The function of the mastoid cavity is still unknown. In the present article a theory of its main function is presented. The mastoid cavity is of vital importance in the non-immunological defence of the middle-ear. It is the most important site for the production of secretions that "wash" the middle-ear free of bacteria and viruses. The mucociliary system and the gravitation force, if the body is in erect position, transports the secretions through the two openings of the ear, the aditus and antrum and the ostia of the Eustachian tube, to be expelled into the epipharynx. Based on this theory pathological disorders of the ear are discussed as well as the effect of ventilation tubes on otosalpingitis and recurrent otitis media. Other plausible mastoid cavity functions are also discussed.