the act of perceiving sound

Man hears sounds when any vibrating object pushes air molecules at a rate between 15 and 15,000 vibrations per second. The waves enter the ear canal and strike the eardrum. The drum's vibrations travel through the middle ear, along a three-bone lever which triples the pressure of the vibrations. These bones pass it to a membrane, the oval window, behind the third bone. The window sends waves through the fluid of the snail-shaped cochlea, agitating hairs which transmit messages along the auditory nerve to the brain. There the stimulation at the end of the chain is heard as sound of a certain pitch and volume.

The sensitive parts of the ear are inside the skull. The fragile eardrum is protected by a muscle, the tensor tympani, which dampens dangerously loud noises, and by the Eustachian tube, an opening to the nose and throat which equalizes the pressure on both sides of the drum. The semicircular canals are nonauditory organs which control balance.

Sound waves are turned into nerve impulses inside the cochlea. This tube is about 1¼ inches long in adult humans. When sound waves move through the perilymph, they move the fibers of the basilar membrane, which lies within the cochlea. These fibers (there are 24,000 of them) cause a vibration in the hair cells, or specialized sense cells, that make up the organ of Corti. The vibration of the hair cells stimulates nerves attached to them. These nerves send messages through the auditory nerve to the temporal lobe, the center of hearing in the brain, where the sounds are classified and interpreted.

The cochlea normally looks like a snail's shell, as shown by the dotted lines, but it has been unwound to show the membrane that extends almost to the tip of the tube.

Other vertebrates have inner ears similar in structure to the human ear. All vertebrates have three semicircular canals, which enable the animal to keep its balance and to detect movement in different directions. Amphibians, birds, and mammals have a cochlea, but fish lack this structure.

PRINT SOURCE
Alan E. Nourse The Body New York: Time Incorporated, 1964

Questions or comments about this page?

This page was last updated on 04/14/2017.