Eye

Eye, organ of vision possessed by all vertebrate and most invertebrate animals. Eyes vary widely in complexity. Many invertebrates have simple cup-shaped eyes containing light-sensitive cells that merely perceive the intensity of light. Insects and crustaceans have compound eyes comprising many hundreds of units that build up a picture composed of minute light and dark spots like a newspaper photograph. Vertebrates and a few invertebrates (such as the squid) have eyes with lenses that focus images onto a light-sensitive surface.

The human eye is roughly spherical and is moved in its socket by 6 muscles. The wall of the eyeball has 3 main layers. The outermost layer is the tough and fibrous sclera, which merges at the front into the cornea, a hard transparent layer. Beneath the sclera is the choroid, which contains a dark pigment to prevent scattering of light within the eye. Toward the front, the choroid forms the ciliary body, whose muscles control the lens. The ciliary body merges with the iris, the colored part of the eye, whose muscles respond to varying light intensity by widening and closing the pupil, the opening in the iris through which lights enters. The innermost layer of the eye is the retina, which contains light-sensitive nerve endings that send signals to the brain through the optic nerve. Since the nerve itself is not sensitive to light, there is a blind spot where it leaves the eye. The blood vessels supplying the retina enter the eye through the center of the optic nerve and spread out over the retina.

Light entering the eye first passes through a thin transparent layer of skin, the conjunctiva, which is lubricated by the tears, secreted by a gland above the eye. The lens lies behind the pupil and focuses an image on the retina. Spread over the surface of the retina are some 130 million minute light-sensitive nerve endings. About 7 million of these, shorter than the others, are called cones. The other nerve endings, called rods, are spaced out much more evenly. The cones are responsible for color vision, but function well only in fairly bright light. The rods operate at much lower levels of lighting, but their image is only in shades of gray. The optic nerve carries the signals via the optic chiasma, a major nerve junction, to the visual part of the brain, the occipital lobe, where the information from the two eyes is combined to give a stereoscopic image.