Nervous System

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2.38   Hearing – The external ear funnels sounds through the ear canal to the eardrum. On the other side of the eardrum is an air-filled cavity called the middle ear. This part of the ear opens to the back of the throat through a tube called the eustacian tube. As sound waves apply pressure to the eardrum, the eardrum vibrates. Three bones located in the middle ear – the hammer, the anvil, and the stirrup – transfer this vibration to a fluid-filled chamber, the cochlea. Inside are tiny hair cells that serve as sound receptors. They respond to the vibrations and send impulses to the brain, which interprets the vibrations as sound.

2.39   Equilibrium – The two other portions of the inner ear, the semicircular canals and the vestibule, are involved with the sense of equilibrium. The semicircular canals detect movement of the head. The vestibule contains sensory cells that respond to changes in head position with respect to gravity. It is the inner ear that contains the sense cells, which enable you to hear, and that produces your sense of equilibrium, or balance and stability in space.

2.40   Hearing loss – Certain conditions, diseases, and types of trauma can result in hearing loss. For example, a build up of wax in the ear or an ear infection can create a barrier to sound waves entering the ear, preventing vibration of the ear drum and the inner-ear bones. Repeated inner-ear infections can result in scarring of the eardrum and damage to the bones of the inner ear. If this occurred, sound waves would not be conducted as well and hearing would be diminished. This type of hearing loss is referred to as conductive deafness.