longissimus thoracis muscle

The longissimus thoracis is the intermediate and largest of the continuations of the sacrospinalis.[WP]. intermediate erector spinae muscle of back; origin, with iliocostalis and from transverse processes of lower thoracic vertebrae; insertion, by lateral slips into most or all of the ribs between angles and tubercles and into tips of transverse processes of upper lumbar vertebrae, and by medial slips into accessory processes of upper lumbar and transverse processes of thoracic vertebrae; action, extends vertebral column; nerve supply, dorsal primary rami of thoracic and lumbar spinal nerves[Stedmans].