26 FIBRINOGEN AND FIBRIN. [BOOK i. 



nitrogen. When fibrinogen is converted into fibrin by means of 

 fibrin ferment, the weight of 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. 



The action of the fibrin ferment on fibrinogen is dependent on 

 other conditions besides temperature ; for instance, the presence 

 of a calcium salt seems to be necessary. If blood be shed into a 

 dilute solution of potassium oxalate, the mixture, which need not 

 contain more than '1 p.c. of the oxalate, remains fluid indefinitely, 

 but clots readily on the addition of a small quantity of a calcium 

 salt. Apparently the oxalate, by precipitating the calcium salts 

 present in the blood, prevents the conversion of the fibrinogen 

 into fibrin. So also a solution of fibrinogen which has been 

 deprived of its calcium salts, by diffusion for instance, will not clot 

 on the addition of fibrin ferment similarly deprived of its calcium 

 salts ; but the mixture clots readily on the addition of a minute 

 quantity of calcium sulphate. We shall have to speak later on of 

 a somewhat analogous part played by calcium salts in the curdling 

 of milk. It may be added that the presence of other neutral 

 salts, such as sodium chloride, appears to influence clotting. 



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 56 C. 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. 



