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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