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. The surface of bones is marked by features that may rise above the surface (projections or roughenings) or be indented below the surface. The features mark attachment points for tendons and ligaments, places where a bone articulates with another, grooves where tendons may lie or an opening for nerves and blood vessels to pass through. These features are given names such as tuberosity, condyle, foramen, tubercle, etc. A bone is connected to an adjacent bone at an articulation (a joint), and they are bound to each other by ligaments. All the joints in the appendicular skeleton are freely moveable (synovial) joints and stabilised by tendons and intracapsular menisci. Their free movement is produced when the muscles attached to them contract and is ensured by the smooth hyaline cartilage that covers the articulating bone surfaces and the lubricating synovial fluid within the joint capsule.