CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 529 



lateral communication between the lymph-vessels the labyrinthine 

 passages of the lymphatic glands do not offer the serious obstacle 

 to the onward flow of the general lymph-stream which they might 

 at first sight be supposed to offer. Nor have we at present any 

 knowledge which would lead us to suppose that any changes in 

 the walls of the lymphatic vessels or of the lymph-capillaries, or in 

 the lymph- spaces, by giving rise in some way to obstacles to the 

 flow of lymph, ever lead to an accumulation of lymph in the latter. 

 (Edema is in nearly all cases due, or due chiefly, to the appearance 

 of lymph in excess in the lymph-spaces and lymph-capillaries ; it 

 appears in these more rapidly than it can be carried away from 

 them by the lymphatic vessels. And to this appearance of lymph 

 in the lymph-spaces and lymph-capillaries as distinguished from 

 the mere flow of lymph along the lymphatic vessels we must now 

 turn. 



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-capillary 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, as we have seen, it 

 is only in particular places that the lymph-capillary immediately 

 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, as we have seen, peculiar and at least not a 

 free and open one. We may confine ourselves at first to the first 

 step, namely to the passage of material from the blood vessel into 

 the lymph-space. 



302. This process we speak of as transudation. What can 

 we say as to its nature ? There are two known physical processes 

 with which we may compare it : diffusion through a membranous 

 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 partition, 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. Filtra- 

 tion on the other hand is the direct result of pressure ; without 

 difference of pressure filtration does not take place; and, the 

 filter remaining of the same nature and in the same condition, the 

 amount of filtrate is dependent on the amount of pressure. May 

 we speak of the process of transudation as a simple process of 



