Medulla

Marrow; pith; hence, essence.

The marrow of bones; the deep or inner portion of an organ or part; as, the medulla, or medullary substance, of the kidney; specifically, the medula oblongata.

A soft tissue, occupying the center of the stem or branch of a plant; pith.

a white fatty substance that forms a medullary sheath around the axis cylinder of some nerve fibers