MICRO-ORGANISMS AS EXCITING AGENTS OF DISEASE. 99 



to such a degree that one-millionth part of a cubic 

 centimetre injected into the animal caused the same 

 typical disease, fatal in eighteen hours, as was obtained 

 by the injection of undiluted blood. The dilution can, 

 however, be carried to a much greater extent by the 

 aid of the methods of cultivation. Pasteur and By cultivation 

 Klebs were the first to show that micro-organisms, sup- 

 posed to be pathogenic, could be cultivated outside the 

 animal body in artificial nutritive material ; that then 

 after the growth of a cultivation a minimal quantity 

 could be placed on a new nutrient material, that a trace 

 of the colonies which then developed could be inoculated 

 on a third soil, and that thus the micro-organism could 

 be cultivated through a whole series of generations. 

 Koch then gave us methods by which it is possible Koch's pure 



,-1 ,. . / . . cultivations. 



to prepare the cultivations of any given organism 

 in such a way that no other organism grows along with 

 it, and that on further transplantations on new soil only 

 the same organism appears. Thus we could for the 

 first time study with certainty, and in an unaltered 

 condition, the organism suspected as the cause of the para- 

 sitic disease, and that for a long time, and in spite of a 

 large series of new transplantations. When an organism 

 has in this way been cultivated through fifty or a hundred 

 generations, it is self-evident that the last generation 

 does not contain any of the materials which adhered toj 

 the original organisms ; it is easy to see that the dilution 

 must be reckoned by trillions, and must ultimately he- 

 incalculable ; any poisonous material originally mixed 

 with it, however intense its action may be, cannot be 

 present in the last cultivation in appreciable amount, and 

 therefore when infection is obtained with the last culti- 

 vation it can only be because the micro-organisms them- 

 selves, which are constantly reproduced at the expense 

 of the nutritive materials, are the real noxious agents. 



As a matter of fact, inoculations with the minutest 

 quantity of the hundredth pure cultivation succeed quite 

 as well as with the original material. In the case of 

 antbrax, of various forms of septicaemia, of glanders, of 

 tuberculosis, &c., Koch was able to carry on pure 



