Internal abdominal oblique muscle

The internal abdominal oblique muscle is a broad, flat muscle of the anterolateral abdominal wall situated deep to the external oblique and superficial to the transversus abdominis. Its fibres run superomedially, perpendicular to those of the external oblique, contributing to the strength and support of the abdominal wall.

Origin

The internal oblique muscle originates from the thoracolumbar fascia, the anterior two-thirds of the iliac crest, and the lateral half of the inguinal ligament.

Insertion

The muscle fibres insert into the inferior borders of ribs 10-12, the linea alba, and the pubic crest and pecten pubis via the conjoint tendon.

Blood Supply

The internal oblique muscle is supplied by the lower posterior intercostal arteries, subcostal artery, deep circumflex iliac artery, and inferior epigastric artery

Innervation

The muscle is innervated by the thoracoabdominal lower intercostal nerves (T7-T11), subcostal nerve (T12), and the iliohypogastric and ilioinguinal nerves (L1).

Actions / Functions

The muscle compresses and supports the abdominal viscera, assists in forced expiration, flexes the trunk bilaterally, and produces ipsilateral trunk rotation and lateral flexion.

Role in the Rectus Sheath

The aponeurosis of internal abdominal oblique muscle splits into two lamina, around the rectus abdominis, above the arcuate line, such that:

• Anterior lamina passes in front of rectus abdominis muscle, while

• Posterior lamina passes behind the rectus abdominis muscle

Thus contributing to anterior and posterior rectus sheath layers (above the arcuate line).

Contribution to the cremaster muscle and fascia

The lower arching fibres of the internal oblique abdominal muscle contribute to formation of cremaster muscle and cremasteric fascia. These structures surround the spermatic cord and are involved in the cremasteric reflex.

Conjoint Tendon (inguinal falx)

The lower aponeurotic fibres of the internal oblique muscle, along with those from the transversus abdominis muscle, form a combined tendon called the conjoint tendon (or the inguinal falx) that arch over the roof of inguinal canal and pass behind it. This reinforces the the posterior wall of the inguinal canal medially.