External oblique

The 'hands in pockets' muscle

Abdomen · outermost lateral flat muscle

The largest and most superficial of the three flat muscles of the side of the abdomen. Its fibres run downwards and forwards, in the direction your fingers point when your hands are in your trouser pockets, from the lower eight ribs to the iliac crest, the pubic bone and the midline. It bends and twists the trunk and squeezes the abdominal contents.

How to spot them

▸Outermost of the three flat muscles: fibres run obliquely down and towards the midline, like hands in pockets
▸Muscle fibres interdigitate on the lower ribs with serratus anterior and, further back, latissimus dorsi
▸A broad aponeurosis in front forms the anterior wall of the inguinal canal and the inguinal ligament
▸Supplied by the lower thoracic nerves T7 to T12

What to know

The largest and outermost of the flat muscles of the side of the abdomen, its fibres running down and forwards like the fingers of a hand in a pocket

External oblique — the 'hands in pockets' muscle.

Its fleshy slips come from the lower ribs, interlocking with the saw-toothed muscle of the chest wall

External oblique — origin from ribs 5 to 12, interdigitating with serratus anterior.

The lower edge of its aponeurosis is folded back on itself between the hip bone and the pubic bone to form a ligament of the groin

External oblique — the inguinal ligament.

Its aponeurosis has a triangular gap just above the pubic bone through which the spermatic cord or round ligament passes

External oblique — the superficial inguinal ring.

Contracting on one side twists the trunk so that the opposite shoulder moves forward, as in a bicycle crunch

External oblique — contralateral rotation of the trunk.

Its rear border, with the broad muscle of the back and the top of the hip bone, encloses a triangle where hernias occasionally appear

External oblique — the lumbar triangle of Petit.

Key ideas

The three flat muscles cross like layers of plywood: the external oblique runs down and in, the internal oblique up and in, and the transversus across, giving a strong wall that resists both twisting and bursting forces.
The muscle on one side and the internal oblique on the other side work as a pair: external oblique on the left and internal oblique on the right together twist the trunk to the right.

Quick-fire facts

•Origin: the external surfaces of ribs 5 to 12
•Insertion: the linea alba, the pubic tubercle and the anterior half of the iliac crest, through a broad aponeurosis
•Action: bilateral contraction flexes the trunk and compresses the abdomen; unilateral contraction bends the trunk to the same side and rotates it to the opposite side
•Nerve supply: thoracoabdominal nerves (T7 to T11) and the subcostal nerve (T12)
•Blood supply: lower posterior intercostal arteries, the subcostal artery and the deep circumflex iliac artery
•The inguinal ligament is the thickened lower border of its aponeurosis, stretching from the anterior superior iliac spine to the pubic tubercle
•Its aponeurosis forms the anterior wall of the inguinal canal, and the superficial inguinal ring is a gap in it
•Its posterior free border runs from the last rib to the iliac crest and forms one side of the lumbar triangle
•The fibres of the three flat muscles run at right angles to each other in successive layers

Drill it — 6 cards

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Sources

TeachMeAnatomy — External oblique