The Miracle of Sight — Why We See
This is the first in a five-part series of web features that explores the
wonderment of eyesight, the care and keeping of eyes, common eye ailments and
treatments, the future of eye care and the emerging hope for the blind.
You don't have to understand how your eyes work to appreciate that they do.
Yet, by studying why we see, one can't help but grow a deepened
appreciation for this amazing sensory ability.
Here's how they work
Two main ingredients make possible the loveliness of vision: light and
eyeballs. One way to grasp the concept is to think of eyes as a film camera,
only better, because our "cameras" are far more flexible and connected
to brains.
The lens cap of our eyes is the cornea, and as such, its job is to protect
the eye and allow light to enter through the pupil, that black dot in the
center. How much light enters is controlled by the iris, the colored ring of
the eye. The iris and pupil work together to perform the aperture function of a
camera: The iris expands and contracts the pupil to allow in more or less
light. This is how our eyes are able to respond to a wider range of light than
any camera or other man-made device, which is why we can see in extremely dark
and extremely bright conditions.
Just behind the iris is the lens, which focuses light onto the retina,
located in the back of the eye. The lens is the part of the eye that sometimes
grows cloudy – called a cataract – and is surgically removed and replaced by an
artificial implant.
Perception is everything
The retina is like camera film onto which the image (defined by light rays)
is exposed. It contains more than 100 million photoreceptor nerve cells called
rods and cones. Rod and cone cells react to different levels of light – cones
respond to high light levels and rods, to low levels. Depending on the amount
of light striking the retina, one kind of cell will become more reactive, while
the other will disengage. The human eye has about 7 million cones and 120
million rods. So it makes sense that nocturnal animals have more rods and fewer
cones. Also, since cones are the cells that distinguish color, many animals
have a limited ability to perceive color. In humans, the condition known as
color blindness is the result of an abnormality or deficiency in the cones.
Back to task, rods and cones change the incoming light rays into electrical
impulses – this is where it gets really complicated and almost magical – and
transports them through the optic nerve to the brain, which then makes sense of
it all. It's interesting to note that the image created by the eye is actually
inverted and the reason we don't notice is because we can't; it's the only
reality we know.
The eye is filled with two nearly transparent liquids that nourish eye cells
in the way that blood nourishes other body cells. The vitreous is a jelly-like
substance that fills the space between the lens and retina. The aqueous humor
fills the space between the lens and cornea, nourishing the lens, iris and
cornea.
Our vision helps make life rich, yet those of us with sight tend to take
it for granted...that is, until there's a problem. Stay tuned for upcoming
articles in this series, which will focus on eye care, some common eye
conditions and current treatments.
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