460 BLOOD AND LYMPH. 



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 peotonization, that is 

 to say, by injecting rapidly into the circulation a certain amount 

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

 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 

 coagulation advocated. They have been called zymoplastic sub- 

 stances, thromboplastic substances, coagulins, cytozyms, thrombo- 

 kinase, etc. It is perhaps most convenient to speak of them in 

 general as thromboplastic substances, since this term does not com- 

 mit us to the manner of their action, but simply implies that they 

 are of importance in the formation of the clot. It has long been 

 known that thromboplastic substance may be extracted from the 

 tissues by the action of ether, or of alcohol and ether, and the 

 author* has shown that the active substance in the ether extracts 

 is one of the phosphatids, corresponding apparently to the fraction 

 which has been designated as kephalin. The closely related lecithin 

 has no thromboplastic power. In aqueous extracts of the tissues 

 the kephalin is held in solution in combination with a protein 

 which is precipitated at a temperature of 60 C. It is probable 

 that this kephalin-protein constitutes the active thromboplastic 

 substance of the tissues. As regards the manner in which it par- 

 ticipates in the process of clotting several views have been pro- 

 posed. According to the theory of Morawitz (see next para- 

 graph) it is concerned, together with the calcium, in activating the 

 prothrombin to thrombin, and, indeed, is necessary to this reaction. 

 According to Howell its function lies in neutralizing the action of 

 antithrombin, hence its effectiveness in causing the clotting of 

 * Howell, "American Journal of Physiology," 31, 1, 1912, and 32, 264, 1913. 



