As a sense organ, the purpose of the human eyeball is to allow the brain to perceive visual images and sensory information. The human eyeball is known as a "camera-type eye" and functions in much the same manner as a camera lens, gathering and transmitting focused light onto a firm surface to produce a visual image.
The human eyeball is comprised of several structures. The cornea is located at the front of the eye and is a transparent structure that serves to focus incoming light. Behind the cornea is a membrane known as the iris, which contains the pupil, a circular opening that can change shape in order to control the amount of light entering the eye.
The retina is found along the rear wall of the eyeball and contains millions of cells knows as rods and cones. These cells are able to translate light into electrical signals that the brain is then able to process into a visual image.
Each eye is controlled by six different muscles. As these muscles contract, they apply force to the eyeball, which allows it to turn and rotate to alter the field of vision or track moving objects. Having two eyes also allows the human brain to perceive relative depth and distance.