DIGESTION AND ABSORPTION 171 



small intestine and by them discharged upon the food mass at 

 about the same time or very soon after it has been mixed with 

 the pancreatic juice. These two digestive juices act more or 

 less together upon the food mass. The intestinal juice is also 

 termed siiccus entericus. 



The three enzymes of the intestinal juice which hydrolyze 

 disaccharoses are : 

 Sucrase or invertase, which hydrolyzes cane sugar (sucrose) 



to glucose and fructose. 

 Maltase, which hydrolyzes malt sugar (maltose) to glucose. 

 Lactase, which hydrolyzes milk sugar (lactose) to glucose 

 and galactose. 



The intestinal juice is alkalhie to methyl orange and con- 

 tains no free hydrochloric or other strong acid. The enzymes 

 present act in media of this reaction. 



The digestion of carbohydrate food is now completed and the 

 final products are glucose, fructose and galactose : the three most 

 common monosaccharoses. These three are all diffusible 

 through the cellular membranes of the digestive tract and as 

 such they pass into the circulatory system ready to take up 

 the metabolic changes by which they form body substance or 

 yield energy. 



DIGESTION OF PROTEINS 



The protein food of animals may consist of eitEer animal or 

 vegetable proteins or both. These different proteins are alike 

 in most respects, and any difference in them shows itself mainly 

 in the products of digestion and does not concern the digestive 

 process itself. The digestive enzymes acting upon protein 

 food act alike, so far as we know, on both classes. 



The chemical changes occurring in the digestion of protein 

 food are analogous to those that take place with carbohydrates, 

 i.e. they are hydrol3^tic. In the laboratory proteins may be 

 hydrolyzed by boiling with acids or alkalies and the products 

 obtained are eventually amino-acids such as glycine (amino- 

 acetic acid), etc. This same hydrolysis of proteins may be 



