What is the primary function of the ciliary muscles?

The primary function of the ciliary muscles is to control the shape of the lens. These muscles are located around the lens and play a crucial role in the process of accommodation, which allows the eye to focus on objects at varying distances. When the ciliary muscles contract, they relax the tension on the zonular fibers connected to the lens, causing the lens to become thicker and more curved, which is necessary for focusing on nearby objects. Conversely, when the ciliary muscles relax, the tension on the zonular fibers increases, flattening the lens for distance vision. This mechanism is vital for clear vision across different distances, making the ciliary muscles essential for proper eye function.

The other options do not accurately describe the function of the ciliary muscles. Controlling the color of the iris relates to the iris muscles, whereas regulating the size of the pupil is primarily managed by the iris as well. The secretion of aqueous humor is a function carried out by the ciliary body, which houses the ciliary muscles, but is distinct from the action of these muscles themselves.