Eye

the organ of vision. In its simplest and essential form an eye is the end of a nerve sensitive to light. In many lower forms of animals it is merely a sensitive spot from which nerves run to the nervous center. Many insects have what is known as a compound eye. The bundle of optic nerves, on reaching the surface, spreads out like a treetop, each fiber terminating in a simple eye or facet. The eye of the ant is composed of about fifty of these fiber ends. There are 4,000 in the eye of the house fly. The prominent, handsome eye of the dragon fly or devil's darning-needle is made up of 20,000 facets. The structure of the human eye may be compared to a two-lens microscope. At the front, there is a body shaped like a watch glass, called the cornea, and a double-convex body, called the crystalline lens. These two are transparent and form an image. The wall of the rear or retina is composed of the fibers of the optic nerve. The image formed by the lenses falls on the retina. The impression is conveyed to the brain. The eyeball is merely a chamber designed to hold the lenses at the necessary distance from the retina. The various muscles and liquids, the iris, the eyelid, the eyelash, and eyebrow are mere accessories that adjust and protect the essential parts. The retina of the white man's eye is scarlet or vermilion; that of the Nubian negro is of a chocolate brown. Man sees more accurately than other animals; at least he is a better judge of shape and distance. See Blindness