Part 3

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.