The Eye

Iris – The iris is the coloured part of the eye. There are two major muscles in the iris, the sphincter muscle which cause it to contract in bright light and the dilator muscle which cause it to dilate in low light, controlling light levels in the eye, very much like the aperture of a camera.

Pupil – The pupil is the dark centre of the eye. The pupil allows light through the eye and its size is controlled by the dilator and sphincter muscles of the iris. The pupils reaction to light is often used to evaluate a patient’s neurological function.

Lens – Situated behind the iris, the lens is transparent and flexible. It also provides about a third of the eye’s power. The inner part of the eye is called the nucleus and is surrounded by a softer material, the cortex. The lens is suspended in place by zonules.
As you get older, the lens gradually hardens, reducing the ability to accommodate or focus.

Cornea – The cornea is the transparent area at the front of the eye which allows light to pass through the eye. By acting as a powerful refracting surface, it provides 70% of the eye’s total focusing power, the remaining 30% provided by the lens. With no blood vessels present in the cornea, it is clear and shiny but with more nerve endings than anywhere else in the body, the cornea is extremely sensitive.

Retina – The retina consists of light sensitive cells known as rods and cones or photoreceptors that convert light into electrical impulses. The impulses travel along the optic nerve to the brain where they are turned into images. There are approximately 6 million cones in the retina which function best in bright light and help us to see colours, and approximately 125 million rods which help us with night vision and peripheral vision.

Sclera – The Sclera is the white part of the eye. It is opaque and very tough and protects the rest of the eye. There are six small muscles connecting it around the eye which provide the eye’s movements. White in younger people, the sclera does tend to yellow with age.

Conjunctiva – The conjunctiva is a thin transparent tissue that covers the outer surface of the eye. Its is fed by virtually invisible blood vessels. It also secretes mucous and oil to help lubricate the eye.

Macula – Located at the centre of the retina, this highly sensitive part of the eye is responsible for detailed central vision. It allows us to perform more detailed tasks such as reading.

Fovea – The fovea is located at the very centre of the macula, as the central point of the retina and consist entirely of cones.

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