This page explains the anatomy of the eye. We also get many requests for information about eye diseases. Sometimes we may talk to you about these eye conditions when you visit us, but we know how difficult it is to absorb everything in one go, so we hope you find these pages helpful.


The eyelids are there to protect the eyes.

Blinking plays an important role in keeping the eyes healthy. When you blink, the layer of tears on the front of the eye is replaced by a fresh layer. This helps protect the eye and keeps the cornea healthy and moisturized.

If something comes towards your eyes at speed you will automatically blink. You cannot prevent this automatic response no matter how hard you try!


The cornea is like a “window” at the front of the eye. It should be clear and does not have any blood vessels.

The cornea is responsible for most of the focusing within the eye. The shape and curvature of the cornea can determine if you are long sighted or short sighted or if you have astigmatism.

It needs oxygen to remain healthy and gets this oxygen from the atmosphere. If you over-wear or abuse contact lenses the cornea can become starved of oxygen and you could lose vision. The cornea is protected and kept moist by a layer of tears. If your tears are not wetting the cornea properly you can get sore eyes and blurry vision.


The sclera is the white part of the eye. This is quite tough and protects the delicate structures inside the eye.

The front part of the sclera is covered by a thin film called the conjunctiva. This is the bit that gets red and inflamed if you get conjunctivitis. The conjunctiva covers the front of the eye, from the cornea, then curls back on itself to cover the inside of the eyelid.


This is the coloured part of the eye.

It is a set of muscles that changes the size of the pupil in response to different light levels and other stimuli. It acts like the shutter in a camera. Your Iris colour is determined by genetics but can be changed by some medication.


This is the hole in the centre of the iris.

The size of the pupil depends on the light level. In dark conditions the pupil will be larger to allow more light to reach the retina. In bright conditions the pupil will get smaller to protect the eye from harmful light and reduce the amount of light reaching the retina.

Some people have naturally small pupils and others naturally large pupils. Pupil size can be affected by drugs (legal and illegal), alcohol, mood and general health. Both pupils are normally equal in size. If not then you should have your eyes examined to make sure that there is nothing wrong.


The lens in the eye is responsible for the minor adjustments in focus that are required to refocus from distance to near and vice versa.

The lens becomes rigid with age and you gradually lose this ability to change focus. This normally becomes apparent around 40 years of age and is called presbyopia. The lens yellows with age and gradually becomes opaque or cloudy with age or injury. This is called a cataract.


This is a jelly-like fluid inside the eye.

It becomes more liquidy and shrinks with age and can pull away from the back of the eye. This can cause flashing lights and floaters (black spots in the vision). Any flashing lights or sudden floaters should be investigated by an optometrist as soon as possible because they could be signs of a retinal detachment. Most people with flashes and floaters do not have detachment BUT it is better to be safe than sorry. If caught in the early stages a retinal detachment can normally be repaired without much loss of vision. If left untreated it can cause significant, and sometimes total, sight loss in that eye!


This is the layer of light sensitive cells (photoreceptors) that covers the inside surface of the eye. It is much like the film in a camera. Light falling on the retina causes a reaction within the photoreceptors causing electrical impulses to be passed along the optic nerve to the visual cortex in the brain. These electrical impulses are then transformed into what we call “vision”.

There are two types of photoreceptors; rods and cones. There are about 125 million rods and cones intermingled across the retina. The rods are more sensitive in low light conditions and are sensitive to movement but are not sensitive to colour. The cones are sensitive to colour but are not very sensitive in low light levels. Cones are responsible for high resolution (detailed) vision.


The macula is a small (2.5-3mm) dimple roughly in the centre of the retina. The centre of the macula is called the fovea and is a rod-free area. There are about 6-7 million cones densely packed into this area and this is the part of the retina responsible for detailed vision, such as recognizing faces or reading. It is also the area used for distinguishing colour.


This is composed of layers of blood vessels that nourish the retina. It lies between the sclera and the retina.


This carries the electrical impulses from the photoreceptors in the retina to the brain. The end of the optic nerve can be seen on a photograph of the retina and is called the optic disc.


Astigmatism is where the cornea is shaped like a rugby ball instead of its normal shape of a football. When someone suffers with astigmatism the cornea has two different curves, one steeper and the other flatter.

Astigmatism affects both distance and near vision and can cause headaches and eyestrain. This blurred vision is due to the light and image focuses on more than one point in the eye.

The visual problems can be corrected using spectacles, contact lenses and some forms of refractive surgery.


A cataract is not a skin over the front of the eye!

It is a progressive change in the lens within the eye. The lens becomes increasingly opaque so the image becomes unclear and out of focus.

This is a natural part of ageing with most patients over the age of 60 showing some signs of cataract formation.

Cataracts can also form as a result of injury or can be triggered by long term use of some medication.

Cataracts can be treated by a routine operation, usually under local anaesthetic. They are not removed by laser. The operation removes the old cloudy lens and replaces it with a new clear plastic lens. Modern surgical techniques mean that you are unlikely to need stitches so recovery is very rapid. New spectacles will be needed after surgery as your prescription will probably be very different, but you may find that you only need reading spectacles. You will normally have to take drops for 2-3 weeks and should finish these before you can have an eye test for new spectacles.

The effect of a cataract

Photograph of the retina before cataract surgery

Same eye after surgery

The photographs above demonstrate the blurring and haziness caused by a cataract.


Diabetic retinopathy is the name for eye disease caused by diabetes. Sight loss is largely preventable if diabetes, blood pressure and cholesterol are all well controlled.

Research has shown that blood pressure and cholesterol control are as important as blood glucose control in preventing eye disease in diabetics.

There is no such thing as “just a bit diabetic” or “not really diabetic”. Even if you control your diabetes with diet or tablets instead of insulin you are still at risk.

If you are diabetic you should have your eyes tested at least once a year to make sure that you are not developing this sight threatening condition. This test should include photographs of the back of the eye taken with a special camera.

Diabetes makes the blood vessels more porous so they become leaky. If this happens unchecked in the eye it can cause severe vision problems.

The early signs of leaky blood vessels often do not cause any vision loss, so you may not know that anything untoward has happened.

If the optometrist sees leaky vessels they can refer you for treatment before you lose any vision. If you wait until you lose sight before you have an eye examination it may be too late for successful treatment.

Click here to see digital images of diabetic eye disease.

There are several schemes for ensuring that diabetics have their eyes examined properly. The location of your G.P.will determine which scheme you fall under. There is no national standard screening service.

The 3 options are:

  • Hospital based – you go to a special clinic at a hospital where you will have drops put into the eyes to dilate the pupils and photographs will be taken of the back of your eyes. The photographs are often assessed at a later date and you will be notified in writing if any further action is needed. The hospital will not perform any other tests so you still need to have a regular eye examination at the optometrists’. It is recommended that you do not drive after you have had the drops as your vision may be blurred.
  • Mobile Screening – this is similar to the hospital based screening apart from the fact that the camera is in a specially adapted van or lorry which comes to the G.P. surgery on a regular basis.
  • Optometry based – this is screening performed by an accredited optometrist who has the appropriate training and equipment. You may be able to have your normal eye examination at the same visit if you come under this type of scheme.

On 31st December 2006 Somerset changed from an Optometry based screening program to a hospital based program. At Sarah Gibson Optometrist we performed the screening for many years, but these changes mean that we are no longer able to offer the service under the NHS.

If you are unable to attend the hospital we have the facility to take photographs and send a full report, including copies of photographs, to both you and your GP. Unfortunately the NHS will no longer pay us a suitable fee to maintain the camera equipment required so this is only available for a private fee of £45.


Glaucoma is a relatively common eye condition that, if untreated, can lead to devastating sight loss.

Click here to see what the Optometrist may see if you have glaucoma

In its most common form, primary open angle glaucoma, (POAG) it is an insidious disease that only causes noticeable symptoms once it is quite advanced. Once the damage is done it cannot be repaired.

If caught early enough POAG can usually be controlled by daily drops, although sometimes surgery is required if the pressure does not respond to drops.

The only way to detect this form of glaucoma is to have regular eye examinations.

Primary open angle glaucoma is characterised by high pressure in the eye, gradual loss of peripheral vision and characteristic changes to the optic disc.

The pressure in the eye is measured using an instrument called a tonometer. There are two methods of tonometry, non contact and contact:

  • Non contact tonometers - the “air puff” ones that are most commonly used by high street opticians. The test is often performed by a clinical or optical assistant.
  • Contact tonometers - Generally require a drop of anaesthetic in the eye before the measurement is taken. These are more common in hospitals although we use this type of tonometer at Sarah Gibson Optometrists. We have found that most people prefer it as it doesn’t make you jump like the air puff.

A pressure of between 10 and 21mmHg is considered to be quite normal, although if someone’s pressure went up from 10 at one visit to 20 at the next we would be concerned. This is why it is important to visit the same optometrist regularly if possible.

The optometrist examines the optic disc when s/he looks at the back of the eye. At Sarah Gibson Optometrists we use digital fundus photography wherever possible so that we can see subtle changes by comparing images over a period of time.

The peripheral vision is measured using a visual field screener. The test involves looking at a central target whilst lights are flashed on and off around it. You will be asked to say how many lights you see and sometimes asked to describe where you saw them. Sometimes the test will involve pressing a buzzer every time you see a light.

The risk of glaucoma increases with age, a high prescription and some medication. If you have a close relative with glaucoma you are considered to be more at risk of developing it. If you are aged over 40 and the parent, child or sibling of someone with glaucoma then you should have an eye examination every year. If there is no family history of glaucoma you should still have your eyes tested at least every 2 years.

There are other forms of glaucoma. Low tension glaucoma, closed angle / acute glaucoma and sub-acute angle closure glaucoma:

  • Low tension glaucoma - This occurs when the peripheral vision is damaged even though the pressure is within what is considered to be normal limits.
  • Closed angle glaucoma - This is a rapid painful rise in the pressure in the eye. Most people who develop this tend to go to casualty rather that the optometrist as it is very painful.
  • Sub-acute angle closure glaucoma This is quite difficult to diagnose as the symptoms and signs are not present all the time. The symptoms can range from recurrent mild discomfort and blurring, which resolves within an hour or so, to a more painful red eye which resolves before you get to the optometrist or doctor. There are signs that may be visible to the optometrist to help them make the diagnosis but often the pressure will be quite normal by the time you get an appointment and any visual field loss minimal in the early stages. Unfortunately similar symptoms are also caused by a large number of other conditions.

For more information on glaucoma visit


Hypermetropia is also known as long sightedness. This means the eyeball is too short so the light and image is focused behind the retina and not on it.

People who are long sighted have problems seeing things close up and in extreme cases they also have problems with distance. They can suffer with eye strain, headaches, and tiredness.

In most cases treatment will depend on symptoms and age. Spectacles and contact lenses are the usual form of correction but some forms of refractive surgery may be suitable.

Hypermetropia is corrected with plus (+) lenses which have thinner edges and a thicker middle.


Age Related Macula Degeneration (AMD) is the largest cause of blindness in the over 60’s in the western world. It is estimated that 25% of people over 60 have some signs of AMD.

AMD causes loss of central vision. It will NOT make you completely blind. It will generally affect your ability to read and to recognise faces. It can make some day to day tasks more difficult in the advanced stages.


With AMD

A good diet, rich in leafy green vegetables is said to help maintain a healthy macula and reduce the incidence of AMD. Food supplements are available which claim to improve macula health by increasing the quantities of appropriate vitamins and minerals in the body.

Smoking is another significant risk factor for AMD, another good reason to quit!

Sun damage is also a significant factor so make sure you wear sunglasses. The damage is often done early in life, so to reduce your chances of developing AMD you should wear good quality sunglasses from an early age.

There are two main forms of AMD: Dry and Wet

This is the most common form of AMD and is a slower more progressive form of the disease. It generally develops over a long period of time and can affect one or both eyes. It cannot be treated at this time.

Generally has a quicker onset that causes more serious blurring and distortion of central vision, which can cause a distortion of straight lines. This distortion is caused by fluid leaking from the blood vessels in the macula area.

Below is a copy of a chart used to determine the presence of macula changes. It is called an amsler chart. The picture on the right is the appearance of that same chart described by someone with AMD.

Treatment options are limited at the moment but new research is looking promising if used in the early stages of the disease.

What AMD looks like in the eye. (Digital fundus photograph)


Myopia is better known as short sightedness. This means that the eyeball is too long and the light and image is focused in front of the retina instead of on it.

People suffering with short sightedness have problems seeing clearly into the distant but can see well close up.

Myopia can either be treated by wearing spectacles, contact lenses or by certain refractive surgical techniques.

Myopia is corrected with minus (-) lenses. These have thicker edges with a thinner middle.


This is when the lens of the eye loses its flexibility and you lose the ability to change focus from long distance to close up. Its effects start to become apparent at around 40 years of age but it is an ongoing process that has been happening since before you were born and will stop only when you do!

As a child you have a large amount of focussing because the lens is very flexible and quite small. This focussing ability is called accommodation.

One theory for the reduction in lens flexibility is this: As you grow the lens grows. Extra layers are created around the nucleus at the centre of the lens. These layers are not sloughed off like skin cells so they build up on top of each other, a bit like the layers of an onion. Because the lens is inside the eye it reaches a point where it cannot get any bigger, so it starts to compress and get more dense. This is when it loses its flexibility.

Optometrists often see people for the first time at around 40 years of age. The following are a few of the comments that people make when they first experience the effects of presbyopia.

I don’t need specs, I just need longer arms.
The print is getting smaller.
I don’t need glasses, I just need more light to see things now.
My distance vision is all blurry after I have been reading.
It takes ages to focus if I look from the TV to my magazine.
My eyes are sore and tired if I have to read for too long.

People who are short sighted often complain that they have to take their glasses off to read.

Presbyopia can be corrected by reading spectacles, bifocals, or varifocals. It can also be corrected by some types of contact lenses.