I 



PHYSIOLOGICAL 351 



be bitten by an adder without suffering evil effects so far as one 

 can see. This heis been corroborated experimentally, and it appears 

 that a hedgehog is not affected by an injection of poison many times 

 stronger than the fatal dose for a rabbit. The hedgehog has natural 

 immunity to the venom of vipers. 



But what does this immunity mean? The first step towards an 

 answer is not difficult, and has been well worked out for the attrac- 

 tive carnivores known as the ichneumon and the mongoose, both 

 of which are inveterate enemies of snakes, and enjoy the same 

 natural immunity as the hedgehog. If some cobra's poison be mixed 

 with mongoose's blood, and if the fluid or serum be injected under 

 a rabbit's skin, nothing happens; but an injection of the same 

 amount of poison without mongoose's serum is immediately fatal. 



Counteracting Substance. — ^The same is true for the hedgehog 

 and adder's poison. Evidently, then, there is some substance or 

 quality naturally present in the blood of mongoose and hedgehog 

 which is able to counteract, neutralise, or somehow take the edge 

 off snake-toxins. 



The hedgehog, the ichneumon, and the mongoose can withstand 

 large injections of poisons; thus the ichneumon can successfully 

 counteract six times the dose that is fatal to a rabbit. But in all 

 cases there is, naturally enough, a limit beyond which the poison 

 is fatal. A few other mammals are known to be immune to snake- 

 poison; thus the American opossum is not affected by the bite of a 

 rattlesnake, and the cat has a high degree 01 immunity to the 

 poison of vipers. A few mammals have a distinct but slight insuscepti- 

 bility to snake-poison; most have none at all. 



The cobra's poison has no effect on other cobras of the same 

 species, and this is a general rule. But when a venomous snake bites 

 a relative that belongs to a different species, the result may be fatal, 

 as is generally the case with different kinds of vipers. A cobra is 

 not much affected by viper's poison, while the viper readily succumbs 

 to a cobra's! All this shows how specific the immunity may be. 

 There are many peculiarities of this sort which do not readily find 

 explanation at present. 



Artificial Immunity. — ^The immunity which is so well illustrated 

 by the hedgehog and the mongoose is natural immunity; but the 

 same quality of insusceptibility may be more or less artificially 

 acquired. One method is to begin by injecting minute doses of the 

 poison and continue with gradual increase in quantity. The blood 

 gets into a condition of resistance or insusceptibility, neutralising 

 doses which would otherwise have been fatal. Thus a man may be 

 immunised to snake-bite. The theory is that the blood is able to 

 prepare a garrison of "anti-toxins" or "anti-bodies" — what's in a 

 name when we do not know what they are, if they are? — sufficient 

 to withstand a strong invasion of toxins. On the same principle, an 



