The spinal column is the foundation to which all the other parts of the skeleton are adapted. It is composed of seven cervical vertebræ, twelve dorsal and five lumbar vertebræ, and is terminated by the sacrum and coccyx. Throughout its whole length runs the vertebral canal, which holds the spinal marrow, and communicates with the cavity of the skull. Each vertebra is composed of a body, two articular processes, two transverse processes, and a spinous process. The body, the anterior portion of the vertebra, is cylindrical, and forms a layer of the column. The articular processes, placed at the sides, serve to unite the vertebræ together; and the transverse processes give attachment to the ligaments, muscles, and in the dorsal regions to the ribs. The spinous process, the posterior portion of the vertebra, forms that series of projections which has given to the vertebral column the name of the spine. The spinous process bifurcates into two plates which complete the ring or vertebral orifice formed by each vertebra, and the open space in which forms a segment of the vertebral canal. Numerous and powerful ligaments combine to unite the vertebræ. Between their bodies fibrous disks in the form of lentils are placed, which adhere intimately to the articular surfaces; they are formed of concentric layers, and near the centre there is a spongy substance saturated with a fluid analogous to the synovia. These disks or intervertebral ligaments, besides binding together the bodies of the vertebræ, serve to diminish the shocks and the pressure to which they are subjected from the weight of the parts of the trunk above them They sink down and become thinner while the body is erect, so that there is a difference in the height between morning and evening of about .02 or .03 of a millimetre; but repose in bed restores to the fibrous disks their primitive thickness.
Between the vertebral plates stretch the yellow ligaments, remarkable for being formed of an elastic tissue which yields to the movements of the spinal column. Other inextensible ligaments envelop the spine at every point, and give great solidity to the whole. The spinal column has three curves— two backward in the cervical and lumbar regions, and one forward in the dorsal region. The ligaments which unite its layers permit only a slight degree of flexibility in the upper dorsal region, but this is a little more extended at the neck and loins, and powerful muscles give it at need great rigidity. And lastly, to its curves, and to the complicated mechanism of its articulations, it owes its great power of vertical resistance.
The head is balanced upon the first cervical vertebra, which is called the atlas; the manner of its articulation with the spinal column permits great extent and freedom of movement, while powerful ligaments and muscles give it great strength.