The Eye—“The Envy of the Computer Scientist”
THE retina is a small membrane that fits over the back of the eye. As thin as paper, it contains over a hundred million neurons arranged in different layers. “The retina,” states the book The Living Body, “is one of the most remarkable pieces of tissue in the human body.” It is “the envy of the computer scientist, performing approximately 10 billion calculations every second,” states Sandra Sinclair in her book How Animals See.
As a camera focuses an image on photographic film, our eye focuses on the retina an image of what we see. Yet, as Dr. Miller explains, camera film “does not even begin to compare with the versatile sensitivity of the retina.” With the same “film” we can see by moonlight or in sunlight 30,000 times more intense. Furthermore, the retina can discern fine details of an object part of which is bathed in light and the rest of which is in shadow. “The camera,” explains Professor Guyton in his Textbook of Medical Physiology, “cannot do this because of the narrow critical range of light intensity required for proper exposure of film.” Hence, photographers need flash equipment.
The “versatile sensitivity of the retina” is due, in part, to 125 million rods. These are sensitive to small amounts of light, making vision possible at night. Then there are about 5.5 million cones that respond to brighter light and make possible detailed color vision. Some cones are most sensitive to red light, others to green and others to blue. Their combined response enables you to see all the colors in this magazine. When all three types of cones are excited equally, the color you see is pure white.
Most animals are limited in their ability to see in color, and many do not see color at all. “Colour vision adds immensely to the joys of life,” says surgeon Rendle Short, adding: “Of all the organs of the body not absolutely essential for life, the eye may be considered the most wonderful.”
Images fall upside down on the retina just as they do on a camera’s film. “Why is the world not upside down to us?” asks Dr. Short. “Because,” he explains, “the brain has developed the habit of reversing the impressions.”
Special glasses have been designed to turn the image around. In scientific experiments, people who wore such glasses saw everything upside down. Then, after a few days, something amazing happened. They began to see normally! “The miraculous teamwork of your eye and your brain is exhibited in a number of ways,” comments The Body Book.
As your eye moves across this line, the cones distinguish the black ink from the white paper. Your retina, however, cannot respond to characters of a man-made alphabet. We learn to give meaning to a string of characters in another part of the brain. A transfer of information is needed.
The retina sends a coded message via a million nerve fibers to a part of your brain situated near the back of your head. “The projections from the retina to the cerebral cortex,” explains the book The Brain, “are highly organized and orderly. . . . If a small light is shined on each different part of the retina, a corresponding part of the visual area [in the brain] will respond.”
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Unlike a camera, because the retina has such a wide range of sensitivity to light, the eye is not dependent on flash equipment
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Your retina has millions of neurons, called cones, which are sensitive to green, red, or blue