The sense of sight, Part 2: Human vision

StoryImage( ‘/Images/Story//Auto-img-1178124129537.jpg’, ”, ‘How we see – The human eye works much like a camera. Light first enters through the transparent cornea. The cornea has a tough outer covering that protects the eye and can quickly repair itself. Light passes through the pupil—the dark opening in the center of the eye. The pupil’s size is controlled by the surrounding iris, which adjusts to let more or less light through, depending on the brightness of the environment. The light then passes through the transparent lens, which adjusts in thickness to improve sharpness of the image. Next, light passes through the clear gel that fills the eyeball, landing on the retina. The retina is the surface that covers the back of the eyeball, and it contains special photoreceptive cells that react to light. These cells are connected to the optic nerve, which transmits signals to the brain for processing into an image.‘);

Human vision has roots back to the beginning of life itself

Eyes are photoreceptive organs; that is, they are capable of detecting light and transmitting information about that light to the brain. In some creatures, eyes simply sense light and dark. More complex eyes like ours can see color and shape, and can work in combination to accurately perceive depth and distances.

Many scientists believe that, despite their amazing diversity, all eyes in nature were derived from a single photoreceptive organ on a prehistoric creature hundreds of millions of years ago. In other words, the eyes of vertebrate animals such as humans, and the eyes of other creatures such as insects, sea worms and snails, all evolved from the same original set of genes. It’s strange to imagine that the light-reflecting eyes of the cat, the multifaceted eyes of a dragonfly, the stalked eyes of the snail, and our eyes might all share genetic ingredients.

Our eyes are paired on the front of our heads, providing us with excellent binocular vision. In prehistoric times, that vision allowed our hominid ancestors to judge depth and distance with the kind of precision needed to successfully hunt and catch prey, and therefore, to survive. In modern times, binocular vision allows humans to do such remarkable things as hitting a 90 m.p.h. fastball, skiing down a treacherously rocky slope, maneuvering a race car around a track, or timing the last step of a long jump.

When we “see,” what we’re actually doing is detecting waves of electro-magnetic radiation (light waves) that are bouncing off various objects around us. The light passes into our eyes and reaches special light-sensitive cells. A chemical response in these cells triggers connected nerves, which transmit signals to the brain for processing. Many things can go wrong in this system, leading to such vision problems as near and farsightedness, color blindness or loss of sight, for example.

from the May 2-8, 2007, issue

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