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.