INSECTS AND HUMAN DISEASE 157 



Kong that we are mainly concerned, for it was here that a 

 Japanese physician named Kitasato discovered Bacillus 

 pestis to be the cause of the disease, and a little later Yersin 

 confirmed his discovery. The bacillus is very minute, nearly 

 as broad as it is long, and when stained its median portion 

 appears as a clear area ; it occurs in great numbers in the 

 swollen lymphatic glands. 



The incubation period of plague varies from twelve 

 hours to fifteen days, but the usual period is three or four 

 days. Sometimes the onset is sudden ; the patient falls as 

 if shot, and dies after a few convulsive gasps. Usually 

 the lymphatic glands of the thigh, groin or armpit, and 

 more rarely of the neck, become swollen and painful, 

 producing the characteristic " buboes," hence the name 

 bubonic plague ; in the severer septicaemic plague the 

 buboes have no time to form, but extensive haemorrhage 

 takes place, the skin becomes mottled with dark patches, 

 and on this account the disease has been called the black 

 death. The most fatal form is pneumonic plague, which 

 attacks the lungs, and the least fatal the ambulatory 

 plague. Fortunately, only two and a half per cent, of the 

 cases are pneumonic, which spreads from man to man by 

 bacilli in the air, but the bubonic and septicaemic forms 

 are only spread from rats to man by the agency of fleas, 

 as was suggested by Ogata in 1897, and shown by 

 Verjbitski and Simond at a later date. 



The home of plague is Central Asia, where it is endemic 

 mainly in the Kurdistan Hills and about the Himalayas. 

 Its spread is little influenced by climate, and in no wise by 

 soil. In Asia, at any rate, the black rat, Mus rattus, is the 

 means of spreading the disease ; as this animal is the true 

 ship rat, it is easy to understand how easily plague is 

 carried from port to port. That its connection with these 

 rodents was known to the ancients has already been 

 mentioned, and, as further evidence, we may remark that 

 the ancient Greeks in Asia Minor worshipped Apollo as 



