COAGULATION OF THE BLOOD 349 



Thrombin may be a protein, but if so then it is protein of unusual 

 properties, for it is not coagulable by heat, and repeated extraction 

 with chloroform appears to remove it from its solution in water. On 

 the other hand it yields the biuret- and Millon reactions and all of 

 the reactions for Tryptophane, and it is coagulable by half-saturation 

 with ammonium sulphate. Putrefaction does not destroy it and in 

 fact often seems to increase its activity. These properties appear to 

 indicate that thrombin may be a protein split-product, possibly a 

 proteose. 



FIG. 19. Fibrin crystals viewed under the ultramicroscope. (After Howell.) 



The nature of Antithrombin is unknown, in plasma it is thermolabile 

 while the antithrombin in leech extracts (Hirudin) is not. It is uncer- 

 tain, however, whether this thermolability may not be due to asso- 

 ciated impurities, as it is in the case of thrombin. On the other hand 

 Antiprothrombin appears to be a phospholipin, McLean having shown 

 that Cuorin from heart-muscle and a phospholipin resembling Jecorin 



