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Marine Biological Laboratory

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Accession No. , ,

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PRACTICAL ANATOMY
OF THE RABBIT

BENSLEY'S

PRACTICAL A7s(AT0MT
OF THE RABBIT

An Elementary

Laboratory Text^Boo\

in Mammalian Anatomy

Eighth Edition
Fully Revised and Edited by

E. HOKHE CRAIGIE, Ph.D.,

Professor of Comparative Anatomy and J^eurology
in the University of Toronto

THE BLAKISTOH COMPAHX

PHILADELPHIA

1948

b)! the University of Toronto Press

Printed in Canada

^^y\CAi

Preface to the Fifth Edition

IN the preparation of the present edition of the Anatomy^
Rabbit, comparatively Httle alteration has been made in the body
of the text. Many minor corrections and emendations have, how-
ever, been recorded.

As explained in previous editions, the main purpose of the text
is to set forth in some kind of practical sequence a plan for the
orderly study of a typical mammal, supplemented by a brief
exposition of the relation of this kind of study to the content and
outlook of cognate biological sciences.

The methods adopted by anatomical instructors are subject to
wide variation, based no doubt upon personal preference and per-
haps to some extent upon habit and upon laboratory facilities
available. This is as it should be, though not capable of being
covered by any system of texts. After all, a student must go
through the process of acquiring a rather complex assortment of
detached pieces of information before a comprehensive view of the
ensemble of mammalian organization may be obtained. There is
thus plenty of room for selection in the order of study, provided
the requirement is met of avoiding the destruction of unidentified
parts. One of the older difficulties of anatomical teaching, perhaps
unintentionally fostered by all text-books, namely thorough, and
often unstimulating, preliminary study of the skeleton before dis-
section, is being overcome by judicious division of skeletal studies
into an introductory survey followed at suitable intervals with
special study during dissection. The method of using a region
under dissection as a basis for topical teaching of related physio-
logical and morphological associations has also much to recom-
mend it.

Teaching responsibility is ever an interesting issue but never-
theless so often overlooked that it becomes an important question
as to whether the opportunities and obligations of anatomical
teaching are being met. For the non-professional student perhaps
the chief consideration is the avoidance of half-assimilated, un-
verified information. For the student looking forward to profes-

vi PREFACE

sional courses, time as well as instructional sequence and useful
selection is equally important. It would be unfortunate if the
high reputation always enjoyed by the anatomical sciences, of
moving safely forward point by point were not used to the extent
possible to counterbalance the rather observable tendency towards
pedagogical confusion.

The writer appreciates the response of various instructors to
his desire for information as to necessary and advisable emenda-
tions of the text. He is under particular obligation to Dr. E. H.
Craigie, University of Toronto, for a critical revision of the account
of the central nervous system which should add materially to the
value of this section from both structural and functional points of
view.

B. A. Bensley
University of Toronto
October 1, 1931

Preface to the Eighth Edition

IN the preparation of the eighth edition of the Practical Anatomy
of the Rabbit use has been made of extensive notes recorded in the
laboratory during successive years of employment of the previous
edition. All relevant questions raised by students or other in-
structors for which an adequate answer was found not to be readily
available in the text have been noted and an attempt has been
made to provide answers for them in the revisions. Criticisms
received have been carefully considered and the whole text has been
searchingly surveyed, with the result that many small changes
have been made, parts (e.g. the description of the larynx) have
been expanded, and a few have been entirely rewritten. New
sections have been added, for example, one on the foetal circulation.
A considerable number of new^ illustrations have been included,
some replacing ones which were felt to be unsatisfactory, but the
total number in the book being further increased from 114 to 124.
Functional aspects as well as developmental ones have been
consistently emphasized.

Since some classes study briefly the microscopic structure along
with the gross anatomy, the early chapter on "General Anatomy"
has been somew^hat expanded. For suggestions regarding this and
for provision of some of the photomicrographs the writer is in-
debted to the co-operation of Dr. V. E. Engelbert. For the new
figure 58 he is indebted to the kindness of Dr. C. C. Macklin.
To these and to other colleagues from whom helpful comments
have been received he wishes to express his gratitude.

E. HoRNE Craigie
Toronto
January, 1948

Contents

Page

IXTRODUCTION xi

PART I. A GENERAL COXSIDERATIOX OF THE
STRUCTURE OF THE RABBIT

Divisions and Methods 1

The Interpretation of Structure 2

Zoological Position 5

General Anatomy 11

The Tissues 13

1. Epithelial tissues 13

2. Connective tissues 18

3. Muscular tissues 25

4. Nervous tissues 29

5. Blood and lymph 32

Special Anatomy 34

Terminology 34

The General Features and Ground Plan of the Or(;an-Systems. 38

Classification of the Organ-Systems 38

General Organization 41

Embryonic Plan of the Systems 43

The Skeletal System 44

The Muscular System '63

The Nervous System 71

The Digestive System 93

The Respiratory System 104

The Blood-Vascular System 109

The Lymphatic System 118

The Urinogenital System 122

The Endocrine System 131

The Serous Cavities 135

Regional Sections 138

PART II. OSTEOLOGY OF THE RABBIT

Divisions of the Skeleton 156

The Vertebral Column 156

Cervical Region 158

Thoracic Region 161

63235

CONTENTS

Lumbar Region 161

Sacral Region 163

Caudal Region 164

The Ribs 164

The Sternum 166

The Skeleton of the Head . 166

The Skull as a Whole 166

The Bones of the Skull 180

The Hyoid Apparatus 197

The Skeleton of the Anterior Limb 198

The Skeleton of the Posterior Limb 206

PART IIL DISSECTION OF THE RABBIT

I. External Features 218

11. The Abdominal Wall 220

III. The Stomach and Spleen 224

IV. The Liver : 231

V. The Intestines 234

Vl. The Urinogenital System 242

The Urinary Organs 242

The Male Genital Organs 245

The Female Genital Organs 250

Vll. The Abdominal Aorta, Inferior Caval Vein, and Sympa-
thetic Trunks 253

VIII. The Anterior Limb 257

Blood-Vessels and Nerves of the Axillary Fossa 261

Blood-Vessels and Nerves of the Arm and Forearm 269

IX. The Posterior Limb 272

Blood-Vessels and Nerves of the Thigh 279

Vessels and Nerves of the Leg and Foot 286

The Lumbosacral Plexus 288

The Articulations of the Posterior Limb 289

X. The Head and Neck 292

XL The Thorax 323

XII. The Vertebral and Occipital Musculature 338

XI 1 1. The Central Nervous System 344

APPENDIX

The Preservation of Material 367

Index 375

<$^

2 ( LI BR A ^V

Introduction

AS a laboratory exercise, the anatomical study of an animal is
chiefly a matter of applying a certain practical method of
exposition, 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 discern-
ment of detail, and because, as a means of obtaining first-hand in-
formation, it is the foundation of laboratory practice. Within the
range of natural science, which limits fact to what is reasonably
demonstrable, laboratory practice takes its place as one of the
principal methods by which first-hand knowledge of anything
concrete is gained and it is important for the student as early as
possible to form the habit of acquiring his knowledge in this way
and of confirming thus information obtained otherwise.

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 as species, may be considered by itself, this provides only a much
restricted point of view and a very superficial study suffices to show
that the structure and function of no living creature can be ade-
quately interpreted apart from the general arrangements of organ-
ized nature and, more especially, from the corresponding features
of similar or nearly related organisms.

This being the case, it is a very pertinent question what is the
best procedure. So far as the present book is concerned, it is expect-
ed that the study of the type will begin with at least a preliminary
survey of the prepared skeleton (Part ii). This will be followed by
dissection (Part ill), in which the^ order by sections will be found
tQ be of less importance than that of details in any particular region
and in which portions of the skeleton related to the part under
examination may be included.

The regional method of approach is indicated rather than the
more complete study of single systems, partly for the sake of

xii INTRODUCTION

economy of material and partly in the belief that this aids under-
standing of the topographical and other interrelations between the
systefks, encouraging the building up of a conception of the indi-
vidual organism as an integrated unit.

The general matter of Part I is purely accessory and, though
necessarily incomplete in many ways, is designed to afiford a com-
prehensive view of the various factors upon which mammalian
structure depends. It will be found that only the first few chapters
are introductory in most respects, the remainder being rather ex-
planatory and hence most valuable if used to supplement the direc-
tions for dissection as this is carried out.

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, for the removal of erroneous
impressions of the position of organs incidental to their displacement
in dissection.

L I B R A - V \ ;«

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, fundamental among which are:
Anatomy, the study of organized structure; Physiology, the study
of function ; and Embryology, the study of development. Anatomy
is an essential foundation for the other branches. Comparative
Anatomy, the comparative study of different 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," originally applied to the study of the
structure of the human body, and still used as referring more
especially to this, has come to be applied to the study of structure
of living organisms 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 naked-eye ap-
pearances, 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 considering 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
to be considered. Thus, dissection is 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 their relations to surrounding parts. The plan
is essentially one of analysis, since conceptions of structure are

2 ANATOMY OF THE RABBIT

based on the recognition of differences, the latter being estimated
by various features, such as form, colour, texture, or position. On
the other hand, because of the class of structure with which it deals,
dissection should also be recognized as a method preliminary to
others involving the use of the microscope. Further, the analysis
should- be followed at each step by a synthesis in the mind of the
student, who must keep before him the unity of the individual
organism as a whole.

THE INTERPRETATION OF STRUCTURE

Gross structure is, in a sense, only an expression of the finer
microscopic structure underlying it. 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
a mass 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 by 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 con-
nected with the outside of the body. It is essential to recognize
that in these, as in the multitude of analogous cases, structure and
function are intimately related and serve to explain each other.

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 purely on that basis.
The factors controlling them lie outside the body of the individual
and comprise environmental influences and inherent characteristics
of the whole race, the interaction of which only through a long
series of gradually changing conditions has directed and determined
the evolution of the type to which the individual belongs. As
applied, to a particular animal, the morphological method consists
in explaining its adult structure by reference either to its embryonic

INTERPRETATION OF STRUCTURE 3

development or to the equivalent conditions in lower forms,
existing or fossil. A recognized principle of embryology is that
known as the 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 complex condition but
also reflects in passing the features of lower forms which presumably
represent its ancestors. These features are mingled with, however,
and sometimes obscured by newer, purely embryonic characteristics
which are not ancestral. Moreover, the adult condition of an ad-
vanced animal is sometimes reached by the retention of a character-
istic which was only embryonic in its predecessors. The application
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 grade of structure comparable with that 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 involves
the distinction of its more general features from the more special
ones and the application to them of ontogenetic and phylogenetic
principles.

The present form common to the individuals of one kind of
animal may be explained only by reference to ancestry. Apart
from influences of accident, the sum of characters of the individual
is the result of development, under more or less fixed environmental
conditions, of the primordial cell which constitutes the fertilized
egg. So long as the environment remains comparable with those nor-
mal for the species, such features as are impressed upon the animal
during growth or maturity are negligible in this connection, the
developmental possibilities of the- fertilized egg having been trans-
mitted to it through the succession of generations. Through this
succession the continuity of life, as the fossil remains of organisms
of the past reveal, has carried onward the structure of the body for
countless millions of years.

4 ANATOMY OF THE RABBIT

With succession has come also modification, as is shown by the
appearance on the earth at different geological times of progressive-
ly more specialized animals, which reveal in a large way the same
kinds of differences observable among primitive and specialized
animals 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 comparative 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 pouches
in the embryonic condition, though these are never used for func-
tional gills, is in itself an important fact bearing on adult structure,
but such a condition also illustrates how extensively 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 re-
semblances 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 re-
semblances 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 — or homologous.
On the other hand, their differences, particularly the differences in
homologous parts, are chiefly marks of divergence in evolution. It
is conceivable that many of the internal features of animals are
the result of a general progressive development brought about by
some inherent force in the constitution of the successive generations
of organisms themselves, more conspicuous in comparison of a

ZOOLOGICAL POSITION 5

series from primitive to specialized types. However, the majority
of their differences are such as have resulted from adaptive modifi-
cations of structure, by which they have become differently ad-
justed to the particular conditions of their accepted habitats.
Adaptation to environment is one great result of the modification
of animal form, and is revealed in part by structural 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. Adap-
tation, in other words, is not a matter of present conditions only.
The rabbit as a gnawing mammal , a lagomorph , for example , is also an
air-breathing, walking vertebrate, and shares these relatively large
and ancient features with many other vertebrates of 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 primi-
tive 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 gradual rather than 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 retrogressive 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.

ZOOLOGICAL POSITION

It will be evident from the foregoing statement that every
specialized animal possesses in its organization a vast assemblage

€ ANATOMY OF THE RABBIT

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 or, ulti-
mately, of all three. The study of an animal as a type or repre-
sentative of a group, however, concerns only in part the features
common to the various members of the latter, 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. What an animal actually repre-
sents is determined by comparison with other forms and is called
its zoological position. This is expressed through the medium of
classification, the latter being arranged to indicate, so far as is
possible, the relationships of organisms one to 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-coloured forms, long-haired Angoras, Lop-
€ar Rabbits, and Belgian "Hares" are more familiar. They are
all descendants of the wild rabbit {Oryctolagus cuniculus, Lepus
cuniculus) of Europe. The latter 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 European common raihhit (Oryctolagus cuniculus) belongs
to the family Leporidae, which contains a large number of closely
related species formerly included under the single genus Lepus,
but now distributed among nine genera with living representatives
and a larger number of extinct ones. In addition to Oryctolagus
cuniculus and its derivatives, the more familiar species are the
European Common Hare (Lepus europaeus) and in North America,
the Cotton-tail Rabbit (Sylvilagus floridanus), the Northern or
Varying Hare (Lepus americanus) , and the Prairie Hare or White-

ZOOLOGICAL posrnoN 7

tailed Jack Rabbit (Lepus campestris). In recent decades, European
hares have been several times introduced and liberated in North
America, and are now established in certain parts of the Eastern
United States and in Ontario, Canada. The two common European
species differ in several well-marked features which form the
basis of the recognized 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 for a time after birth are blind and naked. The
hair is more nearly a running or coursing type, and is distinguished
by longer ears, which are, moreover, tipped with black, longer
hind limbs, and prominent eyes. It inhabits an open "form"
and the young directly after birth are clothed with hair and able
to see. Upwards of one hundred species of Leporidae have been
described in North America. They are variously known as hares
or rabbits. It is generally considered that the American forms,
aside from two aberrant genera, are hares, but in form and habits
the Varying and Prairie Hares of the genus Lepus conform more
closely to the type of the European Hare, while the Cotton-tails,
members of the genus Sylvilagus, make at least some approxi-
mation to the European Rabbit.

The various species constituting this family are distinguished
from the Picas or Tailless Hares {Ochotonidae) of the mountainous
districts of Central Asia and of North America (Rocky Mountains),
by several features, including the imperfect development of the
clavicle, longer ears and limbs, and the presence of a distinct al-
though greatly reduced tail. 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 dis-
tinguishes what was formerly described as the suborder Duplici-
dentata from the suborder Simplicidentata, the latter containing
the majority of rodents and embracing all forms with a single pair
of upper incisors.

Authorities now tend to designate the Duplicidentata, to which
the family Leporidae belongs, as the mammalian order Lagomorpha
and to restrict the order Rodentia, which was formerly considered
to include both the above suborders, to the larger assemblage of
mammals with only one pair of upper incisors, such as squirrels.

8 ANATOMY OF THE RABBIT

marmots, cavies, beavers, mice, porcupines. Fossils of both
lagomorphs and true rodents have been found in Palaeocene rocks
and are reported not to be more similar than are modern repre-
sentatives of the two orders. Their greater resemblance to each
other than to other orders is recognized by a questionable associ-
ation in a group designated "cohort direst In both these orders,
the anterior incisors in both upper and lower jaws are modified to
form chisel-like cutting organs, having their enamel layer disposed
chiefly if not wholly on their front surfaces, so that they remain in
a permanently sharp condition. This modification is associated
with an extensive loss of intermediate teeth, involving posterior
incisors, canines, and anterior premolars. There is also elaboration,
often very considerable, of the remaining premolar and molar teeth,
of the lower jaw, and, indeed, of the parts of the skull generally.
Characteristic of these animals is the extension, both forward and
backward, of the jaw-musculature. The articulation of the lower
jaw has an antero-posteriorly elongated articular process fitting
into a corresponding longitudinal fossa on the skull, the jaw being
able to move forward and backward in addition to vertically and
less from side to side. Further, the teeth are curved and the an-
terior or incisor teeth are provided with open roots, so that their
growth is not limited, as it is in the majority of mammals. The
cheek teeth of the rat and other rodents living on mixed diets have
closed roots, but those of which the food is more difficult to masti-
cate, including the Lagomorpha, have open roots as have the
incisors. In these respects and in the elaboration of the intestine,
especially the caecum, the rodents exhibit the characters of highly
specialized herbivores, but in many particulars they are primitive
types. For example, they tend to retain the five-toed (penta-
dactyl), plantigrade foot, characteristic of primitive mammalia and,
indeed, of terrestrial vertebrates, and exhibit also unelaborated
cerebral hemispheres in the brain.

Like all higher or placental mammalia (Infraclass Eutheria),
the rabbit is viviparous, that is, the young are born in a more or
less advanced stage of development, after 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 mam-

ZOOLOGICAL POSITION 9

malia (Order Marsupialia of the Infraclass Aletatheria) of Australia
and South America, the latter being viviparous but, with one
exception, lacking a placenta. The members of both these groups
(which are associated in the subclass Theria) differ from those of
the order Monotremata (subclass Prototheria) of Australia, which
are not viviparous but oviparous, or egg-laying, like the majority
of reptiles and other lower animals. These three divisions of
mammals, however, are united by the common features of the
class Mammalia. For example, they all are warm-blooded, are
provided with a complete double circulation and a hairy investment
for the surface of the body, and nourish the young for a time after
birth by the secretion of cutaneous glands modified to produce
milk, the mammary glands.

Many of the more general features of the rabbit are not charac-
teristic of any one group but are shared with other terrestrial
vertebrates, including mammals, reptiles, birds, and, in part,
amphibians. Such features are the development of the lungs and
associated respiratory tracts, both the true respiratory 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-
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
significance in the terrestrial vertebrates.

The rabbit is like all Vertebrata or Craniata in the possession
of an axial skeleton formed by the segmented vertebral column and
of an organized head region with differentiated brain, special sense
organs, and enclosing primary skull. Also, the vertebrates exhibit
a basic transverse segmentation (metamerism) of a considerable
part of the body.

Finally, the rabbit agrees with other members of the phylum
Chordata in the ground plan underlying the most general features
of its organs and the position, arrangement, and plan of develop-
ment of its organ-systems. Particularly it possesses a dorsal,

10 ANATOMY OF THE RABBIT

tubular central nervous system; a notochord, a fundamental axial
support which is only embryonic except in the lowest chordates;
and a series of gill clefts, such as occurs in every chordate either
in the adult condition, or in the embryo alone.

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 development,^
i.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 the adherence of human
structure in the vast majority of features to the type of higher
mammals is expressed by the fact that man is also a placental
mammal.

Phylum CHORDATA. Animals with notochord and gills.

fPROTOZOA, Annulata, Mollusca, etc., invertebrate phyla.

Subphylum GRANIATA. (VERTEBRATA) Chordates with organized head

region.
fAcRANiA. Lancelets. Also Ascidians and worm-like Chordates, some-
times separately classified.

Class MAMMALIA. Warm-blooded craniates, with hair coat.
Young nourished from mammary glands.

fCvCLOSTOMATA, PiSCES, AMPHIBIA, RePTILIA, AvES,

lower vertebrate classes.
Subclass THERIA. Viviparous mammals.

IPrototheria. Oviparous mammals.
Infraclass EUTHERIA. Placental mammals.

fMETATHERiA. Viviparous mammals without placenta.

Order LAGOMORPHA. Gnawing placentals, with chisel-like incisors, of which
there is in the upper jaw a small second pair directly behind the
main pair.

fRoDENTiA, Carnivora, Perissodactyla, Primates, etc. Various
placental orders otherwise adapted.
Family LEPORIDAE. Hares and Rabbits.

fOCHOTONIDAE. Picas.

^Equivalent groups.

GENERAL ANATOMY U

GENERAL ANATOMY

Although in every respect a continuous structure and forming
a single organism, the body is differentiated into a large number of
parts, or organs, which are more or less individual in form, com-
position, or function. Organs are arranged for the most part in
systems, each of which is concerned with some general or funda-
mental function, to which several organs may contribute.

In a more general way the body may be considered as an
assemblage of tissues, since these are the materials of which the
organs are composed. Tissues may be defined as layers or aggre-
gations of similarly differentiated cells. They are of several
different kinds and are variously associated in the formation of
organs. Being structures of intermediate rank, they may be
considered either as organ components or as combinations of
specialized cells.

As a body-unit a cell consists of a small mass of living proto-
plasm, containing a central body, the nucleus imbedded in a mass
of cytoplasm. The latter is surrounded or enclosed on its free
border by a cell-membrane. The nucleus is a highly organized
body, having an important function in the reproduction of the
cell and also in its general activity or metabolism. It contains a
characteristic formed material, chromatin, and frequently 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.
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

12 ANATOMY OF THE RABBIT

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 usually associated at some
stage with union or conjugation of two parent cells.

All the cells of the body of a multicellular organism are products
of a single cell, the fertilized egg, but the latter is a product of

Fig. 1. Photomicrograph of a developing ovum
within the ovary of a rabbit, from a section.
X 150. ch, chromatin; cy, cytoplasm; fe, fol-
licular epithelium; nm, nuclear membrane; tf,
theca folliculi; zp, zona pellucida.

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.

In this way, the processes which go on within the body of a
multicellular animal and the structure underlying these processes
are all based upon the same elementary functions of life as those
appearing in one-celled organisms. But the repeated division of

EPITHELIAL TISSUES 13

the fertilized egg, in development toward the adult condition, gives
rise by division of labour 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 differences between these depend partly upon the characters
of the cells composing them and partly upon the nature and
quantity of the material between the latter, the intercellular matrix.

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 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.

1. Epithelial Tissues

Epithelial tissues are distinguished chiefly as surface invest-
ments, such as those of the exterior of the body, and those of the
interior of the alimentary canal, the lungs, the respiratory and
accessory respiratory tracts, and the ducts of the urinogenital
organs. In all epithelia the cellular feature is a prominent one,
the amount of intercellular material being relatively small. With
a few exceptions, they are not penetrated by blood-vessels. As
constituents of lining membranes, they are not conspicuous in
gross structure but they give rise to important derivatives, such as
the hairs and the various kinds of secreting organs or glands.

Epithelium may be simple, i.e. one-layered, or it may be strati-
fied, i.e. composed of several layers of cells. The cells composing
it may be flattened or squamous, cuboidal or isoprismatic, or col-
umnar and are packed together so closely that in free surface view
each is seen to be pressed into a more or less hexagonal form. (See,
for example, the epithelium lining the collecting tubule of the kid-
ney in Fig. 73, p. 126.) The epithelium of the skin (Fig. 2),

ANATOMY OF THE RABBIT

which is known as the epidermis, or scarf-skin, is stratified, the
deepest cells being columnar, formative, growing units which pro-
duce daughter cells that are gradually pushed outward and in-
creasingly flattened so that those at the surface are squamous
and are successively discarded. The several
layers combined produce but a thin mem-
brane. It extends over the entire surface
of the body and connects at certain points
with the epithelia of the internal surfaces.
It is supported by a thick resistant layer
of connective tissue which forms the true
skin or corium. Between the epithelium
and the underlying connective tissue there
is usually a distinct basement membrane,
derived from the inter-cellular substance
of the connective tissue.

In the greater portion of the alimen-
tary tract the lining epithelial layer is sim-
ple or one-layered and is associated with
a thin layer of smooth muscle to form a

From a section of HIUCOUS tuniC (Fig. 15, t.mS.). The CClls

ow rl^i:%:'erJ:ri7s. ^re columuar and have on their free sur-

ttsle°'oftriu';;re;e7d?rm^! ^^^^^ ^ ^^"^ ^^ modified protoplasm, the
f hair follicle; g sebaceous striated bordcr, which is important in ab-

gland; p, papilla; s, hair shaft. ' '^

sorption.

The free surfaces of the epithelial cells sometimes have delicate,
hair-like outgrowths, which may be non-motile (brush border, as
in the proximal convoluted tubules of the kidneys) or motile (cilia,
as in the uterine tubes, respiratory passages, etc.).

Among secondary products of epithelial origin is the coating of
hairs on the surface of the body, the presence of which is a strictly
characteristic mammalian feature. It is a protective investment
arising from the epidermis. Each hair is produced by the modifi-
cation 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 the general
surface of the body of the rabbit the hair follicles are arranged in

Fig. 2.

GLANDULAR EPITHELL\

groups, 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 brings the hair into a more nearly erect position.
Important modifications are displayed in glandular epithelia and
sensory epithelia.

Glandular Epithelia

Epithelial glands are composed of epithelial cells which have
become greatly modified as secreting structures. In some cases the
secreting element is a single cell, a unicellular gland lying directly
in the general layer of epithelium. The mucus-secreting goblet cells
of the intestinal wall are structures of this nature. In other cases
groups of secreting cells form ingrowths
from the main layer, multi-cellular
glands. The lumen or cavity of the
gland, in most cases greatly complicated
through the division of the gland sub-
stance, is connected with the general
surface by a duct which serves to carry
away its secretion. In some cases,
however, the connection of a gland with
the epithelial surface is only 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. Typical epi-
thelial glands are accordingly external
secreting, or exocrine glands, their se-
cretions being discharged through ducts
to free surfaces; while those which lose
their ducts are internal secreting, or
endocrine glands, their products being absorbed directly into
the blood or lymph (cf. p. 131). There are many structures,
such as the suprarenal gland and part of the pituitary body,

Fig. 3. Diagrams of main types
of multicellular glands. A, simple
tubular gland; B, coiled tubular
gland; C, branched tubular gland;
D, simple acinous gland; E,
branched or compound acinous
gland.

16 ANATOMY OF THE RABBIT

commonly described as endocrine glands, which are not necessarily
epithelial in origin and of which the resemblance to ordinary glands
is often only a matter of superficial appearance and of the fact that
both secrete, i.e. produce special substances in their cells and dis-
charge them either on a free surface or towards the blood stream.
Most 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). Both types occur in simple, little
branched, and greatly branched conditions (Figs. 3, 4), and the
tubules of the former type may be elongate and greatly coiled.

Cutaneous glands of two types are commonly present in
mammals in association with the hairs, namely, sudoriferous or
sweat-glands, which are of the tubular type, and sebaceous glands,
which are of the acinous type and produce an oily secretion. 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 the sudoriferous type.
Besides the mucus-secreting cells of the
general epithelium, the glands of the ali-

FiG. 4. From a section of , . , .

the parotid salivary gland of mcntary caual compHsc the important but

the rabbit. A, Duct system , , , i i i r i n i

in relation to body of gland: Icss elaborated glauds oi the Wall, such as
fntedobiiar duct" B° Three the gastrlc glands of the stomach ; and the

individual acini, highly mag- ,, ,, ,, ..i- 1J

nified. greatly elaborated, outstandmg glands

which lie beyond the wall and are con-
nected 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,
buccal, and infraorbital glands^conspicuous structures in the dis-
section 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

GLANDULAR EPnHLLIA

swallowing. In mammals it also contains ferments or enzymes,
chiefly ptyalin, which is capable of transforming starch into 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 im-
portance in digestion. The pancreas is mainly an exocrine gland
of compound acinous structure (Fig. 5). Imbedded among the
acini, however, are numerous small solid groups of endocrine cells,
the islets of Langerhans, which produce the important hormone,
insulin.

Fig. 5. Part of a section of the pancreas of the rabbit. X 130. Two
islets of Langerhans appear in the upper part of the field and a smaller,
less conspicuous one near the bottom. The ancini composing the greater
(exocrine) portion of the gland are well shown and an artery and a vein
also appear at the bottom.

The foregoing are all secretory glands. The term gland, how-
ever, is also applied to excretory organs, such as the kidney, which
remove from the body, with litt]e or no alteration, substances
brought to them by the circulatory system. This process contrasts
with secretion, in which the epithelial cells manufacture new pro-
ducts out of the materials brought to them. There are also cyto-
genic glands, which produce living cells, namely the ovaries, testes,
and lymph glands.

^^\CAc

iBRAf^V

•3;

18 ANATOMY OF THE RABBIT

Sensory Epithelia

There is a close association between the epithelia of the surface
of the body and the nervous tissues. In the adult we may dis-
tinguish as sensory epithelia special aggregations of cells lying in
either a deep or a superficial position, associated more or less
closely with the central nervous system, and functioning for the
reception of stimuli. That is, appropriate agencies set up in them
states of excitation which are then transmitted along sensory
nerves.

They comprise the olfactory epithelium of the nasal cavity,
some of the cells of which are true nerve cells, the gustatory
epithelium of the tongue, and the auditory epithelium of the mem-
branous 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 distinguished
from certain special coverings of internal spaces, the endothelia
and mesothelia. The two latter consist microscopically of thin
pavement-like cells. They differ from epithelia in origin, being
formed, not in connection with originally free surfaces, but in
relation to spaces of the mesoderm or intermediate layer of the
body. Endothelia form the linings of 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, 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 substance or
matrix. The cellular portion is formative, and is much more
conspicuous in the embryonic than in the adult condition. All
connective tissues are products of an embryonic tissue, the mesen-
chyme (Fig. 22, ms.), which consists of branched cells connected
by their outstanding processes and typically suspended in a rela-
tively large amount of tissue fluid. Through the activity of the

CONNECTIVE TISSUES

cells there is formed an intercellular material consisting of either
a homogeneous matrix, or, more frequently, a matrix containing
formed elements of a supportive nature.

Certain types of connective tissue cells occurring in various
parts of the body have in common the function of phagocytosis
and the property of taking up and storing minute particles of
foreign materials brought to them in dilute colloidal solutions.
These cells constitute the reticulo-endothelial or macrophage
system and form an important constituent of the spleen, for
example.

Ordinary Connective Tissues

In the adult condition the ordinary connective tissues, with
few exceptions, consist of the cell basis with three kinds of fibrous
elements, the white, the yellow (Fig. 6) ,
and the reticular fibres all lying in a
ground substance which is partly a
watery fluid and partly more viscid
in nature. White fibres are relatively
coarse, single, unbranched, of various
sizes, and of great strength, each
constituted by a very compact
bundle of fine fibrils. The yellow
fibres are of smaller diameter.
They branch and communicate,
but are not associated to form
bundles. They also differ from

. . I'll 1 • fi^' ^- Areolar connective tissue

white fibres m bemg highly elastic, (subcutaneous tissue) of the rabbit;

_>, • 1 ri 1 • from an embalmed specimen: a dia-

ine reticular fibres are less COnspiC- grammatic representation to be com-
pared with the photograph in Fig. 7:

UOUS in most connective tissues, c.c. connective tissue cell; w.f., bundle

rT->i 1 1 1 1 • ^1 of white fibres; y.f., vellow elastic fibre.

They are related to the white fibres

but form a close-meshed network. The tissue produced in this way
is known as fibrous connective tissue. It occurs in several forms
according to the relative concentration of the different kinds of
fibres or the admixture 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. 7) which connects the skin with the body; but occurs
also in various positions where it has a similar function of joining

ANATOMY OF THE RABBIT

Structures loosely together. Subcutaneous connective tissue is a
white material, the peculiar appearance and properties of which
are due to the fact that the white and yellow fibrous elements are
arranged in a loose felt-like network (Fig. 7) and reticular fibres
are not important components. When stretched, 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.

rv^8%^

Fig. 7. Photomicrograph of stained preparation of areolar connective
tissue — subcutaneous tissue of rabbit. X 150.

Fibrous connective tissue may be greatly modified through the
concentration of any one of the fibrous elements. Concentration
of white fibres is, however, the common modification. This con-
dition is illustrated in the thick connective tissue layer forming the
true skin or corium, but is more conspicuous in the glistening
white tendons (Figs. 8, 37) 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 attach-
ment. The cells are few and are squeezed between the closely pack-
ed w^hite fibres. The structures known anatomically as fasciae are
special sheets of connective tissue covering chiefly individual mus-
cles or muscle groups. Concentration of yellow fibres occurs in

COXXECTIVE TISSUES

8. Photomicrograph of small part of a
longitudinal section of a muscle of a rabbit,
showing part of the tendon, composed of con-
nective tissue fibres, with coarse muscle fibres
joining it obliquely at each side. X 100.

the dorsal ligament of the neck (ligamentum nuchae) where greater
elasticity is required. The ligament is not so conspicuous in the
rabbit as in larger mammals,
where the yellow coloration
is very noticeable. Elastic
membranes composed chiefly
of yellow fibres with scattered
cells occur in the walls of
blood vessels and of dis-
tensible viscera.

Since the fibres are non-
living and cells are few in
tendons, ligaments, and elastic
membranes, vessels also are
few in these situations.

Fat or adipose tissue is a soft form of connective tissue in which
the cells predominate over the intercellular components and are
greatly enlarged by the inclusion of relatively enormous quantities
of fat in the form of globules. Each globule is enveloped by a thin
film of cytoplasm, which is slightly thickened at one side to
contain the flattened nucleus of the cell. Such tissue tends to
occur in certain definite situations, such as in association with the
blood-vessels, but also is found in locations where areolar connective
tissue might be expected to occur. Special fat masses, sometimes
distinguished by unusually dark coloration, occur at the side of the
neck and between the shoulder blades of the rabbit. In the foetus
(cf. Plate VI) these are represented by large masses of vascular
connective tissue. They correspond with the so-called 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, the ciliary body,
and the iris of the eye, is due to the presence of pigment granules,
partly in special connective tissue cells, chromatophores, and partly
in epithelium. The absence of such granules in animals belonging
to species normally coloured constitutes albinism, a condition
indicated by the whiteness of the hair and by the pink colour of
the eyes, the latter being due to the circumstance that the blood-
vessels of the vascular tunic are not concealed by pigment.

22 ANATOMY OF THE RABBIT

Mesothelium, mentioned on page 18 as a type of epithelium, is
related in embryonic origin (from mesenchyme) to connective
tissue. It consists of a single layer of extremely flattened cells
reinforced by some connective tissue fibres and cells. Endothelium
also could be classified here both on account of its origin from
mesenchyme and on account of the fact that its cells, though
squamous, may under certain conditions differentiate into connec-
tive tissue cells.

Skeletal Tissues

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. 9) is a semi-solid and some-
what resilient material of a bluish or pearly coloration. It consists

of a matrix which appears homo-
_ geneous unless certain special tech-

-'772 niques are applied to it and in

Q'^^

V;j

,0.

which the cells are imbedded. The

^-'., ^te^iO' cells are distributed singly, or

: 1^ ; *v^ f^ more often in groups of two to

four, each cell, or occasionally two

cells, being contained in a small

oval space, the cartilage lacuna.

The size of the spaces, and also

their distance apart, are subject

^ V to great variation. The presence

il^X^fSonVT'iokJrSnt- of large numbers of white fibres

IXut a?™:^„'a'ri.n;/v"uof ""^ . in the matrix produces a modi-

fication known as fibro-cartilage.
This occurs in certain definite situations, as in the symphysis of
the pelvis, or in connection with the inter-articular menisci and
at the capsular margins of the joints. Elastic cartilages, such as
the epiglotic cartilage, contain many yellow fibres.

The surface of the mass of cartilage (except the articular surfaces
within joint capsules) has a limiting membrane of fibrous connective
tissue, the perichondrium (Fig. 10) which develops new cartilage,

SKELETAL CONNECTIVE TISSUES

Fie. 10. Photomicrograph of a
stained section of hyaline cartilage
from the ear of the rabbit. X 230.
Shrinkage of the cells make the
lacunae in which they lie conspicuous
and the quantity of matrix is rela-
tively small. The perichondrium ap-
pears at the left side.

adding It to that already present, during embryonic growth and
may do so also in the adult in repair of injury.

In the adult skeleton cartilage is present only in small amount.
It forms the articular surfaces of
joints, the ventral portions or cos-
tal cartilages of the ribs, and a
portion of the nasal septum; it is
also found uniting the basal bones
of the skull and supporting the walls
of the respiratory passages. In the
embryo, however, it forms the en-
tire skeleton, with the exception
of a small portion which, as de-
scribed below, is composed of
membrane bone. In the course of
development, except in the situa-
tions indicated, the cartilage is
replaced by bone.

Vessels and nerves may be absent in cartilage, the small amount

of material exchange re-
quired to maintain the life
of the cells taking place
by diffusion through the
ground substance.

Bone is a compact, re-
sistant, but yet somewhat
elastic tissue, possessing
much greater strength than
cartilage, and therefore
forming a more perfect
skeletal support. As indi-
cated below, its appearance
as a tissue differs some-
what according to its mode
of formation. The more
typical structure (intra-

FiG. 11. Intramembranous bone from a ground lllCmbranOUS boue) is il-

transverse section of a radius (human). X 120. he, i- i

Haversian canal; hi, Haversian lamella; il, inter- lustratecl m the aCCOmpaUy-
.stitial lamella; lac, lacuna.

ANATOMY OF THE RABBIT

ing figure (11) of aground transverse section of the dried shaft of a ra-
dius. Itsdry weight consists of about one-third 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 greatly branched spaces,
the lacunae, which connect with each other and with the outer
surface of the bone by very delicate canaliculi and in which during
life the bone cells are accommodated. In the natural condition,
each bone is enclosed, except on its articular surfaces, by a layer of
connective tissue, the periosteum, derived in the case of replacing
bones from the perichondrium of the original cartilage. During
the period of growth, this membrane contains numerous bone-
forming cells, the osteoblasts, through the activity of which the
bone lamellae are deposited.

Compact bone of the adult develops in the embryo from spicules
which become connected to form a spongy network, the marrow
spaces in which are then gradually reduced by deposition of
successive layers of bone-matrix until they
appear as canals, the Haversian canals, sur-
rounded by concentric lamellae. The con-
centric series of bone-cells and lamellae con-
stitutes a Haversian system and the canal
often contains an artery and a vein with a
little connective tissue. Between the Haver-
sian systems there may be bands of inter-
stitial lamellae and over the outer and inner
surfaces, parallel with them, zones of circum-
ferential (respectively periosteal and endos-
teal) lamellae.

Bone may be formed either with or without
a cartilage basis, being known in the former
case as cartilage or replacing bone, in the lat-
ter as membrane or derm bone. The for-
mer is the more usual, replacing units of
the primary cartilage skeleton, which ap-
pears earlier in the embryo. This it does
both by surrounding and by invading the units

Fig. 12. Inner surface of
proximal end of a dried
femur of a rabbit divided
longitudinally, ct, cancel-
lous bone in proximal
epiphysis; el, epiphysial
line; me, marrow cavity of
shaft, enclosed by compact
bone.

MUSCULAR TISSUES 25

referred to. In only a few regions, as already indicated, the car-
tilage persists throughout life. The difference between the two
types of bone is not fundamental, however, since both are formed
by soft connective tissue, the latter invading a cartilage model
when this exists and destroying it bit by bit previous to the de-
position of bone-matrix.

Membrane bone occurs less extensively than replacing bone. It
is exemplified by the roofing and facial bones of the skull, most
of which have a flattened tabulate form, and by the clavicle. It
is formed in connective-tissue membranes and may contain cartilage,
but does not develop on a cartilage basis. Skeletal units of this
type lie superficial to the other skeletal elements, a feature which
is due to the fact that they represent surface plates which in lower
vertebrates are associated with the skin.

Only in a few cases are the bones of the skeleton solid — 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 blood-vessels and marrow-spaces. Thus in a long
bone (Fig. 12) the central portion or shaft consists of a cylinder of
compact bone surrounding an extensive space, the marrow-cavity,
filled with soft vascular tissue which, in the adult animal, is the
most important region of red blood formation. 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.

It may be noted that, in addition to its function as a mechani-
cally supporting or protecting material, bone serves as a reservoir
•for the important substance, calcium, which can be dissolved in
the blood or redeposited in the bone as the needs of other parts of
the organism may demand.

3. Muscular Tissues

Muscular tissues are the active portions of the individual
muscles which move the skeleton and of the muscle coats of visceral

ANATOMY OF THE RABBIT

Fig. 13. Involun-
tary muscle, from
a section of the
muscular tunic of
the intestine.

organs. Their chief feature consists in the elon-
gation of the cells to form fibres. These fibres
possess the contractile properties of simple proto-
plasm, but with the contraction limited to one
direction. Except in a 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
contraction in both the muscle and its individual
fibres is the shortening of the longitudinal axis and
the increase of the transverse axis. Muscles are
important in the production of heat, which is
liberated not only when the muscle is in action
but also, though less rapidly, in repose.

Two chief types of muscle fibres occur in the
body — 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 compose the individual or
voluntary muscles of the skeleton. Smooth fibres
(Fig. 13) are elongated, spindle-like cells, the
substance of which is longitudinally striated as a
result of the presence within each of numerous
fine myofibrils, 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 autonomic ner-
vous system. They respond slowly to stimula-
tion but are capable of prolonged contraction.
The striated fibres (Fig. 14) are very much lar-
ger, cylindrical structures, the substance of which
possesses characteristic transverse striations in
addition to the longitudinal ones. These are
due to the myofibrils being composed of alter-
FiG 14 Parts of two mating light and dark portions, those of adjacent
preparation of a'piece ^brils being placed side by side so as to give an
musciTo?! rSbit^^'^ appearance of transverse bands. Each fibre is

MUSCULAR TISSUES

enclosed by a loosely attached, viscous, elastic membrane, the sar-
colemma, and contains many nuclei. The presence of the latter
indicates that the fibre is not a single cell but a syncytium, i.e., an
association of cells unseparated by cell boundaries. The muscles
formed by such fibres are under the control of the will, their connec-

FiG. 15. Photomicrograph of a longitudinal section of the pyloric region of
a rabbit. X 10. The end of the pyloric antrum appears below, the beginning of
the duodenum above, m.m., muscularis mucosae; t.m.c, circular layer of the
muscular tunic; t.m.l., longitudinal layer of the muscular tunic; t.ms., mucous
tunic; t.s., tela submucosa; t.sr., serous tunic.

tions being directly with the central nervous system. They com-
prise not only the typical muscles of the skeleton, but also the
special muscles which serve to connect the skeleton with the
skin.

Two kinds of striated muscle fibres occur, red and pale. Those

28 ANATOMY OF 11 1 P: KABBJ r

of (he former kind liavc well (Icfmcd myofibrils and rather in-
coiispicuioiis transverse bands, are usually somewhat small in
diameler, and contain a greater quantity than do the pale fibres of
a reddish, iron-rontaining substance (myoglobin) related to the
haemoglobin of blood. The pale fibres have less cytoplasm, more
cons|)ienous transverse striations, and usually greater diameter,
and (iu'ii- nuclei are more nearly confmed to the inner surface of
(he sarcolenuna. Kvd fibres contract more slowly than white but
aie more resistant to fatigue. The proportion of these two types
xai-ies in differenl muscles, and in many animals (as man) one
never preponderates so greatly as to make an ol)vious difference
in the aj)i)earance of the gross muscles. In the rabbit and many
other am'mals, however, some nuiscles (e.g. the semitendinosus
and the soleus) are definilely red and others (e.g. the adductor
magnus) are pale or white.

The muscular substance of the heart differs from both stri-
ated and smooth muscle in beingcomi)osed of branched anastomosing
fibres, which apparently form a continuous network and which
have (heii- nuclei (HMitrally placed. Like striated nuiscle, it possesses
charact eristic (ransvense markings, but, like involuntary muscle,
it is under I he control of the autonomic nervous system. Certain
differentiated cardiac nuisc le rvWs, the Purkinje fibres, constitute
a conducting system (bundle of His, etc.) which regulates the
contraction of the chambers of the heart.

In the gross, voluntary muscles present a longitudinal striation
which is roughly referred to as the direction of the fibres, and which
is of great value in identification. 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 adja-
cent bundles by a connective tissue enclosure called the
perimysium.

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 nmscles
in its relations to particular pavts. It forms the muscular por-
tion (muscularis mucosae) of the mucous tunic of the alimentary
<-\'mal, and also a separate muscular tunic lying in the outer portion

NERVOUS TISSUKS 29

of its wall (Fig. 15). In the muscular tunic the fibres are arranged
in both circular and longitudinal directions. Involuntary muscle
also forms a small constituent
of many organs, such as cer- \*
tain glands, in which contractil- \

ity is not a chief function. It "^ ^ 4-

is a large constituent of the wall "^ . c.q.

of the urinogenital tubes, par- l^>i! ^. .

ticularly the bladder and the -^v*?-4i.-

uterus. In association with elas-
tic connective tissue, it is an im-
portant constituent of the walls ^""^ ^
of the blood-vessels. ^.r.

The fundamental cause of '•"■- l^'- Ncrvccell from the ventral grey

. column of the spinal cord (cf. Fig. 18): d.,

the shortenmg of muscle fibres dcntrltes; e.g., chromotophile granules; nr.,

ncurite.

is not yet understood nor is

the significance of the transverse striations, when these are {pres-
ent, known.

4. Nervous Tissues

Nervous tissues are the essential components of the central
nervous system and of the outlying nerves and ganglia. They
comprise two kinds of elements — neurons or nerve cells and
neuroglia cells. The former alone carry on the essential nervous
functions, while the latter are supporting structures forming in the
central nervous organs a mass of neutral tissue in which the neurons
are imbedded.

Nerve cells differ greatly in form, but typically each consists of
a cell-body (Fig. Ki) bearing two kinds of processes — a single
axon, neurite, or neuraxis, and a series of branched protoplasmic
processes, the dendrites. The nerve-cell body is characterized
by the presence in its cytoplasm of granular masses, the chro-
matophile or tigroid bodies, or Nissl granules, the size and arrange-
ment of which are distinctive of certain types of nerve cells. These
extend into the dendrites but not into the axon. The dendrites,
which may be greatly elaborated and may be few or many, conduct
nerve impulses towards the cell body and the axon conducts them
away from it. The latter may traverse a relatively enormous dis-

ANATOMY OF THE RABBIT

-a

.— nr

bodies of other neurons,

by terminals of various

Unmyelinated fibres lack

tance before it ends. A nerve fibre consists of an axon with, in
some cases, certain enclosing sheaths. Two kinds of nerve fibres
are distinguished — myelinated and unmyelinated fibres. In the
former, the axon is surrounded by a layer of fatty material, the
medullary or myelin sheath. In peripheral
nerves, a second sheath, the neurilemma, which
is composed of independent cells, encloses the
myelin, and as the latter is interrupted at certain
points, the nodes of Ranvier, the neurilemma
there comes into contact with the axon itself.
Near the peripheral end of the axon, first the
myelin and then the neurilemma disappears,
so that the nerve-ending is devoid of either
covering. The naked portion usually breaks up
into numerous small branches which end on
the dendrites or cell
on muscle fibres, etc.
characteristic forms,
the myelin sheath.

Both types of fibres are present, in varying
proportions, in many peripheral nerves as well as
in the central organs.

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 most nerves is due
to the fatty materials of the myelin sheaths, but nerves are com-
monly found imbedded in a fatty connective tissue which is as-
sociated with the epineurium and is also of white coloration.

Nerve fibres, and hence nerves, are organs of conduction and as
such are designated afferent if they conduct impulses towards the
central nervous system or efferent if they conduct away from it.
Sensory nerves are afferent, while motor nerves are efferent.
Nerves, however, usually contain both afferent and efTerent fibres
and when the proportions of both are considerable, they are de-
scribed as mixed. In the central connections as well as in the

Fig. 17. Parts of two
myelinated nerve fibres
from a teased prepara-
tion of the sciatic nerve
of a rabbit in which
the myeHn sheaths have
been stained dark with
osmic acid. a. axon;
m. myehn sheath; n,
neurilemma; nc. neuri-
lemma cell; nr, node of
Ranvier.

NERVOUS TISSUES 31

peripheral distribution of both afferent and efferent fibres, there is
a marked difference between those associated with the external or
somatic portions of the body and providing for external adjust-
ments to the environment and those connected with internal or
visceral portions and serving for the internal integration of processes
within the body. Consequently somatic and visceral kinds of both
afferent and efferent fibres are distinguished.

cgv

Fig. 18. Photomicrograph of transverse section of the spinal
cord of a rabbit. X 15. The central canal is visible in the middle
of the picture, cgd, dorsal column (horn) of grey matter; cgv,
ventral column (horn) of grey matter; fd, dorsal funiculus of the
white matter; fl, lateral funiculus of the white matter; fmv,
ventral median fissure; fv, ventral funiculus of the white matter;
rd, dorsal root of spinal nerve; rv, position of ventral root of
spinal nerve; smd, dorsal median sulcus.

Nerve fibres differ among themselves in such properties as
calibre, thickness of the sheaths, and reaction to certain stains and
it has been possible to associate some of these differences with the
conduction of certain specific kinds of impulses.

While the nerve-cell bodies mostly lie within the central nervous
organs, some are in the peripheral nerves. The majority of the
latter are grouped into definite masses, each of which is called a
ganglion.

On account of the difference in colour produced by the presence
or absence of myelin, the cellular and the myelinated fibrous

32 ANATOMY OF THE RABBIT

constituents of the central nervous organs produce characteristic
patterns according to their varying concentration. Where cell
bodies, dendrites, and unmyelinated fibres preponderate, the tissue
has a greyish colour and is hence distinguished as grey matter;
while the concentration of myelinated fibres produces an opaque
white appearance similar to that seen in the larger peripheral
nerves, whence the tissue is described as white matter. In the
spinal cord (Fig. 18) the grey matter is disposed as a central core,
the white substance as a peripheral investment. An essentially
similar though much elaborated distribution is found in the basal
portion of the brain (Fig. 123), but the characteristic pattern in
the cerebral hemispheres and in the cerebellum is one in which the
grey substance forms a peripheral, investing, or cortical layer (Figs.
115, 117, 123).

In order that the conduction of excitation waves through the
nerve cells and fibres may be effective, the neurons must be linked
up in functionally useful patterns, the complexity of which is often
almost inconceivably great. Each neuron receives or transmits
impulses from or to others at special points of intimate contact,
the synapses, through which the excitation can pass in only one
direction though within a single neuron it can be propagated in all
directions. The synapses occur almost entirely within the grey
matter.

Neuroglia is a special type of connective tissue unrelated de-
velopmentally to the true connective tissues but having a common
embryonic origin with the nerve cells. The neuroglia cells are
much branched but lack the distinctive features of neurons. Among
the true neuroglia elements occur small, phagocytic, migratory cells
related in origin to ordinary connective tissue. These are known
as microglia.

5. Blood and Lymph

Blood is fundamentally a cellular material, but owing to the
fact that the intercellular matrix takes the form of a liquid medium,
the plasma, in which the cells, or corpuscles, are suspended, its
features largely differ from those of the ordinary tissues of the body.
On account of its liquid character, the appearances presented by
blood in dissection, especially of preserved animals, are almost

BLOOD AND LYMPH 33

negligible, in spite of the importance of its functions. The cellular
components comprise (a) erythrocytes, (b) leucocytes, and {c)
platelets. The erythrocytes or red blood cells are microscopic
biconcave, circular discs of uniform size and definite though not
rigid contour, containing no nuclei in adult mammals. They are
so soft and flexible that they squeeze readily through capillaries of
diameter smaller than their own. They have a yellow colour when
seen singly, or deep red when observed in bulk, on account of the
presence of haemoglobin. The latter material is
the specific carrier of oxygen, with which it
forms a readily dissociated chemical compound.
Arterial, oxygenated blood is bright red, while
venous blood is dark red. The number of red
blood cells is relatively somewhat greater in the ^^^ ^g ^^^ ^^^^^
rabbit than in man, there being over six millions f^fP^efp' pJoSe/^'"'''^'
contained in each cubic millimetre. The cells
are formed first in the yolk-sac wall of the embryo, later in the
spleen and liver, and in the marrow of bones. The leucocytes, also
termed white or colourless blood cells, are amoeboid, nucleate cells,
present in the blood in much smaller numbers than the erythrocytes,
but occurring also in lymph. A number of different kinds is recog-
nized and named. They are classified first as granular or agranular,
the former comprising neutrophiles, eosinophiles, and basophiles,
the latter comprising lymphocytes, monocytes, and some others ap-
parently transitional. They are formed in the lymph glands, in
the spleen, and elsewhere (cf. p. 121). Being capable of passing
through the walls of the smaller vessels, they occur more or less
throughout the tissues, where they have the function of carrying
materials or of ridding the body of injurious substances and bac-
teria. The platelets are minute, non-nucleate masses of cytoplasm
derived chiefly by fragmentation from cells in the bone marrow.
They are related to coagulation of the blood and help to seal up
small openings in the walls of the vessels.

In all multicellular animals, large proportions of the tissues are
necessarily more or less distant from the surfaces of absorption and
excretion. Blood and lymph, circulating through the vascular
system, are the media by which communication with these is kept
up, providing for the transportation of materials essential for the

34 ANATOMY OF THE RABBIT

maintenance of life processes in the tissues. 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, how-
ever, has been shown to vary in composition in different species of
animals, and to be chemically homologous in related ones; and it
can develop substances conferring immunity to bacterial diseases,
which substances also differ in different species and individuals.

In most tissues there occurs a tissue fluid the amount and
character of which differ in different localities. This is derived
primarily from the blood by diffusion through the walls of the
capillaries. All materials passing between the tissue cells and the
blood stream are transmitted through it. It is taken up by the
blind beginnings of the lymphatic capillaries within the tissue,
within which vessels it is known as lymph. This is eventually
poured into veins and thus mixed with the blood.

The peritoneal, pleural, and pericardial fluids and the synovial
fluid in joints are special examples of tissue fluid and the cerebro-
spinal fluid also may be so regarded.

SPECIAL ANATOMY

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.

The terms of the first three groups will be defined so far as
required wherever it seems necessary. The terms of orientation,
however, being based on very general relations of the body, are of
wider application and understanding of them is essential for any

TERMINOLOGY 35

anatomical description to be intelligible. For these reasons they
are selected for definition here to the exclusion of others of a more
restricting or individualizing kind.

In all vertebrates we may recognize a longitudinal axis, corre-
sponding, 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, 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, which is one dividing the body into right
and left halves, and transverse, coronal, and sagittal planes.
Transverse planes are situated at right angles to the longitudinal
axis. Coronal planes are longitudinal and horizontal, at right
angles to the median vertical plane. Sagittal planes are longi-
tudinal and vertical, parallel to and including (as midsagittal) the
median vertical plane.

The median vertical plane is the centre of bilateral symmetry,
each half of the body, as divided by it, being in a general way the
reverse counterpart of the other. Structures or situations partly
in the median plane are unpaired, and are described as median,
while positions 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
reduction 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

36 ANATOMY OF THE RABBIT

whole, or in particular parts, the pairs of opposed 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 distinction from 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,
lateral and medial, or right and left.

The limbs being more or less independent structures, it is proper

to apply to them certain terms not otherwise applicable to the main

portion of the body. The chief terms which are used principally

in this connection are proximal, meaning nearer the centre or base

of attachment, 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 upper and

lower surfaces of the fore foot are described respectively as dorsal

and volar, those of the hind foot as dorsal and plantar (or volar).

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 sarne time,

terms applied in the first instance to the human body, in which the

recognized relations are somewhat different. In contrast with

that of a quadrupedal vertebrate, the human body occupies a

vertical or erect position, 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. In all, however, 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 equivalent to anterior

TERMINOLOGY 37

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
comparative 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.

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 of interest only 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 employing
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 ta
another without considerable qualification.

38 ANATOMY OF THE RABBIT

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. The manner in which the
comparative method is applied in interpretation may be demon-
strated 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 preliminary statement of how the origin of certain out-
standing features of the rabbit may be explained and what grades
of organization they may be presumed to illustrate. It will be
recognized that the lower mammals are in many respects less
specialized than man and must accordingly show in these respects
various stages through which the human species must be assumed
to have passed. Nevertheless, this principle is not applicable to
all parts and cannot be assumed to be true in any particular case
without critical examination.

Classification of the Organ-Systems

The term organ-system is employed in descriptive anatomy to
designate a group of organs which co-operate 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 comprises the skin, and its
derivatives, such as the claws (or nails), hairs, and various glands
of epidermal origin.

(2) The skeletal system comprises the cartilage and bone
elements of the skeleton, with their connections.

(3) The muscular system comprises all contractile structures
of the body. Since, however, the involuntary muscles are arranged
as muscle layers in connection with visceral organs, the muscular

CLASSIFICATION OF ORGAN-SYSTEMS

system is usually considered as including only the individual or
voluntary muscles of the skeleton and skin.

(4) The nervous system comprises 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 and the autonomic nervous system. The
last is made up of a pair of ganglionated sympathetic trunks, with
two series of ganglia, collateral and peripheral, interposed
between these and the visceral organs, and of a set of visceral
ganglia and nerves (parasympathetic system) connected with the
cranial and sacral regions. Associated with the nervous system
are the special sense organs of the head belonging fundamentally
in part to the nervous system and in part to the surface layers of
the body.

Fig. 20. Schematic representation of the chief organ-systems of a
generalized vertebrate as seen in a transverse section of the abdominal
region:

Integument — int.

Skeleton— V, vertebral body; av, vertebral arch; cv, vertebral canal.

Muscular system —sm, skeletal muscle; vm, visceral muscle.

Nervous system — ms, spinal cord, with the central canal, and the dorsal
(posterior) and ventral (anterior) roots^of the spinal nerves; grp, ganglion
of the posterior root; re, ramus communicans to sympathetic trunk; rma
and rmp, ventral and dorsal rami of a spinal nerve; ts, sympathetic
trunk.

Digestive system — i, intestine.

Vascular system — ao, aorta.

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

Serous cavity— cp, general coelom, pleuroperitoneal, or peritoneal
cavity; pv and pp, visceral and parietal parts of the serous tunic — visceral
and parietal peritoneum; mes, mesentery.

ANATOMY OF THE RABBIT

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

(6) The respiratory system comprises the lungs, and respira-
tory passages, namely, the bronchi, the trachea, and the larynx.
\Mlh this system may also be included the accessory respiratory
passages formed by the nasal fossae.

(7) The vascular system comprises the organs of circulation
of the blood and the lymphatic system. The former are the heart,
the arteries, the capillary vessels, and the veins; the latter the
lymph-conducting canals, which, though they ultimately empty
into the veins and do not constitute an independent circulatory
path, are sometimes considered as forming with their associated
lymph glands a separate lymphatic system.

(8) The urinogenital system includes 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 comprise, 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.

Only included in part, or
omitted in this classification are
certain organs which physiologi-
cally at least may be grouped
together because they have general
regulatory and growth-controlling
They constitute the internal secreting, hormone, or

Fig. 21. Rabbit-embryo of 10»/2 days
(4.8 mm.): m., manibular; h., hyoid; 1
and 2, first and second branchial arches;
a.l., anterior Hmb-bud; me., nietameres;
p.l., posterior limb-bud. (After Minot
and Tavlor, in Keibels Normentafeln, V;
Fig. 12.)

functions.

ORGAXIZATIOX OF ORGAN-SYSTEMS 41

endocrine system , and include portions of the reproductive organs
and pancreas, the hypophysis, pineal body, suprarenal, thyreoid,
parathyreoid, and thymus glands.

General Organization

It has already been pointed out that all chordates have a
structural ground plan involving the possession of an axial skeleton
in the form of a notochord, a dorsal tubular central nervous system,
and a series of gill clefts leading from the alimentary canal to the
exterior, and that all vertebrates (which constitute a subphylum
of the chordates) are basically segmented.

The rabbit being a vertebrate, its organ-systems are disposed
in conformity with these fundamental principles.

(1) Axial orientation. Associated with the elongated shape
of the body in most animals there is a general lengthwise arrange-
ment of the principal organ-systems, which thus lie more or less
parallel to one another. This can be observed in invertebrates,
such as the annulate worms, in respect of the more nearly original
systems. In the vertebrates, the presence of the vertebral column
establishes a structural axis, with reference to which the organ-
systems are arranged (Figs. 20, 22).

(2) Metamerism. A large portion of the body, mainly dorso-
lateral 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. This segmentation, or metamerism, does not
appear to any extent on the surface of the adult body, but becomes
evident internally 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. 21, me.) and is founded upon the serial arrange-
ment of parts of the mesoderm (Fig. 22, my., d.m.).

(3) Branchiomerism. This is a secondary segmentation,
superposed upon the primary metamerism by the development of
a series of gill clefts w^hich do not always correspond precisely with
the metameres. It is an adult feature of lower aquatic vertebrates,
such as fishes (Fig. 32), where it is expressed in a series of true gill,
or branchial structures, associated with gill filaments as functional

ANA'l'()M^■ ov rill': RAHin r

irsi)ira(()i-\' organs. In lii^licr Icnwsdial animals it appears as an
onibryonic fcalurr [Vlu;. 21 , in., h., 1, 2) and is to be eonsidered both
as a determinant of adnlt form and as a mark of aqnatic ancestry.
It mulcrlies (he arrani^ement not onl>- of structnres which in the
lower vertel)rates belong- to fnnctional gills (branchial arches in the
restricted sense) but also of modified branchial structures such as
the first visceral, or mandibular arcli (m.) and the second visceral,
or liyoid arch (h.). The modification of these structures in passing

ect.

V fns

\^?':-:--

Fig. 22. Transverse section of a rabbit-embryo of about 10 V2 days,
showing the arrangement of the organ-systems: ao., aorta; ch., notochord;
coe., coelomic cavity; d.m., dorsal mesoderm (myotomic and sclcrotomic
divisions); e., primitive alimentary canal (cnteron); ect., ectoderm; l.b.,
limb-bud; ms., mesenchyme; my., external portion of a myotome; n., neph-
rotome of embryonic kidney; intermediate mass of mesoderm; sp. and so.,
splanchnic (\isceran and somatic (parietal) portions of the ventral mesoderm.

from the embryonic to the adult condition is very great, but their
arrangement 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 certain soft structures, including- the fifth, seventh, ninth, and

EMBRYONIC PLAN OF THE SYSTEMS 43

tenth cranial nerv^es and the chief arterial vessels of the heart,
which are more fully referred to below.

The fundamental significance 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 designated
Chordata. In the various invertebrate phyla are found respiratory
surfaces of many kinds, such as thin surface membranes, external
tufted, or invaginated tubes, and analogous structures, but these
provide for diffusion without perforation of the body tube.

Embryoxic Plan of the Systems

Governed by the broad principles of organization just indicated,
the individual organ-systems are disposed according to a general
plan the main features of which may be outlined as follows:

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

2. The formation at the anterior end of this axial support of
(a) a primary cartilage skull (chondrocraniumj as a support for
the brain, with capsules for 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 rnuscle in a dorsolateral
position along the axis.

4. The formation of the central nervous system as a tube of
nerv^ous matter (neural tubej, lying on the dorsal side of the axial
support, and differentiated into a generalized posterior portion, the
spinal cord, and an expanded and specialized anterior 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 tube, afterAvards connected with the out-
side of the body by accessory respiratory tracts traversing the head.

44 ANATOMY OF THE RABBIT

7. The formation of the circulatory system primarily on an
aquatic plan. This involves the formation of (a) the heart in a
position ventral 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,
in which the upper ends of the aortic arches unite, and which
passes 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.

8. 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.

9. The formation of the kidneys, either as embryonic or as
permanent structures, from an intermediate mass of tissue, lying
in general between the dorsal musculature and the lining of the
coelomic cavity (cf. position of embryonic kidney in Fig. 22).

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

The Skeletal System

The designation "vertebrate" has reference to a common feature
of fishes, amphibians, reptiles, birds, and mammals — the possession
of a backbone or vertebral column, composed of individual seg-
ments, the vertebrae. Vertebrates are, however, more properly
described as animals having an internal skeleton.

Skeletal Architecture

The skeleton being composed of nearly rigid materials, it is
necessary, in order that movement may be possible, that these
should form many separate pieces, designated cartilages or bones
according to the material composing them. In the embryonic
condition, cartilage rudiments form a complete but primitive
skeleton and in some lower vertebrates the skeleton remains entire-
ly cartilaginous throughout life. The latter, however, is probably
a degenerate condition for, although cartilage may actually have
originated earlier than bone in the history of living beings, bone

THE SKELETAL SYSTEM

__.._.t^

if .2 i id I

. 0.3^ o
<-- o - i; "

., "^ c o
•-~ o

rt>

•• " — oj t<

o ag 4J

46 ANATOMY OF THE RABBIT

was already present in many of the oldest known fossil vertebrates.
In most vertebrates, the cartilage rudiments later are largely re-
placed by bone and membrane bones are added. In many cases
the replacement of these elements by bone is not direct, certain
readjustments being necessary both for purposes of growth and to
meet the much more special functional requirements of the adult
skeleton.

The way in which replacing bones are formed on the cartilage
basis explains many peculiarities of the adult skeleton. In the
embryonic condition the cartilage rudiments are enclosed by a
connective tissue sheath, equivalent to the periosteum of a bone
(p. 24), but described as the perichondrium. Like the periosteum,
this sheath contains many osteoblasts, which form bone material
both in the interior of the cartilage (endochondral bone) and on
its surface (intramembranous bone). The formation of endo-
chondral bone proceeds from certain localized areas, known as
centres of ossification, into which active cells of the perichondrium
are carried by vascular ingrowths, the periosteal buds. In the
vicinity of these the cartilage matrix partly dissolves and the cells
thereby set free die and disintegrate. The ingrowing buds extend
into the spaces thus produced, constituting the primary marrow,
and deposit layers of bone-matrix round the irregular calcified
spicules of cartilage-matrix which still remain. Such deposition
of bone material gradually extends from the centres of ossification
through the remainder of the cartilage replacing it. This condition
is partly illustrated in the distal epiphysis of the humerus shown in
Fig. 26 A, where the area of endochondral bone (eb) appears in the
centre of the mass of cartilage.

In 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. 24), may
possess several such centres — a principal one for the chief epiphysis
or actual extremity of the bone and several subsidiary centres for
its outstanding processes. In the shaft the formation of endochon-
dral bone is of short duration. Through the activity of the osteo-

THE SKELETAL SYSTEM

U'. TTia.

rrw.

Fig. 24. Outline sketch of the
proximal end of the femur of a
young animal: cf., principal epi-
physis for the head of the femur.
The accessory epiphyses are for
the great (tr.ma.), lesser (tr.mi.),
and third (tr.t.) trochanters.

blasts lying directly in the perichondrium, or later the periosteum,

a process of formation of intramem-
branous bone goes on, continuing to
the end of the period of growth, and
the result of this peripheral deposition
of bone lamellae is, that the trans-
verse diameter of the bone is greatly
increased. The enlargement of the
marrow-cavity, with which this is
associated, is produced by the ab-
sorption of bone from the interior.

In young animals both the epi-
physial centres and the masses of
cartilage in which they are formed
are sharply marked off from the body of the bone (cf. Fig. 26).
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, into
which the surrounding perichondrium ex-
tends as an ossification ridge. In this
region bone formation takes place, with
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 still be visible. Thus in theMistal 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 figure 12, which represents a divided femur, it may
be seen that the position of the epiphysial lines is indicated by bands

Fig. 25. The occipital portion
of the skull in a three-day-
old rabbit: bo, basi-occipital
bone; ch, occipital portion^ of
chondrocranium; co, occipital
condyle; eo, exoccipital; fm,
foramen magnum; so, supra-
occipital.

ANATOMY OF THE RABBIT

of compact tissue. If the bones of young animals are thoroughly
macerated, the epiphyses are usually found to be readily separable
from the main parts of the bones.

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, nothwithstanding their
possession of several centres of ossification, are to be looked upon
as individual structures, while 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

K^^"^^

Fig. 26. 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; Ip, 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 meniscus.

throughout life, or, as in the occipital region (Fig. 25), 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.

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 fibro-cartilage. The

THE SKELETAL SYSTEM 49

articulations of bones are of two types — immovable articulations
or synarthroses, and movable articulations, diarthroses, 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. Synarthrosis may be formed by ligamentous
union, distinguished as syndesmosis. This is exemplified between
carpal or tarsal bones and between the radius and the ulna. The
articulation of the bones of the skull (except in the basal region as
indicated below), which are fitted together by more or less uneven
edges or surfaces, with usually only a small amount of fibrous
tissue continuous with the periosteum between, is known as suture,
which is thus a special case of syndesmosis. Cartilage union, or
synchondrosis, occurs in certain situations, as in the basal region of
the skull. Union by fibrocartilage, or symphysis, is found in the
articulation of the two sides of the pelvis (symphysis pubis), and
in that of the two halves of the mandible.

In a joint (Fig. 26), the apposed surfaces of the bones are
accurately modelled in relation to each other, and are, moreover,
covered by layers of cartilage, the articular cartilages, which form
joint cushions. 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 continuous with the perichondrium of the articular cartilage
or with the periosteum of the bone. Within the capsule, perichon-
drium is present at the edges of the articular cartilages but thins
out and is absent over the surfaces which actually rub together.
The synovia is secreted by the inner or synovial layer of the 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. 26, B), interarticular cartilages (menisci) are enclosed between
the bone surfaces, and the latter are connected directly by short
ligamentous cords. The various ligaments of a joint permit free
motion of the bones, but only up' to a certain point, which varies
according to the functional needs of each particular joint.

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

I ^ I I ^

v%V — / o

50 ANATOMY OF THE RABBIT

those of the shoulder and hip, a bone is able to move in various
directions about its base of attachment, although actually, in the
limbs, this motion is almost restricted to an anteroposterior di-
rection. 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 arthrodia 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.

Skeletal Regions

The internal skeleton of a vertebrate nearly always consists of
a principal or axial portion and an appendicular portion. The
axial skeleton is formed by the vertebral column, the ribs, the
sternum, and the skeleton of the head; the appendicular skeleton
by the parts devoted to the support of the limbs, though in the
case of terrestrial vertebrates it would be more precise to say that
these are devoted to the support of the body on the limbs and to
locomotion. In both fore and hind limbs, the skeletal support
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 adherence to a common ground plan is
evident from their composition.

The Vertebral Column

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 a characteristic type
of vertebra can be identified for each region. In fishes, which live
in a medium of about the same weight as their bodies, the line of
the vertebral column is straight, and there is little indication of
regional differentiation. In terrestrial vertebrates, on the other
hand, especially in mammals, the vertebrae are not arranged in a
straight line but form a curve dorsad in the trunk or thoraco-

THE VERTEBRAL COLUMN 51

lumbar region. This constitutes an arch between the attachments
to the fore and hind Hmbs for the support of the body in a Hght
medium when the Hmbs raise it from the ground. The anterior and
posterior ends of the body, which project beyond the supporting
limbs, are sustained by the column curving ventrad in the cervical
and caudal regions (with, of course, stout ligaments dorsally). In
man, the curvatures are modified in relation to the assumption of
an erect attitude, a lumbar curve ventrad developing to counteract
the dorsal curve of the thoracic region, and the caudal region is
reduced to a vestige, the coccyx, consisting of coalesced vertebrae.

■»' /„, «'. ^^\ S

Fig. 27. 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 process; s, spinous process; t, transverse process.

The individual vertebra is made up of a massive ventral body,
or centrum, and a dorsal arch, both composed of replacing bone
(Fig. 28). Both body and arch bear processes which extend into
the surrounding muscles and serve for their attachment. The
principal processes are dorsomedian or spinous, lateral or trans-
verse, and in the lumbar region, dorsolateral or mamillary. Less
prominent processes bearing surfaces for mutual articulation are
also present.

Generally speaking every vertebra has three principal functions
— to support the body, to protect the spinal cord, and to provide
a basis of attachment for muscles. In the support and movement
of the body, the ventral part of the vertebra is subjected mainly to
forces compressing it, the dorsal part to forces of tension. Hence

52 ANATOMY OF THE RABBIT

the ventral part, or centrum, is massive and the dorsal arch and

processes are thinner and weaker.

Any vertebra of the rabbit may be
compared with the corresponding vertebra
of any mammal or in a general way with
those of any vertebrate, and will be found
to show resemblances and differences corre-
sponding with those of the precise functions
performed by the bones compared. The
fifth lumbar vertebra of the rabbit, for
example, would be found distinguished by
Fig. 28. Lumbar vertebra of the great development of its proccsscs,

four-day-old rabbit. Cartilage -it r i i

dotted, bone shaded. X5. smcc the latter support powcrful 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 axial support. A corresponding vertebra of the bear will be
found more or less intermediate between the two types (Fig. 27).
The two most cephalic vertebrae are very much modified to
provide for the movements and the support of the head. The
centrum of the first vertebra, or atlas, has lost its connection with
the rest of this part and has fused with the anterior end of the centrum
of the second vertebra, the epistropheus, to form a pivot round which
the ring-like atlas can rotate. The ventral part of the adult atlas is
derived from a small element, the intercentrum, which in other
vertebrae has disappeared. These changes occurred in early
reptiles but reach their most perfect expression in mammals.

The Notochord

The axial line of the vertebrae passes through the centres of the
bodies, the position occupied in the embryo by the notochord
(Fig. 22). Some of the lower aquatic chordates, such as lampreys,
exhibit the notochord in both young and adult conditions, and
show little indication of the development of the elements of verte-
brae. Others, of slightly more advanced grade, such as sharks
(Fig. 29), show the notochord surviving more or less to the adult
condition with the vertebral elements developed round about it.

THE SKULL

Fig. 29. Transverse section
of shark vertebra Ccartilage
stage), from young specimen
of Atlantic dogfish, Acanthias:
i, intercalary cartilage, com-
pleting arch; n, notochord;
V, body of vertebra.

The Skull

Composition of the Skull
The head skeleton of a mammal, usually
but inaptly called the skull, is a complex
of individual bones and cartilage, the
arrangement and functions of which may
be determined with a Httle effort. The
general disposition of the bone elements,
demonstrable in the rabbit or any mam-
mal is as indicated in Fig. 30. Briefly,
there is a linear series of basal segments,
comprising from behind forward basioccip-
ital, basisphenoid, presphenoid, and mes-
ethmoid. The three first-named form the
floor of the brain-case, while the meseth-

moid forms the nasal septum. Associated with the basioccipital
are paired, lateral exoccipital bones, and a supraoccipital element,
together forming an occipital ring enclosing the aperture for trans-
mission of the spinal cord from the cranial cavity (Fig. 25). In
ancestral tetrapods, the basioccipital and exoccipital bones took
part in the formation of a single rounded condyle for articulation
with the first vertebra. In mammals, however, the posterior end
of the basioccipital has become reduced, leaving only the two
lateral components of the original condyle to make this articulation.
Hence the mammal has paired occipital condyles, borne upon the
exoccipital bones only and constituting a stronger joint with motion
practically restricted to the vertical plane. The basisphenoid and
presphenoid bear lateral expansions, respectively the greater and
lesser wings, or alisphenoids and orbitosphenoids, which assist in
the formation of the side walls of the brain-case. The bony capsule
(periotic bone) lodging the internal ear on either side is solidly
built into the lateral cranial wall between the exoccipital and the
alisphenoid, while further forward- the light scroll-Hke surfaces of
the ethmoid bone, or ethmoturbinal, representing the chief, or
olfactory portions of the nasal capsule, are attached on each side
of the base of the mesethmoid. The original extent of the nasal
capsule is, however, more nearly commensurate with the general

ANATOMY OF THE RABBIT

cavity of the nose; and additional turbinal surfaces, comprising
the nasoturbinals and maxilloturbinals, are attached secondarily
to the nasal and maxillary bones, the maxilloturbinals occupying
only the respiratory or non-olfactory portion of the cavity.

To this foundation of cartilage bones there is added a series of
enclosing membrane 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 a median interparietal (absent in the
hare and many other mammals), paired parietal, squamosal, frontal,

mT^'

Fig. 30. 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 (visceral arch I): e, mesethmoid; et, ethmo-
turbinal; ex, exoccipital; f, frontal; hy, hyoid (visceral arch 11); i, incisors;
1, lacrimal; m, molar teeth; mn, mandible; mx, maxilla; mt, maxilloturbinal;
n, nasal; nt, nasoturbinal; os, orbitosphenoicJ; p, premaxilla; pa, parietal; pi,
palatine; pm, premolar teeth; pt, pterygoid; sq, squamosal; so, supraoccipital;
th, thyreohyal (visceral arch III); v, vomer; z, zygomatic.

and nasal bones as roofing structures, 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 protection of the orbit and for 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.
An important though inconspicuous portion of the head skeleton
is formed by the hyoid apparatus supporting the tongue, and

THE SKULL 55

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 three bones of the auditory chain (the series of small
bones in the middle ear — malleus, incus, and stapes) and the
elements just referred to, together with certain replacing or derm
elements, constitute the modified remnant in the mammal of what
is often regarded as a third main division of the internal skeleton,
namely, the visceral skeleton.

Fig. 31. Lateral view of skull of rabbit foetus, 45 mm: cb, co, en,
cranial, orbital, and nasal portions of primary chondrocranium ; fr, frontal;
pa, parietal; pi, palatine; pmx, premapilla; sq, squamosal; st, styloid process;
i, incus; ip, interparietal; m, malleus; mn, mandible; mx, maxilla; na, nasal;
t, tympanic; zy, zygomatic. (Born plate model, after Voit.)

Chondrocranium and Osteocranium

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. 31). As a cartilage skull it is designated as the chondro-
cranium, and after its conversion into bone as the osteocranium.
It is no more than an enclosure foY 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. This, the primary
skull, is designated as the neuro-cranium or cerebral cranium, to

ANATOMY OF THE RABBIT

distinguish it from a second portion of the head skeleton, the
splanchnocranium or visceral cranium, which includes the series
of visceral arches suspended from the ventral surface of the neuro-
cranium. The addition to the primary head skeleton of a large
number of membrane bones results in more or less obscuring 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 cra-
nium, while the true cartilage
or cartilage bone portions of
the former, occurring as the
auditory ossicles, the hyoid,
and the larynx (in part),
although highly modified, re-
main in a more or less in-
dependent relation.

The components are strik-
ingly distinct in the mam-
malian skull during the later
stages of foetal development,
the cartilage of the chondrocranium and the bones ossifying in its
interior forming a basal mass, from which, as a foundation, are sus-
pended 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 incus is attached to the skull, while the mal-
leus 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. 31).

Head Skeleton of Lower Vertebrates

This condition of the developing skull in a mammal finds its
explanation far back in the history of the vertebrates and is made
clear only by the study of some one of the lower fishes such as
shark or sturgeon. In a shark (Fig. 32) the entire internal skeleton
is formed in cartilage which persists throughout life. The principal

Fig. 32. The chondrocranium and visceral
arches of the Atlantic dogfish, Acantliias: ca,
auditory capsule ; ch, chondrocranium ; en, nasal
capsule; h,h', dorsal and ventral segments of
hj'oid arch (II); i, intercalary cartilage of
vertebral column; m,m', dorsal and ventral por-
tions of mandibular arch (I), functional upper
and lower jaws; malleus and incus of mam-
malian ear; or, orbit, depression for optic capsule;
V, vertebra; 1-5, branchial arches.

THE SKULL 57

part of the head skeleton is a massive cartilaginous box (chondro-
cranium), enclosing the brain, and including, as a result of growth
and fusion, the nasal and auditory capsules. The eye capsules are
free, and are accommodated at the side of the cranium in an orbital
depression, to be seen on the skull of all vertebrates. This type
of structure is obviously the basis of the mammalian skull, ex-
cept that in the latter the cartilage mass is more nearly restricted
to its basal portion and at the same time is more specialized
in 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 iaws. Its composition illustrates the

• 1 • r 1 • • r 1 1 Fig. 33. Sha-

phyloe:enetic basis oi the origm or the external green denticles of

r ■, , . 1 • • 1 1 • the smooth dog-

part of the auditory chain in the mammalian fish, Musteius; en-
embryo, the two bones which this part comprises
corresponding with the posterior ends of the two cartilaginous
jaws of the sharks. Such a relation establishes the fact 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 rudimentary state of the primitive tongue,
is nevertheless obviously homologous with the hyoid arch of a
mammal. In most sharks its upper portion has an important
accessory function in the support of the jaws and this part prob-
ably becomes the third bone (stapes) of the auditory chain in the
mammal. 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 the parts of an extensive
system out of which have been formed by specialization part of
the hyoid apparatus and most, if not all, of the laryngeal cartilages
of mammals.

So far as the surface or roofing portion of the skull is concerned,
it is represented in a shark only by dermal teeth or shagreen
denticles (Fig, 33), uniformly distributed in the skin of the body,
and slightly modified in shape and size at the aperture of the mouth
to constitute definite teeth lining the jaws. In sturgeons and re-
lated fishes, however, these structures are already concentrated

58 ANATOMY OF THE RABBIT

into a definite pattern of surface plates, having in general the same
disposition as derm elements in the skull of all higher vertebrates,
and distributed in such a way that they form an almost complete
enclosure for those parts of the chondrocranium which they invest.
The pattern of the dorsal elements is best indicated in fossil am-
phibia 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), enclosing
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, in-
ferior turbinal, and periotic bones;

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

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

(a) The primary mandibular and hyoid- 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.

Dentition

Accessory structures intimately associated with the visceral
skeleton, though related functionally to the digestive system, are

^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) and possibly the other laryngeal cartilages are modified bran-
chial arches, but the structure as a whole is not included with the head skeleton.

THE TEETH 59

the teeth. Those of the rabbit present two characteristically
mammalian features; they are heterodont, or differentiated in
different regions of the jaw; and they are diphyodont, that is to
say, the adult series are permanent teeth, which, excepting those
designated as molars, replace deciduous, or milk teeth of the
young animal. In lower vertebrates, in contrast with this con-
dition, 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, con-
sidering the differentiation of the teeth, it is possible to express
their relations by a dental formula. In the mammalia generally
the teeth are differentiated into incisors, canines, premolars, and
molars, and in placental mammals the full dental formula is indi-
cated as i. I, c, T, pm. |, m. f . The incisors, the most anterior
teeth, are more or less flattened into a chisel-like form; the canines
are sharp, roughly conical, and sometimes elongated into prom-
inent tusks; the premolars and molars, together designated cheek-
teeth, are distinguished chiefly by the occurrence of the former
in both sets and of the latter in the permanent set only. In the
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. f» c. J, pm. f ,
m. f. The specialization of the medial incisors retained by the
rabbit is accomplished by the elimination of enamel from the
posterior surfaces so that the posterior part of the tooth is worn
away by use more rapidly than the anterior layer, which thus
forms a sharp cutting edge. 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 main
part of the oral cavity is separated almost completely from a
small space enclosing the incisor t^eth. This adaptation, however,
is not so perfectly developed in the rabbit as in certain members
of the rodent order proper.

The cheek-teeth of the rabbit are modified for grinding by the
formation of flattened ends with prominent transverse ridges.

60 ANATOMY OF THE RABBIT

Since the chewing motion of the mandible is almost entirely
antero-posterior, these ridges are at right angles to the direction
of movement in this activity.

The Humaft Skull
If the human skull be compared with that of the rabbit or other
mammal, it is seen to differ most markedly in the enormous de-
velopment of the cranial region, and in the anteroposterior com-
pression of the face, with which is associated shortening of the
jaw region, reduction of the nasal cavities, and rotation of the
orbits to a forward position. A most instructive feature is the re-
adjustment of the axes of the skull, coincident with the assumption
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 nearly in a straight line or at least parallel ; while in primates
they tend to form an obtuse angle which is progressively reduced
from lower to higher types, being smallest in man, where it
approximates a right angle.

The Appendicular Skeleton

It will be evident from a study of the limb-skeleton of the rabbit
that there is a general correspondence in structure between its
anterior and posterior divisions. This not only applies to the dis-
tinction 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 be-
tween 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 pelvic girdle is more uniformly 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 the strong attachment to the
sacrum. These features correspond with the usually proportionally
greater thrust of the hind limbs in support of the body-weight and
in locomotion. On the other hand, the pectoral girdle is notable

THE APPENDICULAR SKELETON

chiefly for the strong development of the scapula as opposed to
the weakness of the ventral, pectoral portion. In the rabbit the
clavicle is vestigial, and in many mammals, such as the ungulates,
it is entirely absent. In man, how-
ever, as well as in other mammals
in which the fore limb is capable of
much movement transversely in-
stead of only anteroposteriorly, the
clavicle is well developed and is ar-
ticulated at its ends with the scap-
ula and sternum. In this condition
it acts as a prop upon which the
well-developed pectoral muscles of
adduction move the limb. The
absence of direct articulation with
the vertebral column and the at-
tachment thereto by muscles and
ligaments provide a shock-absorb-
ing apparatus which is of evident
value in a quadruped, like the rab-
bit, w^here locomotion is largely of
a leaping character with the fore
limbs reaching the ground first
after each leap.

Both pectoral and pelvic gir-
dles in terrestrial vertebrates con-
form more or less closely to a tri-
radiate shape if the two halves

are considered individually. The corresponding portions may be
identified (Fig. 34), though in mammals the ventral portion in the
case of the pectoral girdle is greatly reduced. Of the ventral
elements, those commonly present in vertebrates are the coracoid,
which forms the posterior ray, and the procoracoid, the anterior
ray, the latter 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 cora-
coid extension ventrally are identifiable in embryonic marsupials.
Adult marsupials and placentals show only a small hook-like

Fig. 34. Plan of the anterior limb
skeleton in walking vertebrates, the
equivalent elements of the posterior limb
indicated in brackets: si, scapula (ilium),
pp, procoracoid (pubis) ; ci, coracoid
( ischium) ; hf, humerus (femur) ; rt',
radius (tibia); uf, ulna (fibula); rt,
radial carpal (tibial tarsal) ; i, inter-
medium ;.uf, ulnar carpal (fibular tarsal);
c,c, centrals; 1-5, di.stal carpals (tarsals);
m. metacarpals (metatarsals) ; ph, pha-
langes of the digits.

ANATOMY OF THE RABBIT

coracoid process, and the clavicle in either perfect or less perfect
development.

There is no more striking feature of homology than that shown
by the free extremities in 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 vertebrate
as compared with the corresponding elements in the same position
in other forms. The front limb of the rabbit (Fig. 35, A) is slightly

Fig. 35. Homologies of the mammalian limb. A, forefoot, rabbit.
B, forefoot, horse. C, human hand, r, radius; u, ulna; I-V, metacarpa bones.

elongated and semi-digitigrade, the weight being supported on the
tips of the bones of the palm (metacarpals), as shown in Fig. 23.
These modifications make it more efficient for running than a more
primitive limb, which is shorter and plantigrade (having the palm
or sole applied to the ground), though it is less specialized and less
efficient as a running organ than the limb of the horse (Fig. 35, B).
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. The limbs of the
rabbit, of the horse, and of man, however, are all modifications of a
primitive, five-toed limb, sometimes termed the ideal pentadactyl
plantigrade type, in which the palm of the hand or sole of the
foot is placed flat on the ground. The composition of this primitive

THE MUSCULAR SYSTEM 63

limb, traceable in one form or another throughout the higher
vertebrates, and also the serial homologies of the parts are indi-
cated in Fig. 34, while Fig. 36 illustrates the modifications which
have occurred in the bones of the wrist and ankle in a few familiar
species of animals. Such modifications (mainly reduction of the
number of separate bones by fusion) are always greater in the

Carpus Tarsus Carpus Tarsus

Primitive Snapping Turtle

Qo^cP P0OC2P oSgdO 0(^

Rabbit* Cat

^^^'' ^^P ogpo c^p

Ulan Horse

Fig. 36. Diagrams representing the carpal and tarsal bones in a primi-
tive condition and in live adult animals: c, centrale; f, tibiale; i,
intermedium; r, radiale; t, fibulare; u, ulnare; x, postminimus; 1-5,
distal carpals or tarsals. In the tarsals of the snapping turtle four
proximal and central elements are fused. In the carpus of the rabbit the
two centrals are fused and displaced into the distal row, and. as in
all the mammals, distal carpals 4 and 5 are fused to form the hamate
bone. Also the mammals represented have a sesamoid bone, the pisi-
form, added to the proximal row in about the position of the postmini-
mus of the turtle, but the pisiform and postminimus are not homologous.
The tarsus in all cases has the tibiale and intermedium fused as the
talus, and the fourth and fifth distal elements fused as the cuboid.
In the rabbit the first distal tarsal is fused with the second metatarsal.
Other fusions and losses are indicated in the representations of the
other species.

posterior than in the anterior limb. In some kinds of turtles, the
wrist shows the primitive pattern practically unmodified, w^ith an
extra element (the postminimus) which may possibly reflect an
ancient condition when there were more than five digits represented.

The Muscular System
Involuntary Muscle

As would be expected considering the nature of their functions,
the contractile tissues are not arranged in a definite, continuous
system as are most other organ complexes of the body. Smooth
or involuntary muscle fibres, modified mesenchyme cells'^ of the

64 ANATOMY OF THE RABBIT

embryo, which are under the control of the sympathetic nervous
system, form the muscle coats of the dii^estive tube, and are
important not only for its repeated, peristaltic movements, but
also for its elasticity and expansive power. Smooth muscle is
also a constituent 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 blood-
vessels, especially the arteries, where it forms the mechanical organ
of the vasomotor function. This consists in the control of the
diameter of the vessels by vasodilator and vasoconstrictor nerves
connected with the vagus nerve and the sympathetic nervous
system. The constrictive action is stimulated by secretion of the
suprarenal glands. Such regulation of the vessels is important,
first, in maintaining tone of the vessel-walls and therefore blood-
pressure, and, second, in controlling loss of heat from the surface of
the body. Action of the vasomotor nerves may be demonstrated
physiologically in a variety of ways. Transection of the cervical
svm pathetic nerve of one side in the living rabbit is followed by
vasodilatation of the ear, the congestion of which can be seen, and
the heat loss is demonstrable by feeling with the hand. Stimulation
of the cut end which is attached to the head is followed by vaso-
constriction.

Muscle of the Heart

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 are regulated by a minute mass of highly specialized
tissue, the sinu-atrial node, imbedded in the wall near the entrance
of the right superior caval vein. A second, similar mass, the atrio-
\entricular node, receives the impulse from the first and transmits
it through a band of conducting tissue to the muscle of the ventricles.
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 intracardiac nerve connections.
The rate and strength of the beat in the intact animal may be
modified through the vagus and the sympathetic nerves, the

THE MUSCULAR SYSTEM 65

former Inhibiliiiii, the latter acceleratiiii^, as nia\' readily be
demonstrated experlmentalK'.

Voluntary Muscle

The voluntary muscles of the body form the nearest approach
to a continuous system of all contractile tissues. They consist for
the most part of parallel fibres, the association of which into
fasciculi is responsible for the appearance of longitudinal striping
when the gross muscle is viewed from the side, and more or less for
the grained appearance of the cut surface when the muscle is
divided approximately at right angles to the direction of the
striping. The control of action is exercised directly from the
spinal cord or from the brain.

A muscle is typically spindle shaped, consisting of a middle
fleshy portion, termed the belly of the muscle, and of tapering ends
which provide for attachment. The attachment is effected by a
strong band of fibrous connective tissue, the muscle tendon (Figs.
8, 37). Some muscles, such as those of the abdominal wall, are
disposed in the form of flattened sheets, the ends of which are at-
tached by tendons in the shape of broad, thin sheets of connective
tissue, the aponeuroses. 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.

A typical muscle of the skeleton has the disposition of parts
illustrated in Fig. 37 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 attached to the
glenoid border of the scapula, the movable tendon, or tendon of
insertion, to the lower border 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 in raising the forearm and hand from
the ground. The analogous action of the triceps in producing an
exactly opposite movement (extension) of the forearm is similarly
demonstrated. It will be evident that the immediate result of
contraction of the muscles is limited by the form of the joint
between the bones to which they are attached. In this case a

66 ANATOMY OF THE RABBIT

hinge-joint confines motion to one plane, while in the cases 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 recognition of origin and insertion 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 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.

In the study of the skeletal muscles, moreover, it should be
borne in mind that the identification of "origin" and "insertion"
is largely a matter of convention. Actually "fixed" and "movable"
points depend upon the movement being effected at the moment.
An excellent example of the necessity of convention in this respect
is afforded by the human arm in which, as opposed to the ordinary
use of the muscles, most of the relations would be reversed if the
body is considered suspended by the hands, that is in the "bra-
chiate" position commonly assumed by arboreal primates. Also
the action commonly attributed to any muscle is usually an artificial
abstraction, for in life muscles act in groups, not singly, and the
precise effect of any given contraction will be modified by the other
muscles acting at the same time. Finally, it should be noted that a
muscle does not always act as a unit, but sometimes one part may
contract independently of the remainder.

Embryonic Derivation

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 seg-
mented portions of the skeleton. The vertebral muscles themselves
show throughout their attachments to successive vertebrae the
marks of segmental origin and the segmental character is obvious
in the intercostal muscles and in the division of the rectus abdominis
by "tendinous inscriptions." Many others, however, such as those
of the abdomen, to a certain extent those of the limbs, and those

THE MUSCULAR SYSTEM

of the eye show practically no indications thereof, the connective
tissue septa between segments having disappeared during develop-
ment. The fusion indicated in the last sentence may, moreover,
be accompanied by transformation of parts of the resulting sheets
into connective tissue, forming "aponeuroses," and by splitting in
new planes, for example, parallel with the surface.

A transverse septum of connective tissue extends laterally from
the transverse processes and divides the trunk musculature into
dorsal, or epaxial, and ventral, or hypaxial, portions supplied
respectively by dorsal and ventral branches of the spinal nerves.
This division is indicated in Fig. 20. The epaxial portions then
produce the dorsal musculature, the hypaxial forming the pre-
vertebral and lateral musculature of the trunk. In the neck region,
the hypaxial portion is divided in early stages by the gill pouches,
so that epibranchial and hypobranchial groups of muscles are
produced. The muscles of each limb are developed from cells that
have migrated from the ventral ends of several adjacent myotomes
and those of the diaphragm have migrated from myotomes in the
neck (corresponding with which fact, the phrenic nerve has its
origin in the neck) .

Distribution

While the bulk of voluntary muscle is
skeletal, part at least is related to the
skin. This forms a cutaneous sheet,
divisible into the cutaneus maximus and
platysma, and the facial, palpebral, and
auricular muscles of the head. The first
two and the last of these are commonly
developed to a considerable extent in
mammals, though reduced in man. The
trunk musculature comprises 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 appendicular muscles are
distributed in special groups connecting
the limb as a whole with the trunk and

Fig. Z7. Arm muscles of
rabbit from the medial sur-
face, illustrating muscle _ ac-
tion, flexion, and extension:
b, biceps (flexor) ; tr, long
head of triceps (extensor) ; i,
insertion; o, origin; sc, scap-
ula; h, humerus; r, radius;
u, ulna.

ANATOMY OF THE RABBIT

the various segments of the limb with one another,
muscles may be subject to considerable variation.

Individual

Equivalence of Limb Muscles

As already indicated, the skeletons of anterior and posterior
limbs are serially equivalent part for part. This is true also of the
related muscles and their actions upon the parts of the limb. The
respective actual positions of the proximal, middle, and distal
segments, however, are rather different in the front as compared
with the hind limb of a mammal, a condition easily discernible
from the fact that the elbow is directed backward, the knee forward.
Similarly there are peculiarities of the muscles and muscle surfaces,
owing to this difference in position and to a twisting which has
occurred in the anterior limb but not in the posterior one, as ex-

Fig. 38. 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. Radial or tibial side of
limb shaded, ulnar or fibular unshaded.

plained below. There are also conventional ideas prevailing in
anatomy as to flexion and extension, that is bending or straighten-
ing parts of the limb, and as to flexor and extensor muscles and
surfaces. For example, ventral bending of the hand is called flexion
and the reverse movement is extension. A continuation of the
latter movement, however, results in bending the hand dorsally
and may be distinguished as dorsiflexion, which is accomplished by
extensor muscles, the original flexion then being distinguished as
ventral or palmar flexion. At the ankle, the foot in the normal
position of rest is approximately at a right angle or at an acute angle
to the leg. Further bending of the upper surface of the foot towards
the leg is called flexion, or better dorsiflexion, while bringing the

THE MUSCULAR SYSTEM 6^

foot more nearly into line with the leg is extension (or plantar-
flexion). The muscles accomplishing the former movement, how-
ever, are classed as extensor muscles, and in so far as they are
inserted distally in the digits actually do straighten these; while
the muscles which extend the foot at the ankle are flexors and,,
when inserted distally in the digits, do bend the latter.

At the hip joint, movement of the thigh forward is flexion,,
movement backward is extension.

The corresponding surfaces and muscles and in general the
differences presented by fore and hind limbs may be determined
by a study of their embryonic relations, but it is simpler, even if
less accurate, to refer the differences to the common basis of a
more or less primitive or neutral type as illustrated in Fig. 38.
In lower vertebrates, such as reptiles and amphibians, it is easily
observable that the front and hind limbs are more nearly similar
to each other than in mammals, especially in respect of the setting
out from the body of the elbow and knee, so that the limbs are held
nearly at right angles to the body axis. This common tendency is-
further expressed by the existence of a plantigrade condition of
the hand and foot, and by a parallel arrangement of the bones of
the forearm and leg. There is thus an approximation to a neutral
plan as indicated in (A), where the animal is considered to be facing
the observer or the limb viewed from in front. This neutral plan
is, however, never quite realized, because, even in primitive verte-
brates, where the limbs can scarcely be said to support the body^
the adjustments for forward progression have already altered the
respective positions of the segments.

It will be noted that as regards surfaces and angles in the
neutral type, the radial side of the fore limb and the tibial side of
the hind limb, shaded in the diagram, are anterior in position.
Apart from the possible movements of the limb as a whole or of
the segments upon one another, it will be seen that there are
certain angle surfaces, a, h, c, dorsal and lateral in position, which
may be identified as extension angles and certain others, d, e, /,.
ventral and medial, or flexion angles. In the relation of the limb
respectively to dorsal and ventral surfaces of the body, a is likewise
an "abduction" angle w^hile d is an "adduction" angle. (Ab-
duction may signify either the movement of a limb away from the
median plane of the body or the movement of a digit away from

70 ANATOMY OF THE RABBIT

an imaginary extension of the axis of the limb through the hand or
foot. Adduction is the reverse movement. The muscles bringing
about such movements are then known as abductor or adductor
muscles.)

In mammals the limbs are set in rather close to and more nearly
underneath the body, a position better fitted for complete and
permanent support. As compared with the neutral type, the elbow,
as indicated in profile in (B), is rotated backward through ninety
degrees, the hand forward through ninety degrees. The radius
and ulna are crossed upon each other, the radial side of the limb
being lateral at the elbow and medial at the hand. The extension
angle h, at the elbow, is now posterior, that of the wrist, c, anterior.
Many interesting observations may be made by placing the human
arm in the corresponding positions. In most mammals, for ex-
ample, the hand is fixed in a prone position with the radius and
ulna crossed. In man and to a certain extent in some mammals
the hand may be placed in a supine position or the limb held as
in the neutral type. In either case the bones of the forearm are
parallel. The human condition with respect to this character there-
fore appears to be rather primitive, and is in great contrast to
that shown in specialized running animals such as the horse, where
the radius takes over almost the entire support of the forearm and
the ulna becomes simply an accessory of the elbow-joint.

The hind limb of a mammal (C) is rotated in its entirety forward
through ninety degrees. The tibia and fibula retain their parallel
position. The extension and flexion angles retain their mutual
positions but the former become anterior and the latter posterior.
The foot, in its more usual plantigrade condition, presents one of
the striking cases of muscle arrangements in the limbs, in that, for
example, as was pointed out above, an extensor muscle, originating
on the front of the leg and terminating on the dorsum of the toes,
will extend the toes and will bend the foot on the leg at the ankle
joint. In a morphological sense, the foot is not thereby flexed and
the muscles are named and classified accordingly. An attempt has
been made in preceding paragraphs to clarify the apparent contra-
dictions in terminology thus introduced.

From the mechanical point of view, each limb can act both as a
strut and as a lever. When acting as a strut, it exerts forces along

THE NER\^OUS SYSTEM

its own mechanical axis only, as when it is in an approximately
vertical position supporting the weight of the body at rest. When
acting as a lever, it exerts both against the body and against the
ground forces at right angles to its mechanical axis.

As a propulsive mechanism, the limb functions in both these
wa3^s. As a propulsive strut, it is extended by its own intrinsic
musculature and its action may be compared roughly to that of a
pole propellmg a punt. As a lever, the limb is operated by the
muscles which attach it to the body and its effect is somewhat
analogous to that of a double paddle propelling a canoe while the
outer end of its blade remains fixed.

The Nervous System

There is probably no other system of organs in which external
form is so little suggestive of actual function as in the nervous
system. This is perhaps less true of its peripheral portion, con-

FiG. 39. Plan of the central and peripheral connections of a spinal
nerve: an, afiferent (sensory) neuron; avn, afferent visceral neuron; ca, cp,
anterior (ventral) and posterior (dorsal) columns of gray matter; en, efferent
(motor) neuron; esn, visceral efferent (preganglionic sympathetic) neuron;
grp, dorsal root ganglion; i, intestine (visceral organs); m, skeletal muscle;
na, np, anterior (ventral) and posterior (dorsal) rami of spinal nerve; pg,
postganglionic sympathetic neuron; ra, rp, anterior and posterior roots of
spinal nerve; rg, rw, gray and white rami communicantes (sympathetic);
sk, skin; sp, white matter of spinal cord; ts, ganglion of sympathetic trunk;
vm, smooth muscle. Modified, from Herrick.

72 ANATOMY OF THE RABBIT

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 centres where exceedingly complex inter-connections
are made between the nerve elements, and of conducting paths to
and from these centres connecting them with 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-case. It is furthermore
surrounded by connective tissue membranes, meninges. In higher
vertebrates, three of these are differentiated, the dura mater,
which forms a tough external investment; the arachnoidea, which
is a very delicate, somewhat spongy web lying internal to this;
and the pia mater, a thin membrane lying next the nervous matter
and richly supplied with blood-vessels from which branches pene-
trate the latter to provide for its nourishment and respiration.
Still further protection is afforded by the cerebrospinal fluid,
which fills the spaces between these membranes as well as the
cavities within the brain. The nerves, on the other hand, are
distributed freely throughout the body, and though not so ade-
quately protected are more capable of withstanding or repairing
mechanical injury. Also they are commonly found surrounded by
connective tissue where mechanical injury is relatively little likely
to occur. Each nerve has a tough sheath, the perineurium, com-
posed of condensed connective tissue, and similar tissue extends
between the fibres, binding them together in small bundles. Each
has a relatively poor though indispensable blood supply received
through small arteries which anastomose to form a continuous
channel along the nerve.

The central nervous organs contain numerous blood vessels,
the capillaries forming a continuous network the density of which
differs considerably in different parts, but they have no lymphatics.
The tissue spaces are continuous with narrow perivascular spaces
which are nowhere lined with endothelium as are lymphatics.

THE SPIXAL XERVE 73

The larger perivascular spaces acquire a thin lining of tissue from
the arachnoid and pia mater and open into the subarachnoid space
so that the tissue fluid mingles with the cerebrospinal fluid there.
This fluid is mostly secreted by the chorioid plexuses (p. 83) and is
eventually filtered into the venous sinuses which carry the blood
from the brain.

Composition of a Spinal Nerve
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. 39). In the
spinal cord the difference in appearance as between the white and
the grey matter has already been described (p. 32). A spinal
nerve arises by two roots, one of which is dorsal and bears a small
ganglion containing nerve-cell bodies, the other ventral and with-
out a ganglion. Impulses passing through the dorsal root are
centripetal or afferent in that they pass only in the direction of
the central nervous system and 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. Many afferent impulses do not enter consciousness and a
majority of these come from deeper parts. In a similar fashion
the impulses of the ventral root are centrifugal or efferent, in that
they pass only in a 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 redivide
into a dorsal ramus, a ventral ramus, and either a ramus com-
municans or two rami communicantes. Each spinal nerve has a
grey ramus communicans and the thoracic, the first five lumbar
(in the rabbit), and the second to fourth sacral nerves (in the
rabbit) have also a white ramus communicans. The dorsal and
ventral rami are then distributed as somatic nerves to the body
wall, each of them containing fibres from both dorsal and ventral
roots and also fibres (for the blood-vessels) which have come from
the sympathetic ganglia (see below) through the grey communi-
cating rami. The white communicating ramus is a visceral nerve
containing fibres derived from both dorsal and ventral roots and
connecting through autonomic ganglia with the visceral organs.

74 ANATOMY OF THE RABBIT

Thus the grey ramus communicans differs from the other branches
in carrying fibres to rather than from the trunk of the nerve and
in that sense might better be designated as a root.

The Autonomic Nervous System

UnHke 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. Of
these trunk ganglia there are on each side two in the neck of the
rabbit and a segmented series in the thoracic, lumbar, and sacral
regions. The sympathetic trunk is similarly connected with an
unpaired collateral series of ganglia, and through them with certain
peripheral ganglia on the surface of the visceral organs. From
these ganglia, fibres run to the visceral muscles and glands through
plexuses which mostly accompany the blood-vessels. The longi-
tudinal trunks and their connections with the thoracic and lumbar
nerves form the sympathetic portion of the autonomic division of
the peripheral nervous system. The corresponding connections
of the sacral and certain cranial nerves (the third, seventh, ninth,
tenth, and eleventh) constitute the parasympathetic portion of
this division. The latter does not join the sympathetic trunk,
and its fibres end only in the peripheral ganglia. The two divisions
differ also in their responses to certain drugs. Most organs con-
trolled by the autonomic system receive fibres from both, which
usually produce opposite eiTects upon stimulation though in some
cases they co-operate, acting upon different components of the
organ. (For instance, mucous and serous cells in the submaxillary
gland have respectively parasympathetic and sympathetic control.)
It is usual to consider that only the visceral efferent elements
constitute the autonomic division, though visceral afferent fibres
run through the trunks and ganglia along with the efferent com-
ponents and are thus included in the gross anatomical structures.
The cell bodies of the afferent neurons, however, lie in the dorsal
root ganglia of the spinal nerves as indicated in Fig. 39.

From the foregoing, it is apparent that the visceral efferent
peripheral pathway, unlike the somatic one, always involves two
successive neurons. One fibre springs from a cell body in the

FORMATION OF NERVE-PLEXUSES 75

central nervous system and ends in an autonomic ganglion — the
preganglionic fibre — while the other arises from a cell body in the
ganglion and ends in the organ to be controlled — the postganglionic
fibre. Although either a preganglionic fibre or a postganglionic
fibre may traverse several ganglia, the course between spinal cord
and periphery is usually interrupted in only one. The preganglionic
fibres, accompanied by visceral afferent elements, make up the
white communicating rami but the grey rami are composed of
postganglionic fibres destined to accompany the somatic branches
and end in the walls of the blood-vessels, in other smooth muscu-
lature, or in glands.

The sympathetic trunk terminates anteriorly at the base of
the head in a relatively large superior cervical ganglion, which
receives its preganglionic fibres entirely from more posterior levels
by way of the cervical portion of the trunk. The second ganglion
in the rabbit is the inferior cervical, situated at the lower end of the
neck. From the inferior cervical and the first thoracic ganglia,
postganglionic fibres run as delicate grey rami along the vertebral
artery, forming a plexus about the latter and giving off a branch to
each cervical nerve as it crosses the artery.

Plexus Formation

In certain places, peripheral nerves, either spinal or autonomic,
connect with each other so as to form a plexus, or network. This
phenomenon is conspicuously exemplified by the nerves for each
of the limbs (brachial and lumbosacral plexuses). The develop-
ment of these limb-plexuses is probably an outcome of the manner
of origin of the limb-muscles, which involves the fusion of material
from the primary muscle-segments in the embryo and the sub-
sequent differentiation of the resulting mass into units which may
be derived from two or more segments. The originally segmented
nerves then become interconnected in such a way that each
definitive muscle will receive a nerve composed of the ap-
propriate number of fibres belongmg to each segment which has
contributed to its formation. The patterns of the plexuses are
subject to much individual variation.

Observation of these and other pertinent facts has led to the
belief that the relation of nerves to their muscles is constant no

ANATOMY OF THE RABBIT

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REFLEX ACTION

matter what modifications the latter may undergo, though this
principle is not now considered so absolutely rigid as it was formerly
thought to be.

Reflex Action

It is difficult to determine what portion of a muscular con-
traction, even of one which is considered to be purely voluntary,
results from an impulse actually originating in the central nervous
system. The living body, however, affords many examples of
muscular actions as direct responses to immediately previous in-

FiG. 41. Camera lucida tracings of transverse sections of the spinal cord
of a rabbit to show the variations in the shape and in the proportions of grey
matter and of white matter in representative segments: 2nd cervical, 7th
cervical, 5th thoracic, 4th lumbar, 1st sacral, and 1st caudal.

78 ANATOMY OF THE RABBIT

coming stimuli, without conscious experience being a necessary
factor in producing the result. In vertebrates in which the spinal
cord is divided, the lower part thus being separated from the brain,
stimulation of the skin below the level of the section is followed by
co-ordinated movements. These are evidently brought about
through direct connections within the spinal cord between the
dorsal and ventral roots either of one spinal nerve or of neighbour-
ing nerves. This is known as reflex action. Such responses, of
which the well-known scratch reaction of the dog is an example,
occur in all animals. The integration of reflexes and their purpose-
ful control is performed by centres at various levels in a function-
ally superposed series, the cerebral cortex being the ultimate one
and having become increasingly dominant in the mammalian scale.

The Spinal Cord

The spinal cord reflects in its form the basic architectural
pattern of the vertebrate central nervous system, being developed
in the embryo as a tube and retaining this condition throughout
life. The inner part of the wall of the tube is composed of grey
matter, the outer part of white matter. The cavity, how^ever, is
reduced to a very slender central canal while the walls become
enormously thickened by proliferation of the cells and their fibre
extensions, through which are established the nervous functions
of the system, as a connected conducting mechanism. The cord
traverses the vertebral canal, showing slight 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 terminale, by which it is con-
tinued almost to the middle of the length of the tail.

The sudden tapering of the cord into the filum terminale at the
level indicated is a result of growth relatively less than that of the
surrounding parts, the cord in the embryo extending through the
region occupied in the adult by the filum. Such relative shortening
of the cord by retardation of growth is more marked in some
animals than in others, the lower tip of the human spinal cord, for
example, being usually within the first lumbar vertebra.

THE BRx^IN 79

The Brain

Superficial examination of the brain of the rabbit (Fig. 42)
shows that its larger part is formed by the paired cerebral hemi-
spheres. They are closely pressed together on the dorsal side but
separated posteriorly on the ventral side. The external layer, the
grey cortex, is important as the principal part of the physical
substratum of intelligence. In the rabbit, as in all animals which
have not large cerebral hemispheres, the cortex is smooth, con-
trasting with its condition in mammals with larger hemispheres,
where it is corrugated. The amount of this corrugation is roughly
related to the absolute size of the brain. Thus it is considerable
in the cat and dog but reaches a high degree of elaboration in man
and in the whale. At the anterior tips of the cerebral hemispheres
in the rabbit are the slightly expanded olfactory bulbs which can
be seen to be connected backwards with the posteroventral portion
of the hemisphere, known as the pyriform lobe. These parts of the
brain being directly related to the sense of smell, their size in
different mammals corresponds with the degree of development of
that sense and is markedly reduced in man. The development of the
cerebral hemispheres even in lower mammals is such that important

Fig. 42. The brain from the left side: bo, olfactory bulb; c, cerebellum;
fc, paraflocculus cerebelli; h, cerebral hemisphere; Ip, piriform lobe; m,
medulla oblongata; s, spinal cord. Numerals _ indicate the corresponding
cranial nerves: 2, optic; 4, trochlear; 5, trigeminal; 6, abducens; 7, facial;
8, acoustic; 9, glossopharyngeal; 10, vagus; 11, spinal accessory; 12,
hypoglossal.

parts of the brain, notably the diencephalon and mesencephalon
(pp. 80, 81), are concealed dorsally and laterally. However, the
chiasma or crossing of the optic nerves on the ventral aspect of the
diencephalon is evident, while more posteriorly the convergent cords
of the cerebral peduncles may be seen passing backwards on that
of the mesencephalon. The posterior part of the brain is formed
largely by the cerebellum above, the corrugation of which is one

ANATOMY OF THE RABBIT

of its outstanding features, and below by the somewhat tapering
medulla oblongata, which is continuous caudally with the spinal
cord. On the ventral side, the medulla oblongata is crossed in front
by a bridge of fibres, not so conspicuous in the rabbit as in many
mammals and in man, which is known as the pons and which
appears to connect the two sides of the cerebellum. (Actually it
is part of the path to the latter from the cerebral hemisphere.)
These, the outstanding surface features of the brain, afford but a
moderate conception of its details, the nature of which can be made
out only 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 en-
larged and elaborated to include both the highest controlling
centres of the whole nervous mechanism and the special centres of
the nervous mechanism for a variety of functions performed by
organs in the head. It accordingly not only forms a more or less
distinct division, known as the brain or encephalon, as opposed to
the less elaborated spinal cord or spinal medulla, but also develops
a series of paired and unpaired subdivisions containing portions
of the original cavity distended to form ventricles.
The primary divisions of the brain are more or
less similar and homologous in all vertebrates. The
more elaborate condition of the organ in a mammal
may be explained by reference to the general plan
as indicated in Fig. 44, 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. 43) offers one of the best
examples.

The brain as first formed in the embryo appears
as three anterior expansions of the neural tube

Fig. 43. The , . ,. ^ . _, , ., ,

brain of the frog arranged m a Imear series. Iney are aescribea as

from the dorsal , . i i • < • i-

surface: c, cerebei- the primary Cerebral vesicles; or, as pnmary di-

phTi'on; fv, fourth visious of the futurc brain, they are designated in

brai^'^ hemisphere; anteroposterior order as the prosencephalon, mes-

oi. opt?? lobe. ^"'^' encephalon, and rhombencephalon.

THE BRAIN

The first of the primary divisions, the prosencephalon, or
primary forebrain, gives rise during development to a pair of
hollow outgrowths and thus becomes divisible into an anterior
portion, the endbrain or telencephalon, which is largely a
paired structure, and a second portion, unpaired, the diencephalon
or interbrain. The larger, paired portion of the telencephalon is

Fig. 44. Plan of the divisions of the vertebrate brain: A, embryonic;
B, adult, projection from dorsal surface; C, adult, sagittal section. The con-
tour of the mammalian brain is indicated by broken lines.

Primary divisions — PR, prosencephalon; T, telencephalon; DI, dience-
phalon; MS, mesencephalon; RH, rhoihbencephalon ; MT, metencephalon;
MY, myelencephalon; S, spinal cord.

a.c, cerebral aqueduct; b.o, olfactory bulb; c, corpora quadrigemina;
c.a., anterior commissure; cb., cerebellum; cm., mamillary body; c.o., optic
chiasma; c.p., pineal body; f.i., interventricular foramen; h., hypophysis;
h.c, cerebral hemisphere; in., infundibulum; l.t., lamina terminalis; p., pons;
pi., 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.

82 ANATOMY OF THE RABBIT

the basis of the cerebral hemispheres. It contains, as divisions of
the primary cavity, a pair of cavities, the lateral ventricles. Origi-
nally, the whole of the telencephalon was concerned with olfactory
functions, but it has been invaded by an increasing number of
fibres from more posterior parts bearing non-olfactory nerve
impulses. In the frog, a small part of it has already become free
from olfactory connections and in the higher vertebrates the non-
olfactory part becomes increasingly preponderant. In the mam-
malian brain, the olfactory portion, or rhinencephalon, is more or
less definitely marked off from the rest. This portion, sometimes
termed olfactory lobe or olfactory brain, includes the olfactory
bulb and the pyriform lobe, already mentioned, with a number of
related parts.

A primitive cerebral hemisphere like that of the frog or the
young mammalian embryo comprises distinguishable dorsal and
ventral halves, designated respectively pallial and basal. In the
adult mammal, the basal portions have become massive and the
pallium has spread partly over their lateral and ventral surfaces
as well as constituting the dorsal wall of the hemisphere. The
cells of the pallium have formed a highly specialized superficial
layer, the cerebral cortex.

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 each
lateral ventricle through an 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 slender funnel-like
structure, the infundibulum, the tip of the latter being attached
to the pituitary body or hypophysis and its base being connected
with a small grey elevation, the tuber cinereum. Its cavity is the
recessus infundibuli. Immediately in front of the infundibulum
the optic tracts cross each other on the ventral surface of the brain,
forming the optic chiasma, and immediately behind it the floor is
thickened, forming externally a pair of rounded protuberances,
the mamillary bodies. In the brain of the rabbit, the latter bodies
are fused so that superficially they consist of a larger median

THE BRAIN 83

portion with faint lateral elevations appended to it. Collectively,
these ventral structures are considered to form a major division
of the fore-brain, the hypothalamus, the latter consisting of two
portions, namely, an optic portion, comprising the optic chiasma
and some adjacent tissue, and a mamillary portion, including the
mamillary bodies, the tuber cinereum, the infundibulum, and the
hypophysis. The optic portion belongs to the telencephalon and
is better termed telencephalon medium, while the mamillary
portion belongs to the diencephalon.

The more dorsal portion of the diencephalon, containing the
major part of the third ventricle, is sometimes known as the
thalamencephalon, a term now falling into disuse. Its lateral
walls are greatly thickened, while its roof is extremely thin, es-
pecially in its anterior part. Here the actual roof of the ventricle is
formed of a layer of tissue only one cell in thickness, 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 diencephalon bears posteriorly the pineal
body or epiphysis cerebri, an endocrine gland borne upon a stalk which
is attached to certain other small dorsal parts of the brain, the habe--
nulae and habenular commissure. These all together form the
epithalamus. The lateral wall of the third ventricle is formed by
the thalamus, which has become so massive in the mammal that
it bulges medially to fuse with that of the other side and thus to
produce a broad bridge across the middle of the ventricle, the
massa intermedia. In the brain of the rabbit it will be seen that
the thalamus is indicated externally chiefly by a rounded pro-
tuberance, the lateral thalamic tubercle. 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. Postero-
medial to this is a third protuberance, the medial geniculate body.
The medial and lateral geniculate bodies as thus defined constitute
the metathalamus (Fig. 116).

The second of the primary divisions, the mesencephalon, or

84 ANATOMY OF THE RABBIT

midbrain, is noteworthy in a mammal as lacking a cavity large
enough to be designated a ventricle. Instead it has a narrow
canal, funnel-shaped in the rabbit, 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
members of the anterior pair are much larger than the posterior
ones and correspond with the optic lobes of the frog. Its floor is
formed by the cerebral peduncles, the ventral surface of which is
composed mainly of a pair of prominent bundles of nerve fibres
converging from in front, having originated in the cerebral cortex
and passing back into the rhombencephalon.

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

The third primary division, the rhombencephalon, or primary
hindbrain, 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 rhombencephali. It
includes the anterior medullary velum and brachia conjunctiva
(Fig. 122). The rhombencephalon itself is divisible into two por-
tions, especially well defined in the mammalia, namely, the met-
encephalon and the myelencephalon. The former includes the
small brain, or cerebellum, and a ventral region, the pons, which
is marked by a thick transverse band of fibres on the surface. 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 membranes
underlying the latter, each being attached to it along a transverse
line near the middle of its under surface (Fig. 124). One of these,
the anterior medullary velum, is connected forwards with the
mesencephalon, while the other, the posterior medullary velum,
extends back from under the posterior margin of the cerebellum
and covers a triangular space at the caudal end of the ventricle
over which the cerebellum does not reach. The posterior medullary
velum has the same structure as the chorioid plexus of the third
ventricle, but is less well developed.

THE BRAIN

Most of the portions of the brain referred to in the foregoing
paragraphs are not units homogeneous in respect of structure or
function but are made up of constituents of varied significance.
An attempt to indicate briefly the functions of various parts has
been made in connection with the somewhat more detailed descrip-
tion in the directions for dissection (pp. 348-366).

Apart from its principal divisions, which, as indicated above,
are more or less common to all vertebrates, the external form of the
brain in various species is determined by 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 also are significant
in this connection. It will be seen also that the form of the brain
is more or less dependent 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, a
bend in the opposite direction, is at the fourth ventricle; while
the thi-rd, or cervical flexure, is at the point where the myelence-
phalon passes over into the spinal cord. By these flexures the over-
all length of the brain is kept within the dimensions of the cranial
cavity.

The Cranial Nerves

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

mx o

Fig. 45. Branches of the left ophthalmic nerve In the region of the orbit,
dorsal view, after Winckler: f, frontal nerve; 1, lacrimal nerve; mx, maxillary
nerve; n, nasociliary nerve; o, orbital nerve.

ANATOMY OF THE RABBIT
Cerebrum

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THE CRANIAL NERVES

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 — in addition to the autonomic
system, described on page 74. Of these the spinal nerves (p. 73)
are less modified, in both structure and distribution.

N.-^ .--'^^--Y'-^

/ v:- i

Fig. 46. Dissection from the ventral surface of the neck. On the right
side the platysma and depressor conchae posterior are reflected with the skin.
The vagus nerve is in proper relation to the external jugular vein and the
common carotid artery. On the left side the external jugular vein, parotid
and submaxillary glands, and the sternohyoid. sternomastoid._ and cleido-
mastoid muscles are removed, the common carotid displaced medially and the
nerves laterally, but otherwise in proper relation.

be, basioclavicularis: cc, common carotid artery; cm. cleidomastoideus;
c3, c4._ c5, cervical spinal nerves; d, digastricus; fa, anterior facial vein; fp,
posterior facial vein; gcs, superior cervical ganglion; gn, ganglion nodosum;
gp, parotid gland; gs, submaxillary gland; gt, thyreoid^ gland; Icp. deep
cervical lymph gland; m, masseter; my, mylohyoideus; pi, medial insertion
portion of masseter concealing the pterygoideus internus; pi, platysma; rev,
cardiac branch of vagus nerve (n. depressor) ; rdh, descending branch of
hypoglossal nerve; s, stylohyoideus major; sh, sternohyoideus; sm_, sterno-
mastoideus; st. sternothyreoideus; t, thyreohyoideus ; ts, sympathetic trunk;
vje. external jugular vein; vji, internal jugular vein; X, vagus nerve;
XII. hypoglossal nerve. (From dissection by W. H. T. Baillie, drawing by
E. B. Logier.)

88 ANATOMY OF THE RABBIT

The cranial nerves, those arising from the brain and making
exit through the walls of the skull, are comparable in some respects
to the spinal nerves, but in many ways are different in nature in
addition to being in some cases highly specialized. Three pairs,
respectively, olfactory, 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 transmission of afferent impulses of equilibrium.
The optic nerve differs from all others both structurally and de-
velopmentally, being really an outlying part of the brain itself.
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 (p. 41). Although the connections of these
nerves are not fully considered In the dissection as here outlined,
their chief characteristics as branchiomeric structures may be
indicated. The fifth, or trigeminal nerve Is the nerve of the man-
dibular arch and its branches are related to this arch with its
associated structures and to the mouth In a manner comparable
with the relations of a typical branchial nerve to Its gill arch and
the gill cleft in front of it when these structures are present. It
arises in two parts, one of which, the portio major, is sensory, the
other, the portio minor, motor. The portio major splits Into three
main branches, the ophthalmic (Fig. 45), maxillary, and mandibu-
lar nerves, and the portio minor unites with the last of these.
Thus, while the terminal branches of all three divisions are distri-
buted as somatic sensory nerves to the skin of the head, the man-
dibular nerve carries in addition visceral motor fibres for certain
muscles (masticatory group, mylohyoid, and digastric) regarded
as belonging to this, the first visceral arch. Visceral sensory fibres
are carried from the anterior part of the mouth by the lingual
branch of the mandibular nerve and by the palatine branches of
the spheno-palatine ganglion, but both of these, despite their
close peripheral association with the trigeminal, really belong to
the seventh nerve, the connection of the former being through the
chorda tympani, that of the latter through the great superficial
petrosal.

THE SENSE ORGANS 89

The seventh, or facial nerve is the nerve of the second, or hyoid
arch, the gill pouch in front of which is represented by the cavity
of the middle ear. It is distributed chiefly as a motor nerve to the
cutaneous muscles of the head, which are modified visceral muscles
from the region of the hyoid arch, but contains also taste fibres
from the front part of the tongue. The ninth, or glossopharyngeal
nerve, belonging to the third visceral (first branchial) arch, and
the tenth, or vagus, belonging to the fourth and succeeding visceral
arches in lower forms, are distributed as visceral efferent nerves
to the pharyngeal and laryngeal musculature, and as visceral
afferent nerves to various visceral organs, the ninth nerve supply-
ing the gustatory organs of the back part 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 eleventh, or spinal accessory nerve has apparently been
formed by an association of certain motor components separated
off from the vagus with others derived from the anterior spinal
nerves. The spinal elements have a characteristic distribution to
the cleidomastoid, sternomastoid, and trapezius muscles of the
side of the neck and shoulder while the other components join the
vagus and are distributed with it.

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 Sense Organs

The complete mechanism involved in the performance of an
action comprises of necessity a receptor, an afferent conductor, an
adjustor, an efferent conductor, and an effector. The effector may
be a gland or a muscle, both of which have been considered in
previous chapters; the afferent and efferent conductors are the
nerve fibres which run through the peripheral nerves and are usually
partly within the central nervous organs; the adjustor is in the
spinal cord or in the brain; while the receptor is at the periphery
and has been defined as an organ designed to lower the threshold
of excitability for one kind of stimulus and to heighten it for all
others.

90 ANATOMY OF THE RABBIT

Receptors, or sense organs, are classified as exteroceptors,
which receive stimuli from outside the organism, proprioceptors,
which are excited by events in the organism itself, such as con-
traction of muscles or movements of joints, and interoceptors,
which are situated in visceral organs such as those of digestion,
respiration, etc.

The simplest receptors, structurally, appear as free nerve end-
ings in epithelium. These probably transmit impulses which are
interpreted as pain. Several more specialized types of receptors,
also of microscopic size, are stimulated by touch, pressure, or
movement of various parts and others by heat, cold, or chemical
irritation. A slightly enlarged portion of the internal carotid artery
at the very beginning of that vessel, the carotid sinus, contains in
its wall receptors for changes in arterial pressure and a minute
organ between the bases of internal and external carotid arteries,
the carotid body or glomus caroticum, is a receptor for chemical
changes in the blood, both of these giving rise to reflex effects on
blood-pressure and on breathing.

Of all receptors in the mammalian body, the most primitive,
as regards structure are the olfactory cells. These are neuro-
epithelial cells imbedded among the other elements of the nasal
mucous membrane. Each has a free, ciliate outer end and gives rise
at its inner end to an unmyelinated nerve fibre which runs through
the olfactory nerve to terminate in the olfactory bulb.

The gustatory organs, or taste buds, are minute spindle-shaped
groups of differentiated cells imbedded at certain regions in the
stratified epithelial lining of the oral cavity. Sensory and support-
ing cells are distinguishable and round the former are the terminal
ramifications of gustatory nerve fibres.

The Ear
The receptor for sound and for equilibratory stimuli result-
ing from movement or altered position of the head is the inter-
nal ear, which comprises two distinct though connected parts
serving these respective functions. The actual sensory areas occur
in the walls of a system of delicate canals, the membranous
labyrinth, which are contained within corresponding bony canals
imbedded in the petrosal bone. The equilibratory or vestibular por-

THE EAR

tion of the membranous labyrinth includes three semicircular canals,
respectively anterior, posterior, and lateral or horizontal, lying in

planes perpendicular to each other,
so that movement in any direction
will cause a tendency to flow in
the contained fluid (the endo-
lymph) of one or more canals.
Each canal has at one point an
expansion, the ampulla, and all
connect with a larger sac, the
utriculus, the endings of the ves-
tibular nerve fibres being in the
walls of these. The utriculus con-
nects with the endolymphatic duct,
which runs to the cranial cavity
and ends in a blind sac within the
thickness of the dura mater. A
narrow passage connects the utricu-
lus, further, with another relatively
large space, the sacculus, which also
has vestibular nerve endings, and
this in turn is connected by a fine
tube with the cochlear duct, the

external acoustic meatus, terminating at ,. • r i i

the tympanic membrane; m.a.i., internal aUQltOry pOrtlOn 01 the mem bran -

acoustic meatus; s., sacculus; s.e., endo- i i • ^i t^ ^i ^ j_i

lymphatic sac; St., stapes; t.a., auditory ous labyrmth. It appears that the

tube; u., utriculus; v, vestibulum; VIII, , • j_i • 11 .•

acoustic nerve. rcccptors m the ampullae are Stimu-

lated essentially by movement,
those in the utriculus and in the sacculus responding rather to
position (gravitational stimuli).

While the vestibular parts of the labyrinth are completely
surrounded by fluid, the perilymph, in the bony canals, the cochlear
duct is attached to its bony enclosure along one side. It is also
connected with the opposite w^all by a membrane so that the cavity
within the bony cochlear canal is divided into three parallel tubes,
which are coiled in a close spiral and taper gradually towards the
apex. Within the cochlear duct (which, like other parts of the
membranous labyrinth, is filled with endolymph), is the complex
structure containing the actual auditory sensory nerve endings,
the organ of Corti.

Fig. 47. 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 petrotym-
panic bone; t., tympanic portion (bulla
tympani).

c, cochlea; c.s., bony semicircular
canals; c.t., tympanic cavity; d.c, coch-
lear duct; d.e., endolymphatic duct; d.m.,
dura mater; d.s., semicircular ducts; f.c,
cochlear fenestra; f.v., vestibular fen-
estra; i., incus; m, malleus; m.a.e.,

ANATOMY OF THE RABBIT

Vibrations of the ear drum are transferred to a chain of ossicles
in the middle ear (Figs. 47 and 90) the innermost of which fits
loosely into an opening in the wall of the bony labyrinth and thus
passes on the vibrations as pressure changes to the perilymph,
which in turn transmits them to the organ of Corti.

The Eye

The eye is the special organ for the reception of stimulation by
light and consists of a specialized portion of the brain, the retina,
which has grown out on the end of a stalk of nervous tissue to come
close to the surface and has been provided with a mechanism for
focussing light rays upon it, the whole being enclosed in a support-
ing and protective capsule. The capsule is nearly spherical and is
composed of exceedingly dense connective tissue which forms an
opaque white coat, the sclera,
except over the exposed outer
surface of the eye.

On the exposed surface of
the eye, the sclera is suddenly
replaced by a transparent
sheet of modified connective
tissue which is fused with a
thin outer layer corresponding
with the epithelium of the con-
j unctiva (the lining layer on the
inner surface of the eyelids).
This outer layer of the eye is
also perfectly transparent and
along with the transparent
connective tissue constitutes the cornea, a highly refractive curved
window. Internally, separated from the cornea by chambers con-
taining a fluid, the aqueous humour, is suspended the lens, which
is biconvex, somewhat more curved on its inner than on its outer
surface, and composed of modified epithelium. This is suspended
in a very thin capsule by a ring of fibres, the zonula ciliaris, which
fibres are attached at their outer ends to a circular ridge of muscle,
the ciliary body. The zonular fibres are under tension when the
muscle is at rest, keeping the lens slightly flattened, and when the

Fig. 48. Diagram of the parts of the eye in
vertical section: c.a., anterior chamber; c.c,
ciliary body; ch., chorioidea; co., cornea; c.p.,
posterior chamber; 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-
crimal duct; i., iris; 1., lens; n.o., optic nerve;
o.r., optic portion of the retina; p.i., lower eye-
lid; 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.

THE DIGESTIVE SYSTEM 93

muscle contracts this tension is reduced so that the elastic reaction
of the lens causes its curvature to increase. The size of the aperture,
the pupil, through which light reaches the lens is regulated by a
deeply pigmented, muscular diaphragm, the iris.

By the structures described in the previous paragraph, light
rays are brought to a focus upon the retina, the layer of nervous
tissue lining the large cavity which occupies most of the eye. This
cavity is filled with a gelatinous vitreous body.

The outermost layer of the retina (i.e. that nearest the sclera)
is formed by a single row of deeply pigmented epithelial cells, which
prevent the light from passing further. Into these project the rods
and cones,* the actual receptive nerve endings. Internal to the
rods and cones are several layers containing the cell bodies to which
these endings belong and many other nerve cells of various kinds,
together with supporting elements. Thus, in order to reach the
rods and cones, the light must pass through these internal layers,
which are almost perfectly transparent when alive. The most
internal layer of nerve cells gives rise to the fibres of the optic
nerve, and. these converge over the inner surface to one point,
where they turn abruptly outward, penetrating the whole thickness
of the wall of the eye and proceeding to the brain.

In dissection, the nervous portion of the retina separates readily
from the outer pigmented epithelial layer, but the latter is firmly
adherent to a surrounding coat, the chorioid membrane, which is
also deeply pigmented, contains very numerous blood vessels, and
is loosely attached externally to the inner surface of the sclera.
Towards the exposed side of the eye, the chorioid membrane passes
over into the ciliary body and the iris, these three together consti-
tuting the vascular tunic of the eye.

The Digestive System
The digestive system comprises as its chief portions the diges-
tive tube or alimentary canal 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

*The rabbit has very few cones. Correspondingly, it is reported to be
colour-blind, the cones being the colour-sensitive receptors.

94 ANATOMY OF THE RABBIT

liver, and the pancreas. They are parts of an extensive series of
epitheHal glands, otherwise contained within the wall of the tube
and for this reason not appearing as gross structures.

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

1. Oral Cavity 5. Small Intestine

Vestibulum oris Duodenum

Oral cavity proper Mesenterial intestine

Jejunum

2. Pharynx Ileum

Nasal portion

Oral portion 6. Large Intestine

Laryngeal portion Caecum

Vermiform process

3. Oesophagus Colon

Rectum

4. Stomach

Digestion as a Process

The digestive system performs a variety of functions, both
mechanical and chemical, all connected directly or indirectly with
the digestion of food. In the oral cavity, solid food is divided into
small parts by the action of the teeth, and is mixed with salivary
secretion, so that it is more easily swallowed and passed along the
oesophagus to the stomach. The secretion of the oral glands is
thus important chiefly for the lubricating properties of 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 protein, and a coagulating action upon
milk. The liver secretion, known as bile, contains salts which co-
operate with the pancreatic secretion in its action upon fats and
which neutralize the acidity of the gastric secretion, thereby pre-
paring the contents of the intestine for the action of the pancreatic
juice and intestinal enzymes. The bile salts also aid in absorption
of the products of the digestion of fats. The pancreatic secretion
contains a variety of enzymes, degrading proteins and starches, and
breaking fats into fatty acids and glycerin. The microscopic glands

THE LIVER 95

in the lining of the small intestine also secrete enzymes capable of
completing the action of those derived from the previous sources.
The actions of the dissolving enzymes are 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 is the final object of the digestive process, is
accomplished in the lower part of the small intestine and in the
large intestine through the blood-vessels and lymphatics of 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 nutritive quality
of the ingested food. The caecum also provides room for retention
of materials long enough for bacterial action upon cellulose, which
is not otherwise digested, to make resulting products available for
assimilation.

The Liver

The liver has a variety of other functions besides those men-
tioned above. It stores nutritive material in the form of glycogen
("animal starch") and fat, and perhaps also protein, and plays an
important part in fat metabolism. It removes various waste
substances from the blood, eliminating some (bile pigments) in
the bile and preparing others, the nitrogen-containing substances
and toxic bodies absorbed from the colon, to be returned to the
circulation for final excretion by the kidneys. It is one of the
minor situations where red blood corpuscles are developed. From
it, as well as from various other tissues, is obtained a substance
(heparin) which prevents clotting of the blood. In the liver also
there is stored an antianaemic substance, formed by the action of
a specialized digestive enzyme on food protein, which stimulates
the production of red corpuscles in bone marrow.

The liver is primarily a compound tubular gland, but during
development it becomes associated with the vascular system in the
formation of a structure quite peculiar. It is composed of numerous
minute units, the liver lobules, indications of which may often be
distinguished on the surface of a fresh or well-preserved liver.

ANATOMY OF THE RABBIT

Each of these is made up of innumerable cords of epithelial liver
cells arranged in a radial manner around a central vein, which is
a tributary of the hepatic veins draining the organ. Between the
lobules, where several come together, there occurs a branch of each
of the portal vein, the hepatic artery, and the bile duct, lymphatics,
and nerves with a little connective tissue. These branch over the
surface of each lobule, the branches of the vein and the artery both
emptying into numerous sinusoids which pass radially through

Fig 49. Corrosion preparation of the right and left lobes of the liver of
a rabbit, posleroventnil view. The bile duct and the hepatic duct have been
filled with a dark mass and the portal vein and its larger branches with a
pale mass, following which the tissues have been dissolved away. The caudate
lobe is not shown.

THE LIVER

the substance of the lobule and enter the central vein (Fig. 50).
The sinusoids differ from ordinary capillaries in being wider, and
in the more extreme thinness and the irregularity of their endo-
thelial walls, the cells of which are phagocytic and specially closely
adherent to the glandular epithelium. In the cords of hepatic
cells, delicate bile capillaries receive their secretion and carry it
from the centre of the lobule towards the periphery, where they

Fig. 50. Diagram of a cross section of a single liver lobule. The
sinusoids are represented only in the left half of the diagram in order that
the relations of the bile capillaries may be clearer on the right: ah, branch
of hepatic artery; an, anastomosis between two branches of the hepatic artery;
db, intrahepatic bile duct; 1, lymphatic vessel; Ic, lymphatic capillaries;
s, sinusoids; vc, central vein; vp, branch of portal vein.

converge into the tributaries of the intrahepatic bile ducts which
accompany the arteries and veins between the lobules and in turn
unite to form the hepatic ducts. Lymphatic capillaries occur
between the lobules but do not penetrate them. It is claimed that
between the endothelium of the sinusoids and the hepatic cells
there is an extremely thin film of tissue fluid which seeps out to
the periphery of the lobule and is there absorbed by lymphatic
capillaries, but some authorities deny this.

ANATOMY OF THE RABBIT

Form and Symmetry
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 ab-

FiG. 51. Plan of successive embryonic stages in displacement of the
digestive tube and common mesentery from the mid-line position (man) :
a, tr, d, ascending, transverse, and descending colons; r, rectum; si, small
intestine; st, stomach. (Modified from figures by Toldt and Hertwig.)

sorptive 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 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 on its re-
lation to an extensive serous sac — in a mammal 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. 22, 51), 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 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

DIVISIONS OF THE DIGESTIVE SYSTEM 99

mammalia; and in many cases it is further increased by the inde-
pendent elaboration of the blind intestine or caecum. In the rabbit
the combined length of the small and large intestines is approxi-
mately eleven times that of the body.

Principal Divisions

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) depends 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 processes 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 pro-
jecting upward and forward into the oral cavity from its base of
attachment on the hyoid apparatus, but its greater elaboration and
the differentiation of special processes, the circumvallate and foliate
papillae, for the accommodation of the gustatory organs, are fea-
tures characteristic of mammals. The rooif of the oral cavity is
formed by an extensive palatal surface, 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 backward to a point almost directly
above the aperture of the larynx.

The chief features of the pharynx depend on the fact that it
is not merely a simple portion of the digestive tube but is
also related structurally and functionally with the tubes of the res-
piratory 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 fossae, and also
with the middle ear through the internal auditory tube, and a

100

ANATOMY OF THE RABBIT

Fig. 52. The nasopharynx and related parts
of the head as seen in median section (anterior
end to the left): 1, tongue; 2, hyoid; 3, tonsil;
4, epiglottis; 5, entrance to trachea; 6, entrance
to oesophagus; 7, basioccipital bone; 8, soft
palate; 9, pharyngeal aperture of auditory
(Eustachian) tube; 10, cranial cavity; 11,
ethmoturbinal scrolls; 12, nasal cavity; 13, nasal
septum; 14, hard palate; 15, oral cavity; 16,
nasopharynx.

ventral or laryngeal portion, containing the aperture of the
larynx (Fig. 52).

The oesophagus is a slen-
der but greatly expansible
tube leading from the pharynx
to the stomach. In its pas-
sage backward it traverses the
neck and the thorax, and in
both regions occupies a me-
dian position. In the thorax
(Plate VII) it will be obser-
ved to lie between the heart
and the dorsal aorta, thus
exhibiting the original rela-
tion of the digestive tube to
the aortic portion of the vas-
cular system. The function
of the oesophagus is that of a

simple conveyer to the stomach. 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 dis-
placed 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 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 terminal portion, chiefly the transverse colon,
crosses the ventral surface of the duodenum and then turns back-
ward as the descending colon on the dorsal surface of the mesenterial
small intestine (Fig. 51). In the development of this twisted
arrangement and its many variants in different mammals the
superior mesenteric artery has acted more or less as an axis of
rotation (Fig. 53). The duodenum is marked off from the mesen-
terial intestine as an extensive loop containing the major part
of the pancreas and its duct and lying on the right side of the

DIMSIONS OF THE DIGESTIVE SYSTEM

101

dorsal wall of the abdomen. The common bile duct enters its
first portion immediately beyond the pylorus, so that in it materials
received from the stomach are mixed successively with bile and
with pancreatic juice. The mesenterial intestine is a greatly

Fig. 53. Developmental stages in the coiling of the intestine. Redrawn
with modifications after Zietzschmann. A. Primary intestinal loop. B. The
loop twisted through 180°. C. The loop twisted through 360°, intestine
diflferentiated as in rabbit, ac, ascending colon; c, caecum; d, duodenum;
do, descending colon; i, ileum; j, jejunum; 1, liver; s, stomach; tc, transverse
colon.

convoluted portion, lying chiefly on the left side of the abdominal
cavity, and loosely supported by the broad, frill-like mesentery.
Here the digestive processes are advanced greatly and a good deal
of absorption takes place. From the pylorus to the end of the

102 ANATOMY OF THE RABBIT

small intestine there is generally no abrupt change in the character
of the wall, although the first portion of the mesenterial intestine,
that designated as the jejunum, and the duodenum may be con-
sidered 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 rabbit, however,
presents an exception to the general statement at the beginning of
the previous sentence in that the terminal portion of the ileum

Fig. 54. The caecum and vermiform process: c', c", c"', first, second,
and third limbs of the caecum; ca, beginning of the ascending colon; il,
ileum; pv, vermiform process (appendix); sr, sacculus rotundus.

forms a rather conspicuous rounded sacculus rotundus, a structure
not found in other animals.

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

DIVISIONS OF THE DIGESTIVE SYSTEM

103

the latter crosses the body and is flexed backward as the descend-
ing colon. In the rabbit, however, that portion definable .as the
ascending colon, which is the shortest in man, is greatly elongated,
and is composed of five principal limbs, united by flexures. The
last two of these are concealed in dissection from the ventral
surface 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 in the rabbit, its supporting
peritoneum, the descending mesocolon, being fused with that of

^^^%:^^\,^f

ill

Fig. 55. The caecum with its ventral wall removed to show the lumen
and the contained spiral valve: a, ileocolic aperture.

the ascending limb of the duodenal loop so that it is restored to
an approximately median position. The caecum, like the colon,
is much enlarged and particularly elongated, its course as it lies
in the body being comparable to two turns of a left-hand spiral
(Fig. 54). Its blind terminal portfon, the thick-walled vermiform
process, is also relatively large and, until the beginning of this
portion is reached, its internal surface area is further increased
by the presence of a long spiral fold or spiral valve (Fig. 55)
comparable with that present in the intestines of sharks and skates.

104

ANATOMY OF THE RABBIT

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 ana left
colic arteries, comparable to those of man : while on the other hand
the supply to the parts on the right side, the ascending colon,
caecum, and related portions, on account of their great elaboration,

is represented by a large number of

^'k " ^. vessels, branches of a common ileo-

h1 :tf "^^ caecocoHc trunk. Each of these vessels

B 1 ^■BI^^^L. anastomoses with its immediate

m^P § ^Hjil^^m neighbours so that the large intestine

'•■^^ ^^^^^^m is supplied by a continuous series

^(F '^m^t^^ ^^ arterial loops from which smaller

branches are distributed to the in-
testinal walls.

The elaborations of the ascending-
colon and of the caecum, which
contrast markedly with conditions in
carnivorous mammals, (figs. 54, 56),
are highly instructive examples of
adaptation to the character of the
diet.

Fig. 56. Caeca of a cat and of a
man, dorsal view. The former
short, without a vermiform process,
connecting with a smooth colon.
The latter cup-like, with a vermi-
form process proportionally smaller
than that of a rabbit, and having
bands and haustra continuing those
of the colon.

The Respiratory System
In all air-breathing vertebrates, the lungs (Fig. 57) are paired
sacs which arise embryonically as ventral outgrowths of the diges-
tive tube, and are secondarily connected with the outside of the
body 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 respira-
tory system, as represented by the lungs and related tubes, being
developed as a ventral outgrowth of the pharynx, is nominally
ventral to the oesophagus, but in the adult animal this relation is
actually true chiefly of the trachea. In the thorax (Plate VH)
the bronchi are, in general, interposed between the oesophagus and

THE RESPIRATORY SYSTEM 105

the heart, the lungs being expanded laterally into the paired pleural
cavities.

In addition to the carrying of air over the sensory, olfactory
surfaces, other accessory- functions more closely related to respira-
tion are the warming of the air and removal therefrom of particles
of foreign material, both of which are performed by the mucous
membrane of the nose, including that of the turbinated surfaces.
The sole functions of the true respiratory tract and lungs are
respiratory.

Respiration as a Process

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,

- rn \

I W"'lJ ''

Fig. 57. 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, left superior caval vein;
s', s", superior lobes; tr, trachea; vd, right ventricle; vi, inferior caval vein;
vp, pulmonary veins, vs, left ventricle. The right and left pulmonary
arteries and the arch of the aorta are represented too high up — compare
Fig. 62.

106 ANATOMY OF THE RABBIT

for the oxidative phases of metabolism; also in the discharge of
waste gases, principally carbon dioxide, from the 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 lining of the terminal air sacs into

Fig. 58. Photomicrograph of part of a section of the lung of a rabbit 10 micra thick.
X 50. (Macklin and Hartroft.) A .small venule is seen entering a larger one from the
right. The venule is surrounded by alveolar sacs (as), each with small, cup-like
aheoli (a).

the capillaries before it can be taken into the blood cells, the latter
from their flattened shape and very great numbers present a rela-
tively enormous surface for absorption, the process being thereby
facilitated. Moreover, the epithelial lining referred to is so ex-
tremely attenuated that its completeness or even its presence in
the adult mammal is a matter of active dispute. The lungs them-
selves are highly elastic, expansible sacs. They have the structure
of greatly ramified saccular glands, except that the free internal
surfaces are everywhere in contact with air (Fig. 58). The division

THE RESPIRATORY SYSTEM

107

of the trachea into its bronchi, together with the bronchial ramifi-
cations, forms the trunk and main branch portions of a rather
complex system of tubes (Fig. 60), of which the terminal air-spaces
are the final and functional parts. The branching of the blood-
vessels interlaced with the air passages is equally complex (Fig. 59,
63) and these vessels, besides providing for the aeration of the blood,

Fig. 59. Corrosion preparation of the lungs of the rabbit, dorsal view.
The trachea, the bronchi, and most of the more anterior bronchioles have
been filled with a pale mass, the veins with a dark mass, and the arteries with
one of intermediate shade, the tissues then having been dissolved away:
a, aorta; t, trachea; vcd, right superior vena cava; vci, inferior vena cava;
vcs, left superior vena cava; vp, right pulmonary vein.

are said to form a blood-depot which assists in the regulation^of
the relative output of the two sides^ of the heart.

Breathing

What is commonly described as respiration, or the act of
breathing, is a mechanical, muscular process accessory to the

108 ANATOMY OF THE RABBIT

fundamental exchange which really constitutes respiration. 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 inspiration, is brought about by the contraction
of the intercostal and related muscles in such a way that the ribs
are raised and by the simultaneous contraction of the dome-
shaped diaphragm, by which the posterior wall of the thorax is
flattened, and incidentally the abdominal viscera are displaced
backward. On account of the oblique position of the ribs when at
rest, these actions tend to enlarge the thoracic space in all three
dimensions, in consequence of which air passes in from the pharynx
and distends the lungs. The expulsion of air, or expiration, is ac-
complished by relaxation of the muscles mentioned above, assisted
by contraction of the transverse thoracic muscles (p. 323) and the
muscles of the ventral abdominal wall. The precise part played by
the different muscles in the co-ordinated act of breathing has been
shown to vary somewhat in different individual animals.

The action of the diaphragm is controlled directly by the
phrenic nerves, but all respiratory movements are dependent upon
the cervical and thoracic spinal nerves. The excitation of these
nerves is regulated through a respiratory centre in the medulla
oblongata, comprising inspiratory and expiratory portions and
lying mainly under the back part of the fourth ventricle. The
centre is stimulated both directly by carbon dioxide in the blood
and reflexly by afferent impulses conveyed from the lungs, from
chemoreceptors in the blood-vessels, and from other sources through
fibres largely but not entirely in the vagus nerve.

Lungs and Gills
Respiration as a general function is common to all organisms.
Though always 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 aquatic respiration, and the
higher forms by 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 modification from one to

THE BLOOD-VASCULAR SYSTEM

109

Fig. 60. The larger terminal rami-
fications of the left inferior bronchial
ramus, from the dorsal surface; metallic
cast of the interior. Cf. Figs. 57, 59, and
114, i.

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. 21), by
the somewhat later development
of air sacs in addition to these,
and by the gradual elaboration
of the latter in the vertebrate
series from a simple type, as
illustrated in the frog or better
in lower tailed amphibians, to
the greatly branched lung tubes
of mammals and the highly
complex and special arrangements
in birds.

The Blood-Vascular System

In the rabbit, as in all vertebrates, the blood- vascular system
(Fig. 61) embraces a central, muscular organ of propulsion, the heart,
and a series of branched tubes, the blood-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-w^alled collecting vessels — veins.

The chief mammalian feature in this system consists in the
division of the heart into two portions, respectively left and right,
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 systematic circulation.
It is concerned with the distribution of blood to the various
parts of the body, with the exception of the conveyance of blood
to the lungs for aeration (but incljjding the supply of the deeper
tissues of the lungs, through the bronchial arteries). It is estab-
lished 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
anterior portions of the body through paired internal and external

ANATOMY OF THE RABBIT

licail. nut. liiii//.<

jugular and subclavian veins, communicating with the right atrium
of the heart through paired superior cavals^; from the posterior

portions of the body through the unpaired
and 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 con-
cerned with the distribution of the blood to
the lungs for purposes of aeration (Fig. 63).
It is established by the right ventricle, the
pulmonary artery and its paired branches,
and the capillaries of the lungs. The
blood is delivered to the left atrium through
several pulmonary veins. A similar divi-
sion of the circulatory organs occurs as a
homoplastic modification in birds, which,
it will be observed, are also warm-blooded
vertebrates.

In general, the blood which is distribut-
valves^""^ '^^* atrioventricular gd 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 distinguished
by the possession of a second series of capillary vessels ramifying
in the liver. Thus, in the rabbit and other mammals, the blood
distributed to the stomach, spleen, and intestine through the coeliac
and the superior and inferior mesenteric arteries, is collected into
a main intestinal vessel, the portal vein, and the latter passes to the
sinusoids of the liver, which take the place of true capillaries,
differing from them as described on page 97. The liver receives
also oxygenated blood, though in much smaller quantity, through
the hepatic artery and the ultimate branches of this also empty

bf///r wnlt. pnst. litnbs

Fig. 61. The mammalian
circulation: rv, Iv, right and
left ventricles; ra, la, right
and left atria; so, sp, semi-
lunar valves of aorta and
pulmonary artery; vt, vm,

'In many mammalian species, including man, there is a reduction of the left
superior caval vein during development, blood from the left subclavian and jugu-
lar vessels all being diverted through the transverse jugular (p. 297) into the
right superior caval, which thus appears unpaired in the adult.

THE BLOOD-VASCULAR SYSTEM

111

into the sinusoids, which in turn unite in the tributaries of the
hepatic veins. In lower vertebrates and in the embryos of mammals
where the kidney is a mesonephros (p. 126), a second system of
venous capillaries occurs in that organ and is known as the renal
portal system.

* es_ cd

>-vpcl

Fig. 62. Dorsal aspect of heart of rabbit: ad. as, right and left atria;
apd, aps, right and left pulmonary arteries; cd, cs, right and left common
carotid arteries; in, innominate artery; la, arterial ligament (cut); sd, ss,
right and left subclavian arteries; vcd, vcs, right and left superior caval
veins; vci, inferior caval vein; vpd, vps, right and left pulmonary veins.

The ultimate function of the vascular system is connected with
interchange of materials between the tissues and the liquid flowing
in the vessels. This is brought about through the medium of
microscopic capillaries, the delicate walls of which act as semiperme-
able membranes permitting diffusion through them. The gross parts
of the system are concerned with transportation of dissolved
materials from one part of the body to another. The propulsive

112 ANATOMY OF THE RABBIT

action of the heart is muscular and rhythmic; contraction, or
systolic phases, alternating with expansion, or diastolic phases
(cf. p. 64). The flow is maintained in one direction principally by
the atrioventricular valves of the heart and by the semilunar valves
of the aorta and pulmonary arteries, though there are also valves in

ap-^^

L-vVp

Fig. 63. Corrosion preparation of the blood-vessels of the lungs of a
rabbit, dorsal view: a, aorta; ap, pulmonary artery; vc, right superior vena
cava; vp, pulmonary vein.

the course of some of the veins. The arteries are tubes with thicken-
ed 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 and the relative distribution of blood to various parts is

THE BLOODA'ASCULAR SYSTEM

113

regulated both by changes in degree of contraction of the arterial
walls and also by local closing and opening of the capillaries them-
selves. In the return of the blood the veins are largely passive,
acting merely as closed channels connecting the 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 tone in the walls of the vessels and thereby the
pressure of the blood (cf. p. 64).

Many of the pecuHar features of the mammalian circulation
which at first sight do not appear to
be general, but are so in reality, de-
pend on the circumstance that the
complete partition of the organs is a
final stage of a general progressive de-
velopment, observable in air-breath-
ing vertebrates, in which the lungs and
their vascular connections become
perfected for pulmonary respiration.
On the other hand, the vascular
system in its earlier embryonic condi-
tion more especially in its aortic por-
tion, 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 con-
nected with a series of paired bran-
chial aortic arches, traversing the
rudimentary gill structures and thus
passing upward around the sides of
the primitive pharynx. The dorsal aorta is formed by the junction
of the branchial aortic arches, and passes backward as a main dis-
tributing vessel on the ventral side of the axial support. The heart
itself is formed primarily on a plan similar to that in fishes, where
all the blood is received by a 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

Fig. 64. Plan of the branchial
aortic arches. The adult mammalian
vessels- are indicated in black (sys-
temic) or shaded (pulmonary) : 1-6,
primary arches; ao., aorta; a,p.,
pulmonary _ artery; c.e., external
carotid; c.i., internal carotid; d.a.,
ductus arteriosus (Botalli) ; i., in-
nominate artery; s.d., right sub-
clavian; S.S., left subclavian. (From
Weber, after Boas.)

114 ANATOMY OF THE RABBIT

in the accompanying diagram (Fig. 64), the arched condition is re-
tained by the aorta and by the puhnonary artery. It is interesting
to note also that the primary connection of these vessels is repre-
sented in the foetus by an open canal, the ductus arteriosus (Bo-
talli), which closes shortly after birth but is indicated in the adult
by a short fibrous cord between the left pulmonary artery and the
aorta, the arterial ligament (Fig. 57, 1). It will be evident from an
examination of Fig. 64 that this is a vestige of the dorsal part of
the sixth aortic arch on the left side. The adult aortic arch
represents the left one of a pair (the fourth) while that of the
right side is represented only imperfectly by the base of the right
subclavian artery (the innominate artery and the common carotids
being derived from parts of the ventral aorta, as is apparent in
Fig. 64). Hence a condition of asymmetry results, which is ex-
pressed mainly in the sinistral position of the aortic 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 (derived from the ventral aorta of the embryo), and the
dorsal position of the descending part of the aorta (beyond its arch)
as a median vertebral trunk.

The vascular system is noteworthy for several departures from
the condition of symmetry, one of which has already been men-
tioned. In addition, it is seen that in a mammal, as in terrestrial
vertebrates generally, the base of the pulmonary artery (Figs. 57,
62) 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 to divide on the dorsal side of
the latter into its two main branches. Moreover, the separation
of the ventricular portion of the heart into two chambers is as-
sociated 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.

THE FOETAL CIRCULATORY SYSTEM 115

since from its beginning at the posterior end of the abdominal
cavity to its termination on the right atrium it Hes 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
asymmetrical, since the trunk lies to the right of the bodies of the
vertebrae, and is connected at its base with the right superior cava!
vein. These dispositions have been derived from originally sym-
metrical ones through complicated embryological changes.

The Foetal Circulation and Its Transformation
to the Adult Condition

Besides the features of the circulatory s^^stem discussed above,
which are explained by their embryonic origin, others reflect later
stages of prenatal development, when the aeration of the blood and
the absorption of nourishment are accomplished in the peculiar
organ characteristic of the higher, or placental mammals, the
placenta. This organ is formed in part by the wall of the maternal
uterus and in part by an outgrowth (the allantois) from the embry-
onic alimentary canal. It provides for a close interlacement of the
maternal and embryonic bloodvessels so that, without any mixing
of the two blood-streams, an exchange of dissolved material can
occur between them by diffusion. The course of the foetal blood
at this time is represented diagrammatically in figure 65.

After aeration in the placenta, the blood returns to the body
of the foetus through the umbilical vein, unshaded in the diagram,
and is carried by it to the liver, which it traverses through a wide
channel, the ductus venosus. Here it is mixed with unoxygenated
blood from the portal vein and is then emptied into the inferior
vena cava, there mingling with a second stream of unoxygenated
blood coming from the posterior parts of the body. This mixed
blood is indicated in the diagram by stippling. Entering the right
atrium of the heart, such blood mixes very little with that coming
through the superior caval veins but mostly flows directly through
the foramen ovale (Fig. 66), a wide passage leading through the
median septum into the left atrium, whence the blood is pumped
to the left ventricle and out through the aorta.

116

ANATOMY OF THE RABBIT

Fig. 65. Diagram of a ventral view of the blood-
vascular system of a foetal rabbit, shortly before birth
ao, aorta; da, ductus arteriosus; dv, ductus venosus-
IV, inferior vena cava; lea, left common carotid artery;
ipa, left pulmonary artery; p, placenta; pv, portal
vein; rpa, right pulmonary artery; rsv, right superior
vena cava; rv, right ventricle; sma, superior mesen-
teric artery; ua, right umbilical artery; uv, umbilical
vein; va, vitelline artery.

THE FOETAL CIRCULATORY SYSTEM

117

Fig. 66. Diagram of a ventral
view of the course of the blood
through the heart of a rabbit
shortly before birth, ao, aorta; f,
foramen ovale; i, opening of in-
ferior vena cava; 1, opening of left
superior vena cava ; la, left atrium ;
Iv. left ventricle; pa, pulmonary-
artery-; pv, openings of pulmonary
veins; r, right superior vena cava;
ra. right atrium ; rv, right ventricle.

The Linoxygenated blood from the
rep^ions in front of the heart enters
the right atrium through the superior
caval veins and, although there is no
partition to separate it from the
stream entering by the inferior caval,
it is mainly directed through the
right atrioventricular opening to the
right ventricle and so into the pul-
monary artery. The lungs being
non-functional until birth, however,
only a part of this current is carried
to them, the greater portion passing
through the wide ductus arteriosus
(the retained dorsal part of the left
sixth aortic arch) to the aorta. These
vessels carrying unox^^genated blood
appear black in the diagram. Some
of the mixed blood which entered the
aorta from the left ventricle has been
distributed through the carotid arteries to the head and through
the subclavian arteries to the anterior limbs before this final
admixture of unoxygenated blood through the ductus arteriosus
occurs, so that these anterior parts receive blood better ox\genated
than that which reaches the trunk and tail. At its caudal end, the
aorta divides into a pair of large common iliac arteries, the greater
part of the blood from which enters the umbilical arteries and so is
returned to the placenta to have its load of oxygen renewed. The
external iliac artery, which continues into the hind limb, is con-
siderably smaller in the foetus than the umbilical, and the hypogas-
tric (internal iliac) is smaller still.

Radical changes in these dispositions occur at birth. The
placenta is suddenly lost and the flow of blood through the umbilical
vein ceases, this vessel rapidly degenerating to a cord of connective
tissue, the remains of which appear in the adult as the round
ligament of the liver. The wide passage through the liver, the
ductus venosus, also becomes obliterated so that all blood entering
that organ has to flow through its sinusoids to reach the hepatic
veins and enter the vena cava. Since no oxygenated blood now

118 ANATOMY OF THE RABBIT

reaches the Hver except the small flow through the hepatic arteries,
the blood in the inferior caval vein is now nearly devoid of oxygen.
The sudden expansion of the lungs with the first breath results
in an immediate expansion of the pulmonary arteries and a simul-
taneous active contraction of the walls of the ductus arteriosus
forces all the blood from the right atrium to flow through these.
This contraction of the ductus arteriosus is maintained until the
lumen is permanently obliterated and the vessel remains only as a
solid cord, the arterial ligament of the adult.

The increased stream of blood through the lungs returns through
the pulmonary veins to the left atrium, producing altered pressure-
relations there such that the flaps at each side of the foramen ovale
are pressed into contact. Thus an almost immediate functional
closure of the foramen takes place. Later, the flaps fuse and the
position of the foramen is indicated only by a thin area, the fossa
ovalis, in the adult heart. Such closure of the foramen ovale
diverts all the blood entering the right atrium into the right
ventricle, whence it all is pumped to the lungs, as just indicated.

Thus all the regions of mixture of oxygenated and unoxygenated
blood are closed off at birth and thereafter all blood in the right side
of the heart is unoxygenated and all blood in the left chambers of
the heart is oxygenated. Moreover, the oxygenated blood from
the left ventricle is distributed through the aorta and its branches
to all parts of the body without any dilution such as is brought
about by the ductus arteriosus in the foetus, and even the most
posterior parts receive blood with as much oxygen as in that to the
head.

Although the placenta is lost, the basal part of the outgrowth
of the embryonic alimentary canal which produced it remains
and forms the urinary bladder; and the corresponding portions of
the umbilical arteries also remain to supply that organ, though
reduced in relative size and now carrying oxygenated blood.

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 may

be seen by ordinary dissection, the structures which are revealed

THE LYMPHATIC SYSTEM

119

in this way being mainly the lymph glands, or lymph nodes.
Anatomically, the system may be regarded as comprising super-
ficial and deep portions, the superficial nodes occurring under the
skin either singly, as in the head and neck, or more or less grouped,
as in the axillary and inguinal regions, their precise number and

Fig. 67. Some of the lymph vessels and nodes of the anterior part of
the rabbit, according to Jossifow (redrawn): ao, aorta; da, deep axillary
lymph node; do, deep cervical lymph node; dt, thoracic duct; h, lymph vessels
of heart; 1, lymph vessels of lungs; 11, lymph vessels from lips; m, mediastinal
lymph nodes; sa, superficial axillary lymph nodes; sc, superficial cervical
lymph nodes; sm, submaxillary lymph nodes; tj, jugular trunk; ts, subclavian
trunk; vj, external jugular vein.

arrangement being somewhat variable. As deep structures they
are conspicuous in the intestinal mesenteries and in the walls of the
digestive tube, occurring 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 aggre-
gated lymph follicles (Peyer's patches) at various points in the
wall of the small intestine. • .

120 ANATOMY OF THE RABBIT

The conducting portion of the system comprises an extensive
series of canals, beginning as lymphatic capillaries in peripheral
organs, and ending as lymphatic trunks which empty into the great
veins. The lymphatic capillaries are terminal 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 blood-vascular system, between vessels of a
larger order. The lymphatic capillaries begin blindly in the tissue
spaces, where they collect through their walls fluid derived from
the blood plasma by exudation through the walls of the blood
capillaries. The lymphatic capillaries unite to form larger vessels
and these are connected as extensive plexuses, at important points
in which the lymph nodes are distributed. The latter act as
strainers for the lymph, removing bacteria or other foreign particles
so that they will not be conveyed into the blood stream. They
also add to it new white blood cells. From them the vessels convey
the lymph to the lymphatic trunks.

The lymphatic trunks of the anterior portion of the body (Fig.
C)7) 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 or of the common jugular and subclavian (Fig. 111).
The lymphatic vessels of the posterior portion of the body, in-
cluding the intestine, usually largely unite in a lymph-reservoir at
about the level of the first lumbar vertebra, and from this the
lymph flows forward through 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. There are also retrosternal lymph-tracts
accompanying the internal mammary blood-vessels and functional
tests have shown that drainage from the abdominal cavity is
largely through these. They enter the jugular veins of their
respective sides just after the union of external and internal
jugulars. All these trunks are so thin-walled that it is not usually
possible to see them in ordinary dissection.

The lymph or fluid present in lymph vessels and in the spaces
of lymph nodes and the tissue fluid in the tissues of the body are

THE LYAIPHATIC SYSTEM

121

comparable to the fluid part or plasma of the blood. While blood
is a carrier for both in-going and out-going materials of metabolism,
on account of being confined to the capillaries it is not brought into
direct contact with the tissue-cells of the body. The cells are,
however, bathed in tissue fluid, which can permeate the tissues by
diffusion. It contains cells of a type that can migrate through the
walls of vessels and when it enters the lymphatics it becomes lymph.
The lymph is thus a general medium of transmission with special
cell functions. The fat-carrying function of the intestinal lym-
phatics, in which the delivery of
food materials directly to the blood
is the principal consideration, is
doubtless a very special phase of
the transfer mechanism.

The cell contents of lymph ves-
sels and spaces are amoeboid cells
or leucocytes (cf. p. 33). Leuco-
cytes of several kinds are found in
various situations in the body.
Since they are wandering cells,
their situation at any one time re-
veals little of their points of origin.
They are formed originally in lymph
nodes, in the spleen, in the bone
marrow, in endothelial linings, ar.d
by local proliferation of connective
tissue cells. They serve a variety
of purposes of which the phagocytic action, chiefly ingestion and
destruction of bacteria, and disintegration of erythrocytes and
other cell debris, is one of the chief.

Lymph nodes, which, as just indicated, are centres for the
proliferation of certain types of leucocytes, concentration points
for such cells, and local centres of phagocytic action, appear to be
situated strategically with reference to parts of the body served.
In local infection, the activity of leucocytes of various kinds at
the point of injury can be correlated with that of leucocytes in the
nearest lymph nodes and the general relation can be observed in the
appearance and behaviour of the two regions themselves. The

Fig. 68. Homologies of male (A) and
female (B) urinogenital systems: b,
urinary bladder; cc, crura clitoridis; cp,
crura penis; dd, ductus deferens; ep,
epididymis; k, kidney; ov, ovary;" r,
rectum; t, testis; tu, uterine tube; u,
urethra; ut, uterus; ur, ureter; va,
vagina; vs, seminal vesicle; vs', vesti-
bulum, urethra.

122

ANATOMY OF THE RABBIT

spleen is a lymphatic organ, the largest in the body, with func-
tions comparable to those of lymph nodes. It contains large
vessels which act as reservoirs for blood and it also stores iron.

The Urinogenital System
The urinogenital system comprises two primary systems — re-
productive 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. 68),
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

Fic. 69. 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,
urinogenital sinus (vestibulum) ; ut, uterine tube; v, vagina. (Chiefly from
figures of Gegenbaur and Wiedersheim.)

designated in the male as the urethra, but in the female as the
vestibulum, since the structure known from the human relation
as the female urethra is only a urinary canal leading from the
bladder and does not serve as a reproductive duct.

In primitive vertebrates (Fig. 69), the urinary and genital ducts
open into the posterior end of the digestive tube, the latter forming
in this relation a common canal, the cloaca. In terrestrial verte-
brates, 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 transformed into a urinogenital
sinus. This development reaches its extreme in the placental mam-

THE KIDNEYS

123

mals, where the urinogenital sinus becomes completely separated
from the digestive tube, and where the urinary ducts are also trans-
ferred 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 Kidneys

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 intermediate cell mass
(Fig. 22, n.) from which they are formed. During development,
one kidney is often displaced more than the other by the pressure
of adjacent organs so that the symmetrical disposition of the pair
is destroyed. Thus in the human adult
the right kidney is situated lower than
the left on account of the pressure of the
right lobe of the liver. In the rabbit, on
the other hand, the left kidney is dis-
placed further back than the right by
the posterior expansion 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 ex-
panded end of the ureter. In the rabbit
the kidney appears as an almost con-
tinuous mass, in which, however, slight
traces of lobulation can be distinguished. In many mammals,
such as sheep and bear, the organ is composed of distinct and
separable lobules. This condition is clearly shown in the human
kidney during foetal life, and though the organ is much more
concentrated in the adult, the lobulated condition there appears
internally in the division of the ureter into several renal calyces,
each of them connected with a corresponding renal papilla. In the
rabbit, however, there is only a single renal papilla and the ex-
panded end of the ureter, the renal pelvis, is undivided. The pelvis
has a lobulated form not readily displayed in dissection but strlklng-

FiG. 70. The left kidney,
divided horizontally lengthwise,
cut surface of dorsal half: c,
cortical substance; m, medullary
substance; p, renal papilla; u,
ureter.

124 ANATOMY OF THE RABBIT

ly shown in a cast of the cavity (Fig. 71). The suitability of the
term cah'x is evident.

Fig. 71. Medial, dorsal, and posterolateral views of a cast of the renal
pelvis and beginning of the ureter of a rabbit. The deep depression visible
in the middle .of the cast in the posterolateral view is occupied by the renal
papilla. The pelvis is slightly distended by the pressure required to fill it
with the mass.

Internal Structure and Function
When horizontally divided (Fig. 70), the kidney is seen to be
made up of a n:ore vascular and granular external layer, termed the
cortex, and of a somewhat radially striated, central mass, termed
the medulla. Notwithstanding 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 is such that water and certain
soluble substances to be excreted are passed into them from the
blood stream. The primary tubule, or nephron, (Fig. 73, B)
begins in each case in the cortical substance with a cup-like structure,
known as a renal or Malpighian corpuscle. This consists of a double
capsule containing a glomerulus or knot of capillaries from the
renal artery (Fig. 73, A). The blood in the glomerulus is separated
from the cavity between the two layers of the capsule only by a
very thin membrane composed of the lining of the capillary and
that of the capsule, both of which linings are uninterrupted, and
through this membrane fluid is filtered into the cavity of the capsule,
whence it flows into the tubule. The nephron beyond the capsule
is differentiated into portions known as the proximal and the distal
tubules. The first of these comprises a convoluted portion, a
straight portion, a thin portion, and a thick portion, of which the
first with the renal corpuscle lies entirely in the cortex while the

THE KIDNEYS 125

others form a long loop, the loop of Henle, extending into the
medulla and back to the caj^sule. Here commences the distal
tubule, which is tortuous and leads into an initial collecting tubule.
This, in turn, unites with others to form a collecting tubule that
passes through the medulla to enter the pelvis through the surface
of the papilla. Blood capillaries again come into contact with each
tubule at certain points in its course, where further excretion and
selective reabsorption occur. The excreted fluid, urine, contains
characteristic nitrogenous waste niaterials, usually mainly urea
but with smaller quantities of other nitrogenous substances such

A B

Fig. 72. Corrosion preparations of the larger blood vessels in the kidney
of the rabbit: A, veins; B, arteries. The greater density of the vessels in
the cortex as compared with the medulla is evident.

as uric acid or, in most mammals, allantoin. These are formed
mainly in the liver and perhaps elsewhere in the body.

Homologies of Vertehraie Kidneys

Like all other parts of the urinogenital system, the mammalian
kidney affords in its structure and embryonic development a re-
markable illustration of the extent to which the adult form and
relations of an organ may depend upon ancestry, and of the
greatness of the changes which ^may occur before these adult
conditions are attained. In the vertebrate subphylum, three
successive pairs of kidneys have been recognized. They occur in
antero-posterior order in the body, they are of increasing special-
ization, and their order of appearance and functional value are

126

ANATOMY OF THE RABBIT

directly associated with the degree of general speciaUzation of
the groups in which they occur. These organs have been designated

Fig. 73. A. Plan of a single primitive kidney tubule in a
lower vertebrate, the cavity between the two layers of the
capsule and the cavity of the tubule represented in solid black:
gl, glomerulus; np, nephrostome; cl, coelomic epithelium;
d, longitudinal duct; t, main portion of tubule.

B. S'cheme of the parts of the nephrons and their situa-
tions in the mammalian kidney, after Sperber: c, cortex;
iz, inner zone of the medulla; m, medulla; oz, outer zone of
the medulla; re, renal corpuscle; tc, convoluted portion of
proximal tubule; tk, thick segment of proximal tubule; tn,
thin segment of proximal tubule; ts, straight portion of
proximal tubule. The proximal tubules are unshaded, the
distal tubules grey, the collecting tubules black.

as pronephros, mesonephros, and metanephros. The metanephros
is the aduh kidney in mammals, while the other two are embryonic

THE KIDNEYS 127

in that class. The mesonephros is, however, the adult kidney
in fishes and amphibians, where its duct serves in the male as
both reproductive duct and ureter. The presence of this kidney
and of its duct in embryonic mammals determines the form of
the ductus deferens and its connections with the terminal portions
of the urinary system. The pronephros, on the other hand, is in

Fig. 74. Photomicrograph of a small part of a section of the kidney of
a rabbit. X 360. A collecting tubule appears at the right, the structure of
its epithelial lining being distinct. At the left is a glomerulus lying within
its Bowman's capsule and round it are several sections of convoluted tubules.

all vertebrates a vestigial kidney present only in embryos. Its
duct system, however, which opens proximally to the body cavity,
plays an important part in the formation of the oviduct in the
female of all classes. The tubules of the pronephros and often
those of the mesonephros connect with the coelom by ciliated
openings, the nephrostomes, which do not occur in more special-
ized kidneys. Thus the development and structure of the prone-
phros and, in a less perfect way, those of the mesonephros show

128

ANATOMY OF THE RABBIT

that the primary connections of the kid-
ney tubules are with the coelomic cavity
(Fig. 73, B).

The Testis and its Duct

The male gonad, or testis (Fig. 81),
is composed mainly of convoluted semini-
ferous tubules, held together by layers of
connective tissue, and in the walls of these
tubules the male germ cells, the sperma-
tozoa, are developed. In a transverse sec-
tion of a tubule, cells with dark nuclei
appear in the outer or basal row (Fig. 76).
These, krown as spermatogonia, produce
cells (spermatocytes) which are transformed
through several intermediate steps into
spermatozoa the latter then passing from
the tubules to the epididymis. In the rab-
bit, as in all mam.mals, the testis is con-
nected with the peripheral duct system
(Fig. 75) by means of the epididymis and
the ductus deferens, which are parts of
the mesonephric connections of the em-
bryo. While the ductus deferens is a

Fig. 76. Small part of a section of the testis of a
rabbit showing two seminiferous tubules, sc.
spermatocytes; sg, .spermatogonia; sz. heads of
newly formed spermatozoa.

Fig. 75. The male urino-
genital ducts and related
structures viewed from the
lateral surface: a, anal aper-
ture; b, bulbourethral gland;
c, end of crus penis cut from
its attachment to the ischium;
dd, ductus deferens; gi, brown
portion of inguinal gland; gr,
rectal gland; i, white portion
of inguinal gland; 1, pars
libera penis; mi, ischiocaver-
nosus muscle; p, paraprostatic
glands; pr, prostate gland; r,
rectum; s, inguinal space; u,
urethra (membranous por-
tion); ur, ureter (these
stumps should be directed
more backward, to pass under
the deferent ducts) ; v, vesi-
cular gland; vs, seminal
vesicle; vu, urinary bladder.

single tube, the epididymis
consists of an aggregation
of small tubules, lying chief-
ly toward the anterior end
of the testis, but with the
tubules not individually
discernible. In the embryo

THE GENITAL ORGAXS

129

of every vertebrate, the testis is formed in association with the dorsal

rs-.i abdominal wall, but in many

mammals it moves backward
either periodically or perma-
nently in the course of de-
velopment, to a position in a
separate sac derived from the
posteroventral part of the
coelom, the scrotal sac. This
change in position, known as
the descent of the testis, has
been shown to be an adap-
tation for temperature regu-
lation in that organ. It is
controlled by a cord of muscle
and connective tissue, the
gubernaculum, which is re-
tained in the adult rabbit as
a fibrous band attaching the
testis to the end of the scrotal
sac. The migration of the

Fig. 77. The female urinogenital system:
a, aorta; as, internal spermatic artery; au,
umbilical artery; c, clitoris; gp, inguinal gland;
gr rectal gland; h hydatid of uterine tube; hr orS:an determines a UUmbcr
middle hasmorrnoidal artery; i, inferior caval °

vein; lo. ovarian ligament; It, round ligament;
lu. umbilical ligament; ms, mesosalpinx; mt,
mesometrium; o, ovary; ot, ostium tubae; r, of itS blood-VCSScls and pcri-
rcctum; rp, peritoneal recess (rudimentary

vaginal process); tu, uterine tube; u, ureter; tOUCal COUnCCtlOnS.
v.t. uterus; vg, vagina; vs, vestibulum; vu,
urinary bladder.

of peculiarities in the relations

The Ovary and Oviducts

The female gonad or ovary (Fig. 77) lies on the dorsal wall of
the abdominal cavity, thus retaining to a large extent the primitive
position. It has, however, gubernacular connections corresponding
with those of the testis and these are plainly discernible in the adult
animal as the ovarian and round ligaments, of which the latter
is inserted into a small pocket of the abdominal wall simulating
the testis sac.

Though inconspicuous in gross size as compared with the testis,
the ovary is concerned wdth the formation of cells of relatively large
dimensions, the female germ cells or ova (Fig. 1), which, however, are

130 ANATOMY OF THE RABBIT

produced in much smaller numbers than the spermatozoa. The ova
undergo their primary development as single cells in the tissue of
the organ, but at times, through rupture of the enclosing follicles,
they are released at the surface, and thence pass directly into the
open mouth of the uterine tube, the narrow first part of the oviduct.
In this tube the ova may come into contact with spermatozoa,
fusion with one of which constitutes fertilization of the ovum.
(Spermatozoa deposited in the vagina are carried into the uterus
by muscular action of the duct, traverse the uterus by their own
locomotor activity, and are carried up the uterine tube largely by
the cilia lining it.) If fertilized, they begin their segmentation and
further development into an ^.^^ ^^

embryo, the latter becoming J^ At ^^\\/7^ \ /
attached to the wall of the \\// \"/ \"/

more posterior part of the Vh^ W \A

oviduct, which is enlarged to { j '^ 1 1 ^ f ] ^

form the uterus. A placental

Fig. 78. Three stages of specialization of the

connection IS formed by which uterus, a, uterus duplex; B, uterus bicornis;

. , . . , 1 C, uterus simplex, t, uterine tube; u, uterus;

nourishment is carried to the v, vagina.
embryo during the period of

intra-uterine life, in the rabbit about thirty days. The rabbit has
two complete uteri, the cavities of which are connected distally
with the unpaired vagina, and through this with the urinogenital
sinus or vestibulum. 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
fem.ales 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 the
arrangement in many other mammals. Paired oviducts (Fig. 69, A)
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 monotreme stage of mammals,
but in higher forms of the latter the ducts are progressively coa-
lesced. In marsupials the posterior part of each oviduct is differ-
entiated as a vagina, which is still paired, while in placentals the

THE ENDOCRINE SYSTEM 131

vaginae are fused to an unpaired tube. In the rabbit, as in many
lower placentals, there are two complete uteri, and as an organ the
whole structure represents the stage of uterus duplex (Fig. 78, A).
A partly fused condition existing in some mammals, for example
sheep, is known as uterus bicornis (Fig. 78, B), while the com-
pletely fused condition in man is known as uterus simplex (Fig.
78, 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 Endocrine System

In contrast to the organ-systems usually recognized, which have
a structural continuity and are associated with contributory but
more or less separate functions, digestive, nervous, and the like,
there are certain organs which have a detached distribution, belong
structurally and embryonically to different systems and body-
layers, and yet have common general functions in chemical and
physical regulation, including growth. They are described as
internal secreting, ductless, or endocrine glands. Their effects
are exerted through relatively small quantities of very active
substances, hormones, thrown into the circulation. In a physio-
logical sense, they constitute an internal secreting or endocrine
system. They include the suprarenal bodies, the thyreoid, the
parathyreoids, the thymus, the hypophysis, the pineal body, and
portions of the male and female gonads and of the pancreas as
well as the epithelial lining of the duodenum. The occurrence of
such endocrine organs is a feature peculiar to the vertebrates, in
which they supplement nervous regulation, providing a duplicate
mechanism for the maintenance of the all-important balance among
the activities of the parts of the individual organism.

Both the testis and the ovary produce substances which have
a pronounced effect upon metabolism and growth, especially the
development of secondary sex characters in the young, and, in the
adult female, changes connected with pregnancy. A more specific
action is shown, for example, by the pancreas, imbedded in which
are microscopic groups or islands of cells which are quite distinct

132 ANATOiMV OF THE RABBIT

from those forming the main mass of the gland and have no con-
nection with its duct (Fig. 5, p. 17). These islets produce insulin,
a regulator'of oxidation of starches, sugars, and fats.

The duodenal epithelium produces a hormone, named secretin,
which stimulates the secretion of the digestive juice of the pancreas
and also increases the discharge of bile from the liver.

The Suprarenal Gland

Each of the pair of suprareral bodies is double, being composed,
in mammals, of an inner medulla and an outer cortex which differ in
origin, in structure, and in function. The medulla is an aggre-

Fic 79. riiGtoniicrograph of transverse section of suprarenal
body of rabbit, showing cortex and medulla. X 16.

gation of cells which have a common origin with those of the
sympathetic nerve ganglia, and the adrenalin or epinephrine which
it secretes has an effect somewhat similar to that of stimulating
these nerve cells, raising the blood pressure by vasoconstriction,
causing release of glucose into the blood stream, and enabling the
animal to meet emergency conditions. The discharge of adrenalin
has emotional associations and its occurrence during a state of
fear is regarded as an adaptive reaction preparing the animal for
fight or flight. Actually, however, its effect in such circumstances
appears to be rather enervating or even paralyzing.

The cortex is considerably more voluminous and is derived

THE ENDOCRIXE SYSTEM

133

from the lining of the body cavity. The homologous tissue forms
a mass (interrenal gland) associated with the mesonephros in
lower vertebrates. Its secretion is necessary for life. It appears to
be concerned in maintenance of normal functioning of the kidneys,
in the regulation of inorganic substances in the body, especially
sodium and potassium, and in the conservation of muscular strength
and ability to resist fatigue.

The Thyreoid Gland

Like the pancreas, the thyreoid gland develops as an outgrowth
of the lining of the digestive tube, in this case from that of the
pharynx, but it becomes completely detached therefrom. It gives
directly into the blood a secretion (thyroxin) which takes part in
the regulation of growth and stimulates metabolism in the body
generally. This substance has a high content of iodine in combi-
nation with colloid material. Thyreoid deficiency in man is as-
sociated with the conditions known as cretinism and myxoedema,
and hyperactivity with exophthalmic goitre.

The Parathyreoid Gland

The parathyreoid bodies are minute cell-masses lying in, or
immediately outside of, the thyreoid. They are produced embry-

onically from the dorsal part of
the epithelial lining of certain gill
pouches. They have been shown
to have essential functions in con-
nection w^ith calcium-phosphorus
metabolism.

The Thymus Gland
The thymus is developed in
mammals from the ventral part of
the epithelial lining of the third
pair of embryonic pharyngeal gill
pouches. It is thus paired in ori-
gin, but in mammals the masses
of the two sides become associated ventrally and migrate backwards
to a position near the heart. The functions of the thvmus are

Fig. 80. The parts_ of the hypophysis
of the rabbit and adjacent structures as
seen in sagittal section: d, pars distalis
(anterior lobe) ; e, median eminence of
tuber cinereum; i, pars intermedia; 1,
residual lumen; m, mamillary body; n,
infundibular process (neural lobe) ; o,
optic chiasma; r, infundibular recess; s,
infundibular stalk; t, pars tuberalis.

134 ANATOMY OF THE RABBIT

still problematical, but an extract has been shown to increase
fertility and when administered through successive generations to
produce accruing precocity- in development. The gland becomes
lymphoid in character at some time after birth and is more or less
reduced after the animal reaches maturity. There is some evidence
which has been interpreted as indicating antitoxic activity.

The Hypophysis
The hypophysis is embryonically of double origin, while structur-
ally three major divisions are recognized, viz: lobus glandularis,
lobus nervosus, and infundibulum. The first of these comprises a
pars distalis, a pars tuberalis, and a pars intermedia (Fig. 80), and
the infundibulum is subdivided into the median eminence of the
tuber cinereum and the infundibular stem. The glandular lobe is
derived from the epithelium of the roof of the mouth, while the
remaining parts are an outgrowth from the brain. Experimental
removal of the pars distalis retards growth in young animals, the
body retaining an infantile condition, and in normal circumstances
the growth hormone produced is concerned in protein metabolism.
Hyperactivity of this part in human adults is associated with the
condition known as acromegaly, abnormal enlargement of the
hands, feet, or parts of the head. The pars distalis also secretes
hormones concerned in ovulation, in lactation, and in the stimu-
lation of other endocrines. The neural lobe (lobus nervosus) pro-
duces a substance similar in some respects in its action to adrenalin
and, like the latter, related functionally with the activity of the
sympathetic system. It restricts cardiac output and oxygen con-
sumption. Possibly, one component regulates passage of sub-
stances through the walls of capillaries, and another may stimulate
uterine contractions in parturition.

The Pineal Gland

The pineal body is also an unpaired outgrowth of the brain,
being the product of a part which in lower vertebrates displays the
potentiality of developing into either a sense organ (a light-receptor)
or a gland. In mammals it is always an endocrine gland. Its
functions are not yet well understood but it has been reported to
produce a growth-regulating substance, or one accelerating
differentiation.

THE SEROUS CAVITIES 135

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 membra-
nous linings of these cavities so that they are more or less completely
invested by them without interrupting them at any point.

The serous sacs are extensive body-spaces, derivatives of a
primary body cavity or coelom. They are usually considered
loosely 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 (Fig. 24). 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 im-
perfectly 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 the lungs
in terrestrial vertebrates. 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 mammal four large serous spaces, namely, the pericardial, the
peritoneal, 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 pul-
monary pleura, investing the greater part of each organ, the costal
pleura, lining the internal surface of the thorax, and the dia-
phragmatic pleura, covering the anterior surface of the diaphragm,

136

ANATOMY OF THE RABBIT

the sac being completed medially by the mediastinal pleura. The
two last mentioned are 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 or scrotum. The general relation of the cavity to
the abdominal organs is indicated diagrammatically in Fig. 20.

Its lining membrane, the peritoneum,
is divisible into two principal por-
tions, 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 surfaces. 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
relation of the testis to its invest- of two platcs of peritoncum, cnclosins:

ments: ai, inguinal ring; ce, caput .

epididymidis; cr, cremaster muscle; bctWCCn them a thin layer of COU-
dd, ductus deferens; g, guber- , . i i • ••

naculum; mes, mesorchium; ptv and nCCtlVC tlSSUC, the lamma meseuteni
vtv, parietal and visceral layers of • r i • • r

the tunica vaginalis propria; pv, propria, lor the transmission oi ncrvcs,

cavity of the vaginal process; s, , , , , . , ■, ,

integument of the scrotum; sv. blOOO-VCSSelS, and lymph CaualS.
spermatic vessels; t, testis. a • j* ^ j • i- i j.

As indicated m an earlier chapter,
the relations of the abdominal portion of the digestive tube are
greatly modified by its elongation and displacement from a median
position. Thus, while in the embryo the common mesentery
(Fig. 51) is recognizable as a continuous median vertical fold, in
the adult it follows the convolutions of the digestive tube, and is
therefore considered as comprising corresponding parts. In many
cases the relations of these are greatly complicated by secondary

showing

THE SEROUS CAMTIES 137

adhesions. In the rabbit the mesoduodenum, mesentery of the
jejunum, and descending mesocolon will be recognized as parts in
which a more typical arrangement is retained. Moreover, in the an-
terior 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 region of the small and large intestines. The
peritoneum, passing from the dorsal wall, successively invests the
spleen, the stomach, and the liver, and passes over from the
last-named structure to the diaphragm and the ventral body-
wall through the coronary, triangular, and falciform ligaments. A
portion of the peritoneum passing between two organs, such as the
stomach and the liver, rather than connecting either to the body-
Avall, is termed an omentum or sometimes a ligament. Thus the
peritoneal attachments of the stomach are differentiated into the
mesogastrium (connecting the stom.ach with the diaphragm and
dorsal body-wall and divisible posteriorly into gastrosplenic and
phrenicospenic ligaments), the greater omentum (a broad fold
connected to the greater curvature and to the transverse meso-
colon), and the lesser omentum (passing between the lesser cur-
vature and the liver). Similarly, in the posterior part of the body
the peritoneum passes from the rectum to the urinary bladder,
enclosing also in the female the vagina. It is then continued to
the ventral body-wall as the middle umbilical fold. The falciform
ligament and the middle umbilical fold represent respectively the
anterior and the posterior ends of an originally continuous ventral
mesentery, these parts being retained when the rest degenerates
in early foetal life because through them run the umbilical blood-
vessels (pp. 115, 117).

In the male, as indicated in the accompanying diagram (Fig. 81 ) ,
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
— is continuous with the parietal peritoneum of the abdomen. It

138 ANATOMY OF THE RABBIT

thus represents a permanent vaginal process, an outpouching of
the peritoneum which in man becomes closed off. 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, i.e., in a position indicating the original
situation of the testis itself.

In the female the ovary is situated close to 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 56 mm., and may be used either in
connection with the general features of topography as outHned
above, or for the identification of various minor structures appear-
ing 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.

PLATE-FIGURES

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140

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141

DESIGNATIONS FOR PLATE II

Transverse sinus of dura

22.

Cervical flexure.

mater.

23.

Central canal of spinal

Dura mater.

cord.

Pallium of cerebral hemis-

24.

Hypophysis.

phere.

25.

Frontal bone.

Lateral ventricle.

26.

Nasal bone.

Olfactory bulb.

27.

Nasal fossa.

Olfactory tract.

28.

Mesethmoid cartilage.

. Divided olfactory nerve in

29.

Cartilage of vomeronasal

the cribiform plate.

organ.

Chorioid plexus of third

30.

Premaxilla.

ventricle.

31.

Nasopalatine duct and car-

Anterior commissure.

tilage.

Thalamus.

32.

Maxilla.

41. Oral portion of pharynx. _

42. Epiglottis and epiglottic

cartilage.

43. Thyreoid cartilage of

larynx.

44. Laryngeal cavity.

45. 45a. Cricoid cartilage.

46. Oesophagus.

47. Cricothyreoideus muscle.

48. Thyreoid gland.

49. Sternohyoideus muscle.

50. Genioglossus muscle.

51. Geniohyoideus muscle.

52. Mylohyoideus muscle.

10. Optic chiasma.

11. Tuber cinereum.

12. Mamillary body.

13. Superior colliculus.

14. Inferior colliculus.

15. Anterior medullary velum.

16. Cerebral peduncle; cephalic

flexure.

17. Isthmus rhombencephali.

18. Fourth ventricle.

19. Pons; pontine flexure.

20. Cerebellum.

21. Posterior medullary velum.

33. Hard palate (palatine and

maxilla).

34. Presphenoid.

35. Intersphenoidal synchon-

drosis.

36. Basisphenoid; hypophyseal

fossa.

37. Sphenooccipital synchon-

drosis.

38. Basioccipital.
38a. Supraoccipital.

39. Nasal portion of pharynx.

40. Soft palate.

142

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

143

DESIGNATIONS FOR PLATE III

Nasal bone.

14.

Levator alae nasi muscle.

15.

Nasal septum.

16.

Nasoturbinal cartilage.

17.

Maxilloturbinal (concha inferior).

18.

Nasal fossa.

• 19.

Nasolacrimal duct.

20.

Vomeronasal organ and cartilage.

21.

Premaxilla.

22.

10.

Small upper incisor.

Ts.

11.

Large upper incisor.

24.

12.

Nasopalatine ducts.

13.

Oral cavity.

25.

14. Tongue.

Vibrissae.

Caninus muscle.

Terminals of superior maxillary nerve.
18. Buccal glands.

Buccinator muscle.

Terminals of inferior alveolar nerve.

Ouadratus labii inferioris muscle.

Mandible.

Lower incisor.

Meckel's cartilage (primary mandibular
arch).

Mentalis muscle.

144

Ill

A TRANSVERSE SECTION OF THE ANTERIOR NASAL REGION

145

DESIGNATIONS

1. Superior sagittal sinus of dura mater.

2. Lateral ventricle.

3. Cerebral hemisphere.

4. Pia mater.

5. Frontal bone.

6. Cartilage of orbital wing.

7. Mesethmoid cartilage.^

8. Cupula posterior cartilage.

9. Obliquus superior muscle.

10. Ophthalmic vessels and nerves.

11. Levator palpebrae superioris muscle.

12. Rectus medialis muscle.

FOR PLATE IV

25. Nasal tract; choana.

26. Palatine bone.

27. Oral cavity.

28. Palatine nerve.

29. Sphenopalatine ganglion.

30. Infraorbital vein.

31. Internal maxillary artery.

32. Maxillary nerve.

33. Maxilla.

34. Zygomatic bone.

35. Submaxillary duct.

36. Buccinator muscle.

13. Retractor oculi muscle.

14. Rectus inferior muscle.

15. Sclera.

16. Retina and chorioidea.

17. Vitreous body.

18. Lens.

19. Posterior chamber of eye.

20. Anterior chamber.

21. Cornea.

22. Ciliary body and iris.

23. Upper eyelid.

24. Lower eyelid.

37. Masseter muscle.

38. Parotid duct.

39. Facial nerve.

40. External maxillary artery and vein

(anterior facial vein).

41. Platysma muscle.

42. Inferior labial artery and vein.

43. Mandible.

44. Genioglossus muscle.

45. Digastricus muscle.

46. Quadratus labii inferioris muscle.

47. Geniohyoideus muscle.

146

A TRANSVERSE SECTION OF THE ORBITAL REGION

147

DESIGNATIONS FOR PLATE V

1. Parietal bone.

2. Transverse sinus of dura mater.

3. Superior colliculus.

4. Cerebral aqueduct.

5. Isthmus rhombencephali.

6. Pons.

7. Trigeminal nerve.

8. Basilar artery.

9. Facial nerve.

10. Cartilaginous auditory capsule.

11. Cochlea.

21. Longus capitis.

22. Rectus capitis anterior.

23. Oral portion of pharynx.

24. Thyreohyoideus muscle.

25. Sternohyoideus muscle.

26. Greater cornu of hyoid.

27. Stylohyoideus major muscle.

28. Lingual artery.

"■J. Hypoglossal nerve.

3 ) Tendon of digastricus muscle.

3 1 l''.Kternal maxillary artery.

12. Basioccipital bone.

13. Tensor tympani muscle.

14. Tympanic cavity.

15. Malleus.

16. Tributaries of posterior facial vein,

17. Squamosal bone.

18. Cephalic portion of median vertebral vein.

19. Nasal portion of pharynx.

20. Origin of basioclavicularis and levator

scapulae major muscles.

32. Stylohyoideus minor.

33. Styloglossus.

34. Internal maxillary artery.

35. Tympanic bone.

36. Mandible.

37. Submaxillary gland.

38. Anterior facial vein.

39. Internal carotid artery.

148

A TRANSVERSE SECTION OF THE AUDITORY REGION

149

DESIGNATIONS FOR PLATE VI

1. Rhomboidcus minor.

2. Superior portion of trapezius.
2a. Levator scapulae minor.

3. Splenius.

4. Semispinalis capitis.

5. Rectus capitis posterior superficialis.

6. Obliquus capitis major.

7. Arch of epistropheus.

8. Ganglion of posterior root.

9. Longissimus cervicis.

19. Oesophagus.

20. Recurrent nerve.

21. Inferior thyreoid vein.

22. Trachea.

23. Thyreoid gland.

24. Cardiac branch of vagus (n. depressor).

25. Sympathetic trunk.

26. Vagus nerve.

27. Common carotid artery.

28. Internal jugular vein.

10. Longissimus capitis,

11. Vertebral artery and vein.

12. Longus atlantis.

13. Vertebral body.

14. Transverse process (anterior root).

15. Median vertebral vein.

16. Longus colli.

17. Longus capitis.

18. Fat-body.

29. Sternohyoideus muscle.

30. Sternothyreoideus muscle.

31. Sternomastoideus muscle.

32. Descending ramus of hypoglossal nerve.

33. External jugular vein.

34. Basioclavicularis muscle.

35. Levator scapulae major muscle.

36. Cleidomastoideus.

37. Platysma.

150

A TRANSVERSE SECTION OF THE ANTERIOR CERVICAL REGION

151

DESIGNATIONS FOR PLATE VII

Semispinalis dorsi.

24.

Costal pleura.

Longissimus dorsi.

25.

Bone ribs.

Iliocostalis.

26.

Costal cartilage.

Spinal cord.

27.

Sternum.

Ganglion of posterior root and intercostal

28.

Cutaneus maximus muscle.

nerve.

29.

Inferior portion of trapezius.

Tubercle of rib.

30.

Rhomboideus major.

Head of rib.

31.

Inferior angle of scapula.

Sympathetic trunks.

32.

Latissimus dorsi.

Azygos vein.

33.

Serratus posterior.

10.

Thoracic aorta.

34.

Intercostales externi and interni

11.

Oesophagvis.

34a

. Intercostalis internus.

12. 12a. Right and left vagi.

13. Lung.

14. Bronchi.

15. Branches of pulmonary artery.

16. Pulmonary veins.

17. Right atrium.

18. Tricuspid valve.

19. Right ventricle.

20. Left atrium.

21. Left ventricle.

22. Pericardial cavity.

23. Pulmonary pleura.

35. Thoracic portion of serratus anterior.

36. Obliquus externus abdominis.

37. Transversus thoracis.

38. Pectoralis major.

39. Rectus abdominis.

40. Long head of triceps.

Extensor antibrachii parvus.
Medial head of triceps.
Lateral head of triceps.
Distal extremity of humerus.
Proximal portion of radius.

152

VII
A TRANSVERSE SECTION OF THE THORAX

153

DESIGNATIONS FOR PLATE VIII

1. Spinal cord.

2. Vertebral canal.

3. Vertebral body.

4. Sacrospinalis muscle.

5. Quadratus lumborum.

6. Psoas major.

7. Psoas minor.

8. Sympathetic trunk.

18, 18a. Posterior and anterior lobules of left

lobe of liver.

19, 19a. Right lobe of liver.

20, Obliquus internus abdominis and trans-

versus abdominis.

21, Obliquus externus abdominis.

22, Rectus abdominis.

22a. Cutaneus maximus.

9. Abdominal aorta.

10. Inferior caval vein.

11. Descending mesocolon.

12. Ureter.

13. Renal pelvis.

14. Renal papilla.

15. Left kidney.

16. Parietal peritoneum.

17. Visceral peritoneum.

23. Middle umbilical fold.

24. Urinary bladder (canal of foetal allan-

tois).

25. Umbilical arteries.

26. Duodenum.

27. Pancreas and mesoduodenum.

28. Descending colon.

29. Parts of mesenterial small intestine.

30. Caecum.

154

VIII
A TRANSVERSE SECTION OF THE ABDOMEN

155

PART II
Osteology of the Rahhit

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. 88). 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. 180-195) 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 com-
prises 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 particular regions, moreover, resemble each other specially
closely in function and form, so that it is possible to classify them
into cervical (neck), thoracic (chest), lumbar (abdominal), sacral
(hip), and caudal (tail) groups.

A typical vertebra — for the characters of which any one of the
thoracic or lumbar series may be taken (Fig. 82, D-F)- — consists of

156

THE \ ERTEBRAL COLUMN

157

a somewhat massive basal portion, the vertebral body (corpus
vertebrae) or centrum, and of a dorsal, vertebral arch (arcus verte-
brae). The two portions enclose a large aperture, the vertebral

Fig. 82. Representative vertebrae: A, atlas, anterior surface; B, epistro-
pheus, lateral surface; C. fifth cervical vertebra, anterior surface; D, fourth
thoracic, 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., foramen transversarium; f.v., fora-
men 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.tn., tri-
angtilar 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.

foramen (foramen vertebrale). The successive foramina form an
almost complete tube, the vertebral canal (canalis vertebralis),
for the accommodation of the spinal cord.

158 ANATOMY OF THE RABBIT

The body of a vertebra is a cylindrical, or somewhat dorso-
ventrally flattened, mass of bone, which bears at either end an
articular surface for attachment to the body of the adjacent
vertebra. The articular surfaces are borne on thin plate-like
epiphyses, the epiphysial lines being evident even in older animals,
especially in the lumbar region. The arch of a vertebra is composed
of a lateral, vertical portion, the pedicle or root (radix arcus
vertebrae), at each side and a dorsal, transverse portion connecting
the tops of the pedicles, distinguished as the lamina. Each pedicle
is attached to one side of the dorsal surface of the body of the
vertebra so that the latter forms the floor of the vertebral foramen,
the pedicles constituting its sides and the lamina its roof. The
anterior and posterior margins of the pedicle are notched, each
notch or incisure being opposite that of the adjacent vertebra, so
that together they form a rounded aperture, the intervertebral
foramen (foramen intervertebrale), for the passage outward of a
spinal nerve.

The arch of the vertebra bears various projections or processes.
On either side is a horizontal plate of bone, the transverse process
(processus transversus) and, dorsally, there is a median projection,
the spinous process (processus spinosus), all three serving for the
attachment of ligaments which hold the vertebrae together and
for the attachment of the spinal musculature. Special surfaces
for articulation with the adjacent vertebrae are borne on low
articular processes (processus -articulares) 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 of the next vertebra, which are directed more or less
ventrad. A certain amount of movement is permitted by one
surface slipping across the other, the mechanism illustrating the
arthrodia, or gliding-joint.

Cervical Region

The cervical vertebrae (vertebrae cervicales) are seven in

number and serve mainly for the support of the head. As the

latter has to be freely movable in a variety of directions, the

articulations are such as to permit considerable flexibility in this

THE VERTEBRAL COLUMN 159

region and the first two cervical vertebrae (the atlas and the
epistropheus) are specially modified to provide for movements of
the skull. The posterior vertebrae (Fig. 82, C) are dorsoventrally
compressed, with low arches and short spinous processes. In the
seventh vertebra, however, the spinous process begins to be elon-
gated as in the succeeding thoracic vertebrae. In each vertebra
the transverse process is perforated by a costo-transverse foramen
(foramen transversarium), which serves for the passage of the
vertebral artery forward to the head. This aperture divides the
base of the transverse process into a dorsal, or posterior root (radix
posterior) and a ventral, or anterior root (radix anterior). The
development of these parts shows that the anterior root is really
a reduced rib which has become fused to the body and to the
transverse process and is comparable in its general relations to the
normal ribs of the thoracic vertebrae.

Atlas

The first vertebra is the atlas (Fig. 82, 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 possess-
ing special articular surfaces, and in having its transverse process
greatly flattened dorsoventrally. It consists of a ventral half-ring,
the anterior arch (arcus anterior), a dorsal half-ring, the posterior
arch (arcus posterior), and paired lateral masses (massaelaterales),
the last being thickened regions of the bone uniting the arches at
each side and forming the bases of the • transverse processes.
The anterior arch bears on its ventral side a small backwardly-
directed process, the anterior tubercle (tuberculum anterius),
named from its position in the human body, where the ventral
surface is anterior. A similar posterior tubercle (tuberculum
posterius) projects forward on the dorsal surface of the posterior
arch and 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 one of the
convex occipital condyles of the skull. Its posterior surface
bears en either side a smaller, somewhat triangular, inferior
articular facet (facies articularis inferior) for articulation with
the epistropheus. These surfaces take the place of the arch

160 ANATOMY OF THE RABBIT

articulations of ordinary vertebrae. As a result of the flattening
of the transverse process, the costo-transverse foramen is extended
into a tubular canal. The anterior aperture of this is connected by
a shallow groove, the sulcus arteriae vertebralis, with a foramen
perforating the posterior arch (foramen obliquum). Through this
latter foramen, represented in some mammals by separate alar and
intervertebral foramina, the vertebral artery and the first cervical
nerve enter the vertebral canal.

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

Epistropheus

The second vertebra is the epistropheus or axis (Fig. 82, 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 are three main points of attachment for the occipital muscu-
lature, which passes between the head and neck.

Consideration of the form and relations of the articular surfaces
will make it evident that the articulation between skull and atlas
provides chiefly for a nodding movement and that that between
atlas and epistropheus allows mainly a pivoting movement of the
head. Simple bending of the head to one side or the other is the
main movement permitted between the remaining cervical verte-
brae, in which the superior and inferior articular surfaces respec-
tively face mainly dorsad and ventrad.

THE VERTEBRAL COLUMN 161

Thoracic Region

The thoracic vertebrae (vertebrae thoracales) are twelve in
number. They form the backbone of the chest region and provide
attachment for certain muscles of the shoulder and muscles and
ligaments of the neck, but are distinguished chiefly by the possession
of articular pits for the attachment of ribs (Fig. 82, D). A typical
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, but in the remaining
vertebrae, a complete articulating surface is formed by two demi-
facets, one on the vertebra to which the rib belongs, the other on
the vertebra immediately in front. The point of articulation of a
rib with a transverse process is marked on the latter by an oval
facet, the costal pit of the transverse process (fovea costalis trans-
versalis). It is present only in the first ten of the thoracic vertebrae,
the other two having the costal articulations on their bodies only.

In all vertebrae of the thoracic series, the spinous processes are
well developed, mainly for attachment of the dorsal ligament of
the neck, which supports much of the weight of the head. They
increase in length to the third, and then become gradually shorter
but wider, so that their surfaces are, on the whole, slightly increased
in extent. The anterior ten are directed backward, the eleventh is
almost vertical (anticlinal vertebra), while the twelfth is directed
forward, like those of the succeeding lumbar vertebrae. The anti-
clinal vertebra is a centre about which the body bends in such
movements as galloping.

Dorsolaterally, the more posterior vertebrae of the region have
small mamillary processes corresponding with the more conspicuous
ones of the lumbar region.

Lumbar Region
The lumbar vertebrae (vertebrae lumbales) are seven in
number. As they not only support the longer part of the trunk
but also provide the origins for some of the proximal muscles of the
hind limb, they are large vertebrae, conspicuous for their extensive
surfaces and processes for muscular attachment (Fig. 82, E, F).
The transverse processes continue the general line of the ribs of

162

ANATOMY OF THE RABBIT

the thoracic region, being directed forward and downward, as well
as outward. The tip of each is formed by a thin triangular plate
(processus triangularis), which represents a vestigial rib fused with
the original process. At the posterior side of the base of each is a
short, flattened projection, the accessory process (processus
accessorius). The spinous process is notably broad and is directed
forward. The articular processes are rotated upward, so that their
surfaces are directed more nearly toward, or away from, the median

^^w>X-^M

pas. p-i

Fig. 83. The os 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., mamillary
process of first vertebra; pr., promontory; p.s., spinous processes.

plane, instead of to the dorsal or ventral surface, so that the move-
ment provided for is chiefly a bending of the body dorso-ventrally.
The anterior articular surfaces are borne on the bases of stout,
upwardly-directed mamillary processes (processus mamillares)
upon which the powerful muscles of the back originate (m. sacro-
spinalis, p. 339). The latter processes are most characteristic of
the lumbar vertebrae, but, as mentioned above, appear in the
posterior thoracic region as small elevations of the transverse

THE \TRTEBRAL COLUMN 163

processes. Each of the first three of the lumbar vertebrae bears
a median ventral projection, the anterior spinous process (processus
spinosus anterior), for the attachment of the lumbar portion of
the diaphragm.

Sacral Region

The sacral vertebrae (vertebrae sacrales) are four in number
and are modified for the attachment of the pelvic girdle. In con-
trast to the true vertebrae — those united by ligament and articular
surfaces — of the remaining portions of the vertebral column, they
are false vertebrae, being united in the young by synchondroses,
and in the adult coalesced to form a composite structure, the os
sacrum (Fig. 83). 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 where they articulate.
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
in the line of junction of the first and second vertebrae and minute
foramina behind the second and third vertebrae transmit the
dorsal rami of the first to third sacral nerves. 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, roughened tubercles.
In the middle line dorsally the vertebral arches are separated by
conspicuous apertures, the median sacral foramina.

164 ANATOMY OF THE RABBIT

Caudal Region
The caudal or coccygeal vertebrae (vertebrae caudales, s.
coccygeae) are sixteen in number. They are segments of small
size, increasing slightly to the third, and then gradually decreasing
to the end of the column. As only the small tail muscles are
attached to them, they lack any marked projections. The arches
are complete in the first seven. The transverse processes are
vestigial in all except the third. At the end of the column, the seg-
ments 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 (cartilage
costalis) (Fig. 84). From their attachment on the vertebral column
the bone-ribs are directed outward, downward, and backward.
The costal cartilages are directed for the most part inward, down-
ward, and forward. The first costal cartilage forms a pronounced
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. Those unattached 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 sur-
faces 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. Each rib is curved, not uniformly but so that its
greatest convexity, or angle, is at some point toward the dorsal
surface. Passing backward, in succeeding ribs the point of greatest
convexity changes from a mediodorsal to a laterodorsal 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 RIBS

165

cr.c.

O.C.

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
in the last four, the tubercle bears a sharp, dorsally-directed process
for muscular attachment. The slender portion of the rib inter-
vening betw^een 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 the ribs and the sternum with the
assistance of the thoracic vertebrae. It encloses a large space, the
thoracic cavity (cavum thoracis). The latter is conical in shape,
with the apex directed forward.
The dorsoventral diameter of the
cavity is considerably greater than
the transverse diameter. Apart
from the intercostal spaces, the
cavity is open at two points : anteri-
orly, the first thoracic vertebra, the
first rib, and the manubrium sterni
together enclose a small opening,
the superior thoracic aperture
(apertura thoracis superior) ; pos-
teriorly, the seventh and succeed-
ing ribs, together with the posterior
thoracic vertebrae and the xiphoid
process of the sternum, enclose a
much larger opening, the inferior
thoracic aperture (apertura tho-
racis inferior). In the natural con-
dition it is largely closed by the
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).

Fig. 84. The sternum and first rib,
ventral view: 1-7, the true ribs; 8, first
false rib; c.c, head of rib; cl.c, neck of
rib; cr., costal cartilage; cr.c, body of
rib; c.s., body of sternum; m.s., manu-
brium sterni; o.c, bone-rib; p.x., xiphoid
process; t.c, costal tubercle.

166 ANATOMY OF THE RABBIT

As a result of their articulations with the vertebral column,
and of the flexible nature of the costal cartilages, the ribs are
capable of being moved, or rotated, forward. The movement
brings about an increase of the extent of the thoracic cavity, and
is of importance in breathing.

THE STERNUM

The sternum (Fig. 84) consists of a linear series of six segments,
the sternebrae. The first segment is the manubrium sterni. It is
about twdce the length of the middle segments. It is somewhat
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 connected 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 with a broad, thin plate of cartilage attached to its posterior
end.

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 consti-
tuting 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 contain-
ing 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).

The Skull as a Whole

The skull (Figs. 85-88) is a composite structure, consisting of
a large number of elements, which, with the exception of the man-

THE SKULL AS A WHOLE 167

dible, are united by immovable articulations, so that they produce
the effect of a continuous mass. In this, the sutures between com-
ponent bones appear as fine lines of varying distinctness. The
mandible is a more or less independent structure, articulated with
the main body of the skull by a typical movable joint.

The posterior, cranial portion of the skull has a somewhat
conical shape, its apex being directed forward. It is separated from
the anterior, facial portion by a depression on either side of the
skull, the orbital cavity (orbita), which serves for the accommoda-
tion of the optic bulb. Unlike the nasal and auditory 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 an-
terior extremiity 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 surface, the
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 aspect of the occipital bone, with the ex-
ception, chiefly, of the basilar part 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 aper-
ture, the foramen magnum occipitale, for the passage of the central
nervous system from the cranial cavity into the vertebral canal.
On either side of this is a smooth, ridge-like projection, the oc-
cipital condyle (condylus occipitalis), for articulation with the
superior articular pits of the atlas. The single occipital bone of
the adult is formed by the fusion of a dorsal, median supraoccipital,
a ventral, median basioccipital, and paired, lateral erxoccipital bones,
the last of these bearing the main parts of the occipital condyles. At
a little distance lateral to the occipital condyle, the nuchal surface
is continued downward through the medium of a somewhat tri-
angular, pointed jugular or paramastoid process (processus
jugularis) of the exoccipital bone. This structure is separated

168

ANATOMY OF THE RABBIT

from the occipital 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 stylo-
glossus, stylohyoids major and minor, and the digastricus, the
suspensory ligament of the lesser cornu of the hyoid also being

p.s.a.

Fig. 85. 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,
mastoid 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; 1.1., lateral lamina of
pterygoid process; l.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.

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 con-
tinuation 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, some-
what tri-radiate tip of the projection, together with a thin ridge
extending ventrad from it, is the external occipital protuberance

THE SKULL AS A WHOLE 169

(protuberantia occipitalis externa), an important median point of
attachment for the occipital muscles and the dorsal ligament of
the neck.

The ventral wall of the cranial portion is the basal part (basi-
cranium) 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 seg-
ments. It is formed by a linear series of three bones, namely,
from back to front, the basilar portion of the occipital, the body
of the posterior sphenoid, and that of the anterior sphenoid (re-
spectively basioccipital, basisphenoid, and presphenoid bones).
Its extremely narrow, anterior portion forms the roof of a deep
groove which encloses the nasal portion of the pharynx. As view^ed
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
downward 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 w^hich serve for the attachment of the
external and internal pterygoid muscles of the mandible.

The lateral wall of the cranial portion of the skull forms anteri-
orly a large part of the boundary of the orbit. The components
which do this are 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 posterior
sphenoid, or alisphenoid, and two membrane elements, the frontal
bone of the roof of the skull and the squamosal bone. The latter
is distinguishable as the support of the posterior root of the zygo-
matic arch, which projects outward and then downward immedi-
ately behind the orbit. This root is formed by a zygomatic process
(processus zygomaticus) of the squamosal bone, the tip of which
process forms a vertical plate, 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

170

ANATOMY OF THE RABBIT

wall, there is a shallow, horizontal groove, lodging in the natural
condition the temporalis muscle of the mandible, and therefore
representing 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 zygomatic arch and through this foramen the external oph-
thalmic vein emerges from the orbit. The dorsal boundary of

the fossa is formed by a
pronounced ridge, the tem-
poral 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 tympanic bulla
(bulla tympani), formed by
the tympanic bone. It contains
the capacious tympanic cavity
(cavum tympani) and en-
closes the three small bones of
the middle ear, the auditory
ossicles (ossicula auditus),
the relations of which are
more fully dealt with below
(p. 188). The dorsal portion
of the tympanic bulla is con-
tinuous with a short bony
tube which opens at a short
distance dorsally by a large
oval aperture. This tube is
part of a more extensive
canal, the external acoustic
meatus (meatus acusticus
externus) which, in the natu-

FlG.
frontal :

Dorsal surface of the skull: F,
interparietal; L, lacrimal; M,
maxilla; MS, mastoid portion of petrosal
(petromastoid) ; N, nasal; P, parietal; SO,
supraoccipital (squamous portion of occipital) ;
SQ, squamosal; ZY, 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 p.s.p., anterior
and posterior supraorbital processes of frontal;
p.sc, subcutaneous process of lacrimal; p.z.,
zygomatic process of squamosal; p.z.m., zygo-
matic process of maxilla; s.f., frontal spine;
s.m., spina masseterica.

THE SKULL AS A WHOLE 17r

ral condition, leads downward through the base of the external
ear to the tympanic membrane. The tympanic bulla is not
exposed to the cranial cavity. It is applied closely to the external
surface of the periotic or petromastoid bone (os petrosum),
which forms the lateral boundary of the cranial cavity,
and contains the structures of the internal ear. The exter-
nal or mastoid portion of this bone appears in the space enclosed
between the tympanic bulla and the jugular process of the oc-
cipital bone, where it is readily distinguishable by its pitted ap-
pearance. Its ventral portion bears a slender projection, lying
parallel to the jugular process, the mastoid process (processus
mastoideus) which is the point of insertion of one of the neck
muscles (sternomastoid).

A series of foramina, lying partly within the orbit and extending
thence posteriorly along the boundary between the lateral and
ventral walls to the occiput, puts the cranial cavity in communica-
tion with the outside, and serves for the passage of nerves and vessels.
The first and largest of these openings, the optic foramen (foramen
opticum ) , occupies the middle portion of the orbit, and, in the natural
condition, transmits 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 on its course dorsad
into the orbit, while the remaining two, the middle and posterior
sphenoidal foramina, transmit dorsally-directed muscular branches
(massetericotemporal and pterygobuccinator) of the mandibular
nerve. On the medial side of the base of the medial lamina of the
pterygoid process there is a shallow longitudinal groove, represent-
ing the pterygoid canal (canalis pterygoidus) of the human skull.
This accommodates a nerve (the Vidian, from the facial and the
sympathetic). Immediately in front of the tympanic bulla, on

172 ANATOMY OF THE RABBIT

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, which is ventral and transmits
the internal carotid artery into the cranial cavity, the foramen
ovale of the typical mammalian skull, which is the antero-dorsal
part and serves to transmit the mandibular portion of the fifth
nerve. 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 (foramen caroticum internum). It is the anterior end
of a canal transmitting the internal carotid artery prior to the
entry of that vessel to the cranial cavity through the foramen
lacerum; the posterior end of this canal, 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 posteromedial surface of the
tympanic bulla. The second, lateral aperture 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 in communication with the
nasal portion of the pharynx. Associated with the mastoid process
is a small a perture, the stylomastoid foramen (foramen stylomastoi-
deum), the external opening of the facial canal, which serves for the
passage of the facial nerve. This foramen is named from the fact
that in the human skull it is bounded in front by the styloid process,
which is absent in the rabbit, and behind by the mastoid process.
A slit-like aperture , the jugular foramen (foramen j ugulare) , 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

THE SKULL AS A WHOLE

173

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, how-
ever, by an unpaired, lozenge-shaped element, the interparietal

ppm.

,js.m

I p.o.e. so
f.m.o.

Fig. 87. \'entral surface of the skull: .\S, alisphenoid (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); SQ, squamosal; T, tympanic; ZY, zygomatic.

ch, choana; c.hy., hypoglossal canal; c.o., 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 laminea of pterygoid process of posterior
sphenoid; s.m., spina masseterica.

bone, and by the shield-shaped projection, described above, which
is part of the occipital bone.

The facial portion of the skull is constituted largely by the invest-
ing bones of the upper jaw, palate, and mandible, but it encloses

174 ANATOMY OF THE RABBIT

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 the lateral surface of the maxilla, 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 connecting 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 portion 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.

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 over-
hangs 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. Ligaments convert these incisures into
foramina, the anterior for the passage from the orbit of the frontal
nerve and the frontal artery, the posterior for the emergence of the
lacrimal nerve and the lacrimal artery. The anterior wall of the
orbit is formed in part by a loosely articulated element, the lac-

THE SKULL AS A WHOLE 175

rimal bone, the lateral margin of which projects from the orbital rim
as a blunt subcutaneous process (processus subcutaneus) . This bone
is frequently missing from the dried skull unless care has been taken
to keep it in place. On the ventral side of the base of the subcuta-
neous process 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 sphenopalatine 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 (fora-
men 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
by soft tissue in the natural condition, the cavity is open at two
points. Posteriorly it communicates with the ventral surface of
the skull by the choanae, which, in the rabbit, are incompletely
divided. Anteriorly it opens to the outside by the piriform aperture
(apertura piriformis). The cavity is divided into right and left
portions, the nasal fossae.

In a divided skull may be examined the space enclosed by the
cranial portion, the cranial cavity (cavum cranii). Its form
depends on that 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, accommodates the enlarged cerebral
hemispheres. The posterior cranial fossa is a small division

176 ANATOMY OF THE RABBIT

extending backward to the foramen magnum and containing, in
the 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
inward from the dorsal and lateral walls of the skull. This fold
is usually found adhering to the internal surface of the skull,
unless the latter has been very thoroughly cleared, and in all
cases its position is indicated by a low ridge of bone. The marked
difference in diameter between the middle and posterior cranial
fossae is accounted for by the great thickness of the auditory por-
tion 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 prominent elevation, the sella turcica, which is
borne on the body of the posterior sphenoid. The sella turcica
contains a large central depression, the hypophyseal fossa (fossa
hypophyseos), which in the natural condition lodges the hypophysis
or pituitary body. The aperture of the fossa is partly enclosed
laterally by a pair of pointed posterior clinoid processes (processus
clinoidei posteriores), the tips of which are directed forward; and
a corresponding pair of anterior clinoid 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 oblongata. 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
excluded from the cranial cavity. The usually paired optic fora-
mina are here confluent, there being a single aperture for the
transmission of the optic nerves. The posterior ventral boundary
of this aperture contains a broad groove, the sulcus chiasmatis,
which in the intact animal lodges the optic chiasma.

In the anterior cranial fossa the floor is largely formed by a
perforated area, borne on the cribriform plate (lamina cribrosa)

THE SKULL AS A WHOLE

177

of the ethmoid bone, and serving for the transmission of the
divided olfactory nerves. Its median portion projects sHghtly 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-

co.

Fig. 88. The skull in vertical section: B.O., basioccipital (basilar portion
of occipital) ; BS, basisphenoid (body of posterior sphenoid) ; ET, ethmo-
turbinal; F, frontal; I, interparietal; 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 condyle; f.c.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., perpendicular plate of the ethmoid;
m.a.i., internal acoustic meatus; c, 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.

ward with the foramen lacerum by a broad groove, the sulcus
sphenoidalis, which, in the natural condition, lodges 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 cerebelli.
On the lateral wall of the posterior cranial fossa, and enclosed
by the compact, white, petrous portion of the periotic 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 provides accommodation for the parafloc-

178 ANATOMY OF THE RABBIT

cuius, a small stalked appendage of the cerebellum. Ventral to
this fossa, and also somewhat in front of it, a thin ledge of bone
extends over an oval opening, the internal 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 projecting ledge described
above, and resembling superficially the complete common tube
represented by the internal acoustic meatus of the human skull.

In the bisected skull it is seen that the division of the nasal
cavity into right and left fossae is effected chiefly through a median
vertical, cartilaginous plate, 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 vomeronasal organ, 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
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 and
posterior portion, the ethmoturbinal proper, composed of several

THE SKULL AS A WHOLE

179

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 ethmo-
turbinal contains the olfactory sense organs, while that covering
the maxilloturbinal is non-sensory and possesses the 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 distinguished
as sensory (olfactory) turbinals and respiratory turbinals.

The mandible (mandibula) is composed of two portions, united
anteriorly by the symphysis mandibulae. Each half comprises

d.i.

Fig. 89. Lateral surface of the left hand of the mandible: a.m., angle;
cm., 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.

a horizontal portion, forming in conjunction with that of the
opposite 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 dorsal margin the alveoli of the lower teeth. The mandibular
ramus forms a broad plate, the latera^l surface of which is occupied
in the intact animal 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
postero ventral portion through the expansion of the bone to form

180 ANATOMY OF THE RABBIT

the angle (angulus mandibiilae), 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.
Terminal portions of the same structures emerge on the lateral sur-
face through the mental foramen, a little in front of the cheek teeth.
The mandible is described in greater detail starting on page 195.

The Bones of the Skull
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 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. 25) 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 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 BONES OF THE SKULL 181

the sphenooccipital synchondrosis (synchondrosis sphenooccipi-
taHs). 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 petrotympanic
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 exocclpltal is directed dorsad from the basloccipital 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 exocclpltal, with the exception, however,
of its ventral tip, which belongs to the basloccipital. The portion
of the occipital bone connecting the basioccipital and exocclpltal
contains the jugular fossa and the apertures representing 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
basloccipital, in a horizontal plane. Its external surface bears the
nuchal crest and the external occipital protuberance. Paired
lateral wing-like expansions rest upon, and partly overlap, 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 by a
vacuity. The Internal surface bears-a median longitudinal groove,
lodging in the natural condition the 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.

182 ANATOMY OF THE RABBIT

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 an terius). In the rabbit, as in mammals
generalh', 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 naso-
pharynx. It is perforated in its middle by a round aperture, the
foramen cavernosum, w^hich 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 posterior
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 wath both the foramen caver-
nosum and the hypophyseal fossa.

The alisphenoid extends laterad at first, but soon changes its
direction so that its axis becomes 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 along with the basisphenoid, and
to a certain extent with the orbitosphenoid, encloses the superior
orbital fissure. The foramen lacerum is formed by the posterior
margin of its root in association with the petrotympanic.

The external surface of the alisphenoid is convex, both toward
the orbital 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 BONES OF THE SKULL 183

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 exca-
vated, like the adjacent portions of the alisphenoid. It bears a
shallow groove, representing a pterygoid canal (canalis ptery-
goideus), and is perforated by the three apertures described above
as the anterior, middle, and posterior sphenoidal foramina.

The Anterior Sphenoid

The anterior sphenoid (os 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 anteri-
orly with the perpendicular plate of the ethmoid and with the carti-
laginous nasal septum. In the divided skull, or better in one from
which the roof has been removed, the actual dorsal surface of the
bone is seen to be exposed to the cranial cavity only in its posterior
portion, where it is occupied by the sulcus 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 orbito-
sphenoids, 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 orbitosphenoid
proper, and an anterior portion, the ethmoidal process (processus

184 ANATOMY OF THE RABBIT

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 consider-
able portion of the anteroventral wall of the middle cranial fossa.

The ethmoidal process extends forward from the optic foramen.
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.

The Squamosal Bone

The temporal bone, or temporal complex, as recognized from
the human condition, is an association of three elements — squamo-
sal, tympanic, and periotic — which in the human skull are coalesced
to form a single bone. It is 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 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 petrotympanic 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 zygomatic

THE BOXES OF THE SKULL 185

arch is formed by a lateral and afterwards ventral projection, the
zygomatic process of the squamosal. The base of this process
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.

The Petrotympanic Bone

The petrotympanic bone (os petrotympanicum) is a somewhat
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 w4th 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, with the ex-
ception of a small ventral portion which is articulated with the
basioccipital bone, is exposed to the posterior cranial fossa.
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
cranial fossa. The dorsal portion of the bone corresponds in thick-
ness with the wing of the supraoccipital with which it is articulated.
The posterior surface is applied to the anterior surface of the ex-
occipital, 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

18G

ANATOMY OF THE RABBIT

m-so.

stylomastoid foramen lies between the latter and the external
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 antero ventral angle, visible only 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 be-
low in connection with the struc-
tures of the tympanic cavity.
The tympanic portion forms
the spherical, expanded, shell -
like, tympanic bulla, which con-
tains in its interior the tympanic
cavity, and is continuous dorsally
with the bony enclosure of the
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 basioccipital, but the swollen portion
is separated from the latter by a broad groove terminating

7n-ae

Fig. 90. Petrotympanic portion of the
auditory complex of the left side X 3. The
lateral portions of the tympanic bulla and
external acoustic meatus have been removed,
exposing the structures of the tympanic
cavity. MS, mastoid portion; P, petrous
portion; T, tympanic portion (bulla tym-
pani); cm., mastoid cells; c.t., tympanic
cavity; f.c, cochlear fenestra; in., incus;
m.a.e., external acoustic meatus; m.m.,
manubrium of the malleus; m.so., supra-
occipital margin of petromastoid; p.m.,
mastoid process; st., stapes; t.a., aperture of
auditory tube.

THE BOXES OF THE SKULL 187

posteriorly in the jugular fossa and the jugular foramen. Immedi-
ately 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 the tympanic portion. At the anterior end of the groove, commu-
nicating with the foramen 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
relations of these apertures are seen to best advantage when the
petrotympanic is disarticulated from the associated posterior
sphenoid bone. The auditory tube is then seen to lead directly
into the tympanic cavity. A fine bristle may be passed through
the carotid canal from one foramen to the other.

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. 90. The
tympanum or middle ear is enclosed by the tympanic and petro-
mastoid portions of the temporal complex. The attached margin
of the tympanic bulla encloses a roughly triangular area, into the
ventral part of which the petrous portion of the petromastoid
projects as a smooth, white, convex ridge, the promontory (promon-
torium). 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 con-
dition it is closed by a thin membrane which separates the tympanic
cavity from the perilymphatic space containing the membranous
labyrinth. The second aperture, the vestibular fenestra (fenestra

188 ANATOMY OF THE RABBIT

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 opening
to the internal ear as represented by the vestibular fenestra. They
occupy the dorsal angle of the triangular area already described
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 membrane.
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 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 as it is in man.

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 sagittal, coronal,
squamosal, and lambdoidal. The posterolateral angle of the bone
is produced ventrally into a long, curved squamous process (pro-
cessus squamosus), which lies in the angle formed by the tentorium

THE BOXES OF THE SKULL 189

cerebelli and the lateral wall of the middle cranial fossa. It is not
exposed to the external surface of the skull.

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 their
homologues in the human skull, the two bones are separate through-
out 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 part 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 wdth the nasal and 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.

The Ethmoid Bone
The ethmoid bone (os ethmoidale), the chief representative of
the embryonic cartilaginous nasal capsule, is a delicate, greatly

190 ANATOMY OF THE RABBIT

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 paired 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, continu-
ous 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
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
marsupium 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, back-
wardly-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 or
spaces contained by them. They are roughly comparable to the

THE BONES OF THE SKULL 191

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 papy-
racea. In the rabbit, however, the space usually occupied by the
lamina papyracea is partly filled by the lacrimal bone, the eth-
moidal process of the orbitosphenoid, and the sphenoorbital process
of the maxilla.

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. 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.

The Maxilla

The maxilla, the largest element of the facial region, is associ-
ated 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 zygo-
matic 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

192 ANATOMY OF THE RABBIT

the palatine bridge it is applied to the lateral surface of the palatine
bone, and is projected into the orbit as a broad ridge enclosing 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 the 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
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, is
fused with the anterior end of the zygomatic bone. Its ventral
angle bears a prominent masseteric spine for the attachment 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.

THE BONES OF THE SKULL 193

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 boundary of the
incisive foramen. Its dorsal surface forms part of the boundary of
the piriform aperture, the remaining portion of this being formed
b}^ the nasal bone. The palatine process extends backward on the
medial side of the bone, closely appHed 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 conjunc-
tion with that of the corresponding process of the other side, bears
a broad palatine groove (sulcus palatlnus), 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.

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 trans-
verse 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.

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

194 ANATOMY OF THE RABBIT

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.

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 posterior portion
bears a shelf-like projection, the ala vomeris, which assists in the
support of the ethmoturbinal.

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. It consists of a very thin basal portion, somewhat
rectangular on its orbital surface, and of two processes, namely, the
subcutaneous process and the hamulus lacrimalis. The sub-
cutaneous process is the prominent, somewhat thickened, hook-like
projection extending laterad beyond the orbital rim. The hamulus
lacrimalis is a 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 and the lateral end of which separates the hamulus below
from the subcutaneous process above.

The Palatine Bone
The palatine bone (os palatinum) forms the posterior 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 perpendicular
portion (pars perpendicularis) is the vertical plate extending

THE BOXES OF THE SKULL 195

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 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 the 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 (pro-
cessus 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 pterygo-
palatine canal and the sphenopalatine foramen.

The Mandible
The mandible (mandibula) or lower jaw comprises the tw^o
dentary bones (ossa dentalia), which, in the rabbit, as in mammals
generally, are united by a fibrous or fibrocartilaginous connection
(symphysis mandibulae); not coalesced, as in the human skull, to
form a continuous structure. 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

196

ANATOMY OF THE RABBIT

chiefly for the accommodation of the lips, which in this region en-
croach medially on the oral cavity. The medial surface of the
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, and through which the inferior
alveolar vein emerges, lies on this surface at the junction of the
horizontal portion with the ramus. The corresponding 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 an anastomotic vein connecting the inferior

alveolar and the deep facial veins
(p. 298).

The mandibular ramus forms,
in general, an obtuse angle with
the horizontal portion. As in other
herbivores, the ventral part, dis-
tinguished as the angle, is greatly
increased in size at the expense of
the condyloid process and to a still
greater extent of the coronoid pro-
cess, the latter being vestigial. In
addition to a low pterygoid tuber-
osity (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 depres-

s-m

m ct

Fig. 91. Lateral surface of the hyoid
and larynx: c.a., arytenoid cartilage;
c.c, cricoid cartilage; c.i., inferior cornu
of thyreoid cartilage; cm., lesser cornu
of hyoid; c.mj., greater cornu of hyoid;
C.S., superior cornu of thyreoid 'cartilage;
ct., left plate of the thyreoid cartilage;
e, epiglottic cartilage; f.t.s., thyreoid
foramen; l.h., lateral hyothyreoid liga-
ment; l.h.m., median hyothyreoid liga-
ment; m.ct., cricothyreoideus muscle;
o.h., hyoid bone; s.m., stylohyoideus
minor muscle; s.mj., stylohyoideus major
muscle; tr., cartilaginous tracheal rings.

THE HYOID APPARATUS 197

sion for the insertion of the pterygoideus interniis 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 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.

The Hyoid Apparatus
The hyoid bone (os hyoideum) (Fig. 91) is a stout, somewhat
wedge-shaped bone lying in front of the larynx and between the
angles of the mandible. Its ventral portion is connected with the
thyreoid cartilage of the larynx by the median hyothyreoid liga-
ment. With its lateral portion are articulated two independent
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 muscle tendon contains, near the jugular process
a small ossification representing a detached styloid process. The

198 ANATOMY OF THE RABBIT

greater cornu (cornu majus) is a larger element extending obliquely
dorsad, and similarly suspended from the jugular process by the
stylohyoideus major muscle. The connection of the lesser cornu
with the styloid process through the stylohyoideus minor replaces
the stylohyoid ligament of the human skull and the chain of ele-
ments commonly occurring in mammals and other vertebrates in
this region. In most mammals the term "lesser", as applied to it,
is inappropriate. The lesser cornu, the styloid process, and their
connections, together with the hyoid bone itself, indicate 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 connected with the superior
cornu of the thyreoid cartilage of the larynx by the lateral hyo-
thyreoid ligament. This cornu 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. This arrange-
ment may be regarded as providing a shock-absorbing mechanism.

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 positions occupied by the principal parts in the natural
state are shown in Fig. 23.

The Scapula
The scapula (Fig. 92) is a somewhat triangular plate of bone
lying, in the natural position, on the lateral surface of the anterior

THE BONES OF THE ANTERIOR LIMB

199

part of the thorax, with its apex directed downward and forward.
In the rabbit, as in quadruped 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 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 arm-
pit. The corresponding angles are called medial (superior —
between the superior and vertebral borders), inferior, and lateral

/n.v.

Fig. 92. Lateral surface of the left scapula: a.,_ acromion; a.i., a.l., and
a.m., inferior, lateral, and medial angles; e.g., glenoid cavity; c.s., neck of the
scapula; f.s. and f.i., supraspinous and infraspinous fossae; m., meta-
cromion; m.a., m.s., and m.v., axillary, superior, and vertebral borders; p.c,
coracoid process; s.s., scapular spine.

(glenoid — between the superior and axillary borders). 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 ventrally in a free projection, the acromion.
The posterior margin of the acromion bears a backwardly-directed
process, the metacromion (processus hamatus). Through the pre-
sence 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, on the other hand, presents a single large shallow depress-

200 ANATOMY OF THE RABBIT

ion, the subscapular fossa (fossa subscapularls), 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 slender portion — the so-called neck of the scapula (collum
scapulae) — connecting it with the body of the bone. It bears a
concave depression, the glenoid cavity (cavitas glenoidalis), for
articulation 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 projection, the coracoid process (processus cora-
coideus). The free portion of the latter forms a blunt, hook-like
projection mediad. It represents the separate coracoid bone of lower
terrestrial vertebrates (p. 61).

In the fresh condition, the vertebral border of the scapula
bears a plate of cartilage, the suprascapula, which is about three-
quarters of a centimetre wide near the inferior angle and tapers to
a point towards the medial angle.

The Clavicle

The clavicle (clavicula) is imperfectly developed in the rabbit,
consisting of a slender, curved rod of bone, tipped by cartilage,
which lies in the interspace between the manubrium sterni and
the head of the humerus. It occupies only a portion of this inter-
space, being attached medially by the sternoclavicular ligament
and laterally by an acromioclavicular and a very slender coraco-
clavicular ligament, all three of which are considerably elongated.
The sternoclavicular ligament is nearly two millimetres in diameter
and fully twenty millimetres long, the acromioclavicular about
two millimetres by thirty-five millimetres, and the coracoclavicular
about twenty-five millimetres long. According to recent studies,
the so-called cleidohumeral ligament to which certain muscles are
attached (pp. 258 and 264) is merely a persistently fibrous inter-
section between tendons attached to the clavicle before reduction
of the latter occurred but is not a vestige of the clavicle or its
true ligaments.

THE BONES OF THE ANTERIOR LIMB

201

v5 i-

tmi

The Humerus
The humerus (Fig. 93) is typical
of the long bones of the proximal
and middle segments of the fore and
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 articu-
lation. 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 nom-
inally a ball-and-socket joint, or enar-
throsis, 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 inter-
tubercularis), from a much larger
lateral elevation, the lateral tuberosity or greater tubercle (tuber-
culum 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. These tuberosities are
for muscle attachment, mainly for the insertion of muscles moving
the shoulder-joint.

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

e.m.-

Fig. 93. Anterior surface of the
left humerus: c, capitulum; c.h.,
head of humerus; e.l. and e.m.,
lateral and medial epicondyles; f.r.,
radial fossa; s.h., deltoid tuberosity;
s.i., intertubercular groove; t.h.,
trochlea humeri; t.mi. and t.mj.,
lesser and greater tubercles.

202 ANATOMY OF THE RABBIT

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 respectively 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 olecranon process of the
ulna.

Between the radial and olecranon fossae the bone is reduced to
a very thin lamina, which is sometimes pierced by an opening of
very variable size, the supratrochlear foramen.^

The Radius and Ulna

The radius (Fig. 94) 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 crosses 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
radii) is immovably articulated with the ulna. It bears an ex-
tensive articular surface, meeting both trochlea and capitulum of
the humerus, 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. 94) is a somewhat S-shaped bone, the shaft of
which is vertically flattened, so that it possesses two main surfaces,

'In an examination of 255 specimens, this was found in 173, or 68 per cent.

THE BOx\ES OF THE ANTERIOR LIMB

203

Fig. 94. Skeleton of the forearm and hand from the dorsal surface; R.
radius; U, ulna; C, carpus; M, metacarpus; P, phalanages; I-V, metacarpal
bones c, central bone; cp., capitate; c.r., head of radius; f.a.c, carpal
articular surface of radius; h., hamate bone; i.s., semilunar notch of the ulna;
J., luriate bone; mi., lesser multangular; mj., greater multangular; n.,
navicular; ol , olecranon; p.s., styloid process of the ulna; tr., triquetral bone;
u., ungual phalanges.

204 ANATOMY OF THE RABBIT

respectively anterodorsal and posteroventral. The former, in con-
junction 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 semilunaris),
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 bone forms the large pro-
jecting portion of the elbow, the olecranon, which is a strong
process for the insertion of the extensor muscles (anconaei) acting
on the forearm and provides leverage for their action. 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, i.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 prevented from changing
their positions with respect to each other; in other words, the
radius is unable to rotate on an axis formed by the ulna as it does
in man, the forefoot being fixed in a position comparable to that
of pronation in the human hand (cf. p. 70).

The Carpus

The carpus (Fig. 94) 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

THE BONES OF THE ANTERIOR LIMB 205

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 pisi-
form bones. The navicular and lunate are the radiale and inter-
medium of the primary terrestrial limb skeleton (p. 03) and are articu-
lated with the distal extremity of the radius. The triquetral is the
ulnare of the basic pattern and is articulated 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. It is really a sesamoid bone (p. 206)
added to the primary three proximal carpals. The distal row
contains five elements, namely, the greater multangular, lesser
multangular, 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 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. It represents the two lateral
members of the original distal row of carpals (Fig. 36) fused together.

The Metacarpus and Phalanges

The metacarpus (Fig. 94) 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.

206 ANATOMY OF THE RABBIT

Sesamoid Bones

Accessory elements, sesamoid bones (ossa sesamoidea), de-
veloped in the ligaments or tendons 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 articu-
lations 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 LLMB

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. Along with the sacrum,
which is interposed between them dorsally, they constitute the
pelvis. This rigid framework, involving part of the vertebral
column, provides a strong basis for the powerful thrust of the
hind limbs in locomotion and contrasts with the elastic attachment
of the fore limbs to the trunk. The distal portion of the posterior
limb, like that of the anterior, comprises the supports of the free ex-
tremity and 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, however,
are extensively coalesced; and the distal segment twenty-three
elements, of which six form the tarsus, five the metatarsus, and
twelve the phalanges.

The Coxal Bone

The coxal bone (os coxae) (Fig. 95) is a somewhat triradiate
structure with one anterior ray and two posterior ones, the latter
united so that they enclose a large aperture the obturator foramen
(foramen obturatum). The bone is firmly articulated with the
sacrum dorsally and is united ventrally with its fellow of the
opposite side by a thin strip of cartilage containing a small amount
of fibrous material. The latter connection is the pubic symphysis
(symphysis pubis), better ternied in the rabbit the pelvic
symphysis, since it is somewhat more extensive than the correspond-

THE BONES OF THE POSTERIOR LIMB

207

ing articulation of the human pelvis and involves the ischium as
Avell as the pubis.

In the young animal each half of the pelvis consists of three
elements, namely, the ilium, ischium, and pubis. They form the

Fig. 95. Lateral surface of the left coxal bone: IL, ilium; IC, ischium;
P, pubis, a., acetabulum; a.i., iliac wing; c.i., body of ilium; c.is., body of
ischium; c.p., body of pubis; cr., iliac crest; e.i., iliopectineal eminence; f.a.,
acetabular fossa; f.o., obturator foramen; i.a., acetabular notch; i.mi., lesser
sciatic notch; i.mj., greater sciatic notch; l.i., iliopectineal line; p.l., lateral
process of ischial tuberosity; r.i.i., inferior ramus of ischium; r.i.p., inferior
ramus of pubis; r.s.i., superior ramus of ischium; r.s.p., superior ramus of
pubis; s.a.i., inferior anterior spine of the ilium; s.a.s., superior anterior
spine; s.i., ischial spine; s.p., symphysis pubis; s.p.i., inferior posterior spine;
t.i., ischial tuberosity; t.p., pubic tubercle.

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

208 ANATOMY OF THE RABBIT

of the acetabulum, the pubis being excluded through the develop-
ment 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 transverse section, its surface being divided into three areas,
which are respectively medial, or sacral, ventrolateral, or iliac,
and dorsolateral, or gluteal. The corresponding borders are re-
spectively 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), which is the origin of an
extensor muscle of the leg (second portion of the rectus femoris).
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) over which pass the sciatic nerve and
artery as well as the piriformis muscle. The pubic border presents
on its medial side a faint, ridge-like elevation, the iliopectineal
line (linea iliopectinea), which connects 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 sur-
face (facies auricularis), for connection with the sacrum. The
dorsal margin is thin and straight. Posteriorly, where it is asso-
ciated 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
anterodorsal margin of the bone, the latter forming the projecting

THE BONES OF THE POSTERIOR LIMB 209

end of the wing, which is distinguished as the iliac crest (crista
iUaca). 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 anterior elongation of the ilium is an adaptation to the powerful
anterior thrust of the hind limb in progression.

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 ace-
tabuli), 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), a point of muscle origin (p. 275). 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). Through both notches pass muscles which move the
femur (p. 275).

The superior or acetabular ramus of the ischium is the con-
tinuation backAvard 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 sym-
physeal ramus is that part of the ischium which extends from

210 ANATOMY OF THE RABBIT

the superior ramus downward and forward between the obturator
foramen and the symphysis to meet the corresponding ramus of
the pubis.

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 tubercle (tuber-
culum pubicum), and laterally a more extensive elevation, the
iliopectineal eminence (eminentia iliopectinea). The latter is more
conspicuous in older specimens, where it is easily recognizable by
its jagged outline. Its lateral margin is continuous with the
iliopectineal line.

The Femur

The femur (Fig. 96) is a somewhat S-shaped bone, the body
being very slightly curved, with the distal extremity bent down-
ward, forming the articulation of the knee, while the proximal
one, with its various processes, turns slightly upward in association
with the pelvis. In considering the general form, it will be re-
membered that in the natural sitting posture of the rabbit, the
position of the femur is approximately horizontal, the convex
surface of the 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 ex-
tremity 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 process 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 projec-
tion, while the other, the third trochanter (trochanter tertius),
is the smaller lateral crest. On the medial side of the bone, im-

THE BONES OF THE POSTERIOR LIMB

211

mediately distal to the head, there is a triangular elevation, the les-
ser or second trochanter (trochanter minor s. secundus) . Pos-
teriorly, these projections form a smooth surface for muscle attach-
ment, except, however, at the base of the trochanter major, where

Fig. 96. Anterior surface of the left femur: c.I. and cm., lateral and
medial condyles; cl.f., neck of femur; cp.f., articular portion (head); e.l.
and e.m., lateral and medial epicondyles; f.p., patellar surface; t.mi.,
trochanter minor; t.mj., trochanter major, including t.p. and t.t., the first
and third trochanters.

the surface of the bone presents a deep, though narrow, depression,
the trochanteric fossa ( fossa trocharbterica).

The distal extremity bears an extensive surface for articulation
with the tibia. It is divided into two portions, known as the me-
dial and lateral condyles, through the presence of a deep excava-
tion, the intercondyloid fossa (fossa intercondyloidea). Imme-

212

ANATOMY OF THE RABBIT

diately 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 epi-
condyles, for muscular attachment.

The Tibia and Fibula
The tibia (Fig. 97) is the larger of
the two bones of the leg, lying on the
medial side of the fibula, and fused with
the latter, in the rabbit, for more than
one-half of its length. Its proximal ex-
tremity is triangular in section, the main
surfaces being respectively anterolateral,
anteromedial, and posterior. The ante-
rior border is formed by a stout, ridge-
like elevation, the tuberosity of the
tibia (tuberositas tibiae), which in the
natural condition serves for the in-
sertion of the quadriceps femoris, the
extensor tendon by which this group
of muscles is inserted being carried over
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, partly divided SV'afd 'S/T^t/raf fid' mSli
hillock, the intercondyloid eminence
(eminentia intercondyloidea), and also
posteriorly by a depression of the articu-
lar border, the posterior intercondyloid fossa

condyles; f.a.s., proximal artic-
ular surface for the femur; m.l.
and m.m., lateral and medial
malleoli; t.t., tuberosity of tibia.

A corresponding

THE BONES OF THE POSTERIOR LIMB 213

anterior intercondyloid fossa lies in front of the intercondyloid
eminence, but is poorly differentiated.

The fibula (Fig. 97) 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 interosseous ligament of the leg. Its proximal
extremity is connected with the lateral condyle of the tibia by an
elongated epiphysis, the latter, like those of the distal ends of
the radius and ulna, being distinguishable even in older animals.

The combined distal extremities of the tibia and fibula bear
a roughly rectangular articular surface for the tarsus. The tibial
portion of this surface presents two grooves, separated by a ridge,
for articulation with the trochlea tali. On its medial side is a
small projection, the medial malleolus (malleolus medialis) round
which, in the intact animal, passes the insertion tendon of the
extensor hallucis longus muscle.

The fibular portion of the distal extremity projects distad a
little further than the tibial portion, forming the lateral malleolus,
the end of which presents a broad, shallow, transverse depression
for the reception of the convex articular surface of the calcaneus,
while its medial surface articulates with the lateral side of the
trochlea tali. Immediately above it, on the lateral side of the bone,
a prominent projection forms the anterior and lateral boundaries
of a groove which in the natural condition lodges the insertion
tendons of the peroneal muscles.

The Tarsus
The tarsus (Fig. 98) comprises six elements, the tarsal or ankle-
bones (ossa tarsi), which, like the corresponding 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 intermediate position. The proximal row contains two
elements, the talus and calcaneus.^ The talus is medial and also
slightly dorsal in position. It represents the tibial tarsal, or tibiale,
fused with the intermedium (Fig. 36, p. 63) of the primary limb
skeleton. Its proximal end, described as the body (corpus tali),
bears an extensive pulley-like surface, the trochlea tali, for

214

ANATOMY OF THE RABBIT

articulation with the tibia, and forming with the latter 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 ventrolateral border is extensively articulated with the
calcaneus. The latter represents the
ulnar tarsal, or ulnare, of the primary
limb skeleton and 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 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 articu-
lates wdth the cuboid and also with the
navicular.

The intermediate element, the na-
vicular bone comes between proximal
and distal tarsals and is the central bone
of the primary pattern. Thus it does
not correspond with the navicular bone
of the wrist, which is the radiale (p. 205).
It 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 metatarsal bone and
the second and third cuneiform bones, on
the other. It represents the central bone
of the primitive tarsus (Fig. 36) and 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

Fig. 98. The bones of the left
foot, viewed from the dorsal sur-
face: T, tarsus; M, metatarsus;
P, phalanges. II-V, the four
metatarsal bones. cb., cuboid;
cl., calcaneus; c.s., second cunei-
form; c.t., third cuneiform; f.a.,
articular surface for fibular side
of the tibiofibula; n, navicular;
t, talus; t.c., tuber calcanei; t.t.,
trochlea tali.

THE BOXES OF THE POSTERIOR LIMB 215

and tarsus, like other parts of the Hmb, 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 than the cuboid and
articulate 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 formed by fusion of the fourth and fifth distal tarsals and
articulating, 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 (tuber-
ositas OSS. cuboidei), in front of which is a groove for the accom-
modation of the peculiar insertion tendon of the peronaeus primus
muscle.

The Metatarsus and Phalanges

The metatarsus (Fig. 98) comprises five elements, of which
four are fully developed and greatly exceed in size the corresponding
bones of the 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
distinguished 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.

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

216 ANATOMY OF THE RABBIT

the tuberosity of the tibia. On the posterior surface there are
three sesamoid bones, of which one (in the medial head of the
gastrocnemius) lies in association with the medial condyle of the
femur, while the remaining two are associated respectively with
the lateral condyle of the femur (imbedded in the lateral head of
the gastrocnemius and the plantaris) and that of the tibia (con-
tained in the popliteus muscle). 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 Rabhit

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 by embalming followed by arterial
injection (see appendix). These points may be mentioned as ex-
plaining 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 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 ex-
tensive and accurate as that he should employ only good instru-
ments or maintain the proper sequence in dissection.

The method of regional dissection, as here developed, lends
itself particularly to the observation of inter-relations between the
different organ systems and should help the student to keep in
mind the essential dependence of each of these upon the others.

217

218 ANATOMY OF THE RABBIT

At the same time, it requires that a synthesis be made of the
observations in various places in order that a conception of each
system as a whole as well as of the total organism be obtained.

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 head (caput), neck (collum),
trunk (truncus), tail (cauda), and 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 external opening of the mouth is relatively narrow,
having been reduced during development by the growth
forward of tissue from each side to form the cheek.

(d) The nose (nasus) and its ovoid external apertures, the
nostrils (nares anteriores), which connect with the upper
end of the groove dividing the upper lip into right and left
halves and have the skin at their inner margins slightly
folded.

(e) The eye (oculus) and its coverings, the eyelids, including
the upper eyelid (palpebra superior), the lower eyelid
(palpebra inferior), and the third eyelid or nictitating
membrane (palpebra tertia). The third eyelid occupies
the anterior angle of the eye, and is comparable to the
conjunctival fold of the human eye. It is stiffened by a
thin plate of flexible cartilage covered with a layer of
glandular tissue and moulded to the exact curvature of
the surface of the eyeball.

The eyes of the rabbit look more nearly straight laterally than do
those of most mammals, the angle between the visual axes of the two
eyes after death having been found to be over 141°. The fields of vision

EXTERNAL FEATURES 219

of the two eyes at rest overlap in front only about 27° or less. On the
other hand, they also overlap behind. The rabbit is one of the few
mammals in which vision is not solely binocular,

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

{g) Points on the head skeleton, to be identified by feeHng
through the skin; zygomatic arch, supraorbital process,
external occipital protuberance, angle of the mandible,
symphysis of the mandible, and the hyoid l^ne.

3. 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.

(c) The anal aperture (anus), and on either side of it the
inguinal spaces, deep hairless depressions in 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 a small, rod-like structure con-
tained in its ventral w^all. The mammary nipples (papillae
mammarum), eight (to ten) in number, on the ventral
surface of the breast and abdomen.

(/) 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

220 ANATOMY OF THE RABBIT

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
(man us).

(b) The position of the elbow (cubitus) in comparison with
the knee (cf. p. 70).

(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.

II. THE ABDOMINAL WALL

1. Place the animal on its back. Make a median incision of the
skin of the ventral surface extending from the pubic symphysis
to the manubrium sterni, being careful not to cut through more
than the skin itself. Make two transverse incisions through the
skin on the left side, the first passing just behind the arm, the
second just in front of the thigh, both extending round to the
dorsal surface. Work the flaps loose from the surface, using
the handle of the scalpel, until the side of the trunk is well
exposed, but leave them attached to the body. On the right
side of the body it is sufficient to clear the middle line. Identify
the structures as follows:
On the inner surface of the skin:
(a) The thick compact connective tissue forming the corium.

THE ABDOMINAL WALL 221

(b) the hair-follicles imbedded in it, appearing as dots.

(c) The loose subcutaneous tissue (tela subcutanea) by which
the skin is attached. In some animals, large amounts of fat
are at times deposited in this layer. Some fat may be
present in the rabbit but the quantity is not usually great.

(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 tendinous line extending from
the pubic symphysis to the xiphoid process of the sternum.

(/) 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 portion originating on the last-named appears on the
medial surface of the humerus. Insertion: the skin of the
trunk, mainly dorsolaterally but some fibres reaching the
mid-dorsal line so that the muscles of the two sides are
continuous across the back. The fibres are directed
upward (in the natural position of the animal) and back-
ward. The muscle 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 epi-
gastric, a branch of the femoral (p. 279). Passing into the ventral
portion of the cutaneus maximus muscle, it anastomoses forward with
the external thoracic artery, a branch of the lateral thoracic. The
corresponding veins are usually conspicuous in the female, since the
vessels supply the mammary 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. Separate the cutaneus maximus from the surface of the muscle

222 ANATOMY OF THE RABBIT

beneath. Identify the following points of attachment of the
abdominal muscles proper:

(a) The linea alba.

{b) The xiphoid process of the sternum, the ribs, and the
costal arch.

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

(d) 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 xiphoid process, the posterior
ten ribs by separate slips, and the lumbodorsal fascia. Insertion :
the linea alba and the inguinal ligament. The fleshy portion,
or muscle proper, covers the abdomen lateral to the tendinous
portion, or aponeurosis, which appears as a longitudinal,
whitish band attaching the muscle to its insertion. The fleshy
portion and the aponeurosis meet along a slightly curved line
a short distance lateral to the linea alba. The fibres are directed
from an anterior dorsal origin downward and backward, the
more dorsal ones almost directly backward, and the fibres of the
aponeurosis continue the line of the muscular fibres attached
to them. 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 external oblique muscle, and then separate it fully
from the next, which may be distinguished by the markedly
different 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. This separation cannot

THE ABDOMINAL WALL 223

satisfactorily be carried quite to the linea alba as the medial
part of the aponeurosis is fused with that of the internal oblique
beneath it.

Examine the following muscles, proceeding in a similar manner:

(a) The internal oblique muscle (m. obliquus internus abdomi-
nis). 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
dorsal and ventral leaves, containing between them the
thin rectus abdominis muscle. Along the line of cleavage,
which is known as the linea semilunaris, there is often
a small deposit of fat.

(b) The rectus abdominis muscle. Origin: lateral border of
the sternum, including the xiphoid process; also the
ventral surfaces of the first to seventh costal cartilages.
Insertion : at the anterior end of the pubic symphysis. It
is a thin, strap-like muscle, enclosed between two sheets of
the aponeurosis of the internal oblique, and separated from
its fellow of the opposite side by the linea alba.

The artery passing forward, for the most part in this muscle, is the
inferior epigastric, a branch of the external iliac (p. 255). It anas-
tomoses with the superior epigastric artery, a continuation of the
internal mammary (p. 326). 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.

(c) The transverse muscle (m. transversus abdominis), the
deepest muscle of the abdominal wall. Origin: seven
I^^sterior ribs, the tips of the transverse processes of the
lumbar vertebrae by a thin aponeurosis (also termed the
middle layer of the lumbar fascia) , and the inguinal ligament.
Insertion : the linea alba, by ^n aponeurosis which fuses with
the weakly developed dorsal leaf of the aponeurosis of the
internal oblique to form the dorsal wall of the sheath of the
rectus abdominis. The fibres are directed downward and
slightly backward.

224 ANATOMY OF THE RABBIT

5. Divide the remaining portion of the abdominal wall on the left
side, and its whole thickness on the right, by transverse and
longitudinal incisions corresponding with those first made
through the skin, so that the abdominal viscera are fully exposed.
Note on the internal surface of the wall the smooth serous
investment here forming the parietal peritoneum (peritonaeum
parietale).

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. 135). The major portion of , the cavity is abdominal, i.e. it lies
between the diaphragm in front and the margin of the bony pelvic
girdle behind, but it extends into the pelvis, the portion of the
body enclosed by the skeletal ring of that name, 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 urinogential systems.

The general relations of the visceral peritoneum 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 (the visceral peritoneum), similar in appearance to the mem-
brane covering the body-wall, and the extension of this coat into a
mesentery for the attachment of the structure to the dorsal body-
wall. Note the parallel arrangement of the arteries and veins, and
also their frequent anastomoses. Lymphatic vessels (lacteal
vessels) accompany the blood-vessels in the mesentery but, being
transparent, are not readily recognizable. Lymph nodes also occur,
but in this portion of the mesentery they are aggregated near its
dorsal attachment or root (radix mesenterii).

For the general relations of the stomach see p. 100.

1. Displace the posteroventral portion of the liver forward, ex-
posing in this way the ventral surface of the stomach. Without
injuring the enclosing peritoneum, move the organ about

THE STOMACH AND SPLEEN 225

sufficiently to display its contour and 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 ven-
triculi). It lies for the most part to the left of the median
plane.

(d) The cardia or area of junction with the oesophagus, largely
concealed by the lesser omentum (2c, p. 226), a delicate
sheet of peritoneum extending from the cardia to the liver.
Through the semi-transparent omentum it is possible to
to see the pale-coloured osesophagus, which lies dorsal to it,
approaching the stomach from in front and after careful
examination the omentum maybe torn to reveal these parts
more clearly.

(e) The fundus, a sac-like expansion of the stomach to the left
of the cardia.

(J) The pyloric limb (pars pylorica) forms the right portion of
the organ.

(g) 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 muscu-
lar portion of the pyloric limb, known as the pyloric an-
trum (antrum pyloricum).

2. Raise the posterior portion of the stomach and turn it forward.
Note on the dorsal surface of the greater curvature at the left
side a flat elongated body, the spleen (lien). It has sometimes
a pale coloration in the embalmed animal , where the large amount
of contained blood has been washed out by the preserving fluid,
but is dark red in life. On the right side of the artery of the
spleen, enclosed in the peritoneum, will be seen a diffuse, brown-
ish, 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

226 ANATOMY OF THE RABBIT

the dorsal abdominal wall and the diaphragm with the left
side and dorsal surface of the greater curvature of the
stomach. Its posterior portion is divided into two parts by
the spleen. The dorsal part, the phrenicosplenic ligament
(lig. phrenicolienale) connects the spleen with the dorsal
body-wall. The ventral part, the gastrosplenic ligament
(lig. gastrolienale) connects the spleen with the greater
curvature (cf. Fig. 51).

(b) The peritoneum is projected backward from the greater
curvature as a free fold, the greater omentum (omentum
majus, epiploon) (cf. p. 137), which covers the surface of
the intestines to a certain extent. It usually contains fat.
It is composed of four layers, of which two, representing the
ordinary layers of a mesentery, proceed backward from the
surface of the stomach, and at the posterior free edge of
the omentum turn forward as the other two in a more
dorsal position to unite with the transverse mesocolon, the
mesentery supporting the transverse colon (p. 239), a part
of the large intestine. Fig. 51 shows how this arrangement
develops.

(c) 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 omen-
tum, connecting the caudate lobe of the liver with the
lesser curvature.

3. Working on the left side between the dorsal surface of the
stomach and the body-wall, tear away sufficient of the peri-
toneum to expose the first portion of the abdominal aorta as it
emerges from the diaphragm, and runs along the median line
of the dorsal body-wall. In doing so, try to avoid damaging
the slender greater splanchnic nerve (g). Passing in the di-
rection of the stomach is a median ventral branch of the aorta,
the coeliac artery, the distribution of which may be traced

THE STOMACH AXD SPLEEN 227

(section 4 below). The following structures, however, should
first be identified, especially the ganglia {d, e) which are likely
to be damaged in disturbing the peritoneum, and may advan-
tageously be examined first.

(a) The superior mesenteric artery (a. mesenterica superior),
a second and much larger, median branch of the aorta,
given off a little distance behind the coeliac artery and
passing in the direction of the intestine.
{b) The suprarenal gland (gl. suprarenalis) of the left side, a
pale flattened body about a quarter of an inch or more in
length, a short distance medial to the anterior part of the
kidney. If the gland is halved, examination of the cut
surfaces will show it to be composed of a relatively thick
outer cortex and a central medulla. These tw^o portions,
despite their close association, are both developmen tally
and functionally distinct organs (cf. p. 132).
(c) 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 following ganglia lie near

(j(j the root of the mesentery and may

Ca^ \ be concealed by lymph nodes

(p. 237, d) or by fat.
{d) The coeliac ganglion (g. coeliacum),
^ "~ an unpaired, usually somewhat

^ triangular ganglion of the sym-

pathetic nervous system lies a short
Sma'3 distance in front of the superior

Smp -- "^ mesenteric artery (Fig. 99).

{e) The Superior mesenteric ganglion
(?. mesentericum superius) of the

Fig. 99. Coeliac and superior ° _ ...

mesenteric ganglia exposed and Sympathetic SyStCm, whlch IS alsO

viewed from the left side, aa, '^ . . i i i i •

abdominal aorta; ca. coeliac Unpaired, IS a CUrVCd body lymg

artery; eg, coeliac ganglion; sma, . i* i i i • j i

superior mesenteric artery; smg, immediately behind the SUpCriOr

superior mesenteric ganglion; sn, , t j i t

splanchnic nerve. mesenteric artery, its dorsal end

projects forward on the left side of
that vessel and sometimes is nearly separated from the rest

228 ANATOMY OF THE RABBIT

of the ganglion. Delicate nerve strands connect the coeliac
and superior mesenteric ganglia, which are the largest of
those known as collateral (p. 74).
(/) The delicate nerves proceeding from the coeliac and
superior mesenteric ganglia accompany the corresponding
arteries to the respective organs which they supply,
forming the coeliac and superior mesenteric plexuses.

Through these plexuses run postganglionic fibres (p. 75) from the
coeliac and superior mesenteric ganglia and from the ganglia of the
S3^mpathetic trunk, preganglionic fibres destined for the peripheral
ganglia, and visceral afferent fibres. In the pancreas it has been shown
that the gland-cells are innervated by parasympathetic fibres from
cell-bodies in ganglia within the gland and the blood-vessels are con-
trolled entirely by sympathetic fibres with their cell-bodies in the coeliac
and superior mesenteric ganglia.

(g) The (greater) splanchnic nerve (n. splanchnicus major) of
the left side passes backward from its origin in the thorax
(see p. 336), around the reduced left crus of the diaphragm,
and, crossing the aorta obliquely, enters the coeliac and
superior mesenteric ganglia as well as sending branches to
the renal plexus. It is composed of preganglionic fibres
(p. 75).

The ganglia just described transmit to the viscera nerve impulses
received from the spinal cord through the splanchnic nerves but also
probably transmit local reflex impulses received directly from the
viscera.

Experimental section of the nerves in the living animal results in
vaso-dilation, stimulation in vaso-constriction.

The lesser splanchnic nerve is absent as a distinct structure in the
rabbit.

(h) The beginning of the inferior mesenteric artery and the in-
ferior mesenteric ganglion, with the related autonomic nerve
plexus, all of which are described on pages 241-2, may be
noted at this point.

(i) An outlying portion of the pancreas (cf. p. 23G) is seen in
the peritoneum after the branches of the splenic artery
have been severed (4, a). This is the part already point-
ed out in the first paragraph of section 2.

4. Trace the plan of branching of the coeliac artery, beginning
at the point of origin, and exposing the vessels in order.

ARTERIES OF THE STOMACH 229

The details of this pattern vary considerably in different
individuals but the parts supplied by the respective branches are
constant.

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 inferores), are given off from the
anterior wall of the coeliac and are distributed to the diaphragm.
The distribution of the main branches is as follow^s:

{a) The splenic artery (a. lienalis) passes in the direction of
the spleen, giving off small branches (rr. 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 sur-
face, or hilus, of the spleen, it gives off several splenic
branches (rr. lienales) to that organ, and also several more
short gastric arteries, to the greater curvature. Toward the
end of the spleen the splenic artery passes into the free fold of
the greater omentum as the slender omental artery, and near
this point there is given off a large vessel, the left gastro-
epiploic artery (a. gastroepiploica 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 oesophagus, while that on the right bears
a small pyloric branch which anastomoses across the lesser
curvature with the right gastric artery.

230 ANATOMY OF THE RABBIT

The chief nerves of the coeliac plexus accompany the branches
of the artery to the stomach where they are associated with the terminal
ramifications of the vagus (see below). These two represent respectively
the mutually antagonistic sympathetic and parasympathetic divisions
of the nervous system, the latter exciting the former inhibiting gastric
activity.

In dissecting the following arteries, care must be taken
to avoid injuring the bile duct and the portal vein.

(c) The hepatic artery (a. hepatica), the continuation of the
coeliac, passes forward and to the right, giving oH 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
gastroepiploic artery (a. gastroepiploica dextra), traverses
the greater omentum to the greater curvature where it
anastomoses with the left gastroepiploic artery.

After giving ofT 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 gastro-
epiploic 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

THE LIVER 231

portion of the lesser curvature. The right cord passes to
the stomach in a similar manner from the dorsal surface
of the oesophagus. These relations suggest the twisting
which the stomach has undergone in developing its adult
form and position.

5. Cut across the stomach at the pyloric antrum. Divide the
oesophagus, and remove the stomach from the body. Open the
organ by means of an incision extending around the greater
curvature 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 each
other 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 trans-
verse 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 latter being
through the common bile duct, which marks the point at which it
developed as an outgrowth from the embryonic endodermal canal.

For the general relations of the liver, see pp. 94, 95, and following pages.

1. Examine the contour and plan of division as follows:

(a) The anterior surface is convex, applied to the diaphragm:
the posterior surface is concave, fitting the convexity of the
stomach. The organ is thickened in its dorsal portion and
tapers to a thin posteroventral margin.

(b) The liver is partially divided by a deep median cleft into
right and left lobes and each of these comprises distinct
anterior and posterior lobules. Variable indications of
further subdivision sometimes appear, particularly in the

232 ANATOMY OF THE RABBIT

right posterior lobule. The latter lies close to the dorsal
body-wall, is separated from the anterior lobule by a rather
wide space which accommodates the pyloric end of the
stomach, and fits round the anterior end of the right kidney.

(c) The gall bladder (vesica fellea) is an elongated, rather thin-
walled sac situated in a deep depression on the posterior
surface of the right anterior lobule.

(d) The quadrate lobe (lobus quadratus) is a subdivision of the
right lobe lying medial to the gall bladder. Its lateral
limit is sometimes further indicated by a groove extending
ventrally from the depression containing the gall bladder.
It is frequently notched on its medial margin.

(e) The caudate lobe (lobus caudatus ) is a small , well-separated ,
lobe with an almost circular portion fitting against the base
of the left posterior lobule and an extension backwards
which is accommodated in the natural condition in the
space enclosed by the lesser curvature of the stomach.
Dorsally, it blends with the posterior right lobule and
passes over into the common dorsal mass wherein all the
main lobules of the liver meet.

(/) The portal fissure (porta hepatis) is a large depression filled
by the portal vein at its point of entrance and containing
also the primary branches of the hepatic artery and
tributaries of the common bile duct.

2. Trace the peritoneal connections as follows:

(a) The lesser omentum, represented by the hepatoduodenal
ligament and the hepatogastric omentum, previously
divided.

(b) 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. It is a remnant of a primitive ventral
mesentery. The position of this ligament indicates the
line of division of the liver into right and left lobes. The
free curved border of the ligament contains a thin cord, the
round ligament (lig. teres hepatis) , which marks the position

THE LIVER 233

of the umbilical vein in the foetus (p. 115). The corre-
sponding umbilical notch is less conspicuous than in man
because of the highly lobulated condition of the whole
organ in the rabbit.

(c) The coronary ligament (lig. coronarium hepatis), a short
circular fold like a short section of a hollow cylinder, con-
tinuous wdth the dorsal extremity of the falciform, and
connecting the anterior surface of the liver with the middle
of the diaphragm. \Mthin this the inferior vena cava
reaches and pierces the diaphragm.

(d) The left triangular ligament (lig. triangulare sinistrum), a
lateral continuation of the coronary connecting the left
lobe with the diaphragm.

In occasional individuals a smaller right triangular ligament also occurs.

3. Trace the branches of the common bile duct, the hepatic artery,
and the portal vein. These structures traverse the lesser omen-
tum 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 (Fig. 49). The latter receives the cystic duct
(d. cysticus) from the gall bladder. A duct from the quadrate
lobe may join the left hepatic duct. Special ducts from the
right posterior lobule and from the caudate lobe enter the
common bile duct through a short common trunk and an
additional duct from the anterior part of the caudate lobe
may enter the common bile duct directly. The ducts from
the caudate lobe run dorsal to the portal vein. 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
immediately beyond the pylorus.

(b) The hepatic artery (a. hepatica) approaches the liver by
passing forward on the right side of the portal vein ventral
to the bile duct. It distributes branches to the right
posterior lobule, usually two in number, and from one of
these a secondary branch crosses obliquely dorsal to the
portal vein and enters the caudate lobe. At the common

234 ANATOMY OF THE RABBIT

base of the remaining portions of the Hver, the hepatic
artery divides into right and left rami, the right ramus
sending 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,
having been formed by the confluence behind this of the
veins from the intestines and the stomach. 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 falciform, the coronary, and the triangular ligaments,
cutting near {but not into) 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 folloAving:
(a) The inferior vena cava, accommodated in a depression of
the thickened dorsal portion of the organ. The vessel
should be opened lengthwise.
(&) 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 formed by
the union of tributaries from the right anterior lobule and
from both divisions of the left lobe.
(c) The renal impression (impressio renalis), an extensive
excavation of the right posterior lobule for the accommo-
dation 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 distinguish-

THE INTESTINES 235

able in calibre, namely, the small intestine (intestinum tenue) and
the large intestine (intestinum crassum). Both are greatly elon-
gated and convoluted. In examining them, care must be taken to
avoid injury to the blood-vessels and mesenteries, especially the
dorsal 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. 100, 136.
For study of mucous surface, see note p. 367.

1. Beginning at the pylorus, trace the course of the small intestine*
as follows: Its first portion, the duodenum, curves round from
the pylorus to turn back and form a U-shaped loop lying on
the dorsal wall of the adbominal cavity to the right of the
vertebral column. The distal end of this portion of the intestine,
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. At its connection with the caecum,
the small intestine forms a rounded, semi-expanded sac, the
sacculus rotundus, a feature peculiar to the rabbit. The termi-
nal 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 portion is the short part which curves round
from the pylorus to lead into a long descending portion.
The latter is then connected by a short, wavy, transverse
(horizontal) portion with an ascending portion of inter-
mediate length.

(6) The common bile duct, opening on the dorsal wall of the
superior portion.

236 ANATOMY OF THE RABBIT

(d) The pancreas (Fig. 5, p. 17). Its principal portion is
here seen as a diffuse brownish mass lying in the mesoduo-
denum (cf. pp. 94 and 131) sometimes associated with a
considerable amount of fat. Its duct (d. pancreatis) opens
into the posterior portion of the ascending limb.

The extraordinarily diffuse form of the pancreas and, more
particularly, the wide separation of the opening of its duct from
that of the bile duct (these two having a common termination in
many mammals, including man) are associated with the lengthening
of the duodenum as of other parts of the intestine in the herbivore.

(e) The superior pancreaticoduodenal artery, a branch of the
gastroduodenal (see p. 230), passes backward on the first
portion of the descending limb.

(/) The inferior pancreaticoduodenal artery (a. pancreatico-
duodenalis inferior), a branch of the superior mesenteric
(p. 240), enters the mesoduodenum from the left side and
supplies the major portion of the loop. An anterior branch
anastomoses with (e).

3. In the mesenterial small intestine, the following features may
be identified:

(a) 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).

(b) 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 dis-
tinctly 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 colour difTerences are not usually well indicated in
embalmed animals.

(c) 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

THE INTESTINES 237

movement of this part 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.

(d) The mesenteric lymph glands (lymphoglandulae mesen-
tericae) are aggregated a short distance from the dorsal
attachment of the mesentery, where they form a compact
mass covering the left side of the superior mesenteric
artery.

(e) The wall of the sacculus rotundus shows externally a
pattern of fine hexagonal markings, like the surface of a
minute honeycomb, on account of the presence in it of a
large number of lymph follicles. Structures of similar
composition and similarly marked, forming oval areas
about 3 mm. in diameter and 5 mm. in length, or somewhat
larger, may be found along the wall of the small intestine
(aggregated lymph nodules of Payer).

(/) 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:

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.
Strictly speaking, the caecum begins at the opening from the small
intestine, but in the rabbit and many other mammals its peculiar
structure extends a short distance beyond this point along the other
part of the large intestine, the colon. The caecum is so greatly
enlongated in the rabbit that it has become coiled in a spiral
manner and may be considered to consist of three limbs (Fig. 54),
the third terminating in the narrow^ but thick-walled vermiform
process (processus vermiformis) 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

238 ANATOMY OF THE RABBIT

sacculus rotundus, in which position it is distinguished by its
greatly sacculated walls. As noted above, the first part of the
colon of the rabbit has assumed the structure of the caecum,
constituting the ampulla caecalis coli, beyond which the structure
of the beginning of the colon appears suddenly. Such an ampulla
is not present in mammals which, like man, have not an extensively
developed caecum.

The third portion, the straight intestine or rectum (intestinum
rectum), is a small terminal division situated in the middle line
and enclosed for the most part by the pelvis. It is scarcely dis-
tinguishable from the related portion of the colon, so that the point
of disappearance of the latter from the abdominal cavity may be
regarded for convenience as the dividing line between them.

5. In the caecum the following features may be distinguished:

(a) The wall is notably thin and, though otherwise smooth, is
divided by a spirally arranged constriction, the latter de-
noting the position, internally, of a fold of the mucous
tunic, the spiral valve. Both of these features are continued
into the ampulla caecalis coli.

(6) The vermiform process is a narrow, light-coloured 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, beyond the ampulla caecalis, is divisible into ascend-
ing, transverse, and descending 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 forward 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.,
upward in man — see Fig. 51), follows more or less closeh'
the course of the caecum. It is composed of five principal
limbs, each of these being a portion which runs either for-
ward or backward and is united by a flexure to the next,
which has the opposite direction. Three of the limbs are

THE INTESTINES 239

directed for the most part forward, the remaining two back-
ward, and the third includes a pronounced secondary
curvature in a lateral direction.

The first limb of the colon bears three rows of small
sacculations, the haustra, separated by three longitudinal
muscle-stripes, distinguished as the bands of the colon
(taeniae coli). Two of these bands are free, while the third
is enclosed by the supporting peritoneum, the mesocolon.
The two free bands unite toward the anterior end of the
first limb and the third or attached band joins them soon
after, so that the second limb has but one row of haustra
along most of its extent, this row continuing on to the
beginning of the third limb. The modifications described,
which are not observed in carnivores, serve to increase
the storage capacity of the intestine and the area of its
walls and to delay the passage of its contents.

(6) The transverse colon (colon transversum) is a short seg-
ment, beginning forw^ard on the right and crossing the
middle line transversely to the left, where it bends sharply
backward, and is replaced by the descending colon.

(c) The descending colon (colon descendens) passes backward
to a point in front of the pelvis, where it is replaced, with-
out any definite demarcation, by the rectum.

The descending mesocolon, which connects this portion
with the dorsal body-wall, should be noted on account of its
relation to the inferior mesenteric artery and sympathetic
plexuses. It is connected for a considerable distance with
the mesentery of the ascending limb of the duodenum.

7. Displace the caecum, turning it o\'er to the right side of the
animal. Lay out the mesenterial small intestine, so that the
mesentery and its blood-vessels are exposed. Remove the
lymph glands from about the superior mesenteric artery, first
noting their position a short distance from 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.

240 ANATOMY OF THE RABBIT

Trace the branches of the superior mesenteric artery as follows:

(a) The middle colic artery (a. colica media), a small vessel
(frequently two) arising from the left wall and passing to-
the transverse colon.

(6) The inferior pancreaticoduodenal artery (p. 236) 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 proximal branches of the ileocaecocolic artery include:

(1) Small branches to the third, fourth, and fifth limbs of
the ascending colon, each anastomosing with its
neighbours, and the last with the middle colic arter}'.

(2) The appendicular artery (a. appendicularis) to the
vermiform process. This vessel also gives off several
short branches to the immediately adjacent part of the
ileum and a longer branch, arising from the appendicular
near its point of origin, passes along the ileum to
anastomose with an intestinal branch of the superior
mesenteric trunk (top of p. 241).

(3) An anterior ileocaecal artery to the terminal fourth
(anterior part of the 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 the 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 of the ileocaecocolic artery include:

(6) A posterior ileocaecal artery to the middle portion of

THE INTESTINAL BLOOD-X'ESSELS 241

the third Hmb of the caecum and the adjacent portion
of the ileum; anastomosing with (3).

(7) A caecal artery to the second Hmb and the posterior
end of the third Hmb of the caecum.

(8) Terminal branches to the parts of the ileum, caecum,
and colon 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
after the ileocaecocolic artery has left it, and are distributed
to the free portion of the mesenterial small intestine. The
successive vessels are connected by anastomoses the first
connecting also with a branch of the inferior pancreat-
icoduodenal artery. All but two of the intestinal arteries
. arise from one side of the superior mesenteric artery, one
forms the end of the latter, and one springs from its opposite
side. The last anastomoses forward with a branch of the
appendicular artery.

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 colon (anastomosing with the middle colic), and
the superior haemorrhoidal artery (a. haemorrhoidalis superior)
to the posterior portion of the colon and the rectum, continuing
caudad along the dorsal surface of the latter.

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
mesenteric vein (v. mesenterica inferior) collects blood from
the descending colon and rectum and joins the superior mesen-
teric vein to form the portal vein (p. 234). It may be traced
forward in the descending mesocolon, where it crosses the
inferior mesenteric artery almost at right angles, only its more
posterior part accompanying the arteries.

242 ANATOMY OF THE RABBIT

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
appearing in the mesocolon is the abdominal aortic plexus
(plexus aorticus abdominalis). It is connected anteriorly with
the coeliac and superior mesenteric plexuses (p. 213) accom-
panying the corresponding vessels, and with the renal plexuses
accompanying the renal vessels to the kidneys; posteriorly with
the inferior mesenteric and spermatic plexuses about the
inferior mesenteric and internal spermatic arteries, and with
the hypogastric plexus about the pelvic vessels.

11. By division of the rectum close in front of the pelvis and of the
peritoneal attachments, the intestines may be separated and
laid out in an extended condition. The relations to one another
of the ileum, caecum, and colon are studied thus to much
better advantage than in the natural position. The caecum
should be opened lengthwise and the spiral valve examined
(Fig. 55).

12. The lymphatic system can be studied adequately only in specially injected
specimens, though the larger mesenterial lymph glands have been pointed
out in the foregoing directions. The lymphatics from the liver and intestines
converge in lymph nodes, of which there are usually two near the posterior
end of the mesoduodenum, two associated with the portal vein near the lesser
curvature of the stomach, and two near the junction of the splenic and
superior mesenteric veins. These are all connected with the large mass of
lymph nodes near the origin of the superior mesenteric artery, whence an
intestinal trunk empties into one of the lumbar trunks, which run in the
lateral walls of the abdominal aorta.

VI. THE URINOGENITAL SYSTEM

For the general relations of the urinogenital organs, see p. 122.

(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

THE URIXARY ORGAXS 243

fibrous coat immediately surrounding the kidney substance, each
organ is imbedded in a mass of fatty material, the adipose capsule
(capsula adiposa), and is also held in position by the peritoneum,
which is stretched across its ventral surface.

1. If the peritoneum and adipose capsule are removed from the
left kidney, the external features and vascular connections may
be made out as follows:
(a) The general convexity of contour.

(6) The renal hilus (hilus renalis), a concavity of the medial
surface of the organ.

(d) 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.

(e) The renal vein (v. renalis), leaving the kidney at the hilus,
and joining the inferior cava.

The right renal artery leaves the aorta about one-half centimetre
in front of the origin of the left vessel, than which it is considerably
shorter. The two renal veins have similar relative positions, but the
distance between their proximal ends is a little greater.

2. Divide the kidney, beginning the incision at the hilus and re-
moving the ventral half (Fig. 70). Examine the cut surface of
the dorsal half for the following:

(a) The renal pelvis (pelvis renalis), a cavity within the
kidney, 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.
(6) The cortical substance (substantia corticalis) ; distinguish-
able as a narrow peripheral zone of the kidney substance,
(c) The medullary substance (substantia medullaris), forming
the central and medial portion of the kidney, including the
renal papilla. It is distinguished by its radial striations.

244 ANATOMY OF THE RABBIT

(d) 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 imperfectly expressed. The medullary substance, how-
ever, possesses a slightly divided margin.

The cortical substance is of darker coloration than the medullary
in the natural condition, but in embalmed animals the colour relations
are usually reversed.

3. The urinary bladder (vesica urinaria) lies in the ventral pos-
terior portion of the abdominal cavity. It is a muscular sac,
capable of a considerable amount of distension, but usually
found in preserved animals in a 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 dorsolateral surface of
the rectum in the male and from the vagina in the female,
to the bladder, and after investing the latter passes to the
ventral abdominal wall. The peritoneum dorsal to the
bladder forms in the mak a paired retrovesical fold (plica
recto vesicalis), and in the female a similar vesicouterine
fold, the ureter in each case running in the edge of the fold
and a recess of considerable extent (rectovesical or vesi-
couterine pouch) being left between the adjacent structures.
The ventral peritoneum forms a broad median vertical
sheet, the middle umbilical fold (plica umbilicalis media)
between the bladder and the ventral abdominal wall. 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
umbilical arteries in the foetus, where they run beyond the
bladder into the umbilical cord to reach the placenta (Fig.
65) . The middle umbilical fold is often heavily laden with fat.

THE MALE GENITAL ORGANS 245

(b) The umbilical artery (a. umbilicalis), a branch of the
hypogastric, which has not yet been exposed (p. 255),
passes along the side of the bladder to the vertex accom-
panied by the vesical vein. From the umbilical artery
near its beginning, branches are given off to the ureter (a.
ureterica) and related portions of the genital ducts.

The Male Genital Organs

1. Continue the median ventral incision of the skin backward along
the symphysis to the free end of the penis. Reflect the skin on
both sides and clear away the connective tissue so as to expose
fully the body of the penis and its attachments to the ischium,
and on one side continue the exposure to a point beyond the scro-
tum. Note the cremaster muscle (m. cremaster), a thin layer of
muscle fibres forming the outer layer of each scrotal sac (sac
of the testis) after removal of the skin and subcutaneous tissue.
Though situated directly under the latter, it is continuous with
the internal oblique muscle of the abdominal wall, and also
contains fibres from the transverse muscle. It is supplied with
blood from the external spermatic artery (p. 223). Make a
longitudinal incision through this muscle, cutting forward into
the abdominal cavity. After the two flaps are spread apart,
the following features may be made out:

(a) The parietal layer (lamina parietal is) of the tunica vagi-
nalis propria, a layer of peritoneum continuous with that
of the abdominal wall, forms the internal lining of the sac
of the testis (cf. p. 137 and Fig. 75). The sac is widely
open to the abdominal cavity, so that the testis passes
freely from one cavity to the other, a condition more
prirriitive than that when the scrotal sac is closed off.
(6) The male reproductive gland, the testis, and its associated
vessels and duct occupy the cavity of the sac, the testis
being suspended from its dorsal wall. The gland has the
form of an elongated oval ,^ about two to three centimetres
in length and seven or eight millimetres wide in the mature
adult.

246 ANATOMY OF THE RABBIT

ing smooth muscle fibres, intimately associated with the
lower end of the epididymis (e), joins the posterior end of
the testis with the end of the sac.

(d) The visceral layer (lamina visceralis) of the tunica vagi-
nalis 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.

(e) The first portion of the duct of the testis, the epididymis,
is very long, slender, and much coiled, the coils being
bound together by connective tissue to form a thickened
mass, usually imbedded in fat, fitting like a cap over the
anterior end of the testis. It then extends back as a thinner
cord along the side of the latter body and of the guber-
naculum. The thickened anterior part is the caput
epididymidis, the more slender part beside the posterior
end of the testis and the gubernaculum is the cauda
epididymidis, while the still thinner intervening portion
is the corpus epididymidis. The second portion of the
duct, the ductus deferens, leads forward from the cauda
epididymidis, where it is firmly attached to the guber-
naculum. The connection with the epididymis may be
shown by carefully separating the duct from the guber-
naculum and the side of the testis. The ductus deferens
receives its blood supply mainly by the arteria deferentialis,
which originates from the base of the umbilical artery or
from the immediately adjacent part of the common iliac
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 sends
branches to the epididymis and ductus deferens, and then
follows a greatly contorted course to the anteromedial part
of the testis, on the surface of which it then coils back and
forth before finall^^ entering its substance. The tortuous
course of the artery appears to be an arrangement for
slowing the blood flow.

THE MALE GENITAL ORGANS 247

(g) The spermatic vein (v. spermatica) is formed by a net-
work of vessels, the plexus pampiniformis, which surrounds
the internal spermatic artery as it approaches the testis.
The left vein 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 ab-
dominal cavity, the association of the ductus deferens with the sper-
matic vessels to form a spermatic cord (funiculus spermaticus) as in
the human species is very imperfectly expressed.

2. The structure and attachments of the penis should be examined.
Apart from the urethra, the soft-walled tube which traverses
it ventrally and opens at its tip, the body of the penis is formed
chiefly by a pair of hollow fibrous structures, the cavernous
bodies (corpora cavernosa penis).

The cavernous bodies have thick white sheaths (tunicae albu-
gineae) which fuse in a median septum and surround columns of spongy
tissue which can be distended with blood i.e. erectile tissue. The fusion
of the sheaths produces an apparently unpaired, median structure and
the two contained corpora are best seen by cutting the penis transversely
after the study of the organs has been otherwise completed. The wall of
the associated portion of the urethra has a thin layer of similar erectile
tissue.

The cavernous bodies diverge at their proximal ends, the
diverging parts constituting the crura penis, and each crus is
firmly attached to the ventromedial margin of the ischium, a little
posterior to the symphysis, by a short cord of -white fibrous con-
nective tissue.

The crus is partly concealed by a short thick ischiocavernosus
muscle, the origin of which is on the edge of the ischium both
anteromedial and posterolateral to the attachment of the crus.
The penis is also attached to the symphysis by a short but stout
unpaired suspensory ligament (lig. suspensorium) and by a thick
spindle-shaped pubocavernosus muscle lying in a median position
ventral to the ligament and between the two ischiocavernosi.

Strictly speaking, a glans penis, which occurs in many mammals,
is absent in the rabbit and the free extremity of the organ, occupying
the position of that part, should be called simply pars libera. The
glans, properly, is a swollen terminal portion of the erectile tissue
(corpus spongiosum) in the wall of the urethra.

248 ANATOMY OF THE RABBIT

When the study of these parts has been completed, the attach-
ments of the penis should be severed at the posterior border of the
ischium and the symphysis should be divided. Pressing apart the
two halves of the pelvis facilitates examination of the connections
of the deferent ducts with the common urinogenital tube and
related parts. Following this the urinogenital organs and pelvic
portion of the rectum may be dissected out and removed from the
body in a single piece without damage to anything except the
vessels supplying these organs, which must be severed with the
attaching connective tissue. The rectum should then be separated
from the urinogenital structures.

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 vescicle. 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 latter is an elongated, paired
organ enveloping the rectum a short distance in front of the anus and pouring
into it an oily secretion. 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 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.

Immediately dorsolateral to the body of the penis and just under the skin lie
the paired inguinal glands. At each side, a nearly spherical white inguinal
gland and, closely associated, a brown inguinal gland pour their secretions
into the hairless inguinal spaces. The former gland is sebaceous, the latter a
modified sweat gland producing an odoriferous secretion.

The following parts of the urinogenital system may be made out :
(a) The connection of the bladder with 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.
(6) The seminal vesicle (vesicula seminalis) lies on the dorsal
surface of the base of the bladder. It is a flattened median
glandular pouch, the forward-directed tip of which has. a
relatively thick muscular wall and is slightly divided,
corresponding with a bilobed character of the cavity within.

THE MALE GEXITAL ORGANS

249

The organ as a whole is nearly 2.5 cm. long but is largely
covered and compressed dorsally by the vesicular and
prostate glands (Fig. 100). Its thin ventral wall adheres
closely to the expanded terminal portions of the deferent
ducts.

(c) The somewhat dilated final portions (ampullae) of 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, where their point of entrance is
marked by a pair of internal papillae.

(d) The vesicular gland and the prostate gland lie in the dorsal
wall of the more posterior part of the seminal vesicle, the
former anterior to the latter, each enveloped in a connective
tissue capsule. Macroscopically they are very similar and
are associated in a single mass but histologically they are
different and are separated by a thin connective tissue
septum. In the fresh con-
dition, the larger vesicular
gland tends to be dull grey
while the prostate is cream-
coloured.

The vesicular gland has
a pair of ventral ducts, one of
them shown in Fig. 100 which
enter the urethra at either
side of the seminal coUiculus
(vide infra) and the prostate
has four to six minute ducts
at either side opening just
behind them.

The paraprostatic glands
are minute finger-like pro-
jections of the urethral lining
imbedded in the outer part
of its wall at either side
of the base of the seminal
vesicle. Their number is variable

Fig. 100. Diagram of sagittal sec-
tion of urethra with accessory sex
glands and their ducts, from a seven-
months old rabbit. (After Bern and
Krichesky.) am, ampulla of ductus
deferens; bu, bulbourethral gland;
pr, prostate; sc, seminal coUicuIus;
sp, septum between vesicular and
prostate glands; sv, seminal vesicle;
ur, urethra; vg, vesicular gland.

250 ANATOMY OF THE RABBIT

{e) The bulbourethral or Cowper's gland is a bilobed mass
(deep pink in the Hving animal but usually dark coloured
in the embalmed specimen) imbedded in the dorsolateral
walls of the urethra immediately behind the prostate.

The above dcsLTibed glands, including the seminal
vesicle, contribute to the liquid in which the sperms are
transmitted. The seminal vesicle is not a reservoir for
storage of sperms.

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 and
\esicular glands may sometimes be made out. The seminal
vesicle should also be cut open.

The Female Genital Organs
1. The organs may be traced from the abdominal cavity backward,
as follows:

(a) The ovary (ovarium) is a small — in young animals minute
- — elongated, somewhat flattened, structure of greyish or
yellowish coloration lying 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.

(b) The mesovarium, a short fold of peritoneum suspending
the ovary from the body-wall.

(c) 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, giving branches also
to the uterine tube.

(d) The spermatic vein (v. spermatica) leaves the medial side
of the ovary and, crossing the body-wall, enters the inferior
caval vein.

THE FEMALE GEXITAL ORGANS 251

(e) 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 ex-
pansion, 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. One of these is attached to the anterior end
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 collapsed. It
probably is a vestige of a part of the oviduct anterior to the ostium
abdominale.

(/) The mesosalpinx is the peritoneum supporting the uterine
tube. It is continuous with the mesovarium.

(g) The uterus, the second portion of the oviduct; distin-
guished by its greater diameter and muscular walls. The
size of this portion is enormously increased in animals
which contain or have borne young.

(h) 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).

(i) The ovarian ligament (lig. ovarii proprium) is a fine thread
in the edge of a secondary fold of peritoneum which crosses
the mesosalpinx from the posterior end of the ovary to
the anterolateral end of the uterus.

(j) The round ligament (lig. teres uteri) extends in line with
the ovarian ligament behind the uterus. It is a fine fibrous
cord which raises the peritoneum into a fold and which may
be traced from the anterior end of the uterus to the body-
wall ventral to 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. The ovarian and
round ligaments together represent the gubernaculum of
the male.

(k) The uterine artery (a. uterina) runs through the meso-
metrium after originating from the umbilical artery. It

252 ANATOMY OP^ THE RABBIT

supplies the uterus and anastomoses anteriorly with the
most posterior of the branches to the uterine tube from the
internal spermatic artery.

(/) 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 the skin of the ventral surface
should be continued backward to the tip of the clitoris, which
organ appears as a flexible median rod imbedded in the ventral
wall of the vestibulum. Corresponding with the glans penis of
the male, there is a short terminal portion, the glans clitoridis,
covered by a fold of skin. The structure and attachments of
the clitoris should now be examined.

(a) The clitoris, like the penis of the male, is composed mainly
of a pair of cavernous bodies (corpora cavernosa clitoridis),
each consisting of an elongate mass of spongy vascular
(erectile) tissue surrounded by a tough white sheath. The
sheaths of the two cavernous bodies are fused in the
median plane so closely that the double character of the
organ is not evident except at the attached end, where the
two bodies diverge as the crura clitoridis, and are connected
to the posteroventromedial borders of the ischia by short
fibrous cords overlain by muscles.

(b) The unpaired, median pubocavernosus muscle, originating
at the symphysis and the paired ischiocavernosus muscle,
originating on the posterior borders of the ischia, pass to
the base of the clitoris.

(c) The suspensory ligament is a short median cord dorsal to
the pubocavernosus muscle, joining the base of the clitoris
with the posterior end of the symphysis. The crura
clitoridis are largely covered ventrally by the ischio-
cavernosus muscles, attaching the clitoris to the posterior
edges of the ischia at each side.

The attachments of the clitoris should be severed and the
symphysis should be divided. By pressing apart the two sides of
the pelvis and cutting through the skin round the anus and the

THE ABDOMINAL AORTA 253

tissue attaching the organs to the pelvis and the base of the tail,
the urinogenital tube and the rectum may be dissected out in a
single piece, their blood-vessels and the attaching peritoneum being
the only other parts divided. The rectum should then be separated
from the urinogenital organs.

The middle haemorrhoidal artery (a. haemorrhoidalis media), a branch
of the hypogastric (p. 255), supplies the lateral walls of the rectum and the
vestibulum. The internal pudendal artery, accompanied by the corresponding
vein and nerve, passes over the side of the distal part of the vestibulum to the
clitoris after giving off the inferior haemorrhoidal artery to the terminal
portion of the rectum and to the rectal or anal gland, This gland is an elongated,
paired organ, the pair almost surrounding the rectum a short distance in front of
the anus and pouring into it an oily secretion. The rectum is connected with
the base of the tail by the rectocaudal muscle, a somewhat spindle-shaped
involuntary muscle originating on the body of the second caudal vertebra and
inserted a little further forward on the dorsal side of the rectum. The external
and internal anal sphincters are closely related thin muscles enclosing the rectum
and the vestibulum, the former having its origin on the dorsum of the tail.

At each side of the external opening of the vestibulum, just under the skin,
lie the paired inguinal glands. As in the male, each of these comprises a larger,
medial, dark portion and a smaller lateral, white portion, the latter sebaceous,
the former a modified sweat-gland secreting an odoriferous liquid. Ducts from
both empty upon the hairless inguinal spaces.

In the urinogenital ducts, examine the extent of the vagina
backward and its connection with the canal of the bladder (female
urethra) to form the common vestibulum. The canal and the
vestibulum together correspond with the male urethra (cf. Fig. 68,
p. 121). The bulbourethral gland (gl. bulbourethralis) situated
in the dorsal wall of the vestibulum, is similar to that of the male
(cf. p. 250).

If the vestibulum be slit open and the incision be extended into
the bladder and also forward into the left uterus the apertures of
these structures may be examined from the interior. There is a
separate external uterine aperture (orficium externum uteri) open-
ing from each uterus into the vagina.

VII. THE ABDOMINAL AORTA, INFERIOR CAYAL
VEIN, AND SYMPATHETIC TRUNKS

The dissection and removal of the intestines and urinogenital
organs clear the dorsal body-wall for an examination of the

254

ANATOMY OF THE RABBIT

abdominal portion of the aorta, the inferior caval 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 re-
moved with the liver.

The abdominal portion of the aorta, described as the abdominal
aorta (aorta abdominalis) extends from the hiatus aorticus of
the diaphragm to the seventh lumbar vertebra, where it is re-
placed 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 primitive continuation backwards
on the sacrum and the caudal vertebrae is represented by the
greatly reduced median sacral artery.

The branches of the vessel are distributed in two series: (1)
visceral branches (rami viscerales) to the parts of the digestive
tube and the urinogenital or-
gans; and (2) parietal bran-
ches (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), very small
vessels arising by a com-
mon trunk in the hiatus
aorticus and passing to

the diaphragm (usually seen better on the right side).

(b) The suprarenolumbar artery (a. suprarenolumbalis), arising

Fig. 101. Plan of the pelvic blood-vessels.
.Arteries: a, aorta; aei, inferior epigastric;
af, femoral; ah, hypogastric; ahm, middle
haemorrhoidal; ai, sciatic; aic, common iliac;
aie, external iliac; ail, iliolumbar; ao, ob-
turator; as, sacral; au, umbilical. Veins: vci,
inferior cava; vf, femoral; vh, hypogastric;
vh', common hypogastric; vie, external iliac;
vil, iliolumbar.

THE ABDOMINAL AORTA 255

on either side from the renal artery, and passing antero-
laterad to the body-wall, supplying also the suprarenal
body.

Occasionally the suprarenal artery originates separately, either
from the renal artery or from the aorta itself.

(c) 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, each pair
originating as a single trunk which branches to right and left.

(d) 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 and
of the caudal vertebrae 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 paired
trunk, the branches of which pass to the posterior limb, the
wall of the pelvis, and the pelvic viscera. The first branch
is usually the iliolumbar artery, which passes laterad to the
body wall, though the point of origin of this vessel varies
considerably and may be on the aorta itself. After giving off
the iliolumbar artery, the common iliac divides into two
branches, the external iliac and the hypogastric. The con-
nections of these may be traced as follows:

(a) 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.

(b) The hypogastric artery (a. hypogastrica formerly known
also as internal iliac artery^ is the smaller, medial branch,
directed backward on the dorsal wall of the pelvis. Its
course may be traced, care being taken not to injure the
nerves of the lumbosacral plexus. Where it diverges from

256 ANATOMY OF THE RABBIT

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 next branch of the
hypogastric is the obturator artery, which passes postero-
laterad to the pelvic wall. About the same point arises the
medial femoral circumflex artery, which runs more directly
laterad into the muscles of the thigh. Slightly further back,
the hypogastric gives rise to the middle haemorrhoidal
artery to the side of the rectum and the urethra in the male
or to the rectum and the vestibulum in the female. The
hypogastric then leaves the pelvic cavity as the sciatic
artery (a. ischiadica), passing to the lateral side of the
abductor caudae anterior muscle. The sciatic artery re-
appears posteriorly, and divides 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,
the latter a short median trunk receiving the paired hypogastric
veins. 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. 234). 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 six pairs of vessels just
in front of the corresponding first six lumbar arteries, and the
paired iliolumbar vein (v. iliolumbalis). The members of each
of the first two pairs of lumbar veins unite to form a single
short trunk but the more posterior veins enter the vena cava
separately. The suprarenolumbar vein at each side joins the
renal vein or may enter the inferior vena cava directly.

The paired hypogastric vein receives as its largest tributary
the sciatic vein from the back of the thigh. It also receives an
external haemorrhoidal vein and a small obturator vein and

THE ANTERIOR LIMB 257

into either the right or the left vessel opens the unpaired median
sacral vein. The seventh pair of lumbar veins opens into the
dorsal side of the common hypogastric vein.

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 accompanying the umbilical
artery and receiving in the female also the veins of the uterus.

0. The sympathetic trunk (truncus sympathicus). Its lumbar 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 of each trunk comprises two minute
ganglia and an unpaired terminal ganglion unites the two
trunks.

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 at first 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, closely associated with the platysma but lying be-
neath it, the depressor conchae (parotideoauricularis) posterior,

258 ANATOMY OF THE RABBIT

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 some-
times removed 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 by feeling. The muscle directed
forward from it toward the angle of the mandible is the sterno-
mastoideus, one of the muscles of the head. The external jugular
vein lies on its lateral side and is joined by the transverse scapular
vein from the lateral surface of the shoulder. Identify by feeling the
clavicle rudiment and the cleidohumeral ligament attaching it to
the humerus. Find the mid-dorsal line of the neck, indicating the
position of the neck ligament (ligamentum nuchae). Then proceed
to uncover the muscles, beginning with those on the ventral side
and working around to the shoulder.

For the general relations of the muscles of the limbs, see pp. 68-70.

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 manubrium
sterni is the sternomastoideus, one of the muscles of the head.

(b) The basioclavicularis (basiohumeralis). Origin: Basioc-
cipital bone. Insertion: Lateral third of the clavicle and
the cleidohumeral ligament.

(c) The levator scapulae major. Origin: Cartilage union of
basioccipital and basisphenoid (sphenooccipital synchon-
drosis). Insertion: Metacromion.

The superficial cervical artery (p. 325) 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 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.

THE ANTERIOR LIMB 259

(d) The trapezius. Origin in two portions. Superior (cervical)
portion. External occipital protuberance and dorsal liga-
ment of the neck (ligamentum nuchae). Insertion: Meta-
cromion and supraspinous fascia. Inferior (thoracic)
portion. Origin: Spinous processes of the thoracic verte-
brae and the lumbodorsal fascia. Insertion: 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 ma}^ be found nerves from the ventral rami
of the third, fourth, and fifth cervical spinal nerves. The great auri-
cular nerve (n. auricularis magnus) passes from the third to the ear,

(e) 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.

(g) 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 dis-
placed laterad. The operation is facilitated by dividing
the latissimus dorsi, the relations of which should, however,
first be noted (2, a).

(h) The serratus anterior consists of two portions. Cervical
portion : Origin on the transverse processes of the posterior
five cervical vertebrae and the anterior two ribs. Insertion
on about the anterior four-fifths of the medial surface of
the vertebral border of the scapula. Thoracic portion:
Origin on the third to the ninth ribs by separate slips
alternating with those of the external oblique. Insertion
on the posterior two-fifths of the medial surface of the
vertebral border of the scapula, overlapped medially by
the cervical portion and the levator scapulae minor.

260 ANATOMY OF THE Rx\BBIT

The transverse artery of the neck (a. transversa colli) lies on the
medial side of the cervical portion.

The thoracic portion of the serratus anterior ma}^ function as an
aid to breathing when the anterior limb is held firm and the muscle
contracts so as to raise the ribs. In this case the relations of origin and
insertion just described are thus reversed.

2. Muscles arising from the axial skeleton and the pectoral girdle
and inserted on the humerus, for the most part at its proximal
extremity.
Note the axillary lymph glands lying in the fat of the axillary fossa.

{a) 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 in-
sertion end passes to the medial side of the humerus.

ih) The pectoralis primus (p. tenuis). Origin: Manubrium
sterni. Insertion: Deltoid tuberosity.

A branch of the thoracoacromial artery appears between this
muscle and the cleidohumeralis (3, a).

The muscle should be raised from the surface and
divided.
(c) The pectoralis secundus (p. major). Origin: Entire lateral
portion of the sternum. Insertion: Anterior and antero-
medial surfaces of the humerus, beginning below the greater
tubercle and extending to near the boundary between the
middle and distal thirds. The more anterior fibres are
covered by those of {h). The more posterior fibres pass
dorsal to the more anterior ones so that the muscle has a
partly twisted form and its insertion is in two layers or
separate slips. Some of the posterior fibres are inserted
highest on the humerus. By working back from the
clavicle, the muscle can be separated from those beneath
and divided.

The more superficial layer is the thinner and is derived mainly
from the anterior half of the origin. It is attached along a line extend-
ing distad from the greater tubercle along and beyond the medial part
of the humeral spine. The deeper layer, derived mainly from the

THE ANTERIOR LIMB 261

posterior part of the origin, is attached more obliquely, just in front of
the intertubercular groove and along the medial edge of the spine of
the humerus, ending at the tip of that ridge.

(d) The pectoralis tertius (p. minor). Origin consisting of two
portions. First portion: The sternum from its anterior
end to the attachment of the fourth rib. Second portion:
On the manubrium sterni from its anterior end to a point a
little behind the attachment of the first costal cartilage
and on this cartilage, lying dorsal to the first portion.
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 ventral fourth of the
scapular spine, the supraspinous fascia (p. 264), and both
surfaces of the medial angle of the scapula. 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
posterior edge of the first portion of (d) and the thoracoa-
cromial artery passes between them.

(/) The pectoscapularis. Origin: The manubrium sterni at
the point of attachment of the first costal cartilage. In-
sertion as indicated above. A slender muscle dorsal to the
first portion of the p. tertius, which should be divided, and
overlapping ventrally the posterior edge of the second
portion.

Blood-Vessels and Nerves of the Axillary Fossa

After division of the pectorals and the clavicle, the blood-
vessels and nerves of the axillary fossa will be fully exposed.

During the examination of these, the axillary lymph glands may be noted.
In speciall}' injected preparations, these are found to receive superficial and deep
lymphatic vessels from the anterior limb and to drain into the subclavian trunk,
which accompanies the corresponding vein and opens into the superior vena cava.

262 ANATOMY OF THE RABBIT

The axillary artery (a. axillaris), the continuation of the sub-
clavian, crosses from the first rib to the medial surface of the
humerus, after reaching which it is called the brachial artery. Its
branches are:

(a) The transverse scapular (suprascapular) artery (a. trans-
versa 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
supraspinatus muscle (3, e).

(b) 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. primus and the cleidohumeralis. It distributes
branches to these muscles and, taking a position ventral
to the clavicle, passes to the platysma and the skin.

(c) The lateral (long) thoracic artery (a. thoracalis lateralis)
arises from the posterior wall or in common w4th (&),
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 dis-
tributes external mammary branches to the mammary
glands. It anastomoses posteriorly with the superficial
epigastric branch of the femoral.

(d) 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. Perfo-
rating 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 anastomoses covering the abdominal

THE AXTERIOR LIMB 263

region and in the female the mammary glands (pp. 221

and 223).
(e) The circumflex arteries of the head of the humerus. See

p. 269.
(/) The deep artery (a. profunda brachii). See p. 270.

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. 271), 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.

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,
which vary considerably in detail, 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, medians and ulnar
nerves (pp. 271, 272). These nerves are formed principally from a
trunk produced by fusion of the ventral branches of the eighth cervi-
cal and first thoracic nerves, the latter crossing the inner surface of
the first rib to meet the former, but adjacent nerves also contribute.
The radial nerve separates first, the median and ulnar a little more
distally.

The seventh cervical nerve gives a branch running mainly to the
median and also connecting with the common trunk of the median
and ulnar just before it divides into these nerves, and from the same
source a slightly larger branch passes to the radial nerve. From
the last-mentioned branch a subscapular nerve runs to the teres
major, accompanied by a nerve to the latissimus dorsi, which latter
frequently receives also a fascicle from the radial nerve. Another
subscapular nerve to the muscle of that name arises mainly from
the sixth cervical, these two being connected by a loop. A supra-
scapular nerve, formed chiefly from the sixth cervical, passes to

264

ANATOMY OF THE RABBIT

Lateral

the anterior border of the scapula, entering the supraspinatus
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.

3. 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. 271) should be traced be-
fore separating the muscles
of the front of the forearm.
Note the supraspinous
and infraspinous fasciae,
tough sheets of connective
tissue covering the muscles
lying in the supraspinous
and infraspinous fossae of
the scapula respectively.
(a) .The cleidohumeralis.
Origin : Lateral por-
tion of the clavicle and
the cleidohumeral liga-
ment.^ Insertion: An-
terior surface of the
humerus in its distal
third. The muscle is
a continuation of the
basioclavicularis, but
represents the brachial
part of the brachioce-
phalic muscle, which, in
many mammals with
reduced clavicle, ex-
tends from the mastoid

portion of the skull to the front of the arm.
part is the cleidomastoideus.

'Regarding this ligament, see p. 200.

Posterior

Fig. 102. Transverse section through the
distal portion of the arm; semidiagrammatic;
a.b., brachial artery; a.c.r., radial collateral
artery; b., biceps; br.l. and br.m., lateral and
medial heads of the branchialis; d., deltoideus
(insertion); e.a.p., extensor antibrachii parvus;
f., brachial fascia; h., humerus; n.m., median
nerve; n.r., radial nerve; n.u., ulnar nerve;
tr.l.tr.3, long, lateral, and medial heads of the
triceps; v.b., brachial vein; v.c, cephalic vein.

The cervical

THE ANTERIOR LIMB 265

(b) The deltoideus. Acromial portion. Origin: The acromion.
Insertion : Distal portion of the deltoid tuberosity. Scap-
ular portion. Origin: Infraspinous fascia. 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
muscle passes beneath the metacromion, which also
serves as a point of attachment, and is replaced on the
lateral surface of the humerus, beneath the acromial por-
tion, by a thin tendon, through which it is inserted
beside the acromial portion.

* The scapular portion of the deltoideus should be
separated from the infraspinatus and divided, the distal
end being reflected together with the metacromion.

(c) The infraspinatus. Origin : Posterior portion of the lateral
surface of the scapula, including the spine. Insertion:
Greater tubercle of the humerus. The muscle fills the
infraspinous fossa.

(d) 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.

(e) 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.

(g) 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.

(h) The coracobrachialis. Origin: Coracoid process. In-

266 ANATOMY OF THE RABBIT

sertion: Distal portion of the upper third of the humerus
on its medial surface.

4. 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. 37, 38, 102).

A. Extensor (anconaeus) group. The muscles arise for the
most part behind the axis of the humerus, and are inserted on the
olecranon.

(a) The extensor antibrachii parvus (anconaeus quartus).
Origin: Fascia of the medial surface of the humerus. In-
sertion: Medial surface of the olecranon.

The muscle should be divided, or detached from its
origin, and reflected.

(b) The anconaeus minimus (epitrochleonanconaeus). Origin:
Medial epicondyle of the humerus. Insertion: Medial
surface of the olecranon.

(c) The triceps brachii. Origin in three portions. Caput
longum (anconaeus longus) : Ventral portion of the axillary
border of 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. 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) 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.

(b) 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 cleidohumeralis muscles. Insertion: In
common with the biceps.

THE ANTERIOR LIMB 267

5. 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. 103).

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) The extensor carpi radialis longus. Origin: Lateral epi-
condyle. Insertion: Base of the second metacarpal.

(b) The extensor carpi radialis brevis. Origin: Lateral epi-
condyle. Insertion: Base of the third metacarpal. The
muscle is partly fused with the foregoing one, and the
tendons are closely associated on the wrist.

(c) 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).

(d) 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.

(e) The extensor digitorum communis. Origin: Lateral epi-
condyle and proximal end of the ulna. Insertion: By four
tendons on all phalanges of the four lateral digits.

(/) The extensor digiti quarti proprius. Origin: Lateral epi-
condyle. Insertion: Ungual phalanx of the fourth digit.

(g) The extensor digiti quinti proprius. Origin: Lateral epi-
condyle and lateral surface of the ulna. Insertion: Head
of the fifth metacarpal and base of the first phalanx of this
digit.

{h) The extensor carpi ulnaris. Origin: Lateral epicondyle
and proximal portion of the lateral surface of the ulna.
Insertion: 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

268

ANATOMY OF THE RABBIT

Lateral

or posteromedial surface of the radius and ulna. Insertion volar,
the tendons (except that of a) passing under a very strong trans-
verse carpal ligament, which stretches from the navicular and
greater multangular to the pisiform and hamate bones.

(a) The pronator teres. Origin: Medial epicondyle. Insertion:

Ventral surface of the radius.
{h) The flexor carpi radialis. Origin: Medial epicondyle. In-
sertion: Base of the second metacarpal.
{c) The flexor digitorum sublimis. Origin: In common with
the ulnar portion of the profundus from the medial epicon-
dyle; proximal portion
Dorsa/ of the ulua. Insertion:

Bases of the second phal-
anges of the four lateral

Medial digits.

{d) The palmaris. Origin:
Medial epicondyle. In-
sertion: Superficially on
the volar fascia. This
extremely slender muscle
lies between the super-
ficial portion of the pro-
fundus and the flexor
carpi ulnaris.
{e) The flexor digitorum pro-
fundus. Origin in four
portions. Superficial

portion: Medial epicon-
dyle. Radial portion: Ventral surface of the radius. Mid-
dle 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. In
exposing these, care should be taken not to destroy the
flexor digiti quinti (6, a).

The flexor carpi ulnaris. Origin: Medial epicondyle and
medial surface of the olecranon, forming two short but
separate heads. Insertion: Pisiform bone.

Fig. 103. Transverse section of the distal end of
the forearm. Showing the relative positions of the
muscle tendons: ap, abductor poUicis; ar, radial
artery; au, ulnar artery; ecu, extensor carpi ul-
naris; edc, extensor digitorum communis; emp,
extensor digiti quinti proprius; epi, extensor pol-
licis et indicis; eqp, extensor digiti quarti pro-
prius; erb, extensor carpi radialis brevis; erl,
extensor carpi radialis longus; fer, fiexar carpi
radialis; feu, flexor carpi ulnaris; fdp, flexor digi-
torum profundus; fds, flexor digitorum sublimis;
led, dorsal carpal ligament; let, transverse carpal
ligament; nm, median nerve; nu, ulnar nerve; p,
palmaris; r, radius; u, ulna; vc, cephalic vein;
vr, radial vein; \ti, ulnar vein.

(/)

THE ANTERIOR LIMB 269

6. 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. This is a small
muscle superficial to the most lateral division of the tendon
of the flexor digitorum profundus. It represents both the
flexor digiti quinti brevis and the abductor digiti quinti of
some species.

(b) The lumbricales. Three in number. Origin: From the
point of division of the tendon of the flexor digitorum pro-
fundus. Insertion : Medial side of the first phalanx in each
of the third, fourth, and fifth digits.

(c) The adductor digiti quinti, adductor digiti quarti, and ad-
ductor indicis. Three slender muscles. Origin: Close
together on the dorsal part of the tendon sheath of the
flexor digitorum profundus. Insertion: By long thin ten-
dons, respectively, to the radial sides of the fifth and fourth
digits and the ulnar side of the second digit, just dorsal to
the interossei.

(d) The flexor poUicis brevis. A minute muscle. Origin: The
lateral part of the transverse carpal ligament. Insertion:
The outer side of the base of the first phalanx of the pollex.

(e) 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 metacarpo-
phalangeal joints. The fibres of each pair interlace so that
the members cannot be smoothly separated. Differentia-
tion into volar and dorsal interossei is suggested, but they
cannot be dissected apart.

Blood-Vessels and Nerves of the Arm and Forearm

The axillary artery gives rise to posterior and often anterior

branches before turning into the arm as the brachial artery. The

anterior and posterior circumflex arteries to the neck of the humerus

and adjacent muscles may arise as one or several branches, of which

270 ANATOMY OF THE RABBIT

the largest (posterior) passes between the coracobrachialis and the
teres muscles, giving branches to the deltoideus and to the proximal
ends of the lateral and long heads of the triceps. It then continues
(ramus descendens) 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 distal part of the distribution of the vessel just described
corresponds with the distal part of the deep or superior profunda
artery of the human subject. The deep artery (a. profunda brachii)
of the rabbit is a small and variable vessel originating from the
beginning of the brachial artery. It lies behind the radial, median,
and ulnar nerves, accompanying the first for a short distance, and
supplies the long head of the triceps.

The brachial artery (a. brachialis), the continuation of the
axillary, passes distad on the medial surface of the arm between the
biceps and the medial head of the triceps. Crossing to the anterior
surface of its distal extremity, it passes beneath the head of the
pronator teres to the medial surface of the radius, dividing at this
point— a short distance in front of the elbow — into the median and
ulnar arteries. Its chief branches on the arm are the ulnar col-
lateral arteries (superior, middle, and inferior) to the muscles and
the elbow-joint. It soon gives off an interosseous branch which
runs along between radius and ulna.

The median artery (a. medianus) 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. A small radial artery may be given
off before the bifurcation into median and ulnar and may run superfi-
cially distad in the forearm. Toward the distal end of the radius the
median artery crosses the ventral surface of the tendon of the flexor
carpi radialis, and appears in a superficial position on the medial bor-
der of the carpus after giving off a rather large branch, the medianora-
dial artery. Both vessels reach the volar surface of the hand, the
median passing obliquely across the tendon of the flexor digitorum
sublimis and curving laterad to anastomose with the ulnar artery.
It thus forms a volar arch from which branches extend into the
more lateral digits. (It may be noted that the parts supplied in man
by the radial artery are here supplied mainly by the median vessel.)

THE ANTERIOR LIMB 271

The ulnar artery (a. ulnaris) crosses the uhia obHquely 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 median and ulnar veins, which accompany
the corresponding arteries and join one another at the point of
separation of the latter. The median 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 median 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 acromial portion of the deltoideus
and the lateral head of the triceps. It disappears from this surface
in the triangular space enclosed by these muscles and the insertion
of the levator scapulae major, receiving at this point a large tribu-
tary from the shoulder. It appears on the medial surface of the
shoulder at the distal end of the axillary border of the scapula
between the teres major and the subscapularis; entering the axillary
vein at about the same place as the subscapular vein, or 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 medial
head of the triceps, appearing afterwards on the lateral side of the
brachialis in company with the collateral radial artery. A super-
ficial ramus, given off on the distal portion of the arm, accompanies
the cephalic vein : it passes along the surface of the extensor carpi
radialis, dividing into branches for the dorsum of the hand. The
remaining portion is chiefly distributed as the ramus profundus to
the extensor muscles of the forearm.

272 ANATOMY OF THE RABBIT

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 fore-
arm, 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

Dissect on the side opposite to that of injection. The dis-
section 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.

The skin should be removed first from the thigh and back to
the mid-dorsal line, that on the leg and foot being stripped off later,
when the muscles of these parts are to be examined.

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 on account of the fixed condition of the pelvic girdle, they
combine the characters of vertebral and appendicular muscles.

(a) The psoas minor. Origin: Bodies of the four posterior
lumbar vertebrae. Insertion: Pecten of the pubis. The
flat, pointed tendon forms a right-angled cross with a liga-
mentous band which is stretched transversely from the

THE POSTERIOR LIMB 273

middle of the inguinal ligament to the centre of the ventral
surface of the body of the first sacral vertebra. On this
dorsal (sacral) continuation of the inguinal ligament some
of the superficial fibres of the psoas minor tendon are
inserted.

It is necessary to divide the inguinal ligament and reflect
its sacral continuation, together with the tendon of the
psoas minor.

(b) 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.

(c) The iliacus. Origin: Bodies of the last lumbar and first
sacral vertebrae, extending to the sacroiliac union and the
body of the ilium behind it as far back as the ventral border
of the acetabulum. Insertion: 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, though, as an individual
variation, the fifth lumbar nerve may be the most anterior to appear
in this position. The fifth and sixth lumbar nerves usually together
give rise to the greater part of the femoral nerve (p. 280), 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 remaining 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 median line, the fourth lumbar nerve
being divided.

(d) The quadratus lumborum. Origin: Bodies of the posterior
five thoracic vertebrae and the bases of the corresponding
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 surface.

274 ANATOMY OF THE RABBIT

2. Aluscles arising from the pelvic girdle and sacrum and inserted
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 (p. 278), which must be examined and divided. To
begin the dissection, identify the sciatic vein (p. 280) on the
lateral surface of the thigh and cut the fascia along the proximal
part of its course, where it runs in a nearly transverse direction.
This procedure will free the first portion of the biceps in front from
the superficial head of the semimembranosus behind. A continu-
ation of the same incision distad and towards the front of the thigh
along a line which is usually clearly visible, and which delimits the
first portion of the biceps from the second, will separate these two
portions. Now cut along the tendinous line which may be observed
joining the tip of the great trochanter with the sacrum and carry
the incision distad along the intermuscular septum of the lateral
surface of the thigh to the knee. The first head of the biceps,
having been freed both in front and behind by these manipulations,
should now be raised slightly by working the handle of the scalpel
under its distal portion. Divide the muscle, starting at its posterior
margin and being careful not to injure the slender tensor fasciae
cruris muscle and the branches of the sciatic nerve which lie beneath
it. Reflecting the proximal end will expose the short muscles of
the thigh.

Dissect on the lateral surface posteriorly.

(a) 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 POSTERIOR LIMB 275

{b) The glutaeus medius. Anterior border of the wing of the
iHum and the ihac 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 trochanteric fossa.

The muscle should be divided.

(c) 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.

(d) The tensor fasciae latae. Origin: Anterior portion of the
ventral border of the wing of the ilium. Insertion: Broad
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.

(e) The piriformis. Origin : Lateral portions of the second and
third sacral vertebrae. Insertion : Tip of the great trochan-
ter. The muscle passes through the greater sciatic notch.

The muscle should be divided, care being taken to avoid
injury to the nerves and blood-vessels beneath it.
(/) The gemellus superior. Origin: Tendinous from the
ischial spine and fleshy from the body of the ischium im-
mediately in front of it. Insertion: Lateral wall of the
trochanteric fossa, by a thick tendon common to this and
the next two muscles.

The muscle extending from the ischial spine to the sacrum is the
abductor caudae anterior (p. 343).

(g) The obturator intemus. Origin: Internal surface of the
coxal bone all round the edge of the obturator foramen,
extending forward along the medial surface of the ilium to
the sacroiliac articulation, where a few fibres are attached
to the sacrum. Insertion: Trochanteric fossa. The muscle
passes over the ischium in the lesser sciatic notch, only its
thick, white tendon of insertion 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.

276 ANATOMY OF THE RABBIT

(h) The gemellus inferior. Origin: Posterior portion of the
superior ramus of the ischium and the ischial tuberosity.
Insertion: Trochanteric fossa.

(i) 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 remain-
ing ones below the trochanteric fossa.

(j) 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 pectineus and ad-
ductores brevis and longus.

Dissect on the medial surface posteriorly, after examination
and division of the sartorius and the gracilis (p. 278).

(k) The pectineus. Origin: Pecten of the pubis. Insertion:
Immediately below the lesser trochanter.

(/) The adductor brevis. Origin: Anterior portion of the
symphysis pubis. Insertion: Below the pectineus.

(w) 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 in-
serted on the proximal portions of the tibia and fibula (extensors
and flexors of the leg) (Fig. 104).

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 (the stout ligament connecting the patella
to the tuberosity of the tibia).

(a) The rectus femoris. Origin in two portions. First portion:
Superior anterior spine, fused with the tensor fasciae latae.

THE POSTERIOR LIMB

277

and ventral border of the iliac wing. Second portion : By
a stout round tendon from the inferior anterior spine, im-
mediately in front of the acetabulum. This part is cylindri-
cal 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
thickened fascia attached to the lateral surface of the femur
behind the proximal

end of the muscle).

The muscle should
be divided.

(c) The vastus inter-
medius. Origin in two
portions. First por-
tion: Great trochan-
ter, below the origin
of the vastus lateralis.
Second portion : Ante-
rior surface of the
femur.

(d) The vastus medialis.
Origin: Medially,
at the base of the
neck of the femur and
the adjacent portion
of the shaft. Common
insertion (a-d) : Tibial
tuberosity.

B. Flexor group (ham-
string muscles) . With the ex-
ception of the sartorius, the
muscles lie behind the axis of
the femur, and are inserted on
the medial and lateral sur-
faces of the knee-joint and the
corresponding proximal por-

Postenor

Fig. 104. Transverse section through the middle of
the thigh: a.l., adductor longus; a.m., adductor mag-
nus; a.s.m., femoral artery-; b.f. 1 and b.f. 2, first and
second heads of the biceps femoris; f., femur; gr.,
gracilis; n.p., peroneal nerve; n.s.m., greater sap-
henous" nerve ; n.t., tibial nerve; r.f. 1 and r.f. 2, first
and second heads of the rectus femoris; s, sartorius;
sm., semimembranosus; St., semitendinosus; t.f.c,
tensor fasciae cruis; t.f.l., tensor fasciae latae; v.i. 1
and vi. 2, first and second heads of the vastus inter-
medius; v.is., sciatic vein; v.l., vastus lateralis; v.m.,
vastus medialis; v.s.m., great saphenous vein.

278 ANATOMY OF THE RABBIT

tions of the leg. They form the boundaries of the popHteal fossa.

(a) The sartorius. Origin: Posterior portion of the inguinal
ligament, especially its sacral extension. Insertion: Medial
condyle of the tibia. This muscle is an extremely thin and
narrow band of fibres, lying on the more anterior portion
of the medial surface of the thigh. It is fused distally
with the gracilis, and is a flexor only through its connection
with the latter, since its position is that of a rotator.

(b) The gracilis. Origin: The entire extent of the pubic
symphysis. Insertion: Through a broad tendinous expan-
sion ending in the fascia of the proximal portion of the
medial surface of the leg. The muscle forms a broad,
comparatively thin sheet, covering the posterior portion
of the medial surface of the thigh. Its insertion tendon is
perforated by the great saphenous artery and vein and the
greater saphenous nerve.

The sartorius and gracilis should be raised from the
surface and divided.

(c) 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 tri-
angular 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
(see below). 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.

Directions for freeing the biceps and dividing its first portion
have been given on page 274, this procedure being necessary to
expose the short thigh muscles. The freeing of the second portion
should now be completed, an incision first following the sciatic vein to
the posterior margin of the muscle, and the muscle should be divided.

THE POSTERIOR LIMB , 279

(d) 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.

(e) 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 cal-
caneus).

(/) 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.

Blood-Vessels 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 posteriorly 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 the limb, chiefly to the adductores
longus and magnus. A second branch, the lateral circumflex artery
(a. circumflexa femoris lateralis), is given off from the anterolateral
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
intermedins, on the other. It supplies various parts of the quadri-
ceps 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. 221). 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

280 ANATOMY OF THE RABBIT

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 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. 255) 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 distributed 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 correspond-
ing artery. Its tributaries comprise the great saphenous, super-
ficial epigastric, and lateral circumflex, and the deep vein of the
thigh.

The sciatic vein (v. ischiadica) traverses the lateral surface of
the thigh near its posterior margin, lying between the biceps and
the semimembranosus proximally and, more distally, on the pos-
terior margin of the former. At the dorsal border of the ischium,
in front of the ischial tuberosity, before passing inwards to join the
hypogastric vein, it receives the lateral caudal and internal pu-
dendal veins.

The femoral nerve (n. femoralis) arises from the lumbo-sacral
plexus, chiefly from the fifth and sixth lumbar nerves (p. 289). Its
position between the psoas major and iliacus muscles has already
been noted (p. 273). 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 great 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

THE POSTERIOR LIMB 281

muscle, and then turns and extends distad through the thigh, where
it Hes on the lateral surfaces of the adductores magnus and longus.
It distributes 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
branch is the peroneal nerve (n. peronaeus), the posterior branch
the tibial nerve (n. tibialis). The lesser saphenous nerve (p. 288)
is a small branch given off from the tibial above the knee-joint.

For the origin of this and related nerves see p. 289.

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 sciatic artery backward to the ischial tuber-
osity, 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 gluteal nerves originate from a loop connecting the seventh
lumbar and the first sacral nerves and the posterior cutaneous
nerve is described as having the same connection in the rabbit.
The latter nerve may be found, however, connected chiefly with
the second and third sacral nerves (as in various other mammals,
such as the cat) and associated with the pudendal and visceral
branches. The last-mentioned arrangement is shown in figure 106.

The pudendal nerve (n. pudendus) accompanies the sciatic
artery and afterwards the internal pudendal to the penis or clitoris.

The inguinal lymph nodes were observed at an earlier stage of the dissection
(p. 221). These and a popliteal lymph node receive the subcutaneous lymph
vessels of the hind limb, which can be seen only if specially injected. From them,
lymph vessels run to a group of small iliac lymph nodes associated with the
common iliac arteries and veins, into which nodes also the deep lymph vessels
of the limb are emptied and from which arise lumbar trunks running forward
in the lateral walls of the aorta.

282

ANATOMY OF THE RABBIT

In preparation for the muscular dissection of the leg, the in-
sertion 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 popliteal 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
posterolateral border of the leg; also the sciatic vein, with the an-
terior tibial vein, of which it is the continuation, and the accessory
small saphenous vein (p. 287). 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

Postenor

Fig. 105. 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.l., extensor digitorum longus; e.h.l.,
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; pi., plantaria; s, soleus; t., tibia; t.a., tibialis anterior; t.f.c, tensor fasciae
cruis (insertion); v. is., sciatic vein; v.s.m., great saphenous vein; v.s.p., small
saphenous vein; 1-4, the peronaei (primus-quartus).

THE POSTERIOR LIMB 283

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. 105).

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 : The medial condyle of
the tibia just behind the tibial collateral ligament (p. 290)
and the anteromedial surface of the same bone from about the
level of the distal end of the tibial tuberosity to the middle of
the length of the bone. Also the proximal half of the middle
third of the posteromedial border of the tibia. Insertion:
The tendon passes round the medial malleolus of the tibia,
beneath the base and along the medial surface of the second
(first functional) metatarsal, and to the dorsal surface of
the basal phalanx of the corresponding digit, where it
unites with the first tendon of the extensor digitorum
longus.

This muscle is also identified as a tibialis posterior with
displaced insertion tendon and is grouped with the flexors.

The posterior tibial artery, the continuation of the great saphen-
ous, and the tibial nerve accompany the tendon in the malleolar groove.

(b) The tibialis anterior. Origin: Lateral condyle of the tibia
and corresponding surface of the tibial tuberosity. In-
sertion: Base of the second metatarsal. 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.

(c) 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
anterolateral 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 portions for insertion on all the phalanges of the
digits.

284 ANATOMY OF THE RABBIT

The muscle may be displaced by dividing the crural
ligament.

The anterior tibial artery and its peroneal branch lie behind this
muscle, the former in a medial position, in contact with the tibia, the
latter on the peronaei muscles in company with the peroneal nerve.

B. 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) The peronaeus longus (p. primus). Origin: Lateral con-
dyle 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.

(b) 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
metatarsal.

(c) The peronaeus tertius (p. digiti quinti). Origin: The head
of the fibula and the crural interosseous ligament, fused
with the flexor 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.

(d) The peronaeus quartus (p. digiti quarti) . Origin : The fibula
and the interosseous ligament, fused with the peronaeus
brevis and with the flexor digitorum longus. Insertion:
Head of the fourth metatarsal.

C. 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) The triceps surae comprises:

(1) The gastrocnemius. Origin in two portions. Caput
laterale: Lateral condyles of tibia and femur and related

THE POSTERIOR LIMB 285

femoral sesamoid. Caput mediale: The main origin is
on the medial condyle of the femur and its sesamoid, but a
smaller portion of the muscle originates in part with
(immediately dorsal to) the caput laterale and in part by
a flat tendon from the lateral edge of the patella.
(2) The soleus: Origin: By a strong tendon from the
head of the fibula.

Insertion: Through the Achilles' tendon (tendo cal-
caneus). The latter passes over the posterior end of the
tuber calcanei, 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
saphenous nerve.

(b) 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. Each of these parts is perforated near its
termination by a tendon of insertion of the flexor digitorum
longus.

The two muscles should be divided.

(c) 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.

(d) 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 deve^loped digits.

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.

286 ANATOMY OF THE RABBIT

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 adductor indicis and the adductor minimi digiti. Two
extremely slender slips of muscle. Origin : Near the middle
of the dorsal wall of the tendon-sheath of the flexor digi-
torum longus. Insertion : By long, thin tendons respective-
ly to the lateral side of the first phalanx of digit two and
the medial side of that of digit five.

(c) The interossei (metatarsi). Origin: From the dorsal por-
tion of the tendon-sheath of the flexor digitorum longus,
external and distal to the origin of the adductors. Insertion :
Heads of the four metatarsals.

Vessels and Nerves of the Leg and Foot

The great saphenous artery passes distad on the medial sur-
face 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 an-
terior surface of the popliteus, and afterwards the anterior surfaces
of the tibia and fibula by passing between their proximal ends. It
distributes branches to the muscles about the knee-joint, including
a branch to the distal portion of the vastus lateralis, which is given
off near the same point as 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, the peroneal artery, given off in
the upper part of the leg also reaches the dorsum of the foot from
a more lateral position.

THE POSTERIOR LIMB 287

A branch of the popliteal artery supplying the flexor digitorum longus
represents the posterior tibial artery of the human limb. In the latter, the great
saphenous artery is lost and the peripheral part of its distribution (the posterior
tibial artery as described above) has been taken over by the branch indicated.
A comparable arrangement can occur as an individual variation in the rabbit.
In man, the peroneal artery is a branch of the posterior tibial and runs distad
behind the fibula.

The small saphenous artery (a. saphena parva) rises from the
popliteal and 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. It continues in the thick lateral
superficial fascia to the lateral aspect of the calcaneus and ramifies
extensively to the structures about the dorsal, lateral, and plantar
surfaces of the heel, passing mediad across the plantar surface of the
tarsus to anastomose with a small branch of the posterior tibial
artery. A branch given off about the level of the lateral malleolus
accompanies the peroneal tendons to the dorsum of the foot.

The great saphenous vein (v. saphena magna), a large tributary
of the femoral, accompanies the corresponding artery and the
greater saphenous nerve. It is a continuation of the posterior
tibial vein (v. tibialis posterior) from 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, where this vein has been
formed by tributaries accompanying the distal branches of the
small saphenous artery.

The sciatic vein is the continuation of the anterior tibial vein
(v. tibialis anterior), which runs along the lateral surface of the
leg. The anterior tibial receives the accessory small saphenous
vein (v. saphena parva accessoria) from the posterior surface and
drains the dorsum of the foot, passing to the fibular side of the
crural ligament. It reaches the region of the medial malleolus but
does not pass this in the rabbit.

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 to supply the skin.

288 ANATOMY OF THE RABBIT

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 mal-
leolus 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 or sural nerve separates from the tibial
before it reaches the gastrocnemius muscle and accompanies the
small saphenous artery and vein on the posterior margin of the
lateral head of the gastrocnemius. It is distributed in the skin and
fascia of the ankle and heel, one terminal branch passing under the
external malleolus to the lateral and ventral surfaces of the calcaneus.

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
exposed by 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,
traversing the leg at first behind the peronaeus longus and then
around its medial margin to the front of its tendon, where it
becomes associated with the peroneal artery. It passes with the
latter over the fibular side of the crural ligament and branches over
the whole dorsal surface of the foot (a distribution somewhat more
extensive than in most animals). The nerve distributes branches
to the tibialis anterior, to the extensor digitorum longus, and to
the peronaeus muscles.

In man and most mammals, a common peroneal nerve divides into a super-
ficial and a deep branch, but the latter appears to be absent in 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 thus exposed. The posterior portion of the psoas and
iliacus muscles may be picked away with the forceps, and the

THE POSTERIOR LIMB

289

Ivi

m ^^"

ns ^

npci

Fig. 106. Ventral view
of right lumbosacral
plexus, cl, first caudal
nerve; 1 IV- VII, fourth
to seventh lumbar
nerves (ventral rami) ;
nf, femoral nerve; ng,
gluteal nerves; no,
obturator nerve ; np,
pudendal nerve ; npc,
posterior cutaneous
nevye ; ns, sciatic nerve ;
sI-IV, first to fourth
sacral nerves.

abductor caudae anterior muscle (p. 343)
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. 106). It is divisible into a lum-
bar plexus (plexus lumbalis), from which
arises the femoral nerve, and a sacral plexus
(plexus sacralis), from which arises the sciatic
nerve. It is subject to certain variation.

The femoral nerve is formed usually
from the fifth, sixth, and seventh lumbar,
especially from the loop connecting the fifth
and sixth (ansa lumbalis ii). 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 distributed 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

in).

The internal pudendal nerve is formed
from the loop connecting the second and third
sacral nerves (ansa sacralis ii) , but chiefly from
the second, and the posterior cutaneous nerve
may also connect with the same roots.

The Articulations of the Posterior Limb
The more perfect development and larger
size of the joints of the posterior limb make
them much more favourable for examination
than the corresponding parts of the anterior
limb.

The muscular attachments should be re-

290 ANATOMY OF THE RABBIT

moved 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 (ven-
tral) 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 glenoidale)
is the ring of fibrocartilage surrounding the margin of the acetabu-
lum and connecting with the articular capsule.

B. The knee-joint (articulatio genu) is a hinge-joint or gin-
glymus 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 interarticular
fibrocartilages (see section, Fig. 27).

The tibial collateral ligament (lig. collaterale tibiale) is a stout
band of connective tissue stretching from 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 connecting
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
tibial 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, and a pad of soft fat underlies the patellar

THE ARTICULATIONS OF THE POSTERIOR LIMB 291

ligament in such a way as to project into the joint cavity.

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 longus 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 tibia and fibula.

C. The ankle-joint (articulatio talocruralis) is a ginglymus
with a considerable amount of spiral torsion. The articulating
surfaces are formed chiefly by the tibia and talus, but also by the
fibular side of the tibiofibula and the calcaneus. On the tibial side
the calcaneotibial ligament (lig. calcaneotibiale) passes obliquely
anteroventrad and then across the plantar. surface to connect the
medial malleolus with the sustentaculum tali and deep (lateral) to
this a thick talotibial ligament connects the malleolus with the
medial surface of the talus. 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 the short, strong talofibular ligament connecting the

292 ANATOMY OF THE RABBIT

medial side of the lateral malleolus with the lateral and ventral
surfaces of the trochlea tali.

The Bone Marrow

Before the bones are discarded following the foregoing dissection,
an instructive view of the marrowy which is not observed in the
study of dried bones, may be obtained by breaking the femur and
examining its interior. The marrow is a mass of reticular con-
nective tissue with, in long bones such as the femur, a predominance
of fat cells. It contains numerous vessels and is one of the principal
sites of development of erythrocytes as w^ell as producing white
blood cells. The vessels are accompanied by sympathetic (efferent)
and afferent nerve fibres.

X. THE HEAD AND NECK

This dissection includes the various structures of the region,
with the exception 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 median ventral incision of the skin
should be extended forward to the mandibular symphysis and the
skin 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 more posterior part
of the platysma has already been described on page 257. It is a
very thin sheet of muscle originating in the skin over the first two
ribs and extending over the neck and the ventral and lateral aspects
of the head to be inserted in the skin over the cheek. Closely
associated with this muscle and lying immediately beneath it, the
depressor conchae posterior (p. 257) and the depressor conchae
anterior (parotideoauricularis anticus), the latter originating on
the ventrolateral surface of the mandible just in front of the
masseter muscle, meet and are inserted together on the outer part
of the base of the external ear.

The platysma proper is composed mainly of longitudinal fibres inserted in
the skin about the angle of the mouth and the chin. A more superficial la^^er of
roughly vertical and transverse fibres, which nearly always comes away with the
skin, is the sphincter superficialis of the head. A specialized portion of the

THE HEAD AND NECK 293

platysma, the pars zygomatica platysmatis, has acquired an insertion on the
zygomatic bone and is more or less separated from the main muscle. A deeper
transverse layer, the sphincter profundus, is also distinguishable.

In removing the skin of the upper and lower eyelids, the dis-
sector may observe the orbicularis oculi. This is a thin, somewhat
ring-like band of muscle lying directly on the inner surface of the
skin and forming a sphincter round both eyelids, which are closed
by its contraction. The muscle fibres are concentrated at the
anterior and, more particularly, at the posterior angles. The
antagonistic action of raising the upper eyelid is accomplished by
a muscle (levator palpebrae superioris) which arises from the orbital
wall and, at this stage of dissection, is concealed by the projecting
supraorbital process, while the depressor palpebrae inferioris
consists of an extremely delicate group of muscle fibres described
as originating on the zygomatic process of the maxilla and inserted
in the anterior part of the lower eyelid.

Round the mouth, a very thin band of fibres on the skin constitutes the
orbicularis oris, which in the rabbit is inconspicuous and forms a ring inter-
rupted dorsally on account of the cleft upper lip. It is a special portion of the
sphincter profundus mentioned above.

Small lymph glands of irregular occurrence are usually found ventral to
the mandible. These receive the flow from subcutaneous lymphatic vessels and
are drained by other vessels into the superficial lymph glands of the neck, of
which usually two or three are associated with the external jugular veins and
give rise to the jugular trunks. These last empty into the veins near the junction
of the jugulars with the subclavians.

1. On the lateral surface of the head, the following structures may
be made out after removal of the platysma without further dis-
section beyond the clearing of a little fascia.

{a) The parotid gland (gl. parotis), a diffuse, white or brownish
gland lying immediately behind the angle of the mandible.
It expands dorsally to cover the lateral aspect of the
base of the external ear and ventrally beneath the
mandible (Fig. 46). Its duct (d. parotideus) passes forward
across the lateral surface of^the masseter muscle (c) in close
association with the branches of the facial nerve (Fig. 107)
and, perforating the mucous membrane of the cheek, opens
into the oral cavity opposite the last upper molar tooth.

294 ANATOMY OF THE RABBIT

A lymph t;l^i"<^l ^>^ some size is imbedded iii the posterior
aspect of the upi)er part of the parotid.

(b) The chief part of the seventh cranial or facial nerve
(n. faciahs) appears in the anterior portion of the parotid
gh\nd, its branches crossing the masseter. They are dis-
tributed as motor nerves to the cutaneous muscles of the
face, including the platysma.

{c) The masseter muscle. Origin: The zygomatic arch;
tendinous from its anterior angle and fleshy behind. In-
sertion: Lateral surface of the angle of the mandible. This
muscle should not be disturbed at present. It is described
in more detail on i)age 302.

(d) The external maxillary artery (a. maxillaris externa) ap-
pears at tlu; ventral border of the mandible immediately
in front of the masseter. It i)asses dorsad to the region just
in front of the eye, where it ends as the angular artery (a.
angularis). Its chief branches to the anterior portion of the
face are: (1) the submental artery (a. submentalis) to the
chin, a small branch rising near where the external maxillary
crosses the ventral margin of the mandible; (2) the in-
ferior 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. 307).

(e) 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) The subcutaneus faciei, a thin muscle described as origi-
nating on the lateral border of the premaxilla, its frontal
process, and the supraorbital process of the frontal bone
and as being inserted on the skin of the dorsal surface of
the nose, appears to vary in the degree of its development.

rilK HKAD AND xNECK

29.'

(b) The corrugator supercilii, a conspicuous band closely associ-
ated at its ins(;rli(jn with the foregoing and probably
operating in conjunction with the orbicularis oculi in firmly
closing the eye. Origin: Anterodorsal margin of the
zygomatic arch. Insertion: In front of and dorsal to the
upper eyelid.

git

Fio. 107. Lateral view of the head. The insf;rtion portions of the quadratus
labii superioris and the zygomaticus minor, the origin of the caninus, and the
anterior part of the zygomatic arch with the attached portion of the mas.seter
have been removed, as have the more superficial structures. The buccinator
muscle has been divided along two longitudinal lines to reveal the underlying
buccal glands.

b, buccinator muscle; bi, inferior buccal gland; bs, superior buccal gland;
c, caninus, remnant of insertion portion; sc, corrugator supercilii, in.scrtion
portion; dp, parotid duct; fl, facial lymph gland; gi, infraorbital gland; gl,
lymph glands; git. temporal lobe of lacrimal gland; gm, masseteric gland; gms.
superficial mandibular gland; gp, parotid gland; Ian,' levator alae nasi; m, cut
end of zygomatic process of maxilla; mi, sc^Jarable posterior jjart of internal
division of masseter muscle; mt, temporal muscle, origin portion; nf, facial
nerve; nio, infraorbital nerve; qli, quadratus labii inferioris; qls, quadratus
labii superioris; z, zygomatic gland; zm, zygomaticus minor, origin.

(c) The quadratus labii superioris. Origin: The postero-
lateral corner of the nasal bone, the frontal process of the
premaxilla, and the maxillary process of the frontal bone.
Insertion : Skin of the upper lip.^

(d) The zygomaticus minor.. Origin: Anterior surface of the
zygomatic process of the maxilla. Insertion: Skin of the

^The levator alae nasi and zygomaticus minor muscles may he considerefl to
be subdivisions of this muscle.

296 ANATOMY OF THE RABBIT

posterolateral part of the upper lip, in common with (c).

(e) The levator alae nasi. Origin : Ventral part of the maxil-
lary fossa. Insertion: Skin covering the lateral cartilage
of the nose. The muscle is slender and is inserted by a long
tendon which underlies the insertion portions of the two
preceding muscles.

(/) The caninus. Origin: Lateral border of the upper jaw.
Insertion: Hairy portion of the mucous membrane of the
mouth. The muscle is very broad, short, and thin and is
closely applied to the lateral surface of the buccinator.

(g) The buccinator. A broad stout sheet of fibres enclosing
the cheek. Origin: The alveolar borders of the upper jaw
and mandible as well as the anterolateral surface of the
mandible. The insertion portion curves forward into' each
lip to be attached to the lining of the mouth.

{h) The quadratus labii inferioris. Origin: Ventral border of
the mandible. Insertion: Skin of the lower lip.

(i) 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 over-
lies 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 extent should be noted in contrast to the vestigial character
of the ear muscles in man.

Dissection of glands on the lateral aspect of the head (Fig. 107).
The salivary glands are extensively developed in the rabbit. The
largest is the parotid gland, described above (la). The sub-
maxillary gland is described on page 298, the sublingual gland on
page 307, and the zygomatic gland on page 316.

(j) The superior buccal gland, a long, narrow band of loosely
connected lobules internal to the more dorsal part of the
buccinator muscle, which must be divided to expose the
gland. The lobules of the gland are associated with the
superior labial artery and extend from a point just above

THE HEAD AXD NECK 297

the terminal portion of the parotid duct to a position
dorsal to the angle of the mouth.

{k) The larger inferior buccal gland is also elongate but thicker,
and comprises three portions. Two long, slender masses
lie side by side internal to the more ventral part of the
buccinator muscle, which must be cut lengthwise to expose
them. The thicker, more dorsal of the two is replaced
caudally by a third portion which has received the name
masseteric gland.

(/) The superficial mandibular gland, a flattened, oval mass
closely applied to the ventrolateral surface of the mandible,
covered by the anterior part of the platysma, is a cutaneous
gland, from which the ducts open on the skin of the lower

lip.

(m) The facial lymph gland overlies the dorsal edge of the
buccinator muscle and is covered laterally by the zygo-
maticus minor.

3. Dissection on the ventral surface of the neck to free the ex-
ternal 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 be-
hind 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. Ill, p. 327). The last-mentioned vessel crosses ventral to the
common carotid artery and the sternohyoid muscle a short distance
anterior to the tip of the manubrium sterni.

The posterior facial vein is formed in front of the base of the
ear by the union of the superficial temporal vein, which runs
forward immediately dorsal to the external auditory meatus after
emerging from the cranial cavity, and the external ophthalmic
vein, which passes back from the orbit.

The superficial temporal vein receives blood from the brain through the
transverse sinus, which emerges through a foramen between the squamosal and

298 ANATOMY OF THE RABBIT

petromastoid bones near the tip of the squamosal process of the parietal. As it
runs along the lower margin of the temporal muscle, it is joined by one or more
small deep temporal veins from the substance thereof. The posterior facial is
joined by the anterior auricular vein from the ear and then passes downward
through the parotid gland, receiving the small transverse facial vein and being
crossed by the facial nerve. Immediately below the latter it receives the posterior
auricular vein from the ear and the back of the head. At about the same
level it is joined by a deep vessel, the posterior internal maxillary, emerging
from behind the mandible.

In addition to the tributaries described above (p. 294), 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 downward
beneath the masseter muscle. The anterior facial vein receives at
the ventral border of the mandible the internal maxillary vein
(v. maxillaris interna). The latter also begins in the orbit, where
it is connected with the deep facial. It is also identified as the
sublingual vein. 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.

An anastomotic branch connects the deep facial vein with the inferior
alveolar vein through the foramen at the ventral end of the sulcus ascendens of
the mandible. This provides an outlet through the latter vein for the blood from
the former when its passage is obstructed by the pressure of the contracting
masseter and internal pterygoid muscles.

A small, unpaired median submental vein enters the anterior facial of
one side.

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, is 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-coloured duct
(d. submaxillaris) may be seen running upward and slightly
forward to enter the mouth (cf. p. 307). It crosses the
lateral surface of the digastric muscle but is medial to the
external maxillary artery and is approximately paralleled

THE HEAD AND NECK 299

by a branch from this artery to the gland and by a corre-
sponding tributary of the anterior facial vein.

(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, the latter
being overlapped ventrally by a part of the former.

(c) The digastricus. Only its insertion portion is visible (the
origin being by a long, round tendon from the stylohyoid
ligament, and so from the jugular process of the occipital bone

mh \

aem

Sgl

Fig. 108. Medial and somewhat ventral view of the muscles of the right half of the lower
jaw. aem, external mandibular artery; d. digastricus muscle; Ic, longus capitis muscle; Is,
levator scapulae major (with basioclavicularis) muscle ; mh, mylohyoid muscle (right half) ; .
mm, medial insertion-portion of masseter muscle; pa, parotid gland; pi, pter\-goideus
internus muscle; pj, jugular process; pp. pterygoid process; rca, rectus capitis anterior
muscle; sd. duct of submaxillary gland; sg, styloglossus and stylohyoideus minor muscles;
sgl, submaxillary gland (displaced ventrad) ; sm, mandibular symphysis; smj, stylohyoideus
major muscle.

— Fig. 108). It passes forward along the medial surface of
the mandible, on the anterior ventral portion of which surface
it is inserted. In man and some lower primates, a second
fleshy portion occupies the position of the posterior part of
the tendon of origin, w^hence the muscle derives its name.

(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

300 ANATOMY OF THE RABBIT

the opposite side, from the manubrium sterni. Insertion:
Mastoid process of the skull.

(/) 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.

The two muscles are closely associated in the middle
line. They should be separated from each other and
divided.

(g) 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.

(h) The trachea occupies a median position. It is supported
by cartilaginous tracheal rings, each of which is incomplete
dorsally thus allowing brief partial compression.

(i) The thyreoid cartilage of the larynx ; a saddle-shaped
cartilage, composed of right and left thyreoid plates
(Fig. 91) completely fused with each other ventrally.

(j) 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.

(k) The deep cervical lymph gland (lymphoglandula cervicalis
profunda) is a large, elongated, reddish-coloured gland
in the upper portion of the neck, opposite the thyreoid
cartilage.

This gland receives lymph vessels from the root of the tongue,
the pharynx, and the larynx and empties into the jugular trunk through
a vessel which accompanies the internal jugular vein.

(/) The thyreoid gland (gl. thyreoidea) lies on the ventral and
lateral surfaces of the trachea behind the cricoid cartilage.
It is composed of right and left portions connected across

THE HEAD AND NECK 301

the middle line by a thin median portion, the isthmus
(cf. p. 133).

(w) 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
thyreoid gland, the oesophagus, the larynx, and the
cricothyreoid muscle of the larynx (inferior laryngeal
artery). The (superior) laryngeal artery rises from the
upper end of the common carotid or the beginning of the
external carotid artery near the level of the anterior edge
of the thyreoid plate and accompanies the superior laryngeal
nerve through the thyreoid foramen to the interior of the
larynx after sending branches to the oesophagus, the hyo-
thyreoid membrane, and the hyothyreoid, sternothyreoid,
and sternohyoid muscles.

(n) 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.

(o) 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
and the internal jugular vein. It gives off the n. laryngeus
superior to the larynx, this nerve crossing the dorsal side
of the common carotid artery.

The superior laryngeal nerve passes through the thyreoid foramen
(p. 313) into the larynx and supplies sensory fibres to the mucous
membrane of the larynx and motor fibres to the cricothyreoid muscle.
The vagus is a mixed nerve, containing both afferent and efferent
fibres. Its action on the heart is inhibitory, that on the stomach is
excitatory. Section of the nerves increases heart beat.

(p) The ramus descendens of the twelfth cranial or hypo-
glossal nerve (p. 309) crosses the root of the vagus from a
lateral to a medial position. It passes backward on the
ventral surface of the common carotid artery, and is dis-
tinguishable chiefly by its branches to the sternohyoideus
and related muscles.

302 ANATOMY OF THE RABBIT

(q) 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.

Section and stimulation of the sympathetic in the neck is one of the
classic demonstrations of vaso-motor action. The result of section can
be seen in reddening and loss of heat in the ear (vasodilatation), and
contraction of the pupil of the eye. Stimulation has the opposite effect
(vaso-constriction) .

(r) 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, arising at the level of the pos-
terior margin of the thyreoid cartilage. It is an afferent
nerve. Its fibres are said to originate from cells in the upper
pole of the jugular ganglion (a mass of nerve cells forming
two slight swellings on the vagus nerve just before it
emerges from the skull). Those of the left terminate peri-
pherally in the arch of the aorta, those of the right in the
subclavian, being distributed along with other nerve fibres
in the cardiac plexus.

Occurring in the rabbit as a separate nerve, the depressor is im-
portant 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 to reflex action on the blood-vessels (cf . p. 64) ,
while the latter depends upon reflex inhibition by impulses passing
through the vagus, as is shown by the fact that slowing of the heart
does not take place if the vagi also are divided.

(s) The third and fourth cervical nerves may be traced from
their origin in the intervertebral foramina to the muscu-
lature of the neck. They encircle the basioclavicularis
muscle, under cover of the sternomastoideus and cleido-
mastoideus.

5. Dissection of the muscles of mastication and related structures
of the mandible.

(a) The masseter muscle. Origin: The zygomatic arch. In-
sertion: Lateral surface of the angle of the mandible (1, a),
also that of the ramus. Some of the most anterior fibres
curve round the ventral edge of the mandible and pass back
medial to it, covering the ventral part of the internal

THE HEAD AND NECK 303

pterygoid (c). They are inserted along the ventral edge
almost to its posterior extremity.

The masseter muscle consists of external and internal divisions
which are readily separable posteriorly but not anteriorly. The ex-
ternal originates from the lateral surface of the zygomatic arch along
slightly less than the anterior half of its length, tendinous from its
anterior angle and fleshy behind that. Its insertion is near the ventral
edge of the mandible. The main part of the internal division takes
origin from the internal surface of the same part of the arch and is
inserted dorsal to the external division. In the rabbit a readily separable
portion, probably to be included with the internal division, originates
from the remainder of the inner surface of the arch and is inserted on
the lateral surface of the ramus of the mandible.

The orbital structures should be freed from the zygo-
matic arch by passing a knife along its dorsal margin. The
zygomatic processes of the maxilla and of the squamosal
bone should then be divided and the zygomatic arch should
be removed, together with the whole of the masseter
muscle, which should be cleanly cut from its attachment
to the mandible. Care should be taken not to injure the
insertion tendon of the temporalis muscle just internal
to the arch.
(b) The temporalis is a slender muscle, being much smaller in
the rabbit than in many mammals. It originates in the
reduced temporal fossa (sulcus temporalis) of the skull
back to about the posterior margins of the squamosal and
parietal bones and is inserted by a long stout tendon on the
edge and adjacent part of the lateral surface of the reduced
coronoid process. Fibres from the masseter and the su-
perior portion of the external pterygoid muscles are at-
tached along the side of the tendon. The muscle may be
exposed by dividing the temporal portion of the posterior
supraorbital ligament which holds its tendon in place, and
the muscles in front of the external ear if these are still in
position. The temporal muscle itself may then be divided.
On account of the narrowness^ of the space between the two
limbs of the mandible and the great depth of its angle, it is neces-
sary, 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

304 ANATOMY OF THE RABBIT

mandible. A better understanding of the attachments and re-
lations of the muscles of mastication may be obtained, however, if
this is done in steps, as follows.

Divide the mandibular symphysis and free one half of the
mandible from its attachments to the lips and to the lining of the
mouth. Pass a scalpel along the ventral part of the medial surface
of the bone so as to detach the digastricus and mylohyoideus
muscles and also the pterygoideus internus (see below). The
insertion of the pterygoideus externus, which is more dorsally
situated, should remain intact. The tip of the knife should be
kept close to the bone so that the underlying soft parts, except in
that they are divided at their attachments, will be kept uninjured,
the medial surface of the mandible being clean when removed.
The ventral edge of the mandible should then be turned laterad
so as to rotate the bone towards a horizontal position, thereby
exposing the following structures without further injury to any.
(In order that this may be done, the zygomatic process of the
squamosal bone must have been cut through its most dorsal part
when the zygomatic arch was removed, as directed on the previous
page.)

The structures appearing on the cut surface include the in-
sertion of the digastricus and the margin of the mylohyoideus; also
the following parts are exposed.

(c) The pterygoideus internus muscle. Origin: Pterygoid
process of the skull. Insertion: Ventral portion of the
medial surface of the angle.

Still intact and attached to the mandible are :

(d) The pterygoideus externus. The muscle comprises two
portions. Superior head. Origin: Infratemporal surface
of the alisphenoid. Insertion: Medial surface of the ramus
of the mandible and sulcus ascendens.

This portion has also been described as a division of the temporalis.
Inferior head. Origin: Posterior edge and both surfaces
of the lateral plate of the pterygoid process. Insertion:
The depression in front of the neck of the mandible, the
interarticular cartilage of the temporo-mandibular joint,
and the whole medial edge of the head of the mandible.

THE HEAD AND NECK 305

It will be observed that the more powerful of the jaw-muscles
are those which raise and protract the mandible, the combination
of movements in these two directions being particularly important
in animals with rodent habits. Raising is accomplished by the
masseter, particularly its internal division, and the internal
pterygoid, aided by the superior head of the external pterygoid and
the temporalis. Protraction is the work of the masseter, particularly
its external division, and of the inferior head of the external
pterygoid. Retraction is a weaker movement accomplished mainly
by the posterior part of the internal division of the masseter
with some aid from the superior head of the external pterygoid
and a little from the temporalis (all three pulling both upwards and
backwards.) Lowering of the jaw is the work of the digastricus,
aided by a simultaneous forward pull of the inferior portion of the
external pterygoid on the head of the mandible.

(e) The inferior alveolar artery (a. alveolaris inferior) lies be-
tween the two pterygoidei, and enters the mandible through
the mandibular foramen. The corresponding inferior
alveolar vein leaves the mandible at this point.

(/) The inferior alveolar nerve (n. alveolaris inferior) accom-
panies the inferior alveolar artery to the mandible. The
continuation of the nerve is the mental nerve. It appears
at the mental foramen, and passes to the lower lip.

The displaced half of the mandible should now be freed from
the foregoing structures and removed entirely.

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).
The mandibular nerve also gives off anteriorly the stout lingual
nerve to the tongue and posteriorly the slender mylohyoid nerve
to the digastric and mylohyoid muscles. 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 points of origin from the skull and removed.

306 ANATOMY OF THE RABBIT

6. The branches of the common carotid artery may be traced in
the anterior portion of the ventral surface of the neck as follows:

(a) The internal carotid artery (a. carotis interna) is a small
vessel given off from the dorsal wall (pp. 172, 361).
The trunk then passes forward as the external carotid
(a. carotis externa).

The internal carotid artery passes dorsad, medial to the styloglossal
and stylopharyngeal muscles, to the base of the skull and enters the
external carotid foramen, traverses the carotid canal, and enters the
cranial cavity through the foramen lacerum to supply the brain.

At its very beginning, the internal carotid artery is very slightly
distended as the carotid sinus, an organ which, though hardly noticeable
in ordinary dissection, is important physiologically. It is a sensory
receptor, stimulation of which by increased blood-pressure causes
impulses to pass through fibres in the ninth, tenth, and eleventh cranial
nerves and to produce reflex vasodilatation.

In the angle between the origins of the external and the internal
carotid arteries is situated another organ too minute for observation
in the gross, the carotid body or glomus caroticum. This is stimulated
by chemical changes in the blood and sends impulses through a special
branch of the glossopharyngeal nerve to alter blood-pressure, heart-beat,
and respiration. The branch in question, the first to separate from the
ninth nerve as it emerges from the cranium, is the intercarotid nerve or
nerve of Hering.

(b) 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 hypoglossal nerve crosses the ventral surface of the artery and
should be kept intact.

(d) 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

THE HEAD AND NECK 307

divided at the point where it passes around the ventral
border of the mandible to the lateral surface of the face.
(e) 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. 318), giving off the inferior
alveolar artery to the mandible.
(/) The superficial temporal artery (a. temporalis superficialis),
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.
7. Dissection of the tongue and hyoid.

The mylohyoideus should be reflected. Note the position
of the lingual nerve.

Dorsal to the anterior part of the mylohyoideus lies the sublingual
gland (gl. sublingualis minor — the gl. sublingualis major is absent in the
rabbit*) from which several small ducts run dorsad between the geniohyoid
and the hyoglossus muscles to the floor of the mouth. The submaxillary duct
turns forward behind the sublingual gland and runs along its dorsal surface
to open on the floor of the mouth near the mandibular symphysis.

(a) The stylohyoideus major muscle. Origin: Jugular process
of the occipital bone. Insertion: Tip of the greater corn u
of the hyoid. The muscle has been divided.

The superficial temporal and internal maxillary arteries
should be divided.

(b) 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.

(c) The stylohyoideus minor. Origin: Jugular process. In-
sertion: Lesser cornu of the hyoid. A slender muscle
having about the same direction, but ending on the more
dorsal part of the hyoid apparatus.

*In many animals, the major sublingual gland is closely associated with the
anterior end of the submaxillary gland and some authors have so designated
the anterior lobes of the submaxillary in the rabbit. This has been denied
however, on the basis of critical embryological studies.

308 ANATOMY OF THE RABBIT

The remaining muscle is the stylopharyngeus, a thin deHcate
muscle, the insertion of which is on the lateral wall of the pharynx.

In man, the tendon joining the two portions of the digastric muscle
usually passes through the stylohyoid muscle. In the rabbit, the
tendon of the digastric passes between the major and minor stylohyoidei.

id) The geniohyoideus. Unpaired. Origin: Mandibular sym-
physis. Insertion : Ventral surface of the body of the hyoid.

(e) The genioglossus. Origin: Medial surface of the mandible
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 mass of fibres with no skeletal attachments.

{h) 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. It contains fibres
for general sensibility and near its point of origin is joined by the chorda
tympani (p. 322), the latter containing gustatory fibres.

{i) The twelfth cranial or hypoglossal nerve (n. hypoglossus)
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.

(j) The ramus lingualis of the ninth cranial, or glossopharyn-
geal nerve (n. glossopharyngeus), enters the base of the
tongue at a point dorsal to the hypoglossus and between
the stylohyoideus minor and the stylopharyngeus. It is a
gustatory nerve of the tongue.

8. Dissection of the extra-cranial roots of the ninth to twelfth

nerves (Fig. 109).

These nerves, which for the most part have already been exposed,
may be traced to their origin in the jugular and hypoglossal fora-

THE HEAD AND NECK

309

mina. The tympanic bulla should be cleared and the tendons of
origin of tongue muscles removed from the jugular process.

(a) The ninth (glossopharyngeal) nerve is farthest forward.
Its two main branches are the ramus lingualis to the pos-
terior part of the tongue, for taste, and the ramus pharyn-
geus, the latter entering the lateral wall of the pharynx.

(b) The tenth (vagus) nerve bears an elongated ganglionic
enlargement, the plexus ganglioformis or ganglion nodosum.

It lies immediately below the jugular for-
amen . The superior laryngeal nerve and
the ramus cardiacus (depressor nerve)
are given off at the level of the origin of
the internal carotid artery. Within the
jugular foramen of the skull, the vagus
bears a slight enlargement, usually double,
the jugular ganglion, and gives off a mi-
nute auricular branch which connects with
the ninth and seventh nerves and then
emerges from the petromastoid bone just
behind the external acoustic meatus to be
distributed in the external ear.
(c) 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 sternomastoideus and cleidomas-
toideus muscles, giving branches to the
latter, and then passes backward to the
ventral surface of the trapezius to which
it is distributed.
(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.

the

extra-cranial roots of the
IX-XIl cranial nerves and
sympathetic trunk ; ven-
tral surface, right side, the
sympathetic and depressor
nerve shown as displaced
from the dorsal surface of
the artery. 9, 10, 11, 12,
glossopharyngeal, vagus,
spinal accessory, and hypo-
glossal nerves; ac, carotid
artery; c, cervical root of
ramus descendens XII;
gn, ganglion nodosum vagi ;
Is, superior laryngeal; nd,
ramus cardiacus vagi (de-
pressor nerve) ; rd, ramus
descendens hypoglossi ; s,
sympathetic.

310 ANATOMY OF THE RABBIT

The fibres composing the ramus descendens do not originate in
the hypoglossal nucleus in the brain but are derived from the most
anterior cervical nerves and are only secondarily included within the
sheath of the hypoglossal.

(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 caro-
tid arteries to the head.

9. The oral cavity and pharynx.

The 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. If a probe is inserted from the oral cavity backward into
the oesophagus and an incision through the lateral wall is made
following this guide, the internal surface of this portion of the
digestive tube will be exposed sufficiently for the study of its
features. The incision divides the constrictor pharyngis muscle,
a broad band of muscle fibres enclosing the posterior portion of the
pharynx.

The constrictor pharyngis has three heads of origin, not readily distinguished :
(a) a very fine band from the base of the external acoustic meatus, (b) a larger
mass from the tip of the medial lamina of the pterygoid process, and (c) a delicate
band attached in the soft palate.

For the general relations of the oral cavity see p. 99 and Fig. 52.

(a) The oral cavity (cavum oris) is divisible into the oral
cavity proper, and the vestibulum oris, the latter lying
between the alveolar processes and teeth on the one hand
and the lips and cheeks on the other.

(b) The pharynx comprises an oral portion (pars oralis), con-
tinuing the canal of which the first division is the oral
cavity and connecting the latter with the oesophagus, and
a dorsal and anterior nasal portion (pars nasalis) or naso-
pharynx, which lies above the soft palate, and receives the
posterior aperture of the nose. Its ventral and posterior

THE ORAL CAVITY 311

laryngeal portion (pars laryngea), not well-defined, con-
tains the aperture of the larynx, the aditus laryngis.
In the oral cavity:

(a) The hard palate (palatum durum) forms the anterior
portion of the roof; its mucous membrane is thrown into
a series of transverse ridges.

(b) The soft palate (palatum molle) is the thin, narrow, pos-
terior, 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 soft palate contains a system of delicate muscles, including the
tensor veli palatini (attached to the anteroventral part of the tympanic
bulla and to the median process on the bony palatine bridge), the
levator veli palatini (originating on the tympanic bulla and inserted in
the free edge of the soft palate), the uvular muscle (origin on the median
spine of the palatine bridge, insertion in the middle of the soft palate,
underlying the buccal mucous membrane), the pharyngopalatine muscle
(scattered fascicles from the dorsal wall of the pharynx spreading
through the soft tissue of the palate), and the glossopalatine muscle
(a very thin system of fibres curving round from the base of the tongue
into the centre of the palate).

(c) The nasopalatine or incisive ducts (dd. nasopalatini) open
by small slits about a millimetre behind the secondary
incisors, each opening being covered by a slight projection
from its medial margin. The ducts connect the anterior
portion of the nasal cavity with the mouth, and a probe
may readily be passed backward along them from their
oral apertures.

(d) 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, hard portion, which forms a considerable rounded
elevation, and an anterior softer and rougher portion.
Both are covered by closely set fine processes, the papillae
operariae, which correspond with the filiform and conical
papillae of the human tongue. These are most typically

312 ANATOMY OF THE RABBIT

developed on the softer anterior part of the tongue, where
single minute low elevations, the fungiform papillae, are
scattered among them. At the posterior end of the smooth
portion, there are on either side a minute spherical ele-
vation, 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. Microscopic taste
buds occur on the fungiform and, especially, on the vallate
and foliate papillae.

In the pharynx:

(a) The tonsils (tonsillae palatinae) are a pair of rounded masses
of lymph follicles each lying on the anterior wall of a deep
lateral depression, the tonsillar sinus (sinus tonsillaris).
The vertical slit-like aperture of the sinus is bounded by
low anterior and posterior folds.

(b) The epiglottis, a valve-like fold guarding the entrance to
the larynx, projects upward from the floor into the pharyn-
geal cavity, past the edge of the soft palate.

(c) By removing the posterior portion of the soft palate, the
connection of the nasopharynx with the nasal fossae will
be exposed. Also, on the lateral wall of the nasopharynx,
there will be visible the pharyngeal aperture of the auditory
tube (ostium pharyngeum tubae), the other end of which
opens into the middle ear.

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 man-
dible outward. The hyoid apparatus, which supports the base of
the tongue, should be cleared and examined (see p. 197).

The small, unpaired, median vertebral vein may be observed on the ventral
surfaces of the vertebrae. This vessel, formed anteriorly by the veins of the
nasal septum, receives a tributary through the foramen cavernosum from the
basisphenoid bone, is joined by paired vertebral veins, and empties into the
posterior end of the external jugular vein of either the right or the left side.

THE LARYNX 313

The laryngeal cartilages should now be thoroughly and carefully
cleared externally b>- the removal of all soft tissues so that the fol-
lowing parts are clearly seen.

(a) 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 at each side projects
forward as the cornu superior, connected by ligament with
the greater cornu of the hyoid. The corresponding postero-
dorsal 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 and just ventral to this a longitudinal ridge serves
for the attachment of the sternothyreoid, thyreohyoid, and
thyreolaryngeal muscles.

(b) The cricoid cartilage (cartilago cricoidea) is annular,
surrounding the first tracheal ring. Its ventral portion,
the arch of the cricoid cartilage, is situated some distance
caudal to the thyreoid cartilage, the intervening space
being largely occupied by the cricothyreoidei muscles. Its
lateral part slants obliquely anterodorsad and expands into
the dorsal portion, the lamina of the cricoid, which is
partly enclosed at the sides by the posterodorsal angles of
the thyreoid laminae and has a firm ligamentous attach-
ment to these. The lamina of the cricoid extends craniad
and forms the larger part of the dorsal wall of the larynx,
its anterior margin having a blunt median point and slanting
obliquely laterocaudad at either side.

(c) The paired arytenoid cartilages (cartilagines arytenoideae)
lie obliquely one on each side of the anterior tip of the
cricoid plate, closely articulated with its margin. Each ap-
pears curved and somewhat irregularly pear-shaped in dorsal
view, tapering to a point anteromedially. From the lateral
angle of the broader posterior end there is a prominent ven-
tral projection for the attachment of one end of a vocal fold.

314 ANATOMY OF THE RABBIT

(d) The corniculate cartilages (cartilagines corniculatae) are
minute, slender, curved bodies composed of very flexible
elastic cartilac^e and borne on the apices or anterior ex-
tremities of the arytenoid cartilages. Each projects craniad
and lies in the dorsal end of the fold of mucous membrane
extending to the edge of the epiglottis and forming the
margin of the opening from the pharynx, the aditus laryngis.

(e) The epiglottic cartilage (cartilago epiglottica) is a thin,
very flexible, curved plate of elastic cartilage covered only
by mucous membrane. It projects upward into the cavity
of the pharynx just in front of the aditus laryngis and is
attached ventrally to the internal surface of the thyreoid
cartilage. At the base of its posterior surface appears a
pair of small but prominent projections, the hamuli.

(/) The vocal folds (plicae vocales), which are rudimentary in
the rabbit, may be seen as vertical folds of the internal
surface of the larynx, especially prominent when the thyreoid
cartilage is bent downward on the cricoid. Each fold is
attached at one end to the thyreoid, at the other end to an
arytenoid cartilage, and forms the posterior boundary of a
shallow pouch, thelaryngeal ventricle (ventriculus laryngis.)

In the rabbit the two laryngeal ventricles unite in a shallow median
ventral depression which extends to between the hamuli epiglottici.

In addition to the cricothyreoidei, the laryngeal cartilages are
connected by several small muscles, including the cricoarytenoidei
posterior and lateralis, the thyreoarytenoideus and the ar3'tenoideus
transversus, the last named being an unpaired muscle connecting the
arytenoid cartilages. These muscles acting together in various ways
modify the shape of the laryngeal cavity and the degree of tension and
of approximation of the vocal folds.

11. 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, which may be observed in the anterior
part of the medial surface of the lower eyelid, to the lacrimal bone,
will be divided. The supraorbital process of the frontal bone may
advantageously be broken away. The muscles and glands of the
orbit may be made out as follows:

THE ORBIT 315

(a) 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.

{b) 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:
Anterodorsal portion of the eyeball.

The oblique muscles are relatively large in the rabbit,
a feature correlated with the lateral direction of the eyes.

(d) The four recti muscles, superior, inferior, medialis, and
lateralis, 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.

{e) The retractor oculi, or retractor bulbi, muscle (best seen
after removal of the eye) originates on the posterolateral
margin of the optic foramen, internal to the origin of the
lateral rectus muscle, and is connected by a fibrous band
through the foramen with the origin of the corresponding
muscle of the other side. It has the form of a hollow cone
with a cleft along its dorsal wall, in w^hich the optic nerve
lies upon a bed of fatty connective tissue. It is inserted on
the medial portion of the eyeball around the optic nerve.
Although the retractor oculi is described as consisting of
four distinct parts, these are fused in the rabbit so that
they are indicated only by sinuosities in the line of insertion
alternating with the insertions of the recti muscles.

(/) The Harderian gland (gl. Harderiana) is a large, compact,
lobulated gland lying in the anterior portion of the orbit,
internal to the inferior oblique muscle. It is composed, in
the rabbit and the hare, of two parts, a large, pale, grey-
reddish, posteroventral lobe and an almost white antero-
dorsal lobe about one-third the size of the former. Both

316 ANATOMY OF THE RABBIT

lobes open by a common duct on the inner surface of the
third eyelid. In embalmed rabbits, both parts may be
brown so that the colour difference may not be very
noticeable; though in the majority of cases the difference is
extremely conspicuous. The presence of this gland, which
is absent in Primates, is associated with that of a well-
developed third eyelid.

Fig. 110. Lateral view of the left orbit after removal of the eyeball, gi, infra-
orbital gland; gl, lacrimal gland; git, temporal lobe of lacrimal gland; gz,
zygomatic gland; Hr, reddish portion of Harderian gland; Hw, v^diite portion of
Harderian gland; m, cut end of zygomatic process of maxilla; mo, cut end of
inferior oblique muscle.

(g) The lacrimal gland (gl. lacrimalis) is a much smaller, darker
coloured, greatly lobulated structure lying close to the skull
in the temporal angle of the orbit. An outlying portion is
situated in the back part of the temporal foramen, where it
overlies the tendon of the temporal muscle. The gland
communicates by several fine ducts with the caudal part
of the inner surface of the upper eyelid.

Described by some authors as inferior lobe of the lacrimal gland,
a similar mass extends forward immediately internal to the z^^gomatic
arch and near the anterior end of the orbit, where the gland expands
considerably, lies dorsal as well as medial to the arch. This is the
infraorbital gland of most authors. Its duct opens near those of the
lacrimal gland proper, a short distance behind and below the posterior
connection of the two eyelids.

The zygomatic gland (gl. zygomatica — infraorbital gland of
earlier editions) is a rather small, white or yellow gland lying in the

THE EYE 317

anteroventral angle of the orbit immediately medial to the zygomatic
arch and ventral to the anterior end of the infraorbital gland described
above. The gland is one of the salivary series, its duct passing down-
ward and opening through the mucous membrane of the cheek into the
cavity of the mouth.

The application of the terms infraorbital and zygomatic to these
two glands is reversed by some authors.

To examine the structure of the eye, the muscles of the eyeball
should be divided at their insertions, and the whole structure
should be removed. The second cranial or optic nerve (n. 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 into medial and lateral hemispheres by
a circular incision, the lateral hemisphere, which contains the lens,
being again divided vertically. The parts should be examined
under water. The chief structures (Fig. 48, p. 92) comprise:

(a) 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
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
sclera is not so extensively exposed in the rabbit as it is in
man (white of the eye), a condition related to the very
small angle of movement in laterally placed eyes like those
of the rabbit.

(b) 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), composed of
numerous, radially arranged ciliary folds (plicae ciliares)
and 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 ciliary body, which in man contains both radial and circular
muscle, has only the former in the rabbit.

318 ANATOMY OF THE RABBIT

(c) The retina, the innermost layer of the eye, forms a thin
soft membrane covering the inner surface of the chorioidea.
It is divisible into 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 inner surface of which it
covers.

A little distance from the centre of the optic portion
can be recognized the disc or "blind spot", where the nerve
fibres in the retina converge and form the optic nerve.

(d) The transparent lens of the eye is suspended by fine fila-
ments, the zonular fibres, reflected from the margin of the
ciliary body.

When the eye is at rest, the zonular fibres are in a state of tension
sufficient to produce a slight flattening of the lens. Contraction of the
muscles in the ciliary body reduces this tension so that the lens may
become more convex by its own elasticity.

(e) 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 large 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. These two
chambers communicate through the central aperture en-
closed by the free margin of the iris, the pupil (pupilla).

12. Following the removal of the eye, the blood-vessels 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) 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

BLOOD-VESSELS AND NERVES OF THE ORBIT 319

angle of the orbit it gives off the inferior ophthalmic
artery (a. ophthalmica inferior). This vessel passes up-
ward 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.

(b) 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 enters the orbit from the superior orbital fissure in com-
pany with certain parts of the trigeminal (see e, f below). The small
nerves passing through the middle and posterior sphenoidal foramina of
the pterygoid process are the pterygobuccinator and masseterico-
tetnporal nerves, branches of the mandibular, passing to the muscles
of mastication,

(d) The sixth cranial, or abducent nerve (n. abducens), is dis-
tributed to the rectus lateralis and to the retractor oculi.

320 ANATOMY OF THE RABBIT

(e) The ophthalmic nerve (n. ophthahnicus), the first division
of the fifth cranial, or trigeminal nerve (n. trigeminus),
accompanies the inferior ophthahiiic artery on the medial
wall of the orbit. It is entirely sensory. It gives off a
lacrimal nerve, which passes upward through the posterior
foramen of the supraorbital process, after giving off small
branches to the lacrimal gland, and is distributed to the
skin of the upper eyelid. The ophthalmic then passes
forward a short distance and divides into two parts. One
of these, the frontal nerve, leaves the orbit through the
anterior supraorbital foramen and branches in the skin.
The other, the nasociliary nerve, is distributed partly to
the anterior portion of the orbit, while its main division
leaves the orbit as the ethmoidal nerve, which passes
through the small ethmoidal foramen of the orbital part
of the frontal bone to supply the interior of the nose. The
nasociliary nerve is also connected with the minute ciliary
ganglion lying on the optic nerve by a very delicate long
root of the ciliary ganglion.

The lacrimal, frontal, and nasociliary nerves appear as
separate structures in the orbit, their origin being deep
(Fig. 45, p. 85).

(/) 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 to 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
surface 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 splenopalatine nerve will be found on the surface of

THE MIDDLE EAR 321

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 the spheno-
palatine nerve is also connected 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), or
Vidian nerve, is a slender cord which passes backward along the
orbital wall from the posterodorsal angle of the sphenopalatine
ganglion. 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 posteriorly.
One of them, the deep petrosal nerve (n. petrosus profundus), is
connected with the sympathetic plexus of the internal carotid
artery, its fibres coming from the superior cervical ganglion. The
other, the great superficial petrosal nerve (n. petrosus super-
ficialis 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 constitutes the motor root of
the sphenopalatine ganglion, the sensory root being that provided
by the sphenopalatine nerve.

The sphenopalatine ganglion is one of several representing the
parasympathetic division of the autonomic system in the head, and
having motor and sensory roots from the cerebral nerves in addition
to autonomic connections. The series includes the ciliary gang-
lion, which lies on the optic nerve, the sphenopalatine ganglion,
the otic ganglia, associated with the mandibular nerve, and the
submaxillary ganglion, associated with the lingual nerve (Fig. 40) .

13. Examination of the middle ear.

By breaking away the ventrolateral portion of the tympanic
bulla and clearing the surface, the structures 6i the tympanic cavity

322 ANATOMY OF THE RABBIT

may be studied. They are chiefly those already described in
connection with the skeleton (p. 186), 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 connections 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. 47, p. 91)
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
connected 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 endo-
lymphatic duct and sac. 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 peri-
lymphatic space also occupied by a fluid material termed the perilymph.

THE THORAX 323

XL THE THORAX
1. Examination of the thoracic wall.

For this purpose the lateral surface of the thorax may con-
veniently be 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, pectoscapularis, 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 at a 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. 343) 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 extending from the middle of the first rib backward
to the end of the ninth bone rib, or a point on this rib a little more
dorsal, and the third across the ventral surface close in front of the
diaphragm. Do not cut into 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,

324 ANATOMY OF THE RABBIT

from the second to the sixth, at the junctions of the bone ribs with
the costal cartilages.

A very thin layer of muscle fibres passing in the same direction as those of
the internal intercostal muscles has been described as lying immediately internal to
them in the lateral wall of the thorax and is named intracostal (more dorsal) and
subcostal (more ventral) muscles. These, however, are not present as a distinct
layer in the rabbit. They and the transversus thoracis muscle together represent
a thoracic continuation of the transversus abdominis. The main branches of the
intercostal nerves run between these and the internal intercostal muscles.

The artery passing along the ventral wall between the transverse thoracic
muscle and the internal intercostals is the internal mammary (p. 326).

The importance of the above described muscles in the act of breathing is in-
dicated on p. 108. For the general relations of heart and lungs, see pp. 106 and 109.

2. Dissection of structures in the superior thoracic aperture.

The nerves and blood-vessels of this region are concealed by
the thymus gland, a large triangular flattened structure of fatty
consistence, lying partly ventral to the heart and extending forward
from its base to the anterior end of the thorax (cf. p. 133). The
thymus should be carefully raised from behind and dissected 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 from 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.

(a) The arch of the aorta (arcus aortae). Beginning at the
anterior end of the heart, the aorta at first passes forward,
and then describing a curve, in the course of which it lies
slightly to the 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. 330) the first branches are the large vessels arising from
the convex surface of the arch. 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

THE THORAX 325

carotid arises immediately to the left of this vessel or from
its base. The left subclavian arises some distance farther
out along the arch. 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 becomes 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) The vertebral artery (a. vertebralis). This vessel
passes into the costotransverse foramen of the sixth
cervical vertebra, and, traversing the canal formed by
this and the corresponding foramina of the vertebrae
anterior to it, reaches the interior of the cranial cavity.
Its union on the ventral surface of the medulla oblon-
gata with its fellow of the opposite side to form the
basilar artery will be seen at a later stage (p. 360).

(2) The superficial cervical artery (a. cervicalis super-
ficialis) — divided in a previous dissection (p. 258) — is
a small vessel which passes forward and outward be-
neath the insertions of the cleidomastoideus, basio-
clavicularis, 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.

(3) The transverse artery of the neck (a. transversa colli),
also divided in a previous dissection (p. 260), 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 rhomboidei.

(4) The highest intercostal artery (a. intercostalis suprema)

326 ANATOMY OF THE RABBIT

passes backward to the internal surface of the thoracic
wall, giving off the first three (or four) intercostal
arteries in the intercostal spaces, and also small
branches to the oesophagus, the trachea, and the
bronchi.

(5) The internal mammary artery (a. mammaria interna),
the first portion of which runs along the inner surface
of the ventral wall of the thorax and has been removed
with it, passes backward to the ventral abdominal
wall as the superior epigastric artery (a. epigastrica
superior) anastomosing with the inferior epigastric
(p. 223).

{b) 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
passes almost directly backward, crossing the ventral
surface of the right subclavian artery, and enters the an-
terior portion of the right atrium. The left vessel crosses
both the left subclavian artery and the arch of the aorta
and turns mediad over the dorsal surface of the heart to reach
the posteromedial part of the right atrium (Fig. 62, p. 111).
The transverse terminal portion of the left superior caval
vein constitutes the coronary sinus and is retained in
reduced size in species, such as man, where the rest of the
left superior caval vessel degenerates during development.

(c) The vagus nerve on the right side crosses the ventral surface
of the subclavian artery, passing dorsad over the dorsal
surface of the bronchus to the wall of the oesophagus. It
gives off the recurrent nerve (n. recurrens), which curves
round the subclavian artery and passes forward dorsal to
the latter and then along the side of the trachea to the
muscles of the larynx (except the cricothyreoideus). On
the left side the vagus descends dorsal to the superior
vena cava, crosses the ventral surface of the arch of the
aorta to the point of connection of the arterial ligament
(p. 330), where it gives rise to the recurrent branch, and

THE THORAX

327

then continues backwards between the arch of the aorta
and the base of the heart. It passes dorsal to the pulmonary
vessels and the bronchus (where it gives off pulmonary
branches) to continue along the ventrolateral wall of the
oesophagus. The recurrent nerve passes forward dorsal
to the arterial ligament and over the dorsal side of the
aortic arch to proceed craniad along the lateral surface of
the trachea.

Near its beginning, the recurrent branch gives off
efferent rami to the cardiac plexus (plexus cardiacus), a
network of autonomic nerve fibres lying between the aortic
arch and the pulmonary artery and distributed to the heart
(coronary plexus) and the immediately adjacent arteries.

{d) The ramus cardiacus of the vagus has already been observed
running along the dorsal
surface of the common
carotid artery (p. 302).
In front of the subcla-
vian artery, this sensory
branch is at first close-
ly 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. Its
posterior end connects
with the cardiac plexus
•through which its fibres
come from the walls of
the adjacent vessels and

of the heart. ■ r^^- ^^^- ^^^^ of the venous and lymphatic

^ trunks of the anterior portion of the body.

/ \ -T*! I- • / After McClure and Silvester.

(ej Ine phrenic nerve (n. a., azygos vein; ao, aorta; C.S., left superior

I.N,. , , caval vein; d.th., thoracic duct; j.e., j.i., and

pnreniCUS) is a stout j.tr., external, internal, and transverse jugular

J . . I* n r veins; s., left subclavian vein; tr.s., transverse

cord arismg chietly from scapular vein.

328 ANATOMY OF THE RABBIT

the fourth cervical spinal nerve. That of the left side
crosses the ventral surface of the subclavian artery but
dorsal to the subclavian vein, then, lying just lateral to the
superior vena cava, crosses ventral to the aortic arch and
passes along the pericardium to the diaphragm (Fig. 112).
That of the right side passes back along the dorsolateral wall
of the superior caval vein, then along the pericardium, and
accompanies the thoracic portion of the inferior caval vein.
The phrenic nerves control the respiratory movements of the
diaphragm, each turning laterad when it reaches that organ
and being distributed in the muscular portions thereof on its
own side.
(/) 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 gang-
lion enter the cardiac plexus and the sympathetic plexuses
of the subclavian and its branches.

If desired, a useful view of the relations of the nervous
structures may be obtained before proceeding to the next
section by cutting away the left lateral wall of the thorax
and examining from that side the parts described in section
5 (pp. 335-337).

3. Dissection of the heart.

The character and relations of the sac enclosing the heart, the
pericardium, should first be noted. The relation to the heart of the
pericardium proper, or serous pericardium, is similar to that of the
peritoneum and pleura to the visceral organs which they invest
(p. 135). The serous pericardium comprises a parietal layer, which
lines the inner surface of a strong loose sac commonly known as the
pericardium, and a visceral layer, or epicardium, which forms an
intimately attached investment over the outer surface of the heart

THE THORAX

329

substance. The two layers are continuous through sheaths sur-
rounding the vessels which enter and leave the heart. The parietal
layer of the serous pericardium is applied to the inner surface of a
thicker and much tougher sac, the fibrous pericardium. This, in
turn, is partly attached by loose connective tissue to surrounding
organs and partly covered by the serous mediastinal pleurae, the
linings of the cavities for the lungs.

The paired pleural cavities containing the lungs are broadly separated by a
longitudinal vertical partition, the mediastinum or mediastinal septum, the
«pace enclosed by the latter being largely occupied by the heart and by the cavity
oi the pericardium. For a considerable area ventrally the pericardium is loosely

Fig. 112. Interior of the left pleural cavity with the lung removed to show the
mediastinum, lateral view, a, thoracic aorta; d, diaphragm; h, heart in peri-
cardium; 1, remnant of base of lung; Ip, pulmonary ligament; m, posterior
mediastinum; oe, oesophagus; th, thymus gland; vcs, left superior caval vein.
The phrenic nerve is visible crossing tlie pericardium and the mediastinal septum
and branching into the diaphragm, more dorsally the vagus nerve crosses the
lateral surface of the oesophagus, and the sympathetic trunk appears on the
dorsal wall.

applied to the thoracic wall, the intervening space, which is bounded laterally
by the membrane lining the pleural cavities (pleura, p. 334), 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 bfonchi, and the pulmonary blood-vessels,
and caudally forms a thin septum similar to mesentery. In the rabbit and most
quadrupedal mammals, unlike man, the pericardium does not reach the diaphragm,
so that, in the rather narrow space between the caudal end of the pericardium and

330 ANATOMY OF THE RABBIT

the diaphragm, the posterior mediastinum widens into a thin vertical sheet
extending from the dorsal thoracic wall to the internal surface of the sternum
(Fig. 112).

The pericardium should be removed, and the external features
of the heart^ and its great vessels examined. These are as follows:

(a) The posterior, somewhat conical, ventricular portion of the
heart. The left ventricle (ventriculus sinister) may be
distinguished both by its position and by the more solid
character 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.

(b) The pulmonary artery (a. pulmonalis) leaves the base of
the right ventricle, passing forward and to the left and then
dorsad and caudad in a somewhat spiral fashion around the
aorta. On the dorsal surface of the latter it divides into
the right and left pulmonary arteries, one for each lung.
Close to the point of division the left pulmonary artery is
connected with the aorta by a short fibrous cord, the
arterial ligament (lig. arteriosum), representing the foetal
connection of the two vessels through the ductus arteriosus
(pp. 114, 118).

(c) The left coronary artery (a. coronaria sinistra) emerges
from between the root of the pulmonary artery and the left
atrium and divides into two main branches, one passing
backward in or near the anterior longitudinal sulcus, the
other ramifying over the left side of the heart. The right
coronary artery (a. coronaria dextra) runs ventrad between
the pulmonary artery and the right atrium and along the
right atrioventricular groove, giving off branches to supply
the whole right side of the heart. Both vessels supply the
walls of the aorta, of the pulmonary arteries, and of the

^The heart is relatively small in the rabbit, as in most animals not capable of
prolonged severe muscular effort but depending upon hiding for safety. Its
weight is given as about 0.003 of that of the body, which may be compared with
values of about 0.006 in man and 0.01 in a deer.

THE HEART 331

great veins as well as the muscular substance of the heart
itself. They arise from the aortic sinuses (p. 333), the right
artery from the ventral sinus, the left one from the left
dorsal sinus.

The blood distributed to the walls of the heart is collected by four
groupsof freely anastomosing cardiac veins. Those draining the left side
unite in a left cardiac vein, which passes round in the left atrioventricular
groove and enters the caudal part of the left superior vena cava, or the
coronary sinus. The right cardiac vein, receiving the vessels from the
right side, lies in the right atrioventricular groove and opens into the
coronary sinus. The veins draining the dorsal surface of the heart
join the right cardiac vein as it enters the sinus. Minute veins from
the terminal portion of the right ventricle open directly into the right
atrium.

(d) 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), passing from the
medial portions of the lungs, usually unite into two main
vessels on each side and these fourvesselsenter a short, wide,
funnel-shaped diverticulum on the left atrium. (This diver-
ticulum is a feature of more primitive mammals.)

(e) 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 blood-vessels.
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.

Open the right ventricle by a longitudinal incision of the ventral
wall, extending the incision forward into the pulmonary artery.
Open both atria by transverse incisions. By washing out the
cavities, the internal features of the wall, including the arrange-
ment 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.

332 ANATOMY OF THE RABBIT

In the most primitive vertebrate hearts, the ventricular wall is
composed almost entirely of a spongy mass of muscular trabeculae
with a thin layer of compact muscle on the outer surface. There is a
progressive change in the vertebrate series to the mamm.alian condition,
where there is a thick, compact muscular wall with relatively few
internal trabeculae.

(b) The tricuspid valve (valvula tricuspidalls). 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
connected 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 re-
duced 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. Each fold
forms a pocket opening towards the artery and the cavity
of the pocket is a pulmonary sinus. 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
partition separating them may be seen a thin fibrous por-
tion, the fossa ovalis, denoting the position of the embry-
onic 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 greater thickness of the wall the internal structure is not so
easily examined as in the right ventricle. The interventricular

THE LUNGS

333

partition is approximately of the same thickness as the rest of the
wall of the left ventricle.

(a)

(b)

The bicuspid or left atrioventricular valve (valvula bicuspi-
dalis) 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. Opposite

Fig. 113. Diagram of a transverse section of the thorax
of a rabbit, passing through the posterior tip of the heart,
am, anterior mediastinum; ao, aorta; cp, costal pleura;
ep, epicardioum; f, fold of mediastinal pleura containing
the inferior caval vein; Ipl, left pulmonary ligament; m,
medial lobule of inferior lobe of the right lung; ml, middle
lobe the left lung; mp, mediastinal pleura ;_ oe, oesophagus;
p. pericardium; pm, posterior mediastinum; pp, pul-
monary pleura; rpl, right pulmonary ligament; rv, right
ventricle; s, sternum; vc, inferior caval vein.

each, the wall of the aorta is slightly excavated so that the
valve will not adhere to it. The cavity enclosed by each
valve is known as an aortic sinus (of Valsalva) .

4. Examination of the lungs and their connections (Figs. 57, 114).
The removal of the ventral w^all 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:

334 ANATOMY OF THE RABBIT

(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 anterior
surface of the diaphragm as the diaphragmatic pleura,
and over the surface of the lung as the pulmonary pleura,
and entering into the formation of the mediastinum as the
mediastinal pleura.

A secondary fold on the right side of the mediastinal septum con-
tains the inferior vena cava and forms a pocket in which lies the medial
lobule of the inferior lobe of the right lung. The attachment of the
posterior margin of the septum to the diaphragm is displaced to the
left so that it and the secondary fold are approximately symmetrical
and the pleural pocket is median. Posteriorly, the pulmonary pleura
passes from the medial margin of the left lung and from the medial
margins of both inferior lobules of the right lung to the mediastinal
septum and backward to the diaphragm, forming the pulmonary
ligament (lig. pulmonale). These relations may be understood more
clearly by reference to a transverse section such as that represented
diagrammatically in Fig. 113.

{b) The lungs (pulmones) are paired expansible structures, the

surfaces of which are free, except medially, where they are

connected with the respiratory passages and the pulmonary

blood-vessels, and posteromedially, where they are attached

to the mediastinum and to the diaphragm by the pulmonary

ligaments.
(c) The right lung is divided by deep fissures into superior,

middle, and inferior lobes, the

inferior lobe consisting of a

large lateral lobule and a

smaller medial lobule, the last

frequently further subdivided.

The inferior caval vein passes

between these lobules. The

left lung is only about two

. . , r , , • r . 1 • v. Fig. 114. Plan of the respiratory

thirds Ot the size OI the right tubes as seen from the ventral sur-

1 J l.*-U^ U 4-U ,.^:^^1^ face, tr, trachea; br, br', left and

lung and, although the middle ^ight bronchi; ep, eparterial bron-

nr^A I'nf orir^r lrkhi:^c ^ r^ \\7f^]] ^^^^> ^' "^' "^'' ^' ^'' bronchial rami to
and interior lOOeS are wen superior, middle, and inferior lobes;

developed, the superior lobe is iJ;^J-j /Xuief'^ '"""^ '° ^'''''^ ^"^

very imperfectly represented.

The inferior lobe of the left lung is not subdivided.

THE LUNGS 335

(d) The trachea divides at its posterior end into two portions,
the 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 within it. On the right side a small
eparterial bronchus is given off from the right bronchus to
the well-developed superior lobe, entering the lung anterior
to the right pulmonary artery.

(e) The branches of the pulmonary artery and the pulmonary
veins may be traced for a short distance on the medial
portion, or hilus, of each organ. The artery penetrates
deeply almost at once, anterior and then dorsal to the main
bronchus, but some large venous tributaries have a con-
siderable superficial course.

(/) The vagus nerve passes to the dorsal side of the bronchus,
giving off a number of branches, which accompany the
bronchus to the lung.

These branches contain both afferent nerve fibres for the mucous

membrane and efferent fibres to the smooth muscles of the bronchioles,

the latter producing bronchoconstriction when stimulated and assisting

expiration.

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 may be prepared for the next dissection.

5. The following structures may now be traced on the dorsal wall
of the thorax:

(a) The oesophagus. It traverses the thorax in a median
position, entering the diaphragm at the hiatus oesophageus.

(b) 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. In the posterior part of the thorax,
both cords lie dorsolatera,l to the oesophagus and after
passing through the diaphragm in this position the left
nerve crosses 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. 230).

336 ANATOMY OF THE RABBIT

(c) 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 metamerlcally in the intercostal spaces, and pass laterad
to the thoracic wall.

(d) The thoracic portions of the sympathetic trunks lie on the
lateral surfaces of the bodies of the thoracic vertebrae,
the left trunk reaching this position by extending backward,
dorsal to the aorta, from the first thoracic ganglion, to which
it has already been traced. The posterior ganglia give origin
to the splanchnic nerve, the latter usually separating at
about the eighth thoracic ganglion and passing backward
into the abdominal cavity (p. 228).

(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.

(/) The intercostal nerves (nn. intercostales) accompany the
intercostal arteries to the lateral wall of the thorax, their
trunks running mainly between the internal intercostal
muscles and the vestigial intracostal and subcostal muscles
but partly enclosed by the internal intercostals.

(g) The azygos vein (v. azygos) is a small, asymmetrical,
venous trunk lying to the right of the dorsal surface of the
aorta. It receives from both sides 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 at about the level of the second Intercostal
space. The more anterior Intercostal veins are tributaries
of the right and left supreme intercostal veins which open
into the corresponding superior cavals.

The azygos vein lies ventral to the more anterior intercostal
arteries but dorsal to the more posterior ones, the change of relation

THE DIAPHRAGM 337

occurring most frequently at the eighth intercostal space but often in
the ninth, tenth, or eleventh.
(^) The thoracic duct, which is not readily observed in ordinary dis-
section, is formed between the crura of the diaphragm by the union of
the two lumbar l3miphatic trunks and the intestinal trunk. It passes
forward between the azygos vein and the aorta to the level of the
second intercostal space, where it crosses to the left between the aorta
and the oesophagus. It then follows the left superior vena cava and
enters into the junction of the jugular and subclavian veins. A variable
series of lymph nodes lies between the aorta and the oesophagus, re-
ceiving vessels from the organs in the thorax and draining either into
the thoracic duct or separately into the vein. The arrangement of the
lymphatic vessels and their connections with the veins show marked
individual differences.

6. The diaphragm (diaphragma) is a muscular and tendinous

sheet forming the posterior wall of the thorax and separating

the pleural cavities from the peritoneal cavity. It is somewhat

dome-shaped and contraction of its muscles partially flattens

the dome 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 by slips separated by small triangular aponeurotic

areas. The third, or sternal portion, arises from the xiphoid process

of the sternum. The insertion of the muscles of the diaphragm 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 mediastinum and the

pulmonary ligaments. The centrum tendineum is shaped somewhat

like a trefoil, its margin being indented dorsally by the crura and

at each side at the position of the inferior phrenic veins. The fibres

of the costal and sternal portions converge radially to its margin.

The connection of the lumbar portion is somewhat asymmetrical,

the two crura combining ventral to the aorta and ending largely

to the left of the median plane.

338 ANATOMY OF THE RABBIT

The following may be made out on the posterior surface:

(a) The cut margins of the falciform, coronary, and left tri-
angular ligaments, which were severed in the removal of
the liver.

(b) The hiatus aorticus, an aperture enclosed by the dorsal parts
of the two crura and serving for the transmission of the
aorta.

{c) The hiatus oesophageus, more ventral than the foregoing,
and serving for the passage of the oesophagus. The muscle
fibres of the right crus of the lumbar portion diverge at the
dorsal side of this opening, a few passing to the right of it
but the great majority ending in the central tendon to the
left of it.

(d) The foramen venae cavae, situated slightly to the right
and slightly ventral to the hiatus oesophageus. It serves

'for the transmission of the vena cava inferior and is sur-
rounded by the coronary ligament.

(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 latter, and enter
the crura.

The inferior phrenic arteries are minute branches arising at the base
of the coeliac.

(/) 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 small superior phrenic veins run close to the phrenic nerves,
pass forward from the centrum tendineum of the diaphragm along the
mediastinum, ventral to the roots of the lungs, and open into the
superior caval veins.

XII. THE VERTEBRAL AND OCCIPITAL MUSCULATURE

Dissect 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

VERTEBRAL AND OCCIPITAL MUSCULATURE 339

to the last rib, and is inserted on the lateral surfaces of the eight
posterior 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
process of the atlas.

These two muscles should be divided, the serratus posterior
being removed from the surface.

These and the muscles described in the next two sections con-
stitute the epaxial musculature (p. 67).

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-
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 lumbo-
dorsal 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 surface of the ribs. Its
medial border is separated from the middle line by a space
of considerable width, in which the semispinalis and multi-
fidus 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, name-

340 ANATOMY OF THE RABBIT

ly, 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 accessory 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
iliocostalis dorsi. Medially, it receives 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
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.

(b) 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 semi-
spinalis dorsi, is inserted by a series of fleshy slips on the
spinous processes of the more anterior thoracic vertebrae, but
extends to the spinous process of the third or fourth cervical
vertebra. The posterior portion, the multifidus, increases

VERTEBRAL AND OCCIPITAL MUSCULATURE 341

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 longis-
simus 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.

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, composed
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.

342 ANATOMY OF THE RABBIT

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 trans-
verse process of the atlas.

(/) 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:

(These belong to the epibranchial portion of the hypaxial
musculature — p. 67. The hypobranchial portion cornprises the
sternohyoid, sternothyreoid, thyreohyoid, and geniohyoid
muscles.)

(a) The scalenus ventralis or anterior. Origin: Transverse
processes of the four posterior cervical vertebrae. Insertion :
Anterior and lateral surfaces of bony first rib.

(b) The scalenus medius. Origin: Transverse process of the
fifth cervical vertebra. Insertion: Lateral surfaces of the
third to fifth ribs (with sometimes a slip to the second rib) .

(c) The scalenus dorsalis or posterior. Origin: Transverse
processes of the fourth to sixth cervical vertebrae. Insertion :
More dorsal part of first rib. •

The medius is superficial; the ventral more or less
separated from the dorsal by the origin on the first rib of
part of the cervical portion of the vSerratus anterior.

(d) The obliquus thoracis or transversus costarum. Origin:
Lateral surface of the bony first rib just ventral to the
insertion of the scalenus ventralis. Insertion: Side of the
sternum from the attachment of the second costal cartilage
to that of the fourth, by a triangular aponeurosis which is
fused with that of the anterior end of the rectus abdominis

VERTEBRAL AND OCCIPITAL MUSCULATURE 343

muscle. This muscle has been shown to be morphologically
a part of the same sheet as the external oblique muscle of
the abdomen.

The foregoing muscles, comprising the scalenus group,
are destroyed by the removal of the ventral thoracic wall
(p. 323).

(e) 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 ofT
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 trans-
verse processes of the first six cervical vertebrae. Insertion :
Sphenooccipital synchondrosis.

(g) 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.

{h) The rectus capitis anterior. Medial portion of the ventral
surface of the transverse process of the atlas. Insertion:
Basioccipital bone.

4. 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 groove between the
articular and transverse processes and is inserted on succeeding
vertebrae. It appears to continue the multifidus, but corresponds
to the more medial portion of the longissimus.

(c) The abductor caudae anterior. Origin: Ischial spine. In-
sertion: Lateral surface of the sacrum and the transverse processes
of the caudal vertebrae.

(d) The flexor caudae. Origin: Ventral surface of the sacrum and
anterior caudal vertebrae^ Insertion: Ventral surfaces of succeed-
ing vertebrae.

These muscles are also known as sacro-coccygei, dorsalis, lateralis,
and ventralis {a, b, d), and coccygeus (c).

344 ANATOMY OF THE RABBIT

XI I J. THE CENTRAL NERVOUS SYSTEM
1. The spinal cord and nerve roots.

To expose the whole cord or a portion of it from the dorsal
surface, the muscles should be removed from the dorsal aspect and
both sides of the vertebral arches and the latter should be broken
away with bone forceps. The following features may be made oiit
when the extent of exposure is sufficient:

(a) The spinal cord is enclosed in a set of three protective
membranes, the meninges, the outermost of which is a
relatively thick, tough, fibrous sheet, the dura mater.
Unlike that of the brain, the dura mater of the spinal cord
is not firmly attached to the inner surface of the surround-
ing bone and it is exposed in an intact condition by the
removal of the vertebral arches. The epidural space
between this membrane and the inner surface of the bone
contains a little fat.

If the dura mater is now cut open, it is found to be
separated by a narrow space from a much thinner, vascular
membrane, the pia mater, which is closely applied to the
surface of the spinal cord. Between the dura mater and
the pia mater and attached to both lies a very delicate
web of connective tissue, the arachnoidea.

The arachnoidea and the pia mater together are fre-
quently designated leptomeninges, whereas the dura mater
is distinguished as the pachymeninx.

The spaces between these membranes are filled, in the
living condition, with cerebrospinal fluid, the same liquid
which fills also the cavities within the central nervous
organs.

(b) The spinal cord (medulla spinalis) is a thick, subcylindrical,
white cord traversing the vertebral column in the vertebral
canal. Its diameter is not uniform, as it exhibits two slight
enlargements, one in the cervical, the other in the lumbar
region. These enlargements are the regions from which
the nerves to the limbs arise and are due to the increased
number of nerve elements present on that account. At
about the middle of the sacrum, the cord contracts to a

THE SPINAL CORD 345

slender filament, the filum terminale, which may be traced
backward to the middle of the tail, and which is produced
in development by the more rapid growth of the bony
canal than of the spinal cord within it.

A faint median groove, most distinct towards the cephalic
end of the cord, divides it into right and left halves. This
is the dorsal or posterior median sulcus. A short distance
■ to each side of this, the dorsal roots of the spinal nerves
enter the cord along a still fainter groove, the dorsolateral
or posterior lateral sulcus.
(c) The nerve roots have a regional distribution — eight cervical ,
twelve thoracic, seven lumbar, four sacral, and six caudal.

Since the first spinal nerve emerges between the skull
and the atlas, the cervical nerves are numbered to correspond
with the vertebrae lying behind the intervertebral foramina
from which they proceed, though the remaining spinal
nerves are designated according to the vertebrae lying in
front of the corresponding intervertebral foramina. The
nerve transmitted by the intervertebral foramen between the
seventh cervical and first thoracic vertebrae is described as
the eighth cervical.

The disproportionate growth which produces the filum
terminale also carries the connections of the posterior spinal
nerves with the spinal cord to levels further forward than
their emergence from the vertebral column. Hence a group
of these nerves runs backward at each side of the filum
terminale, constituting the formation known as the cauda
equina. These features are less pronounced in the rabbit
than in the human species, in which the spinal cord termin-
ates near the boundary between the first and second lumbar
vertebrae in the adult.
(d) The origin and primary divisions of the nerve roots may be
worked out by removing carefully the lateral portions of
the arches of one or two vertebrae. Each spinal nerve has
a posterior or dorsal root (radix posterior), composed of
afferent nerve fibres (p. 73), which enters the dorsolateral
surface of the spinal cord as a linear series of rootlets. These

346 ANATOMY OF THE RABBIT

extend ventrolaterally dose to the cord and unite just
lateral to it, where the root expands into a spinal ganglion.
The more slender anterior or ventral root (radix anterior) lies
directly ventral to the dorsal root and, like it, consists of a
row of separate filaments. These are composed of efferent
fibres emerging from the ventrolateral surface of the cord.
They converge dorsolaterad to meet and combine with the
dorsal root close to the spinal ganglion, thus forming a
single nerve, which breaks up a little further laterally into
three primary branches, the posterior or dorsal, the anterior
or ventral, and the communicating ramus. Each branch
contains fibres from both roots. The roots lie within the
dura mater and this extends into each intervertebral foramen
there to become continuous with the connective tissue
sheath of the nerve.

The posterior (dorsal) ramus is an inconspicuous branch
(except in the first two cervical nerves) passing to the
dorsal musculature and skin. The anterior (ventral) ramus
is the chief portion of the spinal nerve, the successive
anterior rami appearing as the components of the cervical
and lumbosacral plexuses or as individual spinal nerves.
The ramus communicans is a slender filament passing
ventrad to join the sympathetic trunk. .

Each spinal nerve has a grey ramus communicans and
certain ones have also a white ramus communicans. In
the rabbit, the latter are all the thoracic and the first five
lumbar nerves and similar parasympathetic white rami
occur in the second, third, and fourth sacral nerves (see
pp. 73-75).

2. A small portion of the spinal cord may be excised and examined
(preferably under water) for the following (see Fig. 18, p. 31).

(a) The cord is divided into lateral halves by two median
depressions, the ventral or anterior median fissure (fissura
mediana anterior) and the dorsal or posterior median
sulcus (sulcus medianus posterior).

(b) Each half of the cord is further marked off into three
columns by shallow grooves, the ventrolateral and dorso-

THE SPINAL CORD 347

lateral or anterior and posterior lateral sulci, of which
only the latter are at all distinct. The grooves are marked
by the attachments of the ventral and dorsal nerve roots.
The three columns of each half of the cord are the ventral
(anterior), the lateral, and the dorsal (posterior) funiculi.

(c) On the cut surface the white substance (substantia alba)
is seen to form a peripheral investment enclosing the grey
substance (substantia grisea) of the centre of the cord.
The grey portion is somewhat H-shaped in section, each
half being composed of ventral larger and dorsal smaller
masses, known in section as the horns of the grey matter,
or, as complete structures, the ventral and dorsal grey
columns. These grey columns are situated opposite the
ventrolateral and dorsolateral sulci of the surface and
separate internally the three funiculi mentioned in the
previous paragraph. The white substance is composed of
ascending and descending nerve fibres and, when a large
proportion of these connect the brain with the various levels
of the spinal cord, their number necessarily increases in a
caudocephalic direction. In the rabbit, however, the propor-
tion of such fibres is not great enough for the caudocephalic
increase of the white matter to be marked (Fig. 41, p. 77).
In the median plane is the minute central canal (canalis
centralis), the cavity of the spinal cord.

(d) The ventral (anterior) spinal artery runs along the cord in
the ventral median fissure, giving off branches into the
fissure and smaller branches over the surface.

3. The brain may be exposed by breaking away the supraorbital
processes of the frontal bones 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
lateral wall of the skull on both sides. The part of the operation
requiring most care is the removal of the temporal portion of
the skull by successive steps, exposing first the paraflocculus of
the cerebellum (Fig. 42, p. 79), a small stalked body which is
almost completely enclosed by the dorsal portion of the petro-
sal bone. The entire petrotympanic bone is easily detached, and

348 ANATOMY OF THE RABBIT

if it is 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 if the anterior portion of the cord has not already
been exposed in the previous dissection.

The spinal cord may be divided at about the level of the
third vertebrae. The brain should then be raised very gently
from the ventral wall of the skull and the nerve roots should be
divided with fine, sharp scissors. This operation requires con-
siderable care not to pull upon the nerves since these are strongly
attached at their points of exit from the skull but very lightly
attached to the brain, so that they are in danger of being torn
away from the latter.

The anterior end of the brain may be freed by cutting close to
the bone under and in front of the small anterior expansions
formed by the olfactory bulbs.

The dura mater is adherent to the inner surface of the
cranium but may be stripped away from it in the process of
removal of the bone. Portions which remain attached to the
brain may be cut away with scissors. Such attachment will be
found chiefly along two lines: one, where the membrane extends
down into the longitudinal fissure between the cerebral hemis-
pheres, as the falx cerebri, the other the tentorium cerebelli, a
transverse fold extending inward between the cerebral hemis-
pheres and the cerebellum. These parts contain wide vessels
which receive most of the blood from the brain, the superior
sagittal and the transverse venous sinuses, from the latter of
which the blood passes into the superficial temporal vein (p. 297).
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.

4. The primary divisions of the brain are explained in a previous
chapter (p. 80). The prosencephalon or forebrain, the mesen-
cephalon or midbrain, and the rhombencephalon or hindbrain,
though much elaborated in form, are still to be recognized in
the adult animal. Their superficial features may be identified
as follows:

THE FORE BRAIN 349

THE PROSENCEPHALON:

(a) The greatly enlarged cerebral hemisphere (hemisphaerium
cerebri), the dominant portion which correlates and co-
ordinates the activities of all the rest of the nervous system,
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 each
other by the commissures indicated below. Each hemi-
sphere has a superficial layer of grey matter, the cerebral
cortex, which in larger brains is thrown into numerous
folds but in the rabbit is practically smooth.

(b) The olfactory bulb (bulbus olfactorius) is a small expansion
lying at the anterior end of each hemisphere. Its anterior
and ventral surfaces receive the fascicles of the first cranial
or olfactory nerve, which is not a compact structure but is
represented by numerous separate threads coming from
the mucous membrane of the nose. These may be found
in the skull, where they may be traced into the perforations
of the cribriform plate.

(c) The olfactory bulb is the anterior portion of the 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
olfactory tract (tractus olfactorius) and a somewhat wider
strip of grey matter underlying the tract. This strip
expands caudally into a portion of the brain which, from
its shape, is known as the pyriform lobe (lobus piriformis)
and here the olfactory tract spreads out and most of its
fibres terminate. The lateral margin of the olfactory brain,
which includes the olfactory bulb and the related parts just
described is delimited superficially from the remaining
portions of the cerebral hemisphere by a longitudinal
furrow, the limbic fissure (fissura limbica). The anterior
portion of the furrow, known as the anterior rhinal fissure,
separates the grey matter underlying the olfactory tract
superficially from the narrow anterior end of the non-
olfactory part of the cerebral hemisphere. The correspond-
ing posterior portion of the furrow^ the posterior rhinal
fissure, marks off the pyriform lobe from the posterior,

350 ANATOMY OF THE RABBIT

more expanded part of the cerebral cortex. 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 lateral surface of the cerebral
hemisphere. This represents a rudiment of the lateral
cerebral (Sylvian) fissure, which is a conspicuous feature
in the brain of man and other mammals with convoluted
hemispheres.

Fig. 115. Dissection to show the radia-
tion of the corpus callosum from a dorsal
viewpoint. The longitudinal white line
close to the median plane is the position of
a delicate band of hippocampal fibres, the
medial longitudinal stria.

(d) The corpus callosum is a broad, white commissural band
passing transversely between the hemispheres (Fig. 115) to
connect the cortex of each with that of the other and thus
providing for the co-ordination of their action. Its median
portion lies at the bottom of the longitudinal cerebral fissure
and may be exposed dorsally by pressing apart the medial
margins of the hemispheres so as to open up the fissure.
The foregoing parts belong to the telencephalon, those which
follow belong to the diencephalon (p. 86).

^Though the term is sometimes loosely used, a commissure may be defined as
a structure connecting corresponding parts on opposite sides. It contrasts with
a decussation, which is a system of nerve-fibres crossing the median plane to
connect different parts on the two sides.

THE FOREBRAIN

351

(e) The pineal body (corpus pineale) is a small, somewhat
conical structure lying between the dorsal posterior tips of
the cerebral hemispheres (cf. p. 134). It is connected by a
hollow stalk with the unpaired portion of the brain (the
diencephalon) 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 de-
tached.

By raising and pressing apart the tips of the hemispheres
and pulling away the pineal body with the tissue to which it
is attached, the dorsal surface of the diencephalon will be
sufficiently exposed to make out the following features:

(/) The slit-like aperture appearing in the middle line after
the removal of the pineal body represents the dorsal por-
tion of the third ventricle (ventriculus tertius) (Fig. 116),
the roof of which is formed by tissue just torn away with
the pineal body. This roof consists of a thin membrane
over which lies a dense network of fine blood vessels con-
tained in pia mater, the chorioid plexus. Folds of the
membrane and plexus dip down
into the ventricular cavity and
here cerebrospinal fluid is se-
creted.

(g) The lateral margins of the aper-
ture are largely formed by mi-
nute spindle-shaped masses, one.
on either side, the habenulae.
Their posterior ends are united
by a slender transverse band,
the habenular commissure
(commissura habenularum).
The fibres constituting this band
are faintly traceable forward,
where they form a pair of thin
whitish filaments (medullary

Fig. 116. 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
commissiure; c.p., rostral edge of
posterior commissure; c.s.. superior
colliculus (of mesencephalon) ; g.l.
and g.m., lateral and medial geni-
culate bodies; h., habenula; m.i.,
massa intermedia; p., lateral thal-
amic tubercle; v.t., third ventricle.

352 ANATOMY OF THE RABBIT

striae) composed of fibres from olfactory correlation cen-
tres. The habenulae receive impulses from various olfac-
tory regions of the cerebral hemisphere and tactile and
other related sensory impulses which they correlate with
them, sending resultant excitation impulses to motor cen-
tres, especially those concerned with feeding.

(h) The posterior commissure (commissura posterior) crosses
the posterior portion of the roof immediately behind and
below the habenular commissure, so that only its rostral
edge is visible from above, as indicated in Fig. 116. It is
a composite structure which, like other commissures, con-
nects parts on the two sides of the brain.

(i) The very thick masses of nervous tissue which form the
lateral walls of the third ventricle are the thalami. They
are broadly connected by the intermediate mass (massa
intermedia) or middle commissure, which may be seen
from the dorsal surface crossing and largely filling up
the ventricular cavity. This is not a true commissure
(a band of nerve fibres connecting corresponding structures
bilaterally), but is simply a fusion of the grey matter lining
the walls of the ventricle, brought about by the increased
thickness of the thalami. The latter contain various reflex
centres and serve especially as a relay station through
which pass all impulses proceeding to the cerebral hemis-
phere, except the olfactory ones.

(j) On either side, lateral to the habenula, the dorsal portion
of the thalamus forms a low, somewhat oval projection,
the lateral thalamic tubercle. This swelling represents
superficially a mass of grey matter through which pass
most of the sensory impulses to the hemispheres from
lower parts of the nervous system other than visual and
auditory.

The anterior tubercle of the thalamus is a faint elevation
of very small dimensions lying in the angle enclosed be-
tween the lateral tubercle and the anterior portion of the
aperture of the third ventricle. It also is a relay station for
impulses to the cerebral cortex, in this case mainly ones
coming from olfacto-visceral correlation centres.

THE FOREBRAIN 353

(k) The parts of the metathalamus are distinguishable ex-
ternally as two rounded projections of the lateral surface
at each side of the thalamic region. One of them, the
lateral geniculate body (corpus geniculatum laterale), lies
external to the lateral thalamic tubercle, marked off from
it superficially only by a faint depression, and constitutes
the most lateral part of the thalamic mass. It contains the

Fig. 117. Transverse section of the .forebrain passing through the inter-
ventricular foramina. The drawing represents a section stained by the method
of Weigert, which gives the white nerve fibres a dark colour. The large mass
of fibres dividing the corpus striatum into dorsal (nc) and ventral (gp+pu) parts
(the internal capsule) contains the fibres which form the basis of the cerebral
peduncle and the pvramid further back.

cc, corpus callosum; ch, optic chiasma; clip, hippocampal commissure;
cpl, chorioid plexus of lateral ventricle; cpt, chorioid. plexus of third ventricle,
f, fornix; fi, interventricular foramen; fl, limbic fissure; flc, longitudinal cerebral
fissure; gp, part (globus pallidus) of corpus striatum; h, tapering anterior end
of hippocampus; ha, habenula, anterior tip; hy, hypothalamus; Ip, pynform
lobe; nc, part (caudate nucleus) of corpus striatum; p, cerebral cortex; pu, part
(putamen) of corpus striatum; sm, stria meduUaris thalami; tol, olfactory tract;
vl, lateral ventricle; vt, third ventricle.

vestibule of the hemisphere for visual impulses. The
medial geniculate body is less prominent and occupies a
position immediately medial and posterior to the lateral
geniculate body. It relays auditory excitations to the
cerebral cortex.
(/) The optic tract (tractus opticus) passes obliquely over the
lateral surface of the brain from the ventrally situated
optic chiasma to the lateral geniculate body, part of it

354 ANATOMY OF THE RABBIT

ending there and part continuing to the superior colHculi
of the midbrain. The lateral geniculate body is a relay
station for visual impulses to the cerebral cortex, the fibres
which convey from the eye impulses producing conscious
visual experiences being among those which terminate here.
The fibres to the superior colliculus of the mammal are con-
cerned entirely with the production of reflex adjustments.
(m) On the ventral surface, the optic chiasma (chiasma opti-
cum), forms a conspicuous median cross-like 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. Thus the optic tracts are simply the direct
continuation of the optic nerves after they have crossed
in the chiasma.

While all mammals (and only mammals) have a certain proportion
of optic nerve fibres which do not cross in the chiasma, the number of
these is at a minimum in the rabbit, in which the eyes have an extreme
lateral direction. In mammals generally, the number of uncrossed
fibres is roughly proportional to the amount of overlapping of the
fields of vision of the two eyes.

(n) The hypophysis, or pituitary body, lies immediately behind
the optic chiasma (cf. p. 134). It is a somewhat elongate,
rounded, glandular organ attached to the base of the
brain by a slender stalk.

On account of its enclosure by the walls of the hypo-
physeal fossa, and of the relative weakness of the stalk
connecting it to the brain, the hypophysis is commonly
detached in removal of the brain from the skull, in which
case a slit-like aperture representing the ventral portion of
the third ventricle is exposed.

The meninges fuse in a collar-like ring round the transition between
the hypophyseal body and the infundibular stalk. Thence the dura
continues so as to form a complete lining for the bony hypophyseal
fossa, fused externally with the periosteum and over its whole inner
surface with the fibrous capsule of the gland. Thus the subdural and
subarachnoid spaces do not extend round the gland.

(o) 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-

THE MIDBRAIN 355

bulum, the latter being the slender stalk of the hypophysis
which connects it to the brain. When the hypophysis is
removed, the infundibulum which attaches it to the brain
is, naturally, broken, exposing the cavity in its base as the
slit mentioned above. The region of the tuber cinereum
contains centres concerned chiefly with the correlation of
olfactory with visceral sensory impulses.
{p) The mamillary body (corpus mamillare) forms a con-
spicuous rounded elevation, lying at the posterior end of
the tuber cinereum. The, structure is externally single in
the rabbit, but there is an indication of lateral lobes. It
also receives impulses from the olfactory correlation regions
of the cerebral hemisphere and combines them with others,
chiefly visceral.
The tuber cinereum and the mamillary body belong to the
hypothalamus, a region which includes centres for the control of
the visceral organs through the autonomic system and for the
regulation of highly integrated vegetative functions such as the
metabolism of water, carbohydrate, and fat and the maintenance
of body temperature.

THE mesencephalon:

(a) The boundary between prosencephalon and mesencephalon
is marked dorsally by the anterior edges of a pair of promin-
ent rounded elevations, which are associated with a second
pair just behind them to form the roof of the midbrain.
These four elevations are the corpora quadrigemina. The
anterior pair, distinguished as the colliculi superiores, is
much larger than the posterior pair, the colliculi inferiores.
The superior colliculi correspond to the optic lobes of sub-
mammalian vertebrates and receive many of the fibres of
the optic tracts as well as fibres conveying impulses of
other sensory types to be correlated with the visual ones.
The inferior colliculi are important reflex centres belonging
to the auditory system. _.
ih) The ventral part of the midbrain is shorter anteroposteriorly
than the dorsal one and is occupied by a pair of thick
ridges converging from in front, the cerebral peduncles.

356

ANATOMY OF THE RABBIT

These are separated by a faint median depression, the
interpeduncular fossa, just behind the mamillary body.
The superficial portion of each peduncle is composed of a
broad white band of longitudinal nerve fibres and contains
the main descending pathways carrying impulses from the
cerebral cortex to the cerebellum and to the motor centres
of the brain and spinal cord.
(c) The third cranial, or oculomotor nerve (n. oculomotorius),
which controls the majority of the eye-muscles, emerges
from the ventral surface of the cerebral peduncle.

THE rhombencephalon:

(a) The cerebellum forms a dorsal arch over the anterior part
of the hindbrain and is supported by stout pillars at its
sides. The dorsal part of the arch has become very mas-
sive, is moulded into several lobes, and has a superficial
layer of grey matter, the cerebellar cortex. This is thrown
into numerous transverse folds. The subdivisions recog-
nized include a median portion, the vermis, a cerebellar
hemisphere at each side of this, and a stalked body, the
paraflocculus, arising ventrolateral^ beneath each heml-
phere.

The flocculus is a small fold ventral to the stalk of each para-
flocculus.

Left lateral surface of cerebellum of the rabbit.

Fig. 119. Median section of cerebellum, the cortex stippled. Both figures after
Brodal. Ans, lobulus ansiformis; Ant, lobus anterior; F, flocculus; fp, fissura
prima; fpp, fissura prepyramidalis; fs, fissura secunda; LMM, lobus medius
medianus; N, nodulus; P, pyramis; Pf, paraflocculus; Pm, lobulus para-
medianus; sun, sulcus uvulonodularis; U, uvula; 1, lingula; 2, lobulus centralis;
3, 4, culmen; 1-4, lobus anterior.

THE HINDBRAIN

357

ib)

(c)

The various folds of the cerebellar surface are designated
as indicated in Figs. 118 and 119. The parts named lobulus
ansiformis and lobulus paramedianus together make up the
cerebellar hemisphere.

The cerebellum is concerned chiefly
with the co-ordination of muscular action,
the regulation of the "tone" of the
muscles, and the preservation of the
equilibrium of the body. The cerebellar
hemispheres are specially related to the
cerebral hemispheres and receive large
numbers of fibres (from the pons)
bearing impulses from the latter so
that they may co-ordinate the con-
tractions of muscles activated by direct
impulses from the same source.

The areas of the two crura of the
ansiform lobule indicated by hatching
in Fig, 120 have been shown to be
proportional to the weight of the muscle
masses of the fore and hind limbs
respectively in the rabbit and the re-
mainder of the hemisphere appears to be related to ability
to perform quick, powerful movements of the hind limbs
and trunk.

The anterior medullary velum (velum medullare anterius)
is the thin membrane underlying the anterior portion of
the cerebellum, attached to the ventral surface of the latter
and connecting it w4th the inferior colliculi (Fig. 124). It
forms a small anterior portion of the roof of the fourth
ventricle.

The fourth cranial, or trochlear nerve (n. trochlearis),
which supplies the superior oblique muscle of the eye,
emerges from the anterior medullary velum and runs
transversely on to the lateral surface of the cerebral
peduncle. Here the cut end of the proximal portion of the
nerve is usually to be found.

Fig. 120. Lat-
eral view of the
ansiform lobule
as in Fig. 118,
the superior
crus hatched
t r aiis-verse ly
and the inferior
crus hatched
vertically. The
areas of the two
crura are pro-
portional to the
weights of the
muscles of the
anterior and
posterior limbs
respectively, in-
dicating a prob-
able functional
relation to these
muscles.

358

ANATOMY OF THE RABBIT

{d) The posterior medullary velum (velum medullare pos-
terius) underlies the posterior margin of the cerebellum,
and extends backward over the triangular space enclosed
by the walls of the fourth ventricle. It is a more delicate
membrane than the anterior medullary velum and supports
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 brain, in which case the
interior of the fourth ventricle is exposed,
(e) On the ventral surface (Fig. 121), the pons forms a broad
band extending transversely across the brain and upward
into the supports (peduncles) of the cerebellum, its fibres
being distributed to the cortex of the cerebellar hemispheres.
Its surface is divided into two parts by a median depres-
sion, the sulcus basilaris, occupied by the basilar artery.
It is not really a commissure but is part of the pathway
connecting the cerebral hemispheres with those of the
cerebellum.

The portion of the brain
caudal to the posterior margin
of the pons is the medulla
oblongata.
(/) The anterior median fissure
of the spinal cord ends at the
posterior margin of the pons
in a faint depression, the
foramen caecum.
(g) The anterior funiculus of the
spinal cord is continuous with
a narrow band on the ventral
surface of the hindbrain, the
pyramid, which may be ob-
served extending backwards
on each side of the middle line
from the posterior margin of
the pons. The pyramids are
really the reduced continu-
ation backwards of the fibre-

zvvvy

Fig. 121. The rhombencephalon.
Ventral view (the cerebellum not
figured).

ct, trapezoid body; fc, foramen
caecum; tic, cervical flexure; fma,
anterior median fissure; p, pons;
PC cerebral peduncle (mesence-
phalon); py, pyramid.

Ill, oculomotor nerve; IV, troch-
lear; V^ portio major of the trige-
minus; V^, portio minor; VI, ab-
ducens; VII, facial; VIII, acoustic;
IX-XI, glossopharyngeal, vagus,
and spinal accessory group; XII,
hypoglossal; ci, first cervical spinal.

THE HIXDBRAIN 359

bands on the surface of the cerebral peduncles, which have
been reduced by the ending of many fibres in the pons.

(h) The trapezoid body (corpus trapezoideum) is another,
smaller, superficial transverse band just behind the pons,
and is part of the auditory pathway. It lies in the angle
formed by the lateral margin of the pyramid with the
posterior border of the pons, its fibres passing through
the deeper part of the pyramid (Fig. 123) so that they
are concealed by the latter.

(i) 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, whence they are
directed forward.

The portio major is the common trunk of the ophthalmic, maxillary,
and mandibular nerves, providing for the cutaneous sensibility of most
of the head. The portio minor (motor, to muscles of mastication) joins
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 semilunar ganglion, the latter lying in a depression
at the anterior ventral end of the petrosal bone.

(j) The sixth cranial, or abducent nerve (n. abducens), which
controls the lateral rectus muscle of the eye, is a slender
cord arising by several very delicate rootlets along the
lateral edge of the anterior end of the pyramid.

(k) The seventh cranial, or facial nerve (n. facialis), and the
eighth, or acoustic nerve (n. acusticus), appear to rise 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 which controls
the facial muscles. In addition the nerve receives a sensory filament,
the portio intermedia or intermediate nerve, bearing gustatory and
related impulses. The eighth nerve is purely sensory but comprises
two portions conveying respectively auditory and equilibratory im-
pulses from the corresponding parts of the internal ear. The trapezoid
body is composed of fibres transmitting impulses from the auditory
portion of the nerve, which-^ fibres decussate and eventually reach the
inferior colliculus and the medial geniculate body.

(l) The glossopharyngeus, vagus, and accessorius arise by
several roots arranged in a linear series along the lateral

^(LlBBArvV

\%\ y<2

^ 3SL y

360 ANATOMY OF THE RABBIT

margin of the medulla. The trunk of the accessorius
extends backward on the spinal cord, its roots, about ten
in number, arising as far back as the fifth cervical spinal
nerve. It is an efferent nerve, while the two former are
mixed nerves to various visceral organs.

{m) The twelfth cranial, or hypoglossal nerve (n. hypoglossus),
which controls the movements of the tongue, arises by
several roots from the ventral surface of the more posterior
part of the medulla oblongata at the lateral margin of
the pyramid, its point of origin corresponding to that of
the ventral root of a spinal nerve.
5. The arteries of the brain may be traced on its ventral surface

as follows:

(a) The basilar artery (a. basilaris) is a 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. It passes forward as far as the anterior
edge of the pons, giving off an irregular series of transversely
directed branches on the surface of the brain as well as
median branches which are concealed from view as they
run directly dorsad into the brain substance.

{b) The inferior cerebellar artery (a. cerebelli inferior) is the
largest of the transverse branches arising from the basilar
on the ventral surface of the hindbrain. It originates about
half way along the basilar artery and passes laterad and
up the side of the brain to the posterior part of the cerebel-
lum.

(c) The posterior cerebral artery (a. cerebri posterior) is
a paired vessel formed at the level of the anterior margin
of the pons by the bifurcation of the basilar. It passes
at each side laterad and dorsad to the posterior portion of
the cerebral hemisphere, giving secondary branches to the
diencephalon.

{d) The superior cerebellar artery (a. cerebelli superior) is a
relatively large branch of the posterior cerebral, arising
near the origin of the latter and passing to the anterior
portion of the cerebellum after giving branches to the
midbrain.

THE ARTERIES OF THE BRAIN 361

(e) The cut end of the internal carotid artery Hes on either
side of the tuber cinereum. It turns forward but is con-
nected backwards with the posterior cerebral by a posterior
communicating artery.

(/) The middle cerebral artery (a. cerebri media) is given off
from the internal carotid, branching over the middle por-
tion of the hemisphere to supply most of its lateral and
dorsal surfaces.

(g) The anterior cerebral artery (a. cerebri anterior) is the con-
tinuation of the carotid after the origin of the middle
cerebral artery. It passes to the anterior portion of the
ventral surface of the cerebral hemisphere and to the
olfactory bulb. The anterior cerebral unites with that of
the other side to form a short common trunk between the
hemispheres which redivides into paired vessels supplying
the medial surfaces. A complete anastomotic loop is thus
formed round the hypothalamus by the internal carotid,
anterior cerebral, posterior communicating, and posterior
cerebral arteries. This is the circle of Willis.

The fusion of the anterior cerebral arteries replaces an inter-
connection by an anterior communicating artery, which occurs in man
and many other mammals and occasionally appears in rabbits as an
individual variation.

6. By dividing the supports of the cerebellum on either side, the
entire structure may be removed and the surface may be ex-
posed, as in Fig. 122, for an examination of the structures of
the dorsal surface of the rhombencephalon. The posterior
medullary velum is removed with the cerebellum, but the
anterior medullary velum should be cut so that a small portion
of it remains in place.

(a) The fourth ventricle (ventriculus quartus) is the extensive
space enclosed by the rhombencephalon. It is connected
forwards with the cerebral aqueduct (the cavity in the
midbrain) and backwards with the central canal of the
spinal cord. Its roof is formed principally by the anterior
and posterior medullary vela, these being attached to the
cerebellum close to each other so that they underlie it.

362

ANATOMY OF THE RABBIT

ta -

(b) The rhomboid fossa (fossa rhomboldea) 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 median fissure (sulcus medianus fossae
rhomboideae) , on either side of which the floor is raised into
a low ridge, described as the medial eminence (eminentia
medialis). The posterior end of the fossa forms with the
enclosing wall the some-
what triangular figure des-
cribed as the calamus scrip-
torius.

(c) The lateral supports of the
cerebellum, now represent-
ed by their cut ends, are \
formed by fibre-bands con- ^^'^
necting the cerebellum with
adjacent portions of the fc-
brain. In each there are
three main bands or ped- s
uncles, though these cannot
usually be distinguished in
the cut surfaces. A middle
peduncle, thebrachium pon-
tis, represents the direct
continuation of the pons in-
to the cerebellum, bringing
impulses to the latter from the cerebral cortex. An ante-
rior band, the brachium conjunctivum, contains chiefly
(not exclusively) fibres leading out of the cerebellum and
running into the floor of the midbrain. A third band, the
inferior cerebellar peduncle or restiform body (corpus
restiforme) , comes from behind as a thick ridge continuous
with the dorsal part of the lateral funiculus of the spinal
cord. It forms the lateral wall of the more caudal portion of
the rhomboid fossa. Before turning dorsad into the cere-
bellum it passes under a rounded elevation, the acoustic
tubercle, where part of the auditory nerve ends. The
restiform body conveys impulses from various centres in

Fig. 122. The rhombencephalon. Dorsal
view, after removal of the cerebellum;
be, brachium conjunctivum; bp, brachium
pontis; cl, clava; cli, inferior colliculus
(mesencephalon); cr, restiform body; em,
medial eminence; fc, fasciculus cuneatus;
fg, fasciculus gracilis; fm, median fissure
of the rhomboid fossa; smp, dorsal me-
dian sulcus of the medulla; sip, dorso-
lateral sulcus; ta, acoustic tubercle; vma,
anterior medullary velum.

THE MEDULLA OBLONGATA

363

the spinal cord and medulla oblongata to the cerebellar
cortex.
{d) The dorsal funiculus of the spinal cord, as it passes forward
into the medulla oblongata, is divided into medial and
lateral portions. The medial portion, the fasciculus gracilis,
forms a narrow band terminating rostrally in a club-shaped
expansion, the clava. The lateral portion, the fasciculus
cuneatus, appears to pass into the restiform body but does

Fifi. 123. Transverse section of the hindbrain passing through the posterior
edge of the attachment of the cerebellum. The white nerve fibres are stained
black by the method of Weigert. be, bundles of nerve fibres proceeding to the
brachium conjunctivum; cr, restiform body; fi, flocculus; 1, lingula (part of
vermis cerebelli, see Fig. 119) na, cochlear root of acoustic nerve; nf, root fibres
of facial nerve which emerge from the brain a little further forward; p, pyramid;
pf, paraflocculus; pm, cerebellar hemisphere (lobulus paramedianus) ; t, trape-
zoid body, composed of nerve fibres originating in the acoustic tubercle and
crossing to the opposite side of the brain, where they turn fonvard towards the
inferior colliculus and the medial geniculate body; ta, acoustic tubercle; v, fourth
ventricle; ve, vermis cerebelli.

not actually do so. Both fasciculi end at this level. They
convey from the trunk and limbs impulses of muscle- and
joint-sensibility as well as tactile and related discrimination",
these being then transmitted to more anterior parts of the
brain by deeply situated fibres.

The brain may be divided by a median vertical section, and
the medial surface of one-half (Fig. 124) may then be examined.
In addition to many of the features already observed from other
points of view, the following may be noted:

364

ANATOMY OF THE RABBIT

(a) The deep but extremely narrow cavity formed by the third
ventricle is the most anterior space appearing in the brain,
the paired lateral ventricles, which are reckoned as the
first two without either being specifically designated as first,
lying laterally in the hemispheres. Each of these paired
ventricles is connected with the third ventricle by a narrow
transverse canal, the interventricular foramen (formen
interventriculare), situated a short distance dorsal to the
position of the anterior commissure (Fig. 124).

Fig. 12-1. The brain in median section: a, anterior commissure; ac, cerebral
aqueduct; bo, olfactory bulb; cb, cerebellum; cc, central canal of spinal cord;
ccl, corpus callosum; cf, body of the fornix; cl.i, inferior coUiculus; cl.s,
superior colliculus; cm, mamillary body; co, optic chiasma; cp, pineal body;
fl.c, cervical flexure; h. habenular commissure; he, cerebral hemisphere; hp, hip-
pocampus; inf, infundibulum; It, laminal terminalis; mo, medulla oblongata;
p, posterior commissure; pc, chorioid plexus of the third ventricle; pd.c, cerebral
peduncle; pn, pons; sp, splenium; spl, septum pellucidum; tc, tuber cinereum;
th, thalamus, massa intermedia; vma, anterior medullary velum; vmp, posterior
medullary velum; vq, fourth ventricle; vt, third ventricle; I, olfactory nerve
(origin); II, optic nerve.

(b) The anterior boundary of the third ventricle is formed
ventrally by the narrow transverse wall passing across
from one hemisphere to the other, the lamina terminalis,
in the dorsal portion of which is the small anterior com-
missure (commissura anterior), a connection between the
olfactory portions of the brain on one side and those on
the other. The ventral portion of the ventricle is projected
above the optic chiasma forming the recessus opticus, and
into the infundibulum, forming the recessus infundibuli.

INTERIOR OF THE CEREBRAL HEMISPHERE 365

teriorly but wider posteriorly in the rabbit, the cerebral
aqueduct (aquaeductus cerebri), which connects the third
ventricle with the fourth.

(d) The corpus callosum is shown in section. Anteriorly it
appears to end in a somewhat club-shaped expansion,
though actually it 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 greatly curved longitudinal fibre-
bands, fused for a short distance in the middle line to form the unpaired
body of the fornix (corpus fornicis). They begin in the hippocampus
(p. 366) and end in the mamillary body, conveying to the latter
impulses resulting from the correlation of olfactory with other stimuli.

(e) 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 approaching the medial
surface in each hemisphere and so lying close together in
this region.

8. The cerebral cortex may be removed from part of one hemi-
sphere by carefully scraping away the grey matter over
most of the dorsal and lateral surfaces until the white
surface of the corpus callosum is well exposed. By removing
the corpus callosum, the interior of the hemisphere may
then be examined.

(a) The lateral ventricle (ventriculus lateralis) is the extensive
cavity enclosed by the hemisphere. It reaches 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.

(b) The excised portion of the hemisphere, forming the mod-
erately thick roof and dorsolateral wall, consists largely of
the superficial grey cortex. The extensive portion of the
hemisphere wall containing this cortex and the white matter
immediately under it is termed the pallium.

366 ANATOMY OF THE RABBIT

(c) The floor of the lateral ventricle is formed by two somewhat
oblique, convex ridges. One of these, posterior and medial
in position, is the hippocampus, a part of the pallium which
has become folded inwards to form the ridge observed pro-
jecting into the ventricle. The other, anterior and lateral
in position, has a smaller ventricular exposure but is a
greatly thickened solid mass of nervous matter, the corpus
striatum (Fig. 117). Between the two ridges, the pig-
mented vascular tissue of the chorioid plexus of the lateral
ventricle may be made out. The hippocampus is a region
where olfactory stimuli coming from the pyriform lobe and
from more medial secondary olfactory areas are correlated
with others, chiefly of visceral origin. The corpus striatum
is related to the control of bodily movements.

(d) 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.

9. The passage of the olfactory nerves to the ethmoturbinal sur-
faces 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 connection with the skeleton (pp. 178, 190).

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
coloured injection mass, so that they are more easily traced. The
objects served by embalming are: (1) preserving the body from de-
composition 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 accom-
plished, first, by introducing the preserving fluid through the blood-
vessels, instead of by immersing the animal, as was formerly the
practice ;2 secondly, by using in the preserving fluid such materials

^The methods here given apply only to the preservation of specimens for
ordinary dissection, either singly, or in numbers for a laboratory course; with a
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 text-books may also be suggested.

Owing to imperfect preservation of the contents of the digestive tube,
examination of the inner wall in embalmed animals does not usually reveal its
essential features. It is well to have at hand a demonstration specimen prepared
as follows: Remove stomach and intestines from a freshly killed animal. Wash
out the parts with weak salt solution. Fill, without distending, with 3 per cent
bichromate of potash, formalin-bichromate, or any of the standard fixing agents,
and tie the ends. Immerse the parts in the solution, and after a time examine
by slitting them lengthwise.

2For sometimes convenient but less uniform preservation of small mammals
ordinary immersion methods may be employed, the animal being placed in a
preserving solution after making small incisions in the thoracic and abdominal
walls. Formaldehyde solutions of 2 per cent or better, and graded alcohols be-

367

368 ANATOMY OF THE RABBIT

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 through-
out dissection.

A suitable fluid for the purpose is that recommended by Keiller^
for the preservation of human subjects. The formula is as follows:

Formalin 1.5 parts

Carbolic acid 2,5 "

Glycerin 10.0 "

Water 86.0 "

100.0

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 six 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 six feet of rubber tubing to connect with the
operating 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

ginning with 30 per cent and changing to 60 or 70 per cent are useful for this
purpose. The addition of small quantities of glycerine or carbolic acid or both
improves the brightness and flexibility of the tissues,

^W. Keiller, "On the Preservation of Subjects, etc." (American Journal of
Anatomy, vol. II, 1902-3). Several modifications, apparently successful, have
been employed.

THE PRESERVATION OF MATERIAL 369

completed 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.
A slanting tip, produced by careful grinding on an emery stone
aids insertion into the vessel. 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
aspirator 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.

When large numbers of animals have to be prepared, it is useful
to connect the reservoir by wide rubber tubing to a horizontal
brass pipe six feet long, which is provided with five jets with taps.
A short length of quarter-inch rubber tubing connects each jet to
a Y-tube of glass or brass, which is in turn connected by slightly
smaller tubing to two cannulae. Thus ten animals may be em-
balmed simultaneously.

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 clamps 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

370 ANATOMY OF THE RABBIT

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 representing
the inguinal ligament Ues 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 length-
wise along the structures with 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, its flattened or
collapsed condition, and 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,
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 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.

^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.

THE PRESERVATION OF MATERIAL 371

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 to run in. At the beginning of the process a little
care in arranging the animal will be amply rewarded by con-
venience in dissection. The hind limb on the side opposite the
incision 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 sufflciently 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 apparatus 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.
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 and closed
by a stopper made by tying a short piece of rubber tubing in a knot.
The animal is then set aside, preferably for twenty-four hours to

372 ANATOMY OF THE RABBIT

several days, but if the specimen is wanted for use immediately
the injection of the coloured mass into the vessels 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
colouring material, such as vermilion or a very small quantity of
carmine. There is some advantage in using a 5 per cent or even
stronger formalin and about one part in seven of glycerin 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 glycerin keeps the starch suspended better. 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 in-
jection to run backward and forward in the tube, each time apply-
ing 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 easily be removed, cleaned, and
replaced. The same cannula should always be used.

During recent years, coloured latex has largely replaced other
masses for injection of the blood-vessels in animals to be dissected.
It has the advantage of forming a strong, tough, elastic body
within each vessel. The material may be obtained from commercial
supply houses, which will also provide advice regarding minor
precautions that facilitate its use.

Material prepared according to the directions given above will
keep indefinitely, provided, however, that precautions are taken
to avoid contamination from the surface. These are especially
necessary in view of the thick coating of hairs. It is a good plan,

THE PRESERVATION OF MATERIAL 373

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 per cent
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, or a paraflin-
wax-lined galvanized box with slip over cover, of dimensions
7 X 7 X 24 inches, will be found to be adequate and of proper
proportions for animals of average size. If less costly individual
containers are required, ordinary water-proofed paper sheets or
bags may be employed, the latter being now obtainable through
regular trade channels.

INDEX

When several references occur, the page containing the main
or definitive description is indicated in bold-faced type.

Abduction, 69
Acetabulum, 207
Acromion, 199
Adaptation, 5
Adduction, 69
Aditus laryngis, 311, 314
Adrenalin, 132
Albinism, 21
Allantois, 115
Ampulla caecalis coli, 238
Ansa subclavia, 328
Antrum, pyloric, 225, 231

tympanic (mastoid), 187
Aorta, 113, 114, 117, 226, 254, 324, 326

ventral, 113, 114

Aperture, piriform, 175

thoracic, 165

Aponeurosis, 20, 65, 67

Aqueduct, cerebral (Sylvian), 84, 148,

365
Arachnoidea, 72, 344
Arch, costal, 165

hyoid, 57, 89, 198
mandibular, 57
vertebral, 51
volar, 270
zygomatic, 167, 174
Arches, aortic, 113, 114,
branchial, 57
visceral, 55, 57, 198
Arteries, 109, 113
Artery, alveolar inferior, 305, 307
angular, 294
appendicular, 240
axillary, 262
basilar. 360
brachial, 262, 270
bronchial, 109, 325
caecal, 241

carotid, common, 114, 117, 301,
306, 324

external, 306
internal, 172, 306, 317,
322, 361
caudal, lateral, 256, 280
cerebellar, inferior, 360
superior, 360
cerebral, anterior, 361
middle, 361
posterior, 360
cervical, ascending, 258, 325
superficial, 258, 325
circumflex, lateral of femur, 279
medial of femur, 256
of humerus, 263, 269
coeliac, 226, 229, 254
colic, left, 241

middle, 240
right, 240
Artery.^'collateral, radial, 270
ulnar, 270
communicating, anterior, 361
posterior, 361
coronary, 324, 330
cystic, 234
deep of arm, 263, 270

thigh, 279
deferential, 246
dental, superior, 319
epigastric inferior, 223, 255

superficial, 221, 262,

279
superior, 223, 326
ethmoidal, 319
facial, transverse, 294, 307
femoral, 279
frontal, 174, 319

375

376

INDEX

gastric, left, 229

right, 229, 230
short, 229
gastroduodenal, 230
gastroepiploic, left, 229

right, 229, 230
genu suprema, 279
haemorrhoidal, inferior, 248,
253
middle, 248,
253, 256
superior, 241
hepatic, 230, 233
hypogastric, 117, 255, 280
ileocaecal, 240
ileocaecocolic, 240
iliac, common, 117, 255
external, 255, 279
internal, 255
iliolumbar, 221
infraorbital, 319
innominate, 114, 324
intercostal, 326, 336

highest, 325
intercostalis suprema, 325
intestinal, 241
labial inferior, 294
superior, 294
lacrimal, 174, 319
laryngeal, inferior, 301
superior, 301
lingual, 306
lumbar, 255
malleolar, 286
Artery, mammary, external, 263
internal, 326
maxillary external, 294, 306
internal, 307, 318,
319
median, 270
medianoradial, 270
mesenteric, inferior, 241, 254
superior, 227, 240
254
obturator, 256

occipital, 306
omental, 229
ophthalmic, 317

inferior, 319
palatine, anterior, 319
pancreaticoduodenal, inferior,
236, 240
superior,
230, 236
peroneal, 283, 286, 287
phrenic, inferior, 229

superior, 254, 338
popliteal, 280, 286
pterygopalatine, 319
pudendal, internal, 248, 253,

256, 280
pulmonary, 114, 117, 118, 330,

335
radial, 270
renal, 243

sacral, median, 254, 255
saphenous, great, 280, 282,
283, 286, 287
small, 282, 285, 287
scapular, transverse, 262
sciatic, 256, 280
spermatic, external, 223, 245
internal, 246, 250
sphenopalatine, 319
spinal, ventral (anterior), 347
splenic, 229

subclavian, 114, 117, 324, 325
submental, 294
subscapular, 221, 262
suprarenal, 243, 255
suprarenolumbar, 243, 254
temporal, superficial, 307
thoracic, external, 221, 262

lateral, 221, 262
thoracoacromial, 260, 262
thoracodorsal, 262
thyreoid, superior, 301
tibial, anterior, 284, 286

posterior, 283, 286, 287

INDEX

377

transverse facial, 294

of neck, 260, 325
scapular, 262
ulnar, 271

umbilical, 118, 245, 246
uterine, 251
vertebral, 325, 360
Articulations, 48, 49

of posterior limb, 290
Autonomic nervous system, 74, 321,

355
Atlas, 52, 159
Atrium, 331
Axes of skull, 60
Axis, 160

Bladder, gall, 232

urinary, 118, 244
Blood, 32
Body, carotid, 90, 306

cavernous, 247, 252

ciliary, 92, 317

geniculate, lateral, 353, 354
medial, 353

mamillary, 355

pineal. See gland, pineal

pituitary. See hypophysis

restiform, 362

spongy of urethra, 247

trapezoid, 359

vitreous, 93, 318
Bone, 22, 23

acetabular, 208

alisphenoid, 53, 169, 182

atlas, 159

axis, 160

basioccipital, 53, 167, 169, 180

basisphenoid, 53, 169, 182

calcaneus, 214

carpal, 63, 204

clavicle, 61, 62, 200

coracoid, 61, 200

costal, 164

coxal, 206

dentary, 195

Bone, epistropheus, 160

ethmoid, 53, 58, 177, 178, 189

ethmoturbinal, 53, 178, 190, 191

exoccipital, 53, 167, 180, 181

femur, 210

fibula, 213

frontal, 54, 58, 169, 172, 189

humerus, 201

hyoid, 54, 58, 197

ilium, 207, 208

incisive, 193

incus, 55, 58, 188

inferior turbinal, 58, 191

interparietal, 54, 58, 173, 188

ischium, 207, 209

lacrimal, 54, 58, 174, 194

malar, 174

malleus, 55, 58, 188 {See also

ossicles, auditory)
maxilla, 54, 58, 174, 175, 191
maxilloturbinal, 54, 178, 191
mesethmoid, 178
metacarpal, 205
metatarsal, 215
nasal, 54, 58, 175, 178, 193
nasoturbinal, 54, 178, 190
occipital, 58, 167, 180
orbitosphenoid, 53, 169, 183
palatine, 54, 58, 169, 174, 194
parietal, 54, 58, 172, 188
patella, 215
periotic, 53, 58, 171, 176, 177,

• 184
petromastoid, 171, 187
petrotympanic, 184, 185
phalanx, 205, 215
pisiform, 205
postminimus, 63
premaxilla, 54, 58, 174, 175, 178,

193
presphenoid, 53, 169, 183
procoracoid, 61
pterygoid, 54, 58, 183
pubis, 207, 210
radius, 202

378

INDEX

Bone, sacral, 163

scapula, 61, 198
sesamoid, 206, 215, 290
sphenoid, 58, 170, 182, 183
squamosal, 54, 58, 169, 184
stapes, 55, 57, 58, 188
sternum, 166

supraoccipital, 53, 167, 180, 181
talus, 213
tarsal, 63, 213
temporal, 184
tibia, 212
turbinal, 58
turbinated, 178,191
tympanic, 54, 184, 187
ulna, 202

vomer, 58, 178, 194
zygomatic, 54, 58, 169, 174, 193
Bones, of auditory chain, 54, 55, 56,
57, 170, 188
carpal, 204
limb, 50, 60, 198
metacarpal, 205
metatarsal, 215
phalanges, 205, 215
sesamoid, 206, 215
tarsal, 213
Brachium conjunctivum, 362

pontis, 362
Branchiomerism, 41
Breathing, 107, 166
Bridge, palatine, 174
Bronchus, 335
Bulb, olfactory, 349
Bulla, tympanic, 168, 170, 171, 185, 186

Caecum, 103, 235, 237

Calamus scriptorius, 362

Canal, facial, 172, 178, 186

hypoglossal, 172, 177, 181
infraorbital, 174
nasolacrimal, 175, 192
pterygoid, 171, 183
pterygopalatine, 175, 195
vertebral, 157

Capillaries, 109, 111, 120

lymphatic, 120
Capsule, auditory, 53, 55, 57, 58
joint, 49, 290
nasal, 53, 55, 57, 58
Cartilage, 22

articular, 49
arytenoid, 313
corniculate, 313
costal, 164
cricoid, 300, 313
cupula posterior, 146
epiglottic, 314
laryngeal, 57
of Meckel, 56, 144
mesethmoid, 142, 178
nasopalatine, 142
nasoturbinal, 144
suprascapular, 200
thyreoid, 300, 313
vomeronasal, 142, 144
Cauda equina, 345
Cavity, cranial, 175

glenoid of scapula, 200

skull, 169
nasal, 175

oral. See oral cavity
orbital, 167
peritoneal, 136, 224
pleural, 135, 334
serous, 135, 224
thoracic, 165

tympanic, 136, 158, 170, 174,
175, 186, 187, 300,
Calyx, renal, 123, 124
Cell, 11

Centre, respiratory, 108
Cerebellum, 356
Cerebral hemisphere, 82, 349
Cerebrum, 84
Chiasma, optic, 354
Choana, 175

Chondrocranium, 55, 57, 58
Chorioidea, 317
Chromatophores, 21

INDEX

379

Circle of Willis, 361
Clava, 363
Clitoris, 219, 252
Clivus, 176
Cloaca, 122
Cochlea, 91, 148, 322
Coccyx, 51
Coelom, 135, 224
CoUiculus, inferior, 355
seminalis, 250
superior, 354, 355
Colon, 102, 237, 238
Column, vertebral, 44, 50, 156
Commissure, 350

anterior, 364
habenular, 351
middle, 352
posterior, 352
Condyle, occipital, 167, 181
Conjugation, 12
Connective tissue, 18
Convergence, 4
Cord, spermatic, 247
Cord, spinal. See spinal cord
Cords, vocal. See vocal folds
Corium, 14, 20, 220
Cornea, 92, 317
Corpora lutea, 250

quadrigemina, 355
Corpus callosum, 350, 365

striatum, 366
Cortex, cerebellar, 356

cerebral, 82, 349, 365
Cranium, 166

cerebral, 55, 58
visceral, 55, 58
Crest, nuchal, 168, 181
Crista galli, 177, 190
Crus clitoridis, 252

penis, 247
Cytoplasm, 11

Denticles, shagreen, 57
Descent of the testis, 129
Diaphragm, 66, 67, 108, 140, 337

Diastema, 59, 192, 195
Diencephalon, 83, 350
Digestion, 94
Digits, 220
Divergence, 4.
Duct, bile, 233, 235

CN'stic, 233

deferent, 127, 128, 246, 249

hepatic, 233

incisive. See duct, nasopalatine

nasolacrimal, 175, 314

nasopalatine, 311

pancreatic, 236

parotid, 146, 293

submaxillary. 146, 298, 307

thoracic, 120, 337
Ductusarteriosus(Botalli), 314,317, 318
Ductus venosus, 115, 117
Duodenum, 100, 154, 235
Duplicidentata, 7
Dura mater, 72, 344

Ear, 90, 219, 321, 322
middle, 187

Endothelium, 18, 22

Epaxial musculature, 67, 339

Epibranchial, 78, 342

Epicardium, 135, 328

Epidermis, 14

Epididymis, 128, 246

Epiglottis, 312

Epineurium, 30

Epiphysis, 47

cerebri, 83

Epiploon, 226

Epistropheus, 52, 160

Epithalamus, 83, 86

Epithelium, 13

glandular, 15
nasal, 90, 105, 179
sensory, 17

Erythocytes, 33

Evolution, 4

Excretion, 17

Extension, 68

380

INDEX

Exteroceptors, 90
Eye, 92, 218, 317
Eyelids, 218

Falx cerebri, 348
Fascia, 20

infraspinous, 264
lumbodorsal, 222, 223
supraspinous, 264
Fasciculus cuneatus, 363

gracilis, 363
Fat, 21

Fat masses of neck, 21, 140, 150
Fenestra, cochlear, 187

vestibular, 187
Fertilization, 12
Fibre, nerve, 30
Filum terminale, 78, 345
Fissure, cerebral, lateral, 350

cerebral, longitudinal, 349
limbic, 349

orbital, superior, 171, 177, 182
portal, 232
rhinal, anterior, 349
posterior, 349
Sylvian, 350
Flexion, 68

Flexures of embryonic brain, 85, 142
Flocculus, 356

Fluid, cerebrospinal, 344, 351
Foetal circulation, 115
Fold, middle umbilical, 244
rectovesical, 244
vesicouterine, 244
vocal, 314
Follicle, hair, 14, 221
Follicles, lymph, 119

vesicular ovarian, 250
Foot, 8, 9
Foramen caecum, 358

carotid external, 172

internal, 172, 187
cavernosum, 182
costotransverse, 159, 160
greater palatine, 175

Foramen incisive, 174, 175, 191
infraorbital, 174, 191
interventricular, 364
intervertebral, 158
jugular, 172, 177, 186
lacerum, 172, 177, 182
magnum occipitale, 167, 180
mandibular, 180, 196
mental, 180, 196
obliquum, 160
obturator, 206
optic, 171, 176, 177
ovale, 115, 118, 172, 332
palatine, greater, 175
rotundum, 171
sphenoidal, 171, 183
sphenopalatine, 175, 195
stylomastoid, 172, 186
supratrochlear, 202
thyreoid, 313
transversarium, 159
vertebral, 157
Fornix, 365, 366
Fossa, acetabular, 209

axillary, 219

cranial, 175

hypophyseal, 176

infraspinous, 199

interpeduncular, 356

jugular, 168, 181, 186

mandibular, 169, 185

maxillary, 191

nasal, 174, 175

ovalis, 332

parafloccular, 186

popliteal, 220

pterygoid, 169, 183

rhomboid, 362

subscapular, 200

supraspinous, 199

temporal, 170, 185

trochanteric, 211
Frenulum linguae, 311
Funiculi, 347, 363
Furrow, inguinal, 219

INDEX

381

Ganglia of sympathetic trunk, 74, 257
Ganglion, 31

cervical, inferior, 75, 328
superior, 75, 309,
321
ciliary, 320, 321
coeliac, 227
dorsal root, 73, 346
jugular, 302, 309
mesenteric, inferior, 242
superior, 227
nodosum, 309
otic, 321
semilunar, 359
sphenopalatine, 321
spinal, 73, 346
submaxillary, 321
thoracic, 328
Girdle, pectoral, 61, 198

pelvic, 61, 206
Gland, 15

acinous, 16
alveolar, 16
anal, 248, 253
buccal, inferior, 297
superior, 296
bulbourethral, 250, 253
Cowper's, 250
cutaneous, 16
cytogenic, 16
ductless, 131
endocrine, 15, 131
exocrine, 15
Harderian, 315
hibernating, 21
infraorbital, 316
inguinal, 248, 253
interrenal, 133
lacrimal, 316
lymph, 119, 121

axillary, 260, 261
deep cervical, 300
facial, 297
iliac, 281
inguinal, 221, 281

Gland, lymph, mesenteric, 237, 239,
242
popliteal, 281
superficial cervical, 293

mammary, 16, 221

mandibular, superficial, 297

masseteric, 297

oral, 16

paraprostate, 249

parathyreoid, 133

parotid, 293

pineal, 134, 351

pituitary. See hypophysis

prostate, 249

rectal, 248, 253

salivary, 296

sebaceous, 16

sublingual, 307

submaxillary, 298, 307

sudoriferous, 16

suprarenal, 132, 227

thymus, 133, 324

thyreoid, 133, 300

tubular, 16

types, 15, 16, 17

vesicular, 249

zygomatic, 316
Glans clitoridis, 252

penis, 247
Glomus caroticum, 90, 306
Groove, infraorbital, 175
Gubernaculum, 129, 245

Habenula, 351

Hair, 14

Hallux, 220

Haustra, 239

Heart, 109, 112, 114, 115, 117, 330

Hemisphere, cerebellar, 356, 357

cerebral, 82, 349
Hippocampus, 366
His, bundle of, 28
Homology-, serial, 60

true, 4, 62
Hydatid, 251

382

INDEX

Hyoid apparatus, 197
Hypaxial musculature, 67, 432
Hypobranchial musculature, 67, 432
Hypophysis, 134, 354
Hypothalamus, 83, 355

Ileum, 102, 236

Incisure, supraorbital, 174

Infundibulum, 354

Inguinal space, 219, 248, 253

Insulin, 132

Interoceptors, 90

Intestine, 100, 234

blind. See caecum

large, 237

length, 99

mesenterial, 101, 235, 236

small, 235

straight. See rectum
Iris, 317
Isthmus (rhombencephali), 84

Jejunum, 102, 236
Joint, ankle, 291

hip, 290

knee, 290
Joints, 49

Kidney, 123, 242

Labyrinth of ear, 90, 322
ethmoidal, 190

Lagomorpha, 7

Lamina papyracea, 191
terminalis, 364

Larynx, 300, 312

Lens, 92, 318

Leporidae, 6

Leptomeninges, 344

Leucocytes, 33, 121

Ligament, 20

acromioclavicular, 200
arterial, 114, 118, 330
broad, 138, 251
calcaneohbular, 291

Ligament, calcaneotibial, 291
carpal, 267, 268
cleidohumeral, 200
coracoclavicular, 200
coronary, 233
cruciate of foot, 283
knee, 291
crural, 283

dorsal of the neck, 169
falciform, 232
fibular collateral, 290
gastrosplenic, 226
hepatoduodenal, 226, 232
hyothyreoid, 198
iliofemoral, 290
inguinal, 222

intermuscular, lateral, 277
interosseous of forearm, 202
leg, 284,^291
ischiocapsular, 290
ovarian, 129, 251
patellar, 215, 290
phrenicosplenic, 226
pubocapsular, 290
pulmonary, 334
round, of hip, 290
liver, 232
uterus, 129, 251
sternoclavicular, 166, 200
stylohyoid, 198, 299, 306
suspensory, 247, 252
talofibular, 291
talotibial, 291
tibial collateral, 290.
tibionavicular, 291
triangular, 233
umbilical, middle, 244

Ligamentum nuchae (dorsal ligament
of the neck), 169

Line, epiphysial, 47

superior nuchal, 168
temporal, 170

Lineaalba, 221, 222, 223
semilunaris, 223

Liver, 95, 231

INDEX

383

Lobe, optic, 355

piriform, 349
Lung, 334
Lymph, 34, 120

nodes. See gland, lymph
Lymphatic vessels, 120

Macrophage system, 19
Malleolus, lateral, 213
medial, 213
Mammalia, 9, 10
Mandible, 8, 54, 56, 179, 195
Manubrium sterni, 166
Marrow, 25, 292
Marsupium nasale, 190
Mass, intermediate, 352
Mastoid portion of petrotympanic bone,

171, 185
Meatus, acoustic, external, 170, 185
219
internal, 178, 186
Mediastinum, 329
Medulla oblongata, 358
Membrane, basement, 14

nictitating, 218

serous, 18, 135

tympanic, 322
Meninges, 72, 344
Meniscus, 49

of knee joint, 291
Mesencephalon, 80, 355, 364
Mesenchyme, 18
Mesentery, 135, 224, 236

ventral, 232
Mesocolon, 239
Mesoduodenum, 137, 235
Mesogastrium, 225
Mesonephros, 126, 127
Mesometrium, 138, 251
Mesorchium, 138, 246
Mesosalpinx, 138, 251
Mesothelium, 18, 22, 135
Mesovarium, 138, 250, 251

Metacromion, 199
Metamerism, 41
Metanephros, 126
Metathalamus, 353
Microglia, 32

Mucous membranes (tunics), 14, 231,
236, 237

Muscle, 25, 63

abductor caudae anterior, 343
posterior, 343
digiti quinti, 269
pollicis, 267
adductor brevis, 276
digiti, 251

quarti, 269
quinti, 269
indicis, 269, 286
longus, 276
magnus, 28, 276
minimi digiti, 286
anconaeus, 266

minimus, 266
arrectores pilorum, 15
arytenoideus transversus, 314
auricular, cutaneous, 296
basioclavicularis, 258
biceps brachii, 266

femoris, 274, 278
brachialis, 266
brachiocephalic, 264
buccinator, 296
caninus, 296
cardiac, 28, 64
cleidohumeralis, 264
cleidomastoideus, 258, 264
coccygeal, 343
constrictor pharyngis, 310
coracobrachialis, 265
corrugator supercilii, 294
cremaster, 245
cricoarytenoideus, 314
cricothyreoideus, 300, 313
cutaneus maximus, 221, 257
deltoideus, 265

384

INDEX

Muscle, depressor conchae anterior, 292
posterior,
257. 292
palpebrae inferioris,
293
digastricus, 168, 299, 305
epaxial, 67, 339
extensor, 68

antibrachii parvus,

266
carpi radialis brevis,
267

longus,
267
ulnaris, 267
caudae medialis, 343
digiti quarti proprius
267
quinti proprius
267
digitorum communis,
267
longus,
283, 291
hallucis longus, 283
pollicis et indicis,267
facial, 294
flexor, 68

capri radialis, 268
capri ulnaris, 268
caudae, 343
digiti quinti, 269
digitorum longus, 285
profundus,

268
sublimis, 268
pollicis brevis, 269
gastrocnemius, 284
gemellus inferior, 276
superior, 275
genioglossus, 308
geniohyoideus, 308
glutaeus maximus, 274
medius, 274
minimus, 275

Muscle, gracilis, 274, 278
hamstring, 277
hyoglossus, 308
hypaxial, 67, 342
iliacus, 273

iliocostalis, 340
iliopsoas, 273
infraspinatus, 265
intercostal, 323
interosseus, 269, 286
intertransversarius, 341
intracostal, 324
involuntary, 26, 28, 63
ishciocavernosus, 247, 252
of jaw, 6
of larynx, 314
latissimus dorsi, 222, 260
levator alae nasi, 296
costarum, 336
palpebrae superioris,

315
scapulae major, 258
minor, 259
lingualis, 308
longissimus, 340

capitis, 340
cervicis, 340
costarum, 340
dorsi, 340
longus atlantis, 343
capitis, 343
colli, 343
lumbrical, 269, 286
masseter, 179, 294, 299, 302,

305
mentalis, 296
multifidus, 340
myloh>'oideus, 299
obliquus capitis inferior, 342
major, 150
superior, 341
externus, 222
inferior, 315
internus, 223
superior, 315

INDEX

385

Muscle, thoracis, 342

obturator externus, 276
internus, 275
orbicularis oculi, 293
orbicularis oris, 293
palmaris, 268
papillary, 332
pectineus, 276
pectoralis major (secundus),

222, 260
pectoralis (primus — quartus),

260, 261
pectoscapularis, 261
peronaeus, brevis, 284
longus, 284
quartus, 284
tertius, 284
piriformis, 275
plantaris, 285
platysma, 257, 292
popliteus, 285
pronator teres, 268
psoas major, 273
minor, 272
pterygoid, external, 304, 305
internal, 299, 304,
305
pubocavernosus, 247, 252
quadratus femoris, 276

labii inferioris, 296
superioris, 295
lumborum, 273
quadriceps femoris, 276
rectocaudalis, 248, 253
rectus abdominis, 223

capitis anterior, 343
lateralis, 342
posterior major,
342
minor,
342
superficialis,
341
femoris, 276
inferior, 315

Muscle, lateralis, 315

medialis, 315
superior, 315
red, 28

retractor oculi (bulbi), 315
rhomboideus major, 259
minor, 259
sacrococcygeal, 343
sacrospinalis, 339
sartorius, 274, 278
scalenus anterior, 342
dorsalis, 342
medius, 342
posterior, 342
ventralis, 342
semimembranosus, 279
semispinalis capitis, 341
cervicis, 341
dorsi, 340
semitendinosus, 28, 279
serratus anterior, 222, 259

posterior, 338
soleus, 28, 285
sphincter ani, 248, 253
splenius, 339
stapedius, 322
sternohyoideus, 300
sternomastoideus, 171, 258,

299
sternothyreoideus, 300
styloglosuss, 168, 307
stylohyoideus major, 168, 198
306, 307
minor, 168, 197,
198, 307
st\ lophar} ngeus, 308
subcostal, 324
subcutaneus faciei, 294
subscapularis, 265
supraspinatus, 265
temporal, 170, 303, 305
tensor fasciae cruris, 274, 279
fasciae latae, 275
tympani, 322

386

INDEX

Muscle, teres major, 265
minor, 265
thyreoarytenoideus, 314
th>reohyoideus, 300
tibialis anterior, 283
posterior, 283
transversus abdominis, 223
costarum, 342
thoracis, 323
trapezius, 259
triceps brachii, 266

surae, 284
vastus intermedius, 277
lateralis, 277
medialis, 277
voluntary, 26, 65
white, 28

zygomaticus minor, 295
Myelin, 30
Myofibrils, 26

Nasal epithelium, 105, 179
Nasopharynx, 99, 310, 312
Nephron, 124
Nerve, 30, 73

abducent, 319, 359
accessorius. See nerve, spinal

accessory
acoustic, 178, 359
alveolar, inferior, 305
superior, 320
auricular, 259
chorda tympani, 322
cranial, 88

cutaneous, posterior, 281, 289
depressor. See nerve, vagus,

cardiac branch
ethmoidal, 320
facial, 89, 176, 178, 186, 294,

359
femoral, 273, 280, 289
frontal, 174, 320
glossopharyngeal, 89, 306, 308,
309, 359

Nerve, gluteal, inferior, 281, 289
superior, 281, 289
of Hering, 306
hypoglossal, 89, 301, 308, 309,

360

ramus descendens,
301, 309, 310
infraorbital, 320
intercarotid, 306
intercostal, 336
intermediate, 322, 359
lacrimal, 174, 320
laryngeal, superior, 301, 308
lingual, 305, 308
mandibular, 88, 172, 179, 305,

308, 319
masse terico temporal, 319
maxillary, 320
median, 263, 272
mental, 305
mylohyoid, 305
nasociliary, 320
nasopalatine, 321
obturator, 289
oculomotor, 319, 356
olfactory, 177, 349
ophthalmic, 320
optic, 88, 317, 318, 354
palatine, 146

anterior, 321
peroneal, 28L 284, 288
petrosal, deep, 321

great superficial, 186,
321
phrenic, 327
pterygobuccinator, 319
of the pterygoid canal. See

nerve. Vidian
pudendal, 281, 289
radial, 263, 271
recurrent, 326, 327
saphenous, greater, 280,^287

lesser, 281, 285, 288
sciatic, 280, 289
sphenopalatine, 320, 321

INDEX

387

Nerve, spinal, 73, 345

accessory, 89, 309, 359
splanchnic, 226, 228
subscapular, 263
suprascapular, 263
sural, 288

tenth. See nerve, vagus
tibial, 281, 283, 285, 288
trigeminal, 88, 305, 320, 359
trochlear, 319, 357
ulnar, 263, 272
vagus, 89, 230, 301, 302, 309,
326, 335, 359
cardiac branch, 302, 309

327
recurrent branch, 326,
327
vasomotor, 64
Vidian, 171, 320, 321
Nerve fibre, 30
Nervous system, autonomic, 74, 321,

355. {See also sympathetic trunk)
Neural tube, 80
Neurilemma, 30
Neurocoele, 80
Neurocranium, 55, 58
Neuroglia, 29, 32
Neuron, 29
Nipples, 219
Node, atrioventricular, 64

lymph. See Gland, lymph
of Ranvier, 30
sinuatrial, 64
Notch, acetabular, 209
sciatic, greater, 208

lesser, 209
umbilical, 233
Notochord, 52
Nuchal surface, 167
Nucleus, 11
Nucleolus, 11

Oesophagus, 100, 335
Olecranon, 204

Omentum, 137

greater, 226

hepatogastric, 226, 232

lesser, 226, 232
Ontogeny, 3
Oral cavitN-, 99, 310
Orbit, 167, 169, 175
Organ of Corti, 91

vomeronasal, 178
Ossicles, auditory, 54, 55, 56, 57, 170,

188
Ossification, 46
Osteocranium, 55, 58
Ovary, 129, 131. 250
Oviduct, 130
Ovum, 12, 129

Pachymeninx, 344
Palate, hard, 174, 311
soft, 195, 311
Pallium, 82, 142, 365
Pancreas, 131, 225, 228, 236
Papilla, circumvallate, 99

conical, 311

filiform, 311

foliate, 99, 312

fungiform, 312

operaria, 211

vallate (circumvallate), 99,
312
Paraflocculus, 356
Parasympathetic, 321
Peduncle, cerebellar, 362

cerebral, 355
Pelvis, 206, 224

renal, 123, 243
Penis, 219, 247
Pericardium. 135, 328
Perichondrium, 22, 46, 49
Perimysium, 28
Perineurium, 72
Periosteum, 24, 46, 49
Peritoneum, 136, 224, 244
Petrous portion of petrotympanic bone,
177, 186

388

INDEX

Pever, aggregated lymph nodules of,

237
Pharynx, 99, 310, 312
Phylogeny, 3
Pia mater, 72, 344
Pigment, 21

Placenta, 8, 10, 115, 130
Plate, cribriform, 176, 190
Platelets, 33
Pleura, 135, 329, 334
Plexus, aortic, 242

brachial, 263
cardiac, 327, 328
cervical, 263

chorioid, 83, 351, 358, 366
coeliac, 228, 230
coronary, 327
formation in nerves, 75
ganglioformis, 309
hypogastric, 242
lumbar, 289
lumbosacral, 273, 288
lymphatic, 120
mesenteric, inferior, 242
superior, 228
pampiniformis, 247
renal, 228, 242
sacral, 289
spermatic, 242
Pollex, 220
Pons, 358

Portal system, 110, 111
Pouch, rectovesical, 244

vesicouterine, 244
Prepuce, 219

Process, alveolar, 174, 192
clinoid, 176
condyloid, 180, 197
coracoid, 62, 200
coronoid, 197
ethmoidal, 184
jugular, 167, 181
mastoid, 171, 185
odontoid, 160
orbital, of maxilla, 191, 192

Process, palatine, of maxilla, 191, 192
of premaxilla, 193

paramastoid, 167

pterygoid, 169, 171, 182, 183

pyramidal, 195

sphenoorbital, 191, 192

styloid, 204

supraorbital, 170, 174

thyreohyal, 198

vermiform, 103, 237, 238

xiphoid, 166

zygomatic, 169, 174, 185, 191,
192
Promontory, 163, 187
Pronephros, 126, 127
Proprioceptors, 90
Prosencephalon, 80, 81, 349
Protuberance, external occipital, 168,

181
Purkinje fibres, 28
Pylorus, 225
Pyramid, 358

renal, 244

Recapitulation, 3

Receptors, 90

Rectum, 238, 239

Reflex action, 77

Reproduction, 8

Respiration, 105

Recticulo-endothelial system, 19

Retina, 92, 93, 318

Rhinencephalon, 82

Rhombencephalon, 80, 356, 361

Rib, 164

Rodentia, 7

Rotation of forearm, 70, 204

Sac, scrotal (sac of testis), 129, 136,
137, 219, 223, 224, 245
serous, 135, 224
Sacculus rotundus, 235, 237
Sacrum, 50, 163
Sarcolemma, 27
Sclera, (sclerotic), 92, 317

INDEX

389

Scroll. See bone, turbinated
Scrotum. See sac
Secretin, 132
Secretion, 15, 17
Sella turcica, 176
Sense organs, 89

auditory, 90, 219, 321,

322
gustatory, 90, 99
olfactory, 90
visual, 90, 92, 218, 317
Septum, nasal, 178

pellucidum, 365
Sinus, aortic, 333

carotid, 90, 306

coronary, 326

maxillary, 192

pulmonary, 332

sphenoidal, 182

superior sagittal of dura mater,

348
tonsillar, 312
transverse of dura mater, 172,

181, 186, 297, 348
urinogenital, 122, 130
Sinusoids, 97, 110
Skull, 53, 166

human, 60
Space, epidural, 344

inguinal, 219, 248, 253
perivascular, 72
Specialization, 5
Spermatocytes, 128, 130
Spermatogonia, 128
Spermatozoon, 12, 128
Spinal cord, 31, 78, 344
Splanchnocranium, 56, 58
Spleen, 122, 225, 229
Sternum, 166
Stomach, 100, 224
Sulcus arteriae vertebralis, 160

ascendens of mandible, 197, 298

304
basilaris, 358
caroticus, 182

Sulcus chiasmatis, 183

sphenoidalis, 177, 183
Suprascapula, 200
Sustentaculum tali, 214
Suture, 49

coronal, 188

frontal, 189

harmonic, 194

lambdoidal, 188

sagittal, 188

squamosal, 188
Sympathetic system, 73, 74

trunk, 257, 302, 309, 328,
336
Symphysis, 49
Synapse, 32
Synovia, 49

Taste buds, 90
Teeth, 59

Telencephalon, 81, 350
Tendon, 20, 65
Tendon of Achilles, 285
Tentorium cerebelli, 176, 177
Testis, 128, 129, 131, 245
Thalamencephalon, 83
Thalamus, 352
Thorax, 165
Tissues, 11, 13

adipose, 21

connective, 18

epithelial, 13

fluid, 34

muscular, 25

nervous, 29

skeletal, 22

subcutaneous, 20,'221
Tongue, 311
Tonsil (palatine), 312
Trachea, 300
Tract, olfactory, 349

optic, 353
Trochanters (of femur), 210

390

INDEX

Trochlea humeri, 201
orbital, 315
tali, 213
Trunk, lymphatic, 120, 281

jugular, 293
sympathetic. See Sympa-
thetic trunk
Tube, auditory, 172, 187, 312

Eustachian, 172, 187, 312
neural, 80
uterine, 130, 251
Tuber cinereum, 354, 355
Tubercle, acoustic, 362

pharyngeal, 181
Tuberosity, deltoid, 201
Tubules, renal, 124

seminiferous, 128
Tunica albuginea, 247
Tunica vaginalis propria, 137, 245, 246
Tympanum, 187

Umbilicus, 140

Ureter, 243, 244

Urethra, 122, 244, 247, 248, 250, 253

Uterus, 130, 251, 253

Vagina, 130, 252, 253
Valve, spiral, 103, 238
Valves, atrioventricular, 332, 333
bicuspid, 333
semilunar, 332, 333
tricuspid, 332
Vas deferens. See duct, deferent
Vein, alveolar, inferior, 298, 305
angular, 294
auricular, anterior, 298
posterior, 298
axillary, 263
azygos, 336
brachial, 271
cardiac, 331
caudal, lateral, 280
caval, inferior, 115, 117, 227,
234, 256, 326, 331

Vein superior, 115, 117, 261,

326, 331
cephalic, 263, 271
circumflex, lateral, 280
coronary, 230
deep, of thigh, 280
epigastric, inferior, 257

superficial, 280
facial, anterior, 294, 298
deep, 298
posterior, 297
transverse, 298
femoral, 280
gastroduodenal, 230
gastroepiploic, 230
haemorrhoidal, external, 256
hepatic, 117, 234
hypogastric, common, 256, 257

paired, 256
liac, external, 257
liolumbar, 256
infraorbital, 146
intercostal, 336
jugular, external, 258, 297
internal, 172, 301
transverse, 297
labial, inferior, 294
superior, 294
lumbar, 256
maxillary, external, 146
internal, 298
posterior internal, 298
median, 271
mesenteric, 241
obturator, 256

ophthalmic, external, 170, 297
pancreaticoduodenal, superior,

230
phrenic, inferior, 256, 338

superior, 338
popliteal, 280, 287
portal, 110, 115, 230, 234
pudendal, internal, 280
pulmonary, 331, 335
renal. 243

INDEX

391

Vein, portal, 111

sacral, median, 257
saphenous, great, 280, 282, 287
small, 285, 287
accessory small, 282
287'
scapular, transverse, 258, 297
sciatic, 256, 282, 287
spermatic, 247, 250
splenic, 230
subclavian, 326
sublingual, 298
submental, 298
subscapular, 263
suprarenolumbar, 256
temporal, deep, 298

superficial, 297
thoracic, lateral, 263
thyreoid, inferior, 150
tibial, anterior, 287
posterior, 287
ulnar, 271
umbilical 115, 117
vertebral, 312

median, 144, 148, 150,
312
vesical, 245, 257
Veins, 109, 112, 113
Velum, anterior medullary, 357, 361
posterior medullary, 358, 361

Ventricle, of brain, 80
fourth, 361

of heart, 330, 331, 332
laryngeal, 314
lateral, 364, 365

third, 351, 364
Vertebrae, 50, 51, 156

caudal, 164

cervical, 158

coccygeal, 164

false, 163

lumbar, 161

sacral, 163

thoracic, 161

true, 163
Vertebral column, 44, 50, 156
Vertebrates, 9, 10
Vesicle, cerebral, 80
seminal, 248
Vestibulum, 122, 130, 253
Vestibulum oris, 99, 310
Vibrissae, 15, 144, 218
Vocal folds, 314
Vomeronasal organ, 178
Vulva, 219, 223

Zona pellucida, 11
Zonula ciliaris, 92
Zonular fibres, 318

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Full text of "Bensley's Practical anatomy of the rabbit : an elementary laboratory text-book in mammalian anatomy"

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