236 THE BLOOD 



Hammersten confirmed this, and found that the calcium ions 

 act on the pro-thrombin producing active thrombin. Later work 

 explains the action of calcium ions here and in milk curdling as 

 being due to the non-hydrophilic property of calcium compounds. 



This still does not explain why blood does not clot in the vessels, 

 so physiologists had to postulate the presence of some substance 

 in the blood which would prevent the calcium ions from activating 

 the pro-thrombin. This hypothetical substance they called anti- 

 pro-thrombin. In all cells there is a large or small quantity of 

 thrombokinase, which is the trigger setting off the whole clotting 

 process. When a vessel is ruptured and blood comes into contact 

 with the tissues or into contact with disintegrating blood-cells, 

 it takes from them thrombokinase which neutralises the inhibitory 

 action of the anti-pro-thrombin and so allows the calcium to act. 

 It is now known that this substance from the tissues is not a 

 ferment or kinase. Some people, therefore, prefer to call it 

 thromboplastin. 



The scheme now stands as follows : 



(2) (Clot) Fibrin (synerised gel) + Serum 



t 

 .(Clot) Fibrin enmeshing serum 



t 

 Fibrinogen (sol) + Thrombin (a proteose). 



(a globulin)' t 



(1) Calcium -* anti-pro-thrombin - prothrombin 

 (in blood) (in blood) (in blood) 



* 

 I 



thromboplastin 

 (from tissues, etc.). 



Free calcium ions are necessary for the first step, but apparently 

 not for the second. 



It has been known for long that thromboplastin is a phosphatide, 

 a body like lecithin, containing nitrogen, phosphorus and a fatty 

 acid. That this substance is kephalin has been proved by Howell. 

 Pure kephalin has not been prepared in sufficient quantity to 

 allow of complete analysis but its molecule probably consists of : 



m r . -j (Stearic (saturated) 

 Two fatty acids ' 



unsaturated)> 



+ 

 an amino alcohol, NH 2 -CH 2 -CH 2 -OH 



(amino ethyl alcohol), 



+ 

 a glycero-phosphoric acid. 



