The brain stem consists of the medulla oblongata (or medulla), the pons and the midbrain. The three brain areas each contain cranial nerve nuclei, and the fourth ventricle lies partly in the pons and partly in the medulla. The brain stem may occasionally be referred to as the ‘bulb’ in such terms as the ‘corticobulbar’ tract.
The medulla is around 3 cm long in adult humans and widens rostrally. It is continuous with the spinal cord from just belowthe foramen magnum, at the level of the upper rootlet of the first cranial nerve, and extends through to the lower (caudal) border of the pons. The medulla lies on the basilar part of the occipital bone, and is obscured from view by the cerebellum. Externally, the spinal cord and medulla appear to merge imperceptibly, but internal examination reveals extensive reorganization of white and gray matter at the junction. In the medulla the central canal widens into the fourth ventricle.
From the ventral aspect, the central median fissure appears as a central groove, which is a continuation of that of the spinal cord. The progress of the fissure is interrupted by the decussation (crossing over) of the fiber tracts of the corticospinal tract, where they cross over at the pyramid of the medulla to form the lateral corticospinal tract. Lateral to the pyramids on each side is the olive, made up of a convoluted mass of gray matter called the inferior olivary nucleus. The olive is separated from the pyramids by the rootlets of the hypoglossal nerve (XII). Rootlets of the vagus (X) and the cranial accessory (XI) nerves arise lateral to the olive, the latter two being united with the spinal accessory nerve (XI). The facial (VII) and vestibulocochlear (VIII) nerves arise at the border between the lateral medulla and the pons.
The pons is about 2.5 cm in length. Its name is Latin for ‘bridge’, since it appears to connect the cerebellar emispheres though this is not actually the case. Ventrally, the pons is a sort of relay station, where cerebral cortex fibers terminate ipsilaterally on pontine nuclei, whose axons become the contralateral middle cerebellar peduncles. Thus the ventral (or basal) pons is a sort of massive synaptic junction that connects each cerebral hemisphere with the contralateral cerebellar hemisphere. Functionally, this system maximizes efficiency of voluntary movement.
The ventral surface of the midbrain extends rostrally from the pons to the mamillary bodies, which mark the caudal border of the diencephalon. On either side are prominent swellings called the crus cerebri (basis pedunculi). These are made up of the fiber tracts of the descending pyramidal motor system, and fibers from the cortex to the pons (corticopontine fibers). Although not shown here, the midbrain is penetrated by several small blood vessels in the floor of the interpeduncular fossa, and the area has been named the posterior perforated substance because of these blood vessels. The oculomotor nerve (III) to the eye leaves the brain through the cavernous venous sinus from each side of the interpeduncular fossa. The optic chiasm and optic nerves, together with the diencephalic tuber cinereum are exposed on the ventral surface.