The pigeon retina: Quantitative aspects of the optic nerve and ganglion cell layer

Studies of electron micrographs of pigeon optic nerve showed numerous unmyelinated fibers (29% of total) and more than twice as many total fibers as previously reported. The 2.4 million axons had a mean diameter below 1 μ, close to the limit of resolution of light microscopy. This probably accounts for the large discrepancy between light and electron microscopic counts. Studies of retinal whole mounts showed an irregular distribution of cells in the ganglion cell layer including a minor posterior fovea and an area of increased density in the posterior‐superior quadrant. Sample counts produced an estimate of over 4.8 million cell bodies in this layer. Cytological investigation indicated that over 85% of these cells were neuronal. The considerable excess of neuron cell bodies over axons in thought to be explained by the presence of large numbers of amacrine cells, cells which do not project axons into the optic nerve.