Proximal Attachment 
Splenius cervicis attaches to the spinous processes of T3 to T6.  

Distal Attachment
The muscle ascends, immediately anterior to levator scapulae, to attach to the posterior tubercles of the transverse processes of C1 to C3.  

Splenius cervicis varies in its vertebral attachments, with accessory slips also occurring; rarely, the muscle may be absent.

Nerve Supply
Lateral branches of the dorsal rami of C5 to C7 supply splenius cervicis.

Action
Acting individually, splenius cervicis laterally flexes and slightly rotates the neck to the same side.  Acting together the muscles extend the neck.