Nerve supply of trapezius is by

the and sternocleidomastoid are supplied by spinal accessory nerve. Both of them develop from brachial arch mesoderm. The principal action of trapezius is to rotate scapula during abduction of arm beyond 90deg. Clinically the muscle is tested by asking the patient to shrug his shoulder against resistance. REF: BD Chaurasia 7th edition Page no: 64.