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A soft reddish or yellowish substance filling the medullary cavities and canals of bones. In the medullary cavity of the long bones it is yellow (yellow marrow), and is composed of fat cells supported by retiform tissue. In the articular ends of the long bones and in the other bones the fat cells are fewer, but numerous rounded nucleated cells are present, and smaller reddish cells like the primitive nucleated corpuscles of the embryo, which have been regarded as transition forms between marrow cells and red blood corpuscles (red marrow). Various preparations of m. have been used in anemia.