THE MEDULLA OBLONGATA medulla oblongata is the distal or caudal part of the brain (Figs. 21 and 33). It may be regarded as the expanded intracranial portion of the spinal cord, hence the synonym spinal bulb. It occupies the basilar groove of the occipital bone, posterior to the pons; and is continuous with the spinal cord below the foramen magnum. Dorsally it is in part concealed in the valley of the cerebellum. The vertebral arteries wind forward around it, and form the basilar at its junction with the pons.
medulla is about 2.5 cm. (r in.) long and dorso ventrally is 12-15 mm. thick. Its width at the lower end is 12 mm. (% inch). At the upper extremity it measures from 2-2.5 cm. (o.75-1 inch) in width (Figs. r11 and 112).
Its shape resembles an inverted frustrum of a cone flattened dorso-ventrally at the base. The truncated apex of the frustum, which is nearly circular in outline, is continuous with the spinal cord and the flattened base joins the pons. On the anterior surface a transverse ponto-medullary groove marks the boundary between the medulla and pons. The medulla is a bilateral organ composed of symmetrical halves (Figs. III and 112). In the interior the two halves are united by both gray and white matter in the raphe but on the surface they are partially separated by the anterior and the posterior median fissures (fissura mediana anterior and f. m. posterior).
These fissures are continued through the spinal cord, but neither extends the whole length of the medulla. The anterior median fissure is interrupted in the lower part of the medulla by the crossing of two large tracts of fibers, forming the decus sation of the pyramids; while only through the lower half of the medulla does the posterior median fissure extend. • Origin.—The medulla oblongata is developed from the myel encephalon of the embryo (Figs. 17 and 12o). The myelen cephalic floor and walls thicken and form the greater part of the medulla. Inferiorly, the roof undergoes some thickening but it stretches out into a single layer of epithelium superiorly which is continuous at its upper end with the inferior medullary velum of the cerebellum.
Ventricle.—The common cavity of the posterior brain-vesicle persists in the mature brain as the fourth ventricle (Figs. 18, I 20 and 13o). The fourth is, therefore, the ventricle of the adult rhombencephalon (see p. 263).