Which statement about the iris is true?

Which statement about the iris is true?

• A. It balances the intraocular pressure.
• B. It contains photoreceptors.
• C. It controls the amount of light entering the eye by adjusting pupil size.
• D. It focuses light onto the retina.

Answer: (C)

The iris controls the amount of light entering the eye by adjusting pupil size. It does this with two smooth muscle layers: the circular sphincter pupillae constricts the pupil in bright light, while the radial dilator pupillae widens it in dim light. Parasympathetic input causes constriction and sympathetic input causes dilation, forming the pupillary light reflex that protects the retina from excess light. The iris doesn’t balance intraocular pressure—aqueous humor production and outflow govern pressure. It also doesn’t contain photoreceptors (those are in the retina) or focus light (that’s done by the cornea and lens).

The iris controls the amount of light entering the eye by adjusting pupil size. It does this with two smooth muscle layers: the circular sphincter pupillae constricts the pupil in bright light, while the radial dilator pupillae widens it in dim light. Parasympathetic input causes constriction and sympathetic input causes dilation, forming the pupillary light reflex that protects the retina from excess light. The iris doesn’t balance intraocular pressure—aqueous humor production and outflow govern pressure. It also doesn’t contain photoreceptors (those are in the retina) or focus light (that’s done by the cornea and lens).