EXPERIMENTAL INOCULATION 245 



A fact which at first appeared rather to militate against the 

 pneumococcus being the cause of pneumonia was the discovery 

 of this organism in the saliva of healthy men. This fact was 

 early pointed out by Pasteur, and also by Fraenkel, and the 

 observation has 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 (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 

 can be proved, of course, by inoculation of susceptible animals. 

 Such a fact, however, only indicates the importance of pre- 

 disposing 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 pneumococcus gains an entrance. In relation to 

 this possibility we have the very striking fact that in the 

 irregular forms of pneumonia, secondary to such conditions as 

 typhoid and diphtheria, the pneumococcus is very frequently 

 present, alone or with other organisms. Apparently the effects 

 produced by such bacteria as the b. typhosus and the b. 

 diphtherise can devitalise the lung to such an extent that 

 secondary infection by the pneumococcus is more likely to occur 

 and set up pneumonia. We can therefore understand how 

 much 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 is more difficult to explain why sometimes the pneumo- 

 coccus is associated with a spreading inflammation, as in croupous 

 pneumonia, whilst at other times it is localised to the catarrhal 

 patches in broncho-pneumonia. It is quite likely that in the 

 former condition the organism is possessed of a different order 

 of virulence, though of this we have no direct proof. We have, 

 however, a closely analogous fact in the case of erysipelas ; this 

 disease, we have stated reasons for believing, is produced by 

 a streptococcus which, when less virulent, causes only local 

 inflammatory and suppurative conditions. 



Summary. We may accordingly summarise the facts re- 

 garding the relation of Fraenkel's pneumococcus to the disease 



