Image credit: University of Delaware.
The ciliary muscle is a ring of smooth muscle fibers that is responsible for changing the shape of the lens in the eye to achieve accommodation. Suspensory ligaments connect the ciliary muscle to the lens. When the ciliary muscle is relaxed the ligaments are taut, and the lens is stretched thin enabling it to focus on distant objects. When the ciliary muscle is contracted the suspensory ligaments become less taut, and the lens becomes rounder so that it can focus on objects that are nearby.
The contraction and relaxation of the ciliary muscle is controlled by the autonomic nervous system. Sympathetic nerve fiber stimulation (see sympathetic nervous system) causes relaxation of the muscle, whereas parasympathetic stimulation (see parasympathetic nervous system) causes muscle contraction. The ciliary muscle is part of the ciliary body.