174 MICROBES AND TOXINS 



day, without any nodule developing, a necrosis of the skin 

 over a zone of half to one centimetre ; the necrotic patch 

 becomes detached, and the ulcer heals up and closes without 

 any swelling of the corresponding glands. The same process 

 takes place when, instead of living bacilli, bacilli killed by 

 boiling are injected the second time. 



The tubercle bacilli therefore act in a different fashion in 

 the healthy organism from the organism already tuberculous. 

 Koch observed that a large dose of bacilli killed the 

 tuberculous guinea-pig, whereas a very small dose produced 

 an improvement in their condition and healed up the initial 

 ulceration. He saw in this a principle of treatment. Since 

 the bacilli are not readily absorbed, he replaced them by an 

 extract from cultures ; this was the original tuberculin. 



This substance is practically harmless to non-tuberculous 

 animals, but is fatal to tuberculous animals in a very small 

 dose. In absolutely minimal doses, repeatedly administered, it 

 exerts a curative effect on certain tuberculous lesions. Accord- 

 ing to the size of the dose, it can act as a poison or as a remedy. 

 These are its fundamental properties. 



Local and General Reactions. Koch ascribed the 

 curative effect to the necrotic action which he had noticed 

 after reinoculation of bacilli in tuberculous guinea-pigs. 

 Tuberculin he said in 1890, does not kill the bacilli, it kills 

 the living tuberculous tissues ; it does not even act on tissues 

 already dead, such as the caseous masses. It acts on cells in 

 the same way as the bacillus tuberculosis itself, but the soluble 

 product has a much more extensive radius of activity than the 

 bacillus. 



In what way does this necrotic action become therapeutic ? 

 Because in a necrotic tissue the bacillus is badly nourished 

 and grows feebly ; the dead tissue becomes a sort of slough 

 or sequestrum which the body strives to get rid of. The 

 action of tuberculin is, in a sense, surgical. It might be 

 hoped that every part affected by it might be thrown off, and 

 this is sometimes possible in tuberculosis of the skin and of 

 the lungs. In many cases, however, it is impossible, and it 



