When the ciliary muscle contracts, it releases the tension on the lens caused by the suspensory ligaments. This release of tension makes the lens more spherical, adapting to short range vision. When the ciliary muscle relaxes the ligaments are taut, stretching the lens thin enabling it to focus on distant objects.
The autonomic nervous system controls the contraction and relaxation of the ciliary muscle. Sympathetic nerve fiber stimulation is responsible for muscle relaxation, whereas parasympathetic stimulation causes muscle contraction. The ciliary muscle is an extension of the ciliary body.