The President's Address. By G. S. Woodhead. 123 



culosis, but with the aicUof which the white plague is gradually 

 being driven back. 



Without the Microscope we should still be making but feeble 

 ineffective and ineffectual attempts to stem the death-rate from 

 typhoid fever, cholera, diphtheria, pneumonia, the plague, pneumonic 

 and bubonic, and many other disease of animals, plants, and man. 

 Some of these have been scotched and others killed, and recent 

 discoveries have led us to believe that others may, ere long, follow 

 in their wake. It is not many years since Kitasato, Yersin, Lowson 

 and others tracked down the Bacillus pestis, and in it the cause of 

 plague. Our knowledge of the life-history of this organism has 

 been acquired with great labour, slowly and intermittently, and at 

 each stage the Microscope has been called in to provide the last 

 scrap of evidence, to forge the last link in the chain. 



The disease was known to be either contagious or infective. 

 There must then be an infective agent. This was soon demon- 

 strated in the buboes and blood of patients affected with the 

 bubonic plague, and when the same organism was found in the 

 lungs and blood of the patient affected with the pneumonic plague, 

 a far more mortal disease than the bubonic form, evidence of the 

 etiological identity of the two conditions was complete. 



How is the disease carried from patient to patient ? What 

 relation has the increased mortality of rats, before and during the 

 first stages of an outbreak, to the outbreak itself ? 



The plague bacillus was found in many dead rats. Then it 

 was found that healthy rats might safely be placed near plague- 

 stricken rats if an interchange of fleas could be prevented, but that 

 if an interchange of fleas was allowed the healthy rats became 

 plague-stricken. How the conveyance of the bacillus was effected 

 by the flea still remained doubtful until C. J. Martin demonstrated 

 under the Microscope the blocking of the alimentary canal of a 

 proportion of the fleas that had been fed on mice infected with the 

 Bacillus pestis* In the beautiful specimens exhibited by him at 

 our last conversazione could be seen viscid masses of plague bacilli, 

 which were evidently the result of rapid multiplication of a number 

 that had been ingested by the flea when it took its meal of blood 

 from the infected mouse. This mass, blocking up the alimentary 

 canal, renders the flea a more dangerous foe than an irritated 

 cobra. Ingested blood can no longer pass through the canal, but 

 after being mixed with a number of germs detached from the front 

 portion of the bacillary bolus, it is regurgitated and driven into the 

 next puncture made by the practically starving and irritated flea, 

 operating whether on mouse or man. First and last, the Microscope, 

 and the Microscope only, could provide the special and most im- 

 portant evidence required to make good the case against the plague 



* See full account in Journ. Hygiene, xiii. (1914). Plague Suppl. iii., p. 423. 



