The Iris


The iris is like the diaphragm of a camera, controlling the amount of light entering the eye. This pigmented membrane is located behind the cornea and aqueous humour and in front of the lens. The pupil is the hole in the center of the iris. Its size is controlled by the dilation and constriction of the iris. The dilator muscle causes the iris to dilate, making the pupil larger. This occurs when the eye needs more light. The sphincter muscle causes the iris to close, making the pupil smaller. This happens when the eye needs less light. The iris can be many different colors. Melanocytes, pigment filled cells, give the iris its color. A brown iris has lots of large melanocytes while a blue iris has few.



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