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Columbia University Press
ear
organ of hearing and equilibrium. The human ear consists of outer, middle, and inner parts. The outer ear is the visible portion; it includes the skin-covered flap of cartilage known as the auricle, or pinna, and the opening (auditory canal) leading to the eardrum (tympanic membrane).

The middle ear, separated from the outer ear by the eardrum, contains three small bones, or ossicles. Because of their shapes, these bones are known as the hammer (malleus), anvil (incus), and stirrup (stapes). Air reaches the middle ear through the Eustachian tube, or auditory tube, which connects it to the throat.

The inner ear, or labyrinth, contains the cochlea, which houses the sound-analyzing cells of the ear, and the vestibule, which houses the organs of equilibrium. The cochlea is a coiled, fluid-filled tube divided into the three canals: the vestibular, tympanic, and cochlear canals. The basilar membrane forms a partition between the cochlear canal and the tympanic canal and houses the organ of Corti. Anchored in the Corti structure are some 20,000 hair cells, with filaments varying in length in a manner somewhat analogous to harp strings. These are the sensory hearing cells, connected at their base with the auditory nerve.

The Hearing Process

In the course of hearing, sound waves enter the auditory canal and strike the eardrum, causing it to vibrate. The sound waves are concentrated by passing from a relatively large area (the eardrum) through the ossicles to a relatively small opening leading to the inner ear. Here the stirrup vibrates, setting in motion the fluid of the cochlea. The alternating changes of pressure agitate the basilar membrane on which the organ of Corti rests, moving the hair cells. This movement stimulates the sensory hair cells to send impulses along the auditory nerve to the brain.

It is not known how the brain distinguishes high-pitched from low-pitched sounds. One theory proposes that the sensation of pitch is dependent on which area of the basilar membrane is made to vibrate. How the brain distinguishes between loud and soft sounds is also not understood, though some scientists believe that loudness is determined by the intensity of vibration of the basilar membrane.

In a small portion of normal hearing, sound waves are transmitted directly to the inner ear by causing the bones of the skull to vibrate, i.e., the auditory canal and the middle ear are bypassed. This kind of hearing, called bone conduction, is utilized in compensating for certain kinds of deafness (see deafness; hearing aid), and plays a role in the hearing of extremely loud sounds.

Balance and Orientation

In addition to the structures used for hearing, the inner ear contains the semicircular canals and the utriculus and sacculus, the chief organs of balance and orientation. There are three fluid-filled semicircular canals: two determine vertical body movement such as falling or jumping, while the third determines horizontal movements like rotation. Each canal contains an area at its base, called the ampulla, that houses sensory hair cells. The hair cells project into a thick, gelatinous mass. When the head is moved, the canals move also, but the thick fluid lags behind, and the hair cells are bent by being driven through the relatively stationary fluid. As in the cochlea, the sensory hair cells stimulate nerve impulses to the brain. The sensory hair cells of the saclike utriculus and sacculus project into a gelatinous material that contains lime crystals. When the head is tilted in various positions, the gelatin and crystals exert varying pressure on the sensory cells, which in turn send varying patterns of stimulation to the brain. The utriculus sends indications of the position of the head to the brain and detects stopping and starting.

Disorders of the Ear

One of the most common ear diseases is known as otitis media, a middle ear disorder. Most common among young children, otitis media probably results from Eustachian tubes that are shorter and more horizontal than in adults, allowing infection to spread and preventing fluids in the middle ear from draining. It can bring about permanent hearing loss, although modern medication is generally able to clear up the disease. Other ear diseases include otosclerosis, involving excessive bone growth in the middle ear, and presbycusis, the progressive decay of the inner ear's hearing nerve.