How does the iris control size of pupil?

The iris controls the size of the pupil through the actions of two sets of muscles: the sphincter pupillae and the dilator pupillae.

Sphincter pupillae: This muscle surrounds the pupil and functions to constrict (make the pupil smaller) when stimulated. The sphincter pupillae muscle receives parasympathetic nerve innervation from the ciliary ganglion. When the parasympathetic nervous system is activated, the sphincter pupillae muscle contracts, causing the pupil to constrict. This occurs in response to bright light, during near vision (accommodation), and in certain emotional states such as fear and excitement.

Dilator pupillae: This muscle is located within the iris and functions to dilate (make the pupil larger) when stimulated. The dilator pupillae muscle receives sympathetic nerve innervation from the superior cervical ganglion. When the sympathetic nervous system is activated, the dilator pupillae muscle contracts, causing the pupil to dilate. This occurs in response to low light conditions, far vision, and in certain emotional states such as anger and surprise.

The sphincter pupillae and the dilator pupillae muscles work together to control the size of the pupil in response to changing light conditions and visual demands. In normal conditions, the pupil constricts in bright light and dilates in low light. This helps to regulate the amount of light entering the eye, thus protecting the retina from excessive light and ensuring optimal vision in different lighting conditions.