Laboratory directions for comparative vertebrate anatomy

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MARINE BIOLOGICAL LABORATORY.

Received Augus ut . ■ 1940
Accession No. 46302

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COMPARATIVE VERTEBRATE

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COMPARATIVE VERTEBRATE

ANATOMY

By
I. E. Gray, Ph.D.

Associate Professor of Zoology,
Duke University

P. BLAKISTON'S SON & CO., Inc.

Philadelphia

Copyright, 1938, by P. Blakiston's Son & Co., Inc.

PRINTED IN U. S. A.
BY THE MAPLE PRESS COMPANY, YORK, PA.

This laboratory guide was designed for use In a one-semester
course. It was written for the student — not the instructor —
and was planned to be used independently by the student with
a minimum of aid from the instructor. The laboratory work Is
of course to be supplemented by readings, discussions, and
lectures. Standard laboratory materials, shark, Necturus, and
cat, are the principal animals used; and each organ system is
treated In a comparative manner. Experience has taught that,
although greater storage facilities are sometimes necessary, the
comparative method of teaching vertebrate anatomy Is superior
to other methods. Specific mention of the drawings to be
required of the student is purposely omitted; it seems better
for each instructor to use his own judgment In this respect.

The author makes no claim to originality In writing this
manual. As is the case with most laboratory guides Ideas and
Information were derived from many sources. Special acknowl-
edgment, however, should be made to Professor George Wagner
of the University of Wisconsin, whose methods of presention
have been followed to a considerable extent.

I. E. Gray.

Durham, N. C.

Page

I. Brief Survey of the Vertebrates i

II, The Skeletal System 8

Axial Skeleton 8

Vertebral Column, Sternum, and Ribs 8

Shark 8

Necturus 9

Alligator lo

Cat lO

Skull 13

Shark. . . . . » 13

Ganoid fish 16

Necturus 17

Alligator 18

Cat 19

Appendicular Skeleton 26

Shark 26

Necturus 27

Cat 28

III. The Muscular System 32

Shark 32

Cat 33

IV. The Digestive and Respiratory Systems 44

Comparative Anatomy of Teeth 44

Organs of the Shark 45

Organs of Necturus 47

Organs of the Cat 48

V. The Circulatory System 53

Shark 53

Cat 56

vii

-y^ ■S'OZ

viii CONTENTS

Page

VI. The Urogenital System 65

Shark 65

Necturus 67

Cat . 68

VII. The Nervous System 71

Sense Organs 7'

Ear 71

Eye 72

Brain and Cranial Nerves 74

Shark 74

Cat 11

The principal animals to be used for stud7 in this course
will be the shark {Squalus acanthias), representing a rather
primitive vertebrate; the mudpuppy {Necturus maculosus), an
amphibian representing a stage in the transition of animals
from aquatic life to land life; and the cat {Felis domestica), a
typical land mammal. From time to time reference will be
made to demonstrations of other animals. A more ideal
arrangement would be to study representatives of all groups of
vertebrates, but in a one-semester course time does not permit.
However, the instructor will supply additional material for those
who wish to do extra work.

In as much as one aim of the course is to present anatomy
from the standpoint of evolution, all organ systems will be
studied in a comparative way. The order of procedure will be
to study the same organ system in each vertebrate before taking
up the next organ system.

Do not lose sight of the function of an organ. That the
anatomy of an organ is correlated with its function is a funda-
mental principle in zoology and should be kept in mind. As
you dissect try to imagine how the various parts would act in a
living animal.

The descriptions are based on average normal animals, but
many variations occur. Do not attempt to make the specimen
fit the description. Conduct dissections in the spirit of inves-
tigation. Try to find things for yourself before asking for
help. Of the three ways of acquiring knowledge — observation,
discussion, and reading — observation is the most important in
anatomy. Those who can see only what is pointed out to them
do not make good scientists.

Form the habit from the beginning of looking up the mean-
ing and pronunciation of words you do not know. Also, look
up the derivation of technical terms; it will help in remembering
them.

X INTRODUCTION

The outline is not divided into two-hour studies; the work
is continuous. Do as much as you have time for in one period;
at the next period review briefly the previous day's accompHsh-
ment and then go on with new work. The materials used in
this course are expensive and care must be exercised in using
them. Report damaged skeletons and demonstration speci-
mens immediately. Dissection materials must be kept in their
proper containers when not in use. Do not expect the instructor
to look after your specimens for you.

The keeping of a notebook for recording observations in the
laboratory is expected. One of the best ways of learning the
details of anatomy is to make outline drawings and label them
completely. It is essential, of course, that one think about
the subject while executing the drawing. Drawings should be
made on good quality paper. It is immaterial as to the size
of the paper selected or whether you prefer a loose leaf system
or a bound book, but be consistent. A list of the drawings
required will be posted in the laboratory. This is the minimum
requirement. Your instructor will be glad to correct any
additional drawings you may wish to make.

A thorough knowledge of the work covered in the laboratory
and of the functions of the various structures is expected.
Examinations, oral, written or both, will be given from time to
time. The quality of your dissections is important. Each
student is expected to make his own dissections. Sponging on
another is a sign of lack of ability or of mental laziness. Have
all dissections checked by the instructor at the time they are
made and before organs are removed.

I. BRIEF SURVEY OF THE VERTEBRATES

Much of the work included in this section will be a review for
those who have had a course in General Zoology. However, it
is important as a preface to the work which comes later. A
knowledge of the general classification of the vertebrates is
expected. The following brief outline of the phylum Chordata
is included for purposes of reference.

PHYLUM CHORDATA

Sub-phyla: I. hemichorda — acorn worms
H. UROCHORDA — sea squirts
HI. CEPHALOCHORDA — amphioxus
IV. VERTEBRATA — fishes, frogs, rcptiles, birds,
mammals

Classes of Vertebrata

1. Cyclostomata — lamprey

2. Elasmobranchii — sharks, rays

3. Pisces — fishes

Crossopterygii — fossil fishes related to ganoids
Dipnoi — lung fishes

Ganoidei—ga.no{d fishes: garpike, sturgeon
Teleostei — common bony fishes: perch, trout

4. Amphibia — salamanders, frogs, toads
Stegocephala — a group of fossil amphibians
Caudata — tailed amphibians: salamanders
Salie7itia — tailless amphibians: frogs, toads

5. Reptilia — lizards, snakes, turtles, alligators
Thermomorpha — a group of fossil reptiles
Squamata — lizards, snakes

Testudinata — turtles

Crocodilia — crocodiles, alligators

6. Aves — birds

7. Mammalia — mammals

2 LABORATORY DIRECTIONS

Monotremata — egg-laying mammals: duckbill

Marsupialia — pouched mammals: opossum, kangaroo

Insectivora — moles, shrews

Rodentia — rabbits, squirrels, rats

Carnivora — cats, bears, foxes

Chiroptera — bats

Primates — monkeys, apes, man

Ungulata — hoofed mammals: sheep, horse

Proboscidea — elephant

Cetacea — whales

Amphioxus — A Primitive Chordate

Although amphioxus is not a vertebrate, it is of great
importance in a study of comparative anatomy for it contains
vertebrate structures in a simplified condition and is by many
biologists thought to be similar to the ancestors of the verte-
brates. A knowledge of the anatomy of amphioxus will aid
greatly in understanding the origin of many vertebrate organs.

Examine specimens of amphioxus that have been stained.
The pointed end is posterior. The head is poorly developed,
possibly degenerate. The expanded membrane at the anterior
end is the oral hood, which forms a cavity, the stomadaeum,
within. The mouth lies at the bottom of the cavity. The oral
hood is fringed with cirri or tentacles. A dorsal fin extends along
the back and continues around the posterior end as the caudal
fin. The two folds on the ventral side are the metapleural folds.
These will be mentioned again in connection with the evolution
of paired appendages. The metapleural folds meet at the atri-
opore, a depression on the ventral side serving as an outlet for
water used in respiration. The a7ius is near the posterior end
on the left side. The V-shaped muscle plates on the sides are
called myomeres. These are separated by sheets of connective
tissue known as myocommata (singular, myocomma). Along
either side may be seen the numerous gonads, segmentally
arranged.

The internal structures can perhaps be studied best by
examining cleared mounted specimens and models and charts.
The mouth opens into the pharynx, a wide cavity extending
posteriorly into the straight intestine. The pharynx is per-

COMPARATIVE ANATOMY 3

forated by numerous gill slits. In respiration water passes
through the gill slits into a large cavity, the atrium, which con-
nects with the atriopore. The skeletal structures supporting
the gills are the bra?ichial bars. The liver is a simple unbranched
diverticulum on the ventral side of the intestine. Dorsal to
the intestine lies the notochord, and just above this, the nerve
cord.

Study cross sections of amphioxus cut through different
regions of the body. The outer covering, the epidermis, is
composed of a single layer of cells. The dermis, beneath the
epidermis, is a thin layer of connective tissue. Identify the
myomeres, the thick masses of muscle; notochord; nerve cord;
dorsal fin ray, a connective tissue support for the dorsal fin;
pharynx with its gill slits and branchial bars in sections of the
anterior end; atrium; intestine and liver, in sections behind the
pharynx; metapleural folds. The ciliated groove on the ventral
side of the pharynx is the endostyle, a structure often considered
as the evolutionary forerunner of the thyroid gland.

Lamprey — Class Cyclostomata

Examine whole and dissected lampreys. There are two
dorsal fins and a caudal fin, but paired appendages are absent.
The skin is soft and slimy and contains numerous glands. True
jaws are missing. The large cavity on the ventral side of the
head, lined with horny teeth, is the buccal funnel. The tongue,
also covered with horny teeth, lies at the bottom of the funnel.
There is but one nostril, on the dorsal side of the head. The eyes
are indistinct. Posterior to the eye, on each side, is a row of
seven gill slits. The anus opens on the ventral side near the
caudal fin. Close behind the anus is the urogenital aperature.

In a median sagittal section locate the tongue and its large
retractor muscles; the pharynx and gill slits; the muscular heart;
the large notochord and above it the small brain and spinal cord;
the thin ribbon-like intestine; the liver, close behind the heart;
and further back, the kidneys and gonads suspended from the
dorsal body wall.

Identify these same structures in cross sections of lampreys
cut at various levels. Under the notochord are three blood
vessels, a median aorta and two lateral cardinal veins.

4 LABORATORY DIRECTIONS

Although considered a vertebrate, the lamprey has very
rudimentary vertebrae which consist merely of minute cartilages
on either side of the notochord.

Shark — Class Elasmobranchii

The elasmobranchs are the cartilaginous fishes. By some
they are classified as belonging to an order of Pisces; others
think they are sufficiently diverse to form a separate class by
themselves.

The mouth is ventral in position. At each corner is an oro-
nasal groove, extending toward, but not reaching the nostril.
The sharply pointed teeth are arranged in several rows. They
are really modified scales.

The nares or nostrils on the ventral side of the head are mere
pits and have no connection with the mouth. The opening of
each nostril is wholly or partly divided by a projecting flap of
skin.

The eyelids are not movable. Behind the eyes are a pair of
spiracles. These are modified gill slits and contain small gills.
Do they open into the mouth cavity ? There are five gill slits
on each side, each with its own gill cover. Do the gill slits
connect with the mouth cavity ?

The cloaca is a small cavity on the ventral side about two-
thirds the way back toward the tail. It is the common outlet
of both digestive and urogenital systems. Within it is the
urogenital -papilla with the urogenital pore at its tip.

The fins consist of two dorsal fins, a caudal fin, paired pectoral
fins, and paired pelvic fins. An anal fin just posterior to the
cloaca is present in most fishes, but is absent in this species of
shark. Male elasmobranchs may be distinguished from females
by the presence in males of a clasper on the inner margin of each
pelvic fin.

A sense organ, the lateral line, extends as a shallow groove
along each side of the body. On the dorsal side of the head are
numerous small pores which are part of the lateral line system.
On the ventral side of the head are similar pores not directly
connected with the lateral line organs. These are the ampullae
of Lorenzini and are probably sensory in function. Do they

COMPARATIVE ANATOMY 5

have a definite arrangement? By pressing on the skin a jelly-
like substance can be forced out.

On the top of the head, between the spiracles, are the open-
ings of two small endolymph ducts, which connect with the ears.
These will be mentioned again in the study of the skull and the
sense organs.

The body is covered with small placoid scales, which can best
be located by running a finger over the skin from the tail toward
the head. Examine a demonstration of them under the micro-
scope. They are of special interest because of their homology
with vertebrate teeth.

Garpike — Class Pisces

The body is covered with ganoid scales, a peculiar type of
scale found also on many fossil fishes. In fact, the ganoid fishes
are related to the group of fossil fishes from which the land
vertebrates are thought to have arisen. What fins are present.''
Is the mouth ventral in position as in the shark .^ A gill-cover,
the operculum, covers the gills. A spiracle is absent.

Bony Fishes — Class Pisces

Bony fishes of several different types should be compared
with the garpike. Observe particularly the absence of the
heavily armored condition, the position of the pelvic fins, and
the type of scale. The ctenoid scales are bony plates derived
from the dermis of the skin and are set in pockets. Examine a
scale under the microscope. Are the small teeth on the proximal
or on the distal edge } The concentric rings are lines of growth.
With training the age of a fish may be told, with a fair degree of
accuracy, by the condition of the lines of growth. Are the pelvic
fins at the posterior end of the body in all fishes } The teleosts
are of more recent evolutionary origin than are the other groups
of fishes.

Necturus— Class Amphibia

Although aquatic this amphibian possesses many char-
acteristics of land vertebrates. The skin is scaleless, soft, and
slimy, and contains many glands. If you are not already

6 LABORATORY DIRECTIONS

familiar with the structure of the amphibian skin examine a
demonstration slide. Distinguish between the outer epidermis
and the deeper dermis. The glands, although derived from the
epidermis, lie wholly within the dermis.

The nostrils communicate with the mouth cavity. Compare
with the shark in this respect. The three pairs of external gills
do not correspond to the internal gills of fishes. Gill slits (how
many.?) are present and open into the pharynx. Water,
however, contrary to the condition in fishes, does not ordinarily
pass through the slits during respiration. Watch the respira-
tory movements of the external gills of a living Necturus if one
is available. The paired appendages, although consisting of
arms and legs with digits on the hands and feet, correspond to
the fins of fishes. Claws are absent. Find the cloaca at the
junction between body and tail.

Lizard — Class Reptilia

Lizards are closely related to snakes and belong to the same
order. Their horny scales are derived from the epidermis and
thus are not homologous to the bony scales of fishes. The
presence of a dry scaly skin is characteristic of reptiles; there are
very few skin glands. Claws are present on the digits. The
nostrils, as in amphibians, open by internal nares into the mouth
cavity. There are three eye-lids: an upper, a lower, and a
nictitating membrane (a thin membrane lying in the inner corner
of the eye when not in use). The depression In the skin behind
the eye may be considered as the beginning of an external ear.
The tympanic membrane stretches across the bottom of the
depression, covering the cavity of the middle ear. Fishes and
Necturus possess Internal ears only. The comparative anatomy
of the ear will be discussed later.

Turtle — Class Reptilia

The shell, which consists of a dorsal carapace and a ventral
plastron, is made up of bony scutes covered with epidermal
scales. Are the scales of the same size and shape as the scutes
beneath ? Teeth are absent; the jaws are covered with a horny
epidermis.

COMPARATIVE ANATOMY

Bird — Class Aves

The chief distinguishing character of birds is the presence
of feathers, which are found nowhere else in the animal kingdom.
Examine also the horny beak, the fore-limb modified for flying,
and the scales on the legs. The scales are homologous to the
scales of reptiles. As in reptiles, skin glands are restricted in
number and distribution.

Cat — Class Mammalia

The cat is so familiar that a detailed study of its external
anatomy is not necessary. However, examine a specimen as
opportunity arises and observe particularly the following:

1. The vihrissae or tactile hairs on upper lips, cheeks, and
above the eyes.

2. The absence of hairs in certain regions of the body.

3. The retractile claws.

4. The soft pads on the soles of the feet.

5. The remnant of the nictitating membrane.

II. THE SKELETAL SYSTEM

As the skeleton largely determines the general form of the
body and as locomotion and means of getting food are condi-
tioned by its structure, a study of the skeletal system should
reveal some interesting phases of comparative anatomy.

For convenience the vertebrate skeleton may be divided as
follows :

Axial skeleton
Skull

Cranium (neurocranium)
Visceral skeleton (splanchnocranium)
Vertebral column
Ribs
Sternum
Appendicular skeleton

Pectoral girdle and fore limbs
Pelvic girdle and hind limbs

^iJlxial Skeleton

VERTEBRAL COLUMN, STERNUM AND RIBS

All members of the phylum Chordata at some time during
life possess a notochord. In most vertebrates this skeletal rod
is replaced in later life by a series of bones, the vertebrae,
closely articulated with one another, and forming the vertebral
column or backbone. The presence of this vertebral column
is one of the most distinguishing features of the vertebrates.
Within the chain of vertebrae lies the neural canal, containing
the spinal cord.

Vertebral Column and Ribs of the Shark

The division of the vertebral column into regions is largely,
though not wholly, dependent upon the presence of limbs.
In forms where limbs are lacking (e.g., snake), and in animals
that do not depend on the limbs for support, as in many aquatic

COMPARATIVE ANATOMY 9

species, the divisions of the vertebral column are not very
marked. In the shark there are only two regions, body and
caudal.

Body Vertebrae. — Use a prepared specimen consisting of
several vertebrae. The vertebrae of the shark are compara-
tively simple in structure, composed largely of cartilage with some
deposition of lime, but have no real bone formation. The more
or less cylindrical basal portion of each vertebra is the centrum
or body. Above this is the neural arch forming a roof over the
neural canal, which serves for the passage of the spinal cord.
The centra are amphicoelus; that is, they are concave on each
end. At the bottom of each concavity is a small canal which
leads through the centrum to the concavity on the other end
of the vertebra. The continuous space thus formed Is occupied
by the remains of the notochord. On the ventral side of each
centrum transverse processes extend laterally. Attached to
each of these, during life, is a slender rib. The ribs are usually
absent on the separate preparations of vertebrae, but may be
seen on the mounted skeletons. The neural arches over the
central canal are composed of alternating neural processes and
interneural plates. These are roughly triangular In shape.
Each neural process has Its base resting on the centrum; the
Interneural plates fill In the spaces between successive neural
processes. The interneural plates of each side unite dorsally;
the neural processes may or may not. A foramen for the exit
of the ventral root of a spinal nerve lies In each neural process;
a foramen for the exit of the dorsal root In each Interneural
plate.

Caudal Vertebrae. — The centra, neural processes and
interneural plates are similar to those of body vertebrae. A
haemal arch, which protects the large blood vessels of the tail,
lies ventral to the centrum. There are no transverse processes
and no ribs.

Vertebral Column, Sternum and Ribs of Necturus

Study the specimens mounted in glass jars. The vertebral
column consists of three regions: cervical (one vertebra), body,
and caudal. There is little differentiation of the vertebrae.
In the caudal region haemal arches are present. A single

lo LABORATORY DIRECTIONS

vertebra supports the pelvic girdle. The vertebrae articulate
with one another by over-lapping processes, a condition charac-
teristic of land vertebrates. The centra are amphicoelus as in
the shark.

A sternum, if it may be called such, is represented by a few
cartilages on the ventral side.

The short triangular ribs have two heads, a dorsal tuberculum
and a ventral capitulum, for articulation with the transverse
processes of the vertebrae.

Vertebral Column, Sternum and Ribs of the Alligator

The vertebral column is differentiated into the five regions
characteristic of mammals. There are nine cervical, bearing
ribs; ten thoracic, bearing long ribs which reach the mid-ventral
line; five lumbar; two sacral; and numerous caudal vertebrae.
The sacral vertebrae bear short stout sacral ribs for the support
of the pelvic girdle. Haemal arches are present on some of the
caudal vertebrae. Toward the end of the tail, however, the
various processes have become so reduced that practically only
the centra remain. Alligator vertebrae are procoelus (centra
are concave anteriorly, convex posteriorly).

A sternum is present between the two halves of the pectoral
girdle. It is drawn out posteriorly into long curved cartilages.

Observe the manner of attachment of the ribs. Are tuber-
culum and capitulum present as in Necturus ? Costal (rib)
cartilages make their first appearance in reptiles.

Vertebral Column of the Cat

In the vertebral column of the cat there are five regions:
(i) Cervical vertebrae, seven in number and forming the
skeleton of the neck.

(2) Thoracic or costal vertebrae, thirteen, bearing ribs.

(3) Lumbar vertebrae, seven, without ribs.

(4) Sacral vertebrae, three fused into one mass and supporting
the pelvic girdle.

(5) Caudal vertebrae, about twenty-three forming the skele-
ton of the tail in ordinary cats, but only four or five in the
short-tailed Manx cats.

Thoracic Vertebrae. — All vertebrae are built on the
same general plan, but there are many kinds of modifications.

COMPARATIVE ANATOMY ii

Examine a fourth or fifth thoracic vertebra. Assuming It to
be a typical vertebra, study it In detail; and then compare It
with the vertebrae of other regions.

Centrum. — This is the solid body of the vertebra, ventral
to the neural canal. Each centrum articulates with centra of
adjacent vertebrae. The smooth ends of the centrum (as on
other bones) are the epiphyses. In kittens the epiphyses are
easily pulled oiT, but in adult cats they are often so completely
fused to the rest of the centrum that they are not readily dis-
tinguishable. The centrum is acoelus, meaning that its ends
are not hollowed out to form cavities. On the caudal surface,
at the dorsal-lateral corner on each side, find a small smooth
area bounded by a ridge of bone. These are costal demifacets.
Similar costal demifacets, not bounded by bony ridges, are to be
found on the cranial end of the centrum. The posterior demi-
facet of one vertebra and the anterior demifacet of the succeed-
ing vertebra form the surface for the articulation of the head
of a rib.

Neural Arch. — This is the bony arch above the centrum.
It is continued dorsally into a long neural spine. The cavity
beneath the arch Is the neural canal., which during life contains
the spinal cord. Between the arch and the centrum on the
posterior end Is a notch, the intervertebral foramen, for the exit
of a spinal nerve. From the neural arch on each side extends
laterally a short transverse process, which bears on the ventral
face of Its distal end a smooth tubercular facet for articulation
with the tubercule of a rib. On the cranial border of the neural
arch, between the transverse processes, are two slight projec-
tions, the prezygapophyses (cranial articular processes), each
with a smooth dorsal face, the cranial articular facet. On the
caudal border, at the base of the neural spine, are similar
postzygapophyses (caudal articular processes), but with the caudal
articular facets facing ventrally. What Is the relation of the
caudal facets to the cranial facets .^

Examine the other thoracic vertebrae In a mounted skeleton,
and see how they difi"er in detail from the one studied.

After studying a thoracic vertebra to serve as a type, com-
pare vertebrae from other regions with it.

Atlas. — This is the first cervical vertebra. The centrum
has been appropriated by the second vertebra; its place Is taken

12 LABORATORY DIRECTIONS

by a narrow ventral arch. The transverse processes are large
and wing-like, and are pierced near the base by foramina trans-
versaria for passage of the vertebral artery. The cranial artic-
ular processes for receiving the occipital condyles of the skull
are large. Dorsal to them lie the atlantal foramina for the
continuation of the vertebral artery. The caudal articular
facets articulate with the second vertebra.

Axis. — This is the second cervical vertebra. The projection
at the anterior end of its centrum is the odontoid process (odon-
toid, toothlike), which is really the centrum of the atlas.
Observe particularly: the small transverse processes, the for-
amina transversaria, the large cranial articular facets, the neural
spine, and the caudal articular facets.

Cervical Vertebrae. — Notice the gradual transition in
them. On some the transverse processes arise by two roots, one
from the centrum and one from the arch. Between the two lies
t\\& foramen transversarium for the vertebral artery. The trans-
verse process divides distally into a dorsal projection represent-
ing the transverse process proper, and a ventral costal process
representing a rib. Observe differences between different
cervical vertebrae in this respect.

Lumbar Vertebrae. — These are larger than the thoracic,
the transverse processes are more complicated in structure, the
spinous processes are shorter, and there are no ribs.

Sacral Vertebrae. — These, usually three in number In the
cat, are united In the adult Into a single bone, the sacrum.
What evidence is there of the multiple character of the sacrum ?

Caudal Vertebrae. — These gradually decrease in size,
becoming longer and more slender, and finally are reduced to
mere centra. In life several of the caudal vertebrae, after the
third, bear haemal processes tipped by chevro7i hones to form a
haemal canal. The chevron bones are usually lost In preparing
the skeleton.

Examine a cervical vertebra of a horse. Is the centrum pro-
coelus or opisthocoelus (convex anteriorly, concave posteriorly) .''

Sternum of the Cat

Locate the sternum or breast bone on a mounted skeleton.
It is made up of eight pieces, called sternebrae, distributed over
three regions as follows:

COMPARATIVE ANATOMY 13

1. The maiiubrium or presternum, the arrow-shaped anterior

piece.

2. The mesosternum, made up of six sternebrae.

3. The xiphisternum (xiphos, sword) or metasternum, the
long posterior piece. It ends in a fan-shaped xiphoid cartilage,
which becomes much shriveled on dry skeletons.

Ribs of the Cat
A typical rib, the fifth or sixth, should be studied. It con-
sists of the following parts:

1. A head or capitulum, the end of which articulates with the
centra of two thoracic vertebrae at the costal demifacets.

2. A tubercule or tuberculum, a slight projection close to the
head, for articulation with the transverse process.

3. The 7ieck, the short portion between head and tubercle.

4. The shaft, the long portion between the tubercle and the
costal cartilage. The point of sudden turn in this portion is the
angle.

5. The costal cartilage, the cartilaginous connection between
the ventral end of the shaft and the sternum.

Various mammals have from nine to twenty pairs of ribs.
Distinguish differences between the various pairs in the cat.
Those ribs that connect by their own cartilages to the sternum
are known as true ribs, the rest as false ribs. There are normally
nine pairs of true and four pairs of false ribs in the cat. The
first three pairs of false ribs have cartilages uniting with the
cartilages of the last true ribs. The last pair of ribs have no
such cartilages and are known a.s floating ribs.

SKULL

Skulls occur only in vertebrate animals. In the simpler
species it is convenient to consider the skull as consisting of
two parts: (i) cranium (neurocranium) which surrounds and
protects the brain, and (2) visceral skeleton {splanchnocranium)
which forms the jaws and gill arches where these are present.
In higher vertebrates parts of the visceral skeleton are trans-
formed to serve other purposes.

Cranium of the Shark

On a mounted skeleton identify the cranium and visceral
skeleton. Observe that the visceral skeleton is not a part of

14 LABORATORY DIRECTIONS

the cranium and is held to it merely by ligaments. Secure a
separate cranium for detailed study. Shark crania are fragile
and expensive. They must be handled with great care and must
be kept in their containers when not in use. Do not allow them
to become dry.

Observe that the whole cranium forms one continuous
cartilaginous mass. This is known as a chondrocranium.
Although the cartilage is sometimes strengthened by impregna-
tion of lime, there is no real bone formation in any part of the
shark skeleton.

Find the following outstanding features:

1. Orbits (eye sockets). — The lateral projections in front of
the orbits are the anterior orbital processes; those behind the
orbits are the posterior orbital processes. Between these two
processes and forming a shelf above each orbit is the supra-
orbital crest.

2. Rostrum. — This is the scoop-like projection at the
anterior end of the cranium, opening dorsally. The open cavity
of the rostrum is the anterior fontanelle, which in life is filled
with a gelatinous substance. Normally the rostrum is braced
by rostal bars, extending from its tip to the anterior orbital
processes, but in prepared crania these bars are usually missing.
On the ventral side of the rostrum is a keel.

3. Foramen Magnum, a large opening at the posterior end
of the cranium for the exit of the spinal cord.

4. Occipital Condyles, small projections, one on either
side of the foramen magnum, for articulation with the first
vertebra.

5. Endolymphatic Fossa. — This fossa is the large depres-
sion on the dorsal surface anterior to the foramen magnum.
Within it are two pairs of openings: the anterior smaller ones
are the openings of the endolymphatic ducts; the posterior ones,
the openings of the perilymphatic ducts. These ducts connect
with the internal ears, which lie embedded in the cartilage of the
otic region.

6. Otic Region (ear region). — Extending anteriorly on each
side of the endolymphatic fossa are ridges which indicate the
location of the anterior semicircular ca^ials. Similar ridges
extending posteriorly mark the position of the posterior semi-

COMPARATIVE ANATOMY 15

circular canals. The position of the horizontal semicircular
canal is indicated by a ridge on the lateral side of the cranium
between the posterior orbital process and the post-otic process.
The post-otic process forms the posterior-lateral corner of the
cranium. What is the function of the semicircular canals .f*
They will be dissected when the sense organs are studied.

7. Fenestrae. — These are the large openings on the ventral
surface, one on either side of the keel of the rostrum. In life
they are closed by membranes.

8. Olfactory Capsules. — These large openings on the
anterior end of the cranium, lateral to the fenestrae, are for the
organs of smell. During life they are closed, but the thin
anterior walls are usually destroyed in preparation of the
cranium.

9. Principal Foramina. — The large hole through the
median wall of each orbit is the optic foramen, for passage of the
optic nerve. In the posterior ventral corner of the orbit is a
group of five foramina, the largest of which is for the exit of the
fifth and seventh cranial nerves; the others are for the third and
sixth nerves and for blood vessels. The row of foramina
extending along either side of the dorsal surface is for passage of
branches of the fifth and seventh nerves. The small foramen
posterior to the anterior fontanelle markc the location of the
epiphysis, a small dorsal outgrowth of the brain. On the pos-
terior end of the cranium are four large foramina; the lateral
ones are for the exit of the ninth cranial nerves, the medial ones
for the passage of the tenth cranial nerves. The foramen in
the mid-ventral line, behind the orbits, is for the entrance of the
internal cartoid arteries.

Examine a cranium that has been bisected longitudinally.
Identify the structures previously mentioned, and in addition
locate the foramen for the passage of the eighth nerve to the
ear, the cavity for the brain, and the depression on the floor of
the cranium in which the pituitary gland rests.

Visceral Skeleton of the Shark

For this study use the mounted skeletons. A knowledge of
the visceral arches of the shark Is essential for a clear under-
standing of the modifications that occur in higher vertebrates.

i6 LABORATORY DIRECTIONS

In this species of shark there are seven cartilaginous arches
nearly surrounding the anterior part of the alimentary canal
and supporting its walls. The first two visceral arches are in
close connection with the cranium. The details of structure of
the visceral skeleton are difficult to study on mounted skeletons,
but with the aid of a chart and the following description make
out as much of the anatomy as possible.

1. Mandibular Arch. — Both upper and lower jaws are
derived from the first visceral arch. The upper jaw is a strong
bar of cartilage formed by the union of paired ptery go-quadrate
cartilages meeting in the median line. The lower jaw is made
up wholly of MeckeVs cartilage.

2. Hyoid Arch. — This, the second visceral arch, lies imme-
diately posterior to the lower jaw. Its ventral half forms the
hyoid apparatus supporting the floor of the mouth. The dorsal
half connects the hyoid apparatus and both jaws to the cranium.
The hyoid consists of five cartilages: a single mid- ventral
basihyal; two ceratohyals which are attached to the ends of the
basihyal and extend parallel to the lower jaw; and two hyo-
mandibular cartilages attached to the corners of the cranium
above and supporting both jaws. The ceratohyals and hyo-
mandibulars bear gill rays^ slender cartilaginous rods which
support the first gill cover.

3. Gill Arches. — There are five arches which support the
soft structures of the gills. Theoretically each gill arch should
contain nine separate cartilages, but this ideal arrangement is
never retained by all of the arches.

Read up on the evolution of the visceral skeleton.

Skull of a Ganoid Fish

Examine the head of a gar-pike or other ganoid fish. The
ganoids show considerable advance over the elasmobranchs in
the development of the skull. Large dermal scales cover the
chondrocranium and form the roof and sides of the cranium.
As some of these large scales correspond to the superficial bones
of higher vertebrates, they have received the same names.
The pterygo-quadrate cartilage of the upper jaw and Meckel's
cartilage of the lower jaw have also become encased with
dermal scales, which form the superficial bones of the jaws.

COMPARATIVE ANATOMY 17

The upper jaw, unlike that of the shark, is Inseparably fused
with the cranium and must be considered as part of It. Because
of their manner of origin from the dermis, the superficial bones
of the cranium and jaws are known as dermal, membrane, or
investing holies. Remember that the dermal bones are on the
outside, embedded in the skin, and that a typical cartilaginous
skull is present inside the covering of scales.

A study of the Individual bones will not be attempted.
Notice, however, that they are very numerous. Development
of membrane bones was greatest in the earliest amphibians and
reptiles, but in living land vertebrates the number has been
greatly reduced through loss and fusion.

Skull of an Amphibian

The descriptions are written for Necturus, but with slight
modifications may be applied to Cryptobranchus, an aquatic
amphibian somewhat similar to Necturus but much larger.
Examine both a dry skull and a specially prepared chondro-
cranium of Necturus. Necturus, like the fishes, has in reality
two skulls, a partially ossified chondrocranium on which is
superimposed a roof of membrane bone. Identify the foramen
mag?ni7n and the two occipital condyles. The fine lines at the
points of articulation between bones are known as sutures. The
upper jaw consists of three bones bearing teeth. Beginning
anteriorly these are the premaxillary, the vomer, and the palato-
pterygoid. The large, unpaired bone forming most of the floor
of the cranium is the parasphenoid. On the dorsal side, forming
the roof of the cranium, are the paired fro7itals, articulating
with the premaxillarles anteriorly, and posterior to these, the
parietals. On either side of the foramen magnum Is an exoccipi-
tal bone, each with Its occipital condyle for articulation with
the first vertebra. The otic capsules are partially ossified; the
opisthotic bones lie lateral to the exocclpltals and help form the
posterior corners of the skull, while the prootics are the small
bones in the anterior part of the otic region. Extending
obliquely forward from the opisthotic is the slender squainosal
bone. Beneath this and articulating with the lower jaw is the
quadrate. This bone represents an ossified remnant of the
pterygo-quadrate cartilage of the shark. The cartilage bones

i8 LABORATORY DIRECTIONS

of the Necturus skull, namely, exoccipital, prootic, opisthotic,
and quadrate, are best seen In the chondrocranium preparations.

The lower jaw consists of membrane bones surrounding
Meckel's cartilage. The dentary bone, bearing teeth, constitutes
the outer surface of each half of the jaw. The inner surface is
composed of two bones, the splenial and the angular, the latter
forming the extreme posterior end of jaw below the dentary. In
Cryptobranchus ossification of a portion of Meckel's cartilage
to form the articulating surfaces of the jaws has begun. This
bone is the articular; it articulates with the quadrate bone of
the upper jaw.

Examine the visceral arches on a mounted skeleton of
Necturus. The hyoid arch is the bar of cartilage parallel and
posterior to the lower jaw. The three gill arches are remnants
of the third, fourth and fifth visceral arches. As compared to
the shark the gill arches are not only reduced in number but
also in the cartilages composing them.

Skull of the Alligator

The skull of the alligator is almost completely ossified, the
chondrocanium having been replaced by bone. Although the
alligator skull will not be studied in detail, the following features
should be noted: (i) There are more bones present than in
mammal skulls. (2) There is but one occipital condyle. (3)
There are no teeth on the roof of the mouth as in Necturus,
Cryptobranchus, and many fishes; teeth are borne on the
maxillary, pre-maxillary, and dentary bones only. (4) The
quadrate bone, an ossified remnant of the pterygo-quadrate
cartilage, forms the articulation with the lower jaw. (5) The
lower jaw consists of two halves or rami united in front by a
symphysis. The bones of each half are: the dentary, bearing
teeth and forming the outer two-thirds of each jaw; the splenial,
forming a large part of the inner side; the angular, below the
large mandibular foramen; the sur-angular above the foramen;
the coronoid, a small bone on the Inner surface forming part of
the anterior boundary of the mandibular foramen; and the
articular, which contains a concavity for articulation with the
quadrate of the upper jaw. The articular Is an ossified portion
of Meckel's cartilage. A remnant of Meckel's cartilage still

COMPARATIVE ANATOMY 19

remains as a core within each ramus. What has become of the
gill arches in the reptiles ?

Skull of the Cat

In the cat, as in all mammals, bone has replaced cartilage
almost entirely, at least in adults. There is also a tendency
for bones to unite with one another so that the mammalian
skull contains fewer bones than a similar skull of its amphibian
or reptilian ancestors. The following features serve as
landmarks:

Orbits (eye sockets). — Are they completely surrounded by
bone } Compare with the skulls of other mammals in this
respect.

Temporal fossa (fossa, ditch), a space just behind the orbit
into which a projection from the lower jaw extends.

Zygomatic arch (zyge, yoke), the bar of bone ventral to the
orbit and temporal fossa.

Foramen magnum, the large hole at the posterior end of the
skull through which the spinal cord passes to the brain.

Occipital condyles, bony projections on either side of the
foramen magnum for the articulation of the skull with the
first vertebra.

Nares, the nostrils.

Lambdoidal ridge (from Lambda, the Greek letter L), an
arched ridge of bone extending transversely dorsal to the fora-
men magnum.

Sagittal crest (sagitta, arrow), a ridge extending anteriorly
from the center of the lambdoidal ridge. Both the lambdoidal
ridge and the sagittal crest are Inconspicuous or absent in kitten
skulls.

Tympanic bullae (bulla, bubble), paired swellings on the
ventral side of the skull, anterior to the occipital condyles.
Each contains the bones and sensory part of the ear.

External auditory meatus (meatus, passage), the opening on
the lateral side of each tympanic bulla.

Choanae (choana, funnel), posterior openings of the nasal
passages on the ventral side of the skull.

The bones of the skull will be described In order, as far as
possible, beginning at the posterior end. Each bone should be

20 LABORATORY DIRECTIONS

located on the skull and its boundaries traced. The fine lines
of separation between bones are known as sutures. Be prepared
to demonstrate the articulations of each bone with the others.
Examine also specimens of the separate skull bones. The
separate skull bones are not to be removed from the laboratory
at any time. Do not mark on the hones. Use a needle or metal
probe in tracing sutures; never a pencil.

1. Occipital. — This bone surrounds the foramen magnum
and forms practically the whole of the posterior face of the
skull. It really represents four bones (i basi-, 2 ex-, and i supra-
occipital) which remain separate in many lower vertebrates,
and are separate also in young mammals. The Inner surface
shows depressions corresponding to convolutions of the cerebel-
lum of the brain. On the ventral surface of the skull the
occipital extends anteriorly between the auditory bullae and
meets the sphenoid bone. The condyles of the occipital, for
articulation of the skull with the first vertebra, have already
been mentioned.

2. Parietals. — The paired parietals, meeting in the median
line, form the posterior half of the roof of the cranium. They
extend far down on the sides. The shelf or tentorium on the
inner surface extends down between cerebrum and cerebellum
of the brain and articulates with the sphenoid, a fact not easily
observed in a whole skull.

3. Interparietal. — This small, median, triangular bone
extends anteriorly from the dorsal edge of the occipital as a
wedge between the posterior parts of the parietals. In old cats
it is usually fused completely with the parietals or occipital,
and is then not a distinct and separate bone.

4. Frontals. — Paired frontals, meeting in the median line
and lying immediately anterior to the parietals, form the
remaining portion of the roof of the skull and a large part of
the medial wall of the orbit. From the dorsal-lateral part of
the frontal posterior to the orbit, extends the zygomatic {post-
orbital) process. It extends ventrally toward a similar process
of the malar bone, and may articulate with it. Looking at a
separate frontal bone from its anterior end discloses an opening
into a cavity within the bone. This is t\\(t frontal sinus, which in
life is connected with the nasal cavity.

COMPARATIVE ANATOMY 21

5. Nasals. — The two nasals meet in the median line and
extend from the frontals to the nares.

6. Premaxillaries.— These are paired bones forming with
the nasals the borders of the nasal opening. They form the
anterior portion of the upper jaw and bear on each side three
incisor teeth.

7. Maxillaries. — The paired maxillary bones form the
remainder of the upper jaw on each side, and the anterior and
lateral portions of the roof of the mouth. Each bears one canine
tooth, three premolars, and one molar. The maxillaries extend
dorsally to meet the nasals and frontals and the median surface
of each maxillary forms a large part of the lateral wall of the
nasal cavity. Dorsal to the premolars the maxillary has a
projection, the malar process or zygomatic process for articulation
with the malar bone. The large hole in the maxillary is the
infra-orbital foramen for passage of a nerve and a blood vessel.

8. Malars. — The malars are paired. Each extends pos-
teriorly from the malar process of the maxillary bone and forms
the major portion of the zygomatic arch. Each overlaps and
articulates with the zygomatic process of the temporal bone. A
post-orbital process extends toward and frequently unites with
the similar process of the frontal.

9. Lachrymals. — These thin bony plates in the anterior
wall of each orbit frequently fall out in dry skulls. On the
anterior border of each is the opening of the lachrymal canal
(tear duct), which connects with the nasal cavity.

10. Temporals. — The temporals are also paired, lying ven-
tral to the lateral borders of the parietals, and forming a large
part of the lateral wall of the cranium posterior to the orbit.
The temporal bone of mammals represents a fusion of a number
of bones which are separate in lower forms. The resulting
complicated structure is conveniently divided into four regions,
corresponding in a general way to the parts from which the
temporal has developed: the squamous, the mastoid, the petrous,
and the tympanic.

The squamous region is the thin plate adjoining the parietal.
Anteriorly it gives off a long zygomatic process, which joins the
malar and forms the posterior portion of the zygomatic arch.
On the ventral surface of this process, near its base, is the

22 LABORATORY DIRECTIONS

mandibular fossa, a notch for the articulation of the mandible.

The tympanic region consists of the tympanic bulla ventrally
(already mentioned), and the ring of bone surrounding the
external auditory meatus.

The mastoid region includes the heavy, thickened portion
posterior to the external auditory meatus. It lies between the
squamous region and the bulla.

The petrous region is seen best on the inner surface of the
temporal bone. From this side it is the central mass of very
dense bone (hence the name, meaning stony) between the
squamous region and the tympanic bulla. In it are embedded
the structures of the inner ear.

Turn now to the ventral surface of the skull and again locate
the premaxillaries and the maxillaries. Then look for the
following:

11. Palatines. — The two palatines form that portion of
the roof of the mouth behind the maxillaries. Each palatine
has a vertical plate, forming part of the wall between orbit and
nasal cavity and extending posteriorly to meet processes of the
sphenoid,

12. Vomer. — This is a single bone seen by looking into the
anterior nares. It is a slender, elongated bone in the floor of
the nasal cavity, and has a groove on its dorsal surface. At its
posterior end it articulates with the body of the presphenoid.
Only this posterior end is visible on the ventral surface.

13. Presphenoid. — This bone is made up of two portions,
which are distinct in young kittens and in lower vertebrates.
These parts are the base (presphenoid of lower forms) and the
two wings (orbitosphenoids). The base lies in the roof of the
mouth just posterior to the vomers and is partly covered by
the vomer, the palatine, and the sphenoid bones, so that only a
narrow central strip is visible. The two wings come from nearly
the whole dorsal-lateral angles of the basal portion. They
appear on the surface as part of the median walls of the orbits,
ventral to the frontal bones and posterior to the palatines.
They carry the optic foramina for passage of the optic nerves.

14. Sphenoid. — In man this is joined with the presphenoid
to form a single bone; in the cat, however, it is separate. It is
formed by the union of five bones which are found in lower

COMPARATIVE ANATOMY 23

vertebrates: the basisphenoid and the paired pterygoids and
alisphenoids.

In the adult cat the basisphenoid is on the ventral surface
of the skull between presphenoid and occipital. It is shield
shaped with the apex pointing forward. Its dorsal surface has a
projection, the dorsum sellae. Immediately interior to this
projection is a deep excavation, the sella turcia (meaning Turkish
saddle), in which is lodged the pituitary gland.

The wings are thin plates attached by their median borders
to the lateral surfaces of the basisphenoid. Each wing is com-
posed of a pterygoid process, which extends forward on the roof
of the mouth between the presphenoid and the palatine; and
an alisphenoid process, which extends forward and dorsally,
meeting presphenoid, frontal, parietal, and temporal.

15. Ethmoid. — This is an unpaired much-coiled bone,
partially visible on looking into the nares. Examination of a
separate bone will show that it consists of several parts.

a. The cribriform plate (cribrum, sieve), is so called because
of its many perforations for the passage of the olfactory nerves.
It forms the dividing wall between brain cavity and nasal
cavity.

b. The lamina perpendicularis (lamina, a thin sheet),
extends anteriorly from the center of the cribriform plate and
forms part of the septum between the nostrils.

c. The ethmoturbinates, a complicated mass of thin folded
sheets of bone, nearly fill the nasal cavity. The turbinates are
attached to the vomer, the nasal, and the maxillary bones.

16. Mandible {inferior maxillary). — This forms the lower
jaw. It is made up of two halves immovably united at the
median symphysis. Each side bears three incisor teeth, one
canine, two premolars, and one molar. Between canine and
premolars is a diastema (an open space between teeth).

On the posterior part of the median surface of each ramus
of the mandible Is the inferior dental foramen, carrying nerves
and arteries which emerge again through the two mental foramina
on the external surface just below the diastema.

Posteriorly the mandible is marked by six prominent features :
a. The coronoid fossa, a large fossa on the lateral surface
for the Insertion of the masseter muscle.

24 LABORATORY DIRECTIONS

b. The angular process, a small projection at the posterior-
ventral angle.

c. The inferior notch, just above the angular process.

d. The condyle for articulation with the mandibular fossa
of the temporal bone.

e. The superior notch above the condyle.

f. The large coronoid process at the dorsal angle, which
projects into the temporal fossa of the cranium.

17. Hyoid Bones. — The hyoid apparatus consists of a chain
of slender bones on each side, serving to support the tongue and
larynx. These bones are usually absent on ordinary skeletons;
examine them on a special demonstration specimen.

18. Principle Foramina. — Locate the following foramina,
some of which have already been mentioned:

a. The infraorbital foramen passes through the maxillary
bone just ventral to the anterior end of the malar. It carries
part of the fifth cranial nerve and a blood vessel.

b. The incisor foramina are the two openings on the ventral
surface immediately posterior to the incisor teeth for the
passage of the nasal artery and part of the fifth nerve. What
bones surround them.^

c. The sphenopalatine foramen is the larger of the two open-
ings in the palatine bone in the wall of the orbit. It carries the
sphenopalatine artery and part of the fifth cranial nerve.

d. The posterior palatine foramen is the smaller opening in
the palatine bone, slightly anterior and ventral to the spheno-
palatine foramen, through which the palatine artery and a
branch of the fifth nerve pass.

e. The olfactory foramina are the many perforations of the
cribriform plate of the ethmoid bone.

f. The optic foramen is the most anterior of the four foramina
in the posterior ventral corner of the orbit. In what bone does
it lie ? It carries the optic nerve and meningeal artery.

g. The orbital fissure, the second of the four foramina in the
posterior ventral corner of the orbit, is bounded by the wings
of the presphenoid and sphenoid bones. It carries the third,
fourth, and sixth cranial nerves and part of the fifth.

h. The foramen rotundum, the third of the four foramina in
the corner of the orbit, lies wholly within the sphenoid bone.
It carries part of the fifth cranial nerve.

COMPARATIVE ANATOMY 25

i. The foramen ovale lies immediately posterior to the fora-
men rotundum. It also carries a branch of the fifth nerve.

j. The stylomastoid foramen is a small opening posterior to
the external auditory meatus for the passage of the seventh
cranial nerve. The shallow groove ventral to this foramen is
for the attachment of the hyoid bone.

k. Eustachian tube. At the anterior end of the tympanic
bulla is a foramen which connects with the cavity of the middle
ear within the bulla. It serves for the passage of the Eustachian
tube from the middle ear to the pharynx.

1. The jugular foramen is an opening near the posterior end
of the bulla and serves for the passage of the ninth, tenth, and
eleventh cranial nerves and the jugular vein.

m. The hypoglossal canal may be seen by looking into the
foramen magnum on either side. The canal makes its exit
near the jugular foramen. It carries the twelfth cranial nerve.

n. Internal auditory meatus. Examination of the internal
surface of a separate temporal bone reveals two prominent
openings. The larger one, anterior to the petrous region, is
for the passage of the Eustachian tube, already mentioned.
The other, consisting of a deep pit in the center of the petrous
region, is the internal auditory meatus. The seventh and eighth
cranial nerves enter it. The seventh emerges through the
stylomastoid foramen; the eighth (auditory) passes to the inner
ear by means of several foramina in the bottom of the pit.

Examine a skull bisected longitudinally and one with the
top of the cranium removed. Identify the various parts.
Skulls of different mammals are available in the laboratory for
those who wish to compare them with the cat.

Larynx. — Study the model of the human larynx. The
U-shaped structure at the top is the hyoid bone which serves
as a support for the tongue. The largest cartilage of the larynx
is the thyroid cartilage, which gives the shape to the voice box.
From the notch in the anterior of the thyroid cartilage extends
the epiglottis, which during the act of swallowing, covers over
the opening into the larynx and prevents food entering. The
two small cartilages to which one end of each vocal cord is
attached are the arytenoids. The arytenoid cartilages rest on
the cricoid cartilage, which is shaped somewhat like a signet ring.
Changes in position of the cricoid and arytenoids with respect

26 LABORATORY DIRECTIONS

to each other and to the thyroid, produce the changes in the
tension on the vocal cords. The vocal cords are elastic tissue
covered over with a mucous membrane. The slit-like opening
between the vocal cords is the glottis. The trachea, extending
below the larynx, is supported by cartilaginous rings, which are
open on the dorsal side. The cartilages of the larynx are
derived from modified visceral arches.

appendicular Skeleton

Typically, vertebrates have two pairs of limbs, pectoral and
pelvic, each with its girdle of bones connecting it with the body.
As a matter of adaptation these limbs undergo great modifica-
tion in various forms, and one, or even both pairs, may be
entirely absent. From the amphibians to the mammals the
limbs of all forms are easily recognized as homologous. But
the limbs (fins) of fishes are of an entirely different structure,
and although probably homologous, are not easily compared
with those of higher vertebrates.

Above the fishes the parts of the pectoral girdle and forelimb
may be homologized with the parts of the pelvic girdle and hind
limb. Each part of one is represented by a similar part in the
other.

Girdles and Limbs of the Shark

The pectoral girdle Is the U-shaped arch of cartilage which
supports the pectoral fins. It is made up of right and left
halves united by a ligament in the mid-ventral line. The
articular {glenoid) surface for articulation of the fin is well up
on the side. The part of the girdle dorsal to this is known as the
scapular region, the most dorsal part of which forms a distinct
cartilage, the suprascapular. The part of the girdle ventral
and medial to the glenoid surface is the coracoid region.

The pectoral fin is attached to the pectoral girdle at the
glenoid surface. It consists of: (a) three hasalia, large cartilages
articulating with the glenoid surface; (b) many radialia or fin
rays extending beyond the basalia; and (c) actinotrichia, the
very numerous horny threads extending beyond the radialia and
supporting the greater portion of the fin.

COMPARATIVE ANATOMY 27

Attempts have been made to trace homologies between the
basalia and radialia of sharks and the bones of the arm and the
hand of higher vertebrates.

The pelvic girdle is much simpler than the pectoral and con-
sists merely of a bar of cartilage.

The pelvic fin is also simpler than the pectoral. It consists
of: (a) two basalia, (b) many radialia (in males the posterior one
is much elongated to form the support for the clasper), and (c)
actinotrichia, as In the pectoral fin.

Girdles and Limbs of Necturus

Study a mounted skeleton. The pectoral girdle Is somewhat
primitive and consists mainly of cartilage. Extending dorsally
from the glenoid surface (the point of articulation of the arm)
Is a short scapula, the only part of the girdle composed of bone.
Attached to the distal end of the scapula Is a cartilaginous
suprascapula. The large cartilage extending medially from the
glenoid surface is the coracoid region; the one extending anteriorly
is the procoracoid.

The forelimb is composed of humerus, articulating with the
girdle; ulna and radius, the bones of the forearm; carpus (wrist)
of several cartilages; metacarpus of four bones; and the phalanges
of the digits. The thumb is missing. Compare the specimen
with a chart to understand the relationship of the cartilages and
bones of wrist and hand. Also, compare Necturus with the
chart of the typical vertebrate limb.

The pelvic girdle represents a transition stage between the
cartilaginous condition of the elasmobranchs and the bony girdle
of reptiles. It consists of a pelvic plate. The socket for articu-
lation of the leg is known as the acetabulum. Medial and ante-
rior to the acetabulum lies the pubic region; posterior to this is
the ischial regio7i; attaching the girdle to the vertebral column
is the ilium. Ilium and Ischium are partly bony.

The hind limb Is like the forelimb. It Is composed oi femur,
articulating with the girdle at the acetabulum; tibia and fibula,
bones of the shank; tarsus of several cartilages; metatarsus of
four bones; and phalanges of the digits. The first toe Is
missing.

28 LABORATORY DIRECTIONS

Pectoral and Pelvic Girdles of the Alligator

The pectoral girdle consists of a dorsal scapula; a ventral
coracoid; and a slender interclavicle bone between the two
coracoids. There are no clavicles.

The pelvic girdle is wholly ossified and contains the typical
three regions. The ilium extends dorsally and forms an articu-
lation with the vertebral column; the pubis extends ventrally
and anterior; the ischium ventrally and posterior. The ischia
of the two sides meet and form the ischial symphysis.

Pectoral Girdle and Fore Limb of the Cat

1. Scapula. — This is the shoulder blade. At the ventral
end is the glenoid fossa for reception of the humerus bone of the
arm. A short spine, the coracoid process, projects medially from
the anterior end of this fossa. This is a remnant of the coracoid
bone found in certain lower vertebrates. The prominent keel
on the outer surface of the scapula is the spine. This has two
projections; the distal one is the acromion, the other the
metacromion.

2. Clavicle. — In some mammals, as in man, this is a well
developed bar of bone, extending from the scapula to the
sternum. In the cat, however, it is greatly reduced, being repre-
sented on each side by a short, slender rod of bone entirely
embedded in the muscles, and not articulated with any other
bones. It is usually absent on mounted skeletons.

3. Humerus. — This is the single bone of the upper arm. At
its proximal end is the head, a rounded swelling with a smooth
face for articulation with the glenoid fossa. The prominent
ridge extending from the lateral border of the head is the greater
tuberosity; the less conspicuous ridge on the medial side of the
head is the lesser tuberosity. The depression between the two
is the bicipital groove. From the ventral end of the greater
tuberosity the pectoral ridge extends down the anterior face of the
shaft. Not quite halfway down, it is joined on the lateral side
by the deltoid ridge, which starts at the posterior end of the
greater tuberosity. The distal end of the humerus presents an
eminence on either side, the outer and inner condyles. Above the
inner condyle the shaft is pierced by the supracondyloid foramen,

COMPARATIVE ANATOMY 29

for the passage of a blood vessel and a nerve. Between the two
condyles is the smooth surface for articulation of the ulna and
radius. Above these on the posterior face is a depression, the
olecranon fossa, for reception of the olecranon process of the
ulna.

4. Ulna. — This is the longer of the two bones of the fore-
arm. The notch into which the humerus fits is the greater
sigmoid cavity. Just distal to this is a smaller transverse con-
cavity, the lesser sigmoid cavity, for articulation with the radius.
The portion of the ulna proximal to the greater sigmoid cavity
is the olecranon -process. The projection at the distal end of the
ulna is the styloid process.

5. Radius. — This is the shorter of the two bones of the fore-
arm. Proximally, by means of its head, it articulates with the
humerus and the ulna. A small projection toward the ulna,
slightly distal to the head, is the bicipital tubercle. As in the
ulna, the process at the distal end of the radius is the styloid
process.

6. Carpus. — The seven bones in the wrist make up the
carpus. These are arranged in two rows. The three bones in
the proximal row, beginning on the medial side, are: scapholunar,
cuneiform, and pisiform. In kittens the scapholunar is repre-
sented by three bones; in man by two, the scaphoid and lunar.
The four bones of the distal row, beginning medially, are:
trapezium, trapezoid, magnum, and unciform.

7. Metacarpus. — This consists of the five long bones of the
hand. For convenience they are numbered from i to 5, begin-
ning on the medial side.

8. Phalanges. — The cat has five digits on the front foot,
each of which, with the exception of the thumb, contains three
phalanges; the thumb has two phalanges. The distal phalanx
of each digit has attached to it a retractile claw.

Pelvic Girdle and Hind Limb of the Cat

I. Innominate. — The pelvic girdle consists of two innomi-
nate bones, one on each side, the two joining in the mid-ventral
line to form the pubic sy7nphysis. Each innominate has near the
center of its lateral surface a depression, the acetabulum, for the
reception of the head of the femur. The acetabulum marks

30 LABORATORY DIRECTIONS

the point of union of the three component parts of the innominate.
(In kittens the sutures between them still exist, as well as a
small acetabular bone in the floor of the depression.) The parts
of the innominate are: (a) the ilium, extending anteriorly from
the acetabulum and connecting with the sacrum; (b) the ischium,
posterior to the acetabulum; and (3) the pubis, the ventral bar
of bone adjoining the pubic symphysis.

The large opening between ischium and pubic is the obturator
foramen. It will be referred to frequently when the muscles of
the hind leg are studied.

2. Femur. — This is the single bone of the thigh. It has a
large globular head which fits into the acetabulum. The large
roughened projection lateral to the head is the great trochanter
which serves for the attachment of many muscles. The lesser
trochanter is a small tubercle on the posterior side, a short dis-
tance below the head. The large depression on the posterior
side between the head and great trochanter is the trochanteric
fossa. The prominences on the distal end of the femur are the
lateral and medial condyles, and the groove between the condyles
is the intercondyloid fossa.

3. Patella. — This is the knee cap. It is a sesamoid bone;
that is, it is formed by ossification of part of a tendon. There
are other sesamoid bones in the skeleton, but this is the largest.

4. Tibia. — This is the larger of the two bones of the shank
and is the longest bone of the body. It articulates with the two
condyles of the femur by means of the lateral and medial tuber-
osities. Anteriorly the tibia has a prominent crest which dis-
appears distally. The projection on the distal end of the tibia
is the medial malleolus.

5. Fibula. — ^The fibula is the slender bone of the shank.
It articulates with the tibia at both ends and with the ankle
distally. The expanded proximal end is the head; the promi-
nence at the distal end is the lateral malleolus.

6. Tarsus. — The tarsus is composed of the seven bones in the
ankle. The calcaneus (sometimes calcaneum) is the largest of
the ankle bones and forms the heel. The astragalus forms the
chief articulation between the foot and the tibia. The navicular
or scaphoid is distal to the astragalus and is usually described as
boat-shaped. The four bones in the distal row, beginning

COMPARATIVE ANATOMY 31

laterally, are: the cuboid, and the lateral-, the intermediate-, and
the medial cuneiform.

7. Metatarsus. — This is composed of the five long bones
of the foot, the first of which is rudimentary.

8. Phalanges. — The first toe is absent. Each of the four
remaining digits has three phalanges. As in the fore limb, the
distal phalanx of each digit carries a retractile claw.

III. THE MUSCULAR SYSTEM

Much of the laboratory work for the rest of the course will
consist of dissection. Dissection means separating the parts so
that they may be more easily seen; very little cutting is required.
Blunt instruments and the fingers often make the best dissecting
tools for separating structures. Follow directions carefully.
Be sure that you know what you are going to do before you do it.
In case of doubt, ask the instructor.

Muscles of the Shark

A detailed comparative study of muscles is very unsatis-
factory because of the great variability in arrangement and
function, even among closely related species. However, the
primitive arrangement of muscles should be clearly understood.
In the lowest vertebrates, as in amphioxus, the greater part of the
external musculature consists of a series of zigzag bands on each
side of the body called myomeres, each separated from the next
by a connective tissue septum, the myocomma. Each myomere
is composed of many muscle fibers which run from one myo-
comma to the next.

From the shark remove a narrow strip of skin from the dorsal
to the ventral side in the region of the pelvic fin, and observe the
primitive arrangement of the muscles. In the vicinity of the
lateral line a connective tissue septum divides the myomeres
into a dorsal epaxial region and a ventral hypaxial region. The
muscles of the two sides of the body never overlap but are
separated in the mid-ventral line by a connective tissue partition,
the linea alba.

Where paired limbs occur some of the myomeres are modified
in character and position to form the musculature of the limbs.
Remove the skin from the pelvic girdle and expose a mass of
muscle originating from the myomeres of the body and inserting
on the fin. By cutting through this muscle mass its origin from
the myomeres beneath may be readily seen. In land verte-

32

COMPARATIVE ANATOMY 33

brates, where greater support is needed, almost all myomeres
are so modified that the original segmental arrangement of the
muscles is no longer recognizable.

Muscles of the Cat

A cat will be supplied that has been embalmed in such a way
that it does not need to be immersed in a preserving fluid. It
must be kept in its container, however, with the cover on tight,
when not in use. This is important. The cat will remain
in good condition throughout the semester if properly cared
for.

Skin your own cat. Before proceeding ask the instructor for
directions. It is preferable, though not essential, to skin the
cat a day or two before using it for muscle dissection to allow
the excess embalming fluid to drain off. During the skinning
notice that the skin is held to the muscular wall underneath by a
loose fibrous tissue, the superficial fascia. Observe also that
certain muscles, particularly in the thoracic region, have their
insertions in the skin. If your cat Is a female, the mammary
glands will be noticeable; in some cases milk may be present.

In speaking of the muscular system reference is made to the
voluntary muscles only. Each muscle is attached at each end
to some other structure, most commonly, though not always, to
bone. The less movable end of a muscle is known as the origin;
the more movable end, as the insertion. A muscle may have
more than one point of origin or insertion. Attachment of a
muscle is frequently made by means of a white fibrous cord called
a tendon. A broad flat tendon is known as an aponeurosis.
Occasionally part of a tendon ossifies, forming a sesamoid bone.
(The patella is a good example.)

Surrounding each muscle, and also groups of muscles and
the musculature of the body as a whole, are sheets of connective
tissue known as fasciae. Dissection of muscles consists almost
wholly of tearing and separating these. Fascia frequently serves
also as the origin or Insertion of a muscle.

Muscles are often described by their actions. In this con-
nection the following terms are important:

Flexor, a muscle which bends a joint.

Extensor, a muscle which straightens a joint.

34 LABORATORY DIRECTIONS

Adductor, a muscle which tends to draw a part toward the
median line.

Abductor, a muscle which tends to draw a part away from the
median line.

Rotator, a muscle which rotates a limb, or part of it, on its
long axis.

There are about five hundred voluntary muscles in the cat.
While the descriptions given here are limited to the hind leg,
dissect as many of the other muscles as you wish. The muscles
are described in the order in which they are most conveniently
dissected. Careful comparison of the muscles with the descrip-
tions will usually enable you to make certain that you have the
right muscle. In case of doubt refer to Reighard and Jennings,
''Anatomy of the Cat.'' Always have a skeleton before you.
First find the origin and insertion of the muscle on the skeleton,
and then find it on the specimen. Observe the direction in
which the muscle fibers run; they are not always parallel to the
long axis of the muscle. Trace each muscle to its origin and
insertion, but do not remove a muscle at its origin or its insertion
unless specifically told to do so. However, a muscle may be cut
through the middle to expose underlying muscles. Try not to
cut the larger blood vessels; a few will be mentioned as the dis-
section proceeds.

Demonstrate dissections to the instructor, and be prepared
at any time to point out any muscle and give its origin, insertion,
and action. A clean-cut dissection will have the fat and con-
nective tissue removed so that the muscles stand out clearly.

Superficial Muscles of the Thigh

Remove the fat and superficial fascia from one thigh and
shank. If the cat is a male, be careful not to injure the spermatic
cord which emerges from the body cavity and extends to the
testis. Place the cat ventral side up.

I. Sartorius. — There are only two superficial muscles on
the median side of the thigh; both are broad and fiat. The
sartorius covers the cephalic half of the median surface of the
thigh; the gracilis, to be studied next, covers the caudal half.
Distally these two muscles nearly meet; proximally they diverge
leaving between them a shallow depression known as Scarpa's

COMPARATIVE ANATOMY 35

triangle (a landmark of importance In human anatomy). The
sartorius originates from the ventral border of the ilium, and
is inserted on fascia proximal to the patella, on the patella, and
on the proximal portion of the tibia.

Action: to adduct and rotate the thigh, and to extend the
shank.

2. Gracilis. — This broad flat muscle occupies most of the
medial surface of the thigh not covered by the sartorius. It
originates by a strong tendon from the caudal three-fourths of
the pubic symphysis. Part of this tendon Is common to the
gracilis muscles of both legs and also gives origin to some fibers
of the adductor femoris muscles of both legs. The gracilis
inserts by a thin aponeurosis on fascia on the shank and on the
medial side of the proximal end of tibia.

Action: to adduct and extend the thigh; to flex the shank.

Extending across Scarpa's triangle are the femoral artery
(Injected with yellow coloring material) , femoral vein (dark), and
the femoral nerve (white). If traced toward the foot each will
be found to give off many branches to the muscles.

Turn the cat over. If the superficial fascia has been cleanly
removed, several muscles will be exposed.

3. Biceps Femoris. — This is a large triangular muscle cover-
ing the caudal two-thirds of the lateral surface of the thigh. It
is narrow at the proximal end and widens distally. Before dis-
secting It, read about the next muscle, the tenuisslmus, which
adheres closely to the underside of the biceps femoris and may
be easily destroyed. The biceps femoris originates from a small
area on the posterior end of the Ischium; insertion Is on the lateral
margin of the patella and by means of fascia, on the tibia.
Separate this muscle from the surrounding tissues, being careful
not to Injure other muscles or their tendons. Carefully cut it
transversely through the middle and expose the tenuisslmus
muscle and the large sciatic nerve.

Action: to abduct the thigh and flex the shank.

4. Tenuissimus. — ^Thls Is an extremely slender muscle,
beneath and closely adherent to the biceps femoris. It Is easily
overlooked. It originates from the transverse process of the
second caudal vertebra, passes to the caudal border of the biceps
femoris, and Is Inserted by the same fascia on the tibia.

36 LABORATORY DIRECTIONS

Action: to aid the biceps femoris.

5. Caudo-femoralis. — This is partly covered by the anterior
border of the biceps femoris. It originates from the transverse
processes of the second and third caudal vertebrae. The fibers
converge, and about half way down the thigh end in a slender
tendon, by means of which this muscle is inserted on the lateral
border of the patella.

Action: to abduct the thigh; to extend the shank.

6. Gluteus Maximus. — This short muscle originates ante-
rior to the proximal end of the caudo-femoralis from the trans-
verse processes of the last sacral and first caudal vertebrae and
from dorsal fasciae. The fibers converge and are inserted just
below the great trochanter. The fibers also frequently inter-
mingle with the fibers of the next muscle, the tensor fasciae latae,
making it difficult to find the exact boundary of the muscle.

Action: to abduct the thigh.

7. Tensor Fasciae Latae. — Cut the sartorius muscle
through the middle. Under this and extending around to the
lateral side of the thigh is a heavy, glistening fascia, the fascia
lata (lata, wide). Slit it lengthwise on each side, being careful
not to Injure the quadriceps femoris muscle which lies under-
neath. Probe under the fascia and trace It to its insertion on
the patella. The proximal end of the fascia lata Is attached to a
triangular muscle, the tensor fasciae latae, which originates partly
from the outer ventral border of the ilium, and partly from the
fascia covering the next muscle, the gluteus medlus.

Action: to stretch the fascia lata (as its name implies) and
to extend the shank.

8. Gluteus Medius. — This is the heavy muscle anterior
to the gluteus maximus and largely covered by the tensor fasciae
latae. It originates from the dorsal half of the lateral surface
of the ilium, from dorsal superficial fasciae, and from the tips
of the transverse processes of the last sacral and first caudal
vertebrae. The fibers converge and join a large central tendon
inserted on the great trochanter of the femur.

Action: to abduct the thigh.

Deep Muscles of the Thigh

To expose the deeper muscles, cut through the middle of the
tensor fasciae latae and caudo-femoralis and pin back the proxi-

COMPARATIVE ANATOMY 37

mal ends. Cut the Insertions of the gluteus maximus and
gluteus medius and roll these muscles back out of the way.
Much of the origin of the gluteus medius will have to be removed,
leaving this muscle attached mainly by a small area of dorsal
fascia. There should now be in view a fan-like mass of several
small muscles radiating around the great trochanter.

9. Piriformis. — This is a small triangular muscle. It is
now the only muscle left in this region that lies over the sciatic
nerve. It originates from the transverse processes of the last two
sacral and first caudal vertebrae. The fibers converge and are
inserted by a flat tendon on the great trochanter. Loosen the
insertion of the piriformis and pin the muscle back. Pull the
sciatic nerve to one side to expose the underlying muscles.

Action: to abduct the thigh.

10. Gluteus Minimus. — This is an elongated muscle, origi-
nating from almost the whole ventral half of the lateral surface
of the ilium. It is inserted by means of a strong tendon on the
lateral side of the great trochanter.

Action: to rotate the thigh.

11. Gemellus Superior. — This is a short, broad, triangular
muscle, partly covered by the piriformis, but extending beyond
it on each side. It originates from the dorsal border of the
posterior half of the ilium and the anterior third of the ischium.
The fibers converge rapidly to a tendon that inserts on the great
trochanter.

Action: to abduct the thigh.

12. Obturator Internus. — This muscle lies caudal to the
gemellus superior. Its origin is on the in7ier surface of the
ischium near the symphysis. The fibers pass over the dorsal
border of the ischium; insertion is by means of a strong tendon
in the trochanteric fossa of the femur.

Action: to abduct the thigh.

13. Gemellus Inferior. — This is a flat muscle almost
entirely covered by the obturator internus. Only a small por-
tion shows posterior to the obturator internus. It originates
on the lateral surface of the ischium and inserts on the same
tendon with the obturator internus. The obturator internus
must be cut where it passes over the ischium if much of the
gemellus inferior is to be seen.

Action: to abduct the thigh.

38 LABORATORY DIRECTIONS

14. QuADRATUS Femoris. — ThIs is the most posterior of tiie
fan-like mass of muscles around the great trochanter. It
originates from a triangular area on the lateral surface of. the
ischium, near the posterior end, and inserts on the greater and
lesser trochanters of the femur.

Action: to extend and rotate the thigh.

15. Capsularis. — This is a small muscle entirely covered
by the gluteus minimus. It originates on the lateral surface
of the ilium near the acetabulum, and inserts on the femur distal
to the great trochanter. It may be seen by removing the gluteus
minimus at its insertion. It will be seen again when the quadri-
ceps femoris muscle is dissected.

Action: to rotate the thigh.

There now remain several muscles, the semitendinosus, the
semimembranosus, the adductor femoris, and the quadriceps
femoris, which show on both sides of the thigh, but are more
easily dissected from the medial side. Cut transversely through
the middle of the gracilis, if this has not already been done.

16. Semitendinosus. — This is the long muscle forming the
caudal border of the thigh. The proximal end is on the lateral
side; the distal end is on the medial. It originates on the
posterior end of the ischium, and inserts on the proximal end of
the tibia by means of a tendon.

Action: to flex the shank.

17. Semimembranosus. — This is the thick muscle, with fibers
loosely held together, lying next to the semitendinosus. It
originates from the caudal border of the ischium and inserts on
the medial side of distal end of the femur and the proximal end
of the tibia.

Action: to extend the thigh.

18. Adductor Femoris.— This is a large triangular muscle
between the semimembranosus and the femur. The fibers are
loosely united, making it difficult to find the boundaries of the
muscle. The origin is from the lateral surface of the pubis and
ischium, near the symphysis, and from the tendon of the gracilis
muscle. The insertion covers almost the whole shaft of the
femur.

Action: to extend and adduct the thigh.

COMPARATIVE ANATOMY 39

19. Adductor Longus. — This is a small muscle anterior to
and slightly overlapping the aductor femoris. It originates
with the pectineus from the cranial border of the pubis and
inserts by a thin aponeurosis on the femur.

Action: to adduct the thigh.

20. Pectineus. — ^This is the last and smallest of the muscles
on the caudal side of the femur. It is sometimes entirely fused
with the adductor longus. It originates from the cranial border
of the pubis, and inserts on the femur.

Action: to adduct the thigh.

21. Iliopsoas. — This is the equivalent of two human mus-
cles, the iliacus and the psoas. It emerges from the body cavity
and inserts on the lesser trochanter of the femur. It originates
from the centra and transverse processes of ten vertebrae. Do
not trace it to its origin; it will be seen again in the study of the
internal anatomy.

Action: to rotate flex the thigh.

22. Quadriceps Femoris. — This is the large mass of muscle
covering the whole cranial surface of the femur. It consists
of four parts, each of which may be considered a separate muscle,
with a common insertion on the patella. The vastus lateralis
forms the lateral face of this muscle. It originates on the lateral
side of the great trochanter and shaft of the femur. The vastus
medialis forms the medial face of the quadriceps femoris, and
has its origin from a long area on the median side of the femur.
The rectus femoris lies between the vastus lateralis and vastus
medialis, and originates from a triangular area just anterior to
the acetabulum. It unites with the vastus lateralis about two-
thirds the way down the femur. Cutting the rectus femoris
through the middle will expose part of the vastus intermedius,
which lies beneath it. The vastus intermedius originates from
nearly the whole length of the femur. It is usually difficult to
separate it cleanly from the vastus medialis. At the proximal
end of the quadriceps femoris, between the vastus lateralis and
rectus femoris, the capsularis muscle may be seen.

Action: to extend the shank.

23. Obturator Externus. — Cut transversely through the
middle of the semitendinosus, semimembranosus, and adductor

40 LABORATORY DIRECTIONS

femorls so as to swing the leg forward until the toe points towards
the head. The large, flat, triangular obturator externus muscle
should now be exposed. It originates on the medial border of
the obturator foramen and inserts in the trochanteric fossa.
Action: to rotate the thigh.

Muscles of the Shank and of the Foot

Skin the foot and remove all superficial fascia from both
shank and foot. This will involve removal of the insertions of
some of the thigh muscles. From now on it is important, at the
end of each laboratory period, to wrap the shank and foot with
cheese cloth soaked in embalming fluid, to prevent the muscles
from becoming dry.

Most of the muscles of the shank extend onto the foot; their
tendons should be traced to the points of insertion.

24. Gastrocnemius. — Almost all of the mass of muscle
forming the calf of the leg is the gastrocnemius, composed of two
heads, a caput mediale (medial head) and a caput laterale (lateral
head). Between the caput mediale and caput laterale lies
another muscle, the plantaris. With the handle of a scalpel
separate the muscles, beginning in the depression behind the
knee. The caput mediale will separate easily; the caput laterale
will be difficult to separate from the plantaris. The two heads
of the gastrocnemius unite about one-third the way down the
shank. The caput laterale originates from four places: from
superficial fascia of the shank, from the lateral edge of the
patella, from a sesamoid bone on the lateral side of the head of
the tibia, and from an aponeurosis covering the plantaris. The
caput mediale originates from the distal end of the femur and
from a sesamoid bone on the medial side of the head of the tibia.
The tendons of the two heads of the gastrocnemius and of
another muscle, the soleus, unite to form the large tendo7i of
Achilles^ which inserts on the calcaneus bone.

Action: to extend the foot.

25. Plantaris. — Dissection of the gastrocnemius has already
revealed this muscle. It originates by a tendon (which it shares
with the caput laterale) from the lateral edge of the patella,
and from the sesamoid bone on the lateral side of the head of the
tibia. The tendon of the plantaris forms the core of the tendon

COMPARATIVE ANATOMY 41

of Achilles, which should be carefully split open to demonstrate
this fact. The tendon of the plantaris passes through a groove
and inserts on the ventral side of the calcaneus.
Action: to extend the foot.

26. SoLEUS. — This is the remaining muscle of the calf of the
leg with an insertion on the tendon of Achilles. It lies on the
lateral side and originates from the proximal half of the
fibula.

Action: to extend the foot.

27. Flexor Digitorum Brevis. — This is a flat muscle on the
sole of the foot. It originates from the tendon of the plantaris,
a fact revealed by removal of the plantaris tendon at its insertion
on the calcaneus. Distally the flexor digitorum brevis divides
into four tendons, one going to each toe.

Action: to flex the toes.

28. QuADRATUS Plantae. — Cut through the middle of the
flexor digitorum brevis and expose the wide tendon beneath.
This is the combined tendon of the flexor digitorum longus and
the flexor hallucis longus. On it is inserted the small triangular
quadratus plantae muscle, which originates from the lateral
surface of the calcaneus and cuboid.

Action: to hold the tendon in place.

29. Flexor Digitorum Longus. — Beginning on the sole
of the foot trace back the smaller and more medial branch of the
combined tendons of the flexor digitorum longus and the flexor
hallucis longus until this muscle is reached. The flexor digi-
torlum longus originates from the surface of the tibia and the
head of the fibula. Distally the tendon, after the flexor hallucis
longus has united with it, divides into four branches, one for
each toe.

Action: to flex the toes and extend the foot.

30. Flexor Hallucis Longus. — This muscle lies lateral to
the flexor digitorum longus and greatly exceeds it in size. It
originates from the shaft of the tibia, and from the head and
shaft of the fibula. In five-toed mammals the tendon of the
flexor hallucis longus inserts on the big toe (hallux); in the cat
it unites with the tendon of the flexor digitorum longus.

Action: with the flexor digitorum longus to flex the toes and
extend the foot.

42 LABORATORY DIRECTIONS

31. PoPLiTEUs. — The short triangular muscle behind the
knee, near the proximal end of the flexor digitorum longus, is the
popliteus. It originates by a strong tendon from the lateral side
of the femur; its fibers diverge obliquely and are inserted on the
medial side of the proximal end of the tibia.

Action: to rotate the shank.

32. Tibialis Posterior. — This is a slender muscle com-
pletely covered by the flexor digitorum longus. It originates
from the head of the fibula and from the tibia near its head.
About half way down the tibia it passes into a tendon, which
passes through a groove on the distal end of the tibia and inserts
on the scaphoid and medial cuneiform bones.

Action: to extend the foot.

33. Extensor Digitorum Longus. — On the dorsal side of
the foot is a tendon with four branches going to the toes. Trace
this tendon back through a small ligament on the foot to a
muscle which passes under the large transverse ligament just
above the ankle. This is the extensor digitorum longus which
should be separated from the large muscle lying between it and
the tibia. The extensor digitorum longus originates by a
tendon from the lateral side of the distal end of the femur.
Insertion is on the upper surface of each digit.

Action: to extend the toes and flex the foot.

34. Tibialis Anterior. — This is the long muscle between
the extensor digitorum longus and the tibia, already partly
dissected. It originates from the proximal portions of both
tibia and fibula and inserts by a tendon on the first metatarsal.
The tendon, together with the extensor digitorum longus muscle,
is held down by the transverse ligament.

Action: to flex the foot.

35. Peroneus Longus. — ^There now remain three peroneus
muscles on the lateral side of the shank, between the extensor
digitorum longus and the flexor hallucis longus. The peroneus
longus is the outermost of these muscles. It originates from the
proximal half of the fibula; and soon passes into a tendon, which
continues through a groove on the distal end of the fibula and
inserts on the proximal parts of all the metatarsals.

Action: to flex the foot.

COMPARATIVE ANATOMY 43

36. Peroneus Tertius. — Beneath the peronous longus lies
the slender peroneus tertius muscle, whose tendon passes with
that of the peroneus brevis through the groove on the fibula,
and then to its insertion on the lateral side of the fifth toe. It
originates on the fibula.

Action: to abduct and extend the fifth toe.

37. Peroneus Brevis. — This is the third muscle of the
peroneus group. It originates from the distal two-thirds of the
fibula, which it almost surrounds and its tendon, after passing
over the end of the fibula, inserts on the lateral side of the fifth
metatarsal.

Action: to extend the foot.

There are many small muscles on the foot which have not
been mentioned. No attempt will be made to dissect them.

IV. THE DIGESTIVE AND RESPIRATORY

SYSTEMS

The digestive and respiratory systems are so closely associ-
ated, particularly in lower vertebrates, that it is convenient to
study them together. The digestive system includes all organs
involved in the taking in and digestion of food, and in the dis-
posal of the undigested wastes. In vertebrates this system is
composed of the alimentary canal, a continuous tube extending
from mouth to anus, and a number of digestive glands opening
into it. The organs concerned with obtaining oxygen and giving
oflF carbon dioxide constitute the respiratory system. While
dissecting the digestive and respiratory systems attention will be
called occasionally to other structures most conveniently studied
at this time.

Comparative Anatomy of Teeth

As the teeth of nearly all vertebrates are derived from
placoid scales, they are similar in general structure. They differ
greatly, however, in shape, number, manner of attachment, and
method of succession in various animals. A few representative
species will be studied.

Lamprey. — The tooth-like structures form a circle around
the mouth. They are not real teeth, but are simply projections
of the horny layer of the skin.

Shark. — ^Examine the prepared jaw of a large shark. The
teeth have no roots, but are attached to the tissues. They are
all alike (homodont) except in size. Each functional tooth has
behind it a series of developing reserve teeth. If a functional
tooth is lost, it is immediately replaced by a reserve tooth behind
it. This replacement may take place an unlimited number of
times, a condition known as polyphyodont.

A Bony Fish. — The dentition here is also homodont and
polyphyodont. The teeth are without roots, but are firmly
attached to the surface of the bone (acrodont). In many fishes

44

COMPARATIVE ANATOMY 45

they occur not only on the jaws, but may sometimes be found
on the vomer, palatine, pterygoid, and parasphenoid bones.

Giant Salamander. — Compare with the fish. The teeth
are pleurodont, that is, attached to the inner surface of the jaw.
A similar condition exists in frogs, but our common toads have
no teeth.

Turtle. — Adult turtles never possess teeth although rudi-
ments may exist in the embryo. The jaws are covered instead
by a thick hard horny layer, usually with a very sharp edge.

Alligator. — The teeth have roots embedded in sockets in
the jaw (thecodont). This is unusual in reptiles, but is the
common condition in mammals.

Bird. — Compare with the turtle. No living birds have
teeth, but some fossil forms were abundantly supplied with
them.

Cat. — The teeth are thecodo?it (set in sockets), heterodofit
(of various shapes), and diphyodont ("milk" teeth precede the
permanent set). These conditions are typical of mammals.
The different kinds of teeth found in the cat have already been
studied; review them again if necessary.

Rodent. — The long curved incisors, suited for gnawing, bear
hard enamel on the anterior face only. These teeth grow
throughout the life of the animal.

Dental Formula. — The number and character of the teeth
are constant for members of the same species, but vary greatly
in different species. To express in a brief form the dental con-
dition, a dental formula is used. The formula for the cat is
I 3/3, C i/i, P 3/2, M i/i ; or omitting the letters, as is usually
done, the formula will read

3, I, 3, I

3, h 2, I

Digestive and Respiratory Organs of the Shark

If your shark is a female ask for instructions before opening
the body cavity; if you have a male open the body cavity by
making a longitudinal incision on each side from the pelvic
region to the pectoral, and then making transverse cuts at each
end so as to remove the ventral body wall. Most of the viscera

46 LABORATORY DIRECTIONS

will now be in view. Begin at the corner of the mouth and cut
back through the gill arches on the left side until the body cavity
is reached. Find the following:

1. Pharynx, the region in the back part of the mouth cavity
common to both digestive and respiratory systems. Lay back
the floor of the mouth and study the gills and the gill arches.
The gills exist as rows of delicate gill filaments radiating from the
membrane covering the gill arches. The first arch, situated on
the anterior wall of the first gill slit, is the hyoid arch. It is
termed a hemibranch as it bears gill filaments on the posterior
side only. A holobra?ich has gill filaments on both sides of the
arch. How many holobranchs and hemibranchs are there and
to which arches do they belong ? The hyoid arch, in addition to
bearing gill filaments, supports the tongue. The small carti-
laginous processes on the inner borders of the gill arches are the
gill rakers.

The spiracles, bearing gill filaments and opening into the
pharynx, have already been studied.

2. Oesophagus, an exceedingly short tube connecting the
pharynx with the stomach.

3. Stomach, the large J-shaped organ extending more than
half the length of the body cavity. It may be partly covered
by the liver. The size of the stomach will depend to a large
extent on the amount of food it contains. It is commonly con-
stricted near the middle. Slit open the stomach, wash out
anything it may contain, and examine the inner surface. The
anterior part contains finger-like papillae; further back are
longitudinal folds, which vary in size with the degree of con-
traction of the stomach. In a stomach distended with food the
longitudinal folds may wholly disappear. A pyloric valve is
present on the caudal end of the stomach.

4. Intestine, extending from stomach to anus, and consisting
of three regions. The first part, the duodenu7n, is short; it lies
between the stomach and the second region, the large spiral
valve. The spiral valve constricts posteriorly to form the short
third division of the intestine, the rectum, which opens into the
cloaca. Attached to and opening into the rectum is a finger-like
projection, the rectal gland, the function of which is not known.

COMPARATIVE ANATOMY 47

Slit open the spiral valve and observe the large spiral fold which
serves to retard the passage of food, and thus secures more com-
plete digestion and absorption.

5. Liver, large and oily, and consisting of several lobes united
at the anterior end. The gall bladder, usually recognizable by
its greenish color, lies embedded in the right side of the smallest
lobe of the liver. Trace the hile duct. It enters the intestine at
the point where the duodenum joins the spiral valve.

6. Pancreas, a pale bilobed mass, one part of which lies on
the duodenum and the other, a more elongated portion, lies
between the stomach and spiral valve. The pancreatic duct may
be difficult to find. It is a short, white duct on the dorsal side
of the more compact lobe of the pancreas, sometimes embedded
in the tissue, and enters the anterior end of the spiral valve.

7. Spleen, the dark colored, triangular organ attached to the
stomach. This structure is associated with the organs of
circulation.

Before replacing the shark in its container remove the major
portion of the liver, leaving the base of the liver and the gall
bladder and bile duct intact. Removal of the liver will aid in
preventing the specimens from becoming oily.

Digestive and Respiratory Organs of Necturus

Open the body cavity by making a longitudinal incision from
one side of the anus through the pelvic girdle to the pectoral
girdle. Spread apart the two flaps; cut them off if necessary.
Identify the following organs: liver, gall bladder and bile duct,
stomach, pyloric valve, small intestine, large intestine, cloaca,
pancreas and spleen. Also, locate the lungs. These are a pair
of elongated, thin, semi-transparent structures lying dorsal to the
liver and stomach.

Cut through the lower jaw and gill arches on each side and
expose the mouth cavity and pharynx. Find the posterior nares,
the internal openings of the nasal passages. These are located
at the posterior ends of the rows of teeth and are covered with
small flaps of skin that act as valves. The tongue is supported by
the hyoid arch. The walls of the pharynx are pierced by two
pairs of gill slits. The walls of the gill slits, unlike those of the

48 LABORATORY DIRECTIONS

shark, bear no gill filaments; the gills are external in position.
Find the opening into the oesophagus. On the floor of the
pharynx just anterior to the opening to the oesophagus is a tiny
slit, the glottis, which leads to a small, very primitive larynx.
The glottis is supported by two very small arytenoid cartilages,
probably derived from the seventh visceral arch. The larynx
is joined to the lungs by a short trachea. Slit open a lung and
observe that the interior surface is smooth. This is a primitive
condition.

Necturus has three methods of obtaining oxygen: through
the gills, through the skin, and through the lungs. When using
the lungs air passes through anterior nares, nasal passages,
posterior nares, mouth cavity, pharynx, glottis, larynx, trachea
and lungs.

Digestive and Respiratory Organs of the Cat

Remove the skin and superficial fascia from the head and
neck. On either side of the neck lies the large external jugular
vein. Trace it toward the head. At about the angle of the
jaw it will be seen that the external jugular is formed by the
confluence of two large veins: the posterior facial bringing blood
from muscles and glands in the region of the ear, and the anterior
facial returning blood from the face and tongue. A communi-
cating branch connects the anterior facials of the two sides.
A pair of lymph glands lie along the anterior facial near its
union with the posterior facial. These may be removed.

Salivary Glands. — Remove the skin and superficial fascia
from the head and neck. There are three important salivary
glands which empty their secretions into the mouth cavity.
The parotid gland, the largest of the salivary glands, is a large
lobulated mass at the base of the ear. Trace its white duct
(Stensen's duct) across the surface of the large masseter muscle
to the corner of the mouth, where it opens near the last premolar
tooth. The submaxillary and sublingual glands lie under the
posterior angle of the jaw, somewhat deeply embedded. Do not
mistake the two lymph glands on the surface for them. The
duct (Wharton's duct) of the submaxillary gland extends along
the under side of the mandible and opens on the floor of the
mouth just back of the incisor teeth through a small papilla.

COMPARATIVE ANATOMY 49

The sublingual, a small gland, lies close to the submaxillary and
along its duct. The duct of the sublingual parallels that of the
submaxillary and opens with it.

Thyroid Glands. — Expose the proximal end of the trachea
by removing the superficial muscles of the front of the neck.
The thyroid glands will appear as dark colored bodies on either
side of the trachea. The thyroids are endocrine glands and
have nothing to do with digestive or respiratory systems.

On each side of the trachea and extending parallel to It are
the vagus (tenth cranial) nerve, the common carotid artery carrying
blood to the head, and the internal jugular vein returning blood
from the brain. Behind the trachea lies the oesophagus.

With bone forceps cut through the mandible on each side
behind the corners of the mouth. Continue the cut along one
side of the neck until the first rib Is reached. Lay back the jaw
so as to expose the structures of the mouth and pharynx.

Tongue. ^ — It Is a muscular organ attached at its posterior
end. The dorsal surface contains papillae of four kinds: (a)
filiform, numerous, simple, and sharp pointed; (b) fungiform,
enlarged at the tips and scattered over the middle of the tongue;
(c) fiat, numerous on the sides of the base of the tongue; (d)
circumvallate, blunt and surrounded by a trench. These last
are not numerous; when distinct they are arranged In a V near
the base of the tongue.

Pharynx. — The curtain-like structure on the roof of the
mouth is the soft palate. Embedded In pockets at the ventral
end of the soft palate on each side are the tonsils. Dorsal to
the soft palate and communicating with the nasal cavities by
means of the choanae, Is the nasopharynx. Cut along the roof
of the mouth and pull aside the soft palate; find the openings
of the Eustachian tubes in the dorsal wall of the nasopharynx.

Larynx and Trachea. — These constitute the "wind pipe."
The walls of the trachea are supported by cartilaginous rings
that are Incomplete on the dorsal side. The larynx is the
enlarged upper end of the wind pipe. Its skeletal parts consist
of several cartilages, the largest of which is the thyroid cartilage
(Adam's apple of man). The slit-like opening of the larynx
is the glottis which Is protected from food particles by a carti-
laginous flap, the epiglottis, Immediately In front of It.

50 LABORATORY DIRECTIONS

Cut along the mid-dorsal line of the larynx and continue
the cut posteriorly for a few inches along the dorsal side of the
trachea. Two folds of skin, one on each side, extend from
the base of the epiglottis. These are t\i€^ false vocal cords. The
space in front of them is called the vestibule. A short distance
behind them is a second pair of folds, the true vocal cords.

Survey of the Viscera. — Beginning just anterior to the
pelvic girdle, cut the abdominal wall on each side, being careful
not to injure the internal organs. Continue this cut forward
to the first rib, carefully loosening the body wall from the
diaphragm when this is reached. Cut the ribs with bone forceps.
To expose the contents of the body cavity it will be necessary to
remove this flap. In doing so observe the internal mammary
arteries and veins that extend onto the chest wall. Without
disturbing anything locate: the lungs, enclosing the heart
between them; the muscular diaphragm separating thorax and
abdomen; the liver, commonly with a portion of gall bladder
exposed on its right lobe; the fatty membrane, known as the
great omentum, covering most of the abdominal viscera; the
stomach, sometimes partly exposed to the left of the liver;
the dark colored spleen at the left of the stomach; the many coils
of the intestine; and the bladder, just anterior to the pubic
symphysis. In cats in an advanced stage of pregnancy the two
horns of the uterus will also be prominently in view, often greatly
displacing the other internal organs. The membrane lining the
body cavity is the peritoneum. A similar one, the pleura, lines
the chest cavity.

Observe that the organs are held together and to the body
wall by membranes. The membranes attaching the viscera
to the body wall are known as mese^iteries; those connecting
organs with other organs are omenta (singular, omentum).

Oesophagus. — This part of the alimentary canal is a straight
tube passing from the pharynx through the diaphragm to the
stomach. To see it, pull the left lung toward the right side,
but do not cut more tissues than is necessary. The oesophagus
is collapsed except when it contains food.

Stomach. — The size of the stomach depends to a large extent
on the amount of food it contains; it also varies greatly in differ-
ent individuals. The cardiac end of the stomach is the region

COMPARATIVE ANATOMY 51

where the oesophagus enters; the pyloric end is the part where
the small Intestine begins. The lateral side of the stomach is
known as the greater curvature; the inner side is the lesser
curvature. The entrance to the small intestine is guarded by a
ring-like thickening, the pyloric valve (pyle, gate).

Small Intestine. — The small intestine is divided into three
regions. The first part, the duodenum, begins at the pyloric
valve, runs posteriorly on the right side and then forward again
on the left side. The place where it turns posteriorly on the left
side is arbitrarily chosen as the end, although there is no distinct
line of division. The jejunum (meaning empty) is the much
convoluted part of the intestine following the duodenum and
extending for approximately twenty-five centimeters. Its limits
cannot be exactly defined. Its name refers to the fact that in
human cadavers it is usually found empty. The remaining
part of the small intestine, the ileum (eilein, to roll up), consti-
tutes about two-thirds of the whole. Observe the mesenteries
supporting it. The ileo-colic valve lies at the point where the
ileum joins the large intestine.

Large Intestine. — The large intestine is composed of two
parts, the colon and the rectum. The colon is the enlarged por-
tion of the intestine into which the ileum enters. Near the
point of entrance of the ileum is a blind pouch, the caeciim. The
colon passes anteriorly, then to the left, and then posteriorly
again, forming roughly the ascending, transverse, and descending
colon. The rectum is assumed to begin where the descending
colon reaches the middle line and extends to the anus, which is
surrounded and kept closed by the sphincter ani muscles. Near
the anus are two small anal glands. Do not dissect the last
part of the rectum until the urogenital system has been studied.

Spleen. — This is the dark red body lying along the greater
curvature of the stomach. It is a ductless gland associated with
the circulatory system and is not an organ of digestion.

Liver. — This is the largest gland of the body. It is divided
into several lobes. The gall Madder lies partially embedded in
the liver. Cut away the free ends of the liver and dissect the
bile ducts. This requires care, patience, and technique, as the
ducts are covered with connective tissue which must be removed.
On the duodenum, about an inch from the pyloric valve, is a

52 LABORATORY DIRECTIONS

small swelling, the ampulla of Fater, into which the pancreatic
duct and the common bile duct empty. The common bile duct
(ductus choledochus) is formed by the union of three bile ducts:
two hepatic ducts, one from each side of the liver, and the cystic
duct from the gall bladder.

Pancreas. — This is a flattened irregular gland, consisting
of two lobes, one lying along the duodenum and the other in the
great omentum near the greater curvature of the stomach.
Do not confuse it with the large lymph glands which lie in the
mesenteries. The main pancreatic duct (duct of Wirsung) is
short and wide and opens, with the common bile duct, into the
ampulla of Vater in the duodenum. The pancreatic duct is
formed by two branches, one from each lobe of the pancreas.
To find the pancreatic ducts, begin near the ampulla of Vater
and scrape off the surface tissue of the pancreas until a white
duct appears, then trace this both ways.

Slit open the duodenum and find the opening of the ampulla
of Vater on the inside.

Lungs. — Trace the trachea back into the chest cavity and
find where it divides into two bronchi, one going into each lung.
Work around the blood vessels so as not to injure them. The
lungs are paired, many lobed organs. The right and left lungs
are completely separated, each lying in its own half of the thorax,
with a double septum, the mediastinum, between them. The
right lung is slightly larger and is divided into three smaller
proximal lobes and a large distal one. The left lung is divided
into three main lobes. Trace one of the bronchi as far as possible
into its lung. Within the lung it will be found to break up into
many branches, the bronchioles. Are the bronchioles supported
by cartilaginous rings ?

Thymus Gland. — This lies just anterior to the heart on the
ventral side of the trachea. Superficially it may resemble fatty
tissue. In adult cats it is usually degenerate, but may be easily
seen. The thymus is not part of the respiratory system.

Diaphragm. — This muscular partition between thorax and
abdomen is to be considered as an organ of respiration. It is an
arched muscular structure with the muscle fibers originating
from the body wall, vertebrae, and ribs and converging on a
central tendon in the middle of the diaphragm for their insertion.

V. THE CIRCULATORY SYSTEM

The circulatory system is in reality made up of two systems,
the blood system and the lymphatic system. The blood system
is the one usually referred to In speaking of the circulatory sys-
tem, and includes the heart, blood, and blood vessels. The heart
acts as a muscular pump and forces blood through the vessels
to all parts of the body. The vessels carrying blood away from
the heart are called arteries, and those that bring the blood to the
heart are known as veins. The character of the blood carried
thus has nothing to do with the naming of the vessels. Arteries
and veins are connected by capillaries, thin-walled and micro-
scopic, through which interchange between the blood and the
tissues takes place.

The lymphatic system consists of lymph vessels, sinuses, and
glands and conveys a colorless fluid, lymph, which bathes the
tissues of the body. Some of the lymph glands have already
been found. The lymph vessels are very delicate and no attempt
will be made to dissect them.

Circulatory System of the Shark

The arrangement of blood vessels in the elasmobranchs is
rather primitive and in many respects Is similar to embryonic
stages in the development of the mammalian circulation.
Hence, a comparison of the shark circulatory system with that
of the cat should present an Interesting study.

External Anatomy of the Heart. — Make a median inci-
sion on the ventral side through the pectoral girdle and carefully
continue the incision forward to the mouth. The pericardial
cavity, bounded posteriorly by the transverse septtnn will be
exposed. Clean away the tissues ventral to this cavity and
expose the heart. The ventricle Is the most conspicuous portion
of the heart from a ventral view. Its walls are heavily muscular
and it contains a single cavity. The short, thick-walled vessel
leaving the ventricle Is the co7ius arteriosus. This continues

S3

54 LABORATORY DIRECTIONS

forward as the ventral aorta, which gives off afferent branchial
arteries to the gills. These will be traced later.

The auricle is a thin-walled sac dorsal to the ventricle. It
contains a single cavity that leads to the ventricle. Lift up the
ventricle and find the sinus venosus that lies just anterior to
the transverse septum. It brings blood from the veins and
empties into the auricle.

Veins. — In the preparation of the specimen the arteries were
injected with a colored substance to make them stand out more
clearly. The veins, however, have not been so treated and
will appear dark with clotted blood, or in some cases may be
colorless.

The veins are best dissected by beginning at the sinus venosus
and tracing them backward. They are thin-walled and in places
dilate to form sinuses. Make a transverse incision across the
ventral wall of the sinus venosus. Wash out any clotted blood.
Spread the walls apart and look for the openings of vessels into
the sinus. In the posterior region near the median line find
the apertures of the hepatic sinuses. Stick probes into them and
follow the probes back into the liver and expose these large liver
sinuses.

Entering the sinus venosus from the lateral sides are the large
ducts of Cuvier. These continue partially around the oesophagus
and end in the anterior and posterior cardinal sinuses, which
extend respectively toward the head and toward the tail. The
posterior cardinal sinuses are a pair of very large vessels lying side
by side along the dorsal wall of the body cavity. Posteriorly
they narrow down and extend between the kidneys. Numer-
ous renal veins may be seen entering the posterior cardinals.
Each anterior cardinal sinus extends dorsal to the gill arches in
the direction of the eye, where it connects with a large orbital
sinus. The anterior cardinal sinus Is best dissected by making
an incision dorsal to the gills and when the sinus is located
tracing It each way.

Entering each posterior cardinal sinus is a subclavian vein
which brings blood from the pectoral fin, and a lateral vein which
extends along the lateral side of the abdominal cavity from pelvic
to pectoral regions. (The lateral veins may have been removed
in opening the shark.) The two lateral veins are connected in

COMPARATIVE ANATOMY 55

the pelvic region, where they are joined by the iliac veins from
the fins.

The hepatic portal system Is made up of all the vessels that
carry blood from the digestive organs to the liver. The hepatic
portal vein Is formed by the confluence of the veins from the
rectal gland, spiral valve, stomach, spleen, and pancreas, and
enters the liver dorsal to the median lobe. Within the liver It
forms capillaries. The blood leaves the liver by means of the
hepatic sinuses.

The caudal vein enters the body cavity and divides Into two
renal portal veins, each of which passes to the dorsal side of a
kidney where It gives off small branches Into the kidney. Blood
leaves the kidneys through the renal veins that enter the pos-
terior cardinal veins.

Arteries. — Dissect the afferent branchial arteries that extend
from the ventral aorta to the gills. How many are there?
Remove the mucous membrane from the roof of the mouth and
trace the efferent branchial arteries which receive blood from the
gills. Follow them back Into the gills as far as possible and
observe that they form a series of loops around the margin of
the gills. Short horizontal vessels connect the loops with one
another. From the first loop arises the common carotid artery.
It divides Into external and internal carotids. The internal
carotids from the two sides join to form a single vessel, which
enters the skull to supply the brain. A coronary artery, arising
from one of the efferent branchial vessels, supplies the muscles
of the heart and the conus arteriosus with blood.

Follow the eflFerent arteries inward and posteriorly to where
they unite to form the dorsal aorta. This large vessel extends
nearly the length of the body. Within the body cavity several
vessels arise from the aorta. Sub-clavian arteries extend to the
pectoral fins. A coeliac artery, originating just posterior to
the transverse septum, supplies stomach, liver, Intestine, and
pancreas. Small renal arteries go to the kidneys. An anterior
mesenteric to the Intestines and a gastro-splenic to the stomach,
pancreas, and spleen arise close together at about the level of
the middle of the spiral valve. Further back a posterior mesen-
teric goes to the rectal gland. Iliac arteries go to the pelvic fins.
Parietal arteries arise along the whole length of the aorta. The

56 LABORATORY DIRECTIONS

aorta continues through the haemal arches of the tail vertebrae
as the caudal artery.

Internal Anatomy of the Heart. — Make a longitudinal
incision through the conus arteriosus and ventricle. Make
another incision through the auricle. It should now be possible
to look into the heart and see the valves within. The sinu-
auricular valve appears as two membranous folds bordering the
opening between sinus venosus and auricles. The auricular-
ventricular valve consists of flaps guarding the entrance from
ventricle to auricle and prevents the backflow of blood into the
auricle when the ventricle contracts. Within the conus arterio-
sus are pocket-like semi-lunar valves that prevent blood from
flowing back into the ventricle.

Heart of an Amphibian

If you are not already familiar with the anatomy of a frog
heart, dissect one or study charts showing the structure of an
amphibian heart. Observe that there are two auricles and one
ventricle, whereas the shark has but one auricle and one ven-
tricle. The left auricle is concerned with the pulmonary circu-
lation and contains pure or oxygenated blood that has come
from the organs of respiration. The right auricle, on the other
hand, carries impure or reduced blood that has returned to the
heart from the tissues, and thus is concerned with the systemic
circulation. Blood from the systemic veins passes through the
sinus venosus before entering the right auricle.

CIRCULATORY SYSTEM OF THE CAT
External Anatomy of the Cat Heart

The heart, in both its embryonic development and in its
evolutionary history, begins as a straight tube. Later it becomes
folded upon itself forming first two chambers, then three, and
finally four divisions, reaching its greatest development in the
birds and mammals.

The heart is enclosed in a membranous sac, the pericardium.
This should be cut away, but do not remove the heart until after
all blood vessels have been studied.

COMPARATIVE ANATOMY 57

Ventricles. — The two ventricles constitute the greater
visible portion of the heart. The left is much firmer than the
right, due to its greater muscular development, by which differ-
ence the boundaries between the two ventricles may be fairly
well traced. The apex or free tip of the heart lies wholly within
the boundaries of the left ventricle.

Auricles.— The auricles are thin-walled sacs anterior to the
ventricles. Externally the division between right and left
auricles is only slightly indicated. A large part of each auricle
is taken up by a flap, the auricular appendage.

Venae Cavae. — Two large veins enter the right auricle.
The posterior or inferior vena cava brings blood to the heart from
the posterior part of the body; the anterior or superior vena cava
brings blood from the shoulders and head.

Aorta. — This is the large artery emerging from the left
ventricle. It passes between the auricular appendages and
soon after leaving the heart bends to the left and passes down
the back as the dorsal aorta. It is the beginning of the arterial
system.

Pulmonary Vessels. — The pulmonary artery leaves the
right ventricle to the left of the aorta and carries blood to the
lungs. There are several short pulmonary veins entering the left
auricle that bring blood from the lungs back to the heart.

Coronary Vessels. — These are the vessels that take blood
to and from the tissues of the heart itself. The left coronary
artery extends along the division line between the ventricles
on the ventral side, the right coronary artery between the ven-
tricles on the dorsal side. Trace these arteries to their points
of origin. The left and the right coronary veins parallel the
coronary arteries. The coronary sinus is a vein partially
encircling the heart between the auricles and ventricles on the
dorsal side. It may be covered with fat. It receives the blood
from both right and left coronary veins and empties into the
right auricle.

Veins of the Cat

Considerable variation occurs among blood vessels, particu-
larly as to their points of origin. A blood vessel is therefore

58 LABORATORY DIRECTIONS

best Identified by the course it takes. In dissecting the veins
care must be used not to destroy the arteries, which in general
follow more or less the same course. Only the more important
arteries and veins are mentioned in this outline; if a more com-
plete description is desired consult Reigard and Jennings.

1. Superior Vena Cava. — This large vein was mentioned
previously in the study of the heart. It returns blood from the
head, fore limbs, and cranial part of the body to the right auricle.
Just before entering the heart it is joined by the azygos vein.

2. AzYGOS Vein.— This vessel, if traced backward from its
point of union with the superior vena cava, will be seen to pass
to the right and then backward into the abdomen, where it
originates from several small veins from the dorsal muscles.
The azygos receives a series of intercostal veins from the muscles
of the ribs.

3. Internal Mammary Veins. — A pair of these carrying
blood from the ventral body wall and diaphragm run forward
on the inner surface of the sternum, unite, and enter the superior
vena cava on a level with the third rib.

4. Innominate Veins. — These are the large veins that unite,
about on a level with the first rib, to form the superior vena cava.
Follow the left one.

5. Vertebral Vein. — This appears to be a short vein enter-
ing the innominate on the dorsal side. Bringing blood from the
brain it passes through the foramina transversaria of the first
six cervical vertebrae before leaving the spinal column to join
the innominate. Do not try to trace it within the vertebral
column.

6. Subclavian Vein. — This vein comes from the arm and
enters the innominate anterior to the first rib. Outside of the
thoracic cavity it is known as the axillary vein. Trace it to the
elbow.

7. External Jugular Vein. — This unites with the sub-
clavian to form the innominate. It comes from the region of the
head. Several veins enter it.

8. Internal Jugular Vein. — This vein enters the external
jugular just anterior to the point of union of external jugular
and subclavian. It arises from the occipital region and passes
posteriorly along the side of the trachea in company with the

COMPARATIVE ANATOMY 59

common carotid artery and the vagus nerve. A large lymph
vessel, the thoracic duct, enters the external jugular near the
place of union with the internal jugular. This is sometimes
mistaken for a blood vessel.

9. Transverse Scapula Vein. — This vein returns blood
from the arm and shoulder region. It joins the external jugular
on the lateral side.

10. Anterior Facial and Posterior Facial Veins. — The
external jugular is formed by the union of these two vessels.
The posterior facial brings blood from the dorsal side of the
head, ear, masseter muscle, parotid gland, and adjacent regions.
The anterior facial collects blood from the face, tongue, jaws,
and nasal region. A transverse vein connects the anterior
facials of the two sides.

Turn now to the veins of the posterior part of the body.
Before dissecting them certain of the urogenital organs should
be located. If you have not already done so identify the kidneys,
large bean-shaped organs on the dorsal wall; adrenal glands,
small structures just anterior to the kidneys; urinary bladder in
the lower part of the abdomen; ureters, ducts from the kidneys
to the bladder; uterus, a branched structure of variable size in
the female; spermatic cord and vas deferens of the male. The vas
deferens loops over the ureter near where the latter joins the
bladder.

11. Femoral Vein. — In company with the femoral artery
and the saphenous nerve this vein extends across Scarpa's
triangle on the inner side of the thigh. The femoral vein, after
receiving several smaller veins, enters the body cavity and then
becomes known as the external iliac vein. Trace the femoral
backward down the leg, dissecting out the larger branches.

12. Hypogastric (internal iliac) Vein. — Bringing blood
from various pelvic organs the hypogastric unites with the exter-
nal iliac just inside the body cavity. Tracing it backward it
extends medially and dorsally.

13. Common Iliac Vein. — ^This is formed by the union of
the hypogastric and external iliac veins. The two common
iliac veins unite in the median line.

14. Inferior Vena Cava. — Formed by the union of the two
common iliac veins, it passes directly forward, through the liver.

6o LABORATORY DIRECTIONS

and enters the right auricle. A number of veins enter the
inferior vena cava along its course.

15. Lumbar Veins. — These are several small veins which
carry blood from the dorsal musculature to the vena cava. The
most cranial of the lumbar veins lies anterior to the diaphragm.

16. Ilio-lumbar Veins. — These are the pair of veins enter-
ing the vena cava just anterior to the union of the common
iliac veins. They come from the surface of the neighboring
muscles.

17. Internal Spermatic Vein {internal ovarian in females).
The right spermatic (ovarian) enters the inferior vena cava
about the level of the caudal end of the kidney. The left
spermatic (ovarian) usually enters the left renal vein. In males
the spermatic originates in the testis and passes through the
spermatic cord along with the vas deferens. In females, the
ovarian originates in the ovary and cranial end of the uterus and
passes medially to the vena cava or renal vein.

18. Renal Vein. — Each originates in the kidney as two
branches which fuse together, sometimes before leaving the
kidney, sometimes immediately after. The renals enter the
vena cava as large veins.

19. Adreno-lumbar Veins. — These veins pass over the sur-
face of the muscles in the neighborhood of the kidney and return
blood from the adrenal glands and enter the vena cava at about
the level of the cranial end of the kidney.

20. Hepatic Veins. — These are a variable number of short
veins carrying blood from the liver to the vena cava. They
enter the vena cava just caudal to the diaphragm. To see them
cut into the substance of the liver.

21. Portal System. — The portal vein, the main vein of this
system, breaks up into capillaries within the substance of the
liver. It is formed by the union of several veins leading from
the stomach and intestines. The largest of these are:

a. The superior mesenteric, originating from many branches
and bringing blood from the small and large intestines.

b. The inferior mesenteric, coming directly from the large
intestine.

c. The gastro-splenic, formed by several branches carrying
blood from the stomach and spleen. It unites near the pyloric

COMPARATIVE ANATOMY 6i

end of the stomach with the superior mesenteric to help form
the portal. Near this union the portal also receives several
other veins from the stomach, duodenum, pancreas, and great
omentum.

22. The Pulmonary Veins. — These were mentioned previ-
ously. They arise in the lungs and enter the left auricle.

Arteries of the Cat

1. Pulmonary Artery. — This was observed In the study
of the heart. Soon after leaving the right ventricle it divides
into two branches, one going to each lung.

2. Aorta. — The origin of this vessel from the left ventricle
has already been seen. It curves dorsally and to the left,
forming the aortic arch. It then passes caudal on the left side
of the vertebral column, pierces the diaphragm, and extends
almost the entire length of the abdomen, running more or less
parallel to the Inferior vena cava. According to the region it is
designated thoracic or abdominal aorta. Soon after leaving the
heart the aorta divides into two main trunks to supply the
anterior parts of the body,

3. Innominate Artery. — This Is the larger of the two main
branches of the aorta. A short distance from the heart the
Innominate gives off three large branches. The first of these
is the left common carotid and near It Is the right common carotid.
The two carotids run forward on either side of the trachea and
supply blood to the head and neck. The remaining part of the
innominate is continued as the right subclavian, which takes
blood to the right arm.

4. Left Subclavian Artery. — This is the other main
trunk of the thoracic aorta. It carries blood to the left arm.

It may be seen that the main arteries to the anterior part
of the body arise unsymmetrlcally from the aortic arch. There
are great variations In different species in this respect.

From here on follow the vessels on one side only.

5. Vertebral Artery.- — Arising from the subclavian and
leading directly to the vertebral column. It enters the foramen
transversarlum of the sixth cervical vertebra and passes to the
head. It supplies the muscles of the neck, and the spinal cord
and brain. Do not trace it beyond its entry into the backbone.

62 LABORATORY DIRECTIONS

6. Internal Mammary Artery. — This arises on the ven-
tral side of the subclavian at about the same level as the verte-
bral and runs caudally on the Inner surface of the sternum. It
is best seen when removing the thoracic body wall.

7. Axillary Artery. — Several other branches are given
off from the subclavian before it leaves the thoracic cavity and
becomes the axillary artery. The axillary supplies the arm and
muscles of the shoulder. Trace the main branches.

8. Internal and External Carotid Arteries. — Follow
the common carotid and observe that It passes to the head, giving
off small branches on the way. It eventually gives rise to the
internal carotid, which passes to the tympanic bulla and enters
the skull, and the external carotid, which divides into several
smaller branches supplying the lips, floor of the mouth, tongue
and portions of the face.

Turn again to the aorta and follow it posteriorly.

9. Intercostal Arteries. — There are about ten pairs of
these that leave the aorta after it reaches the dorsal side. They
supply the muscles of the back, Intercostal spaces, and the
spinal cord.

10. Lumbar Arteries. — These correspond to the inter-
costals in the lumbar region. There are several pairs of them.
The first two pairs originate In the thorax.

11. CoELiAC Artery. — This Is a single large branch from
the abdominal aorta arising near the point where the aorta
penetrates the diaphragm. It divides Into three branches.

a. The hepatic artery, arises nearest to the origin of the
coeliac and passes directly to the liver. Near the pylorus it
sends a branch to the stomach and Intestines.

b. The gastric leads to the stomach.

c. The splenic, the largest of the three branches, divides and
supplies the spleen and pancreas.

12. Superior Mesenteric Artery.— This arises from the
aorta just caudal to the coeliac and may be larger than the
coeliac. It sends branches to the small Intestine, pancreas, and
parts of the colon. The branches follow the mesenteries and
are paralleled by veins.

13. Adrenolumbar Artery. — ^Arising from the aorta, one
on either side about two centimeters caudal to the superior
mesenteric, each supplies the muscles of the dorsal body wall.

COMPARATIVE ANATOMY 63

14. Renal Arteries. — These arise from the aorta and pass
directly to the kidneys. Each usually divides before entering
the kidney.

15. Internal Spermatic Arteries (ovarian in the female).
These arise from the aorta posterior to the kidneys. In the
male each passes with the vas deferens to the testis and scrotum;
in the female the ovarian leads to the ovary and cranial end of
the uterus.

16. Inferior Mesenteric Artery. — ^This vessel arises
from the aorta at about the level of the last lumbar vertebra
and supplies the colon and rectum.

17. Ilio-lumbar Arteries. — These two arteries arise from
the aorta about two centimeters caudal to the inferior or mesen-
teric and pass to the iliopsoas muscles along the dorsal body
wall.

18. External Iliac Arteries. — The aorta divides and
forms the two external iliacs which go to the legs. After leaving
the body cavity they are known as femoral arteries.

19. Hypogastric Arteries. — These usually arise from a
continuation of the aorta, but may come from one of the external
iliacs. Their branches are very variable, but supply mainly
the urogenital organs, rectum, and certain pelvic muscles.

20. Sacralis Artery. — This is the continuation of the aorta
into the sacral and tail regions.

21. Profunda Femoris Artery.— Originating from the
femoral artery at the point where it leaves the abdominal cavity,
it divides into branches going to the tissues of the median surface
of the thigh and to the ventral wall of the abdomen. The
remainder of the femoral passes down the leg.

22. Femoral Artery. — The femoral is the large artery of
the leg. Trace the main branches.

Internal Anatomy of the Heart of the Cat

Remove the auricular appendages and the dorsal wall of
each auricle, except where the veins enter. Insert the point of
the scissors into the pulmonary artery and continue the cut
through the right ventricle. Bisect the left ventricle into dorsal
and ventral halves. Remove any clotted blood that may be
present. It should now be possible to clearly see the internal
structures of the heart.

64 LABORATORY DIRECTIONS

In the auricles locate the pulmonary veins and venae cavae.
Near the opening of the inferior vena cava is the entrance of the
coronary sinus. The inter auricular septum possesses a thin area,
th.t fossa ovalis, which may be felt by running a finger over the
septum. In fetal life there is an opening, the foramen ovale^
between the two auricles.

The wall of the right ventricle is distinctly thinner than that
of the left. The columns of muscle on the inner surfaces are
known as traheculae. The opening between right ventricle and
right auricle is guarded by the tricuspid valve, so named because
it consists of three membranous flaps, one dorsal, one ventral,
and one on the inter-ventricular septum. The valve between the
left auricle and left ventricle is the bicuspid or mitral valve.
Numerous strands of connective tissue, the chordae tendineae
join the flaps of the valves to the walls of the ventricles or to
papillary muscles (muscular projections of the walls). The
aorta leaves the left ventricle on the right side; the pulmonary
artery leaves the right ventricle on the left side. The entrances
to both aorta and pulmonary artery are guarded by semilunar
valves.

Study the structure and action of all of the valves of the
heart. Understand how they operate and at what phases of
the heart beat each valve is open and closed. If necessary, con-
sult physiology textbooks.

VI. THE UROGENITAL SYSTEM

In all vertebrates the organs of reproduction and those con-
cerned with the elimination of nitrogenous wastes are so closely
associated that it is convenient to study them together. Usu-
ally part of one system forms also part of the other.

The following directions apply to mature specimens. In
immature animals it is sometimes difhcult to locate all parts
of the urogenital system. After dissecting your own specimen
exchange with someone who has made a dissection of the oppo-
site sex. DO NOT TAKE SOMEONE ELSE'S SPECIMEN
WITHOUT THE OWNER'S PERMISSION.

Urogenital Organs of the Male Shark

The testes are the large bodies in the anterior part of the body
cavity dorsal to the liver. Each is attached to the dorsal body
wall by a mesentery, the mesorchium.

The kidneys are long, slender, brown bodies extending nearly
the length of the body cavity along the dorsal wall. The
posterior ends are thicker than the anterior and probably func-
tion more in excretion. The shark kidney is a mesonephros or
Wolffian body.

Remove the peritoneum from the surface of one kidney; the
long convoluted vas deferens or Wolffia7i duct Is now visible on
the ventral surface of it. Posteriorly the Wolffian duct straight-
ens out and expands to form a seminal vesicle. Anteriorly, small
tubules may be seen passing through the mesorchium from the
testis and entering the anterior part of the kidney. These are
the vasa efferentia^ which join the kidney tubules. The tubules
of the kidney then connect with the Wolffian duct.

At the posterior end of the kidney, near the median line and
partly embedded, is the ureter or accessory mesonephric duct. Do
not mistake the heavy ligament for this.

Cut open the cloaca and find the urogenital papilla and the
opening of the rectum. Open the urogenital papilla and find an

65

66 LABORATORY DIRECTIONS

expanded sac, the urogenital sinus, Into which the seminal
vesicle and ureter empty. Try to find the openings of these.

The elasmobranchs are peculiar in that there are two meso-
nephric ducts. In most animals with mesonephric kidneys a
single duct serves for the passage of both urine and sperm.

Urogenital Organs of the Female Shark

The ovaries are elongated bodies on each side of the anterior
part of the body cavity. Each is held to the dorsal body wall
by a fold of the peritoneum, known as the mesovarium. The
lobulated character of the ovaries is due to sac-like /o//2V/<?j, each
of which contains an egg. The size of the ovaries depends upon
the state of development of these eggs. The ovaries are not
connected with any duct, but the eggs when ripe simply break
through the ovarian wall into the body cavity.

On the inner surface of the ventral body wall, near the
anterior end of the liver, is a loose membrane with a longitudinal
slit. This slit is the ostium tubae, the common opening of the
two oviducts or Mullerian ducts. The oviducts pass dorsally,
one on either side of the liver, and then caudally. The posterior
swollen portion of each duct is the uterus. The ripe eggs that
have entered the body cavity from the ovary, pass through the
ostium tubae, enter the narrow part of the oviduct, and finally
reach the uterus. In viviparous sharks, like this species, the
young develop here. Between uterus and ostium there may be
found a small swelling, the shell gland. The Mullerian ducts
enter the cloaca close together and dorsal to the urinary papilla.

The kidneys of the shark are known as Wolffian bodies or
mesonephroi. They consist of a pair of elongated structures
extending almost the whole length of the body cavity, dorsal
to the peritoneum and near the median line. The posterior
part of each is wider and thicker than the anterior, and appar-
ently is the only part that functions as an excretory organ in
adults.

The ureter or accessory mesonephric duct runs along the
median edge of the posterior part of the kidney, somewhat
embedded in tissue. Beginning at the lateral edge of the kidney
remove the peritoneum to expose this duct. The ducts from

COMPARATIVE ANATOMY 67

the two sides open into the urinary sinus, which lies in the
urinary papilla in the cloaca. The Wolffian duct is difficult to
locate in females. It lies on the ventral side of the anterior
part of the kidney; in immature females it is directly beneath
the oviduct, and in mature adults along the attachment of the
mesentery supporting the oviduct. It empties with the Wolffian
duct into the urinary sinus of the urinary papilla.

Urogenital Organs of the Male Necturus

The testes are elongated and He dorsal to the small intestine.
Each is supported by a mesentery, the mesorchium. The kid-
neys or mesonephroi lie dorsal and posterior to the testes. The
anterior end may not be excretory in function. The Wolffian
or mesonephric ducts, much coiled, lie along the lateral borders of
the kidneys and serve for the passage of both the sperm and the
excretions of the kidneys. The vasa efferentia, tubes which
carry sperm from the testis to the kidney, may be seen as white
ducts in the mesorchium. The sperm pass from the vasa
efferentia through the tubules of the kidney to reach the Wolf-
fian duct. Trace the Wolffian ducts to the cloaca.

Slit open the cloaca and try to find the entrances of the
Wolffian ducts. Find also the entrance of the large intestine
and of the urinary bladder. How does urine get into the bladder ?

Urogenital Organs of the Female Necturus

The ovaries are sac-like structures containing the eggs.
Each is supported by a mesentery, the mesovarium. The large
white coiled tubes running the length of the body cavity, lateral
to the ovaries, are the oviducts. Trace the oviduct forward and
find the thin-walled expanded end, with an opening, the ostium,
leading into it. Posteriorly the oviducts join the cloaca.

The kidneys or mesonephroi lie dorsal to the ovaries and
median to the oviducts. The Wolffian or mesonephric ducts,
much smaller here than in the male, lie along the lateral borders
of the kidneys. To see where the mesonephric ducts and the
oviducts enter the cloaca, free the cloaca from the body wall.
Observe also the entrance of the urinary bladder and of the
intestine into the cloaca.

68 LABORATORY DIRECTIONS

Urogenital Organs of the Cat Common to Both Sexes

Kidneys. — These are paired, bean-shaped structures lying
against the dorsal body wall on the iliopsoas muscle in the
abdomen. The right kidney is somewhat anterior to the left.
As in all other vertebrates the kidneys lie outside the coelom
and are covered with peritoneum on the ventral side only.
Remove the peritoneum and any fat that may be covering the
kidney. The notch on the median side is known as the hilus.
Here may be found the renal artery coming from the dorsal
aorta to the kidney, the renal vein extending from the hilus to the
inferior vena cava, and the ureter.

The ureter is a white tube, sometimes deeply embedded in
fat, extending from the hilus of the kidney to the base of the
urinary bladder. In removing the fat and in tracing the ureter
be very careful not to cut any blood vessels or other ducts. In
male cats watch particularly for the vas deferens, which crosses
the ureter a short distance from its point of attachment to the
bladder. Urine escapes from the bladder to the outside by
means of the urethra.

Split one kidney into dorsal and ventral halves. Observe
that it is covered with a loose fibrous coat, the renal capsule,
which is continuous with the walls of the ureter. The kidney
may be seen to consist of an outer layer, the cortex, and an inner
layer, the medulla. Near the hilus the ureter expands into a
cavity, the pelvis, into which a papilla projects.

Adrenal Bodies. — These are two small ovoid bodies more
or less embedded in fat, one lying near the anterior end of each
kidney. They are endocrine glands and are not part of the
urogenital system.

With bone forceps cut through the pelvic girdle on either
side of the mid-ventral line and remove the central section of
bone. Work carefully so as not to injure the underlying tissues.
In the male be especially careful not to cut the spermatic cords,
which lie externally.

Reproductive Organs of the Male Cat

Testis and Vas Deferens. — The scrotum is the external
pouch of skin, ventral to the anus. It is divided internally

COMPARATIVE ANATOMY 69

into right and left halves, within each of which lies a testis.
Cut open the scrotum so as to expose a testis. If the mem-
branes covering the testis are removed, the epididymis, the first
part of the duct of the testis, may be seen on one side. It
appears as a flattened band, which passes posteriorly into the
vas deferens. The vas deferens together with the spermatic
artery and spermatic vein, lies within the spermatic cord, which
enters the body cavity through the ingui7ial caiial. Within the
body cavity the vas deferens leaves the spermatic artery and
vein, loops over the ureter, and with the vas deferens of the
other side, passes through the prostate gland and enters the
urethra.

Urethra and Penis. — The urethra extends from the base
of the urinary bladder to the outside, passing through the penis
along its ventral side. From the point where the vasa deferentia
enter it, the urethra serves for the passage of both urine and
spermatozoa, and is thus in reality a urogenital sinus. At the
base of the penis are a pair of Cowper^s glands which also pour
their secretions into the urethra. Examine the end of the penis
and observe the fold of integument forming the prepuce, which
ensheathes the roughened glans penis.

Reproductive Organs of the Female Cat

Ovaries. — These are small ovoid bodies lying just posterior
to the kidneys. Occasionally small grayish spots, the Graafian
follicles, may be seen on them; larger corpora lutea follicles, from
which eggs have been discharged, are sometimes present also.
As in other vertebrates, the ovary has no direct connection with
any duct; it does, however, lie in close proximity to the mouth
of the oviduct.

Genital Ducts. — Each oviduct (usually called Fallopian
tube in mammals) begins with an expanded opening, the ostium
tubae, lying lateral to the ovary. From the ostium the oviduct
passes in a curve around the ovary and then expands into the
uterus.

The uterus is continuous with the oviducts and consists of
two long tube-like cornua (horns) which meet near the base of
the bladder to form the body of the uterus. The young develop
in the cornua and not in the body. In pregnant cats the

70 LABORATORY DIRECTIONS

cornua may be greatly swollen. The posterior end of the uterus
is telescoped into the vagina, forming a slight swelling where
the uterus and vagina overlap. The telescoped portion of the
uterus is known as the cervix uteri (meaning neck of the uterus) ;
the opening between uterus and vagina is the os uteri (meaning
mouth of the uterus). These structures may be seen by splitting
open the vagina and the body of the uterus.

The vagina extends posteriorly until it is joined by the
urethra from the bladder to form the urogenital sinus, a small
space common to both the excretory and the reproductive
systems. The external opening of the urogenital sinus lies
ventral to the anus and is known as the vulva. Cut open the
urogenital sinus and find the opening of the urethra.

VII. THE NERVOUS SYSTEM

The nervous system is composed of four parts: (i) the ce7itral
nervous system, composed of brain and spinal cord; (2) the
-peripheral nervous system, consisting of the cranial and spinal
nerves; (3) the sympathetic nervous system, which regulates
involuntary activities; (4) the sense organs. The brain, cranial
nerves, and certain of the sense organs will be dissected.

SENSE ORGANS
Ear of the Shark

Locate again the openings of the endolymphatic ducts on the
dorsal side of the head between the spiracles. It will be remem-
bered from the study of the shark skull that the endolymphatic
ducts connect with the inner ear. Remove the skin from the
head except for a small patch around the openings of the
endolymphatic ducts. This patch will aid as a landmark
in locating the position of ear structures. Clean away the
muscles and connective tissue of one side until the cartilage of
the skull is exposed in the region posterior to the eyes.

Examine a prepared skull of a shark and locate the position
of the endolymphatic fossa and the semicircular canals of the
ear. Turn again to your own specimen and carefully shave
off the cartilage between the endolymphatic ducts and the
spiracle. Be careful not to injure the brain which lies in
the mid-dorsal region, or the nerves that penetrate the cartilage.

Watch for the appearance of the semi-circular canals through
the cartilage. The first canals to be seen will be the anterior
vertical and the posterior vertical canals. Leave them in place,
but dissect the cartilage from around them. Do not injure
the structures at the ends of the canals. Continue the dissec-
tion, watching for the horizontal canal, which lies lateral to and
deeper than the others. The three semi-circular canals connect
with a thin-walled sac, the vestibule, lying beneath them. Leave
the organs embedded in the cranium, but continue remov^al

71

72 LABORATORY DIRECTIONS

of cartilage until all parts are exposed. A good dissection of
the shark ear requires patience and technique.

Both ends of each semi-circular canal join the vestibule.
On one end of each is a small bulb, the ampulla. Observe
that the semi-circular canals are at right angles to one another.
The vestibule is divided into two regions which are not easily
distinguished. The larger dorsal part is the utriculus. The
semi-circular canals arise from this. The ventral portion is the
sacculus to which the endolymph duct is joined.

The ear of the shark, as in other vertebrates, functions for
the reception of sound vibrations and as an organ of equilibrium.
In fishes the ear consists of an inner ear only; there is no middle
ear or external ear such as may be found in higher forms.

Eye of the Shark

Muscles of the Eye. — Remove the tissues from around
the eye of the shark on the same side of the head as the dissection
of the ear. Cut away the cartilage from above the eye, being
careful not to injure the brain or the nerves. If the dissection is
carefully made, six slender muscles that function in moving
the eyeball in various directions may be seen. Four of these
muscles originate from the posterior part of the orbit. These
are the rectus muscles. The two originating from the anterior
part of the orbit are the oblique muscles. The names of the
individual muscles indicate their position.

a. The superior oblique is the more anterior of the two
muscles inserting on the dorsal side of the eye. It is supplied
by the fourth cranial nerve.

b. The superior rectus inserts near the superior oblique on
the dorsal side of the eye. It is supplied by the third cranial
nerve.

c. The inferior oblique Inserts on the ventral side of the eye
and may be seen by moving the eye backward and medially,
or by cutting the superior oblique. It is innervated by the third
cranial nerve.

d. The external rectus Is Inserted on the posterior surface
of the eyeball. It Is supplied by the sixth cranial nerve.

e. The internal rectus Inserts on the anterior end of the eye-
ball and Is Innervated by the third cranial nerve.

COMPARATIVE ANATOMY 73

f. The inferior rectus inserts on the ventral side of the eye.
It may be seen by cutting the other muscles at their insertions.
It is innervated by the third nerve.

On the median side of the eyeball observe the large optic
nerve. Also, in the posterior corner of the orbit among the
rectus muscles, find the optic pedicel^ a cartilaginous stalk for
supporting the eyeball.

Anatomy of the Eyeball. — Remove the eyeball from
the orbit, cutting any tissues that may hold it.

The outermost covering of the eyeball is the sclerotic coat,
composed of very tough connective tissue. The muscles of
the eye are attached to it. A transparent continuation of the
sclerotic coat, the cornea, extends over the front of the eye.
The pupil is the opening that may be seen through the
cornea.

Remove the dorsal wall of the eyeball and observe the struc-
tures within. The large cavity in the posterior part is filled
during life with a fluid, the vitreous humor. The pigmented
layer just within the sclerotic coat is the choroid coat, and the thin
white membrane within this is the retina, the sensory part of
the eye.

The lens is held in place by a suspensory ligament attached
to the choroid coat. The iris, a pigmented extension of the
choroid coat, surrounds the pupil in front of the lens. The iris
controls the amount of light entering the eye by regulating
the size of the pupil. The cavity between iris and cornea is
filled during life with a fluid, the aqueous humor. The aqueous
humor and vitreous humor aid the lens in bending the light
rays so that they focus on the retina.

The structure of the eye is fundamentally the same in all
vertebrates. Examine the model of the human eye for compari-
son with the eye of the shark.

Middle Ear of an Amphibian

In the frog the ear consists of two regions, an inner ear and a
middle ear. In a demonstration dissection of the middle ear
observe the columella, a single bone extending from the tympa-
num across the cavity of the middle ear. What is the evolu-
tionary origin of this bone .^ The cavity of the middle ear and

74 LABORATORY DIRECTIONS

the Eustachian tube, connecting with the mouth cavity, are
homologous to the spiracle of the shark.

Mammalian Ear

Study charts and models of the human ear and be able to
identify the various parts. Notice particularly the three bones
of the middle ear (malleus, incus, and stapes) and the cochlea of
the inner ear. From what are the three bones of the middle
ear derived ? Be sure that you understand how the ear
functions.

BRAIN AND CRANIAL NERVES
Brain of the Shark — Dorsal Surface

Slice away the cartilage of the roof of the cranium so as to
expose the brain. Nerves will become visible as dissection
proceeds. Try not to cut them. They will be identified
later.

The brain and spinal cord are covered by a thin membrane,
the -primitive meninx, which is comparable to the meninges
(three membranes) of mammals.

The large anterior part of the brain is the telencephalon. It
is composed of indistinctly paired cerebral hemispheres, and in
front of these, large swellings, the olfactory lobes. Extending
forward from the olfactory lobes are short stalks, the olfactory
tracts, which end in olfactory bulbs. Short olfactory nerves
extend from the olfactory bulbs to the nasal capsules. The
olfactory lobes, tracts, and bulbs constitute the rhinencephalon.

Posterior to the cerebral hemispheres there is a narrow
depressed region, the diencephalon. A thin layer of blood
vessels, known as the choroid plexus, covers the third ventricle.
The epiphysis, a slender thread-like structure, arises from this
part of the brain. It was probably destroyed in removing
the roof of the cranium.

The mesencephalon or midbrain lies behind the diencephalon.
It consists mainly of two large optic lobes.

The cerebellum or mete7icephalo7i lies posterior to the optic
lobes and somewhat overlaps them. Shallow cross lines give
the cerebellum the appearance of being divided into four
parts.

COMPARATIVE ANATOMY 75

The part of the brain remaining posterior to the cerebellum
is the medulla oblongata or myelencephalon. The medulla is
continuous posteriorly with the spinal cord. If the thin covering
of the medulla, the choroid plexus, is removed, a large triangular
depression is visible. This is the fourth ventricle. The lateral
extensions of the medulla at its anterior end are the restiform
bodies.

Cranial Nerves of the Shark
The ten pairs of cranial nerves of the dogfish need be dis-
sected on one side only. They are numbered, beginning
anteriorly.

I. Olfactory. — The olfactory nerve consists of a bundle
of short fibers extending from the olfactory bulb to the nasal
capsule.' It is a sensory nerve.

II. Optic. — The optic nerve arises from nerve endings in
the retina of the eye and extends to the ventral side of the
diencephalon, where it crosses the nerve from the other side.
This relation will be better understood after the ventral surface
of the brain is studied. It is a sensory nerve.

III. Oculomotor. — This nerve arises from the mesen-
cephalon on the ventral side. It may be seen as a slender thread
entering the orbit back of the optic nerve. It is a motor nerve
and supplies four of the eye muscles, namely, the internal,
inferior, and superior rectus, and the inferior oblique.

IV. Trochlear. — This begins on the dorsal side of the
brain behind the optic lobes, and beneath the anterior part of
the cerebellum. It is very slender. It passes through the wall
of the orbit dorsal to the optic nerve. It is motor in function
and supplies the superior oblique muscle of the eye.

VI. Abducens. — The sixth nerve is best studied before the
fifth. This is a slender nerve arising from near the mid-ventral
line of the medulla. It is a motor nerve and supplies the
external rectus muscle of the eye.

V. Trigeminal. — The fifth, seventh, and eighth nerves
arise very close together from the side of the medulla. The
separate roots of these three nerves are difficult to distinguish.
The trigeminal enters the orbit and immediately divides into
several branches. It is both motor and sensory.

76 LABORATORY DIRECTIONS

a. The superficial opthalmic branch of the trigeminal Is
joined by a similar branch of the seventh nerve to form a large
trunk. It passes anteriorly on the dorsal part of the median
wall of the orbit, penetrates the cartilage of the skull, and Is
distributed to the dorsal side of the head.

b. The infraorbital nerve, made up of the maxillary branch
of the trigeminal and the buccal branch of the seventh, Is a
large white band passing under the eye on the floor of the orbit.
It ends In several branches. The maxillary fibers Innervate
the skin and the ampullae on the ventral side of the head.

c. The mandibular branch arises ventral to the Infraorbital
and passes along the posterior wall of the orbit to supply muscles
of the lower jaw and gill arches, and also to innervate the sense
organs of the skin.

VII. Facial. — This nerve Is both motor and sensory. Its
origin with the fifth nerve has already been mentioned. It
divides Into three branches.

a. The superficial opthalmic is bound in the same sheath
with the similar branch of the fifth, and with it, goes to the sense
organs of the head, supplying part of the lateral line organs.

b. The buccal branch of the seventh and the maxillary branch
of the trigeminal compose the infraorbital trunk, as has already
been mentioned. The buccal also supplies part of the lateral
line organs.

c. The hyomandibular hra.nch.lies just under the skin posterior
to the spiracle. Trace It back toward the brain. It innervates
the sense organs of the head, the muscles of the hyold arch, the
floor of the mouth and the tongue.

VIII. Auditory. — This arises with the fifth and seventh
and supplies the ear. It Is a sensory nerve.

IX. Glossopharyngeal. — The ninth nerve arises from
the medulla posterior to the eighth and leaves the skull at the
posterior-lateral corner. It passes through the auditory capsule
and then divides and goes to the gill region. It Is both motor
and sensory.

X. Vagus. — This nerve originates from the side of the
medulla, posterior to the ninth, by several roots. It Is a large
nerve and widely distributed. After passing through the
posterior wall of the skull it divides Into many branches to

COMPARATIVE ANATOMY TJ

supply part of the lateral line system, gill region, oesophagus,
stomach, and other viscera. It is both motor and sensory in
function.

Brain of the Shark — Ventral Surface

After demonstrating your dissection to the instructor, cut
the cranial nerves and remove the brain from the cranium.
Work carefully so as not to injure the ventral surface.

Identify as many as possible of the structures mentioned
for the dorsal surface.

The optic chiasma is the place where the optic nerves cross
on the ventral side of the diencephalon. The optic tracts extend
posteriorly and dorsally from the optic chiasma and lead to
the optic lobes.

The infundihulum is the large ventral bulge of the dien-
cephalon posterior to the optic chiasma. It consists largely
of two lobes, the lobi inferiores.

The pituitary body may be partly destroyed in the dissection,
as It fits into a depression in the floor of the skull. It extends
caudally from between the lobi inferiores.

Ventricles of the Shark Brain
With a sharp knife or a razor blade remove the dorsal side
of the brain so as to expose the cavities within. The cavities
of the telencephalon are the first and second, or lateral ventricles.
They extend forward into the olfactory bulbs. Posteriorly they
connect by a passage, the foramen of Monro, with the third
ventricle, which is the cavity of the diencephalon. The roof
of the third ventricle is the choroid plexus. The large triangular
cavity in the medulla is the fourth ventricle, previously men-
tioned. It is also covered by a choroid plexus. A narrow
passage, the iter, joins the third and fourth ventricles. The
iter connects with the optic ventricles of the optic lobes and the
cerebellar ventricle of the cereb-ellum.

Brain of the Cat

Remove the eyes and lower jaw and clean the skull down to
the bone. Detach the head, together with a few neck vertebrae,
from the rest of the body. With the bone forceps make a

78 LABORATORY DIRECTIONS

small hole in the skull and pick off the bones piece by piece
from the roof and sides of the head so as to expose the brain
and beginning of the spinal cord. On the ventral side of the
skull cut the membranes around the nerves. The pituitary
body of the brain fits into a depression, the sella turcica, of
the sphenoid bone. If possible cut the sphenoid so as to leave
the pituitary attached to the brain. Keep the brain in a jar
of formalin when not studying it.

Meninges. — These are membranes covering the brain
and spinal cord. There are three in mammals. The outermost
membrane is the dura mater, which is quite thick and tough.
The pia mater is the delicate inner membrane that follows closely
all the folds of the brain surface. Between these two membranes
is a third, the arachnoid, very delicate and hard to see. The
spaces between the membranes are filled with fluids during
life. Amphibians, reptiles, and birds do not have the arachnoid,
while fishes have only one membrane, the primitive meninx.

Dorsal Surface of the Brain. — Remove the dura mater.
The olfactory bulbs appear as relatively small swellings at the
anterior end of the brain. The olfactory nerves going to the
nostrils pass forward from them. The bulbs may have been
broken in removing the brain from the skull.

The large convoluted cerebral hemispheres lie posterior
to the olfactory bulbs and make up a large proportion of the
brain. The convolutions are known as gyri, and the grooves
between the gyri as sulci. The two hemispheres are separated
from each other by a deep longitudinal fissure. By gently
spreading the hemispheres a thick white mass may be seen
at the bottom of the fissure. This is the corpus callosum,
consisting of nerve tracts connecting the two hemispheres.
It will be mentioned again later.

The cerebellum lies directly behind the cerebral hemispheres,
consisting of a median part, the vermis and two lateral hemi-
spheres. Between the cerebellum and the cerebral hemispheres
lies the mesencephalon, concealed by the hemispheres.

The cerebellum overhangs the medulla oblongata posteriorly.
Beneath the vermis of the cerebellum Is the cavity of the fourth
ventricle of the medulla. The fourth ventricle is normally
covered by a vascular membrane, the choroid plexus. The
medulla narrows posteriorly to form the spinal cord.

COMPARATIVE ANATOMY 79

Ventral Surface of the Brain. — Extending posteriorly
from the olfactory bulbs to the middle of the cerebral hemi-
spheres are white olfactory tracts. These end in the pyriform
lobes of the cerebral hemispheres.

In the mid-ventral line is the optic chiasma where the optic
nerves cross. The optic tracts extend back from the chiasma.

The region behind the optic chiasma is the diencephalon.
The median rounded elevation is the tuber cinereum, to which
is attached the pituitary body. If a small aperature shows
in the tuber cinereum, the pituitary is not present; it was
probably torn off in removing the brain. Two small mammillary
bodies, not distinctly separated from the tuber cinereum, lie
behind the pituitary. Just posterior to these, near the median
line, are the roots of the third or oculomotor nerve. Lateral
to the third nerve roots and the mammillary bodies, and partly
covered by the pyriform lobes, are the cerebral peduncles. These
are bundles of fibers connecting the cerebral hemispheres
with the rest of the brain and the spinal cord. The fourth or
trochlear nerves extend as slender threads across the peduncles.

The transverse band of fibers posterior to the peduncles is
the pons Varolii. At the posterior borders of the pons arise the
fifth or trigeminal nerves.

The medulla oblongata occupies the region posterior to the
pons. A median ventral fissure extends its entire length and
continues down the spinal cord. The longitudinal bands of
fibers on either side of the ventral fissure are the pyramids.
Several cranial nerves arise from the medulla. The sixth or
abducens arises from the pyramids near the median line. The
seventh or facial nerve roots are just posterior to the fifth. The
roots of the eighth or auditory nerve are posterior to the seventh.
The ninth or glossopharyngeal and the tenth or vagus arise
near together by several rootlets. The numerous roots of the
eleventh or spinal accessory arise in a line posterior to the vagus.
The roots of the twelfth or hypoglossal emerge from the lateral
borders of the pyramids.

Brain Bisected. — Cut along the longitudinal fissure be-
tween the cerebral hemispheres so as to divide the brain into
right and left halves. Examine the median surface of one half.

The Commissures. — These are bundles of fibers connecting
the two sides of the brain. Find again the corpus callosum.

8o LABORATORY DIRECTIONS

located at the base of the cerebral hemispheres. This is the
largest commissure of the brain. The anterior and posterior
ends are slightly swollen. A band of fibers, the fornix, extends
ventrally from about the middle of the corpus callosum. The
anterior commissure is a small cord of fibers cranial to the ventral
end of the fornix. The middle commissure is a large bundle of
fibers posterior to the fornix. It is located in the middle of
the third ventricle and is part of the thalamus, to be mentioned
later. The posterior commissure is a small group of fibers
posterior to the middle commissure.

The ventricles of the brain of the cat occupy positions similar
to those of other vertebrates. The lateral ventricles are small
cavities in the cerebral hemispheres. They may be found by
making a longitudinal slit through the corpus callosum and
lifting up the cerebral lobes. Each lateral ventricle connects
by a foramen of Monro to the third ventricle, which surrounds
the middle commissure and extends into the pituitary gland.
The iter is a narrow passage under the cerebellum, connect-
ing the third and fourth ventricles. The fourth ventricle is the
cavity of the medulla and is continuous with the canal of the
spinal cord.

Anterior to the cerebellum and dorsal to the iter are two
rounded swellings in each half of the brain. These are the
corpora quadrigemina, corresponding to the optic lobes of lower
vertebrates. The cerebral peduncles form the floor of the iter.

The branched white tracts in the cerebellum constitute
the arbor vitae.

Remove the dorsal wall of the cerebral hemisphere of one
half of the brain. In doing so observe that the outside of the
brain is gray and the inside white. The gray layer is the cortex.
Identify again the corpora quadrigemina. The prominent
elevation anterior to these is the thalamus, a large ganglion.
Anterior to the thalamus is another ganglion, the corpus striatum,
which forms part of the floor of the lateral ventricle.

Examine the series of vertebrate brain models and identify
the principal structures on each. Observe the marked differ-
ences in some of the parts.

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