If you want a basic understanding of how your eyes work, get a camera. You can ignore the tripod, timer, flash attachment, bayonet lens mountings, motor drives, fancy case, and all the other extras, doodads, and add-ons. Just concentrate on the basic Kodak box camera that George Eastman introduced to the world in 1888. The famous box was built to hold two items: a lens and film. God supplied the sunlight, and you decided what to take a picture of.

Your eyes are a million or so times more complicated than that, but if you keep the camera analogy in your mind, you shouldn't have too much trouble following along as we look at the mechanics of vision.

The Lens System

Let's start out with the two external requirements for vision—light and something to look at. When light strikes an object, the light rays are reflected off that object and into your own biological camera. With the original Kodak, the "lens system" was just a piece of ground glass that focused the image on the film.

The same basic thing happens when the light strikes your eye. The image is focused on the retina, which functions like the film. But the human lens system it passes through is more than just a piece of ground glass.

First the light passes through the cornea, a clear lens that covers the front part of the eye in much the same way that a watch crystal covers the face of a watch.

Once through the cornea, the image enters a narrow space known as the anterior chamber. This space is filled with a liquid called the aqueous humor. Next the image passes through the pupil. While the pupil looks black, it is actually a hole in the center of the iris, the colored part of the eye. Once through the pupil, the image is actually "inside" the eye.

The image then passes through the crystalline lens, usually referred to simply as the lens, then on through the vitreous body, the jellylike substance your eyes are filled with.

"Just what do you mean by filled?" you might ask. Well, the average human eye weighs about 7 grams (about 1/4 ounce). Of that, nearly 4 grams are vitreous.

After getting through the vitreous body, the image has traveled approximately an inch through your eyes and finally reaches its target, the retina.

The retina sits like a lining on the inside of the back part of the eye. It contains a million or so light-sensitive cells, called rods and cones. These are what actually "take" the pictures. And they take them constantly. All the time your eyes are open, they're clicking off more shots than a busload of tourists at the Grand Canyon. Even when there isn't any light they're still taking pictures. They just happen to come out black—underexposed.

The light-sensitive rods and cones have long nerve endings that come together like the stems in a bouquet of flowers and form the optic nerve. As the optic nerves of the right eye and left eye travel through the skull, they merge and then separate again to form the right and left optic tracts.

The fibers of the right optic tract "see" the right half of the field of vision for both the right and the left eye. Similarly, the fibers of the left optic tract "see" the left half of the field of vision for both eyes. After a long, circuitous path through the brain, the fibers finally reach the occipital lobes in the very back of the brain. All the images that the optic nerves transmit to the occipital lobes are stored, sorted, and interpreted by the brain.

It's worth remembering that while your eyes take the pictures, it is the brain that actually interprets what you see. When you close your eyes and recall an image or picture out of the past, you are seeing it with your brain. People who have gone blind can still "see" images in their minds that they "photographed" through their eyes before they lost their vision.

Parts and Accessories

You have just completed the short course on how your eyes work. For more specific information, you'll have to keep on reading. We'll go into the various parts of the eye in more detail, and also look at some of the accessories.

Eyelids

To continue the comparison between your eyes and a camera, the eyelids function as combination lens covers and lens cleaners. Aside from any conscious winking or blinking you might do, the subconscious centers of the brain have your eyelids blink at regular intervals of five to ten times a minute to sweep away small foreign particles such as dust, dirt, or dandruff. Blinking also keeps the surface of the eye moist and smooth by coating it evenly with a thin layer of tears.

The lids also help protect the eyes from wind, dust, fingers, snowballs, coat hangers, and other hazards. A protective response known as the blink reflex kicks in whenever you sense danger to your eyes. They automatically close fast and tight at the first indication of a flying fist or the beam of a flashlight.

Conjunctiva

The conjunctiva is a thin, transparent, protective membrane that covers most of the front part of the eye. Similar to the lining inside your nose and mouth, it also lines the inner surface of the eyelid.

The little bumps at the inner corners of the eyes are special folds of conjunctiva that help direct the flow of tears into ducts located at the inner margins of the eyelids. When the conjunctiva is irritated by rubbing, infection, contact lenses, smoke, or dust and dirt, its blood vessels dilate (widen) and the eye appears red or bloodshot. Although bloodshot eyes might not look very nice, in some respects they are good for you. Because the blood vessels are enlarged, more blood can circulate through them, carrying away toxic substances and bringing additional oxygen and white blood cells to the eye to fight off infection.

Not getting enough sleep also can make your eyes bloodshot. One reason for redness is that you tend to rub your eyes when you're tired. Another is that your eyes are probably drier than they should be because they've been open too long. Your eyes get a great deal of their necessary moisture while you're sleeping.

Sclera

Somehow it's doubtful that the phrase "don't fire until you see the sclera!" would have made it into the history books. But since a wise, but worried, soldier named Israel Putnam warned his comrades back in 1775 not to fire until they saw "the whites of their eyes," the exhortation has been preserved by history books, movies, sergeants, and even T-shirts.

The sclera lies beneath the conjunctiva and covers the eye the way horsehide covers a baseball. The only parts of the eye unprotected by the tough, rigid material is the cornea—where light enters the eye—and the hole in the back of the eye where the optic nerves exit the eye to carry the images to the brain. Thanks to its rigidity, the sclera helps the eye maintain its shape and also serves to protect the eye's more delicate inner workings from unfriendly flying objects like footballs or Frisbees.

Cornea

The cornea is the transparent lens that covers the front of the eye. The eye's most optically powerful lens, the cornea helps focus images on the retina.

Although it is firm, it consists mainly of water and protein and is one of your body's few living tissues that contain no nourishing blood vessels. Instead, it receives nourishment from nearby blood vessels in the conjunctiva and sclera and from tears and the aqueous humor. It gets its oxygen directly from the air.

There are two major advantages to the cornea's not having any blood vessels. First, since the cornea can't become bloodshot, it remains clear and easy to see through. Second, because cells in our blood are responsible for "rejecting" transplanted organs, the lack of blood vessels in the cornea makes it easier for the eye to "accept" a cornea transplant.

Iris

Blue-tinted sunglasses will change the colors that you see when you look through them, but blue eyes won't. The color of your iris does not really affect the quality of your vision. Eye color is largely a matter of inheritance, and all the color does is indicate how much pigment the iris contains.

People with blue eyes have relatively little pigment in the iris, brown-eyed people have a lot of pigment, and people with green eyes are somewhere in the middle.

Pupil

Even though it may look like a black dot, the pupil is actually a hole in the center of the iris that lets the light enter the eye. But this is not your ordinary hole.

If you enter a dark room, the pupil enlarges, or dilates, like the diaphragm of a camera. If you go out into the bright sunlight, the pupil constricts, or becomes smaller. This pupillary reflex protects the eye against the entry of too much light.

See for yourself. Stand in front of a mirror and shine a light into just one eye. Then look at both eyes in the mirror and compare.

Light is not the only thing that a pupil responds to, however. When you see something that you like—especially if it's something that you really like—your pupil can get an extra 45 percent bigger, no matter how much or how little light there is.

Your eye doctor may shine a flashlight into each of your eyes to make sure the pupils are the same size and that they constrict with light. Because this reflex is controlled by nerves that extend from the back of the eyes to the back of the brain, unequal pupil size could be a sign of a stroke, brain tumor, concussion, or neurologic infection.

Choroid and Ciliary Body

Both the choroid and the ciliary body are pigmented tissues connected to the iris. They can be seen only through special examining equipment.

The ciliary body produces the watery fluid, known as the aqueous humor, that circulates throughout the eye. It supplies nutrition for the inside of the eye and also provides the pressure that keeps the eye in its normal shape.

Think of the eye in terms of a water balloon. When too much of the fluid is produced, or when it cannot drain out of the eye, you can develop a high–eye-pressure condition known as glaucoma.

The choroid is sandwiched between the sclera and the retina and provides nutrition to the retina through its many tiny blood vessels. It absorbs energy from the photochemical processes that take place within the retina itself. Occasionally the choroid and other pigmented tissues within the eye can become inflamed, leading to a severe visual problem known as uveitis, which will also be discussed later.

Lens or Crystalline Lens

Usually referred to simply as the lens, the crystalline lens is located just behind the pupil, where it is suspended from the ciliary body by tiny fibers. It is enclosed in an elastic capsule that can be stretched to make the lens thicker or thinner.

As the lens thickens, it adds power to the eye and allows us to focus on close objects, such as the small print in a telephone book. This additional focusing power is called accommodation. But the flexibility that makes this accommodation possible diminishes as age increases. So by the time you're in your early forties, you'll probably need reading glasses or bifocals.

Vitreous

The vitreous is the clear, jellylike substance that fills the large cavity in the back two-thirds of the eye. It provides the eye with support and nutrition, but it can also cause problems.

Occasionally, bits of debris from the vitreous float around inside the eye. These "floaters" appear as spots before your eyes and are one of the most common visual complaints. A much more serious problem occurs when strands of the sticky, chewinggum-like elastic vitreous stick to the retina. If a vitreous strand contracts, it can rip the delicate retina, leaving a hole or tear. This can possibly lead to a detached retina, a vision-robbing condition.

Retina

Lying on the inner surface of the back of the eye, the retina consists of photoreceptor cells—the rods and cones—which contain the light-sensitive substance known as rhodopsin. When light enters the eye and excites the photoreceptors, a pattern is produced on the retina that is interpreted by the brain as a visual image.

The rods are responsible for nighttime vision, while the cones are responsible for daytime and color vision. The retinal pigmented epithelium (RPE) is a single layer of cells that plays a critical role in vision by processing incoming light rays. This layer of cells absorbs light adjacent to the rods and cones, stores and converts vitamin A, and engulfs discarded pieces of the rods and cones. The RPE rests on a transparent membrane called Bruch's membrane, which separates it from the choroid. The blood supply for the RPE, the choriocapillaris, is just on the other side of Bruch's membrane and is part of the choroid. The choroid supplies oxygen and other nutrients to the RPE photoreceptors.

Optic Nerve

The optic nerve is what lets the brain know just what the eye sees.

Imagine a coaxial cable that carries a TV image from a video camera to a video recorder and manages to make sure that everything that the camera "sees" is faithfully recorded, down to the most minute detail or subtle blend of colors. Now imagine something a whole lot more complicated than that: The optic nerve consists of approximately 1.2 million separate wires or nerve fibers. If any of those nerve fibers are damaged, your vision is also damaged.

Optic neuritis is a addition in which the optic nerves become inflamed and swollen. When this happens, you could wind up with blurry or double vision, headaches, or dim vision—as if someone had turned down the lights or bleached the colors out of what you are seeing. Luckily, optic neuritis usually heals itself.

Extraocular Muscles

The extraocular muscles allow you to move your eyes and gaze in every direction. There are six different muscles attached to the sclera of each eye, and they are all controlled and coordinated by nerves arising from the brain.

Sometimes a faulty nerve connection makes it difficult—or impossible—for the eyes to move together smoothly and properly. The result is a condition known as strabismus, in which one eye appears to "turn" in or out, up or down. It occurs in about 3 percent of all children. But if it is treated early, the deviating eye can retain good vision.

When strabismus is first noticed, the child must have a thorough eye exam to determine the cause of the deviated eye. Quite often, glasses will help solve the problem. Patching the strong eye may build up the strength in the weak or "lazy" eye. Sometimes surgery can be performed on the eye muscles to straighten the eyes and allow them to work together.

Lacrimal Gland

Located just under the bony rim above the eye, this gland produces the tears that wash over the eye's surface. The tears are spread evenly across the eye by the eyelids and are then collected at the inner corners. They drain away through tiny holes in the eyelid margin. From there they travel into the nasolacrimal sac and into the nasolacrimal duct, a long tube connecting the inner corner of the eye socket with the inside of the nose. This connection explains why eyedrops placed in the eye can produce a bitter taste in the throat within a few minutes.

Eric Rost

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