Which structure controls lens thickness?

• A. Iris
• B. Ciliary body
• C. Cornea
• D. Retina

Answer: (B)

Accommodation depends on the ciliary body, specifically the ciliary muscle, which changes lens shape to adjust focusing. The lens is held by zonular fibers attached to the ciliary body. When the ciliary muscle contracts, the zonules slacken, allowing the lens to become more spherical and thicker, increasing its refractive power for near vision. When the muscle relaxes, the zonules pull on the lens, flattening it for distant vision. The iris only changes pupil size, the cornea’s curvature is stable for a given eye, and the retina is the light-sensing layer; none of these adjust lens thickness. Parasympathetic input to the ciliary muscle mediates this contraction during near focusing.