236 POPULAR SCIENCE MONTHLY 



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 prodi- 

 gious labor which has been expended in the search for a means of pro- 

 tection 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 varia- 

 tions 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: 



