Light In Focus - How The Eye Works: The Aging Eye Preventing And Treating Eye Disease

Content provided by the Faculty of the Harvard Medical School

Light in focus

Just behind the pupil and iris lies the crystalline lens, which is connected at its outer rim to the ciliary body by ligaments called zonules. The lens focuses light rays on the retina, the thin, light-sensitive inner layer at the rear of the eye. This occurs with the help of muscles in the ciliary body, which enable the flexible lens to alter its shape and allow the eye to focus on objects at varying distances. When you look out the window at a tree far away, for instance, the muscles relax and stretch the zonule ligaments, which in turn pull on the lens, causing it to flatten and assume a thin contour. But shift your gaze to something close, such as a computer screen, and the muscles contract and loosen the zonules, which makes the lens thicker and curved more in the middle. The ability of the lens to focus from far to near is called accommodation.

In addition to its focusing function, the ciliary body contains cells on its surface that produce aqueous humor, a watery fluid that provides nutrients to the lens. The aqueous humor is found principally in the space between the iris and the cornea, known as the anterior chamber. The fluid flows to this region from the posterior chamber — the area between the iris and the lens — and then carries waste products from the eye through Schlemm's canal, a circular drainage system located where the clear cornea, white sclera, and colored iris meet. In a healthy eye, this circulation constantly drains and resupplies the aqueous humor, maintaining a balance of fluid in the two chambers.

Behind the lens is the vitreous humor. This clear, stable gel, which looks like raw egg white, supports and fills the rear two-thirds of the eyeball. This gel provides an open pathway for light coming through the lens.

The last part of the middle layer is the choroid, a membrane sandwiched between the sclera and retina in the rear of the eye. The choroid is packed with blood vessels that carry oxygen and other nourishment to the adjacent outer portion of the retina.

Taking up nearly three-quarters of the rear inside surface of the eye wall is the retina, the innermost layer. Somewhat comparable to the film in a camera, the retina is where images are captured and recorded. This mass of unique nerve cells and fibers sends the brain visual messages about the size, shape, color, and distance of the objects you see. The images travel along the optic nerve, which carries the signals to the brain.

Within the retina are about 150 million rods and 7 million cones — specialized cells made up of chemicals that react to different wavelengths in light. Located mainly in the periphery of the retina, the rods do not perceive color. The cones, which do perceive color, are responsible for fine detail in the center of vision. They enable us to read words on a page and recognize a familiar face from across the room. Cones are most active in bright light, while rods are most sensitive in the dark; this is why it is hard to detect colors and fine details in the dark. The cones are located primarily in the macula, a remarkably small part of the retina that gives us sharp central vision. The best vision — for reading or detailed work — comes from the fovea, at the center of the macula. The rest of the retina delivers peripheral vision (side vision), which is less sharply focused.

Last updated:	June 19, 2007