 in human anatomy, 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. ==structure== the sternocleidomastoid passes obliquely across the side of the neck. it is thick and narrow at its central part, but broader and thinner at either end. * the medial or sternal head is a rounded fasciculus, tendinous in front, fleshy behind, which arises from the upper part of the anterior surface of the manubrium sterni, and is directed superiorly, laterally, and posteriorly. * the lateral or clavicular head, composed of fleshy and aponeurotic fibers, arises from the superior border and anterior surface of the medial third of the clavicle; it is directed almost vertically upward. 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. ===innervation=== 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. ===variations=== the sternocleidomastoid varies much in the extent of its origin from the clavicle: in some cases the clavicular head may be as narrow as the sternal; in others it may be as much as  in breadth. when the clavicular origin is broad, it is occasionally subdivided into several slips, separat
Muscles_of_the_head_and_neck