THE CLOTTING OF BLOOD. 35 



tion of other proteids, converted into the less soluble fibrin. Our knowledge 

 of the constitution of proteid bodies is too imperfect to enable us to make 

 any very definite statement as to the exact nature of the change thus 

 effected ; but we may say this much. Fibrinogen and fibrin have about the 

 same elementary composition, fibrin containing a trifle more 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 fer- 

 ment converts the fibrinogen first into a body which we might call soluble 

 fibrin, and then turns this body into a 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. If the calcium 

 salts are taken out of fibrinogen, the fibrinogen no longer coagulates upon 

 the addition of fibrin ferment free from lime salts. Oxalate of potassium 

 added to freshly drawn blood also prevents coagulation, apparently because 

 of the precipitation of the lime salts ; the addition now of calcium salts is 

 promptly followed by coagulation. 



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

 at all events, soon after it has been shed, contains fibrinogen ; and it also 

 seems probable that the clotting comes about because the fibrinogen is con- 

 verted 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 tem- 

 perature of 56 C. as soon as possible after its removal from the living 

 bloodvessels, gives a proteid precipitate and loses its power of clotting. 

 This may be taken to show 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 coagulating 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 bloodvessels. If the blood be received directly from the 

 bloodvessels into alcohol, the aqueous extract prepared as directed above 

 contains no ferment, or merely a trace. Apparently 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 fol- 

 lowing 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 which have already 

 ensued before the temperature is reached ; or in the living circulation there 



