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 pulmonary artery may arise from the aorta, or in common with it; or, the two ventricles may communicate at their bases, and the septum between the aorta and pulmonary artery may be deficient. The pulmonary artery has been known to arise from the left ventricle, and the aorta from the right: in such cases we either find the ductus arteriosus open, or the foramen ovale, or both. The pulmonary artery may arise from the left ventricle, the right being almost obliterated and communicating with the left. The pulmonary artery may give off the subclavian artery. In a case related by Dr. Farre, it had two origins,—one from the right, and the other from the left ventricle; it then gave off the descending aorta, while an ascending aorta arose directly from the heart, and supplied the head and upper extremities. In cyanosis, the pulmonary artery is frequently found contracted or obliterated at its origin. In such cases the blood reaches the lungs by passing first through the aorta, then through the ductus arteriosus, and so into the right and left pulmonary arteries: the bronchial arteries also, by means of their communications with the pulmonary arteries, will contribute to supply the lungs.
The varieties of the commencement of the aorta, which we shall now consider, may be classed into those relating to its situation, its form, its course, and to the branches which arise from it.
Mr. Quain mentions one case in which the arch was situated "but a little below the level of the top of the sternum;" and another, in which it was so low, that " its upper margin corresponded to the middle of the fourth vertebra".