The Collaborative International Dictionary of English v.0.48:

Splenius \Sple"ni*us\, n. [NL.] (Anat.)
   A flat muscle of the back of the neck.
   [1913 Webster]

WordNet (r) 3.0 (2006):

splenius
    n 1: either of two flat muscles that extend from the upper
         vertebrae to the base of the skull and serve to rotate or
         flex or extend the head and neck [syn: splenius,
         splenius muscle]