A Skeleton of the trunk, anterior view

The skeleton of the trunk is made up of the vertebral column, ribs, and sternum. Details on the ribs and sternum are given on p. 134.

C The spinous processes as anatomical landmarks

Posterior view. The spinous processes of the vertebrae appear as variable prominences beneath the skin and provide important landmarks during the physical examination. With few exceptions, they are easily palpated.

• The spinous process of the seventh cervical vertebra, located at the junction of the cervical and thoracic spine. Usually it is the most prominent of the spinous processes, causing the seventh cervical vertebra to be known also as the vertebra prominens.

• The spinous process of the third thoracic vertebra, located on a horizontal line connecting the scapular spines

• The spinous process of the seventh thoracic vertebra, located at the level of the inferior angles of the scapulae

• The spinous process of the twelfth thoracic vertebra, located slightly below the attachment of the last rib

• The spinous process of the fourth lumbar vertebra, located on a horizontal line connecting the highest points of the iliac crests

Note: The spinous processes of the thoracic vertebrae are angled downward (see p. 112), so the spinous process of the fifth thoracic vertebra, for example, is at the level of the sixth thoracic vertebral body.

A The bony spinal column

a Anterior view, b posterior view, c left lateral view.

Note: Phylogenetically, the transverse processes of the lumbar vertebrae are rudimentary ribs. They therefore are known as costal processes (see also p. 108).

C Integration of the spinal column into the pelvic girdle

Skeleton of the trunk with the skull and pelvic girdle, left lateral view. Normally, the spinal column is curved and integrated into the pelvic girdle in such a way that characteristic angles are formed between certain imaginary lines and axes. These angles and lines are useful in the radiographic evaluation of positional abnormalities and deformities of the spine and trunk.

Lumbosacral angle: angle formed by the axes of the L 5 and S 1 vertebrae, averaging 143°. It results from the fact that the sacrum is a fixed component of the pelvic ring (see p. 140) and thus contributes little to straightening the vertebral column. The result is a characteristic sharp angle at the junction of the presacral part of the spinal column with the sacrum.

Sacral angle: angle between the horizontal plane and the superior surface of the sacrum, averaging approximately 30°

Pelvic inclination angle: angle formed by the pelvic inlet plane (connecting the sacral promontory to the upper border of the symphysis) with the horizontal. It measures approximately 60° in upright stance. The pelvic inclination angle increases or decreases as the pelvis is tilted forward or backward (see p. 159). With an ideal pelvic position in upright stance, the anterior and posterior superior iliac spines are at the same horizontal level, and the anterior superior iliac spine is directly above the pubic symphysis. By knowing this, the examiner can easily evaluate the position of the pelvis by using palpable bony landmarks.

Line of gravity: the line of gravity passes through landmarks that include the external auditory canal, the dens of the axis (C 2), the functional-anatomical transition points in the spinal column (between lordosis and kyphosis), and the whole-body center of gravity just anterior to the sacral promontory

A Development of the spinal column (weeks 4–10)

a Schematic cross section, b–e schematic coronal sections (the plane of section in b–e is indicated in a).

a, b The former somites have differentiated into the myotome, dermatome, and sclerotome. The sclerotome cells separate from the other cells at 4 weeks, migrate toward the notochord, and form a cluster of mesenchymal cells around the notochord (anlage of the future spinal column).

c Adjacent cranial and caudal sclerotome segments above and below the intersegmental vessels join together and begin to chondrify in the sixth week, displacing the notochord material superiorly and inferiorly (notochord segments).

d The intervertebral disks with their nucleus pulposus and anulus fibrosus develop between the rudimentary vertebral bodies. Ossification begins at the center of the vertebral bodies in the eighth week of development.

e By fusion of the caudal and cranial sclerotome segments, the segmentally arranged myotomes interconnect the processes of two adjacent vertebral anlages, bridging the gap across the intervertebral disks. This is how the motion segments are formed (see p. 126). The segmental spinal nerve courses at the level of the future intervertebral foramen, and the intersegmental vessels become the nutrient vessels of the vertebral bodies (week 10).

Clinical aspects: If the neural tube or the dorsal portions of the vertebral arches fail to close normally during embryonic development, the result is spina bifida—a cleft spine. In this anomaly, the spinal column is open posteriorly, and the spinous processes are absent (the various forms and manifestations are described in textbooks of embryology). Usually there is a bilateral defect in the vertebral arches (generally affecting the L 4 and L 5 region), known as spondylolysis. This defect may be congenital or acquired (e.g., due to trauma). Acquired cases are common in sports that pose a risk of vertebral arch fractures (javelin throwing, gymnastics, high jumping). If the associated intervertebral disk is also damaged, the vertebral body will begin to slip forward (spondylolisthesis). In cases of congenital spondylolysis (which are associated with varying degrees of spondylolisthesis), the slippage progresses slowly during growth, and the condition tends to stabilize after 20 years of age.

B Accessory ribs

Superior view. a Cervical rib, b lumbar rib.

The presence of anomalous cervical ribs can narrow the scalene interval, causing compression of the brachial plexus and subclavian artery (scalenus syndrome or cervical rib syndrome, see also p. 364). An accessory lumbar rib, on the other hand, has no adverse clinical effects.

C Costal elements in different regions of the spinal column

Superior view. The shape and configuration of the vertebrae are closely related to the development of the ribs and their rudiments (indicated here by color shading).

a Cervical vertebrae: Here the rudimentary rib forms a process called the anterior tubercle. It unites with the posterior tubercle to form the transverse foramen.

b Thoracic vertebrae: Because these vertebrae give attachment to the ribs, their bodies and transverse processes bear corresponding cartilage-covered articular surfaces (costal facets on the transverse processes, also superior and inferior costal facets).

c Lumbar vertebrae: The costal elements in the lumbar spine take the form of “transverse processes,” which are much larger than in the cervical spine. Because of their size, they are also known as costal processes.

d Sacrum: Here the rudimentary rib forms the anterior portion of the lateral part of the sacral vertebra. It is fused to the transverse processes.

The vertebrae in different regions of the spinal column differ not only in their size but also in their special features. While the vertebral bodies gradually become larger from superior to inferior to accommodate the increasing stresses imposed by the gravity and body weight, the vertebral foramina gradually become smaller to match the decreasing diameter of the spinal cord. The arrangement of the vertebral arches and adjacent processes also varies at different levels in the spine (for details see pp. 111, 113, and 115).

A Cervical spine, left lateral view

Of the cervical vertebrae, which number seven in all, the first and second cervical vertebrae (C1 and C2, atlas and axis) differ most conspicuously from the common vertebral morphology. They are specialized for bearing the weight of the head and allowing the head to move in all directions, similar to a ball-and-socket joint. Each of the remaining five cervical vertebrae (C3–C7) has a relatively small body, which presents a more or less square shape when viewed from above, and a large, triangular vertebral foramen (see Cc). The superior and inferior surfaces of the vertebral bodies are saddle-shaped, the superior surfaces bearing lateral uncinate processes that do not appear until about the tenth year of life (see p. 126). The transverse process consists of an anterior and a posterior bar, which terminate laterally in two small tubercles (anterior and posterior tubercles). These bars enclose the transverse foramen, through which the vertebral artery ascends from the C6 to the C1 level. The superior surface of the transverse process of the first three cervical vertebrae bears a broad, deep notch (spinal nerve sulcus), in which lies the emerging spinal nerve at that level. The superior and inferior articular processes are broad and flat. Their articular surfaces are flat and are inclined approximately 45° from the horizontal plane. The spinous processes of the third through sixth cervical vertebrae are short and bifid. The spinous process of the seventh cervical vertebra is longer and thicker than the others and is the first of the spinous processes that is distinctly palpable through the skin (vertebra prominens).

B Cervical vertebrae, left lateral view

A Thoracic spine, left lateral view

The thoracic vertebral bodies gradually become taller and broader from superior to inferior, the lower vertebral bodies assuming a transverse oval shape like that of the lumbar vertebrae. The vertebral foramen is roughly circular and is smaller than in the cervical and lumbar vertebrae. The end plates are rounded and triangular. The spinous processes are long and angled sharply inferiorly, creating an overlapping arrangement that interlinks the thoracic vertebrae. The facets of the inferior articular processes are directed anteriorly, while the facets of the superior articular processes face posterior so that they can articulate with the inferior facets to form the zygapophyseal or facet joints (p. 124). Another feature of the thoracic vertebrae is that their transverse processes are angled backward to allow for articulations with the ribs.

B Thoracic vertebrae, left lateral view

The costal facets are cartilage-covered surfaces that articulate with the corresponding ribs (see p. 139). The bodies of the second through ninth thoracic vertebrae (T1–T9) bear two articular facets on each side — a superior costal facet and an inferior costal facet — such that two adjacent vertebrae combine to articulate with the head of a rib. Thus, a given numbered rib articulates with its own numbered vertebra and the vertebra above. Exceptions to this scheme are ribs 1 and 10 to 12, which articulate only with the vertebral body of the same number. The body of the tenth thoracic vertebra has only superior articular facets, and the tenth rib usually articulates only with the tenth thoracic vertebrae. As mentioned, the transverse processes of the thoracic vertebrae (except for T11 and T12) also bear articular facets for the ribs.

A Lumbar spine, left lateral view

The bodies of the lumbar vertebrae are large and have a transverse oval shape when viewed from above (see C). The massive vertebral arches enclose an almost triangular vertebral foramen, and they unite posteriorly to form a thick spinous process that is flattened on each side. The “transverse processes” of the lumbar vertebrae correspond phylogenetically to rudimentary ribs (see p. 108). Thus, they are more accurately termed costal processes and are not homologous with the transverse processes of the other vertebrae. The thick costal process is fused with the actual transverse process, which is a small, pointed eminence located at the root of each costal process (accessory process, see Cb). The relatively massive superior and inferior articular processes bear slightly inclined articular facets that have a vertical, almost sagittal orientation. The articular facets of the superior articular processes are slightly concave and face medially, while those of the inferior articular processes are slightly convex and face laterally. The mammillary processes on the lateral surfaces of the superior articular processes serve as origins and insertions for the intrinsic back muscles (see Bb and Ca).

B Lumbar vertebrae, left lateral view

A Sacrum and coccyx, anterior (pelvic) view

The sacrum at birth is composed of five separate sacral vertebrae. Postnatally, they fuse to form a single bone that is flattened anteroposteriorly and has a triangular shape when viewed from the front. The base of the sacrum, located at the superior end of the bone, articulates with the body of the fifth lumbar vertebra by a wedge-shaped intervertebral disk. The apex of the sacrum is at the inferior end of the bone and articulates with the coccyx. The anterior or pelvic surface of the sacrum is concave in both the sagittal and transverse planes (see C). Between the anterior sacral foramina are four transverse ridges (transverse lines), which mark the sites of fusion between the five sacral vertebrae. The coccyx consists of three or four rudimentary vertebrae. Only the first coccygeal vertebra exhibits some of the typical structural elements of a fully formed vertebra. A cartilaginous disk usually connects the base of the coccyx to the apex of the sacrum (sacrococcygeal joint). This joint allows passive forward and backward motion of the coccyx, increasing the anteroposterior diameter of the pelvic outlet during childbirth (see p. 141).

B Sacrum and coccyx, posterior view

The fused spinous processes form a jagged bony ridge on the convex dorsal surface of the sacrum, the median sacral crest. This ridge is flanked on each side by the paired medial sacral crests, formed by fusion of the articular processes. The medial crests are continuous below with the rudimentary inferior articular processes of the fifth sacral vertebra (the sacral cornua); they are continuous above with the two superior articular processes that face posteriorly. Between the sacral cornua is an aperture, the sacral hiatus, which is formed by the incomplete vertebral arch of the fifth sacral vertebrae and provides access to the sacral canal (e.g., for anesthesia). Lateral to the posterior sacral foramina are another pair of longitudinal ridges, the lateral sacral crests, formed by the fused transverse processes. The bony union of the transverse processes with the rudimentary ribs forms the thick lateral parts of the sacral wing that flank the body of the sacrum. Each lateral part bears an ear-shaped (“auricular”) surface that articulates with the ilium (see C). In this view, the two small coccygeal cornua representing the superior articular processes and the two rudimentary transverse processes of the coccyx are visible.

E Transverse section through the sacrum

Superior view (the level of the section is shown in A). Fusion of the upper four sacral vertebrae creates four T-shaped bony canals at the level of the intervertebral foramina on each side, providing sites of emergence for the first through fourth sacral nerves. The corresponding ventral and dorsal rami of the spinal nerves exit the bony canals through the anterior and posterior sacral foramina (see p. 543).

b Sagittal section through a motion segment (see p. 126), left lateral view.

c Detail from b.

Except for the outer zone of the anulus fibrosus, the entire disk is in contact superiorly and inferiorly with the hyaline cartilage layer of the adjacent vertebral endplates. The subchondral, bony portion of the end plate consists of compact bone (intervertebral surface) permeated by myriad pores (see c) through which the vessels in the bone marrow spaces of the vertebral bodies can supply nutrients to the disk tissue.

E Load-dependent fluid shifts in the intervertebral disk

a The nucleus pulposus functions as a “water cushion” to absorb transient axial loads on the intervertebral disk. Mechanically, the disk represents a hydrostatic system that is resilient under pressure. It is composed of a tension-resistant sheath (the anulus fibrosus) and a watery, incompressible core, the nucleus pulposus. This core consists of 80 to 85% water, which it can reversibly bind in its paucicellular, gelatinous, mucoviscous tissue (owing to its high content of glycosaminoglycans). The nucleus pulposus is under a very high hydrostatic pressure, particularly when acted upon by gravity and other forces. This pressure can be absorbed by the adjacent cartilaginous end plates and also by the anulus fibrosus (which transforms compressive forces into tensile forces). In this way the nucleus pulposus functions as a “water cushion” or hydraulic press between two adjacent vertebral bodies. Combined with the anulus fibrosus, it acts as an effective shock absorber that can distribute pressures uniformly over the adjacent vertebral end plates.

b Fluid outflow from the intervertebral disk (green arrows) in response to a sustained pressure load (thick red arrows). While transient loads are cushioned by the shock-absorber function of the nucleus pulposus and anulus fibrosus (see a), sustained loads cause a gradual but permanent outflow of fluid from the disk. The turgor and height of the disk are reduced, while the end plates and eventually the bony vertebral elements move closer together (see p. 133 for further details on disk degeneration).

c Fluid uptake by the intervertebral disk (green arrows) when pressure is released (thin red arrows). The process described in b is reversed when the pressure on the disk is released, and the height of the disk increases. This increase is caused by fluid uptake from the subchondral blood vessels in the bone marrow spaces, which are instrumental in disk nutrition (see Dc). As a result of pressure-dependent fluid shifts (convection) in the intervertebral disks, the overall body height decreases temporarily by approximately 1% (1.5–2.0 cm) relative to the initial body height during the course of the day.