many times greater than that of the windpipe, and this enlargement by retarding the motion of the air favors the lodgment of particles on the surface whence they are removed by the action of the cilia.  The entering air is also brought closely in contact with a moist surface at the narrow opening of the larynx.  That bacteria and other foreign substances can enter the lungs in spite of these guards is shown not only by the infections which take place here, but also by the large amount of black carbon deposited in them from the soot contained in the air.

Infection rarely takes place from the surface of the gullet or oesophagus which leads from the mouth to the stomach.  This is due to the smoothness of the surface and to the rapidity with which food passes over it.  Infection by the stomach also is rare, for this contains a strong acid secretion which destroys many of the bacteria which are taken in with the food.  It is found impossible to infect animals with cholera unless the acidity of the stomach contents be neutralized by an alkali.  Many organisms, although their growth in the stomach is inhibited, are not destroyed there and pass into the intestines, where the conditions for infection are more favorable.  This large and very irregular surface is bathed in fluid which is a good culture medium and but a single layer of cells covers it.  The organisms which cause many of the infectious diseases in both man and animals find entrance by means of the alimentary canal, as cholera, dysentery, typhoid fever, chicken cholera, hog cholera.

Infection by the genito-urinary surface is comparatively rare.  The surface openings are usually closed, and the discharge of urine has a mechanical cleansing effect.  The wide tube of the vagina is further protected by a normal bacterial flora which produces conditions hostile to other and pathogenic bacteria.  The most common infections are the sexual diseases, which are due to organisms which find favorable conditions for growth in and on the surface and which are conveyed from a similar surface by sexual contact.

It remains a question whether bacteria can penetrate an intact surface producing no injury at the point of entrance and be carried by the lymph or blood into internal organs where they produce disease.  Internal infections are often found with seemingly intact body surfaces, but it is impossible to exclude the presence of minute or microscopic surface injuries by which the organisms may have entered.  It is also possible that a slight injury at the point of entrance may heal so completely as to leave no trace.