ENTRANCE AND SPREAD OF BACTERIA. 161 



Mode of Entrance and Spread. Many of the organisms 

 of suppuration have a wide distribution in nature, and many 

 also are present on the skin and mucous membranes of 

 healthy individuals. Staphylococci are commonly present 

 on the skin and also occur in the throat and other parts, 

 and streptococci have a similar distribution and can very 

 often be cultivated from the secretions of the mouth in 

 normal conditions. The pneumococcus of Fraenkel and the 

 pneumobacillus of Friedlander have also been found in the 

 mouth and in the nasal cavity in normal conditions, whilst 

 the bacillus coli communis is a normal inhabitant of the 

 intestinal tract. The entrance of these organisms into the 

 deeper tissues when a surface lesion occurs can be readily 

 understood. Their action will, of course, be favoured by 

 any depressed condition of vitality, though the conditions 

 by which this takes place are not yet fully understood. 

 Having gained entrance to the lymphatic spaces of the 

 tissue they may spread by the lymphatic channels, or may 

 gain entrance to the blood directly through the capillary 

 walls. Though in normal conditions the blood is bacterium- 

 free, we must suppose that from time to time a certain 

 number of such organisms gain entrance to it from trifling 

 lesions of the skin or mucous surfaces, the possibilities of 

 entrance from the latter being especially numerous. In 

 most cases they are killed by the action of the healthy 

 serum or cells of the body, and no harm results. It may 

 again be mentioned in this connection that it has been 

 proved experimentally that, even in the case of a pathogenic 

 organism, a considerable number can be destroyed in the 

 blood of a healthy animal If, however, there be a local 

 weakness, they may settle in that part and produce 

 suppuration, and thence other parts of the body may be 

 infected. Such a supposition as this is necessary to explain 

 many cases of suppuration met with clinically. Thus the 

 liability of diseased heart -valves to become infected by 

 organisms and thence to assume an ulcerative character, is 

 well known. Conditions such as suppurative periostitis 

 and osteomyelitis, multiple suppurative arthritis, suppurative 

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