[TA] deep back (erector spinae) muscle; origin, medial side of angles of lower six ribs; insertion, angles of upper six ribs; action, extends, abducts, and rotates thoracic vertebrae; nerve supply, dorsal branches of thoracic nerves. SYN: iliocostalis thoracis (muscle) [TA], pars thoracica musculi iliocostalis lumborum [TA], musculus iliocostalis dorsi, musculus iliocostalis thoracis.