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Title: A Guide for the Dissection of the Dogfish (Squalus Acanthias)

Author: Lawrence Edmonds Griffin

Release date: May 16, 2019 [eBook #59524]

Language: English

Credits: Produced by WebRover, Gina, MWS, Barry Abrahamsen, and the
        Online Distributed Proofreading Team at http://www.pgdp.net
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*** START OF THE PROJECT GUTENBERG EBOOK A GUIDE FOR THE DISSECTION OF THE DOGFISH (SQUALUS ACANTHIAS) ***




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Online Distributed Proofreading Team at http://www.pgdp.net
(This file was produced from images generously made
available by The Internet Archive)







                       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




------------------------------------------------------------------------




                       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 Eugaleus galeus is more frequently named
either Mustelus canis or Galeus canis. The histories of these names and
systematic descriptions of the species are contained in Samuel Garman’s
Monograph 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 mature 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 differences 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 arrangement 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.


                          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 elevated),
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 fin-rays 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 characteristics
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 considerable size in
adults, are modified portions of the pelvic fin used as copulatory
organs. They are named variously _claspers_, myxopterygia, 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 projecting 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 examined under a dissection
microscope. The exact form and arrangement 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 expose the cartilages
for examination. (The labial pocket of Eugaleus is much smaller, and in
front of the corner of the 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 opening 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 considerable
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, deprived of its
respiratory functions.

In the center of the dorsal surface of the head, between the 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 several 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 enlargement, 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 Lorenzini. Their
function is not well understood, but it has been suggested 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 _supra-orbital_, 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 midline 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
_supratemporal_ 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. Similar 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 magnification. 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 denticle 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 surfaces 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 pectoral 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 abdominal 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 backward it enters the
intestine.

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

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, having the
genital glands and ducts only partly developed. In mature females the
_oviducts_ are conspicuous tubes ventral to the kidneys. In young
specimens they appear as slender, white tubes extending 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 opening 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, forming 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 somewhat 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 dissected
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 _mesenteries_. 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 _gastro-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 mesentery,
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 (mesentery 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, joining the mesogaster dorsal to the stomach
and the mesentery above the intestine.

A small division of the right lobe of the liver stands out between 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 dissection.

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 interior. 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 irregular folding of the mucous coat,
depending upon the degree of contraction of the muscular coat.

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, observing
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, exposing a considerable surface
of the valve. Wash well, and observe 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, Wolffian
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 _testes_ 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 forward, and the kidney tissue
overlying it diminishes. At the anterior 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 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
extremities 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 supports 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 covered 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 forward, 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 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, segmentally 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 oviduct 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 uro-genital
sinus.


                           RESPIRATORY ORGANS

Open the anterior gill pouch of the left side by dorsal and ventral 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 prevent 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 _demibranch_. If
the specimen is injected a large blood vessel can be seen through the
skin in the inner border of the demibranch, 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 number of the
demibranchs 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.[1] 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 Eugaleus 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.

Footnote 1:

  See footnote, p. 31.

In front of the pectoral girdle lies a thin-walled sac, the _pericardial
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
pericardial sac anteriorly, while posteriorly it opens into a large
muscular chamber, the _ventricle_. Dorsal to the ventricle, and
projecting 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. Follow the branches
of the aorta outward on the left side and demonstrate 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 variation 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 _inferior jugular vein_. Lateral to this is
frequently a somewhat larger opening of the _anterior cardinal vein_.
This is absent, however, in 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 pectoral
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 receives a
good-sized _pectoral vein_ from the pectoral fin, and sometimes several
smaller veins from the same region. Traced dorsad it is found to open
into a large blood sinus above the liver and oesophagus, 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 midline 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 enlargement 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 kidneys. These are the
_renal portal veins_, conveying blood to the kidneys. 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 cuvierian
duct. Trace it forward along the ventral ends of the gill pouches; it
receives vessels from the arches and finally joins the _hyoidean_
_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 undertaken.

Just back of the spiracle the anterior cardinal receives the _hyoidean
vein_, which passes ventrad along the base of the first demibranch 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. Commencing 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 particularly
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 vessel 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 succeeding 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 midline to unite with a longitudinal artery, the
_hypobranchial artery_, 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 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 diverge,
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
terminations 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 anteriorly along
the vertebral column.

Near the divisions of the common carotids two _anterior vertebral
arteries_ arise from these vessels and pass posteriorly, often
anastomosing 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 branchial, 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 directly 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 he followed
along the dorsal side of the pericardium and then outward 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 intestinal arteries_.
The first gives off a small _hepatic artery_ to the liver 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 _anterior
mesenteric artery_; it passes to the left side of the large intestine
and its branches anastomose more or less with those of the anterior
intestinal artery. The posterior vessel is the _lienogastric_; 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 observe the vertical
slit-like opening into the auricle and the two membraneous 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-ventricular
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.

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 system 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 follow
closely the arteries of the digestive organs. Trace the following 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_ joining 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 required
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 connection with such a review.

If a line be drawn over the dorsal surface of the head connecting 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 depression of the skull below them. These tubes
are the _ducti endolymphatici_, 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 internal 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 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 perichondrium of the
internal surface of the cranium. The _pia mater_ envelops closely the
brain and cord, and contains numerous blood vessels. Between the two is
the _arachnoid space_, traversed by occasional 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 oblongata_
(_myelencephalon_). The roof of the medulla is extremely thin, and is
broken if the cartilage has not been removed with extreme 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 belonging 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 cartilage 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 endolymphatic duct opens,
and three membraneous tubes (_semicircular canals_) external to the
chamber but communicating with it in various 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 horizontal plane (_horizontal or
external semicircular canal_). At the ventral ends of the vertical
canals are nearly spherical enlargements 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 ventral extremity of the anterior
vertical canal and the anterior extremity of the horizontal canal open
beside each other into an anterior projection of the sacculus. The
ventral extremity of the posterior vertical canal opens into the
posterior and lower part of the sacculus. The posterior extremity of
the horizontal canal opens into the posterior side of the sacculus.
During life the utriculo-sacculus and the semicircular canals are
filled with a lymphatic fluid, and the sacculus contains a large
calcareous ear-stone (_otolith_), which is usually dissolved by the
formalin used in preserving the dogfish.

Whitish patches of thickened sensory epithelium may be seen in the
ampullae (_cristate acusticae_) and in the utriculo-saccular chamber
(_maculae acusticae_). Branches of the eighth nerve can be followed to
all these areas.

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

EXTERNAL FEATURES OF THE EYE. Observe the transparent cornea covering
the external surface of the eye; the dark ring of the iris; the central
opening in the iris, the pupil; the conjunctival sac surrounding the
external half of the eyeball. Cut away sufficient of the upper wall of
the cartilaginous orbit to expose the eyeball and its muscles. Note the
considerable amount of soft connective tissue around the eye and explore
the orbital sinus (p. 13). Take notice of the following nerves, in order
to ensure their preservation until the time comes to trace them more
completely. A large nerve crossing the medial side of the orbit, the
superficial ophthalmic; a nerve leaving the cranium opposite the optic
lobe, passing under the superficial ophthalmic to the anterior muscle of
the eyeball, the trochlear; several long ciliary nerves passing to the
eyeball; several other nerves visible in the deep angle of the orbit.

Six muscles move the eye. Four of these arise close together at the deep
postero-medial angle of the orbit. Diverging, they are inserted upon
four sides of the eyeball, and from the position of their insertions are
named the _superior_, _posterior_, _inferior_, and _anterior recti_. Two
muscles arise from the antero-medial angle of the orbit, the _superior
and inferior oblique muscles_.

Between the recti muscles can be seen a mushroom-shaped stalk of
cartilage, the _ophthalmic peduncle_; the eyeball rests against its
expanded end. (There is no peduncle in Eugaleus.)

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

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

The _terminal nerve, Nervus terminalis_, is a slender nerve running
along the medial surface of the stalk of the olfactory lobe. Follow it
backward to its origin on the anterior surface of the cerebral
hemisphere, deep in the median fissure (in Eugaleus on the ventral
surface). Trace it forward over 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 considered 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 immediately
into three parts; two pass to the anterior and superior recti
respectively, while the third passes downward along the posterior
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 partial 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     superficial
                      ophthalmic      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.

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 ophthalmic. 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 surface 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 Lorenzini.

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 divides 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 innervate 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. Several nerves pass
from the hyomandibular trunk and the hyoid branch to the dorsal
superficial constrictors.

4. The _palatine_ branch springs from the base of the hyomandibular
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 anterior 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 thoroughly 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 midline 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 glossopharyngeal 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
perichondrium from the posterior surface of the auditory capsule.

The _vagus nerve_ (or _pneumogastric_) 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.

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 peritoneum. The spinal nerves, lying
against the peritoneum, will be exposed and can be followed easily,
first to their roots, next ventrally.

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

The third, fourth, fifth and sixth spinal nerves pass backward and
ventrad till they reach the level of the articulation of the pectoral
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 olfactory 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 distance back of
the brain. Cut all cranial nerves just inside the cranium and carefully
lift the brain out. Parts of the ventral portion 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 oculo-motor 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 between the
myelencephalon and telencephalon.

The abducens nerves arise on the ventral surface of the myelencephalon
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 opposite sides are
formed anterior to the optic chiasma. The main branches of the carotids
pass backward along the sides of the hypothalamus 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 olfactory 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 prosencephalon 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 passage 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 myelocoel. 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
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 dogfish 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 thoroughly 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 extent as to
resemble soft bone.

The parts of the skeleton are frequently grouped under two heads: the
_axial skeleton_, comprising the skull and vertebral column; 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 occupying 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 composed 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 occupied 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 cartilage 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-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_ extending upward from
each side of the centrum and uniting with each other dorsally. Between
the neural plates of two successive vertebrae 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 relations 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 foramen for the
dorsal root is found at about the middle of the intercalary plate.

Clean and remove some of the caudal vertebrae from the region just back
of the cloaca. In general they have nearly the same structure 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,
diamond-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 cartilage
lodging the brain and the organs of smell, sight, and hearing; (2) the
_jaws_; (3) the _visceral arches_, or skeletons of the gill arches.

(1) The cranium. A blunt prolongation of the anterior extremity 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 enclosed. 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 posterior angles of
the orbit are distinguished as the prae- and postorbital processes.

The portions of the cranium back of the orbit and at the sides 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 centrum 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 capsules. 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 braincase 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 _Meckel’s
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 downward,
outward, and backward; (2) the _ceratohyal_ 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 attachments of the
jaws to the cranium being of soft connective tissue only. Both the
hyomandibular and ceratohyal cartilages bear slender 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 cartilages, probably
homologous with the branchial rays of the gill arches.

The remaining five visceral arches differ little in their construction.
Dorsally, each has a flat, sickle-shaped _pharyngo-branchial_
cartilage attached to the vertebral column by fibrous bands. The
pharyngo-branchials of the last two arches are fused. Ventrad to each
pharyngo-branchial 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 ceratobranchials
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 attached
to the basibranchial directly. The basibranchial is composed 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 cartilages 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 insertion 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 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 transversely
in the ventral wall of the abdomen. To each end is attached 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 portion 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 intermediate 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 delicate
cartilages. The cartilaginous skeleton of the caudal fin consists 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 corresponding myomeres of the two sides,
the exact course of a single myomere, and the direction of the muscle
fibres in a typical myomere. Observe 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 determined.

MUSCULATURE OF THE HEAD AND NECK. On the lateral and ventral 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
extending forward above and below the jaws. The first is recognized as
consisting of two distinct parts, on the dorsal and ventral surfaces of
the head. The dorsal portion is a small curved muscle on the anterior
wall of the spiracle, extending from the external surface 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 transversely, 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 palato-quadrate 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
infra-orbital 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 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 backward, mediad to
the posterior gill pouches, to insert upon the anterior edge of the
scapular portion of the pectoral girdle. The anterior portion of the
trapezius is also inserted upon the epibranchial 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.

[2]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 communes_, 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 attached around the ventral ends
of the gill arches.

Footnote 2:

  The coraco-mandibularis, coraco-hyodeus, coraco-arcualis communis, and
  coraco-branchialis muscles should be dissected as a preliminary
  operation to following the ventral aorta and its branches.

In front of the coraco-arcuales communes are three large longitudinal
muscles. The median, unpaired one, arising from the fascia between the
coraco-arcuales communes and inserted upon the posterior surface of the
lower jaw, is the _coraco-mandibularis_. The other two, which lie dorsal
to and outside of the coraco-mandibularis, 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 coraco-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-branchials 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-branchial.
_The second_ aortic branch passes between the first and second
coraco-branchials. _The third_ aortic branch passes between the second
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 interarcuale 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 interarcuales 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
(_depressor-protractor_) arises from the median portion of the
girdle and is inserted upon the cartilaginous rays in similar
fasciculi. A portion of the lateral body muscles is inserted upon
the scapular portion of the girdle.

MUSCULATURE OF THE PELVIC FIN. Ventral surface: An _adductor_ 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 depressor.

MUSCULATURE OF THE DORSAL FINS. A sheet of muscle is attached 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 musculature 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.


------------------------------------------------------------------------




 ● Transcriber’s Notes:
    ○ Missing or obscured punctuation was silently corrected.
    ○ Typographical errors were silently corrected.
    ○ Inconsistent spelling and hyphenation were made consistent only
      when a predominant form was found in this book.
    ○ Text that was in italics is enclosed by underscores (_italics_).









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