A 45-year-old woman is admitted to the hospital with neck pain. A CT scan reveals a tumor in the left side of her oral cavity. The tumor and related tissues are removed with a radical neck surgical procedure. Two months postoperatively the patient's left shoulder droops quite noticeably. Physical examination reveals distinct weakness in turning her head to the right and impairment of abduction of her left upper limb to the level of the shoulder. Which of the following structures was most likely injured during the radical neck surgery?