646 THE MECHANISM OF ABSORPTION 



permeability to NaCl, and would therefore be in a peculiarly good 

 position to absorb solutions of this substance in any concentra- 

 tion by diffusion and osmosis. And the action of NaF in show- 

 ing absorption could be readily explained by its destroying this 

 one-sided permeability. 



Cohnheim' s Experiments. Besides the experiments already re- 

 ferred to, Cohnheim made a considerable study of the effect of 

 various poisons upon the absorption of dextrose solutions. He 

 found that anisotonic solutions became, during absorption in a 

 normal gut, isotonic solutions of dextrose, either by the absorp- 

 tion of sugar or water, as the case might be ; and he considered 

 that this conversion of an anisotonic into an isotonic dextrose 

 solution was due to diffusion and osmosis. When, however, the 

 epithelium is severely poisoned by NaF or As, it loses its 

 special peculiarities, and becomes more like an ordinary dead 

 permeable membrane. Consequently, NaCl diffuses from the 

 blood into the gut, and the intestine, instead of containing an 

 isotonic solution of dextrose, is filled with an isotonic fluid con- 

 taining a lower concentration of dextrose and a much higher 

 concentration of NaCl. The rate of absorption of this abnormal 

 fluid is very slow. This might be explained on the grounds that 

 normally it is the partial pressure of NaCl in the blood which 

 determines the absorption of water from a dextrose solution. 

 But Cohnheim has brought forward evidence that this physical 

 explanation may not be the whole truth ; for he has shown that 

 the power of taking up water from a dextrose solution may be 

 interfered with without at the same time destroying the one- 

 sided permeability of the epithelium to NaCl. He found that at 

 a certain stage of poisoning with NaF, quinine, or potassium 

 acetate, the percentage of NaCl in the gut might be scarcely 

 raised at all, and yet the absorption of water was greatly interfered 

 with. He looks upon this result as strong evidence that fluids 

 are transported by the vital activity of the epithelium. Ham- 

 burger has pointed out an obvious objection to all these experi- 

 ments with poisons. It is not justifiable to assume tacitly that 

 the action of a poison is confined to the intestinal epithelium 

 and does not affect also the capillary walls and the circulation 

 through the part. Further, we are not in a position to say that 

 the epithelial cells are still normal because the one-sided perme- 

 ability to NaCl is still retained ; and that any observed alteration 



