PROTECTIVE INOCULATION. 231 



should be tried in selected communities, under rigorous scientific 

 conditions. 



At the time the Conference was in session, there was no exper- 

 imental evidence whatever as to the protection afforded by 

 prophylactic inoculations against pneumonic plague, and no 

 experiments of this nature in animals, other than the ones which 

 will be discussed in this paper, hitherto have been reported. The 

 reason for this appears to be obvious from the fact that there 

 has been no other great epidemic of pneumonic plague within 

 modern times, and the question of prophylactic inoculation in 

 man, as a practical means of protection against this disease, 

 during an epidemic, has not hitherto arisen. We, therefore, de- 

 termined to investigate this question experimentally. The as- 

 sumption that, because prophylactic inoculations furnish some 

 degree of protection against bubonic plague they also would be 

 protective against pneumonic-plague infection hardly would seem 

 warranted, since the portal of entry of the infecting agent is so 

 different in the two conditions. Moreover, the plague organism 

 finds in the pulmonary tissues a much more favorable and exten- 

 sive medium for its multiplication and diffusion than it does in 

 the lymphatic glands. In bubonic plague, the lymphatic glands 

 may be said to act as filters against the general invasion of the 

 body by the plague bacillus, while in primary pneumonic plague 

 there is no such mechanism for the defense of the host, the 

 bacilli spreading rapidly throughout the lung and invading the 

 circulation in every instance in a comparatively short time and 

 apparently before the host has had time to produce any ap- 

 preciable quantity of immune substances. The bronchial lym- 

 phatic glands in primary pneumonic plague offer resistance to 

 the invasion of the plague bacillus, and in every case of this 

 disease these glands are very acutely inflamed and frequently 

 almost of a black color from the resulting toxic haemorrhages in 

 the glandular substance. However, by the time the bronchial 

 glands have become involved, the bacteria have already spread 

 so extensively throughout the lung substance that a bactersemia 

 has usually occurred. 



The infection or immunity of the host, for certain bacteria, 

 sometimes depends solely upon the portal of entry of the or- 

 ganism; for example, the same quantity of cholera vibrios, in- 

 jected subcutaneously in man or administered by the mouth, 

 may produce entirely different results. In the former instance, 

 a local and general reaction is obtained, but the bacteria quickly 

 die, while in the latter instance, the vibrios may pass to and 

 multiply in the intestines and Asiatic cholera result. 



