If you are age 50 or older, there's a good chance you're concerned about eventually developing one of the four common eye diseases that affect older people -- cataracts, glaucoma, age-related macular degeneration, and for those with diabetes -- diabetic retinopathy. Yet most of us rarely consider the complex process that enables us to see the world around us -- until our vision is threatened. In this Special Report, Johns Hopkins illustrates how the eye works.
The eye is a complex structure that sends nerve impulses to the brain when stimulated by light rays reflected from an object. The brain then processes these impulses to create the perception of vision. The eye is made up of numerous parts that work together to make vision possible. Here's how the eye works:
When light enters the eye, it first passes through the cornea, a curved, transparent disk that covers the iris and pupil. The iris is the colored part of the eye, composed of connective tissue and muscle. The pupil is the small black opening in the middle of the eye through which light passes.
When exposed to bright light, muscles in the iris contract to make the pupil smaller and to allow less light to enter the eye. In dim light, these muscles expand, pupil size increases, and more light enters the eye. The sclera -- the white of the eye -- is a protective layer that connects with the cornea and encases the rest of the eye.
Light passes through the pupil and the lens, a transparent, elastic structure that stretches and contracts to help focus light on the retina. (The cornea does about 75% of the work of focusing light on the retina; the lens does the rest.) The light then passes though the vitreous humor, a transparent, gel-like substance that fills the portion of the eye between the lens and retina.
Finally, light reaches the retina, a layer of nerves lining the rear of the eye. The retina senses the characteristics of the light and converts such elements as color, shape, and brightness into electrical impulses that are sent to the brain through a collection of nerve fibers called the optic nerve.
The macula, the important central part of the retina, is located close to the site where the optic nerve connects with the eye. The macula is densely packed with cells called cones, which are critical for seeing color and bright images. (Other cells, called rods, are dispersed in different areas of the retina and respond more strongly to dim light.) The macula is also responsible for detailed visual acuity (needed for activities like reading) as well as for viewing images in the center of the visual field, rather than the periphery.
