Aneurisms of the arch of the aorta do not so often terminate fatally by making their way through the anterior parietes of the chest, and opening externally as by bursting internally : when they occur in that part of the arch of the aorta covered by the pericardium, they most usually burst into the sac of that membrane; cases are recorded in which aneurisms of the aorta have burst into the pulmonary artery,* or, taking a direction backwards, have opened into the trachea, oesophagus, or the substance of the lungs. Aneurisms of the thoracic por tion of the aorta sometimes burst into the left pleura, sometimes into the posterior medi astinum : they have been known to point at the left side of the spine, after having caused ab sorption of the heads of the ribs and sides of the bodies of the vertebra. In two cases observed by Laennec and Mr. Chandler, aneu rism of the thoracic aorta burst into the spinal canal. Aneurisms of the abdominal aorta most usually burst into the cellular tissue of the lumbar regions behind the peritoneum, seldom into the sac of that membrane. An aneurism of the abdominal aorta has been observed to make its way backwards by the side of the spine, and point in such a situation as to have been at first mistaken for lumbar abscess.
Branches of the aorta. I. Branches arising from the arch.—From the arch of the aorta five branches are given off; two from its com mencement, the coronary arteries, and three vessels of considerable size (fig. 78 a b c), from the upper part of its transverse portion to supply the head and the upper extremities. The coronary arteries of the heart or the car diac arteries arise from the aorta close to its origin, and immediately above the free borders of the sigmoid valves; they are usually two in number, one for each ventricle.
The right, anterior or inferior coronary artery is often larger, seldom smaller than the left ; it arises from the anterior side of the aorta above the anterior sigmoid valve, coming out from between the roots of the aorta and pulmonary artery, it passes downwards and to the right side in the groove between the right auricle and ventricle, turns round the right edge of the heart until it reaches the groove of the septum on the inferior surface of that organ, when it changes its direction, coursing along that groove until it arrives at the apex of the heart, where it anastomoses with the left coro nary artery ; in its course it gives off to the right and left many tortuous branches arising nearly at right angles, the right branches are smaller and go to the right auricle, the left are larger and belong to the right ventricle, which they traverse in a longitudinal direction to wards its apex. From the origin of the right
coronary artery two small branches are given off, one to the commencement of the pul monary artery and the surrounding fat, which anastomoses behind the pulmonary artery with a branch of the left coronary; the se cond branch anastomoses with the bronchial arteries.
The left posterior or superior coronary artery arises between the left auricle and the posterior surface of the pulmonary artery, de scending to the left between the left auricle and pulmonary artery, and, having reached the groove at the base of the heart, dividing into two or three branches ; one anterior longitu dinal descends along the anterior groove of the septum to the apex of the heart, where it anas tomoses with the termination of the right coronary artery, with which it holds frequent communication by branches which it sends over the anterior surface of the right ventricle, while it sends• some large branches to the left ventricle; this branch at its commencement gives small twigs to the aorta and pulmonary artery. The second branch of the left coronary artery covered by the great coronary vein passes from right to left in the groove between the left auricle and ventricle, to both of which it gives many branches, turns round the left border of the heart, changes its direction, and descends by the side of the right coronary artery to the apex ; the third branch sinks into the substance of the septum and continues its course to the apex ; this branch sometimes arises directly from the aorta ; in this latter case, of course, there will be three coronary arteries arising from the aorta; Meckel has once seen four ; the supernumerary coronary artery does not arise above any particular valve, but usually close to the origin of one of the normal branches. It is rare to find but one coronary artery in the human subject,which corresponds, according to Camper, with the normal con formation in the elephant. The three large branches arising from the transverse portion of the arch of the aorta and sent to the head and upper extremities, will be described in a separate article.
II. Branches of the thoracic aorta.----These may be divided into anterior and lateral. The anterior branches are, the bronchial, esophageal, and posterior mediastinal. The lateral are the inferior or aortic intercostal arteries. The bronchial arteries are usually two in number, one for each lung ; sometimes, however, there are two for each lung, and sometimes the right and left bronchial arise from a common trunk, which usually springs from the first aortic in tercostal of the right side.