THE BLOOD. I/I 



mixed with potassium oxalate solution in such quantity that the 

 mixture contains Ol per cent, of potassium oxalate, the calcium salts 

 of the plasma are precipitated as calcium oxalate, and the blood will 

 not clot. The subsequent addition of calcium chloride is followed by 

 coagulation. But if oxalate serum is added to oxalate blood, clotting 

 will take place, because thrombin is already present in the serum 

 before the potassium oxalate is added. 



The formation of thrombin from thrombogen and calcium salts is 

 brought about, or at least facilitated, by an activating substance called 

 thrombokinase^ which is derived in mammals mainly from the blood 

 platelets. If oxalate plasma from a mammal is allowed to stand for two 

 or three days on ice, a precipitate of platelets collects at the bottom of 

 the vessel. The plasma still contains thrombogen, but will no longer 

 coagulate on the addition of lime salts. It will clot, however, if some 

 of the platelet precipitate or an extract of an animal tissue be added 

 to it along with the lime salts, the extra factor, obtained from the plate- 

 lets or tissues, being thrombokinase. Again, the blood of birds contains 

 no platelets, and will not clot if it is drawn directly from a blood-vessel 

 without contact with the tissues. On the other hand, if it is allowed to 

 flow over the adjacent tissues on its passage from the vessel, or if a 

 little tissue extract is added to it, it will coagulate readily. Thrombo- 

 kinase is therefore present in nearly all the tissues of the body as well 

 as in the platelets, arid this wide distribution facilitates the protective 

 clotting of the blood which takes place on wounded surfaces. The 

 leucocytes have also been supposed to discharge thrombokinase when 

 blood is shed. 



The facts described above in connection \vith the subject of the 

 coagulation of the blood are generally accepted, but the exact interpreta- 

 tion of them put forward by different authors, as well as the nomenclature 

 applied to the different substances concerned, varies somewhat. 



The factors concerned in coagulation may be diagrammatically sum- 

 marised as in the following table : 



Thrombogen Thrombokinase Lime salts Fibrinogen 



Thrombin 



