MEHAU KULYK / SCIENCE PHOTO LIBRARY MEHAU KULYK / SCIENCE PHOTO LIBRARY
Eye anatomy. Historical artwork of blood vessels (red and yellow lines) and circular structures at the front of a human eye. The pupil (central hole) is surrounded by concentric rings. The iris (large ring) is followed by the ciliary processes (narrow ring, yellow), and then the ciliary ring (outer, large ring, red). The ciliary structures are not normally seen, as they are behind the sclera (the white of the eye). Unlike the retina at the back of the eye, the blood vessels for the front of the eye spread out from the edge of the eye, rather than a central spot. The ciliary structures allow focusing of the lens (not seen). The iris relaxes and contracts to change the size of the pupil, through which light passes to enter the eye.
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