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How does the iris of the human eye control the size of the pupil?
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human eye
A horizontal cross section of the human eye, showing the major parts of the eye, including the protective covering of the cornea over the front of the eye.
The iris controls pupil size through two sets of smooth muscles: the sphincter muscle and the dilator muscle. In bright light, the sphincter muscle, a circular band of fibers, constricts the pupil to limit light entry. Conversely, in dim light, the dilator muscle, with fibers arranged radially, contracts to widen the pupil, allowing more light to reach the retina. This action is involuntary and controlled by the autonomic nervous system. The pupil's size also adjusts for focusing on near objects, narrowing to increase depth of focus.12345