Number of Fibres in the Optic Nerve and the Number of Ganglion Cells in the Retina of Anurans

Abstract

USING a light microscope, Bruesh and Arey¹ found that there were about 15,300 myelinated and 13,700 unmyelinated fibres in the optic nerve of the frog (Rana pipiens), and about 10,200 myelinated and 5,500 unmyelinated fibres in the optic nerve of the toad (Bufo americanus). However, in my electron microscope study of the optic nerve of these and other Anurans (Rana catesbeiana, Bufo terrestris and Hyla cinerea) I have found² that the unmedullated axons are in a considerably larger number, having been underestimated by a factor of 30 or more, and that most of them could not have been resolved with the light microscope. My observations, though, do not greatly contradict the earlier numbers of myelinated fibres, for these can be counted with reasonable accuracy after myelin stain. In the present communication I only wish to report about the general arrangement of the unmyelinated axons and their number. The full description of the fine anatomy of the optic nerve is deferred to an extensive article now in preparation.