The human eye is the organ which gives us the sense of sight, allowing us to learn more about the surrounding world than any of the other five senses. We use our eyes in almost everything we do, whether reading, working, watching television, writing a letter, driving a car, and countless other activities. Sight is the most precious of the five senses, and many people fear blindness more than any other disability.

The eye allows us to see and interpret the shapes, colors, and dimensions of objects in the world by processing the light they reflect or give off. The eye is able to see in dim light or bright light, but it cannot see objects when light is absent. The eye changes light rays into electrical signals then sends them to the brain, which interprets these electrical signals as visual images.

The eye is set in a protective cone-shaped cavity in the skull called the orbit or socket and measures approximately one inch in diameter. The orbit is surrounded by layers of soft, fatty tissue which protect the eye and enable it to turn easily. Six muscles regulate the motion of the eye. Among the more important parts of the human eye are the iris, cornea, lens, retina, conjunctiva, the macula, and the optic nerve.

Our ability to "see" starts when light reflects off an object at which we are looking and enters the eye. As it enters the eye, the light is unfocused. Light entering the eye is first bent, or refracted, by the cornea -- the clear window on the outer front surface of the eyeball. The cornea actually provides most of the eye's optical power or light-bending ability. After the light passes through the cornea, it is bent again, to a more finely adjusted focus, by the crystalline lens inside the eye. The lens focuses the light on the retina. This is achieved by tiny muscles in the eyeball that change the shape of the lens, bending or flattening it to focus the light rays. This adjustment in the lens, known as accommodation, is necessary for bringing near and far objects into focus. The process of bending light to produce a focused image on the retina is called refraction. Once the light is focused on the retina, it stimulates cells which send millions of electrochemical impulses along the optic nerve to the brain. The brain interprets the impulses, enabling us to "see" the object.

DEFINITIONS OF EYE RELATED TERMS

Cornea
The cornea is sometimes referred to as the 'window of the eye'. It provides most of the focusing power when light enters your eye. The cornea is composed of 5 layers of tissue. The outer layer (the epithelium), is the eye's protective layer. This layer is made up of highly regenerative cells that have the ability to grow back within 3 days, and therefore, allow for fast healing of superficial injuries. Most of the inner layers provide strength to the eye.  Back

Lens
The lens is the clear structure located behind the pupil. Its primary function is to provide fine-tuning for focusing and reading. The lens performs this function by altering its shape. At about the age of 40-50, the lens becomes less flexible and presbyopia sets in. At about the age of 60 or 70, the lens becomes cloudy and hard (cataract formation), preventing light from entering the eye.  Back

Pupil
The pupil is the 'black circle' that you see in people's eyes. The primary function of the pupil is to control the amount of light entering the eye. When you are in a bright environment, the pupil becomes smaller to allow less light through. When it is dark, the pupil expands to allow more light to reach the back of the eye.  Back

Iris
This is the colored part you see in people's eyes (i.e. blue/green/brown/hazel). The primary function of the iris is to control the size of the pupil. This is achieved through contraction or expansion of the muscles of the iris.  Back

Vitreous Body
This is the clear 'gel like' substance located inside the eye's cavity. Its purpose is to provide a spherical shape to the eye. The vitreous may develop small clumps known as 'floaters', which are more common in nearsighted people than in the rest of the population.  Back

Optic Nerve
The optic nerve carries images from the retina to the brain.  Back

Retina
The retina consists of fine nerve tissue which lines the inside wall of the eyes and acts like the film in a camera. Its function is to convert light into electrical signals that are later sent down the optic nerve to the brain.  Back

Sclera
This is the 'white part' that we see in people's eyes. The sclera's purpose is to provide structure, strength and protection to the eye.  Back