The ansa cervicalis (or ansa hypoglossi in older literature) is a loop of nerves that are part of the cervical plexus.
Branches from the ansa cervicalis innervate all but one of the infrahyoid muscles, including the sternohyoid muscle, the sternothyroid muscle, and the omohyoid muscle. The thyrohyoid muscle is innervated by C1 only.
The superior root (descendens hypoglossi) of the ansa cervicalis is formed by a branch of spinal nerve C1. These nerve fibers travel in the hypoglossal nerve before leaving to form the superior root. The superior root goes around the occipital artery and then descends in the carotid sheath. It sends off branches to the superior belly of the omohyoid muscle, and the upper parts of the sternothyroid and sternohyoid muscles and is then joined by the inferior root.
The inferior root (descendens cervicalis) is formed by fibers from spinal nerves C2 and C3. The inferior root gives off branches to the inferior belly of the omohyoid muscle, and the lower parts of the sternothyroid and sternohyoid muscles.
"Ansa cervicalis." Stedman's Medical Dictionary, 27th ed. (2000). ISBN 0-683-40007-X
Gray's Anatomy: The Anatomical Basis of Clinical Practice. (2005). ISBN 0-443-07168-3