THE BLOOD. 



133 



consist chiefly of fibrin. This forms a superficial stratum differing in 

 appearance from the rest of the clot, and is of a grayish yellow color. 

 This is known as the luffy coat or crusta phlogistica. The buffy coat 

 produced in the manner just described, commonly contracts more than 

 the rest of the clot, on account of the absence of colored corpuscles 



Fig. 117. Reticulum of fibrin, from a drop of human blood, after treatment with rosanilin. 



CRanvier.) 



from its mashes, and because contraction is less interfered with by ad- 

 hesion to the interior of the containing vessel in the vertical than the 

 horizontal direction. A cup-like appearance of the buffy coat results, 

 and the clot is not only buffed but cupped on the surface. 



Formation of Fibrin. That the clotting of blood is due to the grad- 

 ual appearance in it of fibrin may be easily demonstrated. For example, 

 if recently drawn blood be whipped with a bundle of twigs, the fibrin 

 may be withdrawn from the blood before it can entangle the blood cor- 

 puscles within its meshes, as it adheres to the twigs in stringy threads 

 almost free from corpuscles ; the blood from which the fibrin has been 

 withdrawn no longer exhibits the power of spontaneous coagulability. 

 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. It will be most convenient to treat of the question step by 

 step. 



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 being 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 just as though it were the entire blood, producing a clot 

 similar in all respects to blood clot, except that it is almost colorless 



