Movements of the vertebral colummn
The vertebral column is so formed of a number of bones and intervertebral discs as to serve many purposes. It is the axis of the skeleton; upon it the skull is supported; and with it the cavities of the trunk and the limbs are connected. As a fixed column it is capable of bearing great weight, and, through the elastic intervertebral substances, of resisting and breaking the transmission of shocks. Moreover, it is flexible. Now, the range of movements of the column as a whole is very considerable; but the movements between any two vertebrae are slight, so that motions of the spine may take place without any change in the shape of the column, and without any marked disturbance in the relative positions of the vertebrae. It is about the pulpy part of the intervertebral discs, which form a central elastic pivot or ball, upon which the middle of the vertebras rest, that these movements take place.
The amount of motion is everywhere limited by the common vertebral ligaments, but it depends partly upon the width of the bodies of the vertebrae, and partly upon the depth of the discs, so that in the loins, where the bodies are large and wide, and the discs very thick, free motion is permitted; in the cervical region, though the discs are thinner, yet, as the bodies are smaller, almost equally free motion is allowed. As the ball-Uke pulpy part of the intervertebral disc is the center of movement of each vertebra, it is obvious that the motion would be of a rolling character in any direction but for the articular processes, which serve also to give steadiness to the column and to assist in bearing the superincumbent weight. Were it not for these processes, the column, instead of being steady, endowed with the capacity of movement by muscular agency, would be tottering, requiring muscles to steady it. The influence of the articular processes in limiting the direction of inclination will appear from a study of the movements in the three regions of the spine.
In the neck all movements are permitted and are free, except between the second and third cervical vertebrae, where they are slight, owing to the shallow intervertebral disc and the great prolongation of the anterior hp of the inferior surface of the body of the epistropheus, which checks forward flexion considerably. On the whole, however, extension and lateral inclination are more free and extensive in this than in any other region of the column, whilst flexion is more limited than in the lumbar region. Rotatory movements are also free, but take place, on ac- count of the position and inclination of the articular facets, not, as in the thoracic region, round a vertical axis, but round an oblique axis, the articular process of one side gliding upward and forward and that of the opposite side downward and backward.
In the thoracic region, especially near its middle, antero-posterior flexion and extension are very slight; and, as the concavity of the curve here is forward, the flat and nearly vertical surfaces of the articular processes prevent anything like sliding in a curvilinear manner of the one set of processes over the sharp upper edges of the other, which would be necessary for forward flexion. A fair amount of lateral inclination would be permitted but for the impediment offered by the ribs; while the position and direction of the articular processes allows rotation round a vertical axis which passes through the centers of the bodies of the vertebrae. This rotation is not very great, and is freer in the upper than in the lower part of the thoracic region.
In the lumbar region, extension and flexion are very free, especially between the third and fourth and fourth and fifth vertebrae, where the lumbar curve is sharpest; lateral inclination is also very free between these same vertebrae. It has been stated that the shape and position of the articular processes of the lumbar and the lower two or three dorsal are such as to prevent any rotation in these regions; but, owing to the fact that the inferior articular processes are not tightly embraced by the superior, so that the two sets of articular processes are not in contact on both sides of the bodies at the same time, there is always some space in which horizontal motion can occur round an axis drawn through the central part of the bodies and intervertebral discs, but it is very slight. Thus, the motions are most free in those regions of the column which have a convex curve forward, due to the shape of the intervertebral discs, where there are no bony walls surrounding solid viscera, where the spinal canal is largest and its contents are less firmly attached, and where the pedicles and articular processes are more nearly on a transverse level with the posterior surface of the bodies of the vertebrae.
Nor must the uses of the ligamenta flava be forgotten: these useful structures - (1) complete the roofing-in of the vertebral canal, and yet at the same time permit an ever-changing variation in the width of the interlaminar spaces in flexion and extension; (2) they also restore the articulating surfaces to their normal position with regard to each other after movements of the column; (3) and by forming the medial portion of each articular capsule, they take the place of muscle in preventing it from being nipped between the articular surfaces during movement.