vii INFECTION OF ANIMALS WITH PLAGUE 197 



tions, therefore, there was abundant opportunity for 

 transmission of plague by fleas or other blood- sucking 

 insects, such as lice, from the affected animal to its 

 companion living with it in the same small cage ; more- 

 over, ithe animal that sickened with and died of acute 

 plague about the fourth or fifth day had plenty of B. 

 pestis in its blood. It is not to be supposed that common 

 sewer rats such as were used in these experiments were 

 without fleas or lice, nor can it be supposed that the fleas 

 of the rat that died did not leave this rat for the living 

 rat, a practice of fleas noticed and recorded by several 

 observers. On making the post-mortem examination no 

 fleas but only lice could be found on this dead rat, 1 though 

 during life the animal was frequently observed to scratch 

 itself in the manner customary with rats. Notwith- 

 standing, therefore, all these circumstances specially 

 favourable for the transmission of plague, the companion 

 rat remained unaffected. Here was a rat the blood of 

 which was teeming with B. pestis ; further, close at hand 

 was a companion rat ready for any supposed fleas or lice 

 to migrate to, and yet this companion rat remained un- 

 affected by plague. Nevertheless this companion rat, 

 when later on (after lapse of a week) it was cutaneously 

 inoculated into the skin of the root of the tail with a trace 

 of gelatine culture directly made of the blood of its former 

 dead companion, died on the third to fourth day with acute 

 plague (just like other rats after cutaneous inoculation), 

 showing hemorrhagic swollen glands in the groin with 

 dense crowds of B. pestis, the spleen large, dark, and firm, 



1 It should be here added that I have not been able as yet to find " fleas " on 

 sewer rats examined after death — caused spontaneously by plague or after killing 

 them by chloroform ; lice alone, but these in abundance, have been hitherto found 

 on their dead bodies. 



