OBSTACLES TO INFECTION 25 



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 microorganisms 

 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 in this 

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

 and pharyngeal and postnasal lymphadenoid structures, etc., can 

 hardly be over estimated. 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 cer- 

 tain 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 



