Certified Paraoptometric (CPO) Practice Exam 2025

What is known as the muscle inside the eyeball that alters the shape of the crystalline lens?

1. Retinal Muscle

2. Corneal Muscle

3. Ciliary Muscle

4. Orbital Muscle

The correct answer is: Ciliary Muscle

The ciliary muscle is crucial for changing the shape of the crystalline lens, a process known as accommodation. This adjustment is essential for focusing light appropriately on the retina, allowing for clear vision at various distances. When the ciliary muscle contracts, it relaxes the tension on the zonules (the fibers connecting the ciliary body to the lens), enabling the lens to become more rounded for near vision. Conversely, when the ciliary muscle relaxes, the tension increases, flattening the lens for distant vision. The other options do not pertain to the muscle responsible for altering lens shape. The retinal muscle is not a recognized term; rather, it relates to the structural aspects of the retina. The corneal muscle does not exist, as the cornea is primarily epithelial and does not have muscle properties. The orbital muscle typically refers to muscles controlling eye movement rather than those involved in altering the lens's shape. Thus, the ciliary muscle is the accurate choice for this function in the eye.