Anatomy of the eye
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 eye not only provides us with sight, but also allows us to see in three dimensions. This gives us the ability to gauge depth and distance when processing the world around us.
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 fibres. 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.
In the normal eye, light rays entering the eye are accurately focused on the retina and a clear image is formed. Most of the bending or focusing of the light rays occurs at the cornea, with the natural lens inside the eye being responsible for fine adjustments. If light is not focused on the retina then the eye is said to have a refractive error. Common refractive errors include: myopia (or short-sightedness), hyperopia (or long-sightedness), astigmatism, and presbyopia.
In Myopia, light rays come to a focus in front of the retina. This results in blurred vision, especially when looking at objects far away. Myopia results from the length of the eye being too long or the cornea being too steeply curved.
In hyperopia, light rays are focused behind the retina. This results in blurred vision especially when looking at objects that are close. Hyperopia results from the length of the eye being too short or the cornea being too flat.
In astigmatism, the cornea, or window of the eye, has an irregular curvature being shaped more like a rugby ball than a soccer ball. Light rays are focused at different points. Astigmatism is often associated with myopia or hyperopia.
In Presbyopia the problem is not a result of the cornea being incorrectly shaped or irregular compared to the length of the eye, but is due to an aging process occurring in the natural lens of the eye. As we age the lens becomes harder and less pliable and is unable to change its shape to focus (accommodate). After we reach forty years of age we begin to experience problems with focusing close, for example when we read. This is when most people begin to use reading glasses. Because presbyopia is not caused by an abnormal shape of the cornea, but by aging of the lens, it cannot be treated by the excimer laser.
In myopia, hyperopia, and astigmatism the cornea is incorrectly shaped or irregular compared to the length of the eye. The Excimer Laser is able to accurately re-sculpture the cornea to make the curve of the cornea match the length of the eye. Light rays are then focused normally on the retina. In myopia the cornea is flattened, whereas in hyperopia the cornea is made more curved. With astigmatism the surface of the cornea is re-sculptured to a regular curvature.