614 EXPERIMENTS DEALING WITH 



3. The permeability of the capillary wall. 



4. The chemical composition of the blood plasma . P. 



5. The chemical composition of the tissue fluid 0. L. 



The differences in composition between the blood and tissue 

 fluid are responsible for diffusion and osmosis taking place between 

 them. . P . L constitutes a possible absorbing force, 

 which, however, could only act when it is greater than P p. 



It is necessary to consider how far we are able to estimate in 

 the body these various factors. 



Capillary blood pressure cannot be directly measured. We 

 do not know, for instance, how much it is increased by 

 hydrsemic plethora, nor how much it is reduced by a haemorrhage. 

 We can alter it at will, and infer the magnitude of the alteration 

 by simultaneously measuring the pressures in the corresponding 

 artery and vein. 



The pressure of tissue fluid cannot be accurately estimated. 

 We know that it is usually less than the capillary pressure, and 

 that it is derived partly from the capillary pressure and partly 

 from the elasticity of the tissues. Whether it remains constant 

 during alterations of capillary pressure and whether it can be 

 greater than the capillary pressure are unknown. 



The permeability of the capillary wall is a factor which we have 

 no means of estimating. The nearest approach for experimental 

 purposes to such a living membrane are various dead animal or 

 vegetable membranes. But how far the unaltering properties of 

 a dead membrane are like those of a capillary wall, and which 

 membrane is least unlike it, are unknown. The permeability of 

 different experimental membranes varies enormously. For in- 

 stance, Moore and Parker found that fresh peritoneal membrane 

 is permeable to proteids, peritoneum soaked in gelatine is not, 

 vegetable parchment is permeable neither to proteids nor soaps. 

 Nor do we know how far the experimental means used for altering 

 the permeability of the capillary wall have really brought about 

 their result in that way. All that we actually know in that case 

 is the result, that more lymph of a higher concentration has 

 been formed. It is likely that the capillary wall is relatively 

 impermeable to proteids, that the permeability varies in different 

 parts of the body, and even in the same area, as the result of the 

 action of metabolic products upon it. But the exact opposites 

 are also possible. 



