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Mechanism of Sight
The image of an object has to hit the retina directly otherwise the image is lost. The lens help to focus to allow an accurate view as it directs the light to the right places. For nearer objects, the ciliary muscles contract, the ligaments relax and the lens become fatter and more curved. When the object is far away, the ciliary muscles relax and the ligaments contract to pull the lens into a straight thin shape. This helps the image to be made clear in the retina.
Another helper, the pupils are always opening and closing in response to the amount of light made available in the environment. When it’s dark, they expand to let light in and gets smaller when there’s too strong light in order to protect the eye.
When we look at on object, light is reflected of it and enters the eye, falling onto the surface of the retina. The light receptor cells translate these light messages and send them via the optic nerves as electrical messages to the brain. The image on the retina is upside down due to the fact that light travels in straight lines, but the brain turns the image right side up and allows one to see things in an upright manner This co-ordination occurs in the visual centre of the brain, at the back of the head.
Cones help to deciphers colours. There are 3 types, the red, green and blue sensitive types. Rods are sensitive to dim light but not to colour.
Having 2 eyes increases the angle of vision and helps one to judge depth. There is one optic nerve for each eye, each of which links up to one hemisphere of the brain. When the right optic nerve registers an image, it sends it to the left hemisphere of the brain. At the same time, the left optic nerve sends the picture to the right hemisphere. The brain then combines these 2 pictures into 1 single, solid image, known as stereoscopic vision.
Some common problems of the eye include myopia, hypermetropia, presbyopia and astigmatism. Myopia, which is short-sightedness means one can’t see far away things clearly. The eyeball is lengthened so images can not be focused on the retina clearly. Hypermetropia, which is long sightedness means one can’t see objects that are near due to short eyeballs, such that the image falls behind the retina. Presbyopia is the type of long sightedness that elderly people gets as the eye lens gets harder with time such that it can not curve enough to focus the objects. Astigmatism is when the cornea starts to curve unevenly, making things look fuzzy.
Most of these problems can be corrected with glasses or surgery.
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