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We depend on our vision more than any other sense. The eye allows us to see the shapes, colors, and dimensions of objects in the world by processing the light these objects either reflect or give off. The eye is able to see in light that is dim or bright, but it cannot see an object when light is absent. The eye changes light rays into electrical signals and these signals are sent to the brain, which interprets them as visual images.
The eye is set in a protective cone-shaped cavity in the skull called the orbit (or socket) that 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 cornea, the pupil, the iris, the sclera, the lens, the retina, and the optic nerve.
The cornea is the "window of the eye." It provides most of the focusing power when light enters your eye. The cornea is composed of five layers of tissue. The outer layer, called the epithelium, is the eye's protective layer. This layer is made up of cells that have the ability to grow back within 3 days, which allows for fast healing of superficial injuries. Most of the inner layers provide strength to the eye.
The pupil is the "black circle" in the eye. The pupil controls the amount of light entering the eye. When it is dark, the pupil expands to allow more light to reach the back of the eye. When you are in a bright environment, the pupil becomes smaller to allow less light through.
This is the colored part of the eye, surrounding the pupil. The muscles of the iris control the size of the pupil by contracting or expanding.
This is the "white part" of the eye. The sclera's purpose is to provide structure, strength and protection to the eye.
The lens is the clear structure located behind the pupil. It alters its shape to become thinner or thicker, which provides fine-tuning for focusing and reading. Between the ages of 40 and 60, the lens becomes less flexible and presbyopia (difficulty in reading small print) occurs. Between the ages of 60 and 80, the lens sometimes becomes cloudy and hard, preventing light from entering the eye. This is called the formation of a cataract.
The retina consists of fine nerve tissue, which lines the inside wall of the eye and acts like the film in a camera. It transmits images to the brain. When your vision is good, the light rays coming into your eye focus precisely on the retina.
The optic nerve is located in the rear of the eye, and carries images from the retina to the brain. Damage to the optic nerve can occur from glaucoma, which affects your peripheral vision.