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The primary focusing structure of the EYE, located in the center at the front of the eye. The lens is transparent, convex (rounded outward on each side), round, and flexible. A thin membrane encloses the lens.
Tiny muscles at the front edges of its sides, the ciliary muscles, contract to flatten the lens and relax to thicken the lens. These adjustments alter light refraction (the angle at which the lens bends lightwaves entering the eye) to accommodate near and distant vision.
The most common health conditions that affect the lens are PRESBYOPIA, in which the FLEXIBILITY of the lens diminishes with aging, and CATARACT, in which protein deposits cloud the lens and obscure vision. The lens is also vulnerable to accidental injury, particularly from blunt force (such as a baseball) or puncture.
For further discussion of ophthalmologic structure and function please see the overview section “The Eyes.”