There are two types of tear secretion: (1) Basal (basic) tear secretion, which occurs normally without any stimulation. It maintains the cornea and conjunctiva continuously moist, but is reduced in dry eyes (e.g. keratoconjunctivitis sicca) and in elderly individuals. (2) Reflex tear secretion, which is produced in response to a corneal or conjunctival irritant and also depends on psychological factors. Basal secretion and reflex secretion are produced by the lacrimal gland and the accessory glands of Krause and Wolfring. The amount of tears secreted amounts to 14-33g per 24 hours or 0.5-2.2μl/minute, being about 2μl/minute at 15 years of age and less than 1μl/minute at 65 years of age. Note: it was formally thought that basal secretion was produced solely by the accessory glands and reflex secretion by the lacrimal gland but it is now thought that they all contribute to both forms of tear secretion. See lacrimal reflex; basic secretion test; Schirmer's test.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann