Quia in medio ossis. Marrow; ax-ungia de mumia. In anatomy it hath various significations. The white substance of the brain is called medulla, or the medullary part, to distinguish it from the brown or cortical. The continuation of the brain in the spine is distinguished by the epithet spinalis; but medulla strictly means marrow in the bones.
If the marrow be viewed through a microscope, it seems a mass of small globules united like those in the roe of a herring. The distinction between medulla and succus medullaris is useless, for the marrow in the living body is always fluid. The membrana medullae not only lines the internal surface of the bones, but divides the vesicles, or membranous bags, containing the marrow; these very fine minute vessels from which the marrow is secreted are dispersed on these membranes, and are branches from the artery, which enters the bone by its appropriate aperture. The use of the marrow is said to be rendering the bones flexible, for it was with little reason supposed they would soon otherwise become brittle, as happens in syphilitic and scorbutic habits, where it is apparently separated in too small a quantity, too quickly absorbed, or diseased. The membrana medullae is furnished with a nerve which enters with the artery, and with an accompanying vein. The small vessels which secrete the marrow are more than usually diminished by advancing age; and we thus find the marrow bloody in the earlier periods, oily in the middle stages of life, and watery in old age.
The marrow is never sensible but in a diseased state, and is usually absorbed with the rest of the fat in dropsies. In the foetus the bones contain albumen chiefly; and in birds the proportion of marrow is inconsiderable, for the cavities chiefly contain air. Marrow of animals is prescribed in some compositions, but it has no superior efficacy to other fats.