SUSCEPTIBILITY AND IMMUNITY. 241 



the thigh after they had been bruised by mechanical violence. 

 Abarrin and Roger found that white rats, which are not susceptible 

 to anthrax, became infected and frequently died if they were ex- 

 hausted, previous to inoculation, by being compelled to turn a revolv- 

 ing wheel for a considerable time. Pasteur found that fowls, which 

 have a natural immunity against anthrax, become infected and 

 perish if they are subjected to artificial refrigeration after inocula- 

 tion. This has been confirmed by the more recent experiments of 

 Wagner (1891). According to Canalis and Morpurgo, pigeons 

 which are enfeebled by inanition eaily contract anthrax as a result 

 of inoculation. Arloing states that sheep which have been freely 

 bled contract anthrax more easily than others ; and Serafini found 

 that when dogs were freely bled the bacillus of Friedlander, injected 

 into the trachea or the pleural cavity, entered, and apparently mul- 

 tiplied to some extent in the blood, whereas without such previous 

 bleeding they were not to be found in the circulating fluid. Certain 

 anesthetic agents have been shown also to produce a similar result. 

 Platania communicated anthrax to immune animals dogs, frogs, 

 pigeons by bringing them under the influence of curare, chloral, or 

 alcohol; and Wagner obtained similar results in his experiments 

 upon pigeons to which he had administered chloral. In man, clini- 

 cal experience shows that those who are addicted to the excessive use 

 of alcohol are especially liable to contract certain infectious diseases 

 pneumonia, erysipelas, yellow fever, etc. 



The micrococcus of pneumonia is habitually present in the sali- 

 vary secretions of many healthy individuals, and it is evident that 

 an attack of pneumonia does not depend alone upon the presence of 

 this micrococcus, which has, nevertheless, been conclusively shown 

 to be the usual infectious agent in cases of croupous pneumonia. No 

 doubt the introduction of the pathogenic micrococcus to the vulner- 

 able point the lungs is an essential factor in the development of a 

 case of pneumonia, but there is reason to believe that there are other 

 factors equally essential. Thus it is well known that an attack of 

 pneumonia often results from exposure to cold, which may act as an 

 exciting cause ; and, also, that a recent attack of an acute febrile 

 disease especially measles constitutes a predisposing cause. It is 

 generally recognized that malnutrition, want of exercise, insanitary 

 surroundings, and continued respiration of an atmosphere loaded 

 with dust, as in cotton mills, or a recent attack of pneumonia, con- 

 stitute predisposing causes to tubercular infection by way of the 

 lungs. 



While natural immunity may be overcome by the various depress- 

 ing agencies referred to, it is also true that it has only a relative 

 value in the absence of these predisposing causes, and may be over- 

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