30 CLOTTING OF BLOOD. [BOOK i. 



something else, or whether the corpuscles simply in some way or 

 other assist in the transformation of some previously existing con- 

 stituents of the plasma ? Whether the influence exerted by the 

 condition of the vascular wall is exerted directly on the plasma or 

 indirectly on the corpuscles ? Whether, as some have thought, the 

 peculiar bodies of which we shall presently speak under the name 

 of blood platelets have any share in the matter, and if so what ? 

 These questions are too involved and the discussion of them too 

 long to be entered upon here. 



What we do know is that in blood soon after it has been shed, 

 the body which we have called fibrinogen is present as also the 

 body which we have called fibrin ferment, that the latter acting 

 on the former will produce fibrin, and that the appearance of fibrin 

 is undoubtedly the cause of what is called clotting. We seem 

 justified in concluding that the clotting of shed blood is due to 

 the conversion by ferment of fibrinogen into fibrin. The further 

 inference that clotting within the body is the same thing as 

 clotting outside the body and similarly due to the transformation 

 of fibrinogen by ferment into fibrin, though probable, is not proved. 

 We do not yet know the exact nature and condition of the blood 

 within the living blood vessels, and until we know that we cannot 

 satisfactorily explain why blood in the living blood vessels is 

 usually fluid but can at times clot. 



