Muscles of the Abdominal Wall

The abdomen is the region of trunk situated between diaphragm below and the inlet of the pelvis below. It accommodates a large number of organs and its muscles strengthen or give proper shape to the human skeleton.
From exterior to interior the muscles of the anterior abdominal wall are the external oblique, internal oblique, and transversus abdominis, which are aponeurotic in front. In addition to these, it also contains a wide vertical thin sheet called rectus abdominis, which is separated from its compartments by means of linea alba. The posterior abdominal wall consists of psoas major, quadratus lumborum and iliacus muscle.

Anterior Abdominal Wall

As we discuss earlier the anterior abdominal wall consist of the external oblique, internal oblique, and the transversus abdominis. Their details are as follow:

• External Oblique

It's miles a skinny, wide muscular sheet, arise from the outer surfaces of the lower eight ribs, which fans ahead laterally to be inserted into the xiphoid process,  pubic crest (bone), pubic tubercle (bone), linea alba and anterior half of the iliac crest. The aponeurosis of the external oblique incorporates superficial inguinal ring and additionally forms the inguinal ligament, pectineal ligament and the lacunar ligament (connects inguinal ligament to the pectineal ligament).

Nerve supply: It is innervated by lower six thoracic and lumbar nerves.

Action: It compresses abdominal contents and helps in micturition, defecation, and parturition.

• Internal Oblique

It is also a thin muscular sheet, which lies deep to the external oblique, its fibers fan perpendicular to that of external oblique. Internal oblique arises from the lumbar fascia, anterior two third of the iliac crest, lateral two third of the inguinal ligament. They move upward and forward to be inserted into the lower borders of the lower three ribs and their intercostal cartilages, xiphoid process, linea alba and pubic symphysis.

Nerve supply: It is innervated by lower six thoracic and lumbar vertebrae.

Action: Similar as external oblique.

• Transversus Abdominis

It is the deepest muscle among all, which arise from the costal cartilages of the lower six ribs, lumbar fascia, iliac crest and lateral two third of the inguinal ligament, which is inserted into the xiphoid process, linea alba, and pubic symphysis. Its fibers move forward horizontally and contain the deep inguinal ring.

Nerve supply: Lower six thoracic and lumbar nerves.

Action: Similar as external and internal oblique.

• Rectus Abdominis

The rectus abdominis is long strap muscle that arises from the front of the pubic crest and pubic symphysis. It is inserted into the fifth, sixth, and seventh costal cartilages and the xiphoid process. It is divided into three distinct segments by three transverse tendinous intersections: one at the level of xiphoid process, one at the level of the umbilicus and another halfway between these two. These intersections are strongly attached to the anterior wall of the rectus sheath.

Nerve supply: It is innervated by lower six thoracic nerves.

Action: It pulls the ribs and the pelvis in and curves back.

Posterior Abdominal Wall

The posterior abdominal wall consist of following muscles:

• Psoas Major

The psoas major arises from the transverse processes, bodies, and intervertebral discs of 12th thoracic to 5th lumbar vertebrae. Its fibers move downward laterally to be inserted into the lesser trochanter of the femur, It is enclosed in a fibrous sheath that is derived from the lumbar fascia, it forms medial arcuate ligament superiorly.

Nerve supply: It is innervated by the lumbar plexus.

Action: It flexes the thigh on the trunk and if thigh is fixed flexes trunk on the thigh.

• Quadratus lumborum

  It is quadrilateral shaped, flat muscle that lies obliquely with vertebral column. It arises from the iliolumbar ligament, which is the adjoining part of the iliac crest and the transverse process of the lower lumbar vertebrae. It moves upward medially to be embedded into the 12th rib and upper four lumbar vertebrae.

Nerve supply:  It is innervated by the lumbar plexus.

Action: It boosts or recessed the 12th rib for the duration of respiration and laterally flexes the vertebral column to the same side.

• Iliacus 

It is fan-shaped muscle which arises from the upper surface of the iliac fossa. The fibers of iliac fossa join the lateral side of the psoas tendon to be inserted into the lesser trochanter of the femur, which is commonly called iliopsoas muscles.

Nerve supply: This muscle is supplied by femoral nerve, a branch of the lumbar nerve.

Action: It flexes the thigh on the trunk at the hip joint and if thigh is fixed then flexes trunk on the thigh.