As one approaches the most posteroinferior angle of the neck dissection, the first important structure to be seen is the external jugular vein. It is ligated and divided at the posteroinferior corner (Figure 5). Then the posterior cervical triangle can be completely cleaned of its areolar and lymphatic tissues. The spinal accessory nerve should be preserved as long as it is not involved with tumor or enlarged lymph nodes. The spinal accessory nerve must be divided (Figure 6) if clean dissection of this area is impossible. Dissection is carried forward along the superior aspects of the clavicle. The posterior belly of the omohyoid muscle and the transverse cervical artery and vein are visualized (Figure 6). The posterior belly of the omohyoid muscle is severed (Figure 7) in order to allow greater exposure of the deep muscles and the brachial plexus. The phrenic nerve is found lying upon the anterior scalene muscle between the brachial plexus and the internal jugular vein (Figure 8A). To avoid paralysis of the corresponding leaf of the diaphragm, this nerve should be preserved unless it has been invaded by the cancer. The phrenic nerve lies upon the scalenus anticus muscle. Its exposure has been facilitated by the previous transection of the lower end of the sternocleidomastoid muscle. Just medial to the phrenic nerve, the internal jugular vein is seen (Figure 8A). This vessel, which lies within the carotid sheath (Figure 8B), is dissected free (Figure 9), doubly ligated by a stick tie on the inferior ligation, and then divided (Figure 10). By division of the internal jugular vein, avoiding the thoracic duct on the left side, the dissection has been carried down to the prevertebral fascia overlying the deep muscle structures of the neck. The inferior compartment of the neck is then outlined medially by division of the pretracheal fascia just lateral to the strap muscles of the thyroid (Figure 11). This facilitates exposure of the common carotid artery, which permits the dissection to be carried superiorly. With the lateral limits of the dissection defined and the common carotid artery exposed, dissection is started inferiorly and extended superiorly, following the floor of the neck or the prevertebral fascia.