(a) : Small bones are solid. Long bones, such as the humerus and femur, have a cavity, the marrow cavity, at the centre. The substance of the bone is distinguishable into 3 regions : periosteum, matrix and endosteum. The matrix of bone along with the bone forming cells (osteoblasts) is arranged in concentric layers (lamellae) round the small canals which run parallel to the long axis (shaft) of the bone. These canals, called Haversian canals, are interconnected with one another via Volkmann's canals and contain a blood vessel, a nerve and a lymph vessel. Bone cells remain alive and once they have completely surrounded by the hard bone matrix, they are called osteocytes. The osteocytes are embedded in fluid-filled cavities within the concentric lamellae. These cavities are known as lacunae and occur at regular intervals in these concentric layers of bone tissue. The lacunae are connected to one another and to the Haversian canals by a system of interconnecting canals known as canaliculi. Each Haversian canal, its concentric lamellae, lacunae with osteocytes and canaliculi forms a long cylinder and is called a Haversian system. Separate Haversian systems are joined to each other by means of interstitial lamellae.