The vertebral column (spine) forms the central axis of the skeleton. It supports the skull and gives attachment, by way of the ribs, to the thoracic cage and, by way of this cage, to the pectoral girdle and upper limb. By the pelvic girdle it is strongly united to the lower limbs, which serve the double function of support and propulsion. The great strength of the column comes from the size and architecture of the bony elements, the vertebrae, and the ruggedness of the ligaments and muscles that hold them together. This great strength is combined with great flexibility; the column is flexible because it has so many joints so close together. Finally, the vertebral column contains in its cavity the spinal cord, to which it gives protection.
The vertebral column is made up of five parts with individual vertebrae peculiar to each: cervical, thoracic, lumbar, sacral and coccygeal.
In the fetus in utero the column lies flexed in its whole extent, like the letter C. This anterior flexion or concavity is the primary curvature of the column, and it is retained throughout life in the thoracic, sacral and coccygeal parts. After birth secondary extension of the column produces the secondary curvatures with an anterior convexity (i.e. lordosis) in the cervical and lumbar regions, the former associated with muscular support of the head and the latter with that of the trunk (see p. 35).
As the secondary curvatures develop in the neck and lumbar regions the vertebral column is opened out from its original C shape, and elongated into a vertical column characterized by gentle sinuous bends. These bends give a certain resilience to the column, but the actual shock-absorbing factors in the spinal column are the intervertebral discs.
General features of vertebrae
The general features of vertebrae are best exemplified by a thoracic vertebra. It consists of a ventral body and a dorsal vertebral or neural arch; they enclose between them the vertebral foramen (vertebral canal is the collective name given to the whole series of foramina when the vertebrae are strung together as a column). From the neural arch three processes diverge: in the posterior midline, the spinous process or spine, and on either side the transverse processes. That part of the neural arch between spinous process and transverse process is the lamina, that between transverse process and body is the pedicle. The vertical height of the pedicle is less than that of the body, to allow room for passage of the spinal nerve through the intervertebral foramen between the pedicles of adjacent vertebrae (Fig. 6.76). At the junction of lamina and pedicle (i.e. at the root of the transverse process) are articular processes, superior and inferior, which have hyaline cartilage facets for the synovial joints between the neural arches. The direction of the facets determines the nature of the movement possible between adjacent vertebrae.
Each cervical vertebra has a foramen in the transverse process (foramen transversarium or vertebrarterial foramen) and it has no costal facets. Each thoracic vertebra has costal facets on the side of the body. Each lumbar vertebra has neither a foramen in the transverse process nor costal facets. These two features, foramen in the transverse process and presence or absence of costal facets, serve to distinguish cervical, thoracic and lumbar vertebrae.
During its development a vertebra ossifies in three parts, the centrum and the right and left halves of the neural arch. In the thoracic region costal elements develop separately as the ribs, which articulate with the vertebrae. The centrum is not the same thing as the anatomical body of a vertebra. Part of the neural arch is incorporated into the body of the vertebra, and the neurocentral junction lies anterior to the costal facets on the body of a thoracic vertebra; hence these facets lie on the neural arch, and not on the centrum.
Costal elements develop in association with all vertebrae. But, except in the thoracic region, the costal elements are vestigial and fuse with the neural arches to become incorporated into the vertebrae. The foramen in the transverse process of a cervical vertebra is produced by this fusion (Fig. 6.74). The costal element consists of the anterior bar and tubercle, the intertubercular lamella and the posterior tubercle.
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Figure 6.74 Essential characteristics of cervical, thoracic and lumbar vertebrae (as viewed from above). |
A cervical rib is due to the elongation of the costal element of C7 vertebra. It presents as either bony elements or fibrous tissue bands, passing down from C7 vertebra to the first rib (Fig. 6.75). The subclavian artery and lowest root (T1) of the brachial plexus become displaced upwards over such a rib or band, and pressure upon the neurovascular structures from below may cause severe symptoms. The pressure produced by a thin fibrous band may do more harm than that due to a smooth bony rib. The presence of a fibrous band may be inferred if the anterior tubercle of C7 vertebra is enlarged. The patient whose radiograph is shown in Figure 6.75 had symptoms on the right side which were due to a fibrous band, and the well-ossified cervical rib on the left side produced no symptoms. When a cervical rib is well developed the brachial plexus is more likely to be prefixed (i.e. its roots are C4–8), thus preserving the normal nerve to rib relationship.
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Figure 6.75 Radiograph of a left cervical rib. On the right the anterior tubercle of C7 vertebra is greatly enlarged; a fibrous band passed from this tubercle to the first rib. |
The so-called transverse processes of the lumbar vertebrae are in reality costal elements. The true transverse process is contracted into a small mass of bone which is grooved by the medial branch of the posterior ramus of the spinal nerve. Above the groove lies the small mamillary process (on the superior articular facet) and below the groove is found the tiny accessory tubercle (Figs 6.94 and 6.96).
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Figure 6.94 Typical lumbar vertebra, from above. |
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Figure 6.96 L3 vertebra, from behind. |
The five sacral vertebrae are fused into a single bone and so, too, are the five costal elements. The latter produce the lateral mass of the sacrum, lying lateral to the transverse tubercles (lateral crest) on the back of the sacrum and extending between the anterior sacral foramina on the front of the bone to the side of the sacral vertebral bodies (Figs 6.97 and 6.98). The auricular surface for the sacroiliac joint lies wholly on the lateral mass.
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Figure 6.97 Sacrum: pelvic surface. |
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Figure 6.98 Sacrum from behind. The five pedicles and laminae have been cut through on the left to show the sheaths of dura mater around the nerve roots. |
Vertebral joints
Adjacent vertebrae are held together by strong ligaments and small joints. The vertebrae articulate between their bodies and between their neural arches. These joints are very different from each other, and they allow a greater range of movement between the neural arches than between the bodies.
Joints between the bodies
The bodies of adjacent vertebrae are held together by the strong intervertebral disc, and by the anterior and posterior longitudinal ligaments.
An intervertebral disc is a secondary cartilaginous joint, or symphysis. The upper and lower surfaces of each vertebral body are covered completely by a thin plate of hyaline cartilage. These plates are united by a peripheral ring, the annulus fibrosus, which has a narrow outer collagenous zone and a wider inner fibrocartilaginous zone. It consists of concentric laminae, the fibres of which lie at 25–45° with the horizontal plane. Alternate layers of the annulus contain fibres lying at right angles to each other. By this means the annulus is able to withstand strain in any direction. Inside the annulus is a bubble of semiliquid gelatinous substance, the nucleus pulposus, derived from the embryonic notochord. (The notochord extended originally as far cranially as the sella turcica of the skull, but it disappears except in the nucleus pulposus of each intervertebral disc and in the apical ligament of the atlas.) The nucleus pulposus in the embryo lies at the centre of the disc. Subsequent growth of the vertebral bodies and discs occurs in a ventral and lateral direction (the spinal cord prevents a corresponding growth dorsally). Thus in the adult and especially in the lumbar region the nucleus pulposus lies nearest to the back of the disc (Fig. 6.76) and if it herniates through the annulus it will be most likely to do so posteriorly and press on the roots of a spinal nerve near the intervertebral foramen, or on the spinal cord itself.
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Figure 6.76 Bisected vertebral column (lumbar region). The left half is seen from the right, so showing the inside of the vertebral canal, intervertebral discs in section, and the boundaries of two intervertebral foramina. |
The nucleus pulposus accounts for 15% of the whole disc. It contains about 90% water at birth, and this diminishes to about 70% in old age. The water content keeps the nucleus under constant pressure since its mucoprotein (proteoglycan) component has the property of imbibing and retaining water. Imbibition of water by the nucleus accounts for the overnight increase in height of a young adult by 1cm; when upright during the day, water is squeezed out. In old age there is little height change between night and morning; imbibition of water becomes less and the nucleus more fibrous. In astronauts, who have been relieved of gravity, there may be a height increase of several centimetres.
The relationship of nerve roots to intervertebral discs is of clinical importance, and is best understood by considering the lowest disc—the fifth lumbar or lumbosacral disc—which is the one most frequently herniated or prolapsed (‘slipped disc’), with its nucleus pulposus being extruded posterolaterally. At the level of this fifth lumbar disc, the fifth lumbar nerve root within its dural sheath has already emerged from the intervertebral foramen, hugging the pedicle of L5 vertebra and so is not lying low enough to come in contact with the fifth lumbar disc. The roots that lie behind the posterolateral part of this disc are those of the first sacral nerve, and these are the ones liable to be irritated by a prolapse. Thus the general rule throughout the vertebral column is that when a disc herniates (usually posterolaterally rather than in the midline) it may irritate the nerve roots numbered one below the disc: S1 nerve by L5 disc; L5 nerve by L4 disc; and C8 nerve by C6 disc (there are 8 cervical nerve roots and 7 cervical vertebrae). These are the commonest clinical examples.
The posterolateral lip, or uncus, on the upper surface of cervical vertebrae 3 to 7 (see p. 432) may appear to form a joint with the side of the vertebra above because a small cavity may develop in this region (the so-called neurocentral, uncovertebral or Luschka's joint). It is disputed as to whether these are synovial joints, or are due to degenerative changes in the adjacent disc.
The anterior longitudinal ligament extends from the basiocciput of the skull and the anterior tubercle of the atlas to the front of the upper part of the sacrum. It is firmly united to the periosteum of the vertebral bodies, but is less so over the intervertebral discs. It is a flat band, broadening gradually as it passes downwards.
The posterior longitudinal ligament extends from the back of the body of the axis to the anterior wall of the upper sacral canal. It narrows gradually as it passes downwards. It has serrated margins, being broadest over the discs to which it is firmly attached, and narrow over the vertebral bodies to which it is more loosely attached in order to give free exit to the basivertebral veins emerging from the backs of the bodies (Fig. 6.10). At the top the ligament is continued above the body of the axis as the tectorial membrane (see p. 426 and Fig. 6.80).
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Figure 6.80 Ligaments of the atlanto-occipital and atlantoaxial joints. The posterior part of the occipital bone and the laminae of the upper cervical vertebrae have been removed. |
Joints between the arches
The pedicles of adjacent vertebrae are not attached to one another, so leaving a space—the intervertebral foramen—for the emergence of the spinal nerve. All other parts of the neural arch and its processes are joined to their adjacent companions: the articular processes by synovial joints, and the remainder by ligaments, of which the most important are the ligamenta flava and the supraspinous ligament.
The joints between the articular facets of the superior articular processes of one vertebra and the articular facets of the inferior articular processes of the vertebra above are termed the zygapophyseal joints (or simply known as facet joints). They are synovial with a simple capsule which blends medially with a ligamentum flavum. The articular surfaces allow gliding of one on the other; the direction of the surfaces determines the direction of the possible movements between adjacent vertebrae. The joints have a nerve supply from the nerve of their own segmental level and from the nerve of the segment above. One nerve thus supplies two joints; this may be important when considering nerve root pain which can be referred from facet joints. Although most of the weight transmission by the vertebral column takes place via the vertebral bodies and intervening discs, a small amount does occur through these joints.
The paired ligamenta flava are yellowish from their high content of elastic fibres. They join the contiguous borders of adjacent laminae (Fig. 6.76). They are attached above to the front of the upper lamina and below to the back of the lower lamina. Thus adjacent laminae and ligamenta flava overlap each other slightly like the tiles of a roof. The ligamenta extend from the facet joints to the midline where they partially fuse; small veins connecting the internal and external vertebral venous plexuses may pass between a pair of ligamenta. They are stretched by flexion of the spine; in leaning forward their increasing elongation becomes an increasing antigravity support.
The supraspinous ligaments join the tips of adjacent spinous processes (Fig. 6.76). They are strong bands of white fibrous tissue and are lax in the extended spine. They are drawn taut by full flexion, and then support the spine (no action currents can be obtained from the erector spinae muscles when the spine is fully flexed, as in touching the toes). They are indistinct below the L4 spine where the lumbar fascia is thick. In the neck they are replaced by the ligamentum nuchae (see p. 430).
The interspinous ligaments are relatively weak sheets of fibrous tissue uniting spinous processes along their adjacent borders (Fig. 6.76). They are well developed only in the lumbar region. They fuse with the supraspinous ligaments.
The intertransverse ligaments are similar weak sheets of fibrous tissue joining the transverse processes along their adjacent borders.
Vertebral column
In the normal erect posture the vertebral column supports the head and trunk on the pelvis. (The pelvis is supported by the lower limbs in standing and by its own ischial tuberosities in sitting.) This support is maintained by the bodies of the vertebrae and the intervertebral discs, which thus become progressively larger from above downwards. The curvatures of the spine are produced partly by the wedge-shape of the vertebral bodies, but mostly by the wedge-shape of the intervertebral discs. This is particularly noticeable in the lower part of the spine; L5 vertebra is usually wedge-shaped and the disc between it and the sacrum is very thick anteriorly.
The vertebral canal (see p. 421) becomes progressively smaller from above downwards. It is closed anteriorly by the vertebral bodies, the intervertebral discs and the posterior longitudinal ligament and posteriorly by the laminae and the ligamenta flava. Laterally it is occupied by the pedicles, which are narrower than the height of the vertebral bodies. Thus a series of intervertebral foramina is produced between adjacent pedicles which form the upper and lower boundaries of each foramen. In the thoracic and lumbar regions each intervertebral foramen is bounded in front by the lower part of a body of a vertebra (mainly, in the thoracic region; Fig. 6.92) and the adjacent intervertebral disc (mainly, in the lumbar region; Fig. 6.93), and behind by the facet joint and its capsule. In the cervical region, because the pedicle arises from a little lower down the back of the body, a small part of the vertebral body below the disc is also included in this anterior boundary (Fig. 6.88). The intervertebral foramina lodge the spinal nerves and posterior root ganglia and give passage to the spinal arteries and veins.
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Figure 6.93 T11 and T12 vertebrae, from the left. |
Movements of the vertebral column
In general the movements of the spine are simple enough. Flexion and extension, and lateral flexion (abduction) are possible in cervical, thoracic and lumbar regions, though in varying degree in the three parts. Rotation occurs mainly in the thoracic region. Movements of the head occur at the specialized atlanto-occipital and atlantoaxial joints.
Lumbar region
The articular facets lie in an anteroposterior plane; they lock, and greatly limit rotation of the bodies on each other. Flexion and extension are free, and a good deal of lateral flexion is possible.
Thoracic region
The synovial joints between T12 and L1 are lumbar in type (Fig. 6.93); elsewhere the direction of the articular facets on the neural arches is quite different. On any one neural arch the upper facets face backwards and laterally (Fig. 6.74); they lie on the circumference of a circle whose centre lies in the vertebral body. The lower facets face reciprocally forwards and medially. Thus rotation of the bodies on each other is possible, though restricted by the splinting effect of the ribs. As in the lumbar region, flexion and extension occur, as well as ‘lateral flexion’. The thoracic spine is thus the most mobile region of all, but the range of movements is limited by the ribs.
Cervical region
The atlanto-occipital and atlantoaxial joints are specialized for head nodding and head rotation. They are considered below.
The upper articular facets of the other joints face back-wards and upwards; the lower facets face, reciprocally, forwards and downwards. While flexion and extension are free, pure rotation is impossible. Lateral flexion is not a simple movement. The neural arch of the abducted vertebra slides downwards (and therefore backwards) on the concave side and upwards (and therefore forwards) on the convex side, thus inevitably producing slight concomitant rotation.
Special vertebrae and joints
The atlas (Fig. 6.77) lacks a centrum (see p. 422). The vertebral arch has become modified to form a thick lateral mass on each side, joined at the front by a short anterior arch and with a longer posterior arch at the back. The articular facets on the upper and lower surfaces of the lateral mass differ markedly. The upper surface is kidney-shaped and concave for articulation with the occipital condyle, while the lower is round or oval and nearly flat for the lateral atlantoaxial joint. The articular facets are in line with the uncovertebral joints (see p. 424) of the other cervical vertebrae, not with the articular facets on the neural arches. Thus the C1 and C2 nerves send their anterior rami behind, and not in front of, the joints.
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Figure 6.77 Superior surface of the atlas. C1 nerve divides into anterior and posterior rami just behind the atlanto-occipital joint (right side), where it lies in the groove beneath the vertebral artery (left side). |
The axis is characterized by the dens and a large spinous process (Fig. 6.78). The dens (odontoid process) has an articular facet at the front for the joint with the anterior arch of the atlas. It bears no weight. The weight of the skull is transmitted through the lateral mass of the atlas to the superior articular process of the axis which lies immediately lateral to the dens. The lower articulations of the axis are as for the ordinary cervical vertebrae: body to body with intervening disc and the two uncovertebral joints, and the ordinary articular facets on the neural arch. From the axis downwards the weight of the skull is supported by the vertebral bodies. The bifid spinous process is very large, due to the attachments of muscles of the suboccipital triangle above (Fig. 6.84).
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Figure 6.78 Anterior and lateral views of the axis. |
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Figure 6.84 Suboccipital region and the suboccipital triangle. |
The atlanto-occipital and the atlantoaxial joints are adapted to provide freedom of head movement, the former for nodding and lateral flexion, the latter for rotation.
The atlanto-occipital joint is a synovial joint between the convex occipital condyle and the concave facet on the lateral mass of the atlas. Both surfaces are covered with hyaline cartilage. The synovial cavity of the joint is contained in a lax but strong capsule, which is attached to the articular margins of both bones and is innervated by C1 nerve.
The anterior and posterior atlanto-occipital membranes are attached to the upper borders of the respective arches of the atlas and to the outer margins of the foramen magnum (Fig. 6.84). The posterior membrane is deficient at each lateral extremity to allow passage for the vertebral artery and C1 nerve; the lateral margin of the membrane sometimes ossifies, converting the groove for the vertebral artery into a foramen. The membranes are innervated by C1 nerve.
The curved surfaces of the joint are well adapted for head flexion and extension, and allow also for a considerable amount of lateral flexion of the skull on the atlas. In the ordinary erect position the centre of gravity of the skull lies in front of the joint and the head is maintained in position by the tonus of the extensor muscles, notably semispinalis capitis. It is flexed by relaxation of the extensors (i.e. by gravity) and, actively, by longus capitis and the two sternocleidomastoids acting together. The effect of gravity is considerable on account of the weight of the head and, of course, varies with position. Lateral flexion is produced by unilateral contraction of such muscles as sternocleidomastoid, trapezius and splenius capitis. No rotation is possible at the atlanto-occipital joints.
The median atlantoaxial joint is where the dens articulates with the back of the anterior arch of the atlas (Fig. 6.79). The smooth facets seen on the dry bones are covered with hyaline cartilage in life, with a capsule attached to their margins to make this a small synovial joint. The dens is held in position by the transverse ligament; between the two is a relatively large bursa.
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Figure 6.79 Atlas and related structures. 1. Dens of axis with apical and alar ligaments. 2. Mucosa of nasopharynx over levator palati. 3. Internal carotid artery and last four cranial nerves (Roman figures). 4. Prevertebral fascia. 5.Superior oblique. 6. Vertebral artery. 7. Posterior ramus of C1. 8. Medulla/spinal cord junction (denticulate ligament, accessory nerve and roots of C1 not labelled). 9. Rectus capitis posterior major. 10. Spinous process of axis. 11.Rectus capitis posterior minor, from posterior arch of atlas. 12. Posterior atlanto-occipital membrane. 13. Spinal dura and arachnoid. 14. Inferior oblique. 15. Tectorial membrane. 16. Transverse band of cruciform ligament. 17.Anterior ramus of C1 giving branch to hypoglossal nerve and ending in longus capitis. 18. Internal jugular vein. 19. Pharyngobasilar fascia. 20. Anterior atlanto-occipital membrane. |
On each side there is a lateral atlantoaxial joint (also synovial) between the inferior articular facet of the atlas and the superior articular facet of the axis (Fig. 6.81). The joint surfaces are nearly circular and flat with hyaline cartilage lining, and there is a lax capsule supplied by C2 nerve.
Accessory ligaments connect the axis to the occiput, bypassing the atlas: the tectorial membrane, cruciform ligament, apical ligament and the paired alar ligaments.
The tectorial membrane extends upwards in continuity with the posterior longitudinal ligament (Fig. 6.80). It is attached to the back of the body of the axis and diverges upwards to become attached to the basilar part of the occipital bone above the foramen magnum. It lies in front of the spinal dura mater, which is firmly attached to it (Fig. 6.79).
The transverse ligament is a broad strong band that runs across (and grooves) the back of the dens from its attachment on each side to a tubercle in the ‘hilum’ of the kidney-shaped upper articular facet of the atlas. A weaker longitudinal band runs from the back of the body of the axis to the basiocciput and together with the transverse ligament constitutes a cruciform ligament (Fig. 6.80). This lies in contact with the front of the tectorial membrane. They hold the dens in position; rupture of the ligament and membrane allows the dens to dislocate backwards with fatal pressure on the medulla.
The weak apical ligament joins the apex of the dens to the anterior margin of the foramen magnum, and is a fibrous remnant of the notochord (see p. 23).
The alar ligaments lie obliquely one on either side of the apical ligament. From the sides of the dens they diverge upwards to the margins of the foramen magnum. They are very strong and limit rotation of the head.
Movements at the atlantoaxial joints are simply those of rotation about a vertical axis passing through the dens. The atlas rotates by its anterior arch and transverse limb of the cruciform ligament gliding around the dens and by the lower flat facets on its lateral mass gliding on the superior facets of the axis. The head rotates with the atlas; the curved surfaces of the atlanto-occipital joints do not allow independent rotation of occiput on atlas.
The muscles chiefly responsible for rotation are sterno-cleidomastoid, splenius capitis and the inferior oblique.
The atlantoaxial region of the cervical spine can be visualized in transoral anteroposterior radiographs (Fig. 6.81). The transoral route is also utilized in surgical approaches to this region, with upward retraction of the soft palate and division of the posterior wall of the pharynx.
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Figure 6.81 Transoral anteroposterior radiograph. |
Blood supply of the vertebrae
The vertebrae are supplied segmentally by the vertebral, ascending and deep cervical, posterior intercostal, lumbar and lateral sacral arteries, which give multiple small branches to the vertebral bodies.
The richly supplied red marrow of the vertebral body drains through its posterior surface by a pair of large basivertebral veins into the internal vertebral venous plexus, which lies inside the vertebral canal, outside the dura (Fig. 6.110). It drains into the external vertebral venous plexus. This intramuscular plexus, which also receives blood from the neural arch, drains into the regional segmental veins (vertebral, posterior intercostal, lumbar and lateral sacral veins), which in turn drain into brachiocephalic veins, superior vena cava, inferior vena cava and internal iliac veins. Venous communication is thus established in the pelvis with veins draining the pelvic viscera, in the abdomen with the renal veins, in the thorax with the azygos venous system (and thereby with the venous drainage of the breast and bronchus), and in the neck with the inferior thyroid veins. In this way, by reflux blood flow through these largely valveless veins, malignant disease may spread from prostate, kidney, breast, bronchus and thyroid gland to the bodies of the vertebrae.
Extensor muscles of the spine
Running along the whole length of the vertebral column from skull to sacrum is a posterior mass of mainly longitudinal extensor muscles, derived from the outer of the three layers of the body wall and supplied segmentally by posterior rami of spinal nerves. They form a bulge on either side of the midline of the back, often best seen in the lumbar region. In the neck the posteriormost layer is the splenius muscle. Elsewhere the muscles are covered posteriorly by the thoracolumbar fascia.
The deepest muscles are the small interspinales and intertransversales. The remainder form intermediate and superficial masses collectively called transversospinalis and erector spinae, each of which is composed of three groups. Transversospinalis includes the rotatores, multifidus and semispinalis, while erector spinae comprises iliocostalis, longissimus and spinalis.
Deep layer
The interspinales join adjacent borders of spinous processes, alongside the interspinous ligaments. The inter-transversales join adjacent transverse processes; they are best developed in the upper part of the vertebral column.
Transversospinalis (intermediate layer)
As a group these muscles run from transverse processes to spines, hence the name. The rotatores are small, but multifidus and semispinalis form larger muscle masses.
The rotatores are confined to the thoracic spine, the only region where pure rotation occurs. Each extends from the base of a transverse process to the root of the spinous process of the vertebra above (Fig. 6.82).
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Figure 6.82 Rotator spinae and levator costae muscles. |
Multifidus fibres slope upwards from the back of the sacrum, mamillary processes of lumbar vertebrae, transverse processes of thoracic vertebrae and articular processes of cervical vertebrae to the spinous processes of vertebrae two or three above their level of origin. They commence at the upper part of the sacrum and extend to the upper part of the neck.
Semispinalis lies on the surface of multifidus. Its fibres arise from the transverse processes and slope steeply upwards to the spinous processes. It extends from the lower thoracic region to the skull. Semispinalis thoracis extends from the transverse processes of the lower thoracic vertebrae and each part is inserted into the spinous process six or more vertebrae higher. Semispinalis cervicis arises in continuity at a higher level; the uppermost part of the muscle is inserted into the concavity of the bifid spinous process of the axis.
Semispinalis capitis is the most powerful part of this layer. It arises from the transverse processes of the upper six thoracic and the articular processes of the lower four cervical vertebrae and is inserted into the occipital bone near the midline, between the superior and inferior nuchal lines (see p. 506). It lies beneath splenius and trapezius and is the chief extensor of the head. It contains a large plexus of veins within and around it.
Erector spinae (superficial layer)
This forms the most powerful muscle group. It commences below, deep to the lumbar fascia, on the back of the sacrum and the inner side of the iliac crest. The thick mass of fibres passes upwards and slightly outwards dividing as it does so into two main bundles, iliocostalis laterally and longissimus medially (Fig. 6.83). The iliocostalis fibres are inserted by shining tendons into the angles of the lower six ribs. From these attachments new muscle bundles arise and each runs up to be attached to the angle of the sixth rib above. From there further fibres run up to reach the transverse processes of the lower four cervical vertebrae. Iliocostalis forms the most lateral part of erector spinae.
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Figure 6.83 Parts of the left erector spinae muscle. The most lateral mass is iliocostalis. Most of longissimus (whose lower part is seen adjacent to iliocostalis) has been removed to show various parts of spinalis. Prosection in the Anatomy Museum of the Royal College of Surgeons of England. |
The more medial longissimus bundle arising from the sacrum and iliac crest passes up to be inserted into the gutter between transverse processes and ribs; this is longissimus thoracis. At its insertion it is replaced by new fibres on the medial side that pass up to the transverse processes of the cervical vertebrae; this is longissimus cervicis. Longissimus capitis arises from the transverse processes of the lower three or four cervical vertebrae and is inserted into the mastoid process deep to splenius capitis.
The most medial part of erector spinae is the spinalis part. Its fibres run alongside the spinous processes and are small and often indefinite.
Splenius
In the neck, the underlying extensor muscles are bound down by splenius, a flat sheet arising from upper thoracic spinous processes and the supraspinous ligament, and from the ligamentum nuchae. The fibres slope upwards and laterally, deep to trapezius and sternocleidomastoid (Fig. 6.4), and are inserted (as splenius capitis) into the mastoid process and the lateral third of the superior nuchal line, and (as splenius cervicis) into the transverse processes of the upper three or four cervical vertebrae (deep to levator scapulae). The whole muscle is like a bandage that holds down the deeper extensor muscles at the back of the neck. It, like the muscles deep to it, is supplied by posterior rami.
Back of the neck
The back of the neck consists of muscles connecting the skull to the spine and pectoral girdle. In the midline the ligamentum nuchae separates the muscles of the two sides. This is a triangular septum of fibroelastic tissue attached to the external occipital crest, the bifid spines of the cervical vertebrae, the tubercle of the spine of C7 vertebra and the investing layer of deep cervical fascia which encloses the trapezius muscles.
Beneath the trapezius and sternocleidomastoid lies the splenius, and beneath the splenius lie longissimus capitis and semispinalis capitis. When all these are removed the deeper structures of the back of the neck are seen to be divided into upper and lower portions by the prominent backward projection of the massive spinous process of the axis (C2 vertebra). Below this level semispinalis cervicis is seen converging almost vertically upwards to the internal surfaces of the bifid axial spine. Above the spine lie the right and left suboccipital triangles.
Suboccipital triangle
The suboccipital triangle is bounded by rectus capitis posterior major and the superior and inferior oblique muscles (Fig. 6.84). Its floor contains the posterior arch of the atlas and the posterior atlanto-occipital membrane. Across the floor runs the vertebral artery, and through the floor emerges the suboccipital (C1) nerve. Across the roof run the greater occipital (C2) nerve and the occipital artery.
Rectus capitis posterior major arises from the outer surface of the bifid spinous process of C2 vertebra and extends obliquely upwards and outwards to be attached to the lateral part of the area below the inferior nuchal line. Its action is to extend the head and rotate it (with the atlas) back towards its own side.
The inferior oblique (obliquus capitis inferior) is attached between the outer surface of the bifid spine of the axis (below rectus capitis posterior major) and the back of the transverse process of the atlas. Its action is to rotate the atlas (and the skull with it) back towards its own side.
The superior oblique (obliquus capitis superior) extends from the upper surface of the transverse process of the atlas to the lateral part of the occipital bone between superior and inferior nuchal lines. It is a lateral flexor of the skull.
Rectus capitis posterior minor is the only muscle attached to the posterior arch of the atlas, from where it passes vertically upwards to be inserted into the medial part of the area below the inferior nuchal line. It lies deep to rectus capitis posterior major. It weakly extends the head.
All four muscles are supplied by the posterior ramus of C1.
The second part of the vertebral artery (see p. 348) ascends through the foramina in the transverse processes of the upper six cervical vertebrae, anterior to the emerging spinal nerves. It gives a spinal branch into each intervertebral foramen. Its course from C6 to C2 vertebra is vertical. Between the foramina in the transverse processes of the axis and atlas it passes laterally with a pronounced posterior convexity and then loops upwards beside the atlantoaxial joint (Figs 6.84, 6.85, 6.86 and 6.109). This must be to allow for taking up slack during rotation of the atlas on the axis.
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Figure 6.85 Vertebral arteriogram: anteroposterior view. |
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Figure 6.86 Vertebral arteriogram: lateral view. |
On emerging from the foramen in the transverse process of the atlas, the third part of the vertebral artery curves backwards and medially behind the lateral mass of the atlas. Here it lies in the floor of the suboccipital triangle before piercing the lateral angle of the posterior atlanto-occipital membrane. It deeply grooves the posterior arch of the atlas before entering the skull through the foramen magnum.
The vertebral veins exist only in the neck, not inside the cranial cavity which the vertebral arteries enter. Blood from the veins in and around semispinalis capitis and from the muscles of the suboccipital triangle is collected in a plexus of veins that surrounds the second and third parts of the vertebral artery. The thin walls of these veins are adherent to the periosteum of the posterior arch of the atlas and the foramina in the transverse processes. From this plexus two vertebral veins usually emerge, one from the sixth foramen with the vertebral artery and one alone through the foramen in the transverse process of C7 vertebra, and join the brachiocephalic vein at the root of the neck (Fig. 6.9B).
In the groove between artery and bone on the posterior arch of the atlas lies the posterior ramus of the suboccipital (C1) nerve, as it passes backwards to supply the two recti, the two obliques and the upper fibres of semispinalis capitis. The anterior ramus winds round the lateral side of the lateral mass of the atlas between it and the vertebral artery, and passes forwards between rectus capitis lateralis and anterior to join the cervical plexus (Fig. 6.8). Neither branch reaches the skin.
The greater occipital nerve is the long posterior ramus of C2 and emerges below the posterior arch of the atlas. It curls around the lower border of the inferior oblique muscle and passes upwards across the roof of the suboccipital triangle. It pierces semispinalis capitis (first supplying it) and extends up to supply the skin of the scalp up to the vertex.
The occipital artery is a large vessel that passes back along the occipitomastoid suture of the skull deep to digastric and longissimus capitis. It runs across the upper part of the roof of the suboccipital triangle and passes to the scalp. The companion veins form a rich plexus around and within semispinalis capitis.
