1. A small, thin, deep muscle of the abdomen which is a diagonally arranged abdominal muscle on either side of the torso: The internal abdominal oblique muscle runs diagonally opposite and underneath the external oblique.

The internal abdominal oblique muscle has a quadrilateral form originating from the hip bone, the crest of the ilium, and extending to the cartilage of the lower ribs which are the tenth, eleventh, 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 muscles of the abdomen, which are in front and away from the midline, lying under the external oblique muscle in the lateral and ventral part of the abdominal wall; The internal abdominal oblique muscle 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.