92 THE FLEA [CH. 



improbable that this means of infection is of any 

 real importance, even if it may sometimes occur. 

 Experiments in feeding have shown that an animal is 

 unlikely to become infected by swallowing material 

 containing plague bacilli, unless the amount is con- 

 siderable. Moreover we know that infected fleas 

 confined in test-tubes readily convey the disease when 

 allowed to bite an animal. In such cases the situation 

 of the primary bubo corresponds with the area of 

 skin upon which the fleas are placed. That the 

 transmission of plague is due to the bite of the flea 

 seems abundantly clear. 



It has also been suggested that the proboscis of 

 the flea acts as a mechanical instrument for the 

 transference of the bacilli. No doubt the outside 

 surface of the flea's proboscis must become con- 

 taminated, when it sucks the blood of a plague- 

 stricken rat ; but it is difficult to suppose that 

 contamination of the proboscis can explain cases of 

 continued infectivity during which the flea has been 

 feeding regularly upon healthy animals. 



Next, there is a hypothesis that the salivary 

 glands of the flea become infected and that the bacilli 

 are inoculated along with the saliva. The reader 

 will remember that when a flea sucks, a stream of 

 saliva is pumped down the mandibles into the puncture. 

 But this hypothesis is shattered by the fact that plague 

 bacilli are apparently confined to the alimentary 



