The longissimus is the muscle lateral to the semispinalis. It is the longest subdivision of the sacrospinalis that extends forward into the transverse processes of the posterior cervical vertebrae. Longissimus thoracis The Longissimus thoracis is the intermediate and largest of the continuations of the Sacrospinalis. In the lumbar region, where it's as yet blended with the Iliocostalis lumborum, some of its fibers are attached to the whole length of the posterior surfaces of the transverse pro… (More on Longissimus)