318 DISTURBANCES OF CIRCULATION 



perhaps related to the slowness of the clotting of the blood in this condition. 

 Duke" states that when the platelet count falls below 10,000 per cubic mm. there 

 is delayed coagulation and a tendency to purpura; with counts above 40,000 

 there is usually no hemorrhagic tendency."" If the platelet count is reduced arti- 

 ficially (by benzene, diphtheria toxin) a similar purpuric tendency is observed. 

 Poisons that in large doses reduce the platelet count, will increase it if in small 

 doses. 



Calcium Salts. — The exact significance of calcium in fibrin formation is still 

 unsettled. Blood from which the calcium has been precipitated will not coagu- 

 late, and the addition of calcium salts will promptly cause it to do so. The vari- 

 ous hypotheses advanced to explain the way in which calcium influences the 

 clotting process are not in agreement. One hypothesis is that the calcium ions 

 are necessary for the transformation of prothrombin into thrombin (Pekelharing, 

 Hammarsten, Morawitz), the thrombin consisting of a compound of prothrombin, 

 calcium salts, and thrombokinase. Howell considers that no kinase is necessary, 

 the calcium activating the prothrombin whenever it is not inhibited by anti- 

 thrombin. 



Modification of Coagulability. — If blood is drawn into a glass 

 vessel well coated with oil or vaseline, through a cannula similarlj^ pro- 

 tected, no coagulation will take place; but if any unoiled foreign sub- 

 stance enters, even particles of dust, coagulation begins at once. The 

 explanation is that the leucocj^tes do not liberate their coagulating 

 substances until they have been injured by contact with some foreign 

 body, and the experiment proves the importance of this action of the 

 leucocytes, as well as explaining why the blood does not coagulate dur- 

 ing life. The classical experiment of the ligation of a vein without 

 injury to the endothelium, which permits the blood to remain stag- 

 nant for a long period without clotting, depends upon the same fact, 

 namely, that normal endothehum neither liberates coagulin itself nor 

 injures the leucocytes so that they disintegrate. Loeb recalls the 

 observation of Overton that lipoids are important constituents of the 

 cell membranes, and suggests a similarity between the vessel lining 

 and the oiled cannula, but analyses of aortic endothelium have shown 

 a rather low lipin content (8.41-9.25 per cent.), although peritoneal 

 endothehum has much more (13 to 15 per cent.).'''^ The suggestion 

 that the vessel walls contain an anti-coaguUn could not be confirmed by 

 Loeb. Since leucocytes are constantly undergoing disintegration in 

 the blood and tissues under normal conditions, it might be asked why 

 they do not cause clotting then and there. In explanation Loeb ad- 

 vances his observation that the coagulins are destroyed during cell 

 autolysis, and suggests that when leucocytes normally thsintcgrate, the 

 coagulins are first destroyed by autolysis. It has also been shown that 

 the cells and serum contain substances which inhibit or prevent 

 coagulation, and it is possible that these play an important part under 

 normal conditions in preventing coagulation by products of cell 



"Jour. Exp. Med., 1911 (14), 265; Arch. Int. Med., 1912 (10), 445; Jour. 

 Amer. Med. Assoc, 1915 (65), 1600. 



""Gram (Arch. Int. Med., 1920 (25), 325) states that platelet counts below 

 100,000 goncrallv accompany a tendency to bleed. He gives the normal figure as 

 200,000 to 500,000, but usually over 300,000. 



"Tait, (iuart. Jour. Exp. Physiol., 1915 (S), 391. 



