240 BACTERIA IN INFECTIOUS DISEASES. 



micrococcus of fowl-cholera ; and that Koch has 

 proved that a form of induced septicaemia in mice, 

 which he has especially studied, is due to a minute 

 bacillus. 



It has been suggested that the parasitic micro- 

 organism in these diseases is, perhaps, only a second- 

 ary cause, being merely a carrier of the non-living 

 ferment, which is the special poison of the disease. 

 This hypothesis, also, is excluded by inoculation 

 experiments with a pure-culture, sufficiently re- 

 moved from the natural infective material. For 

 the organisms introduced into culture No. 1, as 

 seed, disappear as quickly from successive cultures 

 as does the non-living material with which they are 

 associated, and we may very soon leave them out 

 of the account, although each successive culture- 

 fluid is invaded throughout by their numerous 

 progeny. 



Having determined for a certain infectious dis- 

 ease that its transmissibility depends upon the 

 presence in the infective material of a living micro- 

 organism, the question naturally arises as to the 

 modus opemndi of this parasite. Does it produce 

 death by appropriating something from the vital 

 fluid, or from the tissues invaded by it, e. g., oxy- 

 gen, which is essential for the maintenance of vital 

 processes in the living animal ? Or does it, at the 

 same time that it appropriates material for its own 

 nutrition, evolve some poisonous chemical product 

 which is the immediate cause of the morbid pheno- 

 mena in the infected animal ? Or does it produce 



