<P> The sternocleidomastoid originates from two locations: the manubrium of the sternum and the clavicle . It travels obliquely across the side of the neck and inserts at the mastoid process of the temporal bone of the skull . The sternocleidomastoid is thick and narrow at its centre, and broader and thinner at either end . </P> <P> The sternal head is a round fasciculus, tendinous in front, fleshy behind, arising from the upper part of the front of the manubrium sterni . It travels superiorly, laterally, and posteriorly . </P> <P> The clavicular head is composed of fleshy and aponeurotic fibers, arises from the upper, frontal surface of the medial third of the clavicle; it is directed almost vertically upward . </P> <P> The two heads are separated from one another at their origins by a triangular interval (supraclavicular fossa) but gradually blend, below the middle of the neck, into a thick, rounded muscle which is inserted, by a strong tendon, into the lateral surface of the mastoid process, from its apex to its superior border, and by a thin aponeurosis into the lateral half of the superior nuchal line of the occipital bone . </P>

What structure is located in the upper third of the sternocleidomastoid muscle