How the Eye Works

organ of sight is far more complex and efficient. Not only does the eye focus and snap pictures, but it also works continuously with the brain and nervous system to process ever-changing images and provide you with the visual information you need for doing everything from hitting a golf ball to preparing your taxes.

Eyeball Engineering

The conjunctiva is a thin, colorless membrane that lines the inner surfaces of the eyelids and the front portion of the sclera, the eye's white outer layer. The conjunctiva is so sensitive that when it becomes aware of a foreign body, it automatically triggers a protective reaction, such as tearing or blinking.

Six extraocular muscles regulate each eye's up-and-down, side-to-side, diagonal, and rotational motions. The muscles come in pairs and run from the back of the orbit to the sides of the eyeball, beneath the conjunctival membrane (see image "The Inside Story").

The middle layer, called the uveal tract, comprises the iris, ciliary body, and choroid. The iris — the pigmented segment, which might be blue, green, brown, or another color — forms a ring around the pupil, a black hole in the iris's center. Basically a circular curtain of muscle fibers, the iris controls how much light enters the eye. As with an automatic camera, which adjusts the size of its aperture (opening) to the available light, the involuntary muscles of the iris open to allow more light to enter the pupil in dim light, and close to make the pupil smaller in bright light. A good example of the eye's adaptation is the mildly painful change that occurs when you walk into sunlight after sitting in a dark movie theater. Even subtle alterations in light prompt a response from the eye, and the iris muscles are continually adjusting to the environment.

From:everydayhealth