The lens is the normally clear structure that is located just
posterior to the iris. The primary function of the lens is to focus
light rays on the retina. The lens is attached to the ciliary body
by zonules. These zonules produce tension in response to contraction
of the ciliary muscle. The ciliary muscle is circular, so that when
it contracts its diameter becomes smaller (Figure 30–1).
This decreases the tension on the lens zonules, and the lens becomes
more spherical. This is known as accommodation,
which is what allows the eye to focus at near. As people age, the
lens becomes progressively stiffer. Because of this, the ability
to focus at near gradually deteriorates, to the point that almost all
people require reading glasses during the fourth decade of life.
Focusing of the lens is controlled by the ciliary body.
(Top) When the ciliary body is relaxed the lens is thinner and focuses
light rays from distant objects. (Bottom) When the ciliary muscle
contracts, the lens zonules loosen, and the lens assumes a more
spherical shape. This allows the lens to focus on near objects.
The lens consists of the anterior and
posterior lens capsules, which are basal lamina produced by
the lens epithelium. The lens epithelium lines the inner capsule.
The epithelial cells at the equator of the lens continue to divide
and produce lens fibers throughout life. The central portion of
the lens (the nucleus) is formed by
birth, and the surrounding portion of the lens (the cortex) is produced postnatally (Figure
30–2). The cytoplasm of lens cells contains crystallins, which produce the light-focusing
properties of the lens.
Lens anatomy. The lens is composed of the inner nucleus,
surrounding cortex, and outer capsule. The lens is attached to the
ciliary body by the lens zonules. The iris in front of the lens
forms the pupil.
Cataracts are present if the lens
is not clear. Cataracts may range from mild to severe, and their
effect on vision can range from minimal to profound. Cataracts are
very common in adults as they age, but they are uncommon in children.
However, they do represent a significant cause of visual morbidity
in children, accounting for approximately 10% of childhood
In addition to cataracts, rare lens disorders include aphakia (absence
of the lens), microspherophakia (an
abnormally small, round lens), ectopia lentis (decentration
of the lens), and lens coloboma (a
partial defect in the periphery of the lens due to absence of adjacent
zonules, usually associated with colobomas of the iris and retina).
The lens initially develops as a thickening of the surface ectoderm
overlying the optic vesicle. This tissue invaginates ...