124 RELATIONS OF BACTERIA TO DISEASE. 



healthy tissues can usually resist a certain number of 

 pathogenic organisms of given virulence, and it is only 

 in a few instances that one or two organisms introduced 

 will produce a fatal disease, e.g., the case of anthrax in white 

 mice. The healthy peritoneum of a rabbit can resist and 

 destroy a considerable number of pyogenic micrococci 

 without becoming inflamed, but if a larger dose be intro- 

 duced, inflammation or suppuration will follow. Again, a 

 certain quantity of a particular organism injected subcutane- 

 ously may produce only a local inflammatory change, but 

 in the case of a larger dose the organisms may gain 

 entrance to the blood stream and produce septicaemia. 

 There is in the case of many organisms a minimum lethal 

 dose for a particular animal, which can be determined by 

 experiment only. 



The path of infection may alter the result, serious effects 

 in many instances following especially a direct entrance into 

 the blood stream. Staphylococci injected subcutaneously 

 in a rabbit may produce only a local abscess, but if injected 

 in the same quantity into the blood stream, multiple 

 abscesses in certain organs and death may follow. In the 

 former case the organisms in the subcutaneous tissue 

 produce around them an inflammatory or suppurative area 

 to which they are practically confined. The nature and 

 significance of this local inflammatory change will not be 

 discussed here, but it may be stated that in many cases it 

 is followed by the destruction of the organisms. In some 

 other cases, however, the organisms are very rapidly 

 destroyed in the blood stream, and Klemperer has found 

 that in the dog, subcutaneous injection of the pneumo- 

 coccus produces death more readily than intravenous 

 injection. 



2. The Subject of Infection. Susceptibility and, in 

 inverse ratio, resistance to a particular microbe vary much 

 in different species and in different varieties of the same 

 species. White rats are practically immune against anthrax 

 infection, guinea-pigs are very susceptible to it ; field mice 

 are amongst the most susceptible animals to glanders, while 



