_ They lie parallel to the long axis of the shaft.
_ Each system consists of a central canal, Haversian canal, surrounded by 4–20 concentric lamellae of bony matrix.
_ The Haversian canal is lined by endosteum whose cells have osteogenic potential (osteoprogenitor cells).
_ The canal contains blood vessels, nerves, lymphatics and loose connective tissue.
_ The Haversian canals communicate with each other, with the periosteum and with the internal medullary cavity through transverse/oblique channels called Volkmann’s canals. These canals are not surrounded by concentric bony lamellae. Instead they penetrate through the lamellae of Haversian system (Fig. ).
_ Each Haversian system is formed by successive deposition of bony lamellae around the neurovascular structures in the canal from the periphery inwards. So the diameter of Haversian canal is highly variable. The younger system has a larger canal, whereas the older one has a smaller canal and the most recently formed lamella is closest to the canal.
_ The lamella contains collagen fi bres which run in a spiral manner parallel to each other. However, they run at right angle to those lamellae on either side of it. This arrangement of fi bres gives maximum rigidity and strength.
_ Osteocytes are seen between lamellae in elliptical lacunae .
_ Many tiny canals called canaliculi radiate from the lacunae and anastomose freely with those of other lacunae and Haversian canal. These canaliculi contain fi lopodial processes of osteocytes and come into contact with the fi lopodial processes of neighbouring osteocytes. Thus, a system of complex communicating canaliculi is formed throughout the bony lamellae
with communication with the vascular connective tissue of the Haversian canal, medullary cavity and periosteum. This arrangement helps to keep even the far off osteocytes alive.