The complexus muscle is the ventrolateral division of semispinalis capitis that arises from cervical vertebral articular processes and inserts lateral to the biventercervicis on occipital bone.
Origin: Caudal articular processes of the first thoracic to the third cervical vertebrae.
Insertion: Dorsal nuchal line.
Action: Raises the head and neck, in unilateral action to flex the head and neck laterally.
Nerve: Dorsal branches of the cervicales.