The spinous processes projecting behind, and the transverse processes projecting laterally, form two deep vertical grooves, which lodge the powerful muscles which support and control the movements of the column. In the adult the column presents four curves-two primary, thoracic and sacral, which are also present in the foetus, are due to the shape of the bones, and whose concavity is directed forwards ; and two secondary, cervical and lumbar, which are acquired after the assumption of the erect posture, are due to the shape of the intervertebral discs, and whose convexity is directed forwards. In infancy, prior to the formation of the secondary curves, the spine generally presents a slight general backward convexity, which in rickety children may proceed to form a pronounced kyphosis. In old persons, in whom the discs have atrophied, the spine frequently assumes a slight general backward convexity ; and in cases of rheumatoid arthritis affecting the spine the various joints may become fused, producing a rigid spine, which is frequently fixed in a kyphotic curve. As the spines of the vertebrae vary in length, a line joining them would not repeat the curves of the column. The junction of the fifth lumbar vertebra with the sacrum forms a marked projection-the saero-vertebral angle-which is frequently felt in abdominal palpation. Sometimes the lumbar vertebrae slip forward from the sacrum at this point, owing to defective development of the last lumbar vertebra, producing spondylolisthesis. Even normally a slight lateral curve of the spine in the dorsal region, with the convexity toward the right, and compensatory curves above and below with the convexity to the left, may be present. This lateral curve has been attributed to the greater use of the right arm, and to the pressure of the thoracic aorta on the dorsal vertebrae.
Scoliosis consists of an exaggeration of this lateral curve, the convexity of the curve being generally to the right, while the vertebrae rotate, so that their bodies are directed to the convexity of the curve. The ribs move with the vertebrae, and thus the angles of the ribs on the side of the convexity are prominent, and make the angle of the scapula project, while on the concave side they are depressed. Correspondingly, the front of the chest on the side of the convexity is flattened, while it is prominent on the concave side. The ribs are also separated from one another on the convex side, and hence the shoulder is raised, while they are crowded together on the concave side, the shoulder being depressed. The condition is due to loss of tone of the muscles (erector spinae attached to the transverse processes and ribs, and the multifidus spinae, etc., to the spinous processes) and ligaments, and is frequently associated with flat-foot, which is due to a similar condition, and is frequently an exciting cause.
The vertebral discs, twenty-three in number, make up nearly one-quarter of the length of the spine, act as buffers and springs, form the cervical and lumbar curves, and assist in the formation of the intercentral articulations. These are amphiarthrodial joints, and while the movement permitted in each individually is slight, the spine as a whole is capable of very considerable movement, particularly in the cervical and lumbar regions, where not only antero-posterior but lateral and rotatory movements are possible.
Lordosis, or an exaggeration of the normal lumbar curve, with the convexity forwards, is frequently seen in hip disease, where all the movement permitted in the limb may be derived from the lumbar spine, and not from the hip-joint.
The bodies of the vertebrae are the most frequent site of tubercle of bone. Generally two contiguous bodies are affected, the pus escaping anteriorly between the ligaments to form a psoas or other abscess (according to the position- retro-pharyngeal in the cervical region, psoas in the dorsal or upper lumbar), and the bodies collapsing, forming an angular projection or ' curvature ' of the spine, known as Pott's disease. The bodies of the vertebrae are also markedly eroded -by the pressure of thoracic aneurism, whereas the intervertebral discs are comparatively resistant to such pressure.
Sprains of the column are generally produced by indirect violence, and occur most frequently in the cervical and lumbar regions, owing to the mobility of these parts, and their proximity to the head and pelvis respectively. Sprained back frequently causes pain and inconvenience to a patient for a considerable time after an injury, owing, perhaps, in some cases to implication of some of the spinal joints, and in others to damage to the muscles. The condition may be simulated in rheumatic affections of the muscles-lumbago- and is one which causes considerable difficulty in medicolegal work. Sprains are seldom associated with much destruction of the parts, a fracture dislocation generally occurring in such cases.
Most of the severe damages met with in the vertebral column from injury are of the nature of fracture dislocations, the contained cord adding greatly to the gravity of the condition. The cord, however, occupies a neutral position between the bodies and the spines, and thus frequently escapes, unless an actual displacement of one or more vertebrae occurs. While fracture can occur without dislocation, dislocation very rarely occurs without fracture, except in the case of the first, second, and fifth cervical vertebrae, and of the lumbo-sacral articulation.
Fracture dislocations may be caused by either direct or indirect violence. The latter are by far the more common, and are generally due to violent forced flexion of the spine, the injury occurring about the junction of a flexible and a rigid portion of the column, as at the cervico-dorsal (frequently between the fifth and seventh cervical) from falls on the head, or dorso-lumbar from weights falling on the shoulders. Owing to the forward flexion, the broad vertebral bodies tend to be crushed together, and the narrow tough laminae and spines, with their strong ligaments, tend to be torn apart, while the cord lies in comparative security between these two forces. As a rule, the upper body is displaced down and forwards, complete dislocation being prevented by locking of the posterior processes. The affected vertebral bodies are partially crushed, and the intervertebral discs lacerated, as likewise the anterior and posterior common ligaments in more severe cases. The laminae, or spines, are broken, and the supra- and interspinous and capsular ligaments and ligamenta subflava torn. The articular processes are generally broken in the dorsal region, but escape in the cervical and lumbar regions. Replacement by forcible extension and manipulation is frequently successful, although perhaps most difficult in the lumbal region.
Direct violence may produce an injury at any part of the spine, the mechanism of the indirect form being reversed. A blow on the back tends to crush the posterior segment, and to tear the bodies in front apart; much displacement is uncommon, and the cord, therefore, more frequently escapes. The occipital bone has been dislocated from the atlas, and more commonly, as in hanging, the atlas has been dislocated forwards from the axis, the odontoid process being broken. The spinous processes of the lower cervical and dorsal regions may be broken off by direct violence. Where injuries to the column cause pressure upon the spine, it may be necessary to perform a laminectomy for its relief (q.v.).