Vision

The faculty of sight. When lightrays reach the eye, most of the focusing is done by the cornea, but the eye also has an automatic fine-focusing facility, accommodation, that operates by altering the curvature of the lens. Together, these systems form an image on the retina. The light-sensitive rod and cone cells in the retina convert the elements of this image into nerve impulses that pass into the visual cortex of the brain via the optic nerves. The rods, which are more concentrated at the periphery of the retina, are highly sensitive to light but not to colour. The colour-sensitive cones are concentrated more at the centre of the retina (see colour vision). The brain coordinates the motor nerve impulses to the 6 tiny muscles that move each eye to achieve alignment of the eyes. Accurate alignment allows the brain to fuse the images from each eye, but because each eye has a slightly different view of a given object, the brain obtains information that is interpreted as solidity or depth. This stereoscopic vision is important in judging distance.