In general, bone marrow is found in bones. In long tubular bones of the limbs, there is a hollow cavity in the middle of the bone, called the medullary cavity, and the bone marrow is distributed in the medullary cavity, which is not very tightly integrated with the surrounding bone. In the case of irregular bones such as flat bones, there is no marrow cavity, and there is often a loose mesh of bone, and the bone marrow can be distributed in these loose bones. During infancy, the bone marrow has hematopoietic function and can have a strong differentiation capacity of blood cells. As growth and development proceed, the bone marrow is gradually filled and replaced by fatty tissue, and yellow marrow appears. When the body is stimulated by the corresponding ischemia, the yellow bone marrow can be transformed into red bone marrow and continue its hematopoietic function.