Osteons, or Haversian systems, are the units of structure in mature bone. They are tightly arranged running generally parallel to the long axis of the bone. At the center of each is an Haversian canal which carries blood vessels and nerves. Canaliculi (small canals) connect the Haversian canals with lacunae containing the osteocytes. Osteocytes extend processes into the canaliculi and receive nutrients and O2 and get rid of wastes and CO2 by diffusion through the canaliculi. The lacunae and canaliculi form lamellae or layers of two types: concentric lamellae form circular rings around each Haversian canal, and interstitial lamellae, derived from previous osteons, fill in the spaces between existing osteons.