The vertebral column
The vertebral column or spine is made up of 33 bones, divided into five main areas. Twenty-four of these bones or vertebrae are individual and unfused, whilst the remaining nine are fused (joined) together. The five areas are as follows:
- Cervical vertebrae: 7 unfused bones. These are the bones of the neck, and support the weight of the head by allowing muscle attachment through the transverse (sides) and spinous (rear) processes. The top two vertebrae are called the atlas and axis. These enable the head to move up and down and side to side.
- Thoracic vertebrae: 12 unfused bones. These allow the attachment of the ribs through their transverse (side) processes. The thoracic vertebrae together with the ribs form the ribcage and protect the heart and lungs.
- Lumbar vertebrae: 5 unfused bones. These are the largest of all the vertebrae. They offer a great deal of weight-bearing capacity, while their large processes allow the attachment of muscles. This muscle attachment, together with the intervertebral discs of cartilage, form cartilaginous joints and enable forwards and backwards movement as well as side to side movement of the trunk.
- Sacral vertebrae: 5 fused bones. The sacral vertebrae are collectively known as the sacrum. These 5 fused vertebrae also fuse to the pelvis (ilium bone) forming the sacroiliac joint. Together the sacrum and the pelvis bear and distribute the weight of the upper body.
- Coccyx: 4 fused bones. The coccyx is commonly called the tailbone. It forms the base of the vertebral column and allows muscle attachment.