well-known song expresses it best. “Let’s start at the
very beginning, a very good place to start. When you
read, you begin with A-B-C, when you sing you begin with
Do-Re-Mi.” And when you attempt to understand something
as entirely complex and profoundly miraculous as the eye
and our generation’s miracle of low vision assistive
technology, the place to start is with the organ itself,
that body part that lets us see, read, watch, observe,
witness, perceive, distinguish, notice, glimpse, view,
examine, and behold. Let this article be your
encyclopedia of anatomy, a reference point for sight.
With it as a foundation, become better prepared to delve
into the exciting world of low vision assistive
technology and the diseases that it helps to conquer.
“front window,” the cornea is the thick, nearly circular
structure covering the eye and providing nearly 2/3 of
its ability to focus. Normally clear and with a shiny
surface, the cornea has no blood vessels. Quite
sensitive, there are more nerve endings in the cornea
than anywhere else in the body. One half millimeter
thick in an adult, it has five layers: the epithelium,
Bowman’s Membrane, the stroma, Descemet’s Membrane and
is a layer of cells covering the surface of the cornea.
It’s nearly five to six cell layers thick and quickly
regenerates when an injury occurs. If a wound penetrates
too deeply into the cornea, an opaque scar is sometimes
left that can result in a loss of clarity and luster.
Membrane lies just beneath the epithelium. Tough and
difficult to penetrate, it protects the cornea.
The stroma is
the thickest layer of the cornea and is comprised of
tiny collagen fibers running parallel to each other.
This special formation of collagen fibers gives the
cornea its clarity.
Membrane lies between the stroma and the endothelium and
serves as a protective barrier against infection and
The endothelium is just one cell
layer thick. It pumps water from the cornea, keeping it
clear. If it is damaged or diseased, its cells will not
The conjunctiva is a thin,
transparent surface covering the outer surface of the
eye. It begins at the outer edge of the cornea, covers
the visible part of the eye and lines the inside of the
eyelids. Nourished by tiny blood vessels invisible to
the naked eye, it secretes oils and mucous that moisten
and lubricate the eye.
The sclera is
the white of the eye and is a tough, opaque surface
serving as the eye’s protective outer coat. At the back
of the eye, the optic nerve is attached to the sclera.
In children, the sclera is thinner and more translucent
and tends to take on a bluish cast, but by adulthood, it
takes on more of a yellowish cast.
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
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.
The extra ocular
muscles are six tiny muscles that surround the eye and
control its movements. The four rectus muscles control
the up, down, left and right movements of the eye, and
the two oblique muscles control the inward and outward
Tear film is produced by tiny
glands that surround the eye. Comprised of three layers
(oil, water and mucous), the lower mucous layer serves
as an anchor for the tear film and helps it adhere to
the eye. The middle layer is comprised of water. The
upper oil layer seals the tear film and prevents
evaporation. Tear film keeps the eye moist, provides a
smooth surface for light to pass through the eye,
nourishes the front of the eye and provides protection
from injury and infection.
Eyenatomy 101, a working knowledge of one of life’s
greatest miracles. From the very beginning, when an
object is in the line of sight, through the incredible
process where light rays are reflected from the object
to the cornea, the light rays are bent, refracted and
focused by the cornea, lens, and vitreous, the rays come
to a sharp focus on the retina, light rays are converted
to electrical impulses and transmitted through the optic
nerve to the brain where the image is perceived, it’s an
astonishing journey of possibility. Truly an amazing
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