All About Eyes

Our eyes are the second most complex organ in our bodies. We rely on them every day, yet we don’t give them much thought until something goes wrong and our vision is affected. Find out more about them from the eyelid, to the retina to the optic nerve.

Eyelid

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 moisturised.

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. 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.

Sclera & Conjunctiva

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.

Iris

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.

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.

Lens

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.

Vitreous Humour

This is a jelly-like fluid inside the eye.

It becomes more liquid 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!

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”.

Macula & Fovea

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 the part of the retina responsible for detailed vision, such as recognizing faces or reading. It is also the area used for distinguishing colour.

Choroid

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

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 imaged in detail using an OCT scan of the retina and is called the optic disc.