Cellular inclusion consisting of circular areas filled with fine slender filaments about 10 nanometers in diameter, delimited by a wall of varying complexity (either a single continuous membrane or a tubular network consisting of a fine filamentous material giving the wall a honeycomb appearance). Fibrillary inclusions are found in the cytoplasm of giant cells of Dieters in the lateral vestibular nucleus of the rat; similar structures have been described in the ventral cochlear nucleus, spinal cord, and substantia nigra.