Skeleton and Joints
There are 206 individual bones in the adult skeleton. Each one is an organ and you should know the names of most of them. They are classified as part of either the axial skeleton or the appendicular skeleton. Bone may be termed compact (dense) or spongy (cancellous). Compact bone is found in the shaft (the diaphysis) of long bones and is dense. Microscopically it can be seen to be composed of cylindrical structures called osteons. These are lamellar structures with osteocytes within lacunae that surround a central canal that contains blood vessels. The central canal can exchange material with the lacunae via little channels called canaliculi. Spongy bone is found in the ends (the epiphyses) of long bones and is composed of bony trabeculae separated by spaces. Marrow is found in the shaft of long bones and between the trabeculae of spongy bone. Active marrow produces red and white blood cells by haemopoiesis. Yellow marrow is inactive. Bone is a storage place for calcium and is continually being remodelled by osteoclasts (which remove bone) and osteoblasts (which deposit bone). In the process, Ca is released or stored.