Midlands Technical College states that the medullary cavity is a hollow space in the middle of some bones. The medullary cavities hold bone marrow, an important substance that produces blood cells, among other activities. Wikipedia states that this hollow cavity is also called the medulla or marrow cavity in some cases.
Red bone marrow is found in all of the medullary cavities of infant bones, according to Midlands Technical College; however, red bone marrow is only located in flat bones, such as the pelvis, and the enlarged ends of long bones, such as the femur, in adults. Instead, adults carry yellow marrow in the medullary cavity of the long, shaft-like bones in the arms and legs.
Science Daily explains that bone marrow of both types contains numerous blood vessels and capillaries. The yellow bone marrow contains a relatively high percentage of fat, which is what produces the yellow color. Red bone marrow produces red blood cells, platelets and most types of white blood cells. By contrast, yellow marrow produces only a few types of white blood cells.
Science Daily states that adults have about 5.7 pounds of bone marrow in their bodies. Approximately half of this marrow is red, while the other half is yellow.