Modern Conceptions of Blood Clotting.*

DESPITE the large number of researches which have been made on blood coagulation, no single one of the many theories advanced satisfies the criterion of a tenable hypothesis in which all known facts should be correlated. A better understanding of the fluidity and clotting of blood should lead to important results. On the practical side, we may hope for the elucidation of thrombosis—the formation of clots in the vessels—of the lack of coagulability in haemophilia, of the changes in plasma during shock, and of the fatal intravascular clotting caused by certain snake venoms. On the theoretical side, light should be thrown on the reactions of colloids, on the mechanisms of immunity and of susceptibility, and possibly on the problems of genetics, as haemophilia is transmitted from parent to offspring, and the existence of homologous bloods often depends on family relationship. In this paper the current theories of blood coagulation will be reviewed briefly from the standpoint of investigations made in collaboration with my friends Drs Hewitt, de Souza, and Taylor,' and evidence for a simpler view will be presented. The pioneers in this subject, Hewson and Lister, studied blood in excised veins. More recent inquirers have often formed their conclusions from reactions in test tubes. Much disagreement has thus arisen. Plasma changes so rapidly when shed that reactions in vitro may differ from those in vivo. Many attempts have been made to overcome this difficulty by experimenting with blood kept fluid by anti-coagulants, the substances commonly employed being "peptone," neutralised nucleic acids, soluble oxalates and citrates. Except in the cases of oxalated and citrated bloods, clotting ultimately occurs, and the blood so treated thus differs from normal circulating