External abdominal oblique muscle

At a glance

English

External abdominal oblique muscle

Latin (TA2)

Musculus obliquus externus abdominis

System

Muscle

Region

Trunk

Side

Paired, left and right

Arises from

The eighth rib, hip bone, seventh rib and sixth rib

About the external abdominal oblique muscle

The external oblique is a broad, thin sheet that wraps the side of the abdomen like the outer layer of a three-ply wall; the internal oblique and transversus abdominis lie beneath it. Its fleshy part covers the flank, while in front it gives way to a wide tendinous sheet, the aponeurosis, that covers rectus abdominis and meets its partner from the other side in the midline.

It begins by a row of fleshy slips from the outer surfaces of the lower eight ribs. The upper slips interlock with those of serratus anterior and the lower ones with latissimus dorsi, producing a zig-zag line on the side of the chest in a muscular person. From these slips the fibres run downward and forward, in the same direction as hands slipped into the front pockets of a coat. The hindmost fibres drop almost vertically to the front half of the iliac crest and leave a free back border. The rest become aponeurotic around the midclavicular line, help form the front layer of the rectus sheath and interlace at the linea alba. The lower edge of the aponeurosis rolls inward between the anterior superior iliac spine and the pubic tubercle to form the inguinal ligament, and a split in it near the pubis makes the superficial inguinal ring.

Acting on both sides, the muscle flexes the trunk, pulls the lower ribs down and squeezes the abdominal contents, raising pressure inside the abdomen for coughing, forced breathing out, straining and lifting. Acting on one side, it bends the trunk to the same side and turns it towards the opposite side, working with the internal oblique of that opposite side. It is supplied by the anterior rami of the seventh to eleventh thoracic nerves and the subcostal nerve, and its blood comes from the lower intercostal and subcostal arteries, the epigastric arteries and the deep circumflex iliac artery.

Strains of the obliques are a familiar injury in throwing and batting sports. Weak points in the aponeurosis near the groin are where inguinal hernias appear.

Attachments, action and supply

Origin

Outer surfaces of the lower eight ribs (fifth to twelfth) by fleshy slips.

Insertion

Iliac crest, on the outer lip of its front half, the pubic tubercle and crest, and the linea alba through its aponeurosis.

Action

Compresses the abdomen, flexes the trunk and depresses the lower ribs; on one side bends the trunk to that side and rotates it to the opposite side.

Nerve supply

Thoracoabdominal nerves (anterior rami of T7-T11) and the subcostal nerve (T12).

Blood supply

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