This section is from the book "Anatomy Of The Arteries Of The Human Body", by John Hatch Power. Also available from Amazon: Anatomy of the Arteries of the Human Body, with the Descriptive Anatomy of the Heart.
Each of the subclavian arteries in its second stage is covered anteriorly by the integuments, platysma, cervical aponeurosis, clavicular origin of the sterno-cleido-mastoid muscle; and frequently immediately behind this muscle, by the transverse branch of communication between the anterior and external jugular veins; and by the scalenus anticus muscle which separates the artery from the subclavian vein; the latter vessel lying lower down, and covering the insertion of the muscle. The phrenic nerve is usually enumerated amongst the anterior relations of the subclavian artery in the second stage; and from the obliquity of its course across the anterior surface of the scalenus anticus muscle, until it becomes related to the internal mammary artery, it may be considered, properly speaking, as an anterior relation both to the first and second stages of the artery. Posteriorly the artery is related to the apex of the cone of the pleura and to the scalenus posticus muscle; the brachial plexus of nerves lies on a plane posterior to the artery in this stage, and partly accompanies the artery into its third stage.