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Projection Fibers

brachium, pontis, nucleus, conjunctivum, tract, nuclei and superior

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PROJECTION FIBERS All fibers that leave the cerebellum, or enter it, do so through the brachia, the restiform bodies and the superior medullary velum, hence these are composed of projection fibers. At a higher level the projection fibers are contained in the corpus medullare.

Brachium Conjunctivum (Figs. 56, 102 and Io3).—The brach ium conjunctivum (superior peduncle) is the innermost of the three, at its origin in the anterior cerebellar notch; lateral to it, in the notch, are the restiform body and the brachium pontis; and, in the angle between the brachium conjunctivum and the restiform body, is the vestibular nucleus of Bechterew (the upper part of Deiters's nucleus). The brachium conjunc tivum is joined to its fellow of the opposite side by the superior medullary velum (velum medullare superius). The brachium conjunctivum contains one great tract, the superior cerebello tegmental fasciculus. It is composed chiefly of axones of the dentate nucleus, augmented by a small number from the nuclei emboliformis, globusus and fastigii. This tract of fibers partially buries itself in the dorsal area of the pons, then penetrates the mid-brain and decussates ventral to the inferior quadrigeminal colliculi. It ends largely in the opposite red nucleus, but partly in the thalamus and motor nuclei of the mid-brain, pons and medulla. This cerebello-tegmental fasc iculus of the brachium conjunctivum includes a diencephalic. a mesencephalic, a pontine and, probably, a bulbar part. It forms a link in the mechanism coordinating the movements of locomotion. In the red nucleus this path is relayed to the thalamus by the rubro-thalamic tract and a part of the cerebello tegmental tract enters the thalamus directly, on the same side; so this path may also conduct afferent impulses from the cerebellum which ultimately reach the cortex of the cerebrum.

The brachium conjunctivum contains a small cerebello petal tract which appears to rise in the terminal nuclei of the optic nerve and end in the cerebellum. According to Edinger such a tract is well developed in fishes, reptiles, amphibians and birds. It may be distinguished as the optic nucleo-cerebellar fasciculus.

A few commissural fibers between Bechterew's nuclei are found in the cerebellar end of the brachium conjunctivum, and the fila lateralia pontis traverses the same part of it. The fila lateralis pontis (taania pontis) is a dissociated bundle of the transverse fibers of the pons. Rising in the opposite nucleus pontis, it winds around the isthmus to the brachium con junctivum; according to Horsley, it terminates in the dentate nucleus. The commissural fibers connecting Bechterew's nuclei cross from one brachium to the other through the superior medullary velum.

The superior medullary velum (Fig. 102) arches over the fourth ventricle between the brachia conjunctiva. It is com posed of longitudinal and transverse fibers. One distinct bundle, derived from the spinal cord, passes through it to the worm. This is the ventral spino-cerebellar fasciculus of Gowers. The other longitudinal tract is the tecto-cerebellar fasciculus, which rises in the inferior colliculus of the quadrigeminal lamina (tectum) and terminates in the cerebellum. It connects the olfactory and acustic paths with the great center of coordi nation. The decussating root-fibers of the fourth nerve (troch lear) course transversely through it and also the commissural fibers between Bechterew's nuclei.

Brachium Pontis (Middle Peduncle, Figs. 56, 103 , 107 and 122) . —The brachium pontis comes from the pons, of which it forms the basilar transverse fibers. It enters into the medullary body of the cerebellum lateral to both the brachium con junctivum and the restiform body. According to Klimoff fibers running to the cerebellum make up the entire brachium pontis. These are axones of the nucleus pontis and nucleus ponto-bulbaris, the opposite ones. Most of them run to the cortex of the cerebellar hemisphere; a small number run to the vermis cerebelli. They form a segment in the indirect efferent path contained, above the pons, in the medial and lateral fifths and the intermediate bundle of the basis pedunculi. By far the greater number of fibers in the brachium pontis are crossed fibers.

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