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The Digestive Tract and Body Cavities

Body Cavities

Vertebrates have a coelomic body cavity. This coelomic space is divided anteriorly into a pericardial (heart) cavity and a posterior pleuroperitoneal cavity by the transverse septum, a tough, white membrane. This is the situation in the dogfish and Necturus. In mammals, the thoracic cavity is subdivided into a central pericardial cavity and paired lateral pleural cavities around the lungs. The esophagus runs through this cavity, but we will be looking primarily at the posterior cavity, the abdominal cavity in this lab. These cavities are separated by a muscular diaphragm in mammals.

The posterior pleuroperitoneal, visceral or abdominal cavity, houses the liver, digestive tract, and gonads. This body cavity has muscular walls (mesoderm). The visceral cavity is lined on the inside with a transparent parietal (somatic) peritoneum. It is attached to the muscles of the body wall but it also overlies the urogenital system, which is retroperitoneal. The parietal peritoneum from each side meet dorsally and ventrally to form a double walled mesentery. This splits to line the digestive tract and other organs as the splanchnic (visceral) peritoneum. The peritoneum is serous (wet). The fluid serves as a lubricant to allow frictionless movement of the organs.

The primary mesenteries are dorsal and ventral, although the dorsal mesentery is often interrupted and moved to one side or the other with the organs and the ventral mesentery is reduced to the membranes of the liver and bladder. Mesenteries running from organ to organ are usually called ligaments.

Digestive Organs

The digestive tract is a tube, with coils and branches, which begins at the mouth and ends either at a cloaca or anus. It processes food, which moves by peristalsis through the process of digestion, absorption and elimination. The general pattern is to have an oral cavity, pharynx, esophagus, stomach and intestine. Accessory organs are the pancreas, liver and gallbladder, which arise as evaginations from the embryonic digestive tract. We will be looking at the variations and similarities in the digestive tracts of the dogfish, salamander and mammal in this lab.

Dogfish Digestive Tract

Use scissors to cut through the body wall. Make your longitudinal cut off center from the midventral line and out to the pectoral and pelvic fins, then turn back the flaps. Note the falciform ligament hanging midventrally from the liver in the anterior half of the pleuroperitoneal cavity. You will have to cut this ligament to look into the cavity. Other remnants of the ventral mesentery are the lesser omentum from the liver to the stomach and small intestine and reproductive mesenteries, which will be looked at later. The dorsal mesentery is seen as the greater omentum holding the esophagus and stomach to the dorsal body wall and dorsal mesentery holding the intestine. From the stomach to the spleen is a gastrosplenic ligament.

The oral cavity contains the tongue and teeth. The pharynx is the portion leading past the spiracle and five gill slits and also contains the tongue in dogfish. Note the taste buds on the tongue. To view this area, cut through the left jaw of the dogfish and perpendicularly through the center of the gills to the pectoral fin. Fold a paper towel over the teeth and use your scissors to open the mouth wide. Make sure you cut all the way through to the pharynx. Cut horizontally across to the other pelvic fin, making sure you are posterior to the transverse septum. You do not want to cut into the pericardial cavity. You will have to cut through the esophagus until you get to the gill slits on the other side. Pull the lower jaw open and clear out any debris. The esophagus extends from the pharynx at the transverse septum and is lined with papillae, which form a tight seal to keep water out. Cut into the esophagus to see the papillae and the stomach with its longitudinal folds called rugae. The "U" shaped stomach, with an anterior cardiac limb and a posterior pyloric limb ends in a constricted pyloric sphincter. The small intestine is composed of, anteriorly, a duodenum, and posteriorly, an ileum, which contains a spiral valve. Cut into the ilium to see the valve, which increases the surface area. The large intestine, rectum or colon, a shorter section than the small intestine has a rectal gland entering it. The rectal gland is for salt excretion for osmoregulation. The rectum, the most posterior portion, ends in the anus which projects into the cloaca, a common opening with the urogenital ducts.

Digestive Organs

Liver, composed of three lobes, and a greenish gall bladder. These extend posteriorly from the transverse septum. Note the bile duct, from the gall bladder, which goes to the duodenum along with two hepatic vessels. Pancreas, consisting of two lobes; a ventral lobe overlying the duodenum, and a dorsal lobe, in the curve between pyloric stomach and duodenum. The pancreatic duct is usually difficult to locate, it runs from the junction of the lobes into the duodenum.

Spleen, an organ of the circulatory system (lymphoid tissue), extends posteriorly from the curvature of the stomach.

Necturus Digestive Tract

Use your scissors to make a longitudinal cut off center from the cloaca to the transverse septum. Note the mid-ventral falciform ligament. Cut across the posterior edge of the transverse septum avoiding the pericardial cavity. Like the dogfish, the main digestive tract is in the pleuroperitoneal cavity. The greater omentum extends from the dorsal wall to the stomach and the gastrosplenic ligament goes from the stomach to the spleen. The dorsal mesentery holds the intestines to the dorsal wall; it is called the mesocolon at the large intestine. Other ligaments support the lungs and urogenital tract. Be careful not to destroy these delicate membranes when observing the organs.

Use your scissors to cut through the left jaw and ventral to the gills to the pectoral girdle. Cut horizontally through the esophagus to open up the mouth. Be sure to avoid the pericardial area and any blood vessels. In the oral cavity are teeth, two rows in the upper jaw, and one in the lower. The tongue is better developed than that of the shark. The pharynx has two pairs of gill slits. On the posterior floor of the pharynx lies the glottis, a slit that leads into the trachea. From the pharynx to the stomach is a short, poorly defined esophagus. The straight stomach has internal rugae and ends with a muscular pyloric sphincter. The small intestine is the duodenum anteriorly, and then a long coiled portion with internal small folds called plicae, which increase the surface area. There is a short large intestine, which enters the cloaca via the anus.

Digestive Organs

The large liver is not lobed but weakly scalloped posteriorly. The gall bladder is on the liver near the duodenum. The hepatic ducts and bile duct empties into the duodenum as in the shark, but may be difficult to see. The irregular pinkish mass of the pancreas lies on the hepato-duodenal ligament and is fused into one. Two small ducts lead to the duodenum. The pancreas is both an exocrine gland, producing digestive enzymes and an endocrine gland regulating metabolism. The spleen, a lymphatic organ, lies to the left of the stomach.

Mammal Digestive Tract

In mammals, the coelomic cavity is divided into thoracic and abdominal cavities by the diaphragm. The thoracic cavity will be examined in future labs. Make an incision in the midventral line through the abdominal wall muscles of the cat or rat from the sternum to about an inch anterior to the clitoris or penis. Make lateral incisions at the extremities of this first incision and deflect the flaps. Exposed is the abdominal or visceral cavity. Between the diaphragm and the liver are three ligaments, the most prominent of which is the falciform ligament, which connects the right and left liver halves to the abdominal wall. The greater omentum is double layered and filled with fat in cats. It extends from the greater curvature of the stomach to the dorsal body wall and extends to the pelvic region covering and tucking under the intestine. The portion of this mesentery from the stomach to the spleen is the gastrosplenic ligament. From the lesser curvature of the stomach and duodenum to the liver is the lesser omentum. The dorsal mesentery holds the intestines to the dorsal body wall. Other mesenteries hold the urogenital system in place. Try not to tear any of the mesenteries when doing your dissection.

Glands

During the course of your dissection you will remove or view the following glands. For today’s lab, observe the three salivary glands, the submaxillary, sublingual and parotid glands, which are located in the head and neck region.

Table of Glands and Functions

Gland Location Function

Submaxillary central throat, large salivary

Sublingual anterior on submaxillary, whitish mucous

Parotid towards ear, outside submaxillary, soft salivary

Lacrimal below ear tears

Lymph nodes 4, above submaxillary, dark, round lymphocytes

Thymus above heart, large on young animals lymphatic-disease

immunity

Thyroid side of larynx, dark red ion and calcium

binding

Adrenal above kidney adrenalin or

epinephrines

Digestive System

The digestive tract starts with the mouth and its associated salivary glands. To view the inside of the mouth, cut through the left jaw after prying the mouth open with a folded paper towel over the teeth. Be sure you are not trying to cut through the molars. Bone cutters may need to be used for the cat. To open the lower jaw, cut through the soft palate. In the oral cavity note the differentiated teeth and the mobile tongue. Lift the tongue to see the lingual frenulim. Note the papillae on the surface of the tongue. Feel the roof of the mouth with its anterior hard palate and posterior soft palate. The pharynx extends from the oral cavity to the larynx and allows passage of food and air. The food, when swallowed, travels down the tubular esophagus, which lies dorsal to the larynx. The soft, distensible esophagus penetrates the diaphragm separating the thoracic and abdominal cavities. Food is moved down it by peristalsis. The esophagus ends at the cardiac sphincter. The J-shaped stomach ends at the pyloric sphincter. The convex side is the greater curvature; the concave side is the lesser curvature. The small intestine starts at the pyloric sphincter with the duodenum a long curved piece, then begins looping back and forth, still descending as the jejunum. The ascending, looping portion is the ileum and it ends in a T-junction with the large intestine. The blind end of the T is the caecum or appendix, large in the rat, small in cats and humans. The other end is the ascending portion of the large intestine or colon followed by a short transverse portion and a descending portion, which ends in a muscular rectum hidden under the pelvic girdle. It opens at the anus, controlled by sphincter muscles, at the base of the tail. The enlarged and constricted areas of the colon are due to peristalsis.

Digestive Organs

The dark reddish brown lobes of the liver (five in rat, six in cat) are attached to the diaphragm. The central lobe has a gall bladder in a cat, but it is absent in the rat, although both animals have a bile duct leading to the anterior duodenum. The mesentery called the greater omentum, stretching from the spleen to the duodenum contains the two lobes of the pancreas, which look like pink granular bubble gum. One lobe runs from the pyloric sphincter to the spleen, the other along the edge of the duodenum. They meet anteriorly forming one duct, which enters the duodenum beside or with the bile duct. The dark reddish brown spleen is an elongate lobe on the left side just below the stomach.

Assignment:

Fill in the table with the functions of the digestive tract, comparing the digestive system of a mammal with Necturus and the Dogfish. For equivalent structures, write same, absent or the new structure.

Organ	Function in Mammal	Necturus Equivalent	Dogfish Equivalent
Esophagus
Stomach
Pyloric sphincter
Small intestine
Large intestine
Caecum
Anus
Pancreas
Spleen
Gall bladder
Liver

updated by Sandra Millen, December, 2003 

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