EXPERIMENTAL INOCULATION 235 



The general conclusion to be drawn -from these experiments 

 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 sub- 

 cutaneously in the human subject, it gives rise to a local inflam- 

 matory 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. In this con- 

 nection 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 septicemic processes which may be produced artificially 

 in the more susceptible representatives of the lower animals. 



A fact which at first appeared rather to militate against the 

 pneumococcus being the cause of pneumonia was the discovery 

 by Pasteur and others of this organism in the saliva of healthy 

 men. It can certainly be isolated by inoculation of susceptible 

 animals, from the mouths of a considerable proportion of normal 

 men, from their nasal cavities, etc., being probably in any par- 

 ticular individual more numerous at some times (especially, it is 

 stated, during the winter months, i.e., a little before the period 

 of the greatest prevalence of pneumonia) than at others, and 

 sometimes being entirely absent. This may indicate the import- 

 ance of predisposing causes in the etiology of the disease, which 

 applies in the case of the diseases caused by pyogenic staphylo- 

 cocci, streptococci, the bacillus coli, etc. By such causes the 

 vitality and power of resistance of the lung may be diminished, 

 and then the pneumococcus gain an entrance. We can there- 

 fore understand how less definite devitalising agents such as 

 cold, alcoholic excess, etc., can play an important part in the 

 causation of pneumonia. In this way also other abnormal 

 conditions of the respiratory tract, — a slight bronchitis, — etc., 

 may play a similar part. It must be stated, however, that ac- 

 cording to the Rockefeller investigations (infra) the pneumo- 



