Bone structure. Artwork showing the structure of an adult long bone. At top the bone is covered by articular cartilage (blue). The outer layer is compact, or cortical, bone, which contains blood (red) and lymph vessels and nerves. Within the bone shaft is the medullary cavity and a core of yellow bone marrow, which stores fat. The head of the bone contains spongy, or cancellous, bone, which is characterised by a honeycomb arrangement. Within the spaces is red bone marrow, the site of blood cell production.