bone marrow

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

• noun The soft, fatty, vascular tissue that fills most bone cavities and is the source of red blood cells and many white blood cells.

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

• noun anatomy The fatty vascular tissue that fills the cavities of bones; the place where new blood cells are produced.

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

• noun very tender and very nutritious tissue from marrowbones
• noun the fatty network of connective tissue that fills the cavities of bones