THE THORAX 961 



thyroid body in the neck, it lies in front of the trachea, and would 

 be endangered in perfoiming the low operation of tracheotomy. 



Varieties. — (i) The innominate artery may be shorter or longer than usual. 

 (2) In cases of high bifurcation the artery may so encroach upon the trachea 

 as to be endangered in performing the low operation of tracheotomy. 



Left Common Carotid Artery in the Thorax. — This vessel arises 

 from the upper aspect of the arch of the aorta, just to the left of, 

 and posterior to, the origin of the iimominate artery. It lies in 

 the superior mediastinum, and its direction is upwards and to the 

 left. Having reached the posterior aspect of the left sterno- 

 clavicular joint, it enters upon the cervical part of its course. 



Relations — Anterior. — ^The left half of the manubrium stemi, 

 with the origins of the left stemo-hyoid and sterno-thyroid muscles, 

 but these structures lie at a little distance from the vessel ; the left 

 innominate vein ; and some remains of the thymus bod}'. Pos- 

 terior. — ^The trachea at first, and subsequently the oesophagus 

 (which here deviates slightly to the left of the trachea), and the 

 thoracic duct. Right. — ^The innominate artery at first, and there- 

 after the trachea. Left. — ^The left pneimiogastric and left phrenic 

 nerves, with the superior cervical cardiac branch of the left sympa- 

 thetic and the inferior cervical cardiac branch of the left pneumo- 

 gastric ; and the left pleura and lung. 



The thoracic portion of the left subclavian artery is on the left 

 of, and posterior to, the vessel, but at a little distance from it. 



The thoracic portion of the left common carotid artery gives off 

 no branches. 



First Part of the Left Subclavian Artery in the Thorax.— This 

 vessel arises from the upper aspect of the arch of the aorta, a little 

 to the left of, and posterior to, the origin of the left common carotid 

 artery. It lies deeply in the superior mediastinum, and is almost 

 parallel to the thoracic portion of the left common carotid, its 

 course being nearly vertical. 



Relations — Anterior. — ^The left common carotid artery ; the left 

 pneumogastric and left phrenic nerves, with the superior cervical 

 cardiac branch of the left sympathetic and the inferior cervical 

 cardiac branch of the left pneumogastric, all these nerves lying 

 between it and the left common carotid artery ; and the left in- 

 nominate vein. Posterior. — ^The oesophagus, and the thoracic duct. 

 Right. — ^The trachea, and the left recurrent laryngeal nerve. Left. — 

 The left pleura, and the inner aspect of the left lung, the latter being 

 grooved by the vessel. 



The thoracic portion of the left subclavian artery gives off no 

 branches. 



Varieties of the Aorta — i. Position. — (a) The arch of the aorta may rise 

 IS high as the upper border of tlie manubrium stemi, or it may stop short 

 of the level of the centre of the manubrium, (b) It may be a right aortic 

 arch, instead of a left, in which cases there may, or may not, be a transposition 

 of the viscera. The presence of a right aortic arch is due to its formation 

 from the fourth right arterial arch, and the persistence of the right primitive 

 dorsal aorta. 



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