230 PATHOGENIC BACTERIA. 



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 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, 

 but acts upon the living tuberculous tissue. In the 

 description of the tissue-changes already given it has 

 been shown that the tubercle bacillus effects the coagu- 

 lation-necrosis of the cells, but does not derive its nutri- 

 ment from the dead tissue. As the cells die and are 

 incorporated in the necrotic mass, the bacilli find the 

 conditions of life unfavorable, and likewise seem to die. 

 The active bacilli, therefore, are always found at the mar- 

 gins of the tuberculous tissues, where the cells are fairly 

 active. The necrosis is due to bacillary poisons. When 

 tuberculin is injected into the organism the result is to 

 double the amount of poisonous influence upon the cells 

 surrounding the bacilli, to destroy their vitality, to re- 

 move the favorable conditions of growth from the organ- 

 ism, and to leave it for a time checkmated. 



Virchow, who well understood the action of the tuber- 

 culin, soon showed that as a diagnostic and therapeutic 

 agent in man its use was attended with great danger. 

 The destroyed tissue was absorbed, and with it the bacilli 

 were likewise absorbed and transported to new areas, 

 where a rapid invasion occurred. Old tuberculous lesions 

 which had been encapsulated were softened, broken 

 down, and became sources of dangerous infection to the 



