The Anatomy of the Lens

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. The crystalline lens, a clear disk behind the iris, 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.

Anatomy

The lens is a clear, curved disk that sits behind the iris and in front of the vitreous of the eye. It is the part of the eye that focuses light and images from the outer world, bending them onto 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 made up of 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

Although the lens is thought to give the eye the most focusing power, the outermost structure of the eye, called the cornea, provides most of the focusing power. Behind the cornea is the iris, which creates a round aperture called the pupil. This pupil changes in size to regulate the amount of light that enters the eye. The crystalline lens sits just behind the iris.

Anatomical Variations

Anatomic variations can exist in the natural lens of the eye. Many different known and unknown congenital diseases can affect the lens in isolation or as part of a syndrome. Most often, these congenital defects present in the form of congenital cataracts or clouding of the crystalline lens.

Most congenital cataracts not associated with a syndrome have no identifiable cause, although genetic mutations are a common reason for cataract presentation. Cataracts at birth can present in one eye (unilaterally) or both eyes (bilaterally). Some of the syndromes associated with congenital cataracts include:

• 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 cataract cases are hereditary.

Function

The lens works much like a camera lens, bending and focusing light to produce a clear image. The crystalline lens is a convex lens that creates an inverted image focused on the retina. The brain flips the image back to normal to create what you see around you. In a process called accommodation, the elasticity of the crystalline lens allows you to focus on images at far distances and near with minimal disruption.

Associated Conditions

As you age, your natural lens also ages. Its flexibility is slowly lost, and, over time, the lens also becomes opaque, turning the natural clear lens into a cataract.

When the lens loses elasticity, close-up vision is impacted, resulting in presbyopia. This is common for people over age 40. When this happens, people require reading glasses or glasses with bifocals to view images clearly up close.

As lenses become clouded, a condition called cataracts develops. When this condition becomes severe enough to limit or hinder essential activities of daily living, cataract surgery is performed. In this procedure, a prosthetic lens called an intraocular lens replaces the cloudy natural lens. Your eye healthcare provider

will be able to determine if there is a cataract and when cataract surgery may be warranted.

Tests

The lens of your eye will be checked by your healthcare provider during an eye exam. A comprehensive eye exam consists of a number of different tests that examine the overall health of your eye.

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

• Eye muscle test to check eye movement and control
• Visual acuity test to measure how clearly 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 to allow your healthcare provider to examine the health of your retina and the blood supply to the eye
• Tonometry to measure the amount of pressure inside your eye