combined action of the epithelium and endothelium maintains a constant water content; the epithelium seals the stroma off from the outside, while the endothelium acts as an ion pump to remove water from the stroma. This requires a sufficiently high density of endothelial cells. Endothelial cell density is age-dependent; normally it is approximately 2500 cells per mm2. At cell densities below 300 endothelial cells per mm2, the endothelium is no longer able to pump water out of the cornea, resulting in edema of the corneal stroma and endothelium.
Protection and nerve supply: The cornea is a vital structure of the eye and as a result extremely sensitive. It receives its ample sensory supply from the ophthalmic division of the trigeminal nerve. The slightest tactile sensation causes an eye closing reflex. Any injury to the cornea (erosion, foreign body penetration, or ultraviolet keratoconjunctivitis) exposes sensory nerve endings and causes intense pain with reflexive tearing and involuntary eye closing.
H The triad of involuntary eye closing (blepharospasm), reflexive tearing (epiphora), and pain always suggests a possible corneal injury (see Chapter 18).
Non-ophthalmologists can evaluate the transparency of the cornea (opacities of the stroma and epithelium suggest scarring or infiltration of the epithelium), its surface luster (lack of luster suggests an epithelial defect), and possible superficial corneal injuries (see Fig. 19.1 ). A simple ruler may be used to measure the size of the cornea (see Anatomy), and sensitivity may be tested with a cotton swab (see Fig. 1.11, p. 11).
The ophthalmologist uses instruments to evaluate corneal morphology and function in greater detail.
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