DIGESTION AND ABSORPTION IN THE INTESTINES. 713 



the lumen in accordance with their high osmotic pressure, and, on 

 the other hand, some of the sodium chlorid diffuses into the blood 

 in accordance with the laws of diffusion. It seems equally clear, 

 however, that absorption as it actually takes place is not governed 

 simply by known physical laws. Thus, the animal's own serum,* 

 possessing presumably the same concentration and osmotic pres- 

 sure as the animal's blood, is absorbed completely from a Thiry- 

 Vella loop. So also it has been shown that in the absorption of 

 salts from the intestine f the rapidity of absorption stands in no 

 direct relation to the diffusion velocity. . The energy that controls 

 absorption resides, therefore, in the wall of the intestine, presumably 

 in the epithelial cells, and constitutes a special form of imbibition 

 which is not yet understood. 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, potas- 

 sium arsenate, etc., their absorptive power is diminished and 

 absorption then follows the laws of diffusion and osmosis. J 



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 withdrawn from the blood and stored as glycogen, so that the amount 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. In such cases the excess is likely to be excreted 



* Heidenhain, "Archiv f. die gesammte Physiologie, " 56, 579, 1894. 

 t Wallace and Cushny, " Archiv f . die gesammte Physiologie, " 77, 202, 

 1899. 



JCohnheim, "Zeitschrift f. Biologie," 37, 443, 1899. 



