CHAPTER XII 

 THE CLOTTING OF BLOOD 



IF the blood be allowed to flow freely from a wound, the flow 

 gradually lessens as the blood becomes more viscid, and at length 

 ceases, a clot, or coagulum, being formed at the site of injury. 



As a rule the blood coming from clean-cut wounds clots less readily 

 than that from jagged wounds. Washing and cleaning a wound 

 prolongs the bleeding; on the other hand, contact of the wound with 

 a foreign body such as a piece of rag or of cotton-wool quickens its 

 arrest. . 



The clot serves a double purpose it plugs the bleeding-points, 

 and so prevents the loss of precious blood, and it forms a protection 

 against the entry of harmful organisms into the blood-stream. If 

 the blood be received as it is shed into a perfectly clean vessel and 

 put aside to clot in a quiet place, it will be found that the jelly-like 

 coagulum is at first so solid that the vessel can be turned upside down, 

 and considerable force is required to disengage the clot from the vessel. 

 The clot gradually shrinks in size and squeezes out drops of a clear, 

 almost colourless, fluid known as the serum. The shrinkage slowly 

 continues until at last there remains a shrunken dark red clot at the 

 bottom of the vessel and a quantity of clear straw-coloured serum 

 above it. If the blood be horse's or cat's blood, an upper yellowish 

 layer is also formed, known as the " buffy coat." This consists of 

 the pale corpuscles which are lighter and i-omain on top, the heavier 

 red corpuscles quickly settling down to the bottom in the blood of 

 these animals. 



Coagulation of the blood may be retarded in various ways. The 

 best-known methods are the following: 



(i.) Cold, by receiving blood into a vessel placed on ice. 



(ii.) Contact with the wall of the bloodvessel. If a large vein 

 for example, the jugular vein of the horse be ligatured in two places, 

 and the tube of blood thus formed be excised and hung up, the cor- 

 puscles will sink to the bottom, leaving the unclotted plasma above. 



(iii.) Receiving the blood into a smooth vessel smeared with oil. 



(iv.) Addition to the blood of neutral salts such as magnesium or 

 sodium sulphate. 



(v.) Addition to the blood of a soluble oxalate, citrate, or fluoride. 



(vi.) Addition of a body (hirudin) obtained by extracting the heads 

 of leeches. Certain snake poisons and bacterial toxins also stop 

 coagulation. 



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