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THE BLOOD 19 



The substance which brings about coagulation of the blood 

 is contained in the plasma ; it is not found in the serum, as shown 

 by the fact that the latter is incapable of spontaneous coagula- 

 tion. The difference between plasma and serum, as we have 

 already seen, is that the former contains fibrinogen and the 

 latter does not. _ -. . 



Nor is clotting a function of the red cells, for lymph is capable 

 of clotting, and there are no red cells in it ; and, further, there is 

 an abundance of red cells in defibrinated blood, yet clotting is 

 impossible. 



We have previously learnt that in the plasma both fibrinogen 

 and serum-albumin are present ; the evidence that serum-albumin 

 takes no share in the process of clotting is that serum contains 

 an abundance of serum-albumin, yet spontaneous coagulation is 

 impossible. 



Everything points to the fluid fibrinogen of the plasma being 

 converted into the solid substance fibrin, and in whatever other 

 respects physiologists differ regarding the question of coagulation, 

 all are agreed on this fundamental fact. In the observations 

 above described it has been shown that a solution of pure 

 fibrinogen does not clot spontaneously, but that it at once 

 coagulates on the addition of a drop of serum or washed blood- 

 clot. 



From this fact we learn that the conversion of fluid fibrinogen 

 into solid fibrin cannot occur without the agency of another 

 substance. The fibrinogen will not coagulate of itself ; it requires 

 to be rendered active by something contained in a drop of serum 

 or a washed blood-clot. This substance is generally spoken of 

 as fibrin ferment ; its present-day name is thrombin. The term 

 1 ferment ' was employed since in some respects the substance 

 resembled the class of bodies known as ' ferments,' inasmuch 

 as a very small amount appeared to be capable of acting on 

 an indefinite amount of fibrinogen ; it further resembled a 

 ferment in its action, being closely dependent on temperature. 

 It is now, however, more generally believed that a small amount of 

 the ferment will not act upon an indefinite amount of fibrinogen, 

 the amount of fibrin formed being proportional to the amount of 

 ferment present. 



It was stated above that a drop of serum added to a solution 

 of fibrinogen at once causes clotting. Evidently, then, blood- 

 serum contains in abundance the substance known as ' fibrin 

 ferment ' or ' thrombin.' If the serum be boiled or heated to 

 65 C. (150 F.), a drop of it added to a solution of fibrinogen 

 does not cause clotting ; but this function can be restored by the 

 addition to it of an alkali. This observation should largely, if 

 not entirely, exclude thrombin from being classed as a ferment. 



