The Healthy Tear Film
The tear film of the human eye is extremely important to the vision process. It lubricates the eye surface and keeps it smooth. The tear film consists of two parts. The first part is the aqueous (mostly water) component, produced by the lacrimal gland (below the eyebrow). The aqueous component of the tears flow along the lower edge of the eyelid into the tears and then into the nose.
If water were the only component of the tear film, the film would immediately evaporate as soon as the eye is opened – leaving the surface of the eye unprotected. However, there is more to the tear film. Below the aqueous portion of the tear film lies a mucus layer and above the water lied a thin oily film produced by the meibomian glands of the eyelids, which reduces the rate at which the aqueous layer evaporates. It’s only this oily layer that makes it possible to keep the eyes open for a long time without the tear film breaking open.