Medulla, Latin for marrow. Because of its derivation from the Latin medius, middle or core, medulla often implies a deep region within a structure, such as the renal medulla or the medulla of the adrenal gland.
Medulla was the word Vesalius used for the spinal cord, inspired perhaps by the name the Greeks favored: myelos rachites, "marrow of the spine". Indeed, the soft neural tissue of the spinal cord within the vertebral canal of the spinal column does remind one of marrow present in the hollow shaft of a long bone (in the space appropriately called the medullary cavity).
In spite of being part of the brain, it is the spinal cord that's referred to in medulla oblongata. The term was first used by the German anatomist Lorenz Heister in 1740 and it's not clear why: its meaning does not make a lot of sense ("oblong-shaped spinal cord" or as some translate it, "rather long spinal cord") and there had been a reasonable term in use earlier, the medulla prolongata, which does make sense ("spinal cord extension"). Nonetheless, medulla oblongata became established while medulla prolongata disappeared.
Heister's original use of the term was for the entire brain stem. In 1750, the Swiss anatomist Albrecht von Haller restricted the use to its present meaning: the thumb-size segment of brain stem continuous with the spinal cord. (Haller, considered the father of modern physiology by many medical historians, named the Graafian follicle in honor of Dutch anatomist and poet Reinier De Graaf who discovered the ovarian follicles (but thought they were eggs)
Albrecht von Haller (1708-1777) www.ilmyco.gen.chicago.il.us