The ciliary muscle is a ring of smooth fibers that is located in the ciliary body (the structure in the eye that secretes aqueous humor) just behind the iris. Its primary responsibility consists of changing the shape of the eye lens in order to accommodate for distance vision or near vision.
Accommodation is the ability of the eye to focus on objects differently depending on where they are relation to the eye's lens. Most mammals' eyes do not accommodate, although fish and amphibians often accommodate by moving the lens back and forth so that it is closer and farther from the lens. The ciliary muscle, on the other hand, allows the human eye to accommodate by changing the shape of the lens itself. This muscle, which is composed of radial fibers, circular fibers, and longitudinal fibers, is attached to the lens by thin suspensory ligaments called ciliary zonule. When the ciliary muscle relaxes, it tightens the ciliary zonule, which in turn stretch the lens. The lens becomes thin and is able to better focus on objects in the distance. When the ciliary muscle contracts, the zonule relax, and the lens becomes rounder. This enables it to focus more clearly on nearby objects. In this way, the ciliary muscle is responsible for enabling humans to see clearly both nearby objects and objects that are far away.
The ciliary muscle contracts and relaxes without any conscious effort, and its movement is controlled by the autonomic nervous system (ANS). Sympathetic stimulation causes the muscle to relax, and parasympathetic stimulation causes the muscle to contract. This mechanism usually works perfectly until about middle age, at which point the lens begins to become less flexible, leading to a condition called presbyopia. This condition limits the amount of clear close-range vision that is possible due to the lens's inflexibility.