Sensory Physiology
217
pensation for movements of the head. The control centers
for these compensating movements obtain their information
about head movement from the vestibular system, which we
will describe shortly. Control systems for the other slow move-
ments of the eyes require the continuous feedback of visual
information about the moving object.
Hearing
The sense of hearing is based on the physics of sound and the
physiology of the external, middle, and inner ear, the nerves to
the brain, and the brain parts involved in processing acoustic
information.
Sound
Sound energy is transmitted through a gaseous, liquid, or solid
medium by setting up a vibration of the medium’s molecules,
air being the most common medium. When there are no mole-
cules, as in a vacuum, there can be no sound. Anything capable
of disturbing molecules—for example, vibrating objects—can
serve as a sound source.
Figure 7–33
demonstrates the basic
mechanism of sound production using a tuning fork as an
example. When struck, the tuning fork vibrates, creating dis-
turbances of air molecules that make up the sound wave. The
sound wave consists of zones of compression, in which the
molecules are close together and the pressure is increased,
alternating with zones of rarefaction, where the molecules are
farther apart and the pressure is lower (
Figure 7–33a–d
). As
the air molecules bump against each other, the zones of com-
pression and rarefaction ripple outward, and the sound wave is
transmitted over distance.
A sound wave measured over time (
Figure 7–33e
) con-
sists of rapidly alternating pressures that vary continuously
from a high during compression of molecules, to a low dur-
ing rarefaction, and back again. The difference between the
pressure of molecules in zones of compression and rarefaction
determines the wave’s amplitude, which is related to the loud-
ness of the sound; the greater the amplitude, the louder the
sound. The frequency of vibration of the sound source (i.e.,
the number of zones of compression or rarefaction in a given
time) determines the pitch we hear; the faster the vibration,
the higher the pitch. The sounds heard most keenly by human
ears are those from sources vibrating at frequencies between
1000 and 4000 Hz (hertz, or cycles per second), but the
entire range of frequencies audible to human beings extends
from 20 to 20,000 Hz. Sound waves with sequences of pitches
are generally perceived as musical. The addition of other fre-
quencies, called overtones, to the basic sound wave gives the
sound its characteristic quality, or timbre.
We can distinguish about 400,000 different sounds.
For example, we can distinguish the note A played on a piano
from the same note on a violin. We can also selectively
not
hear sounds, tuning out the background noise of a party to
concentrate on a single voice.
Figure 7–32
A superior view of the muscles that move the eyes to direct the gaze and provide convergence.
Superior oblique
removed on this side
Inferior oblique
(transparent view)
Superior
oblique
Lateral
rectus
Medial
rectus
Superior
rectus
Superior
levator
removed from
both sides
Inferior rectus
Superior rectus
removed on this side
Optic chiasm
Left eye
Right eye
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