髄 means 'marrow'

All four kanji represent centers—with slight differences. An axis (軸) lies inside something (a planet, feather, or scroll), as does marrow (髄) and a core (髄 or 芯). Other objects (e.g., doors) revolve around pivots (枢). The body contains specialized brain centers, as well as an axis, marrow, and a core. More abstractly, an axis occupies the center of one's existence. Also, one feels things to the core, the essence of an activity (e.g., aikido) is vitally important, and 枢 helps symbolize the center of the economy, of power, and more.