The trunk (torso) is the
central part of the body to which the head and the limbs are attached.
Except for the brain, the trunk houses all the vital organs of the human body. The torso
muscles attach to the skeletal core of the trunk, and depending on their
location are divided into two large groups:
·
anterolateral muscles of the
trunk
·
posterior muscles of the trunk
Trunk muscles have a few
very important functions; they contribute to the protection of the thoracic and abdominopelvic viscera and they assist
essential body activities such as breathing, movement, defecation and micturition.
|
Key facts about muscles of the trunk |
|
|
Anterior thoracic muscles |
Pectoralis major, pectoralis
minor, serratus anterior, subclavius, external intercostals, internal
intercostals, innermost intercostals, subcostals, transversus thoracis,
diaphragm |
|
Anterolateral abdominal wall
muscles |
Rectus abdominis, external
abdominal oblique, internal abdominal oblique, transversus abdominis,
pyramidalis, quadratus lumborum |
|
Suboccipital muscles |
Rectus capitis posterior
major, rectus capitis posterior minor, obliquus capitis superior, obliquus capitis
inferior |
|
Posterior superficial
muscles |
Superficial: Latissimus dorsi,
trapezius, rhomboid major and minor, levator scapulae |
|
Posterior deep muscles |
Superficial: Spinotransversales
group (splenius capitis, splenius cervicis) |
The anterior
trunk muscles cover the anterolateral part of the trunk by attaching to the
bony framework of the thoracic cage and pelvis. These muscles
are grouped into the muscles of the thoracic cage and the muscles of the abdominal
wall.
Muscles
of the thoracic cage
The muscles of the thoracic cage
are the pectoralis major, pectoralis minor, serratus anterior, subclavius, intercostal (external,
internal and innermost),
subcostal and transversus
thoracis muscles, including the diaphragm.
These muscles attach the upper
limb to the axial skeleton of
the trunk and support the thoracic cage. They are also involved in movements of
the upper limb and in respiration.
Pectoralis major is a large,
fan-shaped, superficial muscle located on the anterior thoracic wall. It forms
the bulk of the chest area and can be easily seen on the surface in some
people, for example weightlifters. The muscle has three heads giving it three
points of origin:
·
Clavicular head - originates from the
anterior surface of the clavicle (medial
half).
·
Sternocostal head - originates from the
anterior surface of the sternum and
costal cartilages of ribs 1-6.
·
Abdominal (rectus) head - originates from the
anterior layer of the rectus
sheath.
All the fibers converge and insert
on to the crest of greater tubercle of the humerus.
Pectoralis major is innervated by the lateral and medial pectoral nerves, two branches of the brachial
plexus. The pectoralis major is involved in a variety of arm movements,
such as adduction, internal rotation, flexion and extension. It also draws the
scapula anteroinferiorly.
Pectoralis minor
Deep to pectoralis major is the pectoralis
minor muscle. Pectoralis minor originates from the anterior
aspect of ribs 3-5 and
their costal cartilages. It inserts onto the medial border and coracoid process
of the scapula,
pulling it anteriorly and inferiorly on the thoracic wall. In addition, the
muscle also stabilizes the scapula on the thoracic wall. Pectoralis minor is
innervated by the medial pectoral nerve (C8-T1). The muscle can also be
involved in pectoralis
minor syndrome.
Serratus anterior
Serratus
anterior is another large, fan-shaped muscle. It lies on the
lateral aspect of the thorax, deep to the
scapula and the pectoralis major and minor muscles. It consists of three parts:
·
Superior part - originates from ribs 1-2
and the intercostal fascia. It inserts onto the anterior and posterior surfaces
of the superior angle of the scapula.
·
Middle part - originates from ribs 3-6
and inserts onto the anterior surface of the medial border of the scapula.
·
Inferior part - originates from ribs 7-8
(sometimes 9 and 10). It inserts onto the anterior and posterior surfaces of
the inferior angle of the scapula.
Serratus anterior is important in
attaching the scapula to
the thoracic wall. The muscle pulls the scapula in an anterolateral direction
along the thoracic wall. Serratus anterior also rotates the scapula by pulling
its inferior angle laterally. The muscle is innervated by the long thoracic nerve (C5-C7), which is
a branch of the brachial plexus.
The innervation for the serratus
anterior is very easy to remember if you just know the right mnemonics! 'SALT'
stands for 'Serratus Anterior = Long Thoracic' and will help you remember the
name of the nerve, while knowing 'C5, 6, 7 raise
your arms to heaven!' means you'll never forget the nerve roots
associated with it!
Subclavius
Subclavius is
a small, round muscle found inferior to the clavicle. It originates from the
sternal end of the first rib and
its costal cartilage. It inserts onto the middle third of the clavicle on its anteroinferior aspect.
By acting on the sternoclavicular
joint, the subclavius anchors and depresses the clavicle. The muscle
also protects the subclavian vessels and the brachial
plexus which pass deep to it. The latter innervates the
subclavius.
Intercostal muscles
The
next set of torso muscles are found in the intercostal
spaces between the ribs. The intercostal
muscles consist of a group of three layered muscles, from
superficial to deep: external, internal and innermost intercostals.
The fibres of the external intercostal muscles originate from the inferior
border of the ribs. They extend in an inferomedial direction to the superior
border of the rib below. The internal intercostal muscles extend in an inferoposterior
direction from the costal groove of the ribs until the superior border of the
rib below. The innermost
intercostals also originate from the costal groove of the
ribs and insert onto the superior border of the rib below.
Collectively, the intercostal
muscles support the intercostal spaces and thoracic cage. However, they also
have additional individual functions. The external intercostals elevate the
ribs during forced inspiration, expanding the thorax and lungs.
In contrast, the internal and innermost intercostals depress the rib cage
during forced expiration. This decreases the size of the thorax and lungs. All
intercostal muscles are innervated by the intercostal
nerves (T1-T11). These nerves run in the intercostal
groove between the internal and innermost intercostal muscles.
Subcostal muscles
The subcostal
muscles are
strips of muscle located on the internal surface of the lower ribs, sharing a
plane with the innermost intercostals. They extend from the internal surface of
the lower ribs to the internal surface of
a rib located 2-3 levels below. They support the intercostal spaces and thoracic
cage, and depress the ribs during forced expiration. They receive innervation
from the intercostal nerves.
Transversus thoracis
The transversus
thoracis muscle is a thin, radiating muscle located on the
internal aspect of the anterior thoracic wall. It originates from the
inferoposterior surface of the sternum and xiphoid process. In turn, the muscle
inserts on the internal surface of the 2nd to 6th
costal cartilages. Transversus thoracis is a weak depressor of
the rib cage, assisting expiration. It is innervated by the intercostal nerves.
Deepen your knowledge about the
muscles of the thoracic wall with our additional learning materials.
Diaphragm
The diaphragm is
a large, unpaired, dome shaped muscle located within the trunk. It separates
the thoracic and abdominal cavities and facilitates the passage of anatomical
structures via openings called hiatuses. The diaphragm consists of three parts:
·
Sternal part -originates from the
posterior aspect of xiphoid process.
·
Costal part - originates from the
internal surfaces of the lower costal cartilages and ribs 7-12.
·
Lumbar part - originates from the bodies
of vertebrae L1-L3 and intervertebral
discs, the anterior longitudinal ligament and the arcuate ligaments
(medial, lateral).
All fibres converge on a central
tendon in the middle of the trunk, which has no bony insertions. The diaphragm
is the main muscle of breathing, responsible for inspiration. It also depresses
the costal cartilages and receives innervation from the phrenic nerve (C3-C5).
Here is a collection of resources
to improve your understanding of the diaphragm:
Muscles of the
abdominal wall
We’ve finished with the muscles of
the thoracic cage, so let’s descend on the anterior trunk to learn about the
muscles of the abdominal wall. There are five muscles that form the abdominal
part of the anterior trunk. These are the rectus
abdominis, pyramidalis, external
abdominal oblique, internal
abdominal oblique and transversus
abdominis. The first three are classified as vertical muscles and they are located
near the midline. The remaining ones are flat
muscles and they are located more laterally.
Together, these muscles form the
anterolateral boundary of the abdominal cavity. They are involved in movements
of the trunk, mainly flexion and rotation. They also contribute to changing the
intra-abdominal pressure, which is essential for basic bodily functions such as
micturition and defecation.
Rectus abdominis
Rectus abdominis is a long,
strap-like, paired muscle that passes vertically over the entire length of the abdomen.
The muscle originates from the pubic
crest and pubic symphysis.
It inserts onto the xiphoid
process of the sternum and costal cartilages of the 5th to 7th ribs.
Rectus abdominis acts as a flexor
of the trunk and an accessory muscle of expiration. It also compresses the
abdominal contents to protect them. Rectus abdominis can sometimes be seen
beneath the skin in athletic individuals, resulting in what’s commonly referred
to as a ‘six-pack’. It receives innervation via the intercostal and subcostal nerves.
Pyramidalis
Pyramidalis is
a small triangular muscle found in the lower abdomen. It lies anterior to the
rectus abdominis muscle. It extends from the pubic
symphysis and pubic crest to
the linea alba.
The action of this muscle is to tense the linea alba, which is initiated by the subcostal nerve. Pyramidalis is a variable
muscle of the abdominal wall, being absent in about 20% of the population.
Abdominal oblique muscles
It’s time to take a look at the
three flat muscles of the anterolateral abdominal wall. The first two are the
abdominal oblique muscles. These include the external abdominal oblique and the
internal oblique muscles. The former are the most superficial, overlying the
latter.
The external oblique muscles originate from the external
surfaces of ribs 5-12. They insert onto the anterior half of the iliac crest,
pubic tubercle and linea alba.
The internal oblique muscles originate from the anterior
two-thirds of the iliac crest, the iliopectineal arch and the thoracolumbar
fascia. Their insertion is onto the inferior borders of ribs 10-12,
linea alba, the junction with cremaster
muscle and the pectineal line of pubis (via conjoint tendon).
The functions of the abdominal oblique muscles involve trunk flexion and
ipsilateral rotation, as well as compression of abdominal viscera. They also
assist with expiration.
The abdominal oblique muscles are
innervated by the intercostal, subcostal (T7-T12) and iliohypogastric
nerves (L1). The internal oblique muscles are
also innervated by the ilioinguinal
nerve (L1).
Transversus abdominis
The final muscle in the abdominal
area of the anterior trunk is the transversus
abdominis muscle. It lies underneath the internal oblique muscle,
hence it is the deepest flat muscle. Transversus abdominis has an extensive
origin from the costal cartilages of ribs 7-12,
thoracolumbar fascia, anterior two thirds of the iliac
crest and the iliopectineal
arch. It crosses the abdomen in a horizontal direction to
insert onto the linea alba,
aponeurosis of the internal abdominal oblique muscle, pubic crest and pectineal
line of the pubis.
Contraction of transversus
abdominis compresses the abdominal organs, assists expiration and causes
ipsilateral trunk rotation. The muscle is innervated by the intercostal (T7-T11), subcostal (T12), iliohypogastric (L1) and ilioinguinal nerves (L1).
Quadratus lumborum
Quadratus
lumborum is actually a muscle of the posterior wall, but it is
often described as part of the ventral trunk musculature. This muscle
originates from the iliac
crest and iliolumbar
ligament. It inserts onto the inferior border of the 12th rib. As it ascends, it also attaches
to the transverse processes of L1-L4
vertebrae.
Quadratus lumborum is involved in
extending the trunk and flexing it laterally to the ipsilateral side. These
actions are possible due to inputs provided by the subcostal nerve and the anterior rami
of the L1-L4 spinal
nerves.
Want to learn more about the
muscles of the abdominal wall?
Posterior
trunk muscles
Congratulations, you are now a
master in anterior trunk muscles! However, the trunk also has a posterior or
dorsal side, so let’s learn about the trunk musculature located here. The posterior
trunk muscles are more commonly referred to as the muscles of the back. They
are divided into two functional groups:
·
Superficial, or extrinsic muscles of the back. This
group also includes the suboccipital muscles of the neck.
·
Deep, or intrinsic muscles of the back.
Suboccipital
muscles of the neck
The suboccipital
muscles are located deep to trapezius, in the suboccipital
region of the neck (just inferior to the occipital
bone). They include four muscles:
·
Rectus
capitis posterior major, which arises from the spinous process of the axis
(C2). It inserts onto the lateral part of the inferior
nuchal line of the occipital bone.
·
Rectus
capitis posterior minor, which originates from the posterior tubercle of the
posterior arch of the atlas (C1). It extends until the inferior nuchal line of
the occipital bone, medial to rectus capitis posterior minor.
·
Obliquus
capitis superior,
a muscle that arises from the transverse process of C1. It ascends vertically
to insert onto the occipital bone between the superior and inferior nuchal
lines.
·
Obliquus
capitis inferior,
which arises from the spinous process of C2. It passes obliquely to insert onto
the transverse process of C1.
All suboccipital muscles act on
the atlanto-occipital and
atlanto-axial joints, extending and rotating the head. They are innervated by
the posterior ramus of the C1 spinal nerve (suboccipital
nerve).
Superficial muscles of the back
For descriptive purposes, the
muscles of the back are divided into two groups; superficial (extrinsic)
muscles which move the upper limb and deep (intrinsic) muscles which act on the
trunk.
We will start by describing the
superficial muscles of the back. They are arranged into two layers:
·
Superficial layer containing the trapezius,
latissimus dorsi, rhomboid major and minor and levator
scapulae muscles.
·
Intermediate layer possessing serratus
posterior superior and inferior muscles.
Trapezius
Trapezius is
a large, paired, triangular shaped muscle located in the upper back and neck.
The muscle consists of three parts which fan out during their course:
·
Descending part - originates from the medial
third of the superior nuchal line, the external occipital protuberance, nuchal
ligament and the cervical vertebrae. It inserts into the lateral third of the
clavicle. This part of the trapezius supports and draws the scapula
superiomedially. In addition, it extends, rotates (contralateral) and laterally
flexes (ipsilateral) the head
and neck.
·
Transverse part - extends from the spinous
processes of vertebrae T1-T4 (or C7-T3) to the acromion and superior crest of
the spine of the scapula. This part supports and draws the scapula medially.
·
Ascending part - originates from the spinous
processes of vertebrae T5-T12 (or T2-T12). It inserts into the medial end of
the spine of the scapula. The ascending part supports and moves the scapula
inferomedially.
Trapezius receives motor
innervation from the accessory
nerve (CN XI). It also receives sensory innervation
through the anterior rami of spinal nerves
C3-C4 (via cervical
plexus).
Latissimus dorsi
Latissimus
dorsi is an expansive muscle located in the lower region of the
back. It overlies all the back muscles except for trapezius. Latissimus
dorsi consists of four parts:
·
Vertebral part - originates from the
spinous processes of vertebrae T7-T1 and the thoracolumbar fascia.
·
Iliac part - extends from the posterior
third of the iliac crest.
·
CostaI part - originates from ribs 9-12.
·
Scapular part - begins at the inferior
angle of the scapula.
All fibres extend superiorly into
the axilla to insert onto the medial lip of the intertubercular sulcus of the
humerus. Latissimus dorsi has a variety of actions on the arm, including
internal rotation, adduction and extension. It is also an accessory muscle of
inspiration. Latissimus dorsi is innervated by the thoracodorsal nerve (C6-C8).
Rhomboids
Rhomboid major and rhomboid minor are
a group of small, cylindrical muscles found between the vertebral
column and the medial border of the scapula. Rhomboid major extends from the
spinous process of vertebrae T2-T5 until the medial border of the scapula. Rhomboid minor originates from the
nuchal ligament and spinous processes of C7-T1 vertebrae. It inserts at the
root (medial end) of the spine of scapula.
Their main roles are to support
and move the scapula superomedially. They also rotate the glenoid cavity
inferiorly. The rhomboids are innervated by the dorsal scapular
nerve (C5).
Levator scapulae
Levator
scapulae is located deep to the trapezius and superior to the
rhomboid minor muscle. It originates from the transverse processes of C1-C4 vertebrae and attaches to the
medial border of the scapula..
As the name suggests (‘elevator’), levator scapulae primarily elevates the scapula
superomedially. It also rotates the glenoid cavity inferiorly and laterally
flexes the neck (ipsilaterally). Levator scapulae is innervated by the dorsal scapular nerve (C5) and
anterior rami of C3-C4 spinal
nerves.
Serratus posterior
Diving deeper underneath all the
previous superficial extrinsic muscles, one reaches the intermediate layer. he serratus
posterior muscles are two oblique muscles:
·
Serratus posterior superior - originates from the nuchal
ligament and spinous processes of vertebrae C7-T3. It inserts on the superior
borders of ribs 2-5, elevating them. It is innervated by the 2nd to 5th
intercostal nerves.
·
Serratus posterior inferior - begins at the spinous
processes of vertebrae T11-L2 and inserts onto the inferior borders of ribs
9-12. This muscle depresses the ribs under the control of the 9th to 11
intercostal nerves and the subcostal nerve.
Learn more about the superficial
muscles of the back with our learning materials that help you expand and test
your knowledge in no time.
Deep
muscles of the back
The deep
(intrinsic) muscles of the back are a large group of muscles
responsible for maintaining posture and controlling head and spine movements.
They extend along the vertebral column from the pelvis all the way to the
skull. They can be grouped into four layers:
·
Superficial - spinotransversales muscle (splenius
capitis, splenius
cervicis)
·
Intermediate - erector spinae (iliocostalis, longissimus, spinalis)
·
Deep - transversospinal muscles
(semispinalis, multifidus, rotatores
longus and brevis)
·
Deepest - interspinales,
intertransversarii and levatores
costarum
Superficial layer
The superficial layer of intrinsic
back muscles is made up of the spinotransversales muscles, which are splenius
capitis and splenius
cervicis. They are located on the anterolateral aspects of the neck.
Splenius capitis is a deep, broad muscle found in the floor of
the posterior triangle of the neck. It originates from the spinous processes of C7-T3 vertebrae and the nuchal ligament that covers them. It
inserts onto the occipital
bone (lateral part of the superior nuchal line) and the temporal
bone (mastoid process) of the skull. Splenius
capitis extends, laterally flexes and rotates the head ipsilaterally. It is
innervated by lateral branches of posterior rami of spinal nerves C2-C3.
Splenius cervicis is the other muscle in the superficial layer.
It originates below splenius capitis from the spinous processes of T3-T6 vertebrae. It extends superiorly and
inserts onto the transverse processes of C1-C3.
It has similar functions as splenius capitis: extension, lateral flexion and
rotation of the neck. This muscle is innervated by posterior rami branches
which stem from spinal nerves
below C3.
Intermediate layer
The intermediate layer contains
the massive erector
spinae muscles. They lie in a groove on either side
of the vertebral column in the space between the spinous processes of the
vertebrae and the angles of the ribs. The erector spinae group contains three
muscles, located from lateral to medial, as follows: iliocostalis, longissimus
and spinalis.
Iliocostalis forms the lateral column of
erector spinae. It is divided into three regional areas or parts:
·
Iliocostalis cervicis - extends from the angle of
ribs 3-6 until the transverse processes of C4-C6 vertebrae.
·
Iliocostalis thoracis - originates from the angle
of ribs 7-12. It inserts onto the angles of ribs 1-6 and the transverse process
of vertebra C7.
·
Iliocostalis lumborum - originates from the
lateral crest of sacrum, medial end of the iliac crest and the thoracolumbar
fascia. It inserts onto the angle of ribs 5-12, transverse processes of
vertebrae L1-L4 and the adjacent thoracolumbar fascia.
Contraction of iliocostalis
produces extension and lateral flexion (ipsilateral) of the spine.
Longissimus represents the intermediate column
of erector spinae. It is also the largest member of the muscle group.
Longissimus is divided into several regions or parts:
·
Longissimus capitis - originates from the
transverse processes of vertebrae C4-T5 and inserts onto the mastoid process of
the temporal bone.
·
Longissimus cervicis - attaches to the transverse
processes of T1-T5 and transverse processes of C2-C6.
·
Longissimus thoracis - originates from the medial
end of the iliac crest, lateral crest of the sacrum and the spinous and
transverse processes of L1-L5. It inserts onto the transverse processes of
T1-T12 and the angles of ribs 5-12.
As a whole, longissimus extends
and laterally flexes (ipsilateral) the spine. Longissimus capitis specifically
performs the same movements, but on the head instead. The latter also rotates
the head ipsilaterally.
Spinalis is the last and most medial muscle column that makes
up the erector spinae. It is divided into two parts:
·
Spinalis cervicis - originates from the
spinous processes of C7-T1 and the nuchal ligament. The muscle inserts onto the
spinous processes of C2-C4.
·
Spinalis thoracis - originates from the
spinous processes of T11-L2 and attaches onto the spinous processes of T2-T8.
Contraction of the spinalis
muscles extends and laterally flexes the thoracic and cervical spine. All
iliocostalis muscles are innervated by lateral branches of the posterior rami
of the spinal nerves.
Mnemonic
A really easy way to remember the
muscles in the intermediate layer is to learn the mnemonic 'I Like Standing'. It stands for:
(Lateral to medial)
Iliocostalis
Longissimus
Spinalis
Deep layer
The deep layer of back muscles
are a group called the transversospinalis
muscles. From superficial to deep, these include semispinalis,
multifidus and rotatores. These muscles are located underneath the erector
spinae, occupying the space between the spinous and transverse processes of the
spine.
Semispinalis is the most superficial of the
three muscles. This muscle is divided into three parts:
·
Semispinalis
capitis -
originates from the articular processes of vertebrae C4-C7 and the transverse
processes of vertebrae T1-T6. It inserts between the superior and inferior
nuchal lines of the occipital bone.
·
Semispinalis
cervicis -
attaches to the transverse processes of vertebrae T1-T6 and spinous processes
of vertebrae C2-C5.
·
Semispinalis
thoracis -
originates from the transverse processes of vertebrae T6-T10 and inserts onto
the spinous processes of vertebrae C6-T4.
Semispinalis muscles act on the
head, cervical and thoracic spines. They perform extension, rotation and
lateral flexion (ipsilateral) of these structures. Semispinalis capitis is
innervated by descending branches of the greater occipital (C2) and spinal
nerves (C3). Semispinalis cervicis and thoracis are innervated by medial branches
of the posterior rami of spinal nerves.
Multifidus is a very thin muscle situated
underneath semispinalis. This muscle spans a number of vertebrae to attach to
the spinous processes of vertebrae in higher regions. It is divided into three
parts:
·
Multifidus cervicis - originates from the
superior articular processes of vertebrae C4-C7.
·
Multifidus thoracis - originates from the
transverse processes of the thoracic vertebrae.
·
Multifidus lumborum - originates from the
mammillary processes of the lumbar vertebrae, posterior aspect of the sacrum,
posterior superior iliac spine (PSIS) of ilium and the posterior sacroiliac
ligament.
The fibres of all the parts of
multifidus extend superiorly to insert onto the lateral aspect and tips of the
spinous processes of vertebrae 2-5 levels above their origin. The functions of
multifidus involve extension, rotation (contralateral) and lateral flexion
(ipsilateral) of the spine. It is innervated by medial branches of the
posterior rami of the spinal nerves.
The final group of muscles in the
deep layer are the rotatores muscles.
They are located underneath the multifidus and are most developed in the
thoracic region. The rotatores has two parts:
·
Rotatores brevis - originates from the
transverse processes of vertebrae T2-T12 and inserts onto the laminae/spinous
process of a vertebra one level above its origin.
·
Rotatores longus - extends from the
transverse processes of the thoracic vertebrae until the laminae/spinous
process of a vertebra two levels above its origin.
Contraction of multifidus produces
extension and contralateral rotation of the thoracic spine. The stimulus for this
movement comes through the medial branches of the posterior rami of spinal
nerves.
Deepest layer
The last and deepest layer of the
intrinsic muscles of the back is composed of interspinales, intertransversarii
and levatores costarum muscles.
The interspinales muscles connect the
spinous processes of adjacent vertebrae. They are divided into three regional
parts:
·
Interspinales cervicis - originates from the
superior aspect of spinous processes of vertebrae C2-C7 and inserts onto the
inferior aspect of spinous processes of vertebrae C1-C6.
·
Interspinales thoracis has a variable origin from
the superior aspect of the spinous processes of vertebrae T2, T11 and T12. The
insertion is onto the inferior aspect of spinous processes of vertebrae T1, T10
and T11.
·
Interspinales lumborum - originates from the
superior aspects of the spinous processes of vertebrae L2-L5 and inserts
onto the inferior aspects of the spinous processes of vertebrae L1-L4.
Interspinales extend the cervical
and lumbar spine. They are innervated by the posterior rami of spinal nerves.
The intertransversarii are similar to the
interspinales muscles, but connect adjacent transverse processes rather than
spinous processes. There are four groups of these muscles; anterior
cervical intertransversarii, posterior
cervical intertransversarii, intertransversarii
laterales lumborum and intertransversarii mediales lumborum.
They originate from the transverse
processes of cervical and lumbar vertebrae and insert onto the transverse
process of an adjacent vertebra. These muscles assist in lateral flexion of the
spine and also work to stabilize the spine. They are innervated by the
posterior and anterior rami of spinal nerves.
The last muscles in the deepest
layer are levatores costarum.
These muscles extend inferiorly from the transverse processes of the C7-T11
vertebrae to the superior border of the rib located one level below. They
elevate the ribs and rotate the thoracic spine. They are innervated by
posterior rami of spinal nerves T1-T12.
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