FIBRIN FERMENT. 55 



but a little is present. But if some of the ferment be 

 added, then the mixture coagulates rapidly. The ferment 

 may be obtained by adding a large quantity of strong al- 

 cohol to some fresh blood serum. The alcohol precipi- 

 tates albumin, fibrinoplastin, and the ferment. The pre- 

 cipitate is let stay under alcohol for some months, during 

 which time the albumin and fibrinoplastin are altered so 

 as to become insoluble in water. The alcohol is then de- 

 canted off and the residue treated with water which dis- 

 solves the ferment. This solution added to the above 

 mixture containing fibrinogen, fibrinoplastin, and salts, 

 will make it clot. 



Of these four bodies which play a part in the coagula- 

 tion of blood, the fibrinogen primarily determines the 

 quantity of fibrin formed. The ferment acting on it, in 

 some way turns it into fibrin, but does not itself enter 

 into the fibrin. It is not used up in the process, and the 

 amount of fibrin ultimately formed is the same, whether 

 much or little ferment be present. The more ferment the 

 quicker the clotting. The fibrinoplastin in some way 

 makes it easier for the ferment to work. The part the 

 salts play is obscure: probably part of them are necessary 

 constituents of the fibrin, since it leaves' a large proportion 

 of ash when burnt. But when present in large propor- 

 tions they prevent coagulation, probably by hindering the 

 formation of ferment. If fresh blood be mixed with an 

 equal bulk of a saturated solution of magnesium sulphate 

 (Epsom salts) or of common salt, it will not clot; but if 

 this mixture be largely diluted with water, then clotting 

 will take place. 



Exciting Causes of Coagulation. The coagulation of 

 the blood is clearly a physico-chemical process, but it is 

 still not satisfactorily explained why it does not occur in 

 circulating blood inside healthy blood-vessels. It is, in 

 fact, much easier to point out what are not the proxi- 

 mate causes of the coagulation of drawn blood than what 

 are. 



Blood when removed from the Body and received in a 

 vessel comes to rest, cools, and is exposed to the air, from 



