What is contained within the medullary (marrow) cavity?

The medullary cavity, found within the long bones of the body, primarily contains bone marrow, which plays a crucial role in the production of blood cells. In adults, this cavity is typically filled with yellow bone marrow, which is rich in fat cells and serves as an energy reserve. In contrast, in children, red bone marrow is more prevalent, responsible for hematopoiesis, or the formation of red blood cells.

While cartilage, nerves, and blood vessels can be associated with bone structure and function, they are not the primary contents of the medullary cavity. Cartilage is found on the surfaces of bones at joints, nerves are located in the periosteum and surrounding tissues, and blood vessels are present in the area to supply nutrients and remove waste, but they do not fill the medullary cavity itself. Thus, bone marrow is the correct answer as it is the key element within the medullary cavity's structure and function.