26 FIBRINOGEN AND FIBRIN. [BOOK i. 



the fibrin produced is always less than that of the fibrinogen which 

 is consumed, and there is always produced at the same time a 

 certain quantity of another proteid, belonging to the globulin 

 family. There are reasons however why we cannot speak of the 

 ferment as splitting up fibrinogen into fibrin and a globulin ; it 

 seems more probable that the ferment converts the fibrinogen first 

 into a body which we might call soluble fibrin, and then turns this 

 body into veritable fibrin ; but further inquiries on the subject are 

 needed. 



It may be added that among the conditions necessary for the 

 due action of fibrin ferment on fibrinogen, the presence of a 

 certain quantity of some neutral salt seems to be one. In the 

 total absence of all neutral salts the ferment cannot convert the 

 fibrinogen into fibrin. There are some reasons also for thinking 

 that the presence of a lime salt such as calcium sulphate, though 

 it may be in minute quantity only, is essential. 



21. We may conclude then that the plasma of blood when 

 shed, or at all events soon after it has been shed, contains fibrino- 

 gen ; and it also seems probable that the clotting comes about 

 because the fibrinogen is converted into fibrin by the action of 

 fibrin ferment ; but we are still far from a definite answer to the 

 question, why blood remains fluid in the body and yet clots when 

 shed? 



We have already said that blood, or blood plasma, brought up to 

 a temperature of 56C. as soon as possible after its removal from 

 the living blood vessels, gives a proteid precipitate and loses its 

 power of clotting. This may be taken to shew that blood, as it 

 circulates in the living blood vessels, contains fibrinogen as such, 

 and that when the blood is heated to 56 C., which is the coagu- 

 lating point of fibrinogen, the fibrinogen present is coagulated and 

 precipitated, and consequently no fibrin can be formed. 



Further, while clotted blood undoubtedly contains an abundance 

 of fibrin ferment, no ferment, or a minimal quantity only, is present 

 in blood as it leaves the blood vessels. If blood be received directly 

 from the blood vessels into alcohol, the aqueous extract prepared 

 as directed above contains no ferment or merely a trace. Appa- 

 rently the ferment makes its appearance in the blood as the result 

 of changes taking place in the blood after it has been shed. 



We might from this be inclined to conclude that blood clots 

 when shed but not before, because, fibrinogen being always present, 

 the shedding brings about changes which produce fibrin ferment, 

 not previously existing, and this acting on the fibrinogen gives rise 

 to fibrin. But we meet with the following difficulty. A very 

 considerable quantity of very active ferment may be injected into 

 the blood current of a living animal without necessarily producing 

 any clotting at all. Obviously either blood within the blood 

 vessels does not contain fibrinogen as such, and the fibrinogen 

 detected by heating the blood to 56 C. is the result of changes 



