One of the most superficial structures passing through the posterior triangle of the neck is the external jugular vein (Fig. 8.185). This large vein forms near the angle of the mandible, when the posterior branch of the retromandibular and posterior auricular veins join, and descends through the neck in the superficial fascia.
After crossing the sternocleidomastoid muscle, the external jugular vein enters the posterior triangle and continues its vertical descent.