30 CLOTTING OF BLOOD. [BOOK i. 



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 justi- 

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

 conversion by ferment of fibrinogen into fibrin. The further infer- 

 ence that clotting within the body is the same thing as clotting 

 outside the body and similarly due to the transformation of fibrino- 

 gen 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 satis- 

 factorily explain why blood in the living blood vessels is usually 

 fluid but can at times clot. 



