32 



GENERAL ZOOLOGY 



strict usage, applies to the portion of the peritoneum between 

 the intestine and the body wall, but it is often used for the 

 suspensory or supporting membrane of other viscera. 



The form of the entire alimentary tract can be seen if the liver 



and other overlying parts are re- 

 moved, as in Fig. 12. The posterior 

 continuation of the broad, ciliated, 

 buccal cavity is the pharynx, which 

 is also ciliated, and leads in turn to 

 the esophagus, a wide muscular tube, 

 lined by a ciliated epithelium and 

 creased by longitudinal folds. Like 

 the mouth, the esophagus is provided 

 with mucous glands. The esophageal 

 glands, located near the lower end of 

 the esophagus, secrete pepsin. The 

 demarcation between the esophagus 

 and stomach is indicated externally 

 by a constriction, but on the whole 

 it is not very distinctly marked. If 

 unpalatable food reaches the stom- 

 ach, it may be regurgitated by a 

 reversed peristaltic action which 

 turns the stomach inside out, some- 

 times causing it to bulge outside of 

 the mouth. The soft glandular lin- 



Fig. 12.— Alimentary tract of ing of the stomach secretes pepsin 

 Rana pipiens. B, bladder; BD, anc j hydrochloric acid, in addition to 



bile duct; C, colon (large intes- . „ . . ,. ,. 



tine) ; Ca, cardiac end of stomach; mucin. Pepsin is a digestive enzyme 

 Cl, cloaca; D, duodenum; I, which in the presence of hydrochloric 



ileum; L, lung; La, larynx; M, . . • r j mi 



mesentery; Ob, esophagus; P, acid acts upon protein food. Ihe 



pylorus; Sp, spleen. curved stomach lies with its convex 



surface to the left and is attached to the body wall by the 

 mesogaster and to the liver and small intestine by the gastro- 

 hepatoduodenal ligament. These attaching structures are exam- 

 ples of mesenteries. 



The lower end of the stomach leads to the small intestine, a 

 sharp constriction at the pylorus marking the junction. The 

 first part of the small intestine makes a sharp bend at the pylorus, 

 turns forward and parallels the course of the stomach. This is 



