SECTION IV 

 THE COAGULATION OF THE BLOOD 



IN the process of coagulation, while the corpuscles remain to a 

 large extent intact, the plasma becomes solid from the production in 

 it of a network of fibrin, being converted into fibrin plus serum. The 

 question of coagulation involves the consideration of the precursors of 

 fibrin and of the conditions which determine the conversion of these 

 precursors into fibrin. It is evidently impossible to arrive at any 

 conclusions on these points during the few minutes which elapse 

 between the time at which the blood leaves the vessels and the appear- 

 ance of the clot. We must therefore find some means of retarding 

 coagulation so that we may obtain the plasma free from corpuscles 

 and be able to initiate coagulation in this cell-free fluid at will. Having 

 succeeded in staying the process of coagulation, it is always possible to 

 obtain a cell- free plasma either by allowing the blood to settle or, better 

 still, by the employment of a centrifugal machine. Under the influence 

 of centrifugal force the corpuscles are thrown rapidly down to the 

 bottom of the tube and the clear supernatant plasma can be syphoned 



METHODS OF PREVENTING COAGULATION 



(1) So long as the blood is in contact with the uninjured vessel it remains 

 fluid. If the jugular vein of a large animal such as the horse be tied in two places, 

 the blood contained between the ligatures will remain fluid, sometimes for days. 

 If the tube of vein be hung up, the corpuscles sink to the bottom and the plasma 

 in the upper part of the tube can be poured from one vein to another without 

 undergoing coagulation. On pouring it into a glass vessel or bringing it in contact 

 with foreign substances, it undergoes coagulation. 



(2) When an incision is made in the ordinary way into a blood-vessel of a 

 bird the issuing blood clots very rapidly. The clotting is initiated by a substance 

 contained in the tissues surrounding the vessels. If therefore the vessel be isolated 

 and a perfectly clean glass cannula be inserted into it, care being taken not to 

 bring the cannula in contact with any of the surrounding tissues, blood can be 

 drawn off into a sterilised beaker perfectly free from dust and will remain unclotted 

 for days. Such blood can be centrifuged and the cell-free plasma used for experi- 

 ment. The same procedure does not apply to the mammal, where even the most 

 scrupulous care to prevent contamination by the tissue juices will not prevent 

 the blood from clotting on leaving the Vessels. 



(3) Clotting can be excited even in the living vein by introducing into the 

 blood any solid substance which is wetted by the blood. If the contact of the 

 blood with such substances be prevented by receiving it into vessels previously 

 coated with oil or with paraffin and scrupulously free from dust, clotting may 

 often be delayed for many hours. 



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