. and so., splanchnic (visceral) and somatic (parietal)
portions of the ventral mesoderm.

structure in the subdivision of the vertebral column into vertebrae
and the paired, serial arrangement of the related spinal nerves,
vessels and musculature. Metamerism is externally evident in
embryos (Fig. 22) and is anatomically founded upon the serial
arrangement of parts of the mesoderm.

(3) Branchiomerism. This is an adult feature of lower
aquatic vertebrates (Fig. 30), such as fishes, where it appears as a

48 ANATOMY OF THE RABBIT.

succession of true gill or branchial structures, which support gill
filaments as functional respiratory organs. In higher terrestrial
animals it appears as an embryonic feature (Fig. 22, m.h. 1,2) and
is to be considered both as a determinant of adult form and as a
mark of aquatic ancestry. As in lower vertebrates, it underlies not
only structures of branchial significance (branchial arches in the
restricted sense, but also modified branchial structures, including
the first or mandibular arch (m.), and the second or hyoid arch (h.).
So great is the modification of these structures in passing from the
embryonic to the adult condition that the recognition of the ground-
plan is perhaps here of less general importance. It, however,
determines the position and relations of certain skeletal structures,
including the auditory ossicles, the hyoid, and in part the laryngeal
cartilages—a point of some value in the classification of the parts
of the head skeleton. It also determines the succession of certaia
soft structures, including the fifth, seventh, ninth and tenth cranial
nerves; also the chief arterial vessels of the heart, which are more
fully referred to below.

The fundamental importance of branchiomerism lies in the
fact that respiration by means of gill perforations of the pharynx is
characteristic of that branch of the animal kingdom from which all
chordate animals have been derived. In the various invertebrate
phyla are found respiratory surfaces of many kinds, in which,
however, thin surface membranes, external tufted, or invaginated
tubes and analogous structures, provide for diffusion without
perforation of the body tube.

EMBRYONIC PLAN OF THE SYSTEMS.

In the individual organ-systems the main features of the general
plan, as estimated on embryonic development, may be outlined as
follows:

1. The formation of an axial skeletal support, consisting
primarily of a strand of cellular tissue, the notochord, and second-
arily of a segmented cartilaginous, afterwards bony, vertebral

column.

EMBRYONIC PLAN OF THE SYSTEMS. 49

2. The formation of (a) a primary cartilage skull (chondro-
cranium) as a support for the brain and capsules of the special
sense organs (neurocranium or cerebral cranium); and (b) a series
of cartilaginous visceral arches (splanchnocranium or _ visceral
cranium).

3. The formation of the chief skeletal muscle in a dorsolateral
position along the axis.

4. The formation of the central nervous system as a tube of
nervous matter (neural tube), lying on the dorsal side of the axial
support, and differentiated into a general posterior portion, the
spinal cord, and an anterior expended portion, the brain.

5. The formation of the digestive tube as a median structure,
lying directly beneath the axial support, and of special glandular
appendages arising from the epithelium of its wall.

6. The formation of the lungs as paired outgrowths of the ven-
tral wall of the digestive tub-, afterwards connected with the out-
side of the body by accessory respiratory tracts traversing the head.

7. The formation of the circulatory system primarily on an
aquatic plan. This involves the formation of (a) the heart in a
ventral position to the digestive tube and immediately behind the
gills; (b) a ventral aorta, passing forward to the gills, and dividing
into a paired series of branchial aortic arches; (c) a dorsal aorta,
combining the aortic arches, and passing backward along the
ventral surface of the axial support; and (d) a series of paired
veins returning the blood from various parts of the body to the
heart.

g. The formation of the reproductive organs or gonads in
association with the dorsal lining of the coelomic cavity, and their
connection with the outside of the body by modified kidney ducts.

10. The formation of the kidneys, either as embryonic or
permanent structures, from an intermediate mass of tissue, lying in
general between the dorsal musculature and the lining of the coelo-
mic cavity (cf. position of embryonic kidney in Fig. 23).

11. The formation, in the ventral portion of the body, of an
extensive space, the coelomic cavity or coelom, afterwards differen-
tiated into pericardial, pleural and peritoneal portions.

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THE SKELETAL SYSTEM. 51

THE SKELETAL SYSTEM.

‘

The designation ‘“‘vertebrate’’ has reference to a common
feature of higher animals, fishes, amphibians, reptiles, birds and
mammals—the possession of a backbone or vertebral column,
composed of individual segments, the vertebrae. Vertebrates
VERTEBRATE ANIMALS. 2°: however, more properly described

as animals having an internal skeleton.
The vertebral column is part of the principal, axial skeleton
(Fig. 24), otherwise formed by the ribs, sternum, and skeleton of the
head. In addition to the axial skeleton, nearly all vertebrates
possess an appendicular part of the skeleton devoted to the
support of the limbs, or in terrestrial vertebrates, more correctly
speaking, to the support of the body on the limbs, and to locomotion.
In both fore and hind limbs, this consists of a proximal portion
lying within the contour of the body and forming the pectoral
and pelvic girdles, and of a distal portion, lying beyond the general
contour of the body and comprising the skeleton of the free
extremities. The limbs of vertebrates present an extraordinary
range of adaptations, being modified in the various groups into
fins, paddles, wings, and walking or running limbs. In the majority
of cases their adh+rence to a common ground plan is evident from
their composition.

The vertebral column of the rabbit consists of 7 cervical, 12
thoracic, 7 lumbar, 4 sacral, and 14-16 caudal vertebrae. The
vertebrae are found to be gradually modified from any intermediate
part of the column forward or backward, but characteristic vertebrae
VERTEBRAL can be identified for each region. The vertebrae
REGIONS. are not arranged in a straight line; on the other

hand, there is a curvature dorsad in the thoraco-
lumbar region, and corresponding curvatures ventrad in the cervical
and caudal regions. In fishes the line of the vertebral column is
straight, and there is little indication of regional differentiation.
The feature of curvature appears in terrestrial vertebrates, espec-
ially mammals, and is related to the support of the body ina lighter
medium. The curvatures are modified in man, in view of the erect
position, in that there is a lumbar curvature ventrad counteracting
the effect of the dorsal curvature of the thorax, while the caudal

52 ANATOMY OF THE RABBIT.

region is reduced to a vestigial Coccyx, consisting of coalesced
vertebrae.

The individual vertebra is made up of a more massive ventral
body, upon which is set a bridge of bone forming the arch. Both

fi/

ons

Fic. 25. Mid-lumbar vertebrae of bear (A, fifth), rabbit (B, fifth), and man (C,
third): a, inferior articular process; c, body; m, mamillary and superior articular pro-
cess; Ss, spinous process; t, transverse process.

body and arch bear processes which reach into the surrounding
muscles, and serve for their support. The principal processes are
THE FORMS mid-dorsal or spinous,
OF VERTEBRAE, !ateral or transverse,
and, in the lumbar re-
gion, dorsolateral or mamillary. Gener-
ally speaking every vertebra has three
principal functions—to support the body,
to protect the spinal cord, and to offer
support for muscles.
Any vertebra of the rabbit may be com-
WiG.26. Lumbar-vertebra of Dated with the: corresponding ‘vertebrasot
four-day ‘ord rabbit; tulaee any mammal or in a general way with those
of any vertebrate. The fifth lumbar
vertebra of the rabbit, for example, would be found distinguished
by the great development of its processes, since the latter support
powerful muscles used in leaping. The corresponding human
vertebra, or the third as functionally more nearly equivalent, is
weak in muscular expansion, but its body is massive for purposes of

THE SKELETAL SYSTEM. 53

axial support. A corresponding vertebra of the bear will be found
more or less intermediate between the two types (Fig. 25).

The adult vertebra of the rabbit and of higher vertebrates in
general is composed of bone. It arises, however, embryonically in
cartilage (Fig. 26). The transition from cartilage to bone is
CARTILAGE based on the primary condition in lower vertebrates
AND BONE. in which the entire skeleton, vertebrae included,

is formed in cartilage, and may remain in this
condition throughout life.

The axial line of the vertebrae in mammals passes through the
central portions of the bodies. This position is marked, in the
embryonic condition only, by the noto-
chord (Fig. 23). Some of the lower
aquatic vertebrates, such as lampreys,
THE NOTO- exhibit the notochord in
CHORD. both young and adult con-

ditions, and~ show little
indication of the development of the
elements of vertebrae. Others of slightly
more advanced position, such as_ sharks
(Fig. 27), show the notochord, extending
more or less to the adult condition with

the vertebral elements developed round Tee Hine ASAD Ca
b C of shark vertebra (cartilage
about it. stage), from young specimen of

Atlantic dogfish, Acanthias. i,
The head skeleton Ona mammal, intercalary cartilage, complet-

usually but inaptly called the skull, is a ig arch jt notochord wy body
complex of individual bones and cartilage,
the arrangement and functions of which may be determined with
a little effort. The general disposition of the bone elements,
demonstrable in the rabbit or any mammal is as indicated in Fig. 28.
COMPOSITION Briefly, there isa linear series of basal segments,
OF THE SKULL. Comprising from behind forward basioccipital,
basisphenoid, presphenoid, and meseth-
moid. The three first-named form the floor of the brain-case,
while the mesethmoid forms the nasal septum. Associated with
the basioccipital are paired, lateral exoccipital bones, and a
supraoccipital element, together forming an occipital ring en-

54 ANATOMY OF THE RABBIT.

closing the aperture for transmission of the spinal cord from the
cranial cavity. The basisphenoid and presphenoid bear lateral
expansions, respectively the greater and lesser wings, or ali-
sphenoids and orbitosphenoids, which assist in the formation
of the side walls of the brain case. The bone capsule (periotic)
lodging the internal ear on either side is solidly built into the
lateral cranial wall between the exoccipital and alisphenoid, while
further forward the light scroll-like surfaces of the ethmoid bone,
or ethmoturbinal, representing the chief portions of the nasal

Fic. 28. Composition of the mammalian skull. Cartilage dotted, cartilage
bone shaded, derm bones plain. I-XII, cranial nerves; as, alisphenoid; bh,
body of hyoid; bo, basioccipital; bs, basisphenoid; c, canine teeth; cm, Meckel’s
(mandibular) cartilage (1); e, mesethmoid; et, ethmoturbinal; ex, exoccipital;
f, frontal; hy, hyoid (II); i, incisors; 1, lacrimal; m, molar teeth; mn, mandible;
mx, maxilla; mt, maxilloturbinal; n, nasal; nt, nasoturbinal; os, orbitosphenoid;
p, premaxilla; pa, parietal; pl, palatine; pm, premolar teeth; pt, pterygoid;
sq, squamosal; so, supraoccipital; th, thyreohyal (III); v, vomer; z, zygomatic,

capsule is attached on each side of the base of the mesethmoid
The original proportions of the nasal capsule are however more
nearly commensurate with the general cavity of the nose; and
additional turbinal surfaces, comprising the nasoturbinals and
maxilloturbinals are attached secondarily to the nasal and
maxillary bones.

To this foundation there is‘ added a series of enclosing bones,
for the most part thin and superficial, but nevertheless making up
the greater part of the facial portion of the head skeleton as
opposed to the cranial or brain containing portion. The series
comprises the interparietal (absent in the hare and many mam-

THE SKELETAL SYSTEM 5

mals), paired parietals, squamosals, frontals, and nasals as
roofing bones, and paired premaxillary, maxillary, palatine,
pterygoid, and mandibular bones forming the solid supports of
the mouth. Several lateral elements also take part, including the
lacrimal, at the anterior border of the orbit, the zygomatic,
forming the central portion of the corresponding arch, for pro-
tection of the orbit and muscular support, and finally the bladder-
like tympanic bone, which forms the enclosure of the middle ear
and protects the delicate bones of the auditory chain.

Fic. 29. Lateral view of skull of rabbit foetus, 45 mm. cb, co, cn, cranial,
orbital, and nasal portions of primary chondrocranium; fr, frontal; i, incus;
ip, interparietal; m, malleus; mn, mandible; mx, maxilla; na, nasal; pa,
parietal; pl, palatine; pmx, premaxilla; sq, squamosal; st, styloid process; t,
tympanic; zy, zygomatic; Born plate model, after Voit.

An important though inconspicuous portion of the head skeleton
is formed by the hyoid apparatus supporting the tongue, and
certain cartilages of the larynx, with which the hyoid is intimately
associated. The relation of this complex to the skull is indicated
in a mammal by the suspension of the hyoid apparatus from its base.
The malleus, incus and stapes of the auditory chain form with
the elements just described that portion of the visceral skeleton
as modified in the mammalian skull, with the exception, as described
below, of certain replacing or derm elements also considered to
belong to this division.

56 ANATOMY OF THE RABBIT.

The skull consists primarily in the embryo of a cartilage trough,
the extent of which is roughly definable as the area occupied by the
occipital, anterior and posterior sphenoidal, and ethmoidal portions
(Fig. 29.) As a cartilage skull it is designated as the chondro-
CHONDROCRANIUM cranium, and after its conversion into
AND OSTEOCRANIUM. bone as the osteocranium. It is no

more than an enclosure for the brain,
except that it has associated with it the cartilage capsules of the
nasal, visual, and auditory organs, and, in the case of the first
and last of these, the capsules are incorporated with the skull
proper. Thus, the primary skull is designated as the neuro-
cranium or cerebral cranium, io distinguish it from a second
portion of the head _ skel-
eton, the splanchno-
cranium or visceral cran-
ium, which includes the
series of visceral arches
suspended from the ventral
surface of the neurocran-

Fic. 30. Thechondrocranium and visceral arches 1um. The addition to the

of the Atlantic dogfish, Acanthias: ca, auditory : e Ss

capsule; ch, chondrocranium; cn, nasal capsule; primary head skeleton of a
h,h’, dorsal and ventral segments of hyoid arch : rR

(II); i, intercalary cartilage of vertebral column; large number of me mbrane
m,m’, dorsal and ventral poitions of mandibular 2 oe sates

arch (II), functional upper and lower jaws; malleus bones results Im more Or

and incus of mammalian ear; or, orbit, depression
for optic capsule; v, vertebra; I-5, branchial arches.

less confusion of — the
original divisions, since the
membrane portions of the visceral cranium are, with the exception
of the mandible, united by suture with those of the cerebral cranium,
while the true cartilage or cartilage bone portions of the former,
occurring as the auditory ossicles, the hyoid and larnyx (in part),
although highly modified, remain in a more or less independent
relation.

The appearance of the mammalian skull during the later stages
of foetal development is most striking, the cartilage mass of the
chondrocranium, and the bones ossifying in its interior forming a
foundation basal mass, from which are suspended elements of the
same nature, principally auditory and hyoid, in a somewhat arch or
rod like form. The auditory arch is formed by the two more
lateral bones of the auditory chain, incus and malleus, of which the

THE SKELETAL SYSTEM. 57

incus is attached to the skull, while the malleus is extended as the
cartilage of Meckel almost the whole length of the inner surface
of the mandible. The bulk of the skull is formed already by
the surface elements distributed in the characteristic fashion, but
as yet only loosely associated (cf. Fig. 29).

This condition of the developing skull in a mammal finds its
explanation far back in the history of the vertebrates and is only
made clear by the study of some one of the lower fishes such as
HEAD SKELETON OF shark or sturgeon. Ina shark (Fig. 30)
LOWER VERTEBRATES. _ the entire internal skeleton is formed in

cartilage which persists throughout life.
The principal part of the head sk2leton is a massive cartilaginous
box (chondrocranium), enclosing the brain, and including as a
result of growth and fusion the nasal and auditory capsules. The
eye capsules are free, and their accommodation at
the side of the cranium establishes the orbital
depression to be seen on the skull of all vertebrates.
This type of structure is obviously the basis of the
mammalian skull, except that in the latter the
cartilage mass is at once more restricted to its basal
portion, and is more specialized by its replacement
by definite bone centres.

It is, however, in respect of the visceral arches
suspended from the chondrocranium that the structure of the shark
skull is most illuminating. The first or mandibular arch forms
the upper and lower jaw. Its composition reflects that of the
external part of the auditory chain in the mammalian embryo, and
establishes a fact, for a long time scarcely believed by zoologists,
that in the history of mammals this arch has undergone a profound
change of function. The second or hyoid arch, though developed
to an extent out of keeping with the degree of development of the
primitive tongue, is nevertheless obviously homologous with the

Fic. 31. Shagreen
denticles of the
smooth _ dogfish,
Mustelus; enlarged.

hyoid arch of a mammal and in most sharks has an important
accessory function in the support of the lower jaw. Following
the hyoid arch are five ordinary or branchial arches supporting
the filaments of the gills and serving as pillars of the gill apertures.
These arches are then the parts of an extensive system out of which
those portions of the laryngeal cartilage which are of visceral rather
than tracheal origin have been specialized in mammals.

58 ANATOMY OF THE RABBIT

So far as the surface or roofing portion of the skull is concerned,
it is represented in a shark only by dermal teeth or shagreena
denticles (Fig. 31), uniformly distributed in the skin of the body,
and concentrated at the aperture of the mouth into definite teeth
lining the jaws. In sturgeons and related fishes, however, these

structures are already concentrated into a definite
DERM ee ae
BONES. pattern of surface plates, having in general the

same disposition as derm elements in the skull of
all higher vertebrates, and distributed in sucha way that they form
an almost complete enclosure for those parts of the chondro-
cranium which they invest. The pattern of the dorsal elements is
best indicated in fossil amphibia and early reptilia, in which the
plan is almost diagrammatic.

The elements of the head skeleton may be classified as follows:

1. The CEREBRAL CRANIUM (cranium cerebrale or neuro-
cranium), including:

(a) The primary cartilage skull (chondrocranium), en-
closing the brain, and containing in its wall the olfactory
and auditory capsules (embryonic) ;

(b) The secondary bone skull (osteocranium), replacing
(a) and comprising the occipital, sphenoid, ethmoid,
inferior turbinal, and periotic bones;

(c) The associated derm elements, comprising the inter-
parietal, parietal, frontal, nasal, vomerine, lacri-
mal, tympanic,* and squamosal bones.

2. The VISCERAL CRANIUM (cranium viscerale or splanch-
nocranium), including:

(a) The primary mandibular and hyoidt visceral arches

(embryonic) ; ‘

(b) The secondary elements, replacing (a)—the malleus;
incus, and stapes of the auditory chain; the hyoid
bone and its connections with the skull;

(c) The associated derm elements of the face and palate,
comprising the premaxillary, maxillary, zygomatic,
mandibular, palatine and vestigial pterygoid bones.

*The identification of the tympanic as a derm element has been questioned.

{The thyreoid cartilage of the larynx and its connection with the hyoid
(greater cornu) are modified branchial arches, but the structure as a whole is not
included with the head skeleton.

THE SKELETAL SYSTEM. 59

As accessory structures the teeth of the rabbit present two
mammalian features; they are heterodont, or differentiated
according to particular regions; and the adult series, excepting those
designated as molars, are permanent teeth, replacing deciduous,
or milk teeth of the young animal. The condition is thus de-
scribed as diphyodont in comparison with that in lower verte-
brates, where there is usually a multiple tooth change, new teeth
being developed as required (polyphyodont type).
Moreover, in the rabbit, as in all mammalia, the
number is restricted, so that, considering the differentiation of the
teeth, it is possible to express their relations by a dental formula.
Inthe mammalia generally the teeth are differentiated into incisors,
canines, premolars and molars, and in placental mammals the
full dental formula is indicated as i. 3,c. 4, pm. 4,m. 3. Inthe rabbit
as in other rodents, however, the dentition is greatly modified
by the elaboration of two pairs of incisors for gnawing and the
corresponding obliteration of intermediate teeth, the place of the
latter being occupied by an extensive gap, or diastema, in which no
teeth occur. The dental formula of the rabbit is i. 2, c. ®, pm. 3,
m. 3. It will also be observed in this animal that the absence of the
intermediate teeth allows the lips to be approximated behind the
incisors, and since in this region the lips are also provided with
hairs on their internal surfaces, the oral cavity is separated almost
completely from a small space enclosing the incisor teeth. This
adaptation, however, is not so perfectly developed in the rabbit
as in certain others of the rodent order.

The human skull differs most markedly from that of the rabbit
or other mammal in the enormous development of the cranial
region, and in the anteroposterior compression of the face, with
THE HUMAN which is associated, shortening of the Jaw region,
SKULL. reduction of the nasal cavities and rotation of the

orbits to a forward position. A most instructive

DENTITION.

feature is the re-adjustment of the axis, coincident with the assump-
tion of the erect position. In both quadrupedal and_ bipedal
positions the face naturally retains its forward direction. In most
mammals, as in vertebrates generally, the axial line of the cranium,
known as the basicranial axis, and that of the face, the basifacial
axis, tend to be continuous or at least parallel; while in man,

60 ANATOMY OF THE RABBIT.
or in related primates, they tend to form an obtuse angle which
is progressively reduced from lower to higher types.

It will be evident from a study of the limb skeleton of the
rabbit that there is a general correspondence in structure as between
its anterior and posterior divisions. This applies
not only to the distinction of girdle portions, and
the divisibility of the free extremity into. proximal,
middle and distal portions, but extends to very many smaller
details of composition. The relation in structure as between
anterior and posterior limbs is
described as serial homology,
since two structures in the same
animal cannot be homologous in
the usual meaning of the term.

THE LIMB
SKELETON.

The pelvic girdle is more uni-
formly and solidly developed than
the pectoral girdle, as shown by its
strength in the three principal
directions about the point of
attachment of the limb, the great
development of the ventral union,
and strong attachment to =the
\ sacrum. On the other hand, the
pectoral girdle is notable chiefly for
the strong development of the

Fic. 32. Plan af the anterior limb skele-
ton in walking vertebrates, the equivalent
elements of the posterior limb indicated in
brackets: si, scapula (ilium); pp, procora-
coid (pubis); ci, coracoid (ischium); hf,
humerus (femur); rt’, radius (tibia);
uf’, ulna (fibula); rt, radial carpal (tibial
tarsal); i, intermedium; uf, ulnar carpal
(fibular tarsal); c,c, centrals; I-5, distal
carpals (tarsals); m. metacarpals (meta-

scapula as opposed to the weakness
of the ventral, pectoral portion.
In the rabbit the clavicle is at best
vestigial, and in many mammals,
such as the ungulates, it is entirely
absent. In man, however, as well

tarsals); ph, phalanges of the digits. 6
as in other mammals, many of

them primitive, in which the pectoral muscles of adduction are well
developed, together with the corresponding movement of shoulder
and arm, the clavicle is both well developed and articulated at the
two ends with the scapula and sternum.

Both pectoral and pelvic girdles in terrestrial vertebrates
conform more or less closely to a tri-radiate shape if the two halves

THE SKELETAL SYSTEM. 61

are considered individually. The corresponding portions may be
identified (Fig. 32), though in mammals the ventral portion in the
case of the pectoral girdle is greatly reduced. Of the ventral
elements, those commonlypresent in vertebrates are the coracoid,
GROUND PATTERN which forms the posierior ray, and the
IN TERRESTRIAL procoracoid, the anterior ray, the latter
VERTEBRATES. being partly covered in front by a

derm splint, the clavicle. This condition
though not characteristic of mammals generally is_ still found in
monotremes, and rudiments of the coracoid extension ventrally are

A in mt Wwe
of iit
Fé if \\)
1 } tg | p44 |
| | Sais, ( 4 \
| te H\ | W \\
| @ v@ \4
Ko H i I
aes | ' \ re)
vy af
A B Cc

Fic. 33. Homologies of the mammalian limb. A, fore foot, rabbit
B, fore foot, horse. C, human hand. r, radius; u, ulna; I-V, metacarpa
bones.

identifiable in embryonic marsupials. Adult marsupials and
placentals show only a small hook-like coracoid process, together
with the clavicle in perfect or less perfect development.

There is no more striking feature of homology than that shown
by the free extremities in respect of the different forms of vertebrates.
This is true homology because it concerns the resemblances, part
for part, in the anterior or in the posterior limb of any one verte-
brate as compared with the corresponding elements in the same
position in other forms. The front limb of the rabbit (Fig. 33) is
slightly elongated, and semi-digitigrade, making it a more efficient

62 ANATOMY OF THE RABBIT.

organ for running, though not so highly specialized as that of the
horse. The human hand retains a fairly primitive form as to its
general proportions, but is modified into a seizing or grasping type,
the thumb being opposable to the remaining digits. All three kinds
of limbs are, however, in themselves, modifications of a primitive,
five-toed limb, sometimes termed the ideal pentadactyl planti-
grade type, in which the palm of the hand or sole of the foot is
placed flat on the ground. The composition of this primitive limb,
traceable in one form or another throughout the higher vertebrate ;
and also the serial homologies of the parts are indicated in Fig. 32.

THE MUSCULAR SYSTEM.

The contractile tissues are not arranged in a definite continuous
system as is the case with most other organ complexes of the body.
Smooth or involuntary muscle fibres, modified mesenchyme cells
of the embryo, which are under the control of the sympathetic
INVOLUNTARY nervous system, form the muscle coats of the
MUSCLE. digestive tube, and are important not only for

its repeated, peristaltic movements, but also for
its elasticity and expansive power. Smooth muscle is also a con-
stituent of many other visceral organs, especially glands, in the
active secretion of which it appears to play a mechanical part. It is
further distributed through the walls of the bloodvessell, especially
the arteries, where it forms the mechanical organ of the vasomotor
function. This consists inthe control of the diameter of the vessels
by vasodilator and vasoconstrictor nerves connected with the
vague nerve and the sympathetic nervous system. The constrictive
action is stimulated by secretion of the suprarenal glands. Vessel
regulation of this kind is important, first, in maintaining tone and
therefore pressure, and, second, in controlling heat loss from the
surface of the body. Action of the vasomotor nerves may be demon-
strated physiologically in a variety of ways. Transection of the
cervical sympathetic nerve of one side in the living rabbit is
followed by vasodilation of the ear, the congestion of which can be
seen, and the heat loss is demonstrable by feeling with the hand.
Stimulation of the cephalic cut end is followed by vasoconstriction.

THE MUSCULAR SYSTEM. 63

Cardiac muscle, most nearly allied in action to smooth muscle
is the mechanical organ of the rhythmical contraction or beat of the
heart. The contraction takes place according to the succession
of the chambers or the course of the blood, and the rate and strength
of the beat is under the control of the vagus and
sympathetic nerves, the action of which can be
experimentally demonstrated. The excised heart in the case of
lower vertebrates continues to beat for some time automatically
or under stimulation. This behaviour has been interpreted as
purely automatic action of the heart muscle, but may depend upon
intercardiac nerve connections.

The voluntary muscles of the body form the nearest approach
to a continuous system of all contraccile tissues. They consist
VOLUNTARY for the most part of parallel fibres, the association
MUSCLE. of which into fasciculi is responsible for the appear-

ance of longitudinal striping when the gross muscle
is viewed from the side and more or less for the grained appearance
of the cut surface. The attachments, usually of white fibrous
connective tissue occurs as concentrated tendons or as flat thin
aponeuroses. The control of action is exercised directly from
the spinal cord.

A typical muscle of the skeleton has the disposition of parts
illustrated in Fig. 34 by the biceps (a flexor of the forearm) and the
long head of the triceps (an extensor of the forearm). The fixed
tendon, or tendon of origin, of the biceps is in relation to the
A TYPICAL glenoid border of the scapula, the movable tendon,
MUSCLE. or tendon of insertion, with the lower border of

of the ulna. Noting the position of the muscle
in front of the elbow joint, it will be seen that its contraction results
in flexion, i.e., in bringing the forearm into a position nearer the
arm, or of raising the forearm and hand from the ground. The
analogous action of the triceps is similarly demonstrated, and
it will also be evident that the immediate result of contraction
upon the bones is limited by the form of the joint. In this case
a hinge-joint confines motion to one plane, while in the case of the
shoulder and hip, a joint of the ball-and-socket type allows motion
on points at various angles to a plane according to which muscle or
group of muscles may be brought into action.

THE HEART.

64 ‘ ANATOMY OF THE RABBIT.

Voluntary muscle arises chiefly from the segmented areas or
myotomes of the embryo. The extent to which segmentation is
shown in the adult, however, depends for the most part on how far
the definitive muscle is removed from the vertebral
column or segmented portions of the skeleton.
The vertebral muscles themselves show throughout their attach-
ments to successive vertebrae the marks of segmental origin, but
many others, such as those of the abdomen, to a certain extent those
of the limbs, and those of the eye show practically no indications
of their segmental origin.

ORIGIN.

While the bulk of voluntary muscle
is skeletal, part at least is related to the
skin. This forms a eutaneous sheet,

divisible into the
cutaneus maximus
and platysma, commonly developed to a
considerable extent in mammals, but
reduced in man, and into the facial,
palpebral, and auricular muscles of the
head. The trunk musculature com-
prises a special group of cervical and
occipital muscles in relation to the neck
and head, and the general series which
- are more nearly vertebral. The appen-
Fic. 34. Arm muscles of rabbit @icular muscles are distributed in
from the medial surface, illustrating

muscle action, flexionandextension. special groups connecting the limb as a
b, biceps (flexor); tr, long head of

DISTRIBUTION.

triceps (extensor); i. insertion: o, . whole with the trunk and the various
origin; sc, scapula; h, humerus; 3 .
r, radius; u, ulna. segments of the limb with one another.

As already indicated the skeletons
of anterior and posterior limbs are considered to be equivalent part
for part. This is also true of the related muscles, but the respective
EQUIVALENCE positions of the segments of the limb in a
OF LIMB MUSCLES. ™ammal are such that in examining the

muscles, it is important to determine the
equivalence of the bone surfaces. Since also certain conventional
ideas of flexion and extension prevail in human anatomy, it is
equally important to establish the corresponding relation as between
man and a quadrupedal form. The homologous surfaces may be

THE MUSCULAR SYSTEM. 65
determined by a study of the embryonic relations of the limbs,
but may be referred more easily, if not so accurately, to the neutral
plan as illustrated in Fig. 35.

The neutral type (A) may be defined as one in which the
proximal segment (humerus or femur) is situated at right angles
to the median vertical plane, the middle segment directed down-
ward parallel to the median plane, and the distal segment again
at right angles to it. In this condition the bones of the middle
segment are parallel, with the radius, or the tibia, according to the
particular limb, anterior in position. The first digit is also anterior.
The entire anterior surface is indicated in the accompanying dia-
gram by shaded lines. The angles b and c are “extension”’

G
Ur|p

A

¢
| f
Fic. 35. -Schematic representation of the respective positions of the segments

in the mammalian limbs: A, neutral; B, anterior limb; C, posterior limb. Explanation
in text: tr.p., transverse plane.

angles; e and f ‘‘flexion’’ angles; a is an “‘abduction”’ angle,
while d is an ‘“‘adduction”’ angle.

In both limbs of a mammal, the entire appendage is rotated
downward to a position more or less underneath the body. In the
anterior limb (B) the proximal segment is rotated backward, the
middle and distal segments forward. The two divisions thus come
to differ from one another to the extent of two right angles. Also,
in the front limb, the interesting condition is observable that the
radius and ulna are crossed on one another, the position of the
former being chiefly anterior, although its proximal end is lateral,
and its distal end, like the first digit, medial. In the rabbit, as in
most mammals, the hand is thus fixed in a prone position, while in

66 ANATOMY OF THE RABBIT.

man the hand may be rotated to a supine position in which the
radius is wholly lateral, or in which the two bones are parallel.

In the posterior limb (C) all three segments are rotated forward.
Consequently the bones of the middle segment retain their original
parallel position with reference to one another. The extension
angles of the knee and ankle are anterior, the flexion angles posterior.
They differ from the corresponding angles of the anterior limb
only at the middle joint, but here the difference amounts to two right
angles. It will be observed also that the distal or ankle joint of
the posterior limb retains a primitive condition, in which the foot
is placed approximately at right angles to the leg; in other words,
it is of a plantigrade type, one in which the sole of the foot rests
on the ground. Thus the muscles described as flexors in the foot
pass over the heel. They are functionally flexors of the toes, but
extensors of the foot as a whole. Those described as extensors lie
for the most part on the anterior surface of the tibia, and are
turned from their original course to the dorsal surface of the foot.
They are functionally extensors of the toes, but flexors of the foot
as a whole.

THE NERVOUS SYSTEM.

There is probably no system of organs in which external form is
so little suggestive of actual function as is the case with the nervous
system. This is perhaps less true of its peripheral portion, con-
sisting of nerves which can be seen ramifying through all parts of
the body, than of the central portion comprising the brain and
spinal cord. In examining the external form as a preliminary
step to the study of the functional arrangements, it is advisable
to bear in mind that the nervous system is a great correlating
mechanism, consisting of central cellular portions, and of con-
ducting paths to and from these centres to outlying parts of the
body.

In accordance with its prime importance and at the same time
the non-resistant character of the tissue of which it is composed,
the central nervous system is protected within the canal of the
vertebral column and cavity of the brain cases. It is furthermore
surrounded by connective tissue membranes of which one, the

THE NERvousS SYSTEM. 67

dura mater, forms a tough external investment, while another,
the pia mater, is a more delicate, highly vascular membrane lying
next the nervous matter, and amply supplied with blood-vessels tor
the supply of nourishment. The nerves, however, are distributed
freely throughout the body, and though not so adequately pro-
tected are more capable of withstanding mechanical stress, and are
commonly found in connective tissue situations where mechanical
injury is not likely to occur.

Fic. 36. Plan of the central and peripheral connections
of a spinal nerve: an, afferent (sensory) neurone; asn,
afferent sympathetic neurone; ca, cp, anterior (ventral) and
posterior (dorsal) columns of grey matter; en, efferent
(motor) neurone; esn, efferent sympathetic neurone; grp,
dorsal root ganglion; i, intestine (visceral. organs); m,
skeletal muscle; na, np, anterior (ventral) and posterior
(dorsal) rami of spinal nerve; ra, rp, anterior and posterior
roots of spinal nerve; rc, ramuscommunicans (sympathetic) ;
sk, skin; sp, white matter of cord; ts, ganglion of sym-
pathetic trunk. Slightly modified, from Herrick.

The most typical of the structural arrangements of the nervous
system may be made out from a study of the connections of any one
of the paired nerves of the spinal series (Fig. 36). In the spinal
cord the difference in appearance as between the white and the
grey matter has already been described (p. 39). A spinal nerve
arises by two roots, one of which is dorsal and bears a small
ganglion of cellular material, the other ventral and without a

68 ANATOMY OF THE RABBIT.

ganglion. Impulses passing through the dorsal root are centri-
petal or afferent in that they pass only in the direction of the
central nervous system, but they are also in many cases sensory
in that their effects may be consciously experienced. The most
characteristic sensory impulses are those which come from the skin.
Ina similar fashion the impulses of the ventral root are centrifugal
COMPOSITION OF or efferent, in that they pass only in a
‘A SPINAL NERVE. direction away from the central nervous
system, and are in most cases Motor in that

their effects are commonly observed as muscular contraction. The
two roots, however, unite immediately outside the spinal cord, and
subsequently re-divide in such a way that three nerves are formed,
each containing a proportion of both kinds of fibres. Two of these
nerves, known as the dorsal and ventral rami and then distributed
as somatic nerves to the body wall, while a third is distributed as a
visceral nerve, or ramus communicans, having important se-
condary connections in the sympathetic nervous system. Unlike
the somatic nerves, which take a direct course to their terminations,
the communicating rami of each side, unite in a position ventral
to the vertebral column to form a longitudinal sympathetic
trunk consisting of a connected series of ganglia. The latter
is similarly connected witha prevertebral series of the ganglia, and
through them with certain peripheral ganglia on the surface of
the visceral organs. This system forms the sympathetic division
of the peripheral nervous system. Its ganglia are nerve-cell
centres, and its fibres, afferent and efferent, are distributed both
to the visceral organs and to bloodvessels in all parts of the body.
It is difficult to determine what portion of a muscular con-
traction, even if considered to be purely voluntary, actually arises
from an impulse originating in the central nervous system. The
animal body, however, affords many indications of muscular
REFLEX ACTION. actions as responses to previous incoming
stimuli, without conscious experience as a

necessary factor in the result. Stimulation of the skin, in verte-
brates in which the spinal cord is divided, and thus separated from
the brain, is followed by muscular contractions, presumably by
direct connections of individual dorsal and ventral roots, or exten-
sion of the stimulus to neighbouring roots. This is known as
reflex action. Its nature and conditions can be determined by

THE NERVOUS SYSTEM. 69

using spinal or decerebrated frogs, but similar actions, of which the
well-known scratch reaction of the dog is an example, are common
to all animals.

The spinal cord reflects in its form the ground type of the cen-
tral nervous system. Developed in the embryo as a tube, it retains
this condition throughout life. The cavity, however, is reduced
to a slender central canal, lined by a thin epithelial membrane,
THE SPINAL the ependyma, while through proliferation of its
CORD. cells, and their fibre extensions, the walls become

enormously thickened, and the nervous functions
of the system, as a connected conducting mechanism, thereby
established. The cord traverses the vertebral canal, showing slight

Fic. 37. The brain from the left side: bo, olfactory bulb; c¢, cerebellum;
fc, paraflocculus cerebelli; h, cerebral hemisphere; 1p, piriform lobe (olfactory
brain); m, medulla oblongata; s, spinal cord. Numerals indicate the cor-

responding cranial nerves; 2, optic; 4, trochlear; 5, trigeminal; 6, abducens;

7, facial; 8, acoustic; 9, glossopharyngeal; 10, vagus; I1, spinal accessory; 12,

hypoglossal.
enlargements in the cervical and lumbar regions in relation to the
nerve supply of the limbs, and at about the level of the second
sacral vertebra narrows into the slender, thread-like filum ter-
minale, by which it is continued almost to the middle of the length
of the tail.

Superficial examination of the brain of the rabbit (Fig. 37)
shows that its larger part is formed by the paired cerebral hemis-
pheres. They are closely pressed together on the dorsal side
but separated posteriorly on the ventral side. The external layer,
important as forming the cellular cortex, is smooth in the rabbit
as in rodents generally, but is corrugated in higher mammals such
as the cat and dog, while in man it reaches a high degree of elabora-

70 ANATOMY OF THE RABBIT.

tion. The anterior tips of the cerebral hemispheres in the rabbit
are slightly expanded to form the olfactory bulbs, and the con-
nection of the latter with the posteroventral portion of the hemi-
sphere known as the piriform lobe is easily traced. This portion
of the brain is chiefly of interest in mammals because of its relation
to the olfactory sense organs and because of the reduced condition
of the organ in man. The development of the cerebral hemispheres
even in the lower mammals is such that important
parts of the brain, notably the diencephalon and
mesencephalon, are dorsally and laterally concealed. On the
ventral side, however, the chiasma or crossing of the optic nerves is
evident, while more posteriorly the divergent cords of the cerebral
peduncles may be seen passing forwards in the direction of the
hemispheres. The posterior part o the brain is formed largely
by the cerebellum above, the corrugation of which is one of its
outstanding features, and by the somewhat tapering brain-stem
by which the brain is connected with the spinal cord. Its bulk is
formed by the medulla oblongata, which is crossed in front by a
bridge of fibres, not so conspicuous in the rabbit as in many mam-
mals and in man, which is known as the pons and which connects
the two sides of the cerebellum. These, the outstanding surface
features of the brain, afford but a moderate conception of its
details, the nature of which can only be made out by more thorough
examination and by reference to the plan of development of the
organ as a whole.

Like the spinal cord, with which it is continuous, the brain forms
primarily a portion of the neural tube, containing a central cavity
or neurocoele, but unlike the spinal cord it is greatly enlarged and
elaborated both as the general centre of the whole nervous mechan-

THE BRAIN.

ism and also the special centre of the nervous mechanism for a

variety of functions connected with the head. It accordingly forms
not only a more or less distinct division, known as the brain or
encephalon, as opposed to the more general division, the spinal
cord or spinal medulla, but is also divided into a series of paired
and unpaired segments, containing divisions of the original cavity
in the form of ventricles.

The primary divisions of the brain are more or less similar and
homologous in all vertebrates. The more elaborate condition of the

THE NERVOUS SYSTEM. 71

organ in a mammal may be explained by reference to the general
plan as indicated in Fig. 39, which is based upon general features
of form in vertebrates and upon embryonic development. For
comparison in the gross the brain of the frog (Fig. 38) offers one
of the best examples.

The brain as first formed in the embryo appears as an anterior
expanded portion of the neural tube, or rather as three expansions
arranged in a linear series. They are described as the primary
cerebral vesicles; or, as primary divisions of the future brain,
they are designated in anteroposterior order as
the prosencephalon, mesencephalon, and
rhombencephalon.

The first of the primary divisions, the pro-
sencephalon, or primary fore-brain, becomes
divided during development into two portions,
namely, an anterior portioa, the end-brain or
telencephalon, which is largely a paired struc-
ture, and a second portion, unpaired, the dience-
phalon, or inter-brain. The larger paired
portion of the telencephalon is the basis of the
cerebral hemispheres. It contains, as divisions eer ea
of the primary cavity, a pair of cavities, the of the frog from

the dorsal surface.

lateral ventricles. The anterior portion of the ©, cerebellum; 4,

diencephalon; fv,

telencephalon, moreover, becomes differentiated, fourth ventricle; h,
cerebral hemisphere
so that a small terminal olfactory segment, the Oe LS:
rhinencephalon, is more or less _ perfectly
marked off from the rest. In the mammalian brain this part
is chiefly identifiable as the paired olfactory bulb, the latter
being the anterior portion of the olfactory lobe or olfactory brain,
and containing in its interior an extension of the lateral ventricle.
The unpaired portion of the prosencephalon is considered as
belonging in part to the telencephalon and in part to the dience-
phalon. Its cavity, the third ventricle, is connected with the
lateral ventricles through the interventricular foramen. Its
anterior wall is formed by a transverse connection of the cerebral
hemispheres, the lamina terminalis. In all vertebrates this
portion of the brain is remarkable for the manner in which its wall
is differentiated. The ventral portion extends downward as a

72 ANATOMY OF THE RABBIT.

slender funnel-like structure, the infundibulum, the tip of the
latter being in contact with the pituitary body or hypophysis and
its base connected with a small grey elevation, the tuber cinereum.

Fic. 39. Plan of the divisions of the vertebrate brain: A, embryonic; B,
adult, projection from dorsal surface; C, adult, sagittal section. The contour
of the mammalian brain is indicated by broken lines. 2

Primary divisions—PR, prosencephalon; T, telencephalon; DI, diencep-
halon; MS, mesencephalon; RH, rhombencephalon; MT, metencephalon;
MY, myelencephalon; S, spinal cord.

a.c., cerebral aqueduct; b.o., olfactory bulb; c., corpora quadrigemina;
cb., cerebellum; c.m., mamillary body; c.o., optic chiasma; c.p., pineal body;
f.i., interventricular foramen; h., hypophysis; h.c., cerebral hemisphere; in.,
infundibulum; |.t., lamina terminalis; p., pons; pl., chorioid plexus of third
ventricle; p.c., cerebral peduncle; t., thalamus, also indicates position of massa
intermedia; v.l., lateral ventricle; v.m.p., posterior medullary velum; v.q.,
fourth ventricle.

Its cavity is the recessus infundibuli. Immediately in front of
the infundibulum the ventral portions of the optic tracts join to
form the optic chiasma, and immediately behind it the floor is

ht

THE NERVOUS SYSTEM. 73

thickened, forming externally a pair of rounded protuberances,
the mamillary bodies. In the brain of the rabbit this structure
consists superficially of a larger median portion with faint lateral
elevations appended to it. Collectively, these structures are
considered to form a major division, the hypothalamus, the latter
consisting of two portions, namely, an optic portion, comprising
the infundibulum, tuber cinereum, and the optic chiasma, and a
mamillary portion, including the mamillary bodies. The two
portions are commonly classified as belonging respectively to the
telencephalon and the diencephalon, but embryological study places
the boundary of these of the latter divisions at the optic recess or
roughly at the point of the optic chiasma.

The more dorsal portion of the diencephalon, containing the
major part of the third ventricle, is known as the thalamence-
phalon. Its lateral walls are greatly thickened, while its roof
is extremely thin, especially in its anterior part. Here the actual
roof of the ventricle is formed only of a thin layer of tissue, the
epithelial chorioid lamina, but the latter has associated with it a
series of vascular ingrowths of the investing pia mater, the latter
being described in this relation as the chorioid web (tela chorioidea).
The two structures together form a chorioid plexus. This extends
downward into the third ventricle, reaching out also into the
lateral ventricles.

The dorsal portion of the thalamencephalon bears posteriorly
the pineal body, the latter together with certain related structures,
the habenulae and habenular commissure, forming the
epithalamus. The general portion of the thalamencephalon
bordering the third ventricle, and broadly connected across the
latter by the massa intermedia, is the thalamus. In the brain
of the rabbit it will be seen that the thalamus is chiefly indicated
externally by a rounded protuberance, the pulvinar. ‘The latter
is dorsal in position and is imperfectly marked off from a second
protuberance, the lateral geniculate body, lying on its postero-
lateral side. To the medial side of this is a third protuberance, the
medial geniculate body. The medial and lateral geniculate
bodies as thus defined constitute the metathalamus (Fig. 84).

The second of the primary divisions, the mesencephalon, or
mid-brain, is noteworthy in a mammal as lacking a ventricle.

74 ANATOMY OF THE RABBIT.

Its cavity is a narrow canal, the cerebral aqueduct, leading from
the third ventricle backward to the fourth ventricle, or cavity of the
rhombencephalon. Externally, its roof is differentiated into four
rounded elevations, the corpora quadrigemina, of which the
anterior pair are much larger than the posterior ones. Its floor
is chiefly formed by a pair of divergent cords, the cerebral pe-
duncles.

The parts of the mesencephalon and prosencephalon together
constitute the large brain, or cerebrum.

The third primary division, the rhombencephalon, or primary
hind-brain, is a greatly elaborated portion from which arise the
majority of the cranial nerves. The constricted area joining it with
the mesencephalon is known as the isthmus rhombencephaili.
It includes the anterior medullary velum and brachia con-
junctiva (Fig. 86). The rhombencephalon itself is divisible into
two portions, especially well defined in the mamalia, namely, the
metencephalon, or hind-brain, and the myelencephalon, or
after-brain. The former includes the small brain, or cere-
bellum, and a ventral structure of a commissural nature, the pons.
The myelencephalon is a transitional portion connecting the brain
with the spinal cord. The cavity of the rhombencephalon is the
fourth ventricle. It is a peculiarly shaped space, the floor and
lateral walls of which are very greatly thickened, while the roof is
for the most part thin. The roof appears at first sight to be formed
largely by the cerebellum, but is in reality formed by two mem-
branes underlying the latter. One of these, the anterior medul-
lary velum, is connected forwards with the mesencephalon, while
the other, the posterior medullary velum, covers a triangular
space over which the posterior margin of the cerebellum does not
extend. The posterior medullary velum has the same structure
as the chorioid plexus of the third ventricle, but is much more
poorly developed.

Apart from its principal divisions, which, as indicated above,
are more or less common to all vertebrates, the brain depends for
its external form on the elaboration of certain parts in comparison
with others. In the mammalia the cerebral hemispheres and the
cerebellum are the chief form-determinants, although the pons and
the corpora quadrigemina may also be considered in this connection.

<. > Ph oe

75

THe NERvouS SYSTEM

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76 ANATOMY OF THE RABBIT.

It will be seen also that the form of the brain is more or less depen-
dent on the existence at certain places of well-marked flexures
(cf. Plate II). The first of these, the cephalic flexure, is in the
region of the mesencephalon, the anterior portion of the brain being
bent downward; the second, or pontine flexure, is at the fourth
ventricle; while the third, or cervical flexure, is at the point where
the myelencephalon passes over into the spinal cord.

The peripheral nervous system embraces two groups of paired
and, for the most part, metamerically arranged nerves, namely, the
spinal nerves—those arising from the spinal cord and leaving the
vertebral column through the intervertebral foramina; and the
cranial or cerebral nerves—those arising from the brain and
passing through the foramina of the skull. Of these the spinal
nerves (p. 68) are less modified, both in structure and distribution.

The cranial nerves, those arising from the brain, and making
their exit through the walls of the skull, are comparable in some
respects to the spinal nerves, but in many ways are different in
THE CRANIAL nature in addition to being in some cases highly
NERVES. specialized. Three pairs, respectively, ol-

factory, optic, and acoustic, or first,
second, and eighth of the series are afferent nerves from the
special sense organs of smell, sight and hearing, the function of the
acoustic nerve including also maintenance of equilibrium. The
third, fourth and sixth nerves, respectively, oculomotor,
trochlear and abducent, are distributed as somatic motor nerves
to the muscles of the eyeball, but also contain fibres of muscle sense.

Of the remaining cranial nerves the fifth, seventh, ninth and
tenth are branchiomeric. Although the connections of these
nerves are not fully considered in the dissection as here outlined,
their chief characteristic as branchiomeric structures may be
indicated. The fifth, or trigeminal nerve, the nerve of the
mandibular arch, arises in two parts, one of which, the portio major,
is sensory, the other, the portio minor, motor. The portio minor
unites with the third or mandibular division of the portio major.
Thus, the terminal branches of all three divisions, ophthalmic,
maxillary, mandibular, are distributed as somatic sensory nerves
to the skin of the head, and, in addition, the mandibular nerve

THE DIGESTIVE SYSTEM. wir

distributes visceral motor branches to certain muscles (masticatory
group, mylohyoid and digastric) regarded as belonging to this, the
first arch. A visceral sensory connection with the mouth is con-
sidered to be formed by the lingual branch of the mandibular nerve
and by the palatine branches of the spheno-palatine ganglion.
Both are, however, connected with the central nervous system
through the seventh nerve, the former by the chorda tympani,
and the latter by the great superficial petrosal.

The seventh, or facial nerve, is the nerve of the second, or
hyoid arch. It is chiefly distributed as a visceral motor nerve to the
cutaneous muscles of the head, but contains also taste fibres from
the tongue. The ninth, or glossopharyngeal nerve, belonging
to the third arch, the tenth, or vagus, belonging to the fourth and
succeeding arches in lower forms, and the eleventh, or spinal
accessory nerves, the latter apparently related to the vagus as a
motor portion, are distributed as visceral motor nerves to the
pharyngeal and laryngeal musculature, and as visceral sensory
nerves to various visceral organs, the ninth nerve supplying the
gustatory organs of the tongue. The vagus contains a variety of
fibres, both afferent and efferent, the former from the larynx and
respiratory organs, the latter distributed to the organs of circulation
and digestion. The spinal accessory has a characteristic distribu-
tion to the cleidomastoid, sternomastoid and trapezius muscles of
the side of the neck and shoulder. The twelfth, or hypoglossal
nerve has the relation of the ventral or motor portion of a spinal
nerve, and is distributed as a motor nerve to the muscles of the
tongue.

THE DIGESTIVE SYSTEM.

The digestive system comprises as its chief portions the
digestive tube and the digestive glands. The digestive tube is
divisible into several parts, which, with the exception of the caecum
and its vermiform process, are arranged in a linear series. The
digestive glands comprise the oral glands, the liver, and the
pancreas. They are parts of an extensive series of epithelial
glands, otherwise contained within the wall of the tube, and for
this reason not appearing as gross structures.

78 ANATOMY OF THE RABBIT.

The parts of the digestive tube may be classified as follows:

i ORAL "CAVITY: 5. SMALL INTESTINE.
Oral cavity proper. Duodenum.
Vestibulum oris. -Mesenterial intestine.

Jejunum.
2. PHARYNX. Ileum.
Nasal portion.
Oral portion. 6. LARGE INTESTINE.
Laryngeal portion. Caecum.
Vermiform process.
3. OESOPHAGUS. Colon.
Rectum.

4. STOMACH.

The digestive system comprises a variety of functions, both
mechanical and chemical, and connected directly and indirectly
with the digestion of food. In the oral cavity solid food is com-
minuted by the action of the teeth, and is mixed with salivary
DIGESTION AS secretion, so that it is more easily swallowed
A PROCESS. and passed along the cesophagus to the

stomach. The secretion of the oral glands is
thus important chiefly for its mucous element, but that of the
parotid especially contains an enzyme, ptyalin, which is capable
of converting starch into soluble material. Food is further reduced
to a pulp-like mass in the stomach, while the gastric secretion,
containing pepsin and rennin, exercises a dissolving action upon
proteid, and a coagulating action upon milk. The liver secretion,
known as bile, contains, in addition to coloring materials, salts
which exert a splitting action upon fats. The pancreatic secretion _
contains a variety of enzymes, converting proteids and starches,
and breaking fats into fatty acids and glycerin. The actions of the
dissolving enzymes is successive, secretion being dependent to some
extent on antecedent bodies by which the stimulus for secretion is
determined. The preliminary processes of digestion refer in this
way to the mechanical action of food passage along the canal,
and to the provision of converting enzymes. Absorption, which
the final object of the digestive process is accomplished chiefly
in the large intestine through the bloodvessels and lymphatics of

THE DIGESTIVE SYSTEM. 79

the wall. The relatively great extent of the wall, including the
enormous development of the caecum in the rabbit and other
rodents, is related to the comparatively great bulk and low nutri-
tive quality of the ingested food.

In its most general features the digestive system is significant
as an epithelial tube, in which the food is modified, by solution or
otherwise, so that it is capable of being absorbed through the
epithelial surface. In the form of the digestive tube as seen in a
vertebrate, however, a number of gross mechanical features are
evident, such, as, for example, the increase in capacity, or in
absorptive area, through the folding of the mucous membrane,
or the expansion of the wall; or again, the presence of a special
muscular tunic, and its modification at certain places, as in the

Fic. 40. Plan of successive embryonic stagesin displace -
ment of the digestive tube and common mesentery from the
midline position (man): a, tr, d, ascending, transverse, and
descending colons; r, rectum; si, small intestine; st, stomach
Modified from figures by Toldt and Hertwig.

oesophagus, the pyloric limb of the stomach, and the first portion
of the colon. Moreover, many features of the abdominal portion
of the tube, and, indeed, certain of its recognized divisions, depend
FORM AND on its relation to an extensive serous sac—in a
SYMMETRY. ‘™ammal the peritoneal cavity. In this connection

it is to be considered that the digestive tube is
primarily a median structure. It has this relation in the earlier
stages of embryonic development (Figs. 23, 40), and in many of the
lower vertebrates it does not deviate to a great extent from a median
position. In all higher vertebrates, however, the tube becomes
greatly elongated in comparison with the cavity in which it lies, and

80 ANATOMY OF THE RABBIT.

thus becomes extensively displaced to one side or other of the
median plane. This development, while advanced in all mammals,
may be said to reach an extreme in the herbivorous mammalia;
and in many cases it is further increased by the independent
elaboration of the blind intestine or caecum. In the rabbit the
combined length of the small and large intestines is approximately
eleven times that of the body.

In considering the divisions of the digestive tube in the rabbit,
the posterior, or post-cephalic portion, comprising the oesophagus
and succeeding parts, may be distinguished from the anterior, or
cephalic portion, the latter comprising the oral cavity and pharynx.
The former is a free portion
embracing the digestive tube
proper, while the latter is a
fixed portion exhibiting a
variety of general mammalian
features connected with the
organization of the head.

The form of the anterior,
or cephalic portion of the
digestive tube (Plate II) de-

Fic. 41. The nasopharynx and related parts

of the head as seen in median Section: 1, tongue;
2, hyoid; 3, tonsil; 4, epiglottis; 5, entrance to
trachea; 6, entrance to esophagus; 7, basioccipital
bone; 8, soft palate; 9, pharnygeal aperture of
auditory (Eustachian) tube; 10, cranial cavity;
Ii, ethmoturbinal scrolls; 12, nasal cavity; 13,
nasal septum; 14, hard palate; 15, oral cavity.

pends on its fixed relation
with respect to the enclosing
parts of the head-skeleton.
In the rabbit, as in mammals

generally, the oral cavity is
divisible into two portions, of which one is the oral cavity proper,
while the other, the vestibulum oris, is a space enclosed between
the alveolar process of the jaws and the teeth on
the one hand and the cheeks and lips on the other.
As in other vertebrates, the tongue is a muscular
structure projecting upward and forward into the oral cavity from
its base of attachment on the hyoid apparatus, but its greater
elaboration, as well as the differentiation of special processes, the
circumvallate and foliate papillae, for the accommodation of the
gustatory organs, are features of mammalian significance. The
roof of the oral cavity is formed by an extensive palatal surface,

PRINCIPAL
DIVISIONS.

THE DIGESTIVE SYSTEM. 81

comprising the hard palate, and the membranous, or soft palate.
These structures also form the floor of the accessory respiratory
tracts of the nose, the posterior aperture being thus carried back-
ward to a point more directly above the aperture of the larynx.

Meanie

yve 4

ee ey acne

Fic. 42. The stomach, liver, and portal connections. Stomach: c, body;
cr, cardia; f, fundus; py, pyloric limb; du, duodenum. Liver:ir, renal impres-
sion; la, lp, anterior and posterior lobules of the left lobe; la’, lp’,anterior and
posterior lobules of right lobe; Ic, caudate lobe; v, gall bladder. Vessels:
d’, opening of common bile duct (shown as transparency); h, hepatic artery;
p, portal vein; ve, tributaries of coronary vein. The ramifications of the portal
vessels are indicated right (d) and left (s),

The chief features of the pharynx depend on its relation as a
common or general portion of the digestive tube with the tubes of
the respiratory system. It is divisible into an oral portion,
representing the direct connection of the oral cavity with the
oesophagus, a dorsal or nasal portion, connected with the nasal

82 ANATOMY OF THE RABBIT.

fossae, and also with the middle ear through the internal auditory
tube, and a ventral or laryngeal portion, containing the aperture
of the larynx (Fig. 41).

The oesophagus is a slender but greatly expansible tube leading
from the pharynx to the stomach. In its passage backward it
traverses the neck and the thorax, and in both regions occupies a
median position. In the thorax (Plate VII) it will be observed
that it lies between the heart and the dorsal aorta, thus exhibiting
the original relation of the digestive tube ‘to the aortic portion of
the vascular system. The succeeding portions of the digestive
tube are those associated with the peritoneal cavity, and with the
exception of the terminal portion, the rectum, are displaced from a
median position. Consequently, the divisions which are recognized
are based partly on the differential characters of the wall, and partly
on the position of structures more especially in relation to the
supporting peritoneum. Thus, the chief features of the stomach
(Fig. 42) depend on the expansion of the organ and the rotation
of its pyloric end forward and to the right. In the intestinal tract
as a whole the chief, although by no means most conspicuous feature
of position, depends on the looping of the entire structure on itself,
so that the terfhinal portion, chiefly the transverse colon, crosses
the ventral surface of the duodenum and then turns backward on
the dorsal surface of the mesenterial small intestine. The duo-
denum is sharply marked off from the mesenterial intestine as an
extensive loop, containing the major part of the pancreas and its
duct, and lying on the right side of the dorsal wall of the abdomen.
The common bile duct enters its first portion immediately
beyond the pylorus. The mesenterial intestine is a greatly
convoluted portion, lying chiefly on the left side of the abdominal
cavity, and loosely supported by the broad, frill-like mesentery.
From the pylorus to the sacculus rotundus there is no abrupt change
in the character of the wall, although the first portion of the mesen-
terial intestine, that designated as the jejunum, and the duodenum
may be considered together as a more vascular portion with thicker
walls in comparison with the second portion, the ileum, in which
the wall is less vascular and more transparent.

The main portion of the large intestine, the colon, although
greatly specialized, may be considered to consist as in man of

THE DIGESTIVE SYSTEM. 83

ascending, transverse, and descending parts, that is to say the
ascending colon lies on the right side of the body and passes in a
general way from its point of origin on the caecum forward to a
point where it becomes flexed to the left as the transverse colon;
the latter crosses the body and is flexed backward as the descend-
ing colon. In the rabbit, however, that portion definable as the

Fic. 43. The caecum and vermiform process: c’, ¢’’, c’”’, first,
second, and third limbs of the caecum; ca, beginning of the ascend-
ing colon; il, ileum; pv, vermiform process (appendix); sr, sacculus
rotundus.
ascending colon is greatly elongated, and is composed of five
principal limbs, united by flexures. Two of these, in dissection
from the ventral surface, are concealed by the base of the superior
mesenteric artery, since they lie on its right side. The descending
colon is also only nominally related to the left side of the body wall,
its supporting peritoneum, the descending mesocolon, béing closely
connected with the mesoduodenum of the ascending limb of the
duodenal loop. The course of the caecum (Fig. 43) as it lies in
the body is comparable to two turns of a left-hand spiral, its blind

84 ANATOMY OF THE RABBIT.

termination, the vermiform process, being dorsal in position and
directed for the most part backward. It may be observed at this
point that in their vascular supply the more typical divisions,
namely, the transverse and descending colons, have arterial branches,
respectively, the middle and left colic arteries, comparable to those
of man; while on the other hand the right colic relation, on account

Fic. 44. The heart and lungs from the ventral surface: ad,
right atrium; ao, aorta; ap, pulmonary artery; as, left atrium;
d, right superior caval vein; i’, i’, left and right inferior lobes
of lung; 1, aortic ligament; m’, m’’, middle lobes; ml, medial
lobule of right inferior lobe; s’, s’’, superior lobes; tr, trachea; 2
vd, right ventricle; vp, pulmonary veins; vs, left ventricle.

of the great complexity of its parts, is represented by a large»
number of vessels, branches of a common ileocaecocolic trunk.

THE RESPIRATORY SYSTEM.

In all air-breathing vertebrates the lungs (Fig. 44) are paired
sacs which arise embryonically as ventral outgrowths of the digestive
tube, and are secondarily connected with the outside of the body

THE RESPIRATORY SYSTEM. 85

through special perforations of the anterior portion of the head
and through the oral cavity. The principal connection in a
mammal is represented by an extensive nasal cavity bearing on its
lateral walls the olfactory sense-organs. It is distinguished as an
accessory respiratory tract from the true respiratory tract
formed by the trachea and its terminal divisions, the bronchi.
The respiratory system as represented by the lungs and related
tubes, is nominally ventral to the oesophagus, but this relation
is chiefly true of the trachea. In the thorax (Plate VII) the
bronchi are, in general, interposed between the oesophagus and
the heart, the lungs being expanded laterally into the paired
pleural cavities.

In addition to carrying air over the sensory, olfactory surfaces,
the respiratory system has accessory functions in relation to
respiration. The mucous membrane of the nose, including that of
the turbinated surfaces, serves both to warm the air, and to remove
particles of foreign material. The chief function is, however,
respiratory.

In a mammal, respiration is both a physicochemical and a
mechanical process. The former is fundamental, and consists in the
supply of oxygen to the blood, and in this way to the tissues, for
the oxidative phases of metabolism; also in the discharge of waste
RESPIRATION AS gases, principally carbon dioxide, from the
A PROCESS. blood to the air. The absorption and

transport of oxygen is a specific function
of the red blood cells. Though the oxygen, of which a certain
amount always remains in the lungs during the process of breathing,
must pass through the thin epithelial covering of the terminal air
sacs into the capillaries before it can be taken into the blood cells,
the latter from their flattened shape and very great numbers
present a relatively enormous surface for absorption, the process
being thereby facilitated. The lungs themselves are highly elastic,
expansible sacs. They have the structure of greatly ramified
saccular glands, except that the free internal surfaces are every-
where in contact with air. The division of the trachea into its
bronchi, together with the bronchial ramifications, are the trunk
and main branch portions of a rather complex system of tubes
(Fig. 45), of which the terminal air-spaces are the final and func-
tional parts.

86 ANATOMY OF THE RABBIT.

What is commonly described as respiration, or the act of
breathing, is a mechanical, muscular process accessory to respira-
tion. It consists in the expansion of the thorax, so that a partial
vacuum is created and the lungs fill with air, the expansion being
followed by relaxation, in which the air is expelled.
The first portion of this action, known as inspir-
ation, is brought about by the contraction of the intercostal and
related muscles, by which the ribs are raised, and by the contraction
of the dome-shaped diaphragm, by which the posterior wall of the
thorax is flattened, and incidentally the abdominal viscera displaced
backward. Both actions tend to enlarge the thoracic space. The
action of the diaphragm is
controlled directly by the
phrenic nerves, but all re-
spiratory movements are de-
Se pendent upon the cervical
and thoracic spinal nerves,
and there is also a respiratory
control centre in the medulla.
The expulsion of air, expira-
tion, is accomplished without
‘muscular contraction.

BREATHING.

Respiration as a general
Fic. 45. The larger terminal ramifications of function is common _ to all
the left inferior bronchial ramus, from the dorsal 5
surface; metallic cast of the interior ; cf. Figs. organisms. Though always
44 and 83, i. : :
constructed for easy diffusion,
the organs by which the function is discharged differ profoundly
in the various groups. This is true even within the limits of the
vertebrates, where lower forms are characterized by gills for
LUNGS AND aquatic respiration, and the higher forms by
GILLS. lungs for air respiration. The occurrence of a
great variety of intermediate and _ transitional
growth stages, in which gills are replaced by lungs, with no modifi-
cation from one to the other, introduces a peculiar condition into
the history of vertebrates. This condition is characterized by the
appearance of gill structures in the embryos of all higher forms
(Fig. 22) and by the gradual elaboration in the series of air sacs
from a simple type, as illustrated in the frog, to the greatly branched
lung tubes of mammals.

THE VASCULAR SYSTEM. 87

THE VASCULAR SYSTEM.

In the rabbit, as in all vertebrates, the vascular system
(Fig. 46) embraces a central, muscular organ of propulsion, the
PLAN OF THE heart, and a series of branched tubes, the blood-
CIRCULATION. Vessels, the latter being of three different kinds:

(a) thick-walled, elastic, distributing vessels —
arteries ; (b) microscopic terminal canals in the peripheral organs—
capillaries ; and (c) thin-walled collecting vessels—veins.

The chief mammalian feature in this system consists in the
division of the heart into two portions,
each consisting of a receiving chamber, or
atrium, and a driving chamber, or
ventricle, and the arrangement of their
vascular connections in such a way that
two complete circulations are established.
One of these is the long, or systemic
circulation. It is concerned with the
distribution of blood to the various parts
of the body, with the exception of the lungs. """~
It is established by the left ventricle, the
aorta, the carotid and subclavian branches
of its arch, and the parietal and visceral
branches of its thoracic and abdominal
portions. The blood is collected from the body wal post limbs
anterior portions of the body through ee AG. NES AAI

. . irculation. rv, lv, right and
paired internal and external jugular and {eft ventricles; ra, la, right and

. . . . . left tri SCH SD FEES il
subclavian veins, communicating with the atves of gorta and pulmonary

right atrium of the heart through paired Sioventricular ehh are
superior cavals; from the posterior portions

of the body through the unpaired and also asymmetrical inferior
caval vein, the latter passing forward on the right of the median
plane and entering the posterior end of the right atrium. The
second, short, or pulmonary circulation, is concerned with
the distribution of the blood to the lungs for purposes of aeration.
It is established by the right ventricle, the pulmonary artery and
its paired branches, and by the capillaries of the lungs. The blood
is delivered to the left atrium through several pulmonary veins.

head, ant.limbs

-precardial sustemic’
lungs

precaval
vein----

88 ANATOMY OF THE RABBIT.

A similar division of the circulatory organs occurs as a homoplastic
modification in birds, which, it will be observed, are also warm-
blooded vercebrates.

In general, the blood which is distributed to the various parts
of the body passes through but one set of capillary vessels, and is
then returned through the systemic veins to the heart. In all
vertebrates, however, a special portion of the systemic venous
circulation is set aside as the hepatic portal system (Fig. 42),
distinguished by the possession of a second series of capillary
vessels ramifying in the liver. Thus, in the rabbit, the blood
distributed to the stomach, spleen, and intestine through the coeliac,
superior and inferior mesenteric arteries, is collected into a main
intestinal vessel, the portal vein, and the latter, approaching the
liver through the lesser omentum, divides in that organ into a series
of portal capillaries. The portal capillaries, like the systemic
capillaries proceeding from the hepatic artery, unite in the tribu-
taries of the hepatic veins. In lower vertebrates, although not io
the mammalia, a second system of venous capillaries occurs in
connection with the kidneys and is known as the renal portal
system.

The ultimate function of the vascular system is connected with
interchange of materials in the tissues. This is brought about
through the medium of microscopic capillaries, the gross parts
of the system being concerned with transportation of materials
FUNCTIONS OF from one part of the body to another.
CIRCULATORY System. Lhe propulsive action of the heart is

muscular and rhythmic, contraction, or
systolic phases, alternating with expansion, or diastolic phases
(cf. p. 63). The flow is maintained in one direction principally by
atrioventricular valves of the heart, and by the semilunar valves
the of the aorta and pulmonary arteries, though there are also valves
in the course of some of the veins. The arteries are tubes with
thickened elastic walls. They are expanded by the impulse of
blood from the heart, contraction of which is followed by a pulse
wave in the arteries. The passage of blood into the capillaries
takes place more slowly and uniformly, while the arteries contract
to their previous diameter. In the return of the blood the veins
are largely passive, acting merely as closed channels connecting the

THE VASCULAR SYSTEM. 89

capillaries with the heart. The control of the muscular action of
the heart and arteries through the vagus and sympathetic nerves is
an important element in maintaining pressure and tone in the
vascular system (cf. p. 62).

Many of the peculiar features of the mammalian circulation
which at first sight do not appear to be general, but are so in
reality, depend on the circumstance that the complete partition
of the organs is a final stage of a general progressive development,
observable in air-breathing vertebrates, in which the lungs and their
vascular connections becom? per-
fected for pulmonary respiration.
On the other hand, the vascular
system as it appears in the embryo,
more especially its aortic portion,
is arranged according to the type
of branchial respiration as found
in fishes. In this condition the
blood is sent forward from the
heart through a ventral aorta. The
latter is connected with a series of
paired branchial aortic arches,
traversing the rudimentary gill struc-
tures, and thus passing upward

aroun the sides f Fic. 47. Planofthe branchial aortic

PRIMITIVE th v d it d I Q arches. ‘ne adult mammalian vessels
€ rimitive la- are indicated in black (systemic) or

AORTIC ARCHES. P Pp shaded (pulmonary). 1-6, primary
rymx. The dorsal arches; ao., aorta; a.p., pulmonary

. 7 . artery; c.e., external carotid; c.i., in-

aorta is formed by the junction of the ternal carotid; d.a., ductus arteriosus
(Botalli); i., innominate artery; s.d.,

branchial aortic arches, and passes _ right subclavian; s.s., left subclavian.
backward as a main distributing erie eat oe

vessel on the ventral side of the axial support. The heart itself
is formed primarily on a two-chambered plan, similar to that in
fishes, where all the blood is received by single atrium and is
delivered forward to the gills by a single ventricle.

The definitive condition of the chief arterial vessels is arrived
at by an extensive modification of the branchial plan. As indicated
in the accompanying diagram (Fig. 47) the arch condition is retained
by the aorta and by the pulmonary artery, and it is interesting to
note also that the primary connections of these vessels, represented
in the embryo by an open canal, the ductus arteriosus (Botallt), is

90 ANATOMY OF THE RABBIT.

indicated in the adult condition bya short fibrous cord, the arterial
ligament. The adult aortic arch represents only the left one of a
pair, and since that of the right is only represented imperfectly
by the mnominate artery and the base of the subclavian of that side,
a condition of asymmetry results, which is mainly expressed by
the sinistral position of the arch with reference to the oesophagus
(Plate VII). By comparison with the embryonic plan, it is seen
that the primitive features of the heart and the arterial vessels
include the ventral position of the heart itself, the equivalence of
the two atria and of the two ventricles—these structures being
partitioned internally but imperfectly divided externally—the
forward position of the first portion of the aorta, and the position
of the aorta as a median vertebral trunk.

The vascular system is noteworthy for several departures from
the condition of symmetry, one of these having already been
mentioned. In addition, it is seen that in a mammal, as in terres-
trial vertebrates generally, the base of the pulmonary artery
(Fig. 44) is rotated in a spiral fashion about the base of the aorta,
so that from its beginning on the right ventricle it passes across the
ventral surface of the base of the aorta, and divides on the dorsal
side of the latter into its two main branches. Moreover, the
separation of the ventricular portion of the heart into two cham-
bers is associated with an enormous increase in the muscularity of
the wall in the left ventricle, or, in other words, in that portion
which is concerned with the larger, systemic circulation. The
inferior caval vein (Plate VIII), a highly specialized vessel, is
asymmetrical, since from its beginning in the pelvic cavity to its
termination on the right atrium it lies wholly to the right of the
median plane. The azygos vein of the thorax (Plate VII), a.
vessel uniting the majority of the paired intercostal veins, and
interesting as a remnant of the primitive circulation, is also asym-
metrical, since the trunk lies to the right of the bodies of the verte-
brae, and is connected at its base with the right superior caval vein.

THE LYMPHATIC SYSTEM.

The lymphatic system, both in its functional relation and in
origin, is an appendage of the venous portion of the vascular system.
The system is an important one, of which, unfortunately, little

THE LYMPHATIC SYSTEM gI

may be seen by ordinary dissection, the structures which appear in
this way in being the lymph glands, or lymph nodes, centres of cell
formation, occuring in the course of the conducting vessels. These
as superficial structures are found either singly, as in the head and
neck, or more or less grouped, as in the axillary and inguinal spaces.
As deep structures they are conspicuous in the intestinal mesen-
teries, and in the walls of the digestive tube, occuring in the latter
chiefly as continuous masses of lymph follicles, as, for example,
in the walls of the sacculus rotundus, the vermiform process, or the
tonsil; or, again, as aggregated lymph follicles (Peyer's patches)
at various points in the intestinal wall.

The conducting portion of the
system comprises an _ extensive
series of canals, beginning as lym-
phatic capillaries in peripheral
organs, and ending as lymphatic
trunks which empty into the great
veins. The lymphatic trunks of
the anterior portion of the body are
designated from their association
with the corresponding veins as
jugular and subclavian. They
enter the venous system on either
side at the point of junction of the
internal and external jugular veins — semaie i SR

: urinary bladder; cc, crura clitoridis; cp,
ono the common jugular and sub- “Gira penis: hdd, ductus: deferens.” ep.

clavian (Fig. 82). Shae Ree ay Sie asa: a

Wikier lycnphaticuvessela ot! the Wa ceminalvescel yetvestinulum.
posterior portion of the body,
including the intestine, unite to form a common canal, the thoracic
duct. The latter lies for the most part between the aorta and the
vertebral column, and traverses the thorax in this position to enter
the venous system at the same point as the jugular and subclavian
trunks of the left side.

The lymphatic capillaries are terminal, absorptive vessels,
differing from blood capillaries both in the character of their walls
and in their relations to other portions of the system, since they
are not interposed as in the vascular system between vessels of a
larger order. The lymphatic vessels connecting the capillaries

92 ANATOMY OF THE RABBIT.

with the lymphatic trunks form extensive plexuses, in connection
with which the lymph nodes are distributed.

THE URINOGENITAL SYSTEM.

The urinogenital system comprises two primary systems—
reproductive and urinary—differing widely in their central organs,
but associated to a certain extent by having common ducts. In
the rabbit, as indicated in the accompanying diagram (Fig. 48),
this association extends only to the presence in the two sexes of a
urinogenital canal, or urinogenital sinus connecting both urinary
and genital structures with the outside of the body. This canal is
designated in the male as the urethra, but in the female as the

ov

Fic. 49. The principal stages in specialization of the female urinogenital
ducts in vertebrates. A, frog; B, monotreme; C, marsupial. bl, bladder;
cl, cloaca; k, kidney; od, oviduct; ov, ovary; r, rectum* u, ureter; us, urinogen-
ital sinus (vestibulum); ut, uterine tube; v, vagina. Chiefly from figures of
Gegenbaur and Wiedersheim.

vestibulum, since the structure known from the human relation
as the female urethra is only a urinary canal leading from the .
bladder, and in man is not associated with the reproductive ducts.
In primitive vertebrates (Fig. 49), the urinary and genital ducts
open into the posterior end of the digestive tube, the latter forming
URINOGENITAL DUCTS in this relation a common canal, the
IN VERTEBRATES. cloaca. In terrestrial vertebrates, the
urinary bladder is developed as a ventral

outgrowth of the digestive tube, and, except in amphibians, both
sets of ducts undergo a migration from their original position on
to the wall of its canal, the latter being thus trasfnormed into a

THE URINOGENITAL SYSTEM. 93

urinogenital sinus. This development reaches its extreme in the
higher mammalia, where the urinogenital sinus is completely
separated from the digestive tube, and where the urinary ducts are
also transferred from a posterior or hypocystic position on the
wall of the urinogenital sinus to an anterior or epicystic position
on the dorsal wall of the bladder.

The chief organs of the urinary system are the kidneys. They
are paired organs, lying against the dorsal abdominal wall, approxi-
mately in the position of the embryonic inter-
mediate cell mass from which they are formed.
That of the left side is displaced backward, out of the position |
of symmetry, on account of the posterior
development of the greater curvature
of the stomach. The kidneys appear
as solid organs, brownish in colour and
bean-like in general shape, enclosed by
a fibrous coat, and connected medially
with the expanded end of the ureter.
In the rabbit the kidney appears as an
almost continuous mass, in which, how-
ever, slight traces of lobulation can be
distinguished. In many mammals,
such as sheep and bear, the organ is

THE KIDNEYS.

Fic. 50. The left kidney, divic

composed of distinct and separable ed horizontally lengthwise, cut sur-
face of dorsal half. c, cortical sub-

Iopiles «his condition isiclearly shown. Sence, ty medullany, ‘substance;

i x : p, renal papilla; u, ureter.
in the human kidney in foetal life, and

though much more concentrated in the adult, the lobulated con-
dition appears internally in the division
of the ureter into several renal calyces,
each of them connected with a corresponding renal papilla.

When horizontally divided (Fig. 50), the kidney is seen to be
made up of a more vascular and granular external layer, termed the
cortex, and of a somewhat radially striated, central mass, termed
the medulla. In the rabbit, there is a single renal papilla, and
the expended end or pelvis of the ureter is undivided. Notwith-
standing the solid appearance of cortex and medulla, the kidney is
made up of a system of tubules, the relation of which to the vascular
system and to the outside of the body is such that fluid materials

FORM IN MAMMALS.

94 ANATOMY OF THE RABBIT.

to be excreted may pass into them from the blood stream. The
primary tubule (Fig. 51A) begins in each case in the cortical
substance with a cup-like structure, known as a renal or Mal-
pighian corpuscle, containing a network or glomerulus of
minute vessels from the branches of the
renal artery. The blood stream is thus
separated only by a thin membrane from
the cavity of the tubule, the wall of which is complete. The
terminal parts of the tube system have a characteristic course
in the kidney substance, which accounts for the difference in
appearance as between the cortex and
medulla, and are ultimately connected with
common collecting tubules opening on

INTERNAL STRUCTURE
AND FUNCTION.

e the surface of the papilla. The excreted
fluid, urine, contains characteristic nitro-
genous waste materials, usually urea, but
in some cases hippuric acid. These mater-
ials are formed in the liver and perhaps

m

elsewhere in the body.

Like all other parts of the urinogenital
the structure and embryonic
development of the mammalian kidney
affords a remarkable illustration of the
extent to which the adult form of an organ
is dependent upon ancestry. In the verte-

system,

Fic. 51. Kidneytubules. A,
plan of arrangement in adult
mammal: c, cortex; m, medulla;
gl, vascular glomerulus; - tc,
proximal convoluted portion of
tubule; tr, proximal straight por-
tion of tubule (Henle’s loop);

tp, collecting tubule to renal

papilla. From Weber, after v.
Ebner. B, plan of a single
primitive kidney tubule in

lower vertebrates, glomerulus;
np, nephrostome; cl, coelomic
epithelium; t, body of tubule;
d, longitudinal duct.

brate phylum, three pairs of kidneys have
been recognized. They occur in antero-
posterior order in the bedy, either in embryo
or adult, they are of increasing special-
ization, and their order of appearance and

functional value are directly associated with the degree of general

specialization of the groups in which they occur.

HOMOLOGIES OF

VERTEBRATE KIDNEYS, Mesonephros, and metanephros.

mammals, while the mesonephros is embryonic.

These organs
have been designated as pronephros,
The
metanephric kidney is characteristic of
The latter is,

however, the adult kidney in the frog and allied animals, its duct

ia the male serving both as reproductive duct and ureter.

The

THE URINOGENITAL SYSTEM. 95

presence of this kidney in embryo mammals, together with its duct,
determines the form and connections of the ductus deferens with
the terminal portions of the urinary system. The pronephros, on
the other hand, is a vestigial kidney in all vertebrates, but its duct

Fic. 52. The male urinogenital ducts and related
structures, viewed from the lateral surface. After Rau-
ther: a., anal aperture; bu., bulbourethral gland; ci.c.,
corpus cavernosum; d.d., ductus deferens; g.a., anal
(rectal) gland; g.i., inguinal gland; g.p., glans penis; par.,
paraprostatic glands; pr., prostate; r.,rectum; ur., ureter;
u.v., urethra (membranous portion); v.s., seminal vesicle;
v.u., urinary bladder.

system, open proximally to the body cavity plays an important
part in the formation of the oviduct in the female of all classes of
vertebrates. The development of the kidney reveals a condition,
also suggested in a less perfect way by the mesonephros, in which
it is shown that the primary connections of the kidney tubules are

96 _ ANATOMY OF THE RABBIT.

with the coelomic cavity (Fig. 51 B). Their ciliated internal
openings, termed nephrostomes, are not developed in specialized
kidneys, but may be seen, even in the adult condition, in some
lampreys, where they communicate with that portion of the body
cavity enclosing the heart.

In the rabbit, as in all mammals, the male gonad or testis (Fig. 55),
in which the male germ cells,
spermatozoa, are matured,
is connected with the peri-
pheral duct system (Fig. 52)
by means of the epididymis
and the ductus deferens,
parts of the mesonephric
connections of the embryo.
While the ductus deferens is a
single tube, the epididymis
THE consists of an ag-
TESTIS gregation of small
AND _ tubules, lying chiefly
IT'S toward the anterior
puck end of the testis,
but with the tubules not
individually discernible. The
testis is formed in the embryo

Fic. 53. Thefemale urinogenital system: a, all vertebrates a associa-
aorta: as, internal spermatic artery; au, umbilical tion with the dorsal abdo-
artery; c, clitoris; gp, perineal gland; gr, rectal 4 *
gland; h, hydatid of uterine tube; hr, middle minal wall, but in many
hemorrhoidal artery; i, inferior caval vein; lo,
ovarian ligament; It, round ligament; lu, umbilical mammals moves backward to

ligament; ms, mesosalpinx; mt, mesometrium; SIN .
0, Ovary; ot, ostium tube; r, rectum; rp, peritoneal a scrotal position, either

recess (rudimentary vaginal process); cu, uterine f i
ie es AG Meat We artes re ees peciodically ge penn cary
in the course of development.
This change, known as the descent of the testis, is brought about
through the agency of a muscular cord, the gubernaculum, the
connections of which in the rabbit, are retained in the adult condi-
tion. The migration of the organ determines a number of peculiar-
ities in the relations of its bloodvessels and peritoneal connections.
The female gonad or ovary (Fig. 53) lies on the dorsal wall of the

abdominal cavity, thus retaining to a large extent the primitive

THE URINOGENITAL SYSTEM. 97

position. Its gubernacular connections are, however, plainly
discernible in the adult animal by the ovarian and round liga-
ments, the latter being inserted into a small pocket of the abdom-
inal wall simulating the testis sac.

Though inconspicuous in gross size as compared with the testis,
the ovary is concerned with the formation of cells of relatively large
dimensions, the female germ cells or ova. These undergo their
THE OVARY principal development as single cells in the
AND OVIDUCTS. tissue of the organ, but at times, through

rupture of the enclosing follicles, they gain
access to the surface, and thence pass directly into the open mouth
of the uterine tube. If fertilized, they begin their segmentation
and further development into an embryo, the latter becoming
attached to the wall of the
uterus. A placental con-
nection is formed by which
nourishment is carried to the
embryo, during the period of
intra-uterine life, in the rabbit
aboutchieny days. Wheresare «irene: Avuterus dunies B acerus bcos; ©
mwoleomiplete ter, thevcavis (cue oes oy Seeteae ; tubes us uteris:
ties of which are connected
distally with the unpaired vagina, and through this with the
urinogenital sinus. The size and appearance of the uteri depend
upon the age of the animals examined, and upon whether or not
they are pregnant or have borne young. The uteri of pregnant
females are greatly enlarged and vascular. They contain from five
to eight young, the position of which may be easily seen from the
expansion of those parts of the tubes in which they lie.

The paired condition of the uteri in the rabbit is especially
instructive because of its primitive nature as compared with that
in many othermammals. Paired oviducts (Fig. 49A) are the rule
in lower vertebrates, where the function is simply to carry the
eggs to the outside of the body, This condition is retained with

minor modifications to the mototreme stage of mammals, but in
higher forms of the latter the ducts are successively coalesced.
In marsupials there are still paired vaginae, while in placentals the
structure is unpaired. In the rabbit, as in many lower placentals,

98 ANATOMY OF THE RABBIT.

there are two complete uteri, and as an organ the whole structure
represents the stage of uterus duplex (Fig. 54 A). A partly
fused condition existing in some mammals, for example sheep, is
known as uterus bicornis (Fig. 54 B), while the completely fused
condition in man is known as uterus simplex (Fig. 54.C). It is
characterized by the independent opening of the two uterine tubes
into a single uterine cavity. The successive stages of coalescence
are doubtless associated with progressive reduction of the number
of young, the success of the species being determined by greater
perfection of the placental apparatus.

THE SEROUS CAVITIES.

The organs collectively described as visceral are those associated
with the serous cavities They belong to several systems, but
present the common feature of being projected into the membranous
linings of these cavities so that they are more or less completely
invested by them.

The serous sacs are extensive body-spaces, derivatives of a
primary body cavity or coelom. They are usually considered as
containing the visceral organs, but the condition is more accurately
described as one in which the visceral organs encroach, chiefly from
a dorsal position, on the enclosing membranes. The latter are thus
divided into two portions, one of which is distributed as a parietal
or peripheral layer, forming the enclosure of the sac, while the other
is disposed as a visceral layer on the surface of the visceral organs.
The serous sacs are enclosed by thin, moist, serous membranes,
consisting chiefly of mesothelium, which give to the visceral organs
their characteristic appearance. ;

In lower vertebrates, where the diaphragm is absent or imper-
fectly developed, the coelom is divided into two chief portions—the
pericardial cavity, enclosing the heart, and the pleuroperitoneal
cavity, lodging the remaining visceral organs, including in terres-
trial vertebrates the lungs. In the mammalia the pleuroperitoneal
cavity is completely divided into two portions by the diaphragm,
the smaller pleural portion being again divided into right and left
pleural cavities through the presence of certain structures filling
the median portion of the thorax. There are thus recognizable in a

THE SEROUS CAVITIES. 99

mammal four large serous spaces, namely, the pericardial peri-
toneal, and paired pleural cavities.

The pericardial cavity, the smallest of these spaces, is situated
between the paired pleural cavities. Its enclosing membrane, the
pericardium, forms a capacious sac for the heart, and is reflected
directly over the surface of the latter as a thin membrane, the
epicardium.

The pleural cavities are those lodging the lungs, the latter
being projected into them from a medial position. The lining
membrane or pleura is divided into three chief portions—the
pulmonary pleura, investing the greater part of each organ, the
costal pleura, lining the internal surface of the thorax, and the
diaphragmatic pleura, covering the anterior surface of the dia-
phragm. The latter is broadly connected with the pulmonary
pleura through the pulmonary ligament.

The peritoneal cavity, the largest of the serous spaces, com-
prises in a mammal a general portion, the abdominal cavity, and
its posterior extension into the pelvis—in the male also into the
sac of the testis. The general relation of the cavity to the
abdominal organs is indicated diagrammatically in Fig. 21. Its
lining membrane, the peritoneum, is divisible into two portions,
the parietal peritoneum, lining the abdominal wall, and the
visceral peritoneum, investing the visceral organs. Of the latter
the kidneys encroach only to a minor extent on the serous lining,
so that they are covered by peritoneum only on their ventral sur-
faces. The digestive tube, on the other hand, is removed to such
an extent from the abdominal wall that the peritoneum forms a
complete serous coat, and is connected with the parietal peritoneum
of the wall through a thin transparent membrane, the mesentery.
The latter consists of two plates of peritoneum, enclosing between
them a thin layer of connective tissue, the lamina mesenterii
propria, for the transmission of nerves, bloodvessels and lymph
canals.

As indicated above, the relations of the abdominal portion of the
digestive tube are greatly modified by its elongation and displace-
ment from a median position. Thus, while in the embryo the
common mesentery (Fig. 40) is recognizable as a continuous
median vertical fold, in the adult it follows the convolutions of the

100 ANATOMY OF THE RABBIT.

digestive tube, and is therefore considered as divided into corres-
ponding parts. In many cases the relations of these are greatly
complicated by secondary adhesions. In the rabbit the mesoduo-
denum, mesentery, and descending mesocolon will be recog-
nized as parts in which a more typical arrangement is retained.
Moreover, in the anterior portion of the abdominal cavity the
peritoneum is concerned not only
with the investment of two large
visceral structures, the stomach and
the liver, but also with the formation
of a lining for the posterior surface
of the diaphragm. Thus the general
condition is less simple than in the
small and -large mtestines. - The
peritoneum, passing from the dorsal
wall, successively invests the spleen,
the stomach, and the liver, and is
reflected from the last-named struc-
ture to the diaphragm and the ventral
body-wall through the coronary,
triangular, and falciform liga-
ments. Its gastric portion is differ-
entiated into the mesogastrium
Fic. 55. Diagram showing the rela~ (phrenicosplenic and __ gastrolienal

tion of the testis to its investments:

a.i., inguinal ring; c.e., caput epididy- ligaments), the greater omentum,

midis; cr., cremaster muscle; dd, ductus

deferens; zg, gubernaculum; mes, mes- and the lesser omentum. Similarly,

orchium; ptv, and vtv, parietal and
visceral layers of the tunica vaginalis in the posterior part of the body the

propria; pv, cavity of the vaginal*pro-

Bs eae a a peritoneum passes from the rectum
to the urinary bladder, enclosing also

in the female the vagina. Ic is then reflected to the ventral body-

wall as the middle umbilical fold.

In the male, as indicated in the accompanying diagram (Fig. 55),
the peritoneal relations of the testis are greatly modified by the
migration of the organ from an abdominal to a scrotal position.
The entire sac lodging the testis is an evaginated portion of the
abdominal wall, and since in the rabbit the cavity is widely open
throughout life to the abdominal cavity the lining membrane—that
designated as the parietal layer of the tunica vaginalis propria—

REGIONAL SECTIONS. IOI

is continuous with the parietal peritoneum of the abdomen, and thus
represents a permanent vaginal process. Like other structures
of the abdominal cavity, the testis itself is covered by peritoneum,
the latter being designated as the visceral layer of the tunica
vaginalis propria. This investment is connected with the parietal
layer by the mesorchium, and in the rabbit it will be observed
that the latter is chiefly attached forwards on the dorsal wall of the
abdomen, 7.e., in a position indicating the original situation of the
testis itself.

In the female the ovary is closely associated with the dorsal wall
of the abdomen, and its supporting peritoneum, the mesovarium,
is insignificant. Its duct in passing backward, however, becomes
greatly displaced from a dorsal position, and thus comes to be
supported by a broad fold of peritoneum. The latter is considered
to consist of two portions, one, the mesosalpinx, being the support
of the uterine tube, the other, the mesometrium, that of the
uterus. The entire fold, however, forms a continuous structure,
and is known in this relation as the broad ligament.

REGIONAL SECTIONS.

-The following plate-figures (I-VIII) are from characteristic
sections of a rabbit-foetus of 56mm., and may be used either in
connection with the general features of topography as outlined above,
or for the identification of various minor structures appearing in
the dissection.

Certain points regarding the sections are perhaps worthy of
notice. First, in the longitudinal section illustrated in Plates I and
II it will be noticed that paired structures frequently appear; this
being because of the fact that the section is not exactly median, at
least in certain places. Second, in using sections of the foetus for
gross anatomical features it is necessary to make allowance in some
cases for the different proportions of organs, and consequent slight
differences in position, in the foetal as compared with the adult
condition. Finally, many of the features appearing in the original
sections are such as could not be reproduced in the plates, although
they are indicated in the accompanying skeleton figures, and may
be referred to in this way.

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IWILNA AHL AO NOILOAS TVOILYAA NVIGUN V

103

. Transverse sinus of dura
mater.

Dura mater.

. Pallium of cerebral hemis-

phere.

. Lateral ventricle.

. Olfactory bulb.

. Olfactory tract.

6a. Divided olfactory nerve in
the cribriform plate.

7. Chorioid plexus of third
ventricle.

8. Anterior commissure.

Lan

aon. wr

DESIGNATIONS FOR PLATE II.

21.
. Cervical flexure.

. Central canal of spinal cord.
. Hypophysis.

. Frontal bone.

. Nasal bone.

. Nasal fossa.

. Mesethmoid cartilage.

. Cartilage of vomeronasal

Posterior medullary velum.

organ.

. Premaxilla.
. Nasopalatine duct and car-

tilage.

Oral portion of pharynx. |

Epiglottis and epiglottic
cartilage.

Thyreoic cartilage of
larynx.

Laryngeal cavity.

45a. Cricoid cartilage.

Oesophagus.

Cricothyreoideus muscle.

Thyreoid gland.

49. Sternohyoideus muscle.

50. Genioglossus muscle.

51. Geniohyoideus muscle.

4I.
42.

43-

44.
45,
40.
47-
48.

9. Thalamus.

. Optic chiasma.

Ii. Tuber cinereum.

12. Mamillary body.

13. Superior colliculus.

. Inferior colliculus.

15. Anterior medullary velum.

16. Cerebral peduncle; cephalic
flexure.

17. Isthmus rhombencephali.

18. Fourth ventricle.

. Pons; pontine flexure.

. Cerebellum.

38.

2 Sphenooccipital

. Maxilla.
. Hard palate (palatine and

maxilla).

. Presphenoid.
. Intersphenoidal

synchon-
drosis.

. Basisphenoid; hypophyseal

fossa.

synchon-
drosis.

Basioccipital.

38a. Supraoccipital.

39.
40.

Nasal portion of pharynx.
Soft palate.

104

52. Mylohyoideus muscle.

53. Mandible.

54. Occipital musculature,

55. Semispinalis capitis.

56. Rhomboideus minor.

57- Superior portion of trape-
zius.

58. Atlas.

59. Epistropheus.

~ 59a. Odontoid process.

60. Third cervical vertebra.
61. Median vertebral vein.
62. Body of hyoid bone.

A MEDIAN VERTICAL. SECTION OF THE HEAD.

105

O PI ONBPWNH

DESIGNATIONS FOR PLATE III.

. Nasal bone.

. Levatoralae nasi muscle.

. Nasal septum.

. Nasoturbinal cartilage.
Maxilloturbinal (concha inferior).
. Nasal fossa.

. Nasolacrimal duct.

. Vomeronasal organ and cartilage.
. Premaxilla.

. Small upper incisor.

. Large upper incisor.

. Nasopalatine ducts.

. Oral cavity.

NO
H

NNN
wn

N
Uw

106

. Tongue.

. Vibrissae. -
. Caninus muscle.

. Terminals of superior maxillary nerve.

. Buccal glands.

. Buccinator muscle.

. Terminals of inferior alveolar nerve.

. Ouadratus labii inferioris muscle.

. Mandible.

. Lower incisor.

. Meckel’s cartilage (primary mandibular

arch).

. Mentalis muscle.

Ne

THE

II

xLON.

REC

ASAL

N

OR

ERI

ANT

CTION OF

4

I

S

TERSE

SV

A TRAN

7

lO

DESIGNATIONS FOR PLATE IV.

I. Superior sagittal sinus of dura mater. 25. Nasal tract; choana.

2. Lateral ventricle. 26. Palatine bone.

3. Cerebral hemisphere. 27. Oral cavity.

4. Pia mater. 28. Palatine nerve.

5. Frontal bone. 29. Sphenopalatine ganglion.
6. Cartilage of orbital wing. 30. Infraorbital vein.

7. Mesethmoid cartilage. 31. Internal maxillary artery.
8. Cupula posterior cartilage. 32. Maxillary nerve.

9. Obliquus superior muscle. 33. Maxilla.
10. Ophthalmic vessels and nerves. 34. Zygomatic bone.
11. Levator palpebrae superioris muscle. 35. Submaxillary duct.
12. Rectus medialis muscle. 36. Buccinator muscle.

13. Retractor oculi muscle. 37. Masseter muscle.

14. Rectus inferior muscle. 38. Parotid duct.

15. Sclera. 39. Facial nerve.

16. Retina and chorioidea. 40. External maxillary artery and _ vein
17. Vitreous body. (anterior facial vein).

18. Lens. 41. Platysma muscle.

19. Posterior chamber of eye. 2. Inferior labial artery and vein.
20. Anterior chamber. 43. Mandible.

21. Cornea. 44. Genioglossus muscle.

22. Ciliary body and iris. 45. Digastricus muscle.

23. Upper eyelid. 46. Quadratus labiiinferioris muscle.
24. Lower eyelid. 47. Geniohyoideus muscle.

IV.
A TRANSVERSE SECTION OF THE ORBITAL REGION.

109

DESIGNATIONS FOR PLATE V.

1. Parietal bone. 21. Longus capitis.

2. Transverse sinus of dura mater. 22. Rectus capitis anterior.

3. Superior colliculus. 23. Oral portion of pharynx.

4. Cerebral aqueduct. 24. Thyreohyoideus muscle.

5. Isthmus rhombencephali. 25. Sternohyoideus muscle.

6. Pons. 26. Greater cornu of hyoid.

7. Trigeminal nerve. 27. Stylohyoideus major muscle.
8. Basilar artery. 28. Lingual artery.

9. Facial nerve. 29. Hypoglossal nerve.
to. Cartilaginous auditory capsule. 30. Tendon of digastricus muscle.
tr. Cochlea. 31. External maxillary artery.

12. Basioccipital bone. 2. Stylohyoideus minor.
13. Tensor tympani muscle. 33. Styloglossus.
14. Tympanic cavity. 34. Internal maxillary artery.

15. Malleus. : E

16. Tributaries of posterior facial vein. 35- ae ane bone.

17. Squamosal bone. 30. Mandible.

18. Cephalic portion of median vertebral vein. 37- Submaxillary gland.

19. Nasal poction of pharynx. 38. Anterior facial vein.

20. Origin of basioclavicularis and levator 39. Internal carotid artery.
scapulae major muscles.

j
f
}

IIo

=GION.

RE

A TRANSVERSE SECTION OF THE AUDITORY

IIt

DESIGNATIONS FOR PLATE VI.

I. Rhomboideus minor.

2. Superior portion of trapezius.

2a. Levator scapulae minor.

. Splenius.

. Semispinalis capitis.

. Rectus capitis posterior superficialis.
. Obliquus capitis major.

. Arch of epistropheus.

. Ganglion of posterior root.

. Longissimus cervicis.

O OIAMNHW

ro. Longissimus capitis.

I1. Vertebral artery and vein.

12. Longus atlantis.

13. Vertebral body.

14. Tansverse process (anterior root).
15. -Median vertebral vein.

16. Longus colli.

17. Longus capitis.

18. Fat-body.

NNNNNNHNNHNH
MIAMHLWNHHOO

2

30.
Shieh

2

> 33.
34.
35.
36.
37:

112

. Oesophagus.

. Inferior thyreoid nerve.

. Inferior thyreoid vein.

. Trachea.

. Thyreoid gland.

. Cardiac branch of vagus (n. depressor).
. Sympathetic trunk.

. Vagus nerve.

. Common carotid artery.

. Internal jugular vein.

. Sternohyoideus muscle.

Sternothyreoideus muscle.
Sternomastoideus muscle.

Descending ramus of hypoglossal nerve.
External jugular vein.

Basioclavicularis muscle.

Levator scapulae major muscle,
Cleidomastoideus.-

Platysma.

— Se

VI.

A TRANSVERSE SECTION OF THE ANTERIOR CERVICAL REGION.

113

eal

DESIGNATIONS FOR PLATE VII.

. Semispinalis dorsi.

. Longissimus dorsi.

Iliocostalis.

. Spinal cord.

. Ganglion of posterior root and intercostal
nerve.

Tubercle of rib.

. Head of rib.

. Sympathetic trunks,

. Azygos vein.

. Thoracic aorta.

. Oesophagus.

HOD MID nAPWNH

12a. Right and left vagi.
. Lung.

. Bronchi.

. Branches of pulmonary artery.
Pulmonary veins.

. Right atrium.

. Tricuspid valve.

. Right ventricle.

. Left atrium.

. Left ventricle.

. Pericardial cavity.

. Pulmonary pleura.

34.

. Costal pleura.

. Bone ribs.

. Costal cartilage.

. Sternum.

. Cutaneus maximus muscle.

. Inferior portion of trapezius.
. Rhomboideus major.

. Inferior angle of scapula.

. Latissimus dorsi.

. Serratus posterior.

Intercostales externi and interni.

34a. Intercostalis internus.

II4

Wo

. Thoracic portion of serratus anterior.
. Obliquus externus abdominis.
. Transversus thoracis.

side eee

. Pectoralis major.

. Rectus abdominis.

. Anconaeus longus (caput longum of
triceps).

. Extensor antibrachii parvus.

2. Anconaeus medialis.

. Anconaeus lateralis.

. Distal extremity of humerus.
. Proximal portion of radius.

Vil.

A TRANSVERSE SECTION OF THE THORAX,

O MI ONPWNHH

. Spinal cord.

. Vertebral canal.

. Vertebral body.

. Sacrospinalis muscle.

. Quadratus lumborum.
. Psoas major.

. Psoas minor.

. Sympathetic trunk.

. Abdominal aorta.

. Inferior caval vein.

. Descending mesocolon,
. Ureter.

. Renal pelvis.

. Renal papilla.

. Left kidney.

. Parietal peritoneum.

. Visceral peritoneum.

DESIGNATIONS FOR

18,

Nw hd

NNN DH

&

116

CO MPIAMN SW

PLATE VIII.

18a. Posterior and anterior lobules of left
lobe of liver.
toa. Right lobe of liver.

. Obliquus internus abdominis and trans-

versus abdominis.

. Obliquus externus abdominis

Rectusabdominis.

2a. Cutaneus maximus.

. Middle umbilical fold.

. Urinary bladder (canal of foetal allantois).
. Umbilical arteries.

. Duodenum,

Pancreas and mesoduodenum.

. Descending colon.
. Parts of mesenterial small intestine.
. Caecum, Z

VIII.
A TRANSVERSE SECTION OF THE ABDOMEN.

II7

PART II.

OSTEOLOGY OF] THE, RABBIT:

For a practical study of the rabbit’s skeleton, a thoroughly
cleaned, but otherwise rough, unmounted skeleton will be found
most convenient. The skull should be divided with a fine saw at a
little to one side of the median plane, or a second skull may be
provided for this purpose (cf. Fig. 62). The most useful specimens
for reference are: (1) a well-mounted skeleton of the adult animal,
showing the-natural relations of the bones; and (2) a rough skeleton
of a young animal of from one to five weeks, showing the primary
composition of cartilage bones. For the special study of the skull
(pp. 140-159) a disarticulated specimen may be employed, but the
majority of the features may be made out in the intact or divided
skulls. The general account of the skull as given below will be
found to cover most of the osteological points noted in the dissection.

DIVISIONS OF THE SKELETON.

The skeleton is divisible into two main portions, namely, the
axial skeleton and the appendicular skeleton. The former
comprises the vertebral column, the ribs, the sternum, and the
skeleton of the head; the latter, the supports of the anterior and
posterior limbs, and the associated pectoral and pelvic girdles.

THE VERTEBRAL COLUMN.

The vertebral column (columna vertebralis) is formed of a
linear series of segments, the vertebrae. In accordance with its
function as a general support of the body, and also its relations with
the nervous system and the spinal musculature, the vertebrae, with
minor exceptions, are constructed on the same plan. Those of

118

THE VERTEBRAL COLUMN. 119

particular regions also present certain features in common, so that
it is possible to classify them into cervical, thoracic, lumbar, sacral,
and caudal groups.

A typical vertebra—for the characters of which any one of the

Fic. 56. Representative vertebrae: A, atlas, anterior surface; B, epistro
pheus, lateral surface; C, fifth cervical vertebra, anterior surface; D, fourth
dorsal, lateral surface; E, F, second lumbar vertebra, anterior and lateral
surfaces.

a.a., anterior arch of atlas; a.p., posterior arch of atlas; a.v., vertebral arch;
c.v., vertebral body; d., dens epistrophei; f.a.a., anterior articular facet of
epistropheus; f.a.s., superior articular pit of atlas; f.a.s.’, superior articular.
facet of epistropheus; f.c.i., inferior costal demifacet for head of rib; f.c.s.,
superior costal demifacet; f.c.t., costal facet of transverse process; f.d., fovea
dentis; f.i., intervertebral foramen; f.tr.,foramentransversarium; f.v., foramen
vertebrale; 1., lamina of vertebral arch; m.]l., lateral mass of atlas; p.a.,
accessory process of lumbar vertebra; p.a.i., inferior articular process; p.a.s.,
superior articular process; p.m., mamillary process; p.s., spinous process;
p.s.a., anterior spinous process; p.t., transverse process; p.t., triangular
process; r., radix of vertebral arch; r.a., r.p., anterior and posterior radices of
transverse process of cervical vertebra; t.a., t.p., anterior and posterior
tubercles of atlas.

thoracic or lumbar series may be taken (Fig. 56, D-F)—consists of a
basal portion, the vertebral body (corpus vertebrae), and of a
dorsal, vertebral arch (arcus vertebrae). The two portions

120 ANATOMY OF THE RABBIT.

enclose a large aperture, the vertebral foramen (foramen verte-
brale). The successive foramina form an almost complete tube,
the vertebral canal (canalis vertebralis), for the accommodation
of the spinal cord.

The body of a vertebra is a cylindrical, or somewhat dorso-
ventrally compressed, mass of bone, which bears at either end an
articular surface for attachment to the adjacent vertebra. The
articular surfaces are borne on thin plate-like epiphyses, the epiphy-
sial lines being evident even in older animals, especially in the
lumbar region. The dorsal portion of the body bears on either side
the pedicle, or root of the vertebral arch (radix arcus vertebrae),
the dorsal surface of the body forming in this way the floor of the
vertebral foramen. The dorsal portion of the arch, borne on the
pedicle, is distinguished as the lamina. The anterior and posterior
margins of the pedicle are notched, each notch or incisure being
converted, through its association with that of the adjacent verte-
bra, into a rounded aperture, the intervertebral foramen (fora-
men intervertebrale), for the passage outward of a spinal nerve.

The arch of the vertebra is noteworthy for its projections or
processes. On either side is a horizontal plate of bone, the
transverse process (processus transversus), and, dorsally, a
median projection, the spinous process (processus spinosus), all
three serving for the attachment of the vertebrae to one another
by ligaments, and for the attachment of the spinal musculature.
Special articular surfaces, borne on low articular processes
(processus articulares), are found on the anterior and posterior
margins of the arch. The anterior, or Superior articular surfaces
are directed for the most part toward the dorsal surface, and are
overlapped in the natural condition by the inferior articular
surfaces, which are directed toward the ventral surface. A
certain amount of movement is permitted by one surface slipping
across the other, the mechanism illustrating the arthrodia, or
gliding-joint.

The cervical vertebrae (vertebrae cervicales) are seven in
number. The posterior five are similar, while the anterior two are
specially modified in relation to the skull. The posterior vertebrae
(Fig. 56, C) are dorsoventrally compressed, their arches low, and
the spinous process short. In the seventh vertebra, however, the
spinous process begins to be elongated as in the succeeding thoracic

a a re

THE VERTEBRAL COLUMN. 121

vertebrae. In each vertebra the transverse process is perforated
by a costo-transverse foramen (foramen transversarium), the
latter serving for the passage of the vertebral artery forward to the
head. Through the presence of this aperture, the base of the
transverse process is divided into two parts, namely, a dorsal, or
posterior root (radix posterior), and a ventral, or anterior root
(radix anterior). The anterior root is a coalesced rib, and is com-
parable in its general relations to the normal ribs of the thoracic
vertebrae.

The first vertebra is the atlas (Fig. 56, A). It is peculiar in
lacking the vertebral body, the latter being represented by the
odontoid process of the epistropheus (cf. Plate II); also in possessing
special articular surfaces, and in having its transverse process
greatly flattened in the dorsoventral direction. It consists of a
ventral half-ring, the anterior arch (arcus anterior), a dorsal
half-ring, the posterior arch (arcus posterior), with paired lateral
masses (massae laterales) uniting them. The lateral masses also
form the bases of the transverse processes. The anterior arch
bears on its ventral side a small backwardly-directed process, the
anterior tubercle (tuberculum anterius). A similar posterior
tubercle (tuberculum posterius) on the dorsal surface of the
posterior arch is comparable to the spinous process of an ordinary
vertebra. The anterior surface of the atlas bears on either side an
extensive concave smooth surface, the superior articular pit (fovea
articularis superior), for articulation with the convex occipital
condyles of the skull. lis posterior surface bears on either side a
somewhat triangular inferior articular facet (facies articularis
inferior) for articulation with the epistropheus. These surfaces
take the place of the arch articulations of ordinary vertebrae.
Through the compression of the transverse process, the costo-
transverse foramen is converted into a canal. The anterior aper-
ture of this leads by a shallow groove, the sulcus arteriae
vertebralis, into a foramen perforating the posterior arch.
This aperture, represented in some mammals by separate alar
and intervertebral foramina serves to transmit the vertebral artery
and the first cervical nerve.

The space enclosed by the atlas is divided into a dorsal portion,
corresponding to the vertebral foramen of other vertebrae, and a

22 ANATOMY OF THE RABBIT.

ventral portion which in the natural condition lodges the odontoid
process of the epistropheus. The division is effected partly by a
small tubercle on the inner side of each lateral mass, and partly
by a transverse ligament which is stretched between them and over
the dorsal surface of the odontoid process. On the floor of the
ventral portion, a rounded articular surface, the fovea dentis,
marks the point of articulation of the anterior articular facet of the
odontoid process with the inner surface of the anterior arch.

The second vertebra is the epistropheus or axis (Fig. 56, B).
It resembles the succeeding cervical vertebrae more closely than
does the atlas. It is noteworthy for its great size, for the lateral
compression of its arch and spinous process, and for the possession
of a stout forwardly-directed odontoid process, or tooth (dens
epistrophei). It is articulated with the atlas through an anterior
articular facet, borne on the ventral surface of the odontoid
process, and by large paired superior articular facets borne on
its base. The spinous process of this vertebra and the transverse
processes of the atlas form together three main points of attach-
ment for the occipital musculature.

The thoracic vertebrae (vertebrae thoracales) are twelve in
number. They are distinguished chiefly by the possession of
articular pits for the attachment of ribs (Fig. 56, D). A rib is
articulated at two points, namely, one on the body of the vertebra,
the other on the transverse process. The former is marked by a
small round depression, the costal pit (fovea costalis), or costal
facet. In the last two vertebrae the facet is borne wholly on the
vertebral body to which the rib belongs. In the remaining verte-
brae a complete articulating surface is formed by two demifacets,
one being on the vertebra to which the rib belongs, the other on the
vertebra immediately in front. The articulation of a rib-with a
transverse process is marked by an oval facet, the costal pit of the
transverse process (fovea costalis transversalis). It is present
only in the first ten of the thoracic vertebrae.

In all vertebrae of the thoracic series the spinous processes are
well-developed. They increase in length to the third, and then
become gradually shorter, although their surfaces, on the whole, are’
slightly increased in extent. The anterior ten are directed back-
ward, the eleventh is almost vertical, while the twelfth is directed
forward, like those of the succeeding lumbar vertebrae.

THE VERTEBRAL COLUMN. 123

The lumbar vertebrae (vertebrae lumbales) are seven in num-
ber. They are large vertebrae, conspicuous for their extensive
surfaces and processes for muscular attachment (Fig. 56, E, F).
The transverse processes continue the general line of the ribs of the
thoracic region. They are directed forward, as well as outward,
and the tip of each is formed bya thin triangular plate (processus
triangularis), which represents a fused rib. At the posterior side
of the base of each is a short, flattened projection, the accessory

A B

Fic. 57. Theos sacrum: A, ventral (pelvic) surface; B, dorsal surface;
c.v., bodies of coalesced vertebrae; f.a., auricular surface; f.s.a., anterior
sacral foramina; f.s.m., median sacral foramina; f.s.p., posterior sacral
foramina; p.a.s., superior articular process of first vertebra; p.m., ma-
millary process of first vertebra; pr., promontory; p.s., spinous processes.

process (processus accessorius). The spinous process is especially
well-developed, and is directed forward. The articular processes
are rotated upward, so that their surfaces are directed more
nearly toward, or away from, the median plane, instead of to the
dorsal or ventral surface. The anterior articular surfaces are borne
on the bases of stout, upwardly-directed mamillary processes
(processus mamillares). The latter are most characteristic of the
lumbar vertebrae, but may be seen to arise in the posterior thoracic

124 ANATOMY OF THE RABBIT.

region as small elevations of the transverse processes. Each of the
first three of the lumbar vertebrae bears a median ventral pro-
jection, the anterior spinous process (processus spinosus anterior),
for the attachment of the lumbar portion of the diaphragm.

The sacral vertebrae (vertebrae sacrales) are four in number.
In contrast to the true vertebrae—those united by ligament and
articular surfaces—of the remaining portions of the vertebral
column, they are false vertebrae, uvited in the young by synchon-
droses, and in the adult coalesced to form a composite structure, the
os sacrum (Fig. 57). The axis of the sacrum forms an obtuse
angle with that of the lumbar vertebrae, the angle being indicated
by a ventral projection, the promontory (promontorium), formed
by the last lumbar and first sacral vertebrae. The sacrum is the
medium through which the vertebral column—in other words, the
posterior portion of the trunk—is supported on the posterior limbs.
Its anterior dorsal portion bears on either side a roughened area,
the auricular surface (facies auricularis), for articulation with
the pelvic girdle.. This surface is borne for the most part on the
transverse process of the first sacral vertebra.

The sacrum exhibits many features resulting from its formation
through the fusion of originally distinct vertebrae. On the ventral,
or pelvic surface (facies pelvina), the lines of junction may be
traced either between the bodies, or between the transverse pro-
cesses. Four pairs of apertures on this surface, the anterior sacral
foramina (foramina sacralia anteriora), lead into the intervertebral
foramina, and give passage to the sacral spinal nerves. On the
dorsal surface (facies dorsalis) a pair of posterior sacral foramina
lie in the line of junction of the first and second vertebrae. The
spinous processes are evident in all four vertebrae. The combined
articular and mamillary processes are conspicuous only in the
first two, but are represented in the succeeding two by low, rough-
ened tubercles. In the middle line dorsally the vertebral arches
are separated by conspicuous apertures, the median sacral fora-
mina.

The caudal or coccygeal vertebrae (vertebrae caudales,
s. coccygeae) are sixteen in number. There are segments of small
size, increasing slightly to the third, and then gradually decreasing
to the end of the column. The arches are complete in the first

— -

THE RIBs. 125

seven. The transverse processes are vestigial in all except the third.
At the end of the column the segments are reduced to slender
cylinders of bone representing the vertebral bodies.

THE RIBS.

The ribs (costae) are twelve in number on either side. Each
is composed of a dorsal portion, the costal bone (os costale), or
bone-rib, and a ventral portion, the costal cartilage (cartilago
costalis) (Fig. 58). From their attachment on the vertebral
column the bone-ribs are directed outward, downward, and back-
ward. The costal cartilages are directed for the most part inward,
downward, and forward. The first costal cartilage forms a pro-
nounced angle with the corresponding bone-rib. In the succeeding
ribs the angle is gradually replaced by a broad curve.

Ribs are classified as true or sternal ribs (costae verae), and
false or asternal ribs (costae spuriae). The former—comprising
the anterior seven—are those directly attached to the sternum.
The latter—comprising the posterior five

are either indirectly
attached, or unattached. The unattached ribs are designated as
floating ribs.

Generally speaking, the bone-ribs are cylindrical; but the anterior
five or six are more or less flattened, with their main surfaces
respectively medial and lateral. The compression is most marked
in old animals. The first rib is extremely short. The succeeding
ribs increase in length to the sixth, and then decrease to the twelfth.
The arch formed by each rib has its greatest convexity, or angle,
at some point toward the dorsal surface. Passing backward, the
point of greatest convexity changes from a medial to a lateral
position. This, together with the elongation of the more posterior
ribs, results in an enormous increase in the posterior extent of the
thoracic cavity.

The vertebral end, or head of the rib (capitulum costae), is
articulated with the body of the vertebra to which it belongs, and
also, in the case of the first ten, with the vertebra immediately in
front. The articulation with a transverse process is marked by a
small smooth elevation, the costal tubercle (tuberculum costae).
It is present only in the first nine ribs. Except in the first rib, and

126 ANATOMY OF THE RABBIT.

in the last four, the tubercle bears a sharp, dorsally-directed process
for muscular attachment. The slender portion of the rib inter-
vening between the head and tubercle is the neck (collum costae),
the remaining larger portion being distinguished as the body of the
rib (corpus costae).

The bony thorax is formed by

eLc. the ribs and the sternum with the
C.C. ie assistance of the thoracic vertebra.
eae It encloses a large space, the

es thoracic cavity (cavum thoracis).

+h The latter is conical in shape, with
ARK Aan the apex directed forward. The

TS. hy dorsoventral diameter of the cavity
é RR, E is considerably greater than the
| ees transverse diameter. Apart from
IIS the intercostal spaces, the cavity

rN 2 is open at two points: anteriorly,

Ce: y\% S the first thoracic vertebra, the first
aa On 4: rib, and the manubrium_ sterni
PNT, ‘ together enclose a small opening,

3 the superior thoracic aperture

6 (apertura thoracis superior); pos-

> z teriorly, the seventh and succeed-

ing ribs, together with the posterior
thoracic vertebrae and the xiphoid
process of the sternum, enclose a

Fic. 58. The sternum and first rib, much larger DENNIS the inferior
ventral view: 1-7, the true ribs; 8, frst thoracic aperture (apertura thor-

false rib; c.c., head of rib; cl.c., neck of eee :
rib; cr., costal cartilage; cr.c., body of rib; acis inferior). In the natural con-

c.s., body of sternum; m.s., manubrium
sterni; 0.c.,bone-rib; p.x.,xiphoid process; dition it is largely closed by the
t.c., costal tubercle. E
diaphragm. The curved boundary

formed by the ribs in this region is the costal arch (arcus costarum),
the angle formed at the point of attachment of the xiphoid process
being the infrasternal angle (angulus infrasternalis).

Through their articulations with the vertebral column, and the
nature of the costal cartilages, the ribs are capable of being moved,
or rotated, forward. The movement results in an increase of the

extent of the thoracic cavity, and is of importance in respiration.

THE STERNUM 127

THE STERNUM.

The sternum (Fig. 58) consists of a linear series of six segments,
the sternebrae The first segment is the manubrium sterni.
It is about twice the length of the middle segments. It is some-
what triangular in section, two of its surfaces being ventrolateral,
the third dorsal and directed toward the thoracic cavity. To its
anterior tip is attached the sternoclavicular ligament, by which
the greatly reduced clavicle is united with the sternum

The four middle segments are similar in appearance, and form
the body (corpus sterni). The sixth segment, described as the
xiphoid process (processus xiphoideus), is an elongated strip of
bone, to the posterior end of which is attached a broad, thin plate
of cartilage.

The first costal articulation is situated at about the middle of
the manubrium, the remaining six at the points of junction of the
segments. Five of them occur singly, while the sixth and seventh
costal cartilages are attached together at the point of junction of
the last segment of the corpus sterni with the xiphoid process.

THE SKELETON OF THE HEAD.

The head-skeleton comprises: (1) the series of elements constitut-
ing the skull; and (2) the hyoid bone, with its connections. The
skull, or cranium—using that term in a general sense—includes the
cranium proper, that portion enclosing the brain and containing
in its wall the auditory capsules, and the bones of the face (ossa
faciei), the latter including the series of elements related for the
most part to the jaws and palate. The primary relations of the
constituents of the head-skeleton have already been indicated
above (p. 53).

A. THE SKULL AS A WHOLE.

The skull (Figs. 59-63) is a composite structure, consisting of a
large number of elements, which, with the exception of the mandi-
ble, are united. by synarthroses, so that they produce the effect ofa
continuous mass. The mandible is a more or less independent

’

128 ANATOMY OF THE RABBIT.

structure, articulated with the main body of the skull by a typical
joint.

The skull is roughly divisible into two portions, namely, an
anterior, facial portion, and a posterior, cranial portion The
cranial portion has a somewhat conical shape, its apex being directed
forward. It is separated from the facial portion by a depression
on either side of the skull, the orbital cavity (orbita), which serves
for the accommodation of the optic bulb. Unlike the remaining
special sense-organs, the eye is not included within the skull-wall.
The two portions are united both medially and laterally, the lateral .
connection being established by the zygomatic arch (arcus
zygomaticus), which bridges the lateral portion of the orbit. The
facial portion has also a somewhat conical shape, its apex being
formed by the anterior extremity of the upper jaw and the incisor
teeth. Its base is formed in part by the connection with the cranial
portion, as already described, and also by the anterior walls of the
orbits.

The cranial portion exhibits an extensive posterior, nuchal
surface (planum nuchale), situated in general at right angles to the
cervical portion of the vertebral column and also to the dorsal,
lateral, and ventral walls of the skull. This surface includes the
external surface of the occipital bone, with the exception, chiefly,
of the basilar portion of the latter. Its dorsal portion forms an
area of attachment for the spinal and special occipital musculature.
Its ventral portion is perforated by a large aperture, the foramen
magnum occipitale, for the passage of the central nervous system
from the cranial cavity into che vertebral canal. On either side
of this is a smooth, ridge-like projection, the occipital condyle
(condylus occipitalis), for articulation with the superior articular
pitsof the atlas. Ata little distance lateral to the occipital condyle,
the nuchal surface is continued downward through the medium of a
somewhat triangular, pointed jugular or paramastoid process
(processus jugularis). This structure is separated from the occi-
pital condyle by a pronounced notch, the posterior boundary of a
deep narrow excavation, the jugular fossa (fossa jugularis), which
lies between the condyle and the tympanic bulla. The jugular
process serves for the attachment of muscles belonging to the
tongue, hyoid, and mandible, namely, the styloglossus, stylohyoidei

THE SKULL AS A WHOLE. - SL29

major and minor, and the digastricus, the suspensory ligament of the
lesser cornu of the hyoid also being included in the ligament of the
stylohyoideus minor. The nuchal surface is separated from the
dorsal surface of the skull by a shield-shaped promontory and
crest (crista nuchae). The lateral continuation of this crest is the
superior nuchal line (linea nuchae superior). It forms a curved
ridge, the position of which indicates the dorsal limit of the occipital
musculature. The posterior, somewhat tri-radiate tip of the

P. j.

Fic. 59. Lateral surface of the skull: AS, alisphenoid (ala magna); BO, basioccipital
(basilar portion of occipital); BS, basisphenoid (body of posterior sphenoid); F, frontal;
I, interparietal; L, lacrimal; M, maxilla; MS, masteid portion of petrosal (petromastoid) ;
N, nasal; OS, orbitosphenoid (ala parva); P, parietal; PL, palatine; PM, premaxilla;
SO, supraoccipital (squamous portion of occipital); SQ, squamosal; T, tympanic; ZY,
zygomatic.

a.p., piriform aperture of nose; d.i., incisor teeth; d.m., molars; d.pm., premolars;
f.i., infraorbital foramen; f.mx., maxillary fossa; f.o., optic foramen; f.s., stylomastoid
foramen; f.t., temporal fossa; 1.f., lateral lamina of pterygoid process; 1.m., medial lamina;
m.a.e., OSseous portion of external acoustic meatus; p.a., alveolar process. of maxilla;
p.e., ethmoidal portion of orbitosphenoid; p.f., frontal process of premaxilla; p.j., jugular
process of occipital; p.m., mastoid process of mastoid; p.mx., maxillary process of frontal;
p.o., orbital process of maxilla; ‘p.o.e., external occipital protuberance; p.s., squamosal
process of parietal; p.s.a., and p.s.p., anterior and posterior supraorbital processes of
frontal; p.z., zygomatic process of squamosal; p.z.m., zygomatic process of maxilla;
s, sphenoorbital process of maxilla; s.m., spina masseterica; sq., squamosal process of
squamosal.

projection, together with a thin ridge extending ventrad from it, is
the external occipital protuberance (protuberantia occipitalis
externa), an important median point of attachment for the occi-
pital muscles and the ligamentum nuchae.

The ventral wall of the cranial portion is the basal portion
(basicranium) of the entire skull. Its axial line, the basicranial
axis, continues, in general, that of the bodies of the vertebrae,
and its posterior portion is equivalent, morphologically, to vertebral

130 ANATOMY OF THE RABBIT.

segments. It is formed by a linear series of three bones, namely,
the basilar portion of the occipital, the body of the posterior
sphenoid, and that of the anterior sphenoid (respectively basi-
occipital, basisphenoid and presphenoid bones). Its ex-
tremely narrow, anterior portion forms the roof of a deep groove
which encloses the nasal portion of the pharynx. As viewed from
the ventral surface, it is seen to disappear in the facial complex at
some distance dorsal to the posterior margin of the bony palatine
bridge. Laterally, it is separated from the orbit on either side by a
vertical plate formed by the palatine bone, and also by two down-
ward projections of the posterior sphenoid, the medial and lateral
laminae of the pterygoid process (processus pterygoideus).
These structures enclose between them the pterygoid fossa (fossa
pterygoidea), the walls of which serve for the attachment of the
external and internal pterygoid muscles of the mandible.

The lateral wall of the cranial portion forms anteriorly a large
part of the boundary of the orbit. The cranial wall of the orbit is
partly formed by two upward projections of the basicranium,
namely, the lesser or orbital wing of the anterior sphenoid, or
orbitosphenoid and the greater or temporal wing of the pos-
terior sphenoid, or alisphenoid. The remaining portion is formed
by membrane elements, including the frontal bone of the roof of
the skull and the squamosal bone, the latter distinguishable as the
support of the posterior root of the zygomatic arch. Immediately
behind the orbit, the root of the zygomatic arch projects outward
and then downward. It is formed by a zygomatic process (proces-
sus zygomaticus) of the squamosal bone, and the tip of this forms
a vertical plate, which is united by a horizontal suture with the
zygomatic bone. On the ventral side of the process, close to the
cranial wall, is the glenoid cavity or mandibular fossa (fossa
mandibularis), for articulation with the mandible. On its dorsal
side, but more especially on the adjacent portion of the cranial wall,
there is a shallow, horizontal groove, lodging in the natural condi-
tion the temporalis muscle of the mandible, and therefore represent-
ing a greatly reduced temporal fossa (fossa temporalis). In the
natural condition the anterior portion of the groove is converted
into a foramen through the presence of a stout ligament extending
from the posterior supraorbital process to the base of the zygomatie
arch. The dorsal boundary of the fossa is formed by a pronounced

THE SKULL AS A WHOLE.

131

ridge, the temporal line (linea temporalis), the latter forming

also the lateral margin of the roof of the skull in this region.
Behind the posterior root of the zygomatic arch, the external

surface of the lateral wall is largely occupied by the swollen tym-

panic bulla (bulla tympani),
formed by the tympanic bone.
It contains the capacious
tympanic cavity (cavum
tympani) and certain skeletal
structures of the middle ear,
namely, the auditory ossi-
cles (ossicula auditus), the
relations of which are more
fully dealt with below (p. 147).
The dorsal portion of the
tympanic bulla is continuous
with a short bony tube which
opens at a short distance
dorsally by a large oval aper-
ture. This tube is part of a
more extensive canal, the
external acoustic meatus
(meatus acusticus. externus)
which, in the natural condi-
tion, leads downward through
the base of the external ear to
the tympanic membrane.
The tympanic bulla does not
form the lateral wall of the
skull in this region, and is not
exposed to the cranial cavity.
It is applied closely to the
external surface of the perio-
tic or petromastoid bone

poe.

Dorsal surface of the skull: F, frontal;

Fic. 60.
I,interparietal; L, lacrimal; M, maxilla; MS,
mastoid portion of petrosal (petromastoid); N,
nasal; P, parietal; SO, supraoccipital (squamous

portion of occipital); SO, squamosal; JESS
zygomatic.

f.mx., maxillary fossa; f.t., temporal fossa;
l.n.s., superior nuchal line; l.t., temporal line;
p.f., frontal process of premaxilla; p.mx.,
maxillary process of frontal; p.o.e., external
occipital protuberance; p.s.a. and ).s.p.,

anterior and posterior supraorbital processes of
frontal; p.sc., subcutaneous process of lacrimal;
p.z., zygomatic process of squamosal; p.z.m.,
zygomatic process of maxilla; s.f., frontal spine;
s.m., spina masseterica.

(os petrosum), which forms the lateral boundary of the cranial

cavity, and contains the structures of the internal ear.

The external

or mastoid portion of this bone appears in the space enclosed
between the tympanic bulla and the jugular process of the occipital

132 ANATOMY OF THE RABBIT.

bone, where it is readily distinguishable by its pitted appearance.
Its ventral portion bears a slender projection, lying parallel to the
jugular process, the mastoid process (processus mastoideus).

A series of foramina, lying partly within the orbit, and extend-
ing thence posteriorly along the boundary between the lateral and
ventral walls to the occiput, put the cranial cavity in communica-
tion with the outside, and serve for the passage of nerves and vessels.
The first and largest of these, the optic foramen (foramen opti-
cum), occupies the middle portion of the orbit, and transmits, in the
natural condition, the optic nerve. Following this is a vertical
slit-like aperture—not to be confused with the perforations of the
external lamina of the pterygoid process—the superior orbital
fissure (fissura orbitalis superior). It represents both the superior
orbital fissure of the normal mammalian skull and the foramen
rotundum, and provides for the passage outward of the third,
fourth and sixth cranial nerves, together with the first and second
divisions of the fifth. The lateral lamina of the pterygoid process
presents three foramina, of which the largest, anterior, and medial
one, the anterior sphenoidal foramen (alar canal), serves for the
transmission of the internal maxillary artery, while the remaining
two, the middle and posterior sphenoidal foramina, transmit
muscular branches (masseterico-temporal and pterygobuccinnator)
of the mandibular nerve. On the medial side of the base of the
medial lamina of the pterygoid process there is a shallow longitu-
dinal groove, representing the pterygoid canal (canalis ptery-
goideus) of the human skull. Immediately in front of the tympanic
bulla, on the ventral surface of the skull, an irregular aperture,
the foramen lacerum, leads directly into the cranial cavity. It is
incompletely divided into two parts by a slender bony splint. It
contains, in addition to the foramen lacerum, the foramen ovale
of the typical mammalian skull, and serves to transmit the mandib-
ular portion of the fifth nerve and the internal carotid artery.
Looking into the aperture from the front, it is seen to communicate
not only with the cranial cavity, but also with two apertures in the
anterior portion of the auditory complex. One of these—that
toward the middle line—is the internal carotid foramen (fora-
men caroticum internum). It is the anterior end of a canal trans-
mitting the internal carotid artery; the posterior end of this canal,

THE SKULL AS A WHOLE. 133

the point at which the internal carotid artery enters the tympanic
bone, or the external carotid foramen (foramen caroticum
externum), being visible as a rounded aperture lying on the postero-
medial surface of the tympanic bulla. The second, lateral aper-
ture communicating with the foramen lacerum is that of the
auditory (Eustachian) tube (tuba auditiva). It leads into the
tympanic cavity, and in the natural condition the tube places this
cavity in communication with the nasal portion of the pharynx.
Associated with the mastoid process is a small aperture, the stylo-
mastoid foramen (foramen stylomastoideum), the external
opening of the facial canal, which serves for the passage of the facial
nerve. A slit-like aperture, the jugular foramen (foramen
jugulare), lies in the jugular fossa, between the posterior ventral
margin of the tympanic bulla and the occipital condyle. It
transmits the first portion of the internal jugular vein from the
transverse sinus of the dura mater, and also the ninth, tenth, and
eleventh cranial nerves. Finally, immediately in front of the
dorsal portion of the condyle, the occipital segment is perforated
by several small apertures together representing the hypoglossal
canal (canalis hypoglossi), and serving for the transmission of the
hypoglossal nerve.

The roof of the cranial portion is largely formed by two pairs of
thin membrane elements, the frontal and parietal bones. The
former occupy a general position between the orbits, while the latter
are interposed between the frontal bones and the occipital segment.
A small portion of the roof is formed posteriorly, however, by a small
lozenge-shaped element, the interparietal bone, and by the shield-
shaped projection, described above, which is part of the occipital
bone.

The space enclosed by the cranial portion of the skull is the
cranial cavity (cavum cranii). Its form depends on the external
configuration of the brain. It is divisible into three portions,
known as the cranial fossae. The anterior cranial fossa (fossa
cranii anterior) is a small division lodging in the natural condition
the olfactory bulbs of the brain. The middle cranial fossa,
the largest division of the cavity, lodges the enlarged cerebral
hemispheres. The posterior cranial fossa is a small division
extending backward to the foramen magnum and lodging in the

134 0

ANATOMY OF THE RABBIT.

natural condition the cerebellum and related posterior portions of

the brain.

It is partly set off from the middle cranial fossa by a

fold of the dura mater, the tentorium cerebelli, which projects

Ventral surface of the skull: AS, alis-
phenoid (ala magna); B, basioccipital (basilar
portion of occipital); BS, basisphenoid (body of
posterior sphenoid); EXO, exoccipital; M, maxilla;
PL, palatine; PMX, premaxilla; PR, presphenoid
(body of anterior sphenoid); SO, supraoccipital
(squamous portion of occipital); SCG, squamosal;
T, tympanic; ZY, zygomatic. ;

ch, choana; c.hy., hypoglossal canal; CHOKS
occipital condyle; f.c.e., external carotid foramen;
f.in., incisive foramen; f.j., jugular foramen; f.l.,
foramen lacerum; f.m., mandibular fossa; f.m.o.,
foramen magnum; f.p.m., greater palatine foramen;
f.s.a., anterior sphenoidal foramen; m.a.e., osseous
portion of external acoustic meatus; p.j., jugular
process; p.o.e., external occipital protuberance;
p.pl., palatine process of maxilla; p.pm., palatine
process of premaxilla; p.pt., medial and lateral
laminae of pterygoid process of posterior sphenoid;
s.m., spina masseterica.

Fic. 61.

inward from the dorsal and
lateral walls of the skull.
This fold is usually found
adhering to the internal sur-
face of the skull, unless the
latter has been very thor-
oughly cleared, and in all
cases its position is indicated
by a low ridge of bone. ~The
marked difference in diameter
between the middle and _ pos-
terior cranial fossae is ac-
counted for by the great thick-
ness of the auditory portion
of the skull. The anterior
surface of the periotic bone
will be observed to form an
extensive posterior wall for
the middle cranial fossa.
The floor of the middle
and posterior cranial fossae is
not smooth, like the external
base of the skull, but presents
in its anterior portion a pro-
minent elevation, the sella
turcica, which is borne on the
body of the posterior sphe-
noid. It contains a large
central depression, the hypo-
physeal fossa (fossa hypoph-
seos), which in the natural
condition lodges the hypo-
physis or pituitary body. The

aperture of the fossa is partly enclosed laterally by a pair of pointed
posterior clinoid processes (processus clinoidei posteriores), the

on

>

THE SKULL AS A WHOLE. 135

tips of which are directed forward; and a corresponding pair of
anterior clincid processes lie at the anterior end of the fossa,
with the tips directed backward. The posterior, and also dorsal
wall of the fossa, described as the dorsum sellae, leads by an
abrupt curve backward on to the floor of the posterior cranial fossa,
the sloping portion of the floor, or clivus, supporting in the natural
condition the pons and medulla oblogata. Toward the anterior
end of the middle cranial fossa, the lateral walls of the skull are
greatly compressed, so that the anterior portion of the basicranium,
especially the body of the anterior sphenoid, is largely ecxluded
from the cranial cavity. The usually paired optic foramina are
here confluent, there being a single aperture for the transmission
of the optic nerves. The posterior ventral boundary of this aper-
ture contains a broad groove, the sulcus chiasmatis, which lodges
in the natural condition the optic chiasma.

In the anterior cranial fossa the floor is largely formed by a
perforated area, borne on the cribriform plate (lamina cribrosa)
of the ethmoid bone, and serving for the transmission of the
divided olfactory nerves. Its median portion projects slightly into
the cranial fossa as a low ridge, the crista galli, which is interposed
between the tips of the olfactory bulbs.

In the ventrolateral portion of the cranial cavity may be found
the internal openings of the foramina described above, namely, the
superior orbital fissure, the foramen lacerum, the jugular foramen,
and the hypoglossal canal. The superior orbital fissure is almost
ventral in position to the foramen opticum, and is connected back-
ward with the foramen lacerum by a broad groove, the sulcus
sphenoidalis, which lodges in the natural condition the roots of
the fifth nerve. This groove continues to the medial surface of the
periotic bone, where it is bridged over by the tentorium cerebellt.

On the lateral wall of the posterior cranial fossa, and
enclosed by the compact, white, petrous portion of the peroitic
bone, is a series of three apertures leading into its substance.
One of these, much larger than the remaining two, is_ the
parafloccular fossa (fossa parafloccularis). It lodges in the
natural condition the flocculus, a small stalked appendage of the
cerebellum. Ventral of this fossa, and also somewhat in front of it,
a thin lodge of bone extends over an oval opening, the internal

136 ANATOMY OF THE RABBIT.

aperture of the facial canal (canalis facialis), which serves for the
transmission of the seventh cranial (facial) nerve. Immediately
behind and below this aperture is the opening of the internal
acoustic meatus (meatus acusticus internus) for the transmission
of the eighth cranial (acoustic) nerve. The two apertures tend to
be enclosed by a shallow bony ridge, largely formed by the pro-
jecting ledge described above, and resembling superficially the
complete common tube represented by the internal acoustic meatus
of the human skull.

The facial portion of the skull is largely formed by the invest-
ing bones of the upper jaw, palate, and mandible, but it encloses
also the entire olfactory region of the primary skull, including the
nasal fossae and associated turbinal bones. The upper jaw—the
maxilla of the human skull—is formed of two primary, and, in the
rabbit, separate, elements, the maxilla and premaxilla. They
together form the greater portion of the facial region—in the adult
condition also a large portion of the lateral walls of the nasal fossae
—and bear in a ventrolateral position low alveolar processes
(processus alveolares) for the sockets or alveoli of the incisor and
cheek-teeth. The maxilla bears the anterior root of the zygomatic
arch, the latter being formed partly by a short zygomatic process
arising from its lateral surface, by the zygomatic or malar bone,
which is fused with it, and by the corresponding zygomatic process
of the squamosal bone, constituting the posterior root. The
anterior root of the zygomatic arch is perforated by a deep narrow
infraorbital canal (canalis infraorbitalis), which opens on the
facial surface by a vertical slit-like aperture, the infraorbital
foramen. It serves for the transmission of the infraorbital vessels
and nerves from the orbit to the face. :

The ventral portion of the maxilla is associated with the pala-
tine bone to form the hard palate (palatum durum). This
structure is represented chiefly by a bony palatine bridge con-
necting the two sides of the skull between the more anterior cheek-
teeth. It forms a portion of the roof of the oral cavity and a por-
tion of the floor of the nasal cavity. Immediately in front of it,
the palatal surface is perforated by a pair of large incisive foramina
(foramina incisiva), which are broadly open to the nasal fossae.

THE SKULL AS A WHOLE. 137

A considerable portion of the anterior and dorsal wall of the
orbit is formed by the facial complex. Dorsally, the roofing
element of this region, the frontal bone, bears a curved lateral
projection, the supraorbital process (processus supraorbitalis),
which overhangs the orbit. Its narrower base expands into anterior
and posterior tips, which lie parallel to the adjacent portion of the
skull, and enclose with the latter corresponding anterior and
posterior supraorbital incisures. The latter are converted:
by ligament into foramina. The anterior wall of the orbit is formed
in part by a loosely articulated element, the lacrimal bone, the
lateral margin of which projects from the orbital rim as a blunt
subcutaneous process (processus subcutaneus). On the ventral
side of its base is the orbital opening of the nasolacrimal canal
(canalis nasolacrimalis), the bony enclosure of the nasolacrimal
duct, which in the natural condition leads from the corneal surface
of the eye to the anterior portion of the nasal fossa. A smaller
projection forming the ventral boundary of the nasolacrimal
aperture is the hamulus lacrimalis. Finally, in the ventral
anterior angle of the orbit, the bases of the three posterior cheek-
teeth encroach to a considerable extent on the orbital space. They
are separated from the orbital wall by a deep infraorbital groove
(sulcus infraorbitalis), which leads forward into the canal of the
same name. They partly conceal two important apertures of this
region, the orbital opening of the pterygopalatine canal (canalis
pterygopalatinus), leading to the palatal surface, and the spheno-
palatine foramen (foramen sphenopalatinum), leading to the
nasal fossa. The pterygopalatine canal opens ventrally in the
palato-maxillary suture of the hard palate by a rounded aperture,
the greater palatine foramen (foramen palatinum majus).

The nasal cavity (cavum nasi) is enclosed by the maxilla and
premaxilla, with the assistance of paired roofing elements, the
nasal bones. Apart from the incisive foramina, which are closed
in the natural condition, the cavity is open at two points. Poste-
riorly it communicates with the ventral surface of the skull by the
choanae, which, in the rabbit, are incompletely divided. An-
teriorly it opens to the outside by the piriform aperture (apertura
piriformis). The cavity is divided into right and left portions,
the nasal fossae. In the divided skull it is seen thac the division
is effected chiefly through a median vertical, cartilaginous plate,

13g, ANATOMY OF THE RABBIT.

the nasal septum (septum nasi), or cartilaginous portion of the
mesethmoid. This is continuous posteriorly with a _ small
crescentic vertical plate of bone, the perpendicular plate (lamina
perpendicularis) of the ethmoid bone—the bony portion of the
mesethmoid—and the latter is also the terminal element of the
series of median bones constituting the basicranium. Posteriorly,
the ventral portion of the cartilaginous nasal septum is supported
by a vertical bony plate, the vomer, the dorsal margin of which
is grooved to receive it. Anteriorly, the nasal septum bears on
its ventral margin the paired enclosures of the yomeronasal organ,

Fic. 62. The skullin vertical section: BO, basioccipital (basilar portion of occipital) ;
BS, basisphenoid (body of posterior sphenoid); ET, ethmoturbinal; F, frontal; I, inter-
parietal; M, maxilla; MT, maxilloturbinal; N, nasal; NT, nasoturbinal;. P, parietal;
PL, palatine; PMX, premaxilla; PR, presphenoid (body of anterior sphenoid) ; PT, petrous
portion of petromastoid; SO, supraoccipital (squamous portion of occipital); T, tympanic;
V, vomer.

a.p., piriform aperture of nose; c.f., internal aperture of facial canal; c.o., occipital con-
dyle; fc.a., f.c.m. and f.c.p., anterior, middle, and posterior cranial fossae; f.f., parafloccular
fossa; f.h., hypophyseal fossa; f.in., incisive foramen; f.s., sphenopalatine foramen; 1., per-
pendicular plate of the ethmoid; m.a.i., internal acoustic meatus; 0., optic foramen; p.a.,
alveolar process of maxilla; p.d., hard palate; p.o.e., external occipital protuberance;
p.pt., pterygoid process of posterior sphenoid; s.n., nasal septum; t.c., tentorium cerebelli.

which are also supported by the grooved surface formed in the -
middle line by the adjacent dorsal surfaces of the palatine processes
of the premaxilla. The relations of these structures, as well as of the
cartilage supporting the nasopalatine duct, are best seen in very
young animals (cf. Plate III).

The delicate, folded, or scroll-like turbinated bones, charac-
teristic of the nasal cavity, are borne on its posterior and lateral
walls. Occupying the anterior portion of the lateral wall of the
nasal fossa is a finely-ridged mass of bone, the concha inferior, or
maxilloturbinal. It is easily distinguishable from a more dorsal

THE SKULL AS A WHOLE. 139
\

and posterior series of broader folds, which together constitute the
ethmoturbinal. In the rabbit, as in mammals generally, the
latter is divisible into a more dorsal elongated portion attached
to the nasal bone, the nasoturbinal, and a more ventral portion,
also posterior portion, the ethmoturbinal proper, composed of
several shorter folds decreasing in length from above downward.
In the natural condition the turbinated bones bear a considerable
portion of the nasal epithelium, the surface of which is greatly
increased by the folding of the underlying bone. That covering
the ethmoturbinal contains the olfactory sense organs, while that
covering the maxilloturbinal is non-sensory and possesses the

Fic. 63. lLateralsurface of the left half of the mandible: a.m.
angle; c.m., body of mandible; cp.m., articular portion (head)
of mandible; d.i., d.m., and d.pm., incisor, molar and premolar
teeth; f.m., mental foramen; i.m.a. and i.m.p., anterior and

posterior mandibular incisures; p.c., coronoid process; p.cd.,
condyloid process; t.m. and t.pt., masseteric and pterygoid
tuberosities.

mechanical function of freeing the air of the respiratory tracts from
foreign materials, as well as of warming it slightly in its passage.
On this account the respective structures are conveniently distin-
guished as sensory (olfactory) turbinals and respiratory
turbinals.

The mandible (mandibula) is composed of two portions, united
anteriorly by the symphysis mandibulae. Each half comprises
a horizontal portion, forming in conjunction with that of the oppo-
site side the body of the mandible (corpus mandibulae), and a
posterior, vertical portion, the ramus mandibulae, the latter
serving for the insertion of the muscles of mastication and for
articulation with the skull. The body of the mandible bears on its

140 ANATOMY OF THE RABBIT.

dorsal margin the alveoli of the lower teeth. The mandibular
ramus forms a broad plate, the lateral surface of which is occupied
in the natural condition by the masseter muscle, while the medial
surface forms an area of insertion for the external and internal
pterygoids. The surface of the ramus is greatly increased in its
posteroventral portion through the expansion of the bone to
form the angle (angulus mandibulae), or angular apophysis.
The elongated articular surface is borne at the end of a vertical,
or slightly oblique condyloid process (processus condyloideus).
The nerve and vessels of the mandible enter at the mandibular
foramen (foramen mandibulare), the latter being situated on the
medial surface of the bone immediately behind the last cheek-tooth.

B. THE BONES OF THE SKULL.

1. THE OCCIPITAL BONE.

The occipital bone (os occipitale) is the first of the basicranial
segments as numbered from the occipital articulation forward.
It forms the posterior boundary of the skull, and establishes the
connection of the latter with the vertebral column. Its external
surface is identifiable for the most part with the nuchal surface,
but a portion of it falls in the horizontal plane of the basis cranii.
The internal surface is partly exposed to the cranial cavity, and
forms in this relation the posterior, dorsal, and ventral boundaries
of the posterior cranial fossa. The remaining portion is excluded
from the cranial cavity, being applied instead to the broad posterior
surfaces of the petrotympanic bones.

The occipital bone is divisible into four portions, namely, the
basilar portion (pars basilaris) or basioccipital, the paired
lateral portions (partes laterales), or exoccipitals, and the
squamous portion (squama occipitalis), or supraoccipital.
All four portions take part in the formation of the foramen magnum.
In the young animal (Fig. 12) they are represented by separate
elements, formed in a continuous mass of cartilage, and united
for a time by synchondroses, but in the course of growth they
become fused to form a single occipital bone.

THE BONES OF THE SKULL. 141

The basioccipital is that portion lying below and in front of
the foramen magnum. Its main surfaces are respectively dorsal
and ventral. Its anterior margin is united with the posterior
margin of the basisphenoid by a thin, transverse cartilage union,
the sphenooccipital synchondrosis (synchondrosis spheno-
occipitalis). Posteriorly its dorsal and ventral surfaces come
together in a thin concave edge which forms the ventral boundary
of the foramen magnum. Laterally it is bounded by the petro-
tympanic bone and by the lateral portion of the occipital. The
dorsal surface bears a median groove, deeper in its middle portion,
where the lateral margins of the bone are raised to form a pair
of rounded bosses for articulation with the petrotympanic. The
groove represents the sloping portion or clivus of the occipital, and
lodges in the natural condition, as described above, the ventral
portion of the medulla oblongata. The ventral surface presents
a similar groove, in the posterior portion of which there is a small
ridge-like elevation, the pharyngeal tubercle (tuberculum
pharyngeum).

The exoccipital is directed dorsad from the basioccipital in
such a way that it falls in the plane of the nuchal surface. It is
applied to the posterior surface of the petrotympanic bone, and
also extends downward beyond the latter as the jugular process.
The occipital condyle is borne on the exoccipital, with the exception,
however, of its ventral tip, which belongs to the basioccipital.
The portion of the occipital bone connecting the basioccipital and
exoccipital contains the jugular fossa and the apertures repre-
senting the hypoglossal canal. Its anterior margin bears a
jugular incisure (incisura jugularis), forming the occipital
boundary of the jugular foramen, the remaining portion of the
latter being formed by the petrotympanic.

The supraoccipital is the dorsal portion of the bone. Its
dorsal margin is bent sharply forward, so that it tends to fall, like
the basioccipital, in a horizontal plane. Its external surface bears
the nuchal crest and the external occipital protuberance.
A pair of lateral wing-like expansions rest upon, and partly over-
lap, the dorsal margins of the petrotympanic bones. The anterior
boundary is formed by the interparietal, parietal, and squamosal
bones, but in young skulls the squamosal connection is represented

142 ANATOMY OF THE RABBIT.

by a vacuity. The internal-surface bears a longitudinal groove,
lodging in the natural condition the median vermis of the cerebellum.
It is crossed at its anterior end by a shallow transverse groove
(sulcus transversus), which marks the position of the transverse
sinus of the dura mater.

2. THE POSTERIOR SPHENOID.

The sphenoid bone, as identified from the human condition,
is a complex of elements belonging to two segments, namely, the
posterior sphenoid (os sphenoidale posterius) and the anterior
sphenoid (os sphenoidale anterius). In the rabbit, as in mammals
generally, these segments are separate throughout life.

The posterior sphenoid comprises: (1) a median portion, the
body, or basisphenoid; (2) paired dorsolateral expansions, the
greater wings (alae magnae), or alisphenoids; and (3) paired
ventral projections, the pterygoid processes.

The basisphenoid continues the basis cranii forward from the
basioccipital to the body of the anterior sphenoid. It is united
with the latter by the intersphenoidal synchondrosis. Its
surfaces correspond for the most part to those of the basioccipital.
The ventral surface forms the chief part of the bony roof of the
nasopharynx. It is perforated in its middle by a round aperture,
the foramen cavernosum, which leads into the interior of the
bone. The dorsal surface is occupied by the hypophyseal fossa
and related structures, namely, the dorsum sellae and the pos-
terior clinoid processes. On the lateral surface of the base of
the posterior clinoid process a faint groove, the sulcus caroticus,
marks the course of the internal carotid artery. The interior of the —
bone contains a cavity of considerable size, the sphenoidal sinus
(sinus sphenoidalis), which communicates both with the foramen
cavernosum and the hypophyseal fossa.

The alisphenoid extends at first laterad, but soon changes its
direction so that its axis beomes dorsoventral. At the same time
the bone is rotated in such a way that its surfaces tend to fall in a
transverse plane. It is bounded anteriorly by the orbitosphenoid,
dorsally by the squamosal, and posteriorly by the petrotympanic.
The anterior margin of its root encloses with the basisphenoid,

THE BONES OF THE SKULL. 143

and to a certain extent with the orbitosphenoid, the superior
orbital fissure. The foramen lacerum is formed by the pos-
terior margin of its root in association with the petrotympanic.

The external surface of the alisphenoid is convex, both toward
the orbit and toward the ventral surface of the skull. In the
posterior portion of the orbit this surface bears a jagged elevation,
the crista alae magnae. The internal surface forms a portion
of the floor and anteroventral wall of the middle cranial fossa.
At its base a broad groove, the sulcus sphenoidalis, indicates
the position of the root of the fifth nerve and the related semilunar
(Gasserian) ganglion.

The pterygoid process comprises the two plates described
above as the medial and lateral laminae. The former is vertical,
and its medial surface is directed toward the nasopharynx. The
latter is almost horizontal. The medial lamina ends ventrally in a
hooked projection, the hamular process (hamulus pterygoideus).
In the young animal this portion is formed of an elevation of
cartilage tipped by a separate membrane element, the pterygoid
bone. The pterygoid fossa is formed in part by the medial and
lateral laminae and in part by the divided posterior end of the
palatine bone. The posterior basal portion of the lateral lamina is
extensively excavated, like the adjacent portions of the alisphenoid.
It bears a shallow groove, representing a pterygoid canal (canalis
pterygoideus), and is perforated by the three apertures described
above as the anterior, middle, and posterior sphenoidal
foramina.

3. THE ANTERIOR SPHENOID.

The anterior sphenoid (0s sphenoidale anterius) consists of
two portions, namely, a median portion, the body, or presphenoid,
and a pair of lateral expansions, the lesser wings (alae parvae), or
orbitosphenoids.

The presphenoid is a constricted bony splint which continues
the basis cranii forward from the basisphenoid. It is joined
anteriorly with the perpendicular plate of the ethmoid and with the
cartilaginous nasal septum. In the divided skull, or better in one
from which the roof has been removed, the actual dorsal surface

144 ANATOMY OF THE RABBIT.

of the bone is seen to be exposed to the cranial cavity only in its
posterior portion, where it is occupied by the suleus chiasmatis
and the optic foramina. That part of the floor immediately in
front of the optic foramina is formed by the coalesced roots of the
orbitosphenoids, the dorsal surface of the presphenoid being thus
excluded.

The orbitosphenoid forms a long, low plate, lying in the
ventral portion of the orbit, and divided by a shallow notch at the
level of the optic foramen into a posterior portion, the orbito-
sphenoid proper, and an anterior portion, the ethmoidal process
(processus ethmoidalis). The orbitosphenoid proper lies behind
the optic foramen. It is in contact dorsally with the orbital portion
of the frontal, and ventrally with the alisphenoid; it assists the
latter in the formation of the superior orbital fissure. Its posterior
tip is in contact with the squamosal. Its internal surface forms a
considerable portion of the anteroventral wall of the middle cranial
fossa.

The ethmoidal process extends forward from the optic fora-
men. Its dorsal margin is articulated with the orbital portion of
the frontal, and its ventral margin with the orbital portion of the
palatine. Anteriorly it projects toward the lacrimal bone, thus
occupying, in part, a space which, in the typical mammalian skull,
is filled by the lamina papyracea of the ethmoid. Its internal
surface is associated with the ethmoid bone and with the nasal
cavity. It falls for the most part below the level of the cranial
cavity.

4. THE SQUAMOSAL BONE.

The temporal bone, or temporal complex, as recognized
from the human condition, is an association of three elements—
squamosal, tympanic, and periotic—which in the human skull
are coalesced to form a single bone. It ‘s usually described as
consisting of four portions, of which the squamosal and tympanic
portions are. two, while the periotic bone is considered to consist
of two others, one of which, the petrous portion, is a solid white
portion lodging the internal ear, while the second, or mastoid
portion, is a mass of less compact character appearing externally

THE BONES OF THE SKULL. 145

in the wall of the skull. In the rabbit the original elements are not
coalesced, but the periotic and tympanic bones are so closely
associated that it is proper to describe them as forming a petro-
tympanic bone.

The squamosal bone (os squamosum) is a rectangular plate,
forming part of the lateral wall of the cranium, and bearing the
posterior root of the zygomatic arch. It is articulated’ anteriorly
with the orbitosphenoid and with the orbital portion of the frontal,
dorsally with the frontal and parietal, posteriorly with the supra-
occipital and petrotympanic, and ventrally with the alisphenoid.
Its posterior margin bears a prominent, slightly decurved squa-
mosal process (processus squamosus). It lies on the lateral
surface of the petrotympanic immediately above the opening of the
bony external acoustic meatus. The posterior root of the zygo-
matic arch is formed by a lateral and afterwards ventral projection,
the zygomatic process of the squamosal. Its base bears ventrally
the mandibular fossa, and dorsally, in association with the body
of the squamosal, the temporal fossa. The internal surface of the
squamosal forms a considerable portion of the wall of the cranial
cavity, the middle cranial cavity being, in fact, broadest in this
region.

5. THE PETROTYMPANIC BONE.

The petrotympanic bone (os petrotympanicum) is a some-
what oblong bone lying in the lateral wall of the cranium between
the posterior sphenoid and occipital bones. It is chiefly indicated
externally by the tympanic bulla and the bony external acoustic
meatus. It is articulated anteriorly with the alisphenoid and
squamosal, dorsally with the supraoccipital, and posteriorly with
the exoccipital. Except for the presence of the squamosal process
of the squamosal bone, the lateral and ventral surfaces are exposed
to the outside of the skull. The internal surface is exposed to the
posterior cranial fossa, with the exception, however, of a small
ventral portion which is articulated with the basioccipital bone.
Only a small portion of the anterior surface is in contact with the
squamosal bone, the larger part being applied to the tentorium
cerebelli and forming with the latter a posterior wall for the middle

146 ANATOMY OF THE RABBIT.

cranial fossa. The dorsal portion of the bone corresponds in
thickness with the wing of the supraoccipital with which it is
articulated. The posterior surface is applied to the anterior surface
of the exoccipital, and is thus excluded both from the cranial cavity
and from the external surface of the skull. Viewing the skull from
behind, however, it is seen that a small dorsal portion protrudes
in a triangular space formed by the dorsolateral margin of. the
exoccipital and the ventrolateral margin of the supraoccipital wing.
This portion is distinguishable by its pitted character. It forms
the mastoid portion (pars mastoidea) as distinguished from the
solid white petrous portion (pars petrosa), which is exposed to
the cranial cavity, and which contains the structures of the internal
ear. The mastoid portion lies for the most part above the tympanic
cavity, but it is also continued ventrad between the external
acoustic meatus and the exoccipital as the mastoid process.
The stylomastoid foramen lies between the latter and the ex-
ternal acoustic meatus.

The petrous portion, as viewed from its medial surface, is
roughly oblong; it is placed obliquely with reference to the basi-
occipital and basisphenoid. The parafloccular fossa occupies its
posterodorsal portion, and extends into the substance of the bone,
forming a much larger depression than is indicated by the diameter
of its rim. The related dorsal margin of the bone is occupied by a
groove which leads into a canal at its posterior margin. It indicates
the position of the lateral portion of the transverse sinus of the dura
mater. The ventral, thicker portion of the bone, enclosing the
apertures of the internal acoustic meatus and the facial canal,
is also that lodging the vestibulum and cochlea of the internal ear.
A small aperture at its anteroventral angle, only visible when the
petrotympanic is freed from its connections, represents the hiatus
canalis facialis of the human skull. It transmits the great super-
ficial petrosal nerve, a branch of the facial nerve passing to the
sphenopalatine ganglion.

The tympanic surface of the petrous portion is described below
in connection with the structures of the tympanic cavity.

The tympanic portion forms the spherical, expanded, shell-
like, tympanic bulla, which contains in its interior the tympanic
cavity, and is continuous dorsally with the bony enclosure of the

THE BONES OF THE SKULL. 147

external acoustic meatus. The boundary between the two is
indicated externally by a shallow oblique groove, the position of
which indicates roughly that of the tympanic membrane within.
The medioventral margin of the bone is articulated with the basi-
occipital, but the swollen portion is separated from the latter by a
broad groove terminating posteriorly in the jugular fossa and the
jugular foramen. Immediately in front of the jugular fossa, the
rounded aperture of the external carotid foramen, transmitting
in the natural condition the internal carotid artery, leads into the
carotid canal of the interior
of tympanic portion. At the
anterior end of the groove,
communicating with the fora-
men lacerum, is the anterior
opening of the carotid canal,
the internal carotid foramen,
and on its lateral side the much
larger aperture of the auditory
(Eustachian) tube. The rela-
tions of these apertures are seen
to best advantage’ when the
petrotympanic is disarticulated Bea De oe ee Ce eas ee ear
from, the associated posterior 2uditory complex of the left’side x3. The

lateral portions of the tympanic bulla and
sphenoid bone. The auditory external acoustic meatus have been removed,

exposing the structures of the tympanic

tube iS then seen to lead directly cavity. MS, mastoid portion; P, petrous

portion; T, tympanic portion (bulla tympani).

into the tympanic cavity. oN c.m., mastoid cells; c.t., tympanic cavity;

f.c., cochlear fenestra; in., incus; m.a.e.,

fine bristle may be passed external acoustic meatus; m.m., manubrium

of the malleus; m.so., supraoccipital’ margin

through the carotid canal from of petromastoid; p.m., mastoid process; st.,

stapes; t.a., aperture of auditory tube.

one foramen to the other.

6. THE STRUCTURES OF THE TYMPANIC CAVITY

The relations of the tympanic cavity and associated structures
may be studied with advantage in a skull from which the lateral
wall of the tympanic bulla and external acoustic meatus has been
removed, the surface displayed being as indicated in Fig. 64.
The tympanum or middle ear is enclosed by the tympanic and
petromastoid portions of the temporal complex. The attached

148 ANATOMY OF THE RABBIT.

margin of the tympanic bulla encloses a roughly triangular area,
into the ventral part of which the petrous portion of the petro-
mastoid projects as a smooth, white, convex ridge, the pro-
montory (promontorium). Above and behind the promontory the
tympanic cavity is extended toward the mastoid portion of the bone
as the tympanic or mastoid antrum (antrum tympanicum), and
the interior of the mastoid portion is partly occupied by small
extensions of the tympanic antrum, termed the mastoid cells
(cellulae mastoideae). At the anteroventral angle of the area
already described, a deep notch indicates the point of entrance of
the auditory tube. The exposed surface of the petromastoid
presents two apertures, one of which, situated posteroventrally,
is open in the dried skull, and is the cochlear fenestra (fenestra
cochleae). In the natural condition it is closed by a thin membrane
which separates the tympanic cavity from the _ perilymphatic
space containing the membranous labyrinth. The second aper-
ture, the vestibular fenestra (fenestra vestibuli), lies above and
in front of that just described. It is closed by the base of the
stapes.

The auditory ossicles (ossicula auditus) comprise three ele-
ments, namely, the malleus, incus, and stapes, which bridge the
space intervening between the tympanic membrane and the open-
ing to the internal ear as represented by the vestibular fenestra.
They occupy the dorsal angle of the triangular area already de-
scribed, and lie immediately above the promontory. The malleus
is the lateral element. The main portion, termed the head, is
concealed by the projecting edge of the external acoustic meatus.
It bears a stout vertical process, the manubrium mallei, which
in the natural condition lies in contact with the tympanic mem-
brane. The incus is the intermediate element; it is directly
articulated with the malleus, and bears a downwardly-directed
long limb (crus longum), for articulation with the minute head
of the stapes. The latter element is a small stirrup-shaped bone,
occupying an almost transverse position, and articulated at its base
with the margin of the vestibular fenestra.

THE BONES OF THE SKULL. 149

7. THE INTERPARIETAL BONE.

The interparietal (os interparietale) is a small, lozenge-shaped
element, surrounded by the two parietal bones and the supra-
occipital. It is the first of the membrane roofing elements of the
cranium proceeding forward from the supraoccipital, and in the
rabbit’s skull is not fused with the occipital segment.

8. THE PARIETAL BONE.

The parietal bone (os parietale) is a characteristic roofing bone
covering a large portion of the middle cranial fossa. It is somewhat
rectangular in shape, and is connected by serrate sutures with the
surrounding elements and with its fellow of the opposite side, the
sutures producing a characteristic pattern on the external surface
of the skull. The sutures are medial, anterior, lateral, and posterior
in position, and are designated respectively as saggital, coronal,
squamosal, and lambdoidal. The posterolateral angle of the
bone is produced ventrally into a long, curved Squamous process
(processus squamosus), which lies in the angle formed by the ten-
torium cerebelli and the lateral wall of the middle cranial fossa.
It is not exposed to the external surface of the skull.

9. THE FRONTAL BONE.

The frontal bone (os frontale) is a paired element, lying
directly in front of the parietal, and forming with its fellow of the
opposite side the anterior portion of the roof of the cranial cavity
and also a considerable portion of its lateral, orbital wall. Unlike
the condition in the human skull, the two bones are separate
throughout life, so that there is a permanent frontal suture.
Each consists of a frontal portion (pars frontalis), the external or
dorsal surface of which continues that of the parietal, and of an
orbital portion (pars orbitalis), enclosing the dorsal portion of
the orbit. The two parts are connected at the supraorbital
border, with which is also connected the base of the divided
supraorbital process. The anterior end of the frontal portion
is deeply notched where it comes in contact with the nasal and

150 ANATOMY OF THE RABBIT.

premaxillary bones. Two processes are thus formed, one medial,
the other lateral to the nasal. The medial process is associated
with that of the opposite side to form a triangular frontal spine,
while the lateral or maxillary process (processus maxillaris)
projects forward between the nasal and premaxillary bones, on
the one hand and the subcutaneous process of the lacrimal, the
orbital process of the maxilla, and the body of the latter, on the
other.

The orbital portion of the frontal forms a considerable portion
of the orbital wall. Its anterior margin is in contact with the
lacrimal bone, its ventral margin with the slender sphenoorbital
process of the maxilla, the ethmoid process of the orbitosphenoid,
and the orbitosphenoid proper. Its internal surface is divided by a
vertical ridge into anterior and posterior portions, in relation
respectively to the anterior and middle cranial fossae. The anterior
cranial fossa is enclosed by the frontal bones, with the exception,
however, of a small portion of the floor which is formed by the
cribriform plate of the ethmoid.

10. THE ETHMOID BONE.

The ethmoid bone (os ethmoidale), the chief representative of
the embryonic cartilaginous nasal capsule, is a delicate, greatly
sculptured structure, almost completely enclosed by the membrane
bones of the face. Its features may be studied either in the divided
skull, or in one from which the roof of the nasal and cranial cavities
has been removed. It consists of three main portions, namely, the
cribriform plate, the perpendicular plate, and the Paice
lateral masses, or ethmoidal labyrinths.

The cribriform plate (lamina cribrosa) is exposed to the
anterior cranial fossa. It is somewhat heart-shaped, with its apex
in contact with the ethmoidal processes of the orbitosphenoids.
Its lateral portions are perforated by numerous foramina, giving
passage in the natural condition to the branches of the olfactory
nerves. Its median portion forms a low vertical ridge, the crista
galli, continuous in front with the perpendicular plate.

The perpendicular plate (lamina perpendicularis) is the bony,
posterior portion of the nasal septum, and as such is exposed to the

THE BONES OF THE SKULL. I51

nasal cavity. It is united with the cartilaginous nasal septum and
also with the presphenoid. It forms the terminal member of the
chain of bones lying in the basicranial axis.

The ethmoidal labyrinth (labyrinthus ethmoidalis) occupies
for the most part the posterior portion of the nasal fossa, but the
nasoturbinal extends forward to its anterior end, and is attached
for the greater part of its length to the internal surface of the nasal
bone. It is broadest in its middle portion, where it projects into
the space left between the ethmoturbinal proper and the maxillo-
turbinal, and contains at this point a pouch-like cavity, termed the
mMmarsupium nasale. The whole structure is comparable to one
of the folds of the ethmoturbinal proper; but it is frequently seen
to be divided into anterior and posterior parts by a thin vertical
line of cartilage, the anterior division being probably allied to the
maxilloturbinal. Its middle, ventral, portion bears a_ stout,
backwardly-directed uncinate process (processus uncinatus),
which is applied to the medial surface of the maxilla.

The ethmoturbinal proper consists, as described above, of
several shorter scrolls, decreasing in length from above downward.
Like the posterior part of the nasoturbinal, they are attached
directly to the cribriform plate, the perforations of which may be
seen in the divided skull opening into the ethmoidal scrolls
ot spaces contained by them. They are roughly comparable to the
superior and middle turbinated bones of the human skull, but in
the rabbit, as in most mammals, the ethmoturbinal surfaces are
relatively much more extensive than in man.

In the typical mammalian skull the ethmoid bone is exposed
to the orbit, where it forms a thin plate of bone, the lamina
papyracea. In the rabbit, however, the space usually occupied
by the lamina papyracea is partly filled by the lacrimal bone, the
ethmoidal process of the orbitosphenoid and the sphenoorbital
process of the maxilla.

11. THE INFERIOR TURBINATED BONE.

The inferior turbinated bone (concha nasalis inferior), or
maxilloturbinal, is a finely ridged structure, situated anteriorly
in the nasal fossa, and supported by the maxilla and premaxilla.

152 ANATOMY OF THE RABBIT.

It represents the similarly-named structure of the human skull, the
lowermost of three scroll-like bones, of which the remaining two,
the superior and middle turbinated bones, belong to the ethmo-
turbinal. In the natural condition it is covered by a non-olfactory
epithelium, and is thus distinguishable in function as well as in
position from the latter.

12. THE MAXILLA.

The maxilla, the largest element of the facial region, is asso-
ciated with its fellow of the opposite side to form the main portion
of the upper jaw. It consists of a central portion, the body
(corpus maxillae), and of five processes, namely, alveolar, palatine,
orbital, zygomatic, and sphenoorbital. In the adult condition
the zygomatic bone is fused with the maxilla, so that the extent of
the zygomatic process appears to be greatly increased.

The body of the maxilla is greatly fenestrated on its external
surface, the perforated area extending backward to the anterior
rim of the orbit, and thus including the maxillary fossa and the
infraorbital foramen. The dorsal boundary of the bone is
formed by the frontal process of the premaxilla and by the maxillary
process of the frontal. Anteriorly, it is united with the premaxilla,
the ventral part of the suture appearing in the diastema separating
the incisors from the cheek-teeth. The ventral portion of the bone
forms part of the lateral boundary of the incisive foramen.
Behind the palatine bridge it is applied to the lateral surface of the
palatine bone, and is projected into the orbit as a broad ridge en-
closing the alveoli of the four posterior cheek-teeth.

In the divided skull the medial surface of the body of the maxilla
is found to be concealed by the ethmoturbinal. It contains a deep
longitudinal excavation, the maxillary sinus (sinus maxillaris),
widely open to the nasal fossa, but only seen to advantage when the
ethmoturbinal is removed. The lateral wall of che sinus corres-
ponds in position to the fenestrated area of the external surface.
It bears the chief part of the nasolacrimal canal. —

The alveolar process (processus alveolaris) is that portion of
the maxilla lodging the sockets of the cheek-teeth. In the rabbit

a |

—_—

THE BONES OF THE SKULL. 153

it is separated by the diastema, in which no teeth occur, from a
corresponding but imperfectly differentiated process of the pre-
maxilla.

The palatine process (processus palatinus) extends toward
the median plane. It forms with its fellow of the opposite side
about two-thirds of the palatine bridge.

The orbital process (processus orbitalis) is directed obliquely
toward the dorsal surface of the skull. In conjunction with the
lacrimal bone and the maxillary process of the frontal, it forms the
anterior orbital rim. It is continuous with the fenestrated portion
of the body, and its appearance as a process is largely due to its
solid character as compared with the perforated surface lying in
front of it.

The zygomatic process (processus zygomaticus) forms the
anterior root of the zygomatic arch, and in the adult condition
has fused with it the anterior end of the zygomatic bone. Its
ventral angle bears a prominent masseteric spine for the attach-
ment of the ligament of the masseter muscle.

The sphenoorbital process (processus sphenoorbitalis) lies
on the medial wall of the orbit, in a position opposite to the middle
portion of the ridge lodging the posterior cheek-teeth. It forms a
stout buttress, the tip of which is applied to the anteroventral
angle of the frontal bone. In this position it is visible from the
orbit, lying between the lacrimal bone and the ethmoidal process
of the orbitosphenoid.

13. THE PREMAXILLA.

The premaxilla, or incisive bone (os incisivum), forms the
anterior portion of the upper jaw. It comprises a central portion,
the body—including with the latter the scarcely differentiated
alveolar portion containing the large and small incisors—a
frontal process, and a palatine process. The body forms a
portion of the palatal surface of the skull and of the lateral boun-
dary of the incisive foramen. Its dorsal surface forms part of the
boundary of the piriform aperture, the remaining portion of this

154 ANATOMY OF THE RABBIT.

being formed by the nasal bone. The palatine process extends
backward on the medial side of the bone, closely applied on the
palatal surface to its fellow of the opposite side, and forms in this
way a medial boundary for the incisive foramen. Its dorsal
surface, in conjunction with that of the corresponding process of
the other side, bears a broad palatine groove (sulcus palatinus),
lodging a portion of the cartilage of the vomeronasal organ and
nasopalatine duct. The frontal process (processus frontalis)
is a thin bony splint, extending backward between the nasal and
maxillary bones, and terminating between the former and the
maxillary process of the frontal.

14. THE ZYGOMATIC BONE.

The zygomatic bone (os zygomaticum) is a separate element
only in very young animals. In the adult it is fused anteriorly
with the zygomatic process of the maxilla, the position of the
original suture being roughly identifiable as the point where the free
horizontal portion of the zygomatic arch arises from the transverse
zygomatic process. It forms an almost sagittal plate of bone
bridging the orbit and serving for the attachment of the masseter
muscle of the mandible. Its dorsal margin forms posteriorly
a smooth, horizontal articulation with the zygomatic process of the
squamosal, the end of the bone projecting considerably behind the
articulation.

15. THE NASAL BONE.

The nasal bone (os nasale) is a thin, elongated bone forming
the roof of the nasal fossa, and, in conjunction with its fellow of the
opposite side, the dorsal boundary of the piriform aperture. It is
loosely articulated with the maxilla and with the bone of the
opposite side by smooth (harmonic) sutures. The medial margin
is supported by the dorsal edge of the nasal septum. The internal
surface bears the nasoturbinal scroll.

ee

THE BONES OF THE SKULL. 155

16. THE VOMER.

The vomer is the median, somewhat sickle-shaped, vertical
plate of bone separating the ventral portions of the nasal fossae.
It is visible from the palatal surface through the incisive foramina,
but its extent is best shown in the divided skull. It forms a
support for the ventral border of the nasal septum, and its pos-
terior portion bears a shelf-like projection, the ala vomeris, which
assists in the support of the ethmoturbinal.

17. THE LACRIMAL BONE.

The lacrimal bone (os lacrimale) is a small element lying in the
anterior wall of the orbit. It is loosely articulated with the sur-
rounding bones, and in the dried skull is frequently missing unless
care has been taken to keep it in place. It consists of a basal
portion, somewhat rectangular on its orbital surface, and of two
processes, namely, the subcutaneous process and the hamulus
lacrimalis. The subcutaneous process is the prominent hook-like
projection extending laterad beyond the orbital rim. The hamulus
lacrimalis isa small process, directed toward the nasal cavity. It
bears a groove which, in association with a corresponding groove
of the maxillary bone, forms the first portion of the nasolacrimal
canal.

18. THE PALATINE BONE.

The palatine bone (os palatinum) forms the poste ior portion
of the palatine bridge and the major portion of the lateral wall of the
nasopharynx. It consists of two portions—horizontal, and
perpendicular. The horizontal portion (pars horizontalis) is
that lying in the plane of the palatal surface. It is articulated
in front with the palatine process of the maxilla, the suture between
the two bones enclosing the greater palatine foramen, the
ventral termination of the pterygopalatine canal. The perpen-
dicular portion (pars perpendicularis) is the vertical plate ex-
tending backward from the palatine bridge. Its medial surface is
divided by a low ridge into a dorsal portion, in particular relation
to the nasopharynx, and a ventral portion, in relation to the oral

156 ANATOMY OF THE RABBIT.

cavity, the ridge indicating the position of the soft palate. Its
lateral surface is partly applied to the maxilla and partly exposed
to the orbit. Its dorsal margin is articulated with the presphenoid
and with the ethmoidal process of orbitosphenoid, but a small
posterior portion is free, so that the anterior portion of the basi-
sphenoid is visible from the orbit. The free ventral margin forms
posteriorly a thick projecting angle, the pyramidal process
(processus pyramidalis), the base of which is cleft where it articulates
with the medial and lateral laminae of the pterygoid process.
Between the pyramidal process and the alveolus of the last cheek-
tooth there is a conspicuous palatine notch (incisura palatina),
connecting the orbit with the palatal surface. In the entire skull
only the posterior portion of the lateral surface is visible from the
orbit,-the anterior portion being concealed by the projecting bases
of the posterior cheek-teeth. The ridge of bone on which the
alveoli of these teeth are borne is separated from the palatine bone
by the infraorbital groove. The medial wall of the latter,
formed by the palatine bone, contains the orbital opening of the
pterygopalatine canal and the sphenopalatine foramen.

19. THE MANDIBLE.

The mandible (mandibula), or lower jaw, comprises the two
dentary bones (ossa dentalia), which, in che rabbit, as in mammals
generally, are united by a fibrous or fibrocartilaginous connection
(symphysis mandibulag); not coalesced, as in the human skull, to
form a continuous siructure. As indicated above, each of the
dentary bones comprises: (1) a horizontal, tooth-bearing portion
which, in conjunction with that of the opposite side, forms the
body of the mandible (corpus mandibulae); and (2) a posterior,
vertical plate, the mandibular ramus (ramus mandibulae), for
muscle attachment and articulation. The horizontal portion
is deep posteriorly, where it lodges the alveoli of the cheek-teeth.
Anteriorly, in the diastema separating the latter from the incisors,
its dorsal surface is rounded and depressed, the space thus formed
corresponding to a similar space in the upper jaw and serving
chiefly for the accom.nodation of the lips, which in this region en-
croach medially on the oral cavity. The medial surface of the

ae 6 Petes res

ks eae

ee

i a ee

THE BONES OF THE SKULL. 157

horizontal portion forms an acute angle with that of the bone of the
opposite side, except anteriorly, where it bears a roughened area
for articulation with the latter. Running backward from the
symphysis there is a broad horizontal ridge, representing the
mylohyoid line (linea mylohyoidea), the line of attachment of the
mylohyoid muscle. The mandibular foramen, through which,
in the natural condition, the inferior alveolar nerve and artery
gain access to the interior of the bone, lies on this surface at the
junction of the horizontal portion with the ramus. The corres-
ponding mental foramen (foramen mentale), through which
branches of these structures leave the mandible, is situated on the
lateral surface in front of the first premolar. The mandibular
foramen is closely connected with a second aperture lying at the
ventral end of the sulcus ascendens, directly behind the last molar,
and serving for the transmission of a vein connecting the inferior
alveolar and inferior orbital veins.

The mandibular ramus forms in general an obtuse angle
with the horizontal portion. As in other herbivores, the ventral
part, distinguished as the angle, is greatly increased in size at the
expense of the condyloid process and to a still greater extent of the
coronoid process, the latter being vestigial. In addition to a low
pterygoid tuberosity (tuberositas pterygoidea), situated at the
posterior projecting point of the angle, the posterior and ventral
margins of the angle are excavated on the medial side of the bone,
so that they form the boundary of a pronounced, though shallow,
inferior pterygoid depression for the insertion of the ptery-
goideus internus muscle. The area occupied by the pterygoideus
internus is separated by a low ridge from a more dorsally placed
superior depression for the pterygoideus externus muscle. <A
somewhat similar depression, termed the masseteric fossa,
occupies the lateral surface of the angle, its raised ventral margin
terminating posteriorly in the masseteric tuberosity (tuberositas
masseterica). The articular portion, or head of the mandible is
greatly elongated in the anteroposterior direction in accordance
with the anteroposterior action of the lower jaw, this feature being
one which is of general occurrence in the rodent order, and more
fully expressed in the great extension forward and backward of the
attachment areas of the muscles of mastication. The connection

158 ANATOMY OF THE RABBIT.

of the articulating portion with the condyloid process, the
so-called neck of the mandible (collum mandibulae), is a thin
plate of bone, the anterior and posterior margins of which are
barely notched by the anterior and posterior mandibular
incisures. Connecting the anterior incisure with the rim of the
alveolus of the last cheek-tooth there is a deep groove, the sulcus
ascendens, the lateral margin of which is formed by the reduced
coronoid process (processus coronoideus). Its low medial margin
is formed by a bony stay which
extends to the medial surface of
the horizontal portion opposite the
last cheek-tooth, and is continued
forward into the mylohyoid line.
The sulcus ascendens lodges in the
natural. condition the insertion
portion of the greatly reduced
temporalis muscle.

C:
THE HYOID APPARATUS.
BIG O'S. Lateral “surface” cf the
hyoid and roe c.a., arytenoid The hyoid bone (os hyoideum)

Cantilages) /G:c.,/ackicoid sicartilage;s ccs,

inferior cornu of thyreoid cartilage; (Fig 65) is a stout somewhat
c.m., lesser cornu of hyoid; c.mj., greater 2 * ?

cornu of hyoid; c.s., superior cornu of r fac = :
thyreoid cartilage; c.t., left plate of the W edge shaped bone lying il front

thyreoid cartilage; e., epiglottic cartilage; of the larynx and between the
f.t.s., thyreoid foramen; l.h., lateral ae

hyothyreoid ligament; 1.h.m., median angles of the mandible. Its ven-
hyothyreoid ligament; m.ct., cricothy-

reoideus muscle; o.h., hyoid bone; s.m., tral portion is connected with the
stylohyoideus minor muscle; s.mj., stylo-
hyoideus major muscle; tr., cartilaginous thyreoid cartilage of the larynx
tracheal rings. ’ % ra
by the median hyothyreoid
ligament. With its lateral portion are articulated two indepen-
dent elements, termed the lesser and greater cornua. The lesser
cornu (cornu minus) is a small, partly cartilaginous structure,
attached to the anterodorsal angle of the hyoid, and connected
through the stylohyoideus minor muscle with the jugular process
of the skull. The greater cornu (cornu majus) is a larger element
extending obliquely dorsad, and similarly suspended from the
jugular process by the stylohyoideus major muscle. The con-
nection of the lesser cornu with the styloid process through the
stylohyoideus minor replaces the stylohyoid ligament of the human

THE SKELETON OF THE ANTERIOR LIMB. 159

skull and the chain of elements commonly occurring in mammals
and other vertebrates in this region. In most mammals the term
“lesser”, as applied to it, is inappropriate. The muscle tendon
contains near the jugular process a small ossification representing
a detached styloid process. This connection, together with the
hyoid bone itself, indicates the relation of the embryonic hyoid arch,
from which the skeletal structures in question are derived. The
greater cornu belongs to the succeeding visceral arch, and is con-
nected with the superior cornu of the thyreoid cartilage of the
larynx by the lateral hyothyreoid ligament. It is commonly
represented in mammals by a small thyreohyal process.

THE SKELETON OF THE ANTERIOR LIMB.

The skeleton of the anterior limb is divisible into two portions,
namely, a proximal portion, comprising the scapula and the
clavicle, and a distal portion, comprising the supports of the
free extremity. The scapulae and clavicles of the two sides
together form the pectoral girdle. The pectoral girdle is lightly
constructed, and, apart from its muscular connections, which
constitute its main support, is directly attached to the axial skeleton
only through the sternoclavicular ligament.

The skeleton of the free extremity is divisible into proximal,
middle, and distal segments. The proximal segment contains
a single bone, the humerus; the middle segment two elements, the
radius and ulna; while the distal segment comprises, in addition
to the accessory sesamoid bones, twenty-eight elements of the
regular series, of which nine form the carpus, five the metacarpus,
and fourteen the phalanges of the digits.

THE SCAPULA.

The scapula (Fig. 66) is a somewhat triangular plate of bone
lying in the natural position on the lateral surface of the anterior
part of the thorax, with its apex directed downward and forward.
In the rabbit, as in quadrupedal mammals generally, the main
surfaces are respectively medial and lateral, and differ in this respect
from the human condition, in which, from the transverse widening

160 ANATOMY OF THE RABBIT.

of the thorax, the corresponding surfaces are more nearly ventral
and dorsal. Of its three borders, one, the superior border (margo
superior), is directed toward the occiput; another, the vertebral
border (margo vertebralis), toward the vertebral column; and the
third or axillary border (margo axillaris), toward the armpit.
The corresponding angles are medial (superior), inferior, and
lateral (glenoid). The lateral surface bears a stout bony plate,
the scapular spine (spina scapulae), which arises from the body of
the bone through about two-thirds of its extent, and ends ven-
trally in a free projection, the acromion. The posterior margin
of the acromion bears a backwardly-directed process, the meta-

Fic. 66. Lateral surface of the left scapula: a., acromion; a.i., a.l., and a.m.,
inferior; lateral, and medial angles; c.g., glenoid cavity; c.s., neck of the scapula;
f.s. and f.i., supraspinous and infraspinous fossae; m., metacromion; m.a., m.s., and
m.v., axillary, superior, and vertebral borders; p.c., coracoid process; ‘s.s., scapular
spine.

cromion (processus hamatus). Through the presence of the
scapular spine, the lateral surface of the bone is divided into two
areas for muscular attachment. One of these, the supraspinous
fossa (fossa supraspinata), lies in front of the spine, the other,
the infraspinous fossa (fossa infraspinata), behind it. The
infraspinous fossa is the more extensive one. The medial surface,
onthe other hand, presents a single large shallow depression, the
subscapular fossa (fossa subscapularis), which is triangular in
shape and occupies practically the entire surface. The apex or
lateral angle of the scapula, sometimes termed the head of the
bone, is expanded to a considerable extent in comparison with the

THE SKELETON OF THE ANTERIOR LIMB. 161

slender portion—the so-called neck of the scapula (collum scap-
ulae)—connecting it with the body of the bone. It bears a concave
depression, the glenoid cavity (cavitas glenoidalis), for articula-
tion with the humerus. The articulating surface is borne chiefly
on that part of the bone corresponding to the axillary border, but
it also extends in an anterior direction

to the base of an overhanging pro- Sil.

jection, the coracoid process (pro- émi.
cessus coracoideus). The free por-

tion of the latter forms a _ blunt, Z
hook-like projection lying toward the 5
medial surface of the bone.

THE CLAVICLE.

The clavicle (clavicula) is imper-
fectly developed in the rabbit, con-
sisting of a slender, arcuate rod of
bone, tipped by cartilage, which lies
in the interspace between the manu-
brium sterni and the head of the
humerus. It occupies only a portion of
this interspace, being attached me-
dially by the sternoclavicular liga-
ment and laterally by the cleido-
humeral ligament.

Fic. 67. Anterior surface of the left
humerus: c., capitulum; c.h., head of

a ap \\ 0 Pa alxe humerus; e.l. and e.m., lateral and

The humerus (1 Ig. 67) IS ty pical medial epicondyles; f.r., radial fossa;

i Lat. = r a aaa s.h., deltoid tuberosity; s.i., intertuber-

of the long bones of the proximal cular groove; t.h., trochlea humeri;

Bnigvmidcle See ments Of eme tore and (os and tm... lessegrand-“gredter

THE HUMERUS.

hind limbs in consisting of a central

portion, the body or shaft of the bone, and of proximal and
distal extremities for muscle attachment and articulation. The
proximal extremity bears on its medial side a smooth, convex
projection, the head of the humerus (caput humeri), for articulation
with the scapula. The articulation is nominally a ball-and-socket

162 ANATOMY OF THE RABBIT.

joint, or enarthrosis, but the articulating surfaces are somewhat
restricted, and the muscular arrangements of the limb are such
that the range of lateral motion (abduction and adduction) is small.
Immediately in front of the head of the bone there is a small
elevation, the medial tuberosity or lesser tubercle (tuberculum
minus). It is separated by a longitudinal furrow of the anterior
surface, the intertubercular groove (sulcus intertubercularis),
from a much larger lateral elevation, the lateral tuberosity or
greater tubercle (tuberculum majus). Extending distad from

the latter is a triangular area, the deltoid tuberosity (tuberositas |

deltoidea), the tip of which reaches almost to the middle of the bone
and forms a pronounced angle on its anterior surface.

The distal extremity of the humerus bears a grooved articular
surface, the trochlea humeri, for articulation with the radius
and ulna. On its lateral side is a smaller surface, the capitulum
humeri, for articulation with the radius alone. Immediately
above the trochlea the medial and lateral portions of the bone are
thickened to form two areas for muscular attachment. One of these,
the lateral epicondyle (epicondylus lateralis), is a general point
of origin for the extensor muscles of the dorsal surface of the hand,
while the other, the medial epicondyle (epicondylus medialis),
is a similar point of origin for the flexor muscles of the ventral
or volar surface. Between the epicondyles the extremity of the
bone is greatly excavated, so that the projecting portions of the
radius in front and of the ulna behind are received into depressions
of the surface when the forearm is greatly flexed or extended.
On the anterior side is the radial fossa (fossa radialis); on the
posterior side the olecranon fossa (fossa olecrani), so-called because
it accommodates the olecranpn process of the ulna.

THE RADIUS AND ULNA.

The radius (Fig. 68) is the shorter of the two bones of the
forearm, since its proximal extremity does not extend backward
beyond the front of the elbow joint. It is anterodorsal in its general
position, but is crossed on the ulna in such a way that its proximal
extremity tends to be lateral, while its distal extremity is medial.
The proximal extremity, termed the head of the radius (capitulum

Sees?

THE SKELETON OF THE ANTERIOR LIMB.

radii) is immovably articulated with
the ulna. It bears an extensive
articular surface, meeting both the
trochlea and capitulum of the hum-
erus, and thus forming a considerable
portion of the elbow-joint. The body
‘of the bone is solidly united with the
ulna by the interosseous ligament
of the forearm. The distal extremity
is largely formed by an epiphysis,
which is well-marked even in older
animals. It bears a grooved, carpal
articular surface (facies articularis
carpea), for articulation with the
navicular and lunate bones.

The ulna (Fig. 68) is a somewhat
S-shaped bone, the shaft of which is
vertically flattened, so that it pos-
sesses two main surfaces, respectively
anterodorsal and posteroventral. The
former, in conjunction with the
related surface of the radius, continues
the area of origin of the extensor
muscles of the hand from the lateral
epicondyle of the humerus distad on
to the forearm, while the latter has a
similar function with respect to the
flexor muscles. The proximal portion
of the bone is laterally compressed.
It bears a crescentic depression, the
semilunar notch (incisura
lunaris), the articulating surface of
which continues that of the medial
portion of the head of the radius, and
is received into the trochlea humeri.
Behind the elbow-joint the
forms the large projecting portion
of the elbow, the olecranon, which is
astrong process for the insertion of
the extensor muscles (anconaei)

semi-

bone

Fic. 68. Skeleton of the fore-
arm and hand from the dorsal
surface: R, radius; U, ulna; C,
carpus; M, metacarpus; P, pha-
langes; I-V, metacarpal bones; c.,
central bone; cp., capitate; c.r.,
head of radius; f.a.c., carpal artic-
ular surface of radius; h., hamate
bone; i.s., semilunar notch of the
ulna; 1., lunate bone; mi., lesser
multangular; mj., greater multang-
ular; n., navicular; ol., olecranon;
p.s., styloid process of the ulna;
tr., triquetral bone; u., ungual
phalanges,

164 ANATOMY OF THE RABBIT.

acting on the forearm. The distal extremity of the bone is formed
by an epiphysis, similar to but much longer than that of the radius.
It is immovably articulated with the radius, and its tip is formed
by a blunt styloid process (processus styloideus), which is
articulated with the triquetral bone of the carpus.

The elbow-joint is formed by the trochlea and capitulum of the
humerus in conjunction with the semilunar notch of the ulna and
the corresponding articular surface of the head of the radius. It is
a hinge-joint, or ginglymus, permitting motion in one plane, 7.e.,
extension and flexion of the forearm. The trochlear surface of the
humerus, however, has a slight spiral trend, the anterior portion
being medial in comparison with the posterior portion.

Through the immovable articulation provided by the respective
proximal and distal ends of the bones, and also through the inter-
osseous ligament, the radius and ulna are unable to change their
positions with respect to one another; in other words, the radius is
unable to rotate on an axis formed by the ulna, the fore foot being
fixed in a position comparable to that of pronation in the human
hand (cf. p. 65).

THE CARPUS.

The carpus (Fig. 68) comprises nine small elements, the wrist
or carpal bones (ossa carpi), which are interposed between the
forearm and the digits. They are arranged in two main rows,
namely, a proximal row, the elements of which are articulated
with the radius and ulna; and a distal row, the elements of which
are articulated with the five bones of the metacarpus. Enumerated

from the medial side of the wrist laterad, the proximal row contains”

four elements, namely, the navicular, lunate, triquetral and
pisiform bones. The navicular and lunate are articulated
with the distal extremity of the radius; the triquetral with the
styloid process of the ulna. The pisiform bone lies on the ventral
surface of the extremity of the ulna, and is therefore not exposed
to the dorsal surface of the wrist. The distal row contains five
elements, namely, the greater multangular, lesser mult-
angular, central, capitate, and hamate bones. The first,
second, and fourth are in association respectively with the first,
second, and third metacarpals. The central bone lies to the

se

THE SKELETON OF THE POSTERIOR LIMB. 165

lateral side of the articulation at the base of the second metacarpal.
As its name implies, it is originally an element interposed between
the, proximal and distal rows. The hamate is a comparatively
large element associated with the fourth and fifth metacarpals, but
extending also to the articulation of the third, where it tends to
replace the greatly reduced capitate.

THE METACARPUS AND PHALANGES.

The metacarpus (Fig. 68) comprises five stout elements, the
metacarpal bones (ossa metacarpalia), which form the basal
supports of the digits. Each consists, in addition to a main portion
or body, of a flattened proximal end, or base, and a rounded distal
extremity, or head. The four lateral bones are normally developed,
while the first, which belongs to a reduced digit, is of very small size.

The phalanges or bones of the digits are distributed according
to the formula 2, 3, 3, 3,3. They are similar in form to the meta-
carpals, with the exception, however, of the terminal, ungual
phalanges, which are laterally compressed, pointed, and cleft at
their tips for the attachment of the claws.

SESAMOID BONES.

Accessory elements, sesamoid bones (ossa_ sesamoidea)
developed in connection with the ligaments of muscles, are found
on the volar surface of the foot in association with certain of the
joints. They occur in transverse pairs at the metacarpophalangeal
articulations and in linear pairs at the articulations of the second
with the third phalanges. The pisiform bone of the carpus is also
a sesamoid, being formed in the insertion tendon of the flexor carpi
ulnaris muscle.

THE SKELETON OF THE POSTERIOR LIMB.

In the posterior limb the proximal or girdle portion comprises
the paired coxal bones, which are united ventrally at the pelvic
symphysis, thus forming the pelvic girdle. The distal portion—
comprising, as in the anterior limb, the supports of the free ex-

166 ANATOMY OF THE RABBIT.

tremity—is divisible into proximal, middle and distal segments.
The proximal segment contains a single element, the femur; the
middle segment two elements, the tibia and fibula, which, how-
ever, are extensively coalesced;
and the distal segment twenty-
three elements, of which six
form the tarsus, five the meta-
tarsus, and twelve the pha-
langes.

THE COXAL BONE.

The coxal bone (os coxae)
(Fig. 69) is a somewhat trira-
diate structure, the posterior
limbs of which are united, so
that they enclose a large aper-
ture, the obturator foramen
(foramen obturatum). It is
firmly articulated with the
sacrum, and is united with its
fellow of the opposite side by a
thin strip of cartilage containing
a small amount of fibrous
material. The latter connec-
tion is the pubic symphysis
(symphysis pubis), better term-

FIG. 60. Lateral surface of the left

coxal bone: IL, ilium; IS, ischium; P, ed in the rabbit the pelvic
pubis; a., acetabulum; a.i., iliac wing; ° 5 é : -
c.l., body of ilium; c.is., body of ischium; symphysis, since it 1S some-
c.p., body of pubis; cr., iliac crest; e.i., .

iliopectineal eminence; f.a., acetabular what more extensive than the
fossa; f.0., obturator foramen; -a., 5 : 5
acetabular notch; i.mi., lesser ’ sciatic corresponding articulation of
notch; i.mj., greater sciatic notch; 1.1., 5

iliopectineal line; p.l., lateral process of the human pelvis.

ischial tuberosity; r.i.i., inferior ramus of 5

ischium; r.i.p., inferior ramus of pubis; In the young animal each
r.s.il., SUperior ramus of ischium; r.s.p., i 5 5
superior ramus of pubis; s.a.i., inferior half of the pelvis consists of
anterior spine of the ilium; s.a.s., superior

anterior spine; s.i., ischial spine; s.p., three elements, namely, the

symphysis pubis; s.p.i., inferior posterior

spine; t.i., ischial tuberosity; t.p., pubic ilium, ischium, and pubis. :
tubercle.

They form the three rays of the

coxal bone, and are united with one another in the region of the

THE SKELETON OF THE POSTERIOR LIMB. 167

acetabulum, which is the basin-like depression for the articulation
of the pelvis with the femur. Only two of the original elements,
however, actually take part in the formation of the acetabulum,
the pubis being excluded through the development in the acetabular
depression of a small triangular element, the os acetabuli.
Although completely coalesced in the adult condition, and showing
but few traces of their original separation, the three chief elements
are nevertheless described as if distinct.

The ilium (os ilium) is the anterior, also somewhat dorsal,
portion of the bone; that part extending forward from the ace-
tabulum. It comprises a basal portion, the body (corpus oss.
ilium), which includes the anterior portion of the acetabulum and
the cylindrical part of the bone in front of it, and an expanded
portion, the iliac wing (ala oss. ilium), for muscle attachment and
articulation with the sacrum. The body is somewhat triangular
in section, its surface being divided into three areas, which are
respectively medial, or sacral, ventrolateral, or iliac, and dorso-
lateral, or gluteal. The corresponding borders are respectively
ventral, or pubic, lateral, or acetabular, and dorsal, or ischial.
The acetabular border terminates a short distance in front of the
acetabulum in an abruptly truncated projection, the. inferior
anterior spine (spina anterior inferior). The ischial border forms
the anterior half of a long depression of the dorsal surface of the
coxal bone, the greater sciatic notch (incisura ischiadica major).
The pubic border presents on its medial side a faint, ridge-like
elevation, the iliopectineal line (linea iliopectinea), which con-
nects the sharp anterior border of the pubis with the articular
surface for the sacrum.

The wing of the ilium forms a_ shovel-like expansion, the
natural position of which is almost sagittal. Its lateral surface
provides a fairly extensive area for the origin of the gluteal muscles.
Its medial surface is a muscle surface only in its anterior portion,
the posterior portion being occupied by the roughened auricular
surface (facies auricularis), for connection with the sacrum.
The dorsal margin is thin and straight. Posteriorly, where it is
associated with the greater sciatic notch, there is a small projection,
the inferior posterior spine (spina posterior inferior), also termed
the tuber sacrale. Anteriorly it passes by a broad angle into the

168 ANATOMY OF THE RABBIT.

anterodorsal margin of the bone, the latter forming the projecting
end of the wing, which is distinguished as the iliac crest (crista
iliaca). This portion is considerably thicker than the related
dorsal and ventral margins, and also bears on its medial side a
somewhat hook-shaped process Its anteroventral angle is the
superior anterior spine (spina anterior superior) or tuber coxae.
The ventral margin is slightly longer than the dorsal margin, and
is also concave. It is associated with the pubic border of the body
of the ilium; and is not connected with the inferior anterior spine.

The ischium (os ischii) extends backward from the acetabulum,
its axis continuing that of the ilium. It consists of a basal portion,
or body (corpus oss. ischii), a Superior ramus, and an inferior
ramus. The body of the ischium is for the most part cylindrical.
It forms the posterior part of the acetabulum, and presents in
connection with the latter a deep acetabular notch (incisura
acetabuli), which tends to interrupt the articular surface. The
acetabular notch leads forward into a depression of the centre of the
articular basin, the acetabular fossa (fossa acetabuli). In the
natural condition the combined depressions serve for the attach-
ment of the round ligament of the head of the femur. The dorsal
margin of the bone, belonging in part to the body and in part to the
superior ramus, bears a short hook-like projection, the ischial
spine (spina ischiadica). The spine divides this margin into two
parts, one of which forms the posterior half of the greater sciatic
notch, already described, while the other forms a similar, and, in
the rabbit, scarcely less extensive, posterior depression, the lesser
sciatic notch (incisura ischiadica minor).

The superior or acetabular ramus of the ischium is the con-
tinuation backward of the body of the bone. It is a somewhat
flattened plate of bone, the thicker dorsal portion of which ter-
minates in two blunt projections. One of these, the ischial
tuberosity (tuber ischiadicum), forms the posterior end of the
bone, while the other extends in a lateral direction and is described
as the lateral process (processus lateralis). The inferior or
symphseal ramus is that part of the ischium which extends from
the superior ramus downward and forward between the obturator
foramen and the symphysis to meet the corresponding ramus of the
pubis.

THE SKELETON OF THE POSTERIOR LIMB. 169

The pubis (os pubis) consists of a basal portion, or body lying
immediately below the acetabulum, a superior or acetabular
ramus extending from the body to the symphysis, and an inferior
or symphyseal ramus extending backward along the symphysis
to its junction with the ischium. The
anterior margin of the bone, described
as the pecten oss. pubis, is thin and
sharp. Near the symphysis it bears
a minute elevation, the pubic tuber-
cle (tuberculum pubicum), and
laterally a more extensive elevation,
the iliopectineal eminence (emin-
entia iliopectinea). The latter is more
conspicuous in older specimens, where
it is easily recognizable by its jagged
outline. Its lateral margin is con-
tinuous with the iliopectineal line.

THE FEMUR.

The femur (Fig. 70) is a some-
what S-shaped bone, the body being
very slightly arcuate, while of the two
extremities, the distal one is bent
downward, forming the articulation
of the knee, the proximal one, with
its various processes, slightly upward
in association with the pelvis. In

considering the general form, it will — Fic. 70. Anterior surface of the left
I al 1 th: : | ' -a] femur: cl. and c.m., lateral and
xe remembered that in the natura medial condyles; cl.f., neck of femur;
LENG : ; : cp.f., articular portion (head); e.l.,
sitting posture of the rabbit, the and e.m., lateral and medial epicon-

ete ° : dyles; f.p., patellar surface; t.mi.,
position of the femur is approximately trochanter minor; t.mj., trochanter
| Cis é | | 4 Lee f: s3 f | . major, including t.p. and t.t., the first
norizontal, the convex surface Of tne and third trochanters.

shaft, which is equivalent to the
anterior surface in man, being uppermost.

The proximal extremity of the femur bears an extensive rounded
portion or head (caput femoris), for articulation with the pelvic
girdle. This portion is separated from the main part of the

170 ANATOMY OF THE RABBIT.

extremity by a constricted area or neck (collum femoris), so that,
unlike the case of the anterior limb, the points of muscle attach-
ment fall a considerable distance from the point of articulation.
The actual extremity of the bone is formed by a large procéss for
muscular attachment, the great trochanter (trochanter major).
It is divided into two portions, one of which, the first trochanter
(trochanter primus), forms the large terminal, hook-like projection,
while the other, the third trochanter (trochanter tertius), is
the smaller lateral crest. On the medial side of the bone, imme-
diately distal to the head, there is a triangular elevation, the
lesser, or second trochanter (trochanter minor s. secundus).
Posteriorly, these projections form a smooth surface for muscle
attachment, except, however, at the base of the trochanter major,
where the surface of the bone presents a deep, though narrow,
depression, the trochanteric fossa (fossa trochanterica).

The distal extremity bears an extensive surface for articulation
with the tibia. It is divided into two portions, known as the
medial and lateral condyles, through the presence of a deep
excavation, the intercondyloid fossa (fossa intercondyloidea).
Immediately above the condyles, on the anterior surface of the bone,
the intercondyloid fossa is replaced by a broad groove, the patellar
surface (facies patellaris), which, in the natural condition, accom-
modates the convex internal surface of the patella. The medial
and lateral portions of the bone, intervening between the distal
portion of the patellar surface and the tips of the condyles, provide
slightly elevated, roughened surfaces, the medial and lateral
epicondyles, for muscular attachment.

THE TIBIA AND FIBULA.

The tibia (Fig. 71) is the larger of the two bones of the leg, lying
on the medial side of the fibula, and fused with the latter for more
than one-half of its length. Its proximal extremity is triangular
in section, the main surfaces being respectively anterolateral,
anteromedial, and posterior. The anterior border is formed by a
stout ridge-like elevation, the tuberosity of the tibia (tuberositas
tibiae), which in the natural condition serves for the insertion of ©
the quadriceps femoris, the extensor tendon being carried over

@

THE SKELETON OF THE POSTERIOR LIMB. I7I

the knee by the patella and the patellar ligament. The articular
portion is slightly differentiated into medial and lateral condyles
corresponding to those of the distal end of the femur. On the
articular surface the concave areas for the reception of the condyles
of the femur are separated from one
another by a small intervening, tt.
partly divided hillock, the inter- | fas.
condyloid eminence (eminentia 5 .
intercondyloidea), and also poster-
iorly by a depression of the articular
border, the posterior intercondy-
loid fossa. A corresponding anterior
intercondyloid fossa lies in front of
the intercondyloid eminence, but is
poorly differentiated.

The fibula (Fig. 71) is the smaller,
lateral bone of the leg, and in the
rabbit is so extensively fused with the
tibia that scarcely more than a third
of it is distinguishable. The free
portion forms a flattened bony splint,
the medial margin of which is firmly
united with the tibia by the inter-
osseous ligament of the leg. Its
proximal extremity is connected with
the lateral condyle of the tibia by an
elongated epiphysis, the latter, iike
those of the distal ends of the radius
and ulna, being distinguishable even
in older animals.

Thecombined distal extremities © 7 jf 132° ay ee ate oF the

es = i ae i . c.l. and c.m., lateral and medial
of the tibia and fibula bear a roughly Coda leds ihe. Parone! articles

: , : Fat ls em ie aa ¥ surface for the femur; m.l. and
rectangular articular surface for the sae, Tet Sea enna aaa PG STTe
tarsus. The tibial portion of this t.t., tuberosity of tibia.

surface presents two grooves, separ-

ated by a ridge, for articulation with the trochlea tali. On its
medial side is a small projection, the medial malleolus (malleolus
medialis).

172

ANATOMY OF THE RABBIT.

The fibular portion presents a transverse groove for the convex

articular surface of the calcaneus.

Immediately above it, on the

lateral side of the bone, is a prominent projection, the lateral

malleolus (malleolus lateralis).

The bones of the left

FIG. 72.
foot, viewed from the dorsal sur-
face: T, tarsus; M, metatarsus; P,

phalanges. II-V, the four meta-
tarsal bones: cb., cuboid; cl.,
calcaneus; c.s., second cuneiform;
c.t., third cuneiform: f.a., articular
surface for fibular side of the tibo-

fibula; on, navicular; t, talus;
t.c., tuber calcanei; t.t., trochlea
tali.

It forms the anterior boundary
of a groove which in the natural con-
dition lodges the insertion tendons of
the peroneal muscles.

THE TARSUS.

The tarsus (Fig. 72) comprises six
elements, the tarsal, or ankle-bones
(ossa tarsi), which, like the correspond-
ing bones of the carpus, are arranged in
proximal and distal rows. An exception
is to be made, however, for one element,
the navicular, which occupies an inter-
mediate position. The proximal row
contains two elements, the talus and
calcaneus. The talus is medial and
also slightly dorsal in position. Its
proximal end, described as the body
(corpus tali), bears an extensive pulley-
like surface, the trochlea tali, for
articulation with the tibia, these two
surfaces together forming the chief
portion of the ankle-joint. Its distal
end, termed the head of the talus (caput
tali), provides a convex articular surface
for the navicular bone, and is separated
from the larger trochlear portion by a
slightly constricted intermediate portion
or neck (collum tali). Its ventro-
lateral border is extensively articulated
with the calcaneus. The latter is a
cylindrical element, fully twice as long

as the talus, since it is extended backward behind the ankle-joint,

as the tuber calcanei, or bone of the heel.

Its dorsal surface

THE SKELETON OF THE POSTERIOR LIMB. 173

bears a prominent elevation for articulation with the fibular side
of the tibiofibula. Its medial surface bears a flat, shelf-like process,
the sustentaculum tali, which forms a ventral support for the
talus. The distal extremity of the bone articulates with the cuboid
and also with the navicular.

The intermediate element, the navicular bone, is a somewhat
cubical bone, lying on the medial side of the tarsus between the
talus, on the one hand, and the proximal end of the second meta-
tarsal bone and the second and third cuneiform bones, on the other.
Its position is more nearly that of a central element than is the case
with the bone called by this name in the rabbit’s carpus. In this
connection it will be remembered that the carpus and tarsus, like
other parts of the limbs, are primarily constructed on the same plan.

The distal row of the tarsus contains three elements, namely, the
second and third cuneiform bones and the cuboid bone. The
two former, and especially the first, are smaller bones, articulated
respectively with the second (first developed) and third metatarsals.
In the rabbit the first cuneiform bone—the first element of the
distal row in the usual condition—is fused with the proximal end
of the second metatarsal. The cuboid is a larger element articulat-
ing, like the hamate bone of the carpus, with two distal elements,
the fourth and fifth metatarsals. Its ventral surface bears a
transverse elevation, the tuberosity of the cuboid (tuberositas oss.
cuboidei), in front of which is a groove for the accommodation
of the peculiar insertion tendon of the peronaeus primus muscle.

THE METATARSUS AND PHALANGES.

The metatarsus (Fig. 72) comprises five elements, of which
four are fully developed and greatly exceed in size the corresponding
bones of metacarpus, while one, the first metatarsal, is vestigial.
The vestigial element lies on the plantar surface of the foot, for the
most part ventral to the navicular and at the base of the second
metatarsal. In each developed metatarsal there may be distin-
guished a main portion or body, a proximal extremity or base,
and a distal extremity or head, the last-named portion articulating
with the proximal phalanx of the digit. The base of the fifth
metatarsal bears a tuberosity for the insertion of the peronaeus
secundus muscle.

174 ANATOMY OF THE RABBIT.

The phalanges are distributed according to the formula
0, 3, 3, 3, 3, the terminal, ungual phalanges being modified like those
of the anterior limb.

SESAMOID BONES.

The sesamoid bones of the posterior limb occur at the knee-
joint and on the plantar surface of the foot. On the anterior
surface of the knee is the knee-pan or patella, through which, as
indicated above, the tendon of the quadriceps femoris muscle is
carried over the knee and continued as the patellar ligament to the
tuberosity of the tibia. On the posterior surface there are three
sesamoid bones, of which one lies in association with the medial
condyle of the femur, while the remaining two are associated
respectively with the lateral condyle of the femur and that of the
tibia. The sesamoids of the foot are situated at the metatarso-
phalangeal joints and at those connecting the second and third
phalanges.

PART III.

DISSECTION OF THE RABBIT;

The plan of dissection as outlined in the following pages pre-
supposes in the first place that the entire dissection is to be made
on a single specimen, and, second, that the latter has been prepared
for gross dissection according to the method given in the appendix.
These points may be mentioned as explaining many details of
procedure and also to a certain extent the selection in preference
to others of those structures which are more readily made out by
the method employed.

Because of the convenience of dissecting in circumscribed
regions, the plan has been divided, although of necessity very
unequally, into several parts. The order of these is such that the
visceral dissection is introduced at an early stage. The somewhat
more logical plan of completing first the dissection of the anterior
and posterior limbs may be followed, but on account of the fact
that it involves a lengthy muscular dissection to begin with, it is
perhaps not to be recommended.

The account itself aims at a statement of the various structures
as met with in order of dissection and the features by which they
may be identified, rather than at a full description. The student
should make his own observations and prove them by personal
drawings and descriptions of selected parts. In this connection
he will do well to bear in mind that while dissection is nominally a
means of obtaining anatomical information, its chief value as a
laboratory exercise consists in the training to be acquired from
critical observation and analysis. It is therefore of quite as much
practical importance that he should make his observations exten-
sive and accurate as that he should employ only good instruments,
or maintain the proper sequence in dissection.

175

176 ANATOMY OF THE RABBIT.

I. EXTERNAL FEATURES.

The external structures, subdivisions of the body, and super-
ficial skeletal points may be made out as follows.

1. The division of the body into the head (caput), neck
(collum), trunk (truncus), tail (cauda), and the anterior and
posterior limbs or extremities (extremitates).

2. In the head:

(a) The division into a posterior, cranial portion (cranium),

and an anterior, facial portion (facies).

(b) The mouth (os), bounded by the cleft upper lip (labium
superius) and the undivided lower lip (labium inferius).
The large sensory hairs or vibrissae.

(c) The nose (nasus), and its external apertures (nares an-
teriores).

(d) The eye (oculus), and its coverings, the eyelids, including
the upper eyelid (palpebra superior), the lower eyelid
(palpebra inferior), and the third eyelid (palpebra tertia).
The third eyelid occupies the anterior angle of the eye, and
is comparable to the conjunctival fold of the human eye.

(e) The external ear (auricula), and its canal, the external
acoustic meatus (meatus acusticus externus), leading to the
tympanic membrane.

(f) Points on the head skeleton, to be identified by feeling
through the skin; zygomatic arch, supraorbital process,
external occipital protuberance, angle of the mandible,
symphysis of the mandible, and the hyoid bone.

2. In the trunk:

(a) The division into thorax, abdomen, and back, or dorsum.

(b) The inclusion with the trunk of the proximal portions of the
limbs. The angle formed by the anterior limb with the
trunk represents in part the axillary fossa (fossa axillaris).
The depression is much less evident than in man on account
of the different positions of its enclosing folds formed by the
pectorales and latissimus dorsi muscles. A corresponding
inguinal furrow separates the posterior limb from the
abdomen and pelvis.

EXTERNAL FEATURES. 17

“I

(c) The anal aperture (anus), and on either side of it the
inguinal spaces, hairless depressions, on which the ducts
of the inguinal glands open.

(d) In the male: the urinogenital aperture at the extremity of
the penis; the latter enclosed by a fold of integument, the
prepuce (praeputium); the scrotal sacs (scrotum), lateral
sacs of the integument lodging the testes.

(e) In the female: the urinogenital aperture, enclosed by folds
of the integument, forming the vulva. The clitoris, the
homologue of the penis, is contained in its ventral wall. The
mammary nipples (papillae mammarum), eight (to ten) in
number on the ventral surface of the breast and abdomen.

(f) The following skeletal points: on the axial skeleton, the
manubrium sterni, xiphoid process, costal arch, spinous
processes of thoracic and lumbar vertebrae; on the pectoral
girdle, the acromion, clavicle, and respective borders and
angles of the scapula; on the pelvic girdle, the iliac crest,
pubic symphysis, and ischial tuberosity.

4. In the anterior limb:

(a) The division of the free portion into three segments, the
arm (brachium), forearm (antibrachium), and hand
(manus).

(b) The position of the elbow (cubitus) in comparison with the
knee.

(c) The five digits, designated from the medial side as: first
(d. primus), or pollex, second (d. secundus), or index,
third or middle (d. tertius s. medius), fourth (d. quartus),
and fifth (d. quintus s. minimus).

5. In the posterior limb:

(a) The division into three segments, the thigh (femur), leg
(crus), and foot (pes).

(b) The knee (genu), and the popliteal fossa of its posterior
surface, the latter not well defined. The projection of the
heel (calx), and the angle formed by the foot with the leg.

(c) The four digits (dd. secundus-quintus). The vestigial first
digit, or hallux, is not distinguishable externally.

178 ANATOMY OF THE RABBIT.

II. THE ABDOMINAL WALL.

1. Place the animal in the supine position. Make a’median in-
cision of the skin of the ventral surface extending from the pubic
symphysis to the tip of the mandible, being careful not to cut
through more than the skin itself. Make three transverse in-
cisions on the left side, the first to and along the medial surface
of the arm and extending to the elbow, the second midway between
the anterior and posterior limbs, the third to and along the medial
surface of the thigh and extending to the knee. Work the flaps
loose from the surface, using the handle of the scalpel, until the
side of the trunk is well exposed. On the right side of the body
it is sufficient to clear the middle line. Identify the structures
as follows:

On the skin:

(a) The thick compact connective tissue forming the corium.

(b) The imbedded hair-follicles.

(c) The loose subcutaneous tissue (tela subcutanea) by which
the skin is attached.

(d) In the female: the mammary glands (mammae), forming
a layer on the inner surface, and more or less closely aggre-
gated about the mammary nipples.

On the exposed surface:

(e) The linea alba, a white aponeurotic line extending from the
pubic symphysis to the xiphoid process of the sternum.

(f) The cutaneus maximus muscle, a thin sheet of muscle
fibres covering the entire lateral surface of the thorax and
abdomen. Origin: The linea alba, the ventral surface of the
sternum in its posterior portion, and the deltoid tuberosity.
The last-named portion appears on the medial surface of the
humerus. Insertion: The skin of the dorsolateral surface
of the trunk. The fibres are directed upward and backward.
The muscle is continuous across the back with its fellow of
the opposite side, and is extended backward to the dorsum
of the tail. It is used in shaking the skin.

The artery passing forward for a short distance in the inguinal region,
and lying in the subcutaneous tissue, is the superficial epigastric, a

branch of the femoral (p. 224). Passing into the ventral portion of the
cutaneus maximus muscle, it anastomoses forward with the external

THE ABDOMINAL WALL. 179

thoracic artery, a branch of the lateral thoracic. The corresponding veins
are usually conspicuous in the female, since the vessels supply the mam-
mary glands. A second anastomosis in the cutaneus muscle is formed
laterally by a branch of the subscapular artery which passes backward
from the axillary border of the scapula, uniting with an anterior branch
of the iliolumbar artery.

The inguinal lymph nodes (lymphoglandulae inguinales) are small,
oval, brownish bodies lying in the inguinal furrow.

2. Remove the cutaneus maximus from the surface. Identify
the following points of attachment of the abdominal muscles proper:

(a) The linea alba.

(b) The linea semilunaris, a slightly curved line situated
laterally a short distance from the linea alba.

(c) The ribs and the costal arch.

(d) The lumbodorsal fascia (fascia lumbodorsalis), a broad,
white sheet of connective tissue extending over the posterior
thoracic and lumbar regions.

(e) The inguinal ligament (ligamentum inguinale), a stout
white cord, stretched between the symphysis pubis and the
iliac crest.

3. Identify on the surface the external oblique muscle (m.
obliquus externus abdominis). Origin: The posterior ten ribs by
separate slips, the xiphoid process, and the lumbodorsal fascia.
Insertion: The linea alba and the inguinal ligament. The fibres
are directed from an anterior dorsal position downward and back-
ward, the more dorsal ones almost directly backward. Some of the
anterior slips of origin interdigitate with those of the thoracic
portion of the serratus anterior muscle. Some are concealed by the
pectoral muscles.

The muscle crossing the breast from the sternum-to the arm is the pectoralis
major. That passing forward from the lumbodorsal fascia to the medial surface of
the humerus is the latissimus dorsi. “The margins of these muscles may be raised
where they conceal the external oblique.

4. Taking a line between the iliac crest and the xiphoid process,
divide the muscle, and then separate it fully from the next, which
may be distinguished by the cross direction of its fibres. Note the
separate slips of origin and the difference in appearance between
the fleshy portions of the muscle and its ventral tendinous expansion
or aponeurosis; then remove it from the surface.

180 ANATOMY OF THE RABBIT.

Examine the following muscles, proceeding in a similar manner:

(a) The internal oblique muscle (m. obliquus internus
abdominis). Origin: The inguinal ligament, a second sheet
of the lumbodorsal fascia, and the posterior four ribs.
Insertion: The linea alba. The fibres pass downward and
forward. The ventral aponeurosis is much broader than
that of the external oblique. Near the mid-ventral line
it is split into two leaves, coniaining between them the thin
rectus abdominis muscle.

(b) The transverse muscle (m. transversus abdominis).
Origin: Seven posterior ribs, a third sheet of the lumbo-
dorsal fascia, and the inguinal ligament. Insertion: The

linea alba. The fibres are directed downward and slightly
backward.

The rectus abdominis muscle. Origin: Lateral border
of the sternum, including the xiphoid process; also the
ventral surfaces of the second to seventh costal cartilages.
Insertion: At the anterior end of the pubic symphysis. It
is a thin, strap-like muscle, enclosed by the aponeurosis
of the internal oblique, and separated from its fellow of the
opposite side by the linea alba.

(c

SS’

The artery passing forward, for the most part in this muscle, is the
inferior epigastric, a branch of the external iliac (p. 203). It anastomoses
withthe internal mammary (p. 257). It gives off the external spermatic
artery, a small vessel which perforates the abdominal wall and extends
backward, supplying the sac of the testis in the male and ending in the
female in the wall of the vulva.

5. Divide the remaining portion of the wall on the left side, and
its whole thickness on the right, by a transverse incision, so that
the viscera are fully exposed. Note on the internal surface of
the wall the smooth serous investment here forming the parietal
peritoneum (peritonaeum parietale).

THE STOMACH AND SPLEEN. I8I

III. THE STOMACH AND SPLEEN.

The cavity disclosed by the division of the abdominal wall is the
peritoneal cavity (cavum peritonaei), the largest of the four
great serous sacs representing the primary body-cavity, or coelom
(p. 98). The major portion of the cavity is abdominal, but it
extends into the pelvis, and, in the male, also into the scrotal sacs.
Its lining membrane is that appearing on the body-wall as the
parietal peritoneum, noted above, and on the visceral structures
as the visceral peritoneum (peritonaeum viscerale). The
visceral structures here include the major portions of the digestive
and urinogenital systems.

The general relations of the visceral peritonaeum should first be
examined by raising a portion of the small intestine from the left
side of the visceral mass. Note its enclosure by a complete serous
coat, similar in appearance to the membrane covering the body
wall, and the extension of the lacter into a mesentery for the
support of the structure from the dorsal body-wall. Note the
parallel arrangement of the arteries and veins, and also their
frequent anastomoses. Lymphatic vessels (lacteal vessels)
accompany the bloodvessels in the mesentery, but being trans-
parent are not readily recognizable. Lymph nodes also occur,
but in this portion of the mesentery they are aggregated at its
dorsal attachment, or root (radix mesenteril).

‘

For the general relations of the stomach see p. 82 and Fig. 42.

1. Displace the posteroventral portion of the liver forward,
exposing in this way the ventral surface of the stomach. Without
injuring the enclosing peritoneum, examine the contour of the organ
and its divisions, as follows:

(a) The greater curvature (curvatura ventriculi major), its
convex posterior surface.

(b) The lesser curvature (curvatura ventriculi minor), the
contracted, concave anterior surface.

(c) The main portion, or body of the stomach (corpus ventriculi).
It lies for the most part to the left of the median plane.

(d) The cardia, or area of junction with the oesophagus.

(e) The fundus, a small sac-like portion lying to the left of the
cardia

182
(f)
(g)

ANATOMY OF THE RABBIT.

The pyloric limb (pars pylorica) forms the right portion
of the organ.

The pylorus, the point of communication of the stomach
with the intestine (duodenum). It is marked by an annular
constriction, preceding which is a greatly thickened muscular
portion of the pyloric limb, known as the pyloric antrum
(antrum pyloricum).

2. Raise the posterior portion of the stomach and turn it for-

ward.

Note on the dorsal surface of the greater curvature a dark

red elongated body, the spleen (lien). On the right side of its
artery, enclosed in the peritoneum, will be seen a diffuse brownish
glandular mass, a portion of the pancreas. Trace the course of the
peritoneum from the dorsal abdominal wall to the liver, as follows:

(a) A broad fold of peritoneum, the mesogastrium, connects

(b)

(c)

the dorsal abdominal wall and the diaphragm with the left
side and dorsal surface of the greater curvature. Its
posterior portion is divided into two parts by the spleen
The dorsal part, the phrenicosplenic ligament (lig. phreni-
colienale) connects the spleen with the dorsal body-wall.
The ventral part, the gastrosplenic ligament (lig. gastro-
ienale) connects the spleen with the greater curvature
(cf. Fig. 40).

The peritoneum is projected backward from the greater
curvature as a free fold, the greater omentum (omentum
majus), which covers the intestines to a,certain extent. It
usually contains fat. It is composed of four leaves, of which
two come from the surface of the stomach and two others
pass forward in a dorsal position from the free posterior
border to unite with the transverse mesocolon. ;
The lesser omentum (omentum minus) passes from the
lesser curvature and the duodenum to the posterior surface
of the liver. Its thickened margin on the right side forms
the hepatoduodenal ligament (lig. hepatoduodenale)
which carries three important structures, namely, the
common bile duct, the hepatic artery and the portal vein.
Its left portion forms a thin membrane, the hepatogastric
omentum, connecting the caudate lobe with the lesser
curvature.

3.

THE STOMACH AND SPLEEN. 183

Working on the left side between the dorsal surface of the

stomach and the body-wall, remove sufficient of the peritoneum to
expose the first portion of the abdominal aorta as it emerges from
the diaphragm. Passing in the direction of the stomach is a
median ventral branch, the coeliac artery, the distribution of
which may be traced. The following structures, however, should
first be identified, especially the ganglia (d, e) which are likely to
be damaged in disturbing the peritoneum.

(a)

(b)

(c)

(d)
(e)

(f)

(g)

(h)

4.

The superior mesenteric artery (a. mesenterica superior),
a second, also much larger, median branch of the aorta, given
off a little distance behind the coeliac artery and passing in ~
the direction of the intestine.

The suprarenal gland (gl. suprarenalis) of the left side, a
yellowish bean-shaped body of about half an inch in length,
to the medial side of the anterior part of the kidney.

The inferior caval vein (v. cava inferior), a large thin-
walled vessel lying to the right of the aorta. It is not
conspicuous if empty.

The coeliac ganglion (g. coeliacum) of the sympathetic
nervous system lies in front of the superior mesenteric artery.
The superior mesenteric ganglion (g. mesentericum
superius) of the sympathetic system lies immediately behind
the superior mesenteric artery, and also on its lateral walls.
The two ganglia are connected by nerves which cross the
superior mesenteric artery.

The nerves proceeding from the coeliac and superior mesen-
teric ganglia accompany the corresponding arteries, forming
the coeliac and superior mesenteric plexuses.

The (greater) splanchnic nerve (n. splanchnicus major) of
the left side passes backward from its origin in the thorax
(see p. 264), around the reduced left crus of the diaphragm,
and crossing the aorta obliquely enters the superior mesen-
teric ganglion.

A portion of the pancreas (cf. p. 189) is seen in the peri-
toneum after the branches of the splenic artery have been
severed (4, a).

Trace the plan of branching of the coeliac artery, beginning

at the point of origin, and exposing the vessels in order.

184 ANATOMY OF THE RABBIT.

The coeliac artery (a. coeliaca) is a short trunk, its first main
branch, the splenic artery, being given off near its origin from the
aorta. The remaining portion of the vessel passes to the right in
the direction of the lesser curvature, and divides into two
parts, the left gastric and hepatic arteries. Small vessels, the
inferior phrenic arteries (aa. phrenicae inferiores), are given off
from the anterior wall of the coeliac and are distributed to the
diaphragm. The distribution of the main branches is as follows:

(a) The splenic artery (a. lienalis) passes in the direction of the
spleen, giving off small branches (r.r. pancreatici) to the
pancreas and one or more large vessels, the short gastric
arteries (aa. gastricae breves), to the left portion of the
greater curvature. Passing along the concave surface, or
hilus, of the spleen it gives off several splenic branches
(rr. lienales) to that organ, and also several more branches,
comparable to the short gastric arteries; to the greater
curvature. Toward the end of the spleen the artery passes
into the greater omentum, and at this point there is given off
a large vessel, the left gastroepiploic artery (a. gastroe-
piploica sinistra), which passes to the right on the greater
curvature and anastomoses with the right gastroepiploic
artery.

The gastrosplenic ligament, together with its vessels, may be
divided, the spleen being allowed to fall backward toward the
intestine.

(b) The left gastric artery (a- gastrica sinistra) forms a short
trunk, or more commonly a group of vessels, the branches
of which pass in a somewhat radiate manner toward the
lesser curvature of the stomach, reaching in this way both
dorsal and ventral surfaces. Two larger vessels appear on
the ventral surface respectively to the right and left of the
cardia. That on the left distributes small branches (rr.
oesophagei) to the oesphagus, while that on the right bears
a small pyloric branch which anastomoses across the lesser
curvature with the right gastric artery.

The chief nerves of the coeliac plexus accompany the branches of
the artery to the stomach where they connect with the terminal rami-
fications of the vagus (p. 185).

THE STOMACH AND SPLEEN. 185

The following artery may be identified but not traced to its term-
ination until later, so that the bile duct and portal vein are not injured.
(c) The hepatic artery (a. hepatica), the continuation of the
coeliac, passes forward and to the right, giving off small
branches to the pancreas. Its first main branch is the
gastroduodenal artery (a. gastroduodenalis). The latter
is distributed chiefly to the first portion of the intestine as the
superior pancreaticoduodenal artery (a. pancreatico-
duodenalis superior), but a recurrent branch, the right gas-
troepiploic artery (a. gastroepiploica dextra), traverses the
greater omentum to the greater curvature where it anasto-
moses with the left gastroepiploic artery.

After giving off the gastroduodenal artery, the hepatic enters
the lesser omentum on its way to the liver. A small branch,
the right gastric artery (a. gastrica dextra) passes to the
pylorus and anastomoses across the lesser curvature with a
branch of the left gastric artery.

The veins of the stomach and spleen are tributaries of the portal
vein. Accompanying the branches of the splenic artery are the tribu-
taries of the splenic vein (v. lienalis), including the left gastroepiploic
vein. Accompanying the branches of the left gastric artery are the
tributaries of the coronary vein (v. coronaria ventriculi). The splenic
and coronary veins enter the left wall of the portal vein through a short
common trunk.

On the right side of the stomach the superior pancreaticoduo-
denal vein is united with the right gastroepiploic vein to form a
short trunk, the gastroduodenal vein (v. gastroduodenalis), which
enters the right wall of the portal vein. The left gastroepiploic vein
receives tributaries from the dorsal surface of the pyloric antrum.
The abdominal portion of the tenth cranial, or vagus
nerve (n. vagus) may be traced from the oesophagus to the
surface of the stomach. The left cord appears on the left
wall of the oesophagus; crossing the ventral surface of the
latter obliquely to the right, it ramifies on the ventral
portion of the lesser curvature. The right cord passes to the
stomach in a similar manner from the dorsal surface of the
oesophagus.
5. Cut across the stomach at the pyloric antrum. Divide the
oesophagus, and remove the stomach from the body. Open the

186 ANATOMY OF THE RABBIT.

organ by means of an incision extending around the greater curva-
ture to the oesophagus.

On the cut end of the pyloric antrum the mucous and muscular
tunics (cf. Fig. 16) may be distinguished and separated from one
another by dividing the loose tissue of the tela submucosa. On the
surface of the mucous tunic may be seen the gastric areas (areae
gastricae), formed by the longitudinal folds and imperfect transverse
ridges which tend to connect them. They are well marked only
in the contracted condition of the stomach. The mucous tunic
of the stomach is sharply differentiated from that of the oesophagus.

IV. THE LIVER.

The liver (hepar) is noteworthy, first, as being the largest of the
glandular structures of the body, and, secondly, as containing, in
addition to the primary circulation formed by the hepatic artery
and veins, the ramifications of the portal system. It is an appen-
dage of the digestive tube, its connection with the l4tter being
through the common bile duct.

For the general relations of the liver, see p. 23 and Fig. 42.

1. Examine the contour and plan of division as follows:

(a) The convex anterior surface, applied to the diaphragm;
the concave posterior surface fitting the convexity of the
stomach; the thickened dorsal portion of the organ, and its
thin posteroventral margin.

(b) The division of the organ into right and left lobes, and of
each lobe into anterior and posterior lobules.

(c) The gall bladder (vesica fellea), situated on the ee OS
surface of the right anterior lobule.

(d) The quadrate lobe (lobus quadratus), represented by one
or more small divisions of the medial margins of left and right
anterior lobules.

(e) The caudate lobe (lobus caudatus), a small independent
lobe projecting backward from the base of the left posterior
lobule, and accommodated in the natural condition in the
space enclosed by the lesser curvature of the stomach.

(f) The portal fissure (porta hepatis), a large depression at
the point of entrance of the portal vein.

Ze

(a)

(b)

(c)

(d)

THE LIVER. 187

Trace the peritoneal connections as follows:

The lesser omentum, represented by the hepatoduodenal
ligament and the hepatogastric omentum, previously
divided.

The falciform ligament (lig. falciforme hepatis), a broad
median sheet connecting the anterior surface of the liver
with the diaphragm and extending backward to the ventral
abdominal wall. The position of this ligament indicates the
line of division of the liver into right and left lobes. The
free arcuate border of the ligament contains a thin cord, the
round ligament (lig. teres hepatis), which marks the
position of the umbilical vein in the foetus. The corres-
ponding umbilical notch is less conspicuous than in man
because of the highly lobulated condition of the whole organ
in the rabbit.

The coronary ligament (lig. coronarium hepatis), a short
circular fold, continuous with the falciform, and connecting
the anterior surface of the liver with the middle of the
diaphragm.

The left triangular ligament (lig. triangulare sinistrum), a
lateral continuation of the coronary connecting the left lobe
with the diaphragm.

3. Trace the branches of the common bile duct, the hepatic
artery and the portal vein. These structures traverse the lesser
omentum side by side and their branches are similarly arranged.

(a)

The common bile duct (d. choledochus) is formed on the
posterior surface of the liver by the union of a left hepatic
duct (d. hepaticus) with a similar duct from the right
anterior lobule. The latter receives the cystic duct (d.
cysticus) from the gall bladder. Special ducts from the
right posterior lobule and from the caudate lobe enter the
common bile duct through a short common trunk. The
common bile duct passes backward on the right side of the
portal vein, and enters the digestive tube on the dorsal

surface of the first (superior) portion of the duodenum

(b)

immediately beyond the pylorus.
The hepatic artery (a. hepatica) approaches the liver by
passing forward on the right side of the portal vein ventral to

188 ANATOMY OF THE RABBIT.

the bile duct. It distributes branches to the right posterior
lobule and the caudate lobe, and at the base of the liver
proper divides into right and left rami. The right ramus
sends a branch, the cystic artery (a. cystica), to the gall
bladder.

(c) The portal vein (v. portae), a vessel of large calibre, but
usually found in a collapsed condition, enters the lesser
omentum from the dorsal surface of the pyloric antrum. It
distributes branches to the right posterior lobule and the
caudate lobe; then passing directly forward to the base of
the left lobe is distributed to the latter, a right branch being
given off to the right anterior lobule.

4. Divide the lesser omentum with the structures described
above. Divide the coronary and triangular ligaments, cutting
near the liver so as not to injure the central tendon of the
diaphragm, which resembles the coronary ligament. Remove the
liver and examine its dorsal surface for the following:

(a) The inferior vena cava; it is accommodated in a depression
of the thickened dorsal portion of the organ. The vessel
should be opened lengthwise.

(b) The hepatic veins (vv. hepaticae) open almost directly
from the substance of the liver into the inferior cava. They
are typically four in number, there being separate vessels for
the anterior and posterior parts of the right lobe and for the
caudate lobe, in addition to a large vessel for the left lobe.

(c) The renal impression (impressio renalis), an extensive
excavation of the right posterior lobule for the accommoda-
tion of the right kidney.

V. THE INTESTINES.

The posterior portion of the digestive tube, or that portion
extending from the pyloric aperture of the stomach to the anal
aperture, is divisible into two main parts, not wholly distinguishable
in calibre, namely, the small intestine (intestinum tenue), and the
large intestine (intestinum crassum). Both are greatly elongated
and convoluted. In examining them care must be taken to avoid
injury to the bloodvessels and mesenteries, especially the dorsal

THE INTESTINES. 189

attachments of the mesenteries, in which the chief plexuses and
related ganglia of the sympathetic system will afterwards be traced.

(For the general relations of the intestines and mesenteries, see pp. 82, 99.)

I. Beginning at the pylorus, trace the course of the small
intestine, as follows: Its first portion, the duodenum, forms a
U-shaped loop lying on the dorsal wall of the abdominal cavity
to the right of the vertebral column. The distal end of this loop,
when traced from the right side, disappears in the peritoneum and
may then be picked up in a forward position on the left side.of the
mass. This point marks the beginning of the second portion, the
mesenterial small intestine (intestinum tenue mesenteriale),
which may be traced to its termination on the greatly enlarged
caecum. The connection with the caecum is through a rounded
semi-expanded sac, the sacculus rotundus. The _ terminal
portion of the small intestine is somewhat more difficult to follow
on account of the adhesions of its peritoneum with that of the large
intestine.

2. Examine the divisions of the duodenal loop and related

structures, as follows:

(a) The superior, descending, transverse (horizontal), and
ascending portions of the duodenal loop.

(b) The common bile duct, opening on the dorsal wall of the
superior portion.

(c) The mesoduodenum, a fold of peritoneum joining the
various parts of the loop.

(d) The pancreas (Fig. 3 B, p. 22). Its principal portion is
here seen as a diffuse brownish mass lying in the mesoduo-
denum. Its duct (d. pancreatis) opens into the posterior
portion of the ascending limb.

(e) The superior pancreaticoduodenal artery, a branch of
the gastroduodenal (see p. 185), passes backward on the first
portion of the descending limb.

(f{) The inferior pancreaticoduodenal artery (a. pancreatico-
duodenalis inferior), a branch of the superior mesenteric
(p. 192), enters the mesoduodenum from the left side and
supplies the major portion of the loop. An anterior branch
anastomoses with (e).

190

ANATOMY OF THE RABBIT.

3. In the mesenterial small intestine the following features may
be identified:

(a)

(b)

(c)

(d)

(e)

(f)

The lighter coloration, due to the thicker wall and greater
vascularity, of the first or duodenal portion, thus distin-
guished as the jejunum (intestinum jejunum).

The darker coloration, due to the thinner wall, which allows
the contents to show through, and diminished vascularity of
the terminal or caecal portion, thus distinguished as the
ileum (intestinum ileum). The two portions are not
distinctly separable. The circular folds (plicae circulares),
or valvulae conniventes, of the mucous tunic, which in many
mammals contribute to the thickness of the wall in the
duodenum and jejunum, are, in the rabbit, not definitely
expressed.

The mesentery, the peritoneal support of the mesenterial
small intestine, is distinguished in its major portion by its
broad frill-like character, which allows great freedom of
movement to this portion of the digestive tube. Its terminal
portion, however, beginning at a point where the intestine
turns sharply forward on its way to the caecum, is adherent
to the mesocolon.

The mesenteric lymph glands (lymphoglandulae mesen-
tericae) are aggregated at the dorsal root of the mesentery,
where they form a compact mass surrounding the base of the
superior mesenteric artery.

The wall of the sacculus rotundus shows externally a fine
hexagonal pattern on account of the presence in it of a large
number of lymph follicles. Similar structures, forming ovai
areas about 3 mm. in diameter and 5 mm. in length, may be
found along the intestinal wall (aggregated lymph nodules
of Peyer).

The finger-like processes, or villi, of the mucous tunic of the
small intestine may be seen by making an incision of the
wall and examining its internal surface. A small portion of
the wall may be excised and examined under water.

4. Trace the course of the large intestine, beginning at the
sacculus rotundus, as follows:

THE INTESTINES. I9I

Its first portion, the blind intestine or caecum (intestinum
caecum), distinguished by its great size, is connected with the
large intestine proper only in the region of the sacculus rotundus.
The organ (Fig. 43) consists of three limbs, and terminates in the
narrow but thick-walled vermiform process (processus vermi-
formis) or appendix. The latter lies in a dorsal position, and is
directed backward.

The second portion, the colon, comprising the major portion
of the large intestine proper, leaves the caecum in the region of the
sacculus rotundus, in which position it is distinguished by its greatly
sacculated walls.

The third portion, the straight intestine, or rectum (intes-
tinum rectum) is a small terminal division situated in the middle
line and enclosed for the most part by the pelvis. It is scarcely
distinguishable from the related portion of the colon.

5. In the caecum the following features may be distinguished:

(a) The wall, which is otherwise smooth, is divided by a spirally
arranged constriction, the latter denoting the position,
internally, of a fold of the mucous tunic, the spiral valve.

(b) The vermiform process is a narrow, light-colored tube of
about five inches in length, the wall patterned externally
by lymph follicles, in the same way as that of the sacculus
rotundus, and greatly thickened in comparison with that of
the caecum proper.

6. The colon is divisible into ascending, transverse, and de-
scending portions, the relations of which may be traced as follows:

(a) The ascending colon (colon ascendens) passes from its
origin on the caecum to a point forwards on the right side of
the dorsal body-wall. This portion is greatly elongated in
the rabbit, and instead of passing directly forward (i.e.,
in man, upward (see Fig. 40), follows more or less closely
the course of the caecum. It is composed of five principal
limbs, united by flexures. Three of the limbs are directed
for the most part forward, the remaining two backward.

The first limb of the colon bears three rows of small saccu-
lations, the haustra, separated by three longitudinal
muscle stripes, distinguished as the bands of the colon

192

(b)

(c)

ANATOMY OF THE RABBIT.

(taeniae coli). Two of these bands are free, while the third
is enclosed by the supporting peritoneum, the mesocolon.
The two free bands run together toward the anterior end
of the first limb.

The transverse colon (colon transversum) is a short seg-
ment, beginning forwards on the right and crossing ‘the
middle line transversely to the left, where it bends sharply
backward, and is replaced by the descending colon.

The descending colon (colon descendens) passes backward
to a point in front of the pelvis, where it is replaced by the
rectum.

The descending mesocolon, which connects this por-
tion with the dorsal body-wall, should be noted on account
of its relation to the inferior mesenteric artery and sympathe-
tic plexuses. It is connected for a considerable distance with
the mesentery of the ascending limb of the duodenum. _

————— z

7. Displace the caecum, turning it over to the right side of the
animal. Lay out the mesenterial small intestine, so that the mesen-
tery and its bloodvessels are exposed. Remove the lymph glands
from about the base of the superior mesenteric artery, first noting
their position at the root of the mesentery. They receive afferent
lymphatic vessels from the wall of the intestine, and send off efferent
vessels to one another and to the lymphatic trunks.

Trace the branches of the superior mesenteric artery as follows:

(a)

(b)

The middle colic artery (a. colica media), a small vessel
(frequently two) arising from the left wall and passing to the
transverse colon.

The inferior pancreaticoduodenal artery (p. 189) arises at
the same level, but from the right wall.

(c) The ileocaecocolic artery (a. ileocaecocolica), a large

branch, equalling in size the superior mesenteric trunk, is
distributed to the terminal portion of the ileum, the caecum
(including the vermiform process), and the ascending colon.
Its branches are arranged in two series, a proximal group
being given off near the point of origin of the main vessel,
and a distal group, including the terminal portion of the
vessel, at about two inches from the point of origin.

THE INTESTINES. 193

The proximal branches include:

(1) Small branches to the third, fourth, and fifth limbs of the
ascending colon.

(2) The appendicular artery (a. appendicularis) to the
vermiform process. A branch of this vessel, given off
near the point of origin, passes to the ileum, anasto-
mosing with a branch of the superior mesenteric trunk.

(3) An anterior ileocaecal artery to the terminal fourth
(third limb) of the caecum proper and related portion
of the ileum.

(4) An anterior right colic artery to the flexure uniting
the first and second limbs of che ascending colon.

(5) A posterior right colic artery to the second limb of the
ascending colon. This vessel anastomoses with (4) and
with the special branch to the third limb (1).

The distal branches include:

(6) A posterior ileocaecal artery to the middle portion
of the third limb of the caecum and the adjacent portion
of the ileum; anastomosing with (3).

(7) A caecal artery to the posterior portion of the third
limb of the caecum.

(8) Terminal branches to the parts about the sacculus
rotundus; anastomosing with (4).

(d) The intestinal arteries (aa. intestinales), about twenty in
number, are given off from the superior mesenteric artery,
and are distributed to the free portion of the mesenterial
small intestine. The successive vessels are connected by
anastomoses. The end of the Superior mesenteric artery has
the relation of one of the intestinal arteries. It anastomoses
forward with a single branch given off from the side of the
vessel opposite to the intestinal arteries, and the latter in
turn anastomoses forwards with a branch of the appendicular

artery.
8. Locate in the descending mesocolon the inferior mesenteric
artery (a. mesenterica inferior), a small median vessel arising from

the abdominal aorta. It has two main branches—the left colic
artery (a. colica sinistra) to the anterior portion of the descending

194 ANATOMY OF THE RABBIT.

colon (anastomosing with the middle colic), and the superior
haemorrhoidal artery (a. haemorrhoidalis superior) to the
posterior portion of the colon and the rectum.

9g. The superior mesenteric vein (v. mesenterica superior),
the chief tributary of the portal, collects the blood distributed by
the superior mesenteric artery, its tributaries being similar in
arrangement to the branches of the artery. The inferior mesen-
teric vein (v. mesenterica inferior) collects blood from the de-
scending colon and rectum; it may be traced forward in the descend-
ing mesocolon.

10. Sympathetic plexuses. In the descending mesocolon will be
found the inferior mesenteric ganglion (g. mesentericum in-
ferius), a narrow curved body situated in front of the inferior
mesenteric artery. Surrounding the abdominal aorta and appear-
ing in the mesocolon is the abdominal aortic plexus (plexus
aorticus abdominalis). It is connected anteriorly with the coeliac
and superior mesenteric plexuses (p. 183) accompanying the corres-
ponding vessels, and with the renal plexuses accompanying the
renal vessels to the kidneys; posteriorly with the inferior mesen-
teric and spermatic plexuses about the inferior mesenteric
and internal spermatic arteries, and with the hypogastric plexus
about the pelvic vessels.

11. By dividing the rectum close in front of the pelvis, and
severing the peritoneal attachments, the intestines may be separ-
ated and laid out in an extended condition. The relations to one
another of the ileum, caecum, and colon are studied to much better
advantage than in the natural position.

VI. THE URINOGENITAL SYSTEM.

(For the general relations of the urinogenital organs, see p. 92).

A. The Urinary Organs.

The central organs of excretion, the kidneys (renes), occupy an
anterior position on the dorsal wall of the abdomen. The right
kidney is placed a little farther forward than the left, and is largely
covered by the right posterior lobule of the liver. In addition to a
fibrous coat immediately surrounding the kidney substance, each
organ is imbedded in a mass of fatty material, the adipose capsule

THE URINOGENITAL SYSTEM. 195

(capsula adiposa), and is also held in position by the peritoneum,
which is stretched across its ventral surface.

1. By removing the peritoneum and adipose capsule from the
left kidney, the external features and vascular connections may be
made out as follows:

(a)
(b)

The general convexity of contour.
The renal hilus (hilus renalis), a concavity of the medial
surface of the organ.

(c) The ureter, or duct of the kidney, a white tube passing

(d)

(e)

backward from the hilus.

The renal artery (a. renalis), arising from the abdominal
aorta and entering the kidney at the hilus. A branch of this
vessel, the suprarenolumbar artery (a. suprarenolum-
balis), passes to the body-wall in front of the kidney, giving
off a small suprarenal artery to the suprarenal gland.
The renal vein (v. renalis), leaving the kidney at the hilus,
and joining the inferior cava.

2. Divide the kidney, beginning the incision at the hilus and
removing the ventral half (Fig. 50). Examine the cut surface of the
dorsal half for the following:

(a)

(d)

The renal pelvis (pelvis renalis), formed by the expanded
funnel-like end of the ureter, which is fitted into the renal
hilus. A central cone of kidney substance, the renal
papilla (papilla renalis) projects into the pelvis.

The cortical substance (substantia corticalis) ; distinguish-
able as a narrow peripheral zone of the kidney substance.
The medullary substance (substantia medullaris), forming
the central dnd medial portion of the kidney, including the
renal papilla. It is distinguished by its radial striations.
The fibrous coat (tunica fibrosa) of the kidney may be
stripped from the surface.

In the rabbit the kidney is not lobulated. Hence there is a single renal
papilla, and the division of the kidney substance into renal pyramids is imper-
fectly expressed. The medullary substance, however, possesses a slightly divided

margin.

The cortical substance is of darker coloration in the natural condition, but

in embalmed animals the color features are usually reversed.

196 ANATOMY OF THE RABBIT.

3. The urinary bladder (vesica urinaria) lies in the ventral
posterior portion of the abdominal cavity. It is a muscular sac,
capable of a considerable amount of distension, but usually found
in preserved animals ina greatly contracted condition. Its rounded
anterior end, the vertex, projects forward into the abdominal
cavity, while its posterior portion or fundus, narrows to.a canal,
the urethra, which receives on its dorsal wall the apertures of the
genital ducts and those of the related glands. The connections
may be made out as follows:

(a) The peritoneum is reflected from the ventral surface of the
rectum in the male and from the uterus in the female, to the
bladder, and after investing the latter passes to the ventral
abdominal wall. The dorsal peritoneum forms in the male
a double rectovesical fold (plica rectovesicalis), and in the
female a similar vesicouterine fold, a recess of considerable
extent (rectovesical or vesicouterine pouch) being left
becween the adjacent structures.

The ventral peritoneum forms a broad median vertical
sheet, the middle umbilical fold (plica umbilicalis media).
The free edge of this fold, extending from the vertex of the
bladder to the umbilicus, contains a slender cord, the middle
umbilical ligament (lig. umbilicale medium). The latter
marks the position of the peripheral portions of the um-
bilical arteries in the foetus.

(b) The umbilical artery (a. umbilicalis), a branch of the hypo-
gastric, passes along the side of the bladder to the vertex.
From the base of the artery branches are given off to the
ureter (a. urecerica) and related portions of the genital ducts,

B. The Male Genital Organs. ,

I. Continue the median ventral incision of the skin backward
along the symphysis to the penis. Reflect the skin on both sides
to clear the attachments of the penis to the ischium, and on the left
to a point beyond the scrotum. Note the cremaster muscle
(m. cremaster), a thin layer of muscle fibres forming the outer
layer of the sac of the testis. It is continuous with the internal
oblique muscle of the abdominal wall, and also contains fibres

THE URINOGENITAL SYSTEM. 197

from the transverse muscle. It is supplied with blood from the
external spermatic artery. Make a longitudinal incision through
this muscle, cutting forward into the abdominal cavity. On
spreading apart the two flaps the following features may be made out:

(a)

(b)

(c)

(d)

(e)

(f)

The parietal layer (lamina parietalis) of the tunica
vaginalis propria, a layer of peritoneum, continuous with
that of the abdominal wall, forms the internal lining of the
sac of the testis (cf. p. 100 and Fig. 55). The sac is widely
open to the abdominal cavity so that the testis passes
freely from one cavity to the other.

The male reproductive gland, the testis, with its associated
vessels and duct, occupy the cavity of the sac, the testis
being suspended from its dorsal wall.

The gubernaculum, a short thick cord containing smooth
muscle fibres, joins the posterior end of the testis with the
end of the sac.

The visceral layer (lamina visceralis) of the tunica
vaginalis propria forms the peritoneal coat of the testis and
is continuous with the mesorchium, a broad vertical fold of
peritoneum connecting the testis dorsally and anteriorly with
that of the body-wall.

The first portion of the duct of the testis, the epididymis,
usually imbedded in fat, forms a thickened mass at the an-
terior end of the testis, and extends backward as a thinner
cord along its side. The thickened anterior portion is the
caput epididymidis and the contracted posterior portion,
the cauda epididymidis. The second portion, the ductus
deferens, leaves the testis at its posterior end, where it is
firmly attached to the gubernaculum. The connection
with the epididymis may be shown by carefully separating
the duct from the gubernaculum and the side of the testis.
The ductus deferens receives its blood supply from the
umbilical artery.

The internal spermatic artery (a. spermatica interna)
arises from the abdominal aorta, in the neighbourhood of the
inferior mesenteric artery, or opposite the sixth lumbar
vertebra, the left artery usually behind the right. It enters
the anterior end of the testis.

198 ANATOMY OF THE RABBIT.

(g) The spermatic vein (v. spermatica) is formed by a network
of vessels, the plexus pampiniformis, which surrounds the
internal spermatic artery in its posterior portion. It opens
forwards into the inferior caval at the angle formed by the
latter with the renal artery. (That of the right side enters
the inferior caval at about the level of the spermatic arteries.)

Owing to the open communication of the testis sac with the abdom-
inal cavity, the association of the ductus deferens with the spermatic
vessels to form a spermatic cord (funiculus spermaticus) is very
imperfectly expressed.

2. The connections of the deferent ducts with the common
urinogenital tube and related parts may be displayed by dividing
the symphysis and dissecting in the interior of the pelvis. The
structure and attachments of the penis should first be examined.

Apart from its terminal portion, the glans penis, and the
urethra, the body of the penis is chiefly formed by a pair of hollow
fibrous structures, the cavernous bodies (corpora cavernosa penis).
Eack of these is firmly attached to the posterior margin of the
ischium by a white fibrous cord, the crus penis. The latter is
partly concealed by a short thick ischiocavernosus muscle,
the origin of which occupies a similar position on the ischium.
The penis is also attached to the symphysis by a short but stout
suspensory ligament (lig. suspensorium) and by a thick spindle-
shaped pubocavernosus muscle lying between the ischiocaver-
nosi.

The attachments of the penis should be severed at the posterior
border of the ischium and the symphysis divided. By pressing
apart the two sides of the pelvis the urinogenital tubes, with the
penis and the pelvic portion of the rectum, may be dissected out
and removed from the body.

The middle haemorrhoidal artery (a. haemorrhoidalis media), a branch
of the hypogastric, passes to the side of the rectum, to the urethra, and to the
seminal vesicle. The internal pudendal artery (a. pudenda interna), accom-
panied by the corresponding nerve and vein, passes to the side of the penis, giving
off the inferior haemorrhoidal artery to the terminal portion of the rectum
and to the associated rectal or anal gland. The rectum is connected with the
root of the tail by the rectocaudalis muscle, a somewhat spindle-shaped
aggregation of smooth muscle fibres, arising from the body of the second caudal
vertebra, and inserted a short distance forwards on the dorsal surface of the

THE URINOGENITAL SYSTEM 199

rectum. The sphincter ani externus and sphincter ani internus are two
closely related muscles enclosing the rectum and urethra, the former arising from
the dorsum of the tail.

The following parts of the urinogenital system may be made out:

(a) The connection of the bladder wich the outside of the body
through the urethra. It comprises a short prostatic
portion in relation to the genital ducts, a much longer
membranous portion traversing the pelvis, and a terminal
cavernous portion in the penis.

(b) The seminal vesicle (vesicula seminalis) lies on the dorsal
surface of the base of the bladder. It is a flattened median
pouch, the anterior, slightly divided tip of which is directed
forward.

(c) The deferent ducts lie between the seminal vesicle and the
dorsal wall of the bladder. They terminate in the ventral
wall of the seminal vesicle.

(d) The prostate (prostata), a white or yellowish mass of
glandular tissue, lies in the posterior portion of the dorsal
wall of the seminal vesicle. Its ducts, three or four in
number on either side, open into the urethra. Accessory
paraprostatic glands (gl. paraprostaticae), minute finger-
like projections of the urethral wall, lie on either side of the
base of the seminal vesicle.

(e) The bulbourethral gland (1. bulbourethralis) ; situated on
the dorsal wall of the urethra behind the prostate.

3. The internal surface of the dorsal wall of the urethra may be
exposed by a longitudinal incision extending into the bladder. The
crescentic aperture of the seminal vesicle lies immediately in front
of an oval elevation, the colliculus seminalis, on either side of
which some of the minute apertures of the prostate may sometimes
be made out.

C. The Female Genital Organs.

1. The organs may be traced from the abdominal cavity back-
ward, as follows:

(a) The ovary (ovarium) is a small—in young animals minute—

elongated structure of greyish or yellowish coloration lying

200

(b)

(c)

(e)

(f)

(g)

(h)

ANATOMY OF THE RABBIT.

on the dorsal body-wall some distance behind the kidney.
It is readily distinguished by the circular translucent dots
representing the larger vesicular ovarian follicles. In
some cases the darker radiate impressions (corpora lutea)
left by extruded eggs are discernible.

The mesovarium, a short fold of peritoneum suspending
the ovary from the body-wall.

The internal spermatic artery (a. spermatica interna)
arises from the abdominal aorta, immediately behind the
origin of the inferior mesenteric artery, and crosses the body-
wall transversely to the ovary.

The spermatic vein (v. spermatica) leaves the medial side of
of the ovary, and crossing the body-wall enters the inferior
caval vein.

The uterine tube (tuba uterina), the first portion of the
oviduct, distinguishable by its narrow calibre, opens into the
abdominal cavity through a broad funnel-like expansion, the
ostium abdominale tubae uterinae. The margin bears
a large number of short folds and processes, the fimbriae
tubae, which tend to enclose the margin of the ovary.
A single cyst-like hydatid may be seen in the funnel-like
expansion of the tube, but in embalmed animals is usually
found collapsed.

The mesosalpinx is the peritoneum supporting the uterine
tube. It is continuous with the mesovarium.

The uterus, the second portion of the oviduct; distinguished
by its greater diameter and muscular walls. The size of
this portion is enormously increased in animals which contain
or have borne young.

The mesometrium is the supporting peritoneum of the
uterus, and is a continuation of the mesosalpinx. The
mesometrium, mesosalpinx and mesovarium together con-
stitute the broad ligament (lig. latum uteri).

The ovarian ligament (lig. ovarii proprium) crosses the
mesosalpinx, connecting the ovary with the anterolateral
end of the uterus. It is continued by:

THE URINOGENITAL SYSTEM. 201

(j) The round ligament (lig. teres uteri) which may be traced
from the anterior end of the uterus to the body-wall below
the posterior portion of the inguinal ligament, where it is
inserted into a small peritoneal recess, the homologue of the
testis sac of the male.

(k) The vagina is a flattened median tube with muscular walls;
it receives anteriorly the apertures of the right and left uteri.

2. Preparatory to dissecting the urinogenital structures of the
pelvis, the median incision of skin of the ventral surface should be
continued backward to the base of the clitoris, the structure and
attachments of the latter being then examined as follows:

(a) The cavernous bodies (corpora cavernosa clitoridis) form
the body of the structure, as in the male, and are attached to
the posterior border of the ischium through short fibrous
cords, the crura clitoridis.

(b) The suspensory ligament (lig. suspensorium clitoridis) is 4
short median cord joining the base of the clitoris with the
posterior end of the symphysis.

(c) The pubocavernosus and ischiocavernosus muscles pass
from the posterior border of the ischium to the base of the
clitoris on either side, the former being median in position.

The attachments of the clitoris should be severed and the sym-
physis divided. By pressing apart the two sides of the pelvis the
urinogenital tube may be dissected out and removed, together with
the terminal portion of the rectum.

The related structures, the middle haemorrhoidal artery, etc., appearing in
this dissection are as in the male (see note p. 198).

In the urinogenital ducts, examine the extent of the vagina back-
ward and its connection with the canal of the bladder to form the
common vestibulum. The latter is comparable to the male
urethra (cf. p. 91, Fig. 48). The bulbourethral gland (gl.
bulbourethralis) (cf. p. 199) lies on the dorsal wall.

By slitting open the vestibulum and extending the incision into
the bladder and also forward into the left uterus the apertures of
these structures may be examined from the interior. There is an
external uterine aperture (orificium externum uteri) for each
division of the uterus.

202 ANATOMY OF THE RABBIT.

VII. THE ABDOMINAL AORTA, INFERIOR CAVAL VEIN,
AND SYMPATHETIC TRUNKS.

The dissection and removal of the intestines and urinogenital
organs clears the dorsal body-wall for an examination of the
abdominal portion of the aorta, the inferior cava! vein and the
sympathetic trunks. If the inferior cava does not contain blood its
tributaries should be cleared first, in order to keep them from being
damaged; otherwise the branches of the aorta should first be traced.
The anterior portion of the inferior cava has been removed with
the liver.

1. The abdominal portion of the aorta, described as the abdom-
inal aorta (aorta abdominalis) extends from the hiatus aorticus
of the diaphragm to the seventh
lumbar vertebra, where it is
replaced by the paired common
iliac arteries. It passes back-
ward in a median position along
the ventral surfaces of the bodies
of the vertebrae. Its natural
continuation backwards on the
sacrum and the caudal verte-
brae is represented by the great-
ly reduced median sacral artery.

The branches of the vessel are
distributed in two series: (1)
Fic. 73. Plan of the pelvic bloodvessels; visceral branches (rami vis-
Arteries: a, aorta; aei, inferior epigastric;
Meomaolial’ ar icten Ge oeman sta (ore ae ae Ce
aie, external iliac; ail, iliolumbar;ao,obturator; Gigestive tube and the urino-
as, sacral; au, umbilical. Veins: vci, inferior : O
cava; vf, femoral; vh, hypogastric; vh’, genital organs; and (2) parietal
common hypogastric; vie, external iliac; vil; C c
iliolumbar. branches (rami parietales) to
the body-wall.
The visceral branches comprise the paired renal and spermatic
arteries, and the unpaired coeliac, superior mesenteric and
inferior mesenteric arteries, which have already been traced.

The parietal branches comprise:

(a) The superior phrenic arteries (aa. phrenicae superiores),

small vessels arising by a common trunk in the hiatus

(b)

(c)

(d)

THE ABDOMINAL AORTA. 203

aorticus and passing to the diaphragm (usually seen better
on the right side).

The suprarenolumbar artery (a. suprarenolumbalis),
arising on either side from the renal artery, and passing antero-
laterad to the body-wall, supplying also the suprarenal body.
The lumbar arteries (aa. lumbales), seven pairs of vessels
distributed metamerically to the lumbar portion of the
body-wall. Six pairs arise from the dorsal wall of the aorta,
the seventh from the median sacral artery. They arise as
single trunks, branching into right and left.

The median sacral artery (a. sacralis media) arises from the
dorsal wall of the aorta near its posterior end, and passes
backward on the ventral surface of the sacrum in the middle
line. Its first portion is concealed from the ventral surface
by the common hypogastric vein.

2. The common iliac artery (a. iliaca communis) is a short
trunk, the branches of which pass to the posterior limb, the wall of
the pelvis, and the pelvic viscera. After giving off the iliolumbar
artery (a. iliolumbalis), which passes laterad to the body-wall, the
common iliac divides into two branches the connections of which
may be traced as follows:

(a)

(b)

The external iliac artery (a. iliaca externa) is the larger
lateral branch, directed toward the inguinal ligament, over
which it passes to the medial surface of the limb, becoming the
femoral artery. Near its crossing with the ligament it gives
off the inferior epigastric artery (a. epigastrica inferior),
the main portion of which passes forward in the medial
portion of the abdominal wall.

The hypogastric artery (a. hypogastrica) is the smaller
medial branch, directed backward on the dorsal wall of che
pelvis. Its course may be traced without injury to the
nerves of the lumbosacral plexus. At its junction with the
external iliac the vessel gives off the umbilical artery
(a. umbilicalis) to the bladder, or in the female first to the
vagina and uterus (a. uterina). The obturator artery
(a. obturatoria) passes laterad to the pelvic wall and the
middle haemorrhoidal to the side of the rectum and

204 ANATOMY OF THE RABBIT.

urethra. The main vessel leaves the pelvic cavity as the
sciatic artery (a. ischiadica), passing to the lateral side
of the abductor caudae anterior, and reappears posteriorly,
dividing into the internal pudendal and lateral caudal
arteries.

3 The inferior caval vein (v. cava inferior) is formed on the
dorsal surface of the posterior end of the aorta by the union of the
paired external iliac veins with the common hypogastric. From
this position it passes to the right side of the aorta (rarely to the
left) almost to its ventral surface, and then runs forward on the
right side to the diaphragm. Its visceral roots or tributaries
(radices viscerales) comprise the paired renal and spermatic
veins, and the hepatic veins from the liver (p. 188). Its parietal
tributaries (radices parietales) include the inferior phrenic
veins (vv. phrenicae inferiores), which enter the inferior cava from
either side of the diaphragm, the lumbar veins (vv. lumbales),
a series of vessels corresponding to the lumbar arteries, and the
paired iliolumbar vein (v. iliolumbalis).

4. The external iliac vein (v. iliaca externa), the continuation
of the femoral vein of the thigh, approaches the inferior cava from
the dorsal side of the inguinal ligament. It receives the inferior
epigastric vein from the abdominal wall and the vesical vein
from the bladder, the latter including in the female also the veins
of the uterus.

5. The sympathetic trunk (truncus sympathicus). Its lum-
bar and sacral portions, and, with due care, its caudal portions may
be traced on either side by working between the abdominal aorta
(or its continuation, the median sacral artery) and the body-wall.
Except on the ventral surface of the sacrum, the ganglia of opposite
sides lie close together. The lumbar portion of each trunk
comprises seven ganglia with their connections. The ganglia
lie on the lateral surfaces of the lumbar arteries near the points
where the latter disappear dorsally in the body-wall. The rami
communicantes may be found passing from the ganglia toward
the spinal nerve-roots. The sacral portion comprises four ganglia
of which the first is much larger than the others. The caudal
portion comprises two minute ganglia and an unpaired terminal
ganglion uniting the two trunks.

THE ANTERIOR LIMB 205

VIII. THE ANTERIOR LIMB.

For this dissection the skin must first be reflected from the
lateral surface of the limb and the side of the neck to the dorsal
median line. It is advisable to divide the skin at the elbow,
leaving the forearm and hand covered, so that the tendons of the
muscles do not become dried out before they can be examined.

Covering the side and ventral surface of the neck is a broad thin
sheet of muscle, the platysma, replacing the cutaneus maximus
of the trunk. It forms a continuous layer over the dorsal surface
of the neck, at which place it is also continuous with the cutaneus
maximus. Passing forward from the manubrium sterni is a narrow
band of fibres, inseparable from the platysma but lying beneath it,
the depressor conchae (parotideoauricularis) posterior, which
is inserted into the external base of the ear. The entire sheet of
muscle is so closely attached to the skin that it is sometimes re-
moved with the latter. If in place it should be raised from the
surface, separated posteriorly from its attachment, and turned
forward on the head.

The dissection is mainly muscular, but the arteries and nerves
should be kept intact for later examination.

Identify the manubrium sterni. The muscle directed forward
from it toward the angle of the mandible is the sternomastoideus,
one of the muscles of the head. The external jugular vein lies on
its lateral side. Identify the clavicle rudiment and the cleido-
humeral ligament; then proceed to uncover the muscles directed
forward from this region to the head.

For the general muscle relations of the limbs, see p. 64.

*1. Muscles arising from the axial skeleton and inserted on the
scapula and clavicle.
(a) The cleidomastoideus. Origin: Mastoid portion of the
skull. Insertion: Middle portion of the clavicle.
The muscle lying on its medial side and arising from the manu-
brium sterni is the sternomastoideus, one of the muscles of the head.

*The structures of Group 2 may be dissected first if preferred, the serratus
anterior muscle being exposed from the lateral surface and divided together with
the latissimus dorsi.

206

(b)

(c)

(d)

(e)

(f)

(g)

ANATOMY OF THE RABBIT.

The basioclavicularis (basiohumeralis). Origin: Basiocci-
pital bone. Insertion: Lateral third of the clavicle and the
cleidohumeral ligament.

The levator scapulae major. Origin: Cartilage union of
basioccipital and basisphenoid (sphenooccipital synchon-
drosis). Insertion: Metacromion.

The superficial cervical artery (p. 257) passes obliquely forward
and outward under cover of these muscles, ramifying beneath the superior
portion of the trapezius in the fat-mass of the side of the neck. Its
ascending cervical branch passes forward on the lateral surface of the
external jugular vein.

The trapezius. Origin in two portions:

Superior (cervical) portion: External occipital protuber-
ance and dorsal ligament of neck (ligamentum nuchae).
Insertion: Metacromion and supraspinous fascia.

Inferior (thoracic) portion. Origin: Spinous processes of
the thoracic vertebrae and the lumbodorsal fascia. Inser-
tion: Dorsal half of the scapular spine. The muscle forms
a broad triangular sheet on the dorsolateral surface of the
shoulder.

The levator scapulae major, basioclavicularis, and trapezius should
be divided. On the ventrolateral surface of the superior portion of the
trapezius and levator scapulae major may be found nerves from the
ventral rami of the third, fourth and fifth cervical spinal nerves. The
great auricular nerve (n. auricularis magnus) passes from the third to
the ear.

The rhomboideus minor. Origin: Ligamentum nuchae.
Insertion: Anterior two-thirds of the vertebral border of the
scapula.
The levator scapulae minor. Origin: Mastoid and supra-
occipital portions of the skull. Insertion: Medial surface of
the inferior angle of the scapula.

The rhomboideus major Origin: Spinous processes of
the first seven thoracic vertebrae. Insertion: Posterior
third of vertebral border. The rhomboidei are almost
continuous.

By dividing the rhomboidei the scapula may be displaced laterad.
The operation is facilitated by dividing the latissimus dorsi, the re-
lations of which should, however, first be noted (2, a).

THE ANTERIOR LIMB. 207

(h) The serratus anterior. Origin in two portions:

Cervical portion: Transverse processes of the posterior five
cervical vertebrae and the anterior two ribs.

Thoracic portion: Third to ninth ribs by separate slips
alternating with those of the external oblique.

Common insertion: Medial surface of the vertebral border
of the scapula.

The transverse artery of the neck (a. transversa colli) lies on the
medial side of the cervical portion.

2. Muscles arising from the axial skeleton and the pectoral
girdle and inserted on the humerus, for the most part at its proximal
extremity.

(a)

(b)

Note the axillary lymph glands lying in the fat of the axillary
fossa.
The latissimus dorsi. Origin: Lumbodorsal fascia and
four posterior ribs. Insertion: Deltoid tuberosity. A
long flat triangular muscle, covering a considerable
portion of the lateral surface of the thorax; having
its dorsal angle covered by the thoracic portion of
the trapezius. Its insertion end passes to the medial side
of the humerus.

The pectoralis primus (p. tenuis). Origin: Manubrium
sterni. Insertion: Deltoid tuberosity.

A branch of the thoracoacromial artery appears between this
muscle and the deltoideus.

The muscle should be raised from the surface and divided.

The pectoralis secundus (p. major). Origin: Entire
lateral portion of the sternum. Insertion: Anteromedial
surface of the humerus, beginning below the head, and
extending to the boundary between the middle and
distal thirds. The anterior fibres are covered by those
of (b). Some of the posterior fibres are inserted highest
on the humerus.

By working backward from the clavicle the muscle can be detached
from those underneath and divided.

208 ANATOMY OF THE RABBIT.

(d) The pectoralis tertius (p. minor). Origin consisting of two
portions:

First portion: Anterior portion of sternum to attachment of
fourth rib.

Second portion: Manubrium sterni and costal cartilage of
first rib.

Insertion: The superficial fibres of the first portion are
attached to the clavicle. The remaining fibres, combined
with those of the second portion and those of the pecto-
scapularis, pass to the dorsal side of the clavicle and over the
shoulder to be inserted on the scapular spine. The muscle
forms a broad fleshy mass covering the anterodorsal portion
of the shoulder.

(e) The pectoralis quartus. Origin: The sternum, from the
attachment of the fourth to seventh costal cartilages. In-
sertion: Anterior surface of the head of the humerus, passing
thence to its medial side. The muscle overlaps the more
posterior fibres of the first portion of (d).

(f) The pectoscapularis. Origin: The manubrium sterni at
the point of attachment of the first costal cartilage. In-
sertion as indicated above. A slender muscle concealed
by the p. tertius, which should be divided.

BLOODVESSELS AND NERVES OF THE AXILLARY FOSSA.

After division of the pectorals and the clavicle, the bloodvessels
and nerves of the axillary fossa will be fully exposed.

*The axillary artery (a. axillaris), the continuation of the
subclavian, crosses from the first rib to the medial surface
of the humerus, where it is replaced by the brachial artery.
Its branches are:

(a) The transverse scapular (suprascapular) artery (a.
transversa scapulae); it arises from the anterior wall, and
taking a position dorsal to the clavicle accompanies the
p. tertius and pectoscapularis to the front of the shoulder,
where, under cover of these muscles, it passes into the supra-
spinatus muscle.

(b)

(c)

(d)

THE ANTERIOR LIMB. 209

The thoracoacromial artery (a. thoracoacromialis); it
arises from the ventral wall or in common with (c), passes
between the pectorales tertius and quartus, then between
the p. superficialis and the cleidohumeralis. It distributes
branches to these muscles, and taking a position ventral to
the clavicle, passes to the platysma and the skin.

The lateral (long) thoracic artery (a. thoracalis lateralis)
arises from the posterior wall or in common with (b);
distributes branches chiefly to the p. secundus, and sends
a long superficial branch, the external thoracic artery,
backward through the cutaneus maximus muscle. This
vessel is usually conspicuous in the female, where it distri-
butes external mammary branches to the mammary
glands. It anastomoses posteriorly with the superficial
epigastric branch of the femoral.

The subscapular artery (a. subscapularis) is a large
branch given off from the distal portion of the axillary
artery. It distributes branches to the subscapularis muscle,
and sends a thoracodorsal branch into the latissimus
dorsi. Perforating the teres major muscle near the axilla,
it appears on the lateral surface of the shoulder, where it
sends a large branch into the inferior portion of the trapezius,
and a second into the cutaneus maximus. The latter vessel
supplies the proximal end of the long head of the triceps,
but its chief portion passes backward uniting with an
anterior superficial branch of the iliolumbar, and thus
forming one of three superficial anastomoses covering the
abdominal region and in the female the mammary glands.

(e) The circumflex arteries of the head of the humerus, see p. 215.

(f)

The deep artery (a. profunda brachii), see p. 215.

**The axillary vein (v. axillaris) begins at the medial side of the

humerus and crosses the axillary fossa to the first rib whence it is
continued as the subclavian. It receives the lateral thoracic
and subscapular veins, which accompany the corresponding
arteries, and also the cephalic vein (p. 216), which reaches the
medial side of the shoulder from the anterior surface of the arm by
passing between the teres major and subscapular muscles near the
neck of the scapula.

210 ANATOMY OF THE RABBIT.

***The brachial plexus (plexus brachialis) is the network of
nerves formed from the ventral rami of the posterior five cervical
and first thoracic spinal nerves. The cervical nerves also take part
in the formation of the more general cervical plexus embracing all
nerves of the cervical series. The strands of the brachial plexus
cross the axillary fossa and at the medial surface of the humerus
are largely replaced by the three chief trunks of the free extremity,
the radial, median, and ulnar nerves. These nerves are formed
principally from the eighth cervical, but adjacent nerves also
contribute, especially a large bundle which enters the radial from
the seventh cervical, from which combined trunk the stout sub-
scapular nerves are given off to the corresponding muscle. A
suprascapular nerve, formed chiefly from the sixth cervical,
passes to the anterior border of the scapula, entering the supra-
spinatus muscle.

By dividing the axillary nerves and vessels and the two parts of the serratus
anterior muscle, the limb may be removed from the body.

4. Muscles arising from the pectoral girdle and inserted on the
humerus. These muscles act on the humerus through the shoulder-
joint, and except for the unimportant difference in origin are similar
to those of Group 2.

The course of the cephalic vein (p. 216) should be traced before
separating the muscles of the front of the forearm.

Note the supraspinous and infraspinous fasciae cover-
ing the corresponding portions of the lateral surface of the
shoulder.

(a) The cleidohumeralis. Origin: Lateral portion of the
clavicle and the cleidohumeral ligament. Insertion: Anterior
surface of the humerus in its distal third. The muscle is a
continuation of the basioclavicularis, but represents the
brachial part of the brachiocephalic muscle, which, in
many mammals with reduced clavicle, extends from the
mastoid portion of the skull to the front of the arm. The
cervical part is the cleidomastoideus.

THE ANTERIOR LIMB.

(b) The deltoideus:

Acromial portion.

Scapular portion.

portion of deltoid tuberosity.

Origin: Infraspinous fascia.

211

Origin: The Acromion. Insertion: Distal

The end of

the muscle forms a curved line over the dorsal portion of the

infraspinatus, leaving only a
small triangular portion of
the latter exposed. Insertion:
The distal portion of the mus-
cle passes beneath the meta-
cromion, which also serves as
a point of attachment, and is
replaced on the lateral surface
of the humerus, beneath the
acromial portion, by a long
thin tendon, through which
it is inserted.

~The scapular portion of the|

deltoideus should be separated
from the infraspinatus and divid-
‘ed, the distal end being reflected
together with the metacromion.

(d) The infraspinatus.
Origin: Posterior por-
tion of the lateral sur-
face of the scapula, in-
cluding the spine. In-
sertion: Greater tub-
ercle of the humerus.
The muscle fills the
infraspinous fossa.

Anterior

Lateral

Posterior

Fic. 74. Transverse section through the
distal portion of the arm; semidiagrammatic;
a.b., brachial artery; a.c.1., radial collateral
artery; b., biceps; br.l. and br.m., lateral and
medial heads of the brachialis; d., deltoideus
(insertion); e.a.p., extensor antibrachii
parvus; f., bracnial fascia; h., humerus;
n.m, me cian nerve; n.r., radial nerve; n.u.,
ulnar nerve; tr. I-tr.3, long, lateral, and
medial heads of the triceps; v.b., brachial
vein; v.c., cephalic vein.

(e)

The supraspinatus. Origin: Anterior portion of the lateral
surface of the scapula (supraspinous fossa), supraspinous
fascia, and, to a certain extent, the subscapular fascia.
Insertion: Greater tubercle of the humerus.

The extent of this muscle is evident only after removal of the
loosely attached fleshy parts of the pectorals from its surface.

212
(f)

(g)

(h)

(i)

ANATOMY OF THE RABBIT.

The subscapularis. Origin: Entire medial surface of the
scapula. Insertion: Lesser tubercle of the humerus.

The teres major. Origin: Dorsal portion of the axillary
border of the scapula. Insertion: In common with the
latissimus dorsi on the anterior surface of the humerus.

The teres minor. Origin: Ventral portion of the axillary
border of the scapula. Insertion: Greater tubercle.

The muscle is closely associated with the infraspinatus but is
separated from the teres major by the tendon of origin of the long head
of the triceps.

The coracobrachialis. Origin: Coracoid process. —_In-
sertion: Distal portion of the upper third of the humerus on
its medial surface.

5. Muscles arising from the scapula and humerus and inserted
on the proximal ends of the radius and ulna (extensors and flexors
of the forearm), (Figs. 34, 35, 74).

AY

Extensor (anconaeus) group. The muscles arise for the

most part behind the axis of the- humerus, and are inserted on the
olecranon.

(a)

(b)

(c)

The extensor antibrachii parvus (anconaeus quartus).
Origin: Fascia of the medial surface of the humerus. Inser-
tion: Medial surface of the olecranon.

. The muscle should be divided, or detached from its origin, and re-
flected.
The anconaeus minimus (epitrochleonanconaeus).
Origin: Medial epicondyle of the humerus. Insertion:
Medial surface of the olecranon
The triceps brachii. Origin in three poruons:
Caput longum (anconaeus longus): Ventral portion of the
axillary border ot the scapula.
Caput laterale (anconaeus lateralis): Greater tubercle and
related portion of the lateral surface of the humerus.
Caput mediale (anconaeus medialis): Posterior surface of
the humerus.

The three portions are almost separate muscles. Insertion
on the olecranon.

B:

THE ANTERIOR LIMB. 212

Flexor group. The muscles arise in front of the axis of the

humerus and are inserted on the radius and ulna in front of the
elbow-joint.

(a)

(b)

The biceps brachii. Origin: Anterior border of glenoid
cavity. Insertion: Ventromedial surface of the ulna and
medial surface of the radius. The muscle possesses only one
head in the rabbit.

The brachialis. Origin: Anterior and lateral surfaces of
the humerus, divided unequally into a larger lateral and a
smaller medial portion by the insertion tendons of the
deltoideus and abductores muscles. Insertion: In common
with the biceps.

6. Muscles arising from the distal end of the humerus and the
radius and ulna and inserted on the hand (extensors and flexors of
the hand and of the individual digits). The long insertion tendons
pass through perforations of the dorsal carpal and transverse
(ventral) carpal ligaments (Fig. 75).

A.

Extensor group. The muscles have a general area of

origin from the lateral epicondyle of the humerus and the antero-
dorsal, or anterolateral surface of the radius and ulna. Insertion

dorsal.

’

(a)
(b)

(c)

(d)

The extensor carpi radialis longus. Origin: Lateral
epicondyle. Insertion: Base of the second metacarpal.
The extensor carpi radialis brevis. Origin: Lateral
epicondyle. Insertion: Base of the third metacarpal.
The muscle is partly fused with the foregoing one, and the
tendons are closely associated on the wrist

The abductor pollicis. Origin: Anterolateral surface of
the radius and ulna. Insertion: Base of the first meta-
carpal. The muscle is partly concealed by (e). Its tendon
forms a conspicuous cross with those of (a) and (b). ©

The extensor pollicis et indicis. Origin: Anterolateral
surface of the radius and ulna. Insertion: Ungual phalanx
of the pollex and the head of the second metacarpal. Its
tendon is the first of five in the centre of the carpus.

The extensor digitorum communis. Origin: Lateral
epicondyle and proximal end of the ulna. Insertion: By
four tendons on all phalanges of the four lateral digits.

214 ANATOMY OF THE RABBIT.

(f) The extensor digiti quarti proprius.
Insertion: Ungual phalanx of the fourth digit.
(g) The extensor digiti quinti proprius.

epicondyle and lateral surface of the ulna.

epicondyle.

Origin: Latera

Origin: Lateral
Insertion:

Head of the fifth metacarpal and base of the first phalanx of

this digit.

(h) The extensor carpi ulnaris.

or posteromedial surface of the radius and ulna.

Dorsal
vc. oa p J erb
led. Sgt ef en
S IE a 2
CqP.. (5 aD
Weeal emp._f Th Medial
ecu..\\e
au_.\“ fs) hp
vu--~ ‘~_ar
lct- nm

Volar

Fic. 75. Transverse section of the distal end
of the forearm. Showing the relative positions of
the muscle-tendons: ap, abductor pollicis; ar,
radial artery; au, ulnarartel1y; ecu, extensor carpi
ulnaris; ede, extensor digitorum communis;
emp, extensor digiti quinti proprius; epi, extensor
pollicis et indicis; eqp, extensor digiti quarti pro-
prius; erb, extensor carpi radialis brevis; erl,
extensor carpi radialis longus; fer, flexor carpi rad-
ialis; fcu, flexor carpi ulnaris; fdp, flexor digitorum
profundus; fds, flexor digitorum sublimis; lcd,
dorsal carpal ligament; Ict, transverse carpal liga-
ment; nm, median nerve; nu, ulnar nerve; p,
palmaris; r, radius; u, ulna; vc, cephalic vein;
vr, radial vein; vu, ulnar vein.

Origin: Lateral epicondyl
and proximal portion of the lateral surface of the ulna
Insercion: Base of the fifth metacarpal.

B. Flexor group. The muscles have a general area of origin
from the medial epicondyle of the humerus and the posteroventral

(a)

(b)

(c)

Insertion volar.

The pronator teres.
Origin: Medial epi-
condyle. Insertion

Ventral surface of th:
radius.

The flexor carpi rad-
ialis. Origin: Medial
epicondyle. Insertion:
Base of the second
metacarpal.

The flexor digitorum
sublimis. Origin: In
common with the ul-
nar portion of the pro-
fundus from the medial
epicondyle; proximal
portion of the ulna.
Insertion: Bases of the

second phalanges of the four lateral digits.

(d) The palmaris. Origin:

Medial epicondyle.
This extremely slender

Superficially on the volar fascia.
muscle lies between the superficial portion of the profundus

and the flexor carpi ulnaris.

(e) The flexor digitorum profundus.
Superficial portion: Medial epicondyle.
Radial portion: Ventral surface of the radius.

Insertion:

Origin in four portions:

THE ANTERIOR LIMB. 215

Middle portion: Ventral surface of the ulna.

Ulnar portion: Medial epicondyle in common with (c).
Insertion: By five tendons on the bases of the ungual
phalanges.

(f) The flexor carpi ulnaris. Origin: Medial epicondyle and
medial surface of the olecranon, forming two short but
separate heads. Insertion: Pisiform bone.

7. Muscles arising from the bones of the hand and inserted on
the individual digits:

(a) The flexor digiti quinti. Origin: Pisiform bone and ten-
don sheath of the flexor digitorum profundus. Insertion:
Sesamoid bones of the metacarpophalangeal joint of the
fifth digit, extending to the ungual phalanx.

(b) The lumbricales. Origin: From the point of division of
the tendon of the flexor digitorum profundus. Insertion:
First phalanges of the second to fifth digits.

(c) The interossei. Origin: In pairs from the bases of the
second to fifth metacarpals and related portions of the carpal
bones. Insertion: Sesamoid bones of the metacarpophalan-
geal joints.

BLOODVESSELS AND NERVES OF THE ARM AND FOREARM.

The axillary artery gives rise to posterior and often anterior
branches; the circumflex arteries to the head of the humerus
and the deep artery which latter, arising in one or two branches
and passing between the coraco-brachialis and teres muscles to
the lateral surface of the shoulder, gives branches to the deltoideus
and to the proximal ends of the lateral and long heads of the
triceps. The deep artery continues on the lateral side of the
medial head of the triceps and passes to the lateral head of
the brachialis, near the elbow, as the radial collateral artery.

*The brachial artery (a. brachialis), the continuation of the
axillary, passes distad on the medial surface of the arm between the
biceps and the anconaeus medialis. Crossing to the anterior sur-
face of its distal extremity, it passes beneath the head of the
pronator teres to the medial surface of the radius, dividing at this

216 ANATOMY OF THE RABBIT.

point—a short distance in front of the elbow—into the radial and
ulnar arteries. Its chief branches on the arm are the ulnar
collateral arteries (superior, middle, and inferior) to the muscles
and the elbow joint.

The radial artery (a. radialis) passes distad on the ventro-
medial border of the radius, in company with the median nerve,
lying at first between the flexor carpi radialis and the radial portion
of the flexor digitorum profundus. Toward the distal end of the
radius it crosses the ventral surface of the tendon of the flexor carpi
radialis, and appears in a superficial position on the medial border
of the carpus. It reaches the volar surface of the hand after passing
obliquely across the tendon of the flexor digitorum sublimis.

The ulnar artery (a. ulnaris) crosses the ulna obliquely from its
origin, reaching in this way the lateral border of the flexor carpi
ulnaris, along which it passes to the end of the forearm and to the
ulnar side of the pisiform bone. It passes to the ventral surface
of the fifth digit, and then turns across the hand, forming the volar
arch.

**The single brachial vein (v. brachialis) accompanies the brachial
artery and lies behind it. It is formed in front of the elbow by the
union of two vessels, the radial and ulnar veins, which accompany
the corresponding arteries and join one another at the point of
separation of the latter. The radial vein anastomoses with the
radial portion of the cephalic at a point distal to the middle of the
forearm.

The cephalic vein (v. cephalica) is a large superficial vessel
appearing on the dorsal surface of the forearm. From the radial
side of the latter it receives a large tributary which anastomoses
with the radial vein. It is accompanied by branches of the super-
ficial ramus of the radial nerve. It passes to the front of the arm
across the angle of the elbow, lying at first on the anterior margin
of the lateral head of the brachialis, and afterwards on the lateral
surface of the arm between the abductor brachii superior and the
anconaeus lateralis. It disappears from this surface in the trian-
gular space enclosed by these muscles and the insertion of the
levator scapulae major, receiving at this point a large tributary from
the shoulder. It appears on the medial surface of the shoulder at

THE PosTERIOR LIMB. 2uy

the distal end of the axillary border of the scapula between the
teres major and subscapularis, entering the axillary vein at about
the same place as the subscapular vein, of in common with the
latter.

***The radial nerve (n. radialis) passes behind the brachial
artery to the posterior surface of the humerus. It perforates the
anconaeus medialis, appearing afterwards on the lateral side of the
brachialis in company with the collateral radial artery. Asuper-
ficial ramus, given off on the distal portion of the arm, accom-
panies the cephalic vein: it passes along the surface of the extensor
carpi radialis, dividing into branches for the dorsum ot the hand.
The remaining portion is chiefly distributed as the ramus pro-
fundus to the extensor muscles of the forearm.

The median nerve (n. medianus) passes distad along the
medial surface of the arm, lying at first in front of the brachial
artery and then on its medial side. It accompanies the brachial
artery, passing beneath the head of the pronator teres, and then
traverses the forearm, in company with the radial artery, to the
volar surface of the hand.

The ulnar nerve (n. ulnaris) lies behind the brachial artery.
Toward the distal extremity of the humerus it accompanies the
inferior ulnar collateral artery. It passes from the medial surface
of the elbow, between the anconaeus minimus and the base of the
olecranon, to the dorsal surface of the olecranon head of the flexor
carpi ulnaris, and then crosses the ulna obliquely, in company with
the ulnar artery, to the lateral border of the muscle and along it to
the insertion tendon. At the wrist it crosses the dorsal surface of
the tendon, and passing between the tendon of the sublimis and
the pisiform bone, reaches the volar surface of the hand.

IX. THE POSTERIOR LIMB.

Dissection on the side opposite to that of injection. The
dissection is largely a muscular one; to be conducted in the same
way as in the anterior limb. The corresponding muscle groups
should be compared with respect to the difference in orientation
of the equivalent segments.

218

ANATOMY OF THE RABBIT.

1. Muscles arising from the ventral surface of the posterior
thoracic and lumbar vertebrae and inserted on the pelvic girdle,
or on the lesser trochanter of the femur. These muscles are chiefly
distinguished by their vertebral position, on account of which and
the fixed condition of the pelvic girdle they combine the characters
of vertebral and appendicular muscles.

(a)

(b)

(c)

(d)

The psoas minor. Origin: Bodies of the four posterior
lumbar vertebrae. Insertion: Pecten of the pubis. The
flat, pointed tendon forms a square cross with a dorsal
(sacral) continuation of the inguinal ligament, which is
stretched transversely from the middle of the inguinal liga-
ment to the centre of the body of the first sacral vertebra,
and on which the tendon is also inserted.

It is necessary to divide the inguinal ligament and reflect its sacral
continuation, together with the tendon of the psoas minor.
The psoas major. Origin: Internal surfaces of the bases of
the last three ribs and bodies of the corresponding thoracic
vertebrae; also the lumbar vertebrae. Insertion: Lesser
trochanter.

The iliacus. Origin: Bodies of the last lumbar and first
sacral vertebrae, extending to the sacroiliac union. Inser-
tion: With the psoas major on the lesser trochanter. The
two muscles together form the iliopsoas.

The lumbar portion of the lumbosacral plexus, beginning with
the fourth lumbar nerve, lies on the ventral surface of the psoas major
and between the latter and the iliacus. The fifth lumbar is the chief
root of the femoral nerve (p. 225), the trunk of which may be traced
from a position between the two muscles distad over the dorsal surface
of the inguinal ligament to the medial surface of the thigh. The remain-
ing four nerves crossing the dorsal body-wall obliquely are the twelfth
thoracic and first three lumbar.

The psoas major should be freed at its lateral margin and turned

toward the middle line, the fourth lumbar nerve being divided.
The quadratus lumborum. Origin: Bodies of the
posterior five thoracic vertebrae and the bases of the corres-
ponding five ribs; bodies and transverse processes of the
lumbar vertebrae. Insertion: Triangular processes of six
lumbar vertebrae and the posterior ventral angle of the iliac
wing, together with the adjacent portion of its medial sur-
face.

THE POSTERIOR LIMB. 219

2. Muscles arising from the pelvic girdle and sacrum and in-
serted on the femur, for the most part at its proximal extremity.

The muscles of this group enclose the proximal portion of the
femur on its lateral, posterior, and medial sides. They are partly
covered by the flexors of Group 3, namely, the biceps, sartorius, and
gracilis (pp. 222, 223), which must be divided. To begin the
dissection, see directions for removing the biceps (p. 223). The
position of the sciatic vein (p. 225) should be noted.

Dissection on the lateral surface posteriorly.

(a)

(b)

(c)

(d)

The glutaeus maximus. Origin in two fleshy portions,
joined by an aponeurosis:
First portion: Fascia covering the sacrum in its entire length.
This portion is triangular in shape, and is covered posteriorly
by the first head of the biceps.
Second portion: Anteroventral border of the iliac wing,
fused with the tensor fasciae latae and the first head of the
rectus femoris; also from the dorsal border and lumbar fascia
through the aponeurosis mentioned above.
Insertion: Third trochanter. The axis of the first portion
is transverse, that of the second horizontal.

Both portions of the muscle should be divided. The sciatic nerve
and artery are then exposed.
The glutaeus medius. Anterior border of the wing of the
ilium and the iliac crest and fascia of the first two sacral
vertebrae. Insertion: Greater trochanter. Some of the
fibres pass around the medial surface of the tip of the greater
trochanter and are inserted in the lateral wall of the tro-
chanteric fossa.

The muscle should be divided.
The glutaeus minimus. Origin: Entire lateral surface
of the body and wing of the ilium. Insertion: Greater
trochanter.

Remove the entire muscle. The piriformis is in contact with its
dorsal margin posteriorly.
The tensor fasciae latae. Origin: Anterior portion of the
ventral border of the wing of the illum. Insertion: Broad

220

(e)

(f)

(g)

(h)

(i)

(3)

(k)

ANATOMY OF THE RABBIT.

fascia of the lateral surface of the thigh. The muscle is
fused with the first head of the rectus femoris in front, and
with the second portion of the glutaeus maximus behind.
The piriformis. Origin: Lateral portions of the second
and third sacral vertebrae. Insertion: Tip of the great
trochanter.

The muscle should be divided, care being taken to avoid injury to
the nerves and bloodvessels beneath it.
The gemellus superior. Origin: Tendinous from the
ischial spine and fleshy from the body of the ischium imme-
diately in front of it. Insertion: Lateral wall of the tro-
chanteric fossa.

The muscle extending from the ischial spine to the sacrum is the
abductor caudae anterior (p. 270).
The obturator internus. Origin: Internal extent of the
obturator foramen. Insertion: Trochanteric fossa. The
muscle passes over the ischium in the lesser sciatic notch,
only its insertion portion appearing from the lateral surface.
To see its origin, reflect the tendon through the lesser sciatic
notch and examine the muscle from the internal surface
of the pelvis.
The gemellus inferior. Origin: Posterior portion of the
superior ramus of the ischium and the ischial tuberosity.
Insertion: Trochanteric fossa.
The quadratus femoris. Origin: Ventral surface of the
ischial tuberosity and the superior ramus of the ischium
immediately in front of it. Insertion: The superficial fibres
are inserted on and below the third trochanter, the remaining
ones below the trochanteric fossa. :
The obturator externus: Origin: External extent of the
obturator foramen. Insertion: Trochanteric fossa. The
muscle is largely concealed from this surface, but may be
fully displayed by the division of the pectmens and adduc-
tores brevis and longus.
Dissection on the medial surface posteriorly, after division of
the gracilis.
The pectineus. Origin: Pecten of the pubis. Insertion
Immediately below the lesser trochanter.

THE POSTERIOR LIMB. » 221

(1) The adductor brevis. Origin: Anterior portion of the
symphysis pubis. Insertion: Below the pectineus.

(m) The adductor longus. Origin: Posterior portion of the
symphysis and inferior ramus of the ischium. Insertion:
Posterior surface of the shaft of the femur to its distal third.

(n) The adductor magnus. Origin: Ventral surface of the
ischial tuberosity. Insertion: Medial surface of the distal
end of the femur, extending to the medial condyle of the
tibia.

3. Muscles arising from the pelvic girdle and the femur and
inserted on the proximal portions of the tibia and fibula (extensors
and flexors of the leg). (Fig. 76).

A. Extensor group (quadriceps femoris). The muscles lie
for the most part in front of the axis of the femur. They have a
common insertion on the tibial tuberosity through the patella and
the patellar ligament.

(a) The rectus femoris. Origin in two portions:

First portion: Superior anterior spine, fused with the
tensor fasciae latae, and ventral border of the iliac wing.
Second portion: By a stout round tendon from the inferior
anterior spine, immediately in front of the acetabulum.
This part is cylindrical in shape and is almost a separate
muscle.

The two portions of the muscle should be divided.

(b) The vastus lateralis. Origin: Anterior surface of the
great trochanter and the lateral intermuscular ligament.

The muscle should be divided.

(c) The vastus intermedius. Origin in two portions:

First portion: Great trochanter, below the origin of the
vastus lateralis.
Second portion: Anterior surface of the femur.

(d) The vastus medialis. Origin: Medially, at the base of
the collum femoris and adjacent portion of the shaft.
Common insertion (a-d): Tibial tuberosity.

B. Flexor group (hamstring muscles). With the exception of
the sartorius, the muscles lie behind the axis of the femur, and are

222 ANATOMY OF THE RABBIT.

inserted on the medial and lateral surfaces of the knee-joint and the
corresponding proximal portions of the leg. They form the bound-
aries of the popliteal fossa.
(a) The sartorius.
Patenior Origin: — Posterior
portion of the in-
guinal ligament, es-
pecially its sacral
extension. Inser-
tion: Medial con-
dyle of the tibia.
This is an extremely
thin and narrow
band of fibres, lying

Lateral 6
on the more anterior
portion of the med-
ial surface of the
thigh. The muscle
is fused distally with
the gracilis. It has
the position of a
rotator muscle, and
is .only a flexor
through its connec-
tion with the latter.

(b) The gracilis. Ori-
Pees gin: The entire ex-
Fic. 76. Transverse section through the middle tent of the pubic
of the thigh: a.l., adductor longus; a.m., ad- F a
ductor magnus; b.f.1 and b.f.2, first and second symphysis. Insér-
heads of the biceps femoris; f.,femur; gr., gracilis; -
n.p., peroneal nerve; n.s.m, greater saphenous nerve; tion: Through a
n.t., tibial nerve; r.f.1 and r.f.2, first and second 3
heads of the rectusfemoris; s, sartorius; sm.,semimem- broad tendinous ex-
branosus; st., semitendinosus; t.f.c., tensor fasciae 3 . .
cruris; t.f.l., tensor fasciae latae; v.i.1 and v.i. 2, first pansion ending In
and second heads of the vastus intermedius; v.is. 3
sciatic vein; v.l., vastuo lateralis; v.m., vastus medialis; the fascia of the

v.s.m., great saphenousvein.

proximal portion of
the medial surface of the thigh. The muscle forms a broad, com-
paratively thin sheet, covering the posterior portion of the medial
surface of the thigh. Its insertion tendon is perforated

THE POSTERIOR LIMB 223

by the great saphenous artery and vein and the greater saphenous

nerve.

(c)

(d)

The sartorius and gracilis should be raised from the surface and
divided.
The biceps femoris: Origin in two portions:
First portion (caput breve): Spinous processes of three
posterior sacral and three anterior caudal vertebrae. This
portion is triangular in shape, the distal end, or apex of the
triangle, passing into a thin flat tendon which is inserted on
the lateral margin of the patella.

Second portion (caput longum): Dorsal surface of the
ischial tuberosity, fused with the adductor magnus, and the
lateral process, fused with the second, or deep portion of
the semimembranosus. This portion is also triangular, the
base being distal and providing a broad insertion on the
fascia of the proximal third of the lateral surface of the
leg. The first, or superficial head of the semimembranosus,
covers this portion at its origin, which is also crossed by the
sciatic vein.

To free the biceps and divide it: Incision along the apon-
eurotic line joining the tip of the great trochanter with the
sacrum; also distad along the intermuscular septum of the
lateral surface of the thigh from the great trochanter to the
knee. An incision along the first portion of the sciatic
vein will free the superficial head of the semimembranosus
from the biceps. Raise the biceps slightly by working the
handle of the scalpel under its distal portion. Divide the
muscle from its posterior margin, being careful not to injure
the slender tensor fasciae cruris muscle and the branches of
the sciatic nerve which lie beneath it. The first head of the
biceps may be separated and reflected in order 40 €xpose the
short muscles of the thigh. .

The tensor fasciae cruris. Origin: By a long, slender
tendon, from the transverse process of the fourth sacral
vertebra. Insertion: Lateral fascia of the leg. This slender
muscular slip underlies the biceps femoris.

224 ANATOMY OF THE RABBIT.

(ce) The semimembranosus. Origin in two portions:

First (superficial) portion: Fascia covering the first head
of the biceps.

Second (deep) portion: Lateral process of the ischial
tuberosity. Insertion: In common with the gracilis on the
fascia of the proximal portion of the medial surface of the leg.
This fascia is contracted into two ligaments, one of which
carries the insertion of the muscle to the distal end of the
tibial tuberosity, the other to the distal end of the leg, where
it joins the tendon of the heel (tendo calcaneus).

({) The semitendinosus. Origin: Ischial tuberosity. In-
sertion: Medial condyle of the tibia. The muscle is com-
pletely enclosed by the adductor magnus, which must be
split to expose it.

BiOODVESSELS AND NERVES OF THE THIGH.

*The femoral artery (a. femoralis) traverses the medial surface
of the thigh, beginning at the dorsal side of the inguinal ligament,
where it continues the external iliac artery. Immediately distal
to the inguinal ligament it gives off the deep artery of the thigh
(a. profunda femoris). The latter passes to the dorsal side of the
pectineus and adductor brevis muscles and is distributed to the
posterior proximal portion of limb, chiefly to the adductores
longus and magnus. A second. branch, the lateral circumflex
artery (a. circumflexa femoris lateralis), is given off from the
anterior wall. It passes between the second head of the rectus
femoris and vastus lateralis, on the one hand, and the two portions
of the vastus intermedius, on the other. It supplies various parts
of the quadriceps femoris group. A third branch of the femoral,
the superficial epigastric artery (a. epigastrica superficialis),
given off medially, and passing to the abdominal wall, has been
divided (p. 178). At the beginning of the distal third of the thigh,
a small branch, the a. genu suprema, passes over the medial
condyle of the femur to the ‘knee-joint, and at about the point
of origin of this vessel a large branch, the great saphenous
artery (a. saphena magna), arises from the posterior wall. It
passes across the medial surface of the distal end of the adductor
longus, and through the tendon of the gracilis, to the medial

THE POSTERIOR LIMB. 225,

surface of the leg. The femoral artery passes between the adductores
longus and magnus, continuing as the popliteal artery (a.
poplitea).

The hypogastric artery (p. 203) appears in the greater sciatic
notch, continuing thence as the sciatic artery (a. ischiadica).
The vessel passes backward to the dorsal surface of the superior
ramus of the ischium, where it divides into lateral caudal and
internal pudendal branches. Its smaller branches are dis-
tributed to the glutaei and biceps femoris muscles.

**The femoral vein (v. femoralis) traverses the medial surface
of the thigh in company with the femoral artery. It begins at the
proximal end of the lower third of the thigh as a continuation of the
popliteal vein (v. poplitea), which accompanies the corres-
ponding artery. Its tributaries comprise the great saphenous,
superficial epigastric, lateral circumflex, and profunda
femoris veins.

The sciatic vein (v. ischiadica) traverses the lateral surface of
the thigh near its posterior margin, lying at first between the biceps
and semimembranosus and afterwards on the posterior margin
of the former. At the dorsal border of the ischium, in front of the
ischial tuberosity, it receives the lateral caudal and internal
pudendal veins.

***The femoral nerve (n. femoralis) arises from the lumbo-
sacral plexus, chiefly from the fifth lumbar. Its position between
the psoas major and iliacus muscles has already been noted (p. 218).
Immediately beyond the inguinal ligament it divides into two
portions, one of which is distributed to the muscles of the anterior
side of the thigh, while the other, the greater saphenous nerve
(n. saphenus major), passes to the medial surface of the thigh and
leg in company first with the femoral artery and afterwards with
the great saphenous artery.

The sciatic nerve (n. ischiadicus), formed chiefly from the

seventh lumbar and first sacral nerves, appears laterally in the

greater sciatic notch. It passes backward beneath the piriformis
muscle, and then turns distad to the thigh, where it lies on the
lateral surfaces of the adductores magnus and longus. It dis-
tributes branches to the posterior musculature of the thigh. In
the proximal portion of the thigh it divides into two chief branches,
which are closely associated as far as the knee. The anterior

22 Ora ANATOMY OF THE RABBIT.

branch is the peroneal nerve (n. peronaeus), the posterior branch
the tibial nerve (n. tibialis). The lesser saphenous nerve (p. 231)
is a small branch given off from the tibial above the knee-joint.

For the origin of this and related nerves see p. 232.

The superior gluteal nerve (n. glutaeus superior) appears in
the greater sciatic notch, leaving the sciatic close to the inferior
posterior spine of the ilium. It passes between the glutaeus mini-
mus and the lateral surface of the ilium, ending in the tensor fasciae
latae. Its branches are distributed to the glutaei medius and
minimus and the piriformis muscles.

The inferior gluteal nerve (n. glutaeus inferior) perforates the
posterior portion of the piriformis, and is distributed to the glutaeus
maximus.

The posterior cutaneous nerve (n. cutaneus femoris posterior)
accompanies the hypogastric artery backward to the ischial
tuberosity, where it turns to the posterior margin of the thigh and
the medial surface of the sciatic vein, ending in branches to the
skin.

The pudendal nerve (n. pudendus) accompanies the sciatic
artery and afterwards the internal pudendal to the penis or clitoris.

In preparation for the muscular dissection of the leg, the
insertion tendons of the biceps femoris, tensor fasciae cruris,
gracilis, and semimembranosus muscles should be removed
from about the knee-joint. The adductor magnus may be
detached from the medial condyle of the femur, but the pop-
liteal vessels must be kept intact. The superficial blood-
vessels of the leg should be noted, since it is necessary to clear
them away in separating the muscles. They include, medially,
the great saphenous artery and vein, and laterally the branches
of the small saphenous artery to the insertion portions of the
muscles of the thigh, and its continuation on the postero-
lateral border of the leg; and the sciatic vein, together with its
continuation, the anterior tibial vein, and the accessory small
saphenous vein (p. 231). The tibial and peroneal nerves may
be cut, after first noting their position.

4. Muscles arising from the medial and lateral condyles of the
femur and from the proximal portions of the tibia and fibula,
including the tibial condyles; inserted on the foot. The group

THE POSTERIOR LIMB.

227

includes the typical extensors and flexors of the foot, together with
the peronaei muscles, which individually are extensors and flexors,
but collectively have the relation of lateral tractors (Fig. 77).
A. Extensor group. Muscles occupying an anterior position
on the leg and inserted on the dorsum of the foot.

(a) The extensor hallucis longus.

Origin: Middle portion

of the anteromedial surface of the tibia and from the medial

condyle behind the
tibial collateral lig-
ament. Insertion:
The tendon passes
around the medial
malleolus of the
tibia and beneath
the base of the sec-
ond (first functional)
metatarsal, continu-
ing to its dorsal
surface and uniting
with the first tendon
of the extensor digi-
torum longus.

The posterior tibial artery,
the continuation of the
great saphenous, and the
tibial nerve accompany the
tendon in the malleolar
groove.

(b) The tibialis an-
terior. ‘Origin:
Lateral condyle of

the tibia and corresponding surface of the tibial tuberosity.
sertion: Base of the second metatarsal.

Anterior

Medial

Lateral ect
Posterior

FIG. 77. Transverse section of the proximal
portion of the leg: a.s.m., great saphenous
artery; a.s.p., small saphenous artery; a.p.,;
a.t.a., anterior tibial artery; b.f., biceps femoris
(insertion) ; e.d.J., extensor digitorum longus;

extensor hallucis longus; f., fibula; f.d.l.,
flexor digitorum longus; g.l., and g.m., lateral and
medial heads of the gastrocnemius; gr., gracilis
(insertion tendon); n.s., greater saphenous nerve;
n.s.m., lesser saphenous nerve; n.t., tibial nerve;
pl., plantaris; s., soleus; t., tibia; t.a., tibialis
anterior; t.f.c., tensor fasciae cruris (insertion);
v.is., sciatic vein; v.s.m., great saphenous vein;
v.s.p., small saphenous vein; 1-4, the personaei (pri-
mus-quartus).

e.h.l.,

In-
The tendon passes beneath

the obliquely placed crural ligament of the lower portion of the leg.
The muscle should be divided and its head reflected.

The anterior tibial artery and vein, and the peroneal nerve appear

on the anterior surface of the tibia at’the lateral side of the crural liga-

ment.

228

(c)

B:

ANATOMY OF THE RABBIT.

The extensor digitorum longus. Origin: By a flattened
tendon from the lateral portion of the patellar surface of the
femur. This tendon passes through the capsule of the knee-
joint, and the fleshy portion of the muscle lies on the antero-
lateral surface of the tibia. Insertion: The distal tendon
passes beneath the crural ligament, then beneath the cruciate
ligament of the dorsum of the foot, dividing into four por-
tions for insertion on all the phalanges of the digits.
The muscle may be displaced by dividing the crural ligament.

The branches of the anteror tibial artery lie behind this muscle,
one ina medial position, in contact with the tibia, the other on the
peronaei muscles in company with the peroneal nerve.

Peronaeus group. These muscles arise from the lateral

surface of the leg, and are inserted on all surfaces at the lateral side
of the foot. The insertion tendons reach the foot from beneath the
lateral malleolus. The muscles can be separated after the tendons
are released from this position.

(a)

(b)

(c)

(d)

The peronaeus longus (p. primus). Origin: Lateral
condyle of the tibia and head of the fibula. Insertion: End
of the reduced first metatarsal. The tendon crosses the
plantar surface of the foot, passing around the distal end
of the cuboid bone.

The muscle should be divided.
The peronaeus brevis (p. secundus). Origin: Lateral
condyle of the tibia and corresponding surface of the shaft;
also the crural interosseous ligament joining the tibia and
fibula. Insertion: Tuberosity of the base of the fifth meta-
tarsal.
The peronaeus tertius. Origin: The head of the fibula
and the crural interosseous ligament, fused with the extensor
digitorum longus. Insertion: Head of the fifth metatarsal,
and distally, united with the tendon of the extensor digitorum
longus, on the phalanges of this digit.
The peronaeus quartus. Origin: The fibula and the inter-
osseous ligament, fused with the peronaeus brevis and with
the fiexor digitorum longus. Insertion: Head of the fourth
metatarsal.

C:

THE POSTERIOR LIMB. 229

Flexor group. The muscles arise from the medial and

lateral condyles of tibia and femur (the flexor digitorum sublimis
from the posterior surface of the tibia). They lie behind the axis
of the tibia, and are inserted both on the heel and on the plantar
surface of the foot.

(a)

(b)

(c)

(d)

The triceps surae comprises:

(1) The gastrocnemius. Origin: in two portions:

Caput laterale: Lateral condyles of tibia and femur and
related femoral sesamoid.

Caput mediale: Medial condyle of the femur and its sesa-
moid.

(2) The soleus: Origin: By astrong tendon from the
head of the fibula.

Insertion: Through the Achilles’ tendon (tendo calcaneus).
The latter passes over the posterior end of the tuber cal-
canei, and is attached to its ventral surface. The tendon is
covered by that of the plantaris muscle.

The small saphenous artery and vein lie at the posterior margin of

the lateral head of the gastrocnemius in company with the lesser sap-
henous nerve.
The plantaris. Origin: Lateral condyle of the femur and
associated sesamoid. Insertion: The tendon passes over
the heel to the plantar surface of the foot, and divides into
four parts for insertion on the second phalanges of the four
developed digits.

The two muscles should be divided.

The popliteus. Origin: Lateral condyle of the femur:
The tendon passes through the capsule of the knee-joint.
The muscle contains the tibial sesamoid. It crosses the
posterior surface of the tibia obliquely, and is inserted on the
proximal portion of its posteromedial angle.

The flexor digitorum longus. Origin: Lateral condyle
of the tibia and head of the fibula, extending to the posterior
surface of the interosseous ligament and associated portions
of the tibia and fibula. Insertion: The tendon passes
beneath the sustentaculum tali, reaching the plantar surface
of the foot, where it is partly covered by the plantaris ten-
don. It divides into four parts for insertion on the ungual
phalanges of the four developed digits.

230 ANATOMY OF THE RABBIT.

The tibial nerve lies on the medial surface of the head of the plan-
taris and afterwards on the medial surfaces of the popliteus and flexor
digitorum longus.

5. Muscles arising from the foot and inserted on the individual
digits.

(a) The lumbricales. Origin: Tendon of the flexor digitorum
longus. Insertion: Medial surfaces of the first phalanges
of the three lateral digits.

(b) The interossei (metatarsi). Origin: From the dorsal
portion of the tendon-sheath of the flexor digitorum longus.
Insertion: Heads of the four metatarsals.

VESSELS AND NERVES OF THE LEG AND Foot.

*The great saphenous artery passes distad on the medial
surface of the leg, and is continued as the posterior tibial artery
(a. tibialis posterior) around the medial malleolus to the plantar
surface of the foot. Above the ankle-joint it gives off the malleolar
artery (a. malleolaris) to the posterior surface of the distal end of
the tibiofibula.

The popliteal artery, the continuation of the femoral, passes
between the medial head of the gastrocnemius on the one hand and
the lateral head and the plantaris on the other, reaching the anterior
surface of the popliteus, and afterwards the anterior surfaces of the
tibia and fibula by passing between their proximal ends. It dis-
tributes branches to the muscles about the knee-joint, including a
branch to the distal portion of the vastus lateralis, which is given
off at about the point of origin of the small saphenous artery.
It then continues as the anterior tibial artery. The vessel
appears in front of the interosseous ligament of the leg and of the
peronaeus brevis, and continues to the dorsum of the foot after
passing beneath the crural ligament. <A large branch, given off in
the upper part of the leg also reaches the dorsum of the foot from a
more lateral position.

The small saphenous artery (a. saphena parva) appears on the
proximal portion of the posterolateral margin of the leg, running
along the border of the lateral head of the gastrocnemius in company
with the corresponding vein and the lesser saphenous nerve.

THE POSTERIOR LIMB. agi

**The great saphenous vein (v. saphena magna), a large
tributary of the femoral, accompanies the corresponding artery,
and the greater saphenous nerve, and is continued as the posterior
tibial vein (v. tibialis posterior), to the plantar surface of the foot.

The popliteal vein, the root of the femoral, accompanies the
corresponding artery in the popliteal fossa. It receives the small
saphenous vein (v. saphena parva) from the posterior margin
of the lateral head of the gastrocnemius.

The sciatic vein is continued on the lateral surface of the leg as
the anterior tibial vein (v. tibialis anterior). It receives the
accessory small saphenous vein (vy. saphena parva accessoria)
from the posterior surface, and is continued to the dorsum of the
foot passing to the fibular side of the crural ligament.

***The greater saphenous nerve, the posterior branch of the
femoral nerve, accompanies first the femoral artery and afterwards
the great saphenous artery, passing distad to the medial surface
of the leg.

The tibial nerve, the posterior division of the sciatic, passes
between the medial and lateral heads of the gastrocnemius to the
medial surface of the head of the plantaris. It traverses the leg,
lying on the medial surface first of the popliteus and afterwards
of the flexor digitorum longus, and passing beneath the medial
malleolus reaches the plantar surface of the foot. In the proximal
portion of the leg it distributes muscular branches to the flexor
group.

The lesser saphenous nerve (n. saphenus minor) accompanies
the small saphenous artery and vein on the posterior margin of the
lateral head of the gastrocnemius.

The peroneal nerve, the anterior division of the sciatic, passes
distad, lying at first between the insertion of the biceps and the
lateral head of the gastrocnemius, and thus appearing on the surface
after the removal of the former. It perforates the anterior portion
of the lateral head of the gastrocnemius and afterwards the fused
heads of the peronaeus tertius and flexor digitorum longus, travers-
ing the leg at first behind the peronaeus longus and then around its
medial margin to the front of its tendon. It passes to the fibular
side of the crural ligament and thence to the dorsum of the foot.
The nerve distributes branches to the tibialis anterior and extensor
digitorum longus.

232 ANATOMY OF THE RABBIT.

THE LUMBOSACRAL PLEXUS.

The structure of the lumbosacral plexus may be examined by
breaking away the ventral portion of the pelvis, or by dividing the
sacroiliac articulation in such a way that the two sides of the pelvis
may be pressed apart, the ventral or pelvic face of the sacrum being

[

D [o
eaE]

BI
wH
5

Fic: 78. Plan of -the
lumbosacral plexus: L,S,C,
lumbar, sacral, and caudal
vertebrae; nl, ns, corres-
ponding spinal nerves; nf,
femoral nerve; no, obtur-
ator nerve; nis, sciatic
nerve; np, pudendal nerve.

thus exposed. The posterior portions of the
psoas and iliacus muscles may be picked
away with the forceps, and the abductor
caudae anterior muscle (p. 270) may be
detached from its origin on the ischial spine.

The lumbosacral plexus (plexus lumbo-
sacralis) is formed by the ventral roots of the
four posterior lumbar and four sacral spinal
nerves (Fig. 78). - It is divisible into a
lumbar plexus (plexus lumbalis), from which
arises the femoral nerve, and a sacral plexus
(plexus sacralis), from which arises the sciatic
nerve.

The femoral nerve is formed from the
fifth, sixth, and seventh lumbar, especially
from the loop connecting the fifth and sixth
(ansa lumbalis 11). The obturator nerve
(n. obturatorius), which accompanies the
obturator artery, is formed from the fifth,

sixth and seventh lumbars but chiefly from the sixth, and is dis-
tributed to the obturatores, adductores, and gracilis muscles.

The sciatic nerve, together with the superior and inferior
gluteal nerves, arises chiefly from the loop connecting the last
lumbar and first sacral nerves (ansa lumbalis 111).

The internal pudendal nerve is formed from the loop connecting
the second and third sacral nerves (ansa sacralis 11), but chiefly

from the second.

THE ARTICULATIONS OF THE POSTERIOR LIMB.

The more perfect development and larger size of the joints of the
posterior limb make them much more favorable for examination
than the corresponding parts of the anterior limb.

THE POSTERIOR LIMB. 233

The muscular attachments should be removed from about the
articular capsules and the structures examined as follows:

A. The hip-joint (articulatio coxae) is an enarthrosis, formed
by the head of the femur with the parts of the ischium, ilium, and
the os acetabuli enclosing the acetabulum, together with the
articular capsule (capsula articularis) and accessory ligaments.

The articular capsule extends from the acetabular margin to the
proximal end of the neck of the femur. It is strongest on its dorsal
side, but is especially thickened at three points, forming the ischio-
capsular (dorsal), iliofemoral (anterior), and pubocapsular
(ventral) ligaments.

By dividing the capsule, the contents of the joint and the smooth
articular surfaces may be examined; also the attachment of the head
of the femur to the wall of the acetabular fossa through the round
ligament (lig. teres femoris). The glenoid lip (labrum glenoi-
dale) is the ring of fibrocartilage surrounding the ma gin of the
acetabulum and connecting with the articular capsule.

B. The knee-joint (articulatio genu) is a hinge-joint or
ginglymus with a slight spiral trend. It is formed by the articular
surfaces of the medial and lateral condyles of the femur and tibia,
with the associated articular capsule, ligaments, and inarticular
fibrocartilages (see section, Fig. 13).

The tibial collateral ligament (lig. collaterale tibiale) is a
stout band of connective tissue stretching fr»m the medial condyle
of the femur to the posteromedial angle of the medial condyle of the
tibia.

The fibular collateral ligament is a similar structure con-
necting the lateral condyle of the femur with the anterior surface
of the head of the fibula.

The sesamoid bones of the popliteal region have articular sur-
faces taking part in the formation of the joint. That on the medial
condyle of the femur is contained in the medial head of the gastro-
cnemius, that on the lateral condyle of this bone in the lateral head
of the gastrocnemius and the plantaris, and that on the lateral tibia
condyle in the popliteus.

The common tendon of the quadriceps femoris, the patella, and
the patellar ligament are associated with the capsule, forming the
anterior wall of the joint.

234 ANATOMY OF THE RABBIT.

Between the apposed surfaces of the condyles, in the interior of
the joint, there are two short, cruciate ligaments and two thin
plates of fibrocartilage, the medial and lateral menisci. The
anterior cruciate ligament (lig. cruciatum anterius) passes from
the lateral wall of the intercondyloid fossa of the femur to the
anterior end of the intercondyloid eminence of the tibia. The
posterior cruciate ligament passes from the medial wall of the
intercondyloid fossa of the femur to the posterior intercondyloid
fossa of the tibia. The medial meniscus (meniscus medialis),
a thin crescentic plate of fibro-cartilage, lies on the articular surface
of the medial condyle of the tibia, and is connected by ligament with
the anterior and posterior intercondyloid fossae of the bone. The
larger, lateral meniscus lies on the lateral condyle of the tibia,
and is attached by ligament anteriorly to the medial portion of the
articular surface, and posteriorly to the medial wall of the inter-
condyloid fossa of the femur. The tendon of origin of the extensor
digitorum communis traverses the anterior part of the joint on its
way from the patellar surface of the femur to the front of the leg.

The interosseous ligament of the leg (lig. interosseum cruris) forms an
almost complete sheet connecting the uncoalesced portions of tibis and fibula.

C. The ankle-joint (articulatio talocruralis) is a ginglymus with
a considerable amount of spiral torsion. The articulating surfaces
are chiefly formed by the tibia and talus, but also by the fibular side
of the tibiofibula and the calcaneus. On the tibial side the calean-
eotibial ligament (lig. calcaneotibiale) connects the medial
malleolus with the sustentaculum tali. On the fibular side the
calcaneofibular ligament (lig. calcaneofibulare) connects the
posterior portion of the groove for the peronaei muscles forwards
with the lateral surface of the calcaneus, and a second ligament
extends from the anterior margin of the groove backward to the
lateral surface of the calcaneus. The tibionavicular ligament
(lig. tibionaviculare) connects the anterior surface of the distal
end of the tibia with the dorsal surface of the navicular bone. The
joint contains in its interior a short strong ligament connecting the
medial side of the lateral malleolus with the lateral and ventral
surfaces of the trochlea tall.

THE HEAD AND NECK. 235

X. THE HEAD AND NECK.

This dissection includes the various structures of the region, with the excep-
tion of the cervical and occipital musculature and the central nervous system,
which are treated in the succeeding parts, and the special musculature of the ear,
which has been omitted.

To begin the dissection, the skin, which has already been
divided to the mandibular symphysis and partly reflected, should
be separated from the underlying platysma along‘the side of the
head, and reflected until the surface is clear to a point near the
dorsal median line of the skull. The insertion of the platysma on
the cheek, and that of its special portion, the depressor conchae
posterior, on the base of the ear, should be noted. A second band
of muscle, similar in its relation to the platsyma, arises from the
lateral border of the mandible, immediately in front of the masseter
muscle, and is inserted into the base of the ear. This is the de-
pressor conchae (parotideoauricularis) anterior.

In removing the skin of the upper and lower eyelids, two mus-
cles, the orbicularis oculi and the depressor palpebrae infer-
ioris, may be observed. The former is a somewhat circular band
of fibres enclosing both eyelids, the fibres lying directly on the skin,
and being concentrated at the anterior and posterior angles. The
latter is a very slender muscle arising from the zygomatic arch
and inserted into the skin of the lower eyelid. The corresponding
levator palpebrae superioris arises from the orbital wall, and is here
concealed by the projecting supraorbital process.

1. On the lateral surface of the head the following structures
may be made out without further dissection:

(a) The masseter muscle. Origin: The zygomatic arch;
tendinous from its anterior angle. Insertion: Lateral sur-
face of the angle of the mandible.

(b) The parotid gland (gl. parotis), a diffuse, white or brownish
gland lying immediately behind the angle of the mandible.
Its duct (d. parotideus) crosses the masseter and perforating
the mucous membrane of the cheek opens into the oral
cavity.

(c) The chief part of the seventh cranial or facial nerve (n.
facialis) appears in the anterior portion of the parotid gland,

236

(d)

(e)

ANATOMY OF THE RABBIT.

its branches crossing the masseter. They are distributed as
motor nerves to the cutanecus muscles of the face, including
the platysma.

The external maxillary artery (a. maxillaris externa) ap-
pears at the ventral border of the mandible immediately in
front of the masseter. It passes upward to the anterior
margin of the eye, ending in the angular artery (a. angul-
aris). Its chief branches to the anterior portion of the face
are: (1) the submental artery (a. submentalis) to the chin;
(2) the inferior labial artery (a. labialis inferior) to the
lower lip; and (3) the superior labial artery to the
upper lip.

A small vessel, the transverse facial artery, crosses the cheek,

running along the ventral border of the zygomatic arch. It is a branch
of the superficial temporal (p. 242).
The anterior facial vein (v. facialis anterior) accompanies
the external maxillary artery. It begins in front of the eye
as the angular vein, and receives as tributaries the superior
and inferior labial veins.

2. Dissection of the facial muscles. These muscles arise from the
facial portion of the skull, and are inserted into the skin about the
upper and lower lips.

(a)
(b)

(c)

The quadratus labii superioris. Origin: Dorsal portion
of the maxillary fossa. Insertion: Skin of the upper lip.*
The subcutaneus faciei. Origin: Lateral border of the
premaxilla, its frontal process, and the supraorbital process
of the frontal. Insertion: Skin of the dorsal surface of the
nose.

The muscle is closely associated in front of the eye witha conspicuous
band of fibres arising from the anterodorsal margin of the zygomatic
arch. The latter muscle is probably a corrugator supercilii, operating
in conjunction with the orbicularis in firmly closing the eye.

The zygomaticus minor. Origin: Anterior end of the
zygomatic arch. Insertion: Skin of the angle of the mouth.
The levator alae nasi. Origin: Maxillary fossa. Insertion:
Lateral cartilage of the nose. The muscle is very slender,
and is inserted by a long tendon which underlies the in-

*The levator alae nasi, and zygomaticus minor muscles may be considered to
be subdivisions of this muscle.

THE HEAD AND NECK. 2277.

(e) The buccinator is a broad stout band of fibres enclosing the
cheek and attached to the alveolar borders of the upper jaw
and mandible.

({) The caninus. Origin: Lateral border of the upper jaw.
Insertion: Hairy portion of the mucous membrane of the
mouth. The muscle is very broad, but short and extremely
thin.

(g) The quadratus labii inferioris. Origin: Ventral border
of the mandible. Insertion: Skin of the lower lip.

(h) The mentalis. The muscle surrounds the anterior portion
of the mandible behind the incisor teeth. It is attached
externally to the skin of the lower lip through the insertion
portion of the quadratus labii inferioris, which largely
overlies it.

Because of the great size and mobility of the ears, the cutaneous
auricular muscles, comprising some twenty different members, are
especially well developed. These muscles are not individually described,
but their development should be noted in contrast to the vestigial
character of the ear muscles in man.

3. Dissection on the ventral surface of the neck to free the
external jugular vein and its tributaries. The cervical fascia and a
portion of the parotid gland must be removed.

The external jugular vein (v. jugularis externa) is formed
behind the angle of the mandible by the union of the anterior and
posterior facial veins. It passes backward in a superficial position
to the superior thoracic aperture. Its connections in the lower part
of the neck comprise the transverse scapular vein (v. transversa
scapulae) of the shoulder and its union with the vein of the other
side by the transverse jugular vein (v. jugularis transversa) (Fig.
82).

The posterior facial vein (v. facialis posterior) is formed below
the base of the ear by the union of the superficial and deep
temporal veins of the temporal region, the inferior ophthalmic
vein, from the posterior portion of the orbit, and the anterior
auricular vein from the ear. It passes downward in the substance
of the parotid gland, and in its first portion is covered by the
root of the facial nerve. Immediately below the latter it
receives the posterior auricular vein from the ear and back of
the head.

238 ANATOMY OF THE RABBIT.

In addition to the tributaries described above, the anterior
facial vein receives from beneath the anterior margin of the
masseter the deep facial vein (v. facialis profunda). The latter
arises in the lower anterior portion of the orbit, and passes down-
ward beneath the masseter muscle. The anterior facial vein
receives at the ventral border of the mandible the internal max-
illary vein (v. maxillaris interna). The latter also begins in the
orbit, where it is connected with the deep facial. At the medial
surface of the mandible it receives the inferior alveolar vein—to
be seen at a later stage—from the interior of the mandible.

The external jugular vein may be divided and turned forward together with
the parotid gland.

4. Examination of the more superficial structures of the ventral
surface of the head and neck.

(a) The submaxillary gland (gl. submaxillaris), one of the
salivary series, a somewhat compact rounded or oval gland
lying at the medial side of the extreme ventral portion of the
angle of the mandible. Its whitish-colored duct (d. sub-
maxillaris) may be seen running upward and slightly for-
ward to enter the mouth. It crosses the lateral surface of the
digastric muscle but is medial to the external maxillary
artery.

(b) The angle of the mandible is covered by two muscles of
mastication, the masseter lying on the lateral surface, and
the pterygoideus internus on the medial surface.

(c) The digastricus. Only its insertion portion is visible. It
passes forward along the medial surface of the mandible, to
the anterior portion of which it is attached.

(d) The mylohyoideus is a transverse sheet of muscle arising
from the medial surface of the mandible on either side and
inserted on the hyoid bone.

(e) The sternomastoideus. Origin: In common with that
of the opposite side, from the manubrium sterni. Insertion:
Mastoid process of the skull.

(f) The sternohyoideus. Origin: In common with that of the
opposite side, from the dorsal surface of the manubrium and
anterior portion of the body of the sternum, extending to the
third costal articulation. Insertion: Greater cornu of the
hyoid.

Tue HEAD AND NECK. 239

The two muscles are closely associated in the middle line. They

should be separated from one another and divided.

(g)

(h)
(i)
(3)

(k)

(I)

(m)

(n)

(0)

The sternothyreoideus. Origin: In common with the
sternohyoideus. Insertion: Lateral plate of the thyreoid
cartilage of the larynx. The muscle forms a thin band
lying on the side of the trachea. It is continued from the
thyreoid cartilage to the greater cornu of the hyoid as the
thyreohyoideus.

The trachea occupies a median position, and is supported by
cartilaginous tracheal rings.

The thyreoid cartilage of the larynx; a saddle-shaped carti-
lage, composed of right and left thyreoid plates (Fig. 65).
The cricoid cartilage, a thick annular cartilage situated
between the thyreoid cartilage and the first tracheal ring.
It is connected ventrally with the thyreoid cartilage by the
cricothyreoideus muscle.

The deep cervical lymph gland (lymphoglandula cervicalis
profunda) is a large elongated reddish-colored gland in the
upper portion of the neck, opposite the thyreoid cartilage.
The thyreoid gland (gl. thyreoidea) lies on the ventral
surface of the trachea behind the cricoid cartilage. It is
composed of right and left portions connected across the
middle line by a thin median portion, the isthmus.

The common carotid artery (a. carotis communis) passes
forward from the superior thoracic aperture along the side
of the trachea. Its branches on the neck include the
superior thyreoid artery (a. thyreoidea superior), to the
thyrecid gland, and the (superior) laryngeal artery
(a. larnygea). The latter arises at the level of the thyreoid
plate, passing to the larnyx and to the sternohyoid sterno-
thryeoid muscles.

The internal jugular vein (v. jugularis interna) lies to the
lateral side of the common carotid artery, traversing the
neck from the jugular foramen of the skull to the superior
thoracic aperture.

The tenth cranial or vagus nerve (n. vagus) is the largest
of four nerves accompanying the carotid artery. It lies to
the lateral side of the common carotid, between the latter

240

(p)

(q)

(r)

(s)

Se

ANATOMY OF THE RABBIT.

and the internal jugular vein. It gives off the n. laryngeus
superior to the larynx, the latter passing to the dorsal side
of the common carotid artery.

The ramus descendens of the twelfth cranial or hypo-
glossal nerve crosses the root of the vagus from a lateral
to a medial position. It passes backward on the ventral
surface of the artery, and is chiefly distinguishable by its
branches to the sternohyoideus and related muscles.

The cervical portion of the sympathetic trunk lies on the
dorsal surface of the common carotid, and is slightly medial
in relation to the vagus.

The ramus cardiacus of the vagus (n. depressor) lies on the
dorsal surface of the common carotid on the medial side of
the sympathetic trunk. It arises at the level of the posterior
margin of the thyreoid cartilage.

Occurring inthe rabbit as a separate nerve, the depressor is
important experimentally. Stimulation of the proximal end in the
living animal produces fall of blood pressure and retardation of the heart
beat. The former is due toa reflex action on the bloodvessels (cf. p. 62),
while the latter depends upon reflex stimulation of the vagus, since
slowing of the heart does not take place if the vagi are also divided.
The third and fourth cervical nerves may be traced
from their origin in the intervertebral foramina to the
musculature of the neck. They encircle the basioclavicularis
muscle, under cover of the sternomastoideus and
cleidomastoideus.

Dissection of the muscles of mastication and related structures

of the mandible.

(a)

(b)

The masseter muscle. Origin: The entire surface of the
zygomatic arch; tendinous from its anterior angle (spina
masseterica). Insertion: Lateral surface of the angle of the
mandible (1, a).

The orbital structures should be freed from the zygomatic arch by
passing a knife along its dorsal margin. The zygomatic arch may then
be divided anteriorly and posteriorly and removed, together with the
whole insertion of the masseter muscle.

The temporalis is a slender, somewhat triangular muscle
arising from the reduced temporal fossa (sulcus temporalis)
of the skull and inserted by a long stout tendon on the

(c)

(d)

(e)

(f)

THE HEAD AND NECK. 241

medial side of the reduced coronoid process. The muscle
may be exposed by dividing the temporal portion of the
posterior supraorbital ligament which holds its tendon in
place; then divided.

On account of the narrowness of the space lying between the two
limbs of the mandible, and the great depth of its angle, it is necessary,
in order to expose the surface for the deep dissection of the ventral portion
of the head and neck, to remove one-half of the mandible entirely. Hence
the following order:

Divide the mandibular symphysis, and pass a knife along the
medial surface of the bone to,be removed. The tip of the
knife must be kept close to the bone, so that the underlying
soft parts, excepc¢ for being divided, will be kept uninjured.
The medial surface of the mandible should be clean when
removed. The bone may be turned laterad and detached
from the skull at the articulation.

The structures appearing on the cut surface include the
insertion of the digastricus and the margin of the mylohy-
oideus; also:

The pterygoideus internus muscle. Origin: Pterygoid
process of the skull. Insertion: Ventral portion of the
medial surface of the angle.

The pterygoideus externus. Origin: Lateral plate of the
pterygoid process. Insertion: Dorsal portion of the medial
surface of the angle. Both muscles are strongly developed.
The inferior alveolar artery (a. alveolaris inferior) lies be-
tween the two pterygoidei. Its cut end marks the point of
entrance into the mandible through the mandibular foramen.
The corresponding inferior alveolar vein leaves the man-
dible at this point.

The inferior alveolar nerve (n. alveolaris inferior) accom-
panies the inferior alveolar artery to the mandible. The con-
tinuation of the nerve is the mental nerve. It appears at
the mental foramen, and passes to the lower lip.

The origin of the inferior alveolar nerve may be traced. It
arises from the mandibular nerve (n. mandibularis), the
third division of the fifth cranial or trigeminal nerve
(n. trigeminus), which. also gives off anteriorly the stout
lingual nerve to the tongue and posteriorly the slender

242

ANATOMY OF THE RABBIT.

mylohyoid nerve. These structures, together with the
inferior alveolar artery may be freed from their loose
connections with the pterygoidei, so that they may be left
in place for further study. The two pterygoidei may then
be detached at their poimt of origin from the skull and
removed.

6. The branches of the common carotid may be traced in the
anterior portion of the ventral surface of the neck as follows:

(a)

(b)

(c)

(d)

(e)

(f)

The internal carotid (a. carotis interna) is a small vessel
given off from the dorsal wall (pp. 133, 278). The trunk
then passes forward as the external carotid (a. carotis
externa).

The occipital artery (a. occipitalis) passes from the dorsal
wall to the posterior portion of the head.

The stylohyoideus major, a slender muscle arising with the
digastricus from the stylohyoid ligament and inserted on the greater
cornu of the hyoid, should be divided. The tendon of the digastricus
may be reflected.

The lingual artery (a. lingualis) arises from the ventral
wall and passes forward into the tongue.

The hypoglossal nerve crosses the ventral surface of the artery
and should be kept intact.

The external maxillary artery (a. maxillaris externa) is
given off immediately in front of the lingual artery, some-
times in common with it. It passes forward on the medial
surface of the ventral border of the mandible (medial to the
digastricus), giving branches to the submaxillary gland and
to the muscles of mastication. The vessel has been divided
at the point where it passes around the ventral border of the
mandible to the lateral surface of the face. ;
The internal maxillary artery (a. maxillaris interna), one
of the two terminal branches of the external carotid, passes
in the direction of the orbit (p. 251), giving off the inferior
alveolar artery to the mandible.

The superficial temporal artery (a. temporalis super-
ficialis), the second terminal branch, passes dorsad to the
temporal region, supplying the latter and the base of the ear.
The transverse facial artery, which crosses the cheek, is
an anterior branch of this vessel.

(b)

(c)

(d)
(e)

(f)

THE HEAD AND NECK. 243

Dissection of the tongue and hyoid:

The mylohyoideus should be reflected. Note the position of the
lingual nerve.
The stylohyoideus major muscle. Origin: Jugular process
of the occipital bone. Insertion: Tip of the greater cornu
of the hyoid. The muscle has been divided.

The superficial temporal and internal maxillary arteries should be
divided.
The styloglossus. Origin: Jugular process. Insertion:
The muscle passes downward and forward, expanding at the
base of the tongue into a broad sheet, the fibres of which
extend to its anterior tip.

The muscle should be carefully separated from two others on its
dorsomedial side and divided.
The stylohyoideus minor. Origin: Jugular process. In-
sertion: Lesser cornu of the hyoid. A slender muscle hav-
ing about the same direction, but ending on the more dorsal
part of the hyoid apparatus.

The remaining muscle is the stylopharyngeus, a thin delicate
muscle, the insertion of which is on the lateral wall of the pharynx.

The geniohyoideus. Origin: Mandibular symphysis. In-
sertion: Anterior surface of the body of the hyoid: unpaired.
The genioglossus. Origin: Medial surface of the man-
dible immediately behind the symphysis. The fibres pass
upward and slightly backward into the substance of the
tongue.

The hyoglossus. Origin: The body of the hyoid and the
greater and lesser cornua by more or less separate heads.
The muscle passes into the base of the tongue, enclosed on
either side by the styloglossi.

(¢g) The lingualis, or intrinsic muscle of the tongue consists of a

(h)

mass of fibres with no skeletal attachments.
The lingual nerve (n. lingualis), one of the chief branches
of the mandibular, passes forward and downward to the
side of the tongue and enters its substance immediately
below the ventral border of the styloglossus.

The lingual is the sensory nerve of the tongue. Near its point of
origin the lingual nerve receives the chorda tympani (p. 254).

244

(i)

(1)

Fic. 79. Plan of

ANATOMY OF THE RABBIT.

The twelfth cranial or hypoglossal nerve (n. hypo-
glossus) enters the base of the tongue. It lies on the lateral
side of the external carotid artery and on the medial side of
the stylohyoideus major. It is distributed as a motor nerve
to the lingual muscles.
The ramus lingualis of the ninth cranial, or glosso-
pharyngeal nerve (n. glossopharyngeus) enters the base of
the tongue at a point dorsal to the hypoglossus and between
the stylohyoideus minor and the stylo-
pharyngeus.
the tongue.

8. Dissection of the extra-cranial roots. of
the ninth to twelfth nerves (Fig. 79).

These nerves, which for the most part have
already been exposed, may be traced to their
origin in the jugular and hypoglossal foramina.
The tympanic bulla should be cleared and
the tendons of origin of tongue muscles
removed from the jugular process.

(a) The ninth (glossopharyngeal) nerve

It isa gustatory nerve of

the

extra-cranial roots of the
IX-XII cranial nerves and
sympathetic trunk; ventral

surface, right side, the
sympathetic and depressor
nerve shown as displaced
from the dorsal surface of
the’ ‘artery.5-9) (LO; br, 12;
glossopharyngeal, vagus,
spinal accessory, and hypo-
glossal nerves; ac, carotid
artery; c, cervical root of
ramus descendens XII;
gn, ganglion nodosum vagi;

(b)

is farthest forward. Its two main
branches are the ramus lingualis to
the tongue, and the ramus pharyn-
geus, the latter entering the lateral
wall of the pharynx.

The tenth (vagus) nerve bears an
elongated ganglionic enlargement, the

ls, superior laryngeal;
depressor nerve; rd, ramus
descendens hypoglossi;_ s,
sympathetic.

ee plexus ganglioformis (ganglion

nodosum). It lies immediately below
the jugular foramen.

The superior laryngeal nerve and the ramus cardiacus
(depressor nerve) are given off at the level of the origin of the
internal carotid artery.

The eleventh cranial, or spinal accessory nerve (n.
accessorius) is dorsal to the vagus. The nerve passes dorsad
to the medial side of the mastoid attachments of the sterno-
hyoideus and cleidomastoideus muscles, giving branches
to the latter, and then passes backward to the ventral surface
of the trapezius to which it is distributed.

THE HEAD AND NECK. 245

(d) The twelfth (hypoglossal) nerve arises behind the foregoing
nerves, since it comes from the hypoglossal foramina of the
occipital. It crosses their roots, forming a broad curve
on the lateral surface of the root of the external carotid
artery and enters the base of the tongue. The ramus
descendens is given off at about the point where it crosses
the artery. It has a slender root from the third cervical
nerve.

(e) The cervical portion of the sympathetic trunk begins
in the superior cervical ganglion (g. cervicale superius).
It lies to the medial side of the vagus ganglion and of the
internal carotid artery. The nerves proceeding from the
ganglion accompany the branches of the external and
internal carotid arteries to the head.

(i) The oral cavity and pharynx.

he glossopharyngeal nerve and the superior laryngeal nerve

and artery may be divided, and the external carotid artery with
the associated nerves separated from the oesophagus and trachea.
The latter may be displaced downward to a slight extent by
dividing the loose connective tissue along the ventral surface of the
vertebral column. An incision extending from the oral cavity
backward into the oesophagus will expose the internal surface of
this portion of the digestive tube sufficiently to make out its
features. The incision divides the constrictor pharyngis
muscle, a broad band of muscle fibres enclosing the posterior
portion of the pharynx.

For the general relations of the oral cavity see p. 80 and Fig. 41.

The divisions are:

(a) The oral cavity (cavum oris), divisible into the oral cavity
proper, and the vestibulum oris, the latter laying between
the alveolar processes and teeth on the one hand and the
lips on the other.

(b) The pharynx: its oral portion (pars oralis) continues the
oral cavity, and connects it with the oesophagus. Its dorsal,
also anterior, nasal portion (pars nasalis), or nasopharynx,
lies above the soft palate, and receives the posterior aperture
of the nose. Its ventral and posterior laryngeal portion
(pars laryngea), not well-defined, contains the aperture of
the larynx, the aditus laryngis.

246

ANATOMY OF THE RABBIT.

In the oral cavity:

(a)

(b)

(c)

(d)

The hard palate (palatum durum) forms the anterior
portion of the roof; its mucous membrane is thrown into a
series of transverse ridges.

The soft palate (palatum molle) is the thin, narrow,
posterior, membranous portion of the roof. It is very long
in the rabbit, extending from the bony palatine bridge
backward to a point above the laryngeal aperture, where it
ends with a concave free margin.

The nasopalatine or incisive ducts (dd. nasopalatini)
open by minute apertures immediately behind the small
incisors. They connect the anterior portion of the nasal
cavity with the mouth.

The tongue (lingua) projects upward and forward from its
basal attachments on the hyoid into the floor of the mouth.
Its connection with the latter is extended forward in the
middle line by a vertical membranous fold, the frenulum
linguae. Its dorsal surface is divided into a posterior
smooth and hard portion, and an anterior softer and rougher
portion, occupied by fine low elevations, the fungiform
papillae (papillae fungiformes). At the posterior end of the
smooth portion there is on either side a miaute spherical
elevation, set low into the mucous membrane, the vallate
papilla (papilla vallata), and in a more lateral and anterior
position an oval area, the papilla foliata, the surface of
which is marked by fine parallel ridges. Both vallate
and foliate papillae are occupied by microscopic taste-buds.

In the pharynx:

(a)

(b)

The tonsil (tonsilla) appears as a rounded mass of lymph
follicles lying on the anterior wall of a deep lateral depres-
sion, the tonsillar sinus (sinus tonsillaris). The vertical
slit-like aperture of the sinus is bounded by low anterior
and posterior folds.

The epiglottis, a valve-like fold guarding the entrance
to the larynx, projects upward from the floor into the
pharyngeal cavity.

By removing the posterior portion of the soft palate, the
connection of the nasopharynx with the nasal fossae will be

THE HEAD AND NECK. 247

exposed; also on the lateral wall the pharyngeal aperture
of the auditory tube (ostium pharyngeum tubae).

10) Examination of the larynx.

By cutting around the base of the tongue on the opposite side
of the body, the whole structure, together with the hyoid, larynx,
and a portion of the trachea back to about the end of the thyreoid
gland may be removed. This affords a good opportunity of re-
dissecting on the opposite side from the medial surface of the
mandible outward, also of clearing and examining the hyoid appar-
atus, which is not usually available with the prepared skeleton
(Fig. 65). The laryngeal cartilages should be cleared externally
and the parts made out as follows:

(a)

(b)

(c)

(d)

The thyreoid cartilage (cartilago thyreoidea) forms
the largest portion of the structure. It is an unpaired saddle-
shaped cartilage, described as consisting of right and left
laminae. Its anterodorsal angle projects forward as the
cornu superior; connected by ligament with the greater
cornu of the hyoid. The corresponding posterodorsal angle,
the cornu inferior, overlies the dorsolateral portion of the
cricoid cartilage. The anterior dorsal portion of each plate
bears a small thyreoid foramen (foramen thyreoideum) for
the entrance of the superior laryngeal nerve.

The cricoid cartilage (cartilago cricoidea) is an annular
cartilage, partly enclosed by the posterior portions of the
thyreoid laminae and surrounding the first tracheal ring.
Its ventral portion, the arch of the cricoid cartilage, falls
some distance behind the thyreoid cartilage, the intervening
space being largely occupied by the cricothyreoidei
muscles. Its dorsal portion, the lamina of the cricoid,
forms a greatly expanded plate serving for the support of the
arytenoid cartilages.

The paired arytenoid cartilages (cartilagines arytenoi-
deae) lie one on either side of the anterior tip of the cricoid
plate.

The corniculate cartilages (cartilagines corniculatae) are
slender curved terminal cartilages supported by the ary-
tenoids.

248
(e)
(f)

ANATOMY OF THE RABBIT.

The epiglottic cartilage (cartilago epiglottica) is a trian
gular plate supporting the epiglottis.

The rudimentary vocal folds (plicae vocales) may be seen as
vertical folds of the internal surface of the larynx, especially
prominent when the thyreoid cartilage is bent downward on
tho cricoid. Each fold forms the posterior boundary of a
shallow pouch, the laryngeal ventricle (veniriculus
laryngis).

In addition to the criothryreoidei, the laryngeal cartilages are
connected by several small muscles, including the cricoarytenoidei
posterior and lateralis, the thryreoarytenoideus and the arytenoideus
transversus, the last named being an unpaired muscle connecting the
arytenoid cartilages.

The eye and related structures of the orbital cavity.

The eyeball should be carefully separated from the bony orbital

rim.

The first portion of the nasolacrimal duct (d. nasolacrimalis),

passing from its aperture on the medial surface of the lower eyelid
to the lacrimal bone, will be divided. The muscles and glands of the
orbit may be made out as follows:

(a)

(b)

(c)
(d)

(e)

The levator palpebrae superioris. Origin: Wall of
the orbit above the optic foramen. Insertion: Upper
eyelid.

This thin sheet of muscle should be separated from the underlying
rectus superior of the eyeball.
The obliquus superior. Origin: Anterior margin of the
optic foramen. The muscle passes upward on the wall of the
orbit, then beneath a fibrous cord, the trochlea, which bridges
a small portion of the orbital wall and changes the course of
the tendon by a considerable angle. Insertion: Antero-
dorsal portion of the eyeball.
The obliquus inferior. Origin: Lacrimal bone. In-
sertion: Posteroventral portion of the eyeball.
The four recti muscles, superior, inferior, medialis,
and literalis, arise from the boundary of the optic foramen,
and are inserted respectively on the dorsal, ventral, anterior,
and posterior portions of the periphery of the eyeball.
The retractor oculi (best seen after the removal of the eye)
arises in common with the foregoing recti muscles, and is

(f)

(g)

THE HEAD AND NECK. 249
inserted on the medial portion of the eyeball around the
optic nerve. It consists of four distinct parts.

The Harderian gland (gl. Harderiana) is a large compact
gland lying in the anterior portion of the orbit. It is com-
posed of two portions, one of which is greyish red, the other
white. The duct opens on the inner surface of che third
eyelid... ©

The lacrimal gland (gl. lacrimalis) is a much smaller, also
darker, structure lying close to the skull in the temporal
angle of the orbit. It communicates by several fine ducts
with the inner surface of the upper eyelid.

The infraorbital gland (gl.
infraorbitalis) is a diffuse white
or yellowish gland lying in the
anteroventral angle of the orbit
immediately medial to the zygo-
matic arch. The gland is one
of the salivary series, its duct
passing downward and opening
through the mucous membrane
of the cheek into the cavity of
the mouth.

To examine the structure
of the eye, the muscles of
the eyeball should be divid-
ed at their insertions, and
the whole structure re-
moved. The second cra-
nial or optic nerve (n.

Diagram of the parts of the eye in
C.a., anterior chamber; c.c.,
ch., chorioidea; co., cornea; C.p.,
c.r., ciliary portion of the
retina; c.v., vitreous body; d.h., Harderian duct;
d.l.; position of the lacrimal ducts; d.n., nasola-

Fic. 80.
vertical section:
ciliary body;
posterior chamber;

crimal duct; i., iris; 1., lens; n.o., optic nerve;
o.r., optic portion of the retina; p.i., lower
eyelid; p.s., upper eyelid; p.t., third eyelid;
r.b., retractor oculi; r.i., rectus inferior; r.s.,
rectus superior; sc., sclera; Z., Suspensory
zonular fibres of the lens.

opticus) is divided; also the ophthalmic artery, a small branch
of the internal carotid which accompanies the nerve outward from

the optic foramen to the eyeball.

The eye may be divided by a circular incision into medial

being again divided vertically.
under water.

(a)

and lateral hemispheres, the lateral hemisphere, containing the lens,

The parts should: %e examined
The chief structures (Fig. 80) comprise:

The fibrous tunic (tunica fibrosa oculi), the strong peri-
pheral coat enclosing the whole structure. It is divisible
into a medial portion, the sclera, or sclerotic coat, a thick

250

(b)

(c)

(d)

(e)

(f)

ANATOMY OF THE RABBIT.

white investment of fibrous connective tissue enclosing the
greater part of the eyeball, and a smaller transparent lateral
portion, the cornea, covering the exposed surface.

The vascular tunic (tunica vasculosa oculi), the thin middle
coat of the eye; pigmented, except in albino animals. It is
divisible into: (1) a general portion, the chorioidea, lying on
the inner surface of the sclera; (2) a muscular portion, the
ciliary body (corpus ciliare), forming an annular ridge about
the periphery of the lens; and (3) the iris, the latter forming
a circular fold suspended about the periphery of the lens and
on its outer surface.

The retina, the innermost layer of the eye, forms a thin soft
membrane covering the inner surface of the chorioidea. It
is divisible in a larger optic portion, the sensory part of the
eye, and a smaller ciliary portion, lying about the periphery
of the lens and distinguishable by the radiate markings of
its surface, the latter formed by the projecting ridges of the
ciliary body.

The transparent lens of the eye is suspended by fine fila-
ments, the zonular fibres, reflected from the margin of the
ciliary body.

The vitreous body (corpus vitreum), a transparent mass, of
gelatinous consistence, occupies the large space enclosed by
the lens and the retina.

The space enclosed between the surface of the lens and the
cornea is divisible into a larger portion, the anterior
chamber (camera oculi anterior), lying outside of the iris,
and a smaller portion, the posterior chamber (camera
oculi posterior), lying between the iris and the lens. The
central space enclosed by the free margin of the iris is the

pupil (pupilla).

12. Following the removal of the eye, the bloodvessels and
nerves of the orbit may be freed from the remaining portions of the
eye muscles and examined. In order to see their connections in the
anterior angle of the orbit, it is necessary to break away the anterior
root of the zygomatic arch, and also the bony ridge which lodges the
alveoli of the posterior cheek-teeth.

(a)

(c)

THE HEAD AND NECK. 251

The internal maxillary artery enters the orbit through the
anterior sphenoidal foramen in the root of the lateral lamina
of the pterygoid process. At the posterior ventral angle of
the orbit it gives off the inferior ophthalmic artery
(a. ophthalmica inferior). This vessel passes upward and
forward on the medial wall of the orbit, giving branches to
the eye muscles. It divides into two branches, the frontal
artery, which leaves the orbit through the anterior foramen
of the supraorbital process, and the lacrimal artery, which
passes through the corresponding posterior foramen. The
ethmoidal artery, a small branch of the frontal, passes
through the minute ethmoidal foramen of the orbital portion
of the frontal into the nasal cavity.

The internal maxillary artery passes forward along the
ventral boundary of the orbit, and at the opening of the
infraorbital canal gives off a branch, the pterygopalatine
artery, continuing as the infraorbital artery. A small
branch, the superior dental artery (a. dentalis superior) is
given off laterally to the alveoli of the upper teeth.

The infraorbital artery (a. infraorbitalis) passes through
the infraorbital canal to the face.

The pterygopalatine artery (a. pterygopalatina) divides
almost immediately into the anterior palatine artery,
which traverses the pterygopalatine canal to the mucous
membrane of the hard palate, and the sphenopalatine
artery, which enters the nasal cavity by the sphenopalatine
foramen.

The divisions of the third cranial, or oculomotor nerve,
supply the eye muscles, with the exception of the obliquus
superior, rectus lateralis, and retractor oculi.

This nerve eaters the orbit from the superior orbital fissure in
company with certain parts of the trigeminal (e,f). The small nerves
passing through the middle and posterior sphenoidal formina of the
pterygoid process are the pterygobuccinator and masseterico-
temporal nerves, branches of the mandibular, passing to the muscles
of mastication.

The fourth cranial, or trochlear nerve (n. trochlearis), is
distributed to the obliquus superior muscle.

252 ANATOMY OF THE RABBIT.

(d) The sixth cranial, or abducent nerve (n. abducens), is
distributed to the rectus lateralis, and to the retractor oculi.

(e) The ophthalmic nerve (n. ophthalmicus), the first division
of the fifth cranial, or trigeminal nerve (n. trigeminus),
accompanies the inferior ophthalmic artery on the medial
wall of the orbit. It gives off a lacrimal nerve, which
passes upward through the posterior foramen of the supra-
orbital process, and then passing forward a short distance
divides in two parts. One of these, the frontal nerve,
leaves the orbit through the anterior supraorbital foramen,
while the other, the nasociliary nerve, is partly distributed
to the anterior portion of the orbit, and is connected with the
minute ciliary ganglion lying on the optic nerve, forming its
sensory root. Its chief portion leaves the orbit through
the ethmoidal foramen of the orbital portion of the frontal
bone as the ethmoidal nerve.

The lacrimal, frontal and nasociliary nerves appear as
separate structures in the orbit, their origin being deep.

(f) The branches of the maxillary nerve (n. maxillaris), the
second division of the trigeminus, traverse the ventral por-
tion of the orbit passing forward in company with the
internal maxillary artery. They include the spheno-
palatine nerve (n. sphenopalatinus) and the infraorbital
nerve (n. infraorbitalis). The latter gives off superior
alveolar branches co the upper teeth, passing forward
through the infraorbital groove and foramen to the face.

The connections of the sphenopalatine nerve may be examined
by first dividing both nerves at the posterior angle of the orbit; then
separating the slender sphenopalatine nerve from the ventral sur-
face of the cord and turning the principal, infraorbital portion
forward. A third nerve, the nerve of the pterygoid canal, should
remain intact on the orbital wall. If the infraorbital nerve alone
is divided, the sphenopalatine nerve will be found on the surface
of the bone below the nerve of the pterygoid canal, from which
it may be distinguished by its lighter coloration.

The sphenopalatine nerve is continued forward as the anterior

(major) palatine nerve, which passes through the pterygopalatine
canal to the posterior portion of the hard palate, but is also con-

THE HEAD AND NECK. 253

nected with the sphenopalatine ganglion. Nasal rami pass
to the mucous membrane of the nose, and the nasopalatine nerve
enters the nasal region, traversing the surface of the septum and
reaching the anterior portion of the palate through the incisive

foramina.

The nerve of the pterygoid canal (n. canalis pterygoidei) is a
slender cord which passes backward along the orbital wall from the

posterodorsal angle of the gang-
lion. It lies on the medial side of
the sphenopalatine and infraorbital
nerves and on the lateral surface
of the palatine bone. Posteriorly
it enters the groove representing
the pterygoid canal.

This nerve is composed of
two parts, separated poster-
iorly. One of them, the deep
petrosal nerve (n. petrosus
profundus), is connected with
the sympathetic plexus of the
internal carotid artery. The
other, the great superficial
petrosal nerve (n. petrosus
superficialis major) enters the
skull at the foramen lacerum,
passing into the petrosal bone,
in the interior of which it is
connected with the trunk of
the facial. The nerve con-
stitutes the motor root of the
sphenopalatine ganglion, the

Fic. 81. Diagram of the parts of the
ear in vertical projection. To show the
general relations of the structures covered
by the dissection.

p, petrous portion of the petrotympanic
bone; t., tympanic portion (bulla tym-
pani).

c., cochlea; c.s., bony semicircular
canals; c.t., tympanic cavity; d.c.,
cochlear duct; d.e., endolymphatic duct;
d.m., dura mater; d.s., semicircular
ducts; f.c., cochlear fenestra; f.v., vesti-
bular fenestra; i., incus; m, malleus;
m.a.e., external acoustic meatus, termin-
ating at the tympanic membrane; m.a.i.,
internal acoustic meatus; s., sacculus;
s.e., endolymphatic sac; st., stapes; t.a.,
auditory tube; u., utriculus; v, vestibulum;
VIII, acoustic nerve.

sensory root being that provided by the sphenopalatine nerve.

The sphenopalatine ganglion is one of several representing
the sympathetic system in the head, and having motor and
sensory roots from the cerebral nerves in addition to their
sympathetic connections. The series includes the ciliary
ganglion, which lies on the optic nerve, the sphenopalatine
ganglion, the otic ganglia, associated with the mandibular

254 ANATOMY OF THE RABBIT.

nerve, and the submaxillary ganglion, associated with the
lingual nerve.

13. Examination of the middle ear.

By breaking away the ventrolateral portion of the tympanic
bulla and clearing the surface, the structures of the tympanic cavity
may be studied. They are chiefly those already described in
connection with the skeleton (p. 147), but the following soft parts
may be identified.

(a) The tympanic membrane (membrana tympani) is
stretched almost vertically across the lower end of the
external acoustic meatus.

(b) The tensor tympani is a slender muscle, the origin of which
forwards from the alisphenoid is concealed. It is inserted
on the manubrium mallei.

(c) The stapedius is a minute muscle arising from the periotic
bone above the cochlear fenestra and inserted on the stapes.

(d) The chorda tympani is a delicate nerve which crosses the
tympanic cavity, lying between the long crus of the incus
and the manubrium mallei.

The nerve is a continuation of the intermediate nerve, a sensory
root of the facial, which arises independently of the chief or motor root
and joins the facial in the facial canal of the periotic bone. Its peri-
pheral coanections are with the lingual nerve and the submaxillary
ganglion.

(e) The internal carotid artery traverses the carotid canal
of the tympanic bone. By breaking away the posterior
portion of the bulla, the entrance of the vessel into the
external carotid foramen may be seen.

The dissection of the parts of the ear as here outlined includes only the
external and middle portions together with the associated acoustic nerve and its
entrance to the periotic bone. The parts of the internal ear (Fig. 81) are not
readily made out without the use of special methods, though their position may
be estimated by carefully breaking away the surface of the ventral portion of the
periotic. They include (1) the bony labyrinth, consisting of a series of con-
nected spaces lodged in the interior of the petrous bone, and comprising the
cochlea, vestibulum, and the bony semicircular canals; and (2) the mem-
branous labyrinth, consisting of a second series of spaces contained within the
first, and comprising the duct of the cochlea, the sacculus, the utriculus,
and the semicircular ducts, together with their connections and the endolym-
phatic duct andsac. The membranous labyrinth contains the sensory portion
of the ear and its cavity is occupied by a fluid material, the endolymph. The
wall is separated from that of the bony labyrinth by an extensive perilymphatic
space also occupied by a fluid material termed the perilymph.

THE THORAX. 255

XI. THE THORAX.

1. Examination of the thoracic wall. For this purpose the
lateral surface of the thorax may be conveniently cleared, on the side
from which the anterior limb has been removed, by dissecting away
the attachments of the muscles already examined in the previous
dissections. These include the origins of the pectorales, pecto-
scapularis, serratus anterior (thoracic portion), obliquus externus,
and rectus abdominis.

The dorsal portion of the exposed surface is occupied by the
spinal musculature; to be examined ata later stage. On the ventral
portion appear the ribs, and between them, filling the intercostal
spaces, the intercostal muscles. The external intercostals
(mm. intercostales externi) arise from the posterior margins of the
bone ribs, the fibres passing obliquely downward and backward
to be inserted on the anterior margins of the next succeeding ribs.
The internal intercostals (mm. intercostales interni), the fibres
of which are disposed in the opposite direction, are concealed for
the most part from this surface, but appear ventrally between the
costal cartilages, where they are not covered by the external inter-
costals. -They are best examined at a later stage from the interior
of the thorax.

In preparation for the removal of a section of the thoracic wall,
the pectorales should be divided on the opposite side of the thorax,
close to the sternum, so that the limb may be displaced.

The nerves and vessels of the neck must be kept intact until the following
dissection accounts for their thoracic connections.

The scaleni muscles (p. 268) should be examined, since it is necessary to
destroy their costal insertions.

A triangular section of the wall, including the sternum and the
costal cartilages, may be removed by making three incisions, one on
either side of the sternum, extending from the middle of the first
rib backward to the end of the ninth bone rib, and the third across
the ventral surface close in front of the diaphragm.

The transversus thoracis muscle appears on the inner surface
of the section removed. It is a thin sheet of fibres arising from the
body and xiphoid process of the sternum and inserted on the ribs,

256 ANATOMY OF THE RABBIT.

from the second to the sixth, at the junctions of the bone ribs with

the costal cartilages.

The artery passing along the ventral wall between the foregoing muscle and
the internal intercostals is the internal mammary (p. 257).

For the general relations of heart and lungs, see pp. 84 and 87.

2. Dissection of structures in the superior thoracic aperture.

The nerves and bloodvessels of this region are concealed by the
thymus gland, a large triangular flattened structure of fatty
consistence, extending forward from the base of the heart to the
anterior end of the thorax. The thymus should be carefully
scraped away, all vessels except those of the gland itself being kept
intact.

The following structures, including the aortic arch and the
arteries arising from it, the superior caval veins, and the vagus,
phrenic, and sympathetic nerves, cannot be dissected exactly in the
order given below, but must be separated trom one another and
identified as they appear. The left superior caval vein is superficial,
crossing the ventral surface of the aortic arch. Care should be
taken not to injure the nerves (c-f) in exposing the branches of the
subclavian artery.

(ay) The arch of the aorta (arcus aortae). Beginning at the

base of the heart, the aorta passes at first forward, and then
describing a curve, in the course of which it lies slightly to
left of the median plane, turns backward along the ventral
surfaces of the bodies of the thoracic vertebrae. With the
exception of the coronary arteries (p. 260) the first branches
are the large paired vessels arising from the anterior wall.
They comprise the common carotid and subclavian
arteries. On the right side the carotid and subclavian
arise from a short common trunk, the innominate artery
(a. anonyma). The left common carotid arises immediately
to the left of this vessel or from its base.
The subclavian artery (a. subclavia) is the first portion of
the artery of the anterior limb. It passes from its point of
origin laterad to the anterior margin of the first rib, where it
is replaced by the axillary artery. Near its point of origin
it gives off several branches, the relations of which are
subject to considerable variation. They include:

(1)

(2)

(3)

(4)

(5)

(b)

THE THORAX. 257

The vertebral artery (a. vertebralis). This vessel passes
into the costotransverse foramen of the sixth cervical verte-
bra, and traversing the canal formed by this and the corres-
ponding foramina of the remaining cervical vertebrae,
reaches the interior of the cranial cavity. Its union on the
ventral surface of the medulla oblongata with its fellow of
the opposite side to form the basilar artery will be seen at a
later stage (p. 278).

The superficial cervical artery (a. cervicalis superficialis)
—divided in a previous dissection (p. 206)—is a small
vessel which passes forward and outward beneath the in-
sertions of the cleidomastoideus, basioclavicularis and
levator scapulae major muscles, ramifying extensively in the
fat mass of the side of the neck under cover of the superior
portion of the trapezius. Its ascending cervical branch
lies on the lateral side of the external jugular vein.

The transverse artery of the neck (a. transversa colli),
also divided in a previous dissection (p. 207), passes laterad
around the neck of the first rib to the wall of the thorax.
It passes through the loop formed by the eighth cervical and
first thoracic spinal nerves. It runs dorsad, first on the
medial side of the scalenus anterior, then on the medial side
of the cervical portion of the serratus anterior. A strong
branch passes to the inferior angle of the scapula. The
artery supplies the serratus anterior and the rhomboidel.

The a. intercostalis suprema passes backward to the
internal surface of the thoracic wall, giving off the first chree
(or four) intercostal arteries in the intercostal spaces, and
also small branches to the oesophagus and trachea.

The internal mammary artery (a. mammaria interna),
the first portion of which has been removed with the ventral
wall of the thorax, passes backward to the ventral abdominal
wall as the superior epigastric artery (a. epigastrica
superior) anastomosing with the inferior epigastric (p. 203).
The superior caval vein (v. cava superior) is formed at the
base of the neck by the union of the internal and external
jugular veins, the latter vessel receiving at this point the
subclavian vein (v. subclavia). The right superior caval

258

(c)

ANATOMY OF THE RABBIT.

passes almost directly backward, crossing the ventral surface
of the right subclavian artery, and enters the anterior portion
of the right atrium. The left vessel crosses both the left
subclavian artery and the arch of the aorta, reaching the
right atrium from the dorsal surface of the heart.
The vagus nerve. On the right side the nerve crosses the
ventral surface of the subclavian artery, passing dorsad
to the wall of the oeso-
phagus. It gives off
the recurrent nerve
(n. recurrens), the lat-
ter curving around the
subclavian artery and
passing forward along
the side of the trachea
to the larynx. On the
left side the vagus
passes between Ae
arch of the aorta and
the base of the heart
to the ventrolateral
wall of the oesophagus.
The recurrent. nepve
passes forward on the
dorsal side of the arch.
(d) Theramus cardiacus

Fic. 82. Planofthe venous and lymphatic
trunks of the anterior portion of the body, After of the Vagus. In front

McCiure and Silvester. | :
a., azygos vein; ao., aorta; c.s., left superior of the subclavian

caval vein; d.th., thoracic duct; j.e., j.i., and °
j.tr., external, internal and transverse jugular artery the nerve 1s at

veins;

(e)

., left subclavian vein; tr.s., transverse
scapular vein,

first closely associated
with the vagus trunk,
lying on its medial side. On the right side it passes to the
dorsal surface of the subclavian, and on the left to the
dorsal surface of the aortic arch. It ends in the cardiac
plexus (plexus cardiacus), a network of sympathetic nerves
lying between the aortic arch and the pulmonary artery.

The phrenic nerve (n. phrenicus) is a stout cord arising
chiefly from the fourth cervical spinal nerve. That of the

THE THORAX. 259

left side crosses the ventral surface of the subclavian artery
and the aortic arch, passing then along the pericardium to
the diaphragm. That of the right side accompanies the
thoracic portion of the inferior caval vein. The nerve
controls the respiratory movements of the diaphragm.

(f) The sympathetic trunk. At the base of the neck the
cervical portion of the sympathetic trunk enters the inferior
cervical ganglion (g. cervicale inferius). The latter lies
in front of and somewhat dorsal to the subclavian artery.
The first thoracic ganglion lies behind the artery and is
connected with the inferior cervical by the ansa subclavia,
a loop formed by two cords, one of which passes to the dorsal,
the other to the ventral side of the subclavian artery.

The nerves proceeding from the inferior cervical ganglion
enter the cardiac plexus and the sympathetic plexuses of the
subclavian and its branches.

G) Dissection of the heart.

he character and relations of the enclosing serous sac, the
pericardium, should first be noted. Its relation to the heart is
similar to that of the peritoneum and pleura investing other visceral
organs (p. 99). It comprises a parietal layer, that portion com-
monly known as the pericardium, and a visceral layer, the epi-
cardium, which forms an immediate investment for the heart
substance. The parietal layer forms a loose, capacious sac, the
serous membrane being greatly strengthened by the presence of a
thin layer of connective tissue which forms an externa] investment
and is usually considered as part of the pericardium,

The paired pleural cavities containing the lungs are broadiy separated by a
longitudinal vertical partition, the mediastinum or mediastinal septum, the
space enclosed by the latter being largely occupied by the heart and by the cavity
of the pericardium. For aconsiderable area ventrally the pericardium is loosely
applied to the thoracic wall, the intervening space, which is bounded laterally
by the membrane lining the pleural cavities, being known as the anterior
mediastinum. A corresponding dorsal space lying between the heart and the
bodies of the thoracic vertebrae, and also bounded laterally by the pleura, is the
posterior mediastinum. It is occupied by several structures, namely, the
oesophagus, the thoracic aorta, the bronchi, and the pulmonary bloodvessels.

The pericardium should be removed, and the external features
of the heart and its great vessels examined as follows:

260

(a)

(b)

(d)

(e)

ANATOMY OF THE RABBIT.

The posterior, somewhat conical, ventricular portion of the
heart. The left ventricle (ventriculus sinister) may be dis-
tinguished both by its position and by the more solid charac-
ter of its wall. The right ventricle (ventriculus dexter) is
less muscular, and the wall is readily pressed inward. The
line of division is indicated on the ventral surface by a faint
depression, the anterior longitudinal sulcus.

The pulmonary artery (a. pulmonalis) leaves the base ot
the right ventricle, passing forward and to the left in a some-
what spiral fashion around the aorta. On the dorsal sur-
face of the latter it divides into the right and left pulmon-
ary arteries, one foreach lung. At the point of division the
pulmonary artery is connected with the aorta by a short
fibrous cord, the arterial ligament (lig. arteriosum), repre-
senting the foetal connection of the two vessels through the
ductus arteriosus (p. 89).

The left coronary artery (a. coronaria sinistra) passes back-
ward on the ventral surface of the heart, lying in the anterior
longitudinal sulcus. A corresponding right coronary
artery (a. coronaria dextra) passes to the right side of the
heart, lying between the right ventricle and the right
atrium.

The two vessels supply the substance of the heart. They are the
first branches of the aorta, arising from the aortic sinuses at its base.
The left atrium (atrium sinistrum) is the thin-walled
chamber lying to the left at the base of the heart. The
pulmonary veins (venae pulmonales), several on either
side, enter the left atrium, passing from the medial portions
of the lungs.

The right atrium (atrium dextrum) resembles the left in
the character of its wall. It receives the right and left
superior caval veins and the unpaired inferior caval vein.

The heart may be removed by dividing the great bloodvessels.
The arch of the aorta should be removed with the heart by dividing
the vessel at a point beyond the origin of the left subclavian, and
then severing the carotids and subclavians at their bases. This
exposes the surface for the subsequent examination of the posterior
end of the trachea and its connections with the lungs.

THE THORAX. 261

Open the right ventricle by a longitudinal incision of the ventral
wall, extending the incision forward into the pulmonary artery,
also both atria by transverse incisions. By washing out the cavi-
ties, the internal features of the wall, including the arrangement of
the valvular structures, may be examined as follows:

In the right ventricle:

(a) The trabeculae carneae; muscular ridges of the internal
surface of the wall.

(b) The tricuspid valve (valvula tricuspidalis). The thin
membranous flaps composing the valve enclose the atrio-
ventricular aperture, and project into the cavity of the
ventricle. Their margins, which are otherwise free, are con-
nected by slender fibrous cords, the chordae tendineae,
with the papillary muscles (mm. papillares), the latter
being thick muscular projections, of somewhat conical shape,
arising from the opposite walls.

In the rabbit the valve is composed of only two flaps, of which the
ventral one is very free, and has large papillary muscles, while the dorsal
one is closely attached to the wall, with the papillary muscle reduced or
absent. For this reason the term right atrioventricular valve is more
appropriate than “‘tricuspid’’.

(c) The semilunar valves (valvulae semilunares) of the pul-
monary artery are three extremely thin folds guarding the
entrance to the vessel from the right ventricle. Two of the
valves are usually found intact, the third being destroyed on
opening the vessel.

In the atria:

(a) The respective positions of the pulmonary and_ systemic
veins at their points of entrance.

(b) The complete separation of the two chambers. In the par-
tition separating them may be seen a thin fibrous portion
denoting the position of the embryonic foramen ovale.

Open the left ventricle by a ventral longitudinal incision, cutting
well through the tip of the ventricle and extending the incision
across the pulmonary artery and into the aorta. On account
of the great thickness of the wall the internal structure is not so
easily examined as in the right ventricle.

262

(a)

(b)

ANATOMY OF THE RABBIT.

The bicuspid or left atrioventricular valve (valvula bicus-
pidalis) is similar in general structure to the tricuspid valve
of the right ventricle, but is more nearly circular in form,
with stout, closely grouped papillary muscles.

The semilunar valves of the aorta are three in number, and
are similar to those of the pulmonary artery.

4. Examination of the lungs and their connections (Figs. 44,

83).

The removal of the ventral wall of the thorax opens the pleural
cavities by taking away a considerable portion of the costal pleura,
which is adherent to the internal surfaces of the ribs. The chief
features may be made out as follows:

(a) Each pleural cavity (cavum
pleurae) is a closed serous sac,
the lining membrane, or
pleura, being distributed over
the costal surface as the costal
pleura, partly over the ante-
rior surface of the diaphragm

Fic. 83. Plan of the respiratory as the diaphragmatic pleura,
tubes as seen from the ventral surface. .
tr, trachea; br, br’, left and right and Over the surface of the lung
bronchi; ep, eparterial bronchus; s,m,
m’, i, i’, bronchial rami to superior, as the pulmonary pleura.
middle and inferior lobes; ll, Im, E
bronchial rami to lateral and medial Posteriorly the pulmonary
obules.

(b)

(c)

pleura passes from the medial
margin of the left lung to the medial lobule of the right and
thence backward to the diaphragm, forming a broad sheet of
attachment, the pulmonary ligament (lig. pulmonale).
The lungs (pulmones) are paired expansible structures, the
surfaces of which are free, except posteriorly, where they
are attached to the diaphragm through the pulmonary liga-
ment, and medially where they are connected with the
pulmonary bloodvessels and the respiratory tubes.
Each lung is divided into three portions, the superior,
middle, and inferior lobes. On account of the smaller
size of the left lung, the superior lobe is imperfectly develop-
ed. On the right side the inferior lobe is divided into two
portions, the medial and lateral lobules, the inferior caval
vein passing between them.

THE THORAX. 263

(d) The trachea divides at its posterior end into two portions, the

(e)

(f)

right and left bronchi, one for each lung. Each bronchus
is again divided into smaller portions, the bronchial rami,
which penetrate the substance of the organ and redivide into
smaller tubes. On the right side a small eparterial
bronchus is given off from the right bronchus. It passes
to the superior lobe of this side, and lies in front of the right
pulmonary artery.

The branches of the pulmonary artery and the pulmonary
veins may be traced for a short distance on the medial por-
tion, or hilus, of each organ.

The vagus nerve passes to the dorsal side of the bronchus,
giving off a number of branches, which accompany the
bronchus to the lung.

The lungs may be removed, together with a portion of the
trachea, care being taken to leave the vagus nerves in place. The
lungs may then be examined to better advantage, and the surface
also prepared for the next dissection.

5. The following structures may now be traced on the dorsal
wall of the thorax:

(a)
(b)

(c)

The oesophagus. It traverses the thorax in a median posi-
tion, entering the diaphragm at the hiatus oesophageus.
The vagus nerves. The right and left nerves pass backward
along the lateral walls of the oesophagus, and are connected
with one another through nerve plexuses lying on its dorsal
and ventral surfaces. The left cord is that appearing at the
posterior end of the oesophagus in a ventral position and
passing to the ventral surface of the stomach. The right
cord occupies a corresponding dorsal position and passes to
the dorsal surface of the stomach (p. 185).

The thoracic aorta (aorta thoracalis) passes backward on
the ventral surface of the vertebral column, leaving the
thorax through the hiatus aorticus, the latter being the
aperture enclosed by the crura of the diaphragm. Its
branches in the thorax are the intercostal arteries (aa.
intercostales), beginning with the fourth, which are given off
metamerically in the intercostal spaces, and pass laterad to
the thoracic wall.

264 ANATOMY OF THE RABBIT.

(d) The thoracic portions of the sympathetic trunks lie on
the lateral surfaces of the bodies of the thoracic vertebrae.
The posterior ganglia give origin to the splanchnic nerve,
the latter passing backward into the abdominal cavity (p. 183).

(e) The levatores costarum; a series of small muscles arising
from the transverse processes of the vertebrae and the heads
of the ribs and inserted on the anterior margins of the next
succeeding ribs. They assist the intercostals in respiration.

(f) The intercostal nerves (nn. intercostales) accompany the
intercostal arteries to the lateral wall of the thorax.

(¢) The azygos vein (v. azygos) is a small, asymmetrical, venous
trunk lying to the right of the dorsal surface of the aorta.
It receives the majority of the intercostal veins, which
accompany the corresponding arteries and nerves, the
tributaries extending backward to the first lumbar veins.
It opens forward into the right superior caval. The more
anterior intercostal veins are tributaries of the right and
left supreme intercostal veins which open into the corres-
ponding superior cavals.

6. The diaphragm (diaphragma) is a muscular and tendinogus
sheet forming the posterior wall of the thorax and separating the
pleural cavities from the peritoneal cavity. It is somewhat dome-
shaped in the relaxed condition, but in contraction it becomes
flattened in such a way that the space occupied by the lungs is
considerably increased, while the liver and related structures of the
abdominal cavity are displaced backward.

As a muscle the diaphragm arises in three portions. The first,
or lumbar portion, consists of two muscular and fibrous cords,
the crura, the right much larger and stronger than the left, arising
from the anterior spinous processes of the first three lumbar
vertebrae. The second, or costal portion, arises from the internal
surfaces of the posterior ribs. The third, or sternal portion,
arises from the xiphoid process of the sternum. Its insertion is
represented by its own tendinous central portion, or centrum
tendineum, although the latter is virtually attached forward
to the lungs and pericardium through the broad pulmonary liga-
ment. The centrum tendineum is shaped somewhat like a trefoil,
the fibres of the costal and sternal portions radiating outward from
its margin.

THE VERTEBRAL MUSCULATURE. 265

The following may be made out on the posterior surface:

(a) The cut margins of the falciform, coronary, and left trian-
gular ligaments.

(b) The hiatus aorticus, an aperture enclosed by the two crura
and serving for the transmission of the aorta.

(c) The hiatus oesophageus, ventral to the foregoing, and
serving for the passage of the oesophagus.

(d) The foramen venae cavae, situated slightly to the right
and serving for the transmission of the vena cava inferior.

(e) The superior phrenic arteries (aa. phrenicae superiores)
arise from the aorta at about the point of origin of the
eleventh intercostals or from one of the lacter, and enter the
crura.

The inferior phrenic arteries are minute branches arising at the base
of the coeliac.

(f) The inferior phrenic veins (vv. phrenicae inferiores), one
on either side of the foramen venae cavae, at which point

they enter the inferior cava.
The superior phrenic veins pass forward from the diaphragm,
opening into the superior cavals.

XII. THE VERTEBRAL AND OCCIPITAL MUSCULATURE.

Dissection on the dorsal surface of the body from the occiput
backward; also on the lateral and ventral surfaces of the neck.

The serratus posterior muscle lies on the dorsolateral surface
of the thorax. It arises from the dorsal spinous ligament of the
neck (ligamentum nuchae) and from the lumbodorsal fascia back to
the last rib, and is inserted on the lateral surfaces of the eight pos-
terior ribs.

The splenius muscle is a somewhat triangular sheet arising
from the ligamentum nuchae and inserted on the supraoccipital and
mastoid portions of the skull, extending also to the transverse pro-
cess of the atlas.

These two muscles should be divided, the serratus posterior being removed
from the surface.

1. The long muscles of the vertebral column.

Apart from the iliopsoas, psoas minor, and quadratus lum-
borum—muscles of appendicular insertion lying on the ventral
surface of the vertebral column—the vertebral musculature com-

266 ANATOMY OF THE RABBIT.

prises chiefly modified segmental muscles lying on the dorsal
surface, for the most part in the area enclosed by the spinous and
transverse processes of the vertebrae. They include the sacro-
spinalis, semispinalis, and intertransversarii. Their insertions
are extended in part laterad to the ribs. In the cervical region they
are represented by short muscles, separated for the most part from
the thoracic and lumbar portions, and arising by accessory bundles
from the anterior ribs, the corresponding thoracic, and the posterior
cervical vertebrae. In the cervical region the muscles are easily
separated from one another, but in the posterior part of the body
it is necessary to dissect away the tough investment of lumbodorsal
fascia which covers them.

(a) The sacrospinalis. Origin: Crest of the ilium and dorsal
surface of the sacrum; mamillary processes of the six pos-
terior lumbar vertebrae; investing lumbodorsal fascia.
This muscle is the largest and strongest muscle of the body.
It extends forward over the surfaces of the ribs. Its medial
border is separated from the middle line by a space of con-
siderable width, in which the semispinalis and multifidus
muscles are accommodated. In the lumbar region it is
inserted in a continuous mass on the long transverse processes
of the vertebrae and in the interspaces. In the thoracic
region the muscle divides into two portions, namely, a thin
lateral portion, the iliocostalis or longissimus costarum,
and a thick medial portion, the longissimus. The latter
receives in the posterior portion of the thorax strong acces-
sory bundles from the semispinalis muscle on its medial side,
the two muscles being inseparable at this point.

The iliocostalis is inserted laterally on the ribs as the ilio-
costalis dorsi. It receives medially from the ribs a number
of accessory bundles, which are inserted forwards to the
seventh cervical vertebra as the iliocostalis cervicis.

The longissimus is inserted by broad fleshy bands on the
posterior margins of the ribs, medial to the accessory origins
of the iliocostalis, this portion of the muscle forming the
longissimus dorsi. Continuing to the neck it is inserted
on the transverse processes of the three posterior cervical
vertebrae, medial to the origin of the cervical portion of the

(b)

THE VERTEBRAL MUSCULATURE. 267

serratus anterior, but a number of accessory slips carry the
insertion forward to the transverse process of the atlas.
This portion is the longissimus cervicis. A separate
band of fibres arising chiefly from the transverse processes of
the second to fourth thoracic vertebrae joins the lateral,
ventral portion of the splenius, and forms the longissimus
capitis. It is inserted with the splenius on the mastoid
portion of the skull.

The semispinalis and multifidus. The band of muscle
lying between the longissimus and the middle line, is com-
posed of partly fused slips, arising for the most part by very
long tendons from the mamillary and transverse processes,
and inserted forwards on the spinous processes. It is
divisible into two portions, which are superficially separated
by a constricted area lying at the level of the last thoracic
vertebra, this being also the point where the muscle is fused
with the longissimus. The anterior portion, the semispin-
alis dorsi, is inserted by a series of fleshy slips on the spinous
processes of more anterior thoracic vertebrae, but extends to
the spinous process of the third or fourth cervical vertebra.
The posterior portion, the multifidus, increases in size
backward to the sacrum, where it is continuous with the
abductor caudae posterior.

An almost separate muscle, covering the neck as a broad
sheet immediately beneath the splenius and longissimus
capitis, is the semispinalis capitis. It arises from the
transverse processes of the five posterior cervical and the
transverse processes of the first four thoracic vertebrae. It
is lightly attached on a line from the transverse process
of the atlas to the external occipital protuberance, but is
inserted on the lateral surface of the latter. The more
posterior and medial portion of the muscle is composed of
separate slips arising in common with the longissimus
capitis, two closely applied slips, however, at the free margin
of the muscle, arising from the semispinalis dorsi and the
longissimus. The principal, lateral portion is crossed by a
tendinous inscription.

268 ANATOMY OF THE RABBIT.

A second muscle, the semispinalis cervicis, is covered
by the foregoing one. It arises from the articular processes
of the posterior cervical and first thoracic vertebrae, and is
inserted on the spinous processes of the cervical vertebrae,
chiefly on that of the epistropheus.

(c) The intertransversarii are short muscles connecting the
lateral portions of the vertebrae. They are distinguishable
in part by their darker coloration. They increase in size
backwards, being most conspicuous in the lumbar region,
where they form thick muscular pads interposed between
the mamillary and accessory processes. The last slip is
attached to the crest of the ilium.

2. The following muscles constitute an occipital group, com-
posed of short muscles arising from the atlas and axis and inserted
on the atlas and the occipital portion of the skull.

(a) The rectus capitis posterior superficialis. Origin:
Spinous process of the epistropheus. Insertion: External
occipital protuberance.

(b) The obliquus capitis superior. Origin: Transverse
process of the atlas. Insertion: Lateral surface of the
occipital protuberance.

The foregoing muscles should be divided.

(c) The rectus capitis posterior minor. Origin: Posterior
tubercle of the atlas. Insertion: External occipital pro-

tuberance.
(d) The rectus capitis posterior major. Origin: Spinous
process of the epistropheus. Insertion: Laterally on

the supraoccipital bone.

(e) The obliquus capitis inferior. Origin: Spinous process
of the epistropheus. Insertion: Dorsal surface of the
transverse process of the atlas.

(f) The rectus capitis lateralis. Origin: In common with the
obliquus capitis superior, which covers it. Insertion: Base
of the jugular process of the occipital.

3. Muscles of the lateral and ventral surfaces of the neck:

(a) The scalenus anterior. Origin: Transverse processes of
the four posterior cervical vertebrae. Insertion: First rib.

(b)

(c)

(d)

(e)

(f)

(g)

(h)

THE VERTEBRAL MASCULATURE. 269

The scalenus medius. Origin: Tranverse process of the
fifth cervical vertebra. Insertion: Lateral surfaces of the
third to fifth ribs.

The scalenus posterior. Origin: Transverse processes of
the fourth to sixth cervical vertebrae. Insertion: First rib.

The medius is superficial; the anterior more or less separated from
the posterior by the origin of the cervical portion of the serratus anterior.
The scalenus medialis. Origin: The sternum at the
point of attachment of the first rib. Insertion: Middle of
the ventral portion of the first rib.

The foregoing muscles comprising the scalenus group, are destroyed
by the removal of the ventral thoracic wall (p. 255).

The longus colli. Origin: Bodies of the first six thoracic
vertebrae. Insertion: The muscle passes forward on the
ventral surface of the bodies of the vertebrae, giving off
insertion fibres, and also receiving strands of origin. It
terminates on the anterior tubercle of the atlas.

The longus capitis is partly fused with the foregoing
muscle, but its origin is in a more lateral position from the
transverse processes of the first six cervical vertebrae.
Insertion: Sphenooccipital synchondrosis.

The longus atlantis. Origin: Lateral to the longus capitis,
from the transverse processes of the third to sixth cervical
vertebrae. Insertion: Transverse process of the atlas.

The longus capitis should be divided near its insertion.

The rectus capitis anterior. Medial portion of the
ventral surface of the transverse process of the atlas. In-
sertion: Basioccipital bone.

The caudal musculature in the rabbit comprises, in addition to
the posterior extension of the cutaneus maximus, the following axial
muscles:

(a) The extensor caudae medialis. It lies in the furrow
between the spinous and articular processes of the posterior sacral
and anterior caudal vertebrae, and is inserted on the transverse
processes and dorsal surfaces of the caudal vertebrae.

(b) The abductor caudae posterior lies in the grove between
the articular and transverse process and is inserted on succeed-
ing vertebrae. It appears to continue the multifidus, but corres-
ponds to the more medial portion of the longissimus.

270

ANATOMY OF THE RABBIT.

(c) The abductor caudae anterior. Origin: Ischial spine.
Insertion: Lateral surface of the sacrum and the transverse pro-
cesses of the caudal vertebrae.

(d) The flexor caudae. Origin: Ventral surface of the sa-
crum.and anterior caudal vertebrae. Insertion: Ventral surfaces
of succeeding vertebrae.

These muscles are also known as sacro-coccygei, dorsalis, lateralis,
and ventralis (a, b, d,) and coccygeus (c).

XIII. THE CENTRAL NERVOUS SYSTEM.

Ke The spinal cord and nerve roots.

o expose the whole cord or a portion of it from the dorsal

surface, the muscles should be removed on both sides of the verte-
bral arches and the latter broken away with the bone forceps. The
following features may be made out according to the extent of
exposure:

(a)

(b)

(c)

(d)

The spinal cord (medulla spinalis) is a thick cylindrical
white cord traversing the vertebral column in the vertebral
canal. It is of almost uniform diameter, but exhibits two
slight enlargements, one in the cervical, the other in the
lumbar region. At about the middle of the sacrum it
contracts to a slender filament, the filum terminale, which

may be traced backward to the middle of the tail.

The enclosing membranes or meninges of the cord are three
in number, of which two are readily identified. Lining the
internal surface of the bone is a thick fibrous investment,
the dura mater; on the surface of the nervous matter a
thin vascular membrane, the pia mater.

The dura and pia are connected by a loose web of connective
tissue, the arachnoidea.

The regional distribution of the nerve roots—eight cervical,
twelve dorsal, seven lumbar, four sacral, and six caudal.

The cervical nerves are numbered from the vertebrae lying behind
the intervertebral foramina from which they proceed, the remaining
nerves from the vertebrae lying in front of the intervertebral foramina.
The nerve transmitted by the intervertebral foramen between the
seventh cervical and first thoracic vertebrae is described as the eighth
cervical.

The origin and primary divisions of the nerve roots may be
worked out by removing carefully the lateral portions of the

THE CENTRAL NERVOUS SYSTEM. 271

arches of one or two vertebrae. The posterior, or sensory
root (radix posterior), arises from the dorsolateral surface of
cord, and expands immediately into a spinal ganglion (g.
spinale). The more slender anterior, or motor root (radix
anterior), arises by a number of filaments from the ventro-
lateral surface. The combined roots give origin to the
posterior, anterior, and communicating rami. The posterior
ramus is an inconspicuous branch (except in the first two
cervicals) passing to the dorsal musculature and skin. The
anterior ramus is the chief portion of the spinal nerve, the
successive rami appearing as the components of the cervical
and lumbosacral plexuses or as individual spinal nerves.
The ramus communicans is a slender filament which
passes ventrad to join the sympathetic trunk.

2. Asmall portion of the spinal cord may be excised and exam-
ined (preferably under water) for the following: (see p. 38, Fig. 19).

(a) The division of the cord into lateral halves by two median
depressions, the anterior median fissure (fissura mediana
anterior) and the posterior median sulcus (sulcus me-
dianus_ posterior).

(b) The division of each half into three columns by shallow
grooves, the anterior and posterior lateral sulci. The
grooves are marked by the attachments of the anterior and
posterior nerve roots. The three columns of each half of
the cord are the anterior, lateral, and posterior funiculi.

(c) On the cut surface the white substance (substantia alba) is
seen to form a peripheral investment enclosing the grey sub-
stance (substantia grisea) of the centre of the cord. The
grey portion is somewhat H-shaped in secticn, each half
being composed of anterior larger and posterior smaller
masses, known in section as the horns of the grey matter, or
as complete structures, the anterior and posterior grey
columns. In the median plane, but nearer the dorsal than
the ventral surface, is the minute central canal (canalis
centralis), the cavity of the spinal cord.

3. The brain may be exposed by breaking away the supra-
orbital processes of the frontal bone and then removing the roof of
the skull with bone forceps. In order to clear the brain and the
roots of the cerebral nerves, it is necessary to remove the entire

272 ANATOMY OF THE RABBIT.

lateral wall of the skull on both sides. The chief part of the
operation consists in removing the temporal portion of the skull
by successive steps, exposing first the flocculus of the cerebellum,
a small stalked body which is almost completely enclosed by the
dorsal portion of the petrosal. The entire petrotympanic bone
is easily detached, and if removed en masse the paraflocculus and
probably also the roots of the facial and acoustic nerves will be
destroyed.

The arches of the first three or four cervical vertebrae should be
removed, unless the anterior portion of the cord has been exposed
in the previous dissection.

The spinal cord may be divided at about the level of the third
vertebra. The brain should then be raised very gently from the
ventral wall of the skull and the nerve roots divided with a small
sharp knife or with fine scissors. This operation requires con-
siderable care, since the nerves are strongly attached at their points
of exit trom the cranial wall, but very lightly attached to the brain,
so that they are in danger of being torn away.

The anterior end of the brain may be freed by cutting across the
small anterior expansions formed by the olfactory bulbs.

Portions of the dura mater removed with the brain may be cut
away with scissors. It is adherent chiefly along two lines: one
representing the longitudinal fissure between the cerebral hemi-
spheres, the other the tentorium cerebelli. These parts contain the
superior sagittal and transverse venous sinuses.

On the ventral surface of the brain as removed appear the
basilar and internal carotid arteries and their branches. These
vessels should be kept intact for examination at a later stage.

For the primary divisions of the brain see p. 71.

4) The superficial features may be identified as follows:

IN THE PROSENCEPHALON:

(a) The greatly enlarged cerebral hemisphere (hemisphaerium
cerebri) forms with its fellow of the opposite side the largest
portion of the brain. The two structures are separated by
the longitudinal cerebral fissure, but are connected with
one another by the commissures indicated below.

(b) The olfactory bulb (bulbus olfactorius) is a small ex-
pansion lying at the anterior end of each hemisphere. Its

(c)

(e)

THE CENTRAL NERVOUS SYSTEM. 272

cut end represents the point of origin of the first cranial
or olfactory nerve, the various divisions of which may be
found on the skull, where they may be traced into the per-
forations of the cribriform plate.

The olfactory bulb is the anterior portion of the olfactory
lobe, or olfactory brain. When traced backward on the
ventral surface of the brain it is seen to be replaced by a
white band of fibres, the latter forming the olfactory tract
(tractus olfactorius). The tract ends posteriorly in an
expanded portion of the brain, which, from its shape, is
described as the lobus piriformis. The olfactory brain as
thus defined is separated superficially from the remaining
portions of the cerebral hemisphere by a longitudinal furrow,
the limbic fissure (fissura limbica), which passes along its
lateral margin. The anterior portion of the furrow, known
as the anterior rhinal fissure, separates the olfactory tract
laterally from the narrow anterior portion of the cerebral
hemisphere. The corresponding posterior portion of the fur-
row, the posterior rhinal fissure, separates the lobus
piriformis from the posterior enlarged portion of the hemi-
sphere. The slight angle formed at the junction of the
anterior and posterior rhinal fissures is the point of origin
of a faint depression extending upward on the cerebral
hemisphere. It represents a rudimentary, lateral cerebral
(Sylvian) fissure.

The corpus callosum is a broad white commissural band
passing transversely from one hemisphere to the other. Its
median portion may be exposed by pressing apart the
medial margins of the hemispheres at the longitudinal
cerebral fissure.

The pineal body (corpus pineale) is a small, somewhat
conical structure lying between the dorsal posterior tips of
the cerebral hemispheres, and connected by a hollow stalk
with the unpaired portion of the brain (the thalamence-
phalon) lying below it. The connection is concealed by a
mass of pigmented vascular tissue, the beginning of the
chorioid plexus of the third ventricle, and usually also by a
small portion of the dura mater containing part of the
sagittal venous sinus. The latter may be carefully detached.

ANATOMY OF THE RABBIT.

By pressing apart the tips of the hemispheres and removing
the pineal body, the dorsal surface of the thalamencephalon
will be sufficiently exposed to make out the following

The slit-like aperture appearing in the middle line after the
removal of the pineal body represents the dorsal portion of
the third ventricle (ventriculus tertius) (Fig. 84), the roof
of which is formed anteriorly by the superficial portion of the
chorioid plexus (plexus chorioideus), the latter entering the

274
features:
(f)
ventricle at this point.
(g)

Fic. 84.

The lateral margins of the aperture are largely formed by two
minute spindle-shaped masses, one on either side, the
habenulae. Their posterior ends are united by a slender
transverse band, the habenular commissure (com-
missura habenularum). The fibres
ot the commissure are faintly
traceable forward, where they
form a pair of thin whitish fila-
ments (medullary striae).

The posterior commissure
(commissura posterior) crosses
the posterior portion of the roof
immediately behind and below
the habenular commissure.

(h)

Diagram, showing the

arrangement of the parts of the
thalamencephalon as viewed from
the dorsal surface, after removal of

the pineal body: a., anterior
thalamic tubercle; c.h., habenular
commissure; c¢.p., posterior com-
missure; c¢.sS., superior colliculus
(of mesencephalon); g.l. and g.m.,
lateral and medial geniculate bodies;
h., habenula; m.i., massa inter-
media; p., pulvinar; v.t., third
ventricle.

The walls of the third ventricle
are formed by the thalami.
They are broadly connected by
the intermediate mass (massa
intermedia), or middle com-
missure, which may be seen
from the dorsal surface crossing
the cavity.

(j) On either side of the middle line, or of the habenulae, the
dorsal portion of the thalamus forms a low, somewhat oval
projection, the pulvinar or posterior tubercle.

The anterior tubercle of the thalamus is a faint elevation of very
small dimensions lying in the angle enclosed between the pulvinar and
the anterior portion of the aperture of the third ventricle.

(k)

(1)

(m)

(n)

(0)

(p)

THE CENTRAL NERVOUS SYSTEM. 275

The parts of the metathalamus are distinguishable
externally as two rounded projections of the lateral surfaces
of the thalamencephalon. One of them, the lateral geni-
culate body (corpus geniculatum laterale), lies to the
lateral side of the pulvinar, and is only separated from it by
a faint depression of the surface. The medial geniculate
body occupies a more posterior and medial position.

The optic tract (tractus opticus) connects the geniculate
bodies, especially the lateral one, with the ventral surface of
the brain, ending in the optic chiasma.

On the ventral surface, the optic chiasma (chiasma opti-
cum), forms a conspicuous elevation, the posterior portion
of which is traceable into the optic tracts, the anterior
portion into the bases of the second cranial, or optic
nerves.

The hypophysis, or pituitary body, lies immediately
behind the optic chiasma.

On account of its enclosure by the walls of the hypophyseal fossa,
and also its slight attachment to the brain, the hypophysis is commonly
detached in removing the brain from the skull, in which case a slit-like
aperture, representing the ventral portion of the third ventricle is
exposed.

The tuber cinereum is a small elevation of grey matter
appearing on the ventral surface after the removal of the
hypophysis. It is the base of attachment of the infundi-
bulum, the latter being the slender extension of the brain
downward toward the hypophysis.

The mamillary body (corpus mamillare) forms a con-
spicuous rounded elevation, lying at the posterior end of the
tuber cincereum. ‘The structure is single, but there is an
indication of lateral lobes.

IN THE MESENCEPHALON,.

(a)

(b)

The dorsal surface is marked by four elevations, the corpora
quadrigemina. The anterior pair, distinguished as the
colliculi superiores, are much larger than the posterior
pair, the colliculi inferiores.

The ventral surface is occupied by a pair of divergent cords,
the cerebral peduncles (pedunculi cerebri), separated by a
faint median depression, the interpeduncular fossa (fossa
interpeduncularis).

276 ANATOMY OF THE RABBIT.

(c) The third cranial, or oculomotor nerve (n. oculomotorius),
arises from the ventral surface of the cerebral peduncle.

IN THE RHOMBENCEPHALON:

(a) The cerebellum forms an arch over the posterior portion

of the brain, and is supported by stout pillars from its sides.

In addition to the ridges of its surface the whole structure is

divided into several lobes, including the median portion or

vermis, the lateral hemispheres, and the stalked bodies, or

paraflocculi, arising from the
hemispheres.

(b) The anterior medullary
velum (velum medullare an-
terius) is the thin membrane
underlying the anterior portion
of the cerebellum and connect-
ing the latter with the inferior
colliculi. It forms a small an-
terior portion of the roof of the
fourth ventricle.

(c) The fourth cranial, or troch-
lear nerve (n. trochlearis),
arises from the anterior medul-
lary velum. The first portion ot
the nerve is usually to be found
on the lateral surface of the
cerebral peduncle.

Fic. 85. The rhombencephalon. f
Ventral view (the cerebellum not (d) The posterior medullary
figured). y

c.t., trapezoid body; f.c., foramen velum (velum medullare pos-
caecum; fi.c., cervical flexure; f.m.a., . : ae
anterior median fissure; p., pons; terius) underlies the posterior
p.c., cerebral peduncle (mesence- :
pherOn) yD icn De ae: margin of the cerebellum, and

1 oculomotor nerve; IV, troch- :
Heat ay bos etalon OF gERERIEG, extends backward over the tri-
minus; V2, portio minor; VI, ab- b h
ducens; VII, facial; VIII, acoustic. Ss
IX-XI, glossopharyngeal, vagus, and angular space enc osed yee e
Spinal a ccesseLy tOUD ss Cat abo, walls of the fourth ventricle.

glossal; CI, first cervical! spinal. Sie
The structure forms a chorioid

plexus, similar in character to that of the third ventricle but
much less extensive. It is commonly torn away in the
preparation of the braia, in which case the interior of the
fourth ventricle is exposed.

(e)

(f)

(g)

(h)

(i)

(i)

(k)

(1)

(m

)

THE CENTRAL NERVOUS SYSTEM. 277

On the ventral surface (Fig. 85), the pons forms a broad
commissural band extending transversely across the brain
and upward into the supports of the cerebellum, Its surface
is divided into two parts by a median depression, the sulcus
basilaris, occupied by the basilar artery.

The anterior median fissure of the spinal cord ends at the
posterior margin of the pons in a faint depression, the fora-
men caecum.

The anterior funiculus of the cord is largely replaced for-
wards by the pyramid (pyramis), a narrow band extending
forward on either side of the middle line to the posterior
margin of the pons.

The trapezoid body (corpus trapezoideum) lies in the
angle formed by the lateral margin of the pyramid with the
posterior border of the pons.

The fifth cranial, or trigeminal nerve (n. trigeminus),
arises by two roots, a larger sensory root, the portio major,
and a smaller motor root, the portio minor. The two
parts appear at the lateral border of the pons.

The portio major is the common trunk of the ophthalmic, maxillary
and mandibular nerves, the portio minor joining the mandibular, so
that the latter becomes a mixed nerve. The cut end of the portio major
may be identified on the cranial wall and traced forward into the semi-

lunar ganglion, the latter lying in a depression at the anterior ventral
end of the petrosal bone.

The sixth cranial, or abducent nerve (n. abducens), is a
slender cord arising at the anterior end of the pyramid.
The seventh cranial, or facial nerve (n. facialis), and the
eighth, or acoustic nerve (n. acusticus), arise from the
lateral margin of the trapezoid body.

The two nerves are closely associated, the former being slightly
anterior in position. Its chief portion is the motor root. In addition
the nerve receives a sensory filament, the portio intermedia or inter-
mediate nerve.

The glossopharyngeus, vagus, and accessorius arise by
several roots arranged in a linear series along the lateral
margin of the medulla. The trunk of the accessorius ex-
tends backward on the spinal cord, its roots, about ten in
number, arising as far back as the fifth cervical spinal nerve.
The twelfth cranial, or hypoglossal nerve (n. hypoglossus),
arises by several roots from the ventral surface of the medulla

278

ANATOMY OF THE RABBIT.

and at the lateral margin of the pyramid, its point of origin
corresponding to that of the ventral root of a spinal nerve.

4. The arteries of the brain may be traced on its ventral surface

as follows:

(a)

(b)

(c)

(d)

(e)

(g)

5.

The basilar artery (a. basilaris) isa median trunk formed on
the ventral surface of the medulla oblongata by the union of
the vertebral arteries, the latter here represented by their
cut ends.

The inferior cerebellar artery (a. cerebelli inferior) is the
largest of several transverse vessels arising from the basilar
and passing laterad to the cerebellum.

The posterior cerebral artery (a. cerebri posterior) is
formed on either side at the level of the anterior margin of
the pons by the division of the basilar. It passes to the
posterior portion of the cerebral hemisphere.

The superior cerebellar artery (a. cerebelli superior) is a
branch of the posterior cerebral, passing to the anterior
portion of the cerebellum.

The cut end of the internal carotid artery lies on either
side of the tuber cinereum. It is connected backwards with
the posterior cerebral.

The middle cerebral artery (a. cerebri media) is given off
from the internal carotid, passing to the middle portion of
the hemispheres.

The anterior cerebral artery (a. cerebri anterior), the
continuation of the carotid, passes to the anterior portion of
the ventral surface and the olfactory bulb. The anterior
cerebral is connected with that of the other side, a’complete
anastomotic loop being formed around the hypothalamus by
combined branches of the vertebral and carotid arteries.
This is the circle of Willis.

By dividing the supports of the cerebellum on either side,

the entire structure may be removed and the surface exposed, as in
Fig. 86, for an examination of the structures of the dorsal surface of
the rhombencephalon. The posterior medullary velum is removed
with the cerebellum, but a small portion of the anterior medullary
velum should remain in place.

(a)

The fourth ventricle (ventriculus quartus) is the exten-
sive space enclosed by the rhombencephalon. It is con-

(b)

(c)

(d)

THE CENTRAL NERVOUS SYSTEM. 279

nected forwards with the cerebral aqueduct and backwards
with the central canal of the spinal cord. Its roof is formed
principally by the anterior and posterior medullary vela, the
latter being attached to the cerebellum close to one another,
so that they underlie it.

The rhomboid fossa (fossa rhomboidea) is the shallow
depression enclosed by the thick lateral and anterior walls
and floor of the ventricle. The middle line shows a narrow
depression, the posterior
median fissure (fissura
mediana posterior), on either
side of which the floor is
raised into a low ridge, de-
scribed as the medial emi-
nence (eminentia media-
lis). The posterior end of
the fossa forms with the
enclosing wall the somewhat
triangular figure described
as the calamus scriptorius
The lateral supports of the
cerebellum, now represented
by their cut ends, are
formed by fibre-bands pass-
ing into the cerebellum from
adjacent ventral portions of

the- brains Inteach a middle? . grat vems aren race eee
band, the brachium pon- ip "brachim pontis; “cl. chan; li
bis ptenters athe: pons’, atl)liecticrm body. “ean metal Cothcace:
anterior band, the. brach- gracilis; gmp, posteee median fexine of
LUI COMJUNCHVUINS “C=nistcs. Gece: Tcnecn tices ok ke
fersi. ther mesencephalon... satus state dene ee
while a posterior connection

is established with the medulla through the restiform body.
The lateral wall of the fossa is formed by a thick ridge of
nervous matter, convex on its medial side, the restiform
body (corpus restiforme). Its anterior portion bears a large
spherical elevation overlying the origin of the acoustic nerve.

280

ANATOMY OF THE RABBIT.

(e) The posterior funiculus, in passing forward from the cord,

6.

is divided into medial and lateral portions. The medial
portion, the fasciculus gracilis, forms a narrow band ter-
minating forwards in a club-shaped expansion, the clava.
The lateral portion, the fasciculus cuneatus, passes into
the restiform body.

The brain may be divided by a median vertical section, and

one-half examined from the medial surface (Fig. 87). In addition
to many of the features already made out on the surface the
following may be noted:

(a)

(b)

Fic. 87. The brain in median section: a., anterior commissure; a.c., cerebral

aqueduct; b.o., olfactory bulb; cb., cerebellum; c.cl., corpus callosum; c.f.,

body of the fornix; cl.i., inferior colliculus; cl.s., superior colliculus; c.m., mam-
illary body; c.o., optic chiasma; c.p., pineal body; fl.c., cervical flexure; h.,
habenular commissure; h.c., cerebral hemisphere; hp., hippocampus; inf., infundi-
bulum; ].t., laminal terminalis; m.o., medulla oblongata; p., posterior commissure;
p.c., Chorioid plexus of the third ventricle; pd.c., cerebral peduncle; pn., pons;
sp., splenium; s.pl., septum pellucidum; t.c., tuber cinereum; th., thalamus,
massa intermedia; v.m.a., anterior medullary velum; v.m.p., posterior medullary
velum; y.q., fourth ventricle; v.t., third ventricle. I, olfactory nerve (origin) ;
II, optic nerve.

The deep but extremely narrow cavity formed by the third
ventricle is the first space appearing in the brain from the
anterior end, the first two (paired) ventricles lying laterally
in the hemispheres. They are connected with the third
ventricle by a narrow transverse canal, the interventricular
foramen (foramen interventriculare).

The anterior boundary of the third ventricle is formed ven-
trally by the narrow partition separating the two hemi-
spheres, in the dorsal portion of which is the small anterior

(c)

(d)

(e)

We

THE CENTRAL NERVOUS SYSTEM. 281

commissure (commissura anterior). The ventral portion
of the ventricle is projected toward the optic chiasma
forming the recessus opticus, and into the infundibulum,
forming the recessus infundibuli.

The mesencephalon contains no ventricular expansion, its
substance being perforated only by a narrow tube, the cere-
bral aqueduct (aquaeductus cerebri), which connects the
third with the fourth ventricle.

The corpus callosum is shown in section. Anteriorly it
appears to end in a somewhat club-shaped expansion, but in
reality is extended as a thin sheet of fibres downward toward
the lamina terminalis. Posteriorly it bends downward,
forming the splenium, the latter being attached to the
body of the fornix, which lies below it.

The fornix consists of a pair of longitudinal fibre bands, fused fora

short distance in the middle line to form the unpaired body of the
fornix (corpus fornicis). They begin in the mamillary body, and
passing upward as the columns of the fornix (columnae fornicis), meet
in the body of the fornix, and afterwards diverge laterad as the pillars
of the fornix (crura fornicis), ending in the hippocampus.
Between the body of the fornix and the anterior portion of
the corpus callosum is a thin area of the wall, the septum
pellucidum, the lateral ventricles lying close together in
this region.

The nervous matter covering the corpus callosum may be

removed from one hemisphere by first marking out a triangular

area on the dorsolateral surface; then scraping the material care-

fully away until the white surface of the corpus callosum is well
exposed. By removing the corpus callosum the interior of the

hemisphere may be examined.

(a)

(b)

The lateral ventricle (ventriculus lateralis) is the extensive
space enclosed by the hemisphere. It extends forward
into the olfactory bulb and backward into the posterior free
end of the hemisphere, passing a considerable distance
behind the opening of the interventricular foramen.

The excised portion of the hemisphere, forming the moder-
ately thick roof and dorsolateral wall, consists largely of the
peripheral grey cortex described as the pallium,

282

(c)

(d)

ANATOMY OF THE RABBIT.

The floor is formed by a greatly thickened mass of nervous
matter, appearing from the interior of the ventricle in. the
form of two convex ridges. One of these, posterior and
medial in position, is the hippocampus. The other is
smaller, anterior and lateral in position, and is the corpus
striatum. Between the two bodies the pigmented vascular
tissue of the chorioid plexus of the lateral ventricle may be
made out.

On the medial wall, the thickened posterior portion forms
the body of the fornix, immediately in front of which is the
thinner portion of the wall, described above as the septum
pellucidum.

8. The passage of the olfactory nerves to the ethmoturbinal
surfaces may be traced by removing the nasal bones and working
downward toward the cribriform plate, or the remaining portion
of the skull containing the nasal region still intact may be divided
vertically for a more extended examination of the nasal fossae.
The features to be observed are largely those described in con-
nection with the skeleton (pp. 138, 151).

APPENDIX.
THE PRESERVATION OF MATERIAL.*

The method commonly used in the preparation of material for
dissecting purposes consists in first embalming the body with
suitable preserving fluids; afterwards filling the arteries with a
colored injection mass, so that they are more easily traced. The
objects served by embalming are: (1) preserving the body from
decomposition for a sufficient length of time to complete the
dissection; (2) keeping the body as nearly intact as possible; and
(3) having the organs in good condition for study. The point last
mentioned is an important one, since much depends on having the
parts of the animal in such condition that they are easily and
comfortably handled, and also easily observed. The desired results
are accomplished, first, by introducing the preserving fluid through
the bloodvessels, instead of by immersing the animal, as was
formerly the practice; secondly, by using in the preserving fluid
such materials as will leave the organs in a condition as near the
natural one as possible and at the same time keep them moist and
flexible throughout dissection

A suitable fluid for the purpose is that recommended by Keillert
for the preservation of human subjects. The formula is as follows:

Ra@inaclitier. sai ea S eeb 2 5 eee Segte ee 1.5 parts
CarboliceNeiditnse8 aie. oan eS es Dee DR eee
GIN Genie a: wi Me coor ait cen RMR rc NOMOW a
Wicibeteay, SMe pegite aie Geaira dh Phage uae SOA0. a
100.0

*The methods here given apply only to the preservation of specimens for
ordinary dissection, either singly, or in numbers for a laboratory course; witha few
observations on the difficulties which are likely to be experienced. Especially in
the matter of injections, the student who has acquired some knowledge of the
vascular system will be able to make complete injections of the portal system and
also satisfactory injections of the systemic veins, though the latter are somewhat
more difficult on account of the presence of valves in the vessels. Finer vascular
injections and injections of the lymphatic system according to the directions given
in the anatomical textbooks may also be suggested.

tKeiller, W. ‘On the Preservation of Subjects, etc.’
Anatomy), Vol. II., 1902-3.

’

(American Journal of

284 ANATOMY OF THE RABBIT.

A convenient method of making up the fluid, especially when
embalming the animals in numbers, is to prepare the mixture of
formalin, carbolic acid and glycerin as a stock-solution, to be
diluted for use by adding to each part of stock 6 parts of water.
The amount required varies according to the size of the animal, the
flow of the fluid in the vessels, the length of time during which the
animal is left under the action of the fluid, and the height of the
pressure column. Not less than 1500 cc. should be allowed for each
specimen.

The apparatus needed for embalming includes a reservoir for the
fluid, provided with an exit pipe to which a rubber tube may be
attached; about 6 feet of rubber tubing to connect with the operat-
ing table; several three-way pieces to divide the stream in case
several specimens are to be handled at the same time; selected
rubber tubing of the size indicated below to attach the cannulae;
clamps for the tubing; and, finally, glass or metal cannulae for
insertion into the femoral artery.

Glass cannulae suitable for the purpose are readily made by
heating ordinary glass tubing over the Bunsen flame and drawing
it out to the desired thinness. The tubing used for the purpose
should be of about 6 mm. outside diameter. The cannula when com-
pleted should be about 7 cm. long; and its narrow end should have
a uniform diameter of 1.5-2 mm. for about 2 cm. at the tip. The
tip should be touched lightly in the flame in order to round the
margin by fusion, otherwise it might damage the wall of the
vessel.

The rubber tubing used to connect the cannula with the main
tube should be of the best quality of soft rubber, and should have
an inside diameter of 4 mm., i.e., of proper size to slip on and off
the cannula easily, but yet to retain its hold on the latter under
moderate pressure.

The reservoir for holding the embalming fluid may be an aspir-
ator or irrigator bottle, an enamel fountain, percolator or ordinary
funnel. It may have a capacity of one or two quarts. The
capacity, however, is immaterial, so long as the operator keeps
the fluid replaced. The reservoir is suspended in such a way that
it may be moved up and down within a distance of four feet above
the top of the operating table.

THE PRESERVATION OF MATERIAL. 285

At the time of beginning the embalming process the operator
should have before him the reservoir, suspended at a height of
about three feet, and a column of fluid, free from air-bubbles or
foreign material to the tip of the cannula. This condition must
be maintained throughout the operation. If at any time the
pressure falls in the apparatus sufficiently to admit air, or allow
coagulated blood to run back through the cannula, there is almost
certain to be trouble, not only with the specimen under treatment,
but also others which come after. The column of fluid is held back
until the proper time by a clamp placed on the rubber tubing.

The animal is killed by administering ether or illuminating gas.
It is placed on its back on the table, with the head away from the
operator. The skin is first divided by a small incision on the inner
side of the right thigh.* By inserting the fingers well down into
the incision, the skin may be torn backward and toward the ventral
middle line, and at the same time the superficial epigastric vessels
will be carried with the subcutaneous tissue well out of the oper-
ator’s way. Small portions of the inner surface of the thigh and
of the abdominal wall will be exposed. The white cord repre-
senting the inguinal ligament lies in the bottom of the inguinal
furrow. Appearing from beneath the ligament in this position,
and passing to the surface of the thigh are the femoral nerve,
artery, and vein, covered by an exceedingly thin layer of muscle
belonging to the sartorius. The three structures may be separated
from one another, and the muscle pulled away at the same time,
by working lengthwise along the structures with the fine forceps.
The artery must be thoroughly cleared for about 3 cm. from the
inguinal ligament. Care must be exercised in this operation to
avoid breaking its branches or the tributaries of the vein. The
artery lies in front of the vein and is distinguishable by its smaller
size, flattened or collapsed condition, and by its white coloration.
The vein will be found greatly distended with blood. The nerve
lies in front and partly on the lateral side of the artery.

When the femoral artery has been fully exposed, a ligature of
coarse thread, previously moistened, may be passed around its base,

*The embalming may be done from the common carotid artery of the neck, a
vessel much larger than the femoral artery and therefore easier of manipulation.
This is not recommended, however, because of the damage done to various
important structures of the cervical region.

286 ANATOMY OF THE RABBIT.

close to the inguinal ligament. An ordinary single knot may be
placed on the ligature, but must be left loose until the cannula
is inserted. By grasping the bare edge of the artery at about
2 cm. from the ligament, the operator may make a V-shaped
incision in the vessel with fine scissors. The tips of the scissors
are directed toward the ligament. The incision must be clean-cut,
and care must be taken not to cut more than half-way through the
vessel. By taking up the little angular flap with the fine forceps,
the cannula may be worked into the vessel and pushed well down
into it beyond the inguinal ligament. The knot is then tightened
by a gentle even pull on the ends of the thread. The knot should
never be pulled very tight or doubled.

At the moment when the cannula is securely fastened into the
vessel, the clamp is to be removed from the connecting tube and
the fluid allowed torunin. At the beginning of the process a little
care in arranging the animal will be amply rewarded by conven-
ience in dissection. The hind limb on the side opposite the in-
cision should be drawn backward. The front limbs should be
drawn apart, so that the breast is well exposed, and held in this
position by a thick cord, or, better, a stout flexible wire, passing
around the back of the animal. The body should be turned
slightly to the operator’s left.

The animal is sufficiently embalmed in two hours. About
eight animals may easily be kept on the table by one operator,
provided he has at his disposal a sufficient number of cannulae,
one for each specimen, since the first may be taken off the appar-
atus after the eighth has been put on. After some practice the
number can be greatly increased.

Since small difficulties frequently occur in the process, especially
in placing the cannulae and in keeping them clear of obstruction,
a number of points may be mentioned which indicate to the oper-
ator just how the operation is succeeding. The entrance of the
cannula into the artery, in the first place, is usually accompanied
by a slight rise of blood into its tip. General muscle contractions
in the recently killed animal are a safe indication of uniform flow
of the fluid to these and also other parts of the body, and no clogging
of the vessels need be feared. The fluid may usually be observed
running in the cannula, and, of course, falling in the reservoir.

THE PRESERVATION OF MATERIAL. 287

Finally, there are characteristic changes in the body. The abdo-
men becomes greatly distended, the subcutaneous tissue swollen,
the eyes protrude, and there is usually more or less frothing at
the nose. Leakage, either in the area of the incision or at the nose,
is sometimes a sign of too much pressure. In the former case the
leakage is frequently behind the cannula, and may be stopped by
artery forceps. In the latter case there is no recourse but to
confine the fluid to the nasal cavity by tying the nostrils.

After the embalming process the rubber tube is disconnected
from the cannula, the latter being left carefully in place. The
animal is then set aside, preferably for a few hours, but if wanted

for use immediately the injection may be made after several
“minutes, usually with satisfactory results.

The injection mass may be made by mixing ordinary starch and
water to the consistence of thin cream; then adding a finely-ground
coloring material, such as vermilion or a very small quantity of
carmine. There is some advantage in using a 5% formalin instead
of water alone in making up this mass, the arteries having after-
wards a brighter appearance, which is doubtless due partly to
better preservation and partly to the fixing of the starch in the
vessels. The mass must be thoroughly strained before use, in
order to avoid the presence in it of particles which are too large to
go through the cannula. The injection is made with a syringe,
the latter being provided with a rubber tube of the same kind
as that used in the embalming process. The mass is sent in by
applying a gentle, even pressure, and it is sometimes advantageous
to allow the injection to run backward and forward in the tube,
each time applying a little more pressure. When the vessels have
been filled in this way, the tube is clamped. By drawing on one
cord of the ligature the knot is loosened sufficiently to withdraw
the cannula, and by keeping a finger pressed on the end of the
vessel, the knot may then be drawn tight without loss of injection.

It sometimes happens, despite ordinary precautions, that the
cannula becomes clogged either with settled starch or with coagu-
lated blood. In this case it may be easily removed, cleaned, and
replaced. The same cannula should always be used.

Material prepared according to the directions given above will
keep indefinitely, provided, however, that precautions are taken

288 ANATOMY OF THE RABBIT.

to avoid contamination from the surface. These are especially
necessary in view of the thick coating of hairs. It is a good plan,
therefore, to sponge the animal with a preserving fluid which will
penetrate the coat immediately, or if many specimens are being
prepared, to immerse the whole animal for a moment. A suitable
fluid for this purpose is formalin-alcohol, made by adding 2% of
formalin to a mixture of equal parts of ordinary spirit and water.
The alcohol ensures immediate penetration and assists the formalin
in preservation. The fluid should be squeezed out of the coat so
far as practicable. An excess is undesirable because the alcohol
tends to withdraw fluid from the body if the animals are kept for
some time before dissection, but more especially because the fluid
is likely to get into the material during dissection where it has the
effect of removing glycerin, so that the tissues become brittle and
dry rapidly on exposure.

For the storage of material either before or during dissection
no precaution is necessary except that of protecting the body from
undue exposure to evaporation. The animals may be stored in a
spirit tank if raised above the level of the fluid, or may be kept
individually in special prepared boxes for convenience in_ the
laboratory. A zinc-lined copper box with sliding top, of dimen-
sions 6 x 6 x 20 inches, will be found to be adequate and of proper
proportions for animals of average size.

Abdomen, 176
Abdominal aorta, 202
Cavity, 90, I8t.
wall, muscles of, 179
Abduction in limbs, 65
Accessory respiratory tract, 85
See Nose.
Acinous glands, 23
Acoustic meatus, external, 131, 145, 176
internal, 136, 146
Adaptatien, 13
Adduction in limbs, 65
Adipose tissue, 26
Afferent nerves, 38
Albinism, 27
Alveolar glands, 23
Anal aperture, 177
glands, 198
Analogy, 12
Anastomoses of intestinal
Anatomy defined, 9
Ankle, bones of, 172
joint, 234
Aorta, see Arteries,
Aortic arches, branchial, 89
Aperture, anal, 177
of auditory tube, 82, 247
of larynx, 82, 245
of mouth, 80, 176
of nose, external, 176
internal, 137, 165, 245
piriform, 137, 153
of thorax, inferior, superior, 126
urinogenital, 177
of uterine tube, internal, 200
of uterus, external, 201
Aponeuroses, 26
Appendicular skeleton, 60, 118, 159
Arachnoidea, 270
Arch of aorta, 256
Arches, branchial aortic, 89
visceral, 56, 57
Arrectores pilor um, 22
Arterial ligament, 90, 260
Arteries, chief
aorta, abdominal, 202
arch of, 256
thoracic, 263
development of, dorsal, ventral, 49, 89
carotid, 230, 242, 256, 278
coeliac, 183, 184
iliac, 203
intercostal, 257, 263
lumbar, 203
mesenteric, inferior, 193
superior, 183, 192
pulmonary, 87, 260
renal, 195
spermatic, interna], 197, 200
subclavian, 256
See Bloodvessels.
Arthrodia, 33
Articulations, nature of, 32
of ankle, 234
of hip, knee, 233
Asymmetry, 41
in digestive tube, 79
in vascular system, 90

vessels, 181

INDEX

Atria of heart, 87, 260
Auditory epithelium, 24
ossicles, 55, 148
tube, 82, 133, 147, 247
Axial orientation, 46
Axial skeleton, 51, 118
in chordates, 17
Axis: basicranial, 59, 1290
basifacial, 59
Jongitudinal, of body, 40
Position of organ systems with respect to,

46
Axillary fossa, 176
lymph nodes of, 207
vessels and nerves of, 208

Ball and socket joints,
Beat of heart, 88
Biceps muscles, 35, 64, 213, 223
Bilateral symmetry, 41
Bile duct, 82, 187
Biology, 9
Bipennate muscles, 35
Bladder, urinary, 106
Blood, 39
Bloodvessels, general structure of, 36
lining membranes of, 24
of abdomen, 202, wall, 178, 180
of brain, 278
of limbs, anterior, 208, 215
posterior, 224, 230
of neck, 237, 238, 239, 242
of orbit, 251
of pelvis, 203
See Arteries, Veins,
Body, divisions of, 176
posture of, 42
pituitary, 22, 275
Bones, structure, development, types of, 28
articulations of, 32
Brain, general divisions, 71
dissection, 272
flexures of, 76
olfactory, 71, 273
of frog, 71
Branchiomerism, 47
Breathing, 86
Bronchi, 85, 263
Bulb, olfactory, 70, 272
Bulbourethral gland, 199

33

Caecum, 83, 191
Capillary vessels, blood, 87
lymphatic, or
Capsules of joints, 33
of sense organs, 56
Carpus, bones of, 164
Cartilage, structure of, 27
bones, 20; of skull, 53, 56
Caudal vertebrae, 124
Caval vein, inferior, 183, 204, 260
superior, 257, 260
Cavity, abdominal, 90, 181
of central nervous system, 40, 60, 70
of larynx, 248
of mouth, 80, 245

290 INDEX.

Cavity—continued.
of nose, 54, 85, 137, 282
orbital, 57, 128, 248
pericardial, 99, 259
peritoneal, 99, 181
pleural, 99, 262
serous, 98
of skull, 133
of thorax, 126
tympanic, I3I, 147, 171, 254
Cell, structure of, 19
Central nervous system, general divisions, 70
brainse7. 7551272
spinal cord, 39, 69, 270
Cerebral cranium, 56, 58
hemispheres, 69, 71, 272
nerves, see Nervous System.
Cerebellum, 70, 74, 276
Cerebrum, 74, 75
Cervical vertebrae, 120
Characters of animals, 12
Chondrocranium, 56
Chordates, characters of, 17
Chorioid plexus, structure of, 73
of lateral ventricles, 282
of third ventricle, 274
Circulation, organs of, 87
functions, 88
See Vascular and Lymphatic Systems.
pulmonary, 87
portal, 88
systemic, 87
Classitication, purpose of, 14
of organ systems, 44
Clavicle, 161
Clitoris, 177
structure of, 201
Cloaca in lower vertebrates, 92
Cochlea, 254
Ceelom, 08, 181
Colon, 82, 191
Coloration of skin, etc., 26
Column, vertebral, 51, 118
Comparative anatomy defined, 9
method of, If
Conjugation in Protozoa, 20
Connective tissues, 24
Convergence, I2
Condyles of femur, 170
occipital, 128
of tibia, 171
Cord, spermatic, 198
spinal, wea 270
vocal,
Corpora RE aEene 275
Corpus callosum, 273, 281
Coxal bone, 166
Cranial cavity, 133
nerves, see Nervous System.
Craniota, characters of, 17
Cranium, cerebral, visceral, 56, 58
See Skeleton.

Deferent ducts, 96, 197
Dental! formula, 59
Dentition, in rabbit, 59
in rodents, I5
Derm bones, 29, 58
Descent of testis, 96, 100
Descriptive anatomy, 9
terms, 40
Diaphragm, 264
Diarthrosis, 33
Digestion, process, 78
Digestive tube, wall of, 21, 36, 186, 190

Digestive system, 77
glands of, 23; anal, 198; oral, 23; infra-
orbital, 249; parotid, 235; sub-
maxillary, 238
liver, 186; pancreas, 183, 189.
intestines, small, 189; large, 190
mouth, 80, 245
pharynx, 81, 245
oesophagus, 82, 245, 263; connection with
stomach, 181
stomach, 82, 181
Digits, anterior limb, 165, 177
posterior limb, 173, 177
Dissection, method of, ro
of rabbit, 175
Ducts, otf liver, 187
nasopalatine, 246
of oral glands, intraorbital, 2
parotid, 235; submaxillary, 238
of pancreas, 189
of prostate, 199
thoracic, 91
urinogenital, 92, 195, 197, 199
Ductus arteriosus, 89
deferens, 96, 197
Duodenum, 82, 189
Duplicidentata, 15
Dura mater, 67, 270

Ear, external, 176
middle, 131, 147, 254
internal, 254; capsule of, 56
Efferent nerves, 38
Egg, fertilized, 20
Egg-laying mammals, 16
Elastic fibres of connective tissue, 25
Elbow, position of, 65, 177
Embryo, organ systems in, 48
Embryonic kidney, 94
Embryology, 9
method of, If
Enarthrosis, 33
Encephalon, see Brain.
Endothelia, 24
Environment, relations of animals tc, 13
Epicardium, 259
Epicondyles, of femur, 170
of humerus, 162
Epidermis. 21
Epididymis, 197
Epiglottis, 246
Epiphyses of bones, 31
Epist ropheus, I22
Epithelial tissues, 21
Erect posture in man, 42
Eustachian, tube, see Auditory Tube.
Extension in limbs, 65
Eye, 176; dissection, 248
Eyelids, 176

Facets, articular, of vertebra, 122

Fascia, 26

Fat, 26

Femur, 169

Fertilized egg, 20

Fibres, of connective tissue, 25
muscle, 34
nerve, 37

Fibrocartilage, 27

Fibrous connective tissue, 26

Fibula, 170

Flexion in limbs, 65

Flexures of brain, 76

INDEX.

Follicles, of hairs, 21, 178
lymphatic, 91; ofintestine, 190; of tonsil,
246 S
Ovarian, 200
Foot, 177
skeleton of, 172
See Hand.
Foramina of skull, chief, 132
Forearm, 177
muscles of, 213
nerves and vessels of, 215
skeleton oi, 162
Fore brain, 71, 272
Formula, dental, 59
Frog, brain, 71
Function, relation of structure to, 10
Functional organs, 13

Gall bladder, 186
Ganglia
spinal, 67, 68, 271
sympathetic, of head, 253
prevertepral; coeliac, 183
inferior mesenteric, 191; superior
mesenteric, 183
of trunks, cervical, 245, 259
thoracic, 259, 264; lumbar, sacral,
caudal, 204
Gastric glands, 23
General anatomy, 9
Genital organs, external, 177
female, 199
male, 196
Ginglymus, 33
Girdle, pectoral, 60, 159
pelvic, 60, 165
Glands,
of digestive system, 23
anal, 198; gastric, 23; oral, 23, 235,
238, 249; liver, 78, 186; pancreas,
189
epithelial, 22
of lymphatic system, 91
axillary, 207; inguinal, 179
mesenteric, 190
of orbit, Harderian, lacrimal, 249

of skin, 23 :
inguinal, 23; mammary, 16, 23, 177,
178

suprarenal, 183

thymus, 256

thyreoid, 239

of urinogenital system,

bulbourethral, prostate, 199

Gliding joint, 33
Gonads, 96
Grey substance of nervous system, 38
Gross anatomy, 9
Gubernaculum, 96, 197
Gustatory, epithelium, 24; organs, 246

Hairs, structure of, 21

Hand, 177
skeleton of, 164

Hard palate, 81, 246

Hares and rabbits, distinction ot, 15

Head, dissection of, 235
skeleton ot, 127

Heart, 87, 259

Hemispheres of brain, 69, 272
of cerebellum, 276

Hepatic portal system, 88, 186

Heredity, 12

Hind brain, 74

Hinge joints, 33

Hip joint, 233

291

Histology, 9
Homogeny, I2
Homology, 61
serial, 60
Homoplasy, 12
Humerus, 161
Hyoid apparatus, 55, 158
arch, 57
Hypophysis, 72, 275

Ileum, 82, 190
Ilium, 166
Incisors in rodents, I5
Inguinal furrow, 176; lymph modes 01, 179
glands, 23
Insertion of muscles, 63
Integument, see Skin.
Interarticular cartilages, 27, 234
Intercellular substance, 25
Interpretation of structure, Io
Intestines, 78, 82, 188
large, 190
lymphatics of, 91, 181, 190
muscle layecs of, 21
serous coat of, 116
small, 189
Involuntary muscles, 34
Ischium, 166

Jejunum, 190
Joint, structure of, 33
ankle, 234; hip, knee, 233
Jugular veins, external, 237; in.ernal, 239

Kidney, 93, 194

embryonic, 94

homologies in vertebrates, 94
Knee joint, 233

position of, 65, 177

Labyrinth of ear, 234
ethmoidal, 151
Lacrima] apparatus,
Harderian and lacrimal glands, 249
nasolacrimal canal, 137; duct, 248
Lacteal vessels, 181
Larynx, 247
Leg, 177
muscles of, 226
nerves and vessels of, 230
skeleton of, 170
Leporide, characters of, 14
Lepus, genus defined, 14
Ligaments, 26
Limbs, general position of, 65
anterior, divisions, 177
dissection of, 205
skeleton, 159
posterior, divisions, 177
dissection of, 217
skeleton, 165
Lips, 176
Liver, 23, 186
Long axis of body, 40
Lumbar vertebrae, 123
Lumbosacral plexus, 232
Lungs, 84, 262
Lymph, 39
Lymphatic system, 90
follicles of sacculus rotundus, 190
vermiform process, 191; tonsil, 246
glands, axillary, 207; cervical, 239
inguinal, 179; intestinal, 181; mesen»
teric, 190
vessels of mesentery, 181
thoracic duct, 91

292

Mammalia, characters of, 16
Mammary glands, 16, 23, 177, 178
nipples of, 177
Mandible, 139, 156
Mandibular arch, 57
Marsupial mammalia, 16
Meatus, acoustic, external, 131, 145, 176
internal, 136, 146
Medulla oblongata, 70, 75, 280
Membrane bone, 29; of skull, see Derm
mucous, 21
tympanic, 254
Meninges of central nervous system, 67, 270
Mesencephalon, 73, 275
Mesenchyme, 25
Mesenterial small intestine, 82, 189
Mesentery, 100, 190
structure ot, 99
Mesoderm, 47
Metacarpus, 165
Metamerism, 47
Metatarsus, 173
Microscopic anatomy, 9
Mid-brain, 73, 275
Middle ear, see Ear.
Mixed nerves, 38
Monotremata, 16
Morphology, 9
Morphological aspect of structure, 10
Motor nerves, 38
Mouth, cavity of, 80, 245
glands of, see Digestive System.
Mucous membrane, 21
tunic of stomach, 186; of intestine, 21
Multicellular organisms, 20
Muscle, 62, 63
Muscles, structure and types of, 34, 35, 63
of abdominal wall, 178
of face, 235, 236, 237
of limbs, anterior, 205; posterior, 218
of mastication, 240, 241
of neck, 238, see vertebra)
occipital, 268
of skin, 178, 205, 235
of tongue, 243
vertebral, 265

Nasal cavity, 54, 85, 137, 282
Nasopalatine ducts, 246
Nasopharynx, 81, 245
Neck, dissection of, 235
Nerves, structure and types of, 38, 68
Nervous system, 66, 270
central, brain, 69, 272; spinal cord, 69, 270
peripheral,
cranial nerves, 76
I. olfactory, 273
II. optic, 249, 275
III. oculomotor, 251, 276
IV. trochlear, 251, 276
V. trigeminal, 241, 252, 277
VI. abducent, 252, 277
VII. facial, 235, 253, 277
VIII. acoustic, 277
IX. glossopharyngeal, 244, 277,
X. vagus, 185, 239, 240, 258,
263, 277
accessoly, 244, 277
hypoglossal, 240, 244, 245,
277.
spinal nerves, composition of, 68
cervical, 206, 210
lumbar, sacral, 218, 232
thoracic, 264
plexuses of, brachial, cervical,
210; lumbosacral, 218, 232

XI.
XII.

NOX. &

INDEX.

Nervous system—continued.
sympatheticsystem, 68
ganglia, cervical, 245, 259; cceliac,
183; mesenteric, inferior, 194;
superior, 183; of head, 253; see
trunks.
plexuses, abdominal, 183, 194
cardiac, 258
trunks, cervical, 245, 259;lumbar,
sacral, 204; thoracic, 259, 264
Nervous tissues, 37
Nose, apertures of, external, 176; piriform,
137, 153
internal, 137, 245
cavity of, 54, 85, 137, 282
Notochord, in chordates, 17, 53; in rabbit, 47,

48
Ochodontide, 15
Oesophagus, 82, 215, 263
Olfactory brain, 71, 273
epithelium, 24
Omenta, greater, lesser, 182
Ontogeny, I1
Orbicular muscles, 36
Oibit, 57; structures of, 248
Organs, definition of, 19
relative development of, 13
visceral, 98
Organ systems,see Systems.
Origin of muscles, 63
Ossicles of ear, 55, 148
Osteocranium, 56
Osteology of rabbit, 118
Ovary, 97, 199; position of, IOI
Oviduct, 97, 200
Ovum, I9, 97, 200

Palate, 81, 246
Pancreas, 23, 183, 189
Paraflocculus of cerebellum, 276
Parotid gland, 23, 78, 235
Pectoral girdle, 60, 159
Pelvic girdle, 60, 165
Penis, 177; structure of, 198
Pentadactyl limb, 17, 62
Pericardium, 99, 259
Perichondrium, 30
Periosteum, 29
Peritoneal cavity, 990, 181
Peritoneum, general relations of, 99; parietal,
180; visceral, 181
of bladder, 196
of intestines, 181, 185, 189, 192
of kidney, 195
of liver, 187
of ovary, 200
of stomach, 182 -
of testis, 100, 197
Phalanges, digital, of hand, 165; of foot, 173
Pharynx, 81, 245
Phylogeny, It
Physiological aspect of structure, 10
Physiology, 9
Pia mater, 67, 270
Pigmentation of skin, etc., 26
Pineal body, 73, 273
Pituitary body, 72, 275
Placenta, 16 :
Placental mammals, 16
Planes of body, 41
Plantigrade foot, 17, 62
Pleura, pleural cavity, 90, 262
Plexuses, of spinal nerves, brachial, cervical, 210
lumbosacral, 218, 232 ;
of sympathetic nerves, abdominal aortic,
194; cardiac, 258; coeliac, 183; hypogas-

Plexuses—continued.

tric, 194; mesenteric, inferior, 1094;

superior, 183; renal, spermatic, 194

Pons, 74, 277
Popliteal fossa, 177
Portal system, hepatic, 88, 186

renal, 88
Portal vein, 188
Posture of body, in man, 42

in quadrupeds, 42
Prevertebral ganglia, 68
Pronation, 65
Prone position of body, 40
Prosencephalon, 71, 272
Prostate, 199
Protozoa, cells of, 20
Pubis, 166, 169
Pulse, 88
Pulmonary artery, 87, 260; circulation, 87

Quadriceps muscles, 35

Rabbit, races of, 14
Radius, 162
Rami communicantes, 68, 204
Recapitulation, law of, 11
Rectum, 191
Reduced organs, 13
Reflex action, 68
Regional sections, ror
Renal portal system, 88
Replacing bones, 2
Reproduction in Protozoa, 20
Reproductive organs, 96
female, 199
male, 196
Respiration, 85
relation of ribs to, 86; diaphragm, 264
Respiratory system, 84
accessory respiratory tracts, 85
lungs, 84, 86, 262
respiratory tracts, 85
Retrogressive organs, 13
Rhombencephalon, 74, 276
Ribs, 125
Rodents, characters of, 15
Roots of spinal nerves, 67, 270

Sacculus rotundus, 189
Sacral vertebrae, 124
Sacrum, 124
Scapula, 159
Serotum, 177, 196
Sebaceous glands, 23
Sections, regional, ror
Segmentation of body, 47
Seminal vesicle, 199
Sense organs, special, capsules of, 56
epithelium of, 24
See Ear, Eye, Nose.
Sensory epithelia, 24
hairs, 22, 176
nerves, 38
Septum ot nose, 138
of thorax, 259
Serous cavities, 08
membranes, 24, 98
tunic of intestine, 181
Sesamoid bones, 29
ot limbs, anterior, 165; posterior, 174
Simplicidentata, 15
Skeletal system, 51
tissues, 27
Skeleton, appendicular, 60, 1 18, 159
of anterior limb, 159

INDEx. 293

Skeleton—continued.
of posterior limb, 165
axial, 51, 118
skull, 53, 127; primary composition of,
56; bones of, 140; foetal, 55, 50;
human, 59
vertebral column, 51, 118
Skin, structure of, 21, 26, 178
glands of, 23
muscles of, 178, 205, 235
Skull, see Skeleton.
Smooth muscle, 34, 62
Soft palate, 81, 246
Somatic nerves, 38
Special anatomy, 0
Spermatic cord, 198
Sphincter muscles, 36
Spinal cord, 69, 270
nerves, see Nervous System.
Spleen, 182
Sternum, 127
Stomach, 78, 82, 181
structure of wall, 186
Subcutaneous tissue, 25, 178
Subfunctional organs, I3
Sublingual gland, 23
Submaxillary gland, 23, 238
Sudoriferous glands, 23
Supination, 65
Suprarenal gland, 183
Sweat glands, 23
Symmetry, bilateral, 41
Sympathetic nervous system, see Nervous
System.
Symphysis, 32; of mandible, 139
of pelvis, 166
Synarthrosis, 33
Synchondrosis, 32
Systems, organ, defined, ro, 44
classification of, 44
genetal arrangement, 46
in embryo, 48
digestive, 77
lymphatic, 90
nervous, 66
respiratory, 84
skeletal, 51
urinogenital, 92
vascular, 87
Systemic circulation, 87

Tarsus, bones of, 172
Taste buds o1 tongue, 246
Teeth of rabbit, 59; of rodents, 15
Tendon, 26, 63
Tentorium cerebelli, 134
Terminology, 40
Terrestrial vertebrates, 16
Testis, 96, 1907; descent of, 96, t00
Thalamus, 73, 274
Thigh, 177; muscles of, 210
nerves and vessels of, 224
Thoracic aorta, 263
cavity, 126
duct, 91
vertebrae, 122
Thorax, bony, 126
dissection of, 255
Thymus gland, 256
Thyreoid gland, 239
Tibia, 170
Tissues, defined, 20
classification of, 20
connective, 24
epithelial, 21
muscular, 33

294

Tissues—continued.
nervous, 37
skeletal, 27

Tongue, 243, 246

Tonsil. 246

Trachea, 85, 239, 263

Tracts, respiratory, see Respiratory System.

Triceps muscles, 35

Trunk, divisions of, 176
skeleton, 51, 118

Trunks, lymphatic, 91

sympathetic,see Nervous System, Ganglia.

Tube, auditory, 82, 113, 147, 247
digestive, see Digestive System.
neural, 49, 70
uterine, 200

Tubules of kidney, 93

Tunics of digestive tube, mucous, 21, 186
muscular, 34
serous, 186
of stomach, 186

Turbinated bones, 138

Tympanic cavity, 131, 147, 254
membrane, 254

Ulna, 162
Unicellular glands, 22
organisms, 20
Unipennate muscles, 35
Ureter, 93, 195; epicystic position of, 93
Urethra, 92, 196
Urinary organs, 92, 194
Urinogenital aperture, 177
sinus, 92
system, 92, 194
Uterine tube, 200
Uterus, 200; types of, 98

INDEX.

Vagina, 97, 201
Vasomotor nerves, 62
Vascular system, 87
asymmetry in, 90
See Arteries, Bloodvessels, Veins.
Veins, trunk, caval, inferior, 183, 204, 260
superior, 257, 260
hypogastric, iliac, 204
jugular, external, 237; internal, 239
portal, 188
renal, 195
spermatic, 198, 200
Ventricles of brain, 70, 274, 278, 280, 281
of heart, 87, 260
Vermiform process, 191
Vertebral column, 51, 118
Vertebrates, characters of, 17, 51
terrestrial, 16
organ systems in, 46
Vesicle, seminal, 199
Vestibulum, 92, 201
Visceral arches, 56, 57
nerves, 38
cranium, 56, 58
organs, 98
peritoneum, 181
Vocal folds, 248
Voluntary muscles, 34
Vulva, 177

White fibres of connective tissue, 25
White substance of nervous system, 39
Wild rabbits, species of, 14

Wrist, bones of, 164

Yellow fibres of connective tissue, 25

Zoological position of rabbit, 14

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