External abdominal oblique muscle

The external abdominal oblique muscle (a.k.a. the external oblique muscle) is the largest and most superficial of the three flat muscles of the anterolateral abdominal wall.

Attachments

The external oblique muscle arises from the external surfaces of the lower eight ribs (ribs 5 to 12), where its fibres interdigitate with the serratus anterior superiorly and latissimus dorsi inferiorly. The muscle fibres run infero-medially in a characteristic hands-in-pockets direction before transitioning into a broad aponeurosis anteriorly. This aponeurosis contributes extensively to the anterior abdominal wall and inserts into the linea alba, pubic crest, pubic tubercle, and anterior half of the iliac crest.

----Inguinal ligament

Inferiorly, the rolled lower border of the aponeurosis folds inward upon itself to form the inguinal ligament, which extends between the anterior superior iliac spine (ASIS) and pubic tubercle.

Contributions of external oblique aponeurosis to inguinal canal, superficial inguinal ring, and external spermatic fascia

The aponeurosis of the external abdominal oblique muscle forms the anterior wall and the floor of the inguinal canal. There is a triangular opening inside the aponeurosis of the external oblique muscle, located superior to the pubic tubercle called the superficial inguinal ring. Through this opening the spermatic cord (and the round ligament of uterus) emerge. Weakness of the lower abdominal wall or disruption of the aponeurotic fibres may predispose to inguinal hernias.

During the descent of the testes, the aponeurosis of the external oblique muscle gives rise to the external spermatic fascia, which forms the outermost covering of the spermatic cord and testis.

The broad aponeurosis of the external oblique also contributes to the anterior wall of the rectus sheath.

Functions

Functionally, the external abdominal oblique muscle compresses and supports the abdominal viscera, increases intra-abdominal pressure, and assists in forced expiration. Bilateral contraction produces flexion of the trunk, while unilateral contraction causes ipsilateral lateral flexion and contralateral rotation of the trunk. Together with the internal oblique and transversus abdominis muscles, it forms an important dynamic stabiliser of the trunk and pelvis.

Innervation

The external oblique muscle receives motor innervation by the anterior rami of the lower six intercostal nerves (T7 to T11) and the subcostal nerve (T12).