CHAPTEE III. 



INFECTION ATRIA AND THE EXCRETION OF MICRO- 

 ORGANISMS. 



Infection Atria. 



The infection atrium is the primary point of 

 invasion by micro-organisms, or the point or tissue 

 or surface through which they reach internal 

 structures. 



In general, micro-organisms may enter the body 

 through any of its surfaces, except, of course, the 

 serous coverings. Anatomical structure, however, 

 renders the skin, and other surfaces which are 

 clothed with pavement epithelium, quite resistant 

 to penetration, in the absence of wounds. Leav- 

 ing wound infection out of consideration the mu- 

 cous surfaces afford the most frequent points of 

 entrance. 



Certain micro-organisms appear to have a predi- preferred or 

 lection for particular tissues, preferring one point 

 of entrance or primary involvement above all 

 others. Thus, in so far as we know, the typhoid 

 bacillus and the cholera vibrio always produce 

 their primary infection in the intestines, although 

 a hematogenous typhoid is sometimes spoken of, 

 the true atrium escaping detection. The diph- 

 theria bacillus habitually makes its attack in the 

 upper respiratory passages. It is probable that 

 measles and scarlet fever utilize the respiratory 

 tract for the point of invasion, although this can- 

 not be determined positively at present. As will 

 be described later, many microbes have a predilec- 



