Vestibular Nucleus of the Auditory Nerve (n. nervi vestibu laris) (Fig. 6).—This nucleus is made up of three parts: (I) The chief nucleus (Schwalbe); (2) the nucleus of the descending root, and (3) the nucleus of Deiters, which is lateral in position. The superior parts of Schwalbe's and of Deiters's nuclei ex tend into the pons just medial to the restiform body, and the nucleus of Deiters is prolonged dorsally along that body to ward the cerebellum. This dorsal extension of Deiters's nucleus is called Flechsig's or Bechterew's nucleus. We shall recur to the vestibular nucleus in the medulla where the greater part of it is located.
Lesions in the pons are usually attended by crossed paralysis. The paralysis and annesthesia of parts supplied by spinal and by bulbar nerves are on the opposite side, but the fifth, sixth and seventh cerebral nerves of the same side as the lesion are apt to be involved. If the spino-thalamic and anterior spino-cerebellar tracts are involved and not the medial fillet, the pain and temperature sense is lost, but there is no ataxia; if the medial fillet be destroyed and not the spino-thalamic and ventral spino-cerebellar tracts, then the pain and temperature sense is intact, but the muscular sense is lost on the opposite side of the body. The tactile sense is impaired in both cases.
A lesion of the trapezoid body produces almost total deafness; of the lateral fillet, slightly impaired hearing on the same side and nearly complete deafness in the opposite ear. Conjugate deviation occurs when the nucleus of the sixth nerve is affected and strabismus when the root fibers but not the nucleus are involved. The strabismus is external if the lesion be irritative and internal if the root fibers are destroyed. Destructive lesion in the nucleus of the seventh nerve causes complete facial paralysis, Bell's palsy, if the whole nucleus is involved. Also, complete facial paralysis occurs if the root-fibers of the facial nerve be destroyed in the pars secunda or in the genu internum.
Crossed paralysis (hemiplegia alterans) is characteristic of lesions in the mid-brain and pons. Here the pyramidal tract is uncrossed to motor nuclei at lower levels; hence, a lesion de stroying it produces paralysis on the opposite side of the body below the lesion. But the same lesion may destroy the root of the third, fifth, sixth or seventh nerve in its course to the surface of the brain and paralyze the ocular, masticator or facial muscles on the side of the lesion.