60 THE BLOOD. 



cussion concerning its origin under these circumstances. It is 

 now generally agreed that two causes combine to produce it. 



In the first place, the tendency of the red corpuscles to form 

 rouleaux (see p. 68) is much exaggerated in inflammatory 

 blood ; and as their rate of sinking increases with their aggre- 

 gation, there is a ready explanation, at least in part, of the 

 colorless condition of the top of the clot. And in the next 

 place, inflammatory blood coagulates less rapidly than usual, 

 and thus there is more time for the already rapidly sinking 

 corpuscles to subside. The colorless or buffed condition of the 

 upper part of the clot is therefore, readily accounted for; while 

 the cupped appearance is easily explained by the greater power 

 of contraction possessed by the fibrin of inflammatory blood 

 arid by its contraction being now not interfered with by the 

 presence of red corpuscles in its meshes. 



Although the appearance just described is commonly the re- 

 sult of a condition of the blood in which there is an increase 

 in the quantity of fibrin, it need not of necessity be so. For a 

 very different state of the blood, such as that which exists in 

 chlorosis, may give rise to the same appearance ; but in this 

 case the pale layer is due to a relatively smaller amount of red 

 corpuscles, not to any increase in the quantity of fibrin. 



It is thus evident that the coagulation of the blood is due 

 to its fibrin. The cause of the coagulation of the fibrin, how- 

 ever, is still a mystery. 



The theory of Prof. Lister, that fibrin has no natural ten- 

 dency to clot, but that its coagulation out of the body is due 

 to the action of foreign matter with which it happens to be 

 brought into contact, and, in the body, to conditions of the 

 tissues, which cause them to act towards it like foreign matter, 

 is insufficient; because even if it be true, it still leaves unex- 

 plained the manner in which the fibrin, fluid in the living 

 bloodvessels, can, by foreign matter, be thus made solid. If 

 it be a fact, it is a very important one, but it is not an expla- 

 nation. 



The same remark may be applied also to another theory 

 which differs from the last, in that while it admits a natural 

 tendency on the part of the blood to coagulation, it supposes 

 that this tendency in the living body is restrained by some in- 

 hibitory power resident in the walls of the containing vessels. 

 This also may, or may not, be true; but it is only a statement 

 of a possible fact, and leaves unexplained the manner in which 

 living tissue can thus restrain coagulation. 



Dr. Draper believes that coagulation takes place in the liv- 

 ing body, as out of it, or as in the dead ; but in the one case 

 the fibrin is picked out in the course of the circulation by tis- 



