Splenius capitis m

A broad, strap-like muscle in the back of the neck, deep to the trapezius m.  The splenius capitis originates from the spinous processes of C7 - T3, and inserts on the mastoid process and occipital bone.  The action of the muscle is to extend the head (pull it back) or if active unilaterally to turn the head to the side.  It is innervated by posterior rami of C3 and C4