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 evi- 

 dently 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 appearance 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 off. 



METHODS OF PREVENTING COAGULATION 



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

 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 into 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 experiment. The same procedure 

 does not apply to the mammal, where even the most scrupulous care to prevent con- 

 tamination by the tissue juices will not prevent the blood from clotting on leaving the 

 vessels. 



(3) Clotting can be excited even in the li ving 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. 



(4) Cooled plasma. Horses' blood is received directly into a narrow vessel immersed 



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