medulla

(noun) a white fatty substance that forms a medullary sheath around the axis cylinder of some nerve fibers. Synonyms: myelin, myeline.

(noun) lower or hindmost part of the brain; continuous with spinal cord; (`bulb' is an old term for medulla oblongata). Synonyms: medulla oblongata, bulb. Example: "The medulla oblongata is the most vital part of the brain because it contains centers controlling breathing and heart functioning."

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