This week, Tidbits opens your eyes to see the facts about the organs of vision.

There are more than 2 million working parts in the human eye. It weighs just under an ounce and is about an inch across. Just over 16.5% of the eyeball is exposed. The eyes begin to develop just two weeks after conception.

      How do we see images? Light enters the eye through the cornea and goes to the lens. The cornea and lens bend the light to bring the image into focus. The light reaches the retina at the back of the eye, and the retina converts the light into electrical impulses. Behind the retina, the optic nerve carries the impulses to the brain, which interprets what is being seen and brings into a clear image. When an image reaches the retina, it’s upside-down and backward. The brain reorients the image.

    Situated on the retina are 7 million cone cells, nerve cells that enable us to see at least a million colors when they send signals to the brain. Tubular-shaped rods on the outside area of the retina provide black and white vision. There are more than 100 million rod cells, which are up to 1,000 times more responsive to light than the cone cells. It’s the rod cells that help you see in the dark.

•    About 10% of your waking hours are spent blinking your eyes, keeping them lubricated and protected from dust and other particles. You’ll blink between 15 and 20 times a minute, up to 19,000 times a day, more than 5 million times in a year.

    What does it mean to have 20/20 vision? It simply means that a person can read the eye chart from 20 feet away.    20/20 isn’t by any means “perfect” vision. Many people have 20/15 vision, which means they see clearly from 20 feet what the average person can see at 15 feet.   

    Eye color is the result of a variety of pigments in the iris, a two-layer tissue that allows light into the eye and controls the size of your pupil. There are three main pigments in the iris – melanin, pheomelanin, and eumelanin. Melanin is a yellow-brown pigment, pheomelanin is red-orange, and eumelanin, a black-brown hue. Those with brown eyes have melanin in both layers of the iris, while people with lighter eyes have it in just the back layer. Pheomelanin is the source of color for green and hazel eyes, most common in those of Irish and Scottish descent. The darkest pigment, eumelanin, is found in the eyes of Africans, Asians, and South Americans.

Brown is the most common eye color, with more than half of the world’s population having brown eyes. Green eyes are the rarest, just 2% of the world.    In the U.S., brown eyes account for 45%, blue eyes for 27%, hazel for 18%, and green eyes at 9%. Another 1% have another color, such as amber or violet. A rare condition known as heterochromia occurs in people who have two different eye colors.

Those with blue eyes are more sensitive to light because their eyes don’t have as much melanin to block out UV rays.

•    Approximately 1 in 10 men are color blind, having trouble distinguishing between red and green tones. Some can’t tell blues and yellows apart.    The more correct term is “color deficiency,” since with true color blindness, the person sees only shades of black and white, a much rarer condition.

      The eyes’ corneas are the body’s only tissues that don’t contain blood.