A LABORATORY MANUAL

OF THE

ANATOMY OF THE RAT

•tJ^^^

THE MACMILLAN COMPANY '

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TORONTO

A LABORATORY MANUAL '

OF THE

ANATOMY OF THE RAT

BY

HARRISON R. HUNT, Ph.D.

PROFESSOR OF ZOOLOGY
IN THE MICHIGAN AGRICULTURAL COLLEGE

THE MACMILLAN COMPANY
1924

All rights reserved

Copyright, 1924,

By the macmillan company.

Set up and electrotyped.
Published December, 1924.

Printed in the United States of America by

J. J. LITTLE AND IVES COMPANY, NEW YORK

PREFACE

Rats can usually be obtained in large quantities in any
community if proper trapping methods are used; conse-
quently the rat provides an excellent subject for dissection
at colleges in communities where cats or dogs are difficult
to obtain. This manual was written to supply laboratory
guidance in the study of the rat because it is an inexpensive
mammal which is easy to obtain.

The literature on the gross anatomy of this rodent is not
extensive. Much of the material in the manual is therefore
the result of the w^riter's own investigations, which were
carried on at intervals at the University of Mississippi
since the summer of 1920. No attempt has been made in
the text to indicate those sections which are wholly the
result of the author's work or those for which informa-
tion was obtained from other sources.

Instructors can use the manual for elementary classes by
omitting those sections which they deem of lesser im-
portance, or it can be used in its entirety for classes in
advanced vertebrate anatomy. Illustrations have been pur-
posely omitted, because it is better for the student to get
his visual impressions from the animal itself. Either the
wild Norway rat or its albino variety may be used.

Acknowledgment for limited assistance is due to Martin
and Moale's ''Handbook of Vertebrate Dissection," Part
III (1884). The writer extends special credit and thanks to
Mrs. Eunice Chace Greene, formerly of the zoological teach-
ing staff of Smith College, who has very generously read

vi PREFACE

nearly all the manuscript, comparing it with her own
unpublished researches on the rat, and has called the
author's attention to a considerable number of errors, which
have been corrected; she has also supplied much unpub-
lished additional information from her own investigations.
Credit is due to Dr. Albert M. Reese of West Virginia Uni-
versity for having suggested that the author write this
manual, and for much encouragement during its devel-
opment. In addition, the writer wishes to thank Mr.
Frazier Cochrane of Milligan College, and Mr. Joseph W.
Stack of the Michigan Agricultural College, for reading
the manuscript.

There are doubtless a few errors in fact and nomencla-
ture. I will be very grateful to any critical user of the
manual who will report these errors to me.

HARRISON R. HUNT.

East Lansing, Michigan.
November, 1924.

CONTENTS

PAGE

Introduction 1

External Aspect of the Rat 4

The Skeletal System 6

The Muscular System 38

The Salivary Glands 48

The Vascular System 50, 66, 97

The Respiratory System 63

The Digestive System 68

The Urogenital System 84

The Nervous System 103

Index 117

/ 9 r/^

A LABORATORY MANUAL

OF THE

ANATOMY OF THE RAT

A LABORATORY MANUAL OF
THE ANATOMY OF THE RAT

INTRODUCTION

The rat is one of the few wild mammals that thrives in
civilized communities. It reproduces rapidly, consumes or
otherwise destroys millions of dollars worth of property,
and is the known, or probable, carrier of several parasites
harmful to man. Some of these parasites are : Leptospira
ioterohaemorrhagiaey the spirochaete organism of infectious
jaundice ; germs which cause diseases in man following rat
bites, for instance rat-bite fever; the tapeworm Hymeno-
lepis diminuta, and possibly its relative H. na/na; the
round worm, Trichinella spirwlm, which causes trichiniasis ;
and Bacillus pestis, the germ of plague. It is claimed that
the plague, or ''black death," killed 25,000,000 people in
Europe during the fourteenth century, and that plague has
caused the death of nearly 9,000,000 persons in India alone
since 1896. The rapidity and ease with which rats can be
bred has led to their extensive use in experiments by physi-
ologists, geneticists, and others. It is evident, then, that an
elementary knowledge of the habits, functions, and struc-
tures of the rat is of interest to the medical student,
anatomist, agriculturist, and experimental zoologist.

The rat belongs to the order ^Bodentia, which includes
over one-third of all existing species of mammals. Rodents
possess long, chisel-like, curved incisor teeth (usually but
two in the upper jaw, and never more than two in the
lower) , which grow continuously. The enamel is restricted

1

2 ANATOMY OF THE RAT

to the front of the incisor, so that in gnawing the softer
dentine is worn away at the tip faster than the enamel,
thus keeping the tooth sharp. There are no canine teeth.
Rodents are mostly herbivorous in diet, are usually small,
and have clawed digits. The family Muridae, to which
rats and mice belong, is said to include nearly half the
genera of the Rodentia.

Four common species of this family are found in the
United States. The common house mouse {Mus musculus)
is less than two hundred millimeters in length when adult.
The generic designation Epimys has been established for
the rats, but the older term Mus is doubtless more familiar.
Mus rattus rattus (the black, or house rat) is the rat
of the Middle Ages in Europe. It probably reached that
continent some six hundred years before the Norway rat
arrived. The black rat preceded the Norway rat in North
America, and occurs still in scattered localities in the
northern United States. Its diagnostic characteristics are:
the ears when laid forward extend at least to the middle
of the eye ; the tail is longer than the head and body ; and
is dusky all around ; hind foot 33-37 millimeters ; the color
is blue-black above, and slaty below. A color variety, Mus
rattus alexandrinus (roof, snake, or Alexandrian rat) is
"grayish-brown above, white or yellowish white below"
(Lantz). It is said to be common in the southern states
near the coast. Mus norvegicus (the brown, gray, barn,
wharf, sewer, or Norway rat) has, as a result of its adapta-
bility, fecundity, strength, and ferocity, largely replaced
the other species of rats in temperate climates. When well
fed it is said that a female Norway rat can give birth to
from sixty to a hundred young a year. It is more of a bur-
rower than the black or roof rat, and is the common form
found about bams, houses, warehouses, etc., in the United
States. This species' characteristics are: ''Ears moderate.

INTRODUCTION 3

when laid forward barely or not reaching eye ; tail shorter
than (rarely equal to) the length of head and body, darker
above than below; color of body normally grey-brown
above, white below; hind foot 38-46 millimeters" (Lantz).
The common albino rat {Miis n&rvegicus alhinus) is a
variety of the Norway rat, and is now known only as a
domesticated animal. Mus rattiis also has been known
to produce albinos. The large number of wild Norway
rats that can usually be caught in human communities,
together with the ease with which albinos can now be
secured from dealers, makes the Norway rat a convenient
form for laboratory study by large classes. Either the
wild Norway rat or its albino variety can be used for dis-
section in connection with the directions and descriptions
contained in this manual.

EXTERNAL ASPECT OF THE RAT

Kill two adult rats with ether or chloroform. One of
these is to be used for the study of the muscles, the other
for the dissection of the remaining soft parts. Examine
the external characteristics of the dead animal and deter-
mine the species to which it belongs by referring to the
descriptions of species given above. Observe the general
form of the trunk, and the characteristic shape of the
head. Note the exposed, sharp, chisel-like, canine teeth
which the rat uses in gnawing hard objects. The long
hairs, or vibrissae, which project laterally from the ani-
mal's snout doubtless serve as feelers. Rats in the day-
time appear to have poor vision while in the open, for their
movements are then uncertain or slow. But when in con-
tact with a wall, for instance, they run rapidly, indicating
that they are then assisted by tactile stimuli through the
vibrissae. The limbs are comparatively short, each being
terminated by five digits (fingers on the fore limbs, toes on
the hind ones). The thumb (pollex), which is so incon-
spicuous that it may be overlooked at first, bears a flat-
tened, nail-like claw, but the claw at the end of each of the
remaining four fingers is sharpened. The hind foot has
five clawed digits. Numbering these digits from the
median side, the first (hallux) and the fifth are shorter
than the other three. Observe and determine the numbers
of the footpads, or plantar tubercles, on the ventral sur-
faces of the fore and hind feet. In feeding on a morsel of
food, rats are said to assume a sitting posture, and to hold
the food between the fore paws while gnawing at it, like a

4

EXTERNAL ASPECT OP THE RAT 5

squirrel. Note the scales on the tail. How are they
arranged? Are scales characteristic of mammals? In
what groups of vertebrates are scales usually found?

The anus, or opening through which the residual mate-
rials (feces) from the digestive tube pass, is immediately
ventral to the base of the tail. The conspicuous swelling
ventral to the anus of the male rat is the scrotum, which
contains the two testes. Sometimes the testes lie up in the
body cavity instead of in the scrotum, in which case the
size of the latter is very much reduced. The external
opening of the male's urogenital system is located mid-
ventrally near the anterior end of the scrotum. Carefully
insert one blade of a pair of scissors into this orifice and
slit the skin, lengthwise of the animal, for three or four
millimeters. Note that the foreskin (prepuce), which is
hairy externally, surrounds the distal end (glans) of the
penis. Examine now the corresponding region of the
female. There are three openings in the female : the anus,
ventral to the base of the tail; ventral to the anus is
the vaginal orifice, or exit from the female reproductive
organs; below this orifice the opening from the excretory
system at the distal end of a prominent elevation. How do
the relations between these openings in the female rat
compare with the corresponding openings in other female
mammals ?

Carefully observe the hair color of your animal. If you
are working with a wild rat scrutinize the distribution of
the pigment within individual hairs. Is each hair of the
same color throughout ? Compare the hair color of the rat
with that of other wild animals, if they are available either
alive or mounted. Compare the color of the albino's eyes
with the eye color of the wild rat.

If time permits, sketch the lateral view of the dead
animal.

THE SKELETAL SYSTEM

Animals possess two kinds of skeleton. The exosUelet(m,
found among the Arthropods, covers the outside of the
body, protecting the delicate internal organs, and assisting
in locomotion by providing surfaces for the attachment of
muscles. The disadvantage of an exoskeleton, like that of
a crayfish, is that growth must be accompanied by periodic
molts to permit of increase in body size. Vertebrates
possess an endoskeleton, which is completely inclosed by
the soft parts of the body. The endoskeleton supports the
body and thereby preserves its general form. It facili-
tates locomotion by providing levers which are moved by
the contraction of the muscles. The skull protects the
delicate structures of the brain, while the ribs prevent
injury to the organs of the thorax. One important advan-
tage of the endoskeleton is that it does not interfere with
growth, for the bones of the vertebrate body increase in
length, width, and thickness as the soft parts grow.

The tt'Xial skeleton of a vertebrate comprises the skull,
backbone, ribs, and sternum. The appendicular skeleton
consists of the pectoral and pelvic girdles, the fore legs,
hind legs, and feet.

THE SKULL

The skull of the higher vertebrates is a bony case com-
pletely surrounding and protecting the delicate tissues of
the brain. It incloses and protects the olfactory organ,

6

THE SKELETAL SYSTEM 7

and the inner and middle ears. It offers some protection to
the eyes also. The ventral part is modified to form the
upper and lower jaws which are used in grasping, killing,
and chewing the food. The skull may be divided into two
parts, the cranium, which surrounds the brain, and the
visceral skeleton. The latter includes some of the bones of
the face, of the upper and lower jaws, some of the bones
on the lateral surface of the head in the ear region, and
the small sound transmitting bones of the middle ear.
The visceral skeleton of the rat is doubtless a highly modi-
fied heritage from the fish-like ancestors of the mammals.
The chief function of the visceral skeleton of the fishes is
to support the gills and lend strength to the jaws. The
student should familiarize himself, either through works on
evolution, or with the aid of the instructor, with the prob-
able changes by which the visceral skeleton, epiglottis, and
the laryngeal and tracheal cartilages of the mammals have
evolved from the visceral arches of fishes. Wilder 's '* His-
tory of the Human Body" discusses this question.

The parts of the skull develop in two ways. The floor,
and a portion of the sides and back of the cranium consist
of bones which have developed from centers of ossification
in the embryonic cartilage of these regions. These are the
cartilage hones. The roof of the cranium, the remainder of
the sides, the hard palate, and the bones of both jaws are
intramembranous in origin. They are therefore called
membrane hones.

Dorsal aspect of skull. The skull is long and narrow,
gradually decreasing in width from the posterior to the
anterior end. The width of the human skull is relatively
greater, as compared with the length, than the width of the
rat skull. The ratio of cranial width to length, or the
cephalic index, is an important characteristic in the classi-
fication of the races of man. The irregular lines on the

8 ANATOMY OF THE RAT

surface of the rat's skull are the sutures where the adjacent
bones are joined to one another.

The extreme posterior, or nuchal, surface of the skull
is nearly perpendicular to the long axis of the animal, and
joins the dorsal surface almost at a right angle. The
lamhdoidal ridge occupies the region of union of these two
surfaces. It is semicircular in form. Anterior to the
lambdoidal ridge is a second ridge extending ventrally to
the region of the ear. Dorsally this second ridge joins the
temporal line at right angles. This line extends forward
laterally on the dorsal surface of the cranium to the orbit,
where it turns medially and disappears. Compare the rat
with the horse and cat, for instance, where the lambdoidal
ridge is well developed, and with the human skull where
it is absent.

The orhito-temporal fossa is a deep depression on each
side of the mid-region of the cranium, about one-third as
long as the whole cranium. It contains the eye, eye
muscles, and muscles, which help close the jaw. The
zygomatic arch bounds it laterally. This arch is a bow-
shaped bar of bone which extends forward from the
articulation of the lower jaw. At its anterior end this
arch broadens and is pierced by the large infraorbital fis-
sure. Viewed from the anterior end this fissure is shaped
somewhat like a palm-leaf fan, broad dorsally and narrow
ventrally. In other vertebrates the orbito-temporal fossa
is more or less completely separated by bony processes into
two distinct cavities, the orbit for the eye and its muscles,
and the temporal fossa.

The anterior nares [piriform aperture in man] are
located at the anterior extremity of the cranium. Air
enters through these openings and is conveyed through the
nasal passages to the lungs.

The following bones may be seen from the dorsal side:

THE SKELETAL SYSTEM 9

the occipital, interparietal, parietals, frontals, nasals, pre-
maxillaries, maxillaries, zygomatics, squamosals, and lac-
rimals. The position and structure of each of these bones
will be described in detail later.

Exercise I. Draw and label the dorsal view of the skull.

Lateral aspect of the cranium. The nuchal plate bears
ventrally the occipital condyle, by which the skull articu-
lates with the backbone. The jugular process is a long,
pointed, triangular structure lateral to and slightly in
front of the occipital condyle. Between the two is a semi-
circular depression, the jugular fossfl. The mastoid portion
of the petrosal l>one is immediately anterior to the jugular
process, and is located in a pronounced depression of the
lateral part of the occipital bone. The tympanic bulla is a
hollow spheroidal capsule anterior to the jugular process,
opening to the outside through the external acousiic
meatus. This opening is closed in life by the tympanic
memhrane, which transmits sound waves from the outer ear
to the three small bones of the middle ear.

Where the mastoid portion connects anteriorly with the
squamosal bone there is a ventral extension of the latter,
the post-tympanic hook, which helps to hold the tympanic
bulla in place.

The orbito-temporal fossa and zygomatic arch are seen
from the lateral side also. Within the fossa are foramina
through which the nerves emerge from the brain. The
foramen ovale is located directly ventral to the posterior
end of the zygomatic arch, and anterior to the tympanic
bulla. A branch of the fifth cranial nerve emerges here.
The foramen communicates by a perforation of the exter-
nal pterygoid process with the ventral surface of the skull.
Another small foramen is immediately in front of the fora-
men ovale. A few millimeters anterior to the foramen

10 ANATOMY OF THE RAT

ovale is the large anterior lacerated foramen. The optic
foramen for the passage of the second cranial nerve is
anterodorsal to the anterior lacerated foramen.

There is a pronounced groove along the ventral side of
the orbito-temporal fossa. The small postpalatine foramen
communicates near the center of this groove with the ven-
tral surface of the palatine bone. Anterior to this foramen
another foramen establishes a connection between the orbit
and the nasal cavity. A small foramen dorsal to these
openings perforates the cranium just posterior to the crib-
riform plate, which is described later.

The maxillary bone is incompletely ossified medial to
the infraorbital fissure. Consequently in cleaning the skull
the nasal cavity is usually opened at this place.

The convex lateral surface of the premaxillary bone
reveals the course of the root of the enormous curved in-
cisor tooth. The root occupies a semicircular canal (alve-
olus) which extends back to the level of the infraorbital
fissure. Compare the arrangement of the rat's teeth vsdth
that of the cat, dog, and man, for instance. The rat has
no canine teeth, which are so prominent in carnivores, and
no premolars. The three molar teeth are inserted in the
maxillary bone a considerable distance posterior to the in-
cisors. The toothless space between the incisor and molar
teeth is the diastema. A similar arrangement occurs in
the horse, for instance.

Exercise II. Draw and label the lateral view of the cranium.

Ventral view of the skull. The following bones are seen
from this view: the occipital, which is flanked on either
side by the petrosal bone and the tympanic bulla, the
basisphenoid with an alisphenoid fused to each side, the
presphenoid and the paired squamosals, palatines, maxil-
laries, and premaxillaries.

THE SKELETAL SYSTEM 11

At the posterior end of the skull is the foramen magnum,
through which the brain connects with the spinal cord.
On each side of this foramen is the occipital condyle. Two
pairs of bony plates, the pterygoid processels, arise at the
base of each alisphenoid bone. The outer plates are nearly-
horizontal in position. They extend from the posterior end
of the maxillary bone caudad and laterad to the lateral
surface of the tympanic bulla. The two inner pterygoid
processes are vertical to the base of the cranium. Each
extends posteriorly to the opening of the bony canal
through which the Eustachian tube passes from the cavity
of the middle ear (within the tympanic bulla) to the
pharynx. The single posterior naris, or choanal lies be-
tween the anterior extremities of the inner pterygoid proc-
esses. It leads forward into the nasal cavity, which opens
at the anterior end of the head through the two anterior
nares.

The roof of the mouth in front of the choana consists of
three pairs of bones (palatines, maxillaries, and premax-
illaries), which taken together constitute the hard palate.
At the extreme posterior end of the hard palate are the two
small palatine bones, which form the ventral boundary of
the choana. The maxillary bones are considerably larger
than the palatines and are located laterally and anteriorly
to the latter. The lateral, or alveolar, portion of the max-
illary bears the three molar teeth, which are used for grind-
ing food. The maxillaries articulate in front with the pre-
maxillaries. These bear the two large curved incisor teeth.
Canine and premolar teeth are absent in the rat.

Several pairs of foramina appear on the ventral surface.
The twelfth (hypoglossal) cranial nerve emerges through
the hypoglossal canal at the base of the occipital condyle.
The posterior lacerated foramen lies immediately in front
of the hypoglossal canal, at the boundary between the

12 ANATOMY OP THE RAT

petrosal and occipital bones. This foramen transmits the
ninth (glossopharyngeal), tenth (vagus), and eleventh
(accessory) cranial nerves. The carotid foramen is an-
terior to the jugular foramen, between the tympanic bulla
and the basioccipital bone. Near the posterior ends of the
pterygoid processes is a pair of foramina which communi-
cate with one another by a horizontal space in the basi-
sphenoid bone. A large oval foramen lies immediately
anterior to the one just mentioned. Note the communica-
tion of this oval foramen with the superior orbital fissure.
'Ventral to the foramen ovale another large oval foramen
perforates the posterior part of the lateral pterygoid proc-
ess. Note the two slits in the cranium, dorsal and posterior
to the choana, communicating with the anterior lacerated
foramen.

The prepalatine foramina are two conspicuous slits on
the roof of the mouth in the region of the diastema. They
are surrounded by the maxillary and premaxillary bones.
The postpalaMne foramina perforate the roof of the mouth
near the lateral margin of each palatine bone. A bristle
inserted through the foramen will show its communication
with the groove on the floor of the orbito-temporal fossa.

Exercise III. Draw and label the ventral view of the cranium.

Interior of the cranium. Bisect the skull vertically in
a plane slightly at one side of the sagittal plane. Use a
sharp bone saw with small teeth. Care should be exercised
to injure the bones as little as possible.

Two cavities will be observed, the cranial cavity for the
brain, and the nasal cavity. The cranial cavity is sub-
divided further into the cerebellar, cerebral, and olfactory
fossae.

The cerebellar fossa holds the cerebellum. It occupies
the posterior part of the cranium and is partially sepa-

THE SKELETAL SYSTEM 13

rated from the cerebral fossa by an incompletely ossified
lateral partition, the tentormm. The ossified portion of
the tentorium is/ a dorsal extension of the petrosal bone,
which forms a part of the lateral wall of the cerebellar
fossa. The floccular fossa is a relatively large cavity occu-
pying the poster odorsal region of the petrosal bone. Its
opening into the cranial cavity is an oval aperture con-
siderably narrower than the cavity itself. The cerebral
fossa is the largest division of the cranial cavity. It in-
closes the cerebrum. The foramina observed in the study
of the lateral view may be identified easily by thrusting
a bristle through each foramen from the outside. The
olfactory fossa is a laterally compressed cavity opening
behind into the cerebral fossa and bounded in front by the
cribriform plate. This cavity contains the olfactory lobes
of the brain. The branches of the olfactory nerve enter
the nasal cavity through the numerous foramina in the
cribriform plate. In a cranium from which the roof has
been removed, the cribriform plate will be seen to be heart-
shaped.

Though the nasal cavity can be satisfactorily studied
in a longitudinal section of the skull, it can be seen to the
best advantage if the dorsal and lateral bony wall of the
cavity be removed. In doing this care should be taken to
preserve intact the nasal bones and their delicate ventral
processes, and while chipping away the lateral wall to
injure as little as possible the delicate turbinal bones at
the back and sides of the nasal cavity. The nasal cavity
is tubular in form posteriorly {naso-pharyngeal duct), and
terminates at the single posterior naris, or choana. Its
external opening is through the two anterior nares at the
anterior end of the snout. The nasal cavity is bounded
dorsally by the nasal bones, posteriorly by the cribriform
plate and maxillary bones, ventrally by the premaxillary,

14 ANATOMY OF THE RAT

maxillary, and palatine bones. The two long prepalatine
foramina lie in its floor, and it communicates by a foramen
with each orbito-temporal fossa. A perpendicular median
partition, the nasal septum, divides the nasal cavity into
two nasaZ fossae. The septum is partly cartilaginous.

Each nasal fossa contains three very delicate, folded
turbinal bones, which probably correspond to the maxillo-
turbinal, nasoturbinal, and ethmoturbinal bones of other
mammals. The first of these is an elongated thin sheet of
bone attached ventrally to the inner surface of the pre-
maxillary bone. The second is a process of the nasal bone.
Its ventral edge turns outward and upward like the rolled
edge of a stiff piece of paper. The ethmoturbinal bone is
much more extensively convoluted than the other tur-
binals. The convolutions inclose spaces known as ethmoi-
dal cells, whose long axes are, in general, directed antero-
posteriorly. The ethmoturbinals are attached posteriorly
to the cribriform plate. Examination of a skull in which
the bone has been removed from the side of the nasal fossa
shows that in the posterior part of the fossa the turbinal
bones extend from the top to the bottom of the cavity and
posteriorly almost to the optic foramen. The convolutions
of the turbinal bones, together with the flesh covering them,
are sometimes called the nasal lahyrinth. In mammals the
folds supported by the turbinals warm the air and extract
foreign bodies from it. The epithelium covering the ethmo-
turbinals contains olfactory cells which are stimulated by
odorous substances in the air.

Consult a text on comparative anatomy to determine the
variations among vertebrates in the bones of the nasal
cavity. Compare the nasal cavity of the rat with the same
part in the turtle, frog, dogfish, etc. Assisted by a text on
embryology outline the stages in the evolution of the nasal
cavity.

THE SKELETAL SYSTEM 15

Bones of the Cranium

Having examined the general topography of the skull,
the structure of each bone should be studied in greater
detail.*

Occipital Bone (Os Occipitale)

This bone first appears in the mammalian embryo as
four centers of ossification in the chondrocranium (carti-
laginous cranium). One of these centers is dorsal (supra-
occipital), one ventral (basioccipital), and two are lateral
(exoccipitals) to the foramen magnum. These four bones
fuse in mammals to form the occipital. The supra- and
ex-occipital regions in the rat are the posterior wall of
the cranium. The basioccipital is the floor of the cere-
bellar fossa.

The basioccipital region is trapezoidal in shape, broader
behind than in front. Anteriorly it joins the basisphenoid
bone by a suture at right angles to the long axis of the
skull. Posteriorly it forms the ventral boundary of the
foramen magnum. Laterally it comes in contact with, but
is not fused to„ the tympanic bulla and the petrosal
bone. Note the two marked depressions on the ventral sur-
face of the bone which are separated from one another by
a median ridge. The inner surface of the basioccipital,
where it comes in contact with the brain, is concave.

The occipital condyles and the triangular jugular proc-
esses previously mentioned, are parts of the exoccipital
regions. The condyles bound the foramen magnum ven-
trolaterally. They articulate with the atlas, or first neck
vertebra. This articulation permits the head to be raised

* By boiling in water or an aqueous solution of sodium hydroxide
the bones may sometimes be separated along the sutures. Too pro-
longed boiling in the hydroxide solution will make the bones so
fragile that they will easily fall to pieces when handled.

16 ANATOMY OF THE RAT

and lowered vertically without a corresponding movement
of the neck. The hypoglossal canal is anterior to the con-
dyle. The exoccipitals are fused dorsally with the
supraoccipital, ventrally with the basioccipital, and are in
contact anteriorly with the mastoid process of the petrosal
bone.

Viewed from above the dorsal edge of the supraoccipital
region resembles the segment of a circle with its center
anterior to the bone. The ends of the segment terminate
in a dorsoventral ridge where the supraoccipital connects
with the squamosal bone. This ridge is continuous with the
post-tympanic hook. A median vertical ridge (the poste-
rior occipiital proiuberance) bisects the nuchal surface.

Sphenoid Bone (Os Sphenoidale)

The sphenoid is a complicated structure consisting of two
distinct parts, the anterior sphenoid and posterior sphenoid.
Each of these, in turn, consists of three elements, separ-
ated in some of the lower vertebrates, but fused in the
higher.

The three fused elements forming the posterior sphenoid
are the single median hasisphenoid (post-sphenoid) and
the two lateral alisphen^ids. The hasisphenoid is bounded
caudally by the basioccipital, anteriorly by the anterior
sphenoid, and laterally is fused with the alisphenoid bones.
Anterior to its union with the alisphenoids, the hasisphenoid
increases slightly ii> width up to its attachment with the
presphenoid. The pterygoid processes (one pair on each
side), described in the discussion of the ventral view of
the skull, are attached to the ventral surface of each
alisphenoid bone. Dorsal to the lateral pterygoid process
is a depression into which the foramen ovale and two
other foramina open. The alisphenoid expands, dorsal

THE SKELETAL SYSTEM 17

to these foramina, into a flat process which extends to-
ward the base of the zygomatic arch. This process runs
posteriorly to the tympanic bulla but does not fuse with
it. The process joins dorsally the squamosal and frontal
bones. It forms a part of the posterior wall of the or-
bito-temporal fossa, sending a process forward to the
palatine bone and alveolar process of the maxillary. The
failure of the alisphenoid bones to fuse with the pre-
sphenoid^ and anterior part of the basisphenoid, leaves
a pair of long slits dorsal to the choana. Observe the po-
sition of the anterior lacerated foramen.

The three fused constituents of the anterior sphenoid
bone are the single median presphenoid and the two lateral
orbitosphenoids. The presphenoid bone extends anteriorly
from the basisphenoid. In the cerebral fossa its smooth
surface decreases in width anteriorly until it is reduced
to a thin septum between the two optic foramina. The
ventral surface bears a keel which appears as a low
ridge on the roof of the naso-pharyngeal duct. Each
optic foramen is bounded medially by the presphenoid,
laterally by the wedge shaped orbitosphenoid bone. The
base of the latter is fused with the presphenoid. Its apex
is thrust outward between the alisphenoid bone, poste-
riorly, and the orbital process of the frontal bone, ante-
riorly. The shape and size of the orbitosphenoid can be
readily observed from the inside of the cranial cavity.

Squamosal Bone (Os Squamosum)

The squamosal bone resembles a very short-handled
claw hammer if viewed from the lateral surface of the
skull. The "handle'* widens posteriorly, and with a
spur from the occipital bone forms the posttympanic hook.
The ** handle" is bordered dorsaUy by the parietal bone,

18 ANATOMY OF THE RAT

ventrally by the petrous and tympanic bones, and by an
unossified region of the skull. The ''claw" of the hammer
is bounded ventroanteriorly by the alisphenoid bone; pos-
teriorly its tip approaches the tympanic bone. Dorsally
the "hammer" joins the frontal and parietal bones; it is
bounded anteriorly by the orbital process of the frontal
bone.

The squamosal bone bears a pronounced longitudinal
ridge which runs nearly its whole length. Dorsal to the
**claw" the ridge is extended anteroventrally as a curved
spur, the zygomatic process. This process forms a part
of the zygomatic arch. The mandibular fossa, for the ar-
ticulation of the lower jaw, lies ventral to the base of
the zygomatic process.

Tympanic Bone (Os Tympanicum)

The tympanic hvZla contains the tympanic cavity, or
cavity of the middle ear, which opens laterally through
the external acoustic meatus.

Petrosal Bone (Os Petrosum)

The petrosal bone incloses the inner ear, and is seen
on the inner surface of the skull. The mastoid portion
of the petrosal is inserted into the notch at the side of
the occipital bone.

Interparietal Bone (Os Interparietale)

The interparietal is an ovoid bone of membranous ori-
gin inserted between the supraoccipital and the paired
parietals. It meets the nuchal surface nearly at right
angles.

THE SKELETAL SYSTEM 19

Parietal Bone (Os Parietale)

The paired parietal bones are of membranous origin.
They constitute the greater part of the roof and a con-
siderable part of the sides of the cerebral fossa. Each
bone, were it flattened out, would be nearly rectangular.
The true shape is obscured externally by a process of
the squamosal bone which covers the anteroventral part
of the parietal. Note the connections, posteriorly with
the interparietal, anteriorly with the frontal, and later-
ally with the squamosal bones. The curved inner surface
is smooth, but externally the temporal line separates the
dorsal from the lateral region of each bone.

Frontal Bone (Os Frontale)

The two frontal bones are of membranous origin. They
are united in the mid-dorsal line by the frontal suture.
Each bone consists of a frontal and an orbital part. The
former roofs the skull in front of the parietals, the latter
forms the greater part of the side wall of the orbito-tem-
poral fossa. The temporal line sharply separates these
two portions of the bone externally. The frontal part
joins the parietal bone posteriorly at the coronal suture.
This part decreases in width anteriorly and articulates
with the nasal and premaxillary bones. The orbital part
is concave. Its posterior region, partly overlaid by the
squamosal bone, forms a portion of the back wall of the
orbit. Ventrally it joins the alisphenoid, orbitosphenoid,
and maxillary bones — anteriorly the lacrimal and max-
illary bones.

The frontal bones, internally, are the anterior wall of
the cerebral fossa, the lateral wall of the olfactory fossa,
and they partly inclose the nasal cavity posteriorly. An

20 ANATOMY OF THE RAT

internal ridge marks the boundary between the cerebral
and olfactory fossae. The cribriform plate is attached to
the frontal bones between the olfactory fossa and nasal
cavity.

Ethmoid Bone (Os Ethmoidals)

This highly complicated bone, of cartilaginous origin,
is the chief representative of the embrycmic nasal cap-
sule. It includes the perpendicular plate of the nasal
septum, the cribriform plate, and the ethmoturbinal bones.

Maxillary Bone (Maxilla)

The maxillary bone consists chiefly of five processes —
the frontal, zygomatic, alveolar, palatine, and orhitai. It
is a membrane bone. The frontal process is divided by
the large infraorbital foramen into a lateral and median
portion. The latter forms part of the lateral wall of the
nasal cavity and is comparatively thin. The skull is un-
ossified posterior to this part of the frontal process,
which joins the premaxillary bone dorsally and anteriorly.
The lateral portion of the frontal process is a laterally
concave plate connected with the dorsal surface of the
cranium by a bony arch. To the dorsal end of this plate
is attached the zygomatic process, which forms the an-
terior half of the zygomatic arch. The alveolar process
(so called because it contains the alveoli of the teeth) is
a strong bony mass holding the three molar teeth. It
is bounded posteriorly and dorsomedially by the sphenoid
bones, medially by the palatine process and palatine bone.
The palatine process joins its fellow on the opposite side
by a median suture in the roof of the mouth. It forms
part of the hard palate. In front it surrounds the pos-
terior half of the prepalatine foramen. It joins the pala-

THE SKELETAL SYSTEM 21

tine bone posteriorly. The orbital process overlaps the
anteroventral portion of the frontal bone's orbital process.
It forms but a small part of the wall of the orbito-tem-
poral fossa. A pronounced longitudinal groove on the
floor of the fossa partly separates the orbital from the
alveolar process.

Lacrimal Bone (Os Lacrimale)

The lacrimal bone is a thin inconspicuous element, of
membranous origin, lying on the dorsomedial wall of the
infraorbital foramen. It is immediately below the bony
arch which connects the dorsal surface of the skull with
the lateral part of the maxillary bone's frontal process.
The bone is easily overlooked if its sutures are not ex-
posed by careful scraping.

PrEM AXILLARY BoNE (PrEM AXILLA)

The premaxillary bone, of membranous origin, forms
the anterior part of the upper jaw, and the lateral and
ventral walls of the front part of the nasal cavity. With
the nasal bone it surrounds the anterior nares. Its frontal
process is thrust dorsally between the nasal and maxillary
bones, extending caudally to the frontal bone. This pro-
cess is partially overlaid laterally by the maxillary bone.
On the ventral side the palatine process extends hori-
zontally across the roof of the mouth, surrounding the
anterior part of the prepalatine foramen and meeting
its fellow at a median suture. This suture extends back-
ward between the two maxillary, as well as between the
palatine bones. The premaxillary bears an enormous
incisor tooth. Carefully chip away the bone surrounding
this tooth, and note the size and shape of the alveolus.
The palatine process joins the palatine process of the

22 ANATOMY OF THE RAT

maxillary bone. The process is attached to the vomer
along the median suture of the hard palate. The vomer
forms part of the nasal septum. The elongated maxillo-
turhinal hone attaches to the inner surface of the premax-
illary. It lies lengthwise of the nasal cavity and extends
upward from its line of attachment.

Zygomatic Bone (Os Zygomaticum, Malar, Jugal)

The zygomatic bone is a small bow-shaped element of
membranous origin, joining the zygomatic processes of the
maxillary and squamosal bones, thus completing the zygo-
matic arch.

Vomer, or Ploughshare, Bone

This fragile bone, which is membranous in origin, com-
prises the ventral portion of the nasal septum. It unites
ventrally with the premaxillary bone. It splits dorsally
into two longitudinal lamina, between which the cartilag-
inous part of the nasal septum is inserted. The posterior
part of the bone is without a ventral attachment.

Nasal Bone (Os Nasale)

The nasal bones roof the anterior nares and the nasal
cavity back to the frontal bones. The ventral surface is
concave, both the lateral and the median borders bearing
a flange. The median flanges of the two nasals unite to
form a ridge for the attachment of the nasal septum. The
lateral flange articulates with the frontal process of the
premaxillary bone. This flange is widest where the naso-
turhinal hone (see longitudinal section of the skull) is
attached to it. The line of attachment of the nasoturbinal
runs forward and medially from this point. The nasal
bone is a membrane bone.

THE SKELETAL SYSTEM 23

Palatine, or Palate, Bone (Os Palatinum)

The palatine bone, membranous in origin, consists
chiefly of three elements — the horizontal part, and the orhi-
tal and sphenoidal processes. The horizonal part consti-
tutes the posterior region of the hard palate, and there-
fore is the ventral boundary of the choana. It meets its
fellow of the opposite palatine bone at the median suture
along the dorsal side of the mouth cavity. Laterally it
articulates with the alveolar process of the maxillary bone.
The orbital process, which joins the horizontal part nearly
at right angles, is exposed to the orbito-temporal fossa.
It constitutes the greater part of the nasopharyngeal duct 's
lateral wall. The dorsal edge articulates with the pre-
sphenoid bone. The sphenoid process extends backward
to the pterygoid process, and joins dorsally an anterior
extension of the alisphenoid bone.

The Mandible (Mandibula)

The mandible comprises two lateral elements united in
front by connective tissue at the mandibular symphysis.
Each element contains five parts: the body, the ramus,
and three processes attached to the latter posteriorly. The
body of each half of the mandible is prolonged anteriorly
as a strong, curved process bearing one large incisor tooth.
This tooth projects anterodorsally a considerable distance
beyond the bone. Behind the incisor is the diastema, a
toothless region corresponding to the diastema of the upper
jaw. A row of three molar teeth is posterior to the dia-
stema. The two rows of molars are approximately parallel
to each other, but each row forms an acute angle with the
long axis of the bone in which it is inserted. There are
transverse ridges on the grinding surfaces of the molars.
The ramus is the laterally flattened region immediately

24 ANATOMY OF THE RAT

behind the body. Three processes are attached to its pos-
terior end. The coronoid process projects dorsally into the
orbito-temporal fossa. Just behind this is the condyloid
process, bearing an elongated rounded knob which articu-
lates with the mandibular fossa of the squamosal bone.
These articular surfaces in the rodent permit anteropos-
terior motion of the jaw. The angular process is directed
posteriorly. The anterior mandibular incisure is a pro-
nounced depression separating the coronoid from the con-
dyloid process. The posterior mandibular incisure like-
wise separates the condyloid and angular processes. The
mandibular foramen lies at the base of the condyloid
process. The mental foramen lies ventroanteriorly to the
molar teeth. Other foramina occur on the surface of the
mandible. A ridge extends on the lateral surface of the
mandible from the mental foramen backward to the border
of the angular process. The medial surface of this process
is concave. The incisor tooth of the lower jaw, like that
of the upper, is sickle-shaped, and inserted in a large
dental alveolus which extends caudally, ventral to the
molar teeth, nearly to the base of the condyloid process.
The mandible is largely a membrane bone.

Hyoid Bone (Os Hyoideum)

The hyoid is a u-shaped bone at the base of the tongue

composed of a slender transverse body and two pairs of

horn^ (cornua), one pair extending anteriorly, the other

pair posteriorly.

Exercise IV. Make a labeled sketch of each of the bones of the
head.

THE VERTEBRAL COLUMN

The vertebral column, or backbone, is the second division
of the axial skeleton. It is a linear series of numerous

THE SKELETAL SYSTEM 25

vertebrae, each one fastened to its neighbor by connective
tissue. The back-bone supports the neck, trunk, and tail,
and to it are attached the ribs, pelvic girdle and numerous
muscles. It comprises seven cervical, thirteen thoracic, six
lumbar, four sacral, and about twenty-eight caudal
vertebrae.

A typical vertebra possesses a solid, more or less cylin-
drical ventral portion, the centrum, or body. The verte-
bral, or neural, arch is attached to the upper side of the
body, the two inclosing the vertebral foramen, through
which the spinal cord passes. The roots of the vertebral
arches (pedicles) are the two lateral vertical plates of the
vertebral arch. The two horizontal plates forming the
roof of the arch are the laminae. The neural spine, or
spinous process, projects upward from the junction of the
laminae. The vertebra bears on each side a transverse
process. The spinous and transverse processes serve for
the attachment of spinal muscles. Each vertebral arch
bears two pairs of articular processes, or zyg apophyses.
Of these the anterior one, or prezyg apophysis, articulates
with the preceding vertebra ; the posterior surface, or post-
zygapophysis, articulates with the succeeding vertebra.

The essential features of the vertebral column may be
brought out best by describing vertebrae from each region
of it. The seven cervical vertebrae are quite similar to one
another, except the first two, the atlas and epistropheus
(axis). The atlas is essentially a ring of bone. It lacks
a centrum, this element having been separated from the
atlas and attached to the epistropheus as the dens (odon-
toid process). This process rests in a semi-circular depres-
sion, the fovea dentis, on the ventral floor of the atlas. The
dorsal surface of the atlas bears a tubercle comparable to
the spinous process of an ordinary vertebra. The ven-
tral surface bears a similar tubercle. The atlas articulates

26 ANATOMY OF THE RAT

with the occipital condyles of the skull by means of two
lateral concave surfaces, the superior articular foveae, one
on each side. It bears posteriorly a pair of lateral ovoid
surfaces, inferior articular foveae, for articulation with
the epistropheus. A third articular surface connects these
facets ventrally. In man, and presumably in the rat, the
chief movement permitted between the superior articular
foveae and the occipital condyles is a *' nodding" of the
head. In ''shaking" the head the atlas revolves around
the dens as a pivot. The elongated transverse process
of the atlas is flattened dorsoventrally. Its dorsal surface
slopes backward to the posterior end of the vertebra, where
it joins a vertical plate. Between this plate and the in-
ferior articular fovea is the posterior opening of the fora-
men transversarium, through which the vertebral artery
passes. This foramen extends anterodorsally, and com-
municates with the dorsal surface of the atlas by an oval
opening. The artery then turning medially traverses the
atlantal foramen to the inner surface of the atlas.

The centrum of the epistropheus is flattened dorsoven-
trally and prolonged anteriorly by the dens. The dens
articulate with the fovea dentis by the anterior articular
facet on its ventral side. The cranial articular facets, one
on each side of the epistropheus, impinge upon the inferior
articular foveae of the atlas. The postzygapophyses, one
toward the upper end of each pedicle, articulate behind
with the prezygapophyses of the third cervical vertebrae.
The laterally flattened neural spine is considerably higher
than that of any other cervical vertebra. It extends for-
ward over the atlas and backward over the third vertebra.
The foramen transversarium pierces the base of the short
transverse process.

In the fifth cervical vertebra the centrum and vertebral
arch form in side view an obtuse angle with each other.

THE SKELETAL SYSTEM 27

The centrum is dorsoventrally flattened. The spinous
process is a low pointed crest. The prezygapophysis,
borne by the pedicle, is ovoid when viewed from above. Its
articular surface is directed dorsomediad. The postzyga-
pophysis is directly behind it, is similar in shape, and its
articular surface is ventrolaterad. The transverse process,
attached at the base of the pedicle, is pierced in the region
of attachment by the foramen transversarium. The verte-
hrarterial canal comprises the transverse formina of the
cervical vertebrae. It terminates with the sixth cervical
vertebra.

The centrum of the fifth thoracic vertebra is nearly
cylindrical. The spinous process is long and tapering; its
point is directed caudally. There is a notch on the anterior
edge of the laminae. This notch is flanked on each side by
the oval prezygapophyses whose articular surfaces face
dorsally. The postzygapophyses lie behind the prezyga-
pophyses, but their articular surfaces are directed ven-
trally. Each pedicle is deeply notched posteriorly, form-
ing an intervertehral foramen, for the passage of a spinal
nerve. The transverse process of each side is situated
between the anterior and posterior zygapophyses. The
articular surface of the transverse process, facing laterally,
supports the tiihercle of a rib. A depression, the costal
demifacet, will be observed at each end of the centrum
near its dorsal border. The adjacent demifacets of two
successive vertebrae combine to form the articular sur-
face for the head of a rib. The tubercle of this rib articu-
lates with the posterior vertebra of the pair under con-
sideration.

The fourth lumbar vertebra's centrum is cylindrical
ventrally, but flattened above. The spinous process is a
laterally compressed plate which runs the whole length of
the vertebra, is somewhat triangular in shape, and leans

28 ANATOMY OF THE RAT

cranially. The articular surface of the prezygapophysis
faces mediodorsad. The facet of the prominent postzyga-
pophysis faces laterally. The transverse process projects
cranioventrally from the base of the pedicle.

The sacrum consists of four vertebrae more or less com-
pletely fused together. The transverse processes project
horizontally from the centra. They are fused at their
distal ends. The transverse processes of the first two sacral
vertebrae are considerably thicker than the others. They
are expanded laterally, forming the auricular surface,
which articulates with the pelvic girdle. Since this girdle
is attached to the hind legs, the weight of the posterior part
of the body falls on these two sacral vertebrae — hence their
stout construction. The intervertebral formina are located,
as usual, between the vertebrae. The prezygapophyses of
the first sacral vertebra are well developed, for they are
part of a movable joint. The remaining functionless
zygapophyses of the sacrum are rudimentary. They lie
close to the bases of the high spinous processes.

The caudal, or tail, vertebrae show gradual and progres-
sive stages of simplification from the sacrum to the pos-
terior end of the tail. The neural canal is small in the
anterior part of the tail, and soon completely disappears.
The anterior caudal vertebrae have ventral pairs of spines
between which run the caudal blood vessels. There is a
chevron hone in some of the vertebrae which partly in-
closes these vessels on the under side.

Exercise V. Make a labeled sketch of each vertebra which has
been described.

The sternum, or breastbone, comprises seven bony seg-
ments {sternehrae) united end to end. The first, or manu-
brium, is keeled ventrally, and has a broad anterior region,
concave above. The first rib is attached to it. The sue-

TUB SKELETAL SYSTEM 29

ceeding six pairs of ribs are attached between the seg-
ments. The seventh segment (xiphoid process) Ls not asso-
ciated with a pair of ribs. It terminates in a round plate
of cartilage.

Exerolse VI. Draw and label the sternum.

There are thirteen pairs of rihs. A rib contains a dor-
sal and a ventral segment. The anterior seven pairs are
the true ribs, for they are attached to the sternum. The
six posterior pairs are the false ribs. The eighth, ninth,
and tenth pairs attach to the posterior border of the sev-
enth, and by cartilage to the fifth sternebra. The three
posterior ribs are floating ribs, lacking ventral segments.
The ribs are bow-shaped in general, the angle, or region
of greatest convexity, being dorsal. Most of the ribs are
distinctly flattened anteroposteriorly. From the fifth to
the tenth pairs, inclusive, the distal end of the rib is
twisted, so that the broad surface is posterolaterad. Com-
pare the shapes of the different ribs.

The fifth rib may be used to illustrate the articular sur-
faces. The capitulum cosiae (head) is a slight swelling at
the proximal end. Its circular medial surface articulates
with the demifacets of two adjacent vertebrae, as previ-
ously described. The tuberculum costae (tubercle) is a
dorsal enlargement two or three millimeters lateral to the
head. It articulates with the transverse process of the
vertebra, and is separated from the head by the collwm
costae {neck) of the rib.

The thoracic cavity, surrounded on the sides by the ribs,
dorsally and ventrally by the backbone and sternum re-
spectively, resembles an ice cream cone with the apex cut
off. The cranial thoracic aperture is in front, the larger
caudal thoracic aperture behind. Both are circumscribed
by ribs, sternum, and backbone. The thoracic cavity in-

30 ANATOMY OF THE RAT

creases in size, and air enters the lungs, when the ribs are
pulled anteriorly in breathing.

Exercise VII. Sketch one of the longer ribs.

THE ANTERIOR APPENDAGES

The pectoral girdle includes the scapula (shoulder
blade), and the clavicle (collar bone). The former is a
flattened triangular bone with its apex pointing cranioven-
trad and its broad inner surface facing the vertebral
column. A cartilaginous disc connects one end of the clavi-
cle with the acromion process of the scapula. A similar
disc unites the other end with the anterior end of the
sternum. The clavicle is the only bony connection between
the scapula and axial skeleton.

The concave thickened superior margin of the scapula
is directed cephalad. The convex vertebral margin is dor-
sal. The third margin, the axillary^ faces ventrocaudally,
and is nearly straight. The anterior, posterior, and ventral
corners of the bone are designated the medial, inferior, and
lateral angles respectively. At the lateral angle is the
pear-shaped glenoid cavity for articulation with the hu-
merus bone of the upper arm. The short coracoid process
(homologous to the bone of the same name in the lower
animals) projects medially from the cranial end of the
glenoid cavity. The scapular spine is a thin, prominent
plate (forming nearly a right angle with the scapula)
which divides the lateral surface of the scapula into the
supraspinous fossa (anterior) and the concave infraspin-
ous fossa (posterior). The spine tapers dorsally to a point
at the vertebral border; it ends ventraJly in a free pro-
jection, the acromiion process or acromion. The inconspicu-
ous projection on the caudal surface of the latter is prob-
ably the metacromion process. There are two longitu-
dinal ridges on the inner surface of the scapula.

THE SKELETAL SYSTEM 31

The humerus is the bony constituent of the upper arm.
Its proximal end articulates with the scapula; it articu-
lates distally with the radius and ulna of the fore arm.
Between these two extremities is the shaft, or body, of the
bone. The head is a prominent oval enlargement at the
proximal end, which fits into the glenoid cavity, furnishing
the articulation with the scapula. The lesser tuberosity
is a low knob located immediately in front of the head on
the medial surface of the bone. The greater tuberosity
is a dorsoventrally elongated projection on the cranial
surface of the humerus, immediately in front of the head,
extending also on to the lateral surface of the bone. The
intertubercidar groove separates the two tuberosities. The
deltoid tuberosity is a very prominent rough-edged ridge
running, on the ventral surface, from the greater tuberos-
ity about half way to the distal end of the humerus.

The articular surface at the distal end of the bone is
saddle-shaped to receive the corresponding surfaces of the
radius and ulna. This end of the bone is flattened. It
bears two depressions immediately above the articular sur-
face— the radial fossa on the ventral side to receive the
radius during flexion of the arm, and the olecranon fossa
on the dorsal side to accomodate the olecranon process dur-
ing extension. The medial epicondyle is a prominent cylin-
droid process medial to the saddle-shaped articular surface.
The lateral epicondyle is a broad process located laterally
near the distal end of the bone.

The radius and ulna support the fore arm, the former
lying anterior to the latter. The two are firmly bound
together by the interosseus ligament. The body of the
radius bows forward and outward. Dorsally it expands
abruptly, forming the head, which bears an oval surface
for articulation with the humerus. The posterior side of
the head is flattened, and rests against a similar surface

32 ANATOMY OF THE RAT

just below the semilunar notch of the ulna. The radius
is expanded distally to articulate with the wrist, or carpal
bones. Longitudinal grooves cross this expansion; medi-
ally it bears the pointed styloid process.

The ulna is laterally compressed, the lateral surface
bearing a pronounced groove extending from the level of
the head of the radius two-thirds of the way to the distal
end. Like the radius, the ulna bows forward and out-
ward, so that an articulated skeleton looks * * bow-legged. ' '
The ulna terminates distally in the conical, blunt styloid
process for articulation with the wrist bones. The semi-
lunar notch is the saddle-shaped depression near the proxi-
mal end, which, with the head of the radius, articulates
with the humerus. The flat surface at its ventral border
corresponds to the radial notch of other animals ; it articu-
lates with the head of the radius. The olecranon is the
stout projection behind the semilunar notch for the inser-
tion of the extensor muscles of the upper arm.

The joint between the humerus and scapula is of the
**ball and socket" type, permitting of lateral as well as
anteroposterior movement. The elbow joint, however, is of
the hinge type, which permits only of extension and flexion
of the arm.

There are nine carpal or wrist bones. The metacarpal
bones are the relatively long bones in the palm of the
hand. Counting from the medial side of the manus, the
third and fourth are the longest, the fifth is considerably
shorter, while the first is so short as to be readily mistaken
at first glance for one of the carpal bones. Each digit
contains three phalanges, except the thumb (pollex), which
contains two. Each terminal phalanx bears a claw, that of
the thumb being flattened like a nail.

Exercise VIII. Draw and label the lateral view of the scapula
and the bones of the fore limb.

THE SKELETAL SYSTEM 33

THE POSTERIOR APPENDAGES

The two coxal hones (os coxae), sometimes called the in-
nominate hones, comprise the pelvic girdle. Each bone is
firmly united dorsally to the two anterior sacral vertebrae.
Ventrally the two are held together by connective tissue at
the puhic symphysis. The lateral surface of each bone
bears a deep pit, the aeetahulum,, for the reception of the
head of the thigh bone (femur). Unlike the pectoral
girdle the direct attachment of the pelvic girdle to the back-
bone furnishes a rigid support for the strong and active
hind legs of the animal. The coxal bone consists of three
fused elements, the ilium, ischium, and puhis, which radi-
ate from the acetabulum. The distal ends of the pubis and
ischium are also fused, so that these two bones surround
an oval space, the obturator foramen. Though the sutures
are completely obliterated in the adult, they can be very
easily seen in a very young animal. The cartilaginous
boundaries between the three bones in such an animal all
lie in the acetabular wall, though the end of the pubis
forms only a very small part of this wall. The distal
connection of pubis and ischium will be observed at the
ventral extremity of the obturator foramen. The acetabu-
lar wall of the adult is well supported with bone. The
acetabulum itself is funnel-shaped. The inner, or con-
stricted portion, of this funnel expands medially to form
a dorsoventrally elongated pit.

The ilium (os ilium) is the anterior branch of the tri-
radiate coxal bone. It consists of two parts, the hody and
the imng. The former is som^ewhat flattened laterally,
forms the anterior part of the acetabulum, and bears the
anterior inferior spine on its lateral surface just in front
of the acetabulum. The greater sciatic notch is a depres-
sion along the dorsal border of the ilium, extending back

34 ANATOMY OF THE RAT

on to the ischium. Its anterior border is the posterior
inferior spine. The iliopectineal line on the ventral side
of the body connects the sharp anterior border of the pubis
with the wing of the ilium. The wing is the broad, flat,
anterior part of the ilium. The posterior part of its con-
cave medial surface is the auricular surface, which articu-
lates with the sacrum. Anterior to this surface the ilium
bends outward rather abruptly, terminating in a pro-
nounced tuberosity at right angles to the long axis of the
bone. The thickened anterodorsal margin of the bone is the
iliac crest.

The ischium (os ischium), the most posterior component
of the coxal bone, has three parts: a Ijody, a superior
ramus, and an inferior ramus. The anterior end of the
bone forms the posterior side of the acetabulum. The body
expands posteriorly, giving off dorsally the superior and
ventrally the inferior ramus. This part of the ischium is
concave medially, and convex laterally. The inferior
ramus joins the pubis at the ventral extremity of the ob-
turator foramen. There is a longitudinal depression along
the dorsolateral surface of the ischium. This depression is
bounded ventrally by a longitudinal ridge, terminating
posteriorly in a thickening, the sciatic tuber. The upper
edge of the ischium is convex, while the posterior edge is
nearly a straight line. These two edges meet at almost a
right angle.

The pubis (os pubis) is the ventral branch of the coxal
bone. Its superior ramus extends ventrally from the ace-
tabulum. The inferior, or symphyseal, ramus passes back
along the symphysis pubis to join the inferior ramus of
the ischium. The body is located at the region of union
of the two rami. The sharp anterior edge of the bone
bears the iliopectineal eminence. From this the iliopectin-
eal line passes forward.

THE SKELETAL SYSTEM 35

The femur, or thigh bone, consists of a long tody with
processes at each end for attachment of muscles, or for
articulation with other bones. The proximal end is
Y-shaped. The medial branch of the Y is capped with a
hemispherical knob, the head, which fits into the acetabu-
lum. The head is held in place by the round ligament
(ligamentum teres femoris) of the femur, attached to the
center of the articular surface. The head is joined to the
shaft by the narrow cylindrical neok. Head and neck
together resemble a small 'Hoad stool,*' the neck corre-
sponding to the stalk of the ''stool." The massive lateral
branch of the Y, the great trochanter, is continued as a
high thin ridge down the lateral surface of the femur.
This ridge is sometimes referred to as the third trochanter,
and the lateral branch of the Y as the first trochanter, the
two being considered components of the greater trochanter.
The lesser or second trochanter is a knob ventral to the
base of the neck on the flexor surface of the femur. The
intertrochanteric crest connects the greater and lesser tro-
chanters. The trochanteric fossa is a deep depression at
the base of the medial side of the greater trochanter. The
lateral and medial condyles are large ventral swellings at
the distal end of the femur. They are separated from each
other by the deep inter condyloid fossa, which is continued
on to the extensor surface of the bone as the grooved patel-
lar surface for articulation with the patella. The two
condyles articulate with the proximal end of the tibia.

The tihia extends from the knee to the ankle. The fibula
is a slender bone posterior to the tibia. The two are
anchylosed in the lower third of the tibia. These two bones
resemble an archer 's bow, the fibula representing the cord,
the forward bending tibia the wooden bow. The shaft
or body of the tibia decreases in thickness distally. Be-
yond the anchylosis it is cylindrical in cross section, but

36 ANATOMY OF THE RAT

trian^lar proximal to this point. Thus the bone presents
three surfaces: a concave anterolateral surface, a convex
anteromedial surface, and a posterior surface with two
pronounced longitudinal grooves. A ridge, the anterior
crest, is located proximally on the anterior side of the
bone at the angle formed by the lateral and medial sur-
faces. The bone articulates with the femur proximally.
The articular surface is divided into two large concave
condyles, separated from each other by a longitudinal
groove, the inter condyloid fossa. Each of these is opposed
to the convex surface of one of the femoral condyles. The
tibia and fibula articulate distally with the talus (astra-
galus) bone of the heel. The tibial part of the articular
surface contains two longitudinal grooves, separated by a
ridge. The medial malleolus forms the medial boundary
of the inner groove. The fibula bears the lateral malleolus
upon its lateral surface. The slender fibula expands dor-
sally, where it is attached by a ligament to the external
condyle of the tibia.

The patella is a sesamoid bone lying in front of the
knee joint.

The tarsus, or ankle, contains eight bones arranged in
two rows, one distal and one proximal, with one bone
between the rows. The proximal row contains the talus
and calcaneus. The talus articulates with the tibia and
fibula. The calcaneus, or heel bone, is shovel-shaped and
is the largest bone in the tarsus. The ** handle," or heel
process, is directed posteriorly, forming the support for the
heel. When the toes are depressed the trochlea of the talus
acts as a fulcrum, the heel process as a lever moved by the
posterior leg muscles. The length of the heel process makes
it a relatively efficient lever. There are also four sesa-
moid bones in the tarsus. There are five long, cylindrical
metatarsal bones in the foot, those of the great toe, or

THE SKELETAL SYSTEM 37

hallux, and of the little toe being shorter than the remain-
ing three. Each toe, or digit, possesses three phalanges,
except the hallux, which has two. Each terminal phalanx
ends in a curved claw.

Exercise IX. Make a labeled drawing of the lateral view of the
innominate bone and the bones of the hind limb.

THE MUSCULAR SYSTEM

The function of muscular tissue is to produce movement
of parts by contraction. Contraction may or may not be
under the control of the will. The action of voluntary
muscles is consciously controlled; involuntary muscle is
not so controlled, and a person is usually not cognizant of
its activities. These two types of muscle tissue differ also
in microscopic structure, form, and distribution. They
are alike in that the structural unit in each case is the
muscle fiber, which is a cell highly differentiated for the
function of contraction.

Involuntary, or nonstriated, muscles are found in the
walls of the digestive tract, arteries and veins, bronchial
tubes, uterus, bladder, in the glands, etc. The fibers are
either more or less scattered or disposed in sheets. Each
fiber is an elongated, spindle-shaped cell, containing a
single, centrally located nucleus, and a longitudinally
striated cytoplasm. They are under the control of the
sympathetic nervous system, and they contract much less
rapidly than voluntary muscles.

The striated muscle tissue comprises the skeletal mus-
cles for the movement of the arms, legs, trunk, jaw,
and head, the eye muscles for rotating the eyeballs, the
muscles of the face, etc. With the exception of the
striated muscles of the esophagus, the somewhat differ-
ently constructed muscles of the heart, and a few others,
striated muscles are voluntary. The fibers are very
long as compared with their diameter. The sarcolemma,
or thin outer wall of the fiber, incloses the sarcoplasm, in

38

THE MUSCULAR SYSTEM 39

which the longitudinal fibrils are imbedded. Alternate
light and dark regions in these fibrils produce the cross
striated appearance. Each fiber contains many nuclei,
but it is believed by some to be a single cell. The nuclei
occur near the surface in the 'Svhite" muscles, or deeper
in the fiber between the fibrils in the ''red" muscles.
''Red" muscles contract more slowly than the ''white,"
and are more resistant to fatigue.

Striated muscle fibers are aggregated into bundles, fascic-
uli, and these in turn into masses of varying size and
shape, known as muscles. In general these fibers run
parallel to one another, and are bound together into
bundles by connective tissue. The contraction of the
muscle is initiated by a nervous stimulus. The energy for
muscular work is derived from chemical changes in carbo-
hydrates, proteins, and fats. These are conveyed by the
blood to the muscles. Heat is generated by muscular con-
traction. Carbon dioxide and other w^aste products of
muscular work are carried away by the blood.

As a rule a muscle is attached at each end, the less
movable attachment being the origin; the more movable is
the insertion. The attachment may be by means of a
tendon, a non-elastic cord sometimes of considerable length,
or by a sheet of connective tissue, an aponeurosis. A com-
pound muscle has several origins or insertions. Sometimes
parts of a tendon may ossify, producing a sesamoid hone,
such as the patella. The fleshy portion of a muscle is
called the helly.

Most muscles move bones and cartilages, though some
may induce movement in soft parts^ only, such as the facial
muscles which pass from a bone to the easily movable skin.
Some which lie beneath a concave or convex surface merely
tend to straighten out this surface when they contract.
The processes of many of the bones act as levers moved by

40 ANATOMY OF THE RAT

the muscles attached to them, while adjacent articula-
tions of the bones are fulcra. There are three classes of
these levers in the body: (1) those in which the fulcrum
lies between the muscular attachment and the resistance;
(2) the fulcrum at one end, muscle at the other, the weight
between; (3) or the fulcrum may be at one end, with the
muscle attached to the middle, and the weight at the other
end of the bone. Examples of case 2 are rare in man. The
nearer the muscle attachment is to the fulcrum, the greater
must be the force of contraction to move the lever.
Mechanical advantage is secured in machines by having a
small force act through a great distance at the end of a
long lever, moving a heavy object a short distance. In the
musculo-skeletal system this arrangement is generally
reversed, a powerful muscular contraction acting at the
end of a shorter lever to cause rapid movement of a small
weight at the end of a longer lever. Efficiency is sacri-
ficed for speed.

The extensor muscles straighten a member, like the arm
or finger. Flexor muscles bend it. Batators turn it on its
axis. Abductor muscles bend the part away from the
median line of the body or limb ; adductor muscles cause the
reverse movement. As suggested by the names, a levator
muscle lifts a structure, a depressor depresses it. A
sphincter muscle surrounds an opening, which is closed
when the muscle contracts. The function of dilators is to
expand such an orifice.

To prepare a rat for the study of the muscles, lay it on
its back and make a midventral longitudinal cut through
the skin extending from the external genitalia to the tip of
the jaw. Cut no deeper than the skin, to avoid injuring
the underlying muscles. In the same way slit the skin
on the median surface of the front and hind legs, extending
the cuts to the midventral incision. Remove the skin.

THE MUSCULAR SYSTEM 41

Great care should be exercised to avoid removing the
cutaneous muscles, which are located immediately under
the skin. The student should be provided with two rats,
one for the study of the muscles, the second for the dis-
section of the remaining soft parts. Only the larger super-
ficial muscles are included in the following account.

MUSCLES OF THE SKIN

The cutaneous maximus muscle spreads out fanwise from
the region of the arm pit. It covers the back from the
shoulders to tail, the sides of the trunk posterior to the
front leg, and a large part of the ventral surface of the
trunk. Its function is to move the skin, in which it is
inserted.

The platysma is a thin sheet of muscle which radiates
from the posterior angle of the mouth, covers the posterior
side of the face and the lateral surface of the neck, and
sends a branch up in front of the ear. The broad cervical
portion is attached along the middorsal line of the neck.

Exercise X. Sketch the cutaneous maximus and platysma mus-
cles into an outline drawing of the lateral view of the rat.

Carefully remove the two preceding muscles.

SUPERFICIAL MUSCLES OF THE HEAD
(LATERAL VIEW)

Masseter. This muscle consists of two main parts, an
external and an internal. Anteriorly these two are dis-
tinct, but they blend posteriorly.

Internal part. Origin, the whole^ length of the zygomatic
arch. Insertion, the mandible, posterior to the diastema.
Anteriorly the fibers run nearly at right angles to the
long axis of the jaw ; posteriorly they form an acute angle
with this axis.

42 ANATOMY OF THE RAT

External part. Origin. It is attached anteriorly to the
skull in an elongated pit ventral to the base of the zygo-
matic process of the maxillary bone. From here a strong
flat tendon passes backward over the outer surface of the
internal part of the muscle. It originates posteriorly from
the posterior half of the zygomatic arch. The anterior and
posterior regions blend posteriorly vvdth one another and
with the internal part. Insertion, the mandible.

This muscle is a powerful elevator, raising the lower jaw.
The anteroposterior fibers, especially of the external part,
presumably draw the jaw forward in gnawing.

Temporal Muscle. This is a powerful muscle occupying
the posterior part of the orbito-temporal fossa and the side
of the head behind it. Origin : the temporal line, the post-
tympanic hook, the lateral surfaces of the parietal and
squamosal bones, and the posterior wall of the orbito-
temporal fossa. Insertion: the coronoid process and the
other parts of the mandible. It elevates the lower jaw.

The Quadratus Laibii Superiork Muscle is a triangular
structure whose apex is at the anterodorsal angle of the
orbito-temporal fossa; it is inserted at its broad end into
the whisker pad.

SUPERFICIAL MUSCLES OF THE NECK, SHOUL-
DER, AND UPPER FORE LIMB (LATERAL VIEW)

Levator Auris Longus Muscle. It extends from the mid-
dorsal line of the neck as far forward as the back of the
head, to the base of the auricle of the ear. It pulls the
auricle dorsocaudad.

Acromiotrapezius Muscle. Origin, the spines of all the
cervical and first four thoracic vertebrae. Insertion: (1)
the clavicle; (2) the acromion process and lateral edge of
the spinous process of the scapula.

Clavotrapezius Muscle. Origin, the medial half of the

THE MUSCULAR SYSTEM 43

clavicle. Insertion, the lateral part of the crest that runs
dorsally from the external auditory meatus.

Sternomastoideus Muscle. This muscle is anterior, and
parallel to, the clavotrapezius. Origin, the anterior end
of the manubrium. Insertion, by a tendon to the mastoid
bone.

Spinotrapeziiis Mitscle. Origin, (1) the spines of the
fourth to the thirteenth thoracic vertebrae; (2) from the
lumbodorsal fascia from the level of the thirteenth thoracic
to the third lumbar vertebra. Insertion, the scapular
spine. It draws the scapula dorsocaudad.

Latissimus Dorsi Muscle. This is a triangular muscle
attached at one of its angles to the medial side of the
humerus. Origin : the spines of the thoracic vertebrae from
the eighth to the twelfth, inclusive, and the lumbodorsal
fascia to the level of the third lumbar vertebra. Insertion,
by a strong tendon to the medial surface of the humerus.
It is also strongly united to the muscles lateral to it in
the axillary region. The fibers converge to the tendon of
insertion. It pulls the arm caudodorsad.

Acromiodeltoideus Muscle. Origin, the acromion and
metacromion processes of the scalpa, and the ventral border
of the clavicle. Insertion, the deltoid process of the hu-
merus. The acromiodeltoideus is bipinnate ; the fibers from
the clavicle form an acute angle with those from the acro-
mion and metacromion processes where these two groups of
fibers join a connective tissue septum ventral to the hu-
merus. This septum forms the connection of the muscle
with the deltoid process.

Spinodeltoideus Muscle. Origin,^ the infraspinous fascia
and anterior two-thirds of the spine of the scapula. Inser-
tion : it is united ventrally by an aponeurosis with the free
edge of the deltoid tuberosity of the humerus. It flexes the
humerus and rotates it outward.

44 ANATOMY OF THE RAT

Triceps BracMi Muscle. This muscle has three heads —
the caput longum, caput laterale, and caput mediale.

Caput longum. This head lies on the dorsal side of the
upper arm. Origin, the anterior third of the axillary-
margin of the scapula. Insertion, the olecranon.

Caput laterale. This head lies on the lateral surface of
the upper arm. Origin, the larger tubercle of the humerus.
Insertion, the olecranon.

Caput mediale. It lies along the dorsal surface of the
humerus. It is wedge-shaped, being pointed at the anterior
end, but greatly increasing in size posteriorly. Origin, the
dorsal and dorsolateral surfaces of the humerus through-
out practically the whole ventral surface of the muscle.
The pointed anterior end of the muscle reaches nearly to
the head of the humerus. Insertion, the olecranon. The
caput mediale is not seen superficially, but is found when
the caput laterale and caput longum are removed.

SUPERFICIAL MUSCLES OF THE TRUNK

External Oblique Muscle. This is a very extensive thin
muscle situated laterally and ventrally on the thorax and
abdomen. Anterior origin : it is attached along the side of
the thorax by a muscular slip to each rib from the fourth
to the twelfth. Posterior origin, the lumbodorsal fascia.
The anterior slips interdigitate with the posterior slips of the
serratus anterior. Insertion : (1) it fastens to a fascia on the
ventral surface of the thorax and abdomen; (2) is attached
along the whole length of the inguinal ligament (a white,
stout, slender ligament extending along the angle between
the inner surfaces of the hip and body, from the symphysis
pubis to the crest of the ilium) ; (3) a broad branch of the
muscle extends back to the symphysis pubis. Carefully ex-
pose all parts of the origins and insertions and determine
the muscle 's probable action by an inspection of its relations.

THE MUSCULAR SYSTEM 45

SUPERFICIAL MUSCLES OF THE HIND LEG

Biceps Femoris Muscle. This muscle has three portions,
or heads — the anterior, posterior, and accessory. The
origin of the anterior head is the last sacral and the first
caudal vertebra. The posterior and accessory heads origi-
nate at the sciatic tuber of the ischium. All three are
inserted by an extensive fascia into the distal end of the
femur and the proximal two-thirds of the tibia.

Glutaeiis Maximus Muscle. This triangular muscle
originates by fascia from the dorsal border of the ilium,
from the last three sacral vertebrae, and the first caudal
vertebra. It is inseparably united anteriorly with the ten-
sor fasciae latae muscle. Its origin is covered posteriorly
by the anterior head of the biceps femoris. Insertion, the
third trochanter of the femur.

Tensor Fasciae Latae Muscle. Origin, iliac crest. Inser-
tion, fascia lata of the thigh. This muscle is exposed
antero-ventrad to the glutaeus maximus.

Semitendinosus Muscle. Origin, the posterior part of
the sciatic tuber. Insertion, by a tendon on the tuberosity
of the tibia. It joins the tibia from the medial side. It
passes from its dorsal origin down around the posterior
side of the thigh on to the medial surface of the shank.
Action, flexes and rotates the shank.

Exercise XI. Outline the whole body of the rat as seen in the
lateral view, and in this outline draw and label the muscles visible
after the removal of the cutaneous maximus and platysma muscles.

MUSCLES ON THE VENTRAL SIDE OF THE HEAD,
NECK, THORAX, AND ABDOMEN

Place the skinned animal on its back, then dissect and
identify the following muscles.
Digastricws Mtcscle. This muscle lies between the mas-

46 ANATOMY OF THE RAT

seters on the lower jaw. Locate its origin on the skull and
its insertion on the mandibles at, and near, the symphysis.

Sternahyoideus Muscle. The two sternohyoideus muscles
lie on the ventral side of the neck behind the digastricus.
They are in contact with one another along the sagittal
plane of the neck. Origin, the sternum. Insertion, the
hyoid bone.

The sternomastoideus and clavotrapezius muscles have
already been mentioned in connection with the lateral
view. Identify them here.

Omohyoideiis Muscle. This muscle will be observed on
the ventral surface of the neck in the triangular space
bounded by the digastricus, sternohyoideus, and sterno-
mastoideus muscles. Origin, the superior margin of the
scapula. Insertion, the hyoid bone.

The Pectoral Muscles are a prominent group on the ven-
tral side of the thorax. The pectoralis major partially con-
ceals the pectoralis minor. The pectoralis major consists
of a superficial and a deeper portion. The origin of the
superficial portion is the anterior half of the sternum's
manubrium ; it is inserted on the deltoid ridge. The deeper
portion originates on the manubrium of the sternum and
the first and second sternebrae ; the insertion is likewise on
the deltoid ridge.

Pectoralis Minor Muscle. Remove the pectoralis major
to study this muscle. It comprises three parts. The origin
of the first, or most anterior, part is the second to the fifth
sternebrae, inclusive; insertion, on the coracoid process of
the scapula and the lesser tuberosity of the humerus. The
second, or middle, portion originates at the fifth sternebra
and is inserted on the deltoid ridge. The origin of the
third portion is the xiphoid process of the sternum; the
insertion is on the coracoid process of the scapula.

Rectus Ahdomi7iis Muscle. This consists of a pair of

THE MUSCULAR SYSTEM 47

thin strap-like muscles along the ventral side of the abdo-
men and the ventrolateral surface of the thorax. Over
the greater part of the abdomen each muscle unites with its
fellow by means of the linea alba, along the midventral line.
Posterior to the xiphoid process the muscles diverge, run-
ning forward ventrolaterally along the thorax. It retracts
the sternum and compresses the abdomen. Origin, the
posterior part of the pubic symphysis, and the coxal bone
in the immediate vicinity. Insertion, the first rib, the
manubrium of the sternum, and the clavicle.

Exercise XII. Outline the ventral aspect of the body and draw
the muscles into this outline.

THE SALIVARY GLANDS

The glands of the neck should be studied first, because
they or their ducts are likely to be injured during the dis-
section of the blood vessels of this region. Cautiously
remove the skin from the head and neck, taking care to
avoid injuring the underlying muscles, blood vessels, and
glands. Removal of the subcutaneous connective tissue on
the ventral and lateral surfaces of the neck will now reveal
the glands. These are of three types, salivary, lymph, and
lacrimal.

The salivary glands secrete the saliva, which is mixed
with the food in the mouth during mastication. Look for
two of them in the rat, the parotid and the suh maxillary
glands. The parotid is the rather loosely organized, elon-
gated, mass extending from the base of the external ear
along the side of the neck to the ventral surface of it. Find
and trace the parotid duct (Stenson's duct) which trans-
ports the salivary secretion from the gland to the mouth
cavity. The duct is formed by the union of several tribu-
taries from different parts of the gland, traverses the lat-
eral surface of the jaw muscle (masseter), then turns
inward to the mouth cavity. The duct should not be con-
fused with the nerves which cross the surface of the muscle
in this region. Immediately anterior to the dorsal portion
of the parotid is the extra-orbital portion of the lacrimal,
or tear, gland. Locate its duct and trace it to the eye.
The submaxillary glands are the large elongated pair on
the ventral surface of the neck. The median faces of these
glands are in contact with one another, in the sagittal plane

48

THE SALIVARY GLANDS 49

of the animal, throughout most of their length. Trace the
two submaxillary (Wharton's) ducts, each of which leaves
the anterior end of one of the glands and courses cephalad
to the opening on the ventral surface of the buccal cavity.
In following the ducts avoid injury to the blood vessels
of this region. The masses which lie anterior to the sub-
maxillary glands between the rami of the lower jaw are
said to be lymph glands.

Exercise XIII. Sketch a ventrolateral view of the head showing
these glands and their ducts.

THE VASCULAR SYSTEM*

The vascular system conveys food materials and oxygen
to the cells of the body, and carries away from these cells
the waste products of metabolism. The food substances
dissolved in the blood furnish energy for the cell's activi-
ties, and materials for replacing the protoplasm which
has disintegrated during metabolism. The blood stream
tends also to equalize the temperature throughout the
body. Still another function is the conveyance of hor-
mones from their sources to parts of the body whose activi-
ties the hormones accelerate or retard.

The vascular system carries two fluids — blood and lymph.
The former consists of an almost colorless fluid, plasma,
and of red (erythrocytes), and white (leucocytes) blood
cells. When clotting occurs the plasma is resolved into the
fluid serum, which is incapable of clotting under ordinary
conditions, and the solid clot of fihri7i. The capacity of
blood to form a clot is of primary importance, since it
greatly reduces the chance of death through the loss of
blood following injury. The red blood cells contain hemo-
globin, which has the power of combining with oxygen in
the lungs, and carrying it to the tissues, where it is released.
Hemoglobin gives the blood its reddish color when exposed
to oxygen. There are several types of white blood cells.

* The discussion of the vascular system is divided into three parts,
separated from one another by descriptions of other systems. Experi-
ence has shown the convenience of studying the heart and blood
vessels anterior to it early in the dissection, of examining the hepatic
portal system immediately before the abdominal organs are observed,
and of postponing the analysis of the vessels posterior to the heart
until after the study of the abdominal contents.

50

THE VASCULAR SYSTEM 51

Some of them remove necrotic tissue or other detritus, and
therefore function as scavengers. The lymph consists of
plasma and white blood cells. The lymph vessels are the
parts of the vascular system which transport lymph.

The heart receives blood laden with carbon dioxide (ve-
nous blood) from the systemic veins. It pumps this blood
to the lungs through the pulmonary arteries. Carbon diox-
ide is surrendered here and oxygen enters the blood from
the air chambers of the lungs. The oxygenated blood then
returns to the heart through the pulmonary veins, and
is propelled to the body through the systemic arteries.
The branches of the arteries terminate in vessels of micro-
scopic size, the capillaries, in which food and oxygen are
surrendered to the cells of the muscles, glands, etc., and
carbon dioxide enters the blood. The blood from the
capillaries ultimately drains into the systemic veins and
flows back to the heart. The internal structure of the heart
and other details of the circulation will be discussed later.
The arteries and veins in any part of the body may often
be dissected simultaneously. This is the logical procedure,
since the arteries carrying blood to a part and the veins
conducting blood away from it often lie close together.
Considerable care must be exercised in dissecting out blood
vessels, particularly veins, since they are frequently
broken easily. It is better, as a rule, to locate a vessel at
one place, then to follow it from that locality into the
adjoining tissues just as a workman, having exposed a
buried water pipe which he desires to uncover completely,
follows it from the point at which it was first located.
Every attempt to find a vessel anew at a place where it
is concealed by other tissues unnecessarily increases the
chance of breaking it accidentally.

The arteries should be injected with a colored fluid
immediately after killing the rat. This fluid will force the

52 ANATOMY OF THE RAT

blood out of the arteries, through the capillaries and into
the veins, which will therefore be filled, after preservation,
with dark coagulated blood. The fluid may be introduced
through the left ventricle of the heart.

VEINS ANTERIOR TO THE HEART

Skin a rat whose blood vessels have been injected, exer-
cising great care to remove nothing but the skin. Do not
remove the integument between the hind legs on the ven-
tral and posterior surfaces until ready to study the urino-
genital system. Place the skinned animal upon its back
and slit the thorax along a line parallel to the sternum but
seven or eight millimeters to its left, and extending from
the diaphragm to the level of the anterior end of the heart.
Continue the incision dorsally along the junction of dia-
phragm and body wall. Expose the thoracic cavity by
pressing this flap outward. The lungs and heart will be
seen through this aperture. The membranous attachments
of the lungs will not be injured by this dissection if the scis-
sors are not thrust too far into the thoracic cavity. Avoid
extending the incision too far forward, as the blood vessels
anterior to the heart region will be cut.

Observe the membranous sac (pericardium) surrounding
the heart, the large organ in the middle of the cavity.
Note the mesentery-like mediastinal septum extending from
the diaphragm to the anterior end of the thoracic cavity,
and from the pericardium to the sternum. The small pul-
monary lobe just behind the heart lies loosely in a chamber
inclosed by the diaphragm behind, by lateral membranes
extending from the pericardium to the diaphragm, and in
front by the pericardium. The large left lobe of the lung
is attached by a membrane along its dorsal border to the
esophagus, or to the membrane inclosing the pulmonary lobe
mentioned above.

THE VASCULAR SYSTEM 53

Make a second longitudinal incision in the thoracic wall
seven or eight millimeters to the right of the sternum.
Separate the median strip, containing the sternum, from
the diaphragm and turn the strip forward. The right lung
will be exposed. Observe that the dorsal edge of the ex-
treme posterior lobe on the right side is attached by a
mesentery to the esophagus, which, in turn, is similarly
suspended from the dorsal wall of the thoracic cavity.

Observe, also, the thymus gland anterior to the heart.
This gland has no duct. Its secretion enters the blood.
It is therefore an endocrine gland, or glamd of internal
secretion.

The veins of mammals show considerable variation.
Fluctuations from the relations described in this text should
be noted. While dissecting the veins, care should be taken
to avoid injuring the arteries.

Venous blood from the regions of the body anterior to
the heart drains into the right and the left superior vena
cava {precaval veins), which enter the right atrium of the
heart. Trace one of the superior venae cavae cephalad and
note that it is formed by the confluence of the subclavian
and internal jugidar veins. The right and left internal
mammary veins pass from the ventral thoracic wall, each
one parallel to and near the corresponding internal mam-
mary artery, to the precaval vein of the same side. Both
mammary arteries and veins will be seen in the median
strip which has been reflected forward. The azygous vein
empties into the left precava near its union with the heart.
Follow the azygous dorsally at the left of the aorta, thence
caudally parallel to the aorta, noting its tributaries. Is
there a corresponding branch of the right precaval vein?
Trace the internal jugular vein anteriorly to the thyroid
gland.

The external jugidar and cephalic veins unite near the

54 ANATOMY OF THE RAT

middle of the clavicle to form the subclavian. The sub-
clavian, coursing caudally, receives the blood from the
anterior jugular vein, which enters the subclavian just
median to the entrance of the external jugular.

Trace the external jugular vein anteriorly to its origin
near the occipital region of the head. It receives blood,
near its middle point, from the superficial muscles and
skin of the occipital region through the posterior external
Jugular vein.

Cephalad to its union with the posterior external jugular,
the external jugular is joined by the anterior facial vein.
Trace this vein and its tributaries along the ventral and
lateral regions of the head and identify them from the
following description. The supraorhital vein, arising on
the dorsal side of the head where it joins the superficial
temporal (to be described), courses anteriorly and unites
with the nasal vein at the base of the nose to form the
angular vein. This runs ventrally, unites with the supe-
rior labial, and forms the anterior facial. The latter re-
ceives the inferior labial vein, continues ventrally, then
caudally a short distance along the ventro-median border
of the masseter muscle, and finally to its junction with the
external jugular.

Carefully dissect the remaining tributaries of the external ,
jugular vein on the lateral side of the face, neck, and lower
jaw. Identify these vessels from the following descrip-
tion. The superficial temporal vein, arising in the supra-
orbital region, passes ventrally posterior to the eye, receiv-
ing the transverse facial, which courses caudally near the
zygomatic arch. Crossing the posterior root of the zygo-
matic arch, the superficial temporal runs posteriorly, is
joined by the anterior auricular from the base of the
external ear, then unites with the internal maxillary vein
(described below) to form the posterior facial. The pos-

THE VASCULAR SYSTEM 55

terior facial runs through the parotid gland, and is super-
ficial to the carotid artery. The confluence of the pos-
terior auricular vein and the posterior facial forms the
external jugular. The posterior auricular arises behind the
external ear.

The internal maxillary will be found superficially on the
median surface of the angle of the jaw. Here it curves
laterally to join the superficial temporal, thus forming
the posterior facial vein.

Exercise XIV. Make a labeled sketch of the veins anterior to
the heart. This will be combined later (Exercise XXV) with a
similar sketch of the veins posterior to the heart.

ARTERIES ANTERIOR TO THE HEART

The arterial blood leaves the left ventricle of the heart
through the aorta. This turns dorsally and to the left, as
the aortic arch, and runs caudally through the thoracic
and abdominal cavities near the ventral surface of the
spinal column. The thoracic and abdominal sections of
the aorta are called the thoracic and ahdominM aorta,
respectively. The aortic arch of mammals is supposed to be
homologous to the left fourth aortic arch of lower verte-
brates. The branches of the aortic arch in the rat carry
blood to the head, neck, fore limbs, and thoracic wall.
Named in order from right to left, these branches are
the innominate, left common carotid and left subclavian
arteries.

The left subclavian artery runs cephalad from the aortie
arch, carrying blood to the vessels described below.

Left internal mammary artery. Its origin is the left
subclavian artery. It sends branches to the thymus gland,
pericardium, and lung. On the inner surface of the ven-
tral thoracic wall it turns medially to the side of the ster-
num and runs caudally, parallel to the internal mammary

56 ANATOMY OF THE RAT

vein, giving off branches to the internal intercostal muscles.
Trace it posteriorly along the dorsal surface of the rectus
abdominis muscle, to which it sends branches. Several
branches perforate the intercostal muscles and supply the
pectoral and rectus abdominis muscles. Find the right
internal mammary artery. Its branches and course are
substantially the same as for the left internal mammary.

The vertebral artery upon leaving the subclavian passes
cephalad about half a centimeter and enters the verte-
hrarterial canal at the sixth cervical vertebra. This canal
consists of the transverse foramina of the cervical vertebrae.
Running forward through this canal, the artery sends off
branches between the vertebrae. It traverses the transverse
foramen of the atlas, gives off a branch through the large
oval foramen on the dorsal side of this vertebra, passes
through the atlantal foramen, and enters the cranial cavity
by way of the foramen magnum.

Thyro-cervical trunk. It leaves the subclavian artery
medial to the first rib and passes anterolaterally to a position
dorsal to the proximal end of the clavicle, where it divides
into several branches. These branches supply glands on the
surface of the neck, and muscles of the neck region.

Costo-cervical trunk. This vessel leaves the subclavian
artery at the same level as the internal mammary, courses
laterally anterior to the first rib, then sends branches to the
lateral muscles of the thorax and deeper muscles of the neck.

The axillary artery is the continuation of the subclavian
lateral to the first rib. Its branches are as follows.

(1) Lateral thoracic artery. It proceeds caudally from
the axillary artery along the medial surface of the cutane-
ous maximus muscle.

(2) Suhscapiilar artery. It arises from the axillary ar-
tery and divides into branches which supply muscles in
the upper arm and thorax.

THE VASCULAR SYSTEM 57

(3) Deep ariery of the arm. Leaving the distal end of
the axillary artery, it sends branches to muscles of the
upper arm (triceps), then proceeds distally along a spiral
course, and finally sends branches down the anterior and
lateral surfaces of the fore arm.

(4) The hrachial artery, the continuation of the axillary,
passes through the arm pit region, thence down the medial
surface of the upper arm in company with the large
nerves in this part of the arm. It lies throughout its
length close to the posterior edge of the biceps brachii
muscle. The artery provides branches for the muscles of
the elbow region, but the main trunk passes down the
forearm.

The second large arterial trunk leaving the aortic arch
is the left common carotid artery. It originates from the
aortic arch between the innominate artery and the origin
of the left subclavian, and passes forward lateral to the
trachea as far as the anterior end of the neck, where it
splits into the external and internal carotid arteries.

The internal carotid artery proceeds anterodorsally from
its origin and divides into two branches immediately pos-
terior to the tympanic bulla. The dorsal branch of the two
enters the tympanic bulla through the posterior lacerated
foramen, which is situated on the median surface of the
bulla immediately in front of the jugular process. The
ventral branch enters the cranium by way of the carotid
foramen.

The external carotid artery gives off the following
branches.

(1) The occipital artery courses anterodorsally, from its
origin immediately anterior to the internal carotid artery,
passes around the median surface of the jugular process,
thence dorsally to the dorsal musculature of the neck.

(2) The superior thyroid artery runs ventrally from its

58 ANATOMY OF THE RAT

origin cephalad to the occipital artery, then posteriorly,
sending branches to the thyroid gland, and one small branch
to the larynx.

(3) The Ungual artery leaves the external carotid an-
terior to the superior thyroid artery, enters the posterior
region of the tongue, and follows a sinuous course nearly
to the tip in the ventral part of that organ.

(4) The external maxillary or facial artery leaves the
external carotid a short distance anterior to the lingual
artery, and passes anteroventrally along the medial sur-
face of the masseter muscle. Anteriorly it occupies the
groove between the masseter and digastric muscles. It
bifurcates about half a centimeter back of the angle be-
tween the upper and lower lips. The dorsal ramus sup-
plies the snout and lateral wall of the mouth. The ventral
ramus supplies the lower lip. This artery sends vessels to
the lymph and salivary glands of the neck region, to the
masseter muscle, the ventral musculature of the lower jaw,
and the floor of the posterior region of the buccal cavity.

(5) Posterior auricular artery. The external carotid
artery turns abruptly dorsally immediately after giving off
the external maxillary. It turns anteriorly again ven-
trolateral to the tympanic bulla. The posterior auricular
artery arises at the latter turn. It courses dorsally along
the posterior surface of the external auditory meatus to-
the pinna of the ear, to which it sends branches. It also
supplies the muscles adjoining the pinna. Trace the termi-
nal branches of the external carotid.

The innominate artery is the first arterial trunk leaving
the aortic arch. A short distance from its origin the in-
nominate divides into the right common carotid and the
right subclavian arteries. The subclavian passes antero-
laterally to the anterior surface of the first rib, where, in
company with the nerves of the brachial plexus, it leaves

THE VASCULAR SYSTEM 59

the thoracic cavity and becomes the right axillary
artery.

The right internal mammary artery comes off from the
posterior surface of the subclavian just median to the first
rib. Its course is described in connection with the branches
of the left subclavian.

The branches of the right axillary artery closely resemble

those of the left. Verify this by a dissection of the vessels

of the right arm.

Exercise XV. Sketch and label the arteries anterior to the
heart. This drawing will be combined later (Exercise XXVI)
with a similar sketch of the arteries posterior to the heart.

THE HEART

Study the attachments of the pericardium, the trans-
parent membrane which surrounds the heart. If the
thorax has been opened with sufficient care the pericardium
will be unruptured. Insert a blowpipe through a small
incision in the pericardium and inflate it with air. The
pericardial cavity^ the space between the pericardium and
heart, is a part of the body cavity or coelom. The two
other components of the coelom are the thoracic and ab-
dominal cavities.

Remove the pericardium and study the heart in position.
Note the attachments of the aortic arch^ p^ulmonary ar-
teries and veins, the superior venae cavae, and the inferior
vena cava. The heart contains four chambers — two an-
terior atria and two posterior ventricles. The atrial and
ventricular portions of the organ may be identified easily
in a preserved animal. The atria are very dark in color
because they contain masses of clotted blood, while the
ventricles are decidedly lighter. The coronary sulcus is
the deep furrow on the heart's surface which separates the
atria from the ventricles. The right and left auricles (au-

60 ANATOMY OF THE HAT

ricular appendices) are the ventral extensions of the right
and left atria, respectively, Avhich extend ventrally on
each side of the pulmonary artery. Notice on the surface
of the heart the injected coronary artery, which carries
blood to the tissues of the heart, and the coronary veins,
which drain the venous blood back into the general
circulation.

The heart is a muscular pump which propels the blood
throughout the body. Its complete failure to function
means almost instant death. Venous blood returning from
the capillaries of the muscles, brain, digestive organs, kid-
neys, etc. of the body enters the right atrium through the
superior and inferior venae cavae. Contraction of the
right atrium forces the blood into the right ventricle. The
two atria contract simultaneously, but their contraction
alternates with the simultaneous contraction (systole) of
the ventricles. When the right ventricle is full it drives
the blood through the pulmonary artery to the lungs, then
relaxes (diastole) to receive more blood from the atrium.
The excess of carbon dioxide carried by the blood is elimi-
nated in the lung capillaries into the air contained in the
minute air sacs (alveoli) of the lung. At the same time
oxygen from the alveolar air penetrates the walls of the
air sacs and capillaries and unites chemically with the hemo-
glohin, or red coloring matter of the blood. The blood
then traverses the pulmonary veins to the left atrium of
the heart, thence to the left ventricle, and finally out
through the aorta to the tissues of the body. The internal
mechanism of the heart may now be elucidated by an exam-
ination of the dissected organ.

Insert one point of the scissors into the severed end of
the pulmonary artery, carefully slit it lengthwise, continu-
ing the cut along the lateral wall of the right ventricle.
Bend back the cut walls and carefully wash out the coagu-

THE VASCULAR SYSTEM 61

lated blood. Observe the shape of the right ventricular
cavity. It communicates with the pulmonary artery
through the pulmonary orifice, which is surrounded by the
three pocket-like segments of the pulmonary valve. Locate
these valves and determine how they function. The right
atrioventricular, or tricuspid, orifice is the aperture
through which blood enters the right ventricle from the
right atrium. It is guarded by the tricuspid valve. The
inner surface of the ventricle bears muscular protuber-
ances {trahecidae carneae), some of which (the papillary
muscles) are attached to the tricuspid valves by slender
strands, the chordae tendineae. When the ventricle con-
tracts the blood starts to rush back into the atrium, but in
so doing the tricuspid valve is thrust across the aperture
by the blood stream. The edges of the valve would be
forced up into the atrium were it not for the chordae
tendineae and the papillary muscles, which hold the flaps
athwart the opening. Thus blood having once entered the
right ventricle is unable to retreat into the atrium when
the ventricle contracts, but must find its exit through the
pulmonary artery. Insert the scissors' point into the
severed inferior vena cava and open the right atrium by
a cut extending forward to the right superior vena cava
along the anterior surface of the atrium. Locate the open-
ings of the superior and inferior venae cavae, and the
atrioventricular aperture. Find the membranous valves
near the entrance of the superior vena cava.

Slit the ventral wall of the left ventricle lengthwise by
inserting one point of the scissors into the cut end of the
aorta. Extend the incision to the .apex of the heart. Try
not to injure the aortic valve whose semilunar segments
surround the opening of the ventricle into the aorta. The
muscular wall of the left ventricle is considerably thicker
than the wall of the right, for more pressure is required

62 ANATOMY OF THE RAT

to send blood throughout the body than through the pul-
monary circulation. Observe the trabeculae carneae on the
inner surface of the left ventricle's wall. The aortic ori-
fice, surrounded by the aortic valve, is the opening into the
aorta. Part of the valve may have been torn in opening
the ventricle. The mitral, or bicuspid valve surrounds
the mitral orifice, through which blood passes from the
left atrium to the left ventricle. Note the chordae tendi-
neae extending from this valve also to the ventricular wall.
The ventricular septum is the muscular partition between
the cavities of the right and left ventricles. Open the left
atrium, wash out the coagulated blood, and find the orifices
of the pulmonary veins. How many openings are there?
View the partition between the atria (the atrial septum)
by strong transmitted light. The fossa ovalis is the trans-
lucent area which will be seen in this septum. Ask the
instructor to explain its significance.

Exercise XVI. Make a diagrammatic sketch of the internal
anatomy of the heart.

The arteries and veins posterior to the heart will be
studied after the examination of the respiratory, digestive,
and urinogenital systems.

THE RESPIRATORY SYSTEM

The thoracic cavity of mammals is lined with a thin
membrane, the pleura, in the form of two pleural sacs, one
on either side of the median plane. The costal pleura
covers the body wall, the diaphragmatic pleura the anterior
surface of the diaphragm, while the pulmonary pleura is
reflected over the outer surfaces of the lungs. The two
pleural sacs meet in the median plane to form the medias-
tinal septum. The smooth surfaces of the pleura reduce the
friction between the organs of the thorax.

The heart has already been removed. Clear away the
muscle on the ventral side of the neck, exposing the
trachea (or windpipe) and esophagus. The larynx, to be
described later, is at the anterior end of the trachea. The
thyroid gland is an H-shaped gland of internal secre-
tion lying on the trachea just back of the larynx. The
isthmus, which corresponds to the transverse bar of the
H, runs across the ventral surface of the esophagus, join-
ing the two lateral lobes of the gland. Throughout its
length the trachea is in contact with the esophagus. The
latter is dorsal to the larynx, but gradually swerves to the
left until it passes into the thorax on the left side of the
trachea. The heart and the vessels connected with it are
ventral to both esophagus and trachea. The trachea bifur-
cates posteriorly, forming the two primary bronchi. Each
bronchus leads to a lung. Observe the cartilaginous rings
imbedded in the walls of the trachea and bronchi. They
keep the lumen open for the free passage of air. Explain
why the dorsal wall of the trachea is more flexible than
the ventral.

63

64 ANATOMY OF THE RAT

The left lung has but one lobe, the right has four. The
lobe of the left lung is larger than any of the lobes of the
right. Of the lobes of the right lung, three are in a row
on the right side of the thorax. Each of these has a convex
lateral and a concave median surface. The fourth, and
hindmost, lobe of the right lung is approximately pyram-
idal in shape. The concave base of the pyramid lies
against the diaphragm, the apex is directed toward the
heart. It lies for the most part at the left of the median
plane, and may therefore be mistaken for a lobe of the left
lung if its connection with the right bronchus is not dis-
closed. It is inclosed in the membranous sac previously
mentioned.

Exercise XVII. Outline the ventral view of that part of the
dissected animal anterior to the diaphragm, and into this outline
draw the pleural eavityj lungs, esophagus, trachea, thyroid gland,
and larynx.

Trace the left primary bronchus into the left lung.
Note how it divides into the secondary bronchi, these into
the tertiary bronchi, etc. Follow the subdivisions of the
right primary bronchus into the lobes of the right lung.

The volume of the thoracic cavity is increased by the
periodic contraction of the muscles of the diaphragm and
thoracic wall. Atmospheric pressure then forces air into
the lungs through the nasal passages, trachea, and bronchi.

Remove the larynx. In doing so injure the pharynx as
little as possible. The epiglottis is the flap attached to the
anteroventral border of the larynx. When food is being
swallowed the epiglottis folds back over the entrance into
the larynx, thus preventing the passage of food into
the trachea. The vocal cords are the pair of dorsoventral
folds in the larynx. The glottis is the slit between them
leading to the trachea. Dissect away the shreds of muscle
tissue adhering to the larynx, exposing the cartilages in

THE RESPIRATORY SYSTEM 65

its wall — the thyroid, cricoid, and arytenoid cartilages.
This may be done more easily under a dissecting micro-
scope. The thyroid cartilage is a band extending around
the antero ventral and lateral regions of the larynx. Each
lateral extremity gives off an anterior and posterior horn,
the latter joining the cricoid cartilage. The ventral por-
tion of the cartilage bears a median notch on the anterior
border, and is bound to the cricoid cartilage by the crico-
thyroid ligament. The cricoid cartilage has the shape of
a signet ring, completely encircling the air passage. It
is narrow ventrally, but expands dorsally into a hexagonal
plate whose dorsal surface is concave. It is firmly united
to the trachea by connective tissue. The arytenoids are a
pair of small cartilages articulating with the anterior bor-
der of the dorsal plate of the cricoid. Each attaches to the
dorsal end of a vocal cord.

Exercise XVIII. Draw and label the larynx.

THE VASCULAR SYSTEM (Continued)
THE HEPATIC PORTAL SYSTEM

Prepare now to study the contents of the abdominal
cavity, that part of the coelom which lies posterior to the
diaphragm. Slit the abdominal wall in the sagittal plane
from the diaphragm posteriorly nearly to the external
genital organs, which lie between the hind legs. Do not
injure either the external or internal genital organs. Make
an anterior and a posterior cut at right angles to this longi-
tudinal incision, reflect the lateral flaps, exposing the di-
gestive and reproductive systems lying in the abdominal
cavity. Examine the mesenteries supporting the digestive
organs, and observe their veins.

The portal system comprises the veins which carry to
the liver the blood from the capillaries of the intestines,
stomach, pancreas, and spleen. All the tributaries of this
system converge to the portal vein which goes into the
liver immediately ventral to the entrance of the inferior
vena cava into that organ. Trace the ramifications of the
portal system throughout the mesenteries of the stomach
and intestines. Just before it enters the liver the portal
vein receives the pancreatico-duodenal vein from the duo-
denum and pancreas, and the gastro-epiploic vei/n from the
pancreas and greater curvature of the stomach. These two
vessels unite and enter the portal vein by a common trunk.
The portal vein is formed by the confluence of the gastro-
splenic and superior mesenteric veins. The gastro-splenic
is formed by the union of branches from the spleen, pan-
creas, and lesser curvature of the stomach. The superior

66

THE VASCULAR SYSTEM 67

mesenteric vein drains the ascending colon, caecum, and
small intestine. It receives the inferior mesenteric vein
from the descending colon.

Exercise XIX. Make a diagrammatic drawing of the hepatic
portal system.

THE DIGESTIVE SYSTEM

The digestive system consists of the mouth cavity,
pharynx, esophagus, stomach, small and large intestines,
salivary glands, liver and pancreas.

The mouth opening is triangular. It is located on the
anteroventral surface of the head some distance back from
the tip of the snout. The lower lip conceals the greater
part of the two long lower incisor teeth. Externally it is
hairy; internally it is covered with smooth mucous mem-
brane. A fleshy ridge on its inner surface fits into the
space between the incisor teeth. This ridge joins ventrally
a short vertical septum, the frenulum, which passes between
the incisor teeth, connecting the lip with the mandible.
From the angle of the mouth a lobe of integument, covered
with hair, turns upward along the inner surface of the
cheek.

The lateral halves of the upper lip meet in front at an
acute angle pointed toward the nasal septum. The apex
of the angle is connected with the septum by a groove. The
upper lip is therefore cleft, exposing the incisor teeth of
the upper jaw. A frenulum is lacking. The edge of each
upper lip immediately behind the incisor teeth is turned
inward, forming a lobe whose lower hairy surface touches
the dorsal surface of the tongue. This lobe is hairless dor-
sally, and is in contact here with the roof of the mouth.
The lip lobes, tongue, and a fleshy tubercle on the mid-
dorsal surface of the mouth's roof just behind the lobes,
are all in contact with one another when the jaw is closed,
practically separating the mouth opening from the mouth
cavity.

THE DIGESTIVE SYSTEM 69

The teeth have been described in connection with the
bones of the cranium. There are two long pairs of chisel-
like incisor teeth, one pair in the upper and one pair in the
lower jaw. The lower pair protrudes farther into the
mouth cavity than the upper. The upper incisors incline
slightly backward. The lower extend forward from the an-
terior end of the under jaw, curving upward to their
points which lie a little behind the tips of the upper in-
cisors when the jaw is closed. Immediately behind the
incisor teeth is the toothless region, the diastema. The
three upper and three lower molar teeth for grinding the
food torn loose by the incisors, are back of the disastema.
The mouth cavity comprises two parts, the vestibule
lying between the teeth on the one hand and the lips and
cheeks on the other, and the mouth cavity proper. The ves-
tibule is composed of two completely separated parts — the
incisor and the molar. The former lies between the incisor
teeth and the lips. On the lower jaw it is well developed,
but on the upper is almost completely lacking on account
of the cleft in the upper lip. The molar region of the vesti-
bule is a narrow region between the molar teeth and the
cheek. The discontinuity of the two parts of the vestibule
is due to the absence of teeth in the diastema. The folds
of the upper and lower lips and cheek are the only struc-
tures on the lateral side of the mouth in the diastema re-
gion. The mouth cavity proper is widest in the region of
the molar teeth. It decreases in width anteriorly and be-
tween the tongue and soft palate extends back as a nar-
rowed passage opening into the pharynx through the
isthmus faucium at the posterior epd of the soft palate.

The roof of the mouth comprises two regions, the hard
palate supported by the palatine processes of the premax-
illary, maxillary, and palatine bones, and the soft palate,
which consists of a boneless, flexible wall extending back-

70 ANATOMY OF THE KAT

ward from the hard palate. The surface of the soft palate
bears small papillae. The palate is widest at the posterior
part of the molar tooth region, decreasing in width forward
to the inturned lobes of the upper lips, and backward to the
pharynx. A membranous ridge, the palatine arch (pillar
of the fauces), extends ventrally from the posterior end
of the soft palate on each side of the isthmus faucium. The
hard palate bears eight transverse ridges, the most anterior
of which is ventral to the prepalatine foramina. The three
anterior ridges are the largest and lie in front of the molar
teeth. The other five ridges are within and behind the
molar region. These five may assume approximately the
form of a letter w, with the base of the letter forward, or
the form of a bow, or of an obtuse angle whose anteriorly
directed apex lies in the mid-dorsal region of the palate.
Each of the five ridges bears a row of short spines through-
out its length. Other denticulated ridges, which do not
reach the median line of the hard palate, may also be
present. At the posterior end of the hard palate is a
triangular area bearing spines.

Carefully disarticulate the lower jaw. The tongue is
an elongated muscular organ extending from the pharynx
forward to the incisor teeth. It fits snugly into the floor
of the mouth between the molar teeth. Anterior to the
molars it is unattached to the mouth floor. The vertical
frenulum which in some animals connects this anterior
free region of the tongue with the mouth floor is absent.
A median dorsal furrow extends seven or eight millimeters
back from the tip. The tongue is attached to the floor
of the mouth from the anterior molar teeth caudad. The
proximal one-sixth of the organ is attached on both sides
also, so that it alone forms the floor at the back of the
mouth. The tongue unites with the hyoid bone a short
distance in front of the epiglottis.

THE DIGESTIVE SYSTEM 71

Papillae cover the dorsal surface of the anterior three-
quarters of the tongue, the tip, and dorsal portions of the
lateral surfaces. They are entirely lacking on the pos-
terior quarter of the upper surface, ventral portions of
the lateral surfaces, and the under side of the free an-
terior end. The filiform papillae are slender pointed struc-
tures with their tips generally directed backward. On the
anterior half of the dorsal surface these papillae are rather
conspicuous when seen under the dissecting microscope.
In a triangular area about half way between the anterior
and posterior ends of the tongue the filiform papillae are
larger than elsewhere, conical and erect. The filiform
papillae back of this area are so numerous and slender that
here the tongue has a velvety appearance under the dis-
secting microscope. A few, low, cylindrical fungiform
papillae are scattered over the anterior half of the dorsal
surface, chiefly toward the tip.

There is no organized tonsil. This organ is represented
merely by scattered follicular openings on the longitudi-
nally folded side walls of the narrow faucial canal.

Exercise XX. Sketch a dorsal view of the tongue.

The soft palate forms the ventral floor of the naso-
pharynx (pars nasalis). This is a relatively large cavity
which opens anteriorly through the single posterior naris
into the nasal cavity. Toward its posterior end the naso-
pharynx curves downward and opens into the pharynx
proper. The lateral walls are supported by the sphenoid
processes of the palatine bone and the median pterygoid
processes of the alisphenoid, while the basisphenoid and
presphenoid bones support the roX)f. The auditory (Eus-
tachian) tubes from the middle ears open into the naso-
pharynx about half a centimeter from the posterior border
of the soft palate. The opening of each tube is a longi-
tudinal slit near the angle between the lateral and dorsal

72 ANATOMY OF THE RAT

walls, on the same side of the nasopharynx as the ear with
which the tube is connected.

The pharynx proper lies back of the mouth cavity. It
has four openings. Anterodorsally it communicates with
the nasopharynx, anteroventrally with the mouth cavity,
through the isthmus faucium, posteriorly with the esopha-
gus, and ventrally with the larynx and trachea through
the glottis, the entrance to the larynx. The food coming
from the mouth passes through the pharynx to the esopha-
gus. The air entering and leaving the lungs also traverses
the pharynx. It is therefore a part both of the digestive
and respiratory tracts. During respiration the free end
of the epiglottis is raised to allow the passage of air to
and from the larynx. The epiglottis bends backward and
covers the glottis when food is swallowed, preventing the
entrance of food into the trachea. A u-shaped, fleshy ridge
on the dorsal wall of the pharynx, together with the pos-
terior edge of the soft palate, form an ovoid opening into
the nasopharynx which incloses the epiglottis and the glot-
tis when the roof of the pharynx is depressed. Probably
this arrangement permits breathing to continue, without
drawing particles into the respiratory passages, while the
animal is gnawing materials which it does not swallow. A
recess of considerable size lies lateral to this semicircular
ridge.

The esophagus is the tube which conveys food from the
pharynx to the stomach. From its position anteriorly on
the dorsal side of the larynx, it follows the dorsosinistral
surface of the trachea to the level of the first rib, where
it rests against the left surface of the trachea. Entering
the thorax, the esophagus runs backward, dorsal to the
aortic arch and primary bronchus, to the diaphragm, which
it pierces. Posterior to the liver, in the abdominal cavity,
the esophagus enters the stomach.

THE DIGESTIVE SYSTEM 73

Open the abdominal cavity by a median incision from
the diaphragm to the pelvis.

The stomach lies on the left side in the abdominal cavity,
in contact with the concave posterior side of the liver. It
resembles a half of a doughnut in shape. The long axis
forms approximately a right angle with the long axis of
the body. The greater curvature which corresponds to
the outer edge of the doughnut, is ventral. The lesser
curvature y analogous to the rim of the doughnut's hole, is
dorsal. The esophagus enters the stomach on its dorsal
side, through the cardiac orifice, or cardia, near the center
of the lesser curvature. The esophagus is closer to the
right half of the lesser curvature than to the left, thus
entering the stomach obliquely. External inspection of a
full stomach reveals two distinct regions. The wall of
somewhat more than the left half, the cardiac sac, is flexible,
thin, and in a preserved specimen sufficiently transparent
to permit the food it contains to be seen from the outside.
The wall of the right region of the stomach, on the other
hand, is opaque and relatively firm. This part decreases
in size dorsally and communicates with the small intes-
tine through the pylorus. The inner surface of the stomach
comprises two areas corresponding to the two regions
seen from the outside. These may be seen to good advan-
tage by opening the distended organ with an incision ex-
tending throughout the length of the greater curvature.
The inner surface of the cardiac sac at the left is hard,
smooth, and glossy. The wall is tough. If the stomach was
distended with food when the animal died, the wall is
slightly transparent, the transparency increasing as the
tissue dries. In the region at the right the inner surface
is yellowish in color, opaque, brittle in a preserved ani-
mal, and more or less prominently folded, especially near
the pylorus and on the greater curvature. The two re-

74 ANATOMY OF THE RAT

gions are sharply separated by the horny boundary fold,
which extends ventrodextrad, on the anterior and poster-
ior surfaces, from the vicinity of the cardia. The wall of
the cardiac sac is opaque in an empty stomach, and its
inner surface is extensively folded. The esophagus opens
into the cardiac sac, for it lies at the base of the tongue-like
dextral extension of the sac's horny lining. The part
of the boundary fold bordering the right end of this exten-
sion is highly sinuous.

Food passes to the small intestine through the pylorus,
at the upper right hand extremity of the stomach. The
pylorus contains a ring of muscle whose contraction closes
the digestive tube, shutting off communication between the
stomach and intestine. It regulates in this manner the
passage of food from the stomach to intestine. The posi-
tion of the pylorus is indicated externally by a groove
around the alimentary canal. Internally the high ridges
in the glandular part of the stomach are replaced at, or
slightly to the right of, the pylorus by the velvety surface
of the small intestine.

The stomach wall of the mouse (and rat also) are said
to show the following structures. There are two muscle
layers, a thick circular and a thinner longitudinal layer.
The cardiac sac and boundary fold are lined internally by
the statum corneum, or horny layer of epithelial cells.
There are three zones in the glandular tissue at the right
of the boundary fold. The cardial glands surround the
tongue-like extension of the horny epithelium at the right
of the cardia, and extend ventrally along the anterior and
posterior walls. The rennet glands are on the ventral wall
at the right of the boundary fold. The pyloric gland region
lies between the pylorus, and the cardial and rennet glands.
The glands of the mammalian stomach secrete the gastric
juice which initiates the digestion of protein substances in

THE DIGESTIVE SYSTEM 75

the food. The cardiac sac of the rodent, with its homy
lining, is thought to be a storehouse for holding chewed food
pending gastric digestion. It has been suggested that the
dangers involved in feeding have led Rodents to develop
this thin-walled sac in which quickly procured food may
be stored.

The intestine is the part of the alimentary canal in
which the digestion and absorption of food is completed.
It is an extensively coiled tube occupying a large part of
the abdominal cavity. It is physiologically and mor-
phologically divisible into two parts, the small and the
large intestine. The discussion of the mesenterial supports
will be deferred until after the description of the alimen-
tary canal itself. The length of the small intestine may be
about 70 centimeters, more than three times the length of
the animal from the tip of the snout to the base of the
tail. The large intestine, including caecum and rectum,
is about 20 centimeters long. The small intestine com-
prises the duodenum, jejunum, and ileum.

The duodenum lies on the right side of the abdominal
cavity. It is approximately u-shaped, and is therefore
composed of three parts — a descending, a transverse, and
an ascending limb. The first joins the stomach at the
pylorus. From this attachment it turns posterodextrad
in a broad curve, and runs backward about four centi-
meters near the right abdominal wall. The transverse
duodenum is the sharp short curve at the posterior end
of the descending limb, connecting the latter with the
ascending limb. The latter passes forward along the right
side of the colon, and is connected by a mesentery with the
distal edge of the mesocolon. lii a preserved specimen the
duodenum is pinkish or slightly grayish in color. The
gross structure of the internal surface may be observed
to good advantage with a dissecting microscope if a fresh

76 ANATOMY OF THE RAT

duodenum be slit lengthwise, spread out, thoroughly
cleaned, and hardened with formalin. The mucous mem-
brane is folded into very closely packed, relatively short,
flat, transverse ridges. The longer ridges follow a zig-zag
course and their distal edges are lobed. The ridges prob-
ably facilitate digestion by increasing the secretory and
absorptive surface of the intestine.

The jejunum follows the duodenum. It constitutes in
man two-fifths of the small intestine, exclusive of the
duodenum. Its inner surface, like that of the duodenum,
is thickly covered with short, flat, transverse ridges.

The ileum is not sharply separated from the jejunum in
man. It connects the jejunum and the colon. In the rat
it is characterized by its dark color, due to the color of
the contained feces. The mucous membrane of the ileum is
also thrown into numerous closely packed transverse ridges.

The contents of the ileum pass through the ileocolic
valve into a small chamber which communicates by wide
openings both with the colon and the caecum. The latter
is a sac measuring about two and one-half centimeters
on its longest axis. It varies in shape, sometimes resem-
bling an interrogation mark, or assuming the form of a
short spiral. Thus it has a greater, or outer, and a lesser,
or inner, curvature. At one end it opens into the intes-
tines. A mesentery connects the lesser curvature with the
ileum. The caecum contains a mass of dark fecal matter.
When this is removed it will be seen that the inner sur-
face presents smooth areas of considerable extent, but
bears folds also. These folds are transverse along the
inner curvature, but near the proximal end they radiate,
in general, from the opening of the caecum.

From the caecum and ileum the feces enter the ascend-
ing colon. This follows an irregular route forward, ven-
tral to the duodenum but dorsal to loops of the jejunum

THE DIGESTIVE SYSTEM 77

and ileum, to the region of the stomach. Here the colon
(transverse colon) circles dorsally and to the left as far
as the sagittal plane of the body. The descending colon
follows approximately the sagittal plane of the body back
to the rectum. The ascending limb of the duodenum lies
at the right of the descending colon, and both are close
to the dorsal surface of the abdominal cavity. The rectum
is the continuation of the colon through the pelvis. It
terminates posteriorly at the anns^ where the feces are
expelled. There is no sharp boundary between colon and
rectum. The dark fecal matter in the colon forms pellets
shaped like cotton seeds. Wherever these masses occur
the inner surface of the colon is smooth. Elsewhere the
lumen is practically devoid of feces and the surface is
thrown into prominent folds, the colic ridges. The absence
of ridges and the greater diameter of the colon where the
fecal pellets occur shows that at least one purpose of the
ridges is to x>ermit distension of the mucous membrane of
the colon without undue strain upon it.

The liver is a firm, bulky, dark red organ lying in the
anterior region of the abdominal cavity. Its convex,
cranial surface rests against the concave posterior face
of the diaphragm. Ventrally it rests upon the abdominal
wall. Its caudal surface is concave and is in contact with
the stomach and intestines. The organ is deeply cleft
into several very distinct lobes and lobules.

The median lohe occupies a median position in front of
the other lobes of the liver. A longitudinal fissure near
the sagittal plane of the body partly divides this into two
lateral lobules, approximately equal in size, sometimes re-
ferred to as the right and left <^entral lobes. This fissure
extends from the thin posterior edge of the lobe nearly to
the ventral border of the diaphragm, where it receives the
round ligament. Dorsal to this point the two lobules are

78 ANATOMY OF THE RAT

united anteriorly, and are in contact with the diaphragm.
Posteriorly, the fissure extends upward, sometimes reach-
ing the region of the inferior vena cava. The right lobule
passes upward to the right of this vein, the left lobule to
the left. The median lobe is fastened in this region to
the vena cava. Very little, if any, hepatic glandular tissue
connecting this lobe with the rest of the liver, is visible
in a gross dissection.

The left lateral lobe is immediately behind and at the
left of the median lobe of the liver. It attaches to the
other lobes of the liver only at its dorsomedial angle, and
here merely by its ducts, blood vessels, and connective tis-
sue. The anterior surface is strongly convex, the posterior
surface is concave and rests against the stomach and in-
testines. The lobe is thick near the center, tapering to
a thin edge peripherally. The right side pushes in between
the median and right lateral lobes.

The right lateral lobe lies behind and at the right of the
median lobe. A vertical transverse cleft divides it into two
almost completely separated lobules, one anterior, the other
posterior. A bridge of hepatic tissue crossing the dorsal
side of the inferior vena cava connects the two lobules
with one another and with the caudate lobe on the left
side of the abdomen. The posterior lobule is frequently
annexed to the caudate lobe by a strip of hepatic glandu-
lar tissue crossing the ventral surface also of the inferior
vena cava. Both lobules attach to the right surface of the
vena cava. Aside from the glandular bridges mentioned
above, a gross dissection fails to reveal connections of he-
patic tissue with the other lobes of the liver. The anterior
lobule is thick near the center, thinning out toward the
sharp peripheral edges. Its convex anterior surface fits
into a concavity near the right border of the median lobe.
The right lateral surface rests against the diaphragm.

THE DIGESTIVE SYSTEM 79

The caudal surface presents a ventral convexity and a
dorsal concavity to the cranial surface of the posterior
lobule. The concavity receives also the anterior surface
of the right kidney and the accompanying adrenal gland.
A ventral strip of the posterior surface is presented to
the descending limb of the duodenum. The posterior
lobule is approximately pyramidal. The base of the pyra-
mid fastens to the right side of the inferior vena cava.
The triangular surface facing the anterior lobule is slightly
concave medially, decidedly convex laterally, so that the
apex of the pyramid points backward. The renal im-
pression is the concave triangular dorsal surface which re-
ceives the ventral side of the right kidney. A spur of the
lobule runs dorsal in front of the kidney. Occasionally the
posterior lobule is more or less divided into two secondary
lobules.

The caudate lobe (Spigelian lobe) comprises two dis-
tinct semicircular lobules, the anterior and the posterior
lobules. These unite mediad with each other and with
the left side of the inferior vena cava, but are separated
laterally. As previously mentioned, the caudate lobe is
annexed to the right lateral lobe dorsal, and sometimes
ventral, to the inferior vena cava. These lobules fit neatly
into the lesser curvature, and adjacent surfaces, of the
stomach. The anterior lobule is in front of this organ, the
posterior behind it, lying in the lesser peritoneal cavity.
The esophagus fits into a groove, the esophageal impres-
sion, on the caudal surface of the anterior lobule.

The liver and diaphragm are united by several mem-
branous ligaments which radiate outward from the region
where the inferior vena cava pierces the diaphragm. The
falciform (or suspensory) ligament of the liver lies in
the sagittal plane of the body. It connects the diaphragm
ventral to the inferior vena cava with the anterior surface

80 ANATOMY OF THE RAT

of the median lobe. The roimd ligament (ligamentum
teres hepatis) is the ventral thickened border of the fal-
ciform ligament. It enters the dorsal end of the fissure
in the median lobe. The left triangular ligament is a
triangular sheet connecting the dorsal edge of the left
lateral lobe with the diaphragm. It unites with the dia-
phragm along a line running from the vena cava to the
left and to the dorsal margin of the diaphragm. The right
triangular ligament is a comparatively broad sheet con-
necting the dorsal side of the right lateral lobe with the
diaphragm.

The rat's liver possesses no gall bladder for storing the
bile, the digestive secretion of the liver. The hepatic
ducts convey the bile from the liver lobes to the hile duct
(ductus choledochus) which then passes along the duode-
nohepatic omentum, ventral to the portal vein. The duct
traverses the dorsal side of* the pancreas and enters the
mensenterial side of the duodenum about three centimeters
from the pylorus. The liver receives arterial blood from
the hepatic artery, and venous blood from the alimentary
canal through the portal vein. Food substances absorbed
from the alimentary canal are carried to the liver through
this vein. Carbohydrates are stored there in the form
of glycogen. Blood leaving the liver enters the inferior
vena cava through the hepatic veins.

MESENTERIES OF THE ALIMENTARY CANAL

The digestive organs of the vertebrates are connected
with one another and with the walls of the abdominal cav-
ity by membranes. The membranes uniting the organs
with the dorsal abdominal walls are called mesenteries.
Of these the mesogastrium supports the stomach, the mes-
entery proper the intestine, and the mesocolon supports
the colon. The alimentary canal in all vertebrates except

THE DIGESTIVE SYSTEM 81

the Cyclostomes is more or less convoluted, and the parts
are held together by mesentery-like structures. The hepat-
ogastric ligament ties the stomach to the liver, the gas-
trosplenic ligament connects stomach and spleen, the hepat-
oduodenal ligament unites duodenum and liver.

Though the mesenteries of the rat have doubtless evolved
from a simpler arrangement, such as that found in the
dogfish, they are relatively complex, and require careful
study.

The mesogastrium of the rat is expanded into the form
of a sac, the greater omentum, which projects backward
from the stomach region. The rim of this sac unites with
the stomach upon a line extending from the pylorus along
the greater curvature of the stomach, around the left end
and back to the esophagus on the lesser curvature. Dorsad
the greater omentum unites with the body wall between
the adrenal glands. The omental sac thus comprises two
limbs, a dorsal one united anteriorly with the dorsal ab-
dominal wall, and a ventral limb attached to the greater
curvature of the stomach.

The spleen is suspended along its median surface to the
left side of the omental sac. It lies parallel to the greater
curvature of the stomach. The part of the great omen-
tum connecting the spleen and stomach is the gastrosplenic
or gastrolienal ligament. A part of the pancreas is
lodged in the descending limb of the great omentum.

The descending colon lies in the sagittal plane of the
body. It is held in this position throughout its length by
the relatively narrow mesocolon, which joins the dorsal
abdominal wall just beneath the inferior vena cava. The
mesocolon terminates about three' and one-half centimeters
from the anus. Anteriorly it fastens on the left to the
great omentum. The duodenocolic omentum connects the
descending colon and the ascending limb of the duodenum.

82 ANATOMY OF THE RAT

Its attachment extends throughout nearly the whole as-
cending limb of the duodenum. Its connection with the
descending colon is in common with that of the mesocolon.

The mesoduodenum spans the gap between the ascending
and descending limbs of the duodenum. It joins the dorsal
abdominal wall at the anterior end of the duodenal loop,
as described below.

At its anterior end the duodenum turns sharply ventrad
to join the jejunum. The mesentery of the jejunum and
ileum serves also for the ascending colon, and is therefore
both mesentery of the small intestine and a part of the
mesocolon. The central axis of the mesentery carries the
blood vessels which bear blood to and away from the
intestines. The jejunum and ileum are very much longer
than the axis of the mesentery. Therefore the mesentery's
edge attached to them is expanded into many convolutions.
This arrangement gives great freedom for movement of
the small intestines, so that the peristaltic action, which
forces the food along the canal, is not hindered by the
mesenterial attachments. If an anaesthetized animal is
opened, the movements of the small intestines can be
seen.

The ascending colon is about as long as the axial portion
of the mesentery. Therefore this edge of the mesocolon is
not thrown into folds. A short triangular mesentery passes
between the caecum and the distal end of the ileum. The
transverse colon is tied to the mesoduodenum by a short
mesentery. The length of the ascending mesocolon, as
compared with the relatively short mesenterial connections
of the transverse and descending colons, leaves the as-
cending colon relatively free for movement. Proximally
this complex of jejunal and ilial mesenteries and mesocolon,
fuses with the anterior part of the mesoduodenum. The
combined mesenteries then join the dorsal wall of the ab-

THE DIGESTIVE SYSTEM 83

domen between the kidneys and at the left of the inferior
vena cava.

The pancreas secretes a digestive fluid, the pancreatic
juice, which contains three enzymes, trypsin, steapsin, and
amylopsin. The gastric juice of the stomach contains pep-
sin which reduces protein food substances to soluble pep-
tones. These, in turn, the trypsin breaks down into amino
acids, which are absorbed in the intestines. Steapsin de-
composes fats into fatty acids and glycerin. Amylopsin
reduces all digestible carbohydrates to glucose, which is
absorbed and stored in the liver as glycogen.

The pancreas of the rat is irregular in shape and divided
into many large and small lobes. Some of these lobes
anastomose with each other. The pancreas is suspended
in the descending limb of the great omentum, and through-
out the mesoduodenum. It bends dorsally in the mesen-
terial junction of these membranes, and also sends a lobe
to the pyloric region of the stomach. Its left end is
dorsal to the spleen. Pancreatic tissue lies close against
the mesenterial side of the duodenum throughout the
duodenal loop.

The pancreatic juice enters the bile duct, as it crosses

the pancreas, through several small ducts. The openings

of these ducts into the bile duct may be seen if the latter

is slit and its inner surface examined under the dissecting

microscope. The bile duct conducts the pancreatic juice,

with the bile, to the duodenum. These ducts are identified

with difficulty unless a careful dissection be executed

using a microscope.

Exercise XXI. Make an enlarged drawing of the opened ab-
dominal cavity, showing as many as possible of the contained
organs and mesenteries.

THE UROGENITAL SYSTEM

The reproductive and excretory organs are closely asso-
ciated in structure and function, and are therefore col-
lectively termed the urogenital, or urinogenital, system. In
dissecting the urogenital organs be careful not to cut the
blood vessels, which will be studied later.

The two kidneys are the two principal excretory organs
of the body. The renal arteries carry the blood to the
kidneys, where urea, salts, excess water, etc. are removed
from the blood, which then passes to the inferior vena
cava through the renal veins. Each kidney is a bean-shaped
organ, covered ventrally by peritoneum, and attached to
the dorsal wall of the abdomen. The hilum, or depression
where the renal blood vessels and ureter enter, is on the
median surface. The dorsal surface is flattened, the ven-
tral surface is convex. The anterior end of the right kid-
ney fits into the concave surface of the right lateral lobe
of the liver. The left kidney is displaced caudad, so that
its hilum is slightly posterior to the level of the posterior
end of the right kidney. This displacement is probably
due to the presence of the stomach on the left side of
the body. The ureter is the tubule which conveys the urine
from the kidney to the urinary bladder. Find where it
leaves the hilum of the kidney and trace it back to the
bladder.

The suprarenal gland is a somewhat hemispherical body
immediately anterior to each kidney. It lacks a duct. The
secretion {adrenalin) known as an internal secretion, en-
ters the blood, which carries it to those parts of the body
where it performs its functions.

84

THE UROGENITAL SYSTEM 85

Remove both kidneys from the body. With a sharp
razor or scalpel divide one kidney into its dorsal and ven-
tral halves, and bisect the other by a transverse cut passing
through the hilum. The cut surfaces reveal the outer
cortical and inner medullary regions of the kidney. Within
the latter note the dorsoventrally flattened papilla which
projects into the internal cavity, or sinus, of the kidney.
The urine collects in the sinus and drains into the ureter
through its funnel-shaped anterior end, the pelvis, which
is located at the hilum of the kidney. Insert a fine bristle
into the pelvis where it surrounds the tip of the papilla and
by careful probing locate the lumen of the ureter.

Exercise XXII. Draw the cut surfaces of the bisected kidneys.

THE MALE UROGENITAL ORGANS

The spermatozoa, or male sex cells, are produced in the
two testes, which develop in the embryo near the attach-
ment of the mesentery to the dorsal side of the abdominal
cavity. In the lower vertebrates the testes remain through-
out life near their embryonic position, but in most mam-
mals they come to lie in an integumentary sac, the scrotum,
ventral to the anus. This scrotal sac incloses two exten-
sions of the coelom, each of which communicates in the
embryo wdth the abdominal cavity through the inguinal
canal. This canal marks the position of the scrotal evagi-
nation. The testes usually descend from the abdominal
cavity, through the inguinal canals, into the scrotal sac,
though in the whale and elephant they remain in the
abdominal cavity. Such a condition (cryptorchism) oc-
casionally occurs in man. The inguinal canal remains open
in marsupials, bats, rodents (including the rat), insec-
tivores, etc. The descent of the testes in these animals is
temporary, and at the close of the breeding season the
testes are withdrawn into the abdomen by the cremaster

86 ANATOMY OF THE KAT

muscle. Rats dissected in the laboratory frequently show
the testes more or less completely retracted into the
abdomen.

The scrotum of the rat is a prominent swelling, about
four centimeters long in a large male, lying on the ventral
side of the body between the anus and penis. Like the
rest of the integument it is covered with hair. A longi-
tudinal groove on its ventral surface marks the position
of the septu^m scroti, an internal partition which separates
one testis from the other.

Carefully slit the scrotum ventrolaterally on one side
throughout its entire length, using care to cut no deeper
than the integument. Cautiously separate the integument,
which is the scrotum proper, from the underlying layer of
connective tissue. This connective tissue forms a sac
which joins a similar sac surrounding the other testis
to form a median partition, the septum scroti. Slit the
connective tissue sac lengthwise and determine the num-
ber of layers in it. A muscular sac will be observed im-
mediately under the connective tissue. It may be traced
forward to the ventral abdominal muscles. Microscopic
examination of a piece of this muscle will show that it is
two-layered and that the fibers are cross striated. The
fibers of one layer cross those of the other nearly at right
angles.

Each testis is an ovoid body about two centimeters long.
It lies lengthwise of the scrotum. The epididymis is closely
associated with the testis phylogenetically, structurally,
and functionally. It comprises three parts. The enlarged
anterior end, or caput epididymidis (head), curves around
the anterior end of the testis, and is attached ventrally
and mesially to it by a mesentery. The slender corpus
epididymidis (body) passes along the dorsal side of the
testis, connecting the caput epididymidis in front with the

THE UROGENITAL SYSTEM 87

caiida epididymidis behind. The latter expands posteriorly
and curves down around the posterior end of the testis.
The ductus deferens is the conspicuous tube which passes
forward from the median side of the cauda epididymidis,
through the inguinal canal into the abdominal cavity.

The epididymis and ductus deferens are both attached to
a mesentery which extends from the dorsal side of the testis
to the dorsal wall of the scrotal cavity. Anteriorly this
mesentery also incloses the blood vessels which carry blood
to and away from the testis. Fat accumulates in consider-
able quantity in front of the epididymis and within ventral
prolongations of the mesentery which extend through the
inguinal canal into the abdominal cavity.

The mammalian scrotal cavity, like the abdominal cavity
from which it is derived, is lined with peritoneum, the
tunica vaginalis. The testis, like the intestine, is almost
completely invested with peritoneum and is suspended
in the scrotal cavity by the mesentery, which is covered on
both sides by the peritoneal layer. The wall of the testis
is a very tough, fibrous, transparent membrane, the tunica
alhitginea. This incloses a mass of convoluted seminiferous
tubuleSy readily seen through the tunic. Slit the latter
lengthwise of the testis and with a needle tease out some
of the tubules, noting their convolutions. These tubules
produce the spermatozoa.

The epididymis is covered by a tough transparent mem-
brane similar to the tunica albuginea of the testis. Through
the membrane may be seen tubular convolutions. The
ductus deferens becomes coiled when it enters the cauda
epididymidis. Verify this by a careful dissection.

In man the spermatozoa, prodilced by the epithelial lin-
ing of the seminiferous tubules, pass through efferent ducts,
the ductuli efferentes testis, to the caput epididymidis,
where they are collected into a single duct, the duct of the

88 ANATOMY OF THE RAT

epididymis. The convolutions of this tube form the body
and tail of the epididymis. The tube emerges from the
Cauda epididymidis, and as the ductus deferens conducts
the sperm cells to the urethra. This arrangement doubtless
obtains, in all essential details, in the rat also.

The epididymis of Amniotes is homologous to a part of
the mesonephros of the adult Ichthyopsida. The mesoneph-
ros is a kidney and a sperm carrier in the latter group.
The excretory function is retained by the mesonephros in
the mammalian embryo, but in the development of the male
the organ undergoes partial degeneration, so that in the
adult only the sperm-carrying function is retained.

The ductus deferens of the rat, after entering the ab-
dominal cavity through the inguinal canal, turns medianly,
curves around the ventral surface of the ureter, and joins
the urethra.

There are several accessory glandular structures in this
region. The vesicular glands are a pair of conspicuous,
warty structures at the posterior end of the abdominal
cavity. The anterior end of each gland bends abruptly
caudad, so that each gland, as a whole, somewhat resembles
an interrogation point. The gland is hard and brittle in
a preserved animal. Open it and remove some of the
coagulated secretion. It breaks into fine particles when
rubbed between the fingers. This secretion is produced
by the wall of the gland, which is said to contain a strong
mantle of smooth muscle. Find the duct and trace it
caudad to its entrance into the anterior end of the urethra.
These organs were formerly called seminal vesicles, but
they contain no spermatozoa. It is said that the glands
produce an alkaline secretion which is mixed with the
sperms, upon whose movements the secretion probably ex-
erts a favorable influence. It is claimed that extirpation
of the glands reduces the animal's reproductive capacity.

THE UROGENITAL SYSTEM 89

Note the elongated flattened gland lying against the con-
cave surface of the vesicular gland. Find its duct and
trace it to the anterior end of the urethra. Find also the
pair of small glands (probably the ampullar y glands),
each of which encircles the ductus deferens near its en-
trance into the urethra.

The prostate gland comprises two regions. One attaches
to the dorsal and lateral surfaces of the proximal end of
the urethra, and to the dorsal side of the vesicular gland.
The other region consists of two prominent lobes lying
ventral to the urinary bladder, to which they are attached.
The lobes are also fastened to the adjacent body wall.
Dissect away the tough external tunic of connective tissue,
and note that both parts of the gland are a mass of tubules.
The prostatic secretion in man dilutes and lends bulk to
the seminal fluid. It has been suggested that in rodents
the secretion assists the locomotion of the sperm cells,
and also nourishes them.

The urinary bladder is an oval sac attached to the dorsal
surface of the ventral part of the prostate gland. The
bladder narrows proximally to the neck, which joins the
ductus deferentia to form the urethra. Each ureter, or
urinary drainage tube of the kidney, joins the correspond-
ing side of the bladder near the neck. Urine entering the
bladder through the ureters, is stored there temporarily,
then flows through the neck into the urethra, which con-
veys it to the outside. The urethra also carries the sper-
matic fluid during copulation.

The descent of the testis carries the ductus deferens ven-
tral to the ureter. This accounts for the way these tubes
loop around each other in the adult.

A mesentery in the sagittal plane connects the bladder
and posterior part of the prostate gland with the ventral
body wall.

90 ANATOMY OF THE RAT

Divide the pelvic ^rdle along the symphysis pubis, re-
move the ventral part of this girdle, clear away the adja-
cent muscles, and expose the urethra. This runs back
through the girdle ventral to the rectum. The urethra
joins the penis at the posterior end of the pelvic girdle.
At this place the two hulho-urethral (Cowper's) glands
pour their secretion into the urethra through a pair of long
ducts. Each of these glands is ovoid in shape and about
^\e millimeters long. It lies lateral to the rectum and
just behind the posterior border of the ischium. It has
been suggested that the secretion protects the spermatozoa
against the harmful effects of traces of urine.

Dissect away on one side the tissues surrounding the
penis. Note that it has the form of a figure 7, with the
angle anteroventral. Carefully insert one blade of the
scissors into the genital orifice and slit the integument
lengthwise of the body. Press aside the flaps of skin and
note that the terminal section of the penis, the glans, lies
loose in a chamber surrounded by the prepuce or foreskin.
The surface of the glans is slightly rough. Its blunt free
end points backward. The two preputial glands, one on
either side of the glans penis, discharge their secretion near
the posterior edge of the prepuce. Dissect away the sur-
rounding tissue and locate the duct and its opening. Each
gland is dorsoventrally flattened, rounded anteriorly, and
tapers back to the edge of the foreskin. Insert a fine pair
of scissors into the orifice of the duct and slit it length-
wise. Note the relatively very large calibre of the duct
beyond the opening, and the soapy character of the con-
tained secretion. It has been claimed that this secretion
protects the integument in this region from being injured
by the excretions.

The penis is the structure which is used to carry the
seminal fluid from the male to the vagina of the female.

THE UROGENITAL SYSTEM 91

Dissect away the muscle and connective tissue surrounding
it, including the preputial glands and foreskin. The body
of the penis consists of three parts, the two corpora cav-
ernosa penis, and the corpus cavernosum urethrae. The
former comprise the dorsal part of the body when in its
normal position, the latter lies in a deep groove on the
ventral side of the corpora cavernosa penis. Carefully
separate the corpora cavernosa penis from the corpus
cavernosum urethrae. Cut the former transversely. Ob-
serve that they consist of two lateral spongy masses of
tissue, firmly united in the middle, but separated above
by a groove carrying a blood vessel, and below by the
groove inclosing the corpus cavernosum urethrae. Dis-
tally the corpora cavernosa penis terminate in one or two
bones. These bones extend to the tip of the glans dorsal
to the urethra when the glans is turned forward. The
corpora cavernosa penis diverge proximally, forming the
two crura penis. Each cms penis is a tough body firmly
attached to the posterior border of the innominate bone.
The corpus cavernosum urethrae terminates distally at
the tip of the glans penis. Proximally it expands into a
semilunar structure, probably homologous to the hulb of
human anatomy. The ducts of the bulborurethral glands
enter the urethra in front of the bulb. The urethra trav-
erses the whole length of the corpus cavernosum urethrae
and extrudes the urine or seminal fluid through the external
urethral orifice.

Slit the urethra lengthwise from the external urethral
orifice to the region of the prostate gland. The incision
should be lateral in the vicinity of the bladder to avoid
cutting into the neck of the bkdder. Within the penis
the lumen of the urethra is of small calibre, but between
the penis and urinary bladder it expands and contains a
plug of secretion. Note the diverticulum leading off into

92 ANATOMY OF THE RAT

the bulb. Remove the plug of secretion and locate the
ventral orifice through which urine enters from the bladder,
also near this the openings of the ductus deferentia and
ureters. Urethral glands occur in the mucous membrane
of the urethra.

Exercise XXIII. Make a full page drawing of the entire
urogenital system of the male.

THE FEMALE UROGENITAL ORGANS

Examine the anal region of a female rat and note the
three openings. At the base of the tail is the anus, ventral
to it the vaginal orifice, and below this the urinary open-
ing at the summit of a prominent elevation. As a rule
there is but one external opening for the female urogenital
systems in mammals.

Cautiously cut the urinary elevation lengthwise, insert-
ing but one blade of the scissors into the urinary aperture.
This elevation is homologous to the prepuce in the male.
Note that it surrounds a body resembling the glans of the
penis, to which it is probably homologous. One sex among
animals frequently possesses rudimentary structures corre-
sponding to functional reproductive organs in the oppo-
site sex. The penis of the male mammal is represented
in the female by the diminutive clitoris, which is termi-
nated by the glans clitoridis, the homologue of the glans
penis. A pair of prominent glands just beneath the skin
correspond in position to the preputial glands of the male.
Locate the opening of the duct on the inner surface of
the prepuce a short distance in front of the urinary ori-
fice. Insert a bristle into the duct, slit it open, and ex-
amine the secretion, which has a waxy feeling when rubbed
between the fingers.

Clear away the muscles ventral to the pelvic girdle, slit
the symphysis pubis lengthwise, and expose the female ure-

THE UROGENITAL SYSTEM 93

thra. Trace it forward to the urinary bladder, and back
to its entrance on the dorsal side of the preputial cavity.
At the latter end insert a bristle and push it forward to the
urinary bladder.

The urinary bladder is connected with the ventral wall
of the abdomen by the suspensory ligament. As in the
male, it temporarily stores urine which it receives through
a pair of ureters, one from each kidney.

Remove the prepuce, its glans, and the clitoris. Pull
the cut end of the urethra forward. This exposes the
vagina. Separate the vagina from the surrounding tissues
and observe that it lies immediately below the rectum.
With a pair of scissors slit the vagina throughout its
length. The margin of the vaginal orifice is thrown into
longitudinal folds, as is also the internal surface of the
vagina. The mucous membrane of the vagina is said to
be without glands. The two horns of the uterus converge
and join the anterior end of the vagina. The latter re-
ceives the penis of the male during copulation, and is
therefore a temporary receptacle for spermatozoa. Slit
each horn of the uterus a short distance in front of
the vagina. Insert here a blunt probe into the lumen of
each horn until the probe emerges through the external^
orifice (os uterus) into the vagina. There are two ex-
ternal orifices, one for each horn, showing that the fusion
of the posterior part of the horns is superficial and does
not involve the lumen (uterus duplex). The orifices are
very easy to locate in an animal well advanced in preg-
nancy. They are surrounded by five lobes, one large
dorsal lobe, one large ventral, and three smaller ventral
lobes.

Each horn of the uterus is suspended from the dorsal
side of the abdominal cavity by the hroad ligament, which
may contain considerable fat. This ligament extends an-

94 ANATOMY OF THE RAT

terior to the ovary along the lateral surface of the
kidney.

A pair of pockets, corresponding in position to the
inguinal canals of the male, extends backward from the
abdominal cavity. These pockets are probably the homo-
logues of the male vaginal sacs, and therefore probably
correspond to the ddverticula of Nuck in man. The mesen-
tery whose attachment runs along the posterior third of
the broad ligament's lateral surface, and terminates in
this diverticulum, is doubtless the round ligament. Are
these diverticula found in all female rats?

Slit the anterior end of the uterus and locate the open-
ing of the uterine (or Fallopian) tube. This tube carries
the ova from the ovary to the uterus. The aperture is
small but may be found fairly easily, in a uterus contain-
ing large fetuses, near the anterior end of the uterine
horn, for the tube runs in the wall of the uterus a short
distance before opening into it. The ovary, the gonad of
the female, is incased in a tough transparent capsule, the
hursa ovarica. The periovarial space lies between the bursa
and the ovary. Trace the coiled uterine tube. It is said
to be from two and a half to three centimeters long, and
to contain from eight to ten fairly constant major loops.
The tube pierces the bursa ovarica and projects into the
periovarial space as the infimdihulum tuhae. The epi-
thelial lining of the infundibular section of the tube bears
long cilia. This is the only ciliated region of the tube. The
wall of the tube contains muscles, which are best developed
toward the uterine end. Slit open the ovarial capsule and
observe the corpora lutea on the surface of the ovary. Each
corpus luteum marks the point at which an egg bursts
through the wall of the ovary during ovulation. The
ovary is attached dorsally to a strong tendinous band of
connective tissue which extends in the broad ligament from

THE UROGENITAL SYSTEM 95

the anterior end of the uterine horn to the dorsal body-
wall near its union with the diaphragm.

The ovary of most mammals is not inclosed in a capsule.
The ova which break through the wall of the ovary pass
directly to the opening {ostium ahdominale) of the uterine
tube, and are then carried by cilia in the tube to the uterus.
The following events, however, are said to accompany ovu-
lation in the rat. The periovarial cavity is filled with fluid
at the time of ovulation. The bursa ovarica exerts pres-
sure upon this fluid as the result of the contraction of the
muscles located near the infundibulum tubae and the at-
tachments of the uterine tube. Simultaneously muscular
contractions expand the opening (ostium) of the infundi-
bulum tubae. Thus the fluid within the bursa ovarica,
with the contained eggs, is forced into the uterine tube,
where muscular contractions, possibly assisted to some ex-
tent by ciliary action, carry them to the uterus.

The fertilized egg is implanted in the wall of the uterus,
where it develops. Examine a female rat with well ad-
vanced fetuses in the uterus. Note the large size of the
uterus, its abundant blood supply, and segmental char-
acter. Each segment contains a fetus. Slit a segment
lengthwise along its ventral surface and expose the fetus.
This is covered with a thin membrane, the amnion. Care-
fully open the amnion along the back of the fetus. The
fluid in the amniotic cavity protects the embryo from sud-
den jars. Note the discoidal placenta on the dorsal side
of the uterus. The umhilical cord connects the belly of
the embryo with the center of the placenta. The blood
of the fetus passes through the umbilical cord to and from
the placenta, where by osmosis, it receives nutriment from
the maternal blood and surrenders waste materials to the
mother's blood. The blood of fetus and mother do not mix
in the placenta, but are everywhere separated from each

96 ANATOMY OF THE RAT

other by thin membranes. Compare the fetus with the
adult, with other rat fetuses of different ages, and if pos-
sible with fetuses, or models of them, of other mammals.

Exercise XXIV. Make a full-page drawing of the entire uro-
genital system of the female.

THE VASCULAR SYSTEM (Continued)

VEINS POSTERIOR TO THE HEART *

Blood from the parts of the rat posterior to the heart
enters the right auricle through the inferior vena cava
(post cava). Trace the vena cava caudally to the dia-
phragm, which it pierces, then enters the abdominal cavity.
In the thoracic cavity the vein is surrounded by the lobes
of the lungs, and lies at the right of the esophagus.

Study the structure and attachments of the diaphragm.
It comprises a central V-shaped tendinous portion, sur-
rounding which is a muscular region with fibers radiating
from the central tendon to the adjacent body walls. Note
the relative positions of the apertures through which the
inferior vena cava, esophagus, and descending aorta pene-
trate the diaphragm. Observe the number and distribu-
tion of the phrenic veins which traverse the diaphragm,
carrying blood from its substance to the inferior vena cava.
Follow the inferior vena cava posteriorly from the dia-
phragm, exposing its tributaries, which are described
below.

The hepatic veins convey the blood from the liver, which
received it from the portal circulation and hepatic artery,
to the inferior vena cava. The hepatic veins may be ex-
posed by carefully chipping away the brittle tissue of the
liver with a blunt instrument. Determine the number and
location of these veins. The two renal veins, short but of
large calibre, drain the blood from the kidneys into the

* Locate the sympathetic nervous system before dissecting the
arteries and veins posterior to the heart. This system will probably
be injured during the dissection.

97

98 ANATOMY OF THE RAT

inferior vena cava. As the right kidney lies anterior to
the left, so the right renal vein joins the inferior vena cava
anterior to the left renal. Locate and trace the suprarenal
veins, which drain the suprarenal glands. The two ovariam,
veins pass from the vicinity of the ovary, along the broad
ligament of the uterus, in company with the ovarian ar-
teries, thence along the dorsal side of the abdominal cavity.
The left ovarian vein joins the left renal vein, the right
ovarian opens into the inferior vena cava some distance
caudad to the right renal vein. Do you find spermatic
veins corresponding in general location to the ovarian
veins? In other mammals, the cat and man for instance,
right and left spermatic veins are present which open into
the inferior vena cava and left renal veins, respectively.
The right and left iliolumbar veins join the inferior vena
cava in the lumbar region. They accompany the corre-
sponding iliolumbar arteries. Follow the course of each
vein from its mouth caudolaterally across the dorsal sur-
face of the abdominal cavity.

Trace the inferior vena cava caudally to the region
where it is formed by the confluence of the right and left
common iliac veins. Follow each common iliac distally to
its bifurcation into the external iliac and hypogastric {in-
ternal iliac) veins. The latter may be traced dorsally into
the mass of muscle, then posteriorly close to the median
surface of the pelvic girdle. The external iliac vein gives
off several branches, one of which, the femoral vein, ac-
companies the femoral artery along the inner portion of
the thigh. Note any veins which carry blood away from
the urogenital organs. Locate the junction of the middle
sacral vein with the dorsal side of the inferior vena cava
near the union of the latter with the common iliac veins.
Trace the caudal vein posteriorly through the pelvic region
ventral to the sacrum.

THE VASCULAR SYSTEM 99

Carefully clear away the tissues in which the inferior
vena cava is imbedded and find the lumbar veins , which
join the dorsal side of the inferior vena cava. Note their
number, position, and distribution.

Exercise XXV. Make a full-page drawing of the veins anterior
and posterior to the heart, incorporating the sketch required by
Exercise XIV.

ARTERIES POSTERIOR TO THE HEART

The descending aorta is the posterior continuation of the
aortic arch. It courses caudally through the thorax and
abdomen, ventral to the vertebral column, carrying blood
to the body walls, viscera, hind limbs, etc. Its position and
conspicuous size make its identification easy. The main
branches are as follows, beginning at the anterior end of
the series.

The paired intercostal arteries pass dorsally from the
thoracic section of the aorta to the muscles between the ribs.
Determine the number, arrangement, and distribution of
these arteries.

The first large artery leaving the dorsal aorta after it
enters the abdominal cavity is the coeliac. Passing ven-
trally, this divides into three branches: the splenic, the
hepatic, and left gastric arteries. The splenic artery splits
into several branches which terminate in the spleen. It
sends smaller rami to the pancreas and stomach. The he-
patic artery, some distance from its origin, delivers blood to
the following branches. The right gastric artery goes to
the region of the pylorus, thence along the lesser curvature
of the stomach, and anastomoses with the left gastric ar-
tery, which is described below. The right gastro-epiploic
artery passes to the greater curvature of the stomach.
The superior pancreatico-duodenal artery courses along the

100 ANATOMY OF THE RAT

duodenum, to which it sends branches. The right gastro-
epiploic and superior pancreatico-duodenal arteries may
arise from the hepatic artery by a short gastra-duodenal
artery. Still another branch supplies the pancreas. After
giving off the above vessels, the hepatic artery passes an-
teriorly and breaks up into several branches which enter
the liver. The left gastric artery runs to the lesser curva-
ture of the stomach. A large terminal branch supplies
the dorsal surface of the stomach, another goes to the
ventral surface of this organ. Small vessels are given off
to the esophagus. A small branch traverses, to the right,
the lesser curvature of the stomach, and anastomoses with
the right gastric artery, as previously mentioned. Deter-
mine the variability of the branches of the coeliac.

The superior mesenteric artery leaves the dorsal aorta
a short distance posterior to the coeliac in the region of
the renal arteries. It sends numerous branches through
the mesenteries to all the divisions of the small intestine,
to the caecum, and the colon. The branch to the colon
joins the inferior mesenteric artery.

The two large renal arteries carry blood from the dorsal
aorta to the kidneys. The right renal supplies the right
kidney, the left renal the left kidney. Each renal artery
gives off anteriorly a suprarenal artery to the suprarenal
gland on the same side.

The two spermatic arteries of the male rat leave the dor-
sal aorta a short distance caudad to the renals and course
posteriorly along the dorsal surface of the abdominal cav-
ity. The right spermatic finally enters the right testis, the
left spermatic artery the left testis. Expose each artery
throughout its length. The two ovarian arteries of the
female similarly supply the ovaries. They correspond in
position to the spermatic arteries of the male. Each ova-
rian artery anastomoses with the uterine artery, which runs

THE VASCULAR SYSTEM 101

parallel to the horn of the uterus in the broad ligament.
Trace the uterine artery eaudally to its origin.

The two iliolumbar arteries pass laterally in the lumbar
region from the dorsal aorta, along the dorsal surface of
the abdominal cavity, giving off branches to the muscles of
the back. Trace the terminal branches.

The inferior mesenteric artery branches from near the
posterior end of the dorsal aorta, sends branches to the
rectum and descending colon, passes anteriorly along the
latter, and anastomoses with the superior mesenteric artery.
Is this anastomosis always present?

The aorta bifurcates at its posterior extremity, forming
the right and left common iliac arteries. These give off
dorsal branches to the muscles of the back, then divide into
the external and internal iliac (hypogastric) arteries. The
latter, which runs dorsally, divides into two main branches,
one which passes laterally to the proximal region of the
thigh, while the other passes eaudally through the pelvic
region. Dissect out these branches. The external iliac
artery passes to the median surface of the thigh, there be-
coming the femoral artery. This gives off the saphenous
artery, then penetrating the thigh it courses distally to the
foot, supplying branches to the muscles of the thigh and
shank. Trace the saphenous along the median surface of
the shank, and note that its terminal branches enter the
foot.

The middle sacral artery arises from the dorsal aorta
near its posterior end and proceeds eaudally through the
pelvic region into the tail. Trace it and observe its
branches.

The unpaired lumbar arteries pass dorsally from the
abdominal section of the dorsal aorta. They correspond
to the intercostal arteries of the thoracic region. The
lumbar arteries bifurcate ventral to the vertebral column,

102 ANATOMY OF THE RAT

the branches of each artery proceding dorsally on each
side of the column. Determine the number of lumbar ar-
teries, and follow them into the dorsal musculature of the
lumbar region.

Exercise XXVI. Make a full-page drawing of the arteries an-
terior and posterior to the heart, incorporating the sketch required
by Exercise XV,

THE NERVOUS SYSTEM

The nervous system controls most of the activities of the
rest of the organism. It receives through the sense organs
the stimuli from the organs of the body and the external
world, transmits them as nervous impulses, coordinates
them, and originates impulses which it carries to the mus-
cles, glands, etc., stimulating these structures to action. It
comprises the following parts: (1) central nervous system
(brain and spinal cord), (2) peripheral nervous system
(spinal and cranial nerves, and the sympathetic nervous
system), and (3) the sense organs.

THE CENTRAL NERVOUS SYSTEM

The early embryonic central nervous system of verte-
brates is a comparatively simple tube (neural tube). The
anterior portion develops into the brain, the remainder
into the spinal cord. The embryonic brain at first com-
prises three divisions — in front the prosencephalon, then
the mesencephalon, and posteriorly the rhombencephalon.
The prosencephalon further differentiates into the telen-
cephalon (which includes the cerebral hemispheres) and the
diencephalon. The mesencephalon forms dorsally the op^
tic lobes (corpora qMadrigemina) , while the rhomben-
cephalon is transformed into the metencephalon (cerebel-
lum and pons) anteriorly and the myelencephulon (me-
dulla oblongata) posteriorly. The cavities of the brain are
derivatives of the lumen of the neural tube.

The brain fills the cranial cavity. It has already been
adequately preserved by chipping away a part of the roof

103

104 ANATOMY OF THE RAT

of the cranium and immersing the animal in the formalin
solution. Dissect away the muscles on one side of the
head, preserving all nerves found. Remove the dorsal and
posterior walls, and one lateral wall of the cranium, tak-
ing care not to tear the cranial nerves from the brain. Ob-
serve the connective tissue coverings {meninges) of the
brain. The dura mater is the tough membrane just be-
neath the bone. Remove it. The pia mater is the delicate
membrane, containing blood vessels, resting directly upon
the brain. Between these is the arachnoid layer.

The olfactory lo^es are a pair of elongated structures
lying between the eyes at the anterior end of the brain.
The cerehrum is the large heart-shaped section lying im-
mediately behind the olfactory lobes. It consists mostly
of two lateral hemispheres separated by the longitudinal
fissure. The cerebellum is the ovoid structure posterior to
the cerebrum. It includes the median vermis and the right
and left hemispheres. Observe the transverse furrows on
both vermis and lobes. The medulla oblongata succeeds
the cerebellum and joins the spinal cord posteriorly. Com-
pare the cerebral hemispheres of the rat with those of
the dog, cat, man, or other higher mammal. The rat's
cerebrum lacks the grooves (sulci) and ridges (gyri) found
in these other forms.

Clear away the blood vessels on the surface of the brain.
The longitudinal fissure widens posteriorly, exposing the
corpora quadrigemina {optic lo^es). The pineal body {epi-
physis) is the globular structure at the posterior end of the
longitudinal fissure. Carefully press aside the hemispheres
and observe that of the four lobes of the corpora quadri-
gemina, two are on the right, and two are on the left. The
anterior pair are the superior colUculi, the posterior are the
inferior colUculi. The cerebral hemispheres are connected
with each other by the corpus callosum, a broad white

THE NERVOUS SYSTEM 105

sheet which will be observed in front of the corpora quad-
rigemina when the hemispheres are pressed apart.

The paraflocculus projects laterally from the lateral lobe
of the cerebellum. Cautiously remove the petrotympanic
bone, a bit at a time, and note that the paraflocculus occu-
pies the floccular fossa (see the petrotympanic bone). The
paraflocculus resembles a toadstool, the enlarged distal end
lying in the fossa. The flattened floccus proper is antero-
ventral to, and in contact with, the paraflocculus.

Exercise XXVII. Sketch the dorsal view of the brain.

Remove the brain from the skull, being careful to pre-
serve the roots of the cranial nerves. Care should be taken
to keep the paraflocculus intact. The connections between
the cranial nerves and the brain are easily broken. If
considerable care is used in dissecting away the bone
around the foramina of the nerves, many of them may
be removed almost entire on that side of the head where
the muscles were taken away. Separate the brain from
the spinal cord and immerse it in a weak formalin solu-
tion, where it may be studied.

Each olfactory lobe will now be seen to contain two
parts — the olfactory hulh, and the olfactory trmt. Each
bulb is a swelling at the anterior end of the tract, and in
a preserved specimen the former is considerably darker
than the latter. The tract rests in a depression on the
oblique anterior surface of the cerebral hemisphere. By
its distinct whiteness each tract may be followed backward
and outward to the level of the optic chia^ma, where it dis-
appears. Two pear-shaped areas darker than the olfac-
tory tracts extend from the olfactory lobes back to the
optic chiasma. They meet in the median plane, and each
is limited laterally by the olfactory tract.

The optic nerves of mammals cross at the optic cMasma,

106 ANATOMY OF THE RAT

and continue on into the brain as the optic tracts. Thus
light stimuli received by the right eye may pass to the left
side of the brain, and vice versa. The two white and con-
spicuous optic nerves run forward a short distance from
the chiasma before turning outward to enter the orbits.
Observe that the optic tracts diverge behind the chiasma,
and are there connected with each other by a whitish
band.

The infundibulum is the prominence just behind the
optio chiasma. It is in contact with the pinkish hypophy-
sis or pituitary hody, which is covered by a meningeal fold,
so that the hypophysis remains in the cranium when the
brain is removed. The pedunculi cerebri flank the posterior
part of the infundibulum, joining the cerebrum to the
medulla oblongata.

The medulla is wedge-shaped, the expanded anterior end
running forward beneath the cerebellum. Just behind the
infundibulum a band of nervous tissue, the pons, crosses
the ventral surface of the medulla, connecting the cere-
bellar hemispheres. Two pyramids run back from the
pons along the midventral surface of the medulla.

The distribution of many of the cranial nerves can be
worked out on that side of the head from which the mus-
cles have not been removed. There are twelve pairs of
these nerves, as follows.

The first cranial nerves {olfactory) consist of a number
of filaments proceeding from the anterior ends of the ol-
factory lobes, through the cribriform plate of the ethmoid
bone, to the mucous membrane of the nasal cavity. They
will probably not be seen.

The second cranial nerves {optic) have already been
described.

The third nerves {oculomotor) arise from the cerebral
peduncles and extend anteriorly beneath the dura mater

THE NERVOUS SYSTEM 107

on the under side of the brain. They enter the orbits
through the anterior lacerated foramina and innervate the
superior, inferior, and internal rectus muscles, and the in-
ferior obliques. These muscles move the eye.

The fourth nerves (trochlear) emerge from the dorsal
side of the brain behind the posterior coUiculus, run for-
ward along the under surface of the cerebral hemispheres
lateral to the second and sixth nerves, and enter the orbits
through the anterior lacerated foramina. They innervate
the superior oblique muscles of the eye.

Each of the fifth nerves (trigeminal) leaves the pons as
two closely connected roots. It then divides into three
branches. (1) The ophthalmic branch passes throug'h
the anterior lacerated foramen into the orbito-temporal
fossa, where it sends branches to the adjacent integument,
and a small nerve back into the cranium through a fora-
men which is anterodorsal to the optic foramen. (2) The
superior maxillary is a large branch which also enters the
orbito-temporal fossa through the anterior lacerated fora-
men, runs along the dorsal side of the alveolar process of
the maxillary bone, traverses the infraorbital fissure, and
terminates at the roots of the vibrissae and in the skin of
the face. (3) The inferior maxillary nerve passes from
the cranial cavity through the foramen ovale. Imme-
diately after emerging from the cranium it gives off several
branches to the jaw muscles, a lingual ramus to the tongue,
while a large trunk enters the mandibular foramen of
the mandible. This trunk traverses the dental canal,
emerging by the mental foramen to pass to the skin of the
lower jaw and the chin. If time permits, trace the nerve
through the mandible.

The sixth nerves (abducens) start from the anterior re-
gion of the medulla oblongata, course along under the
brain beside the third cranial nerves, emerge into the orbits

108 ANATOMY OF THE RAT

through the anterior lacerated foramen and innervate the
external rectus muscles of the eyes.

The seventh cranial nerves {facial) emerge from the side
of the medulla behind the fifth, and leave the cranium
through the facial canals just behind the external audi-
tory meatuses. They send branches to the lateral surfaces
of the muscles of mastication, where they may easily be
seen when the skin is removed from the head.

The eighth nerves (acoustic) arise from the medulla ob-
longata behind the seventh, penetrate the auditory cap-
sules, and reach the inner ears.

The ninth nerves (glossopharyngeal) arise from the
medulla oblongata close to the tenth. Leaving the skull by
the posterior lacerated foramen, they send branches to the
pharynx and tongue. They are anterior to, and deeper
than, the twelfth nerves, and lie close to the skull.

The tenth nerves (vagus or pneMmo gastric) arise from
the side of the medulla oblongata posterior to the eighth,
and leave the cranium through the posterior lacerated
foramen. Shortly after emerging they give off the 5W-
perior laryngeal nerves to the larynx, then proceed back
to the thoracic cavity along the common carotid artery.
The recurrent laryngeal nerve is given off near the en-
trance of the vagus into the thorax. Trace it forward, on
both sides, along the esophagus to the larynx. On the right
it turns forward from the vagus dorsal to the subclavian
artery, on the left side dorsal to the aortic arch. The tenth
nerves in the thorax lie dorsal to the heart and lungs,
which they are said to innervate. Passing backward along
the esophagus, they penetrate the diaphragm, then inner-
vate the stomach.

The eleventh nerves (accessory) arise from the anterior
region of the spinal cord, and from the medulla enter the
skull through the foramen magnum, unite with the vagus

THE NERVOUS SYSTEM 109

nerves, and with them escape through the posterior lacer-
ated foramen. Trace them to the muscles of the neck.

The twelfth nerves (hypoglossal) leave the medulla be-
hind the first ten cranial nerves, escape from the skull
through the hypoglossal canals, and may be traced easily
to the ventral part of the tongue.

Exercise XXVIII. Draw a ventral view of the brain, showing the
origins of the cranial nerves.

Using a very sharp razor cut the brain in two along a
plane parallel to, and a millimeter or two at the left of,
the longitudinal fissure. Place the larger piece in water
and cautiously dissect away the remnants of the left half
of the brain, exposing the structures described below.

The corpus callosum is the thick commissure connecting
the cerebral hemispheres. Posteriorly it takes the form of
an oval mass (splenium) ; anteriorly it bends downward
(genu), forming the rostrum. This section exposes a re-
gion of the brain not observed on the dorsal side of the
organ — the thalamencephalon, or diencephalon. The dien-
cephalon of the dogfish, frog and other lower vertebrates is
visible dorsally, but in mammals the backward extension
of the cerebral hemispheres, which is in general coincident
with the development of higher intelligence, conceals the
diencephalon.

The third ventricle is the cavity within the diencephalon.
The fornix is dorsal to this ventricle. From its union,
posteriorly, with the corpus callosum, the fornix runs for-
ward and downward anterior to the third ventricle. Ob-
serve the anterior commissure at the lower end of the fornix.
The lamina terminalis is the anterior boundary of the third
ventricle. The conspicuous intermediate mass, or middle
commissure, extends from one lateral wall to the dien-
cephalon to the other through the third ventricle. The m-

110 ANATOMY OF THE RAT

fundihular recess is the extension of the ventricle into the
infundibulum. Posteriorly the ventricle opens into the
cerebral aqueduct (aqueduct of Sylvius), which leads back
through the mesencephalon to the fourth ventricle, the
cavity under the cerebellum and in the medulla oblongata.
The aqueduct may be seen best in a transverse section
through the corpora quadrigemina, where it will be seen
as a narrow vertical slit.

The corpora quadrigemina will be seen above the aque-
duct. (Compare them with the optic lobes of the dog-
fish.) Observe that the superior coUiculi are longer than
the inferior. The isthmus is the deep dorsal fissure that
separates the corpora quadrigemina from the cerebellum.

In the sectioned cerebellum note the deep fissures (sulci)
which separate the prominent folds (gyri). The gray mat-
ter of the cerebellum is on the surface of the gyri, and
so completely penetrates the white matter that the latter
resembles the branches of a tree, and is therefore called the
''arbor vitae.'^

The fourth ventricle is covered dorsally by the anterior^
and the posterior medullary velum, both thin membranes
underlying the cerebellum. The former extends from the
base of the arbor vitae forward to the inferior coUiculus,
while the latter spreads backward.

The cerebellum is strongly attached by the peduncles
to the medulla on each side. Locate these attachments by
dissecting away the peripheral tissue on the under side of
the cerebellum. The lateral fibrous tract {hrachium pontis
or middle peduncle), which runs anteroventrally along the
lateral surface of the medulla, enters the pons. The median
tract {hrachium conjunctivum or anterior peduncle) goes
forward toward the corpora quadrigemina. The posterior
peduncle appears as a ridge passing back to the dorsal
side of the medulla. These tracts may be seen more clearly

THE NERVOUS SYSTEM 111

if they be severed at their union with the cerebellum by
a horizontal stroke with a sharp razor.

Clear away the tissue on the dorsal and lateral side of
the cerebral hemisphere, exposing the corpus callosum.
Trace it into the hemisphere. Cut through the corpus
callosum, expose the cavity of the hemisphere {lateral ven-
tricle), and open this ventricle throughout its extent. The
anterior horn of the ventricle runs ventrally, bounded
medially by the septum pellucidum, and laterally by the
convex corpus striatum. The remainder of the cavity fol-
lows a curvilinear course, at first posteriorly and laterally,
and finally ventrally, terminating near the infundibulum.
From the corpus striatum backward the hippocampus
forms the median wall of the ventricle. The lateral sur-
face of the hippocampus is convex. Viewed from above
it forms an angle of about forty-five degrees with the long
axis of the brain; from the side it is approximately U-
shaped. The lateral and third ventricles communicate
through the interventricular foramina {foramina of
Munro) .

Exercise XXIX. Sketch a sagittal section of the brain.

THE SYIVIP ATRETIC NERVOUS SYSTEM

The sympathetic nervous system regulates the distri-
bution of blood in the body through its control over the
nonstriated muscle tissue in the walls of the blood vessels.
This regulation is accomplished through variation in the
calibre of the vessels, brought about by the contraction
and relaxation of these muscles. Sympathetic fibers in-
nervate also the respiratory, reproductive, digestive, and
other organs, together with their glands. The motor ac-
tivities of the system are beyond the control of the will,
and the stimulation of its sensory structures does not neces-
sarily induce conscious states.

112 ANATOMY OF THE RAT

The sympathetic system of mammals is centralized in a
pair of ganglion-bearing, longitudinal sympathetic trunks,
one on each side of the vertebral column. This trunk con-
nects with the spinal cord through the spinal nerves, and
also sends fibers to sympathetic plexuses in various parts of
the body. These plexuses in turn send nerves to the viscera.
Locate the two longitudinal sympathetic trunks in the rat,
one on each side of the vertebral column. Trace one of
these anteriorly to the head and posteriorly to the pelvic
region. The superior cervical ganglion is a spindle-shaped
enlargement near the bifurcation of the common carotid
artery. The cervical portion of the trunk is dorsal to the
tenth cranial nerve. The inferior cervical ganglion is the
swelling of the trunk at the base of the neck. The thoracic
section lies ventral to the ribs and presents ganglionic en-
largements. The lumbar portion is median to the dorsal
muscle mass in the lumbar region. Locate if possible the
splanchnic nerves, which leave the thoracic ganglia in the
vicinity of the eleventh and twelfth ribs, enter the abdo-
men, and go to the solar plexus. Find other rami of the
sympathetic trunk.

THE SPINAL NERVES

The study of the spinal cord and the spinal nerves lead-
ing from it should be postponed until dissections and draw-
ings of all other parts of the body, including the sympa-
thetic system, have been completed. Expose the spinal
nerves by carefully dissecting away the muscles ventral and
lateral to the backbone, from the occipital region of the
head to the base of the tail. The spinal nerves are desig-
nated according to the region in which they are found,
cervical, thoracic, lumbar, sacral, and coccygeal.

The cervical spinal nerves, eight pairs in all, are in the
neck. The first pair leaves the spinal canal through a fora-

THE NERVOUS SYSTEM 113

men in the neural arch of the atlas. The other seven pairs
escape through the intervertebral foramina. The first,
second, third, and part of the fourth pairs of nerves go to
the neck muscles. The phrenic nerve leaves the fourth
cervical nerve. Trace it caudad to the diaphragm.

Dissect the brachial plexus, the conspicuous bundle of
intercommunicating nerve trunks in the axillary region.
It is formed by the fourth, fifth, sixth, seventh, and eighth
cervical, and the first dorsal nerves. Note the size of these
nerves. The brachial plexus sends the following branches
to the arm, (1) The ulnar nerve goes to the posterior
region of the forearm. (2) The median nerve runs across
the anterior surface of the elbow joint, supplying the mus-
cles of the inner side of the arm. (3) The axillary nerve
passes around the posterior side of the humerus, crosses
the elbow joint, and as the radial nerve proceeds to the
outer side of the arm. Locate, also, the branches going
from the plexus to the thorax.

There are thirteen pairs of thoracic spinal nerves. The
first enters the brachial plexus. The second to the twelfth
inclusive run in the muscle tissue between the ribs. The
thoracic nerves leave the spinal canal through the inter-
vertebral foramina. Except for the first, they innervate
the walls of the thorax and anterior part of the abdomen.

The six pairs of lumbar nerves are associated with the
thick muscle masses in the lumbar and sacral regions. Care-
fully remove these muscles without severing the nerves or
their connections. The first two pairs of lumbar nerves
innervate the abdominal walls, by way of the lumbar plexus.
Find the lumbar plexus, which is formed on each side by
part of the first, second, third, fourth, and fifth lumbar
nerves. The three following nerves arise from it. The
femoral nerve proceeds caudally to the inner aspect of the
hind limb. The obturator nerve passes through the obtura-

114 ANATOMY OF THE RAT

tor foramen of the coxal bone to the adjacent muscles. The
sciatic nerve may be traced from the lumbar plexus to the
deeper posterior muscles of the thigh.

The remainder of the fifth lumbar nerve, and the sixth
lumbar nerve, together with part of the first and second
sacral nerves, form the sacral plexus. There are four
pairs of sacral nerves. Locate the caudal nerve in the
tail and trace it forward to its connections with the sacral
nerves.

Exercise XXX. Sketch the spinal and sympathetic nerves of
one side of the body.

THE SPINAL CORD (SPINAL MEDULLA)

Clear away the muscles surrounding the backbone and
sever the head from the body at the articulations between
the cranium and the atlas. Carefully expose the spinal
cord throughout its length by removing the roof of each
neural arch. The cord is surrounded by tough membranes,
the meninges.

Observe the connections of several of the spinal nerves
with the cord. Each nerve proceeds from the cord as a
pair of roots, a dorsal and ventral. The former bears a
swelling, or ganglion. Trace a pair of roots from their
origin to the point where they unite to form the spinal
nerve. The dorsal ganglia in mammals contain the cell
bodies of sensory neurons which send some of their fibers
through the dorsal root to the spinal cord, and other fibers
distally to the spinal nerve. The dorsal root is, therefore,
sensory in function. The ventral root carries motor fibers
from nerve cells located in the cord itself. Thus each spinal
nerve carries nervous impulses both to and from the spinal
cord.

Observe the cervical and lumbar enlargements of the
cord in the neck and lumbar regions, respectively. The

THE NERVOUS SYSTEM 115

former is the source of the nerves entering the brachial
plexus, the latter of the fibers going to the sciatic plexus.
Remove the cord from the neural canal, strip off the
meninges, and note that the cord rapidly diminishes in
size in the posterior lumbar region, finally terminating in
the slender thread-like filum termmale. The posterior
spinal nerve roots proceed caudally, accompanied by the
filum terminale, for a considerable distance from their at-
tachment to the cord. These roots constitute the ccmda
equina, so called from its resemblance to a horse's tail.

Exercise XXXI. Draw a cross section of the spinal cord to
show the origins of the spinal nerves, the spinal ganglia, etc.

INDEX

Abdominal cavity, 59, 66.
Acetabulum, 33, 35.
Acoustic meatus, 9, 18.
Acromion process, 30.
Alveolus, 10.
Angular process, 24.
Anterior commissure, 109.
Anus, 5, 77, 92.
Aorta, 55, 99.
Aponeurosis, 39.
Appendicular skeleton, 6.
Aqueduct of Sylvius, 110.
''Arbor vitae," 110.
Artery, axillary, 56.

brachial, 57.

carotid, external and internal,
57.
left common, 55, 57.
right common, 58.

coeliac, 99.

common iliac, 101.

costo-cervical, 56.

deep, of arm, 57.

external maxillary, 58.

facial, 58.

femoral, 101.

gastro-duodenal, 100.

hepatic, 80, 99.

hypogastric, 101.

iliac, external, 101.
internal, 101.

iliolumbar, 101.

innominate, 55, 58.

intercostal, 99.

internal mammary, 55, 56, 59.

lateral thoracic, 56.

left gastric, 99.

Artery, lingual, 58.

lumbar, 101.

mesenteric, 100, 101.

ovarian, 100.

pancreatico-duodenal, 99.

posterior auricular, 58.

pulmonary, 51.

renal, 84, 100.

right axillary, 59.
gastric, 99.
gastro-epiploic, 99.

saphenous, 101.

spermatic, 100.

splenic, 99.

subclavian, 55, 58.

subscapular, 56.

suprarenal, 100.

thyrocervical, 56.

uterine, 100.

vertebral, 56.
Arytenoid cartilage, 65.
Astragalus, 36.
Atlas, 25.
Atrial septum, 62.
Atrium, 59.
Auditory tubes, 71.
Auricle, 59.

Auricular surface, 28, 34.
Axial skeleton, 6.
Axis, 25.

Bicuspid valve, 62.

Blaod, 50, 51.

Bone, alisphenoid, 10.

basisphenoid, 10.

carpal, 32.

cartilage, 7.

117

118

INDEX

Bone, chevron, 28.

coxal, 33.

ethmoid, 20.

frontal, 9, 19.

hyoid, 24.

innominate, 33.

interparietal, 9, 18.

jugal, 22.

malar, 22.

maxillary, 9, 10, 20.

membrane, 7.

metacarpal, 32.

metatarsal, 36.

nasal, 9, 22.

occipital, 9, 10, 15.

palatine, 10, 23.

parietal, 9, 19.

petrosal, 9, 10, 18.

premaxillary, 9, 10.

presphenoid, 10.

sphenoid, 16.

squamosal, 9, 10, 17.

turbinal, 14.

tympanic, 18.

vomer, 22.

zygomatic, 9, 22.
Brachial plexus, 113, 115.
Brachium conjunctivum, 110.

pontis, 110.
Brain, 103.
Bronchi, 63, 64.

Caecum, 76.
Calcaneus, 36.
Capillaries, 51.
Caput epididymidis, 86.
Cardia, 73.
Cardiac sac, 73.
Cauda epididymidis, 87.

equina, 115.
Cerebellum, 12, 103, 104.
Cerebral aqueduct, 110.
Cerebrum, 13, 104.
Cervical ganglia, 112.
Choana, 11.

Chordae tendineae, 61.

Clavicle, 30.

Clitoris, 92.

Coelom, 59.

Colliculi, 104.

Colon, 76.

Condyloid process, 24.

Coracoid process, 30.

Coronoid process, 24.

Corpora cavernosa penis, 91.

lutea, 94.

quadrigemina, 103, 104, 110.
Corpus callosum, 104, 109, 111.

cavernosum urethrae, 91.

epididymidis, 86.

striatum, 111.
Cranial cavity, 12.
Cranium, 7.
Cribriform plate, 13.
Cricoid cartilage, 65.
Crura penis, 91,

Diaphragm, 97.
Diastema, 10, 23, 69.
Diastole, 60.
Diencephalon, 103, 109.
Digestive system, 68.
Digit, 4, 37.
Diverticula of Nuck, 94.
Dorsal ganglia, 114.
Duct, bile, 80, 83.

hepatic, 80.

naso-pharyngeal, 13.

parotid, 48.

submaxillary, 49.
Ductus deferens, 87.
Duodenocolic omentum, 81,
Duodenum, 75.
Dura mater, 104.

Endoskeleton, 6.
Epididymis, 86.
Epiglottis, 64, 72.
Epiphysis, 104.
Epistropheus, 25.

INDEX

119

Erythrocytes, 50.
Esophagus, 63, 72.
Eustachian tubes, 71.
Exoskeleton, 6.

Fallopian tube, 94.
Feces, 5, 77.
Femur, 33, 35.
Fetus, 94, 95.
Fibula, 35.
Filuni terminale, 115.
Foramen, anterior lacerated, 10,
107.

atlantal, 26, 56.

carotid, 12,

interventricular, 111.

intervertebral, 27, 113.

jugular, 12.

magnimi, 11, 108.

mandibular, 24.

mental, 24.

of Munro, 111.

obturator, 33.

optic, 10.

ovale, 9.

posterior lacerated, 11, 108.

postpalatine, 10, 12.

prepalatine, 12, 14.

transversariuni, 26.
Fornix, 109.
Fossa, cerebellar, 12.

cerebral, 12.

floccular, 13, 105.

jugular, 9.

mandibular, 18.

nasal, 14.

olecranon, 31.

olfactory, 12.

orbito-temporal, 8.
Fovea dentis, 25.

Gastric juice, 74.
Genu, 109.

Gland (s), ampullary, 89.
bulbo-urethral, 90, 91.

Gland (s), cardial, 74.

Cowper 's, 90.

endocrine, 53.

lacrimal, 48.

parotid, 48.

preputial, 90.

prostate, 89.

pyloric, 74.

rennet, 74.

salivary, 48.

submaxillary, 48.

suprarenal, 84.

thymus, 53.

thyroid, 63.

vesicular, 88.
Glans, 5.

clitoridis, 92.

of penis, 90.
Glenoid cavity, 30, 31.
Glottis, 64, 72.
Greater omentum, 81.
Greater tuberosity of humerus,

31.
Gyri, 110.

HaUux, 4, 37.

Heart, 51, 59.

Hepatic portal system, 66.

Hilum, 84.

Hippocampus, 111.

Humerus, 31.

Hypoglossal canal, 11, 109.

Hypophysis, 106.

Ileocolic valve, 76.

Ileum, 76.

Ilium, 33.

Infraorbital fissure, 8, 107.

Infundibulum, 106.

recess of, 110.
Inguinal canal, 8^.
Intestine, 75.
Ischium, 33, 34.
Isthmus, 110.

faucium, 69.

120

INDEX

Jejunum, 76.
Jugular process, 9.

Kidney, 84, 85, 93.

Lambdoidal ridge, 8.
Lamina, 25.

terminalis, 109.
Larynx, 63, 64.
Lateral malleolus, 36.
Lesser tuberosity, 31.
Ligament, broad, of uterus, 93.

falciform, 79.

gastrosplenic, 81.

round, 35, 77, 80, 94.

suspensory, 93.

triangular, 80.
Linea alba, 47.
Liver, 77.

Lumbar plexus, 113.
Lung, 64.
Lymph, 50.

vessels, 51.

Mandible, 23.
Mandibular symphysis, 23.
Medial malleolus, 36.
Mediastinal septum, 52, 63.
Medulla oblongata, 103, 104, 106.
Meninges, 104, 114.
Mesencephalon, 103, 110.
Mesenteries, 80.
Mesocolon, 80, 81.
Mesoduodenum, 82.
Mesogastrium, 80.
Metacromion process, 30.
Metencephalon, 103.
Middle commissure, 109.
Mitral orifice, 62.

valve, 62.
Mouse, 2.
Mouth cavity, 69.
Muridae, 2.
Mus, 2, 3.
Muscle, abductor, 40.

Muscle, acromiodeltoideus, 43.
acromiotrapezius, 42.
adductor, 40.
biceps femoris, 45.
clavotrapezius, 42.
cremaster, 85.
cutaneous maximus, 41.
depressor, 40.
digastricus, 45.
dilator, 40.
extensor, 40.
external oblique, 44.

rectus, 108.
flexor, 40.

glutaeus maximus, 45.
inferior oblique, 107.
insertion, 39.
latissimus dorsi, 43.
levator, 40.

auris longus, 42.
masseter, 41.
omohyoideus, 46.
origin, 39.
papillary, 61.
pectoralis major, 46.

minor, 46.
platysma, 41.

quadratus labii superioris, 42.
rectus abdominis, 46.
rotator, 40.
semitendinosus, 45.
of skin, 41.
sphincter, 40.
spinodeltoideus, 43.
spinotrapezius, 43.
sternohyoideus, 46.
sternomastoideus, 43.
superior oblique, 107.
temporal, 42.
tensor fasciae latae, 45.
triceps brachii, 44.
Myelencephalon, 103.

Nares, 8, 11.
Nasal cavity, 12.

INDEX

121

Nasal labyrinth, 14.

septum, 14, 20.
Nasopharynx, 71.
Nerve, abducens, 107.

accessory, 12, 108.

acoustic, 108.

axillary, 113.

caudal, 114.

cervical spinal, 112.

cranial, 12.

eighth cranial, 108.

eleventh cranial, 108.

facial, 108.

femoral, 113.

fifth cranial, 107.

first cranial, 106.

fourth cranial, 107.

glossopharyngeal, 12, 108.

hypoglossal, 109.

lumbar, 113.

ninth cranial, 108.

oculomotor, 106.

olfactory, 13, 106.

optic, 105, 106.

pneumogastric, 108.

sacral, 114.

sciatic, 114.

second cranial, 106.

seventh cranial, 108.

sixth cranial, 107.

spinal, 112.

tenth cranial, 108.

third cranial, 106.

thoracic spinal, 113.

trigeminal, 107.

trochlear, 107.

twelfth cranial, 109.

vagus, 12, 108.
Neural arch, 25.

spine, 25.
Nuchal surface, 8.

Occipital condyle, 9, 11.
Odontoid process, 25.
Olecranon, 32.

Olfactory bulb, 105.

lobes, 104.

tract, 105.
Optic chiasma, 105.

lobes, 103, 104.

tracts, 106.
Os uterus, 93.
Ostium abdominale, 95.
Ovary, 94.
Ovulation, 95.

Palate, 11, 69, 71.

Palatine arch, 70.

Pancreas, 83.

Patella, 36.

Pedicle, 25.

Peduncles of cerebellum, 110.

Pelvic girdle, 33.

Penis, 5, 90, 93.

Pericardium, 52, 59.

Phalanges, 32, 37.

Pharynx, 72.

Pia mater, 104.

Pillar of the fauces, 70.

Pineal body, 104.

Pituitary body, 106.

Pleura, 63.

Plexus, sacral, 114.

sciatic, 115.
Pollex, 4.
Pons, 103, 106.
Posttympanic hook, 9, 17.
Postzygapophysis, 25.
Prepuce, 5, 90.
Prezygapophysis, 25.
Prosencephalon, 103.
Pterygoid process, 9, 11.
Pubis, 33, 34.
Pulmonary orifice, 61.

valve, 61.
Pylorus, 73, 74.
Pyramids, 106.

Badius, 31.
Rat, 2, 3,

122

INDEX

Rectum, 77, 93.
Rhombencephalon, 103.
Ribs, 28, 29.
Rodentia, 1.
Rostrum, 109.

Sacrum, 28.
Scapula, 30.
Scapular spine, 30.
Scrotum, 5, 85.
Semilunar notch, 32.
Seminiferous tubules, 87.
Septum pellucidum, 111.

scroti, 86.
Skull, 6.

Spinal cord, 25, 112, 114.
Spinous process, 25.
Spleen, 81.
Splenium, 109.
Sternum, 28.
Stomach, 73.
Styloid process, 32.
Sulci, 110.

Sympathetic trunks, 112.
Systole, 60.

Talus, 36.

Tarsus, 36.

Teeth, incisor, 10, 69.

molar, 10, 69.
Telencephalon, 103.
Temporal line, 8, 19.
Tendon, 39.
Tentorium, 13.
Testes, 5, 85.
Thalamencephalon, 109.
Thorax, 29, 59.
Thyroid cartilage, 65.
Tibia, 35.
Tongue, 70, 71.
Tonsil, 71.

Trabeculae carneae, 61.
Trachea, 63.
Transverse process, 25.
Trochanters, 35,

Tunica albuginea, 87.

vaginalis, 87.
Tympanic bulla; 9, 18.

membrane, 9.

Ulna, 31, 32.
Umbilical cord, 95.
Ureter, 84, 89, 93.
Urethra, 88, 89, 90.

female, 92.
Urethral orifice, external, 91.
Urinary bladder, 89.
Uterine tube, 94.
Uterus, 93.

Vagina, 90, 93.
Vaginal orifice, 5, 92.
Valve, tricuspid, 61.
Vein, angular, 54.

anterior auricular, 54.
facial, 54.
jugular, 54.

azygos, 53.

cephalic, 53.

common iliac, 98.

external iliac, 98.
jugular, 53.

femoral, 98.

gastro -epiploic, 66.

gastrosplenic, 66.

hepatic, 80, 97.

hypogastric, 98.

iliolumbar, 98.

inferior mesenteric, 67.

internal iliac, 98.
jugular, 53.

internal mammary, 53.
maxillary, 54, 55.

lumbar, 99.

ovarian, 98.

pancreaticoduodenal, 66.

portal, 66, 80.

postcaval, 97.

posterior facial, 54.

INDEX

123

Vein, precaval, 53.

pulmonary, 51.

renal, 84, 97.

subclavian, 53.

superficial temporal, 54.

superior mesenteric, 66.

supraorbital, 54.

suprarenal, 98.

transverse facial, 54.
Vena cava, inferior, 97.

superior, 53.
Ventricles of brain, 109, 110, 111.

of heart, 59.

Ventricular septum, 62.
Vermis of cerebellum, 104.
Vertebrae, 25, 26, 27, 28.
Vertebral arch, 25.

column, 24.

foramen, 25.
Vertebrarterial canal, 27, 56.
Vestibule, 69.
Vibrissae, 4.
Visceral skeleton, 7.
Vocal cords, 64.

Zygomatic arch, 8.