THE CIRCULATION 157 



dropsy. We know, indeed, that this must be due to an 

 over-production oi lymph, and not to a diminished 

 absorption of it from the tissues, for ligature of the chief 

 vein of a limb does not lead to its becoming oedematous, 

 but the parts played in the increased production by 

 alterations in (1) capillary pressure, (2) permeability of 

 the wall, (3) tissue-tension, and (4) composition of the 

 blood respectively, have not been unravelled, and it is 

 possible that in different cases each of these is responsible 

 in different degree. We know still less of the opposite 

 condition of diminished lymph formation, or whether, 

 indeed, that ever occurs at all, although more knowledge 

 on this head might explain some anomalies of nutrition 

 which at present puzzle us. There appears to be no 

 doubt that the permeability of the capillary wall varies 

 in different localities. It is least in the limbs and 

 greatest in the liver. This may explain how it is that 

 ascites may be present in mitral stenosis without there 

 being any evidence of dropsy elsewhere. It is possible, 

 too, that the permeability of the wall is increased as the 

 result of the action upon its cells of certain poisons 

 which fail to be excreted in disease of the kidneys, and 

 this may play a part in the production of renal dropsy. 



The Venous Circulation. 



The veins resemble the arteries in their essential 

 structure, but contain relatively less muscular and 

 elastic tissue, and a stronger external fibrous coat, which 

 enables them to withstand the influence of gravity. 

 They attain their maximum distension at a much lower 



