The medullary cavity is the cavity within the diaphysis that houses what?

• A. The space within the joint cavity
• B. A cartilage-lined channel in the bone
• C. The outer layer surrounding the bone
• D. The cavity within the diaphysis that houses bone marrow

Answer: (D)

The medullary cavity is the hollow space inside the shaft (diaphysis) of a long bone. Its main job is to house bone marrow, the soft tissue that produces blood cells in red marrow (more common in children) and stores fat in yellow marrow (more common in adults). This cavity is lined by endosteum and connects to the bone’s vascular system. The other descriptions refer to different structures: the joint cavity is the synovial space, a cartilage-lined channel would be a canal or growth-related passage, and the outer layer around bone is the periosteum. So, the medullary cavity specifically houses bone marrow.