i88 PATHOGENIC BACTERIA. 



with a mixture containing tubercle bacilli the wound 

 ordinarily heals readily, and soon all signs of local dis- 

 turbance other than enlargement of the lymphatic glands 

 of the neighborhood disappear. In about two weeks there 

 occurs at the point of inoculation a slight induration which 

 develops into a hard nodule, then ulcerates, and remains 

 until the death of the animal. If, however, in the course 

 of a short time the animals are reinoculated, the course 

 of the process is altogether changed, for, instead of heal- 

 ing, the wound and the tissue surrounding it assume 

 a dark color and become obviously necrotic, and ulti- 

 mately slough away, leaving an ulcer which rapidly and 

 permanently heals without enlargement of the lymph- 

 glands. 



Having made this observation with injected cultures 

 of the living bacillus, Koch next observed that the same 

 change occurred when the secondary inoculation was 

 made with pure cultures of the dead bacilli. 



It was also observed that if the material used for the 

 secondary injection was not too concentrated and not 

 too often repeated (only every six to forty-eight hours), 

 the animals thus treated improved in condition, and, 

 instead of dying of the tuberculosis induced by the 

 primary injection in from six to ten weeks, continued 

 to live, sometimes (Pfuhl) as long as nineteen weeks. 



Koch also discovered that a 50 per cent, glycerin 

 extract of cultures of the tubercle bacillus produced the 

 same effect as the dead cultures originally used, and 

 gave this substance, tuberculin, to the scientific world 

 for experimental purposes, in the hope that the prolon- 

 gation of life observed in the guinea-pig might be true 

 in the case of man. 



The active substance of the u tuberculin" seems to be 

 an albuminous derivative insoluble in absolute alcohol. 

 It is not a toxalbumin. 



The action of the tuberculin upon the animal organ- 

 ism is peculiar, but readily understandable. // does not 

 exert the slightest influence upon the tubercle bacillus, 



