The human eye is an organ which allows you to see. In the eye light enters the pupil and is focused on the retina by the lens. Light-sensitive nerves called RODS (for Brightness) CONES (for Colour) react to the light. They interact with each other and send messages to the brain that indicate brightness, colour, and contour.

The visual system in the brain is too slow to process information if the images are slipping across the retina at more than a few degrees per second. For humans to be able to see while moving, the brain must compensate for the motion of the head by turning the eyes. Another complication for vision in frontal-eyed animals is the development of a small area of the retina with a very high visual acuity. This area is called the fovea, and covers about 2 degrees of visual angle in people. To get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Eye movements are very important for visual perception, and any failure to make them correctly can lead to serious visual disabilities.

Having two eyes is an added complication, because the brain must point both of them accurately enough that the object of regard falls on corresponding points of the two retinas; otherwise, double vision would occur. The movements of different body parts are controlled by striated muscles acting around joints. The movements of the eye are no exception, but they have special advantages not shared by skeletal muscles and joints, and so are considerably different.

The very back of the eye is lined with a layer called the retina which acts very much like the film of the camera. The retina is a membrane containing photoreceptor nerve cells that lines the inside back wall of the eye. The photoreceptor nerve cells of the retina change the light rays into electrical impulses and send them through the optic nerve to the brain where an
image is perceived. The centre 10% of the retina is called the macula. This is responsible for your sharp vision, your reading vision. The peripheral retina is responsible for the peripheral vision. As with the camera, if the "film" is bad in the eye (i.e. the retina), no matter how good the rest of the eye is, you will not get a good picture.

Iris
Inside the anterior chamber is the iris. This is the part of the eye which is responsible for one’s eye colour. It acts like the diaphragm of a camera, dilating and constricting the pupil to allow more or less light into the eye.

Pupil
The dark opening in the centre of the coloured iris that controls how much light enters the eye. The coloured iris functions like the iris of a camera, opening and closing, to control the amount of light entering through the pupil.

Lens
The part of the eye immediately behind the iris that performs delicate focusing of light rays upon the retina. In persons under 40, the lens is soft and pliable, allowing for fine focusing from a wide variety of distances. For individuals over 40, the lens begins to become less pliable, making focusing upon objects near to the eye more difficult. This is known as
presbyopia.

One Response to “The Human Eyes”

1. kumar Says:
   April 2nd, 2008 at 2:44 pm

   The article is attractive and informative.
   The pictures are very clear and relevant.
   If only a cross-section diagram or picture
   was inserted, the entire article would be more purposeful.
   Good work! Keep it up!

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