Meaning

noun a white fatty substance that forms a medullary sheath around the axis cylinder of some nerve fibers

noun lower or hindmost part of the brain; continuous with spinal cord; (`bulb' is an old term for medulla oblongata)

noun the inner part of an organ or structure in plant or animal

originBorrowed from Latin medulla (“pith, marrow”), perhaps from medius (“middle”).

Etymology from Wiktionary, CC BY-SA 3.0