96 VACCINE AND SEEUM THERAPY 



2. This tolerance, depending, he believes, on the 

 presence of the avirulent bacilli in the system, does 

 not last more than eighteen months after a single 

 vaccination, but it may be maintained by annual 

 revaccinations, which are harmless in themselves. 

 It is only by vaccination on a large scale that the 

 practical value of this method can be accurately 

 estimated, and, if then found effective, here, then, 

 would be a valuable step towards the protection of 

 our animals against the ravages of tuberculosis. 



Tuberculin. — Koch found that by growing the 

 bacilli from six w^eeks to two months in flasks con- 

 taining slightly alkaline veal broth, to which a per- 

 centage of peptones and of glycerine had been added, 

 and freely supplying the cultures with oxygen 

 throughout, it was possible to obtain a fluid contain- 

 ing some, at any rate, of the toxins produced by the 

 organisms. By passing this through a porcelain 

 filter the bacteria were removed, and a solution of 

 the poisons remained. This was concentrated by 

 evaporation to one-tenth of its bulk, and to the fluid 

 thus obtained the name "tuberculin" or ''old 

 tuberculin" was given. There were great hopes 

 that this would have a curative efl'ect on tuberculosis, 

 but these hopes were dashed to the ground when 

 further experience was gained as to the limitations 

 of tuberculin and the inconstancy of the good results 

 produced. In human medicine numerous modifica- 

 tions of Koch's original tuberculin have been employed 

 in the treatment of tuberculosis, but they have all 



