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welcome to the patient information page. Here we have
tried to provide adequate information on various eye
conditions. This information will help you in
understanding some eye problems being experienced by you
or someone you know. Please feel free to write to us if
you may require any more information on any eye problems
that you or your loved one may have. For ease of
navigation we have divided the various eye conditions
and the eye treatments both alphabetically and by the
various specialties they belong to.
To know more please email us.
Vision is a complex sense composed of many elements. The
human eye, elegant in its detail and design, represents
a gateway to the process we call vision. The eyeball, or
globe, is spherical in shape and about 1 inch across. It
houses many structures that work together to facilitate
sight.
The human eye is comprised of layers and internal
structures, each of which performs distinct functions.
The outside layer of the eye is comprised largely of a
tough, white, protective tissue called the sclera. The
sclera helps maintain the shape of the eyeball. At the
front of the eye is an equally tough but clear structure
called the cornea, which is responsible for letting
light into the eye and bending light.
Going from outside to inside, the next layer of the eye
is the choroid, which carries the blood supply necessary
to nourish the eye's internal structures. Finally, there
is the layer called the retina, lining the inside of the
eye, which is sensitive to light and receives
stimulation to its specialized cells.
The eye has a number of protective features. The
eyelids, eyelashes and eyebrows are all designed to
protect the eye from dirt and dust that might enter it
and cause damage. The globe sits inside the orbital
cavity, a bony pocket lined with fatty tissue as a
cushion. Together these provide additional protection
against injury.
Movements of the eyes occur due to the action of Six
muscles which are attached at various points to the
sclera.
In order for vision to take place, a succession of
processes must occur involving the structures within the
eye and the brain:
The first part of this chain is that light rays must
travel through the eye to ultimately focus on the
retina. There are a number of structures involved in the
bending or refracting of light so that it focuses
properly. Light first passes through the clear cornea at
the front of the eye, and then through a watery
substance called the aqueous humor which fills the small
chambers located behind the cornea. As light continues
on its pathway it passes through the pupil, a round
opening in the center of the iris. The iris is the part
of the eye that gives the eye its color.
The next structure light will penetrate is the lens,
another clear, layered structure shaped like a large
lentil (about 10 mm in diameter) that is attached to
muscles which contract or relax to change the shape of
the lens. The changing lens shape helps light to be
focused in response to the need for clarity. (The loss
of this focusing ability as humans age -- a natural
occurrence -- is the reason that many adults over 40
years old need reading glasses.) Once through the pupil
and lens, the light then passes through the larger
posterior (back) portion of the eye that is filled with
a clear, jelly-like substance called the vitreous humor.
From there, the light will come to the retina, where the
rod cells and cone cells will be stimulated to set off a
chain of split-second chemical reactions converting
light to electrical impulses. The cone cells (about 7
million in number) are located in greatest concentration
in the small, central part of the retina called the
macula. This area is responsible for producing sharp,
detail vision and color vision. The rod cells (numbering
about 100 million) are found in the peripheral retina,
away from the macula. These cells provide vision in dim
light.
Even if all of the structures of the eye work perfectly,
what we know as vision cannot happen without the brain's
interpretation of the electrical impulses sent by the
retina. The optic nerve is the bundle of retinal fibers
that exits the back of the eye and transports electrical
impulses to the brain where they are interpreted in the
primary visual cortex.
When all parts of the visual system are working, the
eyes can move together, can adapt to light and dark,
perceive color and accurately evaluate an object's
location in space. They are sensitive to differences in
contrast, and can also provide detail vision, which is
measured as visual acuity. By convention, we know
"normal" visual acuity to be reported as 20/20 or 6/6.
As the bottom number of this expression gets higher, it
tells us that the vision is poorer than "normal." For
example, the start of the range known as "legal
blindness" is represented by the visual acuity finding
of 20/200. One way to understand the meaning of this
finding is that the eye being tested sees at 20 feet
what the "normal" eye would see at 200 feet. People
whose vision is in the category of "legal blindness" may
still be able to use vision to do some of the things
they need to do.
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