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Laboratory 24. Posterolateral Triangle of the Neck
Step 4. The Spinal Accessory Nerve (CN XI)

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Identify the spinal accessory nerve (CN XI) in the fascia of the posterolateral triangle. It passes on the deep surface of the sternocleidomastoid muscle. It then emerges from a point slightly superior to the middle of the posterior edge of the sternocleidomastoid muscle. It crosses the posterolateral triangle, passing inferiorly and posteriorly in the fascia between the sternocleidomastoid and trapezius muscles. About 2 cm above the clavicle, it passes to the deep surface of the trapezius muscle. It innervates the sternocleidomastoid muscle and the trapezius muscle.

Links and References:
Grant's: 8.4
Netter:172