iris defined in 1951 yeariris - iris;
iris - Structure controlling amount of light admitted to vertebrate eye. The iris is the coloured part of human eye. It is a thin sheet of tissue, forming a pigmented diaphragm in front of lens with central opening (pupil) through which light passes to retina. Attached at outer margin to ciliary body. Contains radiating muscles which enlarge pupil and a ring of muscles at its free inner margin which narrows pupil. Light, stimulating retina and optic nerve, reflexly controls size of pupil, narrowing for strong, enlarging for dim light. Accommodation for near vision is also accompanied by narrowing. A structurally and functionally similar iris occurs in Cephalopod eye.
near iris in Knolik
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