Eye

The Human Eyes

How the human eye works

Structure and working of human eyes

Anatomy, physiology and pathology of human eye

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The human eyes produce tears by means of the lacrimal glands, one of which lies above each eyeball. The tears wash the eye and then flow into canals that lead to the lacrimal sac. From this sac, the tears drain through a passage into the nose

Parts of the eye The visible parts of the eyeball are the white sclera and the coloured iris. A membrane called the conjunctiva covers the sclera. The clear cornea lies in front of the iris. The lens is connected to the ciliary body. Inside the eyeball is a clear substance called vitreous humour. The retina, which un­derlies the choroid, changes light rays into electrical signals. The optic nerve carries the signals to the brain. The fovea centralis, a pit in the macula lutea, is the area of sharpest vision.

The Iris has a round opening called the pupil, which regulates the amount of light that enters the eye. In dim light, the dilator muscle pulls the pupil open wider. In bright light, the sphincter muscle tightens around the pupil and makes it smaller.

Structure of retina  The retina has cells called rods and cones, which absorb light rays and change them into electrical of signals. There are more cones than rods in the central area of the retina. The cones are concentrated in the fovea centralis. Nerve fibres attached to the rods and cones join to form the optic nerve.

Myopia occurs if light rays from distant objects meet be­fore they reach the retina. In most cases, distance vision is blurred, but near vision is sharp. Glasses or contacts that have concave lenses bring the rays together at the retina and correct myopia.

Defects of the eye Eye defects are among the most common physical disorders. They include (1) myopia, (21 long­sightedness, and (3) astigmatism. These defects cannot usually be cured, but vision can be made normal in most cases by means of eyeglasses or contact lenses, or, occasionally, by surgery.

Longsightedness occurs if light rays from distant objects reach the retina before they meet In many cases, the lens can accommodate enough for sharp distance vision but not sharp near vision. Glasses or contacts that have convex lenses correct longsighted­ness.

Astigmatism results if all the light rays from an object do not come together at one point in the eye. Most cases of astigmatism produce blurred vision both nearby and at a distance. Eyeglasses or contact lenses that have cy­lindrical lenses correct the defect.

Some animal eyes - Most animals have organs of some kind that sense light In some animals, these organs can only tell light from dark. The eyes of certain other animals can see objects clearly even in dim light.

When to consult an eye doctor

Doctors recommend that everyone have an eye examination shortly after birth and at least every few years until the age of 40. Thereafter, the eyes should be examined about once a year. In addition, the eyes should be examined any time one of the following conditions persists:

Blurred vision.

Constant rubbing of the eyes.

Cross-eye or walleye.

Difficulty seeing either nearby objects or dis­tant ones.

Double vision.

A drooping eyelid.

Excessive tearing by the eyes.

The feeling of having a particle in one eye or both eyes.

Pain in the eyes or brows.

Poorer vision in one eye than the other.

Redness of the eyes.

Seeing flashes of light or spots in front of the eyes.

Seeing halos or rain­bows around lights.

Sensitivity of the eyes to light.

Squinting.

A white or yellow ap­pearance to the pupil.

The eye

Eye is the organ of sight It is our most important organ for finding out about the world around us. We use our eyes in almost everything we do—reading, working, watching films and television, playing games, and count­less other activities. Sight is our most precious sense, and many people fear blindness more than any other disability. Most animals have light-sensitive organs. The sim­plest ones are found in invertebrates, such as jellyfish or flatworms, and consist of small groups of light-sensitive cells on the surface of the body. Insects and crustaceans have compound eyes and can detect movement. The eyes of vertebrates are more complicated and are capa­ble of image formation.

The human eyeball measures only about 25 milli­metres in diameter. Yet the eye can see objects as far away as a star and as tiny as a grain of sand. The eye can quickly adjust its focus between a distant point and a near one. It can be accurately directed toward an object even while the head is moving.

The eye does not actually see objects. Instead, it sees the light they reflect or give off. The eye can see in bright light and in dim light, but it cannot see in no light at all. Light rays enter the eye through transparent tis­sues. The eye changes the rays into electrical signals.

The signals are then sent to the brain, which interprets them as visual images.

This article deals mainly with the human eye. It dis­cusses the parts of the eye, how we see, defects and dis­eases of the eye, and care of the eye. The last section of the article describes more in detail the eyes of various kinds of animals.

Parts of the eye

Each eyeball is set in a protective cone-shaped cavity in the skull. This cavity is called the orbit or socket. Its ridges form the brow and the cheekbone. Fatty tissue in­side the orbit nearly surrounds the eyeball and cushions it against blows. The soft tissue also enables the eye to turn easily in the orbit Six muscles move the eyeball in much the same way that strings move the parts of a pup­pet

The outer parts of the eye include the eyelids, the conjunctiva, the lacrimal glands, and the lacrimal sac. Three layers of tissue form the wall of the eyeball: (1) the sclera and the cornea, (2) the uveal tract, and (3) the ret­ina. Within the wall is a clear, jellylike substance called the vitreous humour. This substance occupies about 80 per cent of the eyeball. It helps maintain the shape of the eye and the pressure within the eyeball.

The outer parts. The front of the eyeball is protected by the eyelids. Eyelashes on the lids screen out some of the dust and other particles that might otherwise enter the eye. Any sudden movement in front of the eye—or anything that touches the eyelashes—causes the lids to blink in a protective reflex action.

The conjunctiva is a membrane that lines the inside of the eyelids and extends over the front of the white part of the eye. It produces mucus, a clear, slimy fluid that lu­bricates the eyeball. The conjunctiva also produces some tears, which help keep the eye clean. However, most tears are made by the lacrimal glands. A lacrimal gland lies at the upper outer corner of each orbit. Every time a person blinks, the eyelids spread a smooth layer of mucus and tears over the eye. These fluids then flow into tiny canals in the lids. The canals lead to the lacrimal sac, a pouch at the lower inner corner of each orbit. From the lacrimal sac, the mucus and tears drain through a passage into the nose. After crying, a person may have to blow the nose to clear this drainage system of the excess amount of tears.

The sclera and the cornea consist of tough tissues that make up the outer layer of the eyeball and give it strength. The sclera covers about five-sixths of the eye­ball, and the cornea about one-sixth. The sclera is the white part of the eye. It has the strength and feel of soft leather. Although the sclera appears to have many blood vessels on its surface, most of these vessels are part of the conjunctiva. The cornea has no blood vessels at all and is relatively dehydrated. As a result, it is transparent. The cornea lies in front of the coloured part of the eye. It enables light rays to enter the eyeball.

The uveal tract is the middle layer of the wall of the eyeball. It has three parts. They are, from front to back;

(1) the iris,

(2) the ciliary body, and

(3) the choroid.

The iris is the coloured disc that lies behind the cor­nea. Its colour comes from a brownish-black substance called melanin. The more melanin there is and the closer it is to the surface of the tissue, the darker the colour of the iris. For example, there is more melanin in brown eyes—and it is closer to the surface—than in blue eyes. In addition to giving the iris colour, melanin absorbs strong light that might otherwise dazzle the eye or cause blurred vision. Melanin is the same substance that gives skin and hair their colour. People called albi­nos have little or no melanin. They have milky-white skin, white hair, and pinkish-grey irises. Their eyes are extremely sensitive to light. Most albinos have very poor vision (see Albino).

At the centre of the iris is a round opening called the pupil, which looks like a black circle. The pupil regu­lates the amount of light that enters the eye. Two mus­cles in the iris automatically adjust the size of the pupil to the level of light. In dim light, the dilator muscle en­larges the pupil. As much light as possible can then enter the eye. In bright light, the sphincter muscle makes the pupil smaller, which prevents too much light from entering the eye. The pupil also becomes smaller when the eye looks at a nearby object, thus bringing the image into sharp focus.

The ciliary body encircles the iris. It is connected by strong fibres to the crystalline lens, which lies directly behind the iris. The lens is a flexible structure about the size and shape of an aspirin tablet. Like the cornea, the lens is transparent because it has no blood vessels and is relatively dehydrated. The muscles of the ciliary body make constant adjustments in the shape of the lens. These adjustments produce a sharp visual image at all times as the eye shifts focus between nearby and distant objects. The ciliary body also produces a clear, watery fluid called aqueous humour. This fluid nourishes and lubricates the cornea and the lens, and it fills the area between them. The ciliary body produces aqueous hu­mour continuously. The old fluid flows into a drainage system at a spongy, circular groove where the cornea and the sclera meet It then travels through the veins of the eyeball into the veins of the neck.

The choroid forms the back of the uveal tract It looks and feels like a blotter soaked with black ink. The cho­roid has many blood vessels. Blood from the choroid nourishes the outer part of the retina.

The retina makes up the innermost layer of the wall of the eyeball. It is about as fragile as a piece of wet tis­sue paper. Light-sensitive cells in the retina absorb light rays and change them into electrical signals. There are two types of these light-sensitive cells— rods and cones. The cells are named after their shape. The retina has about 120 million rods and about 6 million cones.

Bits of pigment (coloured material) in the rods and cones absorb even the smallest amount of light that strikes the retina. The pigment in the rods is called rho- dopsin or visual purple. It enables the eye to see shades of grey and to see in dim light There are three types of pigment in the cones. They enable the eye to see col­ours and to see sharp images in bright light Cyanolabe absorbs blue light Chlorolabe absorbs green light. Erythrolabe absorbs red light These pigments enable us to distinguish more than 200 colours.

Near the centre of the retina is a round area called the macula lutea or macula. The macula consists chiefly of cones. It produces a sharp image of scenes at which the eyes are directly aimed, especially in bright light. The rest of the retina provides peripheral vision—that is, it enables the eyes to see objects to the side while looking straight ahead. Most of the rods lie in this part of the ret­ina. Because rods are more sensitive in the dark than cones, faint objects often can be seen more clearly if the eyes are not aimed directly at them. For example, look­ing to the side of a dim star makes its image fall on the part of the retina that has the most rods and provides the best vision in dim light.

Nerve fibres attached to the rods and cones join at the centre of the retina and form the optic nerve. This nerve consists of about a million fibres. It serves as a flexible cable that connects the eyeball to the brain. In fact, the optic nerve and the retina are actually exten­sions of the brain. The optic nerve carries the electrical signals produced in the retina to the brain, which inter­prets them as visual images.

The point where the optic nerve enters the eye is known as the blind spot It has no rods or cones and therefore cannot respond to light Normally, a person does not notice the blind spot because it covers such a small area and the eyes make so many quick move­ments. In addition, anything the blind spot of one eye cannot see is seen by the other eye. See Blind spot

How we see

Focusing. Light rays that enter the eye must come to a point on the retina for a clear visual image to form. However, the light rays that objects reflect or give off do not naturally move toward one another. Instead, they ei­ther spread out or travel almost parallel. The focusing parts of the eye—the cornea and the lens—bend the rays toward one another. The cornea provides most of the refracting (bending) power of the eye. After light rays pass through the cornea, they travel through the aque­ous humour and the pupil to the lens. The lens bends the rays even closer together before they go through the vitreous humour and strike the retina. Light rays from objects at which the eyes are aimed come together at the fovea centralis, a tiny pit in the centre of the mac­ula. It is the area of sharpest vision. Light rays from ob­jects to the sides strike other areas of the retina.

The refracting power of the lens changes constantly as the eye shifts focus between nearby objects and dis­tant ones. Light rays from nearby objects spread out, and those from distant objects travel nearly parallel. Therefore, the lens must provide greater bending power for the light rays from nearby objects to come to­gether. This additional power is produced by a process called accommodation. In this process, one of the muscles of the ciliary body contracts, thereby relaxing the fibres that connect the ciliary body to the lens. As a, result, the lens becomes rounder and thicker and thus more powerful. When the eye looks at distant objects, the muscle of the ciliary body relaxes.

This action tight­ens the fibres that are connected to the lens, and the lens becomes flatter. For this reason, the eye cannot form a sharp image of a nearby object and a distant one at the same time.

Depth perception is the ability to judge distance and to tell the thickness of objects. The lens’ system of the eye, like the lens of a camera, reverses images. Thus, the images that form on the retina are much like those produced on film in a camera. The images are upside down and reversed left to right They are also flat, as in a photograph. However, the brain interprets the images as they really are. The ability of the brain to interpret ret­inal images right-side up, unreversed, and in depth comes from experience that begins at a person's birth.

The optic nerves from the two eyes meet at the base of the brain at a point called the optic chiasm. At the optic chiasm, half the nerve fibres from each eye cross over and join the fibres from the other eye. Each side of the brain receives visual messages from both eyes. The nerve fibres from the right half of each eye enter the right side of the brain. These fibres carry visual mes­sages from objects that are to a person's left The nerve fibres from the left half of each eye enter the left side of the brain. These fibres carry visual messages from objects that are to a person's right. Thus, if one side of the brain becomes damaged, the opposite side of a person's field of vision may be reduced. Such dam­age may occur as a result of a stroke or tumour.

The eyes are about 6.5 centimetres apart from centre to centre. For this reason, each eye sees things from a slightly different angle and sends slightly different mes­sages to the brain. The difference can be demonstrated by focusing on a nearby object first with one eye closed and then with the other eye closed. The image seen with each eye is slightly different. The brain puts the images together and thus provides depth perception, also called stereoscopic vision or three-dimensional vision. The image formed by the brain has thickness and shape, and the brain can judge the distance of the object.

Normal depth perception requires that the eyes work together in a process called binocular vision or fusion.

In this process, the eye muscles move the eyes so that light rays from an object fall at a corresponding point on each retina. When viewing objects close up, the eyes turn slightly inward. When viewing distant objects, the eyes are almost parallel. If the images being viewed do not fall at a corresponding point on each retina, they will be blurred or be seen as double or the brain will ig­nore one of them.

In most people, visual messages are stronger in one eye and on one side of the brain than the other. Most people are "right-eyed" or "left-eyed,” just as they are right-handed or left-handed. For example, they favour one eye or the other when aiming a camera or a rifle.

Adaptation to light and dark is partly controlled by the pupil. In strong light, the pupil may become as small as a pinhead and so prevent the eye from being dam­aged or dazzled by too much light. In the dark, it can get almost as large as the entire iris, thus letting in as much light as possible. However, the most important part of adaptation to light and dark occurs in the retina.

Light rays are absorbed by pigments in the retina's rods and cones. The pigments consist of protein and vi­tamin A. Vitamin A helps give the pigments their colour. The colour enables the pigments to absorb light. Light changes the chemical structure of the vitamin A and bleaches out the colour in the pigments. This process generates an electrical signal that the optic nerve trans­mits to the brain. After the pigments have been bleached, the vitamin A moves into a part of the retina known as the retinal pigmented epithelium (RPE). The vi­tamin regains its original chemical structure in the RPE and then returns to the rods and cones. There, it joins with protein molecules and forms new pigments.

The renewal of rhodopsin—the pigment that enables the eye to see in dim light—occurs largely in the dark. Immediately after being exposed to bright light, the eyes cannot see well in dim light because of the bleached rhodopsin. It takes about 10 to 30 minutes for rhodopsin to be renewed, depending on how much was bleached. During this time, the eyes become accus­tomed to the dark.

The cone pigments, which provide sharp vision in bright light, take less time than rhodopsin to be re­newed. The eyes become accustomed to bright light much quicker than they do to darkness. The adaptation from darkness to light depends largely on changes in the retina's nerve cells.

Defects of the eye

Defects of the eye are among the most common of all physical disorders. Certain defects cannot be cured, but vision can be made normal by means of eyeglasses or contact lenses. In many developed countries, about half the population wear glasses or contact lenses. However, more than 10 per cent of these people still have subnor­mal vision because the defects are so severe. The most common defects of the eye include (1) myopia, (2) long­sightedness, (3) astigmatism, (4) strabismus, and (5) col­our blindness.

Myopia, also called shortsightedness, is character­ized by blurred distance vision, though near vision re­mains sharp except in extreme cases. In most cases of myopia, the eyeball is too long from front to back. As a result, light rays from distant objects meet before they reach the retina. When the light rays do strike the retina, they form a blurred image. Eyeglasses or contact lenses that are concave can ensure that the light rays come to­gether at the retina and correct most cases of myopia. Concave lenses are thinner in the middle than at the edges. See Myopia.

Longsightedness, also called hyperopia, occurs in most cases because the eyeball is too short from front to back. Unless the lens accommodates, light rays from distant objects reach the retina before they meet, caus­ing a blurred image. The lens of a normal eye remains relatively flat for distance vision and becomes thicker for nearby objects to be brought into focus. In a long­sighted eye, however, the lens must also thicken for sharp distance vision. The longsighted eye receives sharp images of distant objects. But the constant use of the muscles of the ciliary body to adjust the shape of the lens may cause eyestrain and headaches. In addition, the lens may not thicken enough for sharp near vision. Glasses or contact lenses that are convex correct long­sightedness. Convex lenses are thicker in the middle than at the edges. See Longsightedness.

Between the ages of 40 to 50, a person's lens begins to harden and to lose its ability to thicken. This condition is called presbyopia, and it affects nearly all middle- aged and elderly people. By the time a person is about 60, the lens has almost ho flexibility and can barely ac­commodate. Because of presbyopia, most middle-aged and elderly people need glasses for reading and close work. If they wore glasses beforehand, they may need new glasses with bifocal lenses.

Astigmatism is usually caused by a misshapen cor­nea. As a result of the abnormal shape, all the light rays from an object do not come together at one point in the eye. Some rays may focus on the retina. But others may meet before they reach the retina, or they may reach the retina before they meet. Most cases of astigmatism pro­duce blurred vision both nearby and at a distance. In mild cases, there may be eyestrain and headaches but fairly sharp vision. Astigmatism may be combined with myopia, longsightedness, or presbyopia. To correct astigmatism, doctors prescribe glasses or contact lenses that have cylindrical components. Cylindrical lenses have greater bending power in one axis than in others. See Astigmatism.

Strabismus is a defect in which the eyes are not used together. One of the eyes is deviated (turned too far in one direction) all or part of the time. In most cases, the deviated eye is turned either toward the nose, a condi­tion called cross-eye, or toward the side, a condition called walleye. Strabismus occurs most often in young children. In children with strabismus, each eye sees a different part of a scene and sends a greatly different message to the brain. In most cases, the brain tends to ignore the weaker message—the one from the deviated eye. Blurring or double vision also may occur. Many cases of strabismus can be corrected if detected early. Strabismus may be treated by means of glasses, eye drops, surgery on the muscles of one or both eyes, or a patch worn for a time over one of the eyes—in most cases, the nondeviated one. Unless strabismus is cor­rected early, vision in the deviated eye may be perma­nently reduced. This condition is called strabismic am­blyopia or lazy eye. See Strabismus.

Colour blindness. Very few people are unable to distinguish colours at all. In most cases of colour blind­ness, certain colours are confused with others. For ex­ample, a green object may appear brown. Colour blind­ness is caused by abnormalities in the pigments of the retina's cones. Nearly all colour blindness is present at birth. More males than females are colour blind. The condition cannot be corrected, but it does not get worse. See Colour blindness.

Diseases of the eye

Disease may affect any part of the eye. Eye diseases are the largest cause of all blindness. Injuries cause the rest Eye diseases include:

 (1) cataract,

 (2) glaucoma,

(3) diseases of the outer parts,

(4) diseases of the sclera,

(5) diseases of the cornea,

(6) diseases of the uveal tract,

(7) diseases of the retina, and

(8) diseases of the optic nerve.

Cataract is a condition in which part or all of the lens becomes clouded. The clouded part of the lens is also called a cataract Severely reduced vision—or even blindness—results if the cloudiness covers a large area, is dense, or is at the centre of the lens. However, some cataracts cause little or no loss of vision. Most cataracts result from aging. If a cataract causes enough loss of vi­sion that it interferes greatly with a person's daily activi­ties, the lens is removed by surgery. The patient must then wear strong glasses or contact lenses to see well. In many cases, surgeons replace the diseased lens with a plastic intraocular lens. See Cataract

Glaucoma is a disease in which the aqueous humour—the fluid that nourishes the cornea and the lens—does not drain properly. Pressure in the eye in­creases and, if untreated, destroys the optic nerve. In primary open-angle glaucoma, the most common type, vision to the side gradually narrows, and total blindness may eventually result. Primary open-angle glaucoma oc­curs chiefly in people over 40 years old. It is also called chronic simple glaucoma. It goes unnoticed by most people until some vision is lost, though an optician or eye specialist can detect the disease shortly after it de­velops.

Eye specialists treat most cases of open-angle glaucoma with eye drops or pills that reduce pressure in the eye and so halt damage to the optic nerve. The pa­tient must use the medicine throughout life. If the medi­cine is not effective, a new drainage channel for the aqueous humour is made by means of surgery, or the old channels are reopened by using concentrated, high-energy light beams from a device called a laser.

A type of glaucoma called primary narrow-angle glau­coma or acute glaucoma may occur suddenly at any age. Its symptoms include pain in the eyes or forehead and seeing halos or rainbows around lights. Immediate sur­gery or laser treatment may be necessary to prevent blindness. See Glaucoma.

Diseases of the outer parts. Various diseases can affect the outer parts of the eye. A sty is an infection of one of the sacs from which the eyelashes grow. A sty looks like a pimple on the edge of the eyelid. Sties may be treated by applying a moist, warm cloth and by using prescribed antibiotics. In some cases, pus from a sty may have to be drained by means of minor surgery. See Sty.

A chalazion is a blocked gland along the eyelid. If the gland becomes infected, a lump may form under the lid. The infected gland is treated the same way as a sty. After the infection clears up, the chalazion may be removed by minor surgery.

A disease called conjunctivitis is an inflammation of the conjunctiva, the membrane that lines the eyelids and covers part of the eyeball. Some types of conjunctivitis are called pinkeye. The disease may be caused by an in­fection, by an allergy, or by such substances as smoke or smog. The eyes become red and watery, and pus may form. Many types of conjunctivitis are contagious. They are often spread when someone uses the same towel or swims in the same swimming pool as a person who has the disease. Doctors often treat conjunctivitis with eye drops. They also may advise patients to take precautions so as to avoid infecting others with the disease. See Conjunctivitis.

Disorders of the eye's outer parts

The illustrations below show four eye disorders. A sty resem­bles a pimple on the eyelid. A chalazion may form a lump under the eyelid. Conjunctivitis is an inflammation of the conjunctiva. A subconjunctival haemorrhage is a broken blood vessel in the conjunctiva. It is harmless and should clear up in a week or two.

Surgical implantation of a plastic intraocular lens can re­store normal vision in a patient with a cataract. A cataract clouds the eye's lens, causing progressive vision loss. This intraocular lens is being placed in a surgically created hole in the cornea.

A severe form of conjunctivitis called trachoma is caused by a microscopic organism. The disease is treated with drugs or surgery or both. Trachoma is a rare disease in the developed countries, but it is a lead­ing cause of blindness in many developing countries. See Trachoma.

Diseases of the sclera, the white part of the eye, are relatively uncommon. Inflammation of the sclera is called scleritis. Most cases of scleritis are caused by an infection or an allergy. Doctors usually treat the disease with eye drops or pills.

Diseases of the cornea are among the most com­mon eye disorders. The cornea has less protection than any other part of the eyeball. It may be accidentally scratched by a fingernail or by a particle that flies into the eye, and a painful infection can result. Inflammation of the cornea may be caused by the herpes simplex virus, the same virus that causes cold sores. Doctors treat infections of the cornea with antibiotics, often in the form of drops or ointments.

A disease called keratoconus leads to a deformed cornea. As the disease progresses, the cornea becomes cone-shaped. In the early stages, keratoconus can be treated with glasses or contact lenses. In severe cases, the cornea may be removed by surgery and replaced with a cornea from a person who recently died. The re­placement comes from an agency called an eye bank (see Eye bank). This operation is known as a corneal transplant or corneal graft and is used to treat many dis­orders that cause diseased corneas.

Diseases of the uveal tract An inflammation of the uveal tract is called uveitis. Such diseases include iritis, inflammation of the iris; cyclitis, inflammation of the cili­ary body; and choroiditis, inflammation of the choroid.

In many instances, the cause of these diseases cannot be determined. Doctors treat such cases by prescribing medications that reduce the inflammation. The uveal tract may also be attacked by a cancerous tumour called a melanoma. Doctors may remove the tumour or the en­tire eye, or they may use other techniques, such as radi­ation treatments, to prevent the cancer from spreading.

Diseases of the retina. A number of diseases can damage the retina. One of the most common causes of new cases of blindness is diabetic retinopathy. A small proportion of all people who have diabetes have been severely blinded by this disease. After many years of dia­betes, the blood vessels of the retina may leak, close up, or begin to grow. These conditions may cause blood to enter the vitreous humour, the clear, jellylike fluid in­side the eyeball. The blood makes the vitreous opaque (nontransparent), causing blindness. In some cases, a major surgical procedure called a vitrectomy can re­move the blood and restore vision. In another proce­dure, a narrow beam of light from a laser destroys the abnormal vessels of the retina before blood enters the vitreous humour.

Diabetic retinopathy—and many other disorders of the eye—can also result in a detached retina. In this con­dition, the retina pulls away from the choroid. A de­tached retina can be corrected surgically by indenting the wall of the eyeball so that the choroid meets the ret­ina. If this procedure is successful, the retina then re­mains attached to the choroid. The retina itself is too fragile to withstand surgical instruments and so it can­not be pushed back to meet the choroid.

One of a group of diseases called macular degenera­tion may affect the macula, the part of the retina respon­sible for sharp central vision. The patient loses the abil­ity to see objects at which the eyes are directly aimed, though vision to the side remains. Some cases of macu­lar degeneration are inherited. Other cases, called se­nile macular degeneration, are acquired in old age. Spe­cial magnifying devices enable some patients to see well enough to read. In addition, laser treatment can prevent or postpone serious vision loss in some pa­tients, provided such treatment begins as soon as visual problems are noticed.

A group of diseases called retinitis pigmentosa chiefly affects the rods and cones of the retina. One of the first symptoms is night blindness, in which a person sees very poorly or not at all in dim light or at night. Many patients lose vision to the side and have only tun­nel vision. Some gradually become blind. In numerous cases, retinitis pigmentosa is inherited. No effective treatment has been found for retinitis pigmentosa.

Cancer of the retina is called retinoblastoma. In many cases, the disease is inherited and a tumour forms in the retina in children under 5 years of age. In many cases both eyes are affected. Doctors treat the disease with X rays or drugs. In some patients, the eye must be re­moved.

A blood disease called sickle cell anaemia, which oc­curs chiefly among blacks, damages many parts of the body and may eventually result in death. As the disease develops, it can cause a condition known as sickle cell retinopathy. In this condition, the blood vessels of the retina may clot or may begin to grow and bleed. In many cases, blindness results. Some cases of sickle cell retinopathy can be treated successfully by means of a laser or by eye surgery.

A condition called retrolental fibroplasia can occur in some premature infants. This condition, which can lead to blindness, used to be seen in premature babies nursed in incubators, which provide warmth and oxy­gen to help breathing. High concentrations of oxygen contribute to abnormal development of retinal blood vessels causing poor vision or blindness. Today, prema­ture babies in incubators receive only 40 per cent oxy­gen. As a result, the condition is now rarely seen. The condition has also occurred in some premature babies who have not been given high levels of oxygen, cientists are not sure what causes such cases.

Diseases of the optic nerve. The optic nerve can be damaged by optic neuritis (inflammation of the optic nerve), tumours, infection, and other diseases. Not all diseases of the optic nerve can be treated.

Care of the eye

Preventing eye damage. The most common eye in­juries are caused by blows to the eye, by particles that enter the eye, and by chemical burns, explosions, and firearms.

Many such injuries can be prevented. For ex­ample, safety glasses or goggles protect the eyes from particles that may be thrown from electric saws, grind­ing wheels, and other power tools. Some athletes also wear safety glasses to prevent eye injuries caused by balls, rackets, hockey pucks, and other sports equip­ment. In most cases, specks of dust or other particles that enter the eye can be removed by blinking gently or by flushing the eye with water. If the particle does not come out, it should be removed by a doctor. A fragment of glass, metal, or wood—or any particle stuck on the cornea— also should be removed by a doctor. If chemi­cals enter the eye, they should be washed out immedi­ately with large amounts of water and a doctor should be consulted at once.

The eyes can also be damaged through improper use of contact lenses, eye makeup, and sun beds. Infection may develop if contact lenses are not cleaned before they are inserted in the eyes. The hands also should be clean, and saliva should never be used to moisten the lenses before inserting them. Eye infections may also re­sult from sharing eye makeup or from moistening the cosmetics with saliva to apply them. The light of a sun lamp can cause painful burns of the cornea. Special goggles should be worn when using a sun lamp. No one should ever look directly at the sun, even during an eclipse. Looking at the sun with binoculars or a tele­scope can causp severe damage to the retina.

Receiving regular eye examinations. Doctors rec­ommend that everyone have an eye examination shortly after birth and at least every few years until the age of 40. Thereafter, the eyes should be examined about once a year. Eye examinations may be given by an optometrist or by an ophthalmologist. Optometrists are licensed nonmedical professionals who test vision and prescribe glasses and contact lenses to correct defects. Ophthal­mologists, sometimes called oculists, are doctors who specialize in the eye. They test vision, prescribe glasses and contact lenses, and treat eye diseases by means of medication and surgery. Opticians make and sell glasses and contact lenses prescribed by optometrists and oph­thalmologists.

One of the most familiar parts of an eye examination tests visual acuity (sharpness of vision) at a distance. This test involves reading letters from a chart a certain dis­tance away—usually 6 metres. The letters of each line are smaller than those in the line above. A person who can read all the lines that a normal eye can see from 6 metres has normal visual acuity. This vision is expressed as 20/20 (6/6 in the metric system). A higher denomina­tor indicates a visual defect. For example, a person with 20/40 (6/12) vision must be 6 metres from the chart to read all the lines that a normal eye can see from 12 me­tres. Each eye is tested individually, and one eye may have better vision than the other. Optometrists and oph­thalmologists also test near vision and side vision.

Ophthalmologists and optometrists also examine the inside of the eye for any sign of disease. By looking through the pupil with various instruments, these spe­cialists can examine the blood vessels of the retina and the fibres of the optic nerve. The condition of these structures may indicate not only an eye disease but also the advance of such diseases as diabetes and high blood pressure. For this reason, doctors also look into the eye as part of a routine physical examination.

Eyes of animals

Most animals have organs of some kind that sense light. The most elementary of these organs are called eyespots. Eyespots are light-sensitive areas on the bod­ies of flatworms, starfish, and certain other invertebrates (animals without a backbone). The organs can distin­guish between light and dark but they cannot form im­ages.

Other invertebrates have true eyes. Most kinds of in­sects have two large compound eyes. These eyes con­sist of many tiny lenses. Each lens admits light rays from one direction—one bit of the total scene that the insect sees. All the bits combine and form an image. Many adult insects also have three simple eyes set in a triangle between the compound eyes. Each simple eye has one lens. The simple eyes do not form images, but they re­spond to light quickly. Most crustaceans, such as lob­sters, have two compound eyes, and many species also have a simple eye. Spiders have only simple eyes. Most spiders have eight eyes. The eyes are arranged so that spiders can distinguish movement as light from a mov­ing object shifts from one eye to the next.

The eyes of most vertebrates (animals with a back­bone) have a structure similar to that of the human eye. However, the eyes may vary greatly in certain details.

For example, many vertebrates that live in total dark­ness, such as in deep caves, have tiny eyes that can see little or nothing. On the other hand, owls and some other animals that hunt at night have extremely large eyes and pupils, which provide excellent vision in dim light Cats also have good night vision. A mirrorlike structure in the eye called the tapetum lucidum reflects light onto the retina. The structure makes a cat's eyes ap­pear to glow at night when light is reflected off them.

Most birds have three eyelids—an upper lid, a lower lid, and a nictitating membrane, which moves sideways. The animals use the nictitating membrane to blink, and they close the upper and lower lids when sleeping.

Birds have the best visual acuity of all animals, including human beings. For example, a vulture can see a dead an­imal on the ground from a height of up to 4 kilometres.

Related articles:

Astigmatism

Blindness

Cataract

Colour blindness

Conjunctivitis

Disorders of the eye

Glaucoma

Longsightedness

Myopia

Nystagmus

Ophthalmia

Snow blindness

Strabismus

Sty

Trachoma

Other related articles

Aging (Signs of aging)        

Artificial eye

Blind spot     

Glasses

Colour (How we see colour)

Laser ( produce thin beams of light)

Compound eye        

Contact lens 

Lighting

Eye bank

Tears

Ophthalmology

Ophthalmoscope

Optical illusion

Optometry

Outline

Parts of the eye

The outer parts       

The uveal tract

The sclera and the cornea

The retina

How we see

Focusing

Depth perception

Adaptation to light and dark

Defects of the eye

Myopia          

Strabismus

Longsightedness     

Colour blindness

Astigmatism

Diseases of the eye

Cataract        

Diseases of the cornea

Glaucoma     

Diseases of the uveal tract

Diseases of the outer

Diseases of the retina parts          

Diseases of the optic nerve

Diseases of the sclera

Care of the eye

Preventing eye damage

Receiving regular eye examinations

Eyes of animals

Questions

Which parts of the eye focus light rays on the retina?

How do the pigments of the rods and cones differ in their re­sponse to light?

What are compound eyes? Simple eyes'?

What does the optic nerve do?

What makes the cornea transparent?

What is strabismus? A cataract? Glaucoma? 
What part of the eyeball regulates the amount of light that en­ters the eye?

Why do most middle-aged and elderly people need glasses for reading and close work?

What is diabetic retinopathy?

Why is it impossible for the human eye to form a sharp image of a nearby object and a distant one at the same time?

Take note:

Eye bank is a nonprofit agency through which eyes re­moved shortly after death are distributed to specially trained surgeons. These surgeons perform an operation called a corneal transplant on people who are blind from diseases that cause scarring of the cornea. The cor­nea is the transparent outer layer through which light enters the eye (see Eye [Diseases of the corneal). The op­eration consists of replacing the central portion of the scarred cornea with clear corneal tissue.

An eye remains suitable for surgery for only a short time after removal from the body. For this reason, it must be taken to the eye bank without delay. A surgeon must remove the donor's eyes within three hours after death and pack them in special containers just above freezing temperature. The container is rushed to an eye bank, where the eyes are carefully examined to deter­mine if they are suitable for surgery. An eye surgeon who has patients waiting for surgery is notified that an eye is available. The surgeon sends a patient to the hos­pital, and the operation is performed immediately after arrival of the eye.

The first eye bank was formed in 1944 in the United States. Many eye banks have since developed in other cities and countries. People who wish to donate their eyes should register with an eye bank. Prospective do­nors can then sign legal documents to donate their eyes upon death. If a person has not signed such documents, usually the nearest relative must give legal permission for removal of the eyes after death. Some donated eyes may be unsuitable for corneal transplantation. But such eyes can be used for study and research.