The normal tear film.

PURPOSE: To survey briefly current knowledge on the normal preocular tear film.
METHODS: Sections deal with: (1) Formation of the film, its physical dimensions and rates of inflow and outflow of tears, and briefly the pathways of nervous control of tear production in the main lacrimal gland and accessory lacrimal tissue. (2) The protein and electrolyte composition of the aqueous part of the tears derived from the lacrimal gland, as well as the accessory secretions of the meibomian glands and conjunctiva; the 'soluble' or gel-forming mucins from the conjunctival goblet cells are described as well as the membrane-spanning epithelial mucins of the glycocalyx which take part in wettability and mucus binding. (3) The functions of the film, including acting as a nutritional route for the anterior epithelium of the cornea, and its protective roles in ocular lubrication and in scavenging and eliminating invading debris and microorganisms, plus specific antibacterial and immune functions. (4) Problems of structure and stability of the film are discussed, and (5) the wide variety of tests of tear function and quality, with discussion of which tests are suitable for the clinical environment, and which laboratory-based tests can be useful in assessing the individual patient.
RESULTS: The preocular tear film plays a vital role in nourishing, lubricating and protecting the ocular surface. Many tests can be applied in either the clinical or the laboratory setting, to determine whether the tears of the individual patient exert their physiological and antimicrobial functions at the normal level.
CONCLUSIONS: Knowledge of the normal functions of the film provides a basis for later consideration of clinical and surgical treatment of the dry eye.