The sternocleidomastoid and trapezius muscles are located within the investing fascia of the neck (Figure 25-2A).
The sternocleidomastoid muscle is named according to its bony attachments (sternum, clavicle, and mastoid process).
The sternocleidomastoid muscle creates the borders for both the anterior and the posterior triangles of the neck, and is innervated by the spinal accessory nerve (CN XI). This muscle flexes the neck, pulls the chin upward, and assists in elevating the rib cage during inspiration.
The trapezius muscle creates the anterior border of the posterior cervical triangle.
The trapezius muscle attaches to the occipital bone, nuchal ligament, spinous processes of C7-T12, scapular spine, acromion, and the lateral part of the clavicle. The sternocleidomastoid muscle is innervated by CN XI.
V During a physical examination, the functions of the sternocleidomastoid and trapezius muscles are evaluated together because they share the same innervation. During the examination, the physician looks for signs of muscle atrophy or weakness. To test the sternocleidomastoid muscle, the physician will place her hand on the patient's chin and instruct him to rotate his head to the opposite side against resistance. If acting normally, the patient's muscle can be seen and palpated. The physician next will instruct the patient to shrug his shoulders against resistance. Patients with damage to CN XI will have diminished shoulder strength while shrugging on the injured side. ▼
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