The splenius cervicis is a minor muscle situated deep in the back of the neck. A continuation of the splenius capitus above it, this muscle is found beneath the upper trapezius, which is a large superficial muscle of the neck and back that shrugs the shoulders. In the layer of muscles beneath the trapezius, the splenius cervicis is wedged between the rhomboids minor and major, which are just beneath it and more medial to the spine, and the levator scapulae, which is slightly above it and more lateral to the spine on the side of the neck. Its function is to extend, laterally flex, or bend sideways, and rotate the neck.
Originating on the thoracic spine in the upper back, this muscle begins on three of the upper thoracic vertebrae, T3-T6. It specifically arises from the spinous processes of these vertebrae, which are the pair of bony eminences projecting backward from the body of the vertebra that are visible as bumps along the spine. From here the splenius cervicis ascends, its fibers curving slightly outward and then back in to insert high on the cervical spine. Rather than attaching via a tendon, the muscle forms fascicles, or small bundles of muscle fibers wrapped in connective tissue, which fasten the muscle to the transverse processes of the topmost two or three cervical vertebrae. The transverse processes are small sideways projections from the body of the vertebra, meaning that the muscle ends along the side of the neck.
In anatomy drawings, the splenius cervicis is somewhat continuous with the larger splenius capitis directly above it. A primary extensor of the head and neck, the capitis is flat and broad, and it stretches upward and slightly laterally from the cervical and upper thoracic spine to the base of the skull. In other words, its origins correspond with those of the cervicis, but it crosses superficially to the cervicis toward the head, with the fibers of the smaller cervicis crossing beneath it toward their attachment point on the upper cervical spine. This integration means that the splenius cervicis assists in the function of the capitis in extending, laterally flexing, and rotating the head, but to a lesser degree.
While the cervicis can act bilaterally to extend the neck and head, meaning that both sides contract simultaneously, it acts unilaterally, or upon one side at a time, to laterally flex and rotate the head. In doing so, it moves the head toward the side on which the contraction is taking place. During contractions on the right side of the neck, therefore, the splenius cervicis can tilt the head to the right as well as turn the head to the right on the neck.