dorsal pancreas 



pharyngeal pouches 



stomach 



stomach 



dorsal pancreas 



entrol pancreas 

 gall bladder 



5,5 mm 



B 



9.4 mm 



20 mm 



Figure 9-7. Three stages in the development of the liver and pancreas in the pig. (After Patten, 1 946) 



denal loop where it extends forward along the bile duct and 

 hepatic portal stem. The relatively small, elongate-ovoid 

 spleen lies above the stomach in the dorsal mesentery near 

 the posterior end of the body of that structure. 



Alligator The viscera of the alligator are peculiar in that 

 they are enclosed in the muscular peritoneum which is free 

 from the muscular body wall. Muscle fibers radiate, from 

 an origin along the pubis near the acetabulum, over the 

 outer surface of the peritoneal sac to the posterior margin of 

 the liver. These muscles act in drawing the liver mass pos- 

 teriorly. In this way the volume of the pleural space is in- 

 creased and the lungs inflated. In Tupinambis a similar peri- 

 toneal sheet of musculature from the liver margin attaches 

 to the body wall a short distance behind. 



The body of the stomach is thick-walled and muscular; 

 to this is appended a thin-walled pyloric sac (Figure 9-9). 

 The esophagus is a simple tube entering the body of the 

 stomach anteriorly, and the pyloris opens through a narrow 

 passage just posteromedial to this entrance. The small in- 

 testine opens into the middle of the dorsal surface of the 

 pyloric sac. There is a well-developed, closed duodenal loop 

 between the limbs of which lie the pancreas and the bile 

 duct. The small intestine makes several loops posterior to 

 the stomach before opening into the expanded large intestine. 

 There is no caecum and the large intestine extends nearly 

 straight back to the elongate cloaca. The cloaca has three 

 chambers; a posterior coprodeum into which the gut opens, 

 a middle urodeum for the ureters, and a posterior procto- 

 deum for the sex ducts. In the male the penis lies in the 

 floor of the proctodeum. There is no urinary bladder (Fig- 

 ure 10-38). 



The liver is bilobed with the gall bladder to the right of 

 the ventral midline. A process of the liver projects back 

 along the postcaval vein, as it also does in the lizard and 

 the turtle. The pancreas is relatively small and incon- 

 spicuous; it lies largely on the dorsal aspect of the duo- 



denal loop. The spleen is a round mass at the dorsal 

 midline above the stomach. It does not appear to lie in the 

 mesogaster, but veins extend from it to the stomach. 



The ventral septum is not evident even below the liver. 

 The cloaca is attached to the ventral body wall. 



Turtle Although many turtles show some specialization of 

 this tract, Pseudemys appears to have remained relatively un- 

 modified (Figure 9-10). The stomach first curves strongly to 

 the left, where it is partly embedded in the liver, then back 

 and to the right to reach the pyloric valve at the ventral 

 midline. The valve is not conspicuous, but the lining of the 

 stomach diff'ers from that of the duodenum in that it is 

 coarsely folded. 



The duodenum extends to the right side, where it arches 

 dorsally and receives the bile duct from the gall bladder. 

 From here the duodenum passes posteromedially above the 

 transverse limb of the large intestine, then below the de- 

 scending section of the large intestine, to which it is attached 

 by a supporting mesentery. From here the small intestine 

 loops back to the right side where it enters the large intestine. 

 The large intestine has a small saccular caecum, a long trans- 

 verse limb, a loop to the left, then a straight terminal por- 

 tion which extends to the cloaca. 



The liver is massive with several lobules or incisions in its 

 margin. The gall bladder lies far to the right and the bile 

 duct drains almost directly into the mid part of the duodenum. 

 The pancreas lies along the limb of the duodenum which 

 leads from the stomach. The ovoid spleen lies in the mesen- 

 tery just above the caecal end of the large intestine. 



General observations on tetrapods 



The reptile digestive tract is distinctive as compared with 

 the mammal. An omental bursa is lacking, the spleen is 

 smaller and rounder, and it lies in the mesentery dorsal to 

 the stomach or gut and does not show on the left side of 



THE DIGESTIVE TRACT AND ASSOCIATED STRUCTURES • 259 



