CHAPTER 20
Eye
399
Figure 20-8A.
Lens and zonular f
bers.
H&E,
3
34
The
zonular f bers
are also called the
zonules o± Zinn
. The
combination of all the zonular F bers is referred to as the
suspen-
sory ligament
of the eye. These F bers form a connection between
the ciliary body and the equatorial (lateral) region of the lens (see
±ig. 20-11C,D). One end of each zonular F ber is attached to a
ciliary process
, and the other end of the F ber is embedded in the
capsule
the
lens
. The capsule is thicker on the anterior and
lateral surfaces than on the posterior surface. The zonular F bers
have an important function during
accommodation
, which is to
adjust the tension of the lens (thereby changing the curvature of
the lens) to enable an object to be focused on the retina. Some
ciliary muscle F bers are arranged in a circle at the base of the iris.
Contraction of these F bers decreases the diameter of the circle
and, therefore, decreases the tension on the zonular F bers so the
lens becomes more round (the curvature
increases
). Relaxation
of the ciliary muscle increases tension on the zonular F bers and
the lens becomes fl attened (curvature
decreases
).
Zonular
fibers
Ciliary
muscle
Ciliary
processes
Ciliary
body
Iris
A
D. Cui
Subcapsular
epithelium
Anterior lens capsule
Posterior
lens capsule
Lens nucleus
Lens
cortex
Nucleus of
lens fibers
Lens
fibers
Zonular
fibers (relaxed)
Zonular
fibers (stretched)
Lens curvature
increases
Lens curvature
decreases
Posterior
Anterior
B
Figure 20-8B.
A representation o± the lens and its ±unction.
The
lens
is transparent and is composed of a
lens capsule
,
sub-
capsular epithelium
, and
lens f
bers
. The function of the lens is to
focus light on the retina. When
focusing on a distant object
, the
ciliary muscle is relaxed, tension on the zonular F
bers increases,
and the anteroposterior thickness of the lens decreases. To
focus
on a near object
, the ciliary muscle contracts to release the ten-
sion on the zonular F bers and the thickness of the lens increases.
Ciliary muscle contraction also pulls the choroid forward to aid
in focusing objects on the retina. (The actions of the ciliary mus-
cle are also discussed in ±igs. 20-8A and 20-11B.) The ability of
the lens to accommodate decreases with age (
presbyopia
). The
most common disorder associated with the lens is the
cataract
.
Figure 20-8C.
Cataract.
H&E,
3
51
Cataract
is a condition of opacity in the lens of the eye.
Crystallin
protein
in the lens degenerates, becoming insoluble and opaque. In
cataractous lenses, lens F
bers are edematous and sometimes necrotic.
These changes alter the normal continuity of the lens F
bers. The
capsule may become wrinkled. The
inset
photomicrograph shows
a cluster of altered protein in the anterior lens cortex. Cataract is
usually an age-related disorder that causes partial or total blindness
if left untreated. Risk factors include age, smoking, alcohol use,
sunlight exposure, diabetes mellitus, and systemic corticosteroid
use. Cataracts are usually bilateral and progress slowly. The visual
acuity decrease is directly related to the density of the cataract. Types
of cataracts include
senile
cataract
,
congenital
cataract
,
traumatic
cataract
,
toxic
cataract
, and
cataract
associated
with
systemic
disease
. The
senile cataract
is the most common form. Medical
intervention consists of removing the opaciF
ed lens from the eye
and implanting an artiF
cial intraocular lens.
D. Cui
Wrinkled capsule
Abnormal anterior
lens cortex tissue
Normal anterior
lens cortex tissue
Degenerating
protein
Disrupted
lens fibers
Necrosis
C
CLINICAL CORRELATION
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