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Optic disc rim area, pallor, and nerve fiber course.

To the Editor. —An important objective of most examinations of the optic nerve head is to estimate the mass of nerve tissue, which is assumed to reflect the number of retinal ganglion cell axons leaving the eye. Current clinical estimates in the living eye are based on measurements of the cup/disc ratio or the rim area and suffer from a strong dependence on the size of the optic nerve head. The association between the rim area and the disc size has been noticed in several recent articles. Britton et al 1 first confirmed our finding 2 that the rim width is uncorrelated with the disc diameter. This leads, on a pure geometric basis, to a strong correlation ( r =.75) between the rim and disc areas. Large discs, therefore, have large rim areas as well as large cups. Caprioli and Miller, 3 who obtained similar results, rejected the hypothesis that eyes with