Sternocleidomastoid muscle

Sternocleidomastoid muscle

In human anatomy, the sternocleidomastoid (pronounced /ˌstɚ.noˌkli.dəˈmæs.tɔɪ̯d/) 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 acts to flex and rotate the head.

It also acts as an accessory muscle of inspiration, along with the scalene muscles of the neck.

Etymology

It is given the name sternocleidomastoid because it originates at the manubrium of the sternum (sterno-) and the clavicle (cleido-), and has an insertion at the mastoid process of the temporal bone of the skull.

Origin and insertion

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, 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 the ipsilateral accessory nerve. It supplies only motor fibres. The cervical plexus supplies sensation including proprioception.

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 7.5 cm in breadth.

When the clavicular origin is broad, it is occasionally subdivided into several slips, separated by narrow intervals. More rarely, the adjoining margins of the Sternocleidomastoid and Trapezius are in contact. This would leave no posterior triangle.

The Supraclavicularis muscle arises from the manubrium behind the Sternocleidomastoid (also known as the wing and named after a large prehistoric bird) and passes behind the Sternocleidomastoid to the upper surface of the clavicle.

Relations

The sternocleidomastoid is within the investing fascia of the neck, along with the trapezius muscle, with which it shares its nerve supply (the accessory nerve). Anterior to sternocleidomastoid is the anterior cervical triangle. Posterior to it is the posterior cervical triangle.

Many important structures relate to the sternocleidomastoid, including the common carotid artery, accessory nerve, and brachial plexus.

Cultural significance

  • Tom Wolfe's The Bonfire of the Vanities featured a lawyer fond of clenching his sternocleidomastoid muscles.
  • In the Portuguese movie A Canção de Lisboa, the main character, Vasco, who is studying Medicine, is asked in his final exam where the Sternocleidomastoid muscle is located. The name of the muscle became famous in Portugal after this movie.

Additional images

External links

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