Sound Transmission Through The Middle And Inner Ears

Sound waves enter the auditory canal after being funneled and reflected by components of the external ear, or pinna (Fig. 2). The canal extends 2.5 cm into the skull and ends at the tympanic membrane (eardrum), which forms the boundary between the outer and middle ears. The middle ear is an air-filled cavity containing three small bones, the ossicles, which are connected in series to the tympanic membrane. Sound waves in the auditory canal cause the tympanic membrane to vibrate and thus transmit the vibrations to the ossicles. The first ossicle, the malleus ("hammer"), forms a connection between the tympanic membrane and the incus ("anvil"). The incus forms a flexible connection with the stapes ("stirrup"), which is attached to the membrane of the oval window separating the middle ear from the fluid-filled inner ear. The air in the middle ear is normally self contained, held there by a valve in the eustachian tube connecting the middle ear to the mouth cavity. When the air pressure outside the middle ear is suddenly decreased, as during takeoff in an airplane, the higher air pressure in the middle ear presses (sometimes painfully) against the tympanic and oval window membranes. Temporarily opening the valve by yawning or stretching the jaw allows the pressure to equalize and the pain to subside.

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