Long bones are hollow, with a central core which is not strong like the rest of the bone. It contains the bone marrow, one of the most important tissues in the vertebrate body. It produces blood cells for the blood system, and lymphocytes for the immune system.
Diagram of a typical long bone: shows compact (cortical) and cancellous (spongy) bone
microscopic bone structure
Osteons are the small units of which the hardest parts of human bones are made. They are roughly cylindrical, and about 0.2mm wide and a few millimeters long. They are found in the bone in most mammals, and many reptiles, birds and amphibians. Inside the osteons are bone cells called
osteocytes, each living in its own small space. Osteocytes make contact with each other by cytoplasmic processes through a network of tiny canals. This allows the exchange of nutrients and metabolic waste. Collagen fibers in each ring of cells ('lamellae') give them structure.
Osteons have a hole down the middle, called the haversian canal. This canal contains the bone's blood supply. It also contains capillaries, and nerve fibres.
The details of osteon structure varies between bones and parts of bones, from species to species, between sexes, and by age and environmental factors.