Which muscle of the iris is primarily responsible for constricting the pupil?

The sphincter muscle of the iris is primarily responsible for constricting the pupil, a process known as miosis. This muscle encircles the pupil and contracts in response to various stimuli, such as bright light or during near vision, to reduce the amount of light entering the eye and protect the retina. Its contraction decreases the pupil size, ultimately allowing for better focus on close objects and enhancing depth of field.

In contrast, the dilator muscle, which is not the correct answer, is responsible for dilating the pupil (mydriasis), allowing more light to enter, especially in low-light conditions. The medial rectus and superior oblique muscles are extraocular muscles that control eye movement but do not have a role in regulating pupil size. Therefore, focusing on the function and role of the sphincter muscle clarifies its importance in pupil constriction within the context of eye physiology.