ALIMENTARY TRACT AS AN ABSORPTIVE SYSTEM. 267 



side of it, then differences in osmotic pressure are quickly 

 equalized through diffusion of the dissolved particles, and a 

 movement of water can in consequence show itself only 

 imperfectly. 



But the cells of the alimentary mucosa are by no means 

 equally permeable to all dissolved substances. For this rea- 

 son the cells of the alimentary tract (as well as other body 

 cells) obey the laws of osmotic pressure more perfectly when 

 certain substances are dissolved in the fluids present in the 

 alimentary tract than when others are concerned. A partial 

 explanation at least of this selective permeability of cells will 

 be given when we discuss OVERTON and MEYER'S work on the 

 lipoids. 



We have yet another way of influencing the absorption of 

 water besides through differences in osmotic pressures, and 

 that is by changes in the affinity of the colloids for water. As 

 is well known protoplasm is composed of a mixture of col- 

 loids, and experiment has shown that the amount of water 

 absorbed by colloids can be enormously influenced by a num- 

 ber of external conditions. As shown by such experiments 

 as those of HOFMEISTER/ gelatine plates absorb water from 

 the chlorides of the alkali metals. They absorb much more 

 from isotonic solutions of the bromides and nitrates of these 

 same metals. The citrates, sulphates, and tartrates of these 

 metals, on the other hand, cause such swollen plates to give up 

 their water. 2 It almost seems as though in an understanding 

 of the laws governing the absorption of liquids by colloids, and 

 of the secretion of these same liquids when external con- 

 ditions are slightly altered, lies the explanation of much that 

 is obscure in the phenomena of absorption and secretion as 

 they are found in the animal and vegetable organism. 

 Changes in osmotic pressure are certainly by themselves in- 



1 HOFMEISTER: Archiv f. exp. Path, XXVIII, p. 210. 



2 This almost seems analogous to the secretion of water into the 

 alimentary tract under the influence of cathartic salts. 



