102 THE BLOOD AND THE LYMPH 



wires. In this latter case, however, the clot does not form in the usual 

 manner, but the fine threads of fibrin collect on the twigs or wires, leav- 

 ing behind the blood serum with the corpuscles still suspended in it. 

 The fibrin removed in this way may then be washed free of adherent 

 serum. The serum and corpuscles now form defibrinated blood, which 

 is used for many physiological purposes. Clotting is also greatly acceler- 

 ated by allowing the blood to flow over exposed tissues. Something is 

 evidently added to it from the tissues which accelerates the clotting 

 process, this influence being particularly marked in the case of blood 

 of the lower vertebrates. When the blood of the bird, for example, is 

 received through a cannula inserted directly into a vessel with as little 

 injury to the walls as possible, it very slowly clots if at all, but soon 

 does so if the blood is allowed to come into contact with excoriated 

 tissues, or if it is mixed with tissue extract, such as that of muscle. 

 Clotting is considerably accelerated by warming the blood. The ap- 

 plication of a cloth or tampon well wrung out with hot physiological' 

 saline to a wounded surface is a most efficient means of allaying hem- 

 orrhage from vessels too small to ligate. 



The Nature of the Clotting Process 



Plasma obtained by centrifuging blood that has been prevented from 

 clotting by one of the foregoing methods can be made to clot by removing 

 the inhibiting influence; for example, in cooled plasma by warming the 

 blood to room temperature, in salted plasma by diluting it with at least 

 an equal volume of water, and in decalcified plasma by adding a suffi- 

 cient amount of soluble calcium salts to combine with all the added 

 oxalate and leave a small trace of calcium salts in excess. 



The first question concerns the source of the fibrin, and the answer to 

 it is furnished by comparing the composition of bood plasma with that 

 of serum. Though both of these fluids contain the proteins, albumin 

 and globulin, in approximately the same concentrations, the plasma also 

 contains another protein not unlike globulin in most of its reactions, 

 but distinguished from typical globulin in that it is precipitated by 

 half-saturation with sodium chloride, in which typical globulin is solu- 

 ble, and is more readily coagulated by heat. To produce half-saturation 

 of the plasma with sodium chloride, equal volumes of plasma and satu- 

 rated sodium-chloride solution are mixed together. The precipitate of 

 fibrinogen, as the substance is called, is then collected at the bottom of 

 the tube by centrifuging and is washed several times by decantation with 

 half-saturated sodium-chloride solution. The washed precipitate, dis- 

 solved in weak saline solution (preferably containing a trace of bicar- 

 bonate), will then be found to clot under certain conditions. 



