PHYSICS

Scala vestibuli (perilyumh)
Bone
Tectorial
membrane
200Hz
300Hz 100Hz 50Hz
fl~~;t,·, "!
Scala media
(endolymh)
Tectorial
membrane
Basilar membrane
Fig. 9.5. Schematic diagram of
shearing force created between
the hair cells and the tectorial
membrane as a result of basilar
membrane displacement
20
25
30
Fig. 9.6. Theory of von Bekesy:
dependence of the position of
maximum vibration amplitude
on the sound frequency
Scala tympany (perilyumh)
Fig. 9.4. A cross section of the cochlea
section of the cochlea. Essentially the organ contains two fluidfilled
chambers divided by a membrane, called the basilar membrane.
This membrane does not, however, completely separate
the two chambers. A small opening, called the helicotrema, is present
at the distal or innermost part of the system. The basilar
membrane contains the neural detecting celfs, which are small
hair-like cells which are triggered by shear effects occurring
between the fluid and the membrane. Thus, any lateral or
sideways motion of the membrane with respect to the fluid is
detected. The oval window is forced to vibrate by the action of the
ossicles which, in turn, causes an oscillating pressure in the
upper chamber. This pressure can be equilibrated by the membrane
bulging downward. The bulging process produces the shearing
effects which can be detected by the hairs which are located
between the tectorial and basilar membranes (fig. 9.5). The nervous
system then transmits the signal to the brain. According to
von Bekesy's theory, different frequencies will cause a maximum
vibration amplitude at different points along membrane (fig. 9.6).
Birds. Hearing is particularly important when the birds rest or
perch. It also facilitates communication over long distances in
vegetative or arboreal environments where vision may be partially
or completely occluded. Birds (like mammals) have an external
ear and an ear canal which leads to a recessed ear drum. In many
76
birds, the outer opening of the ear canal is surrounded by specialized
feathers in the form of a funnel which act as an efficient
sound collector. The facial ruff of certain owls is shaped either
like a single parabola or as two parabolas side by side, separated
by the beak at the midline. Such a design increases the animal's
acoustic sensitivity. Peak sensitivity for birds occurs at 2 kHz. One
of the most interesting characteristics of a birds' auditory system is
the ability to resolve the small differences that exist in the
frequency of most sounds, called frequency modulation. Birds are
able to detect a frequency change of only 10 - 15 Hz. These slow
or rapid changes in acoustic frequency enable a bird to recognize a
member of its own species. In birds, the ability to determine the
exact location of a sound source depends on a variety of factors.
The barn owl, for instance, is capable of orienting to a sound
source in space with an error of less than 2 degrees.
Fish have a labyrinth, or inner ear which extends their hearing
to higher frequencies. Sound waves travel to chambers where
small granules called otholits are located. These granules stimulate
sensory hair cells and trigger action potentials in the neurons of
the auditory nerve.
The gas bladders of some fish also provide an area of variable
density. Vibrations pass through the gas bladder and travel
through a pathway of small bones called Weber ossicles. These
serve to connect the gas bladder directly with the inner ear
of the fish.
77