[The fixation theory of middle ear muscle function].

The still unidentified function of the middle ear muscles might be explained by the fixation theory. However, this idea, which is being favoured nowadays, namely, that the action of the muscles controls the position of the ossicles for optimal transmission, has never been investigated experimentally except for a few studies 50 and 100 years ago. In 25 temporal bone preparations, the air pressure-induced movements of the ossicles were microscopically measured, at first without and then with a 10 g. load on the tensor tympani muscle and a 5 g. load on the m. stapedius. The fixation hypothesis could not be confirmed, since with increasing pressure the movement-reducing effect of the middle ear muscle pull decreased. This inability of the muscles to compensate higher static air pressures is also demonstrated theoretically. A 10 g. pull of the tensor tympani muscle can only withstand a suction of 30 mm H2O in the ear canal. Hence, further evaluations are discussed that seem to enforce our hypothesis of the joint-preserving function of the middle ear muscles. The required antagonism of the pull of the muscle is accomplished by the change of the direction of movement in the gliding incudo-malleal joint.