156 THE VASCULAR SYSTEM 



cretory products, some in colloidal linkage, some as crystalloidal 

 substance. In these cells there comes about a complex play of the 

 forces of osmosis and surface tension. Supposing the epithelium 

 of the tubules swells owing to their surface energy being great in 

 comparison with the capillaries where the surface tension is great, 

 then the epithelium in its turn will have a higher surface tension 

 than the lumina of the tubules, and this surface energy will cause 

 the transference of liquid from vessel to tubule. We have in the 

 kidney a gelatinous foam-like structure, the meshes of the foam 

 containing fluid, and the meshes being formed of two sets of ramified 

 channels, the one set leading to the venous and the other to the 

 ureter outlet. Blood is driven in pulses into the vascular mesh- 

 work. By such forces as adsorption and surface tension the 

 fluid part of the blood pervades the tubular meshwork, while the 

 living cells concentrate, and extrude vacuoles filled with, urinary 

 excretions. The mystery of the whole process is hidden from us, but 

 we may be sure that an excretory cell in a bath of collecting fluid 

 and provided with an excretory channel form the structural basis of 

 the mechanism, and that the mechanical filtration theories may be 

 relegated to the conceits of a science in its more primitive days. 



It has always been assumed by those who maintain the de- 

 fence of the filtration theory that the capillary pressure must be 

 raised not only when the general venous and arterial pressure rise 

 together, but also when the general venous pressure is raised, so 

 long as there is no fall of arterial pressure compensating for this 

 venous rise. No such assumption can be made in regard to a 

 rise of arterial pressure because of the unknown factor the re- 

 sistance in the arterioles. Now the experiments of Martin Flack 

 and the writer made on the arterial venous and capillary pressure 

 of their own limbs show that the above assumptions are, at any 

 rate in the case of the limbs, not valid. The capillary system is 

 not filled to distension ; there is a large potential space which only 

 gradually fills when the venous return is impeded, and thus the 

 venous pressure the veins being filled by broad paths of low 

 resistance may rise greatly while the capillary pressure in large 

 areas drained by these veins is scarcely altered. The gradual 

 filling of the capillary system, while producing distension of the 

 part, enormously increases the surface exposure of the blood fluid 

 and the surface tension of the capillary films, and these are pro- 

 bably factors of the greatest importance in the formation of lymph, 



