210
Chapter 7
b'
a'
a
b
Glass
Air
Refraction
Refraction
No refraction
Point source
of light
(a)
(b)
Figure 7–23
Focusing point sources of light.
(a) When diverging light rays enter
a dense medium at an angle to its
convex surface, refraction bends them
inward. (b) Refraction of light by the
lens system of the eye. For simplicity,
we show light refraction only at the
surface of the cornea, where the
greatest refraction occurs. Refraction
also occurs in the lens and at other
sites in the eye. Incoming light from
a (above) and b (below) is bent in
opposite directions, resulting in
b
´
being above
a
´ on the retina.
cornea performs the greater part quantitatively of focusing
the visual image on the retina, all
adjustments
for distance are
made by changes in lens shape. Such changes are part of the
process known as
accommodation.
The shape of the lens is controlled by the ciliary muscle
and the tension it applies to the zonular fi bers, which attach
the ciliary muscle to the lens (
Figure 7–24
). The ciliary mus-
cle, which is stimulated by parasympathetic nerves, is circular,
like a sphincter, so that it draws nearer to the central lens as
it contracts. As the muscle contracts, it lessens the tension on
the zonular fi bers. Conversely, when the ciliary muscle relaxes,
the diameter of the ring of muscle increases and the tension
on the zonular fi bers also increases. Therefore, the shape of
the lens is altered by contraction and relaxation of the ciliary
muscle. To focus on distant objects, the ciliary muscle relaxes
and the zonular fi bers pull the lens into a fl attened, oval shape.
Contraction of the ciliary muscles focuses the eye on near
objects, by releasing the tension on the zonular fi bers, which
allows the natural elasticity of the lens to return it to a more
spherical shape (
Figure 7–25
). The shape of the lens deter-
mines to what degree the light waves are refracted and how
they project onto the retina. Constriction of the pupil also
occurs when the ciliary muscle contracts, which helps sharpen
the image further.
As people age, the lens tends to lose elasticity, reducing
its ability to assume a spherical shape. The result is a progres-
sive decline in the ability to accommodate for near vision. This
condition, known as
presbyopia,
is a normal part of the aging
process and is the reason that people around 45 years of age may
have to begin wearing reading glasses or bifocals for close work.
The cells that make up most of the lens lose their inter-
nal membranous organelles early in life and are thus transpar-
ent, but they lack the ability to replicate. The only lens cells
that retain the capacity to divide are on the lens surface, and
as new cells form, older cells come to lie deeper within the
lens. With increasing age, the central part of the lens becomes
denser and stiffer and acquires a coloration that progresses
from yellow to black.
The changes in lens color that occur with aging are
responsible for
cataract,
an opacity (clouding) of the lens that
is one of the most common eye disorders. Early changes in
lens color do not interfere with vision, but vision is impaired as
the process slowly continues. The opaque lens can be removed
surgically. With the aid of an implanted artifi cial lens or com-
pensating corrective lenses, effective vision can be restored,
although the ability to accommodate is lost.
Cornea and lens shape and eyeball length determine the
point where light rays converge. Defects in vision occur if the
eyeball is too long in relation to the focusing power of the lens
(
Figure 7–26a
). In this case, the images of faraway objects
focus at a point in front of the retina. This
nearsighted,
or
myopic,
eye is unable to see distant objects clearly. Near objects
are clear to a person with this condition, but without the nor-
mal rounding of the lens that occurs via accommodation. In
contrast, if the eye is too short for the lens, images of near
objects are focused behind the retina (
Figure 7–26b
). This
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