ARTICULATION OF THE ATLAS WITH THE AXIS 273 



Movements of the Vertebral Column. The movements permitted in the vertebral 

 column are flexion, extension, lateral movement, circumduction, and rotation. 



In flexion (forward flexion), or movement of the vertebral column forward, the anterior 

 common ligament is relaxed, and the intervertebral substances are compressed in front, while 

 the posterior common ligament, the ligamenta subflava, and the inter- and supraspinous liga- 

 ments are stretched, as well as the posterior fibres of the intervertebral disks. The interspaces 

 between the lamina? are widened, and the inferior articular processes of the vertebra? above 

 glide upward upon the articular processes of the vertebrae below. Flexion is the most extensive 

 of all the movements of the vertebral column. 



In extension (backward flexion], or movement of the vertebral column backward, an exactly 

 opposite disposition of the parts take place. This movement is not extensive, being limited 

 by the anterior common ligament and by the approximation of the spinous processes. 



Flexion and extension are greatest in the lower part of the lumbar region between the third 

 and fourth and fourth and fifth lumbar vertebrae; above the third they are much diminished, 

 and reach their minimum in the middle and upper part of the back. They increase again in the 

 neck, the capability of motion backward from the upright position being in this region greater 

 than that of the motion forward, whereas in the lumbar region the reverse is the case. 



In lateral flexion, the sides of the interveitebral disks are compressed, the extent of motion 

 being limited by the resistance offered by the surrounding ligaments and by the approximation 

 of the transverse processes. This movement may take place in any part of the vertebral column, 

 but has the greatest range in the neck and loins. 



Circumduction is limited, and is produced merely by a succession of the preceding movements. 



Rotation is produced by the twisting of the intervertebral substances; this, although only 

 slight between any two vertebrae, produces a considerable extent of movement when it takes 

 place in the whole length of the vertebral column, the front of the upper part of the column 

 being turned to one or the other side. This movement takes place only to a slight extent in the 

 neck, but is more pronounced in the upper part of the thoracic region, and is altogether absent in 

 the lumbar region. It is thus seen that the cervical region enjoys the greatest extent of each 

 variety of movement, flexion and extension, especially, being very extensive. In the thoracic 

 region the three movements of flexion, extension, and circumduction are permitted only to a 

 slight extent, while rotation is very extensive in the upper part and ceases below. In the lumbar 

 region there is extensive flexion, extension, and lateral movement, but no rotation. 



The movements permitted are mainly due to the shape and position of the articulating pro- 

 cesses. In the loins the inferior articulating processes are turned outward and are embraced by 

 the superior; this renders rotation in this region of the vertebral column impossible, while there is 

 nothing to prevent a sliding upward and downward of the surfaces on each other, so as to allow 

 of flexion and extension. In the thoracic region, on the other hand, the articulating processes, by 

 their direction and mutual adaptation, especially at the upper part of the series, permit of rotation, 

 but prevent extension and flexion, while in the cervical region the greater obliquity and lateral 

 slant of the articular processes allow not only flexion and extension, but also rotation. 



The principal muscles which produce flexion are the Sternomastoid, Rectus capitis anticus 

 major, and Longus colli; the Scaleni; the abdominal muscles and the Psoas magnus. Extension 

 is produced by the fourth layer of the muscles of the back, assisted in the neck by the Splenius, 

 Semispinales dorsi et colli, and the Multifidus spinae. Lateral motion is produced by the fourth 

 layer of the muscles of the back, by the Splenius and the Scaleni, the Quadratus lumborum 

 and Psoas magnus, the muscles of one side only acting; and rotation by the action of the fol- 

 lowing muscles of one side only viz., the Sternomastoid, the Rectus capitis anticus major, the 

 Scaleni, the Multifidus spinae, the Complexus, and the abdominal muscles. 



II. Articulation of the Atlas with the Axis (Articulatio Atlantoepistrophica). 



The articulation of the atlas with the axis is of a complicated nature, compris- 

 ing no fewer than four distinct joints. There is a pivot articulation between the 

 odontoid process of the axis and the ring formed between the anterior arch of 

 the atlas and the transverse ligament (Fig. 226). Here there are two joints one 

 in front between the posterior surface of the anterior arch of the atlas and the front 

 of the odontoid process; the other between the anterior surface of the transverse 

 ligament and the back of the process. Between the articular processes of the 

 two bones there is a double arthrodial or gliding joint. The ligaments which 

 connect these bones are the 



Anterior Atlanto-axial. Transverse. 



Posterior Atlanto-axial. Two Capsular. 



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