The retina is a thin, transparent membrane. It is on the light sensitive retina
that images are formed. The retina is the innermost of the three concentric
layers of the eyeball. The retina is enclosed in a dark-brown pigmented capsule.
Light is not transmitted through this capsule. The retina is an extension
of the brain and contains the cells for vision; the rods and cones. It is
here that light stimulus is changed into neuronal stimulation through photochemical
processes. The center of the retina has a small groove. This is the macula
which contains only cones and where vision is sharpest. This rod-free visual
groove lies along the visual axis which is optically the midpoint of the retina.
The cones here are clustered closely together over an area of 5 gmm and enable
visual acuity as well as colour vision. Rods cover the majority of the periphery
and serve for vision in poor light and spacial awareness. The light sensitive
rods and cones are found deep within the retina and not on the surface.