456 BLOOD AND LYMPH. 



desirable, of course, to understand the process of clotting as fully 

 as possible. The problem has proved to be a difficult and com- 

 plex one. 



General Statement of Problem. The clotting of blood is such 

 a prominent phenomenon that it has attracted attention at all 

 times, and as a result numerous theories to account for it have been 

 advanced. Most of these theories have now simply an historical 

 interest. In recent years much experimental work has been done 

 upon the subject, the result of which has been to increase greatly 

 our knowledge of the process; but no complete explanation has yet 

 been reached. It is generally admitted that the essential constit- 

 uent of the clot namely, the fibrin is formed from the fibrinogen 

 normally present in the plasma, and that without this fibrin- 

 ogen clotting is impossible. If, for instance, blood is heated to 

 60 C., a temperature sufficient to precipitate the fibrinogen as a 

 heat coagulum, its power of clotting is lost. Clotting, therefore, is 

 essentially a process of the blood-plasma, as was shown indeed by 

 the old experimenters (Hewson). Moreover, it is also admitted 

 that the conversion of the soluble fibrinogen to the insoluble fibrin 

 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 adding to the clear plasma 

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



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

 Reymond, 1864, pp. 545, 675, and 1862, pp. 428, 533; "Pfliiger's Archiv. f. 

 d. gesammte Physiol.," 6, 413, 1872; "Zur Blutlehre/' Leipzig, 1892 and 1895. 



