The human eye is the organ which gives us the sense of light, 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 of give off.
The eye changes light rays into electrical signals, then sends them to the brain, which interprets these electical signals as visual images. the eyeball 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 or 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.
The cornea is the transparent, dome shaped window covering the front of the eye. It is a powerful refracting surface, providing 2/3 of the eye's focusing power. It provides the window through which we look.
The colored part of the eye is called the iris. It controls light levels inside the eye similar to the aperture on a camera. The round opening in the center of the iris is called the pupil. The iris is embeded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size.
The pupil is the black, circular opening in the center of the iris. It opens and closes in order to regulate the amount of light entering the eyeball.
The purpose of the lens is to focus light onto the back of the eye. The nucleus, the innermost part of the lens is surrounded by softer material called the cortex. The lense is encased in a capsular-like bag and suspended within the eye by tiny guy wires called zonules.
The vitreous is a thick, transparent substance that fills the center of the eye. It is composed mainly of water and comprises about 2/3 of the eye's volume, giving it form and shape.
The conjunctiva is the thin, transparent tissue that covers the outer surface of the eye. It begins at the outer edge of the cornea, covers the visible part of the eye, and lines the inside of the eyelids. It is nourished by tiny blood vessels that are nearly invisible to the naked eye.
The sclera, commonly known as "the white of the eye", is the tough, opaque tissue that serves as the eye's protective outer coat.
The choroid lies between the retina and sclera. It is composed of layers of blood vessels that nourish the back of the eye.
The macula is located roughly in the center of the retina, temporal to the optic nerve. It is a small and highly sensitive part of the retina responsible for detailed central vision. The fovea is the very center of the macula. The macula allows us to appreciate detail and perform tasks that require central vision such as reading.
The retina is a very thin layer of tissue that lines the inner part of the eye. It is responsible for capturing light rays that enter the eye. These light impulses are then sent to the brain for processing, via the optic nerve.
The optic nerve transmits electrical impulses from the retina to the brain. It connects to the back of the eye near the macula. The visible portion of the optic nerve is called the optic disc.