The intermediate of the three flat abdominal muscles, the internal oblique is a thin muscular sheet that fans out anteromedially. Except for its lowermost fibers, which arise from the lateral half of the inguinal ligament, its fleshy fibers run perpendicular to those of the external oblique, running superomedially (like your fingers when the hand is placed over your chest). Its fibers also become aponeurotic at the midclavicular line (MCL) and participate in the formation of the rectus sheath.