Section of the neck at about the level of the sixth cervical vertebra. Showing the arrangement of the deep cervical fascia (splenius capitis labeled at bottom right).
The fibers of the muscle are directed upward and laterally and are inserted, under cover of the sternocleidomastoideus, into the mastoid process of the temporal bone, and into the rough surface on the occipital bone just below the lateral third of the superior nuchal line. The splenius capitis is deep to sternocleidomastoideus at the mastoid process, and to the trapezius for its lower portion. It is one of the muscles that forms the floor of the posterior triangle of the neck.
The splenius capitis muscle is a prime mover for head extension. The splenius capitis can also allow lateral flexion and rotation of the cervical spine.
Anatomy figure: 01:03-01 at Human Anatomy Online, SUNY Downstate Medical Center - "Superficial layer of the extrinsic muscles of the back."
Anatomy figure: 01:05-01 at Human Anatomy Online, SUNY Downstate Medical Center - "Intermediate layer of the extrinsic muscles of the back, deep muscles."