simpler gut pattern of the lizard or alligator could be a ret- 

 rogressive modification of the rotated type of turtle and 

 mammal. 



Bird 



The gut of the bird shows much modification and can 

 only be described in the most general terms. The stomach 

 usually has an anterior glandular expansion which opens 

 into a thick-walled grinding stomach. The grinding stomach 

 is lacking in birds of prey. From the anterior end of the 

 muscular stomach, a small pyloric section extends to the 

 left to the pyloric sphincter. 



The gut has several closed loops. It is folded or coiled 

 within or between these looped sections. The first loop is the 

 duodenal, which involves the entire length of the duodenum. 

 There is only a short, closed medial portion in many lizards; 

 in this respect the alligator is like the bird. The remainder 



of the small intestine, the Meckelian section, may have a 

 median closed loop, a supraduodenal loop dorsal to and at- 

 tached to the posterior end of the stomach, and a small pre- 

 colonic loop just before it joins the large intestine. 



The large intestine usually has bilateral caeca at its an- 

 terior end. These are right and left caeca and are not com- 

 parable to the usual dorsal caecum of the reptile. The caeca 

 may be quite long and expanded or quite rudimentary. The 

 large intestine extends straight back to the cloaca, which is 

 divided into an anterior coprodeum, middle urodeum, and 

 posterior proctodeum. There is no urinary bladder, but 

 there is a dorsal diverticulum from the proctodeum, the 

 bursa of Fabricius. This structure is large in young and 

 small in adults. Some birds have a penis in the floor of the 

 proctodeum. 



The coiling of the gut suggests the mammal in that the 

 posterior part of the small intestine loops up and over the 

 first part, as viewed ventrally, attaching to the stomach. 

 This appears to be a parallelism since, in the mammal, it is 



-lung 



.liver 



opening into duodenum 

 pyloric sac 



esophagus 



gall bladder. 



pancreas 



.stomach 



tip of duodenal loop 



.small intestine entering large 

 intestine 



_ large intestine 



nlo stomach 



tip of first intestinal loop 



B 



tip of first intestinal loop^ 



cloaca 



-ventral mesentery 



Figure 9-9 Ventral view of viscero of the alligator. A, contents of body cavity; B, ventral wall of 

 storr,ach and pyloric sac cot away to show thickness of walls and internal surface contours as well 

 as relationships of other organs to stomach. 



THE DIGESTIVE TRACT AND ASSOCIATED STRUCTURES • 261 



