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One Miraculous Organ
By Frances Maguire Paist, Staff Writer
(Page 2 of 3)
The iris is the colored membrane of the
eye, located between the cornea and the lens. It
controls light levels inside the eye and because of
microscopic pigment cells called melanin, varies in
color from pale blue to dark brown. Flat in perspective,
it divides the front (or anterior) chamber of the eye
from the back (or posterior) chamber, and its color,
textures and patterns are as unique to each individual
as a fingerprint.
Directly in the middle of the iris is the pupil, the
round black hole that changes size with the help of tiny
muscles that control the amount of light entering the
eye. The sphincter muscle, around the very edge of the
pupil, contracts in bright light to constrict the pupil.
The dilator muscle runs radially throughout the pupil to
widen the eye in dim lighting.
The ciliary body lies just behind the iris, and tiny
fiber guy wires called zonules are attached to it. One
of its functions is the production of aqueous humor, the
clear liquid that fills the space between the cornea and
the iris. Aqueous humor nourishes the cornea and the
lens and gives the front of the eye its shape. The
ciliary body also helps change the shape of the lens.
When it contracts, the zonules relax, the lens thickens
and close-up vision is made possible. When it relaxes,
the zonules contract, the lens becomes thinner and
distance vision is made possible.
Vitreous is a thick, transparent substance that fills
the center of the eye. It is composed of water and is
two-thirds of the eye’s volume, providing its form and
shape. Firmly attached to certain areas of the retina,
vitreous has the consistency of egg-white in children
but thins as we age. As it thins, it can separate from
the retina, and this causes the floaters with which
older adults are familiar.
The choroids lie between the retina and the sclera and
are composed of layers of blood vessels that nourish the
back of the eye. They connect with the ciliary body
towards the front of the eye and the edges of the optic
nerve at the back of the eye.
The lens is transparent and convex on both sides.
Located behind the iris, it focuses light rays entering
through the pupil to form an image on the retina. The
nucleus (or innermost part of the lens) is surrounded by
soft material called the cortex. The lens is encased in
a capsular-like bag and suspended within the eye by
zonules (those tiny guy wires mentioned earlier). In
young people, the lens changes shape to accommodate for
near or far vision, but by older adulthood, it is often
no longer capable of the same flexibility and vision
correction becomes necessary.
The retina is a thin, multilayered membrane lining the
back two-thirds of the eye. Composed of millions of
visual cells, it is connected by the optic nerve to the
brain. The retina captures light rays that enter the eye
and sends electrical impulses to the brain that result
in sight. Rods and cones located in the retina function
as photoreceptors. The approximately six million cones
permit us to appreciate color, and the approximately 125
million rods function best in dim light and are
responsible for peripheral and night vision.
The macula is an area of the eye near the center of the
retina where visual perception is most acute. It is
responsible for the critical focusing vision so
necessary for seeing fine detail. One hundred times more
sensitive to detail than the peripheral retina, it’s
often called the “bull’s eye center” of the retina. The
fovea is the most center portion of the macula.
The optic nerve, composed of thousands of nerve fibers
that connect the macula and retina to the brain, carries
electrical impulses to the processing center of the
brain and converts them to sight. The visual portion of
the optic nerve is called the optic disc and is
connected to the back of the eye near the macula.