Lens of the Eye

About the Lens of the Eye

The crystalline lens is a transparent, biconvex structure suspended behind the iris by fine fibers called zonules, which attach it to the ciliary body. It is made of tightly packed lens fiber cells filled with proteins called crystallins and enclosed in an elastic capsule. The lens has no blood vessels or nerves, and its cells lose their nuclei and other organelles as they mature, which helps keep it transparent. Although it provides less focusing power than the cornea, it can change shape, a process called accommodation: when the ciliary muscle contracts, tension on the zonules relaxes and the lens becomes rounder to focus on near objects. New fibers are added throughout life, so the lens grows larger and stiffer with age, leading to presbyopia, the gradual loss of near focusing ability that typically becomes noticeable in the forties. Clouding of the lens is called a cataract, a leading cause of visual impairment worldwide, and it is treated by replacing the lens with an artificial intraocular lens.