Our sense organs 45

less viscous liquid, called the perilymph, and they
are connected at the apex of the cochlea (by the
helicotrema). The scala vestibuli begins at the
oval window, and the scala tympani ends at the
membrane of the round window.
The cochlear duct and the scala vestibuli are separated
by an elastic sheet, called the vestibular
membrane. This membrane reproduces the waveshaped
changes in volume caused by the incident
sound. The endolymph then transmits the vibrations
to the basilar membrane which lies between
the cochlear duct and the scala tympani. Eventually
the round window is reached via the perilymph.
Because of this shortcut the vibrations do
not have to travel all the way round through the
helicotrema. The vestibular and basilar membranes
thus vibrate in unison.
The spiral-shaped bulge of the organ of Corti is
located above the basilar membrane. It consists
of sensory cells, twelve thousand of which,
arranged in rows of three to five, make up the
exterior hair cells. The interior 3,500 hair cells
as well as the structural support cells lie in one
row. The 12,000 sensory cells are arranged on
a 32 mm long lamina in four parallel rows having
a total width of only 1/20 mm. Their geometrical
sequence and distribution resemble that of piano
keys in a linear scale. At one end the cells are
tuned to a maximum frequency of between
10 and 20 kHz, which descends to about 30 Hz at
the other end.
When sounds are received, the basilar membrane
vibrates in sympathy, but the amplitude is inconceivably
minute, only about 10 -11 m. This is equal
to 100 picometres or one thousand millionth of
a cm (one million million pm = 1 metre) which
approaches the size of a few atoms. The tips of
the outer hair cells penetrate a covering membrane
(the tectorial membrane) which projects
into the cochlear duct. Volume changes in this
passage cause the basilar membrane to move relative
to the tectorial membrane so that the sensory
hairs experience a slight shearing pressure.
These stimuli are then transmitted as electrical
signals along the cochlear (auditory) nerve to the
brain. It should be noted that these signals not
28
only travel to the brain, but also in the reverse
direction. For this purpose there are two types of
neural tissue at the base of the hair cells, namely
the afferent (leading to the brain) and the efferent
fibres leading to the hair cells. The reason for
this feedback is not yet understood and this is
only one of the many unsolved puzzles.
There are about 15,000 receptor cells (hair cells)
in the cochlea and they are sensitive to different
sound frequencies (Diagram, page 27). The hair
cells are located in ordered rows on the basilar
membrane, which is a thin wall extending
through the entire cochlea, following all its convolutions.
An incident sound image is separated
into its single component frequencies, each of
which stimulates only a small fraction of the
15,000 sensory cells located at a specific position
on the basilar membrane. The functioning of the
cochlea is highly complex, and its ingenious
structure is not yet fully understood.
Special abilities of the ear: The ear is the most
sensitive human sensory organ. Sounds with frequencies
between about 20 Hz and 16 kHz are
audible. Lower frequencies are felt rather than
heard. All natural sounds are highly complex;
pure tones consisting of a single sinusoidal frequency
are not found in nature. But when they
are produced artificially, they are of great experimental
use. Sounds and noises can be considered
as a mixture of sinusoidal tones having different
frequencies and amplitudes. A tone can be
regarded as being the elementary unit for natural
sounds and noises. A 3 kHz note having an energy
level of only 4 x 10 -17 W/cm 2 can be heard, and in
general audible sound intensities lie in the range
between 10 -16 and 10 -4 W/cm 2 (Diagram, page 23).
Speech detection: Amongst all living things the
gift of speech is unique. Only man has been
endowed by the Creator with this exceptional
means of communication. It essentially requires
four independent organ systems:
– The larynx which produces the sounds
(phonation).
– The mouth and throat which modulate the
sounds produced by the larynx to form recog-