External abdominal oblique muscle

Origin and insertion

External abdominal oblique muscle originates from the external surfaces of ribs 5-12. The attaching fibers interdigitate with those of serratus anterior and latissimus dorsi, forming a zig-zag oblique line on the lateral side of thorax.

From here, the muscle fibers fan out towards the midline and inferior margins of abdomen; the most posterior fibers course almost vertically, while the remainder passes anteromedially. At the midclavicular line medially and spinoumbilical line inferiorly, external abdominal oblique continues as an aponeurosis via which it inserts to the linea alba, pubic tubercle and anterior half of iliac crest.

Recall that the midclavicular line is a vertical axis passing through the middle of the clavicle, while the spinoumbilical is a line connecting the umbilicus and anterior superior iliac spine (ASIS).

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Relations

External abdominal oblique is the largest and the most superficial of the lateral abdominal muscles. It lies beneath the thoracic and abdominal skin, covering the internal abdominal oblique and anterior halves of the ribs and intercostal muscles. Its muscular part contributes to the lateral part of the abdominal wall. Its aponeurotic part, however, contributes to the anterior abdominal wall, as it contributes to the anterior layer of the rectus sheath.

The superior, medial and inferior margins of the muscle relate to their respective attachment points while the posterior margin is free. This is in contrast to the other lateral abdominal muscles, all of which attach to the thoracolumbar fascia at their posterior ends. The portion of the inferior margin of this muscle that extends between the ASIS and pubic tubercle curves posteriorly, forming a thick incurve, or channel called the inguinal ligament (of Poupart)which constitutes the floor of inguinal canal.

Innervation

The external abdominal oblique is innervated by the anterior rami of the thoracic spinal nerves T7-T12. Those are the intercostal nerves T7- T11 that supply the superior most part of the muscle and the subcostal nerve (T12) that innervates the lower part. The muscle receives sensory innervation from the lumbar plexus, via the iliohypogastric nerve (L1).

Blood supply

Blood supply to the upper two thirds of the external abdominal oblique comes the branches of the lower posterior intercostal and subcostal arteries, while the deep circumflex iliac artery supplies the inferior third of the muscle.

Function

External abdominal oblique muscle has a variety of functions depending if it contracts unilaterally or bilaterally. When acting unilaterally and in synergy with the contralateral internal abdominal oblique, it rotates the trunk to the opposite side. Working together with the ipsilateral abdominal and back muscles, it contributes to lateral flexion of the trunk on the same side.

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When contracting bilaterally, the muscle works together with internal abdominal oblique and rectus abdominis to flex the trunk anteriorly. This activity also increases the tone of abdominal wall and positive intra-abdominal pressure, which is a part of various physiological processes; for example forced exhalation, micturition, defecation and labor.

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