eye

How the Eye Works
Vision is a complex sense composed of many elements. The human eye, elegant in its detail and design, represents a gateway to the process we call vision. The eyeball, or globe, is spherical in shape and about 1 inch across. It houses many structures that work together to facilitate sight.

The human eye is comprised of layers and internal structures, each of which performs distinct functions. The outside layer of the eye is comprised largely of a tough, white, protective tissue called the sclera. The sclera helps maintain the shape of the eyeball. At the front of the eye is an equally tough but clear structure called the cornea, which is responsible for letting light into the eye and bending light.
Going from outside to inside, the next layer of the eye is the choroid, which carries the blood supply necessary to nourish the eye's internal structures. Finally, there is the layer called the retina, lining the inside of the eye, which is sensitive to light and receives stimulation to its specialized cells.
The eye has a number of protective features. The eyelids, eyelashes and eyebrows are all designed to protect the eye from dirt and dust that might enter it and cause damage. The globe sits inside the orbital cavity, a bony pocket lined with fatty tissue as a cushion. Together these provide additional protection against injury. Six muscles attach at various points to the sclera and enable the globe to move in many directions inside the orbit.
In order for vision to take place, a succession of processes must occur involving the structures within the eye and the brain:
The first part of this chain is that light rays must travel through the eye to ultimately focus on the retina. There are a number of structures involved in the bending or refracting of light so that it focuses properly. Light first passes through the clear cornea at the front of the eye, and then through a watery substance called the aqueous humor which fills the small chambers located behind the cornea. As light continues on its pathway it passes through the pupil, a round opening in the center of the iris. The iris is the part of the eye that gives the eye its color. It also is made up of specialized muscles that are able to change the size of the pupil from very small (about 2 mm) to large (about 8 mm), regulating the light that is entering.
The next structure light will penetrate is the lens, another clear, layered structure shaped like a large lentil (about 10 mm in diameter) that is attached to muscles which contract or relax to change the shape of the lens. The changing lens shape helps light to be focused in response to the need for clarity. (The loss of this focusing ability as humans age -- a natural occurrence -- is the reason that many adults over 40 years old need reading glasses.) Once through the pupil and lens, the light then passes through the larger posterior (back) portion of the eye that is filled with a clear, jelly-like substance called the vitreous humor. From there, the light will come to the retina, where the rod cells and cone cells will be stimulated to set off a chain of split-second chemical reactions converting light to electrical impulses. The cone cells (about 7 million in number) are located in greatest concentration in the small, central part of the retina called the macula. This area is responsible for producing sharp, detail vision and color vision. The rod cells (numbering about 100 million) are found in the peripheral retina, away from the macula. These cells provide vision in dim light.
Even if all of the structures of the eye work perfectly, what we know as vision cannot happen without the brain's interpretation of the electrical impulses sent by the retina. The optic nerve is the bundle of retinal fibers that exits the back of the eye and transports electrical impulses to the brain where they are interpreted in the primary visual cortex.
When all parts of the visual system are working, the eyes can move together, can adapt to light and dark, perceive color and accurately evaluate an object's location in space. They are sensitive to differences in contrast, and can also provide detail vision, which is measured as visual acuity. By convention, we know "normal" visual acuity to be reported as 20/20. As the bottom number of this expression gets higher, it tells us that the vision is poorer than "normal." For example, the start of the range known as "legal blindness" is represented by the visual acuity finding of 20/200. One way to understand the meaning of this finding is that the eye being tested sees at 20 feet what the "normal" eye would see at 200 feet. People whose vision is in the category of "legal blindness" may still be able to use vision to do some of the things they need to do.
All eyes are not the same, nor are they all perfect. Some eyeballs are too long or have too much focusing power, causing the person to be myopic (nearsighted). Others are too short or have too little focusing power, and the result is hyperopia (farsightedness). Some eyeballs may have uneven curvature, called astigmatism. Options for correcting these "mechanical" problems are standard eyeglasses, contact lenses or refractive surgery. Other problems may be caused by disease or injury, and are not correctable by conventional means. People whose vision is irreversibly impaired due to diseases such as macular degeneration, glaucoma, cataract, diabetic retinopathy and others can be helped by vision rehabilitation.

RETINITIS PIGMENTOSA
What Is Retinitis Pigmentosa?
Retinitis Pigmentosa is the name of a group of retinal dystrophies that cause degeneration of the retina of the eye. Retinitis pigmentosa is a disease of the eye that the affected individual is born with. The word "retinitis" derives from "retina" (a part of the eye) and "itis" (a disease). It is a disease of the retina, though not a contagious one. The word "pigmentosa" refers to an associated discoloration of the retina, which becomes visible to an eye physician on examination. For those people who find retinitis pigmentosa a difficult term to use, the shortened form "RP" serves as a simpler alternative.
What The Retina Is
The retina is located at the back of the eye and is connected to the brain. It is made up of many millions of light-sensitive cells known as photoreceptor cells. These photoreceptor cells have the vital function of transmitting electrical impulses to the brain to enable seeing to take place.
Retinal dystrophies are caused by the gradual breakdown of these photoreceptors. Therefore it is important to understand the structure of the eye (as well as the ear in Usher Syndrome).
EYE ANATOMY AND PHYSIOLOGY

Structure and Function of the Eye
The eye consists of several parts that resemble a camera (see diagram).
sclera - the eye's white outer protective coat, normally seen as the "white of the eye".
cornea - the transparent, curved structure at the front of the eye.
iris - the coloured part of the eye - blue, brown, green, grey etc - that can be seen through the cornea.
pupil - the black part of the eye in the middle of the iris. It constricts or dilates according to the amount of light passing through it.
lens - the transparent disc (with both sides being convex) immediately behind the iris and pupil.
aqueous humour - the transparent fluid (with consistency similar to water) that circulates behind the cornea and in front of the lens.
vitreous humour - the material (like transparent jelly) that fills the eyeball between the lens and the retina.
retina - the light-sensitive layer of millions of nerve cells that line the back of the eyeball. The cells consist of two main groups, called rods and cones due to their appearance under the microscope.
rods - more numerous, spread out over the entire retina with more toward outer edge, respond to low levels of light.
cones - far fewer, concentrated around the retina's centre, respond to colour and to details.
macula - the small centre of the retina, responsible for reading vision.
retinal pigment epithelium - This is a dark coloured layer of cells at the back of the retina responsible for providing oxygen and other nutrients to the rods and cones.
choroid - a large network of blood vessels (behind the retina) that transport oxygen and other nutrients to the retinal pigment cells.
optic disc - a small yellow oval structure in the retina, to which nerve cell connections travel from all the rods and cones.
optic nerve and beyond - the "cord" of nerve cell connections that passes from the eyeball to destinations throughout the brain.
Function of the Eye
When you see an object, the light travels from that object to the cornea, then passes through the aqueous humour, pupil, lens and vitreous humour to reach the retina. During this passage, the light becomes focused onto the macula.
At the macula, the light causes chemical reactions in the cones, that consequently send electrical messages from the eye to the brain. The brain recognises these messages and indicates to you that this particular object has been seen. The cones are therefore responsible for you being able to recognise colours and to read.
The rods are essential for you to see in the dark, and to detect objects to the sides, above and below the object on which you are directly focused. This function prevents you from bumping into obstacles when moving around.
All the retinal cells (rods and cones) are provided with oxygen and other nutrients from the retinal pigment cells (epithelium), which are kept supplied by the rich network of blood vessels in the choroid.
What is responsible for RP
It is thought that one child is born with RP in approximately every 3,000 births in Australia. It is important to recognise that it is no one's fault and that RP can strike in a family with no known history of it. In fact, RP results from an imperfection in a tiny gene that causes an incorrect protein to be supplied to the retina. Over time this causes photoreceptor cells to die and progressive loss of vision results.

Diabetic Eye Disease FAQ
Diabetes is a very serious disease that can cause problems such as blindness, heart disease, kidney failure, and amputations. But by taking good care of yourself through diet, exercise, and special medications, you can control diabetes. And there is more good news. Diabetic eye disease, a complication of diabetes, can be treated before vision loss occurs. All people with diabetes need to get a comprehensive dilated eye exam at least once a year.
What is diabetic eye disease?
Diabetic eye disease refers to a group of eye problems that people with diabetes may face as a complication of this disease. All can cause severe vision loss or even blindness.
Diabetic eye disease includes:
Diabetic retinopathy: Damage to the blood vessels in the retina.
Cataract: Clouding of the lens of the eye.
Glaucoma: Increase in fluid pressure inside the eye that leads to optic nerve damage and loss of vision.
What is the most common diabetic eye disease?
Diabetic retinopathy. This disease is a leading cause of blindness in American adults. It is caused by changes in the blood vessels of the retina. In some people with diabetic retinopathy, retinal blood vessels may swell and leak fluid. In other people, abnormal new blood vessels grow on the surface of the retina. These changes may result in vision loss or blindness.
What are its symptoms?
There are often no symptoms in the early stages of diabetic retinopathy. There is no pain and vision may not change until the disease becomes severe. Blurred vision may occur when the macula (the part of the retina that provides sharp, central vision) swells from the leaking fluid. This condition is called macular edema. If new vessels have grown on the surface of the retina, they can bleed into the eye, blocking vision. Even in more advanced cases, the disease may progress a long way without symptoms. This symptomless progression is why regular eye examinations for people with diabetes are so important.
Who is most likely to get diabetic retinopathy?
Anyone with diabetes. The longer someone has diabetes, the more likely he or she will get diabetic retinopathy. Between 40-45 percent of those with diagnosed diabetes have some degree of diabetic retinopathy.
How is diabetic retinopathy detected?
If you have diabetes, you should have your eyes examined at least once a year. Your eyes should be dilated during the exam, which means eyedrops are used to enlarge your pupils. This dilation allows the eye care professional to see more of the inside of your eyes to check for signs of the disease.
Prevention & Treatment

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This page was last modified in October 2008
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Diabetic Eye Disease FAQ
Diabetes is a very serious disease that can cause problems such as blindness, heart disease, kidney failure, and amputations. But by taking good care of yourself through diet, exercise, and special medications, you can control diabetes. And there is more good news. Diabetic eye disease, a complication of diabetes, can be treated before vision loss occurs. All people with diabetes need to get a comprehensive dilated eye exam at least once a year.
What is diabetic eye disease?
Diabetic eye disease refers to a group of eye problems that people with diabetes may face as a complication of this disease. All can cause severe vision loss or even blindness.
Diabetic eye disease includes:
Diabetic retinopathy: Damage to the blood vessels in the retina.
Cataract: Clouding of the lens of the eye.
Glaucoma: Increase in fluid pressure inside the eye that leads to optic nerve damage and loss of vision.
What is the most common diabetic eye disease?
Diabetic retinopathy. This disease is a leading cause of blindness in American adults. It is caused by changes in the blood vessels of the retina. In some people with diabetic retinopathy, retinal blood vessels may swell and leak fluid. In other people, abnormal new blood vessels grow on the surface of the retina. These changes may result in vision loss or blindness.
What are its symptoms?
There are often no symptoms in the early stages of diabetic retinopathy. There is no pain and vision may not change until the disease becomes severe. Blurred vision may occur when the macula (the part of the retina that provides sharp, central vision) swells from the leaking fluid. This condition is called macular edema. If new vessels have grown on the surface of the retina, they can bleed into the eye, blocking vision. Even in more advanced cases, the disease may progress a long way without symptoms. This symptomless progression is why regular eye examinations for people with diabetes are so important.
Who is most likely to get diabetic retinopathy?
Anyone with diabetes. The longer someone has diabetes, the more likely he or she will get diabetic retinopathy. Between 40-45 percent of those with diagnosed diabetes have some degree of diabetic retinopathy.
How is diabetic retinopathy detected?
If you have diabetes, you should have your eyes examined at least once a year. Your eyes should be dilated during the exam, which means eyedrops are used to enlarge your pupils. This dilation allows the eye care professional to see more of the inside of your eyes to check for signs of the disease.
Prevention & Treatment

Diabetic Eye Disease FAQ: Prevention & Treatment
Can diabetic retinopathy be treated?
Yes. Your eye care professional may suggest laser surgery in which a strong light beam is aimed onto the retina. Laser surgery and appropriate followup care can reduce the risk of blindness by 90 percent. However, laser surgery often cannot restore vision that has already been lost, which is why finding diabetic retinopathy early is the best way to prevent vision loss.
Can diabetic retinopathy be prevented?
Not totally, but your risk can be greatly reduced. The Diabetes Control and Complications Trial (DCCT) showed that better control of blood sugar level slows the onset and progression of retinopathy and lessens the need for laser surgery for severe retinopathy. The study found that the group that tried to keep their blood sugar levels as close to normal as possible also had much less kidney and nerve disease. This level of blood sugar control may not be best for everyone, including some older adults, children under 13, or people with heart disease. So ask your doctor if this program is right for you.
How common are the other diabetic eye diseases?
If you have diabetes, you are also at risk for other diabetic eye diseases, such as cataract and glaucoma. People with diabetes develop cataract at an earlier age than people without diabetes. Cataract can usually be treated by surgery. A person with diabetes is nearly twice as likely to get glaucoma as other adults. And, as with diabetic retinopathy, the longer you have had diabetes, the greater your risk of getting glaucoma. Glaucoma may be treated with medications, laser surgery, or conventional surgery.
What research is being done?
Much research is being done to learn more about diabetic eye disease. For instance, the National Eye Institute is supporting a number of research studies in the laboratory and with patients to learn what causes diabetic retinopathy and how it can be better treated. This research should provide better ways to detect and treat diabetic eye disease and prevent blindness in more people with diabetes.
What can you do to protect your vision?
Finding and treating the disease early, before it causes vision loss or blindness, is the best way to control diabetic eye disease. So if you have diabetes, make sure you get a comprehensive dilated eye examination at least once a year. Remember... Diabetes is a disease that can cause very serious health problems. If you have diabetes:
Know your ABCs: A1C (blood glucose), blood pressure (BP), and cholesterol numbers.
Take your medicines as prescribed by your doctor.
Monitor your blood sugar daily.
Reach and stay at a healthy weight.
Get regular physical activity.
Quit smoking.
Site Policies and Important Links Privacy Policy FOIA Information Quality Guidelines FAQ ]
We welcome your questions and comments. Please send general questions and comments to the NEI Office of Communication, Health Education, and Public Liaison. Technical questions about this website can be addressed to the NEI Website Manager.
U. S. Department of Health and Human Services
National Institutes of Health