The organic matter in bone is made up mostly of collagen fibers , which help to strengthen the bone. Living bone cells called osteocytes inhabit the spaces between the mineral tubular structures. The structure of bones varies with function. The long bones of the arms and legs, for instance, are mainly composed of hard material called compact bone . These bones contain mineral crystals interspersed with protein fibers.
Compact bone is composed of tubular units called Haversian systems .
A Haversian system consists of a central opening called a Haversian canal , a channel or conduit for blood vessels and nerves, surrounded by a tubular unit of compact bone. An irregular network of mineralized spongy bone is found in the hollow interior of compact bone. Red bone marrow , which manufactures red blood cells, white blood cells, platelets, and antibodies is found in the spongy bone at the ends of the long bones, and in the sternum, ribs, vertebrae, and pelvis. The long bones serve as a reserve source of calcium. A drop in blood calcium level stimulates the parathyroid gland to produce a hormone which causes the release of calcium from the bones to make up the deficit.