DICESTIVE OROAXS 



71 



only small particles can pass through. In the duodenum 

 (dii) the mucous membrane is raised into little tuft -like 

 elevations (r 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 juice, oozes from the mucous membrane. It is 



re? 



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 ; muse, muscular layer ; py. pylorus ; pv. r. pyloric 

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

 denum ; r*. longitudinal ridges of ileum ; ret. rectum ; si. stomach. 



this fluid 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 pro- 



As we have seen, the frog is a carnivorous animal. 



