Ill THE CLOTTING OF BLOOD 115 



but for reasons into which we cannot now enter, it is 

 classed with the "ferments," of which we shall have to 

 speak when we come to consider digestion. These fer- 

 ments are characterized by their power, even when 

 present in sniall quantities, of producing great changes in 

 other bodies without themselves entering into the changes. 

 Thus the particular ferment of which we are speaking, 

 and which has been called "fibrin ferment," or 

 "thrombin," produces fibrin, and yet does not itself 

 become part of the fibrin so produced, or at all events 

 only does so to a very slight extent. 



This ferment is apparently not present in healthy blood 

 as it circulates in the living blood-vessels, but makes its 

 appearance when the blood is shed. We do not know 

 exactly from what source it comes, but there are reasons 

 for thinking that it arises from a breaking down of some 

 kind of white corpuscle, or it may be of the blood- 

 platelets. 



This breakdown yields a suljstance, ^^prothrombin," 

 which appears to be united to salts of lime present in the 

 plasma, the resulting substance is thrombin. Yet one 

 other factor is concerned in the process of coagulation, 

 for the prothrombin and the salts of lime do not unite 

 spontaneously. A third substance, a special kind of 

 ferment known as a kinase, effects the union of the lime 

 salts with the prothrombin. This kinase is to be found 

 especially in injured animal tissues ; therefore, while as 

 we have already seen, blood clots very slowly in uninjured 

 blood-vessels, it clots with great readiness when in 

 contact with cut or bruised tissues. Clearly, in this very 

 complicated chemical reaction, or series of reactions, we 

 have a mechanism which is of great service to the body as 

 it provides for the ready clotting of the blood issuing 

 from a wound. The kinase is provided from the lips of 

 the wound. It unites the prothrombin from the shed 

 corpuscles with the lime salts of the plasma forming 

 thrombin. The thrombin in turn forms fibrin out of 



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