Eyelid

The eyelids are vital to the eye's structural defences. Their natural pattern of blinking every few seconds creates a strong barrier between the eye and the external environment. Their reflexive closure in response to threat to the eye also acts as a barrier against injury to the eye. The act of blinking also lubricates the eye by spreading tears formulated in glands in the eyelids across the surface of the eye. The inner surface of the eyelid is lined with a thin mucous membrane called the conjunctiva; this allows the eyelid to pass over the eye without friction. The eyelid also has a row of numerous eyelashes that protect the eye from airborne objects. Eyelids of the upper lid are long and turn upwards whereas those of the lower lid are short and turn downwards