Anatomy of Abdominal Muscles

Muscles of the Anterior Abdominal Wall

• The muscles of the anterior abdominal wall consist of three broad, thin sheets of muscle that are most pronounced laterally and become aponeurotic anteriorly.
• The four main muscles are: external oblique, internal oblique, transversus abdominis, and rectus abdominis.

External Oblique

• Origin: lower eight ribs
• Insertion: Xiphoid process, linea alba, iliac crest, pubic crest, and tubercle
• Action: support and compress abdominal contents, assist in flexion and rotation of trunk, assist in forced expiration, micturition, defecation, and vomiting
• Innervation: lower six thoracic nerves, iliohypogastric and ilioinguinal nerves (L1)

Internal Oblique

• Origin: iliac crest and lateral 2/3 of inguinal ligament
• Insertion: lower three ribs, Xiphoid process, linea alba, and symphysis pubis
• Action: same as external oblique
• Innervation: same as external oblique

Transversus Abdominis

• Origin: lower six costal cartilages, iliac crest, and lateral 2/3 of inguinal ligaments
• Insertion: Xiphoid process, linea alba, and symphysis pubis
• Action: compress abdominal contents
• Innervation: same as external oblique

Conjoint Tendon

• Formed by the lowest tendinous fibers (aponeurosis) of internal oblique and transversus abdominis muscles
• Attaches medially to the linea alba, but has a lateral free border
• Has an essential role in protecting a weak area in the abdominal wall, where a weakening of the conjoint tendon may lead to a direct inguinal hernia

Rectus Abdominis

• Origin: symphysis pubis and pubic crest
• Insertion: 5th, 6th, and 7th costal cartilages and xiphoid process
• Action: compress, flex vertebral column, and accessory muscle of expiration
• Innervation: lower six thoracic nerves

Rectus Sheath

• The composition of the walls of the rectus sheath changes at different levels
• The rectus sheath is generally considered at three levels: above the costal margin, between the costal margin and the arcuate line, and between the level of the arcuate line and the pubis
• The aponeuroses of the external oblique, internal oblique, and transversus abdominis muscles form the rectus sheath

Linea Alba

• A fibrous band that separates the two rectus sheaths from each other
• Extends from the xiphoid process to the symphysis pubis
• Formed by the fusion of the aponeuroses of the lateral muscles of the two sides

Abdominal Aorta

• Enters the abdomen through the aortic aperture of the diaphragm (T12)
• Branches to the diaphragm (inferior phrenic), abdominal wall (lumbar, median sacral), abdominal viscera, kidneys, and ovaries/testes
• Bifurcates at L4 into common iliac arteries to supply the lower limb and pelvis

Arterial Branches of Abdominal Aorta

• Celiac trunk: the first branch of the abdominal aorta, supplying the stomach, spleen, and liver
• Superior mesenteric artery: supplying most of the small intestine and the first half of the large intestine, or colon
• Inferior mesenteric artery: a small, unpaired artery supplying the second half of the large intestine
• Renal arteries: serving the kidneys
• Gonadal arteries: serving the ovaries and testes
• Lumbar arteries: serving the heavy muscles of the abdomen and trunk walls

Inferior Vena Cava

• Forms at L5 vertebral level when the left and right common iliac veins unite
• Drains the structures of the posterior abdominal wall, the kidneys, and the suprarenal glands
• The veins that drain blood from the gut tube pass their blood into the portal system, not the IVC
• Blood from the portal system passes through the liver before entering the IVC via hepatic veins
• The IVC passes from the liver and through the diaphragm at T8 before entering the inferior surface of the right atrium

Learn about the muscles of the anterior abdominal wall, including the external oblique, internal oblique, transversus abdominis, and rectus abdominis.