652 THE MECHANISM OF ABSORPTION 



of normal absorption. His original experiments were performed 

 long before Cohnheim's, and he has since amplified them. The 

 apparatus consists essentially of two horizontal glass tubes con- 

 taining '9 per cent. NaCl solution; these when clamped together 

 are separated only by a disc of intestinal wall. The amount of 

 fluid on the two sides of the membrane could be accurately 

 measured. With absolutely fresh intestine he found that for a 

 little more than ten minutes fluid passed from one side of the 

 membrane to the other in the direction of normal absorption, 

 but that after that interval fluid passed from both sides into 

 the membrane. In this way he claims to have demonstrated a 

 vital transference of normal salt solution to normal salt solution 

 across surviving rabbit's gut at an equality of hydrostatic pressure 

 on the two sides of the membrane. The actual quantities of 

 fluid passing across the membrane were necessarily minute, but 

 the work done could have been performed only by living cells, 

 if the physical condition on the two sides of the membrane were 

 really identical. There is, however, a fact in Reid's experiments 

 which raises a doubt on this point. He could obtain a positive 

 result only when the membrane was taken from an animal in 

 full digestive activity. Gut taken from a fasting animal did 

 not show the phenomenon at all, but from the very beginning 

 of the experiment fluid passed from both sides into the membrane. 

 Considering the rapidity with which salt solution is absorbed by 

 a normal fasting intestine, this total difference between the gut 

 when digesting and fasting is suggestive that there may be some 

 factor other than cell activity at work. The whole phenomenon 

 might equally well be due to osmosis. In the fasting gut as 

 the cells died the osmotic pressure in it would rise and fluid be 

 attracted into it from both sides. But the intestinal wall of 

 an animal in full digestion would contain the products of digestion, 

 and, if these diffused oat into the salt solution more readily 

 on the serous than the epithelial side, a transference of fluid 

 in the direction of normal absorption would be observed until 

 the dying condition of the gut was able to determine a passage 

 of fluid from both sides into the membrane. And that something 

 wholly abnormal is taking place in these experiments is shown 

 by the consideration that the fluid absorbed by a normal 

 intestinal epithelium does not pass right through the wall of 

 the gut and does not appear on its serous surface. 



