The Human Eye
Sight is regarded by many people as the most precious of our senses and it provides more information about our environment than any of the other five senses. Sight allows us to see the shapes, colours, and sizes of objects around us in the world. Our eyes are responsible for this sense of sight and they are used for almost everything that we do: whether we are doing activities such as walking, driving a car, or sports, or whether we are reading, working on computers or watching television. Sight is formed by light rays that are reflected from objects and enter the eye where they are processed and sent to the brain for interpretation.
The eye measures approximately 2.5 cms in diameter and is situated in the front of the skull, in a well protected, cone-shaped cavity called the orbit or eye socket. The eye is surrounded by fatty tissue that adds further protection and allows the eye move easily. Six muscles, attached to the outside of the eye, regulate eye movement.
The most important parts of the eye are the sclera, cornea, iris, lens, vitreous, retina, macula and the optic nerve.
The sclera is the tough, white outer coat of the eye; it provides structure, strength and protection for the inner structures of the eye.
At the front of the eye the sclera becomes transparent and is regularly shaped to form the cornea, or “window of the eye”. The cornea allows light rays to enter the eye and is responsible for most of the bending or focusing of the light rays that is required for the light to be focused accurately on the retina.
The iris is the coloured circle of tissue that lies behind the cornea and is what people commonly refer to as “the colour of the eye”. In the centre of the iris is a circular gap, called the pupil of the eye; it appears black because no light is reflected from the eye, through the pupil. The iris controls the size of the pupil which acts as an aperture. One set of iris muscles contract the pupil, another enlarges (dilates) the pupil allowing more light into the eye.
The lens lies directly behind the iris. It is a flexible, transparent structure and its shape is controlled by muscle fibers. Its role is to make the final focusing adjustments of light rays so they are sharply focused on the retina.
The vitreous is the clear gel-like substance that fills most of the cavity of the eye: it helps to maintain a firm, spherical shape to the eye.
The retina makes up the innermost layer of the wall of the eye and consists of light-sensitive cells that absorb light rays and change them into electrical signals that are passed to the brain and interpreted as visual images.
The Macula & the Optic Nerve
Situated at the back pole of the eye is a specialised area of retina, where the light sensitive cells are highly concentrated, called the macula. This is the area of the retina that we use for fine, intricate visual tasks. Once the light rays have been processed by the retina, the electrical signals are carried by the optic nerve to the brain, which interprets them as visual images.