Sternocleidomastoid muscle
The sternocleidomastoid muscle, also known as sternomastoid and commonly abbreviated as SCM, is a paired muscle in the superficial layers of the anterior portion of the neck; it is one of the largest and most superficial cervical muscles.
The sternocleidomastoid is innervated by the ipsilateral accessory nerve. It supplies only motor fibres. The cervical plexus supplies sensation, including proprioception, via the ventral primary rami of C2 and C3.
The sternocleidomastoid muscle obscures the carotid artery. Typically, the internal carotid artery is found within the carotid triangle, which is bounded by the sternocleidomastoid, omohyoid muscle and posterior digastric muscles.
The sternocleidomastoid muscle has been removed to expose the external carotid artery and internal carotid artery. The omohyoid muscle defines the inferior border of the carotid triangle.