sternocleidomastoid
Concise Oxford English Dictionary © 2008 Oxford University Press:
sternocleidomastoid /ˌstəːnə(ʊ)ˌklʌɪdə(ʊ)ˈmastɔɪd/
▶noun Anatomy each of a pair of long muscles which connect the sternum, clavicle, and mastoid process and serve to turn and nod the neck.
'sternocleidomastoid' also found in these Oxford entries:

