Origins
The word 'marrow' descends from Old English 'mearg' or 'mearh,' meaning the soft tissue within bones. The Old English form comes from Proto-Germanic *mazgą, with cognates in German 'Mark' (marrow, pith), Dutch 'merg' (marrow), Old Norse 'mergr' (marrow), and Gothic 'mazgs.' The Proto-Germanic word derives from PIE *mosgʰo-, meaning 'marrow' or 'brain' — a root that reveals something fundamental about how ancient peoples understood the body.
The PIE root *mosgʰo- made no distinction between bone marrow and brain tissue. Both were understood as the same substance: the soft, vital, fatty matter enclosed within hard protective structures (bones and skull). This conceptual unity is preserved in the divergent meanings of the root's descendants. In the Germanic languages, the word retained the bone-marrow meaning: English 'marrow,' German 'Mark,' Dutch 'merg.' In the Slavic languages, the word shifted to mean exclusively 'brain': Russian 'mozg' (мозг, brain), Polish 'mózg' (brain), Czech 'mozek' (brain). Sanskrit 'majján-' (marrow) preserves the older meaning. The fact that English 'marrow' and Russian 'brain' are the same word — separated by thousands of years of semantic drift — is one of comparative linguistics' most striking demonstrations.
Bone marrow is one of the most physiologically important tissues in the body. Red bone marrow (found in flat bones, vertebrae, and the ends of long bones) is the site of hematopoiesis — the production of all blood cells: red blood cells, white blood cells, and platelets. An adult human produces approximately 200 billion red blood cells per day, all manufactured in the bone marrow. Yellow bone marrow, found in the shafts of long bones, consists primarily of fat cells and serves as an energy reserve.
Figurative Development
The figurative use of 'marrow' to mean 'the essence' or 'the innermost vital part' dates from the fourteenth century. 'To the marrow' means 'to the very core' — 'chilled to the marrow,' 'rotten to the marrow,' 'English to the marrow.' The metaphor is powerful: marrow is the most hidden, most protected, most essential substance in the body, encased in bone.
Bone marrow as food has been valued since prehistory. Archaeological evidence shows that early hominids cracked open animal bones to extract marrow over two million years ago — making marrow one of the oldest foods in the human diet. The calorie-dense fat of bone marrow may have played a crucial role in the evolution of the large human brain. In modern cuisine, roasted bone marrow has experienced a revival as a luxury dish, served on toast or as a sauce enrichment.
Bone marrow transplantation (more precisely, hematopoietic stem cell transplantation) is one of modern medicine's most important procedures. First performed successfully in 1956 by E. Donnall Thomas, bone marrow transplantation can cure leukemia, lymphoma, aplastic anemia, and certain genetic disorders. The procedure involves replacing a patient's diseased marrow with healthy donor marrow — effectively replacing one person's blood-manufacturing system with another's.
Legacy
In British English, 'marrow' also refers to a large green squash (Cucurbita pepo) — the vegetable marrow — so called because its soft, white flesh was thought to resemble bone marrow. This usage dates from the early nineteenth century. The 'marrowfat pea' is similarly named for its large, round, 'marrow-like' seeds.