THE VITAL PROCESSES 



ingly fine and very delicate threads (Fig. \\} throughout the 

 mass of blood that is coagulating, the fibrin first entangles 



the corpuscles and then, by 

 contracting, draws them into 

 the solid mass or clot. 1 The 

 contracting of the fibrin also 

 squeezes out the serum. 

 This liquid contains all the 

 constituents of the plasma 

 except the fibrinogen. 



FIG. ii. Fibrin threads (after Fibrin Ferment and Calcium. 

 Ranvier). These by contracting draw Most difficult of all to answer have 

 the corpuscles together and form the been t h e questions : What causes 

 c ' ot< the blood to coagulate outside of the 



blood vessels and what prevents its coagulation inside of these vessels ? 

 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, calledy&Ww ferment. 

 This substance is not a regular constituent of the blood, but is formed 

 as occasion requires. 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 coagula- 

 tion 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 



1 If the blood be stirred or "whipped" while it is coagulating, the clot may be 

 broken up and the fibrin separated as fast as it forms. The blood which then 

 remains consists of serum and corpuscles and will not coagulate. It is known as 

 " defibrinated " blood. 



