524 



GENERAL PHYSIOLOGY 



employed when the gases or liquids are in direct contact, and 

 osmosis, when they are separated from one another by an organic 

 membrane. We have already become acquainted with this process. 

 If we recall the experiment that illustrated osmosis (p. 104) and 

 vary it somewhat, we can at the same time make clear the great 

 importance which the second factor, the chemism of the cell, possesses 

 in the presence of diffusion or osmosis. If in the larger vessel 

 {Fig. 261) there is a diffusible salt solution, and in the cylinder 

 the solution of a substance that does not diffuse, a certain quantity 

 of salt will diffuse out of its solution into the liquid of the cylinder, 

 while no substance can pass from the cylinder into the larger vessel. 

 If, however, the substance in the cylinder has a chemical affinity 

 for the salt, the salt diffusing into the cylinder goes into chemical 

 combination at once. If the chemical compound thus arising be 



continually removed and replaced 

 by new solution of the same kind 

 as before, the salt solution in the 

 larger vessel will become constantly 

 weaker and weaker, until finally all 

 the salt has diffused into the cylin- 

 der, has become combined and is 

 removed, so that in the large vessel 

 there is nothing but water. 



This case is realized in the process 

 of taking in gaseous and dissolved 

 substances by living substance. The 

 living substance is capable of mix- 

 ing with the gaseous and dissolved 

 food-stuffs, for it has a chemical 

 affinity for them. The cell-mem- 

 brane, if such be present, represents 

 the membrane of the cylinder ; the 

 cell-contents, the contents of the cylinder ; and the gaseous or dis- 

 solved substances, the salt solution of the larger vessel. These sub- 

 stances must be diffusible, if they are to be taken in ; nevertheless, 

 the living substance cannot diffuse through the cell-membrane, 

 since the proteids, etc., belong to the so-called colloid substances. 

 Hence the food-stuffs will pass into the cell, but the living substance 

 cannot pass out. Since the latter has a chemical affinity for the 

 food-stuffs, it must enter into combination with them immediately 

 after their entrance into the cell. But it is continually decompos- 

 ing, giving off substances to the outside, and reforming ; in other 

 words, the food-substances taken in are constantly being consumed, 

 so that a continual balance between the inside and the outside can 

 never take place, and new masses must constantly diffuse in. The 

 output of substances must take place in an analogous manner. 

 Let us, then, imagine a cell, surrounded by a membrane, existing in 



FIG. 261. Dialyzer. 



