• The gustatory center, which evaluates taste, combines
information provided by both the nasal and oral receptors
to determine flavor. If you have a cold and cannot smell, you
usually cannot taste foods very well either.
• Light is refracted (bent) as it passes through the tissues of
the eyeball. The lens focuses an inverted image on the retina.
Ciliary muscles change the shape of the lens, depending on
the distance from the objects being viewed, a process known
as accommodation. Changes in the shape of the eyeball
influence the focal plane of the lens. Abnormalities can be
corrected with contact lenses, eyeglasses, or surgery.
3
Vision Involves Photoreceptors in the Eyes
235
• The eye has accessory structures to protect it (eyebrows
eyelashes, and eyelids), lacrimal apparatus to moisten it,
and six external muscles to move it.
4
The Ear Is Involved in Hearing and
Equilibrium 241
Semicircular canal
Malleus
Incus
Stapes
Round window
auditory canal
Tympanic membrane
• There are two types of equilibrium: static and dynamic. Static
Retina
equilibrium senses the position of the head, and dynamic
equilibrium senses movements of the head. Both are detected
Choroid
by the vestibular apparatus, which is located in the inner ear. In
static equilibrium, gravity pulls on sensory hairs in the saccule
Sclera
and utricle and evokes nerve impulses. In dynamic equilibrium,
the fluid inside the vestibular apparatus moves hair cells within
the semicircular canals and evokes nerve impulses.
• Nerve impulses from the vestibular apparatus travel through
the vestibulocochlear nerve (CN VIM) to the pons, somato-
sensory cortex, and cerebellum. The vestibular nuclei in the
pons integrate this information along with visual information
and proprioceptive information to sense equilibrium.
The layers of the eyeball • Figure 8.8
Cornea
Pupil
Lens
• The ear is composed of the outer, middle, and inner ears.
The outer ear, including the auricle and external auditory
canal shown in the figure below, collects and directs sound
waves to the middle ear, where they are converted to me-
chanical vibrations. Vibrations of the middle ear cause pres-
sure waves in the fluid-filled inner ear. The pressure waves
move hair cells in the organs of Corti that send impulses to
the brain through the vestibulocochlear nerve (CN VIII) to
the temporal lobe of the cerebral cortex.
I______ ,______ I
Optic (II) nerve
The inner ear • Figure 8.11
• The eyeball, as shown, is a three-layered structure
(outer protective layer, middle vascular layer, and
an inner neural layer). Iris muscles control the
opening that lets light through (pupil). The ciliary
body controls the shape of the lens for focusing
the light on photoreceptors in the retina.
• Photoreceptors (rods and cones) contain phot-
opigments that elicit nerve impulses in the retina
and optic nerve. The axons from the lateral por-
tions of each eye cross in the optic chiasm. The
visual pathway ends in the occipital lobe. The right
occipital lobe interprets information about the left
halves of the visual fields of both eyes, while the
left occipital lobe interprets input from the right
halves of the visual fields.
Summary 247
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