COAGULATION. 453 



is accomplished by the agency of a substance, known as thrombin 

 or fibrin ferment, which is not present, in its active form at least, 

 in the blood while in the blood-vessels, but is formed after the 

 blood is shed or under certain abnormal conditions within the 

 blood-vessels. These two important facts we owe mainly to 

 the investigations of Alexander Schmidt,* whose work com- 

 pleted the older observations of Hewson, Buchanan, Denis, 

 and Briicke. 



Preparation of Solutions of Fibrinogen. — Fibrinogen may 

 be obtained readily in solution free from other proteins by the 

 general method first described by Hammarsten. One may use 

 the plasma of horse's blood which has been kept from clotting 

 by prompt cooling, and in which the corpuscles have been thrown 

 down by centrifugalizing or by long standing at low temperature, 

 but it is more convenient, perhaps, to use blood which has been 

 kept from clotting by allowing the blood, as it escapes from the 

 vessels, to run into a solution of sodium oxalate, using an amount 

 such that the final mixture contains 0.1 per cent, of the oxalate. 

 This mixture is centrifugalized, the clear plasma is removed, and 

 the fibrinogen in it is precipitated by adding an equal part of a 

 saturated solution of sodium chlorid. 



The method in some detail is as follows: After addinc; to the clear plasma 

 an equal buUc of a saturated solution of sodium chlorid the resulting pre- 

 cipitate of fibrinogen is centrifugalized, the supernatant hquid is poured 

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

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

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

 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 hapi^ens 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 oi sodium bicarbonate 

 may be added to carry it into solution. If care is taken in wasliing 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. Instead of using a saturated solution 

 of sodium chloride to precipitate the fibrinogen one may employ with ad- 

 vantage a saturated solution of ammonium sulphate. The solution in this 

 case is added to the oxalated plasma in the proportion of 1 part to 4, giving a 

 mixture one-fifth saturated. 



A solution of fibrinogen prepared as above clots readily upon 

 the addition of blood-serum or of other solutions containing throm- 

 bin, and if the preparation has been entirely successful, a genu- 

 ine clot, that is, the precipitation of the fibrinogen in gelatinous 

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



* "Archiv f. Anat., Physiologic, u. wiss. Medicin." Reichert u. du Bois- 

 Revmond, l<Sn4, pp. 545, 675, and 1S62, pp. 428, 533; "Pfliiger's Archiv. f. 

 d. gcsammte PhvsioL," G, 413, 1872: "Zur Blutlehre," Leipzig, 1892 and 1895. 



