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

Similar word(s): myelin, myeline

Definition categories: substance, fat

2. 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

Similar word(s): bulb

Definition categories: body

3. the inner part of an organ or structure in plant or animal

Definition categories: body