External oblique m.

The external oblique muscle is the largest and the most superficial of the three flat muscles of the lateral anterior abdomen.  Medially its tendon forms the broad, flat, external oblique aponeurosis.

The aponeurosis of the external oblique muscle forms the inguinal ligament. The muscle also contributes to the inguinal canal. Just deep to the external oblique is the internal oblique muscle.

The external oblique muscle is innervated by ventral branches of the lower 6 intercostal (thoracoabdominal) nerves and the subcostal nerve on each side. 

The external oblique functions to pull the chest downwards and compress the abdominal cavity, which increases the intra-abdominal pressure as in a valsalva maneuver. It also has limited actions in both flexion and rotation of the vertebral column. One side of the obliques contracting can create lateral flexion. It also contributes in compression of abdomen.