Eye. I INTRODUCTION Eye, light-sensitive organ of vision in animals. The eyes of

« Several structures, not parts of the eyeball, contribute to the protection of the eye.

The most important of these are the eyelids, two folds of skin and tissue, upper andlower, that can be closed by means of muscles to form a protective covering over the eyeball against excessive light and mechanical injury.

The eyelashes, a fringe ofshort hairs growing on the edge of either eyelid, act as a screen to keep dust particles and insects out of the eyes when the eyelids are partly closed. Inside the eyelids is a thin protective membrane, the conjunctiva, which doubles over to cover the visible sclera.

Each eye also has a tear gland, or lacrimal organ,situated at the outside corner of the eye.

The salty secretion of these glands lubricates the forward part of the eyeball when the eyelids are closed and flushes away anysmall dust particles or other foreign matter on the surface of the eye. Normally the eyelids of human eyes close by reflex action about every six seconds, but if dust reaches the surface of the eye and is not washed away, the eyelids blinkmore often and more tears are produced.

On the edges of the eyelids are a number of small glands, the Meibomian glands, which produce a fatty secretion thatlubricates the eyelids themselves and the eyelashes. The eyebrows, located above each eye, also have a protective function in soaking up or deflecting perspiration or rain and preventing the moisture from running intothe eyes.

The hollow socket in the skull in which the eye is set is called the orbit.

The bony edges of the orbit, the frontal bone, and the cheekbone protect the eye frommechanical injury by blows or collisions. V COMPARATIVE ANATOMY The simplest animal eyes occur in the cnidarians and ctenophores, phyla comprising the jellyfish and somewhat similar primitive animals.

These eyes, known as pigmenteyes, consist of groups of pigment cells associated with sensory cells and often covered with a thickened layer of cuticle that forms a kind of lens.

Similar eyes, usuallyhaving a somewhat more complex structure, occur in worms, insects, and mollusks. Two kinds of image-forming eyes are found in the animal world, single and compound eyes.

The single eyes are essentially similar to the human eye, though varyingfrom group to group in details of structure.

The lowest species to develop such eyes are some of the large jellyfish.

Compound eyes, confined to the arthropods ( see Arthropod), consist of a faceted lens, each facet of which forms a separate image on a retinal cell, creating a moasic field.

In some arthropods the structure is moresophisticated, forming a combined image. The eyes of other vertebrates are essentially similar to human eyes, although important modifications may exist.

The eyes of such nocturnal animals as cats, owls, andbats are provided only with rod cells, and the cells are both more sensitive and more numerous than in humans.

The eye of a dolphin has 7,000 times as many rod cellsas a human eye, enabling it to see in deep water.

The eyes of most fish have a flat cornea and a globular lens and are hence particularly adapted for seeing closeobjects.

Birds’ eyes are elongated from front to back, permitting larger images of distant objects to be formed on the retina. VI EYE DISEASES Eye disorders may be classified according to the part of the eye in which the disorders occur. The most common disease of the eyelids is hordeolum, known commonly as a sty, which is an infection of the follicles of the eyelashes, usually caused by infection bystaphylococci.

Internal sties that occur inside the eyelid and not on its edge are similar infections of the lubricating Meibomian glands.

Abscesses of the eyelids aresometimes the result of penetrating wounds.

Several congenital defects of the eyelids occasionally occur, including coloboma, or cleft eyelid, and ptosis, a drooping ofthe upper lid.

Among acquired defects are symblepharon, an adhesion of the inner surface of the eyelid to the eyeball, which is most frequently the result of burns.Entropion, the turning of the eyelid inward toward the cornea, and ectropion, the turning of the eyelid outward, can be caused by scars or by spasmodic muscularcontractions resulting from chronic irritation.

The eyelids also are subject to several diseases of the skin such as eczema and acne, and to both benign and malignanttumors.

Another eye disease is infection of the conjunctiva, the mucous membranes covering the inside of the eyelids and the outside of the eyeball.

See Conjunctivitis; Trachoma. Disorders of the cornea, which may result in a loss of transparency and impaired sight, are usually the result of injury but may also occur as a secondary result ofdisease; for example, edema, or swelling, of the cornea sometimes accompanies glaucoma. The choroid, or middle coat of the eyeball, contains most of the blood vessels of the eye; it is often the site of secondary infections from toxic conditions and bacterialinfections such as tuberculosis and syphilis.

Cancer may develop in the choroidal tissues or may be carried to the eye from malignancies elsewhere in the body.

Thelight-sensitive retina, which lies just beneath the choroid, also is subject to the same type of infections.

The cause of retrolental fibroplasia, however—a disease ofpremature infants that causes retinal detachment and partial blindness—is unknown.

Retinal detachment may also follow cataract surgery.

Laser beams are sometimesused to weld detached retinas back onto the eye.

Another retinal condition, called macular degeneration, affects the central retina.

Macular degeneration is a frequentcause of loss of vision in older persons.

Juvenile forms of this condition also exist. The optic nerve contains the retinal nerve fibers, which carry visual impulses to the brain.

The retinal circulation is carried by the central artery and vein, which lie in theoptic nerve.

The sheath of the optic nerve communicates with the cerebral lymph spaces.

Inflammation of that part of the optic nerve situated within the eye is knownas optic neuritis, or papillitis; when inflammation occurs in the part of the optic nerve behind the eye, the disease is called retrobulbar neuritis.

When the pressure in theskull is elevated, or increased in intracranial pressure, as in brain tumors, edema and swelling of the optic disk occur where the nerve enters the eyeball, a conditionknown as papilledema, or chocked disk. For disorders of the crystalline lens, see Cataract.

See also Color Blindness. VII EYE BANK Eye banks are organizations that distribute corneal tissue taken from deceased persons for eye grafts.

Blindness caused by cloudiness or scarring of the cornea cansometimes be cured by surgical removal of the affected portion of the corneal tissue.

With present techniques, such tissue can be kept alive for several weeks byfreezing the tissue.

Eye banks also preserve and distribute vitreous humor, the liquid within the larger chamber of the eye, for use in treatment of detached retinas.The first eye bank was opened in New York City in 1945.

The Eye Bank Association of America, in Washington, D.C., acts as a clearinghouse for information. Reviewed By:Ira Snow JonesMicrosoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation.

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