1. (

• myelin ,
• myeline ,
• medulla

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

2. (

• medulla_oblongata ,
• medulla ,
• bulb ,

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

3. (

• medulla

) the inner part of an organ or structure in plant or animal ;