The medulla oblongata is the most inferior part of the brainstem. It lies in the posterior cranial fossa facing the clivus of the occipital bone. It is cone-shaped with a truncated apex and a base facing the pons.
The ventral surface of the medulla oblongata contains five grooves lying towards the clivus:
- Anterior median fissure - in the middle of the surface;
- Anterolateral sulcus (2) - exit point for the hypoglossal nerve (CN XII);
- Posterolateral sulcus (2) - exit point for the glossopharyngeal (CN IX), vagus (CN X) and accessory (CN XI) nerves.
The ventral surface presents two paired prominences:
- Pyramid (2) - located between the anterior median fissure and the anterolateral sulcus; the pyramids contain white matter in a form of the corticospinal tract; the fibers of the corticospinal tract form a pyramidal decussation (where the fibers partly cross over to the other side) below the pyramids - near the junction of the medulla and spinal cord;
- Olive (2) - located between the anterolateral and posterolateral sulci; the olives contain the superior and inferior olivary nuclei.
The dorsal surface of the medulla oblongata takes part in forming the rhomboid fossa and is facing the fourth ventricle. Also the dorsal surface contains several sulci:
- Posterior median sulcus - ends midway in the medulla oblongata where the central canal from the spinal cord widens into the fourth ventricle; inferiorly - continuous with the posterior median sulcus of the spinal cord;
- Posterior intermediate sulcus (2) - each (right and left) posterior intermediate sulcus separates the fasciculus gracilis and fasciculus cuneatus (read below).
Apart from these grooves, the dorsal surface has two paired structures referred to as fascicles containing sensory fibers:
- Gracile fascicle or fasciculus gracilis (2) - lies between the posterior median sulcus and posterior intermediate sulcus; it ends with an enlargement called the gracile tubercle containing the gracile nucleus;
- Cuneate fascicle or fasciculus cuneatus (2) - lies between the posterior intermediate sulcus and posterolateral sulcus; it also ends with an enlargement - cuneate tubercle, which contains the cuneate nucleus.
Lateral to the cuneate nucleus is the spinal nucleus of the trigeminal nerve (CN V) that forms the prominence named the trigeminal tubercle.