There is, therefore, a real antagonism between the actively coagulant substance contained in certain VIPERINE venoms and the anticoagulant substance comprised in the venoms of certain other VIPERIDÆ (_Ancistrodon_), belonging to the subfamily CROTALINÆ, and in those of all the COLUBRIDÆ. The conclusion to be deduced from the foregoing facts is that the venoms of COLUBRIDÆ and those of certain VIPERIDÆ are decidedly _anticoagulant_, while the majority of the venoms of VIPERIDÆ, on the contrary, possess strong _coagulant_ properties, even when mixed with blood in infinitesimal doses. The question therefore arises why these _coagulant_ VIPERINE venoms suppress the coagulability of the blood when mixed with it _in vitro_ in strong doses (for example, in doses beginning from 4 milligrammes of _Lachesis_-venom, or 7 milligrammes of the venom of _Vipera russellii_ for 1 c.c. of 1 per cent. citrate rabbit-plasma). The explanation of this apparently contradictory phenomenon is furnished by the intense proteolysis that these VIPERINE venoms exert upon fibrin, in solution or coagulated. This proteolysis actually manifests itself with weak coagulant doses, for the compact clots formed at the outset soon become soft and then dissolve, like a cube of egg-albumen in an experiment in artificial digestion by trypsin. We shall revert to the subject later on. III.--MECHANISM OF THE ANTICOAGULANT ACTION OF VENOMS ON THE BLOOD. The anticoagulant action of the venoms of COLUBRIDÆ and of _Ancistrodon_ upon the blood appears to take effect in the first place upon the fibrin-ferment, and afterwards upon the fibrin by proteolysis. The action on the fibrin-ferment seems manifest when we experiment with anticoagulant venoms which are feebly proteolytic, like the venom of the _Cobra_. I have already stated that a mixture of fresh blood with a sufficient dose of _Cobra_-venom is non-coagulable, as though the blood on issuing from the animal had been mixed with peptone or leech-extract. But, while blood when peptonised or mixed with leech-extract coagulates readily on the subsequent addition of fibrin-ferment, blood mixed with venom remains positively non-coagulable. It is the same with citrate- or oxalate-plasmas, which no longer coagulate when chloride of calcium is added to them, and with 4 per cent. saline plasma on the addition of distilled water.