COAGULATION. 457 



oxalated plasma fibrinogen and prothrombin or inactive thrombin 

 are present, and the addition of calcium salts serves simply to con- 

 vert the prothrombin to thrombin. If fully formed thrombin 

 prepared by any of the methods described is added to oxalated 

 plasma clotting occurs, although no calcium is present. 



Influence of Tissue Extracts Upon Coagulation. — Another im- 

 portant consideration in the normal clotting of blood is the in- 

 fluence of extracts of tissues upon the rapidity of the process. 

 Many observers have shown that certain substances are contained 

 in the tissues in general, including the blood-corpuscles, which 

 tend to accelerate the process of clotting. Arthus, for example 

 found that blood taken directly from the artery of a mammal 

 through a clean tube will clot within a certain time, while if allowed 

 to flow first over the wounded surface, as happens under normal 

 conditions, the time of clotting is much accelerated. This in- 

 fluence of the tissues is shown in an extreme way when we consider 

 the blood of the lower vertebrates, the birds, reptiles, and fishes. 

 If blood is drawn from an artery of one of these animals through 

 a clean tube it clots with great slowness. If such a specimen of 

 blood is centrifugalized promptly the supernatant plasma, when 

 pipetted off, may remain unclotted for many hours or fail to clot 

 at all. If, however, the drawn blood or the centrifugalized plasma 

 is mixed with an extract from the animal's tissues, the muscles, 

 for example, it will clot within a few minutes. This is, of course, 

 what happens in such animals when wounded. The escaping 

 blood oozes over the cut surface and clotting occurs promptly. 

 Mammalian blood differs from that of the lower vertebrates in 

 that it clots within a relatively short time, even if kept from coming 

 in contact with the injured tissues, and this difference may be ex- 

 plained on the view that the accelerating substance furnished 

 by the tissues in the lower vertebrates is supplied in the case 

 of the mammal by the corpuscles in its own blood, most prob- 

 ably by the platelets which, as is well known, disintegrate very 

 rapidly when the blood is shed. The mammalian blood (dog) 

 may, however, be brought into the condition of the bird's blood 

 very easily by the so-called process of peptonization, that is 

 to say, by injecting rapidly into the circulation a certain amoun^ 

 of a solution of Witte's peptone (see below, Antithrombin). 11 

 the injection is successful, the blood when drawn remains fluid 

 for many hours, and, if promptly centrifugalized, the plasma may 

 fail entirely to clot. In such cases the addition of tissue extracts 

 may cause clotting within a few minutes, as in the case of the bird's 

 blood. The substance or substances in the tissues which exhibit 

 this accelerating influence upon clotting have received various names 

 from different observers in accordance with the special theory of 



