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.
In addition to the usual branches, there may be a left vertebral arising between the left carotid and left subclavian, as in the phoca vitulina; or a left vertebral arising beyond the left subclavian; or an inferior thyroid artery, usually the right one, arising between the innominata and left carotid; or a middle thyroid artery, arising in the same situation; or an internal mammary, or a thymic branch, arising from the arch of the aorta.
The right subclavian and carotid arteries may arise separately from the arch, in which case the right subclavian may be the first branch; or the right subclavian may arise between the right and left carotids; or between the left carotid and left subclavian; or beyond the left subclavian: of this last variety there are many cases on record. Mr. Kirby presented to the College of Surgeons, in this city, a preparation in which a piece of fish-bone pierced the right subclavian artery as it passed behind the oesophagus.* This artery may, however, pass between the oesophagus and trachea, or even in front of the latter. In such cases the inferior laryngeal nerve of this side does not curve under the right subclavian artery, but after its origin from the pneumogastric nerve in the lower portion of the neck, it passes directly inwards to the inferior part of the larynx. The first case in which this peculiar course of the right inferior laryngeal nerve was observed, is related by Dr. Stedman.* The next case is related by Dr. Hart, who was moreover the first author that explained the reason of the deviation. He observes, that "in the earlier periods of the existence of the foetus, the rudiment of the head appears as a small projection from the upper and anterior part of the trunk, the neck not being yet developed. The larynx at this time is placed behind the ascending portion of the arch of the aorta, while the brain, as it then exists, is situated so low as to rest on the thymus gland, and front of that vessel. Hence it is that the inferior laryngeal nerves pass back to the larynx, separated by the ascending aorta,—the left going through its arch, while the right goes below the arteria innominata."† Now it can be readily understood how the ascent of the brain, as the neck becomes developed, brings higher up the origins of the recurrent nerves; and the ascent of the larynx on a deeper plane brings up their terminations in that organ, so that they are made to form loops,—the right under the subclavian artery, and the left under the arch of the aorta. It is therefore clear that if the right subclavian artery should come off from the arch beyond and behind the left subclavian, and pass behind the trachea and oesophagus, or even between the two in order to reach the right side, the artery will be situated behind the destination of the nerve, so that the right nerve will pass at once to the larynx without passing under the right subclavian artery. Or again, should the right subclavian artery, arising thus irregularly, pass in front of the trachea in order to reach the right side, and in its course be placed lower down than the ordinary situation of the arteria innominata, the right inferior laryngeal nerve in this case also will go directly to the larynx above the right subclavian artery. It is therefore clear that in all such cases the right subclavian artery will not have the effect of depressing the inferior laryngeal nerve of the right side into the form of a loop underneath the vessel.
* Houston's Catalogue, p. 79.
* Edin. Med. and Surg. Jour, for 1823. † Same Journal, April, 1826.
The left subclavian may be the first branch of the arch on the right side; and after that may arise, in succession, the right subclavian, right carotid, and left carotid arteries. Or, lastly, the vessels may arise from the arch in the following order: left carotid, right carotid, left subclavian, and right subclavian arteries.