The Abdomen

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Main Anatomy 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 (rectus abdominis).
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.

The 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 part.
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 5^th to 12^th ribs.
Insertion: linea alba, pubic tubercle and anterior half of the iliac crest.
Innervation: inferior six thoracic nerves and subcostal nerves.

As the fibres pass medially, they become aponeurotic. This aponeurosis ends medially in the linea alba.
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 inguinal ligament.
Just superior to the medial part of the inguinal ligament, there is an opening in the aponeurosis called the superficial inguinal ring.

The Internal Oblique Muscle (pp. 133, 135)

This is the intermediate layer of the three flat abdominal muscles.

Origin: thoracolumbar fascia, anterior two-thirds of iliac crest, and lateral half of inguinal ligament.
Insertion: inferior borders of 10^th to 12^th 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 pubis.

The Transversus Abdominis Muscle (p. 135)

This is the innermost of the three flat abdominal muscles.

Origin: internal surfaces of 7^th to 12^th 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 tendon.
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 (pp. 135-6)

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 sinks in.
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 (childbirth).
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 side.

If the thorax if fixed, both external oblique muscles tilt the anterior part of the pelvis superiorly and flex the trunk.
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 opposite side.

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 5^th to 7^th costal cartilages.
innervation: ventral rami of inferior six thoracic nerves.

The two muscles are separated by the linea alba and lie close together inferiorly.
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 bulges.
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 during walking.
The fixation of the pelvis enables the thigh muscles to act effectively.
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.

The Linea Alba and Rectus Sheath (pp. 136-7)

The rectus sheath is the strong, incomplete fibrous compartment for the rectus abdominis muscle.
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 abdominal muscles.
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 (pp. 137-8)

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 spaces.
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, and ribs.
It also contains fat, important nerves, vessels, and lymph nodes.

Muscles of the Posterior Abdominal Wall

There are three paired muscles in the posterior abdominal wall that are clinically important: psoas major, iliacus, and quadratus lumborum.

Psoas Major and Iliacus Muscles

Click here to go to the Iliopsoas muscle.

Quadratus 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 12^th rib and tips of lumbar transverse processes.
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 12^th rib during inspiration.