Key Points

• The posterior abdominal wall muscles contribute to respiration and movement of the torso and thigh.
• The posterior abdominal wall is bound superiorly by the diaphragm, and, inferiorly, by the ilia of the pelvis.

Quadratus lumborum:

• Originates on the iliac crest and the transverse process of L5.
• Inserts on the transverse processes of L1-L4, and along the inferior border of rib 12 (intertextual variation regarding the specific origins and insertions of quadratus lumborum exists).
• It depresses and stabilizes rib 12, and laterally flexes the torso.

Diaphragm:

• Originates on the internal surface of the sternum, costal cartilages of ribs 6-12, and lumbar vertebrae L1-L3
  Inserts into the central tendon.
• Contraction of the diaphragm expands the thoracic cavity, which decreases intrapulmonary pressure and induces inspiration (inhalation).

Iliacus:

• Originates on the iliac fossa and lateral sacrum.
• Inserts on the lesser trochanter of the femur.
• Flexes the thigh at the hip.

Psoas major:

• Originates on the lateral aspects and intervertebral discs of vertebrae T12-L5 Inserts, with iliacus, on the lesser trochanter.
• Flexes the thigh at the hip.

Psoas minor:

• Originates on the lateral aspects and intervertebral discs of T12-L1
• Inserts on the pectineal line on the pelvis.
• Weak trunk flexor; it does not travel to the thigh with psoas major, and, therefore, does not act on the thigh.

Key Point:

• The muscles of the anterior abdominal wall comprise thin sheets that compress and protect the abdominal contents, and, therefore, are involved in a range of actions, including defecation and micturition, parturition, and forceful expiration of air (as in heavy breathing, coughing, or yelling).

External oblique:

• Originates from the external surfaces of ribs 5-12
• Inserts on the ilium (the anterior ½ of the iliac crest and the anterior superior iliac spine), the pubic tubercle, and, via its aponeurosis, the linea alba.
• The inferior border of this aponeurosis forms the inguinal ligament, which is a strong cord of connective tissue to which other muscles attach.
• The fibers of the external oblique extend inferomedially from their posterior origins.
• The external obliques compress the abdomen and flexes and rotates the trunk.

Internal oblique:

• Originates on the lateral 2/3rds of the inguinal ligament, the iliac crest, and the thoracolumbar fascia.
• It inserts at the linea alba, pubic crest and pectineal line, and external surfaces of ribs 9-12; it gives rise to an aponeurosis, which inserts at the midline.
• Its fibers run superomedially at an oblique angle.
• The internal oblique works with the external oblique to compress the abdomen and flex and rotate the trunk.

Rectus abdominis:

• Originates on the pubic crest and pubic symphysis of the pelvis.
• Inserts into the xiphoid process of the sternum and on the external surfaces of the costal cartilages of ribs 5-7.
• Rectus abdominis depresses the ribs and flexes the trunk.
• Rectus sheath covers anterior and posterior surfaces of rectus abdominis; it is formed by the aponeuroses of the other abdominal muscles.
• Its fibers run vertically in four segments that are separated by horizontal tendinous intersections.

Transversus abdominis:

• Originates on the lateral 1/3rd of the inguinal ligament, the iliac crest, the thoracolumbar fascia, and the external surfaces of the costal cartilages of ribs 7-12.
• Inserts along the linea alba, pubic crest, and pectineal line.
• Transversus abdominis compresses the abdomen and depresses the ribs.

Anterior scalene

• Originates on the transverse processes of C3-C7
• Inserts on the superior surface of the first rib.
• Elevates the first rib during forced inhalation, or, when the rib is fixed, flexes the neck (moves the head closer to the chest or shoulder).

Middle scalene

• Originates on the transverse processes of C2-C7.
• Inserts with the anterior scalene on the first rib.
• Elevates the first rib and flexes the neck.

Posterior scalene

• Originates on the transverse processes of C4-C6.
• Inserts on the second rib.
• Elevates rib 2; when this rib is fixed, it flexes the neck.

Diaphragm

• Originates on the inferior internal surface of the sternum, costal cartilages of ribs 6-12, and the lumbar vertebrae L1-L3.
• Inserts into the central tendon.
• Expands the thoracic cavity to decrease intrapulmonary pressure and induce inspiration (inhalation).

Three layers of intercostal muscles:

External intercostals

• Originate on the inferior border of the ribs.
• Insert into the superior border of the ribs below.
• Elevate the ribs during inhalation.

Internal and innermost intercostals

• Originate on the superior border of the ribs.
• Insert into the inferior border of the ribs above.
• Depress ribs during forced exhalation (normal exhalation requires no active effort).

Transversus thoracis

• Originates on the posterior surface of the sternum (body and xiphoid process).
• Inserts into the internal surfaces of ribs 2-6.
• Depresses ribs during exhalation.

Vertebral column (aka, spinal column, aka, spine)

• It encloses and protects the spinal cord.
• It provides attachment sites for muscles of the head, neck, and trunk.
• The individual vertebrae are named according to their region and vertical order. For example, T1 is the first vertebra of the thoracic region.
• The S- shaped curve of the vertebral column facilitate its flexibility and resilience.

Regions of the Vertebral Column:

Cervical

• 7 vertebrae within the neck.
• C1 and C2 have unique anatomical features to support the skull.
• Small bodies.
• Transverse foramen for vertebral arteries.
• Spinous processes of cervical vertebrae 2-6 are bifid (aka, bifurcated).

Thoracic

• 12 vertebrae that articulate with ribs to form posterior wall of thoracic cage.
• Bodies have costal facets for articulations with the ribs.
• Spinous processes are typically longer and angled inferiorly.
• Transverse processes have costal facets for articulation with ribs.

Lumbar

• 5 vertebrae of the lower back.
• Large bodies.
• Spinous processes are typically short, and more horizontally oriented.
• Short transverse processes.

Sacral/Coccygeal

• 5 fused sacral bones.
• 3-4 fused coccygeal bones.
• Contribute to posterior wall of the pelvis.

General Features of Vertebrae:

Vertebral body (aka, centrum)

• The disc-shaped weight-bearing portion of the vertebra.

Vertebral (aka, neural) arch

• Pedicles
• Laminae
• Transverse processes
• Spinous process
• Superior and inferior articular processes for articulation with vertebrae above and below

Vertebral foramen

• Space between body and arch
• When stacked, vertebral foramina create vertebral canal, through which spinal cord passes

Intervertebral foramen

• Where spinal nerves pass through vertebral column laterally

Intervertebral disc

• Absorbs shock and acts as a cushion between the vertebrae, and allows for spinal flexibility.
• It comprises:
  • The nucleus pulposus, which is an inner gelatinous substance
  • The anulus fibrosus is the outer ring of collagen and cartilage that encloses the nucleus pulposus

Clinical correlations:

• Distorted spinal curvatures can be caused by poor posture, aging, disease, and other factors.
• Scoliosis, which is a common curvature abnormality, is characterized by a lateral curvature of the vertebral column.
• Herniated intervertebral discs are common; they occur when a fissure in the annulus fibrosus allows the nucleus pulposus to bulge and compress nearby nerves, which can cause deficits and pain.

Deeper layer =

Erector spinae, from medial to lateral:

• Spinalis, Longissimus, and Iliocostalis.

Spinalis:

• Originates on the thoracic vertebrae and the first two lumbar vertebrae (specifically¬, from their spinous processes).
• Inserts along the vertebral column as superiorly as the 2nd and 3rd cervical vertebrae. It sometimes has a cervical portion that inserts on the base of the skull.

Longissimus:

• Originates on the transverse processes of the lumbar, thoracic, and cervical vertebrae
• Inserts along the ribs and vertebral column (as superiorly as C2).
• It also sends a portion of muscle to attach to the mastoid processes on the skull.

Iliocostalis:

• Originates on the sacrum and ilia.
• Inserts along the ribs and vertebral column as superiorly as cervical vertebrae 4-7.

Additional information about the erector spinae:

• These muscles comprise many short segments of muscle fibers.
• The erector spinae muscles are prone to injury as a result of improper heavy lifting.
• When contracted bilaterally, the erector spinae muscles extend the back;
  Unilateral contraction produces lateral flexion of the back (moves the shoulder towards the lateral hip of the same side).
• The erector spinae muscles can be further subdivided according to vertical region:
  For example, the portion of iliocostalis that lies within the lumbar region is referred to as iliocostalis lumborum; the portion of spinalis that lies with the thoracic region is referred to as spinalis thoracis, and so on.

Superficial layer =

Serratus posterior superior:

• Originates on vertebrae C7-T3 (specifically, from their spinous processes and associated connective tissues), extends inferolaterally
• Inserts as thin strips of muscle, on ribs 2-5, lateral to the erector spinae.
• Elevates ribs 2-5 during forced inspiration.

Serratus posterior inferior:

• Originates on vertebrae T12-L3
• Inserts superolaterally on ribs 9-12 (specifically, to their inferior borders).
• Depresses ribs 9-12 during forced expiration.

• Serratus posterior superior and inferior muscles are mirror images of each other.
• The thoracolumbar fascia separates the erector spinae and serratus posterior superior and inferior muscles.

Key points:

• The superficial muscles of the back stabilize and also move the pectoral girdle and humerus.
• The muscles of the superficial back are symmetrically paired, and trapezius lies superficially over the others.

Trapezius:

• Originates on the skull (the external occipital protuberance, superior nuchal line) and the spines and connective tissues of C7-T12.
• Inserts on the lateral 1/3rd of the clavicle, and on the spine and acromion of the scapula.
• Because the fibers of trapezius are convergent, trapezius adducts, rotates, elevates, or depresses the scapula, depending on which fibers contract.

Levator scapulae:

• Originates on the transverse processes of vertebrae C1-C4.
• Inserts on the medial border of the scapula.
• It elevates and rotates the scapula.

Rhomboid minor:

• Originates on the spines of vertebrae C7-T1.
• Inserts on the scapula at the medial border, adjacent to its spine.
• It adducts the scapula.

Rhomboid major

• Originates on the spines of vertebrae T2-T5
• Inserts on the medial border of the scapula.
• It adducts the scapula.

Latissimus dorsi

• Originates on the spines of vertebrae T7-T12, the thoracolumbar fascia (which overlies the lumbar vertebrae and sacrum), ribs 9-12, and the iliac crest (of the ilium).
• Inserts on the floor of the bicipital groove of the humerus.
• Adducts, extends, and medially rotates the humerus, and also depresses the scapula.

Muscles of mastication are involved in chewing.

Temporalis:

• Originates on the temporal fossa of the temporal bone and inserts on the coronoid process and anterior ramus of the mandible; it elevates and retracts the mandible.

Masseter:

• Originates on the zygomatic bone and arch and inserts on the angle and lateral ramus of the mandible, which it elevates.

Medial pterygoid has two heads:

• The deep head originates on the medial aspect of the lateral pterygoid process of the sphenoid and inserts on the medial surface of the angle and ramus of the mandible.
• The superficial head originates on the tuberosity of the maxilla and inserts with the deep head on the mandibular angle.
• The medial pterygoid elevates the mandible and moves it laterally.

The lateral pterygoid also has two heads:

• The upper head originates from the infratemporal fossa of the sphenoid and inserts on the temporomandibular joint capsule and mandible.
• The lower head originates from the lateral surface of the lateral pterygoid plate and inserts with the upper head.
• The lateral pterygoid protrudes the mandible and moves it laterally.

Movements of the mandible at the temporomandibular joint:

• Protrusion moves the mandible anteriorly; the lateral pterygoid muscles produce this movement.
• Retraction moves the mandible posteriorly; the masseter produces this movement.
• Elevation moves the mandible superiorly towards the upper teeth; temporalis, masseter, and the medial pterygoid produce this movement.
• Depression moves the mandible inferiorly.

Orbicularis oris:

• Originates on the skin and other muscles around the mouth and inserts into the skin of the lips; it closes the lips.

Zygomaticus major:

• Originates on the lateral side of the zygomatic bone and inserts into the angle of the mouth; it draws the corner (aka, angle) of the lips laterally and superiorly.

Zygomaticus minor:

• Originates on the anterior aspect of the zygomatic bone and inserts into the skin of the upper lip, which it elevates.

Levator labii superioris:

• Originates on the maxilla (specifically, from the infraorbital margin), and inserts into the skin of the upper lip, which it elevates; it also dilates (flares) the nostrils.

Buccinator:

• Originates on the surfaces of the maxilla and mandible, and inserts into the lips; it compresses the cheeks.

Risorius:

• Originates on connective tissues on the lateral sides of the face and the skin of the cheeks and inserts into the angle of the mouth; it retracts the lips at the angle of the mouth.

Mentalis:

• Originates on the body of the mandible and inserts into the skin of the chin and lower lip, which it elevates and protrudes.

Depressor labii inferioris:

• Originates on the mandible (specifically, from the anterolateral body) and inserts into the skin of the lower lip, which it depresses.

Additional Information:

• These muscles blend into one another
• Cranial nerve VII innervates them.

Nasalis:

• Originates on the maxilla and inserts on the nasal cartilage at the midline; it flares the nostrils.

Corrugator supercilii muscles:

• Originates on the frontal bone (specifically, from the medial supraorbital margin) and insert on the medial half of the eyebrows and laterally on the supraorbital margin on the frontal bone; they depress the eyebrows (and pull them medially).

Orbicularis oculi:

• Originates on the medial margin of the orbit (specifically, from the maxillary, frontal, and lacrimal bones), and inserts into the rim and skin of the orbit and eyelids; it moves the eyelids and surrounding skin to squint and close the eyes.

Frontalis:

• Originates on the frontal bone and skin of the eyebrows, and inserts into the galea aponeurotica (aka, the epicranial aponeurosis); it elevates the eyebrows, which wrinkles the forehead horizontally.

Additional Information:

• Cranial nerve VII (the facial nerve) innervates these muscles.
• Bell’s palsy occurs when CN VII is damaged; Belly’s palsy results in asymmetrical paralysis of the muscles of facial expression.