Common Sensory Nuclei (Fig. 133).—Of the posterior columna series of nuclei, the terminal nucleus of the fifth, a part of the tenth and a part of the terminal nucleus of the ninth nerve receive common sensory impulses, and trasmit them to the opposite thalamus by two routes, viz., through the medial fillet and through the spino-thalamic tract. From the thalamus these impulses are carried to the cortex of the posterior central gyrus and other gyri. Thus is the cortical connection of these nuclei established and each is brought into simple reflex connection with motor nuclei by axones of the terminal nuclei which run chiefly through the medial longitudinal bundle and terminate in the motor nuclei. Coordinated reflex con nection is made with the cerebellar cortex by axones which run through the restiform body. From the cortex of the cerebellum the junction with motor nuclei is established by axones of the cortical cells of Purkinje, which end in the cerebellar nuclei, and the cells of the cerebellar nuclei, which form the cerebello tegmental tracts, assisted by the neurones of Deiters's nucleus, of the thalamus and red nucleus. The cerebello-tegmental fibers go through the brachium conjunctivum and restiform body directly to the cranial motor nuclei; also to thalamus, red nucleus and the nucleus of Deiters; axones of Deiters's nucleus pass through the medial longitudinal bundle to cranial nerve nuclei; and the vestibulo-spinal, rubro-spinal and thalamo spinal tracts terminate in motor nuclei, both spinal and cranial.
Special Sense Nuclei.—The cortical connection of the nucleus tractus solitary, which receives taste impulses from the glosso pharyngeal and intermediate nerves, has been definitely traced by May and Horsley; it is relayed in the thalamus and is established by fibers of the solitary nucleus which extend through the pons and mid-brain and by certain fibers of the internal capsule which end in the cingulate gyrus (?). The cochlear nuclei (ventral and lateral) receive true impulses of hearing and conduct them on toward the cerebral cortex by way of their axones which form the trapezoid body and medul lary striae and then unite in forming the lateral fillet (Fig. n9). The remaining links of the cortical connection are formed by the brachium inferius and the acustic radiation (radiatio acustica). The reflex connection of these nuclei is somewhat indirect. It is established in part by certain fibers of the lateral fillet which end in the quadrigeminal colliculi, together with the anterior tecto-spinal bundle; but is chiefly brought about by the olivary pedicle and the medial longitudinal bundle. The vestibular nuclei—the principal (Schwalbe's), Deiters's and the nucleus of the descending root—concern equilibrium.
They receive impulses from the vestibule and semicircular canals of the internal ear. They have a cerebral and an im portant cerebellar connection. The former is established as far as the thalamus by the opposite medial fillet, and com pleted by the cortical fillet; and the latter is formed by the direct root-fibers running to nucleus fastigii of the cerebellum and by nucleo-cerebellar fibers going from each of the three terminal nuclei through the restiform body to cerebellar cortex. The coordination path is completed by the cortico-nuclear fibers (of Purkinje's cells); the cerebello-tegmental fibers; the thalamo spinal, the rubro-spinal and the vestibulo-spinal tracts and the medial longitudinal bundle. The simple reflex connection of the vestibular nerve is established by axones of the vestibu lar nuclei which run to motor nuclei through medial longitu dinal bundle and vestibulo-spinal tract.
The terminal nuclei of the optic nerve are situated in the lateral geniculate body, the pulvinar of the thalamus and the superior colliculus of the corpora quadrigemina (Fig. 55). Like the ter minal nucleus of the olfactory nerve, these cannot at present be included in the posterior columna series, because the ventral and dorsal zones of the embryonic fore-brain have not been suf ficiently elucidated. If the sulcus hypothalamicus really separate ventral from dorsal zone in the inter-brain, as is claimed by many, it would seem that both the optic and ol factory terminal nuclei might be included in the posterior series; but there is need of further investigation, as this places the whole cerebral hemisphere in the dorsal zone.
The cortical connection of the terminal nuclei of the optic nerve (Fig. 90) is established by fibers of the optic radiation (radiatio occipito-thalamica) which rise in the lateral geniculate body and in the pulvinar of the thalamus and terminate in the cortex of the calcarine region of the occipital lobe. From this cortical center corticifugal fibers run through the occipito thalamic radiation and brachium superius to the superior quad rigeminal colliculus. This colliculus also receives a few fibers directly from the outer root of the optic tract; it thus becomes the center of optic reflexes; and axones of the superior colliculus form the anterior tecto-spinal bundle which completes the connection with opposite motor nuclei. The terminal optic nuclei are connected with the cerebellum by those nucleo-cere bellar fibers which rise in these nuclei; they descend to the cere bellar cortex through the brachium conjunctivum and the re maining links of the coordinating reflex arcs, which join the cerebellar cortex to motor nuclei, are fully set forth in the pre ceding paragraphs.