COAGULATION. 457 



cipitate of fibrinogen is centrifugalized, the supernatant liquid is poured 

 off, the precipitate is washed with a little of a half-saturated solution of 

 sodium chlorid, and then dissolved with stirring in a 2 per cent, solution of 

 sodium chlorid and filtered. This solution is again precipitated by half- 

 saturation with sodium chlorid, centrifugalized, washed, and dissolved 

 as before in a 2 per cent, solution of sodium chlorid. The process may be 

 repeated a third time, and the washed precipitate is finally dissolved in a 

 1 per cent, solution of sodium chlorid. It frequently happens that the third 

 or even the second precipitate will not dissolve in the dilute sodium chlorid, 

 and in that case a drop or two of a 0.5 per cent, solution of sodium bicarbonate 

 may be added to carry it into solution. If care is taken in washing the centrif- 

 ugalized precipitate, two precipitations usually suffice to give a fibrinogen 

 solution, which will not clot spontaneously nor after the addition of calcium 

 salts, but clots promptly with thrombin. 



A solution of fibrinogen prepared as above clots readily upon 

 the addition of blood-serum or of other solutions containing 

 thrombin, and if the preparation has been entirely successful, a 

 genuine clot, that is, the precipitation of the fibrinogen in gelatin- 

 ous form, cannot be obtained from it by any other means. As a 

 matter of fact, solutions of fibrinogen prepared as described some- 

 times clot, although much more slowly, when instead of a throm- 

 bin solution one adds a little calcium chlorid or a solution con- 

 taining calcium chlorid and sodium bicarbonate in about the 

 proportion found in a Ringer's mixture. This latter fact indicates 

 that the fibrinogen solution in such cases contains a trace of some 

 material from which thrombin may be produced. In all probability 

 this material is the antecedent form of thrombin, that is, so-called 

 prothrombin, which, as we shall see, is converted to thrombin by 

 the action of calcium salts. 



Preparation and Properties of Thrombin. Thrombin, or so- 

 called fibrin ferment, is prepared readily by the method first de- 

 scribed by Schmidt. Blood is allowed to clot, and the serum is then 

 precipitated by the addition of a large excess of alcohol (usually 

 twenty volumes). After standing some days or weeks the pre- 

 cipitate is drained off and dried, and is then ground up and ex- 

 tracted with water. The aqueous extract contains proteins, 

 salts, and other things in addition to the thrombin. A solution 

 made in this way causes a prompt coagulation when added to 

 a solution of pure fibrinogen. That the thrombin thus obtained 

 is not present as such in normal blood, but is formed after shed- 

 ding, is indicated by the fact that if the animal's blood is allowed 

 to flow directly from the artery into a large bulk of alcohol, due 

 care being taken in the process, the precipitate thus obtained when 

 subsequently dried and extracted with water yields little or no 

 thrombin. 



Another, perhaps simpler, method of obtaining a strong prep- 

 aration of thrombin is to treat fibrin with an 8 per cent, solution of 

 sodium chlorid (Buchanan-Gamgee) . Fibrin obtained from a 



