DIGESTIVE ORGANS 



71 



intestine is greatly narrowed and only small particles can 

 pass through; In the duodenum {du) the mucous membrane 

 is raised into little tuft-like elevations (/-") ; in the ileum the 

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

 they are absent. 



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

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



ret 



py" 



py 



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. mucous membrane ; 7mtsc. muscular layer ; py. pylorus ; ^y, 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. 



