THE BLOOD. 69 



means of the ferment known to every cheese-maker as 

 " rennet." If a little of this rennet be added to even large 

 quantities of milk, in a very short time, by the action of 

 this ferment, the milk curds. It has not been possible to 

 get fibrin ferment pure, but neither have we been able as 

 yet to get rennet in a similar form. This fibrin ferment is 

 probably not produced in a purposive way by the tablets, 

 but results as a mere element of disintegration when they 

 die. Thus the strings of fibrin in the clotted blood existed 

 in the normal blood in the form of the liquid, fibrinogen. 

 This liquid did not change into fibrin, because no fibrin fer- 

 ment is present in normal blood, for the reason that the 

 blood plates do not disintegrate under such normal con- 

 ditions. It has been noted, though, that if a solution con- 

 taining such a ferment be injected in considerable quantities 

 into the veins of an animal, internal coagulation at once re- 

 sults. The presence of fibrinoplastin seems to facilitate this 

 process, although it takes no direct part in it, for the blood 

 serum has as much fibrinoplastin as it had before the clot- 

 ting took place. Just how the substance may facilitate a 

 chemical process without directly taking part in it does not 

 seem clear, but chemistry offers a number of analogies. 

 Fibrinogen will not, however, change into fibrin, even in 

 the presence of the fibrin ferment, unless there be found in 

 solution in it certain salts, especially common salt and the 

 salts of lime. If by adding a little oxalic acid the lime salts 

 are removed from fresh blood it does not coagulate at all. 

 This fact has led some observers to believe that the fibrin 

 was really a compound of the fibrinogen and the calcium, a 

 calcium-fibrinogenate. 



Possibly this matter of coagulation may more easily be 

 understood by following the method of making an artificial 

 clot. If a solution of pure fibrin, such as may be secured 

 from a hydrocele fluid, be put in a vessel, it will not coagu- 

 late instantaneously at all. If now to this fibrinogen traces 

 of common salt and lime salts be added, and then some 

 fibrin ferment introduced, it will begin to coagulate at once, 



