Ovid: Clinically Oriented Anatomy

Authors: Moore, Keith L.; Dalley, Arthur F.
Title: Clinically Oriented Anatomy, 5th Edition
> Table of Contents > 4 – Back

Overview of Back and Vertebral Column
The back comprises the posterior aspect of the trunk, inferior to the neck and superior to the buttocks (L. nates).
It is the region of the body to which the head, neck, and limbs are attached. The back includes the:
  • Skin and subcutaneous tissue.
  • Muscles: a superficial layer, primarily
    concerned with positioning and moving the limbs, and deeper layers
    (“true back muscles”), specifically concerned with moving or
    maintaining the position of the axial skeleton (posture).
  • Vertebral column: the vertebrae, intervertebral (IV) discs, and associated ligaments (Fig. 4.1).
  • Ribs (in the thoracic region): particularly their posterior portions, medial to the angles of the ribs.
  • Spinal cord and meninges (membranes that cover the spinal cord).
  • Various segmental nerves and vessels.
Because of their close association with the trunk, the
back of the neck and the posterior and deep cervical muscles and
vertebrae are also described in this chapter. The scapulae, although
located in the back, are part of the appendicular skeleton and are
considered with the upper limb (Chapter 6).
Study of the soft tissues of the back is best preceded by examination of the vertebrae and the fibrocartilaginous intervertebral discs that are interposed between the bodies of adjacent vertebrae. The vertebrae and IV discs collectively make up the vertebral column
(spine), which extends from the cranium (skull) to the apex of the
coccyx. The vertebral column forms the skeleton of the neck and back
and is the main part of the axial skeleton (i.e., the articulated bones of the cranium, vertebral column, ribs, and sternum) (Fig. 4.1D).
The adult vertebral column is 72–75 cm long, of which approximately one
quarter is formed by the IV discs, which separate and bind the
vertebrae together (Fig. 4.1D & E). The vertebral column:
  • Protects the spinal cord and spinal nerves.
  • Supports the weight of the body superior to the level of the pelvis.
  • Provides a partly rigid and flexible axis for the body and an extended base on which the head is placed and pivots.
  • Plays an important role in posture and locomotion (the movement from one place to another).
The vertebral column in an adult typically consists of
33 vertebrae arranged in five regions: 7 cervical, 12 thoracic, 5
lumbar, 5 sacral, and 4 coccygeal (Fig. 4.1A–D).
Significant motion occurs only between the 25 superior vertebrae. Of
the 9 inferior vertebrae, the 5 sacral vertebrae are fused in adults to
form the sacrum and, after approximately age 30, the 4 coccygeal vertebrae fuse to form the coccyx. The lumbosacral angle occurs at the junction of, and is formed by, the long axes of the lumbar region of the vertebral column and the sacrum (Fig.4.1D).
The vertebrae gradually become larger as the vertebral column descends
to the sacrum and then become progressively smaller toward the apex of
the coccyx (Fig. 4.1A–D).
The change in size is related to the fact that successive vertebrae
bear increasing amounts of the body’s weight as the column descends.
The vertebrae reach maximum size immediately superior to the sacrum,
which transfers the weight to the pelvic girdle at the sacroiliac
The vertebral column is flexible because it consists of many relatively small bones, called vertebrae (singular = vertebra), that are separated by resilient IV discs (Fig. 4.2). The 25 cervical, thoracic, lumbar, and first sacral vertebrae also articulate at synovial zygapophysial joints (Fig. 4.2D),
which facilitate and control the vertebral column’s flexibility.
Although the movement between two adjacent vertebrae is small, in
aggregate the vertebrae and IV discs uniting them form a remarkably
flexible yet rigid column that protects the spinal cord they surround.


Figure 4.1. Vertebral column and vertebral canal, demonstrating its five regions. A. This anterior view shows the isolated vertebral column. B.
This right lateral view shows the isolated vertebral column. The
isolated vertebrae are typical of each of the three mobile regions.
Note the increase in size of the vertebrae as the column descends. C. This posterior view of the vertebral column includes the vertebral ends of ribs, representing the skeleton of the back. D.
This medial view of the axial skeleton in situ demonstrates its
regional curvatures and its relationship to the cranium (skull),
thoracic cage, and hip bone. The continuous, weight-bearing column of
vertebral bodies and IV discs forms the anterior wall of the vertebral
canal. The lateral and posterior walls of the canal are formed by the
series of vertebral arches. The IV foramina (seen also in part B)
are openings in the lateral wall through which spinal nerves exit the
vertebral canal. The posterior wall is formed by overlapping laminae
and spinous processes, like shingles on a roof. E. This sagittal MRI study shows the primary contents of the vertebral canal. The medullary cone (L. conus medullaris)
is the cone-shaped inferior end of the spinal cord, which typically
ends at the L1–L2 level in adults. The dura mater, the external
covering of the spinal cord (gray), is separated from the spinal cord by a fluid-filled space (black) and from the wall of the vertebral canal by fat (white) and thin-walled veins (not visible here).


Figure 4.2. A “typical” vertebra, represented by L2. A. Functional components include the vertebral body (bone color), a vertebral arch (red), and seven processes: three for muscle attachment and leverage (blue) and four that participate in synovial joints with adjacent vertebrae (yellow). B and C.
Bony formations of the vertebrae are demonstrated. The vertebral
foramen is bounded by the vertebral arch and body. A small superior
vertebral notch and a larger inferior vertebral notch flank the
pedicle. D. The superior and inferior
notches of adjacent vertebrae plus the IV disc that unites them form
the IV foramen for the passage of a spinal nerve and its accompanying
vessels. Note that each articular process has an articular facet where
contact occurs with the articular facets of adjacent vertebrae (B–D).
Structure and Function of the Vertebrae
Vertebrae normally vary in size and other
characteristics from one region of the vertebral column to another and
to a lesser degree within each region; however, their basic structure
is the same. A typical vertebra (Fig. 4.2) consists of a vertebral body, a vertebral arch, and seven processes.1
The vertebral body is the
more massive, roughly cylindrical, anterior part of the bone that gives
strength to the vertebral column and supports body weight. The size of
the vertebral bodies increases as the column descends, most markedly
from T4 inferiorly, as each bears progressively greater body weight.
The vertebral body consists of vascular, trabecular
(spongy, cancellous) bone enclosed by a thin external layer of compact
bone (Fig. 4.3). The trabecular bone is a
meshwork of mostly tall vertical trabeculae intersecting with short,
horizontal trabeculae. The interstices of these trabeculae are occupied
by red marrow that is among the most actively hematopoietic
(blood-forming) tissues of the mature individual. One or more large
foramina in the posterior surface of the body accommodate basivertebral
veins that drain the marrow (Fig. 4.20).
In life, most of the superior and inferior surfaces of
the vertebral body are covered with discs of hyaline cartilage
(vertebral “end plates”), which are remnants of the cartilaginous model
from which the bone develops (Bogduk, 1997). In dried


laboratory and museum skeletal specimens, this cartilage is absent, and
the exposed bone appears spongy, except at the periphery where an epiphysial rim or ring of smooth bone, derived from an anular epiphysis, is fused to the body (Fig. 4.2B).
In addition to serving as growth zones, the anular epiphyses and their
cartilaginous remnants provide some protection to the vertebral bodies
and permit some diffusion of fluid between the IV disc and the
capillaries in the vertebral body. The superior and inferior epiphyses
usually unite with the centrum, the primary ossification center for the central mass of the vertebral body (Fig. 4.2B), early in adult life (at approximately age 25) (see “Ossification of Vertebrae” in this chapter).

The vertebral arch is posterior to the vertebral body and consists of two (right and left) pedicles and laminae (Fig. 4.2A). The pedicles
are short, stout cylindrical processes that project posteriorly from
the vertebral body to meet two broad, flat plates of bone, called laminae, which unite in the midline. The vertebral arch and the posterior surface of the vertebral body form the walls of the vertebral foramen (Fig. 4.2B & C). The succession of vertebral foramina in the articulated vertebral column forms the vertebral canal
(spinal canal), which contains the spinal cord and the roots of the
spinal nerves that emerge from it, along with the membranes (meninges),
fat, and vessels that surround and serve them (Fig. 4.1E). The vertebral notches
are indentations observed in lateral views of the vertebrae superior
and inferior to each pedicle between the superior and inferior
articular processes posteriorly and the corresponding projections of
the body anteriorly (Fig. 4.2C & D). The superior and inferior vertebral notches of adjacent vertebrae and the IV discs connecting them form the intervertebral foramina (Fig. 4.2D),
in which the spinal (posterior root) ganglia are located and through
which the spinal nerves emerge from the vertebral column with their
accompanying vessels.
Seven processes arise from the vertebral arch of a typical vertebra (Fig. 4.2A–C):
  • One median spinous process
    projects posteriorly (and usually inferiorly, typically overlapping the
    vertebra below) from the vertebral arch at the junction of the laminae.
  • Two transverse processes project posterolaterally from the junctions of the pedicles and laminae.
  • Four articular processes (G. zygapophyses)—two superior and two inferior—also arise from the junctions of the pedicles and laminae, each bearing an articular surface (facet).
The former three processes, one spinous and two
transverse, afford attachments for deep back muscles and serve as
levers, facilitating the muscles that fix or change the position of the
The latter four (articular) processes are in apposition
with corresponding processes of vertebrae adjacent (superior and
inferior) to them, forming zygapophysial (facet) joints (Fig. 4.2D).
Through their participation in these joints, these processes determine
the types of movements permitted and restricted between the adjacent
vertebrae of each region. The articular


also assist in keeping adjacent vertebrae aligned, particularly
preventing one vertebra from slipping anteriorly on the vertebra below.
Generally, the articular processes bear weight only temporarily, as
when one rises from the flexed position, and unilaterally when the
cervical vertebrae are laterally flexed to their limit. However, the
inferior articular processes of the L5 vertebra bear weight even in the
erect posture.

Figure 4.3. Internal aspects of vertebral body and vertebral canal.
Vertebral bodies consist largely of spongy bone, with tall, vertical
supporting trabeculae linked by short horizontal trabeculae, covered by
a relatively thin layer of compact bone. Hyaline cartilage end plates
cover the superior and inferior surfaces of the bodies, surrounded by
smooth bony epiphysial rims. The posterior longitudinal ligament,
covering the posterior aspect of the vertebral bodies and linking the
IV discs, forms the anterior wall of the vertebral canal. Lateral and
posterior walls of the vertebral canal are formed by vertebral arches
(pedicles and laminae) alternating with IV foramina and ligamenta flava.
Regional Characteristics of the Vertebrae
Each of the 33 vertebrae is unique. However, most of the
vertebrae demonstrate characteristic features identifying them as
belonging to one of the five regions of the vertebral column (e.g.,
vertebrae having foramina in their transverse processes are cervical
vertebrae). In addition, certain individual vertebrae have
distinguishing features; the C7 vertebra, for example, has the longest
spinous process. It forms a prominence under the skin at the back of
the neck, especially when the neck is flexed.
In each region, the articular facets are oriented on the
articular processes of the vertebrae in a characteristic direction that
determines the type of movement permitted between the adjacent
vertebrae and, in aggregate, for the region. For example, the articular
facets of thoracic vertebrae are nearly vertical, and together define
an arc centered in the IV disc; this arrangement permits rotation and
lateral flexion of the vertebral column in this region (Table 4.2). Regional variations in the size and shape of the vertebral canal accommodate the varying thickness of the spinal cord (Fig 4.1D & E).
Cervical Vertebrae
Cervical vertebrae form the skeleton of the neck (Fig. 4.1).
The smallest of the 24 movable vertebrae, the cervical vertebrae are
located between the cranium and the thoracic vertebrae. Their smaller
size reflects the fact that they bear less weight than do the larger
inferior vertebrae. Although the cervical IV discs are thinner than
those of inferior regions, they are relatively thick compared to the
size of the vertebral bodies they connect. The relative thickness of
the discs, the nearly horizontal orientation of the articular facets,
and the small amount of surrounding body mass give the cervical region
the greatest range and variety of movement of all the vertebral regions.
The distinctive features of cervical vertebrae are illustrated and listed in Table 4.1. The single-most distinctive feature of each cervical vertebra is the oval transverse foramen in the transverse process (L. foramen transversarium).
The vertebral arteries and their accompanying veins pass through the
transverse foramina, except those in C7, which transmit only small
accessory veins. Thus the foramina are smaller in C7 than those in
other cervical vertebrae, and occasionally they are absent. The
transverse processes of cervical vertebrae end laterally in two
projections: an anterior tubercle and a posterior tubercle. The tubercles provide attachment for a laterally placed group of cervical muscles (levator scapulae and scalenes). Grooves on the transverse processes between tubercles (the floor of the groove being formed by a costotransverse bar) accommodate the anterior rami of the cervical spinal nerves (Table 4.1B).




The carotid tubercles of vertebra C6 are so called carotid tubercles
because the common carotid arteries may be compressed here, in the
groove between the tubercle and body, to control bleeding from these
vessels. Bleeding may continue because of the carotid’s multiple
anastomoses of distal branches with adjacent and contralateral
branches, but at a slower rate.

Table 4.1. Cervical Vertebraea
Part Characteristics
Body Small and wider from side to
side than anteroposteriorly; superior surface concave with uncus of
body (uncinate process); inferior surface convex
Vertebral foramen Large and triangular
Transverse processes Transverse foramina small or
absent in C7; vertebral arteries and accompanying venous and
sympathetic plexuses pass through foramina, except C7, which transmits
only small accessory vertebral veins; anterior and posterior tubercles
Articular processes Superior facets directed
superioposteriorly; inferior facets directed inferioanteriorly;
obliquely placed facets are most nearly horizontal in this region
Spinous processes Short (C3–C5) and bifid (C3–C6); process of C6 long, that of C7 is longer (thus C7 called “vertebra prominens”)
aThe C1 and C2 vertebrae are atypical.
Typical intercostal nerves (3rd through 6th) run along the intercostal
spaces posteriorly, between the parietal pleura (serous lining of the
thoracic cavity)
Figure 4.4. Cranial base and C1 and C2 vertebrae. A. Observe the occipital condyles that articulate with the superior articular surfaces (facets) of the atlas (vertebra C1). B.
The atlas, on which the cranium rests, has neither a spinous process
nor a body. It consists of two lateral masses connected by anterior and
posterior arches. C and D. The tooth-like
dens characterizes the axis (vertebra C2) and provides a pivot around
which the atlas turns and carries the cranium. It articulates
anteriorly with the anterior arch of the atlas (“Facet for dens” in
part B) and posteriorly with the transverse ligament of the atlas (see part B).
Vertebrae C3–C7 demonstrate all the features typical of cervical vertebrae indicated in Table 4.1.
They have large vertebral foramina to accommodate the enlargement of
the spinal cord in this region in relation to the innervation of the
upper limbs. The superior borders of the transversely elongated bodies
of the cervical vertebrae are elevated posteriorly and especially
laterally but are depressed anteriorly, resembling somewhat a sculpted
seat (Table 4.1B).
The inferior border of the body of the superiorly placed vertebra is
reciprocally shaped. The adjacent cervical vertebrae articulate in a
way that permits free flexion and extension and some lateral flexion
but restricted rotation. The planar, nearly horizontal articular facets
of the articular processes are also favorable for these movements. The
elevated superolateral margin is the uncus of the body (uncinate process).
The spinous processes of the C3–C6 vertebrae are short and usually
bifid in whites but usually not in people of African descent. C7 is a
prominent vertebra that is characterized by a long spinous process;
because of this prominent process, C7 is called the vertebra prominens. Run your finger along the midline of the posterior aspect of your neck until you feel the prominent C7 spinous process.
The two superior-most cervical vertebrae are atypical. Vertebra C1, also called the atlas, is unique in that it has neither a body nor a spinous process (Fig. 4.4B; Table 4.1A). This ring-shaped bone has paired lateral masses
that serve the place of a body by bearing the weight of the globe-like
cranium in a manner similar to the way that Atlas of Greek mythology
bore the weight of the world on his shoulders. The transverse processes
of the atlas arise from the lateral masses, causing them to be more
laterally placed than those of the inferior vertebrae. This feature
makes the atlas the widest of the cervical vertebrae, thus providing
increased leverage for attached muscles. The kidney-shaped, concave superior articular surfaces of the lateral masses receive two large cranial protuberances called the occipital condyles at the sides of the foramen magnum (Fig. 4.4A). Anterior and posterior arches, each of which bears a tubercle in the center of its external aspect, extend between the lateral masses, forming a complete ring (Fig 4.4B). The posterior arch, which corresponds to the lamina of a typical vertebra, has a wide groove for the vertebral artery on its superior surface. The C1 nerve also runs in this groove.
Vertebra C2, also called the axis,
is the strongest of the cervical vertebrae. C1, carrying the cranium,
rotates on C2, as when a person turns the head to indicate “no.” The
axis has two large, flat bearing surfaces, the superior articular facets, on which the atlas rotates (Fig. 4.4C). The distinguishing feature of the axis is the blunt tooth-like dens
(odontoid process), which projects superiorly from its body. Both the
dens (G. tooth) and the spinal cord inside its coverings are encircled
by the atlas. The dens lies anterior to the spinal cord and serves as
the pivot about which the rotation occurs. The dens is held in position
against the posterior aspect of the anterior arch of the atlas by the transverse ligament of the atlas (Fig. 4.4B).
This ligament extends from one lateral mass of the atlas to the other,
passing between the dens and spinal cord, forming the posterior wall of
the “socket” that receives the dens. Thus it prevents posterior
(horizontal) displacement of the dens and anterior displacement of the
atlas. Either displacement would compromise the portion of the
vertebral foramen of C1 that gives passage to the spinal cord. C2 has a
large bifid spinous process (Fig. 4.4C & D) that can be felt deep in the nuchal groove, the superficial vertical groove at the back of the neck.




Thoracic Vertebrae
The thoracic vertebrae lie in the upper back and provide attachment for the ribs (Fig. 4.1). Thus the primary characteristic features of thoracic vertebrae are the costal facets
for articulation with ribs. The costal facets and other characteristic
features of thoracic vertebrae are illustrated and listed in Table 4.2.
The middle four thoracic vertebrae (T5–T8) demonstrate all the features
typical of thoracic vertebrae. The articular processes of thoracic
vertebrae extend vertically with paired, nearly coronally oriented
articular facets that define an arc centered in the IV disc. This arc
permits rotation and some lateral flexion of the vertebral column in
this region, in fact, the greatest degree of rotation is permitted here
(Table 4.2A).
Attachment of the rib cage combined with the vertical orientation of
articular facets and overlapping spinous processes limits flexion and
extension as well as lateral flexion.
The T1–T4 vertebrae share some features of cervical
vertebrae. T1 is atypical of thoracic vertebrae in that it has a long,
almost horizontal spinous process that may be nearly as prominent as
that of the vertebra prominens. T1 also has a complete costal facet on
the superior edge of its body for the 1st rib and a demifacet on its
inferior edge that contributes to the articular surface for the 2nd rib.
The T9–T12 vertebrae have some features of lumbar
vertebrae, including tubercles similar to the accessory and mammillary
processes of lumbar vertebrae. However, most of the transition in
characteristics from thoracic to lumbar region occurs over the length
of a single vertebra: vertebra T12. Generally, its superior half is
thoracic in character, having costal facets and articular processes
that permit primarily rotatory movement, whereas its inferior half is
lumbar in character, devoid of costal facets and having articular
processes that permit only flexion and extension. Consequently,
vertebra T12 is subject to transitional stresses that cause it to be
the most commonly fractured vertebra.


Table 4.2. Thoracic Vertebrae
Part Characteristics
Body Heart shaped; one or two costal facets for articulation with head of rib
Vertebral foramen Circular and smaller than those of cervical and lumbar vertebrae
Transverse processes Long and strong and extend
posterolaterally; length diminishes from T1 to T12 (T1–T10 have facets
for articulation with tubercle of rib)
Articular processes Superior facets directed
posteriorly and slightly laterally; inferior facets directed anteriorly
and slightly medially; plane of facets lies on arc centered about
vertebral body
Spinous processes Long; slope posteroinferiorly; tips extend to level of vertebral body below
Lumbar Vertebrae
Lumbar vertebrae are located in the lower back between the thorax and sacrum (Fig. 4.1). Characteristic features of the lumbar vertebrae are illustrated and listed in Table 4.3.
Because the weight they support increases toward the inferior end of
the vertebral column, lumbar vertebrae have massive bodies, accounting
for much of the thickness of the lower trunk in the median plane. Their
articular processes extend vertically, with articular facets sagittally
oriented initially (beginning abruptly with the T12–L1 joints) but
becoming more coronally oriented as the column descends. The L5–S1
facets are distinctly coronal in orientation. In the more sagittally
oriented superior joints, the laterally facing facets of the inferior
processes of the vertebra above are “gripped” by the medially facing
facets of the superior processes of the vertebra below, in a manner
that facilitates flexion and extension, allows lateral flexion, but
prohibits rotation (Figs. 4.1 and 4.2).
The transverse processes project somewhat posterosuperiorly as well as
laterally. On the posterior surface of the base of each transverse
process is a small accessory process,
which provides an attachment for the medial intertransverse lumborum
muscle. On the posterior surface of the superior articular processes
are mammillary processes, which give attachment to the multifidus and medial intertransverse muscles (back muscles).
Vertebra L5 is the largest of all movable vertebrae; it
carries the weight of the whole upper body. L5 is distinguished by its
massive body and transverse processes. Its body is markedly deeper
anteriorly; therefore, it is largely responsible for the lumbosacral
angle between the long axis of the lumbar region of the vertebral
column and that of the sacrum (Fig. 4.1D). Body weight is transmitted from L5 vertebra to the base of the sacrum, formed by the superior surface of S1 vertebra (Fig. 4.5A).


The large, triangular, wedged-shaped sacrum is usually composed of five fused sacral vertebrae in adults (Fig. 4.5).
It is located between the hip bones and forms the roof and
posterosuperior wall of the posterior pelvic cavity. The triangular
shape of the sacrum results from the rapid decrease in the size of the
lateral masses of the sacral vertebrae during development. The inferior
half of the sacrum is not weight bearing; therefore, its bulk is
diminished considerably. The sacrum (L. sacred or holy bone) provides
strength and stability to the pelvis and transmits the weight of the
body to the pelvic girdle, the bony ring formed by the hip bones and
sacrum, to which the lower limbs are attached.
The sacral canal is the continuation of the vertebral canal in the sacrum (Fig. 4.5C). It contains the bundle of spinal nerve roots arising inferior to the L1 vertebra, known as the cauda equina
(L. horse tail), that descend past the termination of the spinal cord.
On the pelvic and posterior surfaces of the sacrum between its
vertebral components are typically four pairs of sacral foramina for the exit of the posterior and anterior rami of the spinal nerves (Fig. 4.5A & B). The anterior (pelvic) sacral foramina are larger than the posterior (dorsal) ones.
The base of the sacrum is
formed by the superior surface of the S1 vertebra. Its superior
articular processes articulate with the inferior articular processes of
the L5 vertebra. The anterior projecting edge of the body of the S1
vertebra is the sacral promontory (L. mountain ridge), an important obstetrical landmark (see Chapter 3). The apex of the sacrum, its


tapering inferior end, has an oval facet for articulation with the coccyx.

Table 4.3. Lumbar Vertebrae
Part Characteristics
Body Massive; kidney shaped when viewed superiorly
Vertebral foramen Triangular; larger than in thoracic vertebrae and smaller than in cervical vertebrae
Transverse processes Long and slender; accessory process on posterior surface of base of each process
Articular processes Superior facets directed
posteromedially (or medially); inferior facets directed anterolaterally
(or laterally); mammillary process on posterior surface of each
superior articular process
Spinous processes Short and sturdy; thick, broad, and hatchet shaped
The sacrum supports the vertebral column and forms the
posterior part of the bony pelvis. The sacrum is tilted so that it
articulates with the L5 vertebra at the lumbosacral angle (Fig. 4.1D),
which varies from 130° to 160°. The sacrum is often wider in proportion
to length in the female than in the male, but the body of the S1
vertebra is usually larger in males.
The pelvic surface of the sacrum is smooth and concave (Fig. 4.5A).
Four transverse lines on this surface of sacra from adults indicate
where fusion of the sacral vertebrae occurred. During childhood, the
individual sacral vertebrae are connected by hyaline cartilage and
separated by IV discs. Fusion of the sacral vertebrae starts after age
20; however, most of the IV discs remain unossified up to or beyond
middle life (Williams et al., 1995).
The dorsal surface of the sacrum is rough, convex, and marked by five prominent longitudinal ridges (Fig. 4.5B). The central ridge, the median sacral crest,
represents the fused rudimentary spinous processes of the superior
three or four sacral vertebra; S5 has no spinous process. The intermediate sacral crests represent the fused articular processes, and the lateral sacral crests
are the tips of the transverse processes of the fused sacral vertebrae.
The clinically important features of the dorsal surface of the sacrum
are the inverted U-shaped sacral hiatus and the sacral cornua (L.
horns). The sacral hiatus results from the
absence of the laminae and spinous process of S5 and sometimes S4. The
sacral hiatus leads into the sacral canal. Its depth varies, depending
on how much of the spinous process and laminae of S4 are present. The sacral cornua,
representing the inferior articular processes of S5 vertebra, project
inferiorly on each side of the sacral hiatus and are a helpful guide to
its location.
The superior part of the lateral surface of the sacrum looks somewhat like an auricle (L. external ear, dim. of auris, ear); because of its shape, this area is called the auricular surface (Fig. 4.5B & C).
It is the site of the synovial part of the sacroiliac joint between the
sacrum and ilium. During life, the auricular surface is covered with
hyaline cartilage.


Figure 4.5. Sacrum and coccyx. A.
The base of the adult sacrum is the anterosuperior surface of the
sacrum (i.e., the aspect of the sacrum opposite its apex). The base
includes the articular surface of vertebra S1 (the anterior margin of
which forms the sacral promontory), the sacral canal (the inferior part
of the vertebral canal), and the right and left alae. Only the first of
the four coccygeal vertebrae has transverse processes. B.
The absence of the S4 and S5 spinous processes has resulted in the
formation of a large sacral hiatus. The cornua, or horns, of the sacrum
and coccyx are palpable clinical landmarks. C.
Lateral and anterior orientation drawings of the sacrum in its
anatomical position demonstrate the essentially frontal or coronal
plane and level at which the sacrum has been sectioned to reveal the
sacral canal containing the cauda equina. Spinal ganglia lie within the
IV foramina, as they do at superior vertebral levels. However, the
sacral posterior and anterior rami of the spinal nerves exit via
posterior and anterior (pelvic) sacral foramina, respectively. The
lateral orientation drawing demonstrates the auricular surface that
joins the ilium to form the synovial part of the sacroiliac joint. In
the anatomical position, the S1–S3 vertebrae lie in an essentially
transverse plane, forming a roof for the pelvic cavity.



The coccyx (tail bone) is a
small triangular bone that is usually formed by fusion of the four
rudimentary coccygeal vertebrae, although in some people, there may be
one less or one more (Fig. 4.5). The coccygeal
vertebra 1 (Co1) may be separate. The coccyx is the remnant of the
skeleton of the embryonic tail-like caudal eminence, which is present
in human embryos from the end of the 4th week until the beginning of
the 8th week (Moore and Persaud, 2003). The
pelvic surface of the coccyx is concave and relatively smooth, and the
posterior surface has rudimentary articular processes. Co1 is the
largest and broadest of all the coccygeal vertebrae. Its short
transverse processes are connected to the sacrum, and its rudimentary
articular processes form coccygeal cornua,
which articulate with the sacral cornua. The last three coccygeal
vertebrae often fuse during middle life, forming a beak-like coccyx;
this accounts for its name (G. coccyx,
cuckoo). With increasing age, Co1 often fuses with the sacrum, and the
remaining coccygeal vertebrae usually fuse to form a single bone. The
coccyx does not participate with the other vertebrae in support of the
body weight when standing; however, when sitting it may flex anteriorly
somewhat, indicating that it is receiving some weight. The coccyx
provides attachments for parts of the gluteus maximus and coccygeus
muscles and the anococcygeal ligament, the median fibrous intersection of the pubococcygeus muscles (see Chapter 3).
Ossification of the Vertebrae
Vertebrae begin to develop during the embryonic period as mesenchymal condensations around the notochord (Moore and Persaud, 2003).
Later, these mesenchymal bone models chondrify and cartilaginous
vertebrae form. Typically, vertebrae begin to ossify toward the end of
the embryonic period (8th week), with three primary ossification centers develop


ing in each cartilaginous vertebra: an endochondral centrum, which will eventually constitute most of the body of the vertebra, and two perichondral centers, one in each half of the neural arch (Fig. 4.6A–J & M).
Ossification continues throughout the fetal period. At birth, each
typical vertebra and the superiormost sacral vertebrae consists of
three bony parts united by hyaline cartilage. The inferior sacral
vertebrae and all the coccygeal vertebrae are still entirely
cartilaginous; they ossify during infancy. The halves of the neural
arches articulate at neurocentral joints, which are primary cartilaginous joints (Fig. 4.6G).
The halves of the neural/vertebral arch begin to fuse with each other
posterior to the vertebral canal during the 1st year, beginning in the
lumbar region and then in the thoracic and cervical regions. The neural
arches begin fusing with the centra in the upper cervical region around
the end of the 3rd year, but usually the process is not completed in
the lower lumbar region until after the 6th year.

Five secondary ossification centers
develop during puberty in each typical vertebra: one at the tip of the
spinous process; one at the tip of each transverse process; and two
anular epiphyses (ring epiphyses), one on the superior and one on the
inferior edges of each vertebral body (i.e., around the margins of the
superior and inferior surfaces of the vertebral body) (Fig. 4.6F & I–L). The hyaline anular epiphyses, to which the IV discs attach, are sometimes referred to as epiphysial growth plates and form the zone from which the vertebral body grows in height (see “Cartilage and Bones” in the Introduction).
When growth ceases early in the adult period, the epiphyses usually
unite with the vertebral body. This union results in the characteristic
smooth raised margin, the epiphysial rim, around the edges of the superior and inferior surfaces of the body of the adult vertebra (Figs. 4.2B and 4.3).
All secondary ossification centers have usually united with the
vertebrae by age 25; however, the ages at which specific unions are
made vary.
Exceptions to the typical pattern of ossification occur in vertebrae C1, C2, and C7 (Fig. 4.6A–C) and in the sacrum (Fig. 4.6M & N) and coccyx. For example, 56–58 primary and secondary centers of ossification have been described in the sacrum.
In addition, at all levels, primordial “ribs” (costal elements) appear in association with the secondary ossification centers of the transverse processes (transverse elements).
The costal elements normally develop into ribs only in the thoracic
region; they become part of the transverse process or its equivalent at
other levels.
In the cervical region, the costal element normally remains diminutive as part of the transverse process. Transverse foramina develop as gaps between the two lateral ossification centers, medial to a linking costotransverse bar, which forms the lateral boundary of the foramina (Fig. 4.6A–F).
Also as a result of the cervical transverse processes being formed from
the two developmental elements, the transverse processes of cervical
vertebrae end laterally in an anterior tubercle (from the costal element) and a posterior tubercle
(from the transverse element). The atypical morphology of vertebrae C1
and C2 is also established during development. The centrum of C1
becomes fused to that of C2 and loses its peripheral connection to the
remainder of C1, thus forming the dens (Fig. 4.6C).
Since these first two centra are fused and are now part of C2, no IV
disc is formed between C1 and C2 to connect them together. The part of
the body that remains with C1 is represented by the anterior arch and tubercle of C1.
In the thoracic region, the costal elements separate
from the developing vertebrae and elongate into ribs, and the
transverse elements alone form the transverse processes (Fig. 4.6I).
All but the base of the transverse processes of the lumbar vertebrae develop from the costal element (Fig. 4.6J); this projecting bar of the mature bone is therefore called the costal process. The transverse elements of the lumbar vertebrae form the mammillary processes.
The ala and auricular surfaces of the sacrum are formed
by the fusion of the transverse and costal elements. (For more
information about the ossification of vertebrae, see Williams et al., 1995.)



Variations in the Vertebrae
Most people have 33 vertebrae, but developmental errors
may result in 32 or 34 vertebrae. Estimates of the frequency of
abnormal numbers of vertebrae superior to the sacrum (the normal number
is 24) range between 5% and 12%. Variations in vertebrae are affected
by race, sex, and developmental factors (genetic and environmental). An
increased number of vertebrae occurs more often in males and a reduced
number occurs more frequently in females. Some races show more
variation in the number of vertebrae. Variations in the number of
vertebrae may be clinically important: An increased length of the
presacral region of the vertebral column increases the strain on the
inferior part of the lumbar region of the column owing to the increased
leverage. However, most numerical variations are detected incidentally
during diagnostic medical imaging studies being performed for other
reasons and during dissections and autopsies of persons with no history
of back problems.
Caution is necessary, however, when describing an injury
(e.g., when reporting the site of a vertebral fracture). When counting
the vertebrae, begin at the base of the neck. The number of cervical vertebrae (seven) is remarkably constant
(and not just in humans, but among mammals—even giraffes and snakes
have seven cervical vertebrae!). When considering a numerical
variation, the thoracic and lumbar regions must be considered together
because some people have more than five lumbar vertebrae and a
compensatory decrease in the number of thoracic vertebrae (O’Rahilly, 1986).
Variations in vertebrae also involve the relationship
between the vertebrae and ribs, and the number of vertebrae that fuse
to form the sacrum (Fig. 4.7). The relationship of presacral vertebrae to ribs and/or sacrum may occur higher (cranial shift) or lower (caudal shift)
than normal. Note, however, that a C7 vertebra articulating with a
rudimentary cervical rib(s) is still considered a cervical vertebra.
The same is true for lumbar vertebrae and lumbar ribs. Likewise, an L5
vertebra fused to the sacrum is referred to as a “sacralized 5th lumbar
vertebrae” (see clinical correlation [blue] boxAbnormal Fusion of Vertebrae,” in this chapter).
Vertebral Column
The vertebral column is an
aggregate structure, normally made up of all 33 vertebrae and the
components that unite them into a single structural, functional
entity—the “axis” of the axial skeleton. Because it provides the
semirigid, central “core” about which movements of the trunk occur,
“soft” or hollow structures that run a longitudinal course are subject
to damage or kinking (e.g., the spinal cord, descending aorta, venae
cavae, thoracic duct, and esophagus). Thus they lie in close proximity
to the vertebral axis, where they receive its semirigid support and
torsional stresses on them are minimized.
Joints of the Vertebral Column
The joints of the vertebral column include the:
  • Joints of the vertebral bodies.
  • Joints of the vertebral arches.
  • Craniovertebral (atlantoaxial and atlanto-occipital) joints.
  • Costovertebral joints (see Chapter 1).
  • Sacroiliac joints (see Chapter 3).


Figure 4.6. Ossification of vertebrae.
Typical vertebrae develop from three primary ossification centers that
are present at birth and five secondary ossifications centers that
appear at puberty. Costal elements are usually incorporated into the
transverse processes, except in the thoracic region where they become
ribs. A. Vertebra C1 (atlas) lacks a centrum. B and C. Vertebra C2 (axis) has two centra, one of which forms most of the dens. D–F. The development of “typical” cervical vertebrae is shown, including (D) the primary ossification centers within the hyaline cartilage, (E) a CT scan of the vertebra shown in part D (SC, spinal cord), and (F) the primary and secondary ossification centers. G–I. The development of thoracic vertebrae is shown, including (G)
the three primary ossification centers in a cartilaginous vertebra of a
7-week-old embryo (observe the joints present at this stage), (H) the bony parts of a thoracic vertebra after skeletonization (cartilage removed), and (I) the primary and secondary ossification centers (with ribs developed from costal elements). J–L. The development of the lumbar vertebrae is shown, including (J) the primary and secondary ossification centers, (K) the anular epiphyses separated from the body, and (L) the anular epiphyses in place. M and N.
The development of the sacrum is shown. Note that the ossification and
fusion of sacral vertebrae may not be completed until age 35.
Figure 4.7. Variations in vertebrae and their relationship to ribs. A.
A “cranial shift” is demonstrated, in which there are 13 ribs,
including a cervical rib articulating with vertebra C7 and a diminished
12th rib articulating with vertebra T12. Vertebra L5 is shown partially
incorporated into the sacrum, but such “sacralization” can also be
complete. The lowest sacral segment (S5) is partially segmented. B. The common arrangement of the vertebrae and the position of 1st and 12th ribs are shown. C.
A “caudal shift” is shown, in which the 12th rib is increased in size,
and there is a small lumbar rib. The transverse process of vertebra L4
is increased in size, whereas those of vertebra L5 are greatly reduced.
The first sacral segment is shown partially separated from the rest of
the sacrum, but such “lumbarization” can also be complete. The 1st
coccygeal segment is incorporated into the sacrum—that is, it is


Joints of the Vertebral Bodies
The joints of the vertebral bodies are symphyses (secondary cartilaginous joints) designed for weight bearing and strength. The articulating surfaces of adjacent vertebrae are connected by IV discs and ligaments (Fig. 4.8).
The IV discs provide strong attachments between the vertebral bodies,
uniting them into a continuous semirigid column and forming the
inferior half of the anterior border of the IV foramen. In aggregate,
the discs account for 20–25% of the length (height) of the vertebral
column (Fig. 4.1). As well as permitting
movement between adjacent vertebrae, their resilient deformability
allows them to serve as shock absorbers. Each IV disc consists of an anulus fibrosus, an outer fibrous part, composed of


concentric lamellae of fibrocartilage, and a gelatinous central mass, called the nucleus pulposus.

Figure 4.8 Lumbar vertebrae and IV discs.
This view of the superior lumbar region shows the structure of the
anuli fibrosi of the discs and the structures involved in formation of
IV foramina. The disc forms the inferior half of the anterior boundary
of an IV foramen (except in the cervical region). Thus herniation of
the disc will not affect the spinal nerve exiting from the superior
bony part of that foramen.
The anulus fibrosus is a fibrous ring consisting of concentric lamellae of fibrocartilage forming the circumference of the IV disc (Figs. 4.8, 4.9A). The anuli insert into the smooth, rounded epiphysial rims on the articular surfaces of the vertebral bodies formed by the fused anular epiphyses (Fig. 4.2B).
The fibers forming each lamella run obliquely from one vertebra to
another; the fibers of one lamella typically run at right angles to
those of the adjacent ones. This arrangement, allows some movement
between adjacent vertebrae, while providing a strong bond between them.
The nucleus pulposus (L. pulpa, fleshy) is the central core of the IV disc (Fig. 4.9A).
At birth, these pulpy nuclei are about 88% water and are initially more
cartilaginous than fibrous. Their semifluid nature is responsible for
much of the flexibility and resilience of the IV disc and of the
vertebral column as a whole. Vertical forces deform the discs, which
thus serve as shock absorbers. The pulpy nuclei become broader when
compressed and thinner when tensed or stretched (as when hanging or
suspended) (Fig. 4.9C).
Compression and tension occur simultaneously in the same disc during
anterior and lateral flexion and extension of the vertebral column (Fig. 4.9D).
During these movements, as well as during rotation, the turgid nucleus
acts as a semifluid fulcrum. Because the lamellae of the anulus
fibrosus are thinner and less numerous posteriorly than they are
anteriorly or laterally, the nucleus pulposus is not centered in the
disc but is more posteriorly placed. The nucleus pulposus is avascular;
it receives its nourishment by diffusion from blood vessels at the
periphery of the anulus fibrosus and vertebral body.
There is no IV disc between C1 and C2 vertebrae; the
most inferior functional disc is between L5 and S1 vertebrae. The discs
vary in thickness in different regions; they are thickest relative to
the size of the bodies they connect in the cervical and lumbar regions
(and absolutely thickest in the latter) and thinnest in the superior
thoracic region. Their relative thickness is clearly related to the
range of movement, and their varying shapes produce the secondary
curvatures of the vertebral column. The discs are thicker anteriorly in
the cervical and lumbar regions, and their thickness is most uniform in
the thoracic region.
Uncovertebral “joints” (of
Luschka) are located between the unci of the bodies of C3–C6 vertebrae
and the beveled inferolateral surfaces of the vertebral bodies superior
to them (Fig. 4.10). The joints are at the
lateral and posterolateral margins of the IV discs. The articulating
surfaces of these joint-like structures are covered with cartilage
moistened by fluid contained within an interposed potential space, or
“capsule.” They are considered synovial joints by some; others consider
them to be degenerative spaces (fissures) in the discs occupied


by extracellular fluid. The uncovertebral joints are frequent sites of spur formation, which may cause neck pain.

Figure 4.9. Structure and function of IV discs. A.
The disc consists of a nucleus pulposus and an anulus fibrosus. The
superficial layers of the anulus have been cut and spread apart to show
the direction of the fibers. Note that the combined thickness of the
rings of the anulus is diminished posteriorly—that is, the anulus is
thinner posteriorly. B. The fibrogelatinous nucleus pulposus occupies the center of the disc and acts as a cushion and shock-absorbing mechanism. C. The pulpy nucleus flattens and the anulus bulges when weight is applied, as occurs during standing and more so during lifting. D.
During flexion and extension movements, the nucleus pulposus serves as
a fulcrum. The anulus is simultaneously placed under compression on one
side and tension on the other.
Figure 4.10. Uncovertebral joints.
These small, synovial joint-like structures are between the unci of the
bodies of the lower vertebrae and the beveled surfaces of the vertebral
bodies superior to them. These joints are at the posterolateral margins
of the IV discs.
The anterior longitudinal ligament is a strong, broad fibrous band that covers and connects the anterolateral aspects of the vertebral bodies and IV discs (Fig. 4.11).
The ligament extends from the pelvic surface of the sacrum to the
anterior tubercle of vertebra C1 and the occipital bone anterior to the
foramen magnum. This ligament prevents hyperextension of the vertebral
column, maintaining stability of the joints between the vertebral
bodies. The anterior longitudinal ligament is the only ligament that
limits extension; all other IV ligaments limit forms of flexion.
The posterior longitudinal ligament is a much narrower, somewhat weaker band than the anterior longitudinal ligament (Fig. 4.11).
The posterior longitudinal ligament runs within the vertebral canal
along the posterior aspect of the vertebral bodies. It is attached
mainly to the IV discs and less so to the posterior aspects of the
vertebral bodies from C2 to the sacrum, often bridging fat and vessels
between the ligament and the bony surface. This ligament weakly resists
hyperflexion of the vertebral column and helps prevent or redirect
posterior herniation of the nucleus pulposus. It is well provided with
nociceptive (pain) nerve endings.
Figure 4.11. Relationship of ligaments to vertebrae and IV discs.
The lower thoracic (T9–T12) and upper lumbar (L1–L2) vertebrae, with
associated discs and ligaments, are shown. The pedicles of the T9–T11
vertebrae have been sawn through and their bodies and intervening discs
removed to provide an anterior view of the posterior wall of the
vertebral canal formed by the laminae of the vertebral arches and the
ligamenta flava extending between them. Between the adjacent left or
right pedicles, the inferior and superior articular processes and the
zygapophysial joints between them (from which joint capsules have been
removed) and the lateralmost extent of the ligamenta flava form the
posterior boundaries of IV foramina. The anterior longitudinal ligament
is broad, whereas the posterior longitudinal ligament is narrow.
Although thickest on the anterior aspect of the vertebral bodies (and
often only this thickest part is illustrated), the anterior
longitudinal ligament covers both the anterior and the lateral aspects
of the bodies to the IV foramen.




Joints of the Vertebral Arches
The joints of the vertebral arches are the zygapophysial joints (often called facet joints for brevity). These articulations are plane synovial joints between the superior and the inferior articular processes (G. zygapophyses) of adjacent vertebrae (Figs. 4.9 and 4.11). Each joint is surrounded by a thin, loose joint (articular) capsule. Those in the cervical region are especially thin and loose, reflecting the wide range of movement (Fig. 4.12).
The capsule is attached to the margins of the articular surfaces of the
articular processes of adjacent vertebrae. Accessory ligaments unite
the laminae, transverse processes, and spinous processes and help
stabilize the joints.
The zygapophysial joints permit gliding movements
between the articular processes; the shape and disposition of the
articular surfaces determine the types of movement possible. The range
(amount) of movement is determined by the size of the IV disc relative
to that of the vertebral body. In the cervical and lumbar regions,
these joints bear some weight, sharing this function with the IV discs
particularly during lateral flexion. The zygapophysial joints are
innervated by articular branches that arise from the medial branches of
the posterior rami of spinal nerves (Fig. 4.13).
As these nerves pass posteroinferiorly, they lie in grooves on the
posterior surfaces of the medial parts of the transverse processes.
Each articular branch supplies two adjacent joints; therefore, each
joint is supplied by two nerves.
Figure 4.12. Joints and ligaments of vertebral column. A.
The ligaments in the cervical region are shown. Superior to the
prominent spinous process of C7 (vertebra prominens), the spinous
processes are deeply placed and attached to an overlying nuchal
ligament. B. The ligaments in the thoracic
region are shown. The pedicles of the superior two vertebrae have been
sawn through and the vertebral arches removed to reveal the posterior
longitudinal ligament. Intertransverse, supraspinous, and interspinous
ligaments are demonstrated in association with the vertebrae with
intact vertebral arches.


Accessory Ligaments of the Intervertebral Joints
The laminae of adjacent vertebral arches are joined by broad, pale yellow elastic tissue called the ligamenta flava (L. flavus,
yellow). These yellow ligaments extend almost vertically from the
lamina above to the lamina below, those of opposite sides meeting and
blending in the midline (Fig. 4.11). The
ligaments bind the lamina of the adjoining vertebrae together, forming
alternating sections of the posterior wall of the vertebral canal. The
ligamenta flava are long, thin, and broad in the cervical region,
thicker in the thoracic region, and thickest in the lumbar region.
These ligaments resist separation of the vertebral lamina by arresting
abrupt flexion of the vertebral column and thereby preventing injury to
the IV discs. The strong elastic ligamenta flava help preserve the
normal curvatures of the vertebral column and assist with straightening
of the column after flexing.
Adjoining spinous processes are united by weak, almost membranous interspinous ligaments and strong fibrous supraspinous ligaments (Fig. 4.12A).
The thin interspinous ligaments connect adjoining spinous processes,
attaching from the root to the apex of each process. The cord-like supraspinous ligament, which connects the apices (tips) of the spinous processes from C7 to the sacrum, merges superiorly with the nuchal ligament (L. ligamentum nuchae), the strong, broad, median band at the back of the neck (Fr. nuque, back of neck) (Fig. 4.12A).
Unlike the interspinous and supraspinous ligaments, the nuchal ligament
is composed of thickened fibroelastic tissue, extending from the
external occipital protuberance and posterior border of the foramen
magnum to the spinous processes of the cervical vertebrae. Because of
the shortness of the C3–C5 spinous processes, the nuchal ligament
substitutes for bone in providing muscular


attachments. The intertransverse ligaments,
connecting adjacent transverse processes, consist of scattered fibers
in the cervical region and fibrous cords in the thoracic region (Fig. 4.12B). The intertransverse ligaments in the lumbar region are thin and membranous.

Figure 4.13. Innervation of zygapophysial joints.
The posterior rami arise from the spinal nerves outside the IV foramen
and divide into medial and lateral branches; the medial branch gives
rise to articular branches distributed to the zygapophysial joint at
that level and to the joint one level inferior to its exit. Thus, each
zygapophysial joint receives articular rami from the medial branch of
the posterior rami of two adjacent spinal nerves; the medial branches
of both posterior rami would have to be ablated to denervate a
zygapophysial joint.
Craniovertebral Joints
There are two sets of craniovertebral joints, the
atlanto-occipital joints. formed between the atlas (C1 vertebra) and
the occipital bone of the cranium, and the atlantoaxial joints, formed
between the atlas and axis (C2 vertebra) (Fig. 4.14). The Greek word atlanto
refers to the atlas. The craniovertebral joints are synovial joints
that have no IV discs. Their design gives a wider range of movement
than in the rest of the vertebral column. The articulations involve the
occipital condyles, atlas, and axis.
Atlanto-Occipital Joints
The articulations between the superior articular surfaces of the lateral masses of the atlas and the occipital condyles (Figs. 4.4A & B and 4.14A),
the atlanto-occipital joints, permit nodding of the head, such as the
neck flexion and extension occurring when indicating approval (the
“yes” movement). These joints also permit sideways tilting of the head.
The main movement is flexion, with a little lateral bending and
rotation. They are synovial joints of the condyloid type and have thin,
loose articular capsules. The cranium and C1 are also connected by anterior and posterior atlanto-occipital membranes, which extend from the anterior and posterior arches of C1 to the anterior and posterior margins of the foramen magnum (Figs. 4.14B and 4.15).
The anterior membranes are composed of broad, densely woven fibers
(especially centrally where they are continuous with the anterior
longitudinal ligament); the posterior membranes are broad but
relatively weak. The atlanto-occipital membranes help prevent excessive
movement of these joints.
Atlantoaxial Joints
There are three atlantoaxial articulations (Fig. 4.14A–C): two (right and left) lateral atlantoaxial joints (between the inferior facets of the lateral masses of C1 and the superior facets of C2), and one median atlantoaxial joint
(between the dens of C2 and the anterior arch of the atlas). The
lateral atlantoaxial joints are gliding-type synovial joints, whereas
the median atlantoaxial joint is a pivot joint. Movement at all three
atlantoaxial joints permits the head to be turned from side to side (Fig. 4.14D),
as occurs when rotating the head to indicate disapproval (the “no”
movement). During this movement, the cranium and C1 rotate on C2 as a
unit. During rotation of the head, the dens of C2 is the axis or pivot
that is held in a socket or collar formed anteriorly by the anterior
arch of the atlas and posteriorly by the transverse ligament of the atlas (Fig. 4.14A–C),
a strong band extending between the tubercles on the medial aspects of
the lateral masses of C1 vertebrae. Vertically oriented but much weaker
superior and inferior longitudinal bands pass from the trans



verse ligament to the occipital bone superiorly and to the body of C2
inferiorly. Together, the transverse ligament and the longitudinal
bands form the cruciate ligament (formerly the cruciform ligament), so named because of its resemblance to a cross.

Figure 4.14. Craniovertebral joints and ligaments. A.
Ligaments of the atlanto-occipital and atlantoaxial joints. The
tectorial membrane and the right side of the cruciate ligament of the
atlas have been removed to show the attachment of the right alar
ligament to the dens of vertebra C2 (axis). B.
The hemisected craniovertebral region shows the median joints and
membranous continuities of the ligamenta flava and longitudinal
ligaments in the craniovertebral region. C.
The articulated atlas and axis showing that the median atlantoaxial
joint is formed as the anterior arch and the transverse ligament of the
atlas form a socket for the dens of the axis. D.
During rotation of head, the cranium and atlas rotate as a unit around
the pivot of the dens when the head is turned side to side (the “no”
Figure 4.15. Membranes of craniovertebral joints. A.
Only the thicker, most anterior part of the anterior longitudinal
ligament is included here to demonstrate its superior continuation as
the anterior atlantoaxial membrane and anterior atlanto-occipital
membrane. Laterally, the membranes blend with the articular capsules of
the lateral atlantoaxial and atlanto-occipital joints. B.
The posterior atlanto-occipital and atlantoaxial membranes span the
gaps between the posterior arch of the atlas (C1) and the occipital
bone (posterior margin of the foramen magnum) superiorly and the
laminae of the axis (C2) inferiorly. The vertebral arteries penetrate
the atlanto-occipital membrane before traversing the foramen magnum.
The alar ligaments extend from the sides of the dens to the lateral margins of the foramen magnum (Fig. 4.14A).
These short, rounded cords, approximately 0.5 cm in diameter (just
smaller than a pencil), attach the cranium to vertebra C1 and serve as
check ligaments, preventing excessive rotation at the joints.
The tectorial membrane (Fig. 4.14A & B)
is the strong superior continuation of the posterior longitudinal
ligament across the median atlantoaxial joint through the foramen
magnum to the central floor of the cranial cavity. It runs from the
body of C2 to the internal surface of the occipital bone and covers the
alar and transverse ligaments.




Movements of the Vertebral Column
The range of movement of the vertebral column varies
according to the region and the individual. Some people are capable of
extraordinary movements, such as acrobats who began their training
during early childhood. The normal range of movement possible in
healthy young adults is typically reduced by 50% or more as they age.
The mobility of the vertebral column results primarily
from the compressibility and elasticity of the IV discs. The vertebral
column is capable of flexion, extension, lateral flexion and extension,
and rotation (torsion) (Fig. 4.16). Bending of the vertebral column to the right or left from the neutral (erect) position is lateral flexion (bending); returning to the erect posture from a position of lateral flexion is lateral extension.
The range of movement of the vertebral column is limited by the:
  • Thickness, elasticity, and compressibility of the IV discs.
  • Shape and orientation of the zygapophysial joints.
  • Tension of the joint capsules of the zygapophysial joints.
  • Resistance of the back muscles and ligaments (e.g., the ligamenta flava and the posterior longitudinal ligament).
  • Attachment to the thoracic (rib) cage.
  • Bulk of surrounding tissue.
The back muscles that produce movements of the vertebral
column are discussed later in this chapter. Movements are not produced
exclusively, however, by the back muscles. They are assisted by gravity
and the action of the anterolateral abdominal muscles (see Chapter 2).
Movements between adjacent vertebrae occur at the
resilient nuclei pulposi of the IV discs (serving as the axis of
movement) and at the zygapophysial joints. The orientation of the
latter joints permits some movements and restricts others. With the
exception perhaps of C1–C2, movement never occurs at a single segment
of the column. Although movements between adjacent vertebrae are
relatively small, especially in the thoracic region, the summation of
all the small movements produces a considerable range of movement of
the vertebral column as a whole (e.g., when flexing to touch the toes).
Movements of the vertebral column are freer in the cervical and lumbar
regions than elsewhere. Flexion, extension, lateral flexion, and
rotation of the neck are especially free because the:
  • IV discs, although thin relative to most other discs, are thick relative to the size of the vertebral bodies at this level.
  • Articular surfaces of the zygapophysial joints are relatively large and the joint planes are almost horizontal.
  • Joint capsules of the zygapophysial joints are loose.
  • Neck is relatively slender (with less surrounding soft tissue bulk compared with the trunk).
Flexion of the vertebral column is greatest in the cervical region (Fig. 4.16A).
The sagittally oriented joint planes of the lumbar region are conducive
to flexion and extension. Extension of the vertebral column is most
marked in the lumbar region and is usually more extensive than flexion;
however, the interlocking articular processes here prevent rotation (Table 4.3).
The lumbar region, like the cervical region, has IV discs that are
large (the largest discs occur here) relative to the size of the
vertebral bodies. Lateral flexion of the vertebral column is greatest
in the cervical and lumbar regions (Fig. 4.16B).
The thoracic region, in contrast, has IV discs that are
thin relative to the size of the vertebral bodies. Relative stability
is also conferred on this part of the vertebral column, through its
connection to the sternum by the ribs and costal cartilages. The joint
planes here lie on an arc that is centered on the vertebral body,
permitting rotation in the thoracic region (Fig. 4.16C).
This rotation of the upper trunk, in combination with the rotation
permitted in the cervical region and that at the atlantoaxial joints,
enables the torsion of the axial skeleton that occurs as one looks back
over the shoulder. However, flexion is limited in the thoracic region,
including lateral flexion.
Figure 4.16. Movements of vertebral column. A.
Flexion (forward bending) and extension (backward bending), both in the
median plane, are shown. Flexion and extension are occurring primarily
in the cervical and lumbar regions. B. Lateral flexion (to the right or left in a frontal plane) is shown, also occurring mostly in the cervical and lumbar regions. C.
Rotation around a longitudinal axis, which occurs primarily at the
craniovertebral joints (augmented by the cervical region) and the
thoracic region, is shown.



Curvatures of the Vertebral Column
The vertebral column in adults has four curvatures: cervical, thoracic, lumbar, and sacral (Fig. 4.17). The thoracic and sacral (pelvic) curvatures (kyphoses; singular = kyphosis) are concave anteriorly, whereas the cervical and lumbar curvatures (lordoses; singular = lordosis) are concave posteriorly. The thoracic and sacral curvatures are primary curvatures that develop during the fetal period in relationship to the (flexed) fetal position. Compare the curvatures in Figure 4.17,
noting that the primary curvatures are in the same direction as the
main curvatures of the fetal vertebral column. The primary curvatures
are retained throughout life as a consequence of differences in height
between the anterior and posterior parts of the vertebrae.
The cervical and lumbar curvatures are secondary curvatures
that result from extension from the flexed fetal position. They begin
to appear during the fetal period but do not become obvious until
infancy. Secondary curvatures are maintained primarily by differences
in thickness between the anterior and the posterior parts of the IV
discs. The cervical curvature becomes
fully evident when an infant begins to raise (extend) its head while
prone and to hold its head erect while sitting. The lumbar curvature
becomes apparent when an infant begins to assume the upright posture,
standing and walking. This curvature, generally more pronounced in
females, ends at the lumbosacral angle formed at the junction of L5 vertebra with the sacrum (Fig. 4.1D). The sacral curvature also differs in males and females, that of the female reduced so that the coccyx protrudes less into the pelvic outlet (see Chapter 3).
The curvatures provide additional flexibility
(shock-absorbing resilience) to the vertebral column, further
augmenting that provided by the IV discs. When the load borne by the
vertebral column is markedly increased (as by carrying a heavy backpack
or another person on one’s shoulders), both the IV discs and the
flexible curvatures are compressed (i.e., the curvatures tend to
increase). While the flexibility provided by the IV discs is passive
and limited primarily by the zygapophysial joints and longitudinal
ligaments, that provided by the curvatures is actively (dynamically)
resisted by the contraction of


groups antagonistic to the movement (e.g., the long extensors of the
back resist increased thoracic kyphosis, whereas the abdominal flexors
resist increased lumbar lordosis). Carrying additional weight anterior
to the body’s normal gravitational axis (e.g., abnormally large
breasts, a pendulous abdomen, or carrying a young child) also tends to
increase these curvatures. The muscles that provide resistance to the
increase in curvature often ache when the weight is borne for extended

Figure 4.17. Curvatures of vertebral column.
The four curvatures of the adult vertebral column—cervical, thoracic,
lumbar, and sacral—are contrasted with the C-shaped curvature of the
vertebral column during fetal life, when only the primary () curvatures exist. The secondary () curvatures develop during infancy and childhood.
When sitting, especially in the absence of back support
for long periods of time, one usually “cycles” between back flexion
(slumping) and extension (sitting up straight) to minimize stiffness
and fatigue (Fig. 4.18). This allows
alternation between the active support provided by the extensor muscles
of the back and the passive resistance to flexion provided by ligaments.


Figure 4.18. Flexion and extension while sitting. During prolonged sitting (especially when no back support is provided), flexion (A) and extension (B) occur alternately, transferring the load between muscles (active) and ligaments (passive).


Vasculature of the Vertebral Column
Vertebrae are supplied by periosteal and equatorial branches of the major cervical and segmental arteries and their spinal branches (Fig. 4.19).
Parent arteries of periosteal, equatorial, and spinal branches occur at
all levels of the vertebral column, in close association with it, and
include the following arteries (described in detail in other chapters):
  • Vertebral and ascending cervical arteries in the neck (Chapter 8).
  • The major segmental arteries of the trunk:
  • Posterior intercostal arteries in the thoracic region (Chapter 1).
  • Subcostal and lumbar arteries in the abdomen (Chapter 2).
  • Iliolumbar and lateral and medial sacral arteries in the pelvis (Chapter 3).
Periosteal and equatorial branches
arise from these arteries as they cross the external (anterolateral)
surfaces of the vertebrae. Spinal branches enter the IV foramina and
divide. Smaller anterior and posterior vertebral canal branches
pass to the vertebral body and vertebral arch, respectively, and give
rise to ascending and descending branches that anastomose with the
spinal canal branches of adjacent levels. Anterior vertebral canal
branches send nutrient arteries anteriorly into the vertebral bodies that supply most of the red marrow of the central vertebral body (Bogduk, 1997). The


larger branches of the spinal branches continue as terminal radicular or segmental medullary arteries
distributed to the posterior and anterior roots of the spinal nerves
and their coverings and to the spinal cord, respectively (see “Vasculature of the Spinal Cord and Spinal Nerve Roots,” later in this chapter).

Figure 4.19. Blood supply of vertebrae.
Typical vertebrae are supplied by segmental arteries—here lumbar
arteries. In the thoracic and lumbar regions, each vertebra is
encircled on three sides by paired intercostal or lumbar arteries that
arise from the aorta. The segmental arteries supply equatorial branches
to the vertebral body, and posterior branches supply the vertebral arch
structures and the back muscles. Spinal branches enter the vertebral
canal through the IV foramina to supply the bones, periosteum,
ligaments, and meninges that bound the epidural space and radicular or
segmental medullary arteries that supply nervous tissue (spinal nerve
roots and spinal cord).
Figure 4.20. Venous drainage of vertebral column. A.
The venous drainage parallels the arterial supply and enters the
external and internal vertebral venous plexuses. There is also
anterolateral drainage from the external aspects of the vertebrae into
segmental veins. B. The superior view of a
lumbar vertebra is shown. The vertebral canal contains a dense plexus
of thin-walled valveless veins, the internal vertebral venous plexuses,
which surround the dura mater. Anterior and posterior longitudinal
venous sinuses can be identified in the internal vertebral venous
plexus. Basivertebral veins from the vertebral body drain primarily
into the anterior internal vertebral venous plexus, but they may also
drain to the anterior external plexus.
Spinal veins form venous plexuses along the vertebral column both inside and outside the vertebral canal. These plexuses are the internal vertebral venous plexus (epidural venous plexuses) and external vertebral venous plexuses, respectively (Fig. 4.20).
These plexuses communicate through the intervertebral foramina. Both
plexuses are densest anteriorly and posteriorly and relatively sparse
laterally. The large, tortuous basivertebral veins
form within the vertebral bodies. They emerge from foramina on the
surfaces of the vertebral bodies (mostly the posterior aspect) and
drain into the anterior external and especially the anterior internal
vertebral venous plexuses, which may form large longitudinal sinuses.
The intervertebral veins receive veins from the spinal cord and vertebral venous plexuses as they accompany the spinal


nerves through the IV foramina to drain into the vertebral veins of the neck and segmental (intercostal, lumbar, and sacral) veins of the trunk.

Nerves of the Vertebral Column
Other than the zygapophysial joints (innervated by articular branches of the medial branches of the posterior rami, as


described with these joints), the vertebral column is innervated by (recurrent) meningeal branches of the spinal nerves (Fig. 4.21).
These rarely described or depicted branches are the only branches to
arise from the mixed spinal nerve, arising immediately after it is
formed and before its division into anterior and posterior rami or from
the anterior ramus immediately after its formation. Two to four of
these fine branches arise on each side at all vertebral levels. Close
to their origin, the meningeal branches receive communicating branches
from the nearby gray rami communicantes. As the spinal nerves exit the
IV foramina, most of the meningeal branches run back through the
foramina into the vertebral canal (hence the alternate term recurrent).
However, some branches remain outside the canal and are distributed to
the anterolateral aspect of the vertebral bodies and IV discs. They
supply the periosteum and especially the anuli fibrosi and anterior
longitudinal ligament. Inside the vertebral canal, transverse,
ascending, and descending branches distribute nerve fibers to the:

Figure 4.21. Innervation of periosteum and ligaments of vertebral column and of meninges.
Except for the zygapophysial joints and external elements of the
vertebral arch, the fibroskeletal structures of the vertebral column
(and the meninges) are supplied by the (recurrent) meningeal nerves.
Although usually omitted from diagrams and illustrations of spinal
nerves, these fine nerves are the first branches to arise from all 31
pairs of spinal nerves and are the nerves that initially convey
localized pain sensation from the back produced by acute herniation of
an IV disc or from sprains, contusions, fractures, or tumors of the
vertebral column itself. (Based on

Frick H, Kummer B, Putz R: Wolf-Heidegger’s Atlas of Human Anatomy, 4th ed. Basel, Karger AG, 1990:476.


  • Periosteum (covering the surface of the posterior vertebral bodies, pedicles, and laminae).
  • Ligamenta flava.
  • Anuli fibrosi of the posterior and posterolateral aspect of the IV discs.
  • Posterior longitudinal ligament.
  • Spinal dura mater.
  • Blood vessels within the vertebral canal.
Nerve fibers to the periosteum, anuli fibrosi and
ligaments supply pain receptors; those to the anuli fibrosi and
ligaments also supply receptors for proprioception (the sense of one’s
position). Sympathetic fibers to the blood vessels stimulate
vasoconstriction. Innervation of the dura mater is discussed later in
this chapter.


Figure 4.22. Vertebral column, spinal cord, spinal ganglia, and spinal nerves.
Lateral and posterior views illustrating the relation of the spinal
cord segments (the numbered segments) and spinal nerves to the adult
vertebral column.


Contents of the Vertebral Canal
The spinal cord, spinal nerve roots, and spinal meninges
and the neurovascular structures that supply them are located within
the vertebral canal (Fig. 4.21).
Spinal Cord
The spinal cord is the major
reflex center and conduction pathway between the body and the brain.
This cylindrical structure, slightly flattened anteriorly and
posteriorly, is protected by the vertebrae and their associated
ligaments and muscles, the spinal meninges, and the cerebrospinal fluid
(CSF). The spinal cord begins as a continuation of the medulla oblongata (commonly called the medulla),
the caudal part of the brainstem. In adults, the spinal cord is 42–45
cm long and extends from the foramen magnum in the occipital bone to
the level of the L1 or L2 vertebra (Fig. 4.22). However, its tapering inferior end, the medullary cone,
may terminate as high as T12 vertebra or as low as L3 vertebra. Thus
the spinal cord occupies only the superior two thirds of the vertebral
The spinal cord is enlarged in two regions in relationship to innervation of the limbs. The cervical enlargement
extends from the C4 through T1 segments of the spinal cord, and most of
the anterior rami of the spinal nerves arising from it form the brachial plexus of nerves that innervates the upper limbs (see Chapter 6). The lumbosacral (lumbar) enlargement
extends from T11 through S1 segments of the spinal cord, inferior to
which the cord continues to diminish as the medullary cone. The
anterior rami of the spinal nerves arising from this enlargement make
up the lumbar and sacral plexuses of nerves that innervate the lower limbs (see Chapter 5).
Spinal Nerve Roots
The formation and composition of spinal nerves and nerve
roots are discussed in the Introduction. Readers are urged to read this
information now if they have not done so previously.
The portion of the spinal cord giving rise to the
rootlets and roots that ultimately form one bilateral pair of spinal
nerves is designated a spinal cord segment,
the identity of which is the same as the spinal nerves arising from it.
Cervical spinal nerves (except C8) bear the same alphanumeric
designation as the vertebrae forming the inferior margin of the IV
foramina through which the nerve exits the vertebral canal, whereas the
more inferior spinal nerves bear the same alphanumeric designation as
the vertebrae forming the superior margin of their exit. The first
cervical nerves lack posterior roots in 50% of people, and the
coccygeal nerve may be absent altogether.
In embryos, the spinal cord occupies the full length of the vertebral canal (Moore and Persaud, 2003);
thus spinal cord segments lie approximately at the vertebral level of
the same number, the spinal nerves passing laterally to exit the
corresponding IV foramen. By the end of the embryonic period (8th
week), the tail-like caudal eminence has disappeared, and the number of
coccygeal vertebrae is reduced from six to four segments. The spinal
cord in the vertebral canal of the coccyx atrophies. During the fetal
period, the vertebral column grows faster than the spinal cord; as a
result, the cord “ascends” relative to the vertebral canal. At birth,
the tip of the medullary cone is at the L4–L5 level. Thus, in postnatal
life, the spinal cord is shorter than the vertebral column;
consequently, there is a progressive obliquity of the spinal nerve
roots (Figs. 4.22 and 4.23).
Because the distance between the origin of a nerve’s roots from the
spinal cord and the nerve’s exit from the vertebral canal increases as
the inferior end of the vertebral column is approached, the length of
the nerve roots also increases progressively. The lumbar and sacral
nerve roots are therefore the longest, extending far beyond the
termination of the adult cord at approximately the L2 level to reach
the remaining lumbar, sacral, and coccygeal IV foramina. This loose
bundle of spinal nerve roots arising from the lumbosacral enlargement
and the medullary cone and coursing within the lumbar cistern of CSF caudal to the termination of the spinal cord resembles a horse’s tail, hence its name—the cauda equina (L. horse tail).
Arising from the tip of the medullary cone, the terminal filum descends among the spinal nerve roots in the cauda equina. The terminal filum (L. filum terminale)
is the vestigial remnant of the caudal part of the spinal cord that was
in the tail-like caudal eminence of the embryo. Its proximal end (the pial part or internal terminal filum)
consists of vestiges of neural tissue, connective tissue, and
neuroglial tissue covered by pia mater. The terminal filum perforates
the inferior end of the dural sac, gaining a layer of dura and
continuing through the sacral hiatus as the dural part or external terminal filum (also known as the coccygeal ligament)
to attach to the dorsum of the coccyx. The terminal filum is an anchor
for the inferior end of the spinal cord and the spinal meninges.


Figure 4.23. Spinal cord in situ.
The vertebral arches and the posterior aspect of the sacrum have been
removed to expose the spinal cord in the vertebral canal. The spinal
dural sac has also been opened to reveal the spinal cord and posterior
nerve roots, the termination of the spinal cord between the L1 and the
L2 vertebral level, and the termination of the spinal dural sac at the
S2 segment.


Spinal Meninges and Cerebrospinal Fluid
Collectively, the dura mater, arachnoid mater, and pia mater surrounding the spinal cord constitute the spinal meninges (Fig. 4.24; Table 4.4).
These membranes surround, support, and protect the spinal cord and
spinal nerve roots, including those of the cauda equina, and contain
the cerebrospinal fluid (CSF) in which these structures are suspended.
Spinal Dura Mater
The spinal dura mater,
composed mainly of tough fibrous with some elastic tissue, is the
outermost covering membrane of the spinal cord. The spinal dura mater
is separated from the periosteum-covered bone and the ligaments that
form the walls of the vertebral canal by the epidural space. This space is occupied by the internal vertebral venous plexus embedded in a fatty matrix (epidural fat).
The epidural space runs the length of the vertebral canal, terminating
superiorly at the foramen magnum and laterally at the IV foramina, as
the dura mater adheres to the periosteum surrounding each opening, and
inferiorly, as the sacral hiatus is sealed by the sacrococcygeal
The dura mater forms the spinal dural sac, a long tubular sheath within the vertebral canal (Figs. 4.22 and 4.23).
This sac adheres to the margin of the foramen magnum of the cranium,
where it is continuous with the cranial dura mater. The spinal dural
sac is anchored inferiorly to the coccyx by the terminal filum.
The sac is evaginated by each pair of posterior and anterior roots as
they extend laterally toward their exit from the vertebral canal (Fig. 4.25). Thus tapering lateral extensions of the spinal dura surround each pair of posterior and anterior nerve roots as dural root sheaths, or sleeves (Figs. 4.24, 4.26, and 4.27). Distal to the spinal ganglia, these sheaths blend with the epineurium (outer connective tissue covering of the spinal nerves) that adheres to the periosteum lining the IV foramina.
Figure 4.24. Spinal cord, spinal nerves, and spinal meninges.
Three membranes (the spinal meninges) cover the spinal cord: dura
mater, arachnoid mater, and pia mater. As the spinal nerve roots extend
toward an IV foramen, they are surrounded by a dural root sheath
(sleeve) that is continuous distally with the epineurium of the spinal
Innervation of the Dura Mater
Nerve fibers are distributed to the dura mater by the (recurrent) meningeal nerves (Fig. 4.21).
The function of these afferent and sympathetic fibers is unclear,
although it is known that the afferent fibers supply pain receptors
that are involved in the referred pain characteristic of spinal
disorders and become irritated when there is inflammation of the
meninges (meningitis).
Spinal Arachnoid Mater
The spinal arachnoid mater
is a delicate, avascular membrane composed of fibrous and elastic
tissue that lines the spinal dural sac and its dural root sheaths. It
encloses the CSF-filled subarachnoid space containing the spinal cord,
spinal nerve roots, and spinal ganglia (Figs. 4.24, 4.25, and 4.27).
The arachnoid mater is not attached to the dura mater but is held
against its inner surface by the pressure of the CSF. In a lumbar
spinal puncture, the needle traverses the dura and arachnoid mater
simultaneously. Their apposition is the dura–arachnoid interface (Table 4.4),
often erroneously referred to as the “subdural space.” No actual space
occurs naturally at this site; it is, rather, a weak cell layer (Haines, 2002). Bleeding into this layer creates a pathological space at the dura–arachnoid junction in which a subdural hematoma
is formed. In the cadaver, because of the absence of CSF, the arachnoid
mater falls away from the inner surface of the dura mater and lies
loosely on the spinal cord. The arachnoid


mater is separated from the pia mater on the surface of the spinal cord by the subarachnoid space containing CSF. Delicate strands of connective tissue, the arachnoid trabeculae, span the subarachnoid space connecting the arachnoid mater and pia mater.

Table 4.4. Spaces Associated with the Spinal Meninges
Space Location Contents
Extradural (epidural) Space between periosteum lining bony wall of vertebral canal and dura mater; position of extradural (epidural) herniation Fat (loose connective tissue); internal vertebral venous plexuses; inferior to L2 vertebra, ensheathed roots of spinal nerves
Subarachnoid (leptomeningeal) Naturally occurring space between arachnoid mater and pia mater CSF; radicular, segmental, medullary, and spinal arteries; veins; arachnoid trabeculae
a Although it is common to refer to a “subdural space,” there is no naturally occurring space at the arachnoid–dura junction (Haines, 2002).
Spinal Pia Mater
The spinal pia mater, the
innermost covering membrane of the spinal cord, consists of flattened
cells with long, equally flattened processes that closely follow all
the surface features of the spinal cord (Haines, 2002).
The pia mater also directly covers the roots of the spinal nerves and
the spinal blood vessels. Inferior to the medullary cone, the pia mater
continues as the terminal filum (Fig. 4.22).
The spinal cord is suspended in the dural sac by the terminal filum and especially by the right and left sawtooth denticulate ligaments (L. denticulus, small tooth), which run longitudinally along each side of the spinal cord (Figs. 4.25


and 4.27).
The denticulate ligament consists of a fibrous sheet of pia mater
extending midway between the posterior and the anterior nerve roots
from the lateral surfaces of the spinal cord. The 20–22 processes,
shaped much like shark’s teeth, attach to the inner surface of the
arachnoid-lined dural sac. The superior process of each denticulate
ligament attaches to the cranial dura mater immediately superior to the
foramen magnum, and the inferior process extends from the medullary
cone, passing between the T12 and the L1 nerve roots.

Figure 4.25. Spinal cord within its meninges. The dura mater and arachnoid mater have been split and pinned flat to expose the spinal cord and spinal nerve roots.
Figure 4.26. Inferior end of spinal dural sac.
A laminectomy has been performed (i.e., most of the vertebral arches of
the lumbar and sacral vertebrae have been removed) to show the inferior
end of the dural sac, which encloses the lumbar cistern containing CSF
and the cauda equina. The lumbar spinal ganglia lie within the IV
foramina, but the sacral spinal ganglia (S1–S5) are in the sacral
canal. In the lumbar region, the nerves exiting the IV foramina pass
superior to the IV discs at that level; thus herniation of the nucleus
pulposus tends to impinge on nerves passing to lower levels.
Figure 4.27. Spinal cord, anterior and posterior nerve rootlets and roots, spinal nerves, and spinal meninges.
The structure of the spinal cord (gray and white substance) and the
entrance and exit of nerve fibers via rootlets are demonstrated. The
denticulate ligament is a serrated shelf-like extension of the spinal
pia mater projecting between the posterior and the anterior nerve roots
in a frontal plane from either side of the cervical and thoracic
regions of the spinal cord. Its relatively strong, tooth-like processes
anchor the spinal cord within the subarachnoid space.
Subarachnoid Space
The subarachnoid space is located between the arachnoid mater and the pia mater and is filled with CSF (Figs. 4.24 and 4.25; Table 4.4).
The enlargement of the subarachnoid space in the dural sac, caudal to
the medullary cone and containing CSF and the cauda equina, is the lumbar cistern (Figs. 4.22 and 4.23).
It extends from the L2 vertebra to the second segment of the sacrum.
Dural root sheaths, enclosing spinal nerve roots in extensions of the
subarachnoid space, protrude from the sides of the lumbar cistern (Fig. 4.26).




Figure 4.28. Arterial supply of spinal cord. A and B.
The arteries derive from branches of the vertebral, ascending and deep
cervical, intercostal, lumbar, and lateral sacral arteries. Three
longitudinal arteries supply the spinal cord: an anterior spinal artery
and two posterior spinal arteries. These vessels are reinforced by
blood from the anterior and posterior segmental medullary arteries.
Segmental medullary arteries supply the nerve roots that they course
along and then contribute to the longitudinal arteries. The great
anterior segmental medullary artery (of Adamkiewicz) is the largest
segmental medullary artery, vital to the blood supply of the spinal
cord between its cervical and lumbosacral enlargements. At levels where
there are no segmental medullary arteries, radicular arteries supply
the posterior and anterior roots of the spinal nerves. (Radicular
arteries are shown at only the cervical and thoracic levels, but they
also occur at the lumbar and sacral levels.)


Vasculature of the Spinal Cord and Spinal Nerve Roots
Arteries of the Spinal Cord and Nerve Roots
The arteries supplying the spinal cord are branches of
the vertebral, ascending cervical, deep cervical, intercostal, lumbar,
and lateral sacral arteries (Figs. 4.28 & 4.29). Three longitudinal arteries supply the spinal cord: an anterior spinal artery and paired posterior spinal arteries. These arteries run longitudinally from the medulla of the brainstem to the medullary cone of the spinal cord. The anterior spinal artery, formed by the union of branches of the vertebral arteries, runs inferiorly in the anterior median fissure. Sulcal arteries arise from the anterior spinal artery and enter the spinal cord through this fissure (Fig. 4.29B). The sulcal arteries supply approximately two thirds of the cross-sectional area of the spinal cord (Williams et al., 1995). Each posterior spinal artery is a branch of either the vertebral artery or the posteroinferior cerebellar artery (Figs. 4.28B and 4.29). The posterior spinal arteries commonly form anastomosing channels in the pia mater.
Figure 4.29. Arterial supply and venous drainage of spinal cord and spinal nerve roots. A.
The veins that drain the spinal cord, as well as the internal vertebral
venous plexuses, drain into the intervertebral veins, which in turn
drain into segmental veins. B. The pattern
of the arterial supply of the spinal cord is from three longitudinal
arteries: one anterior lying in the anteromedian position and the other
two lying posterolaterally. These vessels are reinforced by medullary
branches derived from the segmental arteries. The sulcal arteries are
small branches of the anterior spinal artery coursing in the anterior
median fissure.
By themselves, the anterior and posterior spinal
arteries can supply only the short superior part of the spinal cord.
The circulation to much of the spinal cord depends on segmental
medullary and radicular arteries running along the spinal nerve roots.
The anterior and posterior segmental


medullary arteries
are derived from spinal branches of the ascending cervical, deep
cervical, vertebral, posterior intercostal, and lumbar arteries. The
segmental medullary arteries are located chiefly where the need for a
good blood supply to the spinal cord is greatest—the cervical and
lumbosacral enlargements. They enter the vertebral canal through the IV

The great anterior segmental medullary artery
(of Adamkiewicz), which is on the left side in about 65% of people,
reinforces the circulation to two thirds of the spinal cord, including
the lumbosacral enlargement (Fig. 4.28A).
The great anterior segmental medullary artery, much larger than the
other segmental medullary arteries, usually arises from an inferior
intercostal or upper lumbar artery and enters the vertebral canal
through the IV foramen at the lower thoracic or upper lumbar level.
The posterior and anterior roots of the spinal nerves and their coverings are supplied by posterior and anterior radicular arteries (L. radix, root), which run along the nerve roots (Figs. 4.28 and 4.29).
Most spinal nerve roots and proximal spinal nerves and roots are
accompanied by radicular arteries that do not reach the posterior,
anterior, or spinal arteries. Segmental medullary arteries occur
irregularly in the place of radicular arteries; they are really larger
vessels that make it all the way to the spinal arteries. Most radicular
arteries are small and supply only the nerve roots; however, some of
them assist with the supply of superficial parts of the gray matter in
the posterior and anterior horns of the spinal cord.
Veins of the Spinal Cord
In general, the veins draining the spinal cord have a
distribution similar to that of the spinal arteries. There are usually
three anterior and three posterior spinal veins (Fig. 4.29A). The spinal veins are arranged longitudinally, communicate freely with each other, and are drained by up to 12 anterior and posterior medullary and radicular veins. The veins draining the spinal cord join the internal vertebral (epidural) venous plexus in the epidural space (Fig. 4.20).
The internal vertebral venous plexus passes superiorly through the
foramen magnum to communicate with dural sinuses and vertebral veins in
the cranium (see Chapter 7). The internal
vertebral plexus also communicates with the external vertebral venous
plexus on the external surface of the vertebrae.





Muscles of the Back
Most body weight lies anterior to the vertebral column,
especially in obese people; consequently, the many strong muscles
attached to the spinous and transverse processes are necessary to
support and move the vertebral column. There are two major groups of
muscles in the back. The extrinsic back muscles include superficial and intermediate muscles that produce and control limb and respiratory movements, respectively. The intrinsic (deep) back muscles include muscles that specifically act on the vertebral column, producing its movements and maintaining posture.
Extrinsic Back Muscles
The superficial extrinsic back muscles
(trapezius, latissimus dorsi, levator scapulae, and rhomboids) connect
the upper limbs to the trunk and produce and control limb movements (Fig. 4.30A; see also Chapter 6).
Although located in the back region, for the most part these muscles
receive their nerve supply from the anterior rami of cervical nerves
and act on the upper limb. The trapezius receives its motor fibers from
a cranial nerve, the spinal accessory nerve (CN XI).
The intermediate extrinsic back muscles
(serratus posterior) are thin muscles, commonly designated superficial
respiratory muscles, but are more likely proprioceptive rather than
motor in function (Vilensky et al., 2001). They are described with muscles of the thoracic wall (see Chapter 1). The serratus posterior superior lies deep to the rhomboids, and the serratus posterior inferior
lies deep to the latissimus dorsi. Both serratus muscles are innervated
by intercostal nerves, the superior by the first four intercostals and
the inferior by the last four.
Intrinsic Back Muscles
The intrinsic back muscles (muscles of back proper,
deep back muscles) are innervated by the posterior rami of spinal
nerves and act to maintain posture and control movements of the
vertebral column (Figs. 4.30B and 4.31).
These muscles, which extend from the pelvis to the cranium, are
enclosed by deep fascia that attaches medially to the nuchal ligament,
the tips of the spinous processes of the vertebrae, the supraspinous
ligament, and the median crest of the sacrum. The fascia attaches
laterally to the cervical and lumbar transverse processes and to the
angles of the ribs. The thoracic and lumbar parts of the deep fascia
constitute the thoracolumbar fascia. It
extends laterally from the spinous processes and forms a thin covering
for the deep muscles in the thoracic region and a strong thick covering
for muscles in the lumbar region. The deep back muscles are grouped
into superficial, intermediate, and deep layers according to their
relationship to the surface (Tables 4.5, 4.6 and 4.7).
Superficial Layer of Intrinsic Back Muscles
The splenius muscles (L. musculi splenii)
are thick and flat and lie on the lateral and posterior aspects of the
neck, covering the vertical muscles somewhat like a bandage, which
explains their name (L. splenion, bandage) (Fig. 4.31). The splenii arise from the midline and extend superolaterally to the cervical vertebrae (splenius cervicis) and cranium (splenius capitis).
The splenii cover and hold the deep neck muscles in position.
Information on the attachments, nerve supply, and actions of the
superficial layer of intrinsic muscles is provided in Table 4.5.
Intermediate Layer of Intrinsic Back Muscles
The erector spinae muscles
lie in a “groove” on each side of the vertebral column between the
spinous processes centrally and the angles of the ribs laterally (Fig. 4.31). The massive erector spinae is the chief extensor of the vertebral column and is divided into three columns: The iliocostalis forms the lateral column, the longissimus forms the intermediate column, and the spinalis
forms the medial column. Each column is divided regionally into three
parts according to the superior attachments (e.g., iliocostalis
lumborum, iliocostalis thoracis, and iliocostalis cervicis). The common
origin of the three erector spinae columns is through a broad tendon
that attaches inferiorly to the posterior part of the iliac crest, the
posterior aspect of the sacrum, the sacroiliac ligaments, and




sacral and inferior lumbar spinous processes. The erector spinae are
often referred to as the “long muscles” of the back. In general, they
are dynamic (motion-producing) muscles, acting bilaterally to extend
(straighten) the flexed trunk. Information on the attachments, nerve
supply, and actions of the intermediate layer of intrinsic muscles is
provided in Table 4.6.

Figure 4.30. Muscles of back. A.
The superficial extrinsic muscles are shown. The trapezius is reflected
on the left to show the spinal accessory nerve (CN XI), coursing on its
deep surface, and the levator scapulae and rhomboid muscles. These
muscles help attach the upper limb to the trunk. B.
This transverse section of part of the back shows the location of the
intrinsic back muscles and the layers of fascia associated with them.
The combined posterior aponeuroses of the transverse abdominal and
internal oblique muscles splits into two strong sheets, the middle and
posterior layers of the thoracolumbar fascia, which enclose the
intrinsic back muscles.
Figure 4.31. Superficial and intermediate layers of intrinsic back muscles: splenius and erector spinae.
The sternocleidomastoid and levator scapulae muscles are reflected to
reveal the splenius capitis and splenius cervicis muscles. On the right
side, the erector spinae is undisturbed (in situ) and shows the three
columns of this massive muscle. On the left side, the spinalis muscle,
the thinnest and most medial of the erector spinae columns, is
displayed as a separate muscle by reflecting the longissimus and
iliocostalis columns of the erector spinae.
Table 4.5. Superficial Layer of Intrinsic Back Muscles
Muscle Origin Insertion Nerve supply Main action(s)
Splenius Arises from nuchal ligament and spinous processes of C7–T3 or T4 vertebrae Splenius capitis: fibers run superolaterally to mastoid process of temporal bone and lateral third of superior nuchal line of occipital bone
Splenius cervicis: tubercles of transverse processes of C1–C3 or C4 vertebrae
Posterior rami of spinal nerves Acting alone: laterally flex neck and rotate head to side of active muscles Acting together: extend head and neck
Deep Layer of Intrinsic Back Muscles
Deep to the erector spinae is an obliquely disposed group of much shorter muscles called the transversospinal muscle group (L. transversospinalis),
consisting of the semispinalis, multifidus, and rotatores. These
muscles originate from transverse processes of vertebrae and pass to
spinous processes of more superior vertebrae. They occupy the “gutter”
between the transverse and the spinous processes and are attached to
these processes, the laminae between them, and the ligaments linking
them together (Fig. 4.32).
The semispinalis is the
superficial member of the group. As its name indicates, it arises from
approximately half of the vertebral column (spine) (Fig. 4.31). It is divided into three parts according to the superior attachments: semispinalis capitis, semispinalis thoracis, and semispinalis cervicis. Semispinalis capitis is responsible for the longitudinal bulge in the back of the neck near the median plane. The multifidus is the middle layer of the group and consists of short, triangular muscular bundles that are thickest in the lumbar region (Fig. 4.32A). The rotatores,
or rotator muscles, are the deepest of the three layers of
transversospinal muscles and are best developed in the thoracic region.
Details concerning the attachments, innervation, and action of the
transversospinalis group of the deep layer of intrinsic back muscles
are provided in Table 4.7.
The interspinal (L. interspinales), intertransverse (L. intertransversarii), and elevators of ribs (L. levatores costarum)
are minor deep back muscles that are poorly developed in the thoracic
region. The interspinal and intertransverse muscles connect spinous and
transverse processes, respectively. The elevators of the ribs represent
the posterior intertransverse muscles of the neck. Details concerning
the attachments, nerve supply, and actions of the minor muscles of the
deep layer of intrinsic muscles are provided in Table 4.7.
Principal Muscles Producing Movements of the Intervertebral Joints
The principal muscles producing movements of the cervical, thoracic, and lumbar IV joints are summarized in Tables 4.8 and 4.9. Many of the muscles acting on the cervical vertebrae are specifically discussed in Chapter 8.
The back muscles are relatively inactive in the stand-easy position,
but they (especially the shorter deep layer of intrinsic muscles) act
as static postural muscles (fixators, or steadiers) of the vertebral
column, maintaining tension and stability as required for the erect
posture. Note in Table 4.9 that all movements
of the IV joints (i.e., all movements of the vertebral column) except
pure extension involve or are solely produced by the concentric contraction of abdominal muscles. However, bear in mind that in these as in all movements,


the eccentric contraction (controlled relaxation) of the antagonist muscles is vital to smooth, controlled movement (see “Muscle Tissue and the Muscular System” in the Introduction).
Thus it is actually the interaction of anterior (abdominal) and
posterior (back) muscles (as well as the contralateral pairs of each)
that provides the stability and produces motion of the axial skeleton,
much like guy (guide) wires support a pole. Often chronic back strain
(such as that caused by excessive lumbar lordosis) results from
imbalance in this support (lack of tonus of abdominal muscles in the
case of lordosis). To restore that balance, the patient may need to
exercise or eliminate excessive, unevenly distributed weight.

Table 4.6. Intermediate Layer of Intrinsic Back Muscles
Muscle Origin Insertion Nerve supply Main action(s)
Erector spinae Iliocostalis Longissimus Spinalis Arises by a broad tendon from
posterior part of iliac crest, posterior surface of sacrum, sacroiliac
ligaments, sacral and inferior lumbar spinous processes, and
supraspinous ligament
Iliocostalis: lumborum, thoracis, cervicis; fibers run superiorly to angles of lower ribs and cervical transverse processes
thoracis, cervicis, capitis; fibers run superiorly to ribs between
tubercles and angles to transverse processes in thoracic and cervical
regions, and to mastoid process of temporal bone
Spinalis: thoracis, cervicis, capitis; fibers run superiorly to spinous processes in the upper thoracic region and to cranium
Posterior rami of spinal nerves Acting bilaterally: extend vertebral column and head; as back is flexed, control movement by gradually lengthening their fibers
Acting unilaterally: laterally flex vertebral column
Smaller muscles generally have higher densities of muscle spindles (sensors of proprioception that are interdigitated among the muscle’s fibers) than do large muscles. It was assumed






that the higher concentration of spindles occurred because small
muscles produce the most precise movements, such as fine postural
movements or manipulation and, therefore, require more proprioceptive
feedback. The movements described for small muscles are deduced from
the location of their attachments and the direction of the muscle
fibers and from activity measured by electromyography as movements are
performed. Muscles such as the rotatores, however, are so small and are
placed in positions of such relatively poor mechanical advantage that
their ability to produce the movements described is somewhat
questionable. Furthermore, such small muscles are often redundant to
other larger muscles that have superior mechanical advantage. Hence it
has been proposed (Buxton and Peck, 1989)
that the smaller muscles of small–large muscle pairs function more as
“kinesiological monitors,” or organs of proprioception and that the
larger muscles are the producers of motion.

Table 4.7. Deep Layers of Intrinsic Back Muscles
Muscle Origin Insertion Nerve supply Main action(s)
Deep layer
Transversospinal Semispinalis Multifidus Rotatores (brevis and longus) Transverse processes
Semispinalis: arises from transverse processes of C4–T12 vertebrae
arises from posterior sacrum, posterior superior iliac spine of ilium,
aponeurosis of erector spinae, sacroiliac ligaments, mammillary
processes of lumbar vertebrae, transverse processes of T1–T3, articular
processes of C4–C7
Rotatores: arise from transverse processes of vertebrae; best developed in thoracic region
Spinous processes of more superior vertebrae
thoracis, cervicis, capitis; fibers run superomedially to occipital
bone and spinous processes in thoracic and cervical regions, spanning
4–6 segments
Multifidus: thickest in lumbar region; fibers
pass obliquely superomedially to entire length of spinous processes of
vertebrae, located 2–4 segments superior to origin
fibers pass superomedially to attach to junction of lamina and
transverse process or spinous process of vertebra immediately (brevis)
or 2 segments (longus) superior to vertebra of origin
Posterior rami of spinal nerves Extension
Semispinalis: extends head and thoracic and cervical regions of vertebral column and rotates them contralaterally
Multifidus: stabilizes vertebrae during local movements of vertebral column
stabilize vertebrae and assist with local extension and rotatory
movements of vertebral column; may function as organs of proprioception
Minor deep layer
Interspinales Superior surfaces of spinous processes of cervical and lumbar vertebrae Inferior surfaces of spinous processes of vertebra superior to vertebra of origin Posterior rami of spinal nerves Aid in extension and rotation of vertebral column
Intertransversarii Transverse processes of cervical and lumbar vertebrae Transverse processes of adjacent vertebrae Posterior and anterior rami of spinal nervesa Aid in lateral flexion of vertebral column; acting bilaterally, stabilize vertebral column
Levatores costarum Tips of transverse processes of C7 and T1–T11 vertebrae Pass inferolaterally and insert on rib between tubercle and angle Posterior rami of C8–T11 spinal nerves Elevate ribs, assisting respiration; assist with lateral flexion of vertebral column
back muscles are innervated by posterior rami of spinal nerves, but a
few are innervated by anterior rami. Anterior intertransverse muscles
of the cervical region are supplied by anterior rami.
Figure 4.32. Muscles of deep layer of intrinsic back muscles. A.
The transversospinal muscle group is deep to the erector spinae. It
consists chiefly of a large number of small muscles that run obliquely
upward and medially from transverse to spinous processes. The
multifidus and quadratus lumborum muscles are shown as is the
thoracolumbar fascia. The short lumbar rib is articulating with the
transverse process of L1 vertebra. This common variation does not
usually cause a problem; however, those unfamiliar with its possible
presence may think it is a fractured transverse process. B.
Deeper dissection showing the rotatores and costotransverse ligaments.
Two of the three sets of costotransverse ligaments are seen: superior
and lateral. The (medial) costotransverse ligaments extend between the
neck of the rib and the transverse process of the vertebra of the same
number (not shown). The levator muscles, which elevate the ribs,
represent the posterior intertransverse muscles in the thoracic region.
Table 4.8. Principal Muscles Producing Movement of the Cervical Intervertebral Joints
Flexion Extension Lateral bending Rotation (not shown)
Bilateral action of
  Longus coli
Deep neck muscles
1, semispinalis cervicis and iliocostalis cervicis
2, splenius cervicis and levator scapulae
3, splenius capitis
4, multifidus
5, longissimus capitis
6, semispinalis capitis
T, trapezius
Unilateral action of Iliocostalis cervics
  Longissimus capitis and cervics
  Splenius capitis and cervicis
  Intertransverse and scalenes
Unilateral action of
  Semispinalis capitis and cervicis
  Splenius cervicis
Table 4.9. Principal Muscles Producing Movements of the Thoracic and Lumbar Intervertebral (IV) Joints
Flexion Extension Lateral bending Rotation (not shown)
Bilateral action of
  Rectus abdominis
  Psoas major
Bilateral action of
  Erector spinae
  Semispinalis thoracis
Unilateral action of
  Iliocostalis thoracis and lumborum
  Longissimus thoracis
  External and internal oblique
  Quadratus lumborum
  Serratus anterior
Unilateral action of
  External oblique acting synchronously with opposite internal oblique
  Splenius thoracis
(See bracket to rotatores and multifidus: Transversospinalis)


Suboccipital and Deep Neck Muscles
The suboccipital region, the
superior part of the posterior cervical region, is the triangular area
inferior to the occipital region of the head, including the posterior
aspects of vertebrae C1 and C2. The four small muscles of the
suboccipital region lie deep to the trapezius and semispinalis capitis
muscles and consist of two rectus capitis posterior (major and minor)
and two obliquus muscles. All four muscles are innervated by the posterior ramus of C1, the suboccipital nerve. The nerve emerges as the vertebra artery courses deeply between the occipital bone and the atlas (vertebra C1) within the suboccipital triangle.
Details concerning the boundaries and contents of this triangle and the
attachments of the suboccipital muscles are provided and illustrated in
Table 4.10.
Note that the inferior oblique of the head (L. obliquus capitis inferior)
is the only “capitis” muscle that has no attachment to the cranium.
These muscles are mainly postural muscles, but actions are typically
described for each muscle in terms of producing movement of the head.
The muscles are considered to act on the head directly or indirectly
(explaining the inclusion of capitis in
their names) by extending it on vertebra C1 and rotating it on
vertebrae C1 and C2. However, recall the discussion of the small member
of the small–large muscle pair functioning as a kinesiological monitor
for the sense of proprioception. The principal muscles producing
movements of the craniovertebral joints are summarized in Tables 4.11 and 4.12, and the nerves of the suboccipital region and posterior neck are summarized in Table 4.13.


Table 4.10. Suboccipital Muscles and Suboccipital Triangle
Suboccipital muscles
Muscle Origin Insertion
Rectus capitis posterior major Spinous process of vertebra C2 Lateral part of inferior nuchal line of occipital bone
Rectus capitis posterior minor Posterior tubercle of posterior arch of vertebra C1 Medial part of inferior nuchal line of occipital bone
Inferior oblique of head (L. m. obliquus capitis inferior) Spinous process of vertebra C2 Transverse process of vertebra C1
Superior oblique of head (L. m. obliquus capitis superior) Transverse process of vertebra C1 Occipital bone between superior and inferior nuchal lines
Suboccipital triangle
Aspect of Triangle Structures
Superomedial boundary Rectus capitis posterior major
Superolateral boundary Superior oblique of head
Inferolateral boundary Inferior oblique of head
Floor Posterior atlanto-occipital membrane and posterior arch of vertebra C1
Roof Semispinalis capitis
Contents Vertebral artery and suboccipital nerve


Table 4.11. Principal Muscles Producing Movement of the Atlanto-Occipital Joints
Flexion Extension Lateral bending (not shown)
Longus capitis Rectus capitis posterior major and minor Sternocleidomastoid
Rectus capitis anterior Superior oblique of head Superior oblique of head
Anterior fibers of sternocleidomastoid Splenius capitis Rectus capitis lateralis
Suprahyoid and infrahyoid muscles Longissimus capitis Longissimus capitis
  Trapezius Splenius capitis
Table 4.12. Principal Muscles Producing Movement of the Atlanto-Axial Jointsa
Ipsilateralb Contralateral
Inferior oblique of head Sternocleidomastoid
Rectus capitis posterior, major and minor Semispinalis capitis
Longissimus capitis
Splenius capitis
a Rotation is the specialized movement at these joints. Movement of one joint involves the other.
b Same side to which head is rotated.


Table 4.13. Nerves of the Suboccipital Region/Posterior Neck
Nerve Origin Course Distribution
Suboccipital Posterior ramus of spinal nerve C1 Runs between cranium and C1 vertebra to reach suboccipital triangle Muscles of suboccipital triangle
Greater occipital Posterior ramus of spinal nerve C2 Emerges inferior to inferior oblique and ascends to posterior scalp Skin over neck and occipital bone
Lesser occipital Anterior rami of spinal nerves C2–C3 Passes directly to skin Skin of superior posterolateral neck and scalp posterior to ear
Posterior rami, nerves C3–C7 Posterior rami of spinal nerves C3–C7 Pass segmentally to muscles and skin Intrinsic muscles of back and overlying skin (adjacent to vertebral column)






1In contemporary usage, the terms vertebral body and centrum and the terms vertebral arch and neural arch
are often erroneously used as synonyms. Technically, however, in each
case the former is a gross anatomy term applied to parts of the adult
vertebrae, and the latter is an embryology term referring to parts of a
developing vertebra ossifying from primary centers. The vertebral body
includes the centrum and part of the neural arch; the vertebral arch is
thus less extensive than the neural arch, and the centrum is less
extensive than the vertebral body (O’Rahilly, 1986; Williams et al., 1995).
References and Suggested Readings
Bergman RA, Thompson SA, Afifi AK, Saadeh FA: Compendium of Human Anatomic Variation. Text, Atlas, and World Literature. Baltimore, Urban & Schwarzenberg, 1988.
This useful source has been updated and is available from the Virtual Hospital’s Web site Illustrated Encyclopedia of Human Anatomic Variation at (accessed May 2004).
Bogduk N: Clinical Anatomy of the Lumbar Spine and Sacrum, 3rd ed. London: Churchill Livingstone, 1997.
Bogduk N, Macintosh JE: Applied anatomy of the thoracolumbar fascia. Spine 9:164, 1984.
Buxton DF, Peck D: Neuromuscular spindles relative to joint movement complexities. Clin Anat 2:211, 1989.
Clark CR: The Cervical Spine, 3rd ed., Philadelphia: Lippincott-Raven, 1997.
HA, Heilman AE, Stevens JM: Progressive myelopathy secondary to
odontoid fractures: Clinical, radiological, and surgical features. J Neurosurg 78: 579, 1993.
Dvorak J, Schneider E, Saldinger P, Rahn B: Biomechanics of the craniovertebral region: The alar and transverse ligaments. J Orthop Res 6: 452, 1988.


Greer M: Structural malformations. In Rowland LP (ed): Merritt’s Textbook of Neurology, 10th ed. Baltimore, Lippincott Williams & Wilkins, 2000.
Haines DE (ed): Fundamental Neuroscience, 2nd ed. New York, Churchill Livingstone, 2002.
McCormick PC: Intervertebral discs and radiculopathy. In Rowland LP (ed): Merritt’s Textbook of Neurology, 10th ed. Baltimore, Lippincott Williams & Wilkins, 2000.
McCormick PC, Fetell MR, Roland LP: Spinal tumors. In Rowland LP (ed): Merritt’s Textbook of Neurology, 10th ed. Baltimore, Lippincott Williams & Wilkins, 2000.
Moore KL, Persaud TVN: The Developing Human: Clinically Oriented Embryology, 7th ed. Philadelphia, Saunders, 2003.
Murray MJ, Bower TC, Carmichael SW: Anatomy of the anterior spinal artery in pigs. Clin Anat 5:452, 1992.
O’Rahilly R: Gardner-Gray-O’Rahilly Anatomy. A Regional Study of Human Structures, 5th ed. Philadelphia, Saunders, 1986.
Rickenbacher J, Landolt AM, Theiler K: Applied Anatomy of the Back. New York: Springer Verlag, 1985.
Rowland LP, McCormick PC: Lumbar spondylosis. In Rowland LP (ed): Merritt’s Textbook of Neurology, 10th ed. Baltimore, Lippincott Williams & Wilkins, 2000.
Swartz MH: Textbook of Physical Diagnosis. History and Examination, 4th ed. Philadelphia, Saunders, 2002.
Vilensky JA, Baltes M, Weikel L, Fortin JD, Fourie LJ: Serratus posterior muscles: Anatomy, clinical relevance, and function. Clin Anat 14:237, 2001.
Williams PL, Bannister LH, Berry MM, Collins P, Dyson M, Dussek JE, Ferguson MWJ (eds.): Gray’s Anatomy, 38th British ed. New York, Churchill Livingstone, 1995.
Yochum TR, Rowe LJ: Essentials of Skeletal Radiology, 3rd ed. Baltimore, Lippincott Williams & Wilkins, 2004.

This website uses cookies to improve your experience. We'll assume you're ok with this, but you can opt-out if you wish. Accept Read More