72 THE BLOOD. 



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

 however, is still a mystery. 



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

 tendency 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 unexplained the manner 

 in which the fibrin, fluid in the living blood-vessels, can, 

 by foreign matter, be thus made solid. If it be 

 a fact, it is a very important one, but it is not an 

 explanation. 



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 re- 

 strained by some inhibitory 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 

 living 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 circu- 

 lation by tissues which this particular constituent of 

 the blood is destined to nourish ; in the others, it re- 

 mains and becomes evident as a clot. This explanation 

 is ingenious, but requires some kind of proof before it can 

 be adopted. 



Concerning other theories, as for instance, that coagula- 

 tion is due to the escape of carbonic acid, or of ammonia, 

 it need only be said that they have been completely 

 disproved. 



We must therefore, for the present, believe that the 

 cause of the coagulation of the blood has yet to be dis- 



