Have you seen those javelin throwers?How they adjust the angle of their throw in order that their javelin curves beautifully before landing spot on is amazing. Imagine that the javelin is a ray of light coming from an object that you are ‘seeing’. This ray of light has to curve precisely so as to land exactly on your retina so that you can see a sharp image. Who does this to the rays? No, there are no miniature invisible javelin throwers to adjust the angle of the ray; this work is done by none other than your cornea…
The cornea is the transparent outer layer of your eye. It is like a clear dome shaped covering over your iris (coloured part of your eye), pupil (the opening in the centre of your iris) and anterior chamber (fluid filled front section of your eyeball).
The cornea has a diameter of 11.5 mm and a thickness that varies from 0.5 to 0.6 mm in the centre to 0.6 to 0.8 mm in the periphery.
• Epithelium: This layer functions mainly to block the entry of foreign substances and absorb oxygen.
• Bowman’s Layer: This layer is made of strong protein fibres called collagen.
• Stroma: This layer makes up for 90% of the cornea’s thickness and the collagen fibres that it contains, gives it strength, elasticity and form.
• Descemet’s Layer: This serves as a protective barrier against injuries and infections.
• Endothelium: This innermost layer pumps the fluid out of the cornea. Without the extra fluid being pumped out, the cornea would swell up and become too opaque for light to pass through.
The cornea is made up of proteins and cells.
Since the cornea is the front window of your eye, it seems obvious that it needs to be transparent for the light rays to enter. This means that the blood vessels that usually supply oxygen to any tissue of your body cannot be present in the cornea. How does the cornea then get its oxygen? It gets this directly from the air. The oxygen first dissolves in your tears and then diffuses throughout your cornea. It also receives nutrition from the aqueous humor or +the fluid inside your eye.
It prevents foreign materials from entering the eye like germs, dust etc. It also serves as a filter for screening out harmful UV radiation to protect your lens and retina.
Since the cornea has many nerve endings sensitive to touch, chemicals and temperature, anything that touches your cornea reflexly causes it to close your eyelid. In fact your cornea is one of the most sensitive tissues of your body.
The cornea also focuses and controls the light as it enters your eyes. It bends the rays of light onto the lens which then fine-tunes the light onto your retina (light sensitive tissue at the back of your eyes). The cornea is responsible for two thirds of the focusing power of your eyes. Thus, it helps to produce a reduced inverted image of what you see on your retina. The retina then sends this image to your brain for interpretation.
If you were to compare your eye to a camera then the cornea and crystalline lens could be compared to the lens of the camera and your retina to the screen.
It is the shape of this dome that determines whether you will be near-sighted or far-sighted.