504 TRANSUDATIOK [BOOK n. 



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 nitration 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 

 diffusion or a simple process of filtration, that i to say, can all the 

 phenomena of transudation be explained as simply the results of 

 one or other of these physical processes ? Diffusion by itself will 

 not account for the results ; for the proteids of the blood-plasma 

 are indiffusible or very nearly so and yet the lymph contains a 

 considerable quantity of these proteids. We have no satisfactory 

 knowledge of the exact composition of lymph as it exists in the 

 lymph-spaces. In the lymph of the larger lymph-trunks the 

 diffusible saline substances are present in about the same pro- 

 portion, and the indiffusible proteids to about or less than half 

 as much as in blood-serum ; and we may perhaps assume that 

 the lymph in the lymph-spaces contains relatively less proteids 

 but has otherwise the same composition as blood-plasma. Mere 

 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 

 vessels under a certain pressure; we have seen ( 116) that the 

 pressure is roughly speaking about 30 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 filtration. 

 But the transit through the living wall of the blood vessel 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. Sub- 

 stances 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 filter is within limits 

 permanent, and the passages correspondingly 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 



