ii CHARACTERS OF THE B. PESTIS 33 



normally virulent material, or subcutaneous injection with 

 material of less than normal virulence. This form of 

 plague with delayed death and necrotic changes in the 

 bubo, spleen, liver, and lung, I have termed the " subacute 

 form." 



The rabbit is not so well suited for plague experiments, 

 because its susceptibility is not sufficiently high and not 

 sufficiently constant, and therefore a fatal result with a 

 given material cannot be depended on. Body weight 

 seems to make no difference, since a subcutaneous dose 

 may be productive of fatal issue in one rabbit, while the 

 same dose of exactly the same material may cause nothing 

 more than transitory illness in a rabbit of lesser body 

 weight. 



Amongst rats the tame or white rat is the most 

 susceptible, the common brown sewer rat is considerably 

 less so, and therefore the former is for experimental 

 purposes far preferable. Another reason which enables 

 us to dispense with the sewer rat for laboratory purposes 

 is the fact that between 25 to 30 per cent die when kept in 

 captivity, and, further, the fact that they are unpleasant 

 and unsafe to handle ; whereas the tame or white rat (all 

 white, white-black, white-brown, white and plum-coloured) 

 is born and bred in captivity, lives well in cages, and is 

 easily and safely handled. 



Next in plague susceptibility to the white rat is the 

 brown dock and ship rat ; about the same, but a little less 

 perhaps, is the black or plum-coloured ship rat, and still 

 lower in the scale is the brown rat with yellowish cream- 

 coloured belly — the Norwegian rat (which we get from 

 ships coming from Norway). The common sewer rat seems 

 to compare as regards susceptibility with the Norway rat. 



