360 THE LYMPHATIC SYSTEM. 



crystallizable, passes very rapidly and abundantly. The distinction, 

 furthermore, between the two classes of substances is not a complete 

 one, since they may nearly all be made to transude in some degree by 

 increasing the pressure of the column of fluid upon the corresponding 

 side of the membrane ; but the difference between them is often very 

 great in this respect. According to the observations of Liebig, 1 the 

 requisite pressure for different liquids, in passing through the same 

 membrane in a given time, is as follows : 



COMPARATIVE PRESSURE HEQUIRED TO CAUSE TRANSUDATION THROUGH 

 OX-BLADDER. 



Kind of liquid. Height of the mercurial column. 



Water . . 324 millimetres. 



Solution of salt 514 



Oil 920 " 



Aldbhol 1298 " 



Owing to their varying degree of transmissibity through membranes 

 this property has even been employed for the purpose of separating 

 different substances from each other, when mingled together in the 

 liquid form. This process is termed Dialysis. Thus, if a solution 

 containing both gum and sugar be placed in contact with one side of 

 the membranous diaphragm, with pure water on the other, the sugar 

 alone will pass through, while the gum will be left behind. If a mix- 

 ture of albumen and sodium chloride be placed under the same con- 

 ditions, the salt will transude in a pure form leaving the albumen 

 by itself; both substances in this way being purified from each other 

 through the action of the membrane. By the same process it has been 

 found possible to extricate poisonous c^stallizable matters, such as 

 strychnine or arsenious acid, from their admixture with albuminous 

 substances in a state of sufficient purity to allow of their detection by 

 chemical tests. The tissues of an animal membrane, accordingly, may 

 in this way exercise a kind of elective affinity for various substances, 

 and produce a special composition in fluids which have transuded 

 through them. 



Endosmosis and Exosmosis. Beside the elimination of chemical 

 ingredients above described, the phenomena of trans udation often give 

 rise to a change in volume of the fluid on either side of the membranous 

 septum. When an animal membrane is interposed between two different 

 liquids which are imbibed and transmitted by it with different degrees 

 of facility, that which passes most readily will accumulate in larger 

 quantity on the opposite side of the membrane. 



If we take, for example, a solution of salt and an equal volume of 

 distilled water, and inclose them in a glass tube with a fresh animal 

 membrane stretched between, the two liquids being in contact with 

 opposite sides of the membrane, after a time they will have become 



1 Cited in Longet, Trait de Physiologie. Paris, 1861, tome i. p. 384. 



