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Medula Oblongata Surfaces

lateral, anterior, medulla, surface, posterior, sulcus and roots

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MEDULA OBLONGATA SURFACES The medulla oblongata presents four surfaces: The anterior, posterior and two lateral, separated by the anterior lateral and posterior lateral grooves. In the upper medulla the surfaces are clearly defined, but they become less distinct as they de scend to the inferior and nearly circular extremity (Figs. 112 and 124).

The anterior lateral sulcus (s. lateralis anterior) separates the anterior from the lateral surface, and is in line with the ex its of the anterior roots of the spinal nerves. No corresponding groove exists in the cord. From the anterior lateral groove issue the roots of the hypoglossal nerve and the anterior root of the first cervical nerve. The abducent (or sixth) nerve emerges nearly in line with it from the transverse groove be tween the pons and the medulla (Fig. 11 1).

Posterior Lateral Sulcus (s. lateralis posterior).—The posterior lateral sulcus of the medulla separates the lateral from the posterior surface (Figs. 112 and 113). It descends between the olive and the restiform body and is continued through the spinal cord. Through this sulcus into the cord run the posterior roots of the spinal nerves and likewise the sensory roots of the vagus and glossopharyngeal nerves run through it into the medulla; while the motor roots of the ninth and tenth and the cerebral root of the eleventh nerve emerge from the medulla through the posterior lateral sulcus. The roots of the seventh, eighth and intermediate nerves are found at the superior end of the sulcus in the transverse groove between the medulla and pons. The posterior lateral sulcus is not parallel with the axis of the medulla, but bends outward and forward as it ascends. Inferiorly it is obliterated for a short distance by the crossing of the dorsal fasciculus spino-cerebeliaris (direct cerebellar tract) form the lateral to the posterior surface.

The anterior surface (facies anterior) of the medulla, bounded on either side by the anterior lateral sulcus, extends from the transverse sulcus below the pons down to the spinal cord (Fig. II r). It is made up of symmetrical halves united below by the decussation of the lateral (crossed) pyramidal tracts but separated above by the anterior median fissure which termi nates at the inferior end of the pons in a blind foramen (foramen cecum of Vicq d'Azyr). On either side of the median fissure

the anterior surface presents a fusiform eminence, most promi nent near the pons, called the pyramid. The pyramidal tract, which we have already traced through the internal capsule, basis pedunculi and pons, forms the pyramid of the medulla. In the lower part of the medulla the pyramid divides into two tracts, viz., the lateral (or crossed) pyramidal tract and the anterior (or direct) pyramidal tract the former comprising about the medial four-fifths and the anterior pyramidal tract the lateral one-fifth of the pyramid. Frequently we see a small bundle of fibers (cerebropontal fibers) diverge from the pyramid near its middle and, winding backward below the olive, ascend behind it along the restiform body to the ponto-bulbar nucleus. That is the fasciculus circum-olivaris. It is not aways discernible, as it may be submerged. Transverse fibers, called the anterior external arcuate, are also seen crossing the pyramid from within outward. They form a more or less continuous sheet of fibers, which emerges from the anterior median fissure and winds around the medulla to the posterior surface, where its fibers enter the restiform body. The anterior surface is identical with the surface of the two anterior columns of the medulla.

Lateral Surface (facies lateralis, Figs. 107 and rir).—There are two lateral surfaces, a right and a left. Each is bounded by the anterior lateral and the posterior lateral sulcus and is inclosed between the roots of the hypoglossal nerve, ventrally, and those of the ninth, tenth and the cerebral portion of the eleventh, dorsally. Lateral surface is synonymous with the surface of the lateral column. The lateral surface is formed above by the olive, below by the tracts of the lateral column and winding backward over both are the anterior external arcuate fibers.

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