808 PHYSIOLOGY OF DIGESTION AND SECRETION. 



rapidity of absorption stands in no direct relation to the diffusion 

 velocity. The energy that effects the absorption is furnished, there- 

 fore, by the wall of the intestine, presumably by the epithelial cells. 

 That this particular form of energy is connected with the living 

 structure is shown by the fact that when the walls are injured by 

 the action of sodium fluorid, potassium arsenate, etc., their absorp- 

 tive power is diminished and absorption then follows the laws of 

 diffusion and osmosis.* 



Absorption of the Carbohydrates. Our carbohydrate food is 

 absorbed, for the most part, as simple sugars, monosaccharids. 

 As has been said, there is reason to believe that but little sugar is 

 absorbed in the stomach. Cane-sugar and milk-sugar are inverted 

 in the small intestine by invertase and lactase, the first being con- 

 verted to dextrose and levulose, the second to dextrose and galactose. 

 If, however, these substances are fed in excess they are absorbed in 

 part without conversion to simple sugar, and in that case may be 

 eliminated in the urine. The bulk of our carbohydrate food is taken, 

 however, in the form of starch, and the conditions for absorption in 

 this case are more favorable. The time required for the digestion of 

 the starch to maltose and dextrin, and the subsequent inversion of 

 these substances to dextrose, insures a slower and more complete 

 absorption. Five hundred grams or more of starch may be digested 

 and absorbed in the course of the day and it all reaches the blood in 

 the form of dextrose. This dextrose enters the portal vein and is 

 distributed first to the liver. In this organ the excess of sugar is 

 withdrawn from the blood and stored as glycogen, so that the amount 

 of sugar in the general circulation is thereby kept quite constant, 

 about 0.15 per cent. When a large amount of carbohydrate food is 

 eaten, however, it is possible that the liver may not be able to remove 

 the excess completely. In that case the amount of sugar in the gen- 

 eral circulation may be increased above normal, giving a condition 

 of hyperglycemia, and the excess may be excreted in the urine, 

 thus bringing about the condition known as "alimentary glyco- 

 suria." The amount of any carbohydrate that can be eaten' 

 without producing alimentary glycosuria is designated by Hof- 

 meister f as the assimilation limit of that carbohydrate or, to use 

 another terminology, it establishes the degree of tolerance of the 

 animal for the particular carbohydrate employed. If taken 

 beyond this limit there is a physiological excess, and some sugar 

 is lost in the urine. The assimilation limit varies with a great 

 many conditions; but, so far as the different forms of carbo- 

 hydrates are concerned, it is lowest for the milk-sugar and high- 



* Cohnheim, "Zeitschrift f. Biologie," 37, 443, 1899. 

 t Hofmeister, "Archiv. f. exper. Pathol. u. Pharmakol.," 25, 240, 1889, 

 and 26, 355, 1890. 



