238 STRONG AND TEAGUE. 



Therefore, from these two series of experiments, we see that 

 70.6 per cent of the vaccinated animals proved immune when 

 exposed to infection by means of spraying virulent plague bacilli 

 into the air which they breathed, and hence were immune to this 

 method of infection. However, it is necessary to examine closely 

 into the nature of this immunity. 



It is seen that of 36 control unvacciTiated guinea pigs, all 

 except one succumbed to the infection induced by spraying. 

 Upon post-mortem examination, the following changes, which 

 applied to practically all of the control animals, were en- 

 countered. In general, there were marked evidences of plague 

 infection about the tissues of the neck and throat. The sub- 

 cutaneous tissues showed extensive oedema, and there was swell- 

 ing of the lymphatic glands of the neck and of those about the 

 trachea. Usually the glands were not only swollen but more 

 or less h^emorrhagic, and had the appearance of small buboes. 

 Throughout the body, marked evidence of septicaemia was 

 usually present. There were frequently extensive haemorrhages 

 in the intestinal wall. The spleen sometimes showed the typical 

 changes encountered in plague infection with miliary abscesses. 

 Pneumonia was present in only about 23 per cent of the control 

 animals. These changes suggest that the primary point of 

 infection evidently was located in the mucous membranes of 

 the throat and that it did not usually occur in the bronchi or 

 alveoli of the lung. From these lesions it would appear that 

 normal guinea pigs, under the conditions of the experiment in 

 which the spraying was carried on, do not usually develop 

 primary plague pneumonia, but that infection occurs through 

 the mucous membranes of the mouth and throat and infection 

 and sometimes buboes of the glands of the neck and septicaemia 

 result. It would appear that in guinea pigs, either on account of 

 too shallow respirations or the small size of the larynx and 

 trachea, the bacteria are not so likely to penetrate to the smaller 

 bronchi by means of the inspired air. Instead, they are appar- 

 ently arrested by the mucous membrane of the throat. Attention 

 must be called to the fact that the spray employed was not so 

 fine a one as that used in the subsequent experiments on monkeys 

 and that the bacteria were, therefore, sprayed in larger particles 

 in the first two series of experiments. Whatever the reason, 

 however, the fact remains that primary pneumonic infection 

 in the guinea pigs did not usually result. Therefore, the con- 

 clusions that can be drawn regarding the vaccinated guinea pigs 



