42 



wild rats on account of a failure in the supply of white rats, or for the 

 sake of economy. This may be done very satisfactorily, if one bears 

 in mind the fact that a considerable number of wild rats are more or 

 less immune to plague infection, especially when the infectious ma- 

 terial is introduced by KoUe's (cutaneous) method. Therefore, it is 

 always advisable to use three or four wild rats where one white rat 

 would be sufficient. They should be kept in a container of such de- 

 sign that there is no possibihty of their escaping. The inoculation is 

 best conducted with the animal under the influence of ether. 



MODES OF INFECTION. 



Rats may be infected experimentally by the ingestion of contami- 

 nated material, and by the appUcation of virulent plague bacilH to a 

 mucous or a cutaneous surface, or by subcutaneous injection of the or- 

 ganism. 



Practically we may confine our study to inoculation by the cuta- 

 neous method, and to subcutaneous inoculation, when the material is 

 injected in the ordinary manner. A useful modification of the latter 

 method is to make a small pocket under the skin of the abdomen and 

 thrust the suspected material into this pocket. This avoids the ne- 

 cessity of making an emulsion of infectious matter, such as the organs 

 of an animal. The time that elapses between the inoculation of a rat 

 with virulent culture of plague bacilli and its death varies somewhat 

 with the size of the dose and with the mode of inoculation. The fol- 

 lowing table, compiled from work in San Francisco, shows the day of 

 death of a few white rats and a considerable number of wild rats 

 using the strain of B. pestis that was found in the recent epidemic 

 here. Some were inoculated by the cutaneous and some by the sub- 

 cutaneous method: 



The wild rats were all Mus norvegicus. 



The lesions found, when an artificially inoculated rat is examined 

 after death, are in a general way similar to those found in naturally 

 infected rats with certain differences to be mentioned later. 



In order to obtain accurate figures as to the frequency of the various 

 lesions in inoculated rats, I have compiled the data from the records 



