EXPERIMENTAL INOCULATION. 217 



The occurrence in the lung of inflammatory conditions due 

 to other causes does not make it less likely that the great major- 

 ity of cases of acute pneumonia which occur under natural con- 

 ditions have as the causal agent the pneumococcus. For in the 

 latter we have an organism with certain very definite micro- 

 scopic and biological characters, which is certainly present in 

 the great majority of, if not in all, cases of the disease. Its 

 action as a producer of general septicaemia in animals, we have 

 seen, finds a perfectly rational explanation in the different 

 degrees of susceptibility which exist 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 septicasmic processes which may 

 be produced artificially in the more susceptible representatives 

 of the lower animals. 



A fact which has, in the minds of some, rather militated 

 against the pneumococcus being the cause of pneumonia, is the 

 discovery of this organism in the saliva of healthy men. This 

 fact was early pointed out by Pasteur, Sternberg, and also by 

 Fraenkel, and their observations have been confirmed by many 

 other observers. It can certainly be isolated from the mouths 

 of a considerable proportion of normal men, from their nasal 

 cavities, etc., being probably in any particular individual more 

 numerous at some times than at others, and sometimes being 

 entirely absent. This can be proved, of course, by inoculation 

 of susceptible animals. Such a fact, however, does not neces- 

 sarily imply that the pneumococcus is not the cause of pneu- 

 monia. It only indicates the importance of predisposing causes 

 in the etiology of the disease, and it is further to be observed 

 that .we have corresponding facts in the case of the diseases 

 caused by pyogenic staphylococci, streptococci, the bacillus coli, 

 etc. It is probable that by various causes the vitality and power 

 of resistance of the lung are diminished, and that then the pneu- 

 mococcus gains an entrance. In relation to this possibility we 

 have the very striking facts that in the irregular forms of pneu- 

 monia, secondary to such conditions as typhoid and diphtheria, 

 the pneumococcus is very frequently present, alone or with 



