It runs diagonally opposite underneath the external oblique.
It has a quadrilateral form originating from the hip bone (crest of the ilium), and extending to the cartilage of the lower ribs (the tenth, elenenth, and twelfth ribs).
It is innervated by the lower thoracic nerves and supplies the intercostal and lumbar arteries and this muscle protects a weak point in the abdominal wall and works with the external oblique to help twist the torso.2. One of a pair of anterolateral (front and away from the middle line) muscles of the abdomen, lying under the external oblique muscle in the lateral and ventral part of the abdominal wall.
It is smaller and thinner than the external oblique muscle and it functions to compress the abdominal contents and assists in micturition, defecation, emesis, parturition, and forced expiration.
Both muscles acting together serve to flex the vertebral column, drawing the costal cartilages toward the pubis.
One side acting alone bends the vertebral column laterally and rotates it, drawing the shoulder of the opposite side downward.