Vision

Abstract

Sensitivity to light is a widespread functional feature in the animal world. It is present in one- celled animals and in species which do not have a specialised optical apparatus for refraction or any system for image forming. Many species of worms, for example, have light-sensitive cells scattered over the body surface. The next stage in evolutionary development is for light-sensitive cells to be clustered in pits on the body surface. In this way the cells are protected and, at the same time, the grouping contributes to an increased functional precision. As a further step for the protection of these specialised cells, a transparent covering membrane is developed and thus the primitive eye is transformed into a closed cavity. This covering membrane, which makes up the first primitive cornea, also bends the rays of light so that an image is formed. Later a lens develops from the membrane and gives the eye the capacity to vary the bending of the light rays so that it can be adjusted for sharp vision of objects at different distances.