Tarsal conjunctiva of the upper eyelid.

The precorneal tear film with its lacrimal, mucinous, and meibomian secretions is es­ sential for maintaining the structural in­ tegrity of the cornea in general and for en­ suring optical regularity of its anterior surface in particular. Replenishment and mixing of its components is brought about by normal blinking when the upper eyelid tarsal conjunctiva, preceded by the marginal tear strip, sweeps momentarily across the corneal surface. About 25% of normal tear production is presumed lost by evaporation when the eye is open, and lacrimal drainage channels are used only when flow is excessive. The os­ motic pressure of tears remains constant in spite of this evaporative loss, so that mecha­ nisms for maintaining constant tonicity of the precorneal tear film must exist. Altera­ tions in the rate of tear secretion help coun­ teract large changes in osmolarity, but there is no evidence that such changes occur nor­ mally. Indeed, in keratoconjunctivitis sicca patients, there is no correlation between tear flow and tonicity, the latter remaining re­ markably constant in spite of gross de­ ficiency in tear production.Withdrawal of water from the cornea counteracts to some extent the tendency to hypertonicity caused by evaporation causing the cornea to become relatively deturgesced when the eye is open. The corneal stroma, however, offers a high resistance to water flow, both vertically and tangentially, so that a more rapidly re­ sponsive means of adjustment may be op­ erative. In view of the intimate relationship of the upper eyelid tarsal conjunctiva to the corneal epithelium during blinking and in