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 left subclavian artery arises within the cavity of the thorax, from the arch of the aorta, opposite to and to the left side of the second dorsal vertebra, and ascends slightly outwards into the neck, till it reaches the internal margin of the scalenus anticus muscle, where the second stage commences. Like the common carotid artery, the first stage may be divided into two portions,—a thoracic and cervical: the thoracic portion extends from the origin of the vessel from the arch of the aorta to the upper outlet of the thorax; and the cervical extends from this point to the internal margin of the scalenus anticus. In its thoracic portion it is related, internally, to the left carotid artery, which is also situated on a plane anterior to it; to the oesophagus, thoracic duct, and recurrent nerve, which are on a plane posterior to it, and to the internal jugular vein and its junction with the subclavian to form the left vena innominata: these large veins are also situated on a plane anterior to the artery: ex-ternally it is related to the top of the left lung and pleura: anteriorly it is covered by the sternum, sterno-clavicular articulation, and sterno-hyoid and sterno-thyroid muscles: it is overlapped by the left lung and pleura, and it is crossed obliquely near its origin by the left pneumogastric nerve: the phrenic nerve is anterior to and parallel with the artery. The left vertebral vein lies anterior to it, and on the same plane we find the origin of the left vena innominata as already described: posteriorly the artery corresponds to the second dorsal vertebra at its origin, afterwards to a short portion of the spinal column above this vertebra, to the longus colli muscle, and to the sympathetic nerve and its inferior cervical ganglion. The cervical portion is very short: it has anterior to it the parts already mentioned as lying in front of the artery of the right side; in front of it also we find the internal jugular vein, with the vagus and phrenic nerves. The latter nerve, at the inner margin of the scalenus anticus muscle, passes inwards towards the middle line and crosses in front of the artery at the termination of the cervical portion of its first stage; and the terminating portion of the thoracic duct, as it is about to enter the posterior part of the left subclavian vein at its junction with the internal jugular, lies anterior to the artery in this situation.
From the preceding account it follows that the left subclavian artery differs in the following respects from the right: the left subclavian is longer and proportion-ably more slender; it arises within the cavity of the thorax, and from the arch of the aorta; it is situated at the left side of the spine, which here forms a concavity, and it is in close relation with the left side of the second dorsal vertebra: for these reasons it lies much deeper and farther removed from the surface than the right: its direction is also more vertical, and consequently nearly parallel to the pneumogastric and phrenic nerves; it is intimately connected with the oesophagus and thoracic duct and left longus colli muscle, and it is covered in front and externally by the left lung and pleura: the internal jugular vein is nearly parallel with it internally, whilst at the right side the internal jugular crosses in front of the subclavian artery:—lastly, the left subclavian vein lies superior to a considerable portion of the artery in its first stage, and also internal to it; whilst on the right side the vein is inferior to the artery.