Polarised light micrograph of a section through a long bone. The outer layer (upper frame) of a bone consists of compact, or cortical, bone. This is made of concentric layers of collagen-containing matrix (lamellae), around Haversian canals, which contain blood and lymph vessels, and nerves. The lamellae and canals together form a Haversian system, or osteon. Many systems are arranged in columns running parallel to the long axis of the bone. Osteocytes, the cells that maintain the bony matrix, are housed in small spaces known as lacunae (small black dots). The interior (lower frame) of the bone consists of cancellous, or spongy, bone. This is characterised by a honeycomb arrangement, comprising a network of trabeculae. These structures provide support and strength to the bone. The spaces within this tissue contain bone marrow (grey), a blood forming substance. Magnification: x200 when printed at 15cm tall.