The velvety appearance of 

 the inner surface of the small 

 Intestine is due to the multi- 

 tudes of projecting villi. The 

 layer of cells covering these 

 villi absorbs digested food 

 from the food tube. The di- 

 gested food, after some chem- 

 mical changes, diffuses out 

 into special lymph tubes, the 

 lacteals, and finally gets into 

 the blood. The action of the 

 villi may be compared to that 

 of glands; but whereas the 

 movement of materials is from 

 the blood stream to the spe- 

 cial secretions in the case of 

 the glands, it is from the food 

 supply to special blood sub- 

 stances in the case of/the villi 



Villus - 



Network of 



blood 



vessels 



Lacteal, or 

 lymph 

 vessel 



Intestinal 

 gland 



THE LINING OF THE INTESTINE 



The mixture in the intestine now consists of (1) many crystalloids in solu- 

 tion, (2) many colloids in the process of being converted into crystalloids, 

 and (3) solid substances that are not changed under conditions that exist in 

 the gut. 



When the dinner that you have eaten reaches the end of the small in- 

 testine, most of its carbohydrates, proteins and fats have been absorbed by 

 the villi and passed into the lymph and blood. There are left in the intestines 

 chiefly (1) the undigested (mostly indigestible) fibrous and cell-v^all parts 

 of the plant or animal tissues eaten, and (2) the chemically changed mate- 

 rial from the various glands that have poured their products into the food 

 tube along the way. This mass of refuse now passes into the large intestine 

 (see illustration, p. 167). 



The Large Intestine In the large intestine the enzymes of the digestive 

 juices may continue to act for some time. The lining of the intestine con- 

 tinues to absorb fluids, although there are no villi in the large intestine. 

 Finally, the only chemical changes going on are those produced by the mil- 

 lions of bacteria that are present. 



The mass of material that accumulates toward the end of the large in- 

 testine is of no further use to the body. To this refuse are added dead cells 

 from the lining of the intestine and waste materials absorbetl from the 

 surrounding fluids and cells. The refuse, or feces, is normally removed from 

 time to time. Birds, having no large intestines, throw off the refuse about 



171 



