MAMMALIA 



913 



squirrel to squirrel, and, further, that this flea will bite man 

 Further, they record a subacute case of plague in a boy where the 

 infection was believed to be acquired by contact with ground 

 squirrels. With regard to the outbreak in Manchuria and North 

 China, Gray suggested that it started among men who handled the 

 tarbagan {Arctomys bobcB Schreb), which is susceptible to epizootic 

 plague, and that these men on returning to their homes introduced 

 the disease into three provinces, as pneumonic and septicsemic 

 plague, while it was spread by the agency of the breath and personal 

 contact of clothes and belongings by coolies travelling in parties 

 and sleeping together in overcrowded insanitary inns, especially 

 as the cold of the winter induced an indoor existence. These 

 travelling parties infected adult males who stayed at the inns or 

 were travelling, and so it spread to the ordinary population. No 

 infected rats could be found, in 20,000 examined, while isolation of 

 the patients and their contacts, together with eflicient disinfection, 

 were sufficient to diminish the death-rate. Further researches, 

 however, tended to show that, though the tarbagan suffers at times 

 from plague, the epizootic is not extensive, and its direct relationship 

 to human plague negligible. The pneumonic form of plague may 

 be in epidemics, especially in cold weather, but it is also to be noted 

 that, although it starts from association with an epizootic, it tends 

 to die out without being succeeded by a bubonic outbreak, but it 

 may infect rats and so cause a bubonic outbreak. The marmot 

 (Spermophilus citellus) , which is common around Mukden, was sus- 

 ceptible to the infection. There has been an epizootic in Suffolk, 

 and a few cases of bubonic plague in man. 



We therefore have to consider the role of the flea and the role of 

 the vertebrate in plague, and with regard to the latter there does 

 appear to be some such sequence as this : — The enzootic, the epizootic ; 

 and these become in man: — The endemic, the epidemic, the pandemic. 



The fleas we have described in Chapter XXXIV. (p. 857), but it 

 is necessary to say a few words with regard to the rats. 



CLASS MAMMALIA. 



SUBCLASS EUTHERIA. 

 Order Glires Limiceus, 1758. 



Definition. — Eutheria with toes armed with claws. Size usually small or 

 medium. Front teeth chisel-shaped and separated from the grinding teeth 

 by a wide space. 



Classification. — This classification is taken from Swenk : — 



A. Upper front teeth two, both large (suborder Simplicidentata) . 



1. Fur not sprinkled with quills. 



[a) Tail very broad, flat, scaled; hind feet webbed; size large — 

 Castoridcs. 



(&) Tail round or compressed; hind feet not webbed; size small to 

 medium. 



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