24 THE NATURE OF INFECTION 



motility of the organ, however, some of the microorganisms which 

 have been swallowed may readily escape destruction, and on 

 entering the intestinal canal, with its alkaline reaction and numerous 

 nooks and crevices, find suitable conditions for active growth, food 

 material being present in abundance. The main danger to an invader 

 would then evidently come from the numerous saprophytic organisms 

 which have their normal habitat in the very domain in which the 

 newly introduced organism is a stranger. As such it might readily 

 be destroyed or overgrown by the others. If introduced in suffi- 

 ciently large number, however, the invader could unquestionably 

 maintain itself, for a while at least, and actually become a source of 

 danger, but be destroyed in the end by the normal inhabitants of 

 the bowel. On the other hand, the organism might adjust itself to 

 its new environment, lose its dangerous properties in a measure, and 

 continue to exist without harm to the host. This is probably true 

 of a number of the inhabitants of the bowel which we look upon 

 as normal, such as the colon bacillus, certain streptococci, staphylo- 

 cocci, and others. 



Whether or not microorganisms would find the mouth and nasal 

 passages a favorable place for growth under perfectly normal con- 

 ditions might very well be questioned. The normal secretions 

 which find their way into the mouth and nares are undoubtedly 

 possessed of germicidal properties, which are feeble to be sure, 

 but nevertheless existent, and it is doubtful whether micro- 

 organisms could maintain themselves and multiply in those parts 

 which are well irrigated by these secretions. In man, however, 

 there are many nooks and corners where bacteria may lodge and 

 escape the action of the salivary and nasal secretion. The importance 

 of carious teeth, alveolar disease, the crypts of the tonsils, and 

 pharyngeal and postnasal lymphadenoid structures, etc., can hardly 

 be overestimated. Such districts are notorious breeding places of 

 microorganisms, and recognized portals of infection. But, after all, 

 while fully realizing that infection is more apt to occur from certain 

 areas than from others, the question still remains unanswered, why 

 is it that invasion is not invariably followed by infection? 



Obstacles to Infection. The strongest general obstacle to infection 

 no doubt lies in the mechanical integrity of the epithelial lining of 

 the surface of the body and its cavities and ducts, which are in 

 direct or indirect communication with the exterior. This has long 



