Even if you’re not crying, there is a thin, moist, protective film covering your eye: the tear film.
The tear film forms an optical quality interface with the air and scientific measurements suggest that human tear film is around 3 μm thick.
The tear film is composed of three layers from anterior to posterior: the lipid layer, the aqueous layer and the mucous layer.
(1) The lipid layer is the outer layer of the ocular tear film, and mainly comprises of oils and fatty acids secreted from the meibomian glands. Meibomian glands are at the rim of the eyelids inside the tarsal plate. There are about 25 of these vertical glands lined up side by side across the upper eyelid and fewer than this lined up across the lower eyelid.
(2) The aqueous layer is the middle layer and the main part of the tear film in thickness. It is water-based, including nutrients essential to the ocular surface. The aqueous layer is secreted by the lacrimal gland, and there is one lacrimal gland for each eye. The aqueous layer composes the majority of the tear volume, promotes spreading of the tear film, provides anti-infectious agents and controls osmotic regulation.
(3) The mucous layer is the inner layer of the tear film. The mucins are secreted from goblet cells, scattered in the conjunctival epithelium. The mucous layer includes mucins of different types, and some other chemicals. There is a gradual transition from the mucous layer to the aqueous layer, since some mucins are water-soluble.
The tear film is replenished and redistributed during a blink. A blink starts with the rapid closing of the eyelid, and then ends with the upstroke of the eyelid.
The improper condition of any of the three tear film layers can lead to ocular malfunction, and most commonly dry eye syndrome. Symptoms of dry eye syndrome vary but may include eye redness, itching, irritating or even burning feelings.