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.
These arteries are two in number, a right and left. The right subclavian arises from the arteria innominata, and the left from the arch of the aorta: each is usually described as having three stages. In the first stage it ascends from its origin to the internal margin of the scalenus anticus muscle; in the second stage it passes behind that muscle; and in the third it proceeds obliquely downwards and outwards, till it arrives at the lower margin of the first rib, where it changes its name and becomes the axillary artery. In this course the artery forms an arch, the convexity of which looks upwards, and the summit of which is usually opposite to the sixth cervical vertebra. As the subclavian arteries differ in their origins, their relations must necessarily differ in the first stage, and therefore a separate description will be necessary for each; but in the second and third stages their relations are alike.
* Med. and Chirur. Trans., vol. i. p. 229.