ATLAS, the first vertebra of the neck, so named because it sustains the gloGe of the head. It differs in several important circumstances from all the other vertebrae that enter into the composition of the spinal column, because it has distinct and peculiar offices to perform. It has to support the head, and to allow it the power of exercising two different kinds of motion, namely, a motion forwards and backwards, or that of flexion and extension ; and a rotatory motion, or the power of describing a certain portion of a circle, as it does when it turns from side to aide. These motions are accomplished by the peculiar mode in which the head is connected to the atlas, and the atlas to the second vertebra of the neck, the Vertebra dentate, or Axis. The head is so united with the atlas as to form a perfect hinge-joint, that is, a joint which admits of flexion and extension, or a motion forwards and backwards. The second vertebra, the dentata, forming a pivot on which the atlas turns, and therefore called axis, is united with the atlas in such a manner as to constitute a perfect rotation-joint, or a joint which admits of a rotatory motion. The head being firmly con
nected with the atlas and carried round with it whenever the latter turns upon its axis, it is plain that by the combination of the two joints, namely, the hinge-joint and the rotation-joint, the head can bo moved in every direction—forwards, backwards, and from side to side. In the construction of these joints such is the perfection of the mechanism that these combined motions are attained to the utmost extent, and are performed with the greatest ease. The connection of the different parts with each other forms a union of amazing strength and security, and at the same time certain organs of extreme delicacy and of vital importance are effectually guarded from injury. [Spinal Column, under SKELETON.]