The ocular surface: the challenge to enable and protect vision: the Friedenwald lecture.

The surface of the eye is an extraordinary and vital component of vision. The smooth, wet surface of the cornea is the major refractive surface of the visual system, which, along with corneal transparency, enables light to proceed through the lens and onto the retina for photoreceptor activation. The presence of the smooth, wet refractive ocular surface required for vision comes, however, at a cost. Unlike all other wetsurfaced epithelia of the body, the ocular surface is directly exposed to the outside world where it is especially subject to desiccation, injury, and pathogens. As a consequence, numerous protective mechanisms are provided by the Ocular Surface System, to ensure vision.