THE LYMPHATIC SYSTEM. 311 



however, the accumulation of fluid in the interior becomes so ex- 

 cessive as to* burst the shell- membrane, and then the two liquids 

 become mixed indiscriminately together. 



These are the principal conditions by which endosmosis is influ- 

 enced and regulated. Let us now see what is the nature of the 

 process, and upon what its phenomena depend. 



Endosmosis is not dependent upon the simple force of diffusion 

 or admixture of two different liquids. For sometimes, as in the 

 case of albumen and water, all the fluid passes in one direction and 

 none in the other. It is true that the activity of the process de- 

 pends very much, as we have already seen, upon the difference in 

 constitution of the two liquids. With water and a saline solution, 

 for instance, the stronger the solution of salt, the more rapid is the 

 endosmosis of the water. And if two solutions of salt be used, 

 with a membranous septum between them, endosmosis takes place 

 from the weaker solution to the stronger, and is proportionate in 

 activity to the difference in their densities. From this fact, Dutro- 

 chet was at first led to believe that the direction of endosmosis was 

 determined by the difference in density of the two liquids, and that 

 the current of accumulation was always directed from the lighter 

 liquid to the denser. But we now know that this is not the case. 

 For though, with solutions of salt, sugar, and the like, the current 

 of endosmosis is from the lighter to the denser liquid ; in other 

 instances it is the reverse. With water and alcohol, for example, 

 endosmosis takes place, not from the alcohol to the water, but from 

 the water to the alcohol ; that is, from the denser liquid to the lighter. 

 The difference in density of the liquids, therefore, is not the only 

 condition which regulates the direction of the endosmotic current. 

 In point of fact, the process of endosmosis does not depend princi- 

 pally upon the attraction of the two liquids for each other, but 

 upon the attraction of the animal membrane for the two liquids. The 

 membrane is not a passive filter through which the liquids mingle, 

 but it is the active agent which determines their passage. The 

 membrane has the power of absorbing liquids, and of taking them 

 up into its own substance. This power of absorption, belonging to 

 .the membrane, depends upon the organic or albuminous ingredients 

 of which it is composed ; and, with different animal substances, the 

 power of absorption is different. The tissue of cartilage, for exam- 

 ple, will absorb more water, weight for weight, than that of the 

 tendons ; and the tissue of the cornea will absorb nearly twice as 

 much as that of cartilage. 



