DIGESTIVE ORGANS 



intestine is greatly narrowed, so that only small particles 

 can pass through. In the duodenum (dii) the mucous mem- 

 brane is raised into little tuft-like elevations (r) in the ileum 

 the ridges (B, r") become longitudinal again ; in the rectum 

 (ret) they are absent. Another ring-like muscle, or sphincter, 

 is present round the vent. 



When food is taken into the stomach, a fluid, the gastric 

 mice, oozes from the mucous membrane. It is this fluid 



ret 



FIG. 19. Portions of the enteric canal of the Frog in longitudinal section. 

 A, stomach and duodenum ; B, part of ileum and rectum, du. duodenum ; il. ileum ; 

 m. m. mucous membrane J mrisc. muscular layer ; py. pylorus ; py. v. pyloric 

 valve ; r. longitudinal ridges (rugae) of stomach ; r'. transverse ridges of duo- 

 denum ; r". longitudinal ridges of ileum ; ret. rectum ; st. stomach. 



which reduces the slugs, insects, etc., to the pulpy condition 

 referred to above : it is, like the bile and pancreatic fluid, 

 a digestive juice. 



General Properties of Food, We must now devote a 

 ' little attention to the characters of the food itself and to the 

 precise nature of the changes brought about by the digestive 

 process. 



