Dry Eye Syndrome

Introduction to the eye

The eye is shaped like a ball. The tough outer white coat is called the sclera and its surface is covered by a thin layer called the conjunctiva. The outer layer is transparent at the front of the eye, where it becomes the cornea. This is covered by the tear film, which together constitute the first optical surface (or ‘window’) for light entering the eye. Any disturbance to the tear film or cornea – as may be seen in dry eye syndrome – can cause blurring of vision.

Behind the cornea is the iris – the coloured part of the eye – with the pupil forming a hole in its centre. The space between the cornea and the lens is filled with a clear fluid, called aqueous humour, which maintains the pressure in the eye (the intraocular pressure).

On the inside of the back of the eye is the retina, which is the light sensitive layer onto which an image of what is being seen is focussed by the cornea and the lens working together. The central area of the retina, where the most detailed vision is to be found is known as the macula. There is a very high density of cells, further away from this central detailed vision area is the area of the retina which is more sensitive to dim light and which also provides our peripheral vision.

Immediately below the retina is the choroid, which is the layer of the eye that provides the blood supply to the cells of the retina and onto which the retina is attached.

Light that has passed through the front of the eye and is focussed onto the retina is finally converted into a series of complex electrical impulses by retinal photoreceptor cells known as rods and cones. These signals pass along the optic nerve to the back of the brain, where the final image is constructed.