THE EYE

The Eye Explained

The Cornea is the clear surface of the outer eye. It is about ½ mm thick and consists of five layers: epithelium, Bowman’s membrane, stroma, Descemet’s membrane and the endothelium layer. It has two main functions. First, it acts as a barrier preventing germs, dirt and other harmful material from entering the inner eye. Secondly, the cornea acts as the eye’s outermost lens. It functions like a window that controls and focuses the entry of light into the eye. The cornea contributes between 65-75 percent of the eye’s total focusing power and when light strikes the cornea, it bends, or refracts, the incoming light onto the lens.

The Iris gives the eye its colour. This colour is genetically determined. It is made up of three layers of connective tissue and muscle fibers: endothelium, stroma and the epithelium. Its main function is to control the amount of light that is let into the eye. In bright light the muscles contract causing the opening at the centre of the iris (the pupil) to constrict. In dim light the muscles dilate allowing more light into the eye.

The pupil is the opening at the centre of the iris that lets light into the eye. It changes size in response to light levels.

The lens is a transparent structure of about 5 mm thick with a diameter of about 9 mm and is positioned directly behind the iris. It is made of proteins called crystallins. Its function is to focus light onto the retina. The lens is flexible and its curvature is controlled by the nervous system through a system of muscles around the lens. Changing the curvature of the lens allows focus on objects at different distances. The lens is encased in a capsule and suspended within the eye by zonule fibres.

The vitreous gel (also known as the vitreous humour) is a clear, thick, substance that fills the centre of the eye. It is comprised mainly of water and makes up approximately 2/3 of the eye’s volume, giving it form and shape. It is contact with the retina and help keeps the retina in place.

The retina is a multi-layered sensory tissue of neural cells that lines the back of the inside of the eye. It contains 3 layers of nerve cells including the outermost layer of sensory photoreceptor cells that capture light rays and convert them into electrical impulses which are transmitted by the optic nerve to the brain. Photoreceptors comprise two types: rods and cones. Each retina comprises approximately 125 million rods. These are responsible for peripheral vision and function best in dimlight. There are approximately 6.5 million cones in a human eye and these are more concentrated in the macula, most densely in the fovea. Cones are essential for vision in bright light and for seeing colours. The outer layer of the retina is known as the retinal pigment epithelium (RPE) layer. This layer helps nourish the photoreceptor cells and is attached to the choroid which provides the RPE with this nourishment which includes oxygen. The innermost layer of the choroid is known as Bruch’s membrane.

The macula is situated roughly at the centre of the retina. It is the focus for incoming light and as such is responsible for central vision and the ability to see detail. It has a diameter of approximately 1.5mm.

The fovea is a small pit of around 0.3mm near the centre of the macula which has the highest concentration of cone cells and is free of rod cells.

The optic nerve is the nerve that transmits visual information in the form of electrical impulses from the retina to the brain. It connects to the back of the eye near to the macula. The photoreceptor cells of the retina are not present in the optic nerve. As a result this creates a blind spot in our field of vision at the point on the retina where the optic nerve leads back into the brain. This is not normally noticeable because the vision of one eye overlaps with that of the other.