Eyelids - 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!
Cornea - The cornea is like a "window" at the front of the eye, typically it is just over 0.5mm thick. It should be clear at all times and does not have any blood vessels. 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 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, short sighted or if you have any astigmatism.
Iris - This is the coloured part of the eye.
Two sets of muscles change the size of the pupil in response to different light levels and other stimuli. It acts like the shutter in a camera, allowing more light into the eye when dark and less in bright daylight. Your Iris colour depends on the amount of pigment present. This is determined by genetics but can be changed by some medication.
Pupil - 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 because this could indicate neurological problems.
Sclera – This is the white, dense, fibrous outer coating of the eyeball, it 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 layer that gets red and inflamed if you get conjunctivitis. The conjunctiva covers the front of the eye, from the edge of the cornea, it then curls back on itself forming a sealed sack to cover the inside of the eyelid. This is why you can't lose a contact lens "behind the eye".
Lens – this is responsible for the minor adjustments in focus that are required to refocus from distance objects to near and vice versa.
The lens becomes thicker and more 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 naturally yellows with age and can slowly become cloudy or opaque with age or injury. This is called a cataract which can now be surgically replaced with a clear Perspex lens often restoring sight to its former level.
Vitreous - This is a jelly-like fluid inside the eye.
It becomes more fluid 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 a retinal detachment but the much more common "Posterior Vitreous Detachment" or PVD. BUT it is better to be safe than sorry so always seek advice should you experience any symptoms. 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!
Retina - 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.
Macula - 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 6-8 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. As the name implies, this is the area affected by AMD or "Age-related Macula Degeneration" which can have very serious impact on vision as we get older. Fortunately new treatments can help in certain cases, so any visual distortion should be investigated as soon as possible.
Optic Nerve - 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. If the pressure of the fluid inside the eye builds up this can be damaged causing loss of vision. This condition is called Glaucoma and in the early stages there are no symptoms, which is why it is essential to have regular eye examinations even if your vision seems normal.