FROM THE SMALL INTESTINE 645 



therefore show that the vital activity of the epithelial cells 

 controls absorption. 



Heidenhain's Experiments are of importance in the history of 

 intestinal absorption, because they were the first experiments on 

 the subject performed after a knowledge of osmotic phenomena 

 had begun to be applied to physiology. His method of experi- 

 menting has already been described. His discovery that water 

 was absorbed more slowly from a MgS0 4 solution than from 

 a solution of NaCl of equal concentration, and his explanation 

 of the fact, have been already mentioned. He knew by analyses 

 that the MgS0 4 was less rapidly absorbed ; therefore it would be 

 expected on physical grounds that less water would be absorbed 

 from the MgS0 4 solution. In most of his experiments he used 

 solutions of NaCl, and the two other results which he considered 

 were inexplicable by osmosis and diffusion were the following 

 (1) When a hypertonic NaCl solution (1-5 per cent.) was placed 

 in the gut, no preliminary attraction of water into the intestine 

 took place, but both water and salt began to be absorbed at 

 once. He considered that the salt absorption was due to diffu- 

 sion owing to the partial pressure of NaCl being greater in the 

 solution than in the blood (*65 per cent.), but that the simul- 

 taneous absorption of water could not be due to osmosis. (2) 

 When a hypotonic solution (*3 to '5 per cent.), having a less 

 partial pressure of NaCl than the blood, was introduced, not only 

 water but salt also was absorbed. He accounted for the water 

 absorption by osmosis, but he considered that the absorption of 

 salt must be physiological ; and in order to demonstrate this 

 physiological factor he added NaF to his solutions and found that 

 absorption was then interfered with. Heidenhain, however, in 

 this explanation overlooked two important facts. In the first 

 place, if blood serum is separated from a solution of NaCl isotonic 

 with it by a dead animal membrane, which is permeable to NaCl 

 in both directions, the water of the solution does pass into the 

 serum and the solution is absorbed. Starling has demonstrated 

 beyond doubt that the living blood capillaries can absorb an 

 isotonic salt solution, and a physical explanation of the fact has 

 been given, namely, that serum contains substances possessing an 

 osmotic pressure to which the membrane is impermeable. In the 

 second place, Heidenhain was unaware of the fact subsequently 

 discovered by Cohnheim, that the normal gut possesses a one-sided 



