The sternocleidomastoid muscle is one of the largest and most superficial cervical muscles. The primary actions of the muscle are rotation of the head to the opposite side and flexion of the neck. The sternocleidomastoid is innervated by the accessory nerve. It travels obliquely across the side of the neck and inserts at the mastoid process of the temporal bone of the skull by a thin aponeurosis. The sternocleidomastoid is thick and narrow at its center, and broader and thinner at either end.
The sternal head is a round fasciculus, tendinous in front, fleshy behind, arising from the upper part of the front of the manubrium. It travels superiorly, laterally, and posteriorly.
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.
thumb|Sternocleidomastoid labelled
The two heads are separated from one another at their origins by a triangular interval called Sedillot's triangle 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.
Nerve supply
The sternocleidomastoid is innervated by accessory nerve of the same side. It supplies only motor fibres. The cervical plexus supplies sensation, including proprioception, from the ventral primary rami of C2 and C3.
Anatomical landmark
thumbnail|The sternocleidomastoid muscle with the major [[triangles of the neck labelled.]]
The sternocleidomastoid is within the deep fascia of the neck, along with the trapezius muscle, with which it shares its nerve supply (the accessory nerve). It is thick and thus serves as a primary landmark of the neck, as it divides the neck into anterior and posterior cervical triangles (in front and behind the muscle, respectively) which helps define the location of structures, such as the lymph nodes for the head and neck.
Many important structures relate to the sternocleidomastoid, including the common carotid artery, accessory nerve, and brachial plexus.
Clinical significance
Examination of the sternocleidomastoid muscle forms part of the examination of the cranial nerves. It can be felt on each side of the neck when a person moves their head to the opposite side.