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Abdominal external oblique muscle Musculus obliquus externus abdominis

The abdominal external oblique muscle or musculus obliquus externus abdominis is the largest and most superficial of the three flat muscles present in the anterolateral group of abdominal wall, the other two being: the internal oblique and the transversus abdominis.

Its muscle fibers are laterally placed and they diverge in an inferomedial direction. While the large aponeurotic component covers the anterior part of the abdominal wall right till the midline. The aponeurosis intertwines to form the linea alba near the midline. It extends from the xiphoid process till the pubic symphysis. The inguinal ligament is formed from the reinforced thick lower margin of the aponeurosis. It extends between the anterior superior iliac spine laterally, and the pubic tubercle medially. The inguinal ligament folds on itself to form the inguinal canal.
The lacunar ligament is a crescent shaped extension of fibers from the medial end of the inguinal ligament. Its fibers run posterolaterally to attach to the superior ramus of the pubic bone, the pectin pubis. From this ligament, additional fibers extend to form the pectineal ligament, also known as Cooper’s ligament.

The external oblique muscle has muscular slips that originate from the outer inferior surface of the lower eight ribs (costae V - XII). Its insertion is at the lateral lip of the iliac crest, and its aponeurotic portion ends in the midline raphe (linea alba).
This muscle has various functions. It maintains the abdominal tone and compresses its content. During simultaneous contraction of both muscles, the flexion of the trunk results. However, when each muscle acts on its own, the trunk bends to the same side. Moreover, it is responsible for the rotation of the anterior part of the abdomen to the opposite side. It is innervated by the anterior rami of the lower six thoracic spinal nerves (T7 to T12).
The external oblique muscle is supplied by numerous blood vessels: The lower posterior intercostal arteries, the superior and inferior epigastric, the laterally placed superficial and deep circumflex arteries, and the posterior lumbar too. Veins of similar names follow their corresponding arteries and are responsible for the venous drainage.

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