POSTERIOR MEDIASTINUM. 121 



the recurrent laryngeal nerves, which pass upward beside 

 the trachea to the larynx; upon the left side this nerve 

 curves round the arch of the aorta ; on the right it curves 

 round the subclavian artery (p. 56). 



he THORACIC AORTA commences at the lower border of 

 third dorsal vertebra ; it lies at first upon the left side 

 of the vertebral column, and then inclines inward to the 

 median line; as it passes through the diaphragm it rests 

 upon the fronts of the vertebrae. It gives off the follow- 

 ing branches, viz : 



Bronchial, 



(Esophageal, 



Intercostal. 



The bronchial arteries, two or more in number, and very irregular in 

 their origin, are usually found on the anterior aspect of the aorta, just 

 as the arch ceases to make its curve ; they are of considerable size, 

 and pass immediately to the primary bronchi, upon which they ramify 

 in a tortuous manner, giving small twigs to the oesophagus and peri- 

 cardium, and terminating in the parenchyma of the lungs, of which 

 they are the nutrient vessels. 



The cesofjhageal arteries vary in number, and are small branches 

 rising from the front of the aorta, and distributed upon the oesopha^ 



The intercostal arteries, nine in number, upon each side, are given 

 off from the posterior aspect of the aorta. The right intercostals are 

 the longest, the position of the aorta obliging them to arch over the 

 bodies of the vertebrae. They supply each intercostal space except 

 the two upper, which are furnished by a branch from the subclavian 

 (p. 59). Each artery is accompanied by a nerve and vein, the former 

 being the anterior branch of the spinal nerves, and the latter a branch 

 of the vena azygos. The artery occupies the upper part of the inter- 

 costal space, lying in the groove of the lower border of the rib, between 

 the two layers formed by the external and internal intercostal mus- 

 cles ; it passes forward to inosculate with the anterior intercostal 

 branch of the internal mammary, giving off at various parts of its 

 course external branches, which perforate the intercostal space to go 

 to the muscles and integument of the back and thorax. From one of 

 these branches in the dorsal region a spinal twig goes to the interior 

 of the vertebral canal. 



The lungs should now be removed, by dividing the trachea just 

 above its bifurcation. 



The (ESOPHAGUS is a hollow muscular tube, extending 

 from the pharynx to the stomach. The cervical portion is 

 described at p. 62. The thoracic portion enters the chest 

 on the left of the median line, passes beneath the arch of 

 the aorta, to continue on the right side of that trunk to 

 11 



