200 BACTERIA. 



dyes, it retains the color so tenaciously that it can be 

 washed with twenty-five per cent, of nitric acid without 

 parting with its color. In the sputum of a consumptive 

 there are usually other kinds of bacteria besides the 

 bacillus tuberculosis, but these lose their color in the 

 nitric acid. We generally use a red solution for staining 

 the bacillus and then a blue aniline for a contrast stain. 

 The bacilli then appear red on a blue ground. 



To determine whether or no a particular organism is 

 the cause of a given disease, three things are necessary : 

 First, the organism must be found in all cases suffering 

 from that disease. Second, it must be capable of being 

 isolated and cultivated in a pure state outside the body. 

 Third, such cultures, when inoculated into healthy, sus- 

 ceptible animals, must give rise to the same disease in 

 them. For some time it was impossible to grow this 

 bacillus outside of the body and many physicians believed 

 it to be the result rather than the cause of consumption. 

 Old opinions die hard, and in spite of the innumerable 

 medical fads and fashions, the medical profession as a 

 whole is very conservative. But it was found that by 

 adding glycerine to agar or to beef broth as a medium 

 the bacillus would grow readily enough. By injecting 

 such cultures into guinea pigs or rabbits the animals in- 

 variably became tuberculous and died, so that the chain 

 of proof is complete and no one whose opinion is of any 

 value whatever now doubts the causative influence of the 

 organism. This, like other pathogenic species, kills, not 

 by direct destruction of lung tissue but .by the generation 

 of a poison which acts upon the system. I think that 

 almost all who have for the first time attended an au- 

 topsy of one dead from consumption have been struck 

 with the small amount of actual loss of lung tissue. The 

 doctor who gravely informs the anxious friends of a 

 patient that "one lung is all gone and the other nearly 

 so " is certainly not telling the exact truth. We know 



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