ALIMENTARY TRACT AS AN ABSORPTIVE SYSTEM. 275 



A salt solution will, in the course of time, be absorbed by the 

 alimentary tract no matter whether it be isotonic, hypertonic, 

 or hypotonic with the blood, but the rate differs with the 

 different kinds of salts. Different observers agree that 

 when a hypertonic solution is introduced into the intestine 

 its volume increases at first through a diffusion of water 

 into it, while its concentration diminishes. At the same 

 time certain constituents of the blood diffuse into it. 

 After this the volume of solution in the intestine gradually 

 diminishes. These phenomena are usually explained on the 

 basis that the salt diffuses into the blood because its concen- 

 tration in the lumen of the alimentary tract is higher than the 

 concentration of this same salt in the blood, while various 

 constituents of the blood diffuse out into the intestine for 

 the same reason. The initial increase in the volume of the 

 solution is explained in those cases in which it occurs on the 

 basis that the salt under consideration does not diffuse rapidly 

 enough not to allow osmotic differences to make themselves 

 felt through a migration of water. For this reason, too, it 

 is found that the originally hypertonic solution gradually 

 approaches isotonicity with the blood. 



When we deal with the diffusion of a hypotonic solution 

 water diffuses into the blood because of osmotic differences, 

 and the salt solution under these circumstances also ap- 

 proaches isotonicity with the blood. When this occurs the 

 concentration of the salt in the solution undergoing absorp- 

 tion is increased and so is placed in a position to diffuse into 

 the blood, but this lowers the osmotic pressure of the solu- 

 tion in the lumen of the intestine once more, in consequence 

 of which water is again absorbed from it by the blood; and 

 so these processes repeat themselvfcs until all the salt solution 

 is absorbed. 



Solutions isotonic with the blbod are absorbed quite as 

 easily as hypertonic or hypotonii ones. It is somewhat dif- 

 ficult to see what forces are active in bringing about the 

 transport of the solution into the blood^ The explanation 



