USES OF COAGULATION. 53 



which forms a bright red compound with the coloring mat- 

 ter of the red corpuscles. If the clot be turned upside down 

 and left for a short time, the previously dark bottom layer, 

 now exposed to the air, will become bright; and the previ- 

 ously bright top layer, now immersed in the serum, will 

 become dark. 



Uses of Coagulation. The clotting of the blood is so 

 important a process that its cause has been frequently in- 

 Testigated; but as yet it is not perfectly 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 if the lining of these is injured. In this way the 

 mouths of the small vessels opened in a cut arc clogged up, 

 and the bleeding, which would otherwise go on indefinitely, 

 is stopped. So, too, when a surgeon tics up an artery be- 

 fore dividing it, and the tight ligature crushes or tears its 

 delicate inner surface, the blood clots where this 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. 



Why Blood Coagulates. Blood plasma contains in so- 

 lution a proteid substance, fibrinogen. Before clotting oc- 

 curs another substance, fibrin fernient, fflrms in it from the 

 break ing-down of some white corpuscles or, more probably, 

 of the plaques. The ferment changes the fibrinogen into 

 fibrin. This change only takes place when a small quan- 

 tity of neutral salines is present: and it is much facilitated 

 by the presence of a third substance, fibrinoplastin, or 

 paraglobulin, which exists in solution in large amount in 

 the blood plasma. 



Blood serum does not clot of itself at ordinary tempera- 

 tures: it contains fibrinoplastin and fibrin ferment and the 

 requisite salines, but not the fibrinogen; that which origi- 

 nally existed in the plasma having been used up to form 

 fibrin. 



The liquids found in the cavities of the Body which are 



