Blinking is a
semi-autonomic rapid closing of the
eyelid. A single blink is determined by the forceful closing of the eyelid or inactivation of the levator palpebrea superioris and the activation of the orbicularis oculi, not the full open and close. It is an essential function of the
eye that helps spread
tears across and remove irritants from the surface of the
cornea and
conjunctiva. Blink speed can be affected by elements such as fatigue, eye injury, medication, and disease. The blinking rate is determined by the "blinking center", but it can also be affected by external stimulus. When an animal (usually
human) chooses to blink only one eye as a signal to another in a social setting (a form of
body language), it is known as
winking. Some animals (for example,
tortoises and
hamsters) blink their eyes independently of each other.