The twelfth thoracic dorsal spinal rami run posteriorly, adjacent to the zygapophyseal joints between T12 and L1 and divide into medial and lateral branches.

The medial branches emerge between the joints and the medial edges of the superior costotransverse ligaments and the intertransversarii muscles, to pass between semispinalis thoracis and multifidus to reach the spinous processes of T12 and L1.  Here they pierce the serratus posterior inferior and trapezius muscles to terminate in the overlying skin.  The lateral branches emerge between the superior costotransverse ligaments and the intertransversarii muscles and pass between the longissimus thoracis and iliocostalis cervicis muscles to reach the medial border of levatores costarum.

Supply
The twelfth thoracic dorsal spinal rami supply the multifidus, longissimus thoracis, iliocostalis cervicis, levatores costarum and interspinales muscles as well as the skin overlying the twelfth thoracic and first lumbar spinous processes.