Marrow, also called Medulla, a soft, translucent, yellowish or reddish vascular substance, found in the central cavities and in the spongy texture of the bones of man and the higher animals. There are two varieties of marrow, the yellow and the red. The yellow marrow is found principally in the central cavities of the long bones, such as the femur, humerus, and tibia. It consists of a very fine fibrous network, in which are imbedded an abundance of fat vesicles and capillary blood vessels, together with the ramifications of nerves. It is, however, but little or not at all sensitive in its ordinary condition, although when diseased it may, like other parts of the bone, become exceedingly painful. The reddish variety of marrow is found in the interstices of the spongy texture in the extremities of the long bones, in the short and flat bones, and especially in the sternum and the bodies of the vertebra). It is softer in consistency than the preceding, and contains only slight traces of fat. Besides a delicate fibrous tissue, blood vessels, nerves, and an amorphous albuminous matter, it contains, in the bodies of the vertebrae, the sternum, ribs, and cranial bones, small rounded, nucleated cells, the "marrow cells." Marrow does not exist in cartilage, nor in the compact tissue of bone, but is formed in proportion as the medullary cavities are produced by absorption of the original bone.