The Anatomy of the Lens

The lens focuses images and can be affected by cataracts and aging

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The lens is a curved structure in the eye that that bends light and focuses it for the retina to help you see images clearly. A clear disk behind the iris, the crystalline lens is flexible and changes shape to help you see objects at varying distances.

As you age, the lens may become weaker or damaged. Since the lens changes shape to focus on images near or far, it can grow weaker and may not work as well later in life. Learn where the lens is and how it works.

The lens of the eye

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Anatomy

The lens is a clear, curved disk that sits between the surface of the eye and what we like to call the eyeball. It is the part of the eye that takes light and images from the outer world, bending to focus them into the retina.

Structure

The crystalline lens is a clear biconvex layer of the eye that is made up mostly of proteins. As much as 60% of the lens mass is proteins—a concentration higher than almost any other tissue in the body. Four structures make up the crystalline lens:

  • Capsule
  • Epithelium
  • Cortex
  • Nucleus

Made up of collagen and proteins, the lens actually has no direct blood or nerve connections. Instead, it relies on the aqueous humor—the clear fluid between the lens and the cornea—to provide it with energy and carry away waste products.

The lens grows as you age, weighing about 65 milligrams at birth, 160 milligrams by age 10, and 250 milligrams by age 90.

The lens thickens and bends to transmit light from the cornea to the retina with the help of ciliary muscles. The ciliary body produces aqueous humor and bends the lens to refract light. The lens is held in place by zonular fibers, or zonules, that extend from the ciliary body.

Location

The most outer structure of the eye is the cornea, which is the clear surface of the eye that provides most of the focusing power. Behind the cornea are the pupil and the iris, which work together to regulate the amount of light the enters the eye. The crystalline lens sits just behind the cornea, pupil, and iris, and rests at the front of the retina.

Anatomical Variations

In some extreme cases, infants can be born missing one or both eyes (anophthalmia), or with eyes that do not fully develop (microphthalmia). There can also be malformations or problems with the lens itself at birth, most often in the form of congenital cataracts.

Congenital cataracts can be caused by a number of conditions, including:

  • Galactosemia
  • Congenital rubella syndrome
  • Lowe syndrome
  • Down syndrome
  • Pierre-Robin syndrome
  • Hallerman-Streiff syndrome
  • Cerebrohepatorenal syndrome
  • Trisomy 13
  • Conradi syndrome
  • Ectodermal dysplasia
  • Marinesco-Sjogren syndrome.

Congenital cataracts may not be evident for some time, progressing until the lens takes on a cloudy color and the child's sight is impaired. About one-third of congenital cataracts cases are hereditary.

Function

The lens works much like a camera lens, bending and focusing light to produce a clear image. The cornea does most of the focusing for the eye, but the lens provides the final third of this focusing effort, bending and focusing light into the retina for processing and translation into an image at the is at last translated by the optic nerve.

The lens has the ability to change shape and focus images at various distances in a process called accommodation.

Associated Conditions

Over time, the lens can deteriorate. As a flexible structure, the lens can lose elasticity over time, and UV rays can impact the clarity and its ability to transmit light.

As the lens loses elasticity, close-up vision is impacted, resulting in presbyopia. This is common for people over age 40. For lenses that lose elasticity, prescription glasses may help—particularly reading glasses or glasses with bifocals.

For lenses that are damaged or become clouded—a condition called cataracts—prosthetic lenses called intraocular lenses may be an option. These lenses are surgically placed, and you should talk with your doctor if you are interested in cataract surgery.

Tests

The lens of your eye will be checked by your doctor during a standard eye exam. An eye exam consists of a number of different tests that examine the overall health of your eye. What you have trouble seeing during your eye exam will give your doctor some clues as to which parts of the eye aren't working right.

Below are some of the tests your doctor might perform during an eye exam:

  • Eye muscle test to check eye movement and control
  • Visual acuity test to measure how clear you can see
  • Refraction assessment to check how light bends as it passes through the cornea and lens
  • Visual field test to measure your overall field of vision
  • Color vision testing to check for color blindness or deficiency in seeing some colors
  • Slit lamp examination allows your doctor to examine the health of your retina and the blood supply to the eye
  • Tonometry measures the amount of pressure inside your eye
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Article Sources
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