Aeration of the tympanomastoid cavity and the Eustachian tube.

Physiological changes in body position affect the degree of opening and closing of human auditory tubes. Notable changes were observed in lateral body position concerning the opening rate of the auditory tubes, which corresponds to the changes in pressure within the lower portion of the middle ear cavity. This physiological mechanism seems to be related with the anatomical direction of the auditory tube and the natural mechanism for the protection of the middle ear cavity. When a change takes place which moves the compliance of the eardrum to the positive side (e.g. head position change or static lateral body position), observation suggests the possibility of venostatis and production of gas. Measurement of the oxygen tension in the middle ear cavity under atmospheric pressure revealed the existence of constant gas production which maintains the pressure in the middle ear cavity at the same level as atmospheric pressure. This result denies the influx of gas from the atmospheric environment into the middle ear cavity. These results stem from the intrinsic physiologic mechanism of the middle ear cavity, namely maintenance of effective auditory sound conduction (a function to keep eardrum compliance as close to zero as possible), by keeping the pressure within the middle ear cavity below the atmospheric pressure, either at the same level as or slightly higher than that of the environment pressure. Gas emission from the middle ear cavity in the atmospheric environment and a low oxygen tension are a natural physiologic mechanism to protect the middle ear cavity from infection and other hazards.