CHAPTER X. 



EXPERIMENTS UPON THE LIVING ANIMAL. 



To carry out the last of Koch's postulates, and so complete the 

 chain of evidence in favour of the causal relation of micro-organisms 

 to disease, and to study the mode of action of a pathogenic bacterium, 

 it is necessary to introduce into a living animal a pure cultivation 

 of the micro-organism or its chemical products. For this purpose 

 various animals are employed, such as mice, rabbits, guinea-pigs, 

 pigeons, and fowls. 



Inhalation. The animals may be made to inhale an atmosphere 

 impregnated with micro-organisms by means of a spray. In this 

 way Friedlander succeeded in administering the bacteria of pneu- 

 monia to mice ; and the production of tuberculosis by experimental 

 inhalation has thrown light upon the clinical records of cases 

 reported as instances of the infectiousness of phthisis. 



Ingestion. A sheep fed upon potatoes which have been the 

 medium for the cultivation of the anthrax bacillus dies in a few 

 days. Eabbits fed on cabbage sprinkled with a culture of the 

 bacillus of fowl cholera, rapidly succumb to the disease. Animals 

 fed upon the nodules of bovine tuberculosis or upon tubercular 

 flesh and milk will be readily infected. 



Milk, or bread soaked in milk, is a very convenient medium, 

 and from a public health point of view, a most instructive way of 

 administering and testing the effect of pathogenic bacteria. 



Vaccination and Subcutaneous Inoculation. Vaccination may be 

 performed by making a few superficial scratches and inoculating the 

 wound with a sterilised platinum needle charged with the micro- 

 organisms. Another simple method is to take a sterilised scalpel, 

 infect the point with the material to be inoculated, and then make 

 a minute puncture or incision. In either case a situation should be 

 selected, such as the root of the ear, which cannot be licked by the 

 animal after the operation. 



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