120 



THE BLOOD 



as the bufy coat or crusta phlogistica. The buffy coat, produced in the man- 

 ner just described, commonly contracts more than the rest of the clot, on 

 account of the absence of colored corpuscles from its meshes. When the 

 clot is allowed to stand, the white corpuscles which have escaped the clot 

 by ameboid movement settle on its surface often in such numbers that they 

 form a distinct superficial layer, grayish-white in appearance. 



That the clotting of blood is due to the gradual appearance in it of fibrin 

 may be easily demonstrated, For example, if recently drawn blood be 

 whipped with a bundle of twigs or wires, the fibrin may be withdrawn from 

 the blood before it can entangle the blood corpuscles within its meshes. 

 It adheres to the twigs in stringy threads relatively free from corpuscles. 

 The blood from which the fibrin has been withdrawn no longer exhibits the 

 power of spontaneous coagulability and it is now called defibrinated blood. 

 Although these facts have long been known, the closely associated problem 

 as to the exact manner in which fibrin is formed is by no means so simple. 



FlG. 107. Reticulum of Fibrin, from a Drop of Human Blood, after Treatment with 



Rosanilin. (Ranvier.) 



Fibrin is derived from the plasma. Pure plasma may be procured by 

 delaying coagulation in blood by keeping it at a temperature slightly above 

 freezing-point, until the colored corpuscles have subsided to the bottom of 

 the containing vessel. The blood of the horse is specially suited for the 

 purposes of this experiment. A portion of the colorless supernatant plasma, 

 if decanted into another vessel and exposed to the ordinary temperature of 

 the air, will coagulate, producing a clot similar in all respects to blood clot, 

 except that it is colorless from the absence of red corpuscles. If some of 

 the plasma be diluted with twice or three times its bulk of normal saline 

 solution (0.9 per cent.), coagulation is delayed, and the stages of the gradual 

 formation of fibrin in it may be conveniently watched. The viscidity which 

 precedes the complete coagulation may be actually seen to be due to the 

 formation of fibrils of fibrin first of all at the edge of the fluid-containing 



