Humeral bone marrow cavity. Artwork of a sectioned upper arm bone (humerus), one of the long bones of the skeleton. These carry most of the load in the arms and legs. The outer layer is compact bone (cortical bone), with an interior of cancellous bone (spongy bone). The interior of long bones includes the medullary cavity (blood vessel shown) where bone marrow is located. The medullary cavity of long bones contains yellow marrow, with red marrow in each rounded end of the bone (the epiphysis, top). The epiphyseal line lies between the epiphysis and the shaft (diaphysis).