MEDULLA OBLONGATA.
The upper boundary of the medulla oblongata is marked ventrally by the inferior edge of the pons and dorsally by the striae acusticae in the floor of the fourth ventricle ; the lower boundary is indicated by the attachment of the upper root-bundles of the first cervical nerves, or, ventrally, by the lower limit of the pyramidal decussation.
Let us first examine the ventral surface of the medulla (Fig. 75). In the mid line runs the fissura median anterior, which is prolonged into the fissure of the spinal cord bearing the same name, but separated from it by the crossing fibre-bundles of the pyramidal decussation, decussatio pyramidum. Toward the lower edge of the pons, the fissure widens into a small depression, the foramen caecum. On both sides the median fissure is bordered by the pyramid, a slightly convex tapering column, broad above and narrow toward the spinal cord, which appears to pass into the anterior column of the cord. Only a small part of the pyramidal fibres, however, actually maintains a course along the anterior median fissure in the anterior column of the spinal cord, since the greater part crosses the mid-line in the decussatio pyramidum and continues within the lateral column of the cord of the opposite side. The part which continues within the anterior column is known as the anterior pyramidal tract, that within the opposite lateral column as the lateral pyramidal tract. These will receive more detailed attention in the consideration of the fibre-paths (page 161).
The pyramid is bounded on the outer side by the sulcus lateralis anterior, from which emerge the root-bundles of the hypoglossal nerve. Lateral to the sulcus lateralis and adjoining the pyramid is seen the oliva, an ovoid eminence whose thicker end reaches as far as the pons and which narrows below. The sulcus lateralis anterior may be marked, especially in its lower part, by transversely arching strands of fibres, known as the fibrae arcuatae. • Turning now to the dorsal aspect of the medulla (Fig. 83), we note, in the lower part, the sulcus medianus posterior, which above is soon closed by a thin medullary sheet, the obex. At this point, beneath the obex, the central canal of the cord opens into the fourth ventricle. Lateral to the sulcus medianus, next comes the sulcus inter medius posterior, which in the upper part of the medulla runs laterally and then disappears.
Farther outward is the less distinct sulcus lakralis posterior, which likewise turns out ward and may be followed to about the level of the middle of the olive. Between the median and lateral posterior sulci, the posterior column, funiculus posterior, represents the upward prolongation of the corresponding column of the spinal cord. By means of the sulcus intermedius posterior the funiculus is subdivided into two special tracts. On each side of the posterior median fissure, between the latter and the posterior interme diate fissure, lies the fasciculus gracilis, or Golf's column, continued upward from the cord. In the upper part it broadens into the claret and then, again narrowing, proceeds laterally and upward. Between the lateral and intermediate posterior sulci runs the fasciculus cuneatus, the upward prolongation of Burdach's column, which at the level of the clava expands into the tuberculum cuneatum and higher up also bends outward. Lateral to the sulcus lateralis posterior, between it and the sulcus lateralis anterior, the lateral column, funiculus lateralis, ascends from the spinal cord. After reaching the lower end of the olive, the column passes laterally and dorsally, close to the olive, almost as far as the pons. It is separated into a dorsal and a ventral part by a slight furrow, along which emerge the delicate root-fibres of the accessory, vagus and glossopharyngeal nerves. The dorsal part of the funiculus lateralis broadens above and, in the region behind the tuberculum cuneatum, swells into the tuberculum cinereum. Farther above, it passes laterally in company with the upper ends of the column of Goll and of ,Burdach. These upward and laterally directed portions of the column of Goll and of Burdach and the dorsal segment of the funiculus lateralis collectively constitute the corpus restiforme or inferior cerebellar peduncle, also called the crus cerebelli ad medullam oblongatam, that passes to the cerebellum. Medially, the corpus restiforme borders the lateral margin of the fourth ventricle. The fossa rhomboidea, which forms the floor of the fourth ventricle, overlies the dorsal surface of the preceding parts.