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functional anatomy BY CHRISTY CAEL STERNOCLEIDOMASTOID The sternocleidomastoid (SCM) is a large, superficial muscle on the lateral neck. It has two heads and connects the mastoid process to the manubrium of the sternum and medial clavicle. The muscle runs parallel to the ramus of the mandible and forms an upside-down "V" with the posteriorly oriented splenius capitis muscle. The SCM also divides two major sites of caution in the neck. It joins the trachea and inferior mandible to form the anterior triangle and the lateral edge of the upper trapezius and superior clavicle to form the posterior triangle. Accurate palpation of the SCM helps locate these vulnerable regions and avoids damaging the structures contained within. Strong attachments to the temporal bone STERNOCLEIDOMASTOID Attachments • Origin: Manubrium of sternum and medial clavicle • Insertion: Mastoid process of temporal bone Actions • Flexes neck and extends the head and upper cervicals (bilateral action) Lippincott Williams & Wilkins Figure 06.17 Innervation • Accessory nerve • Cranial nerve XI, C2, and C3 Dragonfly Media Group 01/29/08 Boost your practice with ABMP's Website Builder—free for members on ABMP.com 87 CAEL: Functional Anatomy: A Guide of Musculoskeletal Anatomy for Profs • Laterally flexes the head and neck to same side and rotates to opposite side (unilateral action) and an oblique position on the neck make the SCM a powerful prime mover for flexion, lateral flexion, and rotation. Because the muscle inserts posteriorly on the skull, it can extend the head and upper cervicals. Combined neck flexion and head extension creates a forward head motion, jutting the chin forward. Bilateral shortness in the SCM leads to a forward head posture or, if unilateral, a condition called torticollis (a laterally flexed and rotated neck).