Home >> Anatomy Of The Brain >> Sensory Nerves And Sensory to White Matter Of The >> White Matter of Medulla_P1

White Matter of Medulla

fibers, arcuate, external, anterior, tract, pyramidal, fillet and restiform

Page: 1 2 3 4 5

WHITE MATTER OF MEDULLA The medulla is made up of white and gray matter which to gether bound ventrally and laterally the inferior part of the fourth ventricle and surround the upper extremity of the central canal which is continuous with that of the spinal cord.

For the most part the white matter of the medulla is con tinuous with the longitudinal fibers of the pons and restiform bodies above and with the spinal cord below; the bulbar roots of the eighth to the twelfth cerebral nerves and many de cussating or commissural fibers of the reticular substance are also included in the white substance.

Substantia Reticularis.—Superficially, the white matter is collected into great bundles of fibers, such as the pyramids, lateral column and restiform body; but, in the deep parts of the medulla, the white matter enters into a great network called the substantia reticularis, which has gray matter in its meshes (Figs. 123 and 125). It is continuous above with the reticular formation of the pons and below with the fasciculi proprii of the spinal cord. The substantia reticularis contains many scattered fibers, processes of its intrinsic neurones, which form a frequently interrupted and, for the most part, a crossed ascending and descending tract. Transverse and oblique fibers are likewise numerous in the reticular substance. They are chiefly the arcuate fibers. The distinct tracts of longitudi nal fibers contained in it will be noticed later (p. 3 27). The gray substance of the substantia reticularis is composed of the nucleus lateralis inferior, the eighth to the twelfth cerebral nerve nuclei and the inferior olivary nuclei.

Raphe (Figs. 123 and 125) .—The raphe is, primarily, a sagittal lamina of neuroglia derived from the floor-plate of the myelen cephalon. It lies in the median plane and joins the lateral halves of the medulla together. It is very distinct in the superior part of the medulla. Below the level of the olive, it is entirely obliterated by the fillet and pyramidal decussations, The raphe is pierced transversely by decussating and commis sural fibers and is traversed dorso-ventralward by the anterior external arcuate fibers.

The white matter of the medulla is divided by the direction of its fibers into three classes or systems: (r) Transverse fibers; (2) dorso-ventral fibers; and (3) longitudinal fibers.

1. The transverse fibers of the medulla are really more or less oblique in direction and most of them are arched. They

include the fibers of the following: (a) The pyramidal de cussation (decussatio pyramidum) with the decussating fibers of the pyramidal tract to motor nuclei of the medulla; (b) the fillet decussation (decussatio lemniscorum) and decussating afferent fibers from terminal nuclei of the medulla to the fillets, spino-thalamic tract and medial longitudinal bundle; (c) corn missural fibers connecting bulbar nuclei; (d) anterior external arcuate fibers; (e) olivo-cerebellar fibers, and (f) a few reticulo spinal fibers from the lateral nucleus of the medulla. The pyramidal decussation (Fig. 126) is located in the lower half of the medulla. About four-fifths of the pyramid crosses over through it and becomes the lateral pyramidal tract of the spinal cord. The lower level of .the pyramidal decussation marks the boundary between medulla and cord. The medial fillet, the anterior and the posterior external arcuate fibers all rise in the nucleus funiculi gracilis and nucleus funiculi cuneati. The two former run forward and cross over in the middle of the • medulla, forming the fillet decussation (decussatio lemniscorum), after which they separate (Fig. 125). The medial fillet bends upward and ascends between the inferior olives, hence its name in the medulla, stratum interolivare lemnisci. The anterior external arcuate fibers, continuing their ventral direction, issue from the anterior median fissure and anterior surface of the medulla (Fig. 124), then arch backward around the medulla to the restiform body, through which they enter the cerebellum. The arcuate nucleus makes a small contribution to the anterior external arcuate fibers which probably belongs to the cerebro ponto-cerebellar system. The posterior external arcuate fibers run through the restiform body to the cerebellum without de cussating. The external arcuate fibers, which rise with the medial fillet, conduct impulses of the tactile and muscular senses. The olivo-cerebellar tract is a large one (Fig. 123). It comprises many of the internal arcuate fibers of the medulla. Its origin is in the inferior olive of the medulla; decussating it pierces the opposite olive and continues through the restiform body to the cerebellar cortex of the vermis, being reinforced by a few fibers from the olive of the same side. The tract is visible to the naked eye as it enters into the lateral part of the restiform body.

Page: 1 2 3 4 5