FORMATION AND FLEXIBILITY OF THE NORMAL SPINE The vertebral column of the feetus generally forms one single cur vature, the convexity of which is directed backward (Fig. 57). The child retains this position more or less after birth during the first few months of life. If the child is kept in a recumbent position or on a hard bed a slight flattening of the kyphosis may take place, while the use of the feather mattress will increase the kyphotic curvature which the embryonic vertebral col umn presents. An important change takes place in the vertebral column when a child at the age of three or four months begins to lift the head and turn it backwards. The (torso lumbar region of the spine still retains the ex isting kyphotic curvature, but the cervical part now changes from the original kyphotic condi tion into the opposite curvature—a lordosis with the convexity forward (Fig. 58).
A second change in the shape of the spine takes place at the end of the first year when the child begins to stand on its feet and attempts to walk. The muscles of the back, which hith erto have been little used, arc now set in motion. They arise from the sacrum and are inserted at the lower portion of the dorsal region of .the spine. In contracting they force the kyphotic curvature, which until then was formed by the lumbar region of the spine, gradually forward and finally transform it into a marked lordosis. As a result of this total change the vertebral column, which was originally of a total kyphotic shape, now presents an anterior lordotic curvature in the upper cervico dorsal region, the original .posterior kyphosis in the dorsal region, and the anterior lordotic projection in the lumbar region. Thus the funda mental "long S shape" is attained which the spine presents in adults of erect and faultless carriage (Fig. 59).
A continuous and harmonious cooperation of numerous muscles is essential to retain the spine in a normal position. The posterior muscles located on both sides of the spine tend to increase the lordosis, while the anteriorly located abdominal muscles tend to augment the kyphosis. The normal posture is dependent not alone On the form of the spine, but it is subject also to the position of the pelvis.
The connection of the pelvis with the spore by means of the sacro-iliac articulation is rather rigid. For this reason the attitude of the spine is intimately associated with that of the pelvis. The pelvic motions are based on an axis con necting both hips. In a recumbent position the pelvis is at rest, the thighs alone being able to make certain motions which are termed flexion and extension and result from the action of the flexor and the extensor muscles of the hip.
In the erect attitude, with the legs fixed, pelvic motions are made by like muscular action. When the flexor muscles become active the anterior part of the pelvis is tilted downwards :11111 the plane through the superior entrance becomes nearly vertical. This pelvic position Is called increased inclination of the pelvis. The reverse takes place when the extensor muscles of the hip are brought into action. The posterior part of the pelvis is then tilted downwards and the superior pelvic strait appears more or less horizontal. This attitude is termed decreased inclination of the pelvis (Fig. GO).
It is evident that the attitude of the pelvis exerts a marked influence on the carriage of the vertebral column as well as on the position of the trunk. In case of an inclined pelvis, when the superior pelvic strait is more vertical, the trunk would fall forward were it not for the aebion of the lumbar region of the spine, which, owing to a marked lo•dosis, bends the spine back ward (Figs. 015, GI b, file). Decreased inclina tion of the pelvis has the opposite effect. In this case the falling of the trunk backward is pre vented by a marked posterior kyphotic curvature of the lumbar region of the spine forcing the up per part of the spine forward (Figs. Gil, illy). In normal attitudes, according to exaininat ions made by and tichulthess, a line connecting the promontory and upper border of the symphysis forms an angle of fifty degrees with a horizontal plane. To get prompt information as to the normal appearance of the spine and pelvis in a living person, Sehulthess suggests that a vertical line be drawn from the cervical region down to the lowest point of the sacrum. Such a line in a normal case must either slightly touch the dorsal curvature or pass very near it. Besides the shape of the spine and the inclination of the pelvis the other com ponents which contribute to a normal attitude are of minor importance. It is evident that the legs are extended at the hip- and knee-joints, being kept in a position such as to enable a vertical line running upward from Chopart's joint to strike the hip and the ear. Finally, in order to maintain a normal attitude it is necessary to hold the shoulders backward in a position half way between high and low elevation (Figs. G2a, 62b.)