506 TRANSUDATION. [BOOK n. 



cause it, and the femoral vein of a dog may be ligatured without 

 any excessive transudatiou taking place ; yet if, after the ligature, 

 certain changes be induced in the blood excessive transudation 

 occurs in the leg, the vein of which has been ligatured but not 

 elsewhere. Pointing towards the same conclusion is the fact that 

 excessive transudation more readily occurs when a vein is plugged 

 by a thrombus arising from abnormal conditions of the vascular 

 system than when a vein is simply ligatured. And in general 

 we may say, and this is a point to which we shall return, that two 

 things chiefly determine the amount of transudation : the pressure 

 of the blood in the blood vessels, and the condition of the vascular 

 walls in relation to the blood, the latter being at least as important 

 as the former. 



Another aspect of the matter moreover deserves attention. In 

 filtration the movement takes place through the filter in one 

 direction only, whereas in the living body, the passage of material 

 through the capillary wall takes place in two opposite directions. 

 In all the tissues, though more perhaps in certain tissues than in 

 others, the passage from the blood vessel into the lymph-space is 

 accompanied by a passage from the lymph-space into the blood ; 

 while food for the tissue passes in one direction, waste products 

 pass in the other. In a secreting gland the greater part of the 

 lymph corning from the blood vessels, the water and other matters, 

 pass away into the lumen of the alveolus after undergoing changes 

 in the cell ; but even in such a case there is some return from the 

 cells into the blood vessels, carbonic acid for instance if nothing 

 else is given up by the cells to the blood ; and in such organs as a 

 muscle or the liver, the backward stream of material from the tissue 

 to the blood is extensive and important. Moreover this back- 

 ward stream works against pressure ; indeed, as may be seen in a 

 muscle, it is when the blood vessels are dilated and the pressure 

 in the capillaries and small vessels highest, as during and after the 

 contraction of the muscle, that the passage from the tissue into the 

 blood is most energetic. Many of the waste products of the tissue 

 are it is true diffusible, and we might be tempted to say that while 

 the lymph which feeds the tissue traverses the vascular wall by 

 filtration in the direction of pressure the waste products return to 

 the blood against pressure by diffusion ; but such a view cannot at 

 present be regarded as proved ; and if it be true as is maintained 

 by some, that lymph, including the proteids, may at times be 

 re-absorbed from the tissue into the blood vessels, it is distinctly 

 contradicted. We shall have to return to this question when we 

 come to deal with the secretion of urine ; but meanwhile we may 

 adopt the conclusion, which is especially supported by the phe- 

 nomena of disease, that while diffusion and filtration play their 

 respective parts, diffusible substances passing in and out of the 

 blood more readily than indiffusible substances and an increase of 

 pressure tending to promote transudation, the condition of the 



