Orientation of stapedial nerve fibers in the facial nerve trunk.

By staining with toluidine blue and vital staining with methylene blue, the topographical orientation of the stapedial nerve fibers in the facial nerve trunk was visualized in adult guinea pigs. The stapedial nerve with its perineural sheath emerges from the facial nerve trunk in the posteromedial aspect of the upper part of the vertical portion. Toward the horizontal portion, the stapedial nerve fibers gradually ramify, lose their investments, and intermingle with other nerve fibers inside the facial nerve trunk. In the labyrinthine portion, the stapedial nerve fibers are distributed diffusely near the surface along the posterior, medial, and anterior margins of the facial nerve trunk. These findings lead to the conclusion that the stapedial nerve fibers of guinea pigs do not form a single funiculus throughout the intratemporal course of the facial nerve.