THE BLOOD 225 



The circumstances which hasten and those which 

 retard the clotting of the blood has been studied in the 

 utmost detail. It might be inferred that exposure to air 

 would be an influential one but this can hardly be 

 claimed. Much more important is contact with foreign 

 surfaces, and all surfaces other than the lining of the 

 blood-vessels must be regarded as foreign. When blood 

 flows from a cut, it passes over cells which it does not 

 normally bathe and often over those which have been 

 torn or crushed so as to yield peculiar subtances of their 

 own. Then it runs upon the dead skin and perhaps 

 upon clothing or hair. These harsh and extended con- 

 tacts favor coagulation. Blood will not soon clot in a 

 beaker which has been oiled; this measure seems to 

 create a surface more like the natural lining of the 

 system. 



How are we to connect foreign contacts with the pro- 

 duction of thrombin and so of fibrin? To make a very 

 long story short we may say that the damaging effect 

 of the strange surfaces is first felt by the blood-plates. 

 As these disintegrate their constituents must pass into 

 solution in the plasma. Agents from this source help 

 to perfect or render effective the enzyme thrombin. 

 The formation of thrombin does not occur in the absence 

 of calcium (lime) salts in the solution and, accordingly, 

 a simple way of warding off coagulation is to remove the 

 lime salts from fresh blood by adding a small amount of 

 potassium oxalate. Other cells than the blood-plates 

 for example, those of the lacerated tissues outside the 

 vessels may contribute compounds favorable to the 

 coagulation process. 



From what has been said it will be anticipated that 

 clotting may take place in the vessels if conditions arise 

 there which cause the destruction of blood-plates. An 

 injury to an artery or vein, as by tying or clamping, may 

 render the lining so abnormal that it is equivalent to a 

 foreign surface. Fibrin formation will then begin and 

 the clot will build up upon the injured area as a founda- 



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