preventing tuberculosis might now be found. The early years following the cultivation of the tubercle bacillus saw no realization of this hope, and to-day we are still far from the desired goal. However, the prodigious labor which has been expended in the search for a means of protection against infection with the tubercle poison has not been wholly devoid of results.
In an address of this kind it is not practicable to deal with the separate contributions, in detail, which the many workers have made to the subject of immunity in tuberculosis. The most that can be accomplished is to bring together the more important results of all the workers and, after having assembled them, to judge of their value and to consider, possibly, in what important respects they are still imperfect. I can not do better, at the beginning, than to remind you that the successful point of departure has been the discovery that variations in type and in virulence exist among tubercle bacilli. The earlier view which taught that the tubercle bacillus is a micro-organism of uniform and fixed virulence has been shown to be erroneous, first by the discovery of variations according to certain origins, and second by a gradual decline in pathogenic power suffered by certain strains through long cultivation outside the animal body.
The animals which have been of special use for tests of immunity are rabbits, cattle and goats. The guinea-pig, which furnishes an almost ideal animal for the detection of tuberculosis, because of the sensitiveness of its reaction to inoculations with tubercular material, fails, for the same reason, to be a highly suitable animal in which to carry out tests of immunity; and yet it has been employed with some success.
The first important contribution to the subject of experimental immunity in tuberculosis was made by Koch in connection with his researches on tuberculin—a product of the growth in broth of tubercle bacilli, freed from the bacilli and concentrated. In spite of the failure of tuberculin to bring about a favorable issue in all cases of human tuberculosis in which it is administered, it still remains a useful, perhaps the most useful, strictly medicinal agent employed for the treatment of tuberculosis.-But the sum of its useful properties is not embraced in its employment as a therapeutic substance: it is also a diagnostic agent of high value, and its action upon the tuberculous organism is so specific and remarkable that it has proved itself of the greatest importance and aid in the effort to unravel the complicated series of biological phenomena which constitute the tubercular state.
It is possible to increase somewhat the resistance of animals to tubercular infection by previous treatment of tuberculin; but this increase is not remarkable. It is possible to bring about arrest of the tubercular process in the infected organism by means of tuberculin:
