Section through a human femur (thigh bone). Adult long bones have an outer layer of compact, or cortical, bone that contains blood and lymph vessels and nerves. Within the bone shaft is the medullary cavity and a core of bone marrow (removed here), which stores fat. The ends of the bone contain spongy, or cancellous, bone, which is characterised by a honeycomb arrangement. Within the spaces is red bone marrow (not seen), the site of blood cell production.