DANNY BRASS / SCIENCE PHOTO LIBRARY DANNY BRASS / SCIENCE PHOTO LIBRARY
Human eye. A close-up of a healthy brown human eye. The eye has a tough outer coat called the sclera (the white of the eye). The front circular part of the outer coat is the cornea which performs most of the focusing. The coloured part of the eye is the iris which has a central perforation called the pupil which always appears black. It is through the pupil that light enters the eye and falls onto the retina, the light sensitive layer that lines the inside of the back of the eye and on which the images are cast. The eyelid provides protection for the eye, closing instantaneously if anything approaches it.
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