The SCM is the dominant lateral neck muscle visible as a cord from mastoid to sternal notch during contralateral head rotation. The posterior triangle between SCM and trapezius contains the brachial plexus and accessory nerve.
An expanded entry for Sternocleidomastoid.
| Origin | Sternal head: anterior manubrium surface; Clavicular head: medial third of clavicle |
|---|---|
| Insertion | Mastoid process and lateral half of the superior nuchal line |
| Nerve Supply | Accessory nerve (CN XI), motor; C2, C3, proprioceptive |
| Blood Supply | Occipital and posterior auricular arteries |
| Actions |
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|---|
Acting on one side it tilts the head to the same side and turns the face to the opposite side; acting together they flex the neck and can lift the sternum during forced breathing.
SCM is the landmark for most neck surgery. The carotid sheath lies deep to it. CN XI injury from posterior triangle dissection produces SCM and trapezius palsy. SCM is used as a flap for pharyngeal fistula closure.
Visible and palpable as the dominant anterolateral neck cord during resisted contralateral rotation.
Accessory nerve injury during neck dissection producing SCM and trapezius weakness with shoulder droop and inability to shrug.
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