Long bone interior. Artwork of a cutaway sectioned portion of the upper arm bone (humerus). This is one of the long bones of the skeleton, the ones that 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 where bone marrow is located. The rounded end of a long bone is the epiphysis, which includes the epiphyseal line between it and the shaft (diaphysis).