The Spine object represents a quaternion or skeleton spine given four objects: chest control, hip control, chest depth, hip depth. These four objects define a four-point bezier curve along which the vertebrae are traced. On a skeleton spine the vertebrae are bones, on a quaterion spine the vertebrae can be implicit or polygon cubes. On a quaternion spine these four objects also control the spine's curve (each object controls one of the curve's control points).