THE COAGULATION OF THE BLOOD 963 



in character. That the whole blood does not coagulate within the vessels he 

 explains by assuming that the cells of the blood and tissues are covered normally 

 with an ultra-microscopic layer of fibrin. This forms a neutral surface, like a 

 paraffined vessel, which has no thromboplastic effect upon the plasma. 



According to Mellanby the prothrombin in the plasma is constantly associated 

 with the fibrinogen. It may be converted to thrombin either by the action of 

 calcium and 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 fibrinogeu 

 and thrombin. 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 disintegration when placed under abnormal circumstances. 

 For the formation of active thrornbin 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 prothrornbin 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. 



