Final answer:
The sternocleidomastoid muscle originates from the sternum and clavicle, and inserts onto the mastoid process of the temporal bone, assisting in lateral flexion, rotation of the head to the opposite side, and neck flexion.
Step-by-step explanation:
The sternocleidomastoid muscle is an essential muscle located in the neck. It has a dual origin, with one part originating from the sternum and the other from the clavicle. This muscle then inserts onto the mastoid process of the temporal bone behind the ear. The sternocleidomastoid is involved in various head movements, including lateral flexion and rotation of the head to the opposite side when the muscles act unilaterally, and flexion of the head when they contract bilaterally. Additionally, this muscle plays a crucial role in dividing the neck into anterior and posterior triangles when viewed from the side.