THE METABOLISM OF THE CARBOHYDRATES 689 



We have devoted no inconsiderable space to a discussion of assimila- 

 tion limits because of the great interest in diabetic therapy which this 

 procedure has aroused during recent years. We may now turn our 

 attention to a closer analysis of the changes that take place in carbohy- 

 drates during their passage through the animal body. 



DIGESTION AND ABSORPTION 



Digestion. All digestible carbohydrate taken with the food is con- 

 verted by the digestive agencies into the monosaccharides, glucose and 

 levulose, as which it is absorbed into the blood of the portal system. 

 To bring about this resolution of carbohydrate into monosaccharides, 

 several enzymes are employed. The first of these is the ptyalin of saliva. 

 It is not a very powerful enzyme, being capable of acting only on starches 

 that are in a free state, i.e., not surrounded by a cellulose envelope; 

 but even on free starch, ptyalin displays little of its activity during the 

 time the food is in the mouth. After the food is swallowed and becomes 

 deposited in the fundus of the stomach, there is an interval of time 

 lasting until hydrochloric acid has been secreted to such an extent as to 

 permit some of the acid to exist in a free state during which the ptyalin 

 acts on the starch of the swallowed food. During this time the activity 

 of the ptyalin is actually assisted on account of the fact that a slight 

 increase in hydrogen-ion concentration of the digestive mixture accel- 

 erates the action of ptyalin. 



The product of ptyalin digestion is maltose, a disaccharide composed 

 of two molecules of glucose. On entering the intestine, the carbohydrates 

 therefore exist partly as undigested starch, partly as glucose, and partly 

 as maltose. In the favorable environment of the duodenum a much 

 stronger diastatic enzyme called amylopsin very quickly hydrolyzes the 

 starch through dextrine into maltose. The maltose derived from the 

 starch and the unchanged sugars, such as cane sugar, maltose and lac- 

 tose, which have been taken with the food, unless they are present in very 

 high concentration in the intestinal contents, are not immediately ab- 

 sorbed into the blood, but become subject to the action of other enzymes 

 contributed by the intestinal juice namely, the inverting enzymes, one 

 of which exists for each of the disaccharides. By their action maltose 

 is converted into two molecules of glucose by the enzyme maltase; lac- 

 tose, into galactose and glucose by lactase; and cane sugar, into levu- 

 lose and glucose by invertase. It is interesting to note that in animals 

 whose food does not contain one or other of those disaccharides, the cor- 

 responding inverting enzyme is absent from the intestinal juice. The her- 

 bivorous animals, for example, do not take any lactose in their food, and the 



