Introduction to the structure of the eye

The human eyeball is roughly spherical, approximately 2.5 centimeters in diameter, loosely embedded in the fatty tissue of a cone-shaped cavity in the skull. Each eyeball has six external muscles that work in pairs to determine the eye's position. A pair of lateral muscles controls lateral movement; superior and inferior muscles control upward and downward movement; and two diagonal muscles primarily produce rotational movement about the visual axis. Adjusting the action of these two external eye muscles helps maintain visual focus on the point of fixation at any distance.

The outer layer of the eyeball is a protective, fibrous material about one millimeter thick called the sclera. At the front of the eyeball, which accounts for approximately one-sixth of the total area, is a thin, uniformly arranged, transparent fibrous membrane called the cornea. Its radius of curvature is slightly smaller than that of the rest of the outer layer.

The second membrane within the sclera, the choroid, is composed of arteries and capillaries. It supplies nutrients to the eyeball in general and to the nerves in particular. Near the cornea, the choroid is replaced by the ciliary body, the outer surface of which grows the muscles of the eye.

Inside the choroid lies a third layer, the retina. The retina contains nerve cells that respond to light. The part of the retina that lies along the visual axis of the eye, called the fovea, is particularly important because it is here that images of the center of gaze are formed.