The Retina

The retina, which contains all the sensory receptors for the transmission of light, is really part of the brain. The retinal receptors are divided into two main populations - the rods and the cones. The rods function best in dim light; the cones function best under daylight conditions. The cones are far fewer in number than the rods, numbering 6 million, whereas the rods number 125 million.

Cones enable us to see small visual details with great acuity. Vision with the rods is relatively poor but they detect movement and they also become extremely sensitive in low illumination and at night. Colour vision is totally dependent on the integrity of the cones.

The cones form a concentrated area in the retina known as the fovea, which lies in the centre of the macula lutea. Damage to this area can severely reduce the ability to see directly ahead. The rods are distributed in the periphery of the retina.

The junction of the periphery of the retina and the ciliary body is called the ora serrata. In the extreme periphery of the retina there are no cones and only a few rods. The retina is firmly attached to the choroid at the ora serrata. The other site of firm attachment of the retina is at the circumference of the optic nerve. The outer layer of the retina is composed of pigment epithelium cells and lies adjacent to the choroid, insulating the light receptor signals and making vision possible. Retinal detachment occurs as a result of cleavage between its anterior layers and the posterior pigment layer

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     A wide-field retinal image of a normal eye captured with the Optomap^®