External oblique muscle

The external oblique muscle (EOM) is one of the muscles that forms the anterior abdominal wall. Its free inferior border forms the inguinal ligament, and its aponeurotic part contributes to the anterior wall of the inguinal canal. 

Summary

• origin: outer surface of the shaft of the lower 8 ribs
• insertion: upper aponeurotic fibres to the whole length of the linea alba and extends to the pubic crest and the pectineal line; lower fleshy fibres to the outer lip of the anterior end of the iliac crest
• innervation: segmental supply by lower 6 thoracic nerves
• action: contributes in forming the anterior abdominal wall and the action is along with the other abdominal wall muscles

Gross anatomy

The external oblique muscle arises from the outer surface of the middle of the shaft of lower six ribs as fleshy fibres. At the origin upper four slips and lower four slips interdigitate with the serratus anterior and the latissimus dorsi muscles respectively. The muscle gradually becomes aponeurotic, which inserts into the whole length of the linea alba with extension onto the pubic crest and the pectineal line. Lower fleshy fibres attach to the outer lip of the anterior two thirds of the iliac crest. Muscle fibres are directed obliquely downwards and medially. 

The muscle has some important features:

• inferior free border between the anterior superior iliac spine and the pubic tubercle is thickened and rolls inwards to form the Inguinalinguinal ligament
• posterior fleshy free border forms the anterior boundary of the lumbar triangle (of Petit) ¹
• upper part of the aponeurotic layer crosses over the rectus abdominis muscle contributes to forming the anterior layer of the rectus sheath
• lower part of the aponeurotic layer forms the medial half of the anterior wall of the inguinal canal; medial attachment provides a V-shaped gap (superficial inguinal ring) in order to provide passage of the spermatic cord / round ligament

Blood supply

The upper part of the muscle is supplied by the lower intercostal arteries, and the lower part is by the deep circumflex iliac artery branches. Venous drainage follows the arterial supply.

Innervation

• lower six intercostal nerves segmentally (T7-T12) ²

Action

Along with other abdominal wall muscles, EOM flexes the trunk, assists expiration by depressing the ribs, and assists to maintain intra-abdominal pressure in evacuation of luminal contents and supports intra-abdominal viscera.