Direct parasympathetic pathway to the eye: revisited.

Intraocular injections of wheat germ agglutinin horseradish peroxidase appeared to confirm previous experimental and clinical data supporting the existence of a direct, non-synapsing, parasympathetic pathway from the mesencephalon to the eye. Quantitative and qualitative differences between the two pathways were noted. The intraocular injection of the fluorescent dyes Fast blue and Nuclear yellow failed to provide mesencephalic labeling. The implications of these findings are discussed.