Consider, for a moment, the simple things your eyesight makes possible. You can drive home from work, open your freezer, and choose the Hungry Man meatball lasagna instead of the Healthy Choice lemon pepper fish. You can read the directions on the package and press the right buttons on the microwave. You can decide whether the lettuce is too brown to use in a salad. And as your son plays out back, you can keep an eye on him through the kitchen window.
Like our cars, computers, and cell phones, we rely on our eyesight to accomplish everyday tasks, but have only a vague idea of how it works. Vision is a complex sense—a process, really - that requires precise choreography between the brain and the structures of the eye as you interact with the people and objects around you.
To understand vision, first we need to know a thing or two about the anatomy of the eye. Think of the human eye, with its remarkable anatomical design, as a miniature theater in which the process of vision takes place.
The eyeball, or globe, is spherical (round) and measures about 2.5 cm (1 inch) across. It sits in a cone-shaped pocket within the skull called the orbital cavity, or “eye socket.” This bony orbit is cushioned by fatty tissue and protects the eyeball from trauma, such as injuries that may occur during car accidents, falls, and assaults. The eyes are also shielded by the eyelids, which help keep the eyes moist. Along with the eyelashes and eyebrows, they also block or filter out foreign bodies, such as pollen and dust particles.
Layers of the Eye
Each layer and internal structure of the human eye performs a distinct function. The eye has three primary layers: the sclera, the choroid, and the retina.
Outer layer: The sclera and cornea
The outer layer of the eye is made up largely of an envelope of tissue called the sclera—the part most of us call “the white of the eye.” The sclera extends from the cornea, at the front of the eye, to the optic nerve, at the back of the eye, providing a sheath of protection for the eye’s internal structures. The sclera and cornea are known as the fibrous tunic because together they form a snug-fitting jacket that cloaks the eyeball, concealing its amazing contents from would-be intruders, such as bacteria, viruses, and other pathogens.
The transparent, dome-like window at the front of the eye is called the cornea. Similar in appearance to a contact lens, this membrane covers the iris—the pigmented, or colored, part of the eye—and the pupil, a round opening in the center of the iris. The pupil appears to be black because most of the light that passes through it is absorbed by pigmented cells in the tissue at the back of the eye.
Although the cornea may seem no more substantial than a sheet of plastic wrap, it’s actually a highly specialized, finely constructed network of cells and proteins. In addition to serving as a barrier against harmful microbes (germs) and dirt, the cornea has a second important function. It refracts, or bends, light waves to project them onto the lens of the eye, as we’ll explain in a moment. Because the cornea must remain perfectly clear in order to refract light properly, it’s the only tissue in the body that contains no blood vessels.
Middle layer: The choroid
Beneath the sclera lies a rich network of blood vessels called the choroid. It nourishes and supplies oxygen to the retina and other internal structures of the eye. The choroid layer measures approximately 1/2 mm (1/20 of an inch) thick, about the same as a stack of five sheets of paper. The choroid contains light-absorbing pigment cells that minimize reflections in the retina. These cells are responsible for the red-eye effect you sometimes see in snapshots taken with a flashbulb.
Inner layer: The retina
The retina lies below the choroid and makes up the innermost layer, or lining, of the eye. It contains light-sensitive rod cells and cone cells, known as photoreceptor cells because they capture electrical impulses and transmit images along the optic nerve to the brain. Each eye contains an astonishing number of these cone cells—about 6 million of them. They’re concentrated in the macula, the central part of the retina, especially in the fovea, a tiny pit at the center of the macula. This area is responsible for producing color and sharp-detail vision.
The rod cells populate the fringes of the retina. They’re even more numerous than the cone cells, numbering perhaps 100 million. Rod cells are active in darkness and in dim light, although your eyesight in these conditions is monochromatic (that is, not in color) and has poor resolution. However, you can still distinguish shape, size, and movement with some clarity—it’s the rod cells that keep you from tripping over your cocker spaniel when you get up in the middle of the night.
Forming an Image
To illustrate the process of vision, let’s follow an image as it travels through the eye.
Journey of light through the eye
As you look at something—this web page, for instance—light rays pass through the eye to the retina. Several structures help to refract the light so that it focuses properly. The first of these structures, at the front of the eye, is the cornea. The next is a clear fluid called the aqueous humor, which circulates in the chamber behind the cornea and helps to nourish it and maintain its curvature.
As the light continues on its pathway, it passes through the pupil. The iris is a muscle that controls the size of the pupil. It’s made up of specialized fibers that are able to constrict the pupil to about 1 mm or dilate it to about 10 mm (1 mm is about the thickness of a U.S. dime). This narrowing and widening of the pupil, known as the light reflex, regulates the amount of light that enters the eye.
The light rays then reach the crystalline lens, a transparent, elastic disc that’s about the same size (1 cm in diameter) and shape (round and slightly flattened) as a piece of M&Ms candy. The lens is suspended by muscles that can pull taut or slacken like a drawstring pouch, changing the shape of the lens. This adjustment process, known as accommodation, sharpens visual clarity by helping to project light properly onto the retina.
After passing through the lens, light moves through a large chamber filled with clear, viscous matter called the vitreous humor. This gelatin-like substance makes up 80% of the mass of the eyeball and allows it to maintain its spherical shape. The retina marks the finale of light’s movement through the eye. There, visual images are registered by the rod and cone cells, which convert light into electrical impulses and transmit them to the brain via the optic nerve.
Other structures necessary for vision
The process of vision is not complete until the brain interprets the chorus of electrical impulses transmitted by the retina. These signals are conducted through the optic nerve, an elegant bundle of more than a million long, slender retinal fibers. Together, these specialized cells, called ganglion cells, carry visual images from the retina to the brain.
Some ganglion cells are particularly sensitive to movement and contrast, whereas others are more responsive to shape and detail. Still others relay information about color. Depth perception—the ability to tell how far away objects are from you and from one another—occurs as the brain compares parallel signals transmitted by each eye in perfect synchrony.
In a specialized area called the visual cortex, the brain reconstitutes this fanfare of electrical impulses into fully formed images. It’s a process more exquisite and mysterious than the most brilliant symphony ever written. What’s more, this flawless performance is given by our eyes continuously every day, without a single rehearsal or false note. Now, that deserves a standing ovation.