690 METABOLISM 



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 cecum, 

 which is enormously developed for this purpose (page 497). 



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 

 from the portal vein without serious disturbance to the animal. Indeed, 

 the only way by which the problem can be studied is by comparing the 

 blood of the portal circulation with that of the systemic circulation dur- 

 ing the injection of a solution of glucose into one of the smaller branches 

 of the portal vein. 21 



In such experiments it has been found 'that .the percentage of sugar is a little less 

 in the blood of the abdominal vena cava than in that of the portal vein, and is still 

 less in the blood of the systemic veins, susch as the femoral results which justify the 

 conclusion that the barriers responsible for taking out some of the absorbed sugar 

 from the blood exist in the liver and in the muscles. The curve in Fig. 190 will illus- 

 trate to what extent the mechanism operates. 



