648 THE MECHANISM OF ABSORPTION 



which, these substances are absorbed from the intestine. He 

 found that the diffusibilities of dextrose and peptone through 

 parchment were in the proportion of 2 to 1, but that in the 

 intestine peptone was slightly the more rapidly absorbed. In the 

 same way he has shown that the relative diffusibilities of dextrose 

 and maltose through parchment into serum are as 1-8 to 1, and 

 in the intestine as T05 to 1. From these results he concludes 

 that diffusion is not a prepotent factor in absorption. But these 

 experiments are open to two objections. In the first place, the 

 use of peptone and maltose is objectionable, because they 

 certainly undergo fermentative change during absorption, and 

 their observed rate of absorption might depend just as well on 

 diffusion and the rate of ferment action as upon any cell activity. 

 And in the second place, the permeability of two physical mem- 

 branes for a given substance need not be the same, any more 

 than a given membrane need be equally permeable to all sub- 

 stances. Reid's experiments prove that absorption in the gut is 

 not the same thing as diffusion through the parchment, but they 

 do not prove that the absorption rates of different substances 

 does not accurately represent their diffusion rates through the 

 intestinal epithelium. He also performed experiments to show the 

 effect on the absorption of dextrose, maltose, and peptone solu- 

 tions of the removal of the epithelium which is caused by clamp- 

 ing the mesenteric arteries for half-an-hour. He found that the 

 rates of absorption of these substances then more nearly corre- 

 sponded to their diffusion rates through parchment into serum. 

 But, again, all that this necessarily proves is that the intestinal 

 epithelium and parchment are two very different membranes, and 

 that when the epithelium is removed, the rest of the intestinal 

 mucous membrane has properties something like parchment. 



Three series of Reid's experiments are of great importance. 



(1) Absorption of a Dog's own Serum. The one really import- 

 ant set of experiments performed by Heidenhain were those in 

 which he observed the absorption of horse's or another dog's 

 serum from the small intestine. The chief objections which can 

 be raised against his conclusion that absorption in this case could 

 not be a physical process, are that he took no account of the 

 possibility of filtration and imbibition, and that the serum was 

 that of another animal. Reid repeated Heidenhain's experiments, 

 and at the same time appeared to exclude most of the known 



