Certified Paraoptometric (CPO) Practice Exam 2025

What structure controls the shape of the eye?

1. Pupil

2. Lens

3. Iris

4. Ciliary Muscle

The correct answer is: Ciliary Muscle

The structure that primarily controls the shape of the eye is the ciliary muscle. This muscle is responsible for changing the curvature of the lens, allowing for the fine focusing of light onto the retina. When the ciliary muscle contracts, it relaxes the tension on the zonular fibers connected to the lens, enabling the lens to become more rounded for near vision. Conversely, when it relaxes, the tension on the zonular fibers increases, flattening the lens for distance vision. This process is crucial for accommodating vision at various distances. Other structures mentioned, such as the pupil, lens, and iris, play important roles in vision but do not have the same function in controlling the shape of the eye necessary for focusing. The pupil regulates the amount of light entering the eye, the iris controls the size of the pupil, and the lens serves as a transparent structure that changes its shape through the action of the ciliary muscle to help focus light on the retina.