64 THE HUMAN BODY. 



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 

 coagulu'm 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 proteids, we look first to these as a possible source of 

 the fibrin formed during coagulation. Blood drawn from a 

 living animal into one third of its bulk of a cold saturated 

 solution of magnesium sulphate and kept cold will not clot 

 for a long time. The corpuscles slowly sink in the mixture, 

 and after a time considerable quantities of colorless " salted " 

 plasma can be drawn off from its upper part. The salted 

 plasma still contains something which can form fibrin, for if 

 diluted with six or seven times its volume of water it clots in 

 a manner quite similar to pure blood-plasma (though the clot 

 is a little less firm); and also, fibrin can be obtained by 

 whipping it. 



If salted plasma be saturated with sodium chloride it 

 yields a whitish rather sticky precipitate, called plasmim. 

 The remaining liquid is then found to have lost the power of 

 clotting, but if the plasmine be treated with a little dilute 

 saline solution it dissolves, and the solution soon clots, with 

 the formation of fibrin. 



The plasmine is not a single body. If its solution before 

 it clots have sodium chloride added to it in the proportion 

 of about 15#, a white sticky precipitate is formed, and may 

 be collected on a filter; it is a substance named fibrinogen. 

 If more sodium chloride or some magnesium sulphate be 

 added to the filtrate a second white precipitate is obtained : 

 this is par a globulin. 



Paraglobulin dissolves in dilute solutions of common salt: 

 such solutions cannot be made to yield fibrin, though they 

 are coagulated with the formation of coagulated proteid 

 (p. 10) at the temperature 75 C. (167 F). Purified fibrin- 

 ogen also- dissolves in dilute solution of common salt, and 

 such solution is coagulated by heat (56 C. or 133 F.) : but 

 under certain conditions it clots with the formation of true 

 fibrin. During the clotting the fibrinogen disappears, but 



