Methods of Determination of Pathogenic 3Ucrobes. 139 



is necessary, from time to time, to invigorate the 

 stock by making inoculations to susceptible animals, 

 and from these obtain new cultures. 



III. Experimental infections. 



Pathogenic germs, inoculated to animals, may de- 

 termine in the latter troubles of various kinds. These 

 artificial microbic diseases are occasionally charac- 

 teristic of certain germs. J^evertheless, a given 

 germ does not always produce the same morbid con- 

 ditions; these are influenced by the animal species 

 used for the inoculation and may also vary with the 

 individual; the method of inoculation also has its 

 influence, and the symptoms observed may be de- 

 pendent upon this method ; the condition of the virus 

 used as to virulence, and its origin (cultures or 

 pathological products), have also an important bear- 

 ing on the result of the inoculation ; finally, the 

 amount of the virus has an important influence: 

 there are diseases which, inoculated in small doses, 

 are inofiensive or produce immunity whilst large 

 doses more or less rapidly result in death. 



All these reasons indicate that we can draw no ab- 

 solute conclusions as to the results of the inoculation, 

 and that the latter is of no value except when com- 

 bined with other means of diagnosis. 



Experimental inoculations are occasionally made 

 with the object of purifying an impure virus ; in this 

 case advantage is taken of the property possessed by 

 certain germs of developing in a given animal 

 species, whilst, the other microbes with which they 

 may be commingled can not live in this species. 



Subjects of inoculation. — Many species of animals 



