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Joints Of The Vertebral Bodies

The vertebra from the third cervical to the first sacral inclusive are articulated to one another by: (1) a series of cartilaginous joints between the vertebral bodies; and (2) a series of synovial joints between the vertebral arches.

JOINTS OF THE VERTEBRAL COLUMN

THE JOINTS OF THE VERTEBRAL BODIES

The cartilaginous joints between the bodies of the vertebrae allow only slight movement between adjoining bones, but when this slight movement takes place in a number of consecutive joints the total range of movement is considerable. The vertebral bodies are united by anterior and posterior longitudinal ligaments, and by intervertebral discs of fibrocartilage.

The anterior longitudinal ligament (fig. 511) is a strong band of fibers, which extends along the anterior surfaces of the bodies of the vertebrae. It is broader below than above, thicker and narrower in the thoracic than in the cervical and lumbar regions, and somewhat thicker and narrower opposite the bodies of the vertebrae than opposite the intervertebral discs. It is attached, above, to the basilar part of the occipital bone, whence it extends to the anterior tubercle of the atlas, then to the front of the body of the axis and is continued down as far as the upper part of the front of the sacrum. It consists of longitudinal fibers, which are firmly fired to the intervertebral discs and to the margins of the vertebral bodies, but are loosely attached to the middle parts of the bodies. In the latter situation the ligament is thick and fills up the concavities on the anterior surfaces, and makes the front of the vertebral column more even. It is composed of several layers of fibers, of which the most superficial are the longest and extend between four or five vertebrae. The intermediate fibers extend between two or three vertebrae, while the deepest reach from one vertebra to the next. At the sides of the bodies the ligament consists of a few short fibers which connect adjacent vertebrae.

Figure 511

The posterior longitudinal. ligament (figs. 511, 512) is situated within the vertebral canal on the posterior surfaces of the bodies of the vertebrae. Above, it is attached to the body of the axis, and is thence continued downwards to the sacrum; its upper end is continuous with the membranes tectoria. It consists of smooth, glistening fibers, which are attached to the intervertebral discs and to the upper and lower margins of the vertebral bodies, but are separated from the middle parts of the bodies by the emerging basivertebral veins, and by veins which drain these into the anterior internal vertebral plexuses. In the cervical region the ligament is broad and of nearly uniform width, but in the thoracic and lumbar regions it, presents a denticulated appearance, being narrow over the vertebral bodies and broad over the intervertebral discs. It consists of superficial lavers occupying the interval between three or four vertebrae; and deeper layers which extend between adjacent, vertebrae.

The intervertebral discs (figs. 511, 512) are interposed between the adjacent surfaces of the bodies of the vertebra, from the axis to the sacrum, and form the chief bonds of connection between the vertebra. Their shape corresponds with that of the bodies between which they are placed. Their thickness varies in different region-is of the column, and in different parts of the same disc; they are thicker in front than behind in the cervical and lumbar regions, and thus contribute to the anterior convexities of these parts of the column; while they are of nearly uniform thickness in the thoracic region, the anterior concavity of this part of the column being almost entirely due to the shape of the vertebral bodies. They are adherent, by their surfaces, to thin layers of hyaline cartilage which cover the upper and under surfaces of the bodies of the vertebrae. The intervertebral discs are closely connected to the anterior and posterior longitudinal ligaments; in the thoracic region they are joined laterally, by means of the intra-articular ligaments, to the heads of those ribs which articulate with two vertebrae. The intervertebral discs constitute. about one-fourth of the length of the vertebral column, exclusive of the first two vertebrae; but this amount is not equally distributed between the various bones, the cervical and lumbar portions having, in proportion to their length, a much greater amount than the thoracic region, with the result that these parts possess greater pliancy and freedom of movement.

Figure 512

Structure of the intervertebral discs.-Each is composed, at its circumference, of laminae of fibrous tissue and fibrocartilage, forming the annulus fibrosus; and, at its center, of a soft, pulpy, highly elastic substance, of a yellowish colour, which projects considerably above the surrounding level when the disc is divided horizontally. This pulpy substance (nucleus pulposus), especially well developed in the lumbar region, contains the remains of the notochord. The laminae of the annulus fibrosus are arranged concentrically; the peripheral consist of ordinary fibrous tissue the others of white fibrocartilage. The laminae are not quite vertical in their direction, those Rear the circumference being curved outwards and closely approximated, while those nearest the center curve in the opposite direction, and are sometimes more widely separated. The fibers composing the laminae are directed, for the most part, obliquely from above downwards, the fibers of adjacent laminae cross one another, like the limbs of the letter X. This laminar arrangement exists in about the outer half of each disc. The nucleus pulposus consists of a fine fibrous matrix, containing angular cells united to form a reticular structure.

In the cervical region the cartilaginous joints between the bodies of the vertebrae are complicated occasionally by the presence of a small synovial cavity on each side between the bevelled lateral part of the under surface of the body and the lipped lateral margin of the upper surface of the body below. It is this little joint which is enlarged to form the principal joint in the case of the articulation between the atlas and the axis.

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