The superior intercostal artery supplies the first two intercostal spaces from behind. The first thoracic sympathetic ganglion is large and lies behind the vertebral artery. A small twig, the ansa subclavii, passes in front of the subclavian artery to join the lowest cervical ganglion.
Source: Boileau Grant JC. A Method of Anatomy: Descriptive and Deductive. 5th ed. Baltimore, Md: Williams & Wilkins Co; 1952:530.
April 2003. Arch Surg. 2003;138(4):439. doi:10.1001/archsurg.138.4.439