Internal Oblique Muscle
The internal oblique muscle is thinner than the external oblique muscle. Its fibers are perpendicular to the fibers of the external oblique muscle. Its fibers originate from the lateral part of the inguinal ligament, the iliac crest (upper part of the hip bone) and from the thoracolumbar fascia. From here the fibers of this muscle run upwards and towards the center of the body to be inserted into the linea alba (abdominal midline seam) and the lower parts of the lowest three ribs (ribs 10 to 12).
The internal oblique muscle gets its blood supply from the lower posterior intercostal and subcostal arteries, the superior and inferior epigastric arteries, the superficial and deep circumflex arteries and posterior lumbar arteries.
The internal oblique muscle is supplied by the lower five intercostal nerves (their terminal branches) and the subcostal nerve. It also gets additional nerve supply from the ilioinguinal and iliohypogastric nerves.
The internal oblique muscle helps to maintain the tone of the abdomen along with the other muscles of the anterior abdominal wall. It is used to increase the intra-abdominal pressure and for the lateral flexion of the trunk.
Additionally, it opposes the action of diaphragm (dome shaped sheet of muscle separating the thoracic and the abdominal cavity). This antagonizing action helps reduce the volume of the chest cavity during exhalation (breathing out). The contraction of the diaphragm increases the capacity of the chest cavity allowing the expansion of the lungs, filling it with air. The contraction of the internal oblique muscle puts pressure on the abdominal organs, this in turn reduces the volume of the chest cavity resulting in exhalation.
For the lateral flexion and rotation of the torso, internal oblique muscle works with the external oblique muscle of the opposite side. The left internal oblique muscle, for example, contracts and rotates the trunk with the help of the right external oblique muscle in such a way that it brings the right shoulder towards the left hip.