The eyes are complex sense organs that are basically an extension of the brain.

They evolved from primitive light-sensing spots on the surface of invertebrates and in some species have developed many remarkable variations, providing special advantages in various ecological niches. Each eye has a layer of receptors, a lens system for focusing an image on these receptors, and a system of axons for transmitting action potentials to the brain. This chapter describes how these and other components of the eye work.