406 MOVEMENT OF LYMPH. [BOOK n. 



nervous action ; attempts to demonstrate any direct action of 

 the nervous system on the lymphatics have hitherto failed. 



244. The passage of material, namely, of water containing 

 certain substances in solution, from the interior of the blood 

 vessel where they form part of the plasma into the lymph-cap- 

 illary where they are called lymph consists of two steps : the 

 passage from the blood vessel into the lymph-space, and the 

 passage from the lymph-space into the lymph-capillary ; for 

 it is only in particular places that the lymph-capillary imme- 

 diately surrounds the blood vessel. Once arrived in the lymph- 

 capillary the lymph finds an open path along the rest of the 

 lymphatic system, but the connection between the lymph-space 

 and the lymph-capillary is peculiar and at least not a free and 

 open one. 



The passage of material from the blood vessel into the lymph- 

 space we speak of as transudation. What can we say as to the 

 nature of this process ? There are two known physical processes 

 with which we may compare it : diffusion through a membra- 

 nous or other porous partition, and filtration through a similar 

 partition. Diffusion, though influenced by fluid pressure, is 

 not the direct result of fluid pressure but may on the contrary 

 be the cause of differences of pressure on the two sides of a par- 

 tition, and may work against fluid pressure. When a strong 

 solution and a weak solution of salt are separated by a diffusion 

 septum, diffusion takes place whether the columns of fluid be 

 at the same level on the two sides of the septum or at different 

 levels; and if the columns be at the same level to start with, 

 that of the stronger solution soon comes to exceed the other in 

 height, on account of the osmotic flow of water from the weaker 

 into the stronger solution. Filtration on the other hand is the 

 direct result of pressure ; without difference of pressure filtra- 

 tion does not take place ; and, the filter remaining of the same 

 nature and in the same condition, the amount of filtrate is de- 

 pendent on the amount of pressure. May we speak of the pro- 

 cess of transudation as a simple process of diffusion or a simple 

 process of filtration, that is to say, can all the phenomena of 

 transudation be explained as simply the results of one or other 

 of these physical processes? Diffusion by itself will not ac- 

 count for the results ; for the proteids of the blood-plasma are 

 indiffusible or very nearly so and yet the lymph contains a con- 

 siderable quantity of these proteids. We have no satisfactory 

 knowledge of the exact composition of lymph as it exists in the 

 lymph-spaces. In the lymph of the larger lymph-trunks the 

 diffusible saline substances are present in about the same pro- 

 portion, and the indiffusible proteids to about or less than half 

 as much as in blood-serum; and we may perhaps assume that 

 the lymph in the lymph-spaces contains relatively less proteids 

 but has otherwise the same composition as blood-plasm. Merc- 



