The<unk><unk>  muscle (of the abdomen) is the intermediate muscle of the abdomen, lying just underneath the external<unk>  and just above (superficial to) the transverse<unk>  muscle.
Its fibers run perpendicular to the external<unk>  muscle, beginning in the thoracolumbar fascia of the lower back, the anterior 2/3 of the iliac crest (upper part of hip bone) and the lateral half of the inguinal ligament.  The muscle fibers run from these points superiomedially (up and towards midline) to the muscle's insertions on the inferior borders of the 10th through 12th ribs and the linea alba<unk>  midline seam.)
The<unk><unk>  is innervated by the lower intercostal nerves, as well as the iliohypogastric nerve and the ilioinguinal nerve.
The<unk><unk>  performs two major functions.  First, it acts as an antagonist (opponent) to the diaphragm, helping to reduce the volume of the thoracic (chest) cavity during exhalation.  When the diaphragm contracts, it pulls the lower wall of the chest cavity down, increasing the volume of the lungs which then fill with air.  Conversely, when the<unk><unk> s contract they compress the organs of the abdomen, pushing them up into the diaphragm which intrudes back into the chest cavity reducing the volume of the air filled lungs, producing an exhalation.
Secondly, its contraction rotates and side-bends the trunk by pulling the rib cage and midline towards the hip and lower back, of the same side.  It acts with the external<unk>  muscle of the opposite side to achieve this torsional movement of the trunk.  For example, the right<unk><unk>  and the left external<unk>  contract as the torso flexes and rotates to bring the left shoulder towards the right hip.  For this reason, the<unk><unk> s are referred to as "same side rotators."
Abdominal internal oblique muscle
Transversus abdominis muscle
Rectus abdominis muscle
Obturator internus muscle
Superior oblique muscle
Pubococcygeus muscle
Adductor magnus muscle
Serratus posterior superior muscle