The muscle shown in the image is the sternocleidomastoid muscle.
Origin: manubrium and medial aspect of clavicle
Insertion: mastoid process of the temporal bone, superior nuchal line
Innervation: motor innervation via the accessory nerve (Cranial nerve 11), sensory innervation from the cervical plexus
Blood Supply: Superior thyroid artery and occipital artery
Action: If the muscle contracts on one side, then cervical rotation to the opposite side occurs whilst cervical lateral flexion occurs on the same side. With bilateral contraction, cervical flexion occurs and the manubrium is elevated – in this way it assists inspiration, and is one of the accessory muscles of respiration.