30 CLOTTING OF BLOOD. [Book i. 



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 prob- 

 able, 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. 



