ALIMENTATION 115 



Glycerin and a salt of a fatty acid, or soap, constitute the final 

 products of the digestion of fats. 



Since lipase acts more rapidly and effectively when bile is 

 present, it is thought that bile acids alone or with lecithin (a 

 constituent of bile) functions as a coenzyme, which increases 

 lipolytic action. 



Gastric Absorption. — Relatively little absorption of substances 

 takes place in the stomach. Water is not absorbed to any 

 appreciable extent. One experiment with a dog showed that of 

 500 cc. of water given through the mouth 495 cc. entered the 

 duodenum within a half an hour. Alcohol, on the other hand, 

 is absorbed readily by the stomach. Sugars and peptones are 

 not absorbed. 



Absorption in the Small Intestine. — The products of digestion 

 are absorbed principally in the small intestine. In this connec- 

 tion it is necessary to refer again to the structure of the intestinal 

 villi. The mucous lining of the human small intestine throughout 

 the greater part of its length is raised into permanent transverse 

 folds, the valvulae conniventes. Both on the valvulae and 

 between them the mucosa is in the form of closely packed cylin- 

 drical or club-shaped villi. In the duodenum villi are plate- 

 shaped. Each villus is covered with columnar epithelium, 

 beneath which is an incomplete layer of plain muscle. The core 

 of the villus is formed of loose connective tissue, with a centrally 

 located lymph capillary or lacteal, which communicates with 

 larger lymphatic vessels in the submucosa. Peripheral to the 

 lacteal is a network of blood vessels (Fig. 71). 



Absorption and Metabolism of Protein. — The products of 

 protein digestion, amino acids, are absorbed through the epi- 

 thelium of the intestinal mucosa into the blood stream and carried 

 by the blood to different parts of the body. They are stored 

 temporarily, principally in the liver and muscles. Experiments 

 show that with excessive feeding some of the products of protein 

 digestion may enter the lymph stream. Apparently there is no 

 circulating protein to provide the tissues with their protein 

 requirements. Instead it is believed that this function is served 

 by amino acids in the blood, each tissue using what is required to 

 build up its own form of protein. Amino acids that are not used 

 for tissue-building may undergo deaminization. This consists in 

 splitting off a portion of the molecule as urea, leaving a non- 



