476 ABSORPTION. 



cannot as yet be explained upon purely physical principles, 

 it is nevertheless important to ascertain how far physical 

 laws are involved in this process. With this end in view, 

 we shall study the physical phenomena of endosmosis, chiefly 

 with reference to their physiological applications. 



It is now definitely ascertained that the following condi- 

 tions are necessary for the operation of endosmosis and exos- 

 mosis : 



1. That both liquids be capable of " wetting " the inter- 

 posed membrane, or, in other words, that the membrane be 

 capable of imbibing both liquids. If but one of the liquids 

 can wet the membrane, the current can take place in only 

 one direction. 



2. That the liquids be miscible with each other, and be 

 differently constituted. Though it is found that the currents 

 are most active when the liquids are of different densities, this 

 condition is not indispensable ; for currents will take place 

 between solutions of different substances, such as salt, sugar, 

 or albumen, though they may have precisely the same den- 

 sity. 



The physiological applications of the laws of endosmosis 

 can now be more fully appreciated, as it is evident that the 

 above conditions are fulfilled whenever absorption takes 

 place ; with the single exception of the absorption of fats, 

 which has been specially considered. For example, all sub- 

 stances are dissolved or liquefied before they are absorbed, 

 and in this condition are capable of "wetting the walls of the 

 blood-vessels ; and all the liquids absorbed are capable of 

 mixing with the plasma of the blood. 



What makes this application still more complete is the 

 behavior of albumen in endosmotic experiments. In phys- 

 iological absorption, there is always an immense predomi- 

 nance of the endosmotic current, and there is very little 

 transudation, or exosmosis, of albumen, the chief nutritive 

 constituent of the blood. On the other hand, there is a con- 

 stant absorption of albuminose, which is destined to be con- 



