into and from the lungs are covered with cilia; the surface area of
these tubes constantly enlarges as they branch, the sum of the
diameters of the small tubes being 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