Marked structures are as follows: a- Posterior cricoarytenoid b- Thyroarytenoid c- Cricothyroid d- Oblique interarytenoid The external branch of the superior laryngeal nerve is most likely to be injured during ligation of Superior thyroid aery because both of them course together in a neurovascular bundle near the superior lobe of thyroid gland. The only laryngeal muscle supplied by the external branch of superior laryngeal nerve is Cricothyroid.