The Structure of the Eye

How is the eye structured?

The eyeball is about 25mm in diameter. It has a tough outer coat the SCLERA (the white of the eye) and the CORNEA (the transparent front window of the eye).

Because the cornea is steeply curved and transparent it acts like a strong optical lens. It provides about 70% of the eyes' focusing power.

Covering the front of the sclera is a thin, clear membrane called the CONJUNCTIVA which folds forward to become the lining of the inside of the eyelid. The conjunctiva prevents your contact lens, or any other foreign body from being able to disappear around behind the eye.

Behind the cornea is the coloured IRIS. The iris is like a flat disc with a hole, the PUPIL, in the middle. The iris is elastic, muscular tissue that alters the size of the pupil by expanding and contracting. By closing down in bright light and opening up in dull light the pupil helps to regulate the amount of light entering the eye (just like a camera). The pupil normally looks black because light is not reflected back out of the eye, although occasionally in flash photographs, or when an animal is caught in your headlights at night the pupil appears bright red. This is caused by light being reflected off the back of the eye out through the pupil.

Immediately behind the iris is the CRYSTALLINE LENS, this transparent, flexible lens is about 9mm in diameter and 4mm thick and provides about 30% of the focusing power of the eye. The lens is attached to a circular muscle by fine thread-like fibres. By contracting or relaxing, this muscle causes changes in focus by altering the shape of the crystalline lens. In a young eye the lens is very flexible giving a wide and rapid range of focus, however the crystalline lens continues to grow throughout life becoming thicker and harder with age. This results in a loss of flexibility which means we slowly loose our ability to focus on near work, most people notice this change at about age 40-45.

On the inside of the sclera is a layer made up mostly of blood vessels, the CHOROID. The choroidal blood vessels feed the retina.

The innermost layer of the eye is the RETINA which lies on the choroid. The retina is made up of special light sensitive nerve cells called rods and cones. The nerves from these cells run across the inside of the eye and come together into one large bundle, the OPTIC NERVE which emerges from the back of the eye and connects the eye to the brain.

Most of the eye is filled with a clear jelly, the VITREOUS, which is contained within a membrane. The front portion of the eye is filled with a watery fluid, the AQUEOUS.

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