bile duct. 



spleen 



Figure 9-4. 



(Dide/phys). 



large intestine 



Ventral view of the digestive tract of the opossun 



dorsal body wall, or lies retroperitoneally; it is attached to 

 the large intestine in this area as a result of the rotation of 

 Meckel's tract. A good part of the large intestine is usually 

 involved in a rotated loop, e.xcept in the bat and the tree 

 shrew. 



Embryological development In mammals the lining of the 

 digestive tract and its glands is formed of entoderm. The 

 bulk of the wall and a part of the glands are formed from 

 mesoderm. In the early embryo the formation of the head 

 fold produces a forward-projecting pocket of entoderm, the 

 foregut; the similarly produced tail fold gives rise to the 

 hindgut (Figure 9-6). The embryo is gradually separated 

 from the yolk sac by constriction of the yolk stalk. The fore- 

 gut gives rise to pharynx, lung buds, esophagus, stomach, 

 duodenum, and the liver and pancreatic diverticula. The 

 midgut forms much of the small intestine, while the hind- 

 gut forms the large intestine. These are arbitrary assign- 

 ments for rather vague regions. 



The liver and pancreas arise behind the stomach region, 

 which has expanded and arched to the left (Figure 9-7). 

 The liver appears first; it arises as a ventral diverticulum. 

 This diverticulum branches many times giving rise to a 

 mass of cell cords embedded in mesoderm. These cell cords 

 produce the secretory cords of the definitive gland. Another 

 diverticulum appears from the main duct of this gland 

 which grows out posteriorly and gives rise to the gall 

 bladder. 



The pancreas arises from dorsal and ventral diverticula. 

 The first opposite the duct of the liver and the second just 

 to the left of the liver duct or from that duct to the left of 

 the gall bladder evagination. In the process of development 

 the ducts of the dorsal and ventral pancreatic outgrowths 

 cross. Frequently the dorsal pancreas comes to drain through 

 the ventral duct, but sometimes the dorsal duct is preserved 

 as the duct of Santorini. The two pancreatic diverticula and 

 the early gut relationship can be observed in the 10-mm 

 pig. At this stage a loop of the gut attached to the yolk stalk 

 extends out from the body into the umbilicus. This loop is 

 rotated in the course of development. 



The aUantois (Figure 9-6) grows out from the hindgut. As 

 the embryo is separated from the yolk mass, its stalk ex- 

 tends forward and then out through the umbilicus into the 

 extra-embryonic coelom. The base of the allantois forms the 

 bladder. 



Reptiles 



Up to this point the various visceral structures have been 

 described in an anterior to posterior sequence. A more sys- 

 tematic approach is now feasible. The pattern will be as fol- 

 lows: the intestinal tract from anterior to posterior, then the 

 liver, pancreas, and spleen. 



Lizard and Sphenodon The stomach is of a simple tubular 

 form tapering posteriorly and curving to the right (to the 

 left as seen in ventral view) to end at the pyloric valve 

 (Figure 9-8). The duodenal loop of Iguana is open except for 

 its mid section. A closed loop is one in which the two limbs 

 lie in contact, or nearly in contact. Tupinambis has a com- 

 pletely open, arc-like duodenal loop. The bile duct enters 

 the gut anterior to the mid part of the duodenal loop. 

 Frequently two bile ducts are evident. In a large lizard such 

 as Iguana, which is like Tupinambis, the limbs of the stomach 

 loop are joined by the ventral mesentery; the gastrohepatic 

 ligament and the coils of the small intestine lie above this 

 membrane. In most lizards, and Sphenodon, the small in- 

 testine lies posterior to the stomach. The small intestine 

 loops irregularly and opens into the large intestine. 



A small dorsal caecum of the large intestine is usually 

 present at the ileocolic junction. This part of the intestine 

 makes a loop or two and extends back to the cloaca. The 

 large urinary bladder lies below, and extends forward from, 

 the cloaca. The bladder is attached to the ventral body 

 wall. There is no ventral mesentery for the gut, but the dor- 

 sal mesentery is retained and fans out to the various loops 

 and coils of the intestine. The vessels of the hepatic portal 

 system as well as the arterial supply of the gut are seen in 

 this mesentery. 



In Iguana the large intestine is lengthened. Its anterior 

 end is much expanded and sacculate, and there is a caecum 

 extending back along the course of the small intestine. 

 Following the caecum there is a tightly closed colonic loop 



256 • THE VISCERA 



