The cat optic nerve: fibre total count and diameter spectrum.

An electron microscopic examination of two cat optic nerves indicates a mean total count of 193,000 fibres ranging from 0.5 mu to 13.5 mu in diameter. This count, although nearly double any previously reported, supports recent minimum estimates of the retinal ganglion cell population of the cat eye. A radial gradient of packing density exists across the nerve close to the globe; a high density "core" with a unimodal fibre diameter spectrum may be identified as the area centralis outflow and a peripheral low density region with a trimodal diameter spectrum contains the projection of the peripheral retina. Division of the peripheral fibre spectrum suggests the percentages of alpha, beta and gamma ganglion cells in the peripheral retina to be 5%, 42% and 53% respectively.