Phrenic nerve
The phrenic nerve is formed from contributions from the C3, 4, and 5 ventral primary rami. It lies on the anterior scalene muscle, which means that the surgeon would need to watch for this nerve while transecting the anterior scalene. None of the other structures lie upon the anterior scalene. The roots of the brachial plexus come out between the anterior and middle scalenes, and the trunks lie lateral to the anterior scalene. The long thoracic nerve comes off the C5, 6 and 7 roots of the brachial plexus and can be seen piercing the middle scalene muscle. This nerve lies on the surface of serratus anterior in the thorax, innervating that muscle. The sympathetic trunk is found deep to the scalene muscles; it runs in the prevertebral fascia along the front of the vertebral column. The vagus nerve runs within the carotid sheath, superficial to the anterior scalene muscle. It passes between the subclavian artery and vein.