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The eye is a bag of living cells shaped like a
sphere approximately one inch in diameter, and
filled with transparent jelly and pressurized
liquid that keeps it inflated like a balloon.
At the front of the eye is the cornea, which is
a transparent window that allows light to enter.
Behind the cornea is the iris, a diaphragm of muscle
that controls the amount of light entering the
eye. The pupil is the dark hole in the center of the
iris, which becomes larger or smaller as the
iris expands or contracts. Just behind the iris is the
lens, which is a transparent capsule of living cells with the consistency of rubber.
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The lens focuses light onto the retina.
The ciliary muscle surrounds the lens and enables it to change focus. When the ciliary muscle
expands, it pulls on the lens and makes it thinner, bringing distant objects into focus. When
the ciliary muscle contracts, it makes the lens thicker, bringing near objects into focus.
Six extraocular muscles are attached to and surround each eyeball. These enable the eyes to move
and point at the same object at the same time. The power and precision of the extraocular
muscles is amazing; during the course of a typical rapid eye movement (lasting about 1/10 of a second
), the eyeball acccelerates at a tremendous rate and decelerates almost instantly. To be able to
do this, the extraocular muscles are more than 100 times stronger than is necessary to turn the
eyeballs slowly in their sockets! It has also been theorized that the extraocular muscles can change the
length of the eyeball, especially over a long period of time, and may play a secondary role in the
focusing process.
At the back of the eyes is the retina, which is a complex network of nerve cells that convert the
light into electrical impulses, which travel up the optic nerve to the brain.
The partnership between the eyes and the brain is so close and complex that it is impossible
to discuss vision without describing this remarkable interaction. It takes places in five
steps:
1st: electrical impulses from the eye allow the brain to "observe" a scene and choose an object
to inspect more closely. The eyes are not yet directed specifically to the object.
Next, the brain determines the relative position of the object; computes the direction and
power necessary to move the eyes into position to point it; and directs th extraocular muscles
to rotate the eyeballs to the correct position.
3rd: with eyes pointed at the object, the brain directs the ciliary muscles to focus the lenses
and make the images on the retinas as clear as possible.
4th: the retinas process the images for basic information such as outlines, colors, and motion,
and passes them along to the brain via the optic nerve. The brain completes the processing
and derives more information about detail, distance and dimension. Because the eyes are a
few inches apart, each receives a slightly different image-in two dimensions. The brain, like
ac computer, integrates the separate images into a three dimensional representation. The brain
then determines the meaning and significance of the completed image.
Finally, the brain decided if the body needs to respond to the viewed object, and uses the eyes
to coordinate any body movements necessary to carry out the intended action.
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