The vertebral column consists of a series of vertebrae connected to each other by the inter-vertebral discs. Each vertebra - except the first - is composed of a body and an arch, the arch enclosing the spinal cord. The cervical and lumbar part are flexible, the thoracic and sacral part both rather immobile, but of similar configuration. As the rotation in-between thorax and hip is limited to about 15 degree, the thoracic and sacral part of the vertebral column appear always as of similar in form and as "in line".
The first cervical vertebra or "Atlas" carries the head. It has the form of a ring divided into two parts by a ligament. The dorsal part contains the spinal cord, the ventral part the axis (marked as *) an osseous process of the underlying second vertebra.
The articulation in-between the globe of the head and the "Atlas" allows up and down movements of the head to a total of about 20 degree. The articulation in-between the "Atlas" and the second vertebra allows movements to the side of about 30 degree in each direction.