THE LYMPHATIC SYSTEM. 315 



solution for diffusing itself in the exterior fluid. The simplest illus- 

 tration of the process is that of the transudation and evaporation of 

 moisture. If a fresh animal membrane be exposed to the air under 

 ordinary circumstances, it at once begins to lose water by evaporation ; 

 and the loss will continue, under favorable hygrometric conditions, 

 until the whole of the water has disappeared in the atmosphere and 

 the membrane is completely desiccated. But if the membrane be 

 placed with its upper surface in contact with the air, and its lower 

 surface in contact with water or a watery fluid, it no sooner loses a 

 portion of its water by evaporation than it absorbs a corresponding 

 quantity from beneath. There is thus a continual passage of water 

 from the fluid, through the membrane, to the atmosphere, until the 

 whole of it has been exhausted ; the membrane retaining its own 

 proportion of moisture, while losing water by one surface and absorb- 

 ing it by the other. 



A similar interchange will take place if one surface of the membrane 

 is in contact with water and the other with a saline or saccharine solu- 

 tion ; provided the solution be sufficiently concentrated to absorb water 

 from the membrane. Each layer of the membrane absorbs from that 

 next to it sufficient, moisture to replace that which has passed into the 

 solution ; and endosmosis thus goes on from the water to the solution 

 through the animal membrane. 



In this instance, however, there will be a double action. As the 

 membrane has an absorptive power for both the water and the ingredi- 

 ents of the solution, and as these two are diffusible in each other, they 

 will both be transferred in opposite directions. But since the mem- 

 brane absorbs water more readily than the ingredients of the solution, 

 it can supply these ingredients to the water on one side less abun- 

 dantly than it can supply water to the solution on the other. Con- 

 sequently a larger volume of water passes to the solution than vice 

 versa, and endosmosis preponderates over exosmosis. 



It is evident accordingly that, whatever be the relation of the two 

 liquids to each other, the first requisite for their transudation is the 

 absorptive power of the animal membrane. A membrane in contact 

 with two different liquids will nearly always absorb one of them more 

 rapidly than the other ; and if in contact with a solution containing 

 several ingredients, it will take up some of these ingredients in 

 greater, others in smaller proportion. A substance, therefore, which 

 the intervening membrane does not absorb at all, cannot be transferred 

 to the fluid beyond it. The membrane acts as a barrier to exclude 

 ingredients for which it has no absorptive power, but is ready to 

 supply those which it can take up with facility. 



An equally important condition of endosmosis and exosmosis is the 

 diffusibility of different liquids in each other. This subject was inves- 

 tigated by Graham * in the following manner : Glass vessels, filled 



* Annalen der Chemie und Pharmacie. Heidelberg, 1851. Band Ixxvii., p. 56. 



