Eye Works: The Human Eye
The human eye is a unique and vital organ. To an individual, the eye provides more information about his or her surroundings than any other sensory organ. To a physician, the eye offers a noninvasive and immediate view into a patient’s vascular system, allowing for early diagnosis and monitoring of diseases such as hypertension and diabetes.
An adult eye has a diameter of approximately one inch (25mm). It sits in an area called the orbit, which is a cavity in the skull. Only one sixth of the eye's surface is exposed and visible on a person’s face. The eyelids, eyelashes, and eyebrows protect this exposed area from strong light and foreign objects.
The outermost layer of the eyeball, the sclera, is the white part of the eye. The sclera provides structure and strength. The visible portion of the sclera is covered by a thin, clear layer called the conjunctiva. In front of the sclera, and connected to it, is a dome-shaped layer called the cornea. The cornea is a clear layer of tissue that provides most of the focusing power for light entering the eye.
From the cornea, light passes through the pupil, the dark aperture in the center of the iris. The iris is the pigmented area that gives the eye its color. It acts as a diaphragm, and can expand or contract, making the pupil smaller or larger. This action controls how much light enters the eye. The pupil gets smaller when exposed to bright light and larger when in dim light.
The lens of the eye is located behind the iris. The lens changes its shape to help the cornea focus light onto the retina, which is a light-sensitive area of tissue that lines the inside wall of the eye. The central portion of the eye is filled with a gel-like substance called the vitreous, which gives the eye its round shape.
Light entering the eye is focused directly on the retina, which converts the images into electrical signals. Our most acute vision is in the most central part of the retina, known as the macula. Electrical signals processed by the retina are sent along the optic nerve to the brain. The brain then further processes these signals and interprets them as the shapes and colors of vision.