What makes the iris expand and contract?

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The iris helps in controlling the amount of light entering the eye by altering the diameter of the pupil. In low light conditions, the iris increases the pupil’s diameter to increase the amount of light passing through to the retina to improve the clarity of the image. In high light conditions, the iris reduces the pupil’s diameter to reduce the amount of light passing through and avoid damage to the retina.

The contraction and dilation of the pupil by the iris is caused by two muscles. The sphincter papillae are the muscles responsible for the contraction motion (circular) of the pupil, making it narrower. The dilator papillae are the muscles responsible for the dilatory motion (radial) of the pupil, making it larger.

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