Which muscle of the iris constricts in bright light?

The sphincter muscle of the iris is responsible for constricting in response to bright light. When bright light enters the eye, the pupillary light reflex is activated, and the sphincter muscle contracts, leading to miosis, or constriction of the pupil. This response helps protect the retina from excessive light exposure and improves the depth of field, allowing for better visual acuity in well-lit conditions.

In contrast, the dilator muscle of the iris performs the opposite function, causing the pupil to dilate in low light conditions. The longitudinal muscle does not play a direct role in controlling pupil size, while the radial muscle, which is a term often referring to the dilator muscle, also aids in pupil dilation rather than constriction. Understanding these functions emphasizes the symmetrical and antagonistic roles of the iris muscles in regulating light entry into the eye.