DISCOVERY OF THE TUBERCLE BACILLUS. 425 



differentiated from the surrounding tissue. * It was,' 

 he says, 4 in the highest degree' impressive to ooserve 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 

 artificially for long periods of time and through many 

 successive generations. With a speck of matter, for 

 example, from a tuberculous human lung, he infected 

 a substance prepared, after much trial, by him- 

 self, with the view of affording nutriment to the para- 

 site. 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 producing another brood. Generation after gener- 

 ation of bacilli were developed in this way, without the 

 intervention of disease. At the end of the process, 

 which sometimes embraced successive cultivations ex- 

 tending over half a year, the purified bacilli were in- 

 troduced into the circulation of healthy animals of 

 various kinds. In every case inoculation was followed 

 by the reproduction 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 

 infected cases the guinea-pig sickened and lost flesh. 

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



