Encyclopedia of Health and Medicine

The Eyes 67
provides depth perception and accommodates
each eye’s “blind spot.” Some people have the
ability to intentionally move their eyes independent
of each other, though unintentional disparate
movement generally indicates a pathologic condition.
Discordant movement may characterize neurologic
disorders such as progressive supranuclear
palsy (PSP) and TRAUMATIC BRAIN INJURY (TBI).
Abnormal eye movements also accompany
vestibular disorders (disturbances of the balance
mechanisms of the inner EAR).
MUSCLES THAT MOVE THE EYE
• Superior oblique and inferior oblique rotate the eye
primarily in a circular motion.
• Superior rectus and inferior rectus move the eye primarily
up and down.
• Lateral rectus and medial rectus move the eye primarily
side to side.
How the eye “sees” The sclera gives the eye its
shape and rigidity. The front part of the sclera
forms the “white” of the eye, the coloration coming
from the white pigmentation of the fiber cells.
In its center, the sclera becomes transparent, forming
the CORNEA. The middle layer of the eye’s wall
is the choroid, a thin, dark membrane rich in
BLOOD vessels. The choroid loosely attaches to and
nourishes the sclera and the eye’s innermost layer,
the RETINA, where sight becomes vision.
Specialized cells infuse the retina, which lines
the back of the inner eye. These cells, rods and
cones, convert lightwaves into electrical impulses.
Rods are the most plentiful, numbering about 120
million on each retina, and detect light in perceptions
of shades of gray. Cones detect color and
detail; there are about 6 million of them on each
retina. Cones are sensitive to red, green, or blue.
Rods and cones contain photosensitive chemicals
that react to different wavelengths of light. The
reactions alter the electrical charges of the rods
and cones, creating nerve signals. Each minute of
wakefulness thousands of these impulses traverse
the optic nerves, carrying messages the brain then
interprets and assembles as visual images.
The optic nerve, which contains more than a
million nerve fibers, carries these signals to the
brain. The pigmented cells of the retina are rich in
melanin, the same chemical that causes the SKIN to
darken in response to sun exposure. In the retina,
these cells form a “blackout screen” that eliminates
reflection, allowing lightwaves to reach and
activate the rods and cones without interference.
The macula, a small circular area in the center of
the retina, contains the most dense distribution of
cones and handles fine detail vision. The “blind
spot,” the point at which the optic nerve enters
the retina, is the optic disk; it contains no rods or
cones. RETINITIS PIGMENTOSA (hereditary degeneration
of the retina) and RETINAL DETACHMENT (separation
of the retina from the choroid) are among the
conditions that can affect the retina, resulting in
impaired vision and blindness.
The physics of vision Lightwaves pass through
the cornea and the LENS to enter the eye through
the pupil, the opening in the circular muscle that
rings the lens, the iris. The iris is the colored part
of the eye; the pupil in its center appears black
because it reveals the dark interior of the eye. The
iris dilates (increases the size of) the pupil to allow
more light to enter the eye and constricts
(decreases the size of) the pupil to reduce the light
that enters the eye. The cornea and the lens each
refract, or bend, the entering lightwaves. The ciliary
muscles contract and relax to move the lens,
which thickens or flattens, respectively, to
improve focus. After about age 40 the lens gradually
loses its FLEXIBILITY, accounting for age-related
difficulty with near vision (PRESBYOPIA).
Refracted light forms a final focal point that, in
the healthy eye, aligns in a pattern on the retina
at the back of the eye. The mechanics of this
refractory process are such that the image resulting
on the retina is upside down. When interpreting
and assembling nerve signals from the eye, the
brain automatically reverses the image to perceive
it right-side up. Refractive ASTIGMATISM, HYPEROPIA,
and MYOPIA when the final focal point falls short of
or extends beyond the retina, resulting in images
that are out of focus or distorted.
Helping keep the lightwaves from fragmenting
during refraction are two chambers of fluid, the
aqueous humor, which fills the space between the
cornea and the lens (the anterior chamber), and
the vitreous humor, which fills the interior of the
eye. The ciliary processes, specialized folds of the
eye’s choroid layer that extend into the posterior
chamber at the corners of the lens behind the iris,