The osteon or haversian system (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 0.2mm in diameter. They are present in many bones of most mammals and some bird, reptile, and amphibian species.

The osteon or haversian system (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 0.2mm in diameter. They are present in many bones of most mammals and some bird, reptile, and amphibian species.