What substance are the heads of the tubular bone filled with? What substance is the tubular bone cylinder filled with?

Long bones of various, mainly cylindrical shapes, with a significant ratio of length to thickness, are called tubular bones. For example, the femur, ulna, or humerus. The outer layer of the bones is made up of a special tissue called the periosteum, while the heads are made up of articular cartilage. The heads are filled with a porous substance that contains parts of the blood red bone marrow involved in the generation of red cells (hematopoietic tissue). The body of the bone itself is very light, contains a fatty layer of yellow bone marrow, which does not have a hematopoietic function.