Function of the Middle Ear

Abstract

When the vertebrates began to adapt to terrestrial life it became important for their continued existence that they hear airborne sounds over a wide range of intensities and frequencies. The sense organs which, in their aquatic life, served to sense the vibrations in the surrounding water, responded only to relatively intense sounds in air; most of the sound energy was lost by reflection at the interface between air and the fluid in the organs which were sensitive to vibration because of the great difference in mobility (or impedance) of the two media. In fact, about 99.9% of the energy was lost by reflection. The development of the middle ear mechanism, which began in the amphibians, greatly improved the transmission of energy from air to the fluid of the inner ear by acting as an impedance transformer that matched the high impedance of the fluid to the much lower impedance of the air. The middle ear of the frog is a simple type of transformer which consists of a columella that connects the eardrum with the oval window of the inner ear. The transformer action is brought about because the eardrum has a much larger area than the oval window. During the further development of terrestrial animals, the middle ear took the form as we now know it in mammals. The transformer action of the middle ear is mainly accomplished through the wide difference in the areas of the eardrum and the stapes foot-plate and, to a lesser extent, through the lever action of the ossicular chain.