The skeleton is composed of living tissue – bone.
Bones are made up partly of water, about half of it is hard material (mainly calcium) and the rest is made up of living cells.
Bone tissue is a type of dense connective tissue (mineralized osseous tissue). Other types of tissue found in bones include marrow, endosteum, periosteum, nerves, blood vessels and cartilage.
There are five different types of bones in the body, with different composition depending on their function.
- Long bones – having a body which is longer than it is wide, with growth plates at each end, having a hard outer surface of compact bone and a spongy inner known as cancellous bone containing bone marrow.
Short bones – are approximately as wide as they are long and have a primary function of providing support and stability with little movement.
Flat bones – are strong, flat plates of bone with the main function of providing protection to the body’s vital organs and being a base for muscular attachment. Examples are the scapula (shoulder blade) and cranium (skull).
Irregular bones – are bones which do not fall into any other category, due to their non-uniform shape. Examples include the vertebrae (in the spine), sacrum and mandible (lower jaw). They primarily consist of cancellous bone, with a thin outer layer of compact bone.
Sesamoid bones – are usually short or irregular bones imbedded in a tendon. The most obvious example of this is the patella (knee cap) which sits within the quadriceps tendon. Other sesamoid bones are the pisiform (smallest of the 8 wrist bones) and the two small bones at the base of the first metatarsal. Sesamoid bones are usually present in a tendon where it passes over a joint which functions to protect the tendon.