318 PATHOGENIC BACTERIA. 



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 " tuberculin" seems to be 

 an albuminous derivative insoluble in absolute alcohol. 

 It is a proteid substance and gives all the albumin reac- 

 tions. It differs from the toxalbumins in its resistance 

 to heat, being able to resist exposure to 120 C. for hours 

 without change. Tuberculin is almost harmless for 

 healthy animals, but extremely poisonous for tuberculous 

 animals, its injection into them being not infrequently 

 followed by death. 



The method of preparation of tuberculin is rather 

 simple. Flasks exposing a considerable surface of liquid 

 are filled with bouillon containing 4-6 per cent, of glyc- 

 erin. The bouillon is preferably made with veal instead 

 of beef. The surfaces are inoculated with pure cultures 



