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The ciliary body encircles the eye posterior to the colored part of the eye, the iris. It contains both the ciliary muscles and the ciliary processes. The structure is lined by a double-layered coating, the epithelium, which is composed of a non-pigmented and a pigmented layer. A critical structure within the eye, the ciliary body performs three essential functions — it supports the lens, focuses on a near target, and produces inner-eye fluid. The ciliary body receives its parasympathetic nerve supply from the oculomotor nerve, the third cranial nerve.
Connected to the ciliary body is a fibrous circle of strands known as the zonule of Zinn. Zonular fibers emanate from this ring to insert on the capsule of the human lens. The zonular fibers, also collectively known as the suspensory ligament of the crystalline lens, keep the lens in place within the eye. Dislocation or excessive movement of the lens characterizes conditions that contribute to weak or lax zonules, such as Marfan's Syndrome, pseudoexfoliation, and Homocystinuria. Clinical experiments have shown that the ciliary epithelium secretes the zonular fibers beginning at about the third month of fetal life.
Accommodation is the name given to the process of changing the focal length of the eye from the distance to near. This process involves changing the shape of the lens, thickening it and inducing a more steeply curved front surface. When the ciliary muscle contracts, the tension on the zonular fibers decreases, allowing the lens to thicken axially, which increases its power. The eye uses the extra power to focus light from a near target on the retina. Conversely, when the ciliary muscle relaxes, the zonular fibers draw more tightly on the lens periphery, causing the lens to extend outward and thin centrally.
Aqueous humor, a clear, watery substance, fills the anterior portion of the eye. This fluid inflates the front of the eye and gives a measure of firmness to the eye. It also carries nutrients to the structures of the eye and carries away waste products. The non-pigmented epithelium of the ciliary body forms the aqueous at a steady rate. Injuries to the ciliary body can disrupt normal aqueous production, resulting in an overall softening and shriveling of the eye, called phthisis bulbi.
An optic nerve disease associated with increased eye pressure is glaucoma. In rare cases of poorly controlled glaucoma, ophthalmologists may destroy the ciliary body in order to reduce the production of aqueous, a technique called ciliary body ablation. Destruction of the ciliary body may be accomplished through a circumferential application of laser or freezing treatment directly overlying the body. The success rate for controlling glaucoma after this procedure is approximately 60 to 70 percent, with an average recovery period of four to six weeks. Ciliary ablation carries the risks of increased inflammation, bleeding, and exceptionally low eye pressure, or hypotony.