DIGESTIVE SYSTEM IN MAN 



chemical changes in the gut. There is a ferment in the intestinal 

 juice which converts cane sugar into simpler sugars, but this change 

 may also be brought about by the acids of the stomach, and possibly 

 also by the alkali 

 in the intestine. 



123. Absorp- 

 tion. The lin- 

 ing of the small 

 intestine is like 

 delicate velvet. 

 Very small out- 

 growths project 

 into the cavity, 

 so that the sur- 

 face exposed to 

 contact with the 

 food mixture is 

 increased sev- 

 eral hundred 

 times. Each of 

 these tiny pro- 

 jections, called 

 a villus (plural, 

 villi) t has a 

 rather complex 

 structure, as is 

 shown in the di- 

 agram (Fig. 31). 



The villus seems to be a special absorbing and transforming 

 organ. The mixture in the intestine we now know to consist of 

 many crystalloids in solution, many colloids in the process of 

 being converted into crystalloids, and solid substances that are 

 not capable of changing under the conditions that exist in the gut. 



The crystalloids are absorbed into the cells of the villi, so 

 that as the mass moves along in the intestine, more and more 



Large Intestine 

 and Rectum 



FIG. 32. Digestive system in fish and in bird 



The main features of the digestive system are alike in all back- 

 boned animals. In the birds there is a curious pouch connected 

 with the gullet, the crop, in which food may be retained in- 

 definitely and later either swallowed or regurgitated through the 

 mouth. The glandular portion of the stomach, or proventriculus, 

 is distinct from the grinding part, or gizzard 



