white corpuscles, and perhaps also from the blood platelets, when the
blood is exposed to unnatural conditions. The formation of the ferment
leads in turn to the changing of the fibrinogen into fibrin.

Another substance which is necessary to the process of coagulation is the
element calcium. If compounds of calcium are absent from the blood,
coagulation does not take place. These are, however, regular constituents
of healthy blood. Whether the presence of the calcium is necessary to the
formation of the ferment or to the action of the ferment upon the
fibrinogen is unknown.

*Purpose of Coagulation.*—The purpose of coagulation is to check the flow
of blood from wounds. The fact that the blood is contained in and kept
flowing continuously through a system of _connected_ vessels causes it to
escape rapidly from the body whenever openings in these vessels are made.
Clots form at such openings and close them up, stopping in this way the
flow that would otherwise go on indefinitely. Coagulation, however, does
not stop the flow of blood from the large vessels. From these the blood
runs with too great force for the clot to form within the wound.

*Time Required for Coagulation.*—The rate at which coagulation takes place
varies greatly under different conditions. It is influenced strongly by
temperature; heat hastens and cold retards the process. It may be
prevented entirely by lowering the temperature of the blood to near the
freezing point. The presence of a foreign substance increases the rapidity
of coagulation, and it has been observed that bleeding from small wounds
is more quickly checked by covering them with linen or cotton fibers. The
fibers in this case hasten the process of coagulation.

*Quantity of Blood.*—The quantity of blood is estimated to be about one