467
CHAPTER TWELVE
Nervous System III
choroid coat is pulled forward, and the ciliary body shortens.
Both of these actions relax the suspensory ligaments, thick-
ening the lens. In this thickened state, the lens is focused for
viewing objects closer than before
(f g. 12.29
a
)
.
To focus on a distant object, the ciliary muscles relax,
increasing tension on the suspensory ligaments. The lens
thins again (F
g. 12.29
b
).
PRACTICE
33
Describe the outer and middle tunics of the eye.
34
What factors contribute to the transparency of the cornea?
35
How does the shape of the lens change during accommodation?
36
Why would reading for a long time lead to "eye fatigue," while
looking at something distant is restful?
The
iris
is a thin diaphragm mostly composed of connec-
tive tissue and smooth muscle F bers. Seen from the outside, it
is the colored portion of the eye. The iris extends forward from
the periphery of the ciliary body and lies between the cornea
and the lens. It divides the space separating these parts, called
the
anterior cavity,
into an
anterior chamber
(between the cor-
nea and the iris) and a
posterior chamber
(between the iris
and the vitreous humor, occupied by the lens).
The epithelium on the inner surface of the ciliary body
continuously secretes a watery fl uid called
aqueous humor
into the posterior chamber. The fl
uid circulates from this
chamber through the
pupil,
a circular opening in the cen-
ter of the iris, and into the anterior chamber
(f g. 12.30)
.
Aqueous humor F lls the space between the cornea and the
capsule that surrounds the lens. The body of the lens, which
lacks blood vessels, lies directly behind the iris and pupil and
is composed of specialized epithelial cells.
The cells of the lens originate from a single layer of epi-
thelium beneath the anterior portion of the lens capsule. The
cells divide, and the new cells on the surface of the lens cap-
sule differentiate into columnar cells called
lens f
bers,
which
constitute the substance of the lens. Lens F ber production
continues slowly throughout life, thickening the lens from
front to back. Simultaneously, the deeper lens fibers are
compressed toward the center of the structure
(f
g. 12.27)
.
The lens capsule is a clear, membranelike structure
largely composed of intercellular material. It is quite elastic,
a quality that keeps it under constant tension. As a result,
the lens can assume a globular shape. However, the suspen-
sory ligaments attached to the margin of the capsule are also
under tension, and they pull outward, fl
attening the capsule
and the lens
(f g. 12.28)
.
If the tension on the suspensory ligaments relaxes, the elas-
tic capsule rebounds, and the lens surface becomes more con-
vex. This change, called
accommodation
(ah-kom
o-da
shun),
occurs in the lens when the eye focuses to view a close object.
The ciliary muscles relax the suspensory ligaments dur-
ing accommodation. One set of these muscle F bers forms a
circular sphincterlike structure around the ciliary processes.
The fibers of the other set extend back from fixed points
in the sclera to the choroid coat. When the circular muscle
F
bers contract, the diameter of the ring formed by the cili-
ary processes decreases; when the other F
bers contract, the
Ciliary processes
of ciliary body
Retina
Choroid coat
Sclera
Lens
Suspensory
ligaments
FIGURE 12.28
Lens and ciliary body viewed from behind.
FIGURE 12.27
A scanning electron micrograph of the long,
F
attened lens ±
bers (2,650×). Note the ±
ngerlike junctions where one
±
ber joins another.
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