A very common lesion is Arcus senilis, which sometimes comes on in middle life rather than in old age. It consists in a fatty degeneration of the cells and fibres of the cornea along a defined line near the periphery of the cornea, but still separated from the edge by a line of unaffected tissue.

The term Conical cornea or Keratoconus is applied to a condition in which, on account of weakness in the central parts of the cornea, the pressure from within the bulb of the eye forces out this part and so renders the cornea conical. The central part being furthest removed from the source of nutrition most readily suffers when the general condition of the body is reduced, and so yields.