100 



GENERAL ZOOLOGY 



2a 



its walls are collapsed and after a meal its outline is continually 

 changed by waves of contraction, beginning at about the middle 

 and extending to the pylorus. The epithelium of the mucosa 

 consists of a single layer of cells, some of which are mucus- 

 producing goblet cells. As in the frog, the surface of the mucosa 

 is pitted with the mouths of glands whose epithelium is con- 

 tinuous with that of the surface. The glands of the fundus are 

 believed to secrete pepsin and hydrochloric acid. No specific 



m function is attached to the glands 



of the cardiac and pyloric regions. 

 The tunica muscularis of the 

 stomach is composed of a thin 

 outer longitudinal layer and a 

 ,2b thicker inner circular layer. What 

 is sometimes referred to as a third 

 and innermost layer, whose fibers 

 run obliquely, is really a modifica- 

 tion of a portion of the circular 

 layer, resulting from the twisting 

 of the stomach in the course of its 



Fig. 67. — Diagrammatic cross sec- development. At the pylorus the 



tion of the small intestine of Rana c i rcu l ar l ayer J s enlarged to form 

 jnpiens. 1, serous membrane or . . 



visceral peritoneum; 2a, longitudinal a sphincter muscle which acts as a 



muscle; 2b, circular muscle; 3, sub- i)yloric valve 

 mucosa; 4, mucosa; m, mesentery by 



which the intestine is attached to the Intestine. — The superficial basis 

 bodywalL for the division of the intestine 



into large and small portions rests on a difference in diameter. 

 More important differences are found in the function and struc- 

 ture of the mucosa of the two regions. The small intestine is 

 primarily concerned with digestion and absorption, and the large 

 intestine with absorption alone, principally the absorption of 

 water. The duodenum is the first segment of the small intestine. 

 In the frog it extends from the pylorus to the first bend in the 

 intestine and receives the bile duct about midway. The distinc- 

 tion between the duodenum and the remainder of the small 

 intestine, the ileum, or the jejuno-ileum, is largely based on 

 histological structure. The liver and pancreas pour their secre- 

 tions into the duodenum and these secretions play important 

 parts in intestinal digestion. In the frog the bile duct collects 

 the secretion of the pancreas and thus serves as a common bile 



