896 PHYSIOLOGY 



thrombokinase, or by the action of calcium and alkali, or possibly by the action of 

 calcium alone. The latest work on the subject by Rettger has tended somewhat to 

 the simplification of this extremely complex problem. In the first place, he regards 

 the formation of fibrin as non-fermentative in character, thus agreeing with Nolf, 

 fibrin being produced by the simple union of fibrinogen and thrombin, though a very 

 small amount of thrombin may produce a much larger amount (about 200 times its 

 weight) of fibrin. He finds no evidence of the pre-existence in the blood of a thrombin 

 or pro-ferment, and is inclined to regard the action of so-called kinases, which can be 

 extracted from animal tissues, as similar to that of such agents as dust, threads of linen, 

 which can produce a similar coagulating effect in birds' blood. The thrombin he 

 regards as derived from the formed elements at the moment of their rapid disintegra- 

 tion when placed under abnormal circumstances. For the formation of active thrombin 

 a minimal amount of calcium salts must enter into the molecular complex. We thus 

 return to the simpler expression of the processes of coagulation as given by Pekelharing 

 and Hammarsten, the prothrombin which is formed from the platelets and leucocytes 

 by secretion or process of disintegration being activated to thrombin by the calcium 

 salts present, and the thrombin so formed combining quantitatively with the fibrinogen 

 to form fibrin. The prothrombin is not readily destroyed ; it may remain in calcium 

 free serum for days and when activated form thrombin quickly. Thrombin, on the 

 other hand, disappears very rapidly from active serum in consequence of combining 

 with some of the proteins of the serum. This property of combining with the fibrinogen 

 and disappearing from the serum is not shared by the prothrombin. 



