482 THE NERVOUS SYSTEM. 
From the corpus geniculatum internum there proceeds a tract to the gray cortex of 
the superior convolution of the temporal lobe. The whole nervous apparatus is thus 
linked on to the cerebral cortex, and the succession of neurons which build up the entire 
chain are therefore: (1) the bipolar cells of the ganglion spirale ; (2) the neurons of the 
terminal cochlear nuclei; (3) the neurons of the superior olive and the nucleus of the 
lateral fillet ; (4) the neurons of the corpus geniculatum internum. 
It must be borne in mind that all the axons of the cells of the superior olive do not 
join the trapezoid strand. Many leave its dorsal aspect and pass backwards in a group 
called the pedicle of the superior olive, to end in the nucleus of the sixth nerve, and, 
through the posterior longitudinal bundle, in the nuclei of the fourth and third nerves. 
In this way the organ of hearing is brought into connexion with the nuclei, which preside 
over ‘the movements of the eyeballs (Figs. 344, p. 463, and 359, p. 484). 
Central Connexions of the Vestibular Nerve.— Although the central con- 
nexions of the vestibular nerve have been closely studied by many observers, they are 
still very far from being fully understood. he principal nucleus and the nucleus of 
Deiters both stand in intimate relation with the superior vermis of the cerebellum ; and 
in consideration of the fact that the vestibular nerve is the nerve of equilibration, this 
is an important and significant circumstance. The strand which establishes this con- 
nexion is termed by Edinger “the direct sensory cerebellar tract,’ and in all probability 
it is an efferent tract from the cerebellum. Its fibres arise to a large extent in the 
cerebellar roof nuclei of the opposite side, and, crossing the mesial plane, they sweep 
forwards around the outer side of the superior cerebellar peduncle to end in the nucleus 
of Deiters, the chief vestibular nucleus, and very possibly also in the terminal sensory 
nuclei of certain other cranial nerves, such as the trigeminal, vagus, and glosso- 
pharyngeal. 
Until the precise nature of the nucleus of Deiters is discovered, the exact character 
of the central connexions of the vestibular nerve will remain more or less obscure. It 
cannot be regarded as a nucleus specially given over to the vestibular nerve. Composed 
of large cells scattered amidst the bundles of the descending root of the vestibular nerve, 
it only becomes a compact nucleus above the level of that nerve, viz. at the point where 
the restiform body turns backwards into the cerebellum, or, in other words, at the level 
of the emerging facial nerve and the lower end of the abducent nucleus. Here, in the 
outer part of the floor of the fourth ventricle, its cells are gathered together in a crowded 
mass. Deiters himself considered that this nucleus should be regarded as an internode 
between the cerebellum and the spinal cord, and Ferrier and Turner have brought forward 
strong evidence in support of this view. Klimoff attaches a very high importance to the 
direct “sensory” tract of Edinger. From his description, it would appear that he regards 
it as the only cerebellar efferent tract which takes a downward direction. He believes, 
further, that the axons of the cells of Deiters form the anterior marginal tract of Lowenthal, 
which descends in the antero-lateral column of the cord as far as the lumbar region. — It 
is supposed that the fibres of this tract end in the cord in arborescent terminations 
around the motor cells in the ventral horn of gray matter. Opinion is also divided as 
to the composition and nature of the so-called descending root of the vestibular nerve. 
After division of the eighth nerve, Ferrier and Turner were unable to detect any 
degeneration in this root, and they therefore are inclined to call in question its direct 
continuity with the nerve. They consider that in all probability it forms an interuuncial 
connexion between the nucleus of Deiters and the cuneate nucleus, in which Bruce has 
seen its lower end to terminate. 
Facial Nerve (nervus facialis) (Figs. 358 and 359).—The facial nerve is com- 
posed of two distinct parts, viz. a large efferent or motor portion, the facial nerve 
proper, and a small afferent sensory portion termed the pars intermedia of Wrisberg. 
The facial nerve emerges from the brain at the lower border of the pons, immediately 
in front and to the inner side of the auditory nerve, whilst the pars intermedia 
sinks into the upper part of the medulla between the facial and auditory nerves. 
The three nerves, therefore, lie in intimate relation with each other, where they 
are attached to the surface of the brain, and they pass in company into the internal 
auditory meatus. | 
The nucleus of origin of the facial nerve is an oval mass of gray matter, about 
five mm. in length, and containing numerous groups of large multipolar cells. It 
is sunk deeply in the dorsal or tegmental part of the lower portion of the pons 
Varolii, and is placed close to the inner side of the spinal root of the fifth nerve. 
When transverse sections are made through the brain-stem, the facial nucleus 1s 
