Atlantoaxial Articulation
The articulation of the atlas with the axis is of a complicated nature and comprises three joints. There is a pivot-joint between the odontoid process (dens) of the axis and the ring formed by the anterior arch and the transverse ligament of the atlas (fig. 516), and a pair of plane joints between the articular facets of the two bones. The bones are connected by two capsular ligaments and by the transverse ligament of the atlas.
The capsular ligaments are thin and loose; they surround the joints between the articular facets and are lined with synovial membrane. Each is strengthened at its posterior and medial part by an accessory ligament, which is attached below to the body of the axis near the base of the odontoid process, and above to the lateral mass of the atlas near the transverse ligament.
In front the two vertebra are connected by a continuation of the anterior longitudinal ligament (fig. 514). In this position it is a strong membrane, fixed above to the lower border of the anterior arch of the atlas, and below to the front of the body of the axis. It is strengthened in the median plane by a rounded cord, which connects the tubercle on the anterior arch of the atlas to the body of the axis.
Behind, the atlas and axis are joined by a broad, thin membrane (fig. 515) attached above to the lower border of the posterior arch of the atlas, below to the upper edge's of the laminae of the axis; it is in series with the ligamenta flava.
Figure 514
Figure 515
The transverse ligament of the atlas (figs. 516-518) is a thick, strong band which arches across the ring of the atlas and retains the odontoid process of the axis in contact with the anterior arch. It is concave in front, convex behind, broader in the middle than at the ends, and firmly attached on each side to a small tubercle on the medial surface of the lateral mass of the atlas. As it crosses the odontoid process a small longitudinal band is prolonged upwards, and another downwards, from the superficial or posterior fibers of the ligament.
Figure 516
Figure 517
The upper band is attached to, the upper surface of the basilar part of the occipital bone between the apical ligament of the odontoid process and the membrana tectoria; the lower band is attached to the posterior surface of the body of the axis; hence the whole ligament is named the cruciate ligament of the atlas.
The transverse ligament divides the ring of the atlas into two unequal parts of these, the posterior and larger surrounds the spinal cord and its membranes and the spinal roots of the accessory nerves; the anterior and smaller contains the odontoid process. The neck of the process is constricted where it is embraced posteriorly by the transverse ligament, so that this ligament suffices to retain the odontoid process in position after all the others have been divided.
The joint-cavity between the odontoid process and the transverse ligament is often continuous with those of the atlanto-occipital articulations.
Figure 518
Movements.--This articulation with its three joints allows the rotation of the atlas (and with it the skull) upon the axis, the extent of rotation being limited by the alar ligaments.
The opposed articular facets of the atlas and axis are not reciprocally curved; both are slightly convex in their long axes. When therefore, the upper facet glides forwards on the lower it also descends; the fibers of the capsular ligament are relaxed in a vertical direction, and will then permit of movement in an anteroposterior direction. By this means a shorter capsule suffices and the strength of the joint is materially increased.
Muscles producing the movements. The principal muscles by which these movements are produced are the sternornastoid and semispinalis capitis of one side, acting with the longus capitis, splenius, longissimus capitis, rectus capitis posterior major and obliquus capitis inferior of the other side.
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Vertebral Column