74 HAND-BOOK OF PHYSIOLOGY. 



tissues which cause them to act toward it like foreign matter. The lat- 

 ter, on the other hand, supposes that there is a natural tendency on the 

 part of the blood to clot, but that this is restrained in the living body 

 by some inhibitory power resident in the walls of the containing vessels. 



Support was once thought to be given to Briicke's and like theories 

 by cases of injury, in which blood extravasated in the living body has 

 seemed to remain uncoagulated for weeks, or even months, on account of 

 its contact with living tissues. But the supposed facts have been shown to 

 be without foundation. The blood-like fluid in such cases is not uncoag- 

 ulated blood, but a mixture of serum and blood-corpuscles, with a certain 

 proportion of clot in various stages of disintegration. (Morrant Baker.) 



As the blood must contain the substances from which fibrin is formed, 

 and as the re-arrangement of these substances occurs very quickly when- 

 ever the blood is shed, so that it is somewhat difficult to prevent coagula- 

 tion, it seems more reasonable to hold with Briicke, that the blood has a 

 strong tendency to clot, rather than with Lister, that it has no special 

 tendency thereto. 



It has been recently suggested that the reason why blood does not 

 coagulate in the living vessels, is that the factors which we have seen are 

 necessary for the formation of fibrin are not in the exact state required 

 for its production, and that the fibrin ferment is not formed or is not, at 

 any rate, free in the living blood, but that it is produced (or set free) at 

 the moment of coagulation by the disintegration of the colorless corpuscles. 

 This supposition is certainly plausible, but if it be a true one, it must be 

 assumed either that the living blood-vessels exert a restraining influence 

 upon the disintegration of the corpuscles in sufficient numbers to form a 

 clot, or that they render inert any small amount of fibrin ferment which 

 may have been set free by the disintegration of a few corpuscles; as it is 

 certain that corpuscles of all kinds must from time to time disintegrate 

 in the blood without causing it to clot; and, secondly, that shed and 

 defibrinated blood which contains blood corpuscles, broken down and dis- 

 integrated, will not, when injected into the vessels of an animal, produce 

 clotting. There must be a distinct difference, therefore, if only in 

 amount, between the normal disintegration of a few colorless corpuscles in 

 the living uninjured blood-vessels and the abnormal disintegration of a 

 large number which occurs whenever the blood is shed without suitable 

 precaution, or when coagulation is unrestrained by the neighborhood of 

 the living uninjured blood-vessels. 



THE BLOOD COKPUSCLES OK BLOOD-CELLS. 



There are two principal forms of corpuscles, the red and the white, 

 or, as they are now frequently named, the colored and the colorless. 

 In the moist state, the red corpuscles form about 45 per cent, by weight, 



