"Self-knowledge can be obtained only by looking into the mind and virtue of the soul, which is the diviner part of a man, as we see our own image in anothers eye." [Plato, "Alcibiades," I.133]
pupil pu·pil (pyōō'pəl)
The apparently black circular opening in the center of the iris of the eye, through which light passes to the retina.
|pupil (py'pəl) Pronunciation Key
The opening in the center of the iris through which light enters the eye.
in the anatomy of the eye, the opening within the iris through which light passes before reaching the lens and being focused onto the retina. The size of the opening is governed by the muscles of the iris, which rapidly constrict the pupil when exposed to bright light and expand (dilate) the pupil in dim light. Parasympathetic nerve fibres from the third (oculomotor) cranial nerve innervate the muscle that causes constriction of the pupil, whereas sympathetic nerve fibres control dilation. The pupillary aperture also narrows when focusing on close objects and dilates for more distant viewing. At its maximum contraction, the adult pupil may be less than 1 mm (0.04 inch) in diameter, and it may increase up to 10 times to its maximum diameter. The size of the human pupil may also vary as a result of age, disease, trauma, or other abnormalities within the visual system, including dysfunction of the pathways controlling pupillary movement. Thus, careful evaluation of the pupils is an important part of both eye and neurologic exams.
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