External oblique !

The external oblique is the outer layer of the side abdominal wall; it bends the trunk forward and sideways and turns it to the other side.

Official name: External abdominal oblique muscle

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What it is

The outer layer of the side abdominal wall, with fibres running down and forward.

What it does

Bends the trunk forward and to the side, and turns it to the opposite side.

Key facts

Origin: External surfaces of ribs 5–12
Insertion: Linea alba, pubic tubercle, anterior half of iliac crest; lower free border forms the inguinal ligament
Action: Flexes trunk; lateral flexion to same side; rotates trunk to opposite side; compresses abdomen
Nerve: Thoracoabdominal nerves (T7–T11) and subcostal nerve (T12)
Blood supply: Lower posterior intercostal, subcostal, superior and inferior epigastric, deep circumflex iliac arteries

Relations

Superficial: Subcutaneous fat (Camper and Scarpa fascia) with the superficial epigastric vessels, and skin
Deep: Internal oblique, whose fibres cross at roughly a right angle
Above: Its upper slips interlock with serratus anterior on the middle ribs and its lower slips with latissimus dorsi
Behind: Its free posterior border, latissimus dorsi and the iliac crest bound the inferior lumbar triangle (of Petit)
Below: Inguinal ligament; above the pubic tubercle the superficial ring lets out the spermatic cord (or the round ligament of the uterus) with the ilioinguinal nerve

How to find it

In a lean person its slips are seen on the lower lateral ribs, interlocking with serratus anterior. It hardens during a twisting curl-up, when the shoulder moves toward the opposite hip. The superficial ring is felt by pushing a fingertip up along the spermatic cord through the scrotal skin, just above the pubic tubercle.

Examination

Cough impulse at the superficial inguinal ring: a fingertip at the ring feels a bulge or impulse on coughing when an inguinal hernia is present

On imaging

On ultrasound and CT it is the outermost of the three flat muscles of the flank, becoming aponeurosis toward the midline and groin. In the groin, ultrasound with straining shows inguinal hernias: an indirect hernia starts lateral to the inferior epigastric vessels and runs along the canal under its aponeurosis. CT shows lumbar and Spigelian hernias and tears or haematoma after trauma.

Common problems

Delayed or weak activation is seen in some people with chronic low back pain.

How it is diagnosed

Exam; ultrasound can show its activation.

Treatment

Core stability exercise programmes.

Clinical notes

Open inguinal hernia repair opens its aponeurosis along the line of its fibres, taking care of the ilioinguinal nerve lying just beneath
In a gridiron (McBurney) incision its fibres are split in their own direction, not cut, followed by the internal oblique and transversus abdominis
A lumbar hernia can come through the inferior lumbar triangle at its posterior border
In anterior component separation for large midline hernias its aponeurosis is divided just lateral to the rectus sheath

Memory aid

External oblique fibres run like hands slipped into the front trouser pockets: down and toward the midline; the internal oblique runs at right angles to them.

Test yourself

Think of the answer, then tap the question to see it.

What does the lower free border of the external oblique aponeurosis form?

The inguinal ligament, from the ASIS to the pubic tubercle.

What is the superficial inguinal ring?

A triangular gap in the external oblique aponeurosis just above and lateral to the pubic tubercle.

Which way does the external oblique rotate the trunk?

To the opposite side; it also flexes the trunk, flexes it to the same side and compresses the abdomen.

Which part of the inguinal canal does the external oblique aponeurosis form?

The anterior wall, along its whole length.

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2): musculus obliquus externus abdominis TA2 2364
Official Spanish (DeCS): Músculo oblicuo externo del abdomen DeCS D000009
English (TA2): External abdominal oblique muscle
Also called: external oblique muscle

Sources: Terminologia Anatomica (TA2); DeCS; Anatomy, Anterolateral Abdominal Wall Muscles.

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Representative model, for educational purposes only.

External oblique highlighted on the Durgical 3D model