Tears are formed by tiny glands that surround the eye.  The tear film is comprised of three layers:  oil, water, and mucous.  The lower mucous layer serves as an anchor for the tear film and helps it adhere to the eye.  The middle layer is comprised of water.  The upper oil layer seals the tear film and prevents evaporation.    

The tear film serves several purposes:  it keeps the eye moist, creates a smooth surface for light to pass through the eye, nourishes the front of the eye, and provides protection from injury and infection.