Final answer:
In living adults, the shaft of a long bone, known as the diaphysis, is filled with yellow marrow. The diaphysis contains a medullary cavity surrounded by compact bone, with the ends of the bone, the epiphyses, containing red marrow.
Step-by-step explanation:
In living adults, the shaft of a long bone, also known as the diaphysis, is filled with yellow marrow. This hollow region is referred to as the medullary cavity. While the epiphyses, the ends of the bone, contain red marrow rich in blood cells. The medullary cavity's main function is to store fat, and it also serves as a passageway for nutrient arteries that nourish the bone. The walls around the medullary cavity are made of hard and dense compact bone. The marrow within the diaphysis serves an important biological function of fat storage and can provide some nutrients to the body if needed.