. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 1'he Dominating Factor 719 encephalon Thalamus -Mesencephalon ^Median Lemniscus In the thalamus the fibers of the second order terminate and synapse with sensory neurons of the third order which have their cell bodies in one of the thalamic nuclei. The neurites of these third neurons go to the somates- thetic area of the cerebral cortex. These tracts form pathways for sensations of touch and pressure as well as for proprioceptive impulses from the muscles,
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 1'he Dominating Factor 719 encephalon Thalamus -Mesencephalon ^Median Lemniscus In the thalamus the fibers of the second order terminate and synapse with sensory neurons of the third order which have their cell bodies in one of the thalamic nuclei. The neurites of these third neurons go to the somates- thetic area of the cerebral cortex. These tracts form pathways for sensations of touch and pressure as well as for proprioceptive impulses from the muscles, joints, and tendons. (b) Spinothalamic Tracts.— In the case of the spinothalamic tracts the sensory neurons (neu- rons I) synapse with neurons II within the dorsal gray matter of the cord. The neurites of these neurons of the second order cross, through the ventral commissure, into the spinothalamic tracts of the other side of the cord (Fig. 643). The fibers of each of these tracts reach the thalamus, where they synapse with the neurons of the third order which have their cell bodies in one of the thalamic nu- clei. From this nucleus, the neurites extend to the somatesthetic area of the cerebral cortex. It will be recalled that physio- logically the two tracts of this group are not the same. The ven- tral spinothalamic tract is a path- way for sensations of touch and pressure, the lateral spinothalamic for pain and temperature. Also the neurites of the primary sensory neurons associated with each of these tracts take somewhat dif- ferent courses. The neurites carrying touch and pressure impulses run in the dorsal columns before sending collaterals in the dorsal gray horn. The neurites carrying pain and temperature impulses go directly into the gray matter at the level of entrance into the cord. (c) Spinocerebellar Tracts.—The spinocerebellar tracts are important proprioceptive pathways to the cerebellum, the chief center of correlation of. Pons Nucleus Gracilis Nucleus Cun
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