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The sternocleidomastoid (SCM) is a prominent superficial neck muscle that divides the neck into anterior and posterior triangles (see Image. Superficial Neck Anatomy). Originating from the sternal manubrium and medial clavicle, the muscle inserts onto the mastoid process and superior nuchal line. The SCM facilitates lateral neck flexion, contralateral head rotation, and bilateral cervical flexion or extension, depending on cervical spine rigidity, and contributes to inspiration. The muscle's blood supply emanates from branches of the external carotid artery. Innervation is primarily via the accessory nerve (cranial nerve XI), with proprioceptive fibers coming from the cervical plexus. Anatomical variants include differences in origins, insertions, number of muscle heads, and occasional aberrant innervation. The SCM also participates in posture regulation, temporomandibular function, and complex cervicofacial muscle coordination. The SCM serves as a key landmark in the assessment of neck and head pathologies and the execution of surgical procedures such as carotid endarterectomy. Dysfunction or injury of this muscle can contribute to postural abnormalities, torticollis, impaired cervical mobility, and altered temporomandibular function. Electrophysiological studies indicate that SCM activity reflects broader cervicofacial muscular coordination, making its evaluation useful in diagnosing neuromuscular disorders. SCM palpation and functional testing can aid clinicians in identifying muscle hypotrophy, asymmetry, or altered activation patterns associated with chronic neck pain, congenital muscular abnormalities, or secondary musculoskeletal adaptations.