The joints between the articular processes of the vertebrae belong to the plane variety and are enveloped by articular capsules; the laminae, spines and transverse processes are connected by the following ligaments: ligamenta flavum, ligamentum nuchae, intertransverse, supraspinous, interspinous.
The articular capsules are thin and loose, and are attached just beyond the margins of the articular facets of adjacent articular processes; they are longer and looser in the cervical than in the thoracic and lumbar regions.
The ligaments flava (figs. 511, 513) connect the laminae of adjacent vertebrae, and are best seen from the interior of the vertebral canal. Their attachments extend from the articular capsules to the regions where the laminae fuse to form the spine; here their posterior margins come into contact and are to a certain extent united, small intervals being left for the passage of vessels. The ligaments flava consist of yellow elastic tissue, the fibers of which, almost perpendicular in direction, are attached to the lower part of the anterior surface of the lamina above, and to the posterior surface and upper margin of the lamina below. The ligaments are thin, but broad and long in the cervical region; they are thicker in the thoracic region, and thickest in the lumbar region. Their elasticity helps to preserve the upright posture, and to assist the vertebral column in resuming it after flexion.
The supraspinous ligament (fig. 511) is a strong fibrous cord, which connects together the apices of the spines from the seventh cervical vertebra to the sacrum; fibrocartilage is developed in the ligament at its points of attachment to the tips of the spines. It is thicker and broader in the lumbar than in the thoracic region, and intimately blended in both situations with the neighboring fascia. The most superficial fibers of this ligament extend over three or four vertebrae; those more deeply seated pass between two or three vertebrae; while the deepest connect the spines of neighboring vertebrae. In front it is continuous with the interspinous ligaments. Between the spine of the seventh cervical vertebra and the external occipital protuberance its place is taken by the ligamentum nuchae.
The ligamentum nuchae is a fibrous membrane, which, in the neck, is homologous with the supraspinous ligament of the thoracic and lumbar vertebrae. It extends from the external protuberance and external occipital crest to the spine of the seventh cervical vertebra. From its anterior border a fibrous lamina is given off, which is attached to the posterior tubercle of the atlas and to the spines of the cervical vertebrae, and forms a septum between the muscles of the two sides of the neck. In man it is the representative of an important elastic ligament which, in some of the lower animals, serves to sustain the weight of the head.
The interspinous ligaments (fig. 511), thin and membranous, connect adjoining spines, and their attachments extend from the root to the apex of each process. They meet the ligaments flava in front and the supraspinous ligament behind. They are narrow and elongated in the thoracic region broader, thicker, and quadrilateral in form in the lumbar region; and only slightly developed in the neck.
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