CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 503 



in the lymphatic trunks leading from each of the hind feet, the 

 sciatic nerve on one side is divided, the flow of lymph from 

 the foot on that side is greater than on the intact side, but is 

 diminished on stimulation of the peripheral end of the nerve, the 

 diminution being followed by a subsequent increase. The section 

 of the nerve however leads to arterial dilation, the stimulation of 

 the nerve to arterial constriction; and until other reasons be 

 shewn, we may attribute the increased or diminished flow of 

 lymph to an increased or diminished transudation from the fuller 

 or emptier blood vessels. And this interpretation is supported by 

 the fact that when stimulation of the nerve is so conducted as to 

 lead to arterial dilation ( 168) the result is not a diminished but 

 an increased flow of lymph. Again if the cervical sympathetic in 

 a rabbit be divided on one side and a solution of the blue pigment, 

 sulphindigotate of soda, be injected into the venous system, the 

 ear on that side becomes blue before the other, because the 

 pigment passes more rapidly from the blood vessels into the lymph- 

 spaces of the connective tissue and the blueness also passes away 

 sooner because it is sooner washed away by the subsequent 

 uncoloured lymph. But here too the increased transudation may 

 be regarded as simply the result of the greater fulness of the blood 

 vessels. 



302. 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. 



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 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 the 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 



