Thoracic Wall Anatomy

Thoracic Wall

• The thoracic wall defines the thoracic cavity.
• The thoracic wall includes a superior and inferior thoracic aperture.
• The thoracic wall consists of thoracic vertebrae, intervertebral discs, ribs, and the sternum.

Muscles of Thoracic Wall

• Intercostal muscles
• Sternocostalis
• Subcostales

Intercostal Spaces

• They exist between adjacent ribs.
• There are eleven paired intercostal spaces which are filled by intercostal muscles.
• Each intercostal space contains intercostal vessels and nerves that pass between the internal intercostal muscle and innermost intercostal muscle.

Intercostal Muscles

• Three flat muscles are located in each intercostal space.
• These include the external intercostal (most superficial), the internal intercostal (intermediate), and the innermost intercostal (deepest).

External Intercostal Muscle

• It attaches to the inferior border of the rib above and the superior border of the rib below.
• Fibers run downward and forward.
• It spans from the costal cartilages to the external intercostal membrane in each intercostal space.
• It is most active during inspiration, supporting intercostal spaces and moving ribs superiorly.

Internal Intercostal Muscle

• It attaches to the costal groove of the rib above and the superior border of the rib below.
• Fibers run downward and backward.
• It extends from the sternum to the rib angles, continuing as the internal intercostal membrane.
• It is most active during expiration, moving ribs inferiorly and supporting intercostal spaces.

Innermost Intercostal Muscle

• Each muscle attaches to the internal aspects of two adjacent ribs.
• It is internally related to the endothoracic fascia and parietal pleura.
• It depresses the ribs during forced expiration.
• All intercostal muscles are innervated by corresponding intercostal nerves.

Sternocostalis

• It lies on the lower inner surface of the anterior thoracic wall.
• Originates from the lower part of the sternum's posterior surface.
• Its insertion is the costal cartilages from the 2nd to 6th.
• Its action is to depress the costal cartilages.

Subcostales

• It lies on the inner aspect of the posterior part of intercostal spaces.
• Originates from the internal surface of a rib, near its angle.
• Inserts into the internal surface of the second or third rib below.
• Its action depresses ribs during forced expiration.
• Both sternocostalis and subcostales align at the innermost intercostal muscle level and lie deep to the internal intercostal muscles.

Diaphragm

• The diaphragm is a musculotendinous structure.
• It fills the inferior thoracic aperture and separates the thoracic cavity from the abdominal cavity.

Diaphragm Shape

• Includes a muscular periphery and a central tendon (flat tendon).
• The upper surface is convex, facing the thorax.
• The lower surface is concave, facing the abdomen.
• There are two domes; the right is slightly higher due to the liver, supporting the lungs.
• The central tendon supports the heart.

Diaphragm Origin

• It originates from the margin of the inferior thoracic aperture.
• The sternal components originate from the xiphoid process.
• Costal parts come from the lower six costal cartilages and ribs on each side.
• Vertebral origin is from the lumbar vertebrae, via the right and left crus, and arcuate ligaments.

Diaphragm Insertion

• It inserts into a central tendon.
• Nerve supply comes from the right and left phrenic nerves.
• Injury to the phrenic nerve results in diaphragmatic paralysis.
• The diaphragm is the most important muscle for respiration, pulling downwards during contraction to increase thoracic volume during inspiration.
• Then during expiration, the diaphragm relaxes and moves superiorly.

Diaphragm Apertures

• The diaphragm contains apertures to allow structures to pass between the thorax and abdomen.
• The vena caval aperture for the IVC is at the T8 vertebral level.
• The esophageal aperture is at the T10 vertebral level.
• The aortic aperture sits at the T12 vertebral level.

Abdominal Wall

• The abdominal wall encloses the abdominal cavity and divides into anterolateral and posterior walls.

Anterolateral Abdominal Wall Structure

• Skin
• Superficial fascia
• Anterolateral muscles
• Transversalis fascia
• Extraperitoneal fascia
• Parietal peritoneum

Anterolateral Abdominal Wall Muscles

• Divided into flat and vertical muscles.

Flat Muscles

• Three flat muscles are situated laterally on each side of the abdomen, arranged in layers from superficial to deep.
• These include the external oblique, internal oblique, and transversus abdominis.

Vertical Muscles

• Two vertical muscles lie on either side of the midline anteriorly.
• These include the rectus abdominis and pyramidalis muscles.

Flat Muscles Origin

• External oblique muscle originates from the outer surfaces of the lower eight ribs.
• Internal oblique muscle originates from the iliac crest.
• Transversus abdominis muscle originates from the lower six costal cartilages and iliac crest.

Fibre Direction

• External oblique muscle fibers run inferomedially.
• Internal oblique muscle fibers run superomedially.
• Transversus abdominis fibers run horizontally and medially.

Flat Muscles Insertion

• The anterior part of each flat muscle forms an aponeurosis (a broad, flat tendon).
• The aponeuroses of all the flat muscles cover the rectus abdominis muscle, forming the rectus sheath.
• At the midline, the aponeuroses entwine to form the linea alba.
• Flat muscles support and protect the abdominal viscera.
• They facilitate lateral flexion of the trunk.
• They increase intra-abdominal pressure for activities such as defecation or delivery.
• The lower six thoracic nerves and the L1 nerve provide nerve supply.
• The external oblique muscle's aponeurosis forms the inguinal ligament at its lower border.
• A defect in its aponeurosis forms the superficial inguinal ring.

Rectus Abdominis Muscle

• Originates at the symphysis pubis.
• Inserts at the costal margin.
• It is separated from its counterpart by the linea alba.
• Divided into segments by tendinous intersections.
• It is enclosed by the rectus sheath.
• Its action is flexion of the trunk.
• Its nerve supply comes from the lower six thoracic nerves.

Pyramidalis Muscle

• A small, triangular muscle, anterior to the lower part of the rectus abdominis.

Posterior Abdominal Wall Muscles

• The posterior abdominal wall muscles include the psoas major and minor muscles (medially), the quadratus lumborum muscle (laterally), and the iliacus muscle (inferiorly).

Psoas Major

• Originates from the lumbar vertebrae.
• Inserts on the lesser trochanter of the femur.
• Receives nerve supply from the lumbar plexus.
• It flexes the thigh on the trunk or flexes the trunk on the thigh.

Quadratus Lumborum

• Originates from the iliac crest.
• Inserts on the 12th rib.
• Receives nerve supply from the lumbar plexus.
• Depresses and stabilizes the 12th rib.

Iliacus

• Originates from iliac fossa.
• Inserts on the lesser trochanter of the femur, via the iliopsoas.
• Receives nerve supply from branches of the femoral nerve.
• It acts similarly to the psoas major.

Pelvic Muscles

• The Obturator internus muscle is located on the lateral pelvic wall.
• Pelvic diaphragm muscles are formed by the Levator ani and Coccygeus muscles.
• These muscles support and maintain the pelvic viscera in position.