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eyeball
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Diagram of a horizontal section of the human eye showing labeled parts including the cornea, iris, lens, retina, sclera, optic nerve, and blood vessels, with internal structures and muscles visible. Close-up of a child's blue eye, showing detailed iris patterns and fine eyelashes. Side-by-side diagrams compare a nearsighted eye, where light focuses in front of the retina, with a normal eye, where light focuses on the retina. Both eyes are labeled with cornea, lens, retina, and sclera. Side-by-side diagram comparing a farsighted (hyperopic) eye and a normal eye, showing how light focuses behind the retina in hyperopia and directly on the retina in a normal eye, with labeled parts including lens, cornea, sclera, and retina.
At a Glance
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Diagram of a horizontal section of the human eye showing labeled parts including the cornea, iris, lens, retina, sclera, optic nerve, and blood vessels, with internal structures and muscles visible.
cross section of the eye Horizontal cross section of the human eye, showing the structures of the eye, the visual axis (the central point of image focusing in the retina), and the optical axis (the axis about which the eye is rotated by the eye muscles).

eyeball

anatomy
Written by
Daniel M. Albert
Professor of Ophthalmology, University of Wisconsin at Madison. Professor of Ophthalmology, Wisconsin Alumni Research Foundation.
Daniel M. AlbertAll
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Britannica Editors
Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....
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History

eyeball, 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...

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