semispinalis thoracis

noun

semispinalis tho·​ra·​cis -thə-ˈrā-səs 

: a deep longitudinal muscle of the back that arises from the transverse processes of the lower five thoracic vertebrae, is inserted into the spinous processes of the upper four thoracic and lower two cervical vertebrae, and with the semispinalis cervicis acts to extend the spinal column and rotate it toward the opposite side