The eye is a spherical structure about an inch in diameter. It has a clear bulge on the front side, which is the cornea. The wall of the eye beyond the cornea consists of three tissue layers. The outermost layer is the sclera, a tough, protective coating that covers most of the outer surface of the eye that connects to the transparent cornea at the front of the eye. The middle layer is the choroid, a vascular layer that is continuous with the ciliary body and the iris on the front side of the eye. The inner layer is the retina, a light-sensitive tissue that lines the inside back wall of the eye. To understand how the eye works, it is useful to think of a camera. The human eye is more complex than a television camera, but the principles are similar.