70 THE BLOOD. 



contracts ; and what is generally described as one process of 

 coagulation 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 con- 

 traction or condensation 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, ist, the fibrin, or coagulating sub- 

 stance ; 2nd, the serum ; $rd, 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 sanguinis, i.e., the serum and fibrin, can be 

 separated from the red corpuscles before coagulation. 

 Under ordinary circumstances 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 colour throughout, somewhat 

 darker, it may be, at the most dependent part, from accu- 

 mulation 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 greyish white tint, will 

 coagulate without them, and form a white clot consisting 

 of fibrin alone, or of fibrin with entangled white cor- 

 puscles ; 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 

 coloured clot of ordinary character, i.e., of one in which 



