BACTERIA IN DISEASE. 151 



The surfaces covered with thick, stratified epithelium are 

 not likely to be penetrated by bacteria excepting by direct 

 introduction through some wound or other lesion. This, for 

 instance, is true of the skin, the mouth, the vagina and bladder. 

 The infection of bubonic plague appears to be introduced most 

 often by means of wounds in the skin. Bacteria more easily 

 penetrate surfaces having a thin, columnar epithelium such as 

 occurs in the intestines, the middle ear, bronchi and bronchial 

 tubes, uterus and Fallopian tubes. 



The thin, flat epithelial cells of the air-vesicles of the 

 lungs, as would be expected, seem to be passed with com- 

 parative ease. On epithelial surfaces covered with cilia, as 

 in the bronchi and bronchial tubes, the Eustachian tubes, the 

 uterus and Fallopian tubes, the current toward the exterior 

 created by the cilia acts beneficially in removing bacteria. 



The tonsils and lymph-follicles of the intestines, especially 

 the lymphoid tissue of the ileum and the vermiform appendix, 

 are points where bacterial invasion frequently begins. The 

 lymphoid tissue of the appendix may have some influence in 

 predisposing to infection at that point and to appendicitis. On 

 the other hand, it is certain that the progress of many infections 

 is checked by the lymph-nodes. That is repeatedly seen in the 

 ordinary post-mortem wound where the spread of the inflam- 

 mation along the arm is checked suddenly at the elbow of axilla. 

 The participation of the lymphoid structures in most infections 

 is well known. How far this is a conservative process it is 

 impossible to say. 



In most cases of infectious disease a point of entrance 

 for the bacteria may be discovered. As a rule, the invading 

 microbes produce a lesion at the point where they are intro- 

 duced, as in the familiar cases of boils and carbuncles when 

 pyogenic bacteria enter the skin, or of the tubercles found 

 in the lungs when the bacilli lodge in the respiratory tract. 

 However, there are cases of septicemia and pyemia in which 



