THE HEART AND CIRCULATION. 117 



which is transparent and almost colorless, consists of 

 two materials, the blood serum and fibrin. Fibrin does 

 not exist as such in the body nor in freshly shed blood, 

 but there is a substance named fibrinogen which is worked 

 on by another substance, the fibrin ferment, to form 

 fibrin. Both fibrin ferment and fibrinogen can be iso- 

 lated from the blood. 



Coagulability. In the body the blood is perfectly 

 fluid and under normal conditions does not coagulate. 

 But, though fluid when first shed, upon standing it 

 gradually becomes viscid, that is, in two or three minutes, 

 then jelly-like, in five to ten minutes, and grows firmer 

 and firmer until there finally appears around this jelly- 

 like mass or clot a yellowish fluid, the serum. The 

 clot is made up of the corpuscles and fibrin. If some 

 blood is drawn and set on ice until the corpuscles settle, 

 the plasma can then be drawn off, and after it has stood 

 awhile in a warm place coagulation will take place, a mass 

 of fibrin forming in the middle. It takes from one to 

 two hours for clotting to be complete. In very slow 

 clotting at a low temperature the white corpuscles ap- 

 pear in a layer on top of the clot, the buffy coat. 



Of fibrin little is known, but its formation is the 

 most important step in clotting, as its presence is ab- 

 solutely essential. If it is removed by whipping, the 

 blood will not clot. It is a delicate, stringy material, 

 elastic and contractile, and contains certain salts of lime 

 and magnesium, upon whose presence its power of coagu- 

 lation depends. The coagulability of blood differs in 

 different people and is occasionally so little as to make 

 operation dangerous. 



The most favorable temperature for clotting is that 

 of the body, extreme heat preventing it and cold delaying 

 it. That the blood does not clot in the body must be due 

 to some relation between the blood and the walls of the 

 arteries and veins that prevents it, just as the walls of 

 the stomach are not digested by the juices secreted. 

 Though coagulation does not normally take place in the 



