endosteum

from WordNet (r) 3.0 (2006)

endosteum
    n 1: vascular membrane that lines the inner surface of long
         bones

from The Collaborative International Dictionary of English v.0.48

Endosteum \En*dos"te*um\, n. [NL., fr. Gr. ? + ? a bone.]
   (Anat.)
   The layer of vascular connective tissue lining the medullary
   cavities of bone.
   [1913 Webster]