Internal abdominal oblique muscle

At a glance

English

Internal abdominal oblique muscle

Latin (TA2)

Musculus obliquus internus abdominis

System

Muscle

Region

Trunk

Side

Paired, left and right

Arises from

The costal cartilage of tenth rib, eleventh rib, hip bone and tenth rib

About the internal abdominal oblique muscle

The lateral abdominal wall is built from three muscle sheets whose fibres cross one another, rather like plywood. The internal oblique is the middle layer: the external oblique covers it, and transversus abdominis lies deep to it. Its fibres run upward and forward, roughly at right angles to those of the external oblique.

The muscle arises along a curved line low on the trunk: from the thoracolumbar fascia behind, from the front two-thirds of the iliac crest and from the outer half of the inguinal ligament. From this base the hindmost fibres climb to the lower borders of the last three or four ribs and their cartilages. The middle fibres fan forward into a broad aponeurosis that splits around rectus abdominis above the arcuate line, helping to form both layers of the rectus sheath, and ends in the linea alba. The lowest fibres arch over the inguinal canal and join those of transversus abdominis to form the conjoint tendon, which attaches to the pubic crest and pectineal line.

Acting together on both sides, the internal obliques flex the trunk and compress the abdomen. Squeezing the abdominal organs pushes the diaphragm up into the chest, so the muscle helps in forced breathing out, coughing and straining. On one side it bends the trunk sideways and turns it towards the same side, which is why it is called a same-side rotator. It does this together with the external oblique of the opposite side; for example the right internal and left external obliques act as the left shoulder is brought towards the right hip.

Its nerve supply comes from the anterior rami of the seventh to twelfth thoracic nerves and from the first lumbar nerve through the iliohypogastric and ilioinguinal nerves. Its arteries are the lower posterior intercostal and subcostal arteries, the superior and inferior epigastric arteries and the deep circumflex iliac artery. The arching lower fibres help guard the inguinal canal, and weakness here is linked with inguinal hernia.

Attachments, action and supply

Origin

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

Insertion

Lower borders of the tenth to twelfth ribs and their cartilages, the linea alba via its aponeurosis, and the pubic crest and pectineal line via the conjoint tendon.

Action

Compresses the abdomen and flexes the trunk; on one side bends the trunk to that side and rotates it to the same side.

Nerve supply

Thoracoabdominal nerves (T7-T11), subcostal nerve (T12), iliohypogastric and ilioinguinal nerves (L1).

Blood supply

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