THE FLUIDS OF THE BODY 41 



both of filtration and of osmosis. So, at least, it appears to 

 us when we are looking at the result in ignorance of what 

 has happened inside the living cell. The passage from blood 

 to lymph and vice versa through the wall of a capillary vessel 

 is in certain situations or at certain times a mere process of 

 filtration ; at others a process of restricted filtration. If the 

 wall is behaving as a perfect membrane, it is a process of 

 diffusion, or osmosis. It seems unnecessary to regard it, in 

 any case, as a process of secretion. The more widely the 

 capillaries are dilated, the less resistance do they offer to 

 exudation. The narrower their calibre, the greater is the 

 restraint which they place on the escape or entrance of fluid. 

 When the skin of the palm of the hand is not sufficiently thick 

 to protect the soft tissues beneath it from the injurious effects 

 of the prolonged pressure of an oar or an axe, the capillary 

 vessels of the under-skin dilate ; more lymph transudes ; the 

 skin is raised up as a blister. The same thing happens when 

 the capillaries are dilated and paralyzed by scalding water. 

 The fluid of a blister has much the same constitution as 

 blood-plasm, except that it contains less proteid substance. 

 These results might be regarded as purely mechanical the 

 direct effects of pressure or heat upon the membranous 

 capillary wall. But the " vital " element is more important. 

 The capacity of endothelium to act as a barrier depends 

 upon its nutritive condition its vital integrity, as it might 

 be termed ; which no doubt in the last resort means its 

 chemical relation to the fluids which bathe it. Now and again 

 blebs, like blisters, are formed on the skin the herpes which 

 appears about the mouth ; urticaria, which is more generally 

 distributed ; and various other cutaneous disorders. Frequently 

 a connection can be traced between these eruptions and the 

 consumption of a particular food. An attack of urticaria results 

 not uncommonly from eating lobster, mussels, rook-pie, or 

 some few other articles of diet. Various things bad fish, for 

 example may produce the same effect ; but shell-fish have 

 an especially evil reputation. If extract of lobster or of 

 mussels be injected into the blood of an animal, the amount 

 of lymph which leaves the blood is markedly increased. The 

 extract acts as a poison upon the endothelium of the capillary 

 walls. It increases its permeability in all conditions in which 



