THE BLOOD. 21 / 



cium salts to freshly drawn blood will prevent its coagulation. 

 It was formerly held that the calcium compounds enter into a 

 chemical combination as part of the fibrin molecule, but Ham- 

 marsten's researches seem to show clearly that the part of the 

 calcium is in the formation of the ferment. 



In this coagulation it appears that a portion of the original 

 fibrinogen is split off, yielding a product known as fibrin- 

 globulin, which remains in solution ; that is, the whole of the 

 fibrinogen does not coagulate as such. The coagulation may 

 be prevented or greatly retarded by addition of oxalates as 

 just referred to, and also by addition of several other foreign 

 substances, as acids, alkalies, strong solutions of alkali salts, 

 sugar, gum, albumose solutions, glycerol, etc. An excess of 

 carbon dioxide delays coagulation, as shown by the slower 

 coagulation of venous blood. Blood collected from a vein in 

 a polished vessel of porcelain or in a vessel whose sides have 

 been covered with oil or vaseline coagulates slowly. On the 

 other hand collecting in a vessel with a rough surface hastens 

 coagulation, as does any mechanical agitation. It has been 

 shown that a polished platinum wire may be passed through 

 a vein without inducing coagulation, while a thread in the 

 same position will collect a layer of fibrin. 



The various observations which have been made, while not 

 affording a full answer to the question why the blood does not 

 coagulate spontaneously in the living veins or arteries, sug- 

 gest several important reasons to account for this absence of 

 the reaction. One of the factors evidently present in all ordi- 

 nary coagulations is contact with a rough foreign substance. 

 The foreign substance need not be larger than the specks of 

 dust which blood can gather from the air. In leaving a vein or 

 artery blood naturally comes in contact with such particles, 

 and these serve as nuclei for the beginning of coagulation ; 

 much as a minute dust particle may be sufficient to start crys- 

 tallization in a strong solution of alum. In the body the blood 

 is normally in contact with vessels with very smooth walls. 

 If such a vessel be ligatured at two points and the sac thus 



