vn] FLEAS AND PLAGUE 93 



canal of the flea, and that they have never been 

 found in the salivary glands. 



An apparently more probable explanation, that 

 the contents of the stomach (in which as we know 

 the bacilli may multiply) are regurgitated and trans- 

 ferred to the wound by the mouth-parts, is rendered 

 less credible when we remember that there is a 

 valvular arrangement at the opening of the flea's 

 stomach which seems to make such a thing impossible. 



Lastly, there remains the only theory on which 

 we have positive evidence. It is the theory that the 

 bacilli contained in the faeces of the flea are deposited 

 on the skin and then find their way into the wound 

 made by the piercing organ. They may be helped in 

 this by the rubbing and scratching which follow on 

 the bite of the flea. We know, of course, that plague 

 bacilli are present in abundance in the feces of fleas 

 taken from plague-sick rats, and that such faeces 

 are infective to guinea-pigs both by cutaneous and by 

 subcutaneous inoculation. Experiments were made 

 to discover whether the pricks made by fleas were of 

 sufficient size to allow plague bacilli to enter the 

 body, no other damage to the skin being done. Healthy 

 fleas, confined in a test-tube, were allowed to feed on 

 a small part of a guinea-pig's abdomen, the hair of 

 which had been cropped close without injuring the 

 skin. Immediately afterwards a few drops of the 

 septicsemic blood of a rat which had died of plague, 



