Hearing

The sense of hearing is the ability to detect the mechanical vibrations
we call sound

   Sound waves

  • pass down the auditory
         canal     of the outer ear

  • strike the eardrum (tympanic membrane) causing it to
         vibrate

  • these vibrations are transmitted across the middle ear by three tiny linked bones, the ossicles:

  •  hammer (malleus)   anvil (incus)  stirrup (stapes)
     

  • The ossicles also magnify the amplitude of the
         vibrations.

anatomy of the ear ' ear anatomy drawing sketch image illustration, Human Bodyg

 

 

The middle ear is filled with air
and is connected to the outside air by the eustachian
tube, which opens into the nasopharynx. Opening of the tube - during
swallowing or yawning - equalizes the air pressure on either side of the
eardrum.   Allergies or a head cold may
inflame the walls of the eustachian tubes making them less easily opened. Rapid
changes in pressure at such times - such as descending in an aircraft or during
a SCUBA dive, may be quite painful because of the unequal pressure against the
eardrums.

 

The Inner Ear

Vibrations
of the innermost ossicle, the stirrup, are transmitted through a flexible membrane, the oval window to the cochlea of the inner ear.
The cochlea is a tube, about 3.5 cm long, that is coiled like a snail shell and filled with a special fluid called endolymph. Running through the cochlea for its entire length is a plate of bone and an inner tube that is also filled with endolymph.

Running through the cochlea for its entire length is a plate of bone and
an inner tube that is also filled with endolymph.

The organ of Corti lies within the middle chamber of the cochlea. It
contains thousands of hair
cells, which are the actual vibration receptors These impulses travel back along
the auditory nerve (the 8th cranial nerve) to the brain.

 

Many people, especially when
young, can hear sounds with frequencies (pitches) from as low as 16 to as high
as 20,000 hertz (cycles per second). Detection of a
given frequency is a function of the location of the hair cells along the organ
of Corti with the highest frequencies detected near the base of the cochlea,
and the remainder of the sound spectrum detected in a progressive fashion with
the lowest frequencies detected by hair cells near the tip.