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 (rcf) 

 they are absent. 



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

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



f'ct 



py.v 



py 



Fig. 19. — Portions oi the enteric canal of the Frog in I0ngitudin.1l section. 

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

 ;«. ;«. mucous membrane ; i/rttsc, muscular layer ; J>y. pylorus ; /j'. -'. pjdoric 

 valve ; r. longitudinal ridges (rugse) 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. 



