Horn sheath

Cornea (kor'-ne-ah). (L. corneus = horn-like.)The anterior one-sixth of the first tunic of theeyeball. It is transparent, convex, and fittedinto the sclerotic like a watch crystal, having aradius of 7.8 mm. on its anterior surface and 6.5mm. on the posterior surface. It has a verticaldiameter of about 11 mm. and the horizontaldiameter is 12 mm. In thickness it is about 1mm. at the center, while at the periphery it is1.12 mm.

Source: Ophthalmic Dictionary including pronunciation, derivation and definition of the words used in Optometry and Ophthalmology by James J. Lewis, Oph. D. Professor of Optometry in the Northern Illinois College of Ophthalmology and Otology, Chicago.