The bodies of the vertebrae except those of the sacrum and coccyx are separated, and at the same time connected, by the _intervertebral disks_. These are formed of alternating concentric rings of fibrous tissue and fibro-cartilage, with an elastic mass in the centre known as the _nucleus pulposus_. The bodies are also bound together by _anterior_ and _posterior common ligaments_. The odontoid process of the axis fits into a pivot joint formed by the anterior arch of the atlas in front and the _transverse ligament_ behind; it is attached to the basioccipital bone by two strong _lateral check ligaments_, and, in the mid line, by a feebler _middle check ligament_ which is regarded morphologically as containing the remains of the notochord. This _atlanto-axial joint_ is the one which allows the head to be shaken from side to side. Nodding the head occurs at the _occipito-atlantal joint_, which consists of the two occipital condyles received into the cup-shaped articular facets on the atlas and surrounded by capsular ligaments. The neural arches of the vertebrae articulate one with another by the _articular facets_, each of which has a capsular ligament. In addition to these the laminae are connected by the very elastic _ligamenta subflava_. The spinous processes are joined by _interspinous ligaments_, and their tips by a _supraspinous ligament_, which in the neck is continued from the spine of the seventh cervical vertebra to the external occipital crest and protuberance as the _ligamentum nuchae_, a thin, fibrous, median septum between the muscles of the back of the neck. The combined effect of all these joints and ligaments is to allow the spinal column to be bent in any direction or to be rotated, though only a small amount of movement occurs between any two vertebrae. The heads of the ribs articulate with the bodies of two contiguous thoracic vertebrae and the disk between. The ligaments which connect them are called _costo-central_, and are two in number. The anterior of these is the _stellate ligament_, which has three bands radiating from the head of the rib to the two vertebrae and the intervening disk. The other one is the _interarticular ligament_, which connects the ridge, dividing the two articular cavities on the head of the rib, to the disk; it is absent in the first and three lowest ribs.