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 Posterior Scapular, Or Transversalis Colli Artery, larger than the supra-scapular, passes horizontally outwards in front of the anterior scalenus muscle and phrenic nerve; afterwards, in front of the upper part of the brachial plexus and posterior scalenus muscle, in order to arrive at the superior angle of the scapula. In this course it is covered by the sterno-mastoid and trapezius muscles: under cover of this last muscle it gives off the cervicalis superficialis, which ascends on the side and back of the neck, supplies the splenius and trapezius muscles, the integuments and lymphatic glands, and anastomoses with the descending cervical branches of the occipital artery. Having arrived at the superior angle of the scapula, the posterior scapular artery gets under cover of the levator anguli scapulae muscle, to which it sends a few small vessels, and divides into two branches of nearly equal size; one of which, the posterior scapular branch, properly so called, descends along the vertebral margin of the scapula, covered by the rhomboid muscles and levator anguli scapulae, to each of which, and to the serrati and latissimus dorsi, it sends a supply of blood. The other branch descends more internally, being covered by the scapula, and supplies the sub-scapular and serra-tus major anticus muscles. We will occasionally find the posterior scapular branch of this artery arising from the subclavian artery at the commencement of its third stage, passing through the brachial plexus of nerves, and thus arriving at its destination: in this case the cervicalis superficialis will form a distinct branch of the thyroid axis.
In the second part of its course, while under cover of the scalenus anticus muscle, the subclavian artery gives off the cervicalis profunda and superior intercostal arteries, which frequently arise from it by a common trunk.