infection.

The internal surfaces are much more exposed to attack and the
protection is not so efficient. The moisture of these surfaces is both
a protection and a source of danger. It protects by favoring the
lodgment near the orifices of organisms which are in the inspired air,
for when bacteria touch a moist surface they cannot be raised from
this and carried further by air currents. The moisture is a source of
danger in that it favors the growth of bacteria which lodge on the
surface. The respiratory surface which is most exposed to infection
from the air is further protected by the cilia, which are fine
hair-like processes covering the cells of the surface and which by
their constant motion sweep out fine particles of all sorts which
lodge upon them. The cavity of the mouth harbors large numbers of
organisms, many of them pathogenic. It forms a depot from which
bacteria may pass to communicating surfaces and infection from these
may result. Food particles collect in the mouth and provide culture
material, and there are many crypts and irregularities of surface
which oppose mechanical cleaning. Infection of the middle ear, the
most common cause of deafness, takes place by means of the Eustachian
tube which connects the cavity of the ear with the mouth. Organisms
from the mouth can extend into the various large salivary glands by
means of the ducts and give rise to infections. The tonsils,
particularly in children, provide a favorable surface for infection.
The mucous surface extends into these forming deep pockets lined with
very thin epithelium, and in these debris of all sorts accumulates and
provides material favorable for bacterial growth.

The lungs at first sight seem to offer the most favorable surface for
infection. The surface, ninety-seven square yards, is enormous; it is
moist, the epithelial covering is so thin as to give practically no
mechanical protection, large amounts of air constantly pass in and
out, and the surface is in contact with this. They are protected from
infection in many ways. The tubes or bronchi by which the air passes
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. T