The Collaborative International Dictionary of English v.0.48:

Medulla \Me*dul"la\, n. [L.]
   1. Marrow; pith; hence, essence. [Obs.] --Milton.
      [1913 Webster]

   2. (Anat.) The marrow of bones; the deep or inner portion of
      an organ or part; as, the medulla, or medullary substance,
      of the kidney; specifically, the medula oblongata.
      [1913 Webster]

   3. (Bot.) A soft tissue, occupying the center of the stem or
      branch of a plant; pith.
      [1913 Webster]

   4. See medulla oblongata.
      [PJC]

   Medulla oblongata. [L., oblong medulla] (Anat.), the
      posterior part of the brain connected with the spinal
      cord. It includes all the hindbrain except the cerebellum
      and pons, and from it a large part of the cranial nerves
      arise. It controls very largely respiration, circulation,
      swallowing, and other functions, and is the most vital
      part of the brain; -- called also bulb of the spinal
      cord. See Brain.
      [1913 Webster]

WordNet (r) 3.0 (2006):

medulla
    n 1: a white fatty substance that forms a medullary sheath
         around the axis cylinder of some nerve fibers [syn:
         myelin, myeline, medulla]
    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" [syn: medulla oblongata, medulla, bulb]
    3: the inner part of an organ or structure in plant or animal
       [ant: cortex]