On making an autopsy of an animal which has succumbed to intoxication by snake-venom, we find that the blood in the heart and large vessels is sometimes coagulated into a mass, sometimes entirely fluid, and that, in certain cases, it is as black as prune-juice, while in others it is of a fine transparent red colour. These differences in the effects of venom upon the blood are due to the fact that the various venoms contain in variable proportions, besides the _neurotoxic_ substance which represents the true venomous _toxin_, other substances which act, some upon the plasmasia or fibrin-ferment, or upon the fibrin, others upon the red corpuscles, others on the leucocytes, and others again on the endothelium of the blood-vessels. A.--EFFECTS OF VENOM ON THE COAGULATION OF THE BLOOD. It was observed long ago by Fontana[29] that after viper-bites the blood remains fluid, and Brainard[30] on the contrary, pointed out that, in the case of animals that succumb very rapidly after having been bitten by a _Crotalus_, the blood was always found coagulated into a mass, while, when a certain interval of time had elapsed since the bite, it remained fluid. Weir Mitchell[31] explained these differences by the hypothesis that, in cases of rapid death, the blood had not had time to become modified by the venom. Later on it was found by Sir Joseph Fayrer, and subsequently by Halford,[32] in Melbourne, C. J. Martin,[33] in Sydney, G. Lamb,[34] in Bombay, and recently by Noc, in my laboratory, that the venoms of COLUBRIDÆ, especially those of _Naja tripudians_ and AUSTRALIAN species of this family, always leave the blood fluid after death, while the venoms of VIPERIDÆ, on the contrary, are usually coagulant. On the other hand, it was observed by Phisalix,[35] and at an earlier date by Mosso, of Turin, that the venom of _Vipera berus_ causes the blood of the dog to lose its coagulability, while, on the contrary, the same venom is actively coagulant as regards the blood of the rabbit. How are these differences of action to be explained? It was found by Delezenne,[36] who made an excellent study of the phenomena following the injection of peptone, extracts of organs, and other anticoagulant substances into the organism, that those of these substances that render the blood non-coagulable always dissolve the leucocytes, and thus set at liberty two antagonistic bodies which they contain. One of these substances is coagulant and is found retained by the liver, while the other remains in solution in the plasma, and keeps the blood fluid after issuing from the vessels. Now, certain extracts of organs, ricin, abrin and certain venoms in weak doses, retard coagulation, while in large doses, on the contrary, they produce partial or general intravascular clotting.