2. NERVUS VESTIBULI.
The vestibular division of the auditory nerve arises within the ganglion vestibulare or Scarpa's ganglion, located at the bottom of the internal auditory canal. The periph erally directed processes or fibres of these ganglion cells run to the ampullae, the utricle and saccule of the internal ear; the centrally directed fibres enter the brain-stem and divide into ascending and descending branches. The descending branches form a descending vestibular root and end within the nucleus nervi vestibularis spinalis, which extends as far as the posterior column nuclei. The ascending branches end within the nucleus medialis, as well as within the lateral Deiters' nucleus and the upper Bechterew's nucleus. From these end-nuclei, fibres pass to the cerebellar worm as constituents of the tractus nucleo-cerebellaris. A part of the vestibular fibres pass direct to the roof-nucleus of the cerebellum as constituents of the direct sensory cerebellar tract, the fibres giving off collaterals to Deiters' nucleus. The medial nucleus is brought into relation with the superior olive by means of fibres. Perhaps fibres pass also to the formatio reticularis and to the thalamus.
In view of its importance, the system of Deiters' nucleus claims closer attention. This nucleus receives, on the one hand, fibres from the roof-nucleus of the cerebellum ; on the other hand, as we have seen, Deiters' nucleus gives origin to a fibre-bundle that, as the tractus vestibulo-spinalis, passes to the spinal cord.
Within the same nucleus, moreover, also another path, the posterior longitudinal bundle or the jasciculus lcmgitudinalis medialis, takes its origin. The fibres pass from Deiters' nucleus toward the mid-line, some crossing the latter and then dividing into ascending and descending branches. The ascending branches can be followed upward as far as the oculomotor nucleus ; the descending branches pass to the anterior column of the spinal cord. The posterior longitudinal bundle, however, does not consist exclusively of fibres from Deiters' nucleus. Other fibres take origin in the common nucleus of the commissura posterior and of the fasciculus longitudinalis medialis within the forepart of the mid-brain, in front of the oculomotor nucleus. The posterior longitudinal bundle may be traced from its nucleus through the mid-brain, the pons and the medulla oblongata into the spinal cord, during its course giving off numerous collaterals to the nuclei of the nerves supplying the ocular muscles. This bundle is of great importance. It establishes relations of the nuclei of the eye-muscles to one another, among which that of the abducens to the oculomotorius nucleus deserves particular attention. Of especial impor tance is the connection of the abducent nucleus with those cells of the oculomotor nucleus, from which pass the fibres for the rectus internus, since the synergic function of the rectus externus and internus, which consists in the conjugate deflection of the eyes toward one side, can be explained only by the existence of a direct or an indirect connection between these nuclei. Fig. i68 represents the manner in which the coordinate action of
the two muscles may be explained upon an anatomical basis. Connection of the abducens nucleus with that of the oculomotorius, by means of the posterior longitudinal bundle, is positively established. Further, that the nerve-fibres for the rectus internus arise, in greater part, from the cells of the oculomotor nucleus of the opposite side. On the other hand, it is still undecided, whether the efferent paths from the cortical centre for synergic eye-movements are first interrupted in a special centre within the quadrigeminal region, or pass directly into the posterior longitudinal bundle. In any event, this tract undergoes a total or partial decussation before it enters the posterior longitudinal bundle. In Fig. i68, the path from the cortex to the nucleus of the bundle is represented as crossed. In tnis way, the explanation for the following phenomena is supplied. When a cortical centre for eye-movements is stimulated, the left one for example, deviation of both eyes toward the right occurs. On the other hand, in left-sided disease of the cerebral cortex, followed by paralysis of the right half of the body, deflection of both eyes toward the side of the lesion, that is the left, is frequently observed, since, under these conditions, the eye-muscle nerves of the left side functionally predominate. " In lesions of the hemi spheres, if there is conjugate deviation of the eyes, the patient looks toward the injured hemisphere when there is paralysis, or the limbs are contorted during a convulsion" (Grasset). The diagram explains, further, the deviation of the eyes, toward the side opposite to the seat of the lesion, frequently observed in diseases of the pons. For example, if a lesion of the posterior longitudinal bundle lies in the vicinity of the right abducens nucleus, deviation of the eyes toward the right occurs in consequence of the mastery by the nerves controlling the left eye-muscles.
The posterior longitudinal bundle possesses further importance, since it brings the vestibular apparatus and the cerebellum into relation with the nuclei of the eye-muscles and the spinal cord, by means of the fibres arising within Deiters' nucleus. It unites, therefore, the centres concerned in maintaining equilibrium and orientation in space.
It is to be noted, that, since a connection between the superior olive and the abducens nucleus exists, relations of the acoustic nerve, that is of the auditory path, with the abducens, and, by means of the posterior longitudinal bundle, with the other nuclei of the eye-muscle nerves may also be established. These connections explain the occur rence of reflex ocular movements in response to auditory impressions (Fig. 169).