Fontana[147] had remarked that snakes were quite unaffected by the bite of the viper, even when inoculated with the venom hypodermically. Physalix and Bertrand[148] confirmed these statements, and showed that the snake perfectly resisted quantities of viper-venom capable of killing at least 20 guinea-pigs. According to these scientists, this natural immunity is due to the existence in the blood of toxic principles analogous to those of viper's venom--principles that exist in the labial glands of the snake, and pass into the blood and the fluids via the internal secretions. These writers, and also Calmette, have shown that the blood of venomous serpents becomes antitoxic when heated. [147] _Traité sur le Venin de la Vipère_, Florence, 1781. [148] _Archives de Physiologie_, 1894, p. 423. It has been known for a long time that the hedgehog and the mongoos eat certain venomous reptiles, and eagerly hunt for the vipers in particular. When the hedgehog is bitten, which happens quite often despite its dexterity, it resists the viper-venom quite well. Physalix and Bertrand[149] have experimentally demonstrated that the hedgehog withstands a dose of viper-venom capable of killing at least 40 guinea-pigs. Levin[150] has shown that young individuals are less resistant, and it is concluded from this, and perhaps incorrectly so, that the immunity of the hedgehog is naturally acquired, rather than inherent. Bertrand and Physalix have nevertheless shown that on heating the blood of the hedgehog to 88° C. it manifests an antitoxic power toward serpent-venom _in vitro_. [149] _Bull. Muséum Histoire Naturelle_, I, p. 294; _Compt. rend. Soc. de Biol._, 1899. p. 77. [150] _Deutsche med. Woch._, 1898, p. 629. =Artificial Immunity toward Serpent-venom.=--Immunity may be conferred upon every individual by utilizing the method of habituation. This fact was simultaneously elicited by Calmette, Bertrand, and Physalix. To effect the immunity these scientists prepare an antivenomous serum and inject it into animals, giving at first very small quantities of the diluted venom, and gradually increasing the doses, and the periods intervening between the injections. At the end of about two months of this treatment, the immunity has reached its maximum. Certain rabbits, thus slowly inoculated, have been able to withstand 0.04 Gm. of the venom of the naja at a single injection; such rabbits then yield a vaccinal serum.[151] [151] _Annal. de l'Instit. Pasteur_, 1895, p. 229; _Compt. rend. de l'Acad. des Sciences_, CXXII, p. 203.