Chapter 15
The Special Senses
575
15
internal ear against membranes that set up shearing forces that
pull on the tiny hair cells that stimulate nearby neurons that give
rise to impulses that travel to the brain, which interprets them—
and you hear. Before we unravel this intriguing sequence, let us
describe sound, the stimulus for hearing.
Overview: Properties of Sound
Light can be transmitted through a vacuum (for instance, outer
space), but sound depends on an
elastic
medium for its trans-
mission. Sound also travels much more slowly than light. Its
speed in dry air is only about 331 m/s (0.2 mi/s), as opposed to
about 300,000 km/s (186,000 mi/s) for light. A lightning flash
is almost instantly visible, but the sound it creates (thunder)
reaches our ears much more slowly. (For each second between
the lightning bolt and the roll of thunder, the storm is 1/5 mile
farther away.) Te speed of sound is fastest in solids and slowest
in gases, but it is constant in a given medium.
Sound
is a pressure disturbance—alternating areas of high and
low pressure—produced by a vibrating object and propagated by
the molecules of the medium. Consider a vibrating tuning fork
(Figure 15.28a)
. If the tuning fork is struck on the le±, its prongs
will move first to the right, creating an area of high pressure by
compressing the air molecules there. Ten, as the prongs rebound
to the le±, the air on the le± will be compressed, and the region on
the right will be a
rarefied
, or low-pressure, area (since most of its
air molecules have been pushed farther to the right).
As the fork vibrates alternately from right to le±, it produces a
series of compressions and rarefactions, collectively called a
sound
wave
, which moves outward in all directions (Figure 15.28b).
However, the individual air molecules just vibrate back and forth
for short distances as they bump other molecules and rebound.
Because the outward-moving molecules give up kinetic energy
Te
scala vestibuli
(ska
9
lah vĕs-tĭ
9
bu-li), which lies supe-
rior to the cochlear duct, is continuous with the vestibule
and abuts the oval window. Te middle
scala media
is the
cochlear duct itself. Te
scala tympani
, which terminates
at the membrane-covered round window, is inferior to the
cochlear duct.
Since the scala media is part of the membranous labyrinth, it
is filled with endolymph. Te scala vestibuli and the scala tym-
pani, both part of the bony labyrinth, contain perilymph. Te
perilymph-containing chambers are continuous with each other
at the cochlear apex, a region called the
helicotrema
(hel
0
ĭ-ko-
tre
9
mah; “the hole in the spiral”).
Te “roof” of the cochlear duct, separating the scala me-
dia from the scala vestibuli, is the
vestibular membrane
(Fig-
ure 15.27b). Te duct’s external wall, the
stria vascularis
, is
composed of an unusual richly vascularized mucosa that se-
cretes endolymph. Te “floor” of the cochlear duct is composed
of the osseous spiral lamina and the flexible, fibrous basilar
membrane, which supports the spiral organ. (We will describe
the spiral organ when we discuss hearing.) Te
basilar mem-
brane
, which plays a critical role in sound reception, is narrow
and thick near the oval window and gradually widens and thins
as it approaches the cochlear apex. Te
cochlear nerve
, a division
of the
vestibulocochlear nerve
(VIII), runs from the spiral organ
through the modiolus on its way to the brain.
Table 15.2
summarizes the structures of the internal ear and
their functions.
Physiology of Hearing
We can summarize human hearing in a single sprawling sen-
tence: Sounds set up vibrations in air that beat against the
eardrum that pushes a chain of tiny bones that press fluid in the
Area of
high pressure
(compressed
molecules)
Crest
Trough
Distance
Air pressure
Amplitude
Area of
low pressure
(rarefaction)
(a) A struck tuning fork alternately compresses and rarefies the air
molecules around it, creating alternate zones of high and low
pressure.
(b) Sound waves radiate
outward in all directions.
Wavelength
Figure 15.28
Sound: Source and propagation.
The graph in (a) shows that plotting the
oscillating air pressures yields a sine wave. The height (amplitude) of the crests is proportional to
the energy, or intensity, of the sound wave. The distance between two corresponding points on
the wave (crests or troughs) is the wavelength.
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