Long bone structure. Cutaway computer illustration showing the structure of a long bone. Long bones are those that are longer than they are wide and. They support the body and protect the major organs. The outside of the bone consists of a layer of connective tissue called the periosteum. The outer shell is composed of cortical (or compact) bone, surrounding a layer of cancellous (or trabecular) bone (spongy, inset), which contains red bone marrow. The interior of the bone is the medullary cavity, with the inner core being composed of yellow marrow in adults and red marrow in young children.