DISCOVERY OF THE TUBERCLE BACILLUS. 425 



differentiated from the surrounding tissue. " It 

 he says, " in the highest degree impressive to observe in 

 the centre of the tubercle-cell the minute organism 

 which had created it." Transferring directly, by 

 inoculation, the tuberculous matter from diseased 

 animals to healthy ones, he in every instance re- 

 produced the disease. To meet the objection that 

 it was not the parasite itself, but some virus in 

 which it was imbedded in the diseased organ, that was 

 the real contagium, he cultivated his bacilli artificial- 

 ly for long periods of time and through many suc- 

 cessive generations. With a speck of matter, for exam- 

 ple, from a tuberculous human lung, he infected a sub- 

 stance prepared, after much trial, by himself, with 

 the view of affording nutriment to the parasite. In 

 this medium he permitted it to grow and multiply. 

 From the new generation he took a minute sample, and 

 infected therewith fresh nutritive matter, thus pro- 

 ducing another brood. Generation after generation of 

 bacilli were developed in this way, without the inter- 

 vention of disease. At the end of the process, which 

 sometimes embraced successive cultivations extending 

 over half a year, the purified bacilli were introduced into 

 the circulation of healthy animals of various kinds. 

 In every case inoculation was followed by the reproduc- 

 tion and spread of the parasite, and the generation of 

 the original disease. 



Permit me to give, a little more in detail, an 

 account of Koch's experiments. Of six healthy guinea- 

 pigs, four were inoculated with bacilli derived origi- 

 nally from a human lung, which, in fifty-four days, 

 had produced five successive generations. Two of the 

 six animals were not infected. In every one of the in- 

 fected cases the guinea-pig sickened and lost flesh. After 

 thirty-two days one of them died, and after thirty- 



