-3JS THE AORT.l, 



it bends downwards, inclining, at the same time, a little towards the 

 middle line ; and at the lower border of the body of the fourth dorsal 

 vertebra, on its left side, the arch terminates in the descending portion 

 of the vessel. Near the base of the heart the aorta is larger than else- 

 where, and presents externally three small bulgings of nearly equal 

 size, corresponding with the dilatations which form the sinuses of Val- 

 mh'a or s/nuses of the aortic valves, described with the heart. Two of 

 these sinuses are placed anteriorly and one posteriorly, and in the two 

 anterior sinuses are seen the orifices of the two coronary arteries of the 

 heart, the first branches given oft' by the aorta. 



From the difference in the direction and connections of different 

 portions of the arch it is described as consisting of an ascending, a trans- 

 verse, and a descentlin(j portion. 



The ascendinfi portion of the arch of the aorta is placed at its com- 

 mencement behind the sternum, on a level with the lower l)order of the 

 third costal cartilage of tlie left side : and it rises as high as the upper 

 border of the second costal cartilage of the right side. Its length is 

 about two inches or two inches and a quarter ; and its direction is 

 •curved. In most cases there exists along tlie right side a dilatation, 

 named the great sinus of the aorta. This dilatation varies in size in 

 ditferent bodies, and occasionally is not to be detected. 



This portion of the aortic arch is for the greater part of its length 

 enclosed in the same sheath of pericardium witli the ])ulmouary artery 

 in such a manner that both vessels are covered by tlie serous mem- 

 brane, except where they are in contact with each other. 



At its commencement the ascending part of the arch is in contact 

 anteriorly with the puhnonary artery, and with the right auricular 

 appendage ; but, farther up, the aorta passes to the right side and the 

 ])ulmonary artery to the left, and thus the aorta comes into view. 

 It then approaches very near to the sternum, from which it is separated 

 only by the pericardium, by some connective tissue, and by the remains 

 of the thymus gland lodged in the mediaiitinal space ; the descending 

 vena cava lies on the right side, and the pulmonary artery passes back- 

 wards on the left ; while behind are placed the structures forming the 

 root of the right lung. 



The second or transverse part of the arch is covered on the left side 

 by the left pleura and lung, and is placed immediately iu front and to 

 tiie left of the trachea, the oesophagus and tlie thoracic duct. The 

 upper border of the transverse part of the arch has in contact with it 

 the left innominate vein ; and from it are given off' the large arteries 

 (innominate, left carotid, and left subclavian), which are furnished to 

 the head and the ujiper limbs. The lower or concave border overhangs 

 the bifurcation of the pulmonary artery, and is connected with the left 

 branch of that artery by the remains of the ductus arteriosus. This 

 part of the arch is crossed in front by the left pneumogastric phrenic 

 and superficial cardiac nerves ; and the recurrent laryngeal branch of 

 the pneumogastric turns upwards beneath and behind it. 



The descending portion of the arch rests against the left side of the 

 body of the third and fourth dorsal vertebrae, and is covered by the left 

 pleura and the root of the left lung. To the right side of this part of 

 the arch is the oesophagus with the thoracic duct. 



Branches. — The branches given off from the arch of the aorta are 

 five in number. Two of these, named the coronary arteries of the* 



