Page:George Archdall Reid 1896 The present evolution of man.djvu/234

We have seen that the bacilli of anthrax when exposed to a temperature of from 42° to 43° C., gradually lose their virulence and at length cease to cause death; in other words, the toxins which they manufacture gradually become less and less poisonous, till they become so little poisonous that the phagocytes of the infected animal are able to vary fitly in response to appropriate stimulation, that from the toxins, and become capable of destroying the invading bacilli in spite of the presence of those toxins. We have seen also that if we proceed step by step, first injecting into the tissues of a susceptible animal bacilli of little virulence, then after an interval bacilli of greater virulence, we are able at length to confer immunity against the bacilli of the greatest virulence—of virulence so great that they would have infallibly destroyed the animal experimented on had it not been protected by the previous inoculations. The rationale of the process is then clear; the stimulation from the feeble toxins of the first injected bacilli induce such a reaction, such a fit variation in the phagocytes of the host, that they are thereby placed in a position of advantage, owing to which they are able to vary fitly in response to stimulation from the stronger toxins of the second injection, which, but for their position of advantage, would destroy or paralyze them. In like manner, from this second position of advantage they are again able to vary fitly still farther in response to the stimulation of the toxins of the third and most virulent injection. So skin-cells when subjected to long-continued heat or friction vary in response in a fit direction, till they are able to withstand degrees of heat and friction which would be fatal to them had they not attained a position of advantage. So cells of other kinds by an analogous process of slow training are able to withstand nicotine, opium, alcohol, arsenic, &c., when present in their nutrient fluids in a degree