http://www.ck12.org Chapter 20. Sound
FIGURE 20.7
The three main parts of the ear have
different functions in hearing.- The eardrum is like the head of a drum. It’s a thin membrane stretched tight across the end of the ear canal.
The eardrum vibrates when sound waves strike it, and it sends the vibrations on to the middle ear.
Middle Ear
The middle ear contains three tiny bones (ossicles) called the hammer, anvil, and stirrup. If you look at these bones
inFigure20.7, you might notice that they resemble the objects for which they are named. The three bones transmit
vibrations from the eardrum to the inner ear. They also amplify the vibrations. The arrangement of the three bones
allows them to work together as a lever that increases the amplitude of the waves as they pass to the inner ear.
Inner Ear
The stirrup passes the amplified sound waves to the inner ear through the oval window (seeFigure20.7). When the
oval window vibrates, it causes the cochlea to vibrate as well. The cochlea is a shell-like structure that is full of fluid
and lined with nerve cells called hair cells. Each hair cell has tiny hair-like projections, as you can see inFigure
20.8. When the cochlea vibrates, it causes waves in the fluid inside. The waves bend the "hairs" on the hair cells,
and this triggers electrical impulses. The electrical impulses travel to the brain through nerves. Only after the nerve
impulses reach the brain do we hear the sound.
Hearing Loss
All these structures of the ear must work well for normal hearing. Damage to any of them, through illness or injury,
may cause hearing loss. Total hearing loss is called deafness. To learn more about hearing loss, watch the animation
at this URL: http://www.youtube.com/watch?v=YpIptQSEEjY (1:39).