t 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