BACTERIA IN DISEASE. 173 



Tuberculosis may also be transmitted through the placenta; 

 how frequently is still uncertain. Occasionally the exan- 

 thematous fevers are transmitted from the mother to the fetus. 



The surfaces covered with thick, stratified epithelium are not 

 likely to be penetrated by bacteria except 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 comparative 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 in- 

 fections is checked by the lymph-nodes. That is repeatedly 

 seen in the ordinary post-mortem wound where the spread of 

 the inflammation along the arm is checked suddenly at the el- 

 bow or axilla. The participation of the lymphoid structures 

 in most infections is well known. How far this is a conserva- 

 tive 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 introduced, as in 

 the familiar cases of boils and carbuncles when pyogenic 



