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or Motor Projection Fibers Corticifugal

nucleus, pontis, tract, cerebral, nuclei, nerves and figs

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CORTICIFUGAL, OR MOTOR PROJECTION FIBERS The most important tracts of corticifugal or motor projection fibers are the following, namely, the strio-fugal fibers, the fronto pontal tract, the pyramidal tract and the temporo-pontal tract.

The strio-fugal fasciculi are formed by axones of cell-bodies in the globus pallidus which terminate in the nucleus hypo thalamicus, the lateral nucleus of the thalamus, the nucleus ruber and the substantia nigra. The strio-hypothalamic bundle appears to be merely associative in function; the other three belong to long conduction paths. The strio-thalamic fasciculus ends in the lateral nucleus of the thalamus; but is connected with lower centers by the thalamo-spinal fasciculus and, per haps, by the thalamo-olivary fasciculus. The strio-rubral fasc iculus is an important bundle; through this and the rubro spinal fasciculus, the striate body exerts its steadying effect upon the lower motor neurones. The strio-ffigral fasciculus ends in the substantia nigra, whence the nigro-pontal fasciculus origi nates and continues to the nucleus pontis. The two last fasc iculi form the intermediate path (Figs. 64 and 93).

The intermediate path (stratum intermedium pedunculi, Figs. 64, 93, and 113) extends from the corpus striatum through the inferior lamina of the capsule and the deep part of the basis pedunculi to the motor cerebral nuclei and to the nucleus pontis, though it is relayed in the substantia nigra. From the nucleus pontis axones run by way of the brachium pontis to the cortex of the opposite hemisphere of the cerebellum. The intermedi ate path thus forms a segment of an indirect (through the cere bellum) efferent and probably coordinating path.

The fronto-pontal tract (tractus cerebro-cortico-pontalis fron talis, Figs. 62, 93, 94 and r13) rises from the cortex of the frontal lobe anterior to the precentral sulci. It traverses the centrum semiovale, corona radiata, frontal part of the internal capsule and medial one-fifth of the basis pedunculi to the ventral area of the pons, where it terminates in the nucleus pontis (chiefly) and in the nuclei of motor cerebral nerves (Flechsig). It is probably relayed in the thalamus (Beevor and Horsley).

According to Dejerine, the temporo-pontal tract (tractus cere bro-cortico-pontalis temporalis, Figs. 61, 93, 94 and 113) extends from the temporal lobe through the inferior lamina (and poste rior part of the superior lamina) of the internal capsule and lateral one-fifth of the basis pedunculi to the substantia nigra and the nucleus pontis; but according to Spitzka some of its fibers end in the nuclei of motor cerebral nerves. Thus it should be noted

that, with the exception of those fibers to motor nuclei of the cerebral nerves, each of the three tracts above mentioned, viz., the intermediate, fronto-pontal and temporo-pontal, constitutes a segment of an indirect efferent path which is interrupted in the nucleus pontis and then continued by the axones of that nucleus through the brachium pontis of the cerebellum. It is probable, though not surely established, that the fronto-pontal and temporo-pontal tracts are relayed in the corpus striatum or thala mus, as they have been found undegenerated in the base of the peduncle when their cortical origins were destroyed by extensive lesions.

The pyramidal tract (tractus cerebrospinalis pyramidalis) (Figs. 93 and 94) rises in the anterior central gyrus and the pre central part of the paracentral lobule. It is composed of axones from the giant pyramids of Betz of that region. Descending through the corona radiata, genu and anterior two-thirds of the occipital part of the internal capsule, the pyramidal prises the middle three-fifths of the basis pedunculi, enters into the anterior longitudinal fibers of the pons, forms the pyramid of the medulla and the anterior and lateral pyramidal tracts of the spinal cord (Figs. 61,94, 113,124 and 142). The fibers of the py ramidal tract, with a few exceptions, cross over to the opposite side; they end in connection with the motor nuclei of cerebral and spinal nerves. Fibers enter the nucleus of the trochlear (or fourth) nerve chiefly on the same side, and a few descend to the motor nuclei of other cerebral nerves and to the gray matter in the spinal cord without decussation; all other pyramidal fibers terminate on the side opposite to their origin. The fibers from the lower one-third of the anterior central gyrus, which go to the motor nuclei of the cerebral nerves, to a large extent leave the pyramidal tract high up in the peduncle and run for some dis tance through the medial portion of the fillet; they constitute Bechterew's accessory lemniscus. This accessory fillet has been recently traced by Flechsig.

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