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Gray Matter of the Pons

nucleus, pontis, fibers, nuclei, cells and nerve

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GRAY MATTER OF THE PONS In the pons gray matter is found in two situations: (1) In the interstices between the transverse and longitudinal fibers of the pars basilaris pontis, the nuclei pontis; and (2) in the pars dorsalis pontis, where we find the gelatinous gray sheet im mediately beneath the ventricle and the scattered nuclei in the reticular formation.

The nucleus pontis is a mass of gray matter on either side the raphe, containing the bodies of large multipolar nerve cells whose axones run through the brachium pontis of the cere bellum to the cortex on the opposite side. It extends vertically throughout the pons and is continuous with the arcuate nucleus and ponto-bulbar nucleus of the medulla. The nucleus pontis receives the terminals of the descending tracts which form the inner and outer fifths of the basis pedunculi and the inter mediate bundle of the same, and thus connects these tracts with the cerebellum. It forms a relay in the indirect efferent path.

The cells of the nucleus pontis are all emigrants from the rhombic lip of the myelencephalon. Essick has traced their migration during the latter half of the second and all the third and fourth months (Am. Jour. Anat., Vol. 13). This is a superficial migration. It proceeds upward to the level of the root of the trigeminal nerve; there it bends transversely across the pons, marking out the path of the fasciculus obliquus. The growing axons push the cell-bodies along the migrating stream to their destination in the opposite nucleus pontis. Certain cells of opposite polarity are borne along by this active migration a variable distance toward the nucleus pontis; others remain behind in the rhombic lip. The remaining cells of the rhombic lip and the laggard cells that become fixed along the stream by the increasing firmness of the neural tissue, constitute the nucleus ponto bulbaris (corpus ponto-bulbare) described by Esseck in 1907 (Anat. Rec.). The migrating stream, 0.2 mm. wide, flows upward between the facial and intermediate nerves, where the pontcr-bulbar nucleus is found in the mature brain. Through two and a half months the pontine migration

continues. The fibers making up the base of the cerebral peduncle are descending during the same time, hence the basilar part of the pons is built up of alternating layers of longitudinal and transverse fibers. The cells of the nucleus pontis give origin to the transverse fibers, while those of the nucleus ponto-bulbaris originate the fibers of the fasciculus obliquus. Both nuclei receive cerebro-pontal fibers, the former directly from the base of the peduncle and the latter by way of the fasciculus circum-olivaris. To gether the axones of both nuclei form the brachium pontis.

The gray matter of the pars dorsis includes (1) the olivary group of nuclei, viz., the superior olivary nucleus, the nucleus of the corpus trapezoideum, the preolivary nucleus and the semilunar nucleus; (2) the nuclei of the formatio reticularis, viz., the nucleus centralis medius and inferior, and the nucleus lateralis medius; and (3) the nuclei of cerebral nerves— the fifth, sixth and seventh, and a part of the vestibular nucleus of the eighth nerve.

1. Olivary Group.—The superior olivary nucleus (n. olivaris superior) is situated in the lateral part of the formatio reticu laris in the dorsal portion of the corpus trapezoideum (Fig. 114). It lies just ventral to the nucleus of the facial nerve and ventro lateral to the olivary bundle of fibers. The nucleus contains small bodied nerve cells; and in this respect, resembles the olive of the medulla. Its outline is crescentic, convex toward the median line. In size it is microscopic. According to Bruce and Cunningham it is continuous with the nucleus of the lateral fillet. The superior olive constitutes a subordinate relay in the auditory path, receiving fibers from the cochlear nuclei of both sides and contributing fibers to both lateral fillets (Fig. 119).

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