9 



The Viscera 



Use of the terms "digestive" and "respiratory" systems 

 has been avoided in the title of this chapter since parts of 

 other "systems" are also discussed. In a dissection the parts 

 and organs of various systems are observed together and are 

 best described together, leaving their separation into "func- 

 tional units" to a physiological approach. Viscera are defined 

 as the soft contents of the principle body cavities: the en- 

 trails together with the heart, liver, lungs, etc. The kidneys 

 and gonads are described separately in Chapter 10, and a 

 description of the heart will be deferred to Chapter 11. .'^ 

 discussion of gills is included here as a part of the "respira- 

 tory system." 



THE DIGESTIVE TRACT AND 

 ASSOCIATED STRUCTURES 



The digestive tract of the vertebrate appears to have re- 

 mained largely unchanged through the evolution of the 

 several major groups but has shown extreme adaptive plas- 

 ticity in some species of each of these. This seemingly erratic 

 variation has probably discouraged comparative study. 



In studying the digestive tract, most of the observations 

 are made in ventral view, but it seems best to use orienta- 

 tion terms referable to the specimen itself In other words, 

 structures which lie to the observer's left are a part of the 

 right side of the specimen. Those things which are most 

 superficial in the ventral view lie ventral to the structures 

 they conceal. 



J 

 Mammals 



As represented by the cat, rat, or rabbit, the digestive 

 tract of the mammal begins with the mouth, which has lips, 

 teeth, a mobile tongue, and salivary glands. Behind the 

 oral cavity is a short pharynx, into which opens the nasal 

 passage above, and the glottis below. The glottis is the 

 opening into the trachea and lungs. The esophagus is tu- 

 bular and leads to the expanded hook-shaped stomach. The 

 esophagus passes above the mass of lungs and through the 

 muscular diaphragm separating the pleural cavities from the 



abdominal cavity. The esophagus enters the stomach above 

 the large lobed liver. Between the liver lobes on the right 

 side (student's left) is seen the gall bladder. In the dorsal 

 mesentery of the stomach lies a dark-colored body, the 

 spleen (Figure 9-1). Behind the spleen the mesentery bags 

 out as a thin membranous sac studded with fat masses. This 

 bag, the greater omentum or omental bursa, underlies much 

 of the intestinal mass (cat). The stomach ends at a constric- 

 tion, the pyloric valve, and the small intestine begins. 



A short distance along the small intestine from the pyloric 

 valve, the common bile duct and the main duct of the pan- 

 creas, the duct of Wirsung, join and enter the intestine dor- 

 sally. At the point of entry is a small lump, the ampulla of 

 Vater. Internally, the common duct opens through a papilla 

 (of Vater). Somewhat posterior to the ampulla, an accessory 

 duct of the pancreas (duct of Santorini) may enter the 

 small intestine. 



The first part of the small intestine, the duodenum, forms 

 an arc or open loop. In the mesentery within the arc, and 

 next to the gut, is a flattened irregular pancreas. In man, 

 only a part of the pancreas lies in this arc, the larger part 

 lies retroperitoneally, that is, outside of the somatic peri- 

 toneum, in the region above the stomach. In the rabbit the 

 bile duct enters the first part of the duodenum near the 

 stomach, while the pancreatic duct enters the far end of the 

 duodenal arc. The terminal part of the duodenum is closely 

 bound to the dorsal body wall by the cavo-duodenal liga- 

 ment. The extreme is man where this part of the duodenum 

 has become retroperitoneal. The remainder of the small in- 

 testine, the jejunum and ileum, is long and quite looped; 

 this part of the intestine has been called Meckel's tract. 



The posterior end of the small intestine enters the large 

 intestine about in the posterior ventral midline. At this 

 junction, there is an ileocolic valve. The large intestine ex- 

 tends as a pouch or caecum, a short distance back along the 

 course of the small intestine. The caecum is large and coiled 

 in the rabbit, relatively short in the rat, cat, or man. In 

 man the caecum has an appendix. The functional signifi- 

 cance of the vermiform appendix is obscure, but there is no 

 reason for supposing that it is a primitive or degenerate 

 structure. It is evidently a specialized formation, peculiar 

 to anthropoid apes and man. There are paired caeca in 



253 



