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 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