Which structure is the ring of muscle around the pupil that controls its size?

Which structure is the ring of muscle around the pupil that controls its size?

• A. Pupil
• B. Sclera
• C. Lens
• D. Iris

Answer: (D)

The iris is the ring of muscle surrounding the pupil that controls its size. It contains two smooth muscle groups: the sphincter pupillae, which constricts the pupil to make it smaller, and the dilator pupillae, which dilates the pupil to make it larger. In bright light, the parasympathetic system triggers the sphincter pupillae to constrict, protecting the retina from excess light. In dim light, the sympathetic system triggers the dilator pupillae to dilate, allowing more light in. The pupil itself is the opening, the sclera is the white protective outer layer, and the lens sits behind the iris to focus light on the retina.

The iris is the ring of muscle surrounding the pupil that controls its size. It contains two smooth muscle groups: the sphincter pupillae, which constricts the pupil to make it smaller, and the dilator pupillae, which dilates the pupil to make it larger. In bright light, the parasympathetic system triggers the sphincter pupillae to constrict, protecting the retina from excess light. In dim light, the sympathetic system triggers the dilator pupillae to dilate, allowing more light in. The pupil itself is the opening, the sclera is the white protective outer layer, and the lens sits behind the iris to focus light on the retina.