What is the meaning of medulla?

noun

• A white fatty substance that forms a medullary sheath around the axis cylinder of some nerve fibers.
  • synonyms: myelin, myeline
• Lower or hindmost part of the brain; continuous with spinal cord; (`bulb' is an old term for medulla oblongata. )
  • usage: "the medulla oblongata is the most vital part of the brain because it contains centers controlling breathing and heart functioning"
  • synonyms: medulla oblongata, bulb
• The inner part of an organ or structure in plant or animal. .