INFECTIOUS DISEASES. 619 



The constant presence of this micrococcus in the buccal secretions of healthy 

 persons indicates that some other factor is required for the development of an at- 

 tack of pneumonia ; and it seems probable that this other factor acts by reducing 

 the vital resisting power of the pulmonary tissues, and thus making them vulner- 

 able to the attacks of the microbe. This supposition enables us to account for the 

 development of the numerous cases of pneumonia which can not be traced to infec- 

 tion from without. The germ being always present, auto-infection is liable to 

 occur when, from alcoholism, sewer-gas poisoning, crowd-poisoning, or any other 

 depressing agency, the vitality of the tissues is reduced below the resisting point. 

 We may suppose, also, that a reflex vaso-motor paralysis, affecting a single lobe of 

 the lung, for example, and induced by exposure to cold, may so reduce the resist- 

 ing power of the pulmonary tissues as to permit this micrococcus to produce its 

 characteristic effects. Again, we may suppose that a person, whose vital resisting 

 power is reduced by any of the causes mentioned, may be attacked by pneumonia 

 from external infection with material containing a pathogenic variety of the 

 micrococcus having a potency, permanent or acquired, greater than that possessed 

 by the same organism in normal buccal secretions." 



Investigations made since the above was written show that 

 this micrococcus does vary greatly in its pathogenic power when 

 obtained from different sources, and that virulent cultures ob- 

 tained from the blood of inoculated animals become attenuated 

 when they are kept for a short time. This, indeed, is a general rule 

 as regards the best-known pathogenic bacteria; which usually 

 acquire increased virulence when cultivated in the bodies of sus- 

 ceptible animals, and become attenuated as regards their patho- 

 genic potency when they are cultivated for a certain length of 

 time in artificial media. My own experiments with pneumonic 

 sputum were made in January, 1885, and led me to the identifica- 

 tion of the oval coccus, commonly in pairs, which is found in 

 this material, with the coccus which I had previously found in my 

 own saliva (September, 1880), and which was subsequently the ob- 

 ject of extended experimental researches made by me in 1881-1884. 



In my paper read before the Pathological Society of Philadel- 

 phia, in April, 1885, 1 say : " It seems extremely probable that this 

 micrococcus is concerned in the etiology of croupous pneumonia. 

 . . . But this can not be considered as definitely established by 

 the experiments which have thus far been made upon the lower 

 animals." The extended researches of Frankel, Weichselbaum, 

 Netter, Gamale'ia, G. and F. Klemperer, and others, which have 

 been published since, have fully established the etiological role 

 of the micrococcus in question. 



In 1882 Fehleisen isolated the so-called streptococcus of ery- 

 sipelas and proved by experiment that it is the etiological agent 

 in the production of erysipelatous inflammations. At a later date 

 (1884) Rosenbach isolated the micro-organisms commonly con- 

 cerned in traumatic infections and in the production of acute 

 abscesses. Among these was a streptococcus, called by him Strep- 



