60 THE BLOOD 



given, must therefore be taken to represent only an ap- 

 proximate average. 



Coagulation of the Blood. 



When blood is drawn from the body, and left at rest, 

 certain changes ensue, which constitute a kind of rough 

 analysis of it, and are instructive respecting the nature of 

 some of its constituents. After about ten minutes, taking 

 a general average of many observations, it gradually clots 

 or coagulates, becoming solid like a soft jelly. The clot 

 thus formed has at first the same volume and appearance 

 as the fluid blood had, and, like it, looks quite uniform ; 

 the only change seems to be, that the blood which was fluid 

 is now solid. But presently, drops of transparent yellowish 

 fluid begin to ooze from the surface of the solid clot ; and 

 these gradually collecting, first on its upper surface, and 

 then all around it, the clot or " crassamentum" diminished 

 in size, but firmer than it was before, floats in a quantity 

 of yellowish fluid, which is named serum, the quantity of 

 which may continually increase for from twenty-four to 

 forty-eight hours after the clotting of the blood. 



The changes just described may be thus explained. The 

 liquor sanguinis, or liquid part of the blood (p. 56), consists 

 of a thin fluid called serum, holding fibrin in solution.-' 

 The peculiar property of fibrin, as already said, is its ten- 

 dency to become solid when at rest, and in some other 

 conditions. When, therefore, a quantity of blood is drawn 

 from the vessels, the fibrin coagulates, and the blood cor- 

 puscles, with part of the serum, are held, or, as it were, 

 entangled in the solid substance which it forms. 



But after healthy fibrin has thus coagulated, it always 



:; This statement has been left unaltered in the text ; but, as will be 

 seen farther on, it requires modification. (Ei>.) 



