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.