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 aorta has usually at its origin the form of an arch: in some cases, however, this vessel has been observed to have no arch, but to divide soon after its origin into two great trunks, one of which, after having ascended for some distance, gave off three large branches, presenting the form of a cross, one branch, the continuation, which became the left carotid; a right horizontal branch which was the arteria innominata, and a left horizontal, which became the left subclavian: the other great trunk became the descending aorta. This is the natural arrangement in the horse, ass, sheep, goat, camel, and in many other mammalia, especially those having long necks.*
In other cases the aorta bifurcates as above, but each division gives branches to the head, neck, and upper extremity of the corresponding side, and after encircling the trachea and oesophagus, they unite to form the descending aorta. This is analogous to the natural structure in reptiles, and was first described by Hammel.
In a remarkable case described by Malacarne, the aorta arose by a single trunk of large size, and contained five semilunar valves; it then divided into two branches immediately after its origin; these two formed a loop, the sides of which united into one large trunk which became the descending aorta. From each of the two primary branches three branches arose; the first the subclavian, the second the external carotid, and the third the internal carotid. Thus in this case there was no arteria innominata. There are two specimens in the Museum of the Royal College of Surgeons in Ireland of the aorta having four valves at its origin. Mr. Hunter remarks, " I have found in the human subject only two valves to the aorta; but this is very rare."*
* In Abhandlungen der Josephinischen Med. Chir. Acad. Zuwien. Band 1, s. 271-1787.
A very singular case is related by Gintrac, in which the ascending aorta, which arose from the heart, gave branches to the head and upper extremities; while the descending aorta was a continuation of the pulmonary artery.
Lastly, the aorta has been known to arise by two roots, one from the left ventricle and the other from the right.