AORTA* (human anatomy). — (Arteria magna. Fr. aorte. Germ. Aorta, die grosse Schlagader. Gr. cocri.) Hippocrates applied the term ocogTocc to the lower part of the bronchi. Aristotle called the great trunk of the arterial system The aorta, one of the two great arteries which spring from the heart, is the trunk of the arterial system of the general circulation ; it arises from the extreme right part of the base of the left ventricle of the heart, which, from this circumstance, is sometimes called the aortic ventricle. There is a ring of tendinous structure surrounding the aortic opening of the ventricle, which in the stag and some others of the ruminantia is more or less partially ossified; into this ring the muscular fibres of the heart are inserted. The middle tunic of the aorta is divided at its commencement into three semicircular flaps by an equal number of angu lar notches, forming thus a festooned edge which is bordered throughout its whole extent by a marginal tendinous cord. These three semicircular flaps touch the aortic opening of the ventricle at three equidistant points by the centres of their convex edges, where the fibres of their marginal cord become intimately blended with those of the tendinous ring of the aortic opening of the ventricle ; between these points are three triangular intervals, each of which is occupied by a thin tendinous expansion of considerable strength, having one of its sides continuous with the tendon which encircles the aortic opening of the ventricle, and the other two continuous with the marginal ten dinous cord of the fe.tooned commencement of the middle tunic of the aorta.
The convex margins of the sigmoid valves of the aorta are attached to the margins of the semilunar flaps, and are composed of thin ex pansions sent off from their marginal tendinous cord, covered by a reflexion of the lining mem brane common to the heart and arteries. Hence it follows that the fibres of the middle tunic of the aorta are not continuous with the muscular fibres of the ventricle, being sepa rated from them by the tendinous structure above described ; this tendinous connexion is strengthened and supported externally by a layer of dense cellular membrane, which may be regarded as the commencement of the cellular or external tunic of the arterial system. The lining membrane of the heart,
after being reflected over the sigmoid valves, extends itself into the aorta, and becomes continuous with the lining membrane of that vessel. The muscular substance of the heart rises in form of a swollen annular border around the commencement of the aorta for a little distance, and is connected to it by dense cellular membrane. The serous layer of the pericardium passes loosely from the surface of the heart over the aorta ; a quantity of soft adipose substance, which is absent in the fcetus during the earlier months, begins to collect under the serous membrane in this situation, sometimes before, sometimes after birth, and, increasing as life advances, is found in considerable quantity in old age. The fore going description of the connexion of the aorta with the heart has been determined by my own dissections repeatedly performed, and agrees, in its leading particulars, with the account given of it by M. Bedard."' The aorta, arising from the left ventricle of the heart opposite the left side of the body of the fourth thoracic vertebra, ascends at first obliquely forwards, and to the right behind the middle bone of the sternum, until it arrives at the right side opposite the second intercostal space, and behind the sternal articulation of the cartilage of the second rib ; it then stretches backwards and to the left, opposite the junction of the upper and middle portions of the ster num, on a level with the body of the second thoracic vertebra, and it reaches the left side of the body of the third thoracic vertebra, on which there is a slight depression for lodging it; from this point it descends through the posterior mediastinum, advancing in its course downwards from the left side to the front of the bodies of the ver tebra ; it passes through the aortic opening of the diaphragm, enters the abdomen, and on the body of the fourth abdominal vertebra gives off the two primitive iliac arteries, in which it seems at first view to terminate ; the aorta, however, does not end here, but is continued, although greatly reduced in size, under the name of the middle sacral artery, as far as the extremity of the os coccygis.