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Internal oblique abdominal muscle
Last reviewed: 07.07.2025
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The internal oblique muscle of the abdomen (m. obhquus internus abdominis) is located inside the external oblique muscle of the abdomen, forming the second layer of muscles of the abdominal wall. The muscle begins at the intermediate line of the iliac crest, the lumbothacral fascia and the lateral half of the inguinal ligament.
The bundles of the posterosuperior part of the muscle go from below upward and are attached to the cartilages of the lower ribs, having the same direction as the bundles of the internal intercostal muscles. The lower bundles of the muscle fan out upward and downward and pass into a wide aponeurosis along a line drawn from above downwards from the cartilage of the 10th rib to the pubic bone. At the level of this line, the aponeurosis of the internal oblique muscle of the abdomen splits into two plates, embracing the rectus abdominis muscle from the front and back. The lower part of the aponeurosis is a continuation of that part of the muscle that begins on the lateral half of the inguinal ligament.
The bundles of the lower part of the muscle, together with the bundles separated from the transverse abdominal muscle, form part of the spermatic cord and form the muscle that lifts the testicle (m. cremaster).
Function: With bilateral contraction, the internal oblique abdominal muscles flex the spine. With unilateral contraction, together with the external oblique abdominal muscle of the opposite side, the internal oblique abdominal muscle rotates the torso in its direction, lowers the rib. With a strengthened chest, it raises the pelvis.
Innervation: intercostal nerves (ThVI-ThXII), iliohypogastric (ThXII-LI) and ilioinguinal (LI) nerves.
Blood supply: posterior intercostal arteries, superior and inferior epigastric arteries, musculophrenic artery.
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