More information about the eye

At the front of the eye the white sclera is modified to be transparent, the cornea. Behind this there is a circular gap in the choroid: this is the pupil, which can be made smaller or larger by the action of two muscles depending on the amount of light, or whether we’re looking at something close up, or emotion like fear.

The eyeball is divided into two parts by the lens. The larger part behind the lens is filled with a clear, jelly - like substance, the vitreous humour, while the smaller part in front of the lens is filled by a watery fluid, the aqueous humour.

Light passing through the cornea and the pupil enters the lens, which is made up of clear protein contained in a capsule. The shape of the lens can be altered by a circular muscle in order to focus the light on the retina - a process called accommodation. This enables us to see things clearly both close up and at a distance.

The front of the eye, except the cornea, is covered by a lining called the conjunctiva, which passes from the eyeball onto the back of the eyelids, and becomes continuous with the skin of the face. At this point emerge eyelashes, which protect the eye from dust, each with a tiny sebaceous gland.

Tears consist of watery fluid which lubricates the front of the eye and helps to prevent infection. They are produced continuously by glands in the upper outer part of the eyelid - the lacrimal glands - and they drain through two tiny holes, the puncta, at the inner end of the eyelids into the nose, which is why our nose runs when we cry. Blinking helps the fluid to drain.

The two eyes see things slightly differently. This binocular vision gives us the ability to see things in three dimensions. Movements of the two eyes - up, down, side to side or diagonally - are controlled by six muscles. The eyeballs should move in parallel to maintain binocular vision.

Newborn infants do not see very well. Their visual acuity (a measure of what they can see as a fraction of what they should be able to see) is about 6/200. (This basically means that you can see at 6 meters what you should be able to see at 200). This is due to immaturity of the fovea - the part of the back of the eye responsible for central focussing. Normally the visual acuity improves to 6/6 by around 1 year.

They can fix on close objects, and follow them - with a preference for the mother’s face - from birth, and respond to a smile with a smile at 6 weeks, when the eyes should move together.