Vertebral Column Articulations

The ligaments connecting the vertebral column with the cranium may be divided into two sets; those uniting the atlas with the occipital bone, and those connecting the axis with the occipital bone.

1. THE ARTICULATION OF THE ATLAS WITH THE OCCIPITAL BONE

The articulation between the atlas and the occipital bone consists of a pair of condyloid joints. The ligaments connecting the bones are; two capsular, anterior and posterior atlanto-occipital membrane.

The capsular ligaments surround the condyles of the occipital bone and the superior articular facets of the atlas : they are thin and loose and are lined with synovial membrane. Their lateral portions are directed obliquely upwards and medially, and are reinforced by bundles of fibers, which are attached above to the jugular processes of the occipital bone, and below to the bases of the transverse processes of the atlas.

The atlanto-occipital joints frequently communicate with the joint between the odontoid process and the transverse ligament of the atlas.

The anterior atlanto-occipital membrane (fig. 514) is broad, and composed of densely woven fibers which pass between the anterior margin of the foramen magnum above, and the upper border of the anterior arch of the atlas below; laterally it is continuous with the capsular ligaments; in front, it is strengthened in the median plane by the continuation of the anterior longitudinal ligament, a strong, rounded cord, which connects the basilar part of the occipital bone to the tubercle on the anterior arch of the atlas (fig. 514).

Figure 514
Atlanto-occipital and atlanto-axial joints anterior view - Figure 514
The posterior atlanto-occipital membrane (fig. 515), broad but thin, is connected above to the posterior margin of the foramen magnum; below, to the upper border of the posterior arch of the atlas. On each side it arches over the groove for the vertebral artery, and with this groove bounds an opening for the entrance of the artery and the exit of the first cervical nerve. The free border of the membrane, arching over the artery and nerve, is sometimes ossified.

Figure 515
Posterior atlanto-occipital membrane posterior view - Figure 515
Movements.—-The movements permitted in this joint are (a) flexion and extension, which give rise to the ordinary forward and backward nodding of the bead, and (b) slight lateral motion to one or other side.

Muscles producing the movements

    • Flexion.-Longus capitis, Rectus capitis anterior, and Sternomastoid.
    • Extension.—-Recti capitis posteriores major et minor, Obliquus superior, Semispinalis capitis, Splenius capitis, and Trapezius (upper fibers).

Lateral flexion.-Rectus capitis lateralis, Semispinalis capitis, Splenius capitis, Sternomastoid and Trapezius (upper fibers).

2. THE LIGAMENTS CONNECTING THE AXIS WITH THE OCCIPITAL BONE

Membrana tectoria. Two alar. Apical ligament.

The membrana tectoria (figs. 517, 518) is situated within the vertebral canal, It is a broad, strong band, which covers the odontoid process and its ligaments and appears to be a prolongation upwards of the posterior longitudinal ligament of the vertebral column. It is fixed below to the posterior surface of the body of the axis, and, expanding as it ascends, is attached above to the upper surface of the basilar part of the occipital bone, in front of the foramen magnum, blending with the cranial dura mater. It covers the posterior aspect of the cruciate ligament of the atlas.

The alar ligaments of the odontoid process (fig. 517) are two strong, rounded cords, which arise one on each side of the upper part of the odontoid process, and passing obliquely upwards and laterally, are inserted into rough impressions on the medial sides of the condyles of the occipital bone. The alar ligaments limit both rotation and flexion of the cranium and on this account are sometimes called the check ligaments.

The apical ligament of the odontoid process (fig. 518), which extends from the tip of the process to the anterior margin of the foramen magnum, lies between, the alar ligaments, being intimately blended with the deep portion of the anterior atlanto-occipital membrane and with the upper longitudinal band of the cruciate ligament of the atlas. It is regarded as a rudimentary intervertebral disc, and may contain traces of the notochord in its substance.

It must be remembered that, in addition to the ligaments which unite the atlas and axis to the skull, the ligamentum nuchae connects the cervical vertebrae with the cranium.

Figure 517
Ligaments of the neck posterior view spinous processes removed - Figure 517
Figure 518
Occipital bone and C1,C2,C3 vertebrae median sagittal section - Figure 518
Applied Anatomy.-The ligaments of the vertebral column are so strong, and the bones so interlocked by the arrangement of their articulating processes, that dislocation is very uncommon, and, except in the upper part of the neck, rarely occurs unless accompanied by fracture. Dislocation of the occipital bone from the atlas has been recorded only in one or two cases; but dislocation of the atlas from the axis, with rupture of the transverse ligament of the atlas, is much more common; it is the mode in which death is produced in many cases of execution by hanging. Hanging may however produce a fracture through the axis, or a separation through the disc between the axis and the third cervical vertebra. Below the third cervical vertebra, dislocation without fracture occasionally takes place.

 


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