152.The Physiological Stage Although many people think primarily of the outer earwhen we consider hearing, this part functions simply as a reverse megaphone, designed to collect and to bring sound into the internal portions of the ear. The outer ear also plays a role in helping to locate the direction from which sound originates. When sounds, arriving in the form of wave vibrations, are funneled into the auditory canal, a tube-like passage, they reach the eardrumor the tympanic membrane. The eardrum is aptly named because it operates like a miniature drum, vibrating when sound waves hit it. The more intense the sound, the more it vibrates. These vibrations are then transmitted into the middle ear, a tiny chamber containing just three bones called, because of their shapes, the hammer (malleus), the anvil (incus), and the stirrup (stapes). These bones have one function: to transmit vibrations to the oval window, a thin membrane leading to the inner ear. Because of their shape, these three small bones do a particularly effective job. Because they act as a set of levers, not only do they transmit vibrations but also increase their strength. Moreover, because the opening into the middle ear (the ear drum) is considerably larger than the opening out of it (the oval window), the force of sound waves on the oval window becomes amplified. The middle ear, then acts as a tiny mechanical amplifier, making us aware of sounds that would otherwise go unnoticed. The inner ear is a portion of the ear that actually changes the sound vibrations into a form that allows them to be transmitted to the auditory center of the temporal lobe of the brain. It also contains the organs that allow us to locate our position and determine how we are moving through space. When sound enters the inner ear through the oval window, it moves into the cochlea, a coiled tube that looks something like a snail and is filled with fluid. Inside the cochlea is the basil membranethat is covered with hair cells. When the vibrations entering the cochlea bend these hair cells, a neural message is transmitted to the brain. High intensity sounds (decibels beyond 100) do damage to these hair cells.
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