THE METABOLISM OF THE CARBOHYDRATES 657 



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 of those disaccharides, the correspond- 

 ing inverting enzyme is absent from the intestinal juice. The herbivo- 

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

 intestinal juice contains therefore no lactase, although it is present in 

 that of the young animals while still suckling. 



A certain amount of carbohydrate becomes attacked by the intestinal 

 bacteria. These split the monosaccharides into lower fatty acids and 

 gases, such as methane and carbon dioxide. Besides this obviously de- 

 structive process, bacteria also perform a useful function in the digestion 

 of carbohydrates, in that certain strains of them are able to digest cellu- 

 lose, for which no special enzyme is provided. Bacterial digestion is con- 

 sequently essential in herbivorous animals; it takes place in the eecum, 

 which is enormously developed for this purpose (page 463). 



Absorption. The glucose and levulose produced by digestion are 

 absorbed into the blood of the portal system. "When a very large quan- 

 tity of a disaccharide, such as cane sugar, is present in the food, a certain 

 amount of the sugar is absorbed unchanged that is to say, as cane sugar 

 and appears in the blood, from which, since it is an abnormal con- 

 stituent, it is excreted unchanged in the urine. This alimentary glyco- 

 suria is particularly evident when the sugar is taken without any other 

 food; thus, after taking cane sugar in an amount corresponding to 5 

 grams per kilogram body weight, it was found in one and a half hours 

 afterward that the urine of ten out of seventeen healthy individuals con- 

 tained cane sugar. The urine of three of these men, however, also con- 

 tained invert sugar that is, dextrose and levulose. Cane sugar con- 

 tinued to be excreted for from six to seven hours. 



The Sugar Level in the Blood. While no absorption of sugar is going 

 on, the percentage of this substance in the blood of the portal vein is the 

 same as that in the systemic circulation. During absorption the former 

 becomes perceptibly raised to what extent we can not say and in the 

 latter a less marked increase of sugar concentration is usually detectable. 

 Evidently, then, between the point at which the sugar is absorbed and 

 the blood of the systemic circulation, some barrier exists which holds 

 back some of the excess of absorbed sugar. We have very inaccurate 

 information as to how efficiently these barriers hold back the excess of 

 absorbed glucose because of the technical difficulty in collecting blood 



