Part 2

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.