The crystalline lens is a transparent, biconvex structure whose functions are
to maintain its own clarity
to refract light
to provide accommodation
The lens has no blood supply or innervation after fetal development, and it depends entirely on the aqueous humor to meet its metabolic requirements and to carry off its wastes. It lies posterior to the iris and anterior to the vitreous body (Fig 1-1). The lens is suspended in position by the zonules of Zinn, which consist of delicate yet strong fibers that support and attach it to the ciliary body. The lens is composed of the capsule, lens epithelium, cortex, and nucleus (Fig 1-2).
The anterior and posterior poles of the lens are joined by an imaginary line called the optic axis, which passes through them. Lines on the surface passing from one pole to the other are referred to as meridians. The equator of the lens is its greatest circumference.
The lens is able to refract light because its index of refraction—normally about 1.4 centrally and 1.36 peripherally—is different from that of the aqueous and vitreous that surround it. In its nonaccommodative state, the lens contributes about 15–20 diopters (D) of the approximately 60 D of convergent refractive power of the average human eye. The remaining 40 or so diopters of convergent refractive power occur at the air–cornea interface.
The lens continues to grow throughout life. At birth, it measures about 6.4 mm equatorially and 3.5 mm anteroposteriorly and weighs approximately 90 mg. The adult lens typically measures 9 mm equatorially and 5 mm anteroposteriorly and weighs approximately 255 mg. The relative thickness of the cortex increases with age. At the same time, the lens adopts an increasingly curved shape so that older lenses have more refractive power. However, the index of refraction decreases with age, probably as a result of the increasing presence of insoluble protein particles. Thus, the eye may become either more hyperopic or more myopic with age, depending on the balance of these opposing changes.