Eye


Eye
The eye is the part of the body that lets humans and most animals see. It also gives them knowledge of nature's 24-hour cycle of light and darkness. Most animals have two eyes.
The eye is an organ for the purpose of sensing light. The simplest eyes (such as those of some animals and even protein spots in one-celled organisms) register nothing but whether the surrounding area is light or darkness. Some snails, for example, see no image (picture) at all, but they "see" light, enabling them to keep out of direct sunlight. Slightly better eyes are shaped like cups, which lets the organism know from which direction light is coming.
More complex eyes give the full sense of vision, including color, motion, and texture. These eyes have a round shape that makes light rays focus on the back part of the eye, called the "retina". There are (at least) three types of light-sensitive cells in the retina in mammals. Two of them, "rods" and "cones", allow sight (or seeing) by sending signals through the optic nerve to the brain. The third type senses light for a different purpose than seeing. Some special "ganglion" cells in the retina send their information about light to the brain along a different path (the "retinohypothalamic tract"). This information adjusts (synchronizes) the individual's circadian rhythm to nature's light/dark cycle of 24 hours. This system also works for some blind people who cannot see light at all.
Some animals can see light which is outside of the human range of visible light. They can see ultraviolet or infrared light.
The "lens" on the front part of the eye is curved and acts like a camera lens. It can be pulled flatter or rounder by muscles inside the eye. As some people get older, they might not be as able to do this. Many people are born with other sight problems or get them later in life, and they may need eyeglasses (or "contact lenses") to fix the problem.


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