Lacrimation

Lacrimation, or tearing, is the physiological process by which tears are produced and maintained on the surface of the eye to protect, lubricate, nourish, and defend the ocular surface. Tears are produced primarily by the lacrimal gland and spread as a tear film across the cornea and conjunctiva. The tear film is conventionally described as three layers: a superficial lipid layer from Meibomian glands that reduces evaporation, an aqueous layer containing water, electrolytes, proteins, and antimicrobial agents, and a mucin-rich layer produced by goblet cells that helps the tear film adhere to the ocular surface.

Tear production is regulated mainly by the parasympathetic nervous system. Secretomotor fibers reach the lacrimal gland via the greater petrosal nerve and the nerve of the pterygoid canal to the pterygopalatine ganglion, with postganglionic fibers hitchhiking along the zygomatic and lacrimal nerves to stimulate secretion. Sympathetic input modulates blood flow and glandular tone but has a lesser direct effect on tear production. Tears drain away from the ocular surface through the puncta, canaliculi, lacrimal sac, and nasolacrimal duct into the nasal cavity; obstruction along this drainage pathway can cause persistent tearing (epiphora).

Lacrimation serves several functions: lubrication to reduce friction during blinking, delivery of nutrients and growth factors to the corneal surface, washing away debris and irritants, and providing antimicrobial protection through enzymes such as lysozyme and lactoferrin. Tears can be categorized as basal (continuous maintenance), reflex (in response to irritants), and emotional (triggered by psychological stimuli). Abnormal tearing can reflect dry eye, allergic or inflammatory conditions, ocular surface disease, or nasolacrimal apparatus disorders, and is typically assessed with tear film break-up time and related tests.