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Page 33
As has been said, infection consists in the injury of the body by
living organisms which enter it. The body is in relation to the
external world by its surfaces only, and organisms must enter it by
some one of these surfaces. It is true that the bacteria in the
intestine--either those normally present or unusual varieties--may,
under certain circumstances, produce substances which are injurious
when absorbed; but this is not infection, and is analogous to any
other sort of poisoning. Each surface of the body has its own
bacterial flora. Organisms live on the surface either on matter which
is secreted by the surface or they use up an inappreciable amount of
body material. Many of these bacteria are harmless, some are
protective, producing by their growth such changes in the surface
fluids that these become hostile to the existence of other and
pathogenic forms. The surfaces also frequently harbor pathogenic
organisms which await some condition to arise which will permit them
to effect entrance into the tissues.
The surfaces of the body protect from invasion to a greater or less
degree. The skin protects by the impervious horny layer on the
outside, the external cells of which are dead and constantly being
thrown off. Bacteria are always found on and in this layer, but the
conditions for growth here are not very favorable and the surface is
constantly cleansed by desquamation. The new cells to supply the loss
are produced in the deepest layer of the epidermis, and the movement
of cells and fluids takes place from within outwards. The protection
is less perfect about the hairs and the sweat glands. Infection by the
route of the sweat glands is, however, uncommon, for the sweat is a
fluid unfavorable for bacterial growth and the flow acts mechanically
in washing away organisms which may have entered the ducts. Infection
by the route of the hair follicles is common. There is no mechanical
cleansing as by the sweat, the space around the hair is large and the
accumulated secretion of the hair glands and the desquamated cells
furnish a material in which bacteria may grow. Growing as a mass in
this situation, they may produce sufficient toxic material to destroy
adjacent living cells and thus effect entrance. Infection from the eye
is not common, the surface, though moist, is smooth; the eyelashes
around the margin of the lids give some mechanical protection from the
entrance of bacteria contained in dust, and the movements of the lids
and the constant and easily accelerated secretion of tears act
mechanically in removing foreign substances. It is possible that the
mechanical cleansing of the skin by the daily bath may have some
action in preventing 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 d�bris 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. 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.
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