“Alone unharmed of all who till the earth By deadly serpents, dwells the Psyllian race. Potent as herbs their song; safe is their blood, Nor gives admission to the poison germ E’en when the chant has ceased. Their home itself Placed in such venomous tract and serpent-thronged Gained them this vantage, and a truce with death, Else could they not have lived. Such is their trust In purity of blood, that newly born Each babe they prove by test of deadly asp For foreign lineage. So the bird of Jove Turns his new fledglings to the rising sun, And such as gaze upon the beams of day With eyes unwavering, for the use of heaven He rears; but such as blink at Phœbus’ rays Casts from the nest. Thus of unmixed descent The babe who, dreading not the serpent touch, Plays in his cradle with the deadly snake.”[93] The only scientific conclusion to be drawn from the facts and statements that we have just set before the reader is that, under certain circumstances, man can unquestionably acquire the faculty of resisting intoxication by snake-venom, by conferring upon himself a veritable _active immunity_ by means of repeated inoculations of venom. We shall shortly see that the case is the same with regard to animals. PART III. ANTIVENOMOUS SERUM THERAPEUTICS. CHAPTER XIII. _VACCINATION AGAINST SNAKE-VENOM--PREPARATION OF ANTIVENOMOUS SERUM--ITS PREVENTIVE PROPERTIES AS REGARDS INTOXICATION BY VENOM._ So long ago as the year 1887 it was shown by Sewall, in an important paper on “Rattlesnake-Venom,”[94] that it is possible to render pigeons gradually more resistant to the action of this venom by injecting them with doses at first very small, and certainly incapable of producing serious effects, and then with stronger and stronger doses. In this way, although these little animals are very sensitive, he succeeded in making them withstand doses ten times greater than the minimal lethal dose. A little later Kaufmann[95] obtained the same result with the venom of French vipers. He did not, however, succeed in producing tolerance of doses more than two or three times greater than the lethal one. In 1892, at the time of my first experiments with cobra-venom at Saigon,[96] I arrived at the conclusion that it was possible, by means of successive inoculations with heated venoms, to confer on animals a certain degree of resistance to doses invariably lethal to the controls.