Chap, i.] TISSUES AND MECHANISMS OF DIGESTION. 407 



diffusion would not give rise to a fluid of such a nature. Can 

 we speak of transudation then as a filtration ? The blood is 

 undoubtedly flowing through the capillaries and other small ves- 

 sels under a certain pressure ; we have seen (§ 98) that the 

 pressure is roughly speaking about 20 mm. Hg. ; and it would 

 be possible to select such a filter or porous partition as would 

 at about this pressure permit the passage of a certain quantity 

 of the inorganic and crystalline constituents of blood-plasma to 

 pass through in company with a relatively smaller quantity of 

 the proteids and a large quantity of the water, the red and white 

 corpuscles being excluded. Such a filtrate would be more or 

 less of the nature of lymph ; and so far we might be justified 

 in speaking of the transudation of lymph as a process of filtra- 

 tion. But the transit through the living wall of the blood ves- 

 sel is affected by circumstances in a manner so different from 

 the manner in which the same circumstances affect the transit 

 through an ordinary lifeless filter, that we gain but little, and 

 may be led into error by speaking of the process as a filtration. 

 Substances in solution or otherwise pass through a filter when 

 the pressure is sufficient to drive them through the passages 

 furnished by the interstices existing in the substance of the 

 filter. In the case of an ordinary filter the substance of the fil- 

 ter is within limits permanent, and the passages correspond- 

 ingly constant. The living wall of a capillary however is not 

 a constant unchanging thing. The epithelioid plates and other 

 elements which constitute it are alive, and being alive are 

 continually undergoing change and are especially subject to 

 change ; moreover, as we have seen (§§ 22, 23), the vascular 

 walls appear to be continually acting upon and being acted 

 upon by the blood. Hence a change in the blood tends to 

 cause changes in them ; and these changes may materially 

 affect in one direction or another their action as filters. In an 

 ordinary filter increase of pressure necessarily entails increase 

 of filtration ; in a living filter we should expect to find that it 

 may or may not, that variations of pressure may according to 

 circumstances produce very different results as regards the 

 transudation of lymph, and that the latter may vary independ- 

 ently of the former. 



Observations seem to confirm this view. In the first place 

 an increase of blood-pressure does not necessarily increase the 

 transudation of lymph. It is true that when a small artery 

 dilates, by which the pressure in the still smaller branches and 

 capillaries of that artery is, as we have more than once pointed 

 out, increased, more lymph as a rule appears in the lymph- 

 spaces ; indeed it is one of the main purposes of the widening 

 of small arteries to supply the elements of the tissue with more 

 lymph, that is, with more food. But it does not therefore fol- 

 low that under all circumstances widening of the artery should 



