bile duct 



Meckelian 



diaphragm 



gall bladde 



duodenal bulb with Brunner's gland 



hepatic portal vein 

 colonic loop 



—large intestine 



outline (dashed line) of omentum- 



colonic loop--^ 



B 



Figure 9-3. Ventral views of the digestive tract of the Platypus. (A, after Owen 1866; B, after 

 Mitchell, 1905) 



ner's glands characterize the duodenum of higher forms, 

 this has been called the duodenal bulb. From here the gut 

 continues a short distance to the common entrance of the 

 bile and pancreatic ducts. The pancreas lies entirely in the 

 mesentery of the duodenal arc. 



The duodenal arc extends first to the right, then back to 

 the left above the hepatic portal vein (Figure 9-3) and next 

 to the dorsal body wall, where it is attached by the cavo- 

 duodenal ligament. The remainder of the small intestine, 

 Meckel's tract, is very looped and folded; it joins the large 

 intestine at the ventroposterior margin of the gut mass with- 

 out an ileocolic valve. The large intestine has a slim caecum 

 projecting back along the small intestine; it extends ante- 

 riorly below the coils of the small intestine, forms a colonic 

 loop, then passes dorsally anterior and joins with the ter- 

 minal part of the duodenum; from here it turns posteriorly to 

 the cloaca above the loops of the small intestine. 



The marsupial, as represented by the opossum, is much 

 like the monotreme (Figure 9-4). The stomach of members 

 of this group shows much variation and specialization, but 

 in the opossum it is a simple large sac (Figure 9-5). It opens 

 to the right of the esophagus into a sacculate duodenum. 



The bile duct is a short, thick tube which opens into the gut 

 a short distance from the stomach. The duodenal loop 

 passes dorsally across the body cavity to the looped Meckel's 

 tract. 



The small intestine enters the large intestine ventrally 

 and just behind the stomach. The large intestine has a large 

 but simple caecum. In Thalaanus or Notorycles, a caecum and 

 ileocolic valve are lacking. The large intestine extends dor- 

 sally, anterior to the duodenum, then curves posteriorly, to 

 the left of that part of the gut; from here there is a loop to 

 the right, an ansa coli, and then a straight transit to the 

 anus. 



The liver has a large right lobe with two small dorsal 

 lobules and a smaller left lobe with a digitate margin. The 

 spleen is attached to the dorsal mesentery of the stomach, 

 anterior to the omental bursa (greater omentum). The spleen 

 is intermediate in size between that of the monotreme and 

 the placental. 



The digestive system of the mammal can be character- 

 ized by several features. There is a greater omentum; the 

 spleen lies to the left of the stomach and can be seen from 

 below. The terminal part of the duodenum is bound to the 



THE DIGESTIVE TRACT AND ASSOCIATED STRUCTURES • 255 



