COAGULANTS AND ANTI-COAGULANTS 309 



of the skin, one finds tliat tlie platelets gather at the edge of the 

 wound, forming a gelatinous annular mass. As this mass grows 

 in size a fine interlacing network of hbrin appears in it, with 

 platelets at the nodes or crossings of the net. White cells are 

 incorporated, and as the network becomes more felt-like, red cells 

 are entangled and imprisoned. Sometimes the clot seems to be 

 merely a mass of closely-packed platelets, leucocytes and erythro- 

 cytes (with very little fibrin) plugging the wound. 



Anti-coagulants. The removal of any one of the participating 

 substances from the sphere of activity will naturally prevent 

 clotting. Let us take the process step by step. 



(1) Liberation of thromboplastic substance is prevented by 

 carefully drawing the blood through a paraffined tube into a 

 vaselined or waxed vessel. If there is no contact with a water- 

 wettable surface, there will be no clot. One of the old experi- 

 menters found that blood placed with due precautions on a 

 nasturtium leaf would remain liquid for long. It is well known 

 that water runs off the leaf of this plant. 



(1«) Platelets may be reduced below their effective concentration 

 by anti-platelet serum. In this way (and in purpura in man) 

 clotting power may be poor, wounds hard to heal, and capillary 

 bleeding dangerous. 



(2) We have dealt with the inactivation of the calcium above. 



(3) Thrombin may be rendered ineffective by the action of 

 substances such as hirudin, a proteose prepared from the buccal 

 glands of the medicinal leech. It is injected in blood-pressure 

 experiments to prevent clots from forming in the arterial cannula. 



(4) The fibrin may be removed as rapidly as it is formed by 

 agitating the blood with glass beads or whipping it with twigs. 

 The sticky fibrin adheres to the beads or twigs and may be 

 removed. These methods and some others are summarised in 

 Table XLI. 



Differing from all the above anti-coagulants which act whether 

 injected into the blood-stream or added to shed blood, are those 

 which prevent clotting only when injected. Snake venom in 

 amounts as small as 0-00001 gram per kg. suffices. Commercial 

 peptone (mixture of proteoses and peptones) injected into the 

 circulation in the proportion of 0-3 gram per kg. produces a non- 

 coagulable blood for an hour or so. This class of coagulant seems 

 to act by stimulating the liver to manufacture heparin. Peptonisa- 

 tion and the injection of venom may cause some alteration in the 

 state of the thigmocytes. This has not been investigated. 



Coagulants. Extracts of organs rich in kephalin, e.g. thymus, 

 testes, lymph glands, produce intravascular clotting. 



