The structures and functions of the eyes are complex. Each eye constantly adjusts the amount of light it lets in, focuses on objects near and far, and produces continuous images that are instantly transmitted to the brain.
A person may experience swelling in one or both eyelids. Swelling may be painless or accompanied by itching or pain. Eyelid swelling is distinct from bulging eyes, although a few disorders can cause both.
A person who has eye symptoms should be checked by a doctor. However, some eye disorders cause few or no symptoms in their early stages, so the eyes should be checked regularly (every 1 to 2 years or more frequently if there is an eye condition) by an ophthalmologist or an optometrist. An ophthalmologist is a medical doctor who specializes in the evaluation and treatment (surgical and nonsurgical) of eye disorders. An optometrist is a health care practitioner who specializes in the diagnosis and treatment of vision or refractive problems (refractive errors).
The conjunctiva is the membrane that lines the eyelid and loops back to cover the sclera (the tough white fiber layer covering the eye), right up to the edge of the cornea (the clear layer in front of the iris and pupil—see Structure and Function of the Eyes). The conjunctiva helps protect the eye by keeping small foreign objects and infection-causing microorganisms out and by contributing to the maintenance of the tear film.
The cornea is the clear layer in front of the iris and pupil. It protects the iris and lens and helps focus light on the retina. It is composed of cells, protein, and fluid. The cornea looks fragile but is almost as stiff as a fingernail. However, it is very sensitive to touch.
The eyelids play a key role in protecting the eyes. They sweep away debris when the eyes close and help spread moisture (tears) over the surface of the eyes when they open. The eyelids provide a mechanical barrier against injury by closing rapidly when needed.
The small photoreceptor cells of the retina (the inner surface at the back of the eye) sense light and transmit impulses to the optic nerve. The optic nerve from each eye carries impulses to the brain, where visual information is interpreted.
The retina is the transparent, light-sensitive structure at the back of the eye. The cornea and lens focus light onto the retina. The central area of the retina, called the macula, contains a high density of color-sensitive photoreceptor (light-sensing) cells. These cells, called cones, produce the sharpest visual images and are responsible for central and color vision. The peripheral area of the retina, which surrounds the macula, contains photoreceptor cells called rods, which respond to lower light levels but are not color sensitive. The rods are responsible for peripheral vision and night vision.