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 Anterior Or Ascending Portion arises from the base of the left ventricle, anterior and a little to the right side of the left auriculo-ventricular opening, in front of the left side of the body of the fourth dorsal vertebra, and corresponding to the junction of the cartilage of the fourth rib with the sternum at the left side. From its origin it proceeds upwards, forwards, and to the right side, till it reaches the level of the cartilage of the second rib, at its junction with the cartilage connecting the first and second pieces of the sternum. In this course its anterior surface is related to the pericardium, which separates it from the anterior mediastinum and back of the sternum; to the right coronary artery, the infundibulum of the right ventricle, the pulmonary artery at its origin, and to the tip of the right auricular appendix: the posterior surface corresponds to a part of the left auricle and to the right pulmonary- artery; the left surface is related to the pulmonary artery immediately before it divides; and the right surface first rests on a part of the base of the right ventricle between its arterial and auricular openings, and corresponds in the rest of its course to the descending or superior vena cava. The greatest part of this ascending portion is within the pericardium, the serous layer of which forms a sheath common to the aorta and pulmonary artery. This sheath also contains the right inferior cardiac nerve, which lies between these great vessels, in its course to the coronary plexus of the heart; together with the anterior and posterior cardiac plexuses. We may observe, also, that the serous sheath extends higher up on the aorta than on the pulmonary artery, and higher up on its right than on its left side. The fibrous layer of the pericardium is lost a little higher up on the external coat of the artery, by becoming continuous on this vessel with the descending layer of the thoracic fascia.
If we look at the origin of the aorta through the left ventricle, we observe a triangular opening, the area of which is more contracted than any other part of the arch is naturally found; immediately outside this triangular opening we observe three small bulgings or dilatations, called the sinuses of Valsalva; and above it the aorta enlarges and assumes a form nearly cylindrical, but not exactly so, on account of certain deviations to be noticed hereafter. In order to examine its connection with the heart, we may slit up the front of it longitudinally from the left ventricle. We then find that the aorta is united to the heart in the following manner: first—internally by the continuity of their lining membrane; secondly —by the serous layer of the pericardium, forming a sheath passing up on the vessels as already described; thirdly—on removing these two layers of membrane, we find that the proper fibrous tunic of the artery does not present a straight edge to the ventricle, but that it is formed into three distinct arches, the convexities of which are directed towards the heart. Each of the convexities, or festoons, as they are also called, is separated from its fellow by a small triangular interval, the base of which corresponds to the ventricle. The origin of the vessel will thus present three inverted arches, separated from each other by three small triangular spaces. On examining the base of the left ventricle in this situation, we observe the zona tendinosa, which forms the principal medium of connection between it and the aorta. The inferior margin of this zone is imbedded in the muscular fibres of the ventricle, whilst to its superior margin are intimately and strongly attached by condensed areolar tissue, the three convexities already described. Fourthly, when we examine the small triangular intervals between the festoons, after having removed both the serous layer of the pericardium and the lining membrane of the aorta and left ventricle, we perceive that a process of fibrous membrane, prolonged from the superior margin of the zona tendinosa, fills up each of these intervals, and becomes continuous with the "sclerous" or external tunic of the vessel.
The description, therefore, which represents the lining membrane of the artery, and the serous layer of the pericardium, as being "in apposition" in these triangular spaces, is not correct. The processes from the tendinous zone which fill up the intervals between the three convexities may be easily demonstrated: they are by no means so strong as the rest of the ring, but, though very delicate, have considerable resistance, and are separated from the serous layer of the pericardium by areolar tissue continuous with the external tunic of the artery. It is clear, however, that the lining membrane of the aorta and serous layer of the pericardium could not possibly be in apposition in that situation, where the pulmonary artery and aorta are in contact with each other, and where the serous layer of the pericardium does not dip in between these vessels.
On the inside of the aortic opening we find three folds of the lining membrane forming three semilunar valves, the inferior convex margins of which are attached opposite to the convex margins of the three inverted arches; their free or concave margins look upwards, and each of them is strengthened in its centre by a small prominent body termed the corpus Arantii, or corpus sesamoideum. When the aorta contracts, these valves are thrown away from the walls of the artery, inwards towards the centre or area of the aortic opening, and thus prevent the return of blood into the ventricle: this object is supposed to be more completely effected by the corpora Arantii closing up at that instant the small triangular space which would otherwise exist at the common centre of approximation of the three semilunar valves. Corresponding to the outer surfaces of these valves, the aorta presents three pouches or dilatations termed the lesser sinuses of the aorta, or sinuses of Valsalva. These exist at birth, but are better marked in the adult than in the young subject, on account of the constant pressure of the blood during the contraction of the vessel. By the great sinus of the aorta is meant an enlargement of the tube at the upper part of its first stage, where the vessel begins to change its direction. It does not engage the whole circumference of the tube, but is limited to its anterior and right side. It is obviously the effect of the impulse of the blood from the left ventricle, and is therefore better marked in the old than in the young subject.
If a cast of the interior of the aorta be taken in wax or plaster, it will present at its origin three distinct bulgings, corresponding to the sinuses of Valsalva; these bulgings will appear to be separated from each other by three small fissures, which unite in the centre of the area of the artery: the same observation applies to the pulmonary artery.