It will be seen that all we have examined so
far consists of the vibrations in the outer and middle ear.
The vibrations are constantly passed forward, but so far there
is still nothing apart from a mechanical motion. In other
words, there is as yet no sound.
The process whereby these mechanical motions
begin to be turned into sound begins in the area known as
the inner ear. In the inner ear is a spiral-shaped organ filled
with a liquid. This organ is called the cochlea.
The complex structure of the inner ear. Inside this
complicated bone structure is found both the system
that maintains our balance, and also a very sensitive
hearing system that turns vibrations into sound.
The last part of the middle ear is the stirrup
bone, which is linked to the cochlea by a membrane. The mechanical
vibrations in the middle ear are sent on to the liquid in
the inner ear by this connection.
The vibrations which reach the liquid in the
inner ear set up wave effects in the liquid. The inner walls
of the cochlea are lined with small hair-like structures,
called stereocilia, which are affected by this wave effect.
These tiny hairs move strictly in accordance with the motion
of the liquid. If a loud noise is emitted, then more hairs
bend in a more powerful way. Every different frequency in
the outside world sets up different effects in the hairs.
But what is the meaning of this movement of the
hairs? What can the movement of the tiny hairs in the cochlea
in the inner ear have to do with listening to a concert of
classical music, recognizing a friend's voice, hearing the
sound of a car, or distinguishing the millions of other kinds
The answer is most interesting, and once more
reveals the complexity of the design in the ear. Each of the
tiny hairs covering the inner walls of the cochlea is actually
a mechanism which lies on top of 16,000 hair cells. When these
hairs sense a vibration, they move and push each other, just
like dominos. This motion opens channels in the membranes
of the cells lying beneath the hairs. And this allows the
inflow of ions into the cells. When the hairs move in the
opposite direction, these channels close again. Thus, this
constant motion of the hairs causes constant changes in the
chemical balance within the underlying cells, which in turn
enables them to produce electrical signals. These electrical
signals are forwarded to the brain by nerves, and the brain
then processes them, turning them into sound.
|The inner walls of the cochlea in the inner ear are
lined with tiny hairs. These move in line with the wave
motion set up in the liquid in the inner ear by vibrations
coming from outside. In this way, the electrical balance
of the cells to which the hairs are attached changes,
and forms the signals we perceive as "sound."
Science has not been able to explain all the
technical details of this system. While producing these electrical
signals, the cells in the inner ear also manage to transmit
the frequencies, strengths, and rhythms coming from the outside.
This is such a complicated process that science has so far
been unable to determine whether the frequency-distinguishing
system takes place in the inner ear or in the brain.
At this point, there is an
interesting fact we have to consider concerning the motion
of the tiny hairs on the cells of the inner ear. Earlier,
we said that the hairs waved back and forth, pushing each
other like dominos. But usually the motion of these tiny hairs
is very small. Research has shown that a hair motion of just
by the width of an atom can be enough to set off the reaction
in the cell. Experts who have studied the matter give a very
interesting example to describe this sensitivity of these
hairs: If we imagine a hair as being as tall as the Eiffel
Tower, the effect on the cell attached to it begins with a
motion equivalent to just 3 centimeters of the top of the
Just as interesting is the question of how often
these tiny hairs can move in a second. This changes according
to the frequency of the sound. As the frequency gets higher,
the number of times these tiny hairs can move reaches unbelievable
levels: for instance, a sound of a frequency of 20,000 causes
these tiny hairs to move 20,000 times a second.
Everything we have examined so far has shown
us that the ear possesses an extraordinary design. On
closer examination, it becomes evident that this design is
irreducibly complex, since, in order for hearing
to happen, it is necessary for all the component parts of
the auditory system to be present and in complete working
order. Take away any one of these-for instance, the hammer
bone in the middle ear-or damage its structure, and you will
no longer be able to hear anything. In order for you to hear,
such different elements as the ear drum, the hammer, anvil
and stirrup bones, the inner ear membrane, the cochlea, the
liquid inside the cochlea, the tiny hairs that transmit the
vibrations from the liquid to the underlying sensory cells,
the latter cells themselves, the nerve network running from
them to the brain, and the hearing center in the brain must
all exist in complete working order. The system cannot develop
"by stages," because the intermediate stages would serve no
358 Jeff Goldberg,
"The Quivering Bundles That Let Us Hear," Seeing, Hearing,
and Smelling the World, A Report from the Howard Hughes
Medical Institute, p. 38.