Ansa Cervicalis - Running over the outside of the carotid sheath (but sometimes deep to it) is the Ansa Cervicalis, a motor division of the cervical plexus.  It is composed of two limbs, a superior root which is derived from C1, associated with CN XII, and anteriorly placed, and an inferior root which is derived from C2 and C3 and posteriorly placed.  A loop is formed where the superior and inferior roots join and from this loop, branches arise which innervate the infrahyoid muscles (except thyrohyoid).  This is a very delicate structure and time should be spent dissecting the roots and motor branches to each of the infrahyoids.