ALIMENTARY TRACT AS AN ABSORPTIVE SYSTEM. 279 



other substances which are absorbed from the alimentary 

 tract much more rapidly than the inorganic salts. While 

 the absorption of salts from the stomach is still questioned, 

 the rapidity with which alcohol disappears from the gastric 

 contents is truly phenomenal. Urea also is much more 

 rapidly absorbed than salts of a simpler composition. The 

 reason for this is at once apparent when we assume that 

 while the inorganic salts can pass into the blood only through 

 the intercellular spaces, alcohol, urea, etc., are soluble in the 

 lipoids, and so can pass directly through the epithelial cells 

 as well. 



From what has been said it seems, therefore, as though the 

 behavior of the epithelial cells of the gastro-intestinal tract 

 toward certain soluble substances is not unlike the behavior 

 of cells in general, and is dependent upon the same powers of 

 selective solution by their surface membranes. 



We come now, however, to a series of facts for which an 

 adequate physical explanation is entirely lacking. These 

 facts are connected with the predominant permeability of the 

 gastro-intestinal tract in one direction. While, as already stated, 

 some of the constituents of the Klood may diffuse into a solu- 

 tion contained in -the lumen of the intestine, only very little 

 passes out in this way. Salts found in the blood pass only 

 in exceedingly small amounts, if at all, into the intestine, 

 while /these same salts in various concentrations pass easily 

 from the alimentary lumen into the. blood. It is evident that 

 every constituent of the blood, which, like the albumin and 

 the globulin, is unable to pass, into the intestine can in this 

 way become effective in absorbing water, and in consequence 

 lead to a passage of salt from the fluid in the alimentary tract 

 into the blood. This semi-permeability of the absorbing 

 mucosa in one direction only would therefore greatly 

 favor the absorption of substances from the lumen 

 HEIM). 



All our attempted physical explanations of absorption by 

 the alimentary mucosa fail when we approach the observa- 



