CHAP. L] TISSUES AND MECHANISMS OF DIGESTION. 535 



of the liver contains more solids, and among these more proteids, 

 than does the lymph which transudes into the lymphatics of the 

 limbs. This difference cannot be explained by the capillary 

 pressure being greater in the former case ; it is in fact probably less. 

 The difference is due to a difference in the nature and action of 

 the capillary wall. In the second place the transudation passing 

 through the same capillary wall may differ in composition accord- 

 ing to circumstances. An extreme instance of this is seen in 

 the transudation occurring in inflammation ( 183) ; but other less 

 marked instances may be met with ; not only may the total 

 amount of solids and the total amount of proteids vary, but we 

 have reason to think that the kinds of solids or at least the kinds 

 of proteids which go to form the lymph may vary under different 

 circumstances. It is difficult if not impossible to suppose that 

 these differences are brought about by mere differences of pressure. 



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 coming 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, though we do not as yet know exactly the 

 relative quality and quantity of the waste products leaving the 

 organ on the one hand by the lymphatics, and on the other hand 

 by the blood, the backward stream of material from the tissue to 

 the blood is probably extensive and important. Moreover this 

 backward stream works against pressure ; indeed, as may be seen 

 in a muscle, it is when the blood vessels are dilated and the pres- 

 sure in the capillaries highest, as during and after the contraction 

 of the muscle, that the passage from the tissue into the blood is 

 most energetic. 



Putting all these considerations together we find ourselves 

 in the following position. If owing to the obvious influence of 

 pressure we allow ourselves to speak of the transudation of lymph 

 as a process of filtration, we must admit that it is a process widely 

 different from ordinary filtration through a lifeless filter. The 

 capillary wall, if it be a filter, is one so subject to change that 

 irrespective of pressure it will allow now more, now less material to 

 pass, nay, if the reasoning given some while back be valid, will 

 refuse to allow any lymph at all to pass, though considerable 



