COAGULATION OR CLOTTING 85 



but it has lost its power to coagulate, and is known as defibrin- 

 ated blood. 



The value of clotting is that it stops hemorrhage. A serious 

 condition is present in some pathological states where the blood 

 will not clot. The time of clotting varies in different individuals 

 and at different times. Normally the jelly stage sets in in from 

 three to ten minutes, while the formation of serum requires 

 from ten to forty-eight hours. 



Owing to the complexity of the blood, the investigations 

 as to the cause of clotting have given rise to many different 

 views. It is universally admitted that the essential part of 

 the clot, the fibrin network, is derived from the fibrinogen 

 which, as the result of the interaction of numerous factors, 

 is thrown out of solution as elongated threads. One of these 

 factors is thrombin or fibrin ferment, which is not present as 

 such in normal blood, but is formed after blood is shed. The 

 addition of the thrombin to a solution of pure fibrinogen gives 

 a clot. In this reaction calcium plays an essential part, for 

 if removed by the addition of sodium oxalate, blood will not 

 coagulate. Its action is intimately related to the formation 

 of thrombin. Thus, if oxalated serum containing ready-formed 

 thrombin be added to an oxalated solution of fibrinogen, clotting 

 takes place. Calcium is regarded as assisting in the forma- 

 tion of thrombin from some antecedent substance which is 

 called prothrombin or thrombogen. Other inorganic or organic 

 substances may also be concerned in the formation of thrombin. 

 Such substances, residing in the various tissues, have been 

 called zymoplastic substances. In pigeons,, for instance, shed 

 blood which has not come into contact with the wound clots 

 very slowly, although thrombin, calcium, and fibrinogen are 

 present. 



Although spoken of as fibrin ferment, it is doubtful whether 

 thrombin belongs to the group of enzymes. Heating to 65 C. 

 does not destroy thrombin; it merely inactivates it, and its 

 power may be restored by the addition of an alkali. Thrombin 

 does not seem to undergo any essential change in its interaction 

 with fibrinogen, so that it may form a physicochemical rather 

 than a chemical union. The velocity of combination is practi- 

 cally not affected by temperatures between 17 to 38 C., which 



