This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The aorta, as it leaves the left ventricle, begins under the left portion of the sternum opposite the lower border of the third left costal cartilage. This is the location of the aortic semilunar valves as already given. It passes upward toward the right for 5 cm. and then forms an arch, extending backward toward the left, to reach the spine on the left side of the body of the fourth thoracic vertebra. The arch is continued down in front of the spine as the thoracic aorta and pierces the diaphragm in the median line, between the two crura of the diaphragm, opposite the twelfth thoracic vertebra.
The ascending aorta begins behind the left half of the sternum on a level with the lower border of the third costal cartilage. It proceeds upward toward the right until it reaches the level of the lower border of the right second costal cartilage, where the arch begins.
Immediately above its commencement it has three enlargements, called the sinuses of the aorta (Valsalva), which correspond to the semilunar valves. Of the three semilunar valves two are anterior and one is posterior. From behind the two anterior valves come the right and left coronary arteries.
Beyond the valves, in the upper right portion of the arch, the aorta is again dilated, forming the great sinus of the arch of the aorta.
The right limit of the aorta is about even with the right edge of the sternum; sometimes it projects slightly beyond. When it does so it is liable to be wounded by a stab in the second interspace close to the edge of the sternum. On account of the proximity of the aorta to the second interspace, it is here that the stethoscope is placed to hear aortic murmurs. The aorta at this point is covered only by the thin border of the right lung and pleura and the slight remains of the thymus gland. Below, its commencement is overlapped on the right by the auricula dextra (right auricular appendix) of the atrium and on the left by the commencement of the pulmonary artery.
If the aneurism enlarges anteriorly it will show itself first in the second or third interspace. It will bulge the ribs outward in this region. The right lung will be pushed outward and the two layers of the pleura pressed together. It may break externally through the surface or open into the pleural cavity. If it tends to the right it presses on the descending cava and right atrium, thus interfering with the return of the blood from the head and neck and both upper extremities. If it enlarges to the left or backward it may press on the right pulmonary artery and interfere with the free access of blood to the lungs.
The first portion of the aorta is not united with the pericardium, but simply loosely covered by it, so that this portion of the arch is weaker than the other portions, and rupture, with extravasation of blood into the pericardial sac, is not uncommon. An aneurism may also rupture into the superior vena cava.
The arch of the aorta passes anteroposteriorly from the upper border of the second right costal cartilage in front to the left side of the body of the fourth thoracic vertebra behind. It is about 5 cm. (2 in.) long. Its under surface is level with the angle of the sternum (angle of Ludwig), opposite the second costal cartilage. Its upper surface rises as high as the middle of the first piece of the sternum, which is opposite the middle of the first costal cartilage, about 2.5 cm. (1 in.) below the top of the sternum.