COAGULATION OF THE BLOOD 113 



changed into fibrin by the action of a "ferment." But a 

 complete explanation is yet to seek. There are difficulties 

 still which need to be cleared up. In the first place, it is 

 known that if all salts of lime are removed from the blood it 

 will not clot. Therefore lime is necessary to 'one or other 

 of the factors. Either it combines with the fibrinogen at the 

 time when it is converted into fibrin, which seems to be 

 disproved by the fact that no more lime can be found in 

 fibrin than fibrinogen ; or by uniting with an antecedent of the 

 ferment it develops the activity of this substance. Secondly, 

 the action of the nucleo-proteid which is called blood-ferment 

 differs widely from that of the vast majority of substances 

 which are classed as ferments. Ferments are bodies which 

 induce changes in other bodies by mere contact, without 

 themselves taking part in the chemical action, and the 

 change which they induce is usually a hydrolysis, or union 

 with water, but fibrin seems to contain less water than fibrin- 

 ogen. Blood-ferment, like the ferment of rennet which 

 curdles milk, produces an insoluble and not a more soluble 

 substance. It is, however, possible that fibrinogen is changed 

 by the ferment into a substance containing more water in its 

 molecule, and that this substance divides into the insoluble 

 fibrin and some other substance, probably a globulin, which 

 is freely soluble. The fact that the fibrin obtained from a 

 given quantity of fibrinogen weighs considerably less than 

 the fibrinogen indicates that such a cleavage occurs, as it 

 undoubtedly does in the coagulation of milk. Thirdly, if the 

 formation of fibrin is due to a reaction between fibrinogen 

 and fibrin-ferment in the presence of salts of lime, the injec- 

 tion of fibrin-ferment into the circulating blood should 

 invariably produce coagulation, whereas it usually fails to 

 bring about this result. 



The cause of the coagulation of the blood is perhaps the 

 oldest of physiological problems. Its history is typical of 

 the progress of the science, and not less characteristic is its 

 position at the present time. Like most other problems, it 



