THE CLOTTING OF BLOOD. 55 



the quantity of fibrin formed never is quite equal in weight 

 to the fibrinogen which, disappears, so the process is not a 

 mere direct transformation of one substance into the other. 



We are thus led to the conclusion that the natural clot- 

 ting of fresh blood is due to the formation of fibrin from 

 fibrinogen which existed in solution in the plasma of the 

 circulating blood and has been altered in the clotted, giving 

 origin to fibrin. But as normal blood circulating in healthy 

 uninjured blood-vessels does not clot nor do pure solutions 

 of fibrinogen, we have still to seek the exciting cause of the 

 change. 



If to a solution of fibrinogen there be added a few drops 

 of blood or of blood-serum, or of the washings of a blood-clot, 

 fibrin will be forme^; therefore drawn blood and serum and 

 natural clot each contain something which can effect the con- 

 version of fibrinogen into fibrin. This substance is the 

 enzyme named fibrin -ferment. 



The Fibrin-ferment. When blood-serum is treated with 

 several times its volume of strong alcohol its various proteids 

 and most of its salts are precipitated : if the precipitate be 

 left standing in alcohol for some months the proteids become 

 almost entirely insoluble in water, but a few drops of the 

 watery extract cause clotting in a saline solution of fibrin- 

 ogen, and clearly contain some of tho ferment. A very 

 minute quantity of the ferment will cause the conversion of 

 an indefinite quantity of fibrinogen and does not appear to be 

 itself used up in the process: it acts somehow by its mere 

 presence, and the clotting of blood is to be relegated to that 

 obscure group of physico-chemical processes known as cata- 

 lytic. Solutions containing the ferment always give some 

 proteid reactions and it may be a proteid, but this is doubt- 

 ful; for the proteid present may be only an impurity. Watery 

 solutions of ferment completely lose their activity when 

 boiled. 



If fibrinogen be dissolved in the least possible amount of 

 dilute caustic potash and a few drops of as pure as possible a 

 solution of fibrin ferment, freed from its salts by dialysis, 

 be added, clotting does not occur: but it may be brought 

 about by the addition of a very small quantity of a calcium 

 salt. The presence of some calcium seems to be an essential, 

 but the part it plays is unknown. Of the four substances 

 which take part in the coagulation of blood, the fibrinogen 



