Eyeball Engineering - How The Eye Works: Eye Care

Content provided by the Faculty of the Harvard Medical School

Eyeball engineering

Despite its reputation as a delicate organ, the eye is remarkably resilient and hardy, engineered by nature to last from infancy through old age. Shaped like a sphere, the eyeball is about an inch in diameter, with a slight protrusion in front. It sits in a bony, protective socket of the skull, called the orbit, and is surrounded by a cushiony layer of fat, fibrous tissue, and muscles.

Eyelids and eyelashes act like windshield wipers, constantly brushing and blinking away dust and other debris that might otherwise blow into the eye. The lacrimal gland, located behind the upper lid (see Figure 1), produces tears that course over the eye surface and keep it lubricated, nourished, and clear of foreign matter. Tears drain off into the nose through ducts at the eye's innermost corner.

Figure 1: Seeing the light Light enters the eye through the pupil as the iris expands or contracts according to the brightness. Behind the upper lid is the lacrimal gland, which releases tears that lubricate the eye and drain through the lacrimal sac and duct.

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 Figure 2).

Figure 2: Eye anatomy The eyeball is surrounded by ligaments, fat, and muscles and rests in a protective, bony socket called the orbit. Six extraocular muscles control the eyeball's movement. The cornea, a tough, transparent dome that helps focus light, and the sclera, the white portion of the eye, protect the interior of the eye. The optic nerve delivers visual information to the brain.

Three distinct layers of tissue surround the eye and form its wall. The surface layer (approximately one millimeter thick) is made of tough collagen. You see it in the visible part of the eyeball as both the sclera (the white part) and the cornea, a clear, domelike window at the front of the eye.

The middle layer, called the uveal tract, comprises the iris, ciliary body, and choroid (see Figure 3). 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, making 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 iris muscles are continually adjusting to the environment.

Figure 3: The inside story Rays of light pass through the cornea, the anterior chamber, and then through the lens, which focuses images. The lens is nourished by the aqueous humor, a clear, watery solution that circulates from the posterior chamber into the anterior chamber and helps maintain normal pressure. Light reaches the retina after it passes from the lens through the vitreous humor, a clear gel that fills most of the eyeball. The retina has light-sensitive cells that capture images, which are then sent to the brain via the optic nerve. At the retina's center is the macula, a small region that provides sharp, central vision.

Last updated:	June 19, 2007