146 THE MORE CHRONIC INFECTIOUS DISEASES 



per cent, carbolic acid should certainly kill them in 

 sputum in twelve hours; in watery suspension in 

 thirty minutes. Bichloride of mercury is not of value 

 for sputum disinfection, but in strength of 1 to 1000 

 in watery suspension is fatal in one hour. No kind 

 of animal is absolutely resistant to tuberculosis, but 

 there are some that very seldom present the spon- 

 taneous disease, notably dogs and horses. 



There are four forms or varieties of the tubercle 

 bacillus: the human, bovine or cow, bird, and reptil- 

 ian. The first two only concern us, and the distin- 

 guishing features of these groups are of small importance 

 here. The infectiousness of the bovine form for 

 humans has been mentioned. The human form is of 

 very low virulence for the cow, but may infect most 

 of the smaller animals. It has been found impossible 

 to obtain from any of the lower animals a serum which 

 will have a beneficial effect upon the disease in human 

 beings. That is, no serum can be procured which will 

 give a passive immunity. 



Tuberculin. The poisons made in cultures and used 

 for the tuberculin test in the form of Koch's tuber- 

 culin have already been mentioned. There are many 

 forms of tuberculin which are incidentally modelled 

 after Koch's plans. His original was a broth upon 

 which the bacteria had grown, but freed of living forms 

 and reduced by evaporation to one-tenth its original 

 volume. This contained both the endo- and extra- 

 cellular toxins. His later forms consisted of killed 

 bacteria, of a watery extract from them, and lastly, 

 living bacteria so reduced in virulence that they could 

 not produce tuberculosis. These are all tuberculins, 



