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.