From the trigeminal nucleus axones establishing reflex and cor tical relations run: (a) To motor nuclei by way of the medial longitudinal bundle and directly without entering that bundle, forming the middle link of simple reflex arcs, and, reinforced by direct root fibers, to the cerebellar cortex by way of the restiform body, whence the coordinated reflex arcs are completed by the Purkinje, cerebello-tegmental and Deiters's neurones, or by the two former and those of the red nucleus or thalamus. (b) By way of two paths they run toward the cerebral cortex as far as the thalamus. The latter cross the median raphe and probably enter the medial fillet and the spino-thalamic tract. The axones bearing impulses of the muscular sense enter the medial fillet and are continued through it to the lateral nucleus of the thalamus; those fibers which conduct pain and tem perature impressions run through the spino-thalamic tract to the same nucleus. Both sets of fibers also conduct tactile impulses. The medial fillet conducts impulses leading to tactile discrimination of two or more simultaneous contacts and the spino-thalamic tract carries impulses of tactile locali zation of a single contact. From the thalamus the cortical fillet completes the path to the somwsthetic cortex of the cerebrum.
Vestibular Nuclei (nn. nervi vestibularis, Figs. 123 and 124).— These are located partly in the pons as already pointed out, and extend as low as the mid-medulla. They are somatic terminal nuclei. Their function is equilibrium. The principal nucleus (Schwalbe's) is dorso-medial in position and lies beneath the acustic area of the ventricular floor, crossed by the medullary strive. It extends transversely from near the eminentia medialis almost to the restiform body. It appears to receive nearly all the fibers of the vestibular nerve which arborize and terminate about its cells. Axones of the chief nucleus enter the opposite medial fillet and medial longitudinal bundle and the homo lateral restiform body. Lateral to the principal nucleus are the nucleus of Deiters and the nucleus of the descending root. Deiters's nucleus is spread along the medial surface of the resti form body, chiefly in the pons. It becomes a distinct nucleus as the lower border of the pons is approached and grows larger for some distance above that point. In the pons it is bent backward with the restiform body toward the cerebellum. Its upper end is thus placed in the lateral.wall of the fourth ventricle between the restiform body and the brachium conjunctivum, This portion is called the superior nucleus (of Bechterew or Flechsig). Deiters's nucleus is made up of cell-bodies which are large in comparison with those of the principal nucleus. It
receives vestibular nerve fibers and the descending fibers of the fastigio-bulbar tract from cerebellar ganglia and originates fibers that enter the medial longitudinal bundle, the vestibulo spinal tract and the restiform body—all on the same side. The nucleus of Deiters is a relay in the cerebello-spinal path and it constitutes the middle link in static arcs formed by the vestibular nerve, the Deiters's neurones and motor nerves. The nucleus of the descending root is the spinal nucleus of the vestibular nerve. It is composed of cell-bodies scattered through a strand of fibers called the descending root which extends from the level of the principal nucleus down to the nucleus funiculi cuneati (Bruce). It is placed somewhat under cover of the medial border of the restiform body and, with the enveloping descending root, separates this body from the principal nucleus. Certain fibers of the descending root terminate in the nucleus of the same name; others in the nucleus of Deiters. Axones of the spinal nucleus run into the medial longitudinal bundle and restiform body of the same side, proceeding to motor nuclei and the cerebellar cortex.
Cortical Connection.—Axones from the chief vestibular nucleus ascend to the thalamus through the opposite medial fillet whence the path is completed by the cortical fillet.
Simple reflex connections are established, first, by axones of all three nuclei which enter the medial longitudinal bundles and, dividing T-like, ascend and descend to motor nuclei; and second, by axones of Deiters's nucleus which descend to spinal motor nuclei. Coordinated reflex connections are established through the cerebellum as follows: some vestibular fibers run directly through the restiform body to nucleus fastigii; nucleo-cerebellar fibers go from each vestibular nucleus to cerebellar cortex; cortical axones from the Purkinje cells terminate in the cerebellar nuclei from which point the path is double; axones of nucleus fastigii descend to Deiters's nu cleus, whose axones reach the motor nuclei of spinal nerves; and axones of nucleus dentatus proceed through brachium con junctivum to red nucleus and thalamus, from which the rubro spinal tract and thalamo-spinal tract establish a wide con nection with motor nuclei. In both the fastigio-bulbar tract and the brachium conjunctivum there are cerebello-tegmental fibers which go directly to motor nuclei of the mid-brain, pons and medulla; and, therefore, belong to the coordinated reflex mechanism.
Cochlear Nuclei (nn. nervi cochlearis, Fig. 123).—There are two cochlear nuclei, the ventral and the lateral. They concern hearing proper. They receive the terminals of the cochlear nerve and are somatic special sense nuclei.