As we've seen, one of the functions of the nervous system is to carry impulses from sense organs to the central nervous system. In the remainder of this module, we'll look at two of the most important of these sense organs: the eyes and the ears.

The eyes, our organs of sight, are two fluid-filled spheres about 25 mm in diameter. They can function individually, but also operate as a pair to give three-dimensional vision and enable us to judge distance.

Each eye is situated in and protected by its orbital cavity in the skull. The exterior of the eye consists of three layers:

• the sclera – a thick outer coat
• the choroid – a vascular and pigmented layer
• the retina – an inner light-sensitive layer.