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1. INTRODUCTION
2. SURFACE ANATOMY
3. VERTEBRAL COLUMN
4. MUSCLES OF THE BACK
5. SPINAL CORD
6. EMBRYOLOGY
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Back
1. INTRODUCTION
The back forms the axis (central line) of the human
body and consists of the vertebral column, spinal
cord, supporting muscles, and associated tissues
(skin, connective tissues, vasculature, and nerves).
A hallmark of human anatomy is the concept of
“segmentation,” and the back is a prime example.
Segmentation and bilateral symmetry of the back
will become obvious as you study the vertebral
column, the distribution of the spinal nerves, the
muscles of the back, and its vascular supply.
Functionally, the back is involved in three primary
tasks, as follows:
- Support.^ The vertebral column forms the axis
of the body and is critical for upright posture
(standing or sitting), as a support for the head,
as an attachment point and brace for movements
of the upper limbs, and as a support for transfer-
ring the weight of the trunk to the lower limbs.
- Protection.^ The vertebral column protects the
spinal cord and proximal portions of the spinal
nerves before they distribute throughout the
body.
- Movements.^ Muscles of the back function in
movements of the head and upper limbs and in
support and movements of the vertebral column.
2. SURFACE ANATOMY
Fig. 2.1 shows key surface landmarks of the back,
including the following bony landmarks:
- Vertebrae prominens:^ the spinous process of
the C7 vertebra, usually the most prominent
process in the midline at the posterior base of
the neck.
- Scapula:^ a part of the pectoral girdle that sup-
ports the upper limb; note its spine, inferior
angle, and medial border.
- Iliac crests:^ felt best when you place your hands
“on your hips.” An imaginary horizontal line
connecting the iliac crests passes through the
spinous process of vertebra L4 and the inter-
vertebral disc of L4-L5, providing a useful
landmark for a lumbar puncture or an epidural
block (see Clinical Focus 2-11).
- Posterior superior iliac spines:^ an imaginary
horizontal line connecting these two points
passes through the spinous process of S2 (second
sacral segment).
3. VERTEBRAL COLUMN
The vertebral column (spine) forms the central axis
of the human body, highlighting the segmental
nature of all vertebrates, and usually is composed
of 33 vertebrae distributed as follows (Fig. 2.2):
- Cervical:^ seven vertebrae; the first two called
the atlas (C1) and axis (C2).
- Thoracic:^ 12 vertebrae; each articulates with a
pair of ribs.
- Lumbar:^ five vertebrae; large vertebrae for
support of the body’s weight.
- Sacral:^ five fused vertebrae for stability in the
transfer of weight from the trunk to the lower
limbs.
- Coccyx:^ four vertebrae, but variable; Co1 often
is not fused, but Co2-Co4 are fused (a remnant
of the embryonic tail).
The actual number of vertebrae can vary, espe-
cially the number of coccygeal vertebrae.
Viewed from the lateral aspect (Fig. 2.2), one
can identify the following:
- Cervical curvature^ (cervical lordosis): a second-
ary curvature acquired when the infant can
support the weight of the head.
- Thoracic^ curvature^ (thoracic^ kyphosis):^ a
primary curvature present in the fetus (imagine
the spine in the “fetal position”).
- Lumbar curvature^ (lumbar lordosis): a second-
ary curvature acquired when the infant assumes
an upright posture and supports its own weight.
Trapezius m.
Spine of scapula
Infraspinatus m.
Teres major m.
Latissimus dorsi m.
Thoracolumbar fascia
Iliac crest
Posterior superior iliac spine Sacrum
Spinous process of T12 vertebra
Inferior angle of scapula
Medial border of scapula
Deltoid m.
Spinous process of C7 vertebra
Nuchal lig.
External occipital protuberance
FIGURE 2.1 Key Bony and Muscular Landmarks of the Back. (From Atlas of human anatomy, ed 7
Nuchal lig. Plate 161).
Left lateral view Posterior view
C
C2 Cervical vertebrae
Cervical lordosis
Thoracic kyphosis
Lumbar lordosis
Sacral kyphosis
Thoracic vertebrae
T
C
T
Atlas (C1) Axis (C2)
T
L
Lumbar vertebrae
L
L
Sacrum (S1-S5)
Coccyx
Level Corresponding structure
C2-3 Mandible
C3 Hyoid bone
C4-5 Thyroid cartilage
C6 Cricoid cartilage
C7 Vertebra prominens
T3 Spine of scapula
T8 Level that IVC pierces respiratory diaphragm
T10 Xiphisternal junction
T10 Level that esophagus pierces respiratory diaphragm
T12 Level that aorta pierces respiratory diaphragm
L1 End of spinal cord (conus medullaris)
L3 Subcostal plane
L3-4 Umbilicus
L4 Bifurcation of abdominal aorta
L4 Iliac crests
S2 End of dural sac
FIGURE 2.2 Vertebral Column. (From Atlas of human anatomy, ed 7, Plate 162.)
Regional Vertebrae
Cervical Vertebrae
The cervical spine is composed of seven cervical
vertebrae. The first two cervical vertebrae are unique
and called the atlas and axis (Fig. 2.4). The atlas
(C1) holds the head on the neck (the titan Atlas of
Greek mythology held the heavens on his shoulders
as punishment by Zeus). The axis (C2) is the point
of articulation where the head turns on the neck,
providing an “axis of rotation.”
Table 2.1 summarizes key features of the cervical
vertebrae. The cervical region is a fairly mobile
portion of the spine, allowing for flexion and exten-
sion as well as rotation and lateral bending.
Thoracic and Lumbar Vertebrae
The thoracic spine is composed of 12 thoracic
vertebrae (Fig. 2.5 and Table 2.2). The 12 pairs of
ribs articulate with the thoracic vertebrae. This
region of the spine is more rigid and inflexible than
the cervical region.
The lumbar spine is composed of five lumbar
vertebrae (see Figs. 2.3 and 2.5 and Table 2.2). The
lumbar vertebrae are comparatively large for bearing
the weight of the trunk and are fairly mobile, but
not nearly as mobile as the cervical vertebrae.
Sacrum and Coccyx
The sacrum is composed of five fused vertebrae
that form a single, wedge-shaped bone (Fig. 2.
and Table 2.2). The sacrum provides support for
the pelvis. The coccyx is a remnant of the embryonic
- Vertebral foramen^ (canal): a foramen formed
from the vertebral arch and body that contains
the spinal cord and its meningeal coverings.
- Vertebral^ notches:^ superior^ and^ inferior
semicircular features that in articulated vertebrae
form an intervertebral foramen (two semicircular
notches form a circle).
Vertebral body
Vertebral foramen
Pedicle
Transverse process
Superior articular process (^) Lamina
Spinous process
Interver- tebral disc
Pedicle
Superior articular process
Transverse process
Accessory process
Inferior vertebral notch Interver- tebral (neural) foramen
Articular facet for sacrum
Superior vertebral notch
Inferior articular process
FIGURE 2.3 Features of Typical Vertebra, as Represented
by L2 Vertebra (superior view) and Articulated Lumbar
Vertebrae (L1-L5). (From Atlas of human anatomy, ed 7,
Plate 164.)
TABLE 2.1 Key Features of the Cervical Vertebrae (C1-C7)
VERTEBRAE
DISTINGUISHING
CHARACTERISTICS
Atlas (C1) Ringlike bone; superior facet
articulates with occipital bone.
Two lateral masses with facets
No body or spinous process
C1 rotates on articular facets of C2.
Vertebral artery runs in groove on
posterior arch.
Axis (C2) Dens projects superiorly.
Strongest cervical vertebra
C3 to C7 Large, triangular vertebral foramen
Transverse foramen through which
vertebral artery passes (except C7)
Narrow intervertebral foramina
Nerve roots at risk of compression
C3 to C5 Short, bifid spinous process
C6 to C7 Long spinous process
C7 Vertebra prominens; nonbifid
Anterior tubercle
Anterior arch
Transverse process
Transverse foramen
Superior articular surface of lateral mass for occipital condyle
Atlas (C1): superior view
Axis (C2): posterosuperior view
Posterior tubercle
Posterior arch
Vertebral foramen
Lateral mass
Articular facet for dens
Body
Lamina
Transverse process
Transverse foramen
Superior articular facet
Vertebral foramen
Pedicle
Spinous process Lamina
4th cervical vertebra: superior view
7th cervical vertebra: superior view
Body
Dens
Superior articular facet for atlas
Inferior articular process
Transverse process
Posterior articular facet (for transverse ligament of atlas)
Spinous process
FIGURE 2.4 Representative Cervical Vertebrae. (From Atlas of human anatomy, ed 7, Plate 26.)
Clinical Focus 2-
Cervical Fractures
Fractures of the axis (C2) often involve the dens and are classified as types I, II, and III. Type I fractures are usually stable, type II fractures are unstable, and type III fractures, which extend into the body, usually reunite well when immobilized. The hangman fracture, a pedicle fracture of the axis, can be stabilized, if survived, with or without spinal cord damage. A Jefferson fracture is a burst fracture of the atlas (C1), often caused by a blow to the top of the head.
Fracture of dens Type I.^ Fracture of tip Type II. Fracture of base or neck
Fracture of anterior arch
Superior articular facet
Fracture of posterior arch
Inferior articular facet
Superior articular facet
Jefferson fracture of atlas (C1) Each arch may be broken in one or more places
Superior articular facet
Inferior articular facet
Hangman fracture Fracture through neural arch of axis
Type III. Fracture extends into body of axis
TABLE 2.2 Key Features of Thoracic, Lumbar, Sacral, and Coccygeal Vertebrae
VERTEBRAE
DISTINGUISHING
CHARACTERISTICS
Thoracic
(T1-T12)
Heart-shaped body, with facets for rib
articulation
Small circular vertebral foramen
Long transverse processes, with facets for
rib articulation in T1-T
Long spinous processes, which slope
posteriorly and overlap next vertebra
Lumbar
(L1-L5)
Kidney-shaped body, massive for support
Midsized triangular vertebral foramen
Facets face medial or lateral direction,
which permits good flexion and
extension
Spinous process is short, strong, and
horizontal.
L5: largest vertebra with massive
transverse processes
Clinical Focus 2-
Osteoarthritis
Osteoarthritis is the most common form of arthritis and often involves erosion of the articular cartilage of
weight-bearing joints, such as those of the vertebral column.
Cervical spine involvement Lumbar spine involvement
Atlas (C1)
Axis (C2)
C
Extensive thinning of cervical discs and hyperextension deformity. Narrowing of intervertebral foramina. Lateral radiograph reveals similar changes.
Degeneration of lumbar intervertebral discs and hypertrophic changes at vertebral margins with spur formation. Osteophytic encroachment on intervertebral foramina compresses spinal nerves.
Osteophytic encroachment compressing spinal nn.
Bone spurs
Characteristics of Osteoarthritis
Characteristic Description
Etiology Progressive erosion of cartilage in joints of spine, fingers, knee, and hip most commonly
Prevalence Significant after age 65 years
Risk factors Age, female sex, joint trauma, repetitive stress, obesity, genetic, race, previous inflammatory joint disease
Complications In spine, involves intervertebral disc and facet joints, leading to hyperextension deformity and spinal nerve impingement
VERTEBRAE
DISTINGUISHING
CHARACTERISTICS
Sacrum (S1-S5) Large, wedge-shaped bone that transmits
body weight to pelvis
Five fused vertebrae, with fusion
complete by puberty
Four pairs of sacral foramina on dorsal
and ventral (pelvic) side
Sacral hiatus, the opening of sacral
vertebral foramen
Coccyx
(Co1-Co4)
Co1 often is not fused.
Co2 to Co4 are fused.
No pedicles, laminae, or spines
Remnant of our embryonic tail
called the nucleus pulposus, which is surrounded
by concentric lamellae of collagen fibers that
compose the anulus fibrosus (see Clinical Focus
2-6). The inner gelatinous nucleus pulposus
(remnant of the embryonic notochord) is hydrated
and acts as a “shock absorber,” compressing when
load bearing and relaxing when the load is removed.
The outer fibrocartilaginous anulus fibrosus,
arranged in concentric lamellae, is encircled by a
thin ring of collagen and resists compression and
shearing forces.
The lumbar intervertebral discs are the thickest
and the upper thoracic ones are the thinnest
intervertebral discs. The anterior and posterior
longitudinal ligaments help to stabilize these joints
(see Table 2.4).
articular processes (facets) of adjacent vertebrae
and allow for some gliding or sliding movement
(Fig. 2.7 and Table 2.4). These joints slope inferiorly
in the cervical spine (facilitate flexion and exten-
sion), are more vertically oriented in the thoracic
region (limit flexion and extension but allow for
rotation), and are interlocking in the lumbar spine
(they do allow flexion and extension, but not to
the degree present in the cervical spine). Corre-
sponding ligaments connect the spinous processes,
laminae, and bodies of adjacent vertebrae (see Tables
2.3 and 2.4). Strong anterior and posterior longi-
tudinal ligaments run along most of the length of
the vertebral column. Of these two ligaments, the
anterior longitudinal ligament is stronger and
prevents hyperextension (see Figs. 2.6 and 2.7 and
Table 2.4).
The joints of the vertebral bodies (intervertebral
joints) occur between the adjacent vertebral bodies
(see Fig. 2.7 and Table 2.4). The intervertebral joints
are lined by a thin layer of hyaline cartilage with
an intervening intervertebral disc (except between
the first two cervical vertebrae). These stable,
weight-bearing joints also absorb pressure because
the intervertebral disc is between the bodies.
Intervertebral discs are composed of a central
nuclear zone of collagen and hydrated proteoglycans
TABLE 2.3 Key Features of Atlantooccipital and Atlantoaxial Joints
LIGAMENT ATTACHMENT COMMENT
Atlantooccipital (Biaxial Condyloid Synovial) Joint
Articular
capsule
Surrounds facets and
occipital condyles
Allows flexion
and extension
Anterior and
posterior
membranes
Anterior and
posterior arches
of C1 to foramen
magnum
Limit
movement of
joint
Atlantoaxial (Uniaxial Synovial) Joint
Tectorial
membrane
Axis body to
margin of foramen
magnum
Is continuation
of posterior
longitudinal
ligament
Apical Dens to occipital
bone
Is very small
Alar Dens to occipital
condyles
Limits rotation
Cruciate Dens to lateral
masses
Resembles a
cross; allows
rotation
TABLE 2.4 Features of the Zygapophysial and Intervertebral Joints
LIGAMENT ATTACHMENT COMMENT
Zygapophysial (Plane Synovial) Joints
Articular
capsule
Surrounds facets Allows gliding
motion
C5-C6 is most
mobile.
L4-L5 permits
most flexion.
Intervertebral (Secondary Cartilaginous
[Symphyses]) Joints
Anterior
longitudinal
(AL)
Anterior bodies
and intervertebral
discs
Is strong and
prevents
hyperextension
Posterior
longitudinal
(PL)
Posterior bodies
and intervertebral
discs
Is weaker
than AL and
prevents
hyperflexion
Ligamenta
flava
Connect adjacent
laminae of
vertebrae
Limit flexion
and are more
elastic
Interspinous Connect spines Are weak
Supraspinous Connect spinous
tips
Are stronger
and limit
flexion
Ligamentum
nuchae
C7 to occipital
bone
Is cervical
extension of
supraspinous
ligament and is
strong
Intertransverse Connect transverse
processes
Are weak
ligaments
Intervertebral
discs
Between adjacent
bodies
Are secured
by AL and PL
ligaments
Clinical Focus 2-
Osteoporosis
Osteoporosis (porous bone) is the most common bone disease and results from an imbalance in bone resorption
and formation, which places bones at a great risk for fracture.
T
T
T
T
T
L
Axial
Appendicular fractures
caused by minimal trauma
Proximal femur
Proximal humerus
Distal radius
Most common types
Multiple compression fractures of lower thoracic and upper lumbar vertebrae in patient with severe osteoporosis
Vertebral compression fractures cause continuous (acute) or intermittent (chronic) back pain from midthoracic to midlumbar region, occasionally to lower lumbar region.
Osteoporosis is the thinning of the bones. Bones become fragile and loss of height is common as the back bones begin to collapse.
A change in backbone strength over time
Characteristics of Osteoporosis
Characteristic Description
Etiology Postmenopausal women, genetics, vitamin D synthesis deficiency, idiopathic
Risk factors Family history, white female, increasing age, estrogen deficiency, vitamin D deficiency, low calcium intake, smoking, excessive alcohol use, inactive lifestyle
Complications Vertebral compression fractures, fracture of proximal femur or humerus, ribs, and distal radius (Colles’ fracture)
Clinical Focus 2-
Spondylolysis and Spondylolisthesis
Spondylolysis is a congenital defect or an acquired stress fracture of the lamina that presents with no slippage
of adjacent articulating vertebrae (most common at L5-S1). Its radiographic appearance suggests a “Scottie
dog” (terrier) with a collar (fracture site shown as red collar).
Spondylolisthesis is a bilateral defect (complete dislocation, or luxation) resulting in an anterior displacement
of the L5 body and transverse process. The posterior fragment (vertebral laminae and spinous process of L5)
remains in proper alignment over the sacrum (S1). This defect has the radiographic appearance of a dog with
a broken neck (highlighted in yellow, with the fracture in red). Pressure on spinal nerves often leads to low
back and lower limb pain.
Superior articular process (ear of Scottie dog)
Posterior oblique views: Scottie dog profile in yellow and fracture site in red
Pedicle (eye)
Transverse process (head)
Isthmus (neck)
Spinous process and lamina (body)
Inferior articular process (foreleg)
Opposite inferior articular process (hindleg) In spondylolisthesis, Scottie dog appears decapitated.
In simple spondylolysis, Scottie dog appears to be wearing a collar.
Clinical Focus 2-
Intervertebral Disc Herniation
The intervertebral discs are composed of a central nuclear zone of collagen and hydrated proteoglycans called
the nucleus pulposus, which is surrounded by concentric lamellae of collagen fibers that compose the
anulus fibrosus. The nucleus pulposus is hydrated and acts as a “shock absorber,” compressing when load
bearing and relaxing when the load is removed. Over time, the repeated compression-relaxation cycle of the
intervertebral discs can lead to peripheral tears of the anulus fibrosus that allow for the extrusion and herniation
of the more gelatinous nucleus pulposus. This often occurs with age, and the nucleus pulposus becomes
more dehydrated, thus transferring more of the compression forces to the anulus fibrosus. This added stress
may cause thickening of the anulus and tears. Most disc herniations occur in a posterolateral direction because
the anulus fibrosus tears often occur at the posterolateral margins of the disc (rim lesions). Moreover, the
posterior longitudinal ligament reinforces the anulus such that posterior herniations are much less common;
otherwise, the disc would herniate into the vertebral canal and compress the spinal cord or its nerve roots.
Continued
Clinical Focus 2-
Back Pain Associated With the Zygapophysial (Facet) Joints
Although changes in the vertebral facet joints are not the most common cause of back pain (~15%), such
alterations can lead to chronic pain. Although the articular surfaces of the synovial facet joints are not directly
innervated, sensory nerve fibers derived from the posterior rami of spinal nerves do supply the synovial linings
of the capsules surrounding the joints. Two examples of painful conditions associated with facet joints are
degeneration of the articular cartilage and osteophyte overgrowth of the articular processes.
Facet joint Joint capsule
Bilevel innervation of synovial membrane and capsule of facet joint
Facet joint and capsule innervated by posterior rami from two spinal levels
Joint space
Articular cartilage
Superior articular process
Inferior articular process
Synovial membrane
Joint capsule
Innervation of synovial membrane and capsule
Inferior articular process
Superior articular process
Facet joint
Degeneration of articular cartilage with synovial inflammation or capsular swelling may result in referred pain
Capsular swelling
Synovial inflammation Cartilage degeneration
Osteophytic overgrowth Osteophytes of articular processes of facet joint may impinge on nerve root
Clinical Focus 2-
Low Back Pain
Low back pain, the most common musculoskeletal disorder, can have various causes. Physical examination,
although not always revealing a definite cause, may provide clues to the level of spinal nerve involvement and
relative sensitivity to pain. The following causes are identified most often:
- Intervertebral disc rupture and herniation
- Nerve inflammation or compression
- Degenerative changes in vertebral facet joints
- Sacroiliac joint and ligament involvement
- Metabolic bone disease
- Psychosocial factors
- Abdominal aneurysm
- Metastatic cancer
- Myofascial disorders
A. Standing
Body build Posture Deformities Pelvic obliquity Spine alignment Palpate for: muscle spasm trigger zones myofascial nodes sciatic nerve tenderness Compress iliac crests for sacroiliac tenderness
B. Kneeling on
chair
D. Supine
Ankle jerk
C. Seated on table
Straight leg raising
Knee jerk
E. Prone
Spine extension
Sensation on calf and sole
Measure calf circumference
Test for renal tenderness
Palpate for local tenderness or spasm
Walking on heels (tests foot and great toe dorsiflexion)
Walking on toes (tests calf muscles)
Spinal column movements: flexion extension side bending rotation
Straight leg raising: flex thigh on pelvis and then extend knee with foot dorsiflexed (sciatic nerve stretch)
Palpate abdomen; listen for bruit (abdominal and inguinal)
Palpate for peripheral pulses and skin temperature Palpate for flattening of lumbar lordosis during leg raising
Measure leg lengths (anterior superior iliac spine to medial malleolus) and thigh circumferences Test sensation and motor power
F. Rectal and/or pelvic
examination
H. Laboratory studies
Serum Ca^2 ^ and PO 4 , alkaline phosphatase, prostate-specific antigen (males over 40), CBC, ESR, and urinalysis
G. MRI and/or CT and/or
myelogram of
- lumbosacral spine
- abdomen/pelvis
Clinical Focus 2-
Whiplash Injury
“Whiplash” is a nonmedical term for a cervical hyperextension injury, which is usually associated with a rear-end vehicular crash. The relaxed neck is thrown backward, or hyperextended, as the vehicle accelerates rapidly forward. Rapid recoil of the neck into extreme flexion occurs next. Properly adjusted headrests can greatly reduce the occurrence of this hyperextension injury, which often results in stretched or torn cervical muscles and, in severe cases, ligament, bone, and nerve damage.
Hyperextension
Vertebral fractures Tear of anterior longitudinal ligament
Headrest
reduces
hyperextension
Hyperflexion
Vertebral fracture and disc herniation
Tear of interspinous ligament
Blood Supply to the Spine
The spine receives blood from spinal arteries derived
from branches of larger arteries that serve each
midline region of the body (Fig. 2.9). These major
arteries include the following:
- Vertebral arteries:^ arising from the subclavian
arteries in the neck.
- Ascending cervical arteries:^ arising from a
branch of the subclavian arteries.
- Posterior intercostal arteries:^ arising from the
thoracic aorta.
- Lumbar arteries:^ arising from the abdominal
aorta.
- Lateral sacral arteries:^ arising from pelvic
internal iliac arteries.
Spinal arteries arise from these branches and
divide into small posterior branches that supply
the vertebral arch and small anterior branches that
supply the vertebral body (see Fig. 2.9). Also,
longitudinal branches of radicular arteries, which
arise from these spinal arteries, course along the
inside aspect of the vertebral canal and supply the
vertebral column. (Do not confuse these arteries
with those that supply the spinal cord, discussed
later. In some cases, arteries that do supply the
spinal cord also contribute branches that supply
the vertebrae.)
Radicular veins receive tributaries from the
spinal cord and the internal vertebral veins that
course within the vertebral canal; this internal
venous plexus also anastomoses with a network
of external vertebral veins (see Fig. 2.9). The
internal vertebral venous plexus lacks valves,
whereas the external vertebral venous plexus has
recently been shown to possess some valves, direct-
ing blood flow toward the internal venous plexus.
The radicular veins then drain blood from the
vertebral venous plexus to segmental and interver-
tebral veins, with the blood ultimately collecting
in the segmental branches of the following major
venous channels:
- Superior vena cava:^ drains cervical vertebral
region.
- Azygos venous system:^ drains thoracic region.
- Inferior^ vena^ cava:^ this^ large^ vein^ drains
lumbosacral regions of the spine.
4. MUSCLES OF THE BACK
Although the spine is the axis of the human body
and courses down the body’s midline, dividing it
Posterior spinal aa.
Anterior spinal a. Segmental medullary a.
Posterior radicular a.
Spinal branch
Dorsal branch of posterior intercostal a.
Posterior intercostal a.
Thoracic (descending) aorta
Arteries of the spine: Section through thoracic level: anterosuperior view
Anterior radicular a.
External vertebral venous plexus
Internal vertebral (epidural) venous plexus (Batson's veins)
Anterior spinal v.
Basivertebral v. Internal vertebral (epidural) venous plexus
Intervertebral v.
Anterior segmental medullary/ radicular v.
Posterior segmental medullary/ Internal vertebral radicular v. (epidural) venous plexus
Posterior spinal v.
External vertebral venous plexus
Internal vertebral (epidural) venous plexus
Intervertebral v.
FIGURE 2.9 Arteries and Veins of the Spine. (From Atlas of human anatomy, ed 7, Plates 177 and 178.)
into approximately equal right and left halves, it is
not midway between the anterior and posterior
halves of the body. In fact, most of the body’s weight
lies anterior to the more posteriorly aligned vertebral
column. Consequently, to support the body and
spine, most of the muscles associated with the spine
attach to its lateral and posterior processes, assisting
the spine in maintaining an upright posture that
offsets the uneven weight distribution.
The muscles of the back are divided into two
major groups, as follows:
Extrinsic back muscles: involved in movements
of the upper limb and with respiration.
Intrinsic back muscles: involved in movements
of the spine and maintenance of posture.
Extrinsic Back Muscles
The extrinsic muscles of the back are considered
“extrinsic” because embryologically they arise from
hypaxial myotomes (see Fig. 2.22). The extrinsic
back muscles are divided into the following two
functional groups (Fig. 2.10 and Table 2.5):
- Superficial muscles:^ involved in movements
of the upper limb (trapezius, latissimus dorsi,
levator scapulae, two rhomboids), attach the
pectoral girdle (clavicle, scapula, humerus) to
the axial skeleton (skull, ribs, spine).
- Intermediate muscles:^ thin accessory muscles
of respiration (serratus posterior superior and
inferior) that assist with movements of the rib
cage, lie deep to the superficial muscles, and
extend from the spine to the ribs.
Serratus posterior inferior m.
Latissimus dorsi m.
Serratus posterior superior m.
Rhomboid major m. ( cut )
Trapezius m.
Ligamentum nuchae Spinous process of C7 vertebra
Thoracolumbar fascia
Iliac crest
Spinous process of T12 vertebra
Splenius capitis m.
Splenius cervicis m.
Levator scapulae m.
Rhomboid minor m. ( cut )
12th rib
Erector spinae m.
Note: On the right side, the trapezius, latissimus dorsi, and rhomboid muscles were removed to show the intermediate muscles.
Spine of scapula
FIGURE 2.10 Extrinsic Muscles of the Back. (From Atlas of human anatomy, ed 7, Plate 180.)
Intrinsic Back Muscles
The intrinsic back muscles are the “true” muscles
of the back because they develop from epaxial
myotomes (see Fig. 2.22), function in movements
of the spine, and help maintain posture. The intrinsic
muscles are enclosed within a deep fascial layer
that extends in the midline from the medial crest
of the sacrum to the ligamentum nuchae (a broad
extension of the supraspinous ligament that extends
from the spinous process of the C7 vertebra to the
external occipital protuberance of the skull) (Fig.
2.10) and skull, and that spreads laterally to the
transverse processes and angles of the ribs. In the
thoracic and lumbar regions, the deep fascia makes
up a distinct sheath known as the thoracolumbar
fascia (Figs. 2.10 and 2.11; see also Fig 4.31).
In the lumbar region, this fascial sheath has the
following three layers (see also Fig. 4.31):
- Posterior layer:^ extending from the lumbar and
sacral spinous processes laterally over the surface
of the erector spinae muscles.
- Middle^ layer:^ extending^ from^ the^ lumbar
transverse processes to the iliac crest inferiorly
and to the 12th rib superiorly.
- Anterior layer:^ covering the quadratus lumbo-
rum muscle of the posterior abdominal wall and
extending to the lumbar transverse processes,
and iliac crest, and superiorly, forming the lateral
arcuate ligament for attachment of the respira-
tory diaphragm.
The intrinsic back muscles also are among the
few muscles of the body that are innervated by
posterior rami of a spinal nerve. From superficial
to deep, the intrinsic muscles include the following
three layers (Fig. 2.11 and Table 2.5):
- Superficial layer:^ including the splenius muscles
that occupy the lateral and posterior neck
(spinotransversales muscles).
- Intermediate layer:^ including the erector spinae
muscles that mainly extend and laterally bend
the spine.
- Deep layer:^ including the transversospinales
muscles that fill the spaces between the trans-
verse processes and spinous processes.
The intermediate, or erector spinae, layer of
muscles is the largest group of the intrinsic back
muscles and is important for maintaining posture,
extending the spine, and laterally bending the spine.
These muscles are divided into three major groups,
as follows (Fig. 2.11 and Table 2.5):
- Iliocostalis:^ most laterally located and associated
with attachments to the ribs and cervical trans-
verse processes.
Semispinalis thoracis m.
Multifidus thoracis mm.
Multifidus lumborum mm. ( cut )
Longus Brevis Rotatores thoracis mm.
Brevis
Longus Rotatores cervicis mm.
Serratus posterior inferior m.
Multifidus lumborum mm.
Semispinalis capitis m.
The superficial and intermediate (erecter spinae) layers of the intrinsic back muscles
The deep (transversospinal) layer of the intrinsic back muscles
Erector spinae mm.
Iliocostalis m.
Longissimus m.
Spinalis m.
Serratus posterior superior m.
Splenius capitis and splenius cervicis mm.
Superior nuchal line of skull
Longissimus capitis m. Posterior tubercle of atlas (C1)
Thoracolumbar fascia ( cut edge )
Iliocostalis lumborum m.
Longissimus thoracis m.
Spinalis thoracis m.
Iliocostalis thoracis m.
Iliocostalis cervicis m.
Longissimus cervicis m.
Spinalis cervicis m.
Intertransversarius laterales lumborum m.
Interspinalis lumborum m.
Note: Deep dissection shown on right side.
Thoracolumbar fascia (anterior layer)
Thoracolumbar fascia (posterior layer) ( cut )
Brevis Longus Levatores costarum mm.
Levator costarum m.
Interspinalis cervicis m.
FIGURE 2.11 Intrinsic Muscles of the Back. (From Atlas of human anatomy, ed 7, Plates 181 and 182.)
