106 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



The part played by calcium salts is well illustrated by the fact 

 that coagulation is prevented by the decalcification of the blood. 

 This can be accomplished by the addition of a small amount of 

 soluble oxalate or fluoride to the blood immediately it is shed. The 

 calcium of the blood plasma is then immediately precipitated as 

 insoluble calcium oxalate or fluoride, and is thus not available for 

 the transformation of thrombogen into thrombin. The addition of 

 the oxalate or fluoride must be rapidly performed, otherwise time will 

 be given for the conversion of thrombogen into thrombin, and 

 thrombin, when formed, will act upon fibrinogen whether the calcium 

 has been removed or not. In other words, calcium is only necessary 

 for the formation of fibrin-ferment, and not for the action of fibrin- 

 ferment on fibrinogen. Fibrin is thus not a compound of calcium 

 and fibrinogen. 



The action of a soluble citrate is also in a certain sense a 

 decalcifying action, for although calcium citrate is a soluble salt it 

 does not ionise in solution so as to liberate the free calcium ions 

 which are essential for thrombin formation. 



Oxalated blood (or oxalated plasma) will clot when the calcium is 

 once more restored by the addition of a small amount of calcium 

 chloride, but such an addition to fluoride plasma will not induce clot- 

 ting ; thrombin must be added also. In some way sodium fluoride 

 interferes with the formation of thrombin, probably by preventing the 

 liberation of thrombo-kinase from the formed elements of the blood. 



The other activating agent, thrombo-kinase, is in part liberated 

 from the formed elements of the blood, but it is also obtained from 

 many other tissues. If a haemorrhage takes place under ordinary 

 circumstances the blood as it flows from the wound passes over the 

 muscles and skin that have been cut, and rapidly clots owing to the 

 thrombo-kinase supplied by those tissues. If blood is obtained by 

 drawing it off through a perfectly clean cannula into a clean vessel 

 without allowing it to touch the tissues, it remains unclotted for a 

 long time ; in the case of birds' blood this time may extend to man'y 

 days ; but the addition of a small piece of tissue such as muscle, or of 

 an extract of such a tissue, produces almost immediate clotting. If a 

 solution of fibrinogen is prepared and calcium added it will not clot ; 

 if thrombin or a fluid such as serum which contains thrombin is added 

 also it will clot. It will not clot if birds' plasma obtained as above is 

 added to it ; nor if tissue extract is added to it ; but if both are added 

 it will. In other words, the thrombogen of the birds' plasma 

 plus the thrombo-kinase of the tissue extract have the same effect as 

 thrombin. 



