Know your eye

Thursday, April 17, 2008

Anatomy of eye

ISION is the process by which images captured by the eye are interpreted by the brain, and the visible part of the eye is where the process of sight begins.Our vision is a fine-tuned process. All the parts of the eye — and the brain — need to work together so that a person can see correctly.

YOU can check out different parts of the eye by looking at your own eye in the mirror or by looking at (but not touching) a friend's eye.


Everyone has a dominant eye.Here's how you can tell which eye is dominant:

  • Keeping both eyes open,point to an object in the distance and focus on it.Your finger will look blurry.
  • Next close your left eye.Did your finger move or stay on the object?If your finger stayed on the object,that means your right eye is dominant.If it moved,your left eye is dominant.
  • Try again,this time closing your right eye.You should get the opposite result.


The color of the iris comes from a pigment called melanin(pronounced:meh-luh-nun),the same substance that gives skin and hair their color.The more melanin there is and the closer it is to the surface of the tissue,the darker the iris.People with brown eyes have more melanin in their irises than those with blue eyes!


When light passes through the eye's lens and the image hits the retina,the image is actually upside down.So the message that the optic nerve brings to the brain is upside down,too.But luckily,your brain knows how to flip the image over so it's right-side.

THE eye is about as big as a ping-pong ball and sits in a little hollow area (the eye socket also called the orbit) in the skull.The body has several ways of protecting this vulnerable organ.The basic function of the eye is often compared to a simple camera.

PARTS OF EYE(See the diagram above,for more such diagrams click here):

Eyelids:The eyelid protects the front part of the eye. The lid helps keep the eye clean and moist by opening and shutting several times a minute.The eyelid also has great reflexes, which are automatic body responses, that protect the eye. When you step into bright light, for example, the eyelids squeeze together tightly to protect your eyes until they can adjust to the light. And if you flutter your fingers close (but not too close!) to your friend's eyes, you'll be sure to see your friend's eyes blink. Your friend's eyelids shut automatically to protect the eye from possible danger. Their main job is to protect your eyes from dirt, dust and harsh light

Eyelashes: A protective net for your eyes.They work with the eyelids to keep dirt and other unwanted stuff out of your eyes.

The cornea(say: kor-nee-uh) is the curved front surface of the eye,sitting in front of the colored part of the eye. When a ray of light hits this curved surface, it bends. It is here that most of the refraction occurs. The refractive index is approximately 1.376, which is comparable to that of glass or plastic. In other words,it is a refractive zone that guides light to reach the retina.You can hardly see it because it's made of clear tissue. Like clear glass, the cornea gives your eye a clear window to view the world through.

The iris(say: eye-riss) is the coloured part of the eye. When we say a person has blue eyes, we really mean the person has blue irises! The iris has muscles attached to it that change its shape. This allows the iris to control how much light goes through the pupil(say: pyoo-pul).Like a camera shutter,which controls the amount of light coming in to prevent both over- and under-exposure,the iris becomes wider and narrower,changing the size of the pupil to control the amount of light entering the eye.

The pupil(say: pyoo-pul) is the hole(black circle) in the iris through which the light enters. It enlarges when there is less light, so as to allow the maximum possible light to enter. It becomes smaller when you are in the presence of bright light.In other words,it controls the amount of light that enters your eyes.To see how this works, use a small flashlight to see how your eyes or a friend's eyes respond to changes in brightness. The pupils will get smaller when the light shines near them and they'll open wider when the light is gone.

The lens is the transparent structure inside the eye that focuses light rays onto the back of the eyeball — a part called the retina (say: ret-i-nuh).After light enters the pupil, it hits the lens. The lens sits behind the iris and is clear and colorless. The lens works much like the lens of a movie projector at the movies. It is flexible and changes its shape and focal length. When we look at faraway things, the lens is pulled flat by little muscles. When we look at close-up things, the muscles pull in and the lens gets fat and round. This way, the eye is able to look at things near as well as far.

The retina is the nerve layer that lines the back of the eye. When your eye focuses on an object, all the light rays from a single point on that object are bent toward a single point on your retina. The odd thing is that the images are inverted as they impinge on the retina! (This is because the rays of light are bent by a double convex lens!).Retina holds millions of cells that are sensitive to light. The retina takes the light the eye receives and changes it into nerve signals so the brain can understand what the eye is seeing. The retina senses light and creates impulses that are sent through the optic nerve to the brain. The retina is made up of millions of light receptors. These are called rods and cones.

The macula (say: mah-kyuh-luh) is a small, specialized area on the retina. The macula helps our eyes see fine details when we look directly at an object. It contains mainly cones and few rods.

The optic nerve is the nerve that connects the eye to the brain. The optic nerve carries the impulses formed by the retina to the brain, which interprets them as images.

The sclera(say: sklair-uh) is the white part of the eyeball.The sclera is made of a tough material and has the important job of covering most of the eyeball. Think of the sclera as your eyeball's outer coat. Look very closely at the white of the eye, and you'll see lines that look like tiny pink threads. These are blood vessels, the tiny tubes that deliver blood, to the sclera.

The anterior (say: an-teer-ee-ur) chamber is the space between the cornea and the iris. This space is filled with a special transparent fluid that nourishes the eye and keeps it healthy.

The lens is suspended in the eye by a bunch of fibers. These fibers are attached to a muscle called the ciliary (say: sil-ee-air-ee) muscle. The ciliary muscle has the amazing job of changing the shape of the lens. That's right — the lens actually changes shape right inside your eye!In people who have normal vision, the ciliary body flattens the lens enough to bring objects into focus at a distance of 20 feet (6 meters) or more.Young children can see objects at very close range; many people over 45 have to hold objects farther and farther away to see them clearly. This is because the lens becomes less elastic as we age. When you look at things up close, the lens becomes thicker to focus the correct image onto the retina. When you look at things far away, the lens becomes thinner.

The biggest part of the eye sits behind the lens and is called the vitreous (say: vih-tree-us) body. The vitreous body forms two thirds of the eye's volume. This material allows light to pass through to the retina. It also helps the eye keep its round shape.. It's filled with a clear, jelly-like material called the vitreous humor. Ever touch toy eyeballs in a store? Sometimes they're kind of squishy — that's because they're made to feel like they're filled with vitreous humor. In a real eye, after light passes through the lens, it shines straight through the vitreous humor to the back of the eye.

Rods and Cones:
The retina uses special cells called rods and cones to process light. Your retina has about 120(average 90) million rods and 6(average 4.5) million cones.Rods see in black, white, and shades of gray and tell us the form or shape that something has. Rods can't tell the difference between colors, but they are super-sensitive, allowing us to see when it's very dark.Cones sense color and they need more light than rods to work well. Cones are most helpful in normal or bright light. The retina has three types of cones. Each cone type is sensitive to one of three different colors — red, green, or blue — to help you see different ranges of color. Together, these cones can sense combinations of light waves that enable our eyes to see millions of colors.Rods and cones process the light to give you the total picture. You're able to see that your friend has brown skin and is wearing a blue hat while he tosses an orange basketball.Sometimes someone's eyeball shape makes it difficult for the cornea, lens, and retina to work perfectly as a team. When this happens, some of what the person sees will be out of focus.

Optic nerve:Think of the optic nerve as the great messenger in the back of your eye. The rods and cones of the retina change the colors and shapes you see into millions of nerve messages. Then, the optic nerve carries those messages from the eye to the brain, which interprets them as visual images. The portion of the brain that processes visual input and interprets the messages that the eye sends is called the visual cortex (say: kor-teks). The optic nerve serves as a high-speed telephone line connecting the eye to the brain. When you see an image, your eye "telephones" your brain with a report on what you are seeing so the brain can translate that report into "dog," "banana," or "toy," or whatever the case may be.

Tears:The first line of defence .They also have a corrective lens function.In the upper outer corner of each eye socket are the lacrimal (say: lak-ruh-mul) glands, which make tears. Every time you blink your eye, a tiny bit of tear fluid comes out of your upper eyelid. It helps wash away germs, dust, or other particles that don't belong in your eye.Tears also keep your eye from drying out. After they've done their job moistening the eyes, the tears flow into lacrimal canals or lacrimal ducts(tear ducts) in the eyelids, which drain into the lacrimal sac, a pouch in the lower inner corner of each eye socket. Tears then exit through a passage that leads to the nose. You can see the opening of your tear duct if you very gently pull down the inside corner of your eye. When you see a tiny little hole, you've found the tear duct.Your eyes sometimes make more tear fluid than normal to protect themselves. This may have happened to you if you've been poked in the eye, if you've been in a dusty or smoking area, or if you've been near someone who's cutting onions.

For you to see, the eye has to move. Six muscles, called extraocular (say: ek-struh-ah-kyuh-lur) muscles, surround the eyeball in the skull. These muscles act like the strings on a puppet, moving the eye in different directions. The muscles of each eye normally move together at the same time, allowing the two eyes to remain aligned.

The wall of a person's eyeball is made up of three layers, rather like the layers on an onion: The sclera is the outermost protective layer. This tough, fibrous tissue surrounds the eyeball and attaches to the cornea, which is the clear front surface of the eye. What we see as the white of the eye is the sclera. The conjunctiva, a clear mucous membrane that protects the eye from becoming dry, sits over the sclera and also covers the inner surface of the eyelid.
The choroid (pronounced: kor-oyd) is the middle layer that contains blood vessels that deliver oxygen and nutrients to the retina.
The retina,the innermost of the three layers, lines the inside of the eyeball. The retina is a soft, light-sensitive layer of nervous system tissue. The optic nerve carries signals from the retina to the brain, which interprets them as visual images.

Behind the cornea is a watery fluid called the aqueous (pronounced: a-kwee-us) humor. The cornea and aqueous humor form an outer lens that refracts (bends) light on its way into the eye. This is where most of the eye's focusing work is done.


(Click on the image to view full size)

  © Blogger templates The Professional Template by 2008

Back to TOP