Illustration of the thyroid gland (yellow). The gland is composed of an isthmus in the centre, a right and left lobe, a pyramidal lobe and parathyroid glands that vary from individual to individual. The larynx and trachea are at the back and the common carotid arteries are on both sides. The internal and external jugular veins are outside the common carotid arteries and join up with the junction of the subclavian veins and the brachiocephalic veins, then with the cranial vena cava. The laryngeal nerves are on either side of the larynx and pass behind the thyroid gland. On each side, between the jugular veins and common carotid arteries, are the vagus nerves and the left recurrent laryngeal nerve coiled round the arch of the aorta. The recurrent nerve can be seen between the trachea and the right common carotid artery. The right and left phrenic nerves pass in front of the left and right scalene muscles.