nictitation

Nictitation is the act of blinking or winking; in zoological and medical literature it denotes the rapid closing of the eyelids. The term derives from the Latin nictitare, meaning to wink or blink. In humans and many animals, blinking serves to protect the cornea, spread the tear film, and help moisten the eye.

There are several forms: spontaneous blinking, reflex blinking, and voluntary blinking. Reflex blinking is triggered by corneal irritation or bright light; voluntary blinking is deliberate; spontaneous blinking occurs without obvious stimulus and maintains tear film.

Physiology: Blinking is achieved by the orbicularis oculi muscle (facial nerve VII) closing the eyelids, aided by the levator palpebrae superioris when opening. The blink reflex involves a sensory input via the trigeminal nerve (ophthalmic branch V1) and a motor output via facial nerve to the orbicularis oculi. Normal frequency in humans is roughly 10–15 blinks per minute, decreasing with concentration or darkness and increasing with fatigue or ocular dryness.

In animals, nictitation often refers to the action of the nictitating membrane, a translucent third eyelid that can sweep horizontally across the eye to protect or moisten the cornea; this is distinct from the standard eyelid blink but can accompany it in some species.

Clinical relevance: Nictitation is usually normal; excessive blinking (blepharospasm) or a decreased blink rate can indicate ocular dryness, irritation, or neurologic issues. In contemporary clinical usage, "blink" is the preferred term for the general act, with "nictitate" as a verb; "nictitation" is more common in historical or anatomical contexts.