Vision is the most fundamental of human senses. The eye is a very complex and truly amazing organ. It is approximately one inch wide and deep, and 0.9 inches tall. Human eyes allow humans to appreciate all the beauty of the world they live in, to read and gain knowledge, and to communicate their thoughts and desires to each other through visual expression and visual arts. The human eye is wrapped in three layers of tissue: the external layer, formed by the sclera and cornea; the intermediate layer, divided into two parts: anterior (iris and ciliary body) and posterior (choroid); and the internal layer, or the sensory part of the eye, the retina.