HOW THE EYE FUNCTIONS
To appreciate how the eye functions, imagine
yourself as a beam of light reflected from a 100-foot
tree into the eye of a living person. You are the
image of the tree, traveling at the speed of light
and about to enter an obstacle course on your way
to the brain of the observer. Your first encounter
is your passage through the clear convex cornea
which bends (refracts) you and slows you down.
It also shrinks you to manageable size (little
larger than a nickel). Next you squeeze through
a round, adjustable opening, the pupil, formed
by a colorful membrane, the iris, which, if you
are too bright, will reduce your intensity. You
now encounter a rather dense but transparent medium,
the lens, which not only bends you even more, but
unceremoniously turns you upside down and aims
(focuses) you at the back of the eye, the retina,
which you strike after passing through a clear,
sticky, gel-like substance, the vitreous humor.
You are now the inverted image of a 100-foot tree
shrunk to the size of a postage stamp and flattened
against the retina. But not for long! Instantly
you are transformed from a beam of light into an
electrical impulse, and flashed along the optic
nerve from the retina to the brain, where you are
perceived as a 100-foot, three-dimensional, right-side-up
tree. And all of this in a tiny fraction of a second.
To recap: the cornea is the clear, transparent
front covering which admits light and begins the
refractive process, and also keeps foreign particles
from entering the eye. The pupil is an adjustable
opening that controls the intensity of light permitted
to strike the lens, which focuses light through
the vitreous humor, a clear gel-like substance
that fills the back of the eye and supports the
retina. The retina receives the focused image from
the lens, and transforms this image into electrical
impulses that are carried by
the optic nerve to the brain. In this article we are primarily concerned with
the cornea, and an affliction of the cornea known as keratoconus.
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