* The lower portion of the brainstem located between the pons and brainstem. This structure contains several descending and ascending tracts, lower cranial nerve nuclei, a significant proportion of the reticular system of the brainstem and other structures. (NCI) * caudal subdivision of the hindbrain interposed between the spinal cord and the pons; adult derivative of the myelencephalon, the most caudal subdivision of the developing brain; it has two subdivisions the open medulla bordering on the fourth ventricle and the closed medulla which surrounds the rostral part of the central canal. (CSP)