AVENUES OF INFECTION 87 



being a general infection. Owing, however, to the peculiar pathogenic 

 properties of different bacteria and their affinity for the cells of certain 

 tissues, coupled with a peculiar tissue susceptibility for certain bacteria 

 or their products, we find that many diseases have regular avenues of in- 

 fection, and, indeed, in a few instances infection of the human body may 

 be possible only through a particular and definite route. Infections 

 of the gastro-intestinal, respiratory, and genito-urinary tracts and 

 various sinuses with external openings must be considered as being 

 potentially surface infections. The outer layers do not consist merely 

 of the skin and adjacent mucous membranes, but are made up of all 

 layers covering surfaces and channels, which, however indirectly, com- 

 municate with the exterior. In the higher animals there is only one 

 direct channel of communication between the actual interior and the 

 exterior of the body, this being through the Fallopian tube of the female, 

 which normally has so fine a lumen and is so well protected that to all 

 intents and purposes it may be regarded as closed. In certain inflam- 

 matory conditions of the genital organs, and particularly after parturi- 

 tion, the Fallopian tube may be open, and afford a direct route for the 

 transmission of infection from the external parts to the peritoneal 

 cavity. 



Living in and on the actual and potential external surfaces are count- 

 less microorganisms, which are for the most part harmless, a few being, 

 however, actually or potentially dangerous. 



1. The skin and adjacent mucous membranes, particularly in those 

 portions where warmth and moisture abound, are well adapted to bac- 

 terial growth, and their contact with surrounding objects causes a large 

 variety of microorganisms to adhere to them. 



As a result, the bacteriology of the skin is quite complex, since it may 

 lodge microorganisms from the air, from water, and from soil. A group 

 of cocci and diplococci, particularly the Staphylococcus epidermidis 

 albus of Welch, and the various pseudodiphtheria bacilli, are habitually 

 present upon the human skin. When local injury occurs, they may 

 produce minor suppurative lesions, and may be concerned in the pro- 

 duction of certain skin diseases, such as eczema, impetigo contagiosa, 

 the pustules of variola, etc. 



Other microorganisms may find temporary lodgment upon the skin, 

 and are in no sense regular inhabitants. For example, the fingers and 

 hands may become contaminated with colon, typhoid, and tubercle 

 bacilli, pneumococci, etc. 



The skin forms a very important barrier against the entrance of 



