se 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
thirteenth of the entire weight of the body. This for the average
individual is an amount weighing nearly twelve pounds and having a volume
of nearly one and one half gallons. About 46 per cent by volume of this
amount is made up of corpuscles and 54 per cent of plasma. Of the plasma
about 10 per cent consists of solids and 90 per cent of water, as already
stated.

*Functions of the Blood.*--The blood is the great carrying, or
distributing, agent in the body. Through its movements (considered in the
next chapter) it carries food and oxygen to the cells and waste materials
from the cells. Much of the blood may, therefore, be regarded as _freight_
in the process of transportation. The blood also carries, or distributes,
heat. Taking up heat in the warm parts of the body, it gives it off at
places having a lower temperature. This enables all parts of the body to
keep at about the same temperature.

In addition to serving as a carrier, the blood has antiseptic properties,
i.e., it destroys disease germs. While this function is mai