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 Superior Intercostal Artery inclines a little backwards, arches over the top of the lung and pleura, and descends into the thorax, having behind it the neck of the first rib, and the first dorsal nerve, as the latter ascends from the thorax. In front it is covered by the pleura, and on the inside it is separated from the margin of the longus colli muscle by the first thoracic ganglion of the sympathetic nerve. These parts will therefore lie in the following order, commencing at the bodies of the vertebrae and passing outwards:—first, the longus colli muscle; secondly, the first thoracic ganglion of the sympathetic; thirdly, the superior intercostal artery; and fourthly, the first dorsal nerve as it passes obliquely across the neck of the first rib to unite with the last cervical. The artery then, in many, if not in most cases, goes out of the thorax, passing between the first and second ribs, and re-enters between the second and third. This artery gives off the intercostals of the first and second, and sometimes of the third, or more intercostal spaces; these anastomose with branches of the anterior intercostals from the internal mammary artery; a small descending branch communicates with the first aortic intercostal.
The superior intercostal artery is always small, and sometimes deficient.