The human eyelid is a thin soft tissue structure that covers and protects the anterior surface of the eyeball. It comprises several layers, including skin and subcutaneous tissue, orbicularis oculi muscle, tarsal plates, levator apparatus, and conjunctiva. The eyelid also contains meibomian glands, which secrete the lipid part of the tear film. The upper and lower eyelids use skin, muscles, glands, nerves, and other tissues to help keep the eyes mobile, moist, and lubricated, and reflexes allow them to close quickly to prevent injuries.