322 ABSORPTION. 



pass through the membrane can be measured. The bell-glass is generally filled with a 

 liquid capable of attracting a current of water from without, and is immersed in pure 

 water, so that the membrane is completely covered. Under these circumstances, there 

 is a current of water through the membrane, whicli will cause the liquid to mount in the 

 tube, sometimes to the height of several feet ; but, at the same time, there is a feebler 

 current from the interior of the apparatus to the water. Dutrochet called the stronger, 

 the endosmotic current, and the feebler, the exosmotic current. This nomenclature, 

 however, is not strictly accurate ; for, if the position of the liquids be reversed, the 

 stronger current is exosmotic and the feebler is endosmotic. It must be remembered, 

 therefore, that the name endosmosis is always to be understood as applied to the princi- 

 pal current, while the term exosmosis is applied to the current in the opposite direction. 

 This possible inaccuracy of expression has led to the adoption by Graham and others of 

 the term osmosis, as applied generally to the currents which take place through mem- 

 branes ; but the terms first proposed by Dutrochet are most commonly used. 



The phenomena of endosmosis, which, since the publication of the researches of Du- 

 trochet, have been so closely studied by physicists, are chiefly interesting to the physiolo- 

 gist in their application to absorption. While it is true, perhaps, that all the phenomena 

 of physiological absorption cannot as yet be explained upon purely physical principles, 

 it is nevertheless important to ascertain how far physical laws are involved in this pro- 

 cess. 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 conditions are necessary for the 

 operation of endosmosis and exosmosis : 



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

 though it is found that the currents are most active when the liquids are of different den- 

 sities, this condition is not indispensable ; for currents will take place between solutions 

 of different substances, such as salt, sugar, or albumen, when they have precisely the 

 same density. 



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 substances are dissolved or liquefied before they are ab- 

 sorbed, and, in this condition, they are capable of u wetting " the walls of the blood-vessels 

 All the liquids absorbed are capable, also, of mixing with the plasma of the blood. What 

 makes this application still more complete, is the behavior of albumen in endosmotic ex- 

 periments. In physiological absorption, there is always a great predominance of the 

 endosmotic current, and there is very little transudation, or exosmosis, of the albuminoid 

 constituents of the blood. On the other hand, there is a constant absorption of albu- 

 minose, which is destined to be converted into the albuminoid matters of the blood. 



Recognizing the fact, which was, indeed, pointed out clearly by Dutrochet, that albu- 

 men is capable of inducing a more powerful endosmotic current than almost any other 

 liquid, it has been shown that it never itself passes through membranes in the exosmotic 

 current; but that albuminoids, after transformation by digestion into albuminose, or 

 albumen mixed with gastric juice, pass through animal membranes with great facility. 

 The experiments by which these facts are demonstrated are very conclusive and are 

 of the highest physiological importance. On removing part of the shell of an egg, so 

 as to expose its membranes, and immersing it in pure water, the passage of water into 

 the egg was rendered evident by the projection of the distended membranes; but, 

 although the surrounding liquid had become alkaline and the appropriate tests revealed 

 the presence of some of the inorganic constituents of the egg, the presence of albumen 



