COAGULATION OF BLOOD. 59 



serum, are held, or, as it were, entangled in the solid substance 

 which it forms. 



But after healthy fibrin has thus coagulated, it always con- 

 tracts ; and what is generally described as one process of coagu- 

 lation should rather be regarded as consisting of two parts or 

 stages ; namely, first, the simple act of clotting, coagulating, or 

 becoming solid ; and, secondly, the contraction or condensa- 

 tion of the solid clot thus formed. By this second act much 

 of the serum which was soaked in the clot is gradually pressed 

 out ; and this collects in the vessel around the contracted clot. 



Thus, by the observation of blood within the vessels, and of 

 the changes which commonly ensue when it is drawn from them, 

 we may distinguish in it three principal constituents, namely, 

 1st, the fibrin, or coagulating substance ; 2d, the serum ; 3d, 

 the corpuscles. 



That the fibrin is the only spontaneously coagulable material 

 in the blood, may be proved in many ways ; and most simply 

 by employing any means whereby a portion of the liquor san- 

 guinis, i. e., the serum and fibrin, can be separated from the 

 red corpuscles before coagulation. Under ordinary circum- 

 stances coagulation occurs before the red corpuscles have had 

 time to subside ; and thus, from their being entangled in the 

 meshes of the fibrin, the clot is of a deep dark red color through- 

 out, somewhat darker, it may be, at the most dependent part, 

 from accumulation of red cells, but not to any very marked 

 degree. If, however, from any cause, the red cells sink more 

 quickly than usual, or the fibrin contracts more slowly, then, 

 in either of these cases, the red corpuscles may be observed, 

 while the blood is yet fluid, to sink below its surface ; and the 

 layer beneath which they have sunk, and which has usually an 

 opaline or grayish-white tint, will coagulate without them, and 

 form a white clot consisting of fibrin alone, or of fibrin with 

 entangled white corpuscles; for the white corpuscles, being 

 very light, tend upwards towards the surface of the fluid. The 

 layer of white clot which is thus formed rests on the top of a 

 colored clot of ordinary character, i. e., of one in which the 

 coagulating fibrin has entangled the red corpuscles while they 

 were sinking : and, thus placed, it constitutes what has been 

 called a buffy coat. 



When a buffy coat is formed in the manner just described, 

 it commonly contracts more than the rest of the clot does, and, 

 drawing in at its sides, produces a cupped appearance on the 

 top of the clot. 



In certain conditions of the system, and especially when there 

 exists some local inflammation, this buffed and cupped con- 

 dition of the clot is well marked, and there has been much dis- 



