The existence of fibers in the optic nerve which convey efferent impulses from the higher centers to the retina has been a matter of interest and discussion for many years. Histological evidence for these fibers and for the retinal cellular components of such an efferent system has been provided by Dogiel,1 Cajal,2 Johnston,3 and Polyak.4 Physiological evidence has been noted by Arey,5 who discussed the influence of such elements upon retinal light and dark adaptation. The importance of such a system in dark adaptation has been also mentioned by Pfeifer.6 Monakow7 described, as a result of animal experiments, the existence of a centrifugal pathway.
Polyak,8 more recently, in speaking of "the mechanisms of facilitation or inhibition," states that "a structural arrangement whose function seems to be an interference with the synapses between other neurons is exemplified by the horizontal and centrifugal bipolars