[TA] origin, transverse processes of fifth to eleventh thoracic vertebrae; insertion, spinous processes of first four thoracic and fifth and seventh cervical vertebrae; action, extends vertebral column; nerve supply, dorsal primary rami of cervical and thoracic spinal nerves. SYN: musculus semispinalis thoracis [TA], musculus semispinalis dorsi, semispinal muscle of thorax.