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 the semispinalis thoracis and multifidus muscles to reach the spinous processes of T12 and L1.  Occasionally it pierces trapezius and latissimus dorsi to reach and supply skin close to the midline.

Supply
The medial branches of the twelfth thoracic dorsal spinal rami supply the semispinalis thoracis and multifidus muscles and occasionally the skin overlying the eleventh and twelfth thoracic spinous processes.