BACTERIUM TUBERCULOSIS 141 



animal is absolutely resistant to tuberculosis, but there are 

 some that very seldom present the spontaneous disease, 

 notably dogs and horses. 



There are four forms of varieties of the tubercle bacillus: 

 the human, bovine or cow, bird, and reptilian. The first two 

 only concern us, and the distinguishing 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 impos- 

 sible 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 tuberculin 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 free 

 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, the last forms being called vaccines 

 also. Not only are these toxic solutions of value for diag- 

 nosticating tuberculosis, but they may also be used in treat- 

 ment, the purpose being to induce some active immunity to 

 the tubercle bacillus poisons. They are injected under the 

 skin of tuberculous patients, beginning with extremely minute 

 doses, too small to produce the tuberculin reaction described 

 abo\e. We increase the quantity gradually until the patient 

 can endure large amounts. It is maintained that this treat- 

 ment is very beneficial and that a slight immunity is achieved. 

 Opinions vary as to its value, but those who have had longest 

 experience usually testify to its efficacy, although no one 



