their toxins much more rapidly than do the virulent microbes, a thing most unlikely to happen.
As regards a fourth group of zymotic diseases, we can hardly hope anything from the protective inoculation of attenuated microbes, either before or after infection. Included within this group are tuberculosis and leprosy, which run a prolonged indefinite course in the infected individual, and against the microbes of which the phagocytes never seem to acquire markedly increased powers, behaving in this respect like other kinds of cells when subjected to the action of certain vegetable and mineral poisons—e.g. digitalis and mercury—which they are never able to tolerate the better for training. In leprosy and tuberculosis the toxins, judged by their systemic effects, apparently play a very minor part, at least when compared to the part played by them in other diseases—e.g. diphtheria and anthrax. The battle here is not carried on, so far as has been ascertained by the pathogenic organisms, at long range, but is to all intents and purposes a physical struggle between them and the phagocytes, for at all stages of the disease the former may be seen enclosed in the cell- substance of the latter. In leprosy the microbes are invariably or almost invariably the victors, since never, or very rarely, does a sufferer recover from that disease. In tuberculosis the phagocytes are often victorious, since recovery from this disease is common. But victory does not endow the cells with increased power, as it does in such diseases as small-pox, in which the toxins are abundant; stimulation does not cause them here to vary in a fit direction, for he who has once suffered from tuberculosis is as liable as ever to be attacked by the disease. Immunity from tuberculosis and leprosy is therefore entirely or almost entirely of the inborn inherited kind. Some individuals seem
