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Lens (anatomy)

2007 Schools Wikipedia Selection. Related subjects: Health and medicine

   Light from a single point of a distant object and light from a single
   point of a near object being brought to a focus by changing the
   curvature of the lens.
   Enlarge
   Light from a single point of a distant object and light from a single
   point of a near object being brought to a focus by changing the
   curvature of the lens.

   The lens or crystalline lens is a transparent, biconvex structure in
   the eye that, along with the cornea, helps to refract light to focus on
   the retina. Its function is thus similar to a man-made optical lens.

   In humans, the refractive power of the lens in its natural environment
   is approximately 15 dioptres, roughly one-fourth of the eye's total
   power.

   The lens is flexible and its curvature is controlled by ciliary muscles
   through the Zonules. By changing the curvature of the lens, one can
   focus the eye on objects at different distances from it. This process
   is called accommodation. The lens continually grows throughout life,
   laying new cells over the old cells resulting in a stiffer lens. The
   lens gradually loses its accommodation ability as the individual ages,
   the loss of the individual's focussing ability is termed Presbyopia.

   The lens is made of transparent proteins called crystallins. The
   average concentration of lens proteins is about twice than that of
   other intracellular proteins and is thought to play a structural role
   in the lens. It is about 5 mm thick and has a diameter of about 9 mm
   for an adult human (though these figures can vary). The proteins are
   arranged in approximately 20,000 thin concentric layers, with a
   refractive index (for visible wavelengths) varying from approximately
   1.406 in the central layers down to 1.386 in less dense cortex of the
   lens. This index gradient enhances the optical power of the lens. The
   lens is included into the capsular bag, maintained by the zonules of
   Zinn.

   During the fetal stage, the development of the lens is aided by the
   hyaloid artery. In adults, the lens depends entirely upon the aqueous
   and vitreous humors for nourishment.

Diseases

   Cataract in Human Eye- Magnified view seen on examination with a slit
   lamp
   Enlarge
   Cataract in Human Eye- Magnified view seen on examination with a slit
   lamp
     * A cataract is an opacification of the normally transparent
       crystalline lens that leads to blurred vision.
     * Ectopia lentis is a displacement or malposition of the lens from
       its normal location.
     * Nuclear sclerosis is a normal age-related change of the centre of
       the lens, most commonly seen in dogs.

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