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.
The right subclavian artery arises from the arteria innominata at the superior outlet of the thorax, immediately behind and on a level with the upper portion of the right sternoclavicular articulation, corresponding to the interval between the two origins of the sterno-cleido-mastoid muscle; it then passes obliquely upwards and outwards, till it reaches the internal margin of the scalenus anticus muscle. In this part of its course it is covered anteriorly by the integuments, by the platysma myoides, except in the immediate neighborhood of its origin, by the clavicular origin of the sterno-mastoid muscle, and by the cervical fascia, forming the sheath of this muscle; by the sterno-hyoid and sterno-thyroid muscles, the former of which is in more intimate relation to the artery. Between the sterno-mastoid muscle anteriorly, and the sterno-hyoid and sterno-thyroid muscles and scalenus anticus posteriorly, an interval exists in which we find a quantity of loose areolar tissue, together with several veins, one of which, sometimes of considerable size, passes across the posterior surface of the inferior portion of the sterno-mastoid muscle, and establishes a communication between the anterior and external jugular veins: it is sometimes endangered in the operations for wry neck. When these parts have been removed, the artery will be found covered more immediately by the internal jugular vein close to its junction with the subclavian vein to form the right vena innominata: the union between these two veins usually takes place in front of the internal margin and close to the insertion of the scalenus anticus muscle, in which situation the commencement of the vena innominata lies upon a plane anterior and a little inferior to the artery: lower down, on account of their difference of obliquity, they become more distant, the vein lying on the outer side. The vertebral vein as it is about to terminate in the internal jugular, usually passes anterior to the artery. In front of the artery we observe also the superior and middle cardiac nerves descending; and near the origin of the vessel, the pneumogastric nerve, and sometimes its recurrent branch (which in this situation occasionally begins to detach itself from its parent trunk), are situated in front of it. Vieussens describes a plexiform appearance upon the pneumogastric nerve in this situation, corresponding to the origin of the recurrent, and which he calls the plexus gangliformis; these nerves therefore pass between the artery and the vena innominata. The phrenic nerve also forms an anterior relation of the subclavian artery: immediately after this nerve has passed from off the scalenus anticus muscle, it gets under cover of the internal jugular vein close to its junction with the subclavian, and insinuates itself into a small interval which exists between the origin of the thyroid axis and the inner margin of the muscle; and it is in this situation that the nerve lies in front of the right subclavian artery in its first stage: generally speaking, it does not lie in direct contact with the artery, but is borne from off this vessel by the origin of the internal mammary artery, anterior to and across which the nerve usually passes. Sometimes the phrenic nerve lies upon a plane posterior to the internal mammary artery. Posteriorly the first stage of the right subclavian artery is related to the recurrent nerve, inferior cardiac nerve, and still farther back to the trunk of the sympathetic nerve where it forms its inferior cervical ganglion: this ganglion is situated behind the artery close to the origin of the vertebral. The longus colli muscle, with the interposition of some loose areolar tissue, lies behind the artery: the apex of the cone of the pleura lies a little inferior, to the outside, and on a plane posterior to the vessel.