Document Text Contents
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Page 185
Chapter 12: The Sensory System 175
degree to which the densities of the two substances differ. When
distant objects are sighted, the normal curvature of the lens appro-
priately compensates for the refraction due to the differences in den-
sities among the aqueous humor, the lens, and the vitreous humor.
■ Lens accommodation: Light rays from near objects enter the eyeball
at more divergent angles than rays from distant objects. Thus, when
near objects are sighted, muscles pull on the lens to increase its cur-
vature so that the more divergent rays of the close object are properly
refracted upon the retina.
■ Pupil constriction: One function of the pupil is to regulate the
amount of light that enters the posterior cavity so that the retina
receives the appropriate amount of stimulation. In addition, when
near objects are sighted, the pupil constricts (accommodation pupil-
lary reflex) to block the most divergent light rays that cannot be
brought into focus by the accommodation of the lens. Reading
under low levels of light may be difficult because the pupils are
dilated to allow all available light to enter rather than constricted to
improve focusing. Increasing the amount of light improves the abil-
ity to read because the surplus light permits the pupils to constrict
and the lens to focus a more narrow beam of light rays.
■ Eyeball convergence: Both eyes point in the same direction when
viewing a distant object. When near objects are sighted, the eyes
must be directed medially to simultaneously view the object, a pro-
cess called convergence.
Nerve impulses generated by visual stimuli travel along the axons of gan-
glion cells within the two optic nerves. Before entering the brain, axons
representing the medial portions of the visual fields of each eye cross over
at the optic chiasma. After the crossover, the axons, now forming the optic
tract, enter the thalamus. Processed visual stimuli are then carried to visual
areas of the occipital lobes of both cerebral hemispheres by nerve pathways
called the optic radiations. Because of the partial crossover at the optic
chiasma, each cerebral hemisphere receives the lateral portion of the visual
field of the eye on the same side of the body as the cerebral hemisphere, but
the medial portion of the visual field of the eye is interpreted on the oppo-
site side of the body. In addition, because of the action of the lens, the
image that forms on the retina and that is sent to the brain is inverted and
reversed right to left. The brain, however, interprets all of this seemingly
disparate visual information into a coherent perception of the real world.
15_9780470878743-ch12.indd 17515_9780470878743-ch12.indd 175 6/16/11 3:31 PM6/16/11 3:31 PM
Page 186
176 CliffsNotes Anatomy & Physiology Quick Review
Hearing
The organ of hearing, the ear, consists of three major regions, shown
in Figure 12-2.
■ The outer (external) ear consists of the auricle (pinna), a flap of
elastic cartilage that protrudes from the head, and the external audi-
tory canal (meatus), a tube that enters the temporal bone. The canal
is lined with ceruminous glands that secrete cerumen (earwax), a
sticky substance that traps dirt and other foreign objects. The ear-
drum (tympanic membrane), at the internal end of the external
auditory canal, vibrates in response to incident sound waves.
■ The middle ear (tympanic cavity) is an air-filled cavity within the
temporal bone. It contains three small bones, the auditory ossicles.
These bones, called the malleus, incus, and stapes, act as a lever
system that amplifies and transfers vibrations of the eardrum to the
inner ear. The malleus at one end connects to the eardrum, while the
stapes at the other end attaches with ligaments to the oval window,
a small, membrane-covered opening into the inner ear. Synovial
joints connect the incus, the center bone of the auditory ossicles, to
the malleus and stapes on each side. A second membrane-covered
opening to the inner ear, the round window (secondary tympanic
membrane), lies just below the oval window. A third opening leads
to the auditory (Eustachian) tube, which connects the middle ear to
the upper throat. The auditory tube allows pressure differences
between the middle and outer ear to equalize, thus reducing tension
on the eardrum. Two muscles in the middle ear, the tensor tympani
and the stapedius, connect to the malleus and stapes, respectively.
Contraction of these two muscles restricts the movement of the ear-
drum and auditory ossicles, reducing damage that may occur when
they are exposed to excessive vibration from loud noises.
■ The inner (internal) ear, also called the labyrinth, is a system of
double-walled canals. The canals consist of an outer bony (osseous)
labyrinth that encloses an inner membranous labyrinth. Perilymph
fills the space between the two labyrinths, and endolymph fills the
inner labyrinth. This double-layer labyrinth structure is found
throughout the following inner ear structures. This labyrinth is
made of three semicircular canals and a snail-shaped cochlea (see
Figure 12-2).
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Page 369
Notes
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Page 370
Master the basics—fast!
Available wherever books are sold.
®
Hundreds of
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02_9780470878743-ftoc.indd viii02_9780470878743-ftoc.indd viii 6/16/11 3:26 PM6/16/11 3:26 PM
Page 185
Chapter 12: The Sensory System 175
degree to which the densities of the two substances differ. When
distant objects are sighted, the normal curvature of the lens appro-
priately compensates for the refraction due to the differences in den-
sities among the aqueous humor, the lens, and the vitreous humor.
■ Lens accommodation: Light rays from near objects enter the eyeball
at more divergent angles than rays from distant objects. Thus, when
near objects are sighted, muscles pull on the lens to increase its cur-
vature so that the more divergent rays of the close object are properly
refracted upon the retina.
■ Pupil constriction: One function of the pupil is to regulate the
amount of light that enters the posterior cavity so that the retina
receives the appropriate amount of stimulation. In addition, when
near objects are sighted, the pupil constricts (accommodation pupil-
lary reflex) to block the most divergent light rays that cannot be
brought into focus by the accommodation of the lens. Reading
under low levels of light may be difficult because the pupils are
dilated to allow all available light to enter rather than constricted to
improve focusing. Increasing the amount of light improves the abil-
ity to read because the surplus light permits the pupils to constrict
and the lens to focus a more narrow beam of light rays.
■ Eyeball convergence: Both eyes point in the same direction when
viewing a distant object. When near objects are sighted, the eyes
must be directed medially to simultaneously view the object, a pro-
cess called convergence.
Nerve impulses generated by visual stimuli travel along the axons of gan-
glion cells within the two optic nerves. Before entering the brain, axons
representing the medial portions of the visual fields of each eye cross over
at the optic chiasma. After the crossover, the axons, now forming the optic
tract, enter the thalamus. Processed visual stimuli are then carried to visual
areas of the occipital lobes of both cerebral hemispheres by nerve pathways
called the optic radiations. Because of the partial crossover at the optic
chiasma, each cerebral hemisphere receives the lateral portion of the visual
field of the eye on the same side of the body as the cerebral hemisphere, but
the medial portion of the visual field of the eye is interpreted on the oppo-
site side of the body. In addition, because of the action of the lens, the
image that forms on the retina and that is sent to the brain is inverted and
reversed right to left. The brain, however, interprets all of this seemingly
disparate visual information into a coherent perception of the real world.
15_9780470878743-ch12.indd 17515_9780470878743-ch12.indd 175 6/16/11 3:31 PM6/16/11 3:31 PM
Page 186
176 CliffsNotes Anatomy & Physiology Quick Review
Hearing
The organ of hearing, the ear, consists of three major regions, shown
in Figure 12-2.
■ The outer (external) ear consists of the auricle (pinna), a flap of
elastic cartilage that protrudes from the head, and the external audi-
tory canal (meatus), a tube that enters the temporal bone. The canal
is lined with ceruminous glands that secrete cerumen (earwax), a
sticky substance that traps dirt and other foreign objects. The ear-
drum (tympanic membrane), at the internal end of the external
auditory canal, vibrates in response to incident sound waves.
■ The middle ear (tympanic cavity) is an air-filled cavity within the
temporal bone. It contains three small bones, the auditory ossicles.
These bones, called the malleus, incus, and stapes, act as a lever
system that amplifies and transfers vibrations of the eardrum to the
inner ear. The malleus at one end connects to the eardrum, while the
stapes at the other end attaches with ligaments to the oval window,
a small, membrane-covered opening into the inner ear. Synovial
joints connect the incus, the center bone of the auditory ossicles, to
the malleus and stapes on each side. A second membrane-covered
opening to the inner ear, the round window (secondary tympanic
membrane), lies just below the oval window. A third opening leads
to the auditory (Eustachian) tube, which connects the middle ear to
the upper throat. The auditory tube allows pressure differences
between the middle and outer ear to equalize, thus reducing tension
on the eardrum. Two muscles in the middle ear, the tensor tympani
and the stapedius, connect to the malleus and stapes, respectively.
Contraction of these two muscles restricts the movement of the ear-
drum and auditory ossicles, reducing damage that may occur when
they are exposed to excessive vibration from loud noises.
■ The inner (internal) ear, also called the labyrinth, is a system of
double-walled canals. The canals consist of an outer bony (osseous)
labyrinth that encloses an inner membranous labyrinth. Perilymph
fills the space between the two labyrinths, and endolymph fills the
inner labyrinth. This double-layer labyrinth structure is found
throughout the following inner ear structures. This labyrinth is
made of three semicircular canals and a snail-shaped cochlea (see
Figure 12-2).
15_9780470878743-ch12.indd 17615_9780470878743-ch12.indd 176 6/16/11 3:31 PM6/16/11 3:31 PM
Page 369
Notes
27_9780470878743-bindex.indd 35927_9780470878743-bindex.indd 359 6/16/11 3:34 PM6/16/11 3:34 PM
Page 370
Master the basics—fast!
Available wherever books are sold.
®
Hundreds of
practice problems
online @
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