The Articulations of the Bodies of the Vertebra
The ligaments which unite the bodies of the vertebrae are:
Short lateral ligaments.
The intervertebral fibro-cartilages are tough, but elastic and compressible discs of composite structure, which serve as the chief bond of union between the vertebrae. They are twenty-three in number, and are inter- posed between the bodies of all the vertebrae from the epistropheus to the sacrum. Similar discs are found between the segments of the sacrum and coccyx in the younger stages of life, but they undergo ossification at their surfaces and often throughout their whole extent.
Each disc is composed of two portions - a circumferential laminar, and a central pulpy portion; the former tightly surrounds and braces in the latter, and forms somewhat more than half the disc. The fibrous ring [annulus fibrosus] or laminar portion consists of alternating layers of fibrous tissue and fibro-cartilage; the component fibers of these layers are firmly connected with two vertebrae, those of one passing obliquely down and to the right, those of the next down and to the left, making an X-shaped arrangement of the alternate layers. A few of the superficial lamellae project beyond the edges of the bodies, their fibers being connected with the edges of the anterior and lateral surfaces; and some do not completely surround the rest, but terminate at the intervertebral foramina, so that on horizontal section the circumferential portion is seen to be thinner posteriorly. The more" central lamellae are incomplete, less firm, and not so distinct as the rest; and as they near the pulp they gradually assume its characters, becoming more fibro-cartilaginous and less fibrous, and have cartilage cells in their structure.
The pulpy nucleus [nucleus pulposus] or central portion is situated somewhat behind the center of the disc, forming a ball of very elastic and tightly compressed material, which bulges freely when the confining pressure of the laminar portion is removed by either horizontal or vertical section. Thus, it has a constant tendency to spring out of its confinement in the direction of least resistance, and constitutes a pivot round which the bodies of the vertebrae can twist, tilt, or incline. It is yellowish in color, and is composed of fine white and elastic fibers amidst which are ordinary connective-tissue cells, and peculiar cells of various sizes which contain one or more nuclei. Together with the most central laminae, it is separated from immediate contact with the bone by a thin plate of articular cartilage. The central pulp of the intervertebral substance is the persistent part of the notochord.
The intervertebral substances vary in shape with the bodies of the vertebrae they unite, and are widest and thickest in the lumbar region. In the cervical and lumbar regions they are thicker in front than behind, and cause, the convexity forward of the cervical, and increase that of the lumbar; the curve in the thoracic region, almost entirely due to the shape of the bodies, is, however, somewhat increased by the discs. Without the discs the column loses a quarter of its length, and assumes a curve with the concavity forward, most marked a little below the mid-thoracic region. Such is the curve of old age, which is due to the shrinking and drying up of the intervertebral substances. The disc between the epistropheus and third cervical is the thinnest of all; that between the fifth lumbar and sacrum is the thickest, and is much thicker in front than behind. The intervertebral discs are in relation, in front with the anterior longitudinal ligament; behind, with the posterior longitudinal ligament; laterally, with the short lateral; and in the thoracic region, with the interarticular and radiate ligaments.
In the cervical region lateral diarthrodial joints are placed one on each side of the intervertebral discs. They are of small extent and are confined to the intervals between the prominent lateral lips of the upper surface of the body below and the beveled lateral edges of the lower surface of the body above. Situated in front of the issuing spinal nerves and between those parts of the bodies formed from the neural arches, they are homologous with the joints between the atlas and epistropheus, and between the atlas and occipital bone.
The anterior longitudinal ligament commences as a narrow band attached to the inferior surface of the occipital bone in the median line, just in front of the atlanto-occipital ligament, of which it forms the thickened central portion. Attached firmly to the tubercle of the atlas, it passes down as the central portion of the atlanto-epistrophic ligament, in the mid-line, to the front of the body of the epistropheus. It now begins to widen out as it descends, until it is nearly two inches (5 cm.) wide in the lumbar region. Below, it is fixed to the upper segment of the sacrum, becoming lost in periosteum about the middle of that bone; but is again distinguishable in front of the sacro-coccygeal joint, as the anterior sacro-coccygeal ligament.
Its structure is bright, pearly-white, and glistening. Its lateral borders are separated from the lateral bands by clefts through which blood-vessels pass; they are frequently indistinct and are best marked in the thoracic region. It is thickest in the thoracic region, and thicker in the lumbar than the cervical. It is firmly connected with the bodies of the vertebra, and is composed of longitudinal fibers, of which the superficial extend over several, while the deeper pass over only two or three vertebrae. It is connected with the tendinous expansion of the pre- vertebral muscles in the cervical, and the crura of the diaphragm are closely attached to it in the lumbar region.
The posterior longitudinal ligament extends from the occipital bone to the coccyx. It is wider above than below, and commences by a broad attachment to the cranial surface of the basi-occipital. In the cervical region it is of nearly uniform width, and extends completely across the bodies of the vertebrae, upon which it rests quite flat. It does, however, extend slightly further laterally on each side opposite the intervertebral discs. In the thoracic and lumbar regions it is distinctly dentated, being broader over the inter- vertebral substances and the edges of the bones than over the middle of the bodies, where it is a narrow band stretched over the bones without resting on them, the anterior internal vertebral venous plexus being interposed. The narrow median portion consists of longitudinal fibers, some of which are superficial and pass over several vertebrae; and others are deeper, and extend only from one vertebra to the next but one below.
The dentated or broader portions are formed by oblique fibers which, springing from the bodies near the intervertebral foramina, take a curved course downward and back-ward over an intervertebral fibro-cartilage, and reach the narrow portion of the ligament on the center of the vertebra next below; they then diverge to pass over another intervertebral dies to end on the body of the vertebra beyond, near the intervertebral notch. They thus pass over two discs and three vertebrae. Deeper still are other fibers thickening these expansions of the longitudinal ligament, and extending from one bone to the next.
The last well-marked expansion is situated between the first two segments of the sacrum: 'below this, the ligament becomes a delicate central band with rudimentary expansions, being more pronounced again over the sacro-coccygeal joint, and losing itself in the ligamentous tissue at the back of the coccyx. The dura mater is tightly attached to it at the margin of the foramen magnum and behind the bodies of the upper cervical vertebrae, but is separated from it in the rest of its extent by loose cellular tissue which becomes condensed in the sacral region to form the sacro-dural ligament. The filum terminale becomes blended with it at the lower part of the sacrum and back of the coccyx.
The lateral (or short) vertebral ligaments consist of numerous short fibers situated between the anterior and posterior longitudinal ligaments, and passing from one vertebra over the intervertebral disc, to which it is firmly adherent, to the next vertebra below.
The more superficial fibers are more or less vertical, but the deeper decussate and have a crucial arrangement. They are connected with the deep surface of the anterior longitudinal ligament, and so tie it to the edges of the bodies of the vertebrae and to the intervertebral discs. They blend behind with the expansions of the posterior longitudinal ligament, and so complete the casing round each amphiarthrodial joint. In the thoracic region, they overlie the radiate ligament, and in the lumbar they radiate toward the transverse processes. In the cervical region they are less well marked.