medulla

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The inner core of certain organs or body structures, such as the marrow of bone.
• noun The medulla oblongata.
• noun The pith in the stems or roots of certain plants.
• noun The central portion of a thallus in certain lichens and red or brown algae.

from The Century Dictionary.

• noun In anatomy and zoology: Marrow. [Little used.]
• noun The so-called spinal marrow; the spinal cord, or central axis of the nervous system; the myelon: more fully called medulla spinalis.
• noun The hindmost segment of the brain, continuous with the spinal cord; the afterbrain or metencephalon; the oblongata: more fully called medulla oblongata.
• noun The ventral ganglionic chain of the nervous system of some invertebrates, as Vermes, supposed to be analogous to the spinal cord of vertebrates.
• noun The pith of a hair.
• noun The myelin, or white and fatty covering of the axis-cylinder of a nerve.
• noun In botany, the pith of plants.
• noun The semi-fluid endosarc of a protozoön.

from the GNU version of the Collaborative International Dictionary of English.

• noun obsolete Marrow; pith; hence, essence.
• noun (Anat.) The marrow of bones; the deep or inner portion of an organ or part; ; specifically, the medula oblongata.
• noun (Bot.) A soft tissue, occupying the center of the stem or branch of a plant; pith.
• noun See medulla oblongata.
• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The soft inner part of something, especially the pith of a fruit.
• noun anatomy The inner substance of various organs and structures, especially the marrow of bones.
• noun anatomy The medulla oblongata.
• noun botany The internal tissue of a plant.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the inner part of an organ or structure in plant or animal
• noun a white fatty substance that forms a medullary sheath around the axis cylinder of some nerve fibers
• noun lower or hindmost part of the brain; continuous with spinal cord; (`bulb' is an old term for medulla oblongata)