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eyeball eye·ball (ī'bôl')
The globe-shaped portion of the eye surrounded by the socket and covered externally by the eyelids. Also called bulb of eye.
The eye itself.
spheroidal structure containing sense receptors for vision, found in all vertebrates and constructed much like a simple camera. The eyeball houses the retina-an extremely metabolically active layer of nerve tissue made up of millions of light receptors (photoreceptors)-and all of the structures needed to focus light onto it. The sclera, the tough protective outer shell of the eyeball, is composed of dense fibrous tissue that covers four-fifths of the eyeball and provides attachments for the muscles that move the eye. The sclera is itself covered anteriorly by the conjunctiva, a transparent mucous membrane that prevents the eye from drying out. At the front of the eye, the tear film covers the transparent cornea, the "window" through which light passes into the eye. Working in concert with the aqueous humour behind it, the cornea provides the greatest focusing power of the eye. However, unlike the lens, the shape and focusing power of the cornea are not adjustable. Other important structures in the eyeball include the iris and the lens. Much of the eyeball is filled with a transparent gel-like material, called the vitreous humour, that helps to maintain the spheroidal shape