What is the primary function of the sphincter muscle of the iris?

The primary function of the sphincter muscle of the iris is to constrict the pupil. This muscle is located in the iris of the eye and is responsible for reducing the size of the pupil when it contracts. This process occurs in response to bright light or when focusing on nearby objects, helping to control the amount of light that enters the eye and improving visual acuity in well-lit conditions.

By constricting the pupil, the sphincter muscle also contributes to the depth of field, allowing for clearer vision when observing objects at varying distances. In conjunction with the dilator muscles of the iris, which work to dilate the pupil, the sphincter muscle plays a crucial role in maintaining optimal vision by adjusting to changes in light and focusing requirements.