When the ciliary muscle is contracted, the lens becomes more spherical – and has increased focussing power – due to a lessening of tension on the zonular fibres (a). When the ciliary muscles relax, these fibres become taut – pulling the lens out into a flatter shape, which has less focussing power (b).

What will change in the eye when the ciliary muscles contract?

When the ciliary muscle contracts, all zonular tension is reduced. This permits the central lens surface lens to become rounder (increases its focusing power). When the ciliary muscle relaxes, all zonular tension is increased, causing the lens to flatten (decrease in optical power).

Why does the ciliary muscles contract?

When the ciliary muscle contracts in response to parasympathetic stimulation, this reduces tension on the suspensory ligaments, and the capsule of the lens is relaxed. The lens then becomes shorter and fatter and accommodates for viewing near objects.