Sternocleidomastoid muscle: anatomy and the neck

The sternocleidomastoid (SCM) is a long muscle with an oblique course that runs down from the area behind the ear (mastoid process of the temporal bone) to the sternum and clavicle. It is easily visible and palpable on the side of the neck, especially when the person turns the head against resistance. It has two lower heads (sternal and clavicular) and one upper attachment.

What is its function?

When one side contracts, the head tilts toward the same side and rotates toward the opposite side. When both contract together, the neck flexes. It also acts as an accessory muscle of breathing under effort. Its main motor supply comes from the accessory nerve (cranial nerve XI), and the muscle receives sensation from branches of the cervical plexus.

Why is it an anatomical landmark?

The SCM divides the lateral region of the neck into two classic areas:

• anterior triangle, in front of the muscle, where structures such as the larynx, the thyroid and the suprahyoid and infrahyoid muscles are found;
• posterior triangle, behind the muscle, which houses the cervical plexus, part of the brachial plexus and lymph nodes.

Because it is easy to locate, the SCM serves as a map for describing the position of nodules, lymph nodes, vessels and nerves. The great auricular nerve, for example, wraps around the lateral surface of the muscle toward the ear and the jaw region.

How does it figure in HOF assessment?

In the assessment of the lower third and the neck, the SCM comes up in three situations:

• when observing the lateral contour of the neck and the transition to the jaw angle;
• when palpating the neck for lymph nodes or masses before procedures;
• when interpreting bands and bulges that may belong to the platysma or to the SCM itself.

In assessing the double chin and the cervicomental angle, the reading is of the submental region, but the full view of the neck includes the lateral structures.

How do you explain it to the patient?

In Harmy

The app works with a frontal photo of the face and simulates only the marked areas, such as Jawline and Chin. There is no marking for the neck or cervical muscles. Assessment of the neck and the SCM is clinical, done in person by the professional.

Also called: SCM, sternomastoid muscle.