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Focusing On Retina - A Lesson About How Our Eyes Work
The retina is a lining inside the eye covering the
entire back half of the eyeball. With respect to vision, this
is where the real action takes place. Millions of retinal
cells, nature's microchips, constantly flash messages to your brain.
These countless bits of information are carried on nerve fibers,
somewhat like electrical impulses that travel on fiber-optic
strands.
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Your brain decodes these messages, much as
your telephone and television set decode electrical
impulses. For the transmission of information to the brain,
the cells of the retina are divided into two basic cell
types: the rods and the cones. The rods function best in dim
light, the cones under lighted conditions.
There are about 7 million cones and 125 million rods. The
cones are snappy little numbers that spring to attention the
moment light hits them. They handle fine details and color.
The rods, on the other hand, are laggards, taking about ten
minutes to come to attention when you enter a dim or
darkened room. Their job, in addition to helping you see in
the dark, is to distinguish black and white.
Your sight is further refined by the connection
of each rod and cone to a type of cell called a bipolar
cell. |
The job of the bipolar cells is to relay messages to the over
1.9 million ganglion cells, which carry the messages to the
brain. The bipolar cells have a lot of responsibilities. They
are divided into two types, the magno cells, numbering about 10%
of the total, and the parvo cells, which are much more numerous
at 90% of the total.
The magno cells make it possible to distinguish shapes in
indistinct light and to see in the dark. Thanks to these cells,
you can find a quarter on a wood floor in dim light or locate
that black suit in the back of your closet. Loss of magno cells
may impair your ability to read, causing difficulty in
separating the details of letters, spaces, and the breaks
between words. The parvo cells fill in the details, like a
painter adding texture, line, and color to the sketch.
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Since there are fewer ganglion cells than
there are rods and cones, several hundred rods and cones are
connected to each ganglion cell, forming a field around the
cell. If you were to lose ganglion cells, it would affect
your ability to see well in dim light, cause difficulty in
contrasting similar shades of color, and make detail fuzzy.
Cataracts can cause similar symptoms.
The macula, in the center of the retina, contains the
richest supply of ganglion cells. It is the hub of sight. If
you lose cells in this region, it robs you of the ability to
see things in the center of the visual field and compromises
your ability to fixate (to focus the eyes on a specific
point or object). You need to fixate in order to read, for
example. Otherwise, the letters seem to jump around on the
page.
Macular failure is a condition most often associated
with age-related macular degeneration (AMD), a disease that
primarily affects the elderly. |
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With glaucoma, damage to the macula generally
does not occur until the final stages of the disease. Glaucoma
patients experience visual loss in their peripheral areas first.
Unfortunately, there are fewer cells in this area, so if you lose
any of them, your peripheral vision decreases. While this type of
vision loss is not as disabling as the loss of central vision
experienced by people with AMD, it is, nevertheless, worrisome, for
you find your side vision, and your top and bottom vision, falling
away.
In the center of the macula is a tiny body of densely packed cone
cells, each with its own connecting ganglion cell. The fact that
each cone cell in this area gets its own ganglion cell is an
indication of how important this area is for seeing. This body, the
fovea, is your spyglass; it makes you able to spy a squirrel at the
side of the tree, see a statue decorating the corner of a high
building, or look miles into the distance on a clear day.
Birds of prey, such as hawks and vultures, possess not one, but two,
foveas that act as magnifiers, enabling them to locate objects that
even the sharpest of human eyes cannot detect. Such birds can detect
the presence of a creature that might become lunch from 9,000 to
13,000 feet in the air.
Perhaps the most remarkable feature of our sight apparatus is that
the eye produces actual images on the retina. A tiny inverted image
appears continuously during the act of seeing. This image is slowed
down or sped up depending upon the strength of the light source, the
eye's ability to fixate, and the general health of the rods, cones,
and nerve cells. For example, in a dim light, you may find it takes
a bit longer to discern the outlines or details of an object. A
similar situation occurs if you have cataracts or have trouble
fixating because of a loss of cone cells in the eye. |
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