The medulla oblongata
This conical part of the brain extends from the pons to the spinal medulla, which it joins at the foramen magnum. It is approximately 2.5 cm long, and was once known as the bulb, a term which is occasionally used in some clinical conditions, e.g bulbar paralysis.The anterior surface of the medulla oblongata lies against the basilar part of the occipital bone, while the posterior surface is lodged in a groove on the anterior surface of the cerebellum, the vallecula cerebelli. The inferior half is tunnelled by the narrow central canal, which ascends through it from the spinal medulla, and opens out into the fourth ventricle on the posterior surface of the superior half. The medulla oblongata is partially divided by an anterior median fissure which is continuous with that of the spinal medulla, but is interrupted in the lower medulla oblongata, where bundles of fibres from the pyramids cross the midline through it, interdigitating with each other (decussation of the pyramids).
Olive
Posterolateral to the pyramid, and separated from it by the rootlets of the hypoglossal nerve, is the oval elevation of the olive. It is produced by the olivary nucleus , the main part of which has the shape of a crumpled bag of grey matter with its open mouth facing medially. Its efferent fibres emerge from this mouth and pass to the opposite half of the cerebellum via the inferior cerebellar peduncle. The surface of the olive is often crossed by a number of bundles of nerve fibres, passing posteriorly from the region of the anteromedian fissure towards the inferior cerebellar peduncle. These are the anterior external arcuate fibres, and they arise in the arcuate nucleus which lies on the pyramid. This nucleus and the fibres represent an inferior extension of the pontine nuclei and the pontocerebellar fibres which arise from them. Similar fibres arise in the arcuat nucleus, pass posteriroly through the midline of the medulla oblongata, and run across the floor of the fourth ventricle to the cerebellum forming the stiae medullares of the fourth ventricle, which will be seen later.