Histological evidence against the view that the cat's optic nerve contains centrifugal fibres.

1. Degeneration that can be shown by the Nauta-Gygax technique in the orbital part of the cat's optic nerve does not begin until 10 days after intracranial transection of the nerve, though after enucleation of the eye it is conspicuous in 4 days.2. We were not able to tell, by any silver-staining technique applied to an optic nerve at any interval after an operation, whether at that operation the nerve had been cut peripherally only or both peripherally and centrally.3. From these and subsidiary observations we conclude that either the cat's optic nerve contains no centrifugal fibres detectable by silver staining and light microscopy, or, if there are such fibres, they are much less susceptible to prograde (Wallerian) degeneration and much more susceptible to retrograde degeneration than most of the centripetal fibres. The former is the simpler and, we suggest, the more likely conclusion.