BACILLUS TUBERCULOSIS. 261 



strated by the aniline stains as ordinarily employed, 

 nor by the other nuclear stains, organisms at once 

 became quite evident when a small quantity of alkali 

 was added to the solutions of the aniline colouring 

 matters. As later investigations showed, the substi- 

 tution of anilin, toluidin, turpentine, carbolic acid, or 

 ammonia, for the alkali acts in a similar mannner in 

 enabling the staining material to penetrate into the 

 tubercle bacilli ; even without any such addition to the 

 staining fluids, the bacilli may be successfully stained 

 if only the action of the staining material is sufficiently 

 intense to show quantitative differences with respect to 

 the taking up of the colouring matter. It is also of 

 special advantage that the staining material once it has 

 penetrated into the bacilli is held very tenaciously; 

 Koch found that on treatment of sections stained in 

 the alkaline colouring material with strong nitric acid 

 or hydrochloric acid the colouring matter is extracted 

 from cells, nuclei, and all other bacteria, while the 

 tubercle bacilli alone remain stained. In the tissue, 

 which is now colourless, the individual bacilli are re- 

 markably easily recognised on account of their stain; 

 and the picture may be further improved by treating the 

 section, after the employment of the decolourising means, 

 with an ordinary nuclear aniline stain, the tone of which 

 presents a good contrast to the solution first employed 

 for the staining of the tubercle bacilli. The cell nuclei 

 and also the other bacteria (not tubercle bacilli) take on 

 this second stain, so that, for ' example, the tubercle 

 bacilli appear red, the cell nuclei and other bacteria 

 blue, or the former violet, and the latter brown. (As to 

 the modifications and the technique of the method of 

 staining tubercle bacilli, as well as with regard to the 

 exceptions to the exclusive staining of the tubercle 

 bacilli, see the chapter on "Methods." 



The bacilli found in tuberculous organs by the aid Morphological 

 of this extremely sharp and sensitive method are rods ara( 

 1*5 3'5 p. in length; their breadth is constant when 

 the same staining methods are employed, and corre- 

 sponds to that of the bacilli of mouse septicaemia. For 



