I have found that they can even be utilised to vaccinate animals against venom; by injecting weak, non-lethal, and repeated doses of dilute Cobra-blood into guinea-pigs and rabbits, I have succeeded in rendering them immune to doses of Cobra-venom several times greater than the lethal dose.
There is no doubt that it is to these substances that the poisonous and non-poisonous snakes owe the partial immunity that they themselves enjoy with respect to venoms. We know, in fact, that common snakes suffer without danger many bites from vipers (Phisalix and Bertrand[78]), and that the Cobra is relatively little affected by inoculation with its own venom or with that of other Colubridæ, such as Bungarus, or even of Viperidæ, such as Vipera russellii.
This immunity, however, is far from being absolute; I have killed common snakes (Tropidonotus natrix) with doses of viper-venom ten times greater than the lethal one for the rabbit, and Lachesis lanceolatus (from Martinique) with 0·02 gramme of the venom of Naja tripudians.
Phisalix,[79] on his part, has shown that, while it was necessary to inject from 100 to 200 milligrammes of viper-venom into other vipers or common snakes, beneath the skin or into the peritoneum, in order to cause death, the introduction of only 2 to 4 milligrammes of this venom into the brains of these reptiles was sufficient to kill them with the same symptoms of intoxication. This dose, however, is only twenty-five to thirty times greater than the lethal one for the guinea-pig.
The practical lesson to be learnt from the establishment of the foregoing facts is that poisonous snakes of different species must never be placed in the same cage, for these animals sometimes bite each other, and may thus kill one another.
Simon Flexner and Noguchi[80] have studied the action of the serums of Crotalus, Ancistrodon, and a non-poisonous species, the pine snake (Pituophis catenifer), on the venoms of Naja, Ancistrodon, and Crotalus. They found that the serum of Crotalus rapidly dissolves the red corpuscles of man, the dog, rabbit, guinea-pig, sheep, rat, pigeon, and horse.
The serum of the pine snake affects the same red corpuscles, but in a lesser degree. Heating to 58° C. suppresses the hæmolytic power of these serums, but they can be restored to activity by the addition of a very small quantity of the same serum in a fresh condition, of fresh serum derived from other snakes, or of fresh serum from the guinea-pig.
Antivenomous serum also, when added in a suitable dose, entirely suppresses the hæmolytic action of snake-serums; it has, however, greater effect upon the hæmolysin of Cobra-blood than upon that of the blood of other snakes. This observation had previously been made by W. Stephens,[81] and it has been verified by Noc in my laboratory.
Crotalus-serum dissolves the red corpuscles of the mongoose (Herpestes ichneumon) of Jamaica, whose extraordinary resistance to venom is well known. But if variable doses of Ancistrodon-venom and Crotalus-serum be made to act simultaneously upon these corpuscles, the latter are no longer dissolved. Again, if, instead of red corpuscles which are but little sensitive, like those of the mongoose, we employ the highly sensitive corpuscles of the guinea-pig, the result is the same. These experiments are regarded by Flexner and Noguchi as proving that the amboceptors of the toxic serum become fixed, in conformity with Ehrlich’s theory of the lateral chains, upon the receptors of the sensitive erythrocytes, and leave no more receptors free for the fixation of the venom.
The same investigators have endeavoured to determine the respective toxicity of the tissues of the different organs of Crotalus. They found that the most toxic organs are the spleen and the liver; the toxicity of the spinal cord, kidney and muscles is much less. It appears that this toxicity is intimately connected with the quantity of blood that the tissues retain, for the physiological effects observed are identical with those that follow the injection of blood or serum alone.