127 



but rarely in the oesophagus, and then only when the flea was killed 

 immediately after feeding. 



We have in San Francisco examined quite a number of serial sec- 

 tions of plague-infected fleas with the same result as obtained by the 

 Indian Plague Commission. The bacilli are readily demonstrated, 

 sometimes in enormous numbers, in the gizzard, stomach, and in the 

 rectum, but at no time have they been found in the body cavity, the 

 salivary glands, or the ovary. In fact, as we are dealing with a 

 vegetable organism and not an animal organism, like the Plasmodium 

 malaridR, we could hardly expect to find any biologic change, except 

 simple multiplication, occurring in the intermediate host. 



HOW THE FLEA CLEARS ITSELF OF BACILLI. 



Some explanation is necessary as to why the bacilli eventually 

 disappear from the flea, although they seem to multiply during the 

 first few days. It is evident that the peristaltic action of the stomach 

 during the course of digestion forcing the blood at the proper time 

 into the rectum, finally to be ejected from the body, would in itself 

 cause many bacilli to be discharged, but naturally a few would remain 

 to multiply indefinitely. The bacteriacidal action of the blood is 

 soon lost after entering the flea's stomach, but it has been shown by 

 proper staining that the leucocytes after the first feeding with healthy 

 blood contain numbers of Bacillus pestis, and it seems probable that 

 this phagocytic action is important in the cleansing process. It has 

 been shown that after successive feedings on the blood of noninfected 

 animals the power of phagocytosis is increased, and that successive 

 feedings on the fresh blood of animals that have been immunized 

 against plague still further assists and hastens the process. When 

 there is a frequent introduction of fresh normal or immunized blood 

 its bactericidal action is also instrumental in the cleansing process. 



REGIONAL DISTRIBUTION OF FLEAS ON RATS. 



The location of the primary bubo in a case of plague, human or 

 rodent, depends upon the site of inoculation, for that group of glands 

 will first enlarge which has direct lymphatic connection with tho area 

 through which the Bacillus pestis enters the animal organism. The 

 British Indian Plague Commission found that 72 per cent of their 

 naturally infected rats and 61 per cent of the rats experimentally 

 infected by fleas had cervical buboes, while in no instance in over 

 5,000 plague rats was a mesenteric bubo encountered. On the other 

 hand, where plague was induced through feeding healthy rats with 

 the carcasses of plague rats a mesenteric bubo was found in 74.5 per 

 cent of those infected and a cervical bubo in 36 per cent. In San 

 Francisco in naturally infected rats a primary mesenteric bubo has 

 never been seen, and a cervical bubo has been seen but once. These 



