The accessory phrenic nerves are derived from branches of the nerves to the subclavius muscles, which contain fibers from the ventral ramus of C5, and join the phrenic nerves near the first rib (or occasionally the level of the pulmonary hilum).  They may also be derived from the ventral rami of C4 or C6, or from the ansa cervicalis.  They arise lateral to the main nerve and descend posterior (or anterior) to the subclavian veins.