Its function is to protect and clean the external surface of the eye. It sweeps transversely across the eye from the nasal passage area towards the ear. As it does, it releases lacrimal fluid tears on to the surface of the eye through a duct located between the nictitating membrane and the surface of the eye. In this manner, the lacrimal fluid lubricates and protects the surface of the eye by maintaining a constant thin layer of fluid between the two surfaces where friction would occur without wasting excess fluid. Additionally, the leading edge of the nictitating membrane as it sweeps across the cornea acts as a squeegee polishing the cornea, limiting the amount of lacrimal fluid left on the eye, and sweeps foreign objects off the eye before it.
Mammals possess eyes that rotate in the socket.
The entire eye needs to be lubricated, requiring the production of larger amounts of lacrimal fluid. The eyes of reptiles, amphibians, and birds are fixed in the skull and have very limited movement in the socket. Only those surfaces exposed to the elements require lubrication. Humans possess tear ducts on the edge of the eyelid. Their production of tears is distributed across the exposed surface of the eye.