According to About.com, the structural unit of compact bone is the osteon. Osteons are circular, tube-like units that contain rings of compact bone and a Haversian canal through which blood vessels and nerves run.
Osteons consist of a canal and bone rings or layers, Reference.com explains. The rings of bone that surround the canal are called lamellae. These lamellae contain osteoblasts and osteocytes, canaliculi and collagen fibers.
Osteoblasts and osteocytes are bone cells and canaliculi are small canals where nutrients and waste run. Interstitial lamellae separate one osteon from another. This space between osteons is made up of osteons that are being broken down. While Haversian canals and canaliculi run parallel to each other and never cross into other osteons, Volkmann's canals do. These canals run obliquely, connecting osteons together. According to Wikipedia, Volkmann's Canals not only connect osteons, but through them blood vessels run from the outer layer of bone to the inner layers, connecting and communicating with the Haversian canals. These blood vessels help to further nourish osteons.
The densely packed osteons make a hard outer layer of bone, About.com states, that protects the softer, inner layer of cancellous or spongy bone where important functions take place, like the production of red cells and bone marrow.