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A GUIDE FOR THE DISSECTION

OF THE DOGFISH
(SQUALUS ACANTHIAS)

By

LAWRENCE E. GRIFFIN
Professor of Biology in Reed College

THIRD EDITION

Portland, Oregon
1922

Copyright, 1922

by
LAWRENCE E. GRIFFIN

BiOi
LIB -
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A Guide for the Dissection
of the Dogfish

The small sharks which abound along the coasts of the United
States are commonly called "dogfish" by fishermen and others. The
"dogfish" of inland waters belongs to an entirely different group.
Two species of sharks are caught in numbers and used in laboratory
work, the "spiny dogfish" (Squalus acanthias) and the "smooth
dogfish" (Eugaleus galeus). The first' is easily distinguished by
the sharp spine in front of each dorsal fin. Squalus acanthias is
often referred to under the synonym Acanthias vulgaris, while Eu-
galeus galeus is more frequently named either Mustelus canis or
Galeus canis. The histories of these names and systematic de-
scriptions of the species are contained in Samuel Carman's Mono-
graph on the Elasmobranchs.

Several sizes of dogfish are furnished by dealers. We consider
it best to purchase large, fully developed specimens. The small
specimens may be a little more convenient to handle, but the large
ones have the important advantage of being sexually mature, while
blood vessels and nerves are dissected better in large than in small
specimens. Also, a number of structures are very different in ma-
ture animals from their condition in young ones. Dealers should
be requested to furnish fish with fins and tails complete instead of
trimmed. It is an advantage to issue to the class equal numbers of
both sexes.

An entire specimen and an extra head may be required by each
student for a thorough dissection. It would be better if the head
were cut off just behind the pectoral fins, instead of in front of them
as is usually done, so that the vagus and hypobranchial nerves may
be dissected more completely.

The spiny dogfish, which is the particular subject of this guide,
is the species most frequently supplied to laboratories. However,
the spiny and smooth dogfishes are so much alike that the latter
may be easily dissected with these directions. Where marked differ-
ences between the forms exist the structure of Eugaleus is described
separately.

The student of anatomy should realize that dissection is for the
purpose of enabling him to see for himself the structures which
exist, and that no dissection is satisfactory until the anatomical
arrangements mentioned in the text can be completely demonstrated
in his specimen.

The importance of knowing the structure of the elasmobranch
is so great in comparative anatomy and embryology that it is worth
while to make a thorough dissection of one of this class. As the
dogfish is frequently the first major vertebrate form to be studied
in detail, these directions have been written to conform to the needs
of the student who is beginning comparative anatomy. The arrange-
ment of sections in this guide is intended to permit the omission of
some which it may not be considered desirable to include in the
work of a class.

48347

EXTERNAL CHARACTERS

The spindle-shaped body tapers from near the middle toward
both head and tail; the head is flattened on both the dorsal and
ventral sides, while the remainder of the body is nearly round, with
a lateral compression which is not pronounced except in the caudal
portion.

The general color of the back and sides is gray; darkest above,
where the skin is spotted with scattered, small, round, light spots.
The color of the upper parts shades into the yellow white of the
ventral surface.

Can head, trunk, and tail regions be distinguished? If so, what
characters determine the extent of each?

A little above the middle of the side of the body is the lateral
line, (distinguished partly by color, partly by being slightly ele-
vated), which extends from the back of the head to the tail. Cut
through the skin across the lateral line at several points along the
body and notice the canal which lies in the dermis under the lateral
line. This is the lateral line canal, which opens to the surface by
numerous pores (too minute to be seen), and contains a series of
special sense organs along its dorsal and inner surfaces. Near the
base of the caudal fin the lateral line canal passes into a groove
which continues the lateral line to within a short distance of the
edge of the fin. The lateral line canal in its development begins
as a groove along the side of the body which becomes closed by the
fusion of its edge except in this terminal portion.

The open groove does not appear in Eugaleus.

In the midline of the body are two triangular dorsal fins, each
attached to the body for about half its length. The basal portion
of each is thick and muscular, and contains supporting cartilages
embedded in the muscles. The remainder of the fin is flexible and
semi-transparent, horny finrays being faintly visible between the
layers of skin. In front of each fin is a strong spine which seems
to serve both as a cutwater and a weapon of defense. (Eugaleus
has no spines.)

The broad, paired, pectoral fins, having the general characteris-
tics of the dorsals, spring from the ventral edges of the body just
back of the head. A hard bar of cartilage connecting the bases of
the pectoral fins can be felt through the skin of the ventral surface
of the body. This is the ventral part of the pectoral girdle.

Farther back, and also on the ventral surface, are the paired
pelvic fins. The pelvic girdle can be felt through the skin between
the bases of these fins. If the specimen is a male, it will have a
fingerlike process projecting backward from the base and along the
inner side of each pelvic fin. These organs, which attain a consid-
erable size in adults, are modified portions of the pelvic fin used as
copulatory organs. They are named variously claspers, myxop-
terygia, or pterygopodia. A groove runs along the dorsal side of
the clasper from the tip to near the base, where it opens into a long
sac (glandula pterygopodia) extending some distance in front of
the pelvic fins just within the skin of the ventral surface. By feeling
a cartilaginous axis of the clasper can be discovered, which extends
to the tip of the organ. On the dorsal surface and near the end of
the clasper is a sharp grooved spine on the outer side, and a strongly
recurved hook on the inner side, both almost hidden by a flap pro-
jecting from the inner edge of the groove. This flap is stiffened by
a series of small cartilages similar to the radial cartilages in the
bases of the fins.

The caudal fin is asymmetrical, extending along the dorsal and
ventral edges of the posterior end of the body. Observe the upward
bend of the vertebral column which occurs in the tail; it is this
character which marks the heterocercal type of tail.

Eugaleus has a median ventral, or anal, fin a short distance anterior to the
caudal fin.

The mouth is a broad transverse slit upon the ventral surface
of the head. The cartilaginous jaws can be seen and felt just within
the mouth. Both upper and lower jaws are armed with rows of flat,
sharply pointed teeth. Study their arrangement and approximate
number. Jaws of previously dissected specimens should be exam-
ined under a dissection miscroscope. The exact form and arrange-
ment of the functional teeth can then be ascertained easily, and an
examination of the inner surface of either jaw will disclose several
rows of developing teeth. As the young teeth develop they move, a
row at a time, into position on the edge of the jaw; the oldest teeth,
occupying the outer row, are shed at about the same time.

The upper jaw is partly overhung by a lip-like fold of skin. At
each side of the mouth is a pocket, directed obliquely, having no
communication with the mouth. These labial pockets provide places
for the labial cartilages (which can be felt along the medial edges)
when the mouth is closed, and also afford freedom of motion to the
mandible. Cut along the inner edge of the labial pocket and ex-
pose the cartilages for examination. (The labial pocket of Euga-
leus is much smaller, and in front of the corner of die mouth rather
than lateral to it. The two cartilages are completely separated from
each other, the posterior one scarcely reaching the pocket.)

In front of the mouth are the nostrils, their apertures apparently
divided by projecting flaps of the anterior margin. Explore the
cavity of the nostril with a probe to get a good idea of its size and
form.

Between the pelvic fins is the cloaca, a large depression into
which open the alimentary canal, the excretory and genital ducts,
and the abdominal pores. The opening of the alimentary canal, the
anus, is at the anterior end of the cloaca. In preserved specimens
part of the intestine is frequently everted through the anus. A large
fleshy process, bearing a pore at its tip, projects from the dorsal wall
of the cloaca. In the male this is the uro genital papilla; in the
female the urinary papilla. In the female a genital pore, the open-
ing of the oviduct, is found on either side of the papilla. An
abdominal pore, leading into the abdominal cavity, is found on
each side of the cloaca at the posterior margin. These are frequently
closed in young specimens.

The cloaca of Eugaleus has a comparatively small opening upon the ventral
surface, which must be enlarged before the parts described can be seen well.

The laterally placed eyes are without lids; observe the consider-
able difference in the amount of curvature of the dorsal and ventral
margins of the eye.

In Eugaleus there is a fold of skin stretched across the lower part of the
eye which serves as an eye-lid, and corresponds to the so-called "third eye-lid"
or nictitating membrane of other vertebrates.

On each side of the neck are five vertical gill-clefts, each leading
into a large gill-pouch which communicates with the pharynx by an
internal opening. Pass a probe through a gill-cleft into the mouth.

Back of each eye is a small aperture, the spiracle; explore this
cavity with a probe. The spiracle is to be considered a gill-cleft
moved forward upon the head and largely, though not entirely, de-
prived of its respiratory functions.

In the center of the dorsal surface of the head, between the

4

spiracles, are two pores, the external openings of the endolymphatic
ducts which communicate with the internal ear. Large numbers of
smaller pores can be found on all surfaces of the head, some in
groups, some arranged linearly, many scattered. Most of those
arranged in lines lead into the sensory canal system which continues
from the lateral line canal upon the head, while the majority of
the others belong to a separate type of sense organs, the ampullae
of Lorenzini.

Make a cut encircling the pores of the endolymphatic ducts and
close to them. Do not remove this piece of skin. From it make a
median incision forward to the tip of the snout and back as far as
the level of the first gill slit. Starting at this incision work the
skin off from the tissues beneath it. This must be a careful, close
dissection. When the lateral line is reached it will be seen that the
lateral line canal is continued upon the head and is joined by sev-
eral others. By looking through the loosened skin toward the light
the pores can be seen which lead from the canals to the surface. In
the hollow of the skull in front of and above the eye is a large
group of tubules which open through the pores so conspicuous at
this point. At the internal end of each tubule is a slight enlarge-
ment, of denser tissue, with which a delicate nerve strand can often
be seen connected. The nerve strands can be traced to a large
nerve passing above the eye and distributed to the snout. These
tubules are the ampullae of Lorenzini. This group of ampullae,
which may be called the dorsal group, is quite definitely demarked.
It will be noted that the inner ends of the ampullae are grouped in
a much smaller area than their pores.

Between the spiracle and the first gill slit will be found a lateral
group of similar organs. Notice the arrangement of their tubules
and pores. Under the snout are two groups of ampullae on each
side of the midline. The inner ventral group is separated from the
outer ventral group by the lateral bar of the rostral cartilage. Some
of the tubules of the outer ventral group will be found to extend to
pores situated at the sides of and behind the mouth.

An adult fish possesses from 1200 to 1900 ampullae of Loren-
zini. Their function is not well understood, but it has been sug-
gested that they are organs responsive to stimuli of pressure, either
of currents or water, or resulting from depth, or even of deep
tones.

The system of sensory canals consists of the following members
on each side of the head: —

A supraorbitaly passing above the eye to the end of the snout
and bending back on the ventral surface to join the infra-orbital.

An infra-orbital., which branches off from the supra-orbital and
passes ventrad between the eye and the spiracle, then turns forward
along the ventral margin of the orbit, and finally bends toward the
mid-line and extends to the tip of the snout.

A hyomandibular, which leaves the infra-orbital below the eye
and runs back beyond the angle of the mouth.

A short mandibular, on the mandible close to the angle of the
mouth, which is not connected with the other canals.

The canal systems of the right and left sides are connected by a
supra-temporal canal just behind the endolymphatic pores, and
often by an anastomosis of the infra-orbital canals in front of the
mouth.

On some specimens two crescentic rows of pores can be found
between the bases of the pectoral fins, which represent a third type

of sensory organ, the pit organs, closely related genetically to the
sensory canals. Two longer lines of pit organs, (the mandibular
pit organs), will be found a short distance behind the mouth. Sim-
ilar pit organs are found in front of the endolymphatic pores, and
above the anterior part of the lateral line.

Except for a few small areas the entire surface of the body is
covered with small, sharp-pointed denticles (placoid scales) . Each
consists of a diamond-shaped basal plate embedded in the dermis,
from which projects a leaf -like, backward directed spine. A piece
of skin should be removed and examined under a low magnifica-
tion. The dermis is so dense and pigmented that the basal plate is
not easily studied without further manipulation. For this purpose
boil a piece of skin in 5% caustic potash solution until it is softened,
but not till it disintegrates. Then clear it in glycerine. Examine
the individual denticles under a higher magnification. The den-
ticle consists of dentine, the spine being of a much denser structure
than the base. The teeth and the large spines of the fins and claspers
are also composed of dentine and may be considered as modified
placoid scales. Denticles, teeth, and spines are covered with a
shiny, enamel-like layer which, however, does not appear to be true
enamel such as covers the teeth of higher vertebrates. The shape
of the scales and their closeness vary on different regions of the
body, and there are certain regions entirely free from them, namely,
back of the dorsal, pectoral and pelvic fins, the medio-dorsal sur-
faces of the claspers, inside the upper lip and the labial pockets.

DISSECTION OF THE ABDOMINAL VISCERA

Place the dogfish on its back and, commencing at the middle of
the abdomen, make an incision through the body wall a quarter of
an inch to one side of the midline. Carry this forward to the pec-
toral girdle and backward through the pelvic girdle to the cloaca;
not, however, cutting the wall of the cloaca.

A large vein, the lateral vein, runs along the inner surface of
each lateral wall of the abdomen. After identifying these, cut
through the body wall transversely on both sides of the abdomen at
the level of the posterior attachment of the pectoral fin as far as
the lateral vein. Turn the flaps outward and fasten.

The coelom or body-cavity consists of two portions, the abdom-
inal and pericardial cavities. The abdominal cavity, which has now
been opened, extends from the pectoral girdle to the cloaca and
along the sides of the latter; it communicates with the exterior
through the abdominal pores on either side of the cloaca. Pass a
bristle or probe through each abdominal pore into the cloaca.

Without dissecting, identify the following parts and observe
their relations:

The peritoneum, the smooth lining of the body-wall, which is
reflected over the viscera.

The liver, a large, gray organ attached anteriorly and almost
completely divided into two lobes which extend well back along
the sides of the abdominal cavity.

The stomach, lying between the lobes of the liver. Its posterior
end is bent forward upon itself in the form of a U. The two limbs
of the stomach are known as the cardiac (proximal) and pyloric
(distal), respectively. With a second turn to the right and back-
ward it enters the intestine.

The intestine, a large, thin-walled tube extending from the
stomach to the cloaca.

6

The spleen, a dark, triangular mass attached to the posterior
border of the curve of the stomach.

The spleen of Eugaleus is a long, slender body extending from the middle
of the proximal limb of the stomach around the posterior end of that organ
and forward again along the distal limb for two-thirds of the length of the
latter.

The pancreas, a firm white mass the larger part of which lies
dorsal to the posterior end of the stomach. One extremity lies on
the ventral surface of the junction of the stomach and intestine.

The reproductive glands, (ovaries or testes), lying on either side
of the midline dorsal to the anterior portion of the liver; they may
be mistaken for small lobes of the liver.

The reproductive glands of Eugaleus are long bodies lying above the stomach
and intestine. They are fused to each other for almost their entire length.

The kidneys, two long, slender, brownish bodies extending along
the dorsal wall of the abdominal cavity outside the peritoneum, on
either side of the midline.

The dogfish usually furnished for dissection are immature, hav-
ing the genital glands and ducts only partly developed. In ma-
ture females the oviducts are conspicuous tubes ventral to the kid-
neys. In young specimens they appear as slender, white tubes ex-
tending along the inner borders of the kidneys. Anteriorly, the
oviducts pass ventrad over the front of the liver to the ventral wall
of the body; at the same time they unite to form a funnel, the
ostium tubae, which opens into the coelom. Vestigial oviducts open-
ing into the coelom are found in the same position in males.

In males, the vasa deferentia appear as slender, irregularly
coiled white tubules lying near the medial border of the kidneys;
they are much less conspicuous than the oviducts, especially in
young males.

THE ALIMENTARY SYSTEM. In dissecting the following organs,
care should be taken not to break the connections of the organs with
each other or with other parts, or to cut blood vessels. Organs
should not be removed until such procedure is directed.

The mouth and pharynx can be studied to better advantage later
with the dissection of portions of the vascular system.

The oesophagus can be seen above the liver, by pressing that
organ aside, as a somewhat constricted tube entering the anterior
end of the abdominal cavity. It immediately joins the stomach,
which is more or less expanded according to the amount of food
contained in it.

The stomach passes directly back for more than half of the
length of the abdominal cavity, then turns abruptly forward, form-
ing a distal limb about a third as long as the proximal. (Two-
thirds to three-quarters as long in Eugaleus.) The distal limb
ends with a sharp turn to the right, where it is constricted by the
pyloric sphincter, which marks the end of the stomach.

The narrow beginning of the intestine forming the turn to the
right and backward is frequently distinguished as the duodenum.
It leads from the stomach directly into the large intestine, a wide,
straight tube marked externally by a spiral line of several turns.
The large intestine narrows posteriorly, forming a region some-
what arbitrarily termed the rectum, which opens into the cloaca
through the anus.

Dorsal to the rectum and attached to that body is a narrow
spindle-shaped body, the rectal or digitiform gland.

The liver is attached to the anterior wall by a broad base, the
peritoneum being reflected over the entire remaining surface. The

attaching fold of the peritoneum is frequently called the suspensory
ligament. The peritoneum, or coelomic epithelium, can be dis-
sected easily from the surface of the liver or the kidney and its
extreme thinness and delicacy noted. It consists of a single layer
of cells.

Most of the abdominal organs are suspended from the dorsal
wall of the body cavity by delicate membraneous sheets, or mesen-
teries. Similar sheets between the organs are the omenta. The
stomach is suspended by a mesogaster, which extends as a free fold
along the body as far as the anterior mesenteric and lienogastric
arteries. It encloses these, and is attached to the spleen, pancreas,
stomach, and anterior end of the intestine.

The spleen is connected with the stomach by the Castro-splenic
omentum, formed by an extension of the peritoneal coat of the
stomach around the spleen. The liver is connected to the loop of
the stomach by the gastro-hepatic omentum in which are the hepatic
duct, portal vein, and hepatic artery. Near the stomach it is joined
by a fold of the peritoneum from the duodenum, the duodeno-hepatic
omentum, which also unites with the mesogaster.

The rectum and rectal gland are supported by a second median
mesentary, the mesorectum.

In Eugaleus the mesentery extends the entire length of the abdominal
cavity. It forms a broad sheet attached to the anterior end of the proximal
limb of the stomach (mesogaster), to the anterior end of the intestine (mesen-
tery proper), and to the rectum (mesorectum). There is not the reduction of
the mesentery which there is in Squalus. The gonads are suspended from the
lateral faces of the mesentery above the stomach and intestine. The gastro-
hepatic omentum forms a broad sheet between the limbs of the stomach, join-
ing the mesogaster dorsal to the stomach and the mesentery above the intestine.

A small division of the right lobe of the liver stands out be-
tween the main lobes. In this is located a long, narrow gall-bladder.
Open the bladder by a longitudinal ventral incision. The opening
into the bile-duct will be found near the anterior end of the bladder.

In Eugaleus, which does not possess such a median lobe, the gall-bladder
lies hidden in the right lobe of the liver. It can be opened and explored, but
the connection with the duct can usually be demonstrated only by scraping.
Do this later.

The bile-duct passes along the dorsal side of the gall-bladder
and the edge of the gastro-hepatic and duodeno-hepatic omenta to
the junction of the duodenum and large intestine, where it opens
into the alimentary canal. Trace its oblique course through the wall
of the intestine. The bile duct and the collecting (hepatic) ducts
of the liver will be traced in the liver at a later stage of the dissec-
tion.

The pancreas consists of two lobes; a slender lobe lying dorsal
to and parallel with the stomach, and a flattened oval lobe lying
upon the ventral surface of the duodenum, connected with the dorsal
lobe by a slender bar of glandular tissue.

The pancreatic duct passes from the extreme right end of the
duodenal lobe obliquely through the wall of the intestine, opening
into the anterior end of the large intestine. Free the edge of the
lobe from the peritoneum and follow the duct.

Open the proximal limb of the stomach by a ventral incision
which shall not cut any large blood vessels. Wash out the in-
terior. Observe the three coats of the stomach; the outer peritoneal,
the middle muscular, and the inner mucous coats. In the anterior
portion of the stomach the mucous coat projects in the form of large
papillae (absent in Eugaleus). Posterior to these, observe the irreg-
ular folding of the mucous coat, depending upon the degree of con-
traction of the muscular coat.

8

The muscular coat consists of an outer circular and an inner
longitudinal layer of muscle fibres. Separate the two layers from
each other and from the mucous coat; observe the network of
blood vessels between the longitudinal muscles and the mucosa.

Open the pyloric end of the stomach, continuing the cut through
the pylorus into the intestine. Examine the coats as before, observ-
ing especially that an outer layer of longitudinal muscle fibres is
frequently developed, and that the pyloric valve is formed by an
increase in the thickness of the coat of circular fibres.

Cut through the wall of the large intestine along the right side
from its anterior end to the rectum. Do not cut deeper than the
thickness of the wall. Corresponding to the external markings, the
mucous membrane projects internally in a spiral fold, known as
the spiral valve. Separate the wall of the intestine from the edge
of the spiral fold upon both sides of the longitudinal incision, ex-
posing a considerable surface of the valve. Wash well, and ob-
serve the character of the valve, the direction of the folds, and the
manner of the reversal of their direction which usually takes place
in the posterior half of the valve.

Cut across the rectal gland at its middle. Observe the character
of its tissues, and then insert a bristle into the central cavity of the
gland and pass it into the rectum. Open the rectum and note the
point of communication of the two organs.

URINARY AND REPRODUCTIVE ORGANS. The kidneys (mesonephri,
Wolff ian bodies], are slender bodies extending along the entire
length of the dorsal wall of the abdomen. The posterior moiety of
each is thicker and wider than the anterior, which appears to have
largely lost the functions of excretion in adult dogfish. Notice the
position of the kidneys outside the peritoneum.

THE MALE. The tester are white bodies lying to the right and
left of the oesophagus, dorsal to the anterior portion of the liver.
Each is suspended by a fold of the peritoneum, the mesorchium.
(The testes of Galeus are long bodies attached to the sides of the
mesentery.)

Showing through the peritoneum, a much convoluted, white tube
can be seen on the ventral surface of the kidney. This is the
mesonephric or Wolffian duct. In young specimens it may be
nearly straight, lying near the medial border of the kidney. In adult
specimens it can be followed forward as far as the anterior end of
the testis. While the Wolffian duct is the duct of the kidney, and is
joined by tubules of the anterior part of the kidney, it is so modified
in the male that its principal function is to serve as the duct of the
testis, a vas deferens. The collecting tubules of the posterior part
of the kidney join to form a urinary duct which is independent of
the Wolffian duct. The posterior end of the Wolffian duct is straight
and considerably expanded, forming a large seminal vesicle. The
duct becomes more and more closely convoluted as it passes for-
ward, and the kidney tissue overlying it diminishes. At the an-
terior end of the mesonephros the Wolffian duct forms a mass of
tubules, the epididymis. Very small tubules, the vasa efferentia,
pass from the anterior end of the testis to the epididymis. These
are difficult for the student to distinguish.

Cut through the peritoneum along the outer side of one kidney.
Then strip the peritoneum toward the inner border of the kidney.
The urinary duct will usually be closely attached to the peritoneum
and parallel with the Wolffian duct, but nearer the midline of the
body. The urinary duct can be separated from the peritoneum by

9

a little careful work. Numerous small ducts pass from the kidney
into the urinary duct.

Open the uro-genital papilla near its tip and extend the incision
forward so as to open the sac connected with the base of the papilla.
The pore at the tip of the papilla leads into a space within the
papilla itself, the uro-genital sinus, which branches to the left and
right in pouches which extend beyond the posterior ends of the vasa
deferentia. These cornua of the uro-genital sinus are of variable
length, and are often named sperm-sacs. In a mature male they
may be found to be filled with sperm, as may also the seminal
vesicles and the convoluted portion of the Wolffian duct. The
openings of the vasa deferentia into the sinus are large and easily
located. The urinary duct opens into the sinus by a separate pore
just behind the opening of the vas deferens.

Cut open the seminal vesicle and part of the convoluted vas
deferens. The space within is subdivided by transverse folds or
lamellae extending from a longitudinal ridge.

No vasa efferentia can be distinguished in Eugaleus. The anterior extremi-
ties of the kidney and testis of each side come into close contact with each
other and here the vasa efferentia pass from the testis to the vas deferens. The
sperm-sac is a large blind pouch, one or two inches in length, leading out of
the posterior end of the vas deferens, and directed forward along its side. The
vas deferens of Eugaleus is not convoluted.

In the young specimens usually supplied to laboratories the vas
deferens is straight and no seminal vesicle is developed. The vasa
efferentia are more difficult to see; otherwise the relations of the
urinary and genital organs are as in the adult.

The suspensory ligament of the liver is continued posteriorly
along the midline of the ventral body-wall; the dorsal edge sup-
ports a funnel which opens into the abdominal cavity by a long,
narrow mouth. From the anterior end of the funnel two narrow
tubes pass to the right and left over the anterior surface of the
liver. They end blindly in the tissues dorsal to the anterior end of
the liver. These are vestiges of the Muellerian ducts (pronephric
ducts) which form the oviducts of the females.

THE FEMALE. The ovaries are large, white bodies lying at the
sides of the stomach, dorsal to the lobes of the liver. Each is cov-
ered by the peritoneum and suspended by a fold of the same, the
mesovarium. Ova of various sizes may be felt in the tissue of the
ovary, which should be exposed by dissection.

The ovaries of Eugaleus are long slender bodies lying on either side of
the mesogaster, dorsal to the stomach and intestine. Their posterior portions
are fused.

The oviducts (Muellerian ducts) are large tubes suspended from
between the kidneys by a narrow peritoneal band. The posterior
portion of the oviduct, where development of the eggs takes place,
is considerably enlarged. Each oviduct opens separately into the
cloaca by a pore at the side of the urinary papilla. Followed for-
ward, the oviducts pass over the anterior surface of the liver and
following a continuation of the suspensory ligament, bend around
posteriorly and unite. At the point of union they open into the
coelom by a common, large, funnel-shaped aperture, the ostium
tubae.

Cut through the peritoneum along the outer side of one kidney.
Then strip the peritoneum toward the inner side of the kidney.
Numerous small excretory ducts will be seen joining the main
urinary duct (Wolffian duct, mesonephric duct), which runs along
the inner margin of the kidney. Make an incision in the side of

10

the urinary papilla to open the cavity within it, the urinary sinus.
The connection of this with the pore at the tip of the papilla should
be demonstrated. Extend the incision forward. The urinary sinus
divides into right and left cornua which are of considerable size and
lie dorsal to the oviducts. Trace the Wolffian duct to the urinary
sinus and demonstrate its opening into the cornu anterior to the
point where the two cornua unite.

In young specimens the ovaries are small, and the oviducts are
narrow, white tubes lying along the medial margins of the kidneys.

Nephrostomes, short, segmen tally arranged kidney tubules which
open to the coelom by a funicular aperture, are found by a close
examination along the medial border of each kidney. They should
be observed carefully with the aid of a good dissecting lens.
Learn the significance of these structures.

In the course of development two sets of nephridia (kidneys)
are formed. The first (pronephros) develops just back of the head
of the embryo, but does not persist in the adult. Its duct, known
commonly as the Muellerian duct, develops into the functional ovi-
duct of the female, but forms an apparently useless vestige in the
male. The second kidney (mesonephros) develops behind the first
and is the excretory organ of the adult. Its duct (frequently given
the name of Wolffian duct) is the urinary duct in the female, but
functions in the male chiefly as a sperm duct, and therefore is
called the vas deferens. The collecting tubules of the posterior
portion of the kidney of the male unite to form a urinary duct which
opens into the Wolffian duct or the urogenital sinus.

RESPIRATORY ORGANS

Open the anterior gill-pouch of the left side by dorsal and ven-
tral cuts extending from the angles of the cleft, but cutting only as
far as is necessary to see the structures within the pouch. Upon the
medial side the gill pouch opens into the pharynx by a dorso-ventral
slit, guarded by projecting cartilagineous gill-rakers, which pre-
vent particles of food from passing into the gill pouch with the
respiratory current. On both the anterior and posterior wall of the
pouch is a demi branch. If the specimen is injected a large blood
vessel can be seen through the skin in the inner border of the demi-
branch, and small vessels passing from this into the leaflets, where
the interchange of gases between the water and blood takes place.

Open similarly each pouch of the same side, observing the num-
ber of the demi-branchs and their relation to the pouches.

Upon the anterior wall of the spiracle demonstrate a row of
small vestigial gills; being supplied with arterial instead of venous
blood they form what is termed a pseudobranch.

VASCULAR SYSTEM

HEART AND VENTRAL AORTA. Continue the longitudinal incision
through the skin as far as the mandible.* Dissect away the sheet
of muscles between the gill-pouches and the mandible, exposing a
slender muscle which extends from the pectoral girdle to the middle
of the mandible. The thyroid gland lies dorsal to the anterior end
of this muscle, close against the mandible. (The thyroid of Euga-
leus is a broad, flattened structure covering the anterior ends of the
coracohyoideus muscles.) Carefully dissect out the muscles lying
between the branchial pouches of the right and left sides.

*See footnote, p. 31.

11

In front of the pectoral girdle lies a thin walled sac, the peri-
cardial sac. Open it by a median ventral incision. Remove about
one-half inch of the middle of the pectoral girdle, being careful not
to cut the thin-walled part of the heart lying dorsal to it. The
pericardial cavity is a pear-shaped chamber containing the heart,
and lined by the smooth pericardium which is morphologically
equivalent to the peritoneum. At the anterior extremity of the
chamber the pericardium is seen to be reflected backward over the
surface of the heart, thus forming its smooth outer coat.

The heart may be considered as a bent tube, enlarged in certain
regions to form the chambers. Anteriorly and ventrally is a short,
thick-walled tube, the conus arteriosus; this leads out of the peri-
cardial sac anteriorly, while posteriorly it opens into a large mus-
cular chamber, the ventricle. Dorsal to the ventricle, and project-
ing on either side of it is the thin walled auricle. Dorsal to both
ventricle and auricle is the extremely thin-walled sinus venosus. This
is triangular in shape, the apex opening into the posterior side of
the auricle, the base attached to the posterior wall of the pericardial
cavity; the lateral angles are drawn out into the ducti cuvierii,
which receive veins from the anterior and posterior parts of the
body. From the conus arteriosus springs a smaller vessel, the
ventral aorta, which passes forward between the gill pouches. Take
note of the small arteries passing over the surface of the conus and
along the inner ends of the gill pouches, and take care not to cut
them or their branches in the subsequent dissection.

Two pairs of arteries leave the ventral aorta as it emerges from
the pericardial sac. The aorta then passes forward some distance
and finally divides into two branches which pass to either side. Fol-
low the branches of the aorta outward on the left side and demon-
strate their courses. The anterior branch quickly divides into two,
the anterior of these passing along the base of the first demibranch.
The posterior enters the septum between the first and second pouches,
and supplies the second and third demibranchs. The middle branch
of the aorta passes directly to the fourth and fifth demibranchs. The
posterior branch divides almost as it leaves the aorta, its branches
supplying the remaining demibranchs. There is considerable varia-
tion in this branch of the aorta. It usually divides as stated, but it
frequently passes some distance toward the gills before dividing,
and in a considerable number of cases two vessels arise directly
from the aorta instead of one.

The arteries carrying blood from the ventral aorta to the gills
are named the afferent branchial arteries. Observe the relation of
these vessels to the gills.

VENOUS SYSTEM. All the blood of the body is conveyed to the
sinus venosus. The sides of the sinus venosus are extended as large
vessels, already referred to as the ducti cuvierii. Open the sinus
and ducti by a transverse ventral incision. The ducti pass directly
into the lateral veins. Near the middle of the posterior wall of the
sinus is an aperture of varying size, the opening of the hepatic
sinus; there are rarely two openings in Squalus, always two in
Eugaleus. A large opening on the posterior wall of each ductus
leads into the posterior cardinal vein. On the anterior wall of the
ductus, near the sinus venosus, is a small aperture, that of the in-
ferior jugular vein. Lateral to this is frequently a somewhat larger
opening of the anterior cardinal vein. This is absent, however, in

12

the majority of specimens; the anterior cardinals opening into the
anterior ends of the posterior cardinals in about six out of ten cases.

A large cavity, the hepatic sinus, exists in the anterior end of the
liver just posterior to the suspensory ligament. Cut into the liver
at this point until the sinus is found, open it, and observe the large
hepatic veins bringing blood into it from the liver, as well as its
communication with the sinus venosus.

Trace all veins by passing a flexible probe or guarded bristle
along them and then opening the vein with the probe as a guide.
All smaller vessels emptying into those described should be noted.

The lateral veins pass forward to the posterior edge of the pec-
toral girdle, bend sharply dorsad, and enter the lateral extremities
of the ducti cuvierii. Open a lateral vein near the anterior end and
trace it toward the heart. The right and left lateral veins are joined
by a vein passing along the ventral bar of the pectoral girdle. Open
the lateral veins at a point about two inches in front of the pelvic
girdle and trace the veins backward as far as they can be followed.
The blood from the pelvic fins enters the lateral vein through the
femoral vein. The lateral veins finally unite back of the cloaca.

Just before the lateral vein enters the ductus cuvierius it is joined
by a large coracoid vein which runs dorsad and posteriorly along
the posterior edge of the pectoral arch. Follow its course. It re-
ceives a good-sized pectoral vein from the pectoral fin, and some-
times several smaller veins from the same region. Traced dorsad it
is found to open into a large blood sinus above the liver and oesoph-
agus, the cardinal sinus.

In Eugaleus this connecting vein between the lateral vein and the cardinal
sinus is wanting, the pectoral vein opening directly into the lateral.

The ventral cutaneous vein, which runs along the ventral mid-
line of the body wall, should be followed; anteriorly it joins the
vessel uniting the two laterals; posteriorly it divides at the pelvic
arch and anastomoses with the laterals.

Pass a bristle from the sinus venosus into one of the posterior
cardinal veins and trace the vein backward between the kidneys as
far as possible. Open both posterior cardinals in this way, washing
them out and observing that they receive blood from the kidneys by
a series of renal veins, and that they are separate in their posterior
parts, but communicate with each other anteriorly, where they are
greatly expanded; the communicating portions and coincident en-
largement forming the cardinal sinus. The anterior portion of the
cardinal vein receives ovarian or spermatic veins from the female
or male gonad, anterior oviducal veins from the anterior part of
the oviduct, and segmental veins from the corresponding region of
the body wall. There sometimes is more than a single opening from
the posterior cardinal vein into the cuvierian duct.

Cut across the tail an inch behind the cloaca. Two vessels lie
in the cartilaginous arch below the centra of the vertebrae; the
dorsal of the two is the caudal artery, the ventral one is the caudal
vein. Follow the vein forward. Dorsal to the cloaca it divides into
two, which should be followed along the dorsal surfaces of the kid-
neys. These are the renal portal veins, conveying blood to the kid-
neys. Besides collecting the blood of the tail the renal portals also
receive the posterior oviducal and segmental veins. They pass into
the capillaries of the kidneys.

The inferior jugular vein opens into the medial end of the cuvier-
ian duct. Trace it forward along the ventral ends of the gill-pouches;
it receives vessels from the arches and finally joins the hyoidean

13

veins which follow the hyoid arch. At the outer end of the cuvierian
duct there is often a small opening on the anterior wall opposite the
mouth of the posterior cardinal vein. This leads into the anterior
cardinal vein. As mentioned before, in a slight majority of the
cases examined, the anterior cardinal vein opens into the posterior
cardinal vein, not directly into the cuvierian duct. If possible, pass
a bristle into the anterior cardinal. To follow the vein, and usually
this is the best way to find it, make a vertical longitudinal incision
upon the dorsal side of the neck, between the gill pouches and the
mass of muscle lying beside the vertebral column. This will open
the anterior cardinal, which is considerably expanded in this region,
and it may be traced from this point toward the heart and the head.
The anterior cardinal narrows suddenly in front of the anterior gill
pouch, and leads downward to the orbit, where it expands into the
orbital sinus surrounding the eyeball and its muscles. Trace the
anterior cardinal only as far as the opening into the orbital sinus at
this time. Veins from the anterior portion of the head and from
the brain can be followed when the dissection of the eye is under-
taken.

Just back of the spiracle the anterior cardinal receives the hyoi-
dean vein., which passes ventrad along the base of the first demi-
branch and unites with the hyoidean of the opposite side. Ventrally,
it also communicates with the inferior jugular vein.

The principal veins of the body have now been dissected with
the exception of the hepatic portal vein, which it is better to trace
after the arteries of the digestive tract have been studied.

THE EFFERENT BRANCHIAL ARTERIES AND DORSAL AORTA. Com-
mencing at the mouth, cut through the floor of the pharynx close to
the left side of the ventral aorta and the heart. The cut should leave
the gill arches uninjured, and may be continued into the oesophagus.

Examine the interior of the mouth and pharynx, observing par-
ticularly the form and arrangement of the teeth, the spiracular and
branchial clefts, the gill-rakers, and the character of the mucous
coat of the pharynx.

Remove the skin from the roof of the pharynx. This exposes
four pairs of efferent branchial arteries bringing blood from the
gills and uniting in pairs to form the dorsal aorta. Follow each ves-
sel of the left side out to its gill-cleft. At the dorsal end of the gill-
cleft it divides into a large posterior and small anterior branch.
These respectively pass along the posterior and anterior demibranchs
of the gill pouch, receiving fine branches from the gill lamellae,
and finally unite again at the ventral end of the gill-pouch. Thus
a complete loop is formed around the branchial cleft. The posterior
branch of each efferent artery and the anterior branch of the suc-
ceeding one are united by several short vessels. The efferent artery
of the last demibranch possesses only these connections with the
branch next anterior to it, and none with the aorta directly. From
the ventral ends of the efferent loops small vessels pass toward the
mid-line to unite with a longitudinal artery, the hypobranchial ar-
tery, which will be traced farther a little later in the dissection.

In Eugaleus the dorsal aorta extends forward beyond the union of the first
pair of efferent branchials and then divides into small right and left branches
which pass forward and outward to unite with the common carotid arteries.

A common carotid artery leaves the dorsal end of each anterior
efferent branchial loop, passing forward and inward. At the level
of the spiracles it divides into external and internal carotids; the
internal carotid unites with its fellow of the opposite side and enters
the skull. The external carotid arteries run outward and forward

14

around the eyes and are distributed to the regions of the mandible
and snout. Do not, at present, trace them beyond the posterior
edge of the eye.

Another vessel arises from the middle of the anterior side- of
the first efferent branchial loop and runs forward to the spiracle,
where it ends in the capillaries of the pseudobranch. This is the
afferent hyoidean artery. The term pseudobranch is used for the
branchial lamellae of the spiracle rather than demibranch because
of the arterial blood supply of this organ.

Immediately after uniting the internal carotids divide and di-
verge, forming an X-shaped figure. Each anterior limb of the X
again divides into two branches. The lateral branch passes to the
ventral surface of the skull; it presently gives off an anterior twig
(ophthalmic artery] which enters the eye. It then passes on as the
efferent hyoidean artery to the pseudobranch. The inner of the two
branchs mentioned above passes on as the internal carotid, sensu
strictu, and is distributed to the brain.

If the dissection is made with care, the branches of the internal
carotid can all be found without cutting any important nerves. The
branches passing to the eye and brain are best traced to their ter-
minations in connection with the dissection of the nervous system.

Near the union of the first pair of efferent branchial arteries a
small posterior vertebral artery arises from each, and runs anter-
iorly along the vertebral column.

Near the divisions of the common carotids two anterior vertebral
arteries arise from these vessels and pass posteriorly, often anasto-
mosing with the posterior vertebral arteries. These vertebral arteries
are vestiges of the former anterior part of the dorsal aorta (compare
with Eugaleus, in which the dorsal aorta sends forward two vessels
which join the common carotids).

An oesophageal artery springs from the second efferent branch-
ial, and passes back until it enters the wall of the oesophagus. It
also gives off nutrient branches to the second, third, and fourth
gill pouches. The nutrient artery of the first gill pouch arises di-
rectly from the first efferent branchial.

Near the point at which the fourth pair of efferent branchials
join the aorta, two small subclavian arteries leave the aorta and
pass into the pectoral fins. There is some variation in regard to
the point of origin of these vessels; it may be either in front of or
behind the junction of the fourth efferent branchials. with the aorta.

The hypobranchial artery passes along the ventral ends of the
gill pouches. It is either connected with the efferent branchial loops
by short branches, or is formed, in part at least, by short vessels
connecting these loops. The hypobranchials are important nutrient
vessels, supplying the gill pouches and the muscles of the throat
and the oesophagus by means of numerous small arteries; from the
hypobranchials also arise small posterior coronary arteries which
pass to the ventral and posterior walls of the pericardium and the
sinus venosus, and larger anterior coronary arteries supplying the
ventricle and conus arteriosus. The hypobranchials can frequently
be followed along the dorsal side of the pericardium and then out-
ward to junctions with the subclavian arteries.

The coeliac artery (coeliac axis) arises from the aorta just back
of the subclavians. Passing posteriorly and ventrad close to the
right side of the stomach and reaching the gastro-hepatic omentum,
it divides into two branches, the gastro-hepatic and anterior intes-
tinal arteries. The first gives off a small hepatic artery to the liver

15

and a large gastric artery to the cardiac limb of the stomach. The
anterior intestinal artery supplies the pyloric limb of the stomach,
the pancreas, duodenum, and right side of the large intestine.

Small genital arteries, supplying the reproductive glands, arise
from the coeliac near its origin. (In Eugaleus the genital arteries
arise from the anterior and posterior mesenteric arteries.)

At about the middle of the abdominal cavity two arteries arise
close together from the aorta. The anterior of the two is the an-
terior mesenteric artery; it passes to the left side of the large intes-
tine and its branches anastomose more or less with those of the
anterior intestinal artery. The posterior vessel is the lieno-gastric ;
it goes to the spleen, pancreas, and loop of the stomach.

The posterior mesenteric artery leaves the aorta a little distance
back of the lienogastric and passes to the rectal gland, rectum, and
cloaca.

Free the kidney from the body wall along its outer edge and
turn it up so as to expose its dorsal surface. Observe the numerous
parietal arteries (going to the body wall) and renal arteries (to the
kidney), which spring from the dorsal aorta. Branches of the
parietals also pass into the kidney.

A pair of small iliac arteries pass into the pelvic fins.

Oviducal arteries, one or several on each side, arise from the
aorta behind the coeliac artery and pass to the oviduct. Their size
varies largely with the development and physiological condition of
the oviduct.

The aorta is continued in the tail as the caudal artery.

DISSECTION OF THE HEART. Remove the heart together with the
ventral aorta from the body and fasten it, dorsal side up, under
water. Open the sinus venosus with scissors, wash it out, and ob-
serve the vertical slit-like opening into the auricle and the two mem-
braneous valves which guard it.

Continue the cut through the sinu-auricular aperture along the
median dorsal line of the auricle; observe the thin walls of the
auricle and their strengthening by an irregular mesh of muscles,
the musculi pectinati; the shape and position of the auriculo-ventri-
cular aperture; the flaps of the auriculo-ventricular valve. Press
upon the sides of the ventricle and, if possible, observe the mode of
action of the valve.

Cut across the ventricle from the auriculo-ventricular aperture.
Carry another incision from this along the dorsal side of the conus
arteriosus. Observe the small size of the cavity of the ventricle, the
thickness of its walls, and the projecting network of muscles, the
columnae carneae, some of which are attached to the edges of the
auriculo-ventricular valves.

In the conus arteriosus observe the rows of three pocket-like
valves each around the proximal end (semilunar valves), and a
single row of three similar but larger valves at the junction of the
conus and ventral aorta. There is some variability in the number
of rows of valves in the conus of Squalus; there are always three
rows of three valves each in that of Eugaleus.

In the aorta notice the apertures without valves which lead into
the afferent branchial vessels.

HEPATIC PORTAL SYSTEM. The hepatic portal vein is the large
vein entering the liver alongside the hepatic artery and bile duct. It
receives branches from the stomach, pancreas, spleen, intestine, and
rectal gland.

16

At the surface of the liver it divides into two branches, which
enter the two lobes of this organ. Within the liver the hepatic
portal veins branch until a capillary sytem is formed from which
the blood is collected by the hepatic veins and carried into the sinus
venosus.

In general, the branches of origin of the hepatic portal vein fol-
low closely the arteries of the digestive organs. Trace the follow-
ing parts of the system : A posterior intestinal vein, from the rectal
gland and rectum, the large intestine and spiral valve, across to the
end of the pancreas, along the pancreas to the hepatic portal vein;
an anterior intestinal vein, from the large intestine and spiral valve,
along the duodenal lobe of the pancreas; gastric, duodenal, and
pyloric veins joining the veins already traced; a splenic vein join-
ing the posterior intestinal vein.

The liver, with the bile duct, may now be removed from the
body if it is desired to trace the bile duct into the bladder or to
trace the hepatic ducts. This can be done best by gently scraping
away the soft liver tissue until the bladder and ducts are exposed.

THE NERVOUS SYSTEM.

Only the head and anterior part of the trunk will be required for
the dissection of the nervous system. Cut across the body back of
the pectoral fins; the posterior part of the body will not be re-
quired further unless it is desired to study the muscles and skeleton.

The manner of dissecting the brain depends somewhat upon
the specimens at the disposal of the student. If a large head is to
be used especially for the dissection of the cranial nerves, only the
brain, eye and ear need be studied in the present specimen. But in
most cases it will be found best for the student to dissect the first
dogfish as thoroughly as possible, working out the cranial nerves
as well as the brain, and reserving the second head for a thorough
review of the entire nervous system. Chapter III of Herrick and
Crosby's "Laboratory Outline of Neurology" should be used in con-
nection with such a review.

If a line be drawn over the dorsal surface of the head connect-
ing the two spiracles, two small pores will be found near the middle.
These are the external apertures of the ducti lymphatici. Cut
carefully through the skin in a small circle around the pores, and
remove the skin from the remainder of the dorsal surface of the
skull without disturbing the small section containing the pores.
The latter part should now be lifted gently; beneath it will be
seen two delicate tubes passing from the pores to apertures in a de-
pression of the skull below them. These tubes are the ducti endo-
lymphatici, through which a passage exists between the internal ear
and the exterior. As they cannot be preserved in the subsequent
dissection, the pores by which they pass through the skull to the in-
ternal ear should be found now, and a memorandum-sketch made
of the ducts themselves.

DORSAL SURFACE OF THE BRAIN. The roof of the skull should be
removed from over the brain. Use a sharp scalpel and take very
thin slices of cartilage. Do not cut beyond the brain at the sides.
No attempt should be made at this time to expose more than the
dorsal surface of the brain.

Above the anterior end of the brain there is a small median
foramen through the skull, the epiphysial foramen. A strand of
tissue, the epiphysis, leading from this to the surface of the brain,
should be carefully observed and retained. The cartilage should

17

also be cut away from above the portion of the spinal cord next
the skull. Gently wash away any coagulated lymph.

The brain and spinal cord are invested by two membranes
(meninges) . The tough dura mater lines the cavity in which they
lie, clinging closely to the cartilage; in fact it forms the perichon-
drium of the internal surface of the cranium. The pia mater en-
velops closely the brain and cord, and contains numerous blood
vessels. Between the two is the arachnoid space, traversed by oc-
casional fine threads of connective tissue and filled with lymph.

As the spinal cord passes forward into the skull it enlarges and
merges with the posterior portion of the brain, the medulla oblon-
gata (myelencephalon) . The roof of the medulla is extremely
thin, and is broken if the cartilage has not been removed with ex-
treme care, exposing a cavity within, the fourth ventricle.

In front of the medulla, and overlapping its anterior extremity,
is a large oval organ, the cerebellum (metencephalon) . Ventral
to the cerebellum, each side of the medulla is expanded in an ear-
shaped lobe, the corpus restiformis. Anteriorly, the cerebellum
overlaps a pair of rounded lobes, the optic lobes, which together
form the dorsal portion of the midbrain (mesencephalon) .

In front of the optic lobes are two slightly larger lobes united
in their posterior portions but separated anteriorly, the cerebral
lobes or hemispheres. Together they constitute the prosencephalon.
(The prosencephalon is not divided in Eugaleus.) Between the
mesencephalon and the prosencephalon is a depressed region be-
longing to the brain-stem, the diencephalon (thalamencephalon) ,
from which the epiphysis arises. The roof of the diencephalon
also is very thin and is frequently broken during the exposure of
the brain. The cavity seen within the diencephalon is the third
ventricle.

Stalked bodies arising from the antero-lateral angles of the
cerebral hemispheres are the olfactory lobes. The portion of the
brain including the cerebral hemispheres and the olfactory lobes
constitutes the telencephalon.

DISSECTION OF THE INTERNAL EAR. The structures composing
this organ lie in the projecting cartilage at the side of the medulla
(auditory capsule). Remove the cartilage of the auditory capsule
in thin slices and bit by bit, following the ductus endolymphaticus
to the membraneous labyrinth. Dissect away the surrounding car-
tilage leaving the membraneous canals in place, until the entire
labyrinth is exposed. The membraneous labyrinth consists of a
large central sac (utriculo-saccular chamber) into which the endo-
lymphatic duct opens, and three membraneous tubes (semicircular
canals) external to the chamber but communicating with it in vari-
ous ways. Two, one anterior and the other posterior to the sacculus,
lie in a nearly vertical plane (anterior and posterior semicircular
canals) ; one is external to the sacculus and lies in a nearly hori-
zontal plane (horizontal or external semicircular canal). At the
ventral ends of the vertical canals are nearly spherical enlarge-
ments called ampullae. The ampulla of the horizontal canal is at
its anterior end. The dorsal ends of the vertical canals open near
each other into the upper part of the utriculo-sacculus. The ven-
tral extremity of the anterior vertical canal and the anterior ex-
tremity of the horizontal canal open beside each other into an an-
terior projection of the sacculus. The ventral extremity of the pos-
terior vertical canal opens into the posterior and lower part of the
sacculus. The posterior extremity of the horizonal canal opens

18

into the posterior side of the sacculus. During life the utrici
sacculus and the semicircular canals are filled with a lymph
fluid, and the sacculus contains a large calcerous ear-stone (otolii
which is usually dissolved by the formalin used in preserving
dogfish.

Whitish patches of thickened sensory epithelium may be seei
the ampullae (cristate acusticae) and in the utriculo-saccular ch
her (maculae acusticae). Branches of the eighth nerve can be
lowed to all these areas.

A projection of the ventral wall of the utriculo-sacculus is
lagena, the rudiment from which the cochlea of higher anin
developed. It also contains a macula acustica.

EXTERNAL FEATURES OF THE EYE. Observe the transpa:
cornea covering the external surface of the eye; the dark ring
the iris; the central opening in the iris, the pupil; the conjur
val sac surrounding the external half of the eyeball. Cut a
sufficient of the upper wall of the cartilaginous orbit to expose
eyeball and its muscles. Note the considerable amount of soft <
nective tissue around the eye and explore the orbital sinus (p. ]
Take notice of the following nerves, in order to ensure their pre
vation until the time comes to trace them more completely. A h
nerve crossing the medial side of the orbit, the superficial opht
mic; a nerve leaving the cranium opposite the optic lobe, pas:
under the superficial ophthalmic to the anterior muscle of the
ball, the trochlear; several long ciliary nerves passing to the
ball; several other nerves visible in the deep angle of the orbit.

Six muscles move the eye. Four of these arise close toge
at the deep postero-medial angle of the orbit. Diverging, they
inserted upon four sides of the eyeball, and from the positior
their insertions are named the superior, posterior, inferior,
anterior recti. Two muscles arise from the antero-medial angl<
the orbit, the superior and inferior oblique muscles.

Between the recti muscles can be seen a mushroom-shaped s
of cartilage, the ophthalmic peduncle; the eyeball rests againsi
expanded end. (There is no peduncle in Eugaleus.)

THE CRANIAL NERVES. The cranial nerves are twelve pair;
nerves arising from the brain, and thus distinguished from the sp
nerves which arise from the sides of the spinal cord. They are
tributed chiefly to the head and neck, though branches of the vi
nerve go to the viscera and to the sense organs of the lateral ]
Since the nerves are all paired, the distribution of both nerves <
pair being alike, the descriptions will mention but one nerve <
pair. As the cranial nerves are traced dissect away the sides of
cranium down to the foramina penetrated by the nerves, and fol
each nerve from its origin on the brain to the parts innervated b
Features of the dissection which are not found in tracing the n<
of one side should be sought on the other side.

The olfactory nerve. The anterior surface of the olfactory 1
fills a large foramen in the anterior wall of the cranium am
pressed closely against the posterior surface of the nasal sac.
merous small nerves, collectively forming the olfactory nerve, a
from the anterior face of the lobe, penetrate the membraneous i
of the olfactory organ, and are distributed to its highly folded
face.

The terminal nerve, Nervus terminalis, is a slender nerve :
ning along the medial surface of the stalk of the olfactory 1<
Follow it backward to its origin on the anterior surface of the c

19

bral hemisphere, deep in the median fissure (in Eugaleus on the
ventral surface). Trace it forward aver the dorsal surface of the
olfactory lobe to where it enters the nasal sac. The terminal nerve
is a true cranial nerve which has escaped notice until recent years.
It is associated with the olfactory nerve in vertebrates generally from
fishes to men. The fibres of the terminal nerve remain distinct from
those of the olfactory nerve, both in the olfactory organ and in the
brain. Its function is unknown. There is still a division of opinion
among authorities as to whether the terminal nerve should be con-
sidered to be a distinct cranial nerve, or a portion of the olfactory
nerve.

The optic nerve can be seen at the bottom of the orbit between
the eye and the skull, nearly under the superior oblique muscle. It
arises from the ventral side of the diencephalon, passes outward,
penetrates the orbit at its infero-medial angle, and continues directly
outward to the eyeball.

The trochlear nerve, or patheticus, penetrates the wall of the
orbit opposite the optic lobe. Follow it back to its origin from the
dorsal surface of the brain in the depression between the optic
lobes and the cerebellum. Then follow it from the skull to the
superior oblique muscle, which it innervates.

The oculo-motor nerve arises from the ventral surface of the
midbrain, passes outward, and penetrates the orbit on a level with
and just anterior to the origins of the recti muscles. It divides im-
mediately into three parts; two pass to the anterior and superior
recti respectively, while the third passes downward along the pos-
terior surface of the eyeball to the inferior rectus and inferior
oblique muscles. In tracing this nerve the palatine process of the
upper jaw will be seen projecting from below into the orbit.

The trigeminal, facial, and auditory nerves spring from the side
of the medulla below the corpus restiformis. The roots, and some
of the branches, of the trigeminal and facial nerves are so mingled
as to be indistinguishable except by special neurological technique.
The common root of the trigeminal and facial nerves shows a par-
tial division into a dorsal and a ventral portion; the dorsal portion
belongs to the facial nerve, while the ventral root is mixed. The
root of the auditory nerve lies close behind the trigeminal-facial
root, but can be distinguished fairly well. Both the trigeminal and
facial nerves divide into several trunks, namely:

Trigeminal Facial

superficial ophthalmic superficial ophthalmic

deep ophthalmic buccal

maxillary otic

mandibular hyomandibular

The superficial ophthalmic trunks of the two unite in a single
nerve which passes along the inner wall of the orbit above the
muscles of the eye to a foramen in the antero-medial angle of the
orbit, through which it passes to the dorsal surface of the snout.
The superficial ophthalmic nerve of Squalus is composed almost
entirely of fibres of the facial nerve. The superficial ophthalmic
trunk of the trigeminal gives rise to several small nerves leaving
the common trunk near its origin and passing to the skin above the
eye. The superficial ophthalmic trunk of the facial, nearly the
whole of the common nerve, branches profusely to supply the
sensory organs of the dorsal and lateral surfaces of the snout.

The superficial ophthalmic of Eugaleus rises from the dorsal part of the
trigemino-facial root and leaves the cranium by a separate foramen above and
anterior to the roots of the recti muscles.

20

Directly under the origin of the superficial ophthalmic will be
found a comparatively slender nerve, which passes between the
superior and posterior rectus muscles, and forward along the medial
surface of the eyeball; it penetrates the anterior wall of the orbit
by a separate foramen, and emerges under the superficial ophthal-
mic. It is distributed to the skin of the dorsal and lateral surfaces
of the snout. This is the deep ophthalmic (ophthalmicus profundus)
of the trigeminal nerve. A slender branch (posterior ciliary nerve)
passes from the deep ophthalmic near its origin to the posterior sur-
face of the eyeball. Farther forward the same trunk gives off an
anterior ciliary nerve to the anterior part of the eyeball.

A large nerve which crosses the floor of the orbit, beneath the
eyeball, consists of the maxillary trunk of the trigeminal and the
buccal trunk of the facial nerve. These remain associated, even
into the small branches. Near the anterior margin of the orbit the
maxillary-buccal trunk divides into three parts; the smallest and
outer one passes to the surface lateral and anterior to the eye. The
other two dip downward and pass in front of the jaw to the ventral
surface of the snout. Reflect the skin of the ventral surface of the
snout, and by dissection expose these nerves as they emerge from
the orbit. The larger branch runs forward close to the median line
of the snout, giving off numerous twigs; the other, which appears
to be a pure trigeminal branch, is distributed near the angle of the
mouth. The fibres of the maxillary trunk supply the skin, while
those of the buccal go to the canal organs and ampullae of Loren-
zini.

The mandibular trunk of the trigeminal nerve arises beneath
and behind the maxillary. It passes outward in front of the levator
maxillae superioris muscle, sending a few twigs into this muscle,
and turns downward over the palato-quadrate cartilage. It di-
vides here, one part entering the adductor mandibularis muscle, the
other passing downward along the edge of the mandible, innervating
the skin of the lower jaw and the first ventral superficial constrictor
muscle.

The mandibular and maxillary-buccal trunks of Eugaleus are united until
they approach the edge of the orbit, and the palatine branch is much larger;
otherwise the trigemino-facial branches are much as in Squalus.

The hyomandibular trunk of the facial nerve can be found just
beneath the skin behind and close to the spiracle. From here it
can be followed back to the brain. It arises from the ventral part
of the trigemino-facial root, emerging from the cranium through
the hyomandibular canal. It divides into a number of branches
just beyond the spiracle:

1. The external mandibular branch consists of two portions, a
small anterior nerve extending antero-ventrally to the canals above
and below the angle of the mouth, and a larger nerve which passes
laterally and suddenly breaks up into a brush of twigs which in-
nervate the hyoidean group of ampullae.

2. The internal mandibular branch arises at about the same
level as the external mandibular, but under it, passes inward around
the edge of the hyoid cartilage, under the adductor mandibularis
muscle, and then forward along the mandibular cartilage.

3. The hyoid branch separates from the hyomandibular trunk
at about the same level as the preceding nerves, and then passes,
deep in the tissues, around the angle of the jaw to the ventral side
where it is distributed to the superficial constrictor muscles. Sev-
eral nerves pass from the hyomandibular trunk and the hyoid branch
to the dorsal superficial constrictors.

21

4. The palatine branch springs from the base of the hyoman-
dibular trunk inside the hyomandibular canal. It passes outward
and forward, dividing into numerous branches which innervate the
mucous membranes of the mouth. It can be traced completely later.

One or more small nerves proceeding to the pseudobranch and
anterior wall of the spiracle arise near the point of origin of the
palatine branch.

The otic nerve, passing from the root of the facial nerve to the
postorbital canal, is not likely to be found in this dissection.

Observe the enlargement near the base of the hyomandibular
trunk, and within the cartilaginous canal, the geniculate ganglion.
The gasserian ganglion, a component of the trigeminal nerve, lies in
the ventral portion of the trigeminal-facial root, and can now be
located.

The auditory nerve arises close behind the ventral division of
the trigemino-facial root. The root of the auditory nerve encloses
a large auditory ganglion. A vestibular nerve arises from the an-
terior end of the auditory ganglion and passes into the ear capsule,
innervating the upper part of the utriculo-sacculus and the ampullae
of the anterior and horizontal canals. From the posterior part of
the ganglion nerves pass to the ventral part of the sacculus and the
ampulla of the posterior canal. Trace these branches as thor-
oughly as possible.

The abducens nerve emerges from the cranium under and close
to the origin of the posterior rectus muscle, into which muscle it
enters. To expose this nerve the trigeminal, facial and auditory
nerves must be lifted and cut as they pass through the wall of the
cranium. It can be traced obliquely backward and inward, through
a long canal, to its origin near the mid-line of the ventral surface
of the medulla.

The glossopharyngeal nerve passes through the base of the ear
capsule from the side of the medulla to the upper end of the first
branchial pouch. A ganglionic enlargement is found near where it
emerges from the cartilage. Outside the cranium the glossopharyn-
geal divides into a pretrematic branch, passing down in front of
the first gill pouch, and a posttrematic branch running behind the
pouch. The pretrematic branch quickly sends off a pharyngeal
nerve which runs antero-ventrally to the roof of the pharynx. The
pretrematic and posttrematic branches can be followed along the
gill-arch to the ventral side of the pharynx. A fourth branch of
the glossopharyngeal, the supratemporal, springs from the dorsal
side of the ganglion; passing through the ear capsule it runs to the
dorsal surface of the head, where it is distributed to the sense organs
of a short section of the lateral line canal. This small nerve can
be demonstrated by carefully separating the muscles and perichon-
drium from the posterior surface of the auditory capsule.

The vagus nerve (or pneumo gastric) arises by an extensive
series of roots from the side of the medulla. An easily distinguished
ribbon-like portion of the root, the lateral line root, runs forward
as far as the root of the glossopharyngeal. Note the canal by which
the vagus leaves the cranium, and trace the nerve along the inner
side of the anterior cardinal vein.

The principal branches of the vagus are:

1. The supratemporal branch, a small nerve running dorsal
through the posterior part of the ear capsule to the lateral line
canal and other sense organs of the head. It will be found near the
supratemporal branch of the glossopharyngeal.

22

2. The lateral line branch, a large nerve which separates from
the trunk of the vagus just outside the cranium and runs backward
through the muscles, parallel to the vertebral column on a level
with the lateral line. It sends off numerous small twigs to the
sense organs of the lateral line canal.

3. Four branchial nerves, which can be seen through the floor
of the anterior cardinal vein, leave the outer side of the vagus
trunk. Each divides into a pretrematic and posttrematic branch; a.
pharyngeal branch, the last of which is the largest, arises from
each posttrematic.

4. Beyond the branchial nerves the remainder of the vagus
passes backward as the intestinal or visceral trunk, to the end of
the pharynx, where it divides into a number of branches which are
distributed chiefly to the wall of the stomach. Near the point of
this last division the vagus is crossed by the hypobranchial nerve,
which should be noted and preserved.

The occipital nerve penetrates the lateral wall of the cranium
close behind the root of the vagus and enters the canal of the vagus,
along which it passes. On emerging, it sends small branches to the
nearby muscle, while the principal portion runs on to join the
hypobranchial nerve. The occipital nerve will be found to arise
from the ventral surface of the medulla, below and behind the root
of the vagus, by two or more distinctly separated roots, which may
represent distinct nerves.

SPINAL NERVES. The spinal nerves are those nerves which arise
from the sides of the spinal cord. They differ from the cranial
nerves not only in their origin outside the cranium, but also in
that each spinal nerve arises by two roots which spring from the
spinal cord near the dorsal and ventral surfaces. Each root passes
through a foramen in the cartilaginous wall of the neural canal,
the ventral a little anterior to the dorsal, after which they unite to
form the spinal nerve. Between the junction of the roots and its
foramen the dorsal root contains a mass of ganglion cells, which
cause an enlargement known as the dorsal root ganglion. The
typical course of a spinal nerve is around the body to the ventral
surface, giving off branches to the muscles and skin of its segment.
A short distance from the vertebral column the spinal nerves lie just
outside the peritoneum, through which many of them can be seen
and followed to about the level of the lateral vein. At this point
they pass outward into the muscles of the body wall. To dissect
any of the spinal nerves make a longitudinal incision along the
lateral line and separate the dorsal muscle mass from the lateral
muscles for some distance. The dorsal muscles can then be pressed
toward the vertebral column and dissected away from the perito-
neum. The spinal nerves, lying against the peritoneum, will be
exposed and can be followed easily, first to their roots, next ven-
trally.

The hypobranchial nerve, to which attention was called at the
point where it crosses the vagus, is formed by the union of the
principal branches of the occipital and first two spinal nerves. The
third spinal nerve receives a branch from the second, and itself
accompanies the hypobranchial nerve closely without actually be-
coming a part of it. The union of nerves thus formed is known as
a plexus. After crossing the vagus the hypobranchial nerve forks,
one division passing medial to, the other lateral to the anterior
cardinal vein; both run ventrally, following the last gill arch, and
reunite on the lateral wall of the pericardium, forming a trunk

23

which runs forward. At the anterior end of the pericardium this
divides into a dorsal and a ventral branch which innervate the sur-
rounding muscles. The hypobranchial nerve innervates the skin
of the region immediately in front of the pectoral girdle, and the
coraco-arcualis communis, coracomandibularis, coracohyoideus, and
coracobranchialis muscles.

The third, fourth, fifth and sixth spinal nerves pass backward
and ventrad till they reach the level of the articulation of the pec-
toral fin with the girdle. Here they join to form a simple brachial
plexus, from which arise branches proceeding to the musculature
of the dorsal and ventral faces of the fin. The seventh to eleventh
spinal nerves pass downward to the level of the fin, and then branch,
one portion entering the muscles of the ventral body wall, while the
other passes into the depressor muscles of the fin.

The pelvic fin is innervated by eight or nine spinal nerves which
pass backward and downward along the medial edge of the septum
between the myomeres, entering the dorsal side of the fin along its
axis. No plexus is formed.

OLFACTORY ORGAN (NASAL SAC). Dissect away the skin and other
tissues around the nostril so as to expose completely the olfactory
organ; this will be found to be a dark-colored, nearly spherical
mass, of half the diameter of the eye, firmly attached at its base.
By cutting away the cartilage dorsal to the nasal sac its base will be
exposed, and the olfactory bulb will be shown to be closely adherent
to a considerable part of the postero-dorsal surface of the organ.
Numerous short nerves can be demonstrated to pass from the olfac-
tory bulb into the olfactory organ; all these nerves together are
considered as the first cranial or olfactory nerve. Remove the
olfactory organ from the head; divide it by a median longitudinal
cut; observe the arrangement and structure of its double series of
internal folds (lamellae), and the complete median septum.

VENTRAL SURFACE OF THE BRAIN. Cut the cord in two some dis-
tance back of the brain. Cut all cranial nerves just inside the
cranium and carefully lift the brain out. Parts of the ventral por-
tion of the brain lie in a recess beneath the mesencephalon and
must be disengaged very gently.

Identify and examine the ventral parts of the brain. Note the
considerable lateral compression of the mesencephalon. The optic
nerves cross beneath the diencephalon, forming the optic chiasma.
From the sides of the chiasma slightly elevated optic tracts, formed
by the fibres of the optic nerves, can be traced into the optic lobes.

Back of the optic chiasma the projecting ventral portion of the
diencephalon forms the hypothalamus. The posterior lobe of this
structure is the hypophysis or pituitary body.

The oculomotor nerves emerge over the posterior end of the
hypothalamus.

The ventral portion of the mesencephalon is formed by the
cerebral peduncles (crura cerebri) , columns of fibres passing be-
tween the myelencephalon and telencephalon.

The abducens nerves arise on the ventral surface of the myelen-
cephalon near the midline and just back of a line connecting the
roots of the auditory nerves.

The internal carotid arteries reach the brain at the sides of the
hypothalamus. Branches are sent upward and forward over the
surface of the brain. Anastomoses between the vessels of the op-
posite sides are formed anterior to the optic chiasma. The main
branches of the carotids pass backward along the sides of the hypo-

24

thalamus and unite behind this organ. The median artery thus
formed runs along the ventral surface of the myelencephalon and
the spinal cord. Numerous transverse vessels are given off to the
myelencephalon.

Identify the roots of the remaining cranial nerves.

CAVITIES OF THE BRAIN. Divide the brain into exactly equal
halves by a vertical longitudinal cut.

Each lobe of the prosencephalon contains a large cavity. These
are the prosocoels. They are commonly known either as the lateral
ventricles, or the left cavity as the first ventricle and the right as
the second ventricle. The prosocoels are continued into the olfac-
tory lobes, these portions being known as rhinocoels.

The thalamocoel is the cavity within the diencephalon, often
called the third ventricle. The prosocoels communicate with the
thalamocoel by lateral openings, the foramina of Monro. The roof
of the thalamocoel is very thin and is non-nervous; it is frequently
torn during the early dissection. Where the lobes of the prosence-
phalon meet the dorsal wall of the diencephalon this thin roof is
pushed into the prosocoels, carrying with it the pia mater and its
blood vessels, and thus forms vascular ingrowths known as the
choroid plexi. The thalamocoel continues above into the epiphysis
and below into the hypothalamus.

The myelocoel is the large cavity of the myelencephalon. It
also is frequently apparently open to the exterior at the posterior
end by the accidental breaking of the thin, non-nervous dorsal wall
of this region. The myelocoel is also known as the fourth ventricle.

The thalamocoel and myelocoel are connected by a narrow pas-
sage through the mesencephalon, the aqueduct of Sylvius (iter,
mesocoel) .

The optocoels are large cavities within the optic lobes which
open into the aqueduct of Sylvius.

A large metacoel in the metencephalon opens into the myelo-
coel. The myelocoel is also continued into the corpora restiforma;
posteriorly it joins the central canal which extends down the center
of the spinal cord.

DISSECTION OF THE EYE. Remove one of the eyes from its orbit,
and divide it into inner and outer halves by an equatorial cut around
the eyeball (not directly through it, as this tears the lens from its
fastenings). Place the halves under water and observe:

In the inner half:

The posterior chamber, the cavity of the eyeball which has been
opened. During life it is filled by a gelatinous substance, the
vitreous humor.

The retina, a delicate yellowish-white membrane lining the
interior of the eye, loosely attached to the outer coats except at the
point of entrance of the optic nerve.

The choroid coat, a thin, black membrane outside the retina. It
can be pulled away from the outer coat quite easily except near the
optic nerve.

The sclerotic coat, the outer coat of the eye. This is composed
of connective tissue having an almost cartilaginous consistency, is
only slightly pigmented, and is somewhat translucent. The muscles
of the eye are inserted upon the sclerotic.

In the outer half:

The ora serrata, an irregular line along which the retina ends.

The iris, a fold of the choroid extending inward like a shelf, and

25

perforated centrally to form the pupil. Around the iris the choroid
is folded radially into the ciliary processes.

The lens, a spherical body, transparent and elastic during life,
but opaque and hard in preserved specimens. It projects into the
pupil and is suspended from the ciliary processes by a delicate
membrane, the suspensory ligament.

The anterior chamber, in front of the iris and lens, filled with
a watery fluid, the aqueous humor.

The transparent cornea, forming the outer side of the eyeball,
continuous with the sclerotic.

Take out the other eye and cut it in two by a section through the
pupil and optic nerve. Review the relation of the parts.

THE SKELETON

There seems to be no easy way of cleaning the skeleton of dog-
fish which have been preserved in formalin or alcohol, the only
procedure being to cut, pick, and scrape the flesh away from the
skeleton. Time and patience are required, but if these are allowed
there is no reason why all the parts of the skeleton cannot be thor-
oughly studied. Specimens which have been preserved in brine are
more easily skeletonized.

The skeleton is entirely composed of cartilage which, in large
species of elasmobranchs and in old individuals of small species,
becomes impregnated with lime salts, in some cases to such an ex-
tent as to resemble soft bone.

The parts of the skeleton are frequently grouped under two
heads: the axial skeleton, comprising the skull and vertebral col-
umn; and the appendicular skeleton, including the pectoral and
pelvic girdles and the skeleton of the fins.

VERTEBRAL COLUMN. The vertebral column is divided into two
regions, thoracic and caudal, distinguished by the slightly different
character of the vertebrae. Remove the muscle and connective tissue
from the vertebral column for a short distance anterior to the first
dorsal fin. Care is required not to cut away small cartilages occu-
pying the positions of ribs. Now remove from the body about two
inches of the portion of the column exposed with any cartilaginous
parts which may be attached to the vertebrae. The vertical column
is made up of segments, called vertebrae. Each vertebra consists
of a large ventral mass, the centrum, and an arch, the neural arch,
roofing over the dorsal surface of the centrum; the arch is com-
posed of several small plates of cartilage. The opening enclosed by
each centrum and its neutral arch is the vertebral foramen; the
joined vertebral foramina form the neural canal, which is occu-
pied by the spinal cord.

Separate one of the vertebrae from the rest. The centrum is
deeply concave at each end; such a centrum is termed amphicoelous.
At the middle of the centrum the concavities meet and thus a canal
is formed through it. This canal and the spaces between the ends
of adjoining vertebrae are filled by the remains of the notochord, a
rather pulpy structure extending from end to end of the vertebral
column.

The concave faces of the vertebrae consist of much firmer car-
tilage than the remaining portions, sometimes even calcified. Make
a transverse section through the middle of a centrum and observe
the relations of the parts.

On each side of the centrum, near the ventral edge, is a plate-

26

like projection, the transverse process. Attached to the extremity
of this is a slender cartilaginous rib.

Each neural arch is made up of two distinguishable sets of
plates. The first consists of a pair of broad neural plates extend-
ing upward from each side of the centrum and uniting with each
other dorsally. Between the neural plates of two successive verte-
brae is a pair (one on each side) of intercalary plates which also
unite over the neural canal. The intercalary plates are over the
joint between the centra. Neural and intercalary plates together
make the lateral and dorsal walls of the neural canal. The rela-
tions of these plates can sometimes be seen best when the neural
arch is cleaned, then cut away from the centrum, and looked through
toward the light.

In the lower part of each neural plate is a small foramen which
allows the passage of the ventral root of the spinal nerve. A fora-
men for .the dorsal root is found at about the middle of the inter-
calary plate.

Clean and remove some of the caudal vertebrae from the region
just back of the cloaca. In general they have nearly the same struc-
ture and relations as the thoracic vertebrae, but have no transverse
processes and the plates of the neural arches are not so distinct.
There is also added a ventral arch similar in form to the neural
arch. This is the haemal arch, in which lie the caudal aorta and
vein. Its roof is the surface of the centrum, the sides are formed
by pairs of plates which correspond in number to the centra, and
unite with each other ventrally. Between the successive plates are
openings for the passage of branches of the artery and vein.

In this region foramina for the roots of the spinal nerves are
found only in every other pair of neural and intercalary plates.
Toward the tip of the vertebral column the relation of the neural
and intercalary plates to the centra becomes very irregular.

In Eugaleus the roof of the neural arch is formed by a row of small, dia-
mond-shaped plates which fit in between the other two sets. As these plates
correspond morphologically to the neural spines of higher vertebrates, they may
receive that name here. It is probable that the dorsal portion of the arch in
Squalus is composed of similar neural spine elements which have become fused
with the neural and intercalary plates of each side.

SKULL. The skull is entirely cartilaginous, and comprises three
principal divisions: (1) the cranium, an undivided mass of car-
tilage lodging the brain and the organs of smell, sight, and hear-
ing; (2) the jaws; (3) the visceral arches, or skeletons of the gill-
arches.

(1) The cranium. A blunt prolongation of the anterior ex-
tremity of the cranium forms the rostrum, which supports the soft
tissues of the snout. At each side of the base of the rostrum the
cranium widens abruptly. On the anterior face of the widened
portion and below the posterior angles of the rostrum is a pair of
protruding olfactory capsules, in which the olfactory sacs are en-
closed. An oval aperture in the posterior wall of each capsule
opens into the braincase and permits the passage of the olfactory
nerve through the cranium.

Back of the olfactory capsules are large lateral cavities, the
orbits. The dorsal edge of the orbit makes an overhanging ledge,
known as the supra-orbital crest. The projecting anterior and pos-
terior angles of the orbit are distinguished as the prae- and post-
orbital processes.

The portions of the cranium back of the orbit and at the sides

27

of the braincase form large lateral projections (auditory capsules)
containing the organs of hearing.

At the center of the nearly vertical posterior surface of the
cranium is a large opening, the foramen magnum, through which
the spinal cord passes.

At either side of and below the foramen magnum is a smooth
articulatory surface (occipital condyle) articulating with the cen-
trum of the first vertebra.

The flattened ventral surface of the posterior part of the cranium
forms the roof of the mouth, or palate.

In the mid-dorsal line of the cranium, between the prae-orbital
processes, is a small aperture opening into the brain cavity, the
epiphysial foramen. It is closed during life by a tough, fibrous
membrane. The stalk of the epiphysis extends to the under surface
of this membrane.

Between the auditory capsules is a deep depression in the roof
of the cranium in the floor of which can be seen the two small
pores through which the ducti endolymphatici pass into the cap-
sules. Close behind them are two larger openings for the perilymph
ducts.

A pair of foramina passes through the inner edge of the prae-
orbital process ; these permit the passage of the ophthalmic branches
of the trigeminal and facial nerves to the dorsal surface of the
snout. Near the bottom of the inner wall of the orbit is the foramen
of the optic nerve. In the postero-ventral angle of the orbit is the
large trigemino-facial foramen for the exit of branches of the
trigeminal and facial nerves; in front of it is the small oculo-
motor foramen. The extremely small foramen of the trochlear nerve
is almost directly above the optic foramen, near the top of the inner
wall of the orbit. Close below the trigemino-facial foramen is the
small passage for the abducens nerve. Below the abducens foramen
is the transbasal canal. Behind and below the trigemino-facial
foramen are two foramina, through which pass the hyomandibular
branches of the facial nerve. The foramen of the vagus nerve is
close to the foramen magnum, upon the posterior surface of the
cranium. The foramen of the glossopharyngeal nerve is lateral to
that of the vagus, near the postero-lateral angle of the cranium.

The cranium of Eugaleus is much like that of Squalus, except that the
rostrum is formed by three rods, two dorsal and one ventral, which arise from
the front of the brain case and converge anteriorally until they meet and fuse.
The olfactory capsules are much larger and of heavier cartilage than in Squalus.
The auditory region similarly is more prominent.

(2) The jaws. The jaws in reality are the first pair of visceral
or gill-arches, and in spite of the modification which has taken
place this relation can be seen easily in the adult shark. The
upper jaw consists of a pair of palato-quadrate cartilages, united
medially by ligament, and bearing the upper series of teeth. A
large hooked palatine process extends from each palato-quadrate
cartilage upward along the inner wall of the orbit. The lower jaw
likewise consists of a pair of MeckeVs cartilages, united medially
(the union is called the symphysis), and bearing the lower series
of teeth. A pair of small labial cartilages, which support the edges
of the labial pockets, lie at each corner of the mouth.

(3) Visceral arches. The first of the visceral arches is much
larger and heavier than the rest. It is known as the hyoid arch.
Each side of the arch consists of two rods of cartilage: (1) the
hyomandibular cartilage, which articulates with a distinct facet on
the lateral surface of the auditory capsule, and extends from here

28

downward, outward, and backward; (2) the cerato-hyal cartilage,
which is movably articulated to the hyomandibular and extends
downward, forward and inward. The ventral ends of the ceratohyals
are united by a median, plate-like basihyal.

The palato-quadrate and Meckelian cartilages are suspended
from the hyomandibular by several strong ligaments, the direct at-
tachments of the jaws to the cranium being of soft connective tissue
only. Both the hyomandibular and ceratohyal cartilages bear slen-
der rods (branchial rays) on their posterior edges, which support
the anterior wall of the first gill-pouch. Note the position of the
spiracle between the mandibular and hyoid arches. The anterior
wall of the spiracle is strengthened by two small, flat, vertical car-
tilages, probably homologous with the branchial rays of the gill-
arches.

The remaining five visceral arches differ little in their construc-
tion. Dorsally, each has a flat, sickle-shaped pharyngo-branchial
cartilage attached to the vertebral column by fibrous bands. The
pharyngobranchials of the Isfet two arches are fused. Ventrad to
each pharyngobranchial is an epibranchial cartilage. The next
segment of each arch is formed by the ceratobranchial cartilage. All
the epibranchials and ceratobranchials except those of the fifth
arch bear slender branchial rays. The ventral ends of the cerato-
branchials articulate with each other, the first being attached to the
ceratohyal by ligament. The second, third, and fourth arches have
another more ventral series of cartilages, the hypobranchials. The
lower ends of the hypobranchials are attached to a large median
plate, the basibranchial. The fourth ceratobranchial joins the third
hypobranchial, while the ceratobranchials of the fifth arch are at-
tached to the basibranchial directly. The basibranchial is com-
posed of two segments closely united by ligament; the anterior one
narrow, the posterior broad and flat in front, tapering to a sharp
point behind.

Short teeth of cartilage, called gill rakers, project into the
pharynx from the inner edges of the arches.

A dorsal and a ventral series of extra-branchial cartilages, thin,
slender plates, lie on the external side of each gill-arch.

PECTORAL GIRDLE AND FIN. Remove from the body the pectoral
girdle, with the fins attached, and carefully scrape off the muscles
from the cartilaginous parts. It will be found that the support of
the fin is partly of cartilaginous plates and rods, partly of horny
fibres (dermal fin-rays) which overlie the extremities of the car-
tilages and extend to the edges of the fin. These fibres are in two
layers, one beneath the skin of each side. They are formed in the
dermis. A similar arrangement of horny fibres is found in all the
other fins.

The pectoral girdle passes across the ventral surface of the body
and upward on each side to the level of the vertebral column. The
stout ventral bar presents numerous facets for the origin and inser-
tion of muscles. The articular surfaces for the pectoral fins are
well up on the sides of the girdle. The slender dorsal end of each
side of the girdle consists of a separate bar of cartilage, movably
articulated to the lower portion. The ascending limb of the girdle,
from the fin articulations to the base of the cartilage just mentioned,
is called the scapular portion; the small bar is the supra-scapular;
the ventral bar between the fin articulations is the coracoid portion.

The cartilaginous skeleton of the pectoral fin consists primarily
of a row of three basal cartilages, all articulating proximally with
the girdle. The middle basal is much the largest. Distal to the

29

basals are three rows of rod-like radial cartilages, the proximal row
being articulated to the basals.

PELVIC GIRDLE AND FIN. Remove the pelvic girdle from the body
with the pelvic fins attached, and clean away the muscles.

The pelvic girdle consists of an almost straight bar of cartilage,
slightly thicker at its middle than at its ends, which lies trans-
versely in the ventral wall of the abdomen. To each end is at-
tached a long basal cartilage which lies in the fin, close to and
parallel with its inner margin. A proximal series of slender radial
cartilages is attached to the lateral side of the basal; a distal series
of very short radials lies outside of the first series, while the portion
of the fin beyond these is supported by the dermal fin rays.

FIRST DORSAL FIN. Remove the mass of muscles on both sides
of the base of the fin down to the vertebral column. The principal
cartilages of the fin lie in the median connective tissue septum
which separates the dorsal musculature of the two sides of the body.
The basal cartilages of the fin are attached to the vertebral column
by means of this septum. It is best to remove the underlying por-
tion of the column with the fin. The cartilages can then be scraped
perfectly clean. The skeleton of the fin is composed of three rows
of cartilages: (1) a basal row consisting of one very large, flat
plate and two or three smaller ones posterior to it; (2) an inter-
mediate row of several plates of nearly equal size; and (3) a distal
row of several very small plates. The intermediate and distal rows
extend beyond the body musculature into the base of the fin. The
remainder of the fin is supported by the dermal rays. In front of
the cartilages which have been mentioned is the strong spine of
dentine (see p. 5), with its free portion sheathed by an enamel-
like covering.

SECOND DORSAL FIN. Remove this from the body in the same
manner as the first dorsal. Its structure follows the same general
plan, the differences being minor ones of shape, size, and number
of plates. Several thin cartilaginous plates are sometimes formed in
the median septum in front of the spine.

CAUDAL FIN. Only one side of the caudal fin should be cleaned,
as when both sides are cleaned there is danger of breaking the deli-
cate cartilages. The cartilaginous skeleton of the caudal fin con-
sists of a row of slender rods along the dorsal side of the vertebral
column, extending to its tip. There are no cartilaginous elements in
the fin ventral to the vertebral column. By far the greater part of
the caudal fin is supported by the two layers of horny fin-rays only.

MUSCULATURE

Dissect the skin off the head, neck, and body to back of the
pectoral fins. Observe first the musculature of the dorsal side of the
neck and of the body back of the bases of the pectoral fins, noticing
that it is composed of narrow, zigzag bands, called myomeres.
Where these are fully developed they extend from the mid-dorsal
to the mid-ventral line. Note carefully the relation of correspond-
ing myomeres of the two sides, the exact course of a single myomere,
and the direction of the muscle fibres in a typical myomere. Ob-
serve also that the muscles above the level of the vertebral column
form a thick mass, which is frequently referred to as the dorsal
musculature; the muscle below this level may be correspondingly
referred to as the ventral musculature. As the muscles described
below are dissected the mechanical effect of each should be de-
termined.

30

MUSCULATURE OF THE HEAD AND NECK. On the lateral and ven-
tral surfaces of the neck the primary relations of the myomeres are
much modified by the development of numerous special muscles,
yet here and there traces of the metameric arrangement still show.
Immediately beneath the skin is a thin sheet of muscle covering
most of the ventral and lateral surfaces of the throat as far back as
the pectoral girdle. On the ventral surface a triangular space is
left in front of the pectoral bar; on the sides of the neck the sheet
extends back to the last gill-cleft; dorsally, it reaches to the upper
extremities of the gill pouches. This is the constrictor superficialis
muscle. It is attached to fasciae dorsally and ventrally, and to the
extra-branchial cartilages.

The constrictor superficialis consists of six metameric segments.
The four posterior ones are distinctly limited by the gill-slits and
extra-branchial cartilages. The second is anterior to the first gill-
slit, the largest of all, with distinct dorsal and ventral portions ex-
tending forward above and below the jaws. The first is recognized
as consisting of two distinct parts, on the dorsal and ventral sur-
faces of the head. The dorsal portion is a small curved muscle on
the anterior wall of the spiracle, extending from the external sur-
face of the auditory capsule to the inner surface of the lower jaw.
It lies close against the levator marillae superioris (see below). On
the ventral surface of the throat the posterior constrictor muscles
of the two sides are separated by a large triangular area. In front
of this the ventral portions of the first and second constrictors meet
in a median aponeurosis, from which their fibres extend trans-
versely, those of the first to the mandibular cartilage, those of the
second to the hyoidean cartilage. The first constrictor lies ventral
or superficially to, and largely covers the second. Reflect the
first constrictor from the aponeurosis outward, and demonstrate
the two layers of muscle. It will be noted that the second to sixth
constrictors consist of united dorsal, lateral and ventral portions,
while the first is reduced to widely separated dorsal and ventral
parts.

On each side of the head, just outside the angle of the mouth,
is a large, thick muscle arising from the lateral surface of the
cranium, and inserted upon the outer surface of the mandible, the
adductor mandibularis.

In front of the small dorsal constrictor superficialis 1, and
scarcely separated from it, is the strong levator maxillae superioris,
which extends from the lateral surface of the auditory capsule to
the dorsal edge of the palatoquadrate cartilage.

Eugaleus. The dorsal portion of the constrictor superficialis 2 reaches
above the spiracle to the postorbital process. Reflecting it, the adductor
hyomandibulae is seen behind the spiracle, arising from the upper part of the
side of the auditory capsule and inserted on the end of the hyomandibular
cartilage. The levator palpebrae inferioris arises under the origin of the levator
hyomandibulae, and passes forward and downward between the spiracle and
postorbital process, to insert in the posterior end of the lower eyelid. The
depressor palpebrae superioris arises from the fascia dorsal to the spiracle,
passes mediad to the levator palpebrae inferioris, upward and forward, to insert
in the posterior end of the upper eyelid. Remove these muscles. The infraor-
bital canal passes mediad to the muscles of the eyelids. The levator maxillae
superioris lies between the spiracle and the orbit. Behind it is a small slip
of muscle extending from the anterior wall of the spiracle to the lateral surface
of the auditory capsule which seems to represent the constrictor superficialis
dorsalis 1.

A thin sheet of muscle covers the anterior face of each inter-
branchial septum. At the surface these pass into the constrictor

31

superficialis, and are evidently portions of the latter muscle, though
they are named the musculi interbranchiales.

Above the constrictor superficialis, lying on the side of the
neck between it and the dorsal musculature, is a broad trapezius
muscle. It arises from the fascia covering the lateral surface of the
dorsal musculature. Its fibres pass obliquely downward and back-
ward, mediad to the posterior gill-pouches, to insert upon the an-
terior edge of the scapular portion of the pectoral girdle. The
anterior portion of the trapezius is also inserted upon the epi-
branchial of the fifth gill-arch.

Just in front of the mouth is a pair of strong muscles (levator
labialis superioris) , each arising from the ventral surface of the
cranium close to the median line. They pass into strong tendons
which are inserted among the fibres of the ventral portion of the
adductor mandibulae. The muscle mass in front of the mouth and
the lower part of the adductor mandibularis thus form the two
bellies of a digastric muscle, with the tendon between them.

* Remove the ventral portions of the first and second superficial
constrictors and clear the mass of muscles lying between the coracoid
portion of the pectoral girdle and the mandible. Immediately in
front of the girdle are two large muscles, the coraco-arcuales com-
munes, whose fibres run inward and forward. These muscles
cover the ventral surface of the pericardium, to the wall of which
their median fibres are attached, while the lateral fibres are at-
tached around the ventral ends of the gill-arches.

In front of the coraco-arcuales communes are three large longi-
tudinal muscles. The median, unpaired one, arising from the fascia
between the coraco-arcuales communes and inserted upon the pos-
terior surface of the lower jaw, is the cor aco -mandibularis. The
other two, which lie dorsal to and outside of the coraco-mandibu-
laris, are the coraco-hyoidei. They arise from the fasciae covering
the anterior ends of the coraco-arcuales communes and posterior
parts of the coraco-branchiales, and insert upon the basihyal.

Dissect out the coraco-mandibularis and coroaco-hyoidei, noting
particularly the form and place of origin of the latter. Dorsal to
the coraco-hyoidei are the first divisions of the right and left
coraco-branchialis muscles, which arise from fascia covering the
anterior ends of the coraco-arcuales communes, and are inserted
upon the ventral extremity of the ceratohyal cartilage. Notice that
they pass dorsad to the anterior branches of the aorta, and that
the aorta itself can be exposed between them.

Remove the coraco-arcualis communis by dissecting it from
the pectoral girdle and reflecting it forward. The other four
divisions of the coraco-branchialis are now revealed, attached to the
lateral surface of the pericardium and the lateral portion of the
coracoid. The divisions of the muscle are clearly separated only
near their insertions. The second, third, and fourth coraco-branch-
ials are attached to the hypobranchial cartilages of the second, third
and fourth visceral arches. The fifth division is inserted upon the
lateral portion of the basibranchial and the expanded medial end
of the fifth ceratobranchial.

The first aortic branch passes ventral to the first coraco-branch-
ial. The second aortic branch passes between the first and second

*The coraco-mandibularis, coraco-hyodeus, coraco-arcualis com-
munis, and coraco-branchialis muscles should be dissected as a pre-
liminary operation to following the ventral aorta and its branches.

32

coraco-branchials. The third aortic branch passes between the sec-
ond and third coraco-branchials. The fourth aortic branch passes
between the third and fourth coraco-branchials.

Expose the dorsal ends of the gill arches by clearing away
muscles and other tissues between the gill pouches and the spinal
column. Two sets of four small muscles (interarcuales) will be
found connected with the branchial cartilages. The second, third
and fourth medial interarcuales extend from the posterior surfaces
of pharyngo-branchial cartilages 1, 2, and 3, to the dorsal surfaces
of pharyngo-branchials 2, 3, 4, and 5. The first medial inter-
arcuale arises from the under surface of the cranium and inserts on
the upper end of the first pharyngo-branchial.

The lateral interarcuales lie immediately below the medials. The
first has a double origin, most of the fibres arising from the lower
part of the posterior edge of the first pharyngo-branchial; a smaller
bundle from the anterior edge of the second pharyngo-branchial.
Its insertion is along the dorsal surface of the first epibranchial
cartilage. The second and third lateral inter-arcuales are like the
first, but since the fourth and fifth pharyngo-branchials are fused,
the origin of the fourth lateral interarcuale is not divided.

The circular muscles of the oesophagus are strongly attached to
the last pharyngo-branchial.

MUSCULATURE OF THE PECTORAL FIN. The dorsal muscle of the
fin (levator-retractor) arises mostly from the scapular portion of
the girdle, with a small part arising from the fascia covering the
lateral musculature of the body. It is attached in fasciculi to the
dorsal surfaces of the cartilaginous rays. The ventral muscle (de-
pressor-protractor] arises from the median portion of the girdle and
is inserted upon the cartilaginous rays in similar fasciculi. A por-
tion of the lateral body muscles is inserted upon the scapular por-
tion of the girdle.

MUSCULATURE OF THE PELVIC FIN. Ventral surface: An ad-
ductor muscle has origin upon the postero-lateral edge of the girdle;
it is inserted upon the antero-medial surface of the basal cartilage
of the fin. The depressor muscle consists of small fasciculi, each
corresponding to a cartilaginous ray. They arise from the postero-
lateral surface of the basal cartilage and are inserted upon the distal
extremities of the rays.

Dorsal surface: The abductor arises from the fascia covering
the trunk muscles, and inserts upon the fascia covering the intrinsic
muscles of the dorsal side of the fin. These latter (levatores) are
arranged in exactly the same manner as the fasciculi of the de-
pressor.

MUSCULATURE OF THE DORSAL FINS. A sheet of muscle is at-
tached to each side of the anterior dorsal fin, extending nearly up
to the bases of the dermal fin-rays. This muscle passes downward
between the dorsal body musculature of the two sides. Part of the
fibres arise from the fasciae covering the medial surfaces of the
body muscles, part from the basal cartilage of the fin itself. They
are inserted upon the lateral surfaces of the broad cartilaginous fin-
rays. The muscles of the posterior dorsal fin are exactly similar in
arrangement.

MUSCULATURE OF THE CAUDAL FIN. There is no special muscu-
lature for the dorsal portion. A narrow, band-like muscle is found
on each side of the ventral portion, widest above the triangular
ventral lobe. The fibres of this muscle arise upon the flattened,
expanded ends of the haemal spines. They pass obliquely backward
and upward to be inserted in the fascia underlying the skin.

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