One of a pair of large triangular muscles on the thoracic and lumbar areas of the back. The base of the triangle inserts through lumbar aponeuroses to the spines of lumbar and sacral vertebrae and in the supraspinous ligaments, posterior iliac crest, and the lower four ribs. The fibers of the muscle twist as they pass the scapula and converge at the base of the intertubercular groove of the humerus. The latissimus dorsi extends, adducts, and rotates the arm medially; draws the shoulder back and down; and, with the pectoralis major, draws the body up when climbing. It is innervated by the thoracodorsal nerve.