Internal oblique

Musculus obliquus internus abdominis

Also known as: internal abdominal oblique, conjoint tendon, cremaster

The intermediate flat abdominal muscle, with fibres at right angles to the external oblique (superomedially). Its lowest fibres form the cremaster and, with transversus, the conjoint tendon.

Overview

Internal oblique arises from the thoracolumbar fascia, the anterior two-thirds of the iliac crest and the lateral half (or two-thirds) of the inguinal ligament. Its fibres fan superomedially, at right angles to those of external oblique. The posterior fibres insert into the lower borders of ribs 10–12. The rest form an aponeurosis that splits around rectus abdominis above the arcuate line and joins the linea alba. Its lowest fibres arch over the spermatic cord and fuse with the transversus aponeurosis to form the conjoint tendon (falx inguinalis), which inserts on the pubic crest and pecten pubis. The conjoint tendon reinforces the posterior wall of the inguinal canal behind the superficial ring.

As the testis descends, some lower fibres are carried with it to form the cremaster muscle and cremasteric fascia. Cremaster is supplied by the genital branch of the genitofemoral nerve (L1–L2). Like external oblique, internal oblique compresses the abdomen and flexes the trunk, but it rotates the trunk to the same side. The ilioinguinal and iliohypogastric nerves (L1) run between it and transversus abdominis before piercing it.

Parts & subdivisions

Conjoint tendon (falx inguinalis)

Fused lower aponeuroses of the internal oblique and transversus abdominis, inserting on the pubic crest and pecten pubis. It strengthens the medial part of the posterior wall of the inguinal canal.

Cremaster muscle

Loops of internal oblique fibres around the spermatic cord. They raise the testis (cremasteric reflex, L1–L2).

Anatomy

Origin

Thoracolumbar fascia, anterior two-thirds of the iliac crest and the lateral half of the inguinal ligament.

Insertion

Inferior borders of ribs 10–12, linea alba (via the aponeurosis) and the pubic crest and pecten pubis (via the conjoint tendon).

Action

Compresses the abdomen; flexes the trunk; laterally flexes it and rotates it to the same side.

Innervation

Thoraco-abdominal nerves (T7–T11), subcostal nerve (T12), iliohypogastric and ilioinguinal nerves (L1).

Nerve roots

T7–L1

Arterial supply

Lower posterior intercostal and subcostal arteries; superior and inferior epigastric arteries; deep circumflex iliac artery.

Relations

Lies between the external oblique and transversus abdominis. The main neurovascular plane of the abdominal wall lies deep to it, between it and transversus.

Clinical significance

Cremasteric reflex

Stroking the upper medial thigh (ilioinguinal or genitofemoral afferents) causes contraction of cremaster on the same side (L1–L2). The reflex is typically absent in testicular torsion.

Transversus abdominis plane (TAP) block

Local anaesthetic injected into the plane between the internal oblique and transversus abdominis blocks the T7–L1 nerves for analgesia of the abdominal wall.