What are the circular unites called that make up a compact bone?

The circular units that make up a compact bone are called Haversian systems or osteons. Each Haversian system consists of a central Haversian canal surrounded by concentric layers of bone matrix called lamellae. Within the lamellae, small channels called canaliculi connect the osteocytes (bone cells) to each other and to the Haversian canal, allowing for the exchange of nutrients and waste products. Haversian systems are the basic structural units of compact bone and contribute to its strength and rigidity.