They also ascertained that the contents of the eggs of Crotalus are especially rich in poison, and this poison appears to consist for the most part of neurotoxin, since it does not cause hæmorrhages. Phisalix has observed that the ovules of the viper exhibit analogous toxicity.[82]
Summing up what has been stated above, we find that the blood of both poisonous and non-poisonous snakes contains toxic substances, destructible by heating to 68° C., and physiologically distinct from venoms, but like the latter possessing the property of dissolving the red corpuscles of the majority of vertebrates and of producing hæmorrhages.
CHAPTER XI.
NATURAL IMMUNITY OF CERTAIN ANIMALS WITH RESPECT TO SNAKE-VENOMS.
It was long ago pointed out that certain warm-blooded animals, including the mongoose (Herpestes ichneumon, Family Viverridæ), hedgehog (Erinaceus europæus, Family Erinaceidæ), pig (Sus scrofa, Family Suidæ), and some herons (Ajaja, Subfamily Plataleinæ; Cancroma, Subfamily Cancrominæ; Botaurus, Subfamily Ardeinæ; Mycteria, Subfamily Ciconiinæ), known in Colombia under the names Culebrero and Guacabo, exhibit a natural immunity with respect to snake-bites.
Pigs devour vipers with great readiness, and in the region of North America which adjoins the Mississippi and its tributaries they are even trained to destroy the young rattle-snakes and other poisonous serpents with which the valleys of these watercourses are infested.
During my stay in Indo-China I inoculated a young pig, beneath the skin of the back, with a dose of Cobra-venom (10 milligrammes) capable of killing a large-sized dog. The animal withstood the injection, but I am inclined to think that this is not a case of true immunity; it is probable that the pig owes its resistance to venom to the fact that its skin is lined with an enormous layer of adipose tissue, which is but very slightly vascular, and in which absorption takes place very slowly. This opinion is corroborated by my discovery that the serum of this animal is entirely destitute of any antitoxic substance. I mixed a dose of Cobra-venom, lethal for the rabbit, with 3·5 and 8 c.c. of pig-serum. These mixtures killed rabbits in the same time as the controls that received the venom diluted with equal quantities of rabbit-serum or physiological saline solution.
The natural immunity of the mongoose and the hedgehog rests upon more scientifically established facts.
My own experiments upon the immunity of the mongoose were made with six specimens of these little carnivores captured in Guadeloupe (French West Indies), an island in which no poisonous snakes exist; consequently their immunity could not have arisen from their having become accustomed to the bites of venomous reptiles.
I first introduced a mongoose into a cage containing a Naja bungarus (Ophiophagus) of large size. The snake rose up immediately, dilated its hood, and struck savagely at the little animal, which, darting nimbly out of the way, escaped being seized and, frightened for a moment, took refuge in a corner of the cage. Its stupor, however, was but of brief duration, for at the very moment when the hamadryad was preparing to strike at it again, the mongoose, with open mouth and snarling, sprang upon the reptile’s head, bit it hard in the upper jaw and crushed its skull in a few seconds. This scene is in every respect reminiscent of the admirable description given by Rudyard Kipling, in his celebrated “Jungle Book,” of the great war that Rikki-tikki (the Mongoose) fought with Nag (the Cobra) “through the bathrooms of the big bungalow in Segowlee cantonment”:—
“Nag was asleep, and Rikki-tikki looked at his big back, wondering which would be the best place for a good hold. ‘If I don’t break his back at the first jump,’ said Rikki, ‘he can still fight; and if he fights—O Rikki!’ He looked at the thickness of the neck below the hood, but that was too much for him; and a bite near the tail would only make Nag savage.