[TA] flat superficial muscle of the posterior neck, distinguished from the splenius cervicis muscle primarily by its insertion onto the cranium; origin, from ligamentum nuchae of last four cervical vertebrae and supraspinous ligament of first and second thoracic vertebrae; insertion, lateral half of superior nuchal line and mastoid process; action, rotates head and extends neck; nerve supply, dorsal primary rami of second to sixth cervical spinal nerves. SYN: musculus splenius capitis [TA], splenius muscle of head.