476
UNIT THREE
PRACTICE
43
Distinguish between the rods and the cones of the retina.
44
Explain the roles of visual pigments.
45
What factors make stereoscopic vision possible?
46
Trace the pathway of visual impulses from the retina to the
occipital cortex.
12.5
LIFE-SPAN CHANGES
We often F
rst become aware of aging-associated changes
through diminished senses. By age forty, a book may need to
be held farther away from the eyes. By the F
fties, the senses
of smell and taste may begin to diminish, which usually
refl
ects anosmia, a loss of olfactory receptors.
By age sixty, a quarter of the population experiences
noticeable hearing loss, and from ages sixty-F ve to seventy-
four, the percentage reaches a third. Half of all people over
age eighty-F
ve cannot hear adequately. Age-related hearing
loss may be the result of decades of cumulative damage to
the sensitive hair cells of the spiral organ in the inner ear. It
becomes more difF cult to hear high pitches, as well as par-
of the F bers cross over. More speciF cally, the F
bers from the
nasal (medial) half of each retina cross over, whereas those
from the temporal (lateral) sides do not. Thus, F bers from
the nasal half of the left eye and the temporal half of the
right eye form the right
optic tract;
F bers from the nasal half
of the right eye and the temporal half of the left eye form the
left optic tract.
The nerve F bers continue in the optic tracts, and before
they reach the thalamus, a few of them leave to enter nuclei
that function in various visual refl exes. Most of the F bers, how-
ever, enter the thalamus and synapse in its posterior portion
(lateral geniculate body). ±rom this region, the visual impulses
enter nerve pathways called
optic radiations,
and the pathways
lead to the visual cortex of the occipital lobes
(f g. 12.41)
.
Each visual cortex receives impulses from each eye, so a person may
develop partial blindness in both eyes if either visual cortex is injured.
For example, if the right visual cortex (or the right optic tract) is
injured, sight may be lost in the temporal side of the right eye and the
nasal side of the left eye. Similarly, damage to the central portion of
the optic chiasma, where ±
bers from the nasal sides of the eyes cross
over, blinds the nasal sides of both eyes.
±ibers not leading to the thalamus conduct visual
impulses downward into the brainstem. These impulses are
important for controlling head and eye movements associ-
ated with visually tracking an object; for controlling the
simultaneous movements of both eyes; and for controlling
certain visual refl exes, such as those that move the muscles
of the iris.
Optic
chiasma
Optic
radiations
Visual cortex of
occipital lobe
Optic tract
Eye
Optic
nerve
Fibers from
nasal (medial) half
of each retina
crossing over
Lateral
geniculate
body of
thalamus
FIGURE 12.41
The visual pathway includes the optic nerve, optic
chiasma, optic tract, and optic radiations.
Light
waves
Left eye
Right eye
FIGURE 12.40
Stereoscopic vision results from formation of two
slightly di²
erent retinal images.
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