26 THE NATURE OF INFECTION 



been recognized and is well established. Organisms like the staphy- 

 lococcus and the streptococcus can thus exist on the intact skin with- 

 out giving rise to any disturbance whatever; they are evidently 

 unable to penetrate to the deeper structures through their own 

 efforts. The same manifestly holds good for the epithelial struc- 

 tures of the body in general; staphylococci and streptococci exist in 

 the intestinal tract as on the surface of the body without causing 

 any damage. If, however, the epithelial covering at any point is 

 broken and invasion at the point of injury has preceded, or has 

 occurred at the same time, infection of greater or less extent will of 

 necessity follow. In many instances of infection with the organisms 

 in question the break in the continuity of the epithelial covering 

 may be ever so slight, but it is very doubtful whether infection with 

 these organisms ever occurs through an intact epithelial barrier. 



Various attempts have been made to prove that infection with 

 other organisms can take place in this manner, but the experiments 

 do not carry complete conviction. It has thus been argued that the 

 intact skin must be permeable to an organism like the plague bacillus, 

 because the disease invariably develops in guinea-pigs in which the 

 organism has been rubbed into the shaved abdominal surface. Such 

 experiments demonstrate, of course, that infection with the organism 

 in question may be effected through the skin ; but they do not prove 

 by any means that infection takes place through the intact skin. 

 Through the mere process of rubbing slight injuries are unquestion- 

 ably produced, if not directly, then at least indirectly, for we can 

 readily see that the occlusion of hair follicles and the ducts of sweat 

 glands by little plugs of bacteria constitutes an injury just as well as 

 a visible abrasion of the surface. Such little plugs of bacteria may, 

 on the one hand, act as foreign bodies and mechanically damage the 

 more delicate structures with which they come in contact, or the 

 bacteria as such, through their own secretory or degenerative prod- 

 ucts may cause a local destruction of these more delicate structure 

 and thus open a route to infection of the underlying tissues. This 

 possibility must also be considered in cases where infection takes 

 place apparently directly from the very surface of epithelial linings, 

 but is naturally less likely to play a role in so dense a structure as 

 the surface epithelium of the skin, as in the case of the mucous 

 membranes. 



The infection of the urethral mucosa by the gonococcus is fre- 



