282 THE HUMAN BODY 



living animal into a cold vessel and kept just above freezing 

 temperature it does not clot and after a time the corpuscles settle 

 to the bottom leaving a supernatant portion of clear plasma. This 

 plasma retains the power of clotting, as is shown when it is 

 warmed ; but if before it clots it be saturated with sodium chlorid 

 and filtered, the liquid that remains will no longer clot. The 

 precipitate formed by the saturation with sodium chlorid must 

 contain, therefore, some essential in the process of clotting. This 

 precipitate if examined will be found to be a mixture containing 

 all the fibrinogen of the plasma and part of the para globulin. 

 These two substances may be separated by proper treatment, 

 and after this has been done it is found that a solution of the 

 fibrinogen can be made to clot, while one containing only para 

 globulin cannot. During the clotting of the fibrinogen solution 

 all the fibrinogen disappears, giving place to fibrin. 



We are thus led to the conclusion that the natural clotting of 

 fresh blood is due to the formation of fibrin from fibrinogen which 

 existed in solution in the plasma of the circulating blood and has 

 been altered in the clotted, giving origin to fibrin. But as normal 

 blood circulating in healthy uninjured blood-vessels does not clot 

 nor do pure solutions of fibrinogen, we have still to seek the ex- 

 citing cause of the change. 



If to a solution of fibrinogen there be added a few drops of 

 blood or of blood-serum, or of the washings of a blood-clot, fibrin 

 will be formed; therefore drawn blood and serum and natural 

 clot each contain something which can effect the conversion of 

 fibrinogen into fibrin. This substance is thrombin, frequently 

 called also the fibrin ferment. 



Thrombin. When blood-serum is treated with several times 

 its volume of strong alcohol its various proteins and most of its 

 salts are precipitated : if the precipitate be left standing in alcohol 

 for some days the proteins become almost entirely insoluble in 

 water, but a few drops of the watery extract cause clotting in a 

 saline solution of fibrinogen, and clearly contain some of the 

 thrombin. This substance was for a long time believed to be 

 an enzym, hence its name of " fibrin ferment." Recent careful 

 study shows, however, that it does not correspond to enzyms in 

 either of their two cardinal characteristics, namely, the ability 

 of a small amount of the substance to produce a very large amount 



