A film of tears, spread over the eye by a blink, makes the surface of the eye smooth and clear. Without our tear film, good vision would not be possible.
The tear film consists of three layers:
The oily layer, produced by the meibomian glands, forms the outermost surface of the tear film. It's main purpose is to smooth the tear surface and reduce evaporation of tears.
The middle watery layer makes up most of what we ordinarily think of as tears. This layer, produced by the Lachrymal gland, cleanses the eye and washes away foreign particles or irritants.
The inner layer consists of mucus produced by the conjunctiva. Mucus allows the watery layer to spread evenly over the surface of the eye and help the eye remain wet. Without mucus, tears would not adhere to the eye.