102 THE BLOOD AND THE LYMPH 



The next question concerns the nature of the conditions that cause the 

 fibrinogen to clot. When a fibrinogen solution is mixed with a few drops 

 of blood serum, a clot usually forms, which however is not the case when 

 plasma is added or when the serum is heated before adding it. Because 

 a small quantity of serum is capable of causing the clotting of a large 

 quantity of fibrinogen solution or plasma, it is supposed that the active 

 substance present in it is of the nature of a ferment — fibrin ferment or 

 thrombin. It must be pointed out, however, that there is considerable 

 doubt whether this active body is really of the nature of a ferment or 

 enzyme. For example, although heated serum does not cause clotting, 

 thrombin, prepared from serum by the method about to be described, in 

 the absence of inorganic salts can withstand even a boiling temperature. 

 Moreover, true enzymes are characterized by the fact that, like other 

 catalytic agents, a very minute quantity can effect a change in an indef- 

 inite amount of substance without the enzyme becoming used up in the 

 process (page 72). When thrombin is allowed to act upon a fibrinogen 

 solution, on the other hand, it is said that only a fixed amount of fibrin 

 can be formed when a small amount of thrombin is added. Neither does 

 this amount increase when the time of reaction is prolonged. 



Wha'tever may be the significance of the foregoing facts, it is impor 

 taut to know that the clotting substance, thrombin, can be isolated from 

 blood serum in a tolerably pure condition. For this purpose blood 

 serum is allowed to stand under a large volume of alcohol for a week or 

 two; the precipitate is then collected and rubbed up with water, which 

 extracts the thrombin from it, leaving the serum protein in a coagulated 

 state. The resulting watery solution of thrombin may be further pre- 

 cipitated by alcohol, the precipitate washed in alcohol and redissolved 

 in water, yielding ultimately a solution which exhibits very marked co- 

 agulating powers when added to plasma or fibrinogen solution. Throm- 

 bin shows most of the protein reactions but it is not coagulated by heat. 

 As would be expected, a considerable quantity of thrombin remains 

 adherent to the fibrin formed in the process of clotting, and Howell 8 

 describes a very useful method by which it can be separated from fibrin 

 and preserved in a dry condition. Briefly stated, this method consists 

 in allowing washed fibrin to stand overnight under eight per cent 

 sodium-chloride solution, which dissolves the thrombin. The resulting 

 extract is then mixed with an equal volume of acetone, which throws 

 down a precipitate containing the thrombin. To preserve it, the precip- 

 itate is collected on a number of small filter papers, which are subse- 

 quently opened out and dried by exposure to a current of cold air before 

 an electric fan. When the thrombin solutions are desired, the dried pre- 

 cipitates are extracted with a little water. 



