Is it ensure, insure, or assure?
haversian system n.
the chief structural unit of compact (cortical) bone, consisting of concentric bone layers called lamellae, which surround a long hollow passageway, the haversian canal (named for Clopton Havers, a 17th-century English physician). The haversian canal contains small blood vessels responsible for the blood supply to osteocytes (individual bone cells). Osteons are several millimetres long and about 0.2 millimetre (0.008 inch) in diameter; they tend to run parallel to the long axis of a bone.