40 ESSAYS ON BACTERIOLOGY. 



organ invaded and upon the extent of tlie morbid 

 process. Another bacterium may eventually play a 

 similar role and give rise to pneumonia, but, as a rule, 

 it is certainly Froentel's diplococcus that displays here 

 its energy, for which reason it may properly be re- 

 garded as the real micro-organism of genuine croup- 

 ous lung inflammation. 



But how can this view be harmonized with the cir- 

 cumstance that this micro-organism is also a frequent 

 guest in the healthy body; that in the majority of all 

 persons it is domesticated in the mouth, whence it 

 might easily and at all times undertake an excursion 

 into other regions and thus soon produce meningitis, 

 otitis, or something else? Does the "sword of Damo- 

 cles" actually hang at every moment so close to our 

 heads, and must it not appear almost miraculous that 

 anybody at all is spared by this terrible foe? We can 

 account for this certainly very striking fact only by 

 the circumstance that it requires, as a rule, certain 

 preparatory, as yet unknown, factors to enable this 

 bacterium to make its attacks. The healthy cover- 

 ings and tissues of the body resist the micro-organ- 

 isms; only when the native powers of resistance are 

 weakened or neutralized do the foreign invaders take 

 a firm footing and begin their pernicious activity. All 

 the minute gradations of their infectious power (ob- 

 served even under the conditions of their natural ap- 

 pearance, and surely strong enough to determine the 



