The weight of the head is borne on the condyles of the occiput, and a perpendicular let fall from the condyles passes through the points where the spinal curves pass one into the other and thence through the anterior edge of the promontory of the sacrum. Hence if one curve is altered by injury or disease it is of necessity accompanied by a corresponding change in the curve on the opposite side of the perpendicular line. The first is called the primary curve and the other the secondary one. In anteroposterior curvatures these curves are exaggerations of the normal curves but in lateral curvatures they are newly formed because there is, practically, no normal lateral curve in the spinal column. The spine may project abnormally-backward, forming a kyphosis; forward, lordosis; and more or less laterally, scoliosis.

Fig. 478.   Lateral view of the adult spine, showing its curves. (Piersol).

Fig. 478. - Lateral view of the adult spine, showing its curves. (Piersol).

Kyphosis

The vertebrae are supported one above another by two points of contact, a posterior one, formed by the articular processes, and an anterior one, formed by the bodies of the vertebrae separated by the intervertebral disks. Of these two supports, that afforded by the bodies and disks is the more important. The laminae and pedicles with their attached articular processes are frequently fractured, but the shape of the vertebral column is but little altered; even after laminectomy the spine remains comparatively straight. When, however, the bodies of the vertebrae are destroyed, as occurs in tuberculous disease and crushing injuries, the anterior portion of the spine collapses and the parts bend, the spines projecting backward forming a hump (Fig. 479). Thus the angular character of the deformity is explained by the method of construction of the spine.

Besides this angular kyphosis there is another form, due to general weakness. This is seen in rachitic children; owing to a weakness of all the tissues the normal curves become increased and, as in young children, the normal spine has one long general curve with its convexity posteriorly, we find this curve greatly increased, forming a rachitic kyphosis.

Lordosis

When a child is born and for some time thereafter the spine possesses a slight dorsal and a pelvic curve. When it sits up and begins to hold its head erect and look around, the cervical curve develops. Still later when it begins to walk the lumbar curve develops. An increase in the lumbar curve, or lordosis, is caused by general weakness as just described for rachitic kyphosis, or it results from some disease or injury interfering with the lower extremities and thus disturbing the centre of gravity. This occurs in congenital luxation of the hips (Fig. 480), in which the heads of the femurs are set too far back, and also in rachitic deformities of the lower extremities, hip disease, etc. Likewise, if the abdominal viscera are unduly prominent, the thoracic region is carried further back to maintain the balance, and hence a hollow back is produced. Ankylosis of the hip in a flexed position causes lordosis when the limb is brought straight down as in walking. Therefore in cases of lordosis one should remember that it is a secondary condition dependent on diseased conditions of the viscera or extremities and is comparatively rarely an independent affection.

Fig. 479.   Kyphosis or angular anteroposterior curvature, usually due to caries of the bodies of the vertebras.

Fig. 479. - Kyphosis or angular anteroposterior curvature, usually due to caries of the bodies of the vertebras.

Fig. 480.   Lordosis or hollow back, caused by congenital luxation of the hips.

Fig. 480. - Lordosis or hollow-back, caused by congenital luxation of the hips.

Fig. 481.   Scoliosis or lateral curvature of the spine.

Fig. 481. - Scoliosis or lateral curvature of the spine.