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.
This vessel is formed after the portal system, with which it is connected by a dilatation which is the rudiment of the future heart. In the young child it lies nearer the spine than in the adult, on account of the larger size of the thymus gland which lies in front of it, and the comparatively imperfect development, at this period of life, of the trachea and bronchial tubes, which are situated behind it: but as the right bronchus becomes developed, and the thymus gland absorbed, the arch of the aorta advances nearer to the sternum. We also find that in the young subject the arch is situated higher up than in the adult: this is owing to the thorax of the child having less proportional height; and for the same reason the arch is higher in the adult female than in the male. In some cases we find it unnaturally high, independently of the age or sex of the individual. In the old subject the swell of the arch is considerably increased by the development of the great sinus. If a vertical section be made of the arch of the aorta, the convexity of the arch will be found to be thicker than the concavity.
* Dublin Journal, vol. ix.