The Retina

What Is The Retina?

The innermost layer of the eye is the light sensitive layer, the RETINA. The retina is less than a quarter of a millimetre thick and is made up of pigment cells, specialised light sensitive "receptor" cells (rods and cones) and nerve fibres.

What Is The Retina?

The outermost layer of the retina itself is a pigment layer that is only one cell thick. The dark pigment in these cells absorbs light reducing scatter and reflection of light inside the eye. Without this pigment layer glare would constantly interfere with our vision.

Because the eye must work in bright light and dim conditions there are two types of light sensitive "photoreceptor" cells in the retina, rods and cones. These cells are long and thin and are packed like drinking straws with one end embedded in the pigment layer. The receptor cells contain light sensitive chemicals that are derivatives of vitamin A.

In daylight or normal indoor lighting when detail is most important the cone system functions. There are about six million cones in the retina. In the centre of the retina is a special area where the cones are very tightly packed, this small area gives us our fine detailed "central" vision. Cones are thinly scattered throughout the rest of the retina, which is why we are less aware of detail in vision to the sides. There are three complex photopigments in the cones that give us our colour vision. Colour vision defects result if any of these photopigments are absent or abnormal.

In poor light or at night when sensitivity is important our rod vision takes over. There are 100 million rods in the retina and these are linked together so that the areas of the retina with large numbers of rods are good at detecting light but poor at seeing detail. There are no rods in the central area where the cones are tightly packed so our central vision at night is quite poor. The rods are most dense about 15 degrees out from the centre of the retina which is why in dim conditions when only the rods operate, our best vision is obtained just to the side of our straight ahead line of sight. There is only one photopigment in the rods and so in poor light, where cone vision does not work, we see only in black and white.

Altogether about one million nerve fibres come from the light sensitive cones and rods. These are transparent and run over the retina to come together in a bundle forming the optic nerve, it is this large nerve which carries visual information to the brain.

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