<Tr> <Td_colspan="2"> Anatomical terminology (edit on Wikidata) </Td> </Tr> <P> The osteon or haversian system / həˈvɜːr. ʒən / (named for Clopton Havers) is the fundamental functional unit of much compact bone . Osteons are roughly cylindrical structures that are typically several millimeters long and around 0.2 mm in diameter . They are present in many bones of most mammals and some bird, reptile, and amphibian species . </P> <P> Each osteon consists of concentric layers, or lamellae, of compact bone tissue that surround a central canal, the haversian canal . The haversian canal contains the bone's blood supplies . The boundary of an osteon is the cement line . </P> <P> Each haversian canal is surrounded by varying number (5 - 20) of concentrically arranged lamellae of bone matrix . Near the surface of the compact bone, the lamellae are arranged parallel to the surface; these are called circumferential lamellae . Some of the osteoblasts develop into osteocytes, each living within its own small space, or lacuna . Osteocytes make contact with the cytoplasmic processes of their counterparts via a network of small transverse canals, or canaliculi . This network facilitates the exchange of nutrients and metabolic waste . Collagen fibers in a particular lamella run parallel to each other, but the orientation of collagen fibers within other lamellae is oblique . The collagen fiber density is lowest at the seams between lamellae, accounting for the distinctive microscopic appearance of a transverse section of osteons . The space between osteons is occupied by interstitial lamellae, which are the remnants of osteons that were partially resorbed during the process of bone remodeling . </P>

The central canal of an osteon contains what