The internal oblique muscle (of the abdomen) is the intermediate muscle of the abdomen, lying deep to (below) the external oblique and just superficial to (above) the transverse abdominal muscle.
Its fibers run perpendicular to the external oblique 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 point superiomedially (up and towards midline) to the muscle's insertions on the inferior borders of the 10th through 12th ribs and the linea alba (abdominal midline seam). In males, the cremaster muscle is also attached to the internal oblique.
The internal oblique is innervated by the lower intercostal nerves, as well as the iliohypogastric nerve and the ilioinguinal nerve.
The internal oblique 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 internal obliques 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 oblique muscle of the opposite side to achieve this torsional movement of the trunk. For example, the right internal oblique and the left external oblique contract as the torso flexes and rotates to bring the left shoulder towards the right hip. For this reason, the internal obliques are referred to as "same side rotators."
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