The tear film can be thought of as being made up of 3 layers. It is actually more complex than this. The deepest layer is the mucous layer that is secreted by the goblet cells on the conjunctiva. This layer spreads over the corneal surface and acts as the glue that holds the thickest layer, the watery or aqueous layer in place. The aqueous layer is prevented from evaporating and being lost by the lipid layer which is secreted by the meibomian glands that lie on the lid margin in a row behind the eyelashes. When we blink the muscles of the eyelids squeeze the meibomian gland to release their secretions and as the eyes reopen the aqueous and lipid layers are rasped over the ocular surface.