316 THE HUMAN BODY 



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 

 of chemical activity, and the destruction of the substance by 

 heating above 60 C. It has been definitely proven that the 

 amount of fibrinogen that is converted to fibrin bears a direct 

 relationship to the amount of thrombin present, and that throm- 

 bin solutions free from protein impurities can be boiled without 

 destroying the thrombin. 



Source of Thrombin. If fresh blood is drawn directly from the 

 veins of an animal into strong alcohol, and the resulting pre- 

 cipitate treated as described above for preparing thrombin from 

 serum, it yields no thrombin; this substance, therefore, which is 

 present in blood-serum, is absent from the blood within the Body 

 and must be formed after the blood is shed and before the forma- 

 tion of the clot. When the process of clotting is watched under 

 the microscope the fibrin threads will usually be seen to form 

 about certain centers. These centers consist of disintegrating 

 blood-plates, and the observation that fibrin formation proceeds 

 from them in this fashion led to the idea that the blood-plates are 

 in some way associated with the process. 



