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The precorneal tear film in horses: its importance and disorders

      The cornea is the gateway of the images into the eye. It is the most powerful refractive ocular structure and must remain transparent. The optical properties of the cornea are clarity, surface smoothness, and refractive index [
      • Nishida T
      Cornea.
      ,
      • Samuelson D
      Ophthalmic anatomy.
      ]. The health of the cornea is influenced by the aqueous humor, the intraocular pressure, the eyelids, and the preocular tear film. There are three classifications of tear production: basic, reflex, and psychic. Continuous or “basal” tears are produced at a constant level and permit normal functioning of the preocular tear film. Additional tear production is stimulated by the reflex response to any irritation of the cornea, conjunctiva, or nasal mucosa. Human beings are the only species for which psychic tear stimulation has been proven. The precorneal tear film (PTF) is not truly part of the cornea but is anatomically and functionally intimately associated with the cornea. This article focuses on the functional and clinical aspects of the equine PTF. The origin of the tear film components, the structure and functions of the PTF, and the diagnostic procedures aimed at detecting or confirming tear film disorders are reviewed. The main purpose of this review is to stress the clinical importance of the PTF in the maintenance of corneal health and function.
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