The ear is divided into the outer, middle, and inner parts. The outer ear consists of the auricle and the auditory canal. The middle ear, a small chamber lying within the temporal bone and separated from the outer ear by the eardrum (tympanic membrane), houses the ossicles (three tiny bones bearing the names malleus, incus, and stapes) and communicates with the upper throat via the eustachian tube.

The inner ear consists of the outer osseous labyrinth enclosing the inner membranous labyrinth, whose parts are the cochle, semicircular canals, utricle, and saccule. A watery fluid, the perilymph, fills the space between the osseous and membranous labyrinths, and another watery fluid, the endolymph, fills the membranous labyrinth. The organ of Corti, the actual receptor of sound, is situated within the cochlea.

Sound waves striking the eardrum cause that structure to vibrate; this, in turn, causes the malleus, incus, and stapes to vibrate, in that order. Since it is attached to the membranous oval window at the beginning of the inner ear, the vibrating stapes disturbs the perilymph and endolymph sufficienty to stimulate the organ of Corti. The impulses thus generated make their way to the brain's auditory center via the auditory branch of the acoustic nerve and are interpreted as sound. To ensure free movement of the eardrum, the air pressure on its inner and outer sides is equalized- via the eustachian tube.

Balance, or equilibrium, is mantained through the combined operation of the semicircular canals, the utricle, and the saccule. In brief, the three canals possess hairlike receptors that initiate a nerve impulse in response to the movement of the endolymph. Because the canals lie in three different planes, any particular movement causes more stimulation in one canal than in the others, and the resulting nerve impulse (carried by the vestibular branch of the acoustic nerve) is sensed and acted on in the cerebellum. The receptors of the utricle and saccule, triggered mainly by the position of the head, likewise transmit impulses to the cerebellum.