CONDUCTION PATHS OF THE DIENCEPHALON.
The connections which unite the diencephalon with other parts of the brain have, in large part, been presented in the preceding section. To these belong, in the first place, the tractus and thalamic peduncles—of which the tegmental tract and the optic radiation may be again mentioned as of especial importance. Others to be recalled are those fibre-tracts which unite certain parts of the olfactory brain with the thalamencephalon and the hypothalamus—fornix, stria medullaris, basal olfactory bundle, which tracts are joined, moreover, by those passing from the diencephalon to the mid-brain—fasciculus mamillo-tegmentalis, pedunculus corporis mamil laris, tractus habenulo-peduncularis. Further, the connections which join the corpus striatum with the thalamus and with the subthalamic region—the radiatio and Within the pulvinar thalami and the corpus geniculatum laterale, which parts, together with the superior colliculus, constitute the primary visual centre (page 172), the fibres of the tractus opticus end. The corpus geniculatum mediale, with the inferior colliculus, constitutes the primary auditory centre, since within these parts, particularly within the medial geniculate body, the fibres of the lateral fillet end ; the latter, as later to be described (page x79), serves to conduct the impulses from the end-nuclei of the acoustic nerve farther centrally and, hence, represents the primary auditory tract. From the corpus geniculatum laterale and the pulvinar thalami, the optic radiation passes to the cortex of the visual centre in the occipital lobe ; from the corpus geniculatum mediale, the secondary auditory tract passes to the cortex of the auditory centre in the temporal lobe. Within the thalamus, moreover, end certain fibre-strands which come from the cerebellum, the medulla oblongata and the spinal cord. Bundles of fibres, from the nucleus dentatus and in small part also from the nucleus tecti, pass forward from the cerebellum, constituting collectively the superior cerebellar peduncle. The larger part of these fibres, after decussation, reaches the nucleus ruber in the tegmentum of the brain and there s ; the smaller part of the fibres passes directly to the thalamus, joining such as come from the red nucleus—tractus rubro thalamicus. The fibres proceeding from the medulla oblongata and from the spinal cord form the large ascending sensory path, tractus et the detailed origin and course of which will be later considered. For the present suffice it to note,
that this tract, known as the medial fillet or lemniscus medialis, carries fibres from the spinal cord, the nuclei of the posterior column and the end-nuclei of the sensory cerebral nerves. Its termination is principally within the lateral nucleus and the centrum medianum of the thalamus. Impulses from the spinal cord, the nuclei of the posterior column and the olivary nucleus of the medulla may reach the thalamus also by way of the cerebellum and the superior cerebellar peduncle. These paths are shown in Figs. 142 and 147. The tractus is a spinalward coursing path, that connects the thalamus with the olive of the medulla (Fig. 147); it is also termed the central legmental tract. Since the olive sends fibres to the cerebellum, impulses from the thalamus may be conveyed to the cerebellum by this tract.
Probably still other paths proceed from the thalamus downward, to end within the brain, the pons, the medulla and the spinal cord. Such connections, however, are not rately determined. The descending tractus accompanies the tractus spinalis to the spinal cord, within which it courses in the dorsal part of the lateral column. Finally, mention must be made of the system of the ventricular gray, which gray substance covers the medial surface of the thalamus and hypothalamus and the floor of the third ventricle and is continuous with the gray substance surrounding the aquaeductus Sylvii and investing the floor of the fourth ventricle. The fibres from the cells situated within this gray substance pass to all the thalamic nuclei, while delicate longitudinal strands proceed caudalward through the gray in the medulla oblongata and into the spinal cord. This system of longitudinal fibres, which has been already noticed in connection with the fibre-tracts of the rhinencephalon, is the fasciculus longitudinalis dorsalis, or dorsal longitudinal bundle of Schutz, and is closely connected with the nuclei of the cerebral nerves and other ganglia. Concerning the significance of the entire system, we are, at present, insufficiently informed. According to Edinger, it is not unlikely, that all these nuclei and fibres constitute a central apparatus of the sympathetic.