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thumbnail image of human eye life's grand design
eyeglasses and optomotrist's chart
Even "perfect"
vision involves
seeing through a
network of blood
vessels located
in front of the
retina.
Less-than-perfect vision  
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.  
 
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. diagram of optic nerve
The optic nerve
connects to the
brain through a
hole in the
retina, causing a
blind spot.
   
  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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