What is the central cavity within the diaphysis of a long bone that stores bone marrow called?

• A. Epiphysis
• B. Periosteum
• C. Endosteum
• D. Medullary cavity

Answer: (D)

Internal spaces inside the shaft of a long bone are organized to store marrow, and the central cavity in the diaphysis is called the medullary cavity. This cavity runs through the hollow shaft and is the storage site for bone marrow, lined internally by endosteum. In adults, it typically contains yellow marrow made mostly of fat, while red marrow is found in the ends of bones and in certain bones during development. The ends of the bone are the epiphyses, which are separate from this cavity; the periosteum is the outer membrane wrapping the bone, and the endosteum lines the inner surfaces, including the medullary cavity. So the cavity within the diaphysis that stores bone marrow is the medullary cavity.