THORACIC CAVITY 113 



surface of the right pleura and lung. Posterior to the ascend- 

 ing aorta, from below upwards, are both atria, the right 

 pulmonary artery and the right bronchus. To the right are 

 the auricle of the right atrium, below, and the superior vena 

 cava, above; and to the left lie the left auricle, below, and the 

 upper part of the stem of the pulmonary artery, above. 



Branches. Only two branches are given off from the 

 ascending part of the aorta ; they are the right and left 

 coronary arteries. The right springs from the anterior 

 aortic sinus and the left from the left posterior sinus. Their 

 distribution has been described already (p. 84). 



The Arch of the Aorta. The aortic arch commences at the 

 termination of the ascending part of the aorta, at the level of 

 the second costal cartilage, and posterior to the right margin of 

 the sternum, from which it is separated by the anterior part 

 of the mediastinal portion of the right pleura and lung, or 

 by the remains of the thymus (Figs. 46, 57). It runs 

 backwards, to the left, and slightly upwards, through the middle 

 mediastinum and round the left margins of the trachea and 

 oesophagus (see Figs. 23 and 57), to the level of the lower 

 border of the left side of the fourth thoracic vertebra, where it 

 becomes continuous with the descending part of the aorta. It 

 is curved in both the vertical and the horizontal planes, and 

 as it passes backwards and to the left it forms a convexity 

 upwards, and also a convexity which is directed forwards and 

 to the left. Its lower border is connected with the left 

 pulmonary artery by the ligamentum arteriosum, and from its 

 upper border arise the three great vessels which supply the 

 head, neck and superior extremities (Figs. 32, 39). 



Relations. Above, the left innominate vein runs along its 

 upper border, immediately anterior to the inferior parts of the 

 innominate artery, the left common carotid artery and the left 

 subclavian artery, which spring from it ; the innominate artery 

 arises from the apex of the convexity of the arch, posterior 

 and a little to the left of the centre of the manubrium sterni; 

 the left common carotid artery arises close to, and some- 

 times in common with, the innominate artery, whilst the 

 origin of the left subclavian artery is a little more posterior 

 and to the left, separated by a distinct interval from the left 

 common carotid (Fig. 32). 



Below the arch lie (i) the bifurcation of the stem of 

 the pulmonary artery and portions of its right and left 



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