n. In anatomy and zoology: Marrow. [Little used.]n. The so-called spinal marrow; the spinal cord, or central axis of the nervous system; the myelon: more fully called medulla spinalis.n. The hindmost segment of the brain, continuous with the spinal cord; the afterbrain or metencephalon; the oblongata: more fully called medulla oblongata.n. The ventral ganglionic chain of the nervous system of some invertebrates, as Vermes, supposed to be analogous to the spinal cord of vertebrates.n. The pith of a hair.n. The myelin, or white and fatty covering of the axis-cylinder of a nerve.n. In botany, the pith of plants.n. The semi-fluid endosarc of a protozoön.