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 isolated 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 a while 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 appear 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 absolutely 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 coagulation 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
