TUBERCULOSIS. 211 



could give rise to tuberculosis. With all the ordinary 

 nutrient media then at. his command he entirely failed to 

 obtain any growth of the organism outside the animals in 

 which it led its parasitic life. It depended so much upon 

 these conditions of parasitic life that when removed from 

 them it was no longer able to grow and multiply : it might 

 still remain alive ; in fact, Cornil was able to demonstrate 

 that at the ordinary temperature of the room the tubercle 

 bacillus, kept in ordinary Seine water, continued to exist, but 

 not to multiply, for seventy days. At length, however, Koch 

 overcame the difficulties with which he had to contend, in a 

 most ingenious manner, and he succeeded in growing as a 

 saprophyte what had hitherto been demonstrated only as 

 a parasitic organism. He argued that as the tubercular 

 process developed but slowly, he would have to obtain a 

 medium which would remain unaltered for a considerable 

 length of time when placed in a temperature at which the 

 organism could grow and multiply. He was satisfied too, 

 from his early experiments, that only special substances 

 would serve as nutrient material for this fastidious organism, 

 and he ultimately found that solidified blood serum was by 

 far the best medium on which to cultivate it, as it alone of 

 the many substances which he had then tried supplied all the 

 requirements of the organism. This blood serum contains 

 all the elements necessary for the nourishment of the 

 organism ; it remains solid at the normal temperature of 

 the body, at which temperature it may be kept for a long 

 enough time to allow of the development of the slowly 

 growing bacillus, whilst a small amount of water might be 

 left in the test tube along with the serum without dis- 

 solving it, thus serving to supply the moisture requisite 

 for the perfect growth of the bacillus. In consequence of 

 the slowness of the growth above referred to, it is an ex- 

 ceedingly difficult matter to obtain pure cultivations of the 

 tubercle bacillus should it once become mixed with putre- 

 factive bacteria, and it was for long deemed almost an 

 impossibility to separate it from these other forms : this 

 difficulty has now, however, been overcome. Most of Koch's 

 earlier pure cultivations were obtained by taking as seed 

 material, tubercular lymphatic glands from freshly-killed 

 guinea pigs, which had been inoculated, some three or four 

 weeks before, with tuberculous material. 



