Longissimus thoracis is the largest of the continuations of erector spinae, and blends in the lumbar region with iliocostalis lumborum.  

Proximal Attachment
Some fibers attach to the whole length of the posterior surfaces of the transverse processes, the accessory processes of L1 to L5 and to the middle layer of the thoracolumbar fascia.  

Distal attachment
The muscle runs superiorly to attach to the apices of the transverse processes of T1 to T12 and the fourth to twelfth ribs between the tubercle and angle.