590 STANDING. 



the odontoid process of the axis. The articular surfaces on the pedicles of the 

 first and second vertebrae are convex toward each other in the middle, becoming 

 somewhat lower anteriorly and posteriorly. The head is, therefore, highest in 

 the erect position; if it is rotated on the odontoid process, it undergoes a slight 

 spiral movement downward. In this way distortion of the medulla is avoided 

 when the head is strongly rotated. In standing, no muscular action is required 

 to fix these vertebrae, as rotation cannot occur when the muscles of the neck and 

 the flexors and extensors of the head are at rest. 



2. The vertebral column requires fixation by muscles in those sec- 

 tions where its mobility is the greatest ; these are the cervical and lumbar 

 regions. Here fixation is secured by the numerous and strong muscles of 

 the cervical vertebrae, especially those of the neck, and the lumbar mus- 

 cles, especially the strong origins of the extensor dorsi communis, sup- 

 ported by the quadratus lumborum. 



The least movable vertebras are those from the third to the sixth dorsal; the 

 sacrum is completely immovable. For a definite length of the column the mo- 

 bility depends upon the following factors: (a) The number and the thickness of 

 the elastic intervertebral discs. These are most numerous in the cervical region, 

 and are thickest in the lumbar region and relatively also in the lower cervical 

 region. They permit movement in every direction. The intervertebral discs 

 together form one-fourth the entire length of the spinal column. They are com- 

 pressed somewhat by the weight of the body; hence, the body is longest in the 

 morning and after recumbency of some duration. The smaller circumference of 

 the bodies of the cervical vertebrae must be more favorable for their movement 

 on the discs than is the greater size of the lower vertebrae. (6) The position of 

 the processes also materially influences the mobility. The greatly depressed 

 spines of the dorsal vertebras prevent hyperextension. The articular processes 

 of the cervical vertebrae are so situated that their surfaces are directed obliquely 

 from before and above backward and downward. By this means relatively free 

 movement is rendered possible in rotation, lateral inclination, and flexion. In 

 the dorsal region the articular surfaces of the superior articular processes are 

 directed vertically and directly forward, while those of the inferior articular 

 processes are directed directly backward; in the lumbar region the corresponding 

 position is almost vertical and sagittal. In the act of bending backward as far 

 as possible, the most movable points of the spinal column are the lower cervical 

 vertebrae, from, the eleventh dorsal to the second lumbar vertebra, and the two 

 lower lumbar vertebrae. 



3. The center of gravity of the part of the body thus stiffened (the 

 head and the trunk with the arms) is situated on the anterior border of 

 the inferior surface of the eleventh dorsal vertebra. The perpendicular 

 line dropped from the center of gravity passes behind a line joining both 

 hip-joints. Hence, the trunk would fall backward at the hip-joints; but 

 this is prevented by the ilio-femoral ligament, 14 mm. thick, stretched 

 between the anterior inferior spine and the anterior intertrochanteric 

 line, and also by the anterior tense layer of the fascia lata. As ligaments 

 alone are never able to withstand continuous traction, they are mate- 

 rially supported by the ilio-psoas muscle, which is inserted on the lesser 

 trochanter, and also in part by the rectus femoris, whose origin extends 

 upward over the acetabulum to the anterior inferior spine. A lateral 

 movement of the hip-joint, in which one thigh would be abducted and 

 the other adducted, is prevented especially by the large mass of the 

 gluteal muscles, which fix the thigh on the pelvis posteriorly and lat- 

 erally. When the thigh is extended, the ilio-femoral ligament also is 

 able to prevent adduction, aided by the tense fascia lata. 



4. The part of the body that has thus far been made rigid, including 

 the head and the trunk, with the arms and the thighs, and whose center of 

 gravity is situated somewhat lower and only to such a slight degree further 



