Chapter 39: The vertebral column
The vertebral column usually consists of 33 vertebrae: 24 presacral vertebrae (7 cervical, 12 thoracic, and 5 lumbar) followed by the sacrum (5 fused sacral vertebrae) and the coccyx (4 frequently fused coccygeal vertebrae). The 24 presacral vertebrae allow movement and hence render the vertebral column flexible. Stability is provided by ligaments, muscles, and the form of the bones. The abbreviations C., T., L., S., and Co. are used for the regions, and these are sometimes followed by V. for vertebra or N. for nerve.
The adult vertebral column presents four anteroposterior curvatures: thoracic and sacral, both concave anteriorly, and cervical and lumbar, both concave posteriorly (fig. 39-1). The thoracic and sacral curvatures, termed primary, appear during the embryonic period proper, whereas the cervical and lumbar curvatures, termed secondary, appear later (although before birth) and are accentuated in infancy by support of the head and by the adoption of an upright posture.
A typical vertebra consists of (1) a body and (2) a vertebral arch, which has several processes (articular, transverse, and spinous) for articular and muscular attachments. Between the body and the arch is the vertebral foramen: the sum of the vertebral foramina constitutes the vertebral canal, which houses the spinal cord (fig. 39-2). In addition to the transverse and spinous processes, which serve as short levers, the 12 thoracic vertebrae are connected by joints with paired, long levers, namely the ribs.
The bodies of the vertebrae are separated from each other by intervertebral discs. The body is mainly spongy bone and red marrow, but the margins of the upper and lower surfaces consist of a ring of compact bone, the vertebral end-plates. The body is marked on its sides by vascular foramina.
The vertebral arch consists of right and left pedicles (which connect it to the body) and right and left laminae. The transverse processes emerge laterally at the junction of the pedicles and laminae, and the spinous process proceeds posteriorly from the union of the laminae. The superior and inferior articular processes project vertically from the vertebral arches on each side and bear articular facets. When vertebra are in their anatomical position, notches between adjacent pedicles form intervertebral foramina, each of which typically transmitsneural structures including a spinal ganglion and a ventral root of a spinal nerve.
The first cervical nerve emerges between the skull and the atlas, and cervical nerves C2 to 7 continue to leave the vertebral canal above the correspondingly numbered vertebrae. C8 emerges between the C7 and T1 vertebrae, and the remaining spinal nerves leave below the correspondingly numbered vertebrae.
The vertebrae of each region have special characteristics, which are now described.
The seven vertebrae of the neck are characterized by an opening in each transverse process known as a foramen transversarium. The upper six pairs of foramina transversaria transmit the vertebral artery. The C1 vertebra, which supports the skull, is termed the atlas, and C2, which serves as a pivot for the atlas, is termed the axis.
Atlas (fig. 39-3).
The atlas (C1), which has neither body nor spinous process, consists of two lateral masses connected by a short anterior and a longer posterior arch. Each lateral mass presents upper and lower facets, for the occipital condyle of the skull and for the axis, respectively. The transverse processes are long and are vaguely palpable in vivo immediately below the auricle. The anterior arch presents an anterior tubercle in front (for the anterior longitudinal ligament) and a facet posteriorly (for the dens of the axis). The posterior arch is grooved above for the vertebral artery and the small C1 nerve on each side, and it presents a posterior tubercle behind.
Axis (fig. 39-4).
The axis (C2) is characterized by the dens (or odontoid process), which projects upward from the body and articulates with the anterior arch of the atlas. The dens is anchored to the occipital bone (by apical and alar ligaments) and is limited behind by the transverse ligament of the atlas (fig. 39-3). It is frequently claimed that the dens represents the body of the atlas, but this is doubtful. *
Remaining cervical vertebrae (fig. 39-5).
The lower cervical vertebrae, C2 to 6 are typical and present short, bifid spinous processes. The bodies are small and oval in shape. There are superiorly-projecting lips at the superolateral borders of the vertebral bodies. These closely match indentations in the lateral border of inferior aspect of the vertebral body above. Each transverse process, pierced by a foramen transversarium, ends laterally in anterior and posterior tubercles, which are connected by an "intertubercular lamella" or bar. # The bars are grooved by the ventral primary rami of the spinal nerves, which pass posterior to the foramina transversaria. The anterior tubercles of C6 vertebra are large and are termed the carotid tubercles, because the common carotid arteries can be compressed against them. C7 vertebra has a long, non-bifid spinous process and is known as the vertebra prominens. (The spinous processes of C6 and 7 and T1 are usually visible in vivo when the neck is flexed.) The anterior tubercles (costal processes) of C7 vertebra may develop separately as cervical ribs. (Lumbar ribs are less frequent.)
It should be noted that all vertebrae are dorsal, although only 12 are thoracic. The 12 vertebrae of the thorax bear the ribs. The T1 vertebra (like C.V.7) is transitional in appearance. T2 to 8 vertebra are typical thoracic vertebrae with a kidney shaped body (fig. 39-5). Demifacets for the heads of the ribs (see fig. 20-3) are found supeiorly and inferiorly at the junction of the body and pedicle. The transverse processes of T1 to 10 have costal facets for the tubercles of the ribs. The spinous processes are long, slender, and sloping: their tips lie opposite the subjacent body or even at the level of the intervertebral disc below the subjacent body. The various facets for the ribs on T1 and on T9 to 12 veertebral bodies are arranged differently. T11 and 12 are transitional in form, i.e., partly resembling lumbar vertebrae. A humped back is termed kyphosis.
The five vertebrae between the thorax and sacrum are large and present neither foramina transversaria nor costal facets (fig. 39-5). The body is kidney shpaed, and the pedicles and laminae are short and thick. The part of the lamina between the superior and inferior articular processes is known as the pars interarticularis and is liable to injury in some people (resulting in spondylolisthesis, i.e., a slipping forward of the body of one vertebra on the vertebra or sacrum below it). A mamillary process projects posteriorward from the superior articular process. The transverse process, which corresponds to a rib, is long and thin, and an accessory process may project inferiorward from its root. The spinous processes are quadrilateral and project horizontally backward. The fifth lumbar vertebra, usually the largest vertebra, is mainly responsible for the lumbosacral angle (between the lumbar part of the column and the sacrum). Excessive "hollowing" of the back is termed lordosis.
Five (sometimes six) vertebrae are fused in the adult to form the sacrum, which can be felt below the "small of the back." The sacrum articulates above with L5, laterally with the hip bones, and inferiorly with the coccyx. It has a roughly triangular appearance with a pelvic and dorsal surface, a lateral mass on each side, and a base and apex. The pelvic surface, concave and facing antero-inferiorly, presents four paired sacral foramina for the ventral primary rami of sacral nerves 1 to 4. The dorsal surface, convex and facing posterosuperiorly, presents a modified series of spinous processes termed the median sacral crest and four paired sacral foramina for the dorsal primary rami of sacral nerves 1 to 4. Below, the sacrum shows right and left sacral cornua, which bound a variable gap termed the sacral hiatus. An anesthetic for the spinal nerves may be injected extradurally through the sacral hiatus (caudal analgesia). The cornua articulate with corresponding horns on the coccyx. The lateral part or mass of the sacrum, lateral to the sacral foramina, consists of the fused transverse processes (including their costal elements). Its superior surface is frequently termed the ala. The superolateral part of the lateral mass presents the auricular (ear-shaped) surface for articulation with the hip bone (sacro-iliac joint). The surface is limited behind by an area (sacral tuberosity) for interosseous ligaments. The base, formed by the suprior part of the first sacral vertebra, presents a prominent anterior margin termed the promontory (fig. 39-11). Superior articular processes articulate with the fifth lumbar vertebra. The sacral canal (which contains the dura, cauda equina, and filum terminale) extends from the base to the sacral hiatus. The apex of the sacrum may be fused with the coccyx.
Coccyx (fig. 39-10).
The vertebrae (usually four) below the sacrum are variably fused in the adult to form the coccyx, which resembles a miniature sacrum in shape.
Vertebrae develop in mesenchyme and cartilage during the embryonic period proper, and most begin to ossify during fetal life. Typically, a vertebra at birth shows three ossific areas, one for the centrum (defined in fig. 39-14) and one for each half of the neural arch. At about puberty, ossific centers appear in the margins of the upper and lower surfaces of the body (ring epiphyses) and at the tips of the various processes. Developmental failure of half a vertebra (hemivertebra) is one cause of lateral curvature (scoliosis).
Failure of fusion of the halves of one or more neural (future vertebral) arches is termed spina bifida. The meninges alone (meningocele), or the spinal cord and meninges (meningomyelocele), may protrude through the defect (spina bifida cystica). When the defect is skeletal rather than neural, it may be concealed by the skin (although sometimes marked by a tuft of hair) and is termed spina bifida occulta. In the sacrum, this is quite common.
The spinous processes of the vertebrae are palpable in the median furrow of the back. The external occipital protuberance is palpable in adults. The spines of vertebrae C6, C7, and T1 are usually prominent and palpable, and they are made more conspicuous by flexion of the neck and trunk. In the thoracic region, the spinous process of each vertebra extends to the level of at least the body of the vertebra below. The inferior angle of the scapula is frequently at the level of the spinous process of T7. A horizontal plane between the highest points of the iliac crests (supracristal plane) is usually at the level of the spinous process of L4, and this is used as a landmark for lumbar puncture. A needle introduced here should enter the subarachnoid space after 4 to 6 cm. The posterior superior iliac spine is commonly marked by a skin dimple (fig. 39-15).
Kohler, A., and Zimmer, E. A. Borderlands of the Normal and Early Pathologic in Skeletal Roentgenology, 3rd ed" trans, by S. P. Wilk, Grone & Stratton, New York, 1968. This classic study of the entire skeleton is available in a more recent edition in German.
Schmorl, G., and Junghanns, H., The Human Spine in Health and Disease, 2nd ed., trans. by E. F. Besemann, Grone & Stratton, New York, 1971. This well known work is an important account of the normal and abnormal vertebral column.
39-1 How many vertebrae are movable?
39-2 Which curvatures first appear in the vertebral column?
39-3 How many processes characterize a vertebral arch?
39-4 Where are the intervertebral foramina and what do they contain?
39-5 Between which vertebrae does the C8 nerve emerge?
39-6 What are the key features of the cervical, thoracic, lumbar, and sacral vertebrae?
39-7 How many dorsal vertebrae are present in the body?
39-8 What is the pars interarticularis?
39-9 What are the chief contents of the sacral canal?
39-10 What is the ossific status of a typical vertebra at birth?
39-11 How is surgical access to the spinal cord achieved?
39-12 What is spina bifida?
39-13 Which Latin and Greek roots are used with reference to the vertebral column?
Figure 39-1 The primary (1, thoracic; 2, sacral) and secondary (3, cervical; 4, lumbar) curvatures of the vertebral column.
Figure 39-2 The parts of a vertebra (T.V.6) seen from above and from the right side. Adjacent intervertebral notches form intervertebral foramina for the transmission of nerves.
Figure 39-3 The atlas from above. Muscle origins and the superior vertebral artery are shown on the right side. (After Frazer.)
Figure 39-4 lateral and posterosuperior views of the axis.
Figure 39-5 Various vertebrae from lateral, superior, and posterior aspects.
Figure 39-6 The positions, lengths, and directions of (A) the spinous processes and (B) the transverse processes. The vertebrae in black mark the levels at which a change in direction of curvature occurs.
Figure 39-7 Thoracic vertebrae (and C7 and L1). Note the bodies, pedicles, transverse and spinous processes, and costrotransverse joints. (Courtesy of V.C. Johnson, M.D., Detroit, Michigan.)
Figure 39-8 Lumbar vertebrae and female pelvis.
Figure 39-9 Oblique view of the lumbar vertebrae. Note the very small twelfth rib, the joints between the articular processes ofthe lumbar vertebrae (the arrow indicates the joint between L1 and L2), and the sacrum. In this view the outline of a Scotch terrier is formed by the transverse process (snout, overlapping the vertebral body), the superior articular process (ear), and the inferior articular process (forepaw). The neck of the dog corresponds to the important pars interarticularis, injury to which may result in spondylolisthesis.
Figure 39-10 Female sacrum and coccyx. A, Pelvic and, B, dorsal aspects showing muscular and ligamentous attachments. C, Right lateral aspect in the anatomical position.
Figure 39-11 Female and male sacra from above. The superior aspect of the lateral part is the ala.
Figure 39-12 Scheme of horizontal sections of vertebrae, showing what are thought to be corresponding parts. Note that the costal element forms a part of the transverse process of a cervical vertebra. It forms the rib in the thoracic region, most of the transverse process in the lumbar region, and the greater portion of the lateral part of the sacrum. In the cervical vertebra, the posterior tubercle of the transverse process should probably also be shaded as part of the costal element.
Figure 39-13 Variations in vertebrae. B shows the common arrangement. In A, "cranial shift," a cervical rib articulates with C7 and rib 12 is small. In B, L5 is partially "sacralized" and S5 is partially freed. In C, "caudal shift," rib 12 is large and a small lumbar rib is present. S1 vertebra is partially "lumbarized" and Co.1 is incorporated into the sacrum. (After Schinz et al.)
Figure 39-14 The neural arch and centrum (left half of figure), and the vertebral arch and body (right half). The terms centrum and neural arch refer to those parts of a vertebra ossified from primary centers. The terms vertebral arch and body are descriptive terms generally applied to adult vertebrae. The body of a vertebra includes the centrum and part of the neural arch. The vertebral arch, therefore, is less extensive than the neural arch. Note that the rib articulates with the neural arch and not with the centrum.
Figure 39-15 Some surface landmarks of the back. (From Royce, L Surface Anatomy, Davis, Philadelphia, 1965,)
# The part of a cervical vertebra that corresponds to a rib is probably the transverse process lateral to the foramen transversarium and including the anterior and posterior tubercles as well as the "intertubercular lamella" (or so-called costotransverse bar). See A. J. E. Cave, J. Zool., 177:377-393, 1975. Only the anterior tubercle is shaded in figure 39-12.