THE METABOLISM OP THE CARBOHYDRATES 



sorbed into the blood, bul become Bubjecl to the action of other enzym< - 

 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 Bugar, into levu- 

 lose and glucose by invertase. Ii is interesting to note thai in animals 

 whose fund 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 nol take any lactose in their food, and the 

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

 thai of the young animals while still suckling. 



A certain amounl of carbohydrate becomes attacked by the intestinal 

 bacteria. These split the monosaccharides into lower fatty acids and 

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

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

 of carbohydrates, in that certain strains of them are able to di. ellu- 

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

 sequently essential in herbivorous animals: it takes place in the cecum, 

 which is enormously developed for this purpose I page 463 . 



Absorption. The glucose and levulose produced by digestion arc 

 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 

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

 — ami appears in the blood, from which, Bince it is an abnormal con- 

 stituent, it is excreted unchanged in the urine. This alimentary gl] 

 suria is particularly evidenl 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 our and a half hours 

 afterward that the urine of ten OUl 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 he excrete.] for from six to Seven llollfS. 



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 Bystemic circulation. During absorption the former 

 becomes perceptibly raised to what extent we can no1 -ay and in the 

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

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



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



hack some of the excess of absorbed sugar. We have verj inaccui 

 information as to how efficiently these harriers hold back the 

 absorbed glucose because of the technical difficulty in collecting hi 1 



