Thanks to Darryl Hosford credit for his work.
BACK AND NECK
The adult spine consists of 26 bones called vertebrae, arranged in a column (Figures 1
& 2). (In childhood there are 33-35 individual bones, several of which later fuse
together.) These bones are named and numbered from the top downward: seven cervical
vertebrae (C1-C7) in the neck, twelve thoracic vertebrae (T1-T12) in the upper/middle
back, and five lumbar vertebrae (L1-L5) in the small of the back. L5 rests on top of the
sacrum, a large triangular bone made up of five fused vertebral segments. Below the sacrum
is the coccyx (tailbone), a small bone made up of three to five fused segments. Although
the coccyx is at the bottom of the vertebral column, it does not bear weight. The weight
of the body is transmitted through the sacrum to the ilium (pelvic bone). The joints that
connect the sacrum and the ilium are called the sacroiliac joints.
Viewed from the side, the adult spine has three normal curves (Figure 2). In the
cervical and lumbar regions, the convexity of the curve is forward (lordosis); in the
thoracic region, the convexity is rearward (kyphosis). The cervical and lumbar lordoses
are not present in a newborn infant. The cervical lordosis develops as the infant begins
to hold its head up; the lumbar lordosis develops as the child learns to stand erect.
Although vertebrae of different levels have some structural differences, all have basic
features in common (Figure 3). Each vertebra (except C1) has a drum-shaped body which
bears most of the weight. Projecting backward from the vertebral body are two short
pedicles. Extending back from the pedicles are two bony plates called laminae, which meet
in the middle and are fused together. The pedicles and laminae together form a bony arch
enclosing a space called the vertebral foramen. (Foramen is Latin for "hole";
the plural is foramina.) With the vertebrae arranged in a column, the vertebral foramina
line up to form a hollow tube, the spinal canal. Contained within this canal, surrounded
and protected by bone, is the spinal cord.
Several other bony projections or "processes" arise from the vertebral
arches. The spinous process extends straight backward from the point where the two laminae
meet. (The spinous process of C7 or T1 is especially prominent, and can easily be felt at
the nape of the neck.) Two transverse processes (one on each side of the vertebra) extend
laterally from the junctions of pedicle and lamina. The spinous and transverse processes
serve as attachments for muscles and ligaments.
Each vertebra also has two superior (upper) and two inferior (lower) articular
processes, each with an articular (joint) surface called a facet. The inferior facets of
each vertebra articulate with the superior facets of the vertebra below, and so on down
the column. These interlocking facet joints stabilize the spinal column, helping to hold
it in alignment while permitting flexibility. The facet joints are synovial joints (see
also Anatomy of the Joints).
In the thoracic region of the spine, the vertebrae also have costal facets which
articulate with the ribs (Figure 4). This gives the thoracic spine more structural
stability but less flexibility than the cervical and lumbar regions.
The cervical spine also has some special features. The C1 and C2 vertebrae are unique,
and this is reflected in the fact that they are the only individual vertebrae that have
names. C1, the atlas, has no vertebral body and no spinous process. It consists of a ring
of bone, broadened on each side where it bears the weight of the skull. The joints between
the atlas and the base of the skull (the atlanto-occipital joints) allow the head to nod
forward or tilt back.
C2, the axis, has an odontoid ("toothlike") process that projects upward from
the vertebral body. The odontoid process is encircled by the bony ring of the atlas. Held
in place by strong ligaments, it acts as a pivot for the atlas. The joints between the
axis and atlas (the atlanto-axial joints) allow rotation of the head on the neck.
C3-C7 have a more typical structure. However, all seven cervical vertebrae have a
feature that is not shared by other vertebrae: a foramen in each transverse process. The
two vertebral arteries pass through these foramina (except those of C7) on their way to
Interposed between each pair of vertebral bodies are pads of fibrocartilage called
intervertebral discs. The outer part of the disc forms a thick fibrous ring (the annulus
fibrosus) which surrounds and contains a soft, gelatinous center (the nucleus
Between the disc and the adjacent vertebral bodies (above and below) are thin plates of
cartilage (the vertebral end plates). The discs give the spine flexibility and act as
shock absorbers. Since they are firmly fastened (by the vertebral end plates) to the
vertebral bodies, the discs also help hold the spinal column in alignment.
Each disc is named and numbered according to the vertebrae above and below it, for
example, the C5-6 disc is the disc between C5 and C6; the L5-S1 or lumbosacral disc is the
disc between L5 and the sacrum. There is no C1-2 disc, because C1 (the atlas) has no
Spinal cord and nerves:
In addition to providing support, flexibility, and a site for muscle attachments, the
spinal column also houses and protects the spinal cord. The spinal cord is an extension of
the brainstem (the lowest part of the brain), extending from the base of the skull down to
the low back. Along its length, it gives off 31 pairs of spinal nerves, which branch to
form the peripheral nerves of the neck, trunk, and extremities. The origins of the spinal
nerves as they emerge from the spinal cord are called nerve roots.
The spinal cord is covered by three membranes (the meninges), which are continuous with
the membranes that cover the brain. The delicate innermost membrane, called the
the surface of the spinal cord itself. The weblike middle membrane is called the arachnoid
(Greek for "spiderlike"), and the tough outer membrane is the dura. The
subarachnoid space (between the arachnoid and pia) is filled with cerebrospinal fluid
(CSF), which circulates around the spinal cord and brain. The meninges thus form a
fluid-filled sac around the brain and spinal cord, with sleeves for the emerging nerve
The spinal nerves exit from the vertebral column through openings between adjacent
vertebrae. These openings, called intervertebral foramina, are located just in front of
the facet joints. The spinal nerves are named and numbered according to the vertebral
levels at which they exit. There are eight paired cervical nerves (C1-C8), twelve thoracic
(T1-T12), five lumbar (L1-L5), five sacral (S1-S5), and one coccygeal (Co1). Note that
there are eight pairs of cervical nerves, although there are only seven cervical
vertebrae. The C1 nerves exit above the C1 vertebra (between C1 and the base of the
skull). The C2 nerves exit below the C1 vertebra (between C1 and C2); and so on, down to
the C8 nerves, which exit below the C7 vertebra (between C7 and T1). The rest of the
spinal nerves exit below the vertebral segment of the same name and number: the T1 nerve
below the T1 vertebra; the T12 nerve below the T12 vertebra (between T12 and L1); the L5
nerve below the L5 vertebra (between L5 and the sacrum). The first four pairs of sacral
nerves exit through the four pairs of sacral foramina, and so on.
Although the spinal nerves correspond to their respective vertebral levels, the spinal
cord itself is shorter than the vertebral column, extending only as far as the L1
vertebra. Below that level, the spinal canal contains only the roots of the lumbar,
sacral, and coccygeal nerves, as they descend to exit at the appropriate levels. This
bundle of descending nerve roots is called the cauda equina (Latin for "horse's
After emerging from the vertebral column, the lower cervical nerves (C5-C8) and the
first thoracic nerve (T1) form a network called the brachial plexus, which gives rise to
the peripheral nerves of the shoulder and upper extremity. The lumbar and sacral nerves
(with a contribution from T12) also form a network, the lumbosacral plexus, which gives
rise to the nerves of the lower extremity. The other thoracic nerves (T2-T11) do not form
a plexus, but travel independently of each other to supply the trunk.
Muscles of the back:
(The muscles of the neck are described separately below.) There are two main groups of
back muscles, deep and superficial. The deep muscles are responsible for movements of the
spine and for maintaining posture. The superficial muscles are responsible for movements
of the scapulae (shoulder blades) and shoulders.
The deep muscles (sometimes referred to collectively as paraspinal muscles) form a
thick mass on each side of the spine, extending from the base of the skull to the sacrum.
This muscle mass consists of many separate, overlapping muscles of different lengths,
attached to the spinous or transverse processes of different vertebrae. Each individual
muscle can be thought of as a string. Pulling a string (contracting a muscle) causes one
or more vertebrae to tilt or turn on the vertebra below. Working in coordination, these
muscles extend (backward bend) and rotate the spine. (Flexion or forward bending is
accomplished by the muscles of the abdomen.) The deep muscles of the back also maintain
posture by holding the spine erect against gravity.
In the low back, the fascia (fibrous covering) of the deep muscles forms a thick sheet,
the lumbar fascia. This fibrous sheet serves as an attachment for other muscles, including
muscles of the abdominal wall.
Overlying the deep muscles of the back are the superficial muscles (Figure 6). In the
upper back, the most superficial of these is trapezius, a large flat muscle that extends
like a cape over the upper back and the back of the neck (see also Muscles of the neck,
below). Underneath trapezius are the levator scapulae and the rhomboid muscles. Each of
these muscles has its origin (attachment of one end) on the cervical and/or thoracic
spine, and its insertion (attachment of the other end) on the scapula. Acting together in
various combinations, these muscles produce movements of the scapulae (e.g., shrugging the
shoulders, bracing the shoulders back).
Another group of superficial muscles originates on the scapula and inserts on the upper
end of the humerus (upper arm bone). These muscles produce some movements of the upper arm
at the shoulder joint (extending the arm backward; raising the arm straight out to the
side; rotating the arm inward or outward). Included in this group are
supraspinatus, infraspinatus, teres major, and teres minor (all located in the upper back, overlying the
scapula), as well as the deltoid (located over the top of the shoulder and upper arm).
Latissimus dorsi is a large superficial muscle that covers the middle and low back.
Originating on the lower thoracic spine and lumbar fascia, and inserting on the upper end
of the humerus, it also is responsible for some shoulder movements.
Muscles of the neck:
The deep muscles of the neck are a continuation of the deep muscles of the back
(described above). They consist of separate, overlapping muscles of different lengths,
connecting the cervical vertebrae to each other and to the base of the skull. Attached to
the spinous or transverse processes, these muscles extend (backward bend), side bend, and
rotate the head and neck. Another group of muscles connects the fronts of the cervical
vertebrae to each other and the base of the skull; these muscles flex (forward bend) the
head and neck.
The superficial muscles of the neck are shown in Figure 7. The scalene muscles are
located in the sides of the neck. There are three scalenes on each side: scalenus
anterior, medius, and posterior. These muscles originate on the transverse processes of
the cervical vertebrae, and insert on the first and second ribs. When the scalenes of one
side act alone, they bend and rotate the neck to that side. When both sides act together,
they flex the neck slightly.
As mentioned above (in Muscles of the back), the trapezius is a large, flat,
superficial muscle that covers the back of the neck and extends like a cape onto the upper
back. It originates on the base of the skull and the spine, down to T12; it inserts on the
scapulae. The trapezius acts with other muscles to produce scapular movements (shrugging,
bracing the shoulders back). However, if the scapula is kept immobile, the trapezius
produces movements of the head. Contraction of one side of trapezius tilts the head to
that side; contraction of both sides together tilts the head backward.
The two sternocleidomastoids (SCMs) are superficial, strap-like muscles located in the
front of the neck (one on each side). Each SCM originates at the base of the neck on the
sternum (breastbone) and clavicle (collarbone), and inserts behind the ear on the mastoid
process of the skull. When one SCM acts alone, it bends the neck to that side, at the same
time rotating the head toward the opposite side. When both act together, they flex the
neck while extending the head.
Underneath the SCMs, in the front of the neck, is a group of strap-like muscles that
produce movements of the tongue during swallowing.
Other structures of the neck:
The neck contains several visceral (internal organ) structures: the upper part of the
esophagus, the larynx and upper part of the trachea, and the thyroid gland. The neck also
contains major blood vessels (the vertebral arteries, carotid arteries, and jugular
veins), as well as numerous lymph nodes.