Sensory Physiology
211
eye is
farsighted,
or
hyperopic,
and while a person with this
condition has poor near vision, distant objects can be seen if
the accommodation refl
ex is activated to increase the curva-
ture of the lens. The use of corrective lenses for near- and far-
sighted vision is shown in Figure 7–26.
Defects in vision also occur when the lens or cornea
does not have a smoothly spherical surface, a condition known
as
astigmatism
.
Corrective lenses can usually compensate for
these surface imperfections.
The size and shape of a person’s eye over time depends
in part on the volume of the aqueous humor and vitreous
humor. These two fl uids are colorless and permit the trans-
mission of light from the front of the eye to the retina. The
aqueous humor is constantly formed by special vascular tis-
sue that overlies the ciliary muscle and drains away through
a canal in front of the iris at the edge of the cornea. In some
instances, the aqueous humor forms faster than it is removed,
which results in increased pressure within the eye.
Glaucoma,
the leading cause of irreversible blindness, is a disease in
which retinal cells are damaged as a result of increased pres-
sure within the eye.
Just as the aperture of a camera can be varied to alter
the amount of light that enters, the iris regulates the diameter
of the pupil. The color of the iris is of no importance as long
Figure 7–26
Correction of vision defects. (a) Nearsightedness. (b) Farsightedness.
Ciliary
muscle
Zonular
fibers
Lens
Iris
Cornea
Figure 7–24
Ciliary muscle, zonular fi bers, and lens of the eye.
Light rays from distant
objects are nearly parallel.
Relaxed ciliary muscles, tension on
zonular fibers, flattened lens
(a)
(b)
(c)
Light rays from near objects diverge.
Relaxed ciliary muscles
Out of
focus
In
focus
In
focus
Near object with accommodation
Firing of parasympathetic nerves,
contracted ciliary muscles, slackened
zonular fibers, rounded lens
Figure 7–25
Accommodation for near vision. (a) Light rays from distant objects
are more parallel, and they focus onto the retina when the lens is less
curved. (b) Diverging light rays from near objects do not focus on
the retina when the ciliary muscles are relaxed. (c) Accommodation
increases the curvature of the lens, focusing the image of near
objects onto the retina.
Normal sight
(faraway object is clear)
Normal sight
(near object is clear)
Nearsighted
(eyeball too long)
Nearsightedness corrected
Farsighted
(eyeball too short)
Farsightedness corrected
(b)
(a)
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