There are two distinct sets of articulations in the vertebral column:

1. Those between the bodies and intervertebral discs which form synchondroses and which are amphiarthrodial as regards movement.
2. Those between the articular processes which form arthrodial joints.

The ligaments which unite the various parts may also be divided into two sets, viz. - immediate, or those that bind together parts which are in contact; and intermediate, or those that bind together parts which are not in contact.

Immediate.

1.  Those between the bodies and discs.
2. Those between the articular processes.

Intermediate.

1. Those between the laminae.
2. Those between the spinous processes.
3. Those between the transverse processes.

The Articulations of the Bodies of the Vertebra

Class. - False Synchondrosis.

The ligaments which unite the bodies of the vertebrae are:

Intervertebral fibro-cartilages.

Short lateral ligaments.

Anterior longitudinal.

Posterior longitudinal.

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.

The Ligaments Connecting the Articular Processes

Class. - Diarthrosis. Subdivision. - Arthrodia.

The articular capsules which unite these processes are composed partly of yellow elastic tissue and partly of white fibrous tissue. In the cervical region only the medial side of the capsule is formed by the ligamenta flava, which in the thoracic and lumbar regions, however, extend anteriorly to the margins of the intervertebral foramina.

The part formed of white fibrous tissue consists of short, well-marked fibres, which in the cervical region pass obliquely downward and forward over the joint, between the articular processes and the posterior roots of the transverse processes of two contiguous vertebra. In the thoracic region the fibers are shorter, and vertical in direction, and are attached to the bases of the transverse processes; in the lumbar, they are obliquely transverse. The articular capsules in the cervical region are the most lax, those in the lumbar region are rather tighter, and those in the thoracic region are the tightest.

There is one synovial membrane to each capsule.

The Ligaments uniting the Lamina

The ligamenta flava are thick plates of closely woven yellow elastic tissue, interposed between the laminae of two adjacent vertebrae. The first connects the epistropheus with the third cervical, and the last the fifth lumbar with the sacrum. Each ligament extends from the medial and posterior edge of the intervertebral foramen on one side to a corresponding point on the other; above, it is attached close to the inner margin of the inferior articular process and to a well-marked ridge on the inner surface of the laminae as far as the root of the spine; below, it is fixed close to the inner margin of the superior articular process and to the dorsal aspect of the upper edge of the laminae.

Thus each ligamentum flavum, besides filling up the interlaminar space, enters into the formation of two articular capsules; they do so to a greater extent in the thoracic and lumbar regions than in the cervical, where the articular processes are placed wider apart. When seen from the front after removing the bodies of the vertebrae, they are concave from side to side, but convex from above downward; they make a more decided transverse curve than the arches between which they are placed. This concavity is more marked in the thoracic, and still more in the lumbar region than in the cervical; in the lumbar region the ligamenta flava extend a short distance between the roots of the spinous process, blending with the interspinous ligament, and making a median sulcus when seen from the front; there is, however, no separation between the two parts. In the cervical region, where the spines are bifid, there is a median fissure in the yellow tissue which is filled up by fibro-areolar tissue. The ligaments are thickest and strongest in the lumbar region; narrow but strong in the thoracic; thinner, broader, and more membranous in the cervical region.

The Ligaments connecting the Spinous Processes

These include supraspinous ligament, interspinous ligaments, and the liga- mentum nuchae.

The supraspinous ligament extends without interruption as a well-marked band of longitudinal fibers along the tips of the spines of the vertebrae from that of the seventh cervical downward till it ends on the median sacral crest.

Its more superficial fibers are much longer than the deep. The deeper fibers pass over adjacent spines only, while the superficial overlie several. It is connected laterally with the aponeurotic structures of the back; indeed, in the lumbar region, where it is well marked, it appears to result from the interweaving of the tendinous fibers of the several muscles which are attached to the tips of the spinous processes. In the dorsal region it is a round slender cord which is put on the stretch in flexion and relaxed in extension of the back.

The ligamentum nuchae, or the posterior cervical ligament, is the continuation in the neck of the supraspinous ligament, from which, however, it differs considerably. It is a slender vertical septum of an elongated triangular form, extending from the seventh cervical vertebra to the external protuberance and the crest of the occipital bone. Its anterior border is firmly attached to the tips of the spines of all the cervical vertebrae, including the posterior tubercle of the atlas, as well as to the occiput. Its posterior border gives origin to the trapezii, with the tendinous fibers of which muscle it blends. Its lateral, tri- angular surfaces afford numerous points of attachment for the posterior muscles of the head and neck.

In man it is rudimentary, and consists of elastic and white fibrous tissues. As seen in the horse, elephant, ox, and other pronograde mammals, it is a great and important elastic ligament, which even reaches along the thoracic part of the spinal column. In these animals it serves to support the head and neck, which otherwise from their own weight would hang down. Its rudimentary state in man is the direct consequence of his erect position.

The interspinous ligaments are thin membranous structures which extend between the spines, and are connected with the ligamenta flava in front, and the supraspinous ligament behind.

The fibers pass obliquely from the root of one spine to the tip of the next; they thus decussate. They are best marked in the lumbar region, and are replaced by the well-developed interspinales muscles in the cervical region.

The Ligaments connecting the Transverse Processes The intertransverse ligaments are but poorly developed.

In the thoracic region they form small rounded bundles, and in the lumbar they are flat membranous bands, unimportant as bonds of union. They consist of fibers passing between the apices of the transverse processes. In the cervical region they are replaced by the inter-transversarii muscles.

The arterial supply for the column comes from twigs of the vertebral, ascending pharyngeal, ascending cervical, superior and aortic intercostals, lumbar, ilio-lumbar, and lateral sacral.

Nerves

The nerve-supply comes from the spinal nerves of each region.

Movements of the vertebral colummn

The vertebral column is so formed of a number of bones and intervertebral discs as to serve many purposes. It is the axis of the skeleton; upon it the skull is supported; and with it the cavities of the trunk and the limbs are connected. As a fixed column it is capable of bearing great weight, and, through the elastic intervertebral substances, of resisting and breaking the transmission of shocks. Moreover, it is flexible. Now, the range of movements of the column as a whole is very considerable; but the movements between any two vertebrae are slight, so that motions of the spine may take place without any change in the shape of the column, and without any marked disturbance in the relative positions of the vertebrae. It is about the pulpy part of the intervertebral discs, which form a central elastic pivot or ball, upon which the middle of the vertebras rest, that these movements take place.

The amount of motion is everywhere limited by the common vertebral ligaments, but it depends partly upon the width of the bodies of the vertebrae, and partly upon the depth of the discs, so that in the loins, where the bodies are large and wide, and the discs very thick, free motion is permitted; in the cervical region, though the discs are thinner, yet, as the bodies are smaller, almost equally free motion is allowed. As the ball-Uke pulpy part of the intervertebral disc is the center of movement of each vertebra, it is obvious that the motion would be of a rolling character in any direction but for the articular processes, which serve also to give steadiness to the column and to assist in bearing the superincumbent weight. Were it not for these processes, the column, instead of being steady, endowed with the capacity of movement by muscular agency, would be tottering, requiring muscles to steady it. The influence of the articular processes in limiting the direction of inclination will appear from a study of the movements in the three regions of the spine.

In the neck all movements are permitted and are free, except between the second and third cervical vertebrae, where they are slight, owing to the shallow intervertebral disc and the great prolongation of the anterior hp of the inferior surface of the body of the epistropheus, which checks forward flexion considerably. On the whole, however, extension and lateral inclination are more free and extensive in this than in any other region of the column, whilst flexion is more limited than in the lumbar region. Rotatory movements are also free, but take place, on ac- count of the position and inclination of the articular facets, not, as in the thoracic region, round a vertical axis, but round an oblique axis, the articular process of one side gliding upward and forward and that of the opposite side downward and backward.

In the thoracic region, especially near its middle, antero-posterior flexion and extension are very slight; and, as the concavity of the curve here is forward, the flat and nearly vertical surfaces of the articular processes prevent anything like sliding in a curvilinear manner of the one set of processes over the sharp upper edges of the other, which would be necessary for forward flexion. A fair amount of lateral inclination would be permitted but for the impediment offered by the ribs; while the position and direction of the articular processes allows rotation round a vertical axis which passes through the centers of the bodies of the vertebrae. This rotation is not very great, and is freer in the upper than in the lower part of the thoracic region.

In the lumbar region, extension and flexion are very free, especially between the third and fourth and fourth and fifth vertebrae, where the lumbar curve is sharpest; lateral inclination is also very free between these same vertebrae. It has been stated that the shape and position of the articular processes of the lumbar and the lower two or three dorsal are such as to prevent any rotation in these regions; but, owing to the fact that the inferior articular processes are not tightly embraced by the superior, so that the two sets of articular processes are not in contact on both sides of the bodies at the same time, there is always some space in which horizontal motion can occur round an axis drawn through the central part of the bodies and intervertebral discs, but it is very slight. Thus, the motions are most free in those regions of the column which have a convex curve forward, due to the shape of the intervertebral discs, where there are no bony walls surrounding solid viscera, where the spinal canal is largest and its contents are less firmly attached, and where the pedicles and articular processes are more nearly on a transverse level with the posterior surface of the bodies of the vertebrae.

Nor must the uses of the ligamenta flava be forgotten: these useful structures - (1) complete the roofing-in of the vertebral canal, and yet at the same time permit an ever-changing variation in the width of the interlaminar spaces in flexion and extension; (2) they also restore the articulating surfaces to their normal position with regard to each other after movements of the column; (3) and by forming the medial portion of each articular capsule, they take the place of muscle in preventing it from being nipped between the articular surfaces during movement.

Muscles which take part in the movements of the vertebral column

Flexors

When acting with their fellows of the opposite side. Rectus abdominis, infra-hyoid muscles (slightly) sterno-mastoid, external oblique, internal oblique, intercostals, scalenus anterior, psoas major and minor, longus colli, longus capitis (rectus capitis anterior major).

Extensors

When acting with their fellows of the opposite side. Sacro-spinalis, quadratus lumborum, semispinalis, multifidus, rotatores, interspinales, serrati posteriores, the splenius, and with the scapula fixed the levator scapulae and the upper fibers of the trapezius.

Muscles which help to incline the column to their own side.

Sacro-spinalis, quadratus lumborum, semispinalis, multifidus, the intercostals helping to fix the ribs, the external and internal oblique muscles, levatores costarum, serrati posteriores, the scalenes, splenius cervicis, longus colli (oblique part), rotatores, intertransversales, psoas, and with the scapula fixed the levator scapulae and the upper and lower fibers of the trapezius.

Muscles which rotate the column and turn the body to their own side

Splenius cervicis, internal oblique (the ribs being fixed), serratus posterior inferior, and with the scapula fixed the lower fibers of the trapezius.

Muscles which rotate the column and turn the body to the opposite side

Multifidus, semispinalis, external oblique, the lower oblique fibers of the longus colli, and with the scapula and humerus fixed the latissimus dorsi and trapezius.

From Morris's treatise on anatomy.