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Even "perfect"
vision involves
seeing through a
network of blood
vessels located
in front of the
retina. |
Less-than-perfect vision |
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| Incredibly,
this is exactly how the human retina is constructed.
Visual quality is degraded because light scatters as it
passes through several layers of cellular wiring before
reaching the retina. Granted, this scattering has been
minimized because the nerve cells are nearly transparent,
but it cannot be eliminated because of the basic design
flaw. Moreover, the effects are compounded because a network
of vessels, which is needed to supply the nerve cells
with a rich supply of blood, also sits directly in front
of the light-sensitive layer, another feature that no
engineer would propose. |
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| A more serious
flaw occurs because the neural wiring must poke directly
through the wall of the retina to carry the nerve impulses
produced by photoreceptor cells to the brain. The result
is a blind spot in the retina -- a region where
thousands of impulse-carrying cells have pushed the sensory
cells aside. Each human retina has a blind spot roughly
a millimeter in diameter -- one that would not exist if
only the eye were designed with its sensory wiring behind
rather than in front of the photoreceptors. |
The optic nerve
connects to the
brain through a
hole in the
retina, causing a
blind spot. |
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Do these design
problems exist because it is impossible to construct an
eye that is wired properly, so that the light-sensitive
cells face the incoming image? Not at all. Many organisms
have eyes in which the neural wiring is neatly tucked
away behind the photoreceptor layer. The squid and the
octopus, for example, have a lens-and-retina eye quite
similar to our own, but their eyes are wired right-side
out, with no light-scattering nerve cells or blood vessels
in front of the photoreceptors, and no blind spot. |
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