442 PLAGUE. 



material on the conjunctiva or mucous membrane of the nose, 

 and this method of inoculation has been successfully applied 

 in cases where the plague bacilli are present along with other 

 virulent organisms, e.g. in sputum along with pneumococci. 

 Rats and mice can also be infected by feeding either with 

 pure cultures or with pieces of organs from cases of the disease, 

 though in this case infection probably takes place through the 

 mucous membrane of the mouth and adjacent parts, and only 

 to a limited extent, if at all, by the alimentary canal. Monkeys 

 also are highly susceptible to infection, and it has been shown 

 in the case of these animals that, when inoculation is made on 

 the skin surface, for example, by means of a spine charged with 

 the bacillus, the glands in relation to the part may show the 

 characteristic lesion and a fatal result may follow without there 

 being any noticeable lesion at the primary seat. This fact throws 

 important light on infection by the skin in the human subject. 

 The disease may also extensively affect monkeys by natural 

 means during an epidemic. 



Paths and Mode of Infection. — It is now well established that 

 in the great majority of cases plague bacilli enter the system by 

 the skin surface through small wounds, cracks, abrasions, etc., 

 and that there is usually no reaction at the site of entrance. 

 This last fact is in accordance with what has been stated above 

 with regard to experiments on monkeys. The path of infection 

 is shown by the primary buboes, which are usually in the glands 

 through which the skin is drained, those in the groin being the 

 commonest site. Absolute proof of infection by the skin is sup- 

 pHed by several cases in which the disease has been acquired 

 at post-mortem examinations, the lesions of the skin surface 

 being in the majority of these of trifling nature ; in only two 

 was there local reaction at the site of inoculation. In most of 

 these cases the period of incubation has been from two to three 

 days ; under natural conditions of infection the Indian Commis- 

 sion places the average period within five days. In a small 

 proportion of cases infection takes place through the mucous 

 membrane of the nose and mouth, and exceptionally of other 

 parts ; it is still considered doubtful whether the alimentary 

 canal is a path of infection. In primary plague pneumonia, 

 from a consideration of the anatomical changes and the clinical 

 facts, the disease may be said to be produced by the direct pas- 



