The type of bone that is arranged in concentric layers is compact bone. Compact bone is the denser and stronger of the two types of osseous tissue, and it makes up the outer cortex of all bones. If you look at compact bone under a microscope, you will observe a highly organized arrangement of concentric circles that look like tree trunks. Each group of concentric circles makes up the microscopic structural unit of compact bone called an osteon or Haversian system. Each ring of the osteon is made of collagen and calcified matrix and is called a lamella. The repeated pattern of compact bone is arranged in concentric layers of solid bone tissue.