A flat, triangular muscle that covers the back of the neck, shoulders and thorax. The upper and lower fibers are important to the orientation of the shoulder blade (scapula). The upper part, acting alone, elevates the shoulder and braces the shoulder girdle when a weight is carried. The lower part draws the scapula downward. When both of these muscles act together, the scapula can be brought toward the body and the head can draw directly backward.