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The lens is the normally clear structure that is located just posterior to the iris. The primary function of the lens is to focus light rays on the retina. The lens is attached to the ciliary body by zonules. These zonules produce tension in response to contraction of the ciliary muscle. The ciliary muscle is circular, so that when it contracts its diameter becomes smaller (Figure 30–1). This decreases the tension on the lens zonules, and the lens becomes more spherical. This is known as accommodation, which is what allows the eye to focus at near. As people age, the lens becomes progressively stiffer. Because of this, the ability to focus at near gradually deteriorates, to the point that almost all people require reading glasses during the fourth decade of life.

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FIGURE 30–1
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Focusing of the lens is controlled by the ciliary body. (Top) When the ciliary body is relaxed the lens is thinner and focuses light rays from distant objects. (Bottom) When the ciliary muscle contracts, the lens zonules loosen, and the lens assumes a more spherical shape. This allows the lens to focus on near objects.

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The lens consists of the anterior and posterior lens capsules, which are basal lamina produced by the lens epithelium. The lens epithelium lines the inner capsule. The epithelial cells at the equator of the lens continue to divide and produce lens fibers throughout life. The central portion of the lens (the nucleus) is formed by birth, and the surrounding portion of the lens (the cortex) is produced postnatally (Figure 30–2). The cytoplasm of lens cells contains crystallins, which produce the light-focusing properties of the lens.

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FIGURE 30–2
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Lens anatomy. The lens is composed of the inner nucleus, surrounding cortex, and outer capsule. The lens is attached to the ciliary body by the lens zonules. The iris in front of the lens forms the pupil.

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Cataracts are present if the lens is not clear. Cataracts may range from mild to severe, and their effect on vision can range from minimal to profound. Cataracts are very common in adults as they age, but they are uncommon in children. However, they do represent a significant cause of visual morbidity in children, accounting for approximately 10% of childhood vision impairment.

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In addition to cataracts, rare lens disorders include aphakia (absence of the lens), microspherophakia (an abnormally small, round lens), ectopia lentis (decentration of the lens), and lens coloboma (a partial defect in the periphery of the lens due to absence of adjacent zonules, usually associated with colobomas of the iris and retina).

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Embryology

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The lens initially develops as a thickening of the surface ectoderm overlying the optic vesicle. ...

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