DISEASE-RESISTING FUNCTIONS OF THE BLOOD 315 



viously dark red bottom layer, now exposed to the air, becomes 

 bright. 



Uses of Coagulation. The clotting of the blood is so important 

 a process that its cause has been frequently investigated; but it is 

 not yet completely understood. The living circulating blood in 

 the healthy blood-vessels does not clot; it contains no solid fibrin, 

 but this forms in it, sooner or later, when the blood gets by any 

 means out of the vessels or when the lining of these is injured. 

 In this way the mouths of the small vessels opened in a cut are 

 clogged up, and the bleeding, which would otherwise go on in- 

 definitely, is stopped. So, too, when a surgeon ties up an artery 

 before dividing it, the tight ligature crushes or tears its delicate 

 inner surface, and the blood clots where that is injured, and from 

 there a coagulum is formed reaching up to the next highest branch 

 of the vessel. This becomes more and more solid, and by the time 

 the ligature is removed has formed a firm plug in the cut end of 

 the artery, which greatly diminishes the risk of bleeding. 



The Source of Blood-Fibrin. Since fresh blood-plasma contains 

 no fibrin but does contain considerable quantities of other pro- 

 teins, we look first to these as a possible source of the fibrin formed 

 during coagulation. If horse's blood be drawn directly from the 

 living animal into a cold vessel and kept just above freezing 

 temperature it does not clot and after a time the corpuscles settle 

 to the bottom leaving a supernatant portion of clear plasma. This 

 plasma retains the power of clotting, as is shown when it is warmed ; 

 but if before it clots it be saturated with sodium chlorid and filtered, 

 the liquid that remains will no longer clot. The precipitate formed 

 by the saturation with sodium chlorid must contain, therefore, 

 some essential in the process of clotting. This precipitate if 

 examined will be found to be a mixture containing all the fibrinogen 

 of the plasma and part of the globulin. These two substances may 

 be separated by proper treatment, and after this has been done it 

 is found that a solution of the fibrinogen can be made to clot, 

 while one containing only paraglobulin cannot. During the 

 clotting of the fibrinogen solution the fibrinogen disappears, giv- 

 ing place to fibrin. 



We are thus led to the conclusion that the natural clotting of 

 fresh blood is due to the formation of fibrin from fibrinogen which 

 existed in solution in the plasma of the circulating blood and has 



