EXPERIMENTAL INOCULATION. 217 



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. An 

 analogous result is also obtained when, instead of taking 

 animals of different susceptibility, the same species of 

 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 

 septicaemia 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 agent 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- 

 ducer of general septicaemia in animals, we have seen, finds 

 a perfectly rational explanation in the different degrees 

 of susceptibility which exists towards it in different species. 

 In this connection the occurrence of manifestations of 

 general infection associated with pneumonia in man is 

 of the highest importance. We have seen that meningitis 

 and other inflammations are not very rare complications of 

 the disease, and such cases form a link connecting the 

 local disease in the human subject with the general 

 septicaemic processes which may be produced artificially 



