ABSORPTION OF MINERAL SUBSTANCES 



307 



5. ABSORPTION OF MINERAL SUBSTANCES 



Since the easily diffusible salts, like sodium chloride, are absorbed by virtue 

 of the activity of the epithelial cells this must the more be true of the heavy 

 salts which diffuse slowly. 



With regard to the behavior of different salts, Hoeber has shown that solu- 

 tions of different salts isotonic with one another are absorbed at different rates. 

 Since in these experiments solutions so dilute that the salts were almost wholly 

 dissociated were used, their different behavior is to be ascribed to the properties 

 of individual ions. Of the cations K, Na and Li are absorbed with approxi- 

 mately the same rapidity, NH 4 and urea more rapidly, Ca more slowly and Mg 

 slowest of all. Of the anions Cl is absorbed most rapidly, then follow in order 

 Br, I, NO 3 and SO 4 . Now we know that the rate of diffusion of a salt in any 



FIG. 119. Duodenum of the mouse, after Hochhaus and Quincke. The section has been treated 

 with ammonium sulphide. The black granules represent absorbed iron. The animal had 

 eaten cheese impregnated with '' carniferrin " containing three per cent Fe. (Carniferrin is a 

 derivative of carnic acid obtained from meat extract, and contains both phosphoric acid and 

 iron.) 



given solution depends, first upon the degree of dissociation of its molecules, and 

 secondly upon the velocity of migration of the ions. This law applies also in 

 absorption; for the rates of absorption of salts are proportional to their rates 

 of diffusion. However, parallelism between diffusion and absorption is subject 

 to some limitations, which make other auxiliary hypotheses necessary and show 

 once more that the physical factors are not sufficient to explain the behavior of 

 the salts in the intestine. 



In order to follow the absorption of water and of substances soluble in 

 water more closely, a solution of methylene blue has been introduced into an 

 intestinal loop and the mucous membrane studied microscopically. The pigment 

 was found partly in the epithelial cells and partly between them, which means 

 that absorption takes place here both between the cells and through them 

 (Heidenhain). 



The soluble salts, like the carbohydrates, are carried away from the intestine 

 by the portal vessels; only in case the quantity of absorbed fluid is great does 

 a part of it pass out by way of the lymph vessels. 



The absorption of iron deserves special consideration. After it had been 



