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The contracting of the fibrin also squeezes out the serum. This liquid contains all the constituents of the plasma except the fibrinogen. These by contracting draw the corpuscles together and form the clot.
It is supposed to result from the breaking down of the 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.
The formation of the clot and the separation of the serum is due almost entirely to the action of this substance. Fibrinogen is for this reason called the coagulable constituent of the blood. In the plasma the fibrinogen is in a liquid form; but during coagulation it changes into a white, stringy solid, called fibrin. This appears in the clot and is the cause of its formation.
The best explanation offered as yet upon this point is as follows: Fibrinogen does not of itself change into fibrin, but is made to undergo this change by the presence of another substance, called fibrin ferment. This substance is not a regular constituent of the blood, but is formed as occasion requires.
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 serum, which is similar in appearance to the blood plasma, differs from that liquid in one important respect as explained below. *Causes of Coagulation.*—Although coagulation affects all parts of the blood, only one of its constituents is found in reality to coagulate. This is the fibrinogen.
These resemble, in a general way, the white of raw egg, but differ from each other in the readiness with which they coagulate. Fibrinogen coagulates more readily than the others and is the only one that changes in the ordinary coagulation of the blood. The others remain dissolved during this process, but are coagulated by chemical agents and by heat.
Most complex and wonderful of fluids, it contains in unexplained and inscrutable combination salts of iron, lime, soda and potassa, with water, oil, albumen, paraglobulin and fibrinogen, which united form fibrine in fact, at times, some part of everything we eat and all that goes to form our bodies, which it everywhere permeates, vitalizes and sustains.
*Plasma.*—The plasma is a complex liquid, being made up of water and of substances dissolved in the water. The dissolved substances consist mainly of foods for the cells and wastes from the cells. Three kinds of proteids are found in the plasma, called serum albumin, serum globulin, and fibrinogen.
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