Our sense organs 45

denigrated the external human ear, with all its
convolutions, as being pointless and useless, a
degenerate left-over from some alleged evolutionary
history. The little protrusion on the upper
outside edge of the auricle has been known since
then as “Darwin’s tubercle”. Generations of
researchers after Darwin blindly accepted his verdict
on the ear. But in reality, the whole beautiful,
convoluted labyrinth that is our auricle is a precise,
genetically programmed device which delivers
an identical additional signal to the brain
after a time lag of one five thousandth of a second
( = 0.0002 s, see the diagram on page 24). In
effect, one has four ears, two of which are located
slightly higher than the other two. The result
of this finely tuned system is that the brain is
able to process six different values, two of them
being the differences between the two upper
”ears”, two more are those between the two
lower “ears”, and the third pair is that between
the lower “ear” on one side, and the upper “ear”
on the other side (see the diagram on page 25).
The required computations are carried out at
lightning speed in the brain to give us a very
sophisticated “sound image” of our surroundings.
This structure is also crucial in our astounding
ability to voluntarily suppress some sounds to
the enhancement of others.
Middle ear: After travelling along the external
acoustic meatus, incoming sound waves strike
the ear drum, which is set vibrating. The energy
transferred in this way is passed on, as the three
tiny, connected bones in the middle ear (malleus,
incus, and stapes, or hammer, anvil and stirrup)
transmit the sound vibrations through the oval
window into the inner ear. Weighing only about
10 mg, a tiny percent of the mass of the smallest
of coins, these minute bones are the smallest in
the entire human body.
The hearing process involves the transfer of air
vibrations into the liquid medium of the inner ear.
In the normal course of events, the greater part of
the sound energy would be reflected at an air/liquid
boundary, and such losses would play havoc
with the hearing process. To circumvent this, the
Creator used a very ingenious interface structure
which limits reflection losses to a negligible level.
The cochlea
Scala vestibuli
Cochlear duct
Scala tympani
Sensory zone (organ of Corti)
Vestibular membrane
Helicotrema
Round window
Stapes in oval window
This complex mechanism, comprising the ear
drum and the three middle ear bones, exactly
matches the sound wave impedance in air to that
of the inner ear. The effective vibrational area of
the ear drum is about 0.65 cm 2 , which is 20 times
that of the oval window (only 0.032 cm 2 ). This is
equivalent to an amplification factor of 20. In
addition, the leverage afforded by the malleusincus-stapes
linkup provides a further amplification
factor of 3.
Inner ear: It is clear that a massive amplification
factor is involved in the conversion of air waves
(at the ear drum) to the vibrations of the liquid in
the cochlea. The inner ear, comprising both the
balance organ and the cochlea (Latin cóchlea =
snail), is housed in the solid bone of the skull. A
second conversion takes place here; mechanical
vibrations are changed into electric neural
(nerve) impulses.
The cochlear duct is filled with a highly viscous
liquid (Latin viscum = birdlime, sticky, or thick),
called the endolymph. There are two more liquidfilled
spaces on either side, the scala vestibuli
(Latin scala = steps; vestibuli = forecourt), and
the scala tympani (Latin tympanum = drum).
Both of these cavities are filled with a somewhat
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