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The retina is a layer of nervous tissue that covers the back of the eye. The retina consists of several layers, possibly the most important of which is the light sensitive rods and cones. As their name suggests the two types of cells differ in their structure and function. The rods contain a chemical rhodopsin which is broken down by a relatively small amount of light and are thus most sensitive in low light levels providing 'night vision'. The cones are shorter and contain the photochemical iodopsin which requires a greater intensity of light to break it down releasing the stimulus to the nerves which allows vision. Rods and cones are distributed across the retina fairly evenly except for the central macula and foveal areas. Here there are no rods and a greater density of cones, the area appears darker when viewed, and is the area of more detailed, central vision. There is also an area, the optic nerve head where there are no rods or cones, this is the point at which the optic nerve leaves the eye ball and causes a small, blind spot. The retina is nourished by blood vessels contained in the choroid layer and protected by the sclera.

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