The anatomy of the nervous system, from the standpoint of development and function . distributed to the skin or mucous membrane of thehead, and a central branch, which runs through the sensory root (pars major)of the trigeminal nerve into the pons. Here it divides into a short ascendingand a long descending branch. The former terminates in the main sensorynucleus, and the latter in the spinal nucleus of that nerve (Fig. 232). Neuron II.—The fibers of the second order in the sensory paths of the tri-geminal nerve arise from cells located in the main sensory and the spinal nucleus 3o8 THE NERVOU
The anatomy of the nervous system, from the standpoint of development and function . distributed to the skin or mucous membrane of thehead, and a central branch, which runs through the sensory root (pars major)of the trigeminal nerve into the pons. Here it divides into a short ascendingand a long descending branch. The former terminates in the main sensorynucleus, and the latter in the spinal nucleus of that nerve (Fig. 232). Neuron II.—The fibers of the second order in the sensory paths of the tri-geminal nerve arise from cells located in the main sensory and the spinal nucleus 3o8 THE NERVOUS SYSTEM of that nerve; and alter crossing the raphe they run in two tracts to the ventralpart of the lateral nucleus of the thalamus. The ventral secondary afferentpath is located in the ventral part of the reticular formation, close to the spino-thalamic tract in the medulla oblongata and dorsal to the medial lemniscus inthe pons and mesencephalon (Figs. 132, 234). The dorsal tract lies not farfrom the floor of the fourth ventricle and the central gray matter of the cerebral. N Thalamus Medial lemniscusMesencephalon II -1-Medial lemniscus-Pons Dorsal secondary tract N. Vj Ventral secondary tract N. V Main sensory nucleus N. V— Pons N. V Spinal tract N. V I Spinal nucleus N. V -*? Medulla oblongataFig. 232.—Diagram of the exteroceptive pathways associated with the trigeminal nerve. aqueduct. It consists in considerable part of uncrossed fibers and of fibers hav-ing a short course (Wallenberg, 1905; Economo, 1911; Dejerine, 1914). Neuron III.—The afferent impulses are relayed from the thalamus to thecortex of the posterior central gyrus by fibers of the third order, which run throughthe posterior limb of the internal capsule. Their cells of origin are located inthe lateral nucleus of the thalamus. THE GREA1 \l I I K l \ l SYSTEMS .W) The Neural Mechanism for Hearing. The spiral organ of Corti within thecochlea is connected with the auditory center in the cerebral cortex
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