198 ACUTE PNEUMONIA. 



The general conclusion to be drawn from these experi- 

 ments thus is that in highly susceptible animals virulent 

 pneumococci produce a general septicaemia, whereas in more 

 immune species there is an acute local reaction at the point 

 of inoculation, and if the latter be in the lung, then there may 

 result pneumonia, which, of course, is merely a local acute 

 inflammation occurring in a special tissue, but identical in 

 essential pathology with an inflammatory reaction in any 

 other part of the body. When a dose of pneumococci 

 sufficient to kill a rabbit is injected subcutaneously in the 

 human subject, it gives rise to a local inflammatory swelling, 

 with redness and slight rise of temperature, all of which 

 pass off in a few days. It is therefore justifiable to suppose 

 that man occupies an intermediate place in the scale of 

 susceptibility, probably between the dog and the sheep, and 

 that when the pneumococcus gains an entrance to his lungs, 

 the local reaction in the form of pneumonia occurs. 



Analogies to the facts just stated are afforded in the case 

 of other diseases caused by bacteria. Thus, for example, 

 the anthrax bacillus produces in the human subject more 

 marked inflammatory reaction, and is more restricted to the 

 local lesions, than in the much more susceptible guinea- 

 pig, in which it produces a rapidly fatal septicaemia and 

 occurs in enormous numbers throughout the blood. An 

 analogous result is also obtained when, instead of taking 

 animals of different susceptibility, the same species cf 

 animal is used, but the virulence of the organism is altered ; 

 for example, a streptococcus, as already stated, producing at 

 one time an erysipelatous condition, causes an acute septi- 

 caemia when its virulence is increased. 



The occurrence in the lung of inflammatory conditions 

 due to other causes does not make it less likely that the 

 great majority of cases of acute pneumonia which occur 

 under natural conditions have as the causal irritant the 

 pneumococcus. For in the latter we have an organism 

 with certain very definite microscopic and biological 

 characters, which is certainly present in the great majority 

 of, if not in all, cases of the disease. Its action as a pro- 



