Sternocleidomastoid muscle

Origin: Manubrium sterni, medial portion of the clavicle

Insertion: Mastoid process of the temporal bone, superior nuchal line

Artery: Occipital artery and superior thyroid artery

Nerve:

• Motor: accessory nerve
• Sensory: cervical plexus

Action: Acting alone, tilts head to its own side and rotates it so the face is turned towards the opposite side. Acting together, flexes the neck, raises the sternum and assists in forced inspiration. 

Description: 
The Sternocleidomastoideus (Sternomastoid muscle) passes obliquely across the side of the neck. It is thick and narrow at its central part, but broader and thinner at either end.

It arises from the sternum and clavicle by two heads:

• 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 upward, lateralward, and backward.
• 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, 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.