medulla

noun

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

   noun.substance

   Synonyms myelin myeline

   Source: WordNet 3.0

2. lower or hindmost part of the brain; continuous with spinal cord; (`bulb' is an old term for medulla oblongata)

   noun.body

   Examples

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

   Synonyms bulb medulla oblongata

   Source: WordNet 3.0

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

   noun.body

   Antonyms cortex

   Source: WordNet 3.0

4. Marrow; pith; hence, essence.

   noun

   Source: webster1913