The sternocleidomastoid muscle (sternomastoid muscle) is a neck muscle that extends from the upper part of chest bones to the top of the neck along the side. There is one sternocleidomastoid muscle on each side of the neck. As the name implies, it has attachments to the sternum, clavicle and the mastoid process on the lower skull.
The sternocleidomastoid is easily visible at the base of the neck when the head is turned sharply to either side.
The sternocleidomastoid muscle has several actions. The muscle on each side of the neck helps in moving the head towards the same shoulder and rotating the head towards opposite side. Sternocleidomastoid muscles on both sides working together help in bending the neck forward and in forced inspiration.
The sternocleidomastoid muscle is innervated by accessory nerves of C1-C5.
The word sternocleidomastoid is built from the muscle's three attachment points:
- Sterno comes from the sternum.
- Cleido comes from the clavicle, which almost always appears as cleido in compound words.
- Mastoid comes from the upper attachment to the mastoid process, which in turn gets its name from its breast-like shape.