Mucus produced by goblet cells in the conjunctiva.
The mucin layer is what enables the tear film to adhere to the epithelium. Without a healthy mucin layer, the ocular surface will not be "wetted" properly even if plenty of tears are produced.
ABSTRACT: The tear film and ocular mucins. Davidson HJ, Kuonen VJ. Vet Ophthalmol. 2004 Mar-Apr;7(2):71-7.