The semispinalis thoracis muscle is composed by bundles that detach form mamillary processes and course over the lateral face of semispinalis muscle to the dorsal portion of spinous processes.
Origin: Fascia of Longissimus.
Insertion: Spinous processes of the sixth thoracic to sixth cervical vertebra.
Action: Fixes the vertebral column and raises the neck.
Nerve: Medial branch of the dorsal branches of the cervical and thoracic nerves.