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Wonders of the Human Eye

The complexity of perceptual cells beggars the imagination.  In humans, 127,000,000 cells called rods and cones line up in rows as the “seeing” elements that receive light and transmit messages to the brain....  These rod cells are so sensitive that the smallest measurable unit of light, one photon, can excite them.  Under optimum conditions the human eye can detect a candle at a distance of 15 miles.  Yet with rods alone, we would see chiaroscuro, only shades of black and gray, and would not get the focal resolution allowed by the more complex cones.

Squeezed into the dense forest of rods, the larger cones tend to concentrate in the precise spot in the eye where focusing is most acute.  Although cones are one thousand times less sensitive to light, they make possible all perception of colors and fine details....Our assortment of rods and cones lets us see objects at the ends of our noses and also stars light-years away....

Our brains do not receive photographic images of anything.  Rather, some of the 127,000,000 rods and cones get “excited” by light waves and fire off messages into the 1,000,000 fibers of the optic nerve, which coils like a thick television cable back into the recesses of the brain.  Impulses from the retina race along the fibers of the optic nerve, fan out in the brain, and finally slam into the visual cortex, stimulating the miracle of sight.

The cortex has no easy task, since one billion messages a second stream in from the retina.
 
— Paul Brand, M.D. and Philip Yancey, celebrating the complexities of the human eye in In His Image (Zondervan, 1987), 134-135.
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