The Medulla: gross anatomy
The medulla is broad above, where it joins the pons; and narrows down below, where it becomes continuous with the spinal cord. Its length is about 3 cm and its width is about 2 cm at its upper end.
The junction of the medulla and cord is usually described as lying at the level of the upper border of the atlas vertebra. The transition is, in fact, not abrupt but occurs over a certain distance. The medulla is divided into a lower closed part, which surrounds the central canal; and an upper open part, which is related to the lower part of the fourth ventricle. The surface of the medulla is marked by a series of fissures or sulci that divide it into a number of regions. The anterior median fissure and the posterior median sulcus are upward continuations of the corresponding features seen on the spinal cord. On each side the anterolateral sulcus lies in line with the ventral roots of spinal nerves. The rootlets of the hypoglossal nerve emerge from this sulcus. The posterolateral sulcuslies in line with the dorsal nerve roots of spinal nerves, and gives attachment to rootlets of the glossopharyngeal, vagus and accessory nerves. The region between the anterior median sulcus and the anterolateral sulcus is occupied (on either side of the midline) by an elevation called the pyramid. The elevation is caused by a large bundle of fibres that descend from the cerebral cortex to the spinal cord. Some of these fibres cross from one side to the other in the lower part of the medulla, obliterating the anterior median fissure. These crossing fibres constitute the decussation of the pyramids.
Some other fibres emerge from the anterior median fissure, above the decussation, and wind laterally over the surface of the medulla. These are the anterior external arcuate fibres. In the upper part of the medulla, the region between the anterolateral and posterolateral sulci shows a prominent, elongated, oval swelling named the olive. This swelling is about half an inch long. It is produced by a large mass of grey matter called the inferior olivary nucleus. The posterior part of the medulla, between the posterior median sulcus and the posterolateral sulcus, contains tracts that enter it from the posterior funiculus of the spinal cord. These are thefasciculus gracilis lying medially, next to the midline, and the fasciculus cuneatus lying laterally. These fasciculi end in rounded elevations called the gracile and cuneate tubercles. These tubercles are produced by masses of grey matter called the nucleus gracilisand the nucleus cuneatus respectively. Just above these tubercles the posterior aspect of the medulla is occupied by a triangular fossa which forms the lower part of the floor of the fourth ventricle.
This fossa is bounded on either side by the inferior cerebellar peduncle. The lower part of the medulla, immediately lateral to the fasciculus cuneatus, is marked by another longitudinal elevation called the tuberculum cinereum. This elevation is produced by an underlying collection of grey matter called the spinal nucleus of the trigeminal nerve. The grey matter of this nucleus is covered by a layer of nerve fibres that form the spinal tract of the trigeminalnerve.
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