ESSENTIALS OF PHYSIOLOGY. 



The essential change in the coagulation of blood is the conversion 

 of fibrinogen into fibrin. The former substance is no longer present in 

 defibrinated blood or in serum. The process of clotting may therefore 

 be represented diagram matically in this way : 



Plasma 



Corpuscles 



Fibrin 



Serum 



Clot. 



When fibrinogen is freed from other substances by repeated precipi- 

 tation, a solution of the pure substance is found to have lost the 

 property of spontaneous coagulation. If, however, some blood serum 

 is added to such a pure solution, clotting will occur. It is clear, 

 therefore, that at least two substances are necessary for the formation 

 of fibrin, and that one of these is contained in blood serum. If twenty 

 volumes of alcohol are added to one volume of serum, a precipitate of 

 serum proteins is formed, and becomes insoluble in water if allowed to 

 remain under alcohol for some days or weeks. If this precipitate be 

 then dried and extracted with water, the solution so obtained will, if 

 added to a solution of fibrinogen, cause the latter to clot. The watery 

 extract contains a substance of unknown composition, which has been 

 called thrombin. 



We therefore find that the formation of fibrin is due to the inter- 

 action of fibrinogen and thrombin ; and, further, it has been shown 

 that a combination of the two bodies takes place, because, if excess of 

 fibrinogen be present, the amount of fibrin formed is proportional to 

 the amount of thrombin added to it. This latter fact is opposed to the 

 theory formerly held that thrombin belongs to the group of ferments. 

 The untenability of the older theory is further shown by the discovery 

 that the activity of thrombin is not permanently destroyed by the 

 action of heat, Moreover, fibrin can be partly broken down, thrombin 

 being set free, by treating it for some time with 8 per cent, .sodium 

 chloride solution. 



Thrombin itself is not contained in the blood-stream of the living 

 animal. If blood is drawn directly from a blood-vessel into alcohol, 

 it contains no thrombin, so that the latter body must be produced after 

 the blood is shed. It has, in fact, been shown that it is derived from 

 a precursor, which has been called thrombogen, by the combination of 

 the latter substance with calcium salts. If freshly shed blood is 



