ALIMENTARY TRACT AS AN ABSORPTIVE SYSTEM. 277 



Sodium chloride has, of all these salts, the greatest diffusion 

 velocity, and also the greatest absorption velocity. From 

 here downwards the velocities of diffusion and of absorption 

 diminish progressively. In the same unit of time a larger 

 amount of the solution of a salt in the first group will dis- 

 appear from a loop of intestine than of an isotonic solution 

 of a salt contained in the second group, and this will dis- 

 appear sooner than an equal amount of an isotonic solution 

 of a salt found in the third or fourth group. For all salts this 

 does not hold, however. Of the halogen salts of sodium, for 

 example, which have all the same diffusion velocity, the 

 chloride is absorbed most rapidly, then the bromide, and 

 finally the iodide. All fluorides are absorbed exceedingly 

 slowly, as also the carbonates, due no doubt to secondary 

 changes produced in the cells, for the fluorides are proto- 

 plasmic poisons, and the carbonates suffer a hydrolytic disso- 

 ciation with the formation of free OH ions, which are toxic 

 in even very low concentrations. 



HOBER has made interesting observations on the paths of 

 absorption of the salts. From studies with dyes which are in 

 part soluble in the lipoids of the cells, in part insoluble, he 

 has been able to show that salts are for the most part absorbed 

 only intercellularly; that is to say, they pass into the blood 

 not through the epithelial cells of the absorbing mucosa, 

 but through the intercellular spaces. If solutions of the salts 

 of various dyes soluble in the lipoids of the cell such as 

 methylene blue, toluidin blue, or neutral red are introduced 

 into the intestinal tract of frogs, microscopic examination 

 shows that the dyes affect slightly all portions of the ab- 

 sorbing mucosa. But while protoplasm, nucleus, and intercel- 

 lular substance are scarcely colored, granules contained within 

 the protoplasm take up the dyes most intensely. The granu- 

 lar material seems to be, therefore, an excellent solvent for 

 these dyes and probably consists of those substances which 

 are collected under the heading lipoids. When solutions of 

 dyes insoluble in the lipoids such as water-soluble aniline 



