In osteology, 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 between 0.25 mm and 0.35 mm in diameter.[1] Their length is often hard to define,[2] but estimates vary from several millimeters[3] to around 1 centimeter.[1] They are present in many bones of most mammals and some bird, reptile, and amphibian species.