The superficial longitudinal muscle mass on either side of the vertebral column. It is sheathed in the fascia thoracolumbalis and arises in a broad, thick tendon from the sacrum, the ilium, and the lumbar vertebrae. It inserts into the ribs and into certain cervical vertebrae and is innervated by the branches of the dorsal primary divisions of the spinal nerves. It extends and flexes the vertebral column and the head, draws the ribs downward, and bends the trunk to the side.