produce aqueous humor. This watery fluid is
about the consistency of saliva and serves also to
lubricate and nourish the cornea. Aqueous humor
circulates through the anterior chamber between
the cornea and the lens, then drains from the eye
via the drainage angle, a channel between the iris
and the cornea. Dysfunction of the drainage angle
is a hallmark characteristic of GLAUCOMA.
Vitreous humor forms when the eye completes
its development during the final trimester of ges-
tation. A substance similar to water in chemical
composition and to gelatin in consistency, vitreous
humor maintains the eye’s shape and helps keep
the retina smooth and even against the back of
the eye. The volume of vitreous humor increases
as the eye grows though otherwise remains con-
stant (unlike the aqueous humor, which the eye
continuously produces). Around age 40 years the
vitreous humor begins to liquefy as a normal
process of aging, causing VITREOUS DETACHMENT,
which usually has little effect on vision though
can produce FLOATERS (fragments of tissue that
become suspended in the vitreous humor).
VISION IMPAIRMENT- Refractive errorsoccur when the focal point of light-
waves entering the eye fails to align properly on the
RETINA(ASTIGMATISM, nearsightedness, farsightedness). - Functional limitationsresult when corrected vision
remains insufficient to allow participation in activities
or occupations that require sight. - Legal blindnessexists when corrective measures can-
not restore VISUAL ACUITYto 20/200 or VISUAL FIELDto
greater than 20 degrees.
Health and Disorders of the Eyes
More than 150 million Americans have a VISION
IMPAIRMENTthat requires CORRECTIVE LENSES (eye-
glasses or contact lenses)—30 percent of men and
40 percent of women. About 12 million Ameri-
cans have uncorrectable vision impairments that
result in functional limitations; 10 percent of them
meet the criteria for legal blindness. Among those
who have uncorrectable vision impairments, 50
percent are age 65 or older. Though the eyes arise
directly from the evolving brain very early in fetal
development, their formation becomes complete
during the final 12 weeks of PREGNANCY. Infants
born before 32 weeks of gestation are at risk for
RETINOPATHYof prematurity, a leading cause among
children of vision impairments ranging from STRA-
BISMUS(inability to focus both eyes on the same
object) to legal blindness.Traditions in Medical History
As refractive errors are very common, practition-
ers throughout history have tried various and
sometimes hazardous methods for improving or
restoring vision. The earliest documentation of
corrective lenses for this purpose dates to 16th
China. European traders who traveled to China
noted the elderly holding quartz crystal lenses to
see objects close to them. Eyeglasses set in frames
and worn on the face began to appear in Europe
in the 17th and 18th centuries. By the late 19th
century inventors were experimenting with glass
lenses placed directly on the eye. These attempts
produced large, heavy, and ultimately unfeasible
lenses that covered the entire surface of the eye.
The contact lens finally became a reality in the
1950s with the advent of high-tech plastics that
were lightweight, optically clear, and inert (did
not react with body fluids). Subsequent advances
over the next 30 years brought about lenses made
of surgical plastics that allow oxygen to reach the
cornea, much improving comfort and safety. By
the 1990s, daily wear disposable contact lenses
became the standard of contact lens correction.
CATARACT, the clouding and discoloration of the
eye’s lens that develops with aging, has for cen-
turies been the leading cause of blindness in
adults. It also is one of the earliest documented
vision problems for which practitioners used sur-
gical treatments to remedy, perhaps because the
cause of the problem, the cloudiness, was so
apparent. CATARACT EXTRACTION AND LENS REPLACE-
MENThas become so commonplace in contempo-
rary ophthalmology that the procedure is no less
an expectation for restoring vision than are eye-
glasses for correcting refractive errors. In about 20
minutes, the ophthalmologist removes the
clouded lens and replaces it with a synthetic one.
Ancient physicians, lacking the benefits of the
anesthetics that make the surgery painless for
today’s patients, became skilled at “couching” a
cataract in only seconds. The procedure required
the doctor to distract the patient long enough to
puncture the cornea and push the lens out of the68 The Eyes