Definitions of medulla:

• noun:   the inner part of an organ or structure in plant or animal
• noun:   lower or hindmost part of the brain; continuous with spinal cord; (`bulb' is an old term for medulla oblongata)

  Example: "The medulla oblongata is the most vital part of the brain because it contains centers controlling breathing and heart functioning"

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