These impulses from the spinal nerves go to the upper two thirds of the posterior central gyrus, those from the lower extrem ity to the upper third and those from the arms to the middle third (Spiller).
Through Cerebral Nerves and Medial Fillet (Fig. r49).— As crossed fibers from the terminal nuclei of the trigeminal, the vestibular, the glossopharyngeal and the vagus nerves join the medial fillet and run to the thalamus, so muscular and tactile sensations transmitted by those cerebral nerves to their nuclei in the medulla and pons are carried by the medial fillet to the lateral nucleus of the thalamus on the opposite side. The corti cal fillet conducts them to the lower portion of the posterior central gyrus in the sommsthetic area.
II. Paths Conveying Pain, Temperature and Tactile Im pressions. Spinal and Cerebral. Through Spino-thalamic and Ventral Spino-cerebellar Tract (Fig. r5o).—In the spinal cord, medulla and pons these constitute one tract, commonly called Gowers' tract. They separate just below the isthmus, whence the spino-thalamic tract continues to the thalamus and the other turns back to the cerebellum. They appear to form the only paths for pain and temperature impulses. These impulses enter the gray crescent of the cord on both sides through the posterior nerve roots. A large number decussate via the intrinsic axones in the gray commissure; the rest de cussate in the first stage of the ascending tracts, crossing in the white anterior commissure, and run upward through the spino thalamic and ventral spino-cerebellar tracts of the opposite side; they run to the thalamus and to the cortex of the superior worm of the cerebellum. In the cord they ascend along the lateral surface. They run dorsal to the olive in the lateral area of the medulla oblongata, and through the lateral part of the formatio reticularis of the pons to the angle in Gowers' tract situated near the isthmus. From the angle, just below the quadrigeminal bodies, the cerebellar impulses run backward with the tract through the superior medullary velum to the cortex of the vermis cerebelli superior; the remainder run upward to the thalamus, and from that to the posterior central cortex. Coordinating reflex impulses are excited in the cere
bellum, which reach the motor nuclei in the usual ways: through the cortico-nuclear neurones, the cerebello-tegmental tracts, and the vestibulo-spinal, rubro-spinal and thalamo-spinal tracts. These impulses may also pass to the conscious centers of the cere brum. The common course of sensory impulses from the cere bellar to the cerebral cortex is, as already described, through nucleus dentatus and brachium conjunctivum to opposite red nucleus and thalamus. Having arrived in the thalamus, they proceed thence by the cortical fillet to the somesthetic cortex.
Certain fibers of Gowers' tract diverge from the others, in the medulla oblongata, and terminate in the inferior lateral nucleus. Impulses of pain and temperature, following the same course, enter the lateral nucleus, and are carried on through the restiform body to the cerebellum by the tract from the lateral nucleus to the cerebellar cortex, thence to the somesthetic area as previously given.
Through Cerebral Nerves and the Spino-thalaniic Tract (Fig. r5o).—Pain and temperature impulses are transmitted by certain fibers of the vagus, glossopharyngeal and trigeminal nerves to their terminal nuclei. From those nuclei they are conducted by axones which probably enter into the spino thalamic tract and perhaps into the ventral spino-cerebellar tract to the thalamus and to the cerebellar cortex. The path from either point to the posterior central gyrus is now familiar.
The Short Fiber Paths.—What special varieties of common sensation are conducted through these paths is unknown. Under certain conditions perhaps they may carry all varieties. (r) The antero-lateral fasciculus proprius and formatio reticu laris contain ascending axones which may convey sensory im pulses from the gray matter of the cord, received from the poste rior roots of the spinal nerves, and from terminal nuclei in medulla and pons which receive the common sensory fibers of cerebral nerves, upward to the thalamus of the opposite side. The course from the thalamus is by way of the cortical fillet.