Final answer:
The medullary cavity of bones is filled with marrow, specifically yellow marrow in the diaphysis and red marrow in the epiphyses geared towards hematopoiesis.
Step-by-step explanation:
The substance found in the medullary cavity of bones is marrow. The medullary cavity, running the length of the diaphysis of a long bone, is filled with yellow marrow, which consists mostly of fat. However, the epiphyses of long bones contain spongy bone filled with red marrow. Red marrow is responsible for the process of hematopoiesis, the production of blood cells. Compact bone makes up the dense outer layer of the bone and does not fill the medullary cavity. Instead, it surrounds the cavity and provides strength and protection to the bones.