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Index | Bones
of the thoracic wall
Last updated 30 March 2006
The Anterolateral Abdominal Wall
- Most of the abdominal wall is muscular and extends
between the thoracic cage and the bony pelvis.
- There are four important paired
muscles in the anterior abdominal wall: three flat muscles (external
oblique, internal oblique, and transversus abdominis) and
one strap-like muscle
- The combination of muscles and aponeuroses in the
anterior abdominal wall affords considerable protection
to the abdominal viscera, especially when the muscles are
in good physical condition.
- The flat muscles cross each other in such a way (similar
to a three-ply corset) that strengthens the abdominal
wall and diminishes the risk of protrusion of viscera
(herniation) between the muscle bundles.
External Oblique Muscle (pp. 132-3)
- This is the largest and most superficial of the three
flat abdominal muscles. It is located in the
anterolateral aspect of the abdominal wall.
- Its fleshy part forms the anterolateral portion and its aponeurosis forms the anterior
- Its fibres run inferoanteriorly and medially in the same
direction as do the extended digits when they are in
one's side pockets.
- Origin: external surfaces of 5th to 12th
- Insertion: linea alba, pubic tubercle and anterior half
of the iliac crest.
- Innervation: inferior six thoracic nerves and subcostal
- As the fibres pass medially, they become aponeurotic.
This aponeurosis ends medially in the linea
- Inferiorly, it folds back on itself to form the inguinal ligament between the
anterior superior iliac spine and the pubic tubercle.
- Medial to the pubic tubercle, the external oblique
aponeurosis is attached to the pubic
crest. Some fibres of the inguinal ligament
cross the linea alba and attach to the opposite pubic
crest. These fibres form the reflex
- Just superior to the medial part of the inguinal
ligament, there is an opening in the aponeurosis called
the superficial inguinal ring.
Internal Oblique Muscle (pp. 133, 135)
- This is the intermediate layer of the three flat
- Origin: thoracolumbar fascia, anterior two-thirds of
iliac crest, and lateral half of inguinal ligament.
- Insertion: inferior borders of 10th to 12th
ribs, linea alba, and the pubic via the conjoint tendon.
- Innervation: ventral rami of inferior six thoracic and
first lumbar nerves.
- Its fibres also become aponeurotic and the aponeurosis
splits to form a sheath for the rectus abdominis muscle.
- The inferior fibres of the aponeurosis arch over the spermatic cord as it lies in
the inguinal ring to
attach to the pubic crest and pecten pubis.
- The most inferior tendinous fibres of the internal
oblique muscle join the aponeurotic fibres of the
transversus abdominis muscle to form the conjoint tendon, which turns
inferiorly to insert into the pubic crest and pecten
Transversus Abdominis Muscle (p. 135)
- This is the innermost of the three flat abdominal
- Origin: internal surfaces of 7th to 12th
costal cartilages, thoracolumbar fascia, iliac crest, and
lateral third of inguinal ligament.
- Insertion: linea alba with aponeurosis of internal
oblique, pubic crest, and pecten pubis via conjoint
- Innervation: ventral rami of inferior six thoracic and
first lumbar nerves.
- Its fibres run more or less horizontally, except to those
of the internal oblique muscle. Muscle fibres of the
transversus abdominis end in an aponeurosis which
contributes to the formation of the rectus sheath.
Actions of the Three Flat Abdominal Muscles
- The anterolateral abdominal wall is unsupported and
unprotected by bone. However the three-ply structure of
its flat muscles and the extensive aponeuroses form a
strong expandable support, which provides considerable protection for the abdominal viscera.
- Normally, quite rhythmic movements of the anterolateral
abdominal wall accompany respiration. The anterolateral
abdominal wall expands as its muscles relax. During
expiration, the anterolateral abdominal wall passively
- However, in the forced expiration that occurs during
coughing, sneezing, vomiting, and straining, all the
anterior abdominal muscles act strongly in compressing
the abdominal contents.
- Acting together, the
flat abdominal muscles increase the intraabdominal
pressure. This action produces the force required for
defecation, micturition (urination), and parturition
- Acting separately, the
flat abdominal muscles move the trunk. If the pelvis is
fixed, both external oblique muscles can flex the trunk.
Acting separately, one external oblique muscle can
laterally flex the trunk and rotate it to the opposite
- If the thorax if fixed, both external oblique muscles
tilt the anterior part of the pelvis superiorly and flex
- Similarly, when the pelvis is fixed, one internal oblique
muscle can flex the trunk and rotate it to the same side.
- If the thorax is flexed, one internal oblique muscle can
laterally flex the trunk and rotate the pelvis to the
Rectus Abdominis Muscle (p. 136)
- This is a long, broad, strap-like muscle and is the principle vertical muscle of
the anterior abdominal wall.
- Origin: pubic symphysis and pubic crest.
- Insertion: xiphoid process and 5th to 7th
- innervation: ventral rami of inferior six thoracic
- The two muscles are separated by
the linea alba and lie close together
- The rectus abdominis is three times as wide superiorly as
it is inferiorly.
- The lateral border of the rectus muscles and its sheath
are convex and form a clinical important surface marking
known as the linea semilunaris.
- Most of the rectus abdominis muscle is enclosed in the rectus sheath formed by the
aponeuroses of the three flat abdominal muscles.
- The anterior layer of the rectus sheath is firmly
attached to the rectus muscle at three or more tendinous intersections. When
this muscle is tensed in muscular persons, each stretch
of muscle between the tendinous intersections is
indicated by grooves in the skin between the muscle
- They are usually located at the level of the xiphoid
process, umbilicus, and halfway between this structures.
Actions of the Rectus Abdominis Muscles (p.136)
- In addition to helping the other abdominal muscles to
compress the abdominal viscera, these muscles depress the
ribs and stabilise the pelvis
- The fixation of the pelvis enables the thigh muscles to
- Similarly, during lower limb lifts from the supine
position, the rectus abdominis muscles contract to
prevent tilting of the pelvis by the weight of the limbs.
Linea Alba and Rectus Sheath (pp. 136-7)
- The rectus sheath is the strong,
incomplete fibrous compartment for the rectus abdominis
- It forms by the fusion and separation of the aponeurosis
of the flat abdominal muscles.
- At its lateral margin, the internal oblique aponeurosis
splits into two layers, one passing anterior to the
rectus muscle and the other passing posterior to it.
- The anteriorly layer joins with the aponeurosis of the
transverse abdominis muscle to form the posterior wall of
the rectus sheath.
- The fibres of the anterior and posterior wall of the
rectus sheath interlace in the medial line to form a
complex tendinous raphe, called the linea
alba, which is an intermixture of the
aponeurotic fibres of the oblique and transverse
- It is narrow inferior to the umbilicus, but is wide
superior to it. The groove is visible in the skin
superficial to it in thin muscular persons.
- The linea alba lies between the two parts of the rectus
abdominis muscle; the umbilicus is located just inferior
to its midpoint.
- Superior to the costal margin, the posterior wall of the
rectus sheath is deficient because the transversus
abdominis muscle passes internal to the costal cartilages
and the internal oblique muscle is attached to the costal
margin. Hence, superior to the costal margin, the rectus
muscle lies directly on the thoracic wall.
- The inferior one-fourth of the rectus sheath is also
deficient because the internal oblique aponeurosis does
not split to enclose the rectus muscle.
- A crescentic border called the arcuate
line marks the inferior limit of the
posterior wall of the rectus sheath.
- The position of this line is usually midway between the
umbilicus and the pubic crest.
- Inferior to the arcuate line, the aponeuroses of the
three flat muscles pass anterior to the rectus muscle to
form the anterior layer of the rectus sheath.
Nerves of the Anterior Abdominal Wall
- The skin and muscles of the anterior abdominal wall are
supplied mainly by the ventral rami of the inferior six thoracic nerves
(i.e., the continuation of the inferior
intercostal nerves, T7 to T11) and the subcostal nerve (T12).
- The inferior part of the abdominal wall is supplied by
two branches of the ventral ramus of the first lumbar
nerve via the iliohypogastric and ilioinguinal nerves.
- The main trunks of the intercostal
nerves pass anteriorly from the intercostal
spaces and run between the internal oblique and
transversus abdominis muscles.
- The plane between these muscles, known as the neurovascular plane,
corresponds with a similar plane in the intercostal
- The inferior intercostal, subcostal and lumbar arteries
accompany the nerves of this plane. The common nerve
supply of the skin and the muscles of the anterolateral
wall explain why palpating the abdomen with cold hands
causes the muscles of the abdominal wall to contract.
Posterior Abdominal Wall
- The posterior abdominal wall is composed principally of
muscles and fascia attached to the vertebrae, hip bones,
- It also contains fat, important nerves, vessels, and
Muscles of the Posterior Abdominal Wall
- There are three paired muscles in the posterior abdominal
wall that are clinically important: psoas major, iliacus, and
Psoas Major and Iliacus Muscles
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to the Iliopsoas muscle.
Lumborum Muscle (p. 230)
- This quadrilateral muscle forms a thick muscular sheet in
the posterior abdominal wall.
- It lies adjacent to the transverse processes of the
lumbar vertebrae and is broader inferiorly.
- Superior attachments: medial half of inferior border of
12th rib and tips of lumbar transverse
- Inferior attachments: iliolumbar ligament and internal
lip of the iliac crest.
- Innervation: ventral branches of T12 and L1 to L4
- The quadratus lumborum extends and laterally flexes the
vertebral column, and fixes the 12th rib