BLOOD CLOTTING 105 



the coagulation, which it does not do, however, if it is first of all heated 

 to 80 C. and filtered free of the coagulated protein. Moreover, the 

 antagonistic action is quantitative in the sense that a fixed amount of 

 the peptone-plasma inhibits the action of a fixed amount of thrombin. 

 The source of antithrombin in the body appears to be mainly at least 

 the liver, for it has been found: (1) that peptone injection into an animal 

 from which the liver has been removed does not cause antithrombin to 

 be formed (Denney and Minot) ; 10 (2) that peptone injections into the 

 portal vein cause antithrombin to appear in the blood much more rap- 

 idly than when the injection is made into a systemic vessel; and (3) that, 

 when the liver is perfused outside the body with a perfusion fluid con- 

 taining peptone, antithrombin accumulates in the perfusion fluid. 



A fluid containing a high concentration of antithrombin is secreted 

 by the so-called salivary gland at the head end of the leech. The func- 

 tion of the fluid is to prevent clotting of the blood, so that the animal 

 may continue to suck it without interference by clotting. After apply- 

 ing leeches for medicinal purposes it is therefore necessary to wash the 

 wound thoroughly with water so that all traces of the antithrombin may 

 be removed ; otherwise the bleeding may continue for a considerable time. 

 Practical use is made of this effect of the leech to prevent clotting of blood 

 outside the body, or it may be used to inhibit coagulation intra vitam in 

 experiments where clotting would otherwise interfere with their prog- 

 ress; for example, in crossed circulation experiments (page 365) and in 

 experiments in vividiffusion (page 607). For such purposes the leech 

 head is cut off. and extracted either with saline or by treatment with 

 chloroform, which removes other proteins from the saline solution leav- 

 ing a strong antithrombin, known under the trade name of "hirudin." 

 At temperatures about that of the body the action of antithrombin is 

 greatly augmented. In animals like the mammals in which the content 

 of antithrombin is small, this may be important in maintaining the flu- 

 idity of the blood (Howell). Blood containing antithrombin can be 

 made to clot by the addition of thrombin, and therefore of blood serum. 



