Merriam-Webster's Collegiate® Dictionary, 11th Edition
MARROW
I. noun
Etymology: Middle English marowe, from Old English mearg; akin to Old High German marag marrow, Sanskrit majjan Date: before 12th century
1. bone marrow , the substance of the spinal cord,
2. the choicest of food, the seat of animal vigor, the inmost, best, or essential part ; core , vegetable marrow , marrowy adjective
II. noun
Etymology: Middle English marwe, marrow
Date: 1516
one of a pair