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The Merck Manual of Medical Information--Home Edition
Section 20. Eye Disorders
Chapter 216

Eyes and Vision

The structure and function of the eye are complex and fascinating. The eye constantly adjusts the amount of light it lets in, focuses on objects near and far, and produces continuous images that are instantly transmitted to the brain.

Structure and Function

The front of the eye's relatively tough white outer layer (sclera or white of the eye) is covered by a thin membrane (conjunctiva). Light enters through the cornea, a transparent dome on the surface of the eye. Besides serving as a protective covering for the front of the eye, the cornea also helps focus light on the retina at the back of the eye. After passing through the cornea, light enters the pupil, the black area in the middle of the iris--the circular, colored area of the eye. The iris controls the amount of light that enters the eye by opening and closing like the aperture of a camera lens. The iris allows more light into the eye when the environment is dark and allows less light into the eye when the environment is bright. The size of the pupil is controlled by the pupillary sphincter muscle, which opens and closes the iris.

Behind the iris sits the lens. By changing its shape, the lens focuses light onto the retina. For the eye to focus on nearby objects, a small muscle called the ciliary muscle contracts, making the lens thicker and thus stronger. For the eye to focus on distant objects, the same muscle relaxes, making the lens thinner and thus weaker. As people age, the lens typically becomes less flexible, less able to thicken, and thus less able to focus on nearby objects, a condition called presbyopia.

The retina contains the nerves that sense light and the blood supply that nourishes them. The most sensitive part of the retina is a small area called the macula, which has hundreds of nerve endings close together. The high density of nerve endings makes the visual image sharp, just as high-resolution film has more tightly packed grains. The retina then converts the image into electrical impulses, which are carried to the brain by the optic nerve.

The optic nerve connects the retina to the brain in a split pathway. Half the fibers of this nerve cross over to the other side at the optic chiasm, an area just below the front of the brain. The bundles of nerve fibers then come together again just before they reach the back part of the brain, where vision is sensed and interpreted.

click on thumbnail to see full-size imageThe eyeball itself is divided into two segments, each filled with fluid. The front (anterior) segment extends from the cornea to the lens; the back (posterior) segment extends from the back edges of the lens to the retina. The anterior segment is filled with a fluid called the aqueous humor that nourishes its internal structures; the posterior segment contains a gel-like substance called the vitreous humor. These fluids help the eyeball maintain its shape. The anterior segment itself is divided into two chambers. The front (anterior) chamber extends from the cornea to the iris; the back (posterior) chamber extends from the iris to the lens. Normally, the aqueous humor is produced in the posterior chamber, passes through the pupil into the anterior chamber, and then drains out of the eyeball through outflow channels at the edge of the iris.

Muscles, Nerves, and Blood Vessels

Several muscles working together move the eyes. Each muscle is stimulated by a specific cranial nerve. The bony orbit that protects the eye also contains many other nerves. As mentioned, the optic nerve exits through the back of the eye and carries nerve impulses created in the retina to the brain. The lacrimal nerve stimulates the tear glands to produce tears. Other nerves transmit sensation to other parts of the eye and stimulate the muscles of the orbit.

An ophthalmic artery and a retinal artery provide blood to each eye, and an ophthalmic vein and a retinal vein drain blood from it. These blood vessels enter and leave through the back of the eye.

Protective Features

The structures around the eye protect it while allowing it to move freely in all directions. They protect the eye, which is constantly exposed to dust, wind, bacteria, viruses, fungi, and other potentially injurious substances, while allowing it to remain open enough to catch light rays.

The orbits are bony cavities containing the eyeballs, muscles, nerves, blood vessels, fat, and structures that produce and drain tears. The eyelids, thin folds of skin, cover the eyes. They reflexively close quickly to protect the eye from foreign objects, wind, dust, and very bright light. When blinked, the eyelids help spread liquid over the surface of the eyes, and when closed, they help keep the surface moist. Without such moisture, the normally transparent cornea can become dried, injured, and opaque.

click on thumbnail to see full-size imageThe inner surface of the eyelid is a thin membrane (conjunctiva) that loops back to cover the surface of the eye. The eyelashes are short hairs growing from the edge of the eyelid that help protect the eye by acting as a barrier. Small glands at the edge of the eyelid secrete an oily substance that contributes to the tear film and keeps tears from evaporating.

The lacrimal glands, located at the top outer edge of each eye, produce the watery portion of tears. Tears drain from the eyes into the nose through the two nasolacrimal ducts; each of these ducts has openings at the edge of the upper and lower eyelids near the nose. Tears keep the surface of the eye moist and healthy; they also trap and sweep away small particles that enter the eye. Moreover, tears are rich in antibodies that help prevent infection.

Blindness

Both eye injury and disease can affect vision. The clarity of vision is called visual acuity, which ranges from full vision to no vision. As acuity decreases, vision becomes progressively blurred. Acuity is usually measured on a scale that compares a person's vision at 20 feet with that of someone who has full acuity. Thus, a person who has 20/20 vision sees objects 20 feet away with complete clarity, but a person who has 20/200 vision sees at 20 feet what a person with full acuity sees at 200 feet.

Common Causes of Blindness

Cataract

  • Most common cause
  • Can be cured with surgery

Infection

  • Most common preventable cause in the world
  • Not common in United States

Diabetes

  • One of most common causes in United States
  • Often preventable
  • Laser treatment slows vision loss

Macular degeneration

  • Affects central vision, not peripheral vision
  • Preventable and treatable in fewer than 10 percent of people

Glaucoma

  • Highly treatable
  • If treated early, should not lead to blindness

Legal blindness is defined as visual acuity worse than 20/200 even after correction with eyeglasses or contact lenses. Many people who are considered legally blind can distinguish shapes and shadows but not normal detail.

Causes

Blindness can occur for any of the following reasons:

  • light can't reach the retina
  • light rays don't focus properly on the retina
  • the retina can't sense light rays normally
  • the nerve impulses from the retina aren't transmitted to the brain normally
  • the brain can't interpret information sent by the eye.

Several disorders can cause these problems that lead to blindness. A cataract (see page 1042 in Chapter 223, Cataracts) can block light coming into the eye, so that it never reaches the retina. Focusing (refraction) errors (see Chapter 217, Refractive Disorders below) can usually be corrected with prescription lenses but not always completely. A detached retina and hereditary disorders such as retinitis pigmentosa (see page 1047 in Chapter 225, Retinal Disorders) can affect the retina's ability to sense light. Diabetes or macular degeneration (see page 1045 in Chapter 225, Retinal Disorders) can also damage the retina. Disorders of the nervous system such as multiple sclerosis or inadequate blood supply can damage the optic nerve, which carries impulses to the brain. Tumors in nearby structures, such as the pituitary gland, also can damage this nerve. The areas of the brain that interpret visual impulses may be damaged by stroke, tumor, or other disease.

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